Probing functional contributions of microglia and non-parenchymal CNS macrophages in physiology and pathophysiology

Introduction

Brain macrophages have emerged as major players in central nerve system (CNS) physiology and pathophysiology. Much of the recent insight derives from fate mapping, intra-vital imaging, cell ablation and targeted mutagenesis using respective Cre / loxP system-based mouse models. In parallel, advances in flow cytometry and single cell transcriptomics have highlighted the complexity of the brain macrophage compartment ^1,2. Specifically, the latter comprises parenchymal microglia and non-parenchymal, CNS border-associated macrophages (BAM) located in perivascular and meningeal niches, as well as the choroid plexus. In depth understanding of specific functional contributions of these distinct CNS macrophage populations will require the development of novel binary transgenic Cre approaches that allow the study in physiological context.

Material and Methods

Here, we developed novel binary transgenic mouse models relying on the co-expression of split Cre fragments to target specific CNS macrophage populations. Following crossing to animals harboring conditional reporter alleles or a ‘Ribo-tag’ allele that allows translatome profiling ³ we confirmed differential targeting of microglia and BAM.

Results and discussion

According to imaging, flow cytometry analysis, and mRNA expression profiling results, a combination of Cx3cr1^C-Cre and Sall1^N-Cre transgenes allowed for specific targeting of microglia, unlike previously established Cx3cr1^CreER and Sall1^CreERtransgenes (Chappell-Maor et al, submitted). In contrast, Cx3cr1^C-Cre:Lyve1^N-Cre transgenic mice were found to specifically target non-parenchymal macrophages located in meningeal and perivascular regions. We are currently using these animals in combination with the RiboTag approach to define distinct functional contributions of microglia and BAM to CNS pathologies.

Conclusion

Advanced sequencing studies highlight the transcriptomic heterogeneity of macrophage populations in CNS. Taking advantage of a binary transgenic split Cre system, we have started to dissect the functional heterogeneity of macrophage populations in their physiological brain context.