Review Article Roles of Sigma-1 Receptors in Alzheimer's Disease

Int J Clin Exp Med 2015;8(4):4808-4820 www.ijcem.com /ISSN:1940-5901/IJCEM0006695 Review Article Roles of sigma-1 receptors in Alzheimer’s disease Jia-Li Jin*, Min Fang*, Yan-Xin Zhao, Xue-Yuan Liu Department of Neurology, Shanghai Tenth People’s Hospital, Tongji University, School of Medicine, China. *Equal contributors. Received February 4, 2015; Accepted April 3, 2015; Epub April 15, 2015; Published April 30, 2015 Abstract: Alzheimer’s disease (AD) is a progressive neurodegenerative disorder and the leading cause of senile dementia all over the world. Still no existing drugs can effectively reverse the cognitive impairment. However, Sigma-1 (σ-1) receptors have been long implicated in multiple neurological and psychiatric conditions over these years. In this review, we discuss the current understanding of σ-1 receptor functions. Through regulation of lipid rafts, secretases, kinases, neuroceptors and ion channels, σ-1 receptors can influence cellular signal transduction, TCA cycle, oxidative stress, neuron plasticity and neurotransmitter release etc. Based on this, we suggest the key cellular mechanisms linking σ-1 receptor to Alzheimer’s disease. Besides, we detail the evidences showing that σ-1 receptors agonists, being the promising compounds for treatment of cognitive dysfunction, exhibit robust neuroprotection and anti-amnesia effect against Aβ neurotoxicity in the progress of Alzheimer’s disease. The evidence comes from animal models, preclinical studies in humans and full clinical trials. In addition, the questions to be solved regard- ing this receptor are also presented. When concerned with NMDAR, σ-1 receptor activation may result in two totally different influences on AD. Utilization of σ-1 agents early in AD remains an overlooked therapeutic opportunity. This article may pave the way for further studies about sigma-1 receptor on Alzheimer’s disease. Keywords: Sigma-1 receptor, Alzheimer’s disease, pathogenesis, Aβ neurotoxicity, NMDA receptor Introduction observed that σ-1 receptor agonists can significantly reduce AD induced cognitive dysfunc- Characterized by progressive cognitive dys- tion. Thus, we aim at highlighting the prospect function and behavioral impairment, Alzhei- of sigma-1 receptor effects and treatment in mer’s disease (AD) is a neurodegenerative dis- the progress of Alzheimer’s disease. order with insidious onset. So far the most widely accepted pathology of AD comprises Characteristics and biological effects of σ amyloid-β deposition and neurofibrillary tan- receptor gles of hyperphosphorylated tau protein. But no existing drugs can effectively reverse the Sigma (σ) receptor was first identified as sub- cognitive impairment. Recently, σ-1 receptor type of opioid receptor [4]. It independently has shown an emerging new look of improving established a receptor family after Quirion R cognitive function, especially its anti-amnesic proposing its difference from opioid receptor and neuroprotective effects [1]. An early post- and phencyclidine binding site [5]. σ receptors mortem study reported that σ-1 receptor were can be divided into 2 subtypes: σ-1 and σ-2. Still decreased in hippocampus CA1 region of AD there are disputes over the existence of σ-3 patients [2]. Later M. Mishina found σ-1 recep- subtype. σ receptors are abundant in the body, tor loss in the early phase of AD using Positron especially in the central nervous system. It has emission tomography (PET) with (11C) SA4503. high density distribution in the spinal cord, The binding potential was significantly pons, medulla oblongata, red nucleus, cerebel- decreased by 44-60% in the frontal, temporal, lum, hippocampus , medium density distribu- and occipital lobe, cerebellum and thalamus tion in the cerebral cortex and hypothalamus [3]. Based on changes of σ-1 receptor density, and low density distribution in the basal ganglia the following research over these years and thalamus [6]. Study comparing σ1 versus Sigma-1 receptors in Alzheimer’s disease 5-hydroxy tryptamine, gluta- mate, dopamine, norepineph- rine, acetylcholine, γ-amino- butyric acid and so on [15]. Potential mechanisms of σ-1 receptor in the progression of Alzheimer’s disease Despite of the mounting evidence on the etiology and pathogenesis of AD over these decades, the exact cause has not been fully elu- cidated, which may be attrib- uted to the complexity and multiple factors related to it. Here, we suggest the key cellular mechanisms linking σ-1 Figure 1. The possible mechanism of σ-1 receptor in the profession of Al- receptor to Alzheimer’s dis- zheimer’s Disease. ease (Figure 1). σ2 receptor found their dramatic difference in Aβ cascade hypothesis size, distribution and ligand affinity [7]. To date, σ-1 receptor has been cloned g in guinea-pig Considered as multi-gene inherited disease and human [8, 9] and, then, in rat and mouse with genetic heterogeneity, AD can be generally divided into familial AD and sporadic AD. Now [10, 11]. Its gene encodes a protein of 223 three different autosomal dominant gene have amino acid with two transmembrane domains been found to be related to early-onset AD: pre- and a typical endoplasmic reticulum localized senilin-1 (PS-1), presenilin-2 (PS-2) and amyloid signal near the short N terminus [12]. But so far precursor protein (APP). However, only one pre- there is no mammalian protein can specifically disposing gene for late-onset AD is universally bind to this receptor. σ-2 receptor has not yet recognized, namely APOE epsilon 4 (APOEε4). been cloned and little knowledge is known Recently, emerging technologies to analyze the about its relationship with AD. Recently, Izzol et entire genome in large data sets have revealed al. found that Aβ1-42 exhibits synaptic toxicity new genes associated with late-onset AD risk, after binding to the σ-2/PGRMC1 receptor [13]. including ABCA7, BIN1, CASS4, CD33, CD2AP, However, it is generally believed that σ-1 recep- HLA-DRB5-DBR1and so on [16]. Unfortunately, tor plays a more important role in the progres- to date there is no evidence that combines AD sion of Alzheimer’s disease. related genes to σ-1 receptors. Only a few researches on correlation between risk for In normal times, σ-1 receptors mainly localize developing AD and σ-1 receptor gene mutation on the mitochondrial associated endoplasmic polymorphism have been reported. Some typi- reticulum membrane (MAM), forming a Bip cal studies are discussed in subsequent sec- chaperone structure with high sensitivity to the tions of this paper. Among the 4 discovered AD calcium ion. When activated by agonists such genes, mutation of APP, PS-1 and PS-2 has as cocaine or analgesic, σ-1 receptors separate been found to increase Aβ generation while from BiP and translocate from MAM to other APOEε4 results in Aβ deposition. Substantial parts of the cell. Through regulation of inositol evidence suggests the imbalance of amyloid triphosphate (IP3) receptors, N-methyl-D- beta production and clearance is the initial aspartic acid receptor (NMDA) receptors, dopa- event in neuronal degeneration and dementia, which is also the common pathway of other mine (DA) receptors and ion channels, σ-1 cause in the pathogenesis leading to AD. receptors can influence TCA cycle, oxidative stress [14], mitochondrial function, neuron Different mechanisms could be evoked to plasticity and neurotransmitter release such as describe the nature of σ-1 receptor alleviating 4809 Int J Clin Exp Med 2015;8(4):4808-4820 Sigma-1 receptors in Alzheimer’s disease Aβ neurotoxicity: (i) σ-1 receptors may have or antagonists of voltage-dependent calcium effective protection against Aβ toxicity by mod- channels in various learning tests [1]. σ-1 recep- ulating recomposition of lipid rafts, inhibiting tors not only localize in endoplasmatic reticu- Aβ fibrin formation [17]. Substantial amounts of lum, but also in nuclear and plasma mem- the aspartyl protease β-secretase (BACE-1) and branes and on mitochondria [32-36], on which γ-secretase [18] localize in lipid rafts, where Aβ binding sites are involved in the regulation production occurs preferentially. What’s more, of calcium mobilization. Furthermore, σ-1 GM1 ganglioside-bound amyloid β protein receptor protects mitochondria and neural via- (GM1/Aβ), a seed proposed to be involved in bility by maintaining intracellular calcium ho- initiation of amyloid fibril formation, is also meostasis. associated within lipid rafts on the plasma membrane [19, 20]. Chronic activation of σ-1 (v) Overexpression of σ-1 receptors potentiate receptor results in its translocation from the lipid rafts-associated NGF, EGF and BDNF endoplasmic reticulum (ER), within lipid drop- effect [1] and NGF-induced neuroprotection lets, towards the lipid rafts on cytomembrane through regulating PLCγ-DAG-PKC, Ras-Raf- where it may involve in recomposition of lipid MEK-ERK and PI3K-Akt-mTOR signaling path- rafts and modifying cellular functions including ways [37]; Since the discovery of growth factor signal transduction of protein coupled receptor, receptors localizing in lipid rafts [21], sustained biosynthetic or endocytic traffic of protein [21]. activation of σ-1 receptors may be beneficial for Therefore, sustained σ-1 receptor activation cell survival rate and, therefore, facilitate neu- may preferentially translocate to lipid rafts, on roprotection or neuronal recovery against Aβ which binding sites are actively involved in Aβ neurotoxicity. In addition, σ-1 receptors can production and transportation. enhance the axonal and dendritic growth in hippocampus [38]. (ii) Attenuate Aβ25-35-induced Aβ1-42 seeding in hippocampal neurons [22]. Administration of (vi) Mitigate Aβ25-35-induced apoptosis by Aβ25-35 can provoke a significant increase in decreasing expression of proapoptotic gene APP expression and activation of the β- Bax and the death protease caspase-3 where- secretase pathway resulting in the endogenous as preserving antiapoptotic gene Bcl-2 levels, Aβ peptide content in hippocampus, which is resulting in a concomitant enhancement in cell called Aβ seeding [23]. σ-1 receptors can block survival.

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