In Vivo Assessment of Mouse Hindleg Intramyocellular Lipids by1 H-MR Spectroscopy

Rationale and Objectives

1 H-magnetic resonance spectroscopy ( 1 H-MRS) has proved to be the sole in vivo technique able to measure intramyocellular lipids (IMCL) in both humans and animals. Mouse models are now widely used for physiologic studies and drug discovery. However, IMCL assessment using 1 H-MRS is hindered in this animal model by the small muscle size and strong contamination from the extramyocellular lipid (EMCL) signal. The objective of this study was to the use of 1 H-MRS for IMCL quantification in mice at different ages.

Materials and Methods

Noninvasive IMCL quantification was performed at 7 T in tibialis anterior (TA) muscles of healthy male C57/BL6 mice ( n = 9; age, 13.6 ± 1 months), db/db mice ( n = 4), and their C57BL/KSJ control littermates ( n = 4) at 7 and 17 weeks of age.

Results

The IMCL content of diabetic mice TA was significantly higher than their littermates (2.41 ± 0.5 vs. 1.21 ± 0.35, P < .01). An age effect was observed, with TA IMCL levels being lower in older than younger control mice, but increasing between 7 and 17 weeks in the db/db mice.

Conclusions

The feasibility of 1 H-MRS spectroscopy was demonstrated in mice muscle, despite its small size, and used to assess IMCL content in db/db mice.

In skeletal muscle, lipids can be stored extramyocellularly within interstitial adipocytes or intracellularly within muscle cell cytoplasm as droplets in direct contact with mitochondria, serving as a source of energy . Quantifying intramyocellular lipids (IMCL) became important because of their contribution to such metabolic disorders as obesity, insulin resistance, and diabetes . Quantification can be performed invasively using biopsy specimens for biochemical analysis or electron morphometry . However, these methods are not suitable for repeated examination, especially in animal application because of the sacrifice of animals and are physically limited by the small size of the region of interest. In addition, IMCL levels quantified by biochemical analysis may be overestimated because of extramyocellular lipid (EMCL) contamination and therefore necessitate careful microdissection of the muscle to minimize this contamination .

1 H-magnetic resonance spectroscopy ( 1 H-MRS) is the only method so far able to distinguish IMCL from EMCL in vivo. Unlike EMCL methylene group resonance, IMCL methylene group resonance at 1.3 parts per million (ppm) is independent of the angle between muscle fiber and the static magnetic field direction, because of the spherical form of the IMCL droplets within the muscle cell cytoplasm. Aligning muscle fiber parallel to the static magnetic field maximizes the chemical shift difference between the two lipid components , enabling IMCL and EMCL to be quantified separately. 1 H-MRS has been applied successfully in human , dog , rabbit , and rat muscle to investigate certain muscle lipid metabolism disorders, and to study drug, nutrition, and exercise effects . Previous 1 H-MRS studies have also shown that IMCL content depends on strain, age, sex, gender, and fiber type .

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Materials and methods

Experimental Protocol

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Animal Preparation

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MRS Measurements

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Data Processing and Statistical Evaluation

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Results

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Discussion

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In Vivo Assessment of Mouse Hindleg Intramyocellular Lipids by1 H-MR Spectroscopy

Rationale and Objectives

Materials and Methods

Results

Conclusions

Materials and methods

Experimental Protocol

Animal Preparation

MRS Measurements

Data Processing and Statistical Evaluation

Results

Discussion

References

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