A new mouse model of Charcot-Marie-Tooth 2J neuropathy replicates human axonopathy and suggest alteration in axo-glia communication

Abstract

Myelin is essential for rapid nerve impulse propagation and axon protection. Accordingly, defects in myelination or myelin maintenance lead to secondary axonal damage and subsequent degeneration. Studies utilizing genetic (CNPase-, MAG-, and PLP-null mice) and naturally occurring neuropathy models suggest that myelinating glia also support axons independently from myelin. Myelin protein zero (MPZ or P0), which is expressed only by Schwann cells, is critical for myelin formation and maintenance in the peripheral nervous system. Many mutations in MPZ are associated with demyelinating neuropathies (Charcot-Marie-Tooth disease type 1B [CMT1B]). Surprisingly, the substitution of threonine by methionine at position 124 of P0 (P0T124M) causes axonal neuropathy (CMT2J) with little to no myelin damage. This disease provides an excellent paradigm to understand how myelinating glia support axons independently from myelin. To study this, we generated targeted knock-in MpzT124M mutant mice, a genetically authentic model of T124M-CMT2J neuropathy. Similar to patients, these mice develop axonopathy between 2 and 12 months of age, characterized by impaired motor performance, normal nerve conduction velocities but reduced compound motor action potential amplitudes, and axonal damage with only minor compact myelin modifications. Mechanistically, we detected metabolic changes that could lead to axonal degeneration, and prominent alterations in non-compact myelin domains such as paranodes, Schmidt-Lanterman incisures, and gap junctions, implicated in Schwann cell-axon communication and axonal metabolic support. Finally, we document perturbed mitochondrial size and distribution along MpzT124M axons suggesting altered axonal transport. Our data suggest that Schwann cells in P0T124M mutant mice cannot provide axons with sufficient trophic support, leading to reduced ATP biosynthesis and axonopathy. In conclusion, the MpzT124M mouse model faithfully reproduces the human neuropathy and represents a unique tool for identifying the molecular basis for glial support of axons.

Phenotypic spectrum of myelin protein zero-related neuropathies: a large cohort study from five mutation clusters across Italy

“…In 2015, Brügger and colleagues 34 showed that P0 expression was fundamental for the maintenance of paranodal/nodal integrity and axonal function through interaction with the neurofascins, and speculated that this mechanism might be impaired by some MPZ mutations related to late-onset axonal neuropathy. These findings were corroborated by Shackleford and coauthors, 35 who generated a p.Thr124Met knock-in mouse model that showed prominent alterations in non-compact myelin domains such as paranodes, Schmidt-Lanterman incisures and gap junctions, implicated in Schwann cell-axon communication and axonal metabolic support. Nevertheless, this hypothesis remains to be confirmed and verified in its specificity for MPZ mutations.…”

Myelin protein zero mutation‐related hereditary neuropathies: Neuropathological insight from a new nerve biopsy cohort

“…At this point, this remains an observation of unknown causal relation to the MPZ neuropathy; mitochondrial abnormalities are to our knowledge not an established feature of MPZ neuropathy thus far. Nevertheless, mitochondrial alterations were clearly more frequent than generally observed in muscle biopsies and were in line with altered size, function, and distribution of axonal mitochondria in a mouse model of MPZ neuropathy [72]. Furthermore, denervation causes extensive muscle fiber changes including myofibrillar disintegration (target phenomena) and remodeling of the sarcoplasmic reticulum [49], which should also affect muscle fiber mitochondria.…”

“…While CMT1‐associated mutations strongly perturbed intercellular adhesions or were retained in the cytoplasm, CMT2‐associated mutations only moderately reduced intercellular adhesions in cultured cells [ 90 ] or mainly affect axo‐glial interactions [ 60 , 72 , 90 , 91 ], including axo‐glial interactions to stabilize nodes and paranodes [ 72 , 92 ]. In line with a loss of function mutation, CMT2 patient #17 with onset in the 3rd decade of life had a novel c.73_74 delinsA (p.S25TfsX22) variant.…”

Structural context of homomeric interactions in the lg domain of the MPZ (P0) myelin adhesion protein and relation to Charcot-Marie-Tooth disease phenotype variants

“…Moreover, the ‘CMT2-surface’ surrounds the glycosylation site suggesting this region likely interacts with the glycan in a particular way, perhaps to properly position the lg MPZ domain with respect to the membrane. lndeed, mutations in the consensus N-linked glycosylation site lead (N 122 XT) cause late-onset CMT2 phenotypes in humans and mice (Shackleford et al, 2022; Shy et al, 2004) with morphological changes in the areas of non-compacted myelin. Glycosylation has also been found to alter the adhesion activity of MPZ in heterologous systems, suggesting it helps optimally position MPZ for interaction with other components (Filbin & Tennekoon, 1991, 1993).…”