Podocyte expression of the CDK-inhibitor p57 during development and disease.

BACKGROUND: The mature podocyte is a terminally differentiated cell with a limited proliferative capacity. The precise cell cycle proteins necessary for establishing podocyte quiescence during development or permitting podocyte cell cycle re-entry in disease states have not been fully defined. Accordingly, we studied the role of the cyclin dependent kinase (CDK)-inhibitor p57Kip2 (p57) in modulating these processes. METHODS: The expression of p57 protein in relation to markers of DNA synthesis was examined in developing mouse kidneys, and in the passive Heymann nephritis (PHN) and anti-glomerular antibody models of glomerular disease by immunohistochemistry. The role of p57 in glomerulogenesis was explored by examining renal tissue from embryonic p57-/- mice, and the expression of p21, p27 and p57 protein and mRNA was examined in podocytes in vitro. RESULTS: The de novo expression of p57 during glomerulogenesis coincides with the cessation of podocyte proliferation, and the establishment of a mature phenotype, and p57 is expressed exclusively in podocytes in mature glomeruli. However, p57 knockout mice have normal glomerular podocyte development. In addition, mRNA but not protein levels of p57 increased upon differentiation of podocytes in vitro. There was a marked decrease in p57 expression in both animal models of podocyte injury. This was diffuse in PHN, whereas in the murine model, loss of expression of p57 occurred predominantly in proliferating podocytes, expressing proliferating cell nuclear antigen (PCNA). CONCLUSION: Despite the de novo expression of p57 protein coinciding with the cessation of primitive podocyte proliferation during glomerulogenesis, embryonic p57-/- mice glomeruli were histologically normal. Cultured podocytes did not require changes in p57 protein levels to undergo differentiation. These data suggest that p57 alone is not required for podocyte differentiation, and that other cell cycle regulators may play a role. Furthermore, although injury to mature podocytes in experimental glomerular disease is associated with a decrease in p57, the levels of all three members of the Cip/Kip family of CDK inhibitors appear to determine the capability of podocytes to proliferate.

Dissociation of mesangial cell migration and proliferation in experimental glomerulonephritis.

BACKGROUND: Recently, we documented that following in vivo mesangial cell (MC) ablation in the Thy1 model, reconstitution of the mesangium occurs by a coordinated proliferation and migration of Thy1 (OX-7)-positive cells originating from the hilus and extraglomerular mesangium. We investigated the role of basic fibroblast growth factor (bFGF) in the mediation of these events. METHODS: Rats were injected with antithymocyte serum and 48 hours later were pulsed with 3H-thymidine to label proliferating cells. Ninety minutes later, a baseline renal biopsy was obtained, and rats were injected with neutralizing anti-bFGF antibodies or control IgG. Sacrificial biopsies were obtained at 96 hours of disease. Using computer image analysis, biopsies from both time points were quantitated for the number of radiolabeled MC (proliferation) and their mean distance from the hilus (migration). The effect of bFGF on the migration of MCs in culture was examined using a chemotactic assay. RESULTS: At sacrifice, autoradiographs of rats receiving anti-bFGF had significantly fewer radiolabeled MCs as compared with rats receiving control IgG (8.7+/-1.9 vs. 14.7+/-3.5, P = 0.0001), yielding an overall 40% reduction in proliferation. There was no difference, however, in the final distance of radiolabeled MCs from the glomerular hilus in the two groups, indicating that the administration of anti-bFGF did not effect MC migration in this model. In an in vitro chemotactic assay, MCs migrated in response to platelet-derived growth factor (PDGF) BB (20 ng/ml), but did not migrate in response to bFGF at a wide range of concentrations (0.5 to 50 ng/ml). CONCLUSIONS: These studies demonstrate that bFGF is an important mediator of MC proliferation but that it does not significantly influence MC migration. This is the first demonstration showing that the mediators effecting proliferation can be dissociated from those mediating migration in renal injury.

Antibodies to C1q in systemic lupus erythematosus: characteristics and relation to Fc gamma RIIA alleles.

Autoantibodies to the collagen-like region of the first complement component (C1qAB) are found in patients with systemic lupus erythematosus (SLE), particularly those with renal disease. In a cohort of 46 SLE patients with diffuse proliferative glomerulonephritis, we found declining C1qAB titers in 77% of treatment responders and in only 38% of treatment non-responders (P < 0.03). To further characterize this autoantibody, we tested 240 SLE patients for the presence of C1qAB. Positive titers were found in 44% of patients with renal disease and 18% of patients without renal disease (chi2 P < 0.0003). Analysis of IgG subclass revealed IgG2 C1qAB alone in 34%, IgG1 C1qAB alone in 20%, and both IgG1 and IgG2 in 46% of patients. Fewer than 10% of patients had measurable titers of IgG3 or IgG4 C1qAB. The pathogenic role of these IgG2-skewed C1qAB may relate to impaired immune complex clearance by the mononuclear phagocyte system: IgG2 antibodies are efficiently recognized by only one IgG receptor, the H131 allele of Fc gamma RIIa (Fc gamma RIIa-H131). In contrast, Fc gamma RIIa-R131, which is characterized by minimal IgG2 binding, has recently been associated with lupus nephritis. In our C1qAB positive patients, the presence of Fc gamma RIIA-R131 was associated with an increased risk for renal disease. Autoantibodies to C1q may have pathogenic significance in SLE patients with genetic defects in the ability to clear IgG2 containing immune complexes.