Gene Expression Profile of Olfactory Transduction Signaling in an Animal Model of Human Multiple Sclerosis

RNA sequencing of olfactory bulb in Parkinson's disease reveals gene alterations associated with olfactory dysfunction

The olfactory bulb is involved early in the pathophysiology of Parkinson's disease (PD), which is consistent with the early onset of olfactory dysfunction. Identifying the molecular mechanisms through which PD affects the olfactory bulb could lead to a better understanding of the pathophysiology and etiology of olfactory dysfunction in PD. We specifically aimed to assess gene expression changes, affected pathways and co-expression network by whole transcriptomic profiling of the olfactory bulb in subjects with clinicopathologically defined PD. Bulk RNA sequencing was performed on frozen human olfactory bulbs of 20 PD and 20 controls without dementia or any other neurodegenerative disorder, from the Arizona Study of Aging and Neurodegenerative disorders and the Brain and Body Donation Program. Differential expression analysis (19 PD vs 19 controls) revealed 2164 significantly differentially expressed genes (1090 upregulated and 1074 downregulated) in PD. Pathways enriched in downregulated genes included oxidative phosphorylation, olfactory transduction, metabolic pathways, and neurotransmitters synapses while immune and inflammatory responses as well as cellular death related pathways were enriched within upregulated genes. An overrepresentation of microglial and astrocyte-related genes was observed amongst upregulated genes, and excitatory neuron-related genes were overrepresented amongst downregulated genes. Co-expression network analysis revealed significant modules highly correlated with PD and olfactory dysfunction that were found to be involved in the MAPK signaling pathway, cytokine-cytokine receptor interaction, cholinergic synapse, and metabolic pathways. LAIR1 (leukocyte associated immunoglobulin like receptor 1) and PPARA (peroxisome proliferator activated receptor alpha) were identified as hub genes with a high discriminative power between PD and controls reinforcing an important role of neuroinflammation in the olfactory bulb of PD subjects. Olfactory identification test score positively correlated with expression of genes coding for G-coupled protein, glutamatergic, GABAergic, and cholinergic receptor proteins and negatively correlated with genes for proteins expressed in glial olfactory ensheathing cells. In conclusion, this study reveals gene alterations associated with neuroinflammation, neurotransmitter dysfunction, and disruptions of factors involved in the initiation of olfactory transduction signaling that may be involved in PD-related olfactory dysfunction.

Changes in structural plasticity of hippocampal neurons in an animal model of multiple sclerosis

Structural plasticity is critical for the functional diversity of neurons in the brain. Experimental autoimmune encephalomyelitis (EAE) is the most commonly used model for multiple sclerosis (MS), successfully mimicking its key pathological features (inflammation, demyelination, axonal loss, and gliosis) and clinical symptoms (motor and non-motor dysfunctions). Recent studies have demonstrated the importance of synaptic plasticity in EAE pathogenesis. In the present study, we investigated the features of behavioral alteration and hippocampal structural plasticity in EAE-affected mice in the early phase (11 days post-immunization, DPI) and chronic phase (28 DPI). EAE-affected mice exhibited hippocampus-related behavioral dysfunction in the open field test during both early and chronic phases. Dendritic complexity was largely affected in the cornu ammonis 1 (CA1) and CA3 apical and dentate gyrus (DG) subregions of the hippocampus during the chronic phase, while this effect was only noted in the CA1 apical subregion in the early phase. Moreover, dendritic spine density was reduced in the hippocampal CA1 and CA3 apical/basal and DG subregions in the early phase of EAE, but only reduced in the DG subregion during the chronic phase. Furthermore, mRNA levels of proinflammatory cytokines ( Il1β, Tnfα, and Ifnγ) and glial cell markers ( Gfap and Cd68) were significantly increased, whereas the expression of activity-regulated cytoskeleton-associated protein (ARC) was reduced during the chronic phase. Similarly, exposure to the aforementioned cytokines in primary cultures of hippocampal neurons reduced dendritic complexity and ARC expression. Primary cultures of hippocampal neurons also showed significantly reduced extracellular signal-regulated kinase (ERK) phosphorylation upon treatment with proinflammatory cytokines. Collectively, these results suggest that autoimmune neuroinflammation alters structural plasticity in the hippocampus, possibly through the ERK-ARC pathway, indicating that this alteration may be associated with hippocampal dysfunctions in EAE.

Vitamin D levels and olfactory dysfunction in multiple sclerosis patients

OBJECTIVE

The objective was to explore the possible relationship between the serum vitamin D level and olfactory impairment in a population of multiple sclerosis (MS) patients in Guizhou, China.

METHODS

We included 25 patients with MS and 18 healthy controls (HCs) who were recruited from the Affiliated Hospital of Guizhou Medical University from February 2021 to September 2021. The University of Pennsylvania Smell Identification Test (UPSIT) was used to test the patients' sense of smell, and the level of serum 25-hydroxyethylene polyprotein D was measured.

RESULTS

Serum vitamin D levels and UPSIT scores were significantly different between the MS group and the control group (both p < 0.001). Moreover, a significant positive correlation emerged between vitamin D levels and UPSIT scores in MS patients (r = 0.537, p = 0.021).

CONCLUSIONS

The serum vitamin D level may be involved in the regulation of olfactory dysfunction in MS patients in Guizhou, China.

Key Genes in Olfactory Disorder in Experimental Autoimmune Encephalomyelitis Identified by Transcriptomic Analysis of the Olfactory Bulbs

Experimental autoimmune encephalomyelitis (EAE) is an animal model of multiple sclerosis that shows demyelination in the central nervous system and functional deficits, including olfactory impairment. However, the genes related to olfactory impairment in EAE are unknown. We evaluated hub genes of the olfactory bulb in EAE mice. Differentially expressed genes (cut-offs, fold change > 2 and adjusted p < 0.05) and their related pathways in olfactory bulbs were subjected to gene ontology (GO) pathway analysis, gene set enrichment analysis (GSEA). Protein-protein interactions with selected genes were evaluated using the Search Tool for the Retrieval of Interacting Genes/Proteins. Gene regulatory networks (GRNs) which were constructed at the post-transcriptional level, including the genes-transcription factors (TFs) and gene-microRNAs (miRNAs) interaction networks. Twelve hub genes were found, three of which (Ctss, Itgb2, and Tlr2) were validated by RT-qPCR to be related to GO pathways such as immune response and regulation of immune response. GSEA showed that neuron-related genes-including Atp6v1g2, Egr1, and Gap43-and their pathways were significantly downregulated. GRNs analysis of six genes (Ctss, Itgb2, Tlr2, Atp6v1g2, Egr1, and Gap43) revealed 37 TFs and 84 miRNAs were identified as potential regulators of six genes, indicating significant interaction among six genes, TFs, and miRNAs. Collectively, these results suggest that transcriptomic analysis of the olfactory bulb of EAE mice can provide insight into olfactory dysfunction and reveal therapeutic targets for olfactory impairment.

Retinal transcriptome profiling identifies novel candidate genes associated with visual impairment in a mouse model of multiple sclerosis

Visual impairment is occasionally observed in multiple sclerosis (MS) and its animal model, experimental autoimmune encephalomyelitis (EAE). Although uveitis and optic neuritis have been reported in MS and EAE, the precise mechanisms underlying the pathogenesis of these visual impairments remain poorly understood. This study aims to identify differentially expressed genes (DEGs) in the retinas of mice with EAE to identify genes that may be implicated in EAE-induced visual impairment. Fourteen adult mice were injected with myelin oligodendrocyte glycoprotein35-55 to induce the EAE model. Transcriptomes of retinas with EAE were analyzed by RNA-sequencing. Gene expression analysis revealed 347 DEGs in the retinas of mice with EAE: 345 were upregulated, and 2 were downregulated (adjusted p-value < 0.05 and absolute log2 fold change > 1). Gene ontology (GO) analysis showed that the upregulated genes in the retinas of mice with EAE were primarily related to immune responses, responses to external biotic stimuli, defense responses, and leukocyte-mediated immunity in the GO biological process. The expression of six upregulated hub genes (c1qb, ctss, itgam, itgb2, syk, and tyrobp) from the STRING analysis and the two significantly downregulated DEGs (hapln1 and ndst4) were validated by reverse transcription-quantitative polymerase chain reaction. In addition, gene set enrichment analysis showed that the negatively enriched gene sets in EAE-affected retinas were associated with the neuronal system and phototransduction cascade. This study provides novel molecular evidence for visual impairments in EAE and indicates directions for further research to elucidate the mechanisms of these visual impairments in MS.

Transcriptome Profiling in the Hippocampi of Mice with Experimental Autoimmune Encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS), approximates the key histopathological, clinical, and immunological features of MS. Hippocampal dysfunction in MS and EAE causes varying degrees of cognitive and emotional impairments and synaptic abnormalities. However, the molecular alterations underlying hippocampal dysfunctions in MS and EAE are still under investigation. The purpose of this study was to identify differentially expressed genes (DEGs) in the hippocampus of mice with EAE in order to ascertain potential genes associated with hippocampal dysfunction. Gene expression in the hippocampus was analyzed by RNA-sequencing and validated by reverse transcription-quantitative polymerase chain reaction (RT-qPCR). Gene expression analysis revealed 1202 DEGs; 1023 were upregulated and 179 were downregulated in the hippocampus of mice with EAE (p-value < 0.05 and fold change >1.5). Gene ontology (GO) analysis showed that the upregulated genes in the hippocampi of mice with EAE were associated with immune system processes, defense responses, immune responses, and regulation of immune responses, whereas the downregulated genes were related to learning or memory, behavior, and nervous system processes in the GO biological process. The expressions of hub genes from the search tool for the retrieval of interacting genes/proteins (STRING) analysis were validated by RT-qPCR. Additionally, gene set enrichment analysis showed that the upregulated genes in the hippocampus were associated with inflammatory responses: interferon-γ responses, allograft rejection, interferon-α responses, IL6_JAK_STAT3 signaling, inflammatory responses, complement, IL2_STAT5 signaling, TNF-α signaling via NF-κB, and apoptosis, whereas the downregulated genes were related to synaptic plasticity, dendritic development, and development of dendritic spine. This study characterized the transcriptome pattern in the hippocampi of mice with EAE and signaling pathways underpinning hippocampal dysfunction. However, further investigation is needed to determine the applicability of these findings from this rodent model to patients with MS. Collectively, these results indicate directions for further research to understand the mechanisms behind hippocampal dysfunction in EAE.

Effects of In Vivo Gluten Challenge on PBMC Gene Expression Profiles in Diet Treated Celiac Disease

The pathological mechanisms that lead to the onset and reactivation of celiac disease (CD) remain largely unknown. While gluten free diet (GFD) improves the intestinal damage and associated clinical symptoms in majority of cases, it falls short of providing full recovery. Additionally, late or misdiagnosis is also common as CD presents with a wide range of symptoms. Clear understanding of CD pathogenesis is thus critical to address both diagnostic and treatment concerns. We aimed to study the molecular impact of short gluten exposure in GFD treated CD patients, as well as identify biological pathways that remain altered constitutively in CD regardless of treatment. Using RNAseq profiling of PBMC samples collected from treated CD patients and gluten challenged patient and healthy controls, we explored the peripheral transcriptome in CD patients following a short gluten exposure. Short gluten exposure of just three days was enough to alter the genome-wide PBMC transcriptome of patients. Pathway analysis revealed gluten-induced upregulation of mainly immune response related pathways, both innate and adaptive, in CD patients. We evaluated the perturbation of biological pathways in sample-specific manner. Compared to gluten exposed healthy controls, pathways related to tight junction, olfactory transduction, metabolism of unsaturated fatty acids (such as arachidonic acid), metabolism of amino acids (such as cysteine and glutamate), and microbial infection were constitutively altered in CD patients regardless of treatment, while GFD treatment appears to mostly normalize immune response pathways to "healthy" state. Upstream regulator prediction analysis using differentially expressed genes identified constitutively activated regulators relatively proximal to previously reported CD associated loci, particularly SMARCA4 on 19p13.2 and CSF2 on 5q31. We also found constitutively upregulated genes in CD that are in CD associated genetic loci such as MEF2BNB-MEF2B (BORCS8-MEF2B) on 19p13.11 and CSTB on 21q22.3. RNAseq revealed strong effects of short oral gluten challenge on whole PBMC fraction and constitutively altered pathways in CD PBMC suggesting important factors other than gluten in CD pathogenesis.

Upregulation of Cathepsins in Olfactory Bulbs Is Associated with Transient Olfactory Dysfunction in Mice with Experimental Autoimmune Encephalomyelitis

Cathepsins are a family of lysosomal/endosomal proteolytic enzymes that include serine, aspartate, and cysteine proteases. The role of cathepsin in neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease, remains elusive. We evaluated the expression level and localization of different cathepsins in the olfactory bulbs of mice with experimental autoimmune encephalomyelitis (EAE), a model of human multiple sclerosis. Quantitative real-time PCR results and Western blotting analyses revealed that serine, aspartate, and cysteine cathepsins are expressed at significantly higher levels in the olfactory bulbs of mice with EAE in the paralytic stage compared with those of control mice. Immunohistochemical analyses indicated that cathepsin A, D, and S were expressed in the glomerulus layer, external plexiform layer, and mitral cell layer. Furthermore, cathepsins were detected in astrocytes, microglia, inflammatory cells, and vascular cells in the olfactory bulb of EAE mice at the paralytic stage. Collectively, these results suggest that the upregulation of cathepsins in the olfactory bulb of mice with EAE is associated with transient olfactory dysfunction in autoimmune encephalomyelitis.

Glycan changes in the olfactory mucosa of rats with experimental autoimmune encephalomyelitis

Glycans are components of glycoconjugates and function in odorant recognition and cell signaling in the olfactory mucosa. However, little is known about glycan expression in the olfactory mucosa in the presence of neuroinflammatory disorders, which can influence olfaction. We evaluated the changes in glycan in the olfactory mucosa of rats with experimental autoimmune encephalomyelitis (EAE) by histochemical analyses of 21 lectins. In the olfactory mucosa of normal control rats, 16 lectins bound to olfactory sensory neurons, supporting cells, basal cells, nerve and Bowman's glands, and their expression did not significantly change during the course of EAE. In rats with paralytic-stage EAE, five lectins showed different reactivities with the olfactory mucosa compared to those of normal control rats. Of them, Bandeiraea simplicifolia lectin (BSL)-II and BSL-I showed transiently downregulated binding to olfactory sensory neurons and supporting cells in rats with EAE. The reactivities of Lens culinaris agglutinin for the basement membrane, Vicia villosa agglutinin for Bowman's glands and Dolichos biflorus agglutinin for all nuclei were upregulated in the olfactory mucosa of EAE rats. These results suggest that BSL-II-binding N-acetyl-glucosamine and BSL-I-binding N-acetyl-galactose are involved in transient olfactory dysfunction in EAE, which may hamper odor perception and/or signal processing in olfactory sensory neurons.

Microglial and astroglial reaction in the olfactory bulb of mice after Triton X-100 application

Gliosis including microgliosis and astrogliosis is a response to central nervous system inflammation. The purpose of this study was to evaluate whether olfactory bulbs are influenced by intranasal exposure to the detergent Triton X-100, a non-ionic surfactant. In this experiment, we measured olfactory function in mice based on the time needed to identify hidden pellets. Our results found that more time was needed to find the buried pellets by mice exposed to Triton X-100 compared with mice without Triton X-100 exposure, up to day 7. Histopathological examination revealed inflammatory cells in the olfactory mucosa and olfactory bulbs in mice treated with Triton X-100. Western blot analysis revealed significant downregulation of olfactory marker proteins in the olfactory mucosa and bulbs of mice after intranasal exposure to Triton X-100. In the olfactory bulbs of mice exposed to Triton X-100, microgliosis and astrogliosis were evident using immunohistochemistry. Cathepsin D was also upregulated in Iba-1-positive microglia/macrophages and GFAP-positive astrocytes in the olfactory bulbs of mice exposed to Triton X-100. In mice, Triton X-100 induced olfactory sensory neuron death in the nasal cavity and gliosis in olfactory bulbs with concurrent downregulation of olfactory marker protein expression, resulting in transient olfactory dysfunction.