Inhibition of SERCA pumps induces desynchronized RyR activation in overloaded internal Ca2+ stores in smooth muscle cells

A Comparative Study of the Antiemetic Effects of α2-Adrenergic Receptor Agonists Clonidine and Dexmedetomidine against Diverse Emetogens in the Least Shrew (Cryptotis parva) Model of Emesis

In contrast to cats and dogs, here we report that the α2-adrenergic receptor antagonist yohimbine is emetic and corresponding agonists clonidine and dexmedetomidine behave as antiemetics in the least shrew model of vomiting. Yohimbine (0, 0.5, 0.75, 1, 1.5, 2, and 3 mg/kg, i.p.) caused vomiting in shrews in a bell-shaped and dose-dependent manner, with a maximum frequency (0.85 ± 0.22) at 1 mg/kg, which was accompanied by a key central contribution as indicated by increased expression of c-fos, serotonin and substance P release in the shrew brainstem emetic nuclei. Our comparative study in shrews demonstrates that clonidine (0, 0.1, 1, 5, and 10 mg/kg, i.p.) and dexmedetomidine (0, 0.01, 0.05, and 0.1 mg/kg, i.p.) not only suppress yohimbine (1 mg/kg, i.p.)-evoked vomiting in a dose-dependent manner, but also display broad-spectrum antiemetic effects against diverse well-known emetogens, including 2-Methyl-5-HT, GR73632, McN-A-343, quinpirole, FPL64176, SR141716A, thapsigargin, rolipram, and ZD7288. The antiemetic inhibitory ID50 values of dexmedetomidine against the evoked emetogens are much lower than those of clonidine. At its antiemetic doses, clonidine decreased shrews' locomotor activity parameters (distance moved and rearing), whereas dexmedetomidine did not do so. The results suggest that dexmedetomidine represents a better candidate for antiemetic potential with advantages over clonidine.

A re-consideration of neural/receptor mechanisms in chemotherapy-induced nausea and vomiting: current scenario and future perspective

The neural mechanisms and the receptors behind the course of chemotherapy-induced nausea and vomiting (CINV) are well described and considered mechanistically multifactorial, whereas the neurobiology of nausea is not completely understood yet. Some of the anti-neoplastic medications like cisplatin result in biphasic vomiting response. The acute phase of vomiting is triggered mainly via the release of serotonin from the enterochromaffin (EC) cells in the gastrointestinal tract (GIT) and results in stimulation of dorsal vagal complex (DVC) of the vomiting center and the vomiting is initiated by downward communication to the gut via vagal efferents. Agonism of 5HT3 receptors is majorly involved in the mediation of the acute phase. Therefore, antagonists at 5HT3 receptors are effective in the management of acute-phase vomiting episodes. Likewise, Dopamine type 2 (D2) receptors, dopamine neurotransmitter, Muscarinic receptors (M3), GLP1 receptors, and histaminergic receptors (H1) are also implicated in the vomiting act as well. In continuation, Cannabinoid type 1 (CB1) receptors are also recommended and included in the guidelines as agonism of presynaptically located CB1 receptors inhibits the release of excitatory neurotransmitters responsible for vomiting initiation. The delayed phase involves the release of "Substance P" in the gut and results in the stimulation of neurokinin-1 (NK1) receptors centrally in the area postrema (AP) and nucleus tractus solitarius (NTS), subsequently the vomiting response. The current understanding is the existence of overlapping mechanisms of neurotransmitters, serotonin, dopamine, and substance P throughout the time course of CINV. Furthermore, the emetic neurotransmitters are released via calcium ion (Ca++)-dependent mechanisms, implicating the molecular targets of intracellular Ca++ signaling in emetic circuitry. The current review entails the neurobiology of nausea and vomiting induced by cancer chemotherapeutic agents and the recent approaches in the management.

Mechanisms of Nausea and Vomiting: Current Knowledge and Recent Advances in Intracellular Emetic Signaling Systems

Nausea and vomiting are common gastrointestinal complaints that can be triggered by diverse emetic stimuli through central and/or peripheral nervous systems. Both nausea and vomiting are considered as defense mechanisms when threatening toxins/drugs/bacteria/viruses/fungi enter the body either via the enteral (e.g., the gastrointestinal tract) or parenteral routes, including the blood, skin, and respiratory systems. While vomiting is the act of forceful removal of gastrointestinal contents, nausea is believed to be a subjective sensation that is more difficult to study in nonhuman species. In this review, the authors discuss the anatomical structures, neurotransmitters/mediators, and corresponding receptors, as well as intracellular emetic signaling pathways involved in the processes of nausea and vomiting in diverse animal models as well as humans. While blockade of emetic receptors in the prevention of vomiting is fairly well understood, the potential of new classes of antiemetics altering postreceptor signal transduction mechanisms is currently evolving, which is also reviewed. Finally, future directions within the field will be discussed in terms of important questions that remain to be resolved and advances in technology that may help provide potential answers.

Ultra-low doses of the transient receptor potential vanilloid 1 agonist, resiniferatoxin, prevents vomiting evoked by diverse emetogens in the least shrew (Cryptotis parva)

Published studies have shown that the transient receptor potential vanilloid 1 (TRPV1) receptor agonist, resiniferatoxin (RTX), has pro and antiemetic effects. RTX can suppress vomiting evoked by a variety of nonselective emetogens such as copper sulfate and cisplatin in several vomit-competent species. In the least shrew, we have already demonstrated that combinations of ultra-low doses of RTX and low doses of the cannabinoid CB1/2 receptor agonist delta-9-tetrahydrocannabinol (Δ-THC) produce additive antiemetic effects against cisplatin-evoked vomiting. In the current study, we investigated the broad-spectrum antiemetic potential of very low nonemetic doses of RTX against a diverse group of specific emetogens including selective and nonselective agonists of serotonergic 5-hydroxytrptamine (5-HT3) receptor (5-HT and 2-Me-5-HT), dopaminergic D2 receptor (apomorphine and quinpirole), cholinergic M1 receptor (pilocarpine and McN-A-343), as well as the selective substance P neurokinin NK1 receptor agonist GR73632, the selective L-Type calcium channel agonist FPL64176, and the sarcoplasmic endoplasmic reticulum calcium ATPase (SERCA) inhibitor thapsigargin. When administered subcutaneously, ultra-low (0.01 µg/kg) to low (5.0 µg/kg) doses of RTX suppressed vomiting induced by the aforementioned emetogens in a dose-dependent fashion with 50% inhibitory dose values ranging from 0.01 to 1.26 µg/kg. This study is the first to demonstrate that low nanomolar nonemetic doses of RTX have the capacity to completely abolish vomiting caused by diverse receptor specific emetogens in the least shrew model of emesis.

Overexpression of Na+-HCO3- cotransporter contributes to the exacerbation of cardiac remodeling in mice with myocardial infarction by increasing intracellular calcium overload

The role of the cardiac isoform of the electrogenic sodium-bicarbonate ion cotransporter (NBCe1) in cardiac remodeling is not fully understood. The aim of this study was to assess the effects of NBCe1 overexpression on cardiac remodeling induced by myocardial infarction (MI) in mice. We generated NBCe1 transgenic (Tg) mice and NBCe1 overexpressing adult mouse ventricular myocytes (AMVMs) to investigate the role of NBCe1 on post-MI remodeling and calcium kinetics. Tg mice showed a markedly higher mortality rate and larger infarct size after MI. At 6 weeks after MI, the maximum rising rates of left ventricular pressure (dp/dt), contractility index, and the exponential time constant of relaxation (τ) were markedly lower, and there was higher cardiomyocyte apoptosis, in Tg mice compared with WT mice. In cultured AMVMs, overexpression of NBCe1 decreased sarcomere shortening and calcium amplitude. In WT AMVMs, the rates of the rise and decay phase of calcium transients, indicated by the rising time (T_peak, time to peak) and decay time constant (τ_d), and the number of apoptotic cells, were increased following hypoxia, while overexpression of NBCe1 further increased T_peak and cellular apoptosis, but not τ_d. Intracellular resting calcium and sodium concentrations were significantly increased following both hypoxia and NBCe1 overexpression. Co-treatment with S0859, an NBCe1 antagonist, blocked the hypoxia-induced increase in T_peak, τ_d, intracellular resting calcium and sodium concentrations, and apoptosis in cardiomyocytes. These findings indicate that NBCe1 overexpression promotes cardiac remodeling by increasing intracellular calcium overload. Therefore, NBCe1 should be a potential target for treatment of cardiac remodeling.

Synthesis and Biological Characterization of a New Norbormide Derived Bodipy FL-Conjugated Fluorescent Probe for Cell Imaging

Background: Norbormide (NRB) is a selective rat toxicant endowed with vasoconstrictor activity confined to the rat peripheral arteries. In a recent work we used a fluorescent derivative of NRB (NRB-AF12), obtained by coupling the NBD fluorophore to the parent molecule via a linker, in order to gain information about the possible site of action of the unlabeled compound. We found that NRB-AF12 labeled intracellular organelles in both NRB-sensitive and -insensitive cells and we accordingly proposed its use as a scaffold for the development of a new class of fluorescent probes. In this study, we examined the fluorescent properties of a BODIPY FL-conjugated NRB probe (MC009) developed: (A) to verify if NRB distribution could be influenced by the attached fluorophore; (B) to improve the fluorescent performance of NRB-AF12.

Methods: MC009 characteristics were investigated by confocal fluorescence microscopy, in freshly isolated rat caudal artery myocytes (FIRCAM) and in LX2 cells, representative of NRB-sensitive and insensitive cells, respectively.

Main results: In both FIRCAM and LX2 cells MC009 stained endoplasmic reticulum, mitochondria, Golgi apparatus and lipid droplets, revealing the same intracellular distribution as NRB-AF12, and, at the same time, had both improved photostability and gave a more intense fluorescent signal at lower concentrations than was possible with NRB-AF12, which resulted in a better and finer visualization of intracellular structures. Furthermore, MC009 was effective in cellular labeling in both living and fixed cells. At the concentration used to stain the cells, MC009 did not show any cytotoxic effect and did not affect the regular progression of cell cycle and division.

Conclusions: This study demonstrates that the distribution of fluorescently labeled NRB is not affected by the type of fluorophore attached to the parent compound, supporting the idea that the localization of the fluorescent derivatives may reasonably reflect that of the parent compound. In addition, we observed a marked improvement in the fluorescent properties of BODIPY FL-conjugated NRB (MC009) over its NBD-derived counterpart (NRB-AF12), confirming NRB as a scaffold for the development of new, high performance, non-toxic fluorescent probes for the labeling of intracellular structures in both living and fixed cells.

LncRNA ZFAS1 as a SERCA2a Inhibitor to Cause Intracellular Ca2+ Overload and Contractile Dysfunction in a Mouse Model of Myocardial Infarction

RATIONALE

Ca²⁺ homeostasis-a critical determinant of cardiac contractile function-is critically regulated by SERCA2a (sarcoplasmic reticulum Ca²⁺-ATPase 2a). Our previous study has identified ZFAS1 as a new lncRNA biomarker of acute myocardial infarction (MI).

OBJECTIVE

To evaluate the effects of ZFAS1 on SERCA2a and the associated Ca²⁺ homeostasis and cardiac contractile function in the setting of MI.

METHODS AND RESULTS

ZFAS1 expression was robustly increased in cytoplasm and sarcoplasmic reticulum in a mouse model of MI and a cellular model of hypoxia. Knockdown of endogenous ZFAS1 by virus-mediated silencing shRNA partially abrogated the ischemia-induced contractile dysfunction. Overexpression of ZFAS1 in otherwise normal mice created similar impairment of cardiac function as that observed in MI mice. Moreover, at the cellular level, ZFAS1 overexpression weakened the contractility of cardiac muscles. At the subcellular level, ZFAS1 deleteriously altered the Ca²⁺ transient leading to intracellular Ca²⁺ overload in cardiomyocytes. At the molecular level, ZFAS1 was found to directly bind SERCA2a protein and to limit its activity, as well as to repress its expression. The effects of ZFAS1 were readily reversible on knockdown of this lncRNA. Notably, a sequence domain of ZFAS1 gene that is conserved across species mimicked the effects of the full-length ZFAS1. Mutation of this domain or application of an antisense fragment to this conserved region efficiently canceled out the deleterious actions of ZFAS1. ZFAS1 had no significant effects on other Ca²⁺-handling regulatory proteins.

CONCLUSIONS

ZFAS1 is an endogenous SERCA2a inhibitor, acting by binding to SERCA2a protein to limit its intracellular level and inhibit its activity, and a contributor to the impairment of cardiac contractile function in MI. Therefore, anti-ZFAS1 might be considered as a new therapeutic strategy for preserving SERCA2a activity and cardiac function under pathological conditions of the heart.

Thapsigargin-induced activation of Ca(2+)-CaMKII-ERK in brainstem contributes to substance P release and induction of emesis in the least shrew

Cytoplasmic calcium (Ca(2+)) mobilization has been proposed to be an important factor in the induction of emesis. The selective sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase (SERCA) inhibitor thapsigargin, is known to deplete intracellular Ca(2+) stores, which consequently evokes extracellular Ca(2+) entry through cell membrane-associated channels, accompanied by a prominent rise in cytosolic Ca(2+). A pro-drug form of thapsigargin is currently under clinical trial as a targeted cancer chemotherapeutic. We envisioned that the intracellular effects of thapsigargin could cause emesis and planned to investigate its mechanisms of emetic action. Indeed, thapsigargin did induce vomiting in the least shrew in a dose-dependent and bell-shaped manner, with maximal efficacy (100%) at 0.5 mg/kg (i.p.). Thapsigargin (0.5 mg/kg) also caused increases in c-Fos immunoreactivity in the brainstem emetic nuclei including the area postrema (AP), nucleus tractus solitarius (NTS) and dorsal motor nucleus of the vagus (DMNX), as well as enhancement of substance P (SP) immunoreactivity in DMNX. In addition, thapsigargin (0.5 mg/kg, i.p.) led to vomit-associated and time-dependent increases in phosphorylation of Ca(2+)/calmodulin kinase IIα (CaMKIIα) and extracellular signal-regulated protein kinase 1/2 (ERK1/2) in the brainstem. We then explored the suppressive potential of diverse chemicals against thapsigargin-evoked emesis including antagonists of: i) neurokinin-1 receptors (netupitant), ii) the type 3 serotonin receptors (palonosetron), iii) store-operated Ca(2+) entry (YM-58483), iv) L-type Ca(2+) channels (nifedipine), and v) SER Ca(2+)-release channels inositol trisphosphate (IP3Rs) (2-APB)-, and ryanodine (RyRs) (dantrolene)-receptors. In addition, the antiemetic potential of inhibitors of CaMKII (KN93) and ERK1/2 (PD98059) were investigated. All tested antagonists/blockers attenuated emetic parameters to varying degrees except palonosetron, however a combination of non-effective doses of netupitant and palonosetron exhibited additive antiemetic efficacy. A low-dose combination of nifedipine and 2-APB plus dantrolene mixture completely abolished thapsigargin-evoked vomiting, CaMKII-ERK1/2 activation and SP elevation. In addition, pretreatment with KN93 or PD98059 suppressed thapsigargin-induced increases in SP and ERK1/2 activation. Intracerebroventricular injection of netupitant suppressed vomiting caused by thapsigargin which suggests that the principal site of evoked emesis is the brainstem. In sum, this is the first study to demonstrate that thapsigargin causes vomiting via the activation of the Ca(2+)-CaMKII-ERK1/2 cascade, which is associated with an increase in the brainstem tissue content of SP, and the evoked emesis occurs through SP-induced activation of neurokinin-1 receptors.

Kinetics on Demand Is a Simple Mathematical Solution that Fits Recorded Caffeine-Induced Luminal SR Ca2+ Changes in Smooth Muscle Cells

The process of Ca²⁺ release from sarcoplasmic reticulum (SR) comprises 4 phases in smooth muscle cells. Phase 1 is characterized by a large increase of the intracellular Ca²⁺ concentration ([Ca²⁺]_i) with a minimal reduction of the free luminal SR [Ca²⁺] ([Ca²⁺]_FSR). Importantly, active SR Ca²⁺ ATPases (SERCA pumps) are necessary for phase 1 to occur. This situation cannot be explained by the standard kinetics that involves a fixed amount of luminal Ca²⁺ binding sites. A new mathematical model was developed that assumes an increasing SR Ca²⁺ buffering capacity in response to an increase of the luminal SR [Ca²⁺] that is called Kinetics-on-Demand (KonD) model. This approach can explain both phase 1 and the refractory period associated with a recovered [Ca²⁺]_FSR. Additionally, our data suggest that active SERCA pumps are a requisite for KonD to be functional; otherwise luminal SR Ca²⁺ binding proteins switch to standard kinetics. The importance of KonD Ca²⁺ binding properties is twofold: a more efficient Ca²⁺ release process and that [Ca²⁺]_FSR and Ca²⁺-bound to SR proteins ([Ca²⁺]_BSR) can be regulated separately allowing for Ca²⁺ release to occur (provided by Ca²⁺-bound to luminal Ca²⁺ binding proteins) without an initial reduction of the [Ca²⁺]_FSR.

Constitutive PKA activity is essential for maintaining the excitability and contractility in guinea pig urinary bladder smooth muscle: role of the BK channel

The elevation of protein kinase A (PKA) activity activates the large-conductance voltage- and Ca(2+)-activated K(+) (BK) channels in urinary bladder smooth muscle (UBSM) cells and consequently attenuates spontaneous phasic contractions of UBSM. However, the role of constitutive PKA activity in UBSM function has not been studied. Here, we tested the hypothesis that constitutive PKA activity is essential for controlling the excitability and contractility of UBSM. We used patch clamp electrophysiology, line-scanning confocal and ratiometric fluorescence microscopy on freshly isolated guinea pig UBSM cells, and isometric tension recordings on freshly isolated UBSM strips. Pharmacological inhibition of the constitutive PKA activity with H-89 or PKI 14-22 significantly reduced the frequency and amplitude of spontaneous transient BK channel currents (TBKCs) in UBSM cells. Confocal and ratiometric fluorescence microscopy studies revealed that inhibition of constitutive PKA activity with H-89 reduced the frequency and amplitude of the localized Ca(2+) sparks but increased global Ca(2+) levels and the magnitude of Ca(2+) oscillations in UBSM cells. H-89 abolished the spontaneous transient membrane hyperpolarizations and depolarized the membrane potential in UBSM cells. Inhibition of PKA with H-89 or KT-5720 also increased the amplitude and muscle force of UBSM spontaneous phasic contractions. This study reveals the novel concept that constitutive PKA activity is essential for controlling localized Ca(2+) signals generated by intracellular Ca(2+) stores and cytosolic Ca(2+) levels. Furthermore, constitutive PKA activity is critical for mediating the spontaneous TBKCs in UBSM cells, where it plays a key role in regulating spontaneous phasic contractions in UBSM.

Ryanodine receptors as leak channels

Ryanodine receptors are Ca(2+) release channels of internal stores. This review focuses on those situations and conditions that transform RyRs from a finely regulated ion channel to an unregulated Ca(2+) leak channel and the pathological consequences of this alteration. In skeletal muscle, mutations in either CaV1.1 channel or RyR1 results in a leaky behavior of the latter. In heart cells, RyR2 functions normally as a Ca(2+) leak channel during diastole within certain limits, the enhancement of this activity leads to arrhythmogenic situations that are tackled with different pharmacological strategies. In smooth muscle, RyRs are involved more in reducing excitability than in stimulating contraction so the leak activity of RyRs in the form of Ca(2+) sparks, locally activates Ca(2+)-dependent potassium channels to reduce excitability. In neurons the enhanced activity of RyRs is associated with the development of different neurodegenerative disorders such as Alzheimer and Huntington diseases. It appears then that the activity of RyRs as leak channels can have both physiological and pathological consequences depending on the cell type and the metabolic condition.

Endoplasmic reticulum calcium pumps and cancer

Endoplasmic reticulum calcium homeostasis is involved in a multitude of signaling, as well as "house-keeping" functions that control cell growth, differentiation or apoptosis in every human/eukaryotic cell. Calcium is actively accumulated in the endoplasmic reticulum by Sarco/Endoplasmic Reticulum Calcium transport ATPases (SERCA enzymes). SERCA-dependent calcium transport is the only calcium uptake mechanism in this organelle, and therefore the regulation of SERCA function by the cell constitutes a key mechanism to adjust calcium homeostasis in the endoplasmic reticulum depending on the cell type and its state of differentiation. The direct pharmacological modulation of SERCA activity affects cell differentiation and survival. SERCA expression levels can undergo significant changes during cell differentiation or tumorigenesis, leading to modified endoplasmic reticulum calcium storage. In several cell types such as cells of hematopoietic origin or various epithelial cells, two SERCA genes (SERCA2 and SERCA3) are simultaneously expressed. Expression levels of SERCA3, a lower calcium affinity calcium pump are highly variable. In several cell systems SERCA3 expression is selectively induced during differentiation, whereas during tumorigenesis and blastic transformation SERCA3 expression is decreased. These observations point at the existence of a cross-talk, via the regulation of SERCA3 levels, between endoplasmic reticulum calcium homeostasis and the control of cell differentiation, and show that endoplasmic reticulum calcium homeostasis itself can undergo remodeling during differentiation. The investigation of the anomalies of endoplasmic reticulum differentiation in tumor and leukemia cells may be useful for a better understanding of the contribution of calcium signaling to the establishment of malignant phenotypes.

An intelligent sarco-endoplasmic reticulum Ca2+ store: release and leak channels have differential access to a concealed Ca2+ pool

Simultaneous recording of cytosolic and sarco-endoplasmic reticulum (SR/ER) luminal free calcium concentrations ([Ca(2+)](i) and [Ca(2+)](L), respectively) supports the notion that release channels (RyRs and IP(3)Rs) use a concealed Ca(2+) source, likely to be associated with intra-SR/ER Ca(2+) binding proteins, whereas SR/ER Ca(2+) leak channels can only access free luminal Ca(2+). We hypothesize that Ca(2+) is trapped by oligomers of luminal Ca(2+)-binding proteins and that the opening of release channels induces the rapid liberation of this "concealed" Ca(2+) source associated with intra-ER Ca(2+) buffers. Our hypothesis may also clarify why SERCA pumps potentiate Ca(2+) release and explain quantal characteristics and refractory states of Ca(2+) release process.