Disruption of extracellular matrix and perineuronal nets modulates extracellular space volume and geometry

Extracellular matrix (ECM) is a network of macromolecules which has two forms - perineuronal nets (PNNs) and a diffuse ECM (dECM) - both influence brain development, synapse formation, neuroplasticity, CNS injury and progression of neurodegenerative diseases. ECM remodeling can influence extrasynaptic transmission, mediated by diffusion of neuroactive substances in the extracellular space (ECS). In this study we analyzed how disrupted PNNs and dECM influence brain diffusibility. Two months after oral treatment of rats with 4-methylumbelliferone (4-MU), an inhibitor of hyaluronan synthesis, we found downregulated staining for PNNs, hyaluronan, chondroitin sulphate proteoglycans and glial fibrillary acidic protein (GFAP). These changes were enhanced after 4 and 6 months and were reversible after normal diet. Morphometric analysis further indicated atrophy of astrocytes. Using real-time iontophoretic method dysregulation of ECM resulted in increased ECS volume fraction α in somatosensory cortex (SC) by 35 %, from α = 0.20 in control rats to α = 0.27 after the 4-MU diet. Diffusion-weighted magnetic resonance imaging (DW-MRI) revealed a decrease of mean diffusivity (MD) and fractional anisotropy (FA) in the cortex, hippocampus, thalamus, pallidum and spinal cord. This study shows the increase in ECS volume, a loss of FA and changes in astrocytes due to modulation of PNNs and dECM that could affect extrasynaptic transmission, cell to cell communication and neural plasticity.Significance Statement Inhibition of hyaluronan synthesis induced by oral treatment with 4-methylumebelliferone disrupts perineuronal nets (PNNs), diffuse extracellular matrix (dECM), reduces the astrocytic network, increases extracellular space (ECS) volume and changes ECS geometry. The changes of diffusion barriers significantly affect diffusion parameters in the adult brain and spinal cord. Our findings suggest that disruption of ECM allows for more efficient transport of ions, neurotransmitters and neuroactive substances in the ECS and thus ensures broader inter-neuronal communication by extrasynaptic transmission. Disruption of PNNs and an increase in ECS volume can result in enhanced crosstalk between synapses, spill-over of transmitters, formation of new synaptic contacts and thus increased synaptic plasticity.