Sphingolipid/cholesterol regulation of neurotransmitter receptor conformation and function

Role of the transmembrane domain 4/extracellular loop 2 junction of the human gonadotropin-releasing hormone receptor in ligand binding and receptor conformational selection

Recent crystal structures of G protein-coupled receptors (GPCRs) show the remarkable structural diversity of extracellular loop 2 (ECL2), implying its potential role in ligand binding and ligand-induced receptor conformational selectivity. Here we have applied molecular modeling and mutagenesis studies to the TM4/ECL2 junction (residues Pro(174(4.59))-Met(180(4.66))) of the human gonadotropin-releasing hormone (GnRH) receptor, which uniquely has one functional type of receptor but two endogenous ligands in humans. We suggest that the above residues assume an α-helical extension of TM4 in which the side chains of Gln(174(4.60)) and Phe(178(4.64)) face toward the central ligand binding pocket to make H-bond and aromatic contacts with pGlu(1) and Trp(3) of both GnRH I and GnRH II, respectively. The interaction between the side chains of Phe(178(4.64)) of the receptor and Trp(3) of the GnRHs was supported by reciprocal mutations of the interacting residues. Interestingly, alanine mutations of Leu(175(4.61)), Ile(177(4.63)), and Met(180(4.66)) decreased mutant receptor affinity for GnRH I but, in contrast, increased affinity for GnRH II. This suggests that these residues make intramolecular or intermolecular contacts with residues of transmembrane (TM) domain 3, TM5, or the phospholipid bilayer, which couple the ligand structure to specific receptor conformational switches. The marked decrease in signaling efficacy of I177A and F178A also indicates that IIe(177(4.63)) and Phe(178(4.64)) are important in stabilizing receptor-active conformations. These findings suggest that the TM4/ECL2 junction is crucial for peptide ligand binding and, consequently, for ligand-induced receptor conformational selection.

The fusogenic tilted peptide (67-78) of α-synuclein is a cholesterol binding domain

Parkinson's disease-associated α-synuclein is an amyloidogenic protein not only expressed in the cytoplasm of neurons, but also secreted in the extracellular space and internalized into glial cells through a lipid raft-dependent process. We previously showed that α-synuclein interacts with raft glycosphingolipids through a structural motif common to various viral and amyloidogenic proteins. Here we report that α-synuclein also interacts with cholesterol, as assessed by surface pressure measurements of cholesterol-containing monolayers. Using a panel of recombinant fragments and synthetic peptides, we identified two distinct cholesterol-binding domains in α-synuclein. One of these domains, which corresponds to the tilted peptide of α-synuclein (67-78), bound cholesterol with high affinity and was toxic for cultured astrocytes. Molecular modeling suggested that cholesterol binds to this peptide with a tilt angle of 46°. α-synuclein also contains a cholesterol recognition consensus motif, which had a lower affinity for cholesterol and was devoid of toxicity. This motif is encased in the glycosphingolipid-binding domain (34-45) of α-synuclein. In raft-like model membranes containing both cholesterol and glycosphingolipids, the head groups of glycosphingolipids prevented the accessibility of cholesterol to exogenous ligands. Nevertheless, cholesterol appeared to 'signal' its presence by tuning glycosphingolipid conformation, thereby facilitating α-synuclein binding to raft-like membranes. We propose that the association of α-synuclein with lipid rafts involves both the binding of α-synuclein (34-45) to glycosphingolipids, and the interaction of the fusogenic tilted peptide (67-78) with cholesterol. Coincidentally, a similar mechanism is used by viruses (HIV-1, HTLV-I, Ebola) which display a tilted peptide and fuse with host cell membranes through a sphingolipid/cholesterol-dependent process.

Novel drugs from marine microorganisms

Marine microorganisms have expected mounting consideration on the basis of bioactive metabolites and propose an exclusive prospect to both enhance the amount of aquatic natural foodstuffs in clinical trials as well as speed up their progress. This review focuses particularly on those molecules, originated from marine microorganisms, presently in the medical pipeline that have been recognized or highly expected to be identified based on growing incidental evidence. Particularly karlotoxin class compounds, isolated from dinoflagellate Karlodinium veneficum, offer chances to create new molecules for control of cancer and high serum cholesterol levels.

Rapid microfluidic perfusion enabling kinetic studies of lipid ion channels in a bilayer lipid membrane chip

There is growing recognition that lipids play key roles in ion channel physiology, both through the dynamic formation and dissolution of lipid ion channels and by indirect regulation of protein ion channels. Because existing technologies cannot rapidly modulate the local (bio)chemical conditions at artificial bilayer lipid membranes used in ion channel studies, the ability to elucidate the dynamics of these lipid-lipid and lipid-protein interactions has been limited. Here we demonstrate a microfluidic system supporting exceptionally rapid perfusion of reagents to an on-chip bilayer lipid membrane, enabling the responses of lipid ion channels to dynamic changes in membrane boundary conditions to be probed. The thermoplastic microfluidic system allows initial perfusion of reagents to the membrane in less than 1 s, and enables kinetic behaviors with time constants below 10 s to be directly measured. Application of the platform is demonstrated toward kinetic studies of ceramide, a biologically important lipid known to self-assemble into transmembrane ion channels, in response to dynamic treatments of small ions (La(3+)) and proteins (Bcl-x(L) mutant). The results reveal the broader potential of the technology for studies of membrane biophysics, including lipid ion channel dynamics, lipid-protein interactions, and the regulation of protein ion channels by lipid micro domains.

Hypothesis: the role of sterols in autism spectrum disorder

A possible role for sterols in the development of autism spectrum disorder (ASD) has not been proven, but studies in disorders of sterol biosynthesis, chiefly Smith-Lemli-Opitz syndrome (SLOS), enable hypotheses on a causal relationship to be discussed. Advances in genetic technology coupled with discoveries in membrane physiology have led to renewed interest for lipids in the nervous system. This paper hypothesizes on the role of sterol dysfunction in ASD through the framework of SLOS. Impaired sonic hedgehog patterning, alterations in membrane lipid rafts leading to abnormal synaptic plasticity, and impaired neurosteroid synthesis are discussed. Potential therapeutic agents include the development of neuroactive steroid-based agents and enzyme-specific drugs. Future investigations should reveal the specific mechanisms underlying sterol dysfunction in neurodevelopmental disorders by utilizing advanced imaging and molecular techniques.

Modulation of monoamine receptors by adaptor proteins and lipid rafts: role in some effects of centrally acting drugs and therapeutic agents

The monoamines and their cognate receptors are widespread in the central nervous system and are vital for normal brain function. Dysfunction in these systems underlies several psychiatric and neurological disease states, and consequently monoamines are targets of a host of pharmacotherapies. This review provides an overview on how monoamine receptors are regulated by adaptor proteins and lipid rafts with emphasis on interactions in nerve cells. Monoamine receptors have prominent intracellular loops that provide binding sites for adaptor proteins. Receptor function is further modulated by cholesterol and submembranous microdomains termed lipid rafts. These interactions determine several facets of G protein-coupled receptor (GPCR) function including trafficking, localization, and signaling. Possible roles of adaptor proteins and lipid rafts in disease states and in mediating actions of drugs and therapeutic agents are also discussed.

Effect of HFE variants on sphingolipid expression by SH-SY5Y human neuroblastoma cells

C282Y and H63D are two common variants of the hemochromatosis protein HFE. SH-SY5Y human neuroblastoma cells stably transfected to express either wild type HFE (WT-HFE), or the C282Y or H63D allele were analyzed for effect of expression of the mutant proteins on transcription of 14 enzymes involved in sphingolipid metabolism. Cells expressing the C282Y variant showed significant increases (>2-fold) in transcription of five genes and decreases in two compared to that seen for cells expressing WT-HFE, while cells expressing the H63D variant showed an elevation in transcription of one gene and a decrease in two. These changes were seen as alterations in ganglioside composition, cell surface binding by the binding subunit of cholera toxin, expression of sphingosine-kinase-1 and synthesis of sphingosine-1-phosphate. These changes may explain why C282Y-HFE is a risk factor for colon and breast cancer and possibly protective against Alzheimer's disease while H63D-HFE is a risk factor for neurodegenerative diseases.

Lipids and membrane lateral organization

Shortly after the elucidation of the very basic structure and properties of cellular membranes, it became evident that cellular membranes are highly organized structures with multiple and multi-dimensional levels of order. Very early observations suggested that the lipid components of biological membranes might be active players in the creation of these levels of order. In the late 1980s, several different and diverse experimental pieces of evidence coalesced together giving rise to the lipid raft hypothesis. Lipid rafts became enormously (and, in the opinion of these authors, sometimes acritically) popular, surprisingly not just within the lipidologist community (who is supposed to be naturally sensitive to the fascination of lipid rafts). Today, a PubMed search using the key word "lipid rafts" returned a list of 3767 papers, including 690 reviews (as a term of comparison, searching over the same time span for a very hot lipid-related key word, "ceramide" returned 6187 hits with 799 reviews), and a tremendous number of different cellular functions have been described as "lipid raft-dependent." However, a clear consensus definition of lipid raft has been proposed only in recent times, and the basic properties, the ruling forces, and even the existence of lipid rafts in living cells has been recently matter of intense debate. The scenario that is gradually emerging from the controversies elicited by the lipid raft hypothesis emphasizes multiple roles for membrane lipids in determining membrane order, that encompass their tendency to phase separation but are clearly not limited to this. In this review, we would like to re-focus the attention of the readers on the importance of lipids in organizing the fine structure of cellular membranes.

The expanding role of marine microbes in pharmaceutical development

Marine microbes have received growing attention as sources of bioactive metabolites and offer a unique opportunity to both increase the number of marine natural products in clinical trials as well as expedite their development. This review focuses specifically on those molecules currently in the clinical pipeline that are established or highly likely to be produced by bacteria based on expanding circumstantial evidence. We also include an example of how compounds from harmful algal blooms may yield both tools for measuring environmental change as well as leads for pharmaceutical development. An example of the karlotoxin class of compounds isolated from the dinoflagellate Karlodinium veneficum reveals a significant environmental impact in the form of massive fish kills, but also provides opportunities to construct new molecules for the control of cancer and serum cholesterol assisted by tools associated with rational drug design.

Spatial organization of transmembrane receptor signalling

The spatial organization of transmembrane receptors is a critical step in signal transduction and receptor trafficking in cells. Transmembrane receptors engage in lateral homotypic and heterotypic cis-interactions as well as intercellular trans-interactions that result in the formation of signalling foci for the initiation of different signalling networks. Several aspects of ligand-induced receptor clustering and association with signalling proteins are also influenced by the lipid composition of membranes. Thus, lipid microdomains have a function in tuning the activity of many transmembrane receptors by positively or negatively affecting receptor clustering and signal transduction. We review the current knowledge about the functions of clustering of transmembrane receptors and lipid-protein interactions important for the spatial organization of signalling at the membrane.

Molecular insights into amyloid regulation by membrane cholesterol and sphingolipids: common mechanisms in neurodegenerative diseases

Alzheimer, Parkinson and other neurodegenerative diseases involve a series of brain proteins, referred to as ‘amyloidogenic proteins’, with exceptional conformational plasticity and a high propensity for self-aggregation. Although the mechanisms by which amyloidogenic proteins kill neural cells are not fully understood, a common feature is the concentration of unstructured amyloidogenic monomers on bidimensional membrane lattices. Membrane-bound monomers undergo a series of lipid-dependent conformational changes, leading to the formation of oligomers of varying toxicity rich in β-sheet structures (annular pores, amyloid fibrils) or in α-helix structures (transmembrane channels). Condensed membrane nano- or microdomains formed by sphingolipids and cholesterol are privileged sites for the binding and oligomerisation of amyloidogenic proteins. By controlling the balance between unstructured monomers and α or β conformers (the chaperone effect), sphingolipids can either inhibit or stimulate the oligomerisation of amyloidogenic proteins. Cholesterol has a dual role: regulation of protein–sphingolipid interactions through a fine tuning of sphingolipid conformation (indirect effect), and facilitation of pore (or channel) formation through direct binding to amyloidogenic proteins. Deciphering this complex network of molecular interactions in the context of age- and disease-related evolution of brain lipid expression will help understanding of how amyloidogenic proteins induce neural toxicity and will stimulate the development of innovative therapies for neurodegenerative diseases.

Neural membrane signaling platforms

Throughout much of the history of biology, the cell membrane was functionally defined as a semi-permeable barrier separating aqueous compartments, and an anchoring site for proteins. Little attention was devoted to its possible regulatory role in intracellular molecular processes and neuron electrical signaling. This article reviews the history of membrane studies and the current state of the art. Emphasis is placed on natural and artificial membrane studies of electric field effects on molecular organization, especially as these may relate to impulse propagation in neurons. Implications of these studies for new designs in artificial intelligence are briefly examined.

Lipid microdomains and the regulation of ion channel function

Many types of ion channel localize to cholesterol and sphingolipid-enriched regions of the plasma membrane known as lipid microdomains or 'rafts'. The precise physiological role of these unique lipid microenvironments remains elusive due largely to difficulties associated with studying these potentially extremely small and dynamic domains. Nevertheless, increasing evidence suggests that membrane rafts regulate channel function in a number of different ways. Raft-enriched lipids such as cholesterol and sphingolipids exert effects on channel activity either through direct protein-lipid interactions or by influencing the physical properties of the bilayer. Rafts also appear to selectively recruit interacting signalling molecules to generate subcellular compartments that may be important for efficient and selective signal transduction. Direct interaction with raft-associated scaffold proteins such as caveolin can also influence channel function by altering gating kinetics or by affecting trafficking and surface expression. Selective association of ion channels with specific lipid microenvironments within the membrane is thus likely to be an important and fundamental regulatory aspect of channel physiology. This brief review highlights some of the existing evidence for raft modulation of channel function.

Pre-synaptic defects of NPC1-deficient hippocampal neurons are not directly related to plasma membrane cholesterol

Imbalances in brain cholesterol homeostasis have been observed in several neurodegenerative diseases. In Niemann-Pick Type C (NPC) disease, mutations in NPC1 or NPC2 lead to endosomal cholesterol accumulation, neuronal dysfunction and death. Cholesterol in synaptic plasma membranes influences membrane fluidity, curvature, and protein function, and its depletion may adversely affect synaptic vesicle cycling. We have investigated pre-synaptic function in primary hippocampal neurons with altered cholesterol distribution because of NPC1 deficiency or cyclodextrin treatment. In NPC1-deficient neurons grown in serum-free medium, plasma membrane cholesterol was reduced and total synaptic vesicle release during prolonged stimulation was attenuated. In NPC1-deficient neurons cultured in the presence of high-density lipoproteins, plasma membrane cholesterol markedly increased, but the defects in synaptic vesicle release in NPC1-deficient neurons were exacerbated. Treatment with 1 mM methyl-beta-cyclodextrin acutely depleted plasma membrane cholesterol in wild-type neurons to levels below those in NPC1 deficiency, but did not alter synaptic vesicle exo- or endocytosis. Defects only became apparent when higher methyl-beta-cyclodextrin concentrations were used. Our data indicate that synaptic vesicle release can tolerate some degree of plasma membrane cholesterol depletion and suggest that the pre-synaptic defects in NPC1-deficient neurons are not solely caused by a reduction of plasma membrane cholesterol.

How cholesterol constrains glycolipid conformation for optimal recognition of Alzheimer's beta amyloid peptide (Abeta1-40)

Membrane lipids play a pivotal role in the pathogenesis of Alzheimer's disease, which is associated with conformational changes, oligomerization and/or aggregation of Alzheimer's beta-amyloid (Abeta) peptides. Yet conflicting data have been reported on the respective effect of cholesterol and glycosphingolipids (GSLs) on the supramolecular assembly of Abeta peptides. The aim of the present study was to unravel the molecular mechanisms by which cholesterol modulates the interaction between Abeta(1-40) and chemically defined GSLs (GalCer, LacCer, GM1, GM3). Using the Langmuir monolayer technique, we show that Abeta(1-40) selectively binds to GSLs containing a 2-OH group in the acyl chain of the ceramide backbone (HFA-GSLs). In contrast, Abeta(1-40) did not interact with GSLs containing a nonhydroxylated fatty acid (NFA-GSLs). Cholesterol inhibited the interaction of Abeta(1-40) with HFA-GSLs, through dilution of the GSL in the monolayer, but rendered the initially inactive NFA-GSLs competent for Abeta(1-40) binding. Both crystallographic data and molecular dynamics simulations suggested that the active conformation of HFA-GSL involves a H-bond network that restricts the orientation of the sugar group of GSLs in a parallel orientation with respect to the membrane. This particular conformation is stabilized by the 2-OH group of the GSL. Correspondingly, the interaction of Abeta(1-40) with HFA-GSLs is strongly inhibited by NaF, an efficient competitor of H-bond formation. For NFA-GSLs, this is the OH group of cholesterol that constrains the glycolipid to adopt the active L-shape conformation compatible with sugar-aromatic CH-pi stacking interactions involving residue Y10 of Abeta(1-40). We conclude that cholesterol can either inhibit or facilitate membrane-Abeta interactions through fine tuning of glycosphingolipid conformation. These data shed some light on the complex molecular interplay between cell surface GSLs, cholesterol and Abeta peptides, and on the influence of this molecular ballet on Abeta-membrane interactions.

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.

Sphingolipids and gangliosides of the nervous system in membrane function and dysfunction

Simple sphingolipids such as ceramide and sphingomyelin (SM) as well as more complex glycosphingolipids play very important roles in cell function under physiological conditions and during disease development and progression. Sphingolipids are particularly abundant in the nervous system. Due to their amphiphilic nature they localize to cellular membranes and many of their roles in health and disease result from membrane reorganization and from lipid interaction with proteins within cellular membranes. In this review we discuss some of the functions of sphingolipids in processes that entail cellular membranes and their role in neurodegenerative diseases, with an emphasis on SM, ceramide and gangliosides.

Neuronal nicotinic acetylcholine receptor-cholesterol crosstalk in Alzheimer's disease

Alzheimer's disease (AD) is one of the most devastating diseases of the central nervous system (CNS). It is characterized by two neuropathological findings: amyloid plaques and neurofibrillary tangles. AD is also accompanied by an extensive functional deficit in the cholinergic system, involving the neuronal-type nicotinic acetylcholine receptor (AChR). Furthermore there is increasing evidence showing a misregulation of cholesterol metabolism in the development of the disease. Since cholesterol affects AChR protein at multiple levels, the cognitive impairment and other neurological correlates of AD might be partly associated with an abnormal crosstalk between the receptor protein and the sterol in this synaptopathy.