Increased Retinoic Acid Catabolism in Olfactory Sensory Neurons Activates Dormant Tissue-Specific Stem Cells and Accelerates Age-Related Metaplasia

Abstract

The cellular and molecular basis of metaplasia and declining neurogenesis in the aging olfactory epithelium (OE) remains unknown. The horizontal basal cell (HBC) is a dormant tissue-specific stem cell presumed to only be forced into self-renewal and differentiation by injury. Here we analyze male and female mice and show that HBCs also are activated with increasing age as well as non-cell-autonomously by increased expression of the retinoic acid-degrading enzyme CYP26B1. Activating stimuli induce HBCs throughout OE to acquire a rounded morphology and express IP3R3, which is an inositol-1,4,5-trisphosphate receptor constitutively expressed in stem cells of the adjacent respiratory epithelium. Odor/air stimulates CYP26B1 expression in olfactory sensory neurons mainly located in the dorsomedial OE, which is spatially inverse to ventrolateral constitutive expression of the retinoic acid-synthesizing enzyme (RALDH1) in supporting cells. In ventrolateral OE, HBCs express low p63 levels and preferentially differentiate instead of self-renewing when activated. When activated by chronic CYP26B1 expression, repeated injury, or old age, ventrolateral HBCs diminish in number and generate a novel type of metaplastic respiratory cell that is RALDH 2 and secretes a mucin-like mucus barrier protein (FccBP). Conversely, in the dorsomedial OE, CYP26B1 inhibits injury-induced and age-related replacement of RALDH 2 supporting cells with RALDH1 1 ciliated respiratory cells. Collectively, these results support the concept that inositol-1,4,5-trisphosphate type 3 receptor signaling in HBCs, together with altered retinoic acid metabolism within the niche, promote HBC lineage commitment toward two types of respiratory cells that will maintain epithelial barrier function once the capacity to regenerate OE cells ceases.

The association between olfactory dysfunction and cardiovascular disease and its risk factors in middle-aged and older adults

“…www.nature.com/scientificreports/ To explain the possible mechanisms for associations of CVD and its risk factors with olfactory dysfunction, we must understand age-related olfactory neuroepithelial changes. Age-related changes include a decreased number of olfactory receptor neurons, a decrease in basal cell proliferation and replacement of olfactory neuroepithelium with respiratory epithelium [14][15][16] . Although the molecular basis of these changes remains unclear, several studies are investigating the basis.…”

Single or Repeated Ablation of Mouse Olfactory Epithelium by Methimazole

“…Our protocol is the first to describe repeated OE ablation-regeneration cycles. By repeating the injury up to three times, we found a gradual decrease in the regenerative potential of the OE, which is accompanied by the progressive accumulation of two different types of metaplasia (Håglin et al, 2020). Thus, repeated ablationregeneration cycles of the mouse OE following methimazole treatment may be used, for example, to study: i) the exhaustion of the regenerative capacity of neurogenic tissue stem cells, which may mimic aging; ii) the robustness of the re-establishment of a tissue niche when subjected to repeated injury; iii) the plasticity of target neurons in the OB when challenged to re-establish synapses several times over; iv) the role of OE barrier function in protecting against a nasal route of infection as well as uptake of chemical substances to the brain; and v) the roles of different normal and disease variants of genes in these processes since there are many genetically modified mice available for study.…”

“…Repeated ablations by methimazole treatment every 21 days results in the OE no longer regenerating efficiently. Moreover, after repeated ablations, there is a gradual increase in secretory (positive for FcγBP, in green) and ciliated respiratory metaplastic patches (positive for RALDH1/2, in green) in the OE (Håglin et al, 2020). The dashed line marks the location of the basal lamina.…”

“…methimazole doses from 25 mg/kg to 300 mg/kg have been evaluated for OE toxicity (Genter et al, 1995). We conducted a pilot study to elucidate the lowest dose suitable for reproducible OE ablation in mice (Håglin et al, 2020). Single injection of either 75 mg/kg or 100 mg/kg resulted in detachment of the entire OE, whereas 50 mg/kg caused incomplete lesioning since there were residual patches of OE cells that appeared relatively unaffected.…”

“…In the planning stage, decide which methamizole doses you wish to evaluate. We evaluated 50 mg/kg, 75 mg/kg, and 100 mg/kg for adult 2-5 month-old male and female C57BL/6 mice weighing 40-60 g (Håglin et al, 2020 3 animals are analyzed per dose.…”