PROJECT SUMMARY Synapses are fundamental units of communication in the nervous system, where immense diversity in structure and function serve to tune and calibrate information transfer. Defects in the ability of synapses to properly diversify contribute to the etiology of a variety of neurodevelopmental, psychiatric, and neurodegenerative diseases. One means of generating diversity in synaptic function is through molecular heterogeneity, where combinations of distinct genes are expressed at individual synapses to enable specific functional properties. However, it has become increasingly clear, though difficult to resolve, that remarkable synaptic diversity can be achieved from a limited set of molecular machinery. In principle, the Drosophila neuromuscular junction (NMJ) is a uniquely powerful model to address how synaptic diversity is generated given the sophisticated genetic, electrophysiological, and imaging approaches. In this system, two distinct motor neurons converge to co-innervate individual muscle targets, where transmission from a strong and weak input together drive muscle contraction in the motor circuit. However, an inability to selectively isolate transmission from either input has been a major limitation towards understanding synaptic diversity in this system. Here, we propose to use expression of a unique Botulinum NeuroToxin (BoNT) to selectively silence transmission at strong or weak synaptic inputs. Preliminary data suggests that while each neuron is largely composed of the same molecular machinery at active zones, one core component, previously thought to function universally at all active zones, actually subserves dramatically different roles at strong vs weak synapses. We will use BoNT silencing, super resolution imaging, and the latest calcium reporters targeted to release sites to illuminate differences in active zone nanostructure and function between strong and weak synapses. We will also leverage new innovations in CRISPR mutagenesis to dissect the specialized functions of eight core active zone components at strong vs weak synapses. Finally, we will interrogate how these core active zone components are uniquely targeted for modulation and remodeling at strong vs weak synapses in the context of homeostatic synaptic plasticity. Together, these approaches will unlock fundamental insights into how glutamatergic synaptic diversity is established and adaptively modified through plasticity. Ultimately, this understanding will illuminate key mechanisms through which heterogeneous functional properties at glutamatergic release sites are enabled by a limited molecular toolkit.