library_construction_protocol: At 4 weeks after plating, RNA was collected from soma and axon compartments of microfluidic devices using the PicoPureTM RNA Isolation Kit (KIT0204, Applied Biosystems), following manufacturer's instructions. Due to the low RNA concentration obtained from the axon compartment, samples were then pre-amplified using the SuperScript™ IV Single Cell/Low Input cDNA PreAmp Kit (Invitrogen, 11752048), following manufacturer's instructions. After pre-amplification, cDNA samples were purified using the Agencourt AMPure XP beads (Beckman Coulter, A63880) and the DynaMag-2 Magnet (Invitrogen, 12321D). The cDNA products were next checked on Agilent Bioanalyzer. The Nextera XT DNA Library Preparation Kit (Illumina) was used for subsequent library preparation. The resulting libraries were quantified, and quality accessed by Qubit and Agilent TapeStation. The libraries were pooled in equal molarity and sequenced with 2×150bp paired-end configuration on an Illumina NovaSeq 6000 sequencer.