Structural changes in perineuronal nets and their perforating GABAergic synapses precede motor coordination recovery post stroke

Summary Stroke remains one of the leading causes of long-term disability worldwide, and the development of effective restorative therapies is hindered by an incomplete understanding of intrinsic brain recovery mechanisms. Here we explored how perineuronal nets (PNNs), the facet-like extracellular matrix layers surrounding fast-spiking interneurons, contribute to neurological recovery after focal cerebral ischemia in mice with and without induced stroke tolerance. Due to the insufficient resolution of conventional microscopy methods, the contribution of structural changes in PNNs to post stroke brain plasticity remained unknown. Using superresolution stimulated emission depletion (STED) and structured illumination (SR-SIM) microscopy, we revealed that PNN facets become larger and less dense in the post-acute stroke phase. These morphological alterations in PNNs are transient and correlate with the increased surface of contact between activated microglia and PNN-coated neurons. The transient loosening of PNNs after stroke allows for dynamic reorganization of GABAergic axonal terminals on inhibitory interneurons in the motor cortical layer 5. The coherent remodeling of PNNs and their perforating inhibitory synapses precedes the recovery of motor coordination after stroke and depends on the severity of the ischemic injury. Our data suggest a novel mechanism of motor cortical plasticity after stroke, and we propose that prolonging PNN loosening during the post-acute period can extend the opening neuroplasticity window into the chronic stroke phase. Highlights PNNs are degraded partially and transiently post-stroke Transient attenuation of PNNs correlates with GABAergic synapse remodeling Transient attenuation of PNNs precedes functional recovery post stroke Activated microglia preferentially contact PNN-coated neurons post stroke Graphical Abstract