Sialic acids reduction triggers complement C3-mediated microglial response and bipolar cell loss in the mouse retina

Abstract

Sialic acids are sugars located at the cell surface on glycoproteins and lipids, and act as check-point inhibitors of the innate immune and complement systems. Sialic acids are abundant throughout the central nervous system, including the retina. Notably, aging is associated with decreased levels of sialylated gangliosides. However, it remains unknown whether reduced sialylation contributes to retinal inflammation and degeneration. Here, I investigated the retina of mice heterozygous for the null mutant of UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase GNE (Gne+/-), an enzyme crucial for sialic acid biosynthesis, to determine whether decreased sialic acids levels (hyposialylation) contribute to retinal inflammation and degeneration. <br /> The immunostainings results showed decreased expression of trisialic and polysialic acids in most retinal layers in 9-month-old Gne+/- mice compared to wildtype mice (WT). This hyposialylation was correlated with higher microglial expression of the lysosomal marker CD68. The RNA sequencing analysis and semi-quantitative real-time polymerase chain reaction showed increased gene transcription and pathways related to inflammation, complement system, and retinal-associated functions in retinas of Gne+/- mice compared to WT mice. Principally, increased gene transcription was observed in complement factors C3 and C4, and the pro-inflammatory cytokine interleukin 1-beta in 9-month-old Gne+/- mice. In addition, the number of bipolar cells was reduced in 12-month-old Gne+/- mice in comparison to WT mice, suggesting loss of these interneurons that connect photoreceptors with ganglion cells. Interestingly, the transcriptional changes including increased transcription of C4, the higher expression of CD68, as well as the bipolar cell loss were prevented in the Gne+/- mice that were crossed with C3-deficient mice. <br /> Overall, the data from this study indicate that hyposialylation is associated with an enhanced lysosomal response in microglia, an increase in pro-inflammatory markers, and loss of bipolar cells. These retinal changes were dependent on complement factor C3. Thus, these findings confirm that sialylation serves as a checkpoint for the immune system in the mouse retina and suggest that targeting sialylation could be a potential therapeutic approach for inflammatory and degenerative retinal diseases.