The striatum or the V-SVZ was dissected from P4 to P7 WT mice and cultured as previously described (11 (link)). To increase the GFAP and Robo2 expression, immediately after changing the medium to FreeStyle (Thermo Fisher Scientific) containing penicillin/streptomycin (50 U/ml), the striatal astrocytes were incubated under 0 to 2% O2 (AnaeroPack Kenki, Mitsubishi Gas Chemical Company, Tokyo, Japan) for 20 hours and then cultured in Dulbecco’s modified Eagle’s medium containing 10% fetal bovine serum, penicillin/streptomycin (50 U/ml), and 2 mM l-glutamine for 0 to 4 days, depending on the experiment.