Keywords
D-allose; Gal-3; TLR4 signaling; neuroinflammation; ischemic stroke; neurological dysfunction
Introduction
D-allose (PubChem CID: 439507), a C-3 epimer of D-glucose which originally isolated from some plants, is an important typical rare monosaccharide safe for consumption by human beings1. Growing evidence showed that D-allose has an inhibitory effect on the production of reactive oxygen species (ROS) in inflammatory reactions in leukocytes2, an activation effect on programmed cell apoptosis in tumor cells3, an anti-inflammatory effect to protect the kidney4, liver5 and retina6 from ischemia/reperfusion(I/R) injury. Our previous study initially explored the anti-inflammatory effects of D-allose in a model of focal cerebral I/R injury in rats7. However, it remains unclear to date the mechanism of D-allose neuroprotective effect after ischemic stroke (IS) by simultaneously reducing neuroinflammation damage and neuron-specific apoptosis.
Globally, IS is one of the leading causes of death and long-term disability at present and is associated with the shortage of effective treatments despite numerous clinical trials, because the pathophysiology of various cells in injured brain tissue is not yet well understood8-10. Lots of experimental results have indicated that inflammatory response has a direct negative impact on ischemic neurons, a brain function executor, leading to the collapse of organelle structure and the release of pro-apoptotic proteins, and ultimately causes cerebral apoptosis and necrosis11-14. Therefore, protecting neurons from inflammatory damage and apoptosis could be a rational and efficient measure for ameliorating the neurological dysfunction of IS patients.
Galectin-3 (Gal-3), is an important member of β-galactoside- binding lectin family and was first recognized in murine immune cells, and thereafter found in microglial cells, astrocytes and neurons, which was demonstrated to be a novel inflammatory factor participating in the process of inflammation, oxidative stress, cellular proliferation, cell apoptosis and pyroptosis15-19. Given evidence showed that the effect of Gal-3 in neuroinflammation of neurodegeneration and central nerve system (CNS) injury is contentious20. Some studies’ results showed that deletion of the Galectin-3 gene attenuates experimental through decreased expression level of inflammatory cytokines in C57Bl/6 mice during autoimmune encephalomyelitis progression21. The other research indicated that Gal-3 deficiency in microglial cells and astrocytes of C57Bl/6 mice enhanced inflammatory response in Wallerian degeneration (WD)22. Nevertheless, the function of Gal-3 in the apoptotic process is somewhat controversial. Wesley et al reported that Gal-3 in neuro-vascular protection and functional recovery after IS through upregulation of the cerebral blood vessel density and downregulation of neuron apoptotic death23,24. On the contrary, Fukumori T et al showed that tumor cells’ secreted Gal-3 induces T-cells’ apoptosis, which in turn enhanced the effect of Gal-3 anti-apoptosis of tumor cells25. In summary, the potential injury or protective mechanism of Gal-3 on inflammation damage-induced neuronal apoptosis post-IS are largely unknown to date.
Secondary neuroinflammation can occur after acute ischemic stroke. Immune-mediated pro-inflammatory signals rapidly activate various types of inflammatory cells, causing them to infiltrate into areas of ischemia and exacerbate brain damage26. Among all the inflammatory-related proteins and signaling pathways, Toll-like receptor 4 (TLR4) and TLR4-dependent PI3K/AKT signaling are one of the crucial inflammatory pathways found at present, which is related to the pathogenesis and development of many neuroinflammatory diseases, especially protecting the brain from I/R-induced injury27-32. Mietto BS et al observed that a lack of Gal-3 increased both RNA and protein levels for the pro-inflammatory cytokines, as well as for TLR2 and TLR4, which ultimately contributed to the prognosis of WD22. Conversely, Liu et al reported recently that TD139, a specific inhibitor of Gal-3, alleviates TLR4 and MyD88 of microglia in autoimmune uveitis33. Therefore, the relationship between the TLR4/PI3K/AKT signaling axis and Gal-3 in inflammatory damage of neurons induced by IS is unknown. Moreover, like TD139, whether D-allose plays an anti-inflammatory role via inhibiting the expression of the Gal-3 and TLR4 signaling pathway in IS is also unclear.
Taken together, the aims of this study were (1) to determine whether D-allose pretreatment alleviates the inflammation damage and ischemic neuronal apoptosis induced by brain I/R injury; and, if so, (2) to evaluate the involved underlying mechanisms of down-regulation Gal-3 via TLR4/ PI3K/AKT signaling in mediating the protective mechanisms of D-allose.