Research

Synaptic homeostasis at the Drosophila NMJ. Schematic of homeostatic compensation following application of the postsynaptic glutamate receptor antagonist philanthotoxin (PhTx) to the NMJ; Glutamate Receptors (blue ovals), EJP (red traces), and mEJP (purple traces). Application of PhTx initially causes a ~50% decrease in miniature excitatory junction potential (mEJP) amplitude and a parallel ~50% decrease in evoked excitatory junction potential (EJP) amplitude. After 10 min incubation in PhTx, mEJP amplitudes remain depressed while EJP amplitudes increase to baseline values due to an enhancement of presynaptic neurotransmitter release (quantal content).

      We are interested in synapse development, function, and plasticity in general, and in particular how these processes are stably maintained within proper physiological ranges, referred to as homeostatic synaptic plasticity. Homeostatic feedback systems are a ubiquitous form of biological regulation, which recently have been demonstrated to maintain the stability of nervous system function. Homeostatic processes also play crucial roles in the development of the nervous system, tuning synaptic strength and establishing the proper balance of excitation and inhibition. Dysfunction in these systems may contribute to the etiology of schizophrenia, autism, epilepsy, and other complex neurological and psychiatric diseases. Our long term interests are to identify the molecules and elucidate the mechanisms that achieve and maintain the stability of neural function, and to determine how dysfunction in these processes may contribute to human disease.

      It has become clear over the past decade that homeostatic signaling systems are present throughout the nervous system and serve to stabilize neural function during development and throughout life. These processes have been identified in the central and peripheral nervous systems of both vertebrates and invertebrates, indicating that this is a fundamental and evolutionarily conserved form of neural regulation.

A complex homeostatic signaling system: Several presynaptic components of the homeostatic effector system have been identified, including the schizophrenia susceptibility gene dysbindin and the synaptic vesicle protein Snapin. However, several basic questions about this homeostatic system remain unanswered, including the nature of the postsynaptic sensor and the identity of the retrograde signal.

While the phenomenon of the homeostatic regulation of neural function has been extensively demonstrated in a variety of organisms, the molecules and molecular mechanisms involved are relatively unknown. We use the fruit fly Drosophila melanogasteras a model system because of its amenability to advanced genetic, molecular, electrophysiological, imaging, and cell biological approaches.
    The Drosophila neuromuscular junction (NMJ) is a powerful model system to study the homeostatic control of synaptic transmission. Genetic and pharmacological manipulations that reduce postsynaptic (muscle) excitability trigger a retrograde signal to the neuron that increases presynaptic release to precisely compensate for this perturbation and restore normal muscle excitability.

      This implies a complex signaling system that is able to monitor postsynaptic excitability and transduce this information into a retrograde feedback signal to the presynaptic neuron. Effectors in the neuron then must act to increase synaptic vesicle release to precisely compensate and restore proper physiological excitation. Similar types of regulation have been observed in the mammalian central and peripheral nervous systems.

        Using an electrophysiology-based forward genetic screen, we have isolated several novel genes that are required for adaptive plasticity, including one, dysbindin, which has been linked to schizophrenia in humans. Very little is known about the function of Dysbindin. Our genetic and electrophysiological analyses have demonstrated that Dysbindin is required presynaptically for synaptic homeostasis. Genetic and calcium imaging experiments also revealed that Dysbindin functions independently of calcium influx to control synaptic homeostasis, while having a unique role in altering the calcium sensitivity of synaptic vesicle release. We have carried this analysis a step further, in showing that a binding partner of dysbindin, the synaptic vesicle protein Snapin, is also required for homeostatic plasticity, possibly by modulating SNAP-25, a component of the SNARE complex required for vesicle fusion (Fig. 2). We are currently characterizing other novel genes and performing further screens with the goal of illuminating the molecular, cellular, and synaptic mechanisms governing this complex, fundamental and poorly understood process.