The genetics of sphingolipid hydrolases and sphingolipid storage diseases.

The relationship of sphingolipids with human disease first arose from the study of sphingolipid storage diseases over 50 years ago. Most of these disorders are due to inherited deficiencies of specific sphingolipid hydrolases, although a small number also result from defects in sphingolipid transport or activator proteins. Due to the primary protein deficiencies sphingolipids and other macromolecules accumulate in cells and tissues of affected patients, leading to a diverse presentation of clinical abnormalities. Over 25 sphingolipid storage diseases have been described to date. Most of the genes have been isolated, disease-causing mutations have been identified, the recombinant proteins have been produced and characterized, and animal models exist for most of the human diseases. Since most sphingolipid hydrolases are enriched within the endosomal/lysosomal system, macromolecules first accumulate within these compartments. However, these abnormalities rapidly spread to other compartments and cause a wide range of cellular dysfunction. This review focuses on the genetics of sphingolipid storage diseases and related hydrolytic enzymes with an emphasis on the relationship between genetic mutations and human disease.

Gangliosides as apoptotic signals in ER stress response.

Renewed attention has been given lately to gangliosides and to their function as intracellular messengers of the adaptive responses to stress. Gangliosides are vital components of cell membranes; therefore, deleterious consequences can result from changes in their chemical composition and concentration, that is, membrane dynamics and structure can be altered as can the behavior of other membrane proteins. The importance of gangliosides in human health is evident in neurodegenerative diseases associated with defects in their degradation. As key modulators of intracellular calcium flux, gangliosides are involved in cellular processes downstream of calcium signaling. In this review, we focus on the effect of ganglioside accumulation on the endoplasmic reticulum calcium homeostasis and on the integrity of the mitochondrial membranes. We discuss how these events elicit an apoptotic program that ultimately leads to cell death. Owing to interorganelle crosstalk, these events are not necessarily self-contained, and gangliosides may serve as the common factor.

Glycosphingolipids in endocytic membrane transport.

Endocytosis leads to the internalisation of both lipids and proteins and their delivery to specific subcellular locations. This involves sorting processes that are not completely understood, but may involve interactions between lipids and proteins as well as pH and calcium gradients. This article discusses the importance of endocytosis in glycosphingolipid (GSL) synthesis as well as the potential roles of GSLs in endocytic membrane transport. Although the accumulation of GSLs in storage diseases clearly disrupts endocytic transport, increasing evidence also supports a role for GSLs in endocytosis in normal cells.

The pathogenesis of glycosphingolipid storage disorders.

Glycosphingolipid storage disorders are inborn errors of metabolism caused by the defective activity of degradative enzymes in lysosomes. In this review, we summarize studies performed over the past few years attempting to define the secondary and down-stream biochemical and cellular pathways affected in GSL storage disorders that are responsible for neuronal dysfunction, a characteristic of most of these disorders. We focus mainly on the regulation of intracellular calcium homeostasis and phospholipid biosynthesis. These studies may help unravel new roles for glycosphingolipids in the regulation of normal cell physiology, as well as suggest potential new therapeutic options in the glycosphingolipid and other lysosomal storage disorders.

Review: Glycosphingolipids in health and disease.

Glycosphingolipids are ubiquitous membrane constituents that are subdivided in neutral or acidic fractions (gangliosides and sulfatides). Their analysis requires extraction and separation by thin-layer chromatography or high-performance liquid chromatography. Ganglioside composition changes occur in response to variations in cellular morphology and function. Glycosphingolipids are implicated in the pathogenesis of various diseases, including glycosphingolipidoses, peripheral neuropathies caused by anti-ganglioside antibodies, and secretory diarrhea. Gangliosides play a role in the induction of apoptosis. For example, ceramide-induced apoptosis is associated with increased synthesis of a ganglioside, GD3. Gangliosides are also potential diagnostic markers and therapeutic targets for cancer.

Beyond a structural component: sphingolipids in immunology.

Two major classes of lipids participating in signaling cascades in immune cells are known today. One comprises glycerol-based lipids with diacylglycerol as its most prominent member that mediates the activation of classical and novel protein kinase C molecules. The second group contains the sphingolipids, with the best-investigated representatives being sphingosine, sphingosine-1-phosphate, and ceramide. In the last years the latter two molecules have especially received considerable attention for their modulatory capacity in the course of an apoptotic response. Today it is clear that sphingolipids are ubiquitously distributed in all eukaryotic cells, especially in cellular membranes, where they were previously thought to fulfil an exclusively structural role. Recent findings, however, have demonstrated functions beyond this. Sphingolipid specific G-protein coupled receptors were identified and their role as intracellular second messengers has been further elucidated. In addition, glycosphingolipids, in particular, are enriched in certain membrane compartments, known as detergent resistant membranes. These serve as entry sites for several receptor-mediated signaling events by stabilizing receptor/kinase interactions, suggesting an involvement in the initiation of signaling cascades. Altogether, these findings have led to new insights into both the role of these lipids in signaling as well as the underlying pathology of several diseases with imbalances in the sphingolipid metabolism. The development of these disorders has mainly been attributed to the toxic potential of lysosphingolipids up to now. In addition, attempts have been made to develop compounds and drugs containing the sphingolipid backbone for influencing diseases associated with unwanted cell activation (e.g, cancer, inflammatory processes). These novel findings and developments are reviewed in the following.

Inhibition of protein kinase C activity in mucolipidosis type-4: a model for a new pathogenetic mechanism in inborn errors of metabolism.

Inhibition of protein kinase C [PK-C] activity by sphingosine and its derivatives has been suggested to play a role in the pathogenesis of sphingolipidoses. In the present study, PK-C activity and PK-C-mediated phosphorylation of endogenous substrates were studied in skin fibroblasts from patients with mucolipidosis type 4 [ML-4], in which there is accumulation of the phospholipids phosphatidylcholine, lysophosphatidylcholine, phosphatidylethanolamine as well as gangliosides. Cytosolic PK-C activity in 5 ML-4 cell lines was comparable to that in control cells. PK-C activity in the particulate fraction of these cells was 84 +/- 14 pmol 32P/mg protein per min compared with 267 +/- 26 in control cells. Increasing the concentrations of the activating lipids in the reaction mixture did not enhance PK-C activity in ML-4 cells, suggesting a non-competitive inhibition of the kinase. Following partial purification of the enzyme from the particulate fraction PK-C activity increased to 288 +/- 14 and 339 +/- 12 pmol 32P/mg protein per min in ML-4 and control cells, respectively. The phosphorylation pattern of endogenous substrates in the particulate fraction of ML-4 cells differed from that in control cells both in the absence and in the presence of calcium and activating lipids. We suggest that PK-C may be involved in the pathogenesis of sphingolipidoses and that this may represent an example for a new type of pathogenetic mechanisms in inborn errors of metabolism.

[Sphingolipidosis. Recent progress in biochemical diagnosis. New pathogenic approaches]. / Les sphingolipidoses. Actualisation du diagnostic biochimique. Nouvelles approches pathogéniques.

In an increasing number of biochemical variants of the sphingolipidoses, demonstration of the enzyme defect cannot be achieved by the (briefly summarized) conventional techniques. Alternative methods allowing diagnosis of such patients - in particular metabolic studies in living cell cultures - are discussed. The current knowledge and practical impact of molecular biology in the sphingolipidoses field is shortly reviewed. The authors finally discuss the recent findings highlighting in Niemann-Pick disease type C metabolic abnormalities in the cellular processing of cholesterol and their consequences on diagnosis and pathogenesis of this disorder.

[Pathobiochemical aspects of lysosomal enzymes with special reference to lysosomal storage diseases (author's transl)]. / Pathobiochemische Aspekte lysosomolar Enzyme unter besonderer Berücksichtigung lysosomaler Speicherkrankheiten.

Lysosomal hydrolases participate substantially in the degradation of all classes of biological macromolecules. They act physiologically within the lysosome. The enzymes are either primarily included within primary lysosomes or are transported to these cell organelles after secretion and subsequent adsorptive pinocytosis. The involvement of these enzymes in a variety of pathological conditions can be understood on the basis of the known functions of lysosomal hydrolases. Inactivity of one or several of the enzymes causes lysosomal storage disorders. Similar metabolic consequences are found when the enzymes are unable to be concentrated within the lysosome. Lysosomal hydrolases participate, furthermore, in the pathogenesis of numerous diseases. A distinction can be made between lysosomal overload, pathologically-increased enzyme secretion into the extracellular space, and a release of lysosomal enzymes into the cytosol.

The cerebral lipidoses.

The disorders presented consist of those clinical entities in which a reasonably well defined lipid storage material accumulated within nervous tissue. Many other progressive, degenerative disorders are suspected of being storage disorders, but their chemical pathology remains unclear. Collectively this group could be designated the sphingolipidoses. In each case, the disease is a genetic disturbance and transmitted as an autosomal recessive. Sphingolipid storage in each disorder is associated with deficient activity of a specific degradative enzyme or enzyme system, and these deficient enzymes are all lysosomal hydrolases. Lysosomal hydrolases catalyze the breakdown of complex molecules in digestive vacuoles (phagocytic or autophagic) within the cells. Lysosomes show structural latency (requiring osmotic shock or freeze thawing in vitro); their enzymes show maximal activity at acidic pH ranges, and on electron microscopic examination they appear as small, electron-dense intracellular bodies. These hydrolytic enzymes seem to have some form of biological vulnerability in terms of their genetic expression, and this vulnerability underlies the sphingolipidoses. Diagnosis in each case is primarily a clinical problem. The presentation of these disorders, especially in intermediate or advanced forms, is sufficiently distinctive to permit a reasonably accurate diagnosis on the basis of history, physical examination, and routine laboratory data. Patients seen in early stages may be more difficult to recognize but follow-up evaluations usually clarify the problem. Specific enzyme assays are now available for confirmation of the diagnosis in these disorders. A frequent finding in this connection is an increase in the activities of noninvolved lysosomal hydrolases in the storage disorders. Once a case is clinically diagnosed, the clinician has the responsibility of ensuring that proper genetic counseling is made available to the affected families. Considerable supportive care is needed in each case. These patients can survive for prolonged periods, and great stress is placed on their families by these prolonged, hopeless illnesses. Since the disorders affect infants or young children, their parents are usually young adults in their early reproductive years. It is essential that they receive information concerning the risk of subsequent pregnancies. Specific diagnosis of the fetus in early pregnancy can be made now by amniocentesis and enzyme assays on cultured fibroblasts. If the fetus is a homozygote on the basis of enzyme assays, the option of therapeutic abortion should be discussed with the family. For many parents there will be considerable sensitivity to the ethical implications of this course and, if any doubt arises, ethical or pastoral consultation should be sought. Although there are no specific therapeutic approaches, a considerable degree of supportive care can and should be given. In the gangliosidoses and late in the course of the leukodystrophies, seizures will present management problems...