A potential role for bone morphogenetic protein signalling in glial cell fate determination following adult central nervous system injury in vivo David W. Hampton, 1,2 Richard A. Asher, 2 Toru Kondo, 3 John D. Steeves, 1 Matt S. Ramer 1 and James W. Fawcett 2 1 ICORD, University of British Columbia, Vancouver, BC, Canada 2 Cambridge Centre for Brain Repair, University of Cambridge, ED Adrian Building, Forvie Site, Robinson Way, Cambridge CB2 2PY, UK 3 RIKEN Centre for Developmental Biology (CDB), Kobe, Japan Keywords: astrocytes, glial scar, oligodendrocyte precursor cell, noggin, spinal cord injury Abstract Bone morphogenetic proteins (BMPs) and their endogenous inhibitors, including noggin, chordin and follistatin, have roles in pattern formation and fate specification of neuronal and glial cells during nervous system development. We have examined their influence on glial reactions in the injured central nervous system (CNS). We show that penetrating injuries to the brain and spinal cord resulted in the upregulation of BMP-2 ⁄ 4, BMP-7, and noggin, with the latter being expressed almost exclusively by reactive astrocytes at the injury site, and we show that astrocytes in vitro produce noggin. As BMPs have been shown to drive cultured NG2-positive oligodendrocyte precursors (OPCs) towards a multipotential phenotype (type II astrocytes), we investigated the effects of inhibiting noggin with a function-blocking antibody (noggin-FbAb). In vitro, BMP-driven conversion of OPCs to type 2 astrocytes was inhibited by noggin, an effect that was reversed by noggin-FbAb. Noggin-FbAb also increased the number of type 2 astrocytes generated from cultured OPCs exposed to an astrocyte feeder layer, consistent with astrocytes producing both BMPs and noggin. In knife cut injuries in vivo, noggin-FbAb treatment resulted in an increase in the number of NG2-positive cells and small GFAP-positive cells in the injury site, and the appearance of glial cells with the morphological and antigenic characteristics of type 2 astrocytes (as generated in vitro), with coexpression of both GFAP and NG2. This potential conversion of inhibitory OPCs to type 2 astrocyte-like cells in vivo suggests that endogenous BMPs, unmasked by noggin antagonism, might be exploited to manipulate cell fate following CNS trauma. Introduction Bone morphogenetic proteins (BMPs) play an important role in the development of the central nervous system (CNS). During gastrula- tion, they are secreted by ectodermal cells and specify epidermal fate (Zimmerman et al., 1996), and later are involved in dorsoventral patterning of the neural tube (Barth et al., 1999; Lee & Jessel, 1999). In both these situations, it is the interplay between BMPs, their antagonists and BMP receptors that is vital in defining cell fate during development (Helm et al., 2000). In the CNS, BMPs are produced by astrocytes (Mehler et al., 1997; Kondo & Raff, 2004), and are upregulated following injury (Charytoniuk et al., 2000; Setoguchi et al., 2001). Positive in vitro effects of BMPs on CNS cells include neuroprotective and neurotrophic effects (Beck et al., 2001; Chang et al., 2002, 2003; Zuch et al., 2004; Harvey et al., 2005). In vitro, BMPs promote the differentiation of NG2-positive oligo- dendrocyte precursor cells (OPCs) into a form of OPC-derived astrocyte, the type 2 astrocyte, but the existence of these cells in the adult CNS is controversial, although a small proportion of OPCs transplanted into the CNS have been shown to become astrocytes (Belachew et al., 2003; Windrem et al., 2004). It is probable that there are factors that block the actions of BMPs in the CNS, thereby preventing the production of type 2 astrocytes. Noggin is an obvious candidate. It is a member of the BMP antagonist family, which includes chordin, follistatin, and the more recently discovered gremlin and cerberus. Of these, noggin is considered to be the main BMP antagonist [reviewed by Balemans & Van Hul (2002)]. It has been shown that noggin is produced by astrocytes in vitro (Mehler et al., 1997; Kondo & Raff, 2004). However, expression in the adult brain is mostly limited to the ependymal cells, where it is important in maintaining the stem cell niche (Lim et al., 2000) and in influencing the fate of olfactory neurons during migration to the olfactory bulbs (Liu et al., 2004; Peretto et al., 2004). The effects of noggin on the fate of cultured neural precursor cells transplanted into the injured CNS have been examined in two recent experiments, by transfecting these cells to secrete noggin. Setoguchi et al. (2004) reported an increase in neuronal and oligodendrocyte cell marker expression by the transplanted cells, accompanied by functional recovery. However, Enzmann et al. (2005) observed astroglial differentiation of transplanted cells, as well as an increase in inflammatory responses and cavitation. It is possible that in the damaged CNS, the NG2-positive OPCs could be a source of endogenous multipotential progenitors. These cells proliferate massively wherever the CNS is injured, and are therefore available in large numbers in most forms of CNS pathology (Levine et al., 2001). In vitro, it has been possible to generate neurons, astrocytes and oligodendrocytes from OPCs. The first demonstration came from experiments in which the cells were taken through the type 2 astrocyte phenotype by treatment with BMPs (Kondo & Raff, Correspondence: Dr David Hampton, 2 Cambridge Centre for Brain Repair, as above. E-mail: dwh27@cam.ac.uk Received 14 March 2007, revised 29 August 2007, accepted 10 October 2007 European Journal of Neuroscience, Vol. 26, pp. 3024–3035, 2007 doi:10.1111/j.1460-9568.2007.05940.x ª The Authors (2007). Journal Compilation ª Federation of European Neuroscience Societies and Blackwell Publishing Ltd