Inflammasome upregulation and activation in photoreceptors of type 2 diabetes and N-methyl-N-nitrosourea models is attenuated by Wedelolactone

Abstract

Inflammation is known to play an important role in the pathogenesis of diabetic retinopathy (DR) and some aspects of retinal neurodegeneration although the precise underlying mechanisms linking these pathologies are not fully understood. Inflammation and oxidative damage are linked pathogenic processes that are enhanced during obesity and type 2 diabetes mellitus (T2DM). Building on this, there is some recent evidence suggesting an important role of the inflammasome which may link these pathways, especially in T2DM.

With focus on the diabetic retina, activation of the inflammasome in the retina and how it may differ between type 1 diabetes mellitus (T1DM) and T2DM has not yet been determined. There are several types of inflammasome but the so-called AIM2 inflammasome pathway may be the most important in this disease context, since it critically controls the production and maturation of pro-inflammatory cytokines, including IL1B and IL18 via canonical (CASP1) and non-canonical pathways (CASP4/11). Oxidative stress plays an important role in vascular and neurodegenerative changes in DR and has been shown to induce mitochondrial dysfunction in various retinal cells where it can manifest as loss of organelle membrane integrity and porosity, leading to the extravasation of mtDNA. In such scenarios, AIM2 inflammasome signalling is activated in response to DNA-sensing, leading to inflammation and cell death. Wedelolactone has been previously identified as a novel CASP4/11 inhibitor and its potential to modulate the inflammasome cascade may provide protection against diabetes-linked retinal neurodegeneration.

Murine models of T1DM (Ins2Akita and Streptozotocin-induced diabetes) and T2DM (BKsCgm+/+lepr) were used to determine the retinal inflammasome expression profile. This was conducted in conjunction with human ocular samples donated from age-matched controls with no-diabetes, T1DM, T2DM and T2D with DR (T2DM-DR). The role of free fatty acid palmitate was also assessed for its potential to regulate expression of key inflammasome components and activation of this pathway in 661W photoreceptor cells.

In vitro, 661W photoreceptor cells were treated with exogenous DNA (poly(dA:dT)) to determine the role of the AIM2 inflammasome in cell death. In vivo, chemical photoreceptor degeneration was induced via N-methyl-N-nitrosourea injection in C57BL/6J mice. Photoreceptor degeneration was also observed in BKsCgm+/+lepr mice. Wedelolactone treatment was employed in all models to assess its protective role in neurodegeneration of the outer retina.

The inflammasome components including CASP1, CASP4/11 and IL1B are components upregulated in murine and human retina of T2DM but not T1DM. IL1B is seen upregulated in both T1DM and T2DM, although, it shows varying localisation. Palmitic acid upregulates and activates inflammasomal components (NLRP3, CASP11, CASP1, IL1B, ASC) in 661W photoreceptor cells, more so than high glucose treatment. Wedelolactone attenuates poly(dA:dT) and NMU induced photoreceptor cell death, in vitro and in vivo, respectively. Protection of photoreceptors comes through downregulation of the AIM2/CASP11/CASP1/IL1B signalling pathway. The inflammasome pathway is upregulated and activated in the retina of T2DM, but not T1DM. Palmitic acid may play a pivotal role in obesity-induced inflammation and inflammasome activation in the retina of T2DM. Wedelolactone protects injured photoreceptors via downregulation of the AIM2/CASP11/CASP1/IL1B signalling pathway. With further research, Wedelolactone may be a candidate for protecting vision in ocular diseases where photoreceptor cell death occurs.

Date of Award	Jul 2021
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Mei Chen (Supervisor), Heping Xu (Supervisor) & Alan Stitt (Supervisor)