NATIONAL INSTITUTE FOR BASIC BIOLOGY
Our research interest is focused on the regulatory mechanisms of multigene families both in the immune and in the central nervous systems. In the immune system, somatic DNA recombination and gene conversion play important roles in the expression of antigen receptor genes. Gene rearrangement, known as V-(D)-J joining, not only generates a vast diversity in the receptor genes, but also activates a particular member of the gene family by bringing enhancer and promoter elements into close proximity.
For the olfactory system, hundreds of odorant receptor genes have been reported, although it is yet to be studied how this multigene family is regulated for expression. It is known that each member of the gene family is expressed in one of the three or four topographically distinct zones, where olfactory neurons expressing one particular kind of receptor are randomly distributed. Furthermore, it is assumed that a very limited number of the receptor genes (possibly one) is activated in each olfactory neuron.
We are interested in knowing how each neuron select a limited number of receptor genes which are to be expressed. One obvious regulatory mechanism is at the level of transcription. In this mechanism we may need a variety of transcriptional factors for both positive and negative regulations. In the mammalian immune system, somatic DNA recombination is utilized to activate one particular member of the receptor genes, and to exclude one of the two different alleles. It is of interest to study whether such a mechanism is also involved in the regulation of the olfactory system.
In order to study selective expression of the odorant receptor genes, we are characterizing the genomic structure of the murine receptor genes. Although many coding sequences have been reported for the odorant receptor genes, little is known about the 5' and 3' noncoding regions. These regions must contain important DNA elements that regulate the receptor gene expression. We have isolated genomic clones of the odorant receptor genes from the P1 phage library. We are currently analyzing the regulatory regions in these clones. We hope that these studies will reveal the molecular mechanisms for the selective and zonal expression of the odorant receptor genes. We have made transgenic mice which are forced to express one particular member of the odorant receptors in every olfactory neuron. It is of interest to study whether the expression of the transgene represses the endogenous receptor genes. A plasmid construct contains the mouse receptor #28 gene under the control of OMP promoter. Since the OMP gene is expressed specifically in the olfactory epithelium, the #28 receptor is expected to be activated in all mature neurons in the transgenic mouse. These study will give us a new insight into the regulatory mechanisms of the odorant receptor genes.
When olfactory neurons form synapses with other neurons, they send their axons to the specific region called glomeruli in the olfactory bulb. Each olfactory neuron sends only one axon to one of two thousand glomeruli. It is quite amazing that the olfactory neurons are able to find their right target glomerulus every time when they are regenerated. It is interesting to study how the neuronal activity affects the target specificity and selectivity. We are hoping that our transgenic approach will become a usuful clue to answer to these questions.