Sequestration within nuclear chromocenters is not a requirement for silencing olfactory receptor transcription in a placode-derived cell line

Mouse olfaction depends on specialized olfactory sensory neurons (OSNs) that each express only one olfactory receptor protein from among a family of >1000 olfactory receptor (OR) genes encoded in the genome. To investigate epigenetic mechanisms underlying monogenic OR expression, we characterized the nuclear organization of OR loci in an olfactory placode-derived cell line (OP6) derived from a pre-neuronal cell along the OSN lineage. OR loci are significantly enriched within nuclear chromocenters in these cells as compared with control loci tested. However, we observe variability in chromocenter occupancy among different OR loci and from cell-to-cell, suggesting that these associations are transient or context dependent. The lamin B receptor (LBR), whose downregulation is necessary for aggregation of chromocenters and OR genes in mature OSNs, exhibits an unusual non-peripheral expression pattern in OP6 nuclei; upon further OP6 cell differentiation, LBR expression is lost and chromocenters begin to aggregate. However, neither undifferentiated nor differentiated OP6 cells sequester OR genes within the chromocenters, despite the establishment of monogenic OR expression in these cells. These results indicate that sequestration of competing OR loci is not a requirement for monogenic OR expression in OP6 cells, and could indicate that the initial establishment of monogenic OR expression during OSN differentiation in vivo occurs prior to recruitment of OR genes into chromocenters.