The Gender and Isoform Specific Roles of FGF2 in Cardiac Physiology and Remodeling

Abstract

A leading cause of morbidity and mortality in the developed world is cardiovascular disease (CVD). Like many other disease processes the etiology of CVD has origins in both genetic and environmental factors. These factors affect the development of the heart and vasculature and how they respond to physiological and pathological stress. Abnormal heart development can lead to cardiac pathologies that manifest in a shift from normal cardiac geometry and physiology to what is called pathological cardiac remodeling. Often though, pathological remodeling can result from cardiovascular stress even when heart development is normal. Growth factors are essential mediators of cardiac development and physiology and a good number of clinical and experimental studies have implicated growth factors and their signaling effectors as potential therapeutic targets for pathological cardiac remodeling. Of those is Fibroblast Growth Factor 2 (FGF2) which is a potent inducer of fibroblast and cardiomyocyte proliferation in vitro. FGF2 is made in high molecular weight and low molecular weight isoforms (Hi FGF2 and Lo FGF2, respectively). It has already been demonstrated that, in the context of the heart, FGF2 modulates cardiac hypertrophy, cardiac fibrosis and mediates protection against cardiac injury. However, the isoform specific role of FGF2 in cardiac development, physiology and pathological remodeling has not been disclosed, and in this dissertation I address the hypothesis that FGF2 has isoform-specific function in cardiac physiology and remodeling. To test this hypothesis I used mice that are either deficient in Hi FGF2 (Hi KO) or Lo FGF2 (Lo KO) and subjected them to echocardiographic analysis and isoproterenol (Iso) treatment and compared them to wildtype (WT) cohorts. At baseline echocardiographic measurements, female Lo KO hearts are smaller and present with increased peak E-wave velocity, a diminutive A wave, and shortened mitral-flow deceleration time consistent with a restricted filling pattern and myocardial stiffness. Conversely, male Lo KO hearts present with a lower E wave and a higher A-wave velocity and a prolonged isovolumic-relaxation time consistent with impaired left ventricular (LV) relaxation. Female Hi KO hearts display no significant deviation from WT, while male Hi KO hearts exhibit increased systolic function. Hence, a deficiency in Lo FGF2 results in a shift from normal diastolic parameters and geometric measurements which is gender specific. Conversely, a deficiency in Hi FGF2 produces a phenotype in male hearts only. Histological and gravimetric analysis of Lo KO and Hi KO hearts post-Iso treatment reveals that female Lo KO hearts remain smaller even though their cardiomyocytes are hypertrophied while female Hi KO hearts present with a blunted hypertrophic response indicating a hypoplastic myocardium. Male Lo KO hearts present with an exacerbated fibrotic response and increased alpha-smooth muscle actin protein expression while Hi KO hearts exhibit a resistance to the fibrotic response and an induction of atrial natriuretic factor protein expression. Thus, in female hearts Hi FGF2 mediate cardiac hypertrophy while in male hearts Lo FGF2 and Hi FGF2 display an antithetical role in cardiac fibrosis where Lo FGF2 is protective while Hi FGF2 is damaging. Hence, cardiac remodeling following catecholamine overactivation is modulated by FGF2 in isoform- and gender-specific manners. In conclusion, the results presented here provide novel evidence on the interaction of gender and endogenous FGF2 isoforms as modulators of cardiac development, physiology and remodeling. Lo FGF2 signaling is necessary in the male heart for normal myocardial relaxation and for amelioration of the fibrotic response induced by beta-adrenergic stress, while in female hearts Lo FGF2 is necessary for normal cardiac growth and normal myocardial compliance. Hi FGF2 is necessary only in female hearts for mediating the hypertrophic response. Hence, I demonstrate that Lo FGF2 and Hi FGF2 have non-redundant roles in cardiac physiology and remodeling which are gender-specific.