Sphingolactones: Selective and Irreversible Inhibitors of Neutral Sphingomyelinase

Mysterious sphingolipids: metabolic interrelationships at the center of pathophysiology

Metabolic pathways are complex and intertwined. Deficiencies in one or more enzymes in a given pathway are directly linked with genetic diseases, most of them having devastating manifestations. The metabolic pathways undertaken by sphingolipids are diverse and elaborate with ceramide species serving as the hubs of sphingolipid intermediary metabolism and function. Sphingolipids are bioactive lipids that serve a multitude of cellular functions. Being pleiotropic in function, deficiency or overproduction of certain sphingolipids is associated with many genetic and chronic diseases. In this up-to-date review article, we strive to gather recent scientific evidence about sphingolipid metabolism, its enzymes, and regulation. We shed light on the importance of sphingolipid metabolism in a variety of genetic diseases and in nervous and immune system ailments. This is a comprehensive review of the state of the field of sphingolipid biochemistry.

Acceleration of Baylis-Hillman Reaction using Ionic Liquid Supported Organocatalyst

Background:

Baylis-Hillman reaction requires cheap starting materials, easy reaction protocol, and possibility to create the chiral center in the reaction product has increased the synthetic efficacy of this reaction which also suffers from high catalyst loading, low reaction rate, and poor yield.

Objective:

The extensive use of various functional or non-functional ionic liquids (ILs) with organocatalyst acts not only as reaction medium but also as a support to anchor the catalysts to increase the reaction rate of various organic transformations.

Methods:

In this manuscript, we have demonstrated the synthesis of quinuclidine-supported trimethylamine-based functionalized ionic liquid as a catalyst for the Baylis-Hillman reaction.

Results:

We obtained the Baylis-Hillman adducts in good, isolated yield along with low catalyst loading, short reaction time, wide substrate scope, easy product, and catalyst recycling. N- ((E,3S,4R)-5-benzylidene-tetrahydro-4-hydroxy-6-oxo-2H-pyran-3-yl) palmitamide was also successfully synthesized using CATALYST-3 promoted Baylis-Hillman reaction.

Conclusion:

We successfully isolated the 25 types of Baylis-Hillman adducts using three different quinuclidine-supported ammonium-based ionic liquids such as Et3AmQ][BF4] (CATALYST-1), [Et3AmQ][PF6] (CATALYST-2), and [TMAAmEQ][NTf2](CATALYST-3) as new and efficient catalysts. Generally, all the reactions demonstrated higher activity and gave good to high yield in competition with various previously reported homogenous and heterogeneous catalytic systems. Easy catalyst and product recovery followed by 6 times of catalysts recycling were the added advantages of the prosed catalytic system. Tedious and highly active N-((E,3S,4R)-5-benzylidene-tetrahydro- 4-hydroxy-6-oxo-2H-pyran-3-yl) palmitamide derivative was also synthesized using CATALYST- 3 followed by Baylis-Hillman reaction.

Small Molecule Inhibitors Targeting Biosynthesis of Ceramide, the Central Hub of the Sphingolipid Network

Ceramides are composed of a sphingosine and a single fatty acid connected by an amide linkage. As one of the major classes of biologically active lipids, ceramides and their upstream and downstream metabolites have been implicated in several pathological conditions including cancer, neurodegeneration, diabetes, microbial pathogenesis, obesity, and inflammation. Consequently, tremendous efforts have been devoted to deciphering the dynamics of metabolic pathways involved in ceramide biosynthesis. Given that several distinct enzymes can produce ceramide, different enzyme targets have been pursued depending on the underlying disease mechanism. The main objective of this review is to provide a comprehensive overview of small molecule inhibitors reported to date for each of these ceramide-producing enzymes from a medicinal chemistry perspective.

Promotion of endocytosis efficiency through an ATP-independent mechanism at rat calyx of Held terminals

KEY POINTS

At rat calyx of Held terminals, ATP was required not only for slow endocytosis, but also for rapid phase of compensatory endocytosis. An ATP-independent form of endocytosis was recruited to accelerate membrane retrieval at increased activity and temperature. ATP-independent endocytosis primarily involved retrieval of pre-existing membrane, which depended on Ca²⁺ and the activity of neutral sphingomyelinase but not clathrin-coated pit maturation. ATP-independent endocytosis represents a non-canonical mechanism that can efficiently retrieve membrane at physiological conditions without competing for the limited ATP at elevated neuronal activity.

ABSTRACT

Neurotransmission relies on membrane endocytosis to maintain vesicle supply and membrane stability. Endocytosis has been generally recognized as a major ATP-dependent function, which efficiently retrieves more membrane at elevated neuronal activity when ATP consumption within nerve terminals increases drastically. This paradox raises the interesting question of whether increased activity recruits ATP-independent mechanism(s) to accelerate endocytosis at the same time as preserving ATP availability for other tasks. To address this issue, we studied ATP requirement in three typical forms of endocytosis at rat calyx of Held terminals by whole-cell membrane capacitance measurements. At room temperature, blocking ATP hydrolysis effectively abolished slow endocytosis and rapid endocytosis but only partially inhibited excess endocytosis following intense stimulation. The ATP-independent endocytosis occurred at calyces from postnatal days 8-15, suggesting its existence before and after hearing onset. This endocytosis was not affected by a reduction of exocytosis using the light chain of botulinum toxin C, nor by block of clathrin-coat maturation. It was abolished by EGTA, which preferentially blocked endocytosis of retrievable membrane pre-existing at the surface, and was impaired by oxidation of cholesterol and inhibition of neutral sphingomyelinase. ATP-independent endocytosis became more significant at 34-35°C, and recovered membrane by an amount that, on average, was close to exocytosis. The results of the present study suggest that activity and temperature recruit ATP-independent endocytosis of pre-existing membrane (in addition to ATP-dependent endocytosis) to efficiently retrieve membrane at nerve terminals. This less understood endocytosis represents a non-canonical mechanism regulated by lipids such as cholesterol and sphingomyelinase.

Sphingolipids in lung growth and repair

Sphingolipids comprise a class of bioactive lipids that are involved in a variety of pathophysiologic processes, including cell death and survival. Ceramide and sphingosine-1-phosphate (S1P) form the center of sphingolipid metabolism and determine proapoptotic and antiapoptotic balance. Findings in animal models suggest a possible pathophysiologic role of ceramide and S1P in COPD, cystic fibrosis, and asthma. Sphingolipid research is now focusing on the role of ceramides during lung inflammation and its regulation by sphingomyelinases. Recently, sphingolipids have been shown to play a role in the pathogenesis of bronchopulmonary dysplasia (BPD). Ceramide upregulation was linked with vascular endothelial growth factor suppression and decreased surfactant protein B levels, pathways important for the development of BPD. In a murine model of BPD, intervention with an S1P analog had a favorable effect on histologic abnormalities and ceramide levels. Ceramides and S1P also regulate endothelial permeability through cortical actin cytoskeletal rearrangement, which is relevant for the pathogenesis of ARDS. On the basis of these observations, the feasibility of pharmacologic intervention in the sphingolipid pathway to influence disease development and progression is presently explored, with promising early results. The prospect of new strategies to prevent and repair lung disease by interfering with sphingolipid metabolism is exciting and could potentially reduce morbidity and mortality in patients with severe lung disorders.

Novel drugs targeting sphingolipid metabolism

While the evidence for an involvement of sphingolipids (SLs) in a variety of diseases is rapidly increasing, the development of sphingolipid-related drugs is still in its infancy. In fact, the recently FDA-approved fingolimod or FTY-720 (see chapter by J. Pfeilschifter for more information) is the first drug on the market to interfere with sphingolipid signaling. The reasons for this lagging are manifold and within this chapter we try to name some of them. Ceramide is in the center of sphingolipid metabolism. We describe the most important and most recent inhibitors for enzymes controlling cellular ceramide levels.

Natural products as platforms for the design of sphingolipid-related anticancer agents

Modulation of sphingolipid metabolism is a promising strategy for cancer therapy that has already opened innovative approaches for the development of pharmacological tools and rationally designed new drugs. On the other hand, natural products represent a classical and well-established source of chemical diversity that has guided medicinal chemists on the development of new chemical entities with potential therapeutic use. Based on these premises, the aim of this chapter is to provide the reader with a general overview of some of the most representative families of sphingolipid-related natural products that have been described in the recent literature as lead compounds for the design of new modulators of sphingolipid metabolism. Special emphasis is placed on the structural aspects of natural sphingoids and synthetic analogs that have found application as anticancer agents. In addition, their cellular targets and/or their mode of action are also considered.

Neurotrophins regulate cholinergic synaptic transmission in cultured rat sympathetic neurons through a p75-dependent mechanism

The sympathetic nervous system regulates many essential physiological systems, and its dysfunction is implicated in cardiovascular diseases. Mechanisms that control the strength of sympathetic output are therefore potential targets for the management of these disorders. Here we show that neurotrophins rapidly potentiate cholinergic transmission between cultured rat sympathetic neurons. We found that brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF), acting at the p75 receptor, increased the amplitude of excitatory postsynaptic currents (EPSCs). We observed increased amplitude but not frequency of miniature synaptic currents after p75 activation, suggesting that p75 acts postsynaptically to modulate transmission at these synapses. This neurotrophic modulation enhances cholinergic EPSCs via sphingolipid signaling. Application of sphingolactone-24, an inhibitor of neutral sphingomyelinase, blocked the effect of BDNF, implicating a sphingolipid pathway. Furthermore, application of the p75-associated sphingolipid second messengers C(2)-ceramide and d-erythro-sphingosine restricted to the postsynaptic cell mimicked BDNF application. Postsynaptic blockade of ceramide production with fumonisin, a ceramide synthase inhibitor, blocked the effects of BDNF and d-erythro-sphingosine, implicating ceramide or ceramide phosphate as the active signal. Together these data suggest that neurotrophin signaling, which occurs in vivo via release from sympathetic neurons and target tissues such as the heart, acutely regulates the strength of the sympathetic postganglionic response to central cholinergic inputs. This pathway provides a potential mechanism for modulating the strength of sympathetic drive to target organs such as the heart and could play a role in the development of cardiovascular diseases.

New F-18 prosthetic group via oxime coupling

A novel fluorine-18 prosthetic ligand, 5-(1,3-dioxolan-2-yl)-2-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)pyridine [(18)F]2, has been synthesized. The prosthetic ligand is formed in high radiochemical yield (rcy = 71 ± 2%, n = 3) with excellent radiochemical purity (rcp = 99 ± 1%, n = 3) in a short reaction time (10 min). [(18)F]2 is a small, neutral, organic complex, easily synthesized in four steps from a readily available starting material. It can be anchored onto a target molecule containing an aminooxy functional group under acidic conditions by way of an oxime bond. We report herein two examples [(18)F]23 and [(18)F]24, potential imaging agents for β-amyloid plaques, which were labeled with this prosthetic group. This approach could be used for labeling proteins and peptides containing an aminooxy group. Biodistribution in male ICR mice for both oxime labeled complexes [(18)F]23 and [(18)F]24 were compared to that of the known β-amyloid plaque indicator, [(18)F]-AV-45, florbetapir 1. Oximes [(18)F]23 and [(18)F]24 are larger in size and therefore should reduce the blood-brain barrier (BBB) penetration. The brain uptake for oxime [(18)F]23 appeared to be reduced, but still retained some capability to cross the BBB. Oxime [(18)F]24 showed promising results after 2 min post injection (0.48% dose/gram); however, the uptake increased after 30 min post injection (0.92% dose/gram) suggesting an in vivo decomposition/metabolism of compound [(18)F]24. We have demonstrated a general protocol for the fluoride-18 labeling with a new prosthetic ligand [(18)F]2 that is tolerant toward several functional groups and is formed via chemoselective oxime coupling.

Synthesis and evaluation of novel phosphate ester analogs as neutral sphingomyelinase inhibitors

A novel sphingomyelin inhibitor RY221B-a, which contains a bipyridyl moiety as a metal coordination site was designed based upon the mechanism of phosphate ester hydrolysis. RY221B-a was synthesized from N-Boc-sphingosine in three steps via selective etherification using stannyl acetal. Synthesized RY221B-a exhibited relatively-strong inhibitory activity against Bc-SMase (IC(50)=1.2microM).

Total synthesis of (+)-scyphostatin featuring an enantioselective and highly efficient route to the side-chain via Zr-catalyzed asymmetric carboalumination of alkenes (ZACA)

(+)-Scyphostatin (1) was synthesized via (i) construction of a side-chain 3b of > or = 98% purity in 19% yield in eleven steps featuring ZACA reaction, Negishi coupling, and HWE olefination, (ii) an asymmetric synthesis of a fully protected core 4 from 10a, and (iii) a three-step assembly of 1 in 42% yield.

Chemical tools to investigate sphingolipid metabolism and functions

Sphingolipids comprise an important group of biomolecules, some of which have been shown to play important roles in the regulation of many cell functions. From a structural standpoint, they all share a long 2-amino-1,3-diol chain, which can be either saturated (sphinganine), hydroxylated at C4 (phytosphingosine), or unsaturated at C4 (sphingosine) as in most mammalian cells. N-acylation of sphingosine leads to ceramide, a key intermediate in sphingolipid metabolism that can be enzymatically modified at the C1-OH position to other biologically important sphingolipids, such as sphingomyelin or glycosphingolipids. In addition, both ceramide and sphingosine can be phosphorylated at C1-OH to give ceramide-1-phosphate and sphingosine-1-phosphate, respectively. To better understand the biological and biophysical roles of sphingolipids, many efforts have been made to design synthetic analogues as chemical tools able to unravel their structure-activity relationships, and to alter their cellular levels. This last approach has been thoroughly studied by the development of specific inhibitors of some key enzymes that play an important role in biosynthesis or metabolism of these intriguing lipids. With the above premises in mind, the aim of this review is to collect, in a systematic way, the recent efforts described in the literature leading to the development of new chemical entities specifically designed to achieve the above goals.

Inhibitors of sphingolipid metabolism enzymes

Sphingolipids are a family of lipids that play essential roles both as structural cell membrane components and in cell signalling. The cellular contents of the various sphingolipid species are controlled by enzymes involved in their metabolic pathways. In this context, the discovery of small chemical entities able to modify these enzyme activities in a potent and selective way should offer new pharmacological tools and therapeutic agents.