Approximately 50 million people worldwide live with epilepsy, a syndrome characterized by repeated, unprovoked seizures that manifest with a combination of altered behavior and abnormal electric discharges of populations of neurons in the brain. Seizures result from impairment of excitatory-inhibitory (E-I) balance. Enhancement of inhibitory GABAergic function is a common pharmacological strategy. Not surprisingly, GABAergic interneurons in the cortex and hippocampus have been well studied, and their role in dampening excitatory output from these structures is well established. GABAergic interneurons tend to be fast-spiking cells (up to 800Hz!), which compensate for their small number by a high level of activity with each action potential causing GABA release from their terminals. A majority of these fast-spiking neurons are surrounded by a layer of dense extracellular matrix that Golgi termed perineuronal nets (PNNs) over 120 years ago. These are composed of glycosaminoglycanes, negatively charged glycoproteins formed from a superfamily of proteins that cover the cell soma, proximal dendrites and axon initial segment. Their role is not well known but believed to aid in cell differentiation, neural protection and cortical plasticity. During the last grant cycle studying tumor- associated epilepsy, we made an unexpected discovery suggesting that PNNs alter the neuronal membrane capacitance, allowing them to fire at supra-physiological rates. Specially, proteolytic enzymes released from the tumor digest PNNs, thereby increasing membrane capacitance and slowing the firing rates of inhibitory neurons, leading to seizures. We now hypothesize that PNNs may be more generally the target of acquired epilepsy, where proteolysis of extracellular matrix and tissue remodeling are common. Hence, we propose to study PNN integrity and its role in epileptogenesis more broadly across different mouse models of acquired epilepsy and in tissues from epilepsy patients. We hypothesize that PNNs define the placement of astrocytes near synapses to aid uptake of ions and neurotransmitters; that release of matrix-degrading enzymes from reactive astrocytes destroy PNNs, thereby slowing their firing rate. Together these changes may lead to epilepsy. These studies are conceptually novel and may suggest a completely different treatment approach to epilepsy, namely targeting proteolytic enzymes to ameliorate this disease.
Epilepsy resulting from brain infection, tumors or injury presents with tissue remodeling that employs protein- degrading enzymes. These enzymes may damage ?perineuronal nets,? an important protective coat around inhibitory neurons. The loss of this coat slows neuronal firing, making these neurons less inhibitory and giving rise to seizures. This proposal investigates how perineuronal nets may be causally involved in epilepsy and may suggest a completely different therapeutic approach to ameliorate this disease.
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