Impaired motor unit recovery and maintenance in a knock-in mouse model of ALS-associated Kif5a variant

Kinesin family member 5A (KIF5A) is an essential, neuron-specific microtubule-associated motor protein responsible for the anterograde axonal transport of various cellular cargos. Loss of function variants in the N-terminal, microtubule-binding domain are associated with hereditary spastic paraplegia and hereditary motor neuropathy. These variants result in a loss of the ability of the mutant protein to process along microtubules. Contrastingly, gain of function splice-site variants in the C-terminal, cargo-binding domain of KIF5A are associated with amyotrophic lateral sclerosis (ALS), a neurodegenerative disease involving death of upper and lower motor neurons, ultimately leading to degradation of the motor unit (MU; an alpha motor neuron and all the myofibers it innervates) and death. These ALS-associated variants result in loss of autoinhibition, increased procession of the mutant protein along microtubules, and altered cargo binding. To study the molecular and cellular consequences of ALS-associated variants in vivo, we introduced the murine homolog of an ALS-associated KIF5A variant into C57BL/6 mice using CRISPR-Cas9 gene editing which produced mutant Kif5a mRNA and protein in neuronal tissues of heterozygous (Kif5a^{+/c.3005+1G>A}; HET) and homozygous (Kif5a^{c.3005+1G>A/c.3005+1G>A}; HOM) mice. HET and HOM mice appeared normal in behavioral and electrophysiological (compound muscle action potential [CMAP] and MU number estimation [MUNE]) outcome measures at one year of age. When subjected to sciatic nerve injury, HET and HOM mice have delayed and incomplete recovery of the MUNE compared to wildtype (WT) mice suggesting an impairment in MU repair. Moreover, aged mutant Kif5a mice (aged two years) had reduced MUNE independent of injury, and exacerbation of the delayed and incomplete recovery after injury compared to aged WT mice. These data suggest that ALS-associated variants may result in an impairment of the MU to respond to biological challenges such as injury and aging, leading to a failure of MU repair and maintenance. In this report, we present the behavioral, electrophysiological and pathological characterization of mice harboring an ALS-associated Kif5a variant to understand the functional consequences of KIF5A C-terminal variants in vivo.