Diabetes affects millions of people worldwide. It is a leading cause of death, and as a result, is a large area of study. Diabetes is actually a group of diseases characterized by aberrant glucose metabolism. Type I or insulin-dependent diabetes results from immune-mediated destruction of pancreatic beta-cells. Type II or noninsulin-dependent diabetes results in hyperglycemia without loss of endogenous insulin reserve or loss of pancreatic islets. Thus, Type II diabetes is characterized by the presence of insulin resistance, which is often associated with obesity and advancing age and accounts for most cases of diabetes. The use of mouse models and transgenic technology has been important in the understanding of this complex disease.42
Clinically, overt diabetic symptoms include polyuria, polydypsia, and weight loss. One easy pheno-typing assay is monitoring for glucose in the urine. This test should be performed weekly on at-risk mice. Many other phenotyping procedures and tests have been adapted for evaluating diabetic mouse models. It is beyond the scope of this chapter to discuss all of them. Current procedures available to study diabetic models include urinary glucose and ketones, plasma glucose, lactate, ketones, serum lipids (triglycerides, nonesterified fatty acids, total and HDL cholesterol), plasma insulin, leptin, and corticos-terone, assessment of insulin secretion and sensitivity, and glucose disposal by IVGTT with minimal model analysis, measurement of other circulating hormones, including C-peptide, glucagon, GLP-1 (active and total), pancreatic polypeptide, thyroxine and TSH, renal function parameters [urinary albumin excretion, serum creatinine and blood urea nitrogen, in vitro adipocyte metabolism and leptin production, in vitro assessment of insulin secretion from isolated islets, body composition (body weight, percent carcass lipid, and lean mass and percent, and dissected fat pad mass)].
Pathology is important for diabetes characterization and investigation, as it is useful in helping to differentiate true diabetes from other disease processes in the mouse. Islet failure (diabetes) can result from a number of causes, such as pancreatic developmental disorders (dysplasia, atresia, etc.), neoplasia, amyloid infiltration, lipidosis, infectious disease, etc., that must be differentiated from immune-mediated insulitis. Diabetes development is confirmed by histological examination showing destruction of at least 50% of islets.43 By their very nature, diabetic mutants are likely to have immunologic perturbations. Therefore, immune function assays may be needed to thoroughly evaluate a particular mutant strain.
Recently, transgenic mice have become an important resource in the development of a nonobese diabetic model (NOD). This model consists of NOD mice carrying transgenes, gene knockout mutations, and small stretches of allotypic genetic material, enabling them to serve as mouse models for insulin-dependent (Type I) diabetes. Insulin-dependent diabetes is an autoimmune disease in which genetic factors and environmental influences play a role. The resulting immune reaction involves progressive lymphocyte infiltration into the islets and selective destruction of insulin-secreting pancreatic cells.43 There are many important issues to consider when studying this particular mouse model. Dietary management is crucial for maintaining this model. An additional consideration, dependent on environmental conditions, is the onset of disease: 80% of females and 20% of males can be expected to develop diabetes by 12 to 30 weeks of age, depending on the NOD mouse strain. Thus, monitoring blood glucose and urine glucose is crucial. Histopathology involves staining pancreatic samples with H&E and evaluating for insulitis and diabetes. A histology grading scale is often used to evaluate the percentage of islets showing lymphoid infiltration within or around the islets.
Transgenic mice have also been used to explore the correlation between obesity and noninsulin-dependent diabetes. Obesity is often characterized by hyperinsulinemia and insulin resistance. This is often due to increased insulin secretion and reduced insulin clearance. The obese mouse is a good model to investigate this connection. Body condition and weight are important to monitor for this model. Weight can be measured weekly or even daily if indicated. It is critical to house the mice individually to measure body weight and food consumption. Food and water consumption can be most accurately measured via the use of metabolism cages, but estimations can be made using daily or weekly measures of a gram of food consumed subtracted from the amount provided.16 Blood glucose and insulin levels as well as urine glucose should also be determined for these mice. Body weight and the weight of individual organs are also obtained at necropsy. The major organs as well as fat pads should be included. The ratio of each tissue or organ weight relative to body weight should also be calculated. The hepatic lipid content can be assessed on fresh frozen sections with the lipid-specific stain, Oil Red O. Glycogen content can be assessed on fixed paraffin-embedded tissues via periodic acid Schiff reaction.44 The rest of the tissues should be stained with H&E for routine histopathology. This will allow the pathologist to assess for diabetes-related lesions.
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