Structure of human Frizzled5 by fiducial-assisted cryo-EM supports a heterodimeric mechanism of canonical Wnt signaling

Frizzleds (Fzd) are the primary receptors for Wnt morphogens, which are essential regulators of stem cell biology, yet the structural basis of Wnt signaling through Fzd remains poorly understood. Here we report the structure of an unliganded human Fzd5 determined by single-particle cryo-EM at 3.7 Å resolution, with the aid of an antibody chaperone acting as a fiducial marker. We also analyzed the topology of low-resolution XWnt8/Fzd5 complex particles, which revealed extreme flexibility between the Wnt/Fzd-CRD and the Fzd-TM regions. Analysis of Wnt/β-catenin signaling in response to Wnt3a versus a 'surrogate agonist' that cross-links Fzd to LRP6, revealed identical structure-activity relationships. Thus, canonical Wnt/β-catenin signaling appears to be principally reliant on ligand-induced Fzd/LRP6 heterodimerization, versus the allosteric mechanisms seen in structurally analogous class A G protein-coupled receptors, and Smoothened. These findings deepen our mechanistic understanding of Wnt signal transduction, and have implications for harnessing Wnt agonism in regenerative medicine.

Fibrous Topography-Potentiated Canonical Wnt Signaling Directs the Odontoblastic Differentiation of Dental Pulp-Derived Stem Cells

Nanofibrous engineered matrices have significant potential in cellular differentiation and tissue regeneration. Stem cells require specific extracellular signals that lead to the induction of different lineages. However, the mechanisms by which the nanofibrous matrix promotes mesenchymal stem cell (MSC) differentiation are largely unknown. Here, we investigated the mechanisms that underlie nanofibrous matrix-induced odontoblastic differentiation of human dental pulp MSCs (DP-MSCs). An electrospun polystyrene nanofibrous (PSF) matrix was prepared, and the cell responses to the PSF matrix were assessed in comparison with those on conventional tissue culture dishes. The PSF matrix promoted the expression of Wnt3a, Wnt5a, Wnt10a, BMP2, BMP4, and BMP7 in the DP-MSCs, concomitant with the induction of odontoblast/osteoblast differentiation markers, dentin sialophosphoprotein (DSPP), osteocalcin, and bone sialoprotein, whose levels were further enhanced by treatment with recombinant Wnt3a. The DP-MSCs cultured on the PSF matrix also exhibited a high alkaline phosphatase activity and intense Alizarin Red staining, indicating that the PSF matrix promotes odontoblast differentiation. Besides inducing the expression of Wnt3a, the PSF matrix maintained high levels of β-catenin protein and enhanced its translocation to the nucleus, leading to its transcriptional activity. Forced expression of LEF1 or treatments with LiCl further enhanced the DSPP expression. Blocking the Wnt3a-initiated signaling abrogated the PSF-induced DSPP expression. Furthermore, the cells on the PSF matrix increased the DSPP promoter activity. The β-catenin complex was bound to the conserved motifs on the DSPP promoter dictating its transcription. Transplantations of the preodontoblast-seeded PSF matrix to the subcutaneous tissues of nude mice confirmed the association of the PSF matrix with the Wnt3a and DSPP expressions in vivo. Taken together, these results demonstrate the nanofibrous engineered matrix strongly supports odontoblastic differentiation of DP-MSCs by enhancing Wnt/β-catenin signaling.