Prostaglandin E2 Inhibits Histamine-Evoked Ca2+ Release in Human Aortic Smooth Muscle Cells through Hyperactive cAMP Signaling Junctions and Protein Kinase A

In human aortic smooth muscle cells, prostaglandin E₂ (PGE₂) stimulates adenylyl cyclase (AC) and attenuates the increase in intracellular free Ca²⁺ concentration evoked by activation of histamine H₁ receptors. The mechanisms are not resolved. We show that cAMP mediates inhibition of histamine-evoked Ca²⁺ signals by PGE₂. Exchange proteins activated by cAMP were not required, but the effects were attenuated by inhibition of cAMP-dependent protein kinase (PKA). PGE₂ had no effect on the Ca²⁺ signals evoked by protease-activated receptors, heterologously expressed muscarinic M3 receptors, or by direct activation of inositol 1,4,5-trisphosphate (IP₃) receptors by photolysis of caged IP₃. The rate of Ca²⁺ removal from the cytosol was unaffected by PGE₂, but PGE₂ attenuated histamine-evoked IP₃ accumulation. Substantial inhibition of AC had no effect on the concentration-dependent inhibition of Ca²⁺ signals by PGE₂ or butaprost (to activate EP₂ receptors selectively), but it modestly attenuated responses to EP₄ receptors, activation of which generated less cAMP than EP₂ receptors. We conclude that inhibition of histamine-evoked Ca²⁺ signals by PGE₂ occurs through “hyperactive signaling junctions,” wherein cAMP is locally delivered to PKA at supersaturating concentrations to cause uncoupling of H₁ receptors from phospholipase C. This sequence allows digital signaling from PGE₂ receptors, through cAMP and PKA, to histamine-evoked Ca²⁺ signals.

Chemerin Elicits Potent Constrictor Actions via Chemokine-Like Receptor 1 (CMKLR1), not G-Protein-Coupled Receptor 1 (GPR1), in Human and Rat Vasculature

BACKGROUND

Circulating levels of chemerin are significantly higher in hypertensive patients and positively correlate with blood pressure. Chemerin activates chemokine-like receptor 1 (CMKLR1 or ChemR23) and is proposed to activate the "orphan" G-protein-coupled receptor 1 (GPR1), which has been linked with hypertension. Our aim was to localize chemerin, CMKLR1, and GPR1 in the human vasculature and determine whether 1 or both of these receptors mediate vasoconstriction.

METHODS AND RESULTS

Using immunohistochemistry and molecular biology in conduit arteries and veins and resistance vessels, we localized chemerin to endothelium, smooth muscle, and adventitia and found that CMKLR1 and GPR1 were widely expressed in smooth muscle. C9 (chemerin149-157) contracted human saphenous vein (pD₂=7.30±0.31) and resistance arteries (pD₂=7.05±0.54) and increased blood pressure in rats by 9.1±1.0 mm Hg at 200 nmol. Crucially, these in vitro and in vivo vascular actions were blocked by CCX832, which we confirmed to be highly selective for CMKLR1 over GPR1. C9 inhibited cAMP accumulation in human aortic smooth muscle cells and preconstricted rat aorta, consistent with the observed vasoconstrictor action. Downstream signaling was explored further and, compared to chemerin, C9 showed a bias factor=≈5000 for the G_i protein pathway, suggesting that CMKLR1 exhibits biased agonism.

CONCLUSIONS

Our data suggest that chemerin acts at CMKLR1, but not GPR1, to increase blood pressure. Chemerin has an established detrimental role in metabolic syndrome, and these direct vascular actions may contribute to hypertension, an additional risk factor for cardiovascular disease. This study provides proof of principle for the therapeutic potential of selective CMKLR1 antagonists.

Ca(2+) signals evoked by histamine H1 receptors are attenuated by activation of prostaglandin EP2 and EP4 receptors in human aortic smooth muscle cells

BACKGROUND AND PURPOSE

Histamine and prostaglandin E2 (PGE2 ), directly and via their effects on other cells, regulate the behaviour of vascular smooth muscle (VSM), but their effects on human VSM are incompletely resolved.

EXPERIMENTAL APPROACH

The effects of PGE2 on histamine-evoked changes in intracellular free Ca(2+) concentration ([Ca(2+) ]i ) and adenylyl cyclase activity were measured in populations of cultured human aortic smooth muscle cells (ASMCs). Selective ligands of histamine and EP receptors were used to identify the receptors that mediate the responses.

KEY RESULTS

Histamine, via H1 receptors, stimulates an increase in [Ca(2+) ]i that is entirely mediated by activation of inositol 1,4,5-trisphosphate receptors. Selective stimulation of EP2 or EP4 receptors attenuates histamine-evoked Ca(2+) signals, but the effects of PGE2 on both Ca(2+) signals and AC activity are largely mediated by EP2 receptors.

CONCLUSIONS AND IMPLICATIONS

Two important inflammatory mediators, histamine via H1 receptors and PGE2 acting largely via EP2 receptors, exert opposing effects on [Ca(2+) ]i in human ASMCs.

Ca(2+) signalling by P2Y receptors in cultured rat aortic smooth muscle cells

Background and purpose:

P2Y receptors evoke Ca²⁺ signals in vascular smooth muscle cells and regulate contraction and proliferation, but the roles of the different P2Y receptor subtypes are incompletely resolved.

Experimental approach:

Quantitative PCR was used to define expression of mRNA encoding P2Y receptor subtypes in freshly isolated and cultured rat aortic smooth muscle cells (ASMC). Fluorescent indicators in combination with selective ligands were used to measure the changes in cytosolic free [Ca²⁺] in cultured ASMC evoked by each P2Y receptor subtype.

Key results:

The mRNA for all rat P2Y receptor subtypes are expressed at various levels in cultured ASMC. Four P2Y receptor subtypes (P2Y1, P2Y2, P2Y4 and P2Y6) evoke Ca²⁺ signals that require activation of phospholipase C and comprise both release of Ca²⁺ from stores and Ca²⁺ entry across the plasma membrane.

Conclusions and implications:

Combining analysis of P2Y receptor expression with functional analyses using selective agonists and antagonists, we isolated the Ca²⁺ signals evoked in ASMC by activation of P2Y1, P2Y2, P2Y4 and P2Y6 receptors.

Regulation of inositol 1,4,5-trisphosphate receptors by cAMP independent of cAMP-dependent protein kinase

In HEK cells stably expressing type 1 receptors for parathyroid hormone (PTH), PTH causes a sensitization of inositol 1,4,5-trisphosphate receptors (IP₃R) to IP₃ that is entirely mediated by cAMP and requires cAMP to pass directly from type 6 adenylyl cyclase (AC6) to IP₃R2. Using DT40 cells expressing single subtypes of mammalian IP₃R, we demonstrate that high concentrations of cAMP similarly sensitize all IP₃R isoforms to IP₃ by a mechanism that does not require cAMP-dependent protein kinase (PKA). IP₃ binding to IP₃R2 is unaffected by cAMP, and sensitization is not mediated by the site through which ATP potentiates responses to IP₃. In single channel recordings from excised nuclear patches of cells expressing IP₃R2, cAMP alone had no effect, but it increased the open probability of IP₃R2 activated by a submaximal concentration of IP₃ alone or in combination with a maximally effective concentration of ATP. These results establish that cAMP itself increases the sensitivity of all IP₃R subtypes to IP₃. For IP₃R2, this sensitization results from cAMP binding to a novel site that increases the efficacy of IP₃. Using stably expressed short hairpin RNA to reduce expression of the G-protein, Gα_s, we demonstrate that attenuation of AC activity by loss of Gα_s more substantially reduces sensitization of IP₃R by PTH than does comparable direct inhibition of AC. This suggests that Gα_s may also specifically associate with each AC·IP₃R complex. We conclude that all three subtypes of IP₃R are regulated by cAMP independent of PKA. In HEK cells, where IP₃R2 selectively associates with AC6, Gα_s also associates with the AC·IP₃R signaling junction.

IP3 receptors: some lessons from DT40 cells

Inositol-1,4,5-trisphosphate receptors (IP3Rs) are intracellular Ca2+ channels that are regulated by IP3 and Ca2+ and are modulated by many additional signals. These properties allow them to initiate and, via Ca2+-induced Ca2+ release, regeneratively propagate Ca2+ signals evoked by receptors that stimulate formation of IP3. The ubiquitous expression of IP3R highlights their importance, but it also presents problems when attempting to resolve the behavior of defined IP3R. DT40 cells are a pre-B-lymphocyte cell line in which high rates of homologous recombination afford unrivalled opportunities to disrupt endogenous genes. DT40-knockout cells with both alleles of each of the three IP3R genes disrupted provide the only null-background for analysis of homogenous recombinant IP3R. We review the properties of DT40 cells and consider three areas where they have contributed to understanding IP3R behavior. Patch-clamp recording from the nuclear envelope and Ca2+ release from intracellular stores loaded with a low-affinity Ca2+ indicator address the mechanisms leading to activation of IP(3)R. We show that IP3 causes intracellular IP3R to cluster and re-tune their responses to IP3 and Ca2+, better equipping them to mediate regenerative Ca2+ signals. Finally, we show that DT40 cells reliably count very few IP3R into the plasma membrane, where they mediate about half the Ca2+ entry evoked by the B-cell antigen receptor.

Synthetic partial agonists reveal key steps in IP3 receptor activation

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) are ubiquitous intracellular Ca2+ channels. IP(3) binding to the IP(3)-binding core (IBC) near the N terminus initiates conformational changes that lead to opening of a pore. The mechanisms underlying this process are unresolved. We synthesized 2-O-modified IP(3) analogs that are partial agonists of IP(3)R. These are similar to IP(3) in their interactions with the IBC, but they are less effective than IP(3) in rearranging the relationship between the IBC and the N-terminal suppressor domain (SD), and they open the channel at slower rates. IP(3)R with a mutation in the SD occupying a position similar to the 2-O substituent of the partial agonists has a reduced open probability that is similar for full and partial agonists. Bulky or charged substituents from either the ligand or the SD therefore block obligatory coupling of the IBC and the SD. Analysis of DeltaG for ligand binding shows that IP(3) is recognized by the IBC and conformational changes then propagate entirely via the SD to the pore.

Functional ryanodine receptors in the plasma membrane of RINm5F pancreatic beta-cells

Ryanodine receptors (RyR) are Ca²⁺ channels that mediate Ca²⁺ release from intracellular stores in response to diverse intracellular signals. In RINm5F insulinoma cells, caffeine, and 4-chloro-m-cresol (4CmC), agonists of RyR, stimulated Ca²⁺ entry that was independent of store-operated Ca²⁺ entry, and blocked by prior incubation with a concentration of ryanodine that inactivates RyR. Patch-clamp recording identified small numbers of large-conductance (γ_K = 169 pS) cation channels that were activated by caffeine, 4CmC or low concentrations of ryanodine. Similar channels were detected in rat pancreatic β-cells. In RINm5F cells, the channels were blocked by cytosolic, but not extracellular, ruthenium red. Subcellular fractionation showed that type 3 IP₃ receptors (IP₃R3) were expressed predominantly in endoplasmic reticulum, whereas RyR2 were present also in plasma membrane fractions. Using RNAi selectively to reduce expression of RyR1, RyR2, or IP₃R3, we showed that RyR2 mediates both the Ca²⁺ entry and the plasma membrane currents evoked by agonists of RyR. We conclude that small numbers of RyR2 are selectively expressed in the plasma membrane of RINm5F pancreatic β-cells, where they mediate Ca²⁺ entry.

Selective coupling of type 6 adenylyl cyclase with type 2 IP3 receptors mediates direct sensitization of IP3 receptors by cAMP

Interactions between cyclic adenosine monophosphate (cAMP) and Ca²⁺ are widespread, and for both intracellular messengers, their spatial organization is important. Parathyroid hormone (PTH) stimulates formation of cAMP and sensitizes inositol 1,4,5-trisphosphate receptors (IP₃R) to IP₃. We show that PTH communicates with IP₃R via “cAMP junctions” that allow local delivery of a supramaximal concentration of cAMP to IP₃R, directly increasing their sensitivity to IP₃. These junctions are robust binary switches that are digitally recruited by increasing concentrations of PTH. Human embryonic kidney cells express several isoforms of adenylyl cyclase (AC) and IP₃R, but IP₃R2 and AC6 are specifically associated, and inhibition of AC6 or IP₃R2 expression by small interfering RNA selectively attenuates potentiation of Ca²⁺ signals by PTH. We define two modes of cAMP signaling: binary, where cAMP passes directly from AC6 to IP₃R2; and analogue, where local gradients of cAMP concentration regulate cAMP effectors more remote from AC. Binary signaling requires localized delivery of cAMP, whereas analogue signaling is more dependent on localized cAMP degradation.

Gypsy-like retrotransposons in Pyrenophora: an abundant and informative class of molecular markers

This paper describes the development of S-SAP (sequence-specific amplified polymorphism) using a primer derived from the LTR (long terminal repeat) of the Pyggy retrotransposon isolated from Pyrenophora graminea. Fragments were amplified by S-SAP from different Pyrenophora spp., indicating the presence of Pyggy-like sequences in these genomes. The bands were highly polymorphic between isolates and the number of bands differed by as much as 10-fold between species, demonstrating the potential of this method for genetic analysis in fungi. The phylogenetic relationship among the isolates as deduced using S-SAP data is presented, and shows evidence of genetic exchange between P. graminea and P. teres.

Determination of relative proportions of Globodera species in mixed populations of potato cyst nematodes using PCR product melting peak analysis

Summary Using a novel approach based on melting peak analysis of PCR products, we have developed a semi-quantitative assay to measure the relative proportions of Globodera pallida and Globodera rostochiensis in a sample of potato cyst nematodes (PCN). The method depends on a competitive multiplex PCR where the products of each species can be separated by their distinct melting temperatures (T(m)). The melting curves of the products are measured by continual fluorescence monitoring in the presence of the intercalating dye SYBR Green I whilst gradually increasing the temperature. Varying the proportion of cysts of each PCN species gave rise to melting curves with different peak heights, which reflected the relative amounts of each DNA in the sample. By calculating the ratio of the melting peak heights at the Tm of each product and comparing it with standards run under the same conditions, it was possible to estimate the proportion of each product in the mixture. Sensitivity was such that 2% of G. pallida cysts in a mixture could be detected. All results predicted by the analysis were confirmed by agarose gel electrophoresis of the PCR products. The method is rapid, reproducible and following DNA extraction, up to 25 samples can be analysed in an hour.