Renal enlargement: comparative autoradiographic studies of 3H-thymidine uptake in diabetic and uninephrectomized rats

Renal handling of albumin in rats with early stage diabetes: A theoretical analysis

In early diabetic nephropathy (DN), recent studies have shown that albuminuria stems mostly from alterations in tubular function rather than from glomerular damage. Several factors in DN, including hyperfiltration, hypertrophy and reduced abundance of the albumin receptors megalin and cubilin, affect albumin endocytosis in the proximal tubule (PT). To assess their respective contribution, we developed a model of albumin handling in the rat PT that couples the transport of albumin to that of water and solutes. Our simulations suggest that, under basal conditions, ∼75% of albumin is retrieved in the S1 segment. The model predicts negligible uptake in S3, as observed experimentally. It also accurately predicts the impact of acute hyperglycaemia on urinary albumin excretion. Simulations reproduce observed increases in albumin excretion in early DN by considering the combined effects of increased glomerular filtration rate (GFR), osmotic diuresis, hypertrophy, and megalin and cubilin downregulation, without stipulating changes in glomerular permselectivity. The results indicate that in isolation, glucose-elicited osmotic diuresis and glucose transporter upregulation raise albumin excretion only slightly. Enlargement of PT diameter not only augments uptake via surface area expansion, but also reduces fluid velocity and thus shear stress-induced stimulation of endocytosis. Overall, our model predicts that downregulation of megalin and cubilin and hyperfiltration both contribute significantly to increasing albumin excretion in rats with early-stage diabetes. The results also suggest that acute sodium-glucose cotransporter 2 inhibition lowers albumin excretion only if GFR decreases sufficiently, and that angiotensin II receptor blockers mitigate urinary albumin loss in early DN in large part by upregulating albumin receptor abundance. KEY POINTS: The urinary excretion of albumin is increased in early diabetic nephropathy (DN). It is difficult to experimentally disentangle the multiple factors that affect the renal handling of albumin in DN. We developed a mathematical model of albumin transport in the rat proximal tubule (PT) to examine the impact of elevated plasma glucose, hyperfiltration, PT hypertrophy and reduced abundance of albumin receptors on albumin uptake and excretion in DN. Our model predicts that glucose-elicited osmotic diuresis per se raises albumin excretion only slightly. Conversely, increases in PT diameter and length favour reduced albumin excretion. Our results suggest that downregulation of the receptors megalin and cubilin in PT cells and hyperfiltration both contribute significantly to increasing albumin excretion in DN. The model helps to better understand the mechanisms underlying urinary loss of albumin in early-stage diabetes, and the impact of specific treatments thereupon.

Roles of myosin 1e and the actin cytoskeleton in kidney functions and familial kidney disease

Mammalian kidneys are responsible for removing metabolic waste and maintaining fluid and electrolyte homeostasis via selective filtration. One of the proteins closely linked to selective renal filtration is myosin 1e (Myo1e), an actin-dependent molecular motor found in the specialized kidney epithelial cells involved in the assembly and maintenance of the renal filter. Point mutations in the gene encoding Myo1e, MYO1E, have been linked to familial kidney disease, and Myo1e knockout in mice leads to the disruption of selective filtration. In this review, we discuss the role of the actin cytoskeleton in renal filtration, the known and hypothesized functions of Myo1e, and the possible explanations for the impact of MYO1E mutations on renal function.

Biochemical mechanism underlying the pathogenesis of diabetic retinopathy and other diabetic complications in humans: the methanol-formaldehyde-formic acid hypothesis

Hyperglycemia in diabetic patients is associated with abnormally-elevated cellular glucose levels. It is hypothesized that increased cellular glucose will lead to increased formation of endogenous methanol and/or formaldehyde, both of which are then metabolically converted to formic acid. These one-carbon metabolites are known to be present naturally in humans, and their levels are increased under diabetic conditions. Mechanistically, while formaldehyde is a cross-linking agent capable of causing extensive cytotoxicity, formic acid is an inhibitor of mitochondrial cytochrome oxidase, capable of inducing histotoxic hypoxia, ATP deficiency and cytotoxicity. Chronic increase in the production and accumulation of these toxic one-carbon metabolites in diabetic patients can drive the pathogenesis of ocular as well as other diabetic complications. This hypothesis is supported by a large body of experimental and clinical observations scattered in the literature. For instance, methanol is known to have organ- and species-selective toxicities, including the characteristic ocular lesions commonly seen in humans and non-human primates, but not in rodents. Similarly, some of the diabetic complications (such as ocular lesions) also have a characteristic species-selective pattern, closely resembling methanol intoxication. Moreover, while alcohol consumption or combined use of folic acid plus vitamin B is beneficial for mitigating acute methanol toxicity in humans, their use also improves the outcomes of diabetic complications. In addition, there is also a large body of evidence from biochemical and cellular studies. Together, there is considerable experimental support for the proposed hypothesis that increased metabolic formation of toxic one-carbon metabolites in diabetic patients contributes importantly to the development of various clinical complications.

Direct evidence of proximal tubular proliferation in early diabetic nephropathy

Kidney hypertrophy is a common clinical feature in patients with diabetes and is associated with poor renal outcomes. Initial cell proliferation followed by cellular hypertrophy are considered the responsible mechanisms for diabetic kidney hypertrophy. However, whether similar responses against hyperglycemia continue in the chronic phase in diabetes is unclear. We performed lineage tracing analysis of proximal tubular epithelia using novel type 2 diabetic mice with a tamoxifen-inducible proximal tubule-specific fluorescent reporter. Clonal analysis of proximal tubular epithelia demonstrated that the labeled epithelia proliferated in type 2 diabetic mice. Based on the histological analysis and protein/DNA ratio of sorted labeled tubular epithelia, there was no evidence of cellular hypertrophy in type 2 diabetic mice. Lineage tracing and histological analyses of streptozocin-induced type 1 diabetes also revealed that cellular proliferation occurs in the chronic phase of type 1 diabetes induction. According to our study, epithelial proliferation accompanied by SGLT2 upregulation, rather than cellular hypertrophy, predominantly occurs in the hypertrophic kidney in both type 1 and type 2 diabetes. An increased number of SGLT2+ tubular epithelia may be an adaptive response against hyperglycemia, and linked to the hyper-reabsorption of sodium and glucose observed in type 2 diabetes patients.

Non-invasive investigation of early kidney damage in streptozotocin-induced diabetic rats by intravoxel incoherent motion diffusion-weighted (IVIM) MRI

Background

The current study investigated the performance of intravoxel incoherent motion diffusion (IVIM) technology in monitoring early renal injury in streptozotocin rats.

Methods

Forty-eight Sprague-Dawley (SD) rats were divided into a control group and a diabetic mellitus (DM) group. Six rats in each group were randomly selected for MR scans at four different time points (0, 4, 8, and 12 weeks). The IVIM-derived parameters (D, D*, f and ADC values) of the renal cortex (CO), outer and inner stripe of the outer medulla (OS, IS), and internal medulla (IM) were acquired. Changes in each IVIM-derived parameter over time were analyzed, and differences between the two groups at each point were assessed. The associations between the IVIM parameters and IV collagen expression, urine volume (UV), blood urea nitrogen (BUN), and serum creatinine (Scr) were investigated.

Results

The D and D* values of CO and the ADC values of CO, OS, IS and IM displayed significantly different trends between the two groups over time (P<0.05). In addition, significant correlations were discovered between the D* value of CO and UV and BUN (r=0.527, P=0.033; r=0.617, P=0.005), between the ADC value of IM and BUN (r=0.557, P=0.019) and between the f value of IM and BUN (r=0.527, P=0.033). No correlation was found between IVIM parameters and IV collagen expression and Scr.

Conclusions

IVIM is a potential sensitive and noninvasive technology for the simultaneous assessment of early renal cortical and medullary injuries induced by diabetes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12882-021-02530-8.

The effect of the sodium-glucose cotransporter type-2 inhibitor dapagliflozin on glomerular filtration rate in healthy cats

Sodium-glucose cotransporter type-2 inhibitors (SGLT2is) reduce glomerular hyperfiltration in diabetic people with early diabetic nephropathy. The objective of this report was to assess changes in glomerular filtration rate in healthy cats after treatment with a SGLT2i. Eight healthy research adult castrated male cats were used in a randomized, controlled, cross-over study design. We induced isolated renal tubular glucosuria by dosing cats with the SGLT2i dapagliflozin. The cats received by mouth 10 mg dapagliflozin or control every 24 h in each of the 4, 5-d trial periods that were separated by a 7-d washout period. We assessed glomerular filtration rate (iohexol clearance method), serum urea, creatinine, symmetric dimethylarginine, and 24-h sodium and chloride urinary excretion on the fifth day of each trial period. We analyzed the data with a mixed linear model that included the fixed effects of treatment (treated and control) and trial period, and the random effect of the cat. Compared with controls, cats treated with dapagliflozin had a significant increase in mean (±SE) glomerular filtration rate (3.1 ± 0.2 vs 2.5 ± 0.2 mL/kg/min; P = 0.01), whereas there were no significant differences in serum urea, creatinine and symmetric dimethylarginine, and 24-h urine sodium and chloride excretion. We propose that dapagliflozin-mediated delivery of sodium and glucose distal from the proximal convoluted tubule induced compensatory increased sodium absorption at the thick ascending loop of Henle that resulted in decreased sodium delivery to the distal tubule leading to tubuloglomerular feedback-mediated glomerular hyperfiltration. Future studies should determine if SGLT2is' renoprotective effect in people can be enhanced with the addition of a Na⁺-K⁺-Cl^- diuretic and whether dapagliflozin will be useful in mitigating proteinuria and hypertension that follow glomerular hyperfiltration in diabetic companion animals in a similar mechanism as in people.

Activation of PI3K in response to high glucose leads to regulation of SOCS-3 and STAT1/3 signals and induction of glomerular mesangial extracellular matrix formation

Excessive deposition of extracellular matrix (ECM) in the glomerulus contributed by mesangial cells is the hallmark of diabetic nephropathy, eventually leading to glomerulosclerosis. In this study, we examined the regulatory signals involved in the high glucose (HG)-induced overproduction of ECM in rat mesangial cells (RMCs). We disclosed excessive fibronectin and collagen IV production, tyrosine phosphorylation of signal transducer and activator of transcription 1 and 3 (STAT1/3), and up-regulation of suppressor of cytokine signaling-3 (SOCS-3) expression in HG-treated RMCs. STAT1/STAT3 binding element was essential for SOCS-3 promoter activity stimulated by HG. HG was capable of promoting the specific DNA binding activities to an oligonucleotide probe containing the SOCS-3 sequence. The selective phosphoinositide 3-kinase (PI3K) inhibitor LY294002 and dominant negative p85 vector (DNΔp85) transfection effectively abolished these HG-induced responses. Moreover, HG markedly increased the cyclin kinase inhibitor p27^Kip1 protein expression, which could be inhibited by LY294002 or transfection of DNΔp85. Taken together, these results suggest that HG-induced SOCS-3 upregulation depends upon the presence of STAT-binding element in the SOCS-3 promoter, which is specifically activated by STAT1/3. The PI3K/STAT1/3 signaling pathway mediated HG-triggered ECM accumulation and SOCS-3 upregulation in RMCs.

Effects of childbirth on podocyturia in women with normotensive, uncomplicated pregnancies

Changes in hemodynamics and blood pressure occur shortly before and after childbirth regardless of the mode of delivery. This study aimed to test the hypothesis that parturition induces a temporal increase in podocyturia monitored by podocyte-specific protein podocin mRNA expression levels (Pod-mRNA). A total of 105 urine specimens, consisting of 43 and 62 from 18 and 20 otherwise healthy women with vaginal delivery (VD) and elective cesarean delivery (ECS), respectively, were studied. Determination of urine protein and creatinine (Cr) concentrations and quantitative analyses of Pod-mRNA, nephrin mRNA (Nep-mRNA), synaptopodin mRNA (Syn-mRNA), and aquaporin 2 mRNA expression were performed using RT-PCR in pelleted urine samples. Levels of mRNA expression were corrected by urine Cr concentration. Podocyturia increased significantly, concomitant with a significantly decreased Nep:Pod-mRNA ratio (NPR) in the urine, collected immediately before or after childbirth regardless of the delivery mode compared with urine collected before commencement of labor or on postpartum day 3 or later. Podocyturia was significantly negatively correlated with NPR [correlation coefficient (r) = -0.614/-0.750 for VD/ECS women, respectively], as well as the Syn:Pod-mRNA ratio. Systolic blood pressure exceeded 140 mmHg during labor in 50% of VD women, and mean arterial pressure was significantly positively correlated with podocyturia during labor in VD women (r = 0.733). Thus parturition induces a transient increase in urine podocytes with reduced Nep- and Syn-mRNA expressions. Glomerular podocytes with reduced Nep- and Syn-mRNA levels were suggested to be likely to detach from the glomerular basement membrane around childbirth.

Intravoxel incoherent motion and diffusion tensor imaging of early renal fibrosis induced in a murine model of streptozotocin induced diabetes

INTRODUCTION

To assess if parameters in intravoxel incoherent motion (IVIM) and diffusion tensor imaging (DTI) can be used to evaluate early renal fibrosis in a mouse model of diabetic nephropathy.

MATERIALS & METHODS

In a population of 38 male CD1 mice (8weeks old, 20-30g), streptozotocin induced diabetes was created in 20 mice via a single intraperitoneal injection of streptozotocin at 150mg/kg, while 18 mice served as control group. IVIM parameters were acquired at 0, 12 and 24weeks after injection of streptozotocin using a range of b values from 0 to 1200s/mm2. DTI parameters were obtained using 12 diffusion directions and lower b values of 0, 100 and 400s/mm2. DTI and IVIM parameters were obtained using region of interests drawn over the renal parenchyma. Histopathological analysis of the right kidney was performed in all mice. Results were analyzed using an unpaired t-test with P<0.05 considered statistically significant.

RESULTS

Renal cortex fractional anisotropy (FA) was significantly lower in the diabetes group at week 12 as compared with the control group. Renal cortex apparent diffusion coefficient and tissue diffusivity were significantly higher in the diabetes group at week 12 compared with the control group at 12weeks. Blood flow was significantly decreased at the renal medulla at 24weeks. Histopathological analysis confirmed fibrosis in the diabetes group at 24weeks.

CONCLUSION

FA is significantly reduced in diabetic nephropathy. FA might serve a potential role in the detection and therapy monitoring of early diabetic nephropathy.

Alteration of podocyte phenotype in the urine of women with preeclampsia

Podocyte injury has been suggested to induce phenotypic alteration of glomerular podocytes and accelerate the detachment of podocytes from the glomeruli resulting in podocyturia. However, it is not clear whether podocyte phenotypic alteration occurs in the urine of women with preeclampsia (PE). Seventy-seven and 116 pelleted urine samples from 38 and 18 women at various stages of normal and PE pregnancies, respectively underwent quantitative analysis of podocyte-specific or associated protein mRNA expression, including podocin, nephrin, and synaptopodin using RT-PCR. Significant proteinuria in pregnancy (SPIP) is defined as protein:creatinine ratio (P/Cr, mg/mg) ≥0.27 in the urine supernatant. All three urine-pellet mRNAs expression levels were significantly positively correlated with P/Cr levels, suggesting that podocyturia increased with proteinuria. The podocin:nephrin mRNA ratio (PNR) and synaptopodin:nephrin mRNA ratio (SNR) increased significantly with increasing P/Cr, while the podocin:synaptopodin mRNA ratio (PSR) did not change significantly according to P/Cr, resulting in significantly higher PNR and SNR, but not PSR levels, in urine from PE women with than without SPIP. The PNR, SNR, and PSR in urine from PE women before onset of SPIP were comparable to those from controls. Thus, nephrin mRNA expression was reduced in the podocytes recovered from PE women.

Podocyte hypertrophy precedes apoptosis under experimental diabetic conditions

Podocyte hypertrophy and apoptosis are two hallmarks of diabetic glomeruli, but the sequence in which these processes occur remains a matter of debate. Here we investigated the effects of inhibiting hypertrophy on apoptosis, and vice versa, in both podocytes and glomeruli, under diabetic conditions. Hypertrophy and apoptosis were inhibited using an epidermal growth factor receptor inhibitor (PKI 166) and a pan-caspase inhibitor (zAsp-DCB), respectively. We observed significant increases in the protein expression of p27, p21, phospho-eukaryotic elongation factor 4E-binding protein 1, and phospho-p70 S6 ribosomal protein kinase, in both cultured podocytes exposed to high-glucose (HG) medium, and streptozotocin-induced diabetes mellitus (DM) rat glomeruli. These increases were significantly inhibited by PKI 166, but not by zAsp-DCB. In addition, the amount of protein per cell, the relative cell size, and the glomerular volume were all significantly increased under diabetic conditions, and these changes were also blocked by treatment with PKI 166, but not zAsp-DCB. Increased protein expression of cleaved caspase-3 and cleaved poly (ADP-ribose) polymerase, together with increased Bax/Bcl-2 ratios, were also observed in HG-stimulated podocytes and DM glomeruli. Treatment with either zAsp-DCB or PKI 166 resulted in a significant attenuation of these effects. Both PKI 166 and zAsp-DCB also inhibited the increase in number of apoptotic cells, as assessed by Hoechst 33342 staining and TUNEL assay. Under diabetic conditions, inhibition of podocyte hypertrophy results in attenuated apoptosis, whereas blocking apoptosis has no effect on podocyte hypertrophy, suggesting that podocyte hypertrophy precedes apoptosis.

Non-Proteinuric Diabetic Nephropathy

Diabetic nephropathy patients traditionally show significant macroalbuminuria prior to the development of renal impairment. However, this clinical paradigm has recently been questioned. Epidemiological surveys confirm that chronic kidney disease (CKD) diagnosed by a low glomerular filtration rate (GFR) is more common in diabetic patients than in the non-diabetic population but a low number of patients had levels of proteinuria above that which traditionally defines overt diabetic nephropathy (>500 mg/g). The large number of patients with low levels of proteinuria suggests that the traditional clinical paradigm of overt diabetic nephropathy is changing since it does not seem to be the underlying renal lesion in most of diabetic subjects with CKD.

Early-onset diabetic E1-DN mice develop albuminuria and glomerular injury typical of diabetic nephropathy

The transgenic E1-DN mice express a kinase-negative epidermal growth factor receptor in their pancreatic islets and are diabetic from two weeks of age due to impaired postnatal growth of β-cell mass. Here, we characterize the development of hyperglycaemia-induced renal injury in the E1-DN mice. Homozygous mice showed increased albumin excretion rate (AER) at the age of 10 weeks; the albuminuria increased over time and correlated with blood glucose. Morphometric analysis of PAS-stained histological sections and electron microscopy images revealed mesangial expansion in homozygous E1-DN mice, and glomerular sclerosis was observed in the most hyperglycaemic mice. The albuminuric homozygous mice developed also other structural changes in the glomeruli, including thickening of the glomerular basement membrane and widening of podocyte foot processes that are typical for diabetic nephropathy. Increased apoptosis of podocytes was identified as one mechanism contributing to glomerular injury. In addition, nephrin expression was reduced in the podocytes of albuminuric homozygous E1-DN mice. Tubular changes included altered epithelial cell morphology and increased proliferation. In conclusion, hyperglycaemic E1-DN mice develop albuminuria and glomerular and tubular injury typical of human diabetic nephropathy and can serve as a new model to study the mechanisms leading to the development of diabetic nephropathy.

Angiotensin-(1-7) abolishes AGE-induced cellular hypertrophy and myofibroblast transformation via inhibition of ERK1/2

Angiotensin-(1-7) (Ang-(1-7))/AT7-Mas receptor axis is an alternative pathway within the renin-angiotensin system (RAS) that generally opposes the actions of Ang II/AT1 receptor pathway. Advanced glycated end product (AGEs) including glucose- and methylglyoxal-modified albumin (MGA) may contribute to the development and progression of diabetic nephropathy in part through activation of the Ang II/AT1 receptor system; however, the influence of AGE on the Ang-(1-7) arm of the RAS within the kidney is unclear. The present study assessed the impact of AGE on the Ang-(1-7) axis in NRK-52E renal epithelial cells. MGA exposure for 48 h significantly reduced the intracellular levels of Ang-(1-7) approximately 50%; however, Ang I or Ang II expression was not altered. The reduced cellular content of Ang-(1-7) was associated with increased metabolism of the peptide to the inactive metabolite Ang-(1-4) [MGA: 175±9 vs.

CONTROL

115±11 fmol/min/mg protein, p<0.05, n=3] but no change in the processing of Ang I to Ang-(1-7). Treatment with Ang-(1-7) reversed MGA-induced cellular hypertrophy and myofibroblast transition evidenced by reduced immunostaining and protein expression of α-smooth muscle actin (α-SMA) [0.4±0.1 vs. 1.0±0.1, respectively, n=3, p<0.05]. Ang-(1-7) abolished AGE-induced activation of the MAP kinase ERK1/2 to a similar extent as the TGF-β receptor kinase inhibitor SB58059; however, Ang-(1-7) did not attenuate the MGA-stimulated release of TGF-β. The AT7-Mas receptor antagonist D-Ala(7)-Ang-(1-7) abolished the inhibitory actions of Ang-(1-7). In contrast, AT1 receptor antagonist losartan did not attenuate the MGA-induced effects. We conclude that Ang-(1-7) may provide an additional therapeutic approach to the conventional RAS blockade regimen to attenuate AGE-dependent renal injury.

The pathobiology of diabetic vascular complications--cardiovascular and kidney disease

With the increasing incidence of obesity and type 2 diabetes, it is predicted that more than half of Americans will have diabetes or pre-diabetes by 2020. Diabetic patients develop vascular complications at a much faster rate in comparison to non-diabetic individuals, and cardiovascular risk is increased up to tenfold. With the increasing incidence of diabetes across the world, the development of vascular complications will become an increasing medical burden. Diabetic vascular complications affect the micro- and macro-vasculature leading to kidney disease often requiring dialysis and transplantation or cardiovascular disease increasing the risk for myocardial infarction, stroke and amputations as well as leading to premature mortality. It has been suggested that many complex pathways contribute to the pathobiology of diabetic complications including hyperglycaemia itself, the production of advanced glycation end products (AGEs) and interaction with the receptors for AGEs such as the receptor for advanced glycation end products (RAGE), as well as the activation of vasoactive systems such as the renin-angiotensin aldosterone system (RAAS) and the endothelin system. More recently, it has been hypothesised that reactive oxygen species derived from NAD(P)H oxidases (Nox) may represent a common downstream mediator of vascular injury in diabetes. Current standard treatment of care includes the optimization of blood glucose and blood pressure usually including inhibitors of the renin-angiotensin system. Although these interventions are able to delay progression, they fail to prevent the development of complications. Thus, there is an urgent medical need to identify novel targets in diabetic vascular complications which may include the blockade of Nox-derived ROS formation, as well as blockade of AGE formation and inhibitors of RAGE activation. These strategies may provide superior protection against the deleterious effects of diabetes on the vasculature.

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.

Plumbagin ameliorates diabetic nephropathy via interruption of pathways that include NOX4 signalling

NADPH oxidase 4 (Nox4) is reported to be the major source of reactive oxygen species (ROS) in the kidneys during the early stages of diabetic nephropathy. It has been shown to mediate TGFβ1-induced differentiation of cardiac fibroblasts into myofibroblasts. Despite TGFβ1 being recognised as a mediator of renal fibrosis and functional decline role in diabetic nephropathy, the renal interaction between Nox 4 and TGFβ1 is not well characterised. The aim of this study was to investigate the role of Nox4 inhibition on TGFβ1-induced fibrotic responses in proximal tubular cells and in a mouse model of diabetic nephropathy. Immortalised human proximal tubular cells (HK2) were incubated with TGFβ1 ± plumbagin (an inhibitor of Nox4) or specific Nox4 siRNA. Collagen IV and fibronectin mRNA and protein expression were measured. Streptozotocin (STZ) induced diabetic C57BL/6J mice were administered plumbagin (2 mg/kg/day) or vehicle (DMSO; 50 µl/mouse) for 24 weeks. Metabolic, physiological and histological markers of nephropathy were determined. TGFβ1 increased Nox4 mRNA expression and plumbagin and Nox4 siRNA significantly inhibited TGF-β1 induced fibronectin and collagen IV expression in human HK2 cells. STZ-induced diabetic C57BL/6J mice developed physiological features of diabetic nephropathy at 24 weeks, which were reversed with concomitant plumbagin treatment. Histologically, plumbagin ameliorated diabetes induced upregulation of extracellular matrix protein expression compared to control. This study demonstrates that plumbagin ameliorates the development of diabetic nephropathy through pathways that include Nox4 signalling.

Apelin retards the progression of diabetic nephropathy

Apelin and its receptor APJ have pleiotropic effects in mice and humans and play a protective role in cardiovascular diseases at least partially by inhibiting oxidative stress. Our objective was to study the effect of apelin on the progression of kidney disease in mice with established type 1 diabetes. Ove26 mice with type 1 diabetes received daily subcutaneous injections of apelin for 2 or 14 wk. APJ localizes in the glomeruli and blood vessels of kidneys. Renal APJ expression was reduced in diabetic mice but increased after treatment with apelin. Apelin treatment did not affect glycemia, body weight, or blood pressure in diabetic mice. Whole kidney and glomerular hypertrophy, as well as renal inflammation, including monocyte chemoattractant protein 1 and vascular cell adhesion molecule 1 expression, NF-κB activation, and monocyte infiltration, was inhibited after short and long treatment with apelin. Apelin administration significantly reduced albuminuria at 6 mo. Short treatment with apelin was sufficient to reverse the downregulation of the antioxidant enzyme catalase. Expression of angiotensin II and angiotensin type 1 receptor (AT1) in kidneys from diabetic mice treated was not affected by apelin. These findings show for the first time that apelin exerts a protective effect on the diabetic kidney. Short administration is sufficient to reduce kidney and glomerular hypertrophy as well as renal inflammation, but prolonged treatment is required to improve albuminuria. This effect was independent of the activation of the renin angiotensin system but correlated with upregulation of the antioxidant catalase. Apelin may represent a novel tool to treat diabetic nephropathy.

Mechanisms of diabetic complications

It is increasingly apparent that not only is a cure for the current worldwide diabetes epidemic required, but also for its major complications, affecting both small and large blood vessels. These complications occur in the majority of individuals with both type 1 and type 2 diabetes. Among the most prevalent microvascular complications are kidney disease, blindness, and amputations, with current therapies only slowing disease progression. Impaired kidney function, exhibited as a reduced glomerular filtration rate, is also a major risk factor for macrovascular complications, such as heart attacks and strokes. There have been a large number of new therapies tested in clinical trials for diabetic complications, with, in general, rather disappointing results. Indeed, it remains to be fully defined as to which pathways in diabetic complications are essentially protective rather than pathological, in terms of their effects on the underlying disease process. Furthermore, seemingly independent pathways are also showing significant interactions with each other to exacerbate pathology. Interestingly, some of these pathways may not only play key roles in complications but also in the development of diabetes per se. This review aims to comprehensively discuss the well validated, as well as putative mechanisms involved in the development of diabetic complications. In addition, new fields of research, which warrant further investigation as potential therapeutic targets of the future, will be highlighted.

Inhibition of the epidermal growth factor receptor preserves podocytes and attenuates albuminuria in experimental diabetic nephropathy

AIM

Early renal enlargement may predict the future development of nephropathy in patients with diabetes. The epidermal growth factor (EGF)-EGF receptor (EGFR) system plays a pivotal role in mediating renal hypertrophy, where it may act to regulate cell growth and proliferation and also to mediate the actions of angiotensin II through transactivation of the EGFR. In the present study we sought to investigate the effects of long-term inhibition of the EGFR tyrosine kinase in an experimental model of diabetes that is characterized by angiotensin II dependent hypertension.

METHODS

Female heterozygous streptozotocin-diabetic TGR(mRen-2)27 rats were treated with the EGFR inhibitor PKI 166 by daily oral dosing for 16 weeks.

RESULTS

Treatment of TGR(mRen-2)27 rats with PKI 166 attenuated the increase in kidney size, glomerular hypertrophy and albuminuria that occurred with diabetes. The reduction in albuminuria, with EGFR inhibition in diabetic TGR(mRen-2)27 rats, was associated with preservation of the number of glomerular cells staining positively for the podocyte nuclear marker, WT1. Immunostaining for WT1 inversely correlated with glomerular volume in diabetic rats. In contrast to agents that block the renin-angiotensin system (RAS), EGFR inhibition had no effect on either the quantity of mesangial matrix or the magnitude of tubular injury in diabetic animals.

CONCLUSION

These observations indicate that inhibition of the tyrosine kinase activity of the EGFR attenuates kidney and glomerular enlargement in association with podocyte preservation and reduction in albuminuria in diabetes. Accordingly, targeting the EGF-EGFR pathway may represent a therapeutic strategy for patients who continue to progress despite RAS-blockade.

Cell biology and pathology of podocytes

As an integral member of the filtration barrier in the kidney glomerulus, the podocyte is in a unique geographical position: It is exposed to chemical signals from the urinary space (Bowman's capsule), it receives and transmits chemical and mechanical signals to/from the glomerular basement membrane upon which it elaborates, and it receives chemical and mechanical signals from the vascular space with which it also communicates. As with every cell, the ability of the podocyte to receive signals from the surrounding environment and to translate them to the intracellular milieu is dependent largely on molecules residing on the cell membrane. These molecules are the first-line soldiers in the ongoing battle to sense the environment, to respond to friendly signals, and to defend against injurious foes. In this review, we take a membrane biologist's view of the podocyte, examining the many membrane receptors, channels, and other signaling molecules that have been implicated in podocyte biology. Although we attempt to be comprehensive, our goal is not to capture every membrane-mediated pathway but rather to emphasize that this approach may be fruitful in understanding the podocyte and its unique properties.

The proximal tubule in the pathophysiology of the diabetic kidney

Diabetic nephropathy is a leading cause of end-stage renal disease. A better understanding of the molecular mechanism involved in the early changes of the diabetic kidney may permit the development of new strategies to prevent diabetic nephropathy. This review focuses on the proximal tubule in the early diabetic kidney, particularly on its exposure and response to high glucose levels, albuminuria, and other factors in the diabetic glomerular filtrate, the hyperreabsorption of glucose, the unique molecular signature of the tubular growth phenotype, including aspects of senescence, and the resulting cellular and functional consequences. The latter includes the local release of proinflammatory chemokines and changes in proximal tubular salt and fluid reabsorption, which form the basis for the strong tubular control of glomerular filtration in the early diabetic kidney, including glomerular hyperfiltration and odd responses like the salt paradox. Importantly, these early proximal tubular changes can set the stage for oxidative stress, inflammation, hypoxia, and tubulointerstitial fibrosis, and thereby for the progression of diabetic renal disease.

Podocyte membrane vesicles in urine originate from tip vesiculation of podocyte microvilli

Podocyte injury is involved in both the onset and progression of glomerular diseases. Our previous studies revealed that apical cell membranes of podocyte are shed into urine sediment and that urinary podocalyxin is a useful biomarker of podocyte injury. In this study, we examined the origin of urinary podocalyxin. Urine samples and kidney specimens from healthy children (n = 126) and patients with glomerular diseases (n = 77) were analyzed by immunohistologic methods. Immunofluorescence studies demonstrated that urinary podocalyxin was shed as granular structures into both the urine sediment and supernatant. Large amounts of podocalyxin were shed into both the urine sediment (17.2 +/- 3.2 ng/mg creatinine) and the supernatant (172.6 +/- 24.6 ng/mg creatinine) of patients, compared with the small amounts of urinary podocalyxin in healthy controls (sediment, 0.5 +/- 0.1 ng/mg creatinine; supernatant, 24.3 +/- 3.5 ng/mg creatinine). Electron and immunoelectron microscopic examinations showed that podocalyxin-positive vesicles in the sediment (125.6 +/- 8.8 nm) and the supernatant (121.2 +/- 6.4 nm) were similar in size to podocyte microvilli in biopsy specimens (123.6 +/- 8.9 nm), differentiating them from the much smaller urine exosomes (30-80 nm in diameter). Urine podocalyxin-positive vesicles tested negative in immunofluorescence microscopy on both exosomal markers CD24 and CD63. Podocalyxin-positive vesicles also tested negative for cytoskeletal markers, and electron microscopic examination revealed tip vesiculation of microvilli. We conclude that human urinary apical cell membrane vesicles appear to originate not from podocyte exosomes but from tip vesiculation of glomerular podocyte microvilli.

The role of tubular injury in diabetic nephropathy

Diabetic nephropathy is associated with increased mortality in diabetic patients and is a major cause of end-stage renal disease in most countries. Understanding its pathogenesis is important as it may equip us with novel ways in its prevention and in slowing its progression. To date, attempts to unravel the complex pathogenesis and pathophysiology of diabetic nephropathy have mostly focused on the glomerulus. However, recently a lot of data has accumulated that implicates the tubules as playing a key role. This article reviews these data and the light they throw on the role of renal tubules in the pathogenesis of diabetic nephropathy.

Progression of glomerular and tubular disease in pediatrics

Chronic kidney disease may be stimulated by many different etiologies, but its progression involves a common, yet complex, series of events that lead to the replacement of normal tissue with scar. These events include altered physiology within the kidney leading to abnormal hemodynamics, chronic hypoxia, inflammation, cellular dysfunction, and activation of fibrogenic biochemical pathways. The end result is the replacement of normal structures with extracellular matrix. Treatments presently are focused on delaying or preventing such progression, and are largely nonspecific. In pediatrics, such therapy is complicated further by pathophysiological issues that render children a unique population.

Effect of Urtica dioica on morphometric indices of kidney in streptozotocin diabetic rats--a stereological study

The aim of the present study was to investigate the effect of Urtica dioica on Morphometric indices of kidney in diabetic rats. Thirty male adult albino wistar rats of 125-175 g divided into control, diabetic and Urtica dioica treatment groups. In treatment Group, diabetic rats received 100 mg kg(-1) daily hydroalcoholic extract of U. dioica intraperitoneally for 4 weeks. After the animals had been sacrified, the kidneys were removed and fixed by formaldehyde, cut horizontally into 1 mm slices and processed, Stained with H and E. Stereological study performed using light microscope and the image projected on a table of olysa software. Cavalieri principle was used to estimate the volume of cortex, medulla and whole kidney. All the grouped data statistically evaluated using Student's t-test, expressed as the Mean +/- SE. Ration of kidney weight/body weight in diabetes (0.51) and diabetes-extract group (0.67) were higher then control group (0.42). Ratio of kidney volume/body weight in diabetes (350) and diabetes-extract group (348) were higher then control group (323). Volume Ratio of cortex/medulla in diabetes-extract group (1.65) was higher then control (1.34) and diabetes group (1.33). Glomerular area and diameter and proximal tubule diameter in diabetes-Extract group was higher than control and diabetes groups. This study revealed that Urtica dioica has no effect on renal morphometric indices in induced diabetic rats.

Novel mechanisms of protein synthesis in diabetic nephropathy--role of mRNA translation

Ambient protein levels are affected by both synthesis and degradation. Synthesis of a protein is regulated by transcription and messenger RNA (mRNA) translation. Translation has emerged as an important site of regulation of protein expression during development and disease. It is under the control of distinct factors that regulate initiation, elongation and termination phases. Regulation of translation occurs via signaling reactions, guanosine diphosphate-guanosine triphosphate binding and by participation of non-coding RNA species such as microRNA. Recent work has revealed an important role for translation in hypertrophy, matrix protein synthesis, elaboration of growth factors in in vivo and in vitro models of diabetic nephropathy. Studies of translation dysregulation in diabetic nephropathy have enabled identification of novel therapeutic targets. Translation of mRNA is a fertile field for exploration in investigation of kidney disease.

Ornithine decarboxylase inhibitor eliminates hyperresponsiveness of the early diabetic proximal tubule to dietary salt

Heightened sensitivity of the diabetic proximal tubule to dietary salt leads to a paradoxical effect of salt on glomerular filtration rate (GFR) via tubuloglomerular feedback. Diabetic hyperfiltration is a feedback response to growth and hyperreabsorption by the proximal tubule. The present studies were performed to determine whether growth and hyperfunction of the proximal tubule are essential for its hyperresponsiveness to dietary salt and, hence, to the paradoxical effect of dietary salt on GFR. Micropuncture was performed in four groups of inactin-anesthetized Wistar rats after 10 days of streptozotocin diabetes drinking tap water or 1% NaCl. Kidney growth was suppressed with ornithine decarboxylase (ODC) inhibitor, DFMO (200 mg.kg(-1).day(-1)), or placebo. Single nephron GFR (SNGFR) was manipulated by perfusing Henle's loop so that proximal reabsorption (Jprox) could be expressed as a function of SNGFR in each nephron, dissociating primary effects on the tubule from the effects of glomerulotubular balance. Alone, DFMO or high salt reduced SNGFR and suppressed Jprox independent of SNGFR. Suppression of Jprox was eliminated and SNGFR increased when high salt was given to rats receiving DFMO. ODC is necessary for hyperresponsiveness of the proximal tubule to dietary salt and for the paradoxical effect of dietary salt on GFR in early diabetes. This coupling of effects adds to the body of evidence that feedback from the proximal tubule is the principal governor of glomerular filtration in early diabetes.

Sphingolipid signalling in the cardiovascular system: good, bad or both?

Sphingolipids are biologically active lipids that play important roles in various cellular processes and the sphingomyelin metabolites ceramide, sphingosine and sphingosine-1-phosphate can act as signalling molecules in most cell types. With the recent development of the immunosuppressant drug FTY720 (Fingolimod) which after phosphorylation in vivo acts as a sphingosine-1-phosphate receptor agonist, research on the role of sphingolipids in the immune and other organ systems was triggered enormously. Since it was reported that FTY720 induced a modest, but significant transient decrease in heart rate in animals and humans, the question was raised which pharmacological properties of drugs targeting sphingolipid signalling will affect cardiovascular function in vivo. The answer to this question will most likely also indicate what type of drug could be used to treat cardiovascular disease. The latter is becoming increasingly important because of the increasing population carrying characteristics of the metabolic syndrome. This syndrome is, amongst others, characterized by obesity, hypertension, atherosclerosis and diabetes. As such, individuals with this syndrome are at increased risk of heart disease. Now numerous studies have investigated sphingolipid effects in the cardiovascular system, can we speculate whether certain sphingolipids under specific conditions are good, bad or maybe both? In this review we will give a brief overview of the pathophysiological role of sphingolipids in cardiovascular disease. In addition, we will try to answer how drugs that target sphingolipid signalling will potentially influence cardiovascular function and whether these drugs would be useful to treat cardiovascular disease.

Effects of advanced glycation end product modification on proximal tubule epithelial cell processing of albumin

AIM

The goal of this work is to understand the cellular effects of advanced glycation end product (AGE)-modified protein on renal proximal tubule cells.

BACKGROUND

A major function of the proximal tubule is to reabsorb and process filtered proteins. Diabetes is characterized by increased quantities of tissue and circulating proteins modified by AGEs. Therefore in diabetes, plasma proteins filtered at the glomerulus and presented to the renal proximal tubule are likely to be highly modified by AGEs.

METHODS

The model system was electrically resistant polarized renal proximal tubular epithelial cells in monolayer culture. The model proteins comprise a well-characterized AGE, methylglyoxal-modified bovine serum albumin (MGO-BSA), and unmodified BSA.

RESULTS

Renal proximal tubular cells handle MGO-BSA and native BSA in markedly disparate ways, including differences in: (1) kinetics of binding, uptake, and intracellular accumulation, (2) processing and fragmentation, and (3) patterns of electrical conductance paralleling temporal changes in binding, uptake and processing.

CONCLUSION

These differences support the idea that abnormal protein processing by the renal tubule can be caused by abnormal proteins, thereby forging a conceptual link between the pathogenic role of AGEs and early changes in tubular function that can lead to hypertrophy and nephropathy in diabetes.

Growth pattern switch of renal cells and expression of cell cycle related proteins at the early stage of diabetic nephropathy

Renal hypertrophy, partly due to cell proliferation and hypertrophy, has been found correlated to renal function deterioration in diabetes mellitus. We screened the up-regulated cell cycle related genes to investigate cell growth and the expression of cell cycle regulating proteins at the early stage of diabetic nephropathy using STZ-induced diabetic rats. Cyclin E, CDK(2) and P(27) were found significantly up-regulated in diabetic kidney. Increased cell proliferation in the kidney was seen at day 3, peaked at day 5, and returned to normal level at day 30. Cyclin E and CDK(2) expression also peeked at day 5 and P(27) activity peaked at day 14. These findings indicate that a hyperplastic growth period of renal cells is followed by a hypertrophic growth period at the early stage of diabetes. The growth pattern switch may be regulated by cell cycle regulating proteins, Cyclin E, CDK(2), and P(27).

Rapamycin prevents early steps of the development of diabetic nephropathy in rats

BACKGROUND/AIMS

Recent studies suggested the involvement of the Akt/mammalian target of rapamycin (mTOR) pathway in the pathogenesis of diabetic nephropathy. The effect of mTOR blockade by rapamycin in diabetic nephropathy was investigated, but in vivo study of rapamycin treatment in the course of early diabetes is still insufficient. This study was designed to determine the therapeutic effects of rapamycin on diabetic nephropathy at an early stage.

METHODS

Diabetes was induced in Sprague-Dawley rats with streptozotocin, and rapamycin (1 mg/kg) was administered by daily gavage for 4 weeks. Renal structural changes and some factors involved in the early pathogenesis of diabetic nephropathy were tested. The activation level of the Akt/mTOR pathway was also determined.

RESULTS

Rapamycin treatment reduced albuminuria, glomerular enlargement, glomerular basement membrane thickening, renal macrophage recruitment, and levels of renal mRNA expression of proliferating cell nuclear antigen, transforming growth factor-beta1, vascular endothelial growth factor, and monocyte chemoattractant protein-1 without change in blood glucose level and blood pressure in experimental diabetic rats. In addition, treatment with rapamycin also down-regulated the enhanced levels of renal p-Akt, phospho-p70S6 kinase, and phospho-ribosomal S6 protein in diabetic rats.

CONCLUSIONS

Rapamycin treatment can prevent the early renal structural changes of diabetes in experimental rats, and thus halt the early steps of the development of diabetic nephropathy. mTOR blockade might be beneficial for the treatment of diabetic nephropathy.

Kidney growth, hypertrophy and the unifying mechanism of diabetic complications

Michael Brownlee has proposed a 'Unifying Mechanism' of hyperglycemia-induced damage in diabetes mellitus. At the crux of this hypothesis is the generation of reactive oxygen species (ROS), and their impact on glycolytic pathways. Diabetes is the leading cause of chronic kidney failure. In the early phase of diabetes, prior to establishment of proteinuria or fibrosis, comes kidney growth and hyperfiltration. This early growth phase consists of an early period of hyperplasia followed by hypertrophy. Hypertrophy also contributes to cellular oxidative stress, and may precede the ROS perturbation of glycolytic pathways described in the Brownlee proposal. This increase in growth promotes hyperfiltration, and along with the hypertrophic phenotype appears required for hyperglycemia-induced cell damage and the progression of downstream diabetic complications. Here we will evaluate this growth phenomenon in the context of diabetes mellitus.

Cumulative excretion of urinary podocytes reflects disease progression in IgA nephropathy and Schönlein-Henoch purpura nephritis

Recent studies have revealed that podocytopenia leads to glomerular scarring and that the loss of podocytes into the urine may be a cause of podocytopenia. The purpose of this study was to examine whether serial examinations of urinary podocytes (u-podo) could be a useful predictor of disease progression in children with glomerulonephritis. Urine samples and renal biopsy specimens from 20 patients (10 males and 10 females; mean age 11.8 yr; range 4 to 24 yr) with IgA nephropathy (n = 17) and Henoch-Schönlein purpura nephritis (n = 3) were analyzed. Forty-four renal biopsies were performed on 20 patients. Proteinuria (g/d per 1.73 m2), hematuria (score), and u-podo (cells/ml) were examined twice a month in 24 intervals between two biopsies (mean 16.7 mo; range 4 to 58 mo) and average and cumulative values were determined for the intervals. Renal histologic changes were scored on the basis of acute intracapillary, acute extracapillary, acute tubulointerstitial, chronic intracapillary, chronic extracapillary, and chronic tubulointerstitial lesions, as well as glomerulosclerosis. It was found that hematuria, proteinuria, u-podo, and acute lesion scores decreased during the intervals examined, whereas chronic lesion scores increased. Changes in acute histology scores correlated well with hematuria, proteinuria, and u-podo excretion, whereas chronic histology scores and glomerulosclerosis both correlated well with cumulative u-podo excretion. Patients with severe histologic progression of disease also had persistent u-podo excretion. These findings provide additional data to support a potential causative role for prolonged urinary loss of podocytes in disease progression in children with IgA nephropathy and Henoch-Schönlein purpura nephritis.

Primary kidney growth and its consequences at the onset of diabetes mellitus

Diabetes mellitus is a primary contributor to progressive kidney dysfunction leading to end-stage renal disease (ESRD). In the early phase of diabetes, prior to the onset of further complications, both kidney size and glomerular filtration rate (GFR) increase. Glomerular hyperfiltration is considered a risk factor for downstream complications and progression to ESRD. Abnormalities in vascular control have been purported to account for the glomerular hyperfiltration in early diabetes. In this review we discuss a tubulo-centric concept in which tubular growth and subsequent hyper-reabsorption contribute to the onset of glomerular hyperfiltration that demarks the early stage of diabetes. Kidney growth, in this concept, is no longer relegated to a compensatory response to hyperfiltration, but rather plays a primary and active role in its genesis and progression. As such, components of kidney growth, such as the polyamines, may provide a means of early detection of diabetic kidney dysfunction and more effective therapeutic intervention.

Upregulation of nestin, vimentin, and desmin in rat podocytes in response to injury

Podocytes in the renal glomerulus express unusual intermediate filament (IF) proteins for epithelial cells. To gain insight into the role of IF proteins in podocytes, we investigated the expression of nestin, vimentin, and desmin in puromycin aminonucleoside (PAN) nephrosis. A Western blot analysis for nestin, vimentin, and desmin demonstrated their exclusive expression in glomeruli and showed their increase in expression in nephrotic glomeruli. Immunolocalization studies showed nestin and vimentin to be located predominantly in the podocytes in both normal and nephrotic glomeruli and that enhancement of desmin staining only occurred in podocytes. A ribonuclease protection assay showed high levels of vimentin and nestin expression in normal glomeruli and an upregulation of all three IF transcripts in nephrotic glomeruli. One day after the PAN injection, however, the vimentin transcripts were found to already have significantly increased, whereas those of nestin or desmin showed no such increase. These findings indicate that podocytes express three IF proteins, namely, vimentin, desmin, and nestin, which are differentially regulated in response to injury. An upregulation of IF proteins may increase the mechanical stability of cells, thus enabling podocytes to undergo morphological changes on the tensile glomerular capillary wall.

Processing Advanced Glycation End Product-Modified Albumin by the Renal Proximal Tubule and the Early Pathogenesis of Diabetic Nephropathy

Diabetes is characterized by increased quantities of circulating proteins modified by advanced glycation end products (AGEs). Proteins filtered at the glomerulus and presented to the renal proximal tubule are likely to be highly modified by AGEs. The proximal tubule binds, takes up, and catabolizes AGE-modified albumin by pathways different from those of unmodified albumin. These differences were examined in polarized, electrically resistant proximal tubular cells grown in monolayer culture. In patients with type 1 diabetes, urinary excretion of a lysosomal enzyme predicted the development of nephropathy.

Glomerular structural changes in pregnant, diabetic, and pregnant-diabetic rats

Kidneys enlarge both during pregnancy and in diabetes. The enlargement and morphology of glomeruli was studied during pregnancy and in diabetes in order to examine possible similarities, differences, and interactions in the growth in these conditions. Morphometric investigations were performed on glomeruli in pregnant rats, in rats with 2 weeks' diabetes, and in pregnant-diabetic rats. Kidneys were enlarged 22% in the midterm pregnant rats compared with controls, 74% in diabetic rats, and a further 21% in pregnant-diabetic rats. Glomerular volume was enlarged by 26% during midterm pregnancy in normal animals. Diabetes induced an enlargement in glomerular volume of 58% and a further 18% in midterm diabetic animals due to pregnancy. Within the glomerulus, pregnancy in normal animals induced minor non-significant changes. Diabetes induced significant increase in several parameters: mesangial volume and cell volume, capillary and glomerular basement membrane volume, capillary wall surface area, foot process width, filtration slit length, and nuclear number. Pregnancy in diabetic animals induced no significant additional changes. In conclusion, kidney enlargement in pregnancy shows very few glomerular changes in either normal or diabetic animals. Enlargement of glomeruli in diabetes involves hypertrophy and hyperplasia concurrent with several morphological changes within the glomerulus.

Epidermal growth factor receptor inhibition attenuates early kidney enlargement in experimental diabetes

BACKGROUND

Renal enlargement is an early feature of both human and experimental diabetes. Although the precise mechanisms underlying its development are incompletely understood, locally active growth factors have been suggested to have a key role. Having previously documented increased expression of the proproliferative and antiapoptotic growth factor, epidermal growth factor (EGF), in early diabetes-related kidney growth, the present study sought to evaluate its pathogenetic role by blocking its action with a specific inhibitor.

METHODS

Sprague-Dawley rats were randomized to receive streptozotocin (diabetic) or buffer (control) and then further randomized to receive either vehicle or the inhibitor of the EGF receptor tyrosine kinase, PKI 166 (100 mg/kg/day) for 2 days and 3 weeks following streptozotocin administration.

RESULTS

Experimental diabetes was associated with an increase in kidney weight and tubular epithelial cell proliferation as identified by increased expression of proliferating cell nuclear antigen (PCNA) and 5-bromo-2'-deoxyuridine (BrdU) incorporation. PKI 166 resulted in a 30% reduction in kidney weight in diabetic rats (P < 0.01) and reduced tubular epithelial cell proliferation (P < 0.01). In addition, EGF receptor inhibition also led to a 40% increase in tubular epithelial cell apoptosis at 3 weeks (P < 0.01). Diabetes-associated glomerular enlargement was similarly attenuated by PKI 166, although glomerular hyperfiltration was unaffected.

CONCLUSION

These findings suggest that the EGF-EGF receptor (EGFR) axis has a significant role in the development of early diabetes-related kidney growth. The impact of EGFR inhibition on the later development of renal dysfunction, however, remains to be determined.

Glomerular proliferation during early stages of diabetic nephropathy is associated with local increase of sphingosine-1-phosphate levels

In this study, the effects of short-term diabetes (4 days) on rat renal glomerular cells proliferation and the potential involvement of sphingolipids in this process were investigated. Immunohistochemical analysis showed that streptozotocin (STZ)-induced diabetes promoted increased intra-glomerular hyperplasia, particularly marked for mesangial cells. This was associated with a concomitant increase in neutral ceramidase and sphingosine-kinase activities and the accumulation of the pro-proliferative sphingolipid sphingosine-1-phosphate, in glomeruli isolated from kidney cortex of STZ-treated rats. These results suggest a possible involvement of sphingolipid metabolites in the glomerular proliferative response during the early stages of diabetic nephropathy.

Apical cell membranes are shed into urine from injured podocytes: a novel phenomenon of podocyte injury

Previously it was shown that urine from patients with nephritis contains podocytes and their fragments (podocalyxin [PCX]-positive granular structures [PPGS]), reflecting the degree of podocyte injury. The present study was designed to trace PPGS to their origin. Urine samples and renal biopsy specimens from 53 children with nephrotic syndrome and nephritis were examined immunohistochemically. Immunofluorescence studies of kidney sections using an anti-PCX antibody demonstrated that PPGS originated from the glomerulus and flowed into the tubular lumen. Electron microscopic examination revealed that PPGS originated from microvillous or vesicle-like structures on injured podocytes in the glomerulus. For examining the origin of the PPGS, apical, slit-diaphragmatic, and basal portions of the podocytes were specifically stained, revealing that PPGS are composed primarily of apical podocyte membranes. Several newly developed antibodies that are reactive with various segments of the PCX molecule were used to analyze more detailed membrane structures, and it was found that PPGS contained intact PCX molecules, indicating that cell membrane structures are excreted in urine. The quantification of PCX content and podocyte numbers revealed that urinary sediment PCX (u-sed-PCX) content per urinary podocyte was much higher than PCX content per podocyte from isolated glomeruli of normal controls, suggesting that u-sed-PCX are derived from sources other than just the cell debris of detached podocytes. Analysis of the correlation between u-sed-PCX and renal histology revealed that the presence of PPGS reflects acute glomerular injury. In conclusion, podocyte apical cell membranes are shed into the urine from injured podocytes, indicating a previously unrecognized manifestation of podocyte injury.

The physiological and pathophysiological roles of the GH/IGF-axis in the kidney: lessons from experimental rodent models

The growth hormone (GH)/insulin-like growth factor (IGF) system plays an important role in renal development, growth, function and pathophysiology. IGF-I has been associated with renal/glomerular hypertrophy and compensatory renal growth. Potential effects on glomerular size are of interest, since an increase in glomerular size may be permissive for the development of glomerulosclerosis. In an effort to abolish the decline of renal function and possibly to restore the renal structure, different approaches have been tested in experimental models of nephropathy, focusing mainly on early renal changes. The involvement of the GH/IGF system in renal pathophysiology has been studied in much detail in the rat. In view of the growing interest in murine physiology, occurring in large part by genetically modified animals, this review examines those aspects of GH, IGFs, their receptors and binding proteins that relate both to mouse kidney physiology and to a number of conditions characterized by pathophysiological renal changes. A deeper understanding of the role of the GH/IGF system in renal dysfunction may stimulate the development of novel therapeutic approaches aiming at preventing or retarding various kidney diseases.

Bimodal Effect of Advanced Glycation End Products on Mesangial Cell Proliferation Is Mediated by Neutral Ceramidase Regulation and Endogenous Sphingolipids

Advanced glycation end-products (AGE) are generated by chronic hyperglycaemia and may cause diabetic microvascular complications such as diabetic nephropathy. Many factors influence the development of diabetic nephropathy; however, dysregulation of mesangial cell (MC) proliferation appears to play an early and crucial role. In this study, we investigated the effects of AGE on rat MC proliferation and the involvement of sphingolipids in the AGE response. Results show a bimodal effect of AGE on MC proliferation. Thus, low AGE concentrations (<1 microm) induced a significant increase (+26%) of MC proliferation, whereas higher concentrations (10 microm) markedly reduced it (-24%). In parallel, AGE exerted biphasic effects on neutral ceramidase expression and activity. Low AGE concentrations increased neutral ceramidase activity and expression, whereas high AGE concentrations showed opposite effects. Surprisingly, neutral ceramidase modulation did not result in changes of ceramide levels. However, the AGE (10 microm)-inhibitory effect on MC proliferation was associated with accumulation of sphingosine and was specifically prevented by blocking glucosylceramide synthesis, suggesting that the high AGE concentration effects are mediated by sphingosine and/or glycolipids. On the other hand, treatment of cells with low AGE concentrations led to an increase of sphingosine kinase activity and sphingosine-1-phosphate production that drove the increase of MC proliferation. Interestingly, in glomeruli isolated from streptozotocin-diabetic rats, a time-dependent modulation of ceramidase activity was observed as compared with controls. These results suggest that AGE regulate MC growth by modulating neutral ceramidase and endogenous sphingolipids.

Regulation of type II transforming-growth-factor-beta receptors by protein kinase C iota

TGF-beta (transforming growth factor-beta) is implicated in the pathogenesis of diabetic nephropathy. We previously demonstrated that up-regulation of type II TGF-beta receptor (TbetaRII) induced by high glucose might contribute to distal tubular hypertrophy [Yang, Guh, Yang, Lai, Tsai, Hung, Chang and Chuang (1998) J. Am. Soc. Nephrol. 9, 182-193]. We have elucidated the mechanism by using cultured Madin-Darby canine kidney cells. Enhancer assay and electrophoretic-mobility-shift assay were used to estimate the involvement of transcription factors. Western blotting and an in vitro kinase assay were used to evaluate the level and activity of protein kinase. We showed that glucose (100-900 mg/dl) induced an increase in mRNA level and promoter activity of TbetaRII (note: 'mg/dl' are the units commonly used in diabetes studies). The promoter region -209 to -177 appeared to contribute to positive transactivation of TbetaRII promoter by comparing five TbetaRII-promoter-CAT (chloramphenicol acetyl-transferase) plasmids. Moreover, the transcription factor AP-1 (activator protein 1) was significantly activated and specifically binds to TbetaRII promoter (-209 to -177). More importantly, we found that atypical PKC iota might be pivotal for high glucose-induced increase in both AP-1 binding and TbetaRII promoter activity. First, high glucose induced cytosolic translocation, activation and autophosphorylation of PKC iota. Secondly, antisense PKC iota expression plasmids attenuated high-glucose-induced increase in AP-1 binding and TbetaRII promoter activity; moreover, sense PKC iota expression plasmids enhanced these instead. Finally, we showed that antisense PKC iota expression plasmids might partly attenuate a high-glucose/TGF-beta1-induced increase in fibronectin. We conclude that PKC iota might mediate high-glucose-induced increase in TbetaRII promoter activity. In addition, antisense PKC iota expression plasmid effectively suppressed up-regulation of TbetaRII and fibronectin in hyperglycaemic distal-tubule cells.