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

https://doi.org/10.1016/j.jbc.2022.102010Get rights and content
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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/β-catenin signaling pathway, known to promote kidney fibrosis, but not the transforming growth factor β (TGF-β), Notch, Hedgehog, or Yes-associated protein (YAP) signaling pathways in obstructed mouse kidneys, whereas inhibition of FSTL1 lowered Wnt/β-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/β-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/β-catenin pathway.

Keywords

FSTL1
Wnt
FZD
β-catenin
kidney fibrosis

Abbreviations

α-SMA
alpha-smooth muscle actin
AKI
acute kidney injury
BMP
bone morphogenetic protein
CaMKII
Ca2+/calmodulin–dependent protein kinase II
CKD
chronic kidney disease
DMEM
Dulbecco's modified Eagle's medium
EC
extracellular calcium–binding domain
FBS
fetal bovine serum
Fn
fibronectin
FS
follistatin
FSTL1
follistatin-like 1
FSTL3
follistatin-like 3
FZD
Frizzled
HA
hemagglutinin
HEK293T
human embryonic kidney 293T cell line
IMCD3
inner medullary collecting duct 3
IP
immunoprecipitation
JNK
c-Jun N-terminal kinase
LRP
low-density lipoprotein receptor–related protein
MESD
mesoderm development LRP chaperone
NRK-49F
normal rat kidney 49 fibroblast cell
PDGFRβ
platelet-derived growth factor receptor beta
SNAP-FZD4
SNAP-tagged FZD4
SPARC
secreted protein, acidic and rich in cysteine
TGF-β
transforming growth factor beta
UUO
unilateral ureteral obstruction
VWC
von Willebrand factor type C–like
YAP
Yes-associated protein

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