Search for an ancestral mechanism of BMP-mediated patterning

Among the different animal species there is a fascinating diversity of morphologies and their underlying body plans. The greatest morphological diversity is found in a group called Bilateria. Bilateria are animals with two body axes, the primary head-tail and the secondary back-belly axis. Most animal species belong to the Bilateria: vertebrates and sea urchins, insects and crustaceans, scorpions and spiders, mollusks and worms. Cnidaria (corals, sea anemones, jellyfish) are a group of animals, which do not belong to Bilateria, however, they are their closest evolutionary relatives the so-called sister group to Bilateria. Although Cnidaria were initially thought to only have a single body axis and thus not to be bilaterally symmetric, we now know that a subgroup of Cnidaria, called Anthozoa (corals, sea anemones), also exhibit a second body axis. It has therefore been speculated, that there may have been a common ancestor of Cnidaria and Bilateria with a bilaterally symmetric body plan. The sea anemone Nematostella vectensis is the most popular and powerful model organism to study the anthozoan secondary body axis. In order to be formed, the secondary body axis requires the action of signaling molecules as a means of communication between the embryonic cells. The intriguing finding that Bilateria and the bilaterally symmetric cnidarian Nematostella use the same signaling molecules, called BMPs (Bone Morphogenetic Proteins), to regulate their respective secondary body a xes has supported the hypothesis of a bilaterally symmetric cnidarian-bilaterian ancestor. BMP signaling mechanisms regulating body axis patterning have been extensively studied in bilaterian model organisms (mainly in fruit flies and frogs), and it is currently assumed that the way they function in secondary body axis formation in the sea anemone Nematostella are the same as in the fly or frog. Specifically, it is thought that BMPs co-operate with their negative regulator Chordin to achieve their activating function. In the present research project, I aim to characterize the molecular mechanisms of BMP and Chordin function in Nematostella and to find out whether the mechanisms of the secondary axis patterning by BMP signaling are actually equivalent between the sea anemone and Bilateria. To this end, I will use biochemical measurements to understand how BMP and Chordin regulate each other, and fluorescence microscopy to visualize how these molecules move between the cells in the living Nematostella embryo. I am particularly interested in understanding, whether Chordin helps BMPs to reach the right cells in the embryo by promoting BMP stability (avoid signal destruction) or mobility. The exciting research project will provide new insights into how secondary body axis formation in Nematostella is controlled on a sub-cellular level and help us understand how a secondary axis may have been built in the cnidarian-bilaterian ancestor.

At the earliest stages of animal life, the fertilized egg divides and the resulting embryonic cells generate increasingly complex structures, giving rise to axes of symmetry along the developing body called the body axes. In Bilateria, the animal group comprising vertebrates, insects, worms, molluscs, and a variety of other animals, the primary body axis is the head-tail axis and the secondary body axis the back-belly axis. To coordinate development and establish the body axes, signaling proteins are crucial to communicate between the cells of the developing animal. Bone Morphogenetic Proteins (BMPs) are secreted signaling proteins and critical for the proper formation of the secondary body axis in Bilateria. Interestingly, a BMP-dependent second body axis is also found outside the Bilateria, in some animals of the group called Cnidaria: anthozoan cnidarians (sea anemones, corals) have a second body axis, whereas medusozoan cnidarians (jellyfish, hydroids) do not have a second body axis and are radially symmetric. Therefore, studying cnidarian body axes is important to understand when and how many times BMP-dependent second body axes were "invented" during animal evolution. The sea anemone Nematostella vectensis is the non-bilaterian model in which BMP-mediated axis formation is best understood. Still, studies addressing the mechanisms of BMP signaling in this system have been lacking. It is, for example, unknown how BMPs move from the producing cells through the embryo to reach their target cells. It was proposed that the mechanism of BMP action in Nematostella is similar to that in fruit flies or frogs where the negative BMP regulator Chordin plays a crucial role. In this mechanism, called "BMP shuttling", Chordin, which usually attenuates BMP signaling, can also promote BMP signaling by transporting ("shuttling") BMPs towards their target cells. However, another mechanism, in which Chordin acts as local BMP inhibitor but not as facilitator of BMP transport, is also known and had to be considered as valid alternative. My aim in this project was to reveal how Chordin regulates BMP signaling and understand whether Chordin shuttles BMPs in Nematostella. I found that Chordin binds BMPs, forming a complex that associates with the surface of cells in the Nematostella embryo. To test for shuttling, I compared normal Chordin with a modified Chordin that is tethered to the cell surface and therefore cannot move through the embryo. My results show that only normal (mobile) Chordin can promote BMP signaling in cells distant from Chordin-producing cells, whereas tethered (immobile) Chordin cannot. In summary, I could prove that BMP shuttling establishes the Nematostella secondary body axis. This mechanism is highly similar to that acting in secondary axis formation in fruit flies and frogs, indicating that such mechanism may have been acting in the last common ancestor of Cnidaria and Bilateria.