Abstracts

Rationale: Epileptic hyperexcitability fundamentally underlies an imbalance of excitation and inhibition (E-I) in the brain. Multitude of factors, including elevated glutamatergic neurotransmission, diminished or disinhibition of GABAergic neurotransmission, neuronal loss and astroglial dysfunctions are known to alter E-I balance. High-grade glioma patients often exhibit recurrent and unprovoked epileptic seizures as onset symptoms. We have shown previously that upregulation of the glutamate-cysteine antiporter SXC in glioma cells causes excessive glutamate release, which in turn increases excitatory drive and causes excitotoxic neuronal death thereby driving the seizure activity in a mouse model of human glioma-associated epilepsy. Perineuronal nets (PNNs) are specialized extracellular matrix (ECM) assemblies of highly negatively charged proteoglycans that surround PV-expressing fast-spiking, GABAergic interneurons (FSNs). Functionally, PNNs regulate the critical period plasticity and play neuroprotective roles. We hypothesized that glioma releases proteases including matrix metalloproteinase (MMPs), that cause ECM and PNN degradation to alter electrophysiological attributes of fast spiking interneurons thereby contributing to lower the seizure threshold. Methods: Patient-derived glioma cells expression high SXC level were stereotactically implanted into the scid mice cerebral cortex. Mice brains were harvested after 2 weeks and were utilized for immunohistochemical staining of PNNs, and cellular markers (GFAP, NeuN, PV), and in-situ zymography. Acute cortical slices from glioma-implanted mice brains were used for conducting whole cell electrophysiology experiments on PNN-enclosed interneurons and principle neurons in peritumoral cortical areas. Results: Peritumoral cortex exhibit degraded PNNs and an overall lower density of PNN-enclosed FSNs due to glioma-released MMPs and excessive glutamate respectively. Degradation of PNNs increased the membrane capacitance and decreased the spike firing frequency of FSNs. Inhibition of glioma-released MMPs prevented PNN degradation as well as any alterations in the membrane capacitance and spike firing frequency. Experimental degradation of PNNs using Chondroitinase ABC enzyme confirmed that PNN lowers the membrane capacitance of FSNs that allows them to sustain a high spike frequency. Conclusions: Perineuronal nets around FSNs are degraded by glioma-released MMPs thereby directly exposing FSNs to excitotoxic glutamate, leading to cause preferential death of inhibitory interneurons. FSNs with degraded PNNs possess higher membrane capacitance and fail to sustain their fast-spiking property resulting into diminished inhibitory drive. Our study reveals a previously unknown function of PNNs and suggest that other forms of acquired epilepsies may involve a similar dysfunction of FSNs associated with degraded PNNs. Funding: This work was supported by NIHRO1-NS036692, NIH-RO1-NS082851, and NIH-RO1-NS052634.