|T H E N I H C A T A L Y S T||N O V E M B E R - D E C E M B E R 1 9 9 8|
|P E O P L E|
Douglas Bell gave up a promising career as a middle school science teacher and soccer coach to attend graduate school at the University of North CarolinaChapel Hill and carry out research in environmental and molecular mutagenesis. After receiving a Ph.D. in environmental biology in 1988, he became a National Research Council fellow in the Genetic Toxicology Division at the Environmental Protection Agency. In 1990, he joined the Laboratory of Biochemical Risk Analysis at NIEHS, where he is currently head of the Genetic Risk Group.
Environmental risk assessment requires estimation of the differences among humans in their response to toxic chemical exposures. We hypothesize that human genetic variabilities in their biochemical capacities to detoxify chemicals and repair DNA damage are important early susceptibility factors in environmentally induced disease. Thus, individuals with high-risk genotypes will accumulate more genetic damage (such as DNA adducts and somatic mutations) and therefore have a greater risk of developing cancer.
My research has focused on variation in the Phase II metabolism enzymes, particularly the polymorphic forms of N-acetyltransferase (NAT1 and NAT2) and glutathione-S-transferase (GSTM1, GSTT1, GSTP1). During the course of our studies, we characterized new variant alleles in these genes and developed genotyping methods for human population studies. We are studying the proposition that the effects of exposure may be detectable in some genetically defined subpopulations, for example, NAT slow acetylators. Our findings from epidemiological studies support this, suggesting that the NAT genes, particularly NAT1, have an important role in modulating risk for bladder, oral, and colorectal cancer.
In addition, findings on GSTM1 null genotype suggest that this common trait conveys an 80 percent increased risk for bladder cancer, and that the genetic risk depends on cigarette smoking exposure. Similarly, variation in the GSTT1 gene appears to be important in mediating damage from exposures to ethylene oxide and dichloromethane. There are many candidate genes that may influence risk for environmentally induced disease; however, of the approximately 100,000 human genes, only a few dozenthose with easily determined phenotypeshave been explored for polymorphism. We are actively engaged in projects to discover new polymorphisms in genes involved in responses to environmental exposures.
With the encouragement of a mentor at NIEHS, George Lucier, I have established a number of productive collaborative projects, including work with outstanding epidemiology groups from NIEHS, NCI, University of North Carolina, Columbia University (New York), and the Johns Hopkins University (Baltimore). It is our hope that studies incorporating markers of genetic susceptibility and/or exposure will continue to help us understand the distribution of risk in human populations. To the extent that we can quantify risk differences among groups, we can reduce the uncertainty in risk assessment and enhance our ability to reduce the societal costs of environmental hazards. In addition, identifying subpopulations or individuals who might be at particularly high risk from exposures due to inherited or acquired factors should be useful in disease-prevention strategies.
Axons and nerve terminals are unique subcellular structures of the neuron that play a critical role in the development and maintenance of neural connectivity. One of the central tenets in neuroscience is that the protein constituents of these distal neuronal compartments are synthesized in the nerve cell body and are subsequently transported to their ultimate sites of function. Hence, the structure and function of these highly specialized distal domains of the neuron are totally dependent on slow anterograde axoplasmic transport.
In contrast to this viewpoint, work in my laboratory focuses on the hypothesis that de novo protein synthesis occurs within microcompartments in the neuron to include the axon and presynaptic nerve terminal. Our studies use the squid giant axon, which serves as a model invertebrate motor neuron system.
Using this model, my colleagues and I have shown that the axon contains a heterogeneous population of approximately 100200 different mRNAs. These mRNAs are full-length gene transcripts capable of synthesizing protein in a cell-free translation system. We have cloned and characterized several axonal mRNAs that encode B-actin, B-tubulin, spectrin, kinesin, MAP I, neurofilament protein, and enolase.
In addition, we have identified several mRNAs that code for novel proteins. The axonal localization of these mRNA species was definitively demonstrated by in situ hybridization histochemistry, and the presence of these sequences in the polysome fraction was established by reverse transcriptionPCR methodology. Using biochemical labeling experiments and electron spectroscopic phosphate imaging, we were also able to show that the giant axon contained biologically active polyribosomes.
Concurrent with this work, we have demonstrated that protein synthesis occurs in the large presynaptic terminals of squid retinal photoreceptor neurons. This finding was obtained using cell-free translation analysis, high-resolution autoradiography, and electron spectroscopic imaging. Our most recent results suggest that the level of protein synthesis in these presynaptic terminals is affected by calcium ions and, hence, could be regulated by the activity of the terminal itself.
Based upon the information gleaned from this invertebrate model system, we have postulated that key elements of the cytomatrix, molecular motors of the axon transport systems, and proteins involved in energy metabolism are locally synthesized in the distal structural and functional domains of the neuron. In the mature neuron, a local system of protein synthesis could contribute significantly to the maintenance and remodeling of axonal architecture, as well as the dynamic properties of the nerve terminal.
This system might prove especially important in large asymmetric motor and sensory neurons, where the axon and terminal fields are far removed from the cell body.
Currently, my colleagues and I are using differential mRNA display methodology to identify novel constituents of the axonal mRNA population, and we are beginning to explore the mechanisms involved in the intracellular trafficking of axonal mRNAs. These latter studies will involve mRNA-protein binding assays, as well as deletion mutation analysis and microinjection of fluorescently labeled mRNAs into isolated squid giant axon preparations. We hope that these investigations will augment our understanding of the molecular mechanisms that play a key role in neuronal development, regeneration, and plasticity.
Return to Table of Contents