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

NECAB2是一种神经元Ca2+结合蛋白，其特征是Ca2+结合结构域和单加氧酶结构域的结合，其与代谢性谷氨酸和腺苷受体相互作用，从而增加其组成信号。我们研究了NECAB2在亨廷顿病（一种遗传性神经退行性疾病）中的潜在作用，基于以下原因：首先，NECAB2主要在纹状体中表达，而纹状体是亨廷顿病中神经元变性的易发部位。其次，NECAB2与HD中过度活跃的膜受体相互作用。第三，NECAB2的功能被Ca2+改变，Ca2+信号的干扰在HD病理生理中起重要作用。第四，NECAB2的原核同源物具有单加氧酶活性，并参与活性氧的解毒，表明其在真核生物中具有与膜受体不同的类似功能。氧化应激在HD的病理生理中起重要作用。在初步研究中，我们发现在HD小鼠模型中，在纹状体神经元变性之前，NECAB2下调。我们还发现哺乳动物的NECAB2以Ca2+依赖的方式二聚化，并与其原核同源物类似，保护其免受氧化应激。我们知道NECAB2存在于质膜，在那里它与代谢性谷氨酸和腺苷受体相互作用，并增加它们的基础组成信号。当Ca2+通过嗜离子性NMDA通道进入细胞并从细胞内储存释放时，这种相互作用被增加的Ca2+浓度所抑制。我们认为这种细胞质Ca2+浓度的增加导致NECAB2的同二聚化，导致随后的负反馈回路中的受体脱敏。二聚化还激活其单加氧酶活性，这可能用于处理活跃的、受刺激的神经元中产生的活性氧。由于代谢性谷氨酸受体信号在亨廷顿病中过度活跃是一个公认的事实，我假设NECAB2作为一个适应性过程被下调，以阻止或减缓受体信号传导，这可能具有NECAB2无法发挥其酶活性的副作用。这就加速了亨廷顿氏病中棘神经元的退化。为了验证这些假设，我提出以下研究目的：1)通过研究NECAB2缺陷小鼠，阐明NECAB2在神经元活力、受体信号、突触传递和行为中的作用；2)通过研究NECAB2在人类HD中的表达和双转基因NECAB2 -/- x HD小鼠的表型，阐明NECAB2在亨廷顿病中的作用。这项工作是高度相关的，因为它旨在阐明神经生物学和人类疾病的病理生理学的基本方面。

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.