Cell type specific mechanisms of mineralocorticoid receptor mediated renal injury

Field: Medicine (Health Sciences)

The mineralocorticoid receptor (MR) has been found to exhibit both physiological roles in epithelial cells, and pathological roles in non epithelial kidney cells such as macrophages, podocytes and fibroblasts. Recent evidence has shown that activation of MR in these cells promotes inflammation, apoptosis and fibrosis – key pathological processes that contribute to declining renal function, albuminuria and progression of kidney disease. Antagonists of MR have been available for clinical use for decades; however their utility in renal patients has been impeded by hyperkalaemia due to tubular MR blockade. Therefore, an understanding of what cells mediate pathological effects of MR during kidney disease may allow targeted MR blockade, thus avoiding complications of salt disturbance. In order to assess the cell specific effects of MR signalling in kidney disease, we examined renal cells that express MR, such as renal fibroblasts, macrophages and podocytes. Initially, the role of MR activation on renal fibroblast proliferation was examined in vitro (Chapter 2). Rat renal fibroblasts, as well as primary mouse renal fibroblasts harvested from a kidney undergoing fibrosis, were exposed to aldosterone in serum free conditions. Increased proliferation and cell counts were observed in both cell types in a dose-dependent manner. Mechanistic experiments revealed MR activation leading to transactivation of growth factor receptors and subsequent activation of mitogen-activated protein kinases, to increase fibroblast proliferation. These results suggest that aldosterone-induced fibroblast proliferation may be another mechanism by which MR contributes to renal fibrosis. Secondly, we investigated the role of macrophages MR signalling in models of acute glomerulonephritis and diabetic nephropathy. In the glomerulonephritis model (Chapter 3), mice with selective MR deletions in macrophages had reduced overall renal injury as determined by kidney inflammatory infiltrates, glomerular and interstitial fibrosis and preservation of renal function. The degree of renal protection from targeted macrophage MR inhibition was similar to, and often superior to that of systemic MR antagonism. Thus, our findings demonstrate an important role of macrophage MR in the pathogenesis of acute glomerulonephritis, and may be a potential target for targeted MR blockade. Mice were then subjected to streptozotocin induced type 1 diabetes in a model of diabetic nephropathy (Chapter 4). Selective macrophage MR deletion markedly attenuated evidence of glomerular hyperfiltration, with reductions in glomerulomegaly and hypercellularity during diabetic nephropathy. Furthermore, glomerular macrophage infiltration and tubulointerstitial apoptosis were also attenuated in the knock-out mice. Despite these changes, renal function and albuminuria were not affected by macrophage MR deletion, suggesting a limited role for myeloid MR during early diabetic nephropathy. Lastly, we examined the relative importance of podocyte MR signalling in a model of glomerulonephritis (Chapter 3). Mice with selective MR deletions in podocytes developed severe renal injury comparable to wild-type mice, with no protection seen in albuminuria, renal dysfunction or renal fibrosis. These results suggest that podocyte MR signalling plays a limited role in renal injury during acute glomerulonephritis; contrasting to that of myeloid MR signalling. In conclusion, this series of experiments have increased our knowledge of the pathological role of MR activation in non-epithelial renal cells. Our results are unique, in that for the first time, we were able to delineate the effects of MR activation in selective cells, during kidney disease. An understanding of cell specific MR signalling may lead to cell selective MR inhibition, or its downstream signalling pathways, and avoid unwanted adverse effects such as hyperkalaemia. Not only will this improve treatment of patients with kidney disease, it may also have implications for the treatment of cardiovascular and cerebrovascular diseases.