BBB seminar: Di Jiang

Di Jiang
Sars International Center for Marine Molecular Biology, Bergen

Our laboratory is interested in how differentiated cells are organized into complex tissues and organs, and how this process evolves. The ascidian notochord provides an elegant system for studying this process. In contrast to the vertebrate notochord, which consists of thousands of still dividing cells whose positions and movements during development are difficult to trace, ascidian notochord has only 40 cells, which are post-mitotic at the beginning of morphogenesis. During a process called convergence/extension (C/E), a four by ten sheet of differentiated notochord cells are organized into a single column of 40-stacked cells. Following C/E, notochord development continues as individual cells elongate and vacuolize along the anterior/posterior (A/P) axis. Finally, these vacuoles fuse to form a tube that runs the length of the larval tail.

Analysis of a natural mutant ( aimless ) that carries a mutation in ascidian planar cell polarity (PCP) gene prickle demonstrates an essential role for prickle in C/E movement. This work has lead to the discovery of an A/P polarity in individual notochord cells, evidenced by the anterior localization of prickle and strabismus. With the use of aimless mutant, we show a requirement of prickle for the A/P polarity of notochord cells. These findings reveal that the PCP pathway not only operates along the medial/lateral (M/L) axis during C/E, but also regulates notochord morphogenesis along the A/P axis at the elongation phase. The simple morphogenesis of the ascidian embryos, particularly of the notochord, has allowed us to advance our understanding of the role of the PCP pathway in chordate development.

Our recent studies demonstrate that ascidian notochord cells can be viewed as a series of modified epithelial cells with defined polarities. During the morphogenetic movement subsequent to C/E, these cells undergo dramatic cell shape changes, nevertheless the same topological configuration is maintained. The transformation of notochord, from a solid rod of cells tandemly arrayed into a hollow tube, is similar to the tube formation seen, for example, in vasculogenesis. How different molecules and subcellular apparatus within these epithelial-like cells drive and/or accommodate the reshaping of notochord is the central question being addressed in our laboratory.