Preservation of Gi coupling of a chimeric EP3/I-type prostaglandin (IP) receptor

Role of the ERC motif in the proximal part of the second intracellular loop and the C-terminal domain of the human prostaglandin F2alpha receptor (hFP-R) in G-protein coupling control

The human FP-R (F2alpha prostaglandin receptor) is a Gq-coupled heptahelical ectoreceptor, which is of significant medical interest, since it is a potential target for the treatment of glaucoma and preterm labour. On agonist exposure, it mediates an increase in intracellular inositol phosphate formation. Little is known about the structures that govern the agonist-dependent receptor activation. In other prostanoid receptors, the C-terminal domain has been inferred in the control of agonist-dependent receptor activation. A DRY motif at the beginning of the second intracellular loop is highly conserved throughout the G-protein-coupled receptor family and appears to be crucial for controlling agonist-dependent receptor activation. It is replaced by an ERC motif in the FP-R and no evidence for the relevance of this motif in ligand-dependent activation of prostanoid receptors has been provided so far. The aim of the present study was to elucidate the potential role of the C-terminal domain and the ERC motif in agonist-controlled intracellular signalling in FP-R mutants generated by site-directed mutagenesis. It was found that substitution of the acidic Glu(132) in the ERC motif by a threonine residue led to full constitutive activation, whereas truncation of the receptor's C-terminal domain led to partial constitutive activation of all three intracellular signal pathways that had previously been shown to be activated by the FP-R, i.e. inositol trisphosphate formation, focal adhesion kinase activation and T-cell factor signalling. Inositol trisphosphate formation and focal adhesion kinase phosphorylation were further enhanced by ligand binding in cells expressing the truncation mutant but not the E132T (Glu132-->Thr) mutant. Thus C-terminal truncation appeared to result in a receptor with partial constitutive activation, whereas substitution of Glu132 by threonine apparently resulted in a receptor with full constitutive activity.

Cardiospecific Overexpression of the Prostaglandin EP 3 Receptor Attenuates Ischemia-Induced Myocardial Injury

Background— The generation of prostaglandin E 2 (PGE 2 ) is significantly increased in acute myocardial ischemia and reperfusion. PGE 2 , in addition to other prostaglandins, protects the reperfused ischemic myocardium. It has been hypothesized that this cardioprotection is mediated by E-type prostaglandin receptors of the G i -coupled EP 3 subtype.

Methods and Results— We tested this hypothesis by generating transgenic (tg) mice with cardiospecific overexpression of the EP 3 receptor. According to ligand binding, a 40-fold overexpression of the EP 3 receptor was achieved in membranes prepared from tg hearts compared with wild-type (wt) littermates. In isolated cardiomyocytes from tg mice, the forskolin-induced rise in cAMP was markedly attenuated, indicating coupling of the overexpressed EP 3 receptor to inhibitory G proteins (G i ) with constitutive receptor activity. There was no evidence for EP 3 receptor coupling to G q -mediated protein kinase C signaling. Isolated hearts from tg and wt mice were subjected to 60 minutes of no-flow ischemia and 45 minutes of reperfusion. In tg hearts, ischemic contracture was markedly delayed compared with wt hearts, and the ischemia-induced increase in left ventricular end-diastolic pressure was reduced by 55%. Creatine kinase and lactate dehydrogenase release was significantly decreased by 85% and 73%, respectively, compared with wt hearts.

Conclusions— Constitutive prostaglandin EP 3 receptor signaling exerts a protective effect on cardiomyocytes, which is probably G i mediated and results in a remarkable attenuation of myocardial injury during ischemia and reperfusion. Cardioprotective actions of E-type prostaglandins may be mediated by this receptor subtype.

Role of preoptic second messenger systems (cAMP and cGMP) in the febrile response

The present study aimed to test the hypothesis that a decrease in preoptic cAMP mediates fever. To this end, body core temperature (T(c)) of unanesthetized, freely moving rats was monitored by biotelemetry before and after pharmacological modulation of the cAMP pathway, and cAMP levels in the anteroventral third ventricular region (AV3V), where the preoptic region (POA) is located, were determined. We observed that intra-POA administration of the cAMP agonist dibutyryl-cAMP (Db-cAMP, 40 microg) reduced T(c). PGE(2) (the proximal mediator of fever, 200 ng) raised T(c) with a concomitant decrease in AV3V cAMP levels from 22.7+/-1.8 to 17.0+/-1.0 fmol/microg protein. Moreover, PGE(2)-induced fever was impaired by the phosphodiesterase inhibitor aminophylline. In order to verify the interaction between the cAMP- and cGMP-dependent pathways in the POA, we then co-injected Db-cAMP and 8-Br-cGMP into the POA. As a result, 8-Br-cGMP augmented the drop in T(c) evoked by Db-cAMP. Lastly, we observed that intra-POA co-microinjection of the protein kinase A inhibitor (Rp-cAMPS, 1 microg) with the protein kinase G inhibitor (Rp-cGMPS, 1 microg), mimicking the effects of reduced production of cAMP and cGMP, respectively, produced a fever-like response. In summary, the present data support that a decrease in the levels of cAMP and cGMP in the POA is associated with the genesis of fever.