(HSP40) Enhances Axon Regeneration in the Mouse Optic Nerve.

A forward genetics approach was used to identify genomic elements enhancing axon regeneration in the BXD recombinant mouse strains. Axon regeneration was induced by knocking down in retinal ganglion cells (RGCs) using adeno-associated virus (AAV) to deliver an shRNA followed by an intravitreal injection of Zymosan with CPT-cAMP that produced a mild inflammatory response. RGC axons were damaged by optic nerve crush (ONC). Following a 12-day survival period, regenerating axons were labeled by intravitreal injection of Cholera Toxin B (CTB) conjugated with Alexa Fluor 647. Two days later, labeled axons within the optic nerve were examined to determine the number of regenerating axons and the distance they traveled down the optic nerve. The analysis revealed a surprising difference in the amount of axonal regeneration across all 33 BXD strains. There was a 7.5-fold difference in the number of regenerating axons and a 4-fold difference in distance traveled by regenerating axons. These data were used to generate an integral map defining genomic loci modulating the enhanced axonal regeneration. A quantitative trait locus modulating axon regeneration was identified on Chromosome 14 (115 to 119 Mb). Within this locus were 16 annotated genes. Subsequent testing revealed that one candidate gene, , modulates axonal regeneration. encodes Heat Shock Protein 40 (HSP40), which is a molecular chaperone. Knocking down in the high regenerative strain (BXD90) led to a decreased regeneration response, while overexpression of in a low regenerative strain (BXD34) resulted in an increased regeneration response. These findings suggest that not only increases the number of regenerating axons, it also increases the distance those axons travel. This may prove to be critical for functional recovery in large mammals, where the distance axons travel to their target is considerably longer than that of the mouse. Thus, may play a critical role for functional recovery in humans by increasing the number of regenerating axons and the distance the regenerating axons travel.