bioRxiv. 2026 Mar 27. pii: 2026.03.25.714174. [Epub ahead of print]
Elucidating how normal aging increases vulnerability to neurodegeneration remains a major gap in our understanding of disease risk and progression. The dorsal striatum serves as the primary input nucleus of the basal ganglia and is a key region implicated in multiple neurodegenerative diseases (NDDs) (1). In Colon et al. 2025 (2), we examined the impact of normal aging on neuroinflammatory signaling and perineuronal net (PNN) homeostasis within the dorsal striatum. We observed age-associated shifts in the inflammatory landscape and evidence of increased microglial activation, yet PNN homeostasis was largely preserved (2). PNNs are highly organized extracellular matrix (ECM) specializations that preferentially enwrap the soma and proximal dendrites of fast-spiking GABAergic parvalbumin (PV) interneurons, where they contribute to the regulation of synaptic plasticity and provide protection against oxidative stress (3,4). Building on these findings, we developed a working hypothesis to explain the apparent preservation of PNN homeostasis despite an aging-associated pro-inflammatory environment. The shift toward a pro-inflammatory milieu, together with increased gliosis and phagocytic activity, would be expected to impact the maintenance and integrity of perineuronal nets. The observed increase in phagocytosis-related markers may reflect microglia-directed activity as well as contributions from additional central nervous system (CNS) cell populations. Microglia are specialized embryonic-derived myeloid cells that serve as the resident immune cells of the brain and contribute to PNN homeostasis under physiological conditions (5). In Colon et al. 2025, we observed evidence of microgliosis (e.g., morphological changes, Iba1, Trem2 ) along with elevated expression of markers associated with phagocytosis (e.g., Cd68 ) and extracellular matrix-modifying proteases (e.g., Mmp9, Adam17 ) capable of cleaving key PNN components (2). Importantly, Cd68 expression is not exclusive to microglia and has been detected in brain infiltrating macrophages, reactive astrocytes, and neutrophils during inflammation (6-8). Thus, increased Cd68 levels may not solely reflect microglial phagocytic activation but may also reflect astrocyte reactivity and phagocytic phenotypes. Furthermore, astrocytes are the most abundant glial cell in the brain, and they play a major role in maintaining CNS homeostasis by regulating extracellular neurotransmitter concentrations, providing metabolic support, contributing to the synthesis and remodeling of PNN components, and modulating neuronal communication through their involvement in the tetrapartite synapse (9-12). Astrocytes can also phagocytosis microglial debris, myelin, and synapses (7). To better define the cellular source of phagocytic activity and its relationship to PNN remodeling in aging, we performed immunostaining for microglia (Iba1 + ), astrocytes (GFAP + ), phagolysosomal activity (CD68 + ), and PNNs using Wisteria floribunda agglutinin (WFA + ), enabling us to assess the spatial relationship between phagocytosis and PNN components.