Integrin clustering enables anandamide-induced Ca2+ signaling in endothelial cells via GPR55 by protection against CB1-receptor-triggered repression

Enhancement of apamin-sensitive medium afterhyperpolarization current by anandamide and its role in excitability control in cultured hippocampal neurons

Although endocannabinoid anandamide (AEA) plays an important role in synaptic signaling and neuronal survival, the underlying mechanism is not fully understood. Afterhyperpolarization (AHP) is the critical modulator of cell excitability and in turn shapes the neuronal output. Here, we examined the effects of AEA on AHP current and action potential firing in cultured rat hippocampal neurons. In whole-cell patch-clamp recording, AEA applied in the extracellular medium at nanomolar concentration enhanced medium AHP (mAHP) current and spike-frequency adaptation. Activation of apamin-sensitive, small conductance Ca(2+)-activated K(+) (SK) channels, probably SK2 and SK3 as the immunofluorescence analysis indicated, attributed largely to the AEA action on mAHP. Interestingly, AEA-induced potentiation of mAHP current was abolished by inositol 1,4,5-trisphosphate receptors (IP(3)Rs) blockade. However, the potentiation was not affected by inhibiting Ca(2+) influx or Ca(2+) release from internal store through ryanodine receptors. In addition, blockade of CB1, TRPV1 or Gi/o-protein did not attenuate the potentiation. Thus, AEA might enhance the SK mAHP currents mainly in a non-CB1/TRPV1 receptor way. Our study provides the first evidence that a functional cascade might lie among AEA, IP(3)Rs and SK channels, which may keep the membrane excitability stable in a negative-feedback manner.

The orphan G protein-coupled receptor GPR55 promotes cancer cell proliferation via ERK

GPR55 is an orphan G protein-coupled receptor that may be engaged by some lipid ligands such as lysophosphatidylinositol and cannabinoid-type compounds. Very little is known about its expression pattern and physio-pathological relevance, and its pharmacology and signaling are still rather controversial. Here we analyzed the expression and function of GPR55 in cancer cells. Our data show that GPR55 expression in human tumors from different origins correlates with their aggressiveness. Moreover, GPR55 promotes cancer cell proliferation, both in cell cultures and in xenografted mice, through the overactivation of the extracellular signal-regulated kinase cascade. These findings reveal the importance of GPR55 in human cancer, and suggest that it could constitute a new biomarker and therapeutic target in oncology.

Pharmacology, signaling and physiological relevance of the G protein-coupled receptor 55

According to The European Monitoring Centre for Drugs and Drug Addiction (EMCDDA), ∼70 million European adults have consumed cannabis on at least one occasion. Cannabis consumption leads to a variety of psychoactive effects due to the presence of the constituent Δ(9)-tetrahydrocannabinol (Δ(9)-THC). Δ(9)-THC interacts with the endocannabinoid system (ECS), which consists of the seven transmembrane spanning (7TM)/G protein-coupled receptors (GPCRs) CB(1) and CB(2), their respective ligands (endocannabinoids), and enzymes involved in their biosynthesis and degradation. This system plays a critical role in many physiological processes such as learning and memory, appetite control, pain sensation, motor coordination, lipogenesis, modulation of immune response, and the regulation of bone mass. Therefore, a huge effort has been spent trying to fully elucidate the composition and function of the ECS. The G protein-coupled receptor 55 (GPR55) was recently proposed as a novel component of this system; however, its classification as a cannabinoid receptor has been significantly hampered by its complex pharmacology, signaling, and cellular function. GPR55 is phylogenetically distinct from the traditional cannabinoid receptors, but in some experimental paradigms, it is activated by endocannabinoids, phytocannabinoids, and synthetic cannabinoid ligands. However, the most potent compound appears to be a lysophospholipid known as lysophosphatidylinositol (LPI). Here, we provide a comprehensive evaluation of the current pharmacology and signaling of GPR55 and review the proposed role of this receptor in a number of physiological and pathophysiological processes.

International Union of Basic and Clinical Pharmacology. LXXIX. Cannabinoid receptors and their ligands: beyond CB₁ and CB₂

There are at least two types of cannabinoid receptors (CB(1) and CB(2)). Ligands activating these G protein-coupled receptors (GPCRs) include the phytocannabinoid Δ(9)-tetrahydrocannabinol, numerous synthetic compounds, and endogenous compounds known as endocannabinoids. Cannabinoid receptor antagonists have also been developed. Some of these ligands activate or block one type of cannabinoid receptor more potently than the other type. This review summarizes current data indicating the extent to which cannabinoid receptor ligands undergo orthosteric or allosteric interactions with non-CB(1), non-CB(2) established GPCRs, deorphanized receptors such as GPR55, ligand-gated ion channels, transient receptor potential (TRP) channels, and other ion channels or peroxisome proliferator-activated nuclear receptors. From these data, it is clear that some ligands that interact similarly with CB(1) and/or CB(2) receptors are likely to display significantly different pharmacological profiles. The review also lists some criteria that any novel "CB(3)" cannabinoid receptor or channel should fulfil and concludes that these criteria are not currently met by any non-CB(1), non-CB(2) pharmacological receptor or channel. However, it does identify certain pharmacological targets that should be investigated further as potential CB(3) receptors or channels. These include TRP vanilloid 1, which possibly functions as an ionotropic cannabinoid receptor under physiological and/or pathological conditions, and some deorphanized GPCRs. Also discussed are 1) the ability of CB(1) receptors to form heteromeric complexes with certain other GPCRs, 2) phylogenetic relationships that exist between CB(1)/CB(2) receptors and other GPCRs, 3) evidence for the existence of several as-yet-uncharacterized non-CB(1), non-CB(2) cannabinoid receptors; and 4) current cannabinoid receptor nomenclature.

Endocannabinoid-like N-arachidonoyl serine is a novel pro-angiogenic mediator

BACKGROUND AND PURPOSE

N-arachidonoyl serine (ARA-S) is a recently identified endocannabinoid-like lipid with weak affinity for the fully characterized cannabinoid receptors (CB(1) and CB(2)) and the transient receptor potential vanilloid receptor 1 (TRPV-1). ARA-S induces vasodilatation and shows vasoprotective potential via activation of key signalling pathways in endothelial cells. Based on these findings, the effect of ARA-S on endothelial functions was further studied.

EXPERIMENTAL APPROACH

Primary human dermal microvascular endothelial cells (HMVEC) were used to investigate effects of ARA-S (0-10 microM) on certain endothelial functions, using cell proliferation, migration and wound repair models in vitro, and angiogenesis assays in vitro and ex vivo. Selective CB receptor antagonists and specific siRNAs were deployed to block individual CB receptors.

KEY RESULTS

We found that ARA-S stimulated angiogenesis and endothelial wound healing through induction of vascular endothelial growth factor C and its cognate receptor expression in primary HMVEC. Moreover, knock-down of G protein-coupled receptor 55 (GPR55) partly inhibited ARA-S-induced signal transduction and endothelial functions.

CONCLUSIONS AND IMPLICATIONS

Our results indicate that ARA-S is a pro-angiogenic factor in addition to a vessel dilator. The GPR55 receptor may serve as one target of ARA-S.

The GPR55 agonist lysophosphatidylinositol acts as an intracellular messenger and bidirectionally modulates Ca2+ -activated large-conductance K+ channels in endothelial cells

Lysophospholipids are known to serve as intra- and extracellular messengers affecting many physiological processes. Lysophosphatidylinositol (LPI), which is produced in endothelial cells, acts as an endogenous agonist of the orphan receptor, G protein-coupled receptor 55 (GPR55). Stimulation of GPR55 by LPI evokes an intracellular Ca²⁺ rise in several cell types including endothelial cells. In this study, we investigated additional direct, receptor-independent effects of LPI on endothelial large-conductance Ca²⁺ and voltage-gated potassium (BK_Ca) channels. Electrophysiological experiments in the inside-out configuration revealed that LPI directly affects the BK_Ca channel gating properties. This effect of LPI strictly depended on the presence of Ca²⁺ and was concentration-dependent, reversible, and dual in nature. The modulating effects of LPI on endothelial BK_Ca channels correlated with their initial open probability (Po): stimulation at low Po (<0.3) and inhibition at high Po levels (>0.3). In the whole-cell configuration, LPI in the pipette facilitated membrane hyperpolarization in response to low (0.1–2 μM) histamine concentrations. In contrast, LPI counteracted membrane hyperpolarization in response to supramaximal cell stimulation with histamine. These results highlight a novel receptor-independent and direct bidirectional modulation of BK_Ca channels by LPI on endothelial cells. We conclude that LPI via this mechanism serves as an important modulator of endothelial electrical responses to cell stimulation.

The putative cannabinoid receptor GPR55 defines a novel autocrine loop in cancer cell proliferation

Recently, the orphan receptor G protein-coupled receptor 55 (GPR55) has been proposed as a potential cannabinoid receptor, although controversy remains on its physiological roles. Current evidence suggests a role for GPR55 as a receptor for the lysophospholipid lysophosphatidylinositol (LPI). In this study, we show that GPR55 is expressed in several prostate and ovarian cancer cell lines, both at the mRNA and at the protein level, and that it has a critical role in regulating proliferation and anchorage-independent growth. We further show that GPR55 mediates the effects of LPI in prostate and ovarian cancer cells. Indeed we demonstrate that LPI is able to induce calcium mobilization and activation of Akt and extracellular signal-regulated kinase (ERK)1/2 in these cells and that both pharmacological blockade of GPR55 and its downregulation using specific small interfering RNA strongly inhibits these processes. We further identify an autocrine loop by which LPI is synthesized by cytosolic phospholipase A2, pumped out of the cell by the ATP-binding cassette transporter ABCC1/MRP1, and is then able to initialize cascades downstream of GPR55. All together, these data demonstrate a role of LPI and its receptor GPR55 in cancer cells in activating an autocrine loop that regulates cell proliferation. These findings may have important implications for LPI as a novel cancer biomarker and for its receptor GPR55 as a potential therapeutic target.

GPR55-dependent and -independent ion signalling in response to lysophosphatidylinositol in endothelial cells

BACKGROUND AND PURPOSE

The glycerol-based lysophospholipid lysophosphatidylinositol (LPI) is an endogenous agonist of the G-protein-coupled receptor 55 (GPR55) exhibiting cannabinoid receptor-like properties in endothelial cells. To estimate the contribution of GPR55 to the physiological effects of LPI, the GPR55-dependent and -independent electrical responses in this cell type were investigated.

EXPERIMENTAL APPROACH

Applying small interference RNA-mediated knock-down and transient overexpression, GPR55-dependent and -independent effects of LPI on cytosolic free Ca(2+) concentration, membrane potential and transmembrane ion currents were studied in EA.hy296 cells.

KEY RESULTS

In a GPR55-dependent, GDPbetaS and U73122-sensitive manner, LPI induced rapid and transient intracellular Ca(2+) release that was associated with activation of charybdotoxin-sensitive, large conductance, Ca(2+)-activated, K(+) channels (BK(Ca)) and temporary membrane hyperpolarization. Following these initial electrical reactions, LPI elicited GPR55-independent long-lasting Na(+) loading and a non-selective inward current causing sustained membrane depolarization that depended on extracellular Ca(2+) and Na(+) and was partially inhibited by Ni(2+) and La(3+). This inward current was due to the activation of a voltage-independent non-selective cation current. The Ni(2+) and La(3+)-insensitive depolarization with LPI was prevented by inhibition of the Na/K-ATPase by ouabain.

CONCLUSIONS AND IMPLICATIONS

LPI elicited a biphasic response in endothelial cells of which the immediate Ca(2+) signalling depends on GPR55 while the subsequent depolarization is due to Na(+) loading via non-selective cation channels and an inhibition of the Na/K-ATPase. Thus, LPI is a potent signalling molecule that affects endothelial functions by modulating several cellular electrical responses that are only partially linked to GPR55.

Requirement of cannabinoid CB(1) receptors in cortical pyramidal neurons for appropriate development of corticothalamic and thalamocortical projections

A role for endocannabinoid signaling in neuronal morphogenesis as the brain develops has recently been suggested. Here we used the developing somatosensory circuit as a model system to examine the role of endocannabinoid signaling in neural circuit formation. We first show that a deficiency in cannabinoid receptor type 1 (CB(1)R), but not G-protein-coupled receptor 55 (GPR55), leads to aberrant fasciculation and pathfinding in both corticothalamic and thalamocortical axons despite normal target recognition. Next, we localized CB(1)R expression to developing corticothalamic projections and found little if any expression in thalamocortical axons, using a newly established reporter mouse expressing GFP in thalamocortical projections. A similar thalamocortical projection phenotype was observed following removal of CB(1)R from cortical principal neurons, clearly demonstrating that CB(1)R in corticothalamic axons was required to instruct their complimentary connections, thalamocortical axons. When reciprocal thalamic and cortical connections meet, CB(1)R-containing corticothalamic axons are intimately associated with elongating thalamocortical projections containing DGLβ, a 2-arachidonoyl glycerol (2-AG) synthesizing enzyme. Thus, 2-AG produced in thalamocortical axons and acting at CB(1)Rs on corticothalamic axons is likely to modulate axonal patterning. The presence of monoglyceride lipase, a 2-AG degrading enzyme, in both thalamocortical and corticothalamic tracts probably serves to restrict 2-AG availability. In summary, our study provides strong evidence that endocannabinoids are a modulator for the proposed 'handshake' interactions between corticothalamic and thalamocortical axons, especially for fasciculation. These findings are important in understanding the long-term consequences of alterations in CB(1)R activity during development, a potential etiology for the mental health disorders linked to prenatal cannabis use.

Cannabinoid and cannabinoid-like receptors in microglia, astrocytes, and astrocytomas

CB1 and CB2 receptors are activated by a plethora of cannabinoid compounds, be they endogenously-produced, plant-derived or synthetic. These receptors are expressed by microglia, astrocytes and astrocytomas, and their activation regulates these cells' differentiation, functions and viability. Recent studies show that glial cells also express cannabinoid-like receptors, and that their activation regulates different cell functions, but also control cell viability. This review summarizes this evidence, and discusses how selective compounds targeting cannabinoid-like receptors constitute promising therapeutics to manage neuroinflammation and eradicate malignant astrocytomas. Importantly, the selective targeting of cannabinoid-like receptors should provide therapeutic relieve without inducing the typical psychotropic effects and possible addictive properties associated with the use of Delta9-tetrahydrocannabinol, the main psychotropic ingredient produced by the plant Cannabis sativa.

Mitochondrial Ca2+ uptake and not mitochondrial motility is required for STIM1-Orai1-dependent store-operated Ca2+ entry

Store-operated Ca2+ entry (SOCE) is established by formation of subplasmalemmal clusters of the endoplasmic reticulum (ER) protein, stromal interacting molecule 1 (STIM1) upon ER Ca2+ depletion. Thereby, STIM1 couples to plasma membrane channels such as Orai1. Thus, a close proximity of ER domains to the plasma membrane is a prerequisite for SOCE activation, challenging the concept of local Ca2+ buffering by mitochondria as being essential for SOCE. This study assesses the impact of mitochondrial Ca2+ handling and motility on STIM1–Orai1-dependent SOCE. High-resolution microscopy showed only 10% of subplasmalemmal STIM1 clusters to be colocalized with mitochondria. Impairments of mitochondrial Ca2+ handling by inhibition of mitochondrial Na+-Ca2+ exchanger (NCXmito) or depolarization only partially suppressed Ca2+ entry in cells overexpressing STIM1-Orai1. However, SOCE was completely abolished when both NCXmito was inhibited and the inner mitochondrial membrane was depolarized, in STIM1- and Orai1-overexpressing cells. Immobilization of mitochondria by expression of mAKAP-RFP-CAAX, a construct that physically links mitochondria to the plasma membrane, affected the Ca2+ handling of the organelles but not the activity of SOCE. Our observations indicate that mitochondrial Ca2+ uptake, including reversal of NCXmito, is fundamental for STIM1–Orai1-dependent SOCE, whereas the proximity of mitochondria to STIM1-Orai1 SOCE units and their motility is not required.

The endocannabinoid system: its roles in energy balance and potential as a target for obesity treatment

Obesity and cardiometabolic risk continue to be major public health concerns. A better understanding of the physiopathological mechanisms leading to obesity may help to identify novel therapeutic targets. The endocannabinoid system discovered in the early 1990s is believed to influence body weight regulation and cardiometabolic risk factors. This article aims to review the literature on the endocannabinoid system including the biological roles of its major components, namely, the cannabinoid receptors, their endogenous ligands the endocannabinoids and the ligand-metabolising enzymes. The review also discusses evidence that the endocannabinoid system constitutes a new physiological pathway occurring in the central nervous system and peripheral tissues that has a key role in the control of food intake and energy expenditure, insulin sensitivity, as well as glucose and lipid metabolism. Based on the important finding that there is a close association between obesity and the hyperactivity of the endocannabinoid system, interest in blocking stimulation of this pathway to aid weight loss and reduce cardiometabolic risk factor development has become an important area of research. Among the pharmacological strategies proposed, the antagonism of the cannabinoid receptors has been particularly investigated and several clinical trials have been conducted. One challenging pharmacological task will be to target the endocannabinoid system in a more selective, and hence, safe way. As the management of obesity also requires lifestyle modifications in terms of healthy eating and physical activity, the targeting of the endocannabinoid system may represent a novel approach for a multifactorial therapeutic strategy.

Targeting the cannabinoid pathway limits the development of fibrosis and autoimmunity in a mouse model of systemic sclerosis

Our aim was to evaluate the roles of the cannabinoid pathway in the induction and propagation of systemic sclerosis (SSc) in a mouse model of diffuse SSc induced by hypochlorite injections. BALB/c mice injected subcutaneously every day for 6 weeks with PBS or hypochlorite were treated intraperitoneally with either WIN-55,212, an agonist of the cannabinoid receptors 1 (CB1) and receptors 2 (CB2), with JWH-133, a selective agonist of CB2, or with PBS. Skin and lung fibrosis were then assessed by histological and biochemical methods, and the proliferation of fibroblasts purified from diseased skin was assessed by thymidine incorporation. Autoantibodies were detected by ELISA, and spleen cell populations were analyzed by flow cytometry. Experiments were also performed in mice deficient for CB2 receptors (Cnr2(-/-)). Injections of hypochlorite induced cutaneous and lung fibrosis as well as increased the proliferation rate of fibroblasts isolated from fibrotic skin, splenic B cell counts, and levels of anti-DNA topoisomerase-1 autoantibodies. Treatment with WIN-55,212 or with the selective CB2 agonist JWH-133 prevented the development of skin and lung fibrosis as well as reduced fibroblast proliferation and the development of autoantibodies. Experiments performed in CB2-deficient mice confirmed the influence of CB2 in the development of systemic fibrosis and autoimmunity. Therefore, we demonstrate that the CB2 receptor is a potential target for the treatment of SSc because it controls both skin fibroblast proliferation and the autoimmune reaction.

Pharmacological characterization of GPR55, a putative cannabinoid receptor

GPR55 has recently attracted much attention as another member of the cannabinoid family, potentially explaining physiological effects that are non-CB1/CB2 mediated. However, the data gathered so far are conflicting with respect to its pharmacology. We review the primary literature to date on GPR55, describing its discovery, structure, pharmacology and potential physiological functions. The CB1 receptor antagonist/inverse agonist AM251 has been shown to be a GPR55 agonist in all reports in which it was evaluated, as has the lysophospholipid, lysophosphatidylinositol (LPI). Whether GPR55 responds to the endocannabinoid ligands anandamide and 2-arachidonylglycerol and the phytocannabinoids, delta-9-tetrahydrocannabidiol and cannabidiol, is cell type and tissue-dependent. GPR55 has been shown to utilize G(q), G(12), or G(13) for signal transduction; RhoA and phospholipase C are activated. Experiments with mice in which GPR55 has been inactivated reveal a role for this receptor in neuropathic and inflammatory pain as well as in bone physiology. Thus delineating the pharmacology of this receptor and the discovery of selective agonists and antagonists merits further study and could lead to new therapeutics.

Fluorescent ligand binding reveals heterogeneous distribution of adrenoceptors and 'cannabinoid-like' receptors in small arteries

BACKGROUND AND PURPOSE

Pharmacological analysis of synergism or functional antagonism between different receptors commonly assumes that interacting receptors are located in the same cells. We have now investigated the distribution of alpha-adrenoceptors, beta-adrenoceptors and cannabinoid-like (GPR55) receptors in the mouse arteries.

EXPERIMENTAL APPROACH

Fluorescence intensity from vascular tissue incubated with fluorescent ligands (alpha(1)-adrenoceptor ligand, BODIPY-FL-prazosin, QAPB; beta-adrenoceptor ligand, TMR-CGP12177; fluorescent angiotensin II; a novel diarylpyrazole cannabinoid ligand (Tocrifluor 1117, T1117) was measured with confocal microscopy. Small mesenteric and tail arteries of wild-type and alpha(1B/D)-adrenoceptor-KO mice were used.

KEY RESULTS

T1117, a fluorescent form of the cannabinoid CB(1) receptor antagonist AM251, was a ligand for GPR55, with low affinity for CB(1) receptors. In mesenteric arterial smooth muscle cells, alpha(1A)-adrenoceptors were predominantly located in different cells from those with beta-adrenoceptors, angiotensin receptors or cannabinoid-like (GPR55) receptors. Cells with beta-adrenoceptors predominated at arterial branches. Endothelial cells expressed beta-adrenoceptors, alpha-adrenoceptors and cannabinoid-like receptors. Only endothelial alpha-adrenoceptors appeared in clusters. Adventitia was a rich source of G protein-coupled receptors (GPCRs), particularly fibroblasts and nerve tracts, where Schwann cells bound alpha-adrenoceptor, beta-adrenoceptor and CB-receptor ligands, with a mix of separate receptor locations and co-localization.

CONCLUSIONS AND IMPLICATIONS

Within each cell type, each GPCR had a distinctive heterogeneous distribution with limited co-localization, providing a guide to the possibilities for functional synergism, and suggesting a new paradigm for synergism in which interactions may be either between cells or involve converging intracellular signalling processes.

GPR55 ligands promote receptor coupling to multiple signalling pathways

BACKGROUND AND PURPOSE

Although GPR55 is potently activated by the endogenous lysophospholipid, L-alpha-lysophosphatidylinositol (LPI), it is also thought to be sensitive to a number of cannabinoid ligands, including the prototypic CB1 receptor antagonists AM251 and SR141716A (Rimonabant). In this study we have used a range of functional assays to compare the pharmacological activity of selected cannabinoid ligands, AM251, AM281 and SR141716A with LPI in a HEK293 cell line engineered to stably express recombinant, human GPR55.

EXPERIMENTAL APPROACH

We evaluated Ca(2+) signalling, stimulation of extracellular signal regulated kinase (ERK1/2) mitogen activated kinase MAP-kinases, induction of transcriptional regulators that are downstream of GPR55, including nuclear factor of activated T cells (NFAT), nuclear factor-kappaB (NF-kappaB) and cAMP response element binding protein (CREB), as well as receptor endocytosis. In addition, we assessed the suitability of a novel, label-free assay for GPR55 ligands that involves optical measurement of dynamic mass redistribution following receptor activation.

KEY RESULTS

GPR55 linked to a range of downstream signalling events and that the activity of GPR55 ligands was influenced by the functional assay employed, with differences in potency and efficacy observed.

CONCLUSIONS AND IMPLICATIONS

Our data help to resolve some of the issues surrounding the pharmacology of cannabinoid ligands at GPR55 and highlight some differences in effector coupling associated with distinct GPR55 ligands.

Cannabinoid CB1 receptor facilitation of substance P release in the rat spinal cord, measured as neurokinin 1 receptor internalization

The contribution of CB1 receptors in the spinal cord to cannabinoid analgesia is still unclear. The objective of this study was to investigate the effect of CB1 receptors on substance P release from primary afferent terminals in the spinal cord. Substance P release was measured as neurokinin 1 (NK1) receptor internalization in lamina I neurons. It was induced in spinal cord slices by dorsal root stimulation and in live rats by a noxious stimulus. In spinal cord slices, the CB1 receptor antagonists AM251, AM281 and rimonabant partially but potently inhibited NK1 receptor internalization induced by electrical stimulation of the dorsal root. This was due to an inhibition of substance P release and not of NK1 receptor internalization itself, because AM251 and AM281 did not inhibit NK1 receptor internalization induced by exogenous substance P. The CB1 receptor agonist ACEA increased NK1 receptor internalization evoked by dorsal root stimulation. The effects of AM251 and ACEA cancelled each other. In vivo, AM251 injected intrathecally decreased NK1 receptor internalization in spinal segments L5 and L6 induced by noxious hind paw clamp. Intrathecal AM251 also produced analgesia to radiant heat stimulation of the paw. The inhibition by AM251 of NK1 receptor internalization was reversed by antagonists of mu-opioid and GABA(B) receptors. This indicates that CB1 receptors facilitate substance P release by inhibiting the release of GABA and opioids next to primary afferent terminals, producing disinhibition. This results in a pronociceptive effect of CB1 receptors in the spinal cord.

Lysophosphatidylinositol induces rapid phosphorylation of p38 mitogen-activated protein kinase and activating transcription factor 2 in HEK293 cells expressing GPR55 and IM-9 lymphoblastoid cells

Lysophosphatidylinositol (LPI) is an endogenous ligand for GPR55, a putative novel type of cannabinoid receptor. In this study, we first examined the effects of LPI on p38 mitogen-activated protein kinase in HEK293 cells expressing GPR55. LPI induced the rapid phosphorylation of p38 mitogen-activated protein kinase in GPR55-expressing cells. No apparent effect was observed in the vector-transfected cells. The exposure of GPR55-expressing cells to LPI also triggered the phosphorylation of activating transcription factor 2 downstream of the p38 mitogen-activated protein kinase. Treatment of the cells with Y-27632 [a Rho-associated kinase (ROCK) inhibitor] blocked the LPI-induced phosphorylation of p38 mitogen-activated protein kinase and activating transcription factor 2, suggesting that the Rho-ROCK pathway is involved in these cellular responses. Notably, GPR55 was found to be abundantly expressed in lymphoid organs such as the spleen and thymus. We obtained evidence that rapid phosphorylation of p38 mitogen-activated protein kinase and activating transcription factor 2 also takes place in IM-9 lymphoblastoid cells, which naturally express GPR55, after stimulation with LPI. These results suggest that GPR55 and its endogenous ligand LPI play essential roles in the homoeostatic responses to stress signals in several mammalian tissues and cells including certain types of immune cells.

The polarised life of the endocannabinoid system in CNS development

The spatiotemporal expression of cannabinoid receptors and endocannabinoid-metabolising enzymes during brain development guides major developmental processes including neurogenesis, cell differentiation, cell migration, neuronal specification and synaptogenesis.Endocannabinoids (eCBs) play an important role in fine-tuning neurotransmission and have recently been shown to play an important role in brain development. The spatiotemporal expression of cannabinoid receptors and endocannabinoid-metabolising enzymes during development guides major developmental processes including neurogenesis, cell differentiation, cell migration, neuronal specification and synaptogenesis. Furthermore, pharmacological experiments and transgenic animal models have shown the impact of disrupted eCB signalling on normal brain development and revealed the danger of both cannabis abuse and exposure to cannabinoid drugs during embryonic development, childhood and adolescence. In this review, we focus on the dynamic expression of eCB components and the physiological role eCBs play during brain development.

The putative cannabinoid receptor GPR55 affects osteoclast function in vitro and bone mass in vivo

GPR55 is a G protein-coupled receptor recently shown to be activated by certain cannabinoids and by lysophosphatidylinositol (LPI). However, the physiological role of GPR55 remains unknown. Given the recent finding that the cannabinoid receptors CB(1) and CB(2) affect bone metabolism, we examined the role of GPR55 in bone biology. GPR55 was expressed in human and mouse osteoclasts and osteoblasts; expression was higher in human osteoclasts than in macrophage progenitors. Although the GPR55 agonists O-1602 and LPI inhibited mouse osteoclast formation in vitro, these ligands stimulated mouse and human osteoclast polarization and resorption in vitro and caused activation of Rho and ERK1/2. These stimulatory effects on osteoclast function were attenuated in osteoclasts generated from GPR55(-/-) macrophages and by the GPR55 antagonist cannabidiol (CBD). Furthermore, treatment of mice with this non-psychoactive constituent of cannabis significantly reduced bone resorption in vivo. Consistent with the ability of GPR55 to suppress osteoclast formation but stimulate osteoclast function, histomorphometric and microcomputed tomographic analysis of the long bones from male GPR55(-/-) mice revealed increased numbers of morphologically inactive osteoclasts but a significant increase in the volume and thickness of trabecular bone and the presence of unresorbed cartilage. These data reveal a role of GPR55 in bone physiology by regulating osteoclast number and function. In addition, this study also brings to light an effect of both the endogenous ligand, LPI, on osteoclasts and of the cannabis constituent, CBD, on osteoclasts and bone turnover in vivo.

Receptors for acylethanolamides-GPR55 and GPR119

Acylethanolamides are lipid substances widely distributed in the body, generated from a membrane phospholipid precursor, N-acylphosphatidylethanolamine (NAPE). The recent identification of arachidonoyl ethanolamide (anandamide or AEA) as an endogenous cannabinoid ligand has focused attention on acylethanolamides, which has further increased with the subsequent identification of related additional acylethanolamides with signaling function, such as oleoylethanolamide (OEA) and palmitoylethanolamide (PEA). Most of the biological functions of anandamide are mediated by the two G protein-coupled cannabinoid receptors identified to date, CB(1) and CB(2), with the transient receptor potential vanilloid-1 receptor being an additional potential target. There has been increasing pharmacological evidence for the existence of additional cannabinoid receptors, with the orphan G protein-coupled receptor GPR55 being the most actively scrutinized, and is one of the subjects of this review. The other receptor reviewed here is GPR119, which can recognize OEA and PEA. These two acylethanolamides, although structurally related to anandamide, do not interact with classical cannabinoid receptors. Instead, they have high affinity for the nuclear receptor PPARalpha, which is believed to mediate many of their biological effects.

The therapeutic potential of novel cannabinoid receptors

Cannabinoids produce a plethora of biological effects, including the modulation of neuronal activity through the activation of CB(1) receptors and of immune responses through the activation of CB(2) receptors. The selective targeting of either of these two receptor subtypes has clear therapeutic value. Recent evidence indicates that some of the cannabinomimetic effects previously thought to be produced through CB(1) and/or CB(2) receptors, be they on neuronal activity, on the vasculature tone or immune responses, still persist despite the pharmacological blockade or genetic ablation of CB(1) and/or CB(2) receptors. This suggests that additional cannabinoid and cannabinoid-like receptors exist. Here we will review this evidence in the context of their therapeutic value and discuss their true belonging to the endocannabinoid signaling system.

An introduction to the endocannabinoid system: from the early to the latest concepts

A rather complex and pleiotropic endogenous signalling system was discovered in the late 1990s, starting from studies on the mechanism of action of Delta(9)-tetrahydrocannabinol, the major psychoactive principle of the hemp plant Cannabis sativa. This system includes: (1) at least two G-protein-coupled receptors, known as the cannabinoid CB(1) and CB(2) receptors; (2) the endogenous agonists at these receptors, known as endocannabinoids, of which anandamide and 2-arachidonoylglycerol are the best known; and (3) proteins and enzymes for the regulation of endocannabinoid levels and action at receptors. The number of the members of this endocannabinoid signalling system seems to be ever increasing as new non-CB(1) non-CB(2) receptors for endocannabinoids, endocannabinoid-related molecules with little activity at CB(1) and CB(2) receptors, and new enzymes for endocannabinoid biosynthesis and degradation are being identified every year. The complexity of the endocannabinoid system and of its physiological and pathological function is outlined in this introductory chapter, for a better understanding of the subsequent chapters in this special issue.

The endocannabinoid anandamide from immunomodulation to neuroprotection. Implications for multiple sclerosis

Over the last decade, the endocannabinoid system (ECS) has emerged as a potential target for multiple sclerosis (MS) management. A growing amount of evidence suggests that cannabinoids may be neuroprotective during CNS inflammation. Advances in the understanding of the physiology and pharmacology of the ECS have potentiated the interest of several components of this system as useful biological targets for disease management. Alterations of the ECS have been recently implicated in a number of neuroinflammatory and neurodegenerative conditions, so that the pharmacological modulation of cannabinoid (CB) receptors and/or of the enzymes controlling synthesis, transport, and degradation of these lipid mediators is considered an option to treat several neurological diseases. This chapter focuses on our current understanding of the function of anandamide (AEA), its biological and therapeutic implications, as well as a description of its effects on neuroimmune modulation.

Endocannabinoid receptor pharmacology

This chapter will review the basic pharmacology of endocannabinoid receptors. As the best-described cannabinoid receptors are G-protein-coupled receptors (GPCRs), those will be the focus of this chapter. We will start with a basic review of GPCR signaling, as these concepts are critical to understanding the function of cannabinoid receptors. Next, several features of cannabinoid receptor signaling will be presented, with an emphasis on the effectors modulated by cannabinoid receptors. Finally, we will finish with a discussion of cannabinoid receptor agonists and antagonists and future directions. The aim of this chapter is to introduce the cannabinoid receptor pharmacology that will be necessary to appreciate the intricacies of endocannabinoid signaling presented in later chapters.

Endocannabinoid signaling in microglial cells

The endocannabinoid signaling system (eCBSS) is composed of cannabinoid (CB) receptors, their endogenous ligands (the endocannabinoids, eCB) and the enzymes that produce and inactivate these ligands. Neurons use this signaling system to communicate with each other and Delta9-tetrahydrocannabinol (THC), the main psychotropic ingredient of Cannabis sativa, induces profound behavioral effects by impinging on this communication. Evidence now shows that microglia, the macrophages of the brain, also express a functional eCBSS and that activation of CB receptors expressed by activated microglia controls their immune-related functions. This review summarizes this evidence, discusses how microglia might use the eCBSS to communicate with each other and neighboring cells, and argues that compounds selectively targeting the eCBSS expressed by microglia constitute valuable therapeutics to manage acute and chronic neuroinflammation, without inducing the psychotropic effects and underlying addictive properties commonly associated with THC.

Wiring and firing neuronal networks: endocannabinoids take center stage

Endocannabinoids (eCBs) function as retrograde messengers at both excitatory and inhibitory synapses, and control various forms of synaptic plasticity in the adult brain. The molecular machinery required for specific eCB functions during synaptic plasticity is well established. However, eCB signaling plays surprisingly fundamental roles in controlling the acquisition of neuronal identity during CNS development. Recent work suggests that selective recruitment of regulatory signaling networks to CB1 cannabinoid receptors dictates neuronal state-change decisions. In addition, the spatial localization and temporal precision of eCB actions emerges as a novel organizer in developing neuronal networks. Current challenges include fitting novel molecular candidates into regulatory eCB signaling pathways, and defining the temporal dynamics of context-dependent signaling mechanisms underpinning particular neuronal specification events.