Astrocytes make up a sizable population of supporting and structural cells in the brain, with a broad portfolio of activities that are collectively necessary for the normal operation of brain metabolism and neural activity. Like all cell populations, astrocytes are negatively impacted by the accumulating damage and dysfunction of aging, both internal to cells and in the tissue microenvironment. An area of focus for the research community is how aging provokes an ever increasing number of astrocytes into (1) a reactive, inflammatory state that harms brain tissue, but also (2) into a senescent state, which is also a source of inflammatory signaling that becomes detrimental to tissue structure and function when sustained over the long term. Reactivity and senescence may overlap in their contribution to neurodegeneration, and in root causes, but they are distinct issues.
Today’s open access paper reviews what is known of reactivity and senescence in astrocytes, connecting these states to the bigger picture of how loss of cognitive function and onset of neurodegenerative disease emerges from aging. The present understanding of astrocyte biochemistry is, as for all cell types, incomplete. The goal of medicine is ever more precise control over cell state and cell activities, and this drives the scientific endeavor towards assembling an ever more complete understanding of cellular biochemistry. That is a very long term trajectory, however, with an end goal far out of sight of the present day to day work. In the short term, research is a matter of trying to find single genes, single proteins, single interactions in the cell that act as points of control for aspects of behavior, and thus might lead to novel therapies. Medicine remains at a very crude level of cellular control, as illustrated by our struggles with age-related disease.
Brain aging involves progressive disruption of tissue homeostasis and susceptibility to neurodegenerative disorders. Within this context, astrocytes are key determinants of region-specific physiology, given their roles in metabolic support, synapse regulation, proteostasis, neuroinflammation, and blood-brain barrier maintenance. Aging is accompanied by broad transcriptional and functional remodeling in astrocytes, leading to the emergence of distinct cellular states that cannot be defined by classical morphological criteria alone.
This review discusses how aging modifies astrocyte identities toward reactive and senescence-like states. We summarize core features of astrocyte senescence, including altered secretory signaling, impaired neuronal support, and changes in mitochondrial and proteostatic pathways, while integrating recent single-cell and regionally transcriptomic studies that delineate multiple reactive states associated with aging and pathological contexts. We further address evidence that reactivity and senescence are not mutually exclusive endpoints, but may coexist, arise sequentially, or partially overlap depending on timing, brain region, biological sex, and pathological insults. Finally, we define key open questions and experimental priorities required to establish the temporal and causal relationships among astrocyte states.
We argue that resolving these issues is essential for advancing therapeutic strategies that specifically target defined astrocyte phenotypes, rather than nonspecifically suppressing astrocyte activity, in aging and neurodegenerative diseases. In this view, both astrocyte reactivity and senescence represent components of a broader spectrum of astrocyte states, encompassing different degrees of inflammatory signaling, metabolic adjustments, proteostatic imbalance and alteration of homeostatic functions across aging trajectories and disease contexts. Notably, although astrocyte states are becoming increasingly well defined, additional layers of complexity are only beginning to be appreciated.
Importantly, reactive and senescent astrocytes should not be regarded as mutually exclusive identities. Instead, they may coexist within the same tissue, arise sequentially, or partially overlap depending on local conditions and disease stage. This perspective helps reconcile the heterogeneity observed across experimental models and human studies, suggesting that susceptibility to neurodegenerative disease depends not only on the presence of astrocyte dysfunction, but also on how specific astrocyte states interact with neuronal circuits, other glial cells, immune responses, and systemic factors. Moving forward, approaches that combine spatially resolved transcriptomics, longitudinal analyses and functional analyses of defined astrocyte populations will be essential to clarify the temporal and regional dynamics of these states.
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