The T-box family of transcription factors in sea urchin development: Involvement in pattern formation and morphogenesis

Embryonic development requires that an intricate series of patterning events occur in a precise fashion such that the three-dimensional body plan properly arises. These events can involve the localization of maternal determinants and vast arrays of morphogens and cell signaling cascades. Indeed, the embryonic development of most organisms involves a highly coordinated interplay between both of these processes. Additionally, while patterning events are integral development processes, they are not the sole requirement for embryogenesis. A complex series of morphogenetic movements are also needed in order for tissues to be appropriately organized along the body axes.; The sea urchin embryo provides an ideal model in which to study both specification and morphogenetic events. To gain insights into these processes, members of the T-box family of transcription factors were cloned from the sea urchin, Lytechinus variegatus. Orthologues of Brachyury, Tbx2/3, T-brain/Eomesodermin, and Tbx6/16 were identified. Studies on Brachyury showed that protein is localized to distinct animal and vegetal domains of the developing embryo. Vegetally, LvBrac is localized to a torus of nuclei around the blastopore and animally, to the stomodaeum. Vegetal LvBrac expression depends upon autonomous beta-catenin signaling in macromeres and does not require micromere or veg2 inductive signals. LvBrac is necessary for the morphogenetic movements occurring in both expression regions as shown by a dominant interfering construct. At least one LvBrac target may be non-autonomous in action.; LvTbx2/3 protein expression initiates at mesenchyme blastula stage and perdures into pluteus stages. Expression is asymmetric throughout this period being solely localized to aboral territories of the embryo. Aboral specific expression is observed in all three embryonic germ layers. Surgical manipulations and molecular perturbations were performed, the results of which implicate beta-catenin or beta-catenin downstream genes as necessary for LvTbx2/3 expression and in patterning the oral/aboral axis. Further, a proposed role for the ECM in patterning the oral/aboral axis of the sea urchin embryo is supported and extended as treatment with betaAPN, a drug that inhibits collagen crosslinking, prevents both the expression of LvTbx2/3 and the oral ectoderm expression of LvBrac.