Abstract
Purpose: :
The dysregulation of protein acetylation is an integral event in numerous inflammatory and neurodegenerative diseases, including those of the retina. The current studies investigate whether early changes in acetylation, following ischemic and ocular hypertensive insult, play a role in retinal injury and survival.
Methods: :
Changes in the acetylation state of histone-H3 and histone deacetylase (HDAC) activity were measured using rat retinal lysates at 2, 8 and 24 hours following 45 minutes of retinal ischemia, and at 3, 7 and 14 days of ocular hypertensive stress. In addition, caspase-3 activity was measured at 2, 8 and 24 hours in ischemic eyes. To investigate if increases in acetylation can influence retinal injury, the HDAC inhibitor valproic acid (VPA) (100mg/kg; i.p.) or vehicle was administered twice daily on study day 0, 1, 2 and 3, to rats that received 45 minutes retinal ischemia, or twice daily for 28 days following the development of ocular hypertension. The evaluation of retinal structure and function utilized microscopic and electroretinogram analysis.
Results: :
In vehicle-treated rats, retinal ischemia decreased acetyl histone-H3 levels by 15%, 44% and 69% at 2, 8 and 24 hours, compared to contralateral control eyes. Class I HDAC activity was increased at 2, 8 and 24 hours post ischemia by 14.2 ± 5.3%, 15.9 ± 3.1% and 29.6 ± 5.9%, respectively. Active caspase-3 levels were not significantly altered at 2 and 8 hours, but were significantly elevated (229.4 ± 51.1%) at 24 hours post ischemic injury. In ocular hypertensive eyes, acetyl histone-H3 levels were decreased by 5%, 26% and 28% after 3, 7 and 14 days, compared to contralateral control eyes. Class I HDAC activity was increased at 3, 7 and 14 days by 5.1 ± 4.8%, 12.7 ± 2.3% and 13.6 ± 3.2%. The administration of VPA provided a six fold increase in retinal histone-H3 acetylation and a reduction of caspase-3 activity to control levels, at 24 hours post injury. In ischemic and ocular hypertensive models, VPA administration produced significant functional and structural protection.
Conclusions: :
These studies provide evidence that changes in acetylation are early events post ischemic and ocular hypertensive injury, and precede elevations in caspase activity. In addition, these changes can be reversed by hyperacetylation via HDAC inhibition, providing a basis for the development of HDAC inhibitors in the treatment of retinal degeneration and optic neuropathies.
Keywords: neuroprotection • optic nerve • ischemia