Schwann cells restrict axonal diameters via CMTM6

Myelination of axons accelerates nerve impulse propagation 20-100-fold, theoretically allowing rapid nerve conduction with reduced axonal diameters. However, to our knowledge no myelin dependent signal has been described to restrict axonal diameters. Indeed, the only myelin-to-axon signal known to affect axonal size, myelin associated glycoprotein (MAG), actually increases radial axonal growth of myelinated axons in the peripheral nervous system. We hypothesized that many signaling molecules mediating cross-talk between axons and myelinating Schwann cells remain unknown. Thus, we used label-free proteomics of a biochemically fraction enriched for axon/myelin interface to identify novel signaling candidates. By STED-microscopy, immunoblotting and cryo-immuno electron microscopy we confirmed the localization of a novel Schwann cell protein CMTM6 (chemokine-like factor-like MARVEL-transmembrane domain-containing protein 6) at the adaxonal Schwann cell membrane and thus identified a novel myelin constituent in the peripheral nervous system. Genetic disruption of Cmtm6 expression in Schwann cells causes a substantial increase of axonal diameters in various peripheral nerves without impairing myelin biogenesis or axonal integrity. Diameters of non-myelinated axons are also increased when CMTM6 is lacking from Schwann cells. Importantly, radial sorting of axons and myelin biogenesis are not compromised. Notably, increased axonal diameters correlate with accelerated sensory nerve conduction velocity, enhanced sensory responses and perturbed motor performance. It was previously suggested that expression of CMTM6 in cancer cells and interaction with PD-L1 (programmed death ligand 1) limits anti-tumor immunity. Our data however do not support interactions of CMTM6 with PD-L1 in the peripheral nervous system. We could demonstrate that CMTM6-loss of function leading to larger axonal diameters overrides MAG-loss of function, which by itself causes a shift towards reduced axonal diameters. Together we find that Schwann cells utilize adaxonal proteins such as MAG and CMTM6 to regulate radial axonal growth and optimize nervous system function.2021-05-0