The relationship between the free concentrations of Ca2+ and Ca2+-calmodulin in intact cells

Investigating the Impact of Electrostatic Interactions on Calmodulin Binding and Ca2+-Dependent Activation of the Calcium-Gated Potassium SK4 Channel

Ca2+ binding to the ubiquitous Ca2+ sensing protein calmodulin (CaM) activates the intermediate conductance Ca2+-activated SK4 channel. Potential hydrophilic pockets for CaM binding have been identified at the intracellular HA and HB helices in the C-terminal of SK4 from the three published cryo-EM structures of SK4. Single charge reversal substitutions at either site, significantly weakened the pull-down of SK4 by CaM wild-type (CaM), and decreased the TRAM-34 sensitive outward K+ current densities in native HEK293T cells when compared with SK4 WT measured under the same conditions. Only the doubly substituted SK4 R352D/R355D (HB helix) obliterated the CaM-mediated pull-down and thwarted outward K+ currents. However, overexpression of CaM E84K/E87K, which had been predicted to face the arginine doublet, restored the CaM-mediated pull-down of SK4 R352D/R355D and normalized its whole-cell current density. Virtual analysis of the putative salt bridges supports a unique role for the positively charged arginine doublet at the HB helix into anchoring the interaction with the negatively charged CaM glutamate 84 and 87 CaM. Our findings underscore the unique contribution of electrostatic interactions in carrying CaM binding onto SK4 and support the role of the C-terminal HB helix to the Ca2+-dependent gating process.

Regulation of basal autophagy by calmodulin availability

Macroautophagy (hereafter autophagy) is a process that degrades cellular components to maintain homeostasis. The Ca²⁺ sensor calmodulin (CaM) regulates numerous cell functions but is a limiting factor due to its insufficient availability for all target proteins. However, evidence that CaM availability regulates basal autophagy is lacking. Here, we have tested this hypothesis. CaM antagonists W-7, trifluoperazine and CGS9343b cause autophagosome accumulation and inhibit basal autophagic flux in the same manner as does chloroquine. These reagents promote the activity of AMP-activated protein kinase (AMPK) but not that of the mechanistic target of rapamycin (mTOR). Competitive binding assays using CaM sensors with different Ca²⁺ dependencies showed that chloroquine directly binds CaM in a Ca²⁺ -dependent fashion. The CaM antagonists have disparate effects on cytoplasmic Ca²⁺ , triggering from none to robust signals, indicating that their consistent inhibition of autophagy is due to inhibition of CaM and not Ca²⁺ . Chelating intracellular Ca²⁺ reduces the effect of the CaM antagonists to accumulate LC3-II, indicating that they do so by inhibiting CaM-dependent activities at basal Ca²⁺ level. The CaM antagonists cause lysosomal alkalinisation. Consistently, buffering CaM with a high-affinity CaM-binding protein that binds CaM at resting Ca²⁺ level increases lysosomal pH. Enhanced CaM buffering using a chimeric protein that contains two high-affinity CaM-binding sites that can collectively bind CaM at a large range of Ca²⁺ further increases lysosomal pH and increases LC3-II accumulation and AMPK activity, but not that of mTOR. These data demonstrate that CaM availability is required for basal autophagy.

A novel assay to assess the effects of estrogen on the cardiac calmodulin binding equilibrium

AIMS

The Ca²⁺-binding protein calmodulin (CaM) modulates numerous target proteins but is produced insufficiently to bind all of them, generating a limiting CaM equilibrium. Menopause increases cardiac morbidity; however, it is unknown if the cardiac CaM equilibrium is affected by estrogen. We devised an assay to assess the effects of ovariectomy and estrogen treatment on the cardiac CaM equilibrium.

MATERIALS AND METHODS

Sprague-Dawley rats received sham surgery or ovariectomy, followed by 2-week treatment with vehicle or 17β-estradiol. Ca²⁺-saturated left ventricular (LV) lysates were processed through CaM sepharose columns, which retained CaM-binding proteins unoccupied by endogenous CaM. Eluants therefrom were subjected to a competitive binding assay against purified CaM and a CaM biosensor to assess the amounts of unoccupied CaM-binding sites. LV cellular composition was assessed by immunohistochemistry.

KEY FINDINGS

LV eluants processed from sham animals reduce biosensor response by ~32%, indicating baseline presence of unoccupied CaM-binding sites and a limiting CaM equilibrium. Ovariectomy exacerbates the limiting CaM equilibrium, reducing biosensor response by ~65%. 17β-estradiol treatment equalizes the difference between sham and ovariectomized animals. These changes reflect whole tissue responses and are not mirrored by changes in total surface areas of cardiomyocytes and fibroblasts. Consistently, Ca²⁺-dependent, but not Ca²⁺-independent, interaction between CaM and the cardiac inositol trisphosphate receptor (IP₃R) is reduced following ovariectomy and is restored by subsequent 17β-estradiol treatment.

SIGNIFICANCE

Our assay provides a new parameter to assess tissue CaM equilibrium. The exacerbated limiting CaM equilibrium following estrogen loss may contribute to cardiac morbidity and is prevented by estrogen treatment.

NaV1.2 EFL domain allosterically enhances Ca2+ binding to sites I and II of WT and pathogenic calmodulin mutants bound to the channel CTD

Neuronal voltage-gated sodium channel NaV1.2 C-terminal domain (CTD) binds calmodulin (CaM) constitutively at its IQ motif. A solution structure (6BUT) and other NMR evidence showed that the CaM N domain (CaMN) is structurally independent of the C-domain (CaMC) whether CaM is bound to the NaV1.2IQp (1,901-1,927) or NaV1.2CTD (1,777-1,937) with or without calcium. However, in the CaM + NaV1.2CTD complex, the Ca2+ affinity of CaMN was more favorable than in free CaM, while Ca2+ affinity for CaMC was weaker than in the CaM + NaV1.2IQp complex. The CTD EF-like (EFL) domain allosterically widened the energetic gap between CaM domains. Cardiomyopathy-associated CaM mutants (N53I(N54I), D95V(D96V), A102V(A103V), E104A(E105A), D129G(D130G), and F141L(F142L)) all bound the NaV1.2 IQ motif favorably under resting (apo) conditions and bound calcium normally at CaMN sites. However, only N53I and A102V bound calcium at CaMC sites at [Ca2+] < 100 μM. Thus, they are expected to respond like wild-type CaM to Ca2+ spikes in excitable cells.

Investigating the landscape of intracellular [Ca2+] in live cells by rapid photoactivated cross-linking of calmodulin-protein interactions

The Ca²⁺ sensor protein calmodulin interacts in a Ca²⁺-dependent manner with a large number of proteins that among them encompass a diverse assortment of functions and subcellular localizations. A method for monitoring calmodulin-protein interactions as they occur throughout a living cell would thus uniquely enable investigations of the intracellular landscape of [Ca²⁺] and its relationship to cell function. We have developed such a method based on capture of calmodulin-protein interactions by rapid photoactivated cross-linking (t_1/2 ∼7s) in cells stably expressing a tandem affinity tagged calmodulin that have been metabolically labeled with a photoreactive methionine analog. Tagged adducts are stringently enriched, and captured calmodulin interactors are then identified and quantified based on tandem mass spectrometry data for their tryptic peptides. In this paper we show that the capture behaviors of interactors in cells are consistent with the presence of basal microdomains of elevated [Ca²⁺]. Ca²⁺ sensitivities for capture were determined, and these suggest that [Ca²⁺] levels are above ∼1 μM in these regions. Although the microdomains appear to affect capture of most proteins, capture of some is at an apparent Ca²⁺-dependent maximum, suggesting they are targeted to the domains. Removal of extracellular Ca²⁺ has both immediate (5 min) and delayed (30 min) effects on capture, implying that the microdomains are supported by a combination of Ca²⁺ influx across the cell membrane and Ca²⁺ derived from internal stores. The known properties of the presumptive microdomain targeted proteins suggestroles in a variety of Ca²⁺-dependent basal metabolism and in formation and maintenance of the domains.

Biochemical Activity Architectures Visualized-Using Genetically Encoded Fluorescent Biosensors to Map the Spatial Boundaries of Signaling Compartments

All biological processes arise through the coordinated actions of biochemical pathways. How such functional diversity is achieved by a finite cast of molecular players remains a central mystery in biology. Spatial compartmentation-the idea that biochemical activities are organized around discrete spatial domains within cells-was first proposed nearly 40 years ago and has become firmly rooted in our understanding of how biochemical pathways are regulated to ensure specificity. However, directly interrogating spatial compartmentation and its mechanistic origins has only really become possible in the last 20 or so years, following technological advances such as the development of genetically encoded fluorescent biosensors. These powerful molecular tools permit a direct, real-time visualization of dynamic biochemical processes in native biological contexts, and they are essential for probing the spatial regulation of biochemical activities. In this Account, we review our lab's efforts in developing and using biosensors to map the spatial compartmentation of intracellular signaling pathways and illuminate key mechanisms that establish the boundaries of an intricate biochemical activity architecture. We first discuss the role of regulatory fences, wherein the dynamic activation and deactivation of diffusible messengers produce diverse signaling compartments. For example, we used biosensors for the Ca²⁺ effector calmodulin and its downstream target calcineurin to reveal a spatial gradient of calmodulin that controls the temporal dynamics of calcineurin signaling. Our studies using cyclic adenosine monophosphate (cAMP) biosensors have similarly elucidated fenced cAMP domains generated by competing production and degradation pathways, ranging in size from cell-spanning gradients to nanoscale hotspots. Second, we describe the role played by intracellular membranes in creating unique signaling platforms with distinctive pathway regulation, as revealed through studies using subcellularly targeted fluorescent biosensors. Using biosensors to visualize subcellular extracellular response kinase (ERK) pathway activity, for example, led us to discover a local signaling circuit that mediates distinct plasma membrane ERK dynamics versus global ERK signaling. Similarly, our work developing biosensors to monitor the subcellular mechanistic target of rapamycin complex 1 (mTORC1) signaling allowed us to not only clarify the presence of mTORC1 activity in the nucleus but also identify a novel mechanism governing the activation of mTORC1 in this location. Finally, we detail how molecular assemblies enable the precise spatial tuning of biochemical activity, through investigations enabled by cutting-edge advances in biosensor design. We recently identified liquid-liquid phase separation as a major factor in cAMP compartmentation aided by a new strategy for targeting biosensors to endogenously expressed proteins via genome editing, for instance, and have also been able to directly visualize nanometer-scale protein kinase signalosomes using an entirely new class of biosensors specifically developed for the dynamic super-resolution imaging of live-cell biochemical activities. Our work provides key insights into the molecular logic of spatially regulated signaling and lays the foundation for a broader exploration of biochemical activity architectures across multiple spatial scales.

Ca2+-saturated calmodulin binds tightly to the N-terminal domain of A-type fibroblast growth factor homologous factors

Voltage-gated sodium channels (Na_vs) are tightly regulated by multiple conserved auxiliary proteins, including the four fibroblast growth factor homologous factors (FGFs), which bind the Na_v EF-hand like domain (EFL), and calmodulin (CaM), a multifunctional messenger protein that binds the Na_V IQ motif. The EFL domain and IQ motif are contiguous regions of Na_V cytosolic C-terminal domains (CTD), placing CaM and FGF in close proximity. However, whether the FGFs and CaM act independently, directly associate, or operate through allosteric interactions to regulate channel function is unknown. Titrations monitored by steady-state fluorescence spectroscopy, structural studies with solution NMR, and computational modeling demonstrated for the first time that both domains of (Ca²⁺)₄-CaM (but not apo CaM) directly bind two sites in the N-terminal domain (NTD) of A-type FGF splice variants (FGF11A, FGF12A, FGF13A, and FGF14A) with high affinity. The weaker of the (Ca²⁺)₄-CaM-binding sites was known via electrophysiology to have a role in long-term inactivation of the channel but not known to bind CaM. FGF12A binding to a complex of CaM associated with a fragment of the Na_V1.2 CTD increased the Ca²⁺-binding affinity of both CaM domains, consistent with (Ca²⁺)₄-CaM interacting preferentially with its higher-affinity site in the FGF12A NTD. Thus, A-type FGFs can compete with Na_V IQ motifs for (Ca²⁺)₄-CaM. During spikes in the cytosolic Ca²⁺ concentration that accompany an action potential, CaM may translocate from the Na_V IQ motif to the FGF NTD, or the A-type FGF NTD may recruit a second molecule of CaM to the channel.

The regulatory protein 14-3-3β binds to the IQ motifs of myosin-IC independent of phosphorylation

Myosin-IC (Myo1c) has been proposed to function in delivery of glucose transporter type 4 (GLUT4)-containing vesicles to the plasma membrane in response to insulin stimulation. Current evidence suggests that, upon insulin stimulation, Myo1c is phosphorylated at Ser701, leading to binding of the signaling protein 14-3-3β. Biochemical and functional details of the Myo1c-14-3-3β interaction have yet to be described. Using recombinantly expressed proteins and mass spectrometry-based analyses to monitor Myo1c phosphorylation, along with pulldown, fluorescence binding, and additional biochemical assays, we show here that 14-3-3β is a dimer and, consistent with previous work, that it binds to Myo1c in the presence of calcium. This interaction was associated with dissociation of calmodulin (CaM) from the IQ motif in Myo1c. Surprisingly, we found that 14-3-3β binds to Myo1c independent of Ser701 phosphorylation in vitro Additionally, in contrast to previous reports, we did not observe Myo1c Ser701 phosphorylation by Ca2+/CaM-dependent protein kinase II (CaMKII), although CaMKII phosphorylated four other Myo1c sites. The presence of 14-3-3β had little effect on the actin-activated ATPase or motile activities of Myo1c. Given these results, it is unlikely that 14-3-3β acts as a cargo adaptor for Myo1c-powered transport; rather, we propose that 14-3-3β binds Myo1c in the presence of calcium and stabilizes the calmodulin-dissociated, nonmotile myosin.

Evaluating Calmodulin-Protein Interactions by Rapid Photoactivated Cross-Linking in Live Cells Metabolically Labeled with Photo-Methionine

This work addresses the question of how the Ca²⁺ sensor protein calmodulin shapes cellular responses to Ca²⁺ signals. Proteins interacting with affinity tagged calmodulin were captured by rapid (t_1/2 ≈ 7 s) photoactivated cross-linking under basal conditions, after brief removal of extracellular Ca²⁺ and during a cytosolic [Ca²⁺] transient in cells metabolically labeled with a photoreactive methionine analog. Tagged adducts were stringently enriched, and captured proteins were identified and quantified by LC-MS/MS. A set of 489 proteins including 27 known calmodulin interactors was derived. A threshold for fractional capture was applied to define a high specificity group of 170 proteins, including 22 known interactors, and a low specificity group of 319 proteins. Capture of ∼60% of the high specificity group was affected by manipulations of Ca²⁺, compared with ∼20% of the low specificity group. This suggests that the former is likely to contain novel interactors of physiological significance. The capture of 29 proteins, nearly all high specificity, was decreased by the removal of extracellular Ca²⁺, although this does not affect cytosolic [Ca²⁺]. Capture of half of these was unaffected by the cytosolic [Ca²⁺] transient, consistent with high local [Ca²⁺]. These proteins are hypothesized to reside in or near microdomains of high [Ca²⁺] supported by the Ca²⁺ influx.

γ-Oryzanol Improves Cognitive Function and Modulates Hippocampal Proteome in Mice

Rice (Oryza sativa L.) is the richest source of γ-oryzanol, a compound endowed with antioxidant and anti-inflammatory properties. γ-Oryzanol has been demonstrated to cross the blood-brain barrier in intact form and exert beneficial effects on brain function. This study aimed to clarify the effects of γ-oryzanol in the hippocampus in terms of cognitive function and protein expression. Adult mice were administered with γ-oryzanol 100 mg/kg or vehicle (control) once a day for 21 consecutive days following which cognitive behavior and hippocampal proteome were investigated. Cognitive tests using novel object recognition and Y-maze showed that long-term consumption of γ-oryzanol improves cognitive function in mice. To investigate the hippocampal proteome modulated by γ-oryzanol, 2D-difference gel electrophoresis (2D-DIGE) was performed. Interestingly, we found that γ-oryzanol modulates quantitative changes of proteins involved in synaptic plasticity and neuronal trafficking, neuroprotection and antioxidant activity, and mitochondria and energy metabolism. These findings suggested γ-oryzanol as a natural compound able to maintain and reinforce brain function. Although more intensive studies are needed, we propose γ-oryzanol as a putative dietary phytochemical for preserving brain reserve, the ability to tolerate age-related changes, thereby preventing clinical symptoms or signs of neurodegenerative diseases.

An IQSEC2 Mutation Associated With Intellectual Disability and Autism Results in Decreased Surface AMPA Receptors

We have recently described an A350V mutation in IQSEC2 associated with intellectual disability, autism and epilepsy. We sought to understand the molecular pathophysiology of this mutation with the goal of developing targets for drug intervention. We demonstrate here that the A350V mutation results in interference with the binding of apocalmodulin to the IQ domain of IQSEC2. We further demonstrate that this mutation results in constitutive activation of the guanine nucleotide exchange factor (GEF) activity of IQSEC2 resulting in increased production of the active form of Arf6. In a CRISPR generated mouse model of the A350V IQSEC2 mutation, we demonstrate that the surface expression of GluA2 AMPA receptors in mouse hippocampal tissue was significantly reduced in A350V IQSEC2 mutant mice compared to wild type IQSEC2 mice and that there is a significant reduction in basal synaptic transmission in the hippocampus of A350V IQSEC2 mice compared to wild type IQSEC2 mice. Finally, the A350V IQSEC2 mice demonstrated increased activity, abnormal social behavior and learning as compared to wild type IQSEC2 mice. These findings suggest a model of how the A350V mutation in IQSEC2 may mediate disease with implications for targets for drug therapy. These studies provide a paradigm for a personalized approach to precision therapy for a disease that heretofore has no therapy.

Genetically Encoded Fluorescent Biosensors Illuminate the Spatiotemporal Regulation of Signaling Networks

Cellular signaling networks are the foundation which determines the fate and function of cells as they respond to various cues and stimuli. The discovery of fluorescent proteins over 25 years ago enabled the development of a diverse array of genetically encodable fluorescent biosensors that are capable of measuring the spatiotemporal dynamics of signal transduction pathways in live cells. In an effort to encapsulate the breadth over which fluorescent biosensors have expanded, we endeavored to assemble a comprehensive list of published engineered biosensors, and we discuss many of the molecular designs utilized in their development. Then, we review how the high temporal and spatial resolution afforded by fluorescent biosensors has aided our understanding of the spatiotemporal regulation of signaling networks at the cellular and subcellular level. Finally, we highlight some emerging areas of research in both biosensor design and applications that are on the forefront of biosensor development.

Electrostatic control of calcineurin's intrinsically-disordered regulatory domain binding to calmodulin

Calcineurin (CaN) is a serine/threonine phosphatase that regulates a variety of physiological and pathophysiological processes in mammalian tissue. The calcineurin (CaN) regulatory domain (RD) is responsible for regulating the enzyme's phosphatase activity, and is believed to be highly-disordered when inhibiting CaN, but undergoes a disorder-to-order transition upon diffusion-limited binding with the regulatory protein calmodulin (CaM). The prevalence of polar and charged amino acids in the regulatory domain (RD) suggests electrostatic interactions are involved in mediating calmodulin (CaM) binding, yet the lack of atomistic-resolution data for the bound complex has stymied efforts to probe how the RD sequence controls its conformational ensemble and long-range attractions contribute to target protein binding. In the present study, we investigated via computational modeling the extent to which electrostatics and structural disorder facilitate CaM/CaN association kinetics. Specifically, we examined several RD constructs that contain the CaM binding region (CAMBR) to characterize the roles of electrostatics versus conformational diversity in controlling diffusion-limited association rates, via microsecond-scale molecular dynamics (MD) and Brownian dynamic (BD) simulations. Our results indicate that the RD amino acid composition and sequence length influence both the dynamic availability of conformations amenable to CaM binding, as well as long-range electrostatic interactions to steer association. These findings provide intriguing insight into the interplay between conformational diversity and electrostatically-driven protein-protein association involving CaN, which are likely to extend to wide-ranging diffusion-limited processes regulated by intrinsically-disordered proteins.

Oxidation of Methionine 77 in Calmodulin Alters Mouse Growth and Behavior

Methionine 77 in calmodulin can be stereospecifically oxidized to methionine sulfoxide by mammalian methionine sulfoxide reductase A. Whether this has in vivo significance is unknown. We therefore created a mutant mouse in which wild type calmodulin-1 was replaced by a calmodulin containing a mimic of methionine sulfoxide at residue 77. Total calmodulin levels were unchanged in the homozygous M77Q mutant, which is viable and fertile. No differences were observed on learning tests, including the Morris water maze and associative learning. Cardiac stress test results were also the same for mutant and wild type mice. However, young male and female mice were 20% smaller than wild type mice, although food intake was normal for their weight. Young M77Q mice were notably more active and exploratory than wild type mice. This behavior difference was objectively documented on the treadmill and open field tests. The mutant mice ran 20% longer on the treadmill than controls and in the open field test, the mutant mice explored more than controls and exhibited reduced anxiety. These phenotypic differences bore a similarity to those observed in mice lacking calcium/calmodulin kinase IIα (CaMKIIα). We then showed that MetO77 calmodulin was less effective in activating CaMKIIα than wild type calmodulin. Thus, characterization of the phenotype of a mouse expressing a constitutively active mimic of calmodulin led to the identification of the first calmodulin target that can be differentially regulated by the oxidation state of Met77. We conclude that reversible oxidation of methionine 77 in calmodulin by MSRA has the potential to regulate cellular function.

Calmodulin regulates Cav3 T-type channels at their gating brake

Calcium (Cav1 and Cav2) and sodium channels possess homologous CaM-binding motifs, known as IQ motifs in their C termini, which associate with calmodulin (CaM), a universal calcium sensor. Cav3 T-type channels, which serve as pacemakers of the mammalian brain and heart, lack a C-terminal IQ motif. We illustrate that T-type channels associate with CaM using co-immunoprecipitation experiments and single particle cryo-electron microscopy. We demonstrate that protostome invertebrate (LCav3) and human Cav3.1, Cav3.2, and Cav3.3 T-type channels specifically associate with CaM at helix 2 of the gating brake in the I-II linker of the channels. Isothermal titration calorimetry results revealed that the gating brake and CaM bind each other with high-nanomolar affinity. We show that the gating brake assumes a helical conformation upon binding CaM, with associated conformational changes to both CaM lobes as indicated by amide chemical shifts of the amino acids of CaM in 1H-15N HSQC NMR spectra. Intact Ca2+-binding sites on CaM and an intact gating brake sequence (first 39 amino acids of the I-II linker) were required in Cav3.2 channels to prevent the runaway gating phenotype, a hyperpolarizing shift in voltage sensitivities and faster gating kinetics. We conclude that the presence of high-nanomolar affinity binding sites for CaM at its universal gating brake and its unique form of regulation via the tuning of the voltage range of activity could influence the participation of Cav3 T-type channels in heart and brain rhythms. Our findings may have implications for arrhythmia disorders arising from mutations in the gating brake or CaM.

Resident Calmodulin Primes NMDA Receptors for Ca2+-Dependent Inactivation

N-methyl-d-aspartate (NMDA) receptors are glutamate- and glycine-gated channels that flux Na+ and Ca2+ into postsynaptic neurons during synaptic transmission. The resulting intracellular Ca2+ transient is essential to physiological and pathological processes related to synaptic development, plasticity, and apoptosis. It also engages calmodulin (CaM) to reduce subsequent NMDA receptor activity in a process known as Ca2+-dependent inactivation (CDI). Here, we used whole-cell electrophysiology to measure CDI and computational modeling to dissect the sequence of events that underlies it. With these approaches, we estimate that CaM senses NMDA receptor Ca2+ influx at ∼9 nm from the channel pore. Further, when we controlled the frequency of Ca2+ influx through individual channels, we found that a kinetic model where apoCaM associates with channels before their activation best predicts the measured CDI. These results provide, to our knowledge, novel functional evidence for CaM preassociation to NMDA receptors in living cells. This particular mechanism for autoinhibitory feedback reveals strategies and challenges for Ca2+ regulation in neurons during physiological synaptic activity and disease.

Shear-Induced Nitric Oxide Production by Endothelial Cells

We present a biochemical model of the wall shear stress-induced activation of endothelial nitric oxide synthase (eNOS) in an endothelial cell. The model includes three key mechanotransducers: mechanosensing ion channels, integrins, and G protein-coupled receptors. The reaction cascade consists of two interconnected parts. The first is rapid activation of calcium, which results in formation of calcium-calmodulin complexes, followed by recruitment of eNOS from caveolae. The second is phosphorylation of eNOS by protein kinases PKC and AKT. The model also includes a negative feedback loop due to inhibition of calcium influx into the cell by cyclic guanosine monophosphate (cGMP). In this feedback, increased nitric oxide (NO) levels cause an increase in cGMP levels, so that cGMP inhibition of calcium influx can limit NO production. The model was used to predict the dynamics of NO production by an endothelial cell subjected to a step increase of wall shear stress from zero to a finite physiologically relevant value. Among several experimentally observed features, the model predicts a highly nonlinear, biphasic transient behavior of eNOS activation and NO production: a rapid initial activation due to the very rapid influx of calcium into the cytosol (occurring within 1-5 min) is followed by a sustained period of activation due to protein kinases.

Ophiobolin A, a sesterpenoid fungal phytotoxin, displays different mechanisms of cell death in mammalian cells depending upon the cancer cell origin

Herein we have undertaken a systematic analysis of the effects of the fungal derivative ophiobolin A (OphA) on eight cancer cell lines from different tissue types. The LD50 for each cell line was determined and the change in cell size determined. Flow cytometric analysis and western blotting were used to assess the cell death markers for early apoptosis, late apoptosis and necrosis, and the involvement of the caspase signalling pathway. Alterations in calcium levels and reactive oxygen species were assessed due to their integral involvement in intracellular signalling. Subsequently, the endoplasmic reticulum (ER) and mitochondrial responses were investigated more closely. The extent of ER swelling, and the upregulation of proteins involved in the unfolded protein responses (UPR) were seen to vary according to cell line. The mitochondria were also shown to behave differently in response to the OphA in the different cell lines in terms of the change in membrane potential, the total area of mitochondria in the cell and the number of mitochondrial bifurcations. The data obtained in the present study indicate that the cancer cell lines tested are unable to successfully activate the ER stress/UPR responses, and that the mitochondria appear to be a central player in OphA-induced cancer cell death.

Arrhythmogenic calmodulin mutations impede activation of small-conductance calcium-activated potassium current

BACKGROUND

Apamin-sensitive small-conductance calcium-activated potassium (SK) channels are gated by intracellular Ca(2+) through a constitutive interaction with calmodulin.

OBJECTIVE

We hypothesize that arrhythmogenic human calmodulin mutations impede activation of SK channels.

METHODS

We studied 5 previously published calmodulin mutations (N54I, N98S, D96V, D130G, and F90L). Plasmids encoding either wild-type or mutant calmodulin were transiently transfected into human embryonic kidney 293 cells that stably express subtype 2 of SK protein channels (SK2 cells). Whole-cell voltage-clamp recording was used to determine apamin-sensitive current densities. We also performed optical mapping studies in normal murine hearts to determine the effects of apamin in hearts with (n=7) or without (n=3) pretreatment with sea anemone toxin.

RESULTS

SK2 cells transfected with wild-type calmodulin exhibited an apamin-sensitive current density of 33.6 pA/pF (31.4-36.5 pA/pF) (median and confidence interval 25th-75th percentile), which was significantly higher than that observed for cells transfected with N54I (17.0 pA/pF [14.0-27.7 pA/pF]; P = .016), F90L (22.6 pA/pF [20.3-24.3 pA/pF]; P = .011), D96V (13.0 pA/pF [10.9-15.8 pA/pF]; P = .003), N98S (13.7 pA/pF [8.8-20.4 pA/pF]; P = .005), and D130G (17.6 pA/pF [13.8-24.6 pA/pF]; P = .003). The decrease in SK2 current densities was not associated with a decrease in membrane protein expression or intracellular distribution of the channel protein. Apamin increased the ventricular action potential duration at 80% repolarization (from 79.6 ms [63.4-93.3 ms] to 121.8 ms [97.9-127.2 ms]; P = .010) in hearts pretreated with anemone toxin but not in control hearts.

CONCLUSION

Human arrhythmogenic calmodulin mutations impede the activation of SK2 channels in human embryonic kidney 293 cells.

Estrogen Enhances Linkage in the Vascular Endothelial Calmodulin Network via a Feedforward Mechanism at the G Protein-coupled Estrogen Receptor 1

Estrogen exerts many effects on the vascular endothelium. Calmodulin (CaM) is the transducer of Ca(2+) signals and is a limiting factor in cardiovascular tissues. It is unknown whether and how estrogen modifies endothelial functions via the network of CaM-dependent proteins. Here we show that 17β-estradiol (E2) up-regulates total CaM level in endothelial cells. Concurrent measurement of Ca(2+) and Ca(2+)-CaM indicated that E2 also increases free Ca(2+)-CaM. Pharmacological studies, gene silencing, and receptor expression-specific cell studies indicated that the G protein-coupled estrogen receptor 1 (GPER/GPR30) mediates these effects via transactivation of EGFR and subsequent MAPK activation. The outcomes were then examined on four distinct members of the intracellular CaM target network, including GPER/GPR30 itself and estrogen receptor α, the plasma membrane Ca(2+)-ATPase (PMCA), and endothelial nitric-oxide synthase (eNOS). E2 substantially increases CaM binding to estrogen receptor α and GPER/GPR30. Mutations that reduced CaM binding to GPER/GPR30 in separate binding domains do not affect GPER/GPR30-Gβγ preassociation but decrease GPER/GPR30-mediated ERK1/2 phosphorylation. E2 increases CaM-PMCA association, but the expected stimulation of Ca(2+) efflux is reversed by E2-stimulated tyrosine phosphorylation of PMCA. These effects sustain Ca(2+) signals and promote Ca(2+)-dependent CaM interactions with other CaM targets. Consequently, E2 doubles CaM-eNOS interaction and also promotes dual phosphorylation of eNOS at Ser-617 and Ser-1179. Calculations using in-cell and in vitro data revealed substantial individual and combined contribution of these effects to total eNOS activity. Taken together, E2 generates a feedforward loop via GPER/GPR30, which enhances Ca(2+)/CaM signals and functional linkage in the endothelial CaM target network.

The activating role of phospho-(Tyr)-calmodulin on the epidermal growth factor receptor

The existence of a calmodulin (CaM)/phospho-(Tyr)-CaM cycle involved in the regulation of the epidermal growth factor receptor could have important consequences for the control of cell proliferation, as its alteration could potentially result in uncontrolled tumour growth.

Evolutionary and functional perspectives on signaling from neuronal surface to nucleus

Reliance on Ca(2+) signaling has been well-preserved through the course of evolution. While the complexity of Ca(2+) signaling pathways has increased, activation of transcription factors including CREB by Ca(2+)/CaM-dependent kinases (CaMKs) has remained critical for long-term plasticity. In C. elegans, the CaMK family is made up of only three members, and CREB phosphorylation is mediated by CMK-1, the homologue of CaMKI. CMK-1 nuclear translocation directly regulates adaptation of thermotaxis behavior in response to changes in the environment. In mammals, the CaMK family has been expanded from three to ten members, enabling specialization of individual elements of a signal transduction pathway and increased reliance on the CaMKII subfamily. This increased complexity enables private line communication between Ca(2+) sources at the cell surface and specific cellular targets. Using both new and previously published data, we review the mechanism of a γCaMKII-CaM nuclear translocation. This intricate pathway depends on a specific role for multiple Ca(2+)/CaM-dependent kinases and phosphatases: α/βCaMKII phosphorylates γCaMKII to trap CaM; CaN dephosphorylates γCaMKII to dispatch it to the nucleus; and PP2A induces CaM release from γCaMKII so that CaMKK and CaMKIV can trigger CREB phosphorylation. Thus, while certain basic elements have been conserved from C. elegans, evolutionary modifications offer opportunities for targeted communication, regulation of key nodes and checkpoints, and greater specificity and flexibility in signaling.

Neurogranin regulates CaM dynamics at dendritic spines

Calmodulin (CaM) plays a key role in synaptic function and plasticity due to its ability to mediate Ca²⁺ signaling. Therefore, it is essential to understand the dynamics of CaM at dendritic spines. In this study we have explored CaM dynamics using live-cell confocal microscopy and fluorescence recovery after photobleaching (FRAP) to study CaM diffusion. We find that only a small fraction of CaM in dendritic spines is immobile. Furthermore, the diffusion rate of CaM was regulated by neurogranin (Ng), a CaM-binding protein enriched at dendritic spines. Interestingly, Ng did not influence the immobile fraction of CaM at recovery plateau. We have previously shown that Ng enhances synaptic strength in a CaM-dependent manner. Taken together, these data indicate that Ng-mediated enhancement of synaptic strength is due to its ability to target, rather than sequester, CaM within dendritic spines.

Effects of 39 Compounds on Calmodulin-Regulated Adenylyl Cyclases AC1 and Bacillus anthracis Edema Factor

Adenylyl cyclases (ACs) catalyze the conversion of ATP into the second messenger cAMP. Membranous AC1 (AC1) is involved in processes of memory and learning and in muscle pain. The AC toxin edema factor (EF) of Bacillus anthracis is involved in the development of anthrax. Both ACs are stimulated by the eukaryotic Ca²⁺-sensor calmodulin (CaM). The CaM-AC interaction could constitute a potential target to enhance or impair the AC activity of AC1 and EF to intervene in above (patho)physiological mechanisms. Thus, we analyzed the impact of 39 compounds including typical CaM-inhibitors, an anticonvulsant, an anticholinergic, antidepressants, antipsychotics and Ca²⁺-antagonists on CaM-stimulated catalytic activity of AC1 and EF. Compounds were tested at 10 μM, i.e., a concentration that can be reached therapeutically for certain antidepressants and antipsychotics. Calmidazolium chloride decreased CaM-stimulated AC1 activity moderately by about 30%. In contrast, CaM-stimulated EF activity was abrogated by calmidazolium chloride and additionally decreased by chlorpromazine, felodipine, penfluridol and trifluoperazine by about 20–40%. The activity of both ACs was decreased by calmidazolium chloride in the presence and absence of CaM. Thus, CaM-stimulated AC1 activity is more insensitive to inhibition by small molecules than CaM-stimulated EF activity. Inhibition of AC1 and EF by calmidazolium chloride is largely mediated via a CaM-independent allosteric mechanism.

Dynamic visualization of calcium-dependent signaling in cellular microdomains

Cells rely on the coordinated action of diverse signaling molecules to sense, interpret, and respond to their highly dynamic external environment. To ensure the specific and robust flow of information, signaling molecules are often spatially organized to form distinct signaling compartments, and our understanding of the molecular mechanisms that guide intracellular signaling hinges on the ability to directly probe signaling events within these cellular microdomains. Ca(2+) signaling in particular owes much of its functional versatility to this type of exquisite spatial regulation. As discussed below, a number of methods have been developed to investigate the mechanistic and functional implications of microdomains of Ca(2+) signaling, ranging from the application of Ca(2+) buffers to the direct and targeted visualization of Ca(2+) signaling microdomains using genetically encoded fluorescent reporters.

Glatiramer acetate and nanny proteins restrict access of the multiple sclerosis autoantigen myelin basic protein to the 26S proteasome

We recently showed that myelin basic protein (MBP) is hydrolyzed by 26S proteasome without ubiquitination. The previously suggested concept of charge-mediated interaction between MBP and the proteasome led us to attempt to compensate or mimic its positive charge to inhibit proteasomal degradation. We demonstrated that negatively charged actin and calmodulin (CaM), as well as basic histone H1.3, inhibit MBP hydrolysis by competing with the proteasome and MBP, respectively, for binding their counterpart. Interestingly, glatiramer acetate (GA), which is used to treat multiple sclerosis (MS) and is structurally similar to MBP, inhibits intracellular and in vitro proteasome-mediated MBP degradation. Therefore, the data reported in this study may be important for myelin biogenesis in both the normal state and pathophysiological conditions.

Ion channel engineering: perspectives and strategies

Ion channels facilitate the passive movement of ions down an electrochemical gradient and across lipid bilayers in cells. This phenomenon is essential for life and underlies many critical homeostatic processes in cells. Ion channels are diverse and differ with respect to how they open and close (gating) and to their ionic conductance/selectivity (permeation). Fundamental understanding of ion channel structure-function mechanisms, their physiological roles, how their dysfunction leads to disease, their utility as biosensors, and development of novel molecules to modulate their activity are important and active research frontiers. In this review, we focus on ion channel engineering approaches that have been applied to investigate these aspects of ion channel function, with a major emphasis on voltage-gated ion channels.

Calmodulin-controlled spatial decoding of oscillatory Ca2+ signals by calcineurin

Calcineurin is responsible for mediating a wide variety of cellular processes in response to dynamic calcium (Ca(2+)) signals, yet the precise mechanisms involved in the spatiotemporal control of calcineurin signaling are poorly understood. Here, we use genetically encoded fluorescent biosensors to directly probe the role of cytosolic Ca(2+) oscillations in modulating calcineurin activity dynamics in insulin-secreting MIN6 β-cells. We show that Ca(2+) oscillations induce distinct temporal patterns of calcineurin activity in the cytosol and plasma membrane vs at the ER and mitochondria in these cells. Furthermore, we found that these differential calcineurin activity patterns are determined by variations in the subcellular distribution of calmodulin (CaM), indicating that CaM plays an active role in shaping both the spatial and temporal aspects of calcineurin signaling. Together, our findings provide new insights into the mechanisms by which oscillatory signals are decoded to generate specific functional outputs within different cellular compartments.

Biosensor-based approach identifies four distinct calmodulin-binding domains in the G protein-coupled estrogen receptor 1

The G protein-coupled estrogen receptor 1 (GPER) has been demonstrated to participate in many cellular functions, but its regulatory inputs are not clearly understood. Here we describe a new approach that identifies GPER as a calmodulin-binding protein, locates interaction sites, and characterizes their binding properties. GPER coimmunoprecipitates with calmodulin in primary vascular smooth muscle cells under resting conditions, which is enhanced upon acute treatment with either specific ligands or a Ca(2+)-elevating agent. To confirm direct interaction and locate the calmodulin-binding domain(s), we designed a series of FRET biosensors that consist of enhanced cyan and yellow fluorescent proteins flanking each of GPER's submembrane domains (SMDs). Responses of these biosensors showed that all four submembrane domains directly bind calmodulin. Modifications of biosensor linker identified domains that display the strongest calmodulin-binding affinities and largest biosensor dynamics, including a.a. 83-93, 150-175, 242-259, 330-351, corresponding respectively to SMDs 1, 2, 3, and the juxta-membranous section of SMD4. These biosensors bind calmodulin in a strictly Ca(2+)-dependent fashion and with disparate affinities in the order SMD2>SMD4>SMD3>SMD1, apparent K d values being 0.44 ± 0.03, 1.40 ± 0.16, 8.01 ± 0.29, and 136.62 ± 6.56 µM, respectively. Interestingly, simultaneous determinations of biosensor responses and suitable Ca(2+) indicators identified separate Ca(2+) sensitivities for their interactions with calmodulin. SMD1-CaM complexes display a biphasic Ca(2+) response, representing two distinct species (SMD1 sp1 and SMD1 sp2) with drastically different Ca(2+) sensitivities. The Ca(2+) sensitivities of CaM-SMDs interactions follow the order SMD1sp1>SMD4>SMD2>SMD1sp2>SMD3, EC50(Ca(2+)) values being 0.13 ± 0.02, 0.75 ± 0.05, 2.38 ± 0.13, 3.71 ± 0.13, and 5.15 ± 0.25 µM, respectively. These data indicate that calmodulin may regulate GPER-dependent signaling at the receptor level through multiple interaction sites. FRET biosensors represent a simple method to identify unknown calmodulin-binding domains in G protein-coupled receptors and to quantitatively assess binding properties.

The many faces of calmodulin in cell proliferation, programmed cell death, autophagy, and cancer

Calmodulin (CaM) is a ubiquitous Ca(2+) receptor protein mediating a large number of signaling processes in all eukaryotic cells. CaM plays a central role in regulating a myriad of cellular functions via interaction with multiple target proteins. This review focuses on the action of CaM and CaM-dependent signaling systems in the control of vertebrate cell proliferation, programmed cell death and autophagy. The significance of CaM and interconnected CaM-regulated systems for the physiology of cancer cells including tumor stem cells, and processes required for tumor progression such as growth, tumor-associated angiogenesis and metastasis are highlighted. Furthermore, the potential targeting of CaM-dependent signaling processes for therapeutic use is discussed.

A mathematical model of T lymphocyte calcium dynamics derived from single transmembrane protein properties

Fate decision processes of T lymphocytes are crucial for health and disease. Whether a T lymphocyte is activated, divides, gets anergic, or initiates apoptosis depends on extracellular triggers and intracellular signaling. Free cytosolic calcium dynamics plays an important role in this context. The relative contributions of store-derived calcium entry and calcium entry from extracellular space to T lymphocyte activation are still a matter of debate. Here we develop a quantitative mathematical model of T lymphocyte calcium dynamics in order to establish a tool which allows to disentangle cause-effect relationships between ion fluxes and observed calcium time courses. The model is based on single transmembrane protein characteristics which have been determined in independent experiments. This reduces the number of unknown parameters in the model to a minimum and ensures the predictive power of the model. Simulation results are subsequently used for an analysis of whole cell calcium dynamics measured under various experimental conditions. The model accounts for a variety of these conditions, which supports the suitability of the modeling approach. The simulation results suggest a model in which calcium dynamics dominantly relies on the opening of channels in calcium stores while calcium entry through calcium-release activated channels (CRAC) is more associated with the maintenance of the T lymphocyte calcium levels and prevents the cell from calcium depletion. Our findings indicate that CRAC guarantees a long-term stable calcium level which is required for cell survival and sustained calcium enhancement.

Visualizing CaMKII and CaM activity: a paradigm of compartmentalized signaling

Calcium (Ca(2+)) has long been recognized as a crucial intracellular messenger attaining stimuli-specific cellular outcomes via localized signaling. Ca(2+)-binding proteins, such as calmodulin (CaM), and its target proteins are key to the segregation and refinement of these Ca(2+)-dependent signaling events. This review not only summarizes the recent technological advances enabling the study of subcellular Ca(2+)-CaM and Ca(2+)-CaM-dependent protein kinase (CaMKII) signaling events but also highlights the outstanding challenges in the field.

Signaling through myosin light chain kinase in smooth muscles

Ca(2+)/calmodulin-dependent myosin light chain kinase (MLCK) phosphorylates smooth muscle myosin regulatory light chain (RLC) to initiate contraction. We used a tamoxifen-activated, smooth muscle-specific inactivation of MLCK expression in adult mice to determine whether MLCK was differentially limiting in distinct smooth muscles. A 50% decrease in MLCK in urinary bladder smooth muscle had no effect on RLC phosphorylation or on contractile responses, whereas an 80% decrease resulted in only a 20% decrease in RLC phosphorylation and contractile responses to the muscarinic agonist carbachol. Phosphorylation of the myosin light chain phosphatase regulatory subunit MYPT1 at Thr-696 and Thr-853 and the inhibitor protein CPI-17 were also stimulated with carbachol. These results are consistent with the previous findings that activation of a small fraction of MLCK by limiting amounts of free Ca(2+)/calmodulin combined with myosin light chain phosphatase inhibition is sufficient for robust RLC phosphorylation and contractile responses in bladder smooth muscle. In contrast, a 50% decrease in MLCK in aortic smooth muscle resulted in 40% inhibition of RLC phosphorylation and aorta contractile responses, whereas a 90% decrease profoundly inhibited both responses. Thus, MLCK content is limiting for contraction in aortic smooth muscle. Phosphorylation of CPI-17 and MYPT1 at Thr-696 and Thr-853 were also stimulated with phenylephrine but significantly less than in bladder tissue. These results indicate differential contributions of MLCK to signaling. Limiting MLCK activity combined with modest Ca(2+) sensitization responses provide insights into how haploinsufficiency of MLCK may result in contractile dysfunction in vivo, leading to dissections of human thoracic aorta.

A theory of plasma membrane calcium pump stimulation and activity

The ATP-driven Plasma Membrane Calcium pump or Ca(2+)-ATPase (PMCA) is characterized by a high affinity for calcium and a low transport rate compared to other transmembrane calcium transport proteins. It plays a crucial role for calcium extrusion from cells. Calmodulin is an intracellular calcium buffering protein which is capable in its Ca(2+) liganded form of stimulating the PMCA by increasing both the affinity to calcium and the maximum calcium transport rate. We introduce a new model of this stimulation process and derive analytical expressions for experimental observables in order to determine the model parameters on the basis of specific experiments. We furthermore develop a model for the pumping activity. The pumping description resolves the seeming contradiction of the Ca(2+):ATP stoichiometry of 1:1 during a translocation step and the observation that the pump binds two calcium ions at the intracellular site. The combination of the calcium pumping and the stimulation model correctly describes PMCA function. We find that the processes of calmodulin-calcium complex attachment to the pump and of stimulation have to be separated. Other PMCA properties are discussed in the framework of the model. The presented model can serve as a tool for calcium dynamics simulations and provides the possibility to characterize different pump isoforms by different type-specific parameter sets.

Interplay between calmodulin and phosphatidylinositol 4,5-bisphosphate in Ca2+-induced inactivation of transient receptor potential vanilloid 6 channels

The epithelial Ca(2+) channel transient receptor potential vanilloid 6 (TRPV6) undergoes Ca(2+)-induced inactivation that protects the cell from toxic Ca(2+) overload and may also limit intestinal Ca(2+) transport. To dissect the roles of individual signaling pathways in this phenomenon, we studied the effects of Ca(2+), calmodulin (CaM), and phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) in excised inside-out patches. The activity of TRPV6 strictly depended on the presence of PI(4,5)P(2), and Ca(2+)-CaM inhibited the channel at physiologically relevant concentrations. Ca(2+) alone also inhibited TRPV6 at high concentrations (IC(50) = ∼20 μM). A double mutation in the distal C-terminal CaM-binding site of TRPV6 (W695A/R699E) essentially eliminated inhibition by CaM in excised patches. In whole cell patch clamp experiments, this mutation reduced but did not eliminate Ca(2+)-induced inactivation. Providing excess PI(4,5)P(2) reduced the inhibition by CaM in excised patches and in planar lipid bilayers, but PI(4,5)P(2) did not inhibit binding of CaM to the C terminus of the channel. Overall, our data show a complex interplay between CaM and PI(4,5)P(2) and show that Ca(2+), CaM, and the depletion of PI(4,5)P(2) all contribute to inactivation of TRPV6.

Regulation of the ligand-dependent activation of the epidermal growth factor receptor by calmodulin

Calmodulin (CaM) is the major component of calcium signaling pathways mediating the action of various effectors. Transient increases in the intracellular calcium level triggered by a variety of stimuli lead to the formation of Ca(2+)/CaM complexes, which interact with and activate target proteins. In the present study the role of Ca(2+)/CaM in the regulation of the ligand-dependent activation of the epidermal growth factor receptor (EGFR) has been examined in living cells. We show that addition of different cell permeable CaM antagonists to cultured cells or loading cells with a Ca(2+) chelator inhibited ligand-dependent EGFR auto(trans)phosphorylation. This occurred also in the presence of inhibitors of protein kinase C, CaM-dependent protein kinase II and calcineurin, which are known Ca(2+)- and/or Ca(2+)/CaM-dependent EGFR regulators, pointing to a direct effect of Ca(2+)/CaM on the receptor. Furthermore, we demonstrate that down-regulation of CaM in conditional CaM knock out cells stably transfected with the human EGFR decreased its ligand-dependent phosphorylation. Substitution of six basic amino acid residues within the CaM-binding domain (CaM-BD) of the EGFR by alanine resulted in a decreased phosphorylation of the receptor and of its downstream substrate phospholipase Cγ1. These results support the hypothesis that Ca(2+)/CaM regulates the EGFR activity by directly interacting with the CaM-BD of the receptor located at its cytosolic juxtamembrane region.

In calmodulin-IQ domain complexes, the Ca(2+)-free and Ca(2+)-bound forms of the calmodulin C-lobe direct the N-lobe to different binding sites

We have investigated the roles played by the calmodulin (CaM) N- and C-lobes in establishing the conformations of CaM-IQ domain complexes in different Ca(2+)-free and Ca(2+)-bound states. Our results indicate a dominant role for the C-lobe in these complexes. When the C-lobe is Ca(2+)-free, it directs the N-lobe to a binding site within the IQ domain consensus sequence. It appears that the N-lobe must be Ca(2+)-free to interact productively with this site. When the C-lobe is Ca(2+)-bound, it directs the N-lobe to a site upstream of the consensus sequence, and it appears that the N-lobe must be Ca(2+)-bound to interact productively with this site. A model for switching in CaM-IQ domain complexes is presented in which the N-lobe adopts bound and extended positions that depend on the status of the Ca(2+)-binding sites in each CaM lobe and the compositions of the two N-lobe binding sites. Ca(2+)-dependent changes in the conformation of the bound C-lobe that appear to be responsible for directed N-lobe binding are also identified. Changes in the equilibria between extended and bound N-lobe positions may control bridging interactions in which the extended N-lobe is bound to another CaM-binding domain. Ca(2+)-dependent control of bridging interactions with CaM has been implicated in the regulation of ion channel and unconventional myosin activities.

Calmodulin- and Ca2+-dependent facilitation and inactivation of the Cav1.2 Ca2+ channels in guinea-pig ventricular myocytes

The L-type Ca(2+) channel (Ca(V)1.2) shows clear Ca(2+)-dependent facilitation and inactivation. Here we have examined the effects of calmodulin (CaM) and Ca(2+) on Ca(2+) channel in guinea-pig ventricular myocytes in the inside-out patch mode, where rundown of the channels was controlled. At a free [Ca(2+)] of 0.1 microM, CaM (0.15, 0.7, 1.4, 2.1, 3.5, and 7.0 microM) + ATP (2.4 mM) induced channel activities of 27%, 98%, 142%, 222%, 65%, and 20% relative to the control activity, respectively, showing a bell-shaped relationship. Similar results were observed at a free [Ca(2+)] <0.01 microM or with a Ca(2+)-insensitive mutant, CaM(1234), suggesting that apoCaM may induce facilitation and inactivation of the channel activity. The bell-shaped curve of CaM was shifted to the lower concentration side with increasing [Ca(2+)]. A simple model for CaM- and Ca(2+)-dependent modulations of the channel activity, which involves two CaM-binding sites, was proposed. We suggest that both apoCaM and Ca(2+)/CaM can induce facilitation and inactivation of Ca(V)1.2 Ca(2+) channels and that the basic role of Ca(2+) is to accelerate CaM-dependent facilitation and inactivation.

Variations at the semiconserved glycine in the IQ domain consensus sequence have a major impact on Ca2+-dependent switching in calmodulin-IQ domain complexes

We have replaced the semiconserved Gly in the IQ domain consensus sequence with Ala, Arg, or Met in a reference sequence and determined how this affects its complexes with calmodulin. The K(d) for the Ca(2+)-free reference complex is 2.4 +/- 0.3 microM. The Ala and Arg replacements increase this to 5.4 +/- 0.4 and 6.2 +/- 0.5 microM, while the Met increases it to 26.4 +/- 2.5 microM. When Ca(2+) is bound to both calmodulin lobes, the K(d) for the reference complex is not significantly affected, but the K(d) for the Ala variant decreases to 0.9 +/- 0.04 microM, and the values for the Arg and Met variants decrease to 0.4 +/- 0.03 microM. Using mutant calmodulins, we defined the effect of Ca(2+) binding to each lobe, with the C-terminal preceding the N-terminal (C-->N) or vice versa (N-->C). In the C-->N order the first step increases the reference K(d) approximately 5-fold, while it decreases the values for the variants approximately 2- to approximately 10-fold. The second step decreases the K(d) values for the all of the complexes approximately 5-fold, suggesting that the N-terminal lobe does not interact with the semiconserved position after the first step. In the N-->C order the first step increases the K(d) values for the reference complex and Met and Ala variants approximately 15- to approximately 200-fold but does not affect the value for the Arg variant. The second step decreases the K(d) values for the reference and Arg variant approximately 10- and approximately 15-fold and the Ala and Met variants approximately 2000-fold. Thus, both steps in the N-->C order are sensitive to variations at the semiconserved position, while only the first is in the C-->N order. Due to energy coupling, this order is followed under equilibrium conditions.

Calcium-dependent association of calmodulin with the rubella virus nonstructural protease domain

The rubella virus (RUBV) nonstructural (NS) protease domain, a Ca(2+)- and Zn(2+)-binding papain-like cysteine protease domain within the nonstructural replicase polyprotein precursor, is responsible for the self-cleavage of the precursor into two mature products, P150 and P90, that compose the replication complex that mediates viral RNA replication; the NS protease resides at the C terminus of P150. Here we report the Ca(2+)-dependent, stoichiometric association of calmodulin (CaM) with the RUBV NS protease. Co-immunoprecipitation and pulldown assays coupled with site-directed mutagenesis demonstrated that both the P150 protein and a 110-residue minidomain within NS protease interacted directly with Ca(2+)/CaM. The specific interaction was mapped to a putative CaM-binding domain. A 32-mer peptide (residues 1152-1183, denoted as RUBpep) containing the putative CaM-binding domain was used to investigate the association of RUBV NS protease with CaM or its N- and C-terminal subdomains. We found that RUBpep bound to Ca(2+)/CaM with a dissociation constant of 100-300 nm. The C-terminal subdomain of CaM preferentially bound to RUBpep with an affinity 12.5-fold stronger than the N-terminal subdomain. Fluorescence, circular dichroism and NMR spectroscopic studies revealed a "wrapping around" mode of interaction between RUBpep and Ca(2+)/CaM with substantially more helical structure in RUBpep and a global structural change in CaM upon complex formation. Using a site-directed mutagenesis approach, we further demonstrated that association of CaM with the CaM-binding domain in the RUBV NS protease was necessary for NS protease activity and infectivity.

The IQ domains in neuromodulin and PEP19 represent two major functional classes

The affinities of Ca(2+)-saturated and Ca(2+)-free calmodulin for a fluorescent reporter construct containing the PEP19 IQ domain differ by a factor of approximately 100, with K(d) values of 11.0 +/- 1.2 and 1128.4 +/- 176.5 muM, respectively, while the affinities of a reporter containing the neuromodulin IQ domain are essentially identical, with K(d) values of 2.9 +/- 0.3 and 2.4 +/- 0.3 muM, respectively. When Ca(2+) is bound only to the C-terminal pair of Ca(2+)-binding sites in calmodulin, the K(d) value for the PEP19 reporter complex is decreased approximately 5-fold, while the value for the neuromodulin reporter complex is increased by the same factor. When Ca(2+) is bound only to the N-terminal pair of Ca(2+)-binding sites, the K(d) value for the PEP19 reporter complex is unaffected, but the value for the complex with the neuromodulin reporter is increased approximately 12-fold. These functional differences are largely ascribed to three differences in the CaM-binding sequences of the two reporters. Replacement of a central Gly in the neuromodulin IQ domain with a Lys at this position in PEP19 almost entirely accounts for the distinctive patterns of Ca(2+)-dependent stability changes exhibited by the two complexes. Replacement of a Lys immediately before the "IQ" amino acid pair in the neuromodulin sequence with the Ala in PEP19 accounts for the remaining Ca(2+)-dependent differences. Replacement of an Ala in the N-terminal half of the neuromodulin sequence with the Gln in PEP19 accounts for approximately half of the Ca(2+)-independent difference in the stabilities of the two reporter complexes, with the Ca(2+)-independent effect of the Lys replacement accounting for most of the remainder. Since the central Gly in the neuromodulin sequence is conserved in half of all known IQ domains, these results suggest that the presence or absence of this residue defines two major functional classes.

Calmodulin-mediated regulation of the epidermal growth factor receptor

In this review, we first describe the mechanisms by which the epidermal growth factor receptor generates a Ca(2+) signal and, subsequently, we compile the available experimental evidence regarding the role that the Ca(2+)/calmodulin complex, formed after the rise in cytosolic free Ca(2+) concentration, exerts on the receptor. We focus not only on the indirect action that Ca(2+)/calmodulin exerts on the epidermal growth factor receptor, as a result of the activation of distinct calmodulin-dependent kinases, but also, and more extensively, on the direct interaction of Ca(2+)/calmodulin with the receptor. We also describe several mechanistic models that could account for the Ca(2+)/calmodulin-mediated regulation of epidermal growth factor receptor activity. The control exerted by calmodulin on distinct epidermal growth factor receptor-mediated cellular functions is also discussed. Finally, the phosphorylation of this Ca(2+) sensor by the epidermal growth factor receptor is highlighted.