Functional and structural changes in the neuroretina are accompanied by mitochondrial dysfunction in a type 2 diabetic mouse model

Abstract

Background Diabetic retinopathy (DR), one of the leading causes of blindness and vision impairment, is suggested to exhibit functional and structural changes in retinal neurons as the earliest manifestation, which could be used to predict the progression of related angiopathy. While neural function and survival rely on proper mitochondrial function, and a growing body of literature has supported the role of mitochondrial dysfunction in the development of DR, how diabetes affects mitochondrial function in retinal tissue remains elusive. This study primarily aimed to investigate mitochondrial functional changes in a diabetic rodent model. We also characterized the early DR phenotype, in particular, neurodegeneration. Methods C57BLKsJ-db/db (db/db) mice (a type 2 diabetic mouse model) were used with their normoglycemic heterozygous littermates (db/+) serving as controls. Longitudinal changes in retinal function and morphology were assessed with electroretinography (ERG) and optical coherence tomography (OCT), respectively, at 9, 13, 17, and 25 weeks of age. At 25 weeks, the retinas were harvested for immunohistochemistry and ex vivo mitochondrial bioenergetics. Results Decreased ERG responses were observed in db/db mice as early as 13 weeks of age. OCT revealed that db/db mice had significantly thinner retinas than the controls. Immunohistochemistry showed that the retinas of the db/db mice at 25 weeks were thinner at the outer and inner nuclear layers, with lower photoreceptor and cone cell densities compared with the db/+ mice. The number of rod-bipolar cell dendritic boutons and axon terminals was significantly reduced in db/db mice relative to the db/+ mice, suggesting that diabetes may lead to compromised synaptic connectivity. More importantly, the retinas of db/db mice had weaker mitochondrial functions than the controls. Conclusions Our longitudinal data suggest that diabetes-induced functional deterioration and morphological changes were accompanied by reduced mitochondrial function in the retina of db/db mice. These findings suggest that mitochondrial dysfunction may be a contributing factor triggering the development of DR. While the underlying mechanistic cause remains elusive, the db/db mice could be a useful animal model for testing potential treatment regimens targeting neurodegeneration in DR.

Coenzyme Q10 eyedrops conjugated with vitamin E TPGS alleviate neurodegeneration and mitochondrial dysfunction in the diabetic mouse retina

“…The protective effect of CoQ10 against diabetic retinal neuropathy is further supported by the histological data, in which CoQ10 treated db/db mice were found to have thicker ONL and INL and higher photoreceptor and cone soma densities compared with their age-matched controls. Our lab and others have previously demonstrated disruption of synaptic structures in second-order neurons at the outer plexiform layer without apparent decreased bipolar cell viability in diabetic mice ( Hombrebueno et al, 2014 ; Lam et al, 2023 ). In this study, the number of rod-bipolar cell dendritic boutons was found to be significantly preserved in CoQ10 treated db/db mice compared to the controls.…”

“…In addition, mitochondrial dysfunction has been observed in various retinal cells, including retinal endothelial cells, Müller cells, and photoreceptor cells under simulated hyperglycemia ( Trudeau et al, 2010 ; Tien et al, 2017 ; Lam et al, 2022 ). Recently, our lab has demonstrated a compromised mitochondrial biogenesis in the retina of db/db mice, in which functional and structural changes in the neuroretina were observed, compared to their non-diabetic heterozygous littermate ( Lam et al, 2023 ). These findings suggested that mitoprotection may be beneficial in ameliorating DR.…”

“…The in vivo retinal functions were assessed with full field ERGs (ffERGs) at baseline, 1-, 2-, and 4-month after CoQ10 treatment as described previously ( Lam et al, 2023 ). For scotopic ERG, the mice were dark-adapted overnight (at least 16 h) prior to testing in the experimental room.…”

“…Although the rod bipolar cell density of the CoQ10 treated db/db mice was higher than that of the control mice, the difference did not reach statistical significance (Figure 4H, db/db control = 38.01 ± 3.21 vs. CoQ10 treated = 46.60 ± 3.83; Independent sample t-test: t = −1.682, df = 11, p = 0.121). Previously, we reported a reduced number of rod-bipolar cell axon terminals and dendritic boutons in db/db mice, indicating that diabetes leads to a compromised synaptic connectivity of the second-order neurons with other retinal neurons (Lam et al, 2023). Based on these earlier findings, the number of rod-bipolar cell axon terminals and dendritic boutons in the db/db mice after CoQ10 treatment were examined.…”

“…Furthermore, high metabolic activity and constant exposure to damaging light radiation may render the retina more vulnerable to oxidative stress under diabetic conditions. Therefore, there is growing interest in using antioxidants to treat DR. Our previous studies have demonstrated the involvement of mitochondrial dysfunction in DR using both in vitro and in vivo models ( Lam et al, 2022 , 2023 ). Similar findings have also been observed in other retinal cell types, such as retinal endothelial cells and Müller cells under simulated hyperglycemia ( Trudeau et al, 2010 ; Tien et al, 2017 ).…”