Identification of distinct and age-dependent p16High microglia subtypes.

Cells expressing high levels of the cyclin-dependent kinase (CDK)4/6 inhibitor p16 (p16High ) accumulate in aging tissues and promote multiple age-related pathologies, including neurodegeneration. Here, we show that the number of p16High cells is significantly increased in the central nervous system (CNS) of 2-year-old mice. Bulk RNAseq indicated that genes expressed by p16High cells were associated with inflammation and phagocytosis. Single-cell RNAseq of brain cells indicated p16High cells were primarily microglia, and their accumulation was confirmed in brains of aged humans. Interestingly, we identified two distinct subpopulations of p16High microglia in the mouse brain, with one being age-associated and one present in young animals. Both p16High clusters significantly differed from previously described disease-associated microglia and expressed only a partial senescence signature. Taken together, our study provides evidence for the existence of two p16-expressing microglia populations, one accumulating with age and another already present in youth that could positively and negatively contribute to brain homeostasis, function, and disease.