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A pivotal Wnt antagonist role promoting digit joint specification by constraining Wnt activity

ABSTRACT: Bmps and Wnts play opposing roles in several contexts during chondrogenesis and joint formation. Using genetic and genomic approaches, we found instead that canonical Wnts can cooperate with Bmps to enhance pSmad1/5 activity and initiate chondrogenic commitment in digit tip progenitors. 5’HoxdD/D mutant digits are characterized by elevated Bmp and pSmad1/5 activity and subsequent joint loss. We show that expressing stabilized bcatenin (bcatCA) in interdigit mesenchyme rescues 5’HoxdD/D digit joint loss non-autonomously, by inducing secreted Wnt antagonists and normalizing digit tip pSmad1/5 levels. Indeed, genetic removal of Dkk2 in 5’HoxdD/D prevented joint rescue by bcatCA. Furthermore, elevating Wnt activity with Gsk3b antagonists in limb bud culture stabilized pSmad1/5 levels and enhanced Bmp activity. Elevated pSmad1/5, as seen in 5’HoxdD/D, accelerates chondrogenic commitment, impeding a switch of phalanx forming region (PFR) cells in the digit tip to interzone (joint progenitor) fate. We propose that, before progenitors transit into PFR, Wnt antagonists cooperate with Fgfs to prevent precocious pSmad1/5 accumulation by stabilizing Gsk3b to promote Smad-linker phosphorylation and Smad1/5 degradation. Consequently, Wnt antagonists play a key role in modulating the pace of initial commitment of digit progenitors to chondrogenesis, together with Fgfs, and maintain mesenchymal plasticity to balance digit phalanx and joint formation.

ORGANISM(S): Mus musculus

PROVIDER: GSE317957 | GEO | 2026/04/24

REPOSITORIES: GEO

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Project description:A fundamental question in biology is how an undifferentiated field of cells acquires spatial pattern and undergoes coordinated differentiation. The development of the vertebrate limb is an important paradigm for understanding these processes. The skeletal and connective tissues of the developing limb all derive from a population of multipotent progenitor cells located in its distal tip. During limb outgrowth, these progenitors segregate into a chondrogenic lineage, located in the center of the limb bud, and soft connective tissue lineages located in its periphery. We report that the interplay of two families of signaling proteins, fibroblast growth factors (FGFs) and Wnts, coordinate the growth of the multipotent progenitor cells with their simultaneous segregation into these lineages. FGF and Wnt signals act together to synergistically promote proliferation while maintaining the cells in an undifferentiated, multipotent state, but act separately to determine cell lineage specification. Withdrawal of both signals results in cell cycle withdrawal and chondrogenic differentiation. Continued exposure to Wnt however, maintains proliferation and re-specifies the cells towards the soft connective tissue lineages. We have identified target genes that are synergistically regulated by Wnts and Fgfs, and show how these factors actively suppress differentiation and promote growth. Finally, we show how the spatial restriction of Wnt and FGF signals to the limb ectoderm and to a specialized region of it, the apical ectodermal ridge, controls the distribution of cell behaviors within the growing limb, and guides the proper spatial organization of the differentiating tissues. Keywords: transcriptional response to growth factor treatment Cells derived from mouse embryonic stage 11.5 limb buds were cultured and treated with purified Wnt3a protein or vehicle controls. The transcriptional response was detected using spotted cDNA microarrays after 2 hrs or 4 hrs of treatment. 4 biological replicates were used per condition.

Project description:In vitro models allow for the study of developmental processes outside of the embryo. To gain access to the cells mediating digit and joint development, we identified a unique property of undifferentiated mesenchyme isolated from the distal early autopod to autonomously re-assemble forming multiple autopod structures including: digits, interdigital tissues, joints, muscles and tendons. Single cell transcriptomic analysis of these developing structures revealed distinct cell clusters that express canonical markers of distal limb development including: Col2a1, Col10a1, and Sp7 (phalanx formation) Thbs2 and Col1a1, (perichondrium), Gdf5, Wnt5a, and Jun (joint interzone), Aldh1a2 and Msx1 (interdigital tissues), Myod1 (muscle progenitors), Prg4 (articular perichondrium/articular cartilage), and Scx and Tnmd (tenocytes/tendons). Analysis of the gene expression patterns for these signature genes indicates that developmental timing and tissue-specific localization were also recapitulated in a manner similar to the initiation and maturation of the developing murine autopod. Finally, the in vitro digit system also recapitulates congenital malformations associated with genetic mutations as in vitro cultures of Hoxa13 mutant mesenchyme produced defects present in Hoxa13 mutant autopods including digit fusions, reduced phalangeal segment numbers, and poor mesenchymal condensation. These findings demonstrate the robustness of the in vitro digit system to recapitulate digit and joint development. As an in vitro model of murine digit and joint development, this innovative system will provide access to developing limb tissues facilitating studies to discern how digit and articular joint formation is initiated and how undifferentiated mesenchyme is patterned to establish individual digit morphologies. The in vitro digit system also provides a platform to rapidly evaluate treatments aimed at stimulating the repair or regeneration of mammalian digits impacted by congenital malformation, injury, or disease.

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A pivotal Wnt antagonist role promoting digit joint specification by constraining Wnt activity

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