Kaiser T, Feng G. Tmem119-EGFP and Tmem119-CreERT2 transgenic mice for labeling and manipulating microglia. bioRχiv. May 19, 2019
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Kaiser T, Feng G. Tmem119-EGFP and Tmem119-CreERT2 transgenic mice for labeling and manipulating microglia. bioRχiv. May 19, 2019
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NYU Langone
Since Mariko Bennett and Ben Barres published Tmem119 as a marker of microglia distinct from peripheral myeloid cells in 2016, the field has been abuzz with the ability to discern between peripheral immune cells and resident microglia (Bennett et al., 2016). While for many applications it is possible that such distinctions are not important (for instance in disease responses in which there is a similar contribution of both microglia and, say, macrophages), for many developmental and brain-specific immune responses, having such a marker has been imperative.
The advent of these new Tmem119-targeted eGFP and CreERT2 mouse lines provide two key tools for the field. That the eGFP line completely and faithfully labels microglia at different time points of development, and that this fluorescence is stable for FACS purification, will no doubt see the investigation of live-cell imaging and future sequencing experiments beyond what is currently possible (and easily accessible for many labs). One could cross these lines with peripheral immune cell tdTomato reporter lines to determine the CNS migration differences or changes in the integration with disease pathologies in live animals—very exciting. Similarly, the CreERT2 line provides inducible specificity to manipulate a whole host of microglia-specific functions, and is going to enable one to tease the specific functions of resident versus infiltrating immune cells—an exciting prospect to be sure.
The highlights of the mouse include: no change in morphology, gene expression, or numbers in eGFP/CreERT2 lines, high specificity for Tmem119+ CNS-resident microglia, with no expression in other non-microglia monophagocytic cells, e.g., meningeal macrophages. There is some early expression of Tmem119 in non-microglial endothelial cells, but this is itself a feature, not a bug, as it allows the study of microglia and endothelial cells until postnatal day 1 (P1) and microglia specifically after P3 and later.
To highlight how excited the field is, a similar set of lines is being produced by Mariko Bennett, and upon seeing the publication from Kaiser and Feng, her Twitter response was as follows: “‘Scooped’ and can’t even be grumpy because it’s gonna be an awesome resource and is already available at Jax. We’ll be sure to compare the lines as soon as we’re sure ours is ready for prime time.” It will be exciting to cross the Tmem119CreERT2 line with ribotag, bac- and nuc-TRAP lines to further investigations into microglia-specific biology in health and disease.
References:
Bennett ML, Bennett FC, Liddelow SA, Ajami B, Zamanian JL, Fernhoff NB, Mulinyawe SB, Bohlen CJ, Adil A, Tucker A, Weissman IL, Chang EF, Li G, Grant GA, Hayden Gephart MG, Barres BA. New tools for studying microglia in the mouse and human CNS. Proc Natl Acad Sci U S A. 2016 Mar 22;113(12):E1738-46. Epub 2016 Feb 16 PubMed.
Yale University
We ordered the mice for trial in our lab, but have not received them yet. In principle they sound super useful, as they have not knocked out the endogenous Tmem119 gene. This is in contrast to other lines available, such as the CX3CR1, which is widely used but has a loss of function of CX3CR1. LOF of CX3CR1 clearly poses problems for many experiments, as even partial loss of CX3CR1 has significant phenotypes.
The other potentially interesting thing about these mice is that they appear to preferentially target microglia but not peripheral monocytes. This could be very useful in a variety of studies, including in AD and other diseases and injuries, where there is always the controversy as to whether the cells observed in the brain are all microglia or also some peripherally derived cells.
In summary, for us these mice may prove to be highly useful. We are hoping that the GFP brightness in the TMEM119 is large so that we can do live imaging.
Genentech, Inc.
These look useful and I wouldn’t be surprised if many groups took advantage. The current state of the art for this type of approach is an analogous pair of Cx3cr1-driven mice, which have the drawback, as the authors indicate, of also labeling other non-microglia myeloid populations. We used these for a data set included in our Cell Reports manuscript from last year. This new pair of nice looks pretty clean, based on the figures presented (Cd11b:Cd45-high unlabeled in FACS and CD163 meningeal macrophages unlabeled in IHC). If we had used the Tmem119::EGFP mice instead, then I expect our data set would have been even more specific.
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