Injury to the endothelial surface layer induces glomerular hyperfiltration rats with early-stage diabetes

Glomerular endothelial surface layer (ESL) may play a role in the mechanisms of albuminuria in diabetic nephropathy, which lack evidence in vivo. The effects of high glucose on the passage of albumin across the glomerular ESL were analysed in streptozotocin-induced diabetic Sprague-Dawley rats for 4 weeks. Albuminuria and glomerular mesangial matrix were significantly increased in diabetic rats. The passage of albumin across the ESL, as measured by albumin-colloid gold particle density in the glomerular basement membrane (GBM), was increased significantly in diabetic rats. The thickness of the glomerular ESL, examined indirectly by infusing Intralipid into vessels using an electron microscope, was significantly decreased and the GBM exhibited little change in diabetic rats. In summary, the glomerular ESL may play a role in the pathogenesis of albuminuria in rats with early-stage diabetes.

Temporal profile of diabetic nephropathy pathologic changes

Diabetic nephropathy, by far, is the most common cause of end stage renal disease in the US and many other countries. In type 1 diabetes, the natural history of diabetic nephropathy is tightly linked to evolution of classic lesions of the disease, namely glomerular basement membrane thickening, increased mesangial matrix, and reduced glomerular filtration surface density. These lesions progress in parallel and correlate with increased albumin excretion rate and reduced glomerular filtration rate across a wide range of renal function. In fact, the vast majority of the variances of albumin excretion and glomerular filtration rates can be explained by these glomerular lesions alone in type 1 diabetic patients. Although, classic lesions of diabetic nephropathy, indistinguishable from those of type 1 diabetes, also occur in type 2 diabetes, renal lesions are more heterogeneous in type 2 diabetic patients with some patients developing more advanced vascular or chronic tubulointerstitial lesions than diabetic glomerulopathy. More research biopsy longitudinal studies, especially in type 2 diabetic patients, are needed to better understand various pathways of renal injury in diabetic nephropathy.

Podocyte-specific overexpression of metallothionein mitigates diabetic complications in the glomerular filtration barrier and glomerular histoarchitecture: a transmission electron microscopy stereometric analysis

BACKGROUND

We previously demonstrated that cellular and extracellular components of the blood-urine barrier in renal glomeruli are susceptible to damage in OVE transgenic mice, a valuable model of human diabetic nephropathy that expresses profound albuminuria.

METHODS

To test our hypothesis that glomerular filtration barrier damage in OVE mice may be the result of oxidative insult to podocytes, 150-day-old bi-transgenic OVENmt diabetic mice that overexpress the antioxidant metallothionein specifically in podocytes were examined by enzyme-linked immunosorbent assay for albuminuria mitigation and by unbiased transmission electron microscopy (TEM) stereometry for protection from chronic structural diabetic complications.

RESULTS

Although blood glucose and HbA(1c) levels were indistinguishable in OVE and OVENmt animals, albuminuria was significantly reduced (average >7-fold) in OVENmt mice through 8 months of age. Interestingly, the Nmt transgene provided significant glomerular protection against diabetic nephropathic complications outside of the podocyte. Glomerular filtration barrier damage was reduced in OVENmt mice, including significantly increased area occupied by endothelial luminal fenestrations (~13%), significantly reduced glomerular basement membrane (GBM) thickening (~17%) and significantly less podocyte effacement (~18%). In addition, OVENmt mice exhibited significantly reduced glomerular volume (~50%), fewer glomerular endothelial cells (~33%), fewer mesangial cells (~57%) and fewer total glomerular cells (~40%).

CONCLUSIONS

These results provide evidence of oxidative damage to podocytes induces primary diabetic nephropathic features including severe and sustained albuminuria, specific glomerular filtration barrier damage and alterations in glomerular endothelial and mesangial cell number. Importantly, these diabetic complications are significantly mitigated by podocyte targeted metallothionein overexpression.

Mechanisms involved in the genesis of diabetic nephropathy

In recent years, the amount of people suffering from diabetes has increased notably. This increase has led to an augmentation in secondary diseases; amongst them, diabetic nephropathy (DN) is one of the most relevant pathologies. DN provokes a decrease in glomerular filtration rate, which ends up in chronic renal failure. In the developed countries, DN is the first cause of chronic renal failure needing replacement therapy. Thus, this disease is responsible for a remarkable part of the public health costs in the Western countries. All of these facts have encouraged many scientists to perform their research on mechanism causing DN, improving our knowledge of its pathogenesis. In this review we summarize the most relevant facts on the physiopathology of DN.

Alport syndrome and thin basement membrane nephropathy

Both Alport syndrome and thin basement membrane nephropathy (TBMN) can be considered as genetic diseases of the GBM involving the alpha3/alpha4/alpha5 network of type IV collagen. Mutations in any of the COL4A3, COL4A4 or COL4A5 genes can lead to total or partial loss of this network. Males with mutations in the X-linked COL4A5 gene develop Alport syndrome with progressive renal disease and sometimes extra-renal disease. Females who are heterozygous for a COL4A5 mutation are considered to be carriers for X-linked Alport syndrome. Although their clinical course and GBM ultrastructural changes can sometimes mimic TBMN, more often it tends to be more progressive than usually seen in TBMN. Males or females who are heterozygous for COL4A3 or COL4A4 mutations usually manifest as TBMN, with nonprogressive hematuria, while those who are homozygous or combined heterozygotes develop autosomal-recessive Alport syndrome. Thus, individuals with TBMN can be considered to be carriers for autosomal-recessive Alport syndrome, but there remain some exceptions in which patients heterozygous for COL4A3 or COL4A4 mutations develop autosomal-dominant Alport syndrome. Distinguishing between all these groups of patients requires a combination of family history and a renal biopsy for electron microscopic examination of the GBM and immunohistochemical staining of the GBM for the alpha3, alpha4 and alpha5 chains of type IV collagen. Mutational analysis of the COL4A3, COL4A4, and COL4A5 genes, whenever it becomes available, will be a valuable adjunct to the diagnostic workup in these patients. Novel therapeutic approaches may one day provide a treatment or cure for these patients, avoiding the need for transplantation and dialysis.

Altered expression of matrix-related molecules in the development of chronic Thy1.1 nephritis

BACKGROUND/AIM

Matrix production and degradation are critically important in chronic nephritis. Our aim was to investigate the precise expression of matrix-related molecules which is essential for understanding the pathogenesis of renal disease.

METHODS

Chronic nephritis was induced by a single injection of anti-Thy1.1 antibody to unilaterally nephrectomized rats. RNA was extracted from renal cortex and isolated glomeruli 4, 7, and 10 weeks after the antibody injection. Matrix-related gene expressions were measured by polymerase chain reaction. The expression of alpha1(IV) and alpha3(IV) collagens was studied by immunohistochemistry. The gelatinolytic activity in the glomeruli was assayed by gelatin zymography.

RESULTS

Polymerase chain reaction revealed an increase of alpha1(IV) in both glomeruli and renal cortex from nephritic rats. In contrast, the expression of alpha3(IV), normally a component of the glomerular basement membrane, was decreased in nephritic animals. Immunohistochemistry confirmed the finding that alpha1(IV) and alpha3(IV) were up- and downregulated, respectively, in the glomeruli. Gene expression and activity of matrix metalloproteinase 2 were enhanced, while those of matrix metalloproteinase 9 were clearly suppressed in nephritis.

CONCLUSIONS

Downregulation of alpha3(IV) and enhancement of the matrix metalloproteinase-2 activity in the glomeruli may contribute to the glomerular damage by altering the glomerular basement membrane components. Impairment of the glomerular basement membrane integrity may possibly be implicated in irreversible renal dysfunction.

The podocyte's response to injury: role in proteinuria and glomerulosclerosis

The terminally differentiated podocyte, also called glomerular visceral epithelial cell, are highly specialized cells. They function as a critical size and charge barrier to prevent proteinuria. Podocytes are injured in diabetic and non-diabetic renal diseases. The clinical signature of podocyte injury is proteinuria, with or without loss of renal function owing to glomerulosclerosis. There is an exciting and expanding literature showing that hereditary, congenital, or acquired abnormalities in the molecular anatomy of podocytes leads to proteinuria, and at times, glomerulosclerosis. The change in podocyte shape, called effacement, is not simply a passive process following injury, but is owing to a complex interplay of proteins that comprise the molecular anatomy of the different protein domains of podocytes. These will be discussed in this review. Recent studies have also highlighted that a reduction in podocyte number directly causes proteinuria and glomerulosclerosis. This is owing to several factors, including the relative inability for these cells to proliferate, detachment, and apoptosis. The mechanisms of these events are being elucidated, and are discussed in this review. It is the hope that by delineating the events following injury to podocytes, therapies might be developed to reduce the burden of proteinuric renal diseases.