Amino Acids: Basolateral efflux and extracellular homeostasis control in vivo

In all living organisms, amino acids (AA) are essential building blocks, metabolites and signaling molecules. To reach their site of action AA need to pass through polar epithelial cells. Because AA cannot freely diffuse through the cell membrane, specific AA carriers ensure their transport across the apical and the basolateral membrane. Despite numerous flux studies and huge progress in the identification and characterization of those carrier proteins, AA transport across the basolateral membrane remains not fully understood. The best characterized basolateral transporters in the small intestine and kidney proximal tubule are the obligatory exchanger (antiporter) Lat2-4F2hc (Slc7a8) and y+Lat1-4F2hc (Slc7a7). Both require the presence of a "one-way transport" (uniporter) to achieve a net AA efflux. TAT1 (Slc16a10) and LAT4 (Slc43a2) fulfill this requirement with different substrate selectivities: TAT1 facilitates the diffusion of aromatic AA, whereas Lat4 transports branched chain AA, aromatic AA, Met and Pro. The functional cooperation between TAT1 and Lat2-4F2hc has previously been shown using the X. laevis oocyte expression system (Ramadan, Camargo et al. 2007). Furthermore, Lat4 could substitute for TAT1 function. Indeed, TAT1 shows the same localization as LAT2-4F2hc in epithelial cells and is further present in muscle sarcolemma and perivenous hepatocytes. The localization of Lat4 is still unknown, whereas the expression of the two uniporters overlaps only partially. The question addressed in this dissertation is: what is the function of TAT1 and Lat4 in vivo? Using global knock out mouse models we have examined the effect of the absence of the two uniporters on the AA homeostasis and epithelial transport. By inducing specific aminoaciduria in TAT1 defective mice (tat1-/-) under high protein diet, we could confirm the functional collaboration between TAT1 and LAT2-4F2hc in vivo. Furthermore, tat1-/- showed elevated aromatic AA in plasma, skeletal muscles and kidney, but not in liver. Thus, the absence of TAT1 prevents the AA balancing between plasma and liver and the consequent regulation of AA homeostasis. By means of everted gut sacs and microSPECT/CT imaging, we could show that the impairement of AA epithelial transport was caused by an intracellular accumulation of the AA substrates. The mild phenotype of tat1-/- strongly suggests a compensatory role of Lat4, which is also functionally redundant in the oocyte. In fact, Lat4 defective mice (lat4-/-) prematurely died within the first five days of life. After birth, the normal size, skin color and behavior of lat4-/- pups excluded possible prenatal impairments. Normal suckling behavior further excludes possible severe neurological disability. The reduced growth recorded between 24 and 48 hours indicated a disorder in food intake or in the reabsorption. Further investigations should aim to decipher the cause of death. In conclusion, this study analyzes the function of two basolateral uniporters for the first time in vivo. We show that the basolateral transport machinery is still functional in the absence of TAT1, whereas it is impaired in the absence of Lat4. Furthermore, we have shown a new important role of TAT1 in the regulation of the aromatic AA concentrations by the coupling of liver cells and blood plasma. Further research should aim to better characterize the in vivo interplay between AA transporters by the study of combinatorial knock out mice. Finally, the results of the different protein rich diets and their influence on the urinary pattern open new questions about the regulation of AA transporters in vivo.