Smith, HL; (2015) HSJ1 and Motor Neuropathy. Doctoral thesis , UCL (University College London).

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Abstract

Homo sapiens DnaJ 1 (HSJ1) is a neuronal DnaJ protein that functions to target misfolded and aggregated proteins for degradation. The accumulation of misfolded and aggregated protein can be toxic to cells, causing neurodegeneration; therefore, proteins that function in protein quality control, such as HSJ1, play a critical role in maintaining protein homeostasis and promoting cell survival. The protective nature of HSJ1 is highlighted by the ability of the HSJ1a isoform to combat protein aggregation in cell and mouse models of neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS), a disease caused by the progressive loss of motor neurons. Based on the anti-aggregation function of HSJ1a towards ALS-associated proteins, I sought to determine whether HSJ1a could also protect against mutant ubiquilin-2 aggregation, a protein recently implicated in X-linked ALS. I identified wild-type ubiquilin-2 as a binding partner of HSJ1a and demonstrated that HSJ1a retained the ability to bind to ubiquilin-2 mutants, suggesting HSJ1a could potentially modulate mutant ubiquilin-2 aggregation in cells. Recently, three mutations in HSJ1 have been identified in autosomal recessive cases of hereditary motor neuropathies, diseases caused by the degeneration of lower motor neurons. I sought to determine the pathogenicity of the most recently identified variant, HSJ1-Y5C. Using a combination of the heterologous expression of HSJ1-Y5C in SK-N-SH neuroblastoma cells and the examination of endogenous HSJ1-Y5C in patient fibroblasts, I characterised the Y5C substitution as a disease-causing, loss of function mutation that reduces the half-life of HSJ1, leading to a loss of protein. To investigate how the loss of HSJ1 function causes motor neuron degeneration, I examined the cellular phenotype of motor neurons in Hsj1 knockout mice. I demonstrated that a significant 12% of motor neurons degenerate between P15 and P20. Prior to motor neuron loss, motor neurons appear chromatolytic and signs of impaired protein homeostasis are observed, with an increase in protein ubiquitylation, a possible increase in autophagosome formation and activation of the IRE1 and PERK branches of the unfolded protein response. The activation of these cellular stress responses suggests that HSJ1 is a critical component of the protein quality control network in motor neurons.

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