Myeloid Mineralocorticoid Receptor Activation Contributes to Progressive Kidney Disease

Clinical and experimental studies have shown that mineralocorticoid receptor (MR) antagonists substantially reduce kidney injury. However, the specific cellular targets and mechanisms by which MR antagonists protect against kidney injury must be identified. We used conditional gene deletion of MR signaling in myeloid cells (MR^flox/flox LysM^Cre mice; MyMRKO) or podocytes (MR^flox/flox Pod^Cre mice; PodMRKO) to establish the role of MR in these cell types in the development of mouse GN. Accelerated anti–glomerular basement membrane GN was examined in groups of mice: MyMRKO, PodMRKO, wild-type (WT) littermates, and WT mice receiving eplerenone (100 mg/kg twice a day; EPL-treated). At day 15 of disease, WT mice had glomerular crescents (37%±5%), severe proteinuria, and a 6-fold increase in serum cystatin-C. MyMRKO, PodMRKO, and EPL-treated mice with GN displayed proteinuria similar to that in these disease controls. However, MyMRKO and EPL-treated groups had a 35% reduction in serum cystatin-C levels and reduced crescent numbers compared with WT mice, whereas PodMRKO mice were not protected. The protection observed in MyMRKO mice appeared to result predominantly from reduced recruitment of macrophages and neutrophils into the inflamed kidney. Suppression of kidney leukocyte accumulation in MyMRKO mice correlated with reductions in gene expression of proinflammatory molecules (TNF-α, inducible nitric oxide synthase, chemokine (C-C motif) ligand 2, matrix metalloproteinase-12), tubular damage, and renal fibrosis and was similar in EPL-treated mice. In conclusion, MR signaling in myeloid cells, but not podocytes, contributes to the progression of renal injury in mouse GN, and myeloid deficiency of MR provides protection similar to eplerenone in this disease.Mineralocorticoid receptor (MR) antagonists are known to inhibit renal and cardiovascular disease (CVD) by direct blockade of MR in tissues and by reducing hypertension.¹ They can also suppress kidney damage in animal models of GN and diabetic nephropathy without affecting BP.^2–6 In addition, MR antagonists provide added protection against proteinuria and loss of renal function when used with standard antihypertensive therapies in patients with diabetic and nondiabetic CKD.^7–9The clinical use of MR antagonists is limited by the development of hyperkalemia due to the importance of the MR in tubular regulation of salt balance.¹⁰ This consequence of MR blockade in the distal tubule is most evident during renal impairment and can require a reduction in the dosage of MR antagonist or withdrawal of the agent as a therapy.^7,8 The specific renal cell types that are targeted by MR antagonists to reduce injury during kidney disease have not been clearly identified. Establishing the identity of these cells is an important step toward developing more selective inhibitors of MR signaling that do not interfere with tubular cell function.Animal studies demonstrate that the protection afforded by MR antagonists in GN and diabetic nephropathy is associated with reductions in renal inflammation, proteinuria, and glomerular injury.^2,3,5,11 These studies also link MR blockade to suppression of leukocyte recruitment and podocyte injury. This suggests that the major pathologic effects of MR signaling may occur in podocytes and inflammatory cells.Recent in vitro studies have suggested that MR signaling can induce apoptosis in podocytes and oxidative stress in macrophages,^12,13 which supports a role for MR signaling in these cell types in kidney disease. In addition, an MR deficiency in myeloid cells protects against cardiovascular injury and ischemic cerebral infarcts by reducing inflammation and fibrosis.^14–16 However, no in vivo studies have identified whether MR signaling in podocytes or macrophages is specifically important to the development of kidney disease.In this study, we created mice with a selective genetic deficiency of MR in myeloid cells or podocytes and used these strains to evaluate the hypothesis that MR signaling in macrophages or podocytes is required for the development of renal injury in a normotensive model of progressive GN.