Functional genomic screen identifies novel mediators of collagen uptake

Tissue fibrosis occurs when matrix production outpaces matrix degradation. Degradation of collagen, the main component of fibrotic tissue, is mediated through an ex tracellular proteolytic pathway and intracellular pathway of cellular uptake and lysosomal di gestion. Recent studies demonstrate that disruption of the intracellular pathways can exacer bate fibrosis. These pathways are poorly characterized. Here we identify novel mediators of the intracellular pathway of collagen turnover through a genome-wide RNA interference screen in Drosophila S2 cells. Screening of 7505 Drosophila genes conserved among metazo ans identified 22 genes that were required for efficient internalization of type I collagen. These included proteins involved in vesicle transport, the actin cytoskeleton, and signal trans duction. We show further that the flotillin genes have a conserved and central role in collagen uptake in Drosophila and human cells. Short hairpin RNA-mediated silencing of flotillins in human monocyte and fibroblasts impaired collagen uptake by promoting lysosomal degrada tion of the endocytic collagen receptors uPARAP/Endo180 and mannose receptor. These data provide an initial characterization of intracellular pathways of collagen turnover and identify the flotillin genes as critical regulators of this process. A better understanding of these path ways may lead to novel therapies that reduce fibrosis by increasing collagen turnover.