Glycosphingolipids--nature, function, and pharmacological modulation

Second-generation iminoxylitol-based pharmacological chaperones for the treatment of Gaucher disease

A series of O-alkyl iminoxylitol derivatives was synthesized and evaluated as β-glucocerebrosidase (GCase) inhibitors. This structure-activity study shows a dramatic influence of the position of the alkyl chain (α-C1, O2, O3, or O4) on human GCase inhibition. Remarkably, 1,2-shift of the alkyl chain from C1 to O2 was found to maintain high inhibitory potency toward GCase as well as chaperone activity at sub-inhibitory concentration (10 nM). Removal of the stereogenic center at the pseudo-anomeric position led to shorter and more practical synthetic sequences. 2-O-Alkyl iminoxylitol derivatives constitute a new promising class of leads for the treatment of Gaucher disease by means of pharmacological chaperone therapy.

Glycosphingolipids and insulin resistance

Glycosphingolipids are structural membrane components, residing largely in the plasma membrane with their sugar-moieties exposed at the cell's surface. In recent times a crucial role for glycosphingolipids in insulin resistance has been proposed. A chronic state of insulin resistance is a rapidly increasing disease condition in Western and developing countries. It is considered to be the major underlying cause of the metabolic syndrome, a combination of metabolic abnormalities that increases the risk for an individual to develop Type 2 diabetes, obesity, cardiovascular disease, polycystic ovary syndrome and nonalcoholic fatty liver disease. As discussed in this chapter, the evidence for a direct regulatory interaction of glycosphingolipids with insulin signaling is still largely indirect. However, the recent finding in animal models that pharmacological reduction of glycosphingolipid biosynthesis ameliorates insulin resistance and prevents some manifestations of metabolic syndrome, supports the view that somehow glycosphingolipids act as critical regulators, Importantly, since reductions in glycosphingolipid biosynthesis have been found to be well tolerated, such approaches may have a therapeutic potential.

Dihydroceramide desaturase inhibition by a cyclopropanated dihydroceramide analog in cultured keratinocytes

Most mammalian sphingolipids contain a 4,5-(E)-double bond. We report on the chemical synthesis of a dihydroceramide derivative that prevents the introduction of the double bond into sphingolipids. Minimal alteration of the parent structure by formally replacing the hydrogen atoms in the 5- and in the 6-position of the sphinganine backbone by a methylene group leads to an inhibitor of dihydroceramide desaturase in cultured cells. In the presence of 10–50 μM of compound (1), levels of biosynthetically formed dihydroceramide and—surprisingly—also of phytoceramide are elevated at the expense of ceramide. The cells respond to the lack of unsaturated sphingolipids by an elevation of mRNAs of enzymes required for sphingosine formation. At the same time, the analysis of proliferation and differentiation markers indicates that the sphingolipid double bond is required to keep the cells in a differentiated state.

Human glycolipid transfer protein gene (GLTP) expression is regulated by Sp1 and Sp3: involvement of the bioactive sphingolipid ceramide

Glycolipid transfer protein (GLTP) accelerates glycolipid intermembrane transfer via a unique lipid transfer/binding fold (GLTP fold) that defines the GLTP superfamily and is the prototype for functional GLTP-like domains in larger proteins, i.e. FAPP2. Human GLTP is encoded by the single-copy GLTP gene on chromosome 12 (12q24.11 locus), but regulation of GLTP gene expression remains completely unexplored. Herein, the ability of glycosphingolipids (and their sphingolipid metabolites) to regulate the transcriptional expression of GLTP via its promoter has been evaluated. Using luciferase and GFP reporters in concert with deletion mutants, the constitutive and basal (225 bp; ∼78% G+C) human GLTP promoters have been defined along with adjacent regulatory elements. Despite high G+C content, translational regulation was not evident by the mammalian target of rapamycin pathway. Four GC-boxes were shown to be functional Sp1/Sp3 transcription factor binding sites. Mutation of one GC-box was particularly detrimental to GLTP transcriptional activity. Sp1/Sp3 RNA silencing and mithramycin A treatment significantly inhibited GLTP promoter activity. Among tested sphingolipid analogs of glucosylceramide, sulfatide, ganglioside GM1, ceramide 1-phosphate, sphingosine 1-phosphate, dihydroceramide, sphingosine, only ceramide, a nonglycosylated precursor metabolite unable to bind to GLTP protein, induced GLTP promoter activity and raised transcript levels in vivo. Ceramide treatment partially blocked promoter activity decreases induced by Sp1/Sp3 knockdown. Ceramide treatment also altered the in vivo binding affinity of Sp1 and Sp3 for the GLTP promoter and decreased Sp3 acetylation. This study represents the first characterization of any Gltp gene promoter and links human GLTP expression to sphingolipid homeostasis through ceramide.

Pharmacological small molecules for the treatment of lysosomal storage disorders

IMPORTANCE OF THE FIELD

Inherited lysosomal storage diseases often cause severe disability and have a devastating effect on quality of life. Enzyme replacement therapy (ERT) forms a cornerstone in the treatment of lysosomal enzyme deficiencies. Although for some lysosomal disorders ERT is lifesaving, important intrinsic restrictions of the approach are limited access of infused enzyme to less accessible body compartments such as the CNS, the burden of frequent intravenous administration, the emergence of antibodies and the high associated costs. Pharmacological small molecules may overcome these limitations.

AREAS COVERED IN THIS REVIEW

Several novel therapeutic approaches using small molecules are emerging: substrate reduction therapy, pharmacological chaperone therapy, premature nonsense mutation suppressors and proteostasis regulators.

WHAT THE READER WILL GAIN

Based on an extensive literature search up until June 2010, we here review the various therapeutic approaches with small compounds, including those currently in clinical use and those that have entered clinical trials. Compounds that are still in the preclinical phase are also briefly discussed.

TAKE HOME MESSAGE

pharmacological small molecules are a new class of agents that show great promise for the treatment of lysosomal storage disorders.

1-Deoxynojirimycins with dansyl capped N-substituents as probes for Morbus Gaucher affected cell lines

Cyclization by double reductive amination of d-xylo-hexos-5-ulose with methyl 6-aminohexanoate gave (methoxycarbonyl)pentyl-1-deoxynojirimycin. Reaction of the terminal carboxylic acid with N-dansyl-1,6-diaminohexane provided the corresponding chain-extended fluorescent derivative. By reaction with bis(6-dansylaminohexyl)amine, the corresponding branched di-N-dansyl compound was obtained. Both compounds are strong inhibitors of d-glucosidases and could also be shown to distinctly improve, at sub-inhibitory concentrations, the activity of beta-glucocerebrosidase in a Gaucher fibroblast (N370S) cell-line through chaperoning of the enzyme to the lysosome.

The twists and turns of sphingolipid pathway in glucose regulation

Palmitic acid is a saturated fat found in foods that lead to obesity, cardiovascular disease, and Type II diabetes. It is linked to the development of resistance to insulin stimulation in muscle, liver and other organs involved in glucose metabolism, which, in turn, underlines the onset of Type II diabetes. The cellular and molecular mechanisms of this insulin resistance are complex and not completely understood. This article is focused on the role of palmitic acid as a precursor in the synthesis of sphingolipids, a class of lipid molecules that participate in cellular stress response. Recent evidence had indicated that increased dietary supply of palmitate can stimulate the rate of sphingolipid synthesis in "lean" tissues and generate excessive amounts of sphingolipid metabolites that have a negative effect on the insulin signaling cascade. Many experimental results point to the existence of a causative link between sphingolipid synthesis, insulin response, and hyperglycemia. It is not yet clear, however whether ceramides or glycosphingolipids are involved as both have been implicated to be inhibitors of the insulin signaling cascade. Evidence for a coordinated regulation of sphingolipid and tri/diacylglycerol metabolism complicates further the delineation of a single mechanism of sphingolipid effect on glucose homeostasis.

Concise Synthesis of the Unnatural Sphingosine and Psychosine Enantiomer

The accumulation of psychosine (galactosyl sphingosine) has been associated with the pathogenesis of Krabbe disease, however, the exact mechanism of its cytotoxicity remains unclear. Herein, we describe the synthesis of the unnatural enantiomer of erythrosphingosine, psychosine, and related derivatives thereof that would allow for the mechanistic elucidation of the toxicity of psychosine.

Plasma membrane sphingomyelin hydrolysis increases hippocampal neuron excitability by sphingosine-1-phosphate mediated mechanisms

Proteins that control the excitability of neurons, including voltage-dependent ion channels and neurotransmitter receptors, reside in a membrane lipid environment that includes sphingomyelin, but the influence of the metabolism of this lipid on excitability is unknown. Sphingomyelin in the plasma membrane can be cleaved by neutral sphingomyelinases (nSMase) to generate ceramides and sphingosine-1-phosphate (S1P) which have been shown to play a variety of roles in cellular signaling processes. We found that application of nSMase to hippocampal slices results in a selective enhancement in the population spike amplitude, resulting in fEPSP-PS potentiation of the CA3-CA1 schaeffer collateral synapse. Single cell recordings showed that nSMase activity increases action potential frequency in CA1 neurons in a reversible manner. Additional current clamp recordings showed that nSMase reduces the slow after-hyperpolarization after a burst of action potentials. Mass spectrometry-based measurements demonstrated that nSMase activity induces a rapid increase in the levels of ceramides and S1P in cells in hippocampal slices. The ability of nSMase to increase CA1 neuron excitability was blocked by an inhibitor of sphingosine kinase, the enzyme that converts ceramide to S1P. Moreover, direct intracellular application of S1P to CA1 neurons increased action potential firing. Our findings suggest roles for sphingomyelin metabolism and S1P in the positive regulation of the excitability of hippocampal neurons.

Control of metabolism and signaling of simple bioactive sphingolipids: Implications in disease

Simple bioactive sphingolipids include ceramide, sphingosine and their phosphorylated forms sphingosine 1-phosphate and ceramide 1-phosphate. These molecules are crucial regulators of cell functions. In particular, they play important roles in the regulation of angiogenesis, apoptosis, cell proliferation, differentiation, migration, and inflammation. Decoding the mechanisms by which these cellular functions are regulated requires detailed understanding of the signaling pathways that are implicated in these processes. Most importantly, the development of inhibitors of the enzymes involved in their metabolism may be crucial for establishing new therapeutic strategies for treatment of disease.

Role of gangliosides in brain aging and neurodegeneration

Gangliosides are membrane glycosphingolipids bearing sialic acid residues. Within membranes, gangliosides are specifically enriched in highly organized domains, lipid rafts, and are attributed with diverse functions such as intercellular interactions, cell recognition, neurotransmission, and signal transduction. The highest concentration and variability of ganglioside structures are found in the human brain. Specific temporal and regional distribution of brain gangliosides has been reported; moreover, gangliosides may serve as markers of neurodevelopmental stages, aging and neurodegeneration. Brain ganglioside content and composition as well as ganglioside metabolism are altered in Alzheimer’s disease. It appears that the alterations of ganglioside metabolism leading to changes in membrane physico-chemical properties are not merely a consequence of primary pathology, but may also be involved in the early pathogenesis of Alzheimer’s disease through documented effects on APP proteolytic processing and amyloid aggregation. Investigations of glycolipid metabolic alterations which accompany neurodegenerative disorders provide insight into pathogenetic mechanisms and enable recognition of diagnostic markers as well as molecular structures acting as therapeutic tools interfering with cascade of pathological events.