Abstract
Purpose::
Carboxypeptidase E (CPE) is a neuropeptide processing enzyme that generally cleaves the C-terminal end of target peptides. CPE functions to activate neuropeptide precursors and facilitates the sorting of prohormones for the regulated secretory pathway. Immunohistochemical data indicate that CPE is localized to the retinal outer plexiform layer. Intravitreal administration of brain-derived neurotrophic factor (BDNF), a CPE target protein, has been shown to have a neuroprotective role in a cat model of retinal detachment. The purpose of the study is to investigate whether CPE is activated, within the retina, with subsequent processing of BDNF and other downstream stream targets of CPE in a rat model of serous retinal detachment (SRD) induced by retinal venous occlusion (RVO).
Methods::
RVO was induced in one eye of Sprage-Dawley rats by green laser irradiation of all branched retinal veins following infusion with rose bengal. SRD emerged in laser treated eyes within two days. Retinas were harvested at serial time points: day 2 (newly detached), day 4, day 6-8 (following spontaneous re-attachment) and day 13. Total retinal protein was purified. Both pro- and active forms of CPE and BDNF were detected by Western analysis. The contralateral unlasered eye and RVO eyes that had not developed SRD at equivalent time points served as controls.
Results::
Western analysis showed the presence of the precursor of CPE/proCPE (55 Kd) and the two active forms of CPE: membrane-bound (53 Kd) and soluble form (50 Kd) in SRD eyes at day 2, whereas the soluble form was absent from both contralateral and non-detached RVO control eyes. The level of soluble CPE in SRD eyes increased at day 4 relative to day 2 and was undetectable at days 6-8 and day 13. Western analysis revealed similar changes in BDNF. The active form of BDNF (14Kd) was detectd only in SRD eyes at day 2 and was accompanied by a decrease in proBDNF (34Kd). The active form was undetectable by days 6-8 at which point the retina had reattached.
Conclusions::
Our findings suggest that serous retinal detachment may enhance the biosynthesis of soluble CPE in this rat model of retinal detachment, with subsequent activation of BDNF. It is possible that active CPE may act in a "wound-healing" response in retinal neuronal cells during retinal detachment and/or ischemia. Whether the mRNA levels of CPE and BNDF are altered in this model is also of interest. Exogenous CPE could play a neuroprotective role in the clinically setting of retinal detachment and is under investigation.
Keywords: retinal detachment • neuropeptides • neuroprotection