Mitochondrial and Axonal Abnormalities Precede Disruption of the Neurofilament Network in a Model of Charcot-Marie-Tooth Disease Type 2E and Are Prevented by Heat Shock Proteins in a Mutant-Specific Fashion

Mutations in NEFL encoding the light neurofilament subunit (NFL) cause Charcot-Marie-Tooth disease type 2E (CMT2E), which affects both motor and sensory neurons. We expressed the disease-Causing mutants NFLP8R and NFLQ333P in motor neurons of dissociated spinal cord-dorsal root ganglia and demonstrated that they are incorporated into the preexisting neurofilament network but eventually disrupt neurofilaments without causing significant motor neuron death. Importantly, rounding of mitochondria and reduction in axonal diameter occurred before disruption of the neurofilament network, indicating that mitochondrial dysfunction contributes to the pathogenesis of CMT2E, as well as to CMT caused by mitofusin mutations. Heat shock proteins (HSPs) are involved in the formation of the neurofilament network and in protecting cells from misfolded mutant proteins. Cotransfection of HSPB1 with mutated NEFL maintained the neurofilament network, axonal diameter, and mitochondrial length in motor neurons expressing NFLP8R, but not NFLQ333P. Conversely, HSPA1 cotransfection was effective in motor neurons expressing NFLQ333P, but not NFLP8R. Thus, there are NFL mutant-specific differences in the ability of individual HSPs to prevent neurofilament abnormalities, reduction in axonal caliber, and disruption of mitochondrial morphology in motor neurons. These results suggest that FISP inducers have therapeutic potential for CMT2E but that their efficacy would depend on the profile of HSPs induced and the type of NEFL mutation.