Structure and regulation of calcium/calmodulin-dependent protein kinases

Advancements in the study of synaptic plasticity and mitochondrial autophagy relationship

Synapses serve as the points of communication between neurons, consisting primarily of three components: the presynaptic membrane, synaptic cleft, and postsynaptic membrane. They transmit signals through the release and reception of neurotransmitters. Synaptic plasticity, the ability of synapses to undergo structural and functional changes, is influenced by proteins such as growth-associated proteins, synaptic vesicle proteins, postsynaptic density proteins, and neurotrophic growth factors. Furthermore, maintaining synaptic plasticity consumes more than half of the brain's energy, with a significant portion of this energy originating from ATP generated through mitochondrial energy metabolism. Consequently, the quantity, distribution, transport, and function of mitochondria impact the stability of brain energy metabolism, thereby participating in the regulation of fundamental processes in synaptic plasticity, including neuronal differentiation, neurite outgrowth, synapse formation, and neurotransmitter release. This article provides a comprehensive overview of the proteins associated with presynaptic plasticity, postsynaptic plasticity, and common factors between the two, as well as the relationship between mitochondrial energy metabolism and synaptic plasticity.

Transcriptional, biochemical, and immunohistochemical analyses of CaMKKβ/2 splice variants that co-localize with CaMKIV in spermatids

Ca2+/calmodulin-dependent protein kinase kinase (CaMKK) phosphorylates and activates downstream protein kinases, including CaMKI, CaMKIV, PKB/Akt, and AMPK; thus, regulates various Ca2+-dependent physiological and pathophysiological pathways. Further, CaMKKβ/2 in mammalian species comprises multiple alternatively spliced variants; however, their functional differences or redundancy remain unclear. In this study, we aimed to characterize mouse CaMKKβ/2 splice variants (CaMKKβ-3 and β-3x). RT-PCR analyses revealed that mouse CaMKKβ-1, consisting of 17 exons, was predominantly expressed in the brain; whereas, mouse CaMKKβ-3 and β-3x, lacking exon 16 and exons 14/16, respectively, were primarily expressed in peripheral tissues. At the protein level, the CaMKKβ-3 or β-3x variants showed high expression levels in mouse cerebrum and testes. This was consistent with the localization of CaMKKβ-3/-3x in spermatids in seminiferous tubules, but not the localization of CaMKKβ-1. We also observed the co-localization of CaMKKβ-3/-3x with a target kinase, CaMKIV, in elongating spermatids. Biochemical characterization further revealed that CaMKKβ-3 exhibited Ca2+/CaM-induced kinase activity similar to CaMKKβ-1. Conversely, we noted that CaMKKβ-3x impaired Ca2+/CaM-binding ability, but exhibited significantly weak autonomous activity (approximately 500-fold lower than CaMKKβ-1 or β-3) due to the absence of C-terminal of the catalytic domain and a putative residue (Ile478) responsible for the kinase autoinhibition. Nevertheless, CaMKKβ-3x showed the ability to phosphorylate downstream kinases, including CaMKIα, CaMKIV, and AMPKα in transfected cells comparable to CaMKKβ-1 and β-3. Collectively, CaMKKβ-3/-3x were identified as functionally active and could be bona fide CaMKIV-kinases in testes involved in the activation of the CaMKIV cascade in spermatids, resulting in the regulation of spermiogenesis.

Lipidation states orchestrate CLICK-III/CaMKIγ's stepwise association with Golgi and rafts-enriched membranes and specify its functional coupling to STEF-Rac1-dependent neurite extension

CLICK-III/CaMKIγ is a lipid-anchored neuronal isoform of multifunctional Ca2+/calmodulin-dependent protein kinases, which mediates BDNF-dependent dendritogenesis in cultured cortical neurons. We found that two distinct lipidation states of CaMKIγ, namely, prenylation and palmitoylation, controlled its association with detergent-resistant microdomains in the dendrites and were essential for its dendritogenic activity. However, the impact of each lipid modification on membrane targeting/trafficking and how it specifies functional coupling leading to polarized changes in neuronal morphology are not clear. Here, we show that prenylation induces membrane anchoring of CaMKIγ, permitting access to the Golgi apparatus, and a subsequent palmitoylation facilitates association with cholesterol-enriched lipid microdomains or lipid rafts, in particular at the Golgi. To specifically test the role of palmitoylated CaMKγ in neurite extension, we identified and took advantage of a cell system, PC12, which, unlike neurons, conveniently lacked CaMKIγ and was deficient in the activity-dependent release of a neuritogenic growth factor while possessing the ability to activate polarized rafts signaling for morphogenesis. This system allowed us to rigorously demonstrate that an activity-dependent, lipid rafts-restricted Rac activation leading to neuritogenesis could be functionally rescued by dually lipidated CaMKIγ expression, revealing that not only prenylation but also palmitoylation is essential for CaMKIγ to activate a compartmentalized STEF-Rac1 pathway. These results shed light on the significance of recruiting prenylated and palmitoylated CaMKIγ into the coalescing signalosomes at lipid rafts together with Rac1 and its specific GEF and STEF and forming a compartmentalized Ca2+ signaling pathway that underlies activity-dependent neuritogenesis and morphogenesis during axodendritic polarization critical for brain development and circuitogenesis.

Mechanistic insights into the role of calcium in the allosteric regulation of the calmodulin-regulated death-associated protein kinase

Calcium (Ca²⁺) signaling plays an important role in the regulation of many cellular functions. Ca²⁺-binding protein calmodulin (CaM) serves as a primary effector of calcium function. Ca²⁺/CaM binds to the death-associated protein kinase 1 (DAPK1) to regulate intracellular signaling pathways. However, the mechanism underlying the influence of Ca²⁺ on the conformational dynamics of the DAPK1-CaM interactions is still unclear. Here, we performed large-scale molecular dynamics (MD) simulations of the DAPK1-CaM complex in the Ca²⁺-bound and-unbound states to reveal the importance of Ca²⁺. MD simulations revealed that removal of Ca²⁺ increased the anti-correlated inter-domain motions between DAPK1 and CaM, which weakened the DAPK1-CaM interactions. Binding free energy calculations validated the decreased DAPK1-CaM interactions in the Ca²⁺-unbound state. Structural analysis further revealed that Ca²⁺ removal caused the significant conformational changes at the DAPK1-CaM interface, especially the helices α1, α2, α4, α6, and α7 from the CaM and the basic loop and the phosphate-binding loop from the DAPK1. These results may be useful to understand the biological role of Ca²⁺ in physiological processes.

Substrate recognition by Arg/Pro-rich insert domain in calcium/calmodulin-dependent protein kinase kinase for target protein kinases

Calcium/calmodulin-dependent protein kinase kinases (CaMKKs) activate CaMKI, CaMKIV, protein kinase B/Akt, and AMP-activated protein kinase (AMPK) by phosphorylating Thr residues in activation loops to mediate various Ca²⁺ -signaling pathways. Mammalian cells expressing CaMKKα and CaMKKβ lacking Arg/Pro-rich insert domain (RP-domain) sequences showed impaired phosphorylation of AMPKα, CaMKIα, and CaMKIV, whereas the autophosphorylation activities of CaMKK mutants remained intact and were similar to those of wild-type CaMKKs. Liver kinase B1 (LKB1, an AMPK kinase) complexed with STRAD and MO25 and was unable to phosphorylate CaMKIα and CaMKIV; however, mutant LKB1 with the RP-domain sequences of CaMKKα and CaMKKβ inserted between kinase subdomains II and III acquired CaMKIα and CaMKIV phosphorylating activity in vitro and in transfected cultured cells. Furthermore, ionomycin-induced phosphorylation of hemagglutinin (HA)-CaMKIα at Thr177, HA-CaMKIV at Thr196, and HA-AMPKα at Thr172 in transfected cells was significantly suppressed by cotransfection of kinase-dead mutants of CaMKK isoforms, but these dominant-negative effects were abrogated with RP-deletion mutants, suggesting that sequestration of substrate kinases by loss-of-function CaMKK mutants requires the RP-domain. This was confirmed by pulldown experiments that showed that dominant-negative mutants of CaMKKα and CaMKKβ interact with target kinases but not RP-deletion mutants. Taken together, these results clearly indicate that both CaMKK isoforms require the RP-domain to recognize downstream kinases to interact with and phosphorylate Thr residues in their activation loops. Thus, the RP-domain may be a promising target for specific CaMKK inhibitors.

Molecular Mechanisms Underlying Ca2+/Calmodulin-Dependent Protein Kinase Kinase Signal Transduction

Ca²⁺/calmodulin-dependent protein kinase kinase (CaMKK) is the activating kinase for multiple downstream kinases, including CaM-kinase I (CaMKI), CaM-kinase IV (CaMKIV), protein kinase B (PKB/Akt), and 5′AMP-kinase (AMPK), through the phosphorylation of their activation-loop Thr residues in response to increasing the intracellular Ca²⁺ concentration, as CaMKK itself is a Ca²⁺/CaM-dependent enzyme. The CaMKK-mediated kinase cascade plays important roles in a number of Ca²⁺-dependent pathways, such as neuronal morphogenesis and plasticity, transcriptional activation, autophagy, and metabolic regulation, as well as in pathophysiological pathways, including cancer progression, metabolic syndrome, and mental disorders. This review focuses on the molecular mechanism underlying CaMKK-mediated signal transduction in normal and pathophysiological conditions. We summarize the current knowledge of the structural, functional, and physiological properties of the regulatory kinase, CaMKK, and the development and application of its pharmacological inhibitors.

Stimulation of Neurite Outgrowth in Cerebrocortical Neurons by Sodium Channel Activator Brevetoxin-2 Requires Both N-Methyl-D-aspartate Receptor 2B (GluN2B) and p21 Protein (Cdc42/Rac)-Activated Kinase 1 (PAK1)

N-methyl-D-aspartate (NMDA) receptors play a critical role in activity-dependent dendritic arborization, spinogenesis, and synapse formation by stimulating calcium-dependent signaling pathways. Previously, we have shown that brevetoxin 2 (PbTx-2), a voltage-gated sodium channel (VGSC) activator, produces a concentration-dependent increase in intracellular sodium [Na+]I and increases NMDA receptor (NMDAR) open probabilities and NMDA-induced calcium (Ca2+) influxes. The objective of this study is to elucidate the downstream signaling mechanisms by which the sodium channel activator PbTx-2 influences neuronal morphology in murine cerebrocortical neurons. PbTx-2 and NMDA triggered distinct Ca2+-influx pathways, both of which involved the NMDA receptor 2B (GluN2B). PbTx-2-induced neurite outgrowth in day in vitro 1 (DIV-1) neurons required the small Rho GTPase Rac1 and was inhibited by both a PAK1 inhibitor and a PAK1 siRNA. PbTx-2 exposure increased the phosphorylation of PAK1 at Thr-212. At DIV-5, PbTx-2 induced increases in dendritic protrusion density, p-cofilin levels, and F-actin throughout the dendritic arbor and soma. Moreover, PbTx-2 increased miniature excitatory post-synaptic currents (mEPSCs). These data suggest that the stimulation of neurite outgrowth, spinogenesis, and synapse formation produced by PbTx-2 are mediated by GluN2B and PAK1 signaling.

Identification of Candidate Genes Regulating Drought Tolerance in Pearl Millet

Pearl millet is an important crop of the arid and semi-arid ecologies to sustain food and fodder production. The greater tolerance to drought stress attracts us to examine its cellular and molecular mechanisms via functional genomics approaches to augment the grain yield. Here, we studied the drought response of 48 inbreds representing four different maturity groups at the flowering stage. A set of 74 drought-responsive genes were separated into five major phylogenic groups belonging to eight functional groups, namely ABA signaling, hormone signaling, ion and osmotic homeostasis, TF-mediated regulation, molecular adaptation, signal transduction, physiological adaptation, detoxification, which were comprehensively studied. Among the conserved motifs of the drought-responsive genes, the protein kinases and MYB domain proteins were the most conserved ones. Comparative in-silico analysis of the drought genes across millet crops showed foxtail millet had most orthologs with pearl millet. Of 698 haplotypes identified across millet crops, MyC2 and Myb4 had maximum haplotypes. The protein–protein interaction network identified ABI2, P5CS, CDPK, DREB, MYB, and CYP707A3 as major hub genes. The expression assay showed the presence of common as well as unique drought-responsive genes across maturity groups. Drought tolerant genotypes in respective maturity groups were identified from the expression pattern of genes. Among several gene families, ABA signaling, TFs, and signaling proteins were the prospective contributors to drought tolerance across maturity groups. The functionally validated genes could be used as promising candidates in backcross breeding, genomic selection, and gene-editing schemes in pearl millet and other millet crops to increase the yield in drought-prone arid and semi-arid ecologies.

Identification of potential therapeutic and diagnostic characteristics of Alzheimer disease by targeting the miR-132-3p/FOXO3a-PPM1F axis in APP/PS1 mice

Alzheimer disease (AD) is a neurodegenerative disorder, which is characterized by progressive impairment of memory and cognition. Early diagnosis and treatment of AD has become a leading topic of research. In this study, we explored the effects of the miR-132-3p/FOXO3a-PPM1F axis on the onset of AD for possible early diagnosis and therapy. We found that miR-132-3p levels in the hippocampus and blood were drastically decreased in APP/PS1 mice from 9 months of age, and bi-directional manipulation of miR-132-3p levels induced magnified effects on learning memory behaviors, and manifestation of AD-related pathological characteristics and inflammatory cytokines in APP/PS1 mice of relevant ages. The hippocampal PPM1F expression levels were significantly elevated in APP/PS1 mice from 3 months of age, which was correlated with miR-132-3p levels at different ages. Overexpression of PPM1F remarkably accelerated the progression of learning memory deficits and associated pathological factors in APP/PS1 mice. Further, we showed that miR-132-3p modulated the expression of PPM1F via FOXO3a in HT22 cells. Finally, using peripheral blood samples of human study participants, we found that the miR-132-3p and PPM1F expression levels in patients with AD were also altered with prominent correlations. In conclusion, miR-132-3p indirectly regulates PPM1F expression by targeting FOXO3a, which could play an extensive role in contributing to the establishment of early diagnosis, treatment, and pathogenesis of AD.

Conformation-Dependent Reversible Interaction of Ca2+/Calmodulin-Dependent Protein Kinase Kinase with an Inhibitor, TIM-063

Ca²⁺/calmodulin-dependent protein kinase kinase (CaMKK), a Ca²⁺/CaM-dependent enzyme that phosphorylates and activates multifunctional kinases, including CaMKI, CaMKIV, protein kinase B/Akt, and 5'AMP-activated protein kinase, is involved in various Ca²⁺-signaling pathways in cells. Recently, we developed an ATP-competitive CaMKK inhibitor, TIM-063 (2-hydroxy-3-nitro-7H-benzo[de]benzo[4,5]imidazo[2,1-a]isoquinolin-7-one, Ohtsuka et al. Biochemistry 2020, 59, 1701-1710). To gain mechanistic insights into the interaction of CaMKK with TIM-063, we prepared TIM-063-coupled sepharose (TIM-127-sepharose) for association/dissociation analysis of the enzyme/inhibitor complex. CaMKKα/β in transfected COS-7 cells and in mouse brain extracts specifically bound to TIM-127-sepharose and dissociated following the addition of TIM-063 in a manner similar to that of recombinant GST-CaMKKα/β, which could bind to TIM-127-sepharose in a Ca²⁺/CaM-dependent fashion and dissociate from the sepharose following the addition of TIM-063 in a dose-dependent manner. In contrast to GST-CaMKKα, GST-CaMKKβ was able to weakly bind to TIM-127-sepharose in the presence of EGTA, probably due to the partially active conformation of recombinant GST-CaMKKβ without Ca²⁺/CaM-binding. These results suggested that the regulatory domain of CaMKKα prevented the inhibitor from interacting with the catalytic domain as the GST-CaMKKα mutant (residues 126-434) lacking the regulatory domain (residues 438-463) interacted with TIM-127-sepharose regardless of the presence or absence of Ca²⁺/CaM. Furthermore, CaMKKα bound to TIM-127-sepharose in the presence of Ca²⁺/CaM completely dissociated from TIM-127-sepharose following the addition of excess EGTA. These results indicated that TIM-063 interacted with and inhibited CaMKK in its active state but not in its autoinhibited state and that this interaction is likely reversible, depending on the concentration of intracellular Ca²⁺.

Dysregulation of the HOTAIR-miR-152-CAMKIIα Axis in Craniosynostosis Results in Impaired Osteoclast Differentiation

Craniosynostosis is one of the most common craniofacial deformities demanding surgical treatment in infancy. LncRNA HOTAIR has verified its important role in osteogenesis and osteoarthritis. However, whether HOTAIR plays an essential role in the development of craniosynostosis is still unclear. In this study, we aimed to investigate the molecular role of HOTAIR in the osteoclast function and development of craniosynostosis.For osteoclast differentiation, RAW264.7 cells were induced by 50 ng/ml of RANKL and 10 ng/mL M-CSF, followed by TRAP staining. Cell proliferation and apoptosis were assayed by the CCK-8 kit and Annexin V-FITC apoptosis detection kit, respectively. The expression of HOTAIR was determined in PBMCs by qRT-PCR. Protein levels of all those involved genes were measured by Western blot assay. A luciferase reporter assay was used to determine the miRNA target validation. The HOTAIR expression in PBMCs from children with craniosynostosis was significantly downregulated. The results of cell proliferation and apoptosis assays indicated that silencing of HOTAIR could inhibit osteoclast differentiation and increase cell apoptosis. Moreover, the luciferase reporter assay revealed that the regulatory axis and HOTAIR-miR-152-CAMKIIα were the regulatory mechanisms of HOTAIR in the osteoclast function and development of craniosynostosis.In this study, our data showed that HOTAIR could promote osteoclast differentiation by binding miR-152. Furthermore, the HOTAIR/HOTAIR-miR-152-CAMKIIα axis was found to regulate osteoclast differentiation. These results indicate that the HOTAIR plays a crucial role in the development of osteoclasts.

Oligomerization of Ca2+/calmodulin-dependent protein kinase kinase

Ca²⁺/calmodulin-dependent protein kinase kinases (CaMKKα and β) are regulatory kinases for multiple downstream kinases, including CaMKI, CaMKIV, PKB/Akt, and AMP-activated protein kinase (AMPK) through phosphorylation of each activation-loop Thr residue. In this report, we biochemically characterize the oligomeric structure of CaMKK isoforms through a heterologous expression system using COS-7 cells. Oligomerization of CaMKK isoforms was readily observed by treating CaMKK transfected cells with cell membrane permeable crosslinkers. In addition, His-tagged CaMKKα (His-CaMKKα) pulled down with FLAG-tagged CaMKKα (FLAG-CaMKKα) in transfected cells. The oligomerization of CaMKKα was confirmed by the fact that GST-CaMKKα/His-CaMKKα complex from transiently expressed COS-7 cells extracts was purified to near homogeneity by the sequential chromatography using glutathione-sepharose/Ni-sepharose and was observed in a Ca²⁺/CaM-independent manner by reciprocal pulldown assay, suggesting the direct interaction between monomeric CaMKKα. Furthermore, the His-CaMKKα kinase-dead mutant (D293A) complexed with FLAG-CaMKKα exhibited significant CaMKK activity, indicating the active CaMKKα multimeric complex. Collectively, these results suggest that CaMKKα can self-associate in the cells, constituting a catalytically active oligomer that might be important for the efficient activation of CaMKK-mediated intracellular signaling.

Associations Between CAMKK1 Polymorphism rs7214723 and the Prognosis of Patients With Lung Cancer

Background

The 5-year survival rate of patients with lung cancer in China is less than 20% and predicting their prognosis is challenging. We investigated the association between a common non-synonymous single nucleotide polymorphism (SNP), rs7214723, in the Ca²⁺/calmodulin-dependent protein kinase kinase 1 (CAMKK1) gene and the prognosis of patients with lung cancer.

Methods

Genomic DNA was extracted from the blood samples of 839 patients with lung cancer, recruited from Changhai Hospital (n = 536) and Taizhou Institute of Health Sciences (n = 352), and genotyped using the SNPscan technique. The association between patient prognosis and the genotypic data for CAMKK1 was analyzed using a multivariate Cox proportional hazards model adjusted for multiple potential confounders. The CRISPR/Cas9 gene-editing system was used to introduce point mutations in the CAMKK1 rs7214723 of A549 and NCI-H358 cells. Subsequently, Cell proliferation and migration ability were assessed with the Cell Counting Kit-8 and scratch assay. The Annexin V-FITC apoptosis detection kit was used to detect cell apoptosis.

Results

The CAMKK1 rs7214723 recessive CC genotype conferred significantly better overall survival (CC vs. TT + TC: adjusted hazard ratio = 0.78, 95% confidence interval [CI], 0.61-1.00, P = 0.049) than the TT + TC genotypes. Stratified analysis showed that the CAMKK1 rs7214723 CC genotype and recessive CC genotype conferred a significantly decreased risk of death in patients who were male, had a smoking history, or had stage III + IV cancer, compared with the TT and TT + TC genotypes. Relative to the TT + TC genotypes, the rs7214723 recessive CC genotype was also associated with a decreased risk of death in patients aged < 60 years (CC vs. TT + TC: adjusted hazard ratio = 0.59, 95% CI, 0.37-0.93, P = 0.024) and patients with squamous cell carcinoma (CC vs. TT + TC: adjusted hazard ratio = 0.65, 95% CI, 0.44-0.98, P = 0.038). Remarkably, CRISPR/Cas9-guided single nucleotide editing demonstrated that CAMKK1 rs7214723 T > C mutation significantly inhibits cell proliferation and migration and promotes cell apoptosis.

Conclusions

CAMKK1 SNP rs7214723 may be a significant prognostic factor for the risk of death among patients with lung cancer.

Interactions of a Signal Transduction Protein Investigated by Fluorescence Stopped-Flow Kinetics

To understand cellular processes such as biochemical pathways and signaling networks, we need to understand binding and reaction rates of often competing reactions, their dependence on cellular concentrations of participating molecules, and the regulation of these rates through allostery, posttranslational modifications, or other mechanisms. To do so, we break these systems down into their elementary steps, which are almost invariably either unimolecular or bimolecular reactions that frequently occur on sub-second, often sub-millisecond, time scales. Rapid mixing techniques, which generally achieve mixing in less than 2 ms, are generally suitable for the study of such reactions. The application of these techniques to the study of enzyme mechanisms is described in several excellent texts (Cornish-Bowden, Fundamentals of enzyme kinetics, 1995; Gutfreund, Kinetics for the life sciences. Receptors, transmitters and catalysis, 1995); flow techniques are used to study individual steps by monitoring the approach to equilibrium (the pre-steady state) under single turnover conditions.The individual steps in complex biochemical reaction schemes determine how fast systems can respond to incoming signals and adapt to changed conditions [1, 2]. This chapter is concerned with in vitro techniques that have been developed to study fast reactions in solution, and we present the study of various interactions of calmodulin as an example. The kinetic information obtained with these techniques is indispensable for understanding the dynamics of biochemical processes and complements the static structural and thermodynamic information available from X-ray crystallography, NMR, and equilibrium binding studies.

Persulfide Signaling in Stress-Initiated Calmodulin Kinase Response

Significance: Calcium ion (Ca²⁺)/calmodulin (CaM)-dependent protein kinases (CaMKs) are activated by phosphorylation of a crucial threonine residue either by itself (CaMKII) or by upstream kinases, CaMK kinases (CaMKKs) (CaMKI and CaMKIV). CaMKs, present in most mammalian tissues, can phosphorylate many downstream targets, thereby regulating numerous cellular functions. Recent Advances: Aside from canonical post-translational modifications, cysteine-based redox switches in CaMKs affect their enzyme activities. In addition to reactive oxygen species (ROS) and reactive nitrogen species (RNS), reactive sulfur species (RSS) are also recognized as key signaling molecules, regulating protein function through polysulfidation, formation of polysulfides [-S-(S)_n-H] on their reactive cysteine residues. To comprehend the biological significance of RSS signaling-related CaMK regulation, here we introduce a novel concept defining CaMKs as RSS targets in stress responses. The stress responses include an irreversible electrophile attack for CaMKI, inflammation for CaMKII, and endoplasmic reticulum stress for CaMKIV. Critical Issues: Development of various human diseases is associated with increased ROS, RNS, and RSS generation. Therefore, depending on specific pathophysiology, RSS could have very particular effects on CaMK functions. Future Directions: How multiple sources and mutual reactions of ROS, RNS, and RSS are coordinated is obscure. Elucidating the mechanisms through applications of enzymology, chemical biology, and mass spectrometry enables to uncover the complexities of redox regulation of CaMK cascades.

Stabilization of Protein-Protein Interactions between CaMKK2 and 14-3-3 by Fusicoccins

Ca²⁺/calmodulin-dependent protein kinase kinase 2 (CaMKK2) regulates several key physiological and pathophysiological processes, and its dysregulation has been implicated in obesity, diabetes, and cancer. CaMKK2 is inhibited through phosphorylation in a process involving binding to the scaffolding 14-3-3 protein, which maintains CaMKK2 in the phosphorylation-mediated inhibited state. The previously reported structure of the N-terminal CaMKK2 14-3-3-binding motif bound to 14-3-3 suggested that the interaction between 14-3-3 and CaMKK2 could be stabilized by small-molecule compounds. Thus, we investigated the stabilization of interactions between CaMKK2 and 14-3-3γ by Fusicoccin A and other fusicoccanes-diterpene glycosides that bind at the interface between the 14-3-3 ligand binding groove and the 14-3-3 binding motif of the client protein. Our data reveal that two of five tested fusicoccanes considerably increase the binding of phosphopeptide representing the 14-3-3 binding motif of CaMKK2 to 14-3-3γ. Crystal structures of two ternary complexes suggest that the steric contacts between the C-terminal part of the CaMKK2 14-3-3 binding motif and the adjacent fusicoccane molecule are responsible for differences in stabilization potency between the study compounds. Moreover, our data also show that fusicoccanes enhance the binding affinity of phosphorylated full-length CaMKK2 to 14-3-3γ, which in turn slows down CaMKK2 dephosphorylation, thus keeping this protein in its phosphorylation-mediated inhibited state. Therefore, targeting the fusicoccin binding cavity of 14-3-3 by small-molecule compounds may offer an alternative strategy to suppress CaMKK2 activity by stabilizing its phosphorylation-mediated inhibited state.

The Emerging Role of LHb CaMKII in the Comorbidity of Depressive and Alcohol Use Disorders

Depressive disorders and alcohol use disorders are widespread among the general population and are significant public health and economic burdens. Alcohol use disorders often co-occur with other psychiatric conditions and this dual diagnosis is called comorbidity. Depressive disorders invariably contribute to the development and worsening of alcohol use disorders, and vice versa. The mechanisms underlying these disorders and their comorbidities remain unclear. Recently, interest in the lateral habenula, a small epithalamic brain structure, has increased because it becomes hyperactive in depression and alcohol use disorders, and can inhibit dopamine and serotonin neurons in the midbrain reward center, the hypofunction of which is believed to be a critical contributor to the etiology of depressive disorders and alcohol use disorders as well as their comorbidities. Additionally, calcium/calmodulin-dependent protein kinase II (CaMKII) in the lateral habenula has emerged as a critical player in the etiology of these comorbidities. This review analyzes the interplay of CaMKII signaling in the lateral habenula associated with depressive disorders and alcohol use disorders, in addition to the often-comorbid nature of these disorders. Although most of the CaMKII signaling pathway's core components have been discovered, much remains to be learned about the biochemical events that propagate and link between depression and alcohol abuse. As the field rapidly advances, it is expected that further understanding of the pathology involved will allow for targeted treatments.

Backbone resonance assignments of the catalytic and regulatory domains of Ca2+/calmodulin-dependent protein kinase 1D

The CaMK subfamily of Ser/Thr kinases are regulated by calmodulin interactions with their C-terminal regions. They are exemplified by Ca²⁺/calmodulin dependent protein kinase 1δ which is known as CaMK1D, CaMKIδ or CKLiK. CaMK1D mediates intracellular signalling downstream of Ca²⁺ influx and thereby exhibits amplifications of Ca²⁺signals and polymorphisms that have been implicated in breast cancer and diabetes. Here we report the backbone ¹H, ¹³C, ¹⁵N assignments of the 38 kDa human CaMK1D protein in its free state, including both the canonical bi-lobed kinase fold as well as the autoinhibitory and calmodulin binding domains.

Interplay Between Calcium and AMPK Signaling in Human Cytomegalovirus Infection

Calcium signaling and the AMP-activated protein kinase (AMPK) signaling networks broadly regulate numerous aspects of cell biology. Human Cytomegalovirus (HCMV) infection has been found to actively manipulate the calcium-AMPK signaling axis to support infection. Many HCMV genes have been linked to modulating calcium signaling, and HCMV infection has been found to be reliant on calcium signaling and AMPK activation. Here, we focus on the cell biology of calcium and AMPK signaling and what is currently known about how HCMV modulates these pathways to support HCMV infection and potentially contribute to oncomodulation.

Mending Fences: Na,K-ATPase signaling via Ca2+ in the maintenance of epithelium integrity

Na,K-ATPase is a ubiquitous multifunctional protein that acts both as an ion pump and as a signal transducer. The signaling function is activated by ouabain in non-toxic concentrations. In epithelial cells the ouabain-bound Na,K-ATPase connects with the inositol 1,4,5-trisphosphate receptor via a short linear motif to activate low frequency Ca2+ oscillations. Within a couple of minutes this ouabain mediated signal has resulted in phosphorylation or dephosphorylation of 2580 phospho-sites. Proteins that control cell proliferation and cell adhesion and calmodulin regulated proteins are enriched among the ouabain phosphor-regulated proteins. The inositol 1,4,5-trisphosphate receptor and the stromal interaction molecule, which are both essential for the initiation of Ca2+ oscillations, belong to the ouabain phosphor-regulated proteins. Downstream effects of the ouabain-evoked Ca2+ signal in epithelial cells include interference with the intrinsic mitochondrial apoptotic process and stimulation of embryonic growth processes. The dual function of Na,K-ATPase as an ion pump and a signal transducer is now well established and evaluation of the physiological and pathophysiological consequences of this universal signal emerges as an urgent topic for future studies.

Development and Characterization of Novel Molecular Probes for Ca2+/Calmodulin-Dependent Protein Kinase Kinase, Derived from STO-609

Ca²⁺/calmodulin-dependent protein kinase kinase (CaMKK) activates particular multifunctional kinases, including CaMKI, CaMKIV, and 5'AMP-activated protein kinase (AMPK), resulting in the regulation of various Ca²⁺-dependent cellular processes, including neuronal, metabolic, and pathophysiological pathways. We developed and characterized a novel pan-CaMKK inhibitor, TIM-063 (2-hydroxy-3-nitro-7H-benzo[de]benzo[4,5]imidazo[2,1-a]isoquinolin-7-one) derived from STO-609 (7H-benzimidazo[2,1-a]benz[de]isoquinoline-7-one-3-carboxylic acid), and an inactive analogue (TIM-062) as molecular probes for the analysis of CaMKK-mediated cellular responses. Unlike STO-609, TIM-063 had an inhibitory activity against CaMKK isoforms (CaMKKα and CaMKKβ) with a similar potency (K_i = 0.35 μM for CaMKKα, and K_i = 0.2 μM for CaMKKβ) in vitro. Two TIM-063 analogues lacking a nitro group (TIM-062) or a hydroxy group (TIM-064) completely impaired CaMKK inhibitory activities, indicating that both substituents are necessary for the CaMKK inhibitory activity of TIM-063. Enzymatic analysis revealed that TIM-063 is an ATP-competitive inhibitor that directly targets the catalytic domain of CaMKK, similar to STO-609. TIM-063 suppressed the ionomycin-induced phosphorylation of exogenously expressed CaMKI, CaMKIV, and endogenous AMPKα in HeLa cells with an IC₅₀ of ∼0.3 μM, and it suppressed CaMKK isoform-mediated CaMKIV phosphorylation in transfected COS-7 cells. Thus, TIM-063, but not the inactive analogue (TIM-062), displayed cell permeability and the ability to inhibit CaMKK activity in cells. Taken together, these results indicate that TIM-063 could be a useful tool for the precise analysis of CaMKK-mediated signaling pathways and may be a promising lead compound for the development of therapeutic agents for the treatment of CaMKK-related diseases.

Calmodulin-Calcineurin Interaction beyond the Calmodulin-Binding Region Contributes to Calcineurin Activation

Calcineurin (CaN) is a calcium-dependent phosphatase involved in numerous signaling pathways. Its activation is in part driven by the binding of calmodulin (CaM) to a CaM recognition region (CaMBR) within CaN's regulatory domain (RD). However, secondary interactions between CaM and the CaN RD may be necessary to fully activate CaN. Specifically, it is established that the CaN RD folds upon CaM binding and a region C-terminal to CaMBR, the "distal helix", assumes an α-helix fold and contributes to activation [Dunlap, T. B., et al. (2013) Biochemistry 52, 8643-8651]. We hypothesized in that previous study that this distal helix can bind CaM in a region distinct from the canonical CaMBR. To test this hypothesis, we utilized molecular simulations, including replica-exchange molecular dynamics, protein-protein docking, and computational mutagenesis, to determine potential distal helix-binding sites on CaM's surface. We isolated a potential binding site on CaM (site D) that facilitates moderate-affinity interprotein interactions and predicted that mutation of site D residues K30 and G40 on CaM would weaken CaN distal helix binding. We experimentally confirmed that two variants (K30E and G40D) indicate weaker binding of a phosphate substrate p-nitrophenyl phosphate to the CaN catalytic site by a phosphatase assay. This weakened substrate affinity is consistent with competitive binding of the CaN autoinhibition domain to the catalytic site, which we suggest is due to the weakened distal helix-CaM interactions. This study therefore suggests a novel mechanism for CaM regulation of CaN that may extend to other CaM targets.

Cardiac MLC2 kinase is localized to the Z-disc and interacts with α-actinin2

Cardiac contractility is enhanced by phosphorylation of myosin light chain 2 (MLC2) by cardiac-specific MLC kinase (cMLCK), located at the neck region of myosin heavy chain. In normal mouse and human hearts, the level of phosphorylation is maintained relatively constant, at around 30-40% of total MLC2, likely by well-balanced phosphorylation and phosphatase-dependent dephosphorylation. Overexpression of cMLCK promotes sarcomere organization, while the loss of cMLCK leads to cardiac atrophy in vitro and in vivo. In this study, we showed that cMLCK is predominantly expressed at the Z-disc with additional diffuse cytosolic expression in normal adult mouse and human hearts. cMLCK interacts with the Z-disc protein, α-actinin2, with a high-affinity kinetic value of 13.4 ± 0.1 nM through the N-terminus region of cMLCK unique to cardiac-isoform. cMLCK mutant deficient for interacting with α-actinin2 did not promote sarcomeric organization and reduced cardiomyocyte cell size. In contrast, a cMLCK kinase-deficient mutant showed effects similar to wild-type cMLCK on sarcomeric organization and cardiomyocyte cell size. Our results suggest that cMLCK plays a role in sarcomere organization, likely distinct from its role in phosphorylating MLC2, both of which will contribute to the enhancement of cardiac contractility.

The active-site cysteine residue of Ca2+/calmodulin-dependent protein kinase I is protected from irreversible modification via generation of polysulfidation

Ca²⁺/calmodulin (CaM)-dependent protein kinase (CaMK) I is activated by the phosphorylation of a crucial activation loop Thr¹⁷⁷ by upstream kinases, CaMK kinase (CaMKK), and regulates axonal or dendritic extension and branching. Reactive sulfur species (RSS) modulate protein functions via polysulfidation of the reactive Cys residues. Here, we report that the activity of CaMKI was reversibly inhibited via its polysulfidation at Cys¹⁷⁹ by RSS. In vitro incubation of CaMKI with the exogenous RSS donor Na₂S₃ resulted in a dose-dependent inhibition of the phosphorylation at Thr¹⁷⁷ by CaMKK and inactivation of the enzymatic activity. Dithiothreitol (DTT), a small molecule reducing reagent, rescued these inhibitions. Conversely, mutated CaMKI (C179V) was resistant to the Na₂S₃-induced inactivation. In transfected cells expressing CaMKI, ionomycin-induced CaMKI activity was decreased upon treatment with Na₂S₄, whereas cells expressing mutant CaMKI (C179V) proved resistant to this treatment. A biotin-polyethylene glycol-conjugated maleimide capture assay revealed that CaMKI was a target for polysulfidation in cells. Furthermore, the polysulfidation of CaMKI protected Cys¹⁷⁹ from its irreversible modification, known as protein succination. Thus, we propose that CaMKI was reversibly inhibited via polysulfidation of Cys¹⁷⁹ by RSS, thereby protecting it from irreversible modification.

Transport, Bioavailability, Safety, and Calmodulin-Dependent-Phosphodiesterase-Inhibitory Properties of Flaxseed-Derived Bioactive Peptides

The aim of this work was to determine bioavailability and in vivo calmodulin-dependent-phosphodiesterase (CaMPDE)-inhibitory activity of six flaxseed-protein-derived peptides (AGA, AKLMS, QIAK, RWIQ, QQAKQ, and KQLSTGC) after oral administration to Wistar rats. Initial experiments tested the cytotoxicity and cellular-transport potentials of the peptides using Caco-2 cells. The cytotoxicity assay indicated that none of the six peptides had an adverse effect on the proliferation and viability of the Caco-2 cells, whereas the transport assay confirmed peptide translocation across the cell membrane. However, only two of the peptides (AGA and RWIQ) were detected in the rat serum up to 90 min postgavage, with traces of RWIQ persisting in serum 1 week after oral gavage. The six peptides inhibited plasma activity of CaMPDE with AGA (34.63%), QIAK (36.66%), and KQLSTGC (34.21%) being the most effective 30 min after gavage. In contrast, only AGA maintained significant plasma-CaMPDE-activity inhibition (44.35%) after 60 min.

Regulation of Ca2+/calmodulin-dependent protein kinase kinase β by cAMP signaling

BACKGROUND

Ca²⁺/calmodulin-dependent protein kinase kinase (CaMKK) is a pivotal activator of CaMKI, CaMKIV and 5'-AMP-activated protein kinase (AMPK), controlling Ca²⁺-dependent intracellular signaling including various neuronal, metabolic and pathophysiological responses. Recently, we demonstrated that CaMKKβ is feedback phosphorylated at Thr144 by the downstream AMPK, resulting in the conversion of CaMKKβ into Ca²⁺/CaM-dependent enzyme. However, the regulatory phosphorylation of CaMKKβ at Thr144 in intact cells and in vivo remains unclear.

METHODS

Anti-phosphoThr144 antibody was used to characterize the site-specific phosphorylation of CaMKKβ in immunoprecipitated samples from mouse cerebellum and in transfected mammalian cells that were treated with various agonists and protein kinase inhibitors. CaMKK activity assay and LC-MS/MS analysis were used for biochemical characterization of phosphorylated CaMKKβ.

RESULTS

Our data suggest that the phosphorylation of Thr144 in CaMKKβ is rapidly induced by cAMP/cAMP-dependent protein kinase (PKA) signaling in CaMKKβ-transfected HeLa cells, that is physiologically relevant in mouse cerebellum. We confirmed that the catalytic subunit of PKA was capable of directly phosphorylating CaMKKβ at Thr144 in vitro and in transfected cells. In addition, the basal phosphorylation of CaMKKβ at Thr144 in transfected HeLa cells was suppressed by AMPK inhibitor (compound C). PKA-catalyzed phosphorylation reduced the autonomous activity of CaMKKβ in vitro without significant effect on the Ca²⁺/CaM-dependent activity, resulting in the conversion of CaMKKβ into Ca²⁺/CaM-dependent enzyme.

CONCLUSION

cAMP/PKA signaling may confer Ca²⁺-dependency to the CaMKKβ-mediated signaling pathway through direct phosphorylation of Thr144 in intact cells.

GENERAL SIGNIFICANCE

Our results suggest a novel cross-talk between cAMP/PKA and Ca²⁺/CaM/CaMKKβ signaling through regulatory phosphorylation.

Redox regulation of Ca2+/calmodulin-dependent protein kinase IV via oxidation of its active-site cysteine residue

We have recently reported that Ca²⁺/calmodulin (CaM)-dependent protein kinase IV (CaMKIV) is inactivated by reactive sulfur species via polysulfidation of the active-site Cys residue. Here, we show that hydrogen peroxide (H₂O₂) limit CaMKIV activity at the same active-site Cys residue through oxidation and downstream signaling in cells. CaMKIV is phosphorylated at Thr¹⁹⁶ by its upstream CaMK kinase (CaMKK), which induces its full activity. In vitro incubation of CaMKIV with H₂O₂ resulted in reversible inhibition of CaMKK-induced phospho-Thr¹⁹⁶ and the consequent inactivation of CaMKIV. In contrast, mutated CaMKIV (C198V) was refractory to the H₂O₂-induced enzyme inhibition. In transfected cells expressing CaMKIV, Ca²⁺ ionophore-induced CaMKIV phosphorylation at Thr¹⁹⁶ was decreased upon treatment with H₂O₂, whereas cells expressing mutant CaMKIV (C198V) were resistant to H₂O₂ treatment. Modification of free thiol with N-ethylmaleimide revealed that Cys¹⁹⁸ in CaMKIV is a target for S-oxidation. Additionally, the Ca²⁺ influx-induced phospho-Thr¹⁹⁶ of endogenous CaMKIV was also inhibited upon treatment with H₂O₂ in Jurkat T-lymphocytes and cerebellar granule cells. Phosphorylation of cyclic AMP response element-binding protein (CREB) at Ser¹³³, which is downstream of CaMKIV, was also decreased upon treatment with H₂O₂. Thus, our results indicate that oxidation stress regulates cellular function by decreasing the activity of CaMKIV through Cys¹⁹⁸ oxidation.

Loss of Ca2+/Calmodulin Dependent Protein Kinase Kinase 2 Leads to Aberrant Transferrin Phosphorylation and Trafficking: A Potential Biomarker for Alzheimer's Disease

Ca²⁺/calmodulin-dependent protein kinase kinase 2 (CaMKK2) is a serine/threonine kinase that is activated following an increase in the intracellular Ca²⁺ concentration and activates multiple signaling cascades that control physiologically important neuronal processes. CaMKK2 has been implicated in schizophrenia, bipolar disease, neurodegeneration, and cancer. Using isoelectric focusing (IEF) and mass spectrometry-based proteomic analysis, it was found that knockdown (KD) of CaMKK2 in cultured adult primary dorsal root ganglion (DRG) neurons resulted in the reduction of transferrin (TF) phosphorylation at multiple functionally relevant residues which corresponded to loss of an acidic fraction (pH~3-4) of TF. In vitro studies using CRISPR/Cas9 based CaMKK2 knockout (KO) HEK293 and HepG2 cells lines validated previous findings and revealed that loss of CaMKK2 interfered with TF trafficking and turnover. TF is an iron transporter glycoprotein. Abnormal accumulation of iron and/or deregulated Ca²⁺ homeostasis leads to neurodegeneration in Alzheimer's disease (AD). Therefore, it was hypothesized that aberrant CaMKK2 in AD may lead to aberrant phosphorylated transferrin (P-TF: pH~3-4 fraction) which may serve as a hallmark biomarker for AD. A significant reduction of P-TF in the brain and serum of CaMKK2 KO mice and a triple-transgenic mouse model of AD (3xTg-AD) supported this hypothesis. In addition, analysis of early (< 65 years) and late-stage (>65 years) postmortem human AD cerebrospinal fluid (CSF) and serum samples revealed that aberrant P-TF (pH~3-4 fraction) profile was associated with both early and late-stage AD compared to age-matched controls. This indicates P-TF (pH~3-4 fraction) profile may be useful as a minimally invasive biomarker for AD. In addition, this study provides a link between aberrant CaMKK2 with TF trafficking and turnover which provides a novel insight into the neurodegeneration process.

Genetics of micronodular adrenal hyperplasia and Carney complex

Micronodular bilateral adrenal hyperplasia (MiBAH) is a rare cause of adrenal Cushing syndrome (CS). The investigations carried out on this disorder during the last two decades suggested that it could be divided into at least two entities: primary pigmented nodular adrenocortical disease (PPNAD) and isolated micronodular adrenocortical disease (i-MAD). The most common presentation of MiBAH is familial PPNAD as part of Carney complex (CNC) (cPPNAD). CNC, associated with multiple endocrine and non-endocrine neoplasias, was first described in 1985 in 40 patients, 10 of whom were familial cases. In 2000, we identified inactivating germline mutations of the PRKAR1A gene, encoding the regulatory subunit type 1α (RIα) of protein kinase A (PKA), in the majority of patients with CNC and PPNAD. PRKAR1A mutations causing CNC lead to increased PKA activity. Since then, additional genetic alterations in the cAMP/PKA signaling pathway leading to increased PKA activity have been described in association with MiBAH. This review summarizes older and recent findings on the genetics and pathophysiology of MiBAH, PPNAD, and related disorders.

CaMKK2 kinase domain interacts with the autoinhibitory region through the N-terminal lobe including the RP insert

BACKGROUND

Calcium/calmodulin-dependent protein kinase kinase 2 (CaMKK2), a member of the Ca⁠²⁺/calmodulin-dependent kinase (CaMK) family, functions as an upstream activator of CaMKI, CaMKIV and AMP-activated protein kinase. Thus, CaMKK2 is involved in the regulation of several key physiological and pathophysiological processes. Previous studies have suggested that Ca²⁺/CaM binding may cause unique conformational changes in the CaMKKs compared with other CaMKs. However, the underlying mechanistic details remain unclear.

METHODS

In this study, hydrogen-deuterium exchange coupled to mass spectrometry, time-resolved fluorescence spectroscopy, small-angle x-ray scattering and chemical cross-linking were used to characterize Ca²⁺/CaM binding-induced structural changes in CaMKK2.

RESULTS

Our data suggest that: (i) the CaMKK2 kinase domain interacts with the autoinhibitory region (AID) through the N-terminal lobe of the kinase domain including the RP insert, a segment important for targeting downstream substrate kinases; (ii) Ca²⁺/CaM binding affects the structure of several regions surrounding the ATP-binding pocket, including the activation segment; (iii) although the CaMKK2:Ca²⁺/CaM complex shows high conformational flexibility, most of its molecules are rather compact; and (iv) AID-bound Ca²⁺/CaM transiently interacts with the CaMKK2 kinase domain.

CONCLUSIONS

Interactions between the CaMKK2 kinase domain and the AID differ from those of other CaMKs. In the absence of Ca²⁺/CaM binding the autoinhibitory region inhibits CaMKK2 by both blocking access to the RP insert and by affecting the structure of the ATP-binding pocket.

GENERAL SIGNIFICANCE

Our results corroborate the hypothesis that Ca²⁺/CaM binding causes unique conformational changes in the CaMKKs relative to other CaMKs.

Structural Dynamics of the Activation of Elongation Factor 2 Kinase by Ca2+-Calmodulin

Eukaryotic elongation factor 2 kinase (eEF-2K), the only known calmodulin (CaM)-activated α-kinase, phosphorylates eukaryotic elongation factor 2 (eEF-2) on a specific threonine (Thr-56) diminishing its affinity for the ribosome and reducing the rate of nascent chain elongation during translation. Despite its critical cellular role, the precise mechanisms underlying the CaM-mediated activation of eEF-2K remain poorly defined. Here, employing a minimal eEF-2K construct (TR) that exhibits activity comparable to the wild-type enzyme and is fully activated by CaM in vitro and in cells, and using a variety of complimentary biophysical techniques in combination with computational modeling, we provide a structural mechanism by which CaM activates eEF-2K. Native mass analysis reveals that CaM, with two bound Ca²⁺ ions, forms a stoichiometric 1:1 complex with TR. Chemical crosslinking mass spectrometry and small-angle X-ray scattering measurements localize CaM near the N-lobe of the TR kinase domain and the spatially proximal C-terminal helical repeat. Hydrogen/deuterium exchange mass spectrometry and methyl NMR indicate that the conformational changes induced on TR by the engagement of CaM are not localized but are transmitted to remote regions that include the catalytic site and the functionally important phosphate binding pocket. The structural insights obtained from the present analyses, together with our previously published kinetics data, suggest that TR, and by inference, wild-type eEF-2K, upon engaging CaM undergoes a conformational transition resulting in a state that is primed to efficiently auto-phosphorylate on the primary activating T348 en route to full activation.

Characterization of CoPK02, a Ca2+/calmodulin-dependent protein kinase in mushroom Coprinopsis cinerea

We surveyed genome sequences from the basidiomycetous mushroom Coprinopsis cinerea and isolated a cDNA homologous to CMKA, a calmodulin-dependent protein kinase (CaMK) in Aspergillus nidulans. We designated this sequence, encoding 580 amino acids with a molecular weight of 63,987, as CoPK02. CoPK02 possessed twelve subdomains specific to protein kinases and exhibited 43, 35, 40% identity with rat CaMKI, CaMKII, CaMKIV, respectively, and 40% identity with CoPK12, one of the CaMK orthologs in C. cinerea. CoPK02 showed significant autophosphorylation activity and phosphorylated exogenous proteins in the presence of Ca²⁺/CaM. By the CaM-overlay assay we confirmed that the C-terminal sequence (Trp346-Arg358) was the calmodulin-binding site, and that the binding of Ca²⁺/CaM to CoPK02 was reduced by the autophosphorylation of CoPK02. Since CoPK02 evolved in a different clade from CoPK12, and showed different gene expression compared to that of CoPK32, which is homologous to mitogen-activated protein kinase-activated protein kinase, CoPK02 and CoPK12 might cooperatively regulate Ca²⁺-signaling in C. cinerea.

Comparative Analysis of CDPK Family in Maize, Arabidopsis, Rice, and Sorghum Revealed Potential Targets for Drought Tolerance Improvement

Calcium dependent protein kinases (CDPKs) play significant role in regulation of plant growth and development in response to various stresses including drought. A set of 32 CDPK genes identified in maize were further used for searching of orthologs in the model plant Arabidopsis (72) and major food crops such as rice (78) and sorghum (91). We comprehensively studied the phylogenetic relationship, annotations, gene duplications, gene structure, divergence time, 3-D protein structures and tissue-specific drought induced expression of CDPK genes in all four species. Variation in intron frequency in the studied species was one of the reasons for the functional diversity of CDPK genes to various stress responses. Protein kinase and protein kinase C phosphorylation site domains were the most conserved motifs identified in all species. Four groups were identified from the sequence-based phylogenetic analysis, in which maize CDPKs were clustered in group III. Expression data showed that the CDPK genes were highly expressed in leaf of maize, rice, and sorghum whereas in Arabidopsis the maximum expression was observed in root. The expression assay showed 5, 6, 11, and 9 were the commonly and differentially expressed drought-related orthologous genes in maize, Arabidopsis, rice, and sorghum, respectively. 3-D protein structure were predicted for the nine genes (Arabidopsis: 2, maize: 2, rice: 3, and sorghum: 2) showing differential expression in at least three species. The predicted 3-D structures were further evaluated and validated by Ramachandran plot, ANOLEA, ProSA, and Verify-3D. The superimposed 3-D structure of drought-related orthologous proteins retained similar folding pattern owing to their conserved nature. Functional annotation revealed the involvement of CDPK genes in various pathways such as osmotic homeostasis, cell protection, and root growth. The interactions of CDPK genes in various pathways play crucial role in imparting drought tolerance through different ABA and MAPK signaling cascades. These selected candidate genes could be targeted in development of drought tolerant genotypes in maize, rice, and sorghum through appropriate breeding approaches. Our comparative experiments of CDPK genes could also be extended in the drought stress breeding programmes of the related species.

AMP-activated protein kinase-mediated feedback phosphorylation controls the Ca2+/calmodulin (CaM) dependence of Ca2+/CaM-dependent protein kinase kinase β

The Ca²⁺/calmodulin-dependent protein kinase kinase β (CaMKKβ)/5'-AMP-activated protein kinase (AMPK) phosphorylation cascade affects various Ca²⁺-dependent metabolic pathways and cancer growth. Unlike recombinant CaMKKβ that exhibits higher basal activity (autonomous activity), activation of the CaMKKβ/AMPK signaling pathway requires increased intracellular Ca²⁺ concentrations. Moreover, the Ca²⁺/CaM dependence of CaMKKβ appears to arise from multiple phosphorylation events, including autophosphorylation and activities furnished by other protein kinases. However, the effects of proximal downstream kinases on CaMKKβ activity have not yet been evaluated. Here, we demonstrate feedback phosphorylation of CaMKKβ at multiple residues by CaMKKβ-activated AMPK in addition to autophosphorylation in vitro, leading to reduced autonomous, but not Ca²⁺/CaM-activated, CaMKKβ activity. MS analysis and site-directed mutagenesis of AMPK phosphorylation sites in CaMKKβ indicated that Thr¹⁴⁴ phosphorylation by activated AMPK converts CaMKKβ into a Ca²⁺/CaM-dependent enzyme as shown by completely Ca²⁺/CaM-dependent CaMKK activity of a phosphomimetic T144E CaMKKβ mutant. CaMKKβ mutant analysis indicated that the C-terminal domain (residues 471-587), including the autoinhibitory region, plays an important role in stabilizing an inactive conformation in a Thr¹⁴⁴ phosphorylation-dependent manner. Furthermore, immunoblot analysis with anti-phospho-Thr¹⁴⁴ antibody revealed phosphorylation of Thr¹⁴⁴ in CaMKKβ in transfected COS-7 cells that was further enhanced by exogenous expression of AMPKα. These results indicate that AMPK-mediated feedback phosphorylation of CaMKKβ regulates the CaMKKβ/AMPK signaling cascade and may be physiologically important for intracellular maintenance of Ca²⁺-dependent AMPK activation by CaMKKβ.

Calcium/calmodulin-dependent kinase 2 mediates Epac-induced spontaneous transient outward currents in rat vascular smooth muscle

KEY POINTS

The Ca2+ and redox-sensing enzyme Ca2+ /calmodulin-dependent kinase 2 (CaMKII) is a crucial and well-established signalling molecule in the heart and brain. In vascular smooth muscle, which controls blood flow by contracting and relaxing in response to complex Ca2+ signals and oxidative stress, surprisingly little is known about the role of CaMKII. The vasodilator-induced second messenger cAMP can relax vascular smooth muscle via its effector, exchange protein directly activated by cAMP (Epac), by activating spontaneous transient outward currents (STOCs) that hyperpolarize the cell membrane and reduce voltage-dependent Ca2+ influx. How Epac activates STOCs is unknown. In the present study, we map the pathway by which Epac increases STOC activity in contractile vascular smooth muscle and show that a critical step is the activation of CaMKII. To our knowledge, this is the first report of CaMKII activation triggering cellular activity known to induce vasorelaxation.

ABSTRACT

Activation of the major cAMP effector, exchange protein directly activated by cAMP (Epac), induces vascular smooth muscle relaxation by increasing the activity of ryanodine (RyR)-sensitive release channels on the peripheral sarcoplasmic reticulum. Resultant Ca2+ sparks activate plasma membrane Ca2+ -activated K+ (BKCa ) channels, evoking spontaneous transient outward currents (STOCs) that hyperpolarize the cell and reduce voltage-dependent Ca2+ entry. In the present study, we investigate the mechanism by which Epac increases STOC activity. We show that the selective Epac activator 8-(4-chloro-phenylthio)-2'-O-methyladenosine-3', 5-cyclic monophosphate-AM (8-pCPT-AM) induces autophosphorylation (activation) of calcium/calmodulin-dependent kinase 2 (CaMKII) and also that inhibition of CaMKII abolishes 8-pCPT-AM-induced increases in STOC activity. Epac-induced CaMKII activation is probably initiated by inositol 1,4,5-trisphosphate (IP3 )-mobilized Ca2+ : 8-pCPT-AM fails to induce CaMKII activation following intracellular Ca2+ store depletion and inhibition of IP3 receptors blocks both 8-pCPT-AM-mediated CaMKII phosphorylation and STOC activity. 8-pCPT-AM does not directly activate BKCa channels, but STOCs cannot be generated by 8-pCPT-AM in the presence of ryanodine. Furthermore, exposure to 8-pCPT-AM significantly slows the initial rate of [Ca2+ ]i rise induced by the RyR activator caffeine without significantly affecting the caffeine-induced Ca2+ transient amplitude, a measure of Ca2+ store content. We conclude that Epac-mediated STOC activity (i) occurs via activation of CaMKII and (ii) is driven by changes in the underlying behaviour of RyR channels. To our knowledge, this is the first report of CaMKII initiating cellular activity linked to vasorelaxation and suggests novel roles for this Ca2+ and redox-sensing enzyme in the regulation of vascular tone and blood flow.

Reactive sulfur species inactivate Ca2+/calmodulin-dependent protein kinase IV via S-polysulfidation of its active-site cysteine residue

Reactive sulfur species (RSS) modulate protein functions via S-polysulfidation of reactive Cys residues. Here, we report that Ca²⁺/calmodulin (CaM)-dependent protein kinase IV (CaMKIV) was reversibly inactivated by RSS via polysulfidation of the active-site Cys residue. CaMKIV is phosphorylated at Thr¹⁹⁶ by its upstream CaMK kinase (CaMKK), resulting in the induction of its full activity. In vitro incubation of CaMKIV with the exogenous RSS donors Na₂S _n (n = 2-4) resulted in dose-dependent inhibition of the CaMKK-induced phospho-Thr¹⁹⁶ and consequent inactivation of the enzyme activity. Conversely, mutated CaMKIV (C198V) was refractory to the Na₂S _n -induced enzyme inhibition. A biotin-polyethylene glycol-conjugated maleimide capture assay revealed that Cys¹⁹⁸ in CaMKIV represents a target for S-polysulfidation. Furthermore, phosho-Thr¹⁹⁶ and CaMKIV activity were inhibited by incubation with cysteine hydropersulfide, a newly identified RSS that is generated from cystine by cystathionine-γ-lyase. In transfected cells expressing CaMKIV, ionomycin-induced CaMKIV phosphorylation at Thr¹⁹⁶ was decreased upon treatment with either Na₂S₄ or the endoplasmic reticulum (ER) stress inducer thapsigargin, whereas cells expressing mutant CaMKIV (C198V) were resistant to this treatment. In addition, the ionomycin-induced phospho-Thr¹⁹⁶ of endogenous CaMKIV was also inhibited by treatment either with Na₂S₄ or thapsigargin in Jurkat T lymphocytes. Taken together, these data define a novel signaling function for intracellular RSS in inhibiting CaMKIV activity via S-polysulfidation of its Cys¹⁹⁸ during the response to ER stress.

Calmodulin kinases: essential regulators in health and disease

Neuronal activity induces intracellular Ca²⁺ increase, which triggers activation of a series of Ca²⁺ -dependent signaling cascades. Among these, the multifunctional Ca²⁺ /calmodulin-dependent protein kinases (CaMKs, or calmodulin kinases) play key roles in neuronal transmission, synaptic plasticity, circuit development and cognition. The most investigated CaMKs for these roles in neuronal functions are CaMKI, CaMKII, CaMKIV and we will shed light on these neuronal CaMKs' functions in this review. Catalytically active members of CaMKs currently are CaMKI, CaMKII, CaMKIV and CaMKK. Although they all necessitate the binding of Ca²⁺ and calmodulin complex (Ca²⁺ /CaM) for releasing autoinhibition, each member of CaMK has distinct activation mechanisms-autophosphorylation mediated autonomy of multimeric CaMKII and CaMKK-dependent phosphoswitch-induced activation of CaMKI or CaMKIV. Furthermore, each CaMK shows distinct subcellular localization that underlies specific compartmentalized function in each activated neuron. In this review, we first summarize these molecular characteristics of each CaMK as to regulation and subcellular localization, and then describe each biological function. In the last section, we also focus on the emerging role of CaMKs in pathophysiological conditions by introducing the recent studies, especially focusing on drug addiction and depression, and discuss how dysfunctional CaMKs may contribute to the pathology of the neuropsychological disorders. This article is part of the mini review series "60th Anniversary of the Japanese Society for Neurochemistry".

miR-146a facilitates osteoarthritis by regulating cartilage homeostasis via targeting Camk2d and Ppp3r2

Osteoarthritis (OA), characterized by insufficient extracellular matrix synthesis and cartilage degeneration, is known as an incurable disease because its pathogenesis is poorly elucidated. Thus far, limited information is available regarding the pathophysiological role of microRNAs (miRNAs) in OA. In this study, we investigated the specific function of miR-146a in OA pathophysiology using mouse OA models. We found that the articular cartilage degeneration of miR-146a knockout (KO) mice was alleviated compared with that of the wild-type (WT) mice in spontaneous and instability-induced OA models. We demonstrate that miR-146a aggravated pro-inflammatory cytokines induced suppressing the expression of cartilage matrix-associated genes. We further identified calcium/calmodulin-dependent protein kinase II delta (Camk2d) and protein phosphatase 3, regulatory subunit B, beta isoform (Ppp3r2, also known as calcineurin B, type II) were essential targets of miR-146a in regulating cartilage homeostasis. Moreover, we found that surgical-induced OA mice treated with a miR-146a inhibitor significantly alleviated the destruction of articular cartilage via targeting Camk2d and Ppp3r2. These results suggested that miR-146a has a crucial role in maintaining cartilage homeostasis. MiR-146a inhibition in chondrocytes can be a potential therapeutic strategy to ameliorate OA.

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.

Structural Basis for the Recognition of Eukaryotic Elongation Factor 2 Kinase by Calmodulin

Binding of Ca(2+)-loaded calmodulin (CaM) activates eukaryotic elongation factor 2 kinase (eEF-2K) that phosphorylates eEF-2, its only known cellular target, leading to a decrease in global protein synthesis. Here, using an eEF-2K-derived peptide (eEF-2KCBD) that encodes the region necessary for its CaM-mediated activation, we provide a structural basis for their interaction. The striking feature of this association is the absence of Ca(2+) from the CaM C-lobe sites, even under high Ca(2+) conditions. eEF-2KCBD engages CaM largely through the C lobe of the latter in an anti-parallel 1-5-8 hydrophobic mode reinforced by a pair of unique electrostatic contacts. Sparse interactions of eEF-2KCBD with the CaM N lobe results in persisting inter-lobe mobility. A conserved eEF-2K residue (W85) anchors it to CaM by inserting into a deep hydrophobic cavity within the CaM C lobe. Mutation of this residue (W85S) substantially weakens interactions between full-length eEF-2K and CaM in vitro and reduces eEF-2 phosphorylation in cells.

Differential AMP-activated Protein Kinase (AMPK) Recognition Mechanism of Ca2+/Calmodulin-dependent Protein Kinase Kinase Isoforms

Ca(2+)/calmodulin-dependent protein kinase kinase β (CaMKKβ) is a known activating kinase for AMP-activated protein kinase (AMPK). In vitro, CaMKKβ phosphorylates Thr(172) in the AMPKα subunit more efficiently than CaMKKα, with a lower Km (∼2 μm) for AMPK, whereas the CaMKIα phosphorylation efficiencies by both CaMKKs are indistinguishable. Here we found that subdomain VIII of CaMKK is involved in the discrimination of AMPK as a native substrate by measuring the activities of various CaMKKα/CaMKKβ chimera mutants. Site-directed mutagenesis analysis revealed that Leu(358) in CaMKKβ/Ile(322) in CaMKKα confer, at least in part, a distinct recognition of AMPK but not of CaMKIα.

Identification of striated muscle activator of Rho signaling (STARS) as a novel calmodulin target by a newly developed genome-wide screen

To search for novel target(s) of the Ca(2+)-signaling transducer, calmodulin (CaM), we performed a newly developed genome-wide CaM interaction screening of 19,676 GST-fused proteins expressed in human. We identified striated muscle activator of Rho signaling (STARS) as a novel CaM target and characterized its CaM binding ability and found that the Ca(2+)/CaM complex interacted stoichiometrically with the N-terminal region (Ala13-Gln35) of STARS in vitro as well as in living cells. Mutagenesis studies identified Ile20 and Trp33 as the essential hydrophobic residues in CaM anchoring. Furthermore, the CaM binding deficient mutant (Ile20Ala, Trp33Ala) of STARS further enhanced its stimulatory effect on SRF-dependent transcriptional activation. These results suggest a connection between Ca(2+)-signaling via excitation-contraction coupling and the regulation of STARS-mediated gene expression in muscles.

Acute heart failure with cardiomyocyte atrophy induced in adult mice by ablation of cardiac myosin light chain kinase

AIMS

Under pressure overload, initial adaptive hypertrophy of the heart is followed by cardiomyocyte elongation, reduced contractile force, and failure. The mechanisms governing the transition to failure are not fully understood. Pressure overload reduced cardiac myosin light chain kinase (cMLCK) by ∼80% within 1 week and persists. Knockdown of cMLCK in cardiomyocytes resulted in reduced cardiac contractility and sarcomere disorganization. Thus, we hypothesized that acute reduction of cMLCK may be causative for reduced contractility and cardiomyocyte remodelling during the transition from compensated to decompensated cardiac hypertrophy.

METHODS AND RESULTS

To mimic acute cMLCK reduction in adult hearts, the floxed-Mylk3 gene that encodes cMLCK was inducibly ablated in Mylk3(flox/flox)/merCremer mice (Mylk3-KO), and compared with two control mice (Mylk3(flox/flox) and Mylk3(+/+)/merCremer) following tamoxifen injection (50 mg/kg/day, 2 consecutive days). In Mylk3-KO mice, reduction of cMLCK protein was evident by 4 days, with a decline to below the level of detection by 6 days. By 7 days, these mice exhibited heart failure, with reduction of fractional shortening compared with those in two control groups (19.8 vs. 28.0% and 27.7%). Severely convoluted cardiomyocytes with sarcomeric disorganization, wavy fibres, and cell death were demonstrated in Mylk3-KO mice. The cardiomyocytes were also unable to thicken adaptively to pressure overload.

CONCLUSION

Our results, using a new mouse model mimicking an acute reduction of cMLCK, suggest that cMLCK plays a pivotal role in the transition from compensated to decompensated hypertrophy via sarcomeric disorganization.

Ssp1 CaMKK: A Sensor of Actin Polarization That Controls Mitotic Commitment through Srk1 in Schizosaccharomyces pombe

Background

Calcium/calmodulin-dependent protein kinase kinase (CaMKK) is required for diverse cellular functions. Mammalian CaMKK activates CaMKs and also the evolutionarily-conserved AMP-activated protein kinase (AMPK). The fission yeast Schizosaccharomyces pombe CaMKK, Ssp1, is required for tolerance to limited glucose through the AMPK, Ssp2, and for the integration of cell growth and division through the SAD kinase Cdr2.

Results

Here we report that Ssp1 controls the G2/M transition by regulating the activity of the CaMK Srk1. We show that inhibition of Cdc25 by Srk1 is regulated by Ssp1; and also that restoring growth polarity and actin localization of ssp1-deleted cells by removing the actin-monomer-binding protein, twinfilin, is sufficient to suppress the ssp1 phenotype.

Conclusions

These findings demonstrate that entry into mitosis is mediated by a network of proteins, including the Ssp1 and Srk1 kinases. Ssp1 connects the network of components that ensures proper polarity and cell size with the network of proteins that regulates Cdk1-cyclin B activity, in which Srk1 plays an inhibitory role.

Regulation of Ca(2+)/calmodulin-dependent protein kinase phosphatase (CaMKP/PPM1F) by protocadherin-γC5 (Pcdh-γC5)

Ca(2+)/calmodulin-dependent protein kinase phosphatase (CaMKP/PPM1F) is a Ser/Thr protein phosphatase that belongs to the PPM family. It is important to identify an endogenous regulator of CaMKP. Using an Escherichia coli two-hybrid screening method, we identified the C-terminal cytoplasmic fragment of protocadherin γ subfamily C5 (Pcdh-γC5), which was generated by intracellular processing, as a CaMKP-binding protein. Dephosphorylation of phosphorylated Ca(2+)/calmodulin-dependent protein kinase I (CaMKI) by CaMKP was significantly activated by the C-terminal cytoplasmic fragment, Pcdh-γC5(715-944), both in vitro and in cells, suggesting that the C-terminal fragment functions as an endogenous activator of CaMKP. The nuclear translocation of the fragment was blocked by its binding to cytoplasmic CaMKP to form a ternary complex with CaMKI. Taken together, these results strongly suggest that the C-terminal cytoplasmic fragment of Pcdh-γC5 acts as a scaffold for CaMKP and CaMKI to regulate CaMKP activity. These findings may provide new insights into the reversible regulation of CaMKP in cells.

Effects of isoquinolonesulfonamides on action potential secretion coupling in pituitary cells

BACKGROUND

Pituitary lactotrophs fire action potentials spontaneously and the associated voltage-gated calcium influx is sufficient to maintain high and steady prolactin release. Several intracellular proteins can mediate the action of calcium influx on prolactin secretion, including calmodulin-dependent protein kinases. Here, we studied effects of isoquinolonesulfonamides KN-62 and KN-93, calmodulin-dependent protein kinase inhibitors, and KN-92, an inactive analog, on spontaneous electrical activity, voltage-gated calcium influx, cyclic nucleotide production, and basal prolactin release.

METHODS

The effects of these compounds on electrical activity and calcium signaling was measured in single lactotrophs and cyclic nucleotide production and prolactin release were determined in static culture and perifusion experiments of anterior pituitary cells from postpubertal female rats.

RESULTS

KN-62 and KN-93 blocked basal prolactin release in a dose- and time-dependent manner, suggesting that calmodulin-dependent protein kinase could mediate the coupling of electrical activity and secretion. However, a similar effect on basal prolactin release was observed on application of KN-92, which does not inhibit this kinase. KN-93 also inhibited cAMP and cGMP production, but inhibition of prolactin release was independent of the status of cyclic nucleotide production. Single cell measurements revealed abolition of spontaneous and depolarization-induced electrical activity and calcium transients in KN-92/93-treated cells, with a time course comparable to that observed in secretory studies.

CONCLUSIONS

The results suggest that caution should be used when interpreting data from studies using isoquinolonesulfonamides to evaluate the role of calmodulin-dependent protein kinases in excitable endocrine cells, because inactive compounds exhibit comparable effects on action potential secretion coupling to those of active compounds.

Praeruptorin A inhibits in vitro migration of preosteoclasts and in vivo bone erosion, possibly due to its potential to target calmodulin

Excessive activity and/or increased number of osteoclasts lead to bone resorption-related disorders. Here, we investigated the potential of praeruptorin A to inhibit migration/fusion of preosteoclasts in vitro and bone erosion in vivo. Praeruptorin A inhibited the RANKL-induced migration/fusion of preosteoclasts accompanied by the nuclear translocation of NFATc1, a master regulator of osteoclast differentiation. Antimigration/fusion activity of praeruptorin A was also confirmed by evaluating the mRNA expression of fusion-mediating molecules. In silico binding studies and several biochemical assays further revealed the potential of praeruptorin A to bind with Ca(2+)/calmodulin and inhibit its downstream signaling pathways, including the Ca(2+)/calmodulin-CaMKIV-CREB and Ca(2+)/calmodulin-calcineurin signaling axis responsible for controlling NFATc1. In vivo application of praeruptorin A significantly reduced lipopolysaccharide-induced bone erosion, indicating its possible use to treat bone resorption-related disorders. In conclusion, praeruptorin A has the potential to inhibit migration/fusion of preosteoclasts in vitro and bone erosion in vivo by targeting calmodulin and inhibiting the Ca(2+)/calmodulin-CaMKIV-CREB-NFATc1 and/or Ca(2+)/calmodulin-calcineurin-NFATc1 signaling axis.