New tools for studying microglia in the mouse and human CNS

Significance

Microglia are the tissue resident macrophages of the brain and spinal cord, implicated in important developmental, homeostatic, and disease processes, although our understanding of their roles is complicated by an inability to distinguish microglia from related cell types. Although they share many features with other macrophages, microglia have distinct developmental origins and functions. Here we validate a stable and robustly expressed microglial marker for both mouse and human, transmembrane protein 119 (Tmem119). We use custom-made antibodies against Tmem119 to perform deep RNA sequencing of developing microglia, and demonstrate that microglia mature by the second postnatal week in mice. The antibodies, cell isolation methods, and RNAseq profiles presented here will greatly facilitate our understanding of microglial function in health and disease.

Abstract

The specific function of microglia, the tissue resident macrophages of the brain and spinal cord, has been difficult to ascertain because of a lack of tools to distinguish microglia from other immune cells, thereby limiting specific immunostaining, purification, and manipulation. Because of their unique developmental origins and predicted functions, the distinction of microglia from other myeloid cells is critically important for understanding brain development and disease; better tools would greatly facilitate studies of microglia function in the developing, adult, and injured CNS. Here, we identify transmembrane protein 119 (Tmem119), a cell-surface protein of unknown function, as a highly expressed microglia-specific marker in both mouse and human. We developed monoclonal antibodies to its intracellular and extracellular domains that enable the immunostaining of microglia in histological sections in healthy and diseased brains, as well as isolation of pure nonactivated microglia by FACS. Using our antibodies, we provide, to our knowledge, the first RNAseq profiles of highly pure mouse microglia during development and after an immune challenge. We used these to demonstrate that mouse microglia mature by the second postnatal week and to predict novel microglial functions. Together, we anticipate these resources will be valuable for the future study and understanding of microglia in health and disease.