Developmental Signals in Murine Postnatal Cardiac Growth

Cardiovascular disease is the leading cause of death worldwide, and is increasing in an ageing and growing population. Heart failure in adults results from a loss of functional cardiomyocytes (CMs), which are not replaced because the terminally differentiated state of adult CMs severely limits the capacity of the heart to regenerate. Thus, there is a need to develop strategies that enhance endogenous CM proliferation and enable cardiac repair. This thesis describes the characterisation of a surgical model of pressure overload, suprarenal aortic constriction (SAC), which results in hypertension and left ventricular hypertrophy. This model can be used to investigate candidates for cardiac regeneration. Previous work in our laboratory suggested that under hypertensive stress, mice with inhibited signalling of the tyrosine kinase receptor, c-Kit, had improved cardiac function and survival due to CM proliferation. Thus, the expression of c-Kit was evaluated in wild-type mice and was found at very low levels in developing and adult CMs, and was not re-expressed in adults following SAC. Therefore, c-Kit is unlikely to contribute to the maintenance of CM terminal differentiation and is not a suitable therapeutic target for CM proliferation. Another approach to identifying candidates for regeneration is to understand the changes in gene expression and regulatory networks that lead to CM proliferation and terminal differentiation. Characterisation of postnatal cardiac growth identified pivotal postnatal days (P) of CM maturation with high CM proliferation (P2), a transition between CM division and enlargement (P10 and P13), and CM terminal differentiation (P70). A standardised protocol for mice of any age was developed to rapidly purify CMs (~95% purity). Poly(A) RNA was extracted from purified CMs at the aforementioned time points for high-throughput sequencing, which identified ~3,800 differentially expressed mRNAs that underpin CM maturation. Thus, this data has enormous potential to identify molecular targets for cardiac regeneration.