Could Inhibiting a Liver Enzyme Ameliorate Alzheimer’s Pathology in the Brain?

02 Aug 2023

To treat neurodegenerative disease, researchers face the challenge of getting drugs across the blood-brain barrier. But what if targeting a peripheral enzyme could do the job? In the June 26 Neuron online, researchers led by Xin-Hong Zhu at Shenzhen University, China, reported that inhibiting a liver enzyme, soluble epoxide hydrolase (sEH), boosted the amount of protective epoxy fatty acids, particularly 14,15-epoxyeicosatrienoic acid, in the blood of mice. In two different amyloidosis models, plasma 14,15-EET entered the brain, where it stimulated glial cells to mop up amyloid plaques. The intervention also lowered p-tau and improved memory. “Targeting the liver-brain axis … may constitute a promising therapeutic approach for Alzheimer’s prevention,” the authors suggested.

Others saw potential as well. “[The paper] reveals a novel liver-brain interaction pathway that modulates AD neuropathology … [and] raises the exciting possibility that peripheral sEH inhibition may provide therapeutic benefit for treating CNS diseases,” Hui Zheng at Baylor College of Medicine, Houston, wrote to Alzforum (comment below). Bruce Hammock at the University of California, Davis, who collaborates with Zhu, believes his research is top-notch. “Several researchers in the field, using different technologies, have come to the same conclusion regarding epoxy fatty acids resolving inflammation in the CNS,” Hammock wrote.

Plaque Cleanup. Amyloid plaques (bright green) accumulate in the motor cortices (left), hippocampi (middle), and entorhinal cortices (right) of 16-month-old 3xTgAD mice (top), but less so when the liver enzyme sEH is knocked out at 14 months (bottom). [Courtesy of Wu et al., Neuron.]

In recent years, sEH and epoxy fatty acids have attracted attention as potential therapeutic targets for several chronic diseases. In the Alzheimer’s field, Zheng previously found that sEH is elevated in AD brain, as well as in the brains of amyloidosis mice. Inhibiting this enzyme in the brains of 5XFAD mice prevented amyloid deposition, neuroinflammation, and memory problems (Dec 2020 news). Zhu and colleagues reported similar findings (Chen et al., 2020).

Because the liver is the main source of epoxy fatty acids, and a major site for sEH expression, Zhu wondered if targeting the enzyme in this organ would affect the brain. Joint first authors Yu Wu and Jing-Hua Dong made inducible, conditional liver sEH knockouts, and crossed them with 5XFAD mice. When the mice were fed tamoxifen at 5 months of age, sEH production plunged only in the liver. Yet the effects showed up in the brain, where the number of plaques in the hippocampus, entorhinal cortex, and motor cortex plummeted by 90 percent, and insoluble Aβ fell by half. The mice performed better than did control littermates in several behavioral assays, including the Y-maze, novel-object recognition, and fear conditioning.

In a second amyloidosis model, 3xTgAD mice, knocking out the liver enzyme at 14 months of age likewise improved memory and reduced amyloid plaques two months later (image above). It also lowered p-tau by one-third to one-half and tau oligomers, as seen by AT8 and T22 staining. Conversely, overexpressing sEH in the liver of 3xTgAD mice worsened brain pathology, ramping up insoluble Aβ and p-tau and blunting memory.

How did suppressing sEH cause these changes? Using radiolabeling, the authors showed that its substrate 14,15-EET readily crossed the blood-brain barrier. There, it triggered astrocytes to release ApoE, which in turn stimulated microglia to express TREM2 and clear amyloid. Nabil Alkayed at Oregon Health & Science University, Portland, noted that this fits with earlier work suggesting that inhibiting sEH during brain ischemia induces microglia to protect neurons (Wang et al., 2013).

In addition, the authors found that 14,15-EET directly bound to Aβ in vitro, preventing its aggregation and even breaking up Aβ oligomers isolated from 5XFAD brain. The more 14,15-EET in the brains of 5XFAD and 3xTgAD mice, the less plaque they accumulated. Notably, infusing 14,15-EET into the lateral ventricles of 5XFAD or 3xTgAD mice also curbed insoluble Aβ and p-tau and improved memory.

The same liver-brain mechanism may be at work in people, the authors noted. Hepatic sEH activity rises during aging, suppressing plasma 14,15-EET and potentially leaving the brain more vulnerable to amyloid accumulation. The authors found that this fatty acid was lower in people with AD than in healthy elderly. In a small cohort of 35 early AD patients and 15 controls, low plasma 14,15-EET discriminated the groups with an AUC of 0.97, suggesting potential as an early biomarker of the disease.

In addition, sEH, which is encoded by the gene EPHX2, is genetically linked to AD. EPHX2 sits in the CLU locus, a major AD risk site, and a recent GWAS identified it as a top druggable target for Alzheimer’s disease (Su et al., 2023). Other studies suggest sEH may play a role in vascular dementia, Parkinson’s disease, and traumatic brain injury (Nelson et al., 2014; Ren et al., 2018; Dai et al., 2023).

sEH inhibitors are already being tested in Phase 1 clinical trials for various conditions, including hypertension, stroke, and insulin resistance, where they have been well-tolerated so far (e.g., Martini et al., 2022). A company founded by Hammock, EicOsis, tests its inhibitor EC5026 in Phase 1 trials for neuropathic pain.

“The promise of sEH inhibitors in Alzheimer’s disease is that they have proven beneficial in several of these underlying conditions—diabetes, chronic inflammation, vascular disease—and hence can target multiple pathways to prevent brain damage,” Darryl Zeldin at the National Institute of Environmental Health Sciences (NIEHS), Research Triangle Park, North Carolina, wrote to Alzforum. “sEH inhibitors, which have been shown to be safe and effective in early phase human trials, should now be tested in an Alzheimer’s disease population.”—Madolyn Bowman Rogers

Soluble epoxide hydrolase (sEH) is expressed in both peripheral tissues and CNS. Its upregulation has been implicated in multiple neurological conditions, including depression, Parkinson’s disease, and Alzheimer’s disease. Genetic and pharmacological inhibition has been shown to provide therapeutic benefit and these have been attributed to a central mechanism. Here, the authors present an alternative mechanism by which hepatic sEH regulates 14,15-EET, one of the sEH substrates, in the periphery, which then passes the blood-brain barrier to mediate Aβ and tau pathologies and behavior in AD mouse models.

The significance of the work is twofold: One, it reveals a novel liver-brain interaction pathway that modulates AD neuropathology; two, it raises the exciting possibility that peripheral sEH inhibition may provide therapeutic benefit for treating CNS diseases such as AD.

Having said that, the study as presented leaves open several questions that warrant further investigation: 1) We and others have shown that levels of sEH in the brain are increased in AD patients and mouse models (Lee et al., 2019; Ghosh et al., 2020). The strong correlation between sEH inhibition and increased EETs in the brain that we observed supports a central mode of action. Thus the differential contributions of central versus peripheral effects remain to be established. 2) Robust effects of hepatic sEH inhibition on both Aβ and tau pathologies are intriguing and beg the question if these are mediated through common or distinct pathways. 3) A specific increase of 14,15-EET, but not other regioisomers, is also intriguing because these are all sEH substrates and are expected to be affected by sEH inhibition.

Lee HT, Lee KI, Chen CH, Lee TS.Genetic deletion of soluble epoxide hydrolase delays the progression of Alzheimer's disease. J Neuroinflammation. 2019 Dec 17;16(1):267. PubMed.

Ghosh A, Comerota MM, Wan D, Chen F, Propson NE, Hwang SH, Hammock BD, Zheng H.An epoxide hydrolase inhibitor reduces neuroinflammation in a mouse model of Alzheimer's disease. Sci Transl Med. 2020 Dec 9;12(573) PubMed.

The significance of this study is that peripherally administered sEH inhibitors may benefit CNS disorders, especially AD, i.e., there’s no need for the drug to cross the blood-brain barrier, which is a major challenge in designing CNS-targeting drugs. The observation that sEH increases with age is consistent with our previous observation that sEH is increased in the cerebral microvascular endothelium in postmortem human brains from deceased patients who had vascular-type dementia (Nelson et al., 2014). However, in the current paper, the authors observed an age-dependent increase in sEH in the livers of normal mice, as well as in the liver of an AD mouse model. In contrast, the increase in our study was in diseased brain. Therefore, it seems that changes in liver sEH do not necessarily contribute to AD pathology per se, but rather worsen AD by decreasing levels of the anti-inflammatory 14,15-EET, attenuating microglial phagocytosis. The latter observation is consistent with an earlier report showing that sEH inhibition induces a neuroprotective phenotype in activated microglia in the context of global cerebral ischemia (Wang et al., 2013).

That AD patients have reduced 14,15-EET is interesting, as it may provide a biomarker for the early detection of AD. This finding is also consistent with our recent work showing that human polymorphisms in the newly discovered receptor for 14,15-EET, GPR39, are linked to white-matter hyperintensity, an MRI marker of vascular dementia, and that GPR39 gene deletion causes cognitive deficits in mice (Alkayed et al., 2022; Davis et al., 2021; Bah et al., 2022). The authors’ observation that sEH is protective in models of AD is not novel, as similar observations have been previously reported (Ghosh et al., 2020; Griñán-Ferré et al., 2020). The novel findings are the role of hepatic sEH in setting the levels of plasma 14,15-EET, and its therapeutic targeting.

Some sEH inhibitors have made it to human clinical trials, including GSK2256294, which we have recently shown to be safe and well-tolerated in critically ill patients with subarachnoid hemorrhage, and which decreases inflammatory cytokines in CSF from these patients (Martini et al., 2022). However, long-term safety is unknown, which would be a requirement for an AD drug.

Nelson JW, Young JM, Borkar RN, Woltjer RL, Quinn JF, Silbert LC, Grafe MR, Alkayed NJ.Role of soluble epoxide hydrolase in age-related vascular cognitive decline. Prostaglandins Other Lipid Mediat. 2014 Oct;113-115:30-7. Epub 2014 Sep 30 PubMed.

Wang J, Fujiyoshi T, Kosaka Y, Raybuck JD, Lattal KM, Ikeda M, Herson PS, Koerner IP.Inhibition of soluble epoxide hydrolase after cardiac arrest/cardiopulmonary resuscitation induces a neuroprotective phenotype in activated microglia and improves neuronal survival. J Cereb Blood Flow Metab. 2013 Oct;33(10):1574-81. Epub 2013 Jul 3 PubMed.

Alkayed NJ, Cao Z, Qian ZY, Nagarajan S, Liu X, Nelson JW, Xie F, Li B, Fan W, Liu L, Grafe MR, Davis CM, Xiao X, Barnes AP, Kaul S.Control of coronary vascular resistance by eicosanoids via a novel GPCR. Am J Physiol Cell Physiol. 2022 May 1;322(5):C1011-C1021. Epub 2022 Apr 6 PubMed.

Davis CM, Bah TM, Zhang WH, Nelson JW, Golgotiu K, Nie X, Alkayed FN, Young JM, Woltjer RL, Silbert LC, Grafe MR, Alkayed NJ.GPR39 localization in the aging human brain and correlation of expression and polymorphism with vascular cognitive impairment. Alzheimers Dement (N Y). 2021;7(1):e12214. Epub 2021 Oct 14 PubMed.

Bah TM, Allen EM, Garcia-Jaramillo M, Perez R, Zarnegarnia Y, Davis CM, Bloom MB, Magana AA, Choi J, Bobe G, Pike MM, Raber J, Maier CS, Alkayed NJ.GPR39 Deficiency Impairs Memory and Alters Oxylipins and Inflammatory Cytokines Without Affecting Cerebral Blood Flow in a High-Fat Diet Mouse Model of Cognitive Impairment. Front Cell Neurosci. 2022;16:893030. Epub 2022 Jul 6 PubMed.

Ghosh A, Comerota MM, Wan D, Chen F, Propson NE, Hwang SH, Hammock BD, Zheng H.An epoxide hydrolase inhibitor reduces neuroinflammation in a mouse model of Alzheimer's disease. Sci Transl Med. 2020 Dec 9;12(573) PubMed.

Griñán-Ferré C, Codony S, Pujol E, Yang J, Leiva R, Escolano C, Puigoriol-Illamola D, Companys-Alemany J, Corpas R, Sanfeliu C, Pérez B, Loza MI, Brea J, Morisseau C, Hammock BD, Vázquez S, Pallàs M, Galdeano C.Pharmacological Inhibition of Soluble Epoxide Hydrolase as a New Therapy for Alzheimer's Disease. Neurotherapeutics. 2020 Jun 2; PubMed.

Martini RP, Siler D, Cetas J, Alkayed NJ, Allen E, Treggiari MM.A Double-Blind, Randomized, Placebo-Controlled Trial of Soluble Epoxide Hydrolase Inhibition in Patients with Aneurysmal Subarachnoid Hemorrhage. Neurocrit Care. 2022 Jun;36(3):905-915. Epub 2021 Dec 6 PubMed.

This is the third paper that has shown sEH inhibition is beneficial in preventing cognitive decline in Alzheimer’s disease. Importantly, this is the first paper to link sEH in the liver to Alzheimer’s in the brain, though unrelated studies have found connections between liver function and AD progression (Nho et al., 2019). The etiology of AD is largely unknown, though there are known risk factors, such as ApoE4 mutations, diabetes, chronic inflammation, and vascular disease.

The promise of sEH inhibitors in Alzheimer’s is that they have proven beneficial in several of these underlying conditions and hence can target multiple pathways to prevent end-organ damage, in this case, in the brain. Thus, this paper opens the door to a better understanding of the pathogenesis of AD and offers a possible new treatment for the disorder. The sEH inhibitors that have been shown to be safe and effective in early phase human trials should now be tested in an AD population.

Nho K, Kueider-Paisley A, Ahmad S, MahmoudianDehkordi S, Arnold M, Risacher SL, Louie G, Blach C, Baillie R, Han X, Kastenmüller G, Trojanowski JQ, Shaw LM, Weiner MW, Doraiswamy PM, van Duijn C, Saykin AJ, Kaddurah-Daouk R, Alzheimer’s Disease Neuroimaging Initiative and the Alzheimer Disease Metabolomics Consortium.Association of Altered Liver Enzymes With Alzheimer Disease Diagnosis, Cognition, Neuroimaging Measures, and Cerebrospinal Fluid Biomarkers. JAMA Netw Open. 2019 Jul 3;2(7):e197978. PubMed.

This paper has a wealth of data on the 5xFAD and 3xAD Tg models. Overall, it teaches us that manipulation of the peripheral production of specific arachidonic acid-derived oxylipins can ameliorate Alzheimer’s pathology by pleiotropic activities, including reducing BACE1 and Aβ oligomers, with possible direct inhibition of aggregate formation, while also increasing TREM2 and phagocytic amyloid clearance. The most impressive effects come from selectively ablating hepatic Ephx2 after pathology is present: five months in 5xFAD, and 14 months in 3xAD mice. They show a dramatic reduction in pathology to very low levels consistent with clearance and effective treatment of behavioral deficits. These effects are reminiscent of reports from our group and others on curcumin and other immunomodulatory lipid mediators, and highlight the potential of pleiotropic, small molecule interventions in an era of inherently expensive FDA-approved antibody infusions.

This paper does more than this in establishing the impact of bidirectional manipulations of the arachidonic acid/Ephx2 pathway and products that increase or decrease pathology. It shows age-related alterations in hepatic production and circulating levels of 14,15-EET and in DHET/EET ratios in the two animal models, which argue for a treatable, primary, hepatic age change. They also add some data comparing normal control and AD patient levels of these oxylipins to suggest “translational relevance.”

The study has some limitations. 1) It doesn’t include ApoE4 interactions with major effects on TREM2 and lipid mediators. 2) The age-related changes in mice show a jump between ages 5 and 7 months, which equates to normal adults, and only go out to 18 months, while studies on aging mice typically include endpoints at 24 months, or even 28 months and beyond as in the recent paper on taurine and aging (Jun 2023 news). 3) The human data needs to be bolstered, including on MCI measures and aging. I would like a discussion of how their findings relate to multiple, large, unbiased plasma lipidomics studies in humans and mouse models. I am not clear that these unbiased studies have measured these lipids and found, or not found, age or AD changes. In general, they don’t highlight them. 4) Apart from Aβ, the direct targets, vis -à- vis receptors, for the lipid mediators are not identified or discussed.

Despite the limitations, this study adds 3xAD mice and tauopathy data to previous papers from this group, reporting large effects in 5xFAD mice that linked their intervention efficacy to increased lysosomal biogenesis and amyloid clearance in astrocytes.

To make a comment you must login or register.

To make an annotation you must Login or Register.