(AS2) Subarachnoid Hemorrhage Selectively Exposes Membrane-bound Glycans

Introduction: Subarachnoid hemorrhage (SAH) -- the accumulation of blood in the subarachnoid space -- is the most fatal stroke, with a 40% mortality rate. Furthermore, 95% of survivors suffer permanent disabilities. Hippocampal neuroinflammation following SAH has been recognized as a potential cause of post-SAH syndrome, and the complement system, in particular, has been identified as a major player. Levels of C1q, the activating protein of the classical pathway of the complement system, have been found to be significantly higher in the hippocampus (HPC). However, mechanisms of C1q activation remain unknown. We hypothesize that SAH events trigger the cleavage of terminal sialic acids (SA) from cellular surfaces, exposing potential binding sites for C1q.

Methods: To test this hypothesis in the perforation of the circle of Willis circle model of SAH, we employed immunohistochemical staining using various lectins and neuraminidase (NA) treatment to detect and compare SA and other glycans within the hippocampal layers in SAH and Sham mouse brains.

Results: C1q expression is significantly higher in the hippocampal dental gyrus (DG) following SAH, resulting from the remote damage of the entorhinal cortex inducing damage of the perforant pathway, projecting to the hippocampus. Terminal exposure of β-Galactose and N-acetyl-galactosamine significantly increased after SAH in the hippocampal layers suggests the cleavage of terminal SA. Calculation of C1q/SA immunostaining ratio showed an increase ratio in SAH versus Sham animals suggesting increased C1q binding following SAH due to the cleavage of SA. Slice treatment with exogenous NA yielded significantly higher expression of SA in Sham than in SAH animals. This implies that SAH leads to cleavage of SA decreasing number of possible binding sites for NA.

Conclusion : Our findings suggest that while SA cleavage from neuronal surfaces may serve as a potential contributor to neuroinflammation, there are other, unexplored surface markers being cleaved in the event of SAH.