CaMKII phosphorylates a threonine residue in the C-terminal tail of Cav1.2 Ca(2+) channel and modulates the interaction of the channel with calmodulin

Biological function of RNA-binding proteins in myocardial infarction: a potential emerging therapeutic limelight

Myocardial infarction (MI) is currently one of the most fatal cardiovascular diseases worldwide. The screening, treatment, and prognosis of MI are top priorities for cardiovascular centers globally due to its characteristic occult onset, high lethality, and poor prognosis. MI is caused by coronary artery occlusion induced by coronary atherosclerotic plaque blockage or other factors, leading to ischemic necrosis and apoptosis of cardiomyocytes. Although significant advancements have been made in the study of cardiomyocytes at the cellular and molecular levels, RNA-binding proteins (RBPs) have not been extensively explored in the context of MI. RBPs, as key regulators coordinating cell differentiation and tissue homeostasis, exhibit specific functions in gene transcription, RNA modification and processing, and post-transcriptional gene expression. By binding to their target RNA, RBPs coordinate various RNA dynamics, including cellular metabolism, subcellular localization, and translation efficiency, thereby controlling the expression of encoded proteins. Classical RBPs, including HuR, hnRNPs, and RBM family molecules, have been identified as critical regulators in myocardial hypoxia, oxidative stress, pro-inflammatory responses, and fibrotic repair. These RBPs exert their effects by modulating key pathophysiological pathways in MI, thereby influencing specific cardiac outcomes. Additionally, specific RBPs, such as QKI and fused in sarcoma (FUS), are implicated in the apoptotic pathways activated during MI. This apoptotic pathway represents a significant molecular phenotype in MI, offering novel perspectives and insights for mitigating cardiomyocyte apoptosis and attenuating the progression of MI. Therefore, this review systematically summarizes the role of RBPs in the main pathophysiological stages of MI and explores their potential therapeutic prospects.

Regulation of Cardiac Cav1.2 Channels by Calmodulin

Cav1.2 Ca2+ channels, a type of voltage-gated L-type Ca2+ channel, are ubiquitously expressed, and the predominant Ca2+ channel type, in working cardiac myocytes. Cav1.2 channels are regulated by the direct interactions with calmodulin (CaM), a Ca2+-binding protein that causes Ca2+-dependent facilitation (CDF) and inactivation (CDI). Ca2+-free CaM (apoCaM) also contributes to the regulation of Cav1.2 channels. Furthermore, CaM indirectly affects channel activity by activating CaM-dependent enzymes, such as CaM-dependent protein kinase II and calcineurin (a CaM-dependent protein phosphatase). In this article, we review the recent progress in identifying the role of apoCaM in the channel ‘rundown’ phenomena and related repriming of channels, and CDF, as well as the role of Ca2+/CaM in CDI. In addition, the role of CaM in channel clustering is reviewed.

Calmodulin variant E140G associated with long QT syndrome impairs CaMKIIδ autophosphorylation and L-type calcium channel inactivation

Long QT syndrome (LQTS) is a human inherited heart condition that can cause life-threatening arrhythmia including sudden cardiac death. Mutations in the ubiquitous Ca²⁺-sensing protein calmodulin (CaM) are associated with LQTS, but the molecular mechanism by which these mutations lead to irregular heartbeats is not fully understood. Here, we use a multidisciplinary approach including protein biophysics, structural biology, confocal imaging, and patch-clamp electrophysiology to determine the effect of the disease-associated CaM mutation E140G on CaM structure and function. We present novel data showing that mutant-regulated CaMKIIδ kinase activity is impaired with a significant reduction in enzyme autophosphorylation rate. We report the first high-resolution crystal structure of a LQTS-associated CaM variant in complex with the CaMKIIδ peptide, which shows significant structural differences, compared to the WT complex. Furthermore, we demonstrate that the E140G mutation significantly disrupted Ca_v1.2 Ca²⁺/CaM-dependent inactivation, while cardiac ryanodine receptor (RyR2) activity remained unaffected. In addition, we show that the LQTS-associated mutation alters CaM's Ca²⁺-binding characteristics, secondary structure content, and interaction with key partners involved in excitation-contraction coupling (CaMKIIδ, Ca_v1.2, RyR2). In conclusion, LQTS-associated CaM mutation E140G severely impacts the structure-function relationship of CaM and its regulation of CaMKIIδ and Ca_v1.2. This provides a crucial insight into the molecular factors contributing to CaM-mediated arrhythmias with a central role for CaMKIIδ.

The CaMKII phosphorylation site Thr1604 in the CaV1.2 channel is involved in pathological myocardial hypertrophy in rats

Residue Thr1604 in the Ca_V1.2 channel is a Ca²⁺/calmodulin dependent protein kinase II (CaMKII) phosphorylation site, and its phosphorylation status maintains the basic activity of the channel. However, the role of Ca_V1.2 phosphorylation at Thr1604 in myocardial hypertrophy is incompletely understood. Isoproterenol (ISO) was used to induce cardiomyocyte hypertrophy, and autocamtide-2-related inhibitory peptide (AIP) was added as a treatment. Rats in a myocardial hypertrophy development model were subcutaneously injected with ISO for two or three weeks. The heart and left ventricle weights, each of which were normalized to the body weight and cross-sectional area of the myocardial cells, were used to describe the degree of hypertrophy. Protein expression levels were detected by western blotting. CaMKII-induced Ca_V1.2 (Thr1604) phosphorylation (p-Ca_V1.2) was assayed by coimmunoprecipitation. The results showed that CaMKII, HDAC, MEF2 C, and atrial natriuretic peptide (ANP) expression was increased in the ISO group and downregulated by AIP treatment in vitro. There was no difference in the expression of these proteins between the ISO 2-week group and the ISO 3-week group in vivo. Ca_V1.2 channel expression did not change, but p-Ca_V1.2 expression was increased after ISO stimulation and decreased by AIP. In the rat model, p-Ca_V1.2 levels and CaMKII activity were much higher in the ISO 3-week group than in the ISO 2-week group. CaMKII-induced Ca_V1.2 channel phosphorylation at residue Thr1604 may be one of the key features of myocardial hypertrophy and disease development.Abbreviations: CaMKII: Ca2+/calmodulin dependent protein kinase II; p-CaMKII: autophosphorylated Ca2+/calmodulin dependent protein kinase II; CaM: calmodulin; AIP: autocamtide-2-related inhibitory peptide; ECC: excitation-contraction coupling; ISO: isoproterenol; BW: body weight; HW: heart weight; LVW: left ventricle weight; HDAC: histone deacetylase; p-HDAC: phosphorylated histone deacetylase; MEF2C: myocyte-specific enhancer factor 2C; ANP: atrial natriuretic peptide; PKC: protein kinase C

Protein phosphorylation maintains the normal function of cloned human Cav2.3 channels

Ca_v2.3 Ca²⁺ channels are subject to cytosolic regulation, which has been difficult to characterize in native cells. Neumaier et al. demonstrate the role of phosphorylation in the function of these channels and suggest a close relationship between voltage dependence and the phosphorylation state.

R-type currents mediated by native and recombinant Ca_v2.3 voltage-gated Ca²⁺ channels (VGCCs) exhibit facilitation (run-up) and subsequent decline (run-down) in whole-cell patch-clamp recordings. A better understanding of the two processes could provide insight into constitutive modulation of the channels in intact cells, but low expression levels and the need for pharmacological isolation have prevented investigations in native systems. Here, to circumvent these limitations, we use conventional and perforated-patch-clamp recordings in a recombinant expression system, which allows us to study the effects of cell dialysis in a reproducible manner. We show that the decline of currents carried by human Ca_v2.3+β₃ channel subunits during run-down is related to adenosine triphosphate (ATP) depletion, which reduces the number of functional channels and leads to a progressive shift of voltage-dependent gating to more negative potentials. Both effects can be counteracted by hydrolysable ATP, whose protective action is almost completely prevented by inhibition of serine/threonine but not tyrosine or lipid kinases. Protein kinase inhibition also mimics the effects of run-down in intact cells, reduces the peak current density, and hyperpolarizes the voltage dependence of gating. Together, our findings indicate that ATP promotes phosphorylation of either the channel or an associated protein, whereas dephosphorylation during cell dialysis results in run-down. These data also distinguish the effects of ATP on Ca_v2.3 channels from those on other VGCCs because neither direct nucleotide binding nor PIP₂ synthesis is required for protection from run-down. We conclude that protein phosphorylation is required for Ca_v2.3 channel function and could directly influence the normal features of current carried by these channels. Curiously, some of our findings also point to a role for leupeptin-sensitive proteases in run-up and possibly ATP protection from run-down. As such, the present study provides a reliable baseline for further studies on Ca_v2.3 channel regulation by protein kinases, phosphatases, and possibly proteases.

PKA and phosphatases attached to the Ca(V)1.2 channel regulate channel activity in cell-free patches

Calmodulin (CaM) + ATP can reprime voltage-gated L-type Ca(2+) channels (Ca(V)1.2) in inside-out patches for activation, but this effect decreases time dependently. This suggests that the Ca(V)1.2 channel activity is regulated by additional cytoplasmic factors. To test this hypothesis, we examined the role of cAMP-dependent protein kinase A (PKA) and protein phosphatases in the regulation of Ca(V)1.2 channel activity in the inside-out mode in guinea pig ventricular myocytes. Ca(V)1.2 channel activity quickly disappeared after the patch was excised from the cell and recovered to only 9% of that in the cell-attached mode on application of CaM + ATP at 10 min after the inside out. However, immediate exposure of the excised patch to the catalytic subunit of PKA + ATP or the nonspecific phosphatase inhibitor okadaic acid significantly increased the Ca(V)1.2 channel activity recovery by CaM + ATP (114 and 96%, respectively) at 10 min. Interestingly, incubation of the excised patches with cAMP + ATP also increased CaM/ATP-induced Ca(V)1.2 channel activity recovery (108%), and this effect was blocked by the nonspecific protein kinase inhibitor K252a. The channel activity in the inside-out mode was not maintained by either catalytic subunit of PKA or cAMP + ATP in the absence of CaM, but was stably maintained in the presence of CaM for more than 40 min. These results suggest that PKA and phosphatase(s) attached on or near the Ca(V)1.2 channel regulate the basal channel activity, presumably through modulation of the dynamic CaM interaction with the channel.

Abnormal alterations in the Ca²⁺/CaV1.2/calmodulin/caMKII signaling pathway in a tremor rat model and in cultured hippocampal neurons exposed to Mg²⁺-free solution

Voltage-dependent calcium channels (VDCCs) are key elements in epileptogenesis. There are several binding-sites linked to calmodulin (CaM) and several potential CaM-dependent protein kinase II (CaMKII)-mediated phosphorylation sites in CaV1.2. The tremor rat model (TRM) exhibits absence‑like seizures from 8 weeks of age. The present study was performed to detect changes in the Ca2+/CaV1.2/CaM/CaMKII pathway in TRMs and in cultured hippocampal neurons exposed to Mg2+‑free solution. The expression levels of CaV1.2, CaM and phosphorylated CaMKII (p‑CaMKII; Thr‑286) in these two models were examined using immunofluorescence and western blotting. Compared with Wistar rats, the expression levels of CaV1.2 and CaM were increased, and the expression of p‑CaMKII was decreased in the TRM hippocampus. However, the expression of the targeted proteins was reversed in the TRM temporal cortex. A significant increase in the expression of CaM and decrease in the expression of CaV1.2 were observed in the TRM cerebellum. In the cultured neuron model, p‑CaMKII and CaV1.2 were markedly decreased. In addition, neurons exhibiting co‑localized expression of CaV1.2 and CaM immunoreactivities were detected. Furthermore, intracellular calcium concentrations were increased in these two models. For the first time, o the best of our knowledge, the data of the present study suggested that abnormal alterations in the Ca2+/CaV1.2/CaM/CaMKII pathway may be involved in epileptogenesis and in the phenotypes of TRMs and cultured hippocampal neurons exposed to Mg2+‑free solution.

Differential CaMKII regulation by voltage-gated calcium channels in the striatum

Calcium signaling regulates synaptic plasticity and many other functions in striatal medium spiny neurons to modulate basal ganglia function. Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is a major calcium-dependent signaling protein that couples calcium entry to diverse cellular changes. CaMKII activation results in autophosphorylation at Thr286 and sustained calcium-independent CaMKII activity after calcium signals dissipate. However, little is known about the mechanisms regulating striatal CaMKII. To address this, mouse brain slices were treated with pharmacological modulators of calcium channels and punches of dorsal striatum were immunoblotted for CaMKII Thr286 autophosphorylation as an index of CaMKII activation. KCl depolarization increased levels of CaMKII autophosphorylation ~2-fold; this increase was blocked by an LTCC antagonist and was mimicked by treatment with pharmacological LTCC activators. The chelation of extracellular calcium robustly decreased basal CaMKII autophosphorylation within 5min and increased levels of total CaMKII in cytosolic fractions, in addition to decreasing the phosphorylation of CaMKII sites in the GluN2B subunit of NMDA receptors and the GluA1 subunit of AMPA receptors. We also found that the maintenance of basal levels of CaMKII autophosphorylation requires low-voltage gated T-type calcium channels, but not LTCCs or R-type calcium channels. Our findings indicate that CaMKII activity is dynamically regulated by multiple calcium channels in the striatum thus coupling calcium entry to key downstream substrates.

Ca(2+) current facilitation is CaMKII-dependent and has arrhythmogenic consequences

The cardiac voltage gated Ca²⁺ current (I_Ca) is critical to the electrophysiological properties, excitation-contraction coupling, mitochondrial energetics, and transcriptional regulation in heart. Thus, it is not surprising that cardiac I_Ca is regulated by numerous pathways. This review will focus on changes in I_Ca that occur during the cardiac action potential (AP), with particular attention to Ca²⁺-dependent inactivation (CDI), Ca²⁺-dependent facilitation (CDF) and how calmodulin (CaM) and Ca²⁺-CaM dependent protein kinase (CaMKII) participate in the regulation of Ca²⁺ current during the cardiac AP. CDI depends on CaM pre-bound to the C-terminal of the L-type Ca²⁺ channel, such that Ca²⁺ influx and Ca²⁺ released from the sarcoplasmic reticulum bind to that CaM and cause CDI. In cardiac myocytes CDI normally pre-dominates over voltage-dependent inactivation. The decrease in I_Ca via CDI provides direct negative feedback on the overall Ca²⁺ influx during a single beat, when myocyte Ca²⁺ loading is high. CDF builds up over several beats, depends on CaMKII-dependent Ca²⁺ channel phosphorylation, and results in a staircase of increasing I_Ca peak, with progressively slower inactivation. CDF and CDI co-exist and in combination may fine-tune the I_Ca waveform during the cardiac AP. CDF may partially compensate for the tendency for Ca²⁺ channel availability to decrease at higher heart rates because of accumulating inactivation. CDF may also allow some reactivation of I_Ca during long duration cardiac APs, and contribute to early afterdepolarizations, a form of triggered arrhythmias.

Mechanisms underlying the modulation of L-type Ca2+ channel by hydrogen peroxide in guinea pig ventricular myocytes

Although Cav1.2 Ca(2+) channels are modulated by reactive oxygen species (ROS), the underlying mechanisms are not fully understood. In this study, we investigated effects of hydrogen peroxide (H2O2) on the Ca(2+) channel using a patch-clamp technique in guinea pig ventricular myocytes. Externally applied H2O2 (1 mM) increased Ca(2+) channel activity in the cell-attached mode. A specific inhibitor of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) KN-93 (10 μM) partially attenuated the H2O2-mediated facilitation of the channel, suggesting both CaMKII-dependent and -independent pathways. However, in the inside-out mode, 1 mM H2O2 increased channel activity in a KN-93-resistant manner. Since H2O2-pretreated calmodulin did not reproduce the H2O2 effect, the target of H2O2 was presumably assigned to the Ca(2+) channel itself. A thiol-specific oxidizing agent mimicked and occluded the H2O2 effect. These results suggest that H2O2 facilitates the Ca(2+) channel through oxidation of cysteine residue(s) in the channel as well as the CaMKII-dependent pathway.

Nonspecific, reversible inhibition of voltage-gated calcium channels by CaMKII inhibitor CK59

Investigation of kinase-related processes often uses pharmacological inhibition to reveal pathways in which kinases are involved. However, one concern about using such kinase inhibitors is their potential lack of specificity. Here, we report that the calcium-calmodulin-dependent kinase II (CaMKII) inhibitor CK59 inhibited multiple voltage-gated calcium channels, including the L-type channel during depolarization in a dose-dependent manner. The use of another CaMKII inhibitor, cell-permeable autocamtide-2 related inhibitory peptide II (Ant-AIP-II), failed to similarly decrease calcium current or entry in hippocampal cultures, as shown by ratiometric calcium imaging and whole-cell patch clamp electrophysiology. Notably, inhibition due to CK59 was reversible; washout of the drug brought calcium levels back to control values upon depolarization. Furthermore, the IC50 for CK59 was approximately 50 μM, which is only fivefold larger than the reported IC50 values for CaMKII inhibition. Similar nonspecific actions of other CaMKII inhibitors KN93 and KN62 have previously been reported. In the case of all three kinase inhibitors, the IC50 for calcium current inhibition falls near that of CaMKII inhibition. Our findings demonstrate that CK59 attenuates activity of voltage-gated calcium channels, and thus provide more evidence for caution when relying on pharmacological inhibition to examine kinase-dependent phenomena.

Molecular determinants for cardiovascular TRPC6 channel regulation by Ca2+/calmodulin-dependent kinase II

The molecular mechanism underlying Ca(2+)/calmodulin (CaM)-dependent kinase II (CaMKII)-mediated regulation of the mouse transient receptor potential channel TRPC6 was explored by chimera, deletion and site-directed mutagenesis approaches. Induction of currents (ICCh) in TRPC6-expressing HEK293 cells by a muscarinic agonist carbachol (CCh; 100 μm) was strongly attenuated by a CaMKII-specific peptide, autocamtide-2-related inhibitory peptide (AIP; 10 μm). TRPC6/C7 chimera experiments showed that the TRPC6 C-terminal sequence is indispensable for ICCh to be sensitive to AIP-induced CaMKII inhibition. Further, deletion of a distal region (Gln(855)-Glu(877)) of the C-terminal CaM/inositol-1,4,5-trisphosphate receptor binding domain (CIRB) of TRPC6 was sufficient to abolish ICCh. Systematic alanine scanning for potential CaMKII phosphorylation sites revealed that Thr(487) was solely responsible for the activation of the TRPC6 channel by receptor stimulation. The abrogating effect of the alanine mutation of Thr(487) (T487A) was reproduced with other non-polar amino acids, namely glutamine or asparagine, while being partially rescued by phosphomimetic mutations with glutamate or aspartate. The cellular expression and distribution of TRPC6 channels did not significantly change with these mutations. Electrophysiological and immunocytochemical data with the Myc-tagged TRPC6 channel indicated that Thr(487) is most likely located at the intracellular side of the cell membrane. Overexpression of T487A caused significant reduction of endogenous TRPC6-like current induced by Arg(8)-vasopressin in A7r5 aortic myocytes. Based on these results, we propose that the optimal spatial arrangement of a C-terminal domain (presumably the distal CIRB region) around a single CaMKII phosphorylation site Thr(487) may be essential for CaMKII-mediated regulation of TRPC6 channels. This mechanism may be of physiological significance in a native environment such as in vascular smooth muscle cells.

Phylogenetic and in silico structural analysis of the Parkinson disease-related kinase PINK1

Parkinson disease (PD) is the second most common neurodegenerative disorder and is characterized by the loss of dopaminergic neurons in the substantia nigra. Mutations in PINK1 were shown to cause recessive familial PD, and today are proposed to be associated with the disease via mitochondrial dysfunction and oxidative damage. The PINK1 gene comprises eight exons, which encode a ubiquitously expressed 581 amino acid protein that contains an N-terminal mitochondrial targeting domain and a serine/threonine protein kinase. To better understand the relationship between PINK1 and PD we have first analyzed the evolutionary history of the gene showing its late emergence in evolution. In addition, we have modeled the three-dimensional structure of PINK1 and found some evidences that help to explain the effect of some PD-related mutations in this protein's function.

Minor contribution of cytosolic Ca2+ transients to the pacemaker rhythm in guinea pig sinoatrial node cells

The question of the extent to which cytosolic Ca(2+) affects sinoatrial node pacemaker activity has been discussed for decades. We examined this issue by analyzing two mathematical pacemaker models, based on the "Ca(2+) clock" (C) and "membrane clock" (M) hypotheses, together with patch-clamp experiments in isolated guinea pig sinoatrial node cells. By applying lead potential analysis to the models, the C mechanism, which is dependent on potentiation of Na(+)/Ca(2+) exchange current via spontaneous Ca(2+) release from the sarcoplasmic reticulum (SR) during diastole, was found to overlap M mechanisms in the C model. Rapid suppression of pacemaker rhythm was observed in the C model by chelating intracellular Ca(2+), whereas the M model was unaffected. Experimental rupturing of the perforated-patch membrane to allow rapid equilibration of the cytosol with 10 mM BAPTA pipette solution, however, failed to decrease the rate of spontaneous action potential within ∼30 s, whereas contraction ceased within ∼3 s. The spontaneous rhythm also remained intact within a few minutes when SR Ca(2+) dynamics were acutely disrupted using high doses of SR blockers. These experimental results suggested that rapid disruption of normal Ca(2+) dynamics would not markedly affect spontaneous activity. Experimental prolongation of the action potentials, as well as slowing of the Ca(2+)-mediated inactivation of the L-type Ca(2+) currents induced by BAPTA, were well explained by assuming Ca(2+) chelation, even in the proximity of the channel pore in addition to the bulk cytosol in the M model. Taken together, the experimental and model findings strongly suggest that the C mechanism explicitly described by the C model can hardly be applied to guinea pig sinoatrial node cells. The possible involvement of L-type Ca(2+) current rundown induced secondarily through inhibition of Ca(2+)/calmodulin kinase II and/or Ca(2+)-stimulated adenylyl cyclase was discussed as underlying the disruption of spontaneous activity after prolonged intracellular Ca(2+) concentration reduction for >5 min.

Facilitation of murine cardiac L-type Ca(v)1.2 channel is modulated by calmodulin kinase II-dependent phosphorylation of S1512 and S1570

Activity-dependent means of altering calcium (Ca(2)(+)) influx are assumed to be of great physiological consequence, although definitive tests of this assumption have only begun to emerge. Facilitation and inactivation offer two opposing, activity-dependent means of altering Ca(2+) influx via cardiac Ca(v)1.2 calcium channels. Voltage- and frequency-dependent facilitation of Ca(v)1.2 has been reported to depend on Calmodulin (CaM) and/or the activity of Calmodulin kinase II (CaMKII). Several sites within the cardiac L-type calcium channel complex have been proposed as the targets of CaMKII. Here, we generated mice with knockin mutations of alpha(1)1.2 S1512 and S1570 phosphorylation sites [sine facilitation (SF) mice]. Homocygote SF mice were viable and reproduced in a Mendelian ratio. Voltage-dependent facilitation in ventricular cardiomyocytes carrying the SF mutation was decreased from 1.58- to 1.18-fold. The CaMKII inhibitor KN-93 reduced facilitation to 1.28 in control cardiomyocytes. SF mutation negatively shifted the voltage-dependent inactivation and slowed recovery from inactivation, thereby making fewer channels available for activation. Telemetric ECG recordings at different heart rates showed that QT time decreased significantly more in SF than in control mice at higher rates. Our results strongly support the notion that CaMKII-dependent phosphorylation of Cav1.2 at S1512 and S1570 mediates Ca(2+) current facilitation in the murine heart.