PROJECT SUMMARY The hypothesis for aim 1 is that resident sprocs are the source of liver sinusoidal endothelial cells (LSECs) during normal turnover and that resident sprocs reside in a stem cell niche that promotes their quiescence. Aim 1 will use lineage-tracing to determine whether LSECs are derived from resident sprocs, will characterize LSECs and resident sprocs in-depth to identify cell differentiation markers and lineage markers to determine whether LSECs share common origins with other liver cells, will identify signaling pathways known to respond to known stem cell niche ligands, and will characterize the various ligands and cellular elements that compose the stem cell niche for sprocs. The hypothesis for aim 2 is that changes particular to the metabolic syndrome suppress the NO pathway in LSECs and induce capillarization. This aim will characterize the signaling in NAFLD in isolated LSECs in vitro and in LSECs isolated from NAFLD ex vivo. The hypothesis for aim 3 is that the impaired endocytotic function of BM-derived (“capillarized”) LSECs contributes to aberrant clearance of lipids in NAFLD. Aim 3 will examine uptake of lipids in LSECs taken from rats with NAFLD and will use intravital multiphoton fluorescence confocal microscopy to examine lipid uptake in vivo in rats with NAFLD. Interventions will be used to induce maturation of BM-derived LSECs and studies will examine the effect of this on in vitro and in vivo uptake of lipid. Finally the effect of inducing maturation of BM- derived LSECs on serum lipid level will be compared with the effect of a statin. Collectively, these aims will provide a major advance in our understanding of the role of resident sprocs in liver physiology, which will be critical for future approaches to elicit a greater contribution from resident sprocs to the repair of liver injury and to drive liver regeneration. These studies also have the potential to uncover the mechanisms leading to capillarization in NAFLD as a prelude to providing therapeutic strategies to prevent two consequences of capillarization: the contribution of capillarization to hyperlipidemia and the loss of the ability to suppress hepatic stellate cell activation. Finally, these studies should uncover an important mechanism that contributes to elevated lipid levels in NAFLD with its attendant increased risk of cardiovascular mortality and that may lead to novel approaches to treat this aspect of hyperlipidemia.