Astrocytic and microglial phenotypes in focal cortical dysplasia

Abstract

Background: Focal cortical dysplasia (FCD) is a common cause of focal epilepsy, however pathophysiology of epileptogenesis in FCD remains unresolved. Emerging evidence suggests that dysfunctional astrocytes are key players in epilepsy. Recently two phenotypes of astrocytes (A1/A2) and microglia (M1/M2) have been described in neurological diseases with neuroinflammatory and neuroprotective roles. We investigated astrocytic (A1/A2) and microglial (M1/M2) phenotypes in FCD and their role in epileptogenesis.

Material and methods: Histologically confirmed cases of surgically resected FCD IIa (n = 10) and FCD IIb (n = 10) and age and region-matched post-mortem controls (n = 4) were evaluated by immunohistochemistry using C3d and GBP2 (A1-astrocytic markers), pSTAT3 (A2-astrocytes), CD14 (M1-microglia) and CD163 (M2-microglia), caspase 3 (for apoptosis) and phosphorylated-Tau and phosphorylated-neurofilament (for neuronal degeneration). Semi-quantitative assessment for glial phenotypes were correlated with clinical parameters.

Results: Compared to the control group, in FCDIIa, pSTAT3 + A2-astrocytes (mean-44.1 cells/mm²) predominated in absence of C3d/GBP2 + A1-astrocytes. In FCDIIb, A2-astrocytes were significantly higher than A1 (p-value=0.04) [A1 (C3d-15.79 cells/mm²; GBP2-13.67 cells/mm²); A2 (pSTAT3-78.24 cells/mm²)]. Balloon cells in FCDIIb strongly labelled with C3d and GBP2 (A1-phenotype). In both FCDIIa and IIb, pSTAT3 + A2 astrocytes were localised to subpial zone. Increase in both inflammatory CD14 + M1 and reparative CD163 + M2 microglia in perivascular region, was seen in the dysplastic cortex in both FCD IIa (M1- 11.9/mm², M2- 12.4 cells/mm²) and FCD IIb (M1-27.9/mm², M2-18.7/mm²) with M1 >M2 in FCD IIb, though not statistically significant (p-value>0.05). Mean densities of astrocytes (A1, A2) and microglia (M1, M2) did not correlate with any of the clinical parameters. Caspase 3 labelled reactive astrocytes and oligodendrocytes and occasional dysmorphic neurons in both, and BC in FCDIIb.

Conclusions: This is the first study examining astrocytic and microglial phenotypes in FCD IIa and IIb. Identification of specific astrocytic and microglial phenotypes offers novel therapeutic targets for modulation of epileptogenesis, especially in drug resistant epilepsy.

Keywords: A1 and A2 astrocyte; Drug resistant epilepsy; Focal cortical dysplasia; M1 and M2 microglial phenotype.