Calcium release and ionic changes in the sarcoplasmic reticulum of tetanized muscle: an electron-probe study

Structure and Function of Auxin Transporters

Auxins, a group of central hormones in plant growth and development, are transported by a diverse range of transporters with distinct biochemical and structural properties. This review summarizes the current knowledge on all known auxin transporters with respect to their biochemical and biophysical properties and the methods used to characterize them. In particular, we focus on the recent advances that were made concerning the PIN-FORMED family of auxin exporters. Insights derived from solving their structures have improved our understanding of the auxin export process, and we discuss the current state of the art on PIN-mediated auxin transport, including the use of biophysical methods to examine their properties. Understanding the mechanisms of auxin transport is crucial for understanding plant growth and development, as well as for the development of more effective strategies for crop production and plant biotechnology.

Atypical cell death and insufficient matrix organization in long-bone growth plates from Tric-b-knockout mice

TRIC-A and TRIC-B proteins form homotrimeric cation-permeable channels in the endoplasmic reticulum (ER) and nuclear membranes and are thought to contribute to counterionic flux coupled with store Ca2+ release in various cell types. Serious mutations in the TRIC-B (also referred to as TMEM38B) locus cause autosomal recessive osteogenesis imperfecta (OI), which is characterized by insufficient bone mineralization. We have reported that Tric-b-knockout mice can be used as an OI model; Tric-b deficiency deranges ER Ca2+ handling and thus reduces extracellular matrix (ECM) synthesis in osteoblasts, leading to poor mineralization. Here we report irregular cell death and insufficient ECM in long-bone growth plates from Tric-b-knockout embryos. In the knockout growth plate chondrocytes, excess pro-collagen fibers were occasionally accumulated in severely dilated ER elements. Of the major ER stress pathways, activated PERK/eIF2α (PKR-like ER kinase/ eukaryotic initiation factor 2α) signaling seemed to inordinately alter gene expression to induce apoptosis-related proteins including CHOP (CCAAT/enhancer binding protein homologous protein) and caspase 12 in the knockout chondrocytes. Ca2+ imaging detected aberrant Ca2+ handling in the knockout chondrocytes; ER Ca2+ release was impaired, while cytoplasmic Ca2+ level was elevated. Our observations suggest that Tric-b deficiency directs growth plate chondrocytes to pro-apoptotic states by compromising cellular Ca2+-handling and exacerbating ER stress response, leading to impaired ECM synthesis and accidental cell death.

Auxin transport at the endoplasmic reticulum: roles and structural similarity of PIN-FORMED and PIN-LIKES

Auxin is a crucial plant hormone that controls a multitude of developmental processes. The directional movement of auxin between cells is largely facilitated by canonical PIN-FORMED proteins in the plasma membrane. In contrast, non-canonical PIN-FORMED proteins and PIN-LIKES proteins appear to reside mainly in the endoplasmic reticulum. Despite recent progress in identifying the roles of the endoplasmic reticulum in cellular auxin responses, the transport dynamics of auxin at the endoplasmic reticulum are not well understood. PIN-LIKES are structurally related to PIN-FORMED proteins, and recently published structures of these transporters have provided new insights into PIN-FORMED proteins and PIN-LIKES function. In this review, we summarize current knowledge on PIN-FORMED proteins and PIN-LIKES in intracellular auxin transport. We discuss the physiological properties of the endoplasmic reticulum and the consequences for transport processes across the ER membrane. Finally, we highlight the emerging role of the endoplasmic reticulum in the dynamics of cellular auxin signalling and its impact on plant development.

Electrical signals in the ER are cell type and stimulus specific with extreme spatial compartmentalization in neurons

The endoplasmic reticulum (ER) is a tortuous organelle that spans throughout a cell with a continuous membrane containing ion channels, pumps, and transporters. It is unclear if stimuli that gate ER ion channels trigger substantial membrane potential fluctuations and if those fluctuations spread beyond their site of origin. Here, we visualize ER membrane potential dynamics in HEK cells and cultured rat hippocampal neurons by targeting a genetically encoded voltage indicator specifically to the ER membrane. We report the existence of clear cell-type- and stimulus-specific ER membrane potential fluctuations. In neurons, direct stimulation of ER ryanodine receptors generates depolarizations that scale linearly with stimulus strength and reach tens of millivolts. However, ER potentials do not spread beyond the site of receptor activation, exhibiting steep attenuation that is exacerbated by intracellular large conductance K+ channels. Thus, segments of ER can generate large depolarizations that are actively restricted from impacting nearby, contiguous membrane.

Elucidation of the Physiological Functions of Membrane Proteins as Novel Drug Target Candidate Molecules

For pharmaceutical research focused on identifying novel drug target candidate molecules, it is essential to explore unknown biological phenomena, elucidate underlying molecular mechanisms and regulate biological processes based on these findings. Proteins expressed on the plasma membrane and endoplasmic reticulum (ER) membrane play important roles in linking extracellular environmental information to intracellular processes. Stimulating membranous proteins induces various kinds of changes in cells, such as alterations in gene expression levels and enzymatic activities. However, the physiological functions and endogenous ligands of many G-protein-coupled receptors (GPCRs) have not been determined, although GPCRs already constitute a large class of drug-target membrane proteins. Furthermore, the precise physiological roles played by many ER membrane proteins have not been elucidated to date. In this review article, I summarize the results of our recent studies, including the observations that the lipid sensor FFAR4/GPR120 controlled systemic energy homeostasis and that the ER membrane monovalent cation channel trimeric intracellular cation (TRIC)-B and the plasma membrane divalent cation channel transient receptor potential melastatin 7 (TRPM7) regulated bone formation. I further describe the therapeutic significance of these membranous protein-related biological processes.

Imaging the electrical activity of organelles in living cells

Eukaryotic cells are complex systems compartmentalized in membrane-bound organelles. Visualization of organellar electrical activity in living cells requires both a suitable reporter and non-invasive imaging at high spatiotemporal resolution. Here we present hVoSorg, an optical method to monitor changes in the membrane potential of subcellular membranes. This method takes advantage of a FRET pair consisting of a membrane-bound voltage-insensitive fluorescent donor and a non-fluorescent voltage-dependent acceptor that rapidly moves across the membrane in response to changes in polarity. Compared to the currently available techniques, hVoSorg has advantages including simple and precise subcellular targeting, the ability to record from individual organelles, and the potential for optical multiplexing of organellar activity.

Direct Measurements of Intracellular Elemental Composition Utilizing a New Approach to Freezing In Vivo

Many biological experiments can be successfully carried out on tissues isolated under well-defined experimental conditions, but for some physiological or pharmacological studies, knowledge of the in vivo elemental composition and/or structure is essential. We have developed a method for rapid freezing of organs in situ, with a new spring-loaded, hand held, clamping device that holds two apposing melting Freon popsicles (Fig. 5). The melting Freon surfaces that contact the tissue are at the melting point of Freon 22, and these mold to the unevenness of the tissue surface. Rapidly frozen samples of liver, pancreas or mesentery were obtained by placing an anesthetized rat in a cloth sling over an environmental chamber kept at 37°C, 100% humidity (Fig. 5). A small incision is made in the abdomen, and the abdominal wall is manipulated until a lobe of liver protrudes through the incision, while taking care not to touch the surface of the organ. Immediately, the Freon clamper is activated to snap freeze the tissue.

Enhanced activity of multiple TRIC-B channels: an endoplasmic reticulum/sarcoplasmic reticulum mechanism to boost counterion currents

KEY POINTS

There are two subtypes of trimeric intracellular cation (TRIC) channels but their distinct single-channel properties and physiological regulation have not been characterized. We examined the differences in function between native skeletal muscle sarcoplasmic reticulum (SR) K+ -channels from wild-type (WT) mice (where TRIC-A is the principal subtype) and from Tric-a knockout (KO) mice that only express TRIC-B. We find that lone SR K+ -channels from Tric-a KO mice have a lower open probability and gate more frequently in subconducting states than channels from WT mice but, unlike channels from WT mice, multiple channels gate with high open probability with a more than six-fold increase in activity when four channels are present in the bilayer. No evidence was found for a direct gating interaction between ryanodine receptor and SR K+ -channels in Tric-a KO SR, suggesting that TRIC-B-TRIC-B interactions are highly specific and may be important for meeting counterion requirements during excitation-contraction coupling in tissues where TRIC-A is sparse or absent.

ABSTRACT

The trimeric intracellular cation channels, TRIC-A and TRIC-B, represent two subtypes of sarcoplasmic reticulum (SR) K+ -channel but their individual functional roles are unknown. We therefore compared the biophysical properties of SR K+ -channels derived from the skeletal muscle of wild-type (WT) or Tric-a knockout (KO) mice. Because TRIC-A is the major TRIC-subtype in skeletal muscle, WT SR will predominantly contain TRIC-A channels, whereas Tric-a KO SR will only contain TRIC-B channels. When lone SR K+ -channels were incorporated into bilayers, the open probability (Po) of channels from Tric-a KO mice was markedly lower than that of channels from WT mice; gating was characterized by shorter opening bursts and more frequent brief subconductance openings. However, unlike channels from WT mice, the Po of SR K+ -channels from Tric-a KO mice increased as increasing channel numbers were present in the bilayer, driving the channels into long sojourns in the fully open state. When co-incorporated into bilayers, ryanodine receptor channels did not directly affect the gating of SR K+ -channels, nor did the presence or absence of SR K+ -channels influence ryanodine receptor activity. We suggest that because of high expression levels in striated muscle, TRIC-A produces most of the counterion flux required during excitation-contraction coupling. TRIC-B, in contrast, is sparsely expressed in most cells and, although lone TRIC-B channels exhibit low Po, the high Po levels reached by multiple TRIC-B channels may provide a compensatory mechanism to rapidly restore K+ gradients and charge differences across the SR of tissues containing few TRIC-A channels.

No voltage change at skeletal muscle SR membrane during Ca2+ release-just Mermaids on acid

Melzer highlights new work confirming that the sarcoplasmic reticulum transmembrane voltage changes little during Ca²⁺ release

Calcium ions control multiple physiological functions by binding to extracellular and intracellular targets. One of the best-studied Ca²⁺-dependent functions is contraction of smooth and striated muscle tissue, which results from Ca²⁺ ligation to calmodulin and troponin C, respectively. Ca²⁺ signaling typically involves flux of the ion across membranes via specifically gated channel proteins. Because calcium ions are charged, they possess the ability to generate changes in the respective transmembrane voltage. Ca²⁺-dependent voltage alterations of the surface membrane are easily measured using microelectrodes. A well-known example is the characteristic plateau phase of the action potential in cardiac ventricular cells that results from the opening of voltage-gated L-type Ca²⁺ channels. Ca²⁺ ions are also released from intracellular storage compartments in many cells, but these membranes are not accessible to direct voltage recording with microelectrodes. In muscle, for example, release of Ca²⁺ from the sarcoplasmic reticulum (SR) to the myoplasm constitutes a flux that is considerably larger than the entry flux from the extracellular space. Whether this flux is accompanied by a voltage change across the SR membrane is an obvious question of mechanistic importance and has been the subject of many investigations. Because the tiny spaces enclosed by the SR membrane are inaccessible to microelectrodes, alternative methods have to be applied. In a study by Sanchez et al. (2018. J. Gen. Physiol.https://doi.org/10.1085/jgp.201812035) in this issue, modern confocal light microscopy and genetically encoded voltage probes targeted to the SR were applied in a new approach to search for changes in the membrane potential of the SR during Ca²⁺ release.

Tracking the sarcoplasmic reticulum membrane voltage in muscle with a FRET biosensor

The sarcoplasmic reticulum membrane contains ion channels, but it is unknown whether it experiences voltage changes during cellular activity. By expressing voltage-sensitive fluorescence biosensors in this membrane, Sanchez et al. suggest that it remains electrically silent during muscle activation.

Ion channel activity in the plasma membrane of living cells generates voltage changes that are critical for numerous biological functions. The membrane of the endoplasmic/sarcoplasmic reticulum (ER/SR) is also endowed with ion channels, but whether changes in its voltage occur during cellular activity has remained ambiguous. This issue is critical for cell functions that depend on a Ca²⁺ flux across the reticulum membrane. This is the case for contraction of striated muscle, which is triggered by opening of ryanodine receptor Ca²⁺ release channels in the SR membrane in response to depolarization of the transverse invaginations of the plasma membrane (the t-tubules). Here, we use targeted expression of voltage-sensitive fluorescence resonance energy transfer (FRET) probes of the Mermaid family in differentiated muscle fibers to determine whether changes in SR membrane voltage occur during depolarization–contraction coupling. In the absence of an SR targeting sequence, FRET signals from probes present in the t-tubule membrane allow calibration of the voltage sensitivity and amplitude of the response to voltage-clamp pulses. Successful SR targeting of the probes was achieved using an N-terminal domain of triadin, which completely eliminates voltage-clamp–activated FRET signals from the t-tubule membrane of transfected fibers. In fibers expressing SR-targeted Mermaid probes, activation of SR Ca²⁺ release in the presence of intracellular ethyleneglycol-bis(β-amino-ethyl ether)-N,N,N′,N′-tetra acetic acid (EGTA) results in an accompanying FRET signal. We find that this signal results from pH sensitivity of the probe, which detects cytosolic acidification because of the release of protons upon Ca²⁺ binding to EGTA. When EGTA is substituted with either 1,2-bis(o-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid or the contraction blocker N-benzyl-p-toluene sulfonamide, we find no indication of a substantial change in the FRET response caused by a voltage change. These results suggest that the ryanodine receptor–mediated SR Ca²⁺ efflux is well balanced by concomitant counterion currents across the SR membrane.

The Ca2+-ATPase pump facilitates bidirectional proton transport across the sarco/endoplasmic reticulum

Ca²⁺ transport across the sarco/endoplasmic reticulum (SR) plays an essential role in intracellular Ca²⁺ homeostasis, signalling, cell differentiation and muscle contractility. During SR Ca²⁺ uptake and release, proton fluxes are required to balance the charge deficit generated by the exchange of Ca²⁺ and other ions across the SR. During Ca²⁺ uptake by the SR Ca²⁺-ATPase (SERCA), two protons are countertransported from the SR lumen to the cytosol, thus partially compensating for the charge moved by Ca²⁺ transport. Studies have shown that protons are also transported from the cytosol to the lumen during Ca²⁺ release, but a transporter that facilitates proton transport into the SR lumen has not been described. In this article we propose that SERCA forms pores that facilitate bidirectional proton transport across the SR. We describe the location and structure of water-filled pores in SERCA that form cytosolic and luminal pathways for protons to cross the SR membrane. Based on this structural information, we suggest mechanistic models for proton translocation to the cytosol during active Ca²⁺ transport, and into the SR lumen during SERCA inhibition by endogenous regulatory proteins. Finally, we discuss the physiological consequences of SERCA-mediated bidirectional proton transport across the SR membrane of muscle and non-muscle cells.

Dampened activity of ryanodine receptor channels in mutant skeletal muscle lacking TRIC-A

KEY POINTS

The role of trimeric intracellular cation (TRIC) channels is not known, although evidence suggests they may regulate ryanodine receptors (RyR) via multiple mechanisms. We therefore investigated whether Tric-a gene knockout (KO) alters the single-channel function of skeletal RyR (RyR1). We find that RyR1 from Tric-a KO mice are more sensitive to inhibition by divalent cations, although they respond normally to cytosolic Ca²⁺ , ATP, caffeine and luminal Ca²⁺ . In the presence of Mg²⁺ , ATP cannot effectively activate RyR1 from Tric-a KO mice. Additionally, RyR1 from Tric-a KO mice are not activated by protein kinase A phosphorylation, demonstrating a defect in the ability of β-adrenergic stimulation to regulate sarcoplasmic reticulum (SR) Ca²⁺ -release. The defective RyR1 gating that we describe probably contributes significantly to the impaired SR Ca²⁺ -release observed in skeletal muscle from Tric-a KO mice, further highlighting the importance of TRIC-A for normal physiological regulation of SR Ca²⁺ -release in skeletal muscle.

ABSTRACT

The type A trimeric intracellular cation channel (TRIC-A) is a major component of the nuclear and sarcoplasmic reticulum (SR) membranes of cardiac and skeletal muscle, and is localized closely with ryanodine receptor (RyR) channels in the SR terminal cisternae. The skeletal muscle of Tric-a knockout (KO) mice is characterized by Ca²⁺ overloaded and swollen SR and by changes in the properties of SR Ca²⁺ release. We therefore investigated whether RyR1 gating behaviour is modified in the SR from Tric-a KO mice by incorporating native RyR1 into planar phospholipid bilayers under voltage-clamp conditions. We find that RyR1 channels from Tric-a KO mice respond normally to cytosolic Ca²⁺ , ATP, adenine, caffeine and to luminal Ca²⁺ . However, the channels are more sensitive to the inactivating effects of divalent cations, thus, in the presence of Mg²⁺ , ATP is inadequate as an activator. Additionally, channels are not characteristically activated by protein kinase A even though the phosphorylation levels of Ser2844 are similar to controls. The results of the present study suggest that TRIC-A functions as an excitatory modulator of RyR1 channels within the SR terminal cisternae. Importantly, this regulatory action of TRIC-A appears to be independent of (although additive to) any indirect consequences to RyR1 activity that arise as a result of K⁺ fluxes across the SR via TRIC-A.

TRIC-B Mutations Causing Osteogenesis Imperfecta

Trimeric intracellular cation (TRIC) channel subtypes, namely TRIC-A and TRIC-B, are expressed in the endoplasmic/sarcoplasmic reticulum and nuclear envelope, and likely function as monovalent cation channels in various cell types. Our studies using knockout mice so far suggest that TRIC subtypes support Ca²⁺ release from intracellular stores by mediating counter-cationic fluxes. Several genetic mutations within the TRIC-B locus were recently identified in autosomal recessive osteogenesis imperfecta (OI) patients. However, the molecular mechanism by which the mutations cause human disease is not fully addressed. We found that Tric-b-knockout mice exhibit poor bone ossification and thus serve as an OI-model animal. Studies on Tric-b-knockout bones and cultured cell lines derived from the patients currently reveal the main part of the pathophysiological mechanism involved in the TRIC-B-mutated OI form. This mini-review focuses on the essential role of TRIC-B channels in bone ossification.

Sarcoplasmic reticulum Ca2+, Mg2+, K+, and Cl- concentrations adjust quickly as heart rate changes

During systole, Ca²⁺ is released from the sarcoplasmic reticulum (SR) through ryanodine receptors (RyRs) while, simultaneously, other ions (specifically K⁺, Mg²⁺, and Cl^-) provide counter-ion flux. These ions move back into the SR during diastole through the SERCA pump and SR K⁺ and Cl^- channels. In homeostasis, all ion concentrations in different cellular regions (e.g., junctional and non-junctional SR, dyadic cleft, and cytosol) are the same at the beginning and end of the cardiac cycle. Here, we used an equivalent circuit compartment model of the SR and the surrounding cytoplasm to understand the heart rate dependence of SR ion homeostasis. We found that the Ca²⁺, Mg²⁺, K⁺, and Cl^- concentrations in the SR and the cytoplasm self-adjust within just a few heartbeats with only very small changes in Mg²⁺, K⁺, and Cl^- concentrations and membrane voltages (just a few percent). However, those small changes were enough to compensate for the large heart-rate-dependent changes in SR and cytoplasmic Ca²⁺ concentrations in the new steady state. The modeling suggests that ion adaptation to increases in heart rate is inherent to the system and that physiological changes that increase contractility and cardiac output are accommodated by the same self-adjusting mechanism of producing small changes in ion driving forces. Our findings also support the long-held hypothesis that SR membrane potentials are small (~1-2mV).

Mice lacking the intracellular cation channel TRIC-B have compromised collagen production and impaired bone mineralization

The trimeric intracellular cation (TRIC) channels TRIC-A and TRIC-B localize predominantly to the endoplasmic reticulum (ER) and likely support Ca(2+) release from intracellular stores by mediating cationic flux to maintain electrical neutrality. Deletion and point mutations in TRIC-B occur in families with autosomal recessive osteogenesis imperfecta. Tric-b knockout mice develop neonatal respiratory failure and exhibit poor bone ossification. We investigated the cellular defect causing the bone phenotype. Bone histology indicated collagen matrix deposition was reduced in Tric-b knockout mice. Osteoblasts, the bone-depositing cells, from Tric-b knockout mice exhibited reduced Ca(2+) release from ER and increased ER Ca(2+) content, which was associated with ER swelling. These cells also had impaired collagen release without a decrease in collagen-encoding transcripts, consistent with a defect in trafficking of collagen through ER. In contrast, osteoclasts, the bone-degrading cells, from Tric-b knockout mice were similar to those from wild-type mice. Thus, TRIC-B function is essential to support the production and release of large amounts of collagen by osteoblasts, which is necessary for bone mineralization.

Rapid Method To Determine Intracellular Drug Concentrations in Cellular Uptake Assays: Application to Metformin in Organic Cation Transporter 1-Transfected Human Embryonic Kidney 293 Cells

Because of the importance of intracellular unbound drug concentrations in the prediction of in vivo concentrations that are determinants of drug efficacy and toxicity, a number of assays have been developed to assess in vitro unbound concentrations of drugs. Here we present a rapid method to determine the intracellular unbound drug concentrations in cultured cells, and we apply the method along with a mechanistic model to predict concentrations of metformin in subcellular compartments of stably transfected human embryonic kidney 293 (HEK293) cells. Intracellular space (ICS) was calculated by subtracting the [(3)H]-inulin distribution volume (extracellular space, ECS) from the [(14)C]-urea distribution volume (total water space, TWS). Values obtained for intracellular space (mean ± S.E.M.; μl/10(6) cells) of monolayers of HEK cells (HEK-empty vector [EV]) and cells overexpressing human organic cation transporter 1 (HEK-OCT1), 1.21± 0.07 and 1.25±0.06, respectively, were used to determine the intracellular metformin concentrations. After incubation of the cells with 5 µM metformin, the intracellular concentrations were 26.4 ± 7.8 μM and 268 ± 11.0 μM, respectively, in HEK-EV and HEK-OCT1. In addition, intracellular metformin concentrations were lower in high K(+) buffer (140 mM KCl) compared with normal K(+) buffer (5.4 mM KCl) in HEK-OCT1 cells (54.8 ± 3.8 μM and 198.1 ± 11.2 μM, respectively; P < 0.05). Our mechanistic model suggests that, depending on the credible range of assumed physiologic values, the positively charged metformin accumulates to particularly high levels in endoplasmic reticulum and/or mitochondria. This method together with the computational model can be used to determine intracellular unbound concentrations and to predict subcellular accumulation of drugs in other complex systems such as primary cells.

Identification of Small Ankyrin 1 as a Novel Sarco(endo)plasmic Reticulum Ca2+-ATPase 1 (SERCA1) Regulatory Protein in Skeletal Muscle

Small ankyrin 1 (sAnk1) is a 17-kDa transmembrane (TM) protein that binds to the cytoskeletal protein, obscurin, and stabilizes the network sarcoplasmic reticulum in skeletal muscle. We report that sAnk1 shares homology in its TM amino acid sequence with sarcolipin, a small protein inhibitor of the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA). Here we investigate whether sAnk1 and SERCA1 interact. Our results indicate that sAnk1 interacts specifically with SERCA1 in sarcoplasmic reticulum vesicles isolated from rabbit skeletal muscle, and in COS7 cells transfected to express these proteins. This interaction was demonstrated by co-immunoprecipitation and an anisotropy-based FRET method. Binding was reduced ~2-fold by the replacement of all of the TM amino acids of sAnk1 with leucines by mutagenesis. This suggests that, like sarcolipin, sAnk1 interacts with SERCA1 at least in part via its TM domain. Binding of the cytoplasmic domain of sAnk1 to SERCA1 was also detected in vitro. ATPase activity assays show that co-expression of sAnk1 with SERCA1 leads to a reduction of the apparent Ca(2+) affinity of SERCA1 but that the effect of sAnk1 is less than that of sarcolipin. The sAnk1 TM mutant has no effect on SERCA1 activity. Our results suggest that sAnk1 interacts with SERCA1 through its TM and cytoplasmic domains to regulate SERCA1 activity and modulate sequestration of Ca(2+) in the sarcoplasmic reticulum lumen. The identification of sAnk1 as a novel regulator of SERCA1 has significant implications for muscle physiology and the development of therapeutic approaches to treat heart failure and muscular dystrophies linked to Ca(2+) misregulation.

Intracellular Zinc Modulates Cardiac Ryanodine Receptor-mediated Calcium Release

Aberrant Zn²⁺ homeostasis is a hallmark of certain cardiomyopathies associated with altered contractile force. In this study, we addressed whether Zn²⁺ modulates cardiac ryanodine receptor gating and Ca²⁺ dynamics in isolated cardiomyocytes. We reveal that Zn²⁺ is a high affinity regulator of RyR2 displaying three modes of operation. Picomolar free Zn²⁺ concentrations potentiate RyR2 responses, but channel activation is still dependent on the presence of cytosolic Ca²⁺. At concentrations of free Zn²⁺ >1 nm, Zn²⁺ is the main activating ligand, and the dependence on Ca²⁺ is removed. Zn²⁺ is therefore a higher affinity activator of RyR2 than Ca²⁺. Millimolar levels of free Zn²⁺ were found to inhibit channel openings. In cardiomyocytes, consistent with our single channel results, we show that Zn²⁺ modulates both the frequency and amplitude of Ca²⁺ waves in a concentration-dependent manner and that physiological levels of Zn²⁺ elicit Ca²⁺ release in the absence of activating levels of cytosolic Ca²⁺. This highlights a new role for intracellular Zn²⁺ in shaping Ca²⁺ dynamics in cardiomyocytes through modulation of RyR2 gating.

Potassium fluxes across the endoplasmic reticulum and their role in endoplasmic reticulum calcium homeostasis

There are a number of known and suspected channels and exchangers in the endoplasmic reticulum that may participate in potassium flux across its membrane. They include trimeric intracellular cation channels permeable for potassium, ATP-sensitive potassium channels, calcium-activated potassium channels and the potassium-hydrogen exchanger. Apart from trimeric intracellular cation channels, which are specific to the endoplasmic reticulum, other potassium channels are also expressed in the plasma membrane and/or mitochondria, and their specific role in the endoplasmic reticulum has not yet been fully established. In addition to these potassium-selective channels, the ryanodine receptor and, potentially, the inositol 1,4,5-trisphosphate receptor are permeable to potassium ions. Also, the role of potassium fluxes across the endoplasmic reticulum membrane has remained elusive. It has been proposed that their main role is to balance the charge movement that occurs during calcium release and uptake from or to the endoplasmic reticulum. This review aims to summarize current knowledge on endoplasmic reticulum potassium channels and fluxes and their potential role in endoplasmic reticulum calcium uptake and release.

Endogenous and maximal sarcoplasmic reticulum calcium content and calsequestrin expression in type I and type II human skeletal muscle fibres

The relationship between sarcoplasmic reticulum (SR) Ca(2+) content and calsequestrin (CSQ) isoforms was investigated in human skeletal muscle. A fibre-lysing assay was used to quantify the endogenous Ca(2+) content and maximal Ca(2+) capacity of the SR in skinned segments of type I and type II fibres from vastus lateralis muscles of young healthy adults. Western blotting of individual fibres showed the great majority contained either all fast or all slow isoforms of myosin heavy chain (MHC), troponins C and I, tropomyosin and SERCA, and that the strontium sensitivity of the force response was closely indicative of the troponin C isoform present. The endogenous SR Ca(2+) content was slightly lower in type I compared to type II fibres (0.76 ± 0.03 and 0.85 ± 0.02 mmol Ca(2+) per litre of fibre, respectively), with virtually all of this Ca(2+) evidently being in the SR, as it could be rapidly released with a caffeine-low [Mg(2+)] solution (only 0.08 ± 0.01 and <0.07 mmol l(-1), respectively, remaining). The maximal Ca(2+) content that could be reached with SR Ca(2+) loading was 1.45 ± 0.04 and 1.79 ± 0.03 mmol l(-1) in type I and type II fibres, respectively (P < 0.05). In non-lysed skinned fibres, where the SR remained functional, repeated cycles of caffeine-induced Ca(2+) release and subsequent Ca(2+) reloading similarly indicated that (i) maximal SR Ca(2+) content was lower in type I fibres than in type II fibres (P < 0.05), and (ii) the endogenous Ca(2+) content represented a greater percentage of maximal content in type I fibres compared to type II fibres (∼59% and 41%, respectively, P < 0.05). Type II fibres were found on average to contain ∼3-fold more CSQ1 and ∼5-fold less CSQ2 than type I fibres (P < 0.001). The findings are consistent with the SR Ca(2+) content characteristics in human type II fibres being primarily determined by the CSQ1 abundance, and in type I fibres by the combined amounts of both CSQ1 and CSQ2.

Facilitated hyperpolarization signaling in vascular smooth muscle-overexpressing TRIC-A channels

The TRIC channel subtypes, namely TRIC-A and TRIC-B, are intracellular monovalent cation-specific channels and likely mediate counterion movements to support efficient Ca(2+) release from the sarco/endoplasmic reticulum. Vascular smooth muscle cells (VSMCs) contain both TRIC subtypes and two Ca(2+) release mechanisms; incidental opening of ryanodine receptors (RyRs) generates local Ca(2+) sparks to induce hyperpolarization and relaxation, whereas agonist-induced activation of inositol trisphosphate receptors produces global Ca(2+) transients causing contraction. Tric-a knock-out mice develop hypertension due to insufficient RyR-mediated Ca(2+) sparks in VSMCs. Here we describe transgenic mice overexpressing TRIC-A channels under the control of a smooth muscle cell-specific promoter. The transgenic mice developed congenital hypotension. In Tric-a-overexpressing VSMCs from the transgenic mice, the resting membrane potential decreased because RyR-mediated Ca(2+) sparks were facilitated and cell surface Ca(2+)-dependent K(+) channels were hyperactivated. Under such hyperpolarized conditions, L-type Ca(2+) channels were inactivated, and thus, the resting intracellular Ca(2+) levels were reduced in Tric-a-overexpressing VSMCs. Moreover, Tric-a overexpression impaired inositol trisphosphate-sensitive stores to diminish agonist-induced Ca(2+) signaling in VSMCs. These altered features likely reduced vascular tonus leading to the hypotensive phenotype. Our Tric-a-transgenic mice together with Tric-a knock-out mice indicate that TRIC-A channel density in VSMCs is responsible for controlling basal blood pressure at the whole-animal level.

Cytochemical and electron probe X-ray microanalysis studies on the distribution change of intracellular calcium in columella cells of soybean roots under simulated microgravity

The columella cells of soybean roots grown under gravity and simulated microgravity induced by a clinostat were examined using potassium pyroantimonate (PA) and quantitative X-ray microanalysis of cryosections to determine the role of Ca in the regulation of the gravitropic response. Amyloplasts in the columella cells were localized exclusively at the bottom under gravity, but diffusely distributed in the cytoplasmic matrix under simulated microgravity, thus supporting the statolith theory. In the columella cells, PA precipitates containing Ca were diffusely distributed in the cytoplasmic matrix under gravity. Under simulated microgravity, however, they decreased in number and size in the cytoplasmic matrix, whereas increased only in number in the vacuole, indicating that Ca moved from the cytoplasmic matrix into the vacuole. The vacuole of columella cells contained mostly electron-dense granular structures localized along the inner surface of tonoplasts, which closely resembled the tannin vacuole reported in Mimosa pulvinar motor cells. Under simulated microgravity, their configuration changed dramatically from a granular shape to a flat plate. The quantitative X-ray microanalysis of cryosections showed that the vacuolar electron-dense structures contained a large amount of Ca. Under simulated microgravity, the concentration of Ca increased conspicuously in these vacuolar electron-dense structures, concomitantly with a marked decrease of K in the vacuoles and an increase of K in the cell walls. These results suggest that the release of Ca(2+) from, and uptake by, the vacuolar electron-dense structures is closely related to the signal transmission in the gravitropic response and that Ca movement occurs opposite to that of K.

Residual sarcoplasmic reticulum Ca2+ concentration after Ca2+ release in skeletal myofibers from young adult and old mice

Contrasting information suggests either almost complete depletion of sarcoplasmic reticulum (SR) Ca(2+) or significant residual Ca(2+) concentration after prolonged depolarization of the skeletal muscle fiber. The primary obstacle to resolving this controversy is the lack of genetically encoded Ca(2+) indicators targeted to the SR that exhibit low-Ca(2+) affinity, a fast biosensor: Ca(2+) off-rate reaction, and can be expressed in myofibers from adult and older adult mammalian species. This work used the recently designed low-affinity Ca(2+) sensor (Kd = 1.66 mM in the myofiber) CatchER (calcium sensor for detecting high concentrations in the ER) targeted to the SR, to investigate whether prolonged skeletal muscle fiber depolarization significantly alters residual SR Ca(2+) with aging. We found CatchER a proper tool to investigate SR Ca(2+) depletion in young adult and older adult mice, consistently tracking SR luminal Ca(2+) release in response to brief and repetitive stimulation. We evoked SR Ca(2+) release in whole-cell voltage-clamped flexor digitorum brevis muscle fibers from young and old FVB mice and tested the maximal SR Ca(2+) release by directly activating the ryanodine receptor (RyR1) with 4-chloro-m-cresol in the same myofibers. Here, we report for the first time that the Ca(2+) remaining in the SR after prolonged depolarization (2 s) in myofibers from aging (~220 μM) was larger than young (~132 μM) mice. These experiments indicate that SR Ca(2+) is far from fully depleted under physiological conditions throughout life, and support the concept of excitation-contraction uncoupling in functional senescent myofibers.

Mammalian muscle model for predicting force and energetics during physiological behaviors

Muscles convert metabolic energy into mechanical work. A computational model of muscle would ideally compute both effects efficiently for the entire range of muscle activation and kinematic conditions (force and length). We have extended the original Virtual Muscle algorithm (Cheng , 2000) to predict energy consumption for both slow- and fast-twitch muscle fiber types, partitioned according to the activation process (E(a)), cross-bridge cycling (E(xb)) and ATP/PCr recovery (E(recovery)). Because the terms of these functions correspond to identifiable physiological processes, their coefficients can be estimated directly from the types of experiments that are usually performed and extrapolated to dynamic conditions of natural motor behaviors. We also implemented a new approach to lumped modeling of the gradually recruited and frequency modulated motor units comprising each fiber type, which greatly reduced computational time. The emergent behavior of the model has significant implications for studies of optimal motor control and development of rehabilitation strategies because its trends were quite different from traditional estimates of energy (e.g., activation, force, stress, work, etc.). The model system was scaled to represent three different human experimental paradigms in which muscle heat was measured during voluntary exercise; predicted and observed energy rate agreed well both qualitatively and quantitatively.

Regulation of phosphorylase kinase by low concentrations of Ca ions upon muscle contraction: the connection between metabolism and muscle contraction and the connection between muscle physiology and Ca-dependent signal transduction

It had long been one of the crucial questions in muscle physiology how glycogenolysis is regulated in connection with muscle contraction, when we found the answer to this question in the last half of the 1960s. By that time, the two principal currents of muscle physiology, namely, the metabolic flow starting from glycogen and the mechanisms of muscle contraction, had already been clarified at the molecular level thanks to our senior researchers. Thus, the final question we had to answer was how to connect these two currents. We found that low concentrations of Ca ions (10(-7)-10(-4) M) released from the sarcoplasmic reticulum for the regulation of muscle contraction simultaneously reversibly activate phosphorylase kinase, the enzyme regulating glycogenolysis. Moreover, we found that adenosine 3',5'-monophosphate (cyclic AMP), which is already known to activate muscle phosphorylase kinase, is not effective in the absence of such concentrations of Ca ions. Thus, cyclic AMP is not effective by itself alone and only modifies the activation process in the presence of Ca ions (at that time, cyclic AMP-dependent protein kinase had not yet been identified). After a while, it turned out that our works have not only provided the solution to the above problem on muscle physiology, but have also been considered as the first report of Ca-dependent protein phosphorylation, which is one of the central problems in current cell biology. Phosphorylase kinase is the first protein kinase to phosphorylate a protein resulting in the change in the function of the phosphorylated protein, as shown by Krebs and Fischer. Our works further showed that this protein kinase is regulated in a Ca-dependent manner. Accordingly, our works introduced the concept of low concentrations of Ca ions, which were first identified as the regulatory substance of muscle contraction, to the vast field of Ca biology including signal transduction.

Subtype identification and functional characterization of ryanodine receptors in rat cerebral artery myocytes

Ryanodine receptors (RyRs) regulate contractility in resistance-size cerebral artery smooth muscle, yet their molecular identity, subcellular location, and phenotype in this tissue remain unknown. Following rat resistance-size cerebral artery myocyte sarcoplasmic reticulum (SR) purification and incorporation into POPE-POPS-POPC (5:3:2; wt/wt) bilayers, unitary conductances of 110 +/- 8, 334 +/- 15, and 441 +/- 27 pS in symmetric 300 mM Cs(+) were usually detected. The most frequent (34/40 bilayers) conductance (334 pS) decreased to Collapse

Key Words

• calcium-release channels
• vascular smooth muscle

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MESH Headings

• Animals
• Cells, Cultured
• Cerebral Arteries/chemistry
• Cerebral Arteries/cytology
• Cerebral Arteries/physiology
• Female
• Male
• Muscle Cells/chemistry
• Muscle Cells/physiology
• Protein Subunits/analysis
• Protein Subunits/physiology
• Rats
• Rats, Sprague-Dawley
• Ryanodine Receptor Calcium Release Channel/analysis
• Ryanodine Receptor Calcium Release Channel/physiology

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Grants

• AA11560 NIAAA NIH HHS
• R01 HL094378 NHLBI NIH HHS
• HL77424 NHLBI NIH HHS
• HL67061 NHLBI NIH HHS
• HL94378 NHLBI NIH HHS

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Affiliation(s)

Thirumalini Vaithianathan Department Pharmacology, University of Tennessee Health Science Center, Memphis, Tennessee 38163, USA
Damodaran Narayanan
Maria T Asuncion-Chin
Loice H Jeyakumar
Jianxi Liu
Sidney Fleischer
Jonathan H Jaggar
Alejandro M Dopico

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Sarcoplasmic reticulum Ca2+ depletion in adult skeletal muscle fibres measured with the biosensor D1ER

The endoplasmic/sarcoplasmic reticulum (ER/SR) plays a crucial role in cytoplasmic signalling in a variety of cells. It is particularly relevant to skeletal muscle fibres, where this organelle constitutes the main Ca2+ store for essential functions, such as contraction. In this work, we expressed the cameleon biosensor D1ER by in vivo electroporation in the mouse flexor digitorum brevis (FDB) muscle to directly assess SR Ca2+ depletion in response to electrical and pharmacological stimulation. The main conclusions are: (1) D1ER is expressed in the SR of FDB fibres according to both di-8-(amino naphthyl ethenyl pyridinium) staining experiments and reductions in the Förster resonance energy transfer signal consequent to SR Ca2+ release; (2) the amplitude of D1ER citrine/cyan fluorescent protein (CFP) ratio evoked by either 4-chloro-m-cresol (4-CmC) or electrical stimulation is directly proportional to the basal citrine/CFP ratio, which indicates that SR Ca2+ modulates ryanodine-receptor-isoform-1-mediated SR Ca2+ release in the intact muscle fibre; (3) SR Ca2+ release, measured as D1ER citrine/CFP signal, is voltage-dependent and follows a Boltzmann function; and (4) average SR Ca2+ depletion is 20% in response to 4-CmC and 6.4% in response to prolonged sarcolemmal depolarization. These results indicate that significantly depleting SR Ca2+ content under physiological conditions is difficult.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.

Minor sarcoplasmic reticulum membrane components that modulate excitation-contraction coupling in striated muscles

In striated muscle, activation of contraction is initiated by membrane depolarisation caused by an action potential, which triggers the release of Ca(2+) stored in the sarcoplasmic reticulum by a process called excitation-contraction coupling. Excitation-contraction coupling occurs via a highly sophisticated supramolecular signalling complex at the junction between the sarcoplasmic reticulum and the transverse tubules. It is generally accepted that the core components of the excitation-contraction coupling machinery are the dihydropyridine receptors, ryanodine receptors and calsequestrin, which serve as voltage sensor, Ca(2+) release channel, and Ca(2+) storage protein, respectively. Nevertheless, a number of additional proteins have been shown to be essential both for the structural formation of the machinery involved in excitation-contraction coupling and for its fine tuning. In this review we discuss the functional role of minor sarcoplasmic reticulum protein components. The definition of their roles in excitation-contraction coupling is important in order to understand how mutations in genes involved in Ca(2+) signalling cause neuromuscular disorders.

Luminal Mg2+, a key factor controlling RYR2-mediated Ca2+ release: cytoplasmic and luminal regulation modeled in a tetrameric channel

In cardiac muscle, intracellular Ca(2+) and Mg(2+) are potent regulators of calcium release from the sarcoplasmic reticulum (SR). It is well known that the free [Ca(2+)] in the SR ([Ca(2+)](L)) stimulates the Ca(2+) release channels (ryanodine receptor [RYR]2). However, little is known about the action of luminal Mg(2+), which has not been regarded as an important regulator of Ca(2+) release. The effects of luminal Ca(2+) and Mg(2+) on sheep RYR2 were measured in lipid bilayers. Cytoplasmic and luminal Ca(2+) produced a synergistic increase in the opening rate of RYRs. A novel, high affinity inhibition of RYR2 by luminal Mg(2+) was observed, pointing to an important physiological role for luminal Mg(2+) in cardiac muscle. At diastolic [Ca(2+)](C), luminal Mg(2+) inhibition was voltage independent, with K(i) = 45 microM at luminal [Ca(2+)] ([Ca(2+)](L)) = 100 microM. Luminal and cytoplasmic Mg(2+) inhibition was alleviated by increasing [Ca(2+)](L) or [Ca(2+)](C). Ca(2+) and Mg(2+) on opposite sides of the bilayer exhibited competitive effects on RYRs, indicating that they can compete via the pore for common sites. The data were accurately fitted by a model based on a tetrameric RYR structure with four Ca(2+)-sensing mechanisms on each subunit: activating luminal L-site (40-microM affinity for Mg(2+) and Ca(2+)), cytoplasmic A-site (1.2 microM for Ca(2+) and 60 microM for Mg(2+)), inactivating cytoplasmic I(1)-site (approximately 10 mM for Ca(2+) and Mg(2+)), and I(2)-site (1.2 microM for Ca(2+)). Activation of three or more subunits will cause channel opening. Mg(2+) inhibition occurs primarily by Mg(2+) displacing Ca(2+) from the L- and A-sites, and Mg(2+) fails to open the channel. The model predicts that under physiological conditions, SR load-dependent Ca(2+) release (1) is mainly determined by Ca(2+) displacement of Mg(2+) from the L-site as SR loading increases, and (2) depends on the properties of both luminal and cytoplasmic activation mechanisms.

Calsequestrin content and SERCA determine normal and maximal Ca2+ storage levels in sarcoplasmic reticulum of fast- and slow-twitch fibres of rat

Whilst calsequestrin (CSQ) is widely recognized as the primary Ca2+ buffer in the sarcoplasmic reticulum (SR) in skeletal muscle fibres, its total buffering capacity and importance have come into question. This study quantified the absolute amount of CSQ isoform 1 (CSQ1, the primary isoform) present in rat extensor digitorum longus (EDL) and soleus fibres, and related this to their endogenous and maximal SR Ca2+ content. Using Western blotting, the entire constituents of minute samples of muscle homogenates or segments of individual muscle fibres were compared with known amounts of purified CSQ1. The fidelity of the analysis was proven by examining the relative signal intensity when mixing muscle samples and purified CSQ1. The CSQ1 contents of EDL fibres, almost exclusively type II fibres, and soleus type I fibres [SOL (I)] were, respectively, 36 +/- 2 and 10 +/- 1 micromol (l fibre volume)(-1), quantitatively accounting for the maximal SR Ca2+ content of each. Soleus type II [SOL (II)] fibres (approximately 20% of soleus fibres) had an intermediate amount of CSQ1. Every SOL (I) fibre examined also contained some CSQ isoform 2 (CSQ2), which was absent in every EDL and other type II fibre except for trace amounts in one case. Every EDL and other type II fibre had a high density of SERCA1, the fast-twitch muscle sarco(endo)plasmic reticulum Ca2+-ATPase isoform, whereas there was virtually no SERCA1 in any SOL (I) fibre. Maximal SR Ca2+ content measured in skinned fibres increased with CSQ1 content, and the ratio of endogenous to maximal Ca2+ content was inversely correlated with CSQ1 content. The relative SR Ca2+ content that could be maintained in resting cytoplasmic conditions was found to be much lower in EDL fibres than in SOL (I) fibres (approximately 20 versus >60%). Leakage of Ca2+ from the SR in EDL fibres could be substantially reduced with a SR Ca2+ pump blocker and increased by adding creatine to buffer cytoplasmic [ADP] at a higher level, both results indicating that at least part of the Ca2+ leakage occurred through SERCA. It is concluded that CSQ1 plays an important role in EDL muscle fibres by providing a large total pool of releasable Ca2+ in the SR whilst maintaining free [Ca2+] in the SR at sufficiently low levels that Ca2+ leakage through the high density of SERCA1 pumps does not metabolically compromise muscle function.

Dysregulated sarcoplasmic reticulum calcium release: potential pharmacological target in cardiac disease

In the heart, Ca(2+) released from the intracellular Ca(2+) storage site, the sarcoplasmic reticulum (SR), is the principal determinant of cardiac contractility. SR Ca(2+) release is controlled by dedicated molecular machinery, composed of the cardiac ryanodine receptor (RyR2) and a number of accessory proteins, including FKBP12.6, calsequestrin (CASQ2), triadin (TRD) and junctin (JN). Acquired and genetic defects in the components of the release channel complex result in a spectrum of abnormal Ca(2+) release phenotypes ranging from arrhythmogenic spontaneous Ca(2+) releases and Ca(2+) alternans to the uniformly diminished systolic Ca(2+) release characteristic of heart failure. In this article, we will present an overview of the structure and molecular components of the SR and Ca(2+) release machinery and its modulation by different intracellular factors, such as Ca(2+) levels inside the SR as well as phosphorylation and redox modification of RyR2s. We will also discuss the relationships between abnormal SR Ca(2+) release and various cardiac disease phenotypes, including, arrhythmias and heart failure, and consider SR Ca(2+) release as a potential therapeutic target.

Carbonic anhydrases IV and IX: subcellular localization and functional role in mouse skeletal muscle

The subcellular localization of carbonic anhydrase (CA) IV and CA IX in mouse skeletal muscle fibers has been studied immunohistochemically by confocal laser scanning microscopy. CA IV has been found to be located on the plasma membrane as well as on the sarcoplasmic reticulum (SR) membrane. CA IX is not localized in the plasma membrane but in the region of the t-tubular (TT)/terminal SR membrane. CA IV contributes 20% and CA IX 60% to the total CA activity of SR membrane vesicles isolated from mouse skeletal muscles. Our aim was to examine whether SR CA IV and TT/SR CA IX affect muscle contraction. Isolated fiber bundles of fast-twitch extensor digitorum longus and slow-twitch soleus muscle from mouse were investigated for isometric twitch and tetanic contractions and by a fatigue test. The muscle functions of CA IV knockout (KO) fibers and of CA IX KO fibers do not differ from the function of wild-type (WT) fibers. Muscle function of CA IV/XIV double KO mice unexpectedly shows a decrease in rise and relaxation time and in force of single twitches. In contrast, the CA inhibitor dorzolamide, whether applied to WT or to double KO muscle fibers, leads to a significant increase in rise time and force of twitches. It is concluded that the function of mouse skeletal muscle fibers expressing three membrane-associated CAs, IV, IX, and XIV, is not affected by the lack of one isoform but is possibly affected by the lack of all three CAs, as indicated by the inhibition studies.

Skeletal Muscle Fatigue: Cellular Mechanisms

Repeated, intense use of muscles leads to a decline in performance known as muscle fatigue. Many muscle properties change during fatigue including the action potential, extracellular and intracellular ions, and many intracellular metabolites. A range of mechanisms have been identified that contribute to the decline of performance. The traditional explanation, accumulation of intracellular lactate and hydrogen ions causing impaired function of the contractile proteins, is probably of limited importance in mammals. Alternative explanations that will be considered are the effects of ionic changes on the action potential, failure of SR Ca2+release by various mechanisms, and the effects of reactive oxygen species. Many different activities lead to fatigue, and an important challenge is to identify the various mechanisms that contribute under different circumstances. Most of the mechanistic studies of fatigue are on isolated animal tissues, and another major challenge is to use the knowledge generated in these studies to identify the mechanisms of fatigue in intact animals and particularly in human diseases.

Reduced sarcoplasmic reticulum content of releasable Ca2+ in rat soleus muscle fibres after eccentric contractions

AIM

The purpose was to evaluate the effects of fatiguing eccentric contractions (EC) on calcium (Ca2+) handling properties in mammalian type I muscles. We hypothesized that EC reduces both endogenous sarcoplasmic reticulum (SR) content of releasable Ca2+ (eSRCa2+) and myofibrillar Ca2+ sensitivity.

METHODS

Isolated rat soleus muscles performed 30 EC bouts. Single fibres were isolated from the muscle and after mechanical removal of sarcolemma used to measure eSRCa2+, rate of SR Ca2+ loading and myofibrillar Ca2+ sensitivity.

RESULTS

Following EC maximal force in whole muscle was reduced by 30% and 16/100 Hz force ratio by 33%. The eSRCa2+ in fibres from non-stimulated muscles was 45 +/- 5% of the maximal loading capacity. After EC, eSRCa2+ per fibre CSA decreased by 38% (P = 0.05), and the maximal capacity of SR Ca2+ loading was depressed by 32%. There were no effects of EC on either myofibrillar Ca2+ sensitivity, maximal Ca2+ activated force per cross-sectional area and rate of SR Ca2+ loading, or in SR vesicle Ca2+ uptake and release.

CONCLUSIONS

We conclude that EC reduces endogenous SR content of releasable Ca2+ but that myofibrillar Ca2+ sensitivity and SR vesicle Ca2+ kinetics remain unchanged. The present data suggest that the long-lasting fatigue induced by EC, which was more pronounced at low frequencies (low frequency fatigue), is caused by reduced Ca2+ release occurring secondary to reduced SR content of releasable Ca2+.

Kainic acid: insights into excitatory mechanisms causing selective neuronal degeneration

Kainic acid, an acidic pyrolidine isolated from the seaweed Digenea simplex, is the most potent of the commonly used exogenous excitotoxins. The neurotoxic threshold of kainic acid is nearly two magnitudes lower than that of the other receptor-specific agonists, N-methyl-D-aspartic acid and quisqualic acid. Neurophysiological and ligand-binding studies indicate that the neurotoxic action of kainic acid is mediated by a specific receptor which exhibits a remarkably broad phylogenetic distribution in the nervous system of vertebrates and invertebrates. The mechanism of neurotoxicity of kainic acid appears to be indirect and requires the functional integrity of excitatory afferents to vulnerable neurons. Consistent with the excitotoxin hypothesis, kainic acid depletes high-energy phosphates and glucose at sites of neurotoxic action; nevertheless, the proximate cause of neurotoxicity may involve increases in intraneuronal calcium levels and the activation of calcium-dependent proteases. Kainic acid neurotoxicity provides a useful animal model for selective neuronal vulnerability that may shed light on the pathophysiology of a number of neurodegenerative disorders, including Huntington's disease and temporal lobe epilepsy.

Ca2+ stores regulate ryanodine receptor Ca2+ release channels via luminal and cytosolic Ca2+ sites

1. In muscle, intracellular calcium concentration, hence skeletal muscle force and cardiac output, is regulated by uptake and release of calcium from the sarcoplasmic reticulum. The ryanodine receptor (RyR) forms the calcium release channel in the sarcoplasmic reticulum. 2. The free [Ca2+] in the sarcoplasmic reticulum regulates the excitability of this store by stimulating the Ca2+ release channels in its membrane. This process involves Ca2+-sensing mechanisms on both the luminal and cytoplasmic sides of the RyR. In the cardiac RyR, these have been shown to be a luminal Ca2+ activation site (L-site; 60 micromol/L affinity), a cytoplasmic activation site (A-site; 0.9 micromol/L affinity) and a cytoplasmic Ca2+ inactivation site (I2-site; 1.2 micromol/L affinity). 3. Cardiac RyR activation by luminal Ca2+ occurs by a multistep process dubbed 'luminal-triggered Ca2+ feed-through'. Binding of Ca2+ to the L-site initiates brief (1 msec) openings at a rate of up to 10/s. Once the pore is open, luminal Ca2+ has access to the A-site (producing up to 30-fold prolongation of openings) and to the I2-site (causing inactivation at high levels of Ca2+ feed-through). 4. The present paper reviews the evidence for the principal aspects of the 'luminal-triggered Ca2+ feed-through' model, the properties of the various Ca2+-dependent gating mechanisms and their likely role in controlling sarcoplasmic reticulum (SR) Ca2+ release in cardiac muscle. 5. The model makes the following important predictions: (i) there will be a close link between luminal and cytoplasmic regulation of RyRs and any cofactor that prolongs channel openings triggered by cytoplasmic Ca2+ will also promote RyR activation by luminal Ca2+; (ii) luminal Mg2+ (1 mmol/L) is essential for the control of SR excitability in cardiac muscle by luminal Ca2+; and (iii) the different RyR isoforms in skeletal and cardiac muscle will be controlled quite differently by the luminal milieu. For example, Mg2+ in the SR lumen (approximately 1 mmol/L) can strongly inhibit RyR2 by competing with Ca2+ for the L-site, whereas RyR1 is not affected by luminal Mg2+.

Reorganized stores and impaired calcium handling in skeletal muscle of mice lacking calsequestrin-1

Calsequestrin (CS), the major Ca(2+)-binding protein in the sarcoplasmic reticulum (SR), is thought to play a dual role in excitation-contraction coupling: buffering free Ca(2+) increasing SR capacity, and modulating the activity of the Ca(2+) release channels (RyRs). In this study, we generated and characterized the first murine model lacking the skeletal CS isoform (CS1). CS1-null mice are viable and fertile, even though skeletal muscles appear slightly atrophic compared to the control mice. No compensatory increase of the cardiac isoform CS2 is detectable in any type of skeletal muscle. CS1-null muscle fibres are characterized by structural and functional changes, which are much more evident in fast-twitch muscles (EDL) in which most fibres express only CS1, than in slow-twitch muscles (soleus), where CS2 is expressed in about 50% of the fibres. In isolated EDL muscle, force development is preserved, but characterized by prolonged time-to-peak and half-relaxation time, probably related to impaired calcium release from and re-uptake by the SR. Ca(2+)-imaging studies show that the amount of Ca(2+) released from the SR and the amplitude of the Ca(2+) transient are significantly reduced. The lack of CS1 also causes significant ultrastructural changes, which include: (i) striking proliferation of SR junctional domains; (ii) increased density of Ca(2+)-release channels (confirmed also by (3)H-ryanodine binding); (iii) decreased SR terminal cisternae volume; (iv) higher density of mitochondria. Taken together these results demonstrate that CS1 is essential for the normal development of the SR and its calcium release units and for the storage and release of appropriate amounts of SR Ca(2+).

Carbonic anhydrase XIV in skeletal muscle: subcellular localization and function from wild-type and knockout mice

The expression of carbonic anhydrase (CA) XIV was investigated in mouse skeletal muscles. Sarcoplasmic reticulum (SR) and sarcolemmal (SL) membrane fractions were isolated from wild-type (WT) and CA XIV knockout (KO) mice. The CA XIV protein of 54 kDa was present in SR and SL membrane fractions as shown by Western blot analysis. CA activity measurements of WT and KO membrane fractions showed that CA XIV accounts for approximately 50% and 66% of the total CA activities determined in the SR and SL fractions, respectively. This indicates the presence of at least one other membrane-associated CA isoform in these membranes, e.g., CA IV, CA IX, or CA XII. Muscle fibers of the extensor digitorum longus (EDL) muscle were immunostained with anti-CA XIV/FITC and anti-sarco(endo)plasmic reticulum Ca(2+)-ATPase 1/TRITC, with anti-CA XIV/FITC and anti-ryanodine receptor/TRITC, or with anti-CA XIV/FITC and anti-monocarboxylate transporter-4/TRITC. CA XIV was expressed in the plasma membrane and in the longitudinal SR but not in the terminal SR. Isometric contraction measurements of single twitches and tetani and a fatigue protocol applied to fiber bundles of the fast-twitch EDL and of the slow-twitch soleus muscle from WT and KO mice showed that the lack of SR membrane-associated CA XIV did not affect maximum force, rise and relaxation times, and fatigue behavior. Thus, it is concluded that a reduction of the total SR CA activity by approximately 50% in CA XIV KO mice does not lead to an impairment of SR function.

Ca2+ stores regulate ryanodine receptor Ca2+ release channels via luminal and cytosolic Ca2+ sites

The free [Ca2+] in endoplasmic/sarcoplasmic reticulum Ca2+ stores regulates excitability of Ca2+ release by stimulating the Ca2+ release channels. Just how the stored Ca2+ regulates activation of these channels is still disputed. One proposal attributes luminal Ca2+-activation to luminal facing regulatory sites, whereas another envisages Ca2+ permeation to cytoplasmic sites. This study develops a unified model for luminal Ca2+ activation for single cardiac ryanodine receptors (RyR2) and RyRs in coupled clusters in artificial lipid bilayers. It is shown that luminal regulation of RyR2 involves three modes of action associated with Ca2+ sensors in different parts of the molecule; a luminal activation site (L-site, 60 microM affinity), a cytoplasmic activation site (A-site, 0.9 microM affinity), and a novel cytoplasmic inactivation site (I2-site, 1.2 microM affinity). RyR activation by luminal Ca2+ is demonstrated to occur by a multistep process dubbed luminal-triggered Ca2+ feedthrough. Ca2+ binding to the L-site initiates brief openings (1 ms duration at 1-10 s(-1)) allowing luminal Ca2+ to access the A-site, producing up to 30-fold prolongation of openings. The model explains a broad data set, reconciles previous conflicting observations and provides a foundation for understanding the action of pharmacological agents, RyR-associated proteins, and RyR2 mutations on a range of Ca2+-mediated physiological and pathological processes.

Caveolae in smooth muscles: nanocontacts

Smooth muscle cell (SMC) caveolae have been investigated by quantitative and qualitative analysis of transmission electron microscopy (TEM) images of rat stomach, bladder and myometrium, guinea pig taenia coli, human ileum, and rat aortic SMCs. Ultrathin (below 30 nm) serial sections were used for examination of caveolar morphology and their connections with SMC organelles. Average caveolar diameter was smaller in vascular SMCs (70 nm, n=50) than in visceral SMCs (77 nm, n=100), but with the same morphology. Most of the caveolae, featured as flask-shaped plasma membrane (PM) invaginations, opened to the extracellular space through a 20 nm stoma (21, 3nm) having a 7 nm thick diaphragm. A small percentage of caveolae (3%), gathered as grape-like clusters, did not open directly to the extracellular space, but to irregular PM pockets having a 20-30 nm opening to the extracellular space. In visceral SMCs, caveolae were disposed in 4 - 6 rows, parallel to myofilaments, whilst aortic SMCs caveolae were arranged as clusters. This caveolar organization in rows or clusters minimizes the occupied volume, providing more space for the contractile machinery. The morphometric analysis of relative volumes (% of cell volume) showed that caveolae were more conspicuous in visceral than in vascular SMCs (myometrium - 2.40%; bladder - 3.66%, stomach - 2.61%, aorta - 1.43%). We also observed a higher number of caveolae per length unit of cell membrane in most visceral SMCs compared to vascular SMCs (myometrium - 1.06/μm, bladder - 0.74/μm, aorta - 0.57/μm, stomach - 0.48/μm). Caveolae increase the cellular perimeter up to 15% and enlarge the surface area of the plasma membrane about 80% in SMCs. Three-dimensional reconstructions (15μ³) showed that most caveolae, in both visceral and vascular SMCs, have nanocontacts with SR (87%), or with mitochondria (10%), and only 3%, apparently, have no contact with these organelles. Usually, 15 nm wide junctional spaces exist between caveolae and SR, some of them with nanostructural links between each other or with mitochondria: direct contacts (space < 2 nm or none) and molecular links, so called ‘feet’ (about 12 nm electron dense structures between organellar membranes). Direct contacts possibly allow molecular translocation between the two membranes. Electron-dense ‘feet’-like structures suggest a molecular link between these organelles responsible for intracellular Ca²⁺ homeostasis (excitation-contraction coupling or pharmaco-mechan-ical coupling). Close appositions (∼15 nm) have also been observed between caveolae and perinuclear SR cisterna, suggesting that caveolae might be directly implicated in excitation-transcription coupling.

Expression of membrane-bound carbonic anhydrases IV, IX, and XIV in the mouse heart

Expression of membrane-bound carbonic anhydrases (CAs) of CA IV, CA IX, CA XII, and CA XIV has been investigated in the mouse heart. Western blots using microsomal membranes of wild-type hearts demonstrate a 39-, 43-, and 54-kDa band representing CA IV, CA IX, and CA XIV, respectively, but CA XII could not be detected. Expression of CA IX in the CA IV/CA XIV knockout animals was further confirmed using matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Cardiac cells were immunostained using anti-CA/FITC and anti-alpha-actinin/TRITC, as well as anti-CA/FITC and anti-SERCA2/TRITC. Subcellular CA localization was investigated by confocal laser scanning microscopy. CA localization in the sarcolemmal (SL) membrane was examined by double immunostaining using anti-CA/FITC and anti-MCT-1/TRITC. CAs showed a distinct distribution pattern in the sarcoplasmic reticulum (SR) membrane. CA XIV is predominantly localized in the longitudinal SR, whereas CA IX is mainly expressed in the terminal SR/t-tubular region. CA IV is present in both SR regions, whereas CA XII is not found in the SR. In the SL membrane, only CA IV and CA XIV are present. We conclude that CA IV and CA XIV are associated with the SR as well as with the SL membrane, CA IX is located in the terminal SR/t-tubular region, and CA XII is not present in the mouse heart. Therefore, the unique subcellular localization of CA IX and CA XIV in cardiac myocytes suggests different functions of both enzymes in excitation-contraction coupling.

MECHANOSENSITIVE ION CHANNELS IN SKELETAL MUSCLE: A LINK IN THE MEMBRANE PATHOLOGY OF MUSCULAR DYSTROPHY

1. Mechanosensitive (MS) channels are expressed abundantly in skeletal muscle at all stages of development. In recordings from membrane patches, MS channels are constitutively active at the resting potential. The channels are selective for cations and have a large single-channel conductance (approximately 25 pS in physiological saline) and a high Ca2+ permeability (relative permeability of Ca2+ to K+ (PCa/PK) = 7). 2. Mechanosensitive channel activity recorded from the surface of myotubes from dystrophic mdx mice was substantially greater than the activity recorded from wild-type myotubes. Increased channel activity in the mutant results from the induction in a subpopulation of channels of a novel MS gating mode characterized by markedly prolonged channel openings and inactivation in response to membrane stretch. 3. Membrane stretch or a strong depolarization causes an irreversible switch to the stretch-inactivated gating mode in mdx myotubes. A stretch-induced shift in MS channel gating mode may contribute to stretch-induced elevations in [Ca2+]i during the early stages of disease pathogenesis. 4. Abnormalities of MS channel behaviour are also detected in recordings from patches on flexor digitorum brevis fibres acutely isolated from mdx mice. Mechanosensitive channel opening probability is higher in mdx fibres at all developmental stages. In addition, channel numbers are persistently elevated during postnatal development, failing to undergo a normal process of downregulation during the first 3 postnatal weeks. 5. Two distinct mechanisms may contribute to elevations of [Ca2+]i in dystrophin-deficient skeletal muscle: (i) a membrane stress-dependent switch of MS channels into to a prolonged opening mode; and (ii) a loss of developmental downregulation leading to persistent MS channel expression during postnatal muscle development.