Multiple polycomb epigenetic regulatory proteins are active in normal and regenerating adult olfactory epithelium.

Objectives: To investigate epigenetic mechanisms contributing to regulation of cellular renewal and neurogenesis in adult olfactory epithelium (OE).

Study Design: Prospective basic science study.

Methods: Olfactory basal cell cultures were prepared from adult mice per established protocols. in vivo studies were performed using the mouse methimazole lesion-regeneration paradigm. Nasal tissue sections were prepared from adult mice 7 days following lesion, or from unlesioned controls. Polycomb proteins were assessed by Western blot from culture or nasal tissue lysates, and by gene expression studies from cultures. In addition, in vivo expression patterns of Polycomb proteins were examined using immunohistochemistry. Chromosome immunoprecipitation (ChIP) was performed to investigate epigenetic modifications and specific chromatin interactions for Polycomb proteins in olfactory basal cells.

Results: Subunits of Polycomb Repressive Complex 1 (PRC1) and Polycomb Repressive Complex 2 (PRC2) were identified in basal cell cultures and in vivo. In regenerating OE, basal progenitor cells identified by expression of the c-KIT receptor were found to coexpress the PRC2 protein EZH2. Because multiple variants of PRC1 subunits give rise to diverse PRC1 complexes serving different functions, expression of specific PRC1 variants was further examined. We identified PRC1 components including MEL18 (PCGF2) in immature neurons, and confirm BMI1 (PCGF4) expression in mature neurons. Moreover, we identified CBX8 as a neuron-specific PRC1 subunit. ChIP assays from OE cells demonstrated binding of PRC proteins to regulatory regions of specific transcription factors, consistent with PRC-mediated epigenetic silencing mechanisms active in adult OE.

Conclusions: Multiple Polycomb proteins have cell type-specific expression patterns in the adult OE. Findings presented here, together with evidence from prior studies, suggest that PRC-mediated epigenetic silencing contributes to regulation of cellular renewal and tissue homeostasis in the OE. Efforts to define the mechanisms that regulate repair in the OE are essential for development of new therapeutic strategies for olfactory disorders.

Level of Evidence: N/A.