Summary Interactions between developmental signaling pathways govern the formation and function of stem cells. Prostaglandin (PG) E2 regulates vertebrate hematopoietic stem cells (HSC). Similarly, the Wnt signaling pathway controls HSC self-renewal and bone marrow repopulation. Here, we show that wnt reporter activity in zebrafish HSCs is responsive to PGE2 modulation, demonstrating a direct interaction in vivo. Inhibition of PGE2 synthesis blocked wnt-induced alterations in HSC formation. PGE2 modified the wnt signaling cascade at the level of β-catenin degradation through cAMP/PKA-mediated stabilizing phosphorylation events. The PGE2/Wnt interaction regulated murine stem and progenitor populations in vitro in hematopoietic ES cell assays and in vivo following transplantation. The relationship between PGE2 and Wnt was also conserved during regeneration of other organ systems. Our work provides the first in vivo evidence that Wnt activation in stem cells requires PGE2, and suggests the PGE2/Wnt interaction is a master regulator of vertebrate regeneration and recovery.

Understanding pathways controlling cardiac development may offer insights that are useful for stem cell-based cardiac repair. Developmental studies indicate that the Wnt/␤-catenin pathway negatively regulates cardiac differentiation, whereas studies with pluripotent embryonal carcinoma cells suggest that this pathway promotes cardiogenesis. This apparent contradiction led us to hypothesize that Wnt/␤-catenin signaling acts biphasically, either promoting or inhibiting cardiogenesis depending on timing. We used inducible promoters to activate or repress Wnt/␤-catenin signaling in zebrafish embryos at different times of development. We found that Wnt/␤-catenin signaling before gastrulation promotes cardiac differentiation, whereas signaling during gastrulation inhibits heart formation. Early treatment of differentiating mouse embryonic stem (ES) cells with Wnt-3A stimulates mesoderm induction, activates a feedback loop that subsequently represses the Wnt pathway, and increases cardiac differentiation. Conversely, late activation of ␤-catenin signaling reduces cardiac differentiation in ES cells. Finally, constitutive overexpression of the ␤-catenin-independent ligand Wnt-11 increases cardiogenesis in differentiating mouse ES cells. Thus, Wnt/␤-catenin signaling promotes cardiac differentiation at early developmental stages and inhibits it later. Control of this pathway may promote derivation of cardiomyocytes for basic research and cell therapy applications.heart development ͉ mesoderm ͉ Dickkopf-1 ͉ regeneration

In contrast to mammals, lower vertebrates have a remarkable capacity to regenerate complex structures damaged by injury or disease. This process, termed epimorphic regeneration, involves progenitor cells created through the reprogramming of differentiated cells or through the activation of resident stem cells. Wnt/␤-catenin signaling regulates progenitor cell fate and proliferation during embryonic development and stem cell function in adults, but its functional involvement in epimorphic regeneration has not been addressed. Using transgenic fish lines, we show that Wnt/␤-catenin signaling is activated in the regenerating zebrafish tail fin and is required for formation and subsequent proliferation of the progenitor cells of the blastema. Wnt/␤-catenin signaling appears to act upstream of FGF signaling, which has recently been found to be essential for fin regeneration. Intriguingly, increased Wnt/␤-catenin signaling is sufficient to augment regeneration, as tail fins regenerate faster in fish heterozygous for a loss-of-function mutation in axin1, a negative regulator of the pathway. Likewise, activation of Wnt/␤-catenin signaling by overexpression of wnt8 increases proliferation of progenitor cells in the regenerating fin. By contrast, overexpression of wnt5b (pipetail) reduces expression of Wnt/␤-catenin target genes, impairs proliferation of progenitors and inhibits fin regeneration. Importantly, fin regeneration is accelerated in wnt5b mutant fish. These data suggest that Wnt/␤-catenin signaling promotes regeneration, whereas a distinct pathway activated by wnt5b acts in a negative-feedback loop to limit regeneration.