RNAi studies in caenorhabditis elegans reveal that coenzyme Q protects GABA neurons from apoptotic, calcium-dependent degeneration

Abstract

Dissertation under the direction of Professor David M. Miller III Impairment of neurons expressing the neurotransmitter ?-amminobutyric acid (GABA) can result in psychiatric diseases as diverse as schizophrenia, epilepsy, Tourette’s syndrome, and autism. Degeneration of specific GABA neuron populations in the adult brain results in the symptoms of Huntington’s disease and Spinocerebellar ataxias. In order to better understand these neurons in development and aging, we performed RNAi studies in the nematode C. elegans to identify genes that are important for GABA neurons throughout the life cycle. We identified genes that affect movement and GABA neuron morphology. These RNAi targets included genes with no previously known neuronal function. Future studies of these genes should provide clues to the genetic specification of GABA neuron differentiation and function. During the course of these studies, we found that knockdown of the coq-1 enzyme resulted in the age-dependent degeneration of GABA neurons. coq-1 is the initial enzyme in the Coenzyme Q (CoQ) biosynthetic pathway. CoQ is a required component of the mitochondrial electron transport chain and essential for normal energy metabolism. CoQ deficiency in humans causes cerebellar ataxia, and myopathy, indicating that selected tissues are especially sensitive to reduced levels of CoQ. We found that RNAi or genetic ablation of coq-1 expression in C. elegans resulted in a progressive uncoordinated, or Unc, phenotype and degeneration of GABA neurons. Both the degenerative and Unc phenotypes emerge during late larval development and progress in adults. Neuron classes in motor and sensory circuits that utilize other neurotransmitters (dopamine, acetylcholine, glutamate, serotonin) and body muscle cells were unaffected morphologically by RNAi depletion of coq-1. The mechanism of GABA neuron cell death depends on release of intracellular calcium stores, and requires the apoptotic genes ced-3 (caspase) and ced-4 (Apaf-1). Additionally, degeneration requires drp-1, implicating mitochondrial fission machinery in the cell death pathway. We conclude that the neuron specificity and developmental progression of the coq-1 knockdown phenotype in C. elegans resembles that of CoQ deficiency in humans, and therefore may provide a useful model system for studies of this and related neurodegenerative diseases.