Research in the Brewster laboratory centers on the general question of how the central nervous system (CNS, the brain and spinal chord) is formed during embryonic development. We use the zebrafish as a model system as it is a vertebrate that provides many advantages for performing embryonic manipulations, imaging of live embryos and carrying out forward (mutagenesis) and reverse (gene depletion using morpholinos) genetics. More specifically, we are investigating:
1. How cell division and differentiation are controlled in the brain. Differentiation refers to the changes that neural progenitors undergo as they acquire a specific identity and become neurons. In general, once a cell has undergone differentiation it ceases to divide. Division and differentiation have to be well balanced in order for the brain to develop properly. Excessive division can lead to brain cancer and excessive or premature differentiation can cause the loss of specific neuronal cell types. We have found that a cell adhesion molecule called N-cadherin (N-cad) is essential for the control of proliferation and differentiation in the hindbrain. In N-cad mutants, neural progenitors divide excessively and some cells that have undergone differentiation manage to re-enter the cell cycle, resulting in the presence of mitotic neurons. These findings indicate that cadherins negatively regulate the cell cycle in the brain. Furthermore they demonstrate that cell cycle exit and differentiation can be uncoupled in the brain and that cadherins normally function to synchronize these events.
Ongoing projects in the laboratory pertaining to this area of research include the identification of the molecular pathways through which N-cad functions to control cell proliferation and coordinate cell cycle exit and differentiation.
2. How the neural tube, the CNS precursor, is assembled during early embryogenesis. Neural tube defects (NTDs) are the most common severely disabling birth defects in the United States, with a frequency of approximately 1 in every 2000 births. Surprisingly, the genetics underlying NTDs are still poorly understood. In additional to this clinical relevance, studies on neural tube formation offer the opportunity to explore fundamental questions at the interface of Cell and Developmental Biology. We have previously carried out an extensive analysis of the cellular behaviors that drive neurulation in the zebrafish (Hong and Brewster, 2006; Harrington et al., 2009; Harrington et al., 2010). These studies now provide a foundation to explore the molecular mechanisms of neurulation. Briefly, we have found that neurulation in the zebrafish can be considered as a biphasic process that involves: 1. “neural convergence”, the ability of neural progenitor cells to migrate towards the midline and assemble into a chord-like structure and 2. “epithelialization”, the transformation of migratory neural progenitors with mesenchymal properties into stationary epithelial cells that have a clearly defined apico-basal axis. Epithelialization is a multi-step process that involves the formation of apical junctional complexes (tight junctions, adherens junctions) in addition to a complete reorganization of the cytoskeleton. Failure of either neural convergence or epithelialization results in severe NTDs.
Ongoing projects focus on the molecular pathways that control neural convergence and epithelialization. We expect that identification of new genes implicated in either process will pave the way for translational research in preventing these birth defects. Through the study of a mutant called linguini in which the microtubule network is disrupted, we have found that stabilization of the microtubule network is essential for proper neural convergence. We are currently developing tools for analyzing microtubule dynamics in live embryos. We are also searching for molecules that stabilize microtubules during neurulation and have already found several that need further characterization. In addition, we are investigating what signal(s) attract migrating neural progenitor cells towards the midline. With regards to epithelialization, we have found that Pard3, a protein known to regulate various aspect of cell polarity, is essential for transforming the microtubule network from a mesenchymal to an epithelial configuration (Hong et al., 2010). We are now searching for the molecules that function alongside with Pard3 during epithelialization. Furthermore, we are addressing whether there are “master regulators” of epithelialization (molecules that control all aspects of epithelialization) in the neural tube, using a candidate gene approach.