Tyrosine phosphorylation modulates the interaction of calmodulin with its target proteins

New Insights into the Regulation of mTOR Signaling via Ca2+-Binding Proteins

Environmental factors are important regulators of cell growth and proliferation. Mechanistic target of rapamycin (mTOR) is a central kinase that maintains cellular homeostasis in response to a variety of extracellular and intracellular inputs. Dysregulation of mTOR signaling is associated with many diseases, including diabetes and cancer. Calcium ion (Ca²⁺) is important as a second messenger in various biological processes, and its intracellular concentration is tightly regulated. Although the involvement of Ca²⁺ mobilization in mTOR signaling has been reported, the detailed molecular mechanisms by which mTOR signaling is regulated are not fully understood. The link between Ca²⁺ homeostasis and mTOR activation in pathological hypertrophy has heightened the importance in understanding Ca²⁺-regulated mTOR signaling as a key mechanism of mTOR regulation. In this review, we introduce recent findings on the molecular mechanisms of regulation of mTOR signaling by Ca²⁺-binding proteins, particularly calmodulin (CaM).

Calreticulin nuclear translocalization alleviates CaM/CaMKII/CREB signaling pathway to enhance chemosensitivity in HDAC inhibitor-resistant hepatocellular carcinoma cells

Calreticulin (CRT) is located in the endoplasmic reticulum (ER), it helps proteins fold correctly inside the ER, and acts as a modulator of Ca²⁺ homeostasis. Aberrant expression of CRT is implicated in several cancer types, qualifying CRT as a potential therapeutic target. However, it remains unclear how CRT affects specific oncogenic pathways. In this study, we used histone deacetylase inhibitors (HDACis) to establish drug-resistant liver cancer cells and further analyzed the molecular mechanism of development of drug resistance in those cells. The 2D gel electrophoresis and RT-PCR data showed that CRT was downregulated in HDACis-resistant cells by comparing with HA22T parental cells. We previously elucidated the development of drug-resistance in HCC cells via activation of PP1-eIF2α pathway, but not via ER stress pathway. Here, we show that thapsigargin induced ER stress through mechanism other than ER stress downstream protein GRP78-PERK to regulate CRT expression in HDACis-R cells. Moreover, the expression level of CRT was not the main cause of apoptosis in HDACis-resistant cells. Mechanistic studies identified the apoptosis factors in the nucleus-the HDACis-mediated overexpression of CRT, CRT translocation to the cell nucleus, and reduced CaM/CaMKII/CREB pathway-that led to chemosensitivity in HDACis-R HCC cells.

Creating a Selective Nanobody Against 3-Nitrotyrosine Containing Proteins

A critical step in developing therapeutics for oxidative stress-related pathologies is the ability to determine which specific modified protein species are innocuous by-products of pathology and which are causative agents. To achieve this goal, technologies are needed that can identify, characterize and quantify oxidative post translational modifications (oxPTMs). Nanobodies (Nbs) represent exquisite tools for intracellular tracking of molecules due to their small size, stability and engineerability. Here, we demonstrate that it is possible to develop a selective Nb against an oxPTM protein, with the key advance being the use of genetic code expansion (GCE) to provide an efficient source of the large quantities of high-quality, homogenous and site-specific oxPTM-containing protein needed for the Nb selection process. In this proof-of-concept study, we produce a Nb selective for a 3-nitrotyrosine (nitroTyr) modified form of the 14-3-3 signaling protein with a lesser recognition of nitroTyr in other protein contexts. This advance opens the door to the GCE-facilitated development of other anti-PTM Nbs.

Tyrosine nitration on calmodulin enhances calcium-dependent association and activation of nitric-oxide synthase

Production of reactive oxygen species caused by dysregulated endothelial nitric-oxide synthase (eNOS) activity is linked to vascular dysfunction. eNOS is a major target protein of the primary calcium-sensing protein calmodulin. Calmodulin is often modified by the main biomarker of nitroxidative stress, 3-nitrotyrosine (nitroTyr). Despite nitroTyr being an abundant post-translational modification on calmodulin, the mechanistic role of this modification in altering calmodulin function and eNOS activation has not been investigated. Here, using genetic code expansion to site-specifically nitrate calmodulin at its two tyrosine residues, we assessed the effects of these alterations on calcium binding by calmodulin and on binding and activation of eNOS. We found that nitroTyr-calmodulin retains affinity for eNOS under resting physiological calcium concentrations. Results from in vitro eNOS assays with calmodulin nitrated at Tyr-99 revealed that this nitration reduces nitric-oxide production and increases eNOS decoupling compared with WT calmodulin. In contrast, calmodulin nitrated at Tyr-138 produced more nitric oxide and did so more efficiently than WT calmodulin. These results indicate that the nitroTyr post-translational modification, like tyrosine phosphorylation, can impact calmodulin sensitivity for calcium and reveal Tyr site-specific gain or loss of functions for calmodulin-induced eNOS activation.

L-selectin: A Major Regulator of Leukocyte Adhesion, Migration and Signaling

L-selectin (CD62L) is a type-I transmembrane glycoprotein and cell adhesion molecule that is expressed on most circulating leukocytes. Since its identification in 1983, L-selectin has been extensively characterized as a tethering/rolling receptor. There is now mounting evidence in the literature to suggest that L-selectin plays a role in regulating monocyte protrusion during transendothelial migration (TEM). The N-terminal calcium-dependent (C-type) lectin domain of L-selectin interacts with numerous glycans, including sialyl Lewis X (sLe^x) for tethering/rolling and proteoglycans for TEM. Although the signals downstream of L-selectin-dependent adhesion are poorly understood, they will invariably involve the short 17 amino acid cytoplasmic tail. In this review we will detail the expression of L-selectin in different immune cell subsets, and its influence on cell behavior. We will list some of the diverse glycans known to support L-selectin-dependent adhesion, within luminal and abluminal regions of the vessel wall. We will describe how each domain within L-selectin contributes to adhesion, migration and signal transduction. A significant focus on the L-selectin cytoplasmic tail and its proposed contribution to signaling via the ezrin-radixin-moesin (ERM) family of proteins will be outlined. Finally, we will discuss how ectodomain shedding of L-selectin during monocyte TEM is essential for the establishment of front-back cell polarity, bestowing emigrated cells the capacity to chemotax toward sites of damage.

Characterization of calmodulin in the clam Anodonta woodiana: differential expressions in response to environmental Ca2+ and Cd2+

Aims

To explore effect of Ca2+ and Cd2+ on the calmodulin (CaM), one complete cDNA sequence (AwCaM1) was cloned and characterized from the freshwater mussel Anodonta woodiana and its expressions were analyzed.

Materials and methods

The AwCaM1 was cloned from the A. woodiana using the rapid amplification of cDNA ends methods and its expression was determined by real-time PCR.

Results

In the hepatopancreas, AwCaM1 expression was up-regulated with a time and dose dependent pattern in the Ca2+ treated groups (0.01, 0.02, 0.04 and 0.08 mg/L) during experiment observed, and increased more than 56.15% (p<0.05) compared with that of control group. AwCaM1 mRNA level increased more 65.04% (p<0.05) in the Cd2+ treated groups (8 and 16 mg/L). In the gill, AwCaM1 expression increased more than 79.41% (p<0.05) compared with that of control group in all the Ca2+ treated groups, and more than 88.23% (p<0.05) in all the Cd2+ treated groups.

Conclusion

These results indicated that up-regulations of AwCaM1 expression in bivalve A. woodiana are associated with Ca2+ absorb and environmental adaption derived from Ca2+ and Cd2+ treatment.

Src-family tyrosine kinases and the Ca2+ signal

In this review, we shall describe the rich crosstalk between non-receptor Src-family kinases (SFKs) and the Ca²⁺ transient generated in activated cells by a variety of extracellular and intracellular stimuli, resulting in diverse signaling events. The exchange of information between SFKs and Ca²⁺ is reciprocal, as it flows in both directions. These kinases are main actors in pathways leading to the generation of the Ca²⁺ signal, and reciprocally, the Ca²⁺ signal modulates SFKs activity and functions. We will cover how SFKs participate in the generation of the cytosolic Ca²⁺ rise upon activation of a series of receptors and the mechanism of clearance of this Ca²⁺ signal. The role of SFKs modulating Ca²⁺-translocating channels participating in these events will be amply discussed. Finally, the role of the Ca²⁺ sensor protein calmodulin on the activity of c-Src, and potentially on other SFKs, will be outlined as well. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Quantum chemical studies on nucleophilic sites in calcium ion bound zwitterionic calmodulin loops

Quantum chemical calculation on Ca²⁺ bound zwitterionic calmodulin-loops shows terminal capping contributions localized near HOMO and LUMO, which decay exponentially and presence of nucleophilic site at the phenyl-ring O_h of Y99.

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.

Quantum Chemical Studies on Stability and Chemical Activities in Calcium Ion Bound Calmodulin Loops

Quantum chemical (QC) calculations for macromolecules require truncation of the molecule, highlighting the portion of interest due to heavy computation cost. As a result, an estimation of the effects of truncation is important to interpret the energy spectrum of such calculations. We perform density functional theory based QC calculations on calcium ion bound EF-hand loops of Calmodulin isolated from the crystal structure in an implicit solvent. We find that the terminal contributions of neutral capping are negligible across the entire ground-state energy spectrum. The coordination energy range and the nature of hybridization of the coordination state molecular orbitals remain qualitatively similar across these loops. While the HOMO and LUMO of loops in the N-terminal domain are dominated by the acidic aspartates, and the polar/hydrophobic residues, respectively, these levels of the C-terminal domain loops show strong localized electron density on the phenyl rings of the tyrosines. The Fukui index calculation identifies the hydroxyl oxygen in the phenyl ring of Y99 as a potent nucleophile. Our analysis indicates a general way of interpreting the electronic energy spectra to understand stability and functions of large biomolecules where the truncation of the molecule and, hence, the terminal capping effects are inevitable.

Chemical shift perturbations induced by residue specific mutations of CaM interacting with NOS peptides

The regulation of nitric oxide synthase (NOS) by calmodulin (CaM) plays a major role in a number of key physiological and pathological processes. A detailed molecular level picture of how this regulation is achieved is critical for drug development and for our understanding of protein regulation in general. CaM is a small acidic calcium binding protein and is required to fully activate NOS. The exact mechanism of how CaM activates NOS is not fully understood at this time. Studies have shown CaM to act like a switch that causes a conformational change in NOS to allow for the electron transfer between the reductase and oxygenase domains through a process that is thought to be highly dynamic. The interaction of CaM with NOS is modified by a number of post-translation modifications including phosphorylation. Here we present backbone and sidechain (1)H, (15)N NMR assignments of modified CaM interacting with NOS peptides which provides the basis for a detailed study of CaM-NOS interaction dynamics using (15)N relaxation methods.

Characterization and identification of calmodulin and calmodulin binding proteins in hemocyte of the black tiger shrimp (Penaeus monodon)

Calmodulin (CaM), a ubiquitous intracellular calcium (Ca(2+)) sensor in all eukaryotic cells, is one of the well-known signaling proteins. Previously, CaM gene has shown a high transcriptional level in hemocyte of the pathogen infected shrimp, suggesting that shrimp CaM does not only regulate Ca(2+) metabolism, but is also involved in immune response cascade. In the present study, the CaM gene of shrimp Penaeus monodon was identified and the recombinant P.monodon CaM (rPmCaM) was produced and biochemically characterized. The identification of CaM-binding proteins was also performed. The PmCaM cDNA consisted of an open reading frame of 447 bp encoding for 149 amino acid residues with a calculated mass of 16,810 Da and an isoelectric point of 4.09. Tissue distribution showed that the PmCaM transcript was expressed in all examined tissues. The results of gel mobility shift assay, circular dichroism spectroscopy and fluorescence spectroscopy all confirmed that the conformational changes of the rPmCaM were observed after the calcium binding. According to the gene silencing of PmCaM transcript levels, the shrimp's susceptibility to pathogenic Vibrio harveyi infection increased in comparison with that of the control groups. Protein pull-down assay and LC-MS/MS analysis were performed to identify rPmCaM-binding proteins involved in shrimp immune responses and transglutaminase, elongation factor 1-alpha, elongation factor 2 and actin were found. However, by computational analysis, only the first three proteins contained CaM-binding domain. These findings suggested that PmCaM may play an important role in regulation of shrimp immune system.

Characterization of phospho-(tyrosine)-mimetic calmodulin mutants

Calmodulin (CaM) phosphorylated at different serine/threonine and tyrosine residues is known to exert differential regulatory effects on a variety of CaM-binding enzymes as compared to non-phosphorylated CaM. In this report we describe the preparation and characterization of a series of phospho-(Y)-mimetic CaM mutants in which either one or the two tyrosine residues present in CaM (Y99 and Y138) were substituted to aspartic acid or glutamic acid. It was expected that the negative charge of the respective carboxyl group of these amino acids mimics the negative charge of phosphate and reproduce the effects that distinct phospho-(Y)-CaM species may have on target proteins. We describe some physicochemical properties of these CaM mutants as compared to wild type CaM, after their expression in Escherichia coli and purification to homogeneity, including: i) changes in their electrophoretic mobility in the absence and presence of Ca2+; ii) ultraviolet (UV) light absorption spectra, far- and near-UV circular dichroism data; iii) thermal stability in the absence and presence of Ca2+; and iv) Tb3+-emitted fluorescence upon tyrosine excitation. We also describe some biochemical properties of these CaM mutants, such as their differential phosphorylation by the tyrosine kinase c-Src, and their action as compared to wild type CaM, on the activity of two CaM-dependent enzymes: cyclic nucleotide phosphodiesterase 1 (PDE1) and endothelial nitric oxide synthase (eNOS) assayed in vitro.

The tyrosine phosphatase PTPN14 (Pez) inhibits metastasis by altering protein trafficking

Factors secreted by tumor cells shape the local microenvironment to promote invasion and metastasis, as well as condition the premetastatic niche to enable secondary-site colonization and growth. In addition to this secretome, tumor cells have increased abundance of growth-promoting receptors at the cell surface. We found that the tyrosine phosphatase PTPN14 (also called Pez, which is mutated in various cancers) suppressed metastasis by reducing intracellular protein trafficking through the secretory pathway. Knocking down PTPN14 in tumor cells or injecting the peritoneum of mice with conditioned medium from PTPN14-deficient cell cultures promoted the growth and metastasis of breast cancer xenografts. Loss of catalytically functional PTPN14 increased the secretion of growth factors and cytokines, such as IL-8 (interleukin-8), and increased the abundance of EGFR (epidermal growth factor receptor) at the cell surface of breast cancer cells and of FLT4 (vascular endothelial growth factor receptor 3) at the cell surface of primary lymphatic endothelial cells. We identified RIN1 (Ras and Rab interactor 1) and PRKCD (protein kinase C-δ) as binding partners and substrates of PTPN14. Similar to cells overexpressing PTPN14, receptor trafficking to the cell surface was inhibited in cells that lacked PRKCD or RIN1 or expressed a nonphosphorylatable RIN1 mutant, and cytokine secretion was decreased in cells treated with PRKCD inhibitors. Invasive breast cancer tissue had decreased expression of PTPN14, and patient survival was worse when tumors had increased expression of the genes encoding RIN1 or PRKCD. Thus, PTPN14 prevents metastasis by restricting the trafficking of both soluble and membrane-bound proteins.

Structurally distinct Ca(2+) signaling domains of sperm flagella orchestrate tyrosine phosphorylation and motility

Spermatozoa must leave one organism, navigate long distances, and deliver their paternal DNA into a mature egg. For successful navigation and delivery, a sperm-specific calcium channel is activated in the mammalian flagellum. The genes encoding this channel (CatSpers) appear first in ancient uniflagellates, suggesting that sperm use adaptive strategies developed long ago for single-cell navigation. Here, using genetics, super-resolution fluorescence microscopy, and phosphoproteomics, we investigate the CatSper-dependent mechanisms underlying this flagellar switch. We find that the CatSper channel is required for four linear calcium domains that organize signaling proteins along the flagella. This unique structure focuses tyrosine phosphorylation in time and space as sperm acquire the capacity to fertilize. In heterogeneous sperm populations, we find unique molecular phenotypes, but only sperm with intact CatSper domains that organize time-dependent and spatially specific protein tyrosine phosphorylation successfully migrate. These findings illuminate flagellar adaptation, signal transduction cascade organization, and fertility.

Characterization of cDNAs for calmodulin and calmodulin-like protein in the freshwater mussel Hyriopsis cumingii: differential expression in response to environmental Ca(2+) and calcium binding of recombinant proteins

Calmodulin and calmodulin-like protein are two crucial calcium regulators in bivalves. However, molecular characteristics and expression patterns of these genes in the freshwater mussel are poorly understood. In this study, two cDNAs encoding novel calmodulin and calmodulin-like protein (HcCaM and HcCaLP) were cloned and characterized from the freshwater pearl mussel Hyriopsis cumingii. The full-length cDNA of HcCaM was 726 bp, including a 118-bp 5'-untranslated region (UTR), a 447-bp open reading frame (ORF), and a 161-bp 3'-UTR. The 1217-bp HcCaLP cDNA comprised of a 51-bp 5'-UTR, a 447-bp ORF, and a 716-bp 3'-UTR. The potential phosphorylation sites of, Arg(80) and Phe(100) in deduced HcCaM were mutated to Thr(80) and Tyr(100) in HcCaLP. Tissue-specific expression analysis revealed that HcCaM mRNA was prominently expressed in the gill, mantle center, and foot. In contrast, HcCaLP mRNA was mainly expressed in the mantle edge. The recombinant HcCaM and HcCaLP proteins expressed in Escherichia coli showed the typical Ca(2+) dependent electrophoretic shift characterization as CaM and differed in the calcium binding affinity. The calcium stimulation test that lasted 5 weeks implied that HcCaM and HcCaLP had differential expression patterns in response to various environmental Ca(2+) concentrations (0.25-1.25 mM). The expression of HcCaM mRNA was up-regulated by low Ca(2+) concentration (0.25 mM), and the highest expression of HcCaLP mRNA occurred under Ca(2+) concentration of 1 mM.

PTPRZ1 regulates calmodulin phosphorylation and tumor progression in small-cell lung carcinoma

BACKGROUND

Small-cell lung carcinoma (SCLC) is a neuroendocrine tumor subtype and comprises approximately 15% of lung cancers. Because SCLC is still a disease with a poor prognosis and limited treatment options, there is an urgent need to develop targeted molecular agents for this disease.

METHODS

We screened 20 cell lines from a variety of pathological phenotypes established from different organs by RT-PCR. Paraffin-embedded tissue from 252 primary tumors was examined for PTPRZ1 expression using immunohistochemistry. shRNA mediated PTPRZ1 down-regulation was used to study impact on tyrosine phosphorylation and in vivo tumor progression in SCLC cell lines.

RESULTS

Here we show that PTPRZ1, a member of the protein tyrosine- phosphatase receptor (PTPR) family, is highly expressed in SCLC cell lines and specifically exists in human neuroendocrine tumor (NET) tissues. We also demonstrate that binding of the ligand of PTPRZ1, pleiotrophin (PTN), activates the PTN/PTPRZ1 signaling pathway to induce tyrosine phosphorylation of calmodulin (CaM) in SCLC cells, suggesting that PTPRZ1 is a regulator of tyrosine phosphorylation in SCLC cells. Furthermore, we found that PTPRZ1 actually has an important oncogenic role in tumor progression in the murine xenograft model.

CONCLUSION

PTPRZ1 was highly expressed in human NET tissues and PTPRZ1 is an oncogenic tyrosine phosphatase in SCLCs. These results imply that a new signaling pathway involving PTPRZ1 could be a feasible target for treatment of NETs.

Structure and dynamics of calmodulin (CaM) bound to nitric oxide synthase peptides: effects of a phosphomimetic CaM mutation

Nitric oxide synthase (NOS) plays a major role in a number of key physiological and pathological processes. Knowledge of how this is regulated is important. The small acidic calcium binding protein, calmodulin (CaM), is required to fully activate the enzyme. The exact mechanism of how CaM activates NOS is not fully understood. Studies have shown CaM to act like a switch that causes a conformational change in NOS to allow for the transfer of an electron between the reductase and oxygenase domains through a process that is thought to be highly dynamic. To investigate the dynamic properties of CaM-NOS interactions, we determined the solution structure of CaM bound to the inducible NOS (iNOS) and endothelial NOS (eNOS) CaM binding region peptides. In addition, we investigated the effect of CaM phosphorylation. Tyrosine 99 (Y99) of CaM is reported to be phosphorylated in vivo. We have produced a phosphomimetic Y99E CaM to investigate the structural and functional effects that the phosphorylation of this residue may have on nitric oxide production. All three mammalian NOS isoforms were included in the investigation. Our results show that a phosphomimetic Y99E CaM significantly reduces the maximal synthase activity of eNOS by 40% while having little effect on nNOS or iNOS activity. A comparative nuclear magnetic resonance study between phosphomimetic Y99E CaM and wild-type CaM bound to the eNOS CaM binding region peptide was performed. This investigation provides important insights into how the increased electronegativity of a phosphorylated CaM protein affects the binding, dynamics, and activation of the NOS enzymes.

Calreticulin and focal-contact-dependent adhesion

Cell adhesion is regulated by a variety of Ca2+-regulated pathways that depend on Ca2+-binding proteins. One such protein is calreticulin, an ER-resident protein. Calreticulin signalling from within the ER can affect processes outside the ER, such as expression of several adhesion-related genes, most notably vinculin and fibronectin. In addition, changes in the expression level of calreticulin strongly affect tyrosine phosphorylation of cellular proteins, which is known to affect many adhesion-related functions. While calreticulin has been localized to cellular compartments other than the ER, it appears that only the ER-resident calreticulin affects focal-contact-dependent adhesion. In contrast, calreticulin residing outside the ER may be involved in contact disassembly and other adhesion phenomena. Here, we review the role of calreticulin in focal contact initiation, stabilization, and turnover. We propose that calreticulin may regulate cell-substratum adhesion by participating in an "ER-to-nucleus" signalling and in parallel "ER-to-cell surface" signalling based on posttranslational events.

Investigation of phosphorylation site responsible for CaLP (P. fucata) nucleo-cytoplasmic shuttling triggered by overexpression of p21Cip1

Calmodulin (CaM) is a highly conserved and ubiquitous Ca(2+)-binding protein regulating intracellular Ca(2+) concentration by acting as a sensor of this divalent cation in eukaryotic cells. Being such a very important signal sensor, CaM is susceptible to undergo many posttranslational modifications. One of these important modifications is its phosphorylation. Our previous investigations showed that CaM and calmodulin-like protein (CaLP) cloned from Pinctada fucata have many different characteristics in spite of their high similarity to each other. We have narrowed down that the C-terminal domains of CaM and CaLP are responsible for their discrepant subcellular localizations and shuttling of CaLP when it is co-transfected with p21(Cip1), which is commonly considered as an important cell cycle regulating protein. In this study, we first predicted the potential phosphorylation site responsible for the shuttling and confirmed by fluorescence confocal microscopy. Together with fluorescence activated cell sorter analysis, we further investigated the releasing ability of wild type and point mutated CaLP from arrested cell cycle caused by p21(Cip1) overexpression. By performing pull-down analysis and phosphorylation status of CaLP in cytoplasm fraction of transfected COS-7 cells with CaLP alone and phosphorylation status of CaLP in nuclear fraction of co-transfected COS-7 cells with CaLP and p21(Cip), we propose that the CaLP staying in the cytoplasm is in the state of phosphorylation, but when p21(Cip1) is overexpressed in mammalian cells, some signal triggers CaLP dephosphorylation and translocation into the nucleus.

Inhibition of adipogenesis: a new job for the ER Ca2+ pool

Adipogenesis is the process of differentiation of adipocytes from mesenchymal multipotent cells through adipocyte precursors. In this issue, a study by the groups of Opas and Michalak (Szabo, E., Y. Qiu, S. Baksh, M. Michalak, and M. Opas. 2008. J. Cell. Biol. 182:103–116) demonstrates that this process is repressed by increasing intracellular Ca²⁺, which, in turn, is dependent on the expression of calreticulin, the major Ca²⁺-binding protein of the endoplasmic reticulum lumen.

Calreticulin inhibits commitment to adipocyte differentiation

Calreticulin, an endoplasmic reticulum (ER) resident protein, affects many critical cellular functions, including protein folding and calcium homeostasis. Using embryonic stem cells and 3T3-L1 preadipocytes, we show that calreticulin modulates adipogenesis. We find that calreticulin-deficient cells show increased potency for adipogenesis when compared with wild-type or calreticulin-overexpressing cells. In the highly adipogenic crt^−/− cells, the ER lumenal calcium concentration was reduced. Increasing the ER lumenal calcium concentration led to a decrease in adipogenesis. In calreticulin-deficient cells, the calmodulin–Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) pathway was up-regulated, and inhibition of CaMKII reduced adipogenesis. Calreticulin inhibits adipogenesis via a negative feedback mechanism whereby the expression of calreticulin is initially up-regulated by peroxisome proliferator–activated receptor γ (PPARγ). This abundance of calreticulin subsequently negatively regulates the expression of PPARγ, lipoprotein lipase, CCAAT enhancer–binding protein α, and aP2. Thus, calreticulin appears to function as a Ca²⁺-dependent molecular switch that regulates commitment to adipocyte differentiation by preventing the expression and transcriptional activation of critical proadipogenic transcription factors.

Differential calreticulin expression affects focal contacts via the calmodulin/CaMK II pathway

Calreticulin is an ER calcium-storage protein, which influences gene expression and cell adhesion. In this study, we analysed the differences in adhesive properties of calreticulin under- and overexpressing fibroblasts in relation to the calmodulin- and calcium/calmodulin-dependent kinase II (CaMK II)-dependent signalling pathways. Cells stably underexpressing calreticulin had elevated expression of calmodulin, activated CaMK II, activated ERK and activated c-src. Inhibition of calmodulin by W7, and CaMK II by KN-62, caused the otherwise weekly adhesive calreticulin underexpressing cells to behave like the overexpressing cells, via induction of increased cell spreading. Increased vinculin, activated paxillin, activated focal adhesion kinase and fibronectin levels were observed upon inhibition of either the calmodulin or the CaMK II signalling pathways, which was accompanied by an increase in cell spreading and focal contact formation. Both KN-62 and W7 treatment increased cell motility in underexpressing cells, but W7 treatment led to loss of directionality. Thus, both the calmodulin and CaMK II signalling pathways influence cellular spreading and motility, but subtle differences exist in their distal effects on motility effectors.

Frazzled regulation of myosin II activity in the Drosophila embryonic CNS

Frazzled (Fra) is a chemoattractive guidance receptor regulating the cytoskeletal dynamics underlying growth cone steering at the Drosophila embryonic midline. Here, by genetically evaluating the role of Rho GTPases in Fra signaling in vivo, we uncover a Rho-dependent pathway apparently regulating conventional myosin II activity. Midline crossing errors induced by expressing activated Cdc42(v12) or Rac(v12) are suppressed by a heterozygous loss of fra(4) signaling but, in a Fra(wt) gain-of-function condition, no interaction is detected. In contrast, the frequency of crossovers is enhanced approximately 5-fold when Fra(wt) is co-expressed with activated Rho(v14) and this interaction specifically requires the cytoplasmic P3 motif of Fra. Expression of Rho(v14) and activated MLCK (ctMLCK) synergistically increase ectopic crossovers and both require phosphorylation of the regulatory light chain (Sqh) of myosin II. Abelson tyrosine kinase may also help regulate myosin II activity. Heterozygous abl(4) abolishes the midline crossing errors induced by ctMLCK alone or in combination with Fra(wt); suppression of Rho(v14) crossovers is not observed. Interestingly, an interaction between Fra and an activated Abl (Bcr-Abl) also specifically requires the P3 motif. Therefore, the P3 motif of Frazzled appears to initiate Rho and Abl dependent signals to directly or indirectly regulate myosin II activity in growth cones.

GTP avoidance in Tetrahymena thermophila requires tyrosine kinase activity, intracellular calcium, NOS, and guanylyl cyclase

Guanosine 5'-triphosphate (GTP) is a chemorepellent in Tetrahymena thermophila that has been shown to stimulate cell division as well as ciliary reversal. Previous studies have proposed that GTP avoidance is linked to a receptor-mediated, calcium-based depolarization. However, the intracellular mechanisms involved in GTP avoidance have not been previously documented. In this study, we examine the hypothesis that GTP signals through a tyrosine kinase pathway in T. thermophila. Using behavioral assays, enzyme immunosorbent assays, Western blotting, and immunofluorescence, we present data that implicate a tyrosine kinase, phospholipase C, intracellular calcium, nitric oxide synthase (NOS) and guanylyl cyclase in GTP signaling. The tyrosine kinase inhibitor genistein eliminates GTP avoidance in Tetrahymena in behavioral assays. Similarly, pharmacological inhibitors of phospholipase C, NOS, and guanylyl cyclase all eliminated Tetrahymena avoidance to GTP. Immunofluorescence data shows evidence of tyrosine kinase activity in the cilia, suggesting that this enzyme activity could be directly involved in ciliary reversal.

Ca2+-Calmodulin and Janus Kinase 2 Are Required for Activation of Sodium-Proton Exchange by the Gi-Coupled 5-Hydroxytryptamine1aReceptor

The type 1 sodium-proton exchanger (NHE-1) is expressed ubiquitously and regulates key cellular functions, including mitogenesis, cell volume, and intracellular pH. Despite its importance, the signaling pathways that regulate NHE-1 remain incompletely defined. In this work, we present evidence that stimulation of the 5-hydroxytryptamine 1A (5-HT1A) receptor results in the formation of a signaling complex that includes activated Janus kinase 2 (Jak2), Ca2+/calmodulin (CaM), and NHE-1, and which involves tyrosine phosphorylation of CaM. The signaling pathway also involves rapid agonist-induced association of CaM and NHE-1 as assessed by coimmunoprecipitation studies and by bioluminescence resonance energy transfer studies in living cells. We propose that NHE-1 is activated through this pathway: 5-HT1A receptor --> G(i2)alpha and/or G(i3)alpha --> Jak2 activation --> tyrosine phosphorylation of CaM --> increased binding of CaM to NHE-1 --> induction of a conformational change in NHE-1 that unmasks an obscured proton-sensing and/or proton-transporting region of NHE-1 --> activation of NHE-1. The G(i/o)-coupled 5-HT1A receptor now joins a handful of Gq-coupled receptors and hypertonic shock as upstream activators of this emerging pathway. In the course of this work, we have presented clear evidence that CaM can be activated through tyrosine phosphorylation in the absence of a significant role for elevated intracellular Ca2+. We have also shown for the first time that the association of CaM with NHE-1 in living cells is a dynamic process.

Activation of the calcium/calmodulin-dependent protein kinases as a consequence of oxidative stress

Oxygen radicals have diverse effects on cells. In many cases, exposure to reactive oxygen intermediates (ROI) can induce cell death. Conversely, there is also evidence that suggests oxygen radicals can activate signaling pathways that are thought to prevent cell death. In this review, the authors discuss the finding that hydrogen peroxide and ROI-generating treatments trigger the activation of the calcium/calmodulin-dependent kinases (CaM-kinases), and the potential role this activation has in preventing apoptosis. Evidence is presented that CaM-kinase activation occurs by both calcium dependent- and independent-pathways in response to ROIs. In addition, the idea is discussed that ROIs have the potential to lead to the phosphorylation of calmodulin and through this mechanism potentiate the activation of the CaM-kinases. The concept that inhibition of the CaM-kinases as a mechanism to sensitize cells to the damaging effects of ROIs is also presented. Contrasting these studies, evidence is presented that exposure of the CaM-kinases directly to hydrogen peroxide also has the apparent ability to inhibit their activity.

MARCKS is a major PKC-dependent regulator of calmodulin targeting in smooth muscle

Calmodulin (CaM) is a ubiquitous transducer of intracellular Ca(2+) signals and plays a key role in the regulation of the function of all cells. The interaction of CaM with a specific target is determined not only by the Ca(2+)-dependent affinity of calmodulin but also by the proximity to that target in the cellular environment. Although a few reports of stimulus-dependent nuclear targeting of CaM have appeared, the mechanisms by which CaM is targeted to non-nuclear sites are less clear. Here, we investigate the hypothesis that MARCKS is a regulator of the spatial distribution of CaM within the cytoplasm of differentiated smooth-muscle cells. In overlay assays with portal-vein homogenates, CaM binds predominantly to the MARCKS-containing band. MARCKS is abundant in portal-vein smooth muscle ( approximately 16 microM) in comparison to total CaM ( approximately 40 microM). Confocal images indicate that calmodulin and MARCKS co-distribute in unstimulated freshly dissociated smooth-muscle cells and are co-targeted simultaneously to the cell interior upon depolarization. Protein-kinase-C (PKC) activation triggers a translocation of CaM that precedes that of MARCKS and causes multisite, sequential MARCKS phosphorylation. MARCKS immunoprecipitates with CaM in a stimulus-dependent manner. A synthetic MARCKS effector domain (ED) peptide labelled with a photoaffinity probe cross-links CaM in smooth-muscle tissue in a stimulus-dependent manner. Both cross-linking and immunoprecipitation increase with increased Ca(2+) concentration, but decrease with PKC activation. Introduction of a nonphosphorylatable MARCKS decoy peptide blocks the PKC-mediated targeting of CaM. These results indicate that MARCKS is a significant, PKC-releasable reservoir of CaM in differentiated smooth muscle and that it contributes to CaM signalling by modulating the intracellular distribution of CaM.

Phosphorylation of calmodulin fragments by protein kinase CK2. Mechanistic aspects and structural consequences

Calmodulin is phosphorylated in vivo and in vitro by protein kinase CK2 in a manner that is unique among CK2 substrates for being inhibited by the regulatory beta-subunit of the kinase and dramatically enhanced by polybasic peptides. Using synthetic fragments of calmodulin variably encompassing the CK2 phosphorylation sites here we show that individual phosphorylation of Thr79, Ser81, Ser101, and Thr117 is critically influenced by the size and composition of the peptides and that the C-terminal domain of calmodulin is implicated both in down-regulation of calmodulin phosphorylation by the beta-subunit and in its abnormal responsiveness to polylysine. A far-Western blot analysis discloses polylysine-dependent interaction between calmodulin and the N-terminal domain of the beta-subunit. We also show that phosphorylation of Ser81 hampers subsequent phosphorylation of Thr79 and by itself promotes the unfolding of the central helix, whose flexibility is instrumental to the interaction with calmodulin-dependent enzymes. Collectively taken, our data are consistent with a multifaceted regulation of calmodulin phosphorylation through the concerted action of distinct CaM domains, the catalytic and regulatory subunits of CK2, and polycationic effectors mimicking in vivo the effect of polylysine.

NR2B selective NMDA receptor antagonist CP-101,606 prevents levodopa-induced motor response alterations in hemi-parkinsonian rats

Sensitization of NMDA receptors containing the NR2B subunit has been increasingly associated with various forms of synaptic plasticity, including those implicated in the pathogenesis of extrapyramidal motor dysfunction. To determine whether activation of NR2B containing receptors contributes to the development and maintenance of levodopa-induced response changes in parkinsonian animals, we evaluated the effects of the selective NR2B antagonist CP-101,606 on these response alterations in unilateral 6-hydroxydopamine (6-OHDA) lesioned rats. Three weeks of twice-daily levodopa treatment decreased the duration of the rotational response to acute levodopa challenge. The response alteration was associated with an increase in GluR1 (S831) phosphorylation in medium spiny neurons of the dorsolateral striatum. Both the attenuated rotational response and augmented GluR1 phosphorylation were decreased by CP-101,606 treatment. These CP-101,606 effects were observed when the compound was administered either at the end of chronic levodopa treatment (ameliorative effect) or together with the twice-daily levodopa treatment for 3 weeks (preventive effect). Furthermore, concurrent administration of CP-101,606 with levodopa potentiated the ability of levodopa challenge to reverse the 6-OHDA lesion-induced contralateral forelimb movement deficit as measured in a drag test. These results suggest that activation of NR2B subunit containing NMDA receptors contributes to both the development and maintenance of levodopa-induced motor response alterations, through a mechanism that involves an increase in GluR1 phosphorylation in striatal spiny neurons.

Redox regulation of the calcium/calmodulin-dependent protein kinases

Reactive oxygen intermediates (ROI) have been viewed traditionally as damaging to the cell. However, a predominance of evidence has shown that ROI can also function as important activators of key cellular processes, and ROI have been shown to play a vital role in cell signaling networks. The calcium/calmodulin-dependent protein kinases (CaM kinases) are a family of related kinases that are activated in response to increased intracellular calcium concentrations. In this report we demonstrate that hydrogen peroxide treatment results in the activation of both CaM kinase II and IV in Jurkat T lymphocytes. Surprisingly, this activation occurs in the absence of any detectable calcium flux, suggesting a novel means for the activation of these kinases. Treatment of Jurkat cells with phorbol 12-myristate 13-acetate (PMA), which does not cause a calcium flux, also activated the CaM kinases. The addition of catalase to the cultures inhibited PMA-induced activation of the CaM kinases, suggesting that similar to hydrogen peroxide, PMA also activates the CaM kinases via the production of ROI. One mechanism by which this likely occurs is through oxidation and consequential inactivation of cellular phosphatases. In support of this concept, okadaic acid and microcystin-LR, which are inhibitors of protein phosphatase 2A (PP2A), induced CaM kinase II and IV activity in these cells. Overall, these results demonstrate a novel mechanism by which ROI can induce CaM kinase activation in T lymphocytes.

Hypertonicity activates Na+/H+ exchange through Janus kinase 2 and calmodulin

The type 1 sodium-hydrogen exchanger (NHE-1) is a ubiquitous electroneutral membrane transporter that is activated by hypertonicity in many cells. NHE-1 may be an important pathway for Na(+) entry during volume restoration, yet the molecular mechanisms underlying the osmotic regulation of NHE-1 are poorly understood. In the present study we conducted a screen for important signaling molecules that could be involved in hypertonicity-induced activation of NHE-1 in CHO-K1 cells. Hypertonicity rapidly activated NHE-1 in a concentration-dependent manner as assessed by proton microphysiometry and by measurements of intracellular pH on a FLIPR (fluorometric imaging plate reader). Inhibitors of Ca(2+)/calmodulin (CaM) and Janus kinase 2 (Jak2) attenuated this activation, whereas neither calcium chelation nor inhibitors of protein kinase C, the Ras-ERK1/2 pathway, Src kinase, and Ca(2+)/calmodulin-dependent enzymes had significant effects. Hypertonicity also resulted in the rapid tyrosine phosphorylation of Jak2 and STAT3 (the major substrate of Jak2) and CaM. Phosphorylation of Jak2 and CaM were blocked by AG490, an inhibitor of Jak2. Immunoprecipitation studies showed that hypertonicity stimulates the assembly of a signaling complex that includes CaM, Jak2, and NHE-1. Formation of the complex could be blocked by AG490. Thus, we propose that hypertonicity induces activation of NHE-1 in CHO-K1 cells in large part through the following pathway: hypertonicity --> Jak2 phosphorylation and activation --> tyrosine phosphorylation of CaM --> association of CaM with NHE-1 --> NHE-1 activation.

Calmodulin-dependent regulation of hypotonicity-induced translocation of ENaC in renal epithelial A6 cells

Hypotonicity stimulates translocation of epithelial Na(+) channel (ENaC) to the apical membrane from the intracellular store site of ENaC by activating protein tyrosine kinase (PTK) in renal epithelial A6 cells. Based upon the fact that calmodulin shows its action on other enzymes through PTK caused phosphorylation of tyrosine residues of calmodulin itself, we studied whether a calmodulin-dependent pathway is involved in the action of hypotonicity on ENaC. W7, an antagonist of calmodulin, diminished the stimulatory action of hypotonicity on ENaC, irrespective of W7 treatment before or after application of hypotonicity. Calmodulin is known to regulate three pathways: (1) protein phosphatase 2B (PP2B), (2) Ca(2+)/calmodulin-dependent protein kinase II (CaMK II), and (3) myosin light chain kinase (MLCK). Pretreatment with cyclosporin A, an inhibitor of PP2B, did not influence the hypotonicity action on ENaC. The hypotonicity action on ENaC was partially inhibited by pretreatment with KN93, an inhibitor of CaMK II, but not by addition of KN93 after hypotonic stimulation had been applied. ML-7, an inhibitor of MLCK, showed the action similar to KN93. These observations indicate that: (1) the hypotonicity-induced translocation of ENaC depends on CaMK II and MLCK and (2) ENaC translocated to the apical membrane by hypotonicity is maintained in its activity and/or stability at the apical membrane through a calmodulin-dependent pathway.

Participation of the calcium/calmodulin-dependent kinases in hydrogen peroxide-induced Ikappa B phosphorylation in human T lymphocytes

NF-kappaB is an important transcription factor that has a role in a variety of responses such as inflammation, oncogenesis, apoptosis, and viral replication. Oxidative stress is well known to induce the activation of NF-kappaB. Cells can be exposed to either endogenously produced oxidants or oxidants produced by surrounding cells. In addition, ischemia reperfusion and certain cancer therapies such as chemotherapy and photodynamic therapy are thought to result in oxygen radical production. Because of the important role that NF-kappaB has in multiple responses, it is critical to determine the mechanisms by which oxidative stress induces NF-kappaB activity. We report that the calmodulin antagonist W-7 and the calcium/calmodulin-dependent (CaM) kinase inhibitors KN-93 and K252a, can block oxidative stress-induced IkappaB phosphorylation in Jurkat T lymphocytes. Furthermore, KN-93 but not KN-92 can block hydrogen peroxide-induced Akt and IKK phosphorylation. In addition, we found that expression of a kinase-dead CaM-KIV construct in two cell lines inhibits IkappaB phosphorylation or degradation and that expression of CaM-KIV augments hydrogen peroxide-induced IkappaB phosphorylation and degradation. Although the CaM kinases appear to be required for this response, increases in intracellular calcium do not appear to be required. These results identify the CaM kinases as potential targets that can be used to minimize NF-kappaB activation in response to oxidative stress.

Phosphorylation of calmodulin. Functional implications

Calmodulin (CaM) is phosphorylated in vitro and in vivo by multiple protein-serine/threonine and protein-tyrosine kinases. Casein kinase II and myosin light-chain kinase are two of the well established protein-serine/threonine kinases implicated in this process. On the other hand, within the protein-tyrosine kinases involved in the phosphorylation of CaM are receptors with tyrosine kinase activity, such as the insulin receptor and the epidermal growth factor receptor, and nonreceptor protein-tyrosine kinases, such as several members of the Src family kinases, Janus kinase 2, and p38Syk. The phosphorylation of CaM brings important physiological consequences for the cell as the diverse phosphocalmodulin species have differential actions as compared to nonphosphorylated CaM when acting on different CaM-dependent systems. In this review we will summarize the progress made on this topic as the first report on phosphorylation of CaM was published almost two decades ago. We will emphasize the description of the phosphorylation events mediated by the different protein kinases not only in the test tube but in intact cells, the phosphorylation-mediated changes of CaM activity, its action on CaM-dependent systems, and the functional repercussion of these phosphorylation processes in the physiology of the cell.

Cross signaling, cell specificity, and physiology

The literature on intracellular signal transduction presents a confusing picture: every regulatory factor appears to be regulated by all signal transduction cascades and to regulate all cell processes. This contrasts with the known exquisite specificity of action of extracellular signals in different cell types in vivo. The confusion of the in vitro literature is shown to arise from several causes: the inevitable artifacts inherent in reductionism, the arguments used to establish causal effect relationships, the use of less than adequate models (cell lines, transfections, acellular systems, etc.), and the implicit assumption that networks of regulations are universal whereas they are in fact cell and stage specific. Cell specificity results from the existence in any cell type of a unique set of proteins and their isoforms at each level of signal transduction cascades, from the space structure of their components, from their combinatorial logic at each level, from the presence of modulators of signal transduction proteins and of modulators of modulators, from the time structure of extracellular signals and of their transduction, and from quantitative differences of expression of similar sets of factors.

Calcineurin acts via the C-terminus of NR2A to modulate desensitization of NMDA receptors

Phosphatase IIb (calcineurin, CaN) can reduce N-methyl-D-aspartate (NMDA) synaptic responses by enhancing glycine-independent desensitization. We examined the action of CaN on desensitization in recombinant NMDA receptors comprised of NMDA receptor 1 (NR1) and NR2A subunits. The C-terminus of NR2A, but not NR1, was critical for modulation of desensitization by CaN. Alanine-scanning mutagenesis indicated that serines 900 and 929 in NR2A altered desensitization, as did inhibition of tyrosine phosphatases. Our data suggest that dephosphorylation-dependent regulation of the C-terminus of NR2A increases desensitization of NMDA receptors, providing an additional mechanism for modulation of synaptic signals.

Phosphorylation of serine residues affects the conformation of the calmodulin binding domain of human protein 4.1

We have previously characterized the calcium-dependent calmodulin (CaM)-binding domain (Ser76-Ser92) of the 135-kDa human protein 4.1 isoform using fluorescence spectroscopy and chemically synthesized nonphosphorylated or serine phosphorylated peptides [Leclerc, E. & Vetter, S. (1998) Eur. J. Biochem. 258, 567-671]. Here we demonstrate that phosphorylation of two serine residues within the 17-residue peptide alters their ability to adopt alpha helical conformation in a position-dependent manner. The helical content of the peptides was determined by CD-spectroscopy and found to increase from 36 to 45% for the Ser80 phosphorylated peptide and reduce to 28% for the Ser84 phosphorylated peptide; the di-phosphorylated peptide showed 32% helical content. Based on secondary structure prediction methods we propose that initial helix formation involves the central residues Leu82-Phe86. The ability of the peptides to adopt alpha helical conformations did not correlate with the observed binding affinities to CaM. We suggest that the reduced CaM-binding affinities observed for the phosphorylated peptides are more likely to be the result of unfavorable sterical and electrostatic interactions introduced into the CaM peptide-binding interface by the phosphate groups, rather than being due to the effect of phosphorylation on the secondary structure of the peptides.

Bradykinin B2 receptors activate Na+/H+ exchange in mIMCD-3 cells via Janus kinase 2 and Ca2+/calmodulin

We used a cultured murine cell model of the inner medullary collecting duct (mIMCD-3 cells) to examine the regulation of the ubiquitous sodium-proton exchanger, Na+/H+ exchanger isoform 1 (NHE-1), by a prototypical G protein-coupled receptor, the bradykinin B2 receptor. Bradykinin rapidly activates NHE-1 in a concentration-dependent manner as assessed by proton microphysiometry of quiescent cells and by 2'-7'-bis[2-carboxymethyl]-5(6)-carboxyfluorescein fluorescence measuring the accelerated rate of pH(i) recovery from an imposed acid load. The activation of NHE-1 is blocked by inhibitors of the bradykinin B2 receptor, phospholipase C, Ca2+/calmodulin (CaM), and Janus kinase 2 (Jak2), but not by pertussis toxin or by inhibitors of protein kinase C and phosphatidylinositol 3'-kinase. Immunoprecipitation studies showed that bradykinin stimulates the assembly of a signal transduction complex that includes CaM, Jak2, and NHE-1. CaM appears to be a direct substrate for phosphorylation by Jak2 as measured by an in vitro kinase assay. We propose that Jak2 is a new indirect regulator of NHE-1 activity, which modulates the activity of NHE-1 by increasing the tyrosine phosphorylation of CaM and most likely by increasing the binding of CaM to NHE-1.

Regulation and expression of metazoan unconventional myosins

Unconventional myosins are molecular motors that convert adenosine triphosphate (ATP) hydrolysis into movement along actin filaments. On the basis of primary structure analysis, these myosins are represented by at least 15 distinct classes (classes 1 and 3-16), each of which is presumed to play a specific cellular role. However, in contrast to the conventional myosins-2, which drive muscle contraction and cytokinesis and have been studied intensively for many years in both uni- and multicellular organisms, unconventional myosins have only been subject to analysis in metazoan systems for a short time. Here we critically review what is known about unconventional myosin regulation, function, and expression. Several points emerge from this analysis. First, in spite of the high relative conservation of motor domains among the myosin classes, significant differences are found in biochemical and enzymatic properties of these motor domains. Second, the idea that characteristic distributions of unconventional myosins are solely dependent on the myosin tail domain is almost certainly an oversimplification. Third, the notion that most unconventional myosins function as transport motors for membranous organelles is challenged by recent data. Finally, we present a scheme that clarifies relationships between various modes of myosin regulation.

The calmodulin multigene family as a unique case of genetic redundancy: multiple levels of regulation to provide spatial and temporal control of calmodulin pools?

Calmodulin (CaM) is a ubiquitous, highly conserved calcium sensor protein involved in the regulation of a wide variety of cellular events. In vertebrates, an identical CaM protein is encoded by a family of non-allelic genes, raising questions concerning the evolutionary pressure responsible for the maintenance of this apparently redundant family. Here we review the evidence that the control of the spatial and temporal availability of CaM may require multiple regulatory levels to ensure the proper localization, maintenance and size of intracellular CaM pools. Differential transcription of the CaM genes provides one level of regulation to meet tissue-specific, developmental and cell-specific needs for altered CaM levels. Post-transcriptional regulation occurs at the level of mRNA stability, perhaps dependent on alternative polyadenylation and differences in the untranslated sequences of the multiple gene transcripts. Recent evidence indicates that trafficking of specific CaM mRNAs may occur to specialized cellular locales such as the dendrites of neurons. This could allow local CaM synthesis and thereby help generate local pools of CaM. Local CaM activity may be further regulated by post-translational mechanisms such as phosphorylation or storage of CaM in a 'masked' form. The spatial resolution of CaM activity is enhanced by the limited free diffusion of CaM combined with differential affinity for and availability of target proteins. Preserving multiple CaM genes with divergent noncoding sequences may be necessary in complex organisms to ensure that the many CaM-dependent processes occur with the requisite spatial and temporal resolution. Transgenic mouse models and studies on mice carrying single and double gene 'knockouts' promise to shed further light on the role of specificity versus redundancy in the evolutionary maintenance of the vertebrate CaM multigene family.

S-nitrosoglutathione-induced decrease in calcium sensitivity of airway smooth muscle

The purpose of this study was to examine whether the nitric oxide donor S-nitrosoglutathione (GSNO) relaxes canine tracheal smooth muscle (CTSM) strips by decreasing Ca(2+) sensitivity [i.e., the amount of force for a given intracellular Ca(2+) concentration ([Ca(2+)](i))]. We further investigated whether GSNO decreases Ca(2+) sensitivity by altering the relationship between regulatory myosin light chain (rMLC) phosphorylation and [Ca(2+)](i) and the relationship between force and rMLC phosphorylation. GSNO (100 microM) relaxed intact CTSM strips contracted with 45 mM KCl by decreasing Ca(2+) sensitivity in comparison to control strips without significantly decreasing [Ca(2+)](i). GSNO reduced the amount of rMLC phosphorylation for a given [Ca(2+)](i) but did not affect the relationship between isometric force and rMLC phosphorylation. These results show that in CTSM strips contracted with KCl, GSNO decreases Ca(2+) sensitivity by affecting the level of rMLC phosphorylation for a given [Ca(2+)](i), suggesting that myosin light chain kinase is inhibited or that smooth muscle protein phosphatases are activated by GSNO.