Pancreatic protease activation by alcohol metabolite depends on Ca2+ release via acid store IP3 receptors

Toxic alcohol effects on pancreatic acinar cells, causing the often fatal human disease acute pancreatitis, are principally mediated by fatty acid ethyl esters (non-oxidative products of alcohol and fatty acids), emptying internal stores of Ca²⁺. This excessive Ca²⁺ liberation induces Ca²⁺-dependent necrosis due to intracellular trypsin activation. Our aim was to identify the specific source of the Ca²⁺ release linked to the fatal intracellular protease activation. In 2-photon permeabilized mouse pancreatic acinar cells, we monitored changes in the Ca²⁺ concentration in the thapsigargin-sensitive endoplasmic reticulum (ER) as well as in a bafilomycin-sensitive acid compartment, localized exclusively in the apical granular pole. We also assessed trypsin activity in the apical granular region. Palmitoleic acid ethyl ester (POAEE) elicited Ca²⁺ release from both the ER as well as the acid pool, but trypsin activation depended predominantly on Ca²⁺ release from the acid pool, that was mainly mediated by functional inositol 1,4,5- trisphosphate receptors (IP₃Rs) of types 2 and 3. POAEE evoked very little Ca²⁺ release and trypsin activation when IP₃Rs of both types 2 and 3 were knocked out. Antibodies against IP₃Rs of types 2 and 3, but not type 1, markedly inhibited POAEE-elicited Ca²⁺ release and trypsin activation. We conclude that Ca²⁺ release through IP₃Rs of types 2 and 3 in the acid granular Ca²⁺ store induces intracellular protease activation, and propose that this is a critical process in the initiation of alcohol-related acute pancreatitis.     

Endothelial‐smooth muscle cell interactions in the regulation of vascular tone in skeletal muscle

The SMCs of skeletal muscle arterioles are intricately sensitive to changes in membrane potential. Upon increasing luminal pressure, the SMCs depolarize, thereby opening VDCCs, which leads to contraction. Mechanisms that oppose this myogenic tone can involve voltage‐dependent and independent dilator pathways, and can be endothelium‐dependent or independent. Of particular interest are the pathways leading to hyperpolarization of SMCs, as these can potentially evoke both local and conducted dilation. This review focuses on three agonists that cause local and conducted dilation in skeletal muscle: ACh, ATP, and KCl. The mechanisms for the release of these agonists during motor nerve stimulation and/or hypoxia, and their actions to open either Ca²⁺‐activated K⁺ channels (K_Ca) or inwardly rectifying K⁺ channels (K_IR) are described. By causing local and conducted dilation, each agonist has the ability to improve skeletal muscle blood flow during exercise and ischemia.     

Insulin receptor signaling for the proliferation of pancreatic β-cells: Involvement of Ca2+ second messengers,IP3, NAADP and cADPR

Insulin has an autocrine/paracrine role through insulin receptors in pancreatic β-cells. Herein, we show the insulin receptor signaling pathway underlying CD38/ADPR-cyclase activation for NAADP/cADPR formation to induce Ca2+ rise, ultimately resulting in β-cell proliferation. Binding of insulin on insulin receptors leads to the activation of IRS/Akt/PI3K/PLC. Activation of PLC generates IP3 and DAG; the former induces Ca²⁺ release, resulting in activation of CD38/ADPR-cyclase for cADPR production via cGMP-dependent mechanism and the latter activates PKC, resulting in activation of ADPR-cyclase for NAADP synthesis. The NAADP-induced Ca²⁺ signal is required for IP₃-induced Ca²⁺ release from the ER. CD38 plays an important role in insulin receptor signaling in β-cells by reflecting a declined sustained Ca²⁺ signal, cADPR levels, and β-cell proliferation in response to insulin in CD38^-/- islets. However, evidence indicates that a hitherto-unidentified ADPR cyclase in addition to CD38 participates in insulin-induced signaling through cADPR and NAADP synthesis. In conclusion, insulin receptor signaling in β-cells employs three Ca²⁺ signaling messengers, IP₃, NAADP, and cADPR through a complex but concerted action of signaling molecules for Ca2+ signaling, which is involved in the proliferation of the islets.     

The role of ryanodine receptors and consequences of their alterations during cardiac insufficiency

Congestive heart failure (CHF) is a leading cause of death. Although changes to other components contribute, it is generally agreed that much of the contractile deficit is due to reduced Ca²⁺ homeostasis that includes alterations in Ca²⁺ current and action potential characteristics, together with reduced Ca²⁺ transient amplitude. CHF is also associated with progressive skeletal muscle dysfunction. In both cardiac and skeletal muscles, the global increase in myoplasmic Ca²⁺ during depolarization, or Ca²⁺ transient, appears to consist of the summation of large numbers of local, unitary Ca²⁺ release events (ie, Ca²⁺ sparks) resulting from the activity of a cluster of ryanodine receptors (RyRs) (ie, RyR1 or RyR2 in skeletal and cardiac muscles, respectively). RyR2 channels from failing hearts have been shown to be hyperphosphorylated by protein kinase A, leading to dissociation of FK506-binding protein 12.6 and altered RyR2 channel function. After reviewing the alterations occurring in cardiomyocytes, the present report summarizes the intrinsic alterations of Ca²⁺ homeostasis in rat extensor digitorum longus skeletal muscle. They include a weaker and prolonged Ca²⁺ transient that could be attributed to both a lower synchronization of the individual Ca²⁺ sparks and a lower synchronization of these events triggered upon depolarization. As in cardiac muscle, these alterations in sarcoplasmic reticulum function are associated with protein kinase A-induced hyperphosphorylation of RyR1 and a concomitant reduction in FK506-binding protein 12. These specific alterations in RyR1-dependent Ca²⁺ release could play a significant role in the specific force decrements in skeletal muscle as well as in the remodelling that occurs secondary to CHF.     

A mathematical model of vasoreactivity in rat mesenteric arterioles: I. Myoendothelial communication

To study the effect of myoendothelial communication on vascular reactivity, we integrated detailed mathematical models of Ca²⁺ dynamics and membrane electrophysiology in arteriolar smooth muscle (SMC) and endothelial (EC) cells. Cells are coupled through the exchange of Ca²⁺, Cl^?, K⁺, and Na⁺ ions, inositol 1,4,5‐triphosphate (IP₃), and the paracrine diffusion of nitric oxide (NO). EC stimulation reduces intracellular Ca²⁺ ([Ca²⁺ in the SMC by transmitting a hyperpolarizing current carried primarily by K⁺. The NO‐independent endothelium‐derived hyperpolarization was abolished in a synergistic‐like manner by inhibition of EC SK_Ca and IK_Ca channels. During NE stimulation, IP₃diffusing from the SMC induces EC Ca²⁺ release, which, in turn, moderates SMC depolarization and [Ca²⁺]_i elevation. On the contrary, SMC [Ca²⁺]_i was not affected by EC‐derived IP₃. Myoendothelial Ca²⁺ fluxes had no effect in either cell. The EC exerts a stabilizing effect on calcium‐induced calcium release‐dependent SMC Ca²⁺ oscillations by increasing the norepinephrine concentration window for oscillations. We conclude that a model based on independent data for subcellular components can capture major features of the integrated vessel behavior. This study provides a tissue‐specific approach for analyzing complex signaling mechanisms in the vasculature.     

Vascular smooth muscle cell signaling mechanisms for contraction to angiotensin II and endothelin-1

Vasoactive peptides, such as endothelin-1 and angiotensin II, are recognized by specific receptor proteins located in the cell membrane of target cells. After receptor recognition, the specificity of the cellular response is achieved by G-protein coupling of ligand binding to the regulation of intracellular effectors. These intracellular effectors will be the subject of this brief review on contractile activity initiated by endothelin-1 and angiotensin II. Activation of receptors by endothelin-1 and angiotensin II in smooth muscle cells results in phospholipase C activation leading to the generation of the second messengers inositol trisphosphate (IP₃) and diacylglycerol (DAG). IP₃ stimulates intracellular Ca²⁺ release from the sarcoplasmic reticulum and DAG causes protein kinase C activation. Additionally, different Ca²⁺ entry channels, such as voltage-operated, receptor-operated, and store-operated Ca²⁺ channels, as well as Ca²⁺-permeable nonselective cation channels, are involved in the elevation of intracellular Ca²⁺ concentration. The elevation in intracellular Ca²⁺ is transient and initiates contractile activity by a Ca²⁺-calmodulin interaction, stimulating myosin light chain (MLC) phosphorylation. When the Ca²⁺ concentration begins to decline, Ca²⁺ sensitization of the contractile proteins is signaled by the RhoA/Rho-kinase pathway to inhibit the dephosphorylation of MLC phosphatase (MLCP), thereby maintaining force generation. Removal of Ca²⁺ from the cytosol and stimulation of MLCP initiates the process of smooth muscle relaxation. In pathologic conditions such as hypertension, alterations in these cellular signaling components can lead to an overstimulated state causing maintained vasoconstriction and blood pressure elevation.     

Hyperpolarization induced by K+ channel openers inhibits Ca2+ influx and Ca2+ release in coronary artery

The vasodilating mechanisms of the K⁺ channel openers—cromakalim, pinacidil, nicorandil, KRN2391, and Ki4032—were examined by measurement of the cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i) using the fura-2 method in canine or porcine coronary arterial smooth muscle. The five K⁺ channel openers all produced a reduction of [Ca²⁺]_i in 5 and 30 mM KCl physiological salt solution (PSS), the effects of which were antagonized by tetrabutylammonium (TBA) or glibenclamide, but failed to affect [Ca²⁺]_i in 45 and 90 mM MCl-PSS. Cromakalim and Ki4032 only partially inhibited the 30 mM KCl-induced contractures, whereas pinacidil, nicorandil, and KRN2391 nearly abolished contractions produced by high KCl-PSS. The increased [Ca²⁺]_i and force produced by a thromboxane A₂ analogue, U46619, were inhibited by K⁺ channel openers and verapamil. In the absence of extracellular Ca²⁺, U46619 induced a transient increase in [Ca²⁺]_i with a contraction, which is effectively inhibited by cromakalim and Ki4032. Their inhibitory effects were blocked by TBA and counteracted by 20 mM KCl-induced depolarization. Cromakalim and Ki4032 did not affect caffeine-induced Ca²⁺ release. Cromakalim reduced U46619-induced IP₃ production and TBA blocked this inhibitory effect. Thus, cromakalim and Ki4032 are more specific K⁺ channel openers than pinacidil, nicorandil, and KRN2391. The vasodilation related with a reduction of [Ca²⁺]_i produced by K⁺ channel openers is due to the hyperpolarization of the plasma membrane resulting in not only the closure of voltage-dependent Ca²⁺ channels but also inhibition of the production of IP₃ and Ca²⁺ release from intracellular stores related to stimulation of the thromboxane A₂ receptor.     

Local control of Ca-induced Ca release in mouse sinoatrial node cells

Emerging evidence from large animal models implicates Ca²⁺ regulation, particularly intracellular sarcoplasmic reticulum (SR) Ca²⁺ release, as essential for sinoatrial node (SAN) automaticity. However, despite the apparent importance of SR Ca²⁺ release to SAN cell function it is uncertain how SR Ca²⁺ release is controlled in SAN cells from mouse. Understanding mouse SAN SR Ca²⁺ release mechanism will allow improved understanding of results in studies on SAN from genetic mouse models of Ca²⁺ homeostatic proteins. Here we investigated the functional relationship between sarcolemmal Ca²⁺ influx and SR Ca²⁺ release at the level of single SAN cell, using simultaneous patch-clamp current recording and high resolution confocal Ca²⁺ imaging techniques. In mouse SAN cells, both Ca²⁺ channel currents and triggered SR Ca²⁺ transients displayed bell-shaped, graded function with the membrane potential. Moreover, the gain function for Ca²⁺-induced Ca²⁺ release (CICR) displayed a monotonically decreasing function with strong voltage dependence, consistent with a “local control” mechanism for CICR. In addition, we observed numerous discrete Ca²⁺ sparks at the voltage range of diastolic depolarization, in sharp contrast to the much lower frequency of sparks observed at resting potentials. We concluded that the “local control” mechanism of CICR is responsible for both local Ca²⁺ release during diastolic depolarization and the synchronized Ca²⁺ transients observed during action potential in SAN cells.     

Redox Control of Ion Channel Activity in Vascular Endothelial Cells by Glutathione

Oxidized glutathione (GSSG) is endogenously formed within vascular endothelial cells. The bioactivity of GSSG results in the oxidation of protein thiol groups, leading to changes in protein structure-function relationships. When ion channel protein thiols are the target of oxidation by GSSG, important changes in channel conductance, activity, and gating occur. In this review, we focus on two endothelial cell ion channels, the activities of which influence vascular cell signaling and the nitric oxide signaling pathway. The first channel is the GSSG-operated cation channel that depolarizes the endothelial cell, leading to inhibition of capacitative Ca²⁺ entry. The second channel is the inositol 1,4,5-triphosphate (IP₃)-operated Ca²⁺ channel that is responsible for the agonist-stimulated release of Ca²⁺ from IP₃-sensitive endoplasmic reticulum. GSSG acts to deplete IP₃-sensitive Ca²⁺ stores, thereby attenuating the intracellular Ca²⁺ response to agonist stimulation. Together, these effects indicate that glutathione, which is formed endogenously within the cell, is a key physiological modulator of endothelial cell signaling.     

Crystal structure of type I ryanodine receptor amino-terminal β-trefoil domain reveals a disease-associated mutation “hot spot” loop

Muscle contraction and relaxation is regulated by transient elevations of myoplasmic Ca²⁺. Ca²⁺ is released from stores in the lumen of the sarco(endo)plasmic reticulum (SER) to initiate formation of the Ca²⁺ transient by activation of a class of Ca²⁺ release channels referred to as ryanodine receptors (RyRs) and is pumped back into the SER lumen by Ca²⁺-ATPases (SERCAs) to terminate the Ca²⁺ transient. Mutations in the type 1 ryanodine receptor gene, RYR1, are associated with 2 skeletal muscle disorders, malignant hyperthermia (MH), and central core disease (CCD). The evaluation of proposed mechanisms by which RyR1 mutations cause MH and CCD is hindered by the lack of high-resolution structural information. Here, we report the crystal structure of the N-terminal 210 residues of RyR1 (RyR_NTD) at 2.5 Å. The RyR_NTD structure is similar to that of the suppressor domain of type 1 inositol 1,4,5-trisphosphate receptor (IP₃Rsup), but lacks most of the long helix-turn-helix segment of the “arm” domain in IP₃Rsup. The N-terminal β-trefoil fold, found in both RyR and IP₃R, is likely to play a critical role in regulatory mechanisms in this channel family. A disease-associated mutation “hot spot” loop was identified between strands 8 and 9 in a highly basic region of RyR1. Biophysical studies showed that 3 MH-associated mutations (C36R, R164C, and R178C) do not adversely affect the global stability or fold of RyR_NTD, supporting previously described mechanisms whereby mutations perturb protein–protein interactions.     

IP(3)R-mediated Ca(2+) release is modulated by anandamide in isolated cardiac nuclei

Cannabinoids (CBs) are known to alter coronary vascular tone and cardiac performance. They also exhibit cardioprotective properties, particularly in their ability to limit the damage produced by ischaemia reperfusion injury. The mechanisms underlying these effects are unknown. Here we investigate the intracellular localisation of CB receptors in the heart and examine whether they may modulate localised nuclear Ca²⁺ release. In isolated cardiac nuclear preparations, expression of both the inositol 1,4,5-trisphosphate receptor type 2 (IP₃R) and CB receptors (CB₁R and CB₂R) was demonstrated by immunoblotting. Both receptors localised to the nucleus and purity of the nuclear preparations was confirmed by co-expression of the nuclear marker protein nucleolin but absence of cytoplasmic actin. To measure effects of IP₃R and CBR agonists on nuclear Ca²⁺ release, isolated nuclei were loaded with Fluo5N-AM. This dye accumulates in the nuclear envelope. Isolated nuclei responded to IP₃ with rapid and transient Ca²⁺ release from the nuclear envelope. Anandamide inhibited this IP₃-mediated release. Preincubation of nuclear preparations with either the CB₁R antagonist (AM251) or the CB₂R antagonist (AM630) reversed anandamide-mediated inhibition to 80% and 60% of control values respectively. When nuclei were pre-treated with both CBR antagonists, anandamide-mediated inhibition of IP₃-induced Ca²⁺ release was completely reversed. These results are the first to demonstrate the existence of cardiac nuclear CB receptors. They are also the first to show that anandamide can negatively modulate IP₃-mediated nuclear Ca²⁺ release. As such, this provides evidence for a novel key mechanism underlying the action of CBs and CBRs in the heart.     

Defects in T-tubular electrical activity underlie local alterations of calcium release in heart failure

Action potentials (APs), via the transverse axial tubular system (TATS), synchronously trigger uniform Ca²⁺ release throughout the cardiomyocyte. In heart failure (HF), TATS structural remodeling occurs, leading to asynchronous Ca²⁺ release across the myocyte and contributing to contractile dysfunction. In cardiomyocytes from failing rat hearts, we previously documented the presence of TATS elements which failed to propagate AP and displayed spontaneous electrical activity; the consequence for Ca²⁺ release remained, however, unsolved. Here, we develop an imaging method to simultaneously assess TATS electrical activity and local Ca²⁺ release. In HF cardiomyocytes, sites where T-tubules fail to conduct AP show a slower and reduced local Ca²⁺ transient compared with regions with electrically coupled elements. It is concluded that TATS electrical remodeling is a major determinant of altered kinetics, amplitude, and homogeneity of Ca²⁺ release in HF. Moreover, spontaneous depolarization events occurring in failing T-tubules can trigger local Ca²⁺ release, resulting in Ca²⁺ sparks. The occurrence of tubule-driven depolarizations and Ca²⁺ sparks may contribute to the arrhythmic burden in heart failure.The simultaneous and coherent recruitment of cardiomyocytes that occurs at every heartbeat is fundamental to guarantee a proper and healthy contraction of the whole heart. Moreover, mammalian ventricular cardiomyocytes are provided with a complex network of sarcolemmal invaginations called the transverse-axial tubular system (TATS) (1, 2). TATS allows the action potential (AP) to propagate rapidly into the cardiomyocyte core. During the AP, Ca²⁺ enters the cell through depolarization-activated Ca²⁺ channels (dihydropyridine receptors, DHPRs) and triggers Ca²⁺ release (calcium-induced calcium release, CICR) from the sarcoplasmic reticulum (SR) through the ryanodine receptor 2 (RyR2). The free intracellular Ca²⁺ concentration ([Ca²⁺]_i) rises and Ca²⁺ binds to troponin C (TnC), leading to myofilament activation and contraction. The well-organized topographical extension of T-tubules along each sarcomere Z-line profile ensures a homogeneous Ca²⁺ release and, consequently, a synchronous contraction across the whole cardiomyocyte (3).Structural alterations and loss of T-tubules have been found in several human pathological conditions, including chronic heart failure (HF) (4, 5). HF is characterized by weakened heart contraction, which leads to maladaptive remodeling, further weakening cardiac contraction and potentially causing deadly arrhythmias (6). Loss and disorganization of the TATS are early features of cardiomyocyte remodeling in HF, leading to orphaned RyR2 channels and thus determining nonhomogeneous Ca²⁺ release (7, 8). Recently, using random access microscopy in combination with fluorinated voltage-sensitive dyes (VSD) (9), we have probed the electrical activity of multiple TATS elements within isolated cardiomyocytes, highlighting that the presence of a tight electrical coupling between T-tubular system and surface sarcolemma is ensured only by intact TATS (10). In fact, we have demonstrated that in a rat model of postischemic HF, structurally remodeled TATS exhibits abnormal electrical activity, i.e., failure of AP propagation and presence of local spontaneous depolarizations. Tubular AP failures and spontaneous activity can potentially aggravate asynchronous Ca²⁺ release and determine nonhomogeneous myofibril contraction. Simultaneous recording of local Ca²⁺ release and AP in the tubular network is needed to unravel the consequences of these electrical anomalies on intracellular Ca²⁺ dynamics. To address this challenge, here we augment the previous experimental setup by adding the capability to optically measure Ca²⁺ transients simultaneously with AP in several tubular elements. We apply this method to dissect the spatiotemporal relationship between TATS electrical activity and Ca²⁺ release in heart failure.     

The H2O2-Generating Enzyme,Xanthine Oxidase,Decreases Luminal Ca2+ Content of the IP3-Sensitive Ca2+ Store in Vascular Endothelial Cells

Objective: Xanthine oxidase inhibits agonist-stimulated Ca²⁺ signaling in calf pulmonary artery endothelial cells by an H₂O₂-dependent mechanism. We investigated the effect of xanthine oxidase on luminal Ca²⁺ content of the inositol-1,4,5-trisphosphate (IP₃)-sensitive Ca²⁺ store. Methods: Luminal Ca²⁺ content was estimated from the net release of Ca²⁺ activated by 2,5-di-t-butylhydroquinone (BHQ), an inhibitor of microsomal Ca²⁺ pumps. Results: Initially, xanthine oxidase depleted the IP₃-sensitive Ca²⁺ store of releasable Ca²⁺, but with more prolonged incubation, the enzyme also depleted non-IP₃-sensitive stores. In addition, xanthine oxidase inhibited capacitative Ca²⁺ influx. Similar results were observed when thapsigargin was substituted for BHQ. Conclusions: Depletion of luminal Ca²⁺ content within the IP₃-sensitive Ca²⁺ store contributes to xanthine oxidase inhibition of Ca²⁺ signaling in vascular endothelial cells.     

VEGF-induced neoangiogenesis is mediated by NAADP and two-pore channel-2–dependent Ca2+ signaling

Vascular endothelial growth factor (VEGF) and its receptors VEGFR1/VEGFR2 play major roles in controlling angiogenesis, including vascularization of solid tumors. Here we describe a specific Ca²⁺ signaling pathway linked to the VEGFR2 receptor subtype, controlling the critical angiogenic responses of endothelial cells (ECs) to VEGF. Key steps of this pathway are the involvement of the potent Ca²⁺ mobilizing messenger, nicotinic acid adenine-dinucleotide phosphate (NAADP), and the specific engagement of the two-pore channel TPC2 subtype on acidic intracellular Ca²⁺ stores, resulting in Ca²⁺ release and angiogenic responses. Targeting this intracellular pathway pharmacologically using the NAADP antagonist Ned-19 or genetically using Tpcn2^−/− mice was found to inhibit angiogenic responses to VEGF in vitro and in vivo. In human umbilical vein endothelial cells (HUVECs) Ned-19 abolished VEGF-induced Ca²⁺ release, impairing phosphorylation of ERK1/2, Akt, eNOS, JNK, cell proliferation, cell migration, and capillary-like tube formation. Interestingly, Tpcn2 shRNA treatment abolished VEGF-induced Ca²⁺ release and capillary-like tube formation. Importantly, in vivo VEGF-induced vessel formation in matrigel plugs in mice was abolished by Ned-19 and, most notably, failed to occur in Tpcn2^−/− mice, but was unaffected in Tpcn1^−/− animals. These results demonstrate that a VEGFR2/NAADP/TPC2/Ca²⁺ signaling pathway is critical for VEGF-induced angiogenesis in vitro and in vivo. Given that VEGF can elicit both pro- and antiangiogenic responses depending upon the balance of signal transduction pathways activated, targeting specific VEGFR2 downstream signaling pathways could modify this balance, potentially leading to more finely tailored therapeutic strategies.In the adult the formation of new capillaries is an uncommon occurrence mostly restricted to pathological rather than physiological conditions, the majority of blood vessels remaining quiescent once organ growth is accomplished (1). Physiological neoangiogenesis is generally restricted to body sites undergoing regeneration or restructuring (e.g., tissue lesion repair and corpus luteum formation), whereas pathological neoangiogenesis takes place in different diseases ranging from macular degeneration to atherosclerosis, and is vital for the highly noxious development of solid tumors, thus representing a promising target for therapeutic strategies (2). Vascular endothelial growth factors (VEGF), and in particular the family member VEGF-A, are major regulators of angiogenesis and regulate ECs, mainly through the stimulation of VEGF receptor-2 (VEGFR2), a receptor tyrosine kinase, to induce cell proliferation, migration, and sprouting in the early stages of angiogenesis (3, 4). Antiangiogenic agents that target VEGF signaling have become an important component of therapies in multiple cancers, but their use is limited by acquisition of resistance to their therapeutic effects (5, 6). When overall VEGF receptor (VEGFR) signaling is experimentally impaired by the use of blocking antibodies or of specific tyrosine kinase inhibitors, alternative cellular and tissue strategies nullify the success of such interventions (5, 7, 8). Resistance to anti-VEGF therapies may occur through a variety of mechanisms, including evocation of alternative compensatory factors, selection of hypoxia-resistant tumor cells, action of proangiogenic circulating cells, and increased circulating nontumor proangiogenic factors. Moreover, cross-interactions (both cellular and humoral) between ECs and other environmental cues have to be taken into account for the ultimate aim of tailoring therapeutic interventions according to the specific pattern of the angiogenic microenvironment and EC conditions (5–7). The search for novel key downstream effectors is therefore of potential significance in the perspective of angiogenesis control in cancer progression.Autophosphorylation of VEGFR2 upon binding VEGF results in the activation of downstream signaling cascades through complex and manifold molecular interactions that transmit signals leading to angiogenic responses. Stimulation of different EC types via VEGFR2 results in increases in intracellular free calcium concentrations [Ca²⁺]_i (9, 10) and the crucial role of this signaling element in the regulation of EC functions and angiogenesis is recognized (11, 12), and thought to be largely mediated by the phospholipase Cγ (PLCγ)/inositol 1,4,5 trisphosphate (IP₃) signaling pathway (10). It has been reported that IP₃ releases Ca²⁺ from intracellular stores in ECs, increasing [Ca²⁺]_i, and is augmented by store-operated Ca²⁺ influx (13). This signaling primes the endothelium for angiogenesis through the activation of downstream effectors such as endothelial nitric oxide synthase (eNOS), protein kinases C (PKC), and mitogen-activated protein kinases (MAPKs). Indeed, it has been reported that the interplay between IP₃-dependent Ca²⁺ mobilization and store-operated Ca²⁺ entry produces Ca²⁺ signals whose inhibition impairs the angiogenic effect of VEGF (14, 15). Given the complexity of both VEGF and Ca²⁺ signaling, and the crucial finding that VEGF evokes pro- and antiangiogenic responses, it is clear that the specificity of VEGF-evoked Ca²⁺ signatures deserves further investigation.Differences in Ca²⁺ signatures, which are key to determining specific Ca²⁺-dependent cellular responses, rely upon often complex spatiotemporal variations in [Ca²⁺]_i (16). A major determinant of these are based on functionally distinct intracellular Ca²⁺-mobilizing messengers, namely IP₃ and cyclic adenosine diphosphoribose (cADPR), which mobilize Ca²⁺ from the endoplasmic reticulum (ER) stores, and nicotinic acid adenine dinucleotide phosphate (NAADP), which triggers Ca²⁺ release from acidic organelles, such as lysosomes and endosomes (17, 18). NAADP likely targets a channel distinct from IP₃ and ryanodine receptors (RyRs), known as two-pore channels (TPCs) (19–25), and the resulting localized NAADP-evoked Ca²⁺ signals may in some cases be globalized via IP₃ and RyRs through Ca²⁺-induced Ca²⁺ release (26, 27). However, in a few cell types, direct activation of RyRs and Ca²⁺ influx channels by NAADP have also been proposed as alternative mechanisms (28, 29). It has been demonstrated that NAADP-sensitive Ca²⁺ stores are present in the endothelium, and that NAADP is capable of regulating vascular smooth muscle contractility and blood pressure by EC-dependent mechanisms (30). In addition, we have previously demonstrated that NAADP is a specific and essential intracellular mediator of ECs histamine H₁ receptors, evoking [Ca²⁺]_i release and secretion of von Willebrand factor, which requires the functional expression of TPCs (31).In the present work, we identify a novel pathway for VEGFR2 signal transduction whereby receptor activation leads to NAADP and TPC2-dependent Ca²⁺ release from acidic Ca²⁺ stores, which in turn controls angiogenic response in vitro and in vivo. These findings demonstrate, to our knowledge for the first time, the direct relationship between NAADP-mediated Ca²⁺ release and the signaling mechanisms underlying ECs angiogenesis mediated by VEGF.     

The role of Ca2+ release from sarcoplasmic reticulum in the regulation of sinoatrial node automaticity

Summary The role of Ca²⁺ release channels in the sarcoplasmic reticulum in modulating physiological automaticity of the sinoatrial (SA) node was studied by recording transmembrane action potentials and membrane ionic currents in small preparations of the rabbit SA node. Ryanodine, which modifies the conductance and gating behavior of the Ca²⁺ release channels, was used to block Ca²⁺ release from the sarcoplasmic reticulum. Superfusion of 1-mM ryanodine decreased the spontaneous firing frequency as well as the maximal rate of depolarization of the SA, and these reductions reached a steady state within approximately 5min. The action potential recordings revealed that the latter part of diastolic depolarization was depressed and that the take-off potential became less negative. This suggested that the negative chronotropic effect of ryanodine resulted from the blockade of physiological Ca²⁺ release from the sarcoplasmic reticulum. In voltage clamp experiments, using double-microelectrode techniques, ryanodine did not markedly reduce the Ca²⁺ current (I_Ca) but decreased the delayed rectifying K+ current (I_K), the steady-state inward current (I_ss), and the hyperpolarization-activated inward current (I_h). These observations suggest that, even when the function of Ca²⁺ channels in the cell membrane is normally maintained, depression of Ca²⁺ release channels in the sarcoplasmic reticulum would prevent sufficient elevation of the Ca²⁺ concentration in SA node cells for the activation of various ionic currents, and, thus adversely affect the physiological automaticity of this primary cardiac pacemaker.     

Skeletal muscle-specific T-tubule protein STAC3 mediates voltage-induced Ca2+ release and contractility

Excitation–contraction (EC) coupling comprises events in muscle that convert electrical signals to Ca²⁺ transients, which then trigger contraction of the sarcomere. Defects in these processes cause a spectrum of muscle diseases. We report that STAC3, a skeletal muscle-specific protein that localizes to T tubules, is essential for coupling membrane depolarization to Ca²⁺ release from the sarcoplasmic reticulum (SR). Consequently, homozygous deletion of src homology 3 and cysteine rich domain 3 (Stac3) in mice results in complete paralysis and perinatal lethality with a range of musculoskeletal defects that reflect a blockade of EC coupling. Muscle contractility and Ca²⁺ release from the SR of cultured myotubes from Stac3 mutant mice could be restored by application of 4-chloro-m-cresol, a ryanodine receptor agonist, indicating that the sarcomeres, SR Ca²⁺ store, and ryanodine receptors are functional in Stac3 mutant skeletal muscle. These findings reveal a previously uncharacterized, but required, component of the EC coupling machinery of skeletal muscle and introduce a candidate for consideration in myopathic disorders.