Unraveling the molecular interactions between α7 nicotinic receptor and a RIC3 variant associated with backward speech

Recent work putatively linked a rare genetic variant of the chaperone Resistant to Inhibitors of acetylcholinesterase (RIC3) (NM_024557.4:c.262G > A, NP_078833.3:p.G88R) to a unique ability to speak backwards, a language skill that is associated with exceptional working memory capacity. RIC3 is important for the folding, maturation, and functional expression of α7 nicotinic acetylcholine receptors (nAChR). We compared and contrasted the effects of RIC3G88R on assembly, cell surface expression, and function of human α7 receptors using fluorescent protein tagged α7 nAChR and Förster resonance energy transfer (FRET) microscopy imaging in combination with functional assays and 125I-α-bungarotoxin binding. As expected, the wild-type RIC3 protein was found to increase both cell surface and functional expression of α7 receptors. In contrast, the variant form of RIC3 decreased both. FRET analysis showed that RICG88R increased the interactions between RIC3 and α7 protein in the endoplasmic reticulum. These results provide interesting and novel data to show that a RIC3 variant alters the interaction of RIC3 and α7, which translates to decreased cell surface and functional expression of α7 nAChR.

Deletion induced splicing in RIC3 drives nicotinic acetylcholine receptor regulation with implications for endoplasmic reticulum stress in human astrocytes

Nicotinic acetylcholine receptor (nAChR) dysregulation in astrocytes is reported in neurodegenerative disorders. Modulation of nAChRs through agonists confers protection to astrocytes from stress but regulation of chaperones involved in proteostasis with pathological implications is unclear. Resistance to inhibitors of cholinesterase 3 (RIC3), a potential chaperone of nAChRs is poorly studied in humans. We characterized RIC3 in astrocytes derived from an isogenic wild-type and Cas9 edited "del" human iPSC line harboring a 25 bp homozygous deletion in exon2. Altered RIC3 transcript ratio due to deletion induced splicing and an unexpected gain of α7nAChR expression were observed in "del" astrocytes. Transcriptome analysis showed higher expression of neurotransmitter/G-protein coupled receptors mediated by cAMP and calcium/calmodulin-dependent kinase signaling with increased cytokines/glutamate secretion. Functional implications examined using tunicamycin induced ER stress in wild-type astrocyte stress model showed cell cycle arrest, RIC3 upregulation, reduction in α7nAChR membrane levels but increased α4nAChR membrane expression. Conversely, tunicamycin-treated "del" astrocytes showed a comparatively higher α4nAChR membrane expression and upsurged cAMP signaling. Furthermore, reduced expression of stress markers CHOP, phospho-PERK and lowered XBP1 splicing in western blot and qPCR, validated by proteome-based pathway analysis indicated lowered disease severity. Findings indicate (i) a complex RNA regulatory mechanism via exonic deletion induced splicing; (ii) RIC-3 as a disordered protein having contrasting effects on co-expressed nAChR subtypes under basal/stress conditions; and (iii) RIC3 as a potential drug target against ER stress in astrocytes for neurodegenerative/nicotine-related brain disorders. Cellular rescue mechanism through deletion induced exon skipping may encourage ASO-based therapies for tauopathies.

RIC3 variants are not associated with Parkinson's disease in large European, Latin American, or East Asian cohorts

Parkinson’s disease (PD) is a complex neurodegenerative disorder in which both rare and common genetic variants contribute to disease risk. Multiple genes have been reported to be linked to monogenic PD but these only explain a fraction of the observed familial aggregation. Rare variants in RIC3 have been suggested to be associated with PD in the Indian population. However, replication studies yielded inconsistent results. We further investigate the role of RIC3 variants in PD in European cohorts using individual-level genotyping data from 14,671 PD patients and 17,667 controls, as well as whole-genome sequencing data from 1,615 patients and 961 controls. We also investigated RIC3 using summary statistics from a Latin American cohort of 1,481 individuals, and from a cohort of 31,575 individuals of East Asian ancestry. We did not identify any association between RIC3 and PD in any of the cohorts. However, more studies of rare variants in non-European ancestry populations, in particular South Asian populations, are necessary to further evaluate the world-wide role of RIC3 in PD etiology.

RIC3, the cholinergic anti-inflammatory pathway, and neuroinflammation

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels having many functions including inflammation control, as part of the cholinergic anti-inflammatory pathway. Genome wide association studies implicated RIC3, a chaperone of nAChRs, in multiple sclerosis (MS), a neuroinflammatory disease. To understand the involvement of RIC3 in inflammatory diseases we examined its expression, regulation, and function in activated immune cells. Our results show that immune activation leads to dynamic changes in RIC3 expression, in a mouse model of MS and in human lymphocytes and macrophages. We also show similarities in the expression dynamics of RIC3 and CHRNA7, encoding for the α7 nAChR subunit. Homomeric α7 nAChRs were shown to mediate the anti-inflammatory effects of cholinergic agonists. Thus, similarity in expression dynamics between RIC3 and CHRNA7 is suggestive of functional concordance. Indeed, siRNA mediated silencing of RIC3 in a mouse macrophage cell line eliminates the anti-inflammatory effects of cholinergic agonists. Furthermore, we show increased average expression of RIC3 and CHRNA7 in lymphocytes from MS patients, and a strong correlation between expression levels of these two genes in MS patients but not in healthy donors. Together, our results are consistent with a role for RIC3 and for the mechanisms regulating its expression in inflammatory processes and in neuroinflammatory diseases.

Reanalysis of exome sequencing data of intellectual disability samples: Yields and benefits

Recently, with the advancement in next generation sequencing (NGS) along with the improvement of bioinformatics tools, whole exome sequencing (WES) has become the most efficient diagnostic test for patients with intellectual disability (ID). This study aims to estimate the yield of a reanalysis of ID negative exome cases after data reannotation. Total of 50 data files of exome sequencing, representing 50 samples were collected. The inclusion criteria include ID phenotype, and previous analysis indicated a negative result (no abnormality detected). These files were pre-processed and reannotated using ANNOVAR tool. Prioritized variants in the 50 cases studied were classified into three groups, (1) disease-causative variants (2) possible disease-causing variants and (3) variants in novel genes. Reanalysis resulted in the identification of pathogenic/likely pathogenic variants in six cases (12%). Thirteen cases (26%) were classified as having possible disease-causing variants. Candidate genes requiring future functional studies were detected in seven cases (14%). Improvement in bioinformatics tools, update in the genetic databases and literature, and patients' clinical phenotype update were the main reasons for identification of these variants in this study.

NACHO Mediates Nicotinic Acetylcholine Receptor Function throughout the Brain

Neuronal nicotinic acetylcholine receptors (nAChRs) participate in diverse aspects of brain function and mediate behavioral and addictive properties of nicotine. Neuronal nAChRs derive from combinations of α and β subunits, whose assembly is tightly regulated. NACHO was recently identified as a chaperone for α7-type nAChRs. Here, we find NACHO mediates assembly of all major classes of presynaptic and postsynaptic nAChR tested. NACHO acts at early intracellular stages of nAChR subunit assembly and then synergizes with RIC-3 for receptor surface expression. NACHO knockout mice show profound deficits in binding sites for α-bungarotoxin, epibatidine, and conotoxin MII, illustrating essential roles for NACHO in proper assembly of α7-, α4β2-, and α6-containing nAChRs, respectively. By contrast, GABA_A receptors are unaffected consistent with NACHO specifically modulating nAChRs. NACHO knockout mice show abnormalities in locomotor and cognitive behaviors compatible with nAChR deficiency and underscore the importance of this chaperone for physiology and disease associated with nAChRs.