Temporal gene expression profiles of target-ablated olfactory epithelium in mice with disrupted expression of scavenger receptor A: impact on macrophages

Distinct olfactory mucosal macrophage populations mediate neuronal maintenance and pathogen defense

The olfactory mucosa is important for both the sense of smell and as a mucosal immune barrier to the upper airway and brain. However, little is known about how the immune system mediates the conflicting goals of neuronal maintenance and inflammation in this tissue. A number of immune cell populations reside within the olfactory mucosa and yet we have little understanding of how these resident olfactory immune cells functionally interact with the chemosensory environment. Identifying these interactions will allow therapeutic manipulations that treat disorders such as post-viral olfactory dysfunction. Macrophages are the most prevalent immune cell type in the uninflamed olfactory mucosa and here, we identify two distinct tissue macrophage populations in murine olfactory mucosa. P2ry12hi macrophages are transcriptionally specialized for neuron interactions, closely associated with olfactory neuron cell bodies, long-term tissue residents, and functionally specialized to phagocytose cells and debris, including olfactory neurons. Conversely, MHC Class IIhi macrophages are transcriptionally dedicated to cytokine production and antigen presentation, localized primarily within the olfactory lamina propria, more rapidly replaced by blood monocytes, and rapidly produce chemokines in response to viral infection. We further show that these macrophage signatures are present in human olfactory biopsies, and P2ry12-like olfactory macrophages are reduced in patients with long-term smell loss following COVID-19. Together, these data show that two olfactory macrophage populations regulate neurons and initiate the immune response, contributing to our understanding of both olfactory immunity and tissue-resident macrophage biology.

Immune responses in the injured olfactory and gustatory systems: a role in olfactory receptor neuron and taste bud regeneration?

Sensory cells that specialize in transducing olfactory and gustatory stimuli are renewed throughout life and can regenerate after injury unlike their counterparts in the mammalian retina and auditory epithelium. This uncommon capacity for regeneration offers an opportunity to understand mechanisms that promote the recovery of sensory function after taste and smell loss. Immune responses appear to influence degeneration and later regeneration of olfactory sensory neurons and taste receptor cells. Here we review surgical, chemical, and inflammatory injury models and evidence that immune responses promote or deter chemosensory cell regeneration. Macrophage and neutrophil responses to chemosensory receptor injury have been the most widely studied without consensus on their net effects on regeneration. We discuss possible technical and biological reasons for the discrepancy, such as the difference between peripheral and central structures, and suggest directions for progress in understanding immune regulation of chemosensory regeneration. Our mechanistic understanding of immune-chemosensory cell interactions must be expanded before therapies can be developed for recovering the sensation of taste and smell after head injury from traumatic nerve damage and infection. Chemosensory loss leads to decreased quality of life, depression, nutritional challenges, and exposure to environmental dangers highlighting the need for further studies in this area.

Global expression profiling of globose basal cells and neurogenic progression within the olfactory epithelium

Ongoing, lifelong neurogenesis maintains the neuronal population of the olfactory epithelium in the face of piecemeal neuronal turnover and restores it following wholesale loss. The molecular phenotypes corresponding to different stages along the progression from multipotent globose basal cell (GBC) progenitor to differentiated olfactory sensory neuron are poorly characterized. We used the transgenic expression of enhanced green fluorescent protein (eGFP) and cell surface markers to FACS-isolate ΔSox2-eGFP(+) GBCs, Neurog1-eGFP(+) GBCs and immature neurons, and ΔOMP-eGFP(+) mature neurons from normal adult mice. In addition, the latter two populations were also collected 3 weeks after olfactory bulb ablation, a lesion that results in persistently elevated neurogenesis. Global profiling of mRNA from the populations indicates that all stages of neurogenesis share a cohort of >2,100 genes that are upregulated compared to sustentacular cells. A further cohort of >1,200 genes are specifically upregulated in GBCs as compared to sustentacular cells and differentiated neurons. The increased rate of neurogenesis caused by olfactory bulbectomy had little effect on the transcriptional profile of the Neurog1-eGFP(+) population. In contrast, the abbreviated lifespan of ΔOMP-eGFP(+) neurons born in the absence of the bulb correlated with substantial differences in gene expression as compared to the mature neurons of the normal epithelium. Detailed examination of the specific genes upregulated in the different progenitor populations revealed that the chromatin modifying complex proteins LSD1 and coREST were expressed sequentially in upstream ΔSox2-eGFP(+) GBCs and Neurog1-eGFP(+) GBCs/immature neurons. The expression patterns of these proteins are dynamically regulated after activation of the epithelium by methyl bromide lesion.

The conserved scavenger receptor cysteine-rich superfamily in therapy and diagnosis

The scavenger receptor cysteine-rich (SRCR) superfamily of soluble or membrane-bound protein receptors is characterized by the presence of one or several repeats of an ancient and highly conserved protein module, the SRCR domain. This superfamily (SRCR-SF) has been in constant and progressive expansion, now up to more than 30 members. The study of these members is attracting growing interest, which parallels that in innate immunity. No unifying function has been described to date for the SRCR domains, this being the result of the limited knowledge still available on the physiology of most members of the SRCR-SF, but also of the sequence versatility of the SRCR domains. Indeed, involvement of SRCR-SF members in quite different functions, such as pathogen recognition, modulation of the immune response, epithelial homeostasis, stem cell biology, and tumor development, have all been described. This has brought to us new information, unveiling the possibility that targeting or supplementing SRCR-SF proteins could result in diagnostic and/or therapeutic benefit for a number of physiologic and pathologic states. Recent research has provided structural and functional insight into these proteins, facilitating the development of means to modulate the activity of SRCR-SF members. Indeed, some of these approaches are already in use, paving the way for a more comprehensive use of SRCR-SF members in the clinic. The present review will illustrate some available evidence on the potential of well known and new members of the SRCR-SF in this regard.

Cellular and molecular characterization of oxidative stress in olfactory epithelium of Harlequin mutant mouse

Oxidative stress in the olfactory system is a major factor associated with age-related olfactory impairment, although the mechanisms by which this occurs are not completely understood. The Harlequin mutant mouse (Hq/Y), which carries an X-linked recessive mutation in the Aifm1 gene, is a model of progressive oxidative stress-induced neurodegeneration in the cerebellum and retina. To determine whether the Hq/Y mutant mouse is a suitable model of oxidative stress-associated olfactory aging, we investigated cellular and molecular changes in the olfactory epithelium (OE) and olfactory bulb (OB) of 6-month-old male Hq/Y mice compared to those in sex-matched littermate controls (+/Y) and in age- and sex-matched C57BL/6 mice. Immunoreactivity for apoptosis-inducing factor, the protein product of Aifm1, was localized in mature olfactory sensory neurons (mOSNs) in +/Y mice but was rarely detected in Hq/Y mice. Hq/Y mice also exhibited increased lipofuscin autofluorescence and increased immunoreactivity for an oxidative DNA/RNA damage marker in mOSNs and in mitral/tufted cells in the OB and an increased number of cleaved caspase-3 immunoreactive apoptotic cells in the OE. Microarray analysis demonstrated that Aifm1 expression was down-regulated by 80% in the OE of Hq/Y mice compared to that in +/Y mice. Most significantly, regulated genes were classified into functional categories of cell signaling/apoptosis/cell cycle, oxidative stress/aging, and cytoskeleton/extracellular matrix/transport-associated. Analysis with EASE software indicated that the functional categories significantly overrepresented in Hq/Y mice included up-regulated mitochondrial genes and down-regulated cytoskeletal organization- and neurogenesis-related genes. Our results strongly support the Hq/Y mutant mouse being a novel model for mechanistic studies of oxidative stress-associated olfactory aging.

Macrophage scavenger receptors and host-derived ligands

The scavenger receptors are a large family of molecules that are structurally diverse and have been implicated in a range of functions. They are expressed by myeloid cells, selected endothelial cells and some epithelial cells and recognise many different ligands, including microbial pathogens as well as endogenous and modified host-derived molecules. This review will focus on the eight classes of scavenger receptors (class A-H) in terms of their structure, expression and recognition of host-derived ligands. Scavenger receptors have been implicated in a range of physiological and pathological processes, such as atherosclerosis and Alzheimer's disease, and function in adhesion and tissue maintenance. More recently, some of the scavenger receptors have been shown to mediate binding and endocytosis of chaperone proteins, such as the heat shock proteins, thereby playing an important role in antigen cross-presentation.

Apoptosis and the insulin-like growth factor family in the developing olfactory epithelium

Vertebrate olfactory receptor neurons (ORN) are unique in that they are continually replaced throughout life. They die by apoptosis under physiological conditions at all stages during the life cycle, and apoptotic ORN are replaced by their progenitor cells. Apoptosis is linked with neurogenesis, of which pathway is regulated by a number of growth factors and neurotrophic factors. Members of the insulin-like growth factor (IGF) family have an anti-apoptotic effect on ORN, in addition to their ability to promote the proliferation, differentiation, and survival of these neurons. Expression of IGF and related molecules at both mRNA and protein levels in the olfactory epithelium have been reported. In this review article, we focus on apoptosis, IGF, and their related molecules in the developing olfactory epithelium.

Macrophage-mediated neuroprotection and neurogenesis in the olfactory epithelium

Resident and recruited olfactory epithelial macrophages participate in the regulation of the survival, degeneration, and replacement of olfactory sensory neurons (OSNs). We have reported that liposome-encapsulated clodronate (Lip-C) induced selective and statistically significant depletion of macrophages in the OE of sham and 48 h OBX mice (38 and 35%, respectively) that resulted in increased OSN apoptosis and decreased numbers of mature OSNs and proliferating basal cells compared to controls (Lip-O). The aim of this study was to identify molecular mechanisms by which the selective depletion of macrophages in the OE resulted in these cellular changes by using a microarray expression pattern analysis. A 2x2 ANOVA identified 4,085 overall significantly (P < 0.01) regulated genes in the OE of Lip-O and Lip-C sham and 48 h OBX mice, and further statistical analysis using pairwise comparisons identified 4,024 genes that had either a significant (P < 0.01) treatment main effect (n = 2,680), group main effect (n = 778), or interaction effect (n = 980). The mean hybridization signals of immune response genes, e.g., Cxcr4, and genes encoding growth factors and neurogenesis regulators, e.g., Hdgf and Neurod1, respectively, were primarily lower in Lip-C mice compared with Lip-O mice. Apoptosis genes, e.g., Bak1, were also differentially regulated in Lip-C and/or OBX mice. Expression patterns of selected genes were validated with real-time RT-PCR; immunohistochemistry was used to localize selected gene products. These results identified the differential regulation of several novel genes through which alternatively activated macrophages regulate OSN progenitor cell proliferation, differentiation, and maturation, and the survival of OSNs.

Macrophage depletion in the murine olfactory epithelium leads to increased neuronal death and decreased neurogenesis

Apoptosis of olfactory sensory neurons (OSNs) induced by olfactory bulbectomy (OBX) leads to the activation of resident macrophages within the olfactory epithelium (OE). These macrophages phagocytose degenerating OSNs and secrete chemokines, which recruit additional macrophages into the OE, and cytokines/growth factors, which regulate basal cell proliferation and differentiation and maturation of OSNs. In this study we apply for the first time the use of liposome-encapsulated clodronate to selectively deplete macrophages during the OSN degeneration/regeneration cycle in order to elucidate the role(s) of macrophages in regulating cellular mechanisms that lead to apoptosis and neurogenesis. Mice were injected intranasally and intravenously with either liposome-encapsulated clodronate or empty liposomes prior to and after OBX or sham OBX. At 48 hours after surgery the numbers of macrophages in the OE of both sham and OBX clodronate-treated mice were significantly reduced compared to liposome-treated controls (38% and 35%, respectively, P < 0.05). The reduction in macrophage numbers was accompanied by significant decreases in OE thickness (22% and 21%, P < 0.05), the number of mOSNs (1.2- and 1.9-fold, P < 0.05), and basal cell proliferation (7.6- and 3.8-fold, P < 0.005) in sham and OBX mice, respectively, compared to liposome-treated controls. In OBX mice there was also increased immunoreactivity for active caspase-3 in the OE and olfactory nerves of clodronate-treated OBX mice compared to liposome-treated controls. These results indicate that macrophages modulate the OSN population in the normal and target-ablated murine OE by influencing neuronal survival and basal cell proliferation, resulting in neurogenesis and replacement of mature OSNs.