Is caveolin involved in normal proximal tubule function? Presence in model PT systems but absence in situ

Abstract

Zhuang Z, Marshansky V, Breton S, Brown D. Is caveolin involved in normal proximal tubule function? Presence in model PT systems but absence in situ. Am J Physiol Renal Physiol 300: F199-F206, 2011. First published October 27, 2010 doi:10.1152/ajprenal.00513.2010.-Kidney proximal tubule (PT) cells are specialized for the uptake and transport of ions, solutes, peptides, and proteins. These functions are often regulated by hormones that signal at the cell surface and are internalized by clathrin-mediated endocytosis. However, the caveolin/caveolae pathway has also been implicated in normal PT function, often based on data from isolated PTs or PT cells in culture. Although we reported previously that caveolae and caveolin 1 are not detectable in PTs in vivo, reports of caveolin expression and function in PT cells appear periodically in the literature. Therefore, we reexamined caveolin expression in PTs in vivo, in isolated "purified" PTs following collagenase digestion, and in cultured PT cells. Caveolin 1 and 2 protein, mRNA, or immunofluorescence was undetectable in PTs in vivo, but PT cell cultures expressed caveolin 1 and/or 2. Furthermore, caveolin 1 and 2 mRNAs were detected in isolated PTs along with the endothelial markers CD31 and ICAM1. In contrast, no caveolin or endothelial marker mRNAs were detectable in samples isolated from snap-frozen kidneys by laser cut microdissection, which eliminates contamination by other cell types. We conclude 1) caveolin 1 and 2 are not normally expressed by PT cells in situ, 2) caveolin expression is "activated" in cultured PT cells, 3) contamination with non-PT, caveolin-expressing cells is a potential source of caveolin 1 and 2 that must be taken into account when isolated PTs are used in studies to correlate expression of these proteins with PT function. kidney; cell culture; isolated tubules; endocytosis RENAL PROXIMAL TUBULE (PT) cells are highly specialized for the bulk reabsorption of many ions, solutes, and smaller proteins, including receptor ligands and hormones, which penetrate the glomerular filtration barrier and appear in the tubule lumen. Furthermore, PT cells respond to circulating hormones and have distinct transport properties associated with their apical and basolateral membranes. As such, they display a very distinct polarized expression of many channels, transporters, enzymes, and receptors that all contribute toward their physiological function (4 -6, 8, 14, 41, 50). As part of their vectorial transporting and reabsorptive activity, many membrane proteins undergo extensive constitutive and regulated trafficking and recycling (8). In the PT, the endocytotic arm of the trafficking pathway is intimately related to a variety of specialized functions including the reabsorption of filtered molecules from the luminal fluid, as well as downregulation of membrane transporters such as NaPi1a in response to activation of PTH receptors (5), and downregulation of angiotensin II receptors (56). Plasma membrane hormone receptors themselves also undergo endocytosis ...

Vasopressin induces apical expression of caveolin in rat kidney collecting duct principal cells

“…We then examined subcellular Cav1 localization and unexpectedly found that DDAVP treatment profoundly affects Cav1 localization in PCs from BB rats. In untreated animals, Cav1 localization was exclusively associated with the basolateral plasma membrane of PCs, as previously described for SD rats (5,76). However, acute (1 day) exposure to DDAVP induced the appearance of small, scattered spots of Cav1 staining associated with the apical membrane in a number of PCs in CDs from both the IM (Fig.…”

“…In the SD rat kidney, we showed that Cav1 is expressed at the basolateral but not apical membrane of CD PCs, distal convoluted tubule, and connecting segment cells. Cav1 was not detected in intercalated cells of the renal CD or in proximal tubules (5,76 revealed a similar localization pattern in BB rats (Fig. 1) and LE rats (data not shown).…”

“…Cav1 was not detected in intercalated cells of the renal CD or in proximal tubules, where caveolae do not occur either (5,76). Several proposed functions of Cav1 and caveolae in the proximal tubule may need to be reassessed, because they are based almost exclusively on data from proximal tubule cells in culture, which, in contrast to normal PT cells in situ, do show Cav1 expression (76). Cav3 is not expressed at detectable levels in renal epithelia (5).…”

“…As previously reported, there is a relationship between Cav1 and Cav2, given that the latter was found to undergo degradation in the absence of Cav1 (52). Cav1 was not detected in intercalated cells of the renal CD or in proximal tubules, where caveolae do not occur either (5,76). Several proposed functions of Cav1 and caveolae in the proximal tubule may need to be reassessed, because they are based almost exclusively on data from proximal tubule cells in culture, which, in contrast to normal PT cells in situ, do show Cav1 expression (76).…”

“…They have also been implicated in triglyceride homeostasis (36,51), tumorigenesis/tumor supression (13,18,28), apoptosis (33) and cell cycle regulation (19,24) as well as in signal transduction, as Cav1 has been reported to interact with and affect proteins that are crucial for various signaling pathways, such as heterotrimeric G proteins, protein kinases, integrins, and a number of receptors (EGF, insulin, platelet-derived growth factor, p75 nerve growth factor, and transforming growth factor-␤) (11,29,52,63). Caveolae and Cav1, thus, constitute a major endocytotic mecha-nism, the main alternative to clathrin-mediated endocytosis (20,37,70,76).…”