Background Kidney podocytes and their slit diaphragms contribute to prevent urinary protein loss. Previously, we have shown that T cell from patients with systemic lupus erythematosus (SLE) display increased expression of calcium/calmodulin kinase IV (CaMKIV). Pharmacologic inhibition of CaMKIV using spleen cells from lupus prone MRL/lpr mice resulted in decreased expression of CD86. However, the role of CaMKIV in podocytes function in lupus nephritis (LN) remains unknown.
Objectives Here we evaluated the expression of CaMKIV in kidney biopsy specimens from patients with LN and the functional roles in a human podocyte cell line (AB8/13) after exposure to purified immunoglobulin G (IgG) from healthy controls and LN patients.
Methods We treated kidney podocytes for 24 h with LN or normal IgG and then analyzed the gene expression using a DNA microarray. The localization of IgG in podocytes was analyzed by immunofluorescence staining with or without silencing of FcRn, the receptor of IgG. In addition, we silenced CaMKIV in podocytes and we analyzed selected gene expression by real-time polymerase chain reaction. We also examined the expression of the gene CD86 in kidney podocytes of MRL/lpr, MRL/lpr.camkiv−/− and MRL/MPJ mice by in situ hybridization.
Results We found that exposure of podocytes to IgG from LN patients resulted in entry of IgG into the cytoplasm. IgG entered podocytes using the FcRn receptor because when podocytes where treated with FcRn siRNA less IgG was found in the cytoplasm. The DNA microarray studies of podocytes exposed to LN IgG revealed that genes that are related to the activation of immune cells or podocyte damage were upregulated. These genes included CD86, CaMKIV, PTPN22, PDE5A, CD47 and MALT1. Interestingly, CD86 expression decreased after silencing CaMKIV in podocytes. Also, in situ hybridization experiments showed that the expression of CD86 was reduced in podocytes from MRL/lpr.camkiv−/− mice.
Conclusions IgG from LN patients may enter podocytes through the FcRn and causes the upregulation of a distinct set of genes which may alter podocyte function. Upregulation of CaMKIV appears to precede that of genes known to be linked to podocyte damage such as CD86. Blocking of FcRN or inhibition of CaMKIV may prove of clinical use in patients with LN
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Disclosure of Interest : None declared
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