alphaKAP is an anchoring protein for a novel CaM kinase II isoform in skeletal muscle

Maternal oxytocin treatment at birth increases epigenetic age in male offspring

Exogenous oxytocin (OT) is widely used to induce or augment labor with little understanding of the impact on offspring development. In rodent models, including the prairie vole (Microtus ochrogaster), it has been shown that oxytocin administered to mothers can affect the nervous system of the offspring with long lasting behavioral effects especially on sociality. Here, we examined the hypothesis that perinatal oxytocin exposure could have epigenetic and transcriptomic consequences. Prairie voles were exposed to exogenous oxytocin, through injections given to the mother just prior to birth, and were studied at the time of weaning. The outcome of this study revealed increased epigenetic age in oxytocin-exposed animals compared to the saline-exposed group. Oxytocin exposure led to 900 differentially methylated CpG sites (annotated to 589 genes), and 2 CpG sites (2 genes) remained significantly different after correction for multiple comparisons. Differentially methylated CpG sites were enriched in genes known to be involved in regulation of gene expression and neurodevelopment. Using RNA-sequencing we also found 217 nominally differentially expressed genes (p<0.05) in nucleus accumbens, a brain region involved in reward circuitry and social behavior; after corrections for multiple comparisons 6 genes remained significantly differentially expressed. Finally, we found that maternal oxytocin administration led to widespread alternative splicing in the nucleus accumbens. These results indicate that oxytocin exposure during birth may have long lasting epigenetic consequences. A need for further investigation of how oxytocin administration impacts development and behavior throughout the lifespan is supported by these outcomes.

Enhanced capacity for CaMKII signaling mitigates calcium release related contractile fatigue with high intensity exercise

BACKGROUND

We tested whether enhancing the capacity for calcium/calmodulin-dependent protein kinase type II (CaMKII) signaling would delay fatigue of excitation-induced calcium release and improve contractile characteristics of skeletal muscle during fatiguing exercise.

METHODS

Fast and slow type muscle, gastrocnemius medialis (GM) and soleus (SOL), of rats and mouse interosseus (IO) muscle fibers, were transfected with pcDNA3-based plasmids for rat α and β CaMKII or empty controls. Levels of CaMKII, its T287-phosphorylation (pT287-CaMKII), and phosphorylation of components of calcium release and re-uptake, ryanodine receptor 1 (pS2843-RyR1) and phospholamban (pT17-PLN), were quantified biochemically. Sarcoplasmic calcium in transfected muscle fibers was monitored microscopically during trains of electrical excitation based on Fluo-4 FF fluorescence (n = 5-7). Effects of low- (n = 6) and high- (n = 8) intensity exercise on pT287-CaMKII and contractile characteristics were studied in situ.

RESULTS

Co-transfection with αCaMKII-pcDNA3/βCaMKII-pcDNA3 increased α and βCaMKII levels in SOL (+45.8 %, +250.5 %) and GM (+40.4 %, +89.9 %) muscle fibers compared to control transfection. High-intensity exercise increased pT287-βCaMKII and pS2843-RyR1 levels in SOL (+269 %, +151 %) and GM (+354 %, +119 %), but decreased pT287-αCaMKII and p17-PLN levels in GM compared to SOL (-76 % vs. +166 %; 0 % vs. +128 %). α/β CaMKII overexpression attenuated the decline of calcium release in muscle fibers with repeated excitation, and mitigated exercise-induced deterioration of rates in force production, and passive force, in a muscle-dependent manner, in correlation with pS2843-RyR1 and pT17-PLN levels (|r| > 0.7).

CONCLUSION

Enhanced capacity for α/β CaMKII signaling improves fatigue-resistance of active and passive contractile muscle properties in association with RyR1- and PLN-related improvements in sarcoplasmic calcium release.

Reduced O-GlcNAcylation diminishes cardiomyocyte Ca2+ dependent facilitation and frequency dependent acceleration of relaxation

Ca2+ dependent facilitation (CDF) and frequency dependent acceleration of relaxation (FDAR) are regulatory mechanisms that potentiate cardiomyocyte Ca2+ channel function and increase the rate of Ca2+ sequestration following a Ca2+-release event, respectively, when depolarization frequency increases. CDF and FDAR likely evolved to maintain EC coupling at increased heart rates. Ca2+/calmodulin-dependent kinase II (CaMKII) was shown to be indispensable to both; however, the mechanisms remain to be completely elucidated. CaMKII activity can be modulated by post-translational modifications but if and how these modifications impact CDF and FDAR is unknown. Intracellular O-linked glycosylation (O-GlcNAcylation) is a post-translational modification that acts as a signaling molecule and metabolic sensor. In hyperglycemic conditions, CaMKII was shown to be O-GlcNAcylated resulting in pathologic activity. Here we sought to investigate whether O-GlcNAcylation impacts CDF and FDAR through modulation of CaMKII activity in a pseudo-physiologic setting. Using voltage-clamp and Ca2+ photometry we show that cardiomyocyte CDF and FDAR are significantly diminished in conditions of reduced O-GlcNAcylation. Immunoblot showed that CaMKIIδ and calmodulin expression are increased but the autophosphorylation of CaMKIIδ and the muscle cell-specific CaMKIIβ isoform are reduced by 75% or more when O-GlcNAcylation is inhibited. We also show that the enzyme responsible for O-GlcNAcylation (OGT) can likely be localized in the dyad space and/or at the cardiac sarcoplasmic reticulum and is precipitated by calmodulin in a Ca2+ dependent manner. These findings will have important implications for our understanding of how CaMKII and OGT interact to impact cardiomyocyte EC coupling in normal physiologic settings as well as in disease states where CaMKII and OGT may be aberrantly regulated.

Calcium/Calmodulin-Stimulated Protein Kinase II (CaMKII): Different Functional Outcomes from Activation, Depending on the Cellular Microenvironment

Calcium/calmodulin-stimulated protein kinase II (CaMKII) is a family of broad substrate specificity serine (Ser)/threonine (Thr) protein kinases widely expressed in many tissues that is capable of mediating diverse functional responses depending on its cellular and molecular microenvironment. This review briefly summarises current knowledge on the structure and regulation of CaMKII and focuses on how the molecular environment, and interaction with binding partner proteins, can produce different populations of CaMKII in different cells, or in different subcellular locations within the same cell, and how these different populations of CaMKII can produce diverse functional responses to activation following an increase in intracellular calcium concentration. This review also explores the possibility that identifying and characterising the molecular interactions responsible for the molecular targeting of CaMKII in different cells in vivo, and identifying the sites on CaMKII and/or the binding proteins through which these interactions occur, could lead to the development of highly selective inhibitors of specific CaMKII-mediated functional responses in specific cells that would not affect CaMKII-mediated responses in other cells. This may result in the development of new pharmacological agents with therapeutic potential for many clinical conditions.

A Modular Mathematical Model of Exercise-Induced Changes in Metabolism, Signaling, and Gene Expression in Human Skeletal Muscle

Skeletal muscle is the principal contributor to exercise-induced changes in human metabolism. Strikingly, although it has been demonstrated that a lot of metabolites accumulating in blood and human skeletal muscle during an exercise activate different signaling pathways and induce the expression of many genes in working muscle fibres, the systematic understanding of signaling-metabolic pathway interrelations with downstream genetic regulation in the skeletal muscle is still elusive. Herein, a physiologically based computational model of skeletal muscle comprising energy metabolism, Ca2+, and AMPK (AMP-dependent protein kinase) signaling pathways and the expression regulation of genes with early and delayed responses was developed based on a modular modeling approach and included 171 differential equations and more than 640 parameters. The integrated modular model validated on diverse including original experimental data and different exercise modes provides a comprehensive in silico platform in order to decipher and track cause-effect relationships between metabolic, signaling, and gene expression levels in skeletal muscle.

Assessing conservation of alternative splicing with evolutionary splicing graphs

Understanding how protein function has evolved and diversified is of great importance for human genetics and medicine. Here, we tackle the problem of describing the whole transcript variability observed in several species by generalizing the definition of splicing graph. We provide a practical solution to construct parsimonious evolutionary splicing graphs where each node is a minimal transcript building block defined across species. We show a clear link between the functional relevance, tissue regulation, and conservation of alternative transcripts on a set of 50 genes. By scaling up to the whole human protein-coding genome, we identify a few thousand genes where alternative splicing modulates the number and composition of pseudorepeats. We have implemented our approach in ThorAxe, an efficient, versatile, robust, and freely available computational tool.

CaMKII oxidation is a critical performance/disease trade-off acquired at the dawn of vertebrate evolution

Antagonistic pleiotropy is a foundational theory that predicts aging-related diseases are the result of evolved genetic traits conferring advantages early in life. Here we examine CaMKII, a pluripotent signaling molecule that contributes to common aging-related diseases, and find that its activation by reactive oxygen species (ROS) was acquired more than half-a-billion years ago along the vertebrate stem lineage. Functional experiments using genetically engineered mice and flies reveal ancestral vertebrates were poised to benefit from the union of ROS and CaMKII, which conferred physiological advantage by allowing ROS to increase intracellular Ca²⁺ and activate transcriptional programs important for exercise and immunity. Enhanced sensitivity to the adverse effects of ROS in diseases and aging is thus a trade-off for positive traits that facilitated the early and continued evolutionary success of vertebrates.

Natural selection may favor traits underlying aging-related diseases if they benefit the young. Wang et al. find that oxidative activation of CaMKII provides physiological benefits critical to the initial and continued success of vertebrates but at the cost of disease, frailty, and shortened lifespan.

Compartmentation of β2 -adrenoceptor stimulated cAMP responses by phosphodiesterase types 2 and 3 in cardiac ventricular myocytes

BACKGROUND AND PURPOSE

In cardiac myocytes, cyclic AMP (cAMP) produced by both β₁ - and β₂ -adrenoceptors increases L-type Ca²⁺ channel activity and myocyte contraction. However, only cAMP produced by β₁ -adrenoceptors enhances myocyte relaxation through phospholamban-dependent regulation of the sarco/endoplasmic reticulum Ca²⁺ ATPase 2 (SERCA2). Here we have tested the hypothesis that stimulation of β₂ -adrenoceptors produces a cAMP signal that is unable to reach SERCA2 and determine what role, if any, phosphodiesterase (PDE) activity plays in this compartmentation.

EXPERIMENTAL APPROACH

The cAMP responses produced by β₁ -and β₂ -adrenoceptor stimulation were studied in adult rat ventricular myocytes using two different fluorescence resonance energy transfer (FRET)-based biosensors, the Epac2-camps, which is expressed uniformly throughout the cytoplasm of the entire cell and the Epac2-αKAP, which is targeted to the SERCA2 signalling complex.

KEY RESULTS

Selective activation of β₁ - or β₂ -adrenoceptors produced cAMP responses detected by Epac2-camps. However, only stimulation of β₁ -adrenoceptors produced a cAMP response detected by Epac2-αKAP. Yet, stimulation of β₂ -adrenoceptors was able to produce a cAMP signal detected by Epac2-αKAP in the presence of selective inhibitors of PDE2 or PDE3, but not PDE4.

CONCLUSION AND IMPLICATIONS

These results support the conclusion that cAMP produced by β₂ -adrenoceptor stimulation was not able to reach subcellular locations where the SERCA2 pump is located. Furthermore, this compartmentalized response is due at least in part to PDE2 and PDE3 activity. This discovery could lead to novel PDE-based therapeutic treatments aimed at correcting cardiac relaxation defects associated with certain forms of heart failure.

Role of CaMKII and sarcolipin in muscle adaptations to strength training with different levels of fatigue in the set

Strength training promotes a IIX-to-IIA shift in myosin heavy chain (MHC) composition, likely due to changes in sarcoplasmic [Ca²⁺ ] which are sensed by CaMKII. Sarcoplasmic [Ca²⁺ ] is in part regulated by sarcolipin (SLN), a small protein that when overexpressed in rodents stimulates mitochondrial biogenesis and a fast-to-slow fiber type shift. The purpose of this study was to determine whether CaMKII and SLN are involved in muscle phenotype and performance changes elicited by strength training. Twenty-two men followed an 8-week velocity-based resistance training program using the full squat exercise while monitoring repetition velocity. Subjects were randomly assigned to two resistance training programs differing in the repetition velocity loss allowed in each set: 20% (VL20) vs 40% (VL40). Strength training caused muscle hypertrophy, improved 1RM and increased total CaMKII protein expression, particularly of the δ_D isoform. Phospho-Thr²⁸⁷ -CaMKII δ_D expression increased only in VL40 (+89%), which experienced greater muscle hypertrophy, and a reduction in MHC-IIX percentage. SLN expression was increased in VL20 (+33%) remaining unaltered in VL40. The changes in phospho-Thr²⁸⁷ -CaMKII δ_D were positively associated with muscle hypertrophy and the number of repetitions during training, and negatively with the changes in MHC-IIX and SLN. Most OXPHOS proteins remained unchanged, except for NDUFB8 (Complex I), which was reduced after training (-22%) in both groups. The amount of fatigue allowed in each set critically influences muscle CaMKII and SLN responses and determines muscle phenotype changes. With lower intra-set fatigue, the IIX-to-IIA MHC shift is attenuated.

Heterogeneity in human hippocampal CaMKII transcripts reveals allosteric hub-dependent regulation

Calcium/calmodulin-dependent protein kinase II (CaMKII) plays a central role in Ca2+ signaling throughout the body. In the hippocampus, CaMKII is required for learning and memory. Vertebrate genomes encode four CaMKII homologs: CaMKIIα, CaMKIIβ, CaMKIIγ, and CaMKIIδ. All CaMKIIs consist of a kinase domain, a regulatory segment, a variable linker region, and a hub domain, which is responsible for oligomerization. The four proteins differ primarily in linker length and composition because of extensive alternative splicing. Here, we report the heterogeneity of CaMKII transcripts in three complex samples of human hippocampus using deep sequencing. We showed that hippocampal cells contain a diverse collection of over 70 CaMKII transcripts from all four CaMKII-encoding genes. We characterized the Ca2+/CaM sensitivity of hippocampal CaMKII variants spanning a broad range of linker lengths and compositions. The effect of the variable linker on Ca2+/CaM sensitivity depended on the kinase and hub domains. Moreover, we revealed a previously uncharacterized role for the hub domain as an allosteric regulator of kinase activity, which may provide a pharmacological target for modulating CaMKII activity. Using small-angle x-ray scattering and single-particle cryo-electron microscopy (cryo-EM), we present evidence for extensive interactions between the kinase and the hub domains, even in the presence of a 30-residue linker. Together, these data suggest that Ca2+/CaM sensitivity in CaMKII is homolog dependent and includes substantial contributions from the hub domain. Our sequencing approach, combined with biochemistry, provides insights into understanding the complex pool of endogenous CaMKII splice variants.

Phosphorylation of α-dystrobrevin is essential for αkap accumulation and acetylcholine receptor stability

The maintenance of a high density of the acetylcholine receptor (AChR) is the hallmark of the neuromuscular junction. Muscle-specific anchoring protein (αkap) encoded within the calcium/calmodulin-dependent protein kinase IIα (CAMK2A) gene is essential for the maintenance of AChR clusters both in vivo and in cultured muscle cells. The underlying mechanism by which αkap is maintained and regulated remains unknown. Here, using human cell lines, fluorescence microscopy, and pulldown and immunoblotting assays, we show that α-dystrobrevin (α-dbn), an intracellular component of the dystrophin glycoprotein complex, directly and robustly promotes the stability of αkap in a concentration-dependent manner. Mechanistically, we found that the phosphorylatable tyrosine residues of α-dbn are essential for the stability of α-dbn itself and its interaction with αkap, with substitution of three tyrosine residues in the α-dbn C terminus with phenylalanine compromising the αkap-α-dbn interaction and significantly reducing both αkap and α-dbn accumulation. Moreover, the αkap-α-dbn interaction was critical for αkap accumulation and stability. We also found that the absence of either αkap or α-dbn markedly reduces AChRα accumulation and that overexpression of α-dbn or αkap in cultured muscle cells promotes the formation of large agrin-induced AChR clusters. Collectively, these results indicate that the stability of αkap and α-dbn complex plays an important role in the maintenance of high-level expression of AChRs.

Functional implications of CaMKII alternative splicing

Ca²⁺ /calmodulin-dependent protein kinase II (CaMKII) is known to be a crucial regulator in the post-synapse during long-term potentiation. This important protein has been the subject of many studies centered on understanding memory at the molecular, cellular, and organismic level. CaMKII is encoded by four genes in humans, all of which undergo alternative splicing at the RNA level, leading to an enormous diversity of expressed proteins. Advances in sequencing technologies have facilitated the discovery of many new CaMKII transcripts. To date, newly discovered CaMKII transcripts have been incorporated into an ambiguous naming scheme. Herein, we review the initial experiments leading to the discovery of CaMKII and its subsequent variants. We propose the adoption of a new, unambiguous naming scheme for CaMKII variants. Finally, we discuss biological implications for CaMKII splice variants.

Regulation of Multifunctional Calcium/Calmodulin Stimulated Protein Kinases by Molecular Targeting

Multifunctional calcium/calmodulin-stimulated protein kinases control a broad range of cellular functions in a multitude of cell types. This family of kinases contain several structural similarities and all are regulated by phosphorylation, which either activates, inhibits or modulates their kinase activity. As these protein kinases are widely or ubiquitously expressed, and yet regulate a broad range of different cellular functions, additional levels of regulation exist that control these cell-specific functions. Of particular importance for this specificity of function for multifunctional kinases is the expression of specific binding proteins that mediate molecular targeting. These molecular targeting mechanisms allow pools of kinase in different cells, or parts of a cell, to respond differently to activation and produce different functional outcomes.

CaM Kinase: Still Inspiring at 40

The Ca²⁺/calmodulin (CaM)-dependent protein kinase II (CaMKII) was touted as a memory molecule, even before its involvement in long-term potentiation (LTP) was shown. The enzyme has not disappointed, with subsequent demonstrations of remarkable structural and regulatory properties. Its neuronal functions now extend to long-term depression (LTD), and last year saw the first direct evidence for memory storage by CaMKII. Although CaMKII may have taken the spotlight, it is a member of a large family of diverse and interesting CaM kinases. Our aim is to place CaMKII in context of the other CaM kinases and then review certain aspects of this kinase that are of current interest.

Cardiac CaMKII activation promotes rapid translocation to its extra-dyadic targets

Calcium-calmodulin dependent protein kinase IIδ (CaMKIIδ) is an important regulator of cardiac electrophysiology, calcium (Ca) balance, contraction, transcription, arrhythmias and progression to heart failure. CaMKII is readily activated at mouths of dyadic cleft Ca channels, but because of its low Ca-calmodulin affinity and presumed immobility it is less clear how CaMKII gets activated near other known, extra-dyad targets. CaMKII is typically considered to be anchored in cardiomyocytes, but while untested, mobility of active CaMKII could provide a mechanism for broader target phosphorylation in cardiomyocytes. We therefore tested CaMKII mobility and how this is affected by kinase activation in adult rabbit cardiomyocytes. We measured translocation of both endogenous and fluorescence-tagged CaMKII using immunocytochemistry, fluorescence recovery after photobleach (FRAP) and photoactivation of fluorescence. In contrast to the prevailing view that CaMKII is anchored near its myocyte targets, we found CaMKII to be highly mobile in resting myocytes, which was slowed by Ca chelation and accelerated by pacing. At low [Ca], CaMKII was concentrated at Z-lines near the dyad but spread throughout the sarcomere upon pacing. Nuclear exchange of CaMKII was also enhanced upon pacing- and heart failure-induced chronic activation. This mobilization of active CaMKII and its intrinsic memory may allow CaMKII to be activated in high [Ca] regions and then move towards more distant myocyte target sites.

Striatal dopamine 2 receptor upregulation during development predisposes to diet-induced obesity by reducing energy output in mice

Dopaminergic signaling in the striatum, particularly at dopamine 2 receptors (D2R), has been a topic of active investigation in obesity research in the past decades. However, it still remains unclear whether variations in striatal D2Rs modulate the risk for obesity and if so in which direction. Human studies have yielded contradictory findings that likely reflect a complex nonlinear relationship, possibly involving a combination of causal effects and compensatory changes. Animal work indicates that although chronic obesogenic diets reduce striatal D2R function, striatal D2R down-regulation does not lead to obesity. In this study, we evaluated the consequences of striatal D2R up-regulation on body-weight gain susceptibility and energy balance in mice. We used a mouse model of D2R overexpression (D2R-OE) in which D2Rs were selectively up-regulated in striatal medium spiny neurons. We uncover a pathological mechanism by which striatal D2R-OE leads to reduced brown adipose tissue thermogenesis, reduced energy expenditure, and accelerated obesity despite reduced eating. We also show that D2R-OE restricted to development is sufficient to promote obesity and to induce energy-balance deficits. Together, our findings indicate that striatal D2R-OE during development persistently increases the propensity for obesity by reducing energy output in mice. This suggests that early alterations in the striatal dopamine system could represent a key predisposition factor toward obesity.

Analysis of the CaMKIIα and β splice-variant distribution among brain regions reveals isoform-specific differences in holoenzyme formation

Four CaMKII isoforms are encoded by distinct genes, and alternative splicing within the variable linker-region generates additional diversity. The α and β isoforms are largely brain-specific, where they mediate synaptic functions underlying learning, memory and cognition. Here, we determined the α and β splice-variant distribution among different mouse brain regions. Surprisingly, the nuclear variant αB was detected in all regions, and even dominated in hypothalamus and brain stem. For CaMKIIβ, the full-length variant dominated in most regions (with higher amounts of minor variants again seen in hypothalamus and brain stem). The mammalian but not fish CaMKIIβ gene lacks exon v3_N that encodes the nuclear localization signal in α_B, but contains three exons not found in the CaMKIIα gene (exons v1, v4, v5). While skipping of exons v1 and/or v5 generated the minor splice-variants β’, βe and βe’, essentially all transcripts contained exon v4. However, we instead detected another minor splice-variant (now termed βH), which lacks part of the hub domain that mediates formation of CaMKII holoenzymes. Surprisingly, in an optogenetic cellular assay of protein interactions, CaMKIIβH was impaired for binding to the β hub domain, but still bound CaMKIIα. This provides the first indication for isoform-specific differences in holoenzyme formation.

CaMKII regulates the depalmitoylation and synaptic removal of the scaffold protein AKAP79/150 to mediate structural long-term depression

Both long-term potentiation (LTP) and depression (LTD) of excitatory synapse strength require the Ca²⁺/calmodulin (CaM)-dependent protein kinase II (CaMKII) and its autonomous activity generated by Thr-286 autophosphorylation. Additionally, LTP and LTD are correlated with dendritic spine enlargement and shrinkage that are accompanied by the synaptic accumulation or removal, respectively, of the AMPA-receptor regulatory scaffold protein A-kinase anchoring protein (AKAP) 79/150. We show here that the spine shrinkage associated with LTD indeed requires synaptic AKAP79/150 removal, which in turn requires CaMKII activity. In contrast to normal CaMKII substrates, the substrate sites within the AKAP79/150 N-terminal polybasic membrane-cytoskeletal targeting domain were phosphorylated more efficiently by autonomous compared with Ca²⁺/CaM-stimulated CaMKII activity. This unusual regulation was mediated by Ca²⁺/CaM binding to the substrate sites resulting in protection from phosphorylation in the presence of Ca²⁺/CaM, a mechanism that favors phosphorylation by prolonged, weak LTD stimuli versus brief, strong LTP stimuli. Phosphorylation by CaMKII inhibited AKAP79/150 association with F-actin; it also facilitated AKAP79/150 removal from spines but was not required for it. By contrast, LTD-induced spine removal of AKAP79/150 required its depalmitoylation on two Cys residues within the N-terminal targeting domain. Notably, such LTD-induced depalmitoylation was also blocked by CaMKII inhibition. These results provide a mechanism how CaMKII can indeed mediate not only LTP but also LTD through regulated substrate selection; however, in the case of AKAP79/150, indirect CaMKII effects on palmitoylation are more important than the effects of direct phosphorylation. Additionally, our results provide the first direct evidence for a function of the well-described AKAP79/150 trafficking in regulating LTD-induced spine shrinkage.

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

Ca2+/Calmodulin-Dependent Protein Kinase II in Vascular Smooth Muscle

Ca²⁺-dependent signaling pathways are central regulators of differentiated vascular smooth muscle (VSM) contractile function. In addition, Ca²⁺ signals regulate VSM gene transcription, proliferation, and migration of dedifferentiated or "synthetic" phenotype VSM cells. Synthetic phenotype VSM growth and hyperplasia are hallmarks of pervasive vascular diseases including hypertension, atherosclerosis, postangioplasty/in-stent restenosis, and vein graft failure. The serine/threonine protein kinase Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) is a ubiquitous mediator of intracellular Ca²⁺ signals. Its multifunctional nature, structural complexity, diversity of isoforms, and splice variants all characterize this protein kinase and make study of its activity and function challenging. The kinase has unique autoregulatory mechanisms, and emerging studies suggest that it can function to integrate Ca²⁺ and reactive oxygen/nitrogen species signaling. Differentiated VSM expresses primarily CaMKIIγ and -δ isoforms. CaMKIIγ isoform expression correlates closely with the differentiated phenotype, and some studies link its function to regulation of contractile activity and Ca²⁺ homeostasis. Conversely, synthetic phenotype VSM cells primarily express CaMKIIδ and substantial evidence links it to regulation of gene transcription, proliferation, and migration of VSM in vitro, and vascular hypertrophic and hyperplastic remodeling in vivo. CaMKIIδ and -γ isoforms have opposing functions at the level of cell cycle regulation, proliferation, and VSM hyperplasia in vivo. Isoform switching following vascular injury is a key step in promoting vascular remodeling. Recent availability of genetically engineered mice with smooth muscle deletion of specific isoforms and transgenics expressing an endogenous inhibitor protein (CAMK2N) has enabled a better understanding of CaMKII function in VSM and should facilitate future studies.

Vascular CaMKII: heart and brain in your arteries

First characterized in neuronal tissues, the multifunctional calcium/calmodulin-dependent protein kinase II (CaMKII) is a key signaling component in several mammalian biological systems. Its unique capacity to integrate various Ca(2+) signals into different specific outcomes is a precious asset to excitable and nonexcitable cells. Numerous studies have reported roles and mechanisms involving CaMKII in brain and heart tissues. However, corresponding functions in vascular cell types (endothelium and vascular smooth muscle cells) remained largely unexplored until recently. Investigation of the intracellular Ca(2+) dynamics, their impact on vascular cell function, the regulatory processes involved and more recently the spatially restricted oscillatory Ca(2+) signals and microdomains triggered significant interest towards proteins like CaMKII. Heteromultimerization of CaMKII isoforms (four isoforms and several splice variants) expands this kinase's peculiar capacity to decipher Ca(2+) signals and initiate specific signaling processes, and thus controlling cellular functions. The physiological functions that rely on CaMKII are unsurprisingly diverse, ranging from regulating contractile state and cellular proliferation to Ca(2+) homeostasis and cellular permeability. This review will focus on emerging evidence of CaMKII as an essential component of the vascular system, with a focus on the kinase isoform/splice variants and cellular system studied.

The knockdown of αkap alters the postsynaptic apparatus of neuromuscular junctions in living mice

A muscle-specific nonkinase anchoring protein (αkap), encoded within the calcium/calmodulin kinase II (camk2) α gene, was recently found to control the stability of acetylcholine receptor (AChR) clusters on the surface of cultured myotubes. However, it remains unknown whether this protein has any effect on receptor stability and the maintenance of the structural integrity of neuromuscular synapses in vivo. By knocking down the endogenous expression of αkap in mouse sternomastoid muscles with shRNA, we found that the postsynaptic receptor density was dramatically reduced, the turnover rate of receptors at synaptic sites was significantly increased, and the insertion rates of both newly synthesized and recycled receptors into the postsynaptic membrane were depressed. Moreover, we found that αkap shRNA knockdown impaired synaptic structure as postsynaptic AChR clusters and their associated postsynaptic scaffold proteins within the neuromuscular junction were completely eliminated. These results provide new mechanistic insight into the role of αkap in regulating the stability of the postsynaptic apparatus of neuromuscular synapses.

CaMKII content affects contractile, but not mitochondrial, characteristics in regenerating skeletal muscle

Background

The multi-meric calcium/calmodulin-dependent protein kinase II (CaMKII) is the main CaMK in skeletal muscle and its expression increases with endurance training. CaMK family members are implicated in contraction-induced regulation of calcium handling, fast myosin type IIA expression and mitochondrial biogenesis. The objective of this study was to investigate the role of an increased CaMKII content for the expression of the contractile and mitochondrial phenotype in vivo. Towards this end we attempted to co-express alpha- and beta-CaMKII isoforms in skeletal muscle and characterised the effect on the contractile and mitochondrial phenotype.

Results

Fast-twitch muscle m. gastrocnemius (GM) and slow-twitch muscle m. soleus (SOL) of the right leg of 3-month old rats were transfected via electro-transfer of injected expression plasmids for native α/β CaMKII. Effects were identified from the comparison to control-transfected muscles of the contralateral leg and non-transfected muscles. α/β CaMKII content in muscle fibres was 4-5-fold increased 7 days after transfection. The transfection rate was more pronounced in SOL than GM muscle (i.e. 12.6 vs. 3.5%). The overexpressed α/β CaMKII was functional as shown through increased threonine 287 phosphorylation of β-CaMKII after isometric exercise and down-regulated transcripts COXI, COXIV, SDHB after high-intensity exercise in situ. α/β CaMKII overexpression under normal cage activity accelerated excitation-contraction coupling and relaxation in SOL muscle in association with increased SERCA2, ANXV and fast myosin type IIA/X content but did not affect mitochondrial protein content. These effects were observed on a background of regenerating muscle fibres.

Conclusion

Elevated CaMKII content promotes a slow-to-fast type fibre shift in regenerating muscle but is not sufficient to stimulate mitochondrial biogenesis in the absence of an endurance stimulus.

CaMKII isoforms differ in their specific requirements for regulation by nitric oxide

The Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) mediates physiological and pathological functions by its Ca(2+)-independent autonomous activity. Two novel mechanisms for generating CaMKII autonomy include oxidation and S-nitrosylation, the latter requiring both Cys280 and Cys289 amino acid residues in the brain-specific isoform CaMKIIα. Even though the other CaMKII isoforms have a different amino acid in the position homologous to Cys280, we show here that nitric oxide (NO)-signaling generated autonomy also for the CaMKIIβ isoform. Furthermore, although oxidation of the Met280/281 residues is sufficient to generate autonomy for most CaMKII isoforms, oxidation-induced autonomy was also prevented by a Cys289-mutation in the CaMKIIα isoform. Thus, all CaMKII isoforms can be regulated by physiological NO-signaling, but CaMKIIα regulation by oxidation and S-nitrosylation is more stringent.

Muscle-type specific autophosphorylation of CaMKII isoforms after paced contractions

We explored to what extent isoforms of the regulator of excitation-contraction and excitation-transcription coupling, calcium/calmodulin protein kinase II (CaMKII) contribute to the specificity of myocellular calcium sensing between muscle types and whether concentration transients in its autophosphorylation can be simulated. CaMKII autophosphorylation at Thr287 was assessed in three muscle compartments of the rat after slow or fast motor unit-type stimulation and was compared against a computational model (CaMuZclE) coupling myocellular calcium dynamics with CaMKII Thr287 phosphorylation. Qualitative differences existed between fast- (gastrocnemius medialis) and slow-type muscle (soleus) for the expression pattern of CaMKII isoforms. Phospho-Thr287 content of δA CaMKII, associated with nuclear functions, demonstrated a transient and compartment-specific increase after excitation, which contrasted to the delayed autophosphorylation of the sarcoplasmic reticulum-associated βM CaMKII. In soleus muscle, excitation-induced δA CaMKII autophosphorylation demonstrated frequency dependence (P = 0.02). In the glycolytic compartment of gastrocnemius medialis, CaMKII autophosphorylation after excitation was blunted. In silico assessment emphasized the importance of mitochondrial calcium buffer capacity for excitation-induced CaMKII autophosphorylation but did not predict its isoform specificity. The findings expose that CaMKII autophosphorylation with paced contractions is regulated in an isoform and muscle type-specific fashion and highlight properties emerging for phenotype-specific regulation of CaMKII.

Degeneration of neuromuscular junction in age and dystrophy

Functional denervation is a hallmark of aging sarcopenia as well as of muscular dystrophy. It is thought to be a major factor reducing skeletal muscle mass, particularly in the case of sarcopenia. Neuromuscular junctions (NMJs) serve as the interface between the nervous and skeletal muscular systems, and thus they may receive pathophysiological input of both pre- and post-synaptic origin. Consequently, NMJs are good indicators of motor health on a systemic level. Indeed, upon sarcopenia and dystrophy, NMJs morphologically deteriorate and exhibit altered characteristics of primary signaling molecules, such as nicotinic acetylcholine receptor and agrin. Since a remarkable reversibility of these changes can be observed by exercise, there is significant interest in understanding the molecular mechanisms underlying synaptic deterioration upon aging and dystrophy and how synapses are reset by the aforementioned treatments. Here, we review the literature that describes the phenomena observed at the NMJ in sarcopenic and dystrophic muscle as well as to how these alterations can be reversed and to what extent. In a second part, the current information about molecular machineries underlying these processes is reported.

A role for the calmodulin kinase II-related anchoring protein (αkap) in maintaining the stability of nicotinic acetylcholine receptors

αkap, a muscle specific anchoring protein encoded within the Camk2a gene, is thought to play a role in targeting multiple calcium/calmodulin kinase II isoforms to specific subcellular locations. Here we demonstrate a novel function of αkap in stabilizing nicotinic acetylcholine receptors (AChRs). Knockdown of αkap expression with shRNA significantly enhanced the degradation of AChR α-subunits (AChRα), leading to fewer and smaller AChR clusters on the surface of differentiated C2C12 myotubes. Mutagenesis and biochemical studies in HEK293T cells revealed that αkap promoted AChRα stability by a ubiquitin-dependent mechanism. In the absence of αkap, AChRα was heavily ubiquitinated, and the number of AChRα was increased by proteasome inhibitors. However, in the presence of αkap, AChRα was less ubiquitinated and proteasome inhibitors had almost no effect on AChRα accumulation. The major sites of AChRα ubiquitination reside within the large intracellular loop and mutations of critical lysine residues in this loop to arginine increased AChRα stability in the absence of αkap. These results provide an unexpected mechanism by which αkap controls receptor trafficking onto the surface of muscle cells and thus the maintenance of postsynaptic receptor density and synaptic function.

The role of molecular regulation and targeting in regulating calcium/calmodulin stimulated protein kinases

Calcium/calmodulin-stimulated protein kinases can be classified as one of two types - restricted or multifunctional. This family of kinases contains several structural similarities: all possess a calmodulin binding motif and an autoinhibitory region. In addition, all of the calcium/calmodulin-stimulated protein kinases examined in this chapter are regulated by phosphorylation, which either activates or inhibits their kinase activity. However, as the multifunctional calcium/calmodulin-stimulated protein kinases are ubiquitously expressed, yet regulate a broad range of cellular functions, additional levels of regulation that control these cell-specific functions must exist. These additional layers of control include gene expression, signaling pathways, and expression of binding proteins and molecular targeting. All of the multifunctional calcium/calmodulin-stimulated protein kinases examined in this chapter appear to be regulated by these additional layers of control, however, this does not appear to be the case for the restricted kinases.

The role of in vivo Ca²⁺ signals acting on Ca²⁺-calmodulin-dependent proteins for skeletal muscle plasticity

Skeletal muscle fibres are highly heterogeneous regarding size, metabolism and contractile function. They also show a large capacity for adaptations in response to alterations in the activation pattern. A major part of this activity-dependent plasticity relies on transcriptional alterations controlled by intracellular Ca(2+) signals. In this review we discuss how intracellular Ca(2+) fluctuations induced by activation patterns likely to occur in vivo control muscle properties via effects on Ca(2+)-calmodulin-dependent proteins. We focus on two such Ca(2+) decoders: calcineurin and Ca(2+)-calmodulin-dependent protein kinase II. Inherent Ca(2+) transients during contractions differ rather little between slow- and fast-twitch muscle fibres and this difference is unlikely to have any significant impact on the activity of Ca(2+) decoders. The major exception to this is fatigue-induced changes in Ca(2+) transients that occur in fast-twitch fibres exposed to high-intensity activation typical of slow-twitch motor units. In conclusion, the cascade from neural stimulation pattern to Ca(2+)-dependent transcription is likely to be central in maintaining the fibre phenotypes in both fast- and slow-twitch fibres. Moreover, changes in Ca(2+) signalling (e.g. induced by endurance training) can result in altered muscle properties (e.g. increased mitochondrial biogenesis) and this plasticity involves other signalling pathways.

CaMKII in cerebral ischemia

Ischemic insults on neurons trigger excessive, pathological glutamate release that causes Ca²⁺ overload resulting in neuronal cell death (excitotoxicity). The Ca²⁺/calmodulin (CaM)-dependent protein kinase II (CaMKII) is a major mediator of physiological excitatory glutamate signals underlying neuronal plasticity and learning. Glutamate stimuli trigger autophosphorylation of CaMKII at T286, a process that makes the kinase "autonomous" (partially active independent from Ca²⁺ stimulation) and that is required for forms of synaptic plasticity. Recent studies suggested autonomous CaMKII activity also as potential drug target for post-insult neuroprotection, both after glutamate insults in neuronal cultures and after focal cerebral ischemia in vivo. However, CaMKII and other members of the CaM kinase family have been implicated in regulation of both neuronal death and survival. Here, we discuss past findings and possible mechanisms of CaM kinase functions in excitotoxicity and cerebral ischemia, with a focus on CaMKII and its regulation.

Calcium/calmodulin kinase II-dependent acetylcholine receptor cycling at the mammalian neuromuscular junction in vivo

At the mammalian skeletal neuromuscular junction, cycling of nicotinic ACh receptors (nAChRs) is critical for the maintenance of a high postsynaptic receptor density. However, the mechanisms that regulate nAChRs recycling in living animals remain unknown. Using in vivo time-lapse imaging, fluorescence recovery after photobleaching, and biochemical pull down assays, we demonstrated that recycling of internalized nAChRs into fully functional and denervated synapses was promoted by both direct muscle stimulation and pharmacologically induced intracellular calcium elevations. Most of internalized nAChRs are recycled directly into synaptic sites. Chelating of intracellular calcium below resting level drastically decreased cycling of nAChRs. Furthermore we found that calcium-dependent AChR recycling is mediated by Ca(2+)/calmodulin-dependent kinase II (CaMKII). Inhibition of CaMKII selectively blocked recycling and caused intracellular accumulation of internalized nAChRs, whereas internalization of surface receptors remained unaffected. Electroporation of CaMKII-GFP isoforms into the sternomastoid muscle showed that muscle-specific CaMKIIβm isoform is highly expressed at the neuromuscular junction (NMJ) and precisely colocalized with nAChRs at crests of synaptic folds while the CaMKIIγ and δ isoforms are poorly expressed in synaptic sites. These results indicate that Ca(2+) along with CaMKII activity are critical for receptor recycling and may provide a mechanism by which the postsynaptic AChR density is maintained at the NMJ in vivo.

Mouse Emi2 as a distinctive regulatory hub in second meiotic metaphase

The oocytes of vertebrates are typically arrested at metaphase II (mII) by the cytostatic factor Emi2 until fertilization. Regulatory mechanisms in Xenopus Emi2 (xEmi2) are understood in detail but contrastingly little is known about the corresponding mechanisms in mammals. Here, we analyze Emi2 and its regulatory neighbours at the molecular level in intact mouse oocytes. Emi2, but not xEmi2, exhibited nuclear targeting. Unlike xEmi2, separable N- and C-terminal domains of mouse Emi2 modulated metaphase establishment and maintenance, respectively, through indirect and direct mechanisms. The C-terminal activity was mapped to the potential phosphorylation target Tx(5)SxS, a destruction box (D-box), a lattice of Zn(2+)-coordinating residues and an RL domain. The minimal region of Emi2 required for its cytostatic activity was mapped to a region containing these motifs, from residue 491 to the C terminus. The cytostatic factor Mos-MAPK promoted Emi2-dependent metaphase establishment, but Mos autonomously disappeared from meiotically competent mII oocytes. The N-terminal Plx1-interacting phosphodegron of xEmi2 was apparently shifted to within a minimal fragment (residues 51-300) of mouse Emi2 that also contained a calmodulin kinase II (CaMKII) phosphorylation motif and which was efficiently degraded during mII exit. Two equimolar CaMKII gamma isoform variants were present in mII oocytes, neither of which phosphorylated Emi2 in vitro, consistent with the involvement of additional factors. No evidence was found that calcineurin is required for mouse mII exit. These data support a model in which mammalian meiotic establishment, maintenance and exit converge upon a modular Emi2 hub via evolutionarily conserved and divergent mechanisms.

The activation of membrane targeted CaMK-II in the zebrafish Kupffer's vesicle is required for left-right asymmetry

Intracellular calcium ion (Ca2+) elevation on the left side of the mouse embryonic node or zebrafish Kupffer's vesicle (KV) is the earliest asymmetric molecular event that is functionally linked to lateral organ placement in these species. In this study, Ca2+/CaM-dependent protein kinase (CaMK-II) is identified as a necessary target of this Ca2+ elevation in zebrafish embryos. CaMK-II is transiently activated in approximately four interconnected cells along the anterior left wall of the KV between the six- and 12-somite stages, which is coincident with known left-sided Ca2+ elevations. Within these cells, activated CaMK-II is observed at the surface and in clusters, which appear at the base of some KV cilia. Although seven genes encode catalytically active CaMK-II in early zebrafish embryos, one of these genes also encodes a truncated inactive variant (αKAP) that can hetero-oligomerize with and target active enzyme to membranes. αKAP, β2 CaMK-II and γ1 CaMK-II antisense morpholino oligonucleotides, as well as KV-targeted dominant negative CaMK-II, randomize organ laterality and southpaw (spaw) expression in lateral plate mesoderm (LPM). Left-sided CaMK-II activation was most dependent on an intact KV, the PKD2 Ca2+ channel and γ1 CaMK-II; however, αKAP, β2 CaMK-II and the RyR3 ryanodine receptor were also necessary for full CaMK-II activation. This is the first report to identify a direct Ca2+-sensitive target in left-right asymmetry and supports a model in which membrane targeted CaMK-II hetero-oligomers in nodal cells transduce the left-sided PKD2-dependent Ca2+ signals to the LPM.

Effects of zofenopril on cardiac sarcoplasmic reticulum calcium handling

Isolated rat hearts were perfused for 120 minutes in the presence or in the absence of 10 microM zofenoprilat, the active metabolite of zofenopril. At the end of perfusion, cardiac tissue was used to assay sarcoplasmic reticulum (SR) (45)Ca uptake and SR calcium release, which was determined by automatized quick filtration technique after SR vesicle loading with (45)Ca. The expression of genes involved in the control of calcium homeostasis was evaluated by polymerase chain reaction after reverse transcription. In chronic experiments, SR (45)Ca uptake and gene expression were measured in hearts derived from rats treated with 15 mg*kg(-1)*day(-1) zofenopril for 15 days. Acute or chronic zofenopril administration did not produce any change in contractile performance. In acute experiments, SR (45)Ca uptake was significantly increased after exposure to zofenoprilat. The rate constant of calcium-induced calcium release was slightly although not significantly higher, and the calcium leak measured under conditions promoting SR channel closure was significantly increased. In the chronic model, significant increase in the rate of SR (45)Ca uptake was confirmed. Gene expression was not modified, except for decreased phospholamban expression, which is observed in the acute but not in the chronic model. In conclusion, zofenopril increases SR calcium cycling and stimulates active calcium uptake into the SR.

Regulation of CaMKII by phospho-Thr253 or phospho-Thr286 sensitive targeting alters cellular function

Calcium/calmodulin-stimulated protein kinase II (CaMKII) is an important mediator of synaptic function that is regulated by multi-site phosphorylation and targeting through interactions with proteins. A new phosphorylation site at Thr253 has been identified in vivo, that does not alter CaMKII activity, but does alter CaMKII function through interactions with binding proteins. To identify these proteins, as well as to examine the specific effects following Thr253 or Thr286 phosphorylation on these interactions, we developed an in vitro overlay binding assay. We demonstrated that the interaction between CaMKII and its binding proteins was altered by the phosphorylation state of both the CaMKII and the partner, and identified a CaMKII-specific sequence that was responsible for the interaction between CaMKII and two interacting proteins. By comparing CaMKII binding profiles in tissue and cell extracts, we demonstrated that the CaMKII binding profiles varied with cell type, and also showed that overexpression of a CaMKII Thr253 phospho-mimic mutant in human neuroblastoma and breast cancer cells dramatically altered the morphology and growth rates when compared to overexpression of non-phosphorylated CaMKII. This data highlights the importance of the microenvironment in regulating CaMKII function, and describes a potentially new mechanism by which the functions of CaMKII can be regulated.

Alpha-kinase anchoring protein alphaKAP interacts with SERCA2A to spatially position Ca2+/calmodulin-dependent protein kinase II and modulate phospholamban phosphorylation

The sarco-endoplasmic reticulum calcium ATPase 2a (SERCA2a) is critical for sequestering cytosolic calcium into the sarco-endoplasmic reticulum (SR) and regulating cardiac muscle relaxation. Protein-protein interactions indicated that it exists in complex with Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and its anchoring protein alphaKAP. Confocal imaging of isolated cardiomyocytes revealed the colocalization of CAMKII and alphaKAP with SERCA2a at the SR. Deletion analysis indicated that SERCA2a and CaMKII bind to different regions in the association domain of alphaKAP but not with each other. Although deletion of the putative N-terminal hydrophobic amino acid stretch in alphaKAP prevented its membrane targeting, it did not influence binding to SERCA2a or CaMKII. Both CaMKIIdelta(C) and the novel CaMKIIbeta(4) isoforms were found to exist in complex with alphaKAP and SERCA2a at the SR and were able to phosphorylate Thr-17 on phospholamban (PLN), an accessory subunit and known regulator of SERCA2a activity. Interestingly, the presence of alphaKAP was also found to significantly modulate the Ca(2+)/calmodulin-dependent phosphorylation of Thr-17 on PLN. These data demonstrate that alphaKAP exhibits a novel interaction with SERCA2a and may serve to spatially position CaMKII isoforms at the SR and to uniquely modulate the phosphorylation of PLN.

Supramolecular assemblies and localized regulation of voltage-gated ion channels

This review addresses the localized regulation of voltage-gated ion channels by phosphorylation. Comprehensive data on channel regulation by associated protein kinases, phosphatases, and related regulatory proteins are mainly available for voltage-gated Ca2+ channels, which form the main focus of this review. Other voltage-gated ion channels and especially Kv7.1-3 (KCNQ1-3), the large- and small-conductance Ca2+-activated K+ channels BK and SK2, and the inward-rectifying K+ channels Kir3 have also been studied to quite some extent and will be included. Regulation of the L-type Ca2+ channel Cav1.2 by PKA has been studied most thoroughly as it underlies the cardiac fight-or-flight response. A prototypical Cav1.2 signaling complex containing the beta2 adrenergic receptor, the heterotrimeric G protein Gs, adenylyl cyclase, and PKA has been identified that supports highly localized via cAMP. The type 2 ryanodine receptor as well as AMPA- and NMDA-type glutamate receptors are in close proximity to Cav1.2 in cardiomyocytes and neurons, respectively, yet independently anchor PKA, CaMKII, and the serine/threonine phosphatases PP1, PP2A, and PP2B, as is discussed in detail. Descriptions of the structural and functional aspects of the interactions of PKA, PKC, CaMKII, Src, and various phosphatases with Cav1.2 will include comparisons with analogous interactions with other channels such as the ryanodine receptor or ionotropic glutamate receptors. Regulation of Na+ and K+ channel phosphorylation complexes will be discussed in separate papers. This review is thus intended for readers interested in ion channel regulation or in localization of kinases, phosphatases, and their upstream regulators.

Regulation of CaMKII in vivo: the importance of targeting and the intracellular microenvironment

CaMKII (calcium/calmodulin-stimulated protein kinase II) is a multifunctional protein kinase that regulates normal neuronal function. CaMKII is regulated by multi-site phosphorylation, which can alter enzyme activity, and targeting to cellular microdomains through interactions with binding proteins. These proteins integrate CaMKII into multiple signalling pathways, which lead to varied functional outcomes following CaMKII phosphorylation, depending on the identity and location of the binding partner. A new phosphorylation site on CaMKII (Thr253) has been identified in vivo. Thr253 phosphorylation controls CaMKII purely by targeting, does not effect enzyme activity, and occurs in response to physiological and pathological stimuli in vivo, but only in CaMKII molecules present in specific cellular locations. This new phosphorylation site offers a potentially novel regulatory mechanism for controlling functional responses elicited by CaMKII that are restricted to specific subcellular locations and/or certain cell types, by controlling interactions with proteins that are expressed in the cell at that location.

Novel role of calpain-3 in the triad-associated protein complex regulating calcium release in skeletal muscle

Calpain-3 (CAPN3) is a non-lysosomal cysteine protease that is necessary for normal muscle function, as mutations in CAPN3 result in an autosomal recessive form of limb girdle muscular dystrophy type 2A. To elucidate the biological roles of CAPN3 in skeletal muscle, we performed a search for potential substrates and interacting partners. By yeast-two-hybrid analysis we identified the glycolytic enzyme aldolase A (AldoA) as a binding partner of CAPN3. In co-expression studies CAPN3 degraded AldoA; however, no accumulation of AldoA was observed in total extracts from CAPN3-deficient muscles suggesting that AldoA is not an in vivo substrate of CAPN3. Instead, we found CAPN3 to be necessary for recruitment of AldoA to one specific location, namely the triads, which are structural components of muscle responsible for calcium transport and excitation-contraction coupling. Both aldolase and CAPN3 are present in the triad-enriched fraction and are able to interact with ryanodine receptors (RyR) that form major calcium release channels. Levels of triad-associated AldoA and RyR were decreased in CAPN3-deficient muscles compared with wild-type. Consistent with these observations we found calcium release to be significantly reduced in fibers from CAPN3-deficient muscles. Together, these data suggest that CAPN3 is necessary for the structural integrity of the triad-associated protein complex and that impairment of calcium transport is a phenotypic feature of CAPN3-deficient muscle.

Chronic antidepressants induce redistribution and differential activation of alphaCaM kinase II between presynaptic compartments

Changes in synaptic plasticity are involved in pathophysiology of depression and in the mechanism of antidepressants. Ca(2+)/calmodulin (CaM) kinase II, a protein kinase involved in synaptic plasticity, has been previously shown to be a target of antidepressants. We previously found that antidepressants activate the kinase in hippocampal neuronal cell bodies by increasing phosphorylation at Thr(286), reduce the kinase phosphorylation in synaptic membranes, and in turn its phosphorylation-dependent interaction with syntaxin-1 and the release of glutamate from hippocampal synaptosomes. Here, we investigated the chronic effect of different antidepressants (fluoxetine, desipramine, and reboxetine) on the expression and function of the kinase in distinct subcellular compartments in order to dissect the different kinase pools affected. Acute treatments did not induce any change in the kinase. In total tissue extracts chronic drug treatments induced activation of the kinase; in hippocampus (HC), but not in prefrontal/frontal cortex, this was partially accounted for by increased Thr(286) phosphorylation, suggesting the involvement of different mechanisms of activation. In synaptosomes, all drugs reduced the kinase phosphorylation, particularly in HC where, upon fractionation of the synaptosomal particulate into synaptic vesicles and membranes, we found that the drugs induced a redistribution and differential activation of the kinase between membranes and vesicles. Furthermore, a large decrease in the level and phosphorylation of synapsin I located at synaptic membranes was consistent with the observed decrease of CaM kinase II. Overall, antidepressants induce a complex pattern of modifications in distinct subcellular compartments; at presynaptic level, these changes are in line with a dampening of glutamate release.

Activation of Ca(2+)-dependent protein kinase II during repeated contractions in single muscle fibres from mouse is dependent on the frequency of sarcoplasmic reticulum Ca(2+) release

AIM

To investigate the importance and contribution of calmodulin-dependent protein kinase II (CaMKII) activity on sarcoplasmic reticulum (SR) Ca(2+)-release in response to different work intensities in single, intact muscle fibres.

METHODS

CaMKII activity was blocked in single muscle fibres using either the inhibitory peptide AC3-I or the pharmacological inhibitor KN-93. The effect on tetanic force production and [Ca(2+)](i) was determined during work of different intensities. The activity of CaMKII was assessed by mathematical modelling.

RESULTS

Using a standard protocol to induce fatigue (50x 70 Hz, 350 ms duration, every 2 s) the number of stimuli needed to induce fatigue was decreased from 47 +/- 3 contractions in control to 33 +/- 3 with AC3-I. KN-93 was a more potent inhibitor, decreasing the number of contractions needed to induce fatigue to 15 +/- 3. Tetanic [Ca(2+)](i) was 100 +/- 11%, 97 +/- 11% and 67 +/- 11% at the end of stimulation in control, AC3-I and KN-93 respectively. A similar inhibition was obtained using a high intensity protocol (20x 70 Hz, 200 ms duration, every 300 ms). However, using a long interval protocol (25x 70 Hz, 350 ms duration, every 5 s) no change was observed in either tetanic [Ca(2+)](i) or force when inhibiting CaMKII. A mathematical model used to investigate the activation pattern of CaMKII suggests that there is a threshold of active CaMKII that has to be surpassed in order for CaMKII to affect SR Ca(2+) release.

CONCLUSION

Our results show that CaMKII is crucial for maintaining proper SR Ca(2+) release and that this is regulated in a work intensity manner.

Effect of endurance exercise training on Ca2+ calmodulin-dependent protein kinase II expression and signalling in skeletal muscle of humans

Here the hypothesis that skeletal muscle Ca(2+)-calmodulin-dependent kinase II (CaMKII) expression and signalling would be modified by endurance training was tested. Eight healthy, young men completed 3 weeks of one-legged endurance exercise training with muscle samples taken from both legs before training and 15 h after the last exercise bout. Along with an approximately 40% increase in mitochondrial F(1)-ATP synthase expression, there was an approximately 1-fold increase in maximal CaMKII activity and CaMKII kinase isoform expression after training in the active leg only. Autonomous CaMKII activity and CaMKII autophosphorylation were increased to a similar extent. However, there was no change in alpha-CaMKII anchoring protein expression with training. Nor was there any change in expression or Thr(17) phosphorylation of the CaMKII substrate phospholamban with training. However, another CaMKII substrate, serum response factor (SRF), had an approximately 60% higher phosphorylation at Ser(103) after training, with no change in SRF expression. There were positive correlations between the increases in CaMKII expression and SRF phosphorylation as well as F(1)ATPase expression with training. After training, there was an increase in cyclic-AMP response element binding protein phosphorylation at Ser(133), but not expression, in muscle of both legs. Taken together, skeletal muscle CaMKII kinase isoform expression and SRF phosphorylation is higher with endurance-type exercise training, adaptations that are restricted to active muscle. This may contribute to greater Ca(2+) mediated regulation during exercise and the altered muscle phenotype with training.

Differential expression of CaMK-II genes during early zebrafish embryogenesis

CaMK-II is a highly conserved Ca(2+)/calmodulin-dependent protein kinase expressed throughout the lifespan of all vertebrates. During early development, CaMK-II regulates cell cycle progression and "non-canonical" Wnt-dependent convergent extension. In the zebrafish, Danio rerio, CaMK-II activity rises within 2 hr after fertilization. At the time of somite formation, zygotic expression from six genes (camk2a1, camk2b1, camk2g1, camk2g2, camk2d1, camk2d2) results in a second phase of increased activity. Zebrafish CaMK-II genes are 92-95% identical to their human counterparts in the non-variable regions. During the first three days of development, alternative splicing yields at least 20 splice variants, many of which are unique. Whole-mount in situ hybridization reveals that camk2g1 comprises the majority of maternal expression. All six genes are expressed strongly in ventral regions at the 18-somite stage. Later, camk2a1 is expressed in anterior somites, heart, and then forebrain. Camk2b1 is expressed in somites, mid- and forebrain, gut, retina, and pectoral fins. Camk2g1 appears strongly along the midline and then in brain, gut, and pectoral fins. Camk2g2 is expressed early in the midbrain and trunk and exhibits the earliest retinal expression. Camk2d1 is elevated early at somite boundaries, then epidermal tissue, while camk2d2 is expressed in discrete anterior locations, steadily increasing along either side of the dorsal midline and then throughout the brain, including the retina. These findings reveal a complex pattern of CaMK-II gene expression consistent with pleiotropic roles during development.

Roles of the calcineurin and CaMK signaling pathways in fast-to-slow fiber type transformation of cultured adult mouse skeletal muscle fibers

Two Ca2+-dependent signaling pathways, mediated by the Ca2+-activated phosphatase calcineurin and by the Ca2+-activated kinase Ca2+/calmodulin-dependent kinase (CaMK), are both believed to function in fast-to-slow skeletal muscle fiber type transformation, but questions about the relative importance of the two pathways still remain. Here, the differential gene expression during fast-to-slow fiber type transformation was studied using cultured adult flexor digitorum brevis (FDB) fibers and a custom minimicroarray system containing 21 fiber type-specific marker genes. After 3 days of culture, unstimulated fibers showed a generally slower gene expression profile; 3 days of electric field stimulation of cultured FDB fibers with a slow fiber-type pattern transformed the fibers to an even slower gene expression profile. Unstimulated FDB fibers overexpressing constitutively active calcineurin featured a slower gene expression profile, except four genes, indicating that transformation occurred, but was incomplete with activation of the calcineurin pathway alone. In both unstimulated FDB fibers and slow-type electrically stimulated FDB fibers, blocking of CaMK pathway with KN93 generated a faster gene expression profile compared with the negative control KN92, indicating that CaMK pathway functions during the transformation induced by both unstimulated culturing and slow fiber-type electrical stimulation. Moreover, neither the calcineurin nor the CaMK pathway alone could maximally activate the transformation, and coordination of the two pathways is required to accomplish a complete fast-to-slow fiber type transformation.

Multiple downstream proarrhythmic targets for calmodulin kinase II: Moving beyond an ion channel-centric focus

The multifunctional Ca(2+) calmodulin-dependent protein kinase II (CaMKII) has emerged as a pro-arrhythmic signaling molecule. CaMKII can participate in arrhythmia signaling by effects on ion channel proteins, intracellular Ca(2+) uptake and release, regulation of cell death, and by activation of hypertrophic signaling pathways. The pleuripotent nature of CaMKII is reminiscent of another serine-threonine kinase, protein kinase A (PKA), which shares many of the same protein targets and is the downstream kinase most associated with beta-adrenergic receptor stimulation. The ability of CaMKII to localize and coordinate activity of multiple protein targets linked to Ca(2+) signaling set CaMKII apart from other "traditional" arrhythmia drug targets, such as ion channel proteins. This review will discuss some of the biology of CaMKII and focus on work that has been done on molecular, cellular, and whole animal models that together build a case for CaMKII as a pro-arrhythmic signal and as a potential therapeutic target for arrhythmias and structural heart disease.

Regulation and function of Ca2+-calmodulin-dependent protein kinase II of fast-twitch rat skeletal muscle

The activation and function of Ca(2+)-calmodulin-dependent kinase II (CaMKII) in contracting rat skeletal muscle was examined. The increase in autonomous activity and phosphorylation at Thr(287) of CaMKII of gastrocnemius muscle in response to contractions in situ was rapid and transient, peaking at 1-3 min, but reversed after 30 min of contractions. There was a positive correlation between CaMKII phosphorylation at Thr(287) and autonomous CaMKII activity. In contrast to the rapid and transient increase in autonomous CaMKII activity, the phosphorylation of the putative CaMKII substrate trisk95/triadin was rapid and sustained during contractions. There were no changes in CaMKII activity and phosphorylation or trisk95 phosphorylation in the resting contralateral muscles during stimulation. When fast-twitch muscles were contracted ex vivo, CaMKII inhibition resulted in a greater magnitude of fatigue as well as blunted CaMKII and trisk95 phosphorylation, identifying trisk95 as a physiological CaMKII substrate. In summary, skeletal muscle CaMKII activation was rapid and sustained during exercise/contraction and is mediated by factors within the contracting muscle, probably through allosteric activation via Ca(2+)-CaM. CaMKII may signal through trisk95 to modulate Ca(2+) release in fast-twitch rat skeletal muscle during exercise/contraction.