The roles of transmembrane family proteins in the regulation of store-operated Ca2+ entry

Store-operated Ca2+ entry (SOCE) is a major pathway for calcium signaling, which regulates almost every biological process, involving cell proliferation, differentiation, movement and death. Stromal interaction molecule (STIM) and ORAI calcium release-activated calcium modulator (ORAI) are the two major proteins involved in SOCE. With the deepening of studies, more and more proteins are found to be able to regulate SOCE, among which the transmembrane (TMEM) family proteins are worth paying more attention. In addition, the ORAI proteins belong to the TMEM family themselves. As the name suggests, TMEM family is a type of proteins that spans biological membranes including plasma membrane and membrane of organelles. TMEM proteins are in a large family with more than 300 proteins that have been already identified, while the functional knowledge about the proteins is preliminary. In this review, we mainly summarized the TMEM proteins that are involved in SOCE, to better describe a picture of the interaction between STIM and ORAI proteins during SOCE and its downstream signaling pathways, as well as to provide an idea for the study of the TMEM family proteins.

An open pore structure of the Orai channel, finally

Store-operated Orai channels are a primary mechanism for mobilizing Ca²⁺ signals in both non-excitable cells and excitable cells. The structure of the open channel, vital for understanding the mechanism of channel opening, is incompletely understood. We highlight a new study that unveils the structure of a constitutively active Orai mutant and takes us closer towards understanding the molecular basis of Orai channel activation.

Simulation of pH-Dependent, Loop-Based Membrane Protein Gating Using Pretzel

Bacterial porins often exhibit ion conductance and gating behavior which can be modulated by pH. However, the underlying control mechanism of gating is often complex, and direct inspection of the protein structure is generally insufficient for full mechanistic understanding. Here we describe Pretzel, a computational framework that can effectively model loop-based gating events in membrane proteins. Our method combines Monte Carlo conformational sampling, structure clustering, ensemble energy evaluation, and a topological gating criterion to model the equilibrium gating state under the pH environment of interest. We discuss details of applying Pretzel to the porin outer membrane protein G (OmpG).

Intracellular O-linked glycosylation directly regulates cardiomyocyte L-type Ca2+ channel activity and excitation-contraction coupling

Cardiomyocyte L-type Ca²⁺ channels (Ca_vs) are targets of signaling pathways that modulate channel activity in response to physiologic stimuli. Ca_v regulation is typically transient and beneficial but chronic stimulation can become pathologic; therefore, gaining a more complete understanding of Ca_v regulation is of critical importance. Intracellular O-linked glycosylation (O-GlcNAcylation), which is the result of two enzymes that dynamically add and remove single N-acetylglucosamines to and from intracellular serine/threonine residues (OGT and OGA respectively), has proven to be an increasingly important post-translational modification that contributes to the regulation of many physiologic processes. However, there is currently no known role for O-GlcNAcylation in the direct regulation of Ca_v activity nor is its contribution to cardiac electrical signaling and EC coupling well understood. Here we aimed to delineate the role of O-GlcNAcylation in regulating cardiomyocyte L-type Ca_v activity and its subsequent effect on EC coupling by utilizing a mouse strain possessing an inducible cardiomyocyte-specific OGT-null-transgene. Ablation of the OGT-gene in adult cardiomyocytes (OGTKO) reduced OGT expression and O-GlcNAcylation by > 90%. Voltage clamp recordings indicated an ~ 40% reduction in OGTKO Ca_v current (I_Ca), but with increased efficacy of adrenergic stimulation, and Ca_v steady-state gating and window current were significantly depolarized. Consistently, OGTKO cardiomyocyte intracellular Ca²⁺ release and contractility were diminished and demonstrated greater beat-to-beat variability. Additionally, we show that the Ca_v α1 and β2 subunits are O-GlcNAcylated while α2δ1 is not. Echocardiographic analyses indicated that the reductions in OGTKO cardiomyocyte Ca²⁺ handling and contractility were conserved at the whole-heart level as evidenced by significantly reduced left-ventricular contractility in the absence of hypertrophy. The data indicate, for the first time, that O-GlcNAc signaling is a critical and direct regulator of cardiomyocyte I_Ca achieved through altered Ca_v expression, gating, and response to adrenergic stimulation; these mechanisms have significant implications for understanding how EC coupling is regulated in health and disease.

Wasted TMEM16A channels are rescued by phosphatidylinositol 4,5-bisphosphate

Recently there has been a flurry of interest in the regulation of the homo-dimeric calcium-activated chloride channel ANO1 (also known as TMEM16A) by phosphatidylinositol (4,5)-bisphosphate (PI(4,5)P2). These recent studies show that upon Ca2+ binding, PI(4,5)P2 cooperates to maintain the conductive state of ANO1. PI(4,5)P2 does so by binding to sites or modules on the protein's cytosolic side. These findings add a new function to the PI(4,5)P2 repertoire and a new dimension to ANO1 gating.

A Mathematical Model of the Human Cardiac Na+ Channel

Sodium ion channel is a membrane protein that plays an important role in excitable cells, as it is responsible for the initiation of action potentials. Understanding the electrical characteristics of sodium channels is essential in predicting their behavior under different physiological conditions. We investigated several Markov models for the human cardiac sodium channel Na_V1.5 to derive a minimal mathematical model that describes the reported experimental data obtained using major voltage clamp protocols. We obtained simulation results for peak current-voltage relationships, the voltage dependence of normalized ion channel conductance, steady-state inactivation, activation and deactivation kinetics, fast and slow inactivation kinetics, and recovery from inactivation kinetics. Good agreement with the experimental data provides us with the mechanisms of the fast and slow inactivation of the human sodium channel and the coupling of its inactivation states to the closed and open states in the activation pathway.

Determining the target of membrane sterols on voltage-gated potassium channels

Cholesterol, an essential lipid component of cellular plasma membranes, regulates fluidity, mechanical integrity, raft structure and may specifically interact with membrane proteins. Numerous effects on ion channels by cholesterol, including changes in current amplitude, voltage dependence and gating kinetics, have been reported. We have previously described such changes in the voltage-gated potassium channel K_v1.3 of lymphocytes by cholesterol and its analog 7-dehydrocholesterol (7DHC). In voltage-gated channels membrane depolarization induces movement of the voltage sensor domains (VSD), which is transmitted by a coupling mechanism to the pore domain (PD) to open the channel. Here, we investigated whether cholesterol effects were mediated by the VSD to the pore or the PD was the direct target. Specificity was tested by comparing K_v1.3 and K_v10.1 channels having different VSD-PD coupling mechanisms. Current recordings were performed with two-electrode voltage-clamp fluorometry, where movement of the VSDs was monitored by attaching fluorophores to external cysteine residues introduced in the channel sequence. Loading the membrane with cholesterol or 7DHC using methyl-β-cyclodextrin induced changes in the steady-state and kinetic parameters of the ionic currents while leaving fluorescence parameters mostly unaffected in both channels. Non-stationary noise analysis revealed that reduction of single channel conductance rather than that of open probability caused the observed current decrease. Furthermore, confocal laser scanning and stimulated emission depletion microscopy demonstrated significant changes in the distribution of these ion channels in response to sterol loading. Our results indicate that sterol-induced effects on ion channel gating directly target the pore and do not act via the VSD.

Conformational changes during human P2X7 receptor activation examined by structural modelling and cysteine-based cross-linking studies

The P2X7 receptor (P2X7R) is important in mediating a range of physiological functions and pathologies associated with tissue damage and inflammation and represents an attractive therapeutic target. However, in terms of their structure-function relationships, the mammalian P2X7Rs remain poorly characterised compared to some of their other P2XR counterparts. In this study, combining cysteine-based cross-linking and whole-cell patch-clamp recording, we examined six pairs of residues (A44/I331, D48/I331, I58/F311, S60/L320, I75/P177 and K81/V304) located in different parts of the extracellular and transmembrane domains of the human P2X7R. These residues are predicted to undergo substantial movement during the transition of the receptor ion channel from the closed to the open state, predictions which are made based on structural homology models generated from the crystal structures of the zebrafish P2X4R. Our results provide evidence that among the six pairs of cysteine mutants, D48C/I133C and K81C/V304C formed disulphide bonds that impaired the channel gating to support the notion that such conformational changes, particularly those in the outer ends of the transmembrane domains, are critical for human P2X7R activation.

Sialic acids attached to N- and O-glycans within the Nav1.4 D1S5-S6 linker contribute to channel gating

BACKGROUND

Voltage-gated Na+ channels (Nav) are responsible for the initiation and conduction of neuronal and muscle action potentials. Nav gating can be altered by sialic acids attached to channel N-glycans, typically through isoform-specific electrostatic mechanisms.

METHODS

Using two sets of Chinese Hamster Ovary cell lines with varying abilities to glycosylate glycoproteins, we show for the first time that sialic acids attached to O-glycans and N-glycans within the Nav1.4 D1S5-S6 linker modulate Nav gating.

RESULTS

All measured steady-state and kinetic parameters were shifted to more depolarized potentials under conditions of essentially no sialylation. When sialylation of only N-glycans or of only O-glycans was prevented, the observed voltage-dependent parameter values were intermediate between those observed under full versus no sialylation. Immunoblot gel shift analyses support the biophysical data.

CONCLUSIONS

The data indicate that sialic acids attached to both N- and O-glycans residing within the Nav1.4 D1S5-S6 linker modulate channel gating through electrostatic mechanisms, with the relative contribution of sialic acids attached to N- versus O-glycans on channel gating being similar.

GENERAL SIGNIFICANCE

Protein N- and O-glycosylation can modulate ion channel gating simultaneously. These data also suggest that environmental, metabolic, and/or congenital changes in glycosylation that impact sugar substrate levels, could lead, potentially, to changes in Nav sialylation and gating that would modulate AP waveforms and conduction.

Functional insight into development of positive allosteric modulators of AMPA receptors

Positive allosteric modulators of α-amino-3-hydroxy-5-methyl-isoxazole-propionic acid (AMPA) ionotropic glutamate receptors facilitate synaptic plasticity and contribute essentially to learning and memory, properties which make AMPA receptors targets for drug discovery and development. One region at which several different classes of positive allosteric modulators bind lies at the dimer interface between the ligand-binding core of the second, membrane-proximal, extracellular domain of AMPA receptors. This solvent-accessible binding pocket has been the target of drug discovery efforts, leading to the recent delineation of five "subsites" which differentially allow access to modulator moieties, and for which distinct modulator affinities and apparent efficacies are attributed. Here we use the voltage-clamp technique in conjunction with rapid drug application to study the effects of mutants lining subsites "A" and "B" of the allosteric modulator pocket to assess affinity and efficacy of allosteric modulation by cyclothiazide, CX614, CMPDA and CMPDB. A novel analysis of the decay of current produced by the onset of desensitization has allowed us to estimate both affinity and efficacy from single concentrations of modulator. Such an approach may be useful for effective high throughput screening of new target compounds.

The stability of secreted, acid-labile H77/JFH-1 hepatitis C virus (HCV) particles is altered by patient isolate genotype 1a p7 sequences

Secreted infectious particles generated by the genotype 2a JFH-1 hepatitis C virus infectious clone are resistant to acidic pH, whereas intracellular virions remain acid-labile. Thus, JFH-1 particles are thought to undergo pH maturation as they are secreted from the cell. Here, we demonstrate that both infectious intracellular and secreted genotype 1a (H77)/JFH-1 chimaeric particles display enhanced acid sensitivity compared with JFH-1, although pH maturation still occurs upon release. Introduction of p7 sequences from genotype 1a infected HCV patients into the H77/JFH-1 background yielded variable effects on infectious particle production and sensitivity to small molecule inhibitors. However, two selected patient p7 sequences increased the acid stability of secreted, but not intracellular H77/JFH-1 particles, suggesting that p7 directly influences particle pH maturation via an as yet undefined mechanism. We propose that HCV particles vary in acid stability, and that this may be dictated by variations in both canonical structural proteins and p7.

Conserved glycine at position 45 of major cochlear connexins constitutes a vital component of the Ca²⁺ sensor for gating of gap junction hemichannels

Mutations in gap junction (GJ) family of proteins, especially in the connexin (Cx) 26, are responsible for causing severe congenital hearing loss in a significant portion of patients (30-50% in various ethnic groups). Substitution of glycine at the position 45 of Cx26 to glutamic acid (p.G45E mutation) causes the Keratitis-ichthyosis-deafness (KID) syndrome. Previous studies have suggested that this point mutation caused a gain-of-function defect. However, the molecular mechanism of KID syndrome remains unclear. Since glycine at this position is conserved in many Cxs expressed in the cochlea, we tested the hypothesis that glycine at position 45 is an important component of the sensor regulating the Ca(2+) gating of GJ hemichannels. Using reconstituted Cx30, 32 and 43 expressed in the HEK 293 cells, we compared the functions of wild type and p.G45E mutant Cxs. We found that G45E in Cx30 resulted in similar deleterious cellular effects as Cx26 did. Cell death occurred within 24h of transfection, which was rescued by increasing extracellular Ca(2+) concentration ([Ca(2+)]o). Dye loading assay showed that Cx30 G45E, similar to Cx26 G45E, had leaky hemichannels at physiological [Ca(2+)]o (1.2 mM). Higher [Ca(2+)]o reduced the dye loading in a dose-dependent manner. Whole cell membrane current recordings also indicated that G45E caused increased hemichannel activities. p.G45E mutations of Cx32 and 43 also resulted in leaky hemichannels compared to their respective wild types in lower [Ca(2+)]o. Our data in this study provided further support for the hypothesis that glycine at position 45 is a conserved Ca(2+) sensor for the gating of GJ hemichannels among multiple Cx subtypes expressed in the cochlea.