ROR2 regulates self-renewal and maintenance of hair follicle stem cells (original) (raw)

2022, Nature Communications

Hair follicles undergo cycles of regeneration fueled by hair follicle stem cells (HFSCs). While β-catenin-dependent canonical Wnt signaling has been extensively studied and implicated in HFSC activation and fate determination, very little is known about the function of β-catenin-independent Wnt signaling in HFSCs. In this study, we investigate the functional role of ROR2, a Wnt receptor, in HFSCs. By analyzing Ror2-depleted HFSCs, we uncover that ROR2 is not only essential to regulate Wnt-activated signaling that is responsible for HFSC activation and self-renewal, but it is also required to maintain proper ATM/ATR-dependent DNA damage response, which is indispensable for the long-term maintenance of HFSCs. In analyzing HFSCs lacking β-catenin, we identify a compensatory role of ROR2-PKC signaling in protecting β-cateninnull HFSCs from the loss of stem cell pool. Collectively, our study unveils a previously unrecognized role of ROR2 in regulation of stem cell self-renewal and maintenance. In mammals, Wnt signaling functions in tissue morphogenesis, stem cell activation, and tumor development 1,2. The binding of secreted Wnt ligands to receptors and/or co-receptors initiates diverse signaling cascades that can be divided into β-catenin-dependent canonical and β-catenin-independent non-canonical Wnt signaling pathways 1. These pathways might act independently or cooperatively to orchestrate various cellular functions. Canonical Wnt signaling (referred as Wnt/β-catenin signaling) is activated when a Wnt ligand binds to Frizzled (Fzd) and LRP-5/6, which triggers activation of Dishevelled (Dvl) proteins, leading to the inhibition of the destruction complex, composed by Axin, caseine kinase 1α (CK1α), adenomatous polyposis coli (APC) and glycogen synthase kinase 3β (GSK3β), thereby stabilizing β-catenin 3,4. Stabilized β-catenin protein then translocates to the nucleus where it binds to lymphoid-enhancing factor/T-cell factor (LEF/TCF) proteins to activate target gene expression 5. Unlike Wnt/β-catenin signaling, β-catenin-independent Wnt pathways involve multiple intracellular signaling cascades that might be cross-connected. The induction of non-canonical Wnt signaling may trigger the release of intracellular calcium, which in turn activates downstream protein kinases, such as calcium/calmodulin-dependent protein kinase II (CaMKII) and protein kinase C (PKC) 6-8. Non-canonical Wnt signaling can also be transduced via Rho family of the small GTPases, which activate c-Jun N-terminal kinase (JNK) and the downstream activating protein-1 (AP-1) complex for transcriptional regulation, or directly modulate cytoskeleton organization that orchestrates planar cell polarity (PCP) and cell migration 9-13. Receptor tyrosine kinase-like orphan receptor 2 (ROR2) was initially identified along with ROR1 as a tyrosine kinase of the Trk family 14 , and then recognized as one of Wnt (co-)receptors due to its ability to interact with non-canonical Wnts, including Wnt4, Wnt5a, and Wnt11 15,16. Genetic studies show that Ror2 −/− mice displayed striking similarities to Wnt5a −/− mice, suggesting they may function in the same signaling pathway 10,17. In vertebrates, ROR2 is required for Wnt5ainduced cell migration, a function that involves activation of JNK, PKC, actin-binding protein Filamin A and Rho-family of the GTPase 17-21. The interaction of Wnt-ROR2 leads to phosphorylation of Dvl that induces the activation of AP-1 and Rac1 22,23. In addition, ROR2 was shown to interact with and be phosphorylated by CK1and GSK3, both of kinases that also play imperative roles in Wnt/β-catenin signaling 20,24-26. In multiple systems, Wnt5a was shown to inhibit β-catenin-mediated canonical Wnt signaling 27-29. The nature by which ROR2 mediates