Mice expressing only covalent dimeric heparin binding-deficient lipoprotein lipase: muscles inefficiently secrete dimeric enzyme.
Lipoprotein lipase (LpL) hydrolyzes triglycerides of circulating lipoproteins while bound as homodimers to endothelial cell surface heparan sulfate proteoglycans. This primarily occurs in the capillary beds of muscle and adipose tissue. By creating a mouse line that expresses covalent dimers of heparin-binding deficient LpL (hLpLHBM-Dimer) in muscle, we confirmed in vivo that linking two LpL monomers in a head to tail configuration creates a functional LpL. The hLpLHBM-Dimer transgene produced abundant activity and protein in muscle, and the LpL was the expected size of a dimer (approximately 110 kDa). Unlike the heparin-binding mutant monomer, hLpLHBM-Dimer had the same stability as nonmutated LpL. The hLpLHBM-Dimer transgene prevented the neonatal demise of LpL knockout mice; however, these mice were hypertriglyceridemic. Postheparin plasma LpL activity was lower than expected with the robust expression in muscle and was no longer covalently linked. Studies in transfected cells showed that Chinese hamster lung cells, but not COS cells, also degraded tandem repeated LpL into monomers. Thus, although muscle can synthesize tethered, dimeric LpL, efficient production of this enzyme leading to secretion, and physiological function appears to favor secretion of a noncovalent dimer composed of monomeric subunits.