Metabolic cholesterol depletion hinders cell-surface trafficking of the nicotinic acetylcholine receptor

Case Report: Neurological adverse events in subject with myasthenia gravis after PCSK9 inhibitor administration

Background

Myasthenia gravis is a rare chronic autoimmune neuromuscular disorder mainly caused by autoantibodies to the nicotinic acetylcholine receptor. Cholesterol is an essential molecule that affects the distribution and proper functioning of this receptor. Several reports have described the potential worsening of myasthenia gravis in patients treated with statins.

Case presentation

The patient was an obese 72 years old man, past smoker, diagnosed with ischaemic heart disease, type 2 diabetes mellitus and lipid metabolism disorder. Statin treatment was not implemented because of chronic myasthenia gravis and PCSK9i monotherapy [Repatha (evolucamab), 140 mg] was implemented to treat dyslipidaemia. Within 24 h after the first dose of PCSK9i the patient developed severe muscle weakness, joint pain, fever, and general discomfort, lasting for several days. Despite strong advice against the second dose administration, this was self-administered approximately 2 weeks later, leading to report significant worsening of the muscle problems, leading to the patient admittion to the neurology department where he was being treated for myasthenia gravis attack.

Conclusion

Based on the neurologist's conclusion, it can be assumed that in this case, treatment with PCSK9i resulted in significant worsening of the patient's chronic disease.

Carlos Gutiérrez-Merino: Synergy of Theory and Experimentation in Biological Membrane Research

Professor Carlos Gutiérrez-Merino, a prominent scientist working in the complex realm of biological membranes, has made significant theoretical and experimental contributions to the field. Contemporaneous with the development of the fluid-mosaic model of Singer and Nicolson, the Förster resonance energy transfer (FRET) approach has become an invaluable tool for studying molecular interactions in membranes, providing structural insights on a scale of 1-10 nm and remaining important alongside evolving perspectives on membrane structures. In the last few decades, Gutiérrez-Merino's work has covered multiple facets in the field of FRET, with his contributions producing significant advances in quantitative membrane biology. His more recent experimental work expanded the ground concepts of FRET to high-resolution cell imaging. Commencing in the late 1980s, a series of collaborations between Gutiérrez-Merino and the authors involved research visits and joint investigations focused on the nicotinic acetylcholine receptor and its relation to membrane lipids, fostering a lasting friendship.

Interactions between the Nicotinic and Endocannabinoid Receptors at the Plasma Membrane

Compartmentalization, together with transbilayer and lateral asymmetries, provide the structural foundation for functional specializations at the cell surface, including the active role of the lipid microenvironment in the modulation of membrane-bound proteins. The chemical synapse, the site where neurotransmitter-coded signals are decoded by neurotransmitter receptors, adds another layer of complexity to the plasma membrane architectural intricacy, mainly due to the need to accommodate a sizeable number of molecules in a minute subcellular compartment with dimensions barely reaching the micrometer. In this review, we discuss how nature has developed suitable adjustments to accommodate different types of membrane-bound receptors and scaffolding proteins via membrane microdomains, and how this "effort-sharing" mechanism has evolved to optimize crosstalk, separation, or coupling, where/when appropriate. We focus on a fast ligand-gated neurotransmitter receptor, the nicotinic acetylcholine receptor, and a second-messenger G-protein coupled receptor, the cannabinoid receptor, as a paradigmatic example.

Fluorescence Studies of Nicotinic Acetylcholine Receptor and Its Associated Lipid Milieu: The Influence of Erwin London's Methodological Approaches

Erwin London dedicated considerable effort to understanding lipid interactions with membrane-resident proteins and how these interactions shaped the formation and maintenance of lipid phases and domains. In this endeavor, he developed ad hoc techniques that greatly contributed to advancements in the field. We have employed and/or modified/extended some of his methodological approaches and applied them to investigate lipid interaction with the nicotinic acetylcholine receptor (nAChR) protein, the paradigm member of the superfamily of rapid pentameric ligand-gated ion channels (pLGIC). Our experimental systems ranged from purified receptor protein reconstituted into synthetic lipid membranes having known effects on receptor function, to cellular systems subjected to modification of their lipid content, e.g., varying cholesterol levels. We have often employed fluorescence techniques, including fluorescence quenching of diphenylhexatriene (DPH) extrinsic fluorescence and of nAChR intrinsic fluorescence by nitroxide spin-labeled phospholipids, DPH anisotropy, excimer formation of pyrene-phosphatidylcholine, and Förster resonance energy transfer (FRET) from the protein moiety to the extrinsic probes Laurdan, DPH, or pyrene-phospholipid to characterize various biophysical properties of lipid-receptor interactions. Some of these strategies are revisited in this review. Special attention is devoted to the anionic phospholipid phosphatidic acid (PA), which stabilizes the functional resting form of the nAChR. The receptor protein was shown to organize its PA-containing immediate microenvironment into microdomains with high lateral packing density and rigidity. PA and cholesterol appear to compete for the same binding sites on the nAChR protein.

Dysregulation of Neuronal Nicotinic Acetylcholine Receptor-Cholesterol Crosstalk in Autism Spectrum Disorder

Autism spectrum disorder (ASD) is a set of complex neurodevelopmental diseases that include impaired social interaction, delayed and disordered language, repetitive or stereotypic behavior, restricted range of interests, and altered sensory processing. The underlying causes of the core symptoms remain unclear, as are the factors that trigger their onset. Given the complexity and heterogeneity of the clinical phenotypes, a constellation of genetic, epigenetic, environmental, and immunological factors may be involved. The lack of appropriate biomarkers for the evaluation of neurodevelopmental disorders makes it difficult to assess the contribution of early alterations in neurochemical processes and neuroanatomical and neurodevelopmental factors to ASD. Abnormalities in the cholinergic system in various regions of the brain and cerebellum are observed in ASD, and recently altered cholesterol metabolism has been implicated at the initial stages of the disease. Given the multiple effects of the neutral lipid cholesterol on the paradigm rapid ligand-gated ion channel, the nicotinic acetylcholine receptor, we explore in this review the possibility that the dysregulation of nicotinic receptor-cholesterol crosstalk plays a role in some of the neurological alterations observed in ASD.

Nanoscale Sub-Compartmentalization of the Dendritic Spine Compartment

Compartmentalization of the membrane is essential for cells to perform highly specific tasks and spatially constrained biochemical functions in topographically defined areas. These membrane lateral heterogeneities range from nanoscopic dimensions, often involving only a few molecular constituents, to micron-sized mesoscopic domains resulting from the coalescence of nanodomains. Short-lived domains lasting for a few milliseconds coexist with more stable platforms lasting from minutes to days. This panoply of lateral domains subserves the great variety of demands of cell physiology, particularly high for those implicated in signaling. The dendritic spine, a subcellular structure of neurons at the receiving (postsynaptic) end of central nervous system excitatory synapses, exploits this compartmentalization principle. In its most frequent adult morphology, the mushroom-shaped spine harbors neurotransmitter receptors, enzymes, and scaffolding proteins tightly packed in a volume of a few femtoliters. In addition to constituting a mesoscopic lateral heterogeneity of the dendritic arborization, the dendritic spine postsynaptic membrane is further compartmentalized into spatially delimited nanodomains that execute separate functions in the synapse. This review discusses the functional relevance of compartmentalization and nanodomain organization in synaptic transmission and plasticity and exemplifies the importance of this parcelization in various neurotransmitter signaling systems operating at dendritic spines, using two fast ligand-gated ionotropic receptors, the nicotinic acetylcholine receptor and the glutamatergic receptor, and a second-messenger G-protein coupled receptor, the cannabinoid receptor, as paradigmatic examples.

Cholesterol in myasthenia gravis

The cholinergic neuromuscular junction is the paradigm peripheral synapse between a motor neuron nerve ending and a skeletal muscle fiber. In vertebrates, acetylcholine is released from the presynaptic site and binds to the nicotinic acetylcholine receptor at the postsynaptic membrane. A variety of pathologies among which myasthenia gravis stands out can impact on this rapid and efficient signaling mechanism, including autoimmune diseases affecting the nicotinic receptor or other synaptic proteins. Cholesterol is an essential component of biomembranes and is particularly rich at the postsynaptic membrane, where it interacts with and modulates many properties of the nicotinic receptor. The profound changes inflicted by myasthenia gravis on the postsynaptic membrane necessarily involve cholesterol. This review analyzes some aspects of myasthenia gravis pathophysiology and associated postsynaptic membrane dysfunction, including dysregulation of cholesterol metabolism in the myocyte brought about by antibody-receptor interactions. In addition, given the extensive therapeutic use of statins as the typical cholesterol-lowering drugs, we discuss their effects on skeletal muscle and the possible implications for MG patients under chronic treatment with this type of compound.

Lovastatin Differentially Regulates α7 and α4 Neuronal Nicotinic Acetylcholine Receptor Levels in Rat Hippocampal Neurons

Neuronal α7 and α4β2 are the predominant nicotinic acetylcholine receptor (nAChR) subtypes found in the brain, particularly in the hippocampus. The effects of lovastatin, an inhibitor of cholesterol biosynthesis, on these two nAChRs endogenously expressed in rat hippocampal neuronal cells were evaluated in the 0.01-1 µM range. Chronic (14 days) lovastatin treatment augmented cell-surface levels of α7 and α4 nAChRs, as measured by fluorescence microscopy and radioactive ligand binding assays. This was accompanied in both cases by an increase in total protein receptor levels as determined by Western blots. At low lovastatin concentrations (10-100 nM), the increase in α4 nAChR in neurites was higher than in neuronal cell somata; the opposite occurred at higher (0.5-1 µM) lovastatin concentrations. In contrast, neurite α7 nAChRs raised more than somatic α7 nAChRs at all lovastatin concentrations tested. These results indicate that cholesterol levels homeostatically regulate α7 and α4 nAChR levels in a differential manner through mechanisms that depend on statin concentration and receptor localization. The neuroprotective pleomorphic effects of statins may act by reestablishing the homeostatic equilibrium.

Alzheimer's Disease as a Membrane Disorder: Spatial Cross-Talk Among Beta-Amyloid Peptides, Nicotinic Acetylcholine Receptors and Lipid Rafts

Biological membranes show lateral and transverse asymmetric lipid distribution. Cholesterol (Chol) localizes in both hemilayers, but in the external one it is mostly condensed in lipid-ordered microdomains (raft domains), together with saturated phosphatidyl lipids and sphingolipids (including sphingomyelin and glycosphingolipids). Membrane asymmetries induce special membrane biophysical properties and behave as signals for several physiological and/or pathological processes. Alzheimer’s disease (AD) is associated with a perturbation in different membrane properties. Amyloid-β (Aβ) plaques and neurofibrillary tangles of tau protein together with neuroinflammation and neurodegeneration are the most characteristic cellular changes observed in this disease. The extracellular presence of Aβ peptides forming senile plaques, together with soluble oligomeric species of Aβ, are considered the major cause of the synaptic dysfunction of AD. The association between Aβ peptide and membrane lipids has been extensively studied. It has been postulated that Chol content and Chol distribution condition Aβ production and posterior accumulation in membranes and, hence, cell dysfunction. Several lines of evidence suggest that Aβ partitions in the cell membrane accumulate mostly in raft domains, the site where the cleavage of the precursor AβPP by β- and γ- secretase is also thought to occur. The main consequence of the pathogenesis of AD is the disruption of the cholinergic pathways in the cerebral cortex and in the basal forebrain. In parallel, the nicotinic acetylcholine receptor has been extensively linked to membrane properties. Since its transmembrane domain exhibits extensive contacts with the surrounding lipids, the acetylcholine receptor function is conditioned by its lipid microenvironment. The nicotinic acetylcholine receptor is present in high-density clusters in the cell membrane where it localizes mainly in lipid-ordered domains. Perturbations of sphingomyelin or cholesterol composition alter acetylcholine receptor location. Therefore, Aβ processing, Aβ partitioning, and acetylcholine receptor location and function can be manipulated by changes in membrane lipid biophysics. Understanding these mechanisms should provide insights into new therapeutic strategies for prevention and/or treatment of AD. Here, we discuss the implications of lipid-protein interactions at the cell membrane level in AD.

General anesthetics cause mitochondrial dysfunction and reduction of intracellular ATP levels

General anesthetics are indispensable for effective clinical care. Although, the mechanism of action of general anesthetics remains controversial, lipid bilayers and proteins have been discussed as their targets. In this study, we focused on the relationship between cellular ATP levels and general anesthetics. The ATP levels of nematodes and cultured mammalian cells were decreased by exposure to three general anesthetics: isoflurane, pentobarbital, and 1-phenoxy-2-propanol. Furthermore, these general anesthetics abolished mitochondrial membrane potential, resulting in the inhibition of mitochondrial ATP synthesis. These results suggest that the observed decrease of cellular ATP level is a common phenomenon of general anesthetics.

The lipid habitats of neurotransmitter receptors in brain

Neurotransmitter receptors, the macromolecules specialized in decoding the chemical signals encrypted in the chemical signaling mechanism in the nervous system, occur either at the somatic cell surface of chemically excitable cells or at specialized subcellular structures, the synapses. Synapses have lipid compositions distinct from the rest of the cell membrane, suggesting that neurotransmitter receptors and their scaffolding and adaptor protein partners require specific lipid habitats for optimal operation. In this review we discuss some paradigmatic cases of neurotransmitter receptor-lipid interactions, highlighting the chemical nature of the intervening lipid species and providing examples of the receptor mechanisms affected by interaction with lipids. The focus is on the effects of cholesterol, glycerophospholipids and covalent fatty acid acylation on neurotransmitter receptors. We also briefly discuss the role of lipid phase states involving lateral heterogeneities of the host membrane known to modulate membrane transport, protein sorting and signaling. Modulation of neurotransmitter receptors by lipids occurs at multiple levels, affecting a wide span of activities including their trafficking, sorting, stability, residence lifetime at the cell surface, endocytosis, and recycling, among other important functional properties at the synapse.

Diacylglycerol levels modulate the cellular distribution of the nicotinic acetylcholine receptor

Diacylglycerol (DAG), a second messenger involved in different cell signaling cascades, activates protein kinase C (PKC) and D (PKD), among other kinases. The present work analyzes the effects resulting from the alteration of DAG levels on neuronal and muscle nicotinic acetylcholine receptor (AChR) distribution. We employ CHO-K1/A5 cells, expressing adult muscle-type AChR in a stable manner, and hippocampal neurons, which endogenously express various subtypes of neuronal AChR. CHO-K1/A5 cells treated with dioctanoylglycerol (DOG) for different periods showed augmented AChR cell surface levels at short incubation times (30min-4h) whereas at longer times (18h) the AChR was shifted to intracellular compartments. Similarly, in cultured hippocampal neurons surface AChR levels increased as a result of DOG incubation for 4h. Inhibition of endogenous DAG catabolism produced changes in AChR distribution similar to those induced by DOG treatment. Specific enzyme inhibitors and Western blot assays revealed that DAGs exert their effect on AChR distribution through the modulation of the activity of classical PKC (cPKC), novel PKC (nPKC) and PKD activity.

Cytoplasmic domain of δ subunit is important for the extra-synaptic targeting of GABAA receptor subtypes

GABA(A) receptors (GABA(A)Rs) are hetero-pentameric chloride channels and the primary sites for fast synaptic inhibition. We have expressed recombinant γ2 and δ subunits of GABA(A)Rs in cultured hippocampal neurons to analyze the membrane targeting of synaptic and extra-synaptic GABA(A)Rs, a phenomenon not well understood. Our data demonstrate that the synaptic targeting of γ2-containing GABA(A)Rs (γ2-GABA(A)Rs) does not depend on the cytoplasmic loop of γ2 subunit, in parallel with previous findings, showing that the synaptic localization of γ2-GABA(A)Rs requires the TM4 domain of γ2 rather than the large cytoplasmic loop. On the other hand, we showed here that the extrasynaptic targeting of the δ-containing GABA(A)Rs (δ-GABA(A)Rs) depends on the cytoplasmic loop of δ subunit via an active or a passive mechanism. We also show that the amino acid sequences of δ loop is highly conserved across the whole span of vertebrate evolution suggesting an active role of δ loop in extra-synaptic targeting of corresponding receptor subtypes.

Phylogenetic conservation of protein-lipid motifs in pentameric ligand-gated ion channels

Using the crosstalk between the nicotinic acetylcholine receptor (nAChR) and its lipid microenvironment as a paradigm, this short overview analyzes the occurrence of structural motifs which appear not only to be conserved within the nAChR family and contemporary eukaryotic members of the pentameric ligand-gated ion channel (pLGIC) superfamily, but also extend to prokaryotic homologues found in bacteria. The evolutionarily conserved design is manifested in: 1) the concentric three-ring architecture of the transmembrane region, 2) the occurrence in this region of distinct lipid consensus motifs in prokaryotic and eukaryotic pLGIC and 3) the key participation of the outer TM4 ring in conveying the influence of the lipid membrane environment to the middle TM1-TM3 ring and this, in turn, to the inner TM2 channel-lining ring, which determines the ion selectivity of the channel. The preservation of these constant structural-functional features throughout such a long phylogenetic span likely points to the successful gain-of-function conferred by their early acquisition. This article is part of a Special Issue entitled: Lipid-protein interactions.

A novel therapeutic effect of statins on nephrogenic diabetes insipidus

Statins competitively inhibit hepatic 3-hydroxy-3-methylglutaryl-coenzyme A reductase, resulting in reduced plasma total and low-density lipoprotein cholesterol levels. Recently, it has been shown that statins exert additional ‘pleiotropic’ effects by increasing expression levels of the membrane water channels aquaporin 2 (AQP2). AQP2 is localized mainly in the kidney and plays a critical role in determining cellular water content. This additional effect is independent of cholesterol homoeostasis, and depends on depletion of mevalonate-derived intermediates of sterol synthetic pathways, i.e. farnesylpyrophosphate and geranylgeranylpyrophosphate. By up-regulating the expression levels of AQP2, statins increase water reabsorption by the kidney, thus opening up a new avenue in treating patients with nephrogenic diabetes insipidus (NDI), a hereditary disease that yet lacks high-powered and limited side effects therapy. Aspects related to water balance determined by AQP2 in the kidney, as well as standard and novel therapeutic strategies of NDI are discussed.

Cell-surface translational dynamics of nicotinic acetylcholine receptors

Synapse efficacy heavily relies on the number of neurotransmitter receptors available at a given time. In addition to the equilibrium between the biosynthetic production, exocytic delivery and recycling of receptors on the one hand, and the endocytic internalization on the other, lateral diffusion and clustering of receptors at the cell membrane play key roles in determining the amount of active receptors at the synapse. Mobile receptors traffic between reservoir compartments and the synapse by thermally driven Brownian motion, and become immobilized at the peri-synaptic region or the synapse by: (a) clustering mediated by homotropic inter-molecular receptor–receptor associations; (b) heterotropic associations with non-receptor scaffolding proteins or the subjacent cytoskeletal meshwork, leading to diffusional “trapping,” and (c) protein-lipid interactions, particularly with the neutral lipid cholesterol. This review assesses the contribution of some of these mechanisms to the supramolecular organization and dynamics of the paradigm neurotransmitter receptor of muscle and neuronal cells -the nicotinic acetylcholine receptor (nAChR). Currently available information stemming from various complementary biophysical techniques commonly used to interrogate the dynamics of cell-surface components is critically discussed. The translational mobility of nAChRs at the cell surface differs between muscle and neuronal receptors in terms of diffusion coefficients and residence intervals at the synapse, which cover an ample range of time regimes. A peculiar feature of brain α7 nAChR is its ability to spend much of its time confined peri-synaptically, vicinal to glutamatergic (excitatory) and GABAergic (inhibitory) synapses. An important function of the α7 nAChR may thus be visiting the territories of other neurotransmitter receptors, differentially regulating the dynamic equilibrium between excitation and inhibition, depending on its residence time in each domain.

Transient cholesterol effects on nicotinic acetylcholine receptor cell-surface mobility

To what extent do cholesterol-rich lipid platforms modulate the supramolecular organization of the nicotinic acetylcholine receptor (AChR)? To address this question, the dynamics of AChR particles at high density and its cholesterol dependence at the surface of mammalian cells were studied by combining total internal reflection fluorescence microscopy and single-particle tracking. AChR particles tagged with a monovalent ligand, fluorescent α-bungarotoxin (αBTX), exhibited two mobile pools: i) a highly mobile one undergoing simple Brownian motion (16%) and ii) one with restricted motion (∼50%), the rest being relatively immobile (∼44%). Depletion of membrane cholesterol by methyl-α-cyclodextrin increased the fraction of the first pool to 22% and 33% after 15 and 40 min, respectively; the pool undergoing restricted motion diminished from 50% to 44% and 37%, respectively. Monoclonal antibody binding results in AChR crosslinking-internalization after 2 h; here, antibody binding immobilized within minutes ∼20% of the totally mobile AChR. This proportion dramatically increased upon cholesterol depletion, especially during the initial 10 min (83.3%). Thus, antibody crosslinking and cholesterol depletion exhibited a mutually synergistic effect, increasing the average lifetime of cell-surface AChRs∼10 s to ∼20 s. The instantaneous (microscopic) diffusion coefficient D2-4 of the AChR obtained from the MSD analysis diminished from ∼0.001 µm2 s(-1) to ∼0.0001-0.00033 µm2 s(-1) upon cholesterol depletion, ∼30% of all particles falling into the stationary mode. Thus, muscle-type AChR exhibits heterogeneous motional regimes at the cell surface, modulated by the combination of intrinsic (its supramolecular organization) and extrinsic (membrane cholesterol content) factors.

Synaptic membrane rafts: traffic lights for local neurotrophin signaling?

Lipid rafts, cholesterol and lipid rich microdomains, are believed to play important roles as platforms for the partitioning of transmembrane and synaptic proteins involved in synaptic signaling, plasticity, and maintenance. There is increasing evidence of a physical interaction between post-synaptic densities and post-synaptic lipid rafts. Localization of proteins within lipid rafts is highly regulated, and therefore lipid rafts may function as traffic lights modulating and fine-tuning neuronal signaling. The tyrosine kinase neurotrophin receptors (Trk) and the low-affinity p75 neurotrophin receptor (p75^NTR) are enriched in neuronal lipid rafts together with the intermediates of downstream signaling pathways, suggesting a possible role of rafts in neurotrophin signaling. Moreover, neurotrophins and their receptors are involved in the regulation of cholesterol metabolism. Cholesterol is an important component of lipid rafts and its depletion leads to gradual loss of synapses, underscoring the importance of lipid rafts for proper neuronal function. Here, we review and discuss the idea that translocation of neurotrophin receptors in synaptic rafts may account for the selectivity of their transduced signals.

Cholesterol depletion modulates detergent resistant fraction of human serotonin(1A) receptors

Insolubility of membrane components in non-ionic detergents such as Triton X-100 at low temperature is a widely used biochemical criterion to identify, isolate and characterize membrane domains. In this work, we monitored the detergent insolubility of the serotonin(1A) receptor in CHO cell membranes and its modulation by membrane cholesterol. The serotonin(1A) receptor is an important member of the G-protein coupled receptor family. It is implicated in the generation and modulation of various cognitive, behavioral and developmental functions and serves as a drug target. Our results show that a significant fraction (∼28%) of the serotonin(1A) receptor resides in detergent-resistant membranes (DRMs). Interestingly, the fraction of the serotonin(1A) receptor in DRMs exhibits a reduction upon membrane cholesterol depletion. In addition, we show that contents of DRM markers such as flotillin-1, caveolin-1 and GM₁ are altered in DRMs upon cholesterol depletion. These results assume significance since the function of the serotonin(1A) receptor has previously been shown to be affected by membrane lipids, specifically cholesterol. Our results are relevant in the context of membrane organization of the serotonin(1A) receptor in particular, and G-protein coupled receptors in general.

Simvastatin enhances aquaporin-2 surface expression and urinary concentration in vasopressin-deficient Brattleboro rats through modulation of Rho GTPase

Statins are 3-hydroxyl-3-methyglutaryl-CoA reductase inhibitors that are commonly used to inhibit cholesterol biosynthesis. Emerging data have suggested that they also have "pleotropic effects," including modulating actin cytoskeleton reorganization. Here, we report an effect of simvastatin on the trafficking of aquaporin-2 (AQP2). Specifically, simvastatin induced the membrane accumulation of AQP2 in cell cultures and kidneys in situ. The effect of simvastatin was independent of protein kinase A activation and phosphorylation at AQP2-Ser(256), a critical event involved in vasopressin (VP)-regulated AQP2 trafficking. Further investigation showed that simvastatin inhibited endocytosis in parallel with downregulation of RhoA activity. Overexpression of active RhoA attenuated simvastatin's effect, suggesting the involvement of this small GTPase in simvastatin-mediated AQP2 trafficking. Finally, the effect of simvastatin on urinary concentration was investigated in VP-deficient Brattleboro rats. Simvastatin acutely (3-6 h) increased urinary concentration and decreased urine output in these animals. In summary, simvastatin regulates AQP2 trafficking in vitro and urinary concentration in vivo via events involving downregulation of Rho GTPase activity and inhibition of endocytosis. Our study provides an alternative mechanism to regulate AQP2 trafficking, bypassing the VP-vasopressin receptor signaling pathway.

Membrane organization and dynamics of the serotonin1A receptor in live cells

The G-protein coupled receptor (GPCR) superfamily is one of the largest classes of molecules involved in signal transduction across the plasma membrane. The serotonin(1A) receptor is a representative member of the GPCR superfamily and serves as an important target in the development of therapeutic agents for neuropsychiatric disorders such as anxiety and depression. In the context of the pharmacological relevance of the serotonin(1A) receptor, the membrane organization and dynamics of this receptor in the cellular environment assume relevance. We have highlighted results, obtained from fluorescence microscopy-based approaches, related to domain organization and dynamics of the serotonin(1A) receptor. A fraction of serotonin(1A) receptors displays detergent insolubility, monitored using green fluorescent protein, that increases upon depletion of membrane cholesterol. Fluorescence recovery after photobleaching measurements with varying bleach spot sizes show that lateral diffusion parameters of serotonin(1A) receptors in normal cells are consistent with models describing diffusion of molecules in a homogenous membrane. Interestingly, these characteristics are altered in cholesterol-depleted cells. Taken together, we conclude that the serotonin(1A) receptor exhibits dynamic confinement in the cellular plasma membranes. Progress in understanding GPCR organization and dynamics would result in better insight into our overall understanding of GPCR function in health and disease.

Detergent-free isolation and characterization of cholesterol-rich membrane domains from trans-Golgi network vesicles

Cholesterol is an abundant lipid of the trans-Golgi network (TGN) and of certain endosomal membranes where cholesterol-rich microdomains are important in the organization and compartmentalization of vesicular trafficking. Here we describe the development of a rapid method to isolate a cholesterol-rich endomembrane fraction. We show that widely used subcellular fractionation techniques incompletely separate cholesterol-rich membranes, such as the TGN, from organelles, such as late endosomes and lysosomes. To address this issue, we devised a new subcellular fractionation scheme involving two rounds of velocity centrifugation, membrane sonication, and discontinuous sucrose density gradient centrifugation. This strategy resulted in the isolation of a cholesterol and GM1 glycosphingolipid-enriched membrane fraction that was completely cleared of plasma membrane, endoplasmic reticulum, and mitochondria. This buoyant fraction was enriched for the TGN and recycling endosome proteins Rab11 and syntaxin-6, and it was well resolved from cis-Golgi and early and late endosomal membranes. We demonstrate that this technique can give useful insights into the compartmentation of phosphoinositide synthesis, and it facilitates the isolation of cholesterol-rich membranes from a population of TGN-trafficking vesicles.

Material properties of lipid microdomains: force-volume imaging study of the effect of cholesterol on lipid microdomain rigidity

The effect of cholesterol (CHOL) on the material properties of supported lipid bilayers composed of lipid mixtures that mimic the composition of lipid microdomains was studied by force-volume (FV) imaging under near-physiological conditions. These studies were carried out with lipid mixtures of dioleoylphosphatidylcholine, dioleoylphosphatidylserine, and sphingomyelin. FV imaging enabled simultaneous topology and force measurements of sphingomyelin-rich domains (higher domain (HD)) and phospholipid-rich domains (lower domain (LD)), which allowed quantitative measurement of the force needed to puncture the lipid bilayer with or without CHOL. The force required to penetrate the various domains of the bilayer was probed using high- and low-ionic-strength buffers as a function of increasing amounts of CHOL in the bilayer. The progressive addition of CHOL also led to a decreasing height difference between HD and LD. FV imaging further demonstrated a lack of adhesion between the atomic force microscope tip and the HD or LD at loads below the breakthrough force. These results can lead to a better understanding of the role that CHOL plays in the mechanical properties of cellular membranes in modulating membrane rigidity, which has important implications for cellular mechanotransduction.

Cholesterol effects on nicotinic acetylcholine receptor: cellular aspects

Cholesterol is an essential partner of the nicotinic acetylcholine receptor (AChR). It is not only an abundant component of the postsynaptic membrane but also affects the stability of the receptor protein in the membrane, its supramolecular organization and function. In the absence of innervation, early on in ontogenetic development of the muscle cell, embryonic AChRs occur in the form of diffusely dispersed molecules. At embryonic day 13, receptors organize in the form of small aggregates. This organization can be mimicked in mammalian cells in culture.Trafficking to the plasmalemma is a cholesterol-dependent process. Receptors acquire association with the sterol as early as the endoplasmic reticulum and the Golgi apparatus. Once AChRs reach the cell surface, their stability is also highly dependent on cholesterol levels. Acute cholesterol depletion reduces the number of receptor domains by accelerating the rate of endocytosis. In muscle cells, AChRs are internalized via a recently discovered dynamin- and clathrin-independent, cytoskeleton-dependent endocytic mechanism. Unlike other endocytic pathways, cholesterol depletion accelerates internalization and re-routes AChR endocytosis to an Arf6-dependent pathway. Cholesterol depletion also results in ion channel gain-of-function of the remaining cell-surface AChRs, whereas cholesterol enrichment has the opposite effect.Wide-field microscopy shows AChR clusters as diffraction-limited puncta of approximately 200 nm diameter. Stimulated emission depletion (STED) fluorescence microscopy resolves these puncta into nanoclusters with an average diameter of approximately 55 nm. Exploiting the enhanced resolution, the effect of acute cholesterol depletion can be shown to alter the short- and long-range organization of AChR nanoclusters. In the short range, AChRs form bigger nanoclusters. On larger scales (0.5-3.5 mum) nanocluster distribution becomes non-random, attributable to the cholesterol-related abolition of cytoskeletal physical barriers normally preventing the lateral diffusion of AChR nanoclusters. The dependence of AChR numbers at the cell surface on membrane cholesterol raises the possibility that cholesterol depletion leads to AChR conformational changes that alter its stability and its long-range dynamic association with other AChR nanoclusters, accelerate its endocytosis, and transiently affect the channel kinetics of those receptors remaining at the surface. Cholesterol content at the plasmalemma may thus homeostatically modulate AChR dynamics, cell-surface organization and lifetime of receptor nanodomains, and fine tune the ion permeation process.

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.

Cholesterol modulation of nicotinic acetylcholine receptor surface mobility

Nicotinic acetylcholine receptor (AChR) function and distribution are quite sensitive to cholesterol (Chol) levels in the plasma membrane (reviewed by Barrantes in J Neurochem 103 (suppl 1):72-80, 2007). Here we combined confocal fluorescence recovery after photobleaching (FRAP) and confocal fluorescence correlation spectroscopy (FCS) to examine the mobility of the AChR and its dependence on Chol content at the cell surface of a mammalian cell line. Plasma membrane AChR exhibited limited mobility and only ~55% of the fluorescence was recovered within 10 min after photobleaching. Depletion of membrane Chol by methyl-beta-cyclodextrin strongly affected the mobility of the AChR at the plasma membrane; the fraction of mobile AChR fell from 55 to 20% in Chol-depleted cells, whereas Chol enrichment by methyl-beta-cyclodextrin-Chol treatment did not reduce receptor mobility at the cell surface. Actin depolymerization caused by latrunculin A partially restored receptor mobility in Chol-depleted cells. In agreement with the FRAP data, scanning FCS experiments showed that the diffusion coefficient of the AChR was about 30% lower upon Chol depletion. Taken together, these results suggest that membrane Chol modulates AChR mobility at the plasma membrane through a Chol-dependent mechanism sensitive to cortical actin.

Ric-3 chaperone-mediated stable cell-surface expression of the neuronal alpha7 nicotinic acetylcholine receptor in mammalian cells

AIM

Studies of the alpha7-type neuronal nicotinic acetylcholine receptor (AChR), one of the receptor forms involved in many physiologically relevant processes in the central nervous system, have been hampered by the inability of this homomeric protein to assemble in most heterologous expression systems. In a recent study, it was shown that the chaperone Ric-3 is necessary for the maturation and functional expression of alpha7-type AChRs(1). The current work aims at obtaining and characterizing a cell line with high functional expression of the human alpha7 AChR.

METHODS

Ric-3 cDNA was incorporated into SHE-P1-halpha7 cells expressing the alpha7-type AChR. Functional studies were undertaken using single-channel patch-clamp recordings. Equilibrium and kinetic [(125)I]alpha-bungarotoxin binding assays, as well as fluorescence microscopy using fluorescent alpha-bungarotoxin, anti-alpha7 antibody, and GFP-alpha7 were performed on the new clone.

RESULTS

The human alpha7-type AChR was stably expressed in a new cell line, which we coined SHE-P1-halpha7-Ric-3, by co-expression of the chaperone Ric-3. Cell-surface AChRs exhibited [(125)I]alphaBTX saturable binding with an apparent K(D) of about 55 nmol/L. Fluorescence microscopy revealed dispersed and micro-clustered AChR aggregates at the surface of SHE-P1-halpha7-Ric-3 cells. Larger micron-sized clusters were observed in the absence of receptor-clustering proteins or upon aggregation with anti-alpha7 antibodies. In contrast, chaperone-less SHE-P1-halpha7 cells expressed only intracellular alpha7 AChRs and failed to produce detectable single-channel currents.

CONCLUSION

The production of a stable and functional cell line of neuroepithelial lineage with robust cell-surface expression of neuronal alpha7-type AChR, as reported here, constitutes an important advance in the study of homomeric receptors in mammalian cells.

U18666A, a cholesterol-inhibition agent, modulates human neuronal nicotinic acetylcholine receptors heterologously expressed in SH-EP1 cell line

In this study, we evaluate the effects of (3beta)-3-[2-(diethylamino)ethoxy]androst-5-en-17-one dihydrochloride (U18666A), a cholesterol synthesis/transporter inhibitor, on selected human neuronal nicotinic acetylcholine receptors (nAChRs) heterologously expressed in the SH-EP1 cell line using whole-cell patch-clamp recordings. The results indicate that with 2-min pretreatment, U18666A inhibited different nAChR subtypes with a rank-order of potency (IC(50) of whole-cell peak current): alpha4beta2 (8.0 +/- 3.0 nM) > alpha3beta2 (1.7 +/- 0.4 microM) > alpha4beta4 (26 +/- 7.2 microM) > alpha7 (> 100 microM), suggesting this compound is more selective to alpha4beta2-nAChRs. Thus, the pharmacological profiles and mechanisms of U18666A acting on alpha4beta2-nAChRs were investigated in detail. U18666A suppresses both peak and steady state components of whole-cell currents mediated by human alpha4beta2-nAChRs in response to nicotine. In nicotine-induced concentration-response curves, U18666A reduces nicotine-induced current at maximally effective agonist concentrations without influencing nicotine's EC(50) value, suggesting a non-competitive inhibition. U18666A-induced inhibition of nAChR function is concentration-, voltage-, and use-dependent, suggesting an open channel block. Taken into consideration of approximately 10 000-fold enhancement of the potency of U18666A after 2-min pre-treatment, this compound also likely inhibits alpha4beta2-nAChRs through a close channel block. In addition, the U18666A-induced inhibition in alpha4beta2-nAChRs is not mediated by either increased receptor endocytosis or altered cell cholesterol. These data indicate that U18666A is a potent antagonist of alpha4beta2-nAChRs and may be useful as a tool in the functional characterization and pharmacological profiling of nAChRs, as well as a potential candidate for smoking cessation.

Inhibition of cholesterol biosynthesis impairs insulin secretion and voltage-gated calcium channel function in pancreatic beta-cells

Insulin secretion from pancreatic beta-cells is mediated by the opening of voltage-gated Ca2+ channels (CaV) and exocytosis of insulin dense core vesicles facilitated by the secretory soluble N-ethylmaleimide-sensitive factor attachment protein receptor protein machinery. We previously observed that beta-cell exocytosis is sensitive to the acute removal of membrane cholesterol. However, less is known about the chronic changes in endogenous cholesterol and its biosynthesis in regulating beta-cell stimulus-secretion coupling. We examined the effects of inhibiting endogenous beta-cell cholesterol biosynthesis by using the squalene epoxidase inhibitor, NB598. The expression of squalene epoxidase in primary and clonal beta-cells was confirmed by RT-PCR. Cholesterol reduction of 36-52% was observed in MIN6 cells, mouse and human pancreatic islets after a 48-h incubation with 10 mum NB598. A similar reduction in cholesterol was observed in the subcellular compartments of MIN6 cells. We found NB598 significantly inhibited both basal and glucose-stimulated insulin secretion from mouse pancreatic islets. CaV channels were markedly inhibited by NB598. Rapid photolytic release of intracellular caged Ca2+ and simultaneous measurements of the changes in membrane capacitance revealed that NB598 also inhibited exocytosis independently from CaV channels. These effects were reversed by cholesterol repletion. Our results indicate that endogenous cholesterol in pancreatic beta-cells plays a critical role in regulating insulin secretion. Moreover, chronic inhibition of cholesterol biosynthesis regulates the functional activity of CaV channels and insulin secretory granule mobilization and membrane fusion. Dysregulation of cellular cholesterol may cause impairment of beta-cell function, a possible pathogenesis leading to the development of type 2 diabetes.

Ceramides modulate cell-surface acetylcholine receptor levels

The effects of ceramides (Cer) on the trafficking of the nicotinic acetylcholine receptor (AChR) to the plasma membrane were studied in CHO-K1/A5 cells, a clonal cell line that heterologously expresses the adult murine form of the receptor. When cells were incubated with short- (C6-Cer) or long- (brain-Cer) chain Cer at low concentrations, an increase in the number of cell-surface AChRs was observed concomitant with a decrease in intracellular receptor levels. The alteration in AChR distribution by low Cer treatment does not appear to be a general mechanism since the surface expression of the green fluorescent protein derivative of the vesicular stomatitis virus protein (VSVG-GFP) was not affected. High Cer concentrations caused the opposite effects, decreasing the number of cell-surface AChRs, which exhibited higher affinity for [125I]-alpha-bungarotoxin, and increasing the intracellular pool, which colocalized with trans-Golgi/TGN specific markers. The generation of endogenous Cer by sphingomyelinase treatment also decreased cell-surface AChR levels. These effects do not involve protein kinase C zeta or protein phosphatase 2A activation. Taken together, the results indicate that Cer modulate trafficking of AChRs to and stability at the cell surface.

Charged amino acid motifs flanking each extreme of the alphaM4 transmembrane domain are involved in assembly and cell-surface targeting of the muscle nicotinic acetylcholine receptor

The alphaM4 transmembrane domain of the nicotinic acetylcholine receptor (AChR) is flanked by two basic amino acids (His(408) and Arg(429)) located at its cytoplasmic- and extracellular-facing extremes, respectively, at the level of the phospholipid polar head regions of the postsynaptic membrane. A series of single and double alphaM4 mutants (His(408)Ala, Arg(429)Ala, Arg(429)Glu, His(408)Ala/Arg(429)Ala, and His(408)Ala/Arg(429)Glu) of the adult muscle-type AChR were produced and coexpressed with wild-type beta, delta, and epsilon subunits as stable clones in a mammalian heterologous expression system (CHO-K1 cells). The mutants were studied by alpha-bungarotoxin ([(125)I]alpha-BTX) binding, fluorescence microscopy, and equilibrium sucrose gradient centrifugation. Cell-surface [(125)I]alpha-BTX binding diminished approximately 40% in His(408)Ala and as much as 95% in the Arg(429)Ala mutant. Reversing the amino acid charge (e.g., Arg(429)Glu) abolished cell-surface expression of AChR. Fluorescence microscopy disclosed that AChR was retained at the endoplasmic reticulum, with an enhanced occurrence of unassembled AChR species in the mutant clones. Centrifugation analysis confirmed the lack of fully assembled AChR pentamers in all mutants with the exception of His(408)Ala. We conclude that His(408) and Arg(429) in alphaM4 are involved in assembly and cell-surface targeting of muscle AChR. Arg(429) plays a more decisive role in these two processes, suggesting an asymmetric weight of the charged motifs at each extreme of the alpha subunit M4 transmembrane segment. (c) 2006 Wiley-Liss, Inc.

Sphingolipids are necessary for nicotinic acetylcholine receptor export in the early secretory pathway

The nicotinic acetylcholine receptor (AChR) is the prototype ligand-gated ion channel, and its function is dependent on its lipid environment. In order to study the involvement of sphingolipids (SL) in AChR trafficking, we used pharmacological approaches to dissect the SL biosynthetic pathway in CHO-K1/A5 cells heterologously expressing the muscle-type AChR. When SL biosynthesis was impaired, the cell surface targeting of AChR diminished with a concomitant increase in the intracellular receptor pool. The SL-inhibiting drugs increased unassembled AChR forms, which were retained at the endoplasmic reticulum (ER). These effects on AChR biogenesis and trafficking could be reversed by the addition of exogenous SL, such as sphingomyelin. On the basis of these effects we propose a 'chaperone-like' SL intervention at early stages of the AChR biosynthetic pathway, affecting both the efficiency of the assembly process and subsequent receptor trafficking to the cell surface.

Cholesterol depletion induces dynamic confinement of the G-protein coupled serotonin(1A) receptor in the plasma membrane of living cells

Cholesterol is an essential constituent of eukaryotic membranes and plays a crucial role in membrane organization, dynamics, function, and sorting. It is often found distributed non-randomly in domains or pools in biological and model membranes and is thought to contribute to a segregated distribution of membrane constituents. Signal transduction events mediated by seven transmembrane domain G-protein coupled receptors (GPCRs) are the primary means by which cells communicate with and respond to their external environment. We analyzed the role of cholesterol in the plasma membrane organization of the G-protein coupled serotonin(1A) receptor by fluorescence recovery after photobleaching (FRAP) measurements with varying bleach spot sizes. Our results show that lateral diffusion parameters of serotonin(1A) receptors in normal cells are consistent with models describing diffusion of molecules in a homogenous membrane. Interestingly, these characteristics are altered in cholesterol-depleted cells in a manner that is consistent with dynamic confinement of serotonin(1A) receptors in the plasma membrane. Importantly, analysis of ligand binding and downstream signaling of the serotonin(1A) receptor suggests that receptor function is affected in a significantly different manner when intact cells or isolated membranes are depleted of cholesterol. These results assume significance in the context of interpreting effects of cholesterol depletion on diffusion characteristics of membrane proteins in particular, and cholesterol-dependent cellular processes in general.

Lipid raft microdomains and neurotransmitter signalling

Lipid rafts are specialized structures on the plasma membrane that have an altered lipid composition as well as links to the cytoskeleton. It has been proposed that these structures are membrane domains in which neurotransmitter signalling might occur through a clustering of receptors and components of receptor-activated signalling cascades. The localization of these proteins in lipid rafts, which is affected by the cytoskeleton, also influences the potency and efficacy of neurotransmitter receptors and transporters. The effect of lipid rafts on neurotransmitter signalling has also been implicated in neurological and psychiatric diseases.

Agrin elicits membrane lipid condensation at sites of acetylcholine receptor clusters in C2C12 myotubes

The formation of the neuromuscular junction is characterized by the progressive accumulation of nicotinic acetylcholine receptors (AChRs) in the postsynaptic membrane facing the nerve terminal, induced predominantly through the agrin/muscle-specific kinase (MuSK) signaling cascade. However, the cellular mechanisms linking MuSK activation to AChR clustering are still poorly understood. Here, we investigate whether lipid rafts are involved in agrin-elicited AChR clustering in a mouse C2C12 cell line. We observed that in C2C12 myotubes, both AChR clustering and cluster stability were dependent on cholesterol, because depletion by methyl-beta-cyclodextrin inhibited cluster formation or dispersed established clusters. Importantly, AChR clusters resided in ordered membrane domains, a biophysical property of rafts, as probed by Laurdan two-photon fluorescence microscopy. We isolated detergent-resistant membranes (DRMs) by three different biochemical procedures, all of which generate membranes with similar cholesterol/GM1 ganglioside contents, and these were enriched in several postsynaptic components, notably AChR, syntrophin, and raft markers flotillin-2 and caveolin-3. Agrin did not recruit AChRs into DRMs, suggesting that they are present in rafts independently of agrin activation. Consequently, in C2C12 myotubes, agrin likely triggers AChR clustering or maintains clusters through the coalescence of lipid rafts. These data led us to propose a model in which lipid rafts play a pivotal role in the assembly of the postsynaptic membrane at the neuromuscular junction upon agrin signaling.

Influence of cholesterol and lovastatin on alpha-form of secreted amyloid precursor protein and expression of alpha7 nicotinic receptor on astrocytes

The influence of cholesterol and the lovastatin (cholesterol-lowering drug) on secretion of alpha-secretase cleavage product of amyloid precursor protein (APP) and expression of nicotinic acetylcholine receptors (nAChRs) was investigated in human HTB-15 astrocytes. The results showed that exposure of cholesterol to astrocytes inhibited the secretion of alpha-form of secreted APP (alphaAPPs) and reduced cell viability, while lovastatin enhanced the alpha-secretase processing on astrocytes; cholesterol treatment decreased expression of alpha7 nAChR, whereas lovastatin induced an up-regulation of the receptor; the increase in alphaAPPs resulted from lovastatin was partially inhibited by the alpha7 nAChR antagonists, alpha-bungarotoxin or methyllycaconitine; cholesterol or lovastatin did not influence either whole APP level or expression of alpha4 nAChR. We suggest that high dose of cholesterol may inhibit both the activity of alpha-secretase in APP metabolic processing and the expression of alpha7 nAChR, while lovastatin may stimulate alpha-secretase cleavage processing that might be regulated by alpha7 nAChR.

Lipid rafts are involved in C95 (4,8) agrin fragment-induced acetylcholine receptor clustering

During development of the neuromuscular junction, high densities of acetylcholine receptors accumulate beneath the overlying nerve terminal. A defining feature of mature synapses is the sharp demarcation of acetylcholine receptor density, which is approximately 1000-fold higher in the postsynaptic as compared with the contiguous extrasynaptic muscle membrane. These high densities of receptors accumulate by at least four mechanisms, re-distribution of existing surface receptors, local synthesis of new receptors, decreased turnover of synaptic receptors, and limitation of diffusion of sub-neural, aggregated receptors. The limitation of receptor diffusion within the membrane is likely in part due to the anchoring of acetylcholine receptor complexes to components of the cytoskeleton. Here we have tested the idea that lipid rafts--mobile, cholesterol enriched microdomains within the lipid bilayer--are another mechanism by which acetylcholine receptors are clustered in the postsynaptic apparatus. Using mouse C2C12 cells, a muscle cell line, we show that a carboxy terminal 95 amino acid fragment [C95 (4,8)] of the extracellular matrix molecule agrin that is essential for nerve-induced postsynaptic differentiation, promotes the redistribution of acetylcholine receptors into lipid rafts. Disruption of lipid rafts before agrin treatment largely inhibits de novo agrin-induced acetylcholine receptor clustering. Moreover, mature acetylcholine receptor clusters are destabilized if lipid rafts are disrupted. These results show that lipid rafts are important in both the initial clustering and later stabilization of agrin-induced acetylcholine receptor clusters and also suggest that lipid rafts may contribute to the postsynaptic localization of acetylcholine receptors in vivo.

Inhibition of sphingolipid synthesis affects kinetics but not fidelity of L1/NgCAM transport along direct but not transcytotic axonal pathways

Glycosphingolipids are constituents of lipid rafts which might function in sorting apical and axonal cargoes in the trans-Golgi network. In fact, two GPI-linked proteins, Thy1 and PrPC, require lipid raft lipids for sorting to the axon. It was previously shown that inhibition of glycosphingolipid synthesis by FumonisinB1 (FB1) impairs axon outgrowth but not axon specification, leading to the hypothesis that formation of axonally-targeted vesicles is coupled to sphingolipid synthesis. Since the axonal cell adhesion molecule L1/NgCAM can partition into membrane rafts biochemically, we asked whether correct targeting to the axon is FB1-sensitive, similarly to GPI-linked proteins. We previously showed that cultured hippocampal neurons use more than one trafficking pathway to the axon: a transcytotic pathway and a direct pathway. We show here that reducing raft lipid levels does not disrupt axonal targeting of L1/NgCAM along either pathway. Unexpectedly, FB1 selectively slowed the kinetics of surface expression of a truncated NgCAM using the direct pathway, but not of NgCAM using the transcytotic pathway. Therefore, the formation and/or transport of a subset of axonally-targeted vesicles are coupled to sphingolipid synthesis. Our results yield a mechanism for the axon outgrowth defect observed in FB1.

The prion protein requires cholesterol for cell surface localization

The cellular prion protein PrP(c) is attached to the plasma membrane by a glycosyl-phosphatidyl-inositol (GPI-) anchor and is localized in lipid rafts, membrane microdomains characterized by a high content of sphingolipids and cholesterol. Previous studies revealed that perturbation of cholesterol synthesis prevents prion conversion, explained by redistribution of PrP(c) at the plasma membrane. We investigated the influence of inhibition of cholesterol synthesis by the HMG-CoA-reductase inhibitor mevinolin on the trafficking of PrP(c) in neuronal cells. Treatment with mevinolin significantly reduces the amount of surface PrP(c) and leads to its accumulation in the Golgi compartment. Analysis of mutant PrPs highlights the importance of the GPI-anchor for raft localization and provides information about domains implicated in lipid raft association of PrP in the secretory pathway. Our data show that cholesterol is essential for the cell surface localization of PrP(c), known to be necessary for prion conversion.

Mechanisms of general anesthesia: from molecules to mind

Despite the widespread presence of clinical anesthesiology in medical practice, the mechanism by which diverse inhalational agents result in the state of general anesthesia remains unknown. Over recent decades, our understanding of general anesthetic mechanisms has evolved dramatically from early unitary hypotheses, largely due to the development and influence of a myriad of scientific disciplines ranging from molecular biology to cognitive neuroscience. These discoveries have led to a renaissance of investigation into the mechanisms of general anesthetics and have generated both novel answers and questions. In this chapter, we review the major hypotheses of general anesthetic mechanisms of action and present an expanded overview of current investigation into those mechanisms. We also present a framework to aid in thinking about the actions of these agents, highlighting the relationship between putative targets at the molecular level and the more integrated functional changes in behavior and consciousness.

Distinct gamma2 subunit domains mediate clustering and synaptic function of postsynaptic GABAA receptors and gephyrin

Modulation of the concentration of postsynaptic GABA(A) receptors contributes to functional plasticity of inhibitory synapses. The gamma2 subunit of GABA(A) receptor is specifically required for clustering of these receptors, for recruitment of the submembrane scaffold protein gephyrin to postsynaptic sites, and for postsynaptic function of GABAergic inhibitory synapses. To elucidate this mechanism, we here have mapped the gamma2 subunit domains required for restoration of postsynaptic clustering and function of GABA(A) receptors in gamma2 subunit mutant neurons. Transfection of gamma2-/- neurons with the gamma2 subunit but not the alpha2 subunit rescues postsynaptic clustering of GABA(A) receptors, results in recruitment of gephyrin to postsynaptic sites, and restores the amplitude and frequency of miniature inhibitory postsynaptic currents to wild-type levels. Analogous analyses of chimeric gamma2/alpha2 subunit constructs indicate, unexpectedly, that the fourth transmembrane domain of the gamma2 subunit is required and sufficient for postsynaptic clustering of GABA(A) receptors, whereas cytoplasmic gamma2 subunit domains are dispensable. In contrast, both the major cytoplasmic loop and the fourth transmembrane domain of the gamma2 subunit contribute to efficient recruitment of gephyrin to postsynaptic receptor clusters and are essential for restoration of miniature IPSCs. Our study points to a novel mechanism involved in targeting of GABA(A) receptors and gephyrin to inhibitory synapses.

Physical activity in relation to cancer of the colon and rectum in a cohort of male smokers

We examined the association between occupational and leisure physical activity and colorectal cancer in a cohort of male smokers. Among the 29,133 men aged 50-69 years in the Alpha-Tocopherol, Beta-Carotene Cancer Prevention study,152 colon and 104 rectal cancers were documented during up to 12 years of follow-up. For colon cancer, compared with sedentary workers, men in light occupational activity had a relative risk (RR) of 0.60 [95% confidence interval (CI), 0.34-1.04], whereas those in moderate/heavy activity had an RR of 0.45 (CI, 0.26-0.78; P for trend, 0.003). Subsite analysis revealed a significant association for moderate/heavy occupational activity in the distal colon (RR, 0.21; CI, 0.09-0.51) but not in the proximal colon (RR, 0.87; CI, 0.40-1.92). There was no significant association between leisure activity and colon cancer (active versus sedentary; RR, 0.82; CI, 0.59-1.13); however, the strongest inverse association was found among those most active in both work and leisure (RR, 0.33; CI, 0.16-0.71). For rectal cancer, there were risk reductions for those in light (RR, 0.71; CI, 0.36-1.37) and moderate/heavy occupational activity (RR, 0.50; CI, 0.26-0.97; P for trend, 0.04), and no association for leisure activity. These data provide evidence for a protective role of physical activity in colon and rectal cancer.