Transgenic mice in the analysis of metabolic regulation

Effect of a novel proteoglycan PTP1B inhibitor from Ganoderma lucidum on the amelioration of hyperglycaemia and dyslipidaemia in db/db mice

Protein tyrosine phosphatase 1B (PTP1B) is implicated in the negative regulation of the insulin signalling pathway by dephosphorylating the insulin receptor (IR) and IR substrates. Ganoderma lucidum has traditionally been used for the treatment of diabetes in Chinese medicine; however, its anti-diabetic potency and mechanism in vivo is still unclear. Our previously published study reported a novel proteoglycan PTP1B inhibitor, named Fudan-Yueyang-Ganoderma lucidum (FYGL) from G. lucidum, with a half-maximal inhibitory concentration (IC₅₀) value of 5·12 (sem 0·05) μg/ml, a protein:polyglycan ratio of 17:77 and 78 % glucose in polysaccharide, and dominant amino acid residues of aspartic acid, glycine, glutamic acid, alanine, serine and threonine in protein. FYGL is capable of decreasing plasma glucose in streptozotocin-induced diabetic mice with a high safety of median lethal dose (LD₅₀) of 6 g/kg. In the present study, C57BL/6 db/db diabetic mice were trialed further using FYGL as well as metformin for comparison. Oral treatment with FYGL in db/db diabetic mice for 4 weeks significantly (P < 0·01 or 0·05) decreased the fasting plasma glucose level, serum insulin concentration and the homeostasis model assessment of insulin resistance. FYGL also controlled the biochemistry indices relative to type 2 diabetes-accompanied lipidaemic disorders. Pharmacology research suggests that FYGL decreases the plasma glucose level by the mechanism of inhibiting PTP1B expression and activity, consequently, regulating the tyrosine phosphorylation level of the IR β-subunit and the level of hepatic glycogen, thus resulting in the improvement of insulin sensitivity. Therefore, FYGL is promising as an insulin sensitiser for the therapy of type 2 diabetes and accompanied dyslipidaemia.

Murine models in critical care research

INTRODUCTION

Access to genetically engineered mice has opened many new opportunities to address questions relevant to the pathophysiology and treatment of patients in critical conditions. However, the results of studies in mice cannot disregard the unique ability of small rodents to adjust their temperature and high metabolic rate and the corresponding respiratory and circulatory requirements in response to hypoxia.

POINT OF VIEW

Studies performed in mice on questions related to metabolic, circulatory, and respiratory regulation should always be considered in light of the ability of mice to rapidly drop their nonshivering thermogenesis-related metabolism. As an example, it has been recently argued that a moderate level of inhaled hydrogen sulfide may have a potential benefit in patients in coma or shock or during an anoxic or ischemic insult, as this toxic gas dramatically reduces the metabolic rate in resting mice. However, acute hypometabolism has long been described in small mammals in response to hypoxia and is not specific to hydrogen sulfide. More importantly, mice have a specific metabolic rate that is 15-20 times higher than the specific metabolic level of a resting human. This difference can be accounted for by the large amount of heat produced by mice through nonshivering thermogenesis, related to the activity of uncoupling proteins. This mechanism, which is essential for maintaining homeothermia in small mammals, is virtually absent in larger animals, including in adult humans. Accordingly, no direct metabolic effect of hydrogen sulfide is observed in large mammals. We present the view that similar reasoning should be applied when the circulatory or respiratory response to hypoxic exposure is considered. This leads us to question whether a similar strategy could occur in mice in critical conditions other than hypoxia, such as in hypovolemic, septic, or cardiogenic shock.

CONCLUSION

Mouse models developed to understand the mechanisms of protection against hypoxia or ischemia or to propose new therapeutic approaches applicable in critical care patients should be understood in light of the specificity of the metabolic, respiratory, and circulatory responses of mice to a hypoxic insult, since many of these adaptations have no clear equivalent in humans.

Somatostatin receptors and autoimmune-mediated diabetes

Somatostatin (SST) peptide is produced by various SST-secreting cells throughout the body and acts as a neurotransmitter or paracrine/autocrine regulator in response to ions, nutrients, peptides hormones and neurotransmitters. SST is also widely distributed in the periphery to regulate the inflammatory and immune cells in response to hormones, growth factors, cytokines and other secretive molecules. SST peptides are considered the most important physiologic regulator of the islet cell, gastrointestinal cell and immune cell functions, and the importance of SST production levels has been implicated in several diseases including diabetes. The expression of SST receptors has also been found in T lymphocytes and primary immunologic organs. Interaction of SST and its receptors is also involved in T-cell proliferation and thymocyte selection. SSTR gene-ablated mice developed diabetes with morphologic, physiologic and immunologic alterations in the endocrine pancreas. Increased levels of mononuclear cell infiltration of the islets are associated with the increased levels of antigen-presenting cells located in the islets and peripancreatic lymph nodes. Increased levels of SST were also found in antigen-presenting cells and are associated with a significant increase of CD8 expression levels on CD4(+)/CD8(+) immature thymocytes. These findings highlight the crucial role of this neuroendocrine peptide and its receptors in regulating autoimmune functions.

Quantification of hepatic carbohydrate metabolism in conscious mice using serial blood and urine spots

In vivo studies of hepatic carbohydrate metabolism in (genetically modified) conscious mice are hampered by limitations of blood and urine sample sizes. We developed and validated methods to quantify stable isotope dilution and incorporation in small blood and urine samples spotted onto filter paper. Blood glucose and urinary paracetamol-glucuronic acid were extracted from filter paper spots reproducibly and with high yield. Fractional isotopomer distributions of glucose and paracetamol-glucuronic acid when extracted from filter paper spots were almost identical to those isolated from the original body fluids. Rates of infusion of labeled compounds could be adjusted without perturbing hepatic glucose metabolism. This approach was used in mice to find the optimal metabolic condition for the study of hepatic carbohydrate metabolism. In fed mice, no isotopic steady state was observed during a 6-h label-infusion experiment. In 9-h-fasted mice, isotopic steady state was reached after 3 h of label infusion and important parameters in hepatic glucose metabolism could be calculated. The rate of de novo glucose-6-phosphate synthesis was 143 +/- 17 micromol kg(-1) min(-1) and partitioning to plasma glucose was 79.0 +/- 5.2%. In 24-h-fasted mice, abrupt changes were noticed in whole body and in hepatic glucose metabolism at the end of the experiment.

Controlled overexpression of BCKD kinase expression: metabolic engineering applied to BCAA metabolism in a mammalian system

A common metabolic complication of human disease is uncontrolled muscle protein breakdown or cachexia, which occurs in patients with chronic diseases such as cancer, AIDS, renal failure, and diabetes. Increased branched-chain amino acid catabolism is implicated as causal and has stimulated the investigation of methods to regulate the metabolism of these amino acids. Here we demonstrate doxycycline-controlled overexpression of a branched-chain alpha-ketoacid dehydrogenase (BCKD) kinase transgene in mammalian cell culture. This kinase functions to inactivate the BCKD complex by phosphorylation, thus preventing the catabolism of these essential, regulatory metabolites. In this study, doxycycline treatment leads to a 10-fold increase in BCKD kinase protein. The transgene-generated kinase is rapidly incorporated within mitochondria and functions correctly to inactivate the BCKD complex. The maximum reduction in basal BCKD activity achieved was 94%. Unexpectedly, total BCKD activity was also decreased by kinase overexpression despite no observable change in expression of the BCKD catalytic proteins. These results demonstrate that artificial regulation of branched-chain amino acid metabolism is possible through the controlled overexpression of a single endogenous enzyme and suggest the feasibility of clinical applications.

Targeted overactivity of beta cell K(ATP) channels induces profound neonatal diabetes

A paradigm for control of insulin secretion is that glucose metabolism elevates cytoplasmic [ATP]/[ADP] in beta cells, closing K(ATP) channels and causing depolarization, Ca2+ entry, and insulin release. Decreased responsiveness of K(ATP) channels to elevated [ATP]/[ADP] should therefore lead to decreased insulin secretion and diabetes. To test this critical prediction, we generated transgenic mice expressing beta cell K(ATP) channels with reduced ATP sensitivity. Animals develop severe hyperglycemia, hypoinsulinemia, and ketoacidosis within 2 days and typically die within 5. Nevertheless, islet morphology, insulin localization, and alpha and beta cell distributions were normal (before day 3), pointing to reduced insulin secretion as causal. The data indicate that normal K(ATP) channel activity is critical for maintenance of euglycemia and that overactivity can cause diabetes by inhibiting insulin secretion.