Decreased sialylation in the mouse retina elicits complement-related microglia response and bipolar cell loss
Sialylation plays an important role in self-recognition, as well as keeping the complement and innate immune systems in check. It is unclear whether the reduced sialylation as seen during aging and in mice heterozygous for the null mutant of UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (Gne+/-), an essential enzyme for sialic acid biosynthesis, contributes to retinal inflammation and degeneration. We found a reduction of polysialic  acid and trisialic acid expression in several retinal layers in Gne+/- mice at 9 months of age, which was associated with a higher  microglial expression of the lysosomal marker CD68. Furthermore, the total number of rod retinal bipolar cells was reduced in 12 months old Gne+/- mice, demonstrating loss of these retinal interneurons that are involved in connecting photoreceptors and ganglion cells. Whole-genome transcriptome analysis showed up-regulation of the complement, inflammation, and apoptosis pathways in the retinas of Gne+/- mice. Particularly, increased gene transcript levels of the complement factors C3 and C4 and the pro-inflammatory cytokine Il-1ß were observed by semi-quantitative real-time polymerase chain reaction (sqRT-PCR) in 9 months old Gne+/- mice. The Iincreased expression of CD68, as well as the loss of rod bipolar cells, and increased gene transcription of the complement factor C4, were all prevented after crossing Gne+/- mice with complement factor C3-deficient animals.  In conclusion, our data show that retinal hyposialylation in middle-aged Gne+/- mice, at 9 and 12 months of age,  was associated with complement-related inflammation and lysosomal microglia response, as well as rod bipolar cells loss, which was absent in animals with a genetic deletion of complement factor C3. Overall design: To investigate the effect of reduced sialylation on the retinas of middle-ages mice, we studied the retinas of heterozygous Gne gene knock-out mouse model (Gne+/-) which have about 25% overall reduction in sialic acid, and compared them to Gne+/+ wildtype mice. To investigate whether the effects seen in the Gne+/- mice retinas are driven by the complement system, we crossed both the Gne+/+ wildtype mice and the Gne+/- mice with a complete complement knock-out mouse, resulting in Gne+/+ C3-/- and Gne+/- C3-/- mice. RNA-sequencing was performed on nine months old mice.