Calcium-binding proteins and secretion

Alterations in pancreatic β cell function and Trypanosoma cruzi infection: evidence from human and animal studies

The parasite Trypanosoma cruzi causes a persistent infection, Chagas disease, affecting millions of persons in endemic areas of Latin America. As a result of immigration, this disease has now been diagnosed in non-endemic areas worldwide. Although, the heart and gastrointestinal tract are the most studied, the insulin-secreting β cell of the endocrine pancreas is also a target of infection. In this review, we summarize available clinical and laboratory evidence to determine whether T. cruzi-infection-mediated changes of β cell function is likely to contribute to the development of hyperglycemia and diabetes. Our literature survey indicates that T. cruzi infection of humans and of experimental animals relates to altered secretory behavior of β cells. The mechanistic basis of these observations appears to be a change in stimulus-secretion pathway function rather than the loss of insulin-producing β cells. Whether this attenuated insulin release ultimately contributes to the pathogenesis of diabetes in human Chagas disease, however, remains to be determined. Since the etiologies of diabetes are multifactorial including genetic and lifestyle factors, the use of cell- and animal-based investigations, allowing direct manipulation of these factors, are important tools in testing if reduced insulin secretion has a causal influence on diabetes in the setting of Chagas disease. Long-term clinical investigations will be required to investigate this link in humans.

Structural analysis of Ca2+-binding pocket of synaptotagmin 5 C2A domain

Synaptotagmins constitute a family of multifunctional integral membrane proteins found predominantly on vesicles in neural and endocrine tissues. 17 isoforms of synaptotagmin family in mammals have been identified, 7 isoforms among them are known to be able to bind Ca²⁺ via their C2 domains. This study presents the crystal structure of the first C2 domain (C2A domain) of synaptotagmin 5 complexed with Ca²⁺ at 1.90Å resolution. Comparison of the Ca²⁺-binding pocket of synaptotagmin 5 C2A domain with other synaptotagmin C2 domains demonstrated that a serine residue locating at Ca²⁺-binding loop probably responsible to the conformational variation of Ca²⁺-binding pocket, and thus impacts the Ca²⁺-binding mechanism of C2 domain, which is verified by structural analysis of the serine mutant and Ca²⁺-binding assays via isothermal titration calorimetry. Alteration of Ca²⁺-binding mechanism might be correlated with different Ca²⁺ response rates of synaptotagmins, which is the basis of the functions of synaptotagmins in regulating various types of Ca²⁺-triggered vesicle-membrane fusion processes.

Cloning, expression, purification, crystallization and preliminary X-ray diffraction crystallographic study of human synaptotagmin 5 C2A domain

Synaptotagmin acts as the Ca(2+) sensor for neural and endocrine exocytosis. Synaptotagmin 5 has been demonstrated to play a key role in the acquisition of cathepsin D and the vesicular proton ATPase and in Ca(2+)-dependent insulin exocytosis. The C2 domains modulate the interaction of synaptotagmin with the phospholipid bilayer of the presynaptic terminus and effector proteins such as the SNARE complex. This study reports the cloning, expression in Escherichia coli, purification, crystallization and preliminary X-ray analysis of the C2A domain of human synaptotagmin 5 with an N-terminal His(6) tag. The crystals diffracted to 1.90 Å resolution and belonged to the hexagonal space group P6(5), with unit-cell parameters a = b = 93.97, c = 28.05 Å. A preliminary model of the protein structure has been built and refinement of the model is ongoing.

Purification, crystallization and X-ray diffraction analysis of human synaptotagmin 1 C2A-C2B

Synaptotagmin acts as the Ca(2+) sensor for neuronal exocytosis. The cytosolic domain of human synaptotagmin 1 is composed of tandem C2 domains: C2A and C2B. These C2 domains modulate the interaction of synaptotagmin with the phospholipid bilayer of the presynaptic terminus and effector proteins such as the SNARE complex. Human synaptotagmin C2A-C2B has been expressed as a glutathione-S-transferase fusion protein in Escherichia coli. The purification, crystallization and preliminary X-ray analysis of this protein are reported here. The crystals diffract to 2.7 A and belong to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 82.37, b = 86.31, c = 140.2 A. From self-rotation function analysis, there are two molecules in the asymmetric unit. The structure determination of the protein using this data is ongoing.

Autocrine/paracrine stimulation of purinergic receptors in osteoblasts: contribution of vesicular ATP release

Extracellular nucleotides such as ATP and UTP are released in response to mechanical stimulation in different cell systems. It is becoming increasingly evident that ATP release plays a role in autocrine and paracrine stimulation of osteoblasts. Mechanical stimulation, as shear stress, membrane stretch or hypo-osmotic swelling, as well as oscillatory fluid flow, stimulates ATP release from different osteoblastic cell lines. Human osteoblast-like initial transfectant (HOBIT) cells release ATP in response to mechanical stimulation. In the present study, we show that HOBIT cells are activated by nanomolar levels of extracellular ATP, concentrations that can be detected under resting conditions and increase following hypotonic shock. Cell activation by hypotonic medium induced intracellular Ca2+ oscillations, and Egr-1 synthesis and DNA-binding activity. Quinacrine staining of living, resting cells revealed a granular fluorescence, typical of ATP-storing vesicles. Monensin prevented quinacrine staining and considerably inhibited hypotonic-induced ATP release. Finally, elevated levels of cytosolic Ca2+ activated massive ATP release and a dose-dependent loss of quinacrine granules. The contribution of a vesicular mechanism for ATP release is proposed to sustain paracrine osteoblast activation.

Essential role of Ca2+/calmodulin in Early Endosome Antigen-1 localization

Ca2+ is an essential requirement in membrane fusion, acting through binding proteins such as calmodulin (CaM). Ca2+/CaM is required for early endosome fusion in vitro, however, the molecular basis for this requirement is unknown. An additional requirement for endosome fusion is the protein Early Endosome Antigen 1 (EEA1), and its recruitment to the endosome depends on phosphatidylinositol 3-phosphate [PI(3)P] and the Rab5 GTPase. Herein, we demonstrate that inhibition of Ca2+/CaM, by using either chemical inhibitors or specific antibodies directed to CaM, results in a profound inhibition of EEA1 binding to endosomal membranes both in live cells and in vitro. The concentration of Ca2+/CaM inhibitors required for a full dissociation of EEA1 from endosomal membranes had no effect on the activity of phosphatidylinositol 3-kinases or on endogenous levels of PI(3)P. However, the interaction of EEA1 with liposomes containing PI(3)P was decreased by Ca2+/CaM inhibitors. Thus, Ca2+/CaM seems to be required for the stable interaction of EEA1 with endosomal PI(3)P, perhaps by directly or indirectly stabilizing the quaternary organization of the C-terminal FYVE domain of EEA1. This requirement is likely to underlie at least in part the essential role of Ca2+/CaM in endosome fusion.

Calcyclin is an early vasopressin-induced gene in the renal collecting duct. Role in the long term regulation of ion transport

Long-term effects of arginine vasopressin (AVP) in the kidney involve the transcription of unidentified genes. By subtractive hybridization experiments performed on the RCCD(1) cortical collecting duct cell line, we identified calcyclin as an early AVP-induced gene (1 h). Calcyclin is a calcium-binding protein involved in the transduction of intracellular signals. In the kidney, calcyclin was localized at the mRNA level in the glomerulus, all along the collecting duct, and in the epithelium lining the papilla. In RCCD(1) cells and in m-IMCD(3) inner medullary collecting duct cells, calcyclin was evidenced in the cytoplasm. Calcyclin mRNA levels were progressively increased by AVP treatment in RCCD(1) (1.7-fold at 4 h) and m-IMCD(3) (2-fold at 7.5 h) cells. In RCCD(1) cells, calcyclin protein levels were increased by 4 h of AVP treatment. In vivo, treatment of genetically vasopressin-deficient Brattleboro rats with AVP for 4 days induced an increase in both calcyclin and aquaporin-2 mRNA expression. Finally, introduction of anti-calcyclin antibodies into RCCD(1) cells by permeabilizing the plasma membrane prevented the long-term (but not short-term) increase in short-circuit current induced by AVP. Taken together, these results suggest that calcyclin is an early vasopressin-induced gene that participates in the late phase of the hormone response in transepithelial ion transport.

Crystal structure of the cytosolic C2A-C2B domains of synaptotagmin III. Implications for Ca(+2)-independent snare complex interaction

Synaptotagmins are synaptic vesicle-associated, phospholipid-binding proteins most commonly associated with Ca(+2)-dependent exocytotic and Ca(+2)- independent endocytotic events. Synaptotagmin III is a 63.2-kD member of the synaptotagmin homology group; one of its characteristic properties is the ability to bind divalent cations and accessory proteins promiscuously. In the cytosolic portion of this protein, a flexible seven-amino acid linker joins two homologous C2 domains. The C2A domain binds to phospholipid membranes and other accessory proteins in a divalent cation-dependent fashion. The C2B domain promotes binding to other C2B domains, as well as accessory proteins independent of divalent cations. The 3.2 A crystal structure of synaptotagmin III, residues 295-566, which includes the C2A and C2B domains, exhibits differences in the shape of the Ca(+2)-binding pocket, the electrostatic surface potential, and the stoichiometry of bound divalent cations for the two domains. These observations may explain the disparate binding properties of the two domains. The C2A and the C2B domains do not interact; synaptotagmin, therefore, covalently links two independent C2 domains, each with potentially different binding partners. A model of synaptotagmin's involvement in Ca(+2)-dependent regulation of membrane fusion through its interaction with the SNARE complex is presented.

Effect of calcium depletion on the secretion of newly synthesised human chorionic gonadotropin by first trimester human placenta

Depletion of calcium in the extracellular medium used to incubate first trimester human placental minces resulted in a significant decrease in the quantity of immunoreactive hCG in the medium and a corresponding increase in the tissue. In contrast, when secretion of newly synthesised hCG was monitored in the absence of calcium by using a radioactive amino acid precursor, a significant increase in the secretion of newly synthesised hCG in the medium was noticed. This was true of secretion of other proteins also as evidenced by the increase in the trichloroacetic acid precipitable radioactivity in the medium in the absence of calcium. These results suggest that newly synthesised hCG is preferentially released over stored hormone in the absence of calcium.

Human odontoblasts contain S-100 protein-like immunoreactivity

S-100 protein is a group of three closely related isoforms (S-100ao, S-100a, and S-100b). This protein was first described as unique to the nervous system but it has also been identified subsequently in a variety of cell types of neuroectodermal (i.e., melanocytes, glial cells) and non-neuroectodermal origin (i.e., Langerhans cells, adipocytes, chondrocytes). In the present investigation the presence of S-100 protein was studied in human odontoblasts using a specific polyclonal antibody directed against S-100 protein in immunoperoxidase labelling experiments. The S-100 protein was detected in the cytoplasm of odontoblasts. This result suggests that S-100 protein can play a role in odontoblast functions.

Contractile proteins in podocytes: immunocytochemical localization of actin and alpha-actinin in normal and nephrotic rat kidneys

Actin and alpha-actinin immunoreactive sites have been localized at the electron microscope level by the protein A-gold immunocytochemical technique in podocytes of normal and nephrotic rat renal tissues. In normal renal glomeruli, fibrillar networks located in the core of foot processes or bundles of microfilaments interconnecting them were found to be labelled for these two cytoskeletal proteins. On the other hand, in nephrotic renal glomeruli, concomitant with the loss of podocytic foot processes a reorganization of the podocytic cytoskeleton and a concentration of some of its elements into thick uniform bands was observed. Actin and alpha-actinin were revealed in these bands. Control experiments confirmed the specificity of the labelling obtained. Our results suggest that normal podocytes contain an actin-based contractile system that might contribute to the maintenance of the particular cell shape of these cells and that the rearrangement of the podocytic cytoskeleton occurring in the nephrotic syndrome might account for the changes in the foot processes and contribute to the alteration in glomerular function.

Annexins V and VI: major calcium-dependent atrial secretory granule-binding proteins

Atrial natriuretic peptide is stored by atrial myocytes in secretory granules, known as atrial specific granules, and is released from these granules by exocytosis. We have isolated a group of atrial proteins by affinity chromatography that bind to atrial specific granules in a calcium-dependent manner. The two major proteins isolated (32.5 kd and 67 kd) are calcium-binding proteins and have been identified as annexins V and VI by immunoblotting with specific antisera. The calcium dependence of their binding to atrial specific granules has been characterized in vitro and indicates that this interaction takes place at micromolar levels of calcium. In addition, the group of proteins isolated includes another calcium-binding protein of 20 kd, as well as GTP-binding proteins of 22 to 26 kd. Membrane interactions during exocytosis are presumably mediated by the interaction of specific proteins with the granule membrane. The properties of the proteins described here, and their ability to bind to atrial specific granules in a calcium-dependent manner, make them likely candidates in the search for regulatory proteins mediating atrial natriuretic peptide secretion.

Actin cytoskeleton and calcium-ATPase in the process of abomasal mucus secretion in cattle

The distribution of actin filaments in pyloric gland cells of cattle was studied with respect to their functional significance in the process of exocrine secretion by use of rhodamine-phalloidin labelling and immunogold-electron microscopy based on the biotin-streptavidin bridge technique. Actin concentrates on the filamentous network of the luminal-cell cortex. Membranes of secretory vesicles accumulating in the cell cortex are also labelled for actin. The present results support the concept of a barrier function of cortical microfilaments entrapping vesicles and linking them to the cytoskeleton. In addition, intracellular localization of calcium-ATPase activity was determined. Enzyme activity associated with the microfilamentous cortical matrix is supposed to be of cytoskeletal nature indicating participation of myosin (-like) structures in the dynamic secretion event. Deposition on the interior aspect of secretory vesicle membranes points to an ATPase transporting calcium into these organelles and enabling them to participate via storage of the cation in intracellular calcium homeostasis, thereby influencing the functional architecture of the cortical cytoskeleton.

Glucocorticoids inhibit prostaglandin synthesis not only at the level of phospholipase A2 but also at the level of cyclo-oxygenase/PGE isomerase

1. Prostanoid synthesis was induced in bone marrow-derived macrophages by addition of exogenous arachidonic acid to the cell cultures. When the cells were preincubated with dexamethasone (10(-7) and 10(-6) M) overnight, prostaglandin synthesis was inhibited by 66.5 +/- 2.8% and 56.7 +/- 2.9% (mean +/- s.d.; n = 3) respectively. 2. Endogenous membrane bound phospholipase A2 was measured with labelled phospholipids used as substrates. The enzyme activity with phosphatidylcholine and phosphatidylethanolamine as substrates was inhibited by 27.0 +/- 8.3% and 23.3 +/- 11.1% (n = 4) respectively, in dexamethasone-treated macrophages compared to control cells. Neither the distribution of radiolabelled arachidonic acid among the different phospholipid species nor the release of arachidonic acid from prelabelled cells were significantly impaired by pretreatment of the macrophages with dexamethasone (1 microM). 3. The enzyme activity of the cyclo-oxygenase/prostaglandin E (PGE) isomerase was measured in cell membranes from control cells and dexamethasone-treated cells. It was inhibited by 40.0 +/- 8.4% (n = 4) in dexamethasone-treated cells as compared to control cells. Thus, glucocorticoids inhibit not only phospholipase A2 in these cells, but predominantly inhibit arachidonic acid metabolism subsequent to its release from phospholipids.

Vertebrate and yeast calmodulin, despite significant sequence divergence, are functionally interchangeable

Yeast strains relying solely on vertebrate (Xenopus laevis) calmodulin, expressed under control of a yeast (GAL1) promoter, grew at the same rate as yeast cells containing their endogenous calmodulin. Therefore, the ability to perform essential functions has been conserved between yeast and vertebrate calmodulins, suggesting that calmodulin performs the same (or overlapping) roles in yeast as it does in higher eukaryotes. Successful substitution of vertebrate for yeast calmodulin also suggests that the two proteins can adopt similar conformations in vivo, despite the large number of amino acid differences between them (60 out of 148 residues). Strains overproducing either vertebrate or yeast calmodulin about 100-fold and a strain producing a normal level of yeast calmodulin were essentially indistinguishable in many characteristics, including microtubule distribution, rate of secretion, response to mating pheromone, sporulation, and adaptation to nutrient limitation. Calmodulin overproduction did not confer elevated resistance to a phenothiazine drug, trifluoperazine, thought to be a calmodulin-specific inhibitor. These results have important implications for understanding the role of calmodulin in intracellular calcium signaling.

The insulin secretory granule

The insulin secretory granule of the pancreatic B cell is a complex intracellular organelle comprised of a many proteins with different catalytic activities and messenger functions. With the advent of tumour models of the B cells and the application of immunological and molecular cloning techniques considerable progress has been made in recent years towards the elucidation of the structure and function of these granule proteins. A number of examples are selected here for review. Particular emphasis given to how the activities of quite different granule proteins are interdependent and how this contributes to the co-ordination and integration of the organelle's biological functions.

Pollen tube tip growth

This review considers pollen tube growth with regard to current information on pollen tube cytoplasm, wall structure and calcium ion interactions with pollen tubes. Pollen tubes have a marked cytoplasmic Polarity with a number of distinct zones along the tube, each with a characteristic complement of cytoplasmic and nuclear structures. The cytoplasmic structures are characteristic of secretory cells with extensive endoplasmic reticulum cisternae and numerous dictyosomes. The dictyosomes produce secretory vesicles that are mainly directed to the extending tip of the tube, where they provide new plasma membrane and wall components. The rates of secretory vesicle production and delivery have been estimated, allowing quantitative assessments of the rate of delivery of materials to the tip. Pollen tubes contain cytoskeletal components, with microtubules and microfilament strands lying axially in the main tube and diffuse microfilament strands at the tip. The tube wall consists of an outer fibrous layer containing pectins and an inner, more homogeneous layer containing callose and cellulose-like microfibrils, possessing both β-1,4 and β-1,3 linkages. Protein is also present in the wall. The tube tip lacks the inner callosic wall. This type of structure is considered to be different from that of elongating sporophyte tissue cells which are enclosed by a wall containing layers of cellulose microfibrils. Calcium ions are required for pollen tube growth and, in at least some species, act as a chemotropic agent. High concentrations of calcium ions in the external medium inhibit growth. Pollen tubes contain some calcium ions bound to the cell wall and larger amounts located intracellularly, which enter the tube at the tip. This intracellular calcium is present as ions that exist freely within the cytoplasmic Matrix and as ions bound to membrane systems. The highest concentrations in both of these pools are found at the tip and in both they decline towards the base. The structure of the tip and the activity involved in providing components for plasma membrane and Wall assembly provide a basis for considering possible mechanisms of tip growth. Two hypotheses to account for the regulation of tip extension are considered, cell wall control and cytoskeletal control. In the cell wall hypothesis, control depends on an interaction between internal turgor pressure and a plastic cell wall. The mechanical properties of the wall are assumed to be partly dependent on the availability of external calcium ions to crosslink acidic pectin chains. According to this hypothesis, high external calcium ion concentrations cause cessation of tip growth due to increased mechanical resistance of the tip wall. Various observations on plant cell-wall interactions with calcium ions and on experimentally-treated pollen tubes provide evidence that does not support this hypothesis. The cytoskeletal control hypothesis of tip growth depends on the internal tip cytoskeleton to contain the tube tip cytoplasm against the internal turgor pressure during cell wall assembly. The activities and mechanical properties of the cytoskeleton are assumed to depend on the availability of external calcium ions. High external concentrations are believed to cause a state of rigor in the cytoskeleton and hence a cessation of tip growth. Some experimental evidence is presented which suggests that the effects of excess calcium ions are on intracellular processes, and not extracellular ones. The mitochondrial zone behind the tip is believed to maintain the tip calcium ion concentration at an optimal level for growth. Some comparisons are made between tip growth in pollen tubes and that in other tip growing cells. CONTENTS Summary 323 I. Introduction 324 II. Cytoplasm 326 III. Wall structure 332 IV. Calcium 335 V. Tip growth 339 VI. Conclusions 350 References 351.

Effect of cytochalasin D on acinar cell structure and secretion in rat parotid salivary glands in vitro

Cytochalasin D, a microfilament disrupting agent, considerably inhibited isoproterenol-stimulated amylase release from enzymatically dispersed parotid acini. Histologically cytochalasin D caused a loss of microvilli lining acinar lumina and luminal enlargement. Nearly empty vacuoles appeared near the luminal and lateral surface, and the membrane bordering on the vacuoles was often continuous with the plasma membrane. Therefore, the vacuolization probably resulted from an elongation of the membrane lining the lumen. Fluorescence staining with rhodamine-phalloidin showed that cytochalasin D caused disruption of microfilaments. When stimulating the cytochalasin D-treated cells with isoproterenol, the number of secretory granules in the cytoplasm diminished markedly and secretory material was observed in the vacuoles, indicating that inhibition of amylase release by cytochalasin D is not due to blocking of exocytosis but to the retention of amylase discharged into vacuoles. These findings suggest that microfilaments are essential in maintaining the parotid acinar structure but do not play a direct part in the movement of secretory granules and their fusion with the luminal membrane.

The second messenger system(s) mediating the secretion of atrial natriuretic peptide (ANP) from the isolated rat heart during rapid cardiac pacing

This study demonstrates that rapid cardiac pacing elevates Atrial Natriuretic Peptide (ANP) levels, independently from atrial stretch. The second messenger system mediating this response was examined. The phosphoinositide system, generally regarded to be important in mediating ANP release, was shown to play only a modulating role during rapid cardiac pacing. The main mediator would appear to be calcium, and a non-calmodulin dependent, calcium mediated system controlling ANP release during rapid cardiac pacing is suggested.

Cellular Ca2+ homeostasis and Ca2+-mediated cell processes as critical targets for toxicant action: conceptual and methodological pitfalls

Because of the central role of the calcium messenger system in diverse functions of tissues, organs, and cells, Ca2+ homeostasis and function may prove to be critical cellular and molecular targets for a diverse range of toxicants. Experimental proof of these targets as a specific site of toxicant action is challenging and technically difficult as a result of the complexity of Ca2+ homeostatic and Ca2+-mediated processes. However, as the investigation of normal physiological control of Ca2+ and function will continue to be an active and productive area of basic research for several years to come, it is anticipated that these insights will be increasingly applied to the understanding of the mechanisms of action of toxic agents.

Potential anticoagulant activity of lipocortins and other calcium/phospholipid binding proteins

Thrombin generation was determined in the presence of phospholipids, coagulation factors Xm, Vm and II (prothrombin), calcium, and various calcium/phospholipid bindings proteins, including lipocortins I and II, 35 kDa calelectrin, and 32.5 kDa endonexin. All of these proteins induced a dose-dependent inhibition of thrombin generation similar to the inhibition of pig pancreas phospholipase A2. It is suggested that the ability of lipocortins and other related proteins to interact with anionic phospholipids in the presence of Ca++ is responsible for both their anticoagulant and anti-phospholipase A2 activity.

Involvement of a 65 kDa phosphoprotein in the regulation of membrane fusion during exocytosis in Paramecium cells

Antisera were raised against a phosphoprotein of 65 kDa (PP65) from Paramecium cells (shown before to be selectively dephosphorylated during synchronous exocytosis) and specified by immunoblotting. By immunofluorescence PP65 has been localized within the cortex, beneath the cell membrane. This corresponds to data obtained by cell fractionation, applying SDS-PAGE autoradiography to cortices prepared from 32P-prelabeled cells. Antisera against PP65 inhibit exocytosis in vivo (microinjection). Applying anti-PP65 antisera in vitro to cortices we could demonstrate inhibition not only of exocytosis, but also of PP65 dephosphorylation. We conclude that PP65 is involved in the regulation of membrane fusion during exocytosis.

Isolation of two 67 kDa calcium-binding proteins from pig lung differing in affinity for phospholipids and in anti-phospholipase A2 activity

Two 67 kDa proteins adsorbed to membranes in the presence of Ca2+ have been purified to homogeneity from pig lung using conventional procedures, followed by calcium-dependent affinity chromatography on polyacrylamide-immobilized phosphatidylserine. The two proteins were, respectively, excluded (67E) and retained (67R) on the column in the presence of Ca2+. On the basis of amino acid composition and isoelectric point, 67R was identified as 67 kDa calelectrin/calcimedin, whereas 67E could be differentiated from albumin, calregulin, 67 kDa fragment of protein kinase C and surfactant-associated proteins. Only 67R was slightly phosphorylated by protein kinase C, reacted with an antibody raised against 32.5 kDa endonexin and inhibited pig pancreas phospholipase A2 in a way similar to that of lipocortin or endonexin. These data bring further support to the view that inhibition of phospholipase A2 by lipocortin or other related proteins involves interaction with the lipid/water interface. They also provide evidence for a new kind of Ca2+-binding protein (67E), whose role still remains to be determined.