Renal structural changes in insulin-dependent diabetic patients with albuminuria. Comparison of cases with onset of albuminuria after short or long duration

ID1 inhibits USF2 and blocks TGF-β-induced apoptosis in mesangial cells

Mesangial cells (MC) play an essential role in normal function of the glomerulus. Phenotypic changes in MC lead to the development of glomerular diseases such as diabetic nephropathy and glomerulosclerosis. The late phase of diabetic glomerulopathy is characterized by MC death and fibrosis. Current data highlight the transforming growth factor (TGF)-β as a trigger of the pathological changes observed in MC, including death by apoptosis. However, the mechanisms and mediators involved in this process are still poorly understood. Identification of novel elements involved in MC death may provide a better understanding of the pathophysiology of glomerular diseases. Here, we show that bone morphogenetic proteins (BMPs; known antagonists of the profibrotic effects of TGF-β in the kidney) strongly induce inhibitor of DNA binding (ID1) mRNA transcription and protein expression in human MC. ID genes have been implicated in cell survival control and are constitutively expressed in MC. We show that BMPs and ID1 exert an anti-apoptotic effect in MC by inhibition of USF2 transcriptional activity. On the other hand, TGF-β upregulates USF2, increasing BAX (proapoptotic gene) levels and apoptosis rates. Taken together, our results point to a novel molecular pathway that modulates MC apoptosis, which is potentially involved in the pathogenesis of glomerular diseases.

Glucose enhances mesangial cell apoptosis

Mesangial cell apoptosis occurs in experimental diabetic nephropathy, and this correlates with worsening albuminuria. This study examines the mechanism by which glucose modulates mesangial cell apoptosis. Apoptosis was induced in mesangial cells by serum deprivation in the presence of 5 or 25 mM D-glucose, and examined by expression of Annexin-V and disruption of mitochondrial transmembrane potential. Involvement of Bax, Bcl-2 and NF-kappaB were examined by RT-PCR and EMSA. Involvement of TGF-beta1 was sought by determining the effect of recombinant TGF-beta1on apoptosis and the mediators of the apoptotic pathway (Bcl2/Bax and NF-kappaB). Culture of cells in the presence of 25 mM D-glucose (i) enhanced apoptosis stimulated by serum depletion, (ii) enhanced activation of caspase-3, (iii) inhibited NF-kappaB activation, and (iv) decreased Bcl-2:Bax ratio. Inhibition of NF-kappaB using SN50, also increased mesangial cell apoptosis, and decreased Bcl-2:Bax ratio. Addition of TGF-beta1 to mesangial cells mimicked the effect of high glucose reducing NF-kappaB expression and Bcl-2:Bax ratio. Furthermore glucose-mediated enhanced apoptosis was inhibited by the addition of a blocking antibody to TGF-beta1. Exposure of mesangial cells to 25 mM D-glucose stimulated the generation of both total and active TGF-beta1 in the cell culture supernatant, this increase was only significant after 48-72 h, that is at a time point later than enhanced apoptosis. Addition of 25 mM D-glucose, however, increased sensitivity of mesangial cells to TGF-beta1 as assessed by luciferase activity of a Smad sensitive reporter construct. The data suggest that elevated glucose concentration enhanced the pathway leading to apoptosis following serum deprivation. Furthermore, it is likely that this is dependent on glucose-mediated enhanced sensitivity to endogenous TGF-beta1 rather than glucose stimulated de novo TGF-beta1 synthesis.

Ultrastructural morphometry of capillary basement membrane thickness in normal and transgenic diabetic mice

Capillary basement membrane (CBM) thickening is an ultrastructural hallmark in diabetic patients and in animal models of diabetes. However, the wide variety of tissues sampled and diverse methods employed have made the interpretation of thickness data difficult. We showed previously that acellular glomerular BMs in OVE26 transgenic diabetic mice were thickened beyond normal age-related thickening, and in the current study we hypothesized that other microvascular BMs likewise would show increased widths relative to age-matched controls. Accordingly, a series of tissues, including skeletal and cardiac muscle, ocular retina and choriod, peripheral nerve, lung, pancreas, and renal glomerulus was collected from 300-350-day-old normal and transgenic mice. Transmission electron micrographs of cross sections through capillary walls were prepared, and CBM thickness (CBMT) was determined by the "orthogonal intercept" method. Morphometric analyses showed highly variable transgene-related BMT increases in the sampled tissues, with glomerular BM showing by far the greatest increase (+87%). Significant thickness increases were also seen in the retina, pulmonary alveolus, and thoracoabdominal diaphragm. BMT increases were not universal; however, most were modestly widened, and those that were thickest in controls generally showed the greatest increase. Although the pathogenesis of diabetes-related increases in CBM is poorly understood, data in the current study showed that in OVE26 transgenic mice increased BMT was a frequent concomitant of hyperglycemia. Accordingly, it seems likely that hyperglycemia-induced microvascular damage may be a contributing factor in diabetic BM disease, and that microvessel cellular and extracellular heterogeneity may limit the extent of CBM thickening in diverse tissues.

High glucose promotes mesangial cell apoptosis by oxidant-dependent mechanism

Reactive oxygen species are recognized as important mediators of biological responses. Hyperglycemia promotes the intracellular generation of superoxide anion and hydrogen peroxide. In several cell lines, oxidant stress has been linked to the activation of death programs. Here, we report for the first time that high ambient glucose concentration induces apoptosis in murine and human mesangial cells by an oxidant-dependent mechanism. The signaling cascade activated by glucose-induced oxidant stress included the heterodimeric redox-sensitive transcription factor NF-kappaB, which exhibited an upregulation in p65/c-Rel binding activity and suppressed binding activity of the p50 dimer. Recruitment of NF-kappaB and mesangial cell apoptosis were both inhibited by antioxidants, implicating oxidant-induced activation of NF-kappaB in the transmission of the death signal. The genetic program for glucose-induced mesangial cell apoptosis was characterized by an upregulation of the Bax/Bcl-2 ratio. In addition, phosphorylation of the proapoptotic protein Bad was attenuated in mesangial cells maintained at high-glucose concentration, favoring progression of the apoptotic process. These perturbations in the expression and phosphorylation of the Bcl-2 family were coupled with the release of cytochrome c from mitochondria and caspase activation. Our findings indicate that in mesangial cells exposed to high ambient glucose concentration, oxidant stress is a proximate event in the activation of the death program, which culminates in mitochondrial dysfunction and caspase-3 activation, as the terminal event.

The early natural history of nephropathy in type 1 diabetes: II. Early renal structural changes in type 1 diabetes

Renal structural abnormalities are known to precede the development of proteinuria, hypertension, and reduced renal function in patients with type 1 diabetes. The determinants of these early structural abnormalities are, however, largely unknown. The International Diabetic Nephropathy Study (IDNS) has recruited 243 children and adults (aged 10-40 years) in Montreal, Minneapolis, and Paris to identify and quantify these determinants. All study subjects were normotensive and had normal-to-high glomerular filtration rates (GFRs) and urinary albumin excretion rates (AERs) <100 microg/min at study entry. Only 8 of 243 had an AER > or =20 microg/min (microalbuminuria). Two renal biopsies are obtained at a 5-year intervals, with baseline and follow-up measures of renal function, blood pressure (BP), HbA(1c), plasma lipids, and AER. Herein, we examine the baseline renal biopsy morphometric findings in these subjects and in 87 kidney donor control subjects and explore the associations between findings and clinical and demographic variables. The principal morphometric abnormalities were increased glomerular basement membrane (GBM) width and fractional volume of mesangium [Vv(Mes/glom)] and mesangial matrix [Vv(MM/glom)]. The frequency of these abnormalities increased with increasing duration of diabetes but was observed as early as 2-8 years after onset. Diastolic BP (DBP), but not HbA(1c), was directly associated with these abnormalities. Elevated GFR was associated with only a small increase in peripheral glomerular capillary basement membrane filtration surface density. Center differences were detected in renal structural, renal functional, and BP parameters, especially between the Paris and North American centers. GBM width, Vv(Mes/glom), and Vv(MM/glom) are significantly increased even within a few years of onset of type 1 diabetes. These changes are detectable in normoalbuminuric patients and are related to duration, BP, and study site. Changes in these and other morphometric measures over 5-year follow-up should help clarify the roles of glycemia and other determinants of the rates of development of diabetic nephropathy lesions, as well as their relationships to early changes in BP, albumin excretion, and renal function.

Glomerular cell number in normal subjects and in type 1 diabetic patients

BACKGROUND

The number of cells in glomeruli has been a challenging measure, especially in human kidneys, with only a small amount of tissue obtained by biopsy. However, the number of cells and their function are important determinants of renal function in health and disease.

METHODS

Modern morphometric techniques have now provided the means to determine the numerical density (Nv) and number (with a measure of glomerular volume) of endothelial cells, mesangial cells, and podocytes in plastic-embedded renal tissue biopsied from nondiabetic subjects (N = 36) and type 1 diabetic patients (N = 46) over an extended age range from childhood through late adult.

RESULTS

Nv values for all glomerular cells varied only slightly with age and did not change within the range of glomerular lesions of diabetes studied. Thus, the increase in glomerular volume during childhood to a steady level thereafter was the primary determinant of total glomerular cell number. The number of mesangial cells and endothelial cells increased with age, reflecting the increase in all cells, while the podocytes remained unchanged in number over all ages studied (10 to 69 years). Numbers of total glomerular cells, mesangial cells, and endothelial cells were not changed with diabetes, while podocytes were fewer in number in diabetic patients of all ages, with reduced podocyte numbers even in diabetes of short duration.

CONCLUSIONS

The essentially constant glomerular cell density in nondiabetic and diabetic subjects under different circumstances possibly indicates an underlying propensity for the glomerulus to regulate its architecture to maintain a constant number of cells per volume, no matter the size of the glomerulus or the severity of diabetic nephropathy studied in this set of patients. The reductions in podocyte numbers in both younger and older diabetic patients indicate a significant risk for functional abnormalities as diabetic nephropathy progresses. Moreover, these observations do not support the suggestion of marked increases in glomerular cell number (and especially mesangial cells) with the development and progression of diabetic nephropathy.