Identification of distinct and age‐dependent p16^High microglia subtypes

Abstract

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.

Aging microglia

“…In subclustering analysis, p16 + -microglia mostly concentrated in two unknown and distinct clusters (UM1 and UM2), although they were also found in clusters associated with DAM and interferon-response signatures. Although UM1 and UM2 had greater p16 expression, the senescenceassociated genes were more enriched in the DAM cluster, indicating a non-linear correlation between p16 and senescence [88].…”

“…An alternative approach, in which p16 high -expressing cells were isolated based on the expression of a reporter gene under the control of the p16 promoter, showed that aged microglia represented the majority among brain cells [88]. In subclustering analysis, p16 + -microglia mostly concentrated in two unknown and distinct clusters (UM1 and UM2), although they were also found in clusters associated with DAM and interferon-response signatures.…”

Is “cellular senescence” a misnomer?

“…Similarly, in later life, microglia of the brain proliferate and activate, leading to neuroinflammation and, in turn, neurodegeneration. They also increasingly express SA-β-Gal and p16 INK4a , which has been interpreted as cellular senescence [58][59][60]. An alternative possibility is that this reflects inflammatory remodeling activation in these tissue resident macrophages.…”

TREM2-dependent senescent microglia conserved in aging and Alzheimer’s disease

“…In another study, which did not use single-cell cytometry, senescent oligodendrocyte progenitor cells (OPCs) but not astrocytes or microglia, were identified in a mouse model of AD 39 . Of note, P16-positive microglia were also detected in aging 40 . Overall, results from different studies highlight the key role that senescent cells in the brain play in AD.…”