Cardiac hypertrophy and heart failure (HF) remains one of the major health problems in the UK and worldwide. However, advances in their management are limited because the underlying pathological mechanisms are not completely understood. Therefore, it is important to understand novel signalling pathways leading to HF. Myocardial hypertrophy is a crucial pathophysiological process that can lead to the development of HF. Signalling initiated by members of G-protein-coupled receptors (GPCRs) proteins plays an important role in mediating cardiac hypertrophy. One member of this family, the G-protein coupled receptor 99 (GPR99), may have a crucial role in the heart because it acts as a receptor for alpha-ketoglutarate, a metabolite that is elevated in heart failure patients. GPR99 is expressed in the heart, but its precise function during cardiac pathophysiological processes is unknown. The aim of this PhD study is to investigate the role of GPR99 during cardiac hypertrophy. In this study I used in vivo and in vitro approaches to investigate whether GPR99 is directly involved in mediating cardiac hypertrophy. Mice with genetic deletion of GPR99 (GPR99-/-) exhibited a significant increase in hypertrophy following two weeks of transverse aortic constriction (TAC) as indicated by heart weight/tibia length ratio (HW/TL). In addition, GPR99-/- mice displayed increased cardiomyocytes cross-sectional area (CSA) after TAC compared to wild-type (WT) littermates. Hypertrophic markers such as brain natriuretic peptide (BNP) and β-myosin heavy chain (β-MHC) were also elevated in GPR99-/- mice following TAC compared to WT mice. Although interstitial fibrosis was indistinguishable in both genotypes after TAC, a precursor of fibrosis, collagen, type III, alpha1 (COL3A1) was elevated in GPR99-/- mice compared to WT mice after TAC. The baseline cardiac function as indicated by ejection fraction (EF) and fractional shortening (FS) were reduced in GPR99-/- mice compared to WT littermates following TAC. Furthermore, left ventricular end-diastolic diameter (LVEDD), left ventricular end-systolic diameter (LVESD), interventricular septum wall thickness (IVS) and posterior wall thickness at diastole (PW) indicated profound wall thickening and enlargement of the left ventricular (LV) chamber in GPR99-/- mice compared to WT littermates after TAC. In an attempt to examine the mechanism through which GPR99 signals during hypertrophy, I performed molecular analyses based on the data from yeast two hybrid screening showing that GPR99 interacted with COP9 signalosome element 5 (CSN5). Using immunoprecipitation assay, I found that GPR99 formed a ternary complex with CSN5 and non-receptor tyrosine kinase 2 (TYK2). TYK2 is known as a regulator of pro-hypertrophic molecules including signal transducer and activation of transcription 1 (STAT1) and STAT3. I found that the activation of these molecules was increased in GPR99-/- mice following TAC and correspondingly, adenovirus-mediated overexpression of GPR99 in neonatal rat cardiomyocytes (NRCM) blunted TYK2 phosphorylation. In conclusion, my study has identified GPR99 as a novel regulator of pathological hypertrophy via the regulation of the STAT pathway. Identification of molecules that can specifically activate or inhibit this receptor may be very useful in the development of a new therapeutic approach for cardiac hypertrophy in the future.
|Date of Award
|31 Dec 2015
- The University of Manchester
|Delvac Oceandy (Supervisor), Elizabeth Cartwright (Supervisor) & Mamas Mamas (Supervisor)
- Cardiac hypertrophy, heart failure, G-protein-coupled receptors