Different phosphorylation patterns regulate α1D-adrenoceptor signaling and desensitization.

Human α1D-adrenoceptors (α1D-ARs) are a group of the seven transmembrane-spanning proteins that mediate many of the physiological and pathophysiological actions of adrenaline and noradrenaline. Although it is known that α1D-ARs are phosphoproteins, their specific phosphorylation sites and the kinases involved in their phosphorylation remain largely unknown. Using a combination of in silico analysis, mass spectrometry and site directed mutagenesis, we identified distinct α1D-AR phosphorylation patterns during noradrenaline- or phorbol ester-mediated desensitizations. We found that the G protein coupled receptor kinase, GRK2, and conventional protein kinases C isoforms α/ß, phosphorylate α1D-AR during these processes. Furthermore, we showed that the phosphorylated residues are located in the receptor's third intracellular loop (S300, S323, T328, S331, S332, S334) and carboxyl region (S441, T442, T477, S486, S492, T507, S515, S516, S518, S543) and are conserved among orthologues but are not conserved among the other human α1-adrenoceptor subtypes. Additionally, we found that phosphorylation in either the third intracellular loop or carboxyl tail was sufficient to regulate calcium signaling desensitization. By contrast, mutations in either of these two domains significantly altered mitogen activated protein kinase (ERK) pathway and receptor internalization, suggesting that they have differential regulatory mechanisms. Our data provide new insights into the functional repercussions of these posttranslational modifications in signaling outcomes and desensitization.

Protein Kinase C Activation Promotes α1B-Adrenoceptor Internalization and Late Endosome Trafficking through Rab9 Interaction. Role in Heterologous Desensitization.

Upon agonist stimulation, α1B-adrenergic receptors couple to Gq proteins, calcium signaling and protein kinase C activation; subsequently, the receptors are phosphorylated, desensitized, and internalized. Internalization seems to involve scaffolding proteins, such as ß-arrestin and clathrin. However, the fine mechanisms that participate remain unsolved. The roles of protein kinase C and the small GTPase, Rab9, in α1B-AR vesicular traffic were investigated by studying α1B-adrenergic receptor-Rab protein interactions, using Förster resonance energy transfer (FRET), confocal microscopy, and intracellular calcium quantitation. In human embryonic kidney 293 cells overexpressing Discosoma spp. red fluorescent protein (DsRed)-tagged α1B-ARs and enhanced green fluorescent protein--tagged Rab proteins, pharmacological protein kinase C activation mimicked α1B-AR traffic elicited by nonrelated agents, such as sphingosine 1-phosphate (i.e., transient α1B-AR-Rab5 FRET signal followed by a sustained α1B-AR-Rab9 interaction), suggesting brief receptor localization in early endosomes and transfer to late endosomes. This latter interaction was abrogated by blocking protein kinase C activity, resulting in receptor retention at the plasma membrane. Similar effects were observed when a dominant-negative Rab9 mutant (Rab9-GDP) was employed. When α1B-adrenergic receptors that had been mutated at protein kinase C phosphorylation sites (S396A, S402A) were used, phorbol ester-induced desensitization of the calcium response was markedly decreased; however, interaction with Rab9 was only partially decreased and internalization was observed in response to phorbol esters and sphingosine 1-phosphate. Finally, Rab9-GDP expression did not affect adrenergic-mediated calcium response but abolished receptor traffic and altered desensitization. Data suggest that protein kinase C modulates α1B-adrenergic receptor transfer to late endosomes and that Rab9 regulates this process and participates in G protein-mediated signaling turn-off.

Glycogen Synthase Kinase-3 modulates α1A-adrenergic receptor action and regulation.

The possibility that glycogen synthase kinase 3 (GSK3) could modulate α1A-adrenergic receptor (α1A-AR) function and regulation was tested employing LNCaP and HEK293 cells transfected to express the enhanced green fluorescent protein-tagged human α1A-AR. Receptor phosphorylation and internalization, intracellular free calcium, α1A-AR-GSK3 colocalization, and coimmunoprecipitation were studied. The effects of the pharmacological GSK3 inhibitor, SB-216763, and the coexpression of a dominant-negative mutant of this kinase, as well as the signaling, desensitization, and internalization of receptors with S229, S258, S352, and S381 substitutions for alanine or aspartate, were also determined. SB-216763 inhibited agonist- and phorbol myristate acetate (PMA)-mediated α1A-AR phosphorylation, reduced oxymetazoline-induced desensitization, and magnified that induced by PMA. Agonists and PMA increased receptor-GSK3 colocalization and coimmunoprecipitation. Expression of a dominant-negative GSK3 mutant reduced agonist- but not PMA-induced receptor internalization. α1A-AR with the GSK3 putative target sites mutated to alanine exhibited reduced phosphorylation and internalization in response to agonists and increased PMA-induced desensitization. Agonist-induced, but not PMA-induced, receptor-ß arrestin intracellular colocalization was diminished in cells expressing the GSK3 putative target sites mutated to alanine. Our data indicated that GSK3 exerts a dual action on α1A-AR participating in agonist-mediated desensitization and internalization and avoiding PMA-induced desensitization.

The role of ß-arrestins in G protein-coupled receptor heterologous desensitization: A brief story.

G protein-coupled receptors (GPCRs) are transmembrane proteins that have an important impact in a myriad of cellular functions. Posttranslational modifications on GPCRs are a key processes that allow these proteins to recruit other intracellular molecules. Among these modifications, phosphorylation is the most important way of desensitization of these receptors. Several research groups have described two different desensitization mechanisms: heterologous and homologous desensitization. The first one involves the phosphorylation of the receptors by protein kinases, such as PKC, following the desensitization and internalization of the receptor, while the second one involves the phosphorylation of the receptors by GRKs, allowing for the receptor to recruit ß-arrestins to be desensitized and internalized. Interestingly, a few number of studies have described the participation of ß-arrestins during the heterologous desensitization process. Hence, the aim of this review is to briefly explore the role that ß-arrestins play during the heterologous desensitization of several GPCRs.

Distinct phosphorylation sites/clusters in the carboxyl terminus regulate α1D-adrenergic receptor subcellular localization and signaling.

The human α1D-adrenergic receptor is a seven transmembrane-domain protein that mediates many of the physiological actions of adrenaline and noradrenaline and participates in the development of hypertension and benign prostatic hyperplasia. We recently reported that different phosphorylation patterns control α1D-adrenergic receptor desensitization. However, to our knowledge, there is no data regarding the role(s) of this receptor's specific phosphorylation residues in its subcellular localization and signaling. In order to address this issue, we mutated the identified phosphorylated residues located on the third intracellular loop and carboxyl tail. In this way, we experimentally confirmed α1D-AR phosphorylation sites and identified, in the carboxyl tail, two groups of residues in close proximity to each other, as well as two individual residues in the proximal (T442) and distal (S543) regions. Our results indicate that phosphorylation of the distal cluster (T507, S515, S516 and S518) favors α1D-AR localization at the plasma membrane, i. e., substitution of these residues for non-phosphorylatable amino acids results in the intracellular localization of the receptors, whereas phospho-mimetic substitution allows plasma membrane localization. Moreover, we found that T442 phosphorylation is necessary for agonist- and phorbol ester-induced receptor colocalization with ß-arrestins. Additionally, we observed that substitution of intracellular loop 3 phosphorylation sites for non-phosphorylatable amino acids resulted in sustained ERK1/2 activation; additional mutations in the phosphorylated residues in the carboxyl tail did not alter this pattern. In contrast, mobilization of intracellular calcium and receptor internalization appear to be controlled by the phosphorylation of both third-intracellular-loop and carboxyl terminus-domain residues. In summary, our data indicate that a) both the phosphorylation sites present in the third intracellular loop and in the carboxyl terminus participate in triggering calcium signaling and in turning-off α1D-AR-induced ERK activation; b) phosphorylation of the distal cluster appears to play a role in receptor's plasma membrane localization; and c) T442 appears to play a critical role in receptor phosphorylation and receptor-ß-arrestin colocalization.

Sites phosphorylated in human α1B-adrenoceptors in response to noradrenaline and phorbol myristate acetate.

Phosphorylation of the human α1B-adrenergic receptor (fused with the green fluorescent protein) was studied employing the inducible Flp-ln HEK293 T-Rex system for expression. Serine/alanine substitutions were performed in five sites corresponding to those previously identified as phosphorylation targets in the hamster ortholog. Desensitization was decreased in these mutants but receptor phosphorylation was still clearly detected. The protein phosphorylation of the wild-type receptor (fused to the green fluorescent protein) was studied, using mass spectrometry, under baseline and stimulated conditions (noradrenaline or phorbol myristate acetate). Basal phosphorylation was detected at sites located at the intracellular loop 3 and carboxyl terminus, and the number of sites detected increased under agonist activation and stimulation of protein kinase C. The phosphorylation patterns differed under the distinct conditions. Three of the phosphorylation sites detected in this work corresponded to those observed in the hamster receptor. The phosphorylation sites detected included the following: a) at the intracellular loop 3: serines 246, 248, 257, 267, and 277; and threonines 252, 264, and 268, and b) at the carboxyl terminus: serines 396, 400, 402, 406, 423, 425, 427, 455, and 470, and threonines 387, 392, 420, and 475. Our data indicate that complex phosphorylation patterns exist and suggest the possibility that such differences could be relevant in receptor function and subcellular localization.

Role of age and comorbidities in mortality of patients with infective endocarditis.

PURPOSE: The aim of this study was to analyse the characteristics of patients with IE in three groups of age and to assess the ability of age and the Charlson Comorbidity Index (CCI) to predict mortality. METHODS: Prospective cohort study of all patients with IE included in the GAMES Spanish database between 2008 and 2015.Patients were stratified into three age groups:<65 years,65 to 80 years,and ≥ 80 years.The area under the receiver-operating characteristic (AUROC) curve was calculated to quantify the diagnostic accuracy of the CCI to predict mortality risk. RESULTS: A total of 3120 patients with IE (1327 < 65 years;1291 65-80 years;502 ≥ 80 years) were enrolled.Fever and heart failure were the most common presentations of IE, with no differences among age groups.Patients ≥80 years who underwent surgery were significantly lower compared with other age groups (14.3%,65 years; 20.5%,65-79 years; 31.3%,≥80 years). In-hospital mortality was lower in the <65-year group (20.3%,<65 years;30.1%,65-79 years;34.7%,≥80 years;p < 0.001) as well as 1-year mortality (3.2%, <65 years; 5.5%, 65-80 years;7.6%,≥80 years; p = 0.003).Independent predictors of mortality were age ≥ 80 years (hazard ratio [HR]:2.78;95% confidence interval [CI]:2.32-3.34), CCI ≥ 3 (HR:1.62; 95% CI:1.39-1.88),and non-performed surgery (HR:1.64;95% CI:11.16-1.58).When the three age groups were compared,the AUROC curve for CCI was significantly larger for patients aged <65 years(p < 0.001) for both in-hospital and 1-year mortality. CONCLUSION: There were no differences in the clinical presentation of IE between the groups. Age ≥ 80 years, high comorbidity (measured by CCI),and non-performance of surgery were independent predictors of mortality in patients with IE.CCI could help to identify those patients with IE and surgical indication who present a lower risk of in-hospital and 1-year mortality after surgery, especially in the <65-year group.

Dissecting the signaling features of the multi-protein complex GPCR/ß-arrestin/ERK1/2.

G protein-coupled receptors (GPCRs) have emerged as key biological entities that regulate a plethora of physiological processes and participate in the onset and development of many diseases. Moreover, these receptors are important targets of almost 25% of the current therapeutic drugs in the market. Upon agonist binding, GPCRs activate a great number of signaling pathways, resulting in important cellular events like gene transcription, survival, proliferation and differentiation. In order to activate such events, GPCRs interact with a variety of scaffold and molecular entities, particularly with G proteins, but also with ß-arrestins and the extracellular signal-regulated kinases 1 and 2 (ERK1/2) pathway, forming unique signaling modules. The aim of this review is to analyze the signaling features of the multi-protein complex GPCR-ß-arrestin-ERK1/2, a unique signaling module that has received considerable attention from different research groups due to its molecular and physiological roles in diverse cellular contexts.

A549 cells as a model to study endogenous LPA1 receptor signaling and regulation.

Lysophosphatidic acid (LPA) modulates the function of many organs, including the lung. A549 is a lung carcinoma-derived cell line, frequently used as a model for type II pneumocytes. Here we show that these cells expressed messenger RNA coding for LPA1-3 receptors with the following order of abundance: LPA1 > LPA2 > LPA3 and that LPA was able to increase intracellular calcium, extracellular signal-regulated kinases 1/2 phosphorylation, and cell contraction. These effects were blocked by Ki16425, an antagonist selective for LPA1 and LPA3 receptors, and by the LPA1-selective antagonist, AM095. Activation of protein kinase C inhibited LPA-induced intracellular calcium increase. This action was blocked by protein kinase C inhibitors and enzyme down-regulation. Phorbol myristate acetate and AM095, but not Ki16425, decreased the baseline intracellular calcium concentration. Ki16425 blocked the effect of AM095 but not that of phorbol myristate acetate. The data indicate that LPA1 receptors exhibit constitutive activity and that AM095 behaves as an inverse agonist, whereas Ki16425 appears to be a classic antagonist. Furthermore, the LPA agonist, 1-oleoyl-2-O-methyl-rac-glycerophosphothionate, OMPT, induced a weak increase in intracellular calcium, but was able to induce full ERK 1/2 phosphorylation and cell contraction. These effects were blocked by AM095. These data suggest that OMPT is a biased LPA1 agonist. A549 cells express functional LPA1 receptors and seem to be a suitable model to study their signaling and regulation.

Cardiomiopatia de Takotsubo: Um diagnóstico diferencial da síndrome coronariana aguda: revisão de literatura / Takotsubo cardiomyopathy: a differential diagnosis of the acute coronary syndrome: review of the literature

A cardiomiopatia de Takotsubo, também conhecida como síndrome do balonamento apical transitório do ventrículo esquerdo, foi recentemente descrita no Japão, frequentemente desencadeada por intenso estresse, que apresenta características clínicas, eletrocardiográficas e laboratoriais semelhantes à síndrome coronariana aguda. A realização de um estudo hemodinâmico é, portanto, crucial para sua confirmação diagnóstica, desde que a abordagem terapêutica do infarto agudo do miocárdio é diferente da cardiomiopatia de Takotsubo. O objetivo deste artigo é esclarecer a fisiopatologia, diagnóstico e tratamento dessa entidade nosológica. Presume-se que 1 a 2% dos diagnósticos iniciais de síndrome coronariana aguda sejam cardiomiopatia de Takotsubo, que só é confirmada após a realização de cineangiocoronariografia e ventriculografia. A sua fisiopatologia permanece indefinida, mas sabe-se que deve ser tratada diferentemente do infarto agudo do miocárdio e seu prognóstico é mais favorável.

Takotsubo cardiomyopathy, also known as transient apical ballooning syndrome of the left ventricle, was recently described in Japan, often triggered by intense stress, which presents clinical, electrocardiographic and laboratory findings similar to acute coronary syndrome. The realization of a hemodynamic study is therefore crucial for diagnostic confirmation, since the therapeutic approaches to acute myocardial infarction is different from Takotsubo cardiomyopathy.The aim of this paper is to clarify the pathophysiology, diagnosis and treatment of this illness. It is assumed that 1-2% of the initial diagnosis of acute coronary syndrome are Takotsubo cardiomyopathy, which is confirmed only after coronary angiography and ventriculography are performed. Its pathophysiology remains unclear, but it is known it must to be treated differently from acute myocardial infarction and its prognosis is more favorable.