TDP-43 Toxicity Proceeds via Calcium Dysregulation and Necrosis in Aging Caenorhabditis elegans Motor Neurons

Amyotrophic lateral sclerosis (ALS) is a heterogeneous disease with either sporadic or genetic origins characterized by the progressive degeneration of motor neurons. At the cellular level, ALS neurons show protein misfolding and aggregation phenotypes. Transactive response DNA-binding protein 43 (TDP-43) has recently been shown to be associated with ALS, but the early pathophysiological deficits causing impairment in motor function are unknown. Here we used Caenorhabditis elegans expressing mutant TDP-43^A315T in motor neurons and explored the potential influences of calcium (Ca²⁺). Using chemical and genetic approaches to manipulate the release of endoplasmic reticulum (ER) Ca²⁺stores, we observed that the reduction of intracellular Ca²⁺ ([Ca²⁺]_i) rescued age-dependent paralysis and prevented the neurodegeneration of GABAergic motor neurons. Our data implicate elevated [Ca²⁺]_i as a driver of TDP-43-mediated neuronal toxicity. Furthermore, we discovered that neuronal degeneration is independent of the executioner caspase CED-3, but instead requires the activity of the Ca²⁺-regulated calpain protease TRA-3, and the aspartyl protease ASP-4. Finally, chemically blocking protease activity protected against mutant TDP-43^A315T-associated neuronal toxicity. This work both underscores the potential of the C. elegans system to identify key targets for therapeutic intervention and suggests that a focused effort to regulate ER Ca²⁺ release and necrosis-like degeneration consequent to neuronal injury may be of clinical importance.