MECHANISMS OF RETINOID DELIVERY TO TISSUES

The studies proposed in this application are designed to investigate vitamin A (retinoid) storage, metabolism, transport and nutriture in a unique strain of mice (TTR-) which totally lacks immunoreactive transthyretin (TTR). In the circulation, TTR forms a 1 to 1 protein- protein complex with retinol-binding protein (RBP), the sole plasma transport protein for retinol. The formation of the RBP-TTR complex has been postulated to prevent the glomerular filtration and renal catabolism of RBP. Indeed, the plasma levels of both retinol and RBP in the TTR- mice are less than 6% of those measured in wild type mice. Such low plasma levels of retinol and RBP are observed in vitamin A deficiency, in animals which show the clinical symptoms of deficiency and which are within one to two weeks of death. Yet the TTR mice are viable, fertile, and phenotypically normal. We propose to use this unique animal model, which was developed using the forefront molecular genetics approach of targeted gene disruption, to carry out systematic and indepth studies of retinoid physiology. The overall goal of our studies is to gain an understanding of retinoid transport and metabolism in TTR- mice and to explore the possible existence of alternative mechanisms for retinoid delivery to tissues other than through the RBP-TTR complex. Our findings will, by inference, provide great insight into the normal storage, metabolism and transport of retinoids. In Aim l of our studies will compare retinoid storage, metabolism, and transport in inbred and outbred strains of TTR- mice. These studies are important for understanding the possible effects of genetic background on retinoid physiology in the TTR-deficient phenotype. Aim 2 of the proposal is designed to explore tissue retinoid levels in TTR- mice and to investigate the turnover rates of retinol and RBP in these animals. These studies will define plasma retinol and RBP transport and metabolism in the TTR- mice. The effects of dietary vitamin A deficiency and excess on TTR- mice will be examined in Aim 3. These nutritional studies are designed to provide new insights into the possible effects of plasma RBP levels on hypovitaminosis and hypervitimosis A. The contribution which dietary (chylomicron) retinyl esters make towards meeting tissue requirements for retinoids in TTR- and wild type mice will be investigated in Aim 4. The data obtained form these studies will define whether dietary retinoid can serve as a significant and direct source of retinoid for meeting tissue needs. Finally, Aim 5 will explore RBP synthesis and secretion by hepatocytes from TTR- and wild type mice. From these studies we will conclusively determine whether the presence of intracellular TTR plays an important role in mediating and/or regulating RBP synthesis and secretion from hepatocytes.