Therapeutic hypothermia protects photoreceptors through activating Cirbp pathway

Abstract

Therapeutic hypothermia as a physical method to lower the brain temperature of patients has been widely used in clinics as an effective and necessary step during the treatment of acute brain injury or edema. However, due to limitations of the ocular structure, the application of hypothermia in retinal neuroprotection still has an obvious barrier. Here, the neuroprotective mechanism produced by hypothermia in the retina was investigated, with the hopes of deciphering the key molecular targets of the signaling pathway to finally realize the ocular neuroprotection by regulating specific molecular targets. In present study, it was first demonstrated that hypothermia produced significant neuroprotection on photoreceptors (661 W cell) against glucose deprivation (GD)-induced injury in vitro and visible light-induced retinal damage in vivo. The results disclosed that hypothermia (32 °C) was able to attenuate the upregulation of heme oxygenase-1, cleaved Caspase-3, cleaved Caspase-9, and B-cell lymphoma-2-associated X caused by GD, and restored the decline of protective factor B-cell lymphoma-2 as well. Moreover, hypothermia suppressed the excessive generation of intracellular reactive oxygen species and depolarization of mitochondrial membrane potential, and showed marked neuroprotection against GD-induced damage in photoreceptors, which significantly reduced cell death percentage in vitro. In in vivo experiments, it was found that hypothermia was able to protect retinal function against light injury, restoring the decline of a-waves and b-waves in electroretinograms and maintaining the thickness of the retinal outer nuclear layer. Furthermore, hypothermia blocked the visible light-induced cell death pathway in the retina, suppressing poly(ADP-ribose) polymerase-1 activation. More importantly, it was demonstrated that cold-inducible RNA-binding protein (Cirbp) as a key molecular target played an important role in hypothermia-induced neuroprotection, which is the first proof of its function in ophthalmology. In in vitro experiments, hypothermia caused marked expression of Cirbp in photoreceptors. And reducing the expression of Cirbp with specific small interfering RNA was able to block the hypothermia-induced neuroprotection. Consistently, overexpressed Cirbp with Cirbp-gene-modified lentivirus mimicked the neuroprotection against GD-induced injury even under normal temperature (37 °C) conditions. Additionally, the overexpression of Cirbp was detected in hypothermia-treated retinas. These results indicate that hypothermia promotes neuroprotection in photoreceptors via activation of the Cirbp pathway. The study presented here suggests that therapeutic hypothermia may promote neuroprotection in the retina by activating Cirbp, and regulating Cirbp may mimic similar protection even under normal temperature conditions, which might be a specific molecular target in retinal neuroprotection.