Cytokine-induced SOCS expression is inhibited by cAMP analogue: Impact on regeneration in injured retina

https://doi.org/10.1016/j.mcn.2009.04.002Get rights and content

Abstract

Injured adult retinal ganglion cells (RGCs) regrow axons into peripheral nerve (PN) grafted onto cut optic nerve. Survival and regeneration of RGCs is increased by intraocular injections of ciliary neurotrophic factor (CNTF) and axonal regeneration is further enhanced by co-injection of a cyclic AMP analogue (CPT-cAMP). Based on these data, and because cytokine signaling is negatively regulated by suppressor of cytokine signaling (SOCS) proteins, we set out to determine whether CNTF injections increase retinal SOCS expression and whether any changes are attenuated by co-injection with CPT-cAMP. Using quantitative PCR we found increased SOCS1, SOCS2 and SOCS3 mRNA levels at various times after a single CNTF injection. Expression remained high for many days. SOCS protein levels were also increased. In situ hybridization revealed that RGCs express SOCS3 mRNA, and SOCS expression in cultured RGCs was increased by CNTF. Co-injection of CPT-cAMP reduced CNTF induced expression of SOCS1 and SOCS3 mRNA and decreased SOCS3 protein expression. CNTF injection also transiently increased retinal leukemia inhibitory factor (LIF) expression, an effect that was also moderated by CPT-cAMP. We propose that, along with known reparative effects of elevated cAMP on neurons, reducing SOCS upregulation may be an additional way in which cyclic nucleotides augment cytokine-induced regenerative responses in the injured CNS.

Section snippets

Changes in SOCS mRNA expression in PN-grafted retinas after CNTF or combined CNTF/CPT-cAMP injections

There was very low SOCS1–3 mRNA expression in unstimulated, intact retina. The PN–ON graft procedure and intraocular saline injection resulted in only small changes in retinal SOCS1–3 mRNA levels, none of which were significant (Figs. 1A–C). Major changes in expression were, however, seen 2 h to 14 days after CNTF or combined CNTF/CPT-cAMP injections.

Discussion

CNTF stimulates the survival and growth of various types of injured neurons, but its effects are often transient and by no means all neurons in a given population are rescued. In the rat PN-ON graft model for example, in spite of repeated injections of recombinant CNTF, 3–4 weeks after PN transplantation only 15–20% of the adult RGC population remains viable, of which about 25% regenerate an axon (Cui et al., 2003, Park et al., 2004, Hu et al., 2007). The proportion of surviving adult RGCs that

Experimental methods

Young adult (8–10 weeks old) Fischer 344 rats were used in this study. All surgical procedures were approved by the Animal Ethics Committee of The University of Western Australia and conformed to national (NHMRC) guidelines.

Acknowledgments

We thank Greg Cozens for technical assistance. Supported by grants to ARH from the NHMRC and the WA Neurotrauma Research Program.

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    1

    Current address: Children’s Hospital and Department of Neurology, Harvard Medical School, Boston, MA 02215, USA.

    2

    Current address: Bascom Palmer Eye Institute, McKnight Vision Center, University of Miami, Fl 33136, USA.

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