Mesenchymal stromal cell-derived extracellular vesicles attenuate inflammation-associated cognitive impairment and hippocampal microglial-synaptic remodeling

Abstract

Microglia are central regulators of synaptic structure and cognitive function, yet under chronic inflammatory conditions, they can adopt stress-associated states associated with neuronal and synaptic vulnerability. Although inflammation-driven cognitive decline is often attributed to synapse loss, increasing evidence suggests that synaptic remodeling may precede or occur independently of overt synapse elimination. In this study, we combined behavioral testing, immunofluorescence, microglial morphometric analysis, and quantitative transmission electron microscopy to define structural correlates of inflammation-induced cognitive dysfunction and to evaluate the therapeutic potential of mesenchymal stromal cell-derived extracellular vesicles (MSC-EVs). Chronic systemic inflammation was induced in mice by repeated lipopolysaccharide (LPS) administration, followed by intravenous MSC-EV treatment during the inflammatory phase. LPS exposure impaired working memory, object recognition, and spatial recognition memory without affecting locomotor activity. Immunofluorescence revealed increased CD68 immunoreactivity detected with the ED1 antibody clone within IBA1-positive cells, together with increased microglial soma area, reduced arborization area, and an elevated morphological index, consistent with microglial inflammatory remodeling and enhanced endolysosomal/phagolysosomal engagement. Ultrastructural analysis further demonstrated increased phagolysosomal inclusion burden, increased blood vessel-associated microglial profiles, elevated dark microglia-like profiles, reduced neuronal soma profile area, decreased SYN- and PSD95-positive structures, and coordinated pre- and postsynaptic alterations. Total excitatory synapse density showed only a non-significant numerical reduction, indicating synaptic vulnerability characterized primarily by structural remodeling rather than definitive synapse loss. MSC-EV treatment was associated with attenuated microglial inflammatory and morphometric alterations, reduced phagolysosomal and dark microglia-like ultrastructural features, partially restored synaptic marker-positive structures and synaptic ultrastructure, and improved hippocampus-dependent cognition, particularly spatial recognition memory. Together, these findings suggest that MSC-EVs are associated with partial normalization of inflammation-induced microglial, neuronal, and synaptic alterations.