|Ph.D Student||Kehat Rinat|
|Subject||The Role of Acetylcholinesterase in Light-Induced|
Photoreceptor Damege in the Rat Retina
|Department||Department of Medicine||Supervisors||Professor Emeritus Ido Perlman|
|Professor Hermona Soreq|
|Full Thesis text|
Photoreceptors are the light-sensing cells in the retina. Their loss is the primary cause of blindness in degenerative diseases such as retinitis pigmentosa and age related macular degeneration. Exposure of albino rats to bright light provides an established model to study the mechanisms underlying photoreceptor injury.
Acetylcholinesterase (AChE), the acetylcholine (ACh)-hydrolyzing enzyme, apart from terminating synaptic transmission, also plays morphogenic roles, and participates in neuronal stress responses. Stress-induced expression of AChE is accompanied by a shift in the protein C-terminus from the “synaptic” (AChE-S) to the “read-through” (AChE-R) variant, both including the core domain that hydrolyzes ACh. In addition, each of these variants may appear with or without an extended N-terminus. Suppression of AChE-R mRNA overexpression, in stressed mice, by administration of Monarsen, an antisense oligonucleotide, improved histological and functional parameters, suggesting that AChE-R overexpression contributed to neuronal death.
In the retina, in addition to AChE expression in areas of cholinergic transmission, AChE mRNA expression was surprisingly demonstrated in human adult photoreceptors, where no cholinergic activity exists, raising the possibility of stress-related morphogenic function(s).
The goal of this study was to assess the role of AChE in the response of the albino rat retina to short exposure to bright, damaging light and to identify the variant of the expressed AChE.
Adult albino rats were exposed for 24 hours to bright light. AChE mRNA expression was monitored by in-situ hybridization. Histochemistry was used to determine AChE activity, and the presence and identity of the protein was monitored by immunocytochemistry. Monarsen was administered intraperitoneally to minimize light-induced AChE expression. The electroretinogram (ERG) was recorded to assess retinal function.
24 hour exposure to bright light caused severe reduction in the ERG responses and augmented expression of AChE-R mRNA in photoreceptor inner segments (IS), bipolar cells, and ganglion cells. AChE activity increased in IS. The expressed AChE protein had an extended N-terminus (N-AChE). Systemic administration of Monarsen reduced the photic induction of N-AChE, and improved the ERG and morphological parameters compared with light-exposed, saline treated rats.
The findings indicated that the photic-induced N-extended AChE, probably with a readthrough C-terminal, may be causally involved with retinal light damage. The mechanism may be through a reduction of acetylcholine level, by the induced AChE, promoting inflammation and tissue damage. Alternatively, stress-induced AChE could contribute to cellular apoptosis through noncatalytic properties. In that case, excess AChE could induce cell death in noncholinergic neurons, as well.