Distribution of postsynaptic density proteins in rat kidney: relationship to neuronal nitric oxide synthase

The role of nitric oxide in sepsis-associated kidney injury

Sepsis is one of the leading causes of acute kidney injury (AKI), and several mechanisms including microcirculatory alterations, oxidative stress, and endothelial cell dysfunction are involved. Nitric oxide (NO) is one of the common elements to all these mechanisms. Although all three nitric oxide synthase (NOS) isoforms are constitutively expressed within the kidneys, they contribute in different ways to nitrergic signaling. While the endothelial (eNOS) and neuronal (nNOS) isoforms are likely to be the main sources of NO under basal conditions and participate in the regulation of renal hemodynamics, the inducible isoform (iNOS) is dramatically increased in conditions such as sepsis. The overexpression of iNOS in the renal cortex causes a shunting of blood to this region, with consequent medullary ischemia in sepsis. Differences in the vascular reactivity among different vascular beds may also help to explain renal failure in this condition. While most of the vessels present vasoplegia and do not respond to vasoconstrictors, renal microcirculation behaves differently from nonrenal vascular beds, displaying similar constrictor responses in control and septic conditions. The selective inhibition of iNOS, without affecting other isoforms, has been described as the ideal scenario. However, iNOS is also constitutively expressed in the kidneys and the NO produced by this isoform is important for immune defense. In this sense, instead of a direct iNOS inhibition, targeting the NO effectors such as guanylate cyclase, potassium channels, peroxynitrite, and S-nitrosothiols, may be a more interesting approach in sepsis-AKI and further investigation is warranted.

Decreased Podocyte Vesicle Transcytosis and Albuminuria in APC C-Terminal Deficiency Mice with Puromycin-Induced Nephrotic Syndrome

In minimal change nephrotic syndrome, podocyte vesicle transport is enhanced. Adenomatous polyposis coli (APC) anchors microtubules to cell membranes and plays an important role in vesicle transport. To clarify the role of APC in vesicle transport in podocytes, nephrotic syndrome was induced by puromycin amino nucleoside (PAN) injection in mice expressing APC1638T lacking the C-terminal of microtubule-binding site (APC1638T mouse); this was examined in renal tissue changes. The kidney size and glomerular area of APC1638T mice were reduced (p = 0.014); however, the number of podocytes was same between wild-type (WT) mice and APC1638T mice. The ultrastructure of podocyte foot process was normal by electron microscopy. When nephrotic syndrome was induced, the kidneys of WT+PAN mice became swollen with many hyaline casts, whereas these changes were inhibited in the kidneys of APC1638T+PAN mice. Electron microscopy showed foot process effacement in both groups; however, APC1638T+PAN mice had fewer vesicles in the basal area of podocytes than WT+PAN mice. Cytoplasmic dynein-1, a motor protein for vesicle transport, and α-tubulin were significantly reduced in APC1638T+PAN mice associated with suppressed urinary albumin excretion compared to WT+PAN mice. In conclusion, APC1638T mice showed reduced albuminuria associated with suppressed podocyte vesicle transport when minimal change nephrotic syndrome was induced.

Glomerular albumin filtration through podocyte cell body in puromycin aminonucleoside nephrotic rat

It is an a priori concept that protein molecules including albumin are filtrated through the slit membrane between the foot processes of podocytes. However, foot processes are effaced and the number of slit membranes is reduced in nephrotic syndrome, suggesting another pathway of albumin filtration through the foot process cell body. Thus, we investigated the pathway of gold-and fluorescein isothiocyanate (FITC)-labeled albumin filtration in the puromycin aminonucleoside (PAN) model of nephrotic syndrome in the rat. PAN rats at day 7 with established nephrotic proteinuria were injected with 8-nm gold-labeled albumin and FITC-labeled albumin through the jugular vein followed by kidney fixation at 10 or 30 min. Goldlabeled albumin was accumulated in the paramesangial area and in the endosomes of glomerular endothelial cells of both control and PAN rats by electron microscopy. On the other hand, FITC-labeled albumin was detected between foot processes in the control but more in the podocyte cell body in the PAN rat. In conclusion, albumin will be filtrated through the decreased numbers of slit diaphragms; however, albumin can be also taken up in the podocyte, the mesangium, and the glomerular endothelium, suggesting that there might be other routes of glomerular albumin clearance in nephrotic syndrome.

Isoform-specific regulation by N(G),N(G)-dimethylarginine dimethylaminohydrolase of rat serum asymmetric dimethylarginine and vascular endothelium-derived relaxing factor/NO

Asymmetric dimethylarginine (ADMA), which inhibits NO synthase, is inactivated by N(G),N(G)-dimethylarginine dimethylaminohydrolase (DDAH). We tested whether DDAH-1 or -2 regulates serum ADMA (S(ADMA)) and/or endothelium-derived relaxing factor (EDRF)/NO. Small inhibitory (si)RNAs targeting DDAH-1 or -2, or an siRNA control were given intravenously to rats. After 72 hours, EDRF/NO was assessed from acetylcholine-induced, NO synthase-dependent relaxation and 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate for NO activity in isolated mesenteric resistance vessels (MRVs). Expression of mRNA for DDAH-1 versus -2 was 2- and 7-fold higher in the kidney cortex and liver, respectively, whereas expression of DDAH-2 versus -1 was 5-fold higher in MRVs. The proteins and mRNAs for DDAH-1 or -2 were reduced selectively by 35% to 85% in the kidney cortex, liver, and MRVs 72 hours following the corresponding siRNA. S(ADMA) was increased only after siDDAH-1 (266+/-25 versus 342+/-39 [mean+/-SD] nmol x L(-1); P<0.005), whereas EDRF/NO responses and NO activity were not changed consistently by siDDAH-1 but were greatly reduced after siDDAH-2. Mean arterial pressure was not changed significantly by any siRNA. In conclusion, S(ADMA) is regulated by DDAH-1, which is expressed at sites of ADMA metabolism in the kidney cortex and liver, whereas EDRF/NO is regulated primarily by DDAH-2, which is expressed strongly in blood vessels. This implies specific functions of DDAH isoforms.

Double-Edged Action of SOD Mimetic in Diabetic Nephropathy

Oxidative stress plays an important role in the pathogenesis of diabetic complications, and we investigated the effect of superoxide dismutase (SOD) mimetic, tempol, in diabetic nephropathy. Streptozotocin-induced diabetic rats were treated with tempol from 2 weeks until 8 weeks. The expression of NADPH oxidase, catalase, and myeloperoxidase (MPO), superoxide dismutase activity, and production of peroxide and hypochlorite were evaluated. Tempol treatment prevented the increase in NADPH oxidase and peroxide production in the glomeruli of diabetic rat. Catalase was decreased without change in SOD activity, and MPO was enhanced in the kidney of diabetic rats. Tempol treatment stimulated SOD activity and increased the conversion of superoxide to hydrogen peroxide, and hydrogen peroxide on its hand was converted to hypochlorite by the increased MPO. The reduction of peroxide by tempol was followed by the decrease in TGF-beta and mesangial matrix expansion. However, tempol did not reduce hypochlorite or urinary protein excretion. In conclusion, tempol inhibited glomerular matrix expansion via suppression of peroxide production and TGF-beta, but it failed to reduce proteinuria, probably due to the increased hypochlorite production in diabetic nephropathy.

Nitric oxide in the kidney: functions and regulation of synthesis

In the kidney nitric oxide (NO) has numerous important functions including the regulation of renal haemodynamics, maintenance of medullary perfusion, mediation of pressure-natriuresis, blunting of tubuloglomerular feedback, inhibition of tubular sodium reabsorption and modulation of renal sympathetic neural activity. The net effect of NO in the kidney is to promote natriuresis and diuresis. Significantly, deficient renal NO synthesis has been implicated in the pathogenesis of hypertension. All three isoforms of nitric oxide synthase (NOS), namely neuronal NOS (nNOS or NOS1), inducible NOS (iNOS or NOS2) and endothelial NOS (eNOS or NOS3) are reported to contribute to NO synthesis in the kidney. The regulation of NO synthesis in the kidney by NOSs is complex and incompletely understood. Historically, many studies of NOS regulation in the kidney have emphasized the role of variations in gene transcription and translation. It is increasingly appreciated, however, that the constitutive NOS isoforms (nNOS and eNOS) are also subject to rapid regulation by post-translational mechanisms such as Ca(2+) flux, serine/threonine phosphorylation and protein-protein interactions. Recent studies have emphasized the role of post-translational regulation of nNOS and eNOS in the regulation of NO synthesis in the kidney. In particular, a role for phosphorylation of nNOS and eNOS at both activating and inhibitory sites is emerging in the regulation of NO synthesis in the kidney. This review summarizes the roles of NO in renal physiology and discusses recent advances in the regulation of eNOS and nNOS in the kidney by post-translational mechanisms such as serine/threonine phosphorylation.

Role of macula densa neuronal nitric oxide synthase in renal diseases

Macula densa cells have an important role in the regulation of glomerular blood flow and glomerular filtration by its regulation of afferent arteriolar vascular tone. Nitric oxide derived from neuronal nitric oxide synthase (nNOS) in macula densa can dilate afferent arterioles. Macula densa nNOS is important for renin secretion, and its expression is regulated by dietary salt, renal angiotensin II, intracellular pH, and other factors. In salt-sensitive hypertension, nNOS is suppressed, whereas in SHR or in the early phase of diabetes, nNOS is increased in macula densa along with NADPH oxidase, which limits NO bioavailability. Renal damage induced by hypertension, diabetes, and hyperlipidemia could be prevented by enhancement of nNOS in macula densa with ACEI, dipyridamole, alpha(1)-receptor blocker, a low-salt diet, or sodium bicarbonate. Sodium bicarbonate is a safe and clinically available enhancer of nNOS in macula densa that increases glomerular blood flow and prevents the reduction of GFR in radiocontrast nephropathy and chronic renal failure. In conclusion, the enhancement of nNOS in the macula densa can be a promising strategy to prevent reduction of renal function.

Oxidative stress and nitric oxide deficiency in the kidney: a critical link to hypertension?

There is growing evidence that oxidative stress contributes to hypertension. Oxidative stress can precede the development of hypertension. In almost all models of hypertension, there is oxidative stress that, if corrected, lowers BP, whereas creation of oxidative stress in normal animals can cause hypertension. There is overexpression of the p22(phox) and Nox-1 components of NADPH oxidase and reduced expression of extracellular superoxide dismutase (EC-SOD) in the kidneys of ANG II-infused rodents, whereas there is overexpression of p47(phox) and gp91(phox) and reduced expression of intracellular SOD with salt loading. Several mechanisms have been identified that can make oxidative stress self-sustaining. Reactive oxygen species (ROS) can enhance afferent arteriolar tone and reactivity both indirectly via potentiation of tubuloglomerular feedback and directly by microvascular mechanisms that diminish endothelium-derived relaxation factor/nitric oxide responses, generate a cyclooxygenase-2-dependent endothelial-derived contracting factor that activates thromboxane-prostanoid receptors, and enhance vascular smooth muscle cells reactivity. ROS can diminish the efficiency with which the kidney uses O(2) for Na(+) transport and thereby diminish the P(O(2)) within the kidney cortex. This may place a break on further ROS generation yet could further enhance vasculopathy and hypertension. There is a tight relationship between oxidative stress in the kidney and the development and maintenance of hypertension.

Differential localization of mammalian Lin-7 (MALS/Veli) PDZ proteins in the kidney

Lin-7 PDZ proteins, also called MALS or Velis, have been shown to coordinate basolateral membrane expression of various target proteins in renal epithelial cell models. Three different Lin-7/MALS/Veli isoforms, encoded by separate genes, have been identified. Here, we show that each Lin-7/MALS/Veli isoform is expressed in the kidney. Using MALS isoform-specific antibodies in combination with cell-specific marker antibodies, we found the products of the three mammalian Lin-7/MALS/Veli genes are differentially expressed along the length of the nephron. MALS/Veli 1 is predominately expressed in the glomerulus, thick ascending limb of Henle’s loop (TAL), and the distal convoluted tubule (DCT). MALS/Veli 2 is exclusively expressed in the vasa recta. MALS/Veli 3 is largely located in the DCT and collecting duct. The subcellular localization of MALS/Veli proteins can vary, depending on the isoform and the cell type. In contrast to the predominate basolateral location of MALS/Veli 1 in the TAL and DCT and MALS/Veli 3 in the DCT, MALS/Veli 1 is found diffusely throughout the cytosol of intercalated cells. In the collecting duct, MALS/Veli 3 is chiefly located on the basal membrane. Collectively, these results suggest that different MALS/Veli isoforms may carry out cell type-specific functions. The TAL and distal segments appear to have the most significant capacity for a basolateral membrane-targeting mechanism involving different MALS/Veli isoforms.

PDZ binding motif-dependent localization of K+ channel on the basolateral side in distal tubules

Kir5.1, a nonfunctional inwardly rectifying K(+) channel by itself, can form functional channels by assembling with other proteins. We previously showed that Kir5.1 assembled with Kir4.1 and functioned as an acid-base regulator in the kidney. In this study, we examined the intrarenal distribution of Kir5.1 by RT-PCR analysis on dissected nephron segments and immunohistochemical analysis with the specific anti-Kir5.1 antibody. Strong expression of Kir5.1 was detected in distal convoluted tubules, and weak expression was also detected in thick ascending limb of Henle's loop. Colocalization of Kir5.1 with Kir4.1 indicated expression of Kir5.1/Kir4.1 heteromer in these nephron segments. In a renal epithelial cell line, Madin-Darby canine kidney cells, heteromer formation with Kir4.1 changed the localization of Kir5.1 from intracellular components to the cell surface. The COOH-terminal cytoplasmic portion that includes the PDZ binding motif of Kir4.1 was responsible for this intracellular localization. These data suggest the signals on the COOH terminus of Kir4.1, including PDZ binding motif, determine the intracellular localization of Kir5.1/Kir4.1 heteromer in distal tubules.

Molecular mechanisms of membrane polarity in renal epithelial cells

Exciting discoveries in the last decade have cast light onto the fundamental mechanisms that underlie polarized trafficking in epithelial cells. It is now clear that epithelial cell membrane asymmetry is achieved by a combination of intracellular sorting operations, vectorial delivery mechanisms and plasmalemma-specific fusion and retention processes. Several well-defined signals that specify polarized segregation, sorting, or retention processes have, now, been described in a number of proteins. The intracellular machineries that decode and act on these signals are beginning to be described. In addition, the nature of the molecules that associate with intracellular trafficking vesicles to coordinate polarized delivery, tethering, docking, and fusion are also becoming understood. Combined with direct visualization of polarized sorting processes with new technologies in live-cell fluorescent microscopy, new and surprising insights into these once-elusive trafficking processes are emerging. Here we provide a review of these recent advances within an historically relevant context.

Nitric oxide synthesis in the kidney: isoforms, biosynthesis, and functions in health

Nitric oxide (NO) is a gaseous free radical that serves cell signaling, cellular energetics, host defense, and inflammatory functions in virtually all cells. In the kidney and vasculature, NO plays fundamental roles in the control of systemic and intrarenal hemodynamics, the tubuloglomerular feedback response, pressure natriuresis, release of sympathetic neurotransmitters and renin, and tubular solute and water transport. NO is synthesized from L-arginine by NO synthases (NOS). Because of its high chemical reactivity and high diffusibility, NO production by each of the 3 major NOS isoforms is regulated tightly at multiple levels from gene transcription to spatial proximity near intended targets to covalent modification and allosteric regulation of the enzyme itself. Many of these regulatory mechanisms have yet to be tested in renal cells. The NOS isoforms are distributed differentially and regulated in the kidney, and there remains some controversy over the specific expression of functional protein for the NOS isoforms in specific renal cell populations. Mice with targeted deletion of each of the NOS isoforms have been generated, and these each have unique phenotypes. Studies of the renal and vascular phenotypes of these mice have yielded important insights into certain vascular diseases, ischemic acute renal failure, the tubuloglomerular feedback response, and some mechanisms of tubular fluid and electrolyte transport, but thus far have been underexploited. This review explores the collective knowledge regarding the structure, regulation, and function of the NOS isoforms gleaned from various tissues, and highlights the progress and gaps in understanding in applying this information to renal and vascular physiology.

Protein interactions with nitric oxide synthases: controlling the right time, the right place, and the right amount of nitric oxide

Nitric oxide (NO) is a potent cell-signaling, effector, and vasodilator molecule that plays important roles in diverse biological effects in the kidney, vasculature, and many other tissues. Because of its high biological reactivity and diffusibility, multiple tiers of regulation, ranging from transcriptional to posttranslational controls, tightly control NO biosynthesis. Interactions of each of the major NO synthase (NOS) isoforms with heterologous proteins have emerged as a mechanism by which the activity, spatial distribution, and proximity of the NOS isoforms to regulatory proteins and intended targets are governed. Dimerization of the NOS isozymes, required for their activity, exhibits distinguishing features among these proteins and may serve as a regulated process and target for therapeutic intervention. An increasingly wide array of proteins, ranging from scaffolding proteins to membrane receptors, has been shown to function as NOS-binding partners. Neuronal NOS interacts via its PDZ domain with several PDZ-domain proteins. Several resident and recruited proteins of plasmalemmal caveolae, including caveolins, anchoring proteins, G protein-coupled receptors, kinases, and molecular chaperones, modulate the activity and trafficking of endothelial NOS in the endothelium. Inducible NOS (iNOS) interacts with the inhibitory molecules kalirin and NOS-associated protein 110 kDa, as well as activator proteins, the Rac GTPases. In addition, protein-protein interactions of proteins governing iNOS transcription function to specify activation or suppression of iNOS induction by cytokines. The calpain and ubiquitin-proteasome pathways are the major proteolytic systems responsible for the regulated degradation of NOS isozymes. The experimental basis for these protein-protein interactions, their functional importance, and potential implication for renal and vascular physiology and pathophysiology is reviewed.

Angiotensin II blockade restores albumin reabsorption in the proximal tubules of diabetic rats

The kidney plays an important role in protein metabolism. The albumin reabsorption in the proximal tubule is disturbed in the early stage of diabetic nephropathy. We evaluated the effects of angiotensin converting enzyme inhibitor (ACEI) and angiotensin III type 1 receptor blocker (ARB) on albumin reabsorption and expression of megalin, an endocytosis receptor for albumin, in proximal tubules of streptozotocin (STZ)-induced diabetic-rats. Diabetic rats at the second week after STZ injection were treated with quinapril (3 mg/kg/day) or candesartan (0.05 mg/kg/day) for 2 weeks. The tubular reabsorption of fluorescein isothiocyanate (FITC)-labeled albumin was evaluated by immunogold electron microscopy, and megalin expression was investigated by immunohistochemistry and Western blotting. Reabsorption of FITC-labeled albumin and megalin expression were prominently inhibited in the proximal convoluted tubules of diabetic rats compared to the controls. Both quinapril and candesartan restored albumin reabsorption in the proximal tubule due to normalization of megalin expression. Urinary albumin excretion was significantly reduced by both ACEI and ARB treatment. Angiotensin II infusion decreased megalin expression and albumin reabsorption in the proximal tubule. In conclusion, angiotensin II blockade restored albumin reabsorption via amelioration of megalin expression in the proximal tubules of early stage diabetic rats.

Effect of combination therapy with dipyridamole and quinapril in diabetic nephropathy

BACKGROUND/AIMS

Dipyridamole stimulates nitric oxide action via inhibition of phosphodiesterase and also has an antioxidant effect. ACE inhibitor reduces glomerular pressure and enhances NO action via increased bradykinin. Thus, we evaluated the effect of the combination of dipyridamole and ACE inhibitor in diabetic nephropathy.

METHODS

Streptozotocin-induced diabetic rats at 2 weeks were treated with dipyridamole, quinapril or both. The expression of NOS and NAD(P)H oxidase p47phox was investigated using immunohistochemistry and western blot, and urinary albumin, cGMP and lipid peroxidation products (LPO) were measured at 4 weeks.

RESULTS

NAD(P)H oxidase and urinary LPO were significantly enhanced in diabetes, and suppressed by each treatment to the same extent. The nNOS expression in macula densa and eNOS increased significantly with combination therapy compared to quinapril treatment alone contributing to an enhanced urinary excretion of cGMP and to maintain the creatinine clearance. Increased albuminuria in diabetes was reduced more effectively with combination therapy to the control level than with single treatments.

CONCLUSION

Combination therapy with dipyridamole and quinapril suppressed urinary LPO via reduction of NAD(P)H oxidase increase in diabetes. The combination therapy reduced microalbuminuria to the control level and maintained creatinine clearance with enhanced nNOS and eNOS expression compared to quinapril alone.

Angiotensin II and oxidative stress in Dahl Salt-sensitive rat with heart failure

Reactive oxygen species have an important pathogenic role in organ damage. We investigated the role of oxidative stress via nicotinamide adenine dinucleotide phosphate (NAD[P]H) oxidase in the kidney of the Dahl salt-sensitive (DS) rats with heart failure (DSHF). Eleven-week-old DS rats fed an 8%-NaCl diet received either vehicle or imidapril (1 mg/kg per day) for 7 weeks. The renal expression of the NAD(P)H oxidase p47phox and endothelial NO synthase were evaluated. In DSHF rats, associated with increased renal angiotensin II, mRNA and protein expression of NAD(P)H oxidase p47phox were enhanced with an increase in renal lipid peroxidation production (0.33+/-0.03 versus 0.22+/-0.01 nmol/mg protein, P<0.05) and urinary excretion of hydrogen peroxide (26.9+/-6.6 versus 9.5+/-2.1 U/mg creatinine, P<0.01) compared with levels in Dahl salt-resistant rats. The endothelial NO synthase expression was decreased in the kidney. Treatment with imidapril reduced renal angiotensin II and NAD(P)H oxidase expression and the oxidative products (kidney lipid peroxidation product: 0.16+/-0.02, P<0.001; urinary hydrogen peroxide: 3.1+/-0.2, P<0.01 versus DSHF rats). Imidapril significantly decreased albuminuria and reduced glomerulosclerosis without changes in the blood pressure. In conclusion, DSHF rats showed increased oxidative stress in the kidney via NAD(P)H oxidase. Blockade of local angiotensin II with subpressor dose of imidapril inhibited NAD(P)H oxidase and prevented renal damage.

Expression and cellular localization of classic NADPH oxidase subunits in the spontaneously hypertensive rat kidney

Phagocytes generate superoxide anion (O(2)(-)) by a classic, 5-component NADPH oxidase. O(2)(-) contributes to hypertension in spontaneously hypertensive rats (SHR). Therefore, we tested the hypothesis that NADPH oxidase expression is enhanced in the SHR kidney. We also analyzed the localization of NADPH oxidase components in SHR kidney. Renal NADPH oxidase was quantified by reverse transcription-polymerase chain reaction and Western blotting and was localized in SHR and Wistar Kyoto rat (WKY) kidney by immunohistochemistry. The mRNA for 5 subunits of phagocyte NADPH oxidase, and also for MOX1 and RENOX (NOX4), was detected in adult rat kidney. Kidneys of adult (10 weeks old) SHR had a significantly (P<0.01) greater mRNA for p47phox (SHR 0.81 +/- 0.05 versus WKY 0.37 +/- 0.01, arbitrary unit), which was confirmed by Western blotting (SHR 0.58 +/- 0.04 versus WKY 0.42 +/- 0.04, arbitrary unit; P<0.05) and by immunohistochemistry. This higher p47phox protein expression was also detected in young prehypertensive SHR (SHR 0.61 +/- 0.05 versus WKY 0.39 +/- 0.04, arbitrary unit; P<0.01). The 10-week-old SHR contained more modest but significantly (P<0.05) greater protein for p67phox (SHR 0.54 +/- 0.02 versus WKY 0.46 +/- 0.02). Immunostaining localized p47phox, p67phox, and p22phox in vasculature, macula densa, distal convoluted tubule, cortical collecting duct, and outer and inner medullary collecting ducts. The kidney of SHR expresses genes for all the main components of phagocyte NADPH oxidase, RENOX, and MOX1. There is a prominent increase in the SHR kidney of the mRNA, and protein expression of p47phox in the vasculature, macula densa, and distal nephron, which precedes development of hypertension.

Oxidative stress and nitric oxide synthase in rat diabetic nephropathy: effects of ACEI and ARB

BACKGROUND

Angiotensin II (Ang II) can up-regulate nicotinamide adenine dinucleotide phosphate [NAD(P)H] oxidase, whose product superoxide anion (O2-) can interact with nitric oxide (NO) to form peroxynitrite (ONOO-). We tested the hypothesis that Ang II subtype 1 (AT1) receptor activation enhances oxidative stress and nitrotyrosine deposition in the kidneys of rats with diabetes mellitus (DM).

METHODS

After two weeks of streptozotocin-induced DM, rats received either no treatment, an angiotensin-converting enzyme inhibitor (ACEI) or an angiotensin receptor blocker (ARB) for two weeks. At four weeks, renal expression of the p47phox component of NAD(P)H oxidase, endothelial nitric oxide synthase (eNOS), neuronal nitric oxide synthase (nNOS), and nitrotyrosine were evaluated by Western blot and immunohistochemistry and related to plasma lipid peroxidation products (LPO), hydrogen peroxide production in the kidney and 24-hour protein excretion.

RESULTS

Immunoreactive expression of p47phox and eNOS were increased in DM with an increase in plasma LPO, renal hydrogen peroxide production and nitrotyrosine deposition. Expression of nNOS was unaltered. Treatment with either ACEI or ARB prevented all these findings and also prevented significant microalbuminuria. The treatments did not affect the elevated blood sugar, nor did DM or its treatment affect the blood pressure or the creatinine clearance.

CONCLUSION

Early proteinuric diabetic nephropathy increases renal expression of the p47phox component of NAD(P)H oxidase and eNOS with increased indices of systemic and renal oxidative/nitrosative stress. An ACEI or an ARB prevents these changes and prevents the development of proteinuria, independent of blood pressure or blood sugar. This finding indicates a pathogenic role for AT1 receptors in the development of oxidative damage in the kidneys during early DM.

What is brain nitric oxide synthase doing in the kidney?

Brain, or type I, nitric oxide synthase is expressed strongly in the macula densa of the kidney. Functional studies show that it blunts the tubuloglomerular feedback response that causes vasoconstriction of the renal afferent arteriole in response to sodium chloride reabsorption at this site, and regulates renin release from the juxtaglomerular apparatus. Studies published this year have made great progress in defining the mechanism of action of type I nitric oxide synthase on tubuloglomerular feedback. Macula densa-derived nitric oxide can act via both the autocrine and probably the paracrine routes. Nitric oxide bioactivity in the juxtaglomerular apparatus is strongly curtailed by oxidative stress in hypertensive models. The signalling pathways within macula densa cells have been examined in detail. Nitric oxide synthase type I in the macula densa probably adapts renal hemodynamics and possibly renin secretion to changes in blood pressure and salt intake.

A sequence motif responsible for ER export and surface expression of Kir2.0 inward rectifier K(+) channels

Integral membrane proteins are sorted via the secretory pathway. It was proposed that this pathway is non-selective provided that the cargo protein is properly assembled and lacks an endoplasmic reticulum (ER) retention signal. However, recent experimental evidence suggests that efficient export of proteins from the ER to the Golgi complex is not simply a default pathway. Here we demonstrate a novel sequence motif (FxYENEV) in the cytoplasmic C-terminus of mammalian inward rectifier potassium (Kir) channels which determines ER export. This motif is found to be both necessary and sufficient for efficient export from the ER that eventually leads to efficient surface expression of Kir2.1 channels.

Renal expression of constitutive NOS and DDAH: separate effects of salt intake and angiotensin

BACKGROUND

Nitric oxide (NO) is generated from NO synthase (NOS) isoforms. These enzymes can be inhibited by asymmetric dimethylarginine, which is inactivated by N(G)-N(G)-dimethylarginine dimethylaminohydrolase (DDAH). The neuroneal (nNOS) type I and endothelial (eNOS) type III constitutive NOS isoforms are expressed predominantly in the macula densa and microvascular endothelium of the renal cortex, respectively. DDAH is expressed at sites of NOS expression. Since NO may coordinate the renal responses to angiotensin II (Ang II) and changes in salt intake, we tested the hypothesis that salt intake regulates the expression of nNOS, eNOS and DDAH by Ang II acting on type 1 (AT(1)) receptors.

METHODS

Groups (N = 6) of rats were adapted to low-salt (LS) or high-salt (HS) intakes for 10 days. Other groups of LS and HS rats received the AT(1) receptor antagonist losartan for six days (to test the effects of salt independent of AT(1) receptors). A further group of HS rats received an infusion of Ang II for six days (to test the effect of Ang II independent of salt intake).

RESULTS

Compared with HS rats, there was a significant (P < 0.05) increase in LS rats of nNOS protein in kidney and immunohistochemical expression in the macula densa, and of eNOS protein expression and immunohistochemical expression in the microvascular endothelium, and of DDAH protein expression. Losartan prevented these effects of salt on the expression of eNOS or DDAH, both of which were also increased by Ang II infusions in HS rats. In contrast, losartan did not prevent the effects of salt on nNOS expression, which was unresponsive to Ang II infusion. The generation of NO(2)(-) released by slices of renal cortex, in the presence of saturating concentrations of L-arginine, was increased by LS, compared to HS, independent of losartan and by Ang II during HS.

CONCLUSION

The expressions of eNOS in cortical microvascular endothelium and DDAH in kidney are enhanced by Ang II acting on AT(1) receptors. The expression of nNOS in the macula densa is enhanced by salt restriction independent of Ang II or AT(1) receptors.

Constitutive nitric oxide synthesis in the kidney--functions at the juxtaglomerular apparatus

Tubulo-vascular information transfer at the renal juxtaglomerular apparatus (JGA) serves to adjust the biosynthesis and release of renin, the key enzyme of the renin angiotensin system, and to regulate glomerular arteriolar muscle tone. The macula densa serves as a sensor of tubular NaCl. Concentration-dependent salt uptake through the Na-K-2Cl cotransporter located in the apical membrane of macula densa cells triggers a signal transduction cascade that involves the synthesis of nitric oxide (NO) through a type 1 NO synthase (NOS1) which is described with respect to its complex mRNA structure and regulatory aspects. The anatomical and functional targets of the NO-soluble guanylyl cyclase-cGMP pathway at the JGA are reviewed.

Protein-protein interactions controlling nitric oxide synthases

Nitric oxide (NO) biosynthesis is tightly regulated by a variety of mechanisms ranging from transcriptional to post-translational controls. Calmodulin has long been known to be an allosteric modulator of the three major NO synthases (NOS). Recent studies indicate that other proteins directly associate with NOS isoforms and regulate their activity or spatial distribution in the cell. Several proteins residing in or recruited to plasmalemmal caveolae of endothelial cells serve as allosteric regulators of endothelial NOS (eNOS). Caveolins, the resident scaffolding proteins of caveolae, and calmodulin undergo reciprocal Ca2+-dependent association and dissociation with eNOS in the caveolar membrane that inhibits (caveolins) and activates (calmodulin) eNOS activity. Other caveolar proteins appear to contribute to the eNOS-membrane complex, including the bradykinin B2 receptor, the angiotensin AT1 receptor, the CAT1 arginine transporter, and Hsp90. Direct interactions of a variety of proteins bearing PDZ domains with the PDZ domain of neuronal NOS (nNOS) have been shown to influence the subcellular distribution and/or activity of the enzyme in brain and muscle. One of these proteins, PSD-93, co-localizes with a subpopulation of nNOS in the macula densa. Although considerable emphasis has been placed on transcription as the principal step of regulation for inducible NOS (iNOS), our laboratory has recently defined a regulatory interaction of iNOS with Rho family GTPases. While the role of protein-eNOS interactions in the control of vascular tone has been increasingly clarified, the interactions and regulatory importance of protein association with nNOS and iNOS in the vasculature and kidney remains to be explored.

Nitric oxide synthase in the JGA of the SHR: expression and role in tubuloglomerular feedback

The spontaneously hypertensive rat (SHR) has an enhanced tubuloglomerular feedback (TGF) and a diminished buffering by juxtaglomerular apparatus (JGA)-derived NO. We examined the hypothesis that these effects are due to decreases in nitric oxide synthase (NOS) expression or limited availability of L-arginine or tetrahydrobiopterin (BH(4)). SHR had significantly (P < 0.05) greater mRNA abundance (by RT-PCR) or protein (by Western analysis) for neuronal NOS (nNOS, or type I) and endothelial cell NOS (ecNOS, or type III) in renal cortex or isolated glomeruli, respectively. There was prominent expression of ecNOS in glomerular endothelium and nNOS in macula densa. Maximal TGF responses, assessed from changes in proximal stop-flow pressure during orthograde loop of Henle (LH) perfusion, were greater in SHR [Wistar-Kyoto (WKY), 8.1 +/- 0.3 (n = 46) vs. SHR, 10.3 +/- 0.3 mmHg (n = 57); P < 0.001]. Unlike WKY, TGF responses of SHR were unresponsive to microperfusion of the nNOS inhibitor, 7-nitroindazole (7-NI, 10(-4) M) [WKY, 9.5 +/- 0.5 to 13.2 +/- 0.7 (n = 13, P < 0.001) vs. SHR, 11.8 +/- 0.7 to 12.5 +/- 0.6 mmHg (n = 19, not significant)], or to L-arginine (10(-3) M) [WKY, 7.7 +/- 0.8 to 6.3 +/- 0.4 (n = 10, P < 0.05) vs. SHR, 10.4 +/- 0.7 to 10.6 +/- 0.7 mmHg (n = 10, not significant)]. Neither BH(4) (10(-4) M) nor sepiapterin (10(-4) M), its stable precursor, modified TGF responses in WKY or in SHR, nor did they restore a response to microperfusion of 7-NI in SHR. In conclusion, there is a diminished role for NO from nNOS in blunting of TGF in SHR which cannot be ascribed to limited NOS expression or availability of substrate or BH(4).