Renal type a intercalated cells contain albumin in organelles with aldosterone-regulated abundance.

Thomas Buus Jensen,Jeppe Praetorius,Muhammad Umar Cheema,Helle Hasager Damkier,Agata Szymiczek,Franziska Theilig

doi:10.1371/journal.pone.0124902

Abstract

Albumin has been identified in preparations of renal distal tubules and collecting ducts by mass spectrometry. This study aimed to establish whether albumin was a contaminant in those studies or actually present in the tubular cells, and if so, identify the albumin containing cells and commence exploration of the origin of the intracellular albumin. In addition to the expected proximal tubular albumin immunoreactivity, albumin was localized to mouse renal type-A intercalated cells and cells in the interstitium by three anti-albumin antibodies. Albumin did not colocalize with markers for early endosomes (EEA1), late endosomes/lysosomes (cathepsin D) or recycling endosomes (Rab11). Immuno-gold electron microscopy confirmed the presence of albumin-containing large spherical membrane associated bodies in the basal parts of intercalated cells. Message for albumin was detected in mouse renal cortex as well as in a wide variety of other tissues by RT-PCR, but was absent from isolated connecting tubules and cortical collecting ducts. Wild type I MDCK cells showed robust uptake of fluorescein-albumin from the basolateral side but not from the apical side when grown on permeable support. Only a subset of cells with low peanut agglutinin binding took up albumin. Albumin-aldosterone conjugates were also internalized from the basolateral side by MDCK cells. Aldosterone administration for 24 and 48 hours decreased albumin abundance in connecting tubules and cortical collecting ducts from mouse kidneys. We suggest that albumin is produced within the renal interstitium and taken up from the basolateral side by type-A intercalated cells by clathrin and dynamin independent pathways and speculate that the protein might act as a carrier of less water-soluble substances across the renal interstitium from the capillaries to the tubular cells.

Highlights

Albumin is a major plasma protein responsible for the oncotic pressure of the blood [1] and a carrier of substances such as free fatty acids, steroid hormones, bilirubin, and Ca2+ [2]
Three antibodies immunolocalized albumin to a subset of renal tubular cells positioned in later tubular segments
We studied the effects of chlorpromazine and dynasore on fluorescein-albumin uptake to assess whether the uptake into MDCK cells depended on clathrin-mediated endocytosis or broadly on dynamin-dependent endocytosis, respectively

Summary

Introduction

Albumin is a major plasma protein responsible for the oncotic pressure of the blood [1] and a carrier of substances such as free fatty acids, steroid hormones, bilirubin, and Ca2+ [2]. Albumin might be a contaminant it remains possible that albumin is either taken up by cells in the late DCTs, CNTs and CCDs or synthesized in these epithelial cells. The late DCT, CNT and CCD contain several different cell types. Aldosterone is partially bound to albumin and the free fraction of the hormone determines the effect on the target cells, as for other protein-bound hormones. Quantitative mass spectrometry suggested that 24-hours aldosterone administration decreased albumin abundance in the late DCT, CNT and CCD [8]. The cellular identity of the putatively albumin containing cells remains elusive, as mass spectrometry detected albumin in studies of both isolated intercalated cells [9] and non-intercalated late DCT, CNT and CCD cells [8]

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