Chi-Bin Chien
Associate Professor of Neurobiology and Anatomy
B.A. Johns Hopkins University
Ph.D. California Institute of Technology
Research
As the embryonic brain wires itself, a key
step is for axons to navigate to their targets. We are studying
the cellular and molecular basis of axon guidance, using the zebrafish
retinotectal projection as a model. These embryos develop very
quickly and are transparent, allowing us to observe cell behavior
within the living embryo. Furthermore, we can perturb retinal
axons using embryological, genetic, and molecular biological tools.
Most importantly, an elegant genetic screen has yielded mutants
in ~25 genes that are critical for forming the retinotectal pathway.
We are systematically cloning these mutants and then analyzing
the function of the genes. For instance, we found that the astray
gene is a homolog of the roundabout (robo) axon guidance receptor,
making astray the first known vertebrate robo mutant. Using timelapse
analysis of axon behavior, we have shown that astray function
is necessary both to prevent and to correct guidance errors during
normal development. In addition to robo and slit homologs, we
have cloned many other guidance molecules. We can express wildtype
and mutant guidance molecules in the visual system, and can make
beautiful timelapse movies of retinal growth cones navigating
in vivo.
We have recently cloned two other mutants, boxer and nevermind,
and are starting to analyze their functions in detail. Intriguingly,
astray, boxer, and nevermind are all related to known human disease
genes. Finally, we have developed transgenic lines that express
green fluorescent protein (GFP) specifically in retinal axons.
We have begun next-generation screens using these transgenics.

Confocal projection of wildtype (top) and astray (bottom) zebrafish embryos, injected with diI in the dorsonasal retinal quadrant of the left eye. The wildtype axons project only to contralateral optic tectum, while mutant axons project to both tecta, telencephalon and diencephalon, and display multiple midline crossings.
References
1. Lee JS, von der Hardt S, Rusch MA, Stringer SE, Stickney HL, Talbot WS, Geisler R, Nüsslein-Volhard C, Selleck SB, Chien CB*, Roehl H* (2004) Axon sorting in the optic tract requires HSPG synthesis by ext2 ( dackel ) and extl3 ( boxer ). Neuron 44:947-960 *=equal contributions
2. Lee JS, Chien CB (2004) When sugars guide axons: new insights from heparan sulphate proteoglycan mutants. Nature Reviews Genetics 5:923-935
3. Hutson LD, Chien CB (2002) astray/robo2 is required for guidance and error correction in zebrafish retinal axons. Neuron 33:205-217
4. Hutson LD, Chien CB (2002) Wiring the zebrafish: axon guidance and synaptogenesis. Current Opinion in Neurobiology 12:87-92
5. Fricke C, Lee JS, Bonhoeffer F, Geiger-Rudolph S, Chien CB (2001) astray , a zebrafish Roundabout required for retinal axon pathfinding . Science 292:507-510
6. Lee JS, Ray R, Chien CB (2001) Cloning and expression of three zebrafish Roundabout homologs suggest roles in axon guidance and cell migration. Developmental Dynamics 221:216-230
7. Chien CB (1998) Why does the growth cone cross the road? Neuron 20:3-6
8. Chien CB (1996) PY in the fly: receptor-like tyrosine phosphatases in axonal pathfinding. Neuron 16:1065-1068
9. Chien CB, Rosenthal DE, Harris WA, Holt CE (1993) Navigational errors made by growth cones without filopodia in the embryonic Xenopus brain. Neuron 11:237-251


