David Anthony Power

Coordinated Control of Endothelial Nitric-oxide Synthase Phosphorylation by Protein Kinase C and the cAMP-dependent Protein Kinase

Endothelial nitric-oxide synthase (eNOS) is an important regulatory enzyme in the cardiovascular system catalyzing the production of NO from arginine. Multiple protein kinases including Akt/PKB, cAMP-dependent protein kinase (PKA), and the AMP-activated protein kinase (AMPK) activate eNOS by phosphorylating Ser-1177 in response to various stimuli. During VEGF signaling in endothelial cells, there is a transient increase in Ser-1177 phosphorylation coupled with a decrease in Thr-495 phosphorylation that reverses over 10 min. PKC signaling in endothelial cells inhibits eNOS activity by phosphorylating Thr-495 and dephosphorylating Ser-1177 whereas PKA signaling acts in reverse by increasing phosphorylation of Ser-1177 and dephosphorylation of Thr-495 to activate eNOS. Both phosphatases PP1 and PP2A are associated with eNOS. PP1 is responsible for dephosphorylation of Thr-495 based on its specificity for this site in both eNOS and the corresponding synthetic phosphopeptide whereas PP2A is responsible for dephosphorylation of Ser-1177. Treatment of endothelial cells with calyculin selectively blocks PKA-mediated dephosphorylation of Thr-495 whereas okadaic acid selectively blocks PKC-mediated dephosphorylation of Ser-1177. These results show that regulation of eNOS activity involves coordinated signaling through Ser-1177 and Thr-495 by multiple protein kinases and phosphatases.

Array-Based Gene Discovery with Three Unrelated Subjects Shows SCARB2/LIMP-2 Deficiency Causes Myoclonus Epilepsy and Glomerulosclerosis

Action myoclonus-renal failure syndrome (AMRF) is an autosomal-recessive disorder with the remarkable combination of focal glomerulosclerosis, frequently with glomerular collapse, and progressive myoclonus epilepsy associated with storage material in the brain. Here, we employed a novel combination of molecular strategies to find the responsible gene and show its effects in an animal model. Utilizing only three unrelated affected individuals and their relatives, we used homozygosity mapping with single-nucleotide polymorphism chips to localize AMRF. We then used microarray-expression analysis to prioritize candidates prior to sequencing. The disorder was mapped to 4q13-21, and microarray-expression analysis identified SCARB2/Limp2, which encodes a lysosomal-membrane protein, as the likely candidate. Mutations in SCARB2/Limp2 were found in all three families used for mapping and subsequently confirmed in two other unrelated AMRF families. The mutations were associated with lack of SCARB2 protein. Reanalysis of an existing Limp2 knockout mouse showed intracellular inclusions in cerebral and cerebellar cortex, and the kidneys showed subtle glomerular changes. This study highlights that recessive genes can be identified with a very small number of subjects. The ancestral lysosomal-membrane protein SCARB2/LIMP-2 is responsible for AMRF. The heterogeneous pathology in the kidney and brain suggests that SCARB2/Limp2 has pleiotropic effects that may be relevant to understanding the pathogenesis of other forms of glomerulosclerosis or collapse and myoclonic epilepsies.

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 Ca2+ 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 the energy sensor AMP-activated protein kinase in renal physiology and disease

The ultrasensitive energy sensor AMP-activated protein kinase (AMPK) orchestrates the regulation of energy-generating and energy-consuming pathways. AMPK is highly expressed in the kidney where it is reported to be involved in a variety of physiological and pathological processes including ion transport, podocyte function, and diabetic renal hypertrophy. Sodium transport is the major energy-consuming process in the kidney, and AMPK has been proposed to contribute to the coupling of ion transport with cellular energy metabolism. Specifically, AMPK has been identified as a regulator of several ion transporters of significance in renal physiology, including the cystic fibrosis transmembrane conductance regulator (CFTR), the epithelial sodium channel (ENaC), the Na(+)-K(+)-2Cl(-) cotransporter (NKCC), and the vacuolar H(+)-ATPase (V-ATPase). Identified regulators of AMPK in the kidney include dietary salt, diabetes, adiponectin, and ischemia. Activation of AMPK in response to adiponectin is described in podocytes, where it reduces albuminuria, and in tubular cells, where it reduces glycogen accumulation. Reduced AMPK activity in the diabetic kidney is associated with renal accumulation of triglyceride and glycogen and the pathogenesis of diabetic renal hypertrophy. Acute renal ischemia causes a rapid and powerful activation of AMPK, but the functional significance of this observation remains unclear. Despite the recent advances, there remain significant gaps in the present understanding of both the upstream regulating pathways and the downstream substrates for AMPK in the kidney. A more complete understanding of the AMPK pathway in the kidney offers potential for improved therapies for several renal diseases including diabetic nephropathy, polycystic kidney disease, and ischemia-reperfusion injury.

Heparanase Is Involved in the Pathogenesis of Proteinuria as a Result of Glomerulonephritis

The beta-D-endoglycosidase heparanase has been proposed to be important in the pathogenesis of proteinuria by selectively degrading the negatively charged side chains of heparan sulfate proteoglycans within the glomerular basement membrane. A loss of negatively charged heparan sulfate proteoglycans may result in alteration of the permselective properties of the glomerular basement membrane, loss of glomerular epithelial and endothelial cell anchor points, and liberation of growth factors. In this study, therefore, the role of heparanase in passive Heymann nephritis (PHN) was examined. Normal glomeruli showed low-level heparanase expression as determined by immunohistochemistry and Western blot analysis. Days 5, 14, and 28 of PHN were associated with an increase in endothelial and glomerular epithelial cell heparanase. Reverse transcription-PCR confirmed a significant increase in mRNA at day 21 of disease (P < 0.0004). Furthermore, urinary and glomerular heparanase activities were significantly increased at days 5 and 21 of disease, respectively. Western blot analysis of isolated glomeruli separated into membrane- and cytosol-enriched protein fractions showed that the active 58-kD heparanase species was increased but restricted to the cytosol of diseased glomeruli at day 21. The inactive 65-kD precursor, however, was found in membrane and cytosol-diseased fractions, suggesting cell membrane processing. Complement depletion prevented glomerular heparanase expression; in addition, administration of a polyclonal anti-heparanase antibody significantly reduced urinary protein excretion at day 5 of disease to 62 +/- 11 mg/d compared with 203 +/- 43 and 159 +/- 18 mg/d in the normal rabbit serum- and normal saline-treated experimental groups, respectively (P < 0.002). Proteinuria was reduced in the absence of any altered glomerular C5b-9 activity, sheep IgG deposition, or rat anti-sheep antibody titers. These data suggest that heparanase contributes to the pathogenesis of proteinuria in PHN.

Regulation of the renal-specific Na+–K+–2Cl− co-transporter NKCC2 by AMP-activated protein kinase (AMPK)

The renal-specific NKCC2 (Na+-K+-2Cl- co-transporter 2) is regulated by changes in phosphorylation state, however, the phosphorylation sites and kinases responsible have not been fully elucidated. In the present study, we demonstrate that the metabolic sensing kinase AMPK (AMP-activated protein kinase) phosphorylates NKCC2 on Ser126 in vitro. Co-precipitation experiments indicated that there is a physical association between AMPK and the N-terminal cytoplasmic domain of NKCC2. Activation of AMPK in the MMDD1 (mouse macula densa-derived 1) cell line resulted in an increase in Ser126 phosphorylation in situ, suggesting that AMPK may phosphorylate NKCC2 in vivo. The functional significance of Ser126 phosphorylation was examined by mutating the serine residue to an alanine residue resulting in a marked reduction in co-transporter activity when exogenously expressed in Xenopus laevis oocytes under isotonic conditions. Under hypertonic conditions no significant change of activity was observed. Therefore the present study identifies a novel phosphorylation site that maintains NKCC2-mediated transport under isotonic or basal conditions. Moreover, the metabolic-sensing kinase, AMPK, is able to phosphorylate this site, potentially linking the cellular energy state with changes in co-transporter activity.

Renal Structure in Normoalbuminuric and Albuminuric Patients With Type 2 Diabetes and Impaired Renal Function

OBJECTIVE The structural basis of normoalbuminuric renal insufficiency in patients with type 2 diabetes remains to be elucidated. We compared renal biopsy findings in patients with type 2 diabetes and estimated glomerular filtration rate (eGFR) and measured GFR of <60 mL/min/1.73 m2, associated with either normo-, micro-, or macroalbuminuria. RESEARCH DESIGN AND METHODS In patients with normo- (n = 8) or microalbuminuria (n = 6), renal biopsies were performed according to a research protocol. In patients with macroalbuminuria (n = 17), biopsies were performed according to clinical indication. Findings were categorized according to the Fioretto classification: category 1 (C1), normal/near normal; category 2 (C2), typical diabetic nephropathy (DN) with predominantly glomerular changes; and category 3 (C3), atypical with disproportionately severe interstitial/tubular/vascular damage and with no/mild diabetic glomerular changes. RESULTS In our study population (mean eGFR 35 mL/min/1.73 m2), typical glomerular changes (C2) of DN were observed in 22 of 23 subjects with micro- or macroalbuminuria compared with 3 of 8 subjects with normoalbuminuria (P = 0.002). By contrast, predominantly interstitial or vascular changes (C3) were seen in only 1 of 23 subjects with micro- or macroalbuminuria compared with 3 of 8 normoalbuminuric subjects (P = 0.08). Mesangial area increased progressively from normal controls to patients with type 2 diabetes and normo-, micro-, and macroalbuminuria. Varying degrees of arteriosclerosis, although not necessarily the predominant pattern, were seen in seven of eight subjects with normoalbuminuria. CONCLUSIONS Typical renal structural changes of DN were observed in patients with type 2 diabetes and elevated albuminuria. By contrast, in normoalbuminuric renal insufficiency, these changes were seen less frequently, likely reflecting greater contributions from aging, hypertension, and arteriosclerosis.

SCARB2 Mutations in Progressive Myoclonus Epilepsy (PME) Without Renal Failure

Mutations in SCARB2 were recently described as causing action myoclonus renal failure syndrome (AMRF). We hypothesized that mutations in SCARB2 might account for unsolved cases of progressive myoclonus epilepsy (PME) without renal impairment, especially those resembling Unverricht‐Lundborg disease (ULD). Additionally, we searched for mutations in the PRICKLE1 gene, newly recognized as a cause of PME mimicking ULD.

A synthetic heparanase inhibitor reduces proteinuria in passive Heymann nephritis

The beta-D-endoglycosidase heparanase has been proposed to be important in the pathogenesis of proteinuria by acting to selectively degrade the negatively charged side chains of heparan sulfate proteoglycans (HSPG) within the glomerular basement membrane (GBM). A loss of the negatively charged HSPG may result in alteration of the permselective properties of the GBM, loss of glomerular epithelial and endothelial cell anchor points, and liberation of growth factors. This study examined the effect of PI-88, a sulfated oligosaccharide heparanase inhibitor, on renal function, glomerular ultrastructure, and proteinuria. Continuous PI-88 infusion at 25 mg/kg per d did not adversely affect animal behavior, growth, or GFR. Cortical tubular vacuolation, however, was observed by light microscopy, and GBM thickness was significantly reduced in these animals (P < 0.0002). Tissue distribution studies using [(35)S]-labeled PI-88 revealed high levels of radioactivity in the kidney after a single subcutaneous injection of 25 mg/kg, suggesting protracted accumulation; moreover, active PI-88 was detected in urine. In passive Heymann nephritis, PI-88 delivered as a continuous infusion at 25 mg/kg per d significantly reduced autologous-phase proteinuria, at day 14 (P < 0.009), in the absence of altered sheep antibody deposition, C5b-9 deposition, and circulating rat anti-sheep antibody titers. Glomerular vascular endothelial growth factor and fibroblast growth factor expression was unaffected by PI-88 administration. However, PI-88 administration significantly prevented glomerular HSPG loss as demonstrated by quantitative immunofluorescence studies (P < 0.0001) in the absence of altered agrin distribution. These data therefore confirm the importance of heparanase in the development of proteinuria.

New and old markers of progression of diabetic nephropathy

The onset of diabetic nephropathy is characterised by a rise in albumin excretion rate (AER) and/or a transient rise in glomerular filtration rate (GFR) (hyperfiltration). Without intervention AER increases exponentially and there is a linear decrease in GFR after onset of overt nephropathy. In overt nephropathy, AER is a predictor of decline in GFR and the early AER response to antihypertensive therapy correlates with long-term decline in GFR. AER can be measured by immunoassay or by other techniques including HPLC. However, HPLC assays result in higher levels of AER in normal subjects compared with immunoassayable AER. Recent data suggest that there are distinct albuminuric and non-albuminuric pathways to renal impairment in type 1 and type 2 diabetes. In type 2 diabetes, the non-albuminuric pathway may explain a decline in GFR to <60 ml/min/1.73 m(2) in approximately one in four subjects after accounting for the use of renin angiotensin system inhibitors. In established nephropathy (chronic kidney disease (CKD) stages 3 and 4), plasma cystatin C based estimates of GFR are marginally superior to creatinine based estimates. However, cystatin C clearly outperforms creatinine based estimates of GFR decline at GFR levels >60 ml/min/1.73 m(2) (CKD stages 1 and 2). Other potential markers of progression of diabetic nephropathy include transforming growth factor beta (TGFbeta) and connective tissue growth factor (CTGF). However, long-term studies are needed to define their roles as markers of progression. Diabetic nephropathy is likely to be more susceptible to intervention at an early stage and accurate estimation of GFR is already possible with cystatin C. However, improved formulas for estimating GFR based on using creatinine or other markers are still required, because this may still provide the most cost effective approach applicable to existing clinical practice.