Dietary Approach for Post-COVID Treatment of Olfactory Loss

The spread of severe acute respiratory syndrome coronavirus [SARS-CoV-2] has consequently led to the global COVID-19 pandemic. Many patients, whether hospitalized or not, have reported a variety of complications that persist after recovery. The admission for COVID-19 has been associated with anosmia and hyposmia, the inability or decreased ability to smell. Deficiencies in the ability to smell tend to recover within weeks. However, a significant number of cases have been reported in which smell distortions last for several months. Experimental research has identified inflammation as a factor disrupting olfactory neurons. Precisely, local inflammation through cytokine release in sustentacular and horizontal basal cells interferes with the function of olfactory neurons. Further studies have reported that these local inflammatory events are not responsive to common corticoid treatments. Therefore, in order to mediate the recovery of olfaction in COVID-19 patients after viral recovery, this study evaluates vitamin C, vitamin D, vitamin E, and omega-3 polyunsaturated fatty acids to develop a prospective dietary approach with anti-inflammatory properties to reduce the local inflammation of sustentacular and horizontal basal cells.

Transient Effects of Cyclophosphamide on Basal Cell Proliferation of Olfactory Epithelia

Cancer is often treated with broad-spectrum cytotoxic drugs that not only eradicate cancerous cells but also have detrimental side effects. One of these side effects, disruption of the olfactory system, impedes a patient’s ability to smell, perceive flavor, and ultimately may interfere with their nutritional intake and recovery from cancer. Recent studies reported that the chemotherapy drug, cyclophosphamide (CYP), can damage gustatory epithelia and disrupt cell proliferation in olfactory epithelia. In this study, we asked if CYP altered globose and horizontal basal cell proliferation in the murine main olfactory epithelium (MOE) and vomeronasal organ (VNO). We used antibodies for Ki67, a marker strictly associated with cell proliferation, and Keratin 5, a marker for the cytoskeleton of horizontal basal cells. Our results revealed a significant CYP-induced decrease in the number of proliferative cells in both epithelia, especially globose basal cells in the MOE, within the first 1–2 days postinjection. Recovery of cell renewal was apparent 6 days after injection. The immunohistochemical markers showed significantly higher levels of globose and horizontal basal cell proliferation in CYP-injected mice at 14 and 30 days postinjection compared with control mice. The prolonged proliferative activation of globose and horizontal basal cells suggests that, besides altering proliferation of olfactory epithelia, the epithelial substrate needed for successful cell renewal may be adversely affected by CYP.

Acute inflammation regulates neuroregeneration through the NF-κB pathway in olfactory epithelium

Adult neural stem cells/progenitor cells residing in the basal layer of the olfactory epithelium are capable of reconstituting the neuroepithelium even after severe damage. The molecular events underlying this regenerative capacity remain elusive. Here we show that the repair of neuroepithelium after lesioning is accompanied by an acute, but self-limited, inflammatory process. Attenuation of inflammatory cell recruitment and cytokine production by dexamethasone impairs proliferation of progenitor horizontal basal cells (HBCs) and subsequent neuronal differentiation. Using TNF-α receptor-deficient mice, we identify TNF-α signaling as an important contributor to this inflammatory and reparative process, mainly through TNF-α receptor 1. HBC-selective genetic ablation of RelA (p65), the transcriptional activator of the NF-κB pathway, retards inflammation and impedes proliferation at the early stages of regeneration and suggests HBCs directly participate in cross-talk between immune response and neurogenesis. Loss of RelA in the regenerating neuroepithelium perturbs the homeostasis between proliferation and apoptosis while enhancing JNK signaling. Together, our results support a model in which acute inflammation after injury initiates important regenerative signals in part through NF-κB–mediated signaling that activates neural stem cells to reconstitute the olfactory epithelium.

Notch1 maintains dormancy of olfactory horizontal basal cells, a reserve neural stem cell

The remarkable capacity of the adult olfactory epithelium (OE) to regenerate fully both neurosensory and nonneuronal cell types after severe epithelial injury depends on life-long persistence of two stem cell populations: the horizontal basal cells (HBCs), which are quiescent and held in reserve, and mitotically active globose basal cells. It has recently been demonstrated that down-regulation of the ΔN form of the transcription factor p63 is both necessary and sufficient to release HBCs from dormancy. However, the mechanisms by which p63 is down-regulated after acute OE injury remain unknown. To identify the cellular source of potential signaling mechanisms, we assessed HBC activation after neuron-only and sustentacular cell death. We found that ablation of sustentacular cells is sufficient for HBC activation to multipotency. By expression analysis, next-generation sequencing, and immunohistochemical examination, down-regulation of Notch pathway signaling is coincident with HBC activation. Therefore, using HBC-specific conditional knockout of Notch receptors and overexpression of N1ICD, we show that Notch signaling maintains p63 levels and HBC dormancy, in contrast to its suppression of p63 expression in other tissues. Additionally, Notch1, but not Notch2, is required to maintain HBC dormancy after selective neuronal degeneration. Taken together, our data indicate that the activation of HBCs observed after tissue injury or sustentacular cell ablation is caused by the reduction/elimination of Notch signaling on HBCs; elimination of Jagged1 expressed by sustentacular cells may be the ligand responsible.

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

Adult tissue stem cells can serve two broad functions: to participate actively in the maintenance and regeneration of a tissue or to wait in reserve and participate only when activated from a dormant state. The adult olfactory epithelium, a site for ongoing, life-long, robust neurogenesis, contains both of these functional stem cell types. Globose basal cells (GBCs) act as the active stem cell population and can give rise to all the differentiated cells found in the normal tissue. Horizontal basal cells (HBCs) act as reserve stem cells and remain dormant unless activated by tissue injury. Here we show that HBC activation following injury by the olfactotoxic gas methyl bromide is coincident with the down-regulation of protein 63 (p63) but anticipates HBC proliferation. Gain- and loss-of-function studies show that this down-regulation of p63 is necessary and sufficient for HBC activation. Moreover, activated HBCs give rise to GBCs that persist for months and continue to act as bona fide stem cells by participating in tissue maintenance and regeneration over the long term. Our analysis provides mechanistic insight into the dynamics between tissue stem cell subtypes and demonstrates that p63 regulates the reserve state but not the stem cell status of HBCs.