Regulation of Olfactory Epithelial Cell Fate Determination by Polycomb Repressive Complex 2

Abstract

Lining the upper nasal cavity, the olfactory epithelium (OE) serves as an attractive model for investigating barrier epithelial repair and adult neurogenesis. Adult neurogenesis occurs in the mammalian olfactory epithelium to maintain populations of neurons that are vulnerable to injury yet essential for olfaction. Additionally, the OE is comprised of a variety of other cell types: sustentacular barrier cells, Bowman’s glands, and microvillar cells. Multipotent olfactory basal stem cells are capable of regenerating all of the olfactory epithelial cell types. These olfactory basal cells are activated by damage, although mechanisms regulating lineage decisions are not understood. Using mouse lesion models, we focused on defining the role of Polycomb repressive complexes (PRCs) in olfactory neurogenesis. PRC2 has a well-established role in developing tissues, orchestrating transcriptional programs via chromatin modification. PRC2 proteins are expressed in olfactory globose basal cells (GBCs) and nascent neurons. Here, we used a conditional knockout mouse to selectively delete PRC2 subunit EED in olfactory basal cells and utilize lineage tracing, immunohistochemistry, single-cell RNA-sequencing, and chromatin immunoprecipitation to determine the role of PRC2 in olfactory basal cells. Conditional PRC2 loss perturbs lesion-induced neuron production, accompanied by altered histone modifications and misexpression of lineage-specific transcription factors in GBCs. De-repression of Sox9 in PRC2-mutant GBCs is accompanied by increased Bowman’s gland production, defining an unrecognized role for PRC2 in regulating gland versus neuron cell fate. Our findings support a model for PRC2-dependent mechanisms promoting sensory neuronal differentiation in an adult neurogenic niche.Additionally, whether additional stem cell compartments, apart from olfactory basal cells, may contribute to olfactory epithelial reconstitution remains unclear. Early ultrastructural studies and retroviral lineage tracing studies on lesioned mouse OE suggested that Bowman’s glands could be a stem cell population. Furthermore, the nearby and closely related respiratory epithelium in the upper airway contains basal cells that, similar to the OE, regenerate the epithelium after damage, yet the respiratory submucosal glands have been identified as a reserve stem cell population that reconstitutes the respiratory epithelium after severe injury. Using lineage tracing, immunohistochemistry, and single cell RNA-sequencing, we demonstrate that cells within murine submucosal Bowman’s glands can give rise to OE cells after experimental injury. These findings identify a reserve olfactory stem cell with potential implications for regenerative medicine applications for olfactory disorders.