The novel role of the neuropeptides Orexin and QRFP and their involvement in Alzheimer's disease

Abstract

Alzheimer’s disease (AD) is a neurodegenerative disease which affects over 500,000 people in the UK. Worldwide 44 million people are affected by AD and other dementias. Most cases occur over the age of 65 and is characterised by gradual and increasing loss of cognitive function and behavioural abnormalities. The main causes are a build-up of the toxic protein amyloid-β (Aβ) and hyperphosphorylation of the microtubule stabilising protein: tau, leading to neurofibrillary tangles (NFT). These two hallmarks of disease result in neuronal damage and cell death causing associated symptoms and eventually death. Orexins (OX) are neuropeptides which function to regulate the sleep-wake cycle and feeding behaviour. They are produced from a prepro-orexin (PPO) molecule and cleaved into two isoforms: orexin-A (OXA) and orexin-B (OXB). OXA and OXB are the ligands for two G-protein coupled receptors (GPCR): orexin receptor 1 (OX1R) and orexin receptor 2 (OX2R). 50-80,000 OX producing neurons project to many areas of the brain including the lateral hypothalamus (LHA), locus coeruleus (LC), tuberomammillary nucleus (TMN), paraventricular nucleus (PVN) and raphe nuclei and from these areas regulate feeding and appetite and the sleep wake cycle through their receptors. QRFP is a newly discovered neuropeptide which exerts similar orexigenic activity including the control of feeding behaviour. It is the ligand for the GPCR GPR103, both of which are widely expressed in the brain and also in the retina, testes, thyroid, pituitary and prostate. GPR103 also shares 48 and 47% protein sequence homology with OX1R and OX2R respectively. It is in these tissues where it can exert other physiological functions including regulation of feeding, control of the gonadotropic axis and bone formation. The exact expression and signalling characteristics and physiological actions of QRFP/GPR103 are still poorly understood. It is through the physiological functions of the orexigenic system and the clinical symptoms observed in AD which suggests a possible link between the two. For example, in AD one of the main reasons for institutionalisation is the severely dysregulated sleep pattern that is experienced by sufferers. They experience increased nocturnal activity and early awakenings as well as hypersomnia and excessive daytime sleepiness; all of which is beyond what someone of the same age experiences. As well as this AD patients suffer from significant weight loss and a significant negative correlation has been identified between progression of disease and appetite. All of this points towards an involvement of the orexigenic system in AD. AD patients have been found to have a 40% loss of immunoreactive OX neurons and have severe reductions in circulating OXA. This led us to believe that the OX system is of vital importance in AD and could be targeted to ameliorate symptoms. Studies have implicated OX and OXR in memory processes, appetite regulation, and severe disturbances of the sleep-wake cycle all of which are phenotypes of AD. Given that they play a key role in energy homeostasis and physiological behaviour, we hypothesise that OXs and their receptors are implicated in the pathophysiology of AD. Therefore, in this study we will investigate the detailed expression and signalling characteristics of OXR and GPR103 in vitro and in clinical samples In this study we neuronally differentiated two human neuroblastoma cell lines: IMR32 and SH-SY5Y. Neuronally acquired phenotype was confirmed through increased neurite length, increased expression of key neuronal proteins and increases in microtubule-associated protein tau (MAPT), neurogenin1 (NG1) and neuron-specific enolase (NSE) as well as a reduction in the neuronal marker of immaturity; nestin (NES). OXR and GPR103 were confirmed in both cell lines after differentiation at mRNA and protein level and were shown to be fully functional through phosphorylation of extracellular signal-regulated kinases 1/2 (ERK1/2). We also identified possible cross talk of GPR103 with the OXR though addition of selective OXR antagonists, which blocked QRFP induced ERK1/2 phosphorylation. We show for the first time that addition of Aβ42 and zinc sulphate to mimic AD in vitro, results in a significant reduction of OX1R and GPR103 in the cell lines SH-SY5Y and we have performed the first comprehensive study in clinical AD patients which demonstrate a loss of OX1R, OX2R and GPR103 at mRNA and protein level compared to age matched controls in the hippocampus. We performed microarray analysis which identified many genes and pathways regulated by the OXA, OXB and QRFP; including corticotropin-releasing hormone receptor (CRHR1), regulated in development and DNA damage responses 1 (REDD1), erythropoietin (EPO), Bcl-2-like protein 1 (BCL2L11), myb proto-oncogene protein (c-myb), vasoactive intestinal peptide (VIP), endothelin 1 (EDN1) as well as the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-KB) and hypoxia-inducible factor-1α (HIF-1α) pathways. These genes are all implicated in neuroprotection, particularly in AD. This represents the first comprehensive gene expression data in a neuroblastoma cell line for these orexigenic proteins. Collectively these data suggest a potential role of the orexigenic system in neuroprotection and a functional loss of the receptors in AD patients which could confer a loss of neuroprotection through the orexigenic system. 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.