Developing a multiscale mathematical model of protein uptake along the mouse proximal tubule

Abstract

The proximal tubule (PT) efficiently reclaims albumin and other proteins that escape the glomerular filtration barrier to maintain a protein free urine. The recovery of filtered proteins occurs via receptor-mediated endocytosis facilitated by the multiligand receptors megalin and cubilin. PT health is impacted by excess albumin uptake that occurs when the glomerular barrier is breached and by nephrotoxic drugs that enter cells in a megalin/cubilin dependent manner. Understanding where and how endocytic uptake occurs along the PT axis is key to predicting disease outcomes and to devising strategies to protect PT cells from nephrotoxic insults. Transcriptomic and proteomic studies in male rodents have demonstrated a difference in the expression of megalin and cubilin across the S1, S2, and S3 sub-segments that comprise the PT. Based on these differences in expression, we previously developed an axial model of protein uptake in mice which predicts that most uptake under normal conditions occurs in the S1 segment, with later PT segments providing excess uptake capacity under nephrotic conditions. Separately, we created a kinetic model that predicts rates of megalin trafficking rates in S1 segment cells. This model demonstrated that both the actual level and the fraction of total receptor present at the apical surface are critical determinants of endocytic capacity. To integrate our cell-based kinetic model of megalin traffic into the axial model, we used imaging-based approaches to quantify the expression and distribution of megalin and cubilin in SGLT2-positive S1 and SGLT2-negative S2 segments in fixed cortical kidney sections from wild-type C57BL/6 age-matched male and female mice. The fraction of megalin and cubilin at the apical surface was determined by colocalization with the brush border marker, villin, in regions where AP2, a marker for endocytic vesicles, was excluded. These data will be incorporated into our current axial model to develop a multiscale model of protein uptake along the length of the PT. The expanded model will improve our understanding of how endocytic capacity along the PT in vivo is regulated under normal and disease conditions and allow us to determine whether there are sex-based differences in the recovery of filtered proteins. National Institutes of Health T32 DK007052, F31 DK121394, R01 DK125049, R01 DK118726 and ASN Foundation for Kidney Research Pre-Doctoral Fellowship Award This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.