The effect of the Pdgfbʳᵉᵗ mutation on cardiovascular homeostasis

Platelet derived growth factor B (PDGFB)/PDGFRβ signalling is a key regulator of the cardiovascular system in development, health, and disease. During angiogenesis PDGFB regulates pericyte migration and proliferation during development and in tissue repair. Pericytes confer vessel stability, maintain vessel integrity, and control vascular tone. In the field of regenerative medicine, pericytes have also risen to prominence as they were discovered to be the in-situ counterpart of mesenchymal stem/stromal cells (MSCs). Pdgfbret/ret mutantmice are pericyte-deficient. The PDGFB retention motif is deleted, reducing PDGFB retention in the pericellular space and extracellular matrix and therefore pericyte recruitment during angiogenesis. This reduction in vascular pericyte coverage in pdgfbret/ret mice manifests in phenotypic differences compared with pdgfb+/+ and pdgfbret/+ littermates. Of relevance, is the reported development of vascular associated brain calcification in 4-12-month-old pdgfbret/ret mice and alterations to left ventricular structure from 10 to 20 weeks of age. In this thesis, I have investigated the hypothesis that calcification develops throughout the vasculature in pdgfbret/ret mice and that alteration of left ventricular structure leads to a progressive decline in heart function. I also hypothesised that disruption to PDGFB/PDGFRβ signalling in vivo influences the in vitro properties of MSCs. To address my hypotheses, I used a combination of cardiovascular imaging techniques, positron emission tomography/computed tomography (PET/CT) and high-resolution cardiac ultrasound, alongside in vitro MSC osteogenic assays. Contrary to my original hypothesis, results of PET/CT imaging using 18F-NaF, a radioactive tracer with the propensity to bind to hydroxyapatite calcification, did not reveal any areas of ectopic calcification out with the brain in male or female pdgfbret/ret mice. No significant calcification was observed in the aortic arch, the descending thoracic aorta, heart, or kidneys. Indeed, calcification was restricted to the brain at 12 months of age and completely absent from pdgfb+/+ and pdgbret/+ littermates. However, I did observe that the extent and anatomical location of brain calcification was variable in pdgfbret/ret mice. Using high resolution cardiac ultrasound, I next evaluated the cardiac phenotype exhibited in male and female pdgfbret/ret mice over the course of 12 months. The results of this study reveal that both male and female pdgfbret/ret mice exhibit signs of cardiac dilation with increasing left ventricular end diastolic area at 3 months of age, whereas females exhibit an increase in left ventricular mass that may be the result of hypertrophic remodelling or oedema. However, over the course of 6, 9 and 12 months, this phenotype resolves in both male and female pdgfbret/ret mice and normal contractile function is maintained. Further analysis of cardiac structure and function was performed using speckle tracking echocardiography (STE) in a combined male and female cohort of pdgfb+/+, pdgfbret/+ and pdgfbret/ret mice. Using STE, subtle changes to segmental systolic and diastolic cardiac strain were detected at 9 and 12 months of age in pdgfbret/ret mice. These changes may preclude the onset of deterioration of cardiac function in pdgfbret/ret mice and requires further investigation beyond 12 months of age to conclude. Finally, I examined the effects of the pdgfbret mutation on the osteogenic potential of MSCs derived from large coronary vessels and various vascularised tissues including the kidney, the skeletal muscle, and the heart in vitro. Quantification of osteogenic potential saw an increase in calcium deposition in combined pdgfbret MSC tissues. Tissue specific osteogenic potential was highly variable although there was a trend towards an increased osteogenesis in pdgfbret mutant tissues. Together, data from my studies indicate that pdgfbret/ret mice exhibit a transient cardiac phenotype which is ameliorated and have variable levels of calcification exclusive to the brain at 12 months of age. This data also indicates that MSCs derived from pdgfbret mutants have greater osteogenic potential although the tissue specificity of this potential requires further investigation.