Sphingomyelinase in cultured skin fibroblasts from normal and Niemann-Pick type C patients

Effect of dimethylsulfoxide on sphingomyelinase activity and cholesterol metabolism in Niemann-Pick type C fibroblasts

Niemann-Pick type C (NPC) fibroblasts present a large concentration of cholesterol in their cytoplasm due to a still unidentified deficiency in cholesterol metabolism. The influence of dimethylsulfoxide (DMSO) on the amount of intracellular cholesterol was measured in 8 cultures of normal fibroblasts and in 7 fibroblast cultures from NPC patients. DMSO was added to the fibroblast cultures at three different concentrations (1, 2 and 4%, v/v) and the cultures were incubated for 24 h. Sphingomyelinase activity was significantly increased in both groups of cells only when incubated with 2% DMSO (59.4 +/- 9.1 and 77.0 +/- 9.1 nmol h-1 mg protein-1, controls without and with 2% DMSO, respectively: 47.7 +/- 5.2 and 55.8 +/- 4.1 nmol h-1 mg protein-1. NPC without and with 2% DMSO, respectively). However, none of the DMSO concentrations used altered the amount of cholesterol in the cytoplasm of NPC cells (0.704 +/- 0.049, 0.659 +/- 0.041, 0.688 +/- 0.063 and 0.733 +/- 0.088 mg/mg protein, without DMSO, 1% DMSO, 2% DMSO and 4% DMSO, respectively). This finding suggests that sphingomyelinase deficiency is a secondary defect in NPC and shows that DMSO failed to remove the stored cholesterol. These data do not support the use of DMSO in the treatment of NPC patients.

Marked heterogeneity in Niemann-Pick disease, type C. Clinical and ultrastructural findings

Niemann-Pick disease type C (NP-C) is an autosomal recessive lysosomal lipid storage disorder of unknown etiology. Diagnosis of NP-C is based on characteristic clinical findings and reduced fibroblast esterification of LDL-derived cholesterol. We describe three patients who demonstrate the NP-C spectrum of clinical heterogeneity in age of onset, presenting signs, pattern of organ system involvement, and natural history. In addition, electron microscopic analysis of skin biopsy specimens from these patients revealed marked variability in the extent and cellular distribution of intralysosomal storage and was suggestive of the correct diagnosis in only one case. These cases demonstrate both the limitations of electron microscopy for diagnosis of NP-C and the marked clinical variability in patients with this disorder. Practical clinical guidelines for appropriate suspicion of NP-C are presented.

An update of the enzymology and regulation of sphingomyelin metabolism

Sphingomyelin is found in plasma membranes and related organelles (such as endocytic vesicles and lysosomes) of all tissues, as well as in lipoproteins. Abnormalities in sphingomyelin metabolism have been associated with atherosclerosis, cancer and genetically transmitted diseases; however, except for Niemann-Pick disease, little is known about the mechanism for these disorders. Sphingomyelin biosynthesis de novo involves ceramide formation from serine and two mol of fatty acyl-CoA followed by addition of the phosphocholine headgroup. The headgroup appears to come from phosphatidylcholine, but other sources have not been ruled out. Factors that influence the rate of sphingomyelin synthesis include the availability of serine and palmitic acid, plus the relative activities of key enzymes of this pathway. Sphingomyelin turnover involves removal of the headgroup and amide-linked fatty acid by sphingomyelinases and ceramidases, respectively, which have been found in both lysosomes (with acidic pH optima) and plasma membranes (with neutral to alkaline pH optima). The enzymes of sphingomyelin turnover release ceramide and free sphingosine from endogenous substrates, which may have implications for the participation of a sphingomyelin/sphingosine cycle as another 'lipid second messenger' system.

Niemann-Pick disease

Results of the investigation carried out during this decade brought unambigous evidence of biochemical heterogeneity inside the complex of Niemann-Pick disease according to which two entirely different metabolic disorders can be recognized. 1. Niemann-Pick sphingomyelinosis, a clear-cut enzymopathy, the pivotal lesion of which is the deficiency of lysosomal spingomyelinase leading to widespread lysosomal deposition of sphingomyelin liquid crystals. Two main allelic variants are known. The first one, neuronopathic (former type A) known as infantile with rapid course, may also manifest considerably prolonged course or an atypical course with predominantly visceral symptomatology. Patients with the second, visceral, variant (former type B), display mainly slow clinical course and often reach adulthood. With rare exceptions the neuronopathic variant can be biochemically recognized from the visceral one by much lower values of the in vivo sphingomyelin degradation test in the former. 2. The rest of the complex comprising types C-D differs substantially from the sphingomyelinase deficiency group by the remarkable heterogeneity in the lysosomal stored lipid pattern given by differences among the affected cell populations. Sphingomyelin storage could be proved histochemically solely in the histiocytic population together with cholesterol, neutral glycosphingolipids and lysobisphosphatidic acid, whereas the brain neurons displayed only neutral glycosphingolipid storage. There is an increasing evidence of the crucial biochemical lesion in this group being an altered intracellular traffic of exogenously derived cholesterol caused probably by its deficient translocation from lysosomes to other intracellular membrane sites. This leads to decreased cholesterol esterification rate which is the basis of the newly developed diagnostic test. Inconstant depression of sphingomyelinase activity is considered to be a secondary phenomenon. The so-called lactosylceramidosis is a rare variant pertinent to this group. The biochemical nature of type E still awaits clarification. Both groups of Niemann-Pick disease display clinical and especially histochemical features which allows to establish diagnosis in a highly efficient way already at the clinicopathological level.

Biochemical and ultrastructural studies on an Epstein-Barr virus-transformed lymphoid cell line from a Niemann-Pick disease type C patient

Human lymphoid cell lines established from normal subjects and from a Niemann-Pick disease type C patient were investigated from a triple point of view of enzymology, metabolism and ultrastructure: Sphingomyelinase activities, isoenzyme electrofocusing profiles and properties of the major enzyme were quite similar in type C and normal lymphoid cell lines. Similarly, no significant difference was observed in non-specific phosphodiesterases hydrolysing bis(methylumbelliferyl)phosphate and bis(methylumbelliferyl)pyrophosphate. The study of the lipid composition of type C cells showed no obvious accumulation of sphingomyelin or other phospholipid, but only a higher amount of glycolipids (mainly GlcCer and GbOse3Cer), as visualized by bidimensional thin-layer chromatography. Ultrastructural studies demonstrated, in type C cells, the presence of an obvious lysosomal storage of amphiphilic lipids quite similar to that observed in tissues of type C patients. These studies, which demonstrate the validity of lymphoid cell lines as an experimental model system for type C disease, agree with the current opinion that an impairment of sphingomyelin catabolism is not the primary defect in type C disease.

Molecular forms of sphingomyelinase and non-specific phosphodiesterases in Epstein-Barr virus-transformed lymphoid cell lines from Niemann-Pick disease types A and B

Molecular forms of sphingomyelinase and phosphodiesterases from lymphocytes- and Epstein-Barr virus-transformed lymphoid cell lines were separated by preparative electrofocusing in granulated gels. In either type of cell derived from normal individuals, sphingomyelinase focused as a single peak (pI = 5.60 +/- 0.1) while phosphodiesterases hydrolyzing bis(4-methylumbelliferyl)phosphate and bis(4-methylumbelliferyl)diphosphate separated into seven and three molecular forms respectively; one of the latter showed sphingomyelinase as well as phosphodiesterase activities. Lymphoid cell lines derived from patients with Niemann-Pick disease, types A or B, were practically devoid of sphingomyelinase activity; this was not so for the phosphodiesterases which focussed essentially as normal. The protein peak, which in normal cells contained the three activities, had phosphodiesterase but no sphingomyelinase activity in the Niemann-Pick cells. In normal cells, sphingomyelinase and phosphodiesterase activities of this peak showed different responses to heating and several effectors. These data suggest that in lymphoid cell lines, which are a useful model for studies of Niemann-Pick disease, sphingomyelinase and phosphodiesterases are subject to separate genetic coding and that the latter activities are not a reliable measure for diagnosing Niemann-Pick disease.

Biochemical studies in Niemann-Pick disease. III. In vitro and in vivo assays of sphingomyelin degradation in cultured skin fibroblasts and amniotic fluid cells for the diagnosis of the various forms of the disease

Sphingomyelinase activities were assayed in vitro in cultured skin fibroblasts of 61 patients with Niemann-Pick disease (NPD). Residual activities found in type A and B were 1% and 4%, respectively, of the mean control values, i.e. significantly higher in type B. In 27 cases with NPD type C, the mean activity was 42% of that in controls, with residual activities ranging from 15% up to normal. Fifteen pregnancies at risk for NPD type A and B were monitored; 4 affected foetuses were found. The uptake of exogenously added radiolabelled sphingomyelin by cultured cells and metabolism of the choline moiety of this lipid were studied in 35 patients with NPD and 14 controls. No difference of uptake between normal and mutant cells was observed. Normally, 77 +/- 5% of the radioactivity taken up was converted to phosphatidylcholine after 18 h incubation, compared to 5 +/- 2% (n = 7) in NPD type A. A substantially greater hydrolysis (31 +/- 12%; n = 8) occurred in NPD type B, and the test allowed complete discrimination between these two types. In NPD type C, 16 patients showed an abnormally low rate of intracellular sphingomyelin degradation (48 +/- 5%) while 4 others were not distinguishable from controls. There was a correlation (r = 0.76) between the results of the in vitro and in vivo assays, but also between the severity of the clinical symptoms and the impairment in sphingomyelin degradation. For the diagnosis of NPD type C, the in vivo test gave more reproducible and more clearcut results than the in vitro assay.

Studies on the activation of the enzymatic hydrolysis of sphingomyelin liposomes

Two pH optima were observed for the hydrolysis of sphingomyelin liposomes by brain and fibroblast extracts; one at pH 4.2-4.5, the other at pH 7-8. The proportion of the acidic activity in fibroblasts was affected greatly by the culturing conditions. Both the acidic and neutral enzyme activities were deficient in Niemann-Pick Type A fibroblasts, suggesting that both were genetically related. Partially purified activators from normal as well as Gaucher disease spleen stimulated the hydrolysis of sphingomyelin, at both pH values, by fibroblast and brain extracts. After further purification by DE-52 and Sephacryl 200 column chromatography the Gaucher activator retained its ability to stimulate sphingomyelinase and was active as well towards beta-glucocerebrosidase and beta-galactocerebrosidase.