Cardiac injury modulates critical components of prostaglandin E2 signaling during zebrafish heart regeneration.

The inability to effectively stimulate cardiomyocyte proliferation remains a principle barrier to regeneration in the adult human heart. A tightly regulated, acute inflammatory response mediated by a range of cell types is required to initiate regenerative processes. Prostaglandin E2 (PGE2), a potent lipid signaling molecule induced by inflammation, has been shown to promote regeneration and cell proliferation; however, the dynamics of PGE2 signaling in the context of heart regeneration remain underexplored. Here, we employ the regeneration-competent zebrafish to characterize components of the PGE2 signaling circuit following cardiac injury. In the regenerating adult heart, we documented an increase in PGE2 levels, concurrent with upregulation of cox2a and ptges, two genes critical for PGE2 synthesis. Furthermore, we identified the epicardium as the most prominent site for cox2a expression, thereby suggesting a role for this tissue as an inflammatory mediator. Injury also drove the opposing expression of PGE2 receptors, upregulating pro-restorative ptger2a and downregulating the opposing receptor ptger3. Importantly, treatment with pharmacological inhibitors of Cox2 activity suppressed both production of PGE2, and the proliferation of cardiomyocytes. These results suggest that injury-induced PGE2 signaling is key to stimulating cardiomyocyte proliferation during regeneration.

BMP9 Crosstalk with the Hippo Pathway Regulates Endothelial Cell Matricellular and Chemokine Responses.

Endoglin is a type III TGFß auxiliary receptor that is upregulated in endothelial cells during angiogenesis and, when mutated in humans, results in the vascular disease hereditary hemorrhagic telangiectasia (HHT). Though endoglin has been implicated in cell adhesion, the underlying molecular mechanisms are still poorly understood. Here we show endoglin expression in endothelial cells regulates subcellular localization of zyxin in focal adhesions in response to BMP9. RNA knockdown of endoglin resulted in mislocalization of zyxin and altered formation of focal adhesions. The mechanotransduction role of focal adhesions and their ability to transmit regulatory signals through binding of the extracellular matrix are altered by endoglin deficiency. BMP/TGFß transcription factors, SMADs, and zyxin have recently been implicated in a newly emerging signaling cascade, the Hippo pathway. The Hippo transcription coactivator, YAP1 (yes-associated protein 1), has been suggested to play a crucial role in mechanotransduction and cell-cell contact. Identification of BMP9-dependent nuclear localization of YAP1 in response to endoglin expression suggests a mechanism of crosstalk between the two pathways. Suppression of endoglin and YAP1 alters BMP9-dependent expression of YAP1 target genes CCN1 (cysteine-rich 61, CYR61) and CCN2 (connective tissue growth factor, CTGF) as well as the chemokine CCL2 (monocyte chemotactic protein 1, MCP-1). These results suggest a coordinate effect of endoglin deficiency on cell matrix remodeling and local inflammatory responses. Identification of a direct link between the Hippo pathway and endoglin may reveal novel mechanisms in the etiology of HHT.