Endocannabinoid signaling at the periphery: 50 years after THC

Abstract

In 1964, the psychoactive ingredient of Cannabis sativa, Δ(9)-tetrahydrocannabinol (THC), was isolated. Nearly 30 years later the endogenous counterparts of THC, collectively termed endocannabinoids (eCBs), were discovered: N-arachidonoylethanolamine (anandamide) (AEA) in 1992 and 2-arachidonoylglycerol (2-AG) in 1995. Since then, considerable research has shed light on the impact of eCBs on human health and disease, identifying an ensemble of proteins that bind, synthesize, and degrade them and that together form the eCB system (ECS). eCBs control basic biological processes including cell choice between survival and death and progenitor/stem cell proliferation and differentiation. Unsurprisingly, in the past two decades eCBs have been recognized as key mediators of several aspects of human pathophysiology and thus have emerged to be among the most widespread and versatile signaling molecules ever discovered. Here some of the pioneers of this research field review the state of the art of critical eCB functions in peripheral organs. Our community effort is aimed at establishing consensus views on the relevance of the peripheral ECS for human health and disease pathogenesis, as well as highlighting emerging challenges and therapeutic hopes.

Keywords: bone; cardiovascular system; female and male reproductive system; gastrointestinal tract; immune system; liver; localization; muscle; signaling pathways; skin.

Figure 1

Role of ECS in cardiovascular injury/disease. Cardiovascular insult inflicted by ischemia, inflammation or hemodynamic overload leads to increased formation of reactive oxygen and/or nitrogen species (ROS/RNS) and inflammation. These processes trigger activation of ECS in cardiovascular system and infiltrating immune cells. eCBs via activation of CB₁ in cardiomyocytes, endothelial cells, fibroblasts and certain immune cells promote processes facilitating development of cardiovascular dysfunction, inflammation and pathological remodeling. In contrast, eCBs via activation of CB₂ exert opposing protective effects. Moreover, eCBs through their catabolism by FAAH and/or MAGL or oxidation by cyclooxygenases (COXs) or other enzymes may represent a significant source of arachidonic acid (AA) and/or other oxidized eCB metabolites with both pro- and anti-inflammatory effects. Thus, the protective or detrimental effect of eCBs in cardiovascular diseases may largely be context-, time- and pathology-dependent.

Figure 2

The ECS of the gastrointestinal (GI) tract and liver. AEA, 2-AG, and OEA are synthesized in the gut and liver, where they act locally and in the brain. eCBs regulate gut motility at the level of the enteric neural plexuses, they reduce intestinal inflammation through actions on the immune system and they influence intestinal barrier function at the level of epithelium. eCBs and OEA regulate food intake by actions on enteroendocrine cells in the wall of the gut, the vagus nerve and in the brain. In the liver, CB₁ and CB₂ have opposing effects, with CB₁ promoting steatosis, fibrogenesis, apoptosis and proliferation and CB₂ inhibiting these effects.

Figure 3

Cell types and cascade of immunological events that come into play in an adaptive mammalian immune reponse, and respective functional activities that have been implicated as modulated by eCBs. The adaptive immune response in mammals is highly specific to a given pathogen, and can provide long-lasting protection by destroying the invading pathogen and the toxic molecules it produces. Cells that are involved in this response are white blood cells known as B lymphocytes and T lymphocytes. These cells, respectively, play a critical role in humoral and cellular immune responses once activated by antigens (i.e., molecules or linear fragments that are recognized by receptors on the lymphocyte surface). In the humoral immune response, activated B lymphocytes secrete antibodies, or immunoglobulins. These antibodies travel through the bloodstream and bind to the cognate antigen resulting in its inactivation. Such an inactivation may include prevention of its attachment to host cells. In the cellular immune response, T lymphocytes are mobilized when they encounter an antigen-presenting cell such as a dendritic cell or B lymphocyte that has digested an antigen and is displaying antigen fragments bound to its major histocompatability (MHC) molecules. During this response, cytokines (small signaling proteins) facilitate T lymphocyte maturation and the growth of more T lymphocytes. The engendered MHC-antigen complex activates T cell receptors resulting in T lymphocyte secretion of additional cytokines. Some T lymphocytes develop into helper (Th) cells and secrete cytokines that attract other immune cells such as macrophages, granulocytes, and natural killer (NK) cells. Some T lymphocytes become cytotoxic cells (CTLs) and lyse tumor cells or host cells infected with viruses. CD4: cluster of differentiation 4, a glycoprotein found on the cell surface and used as marker for Th cells; CD8: cluster of differentiation 8, a glycoprotein found on the cell surface and used as marker for CTLs; TCR: T cell receptor, a molecule found on the surface of T lymphocytes that recgnizes antigens bound to MHC molecules.

Figure 4

a) Role of ECS in skeletal muscle formation. Simplified representation of the skeletal muscle differentiation process, from myoblasts to myotubes and myofibers and fascicles. Activation of CB₁ by eCBs inhibits myoblast to myotube differentiation, whereas CB₁ blockade, as in cells treated with a CB₁ antagonist or CB₁ siRNA, facilitates this process and in vivo, as in CB₁ null mice, leads to bigger fibers. b) eCB signaling in bone growth and bone remodeling. CB₁ and CB₂, as well as eCB synthetic enzymes, are expressed on hypertrophic chondroblasts in the epiphyseal growth cartilage. Mice lacking CB₂ develop longer femora and vertebral bodies, resulting in a longer stature, whereas stimulation of CB₁ restrains bone growth. Bone remodeling is stimulated in CB₂ deficient mice, but with a net loss of bone mass, thus resulting in an age-related osteoporotic phenotype. CB₂ signaling stimulates proliferation of osteoblast progenitors, and affects differentiation of osteoclasts. CB₁ are most prominently expressed on sympathetic nerve terminals and inhibit release of norepinephrine, thus reducing the sympathetic tone which in turn inhibits bone formation.

Figure 5

a) Schematic diagram showing pregnancy events in mice that are influenced by eCB signaling. Stage-specific eCB signaling during pregnancy is numerically designated (1-11). Implantation and decidualization both are designated by (9), since they are overlapping events. P, placenta; F, fetus. b) Functional activity of ECS elements in human sperm. Far from the oocyte activation of CB₁ reduces sperm motility and viability (1), and activation of TRPV1 inhibits a spontaneous acrosome reaction (AR), that would be useless to fertilize the egg (2). Next to the oocyte activation of CB₁ inhibits zona pellucida (ZP)-induced AR (3), thus avoiding fertilization of the egg by more than one sperm (polyspermy).

Figure 6

Summary of the effects of cutaneous ECS on multiple skin functions. Adapted from ref. 140.