The winner makes it all: cluster competition drives floorplate formation
Every one of us developed from just a single cell: after fusion of egg and sperm, this single initial cell divides to give rise to myriads of cells, which form the tissues and organs of the body. But how does each cell know which cell type to become, and which structure to be part of? A key role is played by groups of cells, the “organizing centers”, which release factors that in turn tell other cells to differentiate, migrate or form a particular structure. But to fully understand development, we need to understand how these organizing centers form in the first place.
In the new study, Elly Tanaka and her team at the Institute of Molecular Biotechnology (IMBA) of the Austrian Academy of Sciences, with co-first authors Teresa Krammer, Hannah Stuart and Elena Gromberg, also affiliated with the neighboring Research Institute of Molecular Pathology (IMP), now shed light onto an alternative mechanism of how a key organizing center in the developing nervous system, the so-called ventral floorplate, forms.
The development of the ventral floorplate is an essential step for the neural tube, which later gives rise to the central nervous system, to pattern and develop correctly. “In the developing organism, this process is regulated by the notochord, a structure that secretes several factors and serves as a compass for floorplate induction”, Krammer, a PhD student in the Tanaka lab and first author of the study, explains. In the embryo, the floorplate develops in the part of the neural tube that is closest to the notochord. However, there must be more to floorplate development: The Tanaka group previously developed organoids of the neural tube, in which the ventral floorplate develops spontaneously, even though no notochord is present. To better understand how organizers develop spontaneously, the Tanaka group investigated how the floorplate forms in these neural tube organoids.
Using fluorescent labelling, the scientists could follow the appearance and behavior of floorplate cells in neural tube organoids. The researchers can observe floorplate cells, as these cells express the factor FOXA2, which they can visualize fluorescently. "In these neural tube organoids, where no notochord is present, these FOXA2-positive floorplate cells initially appeared in a random manner”, Krammer says.
However, no chaos ensues: the researchers identified two mechanisms that ensure that only one ventral floorplate is formed. “FOXA2 expressing cells move inside the organoid to find other FOXA2-positive cells and form multiple clusters”, Krammer adds. “Fascinatingly, these clusters will then compete with each other to decide which one will prevail to become a floorplate.”
To do so, the cell clusters send signals that act long-range between each other, causing this competition. The interplay of these signals, specifically members of the BMP pathway, causes a molecular tug-of-war: In the end, the largest and brightest cell cluster is stronger than the rest, and will form the ventral floorplate. “Knocking out a BMP pathway inhibitor, or increasing BMP signaling, decreased the size of the forming cell cluster and inhibited floorplate formation and neural tube patterning, which indicates that this competitive process is essential to the formation of the floorplate”, Tanaka summarizes.
Having established that BMP signaling plays an important role in floorplate formation in the organoid system, the researchers also wanted to test how relevant these findings are in the organism. “Remarkably, knocking out the same BMP inhibitor in mouse embryos shows similar effects, and a smaller floorplate develops”, Tanaka says.
The results highlight how some embryonic structures can organize regardless of whether other guiding components that provide structural information are present or not. “Our work shows how organizers can form, and that embryonic processes which were thought to be induced by a controlling structure – like the notochord – can actually proceed through regulative wiring of cells”, Tanaka summarizes. “We also show that studying self-organization in an organoid system can give us novel insights into patterning in the developing organism, in our case, into the development of the anterior floorplate.”