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βarrestin (βarr)-1 and -2 (βarrs) (or Arrestin-2 and -3, respectively) are universal G protein-coupled receptor (GPCR) adapter proteins expressed abundantly in extra-retinal tissues, including the myocardium. Both were discovered in the lab of the 2012 Nobel Prize in Chemistry co-laureate Robert Lefkowitz, initially as terminators of signaling from the β-adrenergic receptor (βAR), a process known as functional desensitization. They are now known to switch GPCR signaling from G protein-dependent to G protein-independent, which, in the case of βARs and angiotensin II type 1 receptor (AT1R), might be beneficial, e.g., anti-apoptotic, for the heart. However, the specific role(s) of each βarr isoform in cardiac GPCR signaling and function (or dysfunction in disease), remain unknown. The current consensus is that, whereas both βarr isoforms can desensitize and internalize cardiac GPCRs, they play quite different (even opposing in certain instances) roles in the G protein-independent signaling pathways they initiate in the cardiovascular system, including in the myocardium. The present review will discuss the current knowledge in the field of βarrs and their roles in GPCR signaling and function in the heart, focusing on the three most important, for cardiac physiology, GPCR types (β1AR, β2AR & AT1R), and will also highlight important questions that currently remain unanswered.

barrestin (barr)-1 and -2 are ubiquitously (outside the retina) expressed G-protein-coupled receptor adapter proteins. They uncouple G-protein-coupled receptors from G proteins, internalize the receptor, and subsequently initiate their own wave of signaling independently of G proteins. Angiotensin (Ang) II type 1 receptor (AT 1 R) is a well-established example of a receptor signaling through barrs. Despite the pivotal role of brain AT 1 Rs in the regulation of blood pressure, the involvement of barr-dependent signaling, mediated by AT 1 Rs is not well studied. Particularly, in brain astrocytes very little is known about the effects of barrs in AT 1 R signaling. Herein, we utilized a combination of pharmacological and gene manipulation approaches to investigate the role of barrs in AT 1 R-mediated signaling in isolated brainstem astrocytes from spontaneously hypertensive rats (SHRs) and Wistar rats. We observed that barr1 is the predominant arrestin isoform at the protein level in these cells. Its expression was down-regulated in SHR astrocytes compared to Wistar rat astrocytes. Ang II, contrary to observations in SHR astrocytes where it had no effect, upregulates barr1 protein in Wistar rat astrocytes. We observed differential involvement of barr1 in MAPK activation in brainstem astrocytes of SHR versus Wistar rats. The barr-biased agonist peptide [Sar 1 , Il 4 , Il8] Ang-II (SII), induced AT 1 Rmediated ERK and p38 activation in Wistar rat astrocytes. SII had no effect on ERK and p38 activation in SHRs brainstem astrocytes. Our results indicate, reduced involvement of barr1 in dampening Ang II-induced MAPKs activation and diminished barr1-mediated ERK and p38 activation in SHR brainstem astrocytes. These findings might be mechanistically related to the development of the brain renin-angiotensinaldosterone system hyperactivity, which leads to pathogenesis of the hypertensive state of the SHR model.

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ObjectiveTo investigate the role of βarrestin1 in angiotensin II (AngII) type 1 receptor (AT1R)‐mediated signaling in astrocytes isolated from the brainstem of spontaneously hypertensive rats (SHRs) or normotensive rats.BackgroundThe βarrestins (βarrestin1 and‐2) terminate G protein signaling by G protein‐coupled receptors (GPCRs) and, at the same time, they initiate their own “second wave” of signaling from GPCRs, independently of G proteins. The AT1R is one such GPCR that signals via both G proteins and βarrestins in various cell types. It is unknown whether the AT1R signals via βarrestins in the brain. We sought to examine the involvement of βarrestins in the AT1R‐mediated Extracellular signal‐regulated kinase (ERK1/2) mitogen‐activated protein kinase (MAPK) activation and angiotensinogen (AGT) expression in Spontaneously Hypertensive Rat (SHR)‐versus normotensive rat‐derived primary astrocytes.MethodsPrimary cultures of brainstem astrocytes were isolated from the brains of 2–3 days old pups. 100 nM AngII and 10 μM SII (an AngII peptide analog which is a βarrestin‐biased agonist) were used to study AT1R‐mediated ERK1/2 activation and AGT expression. Protein analysis was done using Western blotting and mRNA measurement was done using real‐time PCR. The results in SHR astrocytes were compared to normotensive Wistar rat astrocytes. Protein analysis was done using Western blotting and mRNA measurement using real time PCR.ResultsOur results indicate that βarrestin1 is the major βarrestin isoform in astrocytes from both SHR and normotensive rat brainstem astrocytes. AngII activates ERK1/2 in SHR astrocytes more robustly than in normotensive rat astrocytes. On the other hand, SII strongly activates ERK1/2 in normotensive rat astrocytes and only minimally in SHR astrocytes. In addition, SII, similarly to AngII, upregulates cytoplasmic AGT protein synthesis in both SHR and normotensive rat astrocytes. However, SII has no significant effects on AGT mRNA levels in brainstem astrocytes from either rat model.ConclusionIn this study, we show that AngII‐AT1R‐induced ERK1/2 activation in SHR astrocytes appears to be almost exclusively G protein‐dependent, contrary to normotensive rat astrocytes. Moreover, the AT1R‐mediated βarrestin1‐dependent signaling pathway plays a role in the regulation of AGT protein synthesis in rat astrocytes derived from the brain stem region in both models.Support or Funding Information* This research was funded by Nova Southeastern University President's Faculty Research & Development Grant and Health Profession Division Grant.