Glomerular prolapse as precursor of one type of segmental sclerosing lesions

Mesangial Injury and Capillary Ballooning Precede Podocyte Damage in Nephrosclerosis

The development of focal and segmental glomerulosclerosis (FSGS) as a consequence of glomerular hypertension resulting from arterial hypertension is widely considered a podocyte disease. However, the primary damage is encountered in the mesangium. In acute settings, mesangial cells disconnect from their insertions to the glomerular basement membrane, causing a ballooning of capillaries and severe changes of the folding pattern of the glomerular basement membrane, of the arrangement of the capillaries, and thereby of the architecture of the tuft. The displacement of capillaries led to contact of podocytes and parietal epithelial cells, initiating the formation of tuft adhesions to Bowman's capsule, the committed lesion to progress to FSGS. In addition, the displacement of capillaries also caused an abnormal stretching of podocytes, resulting in podocyte damage. Thus, the podocyte damage that starts the sequence to FSGS is predicted to develop secondary to the mesangial damage. This sequence was found in two hypertensive rat models of FSGS and in human hypertensive nephrosclerosis.

Focal segmental glomerulosclerosis, excluding atypical lesion, is a predictor of renal outcome in patients with membranous nephropathy: a retrospective analysis of 716 cases

BACKGROUND

Focal segmental lesions (FSLs) are not uncommon in idiopathic membranous nephropathy (IMN). The reported percentage of IMN patients with focal segmental glomerulosclerosis (FSGS) lesions varies widely between studies. The objective of this study was to differentiate atypical FSL (aFSL) from typical FSGS in IMN and to analyse the clinicopathological predictors of primary outcome of IMN patients.

METHODS

A total of 716 patients with biopsy-proven IMN between January 1, 2007 and December 31, 2017 were enrolled in the study. An atypical focal segmental lesion was defined as pure synechia, segmental hyperplasia of podocytes or thickening of the GBM accompanied by proliferation of the mesangial matrix, and absence of typical FSGS. The patients were divided into three groups: patients without FSL (FSL^-), patients with typical FSGS (FSGS⁺), and patients with aFSL (aFSL⁺).The primary outcome was a 50% decline in the initial estimated glomerular filtration rate or end-stage renal disease (ESRD) incidence. Secondary outcomes included all-cause death and ESRD.

RESULTS

FSGS was present in 174 patients, while aFSL was noted in 161 patients. Systolic blood pressure was higher in both aFSL⁺ group and FSGS⁺ groups compared with the FSL^- group. IMN patients without FSL and with aFSL had lower serum creatinine levels than IMN patients with FSGS. Both the FSGS⁺ and aFSL⁺ groups had higher levels of proteinuria and lower albumin levels than the FSL^- group. Renal tissue lesions, including tubulointerstitial fibrosis, glomerular obsolescence, and vascular sclerosis were significantly more severe in the FSGS⁺ group. Cox multivariate analysis showed that older age ≥ 60 years, eGFR< 60 ml/(min·1.73m²), tubulointerstitial fibrosis area ≥ 15% and FSGS at biopsy were independent risk factors for the primary outcome.

CONCLUSIONS

No significant difference in outcome was found between the FSL^- and aFSL⁺ groups, although the patients with aFSL had lower levels of serum albumin and eGFR, higher level of urinary protein, more severe renal lesions with proliferation of the mesangial area,tubulointerstitial fibrosis and vascular sclerosis. FSGS, excluding atypical lesions, was an independent predictor of the primary outcome.

Novel parietal epithelial cell subpopulations contribute to focal segmental glomerulosclerosis and glomerular tip lesions

Beside the classical flat parietal epithelial cells (PECs), we investigated proximal tubular epithelial-like cells, a neglected subgroup of PECs. These cells, termed cuboidal PECs, make up the most proximal part of the proximal tubule and may also line parts of Bowman's capsule. Additionally, a third intermediate PEC subgroup was identified at the junction between the flat and cuboidal PEC subgroups at the tubular orifice. The transgenic mouse line PEC-rtTA labeled all three PEC subgroups. Here we show that the inducible Pax8-rtTA mouse line specifically labeled only cuboidal and intermediate PECs, but not flat PECs. In aging Pax8-rtTA mice, cell fate mapping showed no evidence for significant transdifferentiation from flat PECs to cuboidal or intermediate PECs or vice versa. In murine glomerular disease models of crescentic glomerulonephritis, and focal segmental glomerulosclerosis (FSGS), intermediate PECs became more numerous. These intermediate PECs preferentially expressed activation markers CD44 and Ki-67, suggesting that this subgroup of PECs was activated more easily than the classical flat PECs. In mice with FSGS, cuboidal and intermediate PECs formed sclerotic lesions. In patients with FSGS, cells forming the tip lesions expressed markers of intermediate PECs. These novel PEC subgroups form sclerotic lesions and were more prone to cellular activation compared to the classical flat PECs in disease. Thus, colonization of Bowman's capsule by cuboidal PECs may predispose to lesion formation and chronic kidney disease. We propose that tip lesions originate from this novel subgroup of PECs in patients with FSGS.

FSGS: from pathogenesis to the histological lesion

Focal and segmental glomerulosclerosis (FSGS) is a generic term that in the current terminology can be appropriate for identifying two conditions. First, a morphological pattern of injury characterized by the occlusion of a single or group of glomerular capillary loops by sclerotic material, indicating a precise histological lesion but with a wide range of etiological interpretations. Second, a pauci-immune renal disease called idiopathic or primary FSGS, which is a podocytopathy triggered by an endogenous cytotoxin that recognizes the podocyte as its sole target. Based on the current and past literature, we do not possess practical tools to easily provide a precise classification of an FSGS lesion, although some clues may be of help in everyday clinical practice. Reactive forms, genetic forms, adaptive forms, forms associated with a deregulation of the proliferation and forms secondary to local glomerular inflammation are the etiological classes known to be associated with the development of an FSGS lesion. However, diagnosing each single case based on clinical, serological and histological criteria is still far from easy and mostly depends on the experience of the renal team, which should involve skilled nephrologists and pathologists.

Problems with 'focal segmental glomerulosclerosis'

The term 'focal segmental glomerulosclerosis (FSGS)' has been applied to many different conditions. All classifications of 'FSGS', including those describing 'variants', perpetuate the misconceptions that the entities included have something in common and that the term 'FSGS' has some value. With a rigorous approach to renal biopsies showing segmental lesions, especially with knowledge of clinical circumstances and with detailed analysis of features such as the appearance of lesions and their position within glomeruli, a pathologist can provide information that is clinically more useful than merely the bald diagnosis 'FSGS'. More precise terms should be used. 'Overload changes' can be used to describe the changes seen in reduced renal mass. 'Tip changes' can be seen in many conditions and are not a disease in themselves. 'The glomerular tip lesion as originally defined' means the occurrence of tip changes in otherwise normal glomeruli, in the nephrotic syndrome. 'Early classical segmental sclerosing glomerulopathy' is the combination of tip changes and otherwise abnormal glomeruli, in the nephrotic syndrome. 'Late classical segmental sclerosing glomerulopathy' means segmental lesions at various sites within glomeruli, in the nephrotic syndrome. 'Collapsing glomerulopathy' is distinctive, and its inclusion in classifications emphasises the lack of specificity of 'FSGS'.

Glomerular epithelial stem cells: the good, the bad, and the ugly

Global glomerulosclerosis with loss of podocytes in humans is typical of end-stage renal pathology. Although mature podocytes are highly differentiated and nondividing, converging evidence from experimental and clinical data suggests adult stem cells within Bowman's capsule can rescue some of this loss. Glomerular epithelial stem cells generate podocytes during kidney growth and regenerate podocytes after injury, thus explaining why various glomerular disorders undergo remission occasionally. This regenerative process, however, is often inadequate because of inefficient proliferative responses by glomerular epithelial stem cells with aging or in the setting of focal segmental glomerulosclerosis. Alternatively, an excessive proliferative response by glomerular epithelial stem cells after podocyte injury can generate new lesions such as extracapillary crescentic glomerulonephritis, collapsing glomerulopathy and tip lesions. Better understanding of the mechanisms that regulate growth and differentiation of glomerular epithelial stem cells may provide new clues for prevention and treatment of glomerulosclerosis.

Renal progenitor cells contribute to hyperplastic lesions of podocytopathies and crescentic glomerulonephritis

Glomerular injury can involve excessive proliferation of glomerular epithelial cells, resulting in crescent formation and obliteration of Bowman's space. The origin of these hyperplastic epithelial cells in different glomerular disorders is controversial. Renal progenitors localized to the inner surface of Bowman's capsule can regenerate podocytes, but whether dysregulated proliferation of these progenitors contributes to crescent formation is unknown. In this study, we used confocal microscopy, laser capture microdissection, and real-time quantitative reverse transcriptase-PCR to demonstrate that hypercellular lesions of different podocytopathies and crescentic glomerulonephritis consist of three distinct populations: CD133(+)CD24(+)podocalyxin (PDX)(-)nestin(-) renal progenitors, CD133(+)CD24(+)PDX(+)nestin(+) transitional cells, and CD133(-)CD24(-)PDX(+)nestin(+) differentiated podocytes. In addition, TGF-beta induced CD133(+)CD24(+) progenitors to produce extracellular matrix, and these were the only cells to express the proliferation marker Ki67. Taken together, these results suggest that glomerular hyperplastic lesions derive from the proliferation of renal progenitors at different stages of their differentiation toward mature podocytes, providing an explanation for the pathogenesis of hyperplastic lesions in podocytopathies and crescentic glomerulonephritis.

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

CONTEXT

Etiologic factors and pathways leading to altered podocyte phenotype are clearly numerous and involve the activity of different cellular function.

OBJECTIVE

To focus on recent discoveries in podocyte biology and genetics and their relevance to these human glomerular diseases, named podocytopathies.

DATA SOURCES

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.

CONCLUSIONS

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.

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

CONTEXT

Etiologic factors and pathways leading to altered podocyte phenotype are clearly numerous and involve the activity of different cellular function.

OBJECTIVE

To focus on recent discoveries in podocyte biology and genetics and their relevance to these human glomerular diseases, named podocytopathies.

DATA SOURCES

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.

CONCLUSIONS

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.

The spectrum of focal segmental glomerulosclerosis: new insights

PURPOSE OF REVIEW

Focal segmental glomerulosclerosis (FSGS) is a disease with diverse histologic patterns and etiologic associations. Genetic, toxic, infectious and inflammatory mediators have been identified. This review will focus on new evidence supporting the potential mechanistic basis underlying the histologic variants and their clinical relevance.

RECENT FINDINGS

Evidence from animal models and in-vitro studies suggests that injury inherent within or directed to the podocyte is a central pathogenetic factor. Disruption of signaling from any of the podocyte's specialized membrane domains, including slit diaphragm, apical and basal membranes, or originating at the level of the actin cytoskeleton, may promote the characteristic response of foot process effacement. Irreversible podocyte stress leading to podocyte depletion through apoptosis or detachment is a critical mechanism in most forms of FSGS. In the collapsing variant, podocyte dysregulation leads to podocyte dedifferentiation and glomerular epithelial cell proliferation.

SUMMARY

Translation studies in humans and new evidence from animal models have provided mechanistic insights into the diverse phenotypes of FSGS.

Glomerular tip changes in childhood minimal change nephropathy

Segmental glomerular lesions at the tubular opening, or tip changes, are found in the renal biopsies of adults in many disorders, including some initially considered to show minimal change nephropathy. The hypothesis was that similar tip changes occurred in children. We reviewed a consecutive series of 50 biopsies, diagnosed as minimal change nephropathy, from 49 children. Segmental lesions were found in five biopsies. One biopsy showed lesions at the glomerular hilum. The patient was in remission at follow-up. Four biopsies showed only tip changes. Three patients were in remission, two on no treatment at follow-up, and one on ciclosporin. The other had chronic hepatitis B infection, with persistent proteinuria and segmental lesions at different sites in glomeruli. The other 44 children were nearly all in remission, 18 without treatment at follow-up, and the rest on various immunosuppressants, but one had persistent proteinuria and multiple segmental lesions. Series of childhood minimal change nephropathy, similar to this one, are likely to include cases of the glomerular tip lesion, under the original definition of minimal change nephropathy plus tip changes. This should make little difference in clinical practice, because the clinical course should resemble that of minimal change nephropathy.

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.

Pathways to nephron loss starting from glomerular diseases-insights from animal models

Studies of glomerular diseases in animal models show that progression toward nephron loss starts with extracapillary lesions, whereby podocytes play the central role. If injuries remain bound within the endocapillary compartment, they will undergo recovery or be repaired by scaring. Degenerative, inflammatory and dysregulative mechanisms leading to nephron loss are distinguished. In addition to several other unique features, the dysregulative mechanisms leading to collapsing glomerulopathy are particular in that glomeruli and tubules are affected in parallel. In contrast, in degenerative and inflammatory diseases, tubular injury is secondary to glomerular lesions. In both of the latter groups of diseases, the progression starts in the glomerulus with the loss of the separation between the tuft and Bowman's capsule by forming cell bridges (parietal cells and/or podocytes) between the glomerular and the parietal basement membranes. Cell bridges develop into tuft adhesions to Bowman's capsule, which initiate the formation of crescents, either by misdirected filtration (proteinaceous crescents) or by epithelial cell proliferation (cellular crescents). Crescents may spread over the entire circumference of the glomerulus and, via the glomerulotubular junction, may extend onto the tubule. Two mechanisms concerning the transfer of a glomerular injury onto the tubulointerstitium are discussed: (1) direct encroachment of extracapillary lesions and (2) protein leakage into tubular urine, resulting in injury to the tubule and the interstitium. There is evidence that direct encroachment is the crucial mechanism. Progression of chronic renal disease is underlain by a vicious cycle which passes on the damage from lost and/or damaged nephrons to so far healthy nephrons. Presently, two mechanisms are discussed: (1) the loss of nephrons leads to compensatory mechanisms in the remaining nephrons (glomerular hypertension, hyperfiltration, hypertrophy) which increase their vulnerability to any further challenge (overload hypothesis); and (2) a proteinuric glomerular disease leads, by some way or another, to tubulointerstitial inflammation and fibrosis, accounting for the further deterioration of renal function (fibrosis hypothesis). So far, no convincing evidence has been published that in primary glomerular diseases fibrosis is harmful to healthy nephrons. The potential of glomerular injuries to regenerate or to be repaired by scaring is limited. The only option for extracapillary injuries with tuft adhesion is repair by formation of a segmental adherent scar (i.e., segmental glomerulosclerosis).

Evolution of nephrotic-associated focal segmental glomerulosclerosis and relation to the glomerular tip lesio11See Editorial by Haas,P. 1188

BACKGROUND

Several entities or variants within focal segmental glomerulosclerosis (FSGS) have been described, but their changes with time and interrelationships are undetermined.

METHODS

Changes with time were studied in two series of segmental sclerosing lesions in the nephrotic syndrome, one of 22 specimens from ten patients in a trial, the other of 176 specimens from 121 consecutive patients.

RESULTS

The earliest lesions were probably all at the tubular origin, equivalent to the tip variant of FSGS. In some patients, lesions remained at this site, but progression to renal failure was accompanied by morphologic progression, with development of lesions at various sites, equivalent to FSGS, not otherwise specified (NOS). Progression was more likely if there were large lesions, abnormal mesangium, and extensive acute tubular damage. Patients with lesions at the tubular origin at presentation had a shorter duration of symptoms and less chronic renal damage than those with multiple lesions, were more likely to have a complete response of the nephrotic syndrome, and were less likely to progress to renal failure. Recurrent nephrotic syndrome occurred in 12 of 14 allografts at risk, and was usually accompanied by lesions at the tubular origin, then multiple lesions.

CONCLUSION

At least some patients with FSGS (NOS) have evolved from the tip variant. The tip variant has been considered a distinct entity. Another interpretation is that it includes two conditions, one an early form of classic FSGS, and the other closely related to minimal change nephropathy (MCN), equivalent to the glomerular tip lesion as originally defined.

Glomerular tip lesion: a distinct entity within the minimal change disease/focal segmental glomerulosclerosis spectrum

BACKGROUND

The glomerular tip lesion (GTL) is a distinctive but controversial histopathologic lesion occurring in patients with idiopathic nephrotic syndrome. The relationship of GTL to minimal change disease (MCD) and idiopathic focal segmental glomerulosclerosis (FSGS) has been disputed.

METHODS

In order to define the clinical features and natural history of GTL, we retrospectively reviewed the presenting clinical features, biopsy findings and outcome of 47 cases. Presenting clinical features of GTL were compared to those of controls with MCD (N= 61) or idiopathic FSGS (N= 50).

RESULTS

The cohort of GTL consisted of 45 adults and two children (mean age 47.5 years; range 12 to 79 years), including 76.6% Caucasians and 53% males. At presentation, 93.6% of patients had edema, 89.1% had nephrotic syndrome (mean urine protein 8.31 g, mean serum albumin 2.27 g/dL, and mean cholesterol 340.6 mg/dL), and 34.8% had renal insufficiency. Mean time from onset of renal disease to biopsy was 2.4 months. At biopsy, glomerular segmental lesions included GTL alone in 26%, GTL and peripheral lesions in 6%, GTL and indeterminate lesions in 36%, and GTL with peripheral and indeterminate lesions in 32%. No initial biopsy contained perihilar sclerosis and most (81%) segmental lesions were cellular. Follow-up data were available in 29 patients, of whom 21 received steroids alone and eight received sequential therapy with steroids and a cytotoxic agent. At a mean follow-up of 21.5 months, 58.6% of patients achieved complete remission of nephrotic syndrome, 13.8% had partial remission, and 27.6% had persistent nephrotic proteinuria. Only one patient progressed to end-stage renal disease (ESRD). Predictors of nonremission included severity of proteinuria at presentation and % peripheral lesions. When compared to controls with MCD and idiopathic FSGS, GTL more closely resembled MCD with respect to high incidence of nephrotic syndrome (P < 0.001), severity of proteinuria (P < 0.05), short duration from onset to biopsy (P < 0.001), and absence of chronic tubulointerstitial disease (P < 0.0054).

CONCLUSION

Within the MCD/FSGS spectrum, GTL is a distinctive and prognostically favorable clinical-pathologic entity whose presenting features and outcome more closely approximate those of MCD.

Pathways to recovery and loss of nephrons in anti-Thy-1 nephritis

The present histopathologic study of anti-Thy-1.1 models of mesangioproliferative glomerulonephritis in rats provides a structural analysis of damage development and of pathways to recovery and to nephron loss. As long as the disease remains confined to the endocapillary compartment, the damage may be resolved or recover with a mesangial scar. Irreversible lesions with loss of nephrons emerge from extracapillary processes with crucial involvement of podocytes, leading to tuft adhesions to Bowman's capsule (BC) and subsequent crescent formation. Two mechanisms appeared to be responsible: (1) Epithelial cell proliferation at BC and the urinary orifice and (2) misdirected filtration and filtrate spreading on the outer aspect of the nephron. Both may lead to obstruction of the tubule, disconnection from the glomerulus, and subsequent degeneration of the entire nephron. No evidence emerged to suggest that the kind of focal interstitial proliferation associated with the degeneration of injured nephrons was harmful to a neighboring healthy nephron.