ProperFhl1expression as Wnt signaling is required for chondrogenesis of ATDC5 cells

Cellular diversity of the regenerating caudal fin

Zebrafish faithfully regenerate their caudal fin after amputation. During this process, both differentiated cells and resident progenitors migrate to the wound site and undergo lineage-restricted, programmed cellular state transitions to populate the new regenerate. Until now, systematic characterizations of cells comprising the new regenerate and molecular definitions of their state transitions have been lacking. We hereby characterize the dynamics of gene regulatory programs during fin regeneration by creating single-cell transcriptome maps of both preinjury and regenerating fin tissues at 1/2/4 days post-amputation. We consistently identified epithelial, mesenchymal, and hematopoietic populations across all stages. We found common and cell type-specific cell cycle programs associated with proliferation. In addition to defining the processes of epithelial replenishment and mesenchymal differentiation, we also identified molecular signatures that could better distinguish epithelial and mesenchymal subpopulations in fish. The insights for natural cell state transitions during regeneration point to new directions for studying this regeneration model.

Exostosin-1 enhances canonical Wnt signaling activity during chondrogenic differentiation

OBJECTIVE

Exostosin-1 (Ext1) encodes a glycosyltransferase required for heparan sulfate (HS) chain elongation in HS-proteoglycan biosynthesis. HS chains serve as binding partners for signaling proteins, affecting their distribution and activity. The Wnt/β-catenin pathway emerged as critical regulator of chondrogenesis. Yet, how EXT1 and HS affect Wnt/β-catenin signaling during chondrogenesis remains unexplored.

METHOD

Ext1 was stably knocked-down or overexpressed in ATDC5 chondrogenic cells cultured as micromasses. HS content was determined using ELISA. Chondrogenic markers Sox9, Col2a1, Aggrecan, and Wnt direct target gene Axin2 were measured by RT-qPCR. Proteoglycan content was evaluated by Alcian blue and DMMB assay, canonical Wnt signaling activation by β-catenin Western blot and TOP/FOP assay. ATDC5 cells and human articular chondrocytes were treated with Wnt activators CHIR99021 and recombinant WNT3A.

RESULTS

Ext1 knock-down reduced HS, and increased chondrogenic markers and proteoglycan accumulation. Ext1 knock-down reduced active Wnt/β-catenin signaling. Conversely, Ext1 overexpressing cells, with higher HS content, showed decreased chondrogenic differentiation and enhanced Wnt/β-catenin signaling. Wnt/β-catenin signaling activation led to a down-regulation of Ext1 expression in ATDC5 cells and in human articular chondrocytes.

CONCLUSIONS

EXT1 affects chondrogenic differentiation of precursor cells, in part via changes in the activity of Wnt/β-catenin signaling. Wnt/β-catenin signaling controls Ext1 expression, suggesting a regulatory loop between EXT1 and Wnt/β-catenin signaling during chondrogenesis.

Identification of Novel EZH2 Targets Regulating Osteogenic Differentiation in Mesenchymal Stem Cells

Histone three lysine 27 (H3K27) methyltransferase enhancer of zeste homolog 2 (EZH2) is a critical epigenetic modifier, which regulates gene transcription through the trimethylation of the H3K27 residue leading to chromatin compaction and gene repression. EZH2 has previously been identified to regulate human bone marrow-derived mesenchymal stem cells (MSC) lineage specification. MSC lineage specification is regulated by the presence of EZH2 and its H3K27me3 modification or the removal of the H3K27 modification by lysine demethylases 6A and 6B (KDM6A and KDM6B). This study used a bioinformatics approach to identify novel genes regulated by EZH2 during MSC osteogenic differentiation. In this study, we identified the EZH2 targets, ZBTB16, MX1, and FHL1, which were expressed at low levels in MSC. EZH2 and H3K27me3 were found to be present along the transcription start site of their respective promoters. During osteogenesis, these genes become actively expressed coinciding with the disappearance of EZH2 and H3K27me3 on the transcription start site of these genes and the enrichment of the active H3K4me3 modification. Overexpression of EZH2 downregulated the transcript levels of ZBTB16, MX1, and FHL1 during osteogenesis. Small interfering RNA targeting of MX1 and FHL1 was associated with a downregulation of the key osteogenic transcription factor, RUNX2, and its downstream targets osteopontin and osteocalcin. These findings highlight that EZH2 not only acts through the direct regulation of signaling modules and lineage-specific transcription factors but also targets many novel genes important for mediating MSC osteogenic differentiation.

Estrogen enhances activity of Wnt signaling during osteogenesis by inducing Fhl1 expression

Estrogen is a crucial hormone for osteoclast inhibition and for preventing osteoporosis. However, the hormone's role in osteoblast growth and differentiation remains unclear. The complexity of estrogen's role in guiding osteoblast behavior arises partly from crosstalk with other signaling pathways, including Wnt signaling. In this study, we show that the Wnt agonist, LiCl, induced Fhl1 gene expression, which substantially enhanced osteoblast differentiation. Staining with alizarin red revealed that MC3T3-E1 mineralization was enhanced by overexpression of Fhl1. In addition, Fhl1 promoted the expression of the osteogenic markers, Runt-related transcription factor 2 (Runx2), osteocalcin (OCN), and osteopontin (OPN), whereas MC3T3-E1 cells with gene knockdown of Fhl1 exhibited limited mineralization and expression of Runx2, OCN, and OPN. We further demonstrate evidences from quantitative reverse transcription real-time polymerase chain reaction and reporter assay that Fhl1 expression was synergistically stimulated by estrogen (E2) and LiCl, but reduced by the estrogen-receptor inhibitor fulvestrant (ICI 182,780). However, estrogen could not enhance osteogenesis while Fhl1 expression was knocked down. Because estrogen and Wnt signaling frequently interact in developmental processes, we propose that Fhl1 can be an acting molecule mediating both signaling pathways during osteogenesis.