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Renal fibrosis, the final endpoint of renal disease of any cause, is characterised by myofibroblast deposition of extracellular matrix (ECM) and commonly studied using the unilateral ureteric obstruction (UUO) model. Macrophages are multifunctional cells and involved in renal injury, repair and scarring. Work in other organs has shown that fibrosis is not necessarily irreversible and we established and characterised the murine model of reversible unilateral ureteric obstruction (R-UUO) to investigate the potential reversibility of fibrosis and the underlying mechanism with a particular focus upon the role of macrophages. Reversal of UUO was performed at day 7 and R-UUO kidneys exhibited rapid and profound loss of α-smooth muscle actin (αSMA) positive myofibroblasts over the subsequent 7 days. Loss of αSMA+ myofibroblasts was accompanied by limited and variable degradation of ECM components including collagens I and III. αSMA/TUNEL double staining suggested that some myofibroblasts underwent apoptosis. Infiltrating macrophages were abundant at D7 UUO and persisted at all time points following reversal, however, there was a reduction in the F4/80+ population at D7 Reversal by flow cytometry. Of the F4/80+ population two distinct subpopulations could be identified, F4/80Hi and F4/80Lo cells. The relative contribution of these macrophage populations to the total renal macrophage pool did not alter in obstruction or reversal. The F4/80Hi population was characterised by increased expression of CD11c and decreased expression of CD11b and Ly6C (F4/80HiCD11cHiCD11bLoLy6CLo), whereas the F4/80Lo population was characterised by increased expression of CD11b and Ly6C and decreased expression of CD11c (F4/80LoCD11cLoCD11bHiLy6CHi). CD11b was decreased in both F4/80+ populations during reversal, with altered Ly6C profiles, indicating that the phenotype of the macrophages in each population is different and may change during reversal. Indeed, macrophages isolated by flow cytometry utilising anti-F4/80-APC conjugated antibodies had altered mRNA profiles with D7 reversal associated with decreased mRNA expression of mannose receptor and TGFβ. Previous work in the liver indicates that macrophages may promote or inhibit tissue scarring. In order to ascertain whether macrophages were involved in the loss of αSMA+ myofibroblasts we depleted macrophages after reversal by either administering diphtheria toxin to transgenic CD11b-DTR mice or antagonising colony stimulating factor-1 (CSF-1), a key macrophage mitogen and growth factor, by administering antibodies to the CSF-1 receptor. Both approaches significantly depleted macrophage infiltration but did not retard the loss of αSMA+ myofibroblasts indicating that myofibroblast loss was macrophage independent. Lastly, we investigated the potential role of tissue stiffness in myofibroblast loss following UUO reversal. Primary renal myofibroblasts were cultured from obstructed kidneys, carefully phenotyped and cultured on matrices of differing stiffness. The susceptibility of myofibroblasts to apoptosis increased as the matrix stiffness fell. These data suggest that the altered mechanical microenvironment of the decompressed kidney may be a key stimulus for the macrophage independent loss of myofibroblasts that follows the reversal of UUO.

Hypertension and type two diabetes mellitus (Type 2 DM) are serious chronic illnesses that impact on the lives of millions of people around the world. Various epidemiological studies have shown a relationship between early life events such as intrauterine growth retardation (IUGR) resulting in low birth weight and the development of these chronic illnesses in adult life. To explain the link between these two events, it has been suggested that an ‘insult’ at a critical time point of development can ‘program’ alterations in gene expression, organ size, and cell number. This has been termed “the early life origins of disease’. There is also evidence that these programmed effects are not limited to the first generation but can also be passed to subsequent generations. With changes in lifestyle in modern society, the prevalence of obesity is increasing, in association with problems such as type 2 DM, hypertension, fatty liver, atherosclerosis and the metabolic syndrome. Obesity during pregnancy is linked to problems such as gestational diabetes, hypertension and early miscarriage as well as a higher risk of congenital malformations. Maternal obesity has also been recognised as one of the factors capable of ‘programming’ the offspring, increasing the risk of childhood and adult disorders such as obesity and hypertension. In this thesis I have used two animal models to explore the underlying mechanisms of programming and its intergenerational effects: i) a rat model of prenatal glucocorticoid over-exposure (the dexamethasone-programmed rat) and ii) a mouse model of obesity during pregnancy. Using the dexamethasone-programmed rat, I have shown that prenatal glucocorticoid overexposure reduces fetal and placental weight in the first generation (F1) offspring, in association with alterations in gene expression in placenta and liver. In addition, I have shown effects on fetal and placental weights and gene expression in the second generation (F2) offspring. The observed changes in gene expression in the F2 offspring differ from those in the first generation. Thus, although effects on fetal growth are seen in both generations, the underlying mechanisms appear to be different. We also observed marked parent of origin effects on fetal and placental growth and gene expression in the second generation. In the mouse model of maternal obesity, birth weight was decreased in the F1 offspring. At weaning, the offspring of obese mothers were heavier than controls, however this difference in weight was not persistent. At three months of age, F1 female offspring of obese mothers showed altered expression of hepatic genes important in lipid regulation and metabolism. More striking changes were seen in the F2 generation in which there was an effect of paternal exposure to maternal obesity to decrease birth weight. There were also parent of origin effects on organ weights and insulin levels at six months of age. These results provide evidence for the transmission of programming effects to a second generation in two different programming models and suggest that the mechanisms leading to these effects differ between generations.

This thesis embodies a novel investigation into the cellular role of the specific immune system in fracture repair.

Endothelin-1 (ET-1) binds to endothelin A (ETA) and B (ETB) receptors on vascular smooth muscle cells, resulting in profound vasoconstriction and cellular proliferation. In contrast, activation of endothelial cell (EC) ETB receptors releases nitric oxide (NO) and prostacyclin (PGI2), which are anti-mitotic and mediate vasodilatation. ETB receptors are also responsible for the clearance of ET-1 from the circulation and renal ETB receptors contribute to sodium and water balance. Pharmacological blockade and genetic models featuring total ETB ablation, demonstrate salt sensitive hypertension. However, these do not allow the role of the EC ETB in cardiovascular homeostasis to be determined. Mice featuring loxP sites flanking exons 3 and 4 of the ETB gene (floxed ETB mice: FF/--) were crossed with Tie2-Cre mice (WW/Tie2-Cre), in which the expression of a Cre recombinase cDNA transgene is limited to EC, to generate EC-specific ETB down-regulated mice (FF/Tie2-Cre). Having demonstrated EC-specific down-regulation of ETB receptors using autoradiography, the role and relative contribution of the EC ETB to the regulation of systemic BP, to the clearance of ET-1 from the plasma, as well as to the development of pulmonary arterial hypertension were investigated. Autoradiography revealed significant down-regulation of ETB in EC-rich tissues such as lung of FF/Tie2-Cre animals (8 ± 3 amol.mm-2) compared to controls (80 ± 21 amol.mm-2) (n=4; p<0.05). Levels of ETA expression were preserved despite higher concentrations of plasma ET-1 in the FF/Tie2-Cre samples (12.4 ± 3.0 pg.ml-1) compared to controls (3.0 ± 0.8 pg.ml-1) (n=6; p<0.001). Using radiotelemetry, mean arterial blood pressure of FF/Tie2 mice was not significantly different to that of FF/- controls on low salt (FF/Tie2-Cre: 122.7 ± 1.52 mmHg, n=10; FF/--: 125.7 ± 0.58 mmHg, n=12), normal salt (FF/Tie2-Cre: 133.8 ± 4.0 mmHg, n=10; FF/--: 131.5 ± 3.33 mmHg, n=12) or high salt diet (FF/Tie2-Cre: 149.2 ± 2.71 mmHg, n=10; FF/--: 143.9 ± 2.97 mmHg, n=12). Similarly no differences in SBP, DBP or HR were seen between genotypes. The clearance of an intravenous bolus of radiolabelled ET-1 was significantly impaired in FF/Tie2-Cre mice (0.054 ± 0.006 ml.sec-1) compared to control mice (0.175 ± 0.032 ml.sec-1) (n=5; p<0.01). ETB blockade of control mice reduced ET-1 clearance to that of untreated FF/Tie2-Cre animals (n=4). Two weeks of hypobaric hypoxia induced an exaggerated increase in systolic right ventricular pressure in FF/Tie2-Cre mice (34.4 ± 1.2 mmHg, n=10) compared with FF/-- mice (24.6 ± 1.4mmHg, n=10; p<0.05), associated with an increased right ventricular/ left ventricular + septum ratio in FF/Tie2-Cre mice (normoxia: 0.224 ± 0.009; hypoxia: 0.285 ± 0.017; p<0.01), but not in FF/-- mice. Hypoxia increased the percentage of remodeled vessels in FF/-- mice (normoxia: 5.6 ± 0.6%; hypoxia: 11.4 ± 0.6%; n=6; p<0.001), and this was augmented in FF/Tie2-Cre mice (normoxia: 7.1 ± 0.5%; hypoxia: 18.5 ± 1.2%; n=6; p<0.001). The EC ETB receptor does not play a significant role in the BP response to salt, suggesting that ETB signalling on other cell types is responsible for ETB mediated natriuresis. However, the EC ETB receptor is crucial to the elimination of ET-1 from the circulation and is protective against the development of pulmonary arterial hypertension, most likely by preventing remodeling of small pulmonary arteries.