Intervertebral discs (IVDs) are strong fibrocartilaginous joints that connect adjacent vertebrae of the spine. As discs age they become prone to failure, with neurological consequences that are often severe. Surgical repair of discs treats the result of the disease, which affects as many as one in seven people, rather than its cause. An ideal solution would be to repair degenerating discs using the mechanisms of their normal differentiation. However, these mechanisms are poorly understood. Using the mouse as a model, we previously showed that Shh signaling produced by nucleus pulposus cells activates the expression of differentiation markers, and cell proliferation, in the postnatal IVD. In the present study, we show that canonical Wnt signaling is required for the expression of Shh signaling targets in the IVD. We also show that Shh and canonical Wnt signaling pathways are down-regulated in adult IVDs. Furthermore, this down-regulation is reversible, since re-activation of the Wnt or Shh pathways in older discs can re-activate molecular markers of the IVD that are lost with age. These data suggest that biological treatments targeting Wnt and Shh signaling pathways may be feasible as a therapeutic for degenerative disc disease.

The molecular mechanisms initiating the formation of the lymphatic system, lymphangiogenesis, are still poorly understood. Here we have identified a novel role in lymphangiogenesis for an ETS transcription factor, Etv2/Etsrp, a known regulator of embryonic vascular development. Through the use of fully validated photoactivatable morpholinos we show that inducible Etv2 inhibition in zebrafish embryos at 1 day post-fertilization (dpf) results in significant inhibition of lymphangiogenesis, while development of blood vessels is unaffected. In Etv2-inhibited embryos and larvae, the number of lymphatic progenitors is greatly reduced, the major lymphatic vessel, the thoracic duct, is absent or severely fragmented, and lymphangiogenesis-associated marker expression, including lyve1b, prox1a, and vegfr3/flt4, is strongly downregulated. We also demonstrate that lymphatic progenitors in Etv2 deficient embryos fail to respond to Vegfc signaling. Chromatin immunoprecipitation and sequencing (ChIP-Seq) studies using differentiated mouse embryonic stem (ES) cells as well as luciferase reporter studies in the ES cells and in zebrafish embryos argue that Etv2 directly binds the promoter/enhancer regions of Vegfc receptor Vegfr3/Flt4 and lymphatic marker Lyve1, and promotes their expression. Together these data support a model where Etv2 initiates lymphangiogenesis by directly promoting the expression of flt4 within the posterior cardinal vein.