Orexins, also known as hypocretins, are two neuropeptides secreted from orexin-containing neurons, mainly in the lateral hypothalamus (LH). Orexins orchestrate their effects by binding and activating two G-protein–coupled receptors (GPCRs), orexin receptor type 1 (OX1R) and type 2 (OX2R). Orexin/receptor pathways play vital regulatory roles in many physiological processes, especially feeding behavior, sleep–wake rhythm, reward and addiction and energy balance. Furthermore several reports showed that orexin/receptor pathways are involved in pathological processes of neurological diseases such as narcolepsy, depression, ischemic stroke, drug addiction and Alzheimer’s disease (AD). This review article summarizes the expression patterns, physiological functions and potential molecular mechanisms of the orexin/receptor system in neurological diseases, providing an overall framework for considering these pathways from the standpoints of basic research and clinical treatment of neurological diseases.

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It is well-established that reperfusion following cerebral ischemic injury gives rise to secondary injury accompanied by structural and functional damage. However, it remains unclear how global genes changes in cerebral ischemia-reperfusion injury (IRI). This study investigated global gene expression in the hippocampi of Wistar rats following transient cerebral IRI using an RNA-sequencing strategy. The results revealed ≥2-fold up-regulation of 156 genes and ≥2-fold down-regulation of 26 genes at 24 h post-reperfusion. Fifteen differentially expressed genes were selected to confirm the RNA-sequencing results. Gene expression levels were dynamic, with the peak expression level of each gene occurring at different time points post-reperfusion. Gene Ontology (GO) analysis classified the differentially expressed genes as mainly involved in inflammation, stress and immune response, glucose metabolism, proapoptosis, antiapoptosis, and biological processes. KEGG pathway analysis suggested that IRI activated different signaling pathways, including focal adhesion, regulation of actin cytoskeleton, cytokine-cytokine receptor interaction, MAPK signaling, and Jak-STAT signaling. This study describes global gene expression profiles in the hippocampi of Wistar rats using the middle cerebral artery occlusion (MCAO) model. These findings provide new insights into the molecular pathogenesis of IRI and potential drug targets for the prevention and treatment of IRI in the future.

Orexins are neuropeptides that regulate the sleep-wake cycle and feeding behaviour. QRFP is a newly discovered neuropeptide which exerts similar orexigenic activity, thus playing an important role in energy homeostasis and regulation of appetite. The exact expression and signalling characteristics and physiological actions of QRFP and its receptor GPR103 are poorly understood. Alzheimer’s disease (AD) patients experience increased nocturnal activity, excessive daytime sleepiness, and weight loss. We hypothesised therefore that orexins and QRFP might be implicated in the pathophysiology of AD. We report that the down-regulation of hippocampal orexin receptors (OXRs) and GPR103 particularly in the cornu ammonis (CA) subfield from AD patients suffering from early onset familial AD (EOFAD) and late onset familial AD (LOAD). Using an in vitro model we demonstrate that this downregulation is due to to Aβ-plaque formation and tau hyper-phosphorylation. Transcriptomics revealed a neuroprotective role for both orexins and QRFP. Finally we provide conclusive evidence using BRET and FRET that OXRs and GPR103 form functional hetero-dimers to exert their effects involving activation of ERK1/2. Pharmacological intervention directed at the orexigenic system may prove to be an attractive avenue towards the discovery of novel therapeutics for diseases such as AD and improving neuroprotective signalling pathways.

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Orexin-A and Orexin-B play important roles in many physiological processes in which Orexins orchestrate diverse downstream effects via two G-protein coupled receptors: Orexin1R and Orexin2R. Two alternative C-terminus splice variants of the mouse Orexin receptors mOX2alphaR and mOX2betaR have recently been identified. This study explored the possibility of heterodimerization between mOX2alphaR and mOX2betaR, and investigated novel signal transduction characteristics after stimulation. The dimerization of mOX2alphaR and mOX2betaR was confirmed by BRET and co-immunoprecipitation assays. Meanwhile, in HEK293 cells, co-expression of mOX2alphaR and mOX2betaR resulted in a strengthened increase in activation of ERK1/2, with maximal activation at 5 min and 100 nM. Furthermore, heterodimerization also elicits stronger intracellular Ca2+ elevation after Orexin(s) stimulation, followed by a slower decline in intracellular Ca2+ to a steady endpoint Protein Kinase C Inhibitor significantly inhibited these downstream effects. In addition, the cAMP response element reporter activities were significantly reduced, whereas the serum response element luciferase and the T-lymphocyte activation of nuclear factor-responsive element reporter activity were significantly up-regulated after Orexin(s) stimulation. Besides, Orexin-A/-B induced a significantly higher rate of HEK293 cell proliferation in cells co-expressing mOX2alphaR/mOX2betaR compared to the control group. Taken together, we provide conclusive evidence that mOX2alphaR can form a functional heterodimer with mOX2betaR and this leads to increased PKC and decreased protein kinase A activity by ERK signal pathway leading to a significant increase in cell proliferation. The nature of this signaling pathway has significant implications for the role of Orexin in the regulation of physiological processes including the homeostasis of feeding.

Growth hormone secretagogue receptor 1α (GHSR1a) and Orexin 1 receptor (OX1R) are involved in various important physiological processes, and have many similar characteristics in function and distribution in peripheral tissues and the central nervous system. We explored the possibility of heterodimerization between GHSR1a and OX1R and revealed a signal transduction pathway mechanism. In this study, bioluminescence and fluorescence resonance energy transfer and co-immunoprecipitation (Co-IP) analyses were performed to demonstrate the formation of functional GHSR1a/OX1R heterodimers. This showed that a peptide corresponding to the 5-transmembrane domain of OX1R impaired heterodimer construction. We found that ghrelin stimulated GHSR1a/OX1R heterodimer cells to increase the activation of Gαs protein, compared to the cells that express GHSR1a. Stimulation of GHSR1a/OX1R heterodimers with orexin-A did not alter GPCR interactions with Gα protein subunits. GHSR1a/OX1R heterodimers induced Gαs and downstream signaling pathway activity, including increase of cAMP-response element luciferase reporter activity and cAMP levels. In addition, ghrelin induced a higher proliferation rate in SH-SY5Y cells than in controls. This suggests that ghrelin GHSR1a/OX1R heterodimers promotes an upregulation of a Gαs-cAMP-cAMP-responsive element signaling pathway in vitro and an increase in neuroblastoma cell proliferation.

MicroRNAs are a class of noncoding small RNAs that regulate gene expression by inhibiting target genes at post-transcriptional levels. MicroRNAs have been highlighted in many organs and tissues, including the brain. To identify special microRNAs involved in ischemia-reperfusion injury, we performed a comprehensive small RNA profiling in rat model and the control using Illumina high-throughput sequencing. A total of 9,444,562 and 10,290,391 clean reads were sequenced from two small RNA libraries constructed, respectively. Three hundred fifty-eight known microRNAs were identified, in which 78 microRNAs exhibited significantly differential expression between model and control. In addition, 62 and 68 novel miRNAs were found in model and control, respectively. Comparative analysis showed that 24 novel microRNAs were differentially expressed with greater than six-fold change. The GO annotation suggested that predicted targets of microRNAs were enriched into the category of metabolic process, cell part, cell-extracellular communications, and so on. KEGG pathway analysis suggested that these genes were involved in many important pathways, mainly including signaling transduction, MAPK signaling pathway, NF-κB signaling pathway, and neurotrophin signaling pathway. Our findings provided a deeper understanding to the regulatory mechanism of microRNAs underlying cerebral ischemia, therefore benefitting the improvement of the protection and treatment strategies of this disease.