HRC (histidine-rich Ca2+ binding protein) resides in the lumen of sarcoplasmic reticulum as a multimer

Histidine-rich calcium-binding protein: a molecular integrator of cardiac excitation-contraction coupling

During mammalian cardiomyocyte excitation-contraction coupling, Ca2+ influx through voltage-gated Ca2+ channels triggers Ca2+ release from the sarcoplasmic reticulum (SR) through ryanodine receptor channels. This Ca2+-induced Ca2+ release mechanism controls cardiomyocyte contraction and is exquisitely regulated by SR Ca2+ levels. The histidine-rich calcium-binding protein (HRC) and its aspartic acid-rich paralogue aspolin are high-capacity, low-affinity Ca2+-binding proteins. Aspolin also acts as a trimethylamine N-oxide demethylase. At low intraluminal Ca2+ concentrations, HRC binds to the SR Ca2+-ATPase 2, inhibiting its Ca2+-pumping activity. At high intraluminal Ca2+ levels, HRC interacts with triadin to reduce Ca2+ release through ryanodine receptor channels. This Review analyses the evolution of these Ca2+-regulatory proteins, to gain insights into their roles. It reveals that HRC homologues are present in chordates, annelid worms, molluscs, corals and sea anemones. In contrast, triadin appears to be a chordate innovation. Furthermore, HRC is evolving more rapidly than other cardiac excitation-contraction coupling proteins. This positive selection (or relaxed negative selection) occurs along most of the mammalian HRC protein sequence, with the exception being the C-terminal cysteine-rich region, which is undergoing negative selection. The histidine-rich region of HRC might be involved in pH sensing, as an adaptation to air-breathing, endothermic and terrestrial life. In addition, a cysteine-rich pattern within HRC and aspolin is also found in a wide range of iron-sulfur cluster proteins, suggesting roles in redox reactions and metal binding. The polyaspartic regions of aspolins are likely to underlie their trimethylamine N-oxide demethylase activity, which might be mimicked by the acidic regions of HRCs. These potential roles of HRCs and aspolins await experimental verification.

Sarcoplasmic Reticulum Ca2+ Buffer Proteins: A Focus on the Yet-To-Be-Explored Role of Sarcalumenin in Skeletal Muscle Health and Disease

Sarcalumenin (SAR) is a luminal Ca²⁺ buffer protein with high capacity but low affinity for calcium binding found predominantly in the longitudinal sarcoplasmic reticulum (SR) of fast- and slow-twitch skeletal muscles and the heart. Together with other luminal Ca²⁺ buffer proteins, SAR plays a critical role in modulation of Ca²⁺ uptake and Ca²⁺ release during excitation-contraction coupling in muscle fibers. SAR appears to be important in a wide range of other physiological functions, such as Sarco-Endoplasmic Reticulum Calcium ATPase (SERCA) stabilization, Store-Operated-Calcium-Entry (SOCE) mechanisms, muscle fatigue resistance and muscle development. The function and structural features of SAR are very similar to those of calsequestrin (CSQ), the most abundant and well-characterized Ca²⁺ buffer protein of junctional SR. Despite the structural and functional similarity, very few targeted studies are available in the literature. The present review provides an overview of the role of SAR in skeletal muscle physiology, as well as of its possible involvement and dysfunction in muscle wasting disorders, in order to summarize the current knowledge on SAR and drive attention to this important but still underinvestigated/neglected protein.

The Sarcoplasmic Reticulum of Skeletal Muscle Cells: A Labyrinth of Membrane Contact Sites

The sarcoplasmic reticulum of skeletal muscle cells is a highly ordered structure consisting of an intricate network of tubules and cisternae specialized for regulating Ca2+ homeostasis in the context of muscle contraction. The sarcoplasmic reticulum contains several proteins, some of which support Ca2+ storage and release, while others regulate the formation and maintenance of this highly convoluted organelle and mediate the interaction with other components of the muscle fiber. In this review, some of the main issues concerning the biology of the sarcoplasmic reticulum will be described and discussed; particular attention will be addressed to the structure and function of the two domains of the sarcoplasmic reticulum supporting the excitation-contraction coupling and Ca2+-uptake mechanisms.

Mitochondrial Ca2+ Homeostasis: Emerging Roles and Clinical Significance in Cardiac Remodeling

Mitochondria are the sites of oxidative metabolism in eukaryotes where the metabolites of sugars, fats, and amino acids are oxidized to harvest energy. Notably, mitochondria store Ca²⁺ and work in synergy with organelles such as the endoplasmic reticulum and extracellular matrix to control the dynamic balance of Ca²⁺ concentration in cells. Mitochondria are the vital organelles in heart tissue. Mitochondrial Ca²⁺ homeostasis is particularly important for maintaining the physiological and pathological mechanisms of the heart. Mitochondrial Ca²⁺ homeostasis plays a key role in the regulation of cardiac energy metabolism, mechanisms of death, oxygen free radical production, and autophagy. The imbalance of mitochondrial Ca²⁺ balance is closely associated with cardiac remodeling. The mitochondrial Ca²⁺ uniporter (mtCU) protein complex is responsible for the uptake and release of mitochondrial Ca²⁺ and regulation of Ca²⁺ homeostasis in mitochondria and consequently, in cells. This review summarizes the mechanisms of mitochondrial Ca²⁺ homeostasis in physiological and pathological cardiac remodeling and the regulatory effects of the mitochondrial calcium regulatory complex on cardiac energy metabolism, cell death, and autophagy, and also provides the theoretical basis for mitochondrial Ca²⁺ as a novel target for the treatment of cardiovascular diseases.

Organellar Calcium Handling in the Cellular Reticular Network

Ca²⁺ is an important intracellular messenger affecting diverse cellular processes. In eukaryotic cells, Ca²⁺ is handled by a myriad of Ca²⁺-binding proteins found in organelles that are organized into the cellular reticular network (CRN). The network is comprised of the endoplasmic reticulum, Golgi apparatus, lysosomes, membranous components of the endocytic and exocytic pathways, peroxisomes, and the nuclear envelope. Membrane contact sites between the different components of the CRN enable the rapid movement of Ca²⁺, and communication of Ca²⁺ status, within the network. Ca²⁺-handling proteins that reside in the CRN facilitate Ca²⁺ sensing, buffering, and cellular signaling to coordinate the many processes that operate within the cell.

Ser96Ala genetic variant of the human histidine-rich calcium-binding protein is a genetic predictor of recurrence after catheter ablation in patients with paroxysmal atrial fibrillation

BACKGROUND

Atrial fibrillation (AF) recurrence after radiofrequency catheter ablation (RFCA) still remains a serious issue. Ca2+ handling has a considerable effect on AF recurrence. The histidine-rich calcium-binding protein (HRC) genetic single nucleotide polymorphism (SNP), rs3745297 (T>G, Ser96Ala), is known to cause a sarcoplasmic reticulum Ca2+ leak. We investigated the association between HRC Ser96Ala and AF recurrence after RFCA in paroxysmal AF (PAF) patients.

METHODS AND RESULTS

We enrolled PAF patients who underwent RFCA (N = 334 for screening and N = 245 for replication) and were genotyped for HRC SNP (rs3745297). The patient age was younger and rate of diabetes and hypertension lower in the PAF patients with Ser96Ala than in those without (TT/TG/GG, 179/120/35; 64±10/60±12/59±13 y, P = 0.001; 18.5/ 9.2/8.6%, P = 0.04 and 66.1/50.0/37.1%, P = 0.001, respectively). During a mean 19 month follow-up, 57 (17.1%) patients suffered from AF recurrences. The rate of an Ser96Ala was significantly higher in patients with AF recurrence than in those without in the screening set (allele frequency model: odds ratio [OR], 1.80; P = 0.006). We also confirmed this significant association in the replication set (OR 1.74; P = 0.03) and combination (P = 0.0008). A multivariate analysis revealed that the AF duration, sinus node dysfunction, and HRC Ser96Ala were independent predictors of an AF recurrence (hazard ratio [HR], 1.04, P = 0.037; HR 2.42, P = 0.018; and HR 2.66, P = 0.007, respectively).

CONCLUSION

HRC SNP Ser96Ala is important as a new genetic marker of AF recurrence after RFCA.

The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity

Sudden unexpected cardiac death (SCD) accounts for up to half of all-cause mortality of heart failure patients. Standardized cardiology tools such as electrocardiography, cardiac imaging, electrophysiological and serum biomarkers cannot accurately predict which patients are at risk of life-threatening arrhythmic episodes. Recently, a common variant of the histidine-rich calcium binding protein (HRC), the Ser96Ala, was identified as a potent biomarker of malignant arrhythmia triggering in these patients. HRC has been shown to be involved in the regulation of cardiac sarcoplasmic reticulum (SR) Ca²⁺ cycling, by binding and storing Ca²⁺ in the SR, as well as interacting with the SR Ca²⁺ uptake and release complexes. The underlying mechanisms, elucidated by studies at the molecular, biochemical, cellular and intact animal levels, indicate that transversion of Ser96 to Ala results in abolishment of an HRC phosphorylation site by Fam20C kinase and dysregulation of SR Ca²⁺ cycling. This is mediated through aberrant SR Ca²⁺ release by the ryanodine receptor (RyR2) quaternary complex, due to the impaired HRC/triadin interaction, and depressed SR Ca²⁺ uptake by the sarco/endoplasmic reticulum Ca²⁺ ATPase (SERCA2) pump, due to the impaired HRC/SERCA2 interaction. Pharmacological intervention with KN-93, an inhibitor of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII), in the HRC Ser96Ala mouse model, reduced the occurrence of malignant cardiac arrhythmias. Herein, we summarize the current evidence on the pivotal role of HRC in the regulation of cardiac rhythmicity and the importance of HRC Ser96Ala as a genetic modifier for arrhythmias in the setting of heart failure.

Abnormal calcium cycling and cardiac arrhythmias associated with the human Ser96Ala genetic variant of histidine-rich calcium-binding protein

BACKGROUND

A human genetic variant (Ser96Ala) in the sarcoplasmic reticulum (SR) histidine-rich Ca(2+)-binding (HRC) protein has been linked to ventricular arrhythmia and sudden death in dilated cardiomyopathy. However, the precise mechanisms affecting SR function and leading to arrhythmias remain elusive.

METHODS AND RESULTS

We generated transgenic mice with cardiac-specific expression of human Ala96 HRC or Ser96 HRC in the null background to assess function in absence of endogenous protein. Ala96 HRC decreased (25% to 30%) cardiomyocyte contractility and Ca2+ kinetics compared with Ser96 HRC in the absence of any structural or histological abnormalities. Furthermore, the frequency of Ca2+ waves was significantly higher (10-fold), although SR Ca2+ load was reduced (by 27%) in Ala96 HRC cells. The underlying mechanisms involved diminished interaction of Ala96 HRC with triadin, affecting ryanodine receptor (RyR) stability. Indeed, the open probability of RyR, assessed by use of ryanodine binding, was significantly increased. Accordingly, stress conditions (5 Hz plus isoproterenol) induced aftercontractions (65% in Ala96 versus 12% in Ser96) and delayed afterdepolarizations (70% in Ala96 versus 20% in Ser96). The increased SR Ca2+ leak was accompanied by hyperphosphorylation (1.6-fold) of RyR at Ser2814 by calmodulin-dependent protein kinase II. Accordingly, inclusion of the calmodulin-dependent protein kinase II inhibitor KN93 prevented Ser2814 phosphorylation and partially reversed the increases in Ca2+ spark frequency and wave production. Parallel in vivo studies revealed ventricular ectopy on short-term isoproterenol challenge and increased (4-fold) propensity to arrhythmias, including nonsustained ventricular tachycardia, after myocardial infarction in Ala96 HRC mice.

CONCLUSIONS

These findings suggest that aberrant SR Ca2+ release and increased susceptibility to delayed afterdepolarizations underlie triggered arrhythmic activity in human Ala96 HRC carriers.

AAV-mediated knock-down of HRC exacerbates transverse aorta constriction-induced heart failure

Background

Histidine-rich calcium binding protein (HRC) is located in the lumen of sarcoplasmic reticulum (SR) that binds to both triadin (TRN) and SERCA affecting Ca²⁺ cycling in the SR. Chronic overexpression of HRC that may disrupt intracellular Ca²⁺ homeostasis is implicated in pathogenesis of cardiac hypertrophy. Ablation of HRC showed relatively normal phenotypes under basal condition, but exhibited a significantly increased susceptibility to isoproterenol-induced cardiac hypertrophy. In the present study, we characterized the functions of HRC related to Ca²⁺ cycling and pathogenesis of cardiac hypertrophy using the in vitro siRNA- and the in vivo adeno-associated virus (AAV)-mediated HRC knock-down (KD) systems, respectively.

Methodology/Principal Findings

AAV-mediated HRC-KD system was used with or without C57BL/6 mouse model of transverse aortic constriction-induced failing heart (TAC-FH) to examine whether HRC-KD could enhance cardiac function in failing heart (FH). Initially we expected that HRC-KD could elicit cardiac functional recovery in failing heart (FH), since predesigned siRNA-mediated HRC-KD enhanced Ca²⁺ cycling and increased activities of RyR2 and SERCA2 without change in SR Ca²⁺ load in neonatal rat ventricular cells (NRVCs) and HL-1 cells. However, AAV9-mediated HRC-KD in TAC-FH was associated with decreased fractional shortening and increased cardiac fibrosis compared with control. We found that phospho-RyR2, phospho-CaMKII, phospho-p38 MAPK, and phospho-PLB were significantly upregulated by HRC-KD in TAC-FH. A significantly increased level of cleaved caspase-3, a cardiac cell death marker was also found, consistent with the result of TUNEL assay.

Conclusions/Significance

Increased Ca²⁺ leak and cytosolic Ca²⁺ concentration due to a partial KD of HRC could enhance activity of CaMKII and phosphorylation of p38 MAPK, causing the mitochondrial death pathway observed in TAC-FH. Our results present evidence that down-regulation of HRC could deteriorate cardiac function in TAC-FH through perturbed SR-mediated Ca²⁺ cycling.

The in vivo functions of a histidine-rich protein Hpn in Helicobacter pylori: linking gastric and Alzheimer's diseases together?

Helicobacter pylori causes such gastric diseases as gastritis, peptic ulcerations, gastric cancer and MALT lymphoma. Hpn is a histidine-rich protein abundant in this bacterium and forms amyloid-like oligomers in physiologically relevant conditions. Here we proposed the in vivo functions of this protein with relevance to its physical locations. The collective evidence presented here shed some light on the pathologic mechanisms of H. pylori infections, with emphasis on the bacterial colonization in the gastric environment, pathological effects to the gastric epithelial cells and the possible link to Alzheimer's disease.

Organellar calcium buffers

Ca(2+) is an important intracellular messenger affecting many diverse processes. In eukaryotic cells, Ca(2+) storage is achieved within specific intracellular organelles, especially the endoplasmic/sarcoplasmic reticulum, in which Ca(2+) is buffered by specific proteins known as Ca(2+) buffers. Ca(2+) buffers are a diverse group of proteins, varying in their affinities and capacities for Ca(2+), but they typically also carry out other functions within the cell. The wide range of organelles containing Ca(2+) and the evidence supporting cross-talk between these organelles suggest the existence of a dynamic network of organellar Ca(2+) signaling, mediated by a variety of organellar Ca(2+) buffers.

Histidine-rich calcium binding protein: the new regulator of sarcoplasmic reticulum calcium cycling

The histidine-rich calcium binding protein (HRC) is a novel regulator of sarcoplasmic reticulum (SR) Ca(2+)-uptake, storage and release. Residing in the SR lumen, HRC binds Ca(2+) with high capacity but low affinity. In vitro phosphorylation of HRC affects ryanodine affinity of the ryanodine receptor (RyR), suggesting a functional role of HRC on SR Ca(2+)-release. Indeed, acute HRC overexpression in isolated rodent cardiomyocytes decreases Ca(2+)-induced Ca(2+)-release, increases SR Ca(2+)-load, and impairs contractility. The HRC effects on RyR may be regulated by the Ca(2+)-sensitivity of its interaction with triadin. However, HRC also affects the SR Ca(2+)-ATPase, as shown by HRC overexpression in transgenic mouse hearts, which resulted in reduced SR Ca(2+)-uptake rates, cardiac remodeling and hypertrophy. In fact, in vitro generated evidence suggests that HRC directly interacts with SR Ca(2+)-ATPase2, supporting a dual role of HRC in Ca(2+)-homeostasis: regulation of both SR Ca(2+)-uptake and Ca(2+)-release. Furthermore, HRC plays an important role in myocyte differentiation and in antiapoptotic cardioprotection against ischemia/reperfusion induced cardiac injury. Interestingly, HRC has been linked with familiar cardiac conduction disease and an HRC polymorphism was shown to associate with malignant ventricular arrhythmias in the background of idiopathic dilated cardiomyopathy. This review summarizes studies, which have established the critical role of HRC in Ca(2+)-homeostasis, suggesting its importance in cardiac physiology and pathophysiology.

Junctin and the histidine-rich Ca2+ binding protein: potential roles in heart failure and arrhythmogenesis

Contractile dysfunction and ventricular arrhythmias associated with heart failure have been attributed to aberrant sarcoplasmic reticulum (SR) Ca(2+) cycling. The study of junctin (JCN) and histidine-rich Ca(2+) binding protein (HRC) becomes of particular importance since these proteins have been shown to be critical regulators of Ca(2+) cycling. Specifically, JCN is a SR membrane protein, which is part of the SR Ca(2+) release quaternary structure that also includes the ryanodine receptor, triadin and calsequestrin. Functionally, JCN serves as a bridge between calsequestrin and the Ca(2+) release channel, ryanodine receptor. HRC is a SR luminal Ca(2+) binding protein known to associate with both triadin and the sarcoplasmic reticulum Ca(2+)-ATPase, and may thus mediate the crosstalk between SR Ca(2+) uptake and release. Indeed, evidence from genetic models of JCN and HRC indicate that they are important in cardiophysiology as alterations in these proteins affect SR Ca(2+) handling and cardiac function. In addition, downregulation of JCN and HRC may contribute to Ca(2+) cycling perturbations manifest in the failing heart, where their protein levels are significantly reduced. This review examines the roles of JCN and HRC in SR Ca(2+) cycling and their potential significance in heart failure.

Modulation of the Ryanodine Receptor and Intracellular Calcium

Ryanodine receptors (RyRs)/Ca2+ release channels, on the endoplasmic and sarcoplasmic reticulum of most cell types, are required for intracellular Ca2+ release involved in diverse cellular functions, including muscle contraction and neurotransmitter release. The large cytoplasmic domain of the RyR serves as a scaffold for proteins that bind to and modulate the channel's function and that comprise a macromolecular signaling complex. These proteins include calstabins [FK506-binding proteins (FKBPs)], calmodulin (CaM), phosphodiesterase, kinases, phosphatases, and their cognate targeting proteins. This review focuses on recent progress in the understanding of RyR regulation and disease mechanisms that are associated with channel dysfunction.

Expression and characterization of a histidine-rich protein, Hpn: potential for Ni2+ storage in Helicobacter pylori

Hpn is a small cytoplasmic protein found in Helicobacter pylori, which binds Ni2+ ions with moderate affinity. Consisting of 60 amino acids, the protein is rich in histidine (28 residues, 46.7%), as well as glutamate, glycine and serine residues (in total 31.7%), and contains short repeating motifs. In the present study, we report the detailed biophysical characterization of the multimeric status and Ni2+-binding properties of purified recombinant Hpn under physiologically relevant conditions. The protein exists as an equilibration of multimeric forms in solution, with 20-mers (approx. 136 kDa) being the predominant species. Using equilibrium dialysis, ICP-MS (inductively coupled plasma MS) and UV/visible spectroscopy, Hpn was found to bind five Ni2+ ions per monomer at pH 7.4, with a dissociation constant (K(d)) of 7.1 microM. Importantly, Ni2+ binding to Hpn is reversible: metal is released either in the presence of a chelating ligand such as EDTA, or at a slightly acidic pH (pH for half dissociation, pH1/2 approximately 6.3). Ni2+ binding induces conformational changes within the protein, increasing beta-sheet and reducing alpha-helical content, from 22% to 37%, and 20% to 10% respectively. Growth curves of Escherichia coli BL21(DE3) both with and without the hpn gene performed under Ni2+ pressure clearly implied a role for Hpn to protect the cells from higher concentrations of external metal ions. Similarly, the accumulation of Ni2+ in these cells expressing Hpn from a plasmid was approx. 4-fold higher than in uninduced controls or control cultures that lacked the plasmid. Similarly, levels of Ni2+ in wild-type H. pylori 26695 cells were higher than those in H. pylori hpn-deletion mutant strains. Hpn may potentially serve multiple roles inside the bacterium: storage of Ni2+ ions in a 'reservoir'; donation of Ni2+ to other proteins; and detoxification via sequestration of excess Ni2+.

Sarcoplasmic reticulum: The dynamic calcium governor of muscle

The sarcoplasmic reticulum (SR) provides feedback control required to balance the processes of calcium storage, release, and reuptake in skeletal muscle. This balance is achieved through the concerted action of three major classes of SR calcium-regulatory proteins: (1) luminal calcium-binding proteins (calsequestrin, histidine-rich calcium-binding protein, junctate, and sarcalumenin) for calcium storage; (2) SR calcium release channels (type 1 ryanodine receptor or RyR1 and IP3 receptors) for calcium release; and (3) sarco(endo)plasmic reticulum Ca2+ -ATPase (SERCA) pumps for calcium reuptake. Proper calcium storage, release, and reuptake are essential for normal skeletal muscle function. We review SR structure and function during normal skeletal muscle activity, the proteins that orchestrate calcium storage, release, and reuptake, and how phenotypically distinct muscle diseases (e.g., malignant hyperthermia, central core disease, and Brody disease) can result from subtle alterations in the activity of several key components of the SR calcium-regulatory machinery.

Complementary DNA cloning, genomic characterization and expression analysis of a mammalian gene encoding histidine-rich calcium binding protein

A protein complex present at the junctional sarcoplasmic reticulum (SR) membrane is implicated in the Ca(2+) release process during muscle contraction. The histidine-rich Ca(2+)-binding protein (HRC) is an emerging component associated into the SR protein complex. We cloned cDNAs for rat and monkey HRCs, showing a conserved sequence organization in common with other mammalian HRCs. Genomic analysis revealed that each mammalian HRC gene is present as a single copy in the genome, consisting of 6 exons and 5 introns. Developmental expression analysis using mouse embryos and postnatal hearts demonstrated that Hrc transcription begins at 12.5 days postcoitum and its level increases gradually, reaching an adult level in the range 5-20 days after birth. Comparing the Hrc gene and other SR genes, we found that the timing and pattern of gene expression vary among the SR genes and the full-level expression of these genes is achieved in the heart after postnatal day 20. Collectively, our study provides comprehensive information about the structure and expression of the mammalian HRC gene, together with the comparative expression data of the related SR genes.

Drastic reduction of sarcalumenin in Dp427 (dystrophin of 427 kDa)-deficient fibres indicates that abnormal calcium handling plays a key role in muscular dystrophy

Although the primary abnormality in dystrophin is the underlying cause for mdx (X-chromosome-linked muscular dystrophy), abnormal Ca2+ handling after sarcolemmal microrupturing appears to be the pathophysiological mechanism leading to muscle weakness. To develop novel pharmacological strategies for eliminating Ca2+-dependent proteolysis, it is crucial to determine the fate of Ca2+-handling proteins in dystrophin-deficient fibres. In the present study, we show that a key luminal Ca2+-binding protein SAR (sarcalumenin) is affected in mdx skeletal-muscle fibres. One- and two-dimensional immunoblot analyses revealed the relative expression of the 160 kDa SR (sarcoplasmic reticulum) protein to be approx. 70% lower in mdx fibres when compared with normal skeletal muscles. This drastic reduction in SAR was confirmed by immunofluorescence microscopy. Patchy internal labelling of SAR in dystrophic fibres suggests an abnormal formation of SAR domains. Differential co-immunoprecipitation experiments and chemical cross-linking demonstrated a tight linkage between SAR and the SERCA1 (sarcoplasmic/endoplasmic-reticulum Ca2+-ATPase 1) isoform of the SR Ca2+-ATPase. However, the relative expression of the fast Ca2+ pump was not decreased in dystrophic membrane preparations. This implies that the reduction in SAR and calsequestrin-like proteins plays a central role in the previously reported impairment of Ca2+ buffering in the dystrophic SR [Culligan, Banville, Dowling and Ohlendieck (2002) J. Appl. Physiol. 92, 435-445]. Impaired Ca2+ shuttling between the Ca2+-uptake SERCA units and calsequestrin clusters via SAR, as well as an overall decreased luminal ion-binding capacity, might indirectly amplify the Ca2+-leak-channel-induced increase in cytosolic Ca2+ levels. This confirms the idea that abnormal Ca2+ cycling is involved in Ca2+-induced myonecrosis. Hence, manipulating disturbed Ca2+ handling might represent new modes of abolishing proteolytic degradation in muscular dystrophy.

HRC is a direct transcriptional target of MEF2 during cardiac, skeletal, and arterial smooth muscle development in vivo

The HRC gene encodes the histidine-rich calcium-binding protein, which is found in the lumen of the junctional sarcoplasmic reticulum (SR) of cardiac and skeletal muscle and within calciosomes of arterial smooth muscle. The expression of HRC in cardiac, skeletal, and smooth muscle raises the possibility of a common transcriptional mechanism governing its expression in all three muscle cell types. In this study, we identified a transcriptional enhancer from the HRC gene that is sufficient to direct the expression of lacZ in the expression pattern of endogenous HRC in transgenic mice. The HRC enhancer contains a small, highly conserved sequence that is required for expression in all three muscle lineages. Within this conserved region is a consensus site for myocyte enhancer factor 2 (MEF2) proteins that we show is bound efficiently by MEF2 and is required for transgene expression in all three muscle lineages in vivo. Furthermore, the entire HRC enhancer sequence lacks any discernible CArG motifs, the binding site for serum response factor (SRF), and we show that the enhancer is not activated by SRF. Thus, these studies identify the HRC enhancer as the first MEF2-dependent, CArG-independent transcriptional target in smooth muscle and represent the first analysis of the transcriptional regulation of an SR gene in vivo.

Regulation of myocardial function by histidine-rich, calcium-binding protein

Impaired sarcoplasmic reticulum (SR) Ca release has been suggested to contribute to the depressed cardiac function in heart failure. The release of Ca from the SR may be regulated by the ryanodine receptor, triadin, junctin, calsequestrin, and a histidine-rich, Ca-binding protein (HRC). We observed that the levels of HRC were reduced in animal models and human heart failure. To gain insight into the physiological function of HRC, we infected adult rat cardiac myocytes with a recombinant adenovirus that contains the full-length mouse HRC cDNA. Overexpression (1.7-fold) of HRC in adult rat cardiomyocytes was associated with increased SR Ca load (28%) but decreased SR Ca-induced Ca release (37%), resulting in impaired Ca cycling and depressed fractional shortening (36%) as well as depressed rates of shortening (38%) and relengthening (33%). Furthermore, the depressed basal contractile and Ca kinetic parameters in the HRC-infected myocytes remained significantly depressed even after maximal isoproterenol stimulation. Interestingly, HRC overexpresssion was accompanied by increased protein levels of junctin (1.4-fold) and triadin (1.8-fold), whereas the protein levels of ryanodine receptor, calsequestrin, phospholamban, and sarco(endo)plasmic reticulum Ca-ATPase remained unaltered. Collectively, these data indicate that alterations in expression levels of HRC are associated with impaired cardiac SR Ca homeostasis and contractile function.

Increased Ca2+ storage capacity in the sarcoplasmic reticulum by overexpression of HRC (histidine-rich Ca2+ binding protein)

The histidine-rich Ca(2+) binding protein (HRC) is a high capacity Ca(2+) binding protein in the sarcoplasmic reticulum (SR). Because HRC appears to interact directly with triadin, HRC may play a role in the regulation of Ca(2+) release during excitation-contraction coupling. In this study, we examined the physiological effects of HRC overexpression in rat neonatal cardiomyocytes. Both caffeine-induced and depolarization-induced Ca(2+) release from the SR were increased significantly in the HRC overexpressing cardiomyocytes. Consistently, the Ca(2+) content, normally depleted from the SR in the presence of cyclopiazonic acid (CPA), remained elevated in these cells. In contrast, the density and the ryanodine-binding kinetics of the ryanodine receptor (RyR)/Ca(2+) release channel were slightly reduced or not significantly altered in the HRC overexpressing cardiomyocytes. We suggest that HRC is involved in the regulation of releasable Ca(2+) content into the SR.

Absence of calsequestrin 2 causes severe forms of catecholaminergic polymorphic ventricular tachycardia

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a rare arrhythmogenic disorder characterized by syncopal events and sudden cardiac death at a young age during physical stress or emotion, in the absence of structural heart disease. We report the first nonsense mutations in the cardiac calsequestrin gene, CASQ2, in three CPVT families. The three mutations, a nonsense R33X, a splicing 532+1 G>A, and a 1-bp deletion, 62delA, are thought to induce premature stop codons. Two patients who experienced syncopes before the age of 7 years were homozygous carriers, suggesting a complete absence of calsequestrin 2. One patient was heterozygous for the stop codon and experienced syncopes from the age of 11 years. Despite the different mutations, there is little phenotypic variation of CPVT for the CASQ2 mutations. Of the 16 heterozygous carriers of these various mutations, 14 were devoid of clinical symptoms or ECG anomalies, whereas 2 of them had ventricular arrhythmias at ECG on exercise tests. In line with this, the diagnosis of the probands was difficult because of the absence of a positive family history. In conclusion, these additional three CASQ2 CPVT families suggest that CASQ2 mutations are more common than previously thought and produce a severe form of CPVT. The full text of this article is available at http://www.circresaha.org.

Ca(2+)-dependent interaction of triadin with histidine-rich Ca(2+)-binding protein carboxyl-terminal region

A direct binding of HRC (histidine-rich Ca(2+)-binding protein) to triadin, the main transmembrane protein of the junctional sarcoplasmic reticulum (SR) of skeletal muscle, seems well supported. Opinions are still divided, however, concerning the triadin domain involved, either the cytoplasmic or the lumenal domain, and the exact role played by Ca(2+), in the protein-to-protein interaction. Further support for colocalization of HRC with triadin cytoplasmic domain is provided here by experiments of mild tryptic digestion of tightly sealed TC vesicles. Accordingly, we show that HRC is preferentially phosphorylated by endogenous CaM K II, anchored to SR membrane on the cytoplasmic side, and not by lumenally located casein kinase 2. We demonstrate that HRC can be isolated as a complex with triadin, following equilibrium sucrose-density centrifugation in the presence of mM Ca(2+). Here, we characterized the COOH-terminal portion of rabbit HRC, expressed and purified as a fusion protein (HRC(569-852)), with respect to Ca(2+)-binding properties, and to the interaction with triadin on blots, as a function of the concentration of Ca(2+). Our results identify the polyglutamic stretch near the COOH terminus, as the Ca(2+)-binding site responsible, both for the acceleration in mobility of HRC on SDS-PAGE in the presence of millimolar concentrations of Ca(2+), and for the enhancement by high Ca(2+) of the interaction between HRC and triadin cytoplasmic segment. (c)2001 Elsevier Science.

Interaction of HRC (histidine-rich Ca(2+)-binding protein) and triadin in the lumen of sarcoplasmic reticulum

HRC (histidine-rich Ca(2+) binding protein) has been identified from skeletal and cardiac muscle and shown to bind Ca(2+) with high capacity and low affinity. While HRC resides in the lumen of the sarcoplasmic reticulum, the physiological function of HRC is largely unknown. In the present study, we have performed co-immunoprecipitation experiments and show that HRC binds directly to triadin, which is an integral membrane protein of the sarcoplasmic reticulum. Using a fusion protein binding assay, we further identified the histidine-rich acidic repeats of HRC as responsible for the binding of HRC to triadin. These motifs may represent a novel protein-protein interaction domain. The HRC binding domain of triadin was also localized by fusion protein binding assay to the lumenal region containing the KEKE motif that was previously shown to be involved in the binding of triadin to calsequestrin. Notably, the interaction of HRC and triadin is Ca(2+)-sensitive. Our data suggest that HRC may play a role in the regulation of Ca(2+) release from the sarcoplasmic reticulum by interaction with triadin.

Phospholamban and cardiac contractility

Over the last three decades, our knowledge and understanding of the role of phospholamban and its modulation of sarcoplasmic reticulum (SR) function has advanced significantly. Phospholamban is a key regulator of cardiac contractility and modulates SR Ca2+ sequestration by inhibiting the SR Ca2+-ATPase (SERCA) in its dephosphorylated state. Upon phosphorylation, which is mediated through beta-adrenergic stimulation, the inhibitory effect of phospholamban on the function of SERCA is relieved. This review summarizes recent advances that have been made towards understanding the modulation of SR Ca2+-sequestration by phospholamban through the generation and characterization of genetically altered animal models. It also discusses the role of phospholamban in human heart failure and recent attempts to restore SR function in experimentally induced and human heart failure, which may be translated into future therapeutic approaches in the treatment of this disease.