Olfactory sensory neurons in
the olfactory epithelium synapse
in the olfactory bulb.
Ordering ice cream from a menu of 31 flavors is a difficult exercise in decision making. However, this task is simple compared to the selection of only 1 of 1300 olfactory receptor genes from several chromosomes in each olfactory sensory neuron. How does the neuron turn on 1 gene and turn off 1299 others? Lomvardas et al. report that an enhancer element promotes the expression of olfactory receptor genes across chromosomes—and may limit the number of receptors expressed in each neuron—in a recent article in Cell.
The expression of olfactory receptors on chromosome 14 requires the upstream enhancer element H. The authors used the chromosome conformation capture technique, which ligates closely associated DNA fragments, and found that approximately 25% of the olfactory receptor DNA associated with H elements did not localize to chromosome 14, suggesting that the H enhancer associated with olfactory receptor genes on other chromosomes.
H enhancer elements colocalized with DNA from olfactory receptors on several chromosomes, but only colocalized with one olfactory receptor gene per cell. RNA fluorescent in situ hybridization (FISH) showed that M50 and MOR23 localized to single nuclear spots in olfactory receptor neurons. However, olfactory marker protein (OMP), which is expressed in all olfactory sensory neurons, localized to two spots. These data suggest that both alleles of OMP, but only one allele of the olfactory receptor genes, were transcribed. DNA FISH showed that H-enhancer DNA localized to two spots per cell, one of which colocalized with M50 or MOR23 transcripts. H-enhancer DNA did not colocalize with OMP RNA.
If the H enhancer is necessary for olfactory receptor gene expression, and each cell has two H-enhancer alleles, then two olfactory receptor genes should be expressed in each olfactory sensory neuron. Using bisulfite mapping, the authors determined that methylation silenced one of the H-enhancer alleles. The H enhancer was methylated in olfactory sensory neurons, but not in other neurons. Unlike most genes, which are methylated at CpG sites, H enhancers were most frequently methylated at CpA sites. CpA methylation is rare and requires the DNA methyltransferase Dnmt3a, which is highly expressed in the olfactory epithelium.
The authors generated transgenic mice with several copies of H enhancers, but still found only a single olfactory receptor gene expressed in each olfactory sensory neuron. Once an olfactory receptor gene is expressed, a feedback mechanism blocks the expression of additional receptors, according to the authors. At least 20% of mouse and human olfactory receptor genes are non-coding pseudogenes, which are incapable of initiating this feedback mechanism. Mice with extra copies of H enhancers showed coexpression of olfactory receptor pseudogenes and functional olfactory receptor genes within the same neuron, whereas wild-type mice did not.
Together, these data suggest that the H enhancer crosses chromosomes to limit the choice of olfactory receptor genes. Although it is hard to conceive of another system in the brain that requires the selection of a single gene, the modulation of gene expression across chromosomes may be widespread and could alter our current model of gene regulation.