The Kidney-Specific WNK1 Isoform is Induced by Aldosterone and Stimulates Epithelial Sodium Channel-Mediated Na+ Transport

WNK1 belongs to a unique family of Ser/Thr kinases that have been implicated in the control of blood pressure. Intronic deletions in the WNK1 gene result in its overexpression and lead to pseudohypoaldosteronism type II, a disease with salt-sensitive hypertension and hyperkalemia. How overexpression of WNK1 leads to Na+ retention and hypertension is not entirely clear. Similarly, there is no information on the hormonal regulation of expression of WNK kinases. There are two main WNK1 transcripts expressed in the kidney: the originally described “long” WNK1 and a shorter transcript that is specifically expressed in the kidney (KS-WNK1). The goal of this study was to determine the effect of aldosterone, the main hormonal regulator of Na+ homeostasis, on the transcription of WNK1 isoforms in renal target cells, by using an unique mouse cortical collecting duct cell line that stably expresses functional mineralocorticoid receptors. Our results demonstrate that aldosterone, at physiological concentrations, rapidly induces the expression of the KS-WNK1 but not that of the long-WNK1 in these cells. Importantly, stable overexpression of KS-WNK1 significantly increases transepithelial Na+ transport in cortical collecting duct cells. Similarly, coexpression of KS-WNK1 and the epithelial Na+ channel in Fischer rat thyroid epithelial cells also stimulates Na+ current, suggesting that KS-WNK1 affects the subcellular location or activity but not the expression of epithelial Na+ channel. These observations suggest that stimulation of KS-WNK1 expression might be an important element of aldosterone-induced Na

Induction of the cell survival kinase Sgk1: A possible novel mechanism for α-phenyl-N-tert-butyl nitrone in experimental stroke.

Nitrones (e.g. α-phenyl-N-tert-butyl nitrone; PBN) are cerebroprotective in experimental stroke. Free radical trapping is their proposed mechanism. As PBN has low radical trapping potency, we tested Sgk1 induction as another possible mechanism. PBN was injected (100 mg/kg, i.p.) into adult male rats and mice. Sgk1 was quantified in cerebral tissue by microarray, quantitative RT-PCR and western analyses. Sgk1+/+ and Sgk1-/- mice were randomized to receive PBN or saline immediately following transient (60 min) occlusion of the middle cerebral artery. Neurological deficit was measured at 24 h and 48 h and infarct volume at 48 h post-occlusion. Following systemic PBN administration, rapid induction of Sgk1 was detected by microarray (at 4 h) and confirmed by RT-PCR and phosphorylation of the Sgk1-specific substrate NDRG1 (at 6 h). PBN-treated Sgk1+/+ mice had lower neurological deficit ( p < 0.01) and infarct volume ( p < 0.01) than saline-treated Sgk1+/+ mice. PBN-treated Sgk1-/- mice did not differ from saline-treated Sgk1-/- mice. Saline-treated Sgk1-/- and Sgk1+/+ mice did not differ. Brain Sgk3:Sgk1 mRNA ratio was 1.0:10.6 in Sgk1+/+ mice. Sgk3 was not augmented in Sgk1-/- mice. We conclude that acute systemic treatment with PBN induces Sgk1 in brain tissue. Sgk1 may play a part in PBN-dependent actions in acute brain ischemia

Functionally distinct PI 3-kinase pathways regulate myelination in the peripheral nervous system

Functionally and spatially distinct PI 3-K pathways act either early to promote myelination downstream of axonal Neuregulin1 or late to inhibit myelination downstream of α6β4 integrin and Sgk1

Phosphoinositide lipid second messengers: new paradigms for transepithelial signal transduction

Multiple forms of phosphatidylinositol are generated by differential phosphorylation of the inositol headgroup. These phosphoinositides, specifically PI(4,5)P2, have been implicated as modulators in a variety of transport processes. The data indicate that phosphoinositides can modulate transporters directly or via the activation of down-stream signaling components. The phosphoinositide pathway has been linked to changes in transporter kinetics, intracellular signaling, membrane targeting and membrane stability. Recent results obtained for several of the well-characterized transport systems suggest the need to reassess the role of PI(4,5)P2 and question whether lower abundance forms of the phosphoinositides, notably PI(3,4,5)P3 (PIP3) and PI(3,4)P2, are the pertinent transport regulators. In contrast to PI(4,5)P2, these latter forms represent a dynamic, regulated pool, the characteristics of which are more compatible with the nature of signaling intermediates. A recently described, novel transepithelial signaling pathway has been demonstrated for PIP3 in which a signal initiated on the basolateral membrane is transduced to the apical membrane entirely within the planar face of the inner leaflet of the plasma membrane. The new paradigms emerging from recent studies may be widely applicable to transporter regulation in other cell types and are particularly relevant for signaling in polarized cells

The Kidneys and Aldosterone/Mineralocorticoid Receptor System in Salt-Sensitive Hypertension

Strong evidence supports the ability of the aldosterone/mineralocorticoid receptor (MR) system to dominate long-term blood pressure control. It is also increasingly recognized as an important mediator of cardiovascular and renal diseases, particularly in the presence of excessive salt intake. In a subgroup of individuals with metabolic syndrome, adipocyte-derived aldosterone-releasing factors cause inappropriate secretion of aldosterone in the adrenal glands during salt loading, resulting in the development of salt-induced hypertension and cardiac and renal damage. On the other hand, emerging data reveal that aldosterone is not a sole regulator of MR activity. We have identified the signaling crosstalk between MR and small GTPase Rac1 as a novel pathway to facilitate MR signaling. Such a local control system for MR can also be relevant to the pathogenesis of salt-sensitive hypertension, and future studies will clarify the detailed mechanism for the intricate regulation of the aldosterone/MR cascade

Antiviral activity of the mineralocorticoid receptor NR3C2 against Herpes simplex virus Type 1 (HSV-1) infection

Abstract Analysis of a genome-scale RNA interference screen of host factors affecting herpes simplex virus type 1 (HSV-1) revealed that the mineralocorticoid receptor (MR) inhibits HSV-1 replication. As a ligand-activated transcription factor the MR regulates sodium transport and blood pressure in the kidney in response to aldosterone, but roles have recently been elucidated for the MR in other cellular processes. Here, we show that the MR and other members of the mineralocorticoid signalling pathway including HSP90 and FKBP4, possess anti-viral activity against HSV-1 independent of their effect on sodium transport, as shown by sodium channel inhibitors. Expression of the MR is upregulated upon infection in an interferon (IFN) and viral transcriptional activator VP16-dependent fashion. Furthermore, the MR and VP16, together with the cellular co-activator Oct-1, transactivate the hormone response element (HRE) present in the MR promoter and those of its transcriptional targets. As the MR induces IFN expression, our data suggests the MR is involved in a positive feedback loop that controls HSV-1 infection

Glucocorticoids and renal sodium transport:implications for hypertension and salt-sensitivity

The clinical manifestations of glucocorticoid excess include central obesity, hyperglycaemia, dyslipidaemia, electrolyte abnormalities and hypertension. A century on from Cushing's original case study, these cardinal features are prevalent in industrialized nations. Hypertension is the major modifiable risk factor for cardiovascular and renal disease and reflects underlying abnormalities of Na(+) homeostasis. Aldosterone is a master regulator of renal Na(+) transport but here we argue that glucocorticoids are also influential, particularly during moderate excess. The hypothalamic–pituitary–adrenal axis can affect renal Na(+) homeostasis on multiple levels, systemically by increasing mineralocorticoid synthesis and locally by actions on both the mineralocorticoid and glucocorticoid receptors, both of which are expressed in the kidney. The kidney also expresses both of the 11β-hydroxysteroid dehydrogenase (11βHSD) enzymes. The intrarenal generation of active glucocorticoid by 11βHSD1 stimulates Na(+) reabsorption; failure to downregulate the enzyme during adaption to high dietary salt causes salt-sensitive hypertension. The deactivation of glucocorticoid by 11βHSD2 underpins the regulatory dominance for Na(+) transport of mineralocorticoids and defines the ‘aldosterone-sensitive distal nephron’. In summary, glucocorticoids can stimulate renal transport processes conventionally attributed to the renin–angiotensin–aldosterone system. Importantly, Na(+) and volume homeostasis do not exert negative feedback on the hypothalamic–pituitary–adrenal axis. These actions are therefore clinically relevant and may contribute to the pathogenesis of hypertension in conditions associated with elevated glucocorticoid levels, such as the metabolic syndrome and chronic stress