Dysregulation of Huntingtin-Associated Protein-Mediated Calcium Trafficking in Neurodegenerative Diseases

In neurodegenerative diseases, such as Alzheimer’s (AD) and Huntington’s (HD), calcium (Ca2+) homeostasis is altered resulting in neurodegeneration. Changes of Ca2+-regulating proteins in the plasma membrane, endoplasmic reticulum (ER) and mitochondria are implicated in the neuronal dysfunction in these diseases. Mutations in Ca2+-regulating protein presenilin altered inositol triphosphate receptor (IP3R) function in gating Ca2+ flux on ER, and are the major cause of familial AD. Researchers identified huntingtin (Htt)-associated protein-1 (HAP1) as an IP3R-interacting partner and showed that an IP3R-Hap1-Htt complex mediate neuronal Ca2+ signaling. Mutations in Htt derange cytosolic Ca2+ signaling and lead to HD; however, the underlying mechanism remains elusive. Hap1 is known to associate with microtubule-dependent motor proteins kinesin and dynein/dynactin complex in neurons and involves in intracellular trafficking of receptors and proteins. We hypothesize that the damage of HAP1-related transport machinery impairs the function of Ca2+-binding proteins, resulting in cell death in neurodegenerative patient’s brain. Our preliminary proteomic data showed that kinesin light chain-2 (KLC2), dynein-1 light intermediate chain-2 (Dyn1li2) and Ca2+-binding proteins such as parvalbumin (Pva) and breakpoint cluster region protein (BCR) reduced association with microtubules in HAP1-defecient mouse brain, implicating the requirement of HAP1 for the function of these proteins. Western blots confirmed the participation of HAP1 on BCR trafficking and activation in mouse brain. Furthermore, the distribution of Ca2+ in the brain was changed in Hap1-null mice. In the clinical samples, we found significant reductions of the trafficking proteins, KLC2 and Dyn1li2, in AD patient’s serum. Therefore in this proposed project, we plan on (1) characterizing the physical interactions between HAP1 and Ca2+-binding proteins; (2) elucidating the functions of HAP1 on intracellular Ca2+ trafficking; and (3) analyzing the HAP1-mediated mitochondrial Ca2+ handling. The Hap1-mediated transport machinery may be promising target for the early diagnosis and treatment of neurodegenerative diseases.