Mice deficient in complement C3 are protected against recognition memory deficits and astrogliosis induced by status epilepticus

Introduction

Status epilepticus (SE) can significantly increase the risk of temporal lobe epilepsy (TLE) and cognitive comorbidities. A potential candidate mechanism underlying memory defects in epilepsy may be the immune complement system. The complement cascade, part of the innate immune system, modulates inflammatory and phagocytosis signaling, and has been shown to contribute to learning and memory dysfunctions in neurodegenerative disorders. We previously reported that complement C3 is elevated in brain biopsies from human drug-resistant epilepsy and in experimental rodent models. We also found that SE-induced increases in hippocampal C3 levels paralleled the development of hippocampal-dependent spatial learning and memory deficits in rats. Thus, we hypothesized that SE-induced C3 activation contributes to this pathophysiology in a mouse model of SE and acquired TLE.

Methods

In this study C3 knockout (KO) and wild type (WT) mice were subjected to one hour of pilocarpine-induced SE or sham conditions (control; C). Following a latent period of two weeks, recognition memory was assessed utilizing the novel object recognition (NOR) test. Western blotting was utilized to determine the protein levels of C3 in hippocampal lysates. In addition, we assessed the protein levels and distribution of the astrocyte marker glial fibrillary acidic protein (GFAP).

Results

In the NOR test, control WT + C or C3 KO + C mice spent significantly more time exploring the novel object compared to the familiar object. In contrast, WT+SE mice did not show preference for either object, indicating a memory defect. This deficit was prevented in C3 KO + SE mice, which performed similarly to controls. In addition, we found that SE triggered significant increases in the protein levels of GFAP in hippocampi of WT mice but not in C3 KO mice.

Discussion

These findings suggest that ablation of C3 prevents SE-induced recognition memory deficits and that a C3-astrocyte interplay may play a role. Therefore, it is possible that enhanced C3 signaling contributes to SE-associated cognitive decline during epileptogenesis and may serve as a potential therapeutic target for treating cognitive comorbidities in acquired TLE.

Assessment of tau phosphorylation and β-amyloid pathology in human drug-resistant epilepsy

OBJECTIVE

Epilepsy can be comorbid with cognitive impairments. Recent evidence suggests the possibility that cognitive decline in epilepsy may be associated with mechanisms typical of Alzheimer's disease (AD). Neuropathological hallmarks of AD have been found in brain biopsies surgically resected from patients with drug-resistant epilepsies. These include hyperphosphorylation of the tau protein (p-tau) that aggregate into neuropil threads (NT) or neurofibrillary tangles (NFT), as well as presence of β-amyloid (Aβ) deposits. While recent studies agree on these AD neuropathological findings in epilepsy, some contrast in their correlation to cognitive decline. Thus, to further address this question we determined the abundance of p-tau and Aβ proteins along with their association to cognitive function in 12 cases of refractory epilepsy.

METHODS

Cortical biopsies surgically extracted from the temporal lobes of patients with refractory epilepsy were processed for immunohistology and enzyme-linked immunoassays to assess distribution and levels, respectively, of p-tau (Antibodies: Ser202/Thr205; Thr205; Thr181) and Aβ proteins. In parallel, we measured activation of mechanistic target of rapamycin (mTOR) via p-S6 (Antibodies: Ser240/244; Ser235/236). Pearson correlation coefficient analysis determined associations between these proteins and neurophysiological scores for full scale intelligence quotient (FSIQ).

RESULTS

We found a robust presence of p-tau (Ser202/Thr205)-related NT and NFT pathology, as well as Aβ deposits, and p-S6 (Ser240/244; Ser235/236) in the epilepsy biopsies. We found no significant correlations between FSIQ and p-tau (Thr205; Thr181), Aβ or mTOR markers with FSIQ scores, although some correlation coefficients were modest to strong.

SIGNIFICANCE

These findings strongly support the existence of hyperphosphorylated tau protein and Aβ deposits in patients with human refractory epilepsy. However, their relation to cognitive decline is still unclear and requires further investigation.

Dendritic and Spine Loss in Epilepsy: What Seizures Got to Do With It?

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Getting Excited Through Cyclin: A Role for Endothelial Cdk5 Signaling in Hippocampal Hyperexcitability

Endothelial Cdk5 Deficit Leads to the Development of Spontaneous Epilepsy Through CXCL1/CXCR2-Mediated Reactive Astrogliosis

Liu XX, Yang L, Shao LX, et al. J Exp Med. 2020;217(1):e20180992. DOI: 10.1084/jem.20180992

Blood–brain barrier dysfunction has been suggested to play an important role in epilepsy. However, the mechanism mediating the transition from cerebrovascular damage to epilepsy remains unknown. Here, we report that endothelial cyclin-dependent kinase 5 is a central regulator of neuronal excitability. Endothelial-specific Cdk5 knockout led to spontaneous seizures in mice. Knockout mice showed increased endothelial chemokine (C-X-C motif) ligand 1 (Cxcl1) expression, decreased astrocytic glutamate reuptake through the glutamate transporter 1 (GLT1), and increased glutamate synaptic function. Ceftriaxone restored astrocytic GLT1 function and inhibited seizures in endothelial Cdk5-deficient mice, and these effects were also reversed after silencing Cxcl1 in endothelial cells and its receptor chemokine (C-X-C motif) receptor 2 (Cxcr2) in astrocytes, respectively, in the CA1 by AAV transfection. These results reveal a previously unknown link between cerebrovascular factors and epileptogenesis and provide a rationale for targeting endothelial signaling as a potential treatment for epilepsy.¹

Seizing the Alzheimer's Brain: A Role for Sirtuin 3 in Hyperexcitability

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