Secreted enzyme uptake masks the in vivo phenotype of macrophage-specific lysosomal acid lipase deletion

Background and hypothesis

Lysosomal acid lipase (LAL) is so far the only known intracellular enzyme that is capable of hydrolyzing triglycerides and cholesteryl esters at an acidic pH inside the lysosome. Mutations in the LAL-encoding Lipa gene cause a rare autosomal recessive lysosomal storage disorder in humans with massive lipid accumulation. In mice, the loss of systemic LAL is associated with severe lipid accumulation, particularly in the liver and small intestine, accompanied by infiltration of lipid-filled CD68+-TREM2+ macrophages. We hypothesize that macrophages are among the key players in LAL deficiency and are responsible for lipid accumulation in the affected tissues.

Methods

We generated macrophage (mac)- and macrophage/enterocyte-specific (mac/int-) LAL KO mice and performed morphological, histopathological, and functional analyses under chow- and high-fat/high-cholesterol diet-fed conditions.

Results

We observed that neither macLAL-KO nor mac/int-LAL KO mice replicated the phenotype of whole-body LAL KO mice, as lipoprotein secretion, lipid absorption, and lipid accumulation remained unaffected. However, the absence of macrophage LAL ameliorated diet-induced obesity in both mouse lines. Notably, the lipid accumulation observed in the lysosomes of macrophages from whole-body LAL KO mice was absent in macrophages from macLAL-KO mice, attributable to residual LAL enzyme activity despite genetic ablation. Treatment of macrophages from whole-body LAL KO mice with conditioned medium of hepatocytes from macLAL-KO mice effectively prevented lipid accumulation.

Conclusion

These findings suggest that LAL secreted from hepatocytes, macrophages, and possibly other cell types in vivo corrects the phenotype of cell type-specific LAL deficiency, a key insight for guiding future gene therapy strategies.