The Function of Llgl1 in mediating intercalated disc composition and cardiomyocyte proliferation
Authors: Lovett Jerrell , Lovett Jerrell , O'meara Caitlin 
Published: May 2023

Introduction: The loss of functional cardiomyocytes (CMs) coupled with the limited regenerative capacity of the adult heart leads to the development of systolic heart failure. Thus, there is a great interest in exploring novel therapeutics that promote remuscularization of the heart after injury by stimulating proliferation of pre-existing adult CMs. There is currently poor understanding of how the cell structural changes are regulated to promote cell cycle re-entry, and importantly the factors that facilitate re-integration of CMs into the functional myocardium.Considering the importance of the intercalated disc (ICD) and CM cytoskeletal structure in regulating cell signaling and maintaining cardiac function, we aim to understand the proteins that control CM cytoarchitecture and re-establishment of ICD during cardiac development, regeneration, and repair. In preliminary studies, we generated a panel of zebrafish loss-of-function mutants for candidate genes reported to interact with the Hippo-Yap pathway; a highly conserved signaling pathway with profound influence on cardiomyocyte regeneration. From this screen, we identified Lethal giant larvae protein homolog 1 (Llgl1) as a regulator of CM shape, size, and heart development in the zebrafish heart during development or regeneration. Subsequently, we developed novel genetic mouse model to test the hypothesis that Llgl1 expression in mammalian CMs maintainsintercalated disc composition and suppressescell cycle activity during homeostasis, development, and following injury. Methods and Results: Llgl1’s role on mammalian CM development, morphology, or cell cycle activity is completely unexplored in mammals. We crossed the Llgl1 floxed with the transgenic Myh6CRE mouse wherein the CM-specific Myh6 promoter constitutively drives expression of Cre-Recombinase to specifically delete Llgl1 from CMs. By PCR, we detected the expected recombination event in DNA extracted from hearts of Llgl1fl/fl;Myh6CRE animals. We additionally confirmed our genetic model via qPCR. We found that 21-day old juvenile Llgl1fl/fl Myh6CRE mice had increased yap nuclear localization, a decrease in the intercalated disc proteins, with an increased cell cycle activity compared to their littermate controls. Importantly, Llgl1 depleted mice displayed a significant increase in CM cytokinesis in uninjured mice at a timepoint where proliferation is significantly diminished. Conclusions: Llgl1 depletion in cardiomyocytes modulated intercalated disc composition and increased CM proliferation in juvenile animals. Subsequent studies aim to investigate the cellular and physiological impact of CM deletion of Llgl1 following cardiac injury in neonatal and adult mice. In addition, we seek to determine the mechanism by which Llgl1 regulates yap in mammalian CMs. We hope to gain mechanistic insight of Llgl1 siganaling in CMs, which is necessary to establish Llgl1 as a novel target for treating heart failure. National Heart Lung and Blood Diversity Supplement Fellowship Award This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.