Mechanisms of permanent loss of olfactory receptor neurons induced by the herbicide 2,6-dichlorobenzonitrile: Effects on stem cells and noninvolvement of acute induction of the inflammatory cytokine IL-6

Abstract

We explored the mechanisms underlying the differential effects of two olfactory toxicants, the herbicide 2,6-dichlorobenzonitrile (DCBN) and the anti-thyroid drug methimazole (MMZ), on olfactory receptor neuron (ORN) regeneration in mouse olfactory epithelium (OE). DCBN, but not MMZ, induced inflammation-like pathological changes in OE, and DCBN increased interleukin IL-6 levels in nasal-wash fluid to much greater magnitude and duration than did MMZ. At 24 h after DCBN injection, the population of horizontal basal cells (HBCs; reserve, normally quiescent OE stem cells) lining the DMM became severely depleted as some of them detached from the basal lamina, and sloughed into the nasal cavity along with the globose basal cells (GBCs; heterogeneous population of stem and progenitor cells), neurons, and sustentacular cells of the neuroepithelium. In contrast, the layer of HBCs remained intact in MMZ-treated mice, as only the mature elements of the neuroepithelium were shed. Despite the respiratory metaplasia accompanying the greater severity of the DCBN lesion, residual HBCs that survived intoxication were activated by the injury and contributed to the metaplastic respiratory epithelium, as shown by tracing their descendants in a K5CreErT2∷fl(stop)TdTomato strain of mice in which recombination causes HBCs to express TdTomato in advance of the lesion. But, contrary to published observations with MMZ, the HBCs failed to form ORNs. A role for IL-6 in suppressing ORN regeneration in DCBN-treated mice was rejected by the failure of the anti-inflammatory drug dexamethasone to prevent the subsequent respiratory metaplasia in the DMM, suggesting that other factors lead to HBC neuro-incompetence.

Primary Cilia on Horizontal Basal Cells Regulate Regeneration of the Olfactory Epithelium

“…Another possible explanation is that OSNs in the regenerated OE were derived from residual GBCs, not from HBCs lacking IFT88. This is consistent with the MMI-induced lesion, which can leave residual GBCs (Xie et al, 2013). However, further studies involving lineage tracing are required to conclusively determine whether HBCs lacking cilia contribute to OE regeneration.…”

Transcription factor p63 controls the reserve status but not the stemness of horizontal basal cells in the olfactory epithelium

“…A number of pathways known to be active in the OE, including Wnt, Notch, and EGF, have been shown to affect p63 expression levels in other tissues (21,(43)(44)(45). Moreover, the environment at the time of activation can direct different outcomes, because very severe injury to the OE, worse than the damage occasioned by MeBr exposure here, causes spared HBCs to give rise to metaplastic respiratory epithelium rather than regenerating GBCs and restoring the OE (46). Further elucidation of the role of activating and quiescence-maintaining pathways and more detailed analysis of HBC expression profiles with and without epithelial injury will be necessary to use these cells in regenerative applications.…”

Environmental Toxicants-Induced Immune Responses in the Olfactory Mucosa

“…The inoculation of toxicants into the IOI-transgenic mouse showed that TNF-α-induced inflammation lasted for 6 weeks and compromised the regeneration of OSNs, although the effect was not permanent, suggesting that TNF-α suppresses the proliferative activity of basal cells (12, 61). In contrast, intraperitoneal injection of the herbicide 2,6-dichlorobenzonitrile induced inflammation-like pathological changes in OE and depleted the horizontal basal cells, resulting in permanent loss of OSNs (62). A critical next step is to elucidate the molecular mechanisms underlying the specific loss of OSNs and the resistance of basal and sustentacular cells to olfactory inflammation.…”