Impact of Microglia Isolation and Culture Methodology on Transcriptional Profile and Function
Background: Microglial isolation and culturing methods continue to be explored to maximize cellular yield, purity, responsiveness to stimulation and similarity to in vivo microglia. This study aims to evaluate different microglia isolation methods — 3 variants of microglia isolation from neonatal mice and 2 variants of microglia isolation from adult mice — on transcriptional profile and response to HMGB1. Methods: Microglia from neonatal mice, age 0-3 days (P0-P3) were isolated from mixed glial cultures (MGC). We included three variations of this protocol that differed by use of GM-CSF in culture (No GM-CSF or 500 pg/mL GM-CSF), and days of culture in MGC before microglial separation (10 or 21). Protocols for studying microglia from adult mice age 6-8 weeks included isolation by adherence properties followed by 7 days of culture with 100 ng/mL GM-CSF and 100 ng/mL M-CSF (Vijaya et al., 2023), or acute isolation using CD11b beads (Bordt et al., 2020). Purity, yield, and RNA quality of the isolated microglia were assessed by flow cytometry, hemocytometer counting, Nanodrop, and Bioanalyzer, respectively. Microglial responsiveness to an inflammatory stimulus, HMGB1, was evaluated by measuring TNFa, IL1ß, and IFNß secretion by ELISA and assessing gene expression patterns using bulk RNA sequencing. Results: All five methods demonstrated greater than 90% purity. Microglia from all cultures increased transcription of or secreted TNFa, IL1ß, and IFNß in response to HMGB1. RNA sequencing showed a larger number of differentially expressed genes in response to HMGB1 treatment in microglia cultured from pups than from adult mice, with sparse changes among the 3 MGC culturing conditions.  Conclusion: These findings suggest that while all methods provided high purity, the choice of protocol may significantly influence yield, RNA quality, baseline transcriptional profile and response to stimulation. This comparative study provides valuable insights to inform the choice of microglial isolation and culture method. Overall design: We sequenced 30 samples in total. This includes microglia derived from 5 isolation methods, with 3 biological replicates each and two treatment conditions: Control and HMGB1-treated.