Connexin Hemichannel Block Using Orally Delivered Tonabersat Improves Outcomes in Animal Models of Retinal Disease

Abstract

Increased Connexin43 hemichannel opening is associated with inflammasome pathway activation and inflammation in a range of pathologies including ocular disorders, such as age-related macular degeneration (AMD) and diabetic retinopathy (DR). In this study, the effect on retinal function and morphology of clinically safe doses of orally delivered tonabersat, a small molecule connexin hemichannel blocker, was investigated in the light-damaged retina animal model of dry AMD and in a spontaneous rat model of DR. Clinical parameters (fundus imaging, optical coherence tomography (OCT), and electroretinography) and inflammatory markers (immunohistochemistry for Iba-1 microglial marker, astrocyte marker glial fibrillary acidic protein, and Connexin43 protein expression) were assessed. Tonabersat treatment reduced inflammation in the retina in parallel with preservation of retinal photoreceptor function when assessed up to 3 months post light damage in the dry AMD model. In the DR model, clinical signs, including the presence of aneurysms confirmed using Evans blue dye perfusion, were reduced after daily tonabersat treatment for 2 weeks. Inflammation was also reduced and retinal electrical function restored. Tonabersat regulates assembly of the inflammasome (NLRP3) through Connexin43 hemichannel block, with the potential to reduce inflammation, restore vascular integrity and improve anatomical along with some functional outcomes in retinal disease.

Tonabersat Inhibits Connexin43 Hemichannel Opening and Inflammasome Activation in an In Vitro Retinal Epithelial Cell Model of Diabetic Retinopathy

“…As such, connexin43 hemichannel blockers offer a promising alternative, directly targeting a fundamental upstream pathway in the disease pathogenesis, and one that may result in perpetuating the disease once initiated [ 11 ]. Previous studies have explored the efficacy of connexin43 hemichannel blockers for the treatment of DR in both in vitro and in vivo models [ 19 , 20 , 22 ]. These studies have shown that connexin43 hemichannel block with Peptide5 or tonabersat prevented both inflammatory and vascular signs of the disease.…”

“…These studies have shown that connexin43 hemichannel block with Peptide5 or tonabersat prevented both inflammatory and vascular signs of the disease. Tonabersat has also been shown to block connexin43 hemichannels and in an age-related macular degeneration model, where it was found to inhibit loss of electroretinogram function as well as retinal cell death and inflammation [ 21 , 22 ].…”

“…Another connexin43 hemichannel blocker, tonabersat, is being studied as an alternative to Peptide5. Tonabersat is a small-molecule benzopyran derivative that crosses the blood–brain barrier and has been shown to block connexin43 hemichannels to prevent signs of ocular pathology in several in vivo disease models following systemic and oral delivery [ 21 , 22 ]. However, while tonabersat has been shown to act as a connexin43 hemichannel blocker [ 21 ], its complete mode of action in disease conditions is yet to be proven.…”

Connexins and the Epithelial Tissue Barrier: A Focus on Connexin 26

“…This sequence is highly conserved between Cx43 and Cx37 with a variety of studies revealing Connexin subtype specificity [ 37 , 175 , 176 , 177 , 178 ]. Various platforms have now taken this work forward with the success of extracellular loop peptides, analogues of GAP27, targeting CX43 being successful in clinical trials for patients with ocular conditions including diabetic retinopathy and age related macular degeneration [ 179 , 180 ] ( ). Although there is currently no published information on Cx26 specific peptides, it is likely that targeting this domain could yield peptides capable of uniquely blocking Cx26 activity.…”

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

“…In the light-damaged rat retina, a model of retinal degeneration, hemichannels formed by Cx43 in the choroid, retinal pigment epithelium (RPE) and retina significantly contribute to the inflammatory response [ 12 , 13 , 14 , 15 , 16 ]. It is now understood that a substantial component of inflammation is mediated by the opening of unopposed Cx43 hemichannels [ 2 , 15 , 17 , 18 ], involved in the initiation stages and progression of the inflammatory reaction.…”