RESUMO
The mammalian lymphatic vasculature is important for returning fluids from the extracellular tissue milieu back to the blood circulation. We showed previously that Prox1 dosage is important for the development of the mammalian lymphatic vasculature. The lack of Prox1 activity results in the complete absence of lymphatic endothelial cells (LECs). In Prox1 heterozygous embryos, the number of LECs is reduced because of a decrease in the progenitor pool in the cardinal vein. This reduction is caused by some progenitor cells being unable to maintain Prox1 expression. In this study, we identified Vegfr3, the cognate receptor of the lymphangiogenic growth factor Vegfc, as a dosage-dependent, direct in vivo target of Prox1. Using various mouse models, we also determined that Vegfr3 regulates Prox1 by establishing a feedback loop necessary to maintain the identity of LEC progenitors and that Vegfc-mediated activation of Vegfr3 signaling is necessary to maintain Prox1 expression in LEC progenitors. We propose that this feedback loop is the main sensing mechanism controlling the number of LEC progenitors and, as a consequence, the number of budding LECs that will form the embryonic lymphatic vasculature.
Assuntos
Células Endoteliais/citologia , Células Endoteliais/fisiologia , Células Progenitoras Endoteliais/citologia , Células Progenitoras Endoteliais/fisiologia , Proteínas de Homeodomínio/metabolismo , Proteínas Supressoras de Tumor/metabolismo , Receptor 3 de Fatores de Crescimento do Endotélio Vascular/metabolismo , Animais , Contagem de Células , Embrião não Mamífero , Regulação da Expressão Gênica , Proteínas de Homeodomínio/genética , Vasos Linfáticos/citologia , Vasos Linfáticos/metabolismo , Camundongos , Transdução de Sinais , Proteínas Supressoras de Tumor/genética , Receptor 3 de Fatores de Crescimento do Endotélio Vascular/genéticaRESUMO
Recent advances in self-organizing, 3-dimensional tissue cultures of embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) provided an in vitro model that recapitulates many aspects of the in vivo developmental steps. Using Rax-GFP-expressing ESCs, newly generated Six3-/- iPSCs, and conditional null Six3delta/f;Rax-Cre ESCs, we identified Six3 repression of R-spondin 2 (Rspo2) as a required step during optic vesicle morphogenesis and neuroretina differentiation. We validated these results in vivo by showing that transient ectopic expression of Rspo2 in the anterior neural plate of transgenic mouse embryos was sufficient to inhibit neuroretina differentiation. Additionally, using a chimeric eye organoid assay, we determined that Six3 null cells exert a non-cell-autonomous repressive effect during optic vesicle formation and neuroretina differentiation. Our results further validate the organoid culture system as a reliable and fast alternative to identify and evaluate genes involved in eye morphogenesis and neuroretina differentiation in vivo.