Diaphragm hypertrophy and spinal stress response accompany phrenic myelin defects in a murine model of Charcot-Marie-Tooth Disease Type 1E.

Charcot-Marie-Tooth disease type 1E (CMT1E) is a rare, early-onset dysmyelinating neuropathy linked with point mutations in the peripheral myelin protein 22 (PMP22) gene. Respiratory problems are known to impact neuropathic patients, yet existing studies have not characterized this pathophysiology in corresponding animal models. Trembler J (TrJ) mice carry the same Leu16Pro amino acid substitution in the PMP22 protein that is present in families diagnosed with CMT1E. Utilizing electrophysiological, biochemical and morphological approaches, we examined critical sites of the lower respiratory system, including the diaphragm, phrenic nerve and cervical (C3-C6) region of the spinal cord in adult age-matched wild type (Wt) and heterozygous TrJ mice. In response to high-frequency stimulation of the presynaptic phrenic nerve, diaphragm muscle fibers of neuropathic mice exhibited similar rundown in the release of acetylcholine, but failed to maintain action potentials, suggesting a postsynaptic, muscle-derived deficit in neurotransmission. Although muscle fiber subtype numbers were unaffected in neuropathic mice, cross-sectional areas were enlarged in all fiber subtypes. Additionally, the ubiquitin-proteasome and autophagy pathways were upregulated in muscle, suggesting compensatory remodeling that occurred concurrently with, or because of, impaired neurotransmission. Analyses of phrenic nerve sections revealed highly significant (p < 0.0001) myelin defects, axonal atrophy, and astrogliosis, accompanied by stress granule formation within the grey matter of the cervical spinal cord. These findings identify profound structural and functional deficits of the respiratory system in TrJ mice modeling CMT1E and establish phrenic neuropathy as a mechanism underlying neuronal stress responses in the cervical spinal cord as well as postsynaptic diaphragm dysfunction.