The role of transport proteins for the renal homoeostasis of arginine derivatives

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Proximal tubule cells of the kidney are able to discriminate between structurally and chemically very similar substances leading to selective accumulation or depletion in urine or plasma. For many substances the underlying molecular mechanisms are poorly investigated and understood, so far. Preliminary investigations performed in our laboratory demonstrated, that substances necessary for the body such as the semiessential amino acid L-arginine or its derivative homoarginine are net reabsorbed in the kidney whereas the structurally very similar uremic toxins asymmetric and symmetric dimethylarginine (ADMA and SDMA) appear not to be reabsorbed in a quantitative manner. In Patients with chronic kidney disease ADMA and SDMA accumulate whereas the plasma concentration of homoarginine decreases. Elevated ADMA and SDMA plasma concentrations as well as decreased plasma concentrations of homoarginine are associated with an increased total mortality. It is assumed that especially transport proteins located in the different membrane domains of proximal tubule cells are responsible for the selective reabsorption or secretion. Especially in the case of of chemically similar substances with opposing biological function a better understanding of the underlying mechanisms could aid the search for new therapeutic approaches as well as for the understanding of renally mediated side effects of drugs.Therefore, it is the aim of the proposed project to study the cellular uptake and the vectorial transport of L-arginine and its derivatives using single- and multiple-transfected cell models with recombinant expression of transport proteins expressed in proximal tubule cells. In addition, we aim to analyze whether these transport processes can be modified by drugs, in order to identify effects of potential therapeutic use as well as new mechanisms mediating adverse drug effects. By application of targeted metabolomics and “metabolic-flux” analyses in conditionally immortalized proximal tubular cells (ciPTEC cells), cultured in a 3D cell model, we further aim to analyze the interplay of transport and metabolism in the renal handling of different L-arginine derivatives. Analyses of mRNA and protein expression of relevant proteins involved in transport and metabolism will be conducted in human kidney biopsy samples to study changes in expression during the progression of chronic kidney diseases. These data may also help to identify the mechanisms underlying the paradox decline in plasma homoarginine observed in patients with chronic kidney disease. Taken together, the data obtained are expected to allow the identification of key mechanisms involved in the differential handling of chemically similar substances by the kidney. Moreover, it is aimed to gain data whether these processes can be modified by drugs in a potentially beneficial way, which may have implications for the treatment of patients with chronic kidney disease.