Calcium regulation of endothelin-1 synthesis in rat inner medullary collecting duct

Pharmacological Inhibition of Transient Receptor Potential Vanilloid 4 Reduces Vasogenic Edema after Traumatic Brain Injury in Mice

Vasogenic edema results from blood-brain barrier (BBB) disruption after traumatic brain injury (TBI), and although it can be fatal, no promising therapeutic drugs have been developed as yet. Transient receptor potential vanilloid 4 (TRPV4) is a calcium-permeable channel that is sensitive to temperature and osmotic pressure. As TRPV4 is known to be responsible for various pathological conditions following brain injury, we investigated the effects of pharmacological TRPV4 antagonists on TBI-induced vasogenic edema in this study. A TBI model was established by inflicting fluid percussion injury (FPI) in the mouse cerebrum and cultured astrocytes. Vasogenic brain edema and BBB disruption were assessed based on brain water content and Evans blue (EB) extravasation into brain tissue, respectively. After FPI, brain water content and EB extravasation increased. Repeated intracerebroventricular administration of the specific TRPV4 antagonists HC-067047 and RN-1734 dose-dependently reduced brain water content and alleviated EB extravasation in FPI mice. Additionally, real-time PCR analysis indicated that administration of HC-067047 and RN-1734 reversed the FPI-induced increase in mRNA levels of endogenous causal factors for BBB disruption, including matrix metalloproteinase-9 (MMP-9), vascular endothelial growth factor-A (VEGF-A), and endothelin-1 (ET-1). In astrocytes, TRPV4 level was observed to be higher than that in brain microvascular endothelial cells. Treatment with HC-067047 and RN-1734 inhibited the increase in mRNA levels of MMP-9, VEGF-A, and ET-1 in cultured astrocytes subjected to in vitro FPI. These results suggest that pharmacological inhibition of TRPV4 is expected to be a promising therapeutic strategy for treating TBI-induced vasogenic edema.

Identification of Age-Specific and Common Key Regulatory Mechanisms Governing Eggshell Strength in Chicken Using Random Forests

In today's chicken egg industry, maintaining the strength of eggshells in longer laying cycles is pivotal for improving the persistency of egg laying. Eggshell development and mineralization underlie a complex regulatory interplay of various proteins and signaling cascades involving multiple organ systems. Understanding the regulatory mechanisms influencing this dynamic trait over time is imperative, yet scarce. To investigate the temporal changes in the signaling cascades, we considered eggshell strength at two different time points during the egg production cycle and studied the genotype-phenotype associations by employing the Random Forests algorithm on chicken genotypic data. For the analysis of corresponding genes, we adopted a well established systems biology approach to delineate gene regulatory pathways and master regulators underlying this important trait. Our results indicate that, while some of the master regulators (Slc22a1 and Sox11) and pathways are common at different laying stages of chicken, others (e.g., Scn11a, St8sia2, or the TGF- β pathway) represent age-specific functions. Overall, our results provide: (i) significant insights into age-specific and common molecular mechanisms underlying the regulation of eggshell strength; and (ii) new breeding targets to improve the eggshell quality during the later stages of the chicken production cycle.

Free radical scavenging decreases endothelin-1 excretion and glomerular albumin permeability during type 1 diabetes

Increased renal endothelin-1 (ET-1) production and an ET_A receptor-dependent increase in glomerular albumin permeability (P_alb) accompany type 1 diabetes mellitus (T1D). We hypothesized that T1D-induced oxidative stress contributes to renal ET-1 production and glomerular P_alb Male rats with streptozotocin-induced T1D were provided free access to drinking water without additives (T1D rats) or containing the free radical scavenger tempol (1 mmol/L; T1D+Tempol). After 3 weeks, T1D+Tempol rats displayed lower urinary excretion of thiobarbituric acid reactive substances and glomerular superoxide production (dihydroethidium staining) compared to T1D rats. Urinary ET-1 excretion and inner medullary (but not cortical or outer medullary) prepro-ET-1 mRNA expression were lower in the T1D+Tempol group than in the T1D group. P_alb, measured as the change in volume of isolated glomeruli upon exposure to oncotic gradients of albumin, was significantly lower in the T1D+Tempol group than in the T1D group. Tempol treatment did not alter protein excretion or creatinine clearance. These data support the postulate that oxidative stress contributes to glomerular P_alb and renal ET-1 production during the early phase of type 1 diabetes.

Na +/Ca2+ exchangers and Orai channels jointly refill endoplasmic reticulum (ER) Ca2+ via ER nanojunctions in vascular endothelial cells

We investigated the role of Na⁺/ Ca²⁺ exchange (NCX) in the refilling of endoplasmic reticulum (ER) Ca²⁺ in vascular endothelial cells under various conditions of cell stimulation and plasma membrane (PM) polarization. Better understanding of the mechanisms behind basic ER Ca²⁺ content regulation is important, since current hypotheses on the possible ultimate causes of ER stress point to deterioration of the Ca²⁺ transport mechanism to/from ER itself. We measured [Ca²⁺]_i temporal changes by Fura-2 fluorescence under experimental protocols that inhibit a host of transporters (NCX, Orai, non-selective transient receptor potential canonical (TRPC) channels, sarco/endoplasmic reticulum Ca²⁺ ATPase (SERCA), Na⁺/ K⁺ ATPase (NKA)) involved in the Ca²⁺ communication between the extracellular space and the ER. Following histamine-stimulated ER Ca²⁺ release, blockade of NCX Ca²⁺-influx mode (by 10 μM KB-R7943) diminished the ER refilling capacity by about 40%, while in Orai1 dominant negative-transfected cells NCX blockade attenuated ER refilling by about 60%. Conversely, inhibiting the ouabain sensitive NKA (10 nM ouabain), which may be localized in PM-ER junctions, increased the ER Ca²⁺ releasable fraction by about 20%, thereby supporting the hypothesis that this process of privileged ER refilling is junction-mediated. Junctions were observed in the cell ultrastructure and their main parameters of membrane separation and linear extension were (9.6 ± 3.8) nm and (128 ± 63) nm, respectively. Our findings point to a process of privileged refilling of the ER, in which NCX and store-operated Ca²⁺ entry via the stromal interaction molecule (STIM)-Orai system are the sole protagonists. These results shed light on the molecular machinery involved in the function of a previously hypothesized subplasmalemmal Ca²⁺ control unit during ER refilling with extracellular Ca²⁺.

Osmolar regulation of endothelin-1 production by the inner medullary collecting duct

AIMS

Endothelin-1 (ET-1) is an autocrine inhibitor of collecting duct (CD) Na(+) and water reabsorption. CD ET-1 production is increased by a high salt diet and is important in promoting a natriuretic response. The mechanisms by which a high salt diet enhances CD ET-1 are being uncovered. In particular, elevated tubule fluid flow, as occurs in salt loading, enhances CD ET-1 synthesis. Tubule fluid solute content and interstitial osmolality can also be altered by a high salt diet, however their effect on CD ET-1 alone, or in combination with flow, is poorly understood.

MAIN METHODS

ET-1 mRNA production by a mouse inner medullary CD cell line (mIMCD3) in response to changing flow and/or osmolality was assessed.

KEY FINDINGS

Flow or hyperosmolality (using NaCl, mannitol or urea) individually caused an ~2-fold increase in ET-1 mRNA, while flow and hyperosmolality together increased ET-1 mRNA by ~14 fold. The hyperosmolality effect alone and the synergistic effect of flow + hyperosmolality was inhibited by chelation of intracellular Ca(2+), however were not altered by blockade of downstream Ca(2+)-signaling pathways (calcineurin or NFATc), inhibition of cellular Ca(2+) entry channels (purinergic receptors or polycystin-2), or blockade of the epithelial Na(+) channel. Inhibition of NFAT5 with rottlerin or NFAT5 siRNA greatly reduced the stimulatory effect of osmolality alone and osmolality + flow on mIMCD3 ET-1 mRNA levels.

SIGNIFICANCE

Both flow and osmolality individually and synergistically stimulate mIMCD3 ET-1 mRNA content. These findings may be relevant to explaining high salt diet induction of CD ET-1 production.

P2Y12 Receptor Localizes in the Renal Collecting Duct and Its Blockade Augments Arginine Vasopressin Action and Alleviates Nephrogenic Diabetes Insipidus

P2Y12 receptor (P2Y12-R) signaling is mediated through Gi, ultimately reducing cellular cAMP levels. Because cAMP is a central modulator of arginine vasopressin (AVP)-induced water transport in the renal collecting duct (CD), we hypothesized that if expressed in the CD, P2Y12-R may play a role in renal handling of water in health and in nephrogenic diabetes insipidus. We found P2Y12-R mRNA expression in rat kidney, and immunolocalized its protein and aquaporin-2 (AQP2) in CD principal cells. Administration of clopidogrel bisulfate, an irreversible inhibitor of P2Y12-R, significantly increased urine concentration and AQP2 protein in the kidneys of Sprague-Dawley rats. Notably, clopidogrel did not alter urine concentration in Brattleboro rats that lack AVP. Clopidogrel administration also significantly ameliorated lithium-induced polyuria, improved urine concentrating ability and AQP2 protein abundance, and reversed the lithium-induced increase in free-water excretion, without decreasing blood or kidney tissue lithium levels. Clopidogrel administration also augmented the lithium-induced increase in urinary AVP excretion and suppressed the lithium-induced increase in urinary nitrates/nitrites (nitric oxide production) and 8-isoprostane (oxidative stress). Furthermore, selective blockade of P2Y12-R by the reversible antagonist PSB-0739 in primary cultures of rat inner medullary CD cells potentiated the expression of AQP2 and AQP3 mRNA, and cAMP production induced by dDAVP (desmopressin). In conclusion, pharmacologic blockade of renal P2Y12-R increases urinary concentrating ability by augmenting the effect of AVP on the kidney and ameliorates lithium-induced NDI by potentiating the action of AVP on the CD. This strategy may offer a novel and effective therapy for lithium-induced NDI.

Flow regulation of endothelin-1 production in the inner medullary collecting duct

Collecting duct-derived endothelin (ET)-1 is an autocrine inhibitor of Na(+) and water reabsorption; its deficiency causes hypertension and water retention. Extracellular fluid volume expansion increases collecting duct ET-1, thereby promoting natriuresis and diuresis; however, how this coupling between volume expansion and collecting duct ET-1 occurs is incompletely understood. One possibility is that volume expansion increases tubular fluid flow. To investigate this, cultured IMCD3 cells were subjected to static or flow conditions. Exposure to a shear stress of 2 dyn/cm(2) for 2 h increased ET-1 mRNA content by ∼2.3-fold. Absence of perfusate Ca(2+), chelation of intracellular Ca(2+), or inhibition of Ca(2+) signaling (calmodulin, Ca(2+)/calmodulin-dependent kinase, calcineurin, PKC, or phospholipase C) prevented the flow response. Evaluation of possible flow-activated Ca(2+) entry pathways revealed no role for transient receptor potential (TRP)C3, TRPC6, and TRPV4; however, cells with TRPP2 (polycystin-2) knockdown had no ET-1 flow response. Flow increased intracellular Ca(2+) was blunted in TRPP2 knockdown cells. Nonspecific blockade of P2 receptors, as well as specific inhibition of P2X7 and P2Y2 receptors, prevented the ET-1 flow response. The ET-1 flow response was not affected by inhibition of either epithelial Na(+) channels or the mitochondrial Na(+)/Ca(2+) exchanger. Taken together, these findings provide evidence that in IMCD3 cells, flow, via polycystin-2 and P2 receptors, engages Ca(2+)-dependent signaling pathways that stimulate ET-1 synthesis.

In vivo and ex vivo analysis of tubule function

Analysis of tubule function with in vivo and ex vivo approaches has been instrumental in revealing renal physiology. This work allows assignment of functional significance to known gene products expressed along the nephron, primary of which are proteins involved in electrolyte transport and regulation of these transporters. Not only we have learned much about the key roles played by these transport proteins and their proper regulation in normal physiology but also the combination of contemporary molecular biology and molecular genetics with in vivo and ex vivo analysis opened a new era of discovery informative about the root causes of many renal diseases. The power of in vivo and ex vivo analysis of tubule function is that it preserves the native setting and control of the tubule and proteins within tubule cells enabling them to be investigated in a "real-life" environment with a high degree of precision. In vivo and ex vivo analysis of tubule function continues to provide a powerful experimental outlet for testing, evaluating, and understanding physiology in the context of the novel information provided by sequencing of the human genome and contemporary genetic screening. These tools will continue to be a mainstay in renal laboratories as this discovery process continues and as we continue to identify new gene products functionally compromised in renal disease.

Physiology of endothelin and the kidney

Since its discovery in 1988 as an endothelial cell-derived peptide that exerts the most potent vasoconstriction of any known endogenous compound, endothelin (ET) has emerged as an important regulator of renal physiology and pathophysiology. This review focuses on how the ET system impacts renal function in health; it is apparent that ET regulates multiple aspects of kidney function. These include modulation of glomerular filtration rate and renal blood flow, control of renin release, and regulation of transport of sodium, water, protons, and bicarbonate. These effects are exerted through ET interactions with almost every cell type in the kidney, including mesangial cells, podocytes, endothelium, vascular smooth muscle, every section of the nephron, and renal nerves. In addition, while not the subject of the current review, ET can also indirectly affect renal function through modulation of extrarenal systems, including the vasculature, nervous system, adrenal gland, circulating hormones, and the heart. As will become apparent, these pleiotropic effects of ET are of fundamental physiologic importance in the control of renal function in health. In addition, to help put these effects into perspective, we will also discuss, albeit to a relatively limited extent, how alterations in the ET system can contribute to hypertension and kidney disease.

Role of collecting duct endothelin in control of renal function and blood pressure

Over 26,000 manuscripts have been published dealing with endothelins since their discovery 25 years ago. These peptides, and particularly endothelin-1 (ET-1), are expressed by, bind to, and act on virtually every cell type in the body, influencing multiple biological functions. Among these actions, the effects of ET-1 on arterial pressure and volume homeostasis have been most extensively studied. While ET-1 modulates arterial pressure through regulation of multiple organ systems, the peptide's actions in the kidney in general, and the collecting duct in particular, are of unique importance. The collecting duct produces large amounts of ET-1 that bind in an autocrine manner to endothelin A and B receptors, causing inhibition of Na(+) and water reabsorption; absence of collecting duct ET-1 or its receptors is associated with marked salt-sensitive hypertension. Collecting duct ET-1 production is stimulated by Na(+) and water loading through local mechanisms that include sensing of salt and other solute delivery as well as shear stress. Thus the collecting duct ET-1 system exists, at least in part, to detect alterations in, and maintain homeostasis for, extracellular fluid volume. Derangements in collecting duct ET-1 production may contribute to the pathogenesis of genetic hypertension. Blockade of endothelin receptors causes fluid retention due, in large part, to inhibition of the action of ET-1 in the collecting duct; this side effect has substantially limited the clinical utility of this class of drugs. Herein, the biology of the collecting duct ET-1 system is reviewed, with particular emphasis on key issues and questions that need addressing.

Novel regulation of endothelin-1 promoter activity by protein kinase C

Endothelin-1 (ET-1) is produced in unusually large amounts by the renal collecting duct and acts locally to control renal salt and water excretion and arterial pressure; disorders of collecting duct ET-1 activity can cause marked hypertension. The mechanisms regulating collecting duct ET-1 synthesis are, however, poorly understood. In this study, we investigated the role of protein kinase C (PKC), a known regulator of ET-1 production in endothelial cells, in (1) the control of collecting duct ET-1 production; and (2) the modulation of ET-1 promoter region activity. Cultured rat inner medullary collecting duct (IMCD) cells were studied. Calphostin C, a PKC inhibitor, greatly reduced IMCD ET-1 release. Sustained exposure to phorbol myristate acetate (PMA) also decreased ET-1 secretion. PKC inhibition decreased steady-state ET-1 mRNA content. A brief exposure (15 min) to PMA augmented ET-1 mRNA levels, while prolonged PMA exposure (120 min) reduced ET-1 mRNA content, PKC inhibition did not affect ET-1 mRNA stability. Transfection of ET-1 promoter-luciferase reporter constructs into IMCD cells demonstrated that PKC inhibition reduced activity of only the larger promoter fragments (containing at least 1,725 bp 5' to the ET-1 gene transcription start site). Mutation of a previously identified AP-1 site at -186 in the ET-1 promoter greatly reduced activity of transfected ET-1 promoter-reporter constructs (containing 366 or 1,725 bp 5' to the transcription start site); however, this region appears not to be regulated by PKC in IMCD cells. In summary, PKC stimulates collecting duct ET-1 synthesis via transcriptional activation of the ET-1 promoter. Such transcriptional activation occurs at a heretofore undescribed PKC-regulated region of the ET-1 promoter.

Na delivery and ENaC mediate flow regulation of collecting duct endothelin-1 production

Collecting duct (CD) endothelin-1 (ET-1) is an important autocrine inhibitor of Na and water transport. CD ET-1 production is stimulated by extracellular fluid volume expansion and tubule fluid flow, suggesting a mechanism coupling CD Na delivery and ET-1 synthesis. A mouse cortical CD cell line, mpkCCDc14, was subjected to static or flow conditions for 2 h at 2 dyn/cm(2), followed by determination of ET-1 mRNA content. Flow with 300 mosmol/l NaCl increased ET-1 mRNA to 65% above that observed under static conditions. Increasing perfusate osmolarity to 450 mosmol/l with NaCl or Na acetate increased ET-1 mRNA to ∼184% compared with no flow, which was not observed when osmolarity was increased using mannitol or urea. Reducing Na concentration to 150 mosmol/l while maintaining total osmolarity at 300 mosmol/l with urea or mannitol decreased the flow response. Inhibition of epithelial Na channel (ENaC) with amiloride or benzamil abolished the flow response, suggesting involvement of ENaC in flow-regulated ET-1 synthesis. Aldosterone almost doubled the flow response. Since Ca(2+) enhances CD ET-1 production, the involvement of plasma membrane and mitochondrial Na/Ca(2+) exchangers (NCX) was assessed. SEA0400 and KB-R7943, plasma membrane NCX inhibitors, did not affect the flow response. However, CGP37157, a mitochondrial NCX inhibitor, abolished the response. In summary, the current study indicates that increased Na delivery, leading to ENaC-mediated Na entry and mitochondrial NCX activity, is involved in flow-stimulated CD ET-1 synthesis. This constitutes the first report of either ENaC or mitochondrial NCX regulation of an autocrine factor in any biologic system.

Regulation of blood pressure and salt homeostasis by endothelin

Endothelin (ET) peptides and their receptors are intimately involved in the physiological control of systemic blood pressure and body Na homeostasis, exerting these effects through alterations in a host of circulating and local factors. Hormonal systems affected by ET include natriuretic peptides, aldosterone, catecholamines, and angiotensin. ET also directly regulates cardiac output, central and peripheral nervous system activity, renal Na and water excretion, systemic vascular resistance, and venous capacitance. ET regulation of these systems is often complex, sometimes involving opposing actions depending on which receptor isoform is activated, which cells are affected, and what other prevailing factors exist. A detailed understanding of this system is important; disordered regulation of the ET system is strongly associated with hypertension and dysregulated extracellular fluid volume homeostasis. In addition, ET receptor antagonists are being increasingly used for the treatment of a variety of diseases; while demonstrating benefit, these agents also have adverse effects on fluid retention that may substantially limit their clinical utility. This review provides a detailed analysis of how the ET system is involved in the control of blood pressure and Na homeostasis, focusing primarily on physiological regulation with some discussion of the role of the ET system in hypertension.

Flow regulation of collecting duct endothelin-1 production

Collecting duct (CD) endothelin-1 (ET-1) is an important autocrine inhibitor of CD Na(+) reabsorption. Salt loading is thought to increase CD ET-1 production; however, definitive evidence of this, as well as understanding of the mechanisms transducing this effect, is lacking. Tubule fluid flow increases in response to Na(+) loading; hence, we studied flow modulation of CD ET-1 production. Three days of a high-salt diet increased mouse and rat inner medullary CD (IMCD) ET-1 mRNA expression. Acute furosemide infusion increased urinary ET-1 excretion in anesthetized rats. Primary cultures of mouse or rat IMCD detached in response to flow using a closed perfusion chamber, consequently a CD cell line (mpkCCDcl4) was examined. Flow increased ET-1 mRNA at shear stress rates exceeding 1 dyne/cm(2), with the maximal effect seen between 2 and 10 dyne/cm(2). Induction of ET-1 mRNA was first evident after 1 h, and most apparent after 2 h, of flow. Inhibition of calmodulin or dihydropyridine-sensitive Ca(2+) channels did not alter the flow response; however, chelation of intracellular Ca(2+) or removal of extracellular Ca(2+) largely prevented flow-stimulated ET-1 mRNA accumulation. Downregulation of protein kinase C (PKC) using phorbol 12-myristate 13-acetate, or PKC inhibition with calphostin C, markedly reduced flow-stimulated ET-1 mRNA levels. Flow-stimulated ET-1 mRNA accumulation was abolished by inhibition of phospholipase C (PLC). Taken together, these data indicate that flow increases CD ET-1 production and this is dependent on extracellular and intracellular Ca(2+), PKC, and PLC. These studies suggest a novel pathway for coupling alterations in extracellular fluid volume to CD ET-1 production and ultimately control of CD Na(+) reabsorption.

Cooperative role of ETA and ETB receptors in mediating the diuretic response to intramedullary hyperosmotic NaCl infusion

Acute intramedullary infusion of hyperosmotic NaCl, used to simulate a high-salt diet-induced increase of medullary osmolality, increases urine production and endothelin release from the kidney. To determine whether endothelin mediates this diuretic and natriuretic response, urine flow and Na(+) excretion rate were measured during acute intramedullary infusion of hyperosmotic NaCl in anesthetized rats, with or without endothelin receptor antagonism. Isosmotic NaCl was infused into the left renal medulla during an equilibration period and 30-min baseline period, followed by hyperosmotic NaCl for two additional 30-min periods. Hyperosmotic NaCl infusion significantly increased urine flow of vehicle-treated rats (from 5.9 ± 0.9 to 11.1 ± 1.8 μl/min). Systemic ET(B) receptor blockade enhanced this effect (A-192621; from 7.7 ± 1.1 to 18.7 ± 2.9 μl/min; P < 0.05), ET(A) receptor blockade (ABT-627) had no significant effect alone, but the diuresis was markedly attenuated by combined ABT-627 and A-192621 administration (from 4.4 ± 0.7 to 5.4 ± 0.9 μl/min). Mean arterial pressures overall were not significantly different between groups. Surprisingly, the natriuretic response to hyperosmotic NaCl infusion was not significantly altered by systemic endothelin receptor blockade, and furthermore, intramedullary ET(B) receptor blockade enhanced the diuretic and natriuretic response to hyperosmotic NaCl infusion. ET(A) receptor blockade significantly attenuated both the diuretic and natriuretic responses to hyperosmotic NaCl infusion in ET(B) receptor-deficient sl/sl rats. These results demonstrate an important role of endothelin in mediating diuretic responses to intramedullary infusion of hyperosmotic NaCl. Moreover, these data suggest ET(A) and ET(B) receptors are both required for the full diuretic and natriuretic actions of endothelin.

Endothelin-1 gene regulation

Over two decades of research have demonstrated that the peptide hormone endothelin-1 (ET-1) plays multiple, complex roles in cardiovascular, neural, pulmonary, reproductive, and renal physiology. Differential and tissue-specific production of ET-1 must be tightly regulated in order to preserve these biologically diverse actions. The primary mechanism thought to control ET-1 bioavailability is the rate of transcription from the ET-1 gene (edn1). Studies conducted on a variety of cell types have identified key transcription factors that govern edn1 expression. With few exceptions, the cis-acting elements bound by these factors have been mapped in the edn1 regulatory region. Recent evidence has revealed new roles for some factors originally believed to regulate edn1 in a tissue or hormone-specific manner. In addition, other mechanisms involved in epigenetic regulation and mRNA stability have emerged as important processes for regulated edn1 expression. The goal of this review is to provide a comprehensive overview of the specific factors and signaling systems that govern edn1 activity at the molecular level.

The interdependence of endothelin-1 and calcium: a review

The 21-amino-acid peptide ET-1 (endothelin-1) regulates a diverse array of physiological processes, including vasoconstriction, angiogenesis, nociception and cell proliferation. Most of the effects of ET-1 are associated with an increase in intracellular calcium concentration. The calcium influx and mobilization pathways activated by ET-1, however, vary immensely. The present review begins with the basics of calcium signalling and investigates the different ways intracellular calcium concentration can increase in response to a stimulus. The focus then shifts to ET-1, and discusses how ET receptors mobilize calcium. We also examine how disease alters calcium-dependent responses to ET-1 by discussing changes to ET-1-mediated calcium signalling in hypertension, as there is significant interest in the role of ET-1 in this important disease. A list of unanswered questions regarding ET-mediated calcium signals are also presented, as well as perspectives for future research of calcium mobilization by ET-1.

Identification of two nuclear factor of activated T-cells (NFAT)-response elements in the 5'-upstream regulatory region of the ET-1 promoter

Collecting duct-derived ET-1 regulates salt excretion and blood pressure. We have reported the presence of an inner medullary collecting duct (IMCD)-specific enhancer region in the 5'-upstream ET-1 promoter (Strait, K. A., Stricklett, P. K., Kohan, J. L., Miller, M. B., and Kohan, D. E. (2007) Am. J. Physiol. Renal Physiol. 293, F601-F606). The current studies provide further characterization of the ET-1 5'-upstream distal promoter to identify the IMCD-specific enhancer elements. Deletion studies identified two regions of the 5'-upstream ET-1 promoter, -1725 to -1319 bp and -1319 to -1026 bp, which were required for maximal promoter activity in transfected rat IMCD cells. Transcription factor binding site analysis of these regions identified two consensus nuclear factor of activated T-cells (NFAT) binding sites at -1263 and -1563. EMSA analysis using nuclear extracts from IMCD cells showed that both the -1263 and the -1563 NFAT sites in the ET-1 distal promoter competed for NFAT binding to previously identified NFAT sites in the IL-2 and TNF genes. Gel supershift analysis showed that each of the NFAT binding sites in the ET-1 promoter bound NFAT proteins derived from IMCD nuclear extracts, but they selectively bound different NFAT isoforms; ET-1263 bound NFATc1, whereas ET-1563 bound NFATc3. Site-directed mutagenesis of either the ET-1263 or the ET-1563 sites prevented NFAT binding and reduced ET-1 promoter activity. Thus, NFAT appears to be an important regulator of ET-1 transcription in IMCD cells, and thus, it may play a role in controlling blood pressure through ET-1 regulation of renal salt excretion.

Endothelin-1 inhibits the epithelial Na+ channel through betaPix/14-3-3/Nedd4-2

Epithelial Na+ channels (ENaCs) mediate sodium reabsorption in the cortical collecting duct (CCD), but the regulatory pathways that modulate the activity of these channels are incompletely understood. Here, we observed that endothelin-1 (ET-1) attenuates ENaC activity acutely by reducing the channel's open probability and chronically by decreasing the number of channels in the plasma membrane. To investigate whether beta1Pix, a signaling protein activated by ET-1, mediates ENaC activity, we reconstituted ENaC in CHO cells with or without coexpressed beta1Pix and found that beta1Pix negatively regulates ENaC. Knockdown of betaPix in native principal cells abolished the ET-1-induced decrease in ENaC channel number. Furthermore, we found that betaPix does not decrease ENaC activity through its guanine nucleotide exchange factor (GEF) activity for Rac1 and Cdc42. Instead, coexpression of beta1Pix mutant constructs revealed that beta1Pix affects ENaC activity through binding 14-3-3 proteins. Coimmunoprecipitation experiments supported a physical interaction between beta1Pix and 14-3-3beta in cultured principal cells. Coexpression of 14-3-3beta increased ENaC activity in CHO cells, but concomitant expression of beta1Pix attenuated this increase. Recruitment of 14-3-3beta by beta1Pix impaired the interaction of 14-3-3beta with the ubiquitin ligase Nedd4-2, thereby promoting ubiquitination and degradation of ENaC. Taken together, these results suggest that the inhibitory effects of chronic ET-1 on ENaC result from betaPix interacting with the 14-3-3/Nedd4-2 pathway.

Characterization of vasopressin-responsive collecting duct adenylyl cyclases in the mouse

Little is known about collecting duct adenylyl cyclase (AC) isoforms or regulation in the mouse. We performed RT-PCR for AC isoforms 1-9 in microdissected cortical (CCD) and outer medullary (OMCD) and acutely isolated inner medullary (IMCD) collecting duct. All collecting duct regions contained AC3, AC4, and AC6 mRNA, while CCD and OMCD, but not IMCD, also contained AC5 mRNA. Acutely isolated IMCD expressed AC3, AC4, and AC6 proteins by Western blot analysis. The mIMCD3 cell line expressed AC2, AC3, AC4, AC5, and AC6 mRNA; M-1 CCD cells expressed AC2, 3, 4, and 6, while mpkCCD cell lines contained AC3, AC4, and AC6 mRNA. AVP stimulated cAMP accumulation in acutely isolated mouse IMCD; this was reduced by chelation of extracellular calcium (EGTA) and almost completely abolished by blockade of calmodulin (W-7). Blockade of calmodulin kinase with KN-93 or endoplasmic reticulum calcium ATPase (thapsigargin) also reduced the AVP response. A similar inhibitory effect of W-7, KN-93, and thapsigargin was seen on forskolin-stimulated cAMP content in acutely isolated mouse IMCD. These three agents had the same pattern of blockade of AVP- or forskolin-stimulated AC activity in acutely isolated rat IMCD. AVP responsiveness in primary cultures of mouse IMCD was also reduced by W-7, KN-93, and thapsigargin. Small interfering RNA (siRNA) designed to knock down AC3 or AC6 in primary cultured mouse IMCD significantly reduced AVP-stimulated cAMP accumulation. Together, these data are consistent with a role of AC3 and AC6 in the activation of mouse collecting duct by AVP.

Biology of endothelin receptors in the collecting duct

The collecting duct endothelin (ET) system, involving ET-1 and its two receptors, is involved in the physiologic regulation of renal sodium (Na), water, and acid excretion. Based on in vitro studies and experiments using genetically engineered rodents, the physiology of this system in the collecting duct is being elucidated. Activation of endothelin B (ETB) receptors on principal cells causes inhibition of Na transport through signaling pathways involving src kinase, MAPK1/2, nitric oxide, and possibly prostaglandin E2 (PGE2). Principal-cell ETB receptors also cause inhibition of water transport through protein kinase C-mediated inhibition of AVP-dependent cAMP accumulation. ETB receptors expressed on intercalated cells augment acid secretion, possibly through nitric oxide-dependent mechanisms. The role of endothelin A (ETA) receptors in the collecting duct remains unclear; however, recent evidence suggests that these receptors can exert natriuretic and diuretic effects. Further complexity is lent to this system by studies indicating that ETA and ETB receptors can homo- and hetero-dimerize, with possible functional consequences. This brief review will describe our current state of knowledge about this complex regulatory system in the collecting duct, and will identify clinically relevant issues that need addressing.

Mammalian cell penetration, siRNA transfection, and DNA transfection by supercharged proteins

Nucleic acid reagents, including small interfering RNA (siRNA) and plasmid DNA, are important tools for the study of mammalian cells and are promising starting points for the development of new therapeutic agents. Realizing their full potential, however, requires nucleic acid delivery reagents that are simple to prepare, effective across many mammalian cell lines, and nontoxic. We recently described the extensive surface mutagenesis of proteins in a manner that dramatically increases their net charge. Here, we report that superpositively charged green fluorescent proteins, including a variant with a theoretical net charge of +36 (+36 GFP), can penetrate a variety of mammalian cell lines. Internalization of +36 GFP depends on nonspecific electrostatic interactions with sulfated proteoglycans present on the surface of most mammalian cells. When +36 GFP is mixed with siRNA, protein-siRNA complexes approximately 1.7 mum in diameter are formed. Addition of these complexes to five mammalian cell lines, including four that are resistant to cationic lipid-mediated siRNA transfection, results in potent siRNA delivery. In four of these five cell lines, siRNA transfected by +36 GFP suppresses target gene expression. We show that +36 GFP is resistant to proteolysis, is stable in the presence of serum, and extends the serum half-life of siRNA and plasmid DNA with which it is complexed. A variant of +36 GFP can mediate DNA transfection, enabling plasmid-based gene expression. These findings indicate that superpositively charged proteins can overcome some of the key limitations of currently used transfection agents.