Podocyte injury underlies the progression of focal segmental glomerulosclerosis in the fa/fa Zucker rat.

BACKGROUND: The progression of diabetic nephropathy to chronic renal failure is based on the progressive loss of viable nephrons. The manner in which nephrons degenerate in diabetic nephropathy and whether the injury could be transferred from nephron to nephron are insufficiently understood. We studied nephron degeneration in the fa/fa Zucker rat, which is considered to be a model for non-insulin-dependent diabetes mellitus. METHODS: Kidneys of fa/fa rats with an established decline of renal function and of fa/+ controls were structurally analyzed by advanced morphological techniques, including serial sectioning, high-resolution light microscopy, transmission electron microscopy, cytochemistry, and immunohistochemistry. In addition, tracer studies with ferritin were performed. RESULTS: The degenerative process started in the glomerulus with damage to podocytes, including foot process effacement, pseudocyst formation, and cytoplasmic accumulation of lysosomal granules and lipid droplets. The degeneration of the nephron followed the tuft adhesion-mediated pathway with misdirected filtration from capillaries included in the adhesion toward the interstitium. This was followed by the formation of paraglomerular spaces that extended around the entire glomerulus, as well as via the glomerulotubular junction, to the corresponding tubulointerstitium. This mechanism appeared to play a major role in the progression of the segmental glomerular injury to global sclerosis as well as to the degeneration of the corresponding tubule. CONCLUSIONS: The way a nephron undergoes degeneration in this process assures that the destructive effects remain confined to the initially affected nephron. No evidence for a transfer of the disease from nephron to nephron at the level of the tubulointerstitium was found. Thus, each nephron entering this pathway to degeneration appears to start separately with the same initial injuries at the glomerulus.

How does podocyte damage result in tubular damage?

Severe podocyte damage including detachment from the GBM leads to adhesion of the glomerular tuft to Bowman's capsule, thus to a local loss of the separation of the tuft from the interstitium. Perfused capillaries contained in the tuft adhesion deliver their filtrate no longer into Bowman's space but into the interstitium. In response, interstitial fibroblasts create a cellular cover around the focus of misdirected filtration, interpreted teleologically, aiming at preventing the entry of this fluid into the interstitium. This results in the formation of a crescent-shaped, fluid-filled paraglomerular space overarching the segmental glomerular lesion. Extension of this space over the entire glomerulus leads to global sclerosis; extension of this space via the urinary pole onto the outer aspect of the corresponding tubule leads to the degeneration of the tubule. Since, as we postulate, such misdirected filtration and filtrate spreading is the crucial mechanism of damage progression in 'classic' focal segmental glomerulosclerosis (FSGS), the most characteristic structural injury of FSGS is the merger of the tuft with the interstitium, represented by a tuft adhesion, later a synechia. Therefore, histopathologically, 'classic' FSGS is best defined by an adhesion/synechia of the tuft to Bowman's capsule.

From segmental glomerulosclerosis to total nephron degeneration and interstitial fibrosis: a histopathological study in rat models and human glomerulopathies.

BACKGROUND: Focal segmental glomerulosclerosis (FSGS) is consistently associated with tubular degeneration and interstitial fibrosis, altogether, accounting for the progressive decline in renal function. The mechanisms which link glomerular injury to tubulo-interstitial fibrosis are controversial. The present study describes the step-by-step sequence of histopathological events, i.e. the evolution of the injury from the initial lesion in the glomerulus to total nephron destruction. METHODS: The investigation was performed in male hypertensive Fawn-hooded rats (6-, 9-, and 12-month-old) and 14-month-old Milan normotensive rats. The kidneys were fixed by in vivo perfusion and processed for structural investigation. Autopsy materials from human cases of focal segmental glomerulosclerosis and diabetic nephropathy were also examined. RESULTS: FSGS as seen in rat models consists of collapsed and hyalinized capillaries and mesangial portions which are included within a synechia between the glomerular tuft and Bowman's capsule. In addition, a synechia generally contains glomerular capillaries which are perfused and continue to filter with the filtrate being delivered into the interstitium rather than into Bowman's capsular space. Such filtration creates a paraglomerular space on the outer aspect of the parietal epithelium. This space becomes separated from the interstitium by a dense layer of sheet-like fibroblast processes. Associated with the progression to global sclerosis, this space spreads around the entire circumference of a glomerulus; all 'sclerotic' tuft portions are eventually contained in this space. Starting from the urinary pole this process also involves the proximal tubule, initially by expanding the tubular basement membrane (TBM) and later, by separating the TBM from its epithelium, thus creating a peritubular space by misdirected filtrate spreading. Similar to the situation observed at the glomerulus this space becomes separated from the interstitium by a layer of fibroblast processes. The final degeneration of the nephron occurs via two pathways. Pathway I whereby development to global sclerosis is dominant or develops concurrently with tubular degeneration, eventually terminating in global and cylindrical remnants of extracellular matrix surrounded by abundant fibrous tissue. Pathway II where the degeneration of the tubule is ahead of damage progression in the glomerulus leading to atubular glomerular cysts. CONCLUSION: The present study suggests that severely injured glomeruli may continue to filter with the filtrate spreading along interstitial routes. Fluid added locally to the interstitium from such 'extraterritorial' glomerular capillaries probably is quite different in quantity and composition compared to that from interstitial capillaries. We propose that this kind of abnormal addition of fluid to the interstitium is the essential mechanism accounting for interstitial progression of the disease. Similar histopathological phenomena in human kidneys with focal segmental glomerulosclerosis suggest that the pathogenetic pathways defined in the rat models operate in human disease as well.

Development of vascular pole-associated glomerulosclerosis in the Fawn-hooded rat.

Fawn-hooded hypertensive (FHH) rats constitute a spontaneous model of chronic renal failure with early systemic and glomerular hypertension, proteinuria, and development of focal and segmental glomerulosclerosis. The goal of the present study was to elucidate a step-by-step sequence of histopathologic events leading from an initial glomerular injury to segmental sclerosis. Segmental sclerosis in the FHH rat is consistently associated with the glomerular vascular pole. The initial injury involves the expansion of primary branches of the afferent arteriole. Apposition of those capillaries to Bowman's capsule, together with the degeneration and detachment of corresponding podocytes, allows parietal cells to attach to the naked glomerular basement membrane of this capillary, i.e., allows the formation of a tuft adhesion to Bowman's capsule. The adhesion enlarges to a broad synechia by encroaching to neighboring capillaries, apparently based on progressive podocyte degeneration at the flanks of the adhesion. Capillaries inside the adhesion--before undergoing collapse or hyalinization--appear to stay perfused for some time and to maintain some kind of filtration misdirected toward the cortical interstitium. Thereby, a prominent paraglomerular space comes into existence, enlarging in parallel with the adhesion. Toward the cortical interstitium this space is delimited by a layer of sheetlike fibroblast processes, which has obviously been assembled in response to the formation of this space. Toward the urinary space, the paraglomerular space is demarcated by the parietal epithelium and by the interface between the adhesion and the "intact" tuft remnant. Thus, the sclerotic tuft portions all become enclosed within the paraglomerular space.

Long-term treatment of rats with FGF-2 results in focal segmental glomerulosclerosis.

Long-term treatment (8 and 13 weeks) of rats with FGF-2 led to albuminuria and to increase in serum creatinine indicating the development of chronic renal failure. Histologically, the classic picture of focal segmental glomerulosclerosis (FSGS) was found; males were more severely affected than females. Among the early changes podocyte lesions were most prominent. Surprisingly, mitotic figures in podocytes and a considerable fraction of bi(multi)nucleated podocyte profiles were found in treated animals (roughly 16% in males, 8% in females). Since an increase of cell number of podocytes was not evident, we conclude that FGF-2 stimulates podocytes to re-enter the cell cycle and to undergo mitosis (nuclear division). However, podocytes-probably due to their highly differentiated cell shape in the adult-are unable to complete cell division (cytokinesis) resulting in bi- or multinucleated cells; in others cell division may fail totally leading to podocyte degeneration. Most podocytes in FGF-2-treated rats exhibited degenerative changes including cell body attenuation, extensive pseudocyst formation, widespread foot process effacement, as well as detachments from the glomerular basement membrane (GBM). The development of FSGS in this model is very uniform. In the case of podocyte detachments from peripheral capillaries, parietal cells become attached to naked GBM-areas, establishing the nidus for development of a tuft adhesion to Bowman's capsule. Tuft adhesions grow by encroaching of parietal cells onto adjacent capillary loops, resulting eventually in a solid synechia with collapsed capillaries, that is, what represents segmental sclerosis. The distribution of adhesions on the inner surface of Bowman's capsule appeared to be random, including all locations between the vascular and urinary pole. The two main aspects of this study (inability of podocytes to replicate and development of FSGS based on progressing podocyte degeneration) may be part of a vicious cycle. FGF-2 stimulates podocytes to enter cell division thereby conveying them into a hazardous situation. If a podocyte fails and degenerates it cannot be replaced, aggravating the situation for the remaining cells and possibly increasing their predisposition to respond to mitogenic stimuli. Similar mechanisms may constitute the development of FSGS in other experimental as well as human glomerulopathies.

A role for podocytes to counteract capillary wall distension.

In a previous study of the changes in glomerular structure in the isolated perfused kidney (IPK), perfusion at high pressures lead to an enlargement of the glomerular tuft and to the formation of giant capillaries. The present paper analyzes the morphological and dimensional changes of the peripheral glomerular capillary wall under these circumstances. The enlargement of glomerular capillaries at high pressure perfusion was accompanied by a considerable increase in the surface area of the glomerular basement membrane (GBM). The podocyte as well as the endothelial layer perfectly adapted to the acute challenge in covering increasing GBM area. The interdigitating foot process pattern showed up in an ideal arrangement. The capillary wall expansion was associated with a significant increase in total pericapillary slit area. Compared to the corresponding low pressure groups (65 mm Hg, without and with the application of vasodilators) the slit area increased in the high pressure groups (105 mm Hg, without and with vasodilator) by approximately 50 and 75%, respectively. This increase of the slit area was mainly due to an increase in slit length; the slit width remained fairly constant. These findings indicate that the pericapillary wall is distensible based on a distensibility of the GBM. We suggest that the contractile apparatus of podocyte foot processes regulates the expansion of the GBM.