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The University of Melbourne, Department of Medicine, (AH/NH), Heidelberg Repatriation Hospital, Heidelberg Heights, Victoria 3081 Australia
2To whom correspondence should be addressed. E-mail: sof{at}unimelb.edu.au.
| ABSTRACT |
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KEY WORDS: energy expenditure physical activity obesity transgenics knockout mice
Dramatic developments in the field of molecular genetics over the last 20 years have catapulted the humble rodent to the position of "biomedicines model mammal" (1). The advent of transgenic and gene-targeted models provided powerful and increasingly popular scientific tools with which to study the molecular and cellular mechanisms underlying the many varied metabolic and endocrine disorders, such as obesity and Type II diabetes. The science of ablating a gene or changing its expression has become remarkably simple. However, there are now a number of examples demonstrating that animals containing the same genetic change can exhibit profoundly different phenotypes when present on diverse genetic backgrounds.
The most notable of these for work associated with obesity and Type II diabetes is the mutation in either the leptin gene (Lepob mouse) or the leptin receptor gene (Leprdb mouse). When either of these mutations is present on the C57BL/6J inbred mouse background, a phenotype of massive obesity accompanied by insulin resistance with only transient diabetes is observed (2,3). In Sv129/J mice, which are the most common source of embryonic stem cells for targeted transgenesis, expression of the Leprdb mutation causes extreme hyperinsulinemia and massive obesity (4,5). In contrast, either mutation produces initial obesity and insulin resistance followed by life-shortening diabetes when present on the C57BL/KsJ strain.
Thus it is clear from the above discussion that to analyze properly the resultant phenotype of a specific genetic manipulation, the inherent phenotype of the particular mouse strain employed has to be known. Therefore, although it is now becoming recognized that different inbred strains harbor different susceptibilities to either obesity or diabetes, systematic metabolic characterization of most if not all strains is lacking. The importance of inbred strain background on phenotypic assessment has been well reviewed in the field of neurobiology and behavior (6,7). Although the Jackson Laboratory Mouse Phenome Database (8) contains a great deal of information on many strains, it does not yet have complete data on metabolic parameters pertinent to obesity and diabetes research and response to a high-fat diet. The aim of this study was to characterize the metabolic parameters of 3 of the most commonly used inbred mouse strains, i.e., C57BL/6, DBA/2, and 129T2 and their response to low- and high-fat diets.
| MATERIALS AND METHODS |
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Voluntary physical activity levels were determined in a separate group of mice and were measured by continuous monitoring of the use of the running wheels using a computerized meter as previously described (10). Mice were individually housed for 7 d in rat boxes containing a 15-cm diameter running wheel and had free access to food. The first 2 d were allowed for acclimation, and readings were taken over the following 5 d, averaged, and expressed as cycles/d.
Spontaneous physical activity levels were measured in individually housed mice that had free access to food using an infrared light beam monitor (Columbus Instruments) as previously described (11). Again, mice were allowed 2 d of acclimation to the activity meters; readings were taken over the next 5 d, averaged, and expressed as counts/d.
In wk 6, after the activity measurements were completed, and after 2 h of food deprivation to minimize effects from food digestion, rates of resting energy expenditure, glucose oxidation, and fat oxidation were measured during the light cycle as previously described (12) using indirect calorimetry (Columbus Instruments). Mice were placed in the calorimetry chambers for a 30-min acclimation period followed by measurement of expired oxygen and carbon dioxide every minute over the following 30 min.
Tissue collection. At the end of wk 6, mice were deprived of food overnight and anesthetized with pentobarbitone sodium (Nembutal) administered by i.p. injection (100 mg/kg body weight); 30 min later, the mice were bled from the eye using heparinized capillary tubes. Plasma was collected and stored at 20°C. Epididymal, inguinal subcutaneous, and retroperitoneal adipose tissues were removed and the wet mass was weighed. Because the same person performed the procedure on all mice, the dissection of the adipose tissue depots was standardized.
Measurement of plasma glucose and insulin levels. The glucose oxidase method was used to measure plasma glucose using a GM7Analox glucose analyzer (Helena Laboratories). Plasma insulin was measured by RIA (Linco Research Immunoassay).
Statistical analysis. Data are expressed as means ± SEM. Data were analyzed by 2-way ANOVA with Tukeys Honestly Significant Difference post-hoc test (SPSS 11 for Windows). Comparisons were made between the 2 diets in mice from the same strain and among the 3 mouse strains fed the same diet. Differences were considered significant at P < 0.05.
| RESULTS |
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| DISCUSSION |
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In this study we chose 3 inbred mouse strains commonly used to generate genetically altered mice for the study of metabolic disorders (obesity and diabetes), i.e., C57BL/6, DBA/2, and 129T2. The C57BL/6 and DBA/2 strains were widely used as the hybrid backgrounds for transgenics and, as stated above, the 129 strain was used to derive embryonic stem cells for use in homologous recombination for the production of "knockout" mice. The results show that these seemingly similar mouse strains displayed differences in 2 metabolic parameters and responded differently to a high-fat diet. The first was that weight gain over 6 wk when consuming the control diet and regional fat pad weight regardless of diet were greater in the DBA/2 mice compared with the other 2 strains. An increase in carcass lipid and fat pad weight was shown previously in DBA/2 compared with C57BL/6 mice (13). Intrauterine effects may contribute to the increase in fat accumulation because it was reported that DBA/2 litter sizes are smaller than those of C57BL/6 and 129 (14). Studies showed that mice arising from small litters were heavier and had more carcass fat than mice produced from larger litters (15,16).
The second important finding of this study is that DBA/2 mice displayed decreased ambulatory and voluntary activity as well as reduced resting energy expenditure compared with C57BL/6 but not 129T2 mice; this might partially but not completely explain the increase in adiposity. Moreover, spontaneous and, more profoundly, voluntary activity was decreased in 129T2 mice compared with both C57BL/6 and DBA/2 mice. Although less locomotor activity as assessed by video tracking systems in an open field was reported (17,18), to our knowledge this is the first report of decreased spontaneous and voluntary activity of 129T2 compared with C57BL/6 mice. DBA/2 mice had lower voluntary activity as assessed by running wheels compared with C57BL/6 mice, a finding corroborated by a previous report showing that running duration, distance, and average speed were lower in DBA/2 compared with C57BL/6 mice (19). Together, these data suggest that 129T2 mice are inherently sedentary, and this should be a consideration when physical activity is determined in genetically manipulated models containing the 129T2 background strain. Furthermore, 129T2 mice displayed decreased energy expenditure, which, together with decreased physical activity, should have resulted in increased fat accumulation compared with the other 2 strains. This could be explained by decreased fat uptake by the digestive system as was reported previously for other mouse strains (20).
Furthermore, although none of the spontaneous activities measured were affected in DBA/2 mice, a high-fat diet decreased vertical and ambulatory activity in the C57BL/6 mice, whereas it increased vertical activity in 129T2 mice. Moreover, voluntary activity was decreased in response to a high-fat diet in C57BL/6 and DBA/2 but not 129T2 mice. A decrease in physical activity is associated with increased adiposity and weight gain (11,21,22) and this could explain the increase in body and fat pad weights in C57BL/6 and DBA/2 mice in response to the high-fat diet. The reason for the increase in body and fat pad weights in 129T2 mice as a result of a high-fat diet is not clear. A possible mechanism is enhanced fatty acid transport, which could contribute to the increased weight gain in 129T2 mice consuming the high-fat diet. What is clear, though, is that the 3 strains used in this study have different basal physical activity parameters and respond differently to a high-fat diet.
Finally, dietary manipulation did not affect either fasting plasma glucose or insulin concentrations in any of the 3 strains of mice. In contrast, previous studies demonstrated that insulin resistance and diabetes developed in C57BL/6 mice when a high-fat diet was fed (2325). In fact, the studies by Burcelin and colleagues (2325) differentiated a number of phenotypes in C57BL/6 mice fed a high-fat diet for 9 mo, i.e., lean normoglycaemic, lean diabetic, and obese diabetic. Furthermore, they found that the C57BL/6 mice that remained lean and normoglycemic after consumption of the high-fat diet displayed increased glucose clearance (2325). The reason that we did not a see a change in plasma glucose or insulin concentrations in mice fed the high-fat diet could be due to the length of time (6 mo or longer vs. 6 wk in our study) and type of dietary fat (saturated vs. polyunsaturated used in our study). It was suggested that SFA cause a greater reduction in insulin sensitivity than polyunsaturated fats (26,27). Furthermore, it was shown recently that although a high-fat diet caused glucose intolerance, insulin sensitivity in peripheral tissues was not affected in C57BL/6 mice (28).
Interestingly, plasma glucose concentrations were higher and plasma insulin levels were lower in 129T2 compared with C57BL/6 mice, suggesting differences in pancreatic islet insulin secretory function among strains of mice. In fact, we showed recently that glucose-mediated insulin secretion is enhanced in DBA/2 compared with C57BL/6 mice both in vivo and in vitro (29).
The ease with which the mouse genome can be manipulated makes it an invaluable tool in the discovery of the etiopathogenesis of complex disorders such as obesity and Type 2 diabetes. However, although the methodology has become surprisingly simple, the science and the interpretation have become more complex than ever. This study clearly shows that detailed inherent metabolic information, as well as careful and strategic planning, are required before undertaking the long and arduous road of characterizing a transgenic or "knockout" animal.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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Manuscript received 13 July 2004. Initial review completed 26 August 2004. Revision accepted 24 September 2004.
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