Annexin A4 is a novel direct regulator of adenylyl cyclase type 5.

Annexin A4 (AnxA4), a Ca(2+)- and phospholipid-binding protein, is up-regulated in the human failing heart. In this study, we examined the impact of AnxA4 on ß-adrenoceptor (ß-AR)/cAMP-dependent signal transduction. Expression of murine AnxA4 in human embryonic kidney (HEK)293 cells dose-dependently inhibited cAMP levels after direct stimulation of adenylyl cyclases (ACs) with forskolin (FSK), as determined with an exchange protein activated by cAMP-Förster resonance energy transfer (EPAC-FRET) sensor and an ELISA (control vs. +AnxA4: 1956 ± 162 vs. 1304 ± 185 fmol/µg protein; n = 8). Disruption of the anxA4 gene led to a consistent increase in intracellular cAMP levels in isolated adult mouse cardiomyocytes, with heart-directed expression of the EPAC-FRET sensor, stimulated with FSK, and as determined by ELISA, also in mouse cardiomyocytes stimulated with the ß-AR agonist isoproterenol (ISO) (anxA4a(+/+) vs. anxA4a(-/-): 5.1 ± 0.3 vs. 6.7 ± 0.6 fmol/µg protein) or FSK (anxA4a(+/+) vs. anxA4a(-/-): 1891 ± 238 vs. 2796 ± 343 fmol/µg protein; n = 9-10). Coimmunoprecipitation experiments in HEK293 cells revealed a direct interaction of murine AnxA4 with human membrane-bound AC type 5 (AC5). As a functional consequence of AnxA4-mediated AC inhibition, AnxA4 inhibited the FSK-induced transcriptional activation mediated by the cAMP response element (CRE) in reporter gene studies (10-fold vs. control; n = 4 transfections) and reduced the FSK-induced phosphorylation of the CRE-binding protein (CREB) measured on Western blots (control vs. +AnxA4: 150 ± 17% vs. 105 ± 10%; n = 6) and by the use of the indicator of CREB activation caused by phosphorylation (ICAP)-FRET sensor, indicating CREB phosphorylation. Inactivation of AnxA4 in anxA4a(-/-) mice was associated with an increased cardiac response to ß-AR stimulation. Together, these results suggest that AnxA4 is a novel direct negative regulator of AC5, adding a new facet to the functions of annexins.

SR-targeted CaMKII inhibition improves SR Ca²+ handling, but accelerates cardiac remodeling in mice overexpressing CaMKIIδC.

Cardiac myocyte overexpression of CaMKIIδ(C) leads to cardiac hypertrophy and heart failure (HF) possibly caused by altered myocyte Ca(2+) handling. A central defect might be the marked CaMKII-induced increase in diastolic sarcoplasmic reticulum (SR) Ca(2+) leak which decreases SR Ca(2+) load and Ca(2+) transient amplitude. We hypothesized that inhibition of CaMKII near the SR membrane would decrease the leak, improve Ca(2+) handling and prevent the development of contractile dysfunction and HF. To test this hypothesis we crossbred CaMKIIδ(C) overexpressing mice (CaMK) with mice expressing the CaMKII-inhibitor AIP targeted to the SR via a modified phospholamban (PLB)-transmembrane-domain (SR-AIP). There was a selective decrease in the amount of activated CaMKII in the microsomal (SR/membrane) fraction prepared from these double-transgenic mice (CaMK/SR-AIP) mice. In ventricular cardiomyocytes from CaMK/SR-AIP mice, SR Ca(2+) leak, assessed both as diastolic Ca(2+) shift into SR upon tetracaine in intact myocytes or integrated Ca(2+) spark release in permeabilized myocytes, was significantly reduced. The reduced leak was accompanied by enhanced SR Ca(2+) load and twitch amplitude in double-transgenic mice (vs. CaMK), without changes in SERCA expression or NCX function. However, despite the improved myocyte Ca(2+) handling, cardiac hypertrophy and remodeling was accelerated in CaMK/SR-AIP and cardiac function worsened. We conclude that while inhibition of SR localized CaMKII in CaMK mice improves Ca(2+) handling, it does not necessarily rescue the HF phenotype. This implies that a non-SR CaMKIIδ(C) exerts SR-independent effects that contribute to hypertrophy and HF, and this CaMKII pathway may be exacerbated by the global enhancement of Ca transients.

The signalling pathway of CaMKII-mediated apoptosis and necrosis in the ischemia/reperfusion injury.

Ca(2+)-calmodulin-dependent protein kinase II (CaMKII) plays an important role mediating apoptosis/necrosis during ischemia-reperfusion (IR). We explored the mechanisms of this deleterious effect. Langendorff perfused rat and transgenic mice hearts with CaMKII inhibition targeted to sarcoplasmic reticulum (SR-AIP) were subjected to global IR. The onset of reperfusion increased the phosphorylation of Thr(17) site of phospholamban, without changes in total protein, consistent with an increase in CaMKII activity. Instead, there was a proportional decrease in the phosphorylation of Ser2815 site of ryanodine receptors (RyR2) and the amount of RyR2 at the onset of reperfusion, i.e. the ratio Ser2815/RyR2 did not change. Inhibition of the reverse Na(+)/Ca(2+)exchanger (NCX) mode (KBR7943) diminished phospholamban phosphorylation, reduced apoptosis/necrosis and enhanced mechanical recovery. CaMKII-inhibition (KN-93), significantly decreased phospholamban phosphorylation, infarct area, lactate dehydrogenase release (LDH) (necrosis), TUNEL positive nuclei, caspase-3 activity, Bax/Bcl-2 ratio and Ca(2+)-induced mitochondrial swelling (apoptosis), and increased contractile recovery when compared with non-treated IR hearts or IR hearts pretreated with the inactive analog, KN-92. Blocking SR Ca(2+) loading and release (thapsigargin/dantrolene), mitochondrial Ca(2+) uniporter (ruthenium red/RU360), or mitochondrial permeability transition pore (cyclosporine A), significantly decreased infarct size, LDH release and apoptosis. SR-AIP hearts failed to show an increase in the phosphorylation of Thr(17) of phospholamban at the onset of reflow and exhibited a significant decrease in infarct size, apoptosis and necrosis respect to controls. The results reveal an apoptotic-necrotic pathway mediated by CaMKII-dependent phosphorylations at the SR, which involves the reverse NCX mode and the mitochondria as trigger and end effectors, respectively, of the cascade.

Studies on localization and function of annexin A4a within urinary bladder epithelium using a mouse knockout model.

Annexin A4 (anxA4) is a member of the Ca(2+)-dependent membrane-binding family of proteins implicated in the regulation of ion conductances, Ca(2+) homeostasis, and membrane trafficking. We demonstrate, in mice, that annexins 1-6 are present in whole bladder and exhibit differential expression in the urothelium. An anxA4a-knockout (anxA4a(-/-)) mouse model shows no protein in the urothelium by immunofluorescence and immunoblotting. In wild-type bladders, anxA4a in umbrella cells showed uniform cytoplasmic staining and some association with the nuclear membrane. Application of a hydrostatic pressure to bladders mounted in Ussing chambers resulted in redistribution of anxA4a from cytoplasm to cellular boundaries in the basal and intermediate cells but not in superficial umbrella cells. We hypothesized that anxA4a might be important for barrier function or for stretch-activated membrane trafficking. To test these hypotheses, we conducted a series of functional and morphological analyses on bladders from control and anxA4a(-/-) animals. The transepithelial resistances, water permeabilities, and urea permeabilities of anxA4a(-/-) bladders were not different from controls, indicating that barrier function was intact. Membrane trafficking in response to hydrostatic pressure as measured by capacitance increases was also normal for anxA4a(-/-) bladders. Cystometrograms performed on live animals showed that voiding frequency and intrabladder pressures were also not different. There were no differences in bladder surface morphology or cellular architecture examined by scanning and transmission electron microscopy, respectively. We conclude that loss of anxA4 from the urothelium does not affect barrier function, membrane trafficking, or normal bladder-voiding behavior.

Phosphorylation of calcium calmodulin-dependent protein kinase II following lateral fluid percussion brain injury in rats.

Traumatic brain injury (TBI) can dramatically increase levels of intracellular calcium ([Ca(2+)](i)). One consequence of increased [Ca(2+)](i) would be altered activity and function of calcium-regulated proteins, including calcium-calmodulin-dependent protein kinase II (CaMKII), which is autophosphorylated on Thr(286)(pCaMKII(286)) in the presence of calcium and calmodulin. Therefore, we hypothesized that TBI would result in increased levels of pCaMKII(286), and that such increases would occur early after injury in brain regions known to be damaged following lateral fluid percussion TBI (i.e., hippocampus and cortex). In order to test this hypothesis, immunostaining of CaMKII was examined in rat hippocampus and cortex after lateral fluid percussion (LFP) injury using an antibody directed against pCaMKII(286). LFP injury produced a marked increase in pCaMKII(286) immunostaining in the hippocampus and overlying cortex 30 min after TBI. The pattern of increased immunostaining was uneven, and unexpectedly absent in some hippocampal CA3 pyramidal neurons. This suggests that phosphatase activity may also increase following TBI, resulting in dephosphorylation of pCaMKII(286) in subpopulations of CA3 pyramidal neurons. Western blotting confirmed a rapid increase in levels of pCaMKII(286) at 10 and 30 min after brain injury, and that it was transient and no longer significantly elevated when examined at 3, 8, and 24 h. These results demonstrate that TBI alters the autophosphorylation state of CaMKII, an important neuronal regulator of critical cell functions, including enzyme activities, cell structure, gene expression, and neuronal plasticity, and provide a molecular mechanism that is likely to contribute to cell injury and impaired plasticity after TBI.

CaMKII inhibition targeted to the sarcoplasmic reticulum inhibits frequency-dependent acceleration of relaxation and Ca2+ current facilitation.

Cardiac Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) in heart has been implicated in Ca(2+) current (I(Ca)) facilitation, enhanced sarcoplasmic reticulum (SR) Ca(2+) release and frequency-dependent acceleration of relaxation (FDAR) via enhanced SR Ca(2+) uptake. However, questions remain about how CaMKII may work in these three processes. Here we tested the role of CaMKII in these processes using transgenic mice (SR-AIP) that express four concatenated repeats of the CaMKII inhibitory peptide AIP selectively in the SR membrane. Wild type mice (WT) and mice expressing AIP exclusively in the nucleus (NLS-AIP) served as controls. Increasing stimulation frequency produced typical FDAR in WT and NLS-AIP, but FDAR was markedly inhibited in SR-AIP. Quantitative analysis of cytosolic Ca(2+) removal during [Ca(2+)](i) decline revealed that FDAR is due to an increased apparent V(max) of SERCA. CaMKII-dependent RyR phosphorylation at Ser2815 and SR Ca(2+) leak was both decreased in SR-AIP vs. WT. This decrease in SR Ca(2+) leak may partly balance the reduced SERCA activity leading to relatively unaltered SR-Ca(2+) load in SR-AIP vs. WT myocytes. Surprisingly, CaMKII regulation of the L-type Ca(2+) channel (I(Ca) facilitation and recovery from inactivation) was abolished by the SR-targeted CaMKII inhibition in SR-AIP mice. Inhibition of CaMKII effects on I(Ca) and RyR function by the SR-localized AIP places physical constraints on the localization of these proteins at the junctional microdomain. Thus SR-targeted CaMKII inhibition can directly inhibit the activation of SR Ca(2+) uptake, SR Ca(2+) release and I(Ca) by CaMKII, effects which have all been implicated in triggered arrhythmias.

Nuclear Ca2+/calmodulin-dependent protein kinase II in the murine heart.

Ca(2+) signaling through CaMKII is critical in regulating myocyte function with regard to excitation-contraction-relaxation cycles and excitation-transcription coupling. To investigate the role of nuclear CaMKII in cardiac function, transgenic mice were designed and generated to target the expression of a CaMKII inhibitory peptide, AIP (KKALRRQEAVDAL), to the nucleus. The transgenic construct consists of the murine alpha-myosin heavy chain promoter followed by the expression unit containing nucleotides encoding a four repeat concatemer of AIP (AIP(4)) and a nuclear localization signal (NLS). Western blot and immunohistochemical analyses demonstrate that AIP(4) is expressed only in the nucleus of cardiac myocytes of the transgenic mice (NLS-AIP(4)). The function of cytoplasmic CaMKII is not affected by the expression of AIP(4) in the nucleus. Inhibition of nuclear CaMKII activity resulted in reduced translocation of HDAC5 from nucleus to cytoplasm in NLS-AIP(4) mouse hearts. Loss of nuclear CaMKII activity causes NLS-AIP(4) mice to have smaller hearts than their nontransgenic littermates. Transcription factors including CREB and NFkappaB are not regulated by cardiac nuclear CaMKII. With physiological stresses such as pregnancy or aging (8 months), NLS-AIP(4) mice develop hypertrophy symptoms including enlarged atria, systemic edema, sedentariness, and morbidity. RT-PCR analyses revealed that the hypertrophic marker genes, such as ANF and beta-myosin heavy chain, were upregulated in pregnancy stressed mice. Our results suggest that absence of adequate Ca2+signaling through nuclear CaMKII regulated pathways leads to development of cardiac disease.

Signaling pathways regulating murine cardiac CREB phosphorylation.

Using the mouse Langendorff heart perfusion model, the signaling pathways that regulate cardiac CREB-S133 phosphorylation have been defined. In mouse hearts stimulated with isoproterenol (ISO) (10(-8) M), endothelin-1 (ET-1) (10(-8) M), and phorbol 12-myristate 13-acetate (TPA) (10(-7) M), CREB-S133 phosphorylation was attained only by TPA-treatment. Activation of protein kinase A (PKA) was achieved by ISO. ISO- and ET-1-stimulation activated Ca2+/calmodulin-dependent kinase II (CaMKII). Protein kinase C (PKC) and p90(RSK) were activated with all three stimuli. Inhibition of ERK1/2 with PD98059 (10(-5) M) completely inhibited the activation of p90(RSK), but did not block CREB-S133 phosphorylation in TPA-perfused heart, indicating that PKA, CaMKII, and p90(RSK) do not phosphorylate CREB-S133 in the murine heart. PKC activation is signal specific. Analyses of PKC isoforms suggest that CREB phosphorylation is mediated by PKC epsilon translocating into nucleus only with TPA stimulation. These results, unlike those reported in other tissues, demonstrate that cardiac CREB is not a multi-signal target.

Local InsP3-dependent perinuclear Ca2+ signaling in cardiac myocyte excitation-transcription coupling.

Previous work showed that calmodulin (CaM) and Ca2+-CaM-dependent protein kinase II (CaMKII) are somehow involved in cardiac hypertrophic signaling, that inositol 1,4,5-trisphosphate receptors (InsP3Rs) in ventricular myocytes are mainly in the nuclear envelope, where they associate with CaMKII, and that class II histone deacetylases (e.g., HDAC5) suppress hypertrophic gene transcription. Furthermore, HDAC phosphorylation in response to neurohumoral stimuli that induce hypertrophy, such as endothelin-1 (ET-1), activates HDAC nuclear export, thereby regulating cardiac myocyte transcription. Here we demonstrate a detailed mechanistic convergence of these 3 issues in adult ventricular myocytes. We show that ET-1, which activates plasmalemmal G protein-coupled receptors and InsP3 production, elicits local nuclear envelope Ca2+ release via InsP3R. This local Ca2+ release activates nuclear CaMKII, which triggers HDAC5 phosphorylation and nuclear export (derepressing transcription). Remarkably, this Ca2+-dependent pathway cannot be activated by the global Ca2+ transients that cause contraction at each heartbeat. This novel local Ca2+ signaling in excitation-transcription coupling is analogous to but separate (and insulated) from that involved in excitation-contraction coupling. Thus, myocytes can distinguish simultaneous local and global Ca2+ signals involved in contractile activation from those targeting gene expression.

Targeted inhibition of sarcoplasmic reticulum CaMKII activity results in alterations of Ca2+ homeostasis and cardiac contractility.

Transgenic (TG) mice expressing a Ca2+/calmodulin-dependent protein kinase II (CaMKII) inhibitory peptide targeted to the cardiac myocyte longitudinal sarcoplasmic reticulum (LSR) display reduced phospholamban phosphorylation at Thr17 and develop dilated myopathy when stressed by gestation and parturition (Ji Y, Li B, Reed TD, Lorenz JN, Kaetzel MA, and Dedman JR. J Biol Chem 278: 25063-25071, 2003). In the present study, these animals (TG) are evaluated for the effect of inhibition of sarcoplasmic reticulum (SR) CaMKII activity on the contractile characteristics and Ca2+ cycling of myocytes. Analysis of isolated work-performing hearts demonstrated moderate decreases in the maximal rates of contraction and relaxation (+/-dP/dt) in TG mice. The response of the TG hearts to increases in load is reduced. The TG hearts respond to isoproterenol (Iso) in a dose-dependent manner; the contractile properties were reduced in parallel to wild-type hearts. Assessment of isolated cardiomyocytes from TG mice revealed 40-47% decrease in the maximal rates of myocyte shortening and relengthening under both basal and Iso-stimulated conditions. Although twitch Ca2+ transient amplitudes were not significantly altered, the rate of twitch intracellular Ca2+ concentration decline was reduced by approximately 47% in TG myocytes, indicating decreased SR Ca2+ uptake function. Caffeine-induced Ca2+ transients indicated unaltered SR Ca2+ content and Na+/Ca2+ exchange function. Phosphorylation assays revealed an approximately 30% decrease in the phosphorylation of ryanodine receptor Ser2809. Iso stimulation increased the phosphorylation of both phospholamban Ser16 and the ryanodine receptor Ser2809 but not phospholamban Thr17 in TG mice. This study demonstrates that inhibition of SR CaMKII activity at the LSR results in alterations in cardiac contractility and Ca2+ handling in TG hearts.

Annexin VI regulation of cardiac function.

Annexins are a family of membrane binding proteins that are characterized by a hypervariable amino terminus followed by a series of highly conserved Ca2+-phospholipid binding domains. Annexins function by binding to anionic phospholipid surfaces in a Ca2+-dependent manner. They self-associate to form trimers which further assemble into sheets that cover the membrane surface and alter properties such as fluidity and permeability. This submembranous skeleton alters integral protein functions such as ion transport properties and shields the surface from phospholipid binding proteins such as phospholipases and protein kinase C. Transgenic mouse hearts overexpressing wild type annexin VI (AnxVI673), a dominant-negative truncated annexin VI (residues 1-129, Anx129) and an annexin VI-null mouse (AnxVI-/-) have implicated the protein as a regulator of intracellular Ca2+ homeostasis which affects cardiac function.

CyclinB1 expression is elevated and mitosis is delayed in HeLa cells expressing autonomous CaMKII.

Calcium is a second messenger that is implicated in the regulation of cell cycle transitions. Calmodulin is a ubiquitous protein that translates intracellular calcium signals and activates several enzymes including calcium/calmodulin-dependent protein kinase II (CaMKII). Pharmacological inhibitors and constitutively active mutants have implicated CaMKII in cell cycle mediation. Specifically, constitutively active CaMKII impedes mitosis. In order to elucidate the molecular mechanisms underlying this phenomenon, the effect of constitutively active CaMKII gene expression on cdc2/cyclin B1 was investigated. As seen in previous studies with S. pombe, constitutively active CaMKII-hindered mitosis. However, this report shows that CaMKII does not cause permanent cell cycle arrest but delays progression into mitosis. Constitutive CaMKII expression also leads to elevations in cyclin B1 expression and cdc2 tyrosine-15 phosphorylation, analogous to observations in cells treated with hydroxyurea. Taken together, these data suggest that constitutive CaMKII may delay mitosis by activating a cell cycle checkpoint.

Intron disruption of the annexin IV gene reveals novel transcripts.

Annexin IV (AIV), a Ca2+-dependent membrane-binding protein, is expressed in many epithelia. Annexin IV modifies membrane bilayers by increasing rigidity, reducing water and H+ permeability, promoting vesicle aggregation, and regulating ion conductances, all in a Ca2+-dependent manner. We have characterized a mouse in which a gene trap has been inserted into the first intron of annexin IV. Processing of the primary transcript is disrupted. Northern blot and immunoblot data indicated that annexin IV expression was eliminated in many but not all tissues. Immunohistochemical analysis, however, demonstrated that annexin IV expression was eliminated in some cell types, but was unaltered in others. 5'-Rapid amplification of cDNA ends analysis of intestinal and kidney RNA revealed three transcripts, AIVa, AIVb, and AIVc. AIVa is widely distributed. AIVb is expressed only in the digestive tract. AIVc expression is very restricted. A selected number of epithelial cells of unique morphology demonstrate high concentrations. All three transcripts produce an identical annexin IV protein. The different tissue and cell-specific expression profiles of the three transcripts suggest that regulation of both the annexin IV gene expression and the cellular role of the protein are complex. The AIVa-/- mouse may become a valuable model to further study transcription and the physiological role of annexin IV.

Host-[2]rotaxanes as cellular transport agents.

Host-[2]rotaxanes, containing a diarginine-derivatized dibenzo-24-crown-8 (DB24C8) ether as the ring and a cyclophane pocket or an aromatic cleft as one blocking group, are cell transport agents. These hosts strongly associate with a variety of amino acids, dipeptides, and fluorophores in water (1 mM phosphate buffer, pH 7.0), DMSO, and a 75/25 (v/v) buffer to DMSO solution. All peptidic guests in all solvent systems have association constants (K(A)'s) in the range of 1 x 10(4) to 5 x 10(4) M(-)(1), whereas the K(A) range for the fluorophores is 1 x 10(4) to 9 x 10(5) M(-)(1). Association constants for the cyclophane itself, cyclophane 3, are smaller. These values are in the 1 x 10(3) to 5 x 10(3) M(-)(1) range, which shows that the rotaxane architecture is advantageous for guest binding. Cyclophane-[2]rotaxane 1 efficiently transports fluorescein and a fluorescein-protein kinase C (PKC) inhibitor into eukaryotic COS-7 cells, including the nucleus. Interestingly, cleft-[2]rotaxane 2 does not transport fluorescein as efficiently, even though the results from the fluorescence assays show that both [2]rotaxanes bind fluorescein with the same ability.

Annexin A4 reduces water and proton permeability of model membranes but does not alter aquaporin 2-mediated water transport in isolated endosomes.

Annexin A4 (Anx4) belongs to a ubiquitous family of Ca2+-dependent membrane-binding proteins thought to be involved in membrane trafficking and membrane organization within cells. Anx4 localizes to the apical region in epithelia; however, its physiological role is unclear. We show that Anx4 exhibited binding to liposomes (phosphatidylcholine:phosphatidylserine, 1:1) in the presence of Ca2+ and binding was reversible with EDTA. Anx4 binding resulted in liposome aggregation and a reduction in membrane water permeability of 29% (P < 0.001) at 25 degrees C. These effects were not seen in the presence of Ca2+ or Anx4 alone and were reversible with EDTA. Measurements of membrane fluidity made by monitoring fluorescence anisotropy of 2-(12-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)dodecanoyl-1-hexadecanoyl-sn-glycero-3-phosphocholine (NBD-HPC) demonstrated that Anx4 binding rigidified the outer leaflet of the bilayer (P < 0.001), thus providing a molecular explanation for the inhibition of water flux. To determine whether Anx4 would produce similar effects on physiological membranes we constructed liposomes which recapitulated the lipid composition of the inner leaflet of the MDCK apical membrane. These membranes exhibited reductions to water permeability upon Anx4 binding (19.5% at 25 degrees C, 31% at 37 degrees C; P < 0.01 and P < 0.001, respectively) and to proton permeability (15% at 25 degrees C, 19.5% at 37 degrees C; P < 0.05). Since our in vitro experiments indicated an effect on membrane permeability, we examined localization of Anx4 in the kidney collecting duct, a region of the nephron responsible for concentrating urine through water reabsorbtion. Anx4 was shown to colocalize apically with aquaporin 2 (AQP2) in collecting duct epithelia. To test for the existence of a functional interaction between Anx4 and AQP2 we isolated AQP2-containing endosomes and exposed them to Anx4/Ca2+. Water flux rates were unchanged, indicating Anx4 does not directly regulate AQP2. We conclude that Anx4 can alter the physical properties of membranes by associating with them and regulate passive membrane permeability to water and protons. These properties represent important new functions for Anx4.

Targeted inhibition of Ca2+/calmodulin-dependent protein kinase II in cardiac longitudinal sarcoplasmic reticulum results in decreased phospholamban phosphorylation at threonine 17.

To investigate the role of Ca2+/calmodulin-dependent kinase II in cardiac sarcoplasmic reticulum function, transgenic mice were designed and generated to target the expression of a Ca2+/calmodulin-dependent kinase II inhibitory peptide in cardiac longitudinal sarcoplasmic reticulum using a truncated phospholamban transmembrane domain. The expressed inhibitory peptide was highly concentrated in cardiac sarcoplasmic reticulum. This resulted in a 59.7 and 73.6% decrease in phospholamban phosphorylation at threonine 17 under basal and beta-adrenergic stimulated conditions without changing phospholamban phosphorylation at serine 16. Sarcoplasmic reticulum Ca2+ uptake assays showed that the Vmax was decreased by approximately 30% although the apparent affinity for Ca2+ was unchanged in heterozygous hearts. The in vivo measurement of cardiac function showed no significant reductions in positive and negative dP/dt, but a moderate 18% decrease in dP/dt40, indicative of isovolumic contractility, and a 26.1% increase in the time constant of relaxation (tau) under basal conditions. The changes in these parameters indicate a moderate cardiac dysfunction in transgenic mice. Although the 3 and 4-month-old transgenic mice displayed no overt signs of cardiac disease, when stressed by gestation and parturition, the 7-month-old female mice develop dilated heart failure, suggesting the important role of Ca2+/calmodulin-dependent kinase II pathway in the development of cardiac disease.

Nuclear CaMKII inhibits neuronal differentiation of PC12 cells without affecting MAPK or CREB activation.

Ca(2+)/calmodulin-regulated protein kinase II (CaMKII) mediates many cellular events. The four CaMKII isoforms have numerous splice variants, three of which contain nuclear localization signals. Little is known about the role of nuclear localized CaMKII in neuronal development. To study this process, PC12 cells were transfected to produce CaMKII targeted to either the cytoplasm or the nucleus and then treated with nerve growth factor (NGF). NGF triggers a signaling cascade (MAPK) that results in the differentiation of PC12 cells into a neuronal phenotype, marked by neurite outgrowth. The present study found that cells expressing nuclear targeted CaMKII failed to grow neurites, whereas cells expressing cytoplasmic CaMKII readily produced neurites. Inhibition of neuronal differentiation by nuclear CaMKII was independent of MAPK signaling, as sustained Erk phosphorylation was not affected. Phosphorylation of CREB was also unaffected. Thus nuclear CaMKII modifies neuronal differentiation by a mechanism independent of MAPK and CREB activation.

Interfacial basic cluster in annexin V couples phospholipid binding and trimer formation on membrane surfaces.

Annexin V is an abundant eukaryotic protein that binds phospholipid membranes in a Ca(2+)-dependent manner. In the present studies, site-directed mutagenesis was combined with x-ray crystallography and solution liposome binding assays to probe the functional role of a cluster of interfacial basic residues in annexin V. Four mutants were investigated: R23E, K27E, R61E, and R149E. All four mutants exhibited a significant reduction in adsorption to phospholipid membranes relative to the wild-type protein, and the R23E mutation was the most deleterious. Crystal structures of wild-type and mutant proteins were similar except for local changes in salt bridges involving basic cluster residues. The combined data indicate that Arg(23) is a major determinant for interfacial phospholipid binding and participates in an intermolecular salt bridge that is key for trimer formation on the membrane surface. Together, crystallographic and solution data provide evidence that the interfacial basic cluster is a locus where trimerization is synergistically coupled to membrane phospholipid binding.

Ligand-regulated secretion of recombinant annexin V from cultured thyroid epithelial cells.

The exposure of anionic phospholipids on the external surface of injured endothelial cells and activated platelets is a primary biological signal to initiate blood coagulation. Disease conditions that promote the formation of ectopic thrombi result in tissue ischemia. Annexins, Ca2+-dependent anionic phospholipid binding proteins, are potential therapeutic agents for the inhibition of coagulation. We have designed a transgene that targets secretion of annexin V from cultured thyroid cells under the control of doxycycline. Our results indicate that annexin V in the endoplasmic reticulum (ER)/Golgi lumen does not affect the synthesis, processing, and secretion of thyroglobulin. ER luminal Ca2+ was moderately increased and can be released by inositol 1,4,5-trisphosphate. Our study demonstrates that targeting and secretion of annexin V through the secretory pathway of mammalian cells does not adversely affect cellular function. Regulated synthesis and release of annexin V may exert anticoagulatory and anti-inflammatory effects systemically and may prove useful in further developing therapeutic strategies for conditions including antiphospholipid syndrome.

Mutation of highly conserved arginine residues disrupts the Structure and Function of Annexin V

Background. Annexins are a family of structurally related proteins that bind to phospholipid membranes in a Ca²+-dependent manner. Annexins are characterized by highly conserved canonical domains of approximately 70 amino acids. Anexin V contains four such domains. Each of these domains has a highly conserved arginine (R). Methods. To evaluate the role of the conserved arginines in the molecular structure of annexin V, negatively charged amino acids were substituted for arginines at positions R43, R115, R199, and R274 using site-directed mutagenesis. Results. Mutants R199D and R274E were rapidly degraded when expressed in bacteria, and were not further characterized. R43E exhibited an electrophoretic mobility similar to the wild-type protein, while R115E migrated significantly in a slower fashion, suggesting a less compact conformation, R43E and R115E exhibited much grater susceptibility to proteolytic digestion than the wild type. While Ca²+-dependence for phospholipid binding was similar in both mutants (half-maximal 50-80 µ; Ca²+), R43E and R115E exhibited a phospholipid affinities of the annexins, a phospholipid-dependent clotting reaction, the activated partial thromboplastin time (aPTT), was significantly prolonged by the wild-type protein and mutants R115E and R115A. The aPTT was unaffected by R43E. Conclusions. Our data suggest that mutation of these highly conserved arginine residus in each of the four canonical domains of annexin have differential effects on the phospholipid binding tertiary structure, and proteolytic susceptibility of annexin V. The site I mutation , R43E, produced a large decrease in phospholipid affinity associated with an increase in proteolytic susceptibility. The site II mutation, R115E, produced a small change in phospholipid binding but a significant modification of electrophoretic mobility. Our data suggest that highly conserved arginine residues are required to stabilize the tertiary structure of ammexin V by establishing hydrogen bonds and ionic bridges