Abstract 653: The Role of "Saposin-Like Domain" in Wnt Signaling

Abstract

Hyperlipidemia is a major contributor to atherosclerosis that results in endothelial dysfunction and is driven by several cell types including macrophages and vascular smooth muscle cells. Among the various cytokines and signaling molecules that contribute to the disease, dysregulated Wnt signaling is known to result in uncontrolled cell division within bulging aortic plaques. Wnt proteins are morphogens that signal through interactions with two membrane bound co-receptors: Frizzled and Low-density lipoprotein receptor related protein 5/6. Wnt binding to these receptors transduces a signal, leading to increased cellular levels of ß-catenin that migrates to the nucleus and leads to gene activation. Normally ß-catenin is rapidly degraded, a process that attenuates Wnt signaling. However, in many diseases, including atherosclerosis, ß-catenin levels remain elevated leading to uncontrolled activation of the Wnt pathway genes. Interestingly, while the intracellular events of the Wnt signaling pathway are well studied, questions about how a hydrophobic molecule like Wnt engages with cell surface receptors remain unexplored in the field. A recent X-ray crystal structure revealed Wnt exists as a two-domain protein: a C-terminal “cytokine like domain” and an N-terminal “saposin-like domain” that contains a covalently bound fatty acid. Saposins are a family of proteins that possess membrane / lipid binding properties and undergo a conformational change when they contact membrane surfaces. My hypothesis is that the saposin-like domain is a membrane active component that plays a critical role in initiating Wnt signaling by undergoing a conformational change upon reaching cell membrane in order to promote Wnt-receptor engagement and signaling. I plan to isolate and express the saposin-like domain, which will exist as a water-soluble helical bundle in the absence of lipid. When presented with an appropriate lipid surface, the saposin-like domain will unfurl, thereby exposing a hydrophobic interior that contacts the membrane surface. The ability of the isolated domain to interfere with Wnt signaling will also be determined. Findings from my study reveal the molecular basis underlying initial events of Wnt receptor interaction in atherosclerosis.