Endogenous Tagging Reveals Differential Regulation of Ca(2+) Channels at Single Active Zones during Presynaptic Homeostatic Potentiation and Depression

Neurons communicate through Ca(2+)-dependent neurotransmitter release at presynaptic active zones (AZs). Neurotransmitter release properties play a key role in defining information flow in circuits and are tuned during multiple forms of plasticity. Despite their central role in determining neurotransmitter release properties, little is known about how Ca(2+) channel levels are modulated to calibrate synaptic function. We used CRISPR to tag the Drosophila Ca(V)2 Ca(2+) channel Cacophony (Cac) and, in males in which all Cac channels are tagged, investigated the regulation of endogenous Ca(2+) channels during homeostatic plasticity. We found that heterogeneously distributed Cac is highly predictive of neurotransmitter release probability at individual AZs and differentially regulated during opposing forms of presynaptic homeostatic plasticity. Specifically, AZ Cac levels are increased during chronic and acute presynaptic homeostatic potentiation (PHP), and live imaging during acute expression of PHP reveals proportional Ca(2+) channel accumulation across heterogeneous AZs. In contrast, endogenous Cac levels do not change during presynaptic homeostatic depression (PHD), implying that the reported reduction in Ca(2+) influx during PHD is achieved through functional adaptions to pre-existing Ca(2+) channels. Thus, distinct mechanisms bidirectionally modulate presynaptic Ca(2+) levels to maintain stable synaptic strength in response to diverse challenges, with Ca(2+) channel abundance providing a rapidly tunable substrate for potentiating neurotransmitter release over both acute and chronic timescales. SIGNIFICANCE STATEMENT Presynaptic Ca(2+) dynamics play an important role in establishing neurotransmitter release properties. Presynaptic Ca(2+) influx is modulated during multiple forms of homeostatic plasticity at Drosophila neuromuscular junctions to stabilize synaptic communication. However, it remains unclear how this dynamic regulation is achieved. We used CRISPR gene editing to endogenously tag the sole Drosophila Ca(2+) channel responsible for synchronized neurotransmitter release, and found that channel abundance is regulated during homeostatic potentiation, but not homeostatic depression. Through live imaging experiments during the adaptation to acute homeostatic challenge, we visualize the accumulation of endogenous Ca(2+) channels at individual active zones within 10 min. We propose that differential regulation of Ca(2+) channels confers broad capacity for tuning neurotransmitter release properties to maintain neural communication.