News from the dark side!

The fundamental importance of any given membrane transport protein is something that is stated repeatedly at conferences with each speaker, in turn, using it to justify their career and funding. But how ‘important’ is any particular transport protein in a physiological sense? In reality, it is not something that can be quantified easily. It is certain that some transporters are important, e.g. the Na+,K+-ATPase, because they are expressed ubiquitously. Other transport proteins have been demonstrated to be important because mutations lead to dysfunction and disease, e.g. the CFTR chloride channel, defects in which cause cystic fibrosis. Some transporters appear important because they are expressed abundantly in tissues that are highly amenable to experimentation. Other transporters have apparently become important because they have been known about for many years and have been the focus of much grant funding and many scientific publications. There are, however, some transport proteins that do not fit comfortably into any of the four categories described above. Thus, many transport systems are likely to be essential for success or survival of the organism but little is known about their physiological function. Such transport proteins might be expressed at low levels, or in a small subset of cells within a mixed cell population, or in a tissue or membrane that is simply difficult to work with. Even in today's post-genomic era there remain many transporter-like proteins that have yet to be assigned any known physiological activity. A transport system that does not fit into any of the four categories described above is system T. System T is a Na+-independent, Cl−-independent, low affinity (Km 0.5–7.0 mm for tryptophan) uniporter of the aromatic amino acids tryptophan (hence the T in the name), tyrosine and phenylalanine, plus l-DOPA and derivatives. It was described originally (and named) in a study that characterised tryptophan transport in human erythrocytes (Rosenberg et al. 1980) and its function has since been described in hepatocytes, placenta and pre-implantation conceptuses. The cDNA has been isolated from rat, human and mouse (Kim et al. 2001, 2002; Park et al. 2005; Ramadan et al. 2006) and named TAT1 (T-type amino acid transporter 1). TAT1 is most similar in amino acid sequence to the thyroid hormone transporter MCT8 and the monocarboxylate transporters MCT1–4 and is thus categorised as the tenth member of solute carrier family 16 (SLC16A10). In contrast, most of the well-characterised, plasma-membrane, mammalian amino acid transporters are found in the SLC1, 6, 7, 36, 38 and 43 families. The distribution of TAT1 mRNA varies between species, but in broad terms it is found in small intestine, colon, kidney, liver, skeletal muscle, placenta, heart, spleen, thymus, prostate and pancreas. This relatively broad tissue distribution might suggest key physiological roles, yet it is probably fair to state that, of the so called ‘classical’ amino acid transporters, system T is described in the smallest number of publications. TAT1 protein has been immunolocalised to the basolateral membrane in the small intestine, kidney (proximal tubule) and liver (Kim et al. 2001, 2002; Park et al. 2005; Ramadan et al. 2006). Our knowledge of the roles of basolateral transporters is limited somewhat due to the difficulty in measuring transport, resulting in the basolateral membrane sometimes being described as the ‘dark side’. Although limited knowledge of the membrane distribution of this carrier protein allows us to postulate some of the likely functions, the question still remains: what is it that system T does? One way to determine the physiological function of a transport system is to create a knockout rodent model and to determine the effects on whole animal physiology. In an article in this issue of The Journal of Physiology, Mariotta et al. (2012) adopt such an approach to investigate the physiological roles of system T. The study makes three key observations regarding the physiological roles of this uniporter in amino acid influx in the liver, and efflux in the kidney and small intestine. The tat1−/− mice have increased plasma aromatic amino acid levels and an associated aromatic amino acid uria. The normal levels of hepatic aromatic amino acids in the null mice coupled with the increased plasma levels identify that system T is key in controlling whole body aromatic amino acid homeostasis by mediating influx into hepatocytes (the major site of aromatic amino acid metabolism). The second key role for system T identified by Mariotta et al. (2012) is in the kidney where this carrier mediates the basolateral efflux step of aromatic amino acid reabsorption from the renal filtrate. Lack of system T activity at the proximal tubular basolateral membrane results in an increase in aromatic amino acid levels in the kidney and a major amino acid uria. Interestingly, in animals on a high protein diet, neutral amino acid substrates for the amino acid exchanger system lat2/4f2hc are also lost to urine. Thus, an additional role for system T involves functional cooperation with lat2/4f2hc to maximise amino acid reabsorption under normal circumstances. The third physiological role is observed in the gut, where increased cellular accumulation of aromatic amino acid from lumen in the proximal small intestine of tat1−/− mice demonstrates the role of system T in basolateral release of aromatic amino acids from diet. Despite aromatic amino acids being the precursors of serotonin, nicotinic acid, catecholamines and thyroid hormones, tat1−/− mice grow and reproduce normally suggesting that loss of system T alone may not cause an obvious disease phenotype. Perhaps the real ‘importance’ of system T in small intestine and kidney is that it forms part of a functional network of amino acid transporters to optimise epithelial activity (net solute transport). This complex of basolateral transporters is likely to demonstrate both cooperativity and redundancy and disease phenotypes may appear only with ageing, dietary restriction or other specific stresses. Alternatively, more subtle and complex disease phenotypes may become evident following mutation of multiple transporter genes, as observed with apical amino acid transporters in iminoglycinuria (Broer et al. 2008). The data presented in this investigation provide the first evidence for the physiological roles for the amino acid transporter system T. The importance of the basolateral membrane, and its complement of nutrient transporters, is often under-investigated and so overlooked due to the difficulty in performing the experiments. The availability of the tat1−/− mouse model provides a useful experimental tool for further investigation of the functions of this most neglected of amino acid transporters, where a role in the transepithelial transport of aromatic amino acid-related drugs in the small intestine and kidney should be explored.