Differential TGF-β Signaling in Glial Subsets Underlies IL-6–Mediated Epileptogenesis in Mice

Abstract

TGF-β1 is a master cytokine in immune regulation, orchestrating both pro- and anti-inflammatory reactions. Recent studies show that whereas TGF-β1 induces a quiescent microglia phenotype, it plays a pathogenic role in the neurovascular unit and triggers neuronal hyperexcitability and epileptogenesis. In this study, we show that, in primary glial cultures, TGF-β signaling induces rapid upregulation of the cytokine IL-6 in astrocytes, but not in microglia, via enhanced expression, phosphorylation, and nuclear translocation of SMAD2/3. Electrophysiological recordings show that administration of IL-6 increases cortical excitability, culminating in epileptiform discharges in vitro and spontaneous seizures in C57BL/6 mice. Intracellular recordings from layer V pyramidal cells in neocortical slices obtained from IL-6–treated mice show that during epileptogenesis, the cells respond to repetitive orthodromic activation with prolonged after-depolarization with no apparent changes in intrinsic membrane properties. Notably, TGF-β1–induced IL-6 upregulation occurs in brains of FVB/N but not in brains of C57BL/6 mice. Overall, our data suggest that TGF-β signaling in the brain can cause astrocyte activation whereby IL-6 upregulation results in dysregulation of astrocyte–neuronal interactions and neuronal hyperexcitability. Whereas IL-6 is epileptogenic in C57BL/6 mice, its upregulation by TGF-β1 is more profound in FVB/N mice characterized as a relatively more susceptible strain to seizure-induced cell death.

Theiler's murine encephalomyelitis virus infection of SJL/J and C57BL/6J mice: Models for multiple sclerosis and epilepsy

“…Furthermore, studies have shown that IL-6 alone can induce epileptiform activity in both in vivo and ex vivo models. In a study by Levy et al, mice treated with intraventricular injections of IL-6 were monitored for cortical seizure activity and they displayed significantly more seizure-like activity when compared to control mice (Levy et al, 2015). Single cell recordings performed in cortical rat slice preparations have shown that IL-6 application causes a decrease in evoked inhibitory post synaptic current amplitude without changing evoked excitatory post synaptic current amplitudes (Garcia-Oscos et al, 2012); suggesting IL-6 can drive neurons into a more excitable state.…”

Blood-brain barrier dysfunction in aging induces hyper-activation of TGF-beta signaling and chronic yet reversible neural dysfunction

“…Albumin endocytosis into astrocytes is mediated by binding to the TGFβ receptor II (TGFβR) subunit, which in turn activates the TGFβRI ALK5 (22,28) to induce phosphorylation of Smad2 (pSmad2) and carry out signal transduction of the ALK5-TGFβ signaling cascade. Further, this activation also increases the production of TGFβ1 in astrocytes (Weissberg et al, 2015) and activation of latent TGFβ1 protein from extra-cellular matrix (26), yielding an increase in the canonical ligand of TGFβR and therefore amplification of the TGFβ cascade. Hence, we next investigated the relationship between albumin uptake and TGFβ signaling in the aged mouse brain by quantifying immunolabeled phosphorylated Smad2 protein (pSmad2).…”

“…In both cases, astrocytes act as the primary responders that detect the blood-borne ligands and transduce TGFβ signaling (21)(22)(23). In turn, activated astrocytes release inflammatory cytokines and more TGFβ1 (22,24,25), form glial scars (21), and remodel neural circuits to cause hyperexcitability and dysfunction (22,24,(26)(27)(28). These studies point to the TGFβ signaling pathway as a candidate mechanism that could play a role in pathological outcomes following BBB breakdown in aging.…”

Role of neural barriers in the pathogenesis and outcome of Streptococcus pneumoniae meningitis

“…Among its multiple functions, it modulates T-cell activity, including proliferation and differentiation processes (109,110). The influence of TGF-β on the immune reactivity of the CNS following infection remains to be under debate; it suppresses the production of pro-inflammatory cytokines IL-1β, IL-6 and TNF-α from microglia and macrophages (111,112), but has also been reported to increase the production in cultured astrocytes (113). Absence of TGF-β signaling was demonstrated to facilitate the recruitment of leukocytes and the clearance of S. pneumoniae in the CNS of mice with meningitis, resulting in reduced cerebrovascular complications (114).…”