Publication Date


Document Type

Doctoral Dissertation

Academic Program

Interdisciplinary Graduate Program


Medicine; Cell Biology

First Thesis Advisor

Zheng-Zheng Bao, Ph.D.


Hedgehog Proteins, Axons, Intracellular Signaling Peptides and Proteins, Retinal Ganglion Cells


During development, axons respond to a variety of guidance cues in the environment to navigate to the proper targets. Sonic hedgehog (Shh), a classical morphogen, has been shown to function as a guidance factor that directly acts on the growth cones of various types of axons. We previously found that Shh affects retinal ganglion cell (RGC) axonal growth and navigation in a concentration-dependent manner. However, the signaling pathways that mediate such events are still unclear.

In this thesis, we show that high concentrations of Shh induce growth cone collapse and repulsive turning of the chick RGC through rapid increase of Ca2+ in the growth cone, and specific activation of PKCα and Rho signaling pathways. We further found that integrin linked kinase (ILK) acts as an immediate downstream effector of PKCα. PKCα directly phosphorylates ILK in vitro at two previously unidentified sites threonine-173 and -181. Inhibition of PKCα, Rho, and ILK by pharmacological inhibitors and/or dominant-negative approaches abolished the negative effects of high-concentration of Shh. We provide evidence that Rho likely functions downstream of PKC and suggest that PKC, Rho and ILK may cooperatively mediate the negative effects of high concentrations of Shh. Furthermore, retroviral expression of dominant-negative constructs of PKCα (DN-PKCα) and ILK-double mutants (ILK-DM) resulted in misguidance of RGC axons at the optic chiasm in vivo. These results demonstrate that new signaling pathways composed of PKCα, Rho, and ILK play an important role in Shh-induced axonal chemorepulsion.

In contrast, we show that attractive axonal turning in response to low concentrations of Shh is independent of PKCα, but requires the activity of cyclic nucleotides cAMP. Taken together, our results suggest that the opposing effects of Shh on axon guidance are mediated by different signaling pathways.



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