p38-mitogen activated protein kinases (p38-MAPKs) are stress activated serine/threonine kinases that are activated during several different cardiac pathologies. Classically, studies have focused solely on p38α signaling in the heart. However, there is also high cardiac expression of the p38γ isoform but little is known about its cardiac function. The aim of this study was to elucidate the signaling pathway of p38γ, with a particular focus on its role in the progression of pathological cardiac hypertrophy. Comparisons of cardiac function and structure of wild type (WT) and p38γ knock out (KO) mice, in response to abdominal aortic banding, found that KO mice developed less ventricular hypertrophy than their corresponding WT controls, and have preserved cardiac function. Basal p38γ myocardial staining was primarily localised at the membranes and throughout the cytoplasm. Following aortic constriction, nuclear staining of p38γ increased, but no accumulation of p38α was observed. This suggests that the two isoforms play distinct roles in the heart. To elucidate its signaling pathway, we generated an analogue sensitive p38γ, which is mutated at a gatekeeper residue, to specifically track and identify its endogenous substrates in the myocardium. The mutation allows only the mutant kinase, but not WT kinases, to utilise analogues of ATP that are expanded at the N6 position and contain a detectable tag on the γ-phosphate. Transfer of this tag to substrates allows subsequent isolation and identification. Furthermore, unlike other p38-MAPKs, p38γ contains a C-terminal PDZ domain interacting motif. We have utilised this motif in co pull-down assays to identify interacting proteins of p38γ in the heart. Using these techniques we have identified, amongst other substrates, LDB3 and calpastatin as novel substrates of p38γ and we have determined the residues that are targeted for phosphorylation. Lastly we have shown that phosphorylation of calpastatin reduces its efficiency as a calpain inhibitor in vitro, hence proposing a mechanism by which p38γ may mediate its pro-hypertrophic role.

Protein synthesis (mRNA) is tightly regulated under numerous conditions in cardiomyocytes. It can be activated by hormones such as insulin and also by other agents such as phenylephrine (PE) that activates hypertrophy in the heart. Cardiac hypertrophy involves an increase in the muscle mass of the heart, principally in the left ventricular muscle, and the increase is due to enlarged cell size, not increased cell number. A pivotal element of cardiac hypertrophy is an elevation in the rates of protein synthesis, which drives the increase in cell size causing hypertrophy. Unfortunately, we currently lack the understanding of the basic mechanisms that drives hyperactivated protein synthesis. Cardiac hypertrophy is clinically important because it is a major risk factor for heart failure. It initially serves as an adaptive response to increase cardiac output in response to higher demand, but ultimately leads to deterioration of contractility of the heart if hypertrophy is sustained. The main goal of this research project is to understand how hypertrophic agents, such as phenylephrine (PE), activate protein synthesis using adult rat ventricular cardiomyocytes as a model. Specifically, this study focuses on how the translational initiation is controlled by upstream signalling pathways. / Medicine, Faculty of / Biochemistry and Molecular Biology, Department of / Graduate

Cardiac hypertrophy is an adaptive response in which the heart grows to normalize output during times of increased demand. This increase in size originates from the growth of cardiomyocytes rather than cellular division. Many cellular modifications observed during hypertrophy are reminiscent of apoptosis; caspase proteases, traditionally known for their role in apoptosis, have recently been implicated in non-apoptotic settings including cardiac differentiation. Studies have reported caspase-3 inhibition limits the heart`s ability to undergo pathological hypertrophy in vivo. Data presented here indicate that inhibition of caspase-3 and caspase-8 minimizes hypertrophic growth in primary cardiomyocytes. Phenylephrine induced an increase in cell size, which was attenuated upon addition of caspase inhibitors. These data suggest these proteins may be involved in hypertrophic growth of cardiomyocytes. Furthermore, results suggest that increased caspase activity may not be directly responsible for this effect. Rather, subcellular localization of caspase proteases may contribute to the effects seen during hypertrophy.

This study examined the effect of resistance training proximity to failure on strength, muscle hypertrophy, and fatigue. Fourteen men were randomized into two groups (4-6 rating of perceived exertion-RPE per set or 7-9 RPE per set) and completed an eight-week program. Squat and bench press strength, muscle thickness, subjective fatigue, muscle soreness, and biomarkers (creatine kinase-CK and lactate dehydrogenase-LDH) were assessed. There were no significant differences (p>0.05) in the rate of strength gains and equivalence testing revealed hypertrophy was not statistically similar nor different. All results for indirect markers of muscle damage and fatigue indicated similar recovery between groups within 48 hours; however, a small between group effect size (g=0.39) existed indicating higher session RPE in the 7-9 RPE group across the entire training program. These results suggest that strength and possibly hypertrophy outcomes are similar when training each set to 4-6 RPE or 7-9 RPE in trained men. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2021. / FAU Electronic Theses and Dissertations Collection

Heart failure (HF) is a pathological state indicating insufficient blood supply to the peripheral tissues from the heart. The pathophysiology of HF is multifactorial like cardiac remodeling including cardiac hypertrophy, perivascular fibrosis and apoptosis to compensate for the heart’s inability to pump enough blood. Cardiac hypertrophy is initially adaptive to hemodynamic overload; however, it chronically contributes to heart failure and sudden cardiac death. The extracellular regulatory factors and intracellular signaling pathways involved in the cardiac remodeling are not yet fully clear. PI3-kinase is an important intracellular kinase in organ size control. Cardiac overexpression of Class I PI3-kinase caused heart enlargement in transgenic mice. Autophagy as a dynamic process involving the degradation of damaged mitochondria prevents ROS overproduction which leads to the cardiac remodeling. Therefore, our aim was to study the relationship between PI3-kinases and Ang II-induced cardiac remodeling via an autophagy-dependent mechanism. Ang II significantly increased autophagy with two distinctive phases: an increasing phase at low doses and a decreasing phase at high doses in cardiomyocytes. The Ang II-induced autophagic depression was attenuated by a Class I PI3-kinase inhibitor and potentiated by Class III PI3-kinase inhibitor. Besides, Ang II-induced cardiac hypertrophy and mitochondria ROS generation were attenuated via blockade of Class I PI3-kinase or mTOR. To further validate our in vitro data, we studied the role of Class I PI3-kinase in Ang II-induced cardiac remodeling in vivo. We successfully transferred Lv-DNp85 (Class I PI3-kinase blockade) and Lv-GFP (control) into adult rat hearts and found that cardiac transfer of Lv-DNp85 did not alter Ang II-induced pressor effect, but attenuated Ang II-induced cardiac hypertrophy, perivascular fibrosis and cardiac dysfunction. Ang II-induced cardiac remodeling was associated with impaired autophagy and mitochondrial ROS overproduction, which were significantly attenuated by Lv-DNp85-induced blockade of Class I PI3-kinase. Taken together, these data suggest that Class I PI3-kinase is involved in Ang II-induced impairment of autophagy via Akt/mTOR pathway, leading to mitochondrial ROS overproduction and cardiac remodeling. These results are not only highly significant from a pathophysiological perspective, but also have important pharmacological implications in the control of cardiac hypertrophy to prevent decompensation and failure in cardiac function. / National Institute of Neurological Disorders and Stroke / National Institutes of Health (NIH, NS55008)

Left ventricular hypertrophy (LVH) is more frequently associated with LV dilatation and systolic chamber dysfunction in males than in females. The mechanisms of this effect are uncertain. As excessive adrenergic stimulation may be responsible for LV dilatation and systolic chamber dysfunction in hypertension, in my dissertation I aimed to assess whether gender determines the adverse effects on LV chamber remodeling following 6 months of daily β-adrenergic receptor (AR) stimulation (isoproterenol [ISO] at 0.04 mg.kg-1day-1) in spontaneously hypertensive rats (SHR). LV dilatation was assessed in vivo from LV end diastolic diameter (EDD) (echocardiography) and ex vivo from the volume intercept at 0 mm Hg pressure (V0) of the LV diastolic pressure-volume relationship (isolated, perfused heart technique). LV systolic function was determined in vivo from LV endocardial fractional shortening (FSend) and ex vivo from the slope (LV end systolic elastance [LV Ees]) of the LV end systolic pressure-volume relationship (isolated, perfused heart technique). As compared to saline-treated male SHR (n=13), male SHR receiving ISO for 6 months (n=13) developed an increased LV EDD (Male Saline: 6.56±0.20 mm; Male ISO: 7.78±0.29 mm; p<0.05) and LV V0 (Male Saline: 0.22±0.01 ml; Male ISO: 0.31±0.02 ml; p<0.05). In contrast, ISO administration failed to modify LV EDD (Female Saline, n=13: 6.06±0.15 mm; Female ISO, n=12: 6.33±0.15 mm) or LV V0 (Female Saline: 0.17±0.01ml; Female ISO: 0.17±0.01 ml) in female SHR. In addition, there was a gender-ISO interactive effect on LV Ees (p<0.05; Male Saline: 2268±336 mmHg.ml-1; Male ISO: 1623±164 mmHg.ml-1; Female Saline: 1910±219 mmHg.ml-1; Female ISO: 2302±230 mmHg.ml-1). In conclusion, as compared to female SHR, male SHR are more susceptible to the adverse effects of chronic β-AR activation on LV cavity dimensions and systolic chamber function. These results suggest that the higher prevalence of LV dilatation and systolic chamber dysfunction in males than in females with LVH may be attributed to an increased susceptibility to the adverse effects of adrenergic stimulation.

Cardiac hypertrophy, a compensatory growth response to various physiological and pathological stimuli, involves intricate cytoskeletal changes within cardiomyocytes. However, the molecular mechanisms governing these cytoskeletal processes is not yet well defined. Microtubule affinity-regulating kinase 2 (MARK2) is of particular interest as it plays an important role in controlling microtubule dynamics and cell polarity to define cell shape. In this thesis, we aim to determine whether MARK2 is involved in initiating the morphological alterations that drive cardiac hypertrophy. Our image analysis reveals a significant increase in total MARK2 signal during pathological remodeling, particularly in the initial hours. However, no significant change occurs under physiological hypertrophy. Inhibiting MARK2 significantly impacts cell area and length-to-width ratio during pathological remodeling but has no effect under physiological conditions. Additionally, MARK2 inhibition results in decreased microtubule density and reduced Tau phosphorylation at Serine262 in pathological remodeling cardiomyocytes. Furthermore, our findings indicate an increased number of binucleated cardiomyocytes in pathological hypertrophy, with MARK2 inhibition influencing this parameter. Overall, our findings provide clarity in the role that MARK2 has in driving cytoskeleton alterations to shift cell morphology in pathological cardiac hypertrophy and a unique potential therapeutic in preventing the transition to fulminant heart failure.

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Pathological cardiac hypertrophy (PCH) occurs in response to pathological stimuli affecting the heart such as coronary artery disease, myocardial infarction, or hypertension. PCH is also be independent risk factor for cardiac events and/or sudden death. Despite therapeutic advancements in the treatment of cardiovascular diseases (CVD) and heart failure, deaths due to CVD remain the leading cause of mortality worldwide. Furthermore, treatment of these cardiovascular diseases slows their progression, but individuals eventually progress to heart failure, which has a 5-year survival rate of approximately 50 percent. There is a clear need for development of new therapies that can reverse PCH and the associated damage to the heart. As healthcare improves, populations are living longer, and illness due to age increases. One issue that occurs with aging is loss of normal cardiac function leading to heart failure. This functional decline is accompanied by morphological changes in the heart, including hypertrophy. Although it is well documented that myocardial remodeling occurs with aging, the mechanisms underlying these changes are poorly understood. Growth differentiation factor 11 (GDF11) is a member of the transforming growth factor β (TGF-β) superfamily of proteins, which regulate a number of cellular processes. Shared circulation of a young mouse with an old mouse or a single daily intraperitoneal (IP) injection of GDF11 for 30 days was shown to reverse aging-induced pathological cardiac hypertrophy. This molecule is highly homologous with another TGF-β family member, myostatin, which is a known negative growth regulator of skeletal muscle. We began by attempting to validate published data claiming that a single daily intraperitoneal (IP) injection of 0.1 mg/kg/day of GDF11 could reverse aging induced cardiac hypertrophy. We performed a blinded study during which treated 24-month-old C57BL/6 male mice with a single IP injection of 0.1 mg/kg/day of GDF11for 28 days and monitored changes in cardiac function and structure using echocardiography (ECHO). We also looked for differences in fibrosis, myocyte size, markers of pathological hypertrophy and heart weight. We were unable to find any differences between vehicle treated age mice and GDF11 treated aged mice in any of the measured parameters. While we did find an increase in heart weight between 8-week-old mice and the 24-month-old mice, there was no difference in the heart weight to body weight ratios of these groups of animals. From these data we concluded that our aged- mice did not have pathological hypertrophy and the dose of GDF11 used in this study did not have any effect on cardiac structure or function. Hypertensive heart disease results in changes in cardiac structure and function including left ventricular hypertrophy, systolic and/or diastolic dysfunction. It is also a leading cause of heart failure. Members of the TGF-β superfamily of proteins have been shown to be involved in many of the processes that occur in the heart in response to hypertension, such as the fibrotic response. Although it was previously shown that treatment with 0.1 mg/kg of GDF11 did not prevent pressure overload induced cardiac hypertrophy, we found this dose was too low to alter cardiac structure in our aging study. In addition, a single GDF11 dose is insufficient to fully address this issue. We therefore performed a blinded dose-ranging study to investigate the effects of GDF11 on pressure overload induced cardiac hypertrophy using transverse aortic constriction (TAC) which mimics the effects of chronic hypertension on the heart. In this study, animals received TAC surgery and were assigned to treatment groups so that there were no differences in wall thickness, cardiac function, or pressure gradients across the aortic constriction at the start of the treatments 1 week after TAC. Mice were given 0.5 mg/kg/day of GDF11, 1.0 mg/kg/day GDF11, 5.0 mg/kg/day of GDF11, or vehicle via a single daily IP injection for 14 days. Using these higher doses, we found that GDF11 had dose dependent effects on both cardiac structure and function following TAC. Myocyte cross sectional area was dose-dependently decreased compared to vehicle treated mice in both sham and TAC conditions. Cardiac function was preserved in the 1.0 and 5.0 mg/kg groups treatment groups after TAC. Left ventricular internal chamber dimensions were preserved with the 1.0 mg/kg treatment group. Treatment with GDF11 caused a dose dependent decrease on both body weight and heart weight in both normal and TAC mice, but with an effect on heart weight in the TAC mice that was independent of body weight. However, the 5.0 mg/kg dose caused large reductions in body weight (cachexia) and death. Our results show that GDF11 can reduce pathological hypertrophy and cardiac remodeling after pressure overload, but there is a narrow therapeutic range. / Biomedical Sciences