Nephrin is an essential structural component of the glomerular slit diaphragm (SD), a highly organized intercellular junction that constitutes the ultrafiltration barrier of the kidney. Recent studies have identified two additional
nephrin-interacting SD
proteins (NEPH1 and NEPH2), suggesting that the zipper-like pattern of the SD is formed through complex intra- and intermolecular interactions of these
proteins. However, the complexity of the SD structure suggests that additional SD components remain to be discovered. In this study, we identified
galectin-1 (Gal-1) as a new component of the SD, binding to the ectodomain of
nephrin. Using dual-immunofluorescence and confocal microscopy and dual-immunoelectron microscopy, we found
Gal-1 co-localizing with the ectodomain of
nephrin at the SD in normal human kidney. By immunoprecipitation and surface plasmon resonance, we confirmed a direct molecular interaction between
Gal-1 and
nephrin. Moreover, recombinant
Gal-1 induced
tyrosine phosphorylation of the cytoplasmic domain of
nephrin and activation of the
extracellular signal-regulated kinase 1/2 in podocytes. We also showed that podocytes are a major site of biosynthesis of
Gal-1 in the glomerulus and that the normal expression patterns and levels of
Gal-1 are altered in patients with
minimal change nephrotic syndrome. Finally, in
puromycin aminonucleoside-induced rat
nephrosis, an apparent reduction in the levels of
Gal-1 and
nephrin around the onset of heavy
proteinuria was also revealed. Our data present
Gal-1 as a new extracellular
ligand of
nephrin localized at the glomerular SD, and provide further insight into the complex molecular organization, interaction, and structure of the SD, which is an active site of intracellular signaling necessary for podocyte function.