Aquaporin null phenotypes: the importance of classical physiology.

The biomedical research community has been quick to apply the most modern methods in molecular and cellular biology to the study of human pathological specimens. This approach has fostered the molecular understanding of human disease, while at the same time it has provided unique insight into the normal functions of individual molecules. In settings where disease phenotypes have been less informative, the development of targeted gene disruptions in mice has provided an important alternative approach, which avoids the ambiguities resulting from use of chemical inhibitors. Critical to all of these studies is the existence of precise physiological methods for studying the functions of tissues and organs, but this technology has unfortunately been neglected in recent years because of a misperception that classical physiology is not fashionable. The paper by Schnermann and colleagues (1) in this issue of the Proceedings is an excellent example of how the application of classical physiological measurements to tissues from transgenic mice successfully answered an important biological question: how is water reabsorbed by renal proximal tubules? The human kidney plays an important role in waste removal by filtering approximately 200 liters of plasma per day from which essential solutes are reabsorbed along with most of the water. Renal proximal tubules and descending thin limbs of Henle’s loop are the sites where approximately 80% of this f luid is reabsorbed. The vectorial distribution of salt and sugar transporters at the apical membranes (facing the urinary lumen) or basolateral membranes (facing the interstitium) together create a small-standing osmotic gradient across the tubular epithelium. Thus, the interstitium is slightly hyperosmolar with respect to the urinary lumen, providing the driving force for water reabsorption that is essential for the countercurrent mechanism by which urine is concentrated to osmolalities far above the plasma. It has long been debated whether water is reabsorbed through the renal proximal tubular epithelial cells (transcellular pathway) or through the spaces between cells (paracellular pathway). Fortunately, classical renal physiologists were ready when AQP1 knockout mice became available (1), and they demonstrated that this water channel protein is critical to the transcellular absorption of water by renal proximal tubules. These studies make beautiful sense because the earliest observations that AQP1 resides in apical and basolateral membranes of renal proximal tubules and descending thin limbs (2) provided the essential clue that AQP1 functions as a water transporter (3). Moreover, the abundance of AQP1 at these sites is so striking (ref. 4 and Fig. 1) that calculations predicted AQP1 would fully explain the water permeability of the proximal nephron (5). Surprisingly, the rare humans lacking the Colton blood group antigens were found to bear disrupting mutations in the AQP1 gene (6); however, none exhibited obvious signs of kidney dysfunction. This discrepancy now warrants reanalysis because the studies of kidneys from AQP1 knockout mice were found to have a marked solute concentration defect, and the studies also predicted that compensatory mechanisms will diminish the phenotype in the unstressed animals. Thus, careful water deprivation studies may be needed to uncover renal defects caused by AQP1 deficiency in humans. Interest in this problem by classical physiologists was not simply a coincidence, for investigators in the National Institutes of Health Laboratory of Kidney and Electrolyte Metabolism (LKEM), the institutional home of two authors of ref. 1, have focused their attention on renal transport processes ever since their doors were opened 50 years ago this spring. Previous studies at the LKEM contributed many fundamental discoveries in renal physiology, including development of the isolated perfused tubule by Maurice Burg and his colleagues (7), a technique essential to the analysis of AQP1 knockout mice (1). Several scientific groups are now directing their attention to the aquaporins, a large family of water transport