Environmental enrichment modulates chronic poststroke inflammation and links white matter TREM2-positive microglia in...

Environmental enrichment (EE) has been widely reported to improve functional recovery after stroke, yet its effects on chronic inflammation and white matter pathology remain less well defined. The aim of this study was to investigate whether poststroke EE modulates chronic neuroinflammation and white matter pathology and how these changes relate to functional recovery.

Here, we investigated how EE influences the microglial response and myelin integrity during the recovery phase after photothrombotic stroke in male C57Bl/6J mice. Mice were randomized by using a computer-generated sequence into standard environment (SE) or EE housing conditions 2 days poststroke (SE: n = 6; EE: n = 7 for histological analyses; behavioral data unavailable for 2 SE mice). Sensorimotor performance was assessed at 7, 14, and 21 days poststroke using paw placement, foot fault, and limb symmetry tests, integrated into a composite Neuroscore. Housing in EE improved and sustained behavioral recovery up to 21 days. To explore whether specific microglial subpopulations are involved in tissue reorganization and functional outcome, immunofluorescence was performed for ionized calcium binding adaptor molecule 1 (Iba1), galectin-3 (Gal3), purinergic receptor P2Y12 (P2RY12), cluster of differentiation 68 (CD68), and triggering receptor expressed on myeloid cells 2 (TREM2), together with Black Gold myelin staining, in peri-infarct and white matter regions. Statistical analyses included correlation analyses and analysis of covariance (ANCOVA).

In SE mice, the inflammatory microglial marker Gal3 and the phagocytic marker CD68 correlated positively with infarct size (r = 0.937, p = 0.006; r = 0.845, p = 0.034, respectively), and the accumulation of myelin debris (r = 0.865, p = 0.026) together with loss of myelin coverage (r = -0.907, p = 0.013) followed lesion-driven trajectories. By contrast, these correlations were absent in EE mice (r = -0.028, p = 0.953; r = 0.671, p= 0.099; r = 0.053, p = 0.910; r = 0.292, p = 0.525) indicating that enrichment attenuated lesion-driven inflammatory responses and myelin damage. Importantly, Gal3 expression correlated specifically with peri-infarct myelin debris (r = 0.82, p = 0.048), reinforcing its role as a pro-inflammatory mediator. Finally, correlation analysis between Neuroscore and both microglial and myelin markers revealed a unique positive association between TREM2 coverage in white matter and improved Neuroscore in EE mice (r = 0.890, p = 0.007; ANCOVA p = 0.039).

These results support the idea that EE modulates processes of chronic post-stroke inflammation, peri-infarct myelin debris clearance and demyelination, and highlight white matter TREM2-positive microglia as a potential cellular link to functional recovery.