General Abstract
SynPrime focuses on neuronal signalling in the brain, which occurs at synapses where sending cells release neurotransmitters from synaptic vesicles (SVs) that fuse with the plasma membrane. The speed of SV fusion and the ability of synapses to sustain it at high rates are key requirements for brain function, and dynamic changes of the process are thought to control complex brain processes. Accordingly, many brain disorders involve dysfunctional synapses. We found that patients with mutations in Munc13-1, a 'master-controller' of SVs, suffer from dyskinetic, cognitive, and behavioral defects, and showed that these mutations perturb synapse fine-tuning. We revealed the mechanisms by which Munc13 proteins render SVs 'fusion-ready', so that stimulated synapses respond by immediate SV fusion and recover quickly after phases of strong stimulation. Work on Munc13-related CAPS proteins showed that they control sensory adaptation in the visual system of the brain in vivo. Using newly generated mutant mice, we studied Complexins, which control the efficacy of SV fusion and are linked to multiple brain disorders. We found, in contrast to dogma, that they are positive regulators of SV fusion and not fusion clamps. Using novel methods to characterize synapses in defined functional states, we showed that depressed synapses have less fusion-ready SVs and discovered that Munc13s are tightly controlled by calcium and membrane lipids in vivo, so that their activity is adjusted to synaptic demand. Our work revealed key insights into the physiology and pathophysiology of neurotransmitter signaling.
Background
The speed of SV fusion and the ability of synapses to sustain it at high rates are key requirements for brain function. Plastic changes in these processes have long been thought to control complex brain processes, from sensory adaptation to working memory, but the link between presynaptic plasticity and complex brain functions has long remained hypothetical. A key determinant of presynaptic efficacy is that synapses maintain a 'fusion-ready', primed SV pool that can be replenished rapidly. SV priming is mediated by dedicated proteins (Munc13s, CAPSs, accessory proteins such as Complexins), which are essential for synaptic efficacy and of capacious potential to regulate synaptic plasticity associated with circuit characteristics that control behavior. However, this 'catholic' role of the SV priming machinery has long remained unproven in intact circuits.
Overall Objectives
Our aim was to examine the SV priming machinery in intact circuits and in vivo - specifically to identify the mechanisms of SV priming, of its dynamics, and of defined priming-dependent synaptic plasticity states, and to define the causal links between SV priming, synapse function, synaptic plasticity, and circuit characteristics that determine behavior.
Relevance
Our project made a substantial contribution to a comprehensive description of the role of SV priming in intact circuits. This is important for basic and translational science alike, because all key SV priming proteins are linked to neuropsychiatric diseases.
