Generation of an Inducible Destabilized-Domain Cre Mouse Line to Target Disease Associated Microglia.

The function of microglia during progression of Alzheimer's disease (AD) can be investigated using mouse models that enable genetic manipulation of microglial subpopulations in a temporal manner. We developed mouse lines that express either Cre recombinase (Cre) for constitutive targeting, or destabilized-domain Cre recombinase (DD-Cre) for inducible targeting from the Cst7 locus (Cst7^DD-Cre) to specifically manipulate disease associated microglia (DAM) and crossed with Ai14 tdTomato cre-reporter line mice. Cst7^Cre was found to target all brain resident myeloid cells, due to transient developmental expression of Cst7, but no expression was found in the inducible Cst7^DD-Cre mice. Further crossing of this line with 5xFAD mice combined with dietary administration of trimethoprim to induce DD-Cre activity produces long-term labeling in DAM without evidence of leakiness, with tdTomato-expression restricted to cells surrounding plaques. Using this model, we found that DAMs are a subset of plaque-associated microglia (PAMs) and their transition to DAM increases with age and disease stage. Spatial transcriptomic analysis revealed that tdTomato+ cells show higher expression of disease and inflammatory genes compared to other microglial populations, including non-labeled PAMs. These models allow either complete cre-loxP targeting of all brain myeloid cells (Cst7^Cre), or inducible targeting of DAMs, without leakiness (Cst7^DD-Cre).