Date of Award

1-1-2018

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

College/School/Department

Department of Biological Sciences

Content Description

1 online resource (iii, x, 81 pages) : illustrations (some color)

Dissertation/Thesis Chair

Ben G Szaro

Committee Members

Gregory Lnenicka, Melinda Larsen, Cara Pager

Keywords

Inflammation, Optic nerve regeneration, Protein degradation, Suppressor of cytokine signaling, Ubiqutination, Xenopus laevis, Optic nerve, Cytokines, Cellular signal transduction

Subject Categories

Biology | Cell Biology | Neuroscience and Neurobiology

Abstract

The axons of the optic nerve, like other central nervous system (CNS) axons, tend to lose their capacity to regenerate following an injury in adult amniotes, but these axons are able to regenerate throughout life in anamniotes. In mammals, optic axon regeneration is promoted by inhibiting the increased expression in retinal ganglion cells of a cytokine signaling molecule, Suppressor of Cytokine Signaling 3 (SOCS3), that accompanies injury. In animals capable of regeneration, SOCS3 mRNA expression also increases dramatically in retinal ganglion cells after optic nerve injury, but somehow this increase is insufficient to block regeneration. To gain insights into how this inhibition is overcome, we examined the SOCS3 response to optic nerve injury more closely using the well-characterized optic nerve regeneration of Xenopus laevis as my model system of study. RNA-seq showed that both homeologs of the X. laevis SOCS3 gene increased in mRNA expression after injury, and this increase persisted well into the stage of synaptic refinement of regenerating axons in the optic tectum. In situ hybridization confirmed that this increase in SOCS3 mRNA expression indeed occurred in retinal ganglion cells. However, immunostaining showed that this increase was not reflected in SOCS3 protein expression, suggesting that either the efficiency of translation of the mRNA decreased or degradation of the protein increased with injury. Polysome profiling indicated that the efficiency of SOCS3 mRNA translation increased after injury, and thus, the stable SOCS3 protein levels were most likely due to increased protein degradation. In tumor cells, another member of the SOCS gene family, SOCS2, is known to stimulate SOCS3 degradation by targeting it for ubiquitination. I have found evidence that expression of both SOCS2 protein and mRNA expression increased in Xenopus retinal ganglion cells with injury. Finally, I propose a hypothesis that a similar mechanism could be acting in Xenopus retina to alleviate SOCS3-inhibition of successful optic axon regeneration. The findings from these experiments may provide insights into why successful optic nerve regeneration is unique to anamniotes.

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