Endocannabinoids

Prof Mauro Maccarrone

April 20'th 2009

Endocannabinoids are derivatives of arachidonic acid (and of other poly-unsaturated fatty acids), that have been discovered approximately 15 years ago. Nowadays they are recognized as a new class of lipid signaling molecules, that play several central and peripheral roles. Endocannabinoids are endogenous ligands of brain-type (CB1) and spleen-type (CB2) cannabinoid receptors, that bind also an exogenous ligand like Δ9- tetrahydrocannabinol (THC), the psychoactive component of Cannabis sativa. CB1 receptors have been found mainly in the central nervous system, but they are also present in lymphocytes, ovary, uterine endometrium, testis, vas deferens, urinary bladder, and other peripheral endocrine and neurological tissues. CB2 receptors have been identified mainly in immune cells, but are expressed also in astrocytes and in the brain stem.The best characterized endocannabinoids are N-arachidonoylethanolamine (anandamide, AEA) and 2-arachidonoylglycerol (2-AG), that are prototype members of two types of compounds: the fatty acid amides and the monoacylglycerols, respectively. These lipids are not stored in intracellular compartments, but are synthesised from membrane precursors through multiple biosynthetic pathways; therefore, they are released “on demand” by neurons and peripheral cells. AEA is produced mainly by a transacylase-phosphodiesterase-mediated pathway, starting from the precursor N-arachidonoyl-phosphatidylethanolamine through the action of N-archidonoyl-phosphatidylethanolamine (NAPE) hydrolysis, catalysed by a specific phospholipase D (NAPE-PLD). The biosynthetic pathway of 2-AG provides for rapid hydrolysis of inositol phospholipids by a specific phospholipase C; this enzyme generates diacylglycerol (DAG), which is subsequentially converted to 2-AG by a sn-1-DAG lipase. The biological effects of AEA and 2-AG depend on their life span in the extracellular space, which is limited by a rapid transport through the plasma membrane. Both compounds have been proposed to be taken up by cells through a specific carrier, but the identity of this putative entity is still a controversal issue. Once inside the cells, endocannabinoids can be metabolized by multiple pathways; AEA is a substrate for fatty acid amide hydrolase (FAAH), that breaks the amide bond and releases arachidonic acid and ethanolamine, whereas 2-AG is degraded to arachidonic acid and glycerol mainly by a specific monoacylglycerol lipase (MAGL). As mentioned above, AEA and 2-AG act primarily at cannabinoid receptors. These are seven trans-membrane spanning receptors that belong to the rhodopsin family of G protein-coupled receptors, particularly those of the Gi/o group. The binding of endocannabinoids to CB receptors triggers various signaling pathways, such as the inhibition of adenylyl cyclase, the regulation of ionic currents (inhibition of voltage-gated L, N and P/Q-type Ca2+ channels, activation of K+ channels), the activation of focal adhesion kinase, of mitogen-activated protein kinase (MAPK), and of cytosolic phospholipase A2, and the activation (CB1) or the inhibition (CB2) of nitric oxide synthetase. In addition, recently an unprecedented coupling of CB1 to Gq/11 proteins has been shown, suggesting further diversity of CB1 signaling. Furthermore, there is some evidence that endocannabinoids induce a biological activity via other CB receptors, like a purported CB3 (GPR55) receptor, via non-CB1/non-CB2 receptors, and via non-cannabinoid receptors. In the latter group, type-1 vanilloid receptor (now called transient receptor potential vanilloid 1, TRPV1) has emerged as an important target of AEA, but remarkably not of 2-AG. TRPV1 is a six trans-membrane spanning protein with intracellular N- and C-terminals; this ligand-gated and non-selective cationic channel is activated by molecules derived from plants, such as the pungent component of “hot” red peppers capsaicin, by noxious stimuli like heat and protons, and by peptides contained in spider toxins. AEA is so far the only “endovanilloid” known, behaving as an authentic (though weak) endogenous ligand of TRPV1. In the last 5 years endocannabinoids have emerged as key-mediators of several central and peripheral pathophysiological processes. In fact they act as retrograde neurotransmitters, and as neuroprotective and anti-inflammatory substances, taking part in neuronal circuitries that include dopaminergic, glutamatergic and GABAergic transmission. In addition, they participate in signaling networks that include cytokines (e.g., interleukins, growth factors, interferon-γ and tumor necrosis factor-α), and steroid hormones (e.g., progesterone, 17-β-estradiol and glucocorticoids). Furthermore, only recently new endocannabinoids (like N-arachidonoyldopamine) have been discovered, that along with “endocannabinoid-like” molecules (e.g., N-oleoylethanolamine and N-palmitoylethanolamine) are able to activate unexpected molecular targets like TRPV1, peroxisome proliferator activator receptors (PPAR), and CB3. Against this background, it is not surprising that endocannabinoid signaling is at the basis of neuroinflammatory diseases (like Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, multiple sclerosis and amyotrophic lateral sclerosis), of cancer cell survival and death, of immune response and of metabolic disease. For instance, endocannabinoids have been shown to regulate food intake, and in fact a selective CB1 antagonist is available on several markets as the first anti-obesity drug for humans. Last but not least, human reproduction is under the control of endocannabinoid signaling, that regulates oviductal transport and implantation of embryos (on the female side), as well as spermatogenic output, sperm viability and motility (on the male side).Taken together, endocannabinoids have emerged as widespread signaling molecules, which take part in neuronal circuitries and cytokine-hormone networks that impact to different extents a number of pathophysiological conditions in humans.