Neuronal function of the endocytic adaptor CALM in the sorting of SNAREs and AMPARs

点击翻译按钮获取中文摘要

Brain function critically relies on the proper sorting of crucial membrane proteins. Synaptic vesicle (SV) proteins, for example, have to be retrieved from the presynaptic membrane after neurotransmitter release to locally replenish the SV pool and thereby sustain neurotransmission. Glutamate receptors need to be dynamically inserted and removed from the postsynaptic membrane to allow for synaptic plasticity and, thus, learning and memory. Membrane-anchored enzymes such as g-secretase and their substrates, e.g. the amyloid precursor protein (APP), must follow defined trafficking itineraries to avoid erroneous processing. Consequently, sorting defects give rise to severe neurological diseases ranging from epilepsy to Alzheimer´s disease (AD). The endocytic sorting of transmembrane proteins is often regulated by endocytic adaptor proteins, which link them to the endocytic machinery. The closely related proteins AP180 and CALM are two such endocytic adaptors with vital functions. During the first funding period we showed that AP180 is crucial for the high-fidelity retrieval of the SV SNARE protein Synaptobrevin/VAMP2, an essential component of the neurotransmission machinery. Deletion of AP180 depletes VAMP2 primarily from inhibitory SVs due to their high turnover causing excitatory/inhibitory imbalance, epileptic seizures and premature death. While CALM fulfills overlapping roles in VAMP2 sorting, our preliminary experiments show that its physiological role extends well beyond SV recycling in line with its association with other VAMPs (e.g. VAMP7/8), its localization at pre- and postsynaptic sites, and its strong genetic association with AD in humans. Our preliminary analysis of neuron-specific CALM Knockout (KO) mice has revealed prominent sorting defects: (i) alterations in the vesicular SNARE proteins VAMP7/8, (ii) mislocalization of APP, possibly as a result of impaired VAMP trafficking, and (iii) a surface accumulation of postsynaptic AMPA-type glutamate receptors. Within the proposed research we therefore aim to (i) dissect the physiological role of CALM in VAMP7/8 trafficking and to unravel the organismic consequences of VAMP7/8 missorting in vivo; to (ii) analyze the mechanism underlying the observed alteration in APP localization and processing in CALM KO mice and to understand how neuronal loss of CALM affects AD pathology; and finally to (iii) elucidate how CALM mechanistically facilitates the endocytosis of AMPARs and to delineate how the loss of CALM impacts postsynaptic plasticity. With these lines of research and a combination of live cell imaging, super-resolution microscopy, biochemistry and mouse genetics we will address important questions in molecular neuroscience and substantiate the emerging paradigm that trafficking defects may underlie neurological diseases such as AD. We therefore expect our research in the long run to contribute to the identification of new therapeutic avenues to treat brain diseases.