Reabsorption of protein and other filtered macromolecules via the processes of receptor-mediated and nonspecific fluid-phase endocytosis is a key function of the kidney proximal tubule. Now, Andrew Hall and colleagues report that the different segments of the proximal tubule have specialized endocytotic functions.

“Very little is known about the workings of the proximal tubule endolysosomal system in vivo,” says Hall. “We had noticed subtle differences in endolysosomal system ultrastructure between the S1 and S2 segments of the proximal tubule using electron microscopy and wondered if these differences had functional significance.”

To visualize proximal tubule endocytosis in real time in live mice, the researchers used multiphoton imaging of intravenously injected fluorescently labelled ligands. They also used a tissue clearing method to visualize solute uptake patterns in 3D in large sections of kidney cortex. These analyses showed that uptake of lysozyme and albumin by receptor-mediated endocytosis occurred almost exclusively in S1, whereas fluid-phase endocytosis of dextran occurred in both S1 and S2. However, solute uptake length and proximal tubule morphology were highly variable throughout the kidney. Quantitative PCR and immunostaining confirmed that with the exception of megalin, which binds low molecular weight proteins and albumin, the expression of endolysosomal proteins was much greater in S1 than in S2. Megalin expression was similar in both segments of the proximal tubule.

The researchers conclude that the S1 segment is highly specialized for receptor-mediated endocytosis. “Our findings add to the growing evidence from live imaging studies that S1 and S2 are functionally discrete entities — something that was previously not well appreciated because they appear to be so similar in routine histology,” comments Hall. “They also help to explain topographical patterns of kidney injury in response to endocytosed toxins and suggest that patients with tubular proteinuria most likely have a defect in S1.”