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precursor cells: Defining cellular targets
Journal of the Neurological Sciences 265 (2008) 12 – 16 www.elsevier.com/locate/jns
Promoting remyelination in multiple sclerosis by endogenous adult neural stem/precursor cells: Defining cellular targets Chao Zhao, Malgorzata Zawadzka, Aude J.A. Roulois, Charlotte C. Bruce, Robin J.M. Franklin ⁎ Department of Veterinary Medicine, University of Cambridge, Madingley Road, Cambridge CB3 0ES, UK MS Society Cambridge Centre for Myelin Repair at The Brain Repair Centre, University of Cambridge, Forvie Site, Robinson Way, Cambridge CB2 2PY, UK Received 1 March 2007; received in revised form 1 May 2007; accepted 8 May 2007 Available online 13 June 2007
Abstract Although the treatment of multiple sclerosis has made significant strides in the last decade, the therapeutic enhancement of repair has yet to make the successful translation from laboratory to clinic. Nevertheless, advances in the biology of stem and precursor cells, particularly in relation to myelin damage, make this a realistic proposition during the next decade. Replacing lost myelin (remyelination) is currently thought to be an important clinical objective because of the role it might play in slowing or preventing axonal degeneration. Stem/precursor cell-based strategies for enhancing remyelination can be divided into those in which cells are transplanted into a patient (exogenous or cell therapies) and those in which the patient's own stem/precursor cells are mobilised to more efficiently engage in healing areas of demyelination (endogenous or pharmacological therapies). While the two approaches tend to be regarded separately they are not mutually exclusive. This article focuses on the endogenous approach and reviews the nature and nomenclature of the stem and precursor cells present within the adult CNS that engage in remyelination and that are therefore potential targets for pharmacological manipulation. © 2007 Elsevier B.V. All rights reserved. Keywords: Remyelination; Multiple sclerosis; Precursor cell; Stem cell
1. Introduction Demyelination of central nervous system (CNS) axons is an unusual pathological event in neurological disease in that it can be followed by a highly efficient regenerative process in which axons are reinvested with new myelin sheaths and pre-lesion architecture and function are restored. This process, called remyelination, can proceed to completion not only in laboratory animal models but also in clinical demyelinating disease such as multiple sclerosis (MS) [1]. Remyelination is thought to be mediated primarily by a population of cell specific adult stem/precursor cells, variously called oligodendrocyte precursor/progenitor cells and glial precursor/progenitor cells. These cells are widely ⁎ Corresponding author. Tel.: +44 1223 337642; fax: +44 1223 337610. E-mail address: [email protected] (R.J.M. Franklin). 0022-510X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jns.2007.05.008
distributed throughout white and grey matter in all species that have been examined (including humans) and persist in abundance throughout adulthood [2]. Remyelination represents the best known example of an endogenous population of adult stem/precursor cells engaging in a regenerative process within the diseased CNS [3]. However, it is clear that remyelination sometimes fails leading to persistent demyelination. This may predispose axons to degeneration, a critical and irreversible event which underlies the progressive clinical deterioration and characterises all forms of the disease. For this reason the development of pro-remyelination strategies is an important objective in MS research. Given its efficiency compared to other regenerative processes in the CNS, remyelination ought, in theory, to be the endogenous stem-mediated therapy most amenable to therapeutic enhancement. Rather than having to mobilise a regenerative process that either does not occur or never
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occurs with the required efficiency (for example, in ischaemic or neurodegenerative disease), the challenge is to, at best, promote an ongoing process or, at worst, reactivate one that has the potential to lead to complete healing. 1.1. Which cells mediate remyelination? In developing strategies for enhancing remyelination by endogenous cells it is important to clearly establish the identity and properties of the cells that give rise to remyelinating oligodendrocytes. The current consensus is that remyelination is for the most part mediated by a population of precursor cells which are most frequently referred to as oligodendrocyte precursor (or progenitors) cells (OPCs) [4]. These cells, derived from an extensively studied developmental forebear, express a number of markers of which NG2 and PDGFRα are the most frequently used [5,6], but also include the transcription factors Nkx2.2 and MyT1 which in adult white matter seem to be exclusively expressed by OPCs [6–8]. Other markers such as O4, Olig1 and Olig2 have also been used but since these are also expressed by later stages their use to unequivocally identify OPCs also requires exclusion of later expressed markers. The evidence that these cells are the major source of remyelinating oligodendrocytes is compelling but indirect: there has never been a formal demonstration using lineage tracing strategies that cells carrying these markers give rise to remyelinating oligodendrocytes. The evidence that exists can be summarised as follows: (1) retroviral tracing has been used to show that dividing cells in normal adult white matter (likely but not proven to be adult OPCs) give rise to remyelinating oligodendrocytes [9], (2) transplanted OPCs remyelinate areas of demyelination with great efficiency [10–13], and (3) focal areas of demyelination in which there is death of both oligodendrocytes and OPCs are repopulated by OPCs before the appearance of new oligodendrocytes with a temporal and spatial pattern highly suggestive of the OPCs being the source of the remyelinating cells [6,14,15]. Remyelinating oligodendrocytes can also come from the stem and precursor cells of the adult subventricular zone (SVZ), either from the precursor cells contributing to the rostral migratory stream (RMS) or from the type B, GFAPexpressing stem cells of the SVZ per se [16–18]. These cells contribute to the remyelination of demyelination induced in the corpus callosum, a large transverse tract that resides above and in the close vicinity of the SVZ. This is undoubtedly a phenomenon of considerable biological interest but its relevance to demyelinating disease present throughout the neuraxis is difficult to assess. There are two issues to be resolved. First, since these cells will be competing with the local resident precursor population what relative contribution does each make? Given the abundance of local cells and their rapid response to demyelination, one might predict that the contribution made by the SVZ/RMS-derived cells might be relatively small even within nearby tracts such as the corpus
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callosum. Second, over what area of the brain might SVZ/ RMS-derived cells be able to contribute? It seems likely that their contribution of most white matter tracts, the majority of which are not as close to the SVZ as the corpus callosum, will be negligible or nil. It is therefore by no means certain that strategies aimed at promoting recruitment of remyelinating cells from the SVZ represent a sensible investment of effort in developing endogenous strategies for clinical remyelination enhancement. A topic that has been debated over many years is whether the oligodendrocyte can contribute to remyelination. In lesions where there is death of oligodendrocytes and subsequent repopulation by oligodendrocyte lineage cells then clearly this must be mediated by a cell other than the oligodendrocyte which can neither divide nor migrate. However, the situation where there is survival of oligodendrocyte cell bodies, albeit shorn of their processes and internodes, is less clear. There are two lines of argument that suggest that these cells cannot contribute to remyelination. The first is the behaviour of mature oligodendrocytes transplanted into experimental models of demyelination where, in contrast to the transplantation or OPCs [10], there is no remyelination [19]. The second derives from a relatively complex experimental design in which demyelination is induced by galactocerebroside antibodies and complement, where many oligodendrocytes cell bodies survive, and where the precursor contribution to remyelination has been removed by exposing the white matter to 40 Grays of X-irradiation [20]. In this situation there is no remyelination despite the retention of oligodendrocyte cell bodies within the lesion. In both situations the oligodendrocytes are able to extend the process, some of which are multi-layered and compacted, between but not around the demyelinated axons. Both these arguments are compelling but both carry caveats: are oligodendrocytes that have been maintained in tissue culture or ones that have been exposed to a very heavy dose of X-irradiation, likely to induce significant disturbances in cell function, truly representative of surviving oligodendrocytes in an MS lesion? Although unlikely in the view of these authors the issue of whether a surviving oligodendrocyte can contribute to remyelination remains to be unequivocally resolved. Thus in the vast majority of circumstances remyelination is mediated by adult OPCs. It is therefore worth examining the nature of this cell in some detail. There are two questions worthy of consideration: first, is the term OPC, implying restriction to oligodendrocyte differentiation, the most appropriate one to describe these cells, and second, what is the extent of its phenotypic diversity within the adult CNS? 1.2. OPCs or adult neural stem/precursor cells? Current evidence indicates that the adult OPC is derived from its perinatal forebear [21] and is distinguished from it by a number of reversible differences in its cell biology including cell cycle time and migration rate [22,23]. With
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rare exceptions the perinatal OPC is restricted to oligodendrocyte differentiation during development [24,25], although it exhibits a broader differentiation potential in vitro [26– 28]. In adulthood the OPC may need to differentiate as part of normal cell turnover or growth and certainly needs to do so in response to injury. A picture is emerging suggesting that in both these circumstances the adult OPC might be less restricted in its differentiation potential than its perinatal counterpart. In adult white matter NG2-expressing OPCs appear to be responsible for new myelin internodes that are required in the normal ageing primate brain [29], while in hippocampus, a neurogenic region of the adult brain, NG2+ cells can give rise to new neurons [30], a phenomenon mirrored by transplanted white matter-derived precursors [13,31]. In response to injury there is some evidence to suggest that these cells, as well as giving rise to oligodendrocytes (see above) may also generate astrocytes [32,33] or even Schwann cells [34,35], the myelinating cells of the peripheral nervous system that can remyelinate CNS axons when astrocytes are absent [36,37]. It may therefore be appropriate to abandon the term OPC in favour of glial precursor/progenitor cell or even neural precursor/progenitor cell [13,33,38], terms that better reflect their differentiation potential. However, whether a terminological shift should lead or follow as conceptual shift amongst those working in the field is difficult to judge. The issue of nomenclature has been complicated by the widespread non-specialist use of the generic term ‘stem cell’ as against the specialist terminology driven (one hopes) by biological reasons. However, is there a case that can be made for referring to these cells as adult neural stem cells? They are self-renewing and multipotent, criteria used by some for the assignation of ‘stem cell’. Set against this, the absence of cellular asymmetric division within this population and their rapid proliferative response to injury mean that they have much in common with what transit amplifying progenitor cells in other stem cell-containing tissues such as skin or bone marrow. Since it is valuable to draw a clear distinction between these widely-distributed parenchymal cells of the adult CNS and the true neural stem cells (B-cell) of the SVZ we favour the use of the term adult neural precursor cell. 1.3. Heterogeneity in the adult precursor cell population Within the context of remyelination the adult precursors (OPCs) have tended to be regarded as a single uniform population. However, this view is increasingly at variance with their developmental heterogeneity and physiological diversity in adulthood. It is now clear from many studies that oligodendrocytes are generated through several different developmental pathways and that while the myelinating oligodendrocyte may be a functionally homogenous cell it is derived from multiple routes [39–41]. Even in the spinal cord, where the prevailing view for many years was that precursors of oligodendrocytes originated solely in the ventral spinal cord [42,43] there is now compelling evidence for a dorsal origin
involving different transcriptional control by environmental inducers [44–46]. Moreover, it is clear that cells expressing recognised precursor cells markers (in particular NG2) fulfil a range of important physiological function in both white and grey matter, including regulation of synaptic transmission [47–49]. These roles have prompted some to regard this population as a fifth distinct class of CNS cell rather than simply a stem/precursor cell with functions confined to cell replacement and regeneration, and coined the terms ‘synantocyte’ and ‘polydendrocyte’ to describe them [50,51]. To what extent is this diversity reflected in adulthood? This is a potentially critical question to resolve in the context of developing remyelination-enhancing strategies since it is possible that different precursor cells will make different contributions to remyelination (some more effective than others) and use different regulatory mechanisms. There is currently no direct evidence of adult precursor cells expressing any of the transcription factors that define development heterogeneity, neither is there any evidence of these genes being re-expressed in defined subsets of precursor cells following activation in response to injury, a process that involves increased expression of several developmentally expressed genes [7,52]. However, within adult white matter precursor cells expressing different levels and combination of several precursor markers can be found [53]. In support of adult precursor heterogeneity sub-sets of adult precursors, based on their growth factor responsiveness, can be isolated from adult rat forebrain [54]. Resolving the extent of heterogeneity and the implications that this has for CNS regenerative processes in general and remyelination in particular is an important task over the next few years. Acknowledgements The authors contributing to this article are supported by grants from The UK MS Society (CZ), European Union (European Union Intra European Fellowship — MZ), The Wellcome Trust (AJAR) and the United Kingdom Medical Research Council (CCB). References [1] Patrikios P, Stadelmann C, Kutzelnigg A, Rauschka H, Schmidbauer M, Laursen H, et al. Remyelination is extensive in a subset of multiple sclerosis patients. Brain 2006;129:3165–72. [2] Dawson MRL, Polito A, Levine JM, Reynolds R. NG2-expressing glial progenitor cells: an abundant and widespread population of cycling cells in the adult rat CNS. Mol Cell Neurosci 2003;24:476–88. [3] Franklin RJM. Why does remyelination fail in multiple sclerosis? Nat Rev Neurosci 2002;3:705–14. [4] ffrench-Constant C, Raff MC. Proliferating bipotential glial progenitor cells in adult rat optic nerve. Nature 1986;319:499–502. [5] Dawson MRL, Levine JM, Reynolds R. NG-2-expressing cells in the central nervous system: are they oligodendroglial progenitors? J Neurosci Res 2000;61:471–9. [6] Sim FJ, Zhao C, Penderis J, Franklin RJM. The age-related decrease in CNS remyelination efficiency is attributable to an impairment of both oligodendrocyte progenitor recruitment and differentiation. J Neurosci 2002;22:2451–9.
C. Zhao et al. / Journal of the Neurological Sciences 265 (2008) 12–16 [7] Fancy SPJ, Zhao C, Franklin RJM. Increased expression of Nkx2.2 and Olig2 identifies reactive oligodendrocyte progenitor cells responding to demyelination in the adult CNS. Mol Cell Neurosci 2004;27:247–54. [8] Watanabe M, Hadzic T, Nishiyama A. Transient upregulation of Nkx2.2 expression in oligodendrocyte lineage cells during remyelination. Glia 2004;46:311–22. [9] Gensert JM, Goldman JE. Endogenous progenitors remyelinate demyelinated axons in the adult CNS. Neuron 1997;19:197–203. [10] Groves AK, Barnett SC, Franklin RJM, Crang AJ, Mayer M, Blakemore WF, et al. Repair of demyelinated lesions by transplantation of purified O-2A progenitor cells. Nature 1993;362:453–5. [11] Warrington AE, Barbarese E, Pfeiffer SE. Differential myelinogenic capacity of specific developmental stages of the oligodendrocyte lineage upon transplantation into hypomyelinating hosts. J Neurosci Res 1993;34:1–13. [12] Zhang SC, Ge B, Duncan ID. Adult brain retains the potential to generate oligodendroglial progenitors with extensive myelination capacity. Proc Natl Acad Sci U S A 1999;96:4089–94. [13] Nunes MC, Roy NS, Keyoung HM, Goodman RR, McKhann G, Jiang L, et al. Identification and isolation of multipotential neural progenitor cells from the subcortical white matter of the adult human brain. Nat Med 2003;9:439–47. [14] Levine JM, Reynolds R. Activation and proliferation of endogenous oligodendrocyte precursor cells during ethidium bromide-induced demyelination. Exp Neurol 1999;160:333–47. [15] Watanabe M, Toyama Y, Nishiyama A. Differentiation of proliferated NG2-positive glial progenitor cells in a remyelinating lesion. J Neurosci Res 2002;69:826–36. [16] Nait-Oumesmar B, Decker L, Lachapelle F, Avellana-Adalid V, Bachelin C, Van Evercooren AB. Progenitor cells of the adult mouse subventricular zone proliferate, migrate and differentiate into oligodendrocytes after demyelination. Eur J Neurosci 1999;11:4357–66. [17] Picard-Riera N, Decker L, Delarasse C, Goude K, Nait-Oumesmar B, Liblau R, et al. Experimental autoimmune encephalomyelitis mobilizes neural progenitors from the subventricular zone to undergo oligodendrogenesis in adult mice. Proc Natl Acad Sci U S A 2002;99:13211–6. [18] Menn B, Garcia-Verdugo JM, Yaschine C, Gonzalez-Perez O, Rowitch D, Alvarez-Buylla A. Origin of oligodendrocytes in the subventricular zone of the adult brain. J Neurosci 2006;26:7907–18. [19] Targett MP, Sussman J, Scolding N, O'Leary MT, Compston DAS, Blakemore WF. Failure to remyelinate rat axons following transplantation of glial cells obtained from adult human brain. Neuropathol Appl Neurobiol 1996;22:199–206. [20] Keirstead HS, Blakemore WF. Identification of post-mitotic oligodendrocytes incapable of remyelination within the demyelinated adult spinal cord. J Neuropathol Exp Neurol 1997;56:1191–201. [21] Wren D, Wolswijk G, Noble M. In vitro analysis of the origin and maintenance of O-2Aadult progenitor cells. J Cell Biol 1992;116:167–76. [22] Wolswijk G, Noble M. Identification of an adult-specific glial progenitor cell. Development 1989;105:387–400. [23] Wolswijk G, Noble M. Cooperation between PDGF and FGF converts slowly dividing O-2Aadult progenitors to rapidly dividing cells with characteristics of O-2Aperinatal progenitor cells. J Cell Biol 1992;118:889–900. [24] Levison SW, Goldman JE. Both oligodendrocytes and astrocytes develop from progenitors in the subventricular zone of postnatal rat forebrain. Neuron 1993;10:201–12. [25] Fulton BP, Burne JF, Raff MC. Visualization of O-2A progenitor cells in developing and adult rat optic nerve by quisqualate-stimulated cobalt uptake. J Neurosci 1992;12:4816–33. [26] Raff MC, Miller RH, Noble M. A glial progenitor cell that develops in vitro into an astrocyte or an oligodendrocyte depending on culture medium. Nature 1983;303:390–6. [27] Kondo T, Raff M. Oligodendrocyte precursor cells reprogrammed to become multipotential CNS stem cells. Science 2000;289:1754–7.
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[28] Franklin RJM, Blakemore WF. Glial-cell transplantation and plasticity in the O-2A lineage — implications for CNS repair. Trends Neurosci 1995;18:151–6. [29] Peters A, Sethares C. Oligodendrocytes, their progenitors and other neuroglial cells in the aging primate cerebral cortex. Cereb Cortex 2004;14:995–1007. [30] Belachew S, Chittajallu R, Aguirre AA, Yuan X, Kirby M, Anderson S, et al. Postnatal NG2 proteoglycan-expressing progenitor cells are intrinsically multipotent and generate functional neurons. J Cell Biol 2003;161:169–86. [31] Gaughwin PM, Caldwell MA, Anderson JM, Schwiening CJ, Fawcett JW, Compston DA, et al. Astrocytes promote neurogenesis from oligodendrocyte precursor cells. Eur J Neurosci 2006;23:945–56. [32] Alonso G. NG2 proteoglycan-expressing cells of the adult rat brain: possible involvement in the formation of glial scar astrocytes following stab wound. Glia 2005;49:318–38. [33] Cassiani-Ingoni R, Coksaygan T, Xue H, Reichert-Scrivner SA, Wiendl H, Rao MS, et al. Cytoplasmic translocation of Olig2 in adult glial progenitors marks the generation of reactive astrocytes following autoimmune inflammation. Exp Neurol 2006;201:349–58. [34] Talbott JF, Cao Q, Enzmann GU, Benton RL, Achim V, Cheng XX, et al. Schwann cell-like differentiation by adult oligodendrocyte precursor cells following engraftment into the demyelinated spinal cord is BMP-dependent. Glia 2006;54:147–59. [35] Blakemore WF. The case for a central nervous system (CNS) origin for the Schwann cells that remyelinate CNS axons following concurrent loss of oligodendrocytes and astrocytes. Neuropathol Appl Neurobiol 2005;31:1–10. [36] Blakemore WF. Invasion of Schwann cells into the spinal cord of the rat following local injections of lysolecithin. Neuropathol Appl Neurobiol 1976;2:21–39. [37] Woodruff RH, Franklin RJM. Demyelination and remyelination of the caudal cerebellar peduncle of adult rats following stereotaxic injections of lysolecithin, ethidium bromide and complement/anti-galactocerebroside — a comparative study. Glia 1999;25:216–28. [38] Goldman S. Glia as neural progenitor cells. Trends Neurosci 2003;26:590–6. [39] Spassky N, Olivier C, Perez-Villegas E, Goujet-Zalc C, Martinez S, Thomas J, et al. Single or multiple oligodendroglial lineages: a controversy. Glia 2000;29:143–8. [40] Kessaris N, Fogarty M, Iannarelli P, Grist M, Wegner M, Richardson WD. Competing waves of oligodendrocytes in the forebrain and postnatal elimination of an embryonic lineage. Nat Neurosci 2006;9:173–9. [41] Richardson WD, Kessaris N, Pringle N. Oligodendrocyte wars. Nat Rev Neurosci 2006;7:11–8. [42] Noll E, Miller RH. Oligodendrocyte precursors originate at the ventral ventricular zone dorsal to the ventral midline region in the embryonic rat spinal cord. Development 1993;118:563–73. [43] Pringle NP, Guthrie S, Lumsden A, Richardson WD. Dorsal spinal cord neuroepithelium generates astrocytes but not oligodendrocytes. Neuron 1998;20:883–93. [44] Cai J, Qi Y, Hu X, Tan M, Liu Z, Zhang J, et al. Generation of oligodendrocyte precursor cells from mouse dorsal spinal cord independent of nkx6 regulation and shh signaling. Neuron 2005;45:41–53. [45] Vallstedt A, Klos JM, Ericson J. Multiple dorsoventral origins of oligodendrocyte generation in the spinal cord and hindbrain. Neuron 2005;45:55–67. [46] Chandran S, Kato H, Gerreli D, Compston A, Svendsen CN, Allen ND. FGF-dependent generation of oligodendrocytes by a hedgehogindependent pathway. Development 2003;130(26):6599–609. [47] Bergles DE, Roberts JD, Somogyi P, Jahr CE. Glutamatergic synapses on oligodendrocyte precursor cells in the hippocampus. Nature 2000;405(6783):187–91. [48] Paukert M, Bergles DE. Synaptic communication between neurons and NG2(+) cells. Curr Opin Neurobiol 2006;16(5):515–21.
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[49] Karadottir R, Cavelier P, Bergersen LH, Attwell D. NMDA receptors are expressed in oligodendrocytes and activated in ischaemia. Nature 2005;438:1162–6. [50] Butt AM, Hamilton N, Hubbard P, Pugh M, Ibrahim M. Synantocytes: the fifth element. J Anat 2005;207(6):695–706. [51] Nishiyama A. Polydendrocytes: NG2 cells with many roles in development and repair of the CNS. Neuroscientist 2007;13:62–76. [52] Zhao C, Fancy SPJ, Magy L, Urwin JE, Franklin RJM. Stem cells, progenitors and myelin repair. J Anat 2005;207:251–8.
[53] Kitada M, Rowitch DH. Transcription factor co-expression patterns indicate heterogeneity of oligodendroglial subpopulations in adult spinal cord. Glia 2006;54(1):35–46. [54] Mason JL, Goldman JE. A2B5(+) and O4(+) cycling progenitors in the adult forebrain white matter respond differentially to PDGF-AA, FGF2, and IGF-1. Mol Cell Neurosci 2002;20(1):30–42.
Promoting remyelination in multiple sclerosis by endogenous adult neural stem/precursor cells: Defining cellular targets Chao Zhao, Malgorzata Zawadzka, Aude J.A. Roulois, Charlotte C. Bruce, Robin J.M. Franklin ⁎ Department of Veterinary Medicine, University of Cambridge, Madingley Road, Cambridge CB3 0ES, UK MS Society Cambridge Centre for Myelin Repair at The Brain Repair Centre, University of Cambridge, Forvie Site, Robinson Way, Cambridge CB2 2PY, UK Received 1 March 2007; received in revised form 1 May 2007; accepted 8 May 2007 Available online 13 June 2007
Abstract Although the treatment of multiple sclerosis has made significant strides in the last decade, the therapeutic enhancement of repair has yet to make the successful translation from laboratory to clinic. Nevertheless, advances in the biology of stem and precursor cells, particularly in relation to myelin damage, make this a realistic proposition during the next decade. Replacing lost myelin (remyelination) is currently thought to be an important clinical objective because of the role it might play in slowing or preventing axonal degeneration. Stem/precursor cell-based strategies for enhancing remyelination can be divided into those in which cells are transplanted into a patient (exogenous or cell therapies) and those in which the patient's own stem/precursor cells are mobilised to more efficiently engage in healing areas of demyelination (endogenous or pharmacological therapies). While the two approaches tend to be regarded separately they are not mutually exclusive. This article focuses on the endogenous approach and reviews the nature and nomenclature of the stem and precursor cells present within the adult CNS that engage in remyelination and that are therefore potential targets for pharmacological manipulation. © 2007 Elsevier B.V. All rights reserved. Keywords: Remyelination; Multiple sclerosis; Precursor cell; Stem cell
1. Introduction Demyelination of central nervous system (CNS) axons is an unusual pathological event in neurological disease in that it can be followed by a highly efficient regenerative process in which axons are reinvested with new myelin sheaths and pre-lesion architecture and function are restored. This process, called remyelination, can proceed to completion not only in laboratory animal models but also in clinical demyelinating disease such as multiple sclerosis (MS) [1]. Remyelination is thought to be mediated primarily by a population of cell specific adult stem/precursor cells, variously called oligodendrocyte precursor/progenitor cells and glial precursor/progenitor cells. These cells are widely ⁎ Corresponding author. Tel.: +44 1223 337642; fax: +44 1223 337610. E-mail address: [email protected] (R.J.M. Franklin). 0022-510X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jns.2007.05.008
distributed throughout white and grey matter in all species that have been examined (including humans) and persist in abundance throughout adulthood [2]. Remyelination represents the best known example of an endogenous population of adult stem/precursor cells engaging in a regenerative process within the diseased CNS [3]. However, it is clear that remyelination sometimes fails leading to persistent demyelination. This may predispose axons to degeneration, a critical and irreversible event which underlies the progressive clinical deterioration and characterises all forms of the disease. For this reason the development of pro-remyelination strategies is an important objective in MS research. Given its efficiency compared to other regenerative processes in the CNS, remyelination ought, in theory, to be the endogenous stem-mediated therapy most amenable to therapeutic enhancement. Rather than having to mobilise a regenerative process that either does not occur or never
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occurs with the required efficiency (for example, in ischaemic or neurodegenerative disease), the challenge is to, at best, promote an ongoing process or, at worst, reactivate one that has the potential to lead to complete healing. 1.1. Which cells mediate remyelination? In developing strategies for enhancing remyelination by endogenous cells it is important to clearly establish the identity and properties of the cells that give rise to remyelinating oligodendrocytes. The current consensus is that remyelination is for the most part mediated by a population of precursor cells which are most frequently referred to as oligodendrocyte precursor (or progenitors) cells (OPCs) [4]. These cells, derived from an extensively studied developmental forebear, express a number of markers of which NG2 and PDGFRα are the most frequently used [5,6], but also include the transcription factors Nkx2.2 and MyT1 which in adult white matter seem to be exclusively expressed by OPCs [6–8]. Other markers such as O4, Olig1 and Olig2 have also been used but since these are also expressed by later stages their use to unequivocally identify OPCs also requires exclusion of later expressed markers. The evidence that these cells are the major source of remyelinating oligodendrocytes is compelling but indirect: there has never been a formal demonstration using lineage tracing strategies that cells carrying these markers give rise to remyelinating oligodendrocytes. The evidence that exists can be summarised as follows: (1) retroviral tracing has been used to show that dividing cells in normal adult white matter (likely but not proven to be adult OPCs) give rise to remyelinating oligodendrocytes [9], (2) transplanted OPCs remyelinate areas of demyelination with great efficiency [10–13], and (3) focal areas of demyelination in which there is death of both oligodendrocytes and OPCs are repopulated by OPCs before the appearance of new oligodendrocytes with a temporal and spatial pattern highly suggestive of the OPCs being the source of the remyelinating cells [6,14,15]. Remyelinating oligodendrocytes can also come from the stem and precursor cells of the adult subventricular zone (SVZ), either from the precursor cells contributing to the rostral migratory stream (RMS) or from the type B, GFAPexpressing stem cells of the SVZ per se [16–18]. These cells contribute to the remyelination of demyelination induced in the corpus callosum, a large transverse tract that resides above and in the close vicinity of the SVZ. This is undoubtedly a phenomenon of considerable biological interest but its relevance to demyelinating disease present throughout the neuraxis is difficult to assess. There are two issues to be resolved. First, since these cells will be competing with the local resident precursor population what relative contribution does each make? Given the abundance of local cells and their rapid response to demyelination, one might predict that the contribution made by the SVZ/RMS-derived cells might be relatively small even within nearby tracts such as the corpus
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callosum. Second, over what area of the brain might SVZ/ RMS-derived cells be able to contribute? It seems likely that their contribution of most white matter tracts, the majority of which are not as close to the SVZ as the corpus callosum, will be negligible or nil. It is therefore by no means certain that strategies aimed at promoting recruitment of remyelinating cells from the SVZ represent a sensible investment of effort in developing endogenous strategies for clinical remyelination enhancement. A topic that has been debated over many years is whether the oligodendrocyte can contribute to remyelination. In lesions where there is death of oligodendrocytes and subsequent repopulation by oligodendrocyte lineage cells then clearly this must be mediated by a cell other than the oligodendrocyte which can neither divide nor migrate. However, the situation where there is survival of oligodendrocyte cell bodies, albeit shorn of their processes and internodes, is less clear. There are two lines of argument that suggest that these cells cannot contribute to remyelination. The first is the behaviour of mature oligodendrocytes transplanted into experimental models of demyelination where, in contrast to the transplantation or OPCs [10], there is no remyelination [19]. The second derives from a relatively complex experimental design in which demyelination is induced by galactocerebroside antibodies and complement, where many oligodendrocytes cell bodies survive, and where the precursor contribution to remyelination has been removed by exposing the white matter to 40 Grays of X-irradiation [20]. In this situation there is no remyelination despite the retention of oligodendrocyte cell bodies within the lesion. In both situations the oligodendrocytes are able to extend the process, some of which are multi-layered and compacted, between but not around the demyelinated axons. Both these arguments are compelling but both carry caveats: are oligodendrocytes that have been maintained in tissue culture or ones that have been exposed to a very heavy dose of X-irradiation, likely to induce significant disturbances in cell function, truly representative of surviving oligodendrocytes in an MS lesion? Although unlikely in the view of these authors the issue of whether a surviving oligodendrocyte can contribute to remyelination remains to be unequivocally resolved. Thus in the vast majority of circumstances remyelination is mediated by adult OPCs. It is therefore worth examining the nature of this cell in some detail. There are two questions worthy of consideration: first, is the term OPC, implying restriction to oligodendrocyte differentiation, the most appropriate one to describe these cells, and second, what is the extent of its phenotypic diversity within the adult CNS? 1.2. OPCs or adult neural stem/precursor cells? Current evidence indicates that the adult OPC is derived from its perinatal forebear [21] and is distinguished from it by a number of reversible differences in its cell biology including cell cycle time and migration rate [22,23]. With
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rare exceptions the perinatal OPC is restricted to oligodendrocyte differentiation during development [24,25], although it exhibits a broader differentiation potential in vitro [26– 28]. In adulthood the OPC may need to differentiate as part of normal cell turnover or growth and certainly needs to do so in response to injury. A picture is emerging suggesting that in both these circumstances the adult OPC might be less restricted in its differentiation potential than its perinatal counterpart. In adult white matter NG2-expressing OPCs appear to be responsible for new myelin internodes that are required in the normal ageing primate brain [29], while in hippocampus, a neurogenic region of the adult brain, NG2+ cells can give rise to new neurons [30], a phenomenon mirrored by transplanted white matter-derived precursors [13,31]. In response to injury there is some evidence to suggest that these cells, as well as giving rise to oligodendrocytes (see above) may also generate astrocytes [32,33] or even Schwann cells [34,35], the myelinating cells of the peripheral nervous system that can remyelinate CNS axons when astrocytes are absent [36,37]. It may therefore be appropriate to abandon the term OPC in favour of glial precursor/progenitor cell or even neural precursor/progenitor cell [13,33,38], terms that better reflect their differentiation potential. However, whether a terminological shift should lead or follow as conceptual shift amongst those working in the field is difficult to judge. The issue of nomenclature has been complicated by the widespread non-specialist use of the generic term ‘stem cell’ as against the specialist terminology driven (one hopes) by biological reasons. However, is there a case that can be made for referring to these cells as adult neural stem cells? They are self-renewing and multipotent, criteria used by some for the assignation of ‘stem cell’. Set against this, the absence of cellular asymmetric division within this population and their rapid proliferative response to injury mean that they have much in common with what transit amplifying progenitor cells in other stem cell-containing tissues such as skin or bone marrow. Since it is valuable to draw a clear distinction between these widely-distributed parenchymal cells of the adult CNS and the true neural stem cells (B-cell) of the SVZ we favour the use of the term adult neural precursor cell. 1.3. Heterogeneity in the adult precursor cell population Within the context of remyelination the adult precursors (OPCs) have tended to be regarded as a single uniform population. However, this view is increasingly at variance with their developmental heterogeneity and physiological diversity in adulthood. It is now clear from many studies that oligodendrocytes are generated through several different developmental pathways and that while the myelinating oligodendrocyte may be a functionally homogenous cell it is derived from multiple routes [39–41]. Even in the spinal cord, where the prevailing view for many years was that precursors of oligodendrocytes originated solely in the ventral spinal cord [42,43] there is now compelling evidence for a dorsal origin
involving different transcriptional control by environmental inducers [44–46]. Moreover, it is clear that cells expressing recognised precursor cells markers (in particular NG2) fulfil a range of important physiological function in both white and grey matter, including regulation of synaptic transmission [47–49]. These roles have prompted some to regard this population as a fifth distinct class of CNS cell rather than simply a stem/precursor cell with functions confined to cell replacement and regeneration, and coined the terms ‘synantocyte’ and ‘polydendrocyte’ to describe them [50,51]. To what extent is this diversity reflected in adulthood? This is a potentially critical question to resolve in the context of developing remyelination-enhancing strategies since it is possible that different precursor cells will make different contributions to remyelination (some more effective than others) and use different regulatory mechanisms. There is currently no direct evidence of adult precursor cells expressing any of the transcription factors that define development heterogeneity, neither is there any evidence of these genes being re-expressed in defined subsets of precursor cells following activation in response to injury, a process that involves increased expression of several developmentally expressed genes [7,52]. However, within adult white matter precursor cells expressing different levels and combination of several precursor markers can be found [53]. In support of adult precursor heterogeneity sub-sets of adult precursors, based on their growth factor responsiveness, can be isolated from adult rat forebrain [54]. Resolving the extent of heterogeneity and the implications that this has for CNS regenerative processes in general and remyelination in particular is an important task over the next few years. Acknowledgements The authors contributing to this article are supported by grants from The UK MS Society (CZ), European Union (European Union Intra European Fellowship — MZ), The Wellcome Trust (AJAR) and the United Kingdom Medical Research Council (CCB). References [1] Patrikios P, Stadelmann C, Kutzelnigg A, Rauschka H, Schmidbauer M, Laursen H, et al. Remyelination is extensive in a subset of multiple sclerosis patients. Brain 2006;129:3165–72. [2] Dawson MRL, Polito A, Levine JM, Reynolds R. NG2-expressing glial progenitor cells: an abundant and widespread population of cycling cells in the adult rat CNS. Mol Cell Neurosci 2003;24:476–88. [3] Franklin RJM. Why does remyelination fail in multiple sclerosis? Nat Rev Neurosci 2002;3:705–14. [4] ffrench-Constant C, Raff MC. Proliferating bipotential glial progenitor cells in adult rat optic nerve. Nature 1986;319:499–502. [5] Dawson MRL, Levine JM, Reynolds R. NG-2-expressing cells in the central nervous system: are they oligodendroglial progenitors? J Neurosci Res 2000;61:471–9. [6] Sim FJ, Zhao C, Penderis J, Franklin RJM. The age-related decrease in CNS remyelination efficiency is attributable to an impairment of both oligodendrocyte progenitor recruitment and differentiation. J Neurosci 2002;22:2451–9.
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