Original Articledb/+ Mice as an Alternate Model in Antidiabetic Drug Discovery Research
Introduction
An appropriate animal model of diabetes mellitus would be beneficial in understanding the underlying mechanisms of the disease and in testing novel therapeutic modalities. An ideal model should present physiopathological and clinical features similar to that of human diabetes mellitus. A number of animal models are currently available for antidiabetic drug screening and include db/db mice, NOD mice, ob/ob mice, etc. The best-studied laboratory animals are the obese and insulin-resistant ob/ob (1) and db/db (2) mice, which either lack leptin secretion or present leptin-receptor deficiency.
db/db Mice represent an ideal model of type 2 diabetes mellitus (T2DM) and insulin resistance, which become diabetic due to a mutation in the gene for the leptin receptor. The mutation is autosomal recessive with full penetrance and causes metabolic disturbances in homozygous mice, resembling non-insulin-dependent diabetes mellitus in humans. Abnormal deposition of fat occurs at 3–4 weeks of age and is followed by hyperinsulinemia, hyperglycemia, polyuria, and glycosuria (3). These mice are sterile in nature and are produced by crossing between db/+ mice that do not become diabetic at any stage but are carriers of diabetic characteristics. Studies in the db/+ mice suggest that the leptin signal is apparently attenuated, resulting from a reduced number of molecules of the intact long receptor isoform (4). The db/+ mice have increased plasma leptin levels relative to +/+ mice 5, 6. This suggests that the receptor is not fully recessive with regard to fat mass and that heterozygosity at the leptin receptor may play a role in susceptibility to environmental conditions favoring obesity, such as pregnancy (7).
Although db/db mice represent well-established animal models for T2DM and insulin resistance, breeding and maintenance of these animals is expensive and problematic. Furthermore, only a limited percentage of progeny become diabetic, making this model impractical for the initial screening of a large number of molecules. The cross between db/+ mice leads to ∼15% db/db mice and most of the remaining progeny are db/+ type. The importance of db/+ mice until now is only in breeding. In the present study, attempts have been made to establish these db/+ mice as an alternate model for the initial screening of synthetic antidiabetic molecule/plant extracts. Diabetes was induced in these animals by injecting streptozotocin, and establishment of diabetic conditions was monitored by measuring blood glucose level, lipid profile and activity of the key enzymes of carbohydrate metabolism. Furthermore, the effect of the standard drug metformin on these diabetic parameters was also evaluated to confirm the validity of the model.
Section snippets
Chemicals
Streptozotocin and metformin were purchased from Sigma Chemical Co. (St. Louis, MO). All other chemicals used were of analytical grade.
Animals
Male and female db/+ mice were obtained from the Jackson Laboratory (Bar Harbor, ME) and maintained in the animal house facility of the institute. Mice were housed in groups of five in a temperature, humidity, and light-controlled (12 h light/dark period) colony room. Mice were given ad libitum access to commercial mouse chow and water. Research on animals was
Body Weight
Body weight of animals of each group was measured on alternate days. As evident from the results shown in Figure 1, initially no statistically significant difference was found in the body weight of the normal control and STZ-treated groups, but after day 14 a significant (p <0.01) increase in body weight was observed in normal control group in comparison to untreated diabetic group. Body weight of diabetic mice treated with metformin remained stable in the beginning but was found to increase
Discussion
Investigation of newer formulations with antidiabetic properties is necessary at this time and the mouse has become a popular model for various antidiabetic research programs. The mouse model is genetically well defined, has a short generation time, abundance of natural or targeted mutations, environmental factors can be controlled easily in the laboratory because of their small size, and there are economic advantages. In db/db mice, a single gene mutation in the leptin receptor gene leads to a
Acknowledgments
A.B.S. is the recipient of a Senior Research Fellowship from CSIR, New Delhi for carrying out this study.
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