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 ;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 b-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-b-catenin signaling in OLPs results in profound delay of both developmental myelination and remyelination, based on (1) conditional activation of b-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-b-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.[Keywords: Oligodendrocyte; CNS development; Wnt; multiple sclerosis; remyelination; Olig2] Supplemental material is available at http://www.genesdev.org.

In multiple sclerosis, demyelination of the CNS axons is associated with axonal injury and degeneration, which is now accepted as the major cause of neurological disability in the disease. Although the kinetics and the extent of axonal damage have been described in detail, the mechanisms by which it occurs are as yet unclear; one suggestion is failure of remyelination. The goal of this study was to test the hypothesis that failure of prompt remyelination contributes to axonal degeneration following demyelination. Remyelination was inhibited by exposing the brain to 40 Gy of X-irradiation prior to cuprizone intoxication and this resulted in a significant increase in the extent of axonal degeneration and loss compared to non-irradiated cuprizone-fed mice. To exclude the possibility that this increase was a consequence of the X-irradiation and to highlight the significance of remyelination, we restored remyelinating capacity to the X-irradiated mouse brain by transplanting of GFP-expressing embryo-derived neural progenitors. Restoring the remyelinating capacity in these mice resulted in a significant increase in axon survival compared to non-transplanted, X-irradiated cuprizone-intoxicated mice. Our results support the concept that prompt remyelination protects axons from demyelination-associated axonal loss and that remyelination failure contributes to the axon loss that occurs in multiple sclerosis.

The primate corticospinal tract (CST), the major descending pathway mediating voluntary hand movements, comprises nine or more functional subdivisions. The role of subcomponents other than that from primary motor cortex, however, is not well understood. We have previously shown that following a cervical dorsal rhizotomy (Darian-Smith et al., 2013), CST projections originating from primary somatosensory (S1) and motor (M1) cortex responded quite differently to injury. Terminal projections from the S1 (areas 3b/1/2) shrank to Ͻ60% of the contralateral side, while M1 CST projections remained robust or expanded (Ͼ110%). Here, we asked what happens when a central lesion is added to the equation, to better simulate clinical injury. Monkeys (n ϭ 6) received either a unilateral (1) dorsal root lesion (DRL), (2) or a combined DRL/dorsal column lesion (DRL/DCL), or (3) a DRL/DCL where the DCL was made 4 months following the initial DRL. Electrophysiological recordings were made in S1 4 months postlesion in the first two groups, and 6 weeks after the DCL in the third lesion group, to identify the reorganized region of D1-D3 (thumb, index finger, and middle finger) representation. Anterograde tracers were then injected bilaterally to assess spinal terminal labeling. Remarkably, in all DRL/DCL animals, terminal projections from the S1 and M1 extended bilaterally and caudally well beyond terminal territories in normal animals or following a DRL. These data were highly significant. Extensive sprouting from the S1 CST has not been reported previously, and these data raise important questions about S1 CST involvement in recovery following spinal injury.

This review presents evidence that pain is common after traumatic brain injury. However, while there are many potential mechanisms explaining this problem such as neuroinflammation, excitotoxicity, and axonal degeneration, we have no clear understanding of which of them contribute in individual patients. The authors highlight the priorities for research that will expand our knowledge and that may lead to the rational design of therapies that both reduce pain and provide optimal overall outcomes after traumatic brain injury.

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