Cannabinoid CB1 receptor recognition of endocannabinoids via the lipid bilayer: molecular dynamics simulations of CB1 transmembrane helix 6 and anandamide in a phospholipid bilayer

Structural and functional insights into the G protein-coupled receptors: CB1 and CB2

The cannabinoid receptors CB1 and CB2 mediate a variety of physiological processes and continue to be explored as desirable drug targets. Both receptors are activated by the endogenous endocannabinoids and the psychoactive components of marijuana. Over the years, many efforts have been made to make selective ligands; however, the high degree of homology between cannabinoid receptor subtypes introduces challenges in studying either receptor in isolation. Recent advancements in structure biology have resulted in a surge of high-resolution structures, enriching our knowledge and understanding of receptor structure and function. In this review, of recent cannabinoid receptor structures, key features of the inactive and active state CB1 and CB2 are presented. These structures will provide additional insight into the modulation and signaling mechanism of cannabinoid receptors CB1 and CB2 and aid in the development of future therapeutics.

Endocannabinoid signaling in the central nervous system

It is hard to overestimate the influence of the endocannabinoid signaling (ECS) system on central nervous system (CNS) function. In the 40 years since cannabinoids were found to trigger specific cell signaling cascades, studies of the ECS system continue to cause amazement, surprise, and confusion! CB1 cannabinoid receptors are expressed widely in the CNS and regulate cell-cell communication via effects on the release of both neurotransmitters and gliotransmitters. CB2 cannabinoid receptors are difficult to detect in the CNS but seem to "punch above their weight" as compounds targeting these receptors have significant effects on inflammatory state and behavior. Positive and negative allosteric modulators for both receptors have been identified and examined in preclinical studies. Concentrations of the endocannabinoid ligands, N-arachidonoylethanolamine and 2-arachidonoylglycerol (2-AG), are regulated by a combination of enzymatic synthesis and degradation and inhibitors of these processes are available and making their way into clinical trials. Importantly, ECS regulates many essential brain functions, including regulation of reward, anxiety, inflammation, motor control, and cellular development. While the field is on the cusp of preclinical discoveries providing impactful clinical and therapeutic insights into many CNS disorders, there is still much to be learned about this remarkable and versatile modulatory system.

Endocannabinoids at the synapse and beyond: implications for neuropsychiatric disease pathophysiology and treatment

Endocannabinoids (eCBs) are lipid neuromodulators that suppress neurotransmitter release, reduce postsynaptic excitability, activate astrocyte signaling, and control cellular respiration. Here, we describe canonical and emerging eCB signaling modes and aim to link adaptations in these signaling systems to pathological states. Adaptations in eCB signaling systems have been identified in a variety of biobehavioral and physiological process relevant to neuropsychiatric disease states including stress-related disorders, epilepsy, developmental disorders, obesity, and substance use disorders. These insights have enhanced our understanding of the pathophysiology of neurological and psychiatric disorders and are contributing to the ongoing development of eCB-targeting therapeutics. We suggest future studies aimed at illuminating how adaptations in canonical as well as emerging cellular and synaptic modes of eCB signaling contribute to disease pathophysiology or resilience could further advance these novel treatment approaches.

Mechanisms of endocannabinoid transport in the brain

The endocannabinoids 2-arachidonoylglycerol (2-AG) and anandamide are among the best studied lipid messengers in the brain. By activating cannabinoid receptors in the CNS, endocannabinoids tune synaptic function, thereby influencing a variety of physiological and behavioural processes. Extensive research conducted over the last few decades has considerably enhanced our understanding of the molecular mechanisms and physiological functions of the endocannabinoid system. It is now well-established that endocannabinoids are synthesized by postsynaptic neurons and serve as retrograde messengers that suppress neurotransmitter release at central synapses. While the detailed mechanisms by which endocannabinoids gate synaptic function and behavioural processes are relatively well characterized, the mechanisms governing endocannabinoid transport at central synapses remain ill defined. Recently, several studies have begun to unravel the mechanisms governing intracellular and intercellular endocannabinoid transport. In this review, we will focus on new advances in the mechanisms of intracellular and synaptic endocannabinoid transport in the CNS.

Inhibition of fatty acid amide hydrolase prevents pathology in neurovisceral acid sphingomyelinase deficiency by rescuing defective endocannabinoid signaling

Acid sphingomyelinase deficiency (ASMD) leads to cellular accumulation of sphingomyelin (SM), neurodegeneration, and early death. Here, we describe the downregulation of the endocannabinoid (eCB) system in neurons of ASM knockout (ASM‐KO) mice and a ASMD patient. High SM reduced expression of the eCB receptor CB₁ in neuronal processes and induced its accumulation in lysosomes. Activation of CB₁ receptor signaling, through inhibition of the eCB‐degrading enzyme fatty acid amide hydrolase (FAAH), reduced SM levels in ASM‐KO neurons. Oral treatment of ASM‐KO mice with a FAAH inhibitor prevented SM buildup; alleviated inflammation, neurodegeneration, and behavioral alterations; and extended lifespan. This treatment showed benefits even after a single administration at advanced disease stages. We also found CB₁ receptor downregulation in neurons of a mouse model and a patient of another sphingolipid storage disorder, Niemann–Pick disease type C (NPC). We showed the efficacy of FAAH inhibition to reduce SM and cholesterol levels in NPC patient‐derived cells and in the brain of a NPC mouse model. Our findings reveal a pathophysiological crosstalk between neuronal SM and the eCB system and offer a new treatment for ASMD and other sphingolipidoses.

Advances in methods to characterize ligand-induced ionic lock and rotamer toggle molecular switch in G protein-coupled receptors

Structural biology of GPCRs has made significant progress upon recently developed technologies for GPCRs expression/purification and elucidation of GPCRs crystal structures. The crystal structures provide a snapshot of the receptor structural disposition of GPCRs itself or with cocrystallized ligands, and the results are congruent with biophysical and computer modeling studies reported about GPCRs conformational and dynamics flexibility, regulated activation, and the various stabilizing interactions, such as "molecular switches." The molecular switches generally constitute the most conserved domains within a particular GPCR superfamily. Often agonist-induced receptor activation proceeds by the disruption of majority of these interactions, while antagonist and inverse agonist act as blockers and structural stabilizers, respectively. Several elegant studies, particularly for the β2AR, have demonstrated the relationship between ligand structure, receptor conformational changes, and corresponding pharmacological outcomes. Thus, it is of great importance to understand GPCRs activation related to cell signaling pathways. Herein, we summarize the steps to produce functional GPCRs, generate suitably fluorescent labeled GPCRs and the procedure to use that to understand if ligand-induced activation can proceed by activation of the GPCRs via ionic lock switch and/or rotamer toggle switch mechanisms. Such understanding of ligand structure and mechanism of receptor activation will provide great insight toward uncovering newer pathways of GPCR activation and aid in structure-based drug design.

Ligand-specific homology modeling of human cannabinoid (CB1) receptor

Cannabinoid (CB1) receptor is a therapeutic drug target, and its structure and conformational changes after ligand binding are of great interest. To study the protein conformations in ligand bound state and assist in drug discovery, CB1 receptor homology models are needed for computer-based ligand screening. The known CB1 ligands are highly diverse structurally, so CB1 receptor may undergo considerable conformational changes to accept different ligands, which is challenging for molecular docking methods. To account for the flexibility of CB1 receptor, we constructed four CB1 receptor models based on four structurally distinct ligands, HU-210, ACEA, WIN55212-2 and SR141716A, using the newest X-ray crystal structures of human β₂ adrenergic receptor and adenosine A(2A) receptor as templates. The conformations of these four CB1-ligand complexes were optimized by molecular dynamics (MD) simulations. The models revealed interactions between CB1 receptor and known binders suggested by experiments and could successfully discriminate known ligands and non-binders in our docking assays. MD simulations were used to study the most flexible ligand, ACEA, in its free and bound states to investigate structural mobility achieved by the rearrangement of the fatty acid chain. Our models may capture important conformational changes of CB1 receptor to help improve accuracy in future CB1 drug screening.

Endocannabinoid tone versus constitutive activity of cannabinoid receptors

This review evaluates the cellular mechanisms of constitutive activity of the cannabinoid (CB) receptors, its reversal by inverse agonists, and discusses the pitfalls and problems in the interpretation of the research data. The notion is presented that endogenously produced anandamide (AEA) and 2-arachidonoylglycerol (2-AG) serve as autocrine or paracrine stimulators of the CB receptors, giving the appearance of constitutive activity. It is proposed that one cannot interpret inverse agonist studies without inference to the receptors' environment vis-à-vis the endocannabinoid agonists which themselves are highly lipophilic compounds with a preference for membranes. The endocannabinoid tone is governed by a combination of synthetic pathways and inactivation involving transport and degradation. The synthesis and degradation of 2-AG is well characterized, and 2-AG has been strongly implicated in retrograde signalling in neurons. Data implicating endocannabinoids in paracrine regulation have been described. Endocannabinoid ligands can traverse the cell's interior and potentially be stored on fatty acid-binding proteins (FABPs). Molecular modelling predicts that the endocannabinoids derived from membrane phospholipids can laterally diffuse to enter the CB receptor from the lipid bilayer. Considering that endocannabinoid signalling to CB receptors is a much more likely scenario than is receptor activation in the absence of agonist ligands, researchers are advised to refrain from assuming constitutive activity except for experimental models known to be devoid of endocannabinoid ligands.

Molecular basis for a high-potency open-channel block of Kv1.5 channel by the endocannabinoid anandamide

The endocannabinoid, N-arachidonoylethanolamine (anandamide; AEA) is known to interact with voltage-gated K(+) (Kv) channels in a cannabinoid receptor-independent manner. AEA modulates the functional properties of Kv channels, converting channels with slowly inactivating current into apparent fast inactivation. In this study, we characterize the mechanism of action and binding site for AEA on Kv1.5 channels expressed on HEK-293 cells using the patch-clamp techniques. AEA exhibited high-potency block (IC(50) approximately 200 nM) from the cytoplasmic membrane surface, consistent with open-channel block. Alanine-scanning mutagenesis revealed that AEA interacts with two crucial beta-branching amino acids, Val505 and Ile508 within the S6 domain. Both residues face toward the central cavity and constitute a motif that forms a hydrophobic ring around the ion conduction pathway. This hydrophobic ring motif may be a critical determinant of cannabinoid receptor-independent AEA modulation in other K(+) channel families.

Antagonist-D2S-dopamine receptor interactions in intact recombinant Chinese hamster ovary cells [corrected]

D(2)-type dopamine receptors are major recognition sites for antipsychotic drugs. There are two splice variants: D(2S) and D(2L) with an additional 29 amino acid sequence in the third intracellular loop. Only little comparative information is hitherto available about their pharmacological properties and none of these studies dealt with intact cell systems. This prompted us to investigate the binding properties of [(3)H]-raclopride, a hydrophilic benzamide, and [(3)H]-spiperone, a highly hydrophobic butyrophenone, to intact CHO cells expressing recombinant human D(2L)-receptors. Presently, we have repeated and extended this experimental approach to the human D(2S)-receptors in the same cell system. Except for a slower dissociation of [(3)H]-spiperone from D(2S), the binding properties of these and other antagonists were not significantly different for both isoforms (P > 0.05). The very slow dissociation of the atypical antipsychotic clozapine was surprising in light of its low affinity. Two experiments pointed out the existence of non-competitive interactions between raclopride and spiperone for D(2S) as well as D(2L) (A. Packeu, J. P. De Backer & G. Vauquelin, in preparation). Alongside the different physicochemical properties of these ligands, this finding fits with a model wherein the hydrophilic raclopride approaches the D(2L)-receptor from the aqueous phase, while the hydrophobic spiperone approaches the receptor by lateral diffusion between the membrane lipids. These different modes of approach could imply the existence of topologically distinct ligand binding sites at D(2)-receptors.

Ligands, their receptors and ... plasma membranes

Ligand-receptor interactions are customarily described by equations that apply to solutes. Yet, most receptors are present in cell membranes so that sufficiently lipophilic ligands could reach the receptor by a two-dimensional approach within the membrane. As summarized in this review, this may affect the ligand-receptor interaction in many ways. Biophysicians calculated that, compared to a three-dimensional approach from the liquid phase, such approach could alter the time the ligands need to find a receptor. Biochemists found that ligand incorporation in lipid bilayers modifies their conformation. This, along with the depth at which the ligands reside in the bilayer, will affect the probability of successful receptor interaction. Novel mechanisms were also introduced, including "exosite" binding and ligand translocation between the receptor's alpha-helical transmembrane domains. Pharmacologists focused attention at ligand concentrations in membrane, their adsorption and release rates and the effects thereof on ligand potency and residence time at the receptor.

Transmembrane helical domain of the cannabinoid CB1 receptor

Brain cannabinoid (CB(1)) receptors are G-protein coupled receptors and belong to the rhodopsin-like subfamily. A homology model of the inactive state of the CB(1) receptor was constructed using the x-ray structure of beta(2)-adrenergic receptor (beta(2)AR) as the template. We used 105 ns duration molecular-dynamics simulations of the CB(1) receptor embedded in a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer to gain some insight into the structure and function of the CB(1) receptor. As judged from the root mean-square deviations combined with the detailed structural analyses, the helical bundle of the CB(1) receptor appears to be fully converged in 50 ns of the simulation. The results reveal that the helical bundle structure of the CB(1) receptor maintains a topology quite similar to the x-ray structures of G-protein coupled receptors overall. It is also revealed that the CB(1) receptor is stabilized by the formation of extensive, water-mediated H-bond networks, aromatic stacking interactions, and receptor-lipid interactions within the helical core region. It is likely that these interactions, which are often specific to functional motifs, including the S(N)LAxAD, D(E)RY, CWxP, and NPxxY motifs, are the molecular constraints imposed on the inactive state of the CB(1) receptor. It appears that disruption of these specific interactions is necessary to release the molecular constraints to achieve a conformational change of the receptor suitable for G-protein activation.

Lipid rafts regulate 2-arachidonoylglycerol metabolism and physiological activity in the striatum

Several G protein-associated receptors and synaptic proteins function within lipid rafts, which are subdomains of the plasma membranes that contain high concentrations of cholesterol. In this study we addressed the possible role of lipid rafts in the control of endocannabinoid system in striatal slices. Disruption of lipid rafts following cholesterol depletion with methyl-beta-cyclodestrin (MCD) failed to affect synthesis and degradation of anandamide, while it caused a marked increase in the synthesis and concentration of 2-arachidonoylglycerol (2-AG), as well as in the binding activity of cannabinoid CB1 receptors. Surprisingly, endogenous 2-AG-mediated control of GABA transmission was not potentiated by MCD treatment and, in contrast, neither basal nor 3,5-Dihydroxyphenylglycine-stimulated 2-AG altered GABA synapses in cholesterol-depleted slices. Synaptic response to the cannabinoid CB1 receptor agonist HU210 was however intact in MCD-treated slices, indicating that reduced sensitivity of cannabinoid CB1 receptors does not explain why endogenous 2-AG is ineffective in inhibiting striatal GABA transmission after cholesterol depletion. Confocal microscopy analysis suggested that disruption of raft integrity by MCD might uncouple metabotropic glutamate 5-CB1 receptor interaction by altering the correct localization of both receptors in striatal neuron elements. In conclusion, our data indicate that disruption of raft integrity causes a complex alteration of the endocannabinoid signalling in the striatum.

Antagonist-radioligand binding to D2L-receptors in intact cells

D(2)-dopamine receptors mediate most of the physiological actions of dopamine and are important recognition sites for antipsychotic drugs. Earlier binding studies were predominantly done with broken cell preparations with the tritiated D(2)-receptor antagonists [(3)H]-raclopride, a hydrophilic benzamide, and [(3)H]-spiperone, a highly hydrophobic butyrophenone. Here we compared [(3)H]-raclopride and [(3)H]-spiperone binding properties in intact Chinese Hamster Ovary cells stably expressing recombinant human D(2L)-receptors. Specific binding of both radioligands occurred to a comparable number of sites. In contrast to the rapid dissociation of [(3)H]-raclopride in both medium only and in the presence of an excess of unlabelled ligand [(3)H]-spiperone dissociation was only observed in the latter condition, and it was still slower than in broken cell preparations. However, this could not explain the pronounced difference in the potency of some unlabelled ligands to compete with both radioligands. To integrate these new findings, a model is proposed in which raclopride approaches the receptor from the aqueous phase, while spiperone approaches the receptor by lateral diffusion within the membrane.

Compartmentalization of endocannabinoids into lipid rafts in a dorsal root ganglion cell line

BACKGROUND AND PURPOSE

N-arachidonoyl ethanolamine (AEA) and 2-arachidonoyl glycerol (2-AG) are endogenous cannabinoids binding to the cannabinoid receptors CB1 and CB2 to modulate neuronal excitability and synaptic transmission in primary afferent neurons. To investigate the compartmentalization of the machinery for AEA and 2-AG signalling, we studied their partitioning into lipid raft fractions isolated from a dorsal root ganglion X neuroblastoma cell line (F-11).

EXPERIMENTAL APPROACH

F-11 cells were homogenized and fractionated using a detergent-free OptiPrep density gradient. All lipids were partially purified from methanolic extracts of the fractions on solid phase cartridges and quantified using liquid chromatography tandem mass spectrometry (LC/MS/MS). Protein distribution was determined by Western blotting.

KEY RESULTS

Under basal conditions, the endogenous cannabinoid AEA was present in both lipid raft and specific non-lipid raft fractions as was one of its biosynthetic enzymes, NAPE-PLD. The 2-AG precursor 1-stearoyl-2-arachidonoyl-sn-glycerol (DAG), diacylglycerol lipase alpha (DAGLalpha), which cleaves DAG to form 2-AG, and 2-AG were all co-localized with lipid raft markers. CB1 receptors, previously reported to partition into lipid raft fractions, were not detected in F-11 membranes, but CB2 receptors were detected at high levels and partitioned into non-lipid raft fractions.

CONCLUSIONS AND IMPLICATIONS

The biochemical machinery for the production of 2-AG via the putative diacylglycerol pathway is localized within lipid rafts, suggesting that 2-AG synthesis via DAG occurs within these microdomains. The observed co-localization of AEA, 2-AG, and their synthetic enzymes with the reported localization of CB1 raises the possibility of intrinsic-autocrine signalling within lipid raft domains and/or retrograde-paracrine signalling.

A neuroscientist's guide to lipidomics

Nerve cells mould the lipid fabric of their membranes to ease vesicle fusion, regulate ion fluxes and create specialized microenvironments that contribute to cellular communication. The chemical diversity of membrane lipids controls protein traffic, facilitates recognition between cells and leads to the production of hundreds of molecules that carry information both within and across cells. With so many roles, it is no wonder that lipids make up half of the human brain in dry weight. The objective of neural lipidomics is to understand how these molecules work together; this difficult task will greatly benefit from technical advances that might enable the testing of emerging hypotheses.