Assistant Professor of Biochemistry
Phone: (974) 4492 8349
Smell is an essential sense that allows animals to find food and mates while avoiding predators. In humans, smell is considered an aesthetic sense but olfactory disorders may presage neurological diseases including Parkinson's, Alzheimer's, and schizophrenia. The odorant receptors (ORs), numbering more than 1,000 in the genome, endow an animal with the ability to smell. Critical to the development and function of olfaction is the regulation of the ORs, with each sensory neuron selecting just one OR for expression, at random, from only one allele. While recent experiments have brought the outlines of this process into focus, the core mechanism is obscure. Our long-term goal is to understand how a sensory neuron randomly chooses a single OR gene from among thousands.
The two key questions in the field are: how is random OR choice initiated, and once chosen, how is expression of the OR maintained? Models of initiation of OR choice propose either the infrequent, random transcriptional activation of one OR per window of time, or the action of a unique selection apparatus, able to activate one allele at a time. We, and others, have shown that OR choice is maintained by a feedback mechanism whereby the first functional receptor expressed above a threshold level halts OR selection. Neither the initiation of OR choice nor its maintenance by feedback are understood on a molecular level.
Whole mount (parasagittal) view of a mouse genetically modified (schematic constructs) to express tau-lacZ and GFP from each allele of the P2 odorant receptor gene, subject to anti-b-gal and anti-GFP immunohistochemistry. Neurons express P2 monoallelically (green or red cells) in the olfactory epithelium (oe), and project their axons back into the olfactory bulb (ob) to the glomerulus (gl). Nuclei are counterstained by Toto-3 (blue, Shykind, 2005)
Maintenance of a Single Expressed OR Allele
Model of Switching and the Maintenance of OR Choice. Nonfunctional OR fails to trigger feedback and is shut off (A). Alternate OR is chosen via “switching” (B). Feedback signal is triggered to stabilize singular OR choice for the life of the cell (C).
We, and others, have observed that singular OR choice is maintained by a feedback mechanism, which halts a stochastic process that would proceed to choose multiple receptors per cell. Using a genetic, lineage-marking approach in mice, we showed that expression of functional OR is predominantly stable, but the choice of a non-functional receptor leads to its shutdown and a continuation of the random choice of an alternate OR - a process we termed OR “switching.” Supporting this model, we have ectopically expressed an OR (M71) transgene in >95% of the neurons in the nose, and observe a corresponding suppression of endogenous OR choice resulting in an effective “monoclonalization” of the nose.
The pursuit of the solution to this paradigmatic question of gene regulation has captivated the biomedical research community - not only in the area of neurobiology. The elucidation of its answer will shed light on the establishment and function of this sensory system and also further our understanding of the regulation of the largest gene family in mammals. Additionally, these findings will have central relevance for other examples of stochastic gene choice such as the expression of the lymphocyte antigen receptors and X-chromosome inactivation, and for diverse disease-related process such as trypanosome vsg and malaria var gene switching, and the functional (epigenetic) loss of heterozygosity in cancer; all of which likely depend on complex transcriptional processes.