Chemokine CCL2–CCR2 Signaling Induces Neuronal Cell Death via STAT3 Activation and IL-1β Production after Status Epilepticus

Abstract

Elevated levels of chemokine C-C motif ligand 2 (CCL2) and its receptor CCR2 have been reported in patients with temporal lobe epilepsy and in experimental seizures. However, the functional significance and molecular mechanism underlying CCL2-CCR2 signaling in epileptic brain remains largely unknown. In this study, we found that the upregulated CCL2 was mainly expressed in hippocampal neurons and activated microglia from mice 1 d after kainic acid (KA)-induced seizures. Taking advantage of double-transgenic mice, we demonstrated that CCL2-CCR2 signaling has a role in resident microglial activation and blood-derived monocyte infiltration. Moreover, seizure-induced degeneration of neurons in the hippocampal CA3 region was attenuated in mice lacking CCL2 or CCR2. We further showed that CCR2 activation induced STAT3 (signal transducer and activator of transcription 3) phosphorylation and IL-1β production, which are critical for promoting neuronal cell death after status epilepticus. Consistently, pharmacological inhibition of STAT3 by WP1066 reduced seizure-induced IL-1β production and subsequent neuronal death. Two weeks after KA-induced seizures, CCR2 deficiency not only reduced neuronal loss, but also attenuated seizure-induced behavioral impairments, including anxiety, memory decline, and recurrent seizure severity. Together, we demonstrated that CCL2-CCR2 signaling contributes to neurodegeneration via STAT3 activation and IL-1β production after status epilepticus, providing potential therapeutic targets for the treatment of epilepsy. Epilepsy is a global concern and epileptic seizures occur in many neurological conditions. Neuroinflammation associated with microglial activation and monocyte infiltration are characteristic of epileptic brains. However, molecular mechanisms underlying neuroinflammation in neuronal death following epilepsy remain to be elucidated. Here we demonstrate that CCL2-CCR2 signaling is required for monocyte infiltration, which in turn contributes to kainic acid (KA)-induced neuronal cell death. The downstream of CCR2 activation involves STAT3 (signal transducer and activator of transcription 3) phosphorylation and IL-1β production. Two weeks after KA-induced seizures, CCR2 deficiency not only reduced neuronal loss, but also attenuated seizure-induced behavioral impairments, including anxiety, memory decline, and recurrent seizure severity. The current study provides a novel insight on the function and mechanisms of CCL2-CCR2 signaling in KA-induced neurodegeneration and behavioral deficits.

Microglial proliferation and monocyte infiltration contribute to microgliosis following status epilepticus

“…Here, using a transgenic mouse model, we show that the observed seizure‐induced expansion of microglial population is actually comprised of both local microglial proliferation and monocytes infiltration. This is in line with our recent study as well as another study showing the infiltration of monocytes following chemically‐induced epilepsy pathogenesis (Tian et al, ; Varvel et al, ; Zattoni et al, ). The infiltration of monocytes was shown to be dependent on the CCL2‐CCR2 chemokine signaling pathway and reduced infiltration in CCR2 knockout mice corresponded to reduced neuronal loss, reduced neuroinflammation, and better functional recovery (Tian et al, ; Varvel et al, ).…”

“…This is in line with our recent study as well as another study showing the infiltration of monocytes following chemically‐induced epilepsy pathogenesis (Tian et al, ; Varvel et al, ; Zattoni et al, ). The infiltration of monocytes was shown to be dependent on the CCL2‐CCR2 chemokine signaling pathway and reduced infiltration in CCR2 knockout mice corresponded to reduced neuronal loss, reduced neuroinflammation, and better functional recovery (Tian et al, ; Varvel et al, ). This suggests that in addition to activating resident microglia, infiltrated monocytes also could contribute significantly to pathological progression of epilepsy.…”

“…Here, we found that the degree of microgliosis occurring within the CA1 and CA3 regions of the hippocampus corresponded to the level of neuronal loss and seizure severity. In our previous study, we showed microglial activation was localized to the CA3 region following moderate seizures (Tian et al, ). This is consistent with the notion that the degree of microgliosis directly correlated with seizure activity (Kim et al, ).…”

“…An increasing body of clinical and experimental evidence suggests that neuroinflammation in the brain is a crucial mechanism underlying the pathology of epilepsy (Eyo, Murugan, & Wu, ; Vezzani, ). Microglia are the innate immune cells of the brain and major contributors to neuroinflammation and neuronal loss following status epilepticus (Avignone, Ulmann, Levavasseur, Rassendren, & Audinat, ; Bosco et al, ; Matsuda et al, ; Tian et al, ). Within animal models of epilepsy, microglia become chronically activated with increased cell numbers in the hippocampus (Drage, Holmes, & Seyfried, ; Shapiro, Wang, & Ribak, ).…”

“…Recent studies, however, showed that in addition to resident microglia, infiltrating immune cells (i.e. , monocytes, leukocytes) invade the hippocampus after seizures and contribute significantly to pathogenesis (Tian et al, ; Varvel et al, ; Zattoni et al, ). In this study, we used complimentary techniques to investigate the source of microgliosis, particularly the respective contribution of monocyte infiltration and local proliferation in epilepsy.…”