Target ablation-induced regulation of macrophage recruitment into the olfactory epithelium of Mip-1alpha-/- mice and restoration of function by exogenous MIP-1alpha

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.

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-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.

Oxidative stress in the aging murine olfactory bulb: redox proteomics and cellular localization

A recent proteomics analysis from our laboratory demonstrated that several oxidative stress response proteins showed significant changes in steady-state levels in olfactory bulbs (OBs) of 20- vs. 1.5-month-old mice. Oxidative stress may result in protein oxidation. In this study, we investigated two forms of protein oxidative modification in murine OBs: carbonylation and nitration. Redox proteomics with two-dimensional gel electrophoresis, Western blotting, protein digestion, and mass spectrometry was used to quantify total and specific protein carbonylation and to identify differentially carbonylated proteins and determine the carbonylation status of previously identified proteins in OBs of 1.5- and 20-month-old mice. Immunohistochemistry was used to demonstrate the relative intensity and localization of protein nitration in OBs of 1.5-, 6-, and 20-month-old mice. Total protein carbonylation was significantly greater in OBs of 20- vs. 1.5-month-old mice. Aldolase 1 (ALDO1) showed significantly more carbonylation in OBs from 20- vs. 1.5-month-old mice; heat shock protein 9A and dihydropyrimidinase-like 2 showed significantly less. Several previously investigated proteins were also carbonylated, including ferritin heavy chain (FTH). Nitration, identified by 3-nitrotyrosine immunoreactivity, was least abundant at 1.5 months, intermediate at 6 months, and greatest at 20 months and was localized primarily in blood vessels. Proteins that were specific targets of oxidation were also localized: ALDO1 in astrocytes of the granule cell layer and FTH in mitral/tufted cells. These results indicate that specific carbonylated proteins, including those in astrocytes and mitral/tufted neurons, and nitrated proteins in the vasculature are molecular substrates of age-related olfactory dysfunction.

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.

Temporal mRNA profiles of inflammatory mediators in the murine 1-methyl-4-phenyl-1,2,3,6-tetrahydropyrimidine model of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc). With the exception of a few rare familial forms of the disease, the precise molecular mechanisms underlying PD are unknown. Inflammation is a common finding in the PD brain, but due to the limitation of postmortem analysis its relationship to disease progression cannot be established. However, studies using the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of PD have also identified inflammatory responses in the nigrostriatal pathway that precede neuronal degeneration in the SNpc. To assess the pathological relevance of these inflammatory responses and to identify candidate genes that might contribute to neuronal vulnerability, we used quantitative reverse-transcription polymerase chain reaction (qRT-PCR) to measure mRNA levels of 11 cytokine and chemokine encoding genes in the striatum of MPTP-sensitive (C57BL/6J) and MPTP-insensitive (Swiss Webster, SWR) mice following administration of MPTP. The mRNA levels of all 11 genes changed following MPTP treatment, indicating the presence of inflammatory responses in both strains. Furthermore, of the 11 genes examined only 3, interleukin 6 (Il-6), macrophage inflammatory protein 1 alpha/CC chemokine ligand 3 (Mip-1alpha/Ccl3) and macrophage inflammatory protein 1 beta/CC chemokine ligand 4 (Mip-1beta/Ccl4), were differentially regulated between C57BL/6J and SWR mice. In both mouse strains, the level of monocyte chemoattractant protein 1/CC chemokine ligand 2 (Mcp-1/Ccl2) mRNA was the first to increase following MPTP administration, and might represent a key initiating component of the inflammatory response. Using Mcp-1/Ccl2 knockout mice backcrossed onto a C57BL/6J background we found that MPTP-stimulated Mip-1alpha/Ccl3 and Mip-1beta/Ccl4 mRNA expression was significantly lower in the knockout mice; suggesting that Mcp-1/Ccl2 contributes to MPTP-enhanced expression of Mip-1alpha/Ccl3 and Mip-1beta/Ccl4. However, stereological analysis of SNpc neuronal loss in Mcp-1/Ccl2 knockout and wild-type mice showed no differences. These findings suggest that it is the ability of dopaminergic SNpc neurons to survive an inflammatory insult, rather than genetically determined differences in the inflammatory response itself, that underlie the molecular basis of MPTP resistance.

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

Target ablation [removal of the olfactory bulb (OBX)] induces apoptotic death of olfactory sensory neurons (OSNs) and an immune response in which activation and recruitment of macrophages (ms) into the olfactory epithelium (OE) occupy a central role. Ms phagocytose apoptotic neurons and secrete cytokines/growth factors that regulate subsequent progenitor cell proliferation and neurogenesis. Scavenger receptor A (SR-A) is a pattern recognition receptor that mediates binding of ms to apoptotic cells and other relevant immune response functions. The aim of this study was to determine the impact of the absence of SR-A on the immune response to OBX. The immune response to OBX was evaluated in mice in which functional expression of the m scavenger receptor (MSR) was eliminated by gene disruption (MSR-/-) and wild-type (wt) mice of the same genetic background. OBX induced significant apoptotic death of mature OSNs in the two strains. However, subsequent m infiltration and activation and progenitor cell proliferation were significantly reduced in MSR-/- vs. wt mice. Gene expression profiling at short intervals after OBX demonstrated significant differences in temporal patterns of expression of several gene categories, including immune response genes. Many immune response genes that showed different temporal patterns of expression are related to m function, including cytokine and chemokine secretion, phagocytosis, and m maturation and activation. These studies suggest that impairment of the immune response to OBX in the OE of MSR-/- mice most likely resulted from decreased m adhesion and subsequent reduced infiltration and activation, with a resultant decrease in neurogenesis.

Leptin regulates olfactory-mediated behavior in ob/ob mice

We have investigated olfactory-mediated pre-ingestive behavior in leptin (ob/ob) and leptin receptor (db/db) mutant mice compared to age- and gender-matched wild-type (wt) mice. Olfactory-mediated behavior was tested using a buried food paradigm 5 times/day at 2-h intervals for 6 days. Mean food-finding times of ob/ob and db/db mice were approximately 10 times shorter than those of wt mice. To test the effect of leptin replacement in ob/ob mice, leptin (1 or 5 microg/g body weight in sterile saline) or carrier was injected i.p. once daily prior to testing. Mean food finding times in ob/ob mice injected with carrier or with 1 microg/g leptin were similar and were 2-3 times faster than in wt mice. Mean food finding times in ob/ob mice injected with 5 microg/g leptin tripled compared to carrier-injected ob/ob mice and were of the same order of magnitude as those of wt mice, suggesting functional leptin replacement. A 3-factor repeated measures ANOVA demonstrated significant differences between the 6 cohorts (P = 0.0001), food finding times (P< or = 0.0001), and cohort by day interaction (P< or = 0.0001). Post hoc tests suggested that the ob/ob+5 mug/g leptin cohort performed more like the wt cohort in the food-finding test than like the ob/ob or ob/ob+carrier cohort. Potential local sites of leptin production and action were identified with immunohistochemistry and in situ hybridization in epithelial and gland cells of the olfactory and nasal mucosae. Our results strongly suggest that leptin acting through leptin receptors modulates olfactory-mediated pre-ingestive behavior.

Proteomic identification of differentially expressed proteins in the aging murine olfactory system and transcriptional analysis of the associated genes

Decline in olfactory ability has been associated with aging as well as neurodegenerative disorders. The aim of this study was to gain fundamental insight into molecular events associated with the aging olfactory system. We report a comparative proteomic analysis of the olfactory epithelium (OE) and olfactory bulb (OB) of old (80-week old) and young (6-week old) mice with further analysis of age-related differences in differentially expressed proteins at the mRNA level using real-time RT-PCR. Nine proteins in the OE and 20 in the OB were differentially expressed in old and young mice; of these, aldolase 1, peptidyl prolyl isomerase A, mitochondrial aconitase 2, mitochondrial aldehyde dehydrogenase 2 and albumin 1 were identified in the OE; and ATP synthase isoform 1, enolase 1, ferritin heavy chain, malate dehydrogenase 1, tropomyosin alpha 3 chain and dynamin 1 were identified in the OB. At the transcriptional level, aconitase 2 in the OE and ferritin heavy chain 1 in the OB were differentially expressed with aging, in concordance with the proteomic data. Our results demonstrate an altered proteomic profile of the aged murine olfactory system. The identified proteins fall into three broadly defined functional categories: (i) metabolism, (ii) transport/motility and (iii) stress response. Our transcriptional analysis provides insight into possible mechanisms by which protein expression may be regulated in the OE and OB. The results are discussed in relation to the decrement in olfactory sensitivity with aging.

Temporal profiling of gene expression during neurogenesis and remodeling in the olfactory epithelium at short intervals after target ablation

Neurogenesis in the olfactory epithelium (OE) is induced by olfactory bulbectomy (OBX), which effectively axotomizes olfactory sensory neurons (OSNs) and removes their synaptic targets, resulting in apoptosis. We used Affymetrix high-density oligonucleotide arrays to investigate changes in gene expression during initiation of signaling in pathways that regulate apoptosis and neurogenesis in the murine OE at 2, 8, 16, and 48 hr after bilateral OBX compared to that in sham-operated controls. We focused on regulation of a defined set of genes associated with apoptosis, stem/progenitor cell regulation, and cell cycle progression because of the activation of these processes in OE degeneration and remodeling after OBX. After data scrubbing and categorical analysis, one-way analysis of variance identified 72 genes (4.9% of the present known genes) as being regulated significantly (P < 0.05) at one or more points; 50 were defined as regulated differentially with the false discovery rate at 10%. Significant changes in gene expression occurred in all categories as early as 2 hr post-OBX, with the greatest number of differentially regulated genes at 16 and 48 hr. Hierarchical cluster analysis and correlation coefficients were used to identify similarities in patterns of gene expression changes within and across categories. Validation was carried out with SuperArray macroarrays and real-time RT-PCR. Our results confirmed the participation of many genes in known signaling pathways and identified changes in the expression of 42 genes not identified previously as participating in apoptosis and neurogenesis in the OE. Additionally, our analyses indicated the early involvement of genes regulating cytoskeletal reorganization and angiogenesis in the response to OBX. These studies are an important first step in defining early time-dependent changes in gene expression after target ablation that lead to neurogenesis in the olfactory sensory epithelium.