To compare phagocytic ability of normal and A1 astrocytes, we measured engulfment of purified synaptosomes. A1s engulfed 50–75% fewer synaptosomes than control astrocytes (Fig. 3a,b). Similarly, we found control astrocytes are able to robustly phagocytose myelin debris, but upon conversion to an A1 reactive phenotype almost completely lose this capacity (Fig. 3a,c). This phagocytic deficit corresponded with a 90% decrease in Mertk and 60% decrease in Megf10 mRNA, phagocytic receptors we have previously found mediate synaptic phagocytosis3 (Fig. 3f). To determine whether A1s also display decreased phagocytic ability in vivo, we used Aldh1l1-eGFP transgenic mice and LPS injection (Extended Data Fig. 6) to visualize phagocytosis of Alexa594-conjugated cholera toxin-β, CTB-594, labelled synapses by control and A1 astrocytes. Confocal microscopy was used to visualize engulfed CTB-labelled synapses inside Aldh1l1-eGFP fluorescent astrocytes as we previously reported3. We found A1 reactive astrocytes in the LGN in vivo show the same significant loss of synaptic engulfment ability (around 50% compared to astrocytes in saline-treated control animals) as was seen in our in vitro assay (Fig. 3a,b,d,e). Combined, these data show A1 reactive astrocytes have
