Figure 4.

Protein uptake occurs in regions of proximal tubules with high expression of endolysosomal proteins. (A) One hour post intravenous injection of fluorescently labeled lysozyme, kidneys were perfusion fixed and stained for endolysosomal markers and megalin. Lysozyme (magenta in overlay) was visible in the subapical region of early (#) PCT cells expressing high levels of Rab7 and Lamp1. No uptake was detected in late (*) PCT segments, where expression levels of these markers were low, in spite of a high expression of megalin. Early segments could be observed directly leaving the glomerulus (G), thus confirming their identity. (B) Immuno-staining for the endogenous protein retinol-binding protein (RBP4, magenta in overlay) also revealed an early S1 localization. Scale bars, 10 μm.