title: Secreted protein acidic and rich in cysteine is a matrix scavenger chaperone.
abstract: Secreted Protein Acidic and Rich in Cysteine (SPARC) is one of the major non-structural proteins of the extracellular matrix (ECM) in remodeling tissues. The functional significance of SPARC is emphasized by its origin in the first multicellular organisms and its high degree of evolutionary conservation. Although SPARC has been shown to act as a critical modulator of ECM remodeling with profound effects on tissue physiology and architecture, no plausible molecular mechanism of its action has been proposed. In the present study, we demonstrate that SPARC mediates the disassembly and degradation of ECM networks by functioning as a matricellular chaperone. While it has low affinity to its targets inside the cells where the Ca(2+) concentrations are low, high extracellular concentrations of Ca(2+) activate binding to multiple ECM proteins, including collagens. We demonstrated that in vitro, this leads to the inhibition of collagen I fibrillogenesis and disassembly of pre-formed collagen I fibrils by SPARC at high Ca(2+) concentrations. In cell culture, exogenous SPARC was internalized by the fibroblast cells in a time- and concentration-dependent manner. Pulse-chase assay further revealed that internalized SPARC is quickly released outside the cell, demonstrating that SPARC shuttles between the cell and ECM. Fluorescently labeled collagen I, fibronectin, vitronectin, and laminin were co-internalized with SPARC by fibroblasts, and semi-quantitative Western blot showed that SPARC mediates internalization of collagen I. Using a novel 3-dimensional model of fluorescent ECM networks pre-deposited by live fibroblasts, we demonstrated that degradation of ECM depends on the chaperone activity of SPARC. These results indicate that SPARC may represent a new class of scavenger chaperones, which mediate ECM degradation, remodeling and repair by disassembling ECM networks and shuttling ECM proteins into the cell. Further understanding of this mechanism may provide insight into the pathogenesis of matrix-associated disorders and lead to the novel treatment strategies.
authors.affiliation: Department of Pediatrics, University of Chicago, Chicago, Illinois, United States of America. achlensk@uchicago.edu
authors.forename: Alexandre
authors.initials: A
authors.lastname: Chlenski
authors.forename: Lisa J
authors.initials: LJ
authors.lastname: Guerrero
authors.forename: Helen R
authors.initials: HR
authors.lastname: Salwen
authors.forename: Qiwei
authors.initials: Q
authors.lastname: Yang
authors.forename: Yufeng
authors.initials: Y
authors.lastname: Tian
authors.forename: Andres
authors.initials: A
authors.lastname: Morales La Madrid
authors.forename: Salida
authors.initials: S
authors.lastname: Mirzoeva
authors.forename: Patrice G
authors.initials: PG
authors.lastname: Bouyer
authors.forename: David
authors.initials: D
authors.lastname: Xu
authors.forename: Matthew
authors.initials: M
authors.lastname: Walker
authors.forename: Susan L
authors.initials: SL
authors.lastname: Cohn
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D003094:Collagen / Q000378:metabolism / Q000493:pharmacokinetics; D005109:Extracellular Matrix / Q000378:metabolism*; D016326:Extracellular Matrix Proteins / Q000378:metabolism*; D005110:Extracellular Space / Q000378:metabolism; D005260:Female; D005347:Fibroblasts / Q000166:cytology / Q000378:metabolism; D057809:HEK293 Cells; D006801:Humans; D042541:Intracellular Space / Q000378:metabolism; D008297:Male; D051379:Mice; D057507:Mice, 129 Strain; D018345:Mice, Knockout; D008856:Microscopy, Fluorescence; D018832:Molecular Chaperones / Q000378:metabolism*; D041681:NIH 3T3 Cells; D015676:Osteonectin / Q000235:genetics / Q000378:metabolism* / Q000493:pharmacokinetics; D021381:Protein Transport
chemical_list: D016326:Extracellular Matrix Proteins; D018832:Molecular Chaperones; D015676:Osteonectin; D003094:Collagen; D002118:Calcium