Architecture and function of striatal dopamine release machinery

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Summary Dopamine is an important neuromodulator and pathologies in dopamine signaling are a hallmark of brain diseases such as neurodegeneration, substance abuse, and schizophrenia. Despite these important roles for dopamine, remarkably little is known about the molecular mechanisms of its release. Because dopamine acts as a volume transmitter, it is not clear whether dopamine release involves molecular machinery that warrants spatial and temporal precision for release. Alternatively, dopamine release could be spread over the surface of an axon, which is consistent with volume transmission. The release of classical transmitters relies on an active zone, a highly organized protein structure that contains scaffolding proteins such as RIM and ELKS and determines the precise localization, speed and accuracy of synaptic vesicle exocytosis. The active zone also provides mechanisms for regulation of release during plasticity. Our preliminary experiments reveal that the presynaptic scaffolding protein RIM is absolutely required for dopamine release in the mouse striatum, but that ELKS is dispensable for dopamine release. This is different from classical fast synapses, where knockout of either protein family leads to a reduction of 50-80% of release. We thus hypothesize that dopamine release necessitates mechanistically specialized release sites. This hypothesis is bolstered by superresolution microscopy in striatal brain slices, which shows that several release site scaffolding proteins are clustered inside dopamine axons. We pursue a two-pronged approach to address this central hypothesis. In aim one, we use rigorous conditional mouse genetics and electrophysiology in acute brain slices of the mouse striatum to systematically address the necessity of scaffolding proteins, priming proteins and Ca2+ channel tethers in dopamine release and in co-release of GABA and glutamate from dopamine neurons. This is the first study on the requirements of molecular scaffolds for dopamine secretion and it will lead to a comprehensive assessment of the dopamine release machinery. In aim two, we assess whether scaffolding proteins mediate dopamine secretion as soluble release factors, or whether they are assembled in clustered release sites to target dopamine release to specific membrane domains. The latter possibility is strongly supported by our preliminary data. We will combine superresolution microscopy, subcellular fractionation, electron microscopy and mouse genetics to study the existence and composition of dopamine release sites in the mouse striatum. We will assess how dopamine release sites are associated with vesicle clusters, with receptors for dopamine and for the co-transmitters GABA and glutamate, and with cholinergic innervation, which powerfully triggers dopamine release. These experiments will establish the existence, appearance and composition of dopamine release sites and their structural arrangement into striatal synaptic microcircuits. Our approach is the first comprehensive approach to dissect the secretory pathway for dopamine. We expect to identify new mechanisms that support dopamine release and to uncover general principles for neuromodulation.