The Brewster laboratory is interested in understanding the mechanisms that underlie central nervous system birth defects, such as spina bifida, which results from impaired neural tube formation. In addition, we have recently begun to investigate how the brain responds to low oxygen (hypoxia or anoxia), with the ultimate goal of identifying potential therapeutic targets for stroke. We use the zebrafish as a model system to carry out these studies as this vertebrate organism lends itself well to gene discovery, embryonic and genetic manipulations and live imaging.
1. Studies on neural tube formation. Neural tube defects are the most common severely disabling birth defects in the United States, with a frequency of approximately 1 in every 2000 births. NTDs are thought to be caused by a combination of genetic and environmental factors, neither of which are well understood. We expect that our research on the genetic pathways that control neurulation (neural tube formation) will pave the way for translational research in preventing these birth defects. 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. 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 severe neural tube defects.
Ongoing projects focus on the identification of molecular pathways that control neural convergence. 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 searching for molecules that stabilize microtubules during neurulation and have identified several microtubule-associated proteins and upstream regulators. In addition, we are investigating what signal(s) attract migrating neural progenitor cells towards the midline. Our ultimate goal is to screen patient cohorts with NTDs for causative mutations in the genes we have identified, as a means to identify novel genetic risk factors. We have also begun to investigate environmental factors that alter microtubule stability and cause neural tube defects. These combined approaches are expected to increase our understanding of the genetic and environmental factors that converge to cause NTDs by altering the microtubule cytoskeleton.
2. Studies on anoxia tolerance. Stroke is the third leading cause of death in the US. It is caused by impaired brain function due a temporary blockage of blood flow (ischemia). Cell death in the brain does not occur for several days following ischemic injury, providing a therapeutic window for treatment. However, no drugs have so far been developed that effectively prevent cell death.
Interestingly, some organisms, such as zebrafish embryos, exhibit a high level of tolerance to anoxia, surviving without oxygen for several hours up to a day. While the mechanisms of anoxia tolerance are mostly unknown, it is thought that brain ATP levels are maintained, despite the shutdown of oxydative phosphorylation, by decreasing the rate of ATP consumption. We are currently investigating the role of two proteins, AMP-activated protein kinase (AMPK) and Hypoxia-inducible factor alpha (Hif 1-alpha) in decreasing metabolic rates in the brain in response to anoxia, to conserve ATP levels. In the long term we intend to carry out a genetic screen to identify genes that mediate anoxia tolerance. These approaches will increase our understanding of the mechanisms underlying anoxia tolerance and may lead to the discovery or novel therapeutic targets for treatment of stroke.