Abstract
The vasopressin-regulated urea transporter UT-A1, expressed in kidney inner medullary collecting duct (IMCD) epithelial cells, plays a critical role in the urinary concentrating mechanisms. As a membrane protein, the function of UT-A1 transport activity relies on its presence in the plasma membrane. Therefore, UT-A1 successfully trafficking to the apical membrane of the polarized epithelial cells is crucial for the regulation of urea transport. This review summarizes the research progress of UT-A1 regulation over the past few years, specifically on the regulation of UT-A1 membrane trafficking by lipid rafts, N-linked glycosylation and a group of accessory proteins.
Highlights
Urea is the major end product of amino acid metabolism
The vasopressin-mediated urea transporter (UT)-A1 trafficking to the plasma membrane is crucial for the regulation of UT-A1 activity
It may share some common mechanisms of protein membrane trafficking, recent studies reveal that UT-A1 trafficking may have its own unique regulation
Summary
Urea is the major end product of amino acid metabolism. It is generated from the ornithine cycle in liver, and is excreted by the kidney representing 90% of total nitrogen in urine. Urea reabsorbed in the kidney inner medullary collecting duct (IMCD) contributes to the development of the osmolality in the medullary interstitium. The osmotic gradient, along the corticomedullary axis in the kidney, allows the water and important solutes to be reabsorbed from the renal tubules back into the interstitium and, thereby, generates concentrated urine. UT-A1 is the largest protein isoform and is expressed in the apical plasma membrane of the IMCD [5]. Vasopressin increases kidney urea transport activity by increasing UT-A1 in the apical plasma membrane [10] and UT-A1 phosphorylation [8,11]. Like all transporters at the plasma membrane, UT-A1 function is highly dependent on its membrane trafficking and recent discoveries indicate that lipid rafts, N-linked glycosylation and interacting proteins (such as caveolin, snapin and actin) play key roles in this process
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