We are using the nematode Caenorhabditis elegans to investigate aspects of nerve cell development and function. The wealth of developmental, anatomical, genetic, and molecular information available for C. elegans provides a powerful and multifaceted approach to these studies. Our work has focused on the study of a set of six neurons that are the sensory receptors for gentle touch (the touch cells), to address two questions: 1) How is neuronal cell fate determined? and 2) What is the molecular basis of mechanosensation, a sensory modality that underlies a variety of senses(e.g., touch, hearing, and balance)? We also work on neuronal degeneration, microtubule structure and function, channel structure and function, and, most recently, synapse specification and aging. Facilitating these studies is the development of new experimental methods, such as green fluorescent protein as a gene and protein marker and a novel method to generate subtractive cDNA libraries.
We initially approached touch cell development by mutational analysis, obtaining more than 500 mutations (in 17 genes) that produce a touch insensitive phenotype. These touch genes are needed for the generation, specification, maintenance, and function of the cells. The first three groups contain genes that regulate touch cell development, and the last group (function) contains genes that are developmental targets of this regulation. Many of the genes that regulate touch cell differentiation are transcription factors. In addition we have identified seven other genes that in combination with these genes specify the number and differentiation of the touch cells. Twelve touch genes are needed for touch cell function. The cloning and characterization of these genes have provided the first molecular model for eukaryotic mechanosensation. In this model a channel similar to the epithelial sodium channel in vertebrates is attached to the extracellular matrix via an extracellular gating domain on the channel and is attached intracellularly to a unique form of the microtubule. An implication of this dual tethering is that the channel could be deformed (and opened) by displacement of the microtubules by the touch stimulus. We are currently testing the predictions of this model, biochemically and electrophysiologically. In addition, we have recently developed methods to quantify the forces needed to stimulate the touch cells, which we are adapting to look for mutants that are supersensitive to touch.
The unc-4 gene encodes a homeodomain transcription factor needed for the formation of specific interneuron synapses onto a single class of C. elegans motor neurons. We have identified several genes whose expression is reduced in unc-4 animals using a new subtractive library method. Our working hypothesis is that these genes, several of which encode membrane or secreted proteins, permit appropriate synapse formation, prevent inappropriate synapse formation, or mature or maintain appropriate synapses once they have formed.
The subtractive library method serendipitously allowed us to identify a mutation in a C. elegans catalase gene. Unlike previously known animal catalases, the product of this gene is not localized to peroxisomes, but is found throughout the cytosol. We have shown that this catalase is needed for the increase in adult life-span seen in several C. elegans mutants, suggesting that oxidative damage is an important factor in organismal aging. We are currently searching for other mutants that affect life-span in C. elegans.