Muscle spindle alterations precede onset of sensorimotor deficits in Charcot-Marie-Tooth type 2E

Charcot-Marie-Tooth (CMT) is the most common inherited peripheral neuropathy, affecting approximately 2.8 million people. The CMT leads to distal neuropathy that is characterized by reduced motor nerve conduction velocity, ataxia, muscle atrophy and sensory loss. We generated a mouse model of CMT type 2E (CMT2E) expressing human neurofilament light E396K (hNF-L^E396K ), which develops decreased motor nerve conduction velocity, ataxia and muscle atrophy by 4 months of age. Symptomatic hNF-L^E396K mice developed phenotypes that were consistent with proprioceptive sensory defects as well as reduced sensitivity to mechanical stimulation, while thermal sensitivity and auditory brainstem responses were unaltered. Progression from presymptomatic to symptomatic included a 50% loss of large diameter sensory axons within the fifth lumbar dorsal root of hNF-L^E396K mice. Owing to proprioceptive deficits and loss of large diameter sensory axons, we analyzed muscle spindle morphology in presymptomatic and symptomatic hNF-L^E396K and hNF-L control mice. Muscle spindle cross-sectional area and volume were reduced in all hNF-L^E396K mice analyzed, suggesting that alterations in muscle spindle morphology occurred prior to the onset of typical CMT pathology. These data suggested that CMT2E pathology initiated in the muscle spindles altering the proprioceptive sensory system. Early sensory pathology in CMT2E could provide a unifying hypothesis for the convergence of pathology observed in CMT.

X-linked spinal muscular atrophy in mice caused by autonomous loss of ATP7A in the motor neuron

ATP7A is a copper-transporting P-type ATPase that is essential for cellular copper homeostasis. Loss-of-function mutations in the ATP7A gene result in Menkes disease, a fatal neurodegenerative disorder resulting in seizures, hypotonia and failure to thrive, due to systemic copper deficiency. Most recently, rare missense mutations in ATP7A that do not impact systemic copper homeostasis have been shown to cause X-linked spinal muscular atrophy type 3 (SMAX3), a distal hereditary motor neuropathy. An understanding of the mechanistic and pathophysiological basis of SMAX3 is currently lacking, in part because the disease-causing mutations have been shown to confer both loss- and gain-of-function properties to ATP7A, and because there is currently no animal model of the disease. In this study, the Atp7a gene was specifically deleted in the motor neurons of mice, resulting in a degenerative phenotype consistent with the clinical features in affected patients with SMAX3, including the progressive deterioration of gait, age-dependent muscle atrophy, denervation of neuromuscular junctions and a loss of motor neuron cell bodies. Taken together, these data reveal autonomous requirements for ATP7A that reveal essential roles for copper in the maintenance and function of the motor neuron, and suggest that SMAX3 is caused by a loss of ATP7A function that specifically impacts the spinal motor neuron.

Expressing hNF-LE397K results in abnormal gaiting in a transgenic model of CMT2E

Charcot-Marie-Tooth disease (CMT) is the most commonly inherited peripheral neuropathy. CMT disease signs include distal limb neuropathy, abnormal gaiting, exacerbation of neuropathy, sensory defects and deafness. We generated a novel line of CMT2E mice expressing an hNF-L(E397K) transgene, which displayed muscle atrophy of the lower limbs without denervation, proximal reduction in large caliber axons and decreased nerve conduction velocity. In this study, we showed that hNF-L(E397K) mice developed abnormal gait of the hind limbs. The identification of severe gaiting defects in combination with previously observed muscle atrophy, reduced axon caliber and decreased nerve conduction velocity suggests that hNF-L(E397K) mice recapitulate many of clinical signs associated with CMT2E. Therefore, hNF-L(E397K) mice provide a context for potential therapeutic intervention.

Neuroleptic-induced vacuous chewing movements as an animal model of tardive dyskinesia: a study in three rat strains

Vacuous chewing movements (VCMs) in three different rat strains developed at considerably different rates after 19 weeks of continual haloperidol treatment at an average daily dose of 1.5 mg/kg. Sprague Dawley (SD) rats displayed relatively high rates of VCMs with low variability, compared to Wistar (W) and Long Evan (LE) rats. Atropine decreased but did not abolish VCMs in two of the three strains (LE greater than SD). After haloperidol withdrawal, VCMs remitted gradually in all strains, but least rapidly in the SD rats. In a separate group of SD rats. VCMs were rated weekly from the start of haloperidol treatment and showed considerable interindividual variability. Even after 24 weeks of continuous haloperidol, 12 out of 32 treated rats showed no VCMs at all, while 13 out of 32 had intense movements, analogous to the clinical situation in which only some patients treated with neuroleptics develop tardive dyskinesia. These results indicate that there are individual and strain differences in the development of VCMs, and suggest that there may also be genetically determined differences in the development of tardive dyskinesia.