[tt] Mighty Mouse

[Bryan Bishop]

Paper
http://www.jbc.org/cgi/reprint/M706127200v1.pdf

"Over-expression of the cytosolic form of phosphoenolpyruvate 
carboxykinase (GTP) in skeletal muscle repatterns energy metabolism in 
the mouse"

Transgenic mice, containing a chimeric gene in which the cDNA for 
phosphoenolpyruvate carboxykinase (GTP) (PEPCK-C) (EC 4.1.1.32) was 
linked to the -skeletal actin gene promoter, express PEPCK-C in 
skeletal muscle (1~3 units/g). Breeding two founder lines together 
produced mice with an activity of PEPCK-C of 9 units/g muscle 
(PEPCK-Cmus mice). These mice were seven times more active in their 
cages than controls. On a mouse treadmill, PEPCK-Cmus mice ran up to 6 
km at a speed of 20 m/min while controls stopped at 0.2 km. PEPCK-C mus 
mice had an enhanced running ability, with a VO2 max of 156 ± 8.0 
ml/kg/min, a maximal Respiratory Exchange Ratio (RER) of 0.91 ± 0.03 
and a blood lactate concentration of 3.7 ± 1.0 mM after running for 32 
min at a 25º grade; the values for control animals were 112 ± 21 
ml/kg/min, 0.99 ± 0.08, and 8.1 ± 5.0 mM respectively. The PEPCK-Cmus 
mice eat 60% more than controls, but had half the body weight and 10% 
the body fat as determined by MRI. In addition, the number of 
mitochondria and the content of triglyceride in the skeletal muscle of 
PEPCK-Cmus mice was greatly increased as compared to controls. 
PEPCK-Cmus mice had an extended life span relative to control animals; 
mice up to an age of 2.5 years ran twice as fast as 6-12 month old 
control animals. We conclude that over-expression of PEPCK-C repatterns 
energy metabolism and leads to greater longevity.

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Article
http://www.sciencedaily.com/releases/2007/11/071101162739.htm

Genetically Engineered 'Mighty Mouse' Can Run 6 Kilometers Without 
Stopping

ScienceDaily (Nov. 2, 2007) — Case Western Reserve University 
researchers have bred a line of "mighty mice" (PEPCK-Cmus mice) that 
have the capability of running five to six kilometers at a speed of 20 
meters per minute on a treadmill for up to six hours before stopping.

"They are metabolically similar to Lance Armstrong biking up the 
Pyrenees; they utilize mainly fatty acids for energy and produce very 
little lactic acid," said Richard W. Hanson, the Leonard and Jean 
Skeggs Professor of Biochemistry at Case Western Reserve and the senior 
author of  a journal article on this research. 

These genetically engineered mice also eat 60 percent more than 
controls, but remain fitter, trimmer and live and breed longer than 
wild mice in a control group. Some female PEPCK-Cmus mice have had 
offspring at 2.5 years of age, an amazing feat considering most mice do 
not reproduce after they are one year old. According to Hanson, the key 
to this remarkable alteration in energy metabolism is the 
over-expression of the gene for the enzyme phosphoenolypyruvate 
carboxykinases (PEPCK-C). 

Parvin Hakimi, the article's lead author and a researcher in the Hanson 
lab, developed this new line of PEKCK-C mice over the past five years 
as part of on-going research aimed at understanding the metabolic and 
physiological function of PEPCK-C in skeletal muscle and adipose 
tissue.

The transgenic mice, which now number nearly 500, were derived from six 
founder lines that contain a chimeric gene in which a copy of the cDNA 
for PEPCK-C was linked to the skeletal actin gene promoter, containing 
the 3'-end of the bovine growth hormone gene. The skeletal actin gene 
promoter directs expression of PEPCK-C exclusively to skeletal muscle. 
Various lines of PEPCK-Cmus mice expressed PEPCK-C at different levels, 
but one very active line of PEPCK-Cmus mice had levels of PEPCK-C 
activity of 9 units/gram skeletal muscle, compared to only 0.08 
units/gram in the muscles of control animals. 

It was evident from the beginning that these mice were very different 
from average mice. Hakimi commented, "From a very early age, the 
PEPCK-Cmus mice ran continuously in their cages." She said she could 
identify which mice were from this new line by simply watching their 
level of activity in their home cage. Animal behavior studies later 
demonstrated that the PEPCK-Cmus mice are seven times more active in 
their home cages than controls; in addition, the mice were also 
markedly more aggressive. "The enhanced level of activity noted in the 
PEPCK-Cmus mice extends well beyond two years of age; this is 
considered old-age for mice," the researchers said.

As part of this study, the researchers determined oxygen consumption, 
the production of carbon dioxide and changes in the lactate 
concentrations in the blood of the PEPCK-Cmus mice and controls during 
strenuous exercises on a treadmill, which was set at a 25-degree 
incline. The treadmill speed was increased by 2m/min every minute until 
the mice stopped running. The PEPCK-Cmus mice ran an average of 31.9 
minutes, compared to 19 minutes for the control animals. 

"What is particularly dramatic is the difference in the concentrations 
of lactate in the blood," the researchers said. "At the beginning of 
exercise, the concentration of lactate was similar in two groups of 
mice, but by the end of the exercise period, the control group had 
elevated levels of blood lactate with little change in the levels in 
the PEPCK-Cmus mice." 

They added that this indicates that the PEPCK-Cmus mice relied heavily 
on fatty acids as a source of energy during exercise, while the control 
animals rapidly switched from fatty acid metabolism to using muscle 
glycogen (carbohydrates) as a fuel; this dramatically raised the blood 
lactate levels.

This new mouse line also has an increased content of mitochondria and 
high concentrations of triglycerides in their skeletal muscles, which 
also contributed to the increased metabolic rate and longevity of the 
animals. 

"It is remarkable that the over-expression of a single enzyme involved 
in a metabolic pathway should result in such a profound alteration in 
the phenotype of the mouse," Hakimi and Hanson said. "Understanding the 
biochemical mechanisms responsible for this repatterning of energy 
metabolism will keep us busy for some time to come."

This research article, entitled "Over Expression of the Cytosolic Form 
of Phosphoenolpyruvate Carboxykinase (GTP) in Skeletal Muscle 
Repatterns Energy Metabolism in the Mouse," appeared recently in the 
Journal of Biological Chemistry. 

Other researchers contributing to the research in the article are Jianqi 
Yang, Gemma Casadesus, Duna Massillon, Fatima Tolentino-Silva, Colleen 
K. Nye, Marco E. Cabrera, David R. Hagen, Christopher B. Utter, Yacoub 
Baghdy, David H. Johnson, and David Wilson from Case Western Reserve 
University and John P. Kirwan and Satish C. Kalhan from the Cleveland 
Clinic.

A video of a wild mouse and a mighty mouse on a treadmill is available 
at: http://blog.case.edu/case-news/2007/10/26/mouse.mov

Adapted from materials provided by Case Western Reserve University.

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- Bryan