By Nathifa Nasim, Neurobiology, Physiology & Behavior ‘22
Author’s note: I was recently introduced to this topic and the potential for sEH inhibition in the context of Alzheimer’s while at Dr. Lee-Way Jin’s lab in the MIND Institute. Further research into the topic outside the lab led to the realization of the broader implications of sEH inhibition across numerous neurological disorders through its role in inflammatory pathways. The paper aims to illustrate the therapeutic potential sEH inhibition in neurological disorders through inflammation mediation.
Introduction
Neuroinflammation is a symptom of nearly all neurological disorders, and occurs when the immune system of the brain is activated. Microglia, an immune cell in the brain, release inflammatory mediators typically associated with neuroprotection and neurogenesis. However, incessant inflammation can be associated with harmful effects and neurological diseases. Therefore, treatments that target inflammatory processes are of interest to researchers as they have the potential to alleviate numerous disorders. The soluble epoxide hydrolase enzyme (sEH), an enzyme that modulates various physiological processes through the regulation of inflammatory pathways, is one such possible therapeutic agent. Research on the effect of sEH inhibition provides an avenue for treating different neurological disorders such as brain injury, depression, and Alzheimer’s.
sEH and the Role of EETs
The sEH enzyme is important primarily through its ability to metabolize fatty acids in inflammatory pathways. It is found across mammalian tissues, such as in the brain or the liver, where it is highly expressed. As a member of the epoxide hydrolase family of enzymes, sEH’s C-terminus is responsible for the enzyme’s characteristic epoxide hydrolase activity, converting epoxides by adding a water molecule. The N-terminus, on the other hand, has phosphatase activity instead [2]. Due to the different activities of the two sides of the enzyme, the N-terminal phosphatase domain has been linked to cholesterol metabolism, while the C-terminal domain has been associated with inflammatory and cardiovascular effects.
The epoxide hydrolase activity of sEH is performed on epoxyeicosatrienoic acids (EETs), epoxy fatty acids that act as lipid messengers in the body. Due to their nature as lipid messengers, EETs have numerous roles, such as being anti-inflammatory messengers and increasing vasodilation. An increase in EET levels provides anti-inflammatory, antihypertensive, neuroprotective, and cardioprotective effects [2]. These protective effects of EET may be especially acute in the brain. For example, the inhibition of the cytochrome p450 enzyme results in an absence of EET, which reduces cerebral blood flow by 30 percent, indicating a critical need for EET in brain circulation [4].
Inhibition of sEH, a negative regulator of EET, increases these protective effects of EETs. The enzyme lowers EET activity via conversion to dihydroxyeicosatrienoic acids (DHETs), which have less anti-inflammatory effects, and can be pro-inflammatory instead. Numerous studies have knocked out or selectively inhibited sEH which stabilizes or increases EET activity because it is no longer being converted into DHET. It is through sEH’s metabolism of EETs that it is linked to numerous diseases and various physiological effects, as EETs are critical in controlling inflammation. Research on the possible therapeutic effects of sEH is therefore primarily focused on increasing fatty acid/EET levels through inhibition of sEH.
Inflammation in Brain Injury
Neuroinflammation is a critical aspect of traumatic brain injury (TBI), or brain damage via physical injury. TBI results in cerebral inflammation, which activates microglia to produce post-traumatic inflammatory mediators and reactive oxygen species (ROS). While inflammatory mediators function as an immune response after injury, their overproduction after TBI can lead to neural damage, apoptosis (cell death) and/or brain edema (swelling due to the accumulation of fluids). This additional damage occurring in the brain stimulates an increased microglia response, resulting in a positive feedback mechanism [5].
Inhibition of this post-traumatic stress-induced inflammation is a common mechanism in treating TBI. Given sEH’s role in inflammation, deletion of sEH via genetic knockouts has been shown to improve these effects. Deletion of sEH decreases the number of activated microglia post-TBI, and as a result, reduces the release of inflammatory mediators, thereby reducing edema, apoptosis, and inflammation [5]. The rise in EET levels associated with sEH inhibition also results in anti-inflammatory effects, and possibly an increase in neurotrophic factors, or growth factors that have a neuroprotective effect [5].
sEH and Alzheimer’s
Neuroinflammation is also linked to both the onset and progression of Alzheimer’s. Prolonged activation of glial cells— astrocytes and microglia—increase proinflammatory molecules which lead to a “neurotoxic” environment that aggravates the disease [6]. Alzheimer’s patients have nearly twofold higher sEH levels in the astrocytes, leading to lower EET levels and thus lower anti-inflammatory effects [7]. Given the importance of inflammation in Alzheimer’s, inhibition of sEH should increase EET levels, promoting anti-inflammatory effects crucial to treating the disease.
One experiment treated 5xFAD mice, transgenic mice that express Alzheimer’s phenotypes, with an sEH inhibitor TPPU. The results indicated that not only did the mice have increased EET levels, the inhibitor had also affected gene expression in the hippocampus by downregulating proinflammatory genes. This effectively “calmed” the overactive immune response correlated with Alzheimer’s. Further testing uncovered that these treated mice had fewer, smaller amyloid plaques, or proteins that are considered to play a critical role in Alzheimer’s and are often regarded as a biomarker for the disease, and fewer microglia surrounding these plaques [6]. As prolonged activation of microglia leads to proinflammatory effects and reduced phagocytosis of amyloid plaques, lowering the immune response of these glial cells could theoretically reverse these inflammatory effects. The reduction effects of EETs on ROS may also be involved, as ROS may contribute to neurotoxicity [7]. These results were achieved with TPPU and verified with another sEH inhibitor; it also aligned with results seen in genetic knockouts of sEH [6].
sEH in Peripheral Organs
In addition to the brain, peripheral organs may also play an important role in sEH’s effects on neurological disorders. In addition to astrocytes and other glial cells, sEH is most expressed in the liver. With the liver being the largest gland and responsible for a variety of metabolic activities, hepatic sEH (sEH of the liver) has an influence throughout the body. In a study on major depressive disorder (MDD) by Qin et al., researchers used mice in a chronic mild stress model (CMS), where they were exposed to varying stressors to mimic the effects of depression in an animal model. They found that chronic stress increased sEH levels in mice liver, suggesting hepatic sEH levels are linked to stress and depression [8]. Increased sEH levels in these mice via targeted gene therapy not only led to depressive-like behaviors, but there was also a decrease in proteins that modulate synaptic plasticity, suggesting that sEH parallels the effects of stress at the molecular level. On the other hand, deleting the gene that codes for sEH in the liver induced an antidepressant effect in the CMS mice [8]. In other words, sEH induced depressive-like effects, and inhibiting sEH activity led to antidepressant-like effects, even in stressed model mice. These findings were paralleled in MDD human patients who had lower EET levels compared to the control group, suggesting higher sEH activity in patients with depression, again similar to the Alzheimer’s patients [8].
Conclusion
Given the anti-inflammatory abilities of EETs, further development of sEH inhibitors has the capacity to affect the treatment of multiple neurodegenerative diseases which are associated with inflammation. In addition to those mentioned, other neurological conditions and disorders have also exhibited elevated sEH levels, such as Parkinson’s, schizophrenia, and seizures [6]. As these diseases have different pathologies, the cause for elevated sEH levels may vary and is still under research. Nevertheless, the implication of sEH in a variety of diseases expands the therapeutic range of sEH inhibitors.
As seen in the aforementioned MDD study by Qin et al., peripheral organs’ usage of sEH may also be involved with neurological disorders, such as in the case of hepatic sEH. This not only points to a possible liver-brain axis or connection between the two organs, it also opens another avenue of research into the effects of sEH across the body. Diseases such as depression and heart disease have been implicated with sEH in previous studies, for example, and at present, sEH inhibition has been successful in decreasing blood pressure levels [9]. Development and further research into sEH inhibition and effects therefore have the potential to touch numerous conditions and parts of the human body.
The research regarding Alzheimer’s and TBI both implemented pharmacological sEH inhibitors, in which only the C-terminal hydrolase domain of the enzyme was affected while the N-terminal phosphatase activity was left intact [5,6]. As the N-terminal has been associated with cholesterol metabolism, its role in neurological disorders provides another possible area of study of sEH in treating these disorders. Genetic deletion of the gene that codes for sEH, as seen in the Qin et al. study, deletes both the hydrolase and phosphatase activity of the enzyme. Therefore, studies implementing sEH knockouts may have benefitted from the loss of the phosphatase activity as well as the hydrolase [9]. As a result, further research into different aspects of the enzyme broadens the possibility of its usage and potential.
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