Projects

Presynaptic Protein Dynamics

Presynaptic terminals are simultaneously dynamic and stable structures. They can persist over a period of months while behaving in a highly plastic manner over a time scale of seconds to hours. This plasticity is believed to underlie important brain functions such as learning and memory. The molecules that give rise to neurotransmitter secretion must therefore solve this apparent contradiction of stability and plasticity.

We are currently studying the mechanisms underlying recruitment and retention of a variety of presynaptic molecules using photoactivatable GFP fusion proteins. We image synapses in vivo and quantify the kinetics of protein exchange between synapses for a given molecule. By examining changes in these protein dynamics in a variety of synaptic mutants, we hope to learn how synapses maintain and modulate their molecular identity.

Complexin Function

The critical proteins required for synaptic vesicle fusion are known as SNARE proteins, and the assembly of these three proteins into the SNARE complex is a key step in driving the fusion reaction. Accessory proteins regulate SNARE assembly and therefore neurotransmitter release. One of the main accessory proteins is complexin. This particular regulatory molecule acts as a sort of “gatekeeper” with a role in blocking sporadic unstimulated neurotransmitter release while facilitating release when the signal arrives at the presynaptic terminal. Changes in complexin abundance in the brain have been found in neurodegenerative diseases such as Alzheimer’s, Parkinson’s, and Huntington’s as well as in schizophrenia and other psychiatric disorders.

Complexin is an essential protein in mammals; mice lacking the two major isoforms of complexin die shortly after birth. Thus, it has been difficult to study nervous system function in the complete absence of complexin using rodents. In C. elegans, however, animals lacking complexin are still viable. We found that complexin has a dual role in worm synapses: inhibiting spontaneous (unstimulated) synaptic vesicle fusion while simultaneously promoting stimulated fusion. However, we still don’t know exactly how complexin accomplishes either of these regulatory roles. Our group is currently examining the mechanism of complexin activity using a combination of genetics, imaging, biochemistry, and physiological methods.

Neuronal Modulation by GPCRs

In C. elegans, both Acetylcholine (ACh) and GABA are released in the nerve cord and mediate fast neuromuscular excitation and inhibition during locomotion. These neurotransmitters activate a muscarinic receptor (GAR-2) and the GABAB receptor dimer (GBB-1/2) that detect synaptically released ACh and GABA, respectively. These receptors are expressed on motor neurons and possibly interneurons controlling locomotion.

We are exploring ways of detecting the molecular events that underlie GPCR modulation of the motor neurons as well as rewiring the modulatory circuitry of the worm by missexpressing GPCRs within the nervous system.