Progranulin protects the mouse retina under hypoxic conditions via inhibition of the Toll‑like receptor‑4‑NADPH oxidase 4 signaling pathway

Toll-Like Receptor Signalling Pathways and the Pathogenesis of Retinal Diseases

There is growing evidence that the pathogenesis of retinal diseases such as diabetic retinopathy (DR) and age-related macular degeneration (AMD) have a significant chronic inflammatory component. A vital part of the inflammatory cascade is through the activation of pattern recognition receptors (PRR) such as toll-like receptors (TLR). Here, we reviewed the past and current literature to ascertain the cumulative knowledge regarding the effect of TLRs on the development and progression of retinal diseases. There is burgeoning research demonstrating the relationship between TLRs and risk of developing retinal diseases, utilising a range of relevant disease models and a few large clinical investigations. The literature confirms that TLRs are involved in the development and progression of retinal diseases such as DR, AMD, and ischaemic retinopathy. Genetic polymorphisms in TLRs appear to contribute to the risk of developing AMD and DR. However, there are some inconsistencies in the published reports which require further elucidation. The evidence regarding TLR associations in retinal dystrophies including retinitis pigmentosa is limited. Based on the current evidence relating to the role of TLRs, combining anti-VEGF therapies with TLR inhibition may provide a longer-lasting treatment in some retinal vascular diseases.

Progranulin signaling in sepsis, community-acquired bacterial pneumonia and COVID-19: a comparative, observational study

Background

Progranulin is a widely expressed pleiotropic growth factor with a central regulatory effect during the early immune response in sepsis. Progranulin signaling has not been systematically studied and compared between sepsis, community-acquired pneumonia (CAP), COVID-19 pneumonia and a sterile systemic inflammatory response (SIRS). We delineated molecular networks of progranulin signaling by next-generation sequencing (NGS), determined progranulin plasma concentrations and quantified the diagnostic performance of progranulin to differentiate between the above-mentioned disorders using the established biomarkers procalcitonin (PCT), interleukin-6 (IL-6) and C-reactive protein (CRP) for comparison.

Methods

The diagnostic performance of progranulin was operationalized by calculating AUC and ROC statistics for progranulin and established biomarkers in 241 patients with sepsis, 182 patients with SIRS, 53 patients with CAP, 22 patients with COVID-19 pneumonia and 53 healthy volunteers. miRNAs and mRNAs in blood cells from sepsis patients (n = 7) were characterized by NGS and validated by RT-qPCR in an independent cohort (n = 39) to identify canonical gene networks associated with upregulated progranulin at sepsis onset.

Results

Plasma concentrations of progranulin (ELISA) in patients with sepsis were 57.5 (42.8–84.9, Q25–Q75) ng/ml and significantly higher than in CAP (38.0, 33.5–41.0 ng/ml, p < 0.001), SIRS (29.0, 25.0–35.0 ng/ml, p < 0.001) and the healthy state (28.7, 25.5–31.7 ng/ml, p < 0.001). Patients with COVID-19 had significantly higher progranulin concentrations than patients with CAP (67.6, 56.6–96.0 vs. 38.0, 33.5–41.0 ng/ml, p < 0.001). The diagnostic performance of progranulin for the differentiation between sepsis vs. SIRS (n = 423) was comparable to that of procalcitonin. AUC was 0.90 (95% CI = 0.87–0.93) for progranulin and 0.92 (CI = 0.88–0.96, p = 0.323) for procalcitonin. Progranulin showed high discriminative power to differentiate bacterial CAP from COVID-19 (sensitivity 0.91, specificity 0.94, AUC 0.91 (CI = 0.8–1.0) and performed significantly better than PCT, IL-6 and CRP. NGS and partial RT-qPCR confirmation revealed a transcriptomic network of immune cells with upregulated progranulin and sortilin transcripts as well as toll-like-receptor 4 and tumor-protein 53, regulated by miR-16 and others.

Conclusions

Progranulin signaling is elevated during the early antimicrobial response in sepsis and differs significantly between sepsis, CAP, COVID-19 and SIRS. This suggests that progranulin may serve as a novel indicator for the differentiation between these disorders.

Trial registration: Clinicaltrials.gov registration number NCT03280576 Registered November 19, 2015.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40635-021-00406-7.

MD2 blockade prevents modified LDL-induced retinal injury in diabetes by suppressing NADPH oxidase-4 interaction with Toll-like receptor-4

Modified LDL-induced inflammation and oxidative stress are involved in the pathogenesis of diabetic retinopathy. Recent studies have also shown that modified LDL activates Toll-like receptor 4 (TLR4) to mediate retinal injury. However, the mechanism by which modified LDL activates TLR4 and the potential role of the TLR4 coreceptor myeloid differentiation protein 2 (MD2) are not known. In this study, we inhibited MD2 with the chalcone derivatives L2H17 and L6H21 and showed that MD2 blockade protected retinal Müller cells against highly oxidized glycated-LDL (HOG-LDL)-induced oxidative stress, inflammation, and apoptosis. MD2 inhibition reduced oxidative stress by suppressing NADPH oxidase-4 (NOX4). Importantly, HOG-LDL activated TLR4 and increased the interaction between NOX4 and TLR4. MD2 was required for the activation of these pathways, as inhibiting MD2 prevented the association of NOX4 with TLR4 and reduced NOX4-mediated reactive oxygen species production and TLR4-mediated inflammatory factor production. Furthermore, treatment of diabetic mice with L2H17 significantly reduced LDL extravasation in the retina and prevented retinal dysfunction and apoptosis by suppressing the TLR4/MD2 pathway. Our findings provide evidence that MD2 plays a critical role in mediating modified LDL-induced cell injury in the retina and suggest that targeting MD2 may be a potential therapeutic strategy.

Blocking the activity of a protein involved in triggering inflammation and oxidative stress in the retina may reduce diabetes-induced visual impairment and blindness. Besides hyperglycemia, plasma lipids and lipoproteins have also been proposed as potential risk factors for diabetic retinopathy. However, the precise mechanisms controlling low-density lipoprotein-induced retinal damage are unclear. In experiments on mice, Yi Wang at Wenzhou Medical University in Wenzhou, China, and co-workers found that retinal injury caused by highly oxidized glycated-LDL is mediated by the myeloid differentiation protein 2 (MD2). Blocking MD2 with an inhibitor suppressed two key signaling pathways, reducing the production of reactive oxygen species and inflammatory signaling proteins in the retina. Further investigations are needed, but the team believe MD2 could be a vital therapeutic target for diabetic retinopathy.