Jeremy Swisher

Introduction. Glaucoma is a prevalent disease that affects about 2% of the human population and is the leading cause of blindness (2). This disease usually increases intraocular pressure (IOP), which primarily damages the optic nerve and its associated retinal ganglion cells (RGC). The increase in IOP enhances the entry of calcium into these ganglion cells. The calcium activates the enzyme Calpain which can cleave calcineurin. Calcineurin can then induce apoptosis of the RGC’s by dephosphorylating Bcl-2 associated death promoter (BAD) which is a pro-apoptotic protein. This dephosphorylation leads to cell death by stimulation of cytochrome C from the mitochondria leading to activation of caspases (1,4,5). Stopping the triggered apoptosis is the chief concern of glaucoma. Methods. BRN3a levels were studied using the optic nerve crush model (ONC) with mice. BRN3a is a brain-specific protein that is expressed in retinal ganglion cells (3). Therefore, the levels of this protein were monitored in different scenarios to determine any effects on the retinal ganglion cells. Results. The HDAC3 inhibitor, RGFP9666, indicates that RGC loss is diminished when applied intravitreally. These doses were applied at 2.0 mg/kg every three days, and BRN3a levels were assessed at two and four weeks. At two weeks, the protective effects on RGC’s are clearly seen with the expression of BRN3A6. However, at four weeks, we must look at a different stain such as DAPI, which stains the nucleus of all cells. In this stain, we can see that overall cell loss is still much less with the HDAC3 inhibitor. When using a CD200R1 agonist to impact the same apoptotic pathway, BRNA3a levels were significantly higher3 (161% increase) than the ONC control. Conclusions. There are many intracellular mediators along the apoptotic pathway. A crucial epigenetic regulator involves histone deacetylases. Specifically, HDAC3 was shown to be neurotoxic and specific to RGC apoptosis. It migrates to the nucleus after optic nerve injury and deacetylates to turn off gene transcription, ultimately leading to apoptosis of the RGC. RGFP966 is a potent HDAC3 inhibitor, and it stops HDAC3 from translocating to the nucleus and shutting off the gene transcription (6), thus allowing the RGC to live and preserve vision. Another tested method involves stopping glial cell hyperactivation so that inflammatory cytokines that are released by these cells are unable to initiate apoptosis (3). Using a CD200R1 agonist such as CD200Fc accomplishes this goal by stopping the accumulation of inflammatory cytokines. These cutting-edge techniques show great promise for preserving the vision of glaucoma patients.

1. Bermudez JY, Webber HC, Patel GC, et al. HDAC Inhibitor-Mediated Epigenetic Regulation of Glaucoma- Associated TGFβ2 in the Trabecular Meshwork. Investigative Ophthalmology & Visual Science. 2016; 57(8):3698-3707. doi:10.1167/iovs16-19446. Accessed April 18, 2018
2. Echevarria F, Formichella C, Sappington R. Interleukin-6 Deficiency Attenuates Retinal Ganglion Cell Axonopathy and Glaucoma-Related Vision Loss. Frontiers In Neuroscience. 2017; 11:318. Accessed April20, 2018
3. Huang R, Lan Q, Xu F, et al. CD200Fc Attenuates Retinal Glial Responses and RGCs Apoptosis AfterOptic Nerve Crush by Modulating CD200/CD200R1 Interaction. Journal Of Molecular Neuroscience: MN. 2018; 64(2):200-210. Accessed April 19, 2018.
4. Mac Nair C, Schlamp L, Montgomery A, Shestopalov V, Nickells R. Retinal glial responses to optic nerve crush are attenuated in Bax-deficient mice and modulated by purinergic signaling pathways. Journal of Neuroinflammation. 2016; 13(1), p.93. Accessed April 20, 2018.
5. Schmitt H, Schlamp C, Nickells R. Role of HDACs in optic nerve damage-induced nuclear atrophy of retinal ganglion cells. Neuroscience Letters. 2016; 625:11-15. Accessed April 21, 2018.
6. Schmitt H, Schlamp C, Nickells R. Targeting HDAC3 Activity with RGFP966 Protects Against Retinal Ganglion Cell Nuclear Atrophy and Apoptosis After Optic Nerve Injury. Journal Of Ocular Pharmacology And Therapeutics: The Official Journal Of The Association For Ocular Pharmacology And Therapeutics. 2018; 34(3):260-273. Accessed April 18, 2018.