G-proteins involved in the calcium channel signalling system

A neural mechanism for deprivation state-specific expression of relevant memories in Drosophila

Motivational states modulate how animals value sensory stimuli and engage in goal-directed behaviors. The motivational states of thirst and hunger are represented in the brain by shared and unique neuromodulatory systems. However, it is unclear how such systems interact to coordinate the expression of appropriate state-specific behavior. We show that the activity of two brain neurons expressing leucokinin neuropeptide is elevated in thirsty and hungry flies, and that leucokinin release is necessary for state-dependent expression of water- and sugar-seeking memories. Leucokinin inhibits two types of mushroom-body-innervating dopaminergic neurons (DANs) to promote thirst-specific water memory expression, whereas it activates other mushroom-body-innervating DANs to facilitate hunger-dependent sugar memory expression. Selection of hunger- or thirst-appropriate memory emerges from competition between leucokinin and other neuromodulatory hunger signals at the level of the DANs. Therefore, coordinated modulation of the dopaminergic system allows flies to prioritize the expression of the relevant state-dependent motivated behavior.

T-Type Calcium Channel: A Privileged Gate for Calcium Entry and Control of Adrenal Steroidogenesis

Intracellular calcium plays a crucial role in modulating a variety of functions such as muscle contraction, hormone secretion, gene expression, or cell growth. Calcium signaling has been however shown to be more complex than initially thought. Indeed, it is confined within cell microdomains, and different calcium channels are associated with different functions, as shown by various channelopathies. Sporadic mutations on voltage-operated L-type calcium channels in adrenal glomerulosa cells have been shown recently to be the second most prevalent genetic abnormalities present in human aldosterone-producing adenoma. The observed modification of the threshold of activation of the mutated channels not only provides an explanation for this gain of function but also reminds us on the importance of maintaining adequate electrophysiological characteristics to make channels able to exert specific cellular functions. Indeed, the contribution to steroid production of the various calcium channels expressed in adrenocortical cells is not equal, and the reason has been investigated for a long time. Given the very negative resting potential of these cells, and the small membrane depolarization induced by their physiological agonists, low threshold T-type calcium channels are particularly well suited for responding under these conditions and conveying calcium into the cell, at the right place for controlling steroidogenesis. In contrast, high threshold L-type channels are normally activated by much stronger cell depolarizations. The fact that dihydropyridine calcium antagonists, specific for L-type channels, are poorly efficient for reducing aldosterone secretion either in vivo or in vitro, strongly supports the view that these two types of channels differently affect steroid biosynthesis. Whether a similar analysis is transposable to fasciculata cells and cortisol secretion is one of the questions addressed in the present review. No similar mutations on L-type or T-type channels have been described yet to affect cortisol secretion or to be linked to the development of Cushing syndrome, but several evidences suggest that the function of T channels is also crucial in fasciculata cells. Putative molecular mechanisms and cellular structural organization making T channels a privileged entry for the "steroidogenic calcium" are also discussed.

Endocannabinoid-mediated long-term plasticity requires cAMP/PKA signaling and RIM1alpha

Endocannabinoids (eCBs) have emerged as key activity-dependent signals that, by activating presynaptic cannabinoid receptors (i.e., CB1) coupled to G(i/o) protein, can mediate short-term and long-term synaptic depression (LTD). While the presynaptic mechanisms underlying eCB-dependent short-term depression have been identified, the molecular events linking CB1 receptors to LTD are unknown. Here we show in the hippocampus that long-term, but not short-term, eCB-dependent depression of inhibitory transmission requires presynaptic cAMP/PKA signaling. We further identify the active zone protein RIM1alpha as a key mediator of both CB1 receptor effects on the release machinery and eCB-dependent LTD in the hippocampus. Moreover, we show that eCB-dependent LTD in the amygdala and hippocampus shares major mechanistic features. These findings reveal the signaling pathway by which CB1 receptors mediate long-term effects of eCBs in two crucial brain structures. Furthermore, our results highlight a conserved mechanism of presynaptic plasticity in the brain.

L-type Ca2+ channel blockers promote Ca2+ accumulation when dopamine receptors are activated in striatal neurons

Dopamine (DA) receptor-mediated signal transduction and gene expression play a central role in many brain disorders from schizophrenia to Parkinson's disease to addiction. While trying to evaluate the role of L-type Ca2+ channels in dopamine D1 receptor-mediated phosphorylation of the transcription factor cyclic AMP response element-binding protein (CREB), we found that activation of dopamine D1 receptors alters the properties of L-type Ca2+ channel inhibitors and turns them into facilitators of Ca2+ influx. In D1 receptor-stimulated neurons, L-type Ca2+ channel blockers promote cytosolic Ca2+ accumulation. This leads to the activation of a molecular signal transduction pathway and CREB phosphorylation. In the absence of dopamine receptor stimulation, L-type Ca2+ channel blockers inhibit CREB phosphorylation. The effect of dopamine on L-type Ca2+ channel blockers is dependent on protein kinase A (PKA), suggesting that protein phosphorylation plays a role in this phenomenon. Because of the adverse effect of activated dopamine receptors on L-type Ca2+ channel blocker action, the role of L-type Ca2+ channels in the dopamine D1 receptor signal transduction pathway cannot be assessed with pharmacological tools. However, with antisense technology, we demonstrate that L-type Ca2+ channels contribute to D1 receptor-mediated CREB phosphorylation. We conclude that the D1 receptor signal transduction pathway depends on L-type Ca2+ channels to mediate CREB phosphorylation.

Cloning, expression, and functional analysis of human dopamine D1 receptors

AIM

To construct an HEK293 cell line stably expressing human dopamine D1 receptor (D1R).

METHODS

cDNA was amplified by RT-PCR using total RNA from human embryo brain tissue as the template. The PCR products were subcloned into the plasmid pcDNA3 and cloned into the plasmid pcDNA3.1. The cloned D1R cDNA was sequenced and stably expressed in HEK293 cells. Expression of D1R in HEK293 cells was monitored by the [3H]SCH23390 binding assay. The function of D1R was studied by the cAMP accumulation assay, CRE-SEAP reporter gene activity assay, and intracellular calcium assay.

RESULTS

An HEK293 cell line stably expressing human D1R was obtained. A saturation radioligand binding experiment with [3H]SCH23390 demonstrated that the Kd and Bmax values were 1.5+/-0.2 nmol/L and 2.94+/-0.15 nmol/g of protein, respectively. In the [3H]SCH23390 competition assay, D1R agonist SKF38393 displaced [3H]SCH23390 with an IC50 value of 2.0 (1.5-2.8) micromol/L. SKF38393 increased the intracellular cAMP level and CRE-SEAP activity through D1R expressed in HEK293 cells in a concentration-dependent manner with an EC50 value of 0.25 (0.12-0.53) micromol/L and 0.39 (0.27-0.57) micromol/L at 6 h/0.59 (0.22-1.58) micromol/L at 12 h, respectively. SKF38393 also increased the intracellular calcium level in a concentration-dependent manner with EC50 value of 27 (8.6-70) nmol/L.

CONCLUSION

An HEK293 cell line stably expressing human D1R was obtained successfully. The study also demonstrated that the CRE-SEAP activity assay could be substituted for the cAMP accumulation assay for measuring increase in cAMP levels. Thus, both intracellular calcium measurements and the CRE-SEAP activity assay are suitable for high-throughput screening in drug research.

CNTF inhibits high voltage activated Ca2+ currents in fetal mouse cortical neurones

Neurotrophic factors yield neuroprotection by mechanisms that may be related to their effects as inhibitors of apoptosis as well as their effects on ion channels. The effect of ciliary neurotrophic factor (CNTF) on high-threshold voltage-activated Ca channels in cultured fetal mouse brain cortical neurones was investigated. Addition of CNTF into serum-free growth medium resulted in delayed reduction of the Ca2+ currents. The currents decreased to 50% after 4 h and stabilized at this level during incubation with CNTF for 48 h. Following removal of CNTF the inhibition was completely reversed after 18 h. CNTF reduced the current of all pharmacological subtypes of Ca channels as shown by use of selective blockers of L, N, and P/Q type Ca channels (nifedipine, omega-conotoxin MVIIA, omega-agatoxin IVA). The Ca channel depression was mediated via the CNTF receptor, because enzymatic cleavage of the alpha-subunit glycerophosphatidylinositol anchor of the receptor eliminated the response. The CNTF effect was not elicited through pertussis toxin-sensitive G proteins. Other neurotrophic factors like neurotrophin-3 and insulin-like growth factor-I had no effect on the Ca2+ currents. These results may have important implications for the possible functions of CNTF in the nervous system, such as altered synaptic activity, neuronal excitability and susceptibility to brain ischaemia.

Vasopressin-induced cytoplasmic and nuclear calcium signaling in cultured cortical astrocytes

We sought to determine whether vasopressin V(1a) receptor (V(1a)R) mRNA detected in cortical astrocytes [Brain Res. Mol. Brain Res. 45 (1997) 138] was translated into functional receptors by investigating the effector calcium signaling cascade associated with the vasopressin V(1a) receptor subtype. Analysis of intracellular calcium dynamics using the calcium-sensitive dye fura-2 AM indicated that exposure of cortical astrocytes to V(1) vasopressin receptor agonist, [Phe(2),Orn(8)]-oxytocin, induced a marked dose-dependent increase in intracellular calcium which was abolished by depletion of extracellular calcium. V(1) agonist treatment induced a rapid increase in calcium signal in both the cytoplasm and nucleus, which was followed by an accumulation of the calcium signal in the nucleus, suggesting translocation of cytoplasmic calcium into the nucleus. The nuclear calcium signal was sustained for several seconds followed by translocation back to the cytoplasm. Following the nuclear-to-cytoplasmic calcium translocation, total free intracellular calcium concentration decreased. The dynamic calcium cytoplasmic and nuclear localization was confirmed by laser scanning confocal microscopy coupled with the calcium-sensitive dye fluo-3 AM. To determine the source of calcium, V(1) agonist-induced (45)Ca(2+) uptake and [(3)H]IP(1) accumulation were investigated. V(1) agonist induced significant and rapid uptake of (45)Ca(2+) and a significant dose-dependent increase in [(3)H]IP(1) accumulation in cortical astrocytes. To our knowledge, this is the first documentation of a vasopressin receptor-induced calcium signaling cascade in cortical astrocytes and the first documentation of vasopressin receptor induction of nuclear calcium signaling.

Functional screening of G protein-coupled receptors by measuring intracellular calcium with a fluorescence microplate reader

Ligand binding studies reveal information about affinity to G protein-coupled receptors (GPCRs) rather than functional properties. Increase in intracellular Ca(2+) appears to represent a universal second messenger signal for a majority of recombinant GPCRs. Here, we exploit Ca(2+) signaling as a fast and sensitive functional screening method for a number of GPCRs coupled to different G proteins. Ca(2+) fluorescence measurements are performed using Oregon Green 488 BAPTA-1/AM and a microplate reader equipped with an injector. Buffer alone or test compounds dissolved in buffer are injected into a cell suspension, and fluorescence intensity is recorded for 30 s. Each of the GPCRs tested--G(q)-coupled P2Y(2), G(s)-coupled dopamine D1 and D5, G(i)-coupled dopamine D2L, and G(q/11)-coupled muscarinic acetylcholine M1--yielded a significant rise in intracellular free [Ca(2+)] on agonist stimulation. Agonist stimulation was dose dependent, as shown for ATP or UTP stimulation of P2Y(2) receptors (EC(50) = 1 microM), SKF38393 stimulation of hD1 and hD5 (EC(50) = 18.1 nM and 2.7 nM), and quinpirole at hD2L (EC(50) = 6.5 nM). SCH23390 (at hD1 and hD5) and spiperone, haloperidol, and clozapine (at hD2L) competitively antagonized the Ca(2+) response. Furthermore, the Ca(2+) assay served to screen suramin analogs for antagonistic activity at P2Y(2) receptors. Screening at dopamine receptors revealed LE300, a new lead for a dopamine receptor antagonist. Advantages of the assay include fast and simple 96- or 384-well plate format (high-throughput screening), use of a visible light-excitable fluorescent dye, applicability to a majority of GPCRs, and simultaneous analysis of distinct Ca(2+) fluxes.

Regulation and mechanism of L-type calcium channel activation via V1a vasopressin receptor activation in cultured cortical neurons

We have sought to elucidate the biochemical mechanisms that underlie the memory enhancing properties of the neural peptide vasopressin. Toward that goal we have investigated vasopressin induction of calcium signaling cascades, long held to be involved in long-term memory function, in neurons derived from the cerebral cortex, a brain region associated with long-term memory. Our previous studies demonstrated that in cultured cortical neurons, V1a vasopressin receptor (V1aR) activation resulted in a sustained rise in intracellular calcium concentration that was dependent on calcium influx (Son & Brinton, 1998). To investigate the mechanism of V1aR-induced calcium influx, we investigated V1aR activation of the calcium channel subtype(s) in cortical neurons cultured from Sprague-Dawley rat embryonic day 18 fetuses. The results of these analyses demonstrated that the L-type calcium channel blocker nifedipine blocked 250 nM V1 vasopressin receptor agonist (V1 agonist)-induced calcium influx. Intracellular calcium imaging analyses using fura-2AM demonstrated that blockade of L-type calcium channels prevented the 250 nM V1 agonist-induced rise in intracellular calcium concentration. These results indicate that the influx of extracellular calcium via L-type calcium channels is an essential step in the initiation of the V1 agonist-induced rise in intracellular calcium concentration. To determine the mechanism of V1aR activation of L-type calcium channels, regulatory components of the phosphatidylinositol signaling pathway were investigated. The results of these analyses demonstrated that V1 agonist-induced calcium influx was blocked by both a phospholipase C inhibitor (U-73122) and a protein kinase C inhibitor (bisindolylmaleimide I). Further analysis of V1aR activation of protein kinase C (PKC) demonstrated that V1 agonist induced PKC activity within 1 min of exposure in cultured cortical neurons. These data indicate that in cultured cortical neurons, V1aR activation regulates the influx of extracellular calcium via L-type calcium channel activation through a protein kinase-C-dependent mechanism. The results of these studies provide biochemical mechanisms by which vasopressin could enhance memory function. Those mechanisms include a complex cascade that is initiated by activation of the phosphatidylinositol pathway, activation of protein kinase C, followed by phosphorylation of L-type calcium channels to initiate the influx of extracellular calcium to activate a cascade of calcium-dependent release of intracellular calcium.

Presynaptic specificity of endocannabinoid signaling in the hippocampus

Endocannabinoids are retrograde messengers released by neurons to modulate the strength of their synaptic inputs. Endocannabinoids are thought to mediate the suppression of GABA release that follows depolarization of a hippocampal CA1 pyramidal neuron-termed "depolarization-induced suppression of inhibition" (DSI). Here, we report that DSI is absent in mice which lack cannabinoid receptor-1 (CB1). Pharmacological and kinetic evidence suggests that CB1 activation inhibits presynaptic Ca2+ channels through direct G protein inhibition. Paired recordings show that endocannabinoids selectively inhibit a subclass of synapses distinguished by their fast kinetics and large unitary conductance. Furthermore, cannabinoid-sensitive inputs are unusual among central nervous system synapses in that they use N- but not P/Q-type Ca2+ channels for neurotransmitter release. These results indicate that endocannabinoids are highly selective, rapid modulators of hippocampal inhibition.

Structure and regulation of voltage-gated Ca2+ channels

Voltage-gated Ca(2+) channels mediate Ca(2+) entry into cells in response to membrane depolarization. Electrophysiological studies reveal different Ca(2+) currents designated L-, N-, P-, Q-, R-, and T-type. The high-voltage-activated Ca(2+) channels that have been characterized biochemically are complexes of a pore-forming alpha1 subunit of approximately 190-250 kDa; a transmembrane, disulfide-linked complex of alpha2 and delta subunits; an intracellular beta subunit; and in some cases a transmembrane gamma subunit. Ten alpha1 subunits, four alpha2delta complexes, four beta subunits, and two gamma subunits are known. The Cav1 family of alpha1 subunits conduct L-type Ca(2+) currents, which initiate muscle contraction, endocrine secretion, and gene transcription, and are regulated primarily by second messenger-activated protein phosphorylation pathways. The Cav2 family of alpha1 subunits conduct N-type, P/Q-type, and R-type Ca(2+) currents, which initiate rapid synaptic transmission and are regulated primarily by direct interaction with G proteins and SNARE proteins and secondarily by protein phosphorylation. The Cav3 family of alpha1 subunits conduct T-type Ca(2+) currents, which are activated and inactivated more rapidly and at more negative membrane potentials than other Ca(2+) current types. The distinct structures and patterns of regulation of these three families of Ca(2+) channels provide a flexible array of Ca(2+) entry pathways in response to changes in membrane potential and a range of possibilities for regulation of Ca(2+) entry by second messenger pathways and interacting proteins.

Angiotensin II and calcium channels

Sixty years after its initial discovery, the octapeptide hormone angiotensin II (AngII) has proved to play numerous physiological roles that reach far beyond its initial description as a hypertensive factor. In spite of the host of target tissues that have been identified, only two major receptor subtypes, AT1 and AT2, are currently fully identified. The specificity of the effects of AngII relies upon numerous and complex intracellular signaling pathways that often mobilize calcium ions from intracellular stores or from the extracellular medium. Various types of calcium channels (store- or voltage-operated channels) endowed with distinct functional properties play a crucial role in these processes. The activity of these channels can be modulated by AngII in a positive and/or negative fashion, depending on the cell type under observation. This chapter reviews the main characteristics of AngII receptor subtypes and of the various calcium channels as well as the involvement of the multiple signal transduction mechanisms triggered by the hormone in the cell-specific modulation of the activity of these channels.

Functional coupling of G proteins to endothelin receptors is ligand and receptor subtype specific

1. The aims of the present study were (a) to determine the identity of the G proteins with which the endothelin receptor interacts and whether this interaction is subtype specific and (b) to determine whether agonist exposure can result in specific coupling between the endothelin receptor and G proteins. 2. Coupling between endothelin A (ET(A)) or endothelin B (ET(B)) receptors and G proteins was assessed in two fibroblast cell lines, each expressing one receptor subtype. Four ligands, ET-1, ET-3, SRTXb, and SRTXc, were used for receptor stimulation. The G protein alpha-subunit coupled to the receptor was identified by immunoprecipitation with an antibody against the endothelin receptor and immunoblotting with specific antibodies against different G protein alpha-subunits. 3. Unstimulated ET(A) and ET(B) receptors (ET(A)R and ET(B)R, respectively) were barely coupled to Go(alpha). The unstimulated ET(A)R coimmunoprecipitated with Gi3alpha, whereas the unstimulated ETBR was much less strongly coupled to Gi3alpha. The coupling of ETBR to Gi1Gi2 alpha-subunits was much stronger than the coupling of ET(A)R to these alpha-subunits. Stimulation with the different ET agonists also resulted in differential coupling of G proteins to the receptor subtypes. All four ligands caused a strong increase in coupling of the ET(B)R to Gi3alpha, whereas coupling of the ET(A)R to this subunit was not affected by ET-1 and was even decreased by SRTXc. On the other hand, all four ligands caused a much greater increase in the coupling of ET(A)R to G(q)alpha/G11alpha than in the coupling of ET(B)R to these alpha-subunits. Ligand-induced coupling between the receptors and the Gi1 and Gi2 alpha-subunits is similar for the two receptor subtypes. The same was true for ligand-induced coupling of the receptors to Go(alpha), except that ET-3 increased the coupling of this alpha-subunit to ET(B)R and decreased the coupling to ET(A)R. Taken together, the results of this study show that coupling between ET receptors and G proteins is ligand and receptor subtype specific. 4. It remains to be established whether this diversity of receptor-G protein coupling is of relevance for the various endothelin signaling pathways and/or pathological states.

Cloned delta-opioid receptors in GH(3) cells inhibit spontaneous Ca(2+) oscillations and prolactin release through K(IR) channel activation

Opioid receptors can couple to K(+) and Ca(2+) channels, adenylyl cyclase, and phosphatidyl inositol turnover. Any of these actions may be important in the regulation of neurotransmitter and hormone release from excitable cells. GH(3) cells exhibit spontaneous oscillations of intracellular Ca(2+) concentration ([Ca(2+)](i)) and prolactin release. Activation of cloned delta-opioid receptors stably expressed in GH(3) cells inhibits both spontaneous Ca(2+) signaling and basal prolactin release. The objective of this study was to examine a possible role for K(+) channels in these processes using the patch-clamp technique, fluorescence imaging, and a sensitive ELISA for prolactin. The selective delta receptor agonist [D-Pen(2), D-Pen(2)]enkephalin (DPDPE) inhibited [Ca(2+)](i) oscillations in GH(3) cells expressing both mu and delta receptors (GH(3)MORDOR cells) but had no effect on control GH(3) cells or cells expressing mu receptors alone (GH(3)MOR cells). The inhibition of [Ca(2+)](i) oscillations by DPDPE was unaffected by thapsigargin pretreatment, suggesting that this effect is independent of inositol 1,4,5-triphosphate-sensitive Ca(2+) stores. DPDPE caused a concentration-dependent inhibition of prolactin release from GH(3)MORDOR cells with an IC(50) of 4 nM. DPDPE increased inward K(+) current recorded from GH(3)MORDOR cells but had no significant effect on K(+) currents recorded from control GH(3) cells or GH(3)MOR cells. The mu receptor agonist morphine also had no effect on currents recorded from control cells but activated inward K(+) currents recorded from GH(3)MOR and GH(3)MORDOR cells. Somatostatin activated inward currents recorded from all three cell lines. The DPDPE-sensitive K(+) current was inwardly rectifying and was inhibited by Ba(2+) but not TEA. DPDPE had no effect on delayed rectifier-, Ca(2+)-, and voltage-activated or A-type K(+) currents, recorded from GH(3)MORDOR cells. Ba(2+) attenuated the inhibition of [Ca(2+)](i) and prolactin release by DPDPE, whereas TEA had no effect, consistent with an involvement of K(IR) channels in these actions of the opioid.

Dopamine D2 receptor isoforms expressed in AtT20 cells differentially couple to G proteins to acutely inhibit high voltage-activated calcium channels

The dopamine D2 receptor belongs to the serpentine superfamily of receptors, which have seven transmembrane segments and activate G proteins. D2 receptors are known to be linked, through Galpha(o)- and Galpha(i)-containing G proteins, to several signaling pathways in neuronal and secretory cells, including inhibition of adenylyl cyclase and high voltage-activated Ca2+ channels (HVA-CCs). The dopamine D2 receptor exists in two alternatively spliced isoforms, "long" and "short" (D2L, and D2S, respectively), which have identical ligand binding sites but differ by 29 amino acids in the third intracellular loop, the proposed site for G protein interaction. This has led to the speculation that the two isoforms may interact with different G proteins. We have transfected the AtT20 cell line with either D2L (KCL line) or D2S (KCS line) to facilitate experimentation on the individual isoforms. Both lines show dopamine agonist-dependent inhibition of Q-type HVA-CCs. We combined G protein antisense knock-down studies with multiwavelength fluorescence video microscopy to measure changes in HVA-CC inhibition to investigate the possibility of differential G protein coupling to this inhibition. The initial, rapid, K+ depolarization-induced increase in intracellular Ca2+ concentration is due to influx through HVA-CCs. Our studies reveal that both D2 isoforms couple to Galpha(o) to partially inhibit this influx. However, D2L also couples to Galpha(i)3, whereas D2S couples to Galpha(i)2. These data support the hypothesis of differential coupling of D2 receptor isoforms to G proteins.

Angiotensin II negatively modulates L-type calcium channels through a pertussis toxin-sensitive G protein in adrenal glomerulosa cells

In bovine adrenal glomerulosa cells, angiotensin II and extracellular K+ stimulate aldosterone secretion in a calcium-dependent manner. In these cells, physiological concentrations of extracellular potassium activate both T-type (low threshold) and L-type (high threshold) voltage-operated calcium channels. Paradoxically, the cytosolic calcium response to 9 mM K+ is inhibited by angiotensin II. Because K+-induced calcium changes observed in the cytosol are almost exclusively due to L-type channel activity, we therefore studied the mechanisms of L-type channel regulation by angiotensin II. Using the patch-clamp method in its perforated patch configuration, we observed a marked inhibition (by 63%) of L-type barium currents in response to angiotensin II. This effect of the hormone was completely prevented by losartan, a specific antagonist of the AT1 receptor subtype. Moreover, this inhibition was strongly reduced when the cells were previously treated for 1 night with pertussis toxin. An effect of pertussis toxin was also observed on the modulation by angiotensin II of the K+ (9 mM)-induced cytosolic calcium response in fura-2-loaded cells, as well as on the angiotensin II-induced aldosterone secretion, at both low (3 mM) and high (9 mM) K+ concentrations. Finally, the expression of both Go and Gi proteins in bovine glomerulosa cells was detected by immunoblotting. Altogether, these results strongly suggest that in bovine glomerulosa cells, a pertussis toxin-sensitive G protein is involved in the inhibition of L-type channel activity induced by angiotensin II.

Angiotensin II type 1 receptor activation modulates L- and T-type calcium channel activity through distinct mechanisms in bovine adrenal glomerulosa cells

In adrenal zona glomerulosa cells, calcium entry is crucial for aldosterone production and secretion. This influx is stimulated by increases of extracellular potassium in the physiological range of concentrations and by angiotensin II (Ang II). The high threshold voltage-activated (L-type) calcium channels have been shown to be the major mediators for the rise in cytosolic free calcium concentration, [Ca2+]c, observed in response to a depolarisation by physiological potassium concentrations. Paradoxically, both T- and L-type calcium channels have been shown to be negatively modulated by Ang II after activation by a sustained depolarisation. While the modulation of T-type channels involves protein kinase C (PKC) activation, L-type channel inhibition requires a pertussis toxin-sensitive G protein. In order to investigate the possibility of additional modulatory mechanisms elicited by Ang II on L-type channels, we have studied the effect of PKC activation or tyrosine kinase inhibition. Neither genistein or MDHC, two strong inhibitors of tyrosine kinases, nor the phorbol ester PMA, a specific activator of PKC, affected the Ang II effect on the [Ca2+]c response and on the Ba2+ currents elicited by cell depolarisation with the patch-clamp method. We propose a model describing the mechanisms of the [Ca2+]c modulation by Ang II and potassium in bovine adrenal glomerulosa cells.

Oxidation regulates cloned neuronal voltage-dependent Ca2+ channels expressed in Xenopus oocytes

Functional modifications of neuronal P/Q-type voltage-dependent Ca2+ channels expressed in Xenopus oocytes by oxidation were examined electrophysiologically. Oxidation by external H2O2 enhanced the whole-oocyte currents through the Ca2+ channels composed of the alpha1A, alpha2/delta, and beta3 subunits at negative voltages (<0 mV) without markedly affecting the currents at more positive voltages. Single-channel analysis showed that oxidation accelerates the overall channel opening process. The effect of H2O2 to enhance the Ca2+ channel activity did not require heterologous expression of the alpha2/delta subunit, and it was not mimicked by a cysteine-specific oxidizing agent. The results suggest that oxidative stress may regulate the activity of neuronal Ca2+ channels and that regulation by oxidation may be important in some clinical situations, such as in reperfusion injury after ischemic episodes.

The mitogen-activated protein kinase p38-2 is necessary for the inhibition of N-type calcium current by bradykinin

Calcium influx via voltage-dependent calcium channels (ICa,V) links depolarization of excitable cells to critical cellular processes, such as secretion, contraction, and gene transcription. Fast regulation of ICa,V (<1 sec) by G-protein-coupled receptors is a relatively well-defined mechanism, whereas slow (30-60 sec) actions of transmitters and hormones on the same current remain poorly understood. In NG108-15 cells, the kinetically slow inhibition of N-type ICa,V by bradykinin (BK) requires the sequential activation of two G-proteins, heterotrimeric G13 and monomeric Rac1/Cdc42. We have now defined a role in this pathway for the relatively fast-acting p38 mitogen-activated protein kinase (MAPK). The slow inhibition of ICa,V by BK was suppressed specifically by SB203580, a compound that inhibits the p38 family of MAPKs. BK potently and selectively activated a newly discovered p38 family member, p38-2. These data provide the first evidence that a MAPK is involved in the regulation of ICa,V by a receptor-mediated process.

Involvement of Ca2+ signalling in the vasoactive intestinal peptide and 8-Br-cAMP induction of c-fos mRNA expression

Vasoactive intestinal peptide (VIP) is known to signal via Gs mediated pathways. VIP stimulated c-fos mRNA expression in a clonal GH3 pituitary tumour cell line, GH3Ca, whereas 8-Br-cAMP only moderately induced c-fos expression. The VIP-induced c-fos expression was inhibited in the presence of EGTA, or the L-type Ca2+ channel blockers verapamil and nifedipine. Measurement of intracellular Ca2+ concentration ([Ca2+]i) by Fura-2 indicates that VIP gradually elevates [Ca2+]i, with the maximum level attained at 4 min following hormone addition. No [Ca2+]i increase could be detected in Ca2+ free buffer or in buffer containing nifedipine or verapamil, which suggests that VIP induced Ca2+ entry from L-type Ca2+ channels. 8-Br-cAMP rapidly increased [Ca2+]i, with a maximum concentration attained within 1 min of its addition and the elevated level maintained for 15 min. In the absence of external Ca2+ or in the presence of verapamil or nifedipine, the sustained Ca2+ increase was abolished whereas the transient Ca2+ peak was unaffected. Depletion of the internal calcium pools by thapsigargin (1 microM, 30 min), on the other hand, blocked the rapid transient [Ca2+], rise, suggesting the biphasic [Ca2+]i elicited by 8-Br-cAMP was due to mobilization from internal Ca2+ pool followed by extracellular flow. Interestingly, pretreatment with thapsigargin greatly potentiated the 8-Br-cAMP-stimulated c-fos expression. Pretreatment of cells with cholera toxin (1 microg/ml, 9 h) to deplete Gs proteins abolished VIP stimulated-[Ca2+] elevation, while it had little effect on the 8-Br-cAMP induced [Ca2+]i rise. Our results show that VIP increased Ca2+ influx from L-type channel through a Gs-mediated mechanism and this Ca2+ entry across the plasma membrane plays a major role in the hormone induced c-fos mRNA expression.

The monomeric G-proteins Rac1 and/or Cdc42 are required for the inhibition of voltage-dependent calcium current by bradykinin

Although regulation of voltage-dependent calcium current (ICa,V) by neurotransmitters is a ubiquitous mechanism among nerve cells, the signaling pathways involved are not well understood. We have determined previously that in a neuroblastoma-glioma hybrid cell line (NG108-15), the heterotrimeric G-protein G13 mediates the inhibition of ICa,V produced by bradykinin (BK) via an unknown mechanism. Various reports indicate that G13 can couple to RhoA, Rac1, and Cdc42, which are closely related members of the Rho family of monomeric G-proteins. We have investigated their role as signaling intermediates in the pathway used by BK to inhibit ICa,V. Using immunoblot analysis and the PCR, we found evidence that RhoA, Rac1, and Cdc42 all are expressed in NG108-15 cells. Intracellularly perfused recombinant Rho-GDI (an inhibitor of guanine nucleotide exchange specific for the Rho family) attenuated the inhibition of ICa,V by BK. These findings indicate that activation of RhoA, Rac1, or Cdc42 may be required for the response to BK. To determine whether any of these monomeric G-proteins mediate the response to BK, we have intracellularly applied blocking antibodies specific for each of the candidate proteins. Only the anti-Rac1 antibody blocked the response to BK. In parallel experiments, peptides corresponding to the C-terminal regions of Rac1 and Cdc42 blocked the same response. These data indicate a novel functional contribution of Rac1 and possibly also of Cdc42 to the inhibition of ICa,V by neurotransmitters.

Receptor-mediated modulation of recombinant neuronal class E calcium channels

The modulation of a cloned neuronal calcium channel was studied in a human embryonic kidney cell line (HEK293). The HEK293 cells were stably transfected with the alpha1Ed cDNA, containing the pore forming subunit of a neuronal class E calcium channel. Inward currents of 25 +/- 1.9 pA/pF (n = 79) were measured with the cloned alpha1Ed-subunit. The application of the peptide hormone somatostatin, carbachol, ATP or adenosine reduced the amplitude of Ca2+ and Ba2+ inward currents and exhibited a slowing of inactivation. This inhibitory effect by somatostatin was significantly impaired after pre-incubating the transfected cell line with pertussis toxin (PTX). Internal perfusion of the cells with the G-protein-inactivating agent GDP-beta-S or with the permanently activating agent GTP-gamma-S also attenuated the somatostatin effect. The inhibition indicates that modulation of the alpha1Ed-mediated Ca2+ current involves pertussis toxin-sensitive G-proteins. The block of Ca2+ and Ba2+ inward currents by somatostatin is also found in cells expressing a truncated alpha1Ed-subunit which lacks a 129-bp fragment in the C-terminus. This fragment corresponds to the major structural difference between two native human alpha1E splice variants. As somatostatin inhibits inward currents through both, the cloned alpha1Ed- and the truncated alpha1Ed-DEL-subunit, the hormone-mediated modulation is independent from the presence of the 129-bp insertion in the C-terminus.

Development of G protein-mediated Ca2+ channel regulation in mouse embryonic stem cell-derived neurons

Besides other mechanisms, the influx of Ca2+ into embryonic neurons controls growth and differentiation processes. To study the expression and regulation of voltage-gated Ca2+ channels during early neurogenesis, we measured whole-cell Ca2+ currents (I(Ca)) in neurons developing from pluripotent embryonic stem cells. Various receptor agonists, including somatostatin and baclofen, reversibly inhibited I(Ca) in embryonic stem cell-derived neurons. The effects of somatostatin and baclofen were abolished by pretreatment of cells with pertussis toxin and mimicked by intracellular infusion of guanosine 5'-O-(3-thiotriphosphate), suggesting the involvement of pertussis toxin-sensitive G proteins in I(Ca) inhibition. Investigations at different stages of neuronal differentiation showed that somatostatin efficiently suppressed L- and N-type Ca2+ channels in immature as well as mature neurons. In contrast, inhibition of L- and N-type channels by baclofen was rarely observed at the early stage. In terminally differentiated neurons, responses to baclofen were as prominent as those to somatostatin but were confined to N-type Ca2+ channels. The stage-dependent sensitivity of voltage-gated Ca2+ channels to somatostatin and baclofen was not due to differential expression of G alpha(o) isoforms, as revealed by reverse transcription-polymerase chain reaction and immunofluorescence microscopy. These findings demonstrate that specific neurotransmitters such as somatostatin regulate voltage-gated Ca2+ channels via G proteins during the early stages of neurogenesis, thus providing a mechanism for the epigenetic control of neuronal differentiation.

D2 dopamine receptors reduce N-type Ca2+ currents in rat neostriatal cholinergic interneurons through a membrane-delimited, protein-kinase-C-insensitive pathway

Dopamine has long been known to regulate the activity of striatal cholinergic interneurons and the release of acetylcholine. Yet, the cellular mechanisms by which this regulation occurs have not been elucidated. One way in which dopamine might act is by modulating voltage-dependent Ca2+ channels. To test this hypothesis, the impact of dopaminergic agonists on Ca2+ channels in neostriatal cholinergic interneurons was studied by combined whole cell voltage-clamp recording and single-cell reverse transcription-polymerase chain reactions. Cholinergic interneurons were identified by the presence of choline acetyltransferase mRNA. Nearly, all interneurons tested (90%, n = 17) coexpressed D2 (short and long isoforms) and D1b (D5) dopamine receptor mRNAs. D1a receptor mRNA was found in only a small subset (20%) of the sample and D3 and D4 receptor mRNAs were undetectable. D2 receptor agonists rapidly and reversibly reduced N-type Ca2+ currents. D1b/D1a receptor activation had little or no effect on Ca2+ currents. The D2 receptor antagonist sulpiride blocked the effect of D2 agonists. Dialysis with guanosine-5'-O-(2-thiodiphosphate) or brief exposure to the G protein (Gi/o) alkylating agent N-ethylmaleimide also blocked the D2 modulation. The reduction in N-type currents was neither accompanied by kinetic slowing nor significantly reversed by depolarizing prepulses. The D2 receptor effects were mediated by a membrane-delimited pathway, because the modulation was not seen in cell-attached patches when agonist was applied to the bath and was not disrupted by perturbations in cytosolic signaling pathways known to be linked to D2 receptors. Activation of M2 muscarinic receptors occluded the D2 modulation, suggesting a shared signaling element. However, activation of protein kinase C attenuated the M2 modulation without significantly affecting the D2 modulation. Taken together, our results suggest that activation of D2 dopamine receptors in cholinergic interneurons reduces N-type Ca2+ currents via a membrane-delimited, Gi/o class G protein pathway that is not regulated by protein kinase C. This signaling pathway may underlie the ability of D2 receptors to reduce striatal acetylcholine release.

Alzheimer's PS-1 mutation perturbs calcium homeostasis and sensitizes PC12 cells to death induced by amyloid beta-peptide

Mutations in the presenilin-1 (PS-1) gene on chromosome 14 are linked to autosomal dominant early-onset Alzheimer's disease. The amino acid sequence of PS-1 predicts an integral membrane protein and immunocytochemical studies indicate that PS-1 is localized to endoplasmic reticulum (ER). We report that expression of PS-1 mutation L286V in cultured PC12 cells exaggerates Ca2+ responses to agonists (carbachol and bradykinin) that induce Ca2+ release from ER. Cells expressing L286V exhibit enhanced elevations of [Ca2+]i following exposure to amyloid beta-peptide (A beta) and increased vulnerability to A beta toxicity. An antagonist of voltage-dependent calcium channels (nifedipine), and a blocker of Ca2+ release from ER (dantrolene), counteract the adverse consequences of the PS-1 mutation. By perturbing Ca2+ homeostasis, PS-1 mutations may sensitize neurons to A beta-induced apoptosis.

Functional analysis of cloned opioid receptors in transfected cell lines

Opioids modulate numerous central and peripheral processes including pain perception neuroendocrine secretion and the immune response. The opioid signal is transduced from receptors through G proteins to various different effectors. Heterogeneity exists at all levels of the transduction process. There are numerous endogenous ligands with differing selectivities for at least three distinct opioid receptors (mu, delta, kappa). G proteins activated by opioid receptors are generally of the pertussis toxin-sensitive Gi/Go class, but there are also opioid actions that are thought to involve Gq and cholera toxin-sensitive G proteins. To further complicate the issue, the actions of opioid receptors may be mediated by G-protein alpha subunits and/or beta gamma subunits. Subsequent to G protein activation several effectors are known to orchestrate the opioid signal. For example activation of opioid receptors increases phosphatidyl inositol turnover, activates K+ channels and reduces adenylyl cyclase and Ca2+ channel activities. Each of these effectors shows considerable heterogeneity. In this review we examine the opioid signal transduction mechanism. Several important questions arise: Why do opioid ligands with similar binding affinities have different potencies in functional assays? To which Ca2+ channel subtypes do opioid receptors couple? Do opioid receptors couple to Ca2+ channels through direct G protein interactions? Does the opioid-induced inhibition of vesicular release occur through modulation of multiple effectors? We are attempting to answer these questions by expressing cloned opioid receptors in GH3 cells. Using this well characterized system we can study the entire opioid signal transduction process from ligand-receptor interaction to G protein-effector coupling and subsequent inhibition of vesicular release.

Involvement of a phorbol ester-insensitive protein kinase C in the alpha2-adrenergic inhibition of voltage-gated calcium current in chick sympathetic neurons

alpha2-Adrenoceptors regulate the efficacy at the sympathoeffector junction by means of a feedback inhibition of transmitter release. In chick sympathetic neurons, the mechanism involves an inhibition of N-type calcium channels, and we now present evidence that this effect involves an atypical, phorbol ester-insensitive protein kinase C (PKC). The inhibition of voltage-gated Ca2+ currents by the specific alpha2-adrenergic agonist UK 14,304 was significantly attenuated when the PKC inhibitors PKC(19-36), staurosporine, or calphostin C were included in the internal solution used to fill the patch pipettes, or if staurosporine or calphostin C were applied extracellularly; however, phorbol esters as classical activators of PKC or oleoylacetylglycerol did not mimic the effect of UK 14,304, and chronic exposure to 4-beta-phorbol dibutyrate (PDBu) did not attenuate it, ever though PKCalpha and -epsilon isozymes were translocated to plasma membranes by PDBu. The atypical isozyme PKCzeta was translocated by 100 micrometer AA and this effect was attenuated when PKC(19-36) was added to the patch pipette solution. Our observations indicate that classical, new, and atypical PKC isozymes are present in chick sympathetic neurons and that an atypical, phorbol ester-insensitive PKC is involved in the inhibition of voltage-activated calcium currents by alpha2-adrenoceptor activation.

Modulation of Ca2+ channels by G-protein beta gamma subunits

Calcium ions entering cells through voltage-gated Ca2+ channels initiate rapid release of neurotransmitters and secretion of hormones. Ca2+ currents can be inhibited in many cell types by neurotransmitters acting through G proteins via a membrane-delimited pathway independently of soluble intracellular messengers. Inhibition is typically caused by a positive shift in the voltage dependence and a slowing of channel activation and is relieved by strong depolarization resulting in facilitation of Ca2+ currents. This pathway regulates the activity of N-type and P/Q-type Ca2+ channels, which are localized in presynaptic terminals and participate in neurotransmitter release. Synaptic transmission is inhibited by neurotransmitters through this mechanism. G-protein alpha subunits confer specificity in receptor coupling, but it is not known whether the G alpha or G beta gamma subunits are responsible for modulation of Ca2+ channels. Here we report that G beta gamma subunits can modulate Ca2+ channels. Transfection of G beta gamma into cells expressing P/Q-type Ca2+ channels induces modulation like that caused by activation of G protein-coupled receptors, but G alpha subunits do not. Similarly, injection or expression of G beta gamma subunits in sympathetic ganglion neurons induces facilitation and occludes modulation of N-type channels by noradrenaline, but G alpha subunits do not. In both cases, the G gamma subunit is ineffective by itself, but overexpression of exogenous G beta subunits is sufficient to cause channel modulation.

Stimulation of growth factor receptor signal transduction by activation of voltage-sensitive calcium channels

To understand the mechanisms by which electrical activity may generate long-term responses in the nervous system, we examined how activation of voltage-sensitive calcium channels (VSCCs) can stimulate the Ras/mitogen-activated protein kinase (MAPK) signaling pathway. Calcium influx through L-type VSCCs leads to tyrosine phosphorylation of the adaptor protein Shc and its association with the adaptor protein Grb2, which is bound to the guanine nucleotide exchange factor Sos1. In response to calcium influx, Shc, Grb2, and Sos1 inducibly associate with a 180-kDa tyrosine-phosphorylated protein, which was determined to be the epidermal growth factor receptor (EGFR). Calcium influx induces tyrosine phosphorylation of the EGFR to levels that can activate the MAPK signaling pathway. Thus, ion channel activation stimulates growth factor receptor signal transduction.

Synergistic activation of cAMP and calcium on cAMP-response-element-mediated gene expression in GH3 pituitary tumor cells

Signals responsible for expression of the vasoactive intestinal peptide (VIP)-stimulated prolactin gene in GH3 pituitary tumor cells were examined. Transfection with a deoxyribonucleic acid (DNA) construct containing the chloramphenicol acetyltransferase (CAT) gene fused to the 2.5-kb prolactin 5'-upstream regulatory sequence indicated that VIP stimulated CAT expression. However, this effect could not be mimicked by 8-bromo-cyclic adenosine monophosphate (8-Br-cAMP), and was inhibited by the L-type Ca(2+)-channel blocker verapamil. While KCl had little effect on CAT activity, combined treatment with KCl and 8-Br-cAMP synergistically activated CAT expression. Potentiation between KCl and 8-Br-c-AMP was also seen with c-fos messenger ribonucleic acid (mRNA) expression. In addition, KCl and 8-Br-cAMP synergistically activated cAMP response element (CRE)-mediated CAT expression, and the synergism was abolished by verapamil. In the presence of okadaic acid, cAMP had no significant activation on CRE-driven CAT expression, whereas KCl-stimulated CAT expression was greatly potentiated. These results indicate that cAMP and Ca2+ synergistically activated CRE-driven gene expression through non-overlapping phosphorylation events in GH3 cells.

The human thyrotropin receptor: a heptahelical receptor capable of stimulating members of all four G protein families

Thyrotropin is the primary hormone that, via one heptahelical receptor, regulates thyroid cell functions such as secretion, specific gene expression, and growth. In human thyroid, thyrotropin receptor activation leads to stimulation of the adenylyl cyclase and phospholipase C cascades. However, the G proteins involved in thyrotropin receptor action have been only partially defined. In membranes of human thyroid gland, we immunologically identified alpha subunits of the G proteins Gs short, Gs long, Gi1, Gi2, Gi3, G(o) (Go2 and another form of Go, presumably Go1), Gq, G11, G12, and G13. Activation of the thyrotropin (TSH) receptor by bovine TSH led to increased incorporation of the photoreactive GTP analogue [alpha-32P]GTP azidoanilide into immunoprecipitated alpha subunits of all G proteins detected in thyroid membranes. This effect was receptor-dependent and not due to direct G protein stimulation because it was mimicked by TSH receptor-stimulating antibodies of patients suffering from Grave disease and was abolished by a receptor-blocking antiserum from a patient with autoimmune hypothyroidism. The TSH-induced activation of individual G proteins occurred with EC50 values of 5-50 milliunits/ml, indicating that the activated TSH receptor coupled with similar potency to different G proteins. When human thyroid slices were pretreated with pertussis toxin, the TSH receptor-mediated accumulation of cAMP increased by approximately 35% with TSH at 1 milliunits/ml, indicating that the TSH receptor coupled to Gs and G(i). Taken together, these findings show that, at least in human thyroid membranes, in which the protein is expressed at its physiological levels, the TSH receptor resembles a naturally occurring example of a general G protein-activating receptor.

Effects of fluoride and cholera and pertussis toxins on sensory transduction in the carotid body

The regulation of the chemoreceptor cell function by G proteins has been studied by measuring the release of 3H-labeled catecholamines ([3H]CA) in carotid bodies (CBs) treated with fluoride, cholera toxin (CTX), and pertussis toxin (PTX). Fluoride augmented the basal release of [3H]CA in a dose- (5-20 mM) and Ca(2+)-dependent manner. Nisoldipine (1 microM) and ethylisopropyl amiloride (EIPA; 10 microM) inhibited this effect by approximately 60%, and both drugs combined inhibited it in full. BAY K 8644 (1 microM) doubled the effect of fluoride. The effects of fluoride on the stimulus-evoked release of [3H]CA varied with the type of stimulus and the duration of the treatment. Simultaneous application of fluoride with the stimulus increased by five times the release evoked by hypoxia and by two times that by K+ and dinitrophenol (DNP). Preincubation with fluoride for 1 h caused an inhibition (approximately 70%) of the release evoked by high K+ and veratridine, whereas that evoked by DNP and low PO2 was still augmented (approximately 2 times). Preincubation (4 h) of the CBs with CTX (3 micrograms/ml) reduced by 54% the release of [3H]CA evoked by 35 mM K+ but did not affect that evoked by low PO2 or DNP. A similar treatment with PTX (1 microgram/ml) affected only the release of [3H]CA evoked by DNP, reducing it by 65%. The data show that fluoride, CTX, and PTX have different effects on the release of [3H]CA evoked by high external K+, DNP, and low PO2, indicating that the stimulus-secretion coupling process for each stimulus is differently regulated by G proteins.

ATP and G proteins affect the runup of the Ca2+ current in bovine chromaffin cells

The Ca2+ current recorded by the whole-cell technique in chromaffin cells shows, before the often described rundown, a transient facilitation or runup. Initial current amplitude was 570 +/- 165 pA and then it increased by 49 +/- 23% (n = 19, SD) over 2 +/- 1 min in the absence of adenosine 5'-triphosphate (ATP). In the presence of ATP, this process occurred with the same magnitude but it was slowed in a dose-dependent manner, lasting 17 +/- 2 min with 2 mM ATP (n = 8). Since adenosine 5'-diphosphate (ADP) does not reproduce this ATP effect, a complex series of phosphorylations is likely to intervene and we show that, at least, a cAMP-dependent i.e., cyclic adenosine monophosphate) phosphorylation occurs. Pertussis toxin (PTX) pretreatment yielded an already maximal Ca2+ current (around 1000 pA) at the time of the patch rupture, which only slightly increased thereafter (10%, n = 11). Also, guanosine 5'-diphosphate (GDP) and guanosine 5'-O-(2-thiodiphosphate) (GDP[ beta s]), induced a fast runup, which was absent in the presence of GTP. Furthermore, we show that facilitation does not occur in the presence of dihydrophyridine (DHP) antagonists. Globally, our data suggest that an ATP-dependent phosphorylation stabilizes the inhibitory control exerted by a PTX-sensitive G protein and, as a result, slows down the facilitation of L-type Ca2+ channels. The recruitment of L-type channels can also be facilitated by the application of a DHP agonist or a depolarizing prepulse protocol.l We show that these processes are only effective over a period which parallels the runup and are not additive to it.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhancement by muscarinic agonists of a high voltage-activated Ca2+ current via phosphorylation in a snail neuron

1. In previous work we have shown that in the snail Helix aspersa neuron F1 carbamylcholine (CCh) and other muscarinic agonists enhance the inward current carried through high voltage-activated Ca2+ channels by Ba2+ (HVA-ICa). It was also found that cyclic nucleotides, inositol trisphosphate or arachidonic acid are not involved in this modulation. Moreover, despite the effect of CCh being blocked by intracellular injection of EGTA, neither protein kinase C nor Ca(2+)-calmodulin-dependent protein kinase II appeared to play a role. 2. In the present paper, the intracellular mechanism of this muscarinic modulation was investigated further by studying the effects of inhibitors of Ser-Thr protein phosphatases (PP) on both the HVA-ICa of neuron F1 and its enhancement by CCh. 3. Intracellular injections in the F1 neuron of either microcystin LR or okadaic acid, both inhibitors of PP1 and PP2A, mimic the action of CCh on the HVA-ICa and occlude the effects of CCh on this current. In contrast, cyclosporin A, an inhibitor of PP2B (calcineurin), affects neither the HVA Ca2+ current itself nor its modulation by CCh. 4. The efficacy of PP inhibitors was tested in F1 neurons in which serotonin (5-HT) induces an inward current involving intracellular increases in cAMP and a protein kinase A-dependent closing of K+ channels. We found that intracellular injection of either microcystin LR or okadaic acid mimicked the 5-HT-induced inward current and occluded the effect of further application of 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)

The S-100: a protein family in search of a function

The S-100 is a group of low molecular weight (10-12 kD) calcium-binding proteins highly conserved among vertebrates. It is present in different tissues as dimers of homologous or different subunits (alpha, beta). In the nervous system, the S-100 exists as a mixture composed of beta beta and alpha beta dimers with the monomer beta represented more often. Its intracellular localisation is mainly restricted to the glial cytoplasmic compartment with a small fraction bound to membranes. In this compartment the S-100 acts as a potent inhibitor of phosphorylation on several substrates including the synaptosomal C-Kinase and Tau, a microtubule-associated protein. The S-100 in particular conditions, after binding with specific membrane sites (Kd = 0.2 microM; Bmax = 4.5 nM), is able to modify the activity of adenylate cyclase, probably via G-proteins. In addition, the Ca2+ homeostasis is also modulated by S-100 via an increase of specific membrane conductance and/or Ca2+ release from intracellular stores. "In vitro" and "in vivo" experiments showed that lower (nM) concentrations of extracellular S-100 beta act on glial and neuronal cells as a growth-differentiating factor. On the other hand, higher concentrations of the protein induce apoptosis of some cells such as the sympathetic-like PC12 line. Finally, data obtained from physiological (development, ageing) or pathological (dementia associated with Down's syndrome, Alzheimer's disease) conditions showed that a relationship could be established between the S-100 levels and some aspects of the statii.

Seven helix cAMP receptors stimulate Ca2+ entry in the absence of functional G proteins in Dictyostelium

Surface cAMP receptors (cARs) in Dictyostelium transmit a variety of signals across the plasma membrane. The best characterized cAR, cAR1, couples to the heterotrimeric guanine nucleotide-binding protein (G protein) alpha-subunit G alpha 2 to mediate activation of adenylyl and guanylyl cyclases and cell aggregation. cAR1 also elicits other cAMP-dependent responses including receptor phosphorylation, loss of ligand binding (LLB), and Ca2+ influx through a G alpha 2-independent pathway that may not involve G proteins. Here, we have expressed cAR1 and a related receptor, cAR3, in a g beta- strain (Lilly, P., Wu. L., Welker, D. L., and Devreotes, P. N. (1993) Genes & Dev. 7,986-995), which lacks G protein activity. Both cell lines failed to aggregate, a process requiring the G alpha 2 and G beta- subunits. In contrast, cAR1 phosphorylation in cAR1/g beta- cells showed a time course and cAMP dose dependence indistinguishable from those of cAR1/G beta+ controls. cAMP-induced LLB was also normal in the cAR1/g beta- cells. Finally, cAR1/g beta- cells and cAR3/g beta- cells showed a Ca2+ response with kinetics, agonist dependence, ion specificity, and sensitivity to depolarization agents that were like those of G beta+ controls, although they accumulated fewer Ca2+ ions per cAMP receptor than the control strains. Together, these results suggest that the G beta-subunit is not required for the activation or attenuation of cAR1 phosphorylation, LLB, or Ca2+ influx. It may, however, serve to amplify the Ca2+ response, possibly by modulating other intracellular Ca2+ signal transduction pathways.

Modulation of ion-channel function by G-protein-coupled receptors

Neurotransmitters acting through G-protein-coupled receptors change the electrical excitability of neurons. Activation of receptors can affect the voltage dependence, the speed of gating, and the probability of opening of various ion channels, thus changing the computational state and outputs of a neuron. Each cell expresses many kinds of receptors, and uses several intracellular signaling pathways to modulate channel function in different ways. It has become possible to dissect these pathways by combining pharmacological and biophysical experiments. Recent patch-clamp work in sympathetic neurons will be summarized to illustrate the mechanisms underlying modulation and its significance.

The G protein G13 mediates inhibition of voltage-dependent calcium current by bradykinin

In neuroblastoma-glioma hybrid cells, bradykinin has dual modulatory effects on ion channels: it activates a K+ current as well as inhibits the voltage-dependent Ca2+ current (ICa,V). Both of these actions are mediated by pertussis toxin-insensitive G proteins. Antibodies raised against the homologous Gq and G11 proteins suppress only the activation of the K+ current; this suggested that at least two distinct G protein pathways transduce diverse effects of this transmitter. Here, we show that the inhibition of ICa,V by bradykinin is suppressed selectively by intracellular application of antibodies specific for G13. This novel G protein may play a general role in the inhibition of ICa,V by pathways resistant to pertussis toxin.

AMPA/kainate receptor activation in murine oligodendrocyte precursor cells leads to activation of a cation conductance, calcium influx and blockade of delayed rectifying K+ channels

Studies during the last few years have shown that glial cells can express a large repertoire of neurotransmitter receptors. In this study, we have characterized the properties of a glutamate receptor in oligodendrocytes and their precursor cells from cultures of mouse brain, using the patch-clamp technique to measure ligand-activated currents and a fura-2 imaging system to determine changes in free cytosolic Ca2+ concentration ([Ca2+]i). The precursor cells were identified by their characteristic morphology and their voltage-gated currents as described previously [Sontheimer H. et al. (1989) Neuron 2, 1135-1145]. The ligands kainate, domoate and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), as well as L-glutamate but not trans-1-amino-1,3-cyclopentanedicarboxylate elicited inward currents at a holding potential of -70 mV and the antagonist 6-cyano-7-nitroquinoxaline-2,3-dione blocked the glutamate- and kainate-induced response reversibly, indicating the expression of an AMPA/kainate-type glutamate receptor. The response is due to the activation of a cationic conductance as revealed by analysing the reversal potential of the kainate-activated current. Receptor activation is accompanied by two additional responses: (i) an increase in [Ca2+]i mediated by depolarization and a subsequent activation of voltage-gated Ca2+ channels and (ii) a transient blockade of a delayed rectifying K+ current, but not of the A-type K+ current. The blockade of the K+ current was not due to the increase in [Ca2+]i since it was also observed in Ca(2+)-free bathing solution when no increase in [Ca2+]i was detectable after exposure to kainate. In contrast to precursor cells, oligodendrocytes responded weakly or not at all to glutamate or related ligands. We conclude that glutamate activates a complex pattern of physiological events in the glial precursor cells, which may play a role during the differentiation process of these cells.

Calcium channels: cellular roles and molecular mechanisms

An unexpected variety of different types of Ca2+ channels have been identified using molecular cloning and selective Ca2+ channel toxins. A main focus of current research in this field is to characterize these different channel types and the molecular mechanisms by which Ca2+ channels perform their basic activities of gating, selectivity and modulation. Recent advances demonstrate the roles of different types of Ca2+ channels in muscle and nerve, and provide hints about the structures involved in selectivity and gating.