Follistatin-like 1 (FSTL1) interacts with Wnt ligands and Frizzled receptors to enhance Wnt/ß-catenin signaling in obstructed kidneys in vivo.

Follistatin (FS)-like 1 (FSTL1) is a member of the FS-SPARC (secreted protein, acidic and rich in cysteine) family of secreted and extracellular matrix proteins. The functions of FSTL1 have been studied in heart and lung injury as well as in wound healing; however, the role of FSTL1 in the kidney is largely unknown. Here, we show using single-cell RNA-Seq that Fstl1 was enriched in stromal cells in obstructed mouse kidneys. In addition, immunofluorescence demonstrated that FSTL1 expression was induced in fibroblasts during kidney fibrogenesis in mice and human patients. We demonstrate that FSTL1 overexpression increased renal fibrosis and activated the Wnt/ß-catenin signaling pathway, known to promote kidney fibrosis, but not the transforming growth factor ß (TGF-ß), Notch, Hedgehog, or Yes-associated protein (YAP) signaling pathways in obstructed mouse kidneys, whereas inhibition of FSTL1 lowered Wnt/ß-catenin signaling. Importantly, we show that FSTL1 interacted with Wnt ligands and the Frizzled (FZD) receptors but not the coreceptor lipoprotein receptor-related protein 6 (LRP6). Specifically, we found FSTL1 interacted with Wnt3a through its extracellular calcium-binding (EC) domain and von Willebrand factor type C-like (VWC) domain, and with FZD4 through its EC domain. Furthermore, we show that FSTL1 increased the association of Wnt3a with FZD4 and promoted Wnt/ß-catenin signaling and fibrogenesis. The EC domain interacting with both Wnt3a and FZD4 also enhanced Wnt3a signaling. Therefore, we conclude that FSTL1 is a novel extracellular enhancer of the Wnt/ß-catenin pathway.

Structural characterization of TSC-36/Flik: analysis of two charge isoforms.

Recombinant forms of the glycoprotein TSC-36/Flik were expressed in human cells and used to compare their structural and functional properties with those described for other members of the BM-40/SPARC/osteonectin protein family. TSC-36 was found to occur in two charge isoforms that differ in the extent of sialylation of otherwise identical N-linked, complex type oligosaccharides. Conformational analysis with both circular dichroism and intrinsic fluorescence spectroscopy showed a lack of significant structural changes upon calcium addition or depletion. This finding is in contrast to results obtained for several other BM-40 family members and indicates that the extracellular calcium-binding domain in TSC-36 is non-functional. The lack of conservation of important functional features common to several other members of the BM-40 family indicates that TSC-36, despite its sequence homology to BM-40, has evolved clearly distinct properties.

SC1/hevin. An extracellular calcium-modulated protein that binds collagen I.

SC1, a member of the BM-40 family of extracellular matrix proteins, was recombinantly expressed in a eukaryotic expression system. The full-length protein as well as truncated versions were purified to homogeneity under non-denaturing conditions. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry of full-length SC1 revealed a mass of 87.8 kDa of which 16.8 kDa is contributed by posttranslational modifications. In electron microscopy, after negative staining, SC1 was revealed as a globule attached to a thread-like structure. A calcium dependence of the SC1 conformation could be demonstrated by fluorescence spectroscopy. In the extracellular matrix of cultured osteosarcoma cells SC1 was found associated with collagen I-containing fibrils, and binding of SC1 to reconstituted collagen I fibrils could be demonstrated by immunogold labeling and electron microscopy. SC1 showed a broad expression in a variety of tissues.