Hypertension and albuminuria in chronic kidney disease mapped to a mouse chromosome 11 locus

Animal Models for Studying Protein-Bound Uremic Toxin Removal-A Systematic Review

Protein-bound uremic toxins (PBUTs) are associated with the progression of chronic kidney disease (CKD) and its associated morbidity and mortality. The conventional dialysis techniques are unable to efficiently remove PBUTs due to their plasma protein binding. Therefore, novel approaches are being developed, but these require validation in animals before clinical trials can begin. We conducted a systematic review to document PBUT concentrations in various models and species. The search strategy returned 1163 results for which abstracts were screened, resulting in 65 full-text papers for data extraction (rats (n = 41), mice (n = 17), dogs (n = 3), cats (n = 4), goats (n = 1), and pigs (n = 1)). We performed descriptive and comparative analyses on indoxyl sulfate (IS) concentrations in rats and mice. The data on large animals and on other PBUTs were too heterogeneous for pooled analysis. Most rodent studies reported mean uremic concentrations of plasma IS close to or within the range of those during kidney failure in humans, with the highest in tubular injury models in rats. Compared to nephron loss models in rats, a greater rise in plasma IS compared to creatinine was found in tubular injury models, suggesting tubular secretion was more affected than glomerular filtration. In summary, tubular injury rat models may be most relevant for the in vivo validation of novel PBUT-lowering strategies for kidney failure in humans.

Susceptibility to kidney fibrosis in mice is associated with early growth response-2 protein and tissue inhibitor of metaloproteinase-1 expression

Patients with chronic kidney disease and experimental animal models of kidney fibrosis manifest diverse progression rates. Genetic susceptibility may contribute to this diversity, but the causes remain largely unknown. We have previously described kidney fibrosis with a mild or severe phenotype in mice expressing transforming growth factor-beta1 (TGF-β1) under the control of a mouse albumin promoter (Alb/TGFβ1), on a mixed genetic background with CBAxC57Bl6 mice. Here, we aimed to examine how genetic background may influence kidney fibrosis in TGF-β1 transgenic mice, and in the unilateral ureteral obstruction (UUO) and subtotal nephrectomy (SNX) mouse models. Congenic C57Bl6(B6)-TGFβ and CBAxB6-TGFβ (F1) transgenic mice were generated and survival, proteinuria, kidney histology, transcriptome and protein expressions were analyzed. We investigated the kidneys of B6 and CBA mice subjected to UUO and SNX, and the effects of tissue inhibitor of matrix metalloproteinase-1 (TIMP-1) neutralization on the fibrotic process. CBAxB6-TGFβ mice developed severe kidney fibrosis and premature death, while B6-TGF-β mice had mild fibrosis and prolonged survival. Kidney early growth response factor-2 (EGR2) and TIMP-1 expression were induced only in CBAxB6-TGFβ mice. Similar strain-dependent early changes in EGR2 and TIMP-1 of mice subjected to UUO or SNX were observed. TIMP-1 neutralization in vivo hindered fibrosis both in transgenic mice and the SNX model. EGR2 over-expression in cultured HEK293 cells induced TIMP-1 while EGR2 silencing hindered TGF-β induced TIMP-1 production in HK-2 cells and ureteral obstructed kidneys. Finally, EGR2 and TIMP1 was increased in human kidneys manifesting focal segmental glomerulosclerosis suggesting a correlation between animal studies and patient clinical settings. Thus, our observations demonstrate a strong relationship between genetic background and the progression of kidney fibrosis, which might involve early altered EGR2 and TIMP-1 response, but the relationship to patient genetics remains to be explored.

Phosphorylation of ACTN4 Leads to Podocyte Vulnerability and Proteinuric Glomerulosclerosis

BACKGROUND

Genetic mutations in α-actinin-4 (ACTN4)-an important actin crosslinking cytoskeletal protein that provides structural support for kidney podocytes-have been linked to proteinuric glomerulosclerosis in humans. However, the effect of post-translational modifications of ACTN4 on podocyte integrity and kidney function is not known.

METHODS

Using mass spectrometry, we found that ACTN4 is phosphorylated at serine (S) 159 in human podocytes. We used phosphomimetic and nonphosphorylatable ACTN4 to comprehensively study the effects of this phosphorylation in vitro and in vivo. We conducted x-ray crystallography, F-actin binding and bundling assays, and immunofluorescence staining to evaluate F-actin alignment. Microfluidic organ-on-a-chip technology was used to assess for detachment of podocytes simultaneously exposed to fluid flow and cyclic strain. We then used CRISPR/Cas9 to generate mouse models and assessed for renal injury by measuring albuminuria and examining kidney histology. We also performed targeted mass spectrometry to determine whether high extracellular glucose or TGF-β levels increase phosphorylation of ACTN4.

RESULTS

Compared with the wild type ACTN4, phosphomimetic ACTN4 demonstrated increased binding and bundling activity with F-actin in vitro. Phosphomimetic Actn4 mouse podocytes exhibited more spatially correlated F-actin alignment and a higher rate of detachment under mechanical stress. Phosphomimetic Actn4 mice developed proteinuria and glomerulosclerosis after subtotal nephrectomy. Moreover, we found that exposure to high extracellular glucose or TGF-β stimulates phosphorylation of ACTN4 at S159 in podocytes.

CONCLUSIONS

These findings suggest that increased phosphorylation of ACTN4 at S159 leads to biochemical, cellular, and renal pathology that is similar to pathology resulting from human disease-causing mutations in ACTN4. ACTN4 may mediate podocyte injury as a consequence of both genetic mutations and signaling events that modulate phosphorylation.

GSTM1 Deletion Exaggerates Kidney Injury in Experimental Mouse Models and Confers the Protective Effect of Cruciferous Vegetables in Mice and Humans

BACKGROUND

GSTM1 encodes glutathione S-transferase μ-1 (GSTM1), which belongs to a superfamily of phase 2 antioxidant enzymes. The highly prevalent GSTM1 deletion variant is associated with kidney disease progression in human cohorts: the African American Study of Kidney Disease and Hypertension and the Atherosclerosis Risk in Communities (ARIC) Study.

METHODS

We generated a Gstm1 knockout mouse line to study its role in a CKD model (involving subtotal nephrectomy) and a hypertension model (induced by angiotensin II). We examined the effect of intake of cruciferous vegetables and GSTM1 genotypes on kidney disease in mice as well as in human ARIC study participants. We also examined the importance of superoxide in the mediating pathways and of hematopoietic GSTM1 on renal inflammation.

RESULTS

Gstm1 knockout mice displayed increased oxidative stress, kidney injury, and inflammation in both models. The central mechanism for kidney injury is likely mediated by oxidative stress, because treatment with Tempol, an superoxide dismutase mimetic, rescued kidney injury in knockout mice without lowering BP. Bone marrow crosstransplantation revealed that Gstm1 deletion in the parenchyma, and not in bone marrow-derived cells, drives renal inflammation. Furthermore, supplementation with cruciferous broccoli powder rich in the precursor to antioxidant-activating sulforaphane significantly ameliorated kidney injury in Gstm1 knockout, but not wild-type mice. Similarly, among humans (ARIC study participants), high consumption of cruciferous vegetables was associated with fewer kidney failure events compared with low consumption, but this association was observed primarily in participants homozygous for the GSTM1 deletion variant.

CONCLUSIONS

Our data support a role for the GSTM1 enzyme in the modulation of oxidative stress, inflammation, and protective metabolites in CKD.

Animal Models of Hypertension: A Scientific Statement From the American Heart Association

Hypertension is the most common chronic disease in the world, yet the precise cause of elevated blood pressure often cannot be determined. Animal models have been useful for unraveling the pathogenesis of hypertension and for testing novel therapeutic strategies. The utility of animal models for improving the understanding of the pathogenesis, prevention, and treatment of hypertension and its comorbidities depends on their validity for representing human forms of hypertension, including responses to therapy, and on the quality of studies in those models (such as reproducibility and experimental design). Important unmet needs in this field include the development of models that mimic the discrete hypertensive syndromes that now populate the clinic, resolution of ongoing controversies in the pathogenesis of hypertension, and the development of new avenues for preventing and treating hypertension and its complications. Animal models may indeed be useful for addressing these unmet needs.

T Cells Play a Causal Role in Diastolic Dysfunction during Uremic Cardiomyopathy

BACKGROUND

Uremic cardiomyopathy, characterized by left ventricular hypertrophy, diastolic dysfunction, and impaired myocardial strain, contributes to increased cardiovascular mortality in patients with CKD. Emerging evidence suggests a pathogenic role for T cells during chronic heart failure.

METHODS

To determine whether T cells contribute to uremic cardiomyopathy pathogenesis, we modeled this condition by inducing CKD via 5/6th nephrectomy in mice. We used flow cytometry to assess expression of markers of T cell memory or activation by lymphocytes from CKD mice and controls, as well as lymphocyte capacity for cytokine production. Flow cytometry was also used to quantify immune cells isolated from heart tissue. To test effects of T cell depletion on cardiac function, we gave CKD mice anti-CD3 antibody injections to deplete T cells and compared heart function (assessed by echocardiography) with that of controls. Finally, we correlated T cell phenotypes with structural and functional measures on clinically acquired echocardiograms in children with CKD.

RESULTS

Mice with CKD accumulated T cells bearing markers of memory differentiation (CD44hi) and activation (PD-1, KLRG1, OX40), as reported previously in human CKD. In addition, mice with CKD showed T cells infiltrating the heart. T cell depletion significantly improved both diastolic function and myocardial strain in CKD mice without altering hypertension or degree of renal dysfunction. In children with CKD, increasing frequency of T cells bearing activation markers PD-1 and/or CD57 was associated with worsening diastolic function on echocardiogram.

CONCLUSIONS

CKD results in an accumulation of proinflammatory T cells that appears to contribute to myocardial dysfunction.

VEGF-C attenuates renal damage in salt-sensitive hypertension

Salt-sensitive hypertension is a major risk factor for renal impairment leading to chronic kidney disease. High-salt diet leads to hypertonic skin interstitial volume retention enhancing the activation of the tonicity-responsive enhancer-binding protein (TonEBP) within macrophages leading to vascular endothelial growth factor C (VEGF-C) secretion and NOS3 modulation. This promotes skin lymphangiogenesis and blood pressure regulation. Whether VEGF-C administration enhances renal and skin lymphangiogenesis and attenuates renal damage in salt-sensitive hypertension remains to be elucidated. Hypertension was induced in BALB/c mice by a high-salt diet. VEGF-C was administered subcutaneously to high-salt-treated mice as well as control animals. Analyses of kidney injury, inflammation, fibrosis, and biochemical markers were performed in vivo. VEGF-C reduced plasma inflammatory markers in salt-treated mice. In addition, VEGF-C exhibited a renal anti-inflammatory effect with the induction of macrophage M2 phenotype, followed by reductions in interstitial fibrosis. Antioxidant enzymes within the kidney as well as urinary RNA/DNA damage markers were all revelatory of abolished oxidative stress under VEGF-C. Furthermore, VEGF-C decreased the urinary albumin/creatinine ratio and blood pressure as well as glomerular and tubular damages. These improvements were associated with enhanced TonEBP, NOS3, and lymphangiogenesis within the kidney and skin. Our data show that VEGF-C administration plays a major role in preserving renal histology and reducing blood pressure. VEGF-C might constitute an interesting potential therapeutic target for improving renal remodeling in salt-sensitive hypertension.

MYH9 E1841K Mutation Augments Proteinuria and Podocyte Injury and Migration

Intronic variants of the MYH9 gene that encodes the nonmuscle myosin heavy chain IIA are associated with diabetic nephropathy in European Americans and with sickle cell disease-associated nephropathy. However, the causal functional variants of MYH9 have remained elusive. Rare missense mutations in MYH9 cause macrothrombocytopenia and are occasionally associated with development of nephropathy. The E1841K mutation is among the common MYH9 missense mutations and has been associated with nephropathy in some carriers. To determine the contribution of the E1841K mutation in kidney disease, we studied the effects of the E1841K mutation in mice subjected to high salt or angiotensin II (Ang II) as models of hypertension and in mice subjected to renal mass reduction as a model of CKD. Despite similar levels of BP among wild-type (MYH9^+/+ ) mice and mice heterozygous (MYH9^+/E1841K ) and homozygous (MYH9^{E1841K/E1841K} ) for the mutation in each model, MYH9^{E1841K/E1841K} mice exhibited mildly increased albuminuria in response to high salt; severe albuminuria, nephrinuria, FSGS, and podocyte foot effacement in Ang II-induced hypertension; and early mortality in the renal mass reduction model. Treatment with candesartan during Ang II-induced hypertension attenuated kidney disease development in MYH9^{E1841K/E1841K} mice. In vitro, isolated primary podocytes from MYH9^{E1841K/E1841K} mice exhibited increased lamellipodia formation and reorganization of F-actin stress fibers. Wound healing assays revealed that MYH9^+/+ podocytes had the lowest migration rate, followed by MYH9^+/E1841K then MYH9^{E1841K/E1841K} podocytes. In conclusion, the MYH9 E1841K variant alters podocyte cytoskeletal structure and renders podocytes more susceptible to injury after a damaging stimulus.

Renal Collectrin Protects against Salt-Sensitive Hypertension and Is Downregulated by Angiotensin II

Collectrin, encoded by the Tmem27 gene, is a transmembrane glycoprotein with approximately 50% homology with angiotensin converting enzyme 2, but without a catalytic domain. Collectrin is most abundantly expressed in the kidney proximal tubule and collecting duct epithelia, where it has an important role in amino acid transport. Collectrin is also expressed in endothelial cells throughout the vasculature, where it regulates L-arginine uptake. We previously reported that global deletion of collectrin leads to endothelial dysfunction, augmented salt sensitivity, and hypertension. Here, we performed kidney crosstransplants between wild-type (WT) and collectrin knockout (Tmem27^Y/- ) mice to delineate the specific contribution of renal versus extrarenal collectrin on BP regulation and salt sensitivity. On a high-salt diet, WT mice with Tmem27^Y/- kidneys had the highest systolic BP and were the only group to exhibit glomerular mesangial hypercellularity. Additional studies showed that, on a high-salt diet, Tmem27^Y/- mice had lower renal blood flow, higher abundance of renal sodium-hydrogen antiporter 3, and lower lithium clearance than WT mice. In WT mice, administration of angiotensin II for 2 weeks downregulated collectrin expression in a type 1 angiotensin II receptor-dependent manner. This downregulation coincided with the onset of hypertension, such that WT and Tmem27^Y/- mice had similar levels of hypertension after 2 weeks of angiotensin II administration. Altogether, these data suggest that salt sensitivity is determined by intrarenal collectrin, and increasing the abundance or activity of collectrin may have therapeutic benefits in the treatment of hypertension and salt sensitivity.

Trans-ethnic Fine Mapping Highlights Kidney-Function Genes Linked to Salt Sensitivity

We analyzed genome-wide association studies (GWASs), including data from 71,638 individuals from four ancestries, for estimated glomerular filtration rate (eGFR), a measure of kidney function used to define chronic kidney disease (CKD). We identified 20 loci attaining genome-wide-significant evidence of association (p < 5 × 10(-8)) with kidney function and highlighted that allelic effects on eGFR at lead SNPs are homogeneous across ancestries. We leveraged differences in the pattern of linkage disequilibrium between diverse populations to fine-map the 20 loci through construction of "credible sets" of variants driving eGFR association signals. Credible variants at the 20 eGFR loci were enriched for DNase I hypersensitivity sites (DHSs) in human kidney cells. DHS credible variants were expression quantitative trait loci for NFATC1 and RGS14 (at the SLC34A1 locus) in multiple tissues. Loss-of-function mutations in ancestral orthologs of both genes in Drosophila melanogaster were associated with altered sensitivity to salt stress. Renal mRNA expression of Nfatc1 and Rgs14 in a salt-sensitive mouse model was also reduced after exposure to a high-salt diet or induced CKD. Our study (1) demonstrates the utility of trans-ethnic fine mapping through integration of GWASs involving diverse populations with genomic annotation from relevant tissues to define molecular mechanisms by which association signals exert their effect and (2) suggests that salt sensitivity might be an important marker for biological processes that affect kidney function and CKD in humans.

Role of collectrin, an ACE2 homologue, in blood pressure homeostasis

Collectrin (Tmem27) is a transmembrane glycoprotein that is highly expressed in the kidney and vascular endothelium. It is a homologue of the angiotensin-converting enzyme 2 (ACE2) but harbors no catalytic domain. In the extravascular tissues of the kidney, collectrin is localized to the proximal tubule and collecting duct. Collectrin-deficient mice are featured with hypertension and exaggerated salt sensitivity. These phenotypes are associated with impaired uptake of the nitric oxide precursor L-arginine and the expression of its amino acid transporters, CAT-1 and y(+)LAT1, in endothelial cells. In addition, collectrin-deficient mice display decreased dimerization of nitric oxide synthase and decreased nitric oxide synthesis, but enhanced superoxide generation, suggesting that deletion of collectrin leads to a state of nitric oxide synthase uncoupling. These findings suggest that collectrin plays a protective role against hypertension. The collectrin knockout mouse represents a unique model for hypertension research. Furthermore, collectrin may serve as a novel therapeutic target in the treatment of hypertension.

Epigenetics and the control of the collecting duct epithelial sodium channel

The apical membrane epithelial Na(+) channel subunit (ENaC) in series with the basolateral Na(+)/K(+)-adenosine triphosphatase mediates collecting duct Na(+) reabsorption. Aldosterone induces αENaC gene transcription, which appears to be rate limiting for ENaC activity in this segment. Although this response has long been assumed to be solely the result of liganded nuclear hormone receptors trans-activating αENaC, epigenetic controls of basal and aldosterone-induced transcription of αENaC in the collecting duct recently were described. These epigenetic pathways involve dynamic nuclear repressor complexes targeted to specific subregions of the αENaC promoter and consisting of the histone methyltransferase disrupter of telomeric silencing (Dot)1a together with the transcriptional factor Af9 or the nicotinamide adenine dinucleotide (NAD)-dependent protein deacetylase Sirt1, key co-regulatory proteins, including serum- and glucocorticoid-induced kinase-1 and the putative transcription factor Af17, and targeted chromatin modifications. The complexes, through the action of Dot1a, maintain chromatin associated with the αENaC promoter in a stable hypermethylated state, constraining αENaC transcription under basal conditions. Aldosterone and serum- and glucocorticoid-induced kinase-1, itself, activate αENaC transcription in large part by disrupting or diminishing the Dot1a-Af9 and Dot1a-Sirt1 complexes and their effects on chromatin. Mouse models indicate potential roles of the Dot1a pathways in renal salt excretion and hypertension.

Loss of GSTM1, a NRF2 target, is associated with accelerated progression of hypertensive kidney disease in the African American Study of Kidney Disease (AASK)

Oxidative stress is acknowledged to play a role in kidney disease progression. Genetic variants that affect the capacity to handle oxidative stress may therefore influence the outcome of kidney disease. We examined whether genetic variants of the GSTM1 gene, a member of a superfamily of glutathione S-transferases, influence the course of kidney disease progression in participants of the African American Study of Kidney Disease (AASK) trial. Groups with and without the common GSTM1 null allele, GSTM1(0), differed significantly in the time to a glomerular filtration rate (GFR) event or dialysis (P = 0.04) and in the time to GFR event, dialysis, or death (P = 0.02). The hazard ratios (HR) for the time to a GFR event or dialysis in those with two or one null allele relative to those possessing none were 1.88 [95% confidence interval (CI), 1.07 to 3.30, P = 0.03] and 1.68 (95% CI, 1.00 to 2.84, P < 0.05), respectively. For the time to GFR event, dialysis, or death, the HR for two null alleles was 2.06 (95% CI, 1.20 to 3.55, P = 0.01) and for one null allele 1.70 (95% CI, 1.02 to 2.81, P = 0.04). We demonstrated that GSTM1 directly regulates intracellular levels of 4-hydroxynonenal (4-HNE) in vascular smooth muscle cells. Furthermore, we showed that renal 4-HNE levels and GSTM1 are both increased after reduction of renal mass (RRM) in the mouse. We conclude that GSTM1 is normally upregulated in chronic kidney disease (CKD) in a protective response to increased oxidative stress. A genetic variant that results in loss of GSTM1 activity may be deleterious in CKD.

A major X-linked locus affects kidney function in mice

Chronic kidney disease is a common disease with increasing prevalence in the western population. One common reason for chronic kidney failure is diabetic nephropathy. Diabetic nephropathy and hyperglycemia are characteristics of the mouse inbred strain KK/HlJ, which is predominantly used as a model for metabolic syndrome due to its inherited glucose intolerance and insulin resistance. We used KK/HlJ, an albuminuria-sensitive strain, and C57BL/6J, an albuminuria-resistant strain, to perform a quantitative trait locus (QTL) cross to identify the genetic basis for chronic kidney failure. Albumin-creatinine ratio (ACR) was measured in 130 F2 male offspring. One significant QTL was identified on chromosome (Chr) X and four suggestive QTL were found on Chrs 6, 7, 12, and 13. Narrowing of the QTL region was focused on the X-linked QTL and performed by incorporating genotype and expression analyses for genes located in the region. From the 485 genes identified in the X-linked QTL region, a few candidate genes were identified using a combination of bioinformatic evidence based on genomic comparison of the parental strains and known function in urine homeostasis. Finally, this study demonstrates the significance of the X chromosome in the genetic determination of albuminuria.

Genetic modifiers of hypertension in soluble guanylate cyclase α1-deficient mice

Nitric oxide (NO) plays an essential role in regulating hypertension and blood flow by inducing relaxation of vascular smooth muscle. Male mice deficient in a NO receptor component, the α1 subunit of soluble guanylate cyclase (sGCα1), are prone to hypertension in some, but not all, mouse strains, suggesting that additional genetic factors contribute to the onset of hypertension. Using linkage analyses, we discovered a quantitative trait locus (QTL) on chromosome 1 that was linked to mean arterial pressure (MAP) in the context of sGCα1 deficiency. This region is syntenic with previously identified blood pressure-related QTLs in the human and rat genome and contains the genes coding for renin. Hypertension was associated with increased activity of the renin-angiotensin-aldosterone system (RAAS). Further, we found that RAAS inhibition normalized MAP and improved endothelium-dependent vasorelaxation in sGCα1-deficient mice. These data identify the RAAS as a blood pressure-modifying mechanism in a setting of impaired NO/cGMP signaling.

Genetic modifiers of cardiovascular phenotype caused by elastin haploinsufficiency act by extrinsic noncomplementation

Elastin haploinsufficiency causes the cardiovascular complications associated with Williams-Beuren syndrome and isolated supravalvular aortic stenosis. Significant variability exists in the vascular pathology in these individuals. Using the Eln(+/-) mouse, we sought to identify the source of this variability. Following outcrossing of C57Bl/6J Eln(+/-), two backgrounds were identified whose cardiovascular parameters deviated significantly from the parental strain. F1 progeny of the C57Bl/6J; Eln(+/-)x129X1/SvJ were more hypertensive and their arteries less compliant. In contrast, Eln(+/-) animals crossed to DBA/2J were protected from the pathologic changes associated with elastin insufficiency. Among the crosses, aortic elastin and collagen content did not correlate with quantitative vasculopathy traits. Quantitative trait locus analysis performed on F2 C57; Eln(+/-)x129 intercrosses identified highly significant peaks on chromosome 1 (LOD 9.7) for systolic blood pressure and on chromosome 9 (LOD 8.7) for aortic diameter. Additional peaks were identified that affect only Eln(+/-), including a region upstream of Eln on chromosome 5 (LOD 4.5). Bioinformatic analysis of the quantitative trait locus peaks revealed several interesting candidates, including Ren1, Ncf1, and Nos1; genes whose functions are unrelated to elastic fiber assembly, but whose effects may synergize with elastin insufficiency to predispose to hypertension and stiffer blood vessels. Real time RT-PCR studies show background-specific increased expression of Ncf1 (a subunit of the NOX2 NAPDH oxidase) that parallel the presence of increased oxidative stress in Eln(+/-) aortas. This finding raises the possibility that polymorphisms in genes affecting the generation of reactive oxygen species alter cardiovascular function in individuals with elastin haploinsufficiency through extrinsic noncomplementation.

Af17 deficiency increases sodium excretion and decreases blood pressure

The putative transcription factor AF17 upregulates the transcription of the epithelial sodium channel (ENaC) genes, but whether AF17 modulates sodium homeostasis and BP is unknown. Here, we generated Af17-deficient mice to determine whether deletion of Af17 leads to sodium wasting and low BP. Compared with wild-type mice, Af17-deficient mice had lower BP (11 mmHg), higher urine volume, and increased sodium excretion despite mildly increased plasma concentrations of aldosterone. Deletion of Af17 led to increased dimethylation of histone H3 K79 and reduced ENaC function. The attenuated function of ENaC resulted from decreased ENaC mRNA and protein expression, fewer active channels, lower open probability, and reduced effective activity. In contrast, inducing high levels of plasma aldosterone by a variety of methods completely compensated for Af17 deficiency with respect to sodium handling and BP. Taken together, these data identify Af17 as a potential locus for the maintenance of sodium and BP homeostasis and suggest that a particular histone modification is directly linked to these processes. Af17-mediated regulation of BP is largely, but not exclusively, the result of modulating ENaC, suggesting it has potential as a therapeutic target for the control of BP.

TRPC6 enhances angiotensin II-induced albuminuria

Mutations in the canonical transient receptor potential cation channel 6 (TRPC6) are responsible for familial forms of adult onset focal segmental glomerulosclerosis (FSGS). The mechanisms by which TRPC6 mutations cause kidney disease are not well understood. We used TRPC6-deficient mice to examine the function of TRPC6 in the kidney. We found that adult TRPC6-deficient mice had BP and albumin excretion rates similar to wild-type animals. Glomerular histomorphology revealed no abnormalities on both light and electron microscopy. To determine whether the absence of TRPC6 would alter susceptibility to hypertension and renal injury, we infused mice with angiotensin II continuously for 28 days. Although both groups developed similar levels of hypertension, TRPC6-deficient mice had significantly less albuminuria, especially during the early phase of the infusion; this suggested that TRPC6 adversely influences the glomerular filter. We used whole-cell patch-clamp recording to measure cell-membrane currents in primary cultures of podocytes from both wild-type and TRPC6-deficient mice. In podocytes from wild-type mice, angiotensin II and a direct activator of TRPC6 both augmented cell-membrane currents; TRPC6 deficiency abrogated these increases in current magnitude. Our findings suggest that TRPC6 promotes albuminuria, perhaps by promoting angiotensin II-dependent increases in Ca(2+), suggesting that TRPC6 blockade may be therapeutically beneficial in proteinuric kidney disease.

TGF-alpha mediates genetic susceptibility to chronic kidney disease

The mechanisms of progression of chronic kidney disease (CKD) are poorly understood. Epidemiologic studies suggest a strong genetic component, but the genes that contribute to the onset and progression of CKD are largely unknown. Here, we applied an experimental model of CKD (75% excision of total renal mass) to six different strains of mice and found that only the FVB/N strain developed renal lesions. We performed a genome-scan analysis in mice generated by back-crossing resistant and sensitive strains; we identified a major susceptibility locus (Ckdp1) on chromosome 6, which corresponds to regions on human chromosome 2 and 3 that link with CKD progression. In silico analysis revealed that the locus includes the gene encoding the EGF receptor (EGFR) ligand TGF-α. TGF-α protein levels markedly increased after nephron reduction exclusively in FVB/N mice, and this increase preceded the development of renal lesions. Furthermore, pharmacologic inhibition of EGFR prevented the development of renal lesions in the sensitive FVB/N strain. These data suggest that variable TGF-α expression may explain, in part, the genetic susceptibility to CKD progression. EGFR inhibition may be a therapeutic strategy to counteract the genetic predisposition to CKD.

Genetic analysis of albuminuria in aging mice and concordance with loci for human diabetic nephropathy found in a genome-wide association scan

Aging in the kidney can cause albuminuria, and discovering molecular mechanisms responsible for this might offer a new perspective on the etiology of this abnormality. Haplotype association mapping in the mouse is a novel approach which uses the haplotypes of the relatively closely related mouse inbred strains and the phenotypic variation among these strains to find associations between haplotypes and phenotype. The albumin-to-creatinine ratios, a measure of urinary albumin excretion, were determined in 30 inbred mouse strains at 12, 18, and 24 months of age. Mapping was performed for males and females separately at all three time points using a high density set of 63,222 single-nucleotide polymorphisms to determine genetic loci involved in albuminuria. One significant and eight suggestive loci were found, some of which map to previously identified loci for traits associated with kidney damage in the mouse, but with a much higher resolution thus narrowing their chromosomal location. These nine loci were then compared with genome-wide association scans for diabetic nephropathy (DN) in human type I diabetes. Our study found that two of the nine mouse loci for age-related albuminuria were significantly associated with DN and consistent across male and female strata. This suggests common underlying genes predispose to kidney disease in mice and humans.

Influence of genetic background on albuminuria and kidney injury in Ins2(+/C96Y) (Akita) mice

Previous studies have shown that Akita mice bearing the Ins2(+/C96Y) mutation have significant advantages as a type I diabetes platform for developing models of diabetic nephropathy (DN; Gurley SB, Clare SE, Snow KP, Hu A, Meyer TW, Coffman TM. Am J Physiol Renal Physiol 290: F214-F222, 2006). In view of the critical role for genetic factors in determining susceptibility to DN in humans, we investigated the role of genetic background on kidney injury in Akita mice. To generate a series of inbred Akita mouse lines, we back-crossed the Ins2(C96Y) mutation more than six generations onto the 129/SvEv and DBA/2 backgrounds and compared the extent of hyperglycemia and renal disease with the standard C57BL/6-Ins2(+/C96Y) line. Male mice from all three Akita strains developed marked and equivalent hyperglycemia. However, there were significant differences in the level of albuminuria among the lines with a hierarchy of DBA/2 > 129/SvEv > C57BL/6. Renal and glomerular hypertrophy was seen in all of the lines, but significant increases in mesangial matrix compared with baseline nondiabetic controls were observed only in the 129 and C57BL/6 backgrounds. In F1(DBA/2 x C57BL/6)-Ins2(+/C96Y) mice, the extent of albuminuria was similar to the parental DBA/2-Ins2(+/C96Y) line; they also developed marked hyperfiltration. These studies identify strong effects of genetic background to modify the renal phenotype associated with the Ins2(C96Y) mutation. Identification of these naturally occurring strain differences should prove useful for nephropathy modeling and may be exploited to allow identification of novel susceptibility alleles for albuminuria in diabetes.

Nephron deficit is not required for progressive proteinuria development in the Munich Wistar Frömter rat

The Munich Wistar Frömter (MWF) rat represents a genetic model with an inherited nephron deficit and exhibits mild hypertension and progressive albuminuria, which is more pronounced in males than females. Previously, we demonstrated in a consomic strain that replacement of a quantitative trait locus on chromosome 6 normalized the nephron deficit and suppressed albuminuria development, suggesting a link between the two findings. Here we tested the role of a second major locus linked to albuminuria in MWF on chromosome 8 and generated the consomic strain MWF-8(SHR) by transfer of chromosome 8 from spontaneously hypertensive rats (SHR) into MWF. The early onset of albuminuria at 8 wk of age in MWF (>50-fold increase compared with SHR) was significantly suppressed in consomic animals, and the development of marked proteinuria at 32 wk significantly diminished. Total nephron number in consomic rats (23,771 +/- 1,352) and MWF (27,028 +/- 1,322) were similar and significantly lower (-36%) compared with SHR (36,979 +/- 1,352, P < 0.0001). The development of mild albuminuria in female MWF was also significantly diminished in MWF-8(SHR). Thus, the development of overt and mild albuminuria in male and female MWF rats is not a mandatory consequence of the inherited nephron deficit. The locus on chromosome 8 appears of interest, because its exchange between MWF and SHR protects against the development of albuminuria in MWF-8(SHR) animals despite their inherited nephron deficit and higher systolic blood pressure.