Investigating mechanisms of salt-sensitive hypertension in 11β-HSD2 heterozygote mice

The mineralocorticoid hormone, aldosterone, classically acts via the Mineralocorticoid Receptor (MR) to promote sodium transport in aldosterone target tissues, such as the kidney, thereby controlling long-term electrolyte homeostasis and blood pressure (BP). Aldosterone biosynthesis by the adrenal gland is regulated by a negative feedback loop called the Renin Angiotensin Aldosterone System (RAAS). The glucocorticoid cortisol (corticosterone in rodents), which has a very similar structure to aldosterone, shares with aldosterone an equal affinity for the MR. Typically, plasma cortisol levels are approximately 1000-fold higher than plasma aldosterone, and so the ligand specificity for aldosterone must be imposed on MR by other, non-structural, means. This specificity is important in order to retain electrolyte and BP balance within the control of the RAAS. The co-localisation of the enzyme 11β-Hydroxysteroid Dehydrogenase Type 2 (11β-HSD2) with the MR in aldosterone target tissues provides the MR with the aldosterone specificity it inherently lacks. 11β-HSD2 achieves this by converting active cortisol to its inactive 11-keto metabolite, cortisone (dehydrocorticosterone in rodents). In humans with the monogenetic Syndrome of Apparent Mineralocorticoid Excess (SAME), inactivating mutations in the HSD11B2 gene allows cortisol unregulated access to the MR. Resultant symptoms include severe hypertension and life-threatening hypokalemia. Individuals heterozygous for SAME display no overt phenotypes. However, some studies have associated SAME heterozygosity and loss-of-function polymorphisms within the HSD11B2 gene with essential and/or salt-sensitive hypertension in the general population. Targeted disruption of the Hsd11b2 gene in mice (Hsd11b2-/-) faithfully reproduces with all the major phenotypes of SAME patients. Mice heterozygote for the targeted gene (Hsd11b2+/-) have no phenotype and display a normal BP. In the present study, Hsd11b2+/- mice were used to explore the relationship between reduced 11β-HSD2 enzyme activity and salt-sensitive hypertension. On a high salt diet, Hsd11b2+/- mice were found to have increased BP and impaired natriuresis, compared to wild-type controls (Hsd11b2+/+). Further studies used pharmacological blockade of the Epithelial Sodium Channel (ENaC) and MR to ascertain the contributions of these pathways towards the observed phenotypes. These identified a deregulation of ENaC activity pertaining to an inability to regulate sodium appropriately. Investigations into the contributions of the RAAS and the Hypothalamus Pituitary Adrenal (HPA) axis have revealed valuable insights into their roles in this model. There is an implication that the RAAS has increased sensitivity in Hsd11b2+/-, further exacerbated by increased dietary sodium, and that the regulation of corticosteroids may also be altered. Novel observations have suggested that oxidative stress in response to a high salt diet could also be involved, as a study administering an antioxidant drug in conjunction with a high salt diet prevented the manifestation of a phenotype in Hsd11b2+/-. Finally, the generation of a floxed Hsd11b2 targeting construct for tissue-specific deletion of 11β-HSD2 will allow future studies into the contributions of specific 11β-HSD2 expression sites (such as the kidney) towards the phenotypes of both homozygous and heterozygous mic

The relationship between P2X4 and P2X7: a physiologically important interaction?

Purinergic signaling within the kidney is becoming an important focus in the study of renal health and disease. The effectors of ATP signaling, the P2Y and P2X receptors, are expressed to varying extents in and along the nephron. There are many studies demonstrating the importance of the P2Y2 receptor on kidney function, and other P2 receptors are now emerging as participants in renal regulation. The P2X4 receptor has been linked to epithelial sodium transport in the nephron and expression levels of the P2X7 receptor are up-regulated in certain pathophysiological states. P2X7 antagonism has been shown to ameliorate rodent models of DOCA salt-induced hypertension and P2X4 null mice are hypertensive. Interestingly, polymorphisms in the genetic loci of P2X4 and P2X7 have been linked to blood pressure variation in human studies. In addition to the increasing evidence linking these two P2X receptors to renal function and health, a number of studies link the two receptors in terms of physical associations between their subunits, demonstrated both in vitro and in vivo. This review will analyze the current literature regarding interactions between P2X4 and P2X7 and assess the potential impact of these with respect to renal function

Failure to Downregulate the Epithelial Sodium Channel Causes Salt Sensitivity in Hsd11b2 Heterozygote Mice

In vivo, the enzyme 11β-hydroxysteroid dehydrogenase type 2 influences ligand access to the mineralocorticoid receptor. Ablation of the encoding gene, HSD11B2, causes the hypertensive syndrome of apparent mineralocorticoid excess. Studies in humans and experimental animals have linked reduced 11β-hydroxysteroid dehydrogenase type 2 activity and salt sensitivity of blood pressure. In the present study, renal mechanisms underpinning salt sensitivity were investigated in Hsd11b2(+/-) mice fed low-, standard-, and high-sodium diets. In wild-type mice, there was a strong correlation between dietary sodium content and fractional sodium excretion but not blood pressure. High sodium feeding abolished amiloride-sensitive sodium reabsorption, consistent with downregulation of the epithelial sodium channel. In Hsd11b2(+/-) mice, the natriuretic response to increased dietary sodium content was blunted, and epithelial sodium channel activity persisted. High-sodium diet also reduced renal blood flow and increased blood pressure in Hsd11b2(+/-) mice. Aldosterone was modulated by dietary sodium in both genotypes, and salt sensitivity in Hsd11b2(+/-) mice was associated with increased plasma corticosterone levels. Chronic administration of an epithelial sodium channel blocker or a glucocorticoid receptor antagonist prevented salt sensitivity in Hsd11b2(+/-) mice, whereas mineralocorticoid receptor blockade with spironolactone did not. This study shows that reduced 11β-hydroxysteroid dehydrogenase type 2 causes salt sensitivity of blood pressure because of impaired renal natriuretic capacity. This reflects deregulation of epithelial sodium channels and increased renal vascular resistance. The phenotype is not caused by illicit activation of mineralocorticoid receptors by glucocorticoids but by direct activation of glucocorticoid receptors

The mineralocorticoid receptor (MR) regulates ENaC but not NCC in mice with random MR deletion

Aldosterone binds to the mineralocorticoid receptor (MR) and increases renal Na+ reabsorption via up-regulation of the epithelial Na+ channel (ENaC) and the Na+-K+- ATPase in the collecting system (CS) and possibly also via the NaCl cotransporter (NCC) in the distal convoluted tubule (DCT). However, whether aldosterone directly regulates NCC via MR, or indirectly through systemic alterations remains controversial. We used mice with deletion of MR in ~20% of renal tubule cells (MR/X mice), in which MR-positive (MRwt) and -negative (MRko) cells can be studied sideby- side in the same physiological context. Adult MR/X mice showed similar mRNA and protein levels of renal ion transport proteins to control mice. In MR/X mice, no differences in NCC abundance and phosphorylation was seen between MRwt and MRko cells and dietary Na+ restriction up-regulated NCC to similar extent in both groups of cells. In contrast, MRko cells in the CS did not show any detectable alpha- ENaC abundance or apical targeting of ENaC neither on control diet nor in response to dietary Na+ restriction. Furthermore, Na+-K+-ATPase expression was unaffected in MRko cells of the DCT, while it was lost in MRko cells of the CS. In conclusion, MR is crucial for ENaC and Na+-K+-ATPase regulation in the CS, but is dispensable for NCC and Na+-K+-ATPase regulation in the DCT

Col4a1 mutation in mice causes defects in vascular function and low blood pressure associated with reduced red blood cell volume

Collagen type IV is the major structural component of the basement membrane and COL4A1 mutations cause adult small vessel disease, familial porencephaly and hereditary angiopathy with nephropathy aneurysm and cramps (HANAC) syndrome. Here, we show that animals with a Col4a1 missense mutation (Col4a1+/Raw) display focal detachment of the endothelium from the media and age-dependent defects in vascular function including a reduced response to nor-epinephrine. Age-dependent hypersensitivity to acetylcholine is abolished by inhibition of nitric oxide synthase (NOS) activity, indicating that Col4a1 mutations affect vasorelaxation mediated by endothelium-derived nitric oxide (NO). These defects are associated with a reduction in basal NOS activity and the development of heightened NO sensitivity of the smooth muscle. The vascular function defects are physiologically relevant as they maintain in part the hypotension in mutant animals, which is primarily associated with a reduced red blood cell volume due to a reduction in red blood cell number, rather than defects in kidney function. To understand the molecular mechanism underlying these vascular defects, we examined the deposition of collagen type IV in the basement membrane, and found it to be defective. Interestingly, this mutation also leads to activation of the unfolded protein response. In summary, our results indicate that mutations in COL4A1 result in a complex vascular phenotype encompassing defects in maintenance of vascular tone, endothelial cell function and blood pressure regulation

Multiparametric imaging reveals that mitochondria-rich intercalated cells in the kidney collecting duct have a very high glycolytic capacity

Alpha intercalated cells (αICs) in the kidney collecting duct (CD) belong to a family of mitochondria rich cells (MRCs) and have a crucial role in acidifying the urine via apical V-ATPase pumps. The nature of metabolism in αICs and its relationship to transport was not well-understood. Here, using multiphoton live cell imaging in mouse kidney tissue, FIB-SEM, and other complementary techniques, we provide new insights into mitochondrial structure and function in αICs. We show that αIC mitochondria have a rounded structure and are not located in close proximity to V-ATPase containing vesicles. They display a bright NAD(P)H fluorescence signal and low uptake of voltage-dependent dyes, but are energized by a pH gradient. However, expression of complex V (ATP synthase) is relatively low in αICs, even when stimulated by metabolic acidosis. In contrast, anaerobic glycolytic capacity is surprisingly high, and sufficient to maintain intracellular calcium homeostasis in the presence of complete aerobic inhibition. Moreover, glycolysis is essential for V-ATPase-mediated proton pumping. Key findings were replicated in narrow/clear cells in the epididymis, also part of the MRC family. In summary, using a range of cutting-edge techniques to investigate αIC metabolism in situ, we have discovered that these mitochondria dense cells have a high glycolytic capacity