Martin Chalfie

Martin Chalfie

We use 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 receptor neurons, or TRNs), 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, and channel structure and function, and we develop methodologies to further scientific discovery.

We initially approached TRN development and function by mutational analysis, obtaining more than 450 mutations (in 17 genes) that produce a touch-insensitive phenotype. These touch genes are needed for the generation, specification, and function of the TRNs. The first two 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 TRN differentiation are transcription factors, and we have identified transcription factors that specify, maintain, and restrict cell fate. Some of these proteins act as classical selector factors, which direct the production of cell-characteristic proteins. Other transcription factors restrict and allow the selectors to act. Among these latter transcription factors are repressors of TRN differentiation that allow other cells to differentiate using the same selectors and inhibitors of these repressors that ensure that the TRNs can still differentiate as TRNs. We have also identified another class of transcription factors that do not themselves direct differentiation, but instead reduce stochastic variability and ensure robust TRN differentiation. We call such transcription factors (which include Hox proteins) “guarantors.” In addition to acting as guarantors, Hox transcription factors also specify regionally different subtypes of TRNs, e.g., by causing posterior TRNs to develop as bipolar neurons in distinction to the anterior cells (the apparent TRN ground state), which are monopolar neurons.

Twelve touch genes from our original collection are needed for TRN function. Using genetics, molecular biology, and electrophysiology, we have identified a transduction channel, the MEC-42MEC-10 heterotrimer, that underlies the touch response in the TRNs. Other genes encode proteins that are needed for optimum channel activity. These include the cholesterol-binding membrane protein MEC-2, the channel-specific chaperone MEC-6, and the extracellular matrix proteins MEC-1 and MEC-9. We are currently studying how these and other proteins expressed in these cells transduce touch. Our current model is that the channel complex is associated with the extracellular matrix. This tethering can lead to movement of the complex in the membrane leading to its opening. 

Current projects include investigations into neuronal ensheathment, genes that modulate touch, the role of microtubules and their modifiers in neuronal outgrowth, the role and mechanism of guarantor transcription factors, drug sensitivity and resistance, the structure and molecular function of the MEC-42MEC-10 heterotrimer, and the use of previously sequenced mutagenized genomes (the Million Mutation Project) for gene discovery.