Abstract
Glomerular parietal epithelial cell (PEC) activation, as revealed by de novo expression of CD44 and cell migration toward the injured filtration barrier, is a hallmark of podocyte injury-driven focal segmental glomerulosclerosis (FSGS). However, the signaling pathway that mediates activation of PECs in response to podocyte injury is unknown. The present study focused on CD44 signaling, particularly the roles of two CD44-related chemokines, migration inhibitory factor (MIF) and stromal cell-derived factor 1 (SDF1), and their common receptor, chemokine (C-X-C motif) receptor 4 (CXCR4), in the NEP25/LMB2 mouse podocyte-toxin model of FSGS. In the early phase of the disease, CD44-positive PECs were locally evident on the opposite side of the intact glomerular tuft and subsequently increased in the vicinity of synechiae with podocyte loss. Expression of MIF and SDF1 was first increased in injured podocytes and subsequently transferred to activated PECs expressing CD44 and CXCR4. In an immortalized mouse PEC (mPEC) line, recombinant MIF and SDF1 (rMIF and rSDF1, respectively) individually increased CD44 and CXCR4 mRNA and protein levels. rMIF and rSDF1 stimulated endogenous MIF and SDF1 production. rMIF- and rSDF1-induced mPEC migration was suppressed by CD44 siRNA. However, MIF and SDF1 inhibitors failed to show any impact on proteinuria, podocyte number, and CD44 expression in NEP25/LMB2 mice. Our data suggest that injured podocytes upregulate MIF and SDF1 that stimulate CD44 expression and CD44-mediated migration, which is enhanced by endogenous MIF and SDF1 in PECs. This biphasic expression pattern of the chemokine-CD44 axis in podocytes and PECs may be a novel mechanism of "podocyte-PEC cross-talk" signaling underlying podocyte injury-driven FSGS.