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A requisite component of nervous system development is the achievement of cellular recognition and spatial segregation through competition-based refinement mechanisms. Competition for available axon space by myelinating oligodendrocytes ensures that all relevant CNS axons are myelinated properly. To ascertain the nature of this competition, we generated a transgenic mouse with sparsely labeled oligodendrocytes and establish that individual oligodendrocytes occupying similar axon tracts can greatly vary the number and lengths of their myelin internodes. Here we show that intercellular interactions between competing oligodendroglia influence the number and length of myelin internodes, referred to as myelinogenic potential, and identify the amino-terminal region of Nogo-A, expressed by oligodendroglia, as necessary and sufficient to inhibit this process. Exuberant and expansive myelination/remyelination is detected in the absence of Nogo during development and after demyelination, suggesting that spatial segregation and myelin extent is limited by microenvironmental inhibition. We demonstrate a unique physiological role for Nogo-A in the precise myelination of the developing CNS. Maximizing the myelinogenic potential of oligodendrocytes may offer an effective strategy for repair in future therapies for demyelination.

The progressive loss of CNS myelin in patients with multiple sclerosis (MS) has been proposed to result from the combined effects of damage to oligodendrocytes and failure of remyelination. A common feature of demyelinated lesions is the presence of oligodendrocyte precursors (OLPs) blocked at a premyelinating stage. However, the mechanistic basis for inhibition of myelin repair is incompletely understood. To identify novel regulators of OLP differentiation, potentially dysregulated during repair, we performed a genome-wide screen of 1040 transcription factor-encoding genes expressed in remyelinating rodent lesions. We report that approximately 50 transcription factor-encoding genes show dynamic expression during repair and that expression of the Wnt pathway mediator Tcf4 (aka Tcf7l2) within OLPs is specific to lesioned-but not normal-adult white matter. We report that beta-catenin signaling is active during oligodendrocyte development and remyelination in vivo. Moreover, we observed similar regulation of Tcf4 in the developing human CNS and lesions of MS. Data mining revealed elevated levels of Wnt pathway mRNA transcripts and proteins within MS lesions, indicating activation of the pathway in this pathological context. We show that dysregulation of Wnt-beta-catenin signaling in OLPs results in profound delay of both developmental myelination and remyelination, based on (1) conditional activation of beta-catenin in the oligodendrocyte lineage in vivo and (2) findings from APC(Min) mice, which lack one functional copy of the endogenous Wnt pathway inhibitor APC. Together, our findings indicate that dysregulated Wnt-beta-catenin signaling inhibits myelination/remyelination in the mammalian CNS. Evidence of Wnt pathway activity in human MS lesions suggests that its dysregulation might contribute to inefficient myelin repair in human neurological disorders.

Osteopontin (OPN) is a key immunoregulator in the autoimmune-mediated demyelinating disease multiple sclerosis. OPN may also play a role in the remyelination since it is 1) a ligand for alpha V integrins, several of which regulate the properties of the oligodendrocyte precursor cells (OPCs) primarily responsible for remyelination, and 2) enhances myelin membrane formation in OPC lines. Here we show that OPN is expressed at high levels during remyelination of toxin-induced demyelination. The increased expression is due to mRNA expression in macrophages and follows differences in macrophage responses to demyelination in young and old adult animals. To identify the role of OPN in remyelination focal demyelination was induced in wild-type and OPN(-/-) mice. There was no difference in the rate of remyelination between the two groups indicating that OPN is not a critical component of remyelination.

Olig1 and Olig2 encode basic helix-loop-helix (bHLH) transcription factors that are expressed in both the developing and mature vertebrate central nervous system. While numerous studies have established critical functions for Olig genes during the formation of motor neurons and oligodendrocytes of the ventral neural tube, their roles at later stages of development and in adulthood have remained relatively obscure. Recent studies, however, reveal that in the fetal dorsal spinal cord and neural progenitor cells of the adult brain, Olig expression continues to mark, and may regulate, the formation of oligodendroglia. Studies of Olig expression in human brain tumors and repair of demyelinating lesions suggest the possibility of additional functions in a variety of neurological diseases., (Copyright 2005 Wiley-Liss, Inc.)

There are two components to the treatment of multiple sclerosis (MS); the first is to prevent damage occurring, and the second is to repair the residual damage. While considerable progress has been made in the recent years with the former through the development of anti-inflammatory and immunomodulatory therapies, there are currently no effective repair therapies routinely used in MS patients. This represents a significant gap in the MS clinician's therapeutic armoury. In this article we argue that a clear understanding of the repair mechanisms following CNS demyelination is fundamental to filling this gap. We discuss (1) the cellular events involved in remyelination, (2) changes in transcription factor expression within oligodendrocyte precursor cells associated with their activation in response to demyelination, (3) the role of platelet derived growth factor in the OPC recruitment phase of remyelination, and (4) the significance of the inflammatory response associated with demyelination in creating a signalling environment that favours remyelination.