Glucocorticoids Pro-survival Benefits On Podocyte Injury And Its Regeneration In Glomerular Disease

Background: Focal segmental glomerulosclerosis (FSGS) is the leading cause ofnephrotic syndrome in adults. In this disease, the podocyte, a terminally differentiatedepithelial cell, is the primary target of injury. The subsequent decrease in podocytenumber is a critical determinant underlying the development of glomerulosclerosis.Although FSGS is typically considered a non-immunological disease,immunosuppressive agents, such as glucocorticosteroids, are commonly used as firstline therapy in clinical disease. In addition to immunomodulatory effects, thetherapeutic benefit of glucocorticoids may be in part due to direct cellular actions onpodocytes including limiting apoptosis, actin re-arrangement, down-regulatingpodocyte cytokines, and enhanced slit diaphragm complex proteins synthesis.Several recent lines of evidence suggests that glomerular parietal epithelial cells(PECs) may serve as progenitor cells for podocytes, and thus may have a potentialrole in glomerular repair by replacing, in part or completely, any decrease in podocytenumber. Romagnani reported a subset of parietal progenitors were present in normalhuman glomeruli. Moeller’s group showed that podocytes can derive from PECs inadolescent mice. We, and others, have shown in certain experimental glomerulardiseases such as FSGS, membranous nephropathy and aging nephropathy that PECsbegin to express proteins traditionally considered specific for podocytes, such asWT-1and synaptopodin. Some have called glomerular cells expressing both PEC andpodocyte proteins as glomerular epithelial transitional cells or progenitor cells.However, to date no published reports show that this subpopulation of PECs becomesfully functional podocytes.To our knowledge, the effect of corticosteroids on podocyte number is not known,nor is it known if corticosteroids enhance glomerular repair by affecting the numberof glomerular epithelial progenitor cells. The purpose of this study was to determine ifthe positive effects of glucocorticoids in FSGS mice might be due to an increase in thenumber of glomerular epithelial progenitor cells (defined as glomerular cellsexpressing a PEC and podocyte protein) in experimental FSGS characterized byreduced podocyte number, and also drive parietal epithelial cells transition topodocyte in culture. Objective: To determine if the positive effects of prednisone in FSGS might be due toan increase in the number of glomerular epithelial progenitor cells (defined asglomerular cells expressing a PEC and podocyte protein) in experimental FSGScharacterized by reduced podocyte number.Methodology: Experimental FSGS was induced in mice with a cytotoxicanti-podocyte antibody, resulting in an abrupt decrease in podocyte number by daythree, proteinuria and the development of glomerulosclerosis. Administer dailyprednisone to mice with FSGS, beginning at day3. Immunohistochemistry stainingwas performed on kidney tissues after animals were sacrificed. In vitro, primarymouse PEC cell culture was performed. After final differentiation for14days,immunocytochemistry staining was done and data was quantitated.Results: Administering daily prednisone to mice with FSGS, beginning at day3,significantly increased podocyte number at weeks2and4. Podocyte number did notincrease in control mice with FSGS given DMSO. The increase in podocyte numberin prednisone treated mice correlated significantly with reduced glomerulosclerosis.Prednisone reduced podocyte apoptosis measured by synaptopodin+/caspase-3+double-staining. Additionally, the number of podocyte progenitors, defined as cellsexpressing both a parietal epithelial cell protein and a podocyte protein, wassignificantly increased in prednisone treated mice with FSGS at weeks2and4. Thiswas associated with increased phospho-ERK staining in both parietal epithelial cells(PAX2+/p-ERK+), and in podocyte progenitors (WT-1+/p-ERK+lining Bowman’scapsule). In vitro, cell culture data indicated that PECs begin to express synaptopodin,which is a podocyte specific protein after dexamethasone treatment. This phenomenonhappened at lower dose of dexamethasone and short time incubation.Conclusion: These data show that in this model of experimental FSGS, prednisoneaugments glomerular repair by increasing podocyte number through direct effects onboth glomerular epithelial cells. Prednisone limits podocyte loss by reducingapoptosis, and increases regeneration by augmenting the number of podocyteprogenitors. The data supports a direct glomerular cell action for prednisone inimproving outcomes in FSGS.