We hope to define the cellular and molecular mechanisms that maintain the stability of neural function throughout the life of an organism. To do so, we are combining forward genetic and functional genomic approaches in Drosophila with synaptic electrophysiology and quantitative live imaging. Three ongoing research projects are described here.
I. The homeostatic regulation of synaptic function. It is increasingly apparent that homeostasis and homeostatic signaling represent potent mechanisms that can modulate synaptic function in the nervous system (Marder, 1996; Davis and Goodman, 1998; Turrigiano and Nelson, 2000; Davis and Bezprozvanny, 2001). Synaptic homeostasis refers to the observation that synaptic function can be altered to compensate for a perturbation in the excitability of a postsynaptic cell or a neural network. At the neuromuscular junction (NMJ) of both vertebrates and invertebrates, decreased postsynaptic excitability causes a compensatory increase in presynaptic neurotransmitter release that can restore normal muscle depolarization (Davis et al., 1998; Davis and Goodman, 1998; Davis and Bezprozvanny, 2000; Paradis et al, 2001). We are pursuing experiments to better define the phenomenology and molecular basis of synaptic homeostasis at the Drosophila NMJ. A detailed molecular understanding of synaptic homeostasis may relate to how stable neural activity is achieved and maintained in the nervous system, and how it is altered in neural disease.
II. The molecular mechanisms of synapse stabilization versus disassembly. Throughout the nervous system there is evidence that the refinement and modulation of neural circuitry is driven not only by synapse formation, but also by the regulated disassembly of previously functional synaptic connections. Synapse disassembly is also an early and pivotal event in many forms of neurodegenerative disease. Currently, very little is known about the molecular mechanisms that control synapse stability versus disassembly. Using new high-throughput assays combined with Drosophila genetic and functional genomics we hope to define a core cellular program that is responsible for regulated synapse disassembly with the belief that this process will be informative for understanding synapse disassembly during development, disease and aging. The first genes to be published from this work have recently been implicated as human disease genes in amyotrophic lateral sclerosis (ALS) (Eaton et al., 2002; LaMonte et al., 2002; Puls et al., 2003).
III. The molecular mechanisms of synaptic vesicle recycling. Many of the molecular players involved in synaptic vesicle endocytosis and recycling have been identified biochemically. A remaining challenge is to understand how these molecules are organized into a highly efficient molecular machine capable of high fidelity, compensatory vesicle endocytosis. Standard genetic approaches have been valuable, but are also limited in many ways. For example, proteins that are required for vesicle endocytosis such as clathrin are also essential for cell viability, preventing a standard genetic analysis. Other proteins participate in both exocytosis and endocytosis and standard genetic approaches are unable to dissociate the exocytic from endocytic functions for these molecules. Therefore, we have developed new technologies for the acute disruption of protein function, in vivo, using light (Marek and Davis, 2002; Poskanzer et al., 2003). These new tools are allowing us to dissect the function of molecules in the synaptic vesicle cycle with spatial and temporal resolution of light, a molecular resolution not previously possible.