The human brain is arguably one of the most important, complex, and mysterious organ systems. Based on the precise interconnectivity of the neurons and glial cells that make up the brain, the central nervous system (CNS) controls most aspects of an organism’s physiology, abilities, and even survival.  This complexity of control means that when aspects of brain development go wrong, major problems can arise, and it also means that our understanding of those problems and potential solutions are seriously limited. Successful wiring of the nervous system relies on an elaborate communication system between newly born neurons attempting to send out a long arm-like process called an axon with the cellular and molecular growth environment.  For bilaterally symmetrical organisms the midline represents a major decision point for a new neuron, as it marks a location that the growing axon may have to cross forming what is known as a “commissure,” which serves to connect the two sides of a nervous system. 

How is this decision made during development with such great fidelity that every individuals’ commissures are similarly positioned?  To answer this question we have characterized a simple system to simultaneously observe commissural axons interacting with their growth substrate in the live zebrafish forebrain, as well as directly testing the role of specific molecular mechanisms controlling axon guidance across the midline. The zebrafish model system offers great simplicity paired with a rapidly developing embryonic brain. Additionally, the zebrafish possesses extensive conservation from gene to tissue with all vertebrate species, while also offering a plethora of unique experimental techniques (Image 1). 

This CAREER award enabled the Barresi lab to achieve many outcomes toward a better understanding of the role that astroglial cells play in commissure formation during development of the CNS. Glial cells and neurons are found at roughly equal proportions (50/50), yet little is known about the role of glial cells in the brain and even less known about their own developmental history or their role in helping the nervous system form. This investigation largely focused on the zebrafish forebrain by first characterizing the types of astroglial cells present at the location of developing commissures (Image 2).  We took advantage of the ability to genetically label astroglial cells with fluorescent proteins to visually see these cells in the live embryo. Additionally, cells can be transplanted between zebrafish embryos, which is a technique that allowed us to see small populations of astroglial cells interacting with commissural axons. With this approach, we were able to define four different astroglial morphologies that come in direct contact with midline crossing axons (Image 3). To directly test whether these astroglial cells are important for commissure development, we generated a new genetic line of zebrafish that enables us to chemically induce the death or ablation of only astroglial cells. Using this ablation system we showed that these astroglia are required for many aspects of nervous system development. Astroglia are needed to maintain the pattern of axons throughout the CNS, prevent foreign cells from migrating inside CNS, and to maintain the blood-brain barrier. Astroglia are even required to function as neural stem cells in the embryo to create new neurons and other glial cells that help the CNS grow. Through this work we have also determined the role of molecular guidance cues called Slits and Roundabouts, which are proteins that can tell axons and astroglia where to travel. We demonstrated that Slit1a functions in a novel way to attract the growth of both commissural axons and astroglial cells to the correct midline location to form a commissure in the forebrain. In an effort to move this visually beautiful data toward a more quantitative analysis we created a new computational method (Image 4). This method will provide our work and the greater community statistically rigorous approaches to glean new and credible knowledge about how the vertebrate organism is built.

All of the research for this award was conducted at Smith College, the nation’s largest liberal arts college for women. This award helped the PI to introduce innovative teaching practices to better engage students, such as bringing researchers into the classroom through web conferencing and having students produce educational documentaries, all of which the PI has made publically available. Moreover, we extended the reach of this exciting research to the broader educational community by creating the “Student Scientists” outreach program.  The aim of this program is to enhance Biology curriculum and laboratory instruction in secondary education. Student Scientists is now a truly sustainable outreach program that brings zebrafish embryos, sophisticated microscopy equipment, and teacher trained curriculum to 8 different school districts in the area. Thousands of K12 students have been impacted along with over 20 different teachers. Greater than 60 undergraduates have supported this program and their own professional development through it. 

Last Modified: 07/25/2017
Modified by: Michael J Barresi