Calcium-sensing receptor regulates Kv7 channels via Gi/o protein signalling and modulates excitability of human induced pluripotent stem cell-derived nociceptive-like neurons

BACKGROUND AND PURPOSE

Neuropathic pain, a debilitating condition with unmet medical needs, can be characterised as hyperexcitability of nociceptive neurons caused by dysfunction of ion channels. Voltage-gated potassium channels type 7 (Kv7), responsible for maintaining neuronal resting membrane potential and thus excitability, reside under tight control of G protein-coupled receptors (GPCRs). Calcium-sensing receptor (CaSR) is a GPCR that regulates the activity of numerous ion channels, but whether CaSR can control Kv7 channel function has been unexplored until now.

EXPERIMENTAL APPROACH

Experiments were conducted in recombinant cell models, mouse dorsal root ganglia (DRG) neurons and human induced pluripotent stem cell (hiPSC)-derived nociceptive-like neurons using patch-clamp electrophysiology and molecular biology techniques.

KEY RESULTS

Our results demonstrate that CaSR is expressed in recombinant cell models, hiPSC-derived nociceptive-like neurons and mouse DRG neurons, and its activation induced depolarisation via Kv7.2/7.3 channel inhibition. The CaSR-Kv7.2/7.3 channel crosslink was mediated via the Gi/o protein-adenylate cyclase-cyclicAMP-protein kinase A signalling cascade. Suppression of CaSR function demonstrated a potential to rescue hiPSC-derived nociceptive-like neurons from algogenic cocktail-induced hyperexcitability.

CONCLUSION AND IMPLICATIONS

This study demonstrates that the CaSR-Kv7.2/7.3 channel crosslink, via a Gi/o protein signalling pathway, effectively regulates neuronal excitability, providing a feasible pharmacological target for neuronal hyperexcitability management in neuropathic pain.

Heterozygous RELN missense variants associated with genetic generalized epilepsy

PURPOSE

The RELN gene encodes the secreted glycoprotein Reelin and has important functions in both developing and adult brains. In this study, we aimed to explore the association between the RELN and genetic generalized epilepsy (GGE).

METHODS

We performed whole-exome sequencing on a cohort of 92 patients with GGE. Based on amino acid sequence alignments, allele frequency, pedigree validation and computational modeling, the RELN variants were identified and clinical features of cases were summarized. Cell-based Reelin secretion assays were examined by Western blotting. Alterations of mutant Reelin transport through the secretion pathway were detected by immunofluorescence staining.

RESULTS

Three novel pathogenic RELN variants (3.26%; c.2260C>T/p.R754W, c.2914C>G/p.P972A and c.3029G>A/p.R1010H) were identified. All probands showed adolescence-onset generalized seizures characterized by generalized epileptiform discharges with normal EEG backgrounds, no or mild cognitive impairment, and responded well to anti-seizure medications. All these variants were located in the central regions from 1B to 2A consecutive repeats, and protein modeling demonstrated structural alterations in Reelin. Moreover, we found that these heterozygous missense variants significantly decreased the secretion of mutant proteins in HEK-293T cells, and this impairment was due to the altered transport of mutant Reelin in the secretion pathway.

CONCLUSION

These results suggest that RELN is potentially associated with GGE. The phenotype of GGE caused by RELN variants is relatively mild, and the pathogenic mechanism may involve a loss-of-function.

Autosomal Dominant Hypocalcemia Type 1 and Neonatal Focal Seizures

Autosomal dominant hypocalcemia type 1 (ADH1) is a rare form of hypoparathyroidism that is characterized by gain-of-function mutations in the CASR gene, which provides instructions for producing the protein called calcium-sensing receptor (CaSR). Hypocalcemia in the neonatal period has a wide differential diagnosis. We present the case of a female newborn with genetic hypoparathyroidism (L125P mutation of CASR gene), hypocalcemia, and neonatal seizures due to the potential correlation between refractory neonatal seizures and ADH1. Neonatal seizures were previously described in patients with ADH1 but not in association with the L125P mutation of the CASR gene. Prompt diagnosis and management by a multidisciplinary and an appropriate therapeutic approach can prevent neurological and renal complications.

Calcium-sensing receptor-mediated NLRP3 inflammasome activation in rheumatoid arthritis and autoinflammation

The calcium-sensing receptor (CaSR) is expressed in many cell types - including immune cells and in particular circulating monocytes. Here, the receptor plays an important physiological role as a regulator of constitutive macropinocytosis. This review article provides an overview of the literature on the role of the calcium sensing receptor in the context of inflammatory processes. Special emphasis is laid upon the importance for monocytes in the context of rheumatoid arthritis. We have shown previously, that stimulation of the receptor by increased extracellular Ca²⁺ ([Ca²⁺]_ex) triggers a pro-inflammatory response due to NLRP3 inflammasome assembly and interleukin (IL)-1β release. The underlying mechanism includes macropinocytosis of calciprotein particles (CPPs), which are taken up in a [Ca²⁺]_ex-induced, CaSR dependent manner, and leads to strong IL-1β release. In rheumatoid arthritis (RA), this uptake and the resulting IL-1β release is significantly increased due to increased expression of the receptor. Moreover, increased [Ca²⁺]_ex-induced CPP uptake and IL-1β release is associated with more active disease, while CaSR overexpression has been reported to be associated with cardiovascular complications of RA. Most importantly, however, in animal experiments with arthritic mice, increased local calcium concentrations are present, which in combination with release of fetuin-A from eroded bone could contribute to formation of CPPs. We propose, that increased [Ca²⁺]_ex, CPPs and pro-inflammatory cytokines drive a vicious cycle of inflammation and bone destruction which in turn offers new potential therapeutic approaches.

Investigation into Antiepileptic Effect of Ganoderic Acid A and Its Mechanism in Seizure Rats Induced by Pentylenetetrazole

Ganoderic acid A (GAA) exhibited neuron protection in in vitro epilepsy study, but no study has been done in vivo. Rats were administered (i.p.) pentylenetetrazole daily for 28 days to induce seizure. Rats with grade II or above of epileptic score were divided into three groups and given placebo, sodium valproate, or GAA treatment, respectively, for 7 days. The electrical signals of brain were monitored with electroencephalography (EGG); epileptic behavior was assessed using the Racine scale; morphological changes and apoptosis rate of cortical neurons were assessed with H&E staining and TUNEL staining, respectively. Protein expression of calcium-sensing receptor, p-ERK, p-JNK, and p-p38 in hippocampal tissue and Bcl-2, cleaved caspase-3, and Bax in cortical tissues was observed by Western blot and immunohistochemistry assay, respectively. After GAA treatment, apparent seizure-like EEG with significant arrhythmic disorder and spike waves was reduced or disappeared, and wave amplitude of EEG was reduced significantly. GAA showed similar effect with sodium valproate treatments on epilepsy. There were an apparent improvement of the epileptic behavior and a significant increase in the epileptic latency and shortening of the epileptic duration in the treatment group compared to control. GAA treatment ameliorated the nuclear pyknosis of neurons which appeared seriously in the epilepsy group. GAA treatment significantly reduced the cortical neuron apoptosis of epilepsy and the expression of calcium-sensing receptor, p-P38, p-JNK, cleaved caspase-3, and Bax but increased the expression of both p-ERK and Bcl-2. In conclusion, GAA treatment showed strong antiepileptic effect by decreasing apoptosis in cortical neuron and the expression of calcium-sensing receptor and stimulating the MAPK pathway.

A Fifth of the Protein World: Rossmann-like Proteins as an Evolutionarily Successful Structural unit

The Rossmann-like fold is the most prevalent and diversified doubly-wound superfold of ancient evolutionary origin. Rossmann-like domains are present in a variety of metabolic enzymes and are capable of binding diverse ligands. Discerning evolutionary relationships among these domains is challenging because of their diverse functions and ancient origin. We defined a minimal Rossmann-like structural motif (RLM), identified RLM-containing domains among known 3D structures (20%) and classified them according to their homologous relationships. New classifications were incorporated into our Evolutionary Classification of protein Domains (ECOD) database. We defined 156 homology groups (H-groups), which were further clustered into 123 possible homology groups (X-groups). Our analysis revealed that RLM-containing proteins constitute approximately 15% of the human proteome. We found that disease-causing mutations are more frequent within RLM domains than within non-RLM domains of these proteins, highlighting the importance of RLM-containing proteins for human health.

Calcilytic NPS 2143 Reduces Amyloid Secretion and Increases sAβPPα Release from PSEN1 Mutant iPSC-Derived Neurons

Despite numerous efforts and studies over the last three decades, Alzheimer’s disease (AD) remains a disorder not fully understood and incurable so far. Development of induced pluripotent stem cell (iPSC) technology to obtain terminally differentiated neurons from adult somatic cells revolutionized the study of AD, providing a powerful tool for modelling the disease and for screening candidate drugs. Indeed, iPSC reprogramming allowed generation of neurons from both sporadic and familial AD patients with the promise to recapitulate the early pathological mechanisms in vitro and to identify novel targets. Interestingly, NPS 2143, a negative allosteric modulator of the calcium sensing receptor, has been indicated as a possible therapeutic for AD. In the present study, we assessed the potential of our iPSC-based familial AD cellular model as a platform for drug testing. We found that iPSC-derived neurons respond to treatment with γ-secretase inhibitor, modifying the physiological amyloid-β protein precursor (AβPP) processing and amyloid-β (Aβ) secretion. Moreover, we demonstrated the expression of calcium sensing receptor (CaSR) protein in human neurons derived from healthy and familial AD subjects. Finally, we showed that calcilytic NPS 2143 induced a changing of Aβ and sAβPPα secreted into conditioned media and modulation of CaSR and PSEN1 expression at the plasma membrane of AD neurons. Overall, our findings suggest that NPS 2143 affects important AD processes in a relevant in vitro system of familial AD.

International Union of Basic and Clinical Pharmacology. CVIII. Calcium-Sensing Receptor Nomenclature, Pharmacology, and Function

The calcium-sensing receptor (CaSR) is a class C G protein-coupled receptor that responds to multiple endogenous agonists and allosteric modulators, including divalent and trivalent cations, L-amino acids, γ-glutamyl peptides, polyamines, polycationic peptides, and protons. The CaSR plays a critical role in extracellular calcium (Ca2+ o) homeostasis, as demonstrated by the many naturally occurring mutations in the CaSR or its signaling partners that cause Ca2+ o homeostasis disorders. However, CaSR tissue expression in mammals is broad and includes tissues unrelated to Ca2+ o homeostasis, in which it, for example, regulates the secretion of digestive hormones, airway constriction, cardiovascular effects, cellular differentiation, and proliferation. Thus, although the CaSR is targeted clinically by the positive allosteric modulators (PAMs) cinacalcet, evocalcet, and etelcalcetide in hyperparathyroidism, it is also a putative therapeutic target in diabetes, asthma, cardiovascular disease, and cancer. The CaSR is somewhat unique in possessing multiple ligand binding sites, including at least five putative sites for the "orthosteric" agonist Ca2+ o, an allosteric site for endogenous L-amino acids, two further allosteric sites for small molecules and the peptide PAM, etelcalcetide, and additional sites for other cations and anions. The CaSR is promiscuous in its G protein-coupling preferences, and signals via Gq/11, Gi/o, potentially G12/13, and even Gs in some cell types. Not surprisingly, the CaSR is subject to biased agonism, in which distinct ligands preferentially stimulate a subset of the CaSR's possible signaling responses, to the exclusion of others. The CaSR thus serves as a model receptor to study natural bias and allostery. SIGNIFICANCE STATEMENT: The calcium-sensing receptor (CaSR) is a complex G protein-coupled receptor that possesses multiple orthosteric and allosteric binding sites, is subject to biased signaling via several different G proteins, and has numerous (patho)physiological roles. Understanding the complexities of CaSR structure, function, and biology will aid future drug discovery efforts seeking to target this receptor for a diversity of diseases. This review summarizes what is known to date regarding key structural, pharmacological, and physiological features of the CaSR.

Intractable Generalized Epilepsy and Autosomal Dominant Hypocalcemia: A Case Report

Calcium-sensing receptor gain-of-function mutations are known to cause autosomal dominant hypocalcemia and independently an epilepsy syndrome. We report the unique case of a child with both intractable generalized epilepsy and a chronic abnormality in calcium homeostasis due to a calcium-sensing receptor gene mutation. She is a 16-year-old female who began having staring events around 3 years of age. After her first generalized convulsion at age 5 years, investigations revealed hypocalcemia, hypercalciuria, and central nervous system calcifications. Her electroencephalogram demonstrated generalized epileptiform discharges, a hyperventilation-induced electroclinical seizure, and a photoconvulsive response. She has since been diagnosed with intellectual impairment, behavior disorder, and intractable childhood-onset seizures, the latter of which include eyelid myoclonia with absences. We conclude that calcium-sensing receptor gain-of-function mutations may precipitate an intractable generalized epilepsy syndrome with a comorbid endocrinopathy and that further investigations should be pursued in children with seizures presumed to be provoked by hypocalcemia.

CaSR is required for ischemia-induced proliferation and differentiation of white matter progenitor cells from neonatal rats

This study was designed to investigate whether calcium-sensing receptor (CaSR) could induce immture white matter progenitor cells proliferation and differentiation into oligodendrocyte(OL) precursor cells after oxygen-glucose deprivation (OGD) in vitro. Progenitor cells of immature white matter originating from five-day-old newborn rats were divided into control, OGD, control + CaSR silencing, OGD + CaSR silencing, control + adenosine triphosphate magnesium chloride (ATP-MgCl2) and OGD + ATP-MgCl2 groups. Immunofluorescence, real-time RT-PCR, gene silencing, Hoechst 33342/propidium iodide (PI) and Flow cytometry tests were used to examine the proliferation, differentiation and survival of the white matter progenitor cells in the different treatment groups. The results showed that normal immature white matter progenitor cells have certain ability of self-proliferation and differentiation in vitro. OGD could further induce progenitor cells proliferation and differentiation into O4 + OL precursor cells by activating CaSR, but OGD also induced more necrosis and apoptosis of newborn cells and less MBP + OL formation. The addition of ATP-MgCl2 as an activating agent of CaSR further promoted cell proliferation and differentiation both under normal and OGD conditions and reduced OGD-induced apoptosis and necrosis, while CaSR silenced resulted in minimal cell proliferation, differentiation and survival. This study suggests that CaSR plays an important role in the induction of immature white matter progenitor cells proliferation and differentiation into OL precursor cells after OGD, which may provide a new angle to further study whether CaSR initiates the intrinsic repair potential of immature white matter after ischemia in vivo.

Subtle Brain Developmental Abnormalities in the Pathogenesis of Juvenile Myoclonic Epilepsy

Juvenile myoclonic epilepsy (JME), a lifelong disorder that starts during adolescence, is the most common of genetic generalized epilepsy syndromes. JME is characterized by awakening myoclonic jerks and myoclonic-tonic-clonic (m-t-c) grand mal convulsions. Unfortunately, one third of JME patients have drug refractory m-t-c convulsions and these recur in 70-80% who attempt to stop antiepileptic drugs (AEDs). Behavioral studies documented impulsivity, but also impairment of executive functions relying on organization and feedback, which points to prefrontal lobe dysfunction. Quantitative voxel-based morphometry (VBM) revealed abnormalities of gray matter (GM) volumes in cortical (frontal and parietal) and subcortical structures (thalamus, putamen, and hippocampus). Proton magnetic resonance spectroscopy (MRS) found evidence of dysfunction of thalamic neurons. White matter (WM) integrity was disrupted in corpus callosum and frontal WM tracts. Magnetic resonance imaging (MRI) further unveiled anomalies in both GM and WM structures that were already present at the time of seizure onset. Aberrant growth trajectories of brain development occurred during the first 2 years of JME diagnosis. Because of genetic origin, disease causing variants were sought, first by positional cloning, and most recently, by next generation sequencing. To date, only six genes harboring pathogenic variants (GABRA1, GABRD, EFHC1, BRD2, CASR, and ICK) with Mendelian and complex inheritance and covering a limited proportion of the world population, are considered as major susceptibility alleles for JME. Evidence on the cellular role, developmental and cell-type expression profiles of these six diverse JME genes, point to their pathogenic variants driving the first steps of brain development when cell division, expansion, axial, and tangential migration of progenitor cells (including interneuron cortical progenitors) sculpture subtle alterations in brain networks and microcircuits during development. These alterations may explain "microdysgenesis" neuropathology, impulsivity, executive dysfunctions, EEG polyspike waves, and awakening m-t-c convulsions observed in JME patients.

The Nervous System Relevance of the Calcium Sensing Receptor in Health and Disease

The calcium sensing receptor (CaSR) was first identified in parathyroid glands, and its primary role in controlling systemic calcium homeostasis by the regulation of parathyroid hormone (PTH) secretion has been extensively described in literature. Additionally, the receptor has also been investigated in cells and tissues not directly involved in calcium homeostasis, e.g., the nervous system (NS), where it plays crucial roles in early neural development for the differentiation of neurons and glial cells, as well as in the adult nervous system for synaptic transmission and plasticity. Advances in the knowledge of the CaSR's function in such physiological processes have encouraged researchers to further broaden the receptor's investigation in the neuro-pathological conditions of the NS. Interestingly, pre-clinical data suggest that receptor inhibition by calcilytics might be effective in counteracting the pathomechanism underlying Alzheimer's disease and ischemia, while a CaSR positive modulation with calcimimetics has been proposed as a potential approach for treating neuroblastoma. Importantly, such promising findings led to the repurposing of CaSR modulators as novel pharmacological alternatives for these disorders. Therefore, the aim of this review article is to critically appraise evidence which, so far, has been yielded from the investigation of the role of the CaSR in physiology of the nervous system and to focus on the most recent emerging concepts which have reported the receptor as a therapeutic target for neurodegeneration and neuroblastic tumors.

The calcium-sensing receptor in physiology and in calcitropic and noncalcitropic diseases

The Ca2+-sensing receptor (CaSR) is a dimeric family C G protein-coupled receptor that is expressed in calcitropic tissues such as the parathyroid glands and the kidneys and signals via G proteins and β-arrestin. The CaSR has a pivotal role in bone and mineral metabolism, as it regulates parathyroid hormone secretion, urinary Ca2+ excretion, skeletal development and lactation. The importance of the CaSR for these calcitropic processes is highlighted by loss-of-function and gain-of-function CaSR mutations that cause familial hypocalciuric hypercalcaemia and autosomal dominant hypocalcaemia, respectively, and also by the fact that alterations in parathyroid CaSR expression contribute to the pathogenesis of primary and secondary hyperparathyroidism. Moreover, the CaSR is an established therapeutic target for hyperparathyroid disorders. The CaSR is also expressed in organs not involved in Ca2+ homeostasis: it has noncalcitropic roles in lung and neuronal development, vascular tone, gastrointestinal nutrient sensing, wound healing and secretion of insulin and enteroendocrine hormones. Furthermore, the abnormal expression or function of the CaSR is implicated in cardiovascular and neurological diseases, as well as in asthma, and the CaSR is reported to protect against colorectal cancer and neuroblastoma but increase the malignant potential of prostate and breast cancers.

Novel and de novo mutations in pediatric refractory epilepsy

Pediatric refractory epilepsy is a broad phenotypic spectrum with great genetic heterogeneity. Next-generation sequencing (NGS) combined with Sanger sequencing could help to understand the genetic diversity and underlying disease mechanisms in pediatric epilepsy. Here, we report sequencing results from a cohort of 172 refractory epilepsy patients aged 0-14 years. The pathogenicity of identified variants was evaluated in accordance with the American College of Medical Genetics and Genomics (ACMG) criteria. We identified 43 pathogenic or likely pathogenic variants in 40 patients (23.3%). Among these variants, 74.4% mutations (32/43) were de novo and 60.5% mutations (26/43) were novel. Patients with onset age of seizures ≤12 months had higher yields of deleterious variants compared to those with onset age of seizures > 12 months (P = 0.006). Variants in ion channel genes accounted for the greatest functional gene category (55.8%), with SCN1A coming first (16/43). 81.25% (13/16) of SCN1A mutations were de novo and 68.8% (11/16) were novel in Dravet syndrome. Pathogenic or likely pathogenic variants were found in the KCNQ2, STXBP1, SCN2A genes in Ohtahara syndrome. Novel deleterious variants were also found in West syndrome, Doose syndrome and glucose transporter type 1 deficiency syndrome patients. One de novo MECP2 mutation were found in a Rett syndrome patient. TSC1/TSC2 variants were found in 60% patients with tuberous sclerosis complex patients. Other novel mutations detected in unclassified epilepsy patients involve the SCN8A, CACNA1A, GABRB3, GABRA1, IQSEC2, TSC1, VRK2, ATP1A2, PCDH19, SLC9A6 and CHD2 genes. Our study provides novel insights into the genetic origins of pediatric epilepsy and represents a starting-point for further investigations into the molecular pathophysiology of pediatric epilepsy that could eventually lead to better treatments.

Clinical and genetic study of Tunisian families with genetic generalized epilepsy: contribution of CACNA1H and MAST4 genes

Genetic generalized epilepsies (GGE) (childhood absence epilepsy (CAE), juvenile myoclonic epilepsy (JME) and epilepsy with generalized tonic-clonic seizures (GTCS)) are mainly determined by genetic factors. Since few mutations were identified in rare families with autosomal dominant GGE, a polygenic inheritance was suspected in most patients. Recent studies on large American or European cohorts of sporadic cases showed that susceptibility genes were numerous although their variants were rare, making their identification difficult. Here, we reported clinical and genetic characteristics of 30 Tunisian GGE families, including 71 GGE patients. The phenotype was close to that in sporadic cases. Nineteen pedigrees had a homogeneous type of GGE (JME-CAE-CGTS), and 11 combined these epileptic syndromes. Rare non-synonymous variants were selected in probands using a targeted panel of 30 candidate genes and their segregation was determined in families. Molecular studies incriminated different genes, mainly CACNA1H and MAST4. The segregation of at least two variants in different genes in some pedigrees was compatible with the hypothesis of an oligogenic inheritance, which was in accordance with the relatively low frequency of consanguineous probands. Since at least 2 susceptibility genes were likely shared by different populations, genetic factors involved in the majority of Tunisian GGE families remain to be discovered. Their identification should be easier in families with a homogeneous type of GGE, in which an intra-familial genetic homogeneity could be suspected.

A Forward Genetic Screen in Zebrafish Identifies the G-Protein-Coupled Receptor CaSR as a Modulator of Sensorimotor Decision Making

Animals continuously integrate sensory information and select contextually appropriate responses. Here, we show that zebrafish larvae select a behavioral response to acoustic stimuli from a pre-existing choice repertoire in a context-dependent manner. We demonstrate that this sensorimotor choice is modulated by stimulus quality and history, as well as by neuromodulatory systems-all hallmarks of more complex decision making. Moreover, from a genetic screen coupled with whole-genome sequencing, we identified eight mutants with deficits in this sensorimotor choice, including mutants of the vertebrate-specific G-protein-coupled extracellular calcium-sensing receptor (CaSR), whose function in the nervous system is not well understood. We demonstrate that CaSR promotes sensorimotor decision making acutely through Gα_i/o and Gα_q/11 signaling, modulated by clathrin-mediated endocytosis. Combined, our results identify the first set of genes critical for behavioral choice modulation in a vertebrate and reveal an unexpected critical role for CaSR in sensorimotor decision making.

Genetic susceptibility in Juvenile Myoclonic Epilepsy: Systematic review of genetic association studies

BACKGROUND

Several genetic association investigations have been performed over the last three decades to identify variants underlying Juvenile Myoclonic Epilepsy (JME). Here, we evaluate the accumulating findings and provide an updated perspective of these studies.

METHODOLOGY

A systematic literature search was conducted using the PubMed, Embase, Scopus, Lilacs, epiGAD, Google Scholar and Sigle up to February 12, 2016. The quality of the included studies was assessed by a score and classified as low and high quality. Beyond outcome measures, information was extracted on the setting for each study, characteristics of population samples and polymorphisms.

RESULTS

Fifty studies met eligibility criteria and were used for data extraction. With a single exception, all studies used a candidate gene approach, providing data on 229 polymorphisms in or near 55 different genes. Of variants investigating in independent data sets, only rs2029461 SNP in GRM4, rs3743123 in CX36 and rs3918149 in BRD2 showed a significant association with JME in at least two different background populations. The lack of consistent associations might be due to variations in experimental design and/or limitations of the approach.

CONCLUSIONS

Thus, despite intense research evidence established, specific genetic variants in JME susceptibility remain inconclusive. We discussed several issues that may compromise the quality of the results, including methodological bias, endophenotype and potential involvement of epigenetic factors.

PROSPERO REGISTRATION NUMBER

CRD42016036063.

Molecular Genetics of Epilepsy: A Clinician's Perspective

Epilepsy is a common neurological problem, and there is a genetic basis in almost 50% of people with epilepsy. The diagnosis of genetic epilepsies makes the patient assured of the reasons of his/her seizures and avoids unnecessary, expensive, and invasive investigations. Last decade has shown tremendous growth in gene sequencing technologies, which have made genetic tests available at the bedside. Whole exome sequencing is now being routinely used in the clinical setting for making a genetic diagnosis. Genetic testing not only makes the diagnosis but also has an effect on the management of the patients, for example, the role of sodium channels blockers in SCN1A+ Dravet syndrome patients and usefulness of ketogenic diet therapy in SLC2A1+ generalized epilepsy patients. Many clinicians in our country have no or limited knowledge about the molecular genetics of epilepsies, types of genetic tests available, how to access them and how to interpret the results. The purpose of this review is to give an overview in this direction and encourage the clinicians to start considering genetic testing as an important investigation along with electroencephalogram and magnetic resonance imaging for better understanding and management of epilepsy in their patients.

Novel Mutation in the CASR Gene (p.Leu123Ser) in a Case of Autosomal Dominant Hypocalcemia

Autosomal dominant hypocalcemia, caused by activating mutations of the calcium-sensing receptor (CASR) gene, is characterized by hypocalcemia with an inappropriately low concentration of parathyroid hormone (PTH). In this report, we describe the identification of a novel missense mutation in the CASR gene, in a boy with autosomal dominant hypocalcemia. Polymerase chain reaction (PCR)-single strand and DNA sequencing revealed a heterozygous mutation in CASR gene that causes a leucine substitution for serine at codon 123 (p.Leu123Ser). This mutation was absent in DNA from 50 control patients. In silico studies suggest that the identified variant was likely pathogenic. Sequencing analysis in the mother suggested mosaicism for the same variant, and she was clinically and biochemically unaffected. Clinical manifestations of the index case started with seizures at 14 months of age; cognitive impairment and several neuropsychological disabilities were noted during childhood. Extrapyramidal signs and basal ganglia calcification developed later, namely, hand tremor and rigidity at the age of 7 and 18 years, respectively. Laboratory analysis revealed hypocalcemia, hyperphosphatemia, and low-serum PTH with hypomagnesemia and mild hypercalciuria. After 2 years of treatment with calcium supplements and calcitriol, some brief periods of clinical improvement were reported; as well as an absence of nephrocalcinosis.

Voltage Dependence of a Neuromodulator-Activated Ionic Current

The neuromodulatory inward current (IMI) generated by crab Cancer borealis stomatogastric ganglion neurons is an inward current whose voltage dependence has been shown to be crucial in the activation of oscillatory activity of the pyloric network of this system. It has been previously shown that IMI loses its voltage dependence in conditions of low extracellular calcium, but that this effect appears to be regulated by intracellular calmodulin. Voltage dependence is only rarely regulated by intracellular signaling mechanisms. Here we address the hypothesis that the voltage dependence of IMI is mediated by intracellular signaling pathways activated by extracellular calcium. We demonstrate that calmodulin inhibitors and a ryanodine antagonist can reduce IMI voltage dependence in normal Ca(2+), but that, in conditions of low Ca(2+), calmodulin activators do not restore IMI voltage dependence. Further, we show evidence that CaMKII alters IMI voltage dependence. These results suggest that calmodulin is necessary but not sufficient for IMI voltage dependence. We therefore hypothesize that the Ca(2+)/calmodulin requirement for IMI voltage dependence is due to an active sensing of extracellular calcium by a GPCR family calcium-sensing receptor (CaSR) and that the reduction in IMI voltage dependence by a calmodulin inhibitor is due to CaSR endocytosis. Supporting this, preincubation with an endocytosis inhibitor prevented W7 (N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride)-induced loss of IMI voltage dependence, and a CaSR antagonist reduced IMI voltage dependence. Additionally, myosin light chain kinase, which is known to act downstream of the CaSR, seems to play a role in regulating IMI voltage dependence. Finally, a Gβγ-subunit inhibitor also affects IMI voltage dependence, in support of the hypothesis that this process is regulated by a G-protein-coupled CaSR.

Calcium-Sensing Receptor: A Key Target for Extracellular Calcium Signaling in Neurons

Though both clinicians and scientists have long recognized the influence of extracellular calcium on the function of muscle and nervous tissue, recent insights reveal that the mechanisms allowing changes in extracellular calcium to alter cellular excitability have been incompletely understood. For many years the effects of calcium on neuronal signaling were explained only in terms of calcium entry through voltage-gated calcium channels and biophysical charge screening. More recently however, it has been recognized that the calcium-sensing receptor is prevalent in the nervous system and regulates synaptic transmission and neuronal activity via multiple signaling pathways. Here we review the multiplicity of mechanisms by which changes in extracellular calcium alter neuronal signaling and propose that multiple mechanisms are required to describe the full range of experimental observations.

The importance of de novo mutations for pediatric neurological disease--It is not all in utero or birth trauma

The advent of next generation sequencing (NGS, which consists of massively parallel sequencing to perform TGS (total genome sequencing) or WES (whole exome sequencing)) has abundantly discovered many causative mutations in patients with pediatric neurological disease. A surprisingly high number of these are de novo mutations which have not been inherited from either parent. For epilepsy, autism spectrum disorders, and neuromotor disorders, including cerebral palsy, initial estimates put the frequency of causative de novo mutations at about 15% and about 10% of these are somatic. There are some shared mutated genes between these three classes of disease. Studies of copy number variation by comparative genomic hybridization (CGH) proceded the NGS approaches but they also detect de novo variation which is especially important for ASDs. There are interesting differences between the mutated genes detected by CGS and NGS. In summary, de novo mutations cause a very significant proportion of pediatric neurological disease.

Spectrum of epilepsy - prevalence, impact, and treatment gap: an epidemiological study from Al-Quseir, Egypt

BACKGROUND

Epidemiology continues to be an important research tool in the study of epilepsy and related disorders, providing a better understanding of the frequency, causes, and natural history of the disorder.

OBJECTIVE

To estimate the prevalence of epilepsy in Al-Quseir, Red Sea Governorate, Egypt, and its magnitude of treatment gap.

METHODS

The study was part of a door-to-door study, including every door, to screen all inhabitants in Al-Quseir (33,818 inhabitants) by three specialists of neurology and 15 female social workers (for demographic data collection) using a standardized screening questionnaire. All suspected cases were subjected to detailed history, clinical examination, and electroencephalogram. Neuroimaging studies and estimation of serum drug level were done in select cases if needed.

RESULTS

The study revealed that the lifetime prevalence rate of epilepsy in Al-Quseir is 5.5/1,000, with the highest peak during early childhood, while that of active epilepsy is 3.3/1,000 population. The annual incidence rate is 48/100,000, and the age-specific incidence rate has a U-shaped pattern with two peaks of incidence in early infancy and elderly life. Localization-related epilepsy is the most frequently encountered type (58.8%). The treatment gap of epilepsy in Al-Quseir is 83.8%.

CONCLUSION

The lifetime prevalence of epilepsy in Al-Quseir city, Red Sea Governorate, was 5.5/1000.

Anticonvulsant treatment associated with intractable hypocalcaemia in a female child with hypoparathyroidism

BACKGROUND

We report the case of a female infant with hypoparathyroidism due to an activating mutation in the calcium-sensing receptor gene.

CASE REPORT

The child presented in the neonatal period with clinical seizures associated with severe hypocalcaemia, hyperphosphataemia, low parathyroid hormone levels and elevated urine calcium:creatinine ratios. She required intravenous calcium and phenobarbitone initially, and then oral 1-alfacalcidol (1-AC) and phenobarbitone were started. The patient had intractable hypocalcaemia in the first 5 months of life despite escalating doses of 1-AC. When the phenobarbitone was stopped at 5 months of age she was admitted soon after with symptomatic hypercalcaemia. We postulate that the phenobarbitone increased the metabolism of 1-AC and thus she needed large doses of 1-AC to treat hypocalcaemia until the phenobarbitone was stopped. Her parents had no biochemical abnormalities on testing.

RESULTS

Molecular genetic analysis confirmed that our patient had a de novo missense variant, c.682G>A (p.Glu228Lys) in exon 4 of the calcium-sensing receptor.

CONCLUSION

This case report highlights the importance that clinicians caring for children on vitamin D and its analogues are aware of the interaction with phenobarbitone, which can result in symptomatic hypocalcaemia. 1-AC should be stored at 2-8°C, otherwise it will be rendered inactive.

14-3-3 Proteins Buffer Intracellular Calcium Sensing Receptors to Constrain Signaling

Calcium sensing receptors (CaSR) interact with 14-3-3 binding proteins at a carboxyl terminal arginine-rich motif. Mutations identified in patients with familial hypocalciuric hypercalcemia, autosomal dominant hypocalcemia, pancreatitis or idiopathic epilepsy support the functional importance of this motif. We combined total internal reflection fluorescence microscopy and biochemical approaches to determine the mechanism of 14-3-3 protein regulation of CaSR signaling. Loss of 14-3-3 binding caused increased basal CaSR signaling and plasma membrane levels, and a significantly larger signaling-evoked increase in plasma membrane receptors. Block of core glycosylation with tunicamycin demonstrated that changes in plasma membrane CaSR levels were due to differences in exocytic rate. Western blotting to quantify time-dependent changes in maturation of expressed wt CaSR and a 14-3-3 protein binding-defective mutant demonstrated that signaling increases synthesis to maintain constant levels of the immaturely and maturely glycosylated forms. CaSR thus operates by a feed-forward mechanism, whereby signaling not only induces anterograde trafficking of nascent receptors but also increases biosynthesis to maintain steady state levels of net cellular CaSR. Overall, these studies suggest that 14-3-3 binding at the carboxyl terminus provides an important buffering mechanism to increase the intracellular pool of CaSR available for signaling-evoked trafficking, but attenuates trafficking to control the dynamic range of responses to extracellular calcium.

G protein-coupled receptor mutations and human genetic disease

Genetic variations in G protein-coupled receptor genes (GPCRs) disrupt GPCR function in a wide variety of human genetic diseases. In vitro strategies and animal models have been used to identify the molecular pathologies underlying naturally occurring GPCR mutations. Inactive, overactive, or constitutively active receptors have been identified that result in pathology. These receptor variants may alter ligand binding, G protein coupling, receptor desensitization and receptor recycling. Receptor systems discussed include rhodopsin, thyrotropin, parathyroid hormone, melanocortin, follicle-stimulating hormone (FSH), luteinizing hormone, gonadotropin-releasing hormone (GNRHR), adrenocorticotropic hormone, vasopressin, endothelin-β, purinergic, and the G protein associated with asthma (GPRA or neuropeptide S receptor 1 (NPSR1)). The role of activating and inactivating calcium-sensing receptor (CaSR) mutations is discussed in detail with respect to familial hypocalciuric hypercalcemia (FHH) and autosomal dominant hypocalemia (ADH). The CASR mutations have been associated with epilepsy. Diseases caused by the genetic disruption of GPCR functions are discussed in the context of their potential to be selectively targeted by drugs that rescue altered receptors. Examples of drugs developed as a result of targeting GPCRs mutated in disease include: calcimimetics and calcilytics, therapeutics targeting melanocortin receptors in obesity, interventions that alter GNRHR loss from the cell surface in idiopathic hypogonadotropic hypogonadism and novel drugs that might rescue the P2RY12 receptor congenital bleeding phenotype. De-orphanization projects have identified novel disease-associated receptors, such as NPSR1 and GPR35. The identification of variants in these receptors provides genetic reagents useful in drug screens. Discussion of the variety of GPCRs that are disrupted in monogenic Mendelian disorders provides the basis for examining the significance of common pharmacogenetic variants.

Novel carboxypeptidase A6 (CPA6) mutations identified in patients with juvenile myoclonic and generalized epilepsy

Carboxypeptidase A6 (CPA6) is a peptidase that removes C-terminal hydrophobic amino acids from peptides and proteins. The CPA6 gene is expressed in the brains of humans and animals, with high levels of expression during development. It is translated with a prodomain (as proCPA6), which is removed before secretion. The active form of CPA6 binds tightly to the extracellular matrix (ECM) where it is thought to function in the processing of peptides and proteins. Mutations in the CPA6 gene have been identified in patients with temporal lobe epilepsy and febrile seizures. In the present study, we screened for CPA6 mutations in patients with juvenile myoclonic epilepsy and identified two novel missense mutations: Arg36His and Asn271Ser. Patients harboring these mutations also presented with generalized epilepsy. Neither of the novel mutations was found in a control population. Asn271 is highly conserved in CPA6 and other related metallocarboxypeptidases. Arg36 is present in the prodomain and is not highly conserved. To assess structural consequences of the amino acid substitutions, both mutants were modeled within the predicted structure of the enzyme. To examine the effects of these mutations on enzyme expression and activity, we expressed the mutated enzymes in human embryonic kidney 293T cells. These analyses revealed that Asn271Ser abolished enzymatic activity, while Arg36His led to a ~50% reduction in CPA6 levels in the ECM. Pulse-chase using radio-labeled amino acids was performed to follow secretion. Newly-synthesized CPA6 appeared in the ECM with peak levels between 2-8 hours. There was no major difference in time course between wild-type and mutant forms, although the amount of radiolabeled CPA6 in the ECM was lower for the mutants. Our experiments demonstrate that these mutations in CPA6 are deleterious and provide further evidence for the involvement of CPA6 mutations in the predisposition for several types of epilepsy.

New functional aspects of the extracellular calcium-sensing receptor

PURPOSE OF REVIEW

Variations in extracellular calcium level have a large impact on kidney function. Most of the effects seen are attributed to the calcium-sensing receptor (CaSR), a widely expressed G-protein-coupled cell surface protein with an important function in bone mineral homeostasis. The purpose of this review is to recapitulate the novel functional aspects of CaSR.

RECENT FINDINGS

Results from mouse models demonstrate important functions for CaSR in various tissues. In the kidney, the main role of CaSR is the regulation of calcium reabsorption in the thick ascending limb, independently of its role on parathyroid hormone secretion. CaSR modulates claudin 14, the gatekeeper of paracellular ion transport in the thick ascending limb that is associated with urinary calcium excretion. One intracellular signaling pathway by which CaSR alters tight junction permeability is the calcineurin-NFAT1c-microRNA-claudin14 axis.

SUMMARY

The main function of CaSR in the kidney is the regulation of calcium excretion in the thick ascending limb, independently of parathyroid hormone. CaSR modulates paracellular cation transport by altering expression of the tight junction protein claudin 14. Still more work is needed to fully understand all functions of CaSR in the kidney. Alternative pathways of calcium 'sensing' in the kidney need to be investigated.

Engendering biased signalling from the calcium-sensing receptor for the pharmacotherapy of diverse disorders

The human calcium-sensing receptor (CaSR) is widely expressed in the body, where its activity is regulated by multiple orthosteric and endogenous allosteric ligands. Each ligand stabilizes a unique subset of conformational states, which enables the CaSR to couple to distinct intracellular signalling pathways depending on the extracellular milieu in which it is bathed. Differential signalling arising from distinct receptor conformations favoured by each ligand is referred to as biased signalling. The outcome of CaSR activation also depends on the cell type in which it is expressed. Thus, the same ligand may activate diverse pathways in distinct cell types. Given that the CaSR is implicated in numerous physiological and pathophysiological processes, it is an ideal target for biased ligands that could be rationally designed to selectively regulate desired signalling pathways in preferred cell types.

Chromosomal microarray analysis as a first-tier clinical diagnostic test: Estonian experience

Chromosomal microarray analysis (CMA) is now established as the first-tier cytogenetic diagnostic test for fast and accurate detection of chromosomal abnormalities in patients with developmental delay/intellectual disability (DD/ID), multiple congenital anomalies (MCA), and autism spectrum disorders (ASD). We present our experience with using CMA for postnatal and prenatal diagnosis in Estonian patients during 2009-2012. Since 2011, CMA is on the official service list of the Estonian Health Insurance Fund and is performed as the first-tier cytogenetic test for patients with DD/ID, MCA or ASD. A total of 1191 patients were analyzed, including postnatal (1072 [90%] patients and 59 [5%] family members) and prenatal referrals (60 [5%] fetuses). Abnormal results were reported in 298 (25%) patients, with a total of 351 findings (1-3 per individual): 147 (42%) deletions, 106 (30%) duplications, 89 (25%) long contiguous stretches of homozygosity (LCSH) events (>5 Mb), and nine (3%) aneuploidies. Of all findings, 143 (41%) were defined as pathogenic or likely pathogenic; for another 143 findings (41%), most of which were LCSH, the clinical significance remained unknown, while 61 (18%) reported findings can now be reclassified as benign or likely benign. Clinically relevant findings were detected in 126 (11%) patients. However, the proportion of variants of unknown clinical significance was quite high (41% of all findings). It seems that our ability to detect chromosomal abnormalities has far outpaced our ability to understand their role in disease. Thus, the interpretation of CMA findings remains a rather difficult task requiring a close collaboration between clinicians and cytogeneticists.

The quest for juvenile myoclonic epilepsy genes

Introduced into a specific population, a juvenile myoclonic epilepsy (JME) mutation generates linkage disequilibrium (LD). Linkage disequilibrium is strongest when the JME mutation is of recent origin, still "hitchhiking" alleles surrounding it, as a haplotype into the next thousands of generations. Recombinations decay LD over tens of thousands of generations causing JME alleles to produce smaller genetic displacements, requiring other genes or environment to produce an epilepsy phenotype. Family-based linkage analysis captures rare epilepsy alleles and their "hitchhiking" haplotypes, transmitted as Mendelian traits, supporting the common disease/multiple rare allele model. Genome-wide association studies identify JME alleles whose linkage disequilibrium has decayed through thousands of generations and are sorting out the common disease/common allele versus rare allele models. Five Mendelian JME genes have been identified, namely, CACNB4, CASR, GABRa1, GABRD, and Myoclonin1/EFHC1. Three SNP alleles in BRD2, Cx-36, and ME2 and microdeletions in 15q13.3, 15q11.2, and 16p13.11 also contribute risk to JME.

Calcium-sensing receptor (CaSR) mutations and disorders of calcium, electrolyte and water metabolism

The extracellular calcium-sensing receptor (CaSR) is a family C G-protein-coupled receptor (GPCR) that is expressed at multiple sites, including the parathyroids and kidneys. The human CASR gene, located on chromosome 3q21.1, encodes a 1078 amino acid protein. More than 230 different disease-causing mutations of the CaSR have been reported. Loss-of-function mutations lead to three hypercalcemic disorders, which are familial hypocalciuric hypercalcemia (FHH), neonatal severe hyperparathyroidism and primary hyperparathyroidism. Gain-of-function mutations, on the other hand, result in the hypocalcemic disorders of autosomal dominant hypocalcemia and Bartter syndrome type V. Moreover, autoantibodies directed against the extracellular domain of the CaSR have been found to be associated with FHH in some patients, and also in some patients with hypoparathyroidism that may be part of autoimmune polyglandular syndrome type 1. Studies of disease-causing CASR mutations have provided insights into structure-function relationships and highlighted intra-molecular domains that are critical for ligand binding, intracellular signaling, and receptor trafficking.

Roles of the calcium sensing receptor in the central nervous system

The calcium sensing receptor (CaSR) is expressed by subpopulations of neuronal and glial cells throughout the brain and is activated by extracellular calcium [Formula: see text] . During development, the CaSR regulates neuronal cell growth and migration as well as oligodendroglial maturation and function. Emerging evidence suggests that in nerve terminals, CaSR is implicated in synaptic plasticity and neurotransmission. In this review, we analyze the roles attributed to CaSR in regulating diverse brain functions, including central regulation of body fluid composition and blood pressure. We also discuss the potential relevance of Ca(2+)-sensing in brain by other family C G protein-coupled receptors. Finally, evidence that the CaSR contributes to the pathogenesis of various brain disorders raises the possibility that pharmacological modulators of the CaSR may have therapeutic benefit.

CELF4 regulates translation and local abundance of a vast set of mRNAs, including genes associated with regulation of synaptic function

RNA–binding proteins have emerged as causal agents of complex neurological diseases. Mice deficient for neuronal RNA–binding protein CELF4 have a complex neurological disorder with epilepsy as a prominent feature. Human CELF4 has recently been associated with clinical features similar to those seen in mutant mice. CELF4 is expressed primarily in excitatory neurons, including large pyramidal cells of the cerebral cortex and hippocampus, and it regulates excitatory but not inhibitory neurotransmission. We examined mechanisms underlying neuronal hyperexcitability in Celf4 mutants by identifying CELF4 target mRNAs and assessing their fate in the absence of CELF4 in view of their known functions. CELF4 binds to at least 15%–20% of the transcriptome, with striking specificity for the mRNA 3′ untranslated region. CELF4 mRNA targets encode a variety of proteins, many of which are well established in neuron development and function. While the overall abundance of these mRNA targets is often dysregulated in Celf4 deficient mice, the actual expression changes are modest at the steady-state level. In contrast, by examining the transcriptome of polysome fractions and the mRNA distribution along the neuronal cell body-neuropil axis, we found that CELF4 is critical for maintaining mRNA stability and availability for translation. Among biological processes associated with CELF4 targets that accumulate in neuropil of mutants, regulation of synaptic plasticity and transmission are the most prominent. Together with a related study of the impact of CELF4 loss on sodium channel Na_v1.6 function, we suggest that CELF4 deficiency leads to abnormal neuronal function by combining a specific effect on neuronal excitation with a general impairment of synaptic transmission. These results also expand our understanding of the vital roles RNA–binding proteins play in regulating and shaping the activity of neural circuits.

Epilepsy is a devastating brain disorder whereby a loss of regulation of electrochemical signals between neurons causes too much excitation and ultimately results in an “electrical storm” known as a seizure. Epilepsy can be heritable, but it is usually genetically complex, resulting from a collaboration of many genes. It is also a frequent feature of other common brain diseases, such as autism spectrum disorder and intellectual disability, likely because these diseases have a similar dysregulation of neuronal communication. To understand more about how the brain regulates electrical activity, we focused on an RNA–binding protein called CELF4, because a) mice that lack CELF4 have a complex form of epilepsy that includes features of other neurological diseases and b) this kind of protein has the potential to be a master regulator. We show that CELF4 binds to a vast array of mRNAs, and without CELF4 these mRNAs accumulate in the wrong places and can produce the wrong amount of protein. Moreover, many of these mRNAs encode key players in electrochemical signaling between neurons. Although the defects in individual mRNAs are modest, like a genetically complex disease, together these alterations collude to cause neurological symptoms including recurrent seizures.

The association of primary hyperparathyroidism with pancreatitis

The association between primary hyperparathyroidism (PHPT) and acute or chronic pancreatitis is controversial. For this reason, we conducted a review of the literature over the past 30 years to explore the relationship between these 2 disorders. Ten retrospective studies each with >50 patients diagnosed with PHPT were identified. With the notable exception of 2 studies, the rate of pancreatitis among patients with PHPT was higher than that reported in general among hospitalized patients without PHPT. A higher serum calcium level may contribute to pancreatitis in these cases, along with additional genetic or environmental insults. Hypercalcemia may predispose the pancreatic acinar cell to abnormal, sustained calcium levels, lead to premature pancreatic protease activation, and pancreatitis. Although there was only short-term follow-up, most reports cited that definitive treatment of PHPT by parathyroidectomy led to the resolution of pancreatitis attacks. The published cohorts of patients with PHPT and pancreatitis are subject to bias, because serum calcium screening was not universally performed among all control nonpancreatitis patients to evaluate for PHPT. However, the pooled clinical and experimental data suggest an association between PHPT and pancreatitis and implicate hypercalcemia. For clinicians, it is important to recognize pancreatitis in patients with PHPT and, conversely, to consider PHPT by checking serum calcium levels in patients, who present with an unexplained pancreatitis.

The role of the calcium-sensing receptor in human disease

Following the discovery of the calcium-sensing receptor (CaSR) in 1993, its pivotal role in disorders of calcium homeostasis such as Familial Hypocalciuric Hypercalcemia (FHH) was quickly demonstrated. Since then, it has become clear that the CaSR has immense functional versatility largely through its ability to activate many different signaling pathways in a ligand- and tissue-specific manner. This allows the receptor to play diverse and crucial roles in human physiology and pathophysiology, both in calcium homeostasis and in tissues and biological processes unrelated to calcium balance. This review covers current knowledge of the role of the CaSR in disorders of calcium homeostasis (FHH, neonatal severe hyperparathyroidism, autosomal dominant hypocalcemia, primary and secondary hyperparathyroidism, hypercalcemia of malignancy) as well as unrelated diseases such as breast and colorectal cancer (where the receptor appears to play a tumor suppressor role), Alzheimer's disease, pancreatitis, diabetes mellitus, hypertension and bone and gastrointestinal disorders. In addition, it examines the use or potential use of CaSR agonists or antagonists (calcimimetics and calcilytics) and other drugs mediated through the CaSR, in the management of disorders as diverse as hyperparathyroidism, osteoporosis and gastrointestinal disease.

Sodium leak channels in neuronal excitability and rhythmic behaviors

Extracellular K⁺, Na⁺, and Ca²⁺ ions all influence the resting membrane potential of the neuron. However, the mechanisms by which extracellular Na⁺ and Ca²⁺ regulate basal neuronal excitability are not well understood. Recent findings suggest that NALCN, in association with UNC79 and UNC80, contributes a basal Na⁺ leak conductance in neurons. Mutations in Nalcn, Unc79, or Unc80 lead to severe phenotypes that include neonatal lethality and disruption in rhythmic behaviors. This review discusses the properties of the NALCN complex, its regulation, and its contribution to neuronal function and animal behavior.

A new locus for autosomal dominant generalized epilepsy associated with mild mental retardation on chromosome 3p

PURPOSE

Generalized epilepsies are clinically and genetically heterogeneous syndromes. Idiopathic generalized epilepsy (IGE), which has a strong genetic background, is not associated with any additional clinical features, such as mental retardation (MR). Herein we report results of linkage analysis in a large family with autosomal dominant (AD) generalized epilepsy associated with MR.

METHODS

We identified a four-generation kindred with several affected members with generalized epilepsy without any evidence for secondary causes. Electroencephalography (EEG) studies and magnetic resonance imaging (MRI) results were reviewed when available. We performed a genome-wide linkage analysis.

KEY FINDINGS

Fourteen individuals were classified as affected and an additional three were considered as nonpenetrant obligatory carriers. Thirteen affected individual had a history of generalized tonic-clonic seizures, and absence seizures were reported in nine affected individuals. There was no history of preceding febrile seizures. MR was present in nine affected individuals with epilepsy but the other affected individuals had normal intelligence. Neuroimaging did not reveal any structural abnormalities and EEG studies were consistent with IGE rather than symptomatic generalized epilepsy. Genetic analysis detected a group of markers with logarithmic (base 10) of odds (LOD) score >3 on chromosome 3p spanning a 5.5 Mbp region. Sequencing of several candidate genes, including dynein light chain-A, golgin subfamily a4, leucine rich repeat (in FLII) interacting gene, serine/threonine-protein kinase DCAMKL3 (doublecortin- like and CAM kinase-like 3), laforin (EPM2A) interacting protein 1 (EPM2AIP1, programmed cell death 6 interacting protein, and CLIP-associating protein 2 (cytoplasmic linker-associated protein 2) (hOrbit2) genes did not identify the disease-causing mutations.

SIGNIFICANCE

We report the identification of a genetic locus for generalized epilepsy associated with MR on chromosome 3p. Affected individuals have a form of genetic epilepsy with generalized seizures variably associated with MR. Despite the presence of MR in several affected patients, epilepsy phenotype was not fully consistent with symptomatic epilepsy and suggests a biologic continuum between symptomatic epilepsies and IGE.

The calcium-sensing receptor: a molecular perspective

Compelling evidence of a cell surface receptor sensitive to extracellular calcium was observed as early as the 1980s and was finally realized in 1993 when the calcium-sensing receptor (CaR) was cloned from bovine parathyroid tissue. Initial studies relating to the CaR focused on its key role in extracellular calcium homeostasis, but as the amount of information about the receptor grew it became evident that it was involved in many biological processes unrelated to calcium homeostasis. The CaR responds to a diverse array of stimuli extending well beyond that merely of calcium, and these stimuli can lead to the initiation of a wide variety of intracellular signaling pathways that in turn are able to regulate a diverse range of biological processes. It has been through the examination of the molecular characteristics of the CaR that we now have an understanding of how this single receptor is able to convert extracellular messages into specific cellular responses. Recent CaR-related reviews have focused on specific aspects of the receptor, generally in the context of the CaR's role in physiology and pathophysiology. This review will provide a comprehensive exploration of the different aspects of the receptor, including its structure, stimuli, signalling, interacting protein partners, and tissue expression patterns, and will relate their impact on the functionality of the CaR from a molecular perspective.

Diverse roles of extracellular calcium-sensing receptor in the central nervous system

The G-protein-coupled calcium-sensing receptor (CaSR), upon activation by Ca(2+) or other physiologically relevant polycationic molecules, performs diverse functions in the brain. The CaSR is widely expressed in the central nervous system (CNS) and is characterized by a robust increase in its expression during postnatal brain development over adult levels throughout the CNS. Developmental increases in CaSR levels in brain correlate with myelinogenesis. Indeed, neural stem cells differentiating to the oligodendrocyte lineage exhibit the highest CaSR expression compared with those differentiating to astrocytic or neuronal lineages. In adult CNS, CaSR has broad relevance in maintaining local ionic homeostasis. CaSR shares an evolutionary relationship with the metabotropic glutamate receptor and forms heteromeric complexes with the type B-aminobutyric acid receptor subunits that affects its cell surface expression, activation, signaling, and functions. In normal physiology as well as in pathologic conditions, CaSR is activated by signals arising from mineral ions, amino acids, polyamines, glutathione, and amyloid-beta in conjunction with Ca(2+) and other divalent cationic ligands. CaSR activation regulates membrane excitability of neurons and glia and affects myelination, olfactory and gustatory signal integration, axonal and dendritic growth, and gonadotrophin-releasing hormonal-neuronal migration. Insofar as the CaSR is a clinically important therapeutic target for parathyroid disorders, development of its agonists or antagonists as therapeutics for CNS disorder could be a major breakthrough.

Calcium sensing receptor mutations implicated in pancreatitis and idiopathic epilepsy syndrome disrupt an arginine-rich retention motif

Calcium sensing receptor (CaSR) mutations implicated in familial hypocalciuric hypercalcemia, pancreatitis and idiopathic epilepsy syndrome map to an extended arginine-rich region in the proximal carboxyl terminus. Arginine-rich motifs mediate endoplasmic reticulum retention and/or retrieval of multisubunit proteins so we asked whether these mutations, R886P, R896H or R898Q, altered CaSR targeting to the plasma membrane. Targeting was enhanced by all three mutations, and Ca(2+)-stimulated ERK1/2 phosphorylation was increased for R896H and R898Q. To define the role of the extended arginine-rich region in CaSR trafficking, we independently determined the contributions of R890/R891 and/or R896/K897/R898 motifs by mutation to alanine. Disruption of the motif(s) significantly increased surface expression and function relative to wt CaSR. The arginine-rich region is flanked by phosphorylation sites at S892 (protein kinase C) and S899 (protein kinase A). The phosphorylation state of S899 regulated recognition of the arginine-rich region; S899D showed increased surface localization. CaSR assembles in the endoplasmic reticulum as a covalent disulfide-linked dimer and we determined whether retention requires the presence of arginine-rich regions in both subunits. A single arginine-rich region within the dimer was sufficient to confer intracellular retention comparable to wt CaSR. We have identified an extended arginine-rich region in the proximal carboxyl terminus of CaSR (residues R890 - R898) which fosters intracellular retention of CaSR and is regulated by phosphorylation. Mutation(s) identified in chronic pancreatitis and idiopathic epilepsy syndrome therefore increase plasma membrane targeting of CaSR, likely contributing to the altered Ca(2+) signaling characteristic of these diseases.

Presynaptic external calcium signaling involves the calcium-sensing receptor in neocortical nerve terminals

BACKGROUND

Nerve terminal invasion by an axonal spike activates voltage-gated channels, triggering calcium entry, vesicle fusion, and release of neurotransmitter. Ion channels activated at the terminal shape the presynaptic spike and so regulate the magnitude and duration of calcium entry. Consequently characterization of the functional properties of ion channels at nerve terminals is crucial to understand the regulation of transmitter release. Direct recordings from small neocortical nerve terminals have revealed that external [Ca(2+)] ([Ca(2+)](o)) indirectly regulates a non-selective cation channel (NSCC) in neocortical nerve terminals via an unknown [Ca(2+)](o) sensor. Here, we identify the first component in a presynaptic calcium signaling pathway.

METHODOLOGY/PRINCIPAL FINDINGS

By combining genetic and pharmacological approaches with direct patch-clamp recordings from small acutely isolated neocortical nerve terminals we identify the extracellular calcium sensor. Our results show that the calcium-sensing receptor (CaSR), a previously identified G-protein coupled receptor that is the mainstay in serum calcium homeostasis, is the extracellular calcium sensor in these acutely dissociated nerve terminals. The NSCC currents from reduced function mutant CaSR mice were less sensitive to changes in [Ca(2+)](o) than wild-type. Calindol, an allosteric CaSR agonist, reduced NSCC currents in direct terminal recordings in a dose-dependent and reversible manner. In contrast, glutamate and GABA did not affect the NSCC currents.

CONCLUSIONS/SIGNIFICANCE

Our experiments identify CaSR as the first component in the [Ca(2+)](o) sensor-NSCC signaling pathway in neocortical terminals. Decreases in [Ca(2+)](o) will depress synaptic transmission because of the exquisite sensitivity of transmitter release to [Ca(2+)](o) following its entry via voltage-activated Ca(2+) channels. CaSR may detects such falls in [Ca(2+)](o) and increase action potential duration by increasing NSCC activity, thereby attenuating the impact of decreases in [Ca(2+)](o) on release probability. CaSR is positioned to detect the dynamic changes of [Ca(2+)](o) and provide presynaptic feedback that will alter brain excitability.