Hyper-reflective dots in optical coherence tomography imaging and inflammation markers in diabetic retinopathy.

The aim of this study is to correlate small dot hyper-reflective foci (HRF) observed in spectral domain optical coherence tomography (SD-OCT) scans of an animal model of hyperglycaemia with focal electroretinography (fERG) response and immunolabelling of retinal markers. The eyes of an animal model of hyperglycaemia showing signs of diabetic retinopathy (DR) were imaged using SD-OCT. Areas showing dot HRF were further evaluated using fERG. Retinal areas enclosing the HRF were dissected and serially sectioned, stained and labelled for glial fibrillary acidic protein (GFAP) and a microglial marker (Iba-1). Small dot HRF were frequently seen in OCT scans in all retinal quadrants in the inner nuclear layer or outer nuclear layer in the DR rat model. Retinal function in the HRF and adjacent areas was reduced compared with normal control rats. Microglial activation was detected by Iba-1 labelling and retinal stress identified by GFAP expression in Müller cells observed in discrete areas around small dot HRF. Small dot HRF seen in OCT images of the retina are associated with a local microglial response. This study provides the first evidence of dot HRF correlating with microglial activation, which may allow clinicians to better evaluate the microglia-mediated inflammatory component of progressive diseases showing HRF.

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

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.

Connexin therapeutics: blocking connexin hemichannel pores is distinct from blocking pannexin channels or gap junctions.

Compounds that block the function of connexin and pannexin protein channels have been suggested to be valuable therapeutics for a range of diseases. Some of these compounds are now in clinical trials, but for many of them, the literature is inconclusive about the molecular effect on the tissue, despite evidence of functional recovery. Blocking the different channel types has distinct physiological and pathological implications and this review describes current knowledge of connexin and pannexin protein channels, their function as channels and possible mechanisms of the channel block effect for the latest therapeutic compounds. We summarize the evidence implicating pannexins and connexins in disease, considering their homeostatic versus pathological roles, their contribution to excesive ATP release linked to disease onset and progression.

Connexin43 Mimetic Peptide Improves Retinal Function and Reduces Inflammation in a Light-Damaged Albino Rat Model.

PURPOSE: Drugs that regulate connexin43 (Cx43) gap junction channels can reduce the spread of injury and improve functional outcomes after nervous system trauma. In the eye, Cx43 expression increases in the choroid following light damage. The aim of this study was to investigate whether Cx43 hemichannel block could preserve retinal function postinjury. METHODS: Light damage was induced by exposure of adult albino Sprague-Dawley rats to 2700 Lux light for 24 hours. Intravitreal injections of a Cx43 mimetic peptide hemichannel blocker, Peptide5, or sham were administered 2 hours after the onset and at the end of the light damage period. Retinal function was assessed by electroretinogram and inflammatory responses in the choroid and retina were assessed using immunohistochemistry (ionized calcium binding adaptor molecule 1 [Iba-1], leukocyte common antigen [CD45], glial fibrillary acidic protein [GFAP]). RESULTS: Light-damaged rat eyes had (1) reduced neuronal responses in both the rod and cone pathways and (2) marked inflammatory responses in the choroid and retina. Peptide5 significantly preserved function of photoreceptoral and postphotoreceptoral neurons in these animals. This was evident 24 hours after injury and 2 weeks later, as shown by improved mixed a-wave and mixed b-wave amplitudes, isolated rod PII and PIII amplitudes, and cone PII responses when compared with sham-treated controls. Retinal thinning and inflammation were also significantly reduced in Peptide5-treated eyes when compared with sham-treated controls. CONCLUSIONS: Blocking Cx43 hemichannels after light damage can significantly improve functional outcomes of neurons in both the rod and cone photo-transduction pathways in the light-damaged animal model, likely by reducing choroid inflammation and suppressing the glial-mediated inflammatory response. These data may have relevance for the treatment of conditions such as diabetic retinopathy and age-related macular degeneration.