Virginie Ory

Idiopathic nephrotic syndrome comprises several podocyte diseases of unknown origin that affect the glomerular podocyte, which controls the permeability of the filtration barrier in the kidney to proteins. It is characterized by the daily loss of more than 3 g of protein in urine and the lack of inflammatory lesions or cell infiltration. We found that the abundance of c-mip (c-maf inducing protein) was increased in the podocytes of patients with various acquired idiopathic nephrotic syndromes in which the podocyte is the main target of injury. Mice engineered to have excessive c-mip in podocytes developed proteinuria without morphological alterations, inflammatory lesions, or cell infiltration. Excessive c-mip blocked podocyte signaling by preventing the interaction of the slit diaphragm transmembrane protein nephrin with the tyrosine kinase Fyn, thereby decreasing phosphorylation of nephrin in vitro and in vivo. Moreover, c-mip inhibited interactions between Fyn and the cytoskeletal regulator N-WASP (neural Wiskott-Aldrich syndrome protein) and between the adaptor protein Nck and nephrin, potentially accounting for cytoskeletal disorganization and the effacement of foot processes seen in idiopathic nephrotic syndromes. The intravenous injection of small interfering RNA targeting c-mip prevented lipopolysaccharide-induced proteinuria in mice. Together, these results identify c-mip as a key component in the molecular pathogenesis of acquired podocyte diseases.

It is currently considered that idiopathic minimal change nephrotic syndrome is an immune-mediated glomerular disease. Its association with classical Hodgkin lymphoma minimal change nephrotic syndrome (cHL-MCNS) suggests a molecular link, which remains to be elucidated. We analyzed the expression of cmaf inducing protein (c-mip) in lymphomatous tissues and kidney biopsy samples of patients with cHL-MCNS (n = 8) and in lymphomatous tissues of patients with isolated cHL (n = 9). Because c-mip affects the regulatory loop involving Fyn, we investigated possible structural defects in this signaling pathway, using laser capture microdissection, reverse transcription polymerase chain reaction, and Western blotting. We found that c-mip was selectively expressed in Hodgkin and Reed-Sternberg (HRS) cells and podocytes of patients with cHL-MCNS but is undetectable in patients with isolated cHL. We demonstrated that c-mip was specifically involved in the negative regulation of early proximal signaling through its interaction with phosphoprotein associated with glycosphingolipid-enriched microdomains and Fyn. We showed that the up-regulation of c-mip in cHL-MCNS was associated with a possible Fyn defect in HRS cells and podocytes. Moreover, we showed that c-mip was up-regulated in Fyn-deficient podocytes. c-mip may be a useful marker of cHL-MCNS and its induction reflects the dysregulation of proximal signaling.