Role of Grina/Nmdara1 in the Central Nervous System Diseases.

Glutamate receptor, ionotropic, N-methyl-D-aspartate associated protein 1 (GRINA) is a member of the NMDA receptors (NMDARs) and is involved in several neurological diseases, which governs the key processes of neuronal cell death or the release of neurotransmitters. Upregulation of GRINA has been reported in multiple diseases in human beings, such as major depressive disorder (MDD) and schizophrenia (SCZ), with which the underlying mechanisms remain elusive. In this review, we provide a general overview of the expression and physiological function of GRINA in the central nervous system (CNS) diseases, including stroke, depression ,epilepsy, SCZ, and Alzheimer's disease (AD). Copyright© Bentham Science Publishers; For any queries, please email at epub@benthamscience.net.

INTRODUCTION

NMDARs are one of three pharmacologically distinct subtypes of ionotropic receptors that mediate a majority of excitatory neurotransmissions in the brain [1]. Activation of NMDARs and downstream cellular signaling are important for neuronal development, synaptic plasticity, learning and memory [2], but also contribute to the pathogenesis of diverse neurological disorders, such as AD, epilepsy, stroke and SCZ [3-5]. NMDARs are highly permeable to Ca2+, and Ca2+ influx, which is essential for synaptogenesis, experience-dependent synaptic remodelling and long-lasting changes in synaptic efficacy such as long-term potentiation (LTP) and long-term depression (LTD) [1]. GRINA is a glutamate receptor-associated protein and several studies have demonstrated its dysfunction in the brain which is linked to the occurrence of several CNS diseases. As summarized in Table , the upregulation of GRINA has been reported in the depression and SCZ. Meanwhile, the neuroprotective function of GRINA has been shown to be involved in ischemic stroke and post-ischemic unfolded protein response. Herein, we

reviewed the existing literature about GRINA, deciphered the functions of GRINA through its domains, and discussed its roles in the CNS diseases.

DOMAINS AND EXPRESSION OF GRINA

Human GRINA is located at the chromosome region 8q24.3, near the subtelomere [9], and encodes a 371 amino acid protein with a predicted molecular weight of 41.2 kDa. Protein domain prediction (DOMPRED) revealed a critical region which containing the seven-transmembrane α-helices (Fig. ). In order to understand the characterization of GRINA domains, we analyzed the NCBI’s Conserved Domains Database (CDD) [13]. GRINA displayed two major domains: a Pro-rich domain (fragment 39-139) within an N- terminal tail and an LFG-like domain (fragment 151-367), belonging to the BI-1-like superfamily, across the transmembrane region [14].

Nielsen and colleagues measured the expression of GRINA by Northern blot in different murine tissues [15]. They observed a broad expression pattern, strongly expressed in the brain and kidney, and also in the cortex, cerebellum, hindbrain and basal ganglia, as well as other organs like the testes and spleen. GRINA is expressed throughout the brain but at the highest levels in the hippocampus, suggesting that GRINA is likely to play crucial roles in the hippocampus associated neurological diseases, as discussed in more detail.

Grina in Major Depressive Disorder

Major depressive disorder (MDD) is one of the most prevalent mood disorders and ranks first among all neurological disorders in terms of disability-adjusted life years [16]. Although the principal cause of this disorder is largely unknown, some depressed patients show a remarkable improvement following the administration of NMDARs channel antagonist [17, 18]. A recent study indicated that an NMDAR antagonist ketamine enhances visual sensory evoked potential LTP in patients with MDD [19], and blocks bursting in the lateral habenula to rapidly relieve depression [20]. Similar to ketamine, other NMDAR antagonists, including MK-801 [21] and AP-5 [22] mimicked ketamine’s effect in inducing AMPAR-mediated synaptic potentiation. This finding was hypothesized to indicate that ketamine, via blocking the NMDAR at rest, drives synaptic potentiation, leading to synaptic plasticity changes that might be relevant to the antidepressant actions of NMDAR antagonists [23].

Goswami DB et al., used brain samples from MDD patients and found that GRINA is increased in the prefrontal cortex of MDD subjects [6]. In the same study, a majority of the NMDA receptor subunits (GRINA, GRIN2A, GRINL1A) up-regulated among the suicides with major depression versus the controls or the suicides without history of major depression [7]. These results suggest that the expression of GRINA is associated with the pathophysiology of depression and is a critical approach for novel antidepressant treatments.

Grina in Schizophrenia

Schizophrenia (SCZ) is a chronic neuropsychiatric disorder associated with affective, cognitive, neuromorphological, and molecular abnormalities [24]. Even though the etiology of SCZ is uncertain, it is believed to be a neurodevelopmental disorder that results from a combination of environmental insults and genetic vulnerabilities [25]. NMDA receptor is a major subtype of glutamate receptor that mediates fast synaptic transmission in the CNS. Several studies have shown that NMDA hypofunction is tightly linked to SCZ [26-28]. One study demonstrated the differential effect of NMDA receptor GluN2C and GluN2D subunit ablation on behavior and channel blocker-induced SCZ phenotypes [29]. Notably, NMDA receptor antagonists induced SCZ-like behaviors in animal models [30] and psychosis impairment in normal human subjects [31]. Hao et al., reported that NMDARs may be potential therapeutic targets to prevent disease development during asymptomatic periods of SCZ and may serve as targets for preventive and/or therapeutic strategies for SCZ [32].

The region in the N terminal domain of GRINA (fragment 63-96) shows homology with the 33-mer gliadin peptide [33]. Based on this homology, the 33-mer gliadin peptide would act as a natural antagonist, interfering with GRINA and altering its functions. This biochemical mechanism would be relevant in the extraintestinal manifestations of SCZ [33]. Indeed, about one-third of people with SCZ have elevated IgG antibodies to gliadin (AGA IgG) [14]. Supporting this association, a recent study with 80 healthy controls and 160 patients with SCZ showed that GRINA IgG was higher in SCZ patients than in healthy controls, and that the presence of anti-GRINA antibodies was associated with anti-AGA antibodies [8]. These results support the possible role of GRINA in SCZ. However, further research works required to elucidate their exact mechanisms in SCZ.

Grina in Epilepsy

Epilepsy is one of the most common neurological disorders that are characterized by abrupt, recurrent, and synchronous discharges of the brain [34]. Previous studies have found a surprising number of NMDARs mutations in seizure disorders, causing various childhood epilepsy syndromes [35, 36]. Secondly, a sharp increase in the extracellular concentration of glutamate in the focal hemisphere has been observed immediately prior to the onset of an electrographic seizure [9]. Finally, glutamate antagonists selective for NMDARs act as potent anticonvulsants in a range of epilepsies [37, 38]. These findings suggest that NMDARs appear to be a locus for epilepsy.

A form of inherited epilepsy is benign familial neonatal convulsions (BFNC) localized to chromosome 8. GRINA mapped to 8q24 was considered as a candidate for the epileptic disorder [9]. Another recent study found that the GRINA within the 2.3Mb duplicated segment of chromosome 8q24.3 is associated with severe mental retardation and epilepsy [10]. These findings suggested the potential role of GRINA in ameliorating epilepsy via targeting the balance between inhibition and excitation.

Grina in Stroke

Stroke is the second leading cause of death and the third most common cause of disability worldwide [39]. The two main types of stroke are ischemic and hemorrhagic. Ischemic strokes comprise about 87% of all strokes [40]. Ischemic stroke triggers a complex series of pathophysiological events, including the accumulation of synaptic and extrasynaptic glutamate, ion channel dysfunction, inflammation and so on, eventually leading to neuronal cell death and ischemic brain injury [41, 42]. NMDARs -mediated excitotoxicity is the leading cause of neuronal cell death in ischemic stroke [43]. It is well documented that DAPK1 interaction with NMDA receptor NR2B subunits mediates brain damage in stroke [44]. The genetic mutation of GluN2B protects brain cells against stroke damages [45]. Differential roles of NMDA receptor subtypes in ischemic neuronal cell death and ischemic tolerance were found in other recent research [46, 47]. GRINA is a glutamate receptor-associated protein and several studies have demonstrated that GRINA has a highly potent protective effect, preventing mice from cerebral ischemia-induced cell death [11] and post-ischemic unfolded protein response (UPR) [12].

The LFG-like domain gives GRINA an alternative name LFG1. As commented before, mammalian members of the BI-like superfamily include transmembrane proteins related to cell death and survival [48]. Previous studies have provided evidence for the critical role of TMBIM members in the transient brain ischemia. Endoplasmic reticulum protein BI-1 modulates unfolded protein response signaling and protects against traumatic brain injury and apoptotic cell death [49, 50]. Fas apoptotic inhibitory molecule 2 (Faim2) has been shown to modulate hippocampal neuroplasticity and is neuroprotection in cerebral ischemia [51-53].

Overall, these findings strongly suggest that TMBIM members GRINA, BI-1 and Faim2 could be new therapeutic approaches to decrease excitotoxicity-induced neuronal cell death in stroke.

Grina in Alzheimer’s Disease

Alzheimer’s disease (AD) is the most common neurodegenerative disease and is characterized by cognitive disorder and memory dysfunction in the elderly population, affecting almost 40 million people worldwide [54]. The pathophysiology of AD includes the appearance of senile plaques consisting of Aβ and neurofibrillary tangles containing phosphorylated tau, leading to the substantial loss of synaptic profiles [55]. Accumulating evidence has suggested that NMDARs dysfunction is tightly linked to AD [5, 56, 57]. In a recent study by Dong et al. [58], intracerebroventricularly injected IL-1β induced calcium overload and endoplasmic reticulum stress, and NMDAR antagonist MK801 pretreatment significantly attenuated neuronal apoptosis and NMDAR up-regulation [59]. Another recent study found that NMDARs activation mediated by Aβ is involved in Aβ-induced mitochondrial toxicity and neuronal dysfunction [60]. Overall, these findings strongly suggest the important function of NMDARs in AD.

Marked and sustained changes in intracellular calcium signaling occur prior to cognitive decline and extensive neuronal death in AD [61]. GRINA regulates intracellular calcium homeostasis by interaction with IP3R, modulation the ER Ca2+ release [62, 63]. Recent studies have found that GRINA modulates voltage-gated CaV2.2 Ca2+ channels in a G-protein-like manner [64]. Calcium entry through CaV2.2 channels is a major mechanism triggering transmitter release in certain synapses, indicating that GRINA-mediated cytosolic Ca2+ overload is associated with synaptic transmission in AD. In another study, GRINA was found to contain three potential ALG2-binding motifs (ABM1) that interact with the longest isoform of ALG2. Interestingly, ALG2 is among the top RAR-related orphan receptor A (RORA)-linked genes with an elevated expression in the hippocampus of patients with AD [65]. Furthermore, an alternative splice variant lacking the sequence PPPNPGYPGGPQPPMPPYAQ(fragment 15-34) has been found in AD patients’ cortex (NCBI accession AK294127), but its relevance is still unknown [14]. Recent observations indicate a decreased cancer risk in patients with AD. GRINA modulates aerobic glycolysis and promotes tumor progression in gastric cancer [66]. Based on these findings, it is conceivable that GRINA plays a crucial role in AD.Moreover, further research is required to evaluate whether the overexpression of GRINA can be neuroprotective against Aβ-induced cytosolic Ca2+ overload in animal models.

CONCLUSION

Currently available data indicate the significant regulatory roles of GRINA in the pathogenesis of glutamate receptor-dependent neurological disorders (Fig. ). It has been reviewed here that the GRINA contributes to neuroprotection, synaptic transmission and plasticity due to its two conserved domains (Pro-rich domain and LFG-like domain). Such data provide new insights into the GRINA in neurological diseases, suggesting that the modulation of GRINA will hopefully lead to the development of therapeutically effective drugs. While the focus of this review was the role of GRINA in CNS diseases, more research should be directed towards the DNA binding and vesicle transport, or the pathologic function of GRINA in many other human diseases (gastric cancer, osteoarthritis and celiac disease).

Table 1

Summary of studies describing alterations of GRINA in CNS diseases.

Disease Name	Study Models	Conclusions	References
Depression	Clinical patients	GRINA as novel factors associated with major depressive disorder	Goswami DB et al. [6]
Depression	Clinical patients	GRINA up-regulated among the suicides with major depression	Sequeira A et al. [7]
Schizophrenia	Clinical patients	Persons with schizophrenia had significantly increased levels of GRINA	Čiháková D et al. [8]
Epilepsy	In-vitro cell culture	GRINA mapped to the same region of chromosome 8 as BFNC	Lewis TB et al. [9]
Epilepsy	Clinical patients	GRINA associated with severe mental retardation and epilepsy	Bonaglia MC et al. [10]
Stroke	In-vivo mouse modelIn-vitro cell culture	GRINA involved in EPO-mediated neuroprotection after stroke	Habib P et al. [11]
Stroke	In-vivo mouse modelIn-vitro cell culture	GRINA plays a crucial role in post-ischemic unfolded protein response	Habib P et al. [12]