Retinoid X receptor activation reverses age-related deficiencies in myelin debris phagocytosis and remyelination.

The efficiency of central nervous system remyelination declines with age. This is in part due to an age-associated decline in the phagocytic removal of myelin debris, which contains inhibitors of oligodendrocyte progenitor cell differentiation. In this study, we show that expression of genes involved in the retinoid X receptor pathway are decreased with ageing in both myelin-phagocytosing human monocytes and mouse macrophages using a combination of in vivo and in vitro approaches. Disruption of retinoid X receptor function in young macrophages, using the antagonist HX531, mimics ageing by reducing myelin debris uptake. Macrophage-specific RXRα (Rxra) knockout mice revealed that loss of function in young mice caused delayed myelin debris uptake and slowed remyelination after experimentally-induced demyelination. Alternatively, retinoid X receptor agonists partially restored myelin debris phagocytosis in aged macrophages. The agonist bexarotene, when used in concentrations achievable in human subjects, caused a reversion of the gene expression profile in multiple sclerosis patient monocytes to a more youthful profile and enhanced myelin debris phagocytosis by patient cells. These results reveal the retinoid X receptor pathway as a positive regulator of myelin debris clearance and a key player in the age-related decline in remyelination that may be targeted by available or newly-developed therapeutics.This work was supported by grants from the UK Multiple Sclerosis Society, Wellcome-Trust, NINDS/NIH Intramural Research Program, Health Research Board Scholars Program, Gates-Cambridge Scholarship, and Spanish Ministry of Economy and Competitiveness (SAF2012- 31483).S

Retinoid X Receptor activation reverses age-related deficiencies in myelin debris phagocytosis and CNS remyelination

Remyelination is a regenerative process that occurs through the formation of myelin sheaths by oligodendrocytes, which are recruited as oligodendrocyte progenitor cells (OPCs) after demyelination in diseases such as Multiple Sclerosis (MS).A key environmental factor regulating OPC differentiation is the fate of myelin debris generated during demyelination. Myelin debris contains inhibitors of OPC differentiation and thus its clearance by phagocytic macrophages is an important component of creating a lesion environment conducive to remyelination. The efficiency of debris clearance declines with age, contributing to the age-associated decline in remyelination. Therefore, understanding the mechanisms of the age-related decline in myelin debris phagocytosis is important for devising means to therapeutically reverse the decline in remyelination. The aim of this study was to determine the functional/molecular differences between young and old phagocytes involved in myelin debris clearance, thereby identifying therapeutically modifiable pathways associated with efficient myelin debris phagocytosis. In this study, we show that expression of genes involved in the retinoid X receptor (RXR) and peroxisome proliferator-activated receptor (PPAR) pathways are decreased with ageing in both myelin-phagocytosing human monocytes and mouse macrophages. Disruption of RXR and PPAR using synthetic antagonists in young macrophages mimics ageing by reducing myelin debris uptake. Macrophage-specific RXRα knockout mice revealed that loss of RXR function in young mice caused delayed myelin debris uptake and slowed remyelination. Alternatively, receptor agonists partially restored myelin debris phagocytosis in aged macrophages. The FDA-approved agonists bexarotene and pioglitazone, when used in concentrations achievable in human subjects, caused a reversion of the gene expression profiles in MS patient monocytes to a more youthful profile and enhanced myelin debris phagocytosis by patient cells. Activation of these pathways also enhances immunoregulatory markers on monocytes from MS patients, further suggesting the regeneration-promoting capacity of activating these pathways in phagocytes. These results reveal the RXR/PPAR pathway as a positive regulator of myelin debris clearance and a key player in the age-related decline in remyelination that may be targeted by available or newly-developed therapeutics.This work was supported by the Gates-Cambridge Scholarship and NIH-Cambridge Partnership Progra

Intranasal Delivery of E-Selectin Reduces Atherosclerosis in ApoE−/− Mice

Mucosal tolerance to E-selectin prevents stroke and protects against ischemic brain damage in experimental models of stroke studying healthy animals or spontaneously hypertensive stroke-prone rats. A reduction in inflammation and neural damage was associated with immunomodulatory or “tolerogenic” responses to E-selectin. The purpose of the current study on ApoE deficient mice is to assess the capacity of this stroke prevention innovation to influence atherosclerosis, a major underlying cause for ischemic strokes; human E-selectin is being translated as a potential clinical prevention strategy for secondary stroke. Female ApoE−/− mice received intranasal delivery of E-selectin prior to (pre-tolerization) or simultaneously with initiation of a high-fat diet. After 7 weeks on the high-fat diet, lipid lesions in the aorta, serum triglycerides, and total cholesterol were assessed as markers of atherosclerosis development. We also assessed E-selectin-specific antibodies and cytokine responses, in addition to inflammatory responses that included macrophage infiltration of the aorta and altered gene expression profiles of aortic mRNA. Intranasal delivery of E-selectin prior to initiation of high-fat chow decreased atherosclerosis, serum total cholesterol, and expression of the leucocyte chemoattractant CCL21 that is typically upregulated in atherosclerotic lesions of ApoE−/− mice. This response was associated with the induction of E-selectin specific cells producing the immunomodulatory cytokine IL-10 and immunosuppressive antibody isotypes. Intranasal administration of E-selectin generates E-selectin specific immune responses that are immunosuppressive in nature and can ameliorate atherosclerosis, a major risk factor for ischemic stroke. These results provide additional preclinical support for the potential of induction of mucosal tolerance to E-selectin to prevent stroke

Transcriptional Regulation of Rod Photoreceptor Homeostasis Revealed by In Vivo NRL Targetome Analysis

A stringent control of homeostasis is critical for functional maintenance and survival of neurons. In the mammalian retina, the basic motif leucine zipper transcription factor NRL determines rod versus cone photoreceptor cell fate and activates the expression of many rod-specific genes. Here, we report an integrated analysis of NRL-centered gene regulatory network by coupling chromatin immunoprecipitation followed by high-throughput sequencing (ChIP–Seq) data from Illumina and ABI platforms with global expression profiling and in vivo knockdown studies. We identified approximately 300 direct NRL target genes. Of these, 22 NRL targets are associated with human retinal dystrophies, whereas 95 mapped to regions of as yet uncloned retinal disease loci. In silico analysis of NRL ChIP–Seq peak sequences revealed an enrichment of distinct sets of transcription factor binding sites. Specifically, we discovered that genes involved in photoreceptor function include binding sites for both NRL and homeodomain protein CRX. Evaluation of 26 ChIP–Seq regions validated their enhancer functions in reporter assays. In vivo knockdown of 16 NRL target genes resulted in death or abnormal morphology of rod photoreceptors, suggesting their importance in maintaining retinal function. We also identified histone demethylase Kdm5b as a novel secondary node in NRL transcriptional hierarchy. Exon array analysis of flow-sorted photoreceptors in which Kdm5b was knocked down by shRNA indicated its role in regulating rod-expressed genes. Our studies identify candidate genes for retinal dystrophies, define cis-regulatory module(s) for photoreceptor-expressed genes and provide a framework for decoding transcriptional regulatory networks that dictate rod homeostasis

Global protein conjugation by ubiquitin-like-modifiers during ischemic stress is regulated by microRNAs and confers robust tolerance to ischemia.

Hibernation torpor provides an excellent model of natural tolerance to ischemia. We have previously shown that massive global SUMOylation occurs during hibernation torpor in ground squirrels. We have also shown that overexpression of Ubc9, SUMO-1, or SUMO-2/3 provides protection against ischemic damage in cell lines and cortical neurons exposed to oxygen/glucose deprivation, and in mice exposed to middle cerebral artery occlusion. We have now extended our study to other Ubiquitin-Like-Modifiers (ULMs), which have multiple cellular functions during stress, in order to assess the possibility that they also have roles in tolerance to ischemia. We found that not only SUMO conjugation, but also global protein conjugation by other ULMs including NEDD8, ISG15, UFM1 and FUB1 were significantly increased in the brains of hibernating ground squirrels during torpor. By means of miRNA microarrays of ground squirrel brain samples (from active and torpor phase) we found that the miR-200 family (miR-200a,b,c/miR-141/miR-429) and the miR-182 family (miR-182/miR-183/miR-96) were among the most consistently depressed miRNAs in the brain during the torpor phase as compared to active animals. In addition, we showed that these miRNAs are involved in the expression of various ULM proteins and their global conjugation to proteins. We observed that inhibition of the miR-200 family and/or miR-182 family miRNA activities in SHSY5Y cells increases global protein conjugation by the above ULMs and makes these cells more tolerant to OGD-induced cell death. This is the first report to describe that the natural tolerance to brain ischemia in hibernators is linked to regulation by microRNAs of a broad range of ubiquitin-like modifiers

miR-200 family and miR-182 family members of miRNAs indeed target various ULM and/or ULM related genes.

<p>SHSY5Y cells were co-transfected with the pmirGLO Dual-Luciferase miRNA Target Expression (Promega) constructs into which putative target sites for the various ULMs had been inserted, and with mimics of miRNAs as indicated. Two days later, cells were lysed, luciferase acivities (both firefly and Renilla) were measured, firefly luciferase activities were normalized with Renilla firefly activity, and expressed as relative to control (transfected with empty vector and negative control mimic). Data are shown as the mean±SD of three independent experiments. ***p<0.001, **p<0.01, *p<0.05 compared to control.</p

Differentially regulated miRNA families in the brains of 13-lined ground squirrels during hibernation.

<p>(A) 405 miRNAs which had an absolute mean difference between LH and ACR≥1.25 (using 6 different animal samples for each condition) were grouped into 33 families, and plotted against the frequencies observed. Down-regulated miRNAs were shown in red, and up-regulated miRNAs were shown in green. The number on top of each column is the fold change (ACR vs LH). (negative number = decrease, positive number = increase). (B) A subset of differentially regulated miRNAs detected by miRNA microarray was validated by qPCR using the same RNA samples that had been used for the miRNA microarray. The levels of the miRNAs were normalized by the level of miR-103, which was among the most stably expressed miRNAs with expression levels that did not change during the hibernation bout, and expressed as ratios of LH/ACR.</p