The role of NECAB2 in brain physiology and in Huntington s disease

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NECAB2 is a Neuronal Ca2+-binding protein characterized by the combination of Ca2+-binding domains and a monooxygenase domain which interacts with metabotropic glutamate and adenosine receptors thereby increasing their constitutive signaling. We investigated a potential role of NECAB2 in Huntington's disease (HD), a hereditary neurodegenerative disease, based on the following reasoning: First, NECAB2 is predominantly expressed in the striatum, the predilection site for neuronal degeneration in this disease. Second, NECAB2 interacts with membrane receptors that are over-active in HD. Third, the function of NECAB2 is altered by Ca2+ and disturbed Ca2+ signaling plays a major role in HD pathophysiology. Fourth, prokaryotic homologs of NECAB2 possess monooxygenase activity and are involved in the detoxification of reactive oxygen species, suggesting a similar function in eukaryotes distinct from the interaction with membrane receptors. Oxidative stress plays an important role in the pathophysiology of HD. In preliminary studies, we found that NECAB2 is downregulated in mouse models of HD, preceding the degeneration of striatal neurons. We also showed that mammalian NECAB2 dimerizes in a Ca2+-dependent manner and protects against oxidative stress similar to its prokaryotic orthologs. We know that NECAB2 resides at the plasma membrane where it interacts with metabotropic glutamate and adenosine receptors and increases their basal constitutive signaling. This interaction is inhibited by increased Ca2+ concentrations that occur upon synaptic stimulation when Ca2+ enters the cell through ionotropic NMDA channels and is released from intracellular stores. We think that this increase in the cytosolic Ca2+ concentration causes homodimerization of NECAB2, resulting in subsequent receptor desensitization in a negative feedback loop. The dimerization also activates its monooxygenase activity, which probably serves to process reactive oxygen species generated in active, stimulated neurons. As it is a well-established fact that metabotropic glutamate receptor signaling is over-active in Huntington's disease, I hypothesize that NECAB2 is downregulated as an adaptive process to stall or slow-down receptor signaling, which probably has the side effect that NECAB2 is less able to exert its enzymatic activity. This then accelerates the degeneration of medium spiny neurons in Huntington's disease. To test these hypotheses, I propose studies with the following aims: 1) To clarify the role of NECAB2 on neuronal viability, receptor signaling, synaptic transmission and behavior by studying NECAB2-deficient mice; and 2) to clarify the role of NECAB2 in Huntington's disease by studying expression in human HD and the phenotype of double transgenic NECAB2 -/- x HD mice. This work is of high relevance as it aims to elucidate fundamental aspects of neurobiology and the pathophysiology of a human disease.