Aquaporin 2 (AQP2) is a water channel, expressed in kidney collecting ducts (CD). It contributes importantly to water homeostasis in mammals. Considerable progress from our lab and others has been made in understanding the molecular basis of "conventional" AQP2 water channel trafficking. Interesting observations from AQP2 knock out animals have revealed the presence of severe tubular defects, renal failure and neonatal mortality in addition to the expected polyuria/diabetes insipidus (DI). Tubular abnormalities and early mortality have, however, not been observed in other AQP knockout animals that also have polyuria even to a similar degree. In addition, aberrant expression of AQP2 has been frequently reported in a variety of transgenic animal models and in patients with polycystic kidney diseases with polyuria and coexisting structural and functional defects of the kidney tubules. Therefore, we hypothesize that AQP2 is important for maintaining the structural and functional integrity of kidney tubules besides regulating and maintaining water balance. We have generated compelling evidence that AQP2 is, unexpectedly, not just a water channel, but is also an integrin binding membrane protein that promotes cell migration. We have identified a conserved extracellular Arg-Gly-Asp (RGD) integrin-binding motif in AQP2, through which it interacts with integrin ?1, modulates the trafficking and turnover of integrin ?1 at focal adhesions (FAs). Uncoupling of the AQP2/integrin interaction leads to reduced endocytosis, surface retention of integrin ?1 and defective cell migration and tubule formation in vitro. We wil further characterize the interaction of AQP2 and integrin during vesicular trafficking and its contribution to the turnover of focal adhesions in migrating cells (Aim I);In addition, we will examine regulatory signal(s) that mediates polarized trafficking of AQP2 to the leading edge, providing a biochemical basis of AQP2's promigratory function (Aim 2). These studies incorporate a wide battery of approaches including subcellular fractionation, FRAP (fluorescence recovery after photo bleaching), TIRF (total internal reflection fluorescence), and live cell confocal fluorescence imaging to examine dynamic trafficking of integrin ?1 and AQP2. Finally, we will apply multiple model systems including embryonic kidney organ culture, transgenic zebrafish model and AQP2 knockout animals to investigate the functional significance of the AQP2/integrin interaction (Aim 3). Our hypothesis is that the AQP2/integrin interaction plays a role in mediating cell migration, and in turn contributes to kidney tubulogenesis and tubular remodeling after injury. Our studies, therefore, are designed to explore and characterize an unexpected novel mechanism mediated by AQP2 that contributes to renal epithelial cell migration and tubular repair, and to understand the critical role of AQP2 n maintaining kidney tubular structure and function.
Maintaining normal kidney structure and function is critical for sustaining life in vertebral animals. We have proposed a study of a previously unrecognized function of a water channel protein, aquaporin 2 (AQP2) on cell migration and kidney tubule repair after injury. Our study will provide a mechanistic insight into the pathophysiology of kidne diseases.