Association between polymorphism rs2421943 of the insulin-degrading enzyme and schizophrenia: Preliminary report

BACKGROUND

Insulin-degrading enzyme (IDE) is an important gene in studies of the pathophysiology of type 2 diabetes mellitus (T2DM). Recent studies have suggested a possible link between type 2 diabetes mellitus (T2DM) and the pathophysiology of schizophrenia (SZ). At the same time, significant changes in insulin-degrading enzyme (IDE) gene expression have been found in the brains of people with schizophrenia. These findings highlight the need to further investigate the role of IDE in schizophrenia pathogenesis.

METHODS

We enrolled 733 participants from the Czech Republic, including 383 patients with schizophrenia and 350 healthy controls. Our study focused on the single nucleotide polymorphism (SNP) rs2421943 in the IDE gene, which has previously been associated with the pathogenesis of Alzheimer's disease. The SNP was analyzed using the PCR-RFLP method.

RESULTS

The G allele of the rs2421943 polymorphism was found to significantly increase the risk of developing SZ (p < 0.01) when a gender-based analysis showed that both AG and GG genotypes were associated with a more than 1.55 times increased risk of SZ in females (p < 0.03) but not in males. Besides, we identified a potential binding site at the G allele locus for has-miR-7110-5p, providing a potential mechanism for the observed association.

CONCLUSION

Our results confirm the role of the IDE gene in schizophrenia pathogenesis and suggest that future research should investigate the relationship between miRNA and estrogen influence on IDE expression in schizophrenia pathogenesis.

Antibody-Mediated Inhibition of Insulin-Degrading Enzyme Improves Insulin Activity in a Diabetic Mouse Model

Diabetes is a metabolic disease that may lead to different life-threatening complications. While insulin constitutes a beneficial treatment, its use may be limited due to increased degradation and an increase in side effects such as weight gain and hypoglycemia. Small molecule inhibitors to insulin-degrading enzyme (IDE) have been previously suggested as a potential treatment for diabetes through their ability to reduce insulin degradation and thus increase insulin activity. Nevertheless, their tendency to bind to the zinc ion in the catalytic site of IDE may affect other important metalloproteases and limit their clinical use. Here, we describe the isolation of an IDE-specific antibody that specifically inhibits insulin degradation by IDE. Using phage display, we generated a human IDE-specific antibody that binds human and mouse IDE with high affinity and specificity and can differentiate between active IDE to a mutated IDE with reduced catalytic activity in the range of 30 nM. We further assessed the ability of that IDE-inhibiting antibody to improve insulin activity in vivo in an STZ-induced diabetes mouse model. Since human antibodies may stimulate the mouse immune response to generate anti-human antibodies, we reformatted our inhibitory antibody to a “reverse chimeric” antibody that maintained the ability to inhibit IDE in vitro, but consisted of mouse constant regions, for reduced immunogenicity. We discovered that one intraperitoneal (IP) administration of the IDE-specific antibody in STZ-induced diabetic mice improved insulin activity in an insulin tolerance test (ITT) assay and reduced blood glucose levels. Our results suggest that antibody-mediated inhibition of IDE may be beneficial on improving insulin activity in a diabetic environment.

Dietary curcumin restores insulin homeostasis in diet-induced obese aged mice

Although aging is a physiological process to which all organisms are subject, the presence of obesity and type 2 diabetes accelerates biological aging. Recent studies have demonstrated the causal relationships between dietary interventions suppressing obesity and type 2 diabetes and delaying the onset of age-related endocrine changes. Curcumin, a natural antioxidant, has putative therapeutic properties such as improving insulin sensitivity in obese mice. However, how curcumin contributes to maintaining insulin homeostasis in aged organisms largely remains unclear. Thus, the objective of this study is to examine the pleiotropic effect of dietary curcumin on insulin homeostasis in a diet-induced obese (DIO) aged mouse model. Aged (18-20 months old) male mice given a high-fat high-sugar diet supplemented with 0.4% (w/w) curcumin (equivalent to 2 g/day for a 60 kg adult) displayed a different metabolic phenotype compared to mice given a high-fat high-sugar diet alone. Furthermore, curcumin supplementation altered hepatic gene expression profiling, especially insulin signaling and senescence pathways. We then mechanistically investigated how curcumin functions to fine-tune insulin sensitivity. We found that curcumin supplementation increased hepatic insulin-degrading enzyme (IDE) expression levels and preserved islet integrity, both outcomes that are beneficial to preserving good health with age. Our findings suggest that the multifaceted therapeutic potential of curcumin can be used as a protective agent for age-induced metabolic diseases.

Insulin-Degrading Enzyme Regulates the Proliferation and Apoptosis of Porcine Skeletal Muscle Stem Cells via Myostatin/MYOD Pathway

Identifying the genes relevant for muscle development is pivotal to improve meat production and quality in pigs. Insulin-degrading enzyme (IDE), a thiol zinc-metalloendopeptidase, has been known to regulate the myogenic process of mouse and rat myoblast cell lines, while its myogenic role in pigs remained elusive. Therefore, the current study aimed to identify the effects of IDE on the proliferation and apoptosis of porcine skeletal muscle stem cells (PSMSCs) and underlying molecular mechanism. We found that IDE was widely expressed in porcine tissues, including kidney, lung, spleen, liver, heart, and skeletal muscle. Then, to explore the effects of IDE on the proliferation and apoptosis of PSMSCs, we subjected the cells to siRNA-mediated knockdown of IDE expression, which resulted in promoted cell proliferation and reduced apoptosis. As one of key transcription factors in myogenesis, MYOD, its expression was also decreased with IDE knockdown. To further elucidate the underlying molecular mechanism, RNA sequencing was performed. Among transcripts perturbed by the IDE knockdown after, a downregulated gene myostatin (MSTN) which is known as a negative regulator for muscle growth attracted our interest. Indeed, MSTN knockdown led to similar results as those of the IDE knockdown, with upregulation of cell cycle-related genes, downregulation of MYOD as well as apoptosis-related genes, and enhanced cell proliferation. Taken together, our findings suggest that IDE regulates the proliferation and apoptosis of PSMSCs via MSTN/MYOD pathway. Thus, we recruit IDE to the gene family of regulators for porcine skeletal muscle development and propose IDE as an example of gene to prioritize in order to improve pork production.

Effect of Global Brain Ischemia on Amyloid Precursor Protein Metabolism and Expression of Amyloid-Degrading Enzymes in Rat Cortex: Role in Pathogenesis of Alzheimer's Disease

The incidence of Alzheimer's disease (AD) increases significantly following chronic stress and brain ischemia which, over the years, cause accumulation of toxic amyloid species and brain damage. The effects of global 15-min ischemia and 120-min reperfusion on the levels of expression of the amyloid precursor protein (APP) and its processing were investigated in the brain cortex (Cx) of male Wistar rats. Additionally, the levels of expression of the amyloid-degrading enzymes neprilysin (NEP), endothelin-converting enzyme-1 (ECE-1), and insulin-degrading enzyme (IDE), as well as of some markers of oxidative damage were assessed. It was shown that the APP mRNA and protein levels in the rat Cx were significantly increased after the ischemic insult. Protein levels of the soluble APP fragments, especially of sAPPβ produced by β-secretase, (BACE-1) and the levels of BACE-1 mRNA and protein expression itself were also increased after ischemia. The protein levels of APP and BACE-1 in the Cx returned to the control values after 120-min reperfusion. The levels of NEP and ECE-1 mRNA also decreased after ischemia, which correlated with the decreased protein levels of these enzymes. However, we have not observed any changes in the protein levels of insulin-degrading enzyme. Contents of the markers of oxidative damage (di-tyrosine and lysine conjugates with lipid peroxidation products) were also increased after ischemia. The obtained data suggest that ischemia shifts APP processing towards the amyloidogenic β-secretase pathway and accumulation of the neurotoxic Aβ peptide as well as triggers oxidative stress in the cells. These results are discussed in the context of the role of stress and ischemia in initiation and progression of AD.

Short-Term Calorie Restriction Maintains Plasma Insulin Concentrations along with a Reduction in Hepatic Insulin-Degrading Enzyme Levels in db/db Mice

Maintaining blood insulin levels is important for patients with diabetes because insulin secretion capacity declines with the development of the disease. Calorie restriction (CR) is effective for the improvement of glucose tolerance, but it is not clear whether CR can maintain insulin levels in the late stage of diabetes. We examined the effect of CR on whole-body glucose tolerance and fasting blood insulin concentrations in the late stage of diabetes. Male db/db mice were subjected to either a standard laboratory diet ad libitum for 3 weeks (dbdb group) or 40% CR (dbdb+CR group). CR significantly decreased body mass and epididymal fat weight. Glucose tolerance and fasting glucose levels were significantly improved with 3-week CR. Fasting insulin concentrations were decreased in the dbdb group but were maintained in the dbdb+CR group. CR significantly reduced insulin-degrading enzyme (IDE) levels in the liver, and hepatic IDE levels were significantly positively and negatively correlated with plasma glucose concentrations (area under the curve) after glucose administration and after fasting insulin concentrations, respectively. Therefore, 3-week CR maintained blood insulin levels and improved glucose tolerance with decreased hepatic IDE levels in an animal model of late-stage diabetes.

Insulin-signaling Pathway Regulates the Degradation of Amyloid β-protein via Astrocytes

Alzheimer's disease (AD) has been considered as a metabolic dysfunction disease associated with impaired insulin signaling. Determining the mechanisms underlying insulin signaling dysfunction and resistance in AD will be important for its treatment. Impaired clearance of amyloid-β peptide (Aβ) significantly contributes to amyloid accumulation, which is typically observed in the brain of AD patients. Reduced expression of important Aβ-degrading enzymes in the brain, such as neprilysin (NEP) and insulin-degrading enzyme (IDE), can promote Aβ deposition in sporadic late-onset AD patients. Here, we investigated whether insulin regulates the degradation of Aβ by inducing expression of NEP and IDE in cultured astrocytes. Treatment of astrocytes with insulin significantly reduced cellular NEP levels, but increased IDE expression. The effects of insulin on the expression of NEP and IDE involved activation of an extracellular signal-regulated kinase (ERK)-mediated pathway. The reduction in cellular NEP levels was associated with NEP secretion into the culture medium, whereas IDE was increased in the cell membranes. Moreover, insulin-treated astrocytes significantly facilitated the degradation of exogenous Aβ within the culture medium. Interestingly, pretreatment of astrocytes with an ERK inhibitor prior to insulin exposure markedly inhibited insulin-induced degradation of Aβ. These results suggest that insulin exposure enhanced Aβ degradation via an increase in NEP secretion and IDE expression in astrocytes, via activation of the ERK-mediated pathway. The inhibition of insulin signaling pathways delayed Aβ degradation by attenuating alterations in NEP and IDE levels and competition with insulin and Aβ. Our results provide further insight into the pathological relevance of insulin resistance in AD development.