Calcium as an extracellular signalling molecule: perspectives on the Calcium Sensing Receptor in the brain

Various Cellular Components and Its Signaling Cascades Through the Involvement of Signaling Messengers in Keratinocyte Differentiation

Skin is a highly differentiated tissue, in which various signaling molecules play critical roles in the differentiation and proliferation of keratinocytes. Among these, the second messenger calcium and its gradient across skin layers are pivotal in regulating keratinocyte differentiation. Additionally, a diverse array of cellular signaling molecules has been identified as essential for promoting keratinocyte differentiation, thereby maintaining skin integrity and barrier function. The barrier function of the skin provides essential protection against exogenous stimuli and pathogens while maintaining structural stability. The homeostatic processes of skin differentiation are modulated by these second messengers and various signaling molecules. Thus, this review highlights the components associated with keratinocyte differentiation and their biological and pathophysiological roles, as well as redox-sensitive differentiation factors in the modulation of skin homeostasis. This review aims to enhance our understanding of skin physiology and provide insights that may facilitate the development of novel therapeutic strategies for skin diseases.

The bile acid-sensitive ion channel is gated by Ca2+-dependent conformational changes in the transmembrane domain

The bile acid-sensitive ion channel (BASIC) is the least understood member of the mammalian epithelial Na+ channel/degenerin (ENaC/DEG) superfamily of ion channels, which are involved in a variety of physiological processes. While some members of this superfamily, including BASIC, are inhibited by extracellular Ca2+ (Ca2+ o), the molecular mechanism underlying Ca2+ modulation remains unclear. Here, by determining the structure of human BASIC in the presence and absence of Ca2+ using single particle cryo-electron microscopy (cryo-EM), we reveal Ca2+-dependent conformational changes in the transmembrane domain and β-linkers. Electrophysiological experiments further show that a glutamate residue in the extracellular vestibule of the pore underpins the Ca2+-binding site, whose occupancy determines the conformation of the pore and therefore ion flow through the channel. These results reveal the molecular principles governing gating of BASIC and its regulation by Ca2+ ions, demonstrating that Ca2+ ions modulate BASIC function via changes in protein conformation rather than solely from pore-block, as proposed for other members of this superfamily.

Silicon-Rhodamine Functionalized Evocalcet Probes Potently and Selectively Label Calcium Sensing Receptors In Vitro, In Vivo, and Ex Vivo

The calcium sensing receptor (CaSR) is a ubiquitously expressed G-protein coupled receptor (GPCR) that regulates extracellular calcium signals via the parathyroid glands. CaSR has recently also been implicated in noncalcitropic pathophysiologies like asthma, gut inflammation, and cancer. To date, molecular tools that enable the bioimaging of CaSR in tissues are lacking. Based on in silico analyses of available structure-activity relationship data on CaSR ligands, we designed and prepared silicon-rhodamine (SiR) conjugates of the clinically approved drug evocalcet. The new probes EvoSiR4 and EvoSiR6, with differing linker lengths at the evocalcet carboxyl end, both showed a 6-fold and 3-fold increase in potency toward CaSR (EC50 < 45 nM) compared to evocalcet and the evocalcet-linker conjugate, respectively, in an FLIPR-based cellular functional assay. The specificity of the EvoSiR probes toward CaSR binding and the impact of albumin was evaluated in live cell experiments. Both probes showed strong albumin binding, which facilitated the clearance of nonspecific binding interactions. Accordingly, in zebrafish embryos, EvoSiR4 specifically labeled the high CaSR expressing neuromasts of the lateral line in vivo. EvoSiR4 was also assessed in human parathyroid tissues ex vivo, showing a specific absolute CaSR-associated fluorescence compared to that of parathyroid autofluorescence. In summary, functionalization of evocalcet by SiR led to the preparation of potent and specific fluorescent CaSR probes. EvoSiR4 is a versatile small-molecular probe that can be employed in CaSR-related biomedical analyses where antibodies are not applicable.

Fluorescent Protein-Based Sensors for Detecting Essential Metal Ions across the Tree of Life

Genetically encoded fluorescent metal ion sensors are powerful tools for elucidating metal dynamics in living systems. Over the last 25 years since the first examples of genetically encoded fluorescent protein-based calcium indicators, this toolbox of probes has expanded to include other essential and non-essential metal ions. Collectively, these tools have illuminated fundamental aspects of metal homeostasis and trafficking that are crucial to fields ranging from neurobiology to human nutrition. Despite these advances, much of the application of metal ion sensors remains limited to mammalian cells and tissues and a limited number of essential metals. Applications beyond mammalian systems and in vivo applications in living organisms have primarily used genetically encoded calcium ion sensors. The aim of this Perspective is to provide, with the support of historical and recent literature, an updated and critical view of the design and use of fluorescent protein-based sensors for detecting essential metal ions in various organisms. We highlight the historical progress and achievements with calcium sensors and discuss more recent advances and opportunities for the detection of other essential metal ions. We also discuss outstanding challenges in the field and directions for future studies, including detecting a wider variety of metal ions, developing and implementing a broader spectral range of sensors for multiplexing experiments, and applying sensors to a wider range of single- and multi-species biological systems.

Elucidating the Interactome of G Protein-Coupled Receptors and Receptor Activity-Modifying Proteins

G protein-coupled receptors (GPCRs) are known to interact with several other classes of integral membrane proteins that modulate their biology and pharmacology. However, the extent of these interactions and the mechanisms of their effects are not well understood. For example, one class of GPCR-interacting proteins, receptor activity-modifying proteins (RAMPs), comprise three related and ubiquitously expressed single-transmembrane span proteins. The RAMP family was discovered more than two decades ago, and since then GPCR-RAMP interactions and their functional consequences on receptor trafficking and ligand selectivity have been documented for several secretin (class B) GPCRs, most notably the calcitonin receptor-like receptor. Recent bioinformatics and multiplexed experimental studies suggest that GPCR-RAMP interactions might be much more widespread than previously anticipated. Recently, cryo-electron microscopy has provided high-resolution structures of GPCR-RAMP-ligand complexes, and drugs have been developed that target GPCR-RAMP complexes. In this review, we provide a summary of recent advances in techniques that allow the discovery of GPCR-RAMP interactions and their functional consequences and highlight prospects for future advances. We also provide an up-to-date list of reported GPCR-RAMP interactions based on a review of the current literature. SIGNIFICANCE STATEMENT: Receptor activity-modifying proteins (RAMPs) have emerged as modulators of many aspects of G protein-coupled receptor (GPCR)biology and pharmacology. The application of new methodologies to study membrane protein-protein interactions suggests that RAMPs interact with many more GPCRs than had been previously known. These findings, especially when combined with structural studies of membrane protein complexes, have significant implications for advancing GPCR-targeted drug discovery and the understanding of GPCR pharmacology, biology, and regulation.

Modulation of Cellular Response to Different Parameters of the Rotating Magnetic Field (RMF)-An In Vitro Wound Healing Study

Since the effect of MFs (magnetic fields) on various biological systems has been studied, different results have been obtained from an insignificant effect of weak MFs on the disruption of the circadian clock system. On the other hand, magnetic fields, electromagnetic fields, or electric fields are used in medicine. The presented study was conducted to determine whether a low-frequency RMF (rotating magnetic field) with different field parameters could evoke the cellular response in vitro and is possible to modulate the cellular response. The cellular metabolic activity, ROS and Ca2+ concentration levels, wound healing assay, and gene expression analyses were conducted to evaluate the effect of RMF. It was shown that different values of magnetic induction (B) and frequency (f) of RMF evoke a different response of cells, e.g., increase in the general metabolic activity may be associated with the increasing of ROS levels. The lower intracellular Ca2+ concentration (for 50 Hz) evoked the inability of cells to wound closure. It can be stated that the subtle balance in the ROS level is crucial in the wound for the effective healing process, and it is possible to modulate the cellular response to the RMF in the context of an in vitro wound healing.

Role of the neurovascular unit in the process of cerebral ischemic injury

Cerebral ischemic injury exhibits both high morbidity and mortality worldwide. Traditional research of the pathogenesis of cerebral ischemic injury has focused on separate analyses of the involved cell types. In recent years, the neurovascular unit (NVU) mechanism of cerebral ischemic injury has been proposed in modern medicine. Hence, more effective strategies for the treatment of cerebral ischemic injury may be provided through comprehensive analysis of brain cells and the extracellular matrix. However, recent studies that have investigated the function of the NVU in cerebral ischemic injury have been insufficient. In addition, the metabolism and energy conversion of the NVU depend on interactions among multiple cell types, which make it difficult to identify the unique contribution of each cell type. Therefore, in the present review, we comprehensively summarize the regulatory effects and recovery mechanisms of four major cell types (i.e., astrocytes, microglia, brain-microvascular endothelial cells, and neurons) in the NVU under cerebral ischemic injury, as well as discuss the interactions among these cell types in the NVU. Furthermore, we discuss the common signaling pathways and signaling factors that mediate cerebral ischemic injury in the NVU, which may help to provide a theoretical basis for the comprehensive elucidation of cerebral ischemic injury.

Live-Cell Imaging of Physiologically Relevant Metal Ions Using Genetically Encoded FRET-Based Probes

Essential biochemical reactions and processes within living organisms are coupled to subcellular fluctuations of metal ions. Disturbances in cellular metal ion homeostasis are frequently associated with pathological alterations, including neurotoxicity causing neurodegeneration, as well as metabolic disorders or cancer. Considering these important aspects of the cellular metal ion homeostasis in health and disease, measurements of subcellular ion signals are of broad scientific interest. The investigation of the cellular ion homeostasis using classical biochemical methods is quite difficult, often even not feasible or requires large cell numbers. Here, we report of genetically encoded fluorescent probes that enable the visualization of metal ion dynamics within individual living cells and their organelles with high temporal and spatial resolution. Generally, these probes consist of specific ion binding domains fused to fluorescent protein(s), altering their fluorescent properties upon ion binding. This review focuses on the functionality and potential of these genetically encoded fluorescent tools which enable monitoring (sub)cellular concentrations of alkali metals such as K+, alkaline earth metals including Mg2+ and Ca2+, and transition metals including Cu+/Cu2+ and Zn2+. Moreover, we discuss possible approaches for the development and application of novel metal ion biosensors for Fe2+/Fe3+, Mn2+ and Na+.

Molecular Players of EF-hand Containing Calcium Signaling Event in Plants

Ca2+ is a universal second messenger that plays a pivotal role in diverse signaling mechanisms in almost all life forms. Since the evolution of life from an aquatic to a terrestrial environment, Ca2+ signaling systems have expanded and diversified enormously. Although there are several Ca2+ sensing molecules found in a cell, EF-hand containing proteins play a principal role in calcium signaling event in plants. The major EF-hand containing proteins are calmodulins (CaMs), calmodulin like proteins (CMLs), calcineurin B-like (CBL) and calcium dependent protein kinases (CDPKs/CPKs). CaMs and CPKs contain calcium binding conserved D-x-D motifs in their EF-hands (one motif in each EF-hand) whereas CMLs contain a D-x₃-D motif in the first and second EF-hands that bind the calcium ion. Calcium signaling proteins form a complex interactome network with their target proteins. The CMLs are the most primitive calcium binding proteins. During the course of evolution, CMLs are evolved into CaMs and subsequently the CaMs appear to have merged with protein kinase molecules to give rise to calcium dependent protein kinases with distinct and multiple new functions. Ca2+ signaling molecules have evolved in a lineage specific manner with several of the calcium signaling genes being lost in the monocot lineage.

Enhanced expression of the calcium-sensing receptor in reactive astrocytes following ischemic injury in vivo and in vitro

We recently demonstrated that the G protein-coupled calcium-sensing receptor (CaSR) is associated with the pathogenesis of ischemic stroke and may be involved in vascular remodeling and astrogliosis. To further substantiate the involvement of CaSR in the astroglial reaction common to ischemic insults, we investigated the temporal and cell type-specific expression patterns of CaSR in the hippocampus after transient forebrain ischemia. CaSR was constitutively expressed in neurons of the pyramidal and granule cell layers, whereas increased CaSR immunoreactivity was observed in reactive astrocytes, but not in activated microglia or macrophages, in the CA1 region of the post-ischemic hippocampus. Astroglial induction of CaSR expression was evident on days 3-7 after reperfusion and appeared to increase progressively through day 28, at which time CaSR expression was prominent in astrocytes with a highly reactive hypertrophic phenotype and elevated levels of glial fibrillary acidic protein. This expression pattern was supported by results of immunoblot analyses. Furthermore, CaSR expression was upregulated in rat primary cortical astrocytes exposed to oxygen-glucose deprivation, which undergo reactive gliosis-like changes. Thus, our results demonstrate that selective and long-lasting astroglial induction of CaSR expression is a common characteristic of ischemic injury and suggest its involvement in the ischemia-induced astroglial reaction.

Association of Mutations in SLC12A1 Encoding the NKCC2 Cotransporter With Neonatal Primary Hyperparathyroidism

CONTEXT

Primary hyperparathyroidism with hypercalciuria has not been described in the newborn period.

OBJECTIVE

Our objectives are to identify the genetic basis for neonatal primary hyperparathyroidism in a family with 2 affected children.

SUBJECTS

An African American boy presenting with mild neonatal primary hyperparathyroidism and hypercalciuria was evaluated at The Children's Hospital of Philadelphia. His older brother with neonatal primary hyperparathyroidism had died in infancy of multiple organ failure.

METHODS

We collected clinical and biochemical data and performed exome sequencing analysis on DNA from the patient and his unaffected mother after negative genetic testing for known causes of primary hyperparathyroidism.

RESULTS

Exome sequencing followed by Sanger sequencing disclosed 2 heterozygous mutations, c.1883C>A, p.(A628D) and c.2786_2787insC, p.(T931fsX10), in the SLC12A1 gene, which was previously implicated in antenatal type 1 Bartter syndrome. Sanger sequencing confirmed the 2 mutations in the proband and his deceased brother; both parents were heterozygous for different mutations and an unaffected sister was homozygous for wild-type alleles.

CONCLUSIONS

These results demonstrate a previously unrecognized association between neonatal primary hyperparathyroidism and mutation of SLC12A1, the cause of antenatal Bartter syndrome type 1, and suggest that the loss of sodium-potassium-chloride cotransporter-2 cotransporter activity influences parathyroid gland function.

Ammonia Induces Autophagy through Dopamine Receptor D3 and MTOR

Hyperammonemia is frequently seen in tumor microenvironments as well as in liver diseases where it can lead to severe brain damage or death. Ammonia induces autophagy, a mechanism that tumor cells may use to protect themselves from external stresses. However, how cells sense ammonia has been unclear. Here we show that culture medium alone containing Glutamine can generate milimolar of ammonia at 37 degrees in the absence of cells. In addition, we reveal that ammonia acts through the G protein-coupled receptor DRD3 (Dopamine receptor D3) to induce autophagy. At the same time, ammonia induces DRD3 degradation, which involves PIK3C3/VPS34-dependent pathways. Ammonia inhibits MTOR (mechanistic target of Rapamycin) activity and localization in cells, which is mediated by DRD3. Therefore, ammonia has dual roles in autophagy: one to induce autophagy through DRD3 and MTOR, the other to increase autophagosomal pH to inhibit autophagic flux. Our study not only adds a new sensing and output pathway for DRD3 that bridges ammonia sensing and autophagy induction, but also provides potential mechanisms for the clinical consequences of hyperammonemia in brain damage, neurodegenerative diseases and tumors.

Gαi/o-dependent Ca(2+) mobilization and Gαq-dependent PKCα regulation of Ca(2+)-sensing receptor-mediated responses in N18TG2 neuroblastoma cells

A functional Ca(2+)-sensing receptor (CaS) is expressed endogenously in mouse N18TG2 neuroblastoma cells, and sequence analysis of the cDNA indicates high homology with both rat and human parathyroid CaS cDNAs. The CaS in N18TG2 cells appears as a single immunoreactive protein band at about 150 kDa on Western blots, consistent with native CaS from dorsal root ganglia. Both wild type (WT) and Gαq antisense knock-down (KD) cells responded to Ca(2+) and calindol, a positive allosteric modulator of the CaS, with a transient increase in intracellular Ca(2+) concentration ([Ca(2+)]i), which was larger in the Gαq KD cells. Stimulation with 1 mM extracellular Ca(2+) (Ca(2+)e) increased [Ca(2+)]i in N18TG2 Gαq KD compared to WT cells. Ca(2+) mobilization was dependent on pertussis toxin-sensitive Gαi/o proteins and reduced by 30 μM 2-amino-ethyldiphenyl borate and 50 μM nifedipine to the same plateau levels in both cell types. Membrane-associated PKCα and p-PKCα increased with increasing [Ca(2+)]e in WT cells, but decreased in Gαq KD cells. Treatment of cells with 1 μM Gӧ 6976, a Ca(2+)-specific PKC inhibitor reduced Ca(2+) mobilization and membrane-associated PKCα and p-PKCα in both cell types. The results indicate that the CaS-mediated increase in [Ca(2+)]i in N18TG2 cells is dependent on Gαi/o proteins via inositol-1,4,5-triphosphate (IP3) channels and store-operated Ca(2+) entry channels, whereas modulation of CaS responses involving PKCα phosphorylation and translocation to the plasma membrane occurs via a Gαq mechanism.

The calcium-sensing receptor and integrins modulate cerebellar granule cell precursor differentiation and migration

In the developing cerebellum granule cell precursors (GCPs) proliferate in the external granule cell layer before differentiating and migrating to the inner granule cell layer. Aberrant GCP proliferation leads to medulloblastoma, the most prevalent form of childhood brain cancer. Here, we demonstrate that the calcium-sensing receptor (CaSR), a homodimeric G-protein coupled receptor, functions in conjunction with cell adhesion proteins, the integrins, to enhance GCP migration and cell homing by promoting GCP differentiation. During the second postnatal week a robust peak in CaSR expression was observed in GCPs; reciprocal immunoprecipitation experiments conducted during this period established that the CaSR and β1 integrins are present together in a macromolecular protein complex. Analysis of cell-surface proteins demonstrated that activation of the CaSR by positive allosteric modulators promoted plasma membrane expression of β1 integrins via ERK2 and AKT phosphorylation and resulted in increased GCP migration toward an extracellular matrix protein. The results of in vivo experiments whereby CaSR modulators were injected i.c.v. revealed that CaSR activation promoted radial migration of GCPs by enhancing GCP differentiation, and conversely, a CaSR inhibitor disrupted GCP differentiation and promoted GCP proliferation. Our results demonstrate that an ion-sensing G-protein coupled receptor acts to promote neuronal differentiation and homing during cerebellar maturation. These findings together with those of others also suggest that CaSR/integrin complexes act to transduce extracellular calcium signals into cellular movement, and may function in this capacity as a universal cell migration/homing complex in the developing brain.

Hypocalcemia-induced seizure: demystifying the calcium paradox

Calcium is essential for both neurotransmitter release and muscle contraction. Given these important physiological processes, it seems reasonable to assume that hypocalcemia may lead to reduced neuromuscular excitability. Counterintuitively, however, clinical observation has frequently documented hypocalcemia’s role in induction of seizures and general excitability processes such as tetany, Chvostek’s sign, and bronchospasm. The mechanism of this calcium paradox remains elusive, and very few pathophysiological studies have addressed this conundrum. Nevertheless, several studies primarily addressing other biophysical issues have provided some clues. In this review, we analyze the data of these studies and propose an integrative model to explain this hypocalcemic paradox.

Wnt5a/β-catenin signaling drives calcium-induced differentiation of human primary keratinocytes

It is well established that a gradient of extracellular calcium within the epidermis regulates the differentiation of keratinocytes. However, the molecular mechanisms implicated in this process are not fully understood. RNA interference of the calcium-sensing receptor (CaSR) showed that CaSR is essential in calcium-induced differentiation of normal human epidermal keratinocytes (NHEKs) by increasing the levels of free intracellular calcium, which upregulates the expression of Wnt5a but not Wnt3a, Wnt4, and Dkk-1 in the cells. Subsequently, autocrine Wnt5a promotes the differentiation of NHEKs, determined by increased biosynthesis of keratin-1 and loricrin, whereas proliferation is suppressed. Addition of both Wnt5a and calcium to NHEKs activated the Wnt/β-catenin signaling pathway as indicated by (i) increased stability of β-catenin in the cells, (ii) enhanced β-catenin transcriptional activity, demonstrated by a luciferase-based β-catenin-activated reporter assay, and (iii) augmented Wnt/β-catenin target gene expression. NHEKs depleted for β-catenin had a significantly reduced susceptibility to calcium-induced differentiation. Knockdown of axin 2, an antagonist of β-catenin stability, enhanced the biosynthesis of keratin-1 and loricrin in the cells. Our findings establish a directional crosstalk between CaSR and Wnt/β-catenin signaling in keratinocyte differentiation via Wnt5a that acts as an autocrine stimulus in this process.

Cinacalcet monotherapy in neonatal severe hyperparathyroidism: a case study and review

CONTEXT

Neonatal severe hyperparathyroidism (NSHPT) is a severe form of familial hypocalciuric hypercalcemia characterized by severe hypercalcemia and skeletal demineralization. In most cases, NSHPT is due to biallelic loss-of-function mutations in the CASR gene encoding the calcium-sensing receptor (CaSR), but some patients have heterozygous mutations. Conventional treatment consists of iv saline, bisphosphonates, and parathyroidectomy.

OBJECTIVE

The aim of this project was to characterize the molecular basis for NSHPT in an affected newborn and to describe the response to monotherapy with cinacalcet.

METHODS

Clinical and biochemical features were monitored as cinacalcet therapy was initiated and maintained. Genomic DNA was obtained from the proband and parents. The CASR gene was amplified by PCR and sequenced directly.

RESULTS

The patient was a full-term male who developed hypotonia and respiratory failure soon after birth. He was found to have multiple fractures and diffuse bone demineralization, with a marked elevation in serum ionized calcium (1.99 mmol/L) and elevated serum levels of intact PTH (1154 pg/mL); serum 25-hydroxyvitamin D was low, and fractional excretion of calcium was reduced. The serum calcium level was not reduced by iv saline infusion. Based on an extensive family history of autosomal dominant hypercalcemia, a diagnosis of NSHPT was made, and cinacalcet therapy was initiated with a robust and durable effect. Molecular studies revealed a heterozygous R185Q missense mutation in the CASR in the patient and his father, whereas normal sequences for the CASR gene were present in the patient's mother.

CONCLUSIONS

We describe the first use of cinacalcet as monotherapy for severe hypercalcemia in a newborn with NSHPT. The rapid and durable response to cinacalcet suggests that a trial of calcimimetic therapy should be considered early in the course of NSHPT.

Calcium-sensing receptor in rat vagal bronchopulmonary sensory neurons regulates the function of the capsaicin receptor TRPV1

Extracellular calcium-sensing receptor (CaSR) has been known to play a critical role in the maintainance of systemic Ca(2+) homeostasis. Recent studies have shown that CaSR is also expressed in many tissues that are not directly related to plasma Ca(2+) regulation, such as the central and peripheral nervous system, where the function of this receptor remains to be defined. In this study, we aimed to investigate the expression of CaSR and its potential interaction with transient receptor potential vanilloid receptor type 1 (TRPV1) in rat vagal bronchopulmonary sensory neurons. Our immunohistochemical experiments demonstrated the expression of CaSR in these sensory neurons as well as in trachea and lung parenchyma. Results from our whole-cell patch-clamp recordings in isolated neurons showed that strong activation of CaSR with high concentrations of its agonists, including spermine, NPS R-568 and Ca(2+), inhibited the capsaicin-evoked whole-cell inward current. Blockade of CaSR with its antagonists NPS 2390 and NPS 2143 significantly enhanced the capsaicin-evoked TRPV1 current. These data suggest that CaSR is likely to be involved in the integration of primary bronchopulmonary sensory inputs in physiological and/or pathophysiological conditions.

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.

Measuring intracellular Ca2+ changes in human sperm using four techniques: conventional fluorometry, stopped flow fluorometry, flow cytometry and single cell imaging

Spermatozoa are male reproductive cells especially designed to reach, recognize and fuse with the egg. To perform these tasks, sperm cells must be prepared to face a constantly changing environment and to overcome several physical barriers. Being in essence transcriptionally and translationally silent, these motile cells rely profoundly on diverse signaling mechanisms to orient themselves and swim in a directed fashion, and to contend with challenging environmental conditions during their journey to find the egg. In particular, Ca(2+)-mediated signaling is pivotal for several sperm functions: activation of motility, capacitation (a complex process that prepares sperm for the acrosome reaction) and the acrosome reaction (an exocytotic event that allows sperm-egg fusion). The use of fluorescent dyes to track intracellular fluctuations of this ion is of remarkable importance due to their ease of application, sensitivity, and versatility of detection. Using one single dye-loading protocol we utilize four different fluorometric techniques to monitor sperm Ca(2+) dynamics. Each technique provides distinct information that enables spatial and/or temporal resolution, generating data both at single cell and cell population levels.

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.

Regulation of calcium sensing receptor trafficking by RAMPs

As mentioned earlier in this book, RAMPs were identified as proteins escorting the Calcitonin Receptor-Like Receptor (CRLR) to the plasma membrane (PM) to generate either CGRP (when associated with RAMP1), or adrenomedullin receptors (when associated with RAMP2 or RAMP3). Some years after this initial discovery, it was established that RAMPs can accompany four additional class B G Protein-Coupled Receptors-GPCRs- (PTH1, PTH2, glucagon receptor and VPAC1) to the PM.(1) By demonstrating that the sorting traffic of the Calcium Sensing Receptor (CaSR), a class C GPCR, is positively modulated by RAMP1 and RAMP3,(2) our data extended the concept of RAMPs as escorting molecules to another class of GPCRs.

Calcium Channels in the Development, Maturation, and Function of Spermatozoa

A proper dialogue between spermatozoa and the egg is essential for conception of a new individual in sexually reproducing animals. Ca2+ is crucial in orchestrating this unique event leading to a new life. No wonder that nature has devised different Ca2+-permeable channels and located them at distinct sites in spermatozoa so that they can help fertilize the egg. New tools to study sperm ionic currents, and image intracellular Ca2+ with better spatial and temporal resolution even in swimming spermatozoa, are revealing how sperm ion channels participate in fertilization. This review critically examines the involvement of Ca2+ channels in multiple signaling processes needed for spermatozoa to mature, travel towards the egg, and fertilize it. Remarkably, these tiny specialized cells can express exclusive channels like CatSper for Ca2+ and SLO3 for K+, which are attractive targets for contraception and for the discovery of novel signaling complexes. Learning more about fertilization is a matter of capital importance; societies face growing pressure to counteract rising male infertility rates, provide safe male gamete-based contraceptives, and preserve biodiversity through improved captive breeding and assisted conception initiatives.

Ion channels, phosphorylation and mammalian sperm capacitation

Sexually reproducing animals require an orchestrated communication between spermatozoa and the egg to generate a new individual. Capacitation, a maturational complex phenomenon that occurs in the female reproductive tract, renders spermatozoa capable of binding and fusing with the oocyte, and it is a requirement for mammalian fertilization. Capacitation encompasses plasma membrane reorganization, ion permeability regulation, cholesterol loss and changes in the phosphorylation state of many proteins. Novel tools to study sperm ion channels, image intracellular ionic changes and proteins with better spatial and temporal resolution, are unraveling how modifications in sperm ion transport and phosphorylation states lead to capacitation. Recent evidence indicates that two parallel pathways regulate phosphorylation events leading to capacitation, one of them requiring activation of protein kinase A and the second one involving inactivation of ser/thr phosphatases. This review examines the involvement of ion transporters and phosphorylation signaling processes needed for spermatozoa to achieve capacitation. Understanding the molecular mechanisms leading to fertilization is central for societies to deal with rising male infertility rates, to develop safe male gamete-based contraceptives and to preserve biodiversity through better assisted fertilization strategies.

Role of calcium in lipopolysaccharide-induced calcitonin gene expression in human adipocytes

Severe systemic infections induce ubiquitous calcitonin (CALC) gene expression with release of calcitonin peptides, namely procalcitonin, calcitonin gene-related peptide and adrenomedullin. Using an in vitro model for bacterial infection, we tested the hypothesis that intracellular calcium concentration ([Ca(2+)](i)) is elevated after lipopolysaccharide (LPS) stimulation and is responsible for the LPS-mediated increase in CALC gene expression and protein secretion. In our human adipocyte model, LPS did not show any cytotoxic effects and induced increased CALC-I gene mRNA expression. Additionally, LPS provoked an elevation in [Ca(2+)](i). The LPS-induced increase in CALC-I gene mRNA was partially blocked with verapamil, an L-type calcium channel blocker and blocked almost completely with 2-aminoethoxydiphenyl borate, a blocker of store-operated calcium entry and inositol triphosphate-mediated calcium release. Treatment of cells with substances elevating [Ca(2+)]( i) led to an increased CALC-I mRNA expression level. The combination of LPS with substances raising [Ca(2+)](i) even potentiated this increase. At the same time, elevated [Ca(2+)](i) attenuated the expression level of the CALC-V gene. These findings indicate that, in human adipocytes, changes in [Ca(2+)](i) are involved in LPSregulated expression of CALC genes, thereby strengthening previous findings postulating a crucial role of intracellular calcium homeostasis in the state of bacterial infection and sepsis.

A novel loss-of-function mutation, Gln459Arg, of the calcium-sensing receptor gene associated with apparent autosomal recessive inheritance of familial hypocalciuric hypercalcemia

CONTEXT

Mutations that inactivate one allele of the gene encoding the calcium sensing receptor (CaSR) cause autosomal dominant familial hypocalciuric hypercalcemia (FHH), whereas homozygous mutations cause neonatal severe hyperparathyroidism.

OBJECTIVE

We describe the identification and biochemical characterization of a novel CASR gene mutation that caused apparent autosomal recessive FHH in an extended consanguineous kindred.

DESIGN

The study design involved direct sequence analysis of the CaSR gene, clinical and biochemical analyses of patients, and in vitro immunobiochemical studies of the mutant CaSR.

RESULTS

A novel inactivating mutation (Q459R) was identified in exon 4 of both alleles of the CASR in the proband, who presented with asymptomatic hypercalcemia and hypocalciuria at age 2 yr. The proband's parents were heterozygous for the Q459R mutation consistent with autosomal recessive inheritance of FHH. Among 13 family members that were studied, eight subjects were heterozygous for the Q459R mutation and five had normal genotypes. All heterozygous subjects were asymptomatic and normocalcemic apart from one subject who was mildly hypercalcemic. The Q459R mutant CaSR was normally expressed at the cell membrane but retained only 30-50% of the calcium-dependent activity of the wild-type CaSR.

CONCLUSION

We identified a novel loss-of-function Q459R mutation in the CASR gene that exhibits mildly reduced sensitivity to calcium and that is associated with apparent autosomal recessive transmission of FHH. This study demonstrates the importance of genetic testing in FHH to distinguish between de novo and inherited mutations of the CASR gene and assist in management decisions.

Regulation of calcium-sensing-receptor trafficking and cell-surface expression by GPCRs and RAMPs

The calcium-sensing (CaS) receptor is a G-protein-coupled receptor (GPCR) that is of fundamental importance for extracellular calcium signalling and calcium homeostasis. The CaS receptor detects changes in free, ionized extracellular calcium concentration and initiates pathways that constantly re-adjust levels of circulating calcium. In addition, the CaS receptor is involved in processes such as stem-cell homing and regulation of neuronal-process outgrowth. To perform these functions, the CaS receptor must be appropriately targeted to the plasma membrane so that its large N-terminal calcium-sensing domain is positioned in the extracellular environment to detect dynamic changes in ionic calcium concentration. Here, we provide an overview of the molecular determinants controlling CaS receptor forward traffic and highlight the roles of CaS receptor interactors such as receptor-activity-modifying proteins and subunits of other class C GPCRs in this process.

GISP binding to TSG101 increases GABA receptor stability by down-regulating ESCRT-mediated lysosomal degradation

The neuron-specific G protein-coupled receptor interacting scaffold protein (GISP) is a multidomain, brain-specific protein derived from the A-kinase anchoring protein-9 gene. We originally isolated GISP as an interacting partner for the GABA(B) receptor subunit GABA(B1). Here, we show that the protein tumour susceptibility gene 101 (TSG101), an integral component of the endosomal sorting machinery that targets membrane proteins for lysosomal degradation, also interacts with GISP. TSG101 co-immunoprecipitates with GISP from adult rat brain, and using GST pull-downs, we identified that the eighth coiled-coiled region of GISP is critical for TSG101 association. Intriguingly, although there is no direct interaction between GISP and the GABA(B2) subunit, their co-expression in HEK293 cells increases levels of GABA(B2). GISP also inhibits TSG101-dependent GABA(B2) down-regulation in human embryonic kidney 293 cells whereas over-expression of a mutant GISP lacking the TSG101 binding domain has no effect on GABA(B2) degradation. These data suggest that GISP can function as a negative regulator of TSG101-dependent lysosomal degradation of transmembrane proteins in neurons to promote receptor stability.

GISP binding to TSG101 increases GABA receptor stability by down-regulating ESCRT-mediated lysosomal degradation

The neuron-specific G protein-coupled receptor interacting scaffold protein (GISP) is a multidomain, brain-specific protein derived from the A-kinase anchoring protein-9 gene. We originally isolated GISP as an interacting partner for the GABA(B) receptor subunit GABA(B1). Here, we show that the protein tumour susceptibility gene 101 (TSG101), an integral component of the endosomal sorting machinery that targets membrane proteins for lysosomal degradation, also interacts with GISP. TSG101 co-immunoprecipitates with GISP from adult rat brain, and using GST pull-downs, we identified that the eighth coiled-coiled region of GISP is critical for TSG101 association. Intriguingly, although there is no direct interaction between GISP and the GABA(B2) subunit, their co-expression in HEK293 cells increases levels of GABA(B2). GISP also inhibits TSG101-dependent GABA(B2) down-regulation in human embryonic kidney 293 cells whereas over-expression of a mutant GISP lacking the TSG101 binding domain has no effect on GABA(B2) degradation. These data suggest that GISP can function as a negative regulator of TSG101-dependent lysosomal degradation of transmembrane proteins in neurons to promote receptor stability.

Calcium receptor expression and function in oligodendrocyte commitment and lineage progression: potential impact on reduced myelin basic protein in CaR-null mice

Oligodendrocytes develop from oligodendrocyte progenitor cells (OPCs), which in turn arise from a subset of neuroepithelial precursor cells during midneurogenesis. Development of the oligodendrocyte lineage involves a plethora of cell-intrinsic and -extrinsic signals. A cell surface calcium-sensing receptor (CaR) has been shown to be functionally expressed in immature oligodendrocytes. Here, we investigated the expression and function of the CaR during oligodendrocyte development. We show that the order of CaR mRNA expression as assessed by quantitative polymerase chain reaction is mature oligodendrocyte > neuron > astrocyte. We next determined the rank order of CaR expression on inducing specification of neural stem cells to the neuronal, oligodendroglial, or astrocytic lineages and found that the relative levels of CaR mRNA expression are OPC > neuron > astrocytes. CaR mRNA expression in cells at various stages of development along the oligodendrocyte lineage revealed that its expression is robustly up-regulated during the OPC stage and remains high until the premyelinating stage, decreasing thereafter by severalfold in the mature oligodendrocyte. In OPCs, high Ca(2+) acting via the CaR promotes cellular proliferation. We further observed that high Ca(2+) stimulates the mRNA levels of myelin basic protein in preoligodendrocytes, which is also CaR mediated. Finally, myelin basic protein levels were significantly reduced in the cerebellum of CaR-null mice during development. Our results show that CaR expression is up-regulated when neural stem cells are specified to the oligodendrocyte lineage and that activation of the receptor results in OPC expansion and differentiation. We conclude that the CaR may be a novel regulator of oligodendroglial development and function.

GISP binding to TSG101 increases GABA receptor stability by down-regulating ESCRT-mediated lysosomal degradation

The neuron-specific G protein-coupled receptor interacting scaffold protein (GISP) is a multidomain, brain-specific protein derived from the A-kinase anchoring protein-9 gene. We originally isolated GISP as an interacting partner for the GABA(B) receptor subunit GABA(B1). Here, we show that the protein tumour susceptibility gene 101 (TSG101), an integral component of the endosomal sorting machinery that targets membrane proteins for lysosomal degradation, also interacts with GISP. TSG101 co-immunoprecipitates with GISP from adult rat brain, and using GST pull-downs, we identified that the eighth coiled-coiled region of GISP is critical for TSG101 association. Intriguingly, although there is no direct interaction between GISP and the GABA(B2) subunit, their co-expression in HEK293 cells increases levels of GABA(B2). GISP also inhibits TSG101-dependent GABA(B2) down-regulation in human embryonic kidney 293 cells whereas over-expression of a mutant GISP lacking the TSG101 binding domain has no effect on GABA(B2) degradation. These data suggest that GISP can function as a negative regulator of TSG101-dependent lysosomal degradation of transmembrane proteins in neurons to promote receptor stability.

The calcium-sensing receptor changes cell shape via a beta-arrestin-1 ARNO ARF6 ELMO protein network

G-protein-coupled receptors (GPCRs) transduce the binding of extracellular stimuli into intracellular signalling cascades that can lead to morphological changes. Here, we demonstrate that stimulation of the calcium-sensing receptor (CaSR), a GPCR that promotes chemotaxis by detecting increases in extracellular calcium, triggers plasma membrane (PM) ruffling via a pathway that involves beta-arrestin 1, Arf nucleotide binding site opener (ARNO), ADP-ribosylating factor 6 (ARF6) and engulfment and cell motility protein (ELMO). Expression of dominant negative beta-arrestin 1 or its knockdown with siRNA impaired the CaSR-induced PM ruffling response. Expression of a catalytically inactive ARNO also reduced CaSR-induced PM ruffling. Furthermore, beta-arrestin 1 co-immunoprecipitated with the CaSR and ARNO under resting conditions. Agonist treatment did not markedly alter beta-arrestin 1 binding to the CaSR or to ARNO but it did elicit the translocation and colocalisation of the CaSR, beta-arrestin 1 and ARNO to membrane protrusions. Furthermore, ARF6 and ELMO, two proteins known to couple ARNO to the cytoskeleton, were required for CaSR-dependent morphological changes and translocated to the PM ruffles. These data suggest that cells ruffle upon CaSR stimulation via a mechanism that involves translocation of beta-arrestin 1 pre-assembled with the CaSR or ARNO, and that ELMO plays an essential role in this CaSR-signalling-induced cytoskeletal reorganisation.

Ca2+-dependent regulation of calcitonin gene expression by the transcriptional repressor DREAM

Calcitonin (CT), whose secretion from thyroid glands is regulated by increases in the concentration of extracellular Ca(2+), is a well-known hormone that regulates calcium homeostasis. However, the molecular mechanisms underlying the gene expression dependent on Ca(2+) have not been clarified. The downstream regulatory element (DRE) antagonist modulator (DREAM) was recently identified as a Ca(2+)-dependent transcriptional repressor. In the present study, we investigated the possible involvement of DREAM in the regulation of CT gene expression and secretion. A luciferase assay using TT cells, a thyroid carcinoma cell line, showed that a particular region in the CT gene promoter repressed the promoter activity under basal conditions but induced the activity when the Ca(2+) concentration was increased. We found two DRE sequences in a region located upstream from the transcription start site. Gel retardation assay confirmed that DREAM bound to the CT-DRE and also indicated that DREAM bound to the DRE in a Ca(2+)-dependent manner. We generated stable transfectants of TT cells with wild-type or mutant DREAM, which lacked the responsiveness to Ca(2+) changes. In contrast to the wild type, overexpression of the mutant DREAM inhibited the increase in CT secretion induced by a calcium ionophore. The addition of forskolin to increase cAMP activated the CT promoter, probably by the interaction of DREAM with cAMP-responsive element binding proteins, independent on the activation by Ca(2+). Together, these results suggest that DREAM plays an important role in human CT gene expression in a Ca(2+)- and cAMP-dependent manner.

Extracellular calcium regulates postsynaptic efficacy through group 1 metabotropic glutamate receptors

Bursts of synaptic transmission are known to induce transient depletion of Ca2+ within the synaptic cleft. Although Ca2+ depletion has been shown to lower presynaptic release probability, effects on the postsynaptic cell have not been reported. In this study, we show that physiologically relevant reductions in extracellular Ca2+ lead to a decrease in synaptic strength between synaptically coupled layer 2/3 cortical pyramidal neurons. Using quantal analysis and mEPSP analysis, we demonstrate that a lowered extracellular Ca2+ produces a reduction in the postsynaptic quantal size in addition to its known effect on release probability. An elevated Mg2+ level can prevent this reduction in postsynaptic efficacy at subphysiological Ca2+ levels. We show that the calcium-dependent effect on postsynaptic quantal size is mediated by group 1 metabotropic glutamate receptors, acting via CaMKII (Ca2+/calmodulin-dependent protein kinase II) and PKC. Therefore, physiologically relevant changes in extracellular Ca2+ can regulate information transfer at cortical synapses via both presynaptic and postsynaptic mechanisms.

Receptor-activity-modifying proteins are required for forward trafficking of the calcium-sensing receptor to the plasma membrane

The calcium-sensing receptor (CaSR) is a class III G-protein-coupled receptor (GPCR) that responds to changes in extracellular calcium concentration and plays a crucial role in calcium homeostasis. The mechanisms controlling CaSR trafficking and surface expression are largely unknown. Using a CaSR tagged with the pH-sensitive GFP super-ecliptic pHluorin (SEP-CaSR), we show that delivery of the GPCR to the cell surface is dependent on receptor-activity-modifying proteins (RAMPs). We demonstrate that SEP-CaSRs are retained in the endoplasmic reticulum (ER) in COS7 cells that do not contain endogenous RAMPs whereas they are delivered to the plasma membrane in HEK 293 cells that do express RAMP1. Coexpression of RAMP1 or RAMP3, but not RAMP2, in COS7 cells was sufficient to target the CaSR to the cell surface. RAMP1 and RAMP3 colocalised and coimmunoprecipitated with the CaSR suggesting that these proteins associate within the cell. Our results indicate that RAMP expression promotes the forward trafficking of the GPCR from the ER to the Golgi apparatus and results in mature CaSR glycosylation, which is not observed in RAMP-deficient cells. Finally, silencing of RAMP1 in the endogenously expressing HEK293 cells using siRNA resulted in altered CaSR traffic. Taken together, our results show that the association with RAMPs is necessary and sufficient to transfer the immature CaSR retained in the ER towards the Golgi where it becomes fully glycosylated prior to delivery to the plasma membrane and demonstrate a role for RAMPs in the trafficking of a class III GPCR.