Focused ion beam-scanning electron microscopy links pathological myelin outfoldings to axonal changes in mice lacking Plp1 or Mag.

Healthy myelin sheaths consist of multiple compacted membrane layers closely encasing the underlying axon. The ultrastructure of CNS myelin requires specialized structural myelin proteins, including the transmembrane-tetraspan proteolipid protein (PLP) and the Ig-CAM myelin-associated glycoprotein (MAG). To better understand their functional relevance, we asked to what extent the axon/myelin-units display similar morphological changes if PLP or MAG are lacking. We thus used focused ion beam-scanning electron microscopy (FIB-SEM) to re-investigate axon/myelin-units side-by-side in Plp- and Mag-null mutant mice. By three-dimensional reconstruction and morphometric analyses, pathological myelin outfoldings extend up to 10 µm longitudinally along myelinated axons in both models. More than half of all assessed outfoldings emerge from internodal myelin. Unexpectedly, three-dimensional reconstructions demonstrated that both models displayed complex axonal pathology underneath the myelin outfoldings, including axonal sprouting. Axonal anastomosing was additionally observed in Plp-null mutant mice. Importantly, normal-appearing axon/myelin-units displayed significantly increased axonal diameters in both models according to quantitative assessment of electron micrographs. These results imply that healthy CNS myelin sheaths facilitate normal axonal diameters and shape, a function that is impaired when structural myelin proteins PLP or MAG are lacking.

Progressive axonopathy when oligodendrocytes lack the myelin protein CMTM5.

Oligodendrocytes facilitate rapid impulse propagation along the axons they myelinate and support their long-term integrity. However, the functional relevance of many myelin proteins has remained unknown. Here, we find that expression of the tetraspan-transmembrane protein CMTM5 (chemokine-like factor-like MARVEL-transmembrane domain containing protein 5) is highly enriched in oligodendrocytes and central nervous system (CNS) myelin. Genetic disruption of the Cmtm5 gene in oligodendrocytes of mice does not impair the development or ultrastructure of CNS myelin. However, oligodendroglial Cmtm5 deficiency causes an early-onset progressive axonopathy, which we also observe in global and tamoxifen-induced oligodendroglial Cmtm5 mutants. Presence of the WldS mutation ameliorates the axonopathy, implying a Wallerian degeneration-like pathomechanism. These results indicate that CMTM5 is involved in the function of oligodendrocytes to maintain axonal integrity rather than myelin biogenesis.

CMTM6 expressed on the adaxonal Schwann cell surface restricts axonal diameters in peripheral nerves.

The velocity of nerve conduction is moderately enhanced by larger axonal diameters and potently sped up by myelination of axons. Myelination thus allows rapid impulse propagation with reduced axonal diameters; however, no myelin-dependent mechanism has been reported that restricts radial growth of axons. By label-free proteomics, STED-microscopy and cryo-immuno electron-microscopy we here identify CMTM6 (chemokine-like factor-like MARVEL-transmembrane domain-containing family member-6) as a myelin protein specifically localized to the Schwann cell membrane exposed to the axon. We find that disruption of Cmtm6-expression in Schwann cells causes a substantial increase of axonal diameters but does not impair myelin biogenesis, radial sorting or integrity of axons. Increased axonal diameters correlate with accelerated sensory nerve conduction and sensory responses and perturbed motor performance. These data show that Schwann cells utilize CMTM6 to restrict the radial growth of axons, which optimizes nerve function.

Turning to myelin turnover.

Neural plasticity in the adult central nervous system involves the adaptation of myelination, including the formation of novel myelin sheaths by adult-born oligodendrocytes. Yet, mature oligodendrocytes slowly but constantly turn over their pre-existing myelin sheaths, thereby establishing an equilibrium of replenishment and degradation that may also be subject to adaptation with consequences for nerve conduction velocity. In this short review we highlight selected approaches to the normal turnover of adult myelin in vivo, from injecting radioactive precursors of myelin constituents in the 1960s to current strategies involving isotope labeling and tamoxifen-induced gene targeting.