Research
in the Hart LaboratoryResearch
in the Hart Laboratory
We are using genetic, molecular, cellular and behavioral approaches in C. elegans to study nervous system. The nematode C. elegans is uniquely suited for this analysis- the nervous system consists of only 302 neurons whose location and synaptic connections are all known. Yet, this small nervous system is remarkably similar to vertebrate nervous systems. The C.elegans genome is almost completely sequenced, facilitating cloning and molecular analysis of interesting mutations.
One focus of the lab is how animals detect and respond to external stimuli. We hope to identify both the proteins which act as sensory receptors and to learn how sensory information is encoded by the nervous system as synaptic activity. We address these questions in C. elegans by studying ASH sensory neurons which detect mechanical, osmotic and chemical stimuli. We focus our attention on mutations which perturb response to just one stimulus, leaving intact response to the other stimuli detected by ASH, in order to identify proteins specifically involved in stimulus detection and synaptic signaling. (See the ASH circuit.)Mutations have been identified which specifically prevent response to each of the different stimuli detected by ASH. Only two of these genes have been cloned so far. Analysis of glr-1, a gene required for nose touch response, suggested that synaptic output from ASH differs, either qualitatively or quantitatively, depending on the stimulus detected by ASH. Analysis of the osm-10 gene, required for osmotic avoidance, suggested that osm-10 encodes a novel protein involved in signal transduction expressed in a small subset of sensory neurons- including ASH.(See the osm-10::GFP photo.) We are now characterizing mutations which specifically perturb response to other ASH stimuli-- specifically the volatile repellant 1-octanol. We suspect that genes required for response to just one or two stimuli will be involved in stimulus detection or in synaptic signalling.
Another major focus of the laboratory is using C. elegans as a model system to study Huntington's disease. Huntington's is one of several dominant diseases caused by expansion of a polyglutamine domain in the corresponding protein. This expansion causes neurodegeneration, dementia, chorea and eventual death, but the molecular mechanisms underlying this polyglutamine toxicity are unclear. We have developed a C. elegans model for polyglutamine diseases by expressing N-terminal fragments of either normal human huntingtin or mutant human huntingtin (containing an expanded polyglutamine domain). The mutant, expanded protein is toxic in the C. elegans ASH neurons resulting in neuronal dysfunction and neurodegeneration.
We are using the genetic and molecular tools available
in C. elegans to elucidate the cellular and molecular basis of this
polyglutamine-mediated neurodegeneration. We are identifying and characterizing
mutations which either sensitize or protect the neurons from polyglutamine
toxicity. We are currently characterizing the polyQ enhancer-1 (pqe-1)
gene. Loss of pqe-1 function accelerates polyglutamine induced neurodegeneration
in C. elegans resulting in massive ASH neuron cell death at 3 days-
but this cell death is completely dependent on the presence of an expanded
polyglutamine tract in the affected neurons. The pqe-1 gene encodes
a previously uncharacterized protein and we are now addressing how pqe-1
normally protects neurons from polyglutamine toxicity. We are also screening
for mutations which suppress the toxic effects of expanded polyglutamine
domains. The corresponding genes should encode proteins which are critical
for polyglutamine toxicity. These genes and proteins will also be identified,
characterized and cloned to address their function in polyglutamine toxicity.
Human homologs of these proteins are likely to play critical roles in Huntington's
and other diseases-- and may be targets for therapeutic intervention.
A third focus of the laboratory is neuropeptide neurotransmitters. Diverse neuropeptides play critical roles in all nervous systems, yet neuropeptide signaling pathways are poorly understood at a functional or molecular level. Analysis of glr-1, a gene specifically required for nose touch response, suggested that synaptic output from ASH differs, either qualitatively or quantitatively, depending on the stimulus detected by ASH. And, these results and others suggested that neuropeptides neurotransmitters play a role in the selective signaling of sensory stimuli in the ASH circuit. We are using the genetic and molecular tools available in C. elegans to address neuropeptide function in the ASH circuit and elsewhere. We have identified 32 new C. elegans neuropeptide genes many of which define entirely new families of neuropeptide neurotransmitters found in many species. These C. elegans neuropeptide genes are expressed in primarily or exclusively in unique sets of neurons and their function is being addressed with reverse genetic techniques. Preliminary results suggest that at least two neuropeptide genes are expressed in ASH. Neuropeptide genes will be further characterized and "knocked out" to address their role in nervous system function and development.
References:
A.C. Hart, S. Sims and J.M. Kaplan 1995 Synaptic code for sensory modalities revealed by C. elegans GLR-1 glutamate receptor. Nature 378:82-85.
C. Serra-Pages, Q. G. Medley, M. Tang, A. C. Hart and M. Streuli. 1998. Liprins, a family of LAR transmembrane protein-tyrosine phosphatase-interacting proteins. Journal of Biological Chemistry, 237(25)15611-20.
A.J. Berger, A.C. Hart and J.M. Kaplan. 1998 G alpha s-Induced Neurodegeneration in Caenorhabditis elegans. Journal of Neuroscience 18(8):2871-2880.