Phosphorylation and dephosphorylation of calsequestrin on CK2-sensitive sites in heart

The function and regulation of calsequestrin-2: implications in calcium-mediated arrhythmias

Cardiac arrhythmias are life-threatening events in which the heart develops an irregular rhythm. Mishandling of Ca²⁺ within the myocytes of the heart has been widely demonstrated to be an underlying mechanism of arrhythmogenesis. This includes altered function of the ryanodine receptor (RyR2)-the primary Ca²⁺ release channel located to the sarcoplasmic reticulum (SR). The spontaneous leak of SR Ca²⁺ via RyR2 is a well-established contributor in the development of arrhythmic contractions. This leak is associated with increased channel activity in response to changes in SR Ca²⁺ load. RyR2 activity can be regulated through several avenues, including interactions with numerous accessory proteins. One such protein is calsequestrin-2 (CSQ2), which is the primary Ca²⁺-buffering protein within the SR. The capacity of CSQ2 to buffer Ca²⁺ is tightly associated with the ability of the protein to polymerise in response to changing Ca²⁺ levels. CSQ2 can itself be regulated through phosphorylation and glycosylation modifications, which impact protein polymerisation and trafficking. Changes in CSQ2 modifications are implicated in cardiac pathologies, while mutations in CSQ2 have been identified in arrhythmic patients. Here, we review the role of CSQ2 in arrhythmogenesis including evidence for the indirect and direct regulation of RyR2 by CSQ2, and the consequences of a loss of functional CSQ2 in Ca²⁺ homeostasis and Ca²⁺-mediated arrhythmias.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12551-021-00914-6.

Calsequestrin, a key protein in striated muscle health and disease

Calsequestrin (CASQ) is the most abundant Ca²⁺ binding protein localized in the sarcoplasmic reticulum (SR) of skeletal and cardiac muscle. The genome of vertebrates contains two genes, CASQ1 and CASQ2. CASQ1 and CASQ2 have a high level of homology, but show specific patterns of expression. Fast-twitch skeletal muscle fibers express only CASQ1, both CASQ1 and CASQ2 are present in slow-twitch skeletal muscle fibers, while CASQ2 is the only protein present in cardiomyocytes. Depending on the intraluminal SR Ca²⁺ levels, CASQ monomers assemble to form large polymers, which increase their Ca²⁺ binding ability. CASQ interacts with triadin and junctin, two additional SR proteins which contribute to localize CASQ to the junctional region of the SR (j-SR) and also modulate CASQ ability to polymerize into large macromolecular complexes. In addition to its ability to bind Ca²⁺ in the SR, CASQ appears also to be able to contribute to regulation of Ca²⁺ homeostasis in muscle cells. Both CASQ1 and CASQ2 are able to either activate and inhibit the ryanodine receptors (RyRs) calcium release channels, likely through their interactions with junctin and triadin. Additional evidence indicates that CASQ1 contributes to regulate the mechanism of store operated calcium entry in skeletal muscle via a direct interaction with the Stromal Interaction Molecule 1 (STIM1). Mutations in CASQ2 and CASQ1 have been identified, respectively, in patients with catecholamine-induced polymorphic ventricular tachycardia and in patients with some forms of myopathy. This review will highlight recent developments in understanding CASQ1 and CASQ2 in health and diseases.

Transitions of protein traffic from cardiac ER to junctional SR

The junctional sarcoplasmic reticulum (jSR) is an important and unique ER subdomain in the adult myocyte that concentrates resident proteins to regulate Ca(2+) release. To investigate cellular mechanisms for sorting and trafficking proteins to jSR, we overexpressed canine forms of junctin (JCT) or triadin (TRD) in adult rat cardiomyocytes. Protein accumulation over time was visualized by confocal fluorescence microscopy using species-specific antibodies. Newly synthesized JCTdog and TRDdog appeared by 12-24h as bright fluorescent puncta close to the nuclear surface, decreasing in intensity with increasing radial distance. With increasing time (24-48h), fluorescent puncta appeared at further radial distances from the nuclear surface, eventually populating jSR similar to steady-state patterns. CSQ2-DsRed, a form of CSQ that polymerizes ectopically in rough ER, prevented anterograde traffic of newly made TRDdog and JCTdog, demonstrating common pathways of intracellular trafficking as well as in situ binding to CSQ2 in juxtanuclear rough ER. Reversal of CSQ-DsRed interactions occurred when a form of TRDdog was used in which CSQ2-binding sites are removed ((del)TRD). With increasing levels of expression, CSQ2-DsRed revealed a novel smooth ER network that surrounds nuclei and connects the nuclear axis. TRDdog was retained in smooth ER by binding to CSQ2-DsRed, but escaped to populate jSR puncta. TRDdog and (del)TRD were therefore able to elucidate areas of ER-SR transition. High levels of CSQ2-DsRed in the ER led to loss of jSR puncta labeling, suggesting a plasticity of ER-SR transition sites. We propose a model of ER and SR protein traffic along microtubules, with prominent transverse/radial ER trafficking of JCT and TRD along Z-lines to populate jSR, and an abundant longitudinal/axial smooth ER between and encircling myonuclei, from which jSR proteins traffic.

Altered calsequestrin glycan processing is common to diverse models of canine heart failure

Calsequestrin-2 (CSQ2) is a resident glycoprotein of junctional sarcoplasmic reticulum that functions in the regulation of SR Ca(2+) release. CSQ2 is biosynthesized in rough ER around cardiomyocyte nuclei and then traffics transversely across SR subcompartments. During biosynthesis, CSQ2 undergoes N-linked glycosylation and phosphorylation by protein kinase CK2. In mammalian heart, CSQ2 molecules subsequently undergo extensive mannose trimming by ER mannosidase(s), a posttranslational process that often regulates protein breakdown. We analyzed the intact purified CSQ2 from mongrel canine heart tissue by electrospray mass spectrometry. The average molecular mass of CSQ2 in normal mongrel dogs was 46,306 ± 41 Da, corresponding to glycan trimming of 3-5 mannoses, depending upon the phosphate content. We tested whether CSQ2 glycan structures would be altered in heart tissue from mongrel dogs induced into heart failure (HF) by two very different experimental treatments, rapid ventricular pacing or repeated coronary microembolizations. Similarly dramatic changes in mannose trimming were found in both types of induced HF, despite the different cardiomyopathies producing the failure. Unique to all samples analyzed from HF dog hearts, 20-40 % of all CSQ2 contained glycans that had minimal mannose trimming (Man9,8). Analyses of tissue samples showed decreases in CSQ2 protein levels per unit levels of mRNA for tachypaced heart tissue, also indicative of altered turnover. Quantitative immunofluorescence microscopy of frozen tissue sections suggested that no changes in CSQ2 levels occurred across the width of the cell. We conclude that altered processing of CSQ2 may be an adaptive response to the myocardium under stresses that are capable of inducing heart failure.

Calsequestrin accumulation in rough endoplasmic reticulum promotes perinuclear Ca2+ release

Molecular mechanisms underlying Ca(2+) regulation by perinuclear endoplasmic/sarcoplasmic reticulum (ER/SR) cisternae in cardiomyocytes remain obscure. To investigate the mechanisms of changes in cardiac calsequestrin (CSQ2) trafficking on perinuclear Ca(2+) signaling, we manipulated the subcellular distribution of CSQ2 by overexpression of CSQ2-DsRed, which specifically accumulates in the perinuclear rough ER. Adult ventricular myocytes were infected with adenoviruses expressing CSQ2-DsRed, CSQ2-WT, or empty vector. We found that perinuclear enriched CSQ2-DsRed, but not normally distributed CSQ2-WT, enhanced nuclear Ca(2+) transients more potently than cytosolic Ca(2+) transients. Overexpression of CSQ2-DsRed produced more actively propagating Ca(2+) waves from perinuclear regions than did CSQ2-WT. Activities of the SR/ER Ca(2+)-ATPase and ryanodine receptor type 2, but not inositol 1,4,5-trisphosphate receptor type 2, were required for the generation of these perinuclear initiated Ca(2+) waves. In addition, CSQ2-DsRed was more potent than CSQ2-WT in inducing cellular hypertrophy in cultured neonatal cardiomyocytes. Our data demonstrate for the first time that CSQ2 retention in the rough ER/perinuclear region promotes perinuclear Ca(2+) signaling and predisposes to ryanodine receptor type 2-mediated Ca(2+) waves from CSQ2-enriched perinuclear compartments and myocyte hypotrophy. These findings provide new insights into the mechanism of CSQ2 in Ca(2+) homeostasis, suggesting that rough ER-localized Ca(2+) stores can operate independently in raising levels of cytosolic/nucleoplasmic Ca(2+) as a source of Ca(2+) for Ca(2+)-dependent signaling in health and disease.

Glycosylation of skeletal calsequestrin: implications for its function

Calsequestrin (CASQ) serves as a major Ca(2+) storage/buffer protein in the sarcoplasmic reticulum (SR). When purified from skeletal muscle, CASQ1 is obtained in its glycosylated form. Here, we have confirmed the specific site and degree of glycosylation of native rabbit CASQ1 and have investigated its effect on critical properties of CASQ by comparison with the non-glycosylated recombinant form. Based on our comparative approach utilizing crystal structures, Ca(2+) binding capacities, analytical ultracentrifugation, and light-scattering profiles of the native and recombinant rabbit CASQ1, we propose a novel and dynamic role for glycosylation in CASQ. CASQ undergoes a unique degree of mannose trimming as it is trafficked from the proximal endoplasmic reticulum to the SR. The major glycoform of CASQ (GlcNAc(2)Man(9)) found in the proximal endoplasmic reticulum can severely hinder formation of the back-to-back interface, potentially preventing premature Ca(2+)-dependent polymerization of CASQ and ensuring its continuous mobility to the SR. Only trimmed glycans can stabilize both front-to-front and the back-to-back interfaces of CASQ through extensive hydrogen bonding and electrostatic interactions. Therefore, the mature glycoform of CASQ (GlcNAc(2)Man(1-4)) within the SR can be retained upon establishing a functional high capacity Ca(2+) binding polymer. In addition, based on the high resolution structures, we propose a molecular mechanism for the catecholaminergic polymorphic ventricular tachycardia (CPVT2) mutation, K206N.

The cytosolic protein kinase CK2 phosphorylates cardiac calsequestrin in intact cells

The luminal SR protein CSQ2 contains phosphate on roughly half of the serines found in its C-terminus. The sequence around phosphorylation sites in CSQ2 suggest that the in vivo kinase is protein kinase CK2, even though this enzyme is thought to be present only in the cytoplasm and nucleus. To test whether CSQ2 kinase is CK2, we combined approaches that reduced CK2 activity and CSQ2 phosphorylation in intact cells. Tetrabromocinnamic acid, a specific inhibitor of CK2, inhibited both the CSQ2 kinase and CK2 in parallel across a range of concentrations. In intact primary adult rat cardiomyocytes and COS cells, 24 h of drug treatment reduced phosphorylation of overexpressed CSQ2 by 75%. Down-regulation of CK2α subunits in COS cells using siRNA, produced a 90% decrease in CK2α protein levels, and CK2-silenced COS cells exhibited a twofold reduction in CSQ2 kinase activity. Phosphorylation of CSQ2 overexpressed in CK2-silenced cells was also reduced by a factor of two. These data suggested that CSQ2 in intact cells is phosphorylated by CK2, a cytosolic kinase. When phosphorylation site mutants were analyzed in COS cells, the characteristic rough endoplasmic reticulum form of the CSQ2 glycan (GlcNAc2Man9,8) underwent phosphorylation site dependent processing such that CSQ2-nonPP (Ser to Ala mutant) and CSQ2-mimPP (Ser to Glu mutant) produced apparent lower and greater levels of ER retention, respectively. Taken together, these data suggest CK2 can phosphorylate CSQ2 co-translationally at biosynthetic sites in rough ER, a process that may result in changes in its subsequent trafficking through the secretory pathway.

Phosphorylation of human calsequestrin: implications for calcium regulation

Both cardiac and skeletal calsequestrin (CASQ2 and CASQ1) serve as a major Ca(2+) storage/buffer protein in the sarcoplasmic reticulum (SR) by sequestering and releasing large numbers of Ca(2+) ions during each muscular contraction and relaxation cycle. CASQ isolated from various species often exists in a phosphorylated form, but phosphorylation's role is not yet understood. Here, the authors identified two phosphorylation sites, Ser(385) and Ser(393), for the first time, in human CASQ2 (hCASQ2) by mass-spectroscopy and evaluated the consequences of such phosphorylation. Substitution of these two serines with phosphoserine-mimicking aspartic-acid residues results in a significant increase in helical content, solubility and Ca(2+)-binding capacity above 6 mM [Ca(2+)]. However, neither substitution of Ser(385) nor Ser(393) alone produce any significant changes. Based on the crystal structures of hCASQ2, Ca(2+) binding capacity data, turbidity, and light scattering profiles, it was propose that phosphorylation at these two positions produces a disorder-to-order or coil-to-helix transition of the C-terminus, which in turn provides a more stable network of polyanions. Therefore, considering all the previous reports and the new data, the observed dynamic in vivo phosphorylation of CASQ could provide the basis not only for effective regulation of Ca(2+) buffering capacity, but also for the junctional SR trafficking mechanism.

Rough endoplasmic reticulum to junctional sarcoplasmic reticulum trafficking of calsequestrin in adult cardiomyocytes

Cardiac calsequestrin (CSQ) is synthesized on rough endoplasmic reticulum (ER), but concentrates within the junctional sarcoplasmic reticulum (SR) lumen where it becomes part of the Ca(2+)-release protein complex. To investigate CSQ trafficking through biosynthetic/secretory compartments of adult cardiomyocytes, CSQ-DsRed was overexpressed in cultured cells and examined using confocal fluorescence microscopy. By 48h of adenovirus treatment, CSQ-DsRed fluorescence had specifically accumulated in perinuclear cisternae, where it co-localized with markers of rough ER. From rough ER, CSQ-DsRed appeared to traffic directly to junctional SR along a transverse (Z-line) pathway along which sec 23-positive (ER-exit) sites were enriched. In contrast to DsRed direct fluorescence that presumably reflected DsRed tetramer formation, both anti-DsRed and anti-CSQ immunofluorescence did not detect the perinuclear CSQ-DsRed protein, but labeled only junctional SR puncta. These putative CSQ-DsRed monomers, but not the fluorescent tetramers, were observed to traffic anterogradely over the course of a 48h overexpression from rough ER towards the cell periphery. We propose a new model of CSQ and junctional SR protein traffic in the adult cardiomyocyte, wherein CSQ traffics from perinuclear cisternae, along contiguous ER/SR lumens in cardiomyocytes as a mobile monomer, but is retained in junctional SR as a polymer.

Catecholaminergic polymorphic ventricular tachycardia-related mutations R33Q and L167H alter calcium sensitivity of human cardiac calsequestrin

Two missense mutations, R33Q and L167H, of hCASQ2 (human cardiac calsequestrin), a protein segregated to the lumen of the sarcoplasmic reticulum, are linked to the autosomal recessive form of CPVT (catecholaminergic polymorphic ventricular tachycardia). The effects of these mutations on the conformational, stability and Ca(2+) sensitivity properties of hCASQ2, were investigated. Recombinant WT (wild-type) and mutant CASQ2s were purified to homogeneity and characterized by spectroscopic (CD and fluorescence) and biochemical (size-exclusion chromatography and limited proteolysis) methods at 500 and 100 mM KCl, with or without Ca(2+) at a physiological intraluminal concentration of 1 mM; Ca(2+)-induced polymerization properties were studied by turbidimetry. In the absence of Ca(2+), mutations did not alter the conformation of monomeric CASQ2. For L167H only, at 100 mM KCl, emission fluorescence changes suggested tertiary structure alterations. Limited proteolysis showed that amino acid substitutions enhanced the conformational flexibility of CASQ2 mutants, which became more susceptible to tryptic cleavage, in the order L167H>R33Q>WT. Ca(2+) at a concentration of 1 mM amplified such differences: Ca(2+) stabilized WT CASQ2 against urea denaturation and tryptic cleavage, whereas this effect was reduced in R33Q and absent in L167H. Increasing [Ca(2+)] induced polymerization and precipitation of R33Q, but not that of L167H, which was insensitive to Ca(2+). Based on CASQ2 models, we propose that the Arg(33)-->Gln exchange made the Ca(2+)-dependent formation of front-to-front dimers more difficult, whereas the Leu(167)-->His replacement almost completely inhibited back-to-back dimer interactions. Initial molecular events of CPVT pathogenesis begin to unveil and appear to be different depending upon the specific CASQ2 mutation.

Inefficient glycosylation leads to high steady-state levels of actively degrading cardiac triadin-1

In junctional sarcoplasmic reticulum, binding to cardiac triadin-1 provides a mechanism by which the Ca(2+)-release channel/ryanodine receptor may link with calsequestrin to regulate Ca(2+) release. Calsequestrin and triadin-1 both contain N-linked glycans, but about half of triadin-1 in the heart remains unglycosylated. To investigate mechanisms for this incomplete glycosylation, we overexpressed triadin-1 as a series of glycoform variants in non-muscle cell lines and neonatal heart cells using plasmid and adenoviral vectors. We showed that the characteristic incomplete glycosylation stemmed from properties of the glycosylation sequence that are conserved among triadin splice variants, including the close proximity of Asn(75) to the sarcoplasmic reticulum inner membrane. Although triadin-1 appeared by SDS-PAGE analysis as a 35/40-kDa doublet in all cells, variations occurred in the relative levels of the two glycoforms depending on the cell type and whether overexpression involved a plasmid or adenoviral vector. Treatment of triadin-1 with the proteasome inhibitor MG-132 led to striking changes in the relative levels of triadin-1 that indicated active breakdown of unglycosylated, but not glycosylated, triadin-1. Besides substantial increases in the relative levels of unglycosylated triadin-1, proteasome inhibition led to an accumulation of two new modified forms of triadin-1 that were seen with triadin-1 only when it is not glycosylated on Asn(75). Effects of tunicamycin and endoglycosidase H confirmed that these novel isoforms represent two alternative N-linked glycosylation sites, indicating that an alternative topology occurs infrequently leading to yet other glycoforms with short half-lives.

Different endoplasmic reticulum trafficking and processing pathways for calsequestrin (CSQ) and epitope-tagged CSQ

Cardiac calsequestrin (CSQ) is a protein that traffics to and concentrates inside sarcoplasmic reticulum (SR) terminal cisternae, a protein secretory compartment of uncertain origin. To investigate trafficking of CSQ within standard ER compartments, we expressed CSQ in nonmuscle cell lines and examined its localization by immunofluorescence and its molecular structure from the mass spectrum of total cellular CSQ. In all cells examined, CSQ was a highly phosphorylated protein with a glycan structure predictive of ER-retained proteins: Man9,8GlcNAc2 lacking terminal GlcNAc. Immunostaining was restricted to polymeric ER cisternae. Secretory pathway disruption by brefeldin A and thapsigargin led to altered CSQ glycosylation and phosphorylation consistent with post-ER trafficking. When epitope-tagged forms of CSQ were expressed in the same cells, mannose trimming of CSQ glycans was far more extensive, and C-terminal phosphorylation sites were nearly devoid of phosphate, in complete contrast to the highly phosphorylated wild-type protein that concentrates in all cells tested. Epitope-tagged CSQ also showed a reduced ER staining compared to wild-type protein, with significant staining in juxta-Golgi compartments. Loss of ER retention due to epitope tags or thapsigargin and resultant changes in protein structure or levels of bound Ca(2+) point to CSQ polymerization as an ER/SR retention mechanism.