Movement is controlled by connections established during development between motor neurons and muscle cells. Motor neurons extend processes called axons out from their cell bodies, which leave the spinal cord and extend through the developing embryo to contact the appropriate muscle fibers. This process, referred to as axon guidance, is a fundamental component of development and is essential for survival. Axon guidance is highly precise with growth cones, the growing tip of axons making few navigational errors as they extend along stereotyped routes or pathways. While numerous examples of axon guidance events in vertebrates have been described at the cellular level, less is known about the molecules that control these events. The goal of this research is to elucidate the molecules and mechanisms that control motor axon guidance in vertebrates. The experiments outlined in this proposal address this using zebrafish as a model organism. Zebrafish is an excellent model system for studying vertebrate motor axon guidance due to its relatively simple nervous system, the ability to study embryos at early stages of development when axons are growing to their muscles, and the capability to induce, recover, and clone mutations. One approach is to study mutations that disrupt motor axon guidance in the embryonic zebrafish. The mutation, topped, dramatically and specifically affects the ability of ventral motor axons to reach their target muscles. Instead of progressing ventrally along their pathway, motor axons in topped mutants stall and fail to enter the ventral myotome at the normal time. Topped protein functions in the muscle to enable growth cones to extend along the ventromedial muscle suggesting that it is a cue on the muscle that directs ventrally extending axons. Using the zebrafish genetic map, a region of the genome was mapped that contains a protein, Semaphorin 5A, which is known to function in axon guidance in a different part of the nervous system. Using a series of genetic, cellular, and molecular approaches will elucidate whether semaphorin 5A is the gene disrupted in topped mutants. For example, preliminary evidence shows that decreasing Semaphorin 5A causes the same phenotype as the mutant suggesting that this protein is functioning in this process. The mechanism of Semaphorin 5A action will be addressed and the receptor this protein binds to will be isolated. Studying these mutations and proteins will provide novel insights into the genetic control of vertebrate motor axon guidance and will identify molecules that function in this essential developmental process.
The Beattie lab incorporates undergraduate students who perform experiments and participate in lab meetings. Active efforts to recruit graduate students who are members of under-represented minorities has led to the enrollment of one minority graduate student. In collaboration with local faculty, Dr. Beattie has developed a zebrafish lab course for students at Miami of Ohio, thus exposing them to a level of scientific practice that would normally be unavailable at this primarily undergraduate institution
