Macrophage-Specific Expression of Human Lysosomal Acid Lipase Corrects Inflammation and Pathogenic Phenotypes in lal−/− Mice

Abstract

Lysosomal acid lipase (LAL) hydrolyzes cholesteryl esters and triglycerides to generate free fatty acids and cholesterol in the cell. The downstream metabolites of these compounds serve as hormonal ligands for nuclear receptors and transcription factors. Genetic ablation of the lal gene in the mouse caused malformation of macrophages and inflammation-triggered multiple pathogenic phenotypes in multiple organs. To assess the relationship between macrophages and lal ؊/؊ pathogenic phenotypes, a macrophage-specific doxycycline-inducible transgenic system was generated to induce human LAL (hLAL) expression in the lal ؊/؊ genetic background under control of the 7.2-kb c-fms promoter/intron2 regulatory sequence. Doxycycline-induced hLAL expression in macrophages significantly ameliorated aberrant gene expression, inflammatory cell (neutrophil) influx, and pathogenesis in multiple organs. These studies strongly support that neutral lipid metabolism in macrophages contributes to organ inflammation and pathogenesis. Macrophages produce and secrete cytokines, chemokines, and growth factors that influence gene expression, cell proliferation/differentiation, and apoptosis in organ tissues through paracrine and autocrine mechanisms, making them vitally important to the physiological functions of multiple organs. Many diseases are tightly associated with malformation and malfunction of macrophages within organs. Design and establishment of a system to specifically express "genes of interest" in macrophages in a temporal/spatial fashion will greatly assist in understanding the molecular mechanisms of macrophage differentiation/maturation and the functional roles of macrophages in disease formation. The tetracyclineinducible transgenic mouse system has proven to be very effective in inducing genes of interest in a temporal and spatial manner. In this system, "activator" transgenic mice bear the reverse tetracycline-responsive transactivator (rtta) fusion protein under control of a cell typespecific promoter. The rtta expression is restricted to specific cell types in transgenic mice. In a separate transgenic mouse line, the gene of interest is under control of the tet operator DNA-binding sequence that is linked to a minimal promoter. After crossbreeding, expression of the gene of interest can be precisely controlled by addition or removal of tetracycline or doxycycline in double transgenic mice. This strategy has been proven to be very effective in many organ systems, including the lung.

Critical Roles of Lysosomal Acid Lipase in T Cell Development and Function

“…These observations made us speculating that there was a systemic malformation and malfunction in lal Ϫ/Ϫ immune cells. While the functional role of LAL in myeloid cells have been reported previously, 4,7,8 this report emphasizes characterizing the LAL functional roles in development, maturation, homeostasis, and function of T cells. The thymus and spleen are important immune organs for T cell development and maturation.…”

“…As we reported previously, macrophages are the major cells for lipid storage due to LAL deficiency. 4,8 Indeed, Mac3 antibody (a macrophage marker) staining showed massive macrophage infiltration in the lal Ϫ/Ϫ thymus, which was colo- Figure 2. T cell developmental defect in the lal Ϫ/Ϫ thymus.…”

“…Altered differentiation and dysfunction of myeloid lineage cells (macrophages, neutrophils) have been reported previously to contribute to pathogenic progression in lal Ϫ/Ϫ mice. 4,7,8 In these mice, Kupffer cells (liver macrophages), bronchoalveolar macrophages, and intestinal macrophages appeared abnormal (foamy) with neutral lipid accumulation and aberrant cytokine/chemokine secretion. 4,5,8 In a cell type-specific rescue experiment, a myeloid-specific doxycycline-inducible transgenic mouse system was generated to specifically express wild-type hLAL in lal Ϫ/Ϫ myeloid lineage cells under the control of the c-fms promoter.…”

“…As a result, the pathogenic phenotypes in the lung, liver and small intestine of lal Ϫ/Ϫ mice were partially corrected. 8 On the other hand, the function of neutral lipids in lymphocytes, especially in T cells, has not been investigated in lal Ϫ/Ϫ mice. This report will test the hypothesis that LAL deficiency influences T cell development, homeostasis and function.…”

Dual-promoter lentiviral system allows inducible expression of noxious proteins in macrophages

“…Recently, Yan et al have published a macrophage-specific tetracycline-inducible system for in vivo expression in mice, which utilizes the c-fms (CSF-1 receptor) promoter/intron regulatory element (Yan et al, 2006b). For inducible expression these mice have to be bred with another transgenic mouse, in which rtTA inducible promoter drives expression of "geneofinterest".…”