In vivo retinal imaging is associated with cognitive decline, blood-brain barrier disruption and neuroinflammation in type 2 diabetic mice

Abstract

IntroductionType 2 diabetes (T2D) is associated with chronic inflammation and neurovascular changes that lead to functional impairment and atrophy in neural-derived tissue. A reduction in retinal thickness is an early indicator of diabetic retinopathy (DR), with progressive loss of neuroglia corresponding to DR severity. The brain undergoes similar pathophysiological events as the retina, which contribute to T2D-related cognitive decline.MethodsThis study explored the relationship between retinal thinning and cognitive decline in the LepR db/db model of T2D. Diabetic db/db and non-diabetic db/+ mice aged 14 and 28 weeks underwent cognitive testing in short and long-term memory domains and in vivo retinal imaging using optical coherence tomography (OCT), followed by plasma metabolic measures and ex vivo quantification of neuroinflammation, oxidative stress and microvascular leakage.ResultsAt 28 weeks, mice exhibited retinal thinning in the ganglion cell complex and inner nuclear layer, concomitant with diabetic insulin resistance, memory deficits, increased expression of inflammation markers and cerebrovascular leakage. Interestingly, alterations in retinal thickness at both experimental timepoints were correlated with cognitive decline and elevated immune response in the brain and retina.DiscussionThese results suggest that changes in retinal thickness quantified with in vivo OCT imaging may be an indicator of diabetic cognitive dysfunction and neuroinflammation.

The glucocorticoid receptor as a master regulator of the Müller cell response to diabetic conditions in mice

“…Bogdanov et al [73] describe neurodegeneration as early as 2 months of age -an age when mice just develop hyperglycaemia. However, even though they show a slightly reduced retinal thickness at this early age, they do not show a loss of neuronal cells over the course of ageing from 2 to 6 months, which one would expect to see in a progressive disease like DR. More consistent with our findings, many other studies report no change in retinal layer thickness in db/db mice at approximately 3 months of age, but an onset of retinal thinning and neuronal cell loss at approximately 6-7 months of age [37,44,49]. We found significantly reduced cone numbers, which is in line with other studies that found predominant cone photoreceptor dysfunction in early-stage DR in zebrafish [77] and humans [78,79].…”

“…This alings with reports that BRB breakdown and increased apoptosis occur in older db/ db mice [44,45]. Similarily, while microglial activation was observed in mice aged 6-9 months, we detected proinflammatory microglial signatures in 6-month-old db/ db mice before major cellular architecture changes, consistent with other studies [37,49]. However, we detected typical pro-inflammatory microglial signatures in cells isolated from retinae of 6-months-old db/db mice similar to earlier reports [71], thus, before major changes in their cellular architecture occurred, which is supported by the results of other studies [72].…”

“…We next examined whether microglia activation can be detected as reported by others [ 37 ]. We found no significant difference in the number of Iba1-positive microglia/macrophages in the retinae of db/db and control animals (Fig.…”

“…Our MACS approach has the limitation that Müller cells may be slightly contaminated with astrocytes, which express high levels of GFAP in the homeostatic retina. Thus, the increase in GFAP levels detected in the MACS-purified glial population may also be due to expression changes in astrocytes that has consistently been documented in db/db mice [37,72,74] or may be too low to be detected in Müller cells by immunostaining as also described by others [75]. However, other molecular markers of Müller cell gliosis were found to be dysregulated including Kir4.1, Stat3, S100a1 and Cd44 [47,48].…”

“…Here, we report on the role of Müller cells in the context of DR using the db/db mouse with a leptin receptor gene mutation, which develops type 2 diabetes and exhibits DR-like abnormalities [33,34]. They show elevated insulin levels by 14 days, obesity by 1 month, and hyperglycaemia by 1-2 months of age [35][36][37][38][39]. Features of DR, such as pericyte loss, altered neurovascular coupling, and increased generation of reactive oxygen species, appear at the earliest approximately 1 month after the onset of hyperglycaemia in 3 month old db/db mice [40][41][42], with increased expression of and VEGF, indicative of vascular remodelling, detected in animals at 5 months of age [43].…”