Diabetic Kidney Disease in FVB/NJ Akita Mice: Temporal Pattern of Kidney Injury and Urinary Nephrin Excretion

Akita mice are a genetic model of type 1 diabetes. In the present studies, we investigated the phenotype of Akita mice on the FVB/NJ background and examined urinary nephrin excretion as a marker of kidney injury. Male Akita mice were compared with non-diabetic controls for functional and structural characteristics of renal and cardiac disease. Podocyte number and apoptosis as well as urinary nephrin excretion were determined in both groups. Male FVB/NJ Akita mice developed sustained hyperglycemia and albuminuria by 4 and 8 weeks of age, respectively. These abnormalities were accompanied by a significant increase in systolic blood pressure in 10-week old Akita mice, which was associated with functional, structural and molecular characteristics of cardiac hypertrophy. By 20 weeks of age, Akita mice developed a 10-fold increase in albuminuria, renal and glomerular hypertrophy and a decrease in the number of podocytes. Mild-to-moderate glomerular mesangial expansion was observed in Akita mice at 30 weeks of age. In 4-week old Akita mice, the onset of hyperglycemia was accompanied by increased podocyte apoptosis and enhanced excretion of nephrin in urine before the development of albuminuria. Urinary nephrin excretion was also significantly increased in albuminuric Akita mice at 16 and 20 weeks of age and correlated with the albumin excretion rate. These data suggest that: 1. FVB/NJ Akita mice have phenotypic characteristics that may be useful for studying the mechanisms of kidney and cardiac injury in diabetes, and 2. Enhanced urinary nephrin excretion is associated with kidney injury in FVB/NJ Akita mice and is detectable early in the disease process

Regulation of Thromboxane Receptor (TP) Phosphorylation by Protein Phosphatase 1 (PP1) and PP2A

ABSTRACT To investigate the protein phosphatases that dephosphorylate TP, human embryonic kidney cells (HEK293 cells) stably transfected with 12CA5-tagged TP were treated with TP agonist, washed, and then allowed to recover in the presence or absence of the cell-permeable PP1 and PP2A inhibitors calyculin or okadaic acid (OKA). After recovery, cells were rechallenged with TP agonist and TP responsiveness was assessed by measuring inositol trisphosphate generation. TP responsiveness recovered over a 20-min time period. Recovery of TP responsiveness was inhibited by calyculin and OKA and was associated with dephosphorylation of receptor proteins. To further identify the TP phosphatase, TP phosphorylated in the intact cell were isolated by immunoprecipitation and were used as substrate for protein phosphatases prepared from HEK293 cells. TP were dephosphorylated by whole-cell homogenates. Dephosphorylation of TP was completely inhibited by the PP1 and PP2A inhibitors calyculin and microcystin-LR, suggesting that the decrease in TP phosphorylation was not due to receptor degradation. TP phosphatase activity was partially blocked by 1) inhibitor 2, a specific protein inhibitor of PP1; and 2) OKA at concentrations (1 nM) that specifically inhibit PP2A. TP phosphatase activity did not have an absolute requirement for divalent cations and was found primarily in cytosolic fractions of the cell. These data suggest that PP1-and PP2A-like protein phosphatases dephosphorylate TP. By regulating the phosphorylation state of TP, protein phosphatases may modulate tissue responsiveness to thromboxane

The C-terminus of the thromboxane receptor contributes to coupling and desensitization in a mouse mesangial cell line. J Pharmacol Exp Ther 283

ABSTRACT To investigate regulatory domains of the thromboxane A 2 (TxA 2 ) receptor, we constructed a truncated form of the mouse TxA 2 receptor and expressed it in a mesangial cell line. The mutant receptor lacked 22 amino acids in the C-terminus including four potential phosphorylation sites. Ligand binding of mutant receptors was identical with the wild type. Stimulation with TxA 2 agonist induced increases in inositol trisphosphate (IP3) generation and [Ca ϩϩ ] i by both wild-type and mutant receptors. However, the initial increase in IP3 generation by the mutant receptor was only Ϸ50% of that seen in the wild type. Exposure of wild-type receptors to TxA 2 agonist caused desensitization of IP3 and calcium responses. Pretreatment with TxA 2 agonist caused some desensitization of mutant receptors, but the extent of desensitization was reduced compared with the wild type. The protein kinase C inhibitor staurosporine attenuated TxA 2 -induced desensitization of wild-type receptors, but had little effect on TxA 2 -induced desensitization of mutant receptors. Pretreatment with low concentrations of the phorbol ester, phorbol 12,13-dibutyrate (100 nM), reduced subsequent responsiveness of wild-type but not mutant TxA 2 receptors. In contrast, high-dose phorbol 12,13-dibutyrate (1 M) produced a similar degree of desensitization of both receptor types. These data suggest that: 1) the C-terminus participates in coupling of the TxA 2 receptor to its effector systems; 2) the C-terminus contributes to agonist-specific desensitization of the TxA 2 receptor; and 3) protein kinase C-induced desensitization of the TxA 2 receptor is complex and depends, in part, on C-terminal domains of the TxA 2 receptor

TRPC Channels in Proteinuric Kidney Diseases

Over a decade ago, mutations in the gene encoding TRPC6 (transient receptor potential cation channel, subfamily C, member 6) were linked to development of familial forms of nephrosis. Since this discovery, TRPC6 has been implicated in the pathophysiology of non-genetic forms of kidney disease including focal segmental glomerulosclerosis (FSGS), diabetic nephropathy, immune-mediated kidney diseases, and renal fibrosis. On the basis of these findings, TRPC6 has become an important target for the development of therapeutic agents to treat diverse kidney diseases. Although TRPC6 has been a major focus for drug discovery, more recent studies suggest that other TRPC family members play a role in the pathogenesis of glomerular disease processes and chronic kidney disease (CKD). This review highlights the data implicating TRPC6 and other TRPC family members in both genetic and non-genetic forms of kidney disease, focusing on TRPC3, TRPC5, and TRPC6 in a cell type (glomerular podocytes) that plays a key role in proteinuric kidney diseases

The Effects of Short-Term Treatment with the Prostaglandin E1 (PGE1) Analog Misoprostol on Inflammatory Mediator Production in Murine Lupus Nephritis

MRL-lpr/lpr mice spontaneously develop an autoimmune disease with nephritis similar to human systemic lupus erythematosus. In these animals, treatment with E-series prostaglandins ameliorates renal disease and prolongs survival, perhaps by modulating production of cytokines or eicosanoids. To further define the mechanisms of action of E-series prostaglandins in established murine lupus nephritis, we administered the prostaglandin E1 (PGE1) analog misoprostol to 20-week-old MRL-lpr/lpr mice by twice-daily subcutaneous injection. After 2 days of treatment, misoprostol reduced renal cortical interleukin-1β (IL-1β) mRNA levels compared to vehicle-treated controls (0.19 ± 0.06 (misoprostol) vs 0.50 ± 0.04 (vehicle) densitometry units; P < 0.005). A similar reduction in cortical IL-1β mRNA levels was found in left kidneys harvested from MRL-lpr/lpr mice following 2 days of treatment with misoprostol compared to right kidneys harvested from the same animal prior to the first dose of PGE1 analog (0.12 ± .05 (left) vs 0.39 ± 0.18 (right) densitometry units; P < 0.05). This reduction in cortical IL-1β mRNA levels was not associated with alterations in renal production of thromboxane B2, PGE2, or leukotriene B4 or with significant changes in the severity of renal inflammatory cell infiltrates. Timecourse studies indicated that IL-1β mRNA levels were decreased within 24 hr of initiating misoprostol therapy. This reduction in IL-1β mRNA levels was transient because levels were not reduced after 1 week of treatment with the PGE1 analog. Thus, while the beneficial effects of chronic treatment with E-series prostaglandins in MRL-lpr/lpr mice may be mediated by other mechanisms, these data suggest than an important short-term effect of misoprostol in this model may be suppression of renal cortical IL-1β mRNA levels

A role for leukotrienes in cyclosporine nephrotoxicity

A role for leukotrienes in cyclosporine nephrotoxicity. Nephrotoxicity associated with cyclosporine A (CsA) administration is characterized by marked renal vasoconstriction, interstitial fibrosis, and arteriolar hypertrophy. While the molecular mechanisms of CsA toxicity are not well characterized, previous studies have demonstrated that altered arachidonic acid (AA) metabolism plays a role its pathogenesis. Using a rat renal transplant model, the purpose of this study was to examine the effects of CsA on the 5-lipoxygenase (5-LO) pathway of AA metabolism. The PVG (RT1c) strain of rats underwent kidney transplantation, and recipients of nonrejecting kidney transplants were treated with either 50 mg/kg/day CsA or vehicle (N = 24). To determine the physiologic significance of increased leukotriene (LT) production, the peptidoleukotriene receptor antagonist SKF 106203 was administered to CsA-treated animals for six days. CsA caused a substantial reduction in glomerular filtration rate (GFR) in the transplanted rats compared with the vehicle-treated controls (1.5 ± 0.6 vs. 4.1 ± 0.8 mL/min/kg, P < 0.05). The reduction in renal function was associated with enhanced urinary excretion of the peptidoleukotriene metabolites LTE4 (1431 ± 207 vs. 953 ± 125 pg/24 h, P < 0.05) and N-acetyl-LTE4 (4411 ± 848 vs. 463 ± 70 pg/24 h, P < 0.001). LT receptor blockade had a significant protective effect on renal transplant function in CsA-treated animals (GFR, 4.8 ± 1.1 vs. 1.7 ± 0.9 mL/min/kg, P < 0.05), such that CsA-treated animals that received SKF106203 maintained GFR at levels similar to controls that never received CsA (4.1 ± 0.8 mL/min/kg). Peptidoleukotriene receptor blockade also prevented the histomorphological abnormalities caused by CsA, including tubular vacuolization. These studies identify a critical role for LTs in the pathophysiology of CsA nephrotoxicity and suggest that LT antagonists may be useful in preventing CsA-associated kidney toxicity

Fish oil feeding modulates leukotriene production in murine lupus nephritis

Diets enriched with fish oil (FO) ameliorate kidney disease in the MRL- 1pr/1pr murine model of lupus nephritis. Although the mechanisms of this effect are not known, FO is rich in the polyunsaturated fatty acid eicosapentaenoic acid (EPA) which may have profound effects on eicosanoid metabolism. In MRL- 1pr/1pr mice, FO feeding reduces renal production of cyclooxygenase metabolites. However, EPA may also affect the metabolism of arachidonate by the 5-lipoxygenase (5-LO) pathway and enhanced production of 5-LO metabolites has been implicated in the pathogenesis of kidney disease in MRL- 1pr/1pr mice. We therefore investigated the effects of FO feeding on production of 5-LO metabolites in 20 week old MRL- 1pr/1pr mice. After 8 weeks of dietary supplementation with FO, both renal hemodynamic function and glomerular histology were improved compared to safflower oil (SO) controls. Amelioration of kidney disease was associated with alterations in the pattern of leukotriene production by macrophages and kidneys from FO fed mice. There was a significant decrease in the production of leukotriene B 4 (LTB 4) and tetraene peptidoleukotrienes by peritoneal macrophages isolated from mice given FO compared to control animals. Similarly, dietary supplementation with FO decreased renal production of LTB 4. Reduced production of tetraene leukotrienes was accompanied by a modest increase in the production of pentaene leukotrienes by macrophages from FO fed mice. We speculate that this modulation of leukotriene production by FO feeding may have beneficial effects on renal disease in autoimmune nephritis