The transgenic expression of highly inhibitory monomeric forms of phospholamban in mouse heart impairs cardiac contractility.

Transgenic mice were generated with cardiac-specific overexpression of the monomeric, dominant-acting, superinhibitory L37A and I40A mutant forms of phospholamban (PLN), and their phenotypes were compared with wild-type (wt) mice or 2-fold overexpressors of wt PLN (wtOE). The level of PLN monomer in cardiac microsomes was increased 11-13-fold, and the apparent affinity of the sarco(endo)plasmic reticulum Ca(2+)-ATPase for Ca(2+) was decreased from pCa 6.22 in wt or 6.12 in wtOE to 5.81 in L37A and 5.72 in I40A. Basal physiological parameters, measured in isolated myocytes, indicated a significant reduction in the rates of shortening (+dL/dt) and relengthening (-dL/dt). Hemodynamic measurements indicated that peak systolic pressure was unaffected but that pressure changes (+dP/dt and -dP/dt) were lowered significantly in both mutant lines, and relaxation time (tau) was also lengthened significantly. Echocardiography for both mutants showed depressed systolic function and an increase in left ventricular mass of over 1.4-fold. Significant decreases in left ventricular shortening fraction and velocity of circumferential shortening and increases in ejection time were corrected by isoproterenol. The use of antibodies specific against Ser(16)- and Thr(17)-PLN peptides showed that phosphorylation of both pentameric and monomeric PLN were increased between 1.2- and 2.4-fold in both the L37A and I40A lines but not in the wtOE line. These observations show that overexpression of superinhibitory mutant forms of PLN causes depression of contractile parameters with induction of cardiac hypertrophy, as assessed with echocardiography.

Calreticulin is essential for cardiac development.

Calreticulin is a ubiquitous Ca2+ binding protein, located in the endoplasmic reticulum lumen, which has been implicated in many diverse functions including: regulation of intracellular Ca2+ homeostasis, chaperone activity, steroid-mediated gene regulation, and cell adhesion. To understand the physiological function of calreticulin we used gene targeting to create a knockout mouse for calreticulin. Mice homozygous for the calreticulin gene disruption developed omphalocele (failure of absorption of the umbilical hernia) and showed a marked decrease in ventricular wall thickness and deep intertrabecular recesses in the ventricular walls. Transgenic mice expressing a green fluorescent protein reporter gene under the control of the calreticulin promoter were used to show that the calreticulin gene is highly activated in the cardiovascular system during the early stages of cardiac development. Calreticulin protein is also highly expressed in the developing heart, but it is only a minor component of the mature heart. Bradykinin-induced Ca2+ release by the InsP3-dependent pathway was inhibited in crt-/- cells, suggesting that calreticulin plays a role in Ca2+ homeostasis. Calreticulin-deficient cells also exhibited impaired nuclear import of nuclear factor of activated T cell (NF-AT3) transcription factor indicating that calreticulin plays a role in cardiac development as a component of the Ca2+/calcineurin/NF-AT/GATA-4 transcription pathway.

Endocytic properties of the M-type 180-kDa receptor for secretory phospholipases A2.

Endocytic properties of the M-type 180-kDa receptor for secretory phospholipases A2 (sPLA2) were first investigated in rabbit myocytes that express it at high levels. Internalization of the receptor was shown to be clathrin-coated pit-mediated, rapid (ke = 0.1 min-1), and ligand-independent. The signal sequence for internalization was then identified upon transient and stable expression of various receptor constructs with mutated cytoplasmic sequences. Analysis of the internalization efficiency of the mutants suggested that the NSYY motif encodes the major endocytic signal, with the distal tyrosine residue playing the key role. Amino acid substitutions at the putative casein kinase II phosphorylation site of the receptor did not affect internalization. A chimeric protein composed of the extracellular and transmembrane domains of the rabbit sPLA2 receptor and of the cytoplasmic domain of the structurally homologous human macrophage mannose receptor retained the high affinity for sPLA2 and was internalization competent, exhibiting 50% endocytic activity of the M-type sPLA2 receptor. The results indicate the compatibility of the structural domains of the two parent proteins and provide evidence for the interchangeable character of their internalization signals.

[A family of receptors for secretory phospholipases A2]. / Une famille de récepteurs pour les phospholipases A2 sécrétées.

Venom phospholipases A2 (vPLA2's) display a large spectrum of toxic effects including neurotoxicity, myotoxicity, hypotensive, anticoagulant and proinflammatory effects. We have shown that these different types of effects are apparently linked to the existence of a diversity of very high affinity receptors (Kd values as low as 1.5 pM) for these toxic enzymes. On the other hand, mammalian secretory PLA2's (msPLA2's) are now implicated in many biological functions besides digestion, such as airway and vascular smooth muscle contraction, cell proliferation, and in a variety of diseases associated with inflammation such as rheumatoid arthritis, endotoxic shock, respiratory distress syndrome as well as in cancer diseases.... Several different types of receptors (N and M) have been identified for vPLA2's and one of them (180 kDa, called M) has been cloned in rabbit and man. It is a membrane protein with a N-terminal cystein-rich domain, a fibronectin-like type II domain, eight repeats of a carbohydrate recognition domain, a unique transmembrane and an intracellular C-terminal. When expressed in transfected cells, the rabbit M-type receptor binds both the inflammatory-type and the pancreatic-type msPLA2's with fairly high affinities (Kd approximately -1-10 nM) suggesting that the sPLA2 receptors we have identifying vPLA2's are the normal targets of endogenous msPLA2's involved in a variety of diseases. Residues within or close to the Ca2+ binding loop of pancreatic-type PLA2 are crucially involved in the binding step although the presence of Ca2+ which is essential for the enzymatic activity is not required for binding to the receptor. The domain in charge of sPLA2 binding in the M-type receptor has been identified. The M-type receptor is an endocytic receptor that rapidly internalizes its sPLA2 ligand.

A signal for endoplasmic reticulum retention located at the carboxyl terminus of the plasma membrane Ca(2+)-ATPase isoform 4CI.

The plasma membrane Ca(2+)-ATPase isoform 4b (PMCA4CI) with truncations in the cytoplasmically exposed COOH-terminal tail was expressed in COS and HeLa cells and in Sf9 cells using the baculovirus system. The truncated protein terminating with the acidic sequence Glu1067-Arg1087 was retained within the endoplasmic reticulum (ER), whereas mutants lacking this sequence or having it at a distance from the COOH terminus were delivered to the plasma membrane. Although the truncated protein retained in the endoplasmic reticulum was still able to form a Ca(2+)-dependent phosphoenzyme, it underwent partial degradation. Substitution of glutamic and aspartic residue(s) in the acidic region promoted rescue of the protein to the plasma membrane. The results suggest that the sequence Glu1067-Arg1087 encodes a masked signal for ER retention and for the degradation of the protein. However, its presence at the COOH terminus was not sufficient to induce ER-retention and degradation; when the sequence was attached to the full-length PMCA protein, normal plasma delivery was observed. Evidently, ER retention and degradation required the presence of the sequence in its specific location within the PMCA structure. The degradation of the protein retained in the endoplasmic reticulum occurred through the proteolytic attack at cytoplasmically exposed residues (amino acid sequence 720-750) by a cytoplasmic PEST sequence-related protease different from calpain.

Expression, purification, and properties of the plasma membrane Ca2+ pump and of its N-terminally truncated 105-kDa fragment.

Isoform 4b of the human plasma membrane Ca2+ pump was expressed in COS cells and in the baculovirus system (Sf9 cells). A 105-kDa pump fragment lacking the first two transmembrane domains and the so-called transduction domain was also expressed. The expression level was 2-4 times the background in COS cells and at least 7 times in the baculovirus system. Tests on membranes from both systems showed that the expressed pump was active. The expressed pump and the 105-kDa fragment were isolated from Sf9 cell membranes by calmodulin affinity chromatography. The pump had Ca(2+)-dependent ATPase activity with a calmodulin stimulation factor of 3, formed a La(3+)-stabilized phosphoenzyme, and had a KM (Ca2+) in the presence of calmodulin of about 1 microM. The 105-kDa fragment, assayed by the phosphoenzyme test on COS or Sf9 cell membranes or by ATPase measurements after isolation from Sf9 cells, proved inactive. Laser confocal microscopy on Sf9 cells showed that both the pump and the 105-kDa fragment were apparently associated with the plasma membrane. The expressed pump in COS and Sf9 cells and the endogenous pump in a number of other cell lines had a slower gel mobility (i.e. a higher apparent molecular mass) than the erythrocyte pump.

A monoclonal antibody recognizes an epitope in the first extracellular loop of the plasma membrane Ca2+ pump.

A monoclonal antibody against the human erythrocyte Ca2+ pump (1E4) reacted with the enzyme in intact erythrocytes. Using trypsinized preparations of the pump the antibody only reacted with the N-terminal fragments of 33.5 and 35 kDa. The fragments span from the N terminus (35 kDa) or from residue 19 (33.5 kDa) to residue 314 of the hPMCA4 isoform of the pump. Exhaustive degradation with a number of agents produced smaller peptides which reacted with the antibody. Sequencing analysis on two chymotryptic fragments of 8.8 and 13.5 kDa identified the epitope in an approximately 80-residue domain beginning with Gly-81. Two peptides corresponding to the putative extramembrane portions of this region of the pump were synthesized. The antibody reacted with one of them, spanning residues Phe-121 to Gly-152 and containing the first putative external loop of the pump. Peptides corresponding to overlapping portions of this peptide were synthesized, leading to the location of the epitope in a 13-residue sequence (Glu-130 to Glu-142) in the first predicted extracellular loop (Verma, A. K., Filoteo, A. G., Stanford, D. R., Wieben, E. D., Strehler, E. E., Fischer, R., Heim, R., Vogel, G., Mathews, S., Strehler-Page, M-A., James, P., Vorherr, T., Krebs, J., Penniston, J. T., and Carafoli, E. (1988) J. Biol. Chem. 263, 14152-14159).

Mapping of functional domains in the plasma membrane Ca2+ pump using trypsin proteolysis.

The purified erythrocyte Ca2+ pump has been exposed to trypsin under conditions designed to enrich the fragments of molecular mass 90, 85, 81, and 76 kDa, respectively. In SDS-polyacrylamide gels, these fragments are accompanied by a product of molecular mass about 33 kDa. N- and C-terminal sequencing of the fragments blotted on PVDF membranes has located the four high molecular mass fragments and the 33-kDa fragment within the pump structure. The work has extended previous work on the organization of the calmodulin-interacting domain of the pump (Zurini et al., 1984; Benaim et al., 1984) and has tentatively placed the domain of the pump which interacts with acidic phospholipids between transmembrane helices 2 and 3.

Elucidation of conservative elements of calmodulin-dependent enzymes with the use of monoclonal antibodies.

Monoclonal antibodies against human erythrocyte membrane Ca2+-ATPase were obtained. The binding of monoclonal antibodies to the enzyme resulted in a decrease in the enzyme sensitivity to calmodulin (CaM). The effects of monoclonal antibodies on other CaM-dependent enzymes, namely, on the phosphodiesterase of cAMP, phosphorylase kinase, and Ca2+-CaM-dependent protein kinase II (PK II), were studied. It was found that all four enzymes contain a common antigenic site. However, the inhibitory effect of antibodies was observed only with respect to Ca2+-ATPase and PK II. The kinetics of the binding of monoclonal antibodies and their inhibitory action were investigated. It was shown that the antigenic site is confined to the calmodulin-binding portion of Ca2+-ATPase and PK II.

The amino acid sequence of the phosphorylation domain of the erythrocyte Ca2+ ATPase.

The amino acid sequence of a peptide isolated from a CNBr digest of the erythrocyte Ca2+ ATPase has been determined. It contains a highly conserved phosphorylation site sequence common to all aspartyl-phosphate forming ion motive ATPases which have been sequenced so far.