The role of lysosomal phospholipase A2 in the catabolism of bis(monoacylglycerol)phosphate and association with phospholipidosis.

Bis(monoacylglycerol)phosphate (BMP) is an acidic glycerophospholipid localized to late endosomes and lysosomes. However, the metabolism of BMP is poorly understood. Because many drugs that cause phospholipidosis inhibit lysosomal phospholipase A2 (LPLA2, PLA2G15, LYPLA3) activity, we investigated whether this enzyme has a role in BMPcatabolism. The incubation of recombinant human LPLA2 (hLPLA2) and liposomes containing the naturally occurring BMP (sn-(2-oleoyl-3-hydroxy)-glycerol-1-phospho-sn-1'-(2'-oleoyl-3'-hydroxy)-glycerol (S,S-(2,2',C18:1)-BMP) resulted in the deacylation of this BMP isomer. The deacylation rate was 70 times lower than that of dioleoyl phosphatidylglycerol (DOPG), an isomer and precursor of BMP. The release rates of oleic acid from DOPG and four BMP stereoisomers by LPLA2 differed. The rank order of the rates of hydrolysis were DOPG>S,S-(3,3',C18:1)-BMP>R,S-(3,1',C18:1)-BMP>R,R-(1,1',C18:1)>S,S-(2,2')-BMP. The cationic amphiphilic drug amiodarone (AMD) inhibited the deacylation of DOPG and BMP isomers by hLPLA2 in a concentration-dependent manner. Under these experimental conditions, the IC50s of amiodarone-induced inhibition of the four BMP isomers and DOPG were less than 20 µM and approximately 30 µM, respectively. BMP accumulation was observed in AMD-treated RAW 264.7 cells. The accumulated BMP was significantly reduced by exogenous treatment of cells with active recombinant hLPLA2 but not with diisopropylfluorophosphate-inactivated recombinant hLPLA2. Finally, a series of cationic amphiphilic drugs known to cause phospholipidosis were screened for inhibition of LPLA2 activity as measured by either the transacylation or fatty acid hydrolysis of BMP or phosphatidylcholine as substrates. Fifteen compounds demonstrated significant inhibition with IC50s ranging from 6.8 to 63.3 µM. These results indicate that LPLA2 degrades BMP isomers with different substrate specificities under acidic conditions and may be the key enzyme associated with BMP accumulation in drug-induced phospholipidosis.

Suppression of cytochrome P450 reductase (POR) expression in hepatoma cells replicates the hepatic lipidosis observed in hepatic POR-null mice.

Cytochrome P450 reductase (POR) is a microsomal electron transport protein essential to cytochrome P450-mediated drug metabolism and sterol and bile acid synthesis. The conditional deletion of hepatic POR gene expression in mice results in a marked decrease in plasma cholesterol levels counterbalanced by the accumulation of triglycerides in lipid droplets in hepatocytes. To evaluate the role of cholesterol and bile acid synthesis in this hepatic lipidosis, as well as the possible role of lipid transport from peripheral tissues, we developed a stable, small interfering RNA (siRNA)-mediated cell culture model for the suppression of POR. POR mRNA and protein expression were decreased by greater than 50% in McArdle-RH7777 rat hepatoma cells 10 days after transfection with a POR-siRNA expression plasmid, and POR expression was nearly completely extinguished by day 20. Immunofluorescent analysis revealed a marked accumulation of lipid droplets in cells by day 15, accompanied by a nearly 2-fold increase in cellular triglyceride content, replicating the lipidosis seen in hepatic POR-null mouse liver. In contrast, suppression of CYP51A1 (lanosterol demethylase) did not result in lipid accumulation, indicating that loss of cholesterol synthesis is not the basis for this lipidosis. Indeed, addition of cholesterol to the medium appeared to augment the lipidosis in POR-suppressed cells, whereas removal of lipids from the medium reversed the lipidosis. Oxysterols did not accumulate in POR-suppressed cells, discounting a role for liver X receptor in stimulating triglyceride synthesis, but addition of chenodeoxycholate significantly repressed lipid accumulation, suggesting that the absence of bile acids and loss of farnesoid X receptor stimulation lead to excessive triglyceride synthesis.

Ataxia with loss of Purkinje cells in a mouse model for Refsum disease.

Refsum disease is caused by a deficiency of phytanoyl-CoA hydroxylase (PHYH), the first enzyme of the peroxisomal alpha-oxidation system, resulting in the accumulation of the branched-chain fatty acid phytanic acid. The main clinical symptoms are polyneuropathy, cerebellar ataxia, and retinitis pigmentosa. To study the pathogenesis of Refsum disease, we generated and characterized a Phyh knockout mouse. We studied the pathological effects of phytanic acid accumulation in Phyh(-/-) mice fed a diet supplemented with phytol, the precursor of phytanic acid. Phytanic acid accumulation caused a reduction in body weight, hepatic steatosis, and testicular atrophy with loss of spermatogonia. Phenotype assessment using the SHIRPA protocol and subsequent automated gait analysis using the CatWalk system revealed unsteady gait with strongly reduced paw print area for both fore- and hindpaws and reduced base of support for the hindpaws. Histochemical analyses in the CNS showed astrocytosis and up-regulation of calcium-binding proteins. In addition, a loss of Purkinje cells in the cerebellum was observed. No demyelination was present in the CNS. Motor nerve conduction velocity measurements revealed a peripheral neuropathy. Our results show that, in the mouse, high phytanic acid levels cause a peripheral neuropathy and ataxia with loss of Purkinje cells. These findings provide important insights in the pathophysiology of Refsum disease.

Population genetic analysis of the N-acylsphingosine amidohydrolase gene associated with mental activity in humans.

To understand the evolution of human mental activity, we performed population genetic analyses of nucleotide sequences ( approximately 11 kb) from a worldwide sample of 60 chromosomes of the N-acylsphingosine amidohydrolase (ASAH1) gene. ASAH1 hydrolyzes ceramides and regulates neuronal development, and its deficiency often results in mental retardation. In the region ( approximately 4.4 kb) encompassing exons 3 and 4 of this gene, two distinct lineages (V and M) have been segregating in the human population for 2.4 +/- 0.4 million years (MY). The persistence of these two lineages is attributed to ancient population structure of humans in Africa. However, all haplotypes belonging to the V lineage exhibit strong linkage disequilibrium, a high frequency (62%), and small nucleotide diversity (pi = 0.05%). These features indicate a signature of positive Darwinian selection for the V lineage. Compared with the orthologs in mammals and birds, it is only Val at amino acid site 72 that is found exclusively in the V lineage in humans, suggesting that this Val is a likely target of positive selection. Computer simulation confirms that demographic models of modern humans except for the ancient population structure cannot explain the presence of two distinct lineages, and neutrality is incompatible with the observed small genetic variation of the V lineage at ASAH1. On the basis of the above observations, it is argued that positive selection is possibly operating on ASAH1 in the modern human population.

Leukocyte peroxidase deficiency in a family with a dominant form of Kuf's disease.

Use of a spectrophotomtetric assay of peroxidase with p-phenylenediamine as cosubstrate demonstrated deficient enzymne activity in leukocytes from two patients with a dominantly inherited form of ceroid lipofuscinosis (Kuf's disease) and a clinically hlealthy unaffected sibling. When the reaction was performned in the absence of added hydrogen peroxide, oxidation of the p-phenylenediamnine cosubstrate (indicating the presence of endogenous peroxide) occurred only with enzyme samnples from the three siblings but not with those from a large number of unrelated, unaffected controls. This demonstrates that the deficiency of peroxide) found previously in the recessively inherited infantile and juvenile formns of ceroid lipofuscinosis (Batten-Spielmeyer-Vogt disease) is also present in an adult form with dominant inheritance.

Lactosyl ceramidosis: normal activity for two lactosyl ceramide beta-galactosidases.

Lactosyl ceramide beta-galactosidase activities in the fibroblasts from the previously described patient with so-called "lactosyl ceramidosis" were reexamined with the two recently developed assay methods which appear to measure two genetically distinct enzymes that can degrade this substrate. No deficiency of either of the lactosyl ceramide-cleaving enzymes was observed. In addition, sphingomyelinase activity was only one-sixth of normal, while all other enzymes examined were within the normal ranges.

Tay-Sachs disease: generalized absence of a beta-D-N-acetylhexosaminidase component.

Two hexosaminidase components, separable by starch-gel electrophoresis and possessing both beta-D-N-acetylglucosaminidase and beta-D-N-acetylgalactosaminidase activity, are present in human tissues. One of these, hexosaminidase component A, is absent in brain, liver, kidney, skin, cultured skin fibroblasts, blood plasma, and leukocytes from nine patients with Tay-Sachs disease. Hexosaminidase assay may facilitate the early diagnosis of individuals homozygous for Tay-Sachs disease.

Metachromatic leukodystrophy: diagnosis with samples of venous blood.

Arylsulfatase A and B have been demonstrated in preparations of human leukocytes. The level of activity of arylsulfatase A is markedly decreased in the preparations from patients with metachromatic leukodystrophy. Acid phosphatase and arylsulfatase B activities were normal. The assay of arylsulfatase A in leukocyte preparations can be useful in the diagnosis of metachromatic leukodystrophy while obviating the difficulties of current methods.

N-acetyl-beta-hexosaminidase: role in the degradation of glycosaminoglycans.

Extracts of cultured normal human skin fibroblasts released radioactivity from a (14)C-labeled heptasaccharide prepared by addition of [(14)C]N-acetylgalactosamine to the nonreducing terminus of a hexasaccharide derived from chondroitin 4-sulfate whereas fibroblast extracts from patients with Tay-Sachs and Sandhoff-Jatzkewitz diseases did not. The results suggest that N-acetyl-beta-hexosaminidase A is responsible for degradation of the oligosaccharide substrate.

Deficient activity of hepatic acid lipase in cholesterol ester storage disease.

Absence of lysosomal acid lipase activity in the liver is described in cholesterol ester storage disease and Wolman's disease. This enzyme deficiency may result in the excess hepatic cholesterol ester found in both conditions. However, clinical, genetic, and histopathologic differences suggest that the two conditions are separate diseases not completely explained by deficient enzyme activity.

Enzyme replacement in Fabry's disease, an inborn error of metabolism.

Two patients with Fabry's disease were infused with normal plasma to provide active enzyme (ceramide trihexosidase) for hydrolysis of the plasma substrate, galactosylgalactosylglucosylceramide. Maximum ceramide trihexosidase activity occurred 6 hours after infusion of the plasma, attaining a level approximately 150 percent of that in normal plasma; enzymatic activity was detectable for 7 days. The amount of accumulated substrate in the plasma of these recipients decreased about 50 percent on day 10 after infusion. Thus, periodic replacement of ceramide trihexosidase activity in the plasma of patients with Fabry's disease might lead to consistently lower amounts of substrate in the plasma and a decrease in its rate of accumulation in tissues.

Ceramidase deficiency in Farber's disease (lipogranulomatosis).

Ceramidase activity could not be demonstrated in the kidney and cerebellum from a deceased patient with Farber's disease, whereas the activities of six control acid hydrolase enzymes appeared normal. This enzyme defect presumably accounts for the accumulation that has been described in two patients and may represent the biochemical basis of this disorder.

Lysosomal phospholipase A2 and phospholipidosis.

A lysosomal phospholipase A2, LPLA2, was recently characterized and shown to have substrate specificity for phosphatidylcholine and phosphatidylethanolamine. LPLA2 is ubiquitously expressed but is most highly expressed in alveolar macrophages. Double conditional gene targeting was employed to elucidate the function of LPLA2. LPLA2-deficient mice (Lpla2-/-) were generated by the systemic deletion of exon 5 of the Lpla2 gene, which encodes the lipase motif essential for the phospholipase A2 activity. The survival of the Lpla2-/- mice was normal. Lpla2-/- mouse mating pairs yielded normal litter sizes, indicating that the gene deficiency did not impair fertility or fecundity. Alveolar macrophages from wild-type but not Lpla2-/- mice readily degraded radiolabeled phosphatidylcholine. A marked accumulation of phospholipids, in particular phosphatidylethanolamine and phosphatidylcholine, was found in the alveolar macrophages, the peritoneal macrophages, and the spleens of Lpla2-/- mice. By 1 year of age, Lpla2-/- mice demonstrated marked splenomegaly and increased lung surfactant phospholipid levels. Ultrastructural examination of Lpla2-/- mouse alveolar and peritoneal macrophages revealed the appearance of foam cells with lamellar inclusion bodies, a hallmark of cellular phospholipidosis. Thus, a deficiency of lysosomal phospholipase A2 results in foam cell formation, surfactant lipid accumulation, splenomegaly, and phospholipidosis in mice.

Lipidomics in diagnosis of lipidoses.

A review is presented of the major clinical features of a number of glycolipidoses including Fabry, Gaucher, Tay-Sachs, metachromatic leukodystrophy as well as CeroidLipofucinosis and Sjogren-Larsson syndrome. The possibilities offered by lipidomics for diagnosis and follow-up after enzyme replacement therapy are presented from a practical perspective. The contribution of HPLC coupled with tandem mass spectrometry has considerably simplified the detection and assay of abnormal metabolites. Corresponding internal standards consisting of weighed mixtures of the stable-isotope labeled metabolites required to calibrate and quantitate lipid components of these orphan diseases standards have yet to become commercially available. A lipidomics approach has been found to compare favorably with DNA-sequence analysis for the rapid diagnosis of pre-birth syndromes resulting from these multiple gene defects. The method also seems to be suitable for screening applications in terms of a high throughput combined with a low rate of false diagnoses based on the wide differences in metabolite concentrations found in affected patients as compared with normal subjects. The practical advantages of handling samples for lipidomic diagnoses as compared to enzyme assay are presented for application to diagnosis during pregnancy.

Drug-induced phospholipidosis is caused by blockade of mannose 6-phosphate receptor-mediated targeting of lysosomal enzymes.

Cationic amphiphilic drugs (CADs) cause massive intracellular accumulation of phospholipids, thereby resulting in phospholipidosis (PLD); however, the molecular mechanism underlying CAD-induced PLD remains to be resolved. Here, we found that treatment of normal rat kidney cells with CADs known to induce PLD caused redistribution of a mannose 6-phosphate/IGF-II receptor (MPR300) from the TGN to endosomes and concomitantly increased the secretion of lysosomal enzymes, resulting in a decline of intracellular lysosomal enzyme levels. These results enable the interpretation of why CADs cause excessive accumulation of undegraded substrates, including phospholipids in lysosomes, and led to the conclusion that the impaired MPR300-mediated sorting system of lysosomal enzymes reflects the general mechanism of CAD-induced PLD. In addition, our findings suggest that the measurement of lysosomal enzyme activity secreted into culture medium is useful as a rapid and convenient in vitro early screening system to predict drugs that can induce PLD.

The metabolism of Tay-Sachs ganglioside: catabolic studies with lysosomal enzymes from normal and Tay-Sachs brain tissue.

The catabolism of Tay-Sachs ganglioside, N-acetylgalactosaminyl- (N-acetylneuraminosyl) -galactosylglucosylceramide, has been studied in lysosomal preparations from normal human brain and brain obtained at biopsy from Tay-Sachs patients. Utilizing Tay-Sachs ganglioside labeled with (14)C in the N-acetylgalactosaminyl portion or (3)H in the N-acetylneuraminosyl portion, the catabolism of Tay-Sachs ganglioside may be initiated by either the removal of the molecule of N-acetylgalactosamine or N-acetylneuraminic acid. The activity of the N-acetylgalactosamine-cleaving enzyme (hexosaminidase) is drastically diminished in such preparations from Tay-Sachs brain whereas the activity of the N-acetylneuraminic acid-cleaving enzyme (neuraminidase) is at a normal level. Total hexosaminidase activity as measured with an artificial fluorogenic substrate is increased in tissues obtained from patients with the B variant form of Tay-Sachs disease and it is virtually absent in the O-variant patients. The addition of purified neuraminidase and various purified hexosaminidases exerted only a minimal synergistic effect on the hydrolysis of Tay-Sachs ganglioside in the lysosomal preparations from the control or patient with the O variant of Tay-Sachs disease.

Enzyme deficiency in cholesteryl ester storage idisease.

Cholesteryl ester storage disease has been shown to involve severe deficiency of acid cholesteryl ester hydrolase and triglyceride lipase activity in liver, spleen, and lymph node. The cholesteryl ester hydrolase was also deficient in aorta. Tissue storage of both cholesteryl esters and triglycerides is generalized. Both the lipid and enzymatic changes are very similar to those in Wolman's disease.

Ceramidase and ceramide synthesis in human kidney and cerebellum. Description of a new alkaline ceramidase.

It has been shown that tissues of patients with Farber's disease characteristically lack acid (pH 4.0) ceramidase. In normal cerebellum, however, ceramide cleavage and the reverse reaction, free fatty acid-dependent ceramide synthesis, both occur not only at pH 4.0 but also at pH 9.0, although normal kidney exhibits these activities only at pH 4.0. Both tissues are capable of snythesizing ceramide via an acyl-COA-dependent pathway at neutral pH. The synthetic analog of ceramide, N-oleoyl-ethanolamine, is a potent inhibitor of ceramidase.

Hexosaminidase C in Tay-Sachs and Sandhoff disease.

1. Hexosaminidase C has been purified from human placenta. Complete separation from hexosaminidases A and B was achieved. 2. The following properties of hexosaminidase C differ from those of the A and B isozymes. Presence in the supernatant rather than the lysosomes, neutral pH optimum, higher molecular weight, lack of activity on beta-N-acetylgalactosamine derivatives, and lack of immunological relationship. 3. Hexosaminidase C is active in patients deficient in hexosaminidases A and B and can be recognized by its characteristic electrophoretic mobility. It is concluded that the genetic origin of hexosaminidase C is probably different from that of hexosaminidases A and B.