Neuromol Med (2014) 16:490–498 DOI 10.1007/s12017-014-8297-7
ORIGINAL PAPER
Growth Factors and Synaptic Plasticity in Relapsing–Remitting Multiple Sclerosis Francesco Mori • Carolina G. Nicoletti • Silvia Rossi • Caterina Motta • Hajime Kusayanagi • Alessandra Bergami • Valeria Studer • Fabio Buttari Francesca Barbieri • Sagit Weiss • Robert Nistico` • Gianvito Martino • Roberto Furlan • Diego Centonze
•
Received: 16 December 2013 / Accepted: 13 March 2014 / Published online: 27 March 2014 Ó Springer Science+Business Media New York 2014
Abstract During multiple sclerosis (MS) inflammatory attacks, and in subsequent clinical recovery phases, immune cells contribute to neuronal and oligodendroglial cell survival and tissue repair by secreting growth factors. Animal studies showed that growth factors also play a substantial role in regulating synaptic plasticity, and namely in long-term potentiation (LTP). LTP could drive clinical recovery in relapsing patients by restoring the excitability of denervated neurons. We recently reported that maintenance of synaptic plasticity reserve is crucial to contrast clinical deterioration in MS and that the plateletderived growth factor (PDGF) may play a key role in its regulation. We also reported that a Hebbian form of LTPlike cortical plasticity, explored by paired associative stimulation (PAS), correlates with clinical recovery from a relapse in MS. Here, we explored the role of PDGF in clinical recovery and in adaptive neuroplasticity in
F. Mori C. G. Nicoletti S. Rossi C. Motta H. Kusayanagi V. Studer F. Buttari F. Barbieri S. Weiss D. Centonze (&) Clinica Neurologica, Dipartimento di Medicina dei Sistemi, Universita` Tor Vergata, Via Montpellier 1, 00133 Rome, Italy e-mail: [email protected] F. Mori C. G. Nicoletti S. Rossi C. Motta H. Kusayanagi V. Studer F. Buttari F. Barbieri S. Weiss R. Nistico` D. Centonze IRCCS Fondazione Santa Lucia, 00143 Rome, Italy A. Bergami G. Martino R. Furlan Neuroimmunology Unit, Division of Neuroscience, Institute of Experimental Neurology (INSpe), San Raffaele Scientific Institute, 20132 Milan, Italy R. Nistico` Dipartimento di Fisiologia e Farmacologia, Universita` La Sapienza, 00185 Rome, Italy
123
relapsing–remitting MS (RR-MS) patients. We found a correlation between the cerebrospinal fluid (CSF) PDGF concentrations and the extent of clinical recovery after a relapse, as full recovery was more likely observed in patients with high PDGF concentrations and poor recovery in subjects with low PDGF levels. Consistently with the idea that PDGF-driven synaptic plasticity contributes to attenuate the clinical consequences of tissue damage in RR-MS, we also found a striking correlation between CSF levels of PDGF and the amplitude of LTP-like cortical plasticity explored by PAS. CSF levels of fibroblast growth factor, granulocyte colony-stimulating factor and granulocyte–macrophage colony-stimulating factor did not correlate with clinical recovery nor with measures of synaptic transmission and plasticity. Keywords Recovery
CSF PDGF TMS PAS Relapse
Introduction In the early and acute phases of multiple sclerosis (MS), T cells accumulate in the CNS and release various, predominantly proinflammatory cytokines that mediate tissue destruction (Kierdorf et al. 2010). However, during the course of the disease and in subsequent clinical recovery phases, immune cells may also contribute to neuronal and oligodendroglial cell survival and tissue repair by secreting different growth factors (Schwartz et al. 1999; Kerschensteiner et al. 2003). Release of growth factors in MS brains could serve several functions, including the modulation of microglial activity, and the stimulation of oligodendrocyte proliferation or regeneration within lesioned areas (Webster 1997; Chadi and Fuxe 1998; Messersmith et al. 2000;
Neuromol Med (2014) 16:490–498
Erlandsson et al. 2001; Frost et al. 2003; Ponomarev et al. 2007; Rottlaender et al. 2011). Apart from regulating inflammatory processes, promoting neuronal survival and remyelination, growth factors also play a substantial role in synaptic transmission and plasticity, and namely in long-term potentiation (LTP), a form of neuroplasticity consisting in use-dependent and enduring enhancement of excitatory synaptic transmission (Mattson et al. 1993; Schneider et al. 2005; Boyd et al. 2010; Peng et al. 2010). The mechanisms of synaptic plasticity and of neuronal survival largely overlap at receptor and post-receptor levels (Bartlett and Wang 2013), and in fact, LTP disruption is a common feature of the early phases of many neurodegenerative disorders (Bartlett and Wang 2013). In MS, as well as in other neurological diseases, synaptic transmission adaptations and LTP might also be crucial to favor clinical compensation of tissue damage, as increased synaptic responsiveness in surviving neurons is potentially able to restore membrane excitability of neurons that have lost part of their synaptic inputs (Morgen et al. 2004; Mezzapesa et al. 2008). In line with this hypothesis, LTP induction is greatly facilitated in response to ischemic brain damage in rodents (Hagemann et al. 1998), and its occurrence in the periinfarct area is associated with better clinical outcome, both in rodents (Centonze et al. 2007) and in humans (Di Lazzaro et al. 2010). Consistently, we have recently found that cortical LTP reserve, explored by means of transcranial magnetic stimulation (TMS), plays a major role in the compensation of neurological deficits, as this form of synaptic plasticity is still possible and even favored in stable MS patients, while it is exhausted in progressive forms of the disease (Mori et al. 2013a). In addition, cortical plasticity reserve, as explored by means of paired associative stimulation (PAS), another paradigm of TMS exploring Hebbian-LTP also correlates with the extent of clinical recovery after a relapse in MS patients (Mori et al. 2013b). In animal studies, synaptic transmission and LTP are regulated by various growth factors and, among these, platelet-derived growth factor (PDGF) (Peng et al. 2010), fibroblast growth factor (FGF) (Mattson et al. 1993), granulocyte colony-stimulating factor (G-CSF) (Schneider et al. 2005) and granulocyte–macrophage colony-stimulating factor (GM-CSF) (Boyd et al. 2010) are released by inflammatory cells and have therefore the potential to influence synaptic functioning and the clinical manifestations in MS. Thus, the present investigation aimed at exploring the role of PDGF, FGF, G-CSF and GM-CSF in adaptive neuroplasticity and clinical recovery in relapsing–remitting MS (RR-MS) patients. Our results point to PDGF as an essential determinant in LTP induction and in clinical
491
compensation of acute inflammatory damage in the brains of RR-MS patients.
Patients and Methods The study was approved by the Ethics Committee of the University Hospital Tor Vergata, Rome. All the subjects gave their written informed consent to the study. Determination of Growth Factors in the Cerebrospinal Fluid (CSF) To determine PDGF, FGF, G-CSF and GM-CSF concentrations in RR-MS, we used the CSF collected from 84 RRMS patients and from 47 age- and gender-matched controls who were admitted to the neurology clinic of the University Hospital Tor Vergata of Rome between 2009 and 2012. After their admittance, patients underwent, in sequence, brain (and in selected cases also spinal) magnetic resonance imaging (MRI) scan and CSF withdrawal within 24 h. Corticosteroids or other MS-specific immunoactive therapies were initiated later when appropriate. The diagnosis of MS was established by clinical, laboratory and MRI parameters, and matched published criteria (Polman et al. 2011). In all instances, patients underwent detection of oligoclonal banding in the CSF. As controls, we used the CSF from individuals without inflammatory or degenerative diseases of the central or peripheral nervous system who underwent lumbar puncture because of a clinical suspect of acute peripheral neuropathy, meningitis or subarachnoidal hemorrhage, which were not confirmed. Immediately after withdrawal, the CSF was centrifuged and stored at -80 °C until analyzed. Platelet-derived growth factor, FGF, G-CSF and GMCSF concentrations were analyzed using Bio-Plex Multiplex Cytokine Assay (Bio-Rad Laboratories), according to manufacturer instructions. Growth factor concentrations were calculated according to a standard curve generated for the specific target and expressed as pg/ml. When the concentrations were below the detection threshold, they were assumed to be 0 pg/ml. MS Subjects Multiple sclerosis disease onset was defined as the first episode of focal neurological dysfunction indicative of MS. Disease duration was estimated as the number of years from onset to the last assessment of disability, performed at the time of CSF withdrawal. Relapses were defined as the development of new or recurrent neurological symptoms not associated with fever or infection lasting at least 24 h. Disability was determined
123
492
by a specially trained (Neurostatus training and documentation DVD for a standardized neurological examination and assessment of Kurtzke’s functional systems and Expanded Disability Status Scale for MS patients. Basel, Switzerland: Neurostatus, 2006. Available at http://www. neurostatus.net) and certified examining neurologist using Expanded Disability Status Scale (EDSS), a ten-point disease severity score derived from nine ratings for individual neurological domains. In a sample of 21 MS patients, CSF withdrawal and TMS were done during a clinical relapse, and in 14 of these patients, disability assessment was repeated three months later. All patients received standard corticosteroid treatment (3–5 daily i.v. infusions of 1,000 mg methylprednisolone) during the relapse, which in all cases was initiated after CSF withdrawal, clinical and neurophysiological assessments. Based on the difference between the second and the first EDSS assessments, they were classified into a ‘‘complete recovery’’ group (EDSS returned to prerelapse score) and an ‘‘incomplete recovery’’ group (EDSS higher than pre-relapse score). MRI Acquisition and Analysis Three Tesla MRI scans consisted of dual-echo proton density, FLAIR, T2-weighted spin-echo images, and precontrast and post-contrast T1-weighted spin-echo images. All images were acquired in the axial orientation with 3-mm-thick contiguous slices. The presence of gadoliniumenhancing (Gd?) (0.2 ml/kg e.v.) lesions was assessed by a neuroradiologist who was unaware of the patient’s clinical details. Intracortical Circuits in the Motor Cortex The output of the primary motor cortex (M1) can be objectively measured in the form of a motor evoked potential (MEP) from surface electromyographic (EMG) recording electrodes, positioned on the skin overlying the targeted muscles in response to a single suprathreshold TMS pulse delivered to M1. The MEP amplitude elicited by stimulation of M1 can be modulated by a preceding conditioning pulse (CS). Depending on the intensity and inter-stimulus interval (ISI) used, a CS can produce either a facilitation or inhibition of the MEP elicited from the test stimulus (TS) over M1. To explore both excitatory and inhibitory intracortical circuits, we tested through paired-pulse (pp) TMS, shortinterval intracortical inhibition (SICI; mediated by intrinsic GABAAergic circuits) (Kujirai et al. 1993), intracortical facilitation (ICF; believed to follow the preferential recruitment of intrinsic excitatory fibers) (Kujirai et al. 1993), short intracortical facilitation (SICF; likely
123
Neuromol Med (2014) 16:490–498
mediated by excitatory cortical interneurons) (Ziemann et al. 1998) and long-interval intracortical inhibition (LICI; mediated by local GABAB pathways) (Valls-Sole´ et al. 1992) of the left M1. One figure-of-eight coil, external diameter 70 mm, was held tangentially to the scalp over the motor ‘‘hot spot’’ for right FDI muscle. Stimulation intensity for TS was adjusted in each experiment to evoke an MEP of about 1 mV peakto-peak amplitude in the relaxed right first dorsal interosseus muscle (FDI). SICI and ICF were tested using ppTMS with a subthreshold CS preceding a suprathreshold TS (Kujirai et al. 1993). CS stimulus was set at 80 % AMT. Three conditions were presented in a random order: control (TS given alone) and two pp conditions (TS preceded by CS) at one of two different ISI (2 and 15 ms). For SICF, the intensity of CS was set to 90 % RMT. Four randomly intermixed conditions were presented in a random order: TS given alone and three conditions with the TS followed by CS at one of three different ISIs (1.5, 2.7, and 4.5 ms) (Hanajima et al. 2002). For LICI, the intensity of CS was set at 120 % RMT. Two conditions were presented in a random order: control (TS given alone) and one pp condition (TS preceded by CS) at 100 ms ISI (Valls-Sole´ et al. 1992). For each experiment, 10 responses were collected for the TS alone and for conditioned MEPs at each ISI. Changes in MEP amplitude at each ISI were expressed as the percentage of the mean unconditioned MEP amplitude. EMG Electromyographic traces were recorded from the right first dorsal interosseus muscle (FDI) with surface cup electrodes. The active electrode was placed over the muscle belly and the reference electrode over the metacarpophalangeal joint of the index finger. Responses were amplified with a Digitimer D360 amplifier (Digitimer, Welwyn Garden City, Hertfordshire, UK) through filters set at 20 Hz and 2 kHz with a sampling rate of 5 kHz, then recorded by a computer with SIGNAL software (Cambridge Electronic Devices, Cambridge, UK). MEPs were evoked through a figure-of-eight coil with external loop diameter of 70 mm connected to a Magstim 2002 magnetic stimulator (Magstim Company, Whitland, Wales, UK). Coil position was adjusted to find the optimal scalp site to evoke motor responses in the contralateral FDI, the motor ‘‘hot spot,’’ at the beginning of each experimental session and marked over the patient’s scalp with a pencil. The coil was held tangentially to the scalp surface with the handle pointing posteriorly and laterally at about 45° with respect to the midsagittal axis of the head.
Neuromol Med (2014) 16:490–498
Paired Associative Stimulation (PAS) To explore LTP-like plasticity, PAS (Stefan et al. 2000) was delivered using pairs of median nerve electrical and single TMS pulses over the abductor pollicis brevis (APB) hot spot at an ISI of 25 ms. The intensity of the TMS was set to evoke an MEP of 1 mV ca. peak-to-peak amplitude in the APB, while the intensity of the median nerve stimulus (0.2 ms duration) was set at three times perceptual threshold. Two hundred pairs of stimuli were given at a rate of 0.25 Hz. MEPs were recorded from the medianinnervated APB muscle at baseline and 30 min after PAS. MEP’s amplitudes were then averaged at each time point and normalized to the mean baseline amplitude. Data Analysis Differences among two groups were compared by univariate analysis using Student’s t test for continuous variables and Fisher’s exact test for categorical variables. A multiple regression model was used to assess the effect of clinical and biochemical predictors on the LTP responses to the PAS protocol in our patients. As predicted variable, we used the MEP amplitude change 30 min after PAS. Five clinical variables (age, gender, years of disease, presence of gadolinium-enhancing (Gd?) lesions and EDSS) and CSF levels of four different growth factors (PDGF, FGF, GM-CSF and G-CSF) were tested as predictor variables. Gender and presence of Gd? lesions were coded as dummy variables. Predictors were entered into the model using a forward stepwise method. Predictors were tested for collinearity using the variance inflation factor (VIF). The relationship between growth factors and ppTMS protocols was evaluated calculating the Pearson’s correlation coefficients. Statistical tests were considered significant if p \ 0.05. Data, expressed as mean ± standard error (SE), were considered significant at the 0.05 level.
Results Concentrations of Growth Factors in the CSF of Control and MS Patients The two groups (controls and RR-MS) did not differ in terms of the demographic characteristics (Table 1). We first measured growth factor levels in the CSF of MS patients. PDGF, FGF, G-CSF and GM-CSF were not significantly altered in the population of MS subjects, compared to control individuals (p [ 0.05 for each comparison; Fig. 1a). Of note, a trend to PDGF levels increase, although
493 Table 1 Demographic and clinical characteristics of enrolled subjects Total
Control
RR-MS
p value
Number
131
47
84
Sex (F/M)
84/47
27/20
57/27
Age (yrs)
35.1 ± 9.0
36.3 ± 10.2
34.4 ± 8.2 0.2
Disease duration (yrs)
na
na
3.0 ± 5.1
0.3
EDSS
na
na
1.2 ± 1.0
EDSS range
na
na
0–5.0
RR-MS relapsing–remitting multiple sclerosis, F female, M male, yrs years, EDSS Expanded Disability Status Scale, na not applicable
not statistically significant (p = 0.11), was observed among RR-MS subjects. Upon grouping of MS patients according to the presence of Gd? lesions at the MRI, without differences in terms of demographic characteristics (Gd? MS group: n = 37, 28 females and 9 males, aged 22–55 years; Gd- MS group: n = 47, 30 females and 17 males, aged 20–51 years), GMCSF levels were higher in Gd? MS patients than in GdMS patients (p \ 0.01). No significant alterations of the CSF contents of the other growth factors were revealed by grouping Gd- and Gd? patients (p [ 0.05 for each comparison; Fig. 1b). Growth Factors and Clinical Recovery in Relapsing MS Patients The potential involvement of growth factors in post-relapse clinical recovery (Vana et al. 2007; Mori et al. 2013a) was addressed in 21 RR-MS patients, who underwent CSF withdrawal and growth factors measurement during a clinical relapse. This group of patients was clinically assessed during the relapse, and 14 of these patients were clinically reassessed in a scheduled follow-up visit three months later, while seven patients abandoned the study before completion, missing the scheduled follow-up visit for personal reasons (Table 2). Of the 14 patients who completed the study, seven showed a full symptom recovery and seven patients only partially recovered 12 weeks after the relapse. A t test for independent variables showed that PDGF concentrations were significantly higher in the CSF of patients that completely recovered than in the CSF of patients with incomplete symptom recovery after the relapse (F = 30.94, p \ 0.01; Fig. 2), indicating that full symptom recovery was more likely in patients with high PDGF concentrations. Conversely, FGF, G-CSF and GM-CSF did not differ between the two groups.
123
494
Neuromol Med (2014) 16:490–498
Fig. 1 Growth factor levels in the CSF of MS subjects. a Growth factor levels were not significantly different between RR-MS and control groups, although a slight increase in PDGF levels was
observed among RR-MS subjects. b The graph shows that, upon grouping of MS patients according to the presence of Gd? lesions, GM-CSF levels were higher in Gd? MS subjects. *p \ 0.01
Growth Factors and Cortical Excitability in MS
however, did not reach statistical significance (mean post-/ pre-MEP amplitude = 1.07, p = 0.56). Multiple regression analysis, however, revealed that PAS after effects are influenced by age (b = -0.51, p \ 0.05) and PDGF CSF levels (b = 0.30, p \ 0.05) (adjusted R2 = 0.25, p \ 0.05) (Fig. 3a, b). Gd? lesions, EDSS, disease duration, gender and CSF levels of FGF, GM-CSF and G-CSF were also tested and excluded from the model being not significant predictors. VIF \ 5 showed there was no collinearity between predictors. These results indicate therefore that PDGF may favor LTP induction in MS patients and are consistent with in vitro data showing that this growth factor enhances LTP in hippocampal brain slices (Peng et al. 2010; Mori et al. 2013a).
Animal experiments showed that growth factors can influence synaptic transmission; moreover, LTP is a complex phenomenon, and changes in glutamate-mediated synaptic drive or GABA-mediated inhibition alter its induction (Cooke and Bliss 2006; Matsuyama et al. 2008; Gong et al. 2009). Thus, we tried to investigate whether growth factors may alter basal synaptic transmission during a relapse through ppTMS experiments in cortical neurons of 21 RR-MS patients (Table 2). T test for paired samples showed a significant effect of ISI for ICF (mean MEP amplitude = 1.29 ± 0.08; p = 0.002), SICI (mean MEP amplitude = 0.63 ± 0.09; p = 0.001) and LICI (mean MEP amplitude = 0.02 ± 0.04; p \ 0.001). For SICF, a repeated-measures ANOVA showed a significant effect of ISI at 1.5 ms (F = 26.65; p \ 0.001) and 2.7 ms (F = 21.45; p \ 0.001) but not at 4.5 ms (F = 0.32; p = 0.56). Correlation analysis, however, showed that SICI, ICF, SICF and LICI were not related to CSF levels of PDGF, FGF, G-CSF and GM-CSF (Table 3). Growth Factors and PAS-Induced LTP in MS Repeated-measures ANOVA showed a trend to increased mean motor cortex excitability 30 min after PAS that,
123
Discussion The main findings of the present study are the association between high CSF PDGF levels and full symptom recovery after a relapse and the correlation between CSF PDGF levels and PAS-induced LTP in RR-MS. These results are in accordance with previous studies showing that both LTP and PDGF may contribute to limit
Neuromol Med (2014) 16:490–498 Table 2 Characteristics of the subjects enrolled in the PAS experiments
495
Patient
Gender
Age
Recovery
1
M
34
Incomplete
2
2
0
–
2
F
19
NA
1
NA
0
–
3
M
37
Complete
1
0
0
?
4
M
35
NA
1.5
NA
0
–
5
F
36
Incomplete
1.5
1
0
?
6
M
41
Incomplete
2
1
4
–
7
M
45
NA
6
NA
4
–
8
M
42
NA
2
NA
0
–
9
M
39
Incomplete
1.5
1
15
–
10
M
46
NA
3
NA
11
–
11 12
F M
32 28
Complete Incomplete
1 2
0 2
0 0
– ?
13
F
29
NA
1
NA
0
?
14
M
26
NA
1.5
NA
0
–
15
F
24
Complete
3
0
0
?
16
F
27
Complete
1.5
0
0
?
17
F
26
Complete
1
0
1
–
18
F
48
Incomplete
2
2
1.5
–
19
F
42
Complete
2
0
9
–
20
M
43
Complete
1
0
22
–
21
M
23
Incomplete
2
1
0
–
Fig. 2 PDGF levels in the CSF of RR-MS patients predicts symptom recovery after a relapse. PDGF was significantly increased in patients that completely recovered after the relapse, compared to patients with poor recovery. *p \ 0.01
MS disease progression. Indeed, the amount of LTP induced by PAS in RR-MS patients at the time of a relapse showed a correlation with the extent of clinical recovery
EDSS at onset
EDSS 3 months after onset
Disease duration (years)
Gd±
12 weeks later, in the post-relapse phase (Mori et al. 2013b). PDGF, on the other side, showed to act as a key factor in the recovery phase of chronic demyelinating damage in mice (Vana et al. 2007), while its concentration in the CSF of RR-MS patients showed to be associated with limited clinical expression of new brain lesion formation (Mori et al. 2013a). In the present study, we confirm the results of a previous report where PDGF levels in the CSF of RR-MS patients showed a correlation with another form of LTP, induced through a different TMS protocol, named theta burst stimulation (TBS) (Mori et al. 2013a). Based on these findings, we argue that PDGF may contribute to the compensation of brain damage in RR-MS by restoring the excitability of those neurons that have lost part of their synaptic inputs through the induction of synaptic LTP. The mechanism by which PDGF enhances LTP is only marginally understood, but animal studies showed that PDGF receptors are widely expressed in the CNS (Sasahara et al. 1991; Gozal et al. 2000) and that PDGF modulates LTP induction at excitatory synapses. Indeed, when mice slices of hippocampus were pre-incubated with PDGF, the magnitude of LTP measured between 50 and 60 min after TBS was significantly higher (162 ± 9 %, n = 6) than in control slices (134 ± 9 %, n = 6) (p \ 0.05) (Mori et al. 2013a). Moreover, a previous report by Peng et al. (2010) showed that PDGF gives rise to LTP inducing the
123
496 Table 3 Correlation matrix between ppTMS experiments and CSF levels of PDGF, FGF, G-CSF and GM-CSF in 21 RRMS patients
Neuromol Med (2014) 16:490–498
ICF (10 ms ISI)
SICI (2 ms ISI)
LICI (100 ms ISI)
SICF (1.5 ms ISI)
SICF (2.7 ms ISI)
SICF (4.5 ms ISI)
Pearson’s R
0.15
0.13
-0.11
0.09
0.18
0.20
Significance
n.s.
n.s.
n.s.
n.s.
n.s.
n.s.
Pearson’s R
-0.45
0.04
0.08
0.20
0.15
0.10
Significance
n.s.
n.s.
n.s.
n.s.
n.s.
n.s.
Pearson’s R
0.29
0.23
-0.42
0.24
-0.18
0.14
Significance
n.s.
n.s.
n.s.
n.s.
n.s.
n.s.
0.36 n.s.
0.51 n.s.
0.35 n.s.
-0.17 n.s.
0.25 n.s.
-0.10 n.s.
PDGF
FGF
G-CSF
GM-CSF Pearson’s R Significance
Fig. 3 Correlations between PAS-induced LTP age, and PDGF CSF levels in RR-MS patients. PAS-induced LTP showed a an indirect correlation with age and b a direct correlation with CSF levels of PDGF. b = beta coefficients of the multiple regression model with predicted variable = PAS, predictors = age and CSF PDGF levels
expression of Arc/Arg3.1 in hippocampal slices (Peng et al. 2010). The ability of PDGF to favor LTP, and its association with clinically stable MS course, is consistent with its
123
prosurvival effects, because the intracellular signaling pathways that mediate LTP and neuronal survival significantly overlap (Bartlett and Wang 2013). Experimental models also showed that PDGF promotes neuronal differentiation (Erlandsson et al. 2001), reduces apoptosis, improves oligodendrocyte density during acute demyelination, reduces apoptosis and improves remyelination during the recovery period after chronic demyelination (Vana et al. 2007). Progressive forms of MS, characterized by ineluctable clinical worsening, showed reduced synaptic plasticity in response to TBS, while clinically stable RR-MS were associated with a preserved and even enhanced LTP as compared to healthy controls (Mori et al. 2013a). At this regard, it is also interesting to notice that PDGF concentration in the CSF of MS patients decreases with disease duration (Harirchian et al. 2012) and that progressive MS is usually diagnosed in people in their forties, older than the average age for RRMS (Confavreux and Vukusic 2006). Platelet-derived growth factor neuroprotective action may also result from inhibition of the NMDA receptordependent Ca2? overload (Egawa-Tsuzuki et al. 2004; Tseng and Dichter 2005; Ishii et al. 2006), as PDGF selectively inhibits the NR2B-containing NMDA receptors (Beazely et al. 2009), more closely associated with longterm synaptic depression and the excitotoxic neuronal death (Bartlett and Wang 2013). At this regard, it is interesting to consider that demyelination and axonal damage can block or reduce conduction velocity through nerve fibers. As PAS, considered a form of Hebbian plasticity, depends on the synchronicity of different synaptic inputs on a same neuron (Stefan et al. 2000), it may also be argued that PDGF contributes to its preservation, through remyelination and neuroprotection. However, a previous study, exploring PAS in MS patients (Zeller et al. 2010), reported that despite worse
Neuromol Med (2014) 16:490–498
performance in functional motor tests, prolonged corticomuscular latency and decreased NAA/Cr, PAS-induced enhancement of cortico-spinal excitability was comparable between patients and controls. PAS-induced plasticity, in fact, did not correlate with motor impairment or measures of CNS injury, indicating that its induction does not seem to be highly influenced by preexisting brain damage (Zeller et al. 2010) and may thus represent a reliable technique to study LTP and its modulation by soluble molecules in MS. Our results also found that PAS is affected by age. This is in accordance with previous research reporting that PAS effects show a large inter-individual variability, with a significant fraction of individuals presenting a decrease in MEP amplitude rather than an increase (Mu¨ller-Dahlhaus et al. 2008). This variability has been attributed to many factors, including gender, age (Mu¨ller-Dahlhaus et al. 2008), and genetics (Cheeran et al. 2008), and our study indicates for the first time that also PDGF may be among these.
Conclusions Remyelination and restoration of axonal function contribute to recovery (Smith and McDonald 1999) and neuronal plasticity represents an important additional substrate for recovery from, and compensation of, MS-induced neuronal damage when tissue repair at the cellular level is incomplete (Morgen et al. 2004; Mezzapesa et al. 2008). PDGF, apparently facilitating both neuronal survival and synaptic plasticity, may thus represent a potential therapeutic target in MS. LTP is also considered the biological substrate of learning and memory, and PDGF effects on LTP could also influence cognitive abilities in MS patients. Indeed, PDGF has also been implicated in cognitive and socio-emotional functions (Nguyen et al. 2011). Prospective clinical trials might unveil the possible impact of PDGF on recovery from relapses and disease progression in MS patients. Modulation of PDGF signaling could represent the target of new therapeutic strategies aimed at preventing the occurrence of long-term disability in MS. Acknowledgments This study was supported by a Grant from Fondazione Italiana Sclerosi Multipla to DC (FISM Special Project 2012/S/2). Conflict of interest of interest.
The authors declare that they have no conflict
References Bartlett, T. E., & Wang, Y. T. (2013). The intersections of NMDARdependent synaptic plasticity and cell survival. Neuropharmacology, 74, 59–68.
497 Beazely, M. A., Lim, A., Li, H., Trepanier, C., Chen, X., Sidhu, B., et al. (2009). Platelet-derived growth factor selectively inhibits NR2B-containing N-methyl-D-aspartate receptors in CA1 hippocampal neurons. Journal of Biological Chemistry, 284(12), 8054–8063. Boyd, T. D., Bennett, S. P., Mori, T., Governatori, N., Runfeldt, M., Norden, M., et al. (2010). GM-CSF upregulated in rheumatoid arthritis reverses cognitive impairment and amyloidosis in Alzheimer mice. Journal of Alzheimer’s Disease, 21(2), 507–518. Centonze, D., Rossi, S., Tortiglione, A., Picconi, B., Prosperetti, C., De Chiara, V., et al. (2007). Synaptic plasticity during recovery from permanent occlusion of the middle cerebral artery. Neurobiology of Diseases, 27(1), 44–53. Chadi, G., & Fuxe, K. (1998). Analysis of trophic responses in lesioned brain: Focus on basic fibroblast growth factor mechanisms. Brazilian Journal of Medical and Biological Research, 31(2), 231–241. Cheeran, B., Talelli, P., Mori, F., Koch, G., Suppa, A., Edwards, M., et al. (2008). A common polymorphism in the brain-derived neurotrophic factor gene (BDNF) modulates human cortical plasticity and the response to rTMS. Journal of Physiology, 586(23), 5717–5725. Confavreux, C., & Vukusic, S. (2006). Age at disability milestones in multiple sclerosis. Brain, 129(3), 595–605. Cooke, S. F., & Bliss, T. V. (2006). Plasticity in the human central nervous system. Brain, 129(7), 1659–1673. Di Lazzaro, V., Profice, P., Pilato, F., Capone, F., Ranieri, F., Pasqualetti, P., et al. (2010). Motor cortex plasticity predicts recovery in acute stroke. Cerebral Cortex, 20(7), 1523–1528. Egawa-Tsuzuki, T., Ohno, M., Tanaka, N., Takeuchi, Y., Uramoto, H., Faigle, R., et al. (2004). The PDGF B-chain is involved in the ontogenic susceptibility of the developing rat brain to NMDA toxicity. Experimental Neurology, 186(1), 89–98. Erlandsson, A., Enarsson, M., & Forsberg-Nilsson, K. (2001). Immature neurons from CNS stem cells proliferate in response to platelet-derived growth factor. Journal of Neuroscience, 21(10), 3483–3491. Frost, E. E., Nielsen, J. A., Le, T. Q., & Armstrong, R. C. (2003). PDGF and FGF2 regulate oligodendrocyte progenitor responses to demyelination. Journal of Neurobiology, 54(3), 457–572. Gong, N., Li, Y., Cai, G. Q., Niu, R. F., Fang, Q., Wu, K., et al. (2009). GABA transporter-1 activity modulates hippocampal theta oscillation and theta burst stimulation-induced long-term potentiation. Journal of Neuroscience, 29(50), 15836–15845. Gozal, D., Simakajornboon, N., Czapla, M. A., Xue, Y. D., Gozal, E., Vlasic, V., et al. (2000). Brainstem activation of platelet-derived growth factor-beta receptor modulates the late phase of the hypoxic ventilatory response. Journal of Neurochemistry, 74(1), 310–319. Hagemann, G., Redecker, C., Neumann-Haefelin, T., Freund, H. J., & Witte, O. W. (1998). Increased long-term potentiation in the surround of experimentally induced focal cortical infarction. Annals of Neurology, 44(2), 255–258. Hanajima, R., Ugawa, Y., Terao, Y., Enomoto, H., Shiio, Y., Mochizuki, H., et al. (2002). Mechanisms of intracortical I-wave facilitation elicited with paired-pulse magnetic stimulation in humans. Journal of Physiology, 538(1), 253–261. Harirchian, M. H., Tekieh, A. H., Modabbernia, A., Aghamollaii, V., Tafakhori, A., Ghaffarpour, M., et al. (2012). Serum and CSF PDGF-AA and FGF-2 in relapsing–remitting multiple sclerosis: A case–control study. European Journal of Neurology, 19(2), 241–247. Ishii, Y., Oya, T., Zheng, L., Gao, Z., Kawaguchi, M., Sabit, H., et al. (2006). Mouse brains deficient in neuronal PDGF receptor-beta develop normally but are vulnerable to injury. Journal of Neurochemistry, 98(2), 588–600.
123
498 Kerschensteiner, M., Stadelmann, C., Dechant, G., Wekerle, H., & Hohlfeld, R. (2003). Neurotrophic cross-talk between the nervous and immune systems: Implications for neurological diseases. Annals of Neurology, 53(3), 292–304. Kierdorf, K., Wang, Y., & Neumann, H. (2010). Immune-mediated CNS damage. Results and Problems in Cell Differentiation, 51, 173–196. Kujirai, T., Caramia, M. D., Rothwell, J. C., Day, B. L., Thompson, P. D., Ferbert, A., et al. (1993). Corticocortical inhibition in human motor cortex. Journal of Physiology, 471, 501–519. Matsuyama, S., Taniguchi, T., Kadoyama, K., & Matsumoto, A. (2008). Long-term potentiation-like facilitation through GABAA receptor blockade in the mouse dentate gyrus in vivo. Neuroreport, 19(18), 1809–1813. Mattson, M. P., Kumar, K. N., Wang, H., Cheng, B., & Michaelis, E. K. (1993). Basic FGF regulates the expression of a functional 71 kDa NMDA receptor protein that mediates calcium influx and neurotoxicity in hippocampal neurons. Journal of Neuroscience, 13, 4575–4588. Messersmith, D. J., Murtie, J. C., Le, T. Q., Frost, E. E., & Armstrong, R. C. (2000). Fibroblast growth factor 2 (FGF2) and FGF receptor expression in an experimental demyelinating disease with extensive remyelination. Journal of Neuroscience Research, 62(2), 241–256. Mezzapesa, D. M., Rocca, M. A., Rodegher, M., Comi, G., & Filippi, M. (2008). Functional cortical changes of the sensorimotor network are associated with clinical recovery in multiple sclerosis. Human Brain Mapping, 29(5), 562–573. Morgen, K., Kadom, N., Sawaki, L., Tessitore, A., Ohayon, J., McFarland, H., et al. (2004). Training-dependent plasticity in patients with multiple sclerosis. Brain, 127(11), 2506–2517. Mori, F., Kusayanagi, H., Nicoletti, C. G., Weiss, S., Marciani, M. G., & Centonze, D. (2013a). Cortical plasticity predicts recovery from relapse in multiple sclerosis. Multiple Sclerosis. doi:10. 1177/1352458513512541. Mori, F., Rossi, S., Piccinin, S., Motta, C., Mango, D., Kusayanagi, H., et al. (2013b). Synaptic plasticity and PDGF signaling defects underlie clinical progression in multiple sclerosis. Journal of Neuroscience, 33(49), 19112–19119. Mu¨ller-Dahlhaus, J. F., Orekhov, Y., Liu, Y., & Ziemann, U. (2008). Interindividual variability and age-dependency of motor cortical plasticity induced by paired associative stimulation. Experimental Brain Research, 187(3), 467–475. Nguyen, P. T., Nakamura, T., Hori, E., Urakawa, S., Uwano, T., Zhao, J., et al. (2011). Cognitive and socio-emotional deficits in platelet-derived growth factor receptor-b gene knockout mice. PLoS One, 6(3), e18004. Peng, F., Yao, H., Bai, X., Zhu, X., Reiner, B. C., Beazely, M., et al. (2010). Platelet-derived growth factor-mediated induction of the synaptic plasticity gene Arc/Arg3.1. Journal of Biological Chemistry, 285(28), 21615–21624. Polman, C. H., Reingold, S. C., Banwell, B., Clanet, M., Cohen, J. A., Filippi, M., et al. (2011). Diagnostic criteria for multiple
123
Neuromol Med (2014) 16:490–498 sclerosis: 2010 Revisions to the McDonald criteria. Annals of Neurology, 69(2), 292–302. Ponomarev, E. D., Shriver, L. P., Maresz, K., Pedras-Vasconcelos, J., Verthelyi, D., & Dittel, B. N. (2007). GM-CSF production by autoreactive T cells is required for the activation of microglial cells and the onset of experimental autoimmune encephalomyelitis. Journal of Immunology, 178(1), 39–48. Rottlaender, A., Villwock, H., Addicks, K., & Kuerten, S. (2011). Neuroprotective role of fibroblast growth factor-2 in experimental autoimmune encephalomyelitis. Immunology, 133(3), 370–378. Sasahara, M., Fries, J. W., Raines, E. W., Gown, A. M., Westrum, L. E., Frosch, M. P., et al. (1991). PDGF B-chain in neurons of the central nervous system, posterior pituitary, and in a transgenic model. Cell, 64(1), 217–227. Schneider, A., Kru¨ger, C., Steigleder, T., Weber, D., Pitzer, C., Laage, R., et al. (2005). The hematopoietic factor G-CSF is a neuronal ligand that counteracts programmed cell death and drives neurogenesis. Journal of Clinical Investigation, 115(8), 2083–2098. Schwartz, M., Moalem, G., Leibowitz-Amit, R., & Cohen, I. R. (1999). Innate and adaptive immune responses can be beneficial for CNS repair. Trends in Neurosciences, 22(7), 295–299. Smith, K. J., & McDonald, W. I. (1999). The pathophysiology of multiple sclerosis: The mechanisms underlying the production of symptoms and the natural history of the disease. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, 354(1390), 1649–1673. Stefan, K., Kunesch, E., Cohen, L. G., Benecke, R., & Classen, J. (2000). Induction of plasticity in the human motor cortex by paired associative stimulation. Brain, 123(3), 572–584. Tseng, H. C., & Dichter, M. A. (2005). Platelet-derived growth factorBB pretreatment attenuates excitotoxic death in cultured hippocampal neurons. Neurobiology of Diseases, 19(1–2), 77–83. Valls-Sole´, J., Pascual-Leone, A., Wassermann, E. M., & Hallett, M. (1992). Human motor evoked responses to paired transcranial magnetic stimuli. Electroencephalography and Clinical Neurophysiology, 85(6), 355–364. Vana, A. C., Flint, N. C., Harwood, N. E., Le, T. Q., Fruttiger, M., & Armstrong, R. C. (2007). Platelet-derived growth factor promotes repair of chronically demyelinated white matter. Journal of Neuropathology and Experimental Neurology, 66(11), 975–988. Webster, H. D. (1997). Growth factors and myelin regeneration in multiple sclerosis. Multiple Sclerosis, 3(2), 113–120. Zeller, D., Aufm Kampe, K., Biller, A., Stefan, K., Gentner, R., Schu¨tz, A., et al. (2010). Rapid-onset central motor plasticity in multiple sclerosis. Neurology, 74(9), 728–735. Ziemann, U., Tergau, F., Wassermann, E. M., Wischer, S., Hildebrandt, J., & Paulus, W. (1998). Demonstration of facilitatory I wave interaction in the human motor cortex by paired transcranial magnetic stimulation. Journal of Physiology, 511(1), 181–190.
ORIGINAL PAPER
Growth Factors and Synaptic Plasticity in Relapsing–Remitting Multiple Sclerosis Francesco Mori • Carolina G. Nicoletti • Silvia Rossi • Caterina Motta • Hajime Kusayanagi • Alessandra Bergami • Valeria Studer • Fabio Buttari Francesca Barbieri • Sagit Weiss • Robert Nistico` • Gianvito Martino • Roberto Furlan • Diego Centonze
•
Received: 16 December 2013 / Accepted: 13 March 2014 / Published online: 27 March 2014 Ó Springer Science+Business Media New York 2014
Abstract During multiple sclerosis (MS) inflammatory attacks, and in subsequent clinical recovery phases, immune cells contribute to neuronal and oligodendroglial cell survival and tissue repair by secreting growth factors. Animal studies showed that growth factors also play a substantial role in regulating synaptic plasticity, and namely in long-term potentiation (LTP). LTP could drive clinical recovery in relapsing patients by restoring the excitability of denervated neurons. We recently reported that maintenance of synaptic plasticity reserve is crucial to contrast clinical deterioration in MS and that the plateletderived growth factor (PDGF) may play a key role in its regulation. We also reported that a Hebbian form of LTPlike cortical plasticity, explored by paired associative stimulation (PAS), correlates with clinical recovery from a relapse in MS. Here, we explored the role of PDGF in clinical recovery and in adaptive neuroplasticity in
F. Mori C. G. Nicoletti S. Rossi C. Motta H. Kusayanagi V. Studer F. Buttari F. Barbieri S. Weiss D. Centonze (&) Clinica Neurologica, Dipartimento di Medicina dei Sistemi, Universita` Tor Vergata, Via Montpellier 1, 00133 Rome, Italy e-mail: [email protected] F. Mori C. G. Nicoletti S. Rossi C. Motta H. Kusayanagi V. Studer F. Buttari F. Barbieri S. Weiss R. Nistico` D. Centonze IRCCS Fondazione Santa Lucia, 00143 Rome, Italy A. Bergami G. Martino R. Furlan Neuroimmunology Unit, Division of Neuroscience, Institute of Experimental Neurology (INSpe), San Raffaele Scientific Institute, 20132 Milan, Italy R. Nistico` Dipartimento di Fisiologia e Farmacologia, Universita` La Sapienza, 00185 Rome, Italy
123
relapsing–remitting MS (RR-MS) patients. We found a correlation between the cerebrospinal fluid (CSF) PDGF concentrations and the extent of clinical recovery after a relapse, as full recovery was more likely observed in patients with high PDGF concentrations and poor recovery in subjects with low PDGF levels. Consistently with the idea that PDGF-driven synaptic plasticity contributes to attenuate the clinical consequences of tissue damage in RR-MS, we also found a striking correlation between CSF levels of PDGF and the amplitude of LTP-like cortical plasticity explored by PAS. CSF levels of fibroblast growth factor, granulocyte colony-stimulating factor and granulocyte–macrophage colony-stimulating factor did not correlate with clinical recovery nor with measures of synaptic transmission and plasticity. Keywords Recovery
CSF PDGF TMS PAS Relapse
Introduction In the early and acute phases of multiple sclerosis (MS), T cells accumulate in the CNS and release various, predominantly proinflammatory cytokines that mediate tissue destruction (Kierdorf et al. 2010). However, during the course of the disease and in subsequent clinical recovery phases, immune cells may also contribute to neuronal and oligodendroglial cell survival and tissue repair by secreting different growth factors (Schwartz et al. 1999; Kerschensteiner et al. 2003). Release of growth factors in MS brains could serve several functions, including the modulation of microglial activity, and the stimulation of oligodendrocyte proliferation or regeneration within lesioned areas (Webster 1997; Chadi and Fuxe 1998; Messersmith et al. 2000;
Neuromol Med (2014) 16:490–498
Erlandsson et al. 2001; Frost et al. 2003; Ponomarev et al. 2007; Rottlaender et al. 2011). Apart from regulating inflammatory processes, promoting neuronal survival and remyelination, growth factors also play a substantial role in synaptic transmission and plasticity, and namely in long-term potentiation (LTP), a form of neuroplasticity consisting in use-dependent and enduring enhancement of excitatory synaptic transmission (Mattson et al. 1993; Schneider et al. 2005; Boyd et al. 2010; Peng et al. 2010). The mechanisms of synaptic plasticity and of neuronal survival largely overlap at receptor and post-receptor levels (Bartlett and Wang 2013), and in fact, LTP disruption is a common feature of the early phases of many neurodegenerative disorders (Bartlett and Wang 2013). In MS, as well as in other neurological diseases, synaptic transmission adaptations and LTP might also be crucial to favor clinical compensation of tissue damage, as increased synaptic responsiveness in surviving neurons is potentially able to restore membrane excitability of neurons that have lost part of their synaptic inputs (Morgen et al. 2004; Mezzapesa et al. 2008). In line with this hypothesis, LTP induction is greatly facilitated in response to ischemic brain damage in rodents (Hagemann et al. 1998), and its occurrence in the periinfarct area is associated with better clinical outcome, both in rodents (Centonze et al. 2007) and in humans (Di Lazzaro et al. 2010). Consistently, we have recently found that cortical LTP reserve, explored by means of transcranial magnetic stimulation (TMS), plays a major role in the compensation of neurological deficits, as this form of synaptic plasticity is still possible and even favored in stable MS patients, while it is exhausted in progressive forms of the disease (Mori et al. 2013a). In addition, cortical plasticity reserve, as explored by means of paired associative stimulation (PAS), another paradigm of TMS exploring Hebbian-LTP also correlates with the extent of clinical recovery after a relapse in MS patients (Mori et al. 2013b). In animal studies, synaptic transmission and LTP are regulated by various growth factors and, among these, platelet-derived growth factor (PDGF) (Peng et al. 2010), fibroblast growth factor (FGF) (Mattson et al. 1993), granulocyte colony-stimulating factor (G-CSF) (Schneider et al. 2005) and granulocyte–macrophage colony-stimulating factor (GM-CSF) (Boyd et al. 2010) are released by inflammatory cells and have therefore the potential to influence synaptic functioning and the clinical manifestations in MS. Thus, the present investigation aimed at exploring the role of PDGF, FGF, G-CSF and GM-CSF in adaptive neuroplasticity and clinical recovery in relapsing–remitting MS (RR-MS) patients. Our results point to PDGF as an essential determinant in LTP induction and in clinical
491
compensation of acute inflammatory damage in the brains of RR-MS patients.
Patients and Methods The study was approved by the Ethics Committee of the University Hospital Tor Vergata, Rome. All the subjects gave their written informed consent to the study. Determination of Growth Factors in the Cerebrospinal Fluid (CSF) To determine PDGF, FGF, G-CSF and GM-CSF concentrations in RR-MS, we used the CSF collected from 84 RRMS patients and from 47 age- and gender-matched controls who were admitted to the neurology clinic of the University Hospital Tor Vergata of Rome between 2009 and 2012. After their admittance, patients underwent, in sequence, brain (and in selected cases also spinal) magnetic resonance imaging (MRI) scan and CSF withdrawal within 24 h. Corticosteroids or other MS-specific immunoactive therapies were initiated later when appropriate. The diagnosis of MS was established by clinical, laboratory and MRI parameters, and matched published criteria (Polman et al. 2011). In all instances, patients underwent detection of oligoclonal banding in the CSF. As controls, we used the CSF from individuals without inflammatory or degenerative diseases of the central or peripheral nervous system who underwent lumbar puncture because of a clinical suspect of acute peripheral neuropathy, meningitis or subarachnoidal hemorrhage, which were not confirmed. Immediately after withdrawal, the CSF was centrifuged and stored at -80 °C until analyzed. Platelet-derived growth factor, FGF, G-CSF and GMCSF concentrations were analyzed using Bio-Plex Multiplex Cytokine Assay (Bio-Rad Laboratories), according to manufacturer instructions. Growth factor concentrations were calculated according to a standard curve generated for the specific target and expressed as pg/ml. When the concentrations were below the detection threshold, they were assumed to be 0 pg/ml. MS Subjects Multiple sclerosis disease onset was defined as the first episode of focal neurological dysfunction indicative of MS. Disease duration was estimated as the number of years from onset to the last assessment of disability, performed at the time of CSF withdrawal. Relapses were defined as the development of new or recurrent neurological symptoms not associated with fever or infection lasting at least 24 h. Disability was determined
123
492
by a specially trained (Neurostatus training and documentation DVD for a standardized neurological examination and assessment of Kurtzke’s functional systems and Expanded Disability Status Scale for MS patients. Basel, Switzerland: Neurostatus, 2006. Available at http://www. neurostatus.net) and certified examining neurologist using Expanded Disability Status Scale (EDSS), a ten-point disease severity score derived from nine ratings for individual neurological domains. In a sample of 21 MS patients, CSF withdrawal and TMS were done during a clinical relapse, and in 14 of these patients, disability assessment was repeated three months later. All patients received standard corticosteroid treatment (3–5 daily i.v. infusions of 1,000 mg methylprednisolone) during the relapse, which in all cases was initiated after CSF withdrawal, clinical and neurophysiological assessments. Based on the difference between the second and the first EDSS assessments, they were classified into a ‘‘complete recovery’’ group (EDSS returned to prerelapse score) and an ‘‘incomplete recovery’’ group (EDSS higher than pre-relapse score). MRI Acquisition and Analysis Three Tesla MRI scans consisted of dual-echo proton density, FLAIR, T2-weighted spin-echo images, and precontrast and post-contrast T1-weighted spin-echo images. All images were acquired in the axial orientation with 3-mm-thick contiguous slices. The presence of gadoliniumenhancing (Gd?) (0.2 ml/kg e.v.) lesions was assessed by a neuroradiologist who was unaware of the patient’s clinical details. Intracortical Circuits in the Motor Cortex The output of the primary motor cortex (M1) can be objectively measured in the form of a motor evoked potential (MEP) from surface electromyographic (EMG) recording electrodes, positioned on the skin overlying the targeted muscles in response to a single suprathreshold TMS pulse delivered to M1. The MEP amplitude elicited by stimulation of M1 can be modulated by a preceding conditioning pulse (CS). Depending on the intensity and inter-stimulus interval (ISI) used, a CS can produce either a facilitation or inhibition of the MEP elicited from the test stimulus (TS) over M1. To explore both excitatory and inhibitory intracortical circuits, we tested through paired-pulse (pp) TMS, shortinterval intracortical inhibition (SICI; mediated by intrinsic GABAAergic circuits) (Kujirai et al. 1993), intracortical facilitation (ICF; believed to follow the preferential recruitment of intrinsic excitatory fibers) (Kujirai et al. 1993), short intracortical facilitation (SICF; likely
123
Neuromol Med (2014) 16:490–498
mediated by excitatory cortical interneurons) (Ziemann et al. 1998) and long-interval intracortical inhibition (LICI; mediated by local GABAB pathways) (Valls-Sole´ et al. 1992) of the left M1. One figure-of-eight coil, external diameter 70 mm, was held tangentially to the scalp over the motor ‘‘hot spot’’ for right FDI muscle. Stimulation intensity for TS was adjusted in each experiment to evoke an MEP of about 1 mV peakto-peak amplitude in the relaxed right first dorsal interosseus muscle (FDI). SICI and ICF were tested using ppTMS with a subthreshold CS preceding a suprathreshold TS (Kujirai et al. 1993). CS stimulus was set at 80 % AMT. Three conditions were presented in a random order: control (TS given alone) and two pp conditions (TS preceded by CS) at one of two different ISI (2 and 15 ms). For SICF, the intensity of CS was set to 90 % RMT. Four randomly intermixed conditions were presented in a random order: TS given alone and three conditions with the TS followed by CS at one of three different ISIs (1.5, 2.7, and 4.5 ms) (Hanajima et al. 2002). For LICI, the intensity of CS was set at 120 % RMT. Two conditions were presented in a random order: control (TS given alone) and one pp condition (TS preceded by CS) at 100 ms ISI (Valls-Sole´ et al. 1992). For each experiment, 10 responses were collected for the TS alone and for conditioned MEPs at each ISI. Changes in MEP amplitude at each ISI were expressed as the percentage of the mean unconditioned MEP amplitude. EMG Electromyographic traces were recorded from the right first dorsal interosseus muscle (FDI) with surface cup electrodes. The active electrode was placed over the muscle belly and the reference electrode over the metacarpophalangeal joint of the index finger. Responses were amplified with a Digitimer D360 amplifier (Digitimer, Welwyn Garden City, Hertfordshire, UK) through filters set at 20 Hz and 2 kHz with a sampling rate of 5 kHz, then recorded by a computer with SIGNAL software (Cambridge Electronic Devices, Cambridge, UK). MEPs were evoked through a figure-of-eight coil with external loop diameter of 70 mm connected to a Magstim 2002 magnetic stimulator (Magstim Company, Whitland, Wales, UK). Coil position was adjusted to find the optimal scalp site to evoke motor responses in the contralateral FDI, the motor ‘‘hot spot,’’ at the beginning of each experimental session and marked over the patient’s scalp with a pencil. The coil was held tangentially to the scalp surface with the handle pointing posteriorly and laterally at about 45° with respect to the midsagittal axis of the head.
Neuromol Med (2014) 16:490–498
Paired Associative Stimulation (PAS) To explore LTP-like plasticity, PAS (Stefan et al. 2000) was delivered using pairs of median nerve electrical and single TMS pulses over the abductor pollicis brevis (APB) hot spot at an ISI of 25 ms. The intensity of the TMS was set to evoke an MEP of 1 mV ca. peak-to-peak amplitude in the APB, while the intensity of the median nerve stimulus (0.2 ms duration) was set at three times perceptual threshold. Two hundred pairs of stimuli were given at a rate of 0.25 Hz. MEPs were recorded from the medianinnervated APB muscle at baseline and 30 min after PAS. MEP’s amplitudes were then averaged at each time point and normalized to the mean baseline amplitude. Data Analysis Differences among two groups were compared by univariate analysis using Student’s t test for continuous variables and Fisher’s exact test for categorical variables. A multiple regression model was used to assess the effect of clinical and biochemical predictors on the LTP responses to the PAS protocol in our patients. As predicted variable, we used the MEP amplitude change 30 min after PAS. Five clinical variables (age, gender, years of disease, presence of gadolinium-enhancing (Gd?) lesions and EDSS) and CSF levels of four different growth factors (PDGF, FGF, GM-CSF and G-CSF) were tested as predictor variables. Gender and presence of Gd? lesions were coded as dummy variables. Predictors were entered into the model using a forward stepwise method. Predictors were tested for collinearity using the variance inflation factor (VIF). The relationship between growth factors and ppTMS protocols was evaluated calculating the Pearson’s correlation coefficients. Statistical tests were considered significant if p \ 0.05. Data, expressed as mean ± standard error (SE), were considered significant at the 0.05 level.
Results Concentrations of Growth Factors in the CSF of Control and MS Patients The two groups (controls and RR-MS) did not differ in terms of the demographic characteristics (Table 1). We first measured growth factor levels in the CSF of MS patients. PDGF, FGF, G-CSF and GM-CSF were not significantly altered in the population of MS subjects, compared to control individuals (p [ 0.05 for each comparison; Fig. 1a). Of note, a trend to PDGF levels increase, although
493 Table 1 Demographic and clinical characteristics of enrolled subjects Total
Control
RR-MS
p value
Number
131
47
84
Sex (F/M)
84/47
27/20
57/27
Age (yrs)
35.1 ± 9.0
36.3 ± 10.2
34.4 ± 8.2 0.2
Disease duration (yrs)
na
na
3.0 ± 5.1
0.3
EDSS
na
na
1.2 ± 1.0
EDSS range
na
na
0–5.0
RR-MS relapsing–remitting multiple sclerosis, F female, M male, yrs years, EDSS Expanded Disability Status Scale, na not applicable
not statistically significant (p = 0.11), was observed among RR-MS subjects. Upon grouping of MS patients according to the presence of Gd? lesions at the MRI, without differences in terms of demographic characteristics (Gd? MS group: n = 37, 28 females and 9 males, aged 22–55 years; Gd- MS group: n = 47, 30 females and 17 males, aged 20–51 years), GMCSF levels were higher in Gd? MS patients than in GdMS patients (p \ 0.01). No significant alterations of the CSF contents of the other growth factors were revealed by grouping Gd- and Gd? patients (p [ 0.05 for each comparison; Fig. 1b). Growth Factors and Clinical Recovery in Relapsing MS Patients The potential involvement of growth factors in post-relapse clinical recovery (Vana et al. 2007; Mori et al. 2013a) was addressed in 21 RR-MS patients, who underwent CSF withdrawal and growth factors measurement during a clinical relapse. This group of patients was clinically assessed during the relapse, and 14 of these patients were clinically reassessed in a scheduled follow-up visit three months later, while seven patients abandoned the study before completion, missing the scheduled follow-up visit for personal reasons (Table 2). Of the 14 patients who completed the study, seven showed a full symptom recovery and seven patients only partially recovered 12 weeks after the relapse. A t test for independent variables showed that PDGF concentrations were significantly higher in the CSF of patients that completely recovered than in the CSF of patients with incomplete symptom recovery after the relapse (F = 30.94, p \ 0.01; Fig. 2), indicating that full symptom recovery was more likely in patients with high PDGF concentrations. Conversely, FGF, G-CSF and GM-CSF did not differ between the two groups.
123
494
Neuromol Med (2014) 16:490–498
Fig. 1 Growth factor levels in the CSF of MS subjects. a Growth factor levels were not significantly different between RR-MS and control groups, although a slight increase in PDGF levels was
observed among RR-MS subjects. b The graph shows that, upon grouping of MS patients according to the presence of Gd? lesions, GM-CSF levels were higher in Gd? MS subjects. *p \ 0.01
Growth Factors and Cortical Excitability in MS
however, did not reach statistical significance (mean post-/ pre-MEP amplitude = 1.07, p = 0.56). Multiple regression analysis, however, revealed that PAS after effects are influenced by age (b = -0.51, p \ 0.05) and PDGF CSF levels (b = 0.30, p \ 0.05) (adjusted R2 = 0.25, p \ 0.05) (Fig. 3a, b). Gd? lesions, EDSS, disease duration, gender and CSF levels of FGF, GM-CSF and G-CSF were also tested and excluded from the model being not significant predictors. VIF \ 5 showed there was no collinearity between predictors. These results indicate therefore that PDGF may favor LTP induction in MS patients and are consistent with in vitro data showing that this growth factor enhances LTP in hippocampal brain slices (Peng et al. 2010; Mori et al. 2013a).
Animal experiments showed that growth factors can influence synaptic transmission; moreover, LTP is a complex phenomenon, and changes in glutamate-mediated synaptic drive or GABA-mediated inhibition alter its induction (Cooke and Bliss 2006; Matsuyama et al. 2008; Gong et al. 2009). Thus, we tried to investigate whether growth factors may alter basal synaptic transmission during a relapse through ppTMS experiments in cortical neurons of 21 RR-MS patients (Table 2). T test for paired samples showed a significant effect of ISI for ICF (mean MEP amplitude = 1.29 ± 0.08; p = 0.002), SICI (mean MEP amplitude = 0.63 ± 0.09; p = 0.001) and LICI (mean MEP amplitude = 0.02 ± 0.04; p \ 0.001). For SICF, a repeated-measures ANOVA showed a significant effect of ISI at 1.5 ms (F = 26.65; p \ 0.001) and 2.7 ms (F = 21.45; p \ 0.001) but not at 4.5 ms (F = 0.32; p = 0.56). Correlation analysis, however, showed that SICI, ICF, SICF and LICI were not related to CSF levels of PDGF, FGF, G-CSF and GM-CSF (Table 3). Growth Factors and PAS-Induced LTP in MS Repeated-measures ANOVA showed a trend to increased mean motor cortex excitability 30 min after PAS that,
123
Discussion The main findings of the present study are the association between high CSF PDGF levels and full symptom recovery after a relapse and the correlation between CSF PDGF levels and PAS-induced LTP in RR-MS. These results are in accordance with previous studies showing that both LTP and PDGF may contribute to limit
Neuromol Med (2014) 16:490–498 Table 2 Characteristics of the subjects enrolled in the PAS experiments
495
Patient
Gender
Age
Recovery
1
M
34
Incomplete
2
2
0
–
2
F
19
NA
1
NA
0
–
3
M
37
Complete
1
0
0
?
4
M
35
NA
1.5
NA
0
–
5
F
36
Incomplete
1.5
1
0
?
6
M
41
Incomplete
2
1
4
–
7
M
45
NA
6
NA
4
–
8
M
42
NA
2
NA
0
–
9
M
39
Incomplete
1.5
1
15
–
10
M
46
NA
3
NA
11
–
11 12
F M
32 28
Complete Incomplete
1 2
0 2
0 0
– ?
13
F
29
NA
1
NA
0
?
14
M
26
NA
1.5
NA
0
–
15
F
24
Complete
3
0
0
?
16
F
27
Complete
1.5
0
0
?
17
F
26
Complete
1
0
1
–
18
F
48
Incomplete
2
2
1.5
–
19
F
42
Complete
2
0
9
–
20
M
43
Complete
1
0
22
–
21
M
23
Incomplete
2
1
0
–
Fig. 2 PDGF levels in the CSF of RR-MS patients predicts symptom recovery after a relapse. PDGF was significantly increased in patients that completely recovered after the relapse, compared to patients with poor recovery. *p \ 0.01
MS disease progression. Indeed, the amount of LTP induced by PAS in RR-MS patients at the time of a relapse showed a correlation with the extent of clinical recovery
EDSS at onset
EDSS 3 months after onset
Disease duration (years)
Gd±
12 weeks later, in the post-relapse phase (Mori et al. 2013b). PDGF, on the other side, showed to act as a key factor in the recovery phase of chronic demyelinating damage in mice (Vana et al. 2007), while its concentration in the CSF of RR-MS patients showed to be associated with limited clinical expression of new brain lesion formation (Mori et al. 2013a). In the present study, we confirm the results of a previous report where PDGF levels in the CSF of RR-MS patients showed a correlation with another form of LTP, induced through a different TMS protocol, named theta burst stimulation (TBS) (Mori et al. 2013a). Based on these findings, we argue that PDGF may contribute to the compensation of brain damage in RR-MS by restoring the excitability of those neurons that have lost part of their synaptic inputs through the induction of synaptic LTP. The mechanism by which PDGF enhances LTP is only marginally understood, but animal studies showed that PDGF receptors are widely expressed in the CNS (Sasahara et al. 1991; Gozal et al. 2000) and that PDGF modulates LTP induction at excitatory synapses. Indeed, when mice slices of hippocampus were pre-incubated with PDGF, the magnitude of LTP measured between 50 and 60 min after TBS was significantly higher (162 ± 9 %, n = 6) than in control slices (134 ± 9 %, n = 6) (p \ 0.05) (Mori et al. 2013a). Moreover, a previous report by Peng et al. (2010) showed that PDGF gives rise to LTP inducing the
123
496 Table 3 Correlation matrix between ppTMS experiments and CSF levels of PDGF, FGF, G-CSF and GM-CSF in 21 RRMS patients
Neuromol Med (2014) 16:490–498
ICF (10 ms ISI)
SICI (2 ms ISI)
LICI (100 ms ISI)
SICF (1.5 ms ISI)
SICF (2.7 ms ISI)
SICF (4.5 ms ISI)
Pearson’s R
0.15
0.13
-0.11
0.09
0.18
0.20
Significance
n.s.
n.s.
n.s.
n.s.
n.s.
n.s.
Pearson’s R
-0.45
0.04
0.08
0.20
0.15
0.10
Significance
n.s.
n.s.
n.s.
n.s.
n.s.
n.s.
Pearson’s R
0.29
0.23
-0.42
0.24
-0.18
0.14
Significance
n.s.
n.s.
n.s.
n.s.
n.s.
n.s.
0.36 n.s.
0.51 n.s.
0.35 n.s.
-0.17 n.s.
0.25 n.s.
-0.10 n.s.
PDGF
FGF
G-CSF
GM-CSF Pearson’s R Significance
Fig. 3 Correlations between PAS-induced LTP age, and PDGF CSF levels in RR-MS patients. PAS-induced LTP showed a an indirect correlation with age and b a direct correlation with CSF levels of PDGF. b = beta coefficients of the multiple regression model with predicted variable = PAS, predictors = age and CSF PDGF levels
expression of Arc/Arg3.1 in hippocampal slices (Peng et al. 2010). The ability of PDGF to favor LTP, and its association with clinically stable MS course, is consistent with its
123
prosurvival effects, because the intracellular signaling pathways that mediate LTP and neuronal survival significantly overlap (Bartlett and Wang 2013). Experimental models also showed that PDGF promotes neuronal differentiation (Erlandsson et al. 2001), reduces apoptosis, improves oligodendrocyte density during acute demyelination, reduces apoptosis and improves remyelination during the recovery period after chronic demyelination (Vana et al. 2007). Progressive forms of MS, characterized by ineluctable clinical worsening, showed reduced synaptic plasticity in response to TBS, while clinically stable RR-MS were associated with a preserved and even enhanced LTP as compared to healthy controls (Mori et al. 2013a). At this regard, it is also interesting to notice that PDGF concentration in the CSF of MS patients decreases with disease duration (Harirchian et al. 2012) and that progressive MS is usually diagnosed in people in their forties, older than the average age for RRMS (Confavreux and Vukusic 2006). Platelet-derived growth factor neuroprotective action may also result from inhibition of the NMDA receptordependent Ca2? overload (Egawa-Tsuzuki et al. 2004; Tseng and Dichter 2005; Ishii et al. 2006), as PDGF selectively inhibits the NR2B-containing NMDA receptors (Beazely et al. 2009), more closely associated with longterm synaptic depression and the excitotoxic neuronal death (Bartlett and Wang 2013). At this regard, it is interesting to consider that demyelination and axonal damage can block or reduce conduction velocity through nerve fibers. As PAS, considered a form of Hebbian plasticity, depends on the synchronicity of different synaptic inputs on a same neuron (Stefan et al. 2000), it may also be argued that PDGF contributes to its preservation, through remyelination and neuroprotection. However, a previous study, exploring PAS in MS patients (Zeller et al. 2010), reported that despite worse
Neuromol Med (2014) 16:490–498
performance in functional motor tests, prolonged corticomuscular latency and decreased NAA/Cr, PAS-induced enhancement of cortico-spinal excitability was comparable between patients and controls. PAS-induced plasticity, in fact, did not correlate with motor impairment or measures of CNS injury, indicating that its induction does not seem to be highly influenced by preexisting brain damage (Zeller et al. 2010) and may thus represent a reliable technique to study LTP and its modulation by soluble molecules in MS. Our results also found that PAS is affected by age. This is in accordance with previous research reporting that PAS effects show a large inter-individual variability, with a significant fraction of individuals presenting a decrease in MEP amplitude rather than an increase (Mu¨ller-Dahlhaus et al. 2008). This variability has been attributed to many factors, including gender, age (Mu¨ller-Dahlhaus et al. 2008), and genetics (Cheeran et al. 2008), and our study indicates for the first time that also PDGF may be among these.
Conclusions Remyelination and restoration of axonal function contribute to recovery (Smith and McDonald 1999) and neuronal plasticity represents an important additional substrate for recovery from, and compensation of, MS-induced neuronal damage when tissue repair at the cellular level is incomplete (Morgen et al. 2004; Mezzapesa et al. 2008). PDGF, apparently facilitating both neuronal survival and synaptic plasticity, may thus represent a potential therapeutic target in MS. LTP is also considered the biological substrate of learning and memory, and PDGF effects on LTP could also influence cognitive abilities in MS patients. Indeed, PDGF has also been implicated in cognitive and socio-emotional functions (Nguyen et al. 2011). Prospective clinical trials might unveil the possible impact of PDGF on recovery from relapses and disease progression in MS patients. Modulation of PDGF signaling could represent the target of new therapeutic strategies aimed at preventing the occurrence of long-term disability in MS. Acknowledgments This study was supported by a Grant from Fondazione Italiana Sclerosi Multipla to DC (FISM Special Project 2012/S/2). Conflict of interest of interest.
The authors declare that they have no conflict
References Bartlett, T. E., & Wang, Y. T. (2013). The intersections of NMDARdependent synaptic plasticity and cell survival. Neuropharmacology, 74, 59–68.
497 Beazely, M. A., Lim, A., Li, H., Trepanier, C., Chen, X., Sidhu, B., et al. (2009). Platelet-derived growth factor selectively inhibits NR2B-containing N-methyl-D-aspartate receptors in CA1 hippocampal neurons. Journal of Biological Chemistry, 284(12), 8054–8063. Boyd, T. D., Bennett, S. P., Mori, T., Governatori, N., Runfeldt, M., Norden, M., et al. (2010). GM-CSF upregulated in rheumatoid arthritis reverses cognitive impairment and amyloidosis in Alzheimer mice. Journal of Alzheimer’s Disease, 21(2), 507–518. Centonze, D., Rossi, S., Tortiglione, A., Picconi, B., Prosperetti, C., De Chiara, V., et al. (2007). Synaptic plasticity during recovery from permanent occlusion of the middle cerebral artery. Neurobiology of Diseases, 27(1), 44–53. Chadi, G., & Fuxe, K. (1998). Analysis of trophic responses in lesioned brain: Focus on basic fibroblast growth factor mechanisms. Brazilian Journal of Medical and Biological Research, 31(2), 231–241. Cheeran, B., Talelli, P., Mori, F., Koch, G., Suppa, A., Edwards, M., et al. (2008). A common polymorphism in the brain-derived neurotrophic factor gene (BDNF) modulates human cortical plasticity and the response to rTMS. Journal of Physiology, 586(23), 5717–5725. Confavreux, C., & Vukusic, S. (2006). Age at disability milestones in multiple sclerosis. Brain, 129(3), 595–605. Cooke, S. F., & Bliss, T. V. (2006). Plasticity in the human central nervous system. Brain, 129(7), 1659–1673. Di Lazzaro, V., Profice, P., Pilato, F., Capone, F., Ranieri, F., Pasqualetti, P., et al. (2010). Motor cortex plasticity predicts recovery in acute stroke. Cerebral Cortex, 20(7), 1523–1528. Egawa-Tsuzuki, T., Ohno, M., Tanaka, N., Takeuchi, Y., Uramoto, H., Faigle, R., et al. (2004). The PDGF B-chain is involved in the ontogenic susceptibility of the developing rat brain to NMDA toxicity. Experimental Neurology, 186(1), 89–98. Erlandsson, A., Enarsson, M., & Forsberg-Nilsson, K. (2001). Immature neurons from CNS stem cells proliferate in response to platelet-derived growth factor. Journal of Neuroscience, 21(10), 3483–3491. Frost, E. E., Nielsen, J. A., Le, T. Q., & Armstrong, R. C. (2003). PDGF and FGF2 regulate oligodendrocyte progenitor responses to demyelination. Journal of Neurobiology, 54(3), 457–572. Gong, N., Li, Y., Cai, G. Q., Niu, R. F., Fang, Q., Wu, K., et al. (2009). GABA transporter-1 activity modulates hippocampal theta oscillation and theta burst stimulation-induced long-term potentiation. Journal of Neuroscience, 29(50), 15836–15845. Gozal, D., Simakajornboon, N., Czapla, M. A., Xue, Y. D., Gozal, E., Vlasic, V., et al. (2000). Brainstem activation of platelet-derived growth factor-beta receptor modulates the late phase of the hypoxic ventilatory response. Journal of Neurochemistry, 74(1), 310–319. Hagemann, G., Redecker, C., Neumann-Haefelin, T., Freund, H. J., & Witte, O. W. (1998). Increased long-term potentiation in the surround of experimentally induced focal cortical infarction. Annals of Neurology, 44(2), 255–258. Hanajima, R., Ugawa, Y., Terao, Y., Enomoto, H., Shiio, Y., Mochizuki, H., et al. (2002). Mechanisms of intracortical I-wave facilitation elicited with paired-pulse magnetic stimulation in humans. Journal of Physiology, 538(1), 253–261. Harirchian, M. H., Tekieh, A. H., Modabbernia, A., Aghamollaii, V., Tafakhori, A., Ghaffarpour, M., et al. (2012). Serum and CSF PDGF-AA and FGF-2 in relapsing–remitting multiple sclerosis: A case–control study. European Journal of Neurology, 19(2), 241–247. Ishii, Y., Oya, T., Zheng, L., Gao, Z., Kawaguchi, M., Sabit, H., et al. (2006). Mouse brains deficient in neuronal PDGF receptor-beta develop normally but are vulnerable to injury. Journal of Neurochemistry, 98(2), 588–600.
123
498 Kerschensteiner, M., Stadelmann, C., Dechant, G., Wekerle, H., & Hohlfeld, R. (2003). Neurotrophic cross-talk between the nervous and immune systems: Implications for neurological diseases. Annals of Neurology, 53(3), 292–304. Kierdorf, K., Wang, Y., & Neumann, H. (2010). Immune-mediated CNS damage. Results and Problems in Cell Differentiation, 51, 173–196. Kujirai, T., Caramia, M. D., Rothwell, J. C., Day, B. L., Thompson, P. D., Ferbert, A., et al. (1993). Corticocortical inhibition in human motor cortex. Journal of Physiology, 471, 501–519. Matsuyama, S., Taniguchi, T., Kadoyama, K., & Matsumoto, A. (2008). Long-term potentiation-like facilitation through GABAA receptor blockade in the mouse dentate gyrus in vivo. Neuroreport, 19(18), 1809–1813. Mattson, M. P., Kumar, K. N., Wang, H., Cheng, B., & Michaelis, E. K. (1993). Basic FGF regulates the expression of a functional 71 kDa NMDA receptor protein that mediates calcium influx and neurotoxicity in hippocampal neurons. Journal of Neuroscience, 13, 4575–4588. Messersmith, D. J., Murtie, J. C., Le, T. Q., Frost, E. E., & Armstrong, R. C. (2000). Fibroblast growth factor 2 (FGF2) and FGF receptor expression in an experimental demyelinating disease with extensive remyelination. Journal of Neuroscience Research, 62(2), 241–256. Mezzapesa, D. M., Rocca, M. A., Rodegher, M., Comi, G., & Filippi, M. (2008). Functional cortical changes of the sensorimotor network are associated with clinical recovery in multiple sclerosis. Human Brain Mapping, 29(5), 562–573. Morgen, K., Kadom, N., Sawaki, L., Tessitore, A., Ohayon, J., McFarland, H., et al. (2004). Training-dependent plasticity in patients with multiple sclerosis. Brain, 127(11), 2506–2517. Mori, F., Kusayanagi, H., Nicoletti, C. G., Weiss, S., Marciani, M. G., & Centonze, D. (2013a). Cortical plasticity predicts recovery from relapse in multiple sclerosis. Multiple Sclerosis. doi:10. 1177/1352458513512541. Mori, F., Rossi, S., Piccinin, S., Motta, C., Mango, D., Kusayanagi, H., et al. (2013b). Synaptic plasticity and PDGF signaling defects underlie clinical progression in multiple sclerosis. Journal of Neuroscience, 33(49), 19112–19119. Mu¨ller-Dahlhaus, J. F., Orekhov, Y., Liu, Y., & Ziemann, U. (2008). Interindividual variability and age-dependency of motor cortical plasticity induced by paired associative stimulation. Experimental Brain Research, 187(3), 467–475. Nguyen, P. T., Nakamura, T., Hori, E., Urakawa, S., Uwano, T., Zhao, J., et al. (2011). Cognitive and socio-emotional deficits in platelet-derived growth factor receptor-b gene knockout mice. PLoS One, 6(3), e18004. Peng, F., Yao, H., Bai, X., Zhu, X., Reiner, B. C., Beazely, M., et al. (2010). Platelet-derived growth factor-mediated induction of the synaptic plasticity gene Arc/Arg3.1. Journal of Biological Chemistry, 285(28), 21615–21624. Polman, C. H., Reingold, S. C., Banwell, B., Clanet, M., Cohen, J. A., Filippi, M., et al. (2011). Diagnostic criteria for multiple
123
Neuromol Med (2014) 16:490–498 sclerosis: 2010 Revisions to the McDonald criteria. Annals of Neurology, 69(2), 292–302. Ponomarev, E. D., Shriver, L. P., Maresz, K., Pedras-Vasconcelos, J., Verthelyi, D., & Dittel, B. N. (2007). GM-CSF production by autoreactive T cells is required for the activation of microglial cells and the onset of experimental autoimmune encephalomyelitis. Journal of Immunology, 178(1), 39–48. Rottlaender, A., Villwock, H., Addicks, K., & Kuerten, S. (2011). Neuroprotective role of fibroblast growth factor-2 in experimental autoimmune encephalomyelitis. Immunology, 133(3), 370–378. Sasahara, M., Fries, J. W., Raines, E. W., Gown, A. M., Westrum, L. E., Frosch, M. P., et al. (1991). PDGF B-chain in neurons of the central nervous system, posterior pituitary, and in a transgenic model. Cell, 64(1), 217–227. Schneider, A., Kru¨ger, C., Steigleder, T., Weber, D., Pitzer, C., Laage, R., et al. (2005). The hematopoietic factor G-CSF is a neuronal ligand that counteracts programmed cell death and drives neurogenesis. Journal of Clinical Investigation, 115(8), 2083–2098. Schwartz, M., Moalem, G., Leibowitz-Amit, R., & Cohen, I. R. (1999). Innate and adaptive immune responses can be beneficial for CNS repair. Trends in Neurosciences, 22(7), 295–299. Smith, K. J., & McDonald, W. I. (1999). The pathophysiology of multiple sclerosis: The mechanisms underlying the production of symptoms and the natural history of the disease. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, 354(1390), 1649–1673. Stefan, K., Kunesch, E., Cohen, L. G., Benecke, R., & Classen, J. (2000). Induction of plasticity in the human motor cortex by paired associative stimulation. Brain, 123(3), 572–584. Tseng, H. C., & Dichter, M. A. (2005). Platelet-derived growth factorBB pretreatment attenuates excitotoxic death in cultured hippocampal neurons. Neurobiology of Diseases, 19(1–2), 77–83. Valls-Sole´, J., Pascual-Leone, A., Wassermann, E. M., & Hallett, M. (1992). Human motor evoked responses to paired transcranial magnetic stimuli. Electroencephalography and Clinical Neurophysiology, 85(6), 355–364. Vana, A. C., Flint, N. C., Harwood, N. E., Le, T. Q., Fruttiger, M., & Armstrong, R. C. (2007). Platelet-derived growth factor promotes repair of chronically demyelinated white matter. Journal of Neuropathology and Experimental Neurology, 66(11), 975–988. Webster, H. D. (1997). Growth factors and myelin regeneration in multiple sclerosis. Multiple Sclerosis, 3(2), 113–120. Zeller, D., Aufm Kampe, K., Biller, A., Stefan, K., Gentner, R., Schu¨tz, A., et al. (2010). Rapid-onset central motor plasticity in multiple sclerosis. Neurology, 74(9), 728–735. Ziemann, U., Tergau, F., Wassermann, E. M., Wischer, S., Hildebrandt, J., & Paulus, W. (1998). Demonstration of facilitatory I wave interaction in the human motor cortex by paired transcranial magnetic stimulation. Journal of Physiology, 511(1), 181–190.
