Perineuronal nets are phagocytosed by MMP‐9 expressing microglia and astrocytes in the SOD1G93A ALS mouse model

Abstract Aims Perineuronal nets (PNNs) are an extracellular matrix structure that encases excitable neurons. PNNs play a role in neuroprotection against oxidative stress. Oxidative stress within motor neurons can trigger neuronal death, which has been implicated in amyotrophic lateral sclerosis (ALS). We investigated the spatio‐temporal timeline of PNN breakdown and the contributing cellular factors in the SOD1 G93A strain, a fast‐onset ALS mouse model. Methods This was conducted at the presymptomatic (P30), onset (P70), mid‐stage (P130), and end‐stage disease (P150) using immunofluorescent microscopy, as this characterisation has not been conducted in the SOD1 G93A strain. Results We observed a significant breakdown of PNNs around α‐motor neurons in the ventral horn of onset and mid‐stage disease SOD1 G93A mice compared with wild‐type controls. This was observed with increased numbers of microglia expressing matrix metallopeptidase‐9 (MMP‐9), an endopeptidase that degrades PNNs. Microglia also engulfed PNN components in the SOD1 G93A mouse. Further increases in microglia and astrocyte number, MMP‐9 expression, and engulfment of PNN components by glia were observed in mid‐stage SOD1 G93A mice. This was observed with increased expression of fractalkine, a signal for microglia engulfment, within α‐motor neurons of SOD1 G93A mice. Following PNN breakdown, α‐motor neurons of onset and mid‐stage SOD1 G93A mice showed increased expression of 3‐nitrotyrosine, a marker for protein oxidation, which could render them vulnerable to death. Conclusions Our observations suggest that increased numbers of MMP‐9 expressing glia and their subsequent engulfment of PNNs around α‐motor neurons render these neurons sensitive to oxidative damage and eventual death in the SOD1 G93A ALS model mouse.