Function of Exchangeable Apolipoproteins
Ryan Research Program Summary
It is postulated that this feature may influences apoE4 interaction with the amyloid ß peptide, sensitivity to proteolysis or its lipid accrual and receptor binding activities. In other studies the ability of apoE to modulate the signaling morphogen ”Wnt” is under investigation. ApoE binding to the Wnt co-receptor LDLR-related protein 5/6 (LRP 5/6) is under investigation using culture cells and surface plasmon resonance spectroscopy.
In project 2 we are exploring the mechanism whereby the low abundance apolipoprotein, apoA-V, modulates plasma triacylglycerol levels. Structure-function and site directed mutagenesis experiments have been used to show that apoA-V interacts with heparan sulfate proteglycans (HSPG). Other experiments indicate apoA-V function intracellularly to modulate triglyceride trafficking. Current research efforts are designed to test the hypothesis that apoA-V expression drives cytosolic lipid droplet assembly at the expense of lipoprotein secretion (Figure 2).
In the plasma compartment, despite its exceedingly low concentration, apoA-V exerts a profound effect on triglyceride homeostasis. Results show that, as a component of triglyceride rich lipoproteins, apoA-V binds to glycosylphosphatidylinositol high density lipoprotein binding protein 1, thereby enhancing lipoprotein lipase mediated lipolysis of lipoprotein associated triacylglycerol.
In project 3 we are investigating the ability of apolipoproteins to solubilize phospholipid dispersions, generating a homogeneous population of water-soluble, nanometer scale lipid particles, termed nanodisks. One goal of this research is to incorporate hydrophobic biomolecules and employ the resulting particles as water-soluble transport / delivery vehicles. Recent success incorporating the polyene antibiotic amphotericin B, the isoprenoid all trans retinoic acid and the polyphenol, curcumin, illustrate the potential utility of these complexes.
Cell culture and in vivo studies in mice have revealed that nanodisk-associated biomolecules retain their biological activity and can be targeted to cell surface receptors via their intrinsically associated apolipoprotein scaffold component. An example of the potential utility of nanodisk technology relates to Barth Syndrome.
Barth Syndrome (BTHS) is an X-linked recessive disease manifest in young boys that results from mutations in the TAZ (tafazzin) gene locus. The TAZ gene product is a phospholipid transacylase that functions in remodeling cardiolipin molecular species. Cardiolipin is a specialized phospholipid that has unique structural properties and a subcellular location that is largely confined to the inner membrane of mitochondria. Cardiolipin serves a structural role in this membrane where is provides an optimal environment for proteins involved in the electron transport chain. These proteins function in ATP production and Barth Syndrome patients present with ultrastructural changes to mitochondria and metabolic abnormalities consistent with disrupted oxidative metabolism. We hypothesize that exogenous cardiolipin delivered to a TAZ knockdown HL60 cell culture model of BTHS will restore these cells to a normal phenotype. We predict that cardiolipin will be transported to mitochondria and utilized in lieu of de novo synthesized / remodeled cardiolipin. The ultimate goal of this project is to develop a treatment strategy based on administration of a water-soluble cardiolipin /protein formulation to BTHS patients as a means to bypass defective tafazzin enzyme activity. Overall, this concept may be considered lipid replacement therapy and proposed studies are designed to determine if cardiolipin delivery can ameliorate characteristic phenotypic features of this disease.
Friday, May 9, 2014 3:31 PM