Distinct roles for the Charcot-Marie-Tooth disease-causing endosomal regulators Mtmr5 and Mtmr13 in axon radial sorting and Schwann cell myelination

Abstract

The form of Charcot-Marie-Tooth type 4B (CMT4B) disease caused by mutations in myotubularin-related 5 (MTMR5; also called SET Binding Factor 1; SBF1) shows a spectrum of axonal and demyelinating nerve phenotypes. This contrasts with the CMT4B subtypes caused by MTMR2 or MTMR13 (SBF2) mutations, which are characterized by myelin outfoldings and classic demyelination. Thus, it is unclear whether MTMR5 plays an analogous or distinct role from that of its homolog, MTMR13, in the peripheral nervous system (PNS). MTMR5 and MTMR13 are pseudophosphatases predicted to regulate endosomal trafficking by activating Rab GTPases and binding to the phosphoinositide 3-phosphatase MTMR2. In the mouse PNS, Mtmr2 was required to maintain wild type levels of Mtmr5 and Mtmr13, suggesting that these factors function in discrete protein complexes. Genetic elimination of both Mtmr5 and Mtmr13 in mice led to perinatal lethality, indicating that the two proteins have partially redundant functions during embryogenesis. Loss of Mtmr5 in mice did not cause CMT4B-like myelin outfoldings.However, adult Mtmr5 -/mouse nerves contained fewer myelinated axons than control nerves, likely as a result of axon radial sorting defects. Mtmr5 levels were highest during axon radial sorting, whereas Mtmr13 levels rose as myelin formed, and remained high through adulthood.Our findings suggest that Mtmr5 and Mtmr13 ensure proper axon radial sorting and Schwann cell myelination, respectively, perhaps through their direct interactions with Mtmr2. This study enhances our understanding of the non-redundant roles of the endosomal regulators MTMR5 and MTMR13 during normal peripheral nerve development and disease.During the development of the peripheral nervous system (PNS), Schwann cells associate with bundles of mixed diameter axons. In a process known as radial sorting, Schwann cells segregate large diameter axons (>1 μm) from smaller axons, which remain in bundles (1, 2). Once sorted into 1:1 associations with Schwann cells, large caliber axons are wrapped in a specialized, multilayer membrane sheath called myelin (3). Myelin facilitates rapid, saltatory nerve impulse conduction by clustering sodium channels at nodes of Ranvier, and by reducing axonal capacitance (4). Small caliber axons (<1 μm) remain unmyelinated and are supported by Schwann cells in Remak bundles (2). Both myelinating and Remak Schwann cells provide metabolic support to maintain axon integrity (3, 5-8). Both radial sorting and subsequent myelination require coordinated cytoskeleton reorganization, and receptor signaling from the adaxonal and abaxonal Schwann cell surfaces (1, 3). The precise control of downstream signaling is thought to be mediated in part by receptor trafficking through the endolysosomal pathway (9-11).Genomic studies have strongly suggested a link between abnormal endosomal trafficking and demyelinating Charcot-Marie-Tooth (CMT) disease (12). CMT is the most common inherited neurological disorder, affecting 1 in 2500 people (13). Axonal and demyelinating forms of CMT are d...

Characterization of a novel zebrafish model ofMTMR5-associated CMT4B3

“…Each MTMR is associated with distinct patient neuropathies, and distinct nerve pathologies in animal models. Mtmr2 -/-and Mtmr13 -/-mice showed classic CMT4B myelin outfolding in the PNS (Azzedine et al, 2003, Tersar et al, 2007, Robinson et al, 2008, whilst adult Mtmr5 -/-mice showed reduced number of myelinated axons likely due to disruption in axon radial sorting (Mammel et al, 2022). Our 14dpf mtmr5 -/-zebrafish larvae show dysmyelination at the axon plasma membrane, in keeping consistent with both the Mtmr5 KO mice and patient pathology.…”

“…4G-L). The reduced overall body size is consistent with smaller body size reported in Mtmr5 -/-adult mice (Mammel et al, 2022).…”

“…3D-E). In all, the loss of mtmr5 does not affect survival or cause significant movement defects, similar to observations from Mtmr5 -/-mice (Mammel et al, 2022).…”

“…MTMR5 is a pseudophosphatase that binds to its active family member MTMR2 (Mammel et al, 2022). MTMR2 subcellular localization is either diffuse or cytoplasm-specific depending on cell type (Previtali et al, 2007), and its interaction with MTMR5 (Kim et al, 2003).…”

“…Mtmr5 knockout mice have been generated; they have defective spermatogenesis (Firestein et al, 2002, Mammel et al, 2022, and peripheral axon demyelination without overt axon defects or brain involvement (Mammel et al, 2022). Zebrafish serve as an excellent alternative model organism for studying CMT4B3.…”