Divergent roles of Smad3 and PI3-kinase in murine adriamycin nephropathy indicate distinct mechanisms of proteinuria and fibrogenesis

Multiple transforming growth factor (TGF)-β-induced fibrogenic signals have been described in vitro. To evaluate mechanisms in vivo, we used an adriamycin nephropathy model in 129x1/Svj mice that display massive proteinuria by day 5 to7 and pathological findings similar to human focal segmental glomerulosclerosis by day 14. TGF-β mRNA expression increased after day 7 along with nuclear translocation of the TGF-β receptor-specific transcription factor Smad3. Inhibiting TGF-β prevented both pathological changes and type-I collagen and fibronectin mRNA expression, but proteinuria persisted. Renal Akt was phosphorylated in adriamycin-treated mice, suggesting PI3-kinase activation. Expression of mRNA for the p110γ isozyme of PI3-kinase was specifically increased and p110γ colocalized with nephrin by immunohistochemistry early in disease. Nephrin levels subsequently decreased. Inhibition of p110γ by AS605240 preserved nephrin expression and prevented proteinuria. In cultured podocytes, adriamycin stimulated p110γ expression. AS605240, but not a TGF-β receptor kinase inhibitor, prevented adriamycin-induced cytoskeletal disorganization and apoptosis, supporting a role for p110γ in podocyte injury. AS605240, at a dose that decreased proteinuria, prevented renal collagen mRNA expression in vivo but did not affect TGF-β-stimulated collagen induction in vitro. Thus, PI3-kinase p110γ mediates initial podocyte injury and proteinuria, both of which precede TGF-β-mediated glomerular scarring

Tubulointerstitial injury and the progression of chronic kidney disease

In chronic kidney disease (CKD), once injury from any number of disease processes reaches a threshold, there follows an apparently irreversible course toward decline in kidney function. The tubulointerstitium may play a key role in this common progression pathway. Direct injury, high metabolic demands, or stimuli from various other forms of renal dysfunction activate tubular cells. These, in turn, interact with interstitial tissue elements and inflammatory cells, causing further pathologic changes in the renal parenchyma. The tissue response to these changes thus generates a feed-forward loop of kidney injury and progressive loss of function. This article reviews the mechanisms of this negative cycle mediating CKD

Advances in the biology and genetics of the podocytopathies: implications for diagnosis and therapy

Etiologic factors and pathways leading to altered podocyte phenotype are clearly numerous and involve the activity of different cellular function. To focus on recent discoveries in podocyte biology and genetics and their relevance to these human glomerular diseases, named podocytopathies. Genetic mutations in genes encoding for proteins in the nucleus, slit diaphragm, podocyte cytoplasm, and cell membrane are responsible for podocyte phenotype and functional abnormalities. Podocyte injury may also derive from secondary stimuli, such as mechanical stress, infections, or use of certain medications. Podocytes can respond to injury in a limited number of ways, which include (1) effacement, (2) apoptosis, (3) arrest of development, and (4) dedifferentiation. Each of these pathways results in a specific glomerular morphology: minimal change nephropathy, focal segmental glomerulosclerosis, diffuse mesangial sclerosis, and collapsing glomerulopathy. Based on current knowledge of podocyte biology, we organized etiologic factors and morphologic features in a taxonomy of podocytopathies, which provides a novel approach to the classification of these diseases. Current and experimental therapeutic approaches are also discussed

A proposed taxonomy for the podocytopathies: a reassessment of the primary nephrotic diseases

A spectrum of proteinuric glomerular diseases results from podocyte abnormalities. The understanding of these podocytopathies has greatly expanded in recent years, particularly with the discovery of more than a dozen genetic mutations that are associated with loss of podocyte functional integrity. It is apparent that classification of the podocytopathies on the basis of morphology alone is inadequate to capture fully the complexity of these disorders. Herein is proposed a taxonomy for the podocytopathies that classifies along two dimensions: Histopathology, including podocyte phenotype and glomerular morphology (minimal-change nephropathy, focal segmental glomerulosclerosis, diffuse mesangial sclerosis, and collapsing glomerulopathy), and etiology (idiopathic, genetic, and reactive forms). A more complete understanding of the similarities and differences among podocyte diseases will help the renal pathologist and the nephrologist communicate more effectively about the diagnosis; this in turn will help the nephrologist provide more accurate prognostic information and select the optimal therapy for these often problematic diseases. It is proposed that final diagnosis of the podocytopathies should result from close collaboration between renal pathologists and nephrologists and should whenever possible include three elements: Morphologic entity, etiologic form, and specific pathogenic mechanism or association

ECM degradation by cultured human mesangial cells is mediated by a PA/plasmin/MMP-2 cascade

ECM degradation by cultured human mesangial cells is mediated by a PA/plasmin/MMP-2 cascade. We examined the role of the plasminogen activator/plasmin system in extracellular matrix (ECM) degradation by human mesangial cells cultured on thin films of 125I-labeled ECM (Matrigel). ECM degradation (release of 125I into the medium) was dependent on exogenous plasminogen, proportional to the number of mesangial cells and amount of plasminogen added, and coincident witji the appearance of plasmin in the medium. ECM degradation was completely blocked (P < 0.001) by two plasmin inhibitors, α-2-antiplasmin (40 µg/ml) and aprotinin (216 KIU/ml), and partially reduced (-33 ± 1.8%, P < 0.01) by TIMP-1 (40 µg/ml), a specific inhibitor of matrix metallo-proteinases. Zymography of medium obtained from cells cultured in the absence of plasminogen revealed the presence of latent matrix metallo-proteinase-2 (MMP-2) which was converted to a lower molecular weight, active form in the presence of mesangial cells and plasminogen. Northern analysis of poly A + RNA prepared from cultured human mesangial cells revealed mRNA for tissue-type plasminogen activator (tPA), urokinase-type plasminogen activator (uPA), plasminogen activator inhibitor-1 (PAI-1), and uPA receptor (uPAR). The presence of uPA protein in medium obtained from cultured human mesangial cells was demonstrated by Western blotting and ELISA which revealed a large molar excess of PAI-1 (1.2 ± 0.1 × 10-9M) over uPA (1.2 ± 0.1 × 10-12M) and tPA (0.19 ± 0.04 × 10 -9M). ECM degradation was reduced by a monoclonal antibody (MAb) against human tPA (-54 ± 8.6%) or human uPA (-39 ± 5.2%) compared to cells treated with identical amounts of non-specific monoclonal IgG (P < 0.01). In contrast, MAb against human PAI-1 increased ECM degradation four-fold (P < 0.001). A MAb against human uPAR had no significant effect on ECM degradation. Taken together, our results indicate that ECM degradation by cultured human mesangial cells is mediated by a proteinase cascade. This cascade is initiated by tPA and generates plasmin and active MMP-2, which together carry out the degradation of the ECM. We postulate that decreased activity of this cascade may represent a final common pathway contributing to glomerular ECM accumulation in progressive renal disease