Engulfment of Axon Debris by Microglia Requires p38 MAPK Activity

Abstract

The clearance of debris after injuries to the nervous system is a critical step for restoration of the injured neural network. Microglia are thought to be involved in elimination of degenerating neurons and axons in the central nervous system (CNS), presumably restoring a favorable environment after CNS injuries. However, the mechanism underlying debris clearance remains elusive. Here, we establish an in vitro assay system to estimate phagocytosis of axon debris. We employed a Wallerian degeneration model by cutting axons of the cortical explants. The cortical explants were co-cultured with primary microglia or the MG5 microglial cell line. The cortical neurites were then transected. MG5 cells efficiently phagocytosed the debris, whereas primary microglia showed phagocytic activity only when they were activated by lipopolysaccharide or interferon-␤. When MG5 cells or primary microglia were co-cultured with degenerated axons, p38 mitogen-activated protein kinase (MAPK) was activated in these cells. Engulfment of axon debris was blocked by the p38 MAPK inhibitor SB203580, indicating that p38 MAPK is required for phagocytic activity. Receptors that recognize dying cells appeared not to be involved in the process of phagocytosis of the axon debris. In addition, the axons undergoing Wallerian degeneration did not release lactate dehydrogenase, suggesting that degeneration of the severed axons and apoptosis may represent two distinct self-destruction programs. We observed regrowth of the severed neurites after axon debris was removed. This finding suggests that axon debris, in addition to myelin debris, is an inhibitory factor for axon regeneration.

Phagocytosis of neuronal debris by microglia is associated with neuronal damage in multiple sclerosis

“…Our data support earlier studies showing that phagocytosis and degradation of apoptotic neurons by microglia was accomplished within 2 h (Parnaik et al, 2000). Rapid removal of degenerated myelin sheaths and axonal debris is required for axon regeneration and remyelination (Tanaka et al, 2009). In addition, persistence of tissue debris is more likely to lead to autoimmunity.…”

Osteopontin: Correlation with phagocytosis by brain macrophages in a rat model of stroke

“…Therefore, OPN appears not to be involved in the process of phagocytosis of the axon and myelin debris. In support of this, Tanaka et al (2009) suggested that ligands and receptors for phagocytosis of the dying neurons might be different from those for phagocytosis of the axon and myelin debris.…”

Iba1+/NG2+ Macrophage-Like Cells Expressing a Variety of Neuroprotective Factors Ameliorate Ischemic Damage of the Brain

“…BINCs may be involved in the removal of degenerated tissue debris by phagocytosis, since they bear phagosomes that internalize degenerated materials, as has been shown by immunoelectronmicroscopic observations . Removal of cell or tissue debris is shown to facilitate the regeneration of neural processes (Tanaka et al, 2009). BINCs or highly proliferative microglia-like cells can differentiate into new neurons and glial cells, as described in several reports (Butovsky et al, 2007;Niidome et al, 2008;Yokoyama et al, 2006), although it is difficult to evaluate the significance of this for regeneration of damaged brain tissue.…”