Involvement of the 90-kDa Heat Shock Protein (Hsp-90) in CB2 Cannabinoid Receptor-Mediated Cell Migration: A New Role of Hsp-90 in Migration Signaling of a G Protein-Coupled Receptor

Cannabinoid receptor CB1 and CB2 interacting proteins: Techniques, progress and perspectives

Cannabinoid receptors 1 and 2 (CB₁ and CB₂) are implicated in a range of physiological processes and have gained attention as promising therapeutic targets for a number of diseases. Protein-protein interactions play an integral role in modulating G protein-coupled receptor (GPCR) expression, subcellular distribution and signaling, and the identification and characterization of these will not only improve our understanding of GPCR function and biology, but may provide a novel avenue for therapeutic intervention. A variety of techniques are currently being used to investigate GPCR protein-protein interactions, including Förster/fluorescence and bioluminescence resonance energy transfer (FRET and BRET), proximity ligation assay (PLA), and bimolecular fluorescence complementation (BiFC). However, the reliable application of these methodologies is dependent on the use of appropriate controls and the consideration of the physiological context. Though not as extensively characterized as some other GPCRs, the investigation of CB₁ and CB₂ interacting proteins is a growing area of interest, and a range of interacting partners have been identified to date. This review summarizes the current state of the literature regarding the cannabinoid receptor interactome, provides commentary on the methodologies and techniques utilized, and discusses future perspectives.

The Role of Heat Shock Proteins in Regulating Receptor Signal Transduction

Heat shock proteins (Hsp) are a class of stress-inducible proteins that mainly act as molecular protein chaperones. This chaperone activity is diverse, including assisting in nascent protein folding and regulating client protein location and translocation within the cell. The main proteins within the Hsp family, particularly Hsp70 and Hsp90, also have a highly diverse and numerous set of protein clients, which when combined with the high expression levels of Hsp proteins (2%-6% of total protein content) establishes these molecules as "central regulators" of cell protein physiology. Among the client proteins, Hsps regulate numerous signal-transduction and receptor-regulatory kinases, and indeed directly regulate some receptors themselves. This also makes the Hsps, particularly Hsp90, central regulators of signal-transduction machinery, with important impacts on endogenous and drug ligand responses. Among these roles, Hsp90 in particular acts to maintain mature signaling kinases in a metastable conformation permissive for signaling activation. In this review, we will focus on the roles of the Hsps, with a special focus on Hsp90, in regulating receptor signaling and subsequent physiologic responses. We will also explore potential means to manipulate Hsp function to improve receptor-targeted therapies. Overall, Hsps are important regulators of receptor signaling that are receiving increasing interest and exploration, particularly as Hsp90 inhibitors progress toward clinical approval for the treatment of cancer. Understanding the complex interplay of Hsp regulation of receptor signaling may provide important avenues to improve patient treatment.

Immunolocalization of cannabinoid receptor type 1 and CB2 cannabinoid receptors, and transient receptor potential vanilloid channels in pterygium

Cannabinoids, as multi-target mediators, activate cannabinoid receptors and transient receptor potential vanilloid (TRPV) channels. There is evidence to support a functional interaction of cannabinoid receptors and TRPV channels when they are coexpressed. Human conjunctiva demonstrates widespread cannabinoid receptor type 1 (CB1), CB2 and TRPV channel localization. The aim of the present study was to investigate the expression profile for cannabinoid receptors (CB1 and CB2) and TRPV channels in pterygium, an ocular surface lesion originating from the conjunctiva. Semi-serial paraffin-embedded sections from primary and recurrent pterygium samples were immunohistochemically examined with the use of specific antibodies. All of the epithelial layers in 94, 78, 96, 73 and 80% of pterygia cases, exhibited CB1, CB2, TRPV1, TRPV2 and TRPV3 cytoplasmic immunoreactivity, respectively. The epithelium of all pterygia cases (100%) showed strong, mainly nuclear, TRPV4 immunolocalization. In the pterygium stroma, scattered cells demonstrated intense CB2 immunoreactivity, whereas vascular endothelial cells were immunopositive for the cannabinoid receptors and all TRPV channels. Quantitative analyses of the immunohistochemical findings in epithelial cells demonstrated a significantly higher expression level in conjunctiva compared with primary pterygia (P=0.04) for CB1, but not for CB2 (P>0.05). Additionally, CB1 and CB2 were significantly highly expressed in primary pterygia (P=0.01), compared with recurrent pterygia. Furthermore, CB1 expression levels were significantly correlated with CB2 expression levels in primary pterygia (P=0.005), but not in recurrent pterygia (P>0.05). No significant difference was detected for all TRPV channel expression levels between pterygium (primary or recurrent) and conjunctival tissues (P>0.05). A significant correlation between the TRPV1 and TRPV3 expression levels (P<0.001) was detected independently of pterygium recurrence. Finally, TRPV channel expression was identified to be significantly higher than the expression level of cannabinoid receptors in the pterygium samples (P<0.001). The differentiated expression of cannabinoid receptors in combination with the presence of TRPV channels, in primary and recurrent pterygia, imply a potential role of these cannabinoid targets in the underlying mechanisms of pterygium.

Hyponatremia and V2 vasopressin receptor upregulation: a result of HSP90 inhibition

Purpose

Small-molecule inhibitors of heat-shock protein 90 (HSP90) have been under development as chemotherapeutic agents. The adverse events reported from early clinical trials included hyponatremia. Given the limited number of patients enrolled, the number of hyponatremia incidents was remarkable and repeatedly, the event was judged as severe. Inappropriate V2 vasopressin receptor stimulation is an established cause of hyponatremia. We explored the hypothesis that HSP90 inhibition produces hypersensitivity to vasopressin by upregulating V2-receptors.

Methods

Experiments were carried out in cell culture using HEK293 cells with heterologous expression of the human V2-receptor and HELA cells with an endogenous V2-receptor complement. We tested the effect of HSP90 inhibition by three structurally unrelated compounds (alvespimycin, luminespib, radicicol) and asserted its specificity in cells depleted of cytosolic HSP90 (by RNA interference). Assays encompassed surface V2-receptor density and vasopressin-stimulated formation of cyclic AMP (cAMP).

Results

The results demonstrate a twofold increase in cell-surface receptor density following pre-incubation with each of the HSP90 inhibitors. The effect had a concentration-dependence consistent with the individual potencies to inhibit HSP90. Similarly, depletion of cytosolic HSP90 increased surface-receptor density and at the same time, reduced the inhibitor effect. Upregulated V2-receptors were fully functional; hence, in culture treated with an HSP90 inhibitor, addition of vasopressin resulted in higher levels of cAMP than in controls.

Conclusion

Since formation of cAMP is the first signalling step in raising water permeability of the collecting duct epithelia, we suggest that V2-receptor upregulation generates hypersensitivity to vasopressin linking HSP90 inhibition to the development of hyponatremia.

The CB2 receptor and its role as a regulator of inflammation

The CB2 receptor is the peripheral receptor for cannabinoids. It is mainly expressed in immune tissues, highlighting the possibility that the endocannabinoid system has an immunomodulatory role. In this respect, the CB2 receptor was shown to modulate immune cell functions, both in cellulo and in animal models of inflammatory diseases. In this regard, numerous studies have reported that mice lacking the CB2 receptor have an exacerbated inflammatory phenotype. This suggests that therapeutic strategies aiming at modulating CB2 signaling could be promising for the treatment of various inflammatory conditions. Herein, we review the pharmacology of the CB2 receptor, its expression pattern, and the signaling pathways induced by its activation. We next examine the regulation of immune cell functions by the CB2 receptor and the evidence obtained from primary human cells, immortalized cell lines, and animal models of inflammation. Finally, we discuss the possible therapies targeting the CB2 receptor and the questions that remain to be addressed to determine whether this receptor could be a potential target to treat inflammatory disease.

Novel indole-based compounds that differentiate alkylindole-sensitive receptors from cannabinoid receptors and microtubules: Characterization of their activity on glioma cell migration

Indole-based compounds, such as the alkyl-indole (AI) compound WIN55212-2, activate the cannabinoid receptors, CB₁ and CB₂, two well-characterized G protein-coupled receptors (GPCR). Reports indicate that several indole-based cannabinoid agonists, including WIN55212-2, lack selectivity and interact with at least two additional targets: AI-sensitive GPCRs and microtubules. Studying how indole-based compounds modulate the activity of these 4 targets has been difficult as selective chemical tools were not available. Here we report the pharmacological characterization of six newly-developed indole-based compounds (ST-11, ST-23, ST-25, ST-29, ST-47 and ST-48) that exhibit distinct binding affinities at AI-sensitive receptors, cannabinoid CB₁ and CB₂ receptors and the colchicine site of tubulin. Several compounds exhibit some level of selectivity for AI-sensitive receptors, including ST-11 that binds AI-sensitive receptors with a K_d of 52nM and appears to have a weaker affinity for the colchicine site of tubulin (K_d=3.2μM) and does not bind CB₁/CB₂ receptors. Leveraging these characteristics, we show that activation of AI-sensitive receptors with ST-11 inhibits both the basal and stimulated migration of the Delayed Brain Tumor (DBT) mouse glioma cell line. Our study describes a new series of indole-based compounds that enable the pharmacological and functional differentiation of alkylindole-sensitive receptors from cannabinoid receptors and microtubules.

Cannabinoid Receptors Are Overexpressed in CLL but of Limited Potential for Therapeutic Exploitation

The cannabinoid receptors 1 and 2 (CNR1&2) are overexpressed in a variety of malignant diseases and cannabinoids can have noteworthy impact on tumor cell viability and tumor growth. Patients diagnosed with chronic lymphocytic leukemia (CLL) present with very heterogeneous disease characteristics translating into highly differential risk properties. To meet the urgent need for refinement in risk stratification at diagnosis and the search for novel therapies we studied CNR expression and response to cannabinoid treatment in CLL. Expression levels of CNR1&2 were determined in 107 CLL patients by real-time PCR and analyzed with regard to prognostic markers and survival. Cell viability of primary CLL cells was determined in suspension and co-culture after incubation in increasing cannabinoid concentrations under normal and reduced serum conditions and in combination with fludarabine. Impact of cannabinoids on migration of CLL cells towards CXCL12 was determined in transwell plates. We found CNR1&2 to be overexpressed in CLL compared to healthy B-cells. Discriminating between high and low expressing subgroups, only high CNR1 expression was associated with two established high risk markers and conferred significantly shorter overall and treatment free survival. Viability of CLL primary cells was reduced in a dose dependent fashion upon incubation with cannabinoids, however, healthy cells were similarly affected. Under serum reduced conditions, no significant differences were observed within suspension and co-culture, respectively, however, the feeder layer contributed significantly to the survival of CLL cells compared to suspension culture conditions. No significant differences were observed when treating CLL cells with cannabinoids in combination with fludarabine. Interestingly, biologic activity of cannabinoids was independent of both CNR1&2 expression. Finally, we did not observe an inhibition of CXCL12-induced migration by cannabinoids. In contrast to other tumor entities, our data suggest a limited usability of cannabinoids for CLL therapy. Nonetheless, we could define CNR1 mRNA expression as novel prognostic marker.

A G protein-coupled receptor and the intracellular synthase of its agonist functionally cooperate

The GPCR DP1 promotes the activity of L-PGDS, the enzyme that produces the DP1 agonist PGD₂, while at the same time L-PGDS promotes the export and activity of DP1 in response to PGD₂.

Export of newly synthesized G protein–coupled receptors (GPCRs) remains poorly characterized. We show in this paper that lipocalin-type prostaglandin D₂ (PGD₂) synthase (L-PGDS) interacts intracellularly with the GPCR DP1 in an agonist-independent manner. L-PGDS promotes cell surface expression of DP1, but not of other GPCRs, in HEK293 and HeLa cells, independent of L-PGDS enzyme activity. In addition, formation of a DP1–Hsp90 complex necessary for DP1 export to the cell surface is dependent on the interaction between L-PGDS and the C-terminal MEEVD residues of Hsp90. Surprisingly, PGD₂ synthesis by L-PGDS is promoted by coexpression of DP1, suggesting a possible intracrine/autocrine signaling mechanism. In this regard, L-PGDS increases the formation of a DP1–ERK1/2 complex and increases DP1-mediated ERK1/2 signaling. Our findings define a novel cooperative mechanism in which a GPCR (DP1) promotes the activity of the enzyme (L-PGDS) that produces its agonist (PGD₂) and in which this enzyme in turn acts as a cofactor (of Hsp90) to promote export and agonist-dependent activity of the receptor.

Novel, gel-free proteomics approach identifies RNF5 and JAMP as modulators of GPCR stability

The maturation and folding of G protein-coupled receptors are governed by mechanisms that remain poorly understood. In an effort to characterize these biological events, we optimized a novel, gel-free proteomic approach to identify partners of the β2-adrenergic receptor (β2AR). In addition to a number of known interacting proteins such as heterotrimeric G protein subunits, this allowed us to identify proteins involved in endoplasmic reticulum (ER) QC of the receptor. Among β2AR-associated proteins is Ring finger protein 5 (RNF5), an E3 ubiquitin ligase anchored to the outer membrane of the ER. Coimmunoprecipitation assays confirmed, in a cellular context, the interaction between RNF5 and the β2AR as well as the prostaglandin D2 receptor (DP). Confocal microscopy revealed that DP colocalized with RNF5 at the ER. Coexpression of RNF5 with either receptor increased levels of their expression, whereas small interfering RNA-mediated knockdown of endogenous RNF5 promoted the opposite. RNF5 did not modulate the ubiquitination state of β2AR or DP. Instead, RNF5 ubiquitinated JNK-associated membrane protein (JAMP), a protein that recruits the proteasome to the ER membrane and that is negatively regulated by RNF5-mediated ubiquitination. JAMP coimmunoprecipitated with both β2AR and DP and decreased total receptor protein levels through proteasomal degradation. Expression of DP, a receptor largely retained in the ER, promoted proteasome recruitment by JAMP. Degradation of both receptors via JAMP was increased when RNF5 was depleted. Our data suggest that RNF5 regulates the turnover of specific G protein-coupled receptors by ubiquitinating JAMP and preventing proteasome recruitment.

Regulation of G protein signaling by the 70kDa heat shock protein

G protein-coupled receptors (GPCRs) transduce extracellular signals to the interior of the cell by activating membrane-bound guanine nucleotide-binding regulatory proteins (G proteins). An increasing number of proteins have been reported to bind to and regulate GPCRs. We report a novel regulation of the alpha(2A) adrenergic receptor (α(2A)-R) by the ubiquitous stress-inducible 70kDa heat shock protein, hsp70. Hsp70, but not hsp90, attenuated G protein-dependent high affinity agonist binding to the α(2A)-R in Sf9 membranes. Antagonist binding was unchanged, suggesting that hsp70 uncouples G proteins from the receptor. As hsp70 did not bind G proteins but complexed with the α(2A)-R in intact cells, a direct interaction with the receptor seems likely. In the presence of hsp70, α(2A)-R-catalyzed [(35)S]GTPγS binding was reduced by approximately 70%. In contrast, approximately 50-fold higher concentrations of hsp70 were required to reduce agonist binding to the stress-inducible 5-hydroxytryptamine(1A) receptor (5-HT(1A)-R). In heat-stressed CHO cells, the α(2A)-R was significantly uncoupled from G proteins, coincident with an increased localization of hsp70 at the membrane. The contrasting effect of hsp70 on the α(2A)-R compared to the 5-HT(1A)-R suggests that during stress, upregulation of hsp70 may attenuate signaling from specific GPCRs as part of the stress response to foster survival.

Δ9-THC increases endogenous AHA1 expression in rat cerebellum and may modulate CB1 receptor function during chronic use

To characterize the long-term effects of adolescent marijuana abuse, we performed a proteomic analysis of cerebellar extracts from adult female rats with and without ovariectomy that were treated with Δ9-THC for 40 days during adolescence. Six proteins were found to significantly differ among the four treatment groups, with Δ9-THC and ovariectomy (OVX) decreasing the mitochondrial proteins, pyruvate carboxylase and NADH dehydrogenase, whereas the levels of putative cytosolic molecular chaperones NM23B, translationally controlled tumor protein, DJ-1 and activator of heat-shock 90kDa protein ATPase homolog 1 (AHA1) were increased. We further analyzed the effects of AHA1, a HSP90 co-chaperone, on CB1R and CB2R trafficking and signaling in transfected HEK293T and Neuro-2A cells. In HEK293T cells, AHA1 over-expression enhanced plasma membrane levels of CB1R and increased CB1R-mediated effects on cAMP levels and on MAPK phosphorylation. AHA1 over-expression also enhanced cell surface levels of endogenous CB1R and the effects of Δ9-THC on the cAMP levels in Neuro-2A cells. In contrast, over-expression of AHA1 did not affect the subcellular localization and signaling of CB2R. Our data indicate that chronic Δ9-THC administration in adolescence altered the endogenous levels of specialized proteins in the cerebellum, such as AHA1, and that this protein can change CB1R cell surface levels and signaling.

Modulation of α(2C) adrenergic receptor temperature-sensitive trafficking by HSP90

Decreasing the temperature to 30°C is accompanied by significant enhancement of α(2C)-AR plasma membrane levels in several cell lines with fibroblast phenotype, as demonstrated by radioligand binding in intact cells. No changes were observed on the effects of low-temperature after blocking receptor internalization in α(2C)-AR transfected HEK293T cells. In contrast, two pharmacological chaperones, dimethyl sulfoxide and glycerol, increased the cell surface receptor levels at 37°C, but not at 30°C. Further, at 37°C α(2C)-AR is co-localized with endoplasmic reticulum markers, but not with the lysosomal markers. Treatment with three distinct HSP90 inhibitors, radicicol, macbecin and 17-DMAG significantly enhanced α(2C)-AR cell surface levels at 37°C, but these inhibitors had no effect at 30°C. Similar results were obtained after decreasing the HSP90 cellular levels using specific siRNA. Co-immunoprecipitation experiments demonstrated that α(2C)-AR interacts with HSP90 and this interaction is decreased at 30°C. The contractile response to endogenous α(2C)-AR stimulation in rat tail artery was also enhanced at reduced temperature. Similar to HEK293T cells, HSP90 inhibition increased the α(2C)-AR contractile effects only at 37°C. Moreover, exposure to low-temperature of vascular smooth muscle cells from rat tail artery decreased the cellular levels of HSP90, but did not change HSP70 levels. These data demonstrate that exposure to low-temperature augments the α(2C)-AR transport to the plasma membrane by releasing the inhibitory activity of HSP90 on the receptor traffic, findings which may have clinical relevance for the diagnostic and treatment of Raynaud Phenomenon.

Cannabinoid receptor 2 and its agonists mediate hematopoiesis and hematopoietic stem and progenitor cell mobilization

Endocannabinoids are arachidonic acid derivatives and part of a novel bioactive lipid signaling system, along with their G-coupled cannabinoid receptors (CB₁ and CB₂) and the enzymes involved in their biosynthesis and degradation. However, their roles in hematopoiesis and hematopoietic stem and progenitor cell (HSPC) functions are not well characterized. Here, we show that bone marrow stromal cells express endocannabinoids (anandamide and 2-arachidonylglycerol), whereas CB₂ receptors are expressed in human and murine HSPCs. On ligand stimulation with CB₂ agonists, CB₂ receptors induced chemotaxis, migration, and enhanced colony formation of bone marrow cells, which were mediated via ERK, PI3-kinase, and Gαi-Rac1 pathways. In vivo, the CB₂ agonist AM1241 induced mobilization of murine HSPCs with short- and long-term repopulating abilities. In addition, granulocyte colony-stimulating factor -induced mobilization of HSPCs was significantly decreased by specific CB₂ antagonists and was impaired in Cnr2(-/-) cannabinoid type 2 receptor knockout mice. Taken together, these results demonstrate that the endocannabinoid system is involved in hematopoiesis and that CB₂/CB₂ agonist axis mediates repopulation of hematopoiesis and mobilization of HSPCs. Thus, CB₂ agonists may be therapeutically applied in clinical conditions, such as bone marrow transplantation.

Functional consequences of nonsynonymous single nucleotide polymorphisms in the CB2 cannabinoid receptor

OBJECTIVE

To test the hypothesis that the two nonsynonymous single nucleotide polymorphisms at the CB2 cannabinoid receptor gene may have functional consequences on human CB2.

METHODS

Q63R, H316Y, and Q63R/H316 mutations were made in recombinant human CB2 by the method of site-directed mutagenesis. After these mutant CB2 receptors were stably transfected into HEK293 cells, ligand binding, ligand-induced activity, and constitutive activity assays were performed to test the functional significance of these mutations.

RESULTS

In general, our results showed that the CB2 polymorphic receptors are able to bind cannabinoid ligands and mediate signal transduction. However, in ligand-induced cyclic AMP accumulation assays, the cannabinoid agonists WIN55212-2 and 2-arachidonoylglycerol had reduced efficacy in cells expressing the polymorphic receptors as compared with the CB2 wild-type receptor. Furthermore, in constitutive activity assays, the H316Y and Q63R/H316Y polymorphic receptors exhibited higher constitutive activity than the CB2 wild-type receptor.

CONCLUSION

Our data shows that the presence of the polymorphisms at both positions 63 and 316 produce alterations in the CB2 receptor functions. Moreover, these findings strengthen the idea that the CB2 polymorphic receptors may contribute to the etiology of certain diseases.

Temporal proteomics profiling of lipid rafts in CCR6-activated T cells reveals the integration of actin cytoskeleton dynamics

Chemokines orchestrate leukocyte migration toward sites of inflammation and infection and target leukocytes via chemokine receptors such as CCR6, a subfamily of the seven-transmembrane G-protein-coupled receptors. Lipid rafts are cholesterol and sphingolipid-enriched liquid-ordered membrane microdomains thought to serve as scaffolding platforms for signal transduction. To globally understand the dynamic changes of proteins within lipid rafts upon CCR6 activation in T cells, we quantitatively analyzed the time-dependent changes of lipid raft proteome using our recently reported membrane proteomics strategy combining gel-assisted digestion, iTRAQ labeling and LC-MS/MS. To our knowledge, the error-free identification of 852 proteins represents the first data set of the raft proteome in T cells upon chemokine receptor activation, including 354 previously annotated raft proteins and 85 dynamically recruited proteins that are potential raft-associated proteins. The temporal profiles revealed that many proteins involved in the actin cytoskeleton rearrangement are actively recruited into lipid rafts upon CCR6 activation. We further confirmed the proteomics results by Western blotting and used small interfering RNA-mediated knockdown to evaluate their roles upon CCR6 activation. In sum, we employed quantitative proteomic strategy to analyze raft proteome and identified many molecules actively involved in the control of actin assembly and disassembly regulating CCR6 activation-induced cell migration.

Functionally selective cannabinoid receptor signalling: therapeutic implications and opportunities

The CB(1) and CB(2) cannabinoid receptors are G protein-coupled receptors (GPCRs) recognized by a variety of endogenous ligands and activating multiple signalling pathways. This multiplicity of ligands and intracellular transduction mechanisms supports a complex control of physiological functions by the endocannabinoid system, but requires a finely tuned regulation of the signalling events triggered on receptor activation. Here we review the diverse signalling pathways activated by the cannabinoid receptors and discuss the mechanisms allowing for specificity in the associated functional responses triggered by endogenous or exogenous ligands. At variance with the classical concept that all agonists at a given GPCR induce a similar repertoire of downstream events in all tissues, we also summarize the experimental evidence supporting the existence of functional selectivity and protean agonism at cannabinoid receptors. By placing emphasis on the ligand- or constitutive activity-dependent specifications of receptor-G protein coupling, these concepts explain how distinct cannabinoid ligands may activate specific downstream mediators. Finally, although both the diversity and specificity in cannabinoid signalling are now established in vitro, few data are available from in vivo studies. Therefore, we conclude this review by examining the experimental evidence supporting the physiological relevance of this complexity in the cannabinoid system. The ability to selectively manipulate physiological functions, through activation of defined signalling cascades, will in all likelihood help in the development of efficacious and safe cannabinoid-based therapeutics for a variety of indications.

G protein signaling controls the differentiation of multiple cell lineages

G protein-coupled receptors (GPCRs) detect a great diversity of extracellular stimuli ranging from hormonal peptides, chemokines, neurotransmitters, lipids, nucleotides, amino acids, biogenic amines to ions. G protein-coupled pathways regulate a rich collection of biological processes involved in normal physiological function of the body as well as in pathological progression of diseases. In addition to their function in postmitotic steady-state tissues, GPCRs have been implicated in the differentiation of stem cells and tissue specific progenitor cells during development. Examples of these include the functions of nucleotides and neuropeptides in neuronal differentiation and axon growth, chemokines in lymphocyte differentiation and activation, and other GPCR-mediated processes in the differentiation of adipocytes, osteoblasts and smooth muscle cells. This review summarizes the recent advances in our understanding of the importance of GPCR-linked signaling cascades in the differentiation of different cell lineages.

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.