To Wnt5b or not to Wnt5b: Understanding the role of Wnt5b in lymphatic development

Abstract

Dysregulation of the lymphatic system is implicated in pathological conditions ranging from lymphoedema to cancer metastasis, however therapeutically modulating lymphatic growth is impeded by our basic understanding of how the lymphatic system develops. The majority of lymphatic vessels arise from veins, where a subset of venous endothelial cells begin to express Prox1a, a transcription factor that regulates lymphatic fate. The mechanisms that cause some venous cells to induce prox1a is largely unknown. Recently, Wnt5b has been shown to induce lymphatic fate in the posterior cardinal vein, which generates the trunk lymphatics of the zebrafish. Wnt5b is one of a large family of developmental patterning molecules which share common signalling pathways, simplified as either canonical or non-canonical signalling pathways. Wnt5b was shown to induce prox1a expression, and thus lymphatic fate, through the canonical signalling pathway in the zebrafish trunk, however whether Wnt5b is ubiquitously required for lymphatic specification and induction of Prox1a had yet to be established. This study sought to determine whether Wnt5b is required for lymphatic development of the facial lymphatics of the zebrafish. Transgenic zebrafish embryos which marked the facial lymphatics were injected with anti-sense morpholino oligonucleotides to knock down wnt5b, or crossed with mutants (axin1tm213) which overactivated the canonical Wnt signalling pathway. The growth of the facial lymphatic sprout, the first component of the facial lymphatics, was examined in live-imaged transgenic embryos, and was shown to be impaired by wnt5b knock down, but not affected by the axin1tm213 mutation. Fixed embryos immunostained for presence of Prox1 positive lymphatic precursors in the primary head sinus showed that wnt5b knock down or overactivation of the canonical Wnt signalling pathway with the axin1tm213 mutant did not affect the number of lymphatic precursors. This study is the first to suggest that lymphatic vessels originating from the same tissue type respond differently to Wnt signalling and provides evidence to support the recent hypothesis that lymphatic specification is location-dependent. The location-dependent regulation of lymphatic specification suggested in this study highlights the intricate nature of lymphatic development, which might continue to affect our ability to find novel strategies to therapeutically modulate lymphatic growth.