Soluble epoxide hydrolase inhibition alleviates neuropathy in Akita (Ins2 Akita) mice
Introduction
Diabetes is a growing worldwide epidemic and results in many comorbidities including hypertension, nephropathy, retinopathy and neuropathy [1]. Diabetic neuropathy has a profound negative impact on patients and available therapies have severe side effects and are often counter indicated for other comorbid conditions [2]. There is a great need for new therapeutics as well as relevant models to test them that have a potential for translation to man.
The Akita mouse model was discovered in the late 1990s and has been developed as a non-obese, maturity-onset of diabetes of the young (MODY) mouse model for Type I diabetes (Yoshioka 1997). It has important advantages compared to other models of diabetes used to study neuropathy, among other disorders. First, it is an autosomal dominant murine Ins2C96Y mutation with a progressive disease into adulthood which has a syntenic chromosomal conversion to human, and thus, is similar to the development of disease in humans. This obviates much of the argumentation around using streptozocin to chemically induce diabetes since streptozocin has been at times suggested to be neurotoxic [3]. Second, it is a non-obese model, and therefore is not subject to the complications of impaired motor function for behavioral tasks as observed with ob/ob or db/db mouse models. Third, it allows for an investigation of a dimorphic phenotype. The heterozygous Akita mice, particularly the males, progress to pernicious hyperglycemia and resulting secondary pathologies of diabetes providing a unique opportunity to study the physiopathologies of the disease and therapeutic interventions to alleviate them. Homozygous Ins2 Akita males are subject to high mortality at a young age [4] while the homozygous wildtype littermates are used as asymptomatic controls. Here we employ this mouse model to study the effects of enzyme inhibition on the developed diabetic neuropathy and probe differences in the sexually dimorphic expression of the phenotype to answer basic questions about the pathology in these animals.
The experiments here use the naturally occurring Akita mouse model to test the hypothesis that inhibiting the soluble epoxide hydrolase enzyme (sEH) mediates analgesia against the chronic pain of diabetic neuropathy. The sEH is a master regulatory enzyme in the arachidonic acid (ARA) cascade downstream of the cytochrome P450 enzymes that act on long chain polyunsaturated fatty acids (PUFA) as substrates. The sEH transforms epoxy-fatty acids (EpFA) into the vicinal diols of several classes of lipids including both omega-3 and 6 derived metabolites. The EpFA are stabilized in vivo by the inhibition of the sEH and have demonstrated anti-inflammatory, antihypertensive, and analgesic properties [5], [6], [7]. The EpFA have also demonstrated efficacy against chemically induced diabetic neuropathy [8]. Here we use the Akita mouse model to inquire if sEH inhibition is analgesic in this naturally progressing disease, and importantly, if there are sexual dimorphisms in the pain responses. We used the conditioned place preference assay to investigate if sEHI block neuropathic pain. We then tested both male and female Akita mice for sEH enzyme activity and examined the oxylipin profile in the same groups.
Section snippets
Animals
All procedures and animal care adhered the National Institutes of Health guide for the care and use of Laboratory animals (NIH Publications 8th Edition 2011) and were performed in accordance with the protocols approved by the Animal Use and Care Committee (IACUC) of the University of California, Davis. Great care was taken to minimize suffering of the animals and to reduce the number of animals used. Experiments on heterozygous Akita and wildtype littermate controls used mice bred from wild
Inhibiting sEH induces a CPP response in Akita mice with developed neuropathy
For all the behavioral tests the Akita mice were cohoused with their littermate controls for the duration of the experiments and randomly assigned to treatment or vehicle groups and tested intermingled for the studies. Only at sacrifice were the individual mice identified as Akita or littermate control and added to the appropriate groups. The Akita mouse model was used for these experiments because the Akita diabetic mice are lean, motor competent mice that can be assessed in these assays as a
Discussion
The sEH inhibitors (sEHI) are analgesic in several models of pain including inflammatory lipopolysaccharide and carrageenan induced pain which were some of the first models tested with sEHI [16], [17], [18]. Later the investigation broadened into testing neuropathic pain such as streptozocin induced diabetic neuropathy [7], [19]. However, there is debate whether this chemically induced model compares to the human disease [3]. Thus, we report here the naturally progressing Akita MODY model which
Conflict of interest
The University of California holds patents on the sEH inhibitors used in this study as well as their use to treat inflammation, inflammatory pain, and neuropathic pain. BD Hammock is a co-founder and K Wagner is an employee of EicOsis L.L.C., a startup company advancing sEH inhibitors into the clinic.
Acknowledgements
This work was supported by the National Institute of Environmental Health Sciences (NIEHS) Grant R01 ES002710, NIEHS Superfund Research Program P42 ES004699, National Institute of Neurological Disorders and Stroke (NINDS) U54 NS079202-01 and Grants NIEHST32ES007059, NIH 5T32DC008072-05 and 5T32HL086350-08 (to K.W.). This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. A.V.G supported by a Hellman Fellowship. The content is
References (37)
- et al.
National, regional, and global trends in fasting plasma glucose and diabetes prevalence since 1980: systematic analysis of health examination surveys and epidemiological studies with 370 country-years and 2·7 million participants
Lancet
(2011) - et al.
Soluble epoxide hydrolase inhibition is antinociceptive in a mouse model of diabetic neuropathy
J. Pain
(2014) Diabetic painful and insensate neuropathy: pathogenesis and potential treatments
Neurotherapeutics
(2009)- et al.
Inhibition of soluble epoxide hydrolase reduces LPS-induced thermal hyperalgesia and mechanical allodynia in a rat model of inflammatory pain
Life Sci.
(2006) - et al.
Soluble epoxide hydrolase inhibition reveals novel biological functions of epoxyeicosatrienoic acids (EETs)
Prostaglandins Other Lipid Mediat.
(2007) - et al.
Naturally occurring monoepoxides of eicosapentaenoic acid and docosahexaenoic acid are bioactive antihyperalgesic lipids
J. Lipid Res.
(2010) - et al.
Distribution and properties of a mammalian soluble epoxide hydrase
Biochem. Pharmacol.
(1980) - et al.
Epoxide metabolism in the liver of mice treated with clofibrate (ethyl-alpha-(p-chlorophenoxyisobutyrate)), a peroxisome proliferator
Toxicol. Appl. Pharmacol.
(1985) - et al.
Hormonal regulation of the developmental pattern of epoxide hydrolases. Studies in rat liver
Biochem. Pharmacol.
(1989) - et al.
Differential regulation of soluble epoxide hydrolase by clofibrate and sexual hormones in the liver and kidneys of mice
Biochem. Pharmacol.
(1995)