Renal injury in streptozotocin-diabetic Ren2-transgenic rats is mainly dependent on hypertension, not on diabetes

Blood pressure-independent renoprotective effects of small interference RNA targeting liver angiotensinogen in experimental diabetes

BACKGROUND AND PURPOSE

Small interfering RNA (siRNA) targeting liver angiotensinogen lowers blood pressure, but its effects in hypertensive diabetes are unknown.

EXPERIMENTAL APPROACH

To address this, TGR (mRen2)27 rats (angiotensin II-dependent hypertension model) were made diabetic with streptozotocin over 18 weeks and treated with either vehicle, angiotensinogen siRNA, the AT₁ antagonist valsartan, the ACE inhibitor captopril, valsartan + siRNA or valsartan + captopril for the final 3 weeks. Mean arterial pressure (MAP) was measured via radiotelemetry.

KEY RESULTS

MAP before treatment was 153 ± 2 mmHg. Diabetes resulted in albuminuria, accompanied by glomerulosclerosis and podocyte effacement, without a change in glomerular filtration rate. All treatments lowered MAP and cardiac hypertrophy, and the largest drop in MAP was observed with siRNA + valsartan. Treatment with siRNA lowered circulating angiotensinogen by >99%, and the lowest circulating angiotensin II and aldosterone levels occurred in the dual treatment groups. Angiotensinogen siRNA did not affect renal angiotensinogen mRNA expression, confirming its liver-specificity. Furthermore, only siRNA with or without valsartan lowered renal angiotensin I. All treatments lowered renal angiotensin II and the reduction was largest (>95%) in the siRNA + valsartan group. All treatments identically lowered albuminuria, whereas only siRNA with or without valsartan restored podocyte foot processes and reduced glomerulosclerosis.

CONCLUSION AND IMPLICATIONS

Angiotensinogen siRNA exerts renoprotection in diabetic TGR (mRen2)27 rats and this relies, at least in part, on the suppression of renal angiotensin II formation from liver-derived angiotensinogen. Clinical trials should now address whether this is also beneficial in human diabetic kidney disease.

Modeling Human Diabetic Kidney Disease by Combining Hyperglycemia and Hypertension in a Transgenic Rodent Model

Traditional animal models mimic only the earliest stages of human diabetic nephropathy (DN), which limits their utility to dissect the pathogenesis of progressive disease or test novel therapeutics. In this chapter we describe in detail the experimental procedures required to conduct the Cyp1a1mRen2 rodent model, in which hyperglycemia and renin-dependent hypertension synergize to promote moderate proteinuria, renal fibrosis, and induction of many of the transcriptomic changes observed in the kidney of patients with progressive DN.

Renal disease pathophysiology and treatment: contributions from the rat

The rat has classically been the species of choice for pharmacological studies and disease modeling, providing a source of high-quality physiological data on cardiovascular and renal pathophysiology over many decades. Recent developments in genome engineering now allow us to capitalize on the wealth of knowledge acquired over the last century. Here, we review rat models of hypertension, diabetic nephropathy, and acute and chronic kidney disease. These models have made important contributions to our understanding of renal diseases and have revealed key genes, such as Ace and P2rx7, involved in renal pathogenic processes. By targeting these genes of interest, researchers are gaining a better understanding of the etiology of renal pathologies, with the promised potential of slowing disease progression or even reversing the damage caused. Some, but not all, of these target genes have proved to be of clinical relevance. However, it is now possible to generate more sophisticated and appropriate disease models in the rat, which can recapitulate key aspects of human renal pathology. These advances will ultimately be used to identify new treatments and therapeutic targets of much greater clinical relevance.

Summary: This Review highlights the key role that the rat continues to play in improving our understanding of the etiologies of renal pathologies, and how these insights have opened up new therapeutic avenues.

Insulin stimulates uric acid reabsorption via regulating urate transporter 1 and ATP-binding cassette subfamily G member 2

Accumulating data indicate that renal uric acid (UA) handling is altered in diabetes and by hypoglycemic agents. In addition, hyperinsulinemia is associated with hyperuricemia and hypouricosuria. However, the underlying mechanisms remain unclear. In this study, we aimed to investigate how diabetes and hypoglycemic agents alter the levels of renal urate transporters. In insulin-depleted diabetic rats with streptozotocin treatment, both UA excretion and fractional excretion of UA were increased, suggesting that tubular handling of UA is altered in this model. In the membrane fraction of the kidney, the expression of urate transporter 1 (URAT1) was significantly decreased, whereas that of ATP-binding cassette subfamily G member 2 (ABCG2) was increased, consistent with the increased renal UA clearance. Administration of insulin to the diabetic rats decreased UA excretion and alleviated UA transporter-level changes, while sodium glucose cotransporter 2 inhibitor (SGLT2i) ipragliflozin did not change renal UA handling in this model. To confirm the contribution of insulin in the regulation of urate transporters, normal rats received insulin and separately, ipragliflozin. Insulin significantly increased URAT1 and decreased ABCG2 levels, resulting in increased UA reabsorption. In contrast, the SGLT2i did not alter URAT1 or ABCG2 levels, although blood glucose levels were similarly reduced. Furthermore, we found that insulin significantly increased endogenous URAT1 levels in the membrane fraction of NRK-52E cells, the kidney epithelial cell line, demonstrating the direct effects of insulin on renal UA transport mechanisms. These results suggest a previously unrecognized mechanism for the anti-uricosuric effects of insulin and provide novel insights into the renal UA handling in the diabetic state.

Rodent models of diabetic nephropathy: their utility and limitations

Diabetic nephropathy is the most common cause of end-stage renal disease. Therefore, novel therapies for the suppression of diabetic nephropathy must be developed. Rodent models are useful for elucidating the pathogenesis of diseases and testing novel therapies, and many type 1 and type 2 diabetic rodent models have been established for the study of diabetes and diabetic complications. Streptozotocin (STZ)-induced diabetic animals are widely used as a model of type 1 diabetes. Akita diabetic mice that have an Ins2+/C96Y mutation and OVE26 mice that overexpress calmodulin in pancreatic β-cells serve as a genetic model of type 1 diabetes. In addition, db/db mice, KK-Ay mice, Zucker diabetic fatty rats, Wistar fatty rats, Otsuka Long-Evans Tokushima Fatty rats and Goto-Kakizaki rats serve as rodent models of type 2 diabetes. An animal model of diabetic nephropathy should exhibit progressive albuminuria and a decrease in renal function, as well as the characteristic histological changes in the glomeruli and the tubulointerstitial lesions that are observed in cases of human diabetic nephropathy. A rodent model that strongly exhibits all these features of human diabetic nephropathy has not yet been developed. However, the currently available rodent models of diabetes can be useful in the study of diabetic nephropathy by increasing our understanding of the features of each diabetic rodent model. Furthermore, the genetic background and strain of each mouse model result in differences in susceptibility to diabetic nephropathy with albuminuria and the development of glomerular and tubulointerstitial lesions. Therefore, the validation of an animal model reproducing human diabetic nephropathy will significantly facilitate our understanding of the underlying genetic mechanisms that contribute to the development of diabetic nephropathy. In this review, we focus on rodent models of diabetes and discuss the utility and limitations of these models for the study of diabetic nephropathy.

An Update on the Use of Animal Models in Diabetic Nephropathy Research

In the current review, we discuss limitations and recent advances in animal models of diabetic nephropathy (DN). As in human disease, genetic factors may determine disease severity with the murine FVB and DBA/2J strains being more susceptible to DN than C57BL/6J mice. On the black and tan, brachyuric (BTBR) background, leptin deficient (ob/ob) mice develop many of the pathological features of human DN. Hypertension synergises with hyperglycemia to promote nephropathy in rodents. Moderately hypertensive endothelial nitric oxide synthase (eNOS(-/-)) deficient diabetic mice develop hyaline arteriosclerosis and nodular glomerulosclerosis and induction of renin-dependent hypertension in diabetic Cyp1a1mRen2 rats mimics moderately severe human DN. In addition, diabetic eNOS(-/-) mice and Cyp1a1mRen2 rats recapitulate many of the molecular pathways activated in the human diabetic kidney. However, no model exhibits all the features of human DN; therefore, researchers should consider biochemical, pathological, and transcriptomic data in selecting the most appropriate model to study their molecules and pathways of interest.

Renal protection by low dose irbesartan in diabetic nephropathy is paralleled by a reduction of inflammation, not of endoplasmic reticulum stress

Diabetes can disrupt endoplasmic reticulum (ER) homeostasis which leads to ER stress. ER stress-induced renal apoptosis seems to be involved in the development of diabetic nephropathy. The present study was designed to investigate the contribution of reduced ER stress to the beneficial effects of an angiotensin receptor blocker. Insulin-dependent diabetes mellitus was induced by streptozotocin injections to hypertensive mRen2-transgenic rats. After 2weeks animals were treated with 0.7mg/kg/day irbesartan. Blood glucose, blood pressure and protein excretion were assessed. Expression of ER stress markers was measured by real-time PCR. Immunohistochemistry was performed to detect markers of ER stress, renal damage and infiltrating cells. Glomerulosclerosis and apoptosis were evaluated. Diabetic mRen2-transgenic rats developed renal injury with proteinuria, tubulointerstitial cell proliferation as well as glomerulosclerosis and podocyte injury. Moreover, an increase in inflammation, podocyte ER stress and apoptosis was detected. Irbesartan somewhat lowered blood pressure and reduced proteinuria, tubulointerstitial cell proliferation and glomerulosclerosis. Podocyte damage was ameliorated but markers of ER stress (calnexin, grp78) and apoptosis were not reduced by irbesartan. On the other hand, inflammatory cell infiltration in the tubulointerstitium and the glomerulus was significantly attenuated. We conclude that irbesartan reduced renal damage even in a very low dose. The beneficial effects of low dose irbesartan were paralleled by a reduction of blood pressure and inflammation but not by a reduction of ER stress and apoptosis. Thus, sustained endoplasmic reticulum stress in the kidney does not necessarily lead to increased inflammation and tubulointerstitial or glomerular injury.

Pathophysiology of the diabetic kidney

Diabetes mellitus contributes greatly to morbidity, mortality, and overall health care costs. In major part, these outcomes derive from the high incidence of progressive kidney dysfunction in patients with diabetes making diabetic nephropathy a leading cause of end-stage renal disease. A better understanding of the molecular mechanism involved and of the early dysfunctions observed in the diabetic kidney may permit the development of new strategies to prevent diabetic nephropathy. Here we review the pathophysiological changes that occur in the kidney in response to hyperglycemia, including the cellular responses to high glucose and the responses in vascular, glomerular, podocyte, and tubular function. The molecular basis, characteristics, and consequences of the unique growth phenotypes observed in the diabetic kidney, including glomerular structures and tubular segments, are outlined. We delineate mechanisms of early diabetic glomerular hyperfiltration including primary vascular events as well as the primary role of tubular growth, hyperreabsorption, and tubuloglomerular communication as part of a "tubulocentric" concept of early diabetic kidney function. The latter also explains the "salt paradox" of the early diabetic kidney, that is, a unique and inverse relationship between glomerular filtration rate and dietary salt intake. The mechanisms and consequences of the intrarenal activation of the renin-angiotensin system and of diabetes-induced tubular glycogen accumulation are discussed. Moreover, we aim to link the changes that occur early in the diabetic kidney including the growth phenotype, oxidative stress, hypoxia, and formation of advanced glycation end products to mechanisms involved in progressive kidney disease.

Renin inhibition in the treatment of diabetic kidney disease

Inhibition of the RAAS (renin–angiotensin–aldosterone system) plays a pivotal role in the prevention and treatment of diabetic nephropathy and a spectrum of other proteinuric kidney diseases. Despite documented beneficial effects of RAAS inhibitors in diabetic patients with nephropathy, reversal of the progressive course of this disorder or at least long-term stabilization of renal function are often difficult to achieve, and many patients still progress to end-stage renal disease. Incomplete inhibition of the RAAS has been postulated as one of reasons for unsatisfactory therapeutic responses to RAAS inhibition in some patients. Inhibition of renin, a rate-limiting step in the RAAS activation cascade, could overcome at least some of the abovementioned problems associated with the treatment with traditional RAAS inhibitors. The present review focuses on experimental and clinical studies evaluating the two principal approaches to renin inhibition, namely direct renin inhibition with aliskiren and inhibition of the (pro)renin receptor. Moreover, the possibilities of renin inhibition and nephroprotection by interventions primarily aiming at non-RAAS targets, such as vitamin D, urocortins or inhibition of the succinate receptor GPR91 and cyclo-oxygenase-2, are also discussed.

Differential regulation of circulating and renal ACE2 and ACE in hypertensive mRen2.Lewis rats with early-onset diabetes

We examined the impact of early diabetes on the circulating and kidney renin-angiotensin system (RAS) in male and female mRen2.Lewis (mRen2) hypertensive rats. Diabetes (DB) was induced by streptozotocin (STZ; 65 mg/kg) at 11 wk of age for 4 wk without insulin replacement. Systolic blood pressures were not increased in DB males or females compared with controls (CON). Circulating angiotensin-converting enzyme 2 (ACE2) increased ninefold (P < 0.05) in DB females and threefold (P < 0.05) in DB males, but circulating ACE and ANG II were higher in the DB groups. Serum C-reactive protein was elevated in DB females but not DB males, and the vascular responses to acetylcholine and estradiol were attenuated in the DB females. Proteinuria, albuminuria, and angiotensinogen excretion increased to a similar extent in both DB females and males. Glomerular VEGF expression also increased to a similar extent in both DB groups. Renal inflammation (CD68(+)cells) increased only in DB females although males exhibited greater inflammation that was not different with DB. Cortical ACE2 did not change in DB females but was reduced (30%) in DB males. Renal neprilysin activity (>75%, P < 0.05) was markedly reduced in the DB females to that in the DB and CON males. ACE activity was significantly lower in both female (75%, P < 0.05) and male (50%; P < 0.05) DB groups, while cortical ANG II and Ang-(1-7) levels were unchanged. In conclusion, female mRen2 rats are not protected from vascular damage, renal inflammation, and kidney injury in early STZ-induced diabetes despite a marked increase in circulating ACE2 and significantly reduced ACE within the kidney.

Hyperglycemia and renin-dependent hypertension synergize to model diabetic nephropathy

Rodent models exhibit only the earliest features of human diabetic nephropathy, which limits our ability to investigate new therapies. Hypertension is a prerequisite for advanced diabetic nephropathy in humans, so its rarity in typical rodent models may partly explain their resistance to nephropathy. Here, we used the Cyp1a1mRen2 rat, in which the murine renin-2 gene is incorporated under the Cytochrome P4501a1 promoter. In this transgenic strain, administration of low-dose dietary indole-3-carbinol induces moderate hypertension. In the absence of hypertension, streptozotocin-induced diabetes resulted in a 14-fold increase in albuminuria but only mild changes in histology and gene expression despite 28 weeks of marked hyperglycemia. In the presence of induced hypertension, hyperglycemia resulted in a 500-fold increase in albuminuria, marked glomerulosclerosis and tubulointerstitial fibrosis, and induction of many of the same pathways that are upregulated in the tubulointerstitium in human diabetic nephropathy. In conclusion, although induction of diabetes alone in rodents has limited utility to model human diabetic nephropathy, renin-dependent hypertension and hyperglycemia synergize to recapitulate many of the clinical, histological, and gene expression changes observed in humans.

Differential contribution of diabetes and the Ren2 gene to glomerular pathology in diabetic (mREN-2)27 rats

The effect of diabetes mellitus vs the effect of the Ren2 gene on the glomerular pathology of (mREN-2)27 heterozygous male rats is controversial. As discrete diabetes-induced glomerular lesions may have been overlooked, we performed a detailed morphometric analysis of glomeruli in diabetic and non-diabetic heterozygous male (mREN-2)27 rats and their normotensive (non-diabetic and diabetic Sprague-Dawley) controls. Glomeruli were scored by light microscopy for nine discrete histological parameters, some of which were graded for extent and/or severity. Mesangiolysis, segmental hypocellularity, and severe tuft-to-capsule adhesions were specific to diabetes; severe mesangial matrix expansion, glomerulosclerosis, thickening of Bowman's capsule, and dilatation of the urinary space were specific to the Ren2 gene. Hyalinosis and hypercellularity were associated with both diabetes and the Ren2 gene: the effect was additive for hyalinosis and synergistic for hypercellularity. The histological parameters were then combined with two physiological indices (systolic blood pressure and proteinuria) and principle components analysis (PCA) was used to detect correlations between the variables. Four discrete patterns of pathology were identified; three were statistically associated with diabetes and/or the Ren2 gene. These findings suggest that both diabetes and the Ren2 gene make significant, albeit different, contributions to the glomerular pathology of diabetic heterozygous male (mREN-2)27 rats. Despite defining the contribution of diabetes, our work does not support the (mREN-2)27 rat as a model of diabetic nephropathy (DN). Rather, it suggests that these animals remain useful for investigating a particular and limited constellation of DN features.

Renal autoregulation and passive pressure-flow relationships in diabetes and hypertension

We investigated renal hemodynamics in isolated, perfused kidneys from rat models of diabetes and hypertension. Autoregulation and passive vascular responses were measured using stepped pressure ramps in the presence of angiotensin II (pEC50) or papaverine (0.1 mM), respectively. Male diabetic heterozygote m(Ren2)27 rats were compared with three male control groups: nondiabetic, normotensive Sprague-Dawley (SD) rats; nondiabetic, hypertensive heterozygote m(Ren2)27 rats; and diabetic, normotensive SD rats. Kidney function (proteinuria, creatinine clearance) was monitored before induction and at monthly intervals. Vascular function was measured in vitro in rats of induction age (6-8 wk) and at 2 and 4 mo postinduction. Renal flow correlated with age, but not diabetes or the Ren2 gene. Kidney weight-specific and body weight-specific renal flow differed between diabetic and nondiabetic rats because diabetic rats had higher kidney but lower body weights. Kidneys from all groups showed effective autoregulation in the presence of angiotensin II. The autoregulatory pressure threshold of m(Ren2)27 rats was higher, and the autoregulation pressure range was wider, compared with SD rats. When vascular smooth muscle activity was blocked with papaverine, pressure-flow responses differed between groups and with time. The m(Ren2)27 rat groups showed higher renal vascular resistance at lower pressures, suggesting greater vascular stiffness. In contrast, diabetic SD rat kidneys demonstrated reduced vessel stiffness. Flow was impaired in diabetic m(Ren2)27 rats at 4 mo, and this correlated with a decline in creatinine clearance. The results suggest that the characteristic late decline in renal filtration function in diabetes- and hypertension-related renal disease follows changes in renal vascular compliance.

Role of kidney ADP-ribosyl cyclase in diabetic nephropathy

The role of ADP-ribosyl cyclases (ADPR-cyclases) in diabetic nephropathy was investigated. ADPR-cyclases synthesize cADP-ribose (cADPR), a Ca(2+)-mobilizing second messenger, and are stimulated by G protein-coupled receptors. We have previously reported that ADPR-cyclases can be activated by ANG II and showed that a specific kidney ADPR-cyclase inhibitor, 4,4'-dihydroxyazobenzene (DHAB), can protect ANG II-mediated mesangial cell growth (Kim SY, Gul R, Rah SY, Kim SH, Park SK, Im MJ, Kwon HJ, Kim UH. Am J Physiol Renal Physiol 294: F982-F989, 2008). In this study, we examined the preventive effect of DHAB on glomerular injury in streptozotocin (STZ)-induced diabetic mice. Male mice were randomly assigned to normal control and diabetic groups of comparable age. A diabetic group received 45 microg/kg of DHAB for 6 wk via daily intraperitoneal injections. Several nephropathy parameters were improved in the DHAB-treated diabetic group compared with the diabetic group, including urinary albumin (diabetic, 44.6 +/- 5.1 vs. treated, 33.9 +/- 3.9 microg/day), creatinine clearance (diabetic, 0.72 +/- 0.03 vs. treated, 0.83 +/- 0.04 ml.min(-1).100 g(-1)), ratio of kidney to body weight (diabetic, 2.5 +/- 0.04 vs. treated, 1.4 +/- 0.04), and mesangial matrix expansion (diabetic, 13.9 +/- 2.2 vs. treated, 8.5 +/- 2.0%). These results indicate that kidney function in STZ-induced diabetes was improved by DHAB administration. Furthermore, DHAB inhibited phosphorylation of Akt and nuclear factor of activated T cell 3 nuclear translocation, as well as ADPR-cyclase activity and cADPR production, which were increased in the kidneys of the diabetic group. In addition, DHAB treatment decreased fibrosis marker protein expression and glomerular hypertrophy in the diabetic kidney. These findings indicate a crucial role that ADPR-cyclase signaling plays in the renal pathogenesis of diabetes and provide a therapeutic tool for the treatment of renal diseases.

Autonomic renal denervation ameliorates experimental glomerulonephritis

Increasing evidence indicates that inflammation of visceral organs is significantly affected by the autonomic nervous system. Such neuroimmune interactions have not been studied in the kidney. Here, we show that the rat kidney is innervated by both tyrosine hydroxylase-positive sympathetic efferent nerve fibers and calcitonin gene-related peptide-positive primary afferent nerve fibers, both of which are found in proximity to macrophages and dendritic cells. Complete surgical bilateral renal denervation was performed 2 d before glomerulonephritis was induced by injecting the monoclonal anti-Thy-1.1 antibody OX-7. Denervation significantly reduced albuminuria, mesangiolysis, formation of microaneurysms, deposition of glomerular collagen IV, and expression of TGF-beta compared with sham-operated controls. Accordingly, inflammation, identified by accumulation of interstitial macrophages and renal expression of TNF-alpha, and mesangial cell proliferation were significantly reduced. These findings indicate that autonomic renal denervation ameliorates and, by inference, innervation exacerbates acute inflammation in the kidney; therefore, neurotransmitters or neuropeptides and their receptors might represent novel targets for the treatment of acute glomerulonephritis.

Rodent models of streptozotocin-induced diabetic nephropathy

Streptozotocin-induced pancreatic injury is commonly used for creating rodent models of type 1 diabetes which develop renal injury with similarities to human diabetic nephropathy. This model can be established in genetically modified rodents for investigating the role of molecular mechanisms and genetic susceptibility in the development of diabetic nephropathy. In this report, the authors describe and compare the current protocols being used to establish models of diabetic nephropathy in rat and mouse strains using streptozotocin. The authors also list some of the histological criteria and biochemical measurements which are being used to validate these models. In addition, our review explains some of the key aspects involved in these models, including the impact of streptozotocin-dosage, uninephrectomy, hypertension and genetically modified strains, which can each affect the development of disease and the interpretation of findings.

Can rodent models of diabetic kidney disease clarify the significance of early hyperfiltration?: recognizing clinical and experimental uncertainties

In the past, hyperfiltration and increased glomerular capillary pressure have been identified as important determinants of the development of DN (diabetic nephropathy). Recently, some basic research and clinical reviews on DN have omitted identifying hyperfiltration as an important risk factor. At the same time, different rodent models of DN have been described without and with documented hyperfiltration. In the present review, the importance of hyperfiltration is reassessed, reviewing key clinical and research studies, including the first single nephron studies in a mouse model of DN. From clinical studies of Type 1 and Type 2 diabetes mellitus, it is clear that many patients do not have early hyperfiltration and, even when present, its contribution to subsequent DN remains uncertain. Key mechanisms underlying hyperfiltration in rodent models are reviewed. Findings on intrarenal NO metabolism and the control of single-nephron GFR (glomerular filtration rate) in rodent models of DN are also presented. Characterization of valid experimental models of DN should include a careful delineation of the absence or presence of early hyperfiltration, with special efforts made to establish the specific role hyperfiltration may play in the emergence of DN.