Adducin Regulates Sarcomere Disassembly During Cardiomyocyte Mitosis.

BACKGROUND: Recent interest in understanding cardiomyocyte cell cycle has been driven by potential therapeutic applications in cardiomyopathy. However, despite recent advances, cardiomyocyte mitosis remains a poorly understood process. For example, it is unclear how sarcomeres are disassembled during mitosis to allow the abscission of daughter cardiomyocytes. METHODS: Here, we use a proteomics screen to identify adducin, an actin capping protein previously not studied in cardiomyocytes, as a regulator of sarcomere disassembly. We generated many adeno-associated viruses and cardiomyocyte-specific genetic gain-of-function models to examine the role of adducin in neonatal and adult cardiomyocytes in vitro and in vivo. RESULTS: We identify adducin as a regulator of sarcomere disassembly during mammalian cardiomyocyte mitosis. α/γ-adducins are selectively expressed in neonatal mitotic cardiomyocytes, and their levels decline precipitously thereafter. Cardiomyocyte-specific overexpression of various splice isoforms and phospho-isoforms of α-adducin in identified Thr445/Thr480 phosphorylation of a short isoform of α-adducin as a potent inducer of neonatal cardiomyocyte sarcomere disassembly. Concomitant overexpression of this α-adducin variant along with γ-adducin resulted in stabilization of the adducin complex and persistent sarcomere disassembly in adult mice, which is mediated by interaction with α-actinin. CONCLUSIONS: These results highlight an important mechanism for coordinating cytoskeletal morphological changes during cardiomyocyte mitosis.

Medical student residency preferences and motivational factors: a longitudinal, single-institution perspective.

BACKGROUND: A high proportion of medical school graduates pursue specialties different from those declared at matriculation. While these choices influence the career paths, satisfaction, and potential regret students will experience, they also impact the supply and demand ratio of the shorthanded physician workforce across many specialties. In this study, we investigate how the choice of medical specialty and the factors motivating those choices change between the beginning and end of medical school training. METHODS: A questionnaire was administered annually from 2017 to 2020 to a cohort of medical students at the University of Connecticut to determine longitudinal preferences regarding residency choice, motivational factors influencing residency choice, future career path, and demographic information. RESULTS: The questionnaire respondent totals were as follows: n = 76 (Year 1), n = 54 (Year 2), n = 31 (Year 3), and n = 65 (Year 4). Amongst newly matriculated students, 25.0% were interested in primary care, which increased ~ 1.4-fold to 35.4% in the final year of medical school. In contrast, 38.2% of matriculated students expressed interest in surgical specialties, which decreased ~ 2.5-fold to 15.4% in the final year. Specialty choices in the final year that exhibited the largest absolute change from matriculation were orthopedic surgery (- 9.9%), family medicine (+ 8.1%), radiology (+ 7.9%), general surgery (- 7.2%), and anesthesiology (+ 6.2%). Newly matriculated students interested in primary care demonstrated no differences in their ranking of motivational factors compared to students interested in surgery, but many of these factors significantly deviated between the two career paths in the final year. Specifically, students interested in surgical specialties were more motivated by the rewards of salary and prestige compared to primary care students, who more highly ranked match confidence and family/location factors. CONCLUSIONS: We identified how residency choices change from the beginning to the end of medical school, how certain motivational factors change with time, how these results diverge between primary care and surgery specialty choice, and propose a new theory based on risk-reward balance regarding residency choice. Our study promotes awareness of student preferences and may help guide school curricula in developing more student-tailored training approaches. This could foster positive long-term changes regarding career satisfaction and the physician workforce.

Sarcomere function activates a p53-dependent DNA damage response that promotes polyploidization and limits in vivo cell engraftment.

Human cardiac regeneration is limited by low cardiomyocyte replicative rates and progressive polyploidization by unclear mechanisms. To study this process, we engineer a human cardiomyocyte model to track replication and polyploidization using fluorescently tagged cyclin B1 and cardiac troponin T. Using time-lapse imaging, in vitro cardiomyocyte replication patterns recapitulate the progressive mononuclear polyploidization and replicative arrest observed in vivo. Single-cell transcriptomics and chromatin state analyses reveal that polyploidization is preceded by sarcomere assembly, enhanced oxidative metabolism, a DNA damage response, and p53 activation. CRISPR knockout screening reveals p53 as a driver of cell-cycle arrest and polyploidization. Inhibiting sarcomere function, or scavenging ROS, inhibits cell-cycle arrest and polyploidization. Finally, we show that cardiomyocyte engraftment in infarcted rat hearts is enhanced 4-fold by the increased proliferation of troponin-knockout cardiomyocytes. Thus, the sarcomere inhibits cell division through a DNA damage response that can be targeted to improve cardiomyocyte replacement strategies.

A Contraction Stress Model of Hypertrophic Cardiomyopathy due to Sarcomere Mutations.

Thick-filament sarcomere mutations are a common cause of hypertrophic cardiomyopathy (HCM), a disorder of heart muscle thickening associated with sudden cardiac death and heart failure, with unclear mechanisms. We engineered four isogenic induced pluripotent stem cell (iPSC) models of ß-myosin heavy chain and myosin-binding protein C3 mutations, and studied iPSC-derived cardiomyocytes in cardiac microtissue assays that resemble cardiac architecture and biomechanics. All HCM mutations resulted in hypercontractility with prolonged relaxation kinetics in proportion to mutation pathogenicity, but not changes in calcium handling. RNA sequencing and expression studies of HCM models identified p53 activation, oxidative stress, and cytotoxicity induced by metabolic stress that can be reversed by p53 genetic ablation. Our findings implicate hypercontractility as a direct consequence of thick-filament mutations, irrespective of mutation localization, and the p53 pathway as a molecular marker of contraction stress and candidate therapeutic target for HCM patients.