Pim1 maintains telomere length in mouse cardiomyocytes by inhibiting TGFβ signalling

AIMS

Telomere attrition in cardiomyocytes is associated with decreased contractility, cellular senescence, and up-regulation of proapoptotic transcription factors. Pim1 is a cardioprotective kinase that antagonizes the aging phenotype of cardiomyocytes and delays cellular senescence by maintaining telomere length, but the mechanism remains unknown. Another pathway responsible for regulating telomere length is the transforming growth factor beta (TGFβ) signalling pathway where inhibiting TGFβ signalling maintains telomere length. The relationship between Pim1 and TGFβ has not been explored. This study delineates the mechanism of telomere length regulation by the interplay between Pim1 and components of TGFβ signalling pathways in proliferating A549 cells and post-mitotic cardiomyocytes.

METHODS AND RESULTS

Telomere length was maintained by lentiviral-mediated overexpression of PIM1 and inhibition of TGFβ signalling in A549 cells. Telomere length maintenance was further demonstrated in isolated cardiomyocytes from mice with cardiac-specific overexpression of PIM1 and by pharmacological inhibition of TGFβ signalling. Mechanistically, Pim1 inhibited phosphorylation of Smad2, preventing its translocation into the nucleus and repressing expression of TGFβ pathway genes.

CONCLUSION

Pim1 maintains telomere lengths in cardiomyocytes by inhibiting phosphorylation of the TGFβ pathway downstream effectors Smad2 and Smad3, which prevents repression of telomerase reverse transcriptase. Findings from this study demonstrate a novel mechanism of telomere length maintenance and provide a potential target for preserving cardiac function.

Abstract 548: Cellular Contributions to Neonatal Cardiac Regeneration

While the neonatal mouse heart possesses a remarkable ability to regenerate up to a week after birth, the adult mammalian heart is susceptible to irreversible scar formation that impedes cardiac function. Such a scar is formed by the precocious deposition of extracellular matrix (ECM) by resident cardiac fibroblasts (CFs). Unlike their adult counterparts, neonatal CFs in the regenerative widow may have a unique phenotype that contributes to cardiac regeneration and scar resolution. Indeed, the neonatal cardiac ECM secreted by CFs is reported to stimulate regeneration, yet the underlying mechanisms of CF-mediated cardiac repair in the neonate has not been examined. Here, we present a strategy to establish the role of tissue resident CFs in mouse neonatal cardiac regeneration through selective cell depletion and RNA sequencing. Through the initial analysis of published RNA sequencing data, we identified an enrichment of pro-regenerative molecules such as amphiregulin (Areg) during the neonatal regenerative window. Areg, an epidermal growth factor ligand, has been shown to paradoxically stimulate both cardiac repair and pathological fibrosis after adult cardiac injury. To assess its impact on the neonatal cardiac injury response, we developed an adeno-associated virus serotype 9 with the complete coding sequence of mouse Areg (AAV9:Areg) controlled by the cardiomyocyte-specific cardiac troponin T promoter. In vivo , AAV9:Areg treated mice accumulate BrdU+ (proliferative) non-myocytes adjacent to Areg-expressing cardiomyocytes. Ongoing studies are aimed at evaluating the contribution of CFs to neonatal cardiac repair, including whether Areg-dependent cellular changes impact CFs in the neonatal regenerative window in a manner distinct from that in the adult injury response.

Abstract 435: P53 Temporally Regulates the Myofibroblast Activation Response in Pressure Overload

Heart disease is invariably accompanied by the accumulation of resident cardiac fibroblasts (CF) that secrete extracellular matrix and drive the progression of fibrosis and cardiac dysfunction in heart failure. We recently reported that CF exhibit a unique transcriptional response in physiological and pathological remodeling that may underlie the cardioprotective benefits of exercise, and that p53 targets represent the most dysregulated family between the two. To further investigate the role of p53 in pathological cardiac remodeling, we deleted floxed p53 alleles in adult CF using the tamoxifen-inducible Tcf21 MerCreMer mouse (p53-CFKO). p53-CFKO mice initially retain normal cardiac function through 14 days of pressure overload, but eventually progress to heart failure more rapidly than do p53-CFWT, which show gradual functional decline throughout. CF accumulation and fibrosis at these time points match the functional results closely, with p53-CFKO mice protected from both at 14 days relative to p53-CFWT. Conversely, deletion of p53 in myofibroblasts (MF) using the tamoxifen-inducible Postn MerCreMer mouse (p53-MFKO) resulted in no delay in functional loss nor in CF accumulation and fibrosis, suggesting a distinct temporal window of p53 activity in CF exists in pressure overload. In vitro experimentation confirmed that adenovirally-mediated deletion of p53 could delay, but not reverse, myofibroblast activation - and that this irreversible activation is due to induction of Cdkn2a -mediated cell cycle arrest. Furthermore, unbiased single-cell transcriptomic analysis of CF from pressure-overloaded Tcf21 MerCreMer mice at the 14-day functional inflection point revealed p53-CFKO mice have a unique CF cluster defined primarily by a mitotic signature, and this cluster comes at the direct expense of the myofibroblast population. Clusters representing migrating and quiescent CF were unchanged in response to p53. This work describes an inverse correlation of proliferative capacity and myofibroblast activation that indicates p53 exerts master control of the timing of CF accumulation and pathological fibrosis in heart disease.

Abstract 269: Epicardial Deletion of Myocardin-related Transcription Factors Prevent Pathological Cardiac Remodeling After Ischemic Injury

Epicardium-derived progenitor cell (EPDC) activation after injury is a cardio-restorative mechanism in the zebrafish heart, but this response is limited in adult mammals. Identification of mechanisms that regulate adult EPDC function is critical to enhance epicardium-mediated cardiac repair. We previously found that myocardin-related transcription factors (MRTFs) regulate EPDC mobilization and coronary vessel maturation during development. Adult mice lacking both MRTF-A and -B in the epicardium (DKO) were evaluated at baseline and after myocardial infarction (MI) in this study. In non-injured DKO hearts, MRTF expression is reduced in epicardium-derived lineages such as fibroblasts and pericytes, but is unchanged in cardiomyocytes and endothelial cells. DKO mice display increased cellular retention in the epicardium as compared to wildtype mice (WT), consistent with observations during embryonic development. While WT mice demonstrate a decline in cardiac function after MI, ejection fraction is preserved in DKO mice, which also display reduced fibrotic scarring and cardiomyocyte hypertrophy. In order to determine the expression levels of pro-fibrotic genes in epicardial-derived lineages, we isolated CD31-NG2+PDGFRβ+ (pericytes), CD31-NG2-PDGFRβ- (fibroblasts) and CD31+ (endothelial cells) populations from sham and 7-day post-MI hearts. Postn (11-fold), Col1a1 (4-fold), Col3a1 (4.8-fold) and Acta2 (5-fold) are similarly activated in pericyte and fibroblast populations after MI. Of note, pericytes express abundant levels of fibronectin (38-fold vs. 16-fold in fibroblasts) and pro-inflammatory interleukin-1β (41-fold vs 12-fold in fibroblasts) suggesting that pericytes may also contribute to inflammation and fibrosis after cardiac injury. Indeed, NG2+ pericytes are dramatically reduced in remote (5-fold) and damaged regions (3.3-fold) of DKO hearts compared to WT. From this data, we hypothesize that MRTF deletion alters pro-migratory and fibrotic gene programs and reduces inflammatory signals in EPDCs decreasing the presence of scar and improving cardiac function after ischemic injury.

Abstract 1: Sprr2b Drives Proliferation of Cardiac Fibroblasts by Relieving p53-mediated Cell Cycle Inhibition

Pathological cardiac remodeling is initially a compensatory attempt to increase cardiac output, but ultimately leads to the development of fibrosis, a form of scarring that contributes to heart failure (HF). In contrast, physiological cardiac remodeling in response to exercise is not associated with the development of fibrosis and typically remains compensatory. Understanding how cardiac fibroblasts (CF), the primary source of extracellular matrix in the heart, respond to pathological and physiological cues might lead to novel approaches to limit the maladaptive effects of pathological cardiac remodeling. We performed RNA sequencing to define genes that are differentially regulated in CF during physiological (swimming) or pathological (pressure overload) remodeling. This study revealed that cardiac expression of the s mall pr oline r ich 2b ( Sprr2b) gene is restricted to CFs and is significantly elevated in disease and lost in exercise. We demonstrate that SPRR2B drives CF proliferation, but not myofibroblast differentiation, in response to pathological cues. SPRR2B facilitates an interaction between MDM2 and USP7, a nuclear deubiquitinase that leads to proteasomal degradation of p53. SPRR2B-USP7-MDM2 complex formation and p53 degradation is at least partially dependent upon phosphorylation of SPRR2B by Src-family NRTKs. SPRR2B thus relieves p53-mediated constraints on cell cycle progression in response to Src-dependent signaling, leading to CF accumulation. Importantly, SPRR2B expression is elevated in cardiac tissue from human HF patients relative to individuals without heart disease and positively correlates with a proliferative, activated gene expression profile in HF patient CF. Treatment of human HF fibroblasts with IGF-1/H 2 O 2 to mimic physiological cues significantly abrogated SPRR2B expression and increased expression of p53-dependent cell cycle checkpoint genes, which correlated with a less activated phenotype. Taken together, this study defines a unique tissue-specific role of Sprr2b in driving pathological CF cell cycle progression that may underlie the development of cardiac fibrosis.

Abstract 20: Caridomoyocyte Biology Revealed by Fluorescence Ubiquitination-based Cell-cycle Indicators

Existing myocyte contribution to new myocyte formation remains an active area of investigation. Novel experimental methodology is needed to faithfully label cardiomyocyte cell-cycle activity after birth and following injury. The Fluorescence Ubiquitination Cell Cycle Indicator (FUCCI) system can be used to aid visualization of cell cycle activity and progression by monitoring the inverse oscillation dynamics of fluorescently tagged cell cycle fusion proteins AzG-hGeminin and mKO2-hCdt1. Using this system, we hypothesize that cardiomyocytes retain the capacity to cycle throughout postnatal development and re-enter the cell cycle following acute myocardial infarction injury (MI).

A novel cardiac specific FUCCI transgenic mouse model, αMHC-FUCCI, was developed to study cell-cycle dynamics of cardiomyocytes. αMHC-FUCCI hearts were collected throughout postnatal (PN) development to examine cardiomyocyte cell-cycle. Similarly, adult αMHC-FUCCI mice were subjected to MI, injected daily with BrdU and harvested at 3, 7, 10, 14 and 21 days post-MI for further analysis.

Peak incidence of single mKO2-hCdt1 (7%, G 1 ) and AzG-hGem (2%, S/G 2 /M) fluorescence in cardiomyocytes occurs at PN7 and decreases over time as as confirmed by colocalization with BrdU and/or mitotic marker phospho-histone 3. Interestingly, continued mitotic activity exists at PN14 as observed by AzG+/pH3+ myocytes and concurrent mKO2+/AzG+ fluorescence is observed in 60% of adult myocardium by at one month. Together, these results indicate cardiac myocytes remain active at least two weeks after birth and transition into a G1/S phase as opposed to a mitotic exit (G0) as adults. Intriguingly, BrdU+ label is only detected in the non-myocyte interstitial population in and around the border zone through the first two weeks post-MI. BrdU+/AzG+ and or/mKO+ myocytes are detectable at 21days post-MI, indicating a lag in cardiomyocyte cell cycle re-entry. These results suggest myocytes retain the ability to re-enter the cell cycle at low levels three weeks post-MI. Future studies will analyze cardiomyocyte cell-cycle biology in response to diffuse injury and will further elucidate the mechanism behind myocardial regeneration.

Abstract 59: Preservation of Myocardial Structure Is Enhanced by Pim-1 Engineering of Bone Marrow Cells

Rationale: Bone marrow derived cells to treat myocardial injury improve cardiac function and support beneficial cardiac remodeling. However, survival of stem cells is limited due to low proliferation of transferred cells.

Objective: Demonstrate long-term potential of c-kit+ bone marrow stem cells (BMCs) enhanced with Pim-1 kinase to promote positive cardiac remodeling.

Methods and Results: Lentiviral modification of c-kit+ BMCs to express Pim-1 (BMCeP) increases proliferation and expression of pro-survival proteins relative to BMCs expressing GFP (BMCe). Intramyocardial delivery of BMCeP at time of infarction supports improvements in anterior wall dimensions and prevents left ventricle dilation compared to hearts treated with vehicle alone. Reduction of the akinetic left ventricular wall was observed in BMCeP treated hearts at 4 and 12 weeks after infarction. Early recovery of cardiac function in BMCeP-injected hearts facilitated modest improvements in hemodynamic function up to 12 weeks post infarction between cell treated groups. Persistence of BMCeP is improved relative to BMCe within the infarct together with increased recruitment of endogenous c-kit+ cells. Delivery of BMC populations promotes cellular hypertrophy in the border and infarcted regions coupled with an up regulation of hypertrophic genes. Thus, BMCeP treatment yields improved structural remodeling of infarcted myocardium compared to control BMCs.

Conclusions: Genetic modification of BMCs with Pim-1 may serve as a therapeutic approach to promote recovery of myocardial structure. Future approaches may take advantage of salutary BMC actions in conjunction with other stem cell types to increase efficacy of cellular therapy and improve myocardial performance in the injured myocardium.