Olfactory receptor coding sequences cause silencing of episomal constructs in multiple cell lines

The mammalian olfactory system consists of sensory neurons with specialized odorant-binding capability accomplished by mutually exclusive odorant receptor (OR) expression. Mutually exclusive OR expression is a complex multi-step process regulated by a number of cis and trans factors, including pan-silencing of all OR genes preceding the robust and stable expression of the one OR selected in each sensory neuron. We transfected two olfactory-placode-derived cell lines modeling immature odorant sensory neurons, as well as the GD25 fibroblast cell line, with episomes containing CMV-driven GFP and TK-driven hygromycin reporter genes. We inserted various coding sequences, along with an IRES, immediately upstream of the GFP gene to produce bicistronic mRNAs driven from the local CMV promoter. We found that the presence of several OR coding sequences resulted in significantly diminished episomal expression of GFP in all three cell lines. These findings suggest that OR coding sequences have intrinsic self-silencing capability that might facilitate mutually exclusive OR expression in olfactory sensory neurons by making it less likely that multiple ORs acquire an above-threshold level of expression at once.