Abstract
Dong X, Fan J, Lin D, Wang X, Kuang H, Gong L, Chen C, Jiang J, Xia N, He D, Shen W. J Neuroinflammation. 2022;19(1):226. doi:10.1186/s12974-022-02587-8
Evidence from experimental and clinical studies implicates immuno-inflammatory responses as playing an important role in epilepsy-induced brain injury. Captopril, an angiotensin-converting enzyme inhibitor (ACEi), has previously been shown to suppress immuno-inflammatory responses in a variety of neurological diseases. However, the therapeutic potential of captopril on epilepsy remains unclear. In the present study, Sprague Dawley (SD) rats were intraperitoneally subjected to kainic acid (KA) to establish a status epilepticus. Captopril (50 mg/kg, i.p.) was administered daily following the KA administration from day 3 to 49. We found that captopril efficiently suppressed the KA-induced epilepsy, as measured by electroencephalography. Moreover, captopril ameliorated the epilepsy-induced cognitive deficits, with improved performance in the Morris water maze, Y-maze and novel objective test. RNA sequencing (RNA-seq) analysis indicated that captopril reversed a wide range of epilepsy-related biological processes, particularly the glial activation, complement system-mediated phagocytosis and the production of inflammatory factors. Interestingly, captopril suppressed the epilepsy-induced activation and abnormal contact between astrocytes and microglia. Immunohistochemical experiments demonstrated that captopril attenuated microglia-dependent synaptic remodeling presumably through C3–C3ar-mediated phagocytosis in the hippocampus. Finally, the above effects of captopril were partially blocked by an intranasal application of recombinant C3a (1.3 μg/kg/day). Our findings demonstrated that captopril reduced the occurrence of epilepsy and cognitive impairment by attenuation of inflammation and C3-mediated synaptic phagocytosis. This approach can easily be adapted to long-term efficacy and safety in clinical practice.
Commentary
Inflammation is a common histopathological hallmark that occurs in different types of epilepsy independent of their etiology. Extensive research in human and preclinical models suggests that neuroinflammation parallels the generation of spontaneous recurrent seizures as well as the cognitive impairments that can be comorbid with epilepsy. 1 Studies utilizing pharmacogenetic-mediated reductions of proinflammatory signals have shown favorable outcomes in seizure reduction and memory improvement, thereby suggesting a critical role for neuroinflammation in the underlying causes of epilepsy. 1,2 Immune and glial cells produce and release inflammatory molecules that can help resolve an acute injury. However, if long-lasting, neuroinflammation can precipitate neuronal losses along with the pathological neural circuit remodeling that can trigger and maintain neuronal hyperexcitability that translates into seizures. Despite this knowledge, it is still uncertain which immune molecules and associated glial or immune cell responses participate directly in epileptogenic processes. The study by Dong et al interrogated the role of inflammation and immune complement signaling in the generation of epileptic networks in a mouse model of kainic acid (KA)-induced status epilepticus (SE) and acquired temporal lobe epilepsy. 3
In the brain, the immune complement cascade facilitates the generation of different immune signaling molecules needed to “give instructions” for microglial cells, the resident immune cells of the brain, to perform specific functions such as brain patrolling, proliferation, phagocytosis, and/or inflammation. 4 Altered complement signaling is linked to the neuropathology of neurological disorders such as Alzheimer’s disease, multiple sclerosis, and stroke, among others. 4 In human and experimental epilepsy, increases in complement signaling through the central protein of the pathway known as complement C3 have been reported (for review see the study by Wyatt-Johnson and Brewster 5 ); though their role in epilepsy is still not known. To address this question, Dong et al utilized captopril as a potential suppressor of C3 signaling, inflammation, and KA-induced epileptogenesis. 3
Captopril, an inhibitor of the angiotensin-converting enzyme (ACE), prevents angiotensin I from converting to angiotensin II and impedes degradation of vasodilatory prostaglandins. Because of its vasodilation effects, captopril is typically used for the treatment of hypertension. One recently identified substrate for ACEs is the complement molecule C3. 6 Angiotensin-converting enzymes were found to cleave the C3f fragment, 6 which in turn signals to increase synthesis of the C3 molecule. 3 Based on this observation, the authors hypothesized that captopril would reduce SE-induced increases in neuroinflammation and prevent the subsequent development of epilepsy via modulation of C3 signaling. The experimental design consisted of daily administration of captopril or saline (control) between 3 days and 7 weeks after SE to different groups of mice, and monitoring seizure activity with EEG recordings and measuring cognitive function through a battery of behavioral tests. These pathophysiological assessments were followed by measuring the impact of captopril treatment on the mRNA expression of neuroinflammatory markers along with histological evaluations of microglial and astrocyte responses in hippocampi of these mice.
Dong et al reported that in KA-treated mice, captopril reduced the seizure frequency and duration, and prevented the development of memory deficits compared to mice treated with KA+saline. Subsequent RNA-seq analyses revealed substantial alterations in genes associated with inflammatory processes in hippocampi of KA-treated mice compared to control mice (no KA). Most of these KA-induced gene transcription changes (one third) were attenuated in hippocampi of KA+captopril mice. The findings are supported by well-established evidence that immunomodulators as well as ACE inhibitors that suppress neuroinflammation can help control epilepsy and cognitive comorbid conditions. 7
Further, the study also reports that captopril had a measurable effect on changing the expression of least 240 genes, that occurred in parallel to reductions in microglial phagocytosis-related signaling, complement C3, astrogliosis, and microgliosis. Given these observations along with evidence that microglia can phagocytose synaptic elements in a complement C3-dependent manner, 8 the authors hypothesized that captopril may block SE-induced unwanted C3-mediated synaptic phagocytosis. Captopril reduced microglial CD68 signal and rescued expression of the synaptic protein synapsin in hippocampi of KA+captopril mice. Due to the close proximity between CD68, microglia, and synapsin, and reduced immunoreactivity of C3 in astrocytes, the authors concluded that captopril attenuated microglial synaptic phagocytosis. A limitation of this conclusion is that a spatial correlation between microglia and synaptic proteins does not necessarily translate to a functional impact, and that a comprehensive analysis utilizing high-resolution microscopy to measure microglial engulfment and quantification of synaptic cargo is necessary to establish that phagocytosis processes are at play (see the study by Abiega et al 9 ).
A challenging aspect for the interpretation of the findings by Dong et al is that both ACE and complement signaling have a broad range of substrates or downstream targets 6 that can directly or indirectly impact inflammation. 4,7 For example, in addition to the control of blood pressure, captopril has been shown to prevent cognitive deficits and have neuroprotective functions in different disease models through inhibition of ROS-NO pathway or modulation of intracellular signaling cascades such as PI3K, ERK, or GSK3β pathways. 10 –12 Activation of these pathways has been widely reported in association with seizures and epilepsy. Therefore, to establish a possible causal role for captopril-induced attenuation of C3 signaling and the improvement of the pathophysiology, it is necessary to determine the extent at which captopril modulated C3 cleavage and activation in parallel to these other signaling molecules. It is noted that in an attempt to establish a role for C3 in the SE-induced epilepsy, the investigators used intranasal recombinant C3a treatment to increase C3 levels in KA+captopril mice. While this resulted in a reversal of captopril’s beneficial effects on the development of cognitive deficits, synaptic pathology, and epilepsy, the brain levels of the C3 protein or activation of downstream complement targets were not measured. Thus, it is still unclear how C3a and downstream complement-mediated immune responses contribute to epileptogenesis.
In sum, regardless of captopril’s mechanisms of action, the findings by Dong et al support that this ACE inhibitor has a beneficial effect on reducing both the seizure burden and cognitive defects induced by SE in mice. Even though further investigation is still required to identify the exact downstream signaling pathways (e.g., angiotensin, inflammation, or complement) that contribute to captopril’s valuable effect on seizure control, this evidence suggests that ACE inhibitors may be a promising treatment strategy for epilepsy.
