Differential Action of Connexin Hemichannel and Pannexin Channel Therapeutics for Potential Treatment of Retinal Diseases

Abstract

Dysregulation of retinal function in the early stages of light-induced retinal degeneration involves pannexins and connexins. These two types of proteins may contribute to channels that release ATP, leading to activation of the inflammasome pathway, spread of inflammation and retinal dysfunction. However, the effect of pannexin channel block alone or block of both pannexin channels and connexin hemichannels in parallel on retinal activity in vivo is unknown. In this study, the pannexin channel blocker probenecid and the connexin hemichannel blocker tonabersat were used in the light-damaged rat retina. Retinal function was evaluated using electroretinography (ERG), retinal structure was analyzed using optical coherence tomography (OCT) imaging and the tissue response to light-induced injury was assessed immunohistochemically with antibodies against glial fibrillary acidic protein (GFAP), Ionized calcium binding adaptor molecule 1 (Iba-1) and Connexin43 (Cx43). Probenecid did not further enhance the therapeutic effect of connexin hemichannel block in this model, but on its own improved activity of certain inner retina neurons. The therapeutic benefit of blocking connexin hemichannels was further evaluated by comparing these data against results from our previously published studies that also used the light-damaged rat retina model. The analysis showed that treatment with tonabersat alone was better than probenecid alone at restoring retinal function in the light-damaged retina model. The results assist in the interpretation of the differential action of connexin hemichannel and pannexin channel therapeutics for potential treatment of retinal diseases.

Orally Delivered Connexin43 Hemichannel Blocker, Tonabersat, Inhibits Vascular Breakdown and Inflammasome Activation in a Mouse Model of Diabetic Retinopathy

“…Furthermore, in another study carried out in the same inflammatory NOD mouse model used here, we showed that blocking connexin43 hemichannels via intravitreal Peptide5 protected against Müller cell and microglial activation [ 11 ]. The effects of oral tonabersat on retinal inflammation has also been shown in other models of DR and AMD [ 9 , 41 , 42 , 43 ]. Mat Nor et al [ 43 ] showed in a retinal light damage model of dry AMD as well as in a hyperglycaemic rat DR model that oral tonabersat treatment reduced the number of Iba-1+ cells and GFAP expression within retinal Müller cells.…”

“…Tonabersat (0.8 mg/kg; MedChemExpress, Princeton, NJ, USA) was delivered via oral gavage prior to ocular assessments for the safety study. This dose had been optimised in previous studies [ 9 , 43 ]. For the efficacy study, tonabersat was delivered 2 h before administration of intravitreal cytokines, in line with previous clinical trial data suggesting that the t max of the drug can be achieved between 0.5 and 3 h [ 24 ].…”

“…Several studies have shown that blocking pathological connexin43 hemichannels is an effective way to prevent inflammasome activation and its perpetuation in DR [ 8 , 9 , 10 , 11 , 12 , 13 ]. In an in vitro study developed in retinal pigment epithelial (RPE) cells using a combination of high glucose and pro-inflammatory cytokines, IL-1β, and tumour necrosis factor-alpha (TNF-α), it was shown that blocking connexin43 hemichannels using Peptide5 inhibited ATP-dependent activation of the NLRP3 inflammasome [ 10 ].…”