The role of mannosylated enzyme and the mannose receptor in enzyme replacement therapy

Abstract

Lysosomal acid lipase (LAL) is the critical enzyme for the hydrolysis of triglycerides (TGs) and cholesteryl esters (CEs) in lysosomes. LAL defects cause Wolman disease (WD) and CE storage disease (CESD). An LAL null (lal-/-) mouse model closely mimics human WD/CESD, with hepatocellular, Kupffer cell and other macrophage, and adrenal cortical storage of CEs and TGs. The effect on the cellular targeting of high-mannose and complex oligosaccharide-type oligosaccharide chains was tested with human LAL expressed in Pichia pastoris (phLAL) and CHO cells (chLAL), respectively. Only chLAL was internalized by cultured fibroblasts, whereas both chLAL and phLAL were taken up by macrophage mannose receptor (MMR)-positive J774E cells. After intraperitoneal injection into lal-/- mice, phLAL and chLAL distributed to macrophages and macrophage-derived cells of various organs. chLAL was also detected in hepatocytes. Ten injections of either enzyme over 30 d into 2- and 2.5-mo-old lal-/- mice produced normalization of hepatic color, decreased liver weight (50%-58%), and diminished hepatic cholesterol and TG storage. Lipid accumulations in macrophages were diminished with either enzyme. Only chLAL cleared lipids in hepatocytes. Mice double homozygous for the LAL and MMR deficiences (lal-/-;MMR-/-) showed phLAL uptake into Kupffer cells and hepatocytes, reversal of macrophage histopathology and lipid storage in all tissues, and clearance of hepatocytes. These results implicate MMR-independent and mannose 6-phosphate receptor-independent pathways in phLAL uptake and delivery to lysosomes in vivo. In addition, these studies show specific cellular targeting and physiologic effects of differentially oligosaccharide-modified human LALs mediated by MMR and that lysosomal targeting of mannose-terminated glycoproteins occurs and storage can be eliminated effectively without MMR.

Figure 1

HPLC profiles of AA-derivatized phLAL-derived (A) and chLAL-derived (B) oligosaccharides. The proposed structures of oligosaccharides are listed below each respective graph. LU = Luminescence units.

Figure 1

HPLC profiles of AA-derivatized phLAL-derived (A) and chLAL-derived (B) oligosaccharides. The proposed structures of oligosaccharides are listed below each respective graph. LU = Luminescence units.

Figure 2

LAL activity in plasma (A) and LAL activity recovery in liver (B) of lal^−/− mice that received IP injections of hLAL. Plasma was collected from phLAL- or chLAL-injected mice at 0, 10, 30, 60, 120, 180, and 240 min after injection. Livers were collected at 240 min after injection. LAL activities in plasma and liver extract were determined using 4-MUO as the substrate.

Figure 3

Tissue cholesterol and TGs in lal^−/− mice administered PBS or chLAL. Total cholesterol or total TGs per liver (A) or per spleen (B) and lipid concentration in intestine (C) was compared between PBS control mice and phLAL- or chLAL-injected mice (n=6). The P values were from unpaired t tests between PBS and chLAL-treated samples. *P<.05. ***P<.001. WT = wild type.

Figure 4

Gross pathology of age-matched PBS and phLAL-treated lal^−/−;MMR^−/− mice. Ventral views (left panel) show the yellow fatty liver in PBS control lal^−/−;MMR^−/− mice. In phLAL-treated lal^−/−;MMR^−/− mice, the liver had essentially normal color. Gross view of livers, spleens, kidneys, and mesenteric lymph nodes (right panel, top to bottom) from PBS and phLAL-treated lal^−/−;MMR^−/− mice. Notice that the color of these organs was nearly normal in phLAL-treated lal^−/−;MMR^−/− mice.

Figure 5

Histological analyses of wild-type, lal^−/−;MMR^+/+, and lal^−/−;MMR^−/− mice administered PBS, phLAL, or chLAL. Shown are representative H+E-stained tissue sections from age-matched wild-type (lal^+/+) mice (A, F, and K); from lal^−/−;MMR^+/+ mice injected with PBS (B, G, and L), chLAL (C, H, and M), or phLAL (D, I, and N); and from lal^−/−;MMR^−/− mice treated with phLAL (E, J, and O). The tissues are liver (A–E), spleen (F–J), and small intestine (K–O). Arrows indicate hepatocytes, arrowheads indicate Kupffer cells, and the asterisk (*) indicates intestinal macrophages. The numbers of remaining Kupffer storage cells in the phLAL- or chLAL-treated mouse livers are similar, whereas the hepatocytes in the chLAL-treated lal^−/−;MMR^+/+ mice and the phLAL-treated lal^−/−;MMR^−/− mice are essentially cleared of storage. Hepatocyte storage remained in the phLAL-treated lal^−/−;MMR^+/+ mice. In spleen and small intestine, the storage macrophages were nearly eliminated in all phLAL-treated mice. Original magnification, ×200.

Figure 6

Anti-hLAL immunohistochemical staining of lal^−/−;MMR^+/+ (A–C) and lal^−/−;MMR^−/− (D) mouse livers. lal^−/−;MMR^+/+ mice were given IP injections of PBS (A), phLAL (B), and chLAL (C), and lal^−/−;MMR^−/− mice were given IP injections of phLAL (D). The amount of hLAL given to each mouse was 79 μg. Mice were killed 4 h after injection. Liver and spleen were processed for immunohistochemical staining by use of anti-hLAL antibody. Positive signals were in Kupffer cells of the livers (B–D, arrowheads) and in hepatocytes (C and D, arrows). Original magnification, ×400.

Figure 7

Comparison of plasma LAL activity between wild-type and MMR^−/− mice after a single injected dose of hLAL. Age-matched wild-type (lower line) and MMR^−/− (upper line) mice received a single dose (60 μg phLAL per 25 g body weight) via IP injection. Plasma was collected from mice at 0, 5, 20, 30, 45, 60, 75, 90, 105, 120, 180, and 240 min after injection. LAL activities in plasma were determined using 4-MUO as the substrate. Plasma volume was calculated as 3.5% of body weight. The percentage activity remaining in plasma, with respect to input LAL activity, was plotted against time.

Figure 8

Anti-hLAL immunohistochemical staining of lal^−/−;MMR^+/+ (A and C) and lal^−/−;MMR^−/− (B and D) mouse kidneys. Kidney sections from PBS-injected mice (A and B) and hLAL-injected mice (C and D) were processed for immunohistochemical staining by use of anti-hLAL antibody. Only lal^−/−;MMR^−/− mice had positive uptake of hLAL in kidneys and only in the proximal renal tubular epithelial cells (D). Original magnification, ×200.

Figure 9

Tissues recovery of phLAL. A, LAL activity in tissue extracts of liver, spleen, and kidney from lal^−/−;MMR^−/− mice that received 48 U or 72 U of phLAL and were harvested 4 h after injection. B, LAL activity recovery in liver, spleen, and kidney as a percentage of total injected LAL activity in lal^−/−;MMR^−/− mice.

Figure 10

Cellular uptake of chLAL and phLAL in murine macrophages and human fibroblasts. A, Western blot analysis of cellular uptake of chLAL and phLAL. Varying amounts of chLAL (unblackened and blackened circles) or phLAL (diamonds) were placed in media surrounding murine macrophage J774E cells (unblackened circles) or human fibroblasts (blackened circles and diamonds) for 24 h. After the media was replaced with fresh media without supplemented LAL, the cells were harvested (at 48 h) and were processed for western blot analysis. The intracellular form of hLAL was evaluated by densitometry scanning. Uptake of chLAL occurred in mannose receptor–positive J774E macrophages (unblackened circles) and M6P receptor–positive human fibroblasts from an LAL-deficient patient (blackened circles). No uptake was observed in human fibroblasts incubated with phLAL (diamonds). B, Competition of cellular uptake of chLAL in human fibroblasts by M6P. Various amounts of M6P and 12 μg of chLAL were coincubated with human fibroblasts for 24 h. Cells were processed in the same way as described for panel A. The densitometry signal of intracellular hLAL at 0 mM of M6P was defined as 100%. Notice that ∼1.5 mM of M6P inhibited ∼50% of chLAL uptake in fibroblasts.