RESEARCH HIGHLIGHTS
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TLR5 opens the door to neuropathic-pain treatment QX-314– flagellin reversed mechanical allodynia in mouse models of diabetic neuropathy and nerve injury
Firing of large Aβ nerve fibres in response to innocuous mechanical stimulation is thought to play a key part in mechanical allodynia, a hallmark of neuropathic pain. A new study published in Nature Medicine has found that activation of Toll-like receptor 5 (TLR5) on Aβ fibres enables entry of a lidocaine derivative into these cells and improves neuropathic pain in mouse models. TLRs are a group of evolutionarily conserved receptors that have an important role in initiating an immune response to pathogens. Although they are typically expressed on immune cells, recent research has identified members of the TLR family on primary sensory neurons in dorsal root ganglia (DRG), where they modulate sensory functions, including those involved in pain. Using quantitative PCR, in situ hybridization and immunohistochemistry, the authors of the current
Ikon Images/Alamy Stock Photo
study found that TLR5 is expressed in medium-to-large diameter mouse neurons and shows a high level of co-expression with NF200 — a marker of myelinated group A fibres. Moreover, TLR5 was found not only on cell bodies located in the DRG, but also on sensory terminals and in the deep laminae of the spinal horn, where Aβ fibres terminate. Next, the authors asked whether the specific expression of TLR5 on Aβ fibres might be exploited to selectively target this neuron population to treat neuropathic pain. They used the lidocaine derivative QX-314, which, unlike most anaesthetics, is unable to cross the cell membrane and so does not cause general suppression of all neurons. Previous work by others had shown that concomitant administration of this drug with capsaicin — which specifically activates smalldiameter unmyelinated group C fibres — enabled QX-314 entry into group C fibres through the transient receptor potential cation channel TRPV1. Here, the researchers used the TLR5 ligand flagellin, a major component of bacterial flagella, to open the door to QX-314 entry into Aβ fibres. In cell culture, co-application of QX-314 and flagellin significantly inhibited action potential generation in large-diameter A-fibre neurons, but not in small-diameter C-fibre neurons. Moreover, the inhibitory response was lost in group A fibres from Tlr5-knockout mice. The effect of QX-314–flagellin translated to A-fibre neurons isolated from non-diseased human donors at
NATURE REVIEWS | DRUG DISCOVERY
postmortem, suggesting the potential for clinical application. In this regard, the authors also note that flagellin is well-tolerated by animals. To test the approach in vivo, the team used intraplantar injection of QX-314–flagellin into anaesthetised mice and recorded compound action potentials in the sciatic nerve in response to electrical stimulation of the hind paw. Compared with vehicletreated mice, the QX-314–flagellintreated group showed significant inhibition of stimulus-evoked Aβ fibre firing. Chemotherapy can cause dose-limiting peripheral neuropathy and neuropathic pain in patients with cancer. Therefore, the authors also applied their treatment to mice that had received the chemotherapy drug paclitaxel for 1 week. These mice demonstrated mechanical allodynia in the form of reduced thresholds for hind-paw withdrawal in response to stimulation with von Frey hairs. Importantly, intraplantar QX-314– flagellin injection led to reversal of this sensitivity for several hours. In addition, QX-314–flagellin reversed mechanical allodynia in mouse models of diabetic neuropathy and nerve injury. Further study is needed to clarify the mechanism by which flagellinmediated TLR5 activation increases membrane permeability. Katie Kingwell ORIGINAL RESEARCH PAPER Xu, Z.-Z. et al. Inhibition of mechanical allodynia in neuropathic pain by TLR5-mediated A-fiber blockade. Nat. Med. 21, 1326–1331 (2015)
VOLUME 14 | DECEMBER 2015 © 2015 Macmillan Publishers Limited. All rights reserved
PA I N
TLR5 opens the door to neuropathic-pain treatment QX-314– flagellin reversed mechanical allodynia in mouse models of diabetic neuropathy and nerve injury
Firing of large Aβ nerve fibres in response to innocuous mechanical stimulation is thought to play a key part in mechanical allodynia, a hallmark of neuropathic pain. A new study published in Nature Medicine has found that activation of Toll-like receptor 5 (TLR5) on Aβ fibres enables entry of a lidocaine derivative into these cells and improves neuropathic pain in mouse models. TLRs are a group of evolutionarily conserved receptors that have an important role in initiating an immune response to pathogens. Although they are typically expressed on immune cells, recent research has identified members of the TLR family on primary sensory neurons in dorsal root ganglia (DRG), where they modulate sensory functions, including those involved in pain. Using quantitative PCR, in situ hybridization and immunohistochemistry, the authors of the current
Ikon Images/Alamy Stock Photo
study found that TLR5 is expressed in medium-to-large diameter mouse neurons and shows a high level of co-expression with NF200 — a marker of myelinated group A fibres. Moreover, TLR5 was found not only on cell bodies located in the DRG, but also on sensory terminals and in the deep laminae of the spinal horn, where Aβ fibres terminate. Next, the authors asked whether the specific expression of TLR5 on Aβ fibres might be exploited to selectively target this neuron population to treat neuropathic pain. They used the lidocaine derivative QX-314, which, unlike most anaesthetics, is unable to cross the cell membrane and so does not cause general suppression of all neurons. Previous work by others had shown that concomitant administration of this drug with capsaicin — which specifically activates smalldiameter unmyelinated group C fibres — enabled QX-314 entry into group C fibres through the transient receptor potential cation channel TRPV1. Here, the researchers used the TLR5 ligand flagellin, a major component of bacterial flagella, to open the door to QX-314 entry into Aβ fibres. In cell culture, co-application of QX-314 and flagellin significantly inhibited action potential generation in large-diameter A-fibre neurons, but not in small-diameter C-fibre neurons. Moreover, the inhibitory response was lost in group A fibres from Tlr5-knockout mice. The effect of QX-314–flagellin translated to A-fibre neurons isolated from non-diseased human donors at
NATURE REVIEWS | DRUG DISCOVERY
postmortem, suggesting the potential for clinical application. In this regard, the authors also note that flagellin is well-tolerated by animals. To test the approach in vivo, the team used intraplantar injection of QX-314–flagellin into anaesthetised mice and recorded compound action potentials in the sciatic nerve in response to electrical stimulation of the hind paw. Compared with vehicletreated mice, the QX-314–flagellintreated group showed significant inhibition of stimulus-evoked Aβ fibre firing. Chemotherapy can cause dose-limiting peripheral neuropathy and neuropathic pain in patients with cancer. Therefore, the authors also applied their treatment to mice that had received the chemotherapy drug paclitaxel for 1 week. These mice demonstrated mechanical allodynia in the form of reduced thresholds for hind-paw withdrawal in response to stimulation with von Frey hairs. Importantly, intraplantar QX-314– flagellin injection led to reversal of this sensitivity for several hours. In addition, QX-314–flagellin reversed mechanical allodynia in mouse models of diabetic neuropathy and nerve injury. Further study is needed to clarify the mechanism by which flagellinmediated TLR5 activation increases membrane permeability. Katie Kingwell ORIGINAL RESEARCH PAPER Xu, Z.-Z. et al. Inhibition of mechanical allodynia in neuropathic pain by TLR5-mediated A-fiber blockade. Nat. Med. 21, 1326–1331 (2015)
VOLUME 14 | DECEMBER 2015 © 2015 Macmillan Publishers Limited. All rights reserved
