Microtubule-Mediated Regulation of β ₂ AR Translation and Function in Failing Hearts

Background: β 1 AR (beta-1 adrenergic receptor) and β 2 AR (beta-2 adrenergic receptor)-mediated cyclic adenosine monophosphate signaling has distinct effects on cardiac function and heart failure progression. However, the mechanism regulating spatial localization and functional compartmentation of cardiac β-ARs remains elusive. Emerging evidence suggests that microtubule-dependent trafficking of mRNP (messenger ribonucleoprotein) and localized protein translation modulates protein compartmentation in cardiomyocytes. We hypothesized that β-AR compartmentation in cardiomyocytes is accomplished by selective trafficking of its mRNAs and localized translation. Methods: The localization pattern of β-AR mRNA was investigated using single molecule fluorescence in situ hybridization and subcellular nanobiopsy in rat cardiomyocytes. The role of microtubule on β-AR mRNA localization was studied using vinblastine, and its effect on receptor localization and function was evaluated with immunofluorescent and high-throughput Förster resonance energy transfer microscopy. An mRNA protein co-detection assay identified plausible β-AR translation sites in cardiomyocytes. The mechanism by which β-AR mRNA is redistributed post–heart failure was elucidated by single molecule fluorescence in situ hybridization, nanobiopsy, and high-throughput Förster resonance energy transfer microscopy on 16 weeks post–myocardial infarction and detubulated cardiomyocytes. Results: β 1 AR and β 2 AR mRNAs show differential localization in cardiomyocytes, with β 1 AR found in the perinuclear region and β 2 AR showing diffuse distribution throughout the cell. Disruption of microtubules induces a shift of β 2 AR transcripts toward the perinuclear region. The close proximity between β 2 AR transcripts and translated proteins suggests that the translation process occurs in specialized, precisely defined cellular compartments. Redistribution of β 2 AR transcripts is microtubule-dependent, as microtubule depolymerization markedly reduces the number of functional receptors on the membrane. In failing hearts, both β 1 AR and β 2 AR mRNAs are redistributed toward the cell periphery, similar to what is seen in cardiomyocytes undergoing drug-induced detubulation. This suggests that t-tubule remodeling contributes to β-AR mRNA redistribution and impaired β 2 AR function in failing hearts. Conclusions: Asymmetrical microtubule-dependent trafficking dictates differential β 1 AR and β 2 AR localization in healthy cardiomyocyte microtubules, underlying the distinctive compartmentation of the 2 β-ARs on the plasma membrane. The localization pattern is altered post–myocardial infarction, resulting from transverse tubule remodeling, leading to distorted β 2 AR-mediated cyclic adenosine monophosphate signaling.