The Endoterpenoid System: A Membrane‐Based Logic Framework for Integrating Cannabinoid, Olfactory, and Lipid‐Sensitive Receptor Crosstalk

ABSTRACT

The Endoterpenoid System (ETS) is a proposed new membrane‐based framework that ensembles classical endocannabinoid signaling (ECS) with ectopically expressed olfactory GPCRs, ion channels, and microbially derived terpenoids. Integrated with this model is the proposed endoterpenoidome (eTBome) machine: a chemically diverse pool of endogenous cannabinoids, dietary terpenoids, and microbial metabolites whose lipophilicity drives partitioning into cholesterol‐rich lipid rafts. Within these nanoscale microdomains, ETS components undergo co‐localization and co‐expression, enabling receptor heteromerization and β‐arrestin‐scaffolded signalosomes that facilitate dynamic crosstalk and emergent responses such as bistable apoptotic switches and context specific modulation of Gᵢ/o, Gₛ/olf, and Gq pathways. We detail molecular mechanisms underpinning ETS behavior, including lipid mediated allostery, scaffold protein recruitment, and lateral diffusion effects, and highlight the gut microbiota as pivotal architects of the eTBome through terpene synthase‐driven production of bioactive ligands. Germ free and antibiotic treated animal models underscore the dependence of ETS ligand pools and receptor distributions on microbial colonization. This article proposes key avenues for experimental validation, including high resolution structural studies of CB‐OR heteromers, quantitative lipidomic mapping of eTBome distributions, live cell super resolution imaging of receptor nanoclusters, and gnotobiotic manipulations of microbial terpene synthesis. By introducing them as proposals to the scientific community and by integrating ECS/eCBome biology with olfactory receptor (OR) and microbial metabolite research, the ETS/eTBome concept offers a cohesive hypothesis for lipid‐driven GPCR crosstalk and indicated non‐standard targets for therapeutic modulation in cancer, neuroinflammation, and metabolic disorders.

Significance Statement

Here, we define the Endoterpenoid System (ETS) as a membrane‐focused signaling framework that integrates endocannabinoids, diet‐derived and microbial terpenoids, lipid metabolism, and GPCR interactions within cholesterol‐rich lipid raft microdomains. This study, which introduces the concepts of the ETS and the Endoterpenoidome (eTBome), offers a holistic perspective that goes beyond the classical endocannabinoid system and establishes a conceptual foundation for future studies on mechanisms, biomarker discovery, and therapeutic development in cancer, neurodegenerative, metabolic, and inflammatory diseases.

Proposed conceptual framework of the Endoterpenoid System (ETS): Here, the ETS is proposed as a hypothesized biological framework in which signaling does not occur in isolation, but rather within highly organized “communication hubs” on cell membranes. Within this framework, the spatial and temporal coordination of these components within cholesterol‐rich membrane microdomains may facilitate inter‐receptor interactions, heteromerization, and ligand‐dependent interactions involving cannabinoid receptors, olfactory receptors, TRP channels, and other GPCRs. While it is proposed that these interactions influence signaling pathways, including those associated with Gi/o‐, Gs/olf‐, and Gq‐related pathways, β‐arrestin‐dependent signaling, and cAMP‐related signaling dynamics, receptor function may depend on membrane organization and microdomain integrity. Notwithstanding its demonstration in Class A, the evidentiary basis of this phenomenon may encompass the participation of additional GPCR classes, pointing toward the broader theoretical reach of the hypothesis. The figure is a conceptual model intended to encourage future experimental research rather than a representation of established molecular mechanisms.

Abbreviations

2‐AG

2‐arachidonoylglycerol

AEA

anandamide

CB1

cannabinoid receptor 1

CB2

cannabinoid receptor 2

eCBome

endocannabinoidome

ECS

endocannabinoid system

eTBome

endoterpenoidome

ETS

endoterpenoid system

OR

olfactory receptor

TRPV1

transient receptor potential vanilloid 1

1. Introduction

Despite decades of research into the endocannabinoid system (ECS) and independent characterization of ectopically expressed olfactory receptors (ORs), a unifying framework that situates these lipid‐sensitive GPCRs within a common membrane context has remained elusive (Di Marzo et al. 2004). The Endoterpenoid system (ETS) addresses this gap by proposing that cannabinoid receptors (CB1, CB2) and non‐olfactory ORs are embedded within dynamic, cholesterol‐rich membrane microdomains commonly referred to as lipid rafts, which serve as platforms for receptor co‐expression and direct functional crosstalk (Lingwood and Simons 2010; Sarnataro et al. 2005).

Central to ETS is the chemically diverse endoterpenoidome (eTBome), comprising endogenous cannabinoids such as anandamide (AEA) and 2‐arachidonoylglycerol (2‐AG), alongside dietary and microbiota‐derived terpenoids, including geraniol, limonene, citronellal, nerol, and citral whose inherent lipophilicity drives membrane partitioning and facilitates promiscuous engagement of GPCR targets (Kim et al. 2017).

Classical endocannabinoid signaling is characterized by on‐demand synthesis of AEA and 2‐AG from membrane phospholipids and their rapid hydrolysis by fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL), enabling tight spatial and temporal control of Gᵢ/o‐ and Gq‐related pathways. Whereas ORs, though first defined in olfactory sensory neurons, are now recognized in diverse non‐olfactory tissues, where they detect volatile and non‐volatile ligands and modulate cellular functions via Golf/cAMP signaling cascades (Flegel et al. 2013). The ETS hypothesis thus extends classical ECS biology by integrating a chemically diverse eTBome into a membrane‐based logic architecture, in which lipid rafts coordinate ligand synthesis, receptor heteromerization, and downstream signal integration to produce context‐dependent signaling outputs.

The ETS is emerging as a unifying framework for understanding how lipid‐derived molecules such as cannabinoids and terpenoids modulate signaling through co‐expressed G protein‐coupled receptors (GPCRs) embedded in cholesterol‐rich domains of the cell membrane. A fundamental feature of the ETS is that receptor activation does not necessarily correspond to receptor occupancy. This concept is best illustrated by the pharmacological behavior of Δ⁹‐tetrahydrocannabinol (THC), the principal psychoactive constituent of cannabis, which binds to cannabinoid receptor 1 (CB1) but does not fully activate it. Instead, THC acts as a partial agonist, producing submaximal signaling output even when all available receptors are occupied. The idea that ligand binding and receptor activation are distinct properties challenges classical models of GPCR pharmacology, where maximal receptor occupancy was assumed to lead to maximal downstream signaling.

CB receptors and ectopically expressed ORs often co‐exist within the same lipid microdomains, especially lipid rafts, dynamic nanodomains enriched in cholesterol and sphingolipids that serve as organizing platforms for membrane proteins. In various cell types, CB1 receptors partition into detergent‐resistant, cholesterol‐rich microdomains alongside caveolin 1, regulating G‐protein coupling and attenuating G_s/adenylyl cyclase signaling. OR51E2, known as Olfr78 in mice, likewise localizes to cholesterol‐rich lipid raft microdomains, a distribution critical for its sensing of short‐chain fatty acids and regulation of renin secretion. Disruption of these rafts by cholesterol depletion impairs OR51E2 signaling, underscoring the importance of the local lipid environment.

Both CB receptors and ORs are Class A GPCRs sharing conserved activation motifs and responding to lipid‐like ligands that partition into the membrane. Within the ETS, they function as cooperative sensors that integrate chemical information from endogenous metabolism, dietary intake, and microbial metabolism through graded, context‐sensitive signaling. In summary, the ETS provides a flexible molecular infrastructure for encoding and integrating lipidic signals via a network of co‐expressed and co‐localized GPCRs. THC's behavior as a partial agonist illustrates how receptor activation is more nuanced than occupancy alone, and how partial agonism can act as signal filtering. When combined with olfactory GPCRs, membrane microdomains, and terpenoid diversity, the ETS emerges as a biochemical logic system capable of encoding complex internal and external states through the interplay of receptor occupancy, intrinsic efficacy, and membrane dynamics. The main components and functional roles of ETS are summarized in Table 1. Accordingly, ETS should be treated as a nomination framework rather than as proof that any specific cannabinoid‐olfactory‐TRP assembly has already been validated. A prespecified MECS‐BER framework formalises this inferential boundary by requiring one membrane prior, one locked proximal outcome and one nominated cannabinoid GPCR, ectopic olfactory GPCR and TRP‐channel arm before k‐based adjudication of independent, pariwise or higher‐order behaviour (Yarar 2026).

TABLE 1.

Core components of the ETS and their functional roles.

Component	Subtypes/examples	Functional role	Mechanism/pathway
Endocannabinoids	AEA, 2‐AG	Synaptic inhibition, inflammation, apoptosis	Gi/o and Gq signaling, cAMP suppression, Ca2+ release
Olfactory receptors (ORs)	OR51E2, OR51E1, OR2J3	Cell differentiation, apoptosis	Gs/olf‐cAMP axis, GLP‐1 secretion in enteroendocrine cells
Terpenoids	β‐Caryophyllene, Limonene, Perillyl alcohol (POH), CBG	Pro‐apoptotic, anti‐inflammatory effects	Direct/indirect receptor agonism/antagonism, lipid‐raft partitioning
Accessory GPCRs	GPR55, GPR18	Cell motility, chemotaxis, inflammation	Ca2+ flux, Gi/o or Gq mediated signaling
Ion channels	TRPV, TRPM, TRPA	Pain perception, osmotic pressure, volume, stretch, and vibration	Activated by AEA, heat, or capsaicin induces Ca2+ influx
Enzymes	N‐acyl phosphatidylethanolamine‐specific phospholipase D (NAPE‐PLD), Diacylglycerol lipase (DAGL), FAAH, MAGL	Endocannabinoid synthesis and degradation	On‐demand synthesis/degradation within lipid rafts
Transport proteins	FABP5, FABP7	AEA transport	Shuttling AEA to degradation sites
Microbial terpenoids	β‐ionone, ginsenoside metabolites, POH	Metabolic regulation, immune modulation	Microbiota‐derived; integrate into rafts and modulate GPCRs

2. Theoretical Framework and Materials and Methods

2.1. Literature Search

A systematic review of the literature was conducted in PubMed, Scopus and Web of Science through June 2025, using the search terms “endocannabinoid,” “olfactory receptor,” “lipid raft,” “terpenoid,” and “GPCR heteromerization.” Titles and abstracts were screened for relevance; full‐text articles were then reviewed to extract data on receptor expression, membrane microdomain biology, terpenoid chemistry, and microbial terpene synthesis.

2.2. Phylogenomic Analyses

Phylogenomic insights were synthesized from published datasets on isoprenoid biosynthesis and GPCR gene expansions (Lange et al. 2000; Niimura and Nei 2005; Elphick and Egertová 2005). These studies formed the basis for tracing the evolutionary origins of the mevalonate (MVA) versus 2‐C‐methyl‐D‐erythritol 4‐phosphate (MEP) pathways, cannabinoid receptors, and ectopic OR lineages.

2.3. Structural and Biophysical Data Integration

Key structural motifs (DRY/NPxxY, cholesterol recognition/interaction amino acid consensus (CRAC)/CARC (inverse CRAC motif), palmitoylation sites) and lipid–protein interactions were compiled from crystallographic and molecular dynamics studies (Paila and Chattopadhyay 2010; Oates and Watts 2011; Simons and Toomre 2000). These data underlie membrane‐based logic architecture, including ligand partitioning and receptor heteromerization mechanisms of the ETS.

2.4. Microbiome Terpene Synthase Mapping

We surveyed recent metagenomic and functional microbiology studies to identify terpene synthase (TS) homologs in human gut bacteria and to trace their roles in converting dietary terpenoids into bioactive eTBome components. For example, Lactobacillus and Bacteroides species harbor TS genes that produce diverse monoterpenes and sesquiterpenes, Mycobacterium‐type P450s in the gut oxidize limonene to POH, and microbial CCD1 enzymes cleave β‐carotene into β‐ionone, all of which partition into host lipid rafts and engage GPCR targets (Sinha et al. 2025; van Beilen et al. 2005).

2.5. Integrative Model Construction

Extracted data were iteratively synthesized to develop the ETS hypothesis. We mapped co‐localization, co‐expression, crosstalk mechanisms and downstream signal integration—forming a cohesive framework that links ECS/eCBome biology, olfactory GPCRs, accessory lipid receptors, and microbial terpenoids within cholesterol‐rich microdomains.

2.6. Selective Pressures Driving Terpenoid–Receptor Co‐Evolution

The colonization of terrestrial habitats exposed vertebrates to a rich chemical milieu of plant‐derived monoterpenes and sesquiterpenes, exerting strong selective pressure for OR repertoire expansion and diversification. Concurrently, the necessity to regulate membrane‐derived endocannabinoid mediators with high precision enforced the retention and functional specialization of CB1/CB2 receptors. These parallel evolutionary trajectories shaped by environmental terpenoid landscapes on one hand and endogenous lipid signaling demands on the other laid the genomic foundation for the co‐expression and functional crosstalk of ORs and CB receptors that underpins the ETS paradigm (Niimura and Nei 2005).

3. Theoretical Framework: ETS and Endoterpenoidome

3.1. Conceptual Origins of the ETS

The conceptual roots of terpenoid biology trace back to the 19th century, when Kekulé (1866) introduced the term terpene and Williams (1860) identified isoprene as its five‐carbon backbone. Wallach (1885) formalized the “isoprene rule,” explaining how terpenes are assembled via repetitive isoprene units. Ruzicka (1953) expanded this with the “biogenetic isoprene rule,” reclassifying cholesterol and steroid hormones as triterpenoids. This framework culminated in Lynen's (1967) discovery of the mevalonate pathway and the biosynthesis of IPP/DMAPP the universal C₅ precursors of all isoprenoids, and Bloch's (1992) mapping of their downstream conversion into sterols and hormones, thus linking primary terpene biosynthesis with complex lipid signaling (Ciechanover et al. 1980). While classical terpenoid research emphasized metabolic pathways and structural classes, the Endoterpenoid System (ETS) reframes these findings within a signaling framework. ETS posits that endogenously produced terpenoids ranging from cannabinoids to lesser‐known monoterpenes act as context‐sensitive signaling molecules, not just metabolic products. Through ligand partitioning, GPCR heteromerization, and microbial modulation within lipid raft microdomains, ETS highlights a membrane‐based logic that coordinates adaptive cellular responses. Receptor heteromerization and pathways in ETS are summarized in Table 2.

TABLE 2.

Receptor heteromerization and pathway‐specific outputs in ETS.

Receptor complex	Agonists	Activated pathway	Functional outcome	Experimental Model/Context
CB1 + CB2 heteromer	AEA, 2‐AG, THC (partial agonist)	Gi/o ↑ ↓ cAMP, β‐arrestin MAPK ↑ (ERK)	Apoptosis, neuromodulation	Glioma invasion suppression
CB2 + OR51E2 candidate co‐localisation/crosstalk	β‐Caryophyllene	Gi/o ↑ ↓ cAMP, β‐arrestin ↑ ERK1/2	MMP2 suppression, reduced invasiveness	Glioma cell models
Olfr78/OR51E2 monomer activation	Acetate	Gs/olf ↑ ↑ cAMP	Renin secretion, cell differentiation	Enteroendocrine tissue
CB1 monomer activation	THC, AEA	Gi/o ↑ ↓ cAMP	Partial agonism, synaptic regulation	Brain regions (hippocampus, cortex, etc.)
TRPV1 activation	AEA, capsaicin	Ca2+ influx	Pain signaling, vasodilation	Neuronal and vascular systems
GPR55 activation	Lysophosphatidylinositol, THC, and AEA	Gq ↑ PLCβ ↑ IP3 ↑ Ca2+ release	Migration, proliferation, inflammation	Immune and cancer cell models

Lipid rafts are nanoscopic, cholesterol‐ and sphingolipid‐enriched membrane microdomains that serve as platforms concentrating receptors, G‐proteins, and signaling enzymes, thereby modulating their lateral organization and potential interactions. Early biochemical and imaging studies suggested that these domains facilitate selective partitioning of GPCRs and their downstream effectors, tuning signal amplitude and specificity (Simons and Ikonen 1997; Brown and London 1998). Within this context, the ETS model proposes that CB1 and CB2 receptors co‐localize with ectopically expressed olfactory GPCRs (e.g., OR51E2, OR51E1) in overlapping raft microdomains, potentially enabling transient heteromerization and lateral protomer–protomer crosstalk. Super‐resolution microscopy studies have observed “hot spots” of receptor clustering that appear sensitive to ligand binding and membrane lipid composition, suggesting dynamic regulation, though broader validation across all ETS components remains ongoing (Milhiet et al. 2006). This spatial organization is functionally reminiscent of Boolean logic, whereby the juxtaposition of receptors and enzymes within rafts could encode context‐sensitive signaling outcomes.

3.2. Defining the eTBome Chemical Space

By analogy to the eCBome, the ensemble of endogenous cannabinoids and related lipid mediators, the eTBome is defined as the proposed collection of lipophilic terpenoid ligands whose propensity to partition into cholesterol‐rich raft domains may orchestrate coordinated GPCR engagement. Core members include the on‐demand synthesized endocannabinoids AEA and 2‐AG, hydrolyzed by FAAH and MAGL to terminate signaling (Nomura et al. 2010; Cravatt et al. 1996). Semi‐endogenous terpenophenolics such as Cannabigerol (CBG) have been reported to act as partial agonists or allosteric modulators at both CB and OR protomers in vitro, suggesting a modulatory role (Mendiguren et al. 2023). Dietary and microbiota derived monoterpenoids; including limonene, citronellal, nerol, citral, and geraniol have been shown to engage ORs such as OR51E2 in cell based assays, inducing differentiation and apoptotic pathways, though in vivo confirmation is limited (Pronin and Slepak 2021; Maßberg et al. 2016). β‐Caryophyllene has selectively activated CB2 in preclinical models, modulating inflammatory responses (Gertsch et al. 2008). Collectively, these ligands define a dynamic chemical space whose raft partition coefficients and membrane‐perturbing properties are proposed to bias GPCR heteromerization and allosteric modulation, influencing downstream cascades such as G_i/o‐cAMP, Gq‐Ca²⁺, and MAPK pathways. Gut bacteria convert dietary ginsenosides into bioactive aglycones that engage host receptors. In high‐fat diet–fed rodents, Rb1, CK, and PPD each reshape the gut microbiota, enhance bile acid–FXR signaling, and modulate Peroxisome proliferator‐activated receptor gamma (PPARγ) pathways to improve lipid metabolism and reduce inflammation (Zhang et al. 2024). Thymoquinone is a quinone from Nigella sativa . It exerts anti‐inflammatory and anticancer effects via PPARγ activation and inhibition of Akt‐mediated survival pathways in tumor cells; it also modulates TRP channel activity in sensory neurons (Woo et al. 2011).

Enzymatic and transport machinery runs via synthesis enzymes. NAPE‐PLD, ABHD4 and GDE1 liberate AEA from N‐arachidonoyl PE, while DAGLα/β produce 2‐AG from DAG; both enzyme families localize to lipid rafts, coupling on‐demand ligand synthesis directly to nearby GPCRs (Zhang et al. 2024). As degradation enzymes, FAAH hydrolyzes AEA into arachidonic acid and ethanolamine within ordered domains, and MAGL degrades 2‐AG to regulate CB2‐mediated signaling in inflammation and cancer (Nomura et al. 2010). Fatty acid‐binding proteins (FABPs), notably FABP5 and FABP7, shuttle AEA between the plasma membrane and degradative enzymes. Pharmacological inhibition of FABPs elevates AEA levels and produces anti‐inflammatory and analgesic effects (Berger et al. 2012).

3.3. System Boundaries and Modularity

The core ETS module comprises CB1, CB2, and select OR heteromers within cholesterol‐enriched raft domains, supplemented by accessory submodules, including orphan lipid‐sensitive GPCRs (e.g., GPR55, GPR18), polymodal ion channels (TRPV1), and enzymatic hubs (NAPE‐PLD, DAGL, FAAH, MAGL). Single‐molecule tracking has visualized transient clustering of these entities in live cells, suggesting rapid assembly/disassembly cycles in response to ligand availability and membrane order (Drbal et al. 2007). In glioma models, evidence indicates that lipid‐mediated cross‐talk between CB2 and OR51E2 can modulate MMP‐2 expression, correlating with reduced invasiveness in vitro (Cho and Koo 2021). This modular architecture is hypothesized to permit context‐specific recombination of ETS components, potentially generating diverse signaling phenotypes tailored to particular tissues or disease states.

4. Molecular Components of the ETS

4.1. Cannabinoid Receptors (CB1, CB2)

CB1 and CB2 are Class A GPCRs sharing conserved DRY (TM3) and NPXXY (TM7) motifs, as well as CARC sequences that influence their conformational dynamics within lipid rafts (Fantini and Barrantes 2013). CB1 is broadly expressed in the hippocampus, cortex, and cerebellum, modulating synaptic neurotransmitter release, whereas CB2 predominates in immune cells and microglia, with upregulation observed in pathological contexts such as glioblastoma (Grotenhermen 2003). Endocannabinoids AEA and 2‐AG are synthesized on demand by NAPE‐PLD and DAGL, respectively, and subsequently degraded by FAAH and MAGL, providing temporal control of signaling (Reisenberg et al. 2012). The interplay of these enzymes and receptors within rafts is thought to underlie nuanced regulation of neuronal excitability, immune responses, and tumor microenvironment (TME) dynamics. Each of these receptor types integrates within lipid rafts alongside metabolic enzymes and terpenoid ligands, forming the modular core of the ETS and enabling context‐dependent crosstalk and signal integration.

4.2. Olfactory Receptors (e.g., OR51E2, OR51E1, OR2J3)

ORs are Class A GPCRs originally characterized in olfactory sensory neurons but now recognized as ectopically expressed across diverse non‐olfactory tissues; including prostate, colon, skin, kidney, and TMEs where they couple via Golf to adenylate cyclase and regulate proliferation, migration, and apoptosis (Kang and Koo 2012). In cell based models, activation of OR51E2 by propionate has been shown to induce differentiation and apoptotic signaling (Pluznick et al. 2013), highlighting a potential mechanism for therapeutic targeting, though further in vivo studies are required to confirm these effects. These ectopic ORs (Altundag et al. 2025; Altundag and Harbi 2025) enrich the ETS by providing additional ligand–receptor interfaces for dietary and microbiota‐derived terpenoids.

4.3. Accessory GPCRs (GPR55, GPR18) and Ionotropic Partners (TRPV1)

GPR55 responds to lysophosphatidylinositol and certain phytocannabinoids, modulating Ca²⁺ flux and cell motility in immune and cancer cells. GPR18, activated by N‐arachidonoylglycine, appears to influence inflammatory chemotaxis (McHugh and Ross 2009). The polymodal TRPV1 ion channel senses heat, capsaicin, and has been reported to be activated by anandamide, coupling metabotropic endocannabinoid signaling to rapid Ca²⁺ influx and vasodilatory responses (Caterina et al. 1997). These accessory elements co‐reside with core ETS receptors and enzymes in raft domains, facilitating multilayered and context‐dependent crosstalk.

5. Ligand Classes

5.1. Endocannabinoids (AEA, 2‐AG)

Anandamide (AEA) is synthesized on demand from N‐arachidonoyl phosphatidylethanolamine (NAPE) via NAPE‐PLD, ABHD4, and GDE1 pathways and functions as a partial agonist at CB1 and CB2, with additional activity at TRPV1 and PPARγ; its rapid hydrolysis by FAAH‐localized within cholesterol‐rich rafts ensures tight temporal control of receptor engagement (Cravatt et al. 1996; Maccarrone 2017; Wang and Ueda 2009). 2‐Arachidonoylglycerol (2‐AG) is generated from diacylglycerol (DAG) by DAGLα/β, acts as a full agonist at both CB1 and CB2, and is primarily degraded by MAGL within or adjacent to lipid rafts, shaping local receptor activation (Nomura et al. 2010; Justinová et al. 2011; Prandi et al. 2018).

5.2. Terpenoid Ligands (CBG, β‐Caryophyllene, Perillyl Alcohol, Limonene)

Cannabigerol (CBG), a terpenophenolic phytocannabinoid biosynthesized from geranyl pyrophosphate and olivetolic acid, partially agonizes CB2 and modulates TRP and PPARγ channels (De Petrocellis et al. 2011; Fellermeier and Zenk 1998). β‐Caryophyllene, a bicyclic sesquiterpene (log P ≈ 4), selectively activates CB2 (K_i ≈ 155 nM), signals via G_i/o, and suppresses inflammatory cytokine release in immune cells (Gertsch et al. 2008). POH, a monocyclic monoterpene derived from limonene, is relevant to the ETS ligand‐field concept because of its membrane‐partitioning properties and glioma‐facing biological activity; however, direct OR51E2 or GPR55 engagement should not be inferred without receptor‐specific validation (da Fonseca et al. 2008).

5.3. Structural and Lipid‐Interaction Features

Class A GPCRs contain conserved cholesterol recognition/interaction amino acid consensus (CRAC) and CARC motifs, as well as palmitoylation sites on intracellular loops, which directly bind cholesterol and allosterically modulate receptor conformations while driving partitioning into ordered raft domains (Jakubík and El‐Fakahany 2021). Terpenoid lipophilicity (log P ≈ 3‐5) underlies their preferential partitioning into liquid‐ordered, cholesterol‐rich microdomains (rafts), where longer residence times enhance local ligand concentration and fine‐tune GPCR activation kinetics (Vermaas et al. 2018). Caveolin 1 and flotillin form scaffolding platforms that cluster GPCRs and metabolic enzymes into stable nanodomains. Caveolin 1 directly interacts with GPCRs and G proteins to regulate receptor trafficking and signaling (McFarland et al. 2004; Allen et al. 2007; Ostrom and Insel 2004). Each component from microbial terpenoid inputs to enzymatic gatekeepers and lipid–protein interaction motifs converge within cholesterol‐rich microdomains to orchestrate the cross‐talk, co‐expression and co‐localization of ECS/eCBome receptors and accessory GPCRs. This integrated ETS and its eTBome model explains emergent signaling behaviors that traditional receptor‐centric paradigms cannot.

5.4. Membrane Microdomains and Spatial Organization

Lipid rafts are liquid‐ordered nanodomains (10‐200 nm) enriched in cholesterol and sphingolipids that phase‐separate from the surrounding bilayer to form discrete signaling platforms. Cholesterol's rigid sterol ring intercalates between saturated sphingolipid acyl chains, increasing membrane thickness and order, while sphingolipids stabilize the domain through tight acyl chain packing. These physicochemical properties generate distinct lateral pressure profiles and viscosity landscapes that allosterically modulate GPCR conformations and ligand affinity (Lingwood and Simons 2010; Paila and Chattopadhyay 2010). Historically defined by resistance to detergent extraction, rafts in living cells are now visualized as dynamic, transient assemblies by single‐molecule and super‐resolution imaging (Kasai and Kusumi 2014). Small lipophilic terpenoids preferentially partition into these domains and modulate membrane curvature, surface tension, and phase separation dynamics (Turina et al. 2006; Zunino et al. 2011). Within lipid rafts, GPCR protomers such as CB1, CB2 and ORs exhibit confined lateral mobility and prolonged dwell times, as revealed by single‐molecule tracking (Suzuki et al. 2005). Covalent lipid modifications, especially S‐palmitoylation of juxtamembrane cysteines and prenylation of C‐terminal CAAX motifs, alongside conserved CRAC/CARC cholesterol‐binding sequences, collectively steer GPCRs into liquid‐ordered, cholesterol‐rich microdomains. S‐Palmitoylation has been shown to dynamically regulate raft affinity for a broad set of transmembrane proteins, including GPCRs, by enhancing their partitioning into ordered phases (Chini and Parenti 2009; Levental et al. 2010). Prenylation similarly promotes membrane anchoring and microdomain targeting of signaling proteins. Critically, CRAC and its mirror CARC motif bind cholesterol directly, allosterically modulating receptor conformation and further enriching GPCRs within ordered raft regions (Fantini and Barrantes 2013). Small, lipophilic terpenoids (e.g., monoterpenes) intercalate into liquid‐ordered raft domains, perturbing bilayer curvature and increasing line tension at phase boundaries, which in turn nucleates nanoscale receptor clusters in regions of differential lipid packing (Turina et al. 2006). These curvature‐driven nanoclusters create energetically favorable interfaces that promote GPCR heteromer formation, by bringing distinct protomers into close proximity within curved membrane “hotspots” (Meinhardt et al. 2013). Anionic lipids such as phosphatidylinositol 4,5‐bisphosphate (PIP₂) and phosphatidylserine (PS) recruit peripheral proteins by binding basic patches on GPCR intracellular loops, organizing signaling complexes outside classical rafts (Yen et al. 2018). Moreover, cholesterol binding at conserved non‐annular sites on GPCR transmembrane helices acts as an allosteric modulator, stabilizing active or inactive conformations and dictating raft partitioning (Jakubík and El‐Fakahany 2021; Taghon et al. 2021). Co‐localization within lipid rafts stabilizes heteromeric GPCR interfaces and biases downstream signaling outputs. CB1‐CB2 heteromers in rafts exhibit β‐arrestin–biased signaling distinct from monomeric receptors (Callén et al. 2012). Enhanced terpenoid partitioning generates microdomain hotspots that elevate local ligand concentration, increase receptor residence time, and sharpen signaling kinetics—parameters critical for context‐specific ETS functions (Vermaas et al. 2018).

5.5. Molecular Mechanisms Underpinning Receptor Co‐Expression and Crosstalk

Within cholesterol‐rich microdomains, co‐expressed GPCR protomers physically associate to form heteromeric complexes whose pharmacology diverges from monomers. In neuronal tissue, CB1 and CB2 assemble into functional heteromers: agonist binding to one protomer allosterically shifts the partner's G‐protein coupling bias, producing unique downstream responses (Menéndez‐Pérez et al. 2024; Rozenfeld and Devi 2011). A graphical summary of the molecular signaling pathways of ETS is shown in Figures 1 and 2. Upon agonist‐induced GRK phosphorylation, distinct phosphorylation “barcodes” on CB1 versus OR protomers recruit β‐arrestin2, which scaffolds MAPKs (ERK1/2) and Akt into nanoscale signalosomes at the receptor complex. These β‐arrestin assemblies sustain extra‐nuclear ERK signaling and drive pro‐apoptotic transcriptional programs unique to heteromeric clusters (Defea 2008; Bono et al. 2023). Co‐activation of CB‐OR heteromers within ETS microdomains generates integrated second‐messenger signatures that are distinct from monomeric receptor signaling. CB1 engagement of Gᵢ/o proteins inhibits adenylyl cyclase, reducing cAMP levels. In CB1‐CB2 heteromers this Gi/o‐mediated suppression is potentiated compared to monomers, yielding unique cAMP dynamics namely Gᵢ/o‐cAMP Axis (Lu and Mackie 2016). Ectopic ORs (e.g., OR51E2) couple via G_s/olf to stimulate adenylyl cyclase, elevating cAMP in enteroendocrine and epithelial cells on the base of G_s/olf‐cAMP Axis. This G_s/olf‐cAMP signaling has been demonstrated in intestinal OR activation promoting GLP‐1 secretion (Han et al. 2018; Jones and Reed 1989; Kaneko‐Goto et al. 2013). Concurrent activation of Gq‐coupled receptors induces phospholipase Cβ, generating IP₃, and releasing Ca²⁺ from ER stores often at mitochondria‐associated membranes to provoke mitochondrial permeability transition and apoptosis on the route of Gq‐Ca²⁺ Axis (Berridge et al. 2003).

FIGURE 1.

ETS signaling integration by CB‐OR heteromerization in lipid rafts. The figure shows the common signaling pathways of cannabinoid receptors (CB1/CB2), olfactory receptor OR51E2 and other lipid‐sensitive receptors (GPR55, TRPV1/2) co‐localized in lipid raft microdomains within the endoterpenoid system (ETS). The CB‐OR complex formed by heteromerization of CB1/CB2 and OR51E2 induces MAPK (ERK1/2, p38) activation via β‐arrestin, while at the same time reducing cAMP levels via adenylate cyclase inhibition via Gi/o. This bidirectional effect leads to activation of apoptotic signaling pathways (BAD‐ROS) and determination of cellular fate through Bcl‐2 modulation. While cell migration, angiogenesis, and stem cell properties are regulated through Wnt/β‐Catenin and PI3K/Akt/mTORC1/2 axes via GPR55, TRPV1/2 channels regulate the ETS signaling pathway both directly and through CB‐OR interaction. In this mechanism, we integrally demonstrate the multicomponent structure of ETS and contextual cellular responses through co‐expression/heteromerization.

FIGURE 2.

Crosstalk and receptor‐ligand mechanism in ETS. The figure shows the interactions of lipid‐derived molecules in the endoterpenoid system (ETS), particularly endocannabinoids, dietary terpenoids, and microbial enzymes, on co‐localized G‐protein coupled receptors (GPCRs) and their effect on intracellular signaling pathways. GPCRs such as CB1/CB2, olfactory receptors (ORs), GPR55, and TRPV1, located in cholesterol‐rich microdomains, co‐localize and produce an integrated response through ligand‐level diversity, partial agonism (e.g., the effect of THC on CB1), crosstalk between receptors (CB1‐OR interaction), and membrane dynamics. Receptors such as TRPV1 and GPR55 contribute to the inflammatory process, while alternative MAPK/p38 pathways through β‐arrestin increase the diversity of receptor signaling. Thus, the ETS shapes homeostatic and immune responses by processing both intrinsic and peripheral lipid signals as a membrane‐based logic circuit.

5.6. MAPK and β‐Arrestin Scaffolding

Agonist‐induced GRK phosphorylation “barcodes” on CB‐OR heteromers recruit β‐arrestin2, which scaffolds c‐Raf, MEK1, and ERK1/2 into nanoscale signalosomes. This β‐arrestin‐dependent MAPK activation sustains extranuclear ERK signaling and drives pro‐apoptotic transcriptional programs unique to heteromeric clusters (Kahsai et al. 2023).

5.7. Functional Example

In glioma models, simultaneous activation of CB2‐OR51E2 heteromers leads to a coordinated downregulation of MMP‐2 via G_i/o‐mediated cAMP suppression and MAPK pathway modulation, resulting in markedly reduced invasion, an effect that does not occur when either receptor is activated alone (Cho and Koo 2021; Chung et al. 2022). Collectively, these convergent cascades illustrate how ETS microdomains integrate co‐expression, co‐localization, receptor cross‐talk, and lipid‐mediated allostery to transform the chemically diverse eTBome inputs into precise, context‐specific cellular outputs, expanding upon classical ECS and eCBome paradigms. Putting these together under the ETS framework justifies the hypothesis that dual activation of a CB2‐OR51E2 heteromer could coordinately engage Gi/o‐cAMP and MAPK circuits to suppress MMP‐2 and inhibit invasion effects not recapitulated by stimulating either receptor alone. Importantly, individual activation of either CB2 or OR51E2 alone fails to alter MMP‐2 levels or inhibit glioma invasion, demonstrating that heteromer‐mediated cross‐talk is essential for this anti‐invasive response.

6. Discussion

The ETS and its chemically diverse eTBome offers a novel, membrane‐centered framework that integrates classical endocannabinoid signaling (ECS/eCBome), ectopically expressed olfactory GPCRs, accessory lipid‐sensitive receptors (e.g., GPR55, TRPV1), and microbiota‐derived terpenoids within dynamic cholesterol‐rich microdomains. This model moves beyond traditional linear receptor‐ligand interactions by positing a multi‐receptor logic network, wherein ligand partitioning, receptor heteromerization, and β‐arrestin–scaffolded signalosomes collectively generate context‐sensitive, emergent signaling behaviors. These include bistable apoptotic switches, non‐linear crosstalk, and functional receptor redistribution phenomena that are poorly explained by single‐receptor pharmacology. A central strength of the ETS lies in its integrative scope and modularity. By situating CB1/CB2, ORs (e.g., OR51E2), and orphan GPCRs within a unified lipid raft architecture, the model explains promiscuous ligand binding, lateral diffusion, and cooperative receptor activation in terms of thermodynamic and spatial constraints. These insights are grounded in established principles such as phase separation, line tension, and cholesterol‐mediated conformational control each of which has been validated in the structural biology of Class A GPCRs (Fantini et al. 2016). Importantly, the gut microbiota emerges not as a peripheral modifier, but as a primary architect of the eTBome, shaping the host's signaling environment by generating structurally diverse, membrane‐partitioning terpenoids. These microbial metabolites ranging from ginsenoside derivatives to POH and β‐ionone modulate not only CB and OR receptors but also TRP channels and PPARγ, expanding the signaling versatility of ETS. Microbial erosion, such as from antibiotics or dietary imbalances, is thus predicted to disrupt receptor clustering and signal integration within lipid rafts potentially altering outcomes in inflammation, metabolism, and neuroplasticity. The translational relevance of ETS is further underscored by recent findings in cancer biology and neuroimmune regulation. For example, CB2‐OR51E2 heteromer activation has been associated with MMP2 downregulation and reduced glioma invasiveness, a functional phenotype that neither receptor achieves in isolation (Blázquez et al. 2008; Kim and Ghil 2025). Likewise, microbiota‐ETS interactions modulate GLP‐1 secretion via intestinal ORs (Lund et al. 2018), highlighting the system's capacity to influence gut‐brain hormonal axes. However, the ETS model faces several important limitations that must be addressed to advance its experimental validation; structural characterization remains hypothetical: no high‐resolution structures of CB‐OR heteromers within native lipid bilayers are currently available. Quantitative partitioning data such as terpenoid membrane affinity (log P), local concentration gradients, and dynamic residence times within lipid rafts are yet to be systematically measured in vivo. Functional demonstrations of ETS logic in intact tissue contexts are limited; most data derive from overexpression systems, immortalized cell lines, or ex vivo models. Overcoming these limitations will require an interdisciplinary effort: combining cryo‐EM, super‐resolution live‐cell imaging, quantitative lipidomics, and gnotobiotic microbiota modulation to resolve the spatiotemporal dynamics of ETS components under physiological and pathological conditions. Rather than proposing isolated receptor functions, the ETS redefines lipid signaling as a networked logic system embedded within membrane microarchitecture, regulated by both endogenous biosynthesis and microbial ecology. This paradigm provides a versatile platform for explaining nonlinear signaling behaviors, receptor synergy, and context‐sensitive pharmacodynamics laying the groundwork for next‐generation therapeutics targeting multi‐receptor ensembles in cancer, inflammation, and neurometabolic disease. Future ETS studies can therefore pain receptor and lipid‐raft mapping with MECS‐BER‐style adjudication; define the editable membrane state, lock the proximal output, perturb the nominated CB, OR and TRP arms across a complete 2 x 2 x 2 response, surface, and interpret any residual κ term only against a pairwise‐only null. This would separate receptor inventory and co‐localisation from functional integration and would keep higher‐order ETS claims conditional on a defined membrane state and one measurable local output (Yarar 2026).

7. Conclusions

The Endoterpenoid system (ETS) and its chemically diverse endoterpenoidome (eTBome) establish a unifying, membrane‐based logic that builds upon the classical endocannabinoid system (ECS) and its broader endocannabinoidome (eCBome). Within cholesterol‐rich lipid rafts, co localization and co expression of cannabinoid receptors, ectopic olfactory GPCRs, and accessory lipid‐sensitive receptors (e.g., GPR55, TRPV1) enable dynamic crosstalk and emergent signaling behaviors such as bistable apoptotic switches and context specific modulation that elude traditional single receptor paradigms. By harnessing ligand partitioning, receptor heteromerization, and β‐arrestin scaffolded signalosomes, the ETS model integrates endogenous cannabinoids of the eCBome with dietary and microbially derived terpenoids of the eTBome, forging a dynamic crosstalk network. The gut microbiota emerges as pivotal architects of the eTBome, supplying unique terpenoid ligands and transforming diet derived compounds to modulate ECS/ETS ligand pools and gut–brain communication Although high resolution structures of CB–OR heteromers in native like bilayers and quantitative biophysical parameters remain to be determined, the ETS paradigm opens novel therapeutic avenues: targeting membrane lipid composition, heteromer interfaces, or microbial terpenoid production to modulate pathological ECS/ETS signaling in cancer, neuroinflammation, and metabolic disorders. The ETS is proposed as a novel, testable hypothesis: a membrane‐based logic framework in which classical endocannabinoid receptors (CB1/CB2), ectopic olfactory GPCRs, accessory lipid‐sensitive receptors, and microbial terpenoids co localize within cholesterol‐rich rafts to enable co expression, co localization, and finely tuned crosstalk. The eTBome is proposed as the chemically promiscuous pool of endogenous, dietary, and microbial terpenoids whose raft partitioning drives receptor heteromerization and β‐arrestin scaffolded signalosomes to produce emergent signaling behaviors that extend beyond the classical ECS/eCBome paradigm. By defining ETS and eTBome, a unified lens for understanding how lipid microdomain architecture, ligand diversity, and receptor assemblies integrate to generate context specific cellular outcomes from synaptic modulation to tumor cell apoptosis is provided. This new hypothesis not only reconciles formerly disparate observations across cannabinoid, olfactory, and microbial signaling but also opens avenues for structure function studies, advanced lipidomics, live cell imaging, and microbiome‐based interventions aimed at modulating pathological ETS signaling in disease. If membrane lipids can reshape the behaviour of individual receptors, then the same membrane organization may also reshape receptor crosstalk and shared downstream outputs. On this view, membrane state constrains the signaling vocabulary of individual proteins while also governing the local language of receptor communication through its effects on proximity, access to shared machinery, and the conditions under which co‐expression progresses into coordinated crosstalk.

Declaration of Transparency

The authors, reviewers and editors affirm that in accordance to the policies set by the Journal of Neuroscience Research, this manuscript presents an accurate and transparent account of the study being reported and that all critical details describing the methods and results are present.

Author Contributions

Erhan Yarar: conceptualization, methodology, investigation, literature review, writing – original draft, visualization, supervision. Emirhan Harbi: Literature review, writing – review and editing, visualization. Aytug Altundag: writing – review and editing, validation. Christopher E. Mason: writing – review and editing, validation. Michael Aschner: writing – review and editing, validation.

Funding

The authors have nothing to report.

Ethics Statement

The authors have nothing to report.

Consent

The authors have nothing to report.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Data S1: Transparent Science Questionnaire for Authors.

Data Availability Statement

Data sharing not applicable to this article as no datasets were generated or analysed during the current study.