RESEARCH INTERESTS

Molecular mechanisms of axon growth and guidance during nervous system development. We are interested specifically in how axon guidance receptors specify attractive and repulsive signals and transmit these signals to the navigating growth cone to generate a directed motile response.  A second major focus of the lab is to investigate how transcription factor codes for motor neuron identity control the expression of specific axon guidance receptors.

RESEARCH TECHNIQUES

Drosophila genetics, transgenic expression of axon guidance molecules, molecular and cellular biology, cell and tissue culture, biochemistry, light, fluorescent and confocal microscopy of the Drosophila embryonic CNS, FACS sorting of Drosophila motor neurons, micro-array analysis.

A confocal micrograph of several segments of the Drosophila embryonic Central Nervious System showing a subset of interneurons (green) with axon projections crossing the CNS midline. The entire axon scaffold is shown in red.

Three dimensional reconstruction of a Drosophila embryo  triple labeled for components of the midline and neuromuscular  system.  The midline glia are detected by fluorescence mRNA in situ  to Netrin (green), a subset of CNS interneurons and all motor axon  projections are detected by an antibody to the cell adhesion molecule  Fasciclin II (red) and the muscles are detected with an antibody to  muscle myosin (blue).

RESEARCH SUMMARY

How axons in the developing nervous system successfully navigate to their correct targets is a fundamental problem in neurobiology. Understanding the mechanisms that mediate axon guidance will give important insight into how the nervous system is correctly wired during development and may have implications for therapeutic approaches to developmental brain disorders and nerve regeneration. Achieving this understanding will require unraveling the molecular logic that ensures the proper expression and localization of axon guidance cues and receptors, and elucidating the signaling events that regulate the growth cone cytoskeleton in response to guidance receptor activation. Axonal growth cones are guided by both attractants and repellents, and these signals can be either short or long-range. The Slit ligand and Roundabout (Robo) receptors, and the Netrin ligand and DCC/UNC5 receptors are two important evolutionary conserved ligand/receptor systems that contribute to proper connectivity in both the vertebrate and invertebrate nervous systems. The goal of our research is to dissect the signaling mechanisms downstream of attractive and repulsive guidance receptors. The midline of the Drosophila CNS provides an ideal system to address these questions. In Drosophila, both Netrins (NetA and NetB) are expressed by the same midline cells, where they function largely as attractants. This attractive function is mediated by Frazzled (Fra), a member of the DCC/UNC-40 family of Netrin receptors. Slit, on the other hand, functions as a midline repellent. This repulsive function is mediated largely by Robo receptors. These molecules are also known to influence neuronal and mesodermal cell migration, suggesting that determining their function may have broad implications for understanding diseases of nervous system development, many of which have their root in defective cell migration and/or axon guidance. The research in my laboratory addresses the dynamics of axon guidance receptor expression and signaling, and exploits the powerful genetic and molecular approaches available in Drosophila.

KEY WORDS Axon guidance, Developmental neuroscience, Attraction, Repulsion


KEY REFERENCES

Garbe, D.S., Das, A, Dubreuil, R.R. and Bashaw, G.J. (2007). Beta Spectrin functions independently of Ankyrin to regulate the establishment and maintenance of axon connections in the Drosophila embryonic CNS. Development, 134, 273-284.Online PDF file.

Yang, L and Bashaw, G.J. (2006). Son of Sevenless directly links the Robo receptor to Rac activation to control axon repulsion at the midline. Neuron, 52, 595-607. Online PDF file.

Labrador, J.P., O’Keefe, D., Yoshikawa, S., McKinnon, R.D., Thomas, J.B. and Bashaw, G.J. (2005). The Homeobox Transcription Factor Even-skipped Regulates Netrin-Receptor Expression to Control Dorsal Motor-Axon Projections in Drosophila. Current Biology 15, 1413-1419. Online PDF file.

Hu, H., Li, M., Labrador, J.P., McEwen, J., Lai, E.C., Goodman, C.S. and Bashaw, G.J. (2005) Cross GAP/Vilse Links the Roundabout Receptor to Rac to Regulate Midline Repulsion. PNAS 102, 4613-4618. Online PDF file.

Garbe, D.S. and Bashaw, G.J. (2004). Axon Guidance at the Midline: From Mutants to Mechanisms. Critical Reviews in Biochemistry and Molecular Biology 39, 319-341. Online PDF file.

Fan, X., Labrador, J.P., Hing, H. and Bashaw, G.J. (2003). Slit Stimulation Recruits Dock and Pak to the Roundabout Receptor and Increases Rac Activity to Regulate Axon Repulsion at the CNS Midline. Neuron 40, 113-127. Online PDF file.