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Caroline Yu, Sahil Shah, Joana Galvao, Melissa Atkins, Michael Kapiloff, Jeffrey L Goldberg; MEF2D activation by mAKAP signalosomes promotes neurite outgrowth and RGC survival. Invest. Ophthalmol. Vis. Sci. 2018;59(9):6150.
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© ARVO (1962-2015); The Authors (2016-present)
What are the molecular mechanisms that control neuronal survival after injury? Myocyte enhancer factor-2 (MEF-2) is a family of highly conserved transcription factors that have been shown to be important in neuronal activity- and neurotrophin- dependent survival and is a binding partner of the scaffold protein A-kinase anchor protein-6 (AKAP6). Several other enzymes also bind AKAP6 are known to mediate posttranslational modifications of MEF2D. Based on this understanding, we hypothesize that AKAP6 signalosomes mediate post translational modifications of MEF2D to promote retinal ganglion cell (RGC) survival.
We constructed several mutant proteins, including S44A and K439R, which are hypothesized to be constitutively active, and S44E, which is hypothesized to be transcriptionally inactive. To examine the effect on neurite growth, we transfected primary cortical neurons and total axon length was measured in the presence and absence of extracellular stimulation. The AAV-2 packaged constructs were delivered via intravitreal injections, and the number of surviving RGC’s was quantified two weeks after optic nerve crush (ONC).
Expression of MEF2D proteins containing the activating mutations (S44A or K439R) led to significantly greater neurite extension in comparison to both MEF2D wildtype and the negative control in the absence of extracellular stimulation. Interestingly, in the setting of extracellular stimulation, the sumoylated, phospho-mimetic mutant (S44E) leads to decreased depolarization-induced neurite extension, suggesting that this mutant was able to inhibit activity dependent neurite extension. After ONC, we found that overexpression of wild type MEF2D led to increased RGC survival.
In this study, we investigated the effect of MEF2D and several of its post-translational modifications on RGC survival and neurite extension. Not only does overexpression of MEF2D promote neuronal survival in vivo, we found that posttranslational modifications known to be mediated by AKAP6 signalosomes are associated with greater neurite extension in vitro, suggesting that AKAP6 may play an indirect role in neuronal survival via regulation of MEF2D activity. Understanding the role of MEF2D, AKAP6, and their regulation is important for the development of therapeutic strategies for glaucoma and other neurodegenerative diseases.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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