Removing the dead: Olfactory ensheating cells phagocytose axonal debris

Abstract

Olfactory ensheathing cells (OECs) are the glial cells of the olfactory system. Their primary role is thought to be to provide support and guidance for primary olfactory axons. However, OECs are known to phagocytose bacteria and express immune markers and thus they may help to maintain a healthy environment. Interestingly, following widespread death of primary olfactory axons, there is minimal mobilisation of macrophages but yet the axonal debris is rapidly cleared. Purpose: We have therefore investigated whether OECs are the cells that are primarily responsible for removal of axonal debris. Methods: We cultured red fluorescent OECs from S100beta-DsRed mice and green fluorescent primary olfactory neurons from OMP-ZsGreen mice. Results: In explant cultures of DsRed-OECs and ZsGreen-neurons, OECs clearly contained green fluorescent axonal debris. When cellular debris from green fluorescent neurons was added to cultured OECs, the OECs extended pseudopodia and rapidly phagocytosed the axonal debris (n=5). We examined sections through the olfactory system in healthy animals throughout development (E15 to adult, n=5 animals at each time point) and found that with increasing age OECs contained increasing levels of axonal debris. Following degeneration of olfactory neurons by injection of methimazole, OECs had significantly more axonal debris (30-50% more, n=5, p<0.005) compared to controls. In comparison, macrophages within the olfactory system did not display increased levels of axonal debris, indicating that OECs, rather than macrophages, are the cells that are primarily responsible for removal of axonal debris. Conclusion: These results clearly demonstrate that OECs actively phagocytose cellular debris and thus is another mechanism by which they maintain the health of the olfactory system.