Author:

Keywords:

Science & Technology, Life Sciences & Biomedicine, Neurosciences, Neurosciences & Neurology, 1109 Neurosciences, Neurology & Neurosurgery, 3209 Neurosciences

Abstract:

Functional interactions between neurons, glia and the vascular endothelium of the blood-brain barrier (BBB), together forming the neurovascular unit (NVU), are affected in neurodegenerative and neuroinflammatory disorders of the central nervous system (CNS). Although matrix metalloproteinase-3 (MMP-3) has been associated with neuroinflammation and BBB disruption, its explicit involvement in the neuron-glia-vasculature crosstalk in the NVU has not been studied. As the neurosensory retina is an eminent model to study neuron-glia (dys)function, we investigated the role of this proteinase in an experimental mouse model of retinal neurodegeneration induced by optic nerve crush (ONC). Quantitative polymerase chain reaction (qPCR), immunohistochemistry and western blot analysis, revealed MMP-3 mRNA and protein expression to be highly upregulated in the radial processes and end-feet of the Müller glia/astrocytes at early time points after ONC. This augmented MMP-3 expression upon axonal injury seems to play a prominent role in the preservation and even tightening of the glia limitans, as evidenced by an increased mRNA and protein expression of specific astrocytic tight junction molecules (claudin1, claudin4, occludin and ZO-1) in wild type (WT) but not in MMP-3 deficient (Mmp3-/-) retinas. Our data further show that this tightening is important to attenuate neuroinflammatory processes upon ONC. Indeed, Mmp3-/- mice showed an increased intravascular leukocyte adhesion, visualized by FITC-Concanavalin A lectin, as well as an increased myeloid cell infiltration, which was evaluated by flow cytometry. In addition, investigating retinal mRNA and protein levels via respectively qPCR and Bioplex analysis of inflammatory cytokines and chemokines revealed higher expression levels in Mmp3-/- mice compared to WT animals after ONC. Interestingly, these neuroinflammatory features have a prominent impact on retinal ganglion cell (RGC) survival analyzed using Brn3a immunostaining on retinal whole mounts, as MMP-3 deficiency/inhibition leads to an acceleration of neurodegeneration after ONC. Overall, our findings emphasize the relevance of MMP-3 in attenuating neuroinflammatory responses at the level of the retinal NVU by tightening the glia limitans and as such protecting RGCs from neurodegeneration. Hence, our study points to a novel and intriguing role of MMP-3 in preserving the integrity of the retinal NVU upon axonal injury.