Incorporation of Galpha(z)-specific sequence at the carboxyl terminus increases the promiscuity of galpha(16) toward G(i)-coupled receptors

Although the promiscuous nature of G(16) allows it to interact with numerous G protein-coupled receptors, several G(i)-linked receptors are incapable of activating phospholipase C via G(16). A series of chimeras between Galpha(16) and Galpha(z) were constructed and assayed for their ability to mediate receptor-induced stimulation of phospholipase C. Two Galpha(16/z) chimeras harboring 25 or 44 Galpha(z)-specific sequences at their C termini (named 16z25 and 16z44) were capable of responding to 14 different G(i)-coupled receptors tested, including those that were either unable to associate with Galpha(16) (melatonin Mel1c) or activate Galpha(16) weakly (micro-opioid and type 1 somatostatin). Agonist-induced stimulation of phospholipase C was more efficiently mediated (higher maximal and lower EC(50) value) by 16z44 than by Galpha(16). Both 16z25 and 16z44 were also coupled to G(s)- and G(q)-linked receptors. Incorporation of Galpha(z) sequence at the N terminus of Galpha(16) did not further enhance the ability of the chimeras to interact with G(i)-coupled receptors. Expression of the various chimeras was verified by immunodetection and functional analysis of their constitutively activated mutants. These results show that the incorporation of alpha4/beta6 and alpha5 regions of Galpha(z) into a Galpha(16) backbone can improve the recognition of G(i)-coupled receptors. Galpha(16/z) chimeras with expanded capability to interact with G(i)-linked receptors may be used to link orphan receptors to the stimulation of phospholipase C.

Measurements of the bulk, C-axis electromechanical coupling constant as a function of AlN film quality

Piezoelectric thin film AlN has great potential for on-chip devices such as thin-film resonator (TFR)-based bandpass filters. The AlN electromechanical coupling constant, K(2), is an important material parameter that determines the maximum possible bandwidth for bandpass filters. Using a previously published extraction technique, the bulk c-axis electromechanical coupling constant was measured as a function of the AlN X-ray diffraction rocking curve [full width at half maximum (FWHM)]. For FWHM values of less than approximately 4 degrees , K (2) saturates at approximately 6.5%, equivalent to the value for epitaxial AlN. For FWHM values >4 degrees , K(2) gradually decreases to approximately 2.5% at a FWHM of 7.5 degrees . These results indicate that the maximum possible bandwidth for TFR-based bandpass filters using polycrystalline AlN is approximately 80 MHz and that, for 60-MHz bandwidth PCS applications, an AlN film quality of >5.5 degrees FWHM is required.

The effect of protein kinase C activation on G(z)-mediated regulation of type 2 and 6 adenylyl cyclases

Three serine-to-alanine mutants of the alpha subunit of the heterotrimeric G protein G(z) (alpha(z)) were examined for their signaling properties in the presence of phorbol ester treatment. All three alpha(z) mutants resembled wild-type alpha(z) in their abilities to inhibit alpha(s)-stimulated type 6 adenylyl cyclase (AC6) and phorbol ester treatment reduced their magnitudes of inhibition. Depending on the permissive condition, the betagamma-mediated stimulation of type 2 adenylyl cyclase (AC2) was differentially regulated by alpha(z) and the three mutants. Mutation of Ser(27) but not Ser(16) of alpha(z) affected the efficient release of betagamma subunits upon receptor activation and abolished the stimulation of phosphorylated but not alpha(s)-stimulated AC2.

A cervical ectopic pregnancy managed by medical treatment and angiographic embolization

Medical treatment in the form of systemic methotrexate with or without local methotrexate/potassium chloride is effective for early cervical pregnancy. It should be the treatment of choice in suitable cases. Haemorrhagic complications can be effectively managed by angiographic embolization. A case report illustrating these points is presented.

Disruption of receptor-mediated activation of G protein by mutating a conserved arginine residue in the switch II region of the alpha subunit

A naturally occurring point mutation (R231H) within one of the major 3gamma-binding surface (switch II region) on the a subunit of Gs (alpha(s)) has previously been found to disrupt receptor-mediated activation of Gs. The disruption caused by mutating this conserved residue may be a general phenomenon for all a subunits. Homologous mutants of the alpha subunit of Gz [alpha(z); a negative regulator of adenylyl cyclase (AC)] and G16 (alpha16; a stimulator of phospholipase C) were constructed and examined for receptor-mediated regulation of their corresponding effectors. The mutant alphazR209H cannot be fully activated by the delta-opioid receptor, as indicated by the impairment of the inhibition of alpha(s)-stimulated AC and betagamma-mediated stimulation of AC type II (AC2). Similarly, the mutant alpha16R216H lost the ability to mediate receptor-induced activation of phospholipase C and AC2. The receptor coupling efficacy and promiscuity of alpha16R216H were eradicated. The mutation of the conserved arginine has no observable effect on the constitutive activities of the GTPase-deficient derivatives of both alpha(z) and alpha16. The alpha subunit of Gt1 (transducin; alphat1) attenuated betagamma-mediated stimulation of AC2 by sequestrating free betagamma subunits, but the mutant alphat1R204H showed reduced ability to scavenge betagamma-mediated AC2 activation. Presumably, mutation of the conserved arginine disrupted the subunit interactions in addition to the impairment of receptor interaction.

Chronic activation of ORL1 receptor induces supersensitization of adenylyl cyclase

The opioid receptor-like (ORL1) receptor regulates a variety of effectors shared by its close relatives, the opioid receptors. Supersensitization of adenylyl cyclase (AC) is a hallmark of cellular tolerance induced by chronic activation of opioid receptors. To examine if chronic activation of the ORL1 receptor leads to a similar adaptation, a HEK293 cell line stably expressing the human ORL1 receptor (293/ORL1) was established. Chronic treatment of 293/ORL1 cells with nociceptin/OFQ resulted in enhanced AC activity in response to forskolin stimulation. The AC supersensitivity was blocked by pertussis toxin, indicating the involvement of Gi/Go proteins. Likewise, chronic activation of endogenous ORL1 receptors in the neuroblastoma SK-N-SH cells led to Gi/Go-mediated AC supersensitization.

Alanine-23 of transducin alpha subunit is involved in defining the affinity for betagamma complex

Transducin a subunit (alpha(t1)) shows extraordinarily high affinity to G protein beta(gamma) complex. One of the beta(gamma)-binding regions on alpha(t1) is the amino-terminal helix. Alanine-23 is uniquely found on alpha(t1) but not other members of the G(i)-subfamily. Mutation of alanine-23 into serine reduced the ability of alpha(t1) to sequester beta(gamma)-mediated stimulation of type II adenylyl cyclase. The functional impairment is independent to the protein expression levels. Molecular modeling indicated that the hydrophobic interaction between the side chains of alanine-23 of alpha(t1) and leucine-55 of the beta1 subunit could be disrupted by the introduction of a hydroxyl group. This study showed that alanine-23 of alpha(t1) is probably involved in defining its affinity for the beta(gamma) complex.

Alanine-23 of transducin alpha subunit is involved in defining the affinity for betagammma complex

Transducin alpha subunit (alpha(t)1) shows extraordinarily high affinity to G protein betagamma complex. One of the betagamma-binding regions on alphat1 is the amino-terminal helix. Alanine-23 is uniquely found on alphat1 but not other members of the Gi-subfamily. Mutation of alanine-23 into serine reduced the ability of alpha(t)1 to sequester betagamma-mediated stimulation of type II adenylyl cyclase. The functional impairment is independent to the protein expression levels. Molecular modeling indicated that the hydrophobic interaction between the side chains of alanine-23 of alpha(t)1 and leucine-55 of the beta1 subunit could be disrupted by the introduction of a hydroxyl group. This study showed that alanine-23 of alpha(t)1 is probably involved in defining its affinity for the betagamma complex.

Stimulation of phospholipase C by the cloned mu, delta and kappa opioid receptors via chimeric G alpha(q) mutants

Opioid receptors (mu, delta and kappa) are known to regulate diverse physiological functions and yet, at the molecular level, they are coupled to a seemingly identical set of G proteins. A recent study has discerned subtle differences between the opioid receptors in their ability to activate the pertussis toxin-insensitive G16. Differences in microarchitecture might be magnified when these receptors are provided with 'non-native' partners. Here, we examined whether the opioid receptors can interact productively with a set of chimeric Galphaq subunits which are known to link many Gi-coupled receptors to phosphoinositide-specific phospholipase C (PI-PLC). The qi5, qo5 and qz5 chimeras have the last five residues of Galphaq replaced by those of Galphai, Galphao and Galphaz, respectively. Except for mu-receptor and qo5, each pair of opioid receptor and Galphaq chimera allowed opioid agonists to stimulate PI-PLC in transfected COS-7 cells. The Galphaq chimera-mediated responses were ligand selective, agonist dose dependent and saturable. The most robust responses were obtained with kappa-receptor and qi5 or qz5, whereas the coupling of delta- and mu-receptors to Galphaq chimeras produced much weaker responses. Among the Galphaq chimeras, qo5 was less efficiently coupled to the opioid receptors. As revealed by radioligand binding assays and immunoblot analysis, differences in the efficiency of coupling were not due to variations in the expression of receptors and Galphaq chimeras. Differences in the magnitude of PI-PLC responses are thus likely to represent structural incompatibility between opioid receptors and Galphaq chimeras, suggesting that each opioid receptor possesses unique structural surfaces for the binding of G proteins.

Qualitative and quantitative magnetic resonance imaging in haemoglobin H disease: screening for iron overload

OBJECTIVES

To evaluate the clinical utility of magnetic resonance imaging (MRI) in screening for iron overload in non-transfusion dependent Haemoglobin (Hb) H disease.

PATIENTS AND METHODS

Thirty-six non-transfusion dependent HbH patients were evaluated with axial spin echo T1 and gradient echo T2 MRI of the abdomen and heart. The ratios of signal intensities (SIR) of the liver, spleen, pancreas and heart to paraspinous muscles were calculated. SIR <1 was taken as indicative of iron overload. Qualitative grading (0-4 scale) of iron overload was also performed. The relationship between T1 and T2 SIR and serum ferritin, and that between qualitative grading and serum ferritin were examined using standard statistical methods. Comparisons were also made between qualitative grading and quantitative T1 and T2 SIR data in diagnosing iron overload. Six patients underwent liver biopsies.

RESULTS

T2 SIR was more sensitive in detecting iron overload than T1 SIR. Thirty-three livers, 13 spleens, six pancreas and one heart were diagnosed as having iron overload with T2 SIR, including three patients with normal serum ferritin. A positive diagnosis by T2 SIR was more closely related to that of qualitative grading than T1 SIR. Serum ferritin was negatively correlated with hepatic SIR (T1 and T2), and with T2 SIR of the spleen and pancreas, even after adjustment for age. Liver haemosiderosis was confirmed in all six patients who underwent liver biopsies. Liver iron concentration of only one and a half times the normal was found in one patient with positive MR findings.

CONCLUSION

MR is a non-invasive, effective method for early detection of iron overload particularly in the liver and spleen. Qualitative grading and quantitative T2 SIR data are equivalent in diagnosing iron overload. Routine screening of non-transfusion dependent HbH patients will identify high risk patients in whom early therapeutic intervention may prevent further complications and morbidity.

GalphaL1 (Galpha14) couples the opioid receptor-like1 receptor to stimulation of phospholipase C

In most tissues and cells the opioid receptor-like (ORL1) receptor regulates effectors primarily through the pertussis toxin (PTX)-sensitive guanine nucleotide-binding regulatory proteins (G proteins) Gi/Go. Many Gi-coupled receptors possess additional capability to interact with one or more PTX-insensitive G proteins. Using the betagamma-induced stimulation of type 2 adenylyl cyclase as a readout, we screened the ability of ORL1 receptor to interact with a panel of PTX-insensitive G proteins. In the presence of PTX, activation of the ORL1 receptor resulted in the stimulation of type 2 adenylyl cyclase only in HEK 293 cells coexpressing the alpha subunit of Gz, G12, G14, or G16, but not in cells coexpressing G11, G13, or Gq. Coupling to both Gz and G16 was expected because close relatives of the ORL1 receptor, the opioid receptors, are known to couple productively to these G proteins. ORL1 receptor coupling to either G12 or G14 has not been demonstrated. As predicted by the type 2 adenylyl cyclase assays, activation of the ORL1 receptor resulted in the formation of inositol phosphates in COS-7 cells transiently cotransfected with Galpha14. The ORL1 receptor-mediated stimulation of phospholipase C was found to be Galpha14 dependent, agonist dose dependent, ligand selective, and PTX insensitive. We conclude that G14 can link the ORL1 receptor to regulation of phopholipase C.

Chimeric Galpha(q) mutants harboring the last five carboxy-terminal residues of Galpha(i2) or Galpha(o) are resistant to pertussis toxin-catalyzed ADP-ribosylation

Three widely-used Galpha(q) chimeras harboring the last five residues of Galpha(i), Galpha(o) and Galpha(z) (qi5, qo5 and qz5) were examined for their ability to serve as substrates for pertussis toxin (PTX)-catalyzed ADP-ribosylation. In COS-7 cells coexpressing one of the three opioid receptors (mu, delta, and kappa) and a Galpha(q) chimera, agonist-induced stimulation of phosphoinositide-specific phospholipase C (PI-PLC) was largely insensitive to PTX treatment. Only the qi5-mediated stimulation of PI-PLC by kappa-opioids was partially inhibited by PTX. In betagamma-release assays, PTX treatment did not affect the ability of opioid receptors to activate these chimeras. [32P]ADP-ribosylation labeled Galpha(i/o) but not qi5 or qo5, although the expression of these chimeras was confirmed by immunodetection. Thus, Galpha(q) chimeras with a Galpha(i/o)-like tail are insensitive to PTX treatment.

Molecular and cellular analyses of melatonin receptor-mediated cAMP signaling in rat corpus epididymis

By using 2-[125I]iodomelatonin receptor binding studies, we have previously demonstrated high affinity melatonin receptors, the binding activities of which are regulated by testosterone, in the corpus epididymis of rats. In this report, some of the basic molecular and cellular characteristics of these high affinity melatonin receptors in rat corpus epididymis were analyzed. MEL1A and MEL1B receptor mRNAs were expressed by rat corpus epididymal epithelial cells as revealed by in situ hybridization. Functionally, these high affinity melatonin receptors are negatively coupled to adenylyl cyclase via pertussis toxin (PTX) sensitive Gi protein and the inhibitory effects of melatonin on forskolin-stimulated cAMP accumulation were enhanced by 5alpha-dihydrotestosterone (5alpha-DHT). Interestingly, opposing interactions between melatonin and beta-adrenergic receptor signaling in rat epididymal epithelial cells were observed with melatonin inhibiting norepinephrine- and isoproterenol-stimulated cAMP accumulation. In conclusion, our data support a modulatory action of melatonin, mediated via pertussis toxin-sensitive Gi-coupled MEL1A and MEL1B receptors, in androgenic and adrenergic regulation of rat corpus epididymal epithelial cell functions.

Pertussis toxin-insensitive signaling of the ORL1 receptor: coupling to Gz and G16 proteins

Nociceptin/OFQ is the endogenous ligand for the G protein-coupled opioid receptor-like (ORL1) receptor. To elucidate the cellular functions of the ORL1 receptor, we examined its ability to interact with Gz and G16, two pertussis toxin (PTX)-insensitive G proteins that are known molecular partners for the opioid receptors. In HEK 293 cells transiently expressing the ORL1 and dopamine D1 receptors, nociceptin/OFQ dose-dependently inhibited dopamine-stimulated cyclic AMP (cAMP) accumulation in a PTX-sensitive manner. However, PTX failed to block the nociceptin/OFQ-induced inhibition of dopamine-stimulated cAMP accumulation in HEK 293 cells co-expressing the alpha-subunit of Gz. This result indicates functional interaction between the ORL1 receptor and Gz. A similar result was obtained with retinoic acid-differentiated SH-SY5Y cells, which endogenously express both the ORL1 receptor and Gz. When the ORL1 receptor was transiently co-expressed in COS-7 cells with the alpha-subunit of G16, nociceptin/OFQ dose-dependently stimulated the formation of inositol phosphates. Nociceptin-induced stimulation of phospholipase C was absolutely dependent on the co-expression of alpha16 and exhibited the appropriate ligand selectivity. In terms of its ability to interact with PTX-insensitive G proteins, the ORL1 receptor behaves very much like the opioid receptors.

The calcitonin gene-related peptide-induced acetylcholinesterase synthesis in cultured chick myotubes is mediated by cyclic AMP

In vertebrate neuromuscular junctions, post-synaptic specialization includes aggregation of acetylcholine receptors (AChRs) and acetylcholinesterase (AChE). The motor nerve provides soluble factors and electrical activity to achieve this striking localization of AChRs/AChE. Calcitonin gene-related peptide (CGRP), a neuropeptide synthesized by motor neurons, is able to stimulate the expression of AChR in cultured myotubes. Similar to AChR regulation, synthesis of AChE in cultured chick myotubes is also stimulated by CGRP. Application of CGRP onto cultured myotubes stimulated the accumulation of intracellular cyclic AMP (cAMP) as well as the expression of AChE mRNA and protein. However, the enzymatic activity of AChE remained unchanged. In cultured myotubes, various drugs affecting the intracellular level of cAMP, such as N6,O2'-dibutyryladenosine 3',5'-cyclic monophosphate, cholera toxin, and forskolin, could mimic the effect of CGRP in stimulating the expression of AChE. When myotubes were transfected with cDNA encoding constitutively active mutant Galpha(s), the intracellular cAMP synthesis was increased. The increase in cAMP level was in parallel with an increase in the expression of AChE, whereas transfection of active mutant Galpha(i) cDNA decreased the cAMP level as well as the AChE expression. In addition, expression of collagen-tailed AChE was up-regulated by the cAMP pathway. These findings indicated that CGRP-induced AChE regulation is mediated by the cAMP pathway and represented the first evidence to suggest that the regulation of mRNA synthesis of AChR and AChE can be mediated by the same neuron-derived factor.

Differential coupling of mu-, delta-, and kappa-opioid receptors to G alpha16-mediated stimulation of phospholipase C

The mu-opioid receptor has recently been shown to stimulate phosphoinositide-specific phospholipase C via the pertussis toxin-sensitive G16 protein. Given the promiscuous nature of G16 and the high degree of resemblance of signaling properties of the three opioid receptors, both delta- and kappa-opioid receptors are likely to activate G16. Interactions of delta- and kappa-opioid receptors with G16 were examined by coexpressing the opioid receptors and G alpha16 in COS-7 cells. The delta-selective agonist [D-Pen2,D-Pen5] enkephalin potently stimulated the formation of inositol phosphates in cells coexpressing the delta-opioid receptor and G alpha16. The delta-opioid receptor-mediated stimulation of phospholipase C was absolutely dependent on the coexpression of G alpha16 and exhibited appropriate ligand selectivity and dose dependency. Similar transfection studies revealed only weak stimulation by the mu-opioid receptor, whereas the kappa-opioid receptor produced moderate phospholipase C activity. G alpha16 thus appeared to interact differentially with the three opioid receptors. Radioligand binding assays indicate that the mu-opioid receptor was expressed at a lower level than those of the delta- and kappa-opioid receptors. To examine if differential coupling to G alpha16 is prevalent, a panel of Gs- or Gi-coupled receptors was coexpressed with G alpha16 in COS-7 cells and assayed for agonist-induced stimulation of phospholipase C. Activation of alpha2- and beta2-adrenergic, dopamine D1 and D2, adenosine A1, somatostatin-1 and -2, C5a, formyl peptide, and luteinizing hormone receptors all resulted in stimulation of phospholipase C, with maximal stimulations ranging from 1.5- to almost 17-fold. These findings suggest that the promiscuous G alpha16 can in fact discriminate among different receptors and that such preferential interaction might in part be due to the abundance of receptors.

Regulation of calcium influx and phospholipase C activity by indoleamines in dinoflagellate Crypthecodinium cohnii

Exogenous indoleamines such as melatonin and 5-methoxytryptamine have been shown to induce cyst formation (encystment) in many species of dinoflagellate. Induction of inositol phosphates formation by indoleamine has previously been demonstrated in Crypthecodinium cohnii. In addition, depletion of extracellular Ca2+ blocks the indoleamine-induced encystment. In the present study, 12 indoleamines (including melatonin and related compounds) were examined for their abilities to induce Ca2+ influx, inositol phosphates formation, and encystment in C. cohnii. The results showed that melatonin, 5-methoxytryptamine, and the peptide toxin mastoparan stimulated 45Ca2+ influxes in dose- and time-dependent manners. The EC50 values of 5-methoxytrypramine and mastoparan to stimulate 45Ca2+ uptake were 2 mM and 35 microM, respectively. The 5-methoxytryptamine- and mastoparan-induced 45Ca2+ influx were partially attenuated by the calcium channel blockers, verapamil and ruthenium red. A series of indoleamines were examined for their structure-activity relationship on the induction of encystment and formation of inositol phosphates. Melatonin-induced inositol phosphates formation was completely blocked by U73122, indicating the possible involvement of phospholipase C. Taken together, we conclude that indoleamines may induce encystment of the dinoflagellate C. cohnii via parallel activation of phospholipase C and Ca2+ influx signaling pathways. However, activation of phospholipase C and Ca2+ influx are not always necessary or sufficient for inducing encystment. Also, these data provided the first direct evidence of a Ca2+ influx regulating mechanism in dinoflagellate C. cohnii.

Structure and function of the pertussis-toxin-insensitive Gz protein

Gz is the only pertussis-toxin-insensitive member of the inhibitory G protein subfamily. The unique pattern of tissue distribution of Gz suggests it may carry out tissue-specific functions, albeit it appears to share the same profile of G-protein-coupled receptors with Gi. The knowledge of the structural elements of alpha z for receptor coupling and specificity has been enriched by constructing chimeric molecules. Biochemical characteristics of alpha z are considerably different from other G protein alpha-subunits. The regulation of the GTP hydrolysis activity of alpha z by various GTPase-activating proteins and the functional impact of the PKC-mediated phosphorylation of alpha z are discussed. Different routes of signaling pathways that Gz could engage in have been explored. Furthermore, the possible involvement of Gz in retrograde axonal transport and various immune responses shed lights in understanding the physiological importance of Gz.

Role of amino- and carboxyl-terminal regions of G(alphaZ) in the recognition of Gi-coupled receptors

Many Gi-coupled receptors are known to interact with the pertussis toxin (PTX)-insensitive Gz protein. Given that the alpha subunits of Gi and Gz share only 60% identity in their amino acid sequences, their receptor-interacting domains must be highly similar. By swapping the carboxyl termini of alpha i2 and alpha z with each other or with those of alpha t, alpha12, and alpha13, we examined the relative contributions of the carboxyl-end 36 amino acids of the alpha chains toward receptor recognition. Chimeric alpha chains lacking the site for PTX-catalyzed ADP-ribosylation were coexpressed with the type II adenylyl cyclase (AC II) and one of several Gi-coupled receptors (formyl peptide, dopamine D2, and delta-opioid receptors) in human embryonic kidney 293 cells. The alpha i2/alpha z chimera was able to interact with both aminergic and peptidergic receptors, resulting in betagamma-mediated stimulation of AC II in the presence of agonists and PTX. Functional and mutational analyses of alpha i2/alpha z revealed that this chimera can inhibit the endogenous ACs of 293 cells. Similarly, the alpha z/alpha i2 chimera seemed to retain the abilities to interact with receptors and inhibit cAMP accumulation. Fusion of the carboxyl-terminal 36 amino acids of alpha z to a backbone of alpha t1 produced a chimera, alpha t1/alpha z, that did not couple to any of the Gi-coupled receptors tested. Interestingly, an alpha13/alpha z chimera (with only the last five amino acids switched) displayed differential abilities to interact with receptors. Signals from aminergic, but not peptidergic, receptors were transduced by alpha13/alpha z. A similar construct, alpha12/alpha z, behaved just like alpha13/alpha z. These results indicated that "alpha i-like" or "alpha z-like" sequences at the carboxyl termini of alpha subunits are not always necessary or sufficient for specifying interaction with Gi-coupled receptors.

Calcium ion dependency and the role of inositol phosphates in melatonin-induced encystment of dinoflagellates

The unicellular eukaryotic dinoflagellates shed their flagella and form a new pellicle cyst wall in response to environmental stress. This encystment process can also be induced by indoleamines such as melatonin and 5-methoxytryptamine. To decipher the complex signaling events which lead to encystment, we have investigated the functional roles of Ca2+ and inositol phosphates in indoleamine-induced encystment of the dinoflagellates Alexandrium catenella and Crypthecodinium cohnii. Pretreatment with EGTA, but not with EDTA, effectively blocked the indoleamine-induced encystment of A. catenella in a dose-dependent manner. Conversely, agents that facilitate the influx of Ca2+ (Bay K 8644, A23187 and ionomycin) dose-dependently induced encystment of A. catenella. Endoplasmic Ca2+-ATPase inhibitors such as thapsigargin and the peptide toxin melittin also induced encystment of A. catenella. These results suggest that an elevation of intracellular [Ca2+] may be involved in the encystment response. In terms of the regulation of phospholipase C, melatonin dose- and time-dependently stimulated the formation of inositol phosphates in C. cohnii. The rank order of potency for several indoleamines to stimulate inositol phosphates formation was 2-iodomelatonin &gt; 5-methoxytryptamine &gt; or = melatonin &gt;&gt; N-acetylserotonin &gt; 5-hydroxytryptamine. This rank order was the same as for the indoleamine-induced encystment of C. cohnii as previously reported. Our results indicate that indoleamine-induced activation of phospholipase C and elevation of intracellular [Ca2+] may be proximal steps in the signal transduction pathway leading to encystment in dinoflagellates. Moreover, this is the first demonstration of the possible involvement of Ca2+ and inositol phosphates as second messengers in dinoflagellates.

Functional role of amino-terminal serine16 and serine27 of G alphaZ in receptor and effector coupling

The alpha subunit of Gz (alpha(z)) harbors two N-terminal serine residues (at positions 16 and 27) that serve as protein kinase C-mediated phosphorylation sites. The cognate residues in the alpha subunit of Gt1 provide binding surfaces for the beta1 subunit. We used three serine-to-alanine mutants of alpha(z) to investigate the functional importance of the two N-terminal serine residues. Wild-type or mutant alpha(z) was transiently coexpressed with different receptors and adenylyl cyclase isozymes in human embryonic kidney 293 cells, and agonist-dependent regulation of cyclic AMP accumulation was examined in a setting where all endogenous alpha subunits of G1 were inactivated by pertussis toxin. Replacement of one or both serine residues by alanine did not alter the ability of alpha(z) to interact with delta-opioid, dopamine D2, or adenosine A1 receptors. Its capacity to inhibit endogenous and type VI adenylyl cyclases was also unaffected. Functional release of betagamma subunits from the mutant alpha(z) subunits was not impaired because they transduced betagamma-mediated stimulation of type II adenylyl cyclase. Constitutively active mutants of all four alpha(z) subunits were constructed by the introduction of a Q205L mutation. The activated mutants showed differential abilities to inhibit human choriogonadotropin-mediated cyclic AMP accumulation in luteinizing hormone receptor-transfected cells. Loss of both serine residues, but not either one alone, impaired the receptor-independent inhibition of adenylyl cyclase by the GTPase-deficient mutant. Thus, replacement of the amino-terminal serine residues of alpha(z) has no apparent effect on receptor-mediated responses, but these serine residues may be essential for ensuring transition of alpha(z) into the active conformation.

Receptor-induced beta gamma release from fatty acylation-deficient mutants of G alpha z

The neuronal-specific G protein Gz is known to interact with a large variety of receptors for neurotransmitters and hormones. Fatty acylations on the N-terminus of the alpha subunit of Gz (alpha z) provide anchorage to the plasma membrane. Fatty acylation-deficient mutants of alpha z have previously been shown to exhibit altered signaling properties. Since the N-terminus of alpha z is likely to play a critical role in beta gamma binding, we examined the ability of these mutants to interact with beta gamma subunits by means of receptor-mediated stimulation of beta gamma-sensitive type II adenylyl cyclase. Our results indicate that lack of myristoylation, but not lack of palmitoylation, impaired the ability of alpha z to mediate receptor-induced release of beta gamma subunits.

Inverse agonistic effect of ICI-174,864 on the cloned delta-opioid receptor: role of G protein and adenylyl cyclase activation

Previous studies have established that the delta-selective antagonist ICI-174,864 exhibits negative intrinsic activity at the delta-opioid receptors in NG108-15 membranes. To determine whether ICI-174,864 can function as a true inverse agonist in intact cells, its ability to stimulate cAMP accumulation was examined in a human embryonic kidney 293 cell line (293/DOR) expressing the cloned murine delta-opioid receptor. Forskolin-stimulated cAMP accumulation in the 293/DOR cells was dose-dependently suppressed by the delta-selective agonist [D-Pen2, D-Pen5]enkephalin, and such inhibition was abolished by pertussis toxin or the opiate antagonist naloxone. In contrast, ICI-174,864 significantly potentiated the forskolin response. The ICI-174,864-induced enhancement of the forskolin response exhibited dose-dependency and was antagonized by [D-Pen2,D-Pen5]enkephalin and blocked by pertussis toxin. Neither ICI-174,864 nor pertussis toxin elevated the basal level of cAMP accumulation in the absence of forskolin. Other opiate antagonists, such as naloxone and naltrindole, were ineffective in enhancing the forskolin-stimulated cAMP accumulation. Elevation of cAMP levels in response to the activation of Gs (through either ligand-bound receptor or point mutation on alpha(s)) was also potentiated by ICI-174,864. Our results indicate that ICI-174,864 behaves as an inverse agonist in human embryonic kidney 293 cells stably expressing the delta-opioid receptor. The inverse agonistic effect of ICI-174,864 seemed to require Gi proteins and was clearly manifested when adenylyl cyclase was activated.

CGP 52608-induced cyst formation in dinoflagellates: possible involvement of a nuclear receptor for melatonin

Melatonin has been shown to regulate gene transcription through RZR/ROR nuclear receptors in mammalian cells. Thiazolidine dione CGP 52608 is a selective agonist of RZR/ROR receptors with little or no affinity for the cell surface G protein-coupled melatonin receptors. In this study, we addressed whether nuclear signaling may be involved in indoleamine-induced encystment of the unicellular dinoflagellates by examining their responses to CGP 52608. Three species of dinoflagellates (Alexandrium catenella, Amphidinium carterae, and Crypthecodinium cohnii) encysted in the presence of CGP 52608 and the responses were reversible and dose-dependent. Since a previous study has implicated the involvement of G proteins in mediating indoleamine-induced encystment of dinoflagellates, we explored the possibility of cross-talks between G protein-dependent and nuclear signaling pathways. The responses of A. catenella to either mastoparan (a direct activator of mammalian G proteins) or indoleamines were assessed in the presence or absence of CGP 52608. Interestingly, CGP 52608 synergized with either indoleamines or mastoparan to produce a more rapid encystment response. These findings suggest that nuclear signaling may be involved in the indoleamine-induced encystment of dinoflagellates.

Gi-mediated stimulation of type II adenylyl cyclase is augmented by Gq-coupled receptor activation and phorbol ester treatment

Synergism between Gs- and Gi- or Gq-dependent signaling pathways has been demonstrated in the stimulation of type II adenylyl cyclase (AC-II). Provision of activated alpha s is known to allow numerous Gi-coupled receptors to stimulate AC-II and to potentiate the responses to Gq-coupled receptors. To explore possible interactions between Gi- and Gq-coupled receptors that are independent of alpha s, the activity of AC-II was determined after the activation of Gi- and Gq-regulated pathways. Human embryonic kidney 293 cells were transiently cotransfected with cDNAs encoding AC-II and various G-protein-coupled receptors. Agonist-bound Gi-coupled receptors (including the formyl peptide, dopamine-D2, and delta-opioid receptors) stimulated AC-II activity in the absence of activated alpha s, provided that the cells were treated with 100 nM phorbol 12-myristate 13-acetate. Activation of protein kinase C (PKC) thus appears to relieve the requirement for the presence of activated alpha s. Stimulation of PKC via Gq-coupled receptors also allowed Gi-coupled receptors to activate AC-II. Coexpression of the m1 muscarinic receptor with the dopamine-D2 receptor permitted dopamine to stimulate AC-II in the presence of carbachol. The phorbol ester-permissive and alpha s-independent stimulation was mediated by G-protein beta gamma subunits because it was blocked by the beta gamma scavengers alpha t and beta-adrenergic receptor kinase. These results show that AC-II can efficiently integrate signals generated by Gq- and Gi-coupled receptors via a mechanism that is independent of Gs.