Molecular Mechanisms of Synapse Development and Plasticity

National Institute Of Mental Health

Investigators

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Abstract

1. The BAD-BAX-Caspase-3 cascade modulates synaptic vesicle pools via autophagy Synaptic vesicles undergo exocytosis to release neurotransmitters when the presynaptic membrane is depolarized by action potentials. After exocytosis, vesicles are retrieved by endocytosis and recycled to form new synaptic vesicles. The number of synaptic vesicles at a synapse is an important factor determining synaptic strength, which is dynamically regulated during development and by experience. A stable, yet flexible, pool of synaptic vesicles is critical to ensure the reliability and adaptability of neural circuits. Although synaptic vesicles have been intensively studied, how their number is maintained and regulated remains largely unclear. Emerging evidence suggests a role of macroautophagy (autophagy hereinafter) in the life cycle of synaptic vesicles. Rab26, a small GTPase enriched on synaptic vesicles, also localizes to autophagosomes, and its overexpression induces large vesicular clusters containing synaptic-vesicle and lysosomal proteins; activation of autophagy by inhibitors of mammalian target of rapamycin (mTOR) reduces the number of synaptic vesicles. Autophagy is a cellular process through which proteins and organelles are delivered to lysosomes for degradation. Autophagy removes defective proteins and organelles under physiological conditions and serves as a prosurvival mechanism by recycling nonessential cellular constituents under cellular stress, such as starvation and growth factor withdrawal. We previously showed that the BAD-BAX-caspase-3 cascade is a canonical apoptosis pathway and has a nonapoptotic function in long-term synaptic depression at the postsynaptic site. During this reporting period, we show that, at the presynaptic site of hippocampal neurons, the BAD-BAX-caspase-3 pathway inhibits autophagy, which in turn controls the size of synaptic vesicle pools. Moreover, the regulation of autophagy by caspase-3 influences activity-induced depletion, and recovery of synaptic vesicle pools and facilitates learning and memory. This study identifies a new mechanism for the control of synaptic vesicle pools, and a new, nonapoptotic function of the BAD-BAX-caspase-3 pathway in presynaptic terminals. Additionally, it indicates that autophagy is not only a homeostatic mechanism to maintain the integrity of cells and tissues, but also a process engaged by neural activity to regulate synaptic vesicle pools for optimal synaptic responses, learning, and memory. 2. Dysbindin-1 regulates mitochondrial fission and gamma oscillations Mitochondria are vital organelles in eukaryotic cells. Neuronal mitochondria are motile and undergo fission and fusion. Mitochondrial fission is mediated by the dynamin related GTPase Drp1. During mitochondrial fission, cytosolic Drp1 is recruited to mitochondria, binds to adaptors on outer mitochondrial membranes, and assembles into oligomers to constrict and sever mitochondrial membranes. The ratio of mitochondrial fission to fusion is increased by neural depolarization. Mitochondria generate 90% of cellular ATP in neurons. They are essential for synaptic transmission, an energy-demanding process accounting for 41% of ATP consumed in the rat cortex and even more in the primate cortex. A neurons energy expenditure is proportional to the frequency of synaptic transmission. Neural activities at higher frequencies are more mitochondria-dependent. Gamma oscillations are high-frequency (20100 Hz), synchronized activities of neural populations that require strong mitochondrial functions. They are present in many brain areas and underlie the precise timing of neuronal discharges and coherent binding of neural ensembles for information processing during cognitive functions. The mechanism for generating gamma oscillations is best characterized in the hippocampal CA3 region where both excitatory and inhibitory synaptic transmissions are required for gamma oscillations, and the oscillating local field potentials (LFPs) mainly originate from perisomatic inhibitory currents in pyramidal neurons. The power of gamma oscillations is dependent on the ability of excitatory neurons to rapidly repolarize postsynaptic membranes after each synaptic input. Pumping out ions flowing into excitatory postsynaptic neurons to repolarize membrane potentials accounts for 50% of ATP used for synaptic transmission. Despite the recognized importance of mitochondria in gamma oscillations, little is known about how mitochondria accommodate the rapid, local increase in energy demands imposed by gamma oscillations at the postsynaptic site of excitatory neurons. Dysbindin-1 is a coiled-coil domain-containing protein decreased in the brains of people with schizophrenia. It regulates the subcellular distribution of dopamine D2, D3 receptors, the NMDA receptor subunit GluN2A, necdin, and BDNF, as well as glutamate release, neural excitability, and synaptic plasticity. Dysbindin-1 is present on mitochondrial outer membranes. It is unknown, however, whether dysbindin-1 affects mitochondria. During this reporting period, we show that dysbindin-1 regulates mitochondrial fission by promoting Drp1 oligomerization on mitochondria. Neuronal activation in the gamma band induces the translocation of dysbindin-1 to mitochondria where it interacts with Drp1 and the Drp1 receptor Mid49 and Mid51 to increase Drp1 oligomerization and mitochondrial fission. This process is required for gamma oscillations. Dysbindin-1 null mice (sdy) have reduced mitochondrial fission, which leads to deficits in gamma oscillations and novel object recognition (NOR). Notably, these deficits can be alleviated by increasing mitochondrial fission with a light-inducible mitochondrial fission system. These findings reveal an unsuspected role of dysbindin-1 in mitochondrial dynamics and gamma oscillations This study sheds new light on the physiological function of mitochondrial fission and cellular mechanisms of gamma oscillations. 3. The Dorsal Raphe regulates the duration of attack through the medial orbitofrontal cortex and medial amygdala The dorsal raphe (DR) nucleus is one of the raphe nuclei located on the midline of the brainstem. It is a phylogenetically conserved structure and plays a role in various types of aggressive behaviors, such as maternal and territorial aggression in rodents. The role of the DR in aggression is complex and context dependent. For example, infusion of glutamate in the DR increases the frequency of attack bites against a conspecific, but with no effect on threatening behavior. Conversely, infusion of glutamate receptor agonists increases bite latency and decreases bite frequency in maternal aggression, with no effect on chasing behavior. Knockdown of tyrosine receptor kinase receptors in DR neurons decreases latency to attack. Prepartum lesion of the DR decreases the frequency of attack in maternal aggression, while postpartum lesion of the DR decreases the duration of an individual attack bout. The DR contains a heterogenous population of neurons that release one or a combination of the neurotransmitters including serotonin (5-hydroxytryptamine or 5-HT), dopamine, glutamate and GABA. The prefrontal cortex and amygdala are densely innervated by the DR. During this review period, using an in vivo optogenetic approach, we show that CaMKIIa neurons in the DR are activated by attack and that these neurons modulate the duration of attack behavior toward an intruder through two projection areas. Specifically, the DR-medial OFC (MeOC) pathway prolongs an already occurring attack, while the DR-MeA pathway shortens it. These findings reveal two DR-mediated neurocircuits that have divergent functions in aggressive behavior.