Characterization of the progressive neuroaxonal dystrophy and subsequent gait abnormalities in the Hspa8 V95E knock-in rats

Neuroaxonal dystrophy is a group of genetic neurodegenerative diseases that are histopathologically characterized by swollen axons (spheroids). We previously established a novel rat model of neuroaxonal dystrophy harboring a V95E missense mutation in the heat shock protein family A (HSP70) member 8 ( Hspa8 ) gene that originally emerged by ENU-induced random mutagenesis. Hspa8 V95E mutant rats show marked spheroid formation in the central nervous system and develop progressive hind limb ataxia. However, the detailed pathology of Hspa8 V95E mutant rats remains to be elucidated. In the current study, we examined the characteristics of gait abnormalities and the detailed distribution of spheroids in the central nervous system of Hspa8 V95E knock-in (KI) rats. Using footprint tests, we demonstrated the increased ratio of step width to step length (outward step) of the hind limbs in Hspa8 V95E KI rats from 3 weeks of age. In Hspa8 V95E KI rats aged 15 weeks, spheroids were predominantly distributed in the proprioceptive sensory pathway from the lower body, including the gracile fasciculus and the nucleus. The number of spheroids increased with age and was accompanied by glial reactions in the dorsal funiculus. Moreover, we observed the accumulation of kinesin light chain 1; kinesin heavy chain isoforms 5A, -5B, and -5C; and microtubule-associated protein light chain 3 B in the spheroids and degenerated axons of Hspa8 V95E KI rats at 13 weeks of age. Collectively, our data suggest the involvement of impaired axonal transport and autophagy in spheroid formation in Hspa8 V95E mutant rats.