Different kinases regulate activation of voltage-dependent calcium channels by depolarization in GH3 cells

Regulation of the Ca2+ channel α2δ-1 subunit expression by epidermal growth factor via the ERK/ELK-1 signaling pathway

Voltage-gated Ca²⁺ (Ca_V) channels are expressed in endocrine cells where they contribute to hormone secretion. Diverse chemical messengers, including epidermal growth factor (EGF), are known to affect the expression of Ca_V channels. Previous studies have shown that EGF increases Ca²⁺ currents in GH3 pituitary cells by increasing the number of high voltage-activated (HVA) Ca_V channels at the cell membrane, which results in enhanced prolactin (PRL) secretion. However, little is known regarding the mechanisms underlying this regulation. Here, we show that EGF actually increases the expression of the Ca_Vα₂δ-1 subunit, a key molecular component of HVA channels. The analysis of the gene promoter encoding Ca_Vα₂δ-1 (CACNA2D1) revealed binding sites for transcription factors activated by the Ras/Raf/MEK/ERK signaling cascade. Chromatin immunoprecipitation and site-directed mutagenesis showed that ELK-1 is crucial for the transcriptional regulation of CACNA2D1 in response to EGF. Furthermore, we found that EGF increases the membrane expression of Ca_Vα₂δ-1 and that ELK-1 overexpression increases HVA current density, whereas ELK-1 knockdown decreases the functional expression of the channels. Hormone release assays revealed that Ca_Vα₂δ-1 overexpression increases PRL secretion. These results suggest a mechanism for how EGF, by activating the Ras/Raf/MEK/ERK/ELK-1 pathway, may influence the expression of HVA channels and the secretory behavior of pituitary cells.

Convergent Met and voltage-gated Ca2+ channel signaling drives hypermigration of Toxoplasma-infected dendritic cells

Ras-Erk MAPK signaling controls many of the principal pathways involved in metazoan cell motility, drives metastasis of multiple cancer types and is targeted in chemotherapy. However, its putative roles in immune cell functions or in infections have remained elusive. Here, using primary dendritic cells (DCs) in an infection model with the protozoan Toxoplasma gondii, we show that two pathways activated by infection converge on Ras-Erk MAPK signaling to promote migration of parasitized DCs. We report that signaling through the receptor tyrosine kinase Met (also known as HGF receptor) contributes to T. gondii-induced DC hypermotility. Furthermore, voltage-gated Ca2+ channel (VGCC, subtype CaV1.3) signaling impacted the migratory activation of DCs via calmodulin-calmodulin kinase II. We show that convergent VGCC signaling and Met signaling activate the GTPase Ras to drive Erk1 and Erk2 (also known as MAPK3 and MAPK1, respectively) phosphorylation and hypermotility of T. gondii-infected DCs. The data provide a molecular basis for the hypermigratory mesenchymal-to-amoeboid transition (MAT) of parasitized DCs. This emerging concept suggests that parasitized DCs acquire metastasis-like migratory properties that promote infection-related dissemination.

Inhibition of Notch signaling attenuates pituitary adenoma growth in Nude mice

Preclinical and clinical studies support that Notch signaling may play an important oncogenic role in cancer, but there is scarce information for pituitary tumors. We therefore undertook a functional study to evaluate Notch participation in pituitary adenoma growth. Tumors generated in Nude mice by subcutaneous GH3 somatolactotrope cell injection were treated in vivo with DAPT, a γ-secretase inhibitor, thus inactivating Notch signaling. This treatment led to pituitary tumor reduction, lower prolactin and GH tumor content and a decrease in angiogenesis. Furthermore, in silico transcriptomic and epigenomic analyses uncovered several tumor suppressor genes related to Notch signaling in pituitary tissue, namely Btg2, Nr4a1, Men1, Zfp36 and Cnot1. Gene evaluation suggested that Btg2, Nr4a1 and Cnot1 may be possible players in GH3 xenograft growth. Btg2 mRNA expression was lower in GH3 tumors compared to the parental line, and DAPT increased its expression levels in the tumor in parallel with the inhibition of its volume. Cnot1 mRNA levels were also increased in the pituitary xenografts by DAPT treatment. And the Nr4a1 gene was lower in tumors compared to the parental line, though not modified by DAPT. Finally, because DAPT in vivo may also be acting on tumor microenvironment, we determined the direct effect of DAPT on GH3 cells in vitro. We found that DAPT decreases the proliferative, secretory and migration potential of GH3 cells. These results position selective interruption of Notch signaling as a potential therapeutic tool in adjuvant treatments for aggressive or resistant pituitary tumors.

Src-family tyrosine kinases and the Ca2+ signal

In this review, we shall describe the rich crosstalk between non-receptor Src-family kinases (SFKs) and the Ca²⁺ transient generated in activated cells by a variety of extracellular and intracellular stimuli, resulting in diverse signaling events. The exchange of information between SFKs and Ca²⁺ is reciprocal, as it flows in both directions. These kinases are main actors in pathways leading to the generation of the Ca²⁺ signal, and reciprocally, the Ca²⁺ signal modulates SFKs activity and functions. We will cover how SFKs participate in the generation of the cytosolic Ca²⁺ rise upon activation of a series of receptors and the mechanism of clearance of this Ca²⁺ signal. The role of SFKs modulating Ca²⁺-translocating channels participating in these events will be amply discussed. Finally, the role of the Ca²⁺ sensor protein calmodulin on the activity of c-Src, and potentially on other SFKs, will be outlined as well. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Calcium response of KCl-excited populations of ventricular myocytes from the European sea bass (Dicentrarchus labrax): a promising approach to integrate cell-to-cell heterogeneity in studying the cellular basis of fish cardiac performance

Climate change challenges the capacity of fishes to thrive in their habitat. However, through phenotypic diversity, they demonstrate remarkable resilience to deteriorating conditions. In fish populations, inter-individual variation in a number of fitness-determining physiological traits, including cardiac performance, is classically observed. Information about the cellular bases of inter-individual variability in cardiac performance is scarce including the possible contribution of excitation-contraction (EC) coupling. This study aimed at providing insight into EC coupling-related Ca(2+) response and thermal plasticity in the European sea bass (Dicentrarchus labrax). A cell population approach was used to lay the methodological basis for identifying the cellular determinants of cardiac performance. Fish were acclimated at 12 and 22 °C and changes in intracellular calcium concentration ([Ca(2+)]i) following KCl stimulation were measured using Fura-2, at 12 or 22 °C-test. The increase in [Ca(2+)]i resulted primarily from extracellular Ca(2+) entry but sarcoplasmic reticulum stores were also shown to be involved. As previously reported in sea bass, a modest effect of adrenaline was observed. Moreover, although the response appeared relatively insensitive to an acute temperature change, a difference in Ca(2+) response was observed between 12- and 22 °C-acclimated fish. In particular, a greater increase in [Ca(2+)]i at a high level of adrenaline was observed in 22 °C-acclimated fish that may be related to an improved efficiency of adrenaline under these conditions. In conclusion, this method allows a rapid screening of cellular characteristics. It represents a promising tool to identify the cellular determinants of inter-individual variability in fishes' capacity for environmental adaptation.

Novel pituitary actions of TAC3 gene products in fish model: receptor specificity and signal transduction for prolactin and somatolactin α regulation by neurokinin B (NKB) and NKB-related peptide in carp pituitary cells

TAC3 is a member of tachykinins, and its gene product neurokinin B (NKB) has recently emerged as a key regulator for LH through modulation of kisspeptin/GnRH system within the hypothalamus. In fish models, TAC3 not only encodes NKB but also a novel tachykinin-like peptide called NKB-related peptide (NKBRP), and the pituitary actions of these TAC3 gene products are still unknown. Using grass carp as a model, the direct effects and postreceptor signaling for the 2 TAC3 products were examined at the pituitary level. Grass carp TAC3 was cloned and confirmed to encode NKB and NKBRP similar to that of other fish species. In carp pituitary cells, NKB and NKBRP treatment did not affect LH release and gene expression but up-regulated prolactin (PRL) and somatolactin (SL)α secretion, protein production, and transcript expression. The stimulation by these 2 TAC3 gene products on PRL and SLα release and mRNA levels were mediated by pituitary NK2 and NK3 receptors, respectively. Apparently, NKB- and NKBRP-induced SLα secretion and transcript expression were caused by adenylate cyclase/cAMP/protein kinase A, phospholipase C/inositol 1,4,5-triphosphate/protein kinase C and Ca(2+)/calmodulin/Ca(2+)/calmodulin-dependent protein kinase II activation. The signal transduction for the corresponding responses on PRL release and mRNA expression were also similar, except that the protein kinase C component was not involved. These findings suggest that the 2 TAC3 gene products do not play a role in LH regulation at the pituitary level in carp species but may serve as novel stimulators for PRL and SLα synthesis and secretion via overlapping postreceptor signaling mechanisms coupled to NK2 and NK3 receptors, respectively.

Rutin potentiates calcium uptake via voltage-dependent calcium channel associated with stimulation of glucose uptake in skeletal muscle

Rutin is a flavonoid with several pharmacological properties and it has been demonstrated that rutin can modulate glucose homeostasis. In skeletal muscle, an increase in intracellular calcium concentration may induce glucose transporter-4 (GLUT-4) translocation with consequent glucose uptake. The aim of this study was to investigate the effect of rutin and intracellular pathways on calcium uptake as well as the involvement of calcium in glucose uptake in skeletal muscle. The results show that rutin significantly stimulated calcium uptake through voltage-dependent calcium channels as well as mitogen-activated kinase (MEK) and protein kinase A (PKA) signaling pathways. Also, rutin stimulated glucose uptake in the soleus muscle and this effect was mediated by extracellular calcium and calcium-calmodulin-dependent protein kinase II (CaMKII) activation. In conclusion, rutin significantly stimulates calcium uptake in rat soleus muscles. Furthermore, the increase in intracellular calcium concentration is involved in DNA activation by rutin. Also, rutin-induced glucose uptake via CaMKII may result in GLUT-4 translocation to the plasma membrane, characterizing an insulin-independent pathway. These findings indicate that rutin is a potential drug candidate for diabetes therapy.

Constitutive activity of the A2A adenosine receptor and compartmentalised cyclic AMP signalling fine-tune noradrenaline release

Neuroblastoma SH-SY5Y (SH) cells endogenously express A(2A) adenosine receptors and can be differentiated into a sympathetic neuronal phenotype, capable of depolarisation-dependent noradrenaline release. Using differentiated SH culture, we here explored the link between A(2A)-receptor signalling and neurotransmitter release. In response to the receptor agonist CGS21680, the cells produced cyclic AMP (cAMP), and when depolarised, they released increased amounts of noradrenaline. An A(2A)-receptor antagonist, XAC, as well as an inhibitor of cAMP-dependent protein kinase A (PKA), H89, depressed agonist-dependent release. In the presence of XAC or H89, noradrenaline release was found to be below basal values. This suggested that release facilitation also owes to constitutive receptor activity. We demonstrate that even in the absence of an agonist, the native A(2A)-receptor stimulated cAMP production, leading to the activation of PKA and enhanced noradrenaline release. Ancillary, non-cAMP-dependent effects of the receptor (i.e. phosphorylation of CREB, of Rabphilin3A) were refractory to constitutive activation. PKA-dependent facilitation of noradrenaline release was recapitulated with membrane-permeable 8-Br-cAMP; in addition to facilitation, 8-Br-cAMP caused marked inhibition of release, an effect not observed upon receptor activation. Inhibition by receptor-independent cAMP was likely due to suppression of voltage-dependent calcium current (VDCC) and increased activity of Src-family kinases. Receptor-mediated release facilitation was reproduced in the presence of tetrodotoxin (blocking action potentials); hence, the signalling occurred at the active zone comprising release sites. Our findings thus support (1) presynaptic localisation of the A(2A)-receptor and (2) suggest that compartmentalised pathways transmit cAMP signalling in order to facilitate depolarisation-dependent neurotransmitter release.

Decrease of prestin expression by increased potassium concentration in organotypic cultures of the organ of Corti of newborn rats

Prestin is the motor protein of the outer hair cells of the organ of Corti and a key factor in ensuring a high sensitivity level of mammalian hearing. In the present study, we examined the effects of increased extracellular potassium (K(+)) concentration on the expression of prestin mRNA and the transcription factors Gata-3 and Carf in the organotypic culture of the organ of Corti of newborn rats. Mannitol and NaCl were used to analyze possible effects of hyperosmotic stress or ion-specific changes, respectively. An increase in prestin expression by a factor of 1.5-2.0 was seen in cultures grown in the presence of 5mM K(+). Potassium concentration of 35 and 55 mM induced a parallel decrease in prestin and Carf expression, but Gata-3 expression increased. Mannitol had no effect on gene expression whereas increased NaCl concentrations decreased prestin, but not Carf expression. The data suggest that chronic depolarization might decrease the prestin expression and possibly contribute to hearing loss and tinnitus.

Novel adenosine and cAMP signalling pathways in migrating glial cells

This study was aimed at characterizing the effect of purinergic transmission on migration of embryonic ciliary ganglion satellite glial cells. Application of adenosine significantly decreased the rate of migration of glial cells whereas no differences were observed in the presence of ATP. The A(2B) receptor antagonist reverted this action, but application of an A(2A) receptor antagonist or a cAMP-protein kinase inhibitor had no effect on the agonist's stimulation. Forskolin, which stimulates adenylate cyclase activity, and the cAMP analogue 8-CPT-2'-O-Me-cAMP, which selectively activates the guanine exchange factor Epac1, mimicked the effect of adenosine. In addition, intracellular calcium measurements studies revealed that application of either adenosine or ATP induced an increase in [Ca(2+)]i and that the adenosine-induced [Ca(2+)]i response was due to Ca(2+) entry and was blocked by an A(2A) receptor antagonist, SCH 58261, or by high Gd(3+) concentrations. Furthermore, forskolin, but not 8-CPT-2'-O-Me-cAMP, activated the Ca(2+) entry which was blocked by Gd(3+) and was independent of cAMP-protein kinase activity. These results demonstrate the involvement of purinergic P1 signalling in the regulation of cellular migration, and point to the importance of adenosine as a negative modulator of migration of peripheral developing glial cells and as an activator of Ca(2+) entry.

Multiple kinase pathways regulate voltage-dependent Ca2+ influx and migration in oligodendrocyte precursor cells

It is becoming increasingly clear that voltage-operated Ca(2+) channels (VOCCs) play a fundamental role in the development of oligodendrocyte progenitor cells (OPCs). Because direct phosphorylation by different kinases is one of the most important mechanisms involved in VOCC modulation, the aim of this study was to evaluate the participation of serine-threonine kinases and tyrosine kinases (TKs) on Ca(2+) influx mediated by VOCCs in OPCs. Calcium imaging revealed that OPCs exhibited Ca(2+) influx after plasma membrane depolarization via L-type VOCCs. Furthermore, VOCC-mediated Ca(2+) influx declined with OPC differentiation, indicating that VOCCs are developmentally regulated in OPCs. PKC activation significantly increased VOCC activity in OPCs, whereas PKA activation produced the opposite effect. The results also indicated that OPC morphological changes induced by PKC activation were partially mediated by VOCCs. Our data clearly suggest that TKs exert an activating influence on VOCC function in OPCs. Furthermore, using the PDGF response as a model to probe the role of TK receptors (TKr) on OPC Ca(2+) uptake, we found that TKr activation potentiated Ca(2+) influx after membrane depolarization. Interestingly, this TKr modulation of VOCCs appeared to be essential for the PDGF enhancement of OPC migration rate, because cell motility was completely blocked by TKr antagonists, as well as VOCC inhibitors, in migration assays. The present study strongly demonstrates that PKC and TKrs enhance Ca(2+) influx induced by depolarization in OPCs, whereas PKA has an inhibitory effect. These kinases modulate voltage-operated Ca(2+) uptake in OPCs and participate in the modulation of process extension and migration.

Expression, regulation and biological actions of growth hormone (GH) and ghrelin in the immune system

Immune and neuroendocrine systems have bidirectional communications. Growth hormone (GH) and an orexigenic hormone ghrelin are expressed in various immune cells such as T lymphocytes, B lymphocytes, monocytes and neutrophils. These immune cells also bear receptors for hormones: growth hormone receptor (GHR) for GH and growth hormone secretagogue receptor (GHS-R) for ghrelin. The expression of GH in immune cells is stimulated by ghrelin as in anterior pituitary cells, whereas the regulation of GH secretion in the immune system by other peptides seems to be different from that in the anterior pituitary gland. Cytokines and mitogens enhance GH secretion from immune cells. GH has several biological actions in the immune system: enhancing thymopoiesis and T cell development, modulating cytokine production, enhancing B cell development and antibody production, priming neutrophils and monocytes for superoxide anion secretion, enhancing neutrophil adhesion and monocyte migration and anti-apoptotic action. Biological actions of ghrelin include attenuation of septic shock and anti-inflammatory actions, modulating phagocytosis, and enhancing thymopoiesis. The effect of ghrelin may be direct or through GH production, and that of GH may be direct or through insulin like growth factor-I (IGF-I) production. Elucidation of the roles of GH and ghrelin in the immune system may shed light on the treatment and prevention of immunological disorders such as AIDS and organ damages due to obesity/ageing-related chronic inflammation.

PTPepsilon has a critical role in signaling transduction pathways and phosphoprotein network topology in red cells

Protein tyrosine phosphatases (PTPs) are crucial components of cellular signal transduction pathways. Here, we report that red blood cells (RBCs) from mice lacking PTPepsilon (Ptpre(-/-)) exhibit (i) abnormal morphology; (ii) increased Ca(2+)-activated-K(+) channel activity, which was partially blocked by the Src family kinases (SFKs) inhibitor PP1; and (iii) market perturbation of the RBC membrane tyrosine (Tyr-) phosphoproteome, indicating an alteration of RBC signal transduction pathways. Using the signaling network computational analysis of the Tyr-phosphoproteomic data, we identified seven topological clusters. We studied cluster 1 containing Fyn, SFK, and Syk another tyrosine kinase. In Ptpre(-/-)mouse RBCs, the activity of Fyn was increased while Syk kinase activity was decreased compared to wild-type RBCs, validating the network computational analysis, and indicating a novel signaling pathway, which involves Fyn and Syk in regulation of red cell morphology.

Phosphorylation and ankyrin-G binding of the C-terminal domain regulate targeting and function of the ammonium transporter RhBG

RhBG, a human member of the Amt/Mep/Rh/superfamily of ammonium transporters, has been shown to facilitate NH(3) transport and to be anchored to the basolateral plasma membrane of kidney epithelial cells, via ankyrin-G. We showed here that triple alanine substitution of the (419)FLD(421) sequence, which links the cytoplasmic C-terminal domain of RhBG to ankyrin-G, not only disrupted the interaction of RhBG with the spectrin-based skeleton but also delayed its cell surface expression, decreased its plasma membrane stability, and abolished its NH(3) transport function in epithelial cell lines. Similarly, we demonstrated that both anchoring to the membrane skeleton and ammonium transport activity are regulated by the phosphorylation status of the C-terminal tail of RhBG. Tyrosine 429, which belongs to the previously reported YED basolateral targeting signal of RhBG, was demonstrated to be phosphorylated in vitro using purified Src and Syk kinases and ex vivo by analyzing the effect of pervanadate treatment on wild-type RhBG or Y429A mutants. Then, we showed that Y429D and Y429E mutations, mimicking constitutive phosphorylation, abolished NH(3) transport and enhanced Triton X-100 solubilization of RhBG from the cell membrane. In contrast, the nonphosphorylated/nonphosphorylatable Y429A and Y429F mutants behaved the same as wild-type RhBG. Conversely, Y/A or Y/F but not Y/E or Y/D mutations of residue 429 abolished the exclusive basolateral localization of RhBG in polarized epithelial cells. All these results led to a model in which targeting and ammonium transport function of RhBG are regulated by both phosphorylation and membrane skeleton binding of the C-terminal cytoplasmic domain.