ADAM17 Regulates p75NTR-Mediated Fibrinolysis and Nerve Remyelination.

We previously reported that a-disintegrin and metalloproteinase (ADAM)17 is a key protease regulating myelin formation. We now describe a role for ADAM17 during the Wallerian degeneration (WD) process. Unexpectedly, we observed that glial ADAM17, by regulating p75NTR processing, cell autonomously promotes remyelination, while neuronal ADAM17 is dispensable. Accordingly, p75NTR abnormally accumulates specifically when ADAM17 is maximally expressed leading to a downregulation of tissue plasminogen activator (tPA) expression, excessive fibrin accumulation over time, and delayed remyelination. Mutant mice also present impaired macrophage recruitment and defective nerve conduction velocity (NCV). Thus, ADAM17 expressed in Schwann cells, controls the whole WD process, and its absence hampers effective nerve repair. Collectively, we describe a previously uncharacterized role for glial ADAM17 during nerve regeneration. Based on the results of our study, we posit that, unlike development, glial ADAM17 promotes remyelination through the regulation of p75NTR-mediated fibrinolysis.SIGNIFICANCE STATEMENT The α-secretase a-disintegrin and metalloproteinase (ADAM)17, although relevant for developmental PNS myelination, has never been investigated in Wallerian degeneration (WD). We now unravel a new mechanism of action for this protease and show that ADAM17 cleaves p75NTR, regulates fibrin clearance, and eventually fine-tunes remyelination. The results presented in this study provide important insights into the complex regulation of remyelination following nerve injury, identifying in ADAM17 and p75NTR a new signaling axis implicated in these events. Modulation of this pathway could have important implications in promoting nerve remyelination, an often-inefficient process, with the aim of restoring a functional axo-glial unit.

Prostaglandin D2 synthase modulates macrophage activity and accumulation in injured peripheral nerves.

We have previously reported that prostaglandin D2 Synthase (L-PGDS) participates in peripheral nervous system (PNS) myelination during development. We now describe the role of L-PGDS in the resolution of PNS injury, similarly to other members of the prostaglandin synthase family, which are important for Wallerian degeneration (WD) and axonal regeneration. Our analyses show that L-PGDS expression is modulated after injury in both sciatic nerves and dorsal root ganglia neurons, indicating that it might play a role in the WD process. Accordingly, our data reveals that L-PGDS regulates macrophages phagocytic activity through a non-cell autonomous mechanism, allowing myelin debris clearance and favoring axonal regeneration and remyelination. In addition, L-PGDS also appear to control macrophages accumulation in injured nerves, possibly by regulating the blood-nerve barrier permeability and SOX2 expression levels in Schwann cells. Collectively, our results suggest that L-PGDS has multiple functions during nerve regeneration and remyelination. Based on the results of this study, we posit that L-PGDS acts as an anti-inflammatory agent in the late phases of WD, and cooperates in the resolution of the inflammatory response. Thus, pharmacological activation of the L-PGDS pathway might prove beneficial in resolving peripheral nerve injury.

Prostaglandin D2 synthase/GPR44: a signaling axis in PNS myelination.

Neuregulin 1 type III is processed following regulated intramembrane proteolysis, which allows communication from the plasma membrane to the nucleus. We found that the intracellular domain of neuregulin 1 type III upregulated the prostaglandin D2 synthase (L-pgds, also known as Ptgds) gene, which, together with the G protein-coupled receptor Gpr44, forms a previously unknown pathway in PNS myelination. Neuronal L-PGDS is secreted and produces the PGD2 prostanoid, a ligand of Gpr44. We found that mice lacking L-PGDS were hypomyelinated. Consistent with this, specific inhibition of L-PGDS activity impaired in vitro myelination and caused myelin damage. Furthermore, in vivo ablation and in vitro knockdown of glial Gpr44 impaired myelination. Finally, we identified Nfatc4, a key transcription factor for myelination, as one of the downstream effectors of PGD2 activity in Schwann cells. Thus, L-PGDS and Gpr44 are previously unknown components of an axo-glial interaction that controls PNS myelination and possibly myelin maintenance.