Nf2 Mutation in Schwann Cells Delays Functional Neural Recovery Following Injury.

Merlin is the protein product of the NF2 tumor suppressor gene. Germline NF2 mutation leads to neurofibromatosis type 2 (NF2), characterized by multiple intracranial and spinal schwannomas. Patients with NF2 also frequently develop peripheral neuropathies. While the role of merlin in SC neoplasia is well established, its role in SC homeostasis is less defined. Here we explore the role of merlin in SC responses to nerve injury and their ability to support axon regeneration. We performed sciatic nerve crush in wild-type (WT) and in P0SchΔ39-121 transgenic mice that express a dominant negative Nf2 isoform in SCs. Recovery of nerve function was assessed by measuring mean contact paw area on a pressure pad 7, 21, 60, and 90 days following nerve injury and by nerve conduction assays at 90 days following injury. After 90 days, the nerves were harvested and axon regeneration was quantified stereologically. Myelin ultrastructure was analyzed by electron microscopy. Functional studies showed delayed nerve regeneration in Nf2 mutant mice compared to the WT mice. Delayed neural recovery correlated with a reduced density of regenerated axons and increased endoneurial space in mutants compared to WT mice. Nevertheless, functional and nerve conduction measures ultimately recovered to similar levels in WT and Nf2 mutant mice, while there was a small (∼17%) reduction in the percent of regenerated axons in the Nf2 mutant mice. The data suggest that merlin function in SCs regulates neural ultrastructure and facilitates neural regeneration, in addition to its role in SC neoplasia.

Merlin status regulates p75(NTR) expression and apoptotic signaling in Schwann cells following nerve injury.

After nerve injury, Schwann cells (SCs) dedifferentiate, proliferate, and support axon regrowth. If axons fail to regenerate, denervated SCs eventually undergo apoptosis due, in part, to increased expression of the low-affinity neurotrophin receptor, p75(NTR). Merlin is the protein product of the NF2 tumor suppressor gene implicated in SC tumorigenesis. Here we explore the contribution of merlin to SC responses to nerve injury. We find that merlin becomes phosphorylated (growth permissive) in SCs following acute axotomy and following gradual neural degeneration in a deafness model, temporally correlated with increased p75(NTR) expression. p75(NTR) levels are elevated in P0SchΔ39-121 transgenic mice that harbor an Nf2 mutation in SCs relative to wild-type mice before axotomy and remain elevated for a longer period of time following injury. Replacement of wild-type, but not phospho-mimetic (S518D), merlin isoforms suppresses p75(NTR) expression in primary human schwannoma cultures which otherwise lack functional merlin. Despite elevated levels of p75(NTR), SC apoptosis following axotomy is blunted in P0SchΔ39-121 mice relative to wild-type mice suggesting that loss of functional merlin contributes to SC resistance to apoptosis. Further, cultured SCs from mice with a tamoxifen-inducible knock-out of Nf2 confirm that SCs lacking functional merlin are less sensitive to p75(NTR)-mediated cell death. Taken together these results point to a model whereby loss of axonal contact following nerve injury results in merlin phosphorylation leading to increased p75(NTR) expression. Further, they demonstrate that merlin facilitates p75(NTR)-mediated apoptosis in SCs helping to explain how neoplastic SCs that lack functional merlin survive long-term in the absence of axonal contact.