COOH-terminal Disruption of Lipoprotein Lipase in Mice Is Lethal in Homozygotes, but Heterozygotes Have Elevated Triglycerides and Impaired Enzyme Activity

Trey Coleman,Richard L Seip,Jeffrey M Gimble,Denise Lee,Nobuyo Maeda,Clay F Semenkovich

doi:10.1074/jbc.270.21.12518

Abstract

The role of the enzyme lipoprotein lipase (LPL) in atherosclerosis is uncertain. To generate an animal model of LPL deficiency, we targeted the LPL gene in embryonic stem cells with a vector designed to disrupt the COOH terminus of the protein and used these cells to generate LPL-deficient mice. Germ line transmission of the disrupted LPL allele was achieved with two chimeric males, and offspring from each of these animals were phenotypically identical. Pups homozygous (-/-) for LPL deficiency died within 48 h of birth with extreme elevations of serum triglycerides (13,327 mg/dl) associated with essentially absent LPL enzyme activity in heart and carcass. Newborn heterozygous (+/-) LPL-deficient pups had lower LPL enzyme activity and higher triglycerides (370 versus 121 mg/dl) than wild type (+/+) littermates. Adult heterozygotes had higher triglycerides than wild type mice with ad libitum feeding (236 mg/dl for +/- versus 88 mg/dl for +/+) and after fasting for 4 h (98 mg/dl for +/- versus 51 for +/+) or 12 h (109 mg/dl for +/- versus 56 mg/dl for +/+). Triglycerides were present as very low density lipoprotein particles and chylomicrons, but high density lipoprotein cholesterol levels were not decreased in +/- animals. Plasma heparin-releasable LPL activity was 43% lower in +/- versus +/+ adult animals. LPL activity, mRNA, and protein were lower in the tissues of +/- versus +/+ mice. Homozygous LPL deficiency caused by disruption of the COOH terminus of the enzyme is lethal in mice. Heterozygous LPL deficiency caused by this mutation is associated with mild to moderate hypertriglyceridemia without affecting static HDL cholesterol levels. Heterozygous LPL-deficient mice could be useful for determining if hypertriglyceridemia, independently or in combination with other discrete defects, influences atherosclerosis.

Highlights

Homozygous lipoprotein lipase (LPL) deficiency caused by disruption of the COOH terminus of the enzyme is lethal in mice
Heterozygous LPL deficiency caused by this mutation is associated with mild to moderate hypertriglyceridemia without affecting static HDL cholesterol levels
In addition to hydrolyzing triglycerides in VLDL and chylomicrons, the LPL protein may function as an apolipoprotein, associating with the surface of various lipoproteins to promote binding to the low density lipoprotein (LDL) receptor-related protein/nj-macroglobulin receptor [4], LDL receptor [5, 6], and extracellular proteoglycans [7, 8]

Summary

To whom correspondence should be addressed

The LPL protein can be functionally divided into two major domains: an NH2terminal domain containing the catalytically active site, and a COOH-terminal domain [9] The latter is essential for catalytic activity [10], binds lipoproteins [11], and is probably responsible for the LPL-mediated catabolism of triglyceride-rich lipoproteins by the LDL receptor-related protein/oj-macroglobulin receptor [12]. Heterozygous LPL deficiency could have phenotypically different effects depending on genetic and physiological backgrounds This might explain why some families with heterozygous LPL deficiency manifest familial hypertriglyceridemia [19], while others have lipid abnormalities associated with increased atherosclerotic risk [20]. With a goal of generating an animal model of LPL deficiency within a homogeneous genetic background suitable for studying atherosclerosis, we have inactivated the LPL gene in mice by homologous recombination using a targeting vector designed to disrupt the COOH-terminal domain of the LPL protein

EXPERIMENTAL PROCEDURES

RESULTS

I 5 Carcass c

DISCUSSION

Females