January Journal Club: uncovering the function of perineuronal nets

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Looking back at research landmarks of the already past year is a good starting point for this 2019. The unstoppable and continuous advance on scientific research gives its many fruits in a broad spectrum of fields. Today we would like to highlight the discoveries of a group of Virginia-based scientists in the field of the Central Nervous System and Epilepsy.

The research team led by Professor Harald Sontheimer uncovered the function of perineuronal nets (PNNs) while studying tumor-associated epilepsy. PNNs are complex lattice-like extracellular matrix (ECM) assemblies whose exact functions remained unclear until now.

Epilepsy is characterized by an aberrant imbalance in the excitatory and inhibitory neuronal drives. In the case of tumor-associated epilepsy, the excitatory neurotransmitter glutamate (Glu) is notably released by the glioma cells, killing surrounding neurons through its excitotoxicity properties. The reduced inhibitory GABAergic transmission is also a contributor to this imbalance. It can be explained by the loss of density of fast spiking GABAergic interneurons (FSNs) and a reduced inhibitory potential of the remaining. It is known that a vast majority of these FSNs are surrounded by PNNs.

To determine how the glioma-secreted molecules affect the neurons surrounding the tumor and how these changes cause the progression of epilepsy, the authors used a glioma model and analyzed the changes in the peritumoral cortex (PTC). The observed distance-dependent loss of FSNs and degradation of the surrounding PNNs was explained by the participation of tumor-secreted molecules, Glu and matrix metalloproteinases (MMPs) respectively (Fig 1*). MMPs are a large family of zinc-dependent proteins with endopeptidase function. The PNN degradation was correlated with FSNs reduced firing frequency and thus reduced GABA release (which accounts for less inhibitory drive). Biophysical characteristics of inhibitory FSNs were rescued if PNN degradation was prevented.

Fig 1. PNN degradation by glioma-released proteases. 
Representative images showing gelatinase activity (DQG-green), DAPI (blue), and PNN (WFA-yellow) immunofluorescence in the glioma-injected brain (GBM22), in GBM22 treated with a broad-spectrum inhibitor of MMPs (GBM22+GM6001) and in sham sections. PNNs is partially rescued upon inhibition of MMP activity.

 

 

Fig 2. Altered properties of inhibitory neurons in the PTC. 
Membrane capacitance (Cm) of inhibitory FSNs is significantly increased in relation to the contralateral half and sham. The inhibition of MMPs (+GM6001) results in a decrease of the the Cm in a similar manner to sham-treated slices
.

 

Taken together, the authors demonstrated that the perineuronal nets act as electrostatic insulators decreasing membrane capacitance of FSNs (Fig 2*), allowing them to sustain the maximum firing frequency necessary for their inhibitory physiologic functions. Degradation of PNNs and the consequent loss of inhibitory tone is enough to explain epileptiform hyperexcitability.

More research needs to be done in order to fully characterize the role of PNNs in other types of acquired epilepsy, but the authors suggest that dysfunction of PNNs may be a common feature in other forms of this same disease. These findings further validate Iproteos efforts to tackle the progression of epilepsy through the selective inhibition of gelatinases. Gelatinases, also known as MMP-2 and MMP-9, are members of the MMPs. As it has been previously explained, amongst their substrates are found structural constituents of the PNNs. Increased activity of gelatinases had been previously described in CNS pathological processes such as neurodegeneration, inflammation and epilepsy.

The study determines that changes in the PNN explained by increased gelatinase activity are correlated with the observed changes in FSN, what further validates MMP-2 and MMP-9 as a promising target to protect the integrity of PNN and, therefore, maintain the necessary inhibitory tone to avoid the occurrence of seizures and even stop the progression of the disease.

Iproteos proprietary platform IPROTech has allowed the development of a blood-brain barrier permeable and stable compound with high specificity for MMP-2 and MMP-9. This small molecule inhibitor has already been tested in three epilepsy animal models with very positive results and arises as a game-changing therapeutic approach to modify the progression of epilepsy.

 

* Figures from Bhanu P. Tewari et al. Nature Communications 2018, 9(1):4724

 

References:

Tewari B, Chaunsali L, Campbell S, Patel D, Goode A, Sontheimer H. Perineuronal nets decrease membrane capacitance of peritumoral fast spiking interneurons in a model of epilepsy. Nature Communications. 2018;9(1):4724.

Virginia Tech. "Scientists solve century-old neuroscience mystery; answers may lead to epilepsy treatment." ScienceDaily. ScienceDaily, 9 November 2018. <www.sciencedaily.com/releases/2018/11/181109073036.htm>.

Wilczynski G, Konopacki F, Wilczek E, Lasiecka Z, Gorlewicz A, Michaluk P et al. Important role of matrix metalloproteinase 9 in epileptogenesis. J Cell Biol. 2008;180(5):1021-35.