involved in this process. L-selectin (LAM-1, MEL-14) is expressed constitutively on neutrophils, of paralysis and leucocytic infil- monocytes and lymphocytes and is tration in the CNS in 75% (12 out probably involved in transient and of 16) of the animals. In the four reversible binding to the endoanimals that did suffer paralysis, thelium, which itself expresses the onset was delayed and the E-selectin (ELAM-1) and P-selectin (PADGEM, GMP-140, CD62) 8. severity was reduced. In order to migrate across the The animal model used by Yednock et al. 1 is known as endothelium, however, leucocytes experimental allergic encephalo- must first become attached firmly myelitis (EAE). This is an auto- to the endothelial wall. This apimmune inflammatory condition of pears to occur after leucocyte acthe CNS, which is induced in sus- tivation by factors such as cytoceptible strains of rat by injecting kines, and the subsequent binding myelin basic protein into the hind of activation-dependent adhesion footpads. The condition can also be receptors on leucocytes with those induced by injecting activated lym- on endothelial cells. These recepphocytes specific for myelin basic tors and their counter-receptors protein into naive rats; this was the are, generally, members of the method chosen in the study by integrin and immunoglobulin superYedrmck et aL In both cases, families. Although they are conthe result is a single episode of stitutively expressed at low levels ascending paralysis due to inflam- on naive lymphocytes, their inmarion within the CNS. The paral- volvement in cell adhesion depends ysis starts at the tip of the tail and on activation. Expression of their reaches the hind legs over sev- ligands on the surface of endothelial eral days, after which the animals cells is induced over a period of usually recover. Histologically, lym- 4-24 hours by inflammatory stimuli phocytes and monocytes enter the such as the cytokines interleukin 1 spinal cord and brain and initiate (lL-1) and tumour necrosis factor o~ inflammation in those regions2~. (TNF-a) 9. The integrin family of adhesion During the inflammatory cascade, T cells interact with local antigen- receptors consists of at least 13 presenting cells and are stimulated a[3 heterodimers, which are highly to produce inflammatory cytoldnes. conserved between mammalian These cell-cell interactions are species. The ligand specificity of mediated by surface molecules that each integrin is determined by both are members of the integrin and the o~ and the [3 chains, and three immunoglobulin superfamilies, and subfamilies have been defined to previous studies have shown that date, whose members share comblocking these interactions with mon [3 chains. The iS2 integrins are involved monoclonal antibodies can prevent EAE s,6. primarily in cell-cell adhesion and adhesion to immobilized products of the complement cascade 1°. Leucocyte-endothelial-cell adhesion in inflammation The best-characterized example Current data indicate that three of these molecules is lymphokey events lead to the extra- cyte function-associated antigen 1 vasation of leucocytes; these are (LFA-1; also known as CDllasummarized in Fig. 1. CD18), which plays important The first event involves changes roles in lymphocyte activation and at the endothelial surface that facili- proliferation, and probably also tate leucocyte binding via consti- in endothelial-cell-leucocyte intertutively expressed leucocyte ad- actions during leucocyte recruithesion receptors. Evidence from ment. The ligands for LFA-1 are experiments in vivo and in vitro intercellular adhesion molecule 1 suggests that lectin-like adhesion (ICAM-1; also known as CD54) molecules, called selectins*, are and ICAM-2. The distribution of ICAM-1 is limited, but it can be * Selectins constitute a family of cell ad- induced during inflammation on a hesion receptors previously known by a wide variety of cells, including the variety of names, shown in parentheses, endothelium, whereas ICAM-2, owing to their independent discovery by different laboratories. The molecules have which also has a limited distribeen recently termed 'selecfins' to indicate bution, may not be affected by their involvement in cell-surface recognition 7, inflammatory stimuli n.
Leucocyterecruitmentandinflammationin the CN$ D. 1. Sloan, M. J. Wood and H. M. ChaHton Dept of Human Anatomy, University of Oxford, South ParksRoad, Oxford, UK OXl 3QX.
276
he recruitment of circulating leucocytes into inflammatory T lesions requires interactions at the surface of the vascular endothelium, which usually occur in the post-capillary venules, where leucocyte flow is the slowest. These interactions are mediated by cell-surface adhesion receptors and their counter-receptors on leucocytes and endothelial cells, and are helped by chemoattractants, activating factors and products of the plasma protein cascades. Recently, a monoclonal antibody to an adhesion molecule on leucocytes, a4~l integrin (also known as VLA-4, CD49d-CD29), has been used successfully to prevent experimentally induced inflammation in the CNS of rats. Inflammatory responses have the capacity to cause considerable cellular damage in their immediate environment,, which in the CNS can produce long-term debilitation or even death. For example, infection with measles virus can lead to the development of subacute sclerosing panencephalitis, which is often fatal (in up to 20% of cases); the progressive increase in neurological deficits in multiple sclerosis is also probably due to damage caused by inflammatory lesions in the CNS. These examples represent chronic conditions, but in cases of acute CNS trauma, such as spinal cord injury, the inflammatory cascade is also likely to contribute significantly to neuronal death and scarring. Thus it is highly desirable to develop therapeutic strategies aimed at reducing inflammation in the CNS. In a recent report, Yednock et al. 1 have shown that activationdependent adhesion molecules may provide potential targets for monoclonal antibody therapy to prevent inflammation in the CNS. Using an in vitro binding assay, they showed that adhesion of a human monocytic cell line to inflamed blood vessels from rat CNS was blocked by prior incubation of the monocytes with antibodies to the integrin oq or ~1 chains. In vivo, a single injection of antibody against a4~l integrin (more than 1 mg), given to rats two days after the induction of inflammation by the transfer of activated lymphocytes, prevented the development
© 1992. Elsevier Science PublishersLtd, (UK)
TINS, VoL 15, No. 8, 1992
The [31 and ~3 integrins are A involved in cell adhesion to components of the extracellular matrix, such as fibronectin, collagen and laminin, and may be important for retaining leucocytes at inflamed Lymphocyte sites. The oq~t integrin, targeted Neutrophil in the study by Yednock et al. 1, has two distinct binding sites: one for fibronectin; and one for its putative ~v ~. Endothelialsurface immunoglobulin-family ligand, vascular cell adhesion molecule 1 (VCAM-1)12. Like LFA-1, 0 ~ 4 ~ 1 integrin is expressed constitutively on neutrophils, monocytes and lymphocytes, and is continuously expressed at higher levels after lymphocyte activation. Like C ICAM-1, VCAM-1 may be induced B on endothelial cells by inflammatory I FA-1 Lymphocyt cytokines. + The results of Yednock et al. 1 Monocyte Complement imply a degree of exclusivity to the Cytokines role of oq131 integrin in the inChemoattractants Attachment and LFI /, flammatory response in EAE. It "~ration might be expected, considering the function of LFA-1 and that ICAM1, the ligand for LFA-1, has been found on inflamed CNS blood vessels of mice with EAE 13, that treatment with antibodies to LFA-1 ICAM-1 ICAM-2 VCAM-1/ would also affect the adhesion of lymphocytes to inflamed vascular endothelium in the in vitro assay, INFLAMMATION but apparently this was not the case. It may be important that the Fig. 1. (A) Leucocytes moving through blood vessels engage in transient and antibody used to block LFA-1 ac- reversible binding at the surface of the endothelium, a process that is probably tivity in this study was MRC OX- mediated by the selectin adhesion receptors (L-selectin, P-selectin and 52. This antibody, together with E-selectin). (B) A ntJmber of factors, which may include cytokines, chemoMRC OX-19 (CD5), has been used attractants, products of the plasma protein cascades (complement proteins), as a universal T cell marker in and even cell-cell contact, activate leucocytes and the endothefium to express rats 14, and it does not label all or increase the expression (+) of integrin and immunoglobufin-family adlymphocytes as would be expected hesion receptors in response to inflammation. The broken line outlines the of an antibody to LFA-1. Thus, it 014~1 integrin targeted in the study of Yednock et al. 1 (C) The activation may have been inappropriate to process results in the strong and sustained attachment of leucocyles the endothelium by the binding of adhesion receptors ot4fil integrin use MRC OX-52 to block LFA-1. to VCA/H-1, and of LFA-I to ICAM-1 and ICANI-2, prior to their migration Nevertheless, the data of Yednock into the inflamed lesion. et aL 1 provide strong evidence of an important, although not exclusive, role for oq[3t integrin in from other studies suggests that merits suggest that monoclonal leucocyte adhesion to the endo- combinations of two or more anti- antibody treatment may offer more thelium (possibly via VCAM-1) bodies specific for separate targets than just a short-term therapy during inflammation in the CNS. may give even better results than to inflammatory exacerbations. targeting a single antigen16. For Maybe in appropriate combinations Antibody treatment of CNS example, a recent report has de- and under particular circuminflammation scribed the treatment of mice for stances, a cocktail of antibodies Evidence to support the role of six days with a combination of two could be used to prevent further selectins and integrins in leucocyte monoclonal antibodies to the LFA- inflammatory episodes, perhaps by recruitment during inflammation 1-ICAM-1 adhesion receptor pair, inducing tolerance to the stimulatcomes from other in vivo antibody- which resulted in the long-term ing antigen. blocking experiments, in which survival of incompatible heart Alternatively, if long-term treatmonocytes were prevented from transplants 17. This information is ment is required for chronic inflamentering the inflamed peritoneum relevant to this discussion because matory conditions, such as subin mice by injections of antibodies graft rejection is also an inflam- acute sclerosing panencephalitis or against L-selectin or aM[32 integ- matory response that relies upon multiple sclerosis, blocking integrin (also known as Mac-l, CDllb- the recruitment of effector leuco- fin function could result in serious CD18) ~s. Furthermore, evidence cytes. Therefore, these develop- complications by inhibiting normal TINS, Vol. 15, No. 8, 1992
277
leucocyte recruitment during infection. In this regard, a congenital deficiency in the [~2 integrin chain, and therefore in LFA-1, results in a condition called leucocyte adhesion deficiency, which is lethal unless treated by bone marrow transplantation in early life 1°. It has been proposed that the use of combinations of pairs of adhesion receptors and activation signals by leucocytes could explain the selective recruitment of certain leucocyte subsets, such as memory lymphocytes, to particular lymph nodes or sites of infiammarion, and the preferential recruitment of neutrophils early in the inflammatory process 15. This may be of particular interest in multiple sclerosis, for example, where relapse and remission are common features of the disease. Finally, with respect to acute CNS trauma, the prospect of anti-
inflammatory treatment by blocking leucocyte recruitment is particularly attractive as it is likely to require only a short period of treatment and should not lead to side effects, such as an immune response to the therapeutic antibody. In addition, short-term treatment should not leave patients vulnerable to the effects of infection by diminishing their normal immune response. Selected references 1 Yednock, T. A. et al. (1992) Nature 356, 63-66 2 Traugott, U., McFarlin, D. E. and Raine, C. S. (1986) Cell Immunol. 99, 395-410 3 Day, M. J., Tse, A. G. D., Puklavec,
M., Sirnrnonds, S. J. and Mason, D. W. (1992) J. Exp./Vled. 175,655-659 4 Brostoff, S. W. and Mason, D. W. (1984) J. Immunol. 133, 1938-1942 5 Waldor, M. K. et aL (1985) Science 227, 415-417
Programmedcell death: the paths to suidde Jennifer Altman
g A p o p t o s i s ' is rapidly becoming Z"Ika fashionable word in neuroLondon, biology, although it is being applied UK N16 SUN. to a well-known phenomenon programmed cell death during development. This term brings to neurobiology the idea that cell death may be caused by promoting the expression of genes that code for so-called 'death' or 'suicide' proteins. The search is now on, not only for these genes and their products, but also for the signals that activate them. The word apoptosis was introduced in 1972 (Ref. 1) to distinguish cells that die in an orderly fashion, as a part of the normal biological processes in developing and adult tissues, from those dying through necrosis, the catastrophic disintegration of cells in response to pathological insults. Thus apoptosis, derived from the Greek word meaning 'falling off' of leaves or petals, can be seen as the natural counterpart to cell survival. It is thought to involve its own orchestrated biochemical cascades, which have been dubbed 'the death program '2. A key question in the nervous system is whether any of the cell death that occurs in neurodegenerative diseases in adults is due to apoptosis rather than necrosis. If so, then
37 Lordship Park,
278
investigating naturally occurring cell death during development may provide clues to how neurons at risk can be rescued. What triggers the death program? In a recent review in Nature, Raffa takes the extreme position that all cells depend on external signals for their survival when these signals are removed, the cell dies. A possible exception to this is observed in the nematode Caenorhabditis elegans, in which some cells seem to trigger their own death program soon after they are born (see review by Driscoll and Chalfie4). The expression of two genes, ced-3 and ced-4, is required for the cells to die, but it is not yet known whether the genes are activated autonomously or if an external signal is needed 5. Horvitz and his colleagues have now identified a counterpart to these 'death genes', a cell-survival gene, ced-9 (Ref. 6). In the nematode cells where this gene is active, it blocks the effects of ced-3 and ced-4. Horvitz et al. 6 point out that ced-9 has a parallel in humans: the proto-oncogene, bcl-2, overexpression of which prevents or delays apoptosis in B and T lymphocytes. Why ced-9 should be inactive in cells that are destined to die during development is still a mystery.
© 1992.ElsevierSciencePublishersLtd,(UK)
6 Steinman, L., Rosenbaurn, J. T., Sriram, S. and McDevitt, H. O. (1981) Proc. Natl Acad. Sci. USA 78, 7111-7114 7 Bevilacqua, M. et al. (1991) Cell 67, 233 8 Lawrence, M. B. and Springer, T. A. (1991) Cell 65,859-873 9 Pober, J. S. and Cotran, R. S. (1990) Physiol. Rev. 70, 427-451 10 Springer, T. A. (1990) Nature 346, 425-434 11 Staunton, D. E., Dustin, M. L. and Springer, T. A. (1989) Nature 339, 61-64 12 Elices, M. J. et al. (1990) Cell 60, 577-584 13 Cannella, B., Cross, A. H. and Raine, C. S. (1990) J. Exp. Nled. 172, 1521-1524 14 Mason, D. W. et al. (1986) Neuroscience 19, 685-694 15 Butcher, E. C. (1991) Cell 67, 1033-1036 16 Cobbold, S. P., Martin, G., Qin, S. and Waldmann, H. (1986) Nature 323, 164-166 17 Isobe, M., Yagita, H., Okumura, K. and lahara, A. (1992) Science 255, 1125-1127
In the vertebrate nervous system, many sensory, motor and sympathetic neurons die during a critical stage of development. These neurons depend for survival on the presence of neurotrophic factors, such as nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), both of which support the growth of sensory and sympathetic neurons (see review in Ref. 7). (The survival factor for motor neurons has still not been identified, although there is good evidence that one exists 8. ) It has been proposed that growing axons compete for limiting amounts of neurotrophin that is produced by their target tissues those neurons that fail to obtain a sufficient supply of growth factor are doomed to die 9. Cells cultured at this critical stage in development require the appropriate neurotrophic factor in order to survive, and thus they provide a useful model system for examining the mechanisms by which withdrawal of neurotrophin could trigger cell death. The first evidence to suggest that suicide proteins might be involved in this process was the demonstration that, after depriving sympathetic neurons in vitro of their trophic factor, NGF, neurons can be prevented from dying by applying substances that inhibit synthesis of RNA and protein 1°. This has been TINS, Vol. 15, NO. 8, 1992
Leucocyterecruitmentandinflammationin the CN$ D. 1. Sloan, M. J. Wood and H. M. ChaHton Dept of Human Anatomy, University of Oxford, South ParksRoad, Oxford, UK OXl 3QX.
276
he recruitment of circulating leucocytes into inflammatory T lesions requires interactions at the surface of the vascular endothelium, which usually occur in the post-capillary venules, where leucocyte flow is the slowest. These interactions are mediated by cell-surface adhesion receptors and their counter-receptors on leucocytes and endothelial cells, and are helped by chemoattractants, activating factors and products of the plasma protein cascades. Recently, a monoclonal antibody to an adhesion molecule on leucocytes, a4~l integrin (also known as VLA-4, CD49d-CD29), has been used successfully to prevent experimentally induced inflammation in the CNS of rats. Inflammatory responses have the capacity to cause considerable cellular damage in their immediate environment,, which in the CNS can produce long-term debilitation or even death. For example, infection with measles virus can lead to the development of subacute sclerosing panencephalitis, which is often fatal (in up to 20% of cases); the progressive increase in neurological deficits in multiple sclerosis is also probably due to damage caused by inflammatory lesions in the CNS. These examples represent chronic conditions, but in cases of acute CNS trauma, such as spinal cord injury, the inflammatory cascade is also likely to contribute significantly to neuronal death and scarring. Thus it is highly desirable to develop therapeutic strategies aimed at reducing inflammation in the CNS. In a recent report, Yednock et al. 1 have shown that activationdependent adhesion molecules may provide potential targets for monoclonal antibody therapy to prevent inflammation in the CNS. Using an in vitro binding assay, they showed that adhesion of a human monocytic cell line to inflamed blood vessels from rat CNS was blocked by prior incubation of the monocytes with antibodies to the integrin oq or ~1 chains. In vivo, a single injection of antibody against a4~l integrin (more than 1 mg), given to rats two days after the induction of inflammation by the transfer of activated lymphocytes, prevented the development
© 1992. Elsevier Science PublishersLtd, (UK)
TINS, VoL 15, No. 8, 1992
The [31 and ~3 integrins are A involved in cell adhesion to components of the extracellular matrix, such as fibronectin, collagen and laminin, and may be important for retaining leucocytes at inflamed Lymphocyte sites. The oq~t integrin, targeted Neutrophil in the study by Yednock et al. 1, has two distinct binding sites: one for fibronectin; and one for its putative ~v ~. Endothelialsurface immunoglobulin-family ligand, vascular cell adhesion molecule 1 (VCAM-1)12. Like LFA-1, 0 ~ 4 ~ 1 integrin is expressed constitutively on neutrophils, monocytes and lymphocytes, and is continuously expressed at higher levels after lymphocyte activation. Like C ICAM-1, VCAM-1 may be induced B on endothelial cells by inflammatory I FA-1 Lymphocyt cytokines. + The results of Yednock et al. 1 Monocyte Complement imply a degree of exclusivity to the Cytokines role of oq131 integrin in the inChemoattractants Attachment and LFI /, flammatory response in EAE. It "~ration might be expected, considering the function of LFA-1 and that ICAM1, the ligand for LFA-1, has been found on inflamed CNS blood vessels of mice with EAE 13, that treatment with antibodies to LFA-1 ICAM-1 ICAM-2 VCAM-1/ would also affect the adhesion of lymphocytes to inflamed vascular endothelium in the in vitro assay, INFLAMMATION but apparently this was not the case. It may be important that the Fig. 1. (A) Leucocytes moving through blood vessels engage in transient and antibody used to block LFA-1 ac- reversible binding at the surface of the endothelium, a process that is probably tivity in this study was MRC OX- mediated by the selectin adhesion receptors (L-selectin, P-selectin and 52. This antibody, together with E-selectin). (B) A ntJmber of factors, which may include cytokines, chemoMRC OX-19 (CD5), has been used attractants, products of the plasma protein cascades (complement proteins), as a universal T cell marker in and even cell-cell contact, activate leucocytes and the endothefium to express rats 14, and it does not label all or increase the expression (+) of integrin and immunoglobufin-family adlymphocytes as would be expected hesion receptors in response to inflammation. The broken line outlines the of an antibody to LFA-1. Thus, it 014~1 integrin targeted in the study of Yednock et al. 1 (C) The activation may have been inappropriate to process results in the strong and sustained attachment of leucocyles the endothelium by the binding of adhesion receptors ot4fil integrin use MRC OX-52 to block LFA-1. to VCA/H-1, and of LFA-I to ICAM-1 and ICANI-2, prior to their migration Nevertheless, the data of Yednock into the inflamed lesion. et aL 1 provide strong evidence of an important, although not exclusive, role for oq[3t integrin in from other studies suggests that merits suggest that monoclonal leucocyte adhesion to the endo- combinations of two or more anti- antibody treatment may offer more thelium (possibly via VCAM-1) bodies specific for separate targets than just a short-term therapy during inflammation in the CNS. may give even better results than to inflammatory exacerbations. targeting a single antigen16. For Maybe in appropriate combinations Antibody treatment of CNS example, a recent report has de- and under particular circuminflammation scribed the treatment of mice for stances, a cocktail of antibodies Evidence to support the role of six days with a combination of two could be used to prevent further selectins and integrins in leucocyte monoclonal antibodies to the LFA- inflammatory episodes, perhaps by recruitment during inflammation 1-ICAM-1 adhesion receptor pair, inducing tolerance to the stimulatcomes from other in vivo antibody- which resulted in the long-term ing antigen. blocking experiments, in which survival of incompatible heart Alternatively, if long-term treatmonocytes were prevented from transplants 17. This information is ment is required for chronic inflamentering the inflamed peritoneum relevant to this discussion because matory conditions, such as subin mice by injections of antibodies graft rejection is also an inflam- acute sclerosing panencephalitis or against L-selectin or aM[32 integ- matory response that relies upon multiple sclerosis, blocking integrin (also known as Mac-l, CDllb- the recruitment of effector leuco- fin function could result in serious CD18) ~s. Furthermore, evidence cytes. Therefore, these develop- complications by inhibiting normal TINS, Vol. 15, No. 8, 1992
277
leucocyte recruitment during infection. In this regard, a congenital deficiency in the [~2 integrin chain, and therefore in LFA-1, results in a condition called leucocyte adhesion deficiency, which is lethal unless treated by bone marrow transplantation in early life 1°. It has been proposed that the use of combinations of pairs of adhesion receptors and activation signals by leucocytes could explain the selective recruitment of certain leucocyte subsets, such as memory lymphocytes, to particular lymph nodes or sites of infiammarion, and the preferential recruitment of neutrophils early in the inflammatory process 15. This may be of particular interest in multiple sclerosis, for example, where relapse and remission are common features of the disease. Finally, with respect to acute CNS trauma, the prospect of anti-
inflammatory treatment by blocking leucocyte recruitment is particularly attractive as it is likely to require only a short period of treatment and should not lead to side effects, such as an immune response to the therapeutic antibody. In addition, short-term treatment should not leave patients vulnerable to the effects of infection by diminishing their normal immune response. Selected references 1 Yednock, T. A. et al. (1992) Nature 356, 63-66 2 Traugott, U., McFarlin, D. E. and Raine, C. S. (1986) Cell Immunol. 99, 395-410 3 Day, M. J., Tse, A. G. D., Puklavec,
M., Sirnrnonds, S. J. and Mason, D. W. (1992) J. Exp./Vled. 175,655-659 4 Brostoff, S. W. and Mason, D. W. (1984) J. Immunol. 133, 1938-1942 5 Waldor, M. K. et aL (1985) Science 227, 415-417
Programmedcell death: the paths to suidde Jennifer Altman
g A p o p t o s i s ' is rapidly becoming Z"Ika fashionable word in neuroLondon, biology, although it is being applied UK N16 SUN. to a well-known phenomenon programmed cell death during development. This term brings to neurobiology the idea that cell death may be caused by promoting the expression of genes that code for so-called 'death' or 'suicide' proteins. The search is now on, not only for these genes and their products, but also for the signals that activate them. The word apoptosis was introduced in 1972 (Ref. 1) to distinguish cells that die in an orderly fashion, as a part of the normal biological processes in developing and adult tissues, from those dying through necrosis, the catastrophic disintegration of cells in response to pathological insults. Thus apoptosis, derived from the Greek word meaning 'falling off' of leaves or petals, can be seen as the natural counterpart to cell survival. It is thought to involve its own orchestrated biochemical cascades, which have been dubbed 'the death program '2. A key question in the nervous system is whether any of the cell death that occurs in neurodegenerative diseases in adults is due to apoptosis rather than necrosis. If so, then
37 Lordship Park,
278
investigating naturally occurring cell death during development may provide clues to how neurons at risk can be rescued. What triggers the death program? In a recent review in Nature, Raffa takes the extreme position that all cells depend on external signals for their survival when these signals are removed, the cell dies. A possible exception to this is observed in the nematode Caenorhabditis elegans, in which some cells seem to trigger their own death program soon after they are born (see review by Driscoll and Chalfie4). The expression of two genes, ced-3 and ced-4, is required for the cells to die, but it is not yet known whether the genes are activated autonomously or if an external signal is needed 5. Horvitz and his colleagues have now identified a counterpart to these 'death genes', a cell-survival gene, ced-9 (Ref. 6). In the nematode cells where this gene is active, it blocks the effects of ced-3 and ced-4. Horvitz et al. 6 point out that ced-9 has a parallel in humans: the proto-oncogene, bcl-2, overexpression of which prevents or delays apoptosis in B and T lymphocytes. Why ced-9 should be inactive in cells that are destined to die during development is still a mystery.
© 1992.ElsevierSciencePublishersLtd,(UK)
6 Steinman, L., Rosenbaurn, J. T., Sriram, S. and McDevitt, H. O. (1981) Proc. Natl Acad. Sci. USA 78, 7111-7114 7 Bevilacqua, M. et al. (1991) Cell 67, 233 8 Lawrence, M. B. and Springer, T. A. (1991) Cell 65,859-873 9 Pober, J. S. and Cotran, R. S. (1990) Physiol. Rev. 70, 427-451 10 Springer, T. A. (1990) Nature 346, 425-434 11 Staunton, D. E., Dustin, M. L. and Springer, T. A. (1989) Nature 339, 61-64 12 Elices, M. J. et al. (1990) Cell 60, 577-584 13 Cannella, B., Cross, A. H. and Raine, C. S. (1990) J. Exp. Nled. 172, 1521-1524 14 Mason, D. W. et al. (1986) Neuroscience 19, 685-694 15 Butcher, E. C. (1991) Cell 67, 1033-1036 16 Cobbold, S. P., Martin, G., Qin, S. and Waldmann, H. (1986) Nature 323, 164-166 17 Isobe, M., Yagita, H., Okumura, K. and lahara, A. (1992) Science 255, 1125-1127
In the vertebrate nervous system, many sensory, motor and sympathetic neurons die during a critical stage of development. These neurons depend for survival on the presence of neurotrophic factors, such as nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), both of which support the growth of sensory and sympathetic neurons (see review in Ref. 7). (The survival factor for motor neurons has still not been identified, although there is good evidence that one exists 8. ) It has been proposed that growing axons compete for limiting amounts of neurotrophin that is produced by their target tissues those neurons that fail to obtain a sufficient supply of growth factor are doomed to die 9. Cells cultured at this critical stage in development require the appropriate neurotrophic factor in order to survive, and thus they provide a useful model system for examining the mechanisms by which withdrawal of neurotrophin could trigger cell death. The first evidence to suggest that suicide proteins might be involved in this process was the demonstration that, after depriving sympathetic neurons in vitro of their trophic factor, NGF, neurons can be prevented from dying by applying substances that inhibit synthesis of RNA and protein 1°. This has been TINS, Vol. 15, NO. 8, 1992
