Elsevier

Biochimie

Volume 90, Issue 6, June 2008, Pages 838-842
Biochimie

Review
Born to run; the story of the PEPCK-Cmus mouse

https://doi.org/10.1016/j.biochi.2008.03.009Get rights and content

Abstract

In order to study the role of the cytosolic form of phosphoenolpyruvate carboxykinase (GTP) (EC 4.1.1.32) (PEPCK-C) in skeletal muscle, PEPCK-Cmus mice were created by introducing the cDNA for the enzyme, linked to the human α-skeletal actin gene promoter, into their germ line. Two founder lines generated by this procedure were bred together, creating a line of mice that have 9.0 units/g skeletal muscle of PEPCK-C, as compared to 0.080 units/g in muscle from control animals. The mice were more active than controls in their cages and could run for up to 5 km, at a speed of 20 m/min without stopping (control mice run for 0.2 km at the same speed). Male PEPCK-Cmus mice are extremely aggressive, as well as hyperactive. During strenuous exercise, they use fatty acids as a fuel more efficiently than do controls and produce far less lactate than do control animals, perhaps due to the greatly increased number of mitochondria in their skeletal muscle. PEPCK-Cmus mice also store up to five-times more triglyceride in their skeletal muscle, but have only marginal amounts of triglyceride in their adipose tissue depots, despite eating 60% more than controls. The concentration of leptin and insulin the blood of 8–12 months of PEPCK-Cmus mice is far lower than noted in the blood of control animals of the same age. These mice live longer than controls and the females remain reproductively active for as long as 35 months. The possible reasons for the profound alteration in activity and longevity caused the introduction of a simple metabolic enzyme into the skeletal muscle of the mice will be discussed.

Section snippets

Rationale for the generation of the PEPCK-Cmus mice

PEPCK-C is a well studied enzyme, whose role in intermediary metabolism is seemingly established; it is present in the liver, kidney cortex and brown and white adipose tissue and is involved in gluconeogenesis and/or glyceroneogenesis [1]. However, a closer look at the biology of this enzyme indicates anomalies that are not explained by our current understanding of its metabolic role. PEPCK-C is present in a wide variety of mammalian tissues that do not make glucose or where glyceroneogenesis

The surprising results of over-expressing PEPCK-C in skeletal muscle

Four founder lines of transgenic mice, which contained a chimeric gene composed of the α-skeletal actin gene promoter, driving the expression of the cDNA for PEPCK-C, linked to the 3′ end of the bovine growth hormone gene (see Ref. [4] for details of this construct), did not have an overtly different phenotype than littermate controls. This was not surprising, since we expected that the major effect of over-expressing the gene would be subtle and would require detailed biochemical analysis of

The PEPCK-Cmus mice have extended longevity and reproductive capacity

A second surprising result was the apparent extend longevity of the PEPCK-Cmus mice; they lived almost 2 years longer than the controls and had normal litters of pups at 30–35 months of age (most mice stop being reproductively active at 12–18 months). We use the word “apparent” because we have not as yet carried out a detailed aging study, involving multiple mice, which are followed at regular intervals over their lifetime; this type of study is currently in underway in our laboratory so

Changes in hormone and cytokines in the blood of PEPCK-Cmus mice and the possible metabolic consequences

We have carried out a detail analysis of the levels of hormones and cytokines in the blood of the PEPCK-Cmus mice and noted very low levels of insulin, leptin and MCP-1 as compared to control animals (Hakimi, Berger, Tracy and Hanson, unpublished data). These factors all suppress appetite by directly influencing the hypothalamus, so that a markedly reduced circulating concentration of these proteins would explain the higher level of food intake in the PEPCK-Cmus mice, as compared to controls.

A model to explain the metabolic alterations observed in the PEPCK-Cmus mice

From the available information on the PEPCK-Cmus mice, it is possible to speculate on a potential mechanism to explain this complex phenotype (see Fig. 1). This requires several assumptions that need validation, but there is enough evidence in hand to make this proposed mechanism plausible. The first assumption is that the overexpression of PEPCK-C occurs only in skeletal muscle and that it functions as an enzyme and not as a transcription factor; this limits the effect directly to a metabolic

PEPCK-C and energy metabolism

The general schema outlined above does not answer a fundamental question concerning the PEPCK-Cmus mice; how does a single enzyme, expressed at exceedingly high levels in skeletal muscle, initiate such a global alteration in energy metabolism? What is the initiating event? Our current thinking on this issue focuses on the key role of PEPCK-C in citric acid cycle dynamics. Hans Krebs first described the citric acid cycle in 1937, yet the metabolic role of the cycle continues to intrigue

Final words

We are often asked if the remarkable physical activity of the PEPCK-Cmus mice, their longevity and reproductive vigor has direct application to human performance. Can we introduce the gene for PEPCK-C into human skeletal muscle and see a similar alteration in metabolism and behavior? Performance sports, such as bicycling, clearly would benefit from the type of activity noted in our PEPCK-Cmus mice. To be able to use fatty acid as a fuel for long periods of strenuous exercise while generating

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Supported by grants DK058620 and DK025541 from the National Institutes of Health and by the National Cancer Institute Centers for Transdisciplinary Research on Energy and Cancer (TREC) (U54 CA116867) and by the Sponsored Aging Cancer Research Program Development (P20 CA103736).

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