The role of the CB2 receptor and human CB2 receptor variants in neuropathic pain
Final Report Abstract
Within the present project, we have investigated the function of cannabinoid CB2 receptors using genetic mouse models. Our initial focus was on the role of these receptors in bone remodeling. We have discovered a new functional polymorphism in the human CB2-encoding CNR2 gene, which resulted in a Gln -> Arg exchange in position 63 (Gln63Arg). This amino acid is positioned close to the first transmembrane domaine in the first intracellular loop. It has been shown that this variance modulates the signaling properties of the receptor. Importantly, using a sample of French patient suffering from postmenopausal osteoporosis and age-matched controls, we found that the 63Arg variant was associated with osteoporosis. This finding was congruent with the demonstration that a genetic deletion of CB2 resulted in an age-dependent low bone mass phenotype that showed several hallmarks of postmenopausal osteoporosis. The genetic findings were confirmed by us and others in cohorts of Russian and Japanese origin. Later studies indicated that this polymorphism is also associated with psychiatric disorders. The human genetic findings prompted a number of studies into the role of the endocannabinoid system in bone turnover and opened a completely new research chapter. We have actively participated in these efforts in a fruitful collaboration with Professor Itai Bab. It is now well established that the endocannabinoid system modulates various aspects of bone biology, including activity and proliferation of bone cells, the interaction between bone cells and bone-innervating vegetative nerve fibers, and the regulation of bone elongation. In the second funding period our interest shifted towards the role of CB2 signaling in neuropathic pain. This is a serious medical condition, where an injury to sensory neurons increases the sensitivity to pain, or can even be painful by itself. Neuropathic pain is very difficult to treat and often refractory to most pharmacotherapies. CB2 signaling may contribute to two distinct aspects of the underlying pathomechanism. The first one involves an activation of immune responses after nerve injury. Activation of microglia cells in the spinal cord projection area in particular has been linked to many of the physiological changes observed under neuropathic pain conditions. The microglia response is aggravated in animals lacking CB2 receptors and attenuated by CB2 activation. These findings are highly relevant in the context of a previous discovery together with Jürg Gertsch and his colleagues, where we found that ß-caryophyllene, a plant sesquiterpene, is a selective CB2 agonist. ß-caryophyllene is a common constituent of the essential oils of many spices and food plants. It is consumed in quantities sufficient for CB2 activation. We found that oral administration of ßcaryophyllene dose-dependently reduced inflammatory responses and pain symptoms after peripheral nerve injury. The efficacy of ß-caryophyllene was superior to synthetic CB2 agonists. These findings indicated that nutrient factors may be relevant to the manifestation of neuropathic pain symptoms. It is still unclear, if CB2 receptors on dorsal root ganglia neurons also contribute to the analgesic effects of CB2 agonists, because little is known about the physiological responses to neuronal CB2 activation. Together with DIetmar Schmitz and his colleagues, we have recently demonstrated that CB2 activation results in a long-lasting hyperpolarization of hippocampal principal neurons. It now remains to be determined if CB2 receptors on sensory neurons can produce similar effects and thus modulate physiological manifestations of sensory stimulation.
Publications
- (2008) Anandamide effects on 5-HT3 receptors in vivo. Europ. J. Pharmacol., 596: 98-101
Racz I, Bilkei-Gorzo A, Markert A, Stamer F, Gothert M, Zimmer A
(See online at https://doi.org/10.1016/j.ejphar.2008.08.012) - (2008). Crucial role of CB2 cannabinoid receptor in the regulation of central immune responses during neuropathic pain. J.Neurosci., 28: 12125-12135
Racz I, Nadal X, Alferink J, Banos JE, Rehnelt J, Martin M, Pintado B, Gutierrez-Adan A, Sanguino E, Manzanares J, Zimmer A, Maldonado R
(See online at https://doi.org/10.1523/JNEUROSCI.3400-08.2008) - (2008). Interferon-gamma is a critical modulator of CB2 cannabinoid receptor signaling during neuropathic pain. J.Neurosci., 28: 12136-12145
Racz I, Nadal X, Alferink J, Banos JE, Rehnelt J, Martin JM, Pintado B, Gutierrez-Adan A, Sanguino E, Manzanares J, Bellora N, Zimmer A, Maldonado R
(See online at https://doi.org/10.1523/JNEUROSCI.3402-08.2008) - (2009). The cannabinoid receptor type 2 (CNR2) gene is associated with hand bone strength phenotypes in an ethnically homogeneous family sample. Hum. Genet., 126(5): 629-36
Karsak M, Malkin I, Toliat MR, Kubisch C, Nürnberg P, Zimmer A, Livshits G
(See online at https://doi.org/10.1007/s00439-009-0708-8) - (2010). A novel peripherally restricted cannabinoid receptor antagonist, AM6545, reduces food intake and body weight, but does not cause malaise, in rodents. Br J Pharmacol 161: 629–642
Cluny NL, Vemuri VK, Chambers AP, Limebeer CL, Bedard H, Wood JT, et al.
(See online at https://doi.org/10.1111/j.1476-5381.2010.00908.x) - (2010). Activation of cannabinoid 2 receptors protects against cerebral ischemia by inhibiting neurophil recruitment. FASEB J., 24(3):788-98
Murikinati S, Jüttler E, Keinert T, Ridder DA, Muhammad S, Waibler Z, Ledent C, Zimmer A, Kalinke U, Schwaninger M
(See online at https://doi.org/10.1096/fj.09-141275) - (2010). CB(1) receptor inhibition leads to decreased vascular AT1 receptor expression, inhibition of oxidative stress and improved endothelial function. Basic Res Cardiol., 105(4):465-77
Tiyerili V, Zimmer S, Jung S, Wassmann K, Naehle CP, Lütjohann D, Zimmer A, Nickenig G, Wassmann S
(See online at https://doi.org/10.1007/s00395-010-0090-7) - (2010). Early onset of aging-like changes is restricted to cognitive abilities and skin structure in Cnr1-/- mice. Neurobiol Aging
Bilkei-Gorzo A, Drews E, Albayram O, Piyanova A, Gaffal E, Tueting T, Michel K, Mauer D, Maier W, Zimmer A
(See online at https://doi.org/10.1016/j.neurobiolaging.2010.07.009) - (2014). The cannabinoid CB receptor-selective phytocannabinoid beta-caryophyllene exerts analgesic effects in mouse models of inflammatory and neuropathic pain. European Neuropsychopharmacology 24: 608–620
Klauke A-L, Racz I, Pradier B, Markert A, Zimmer AM, Gertsch J, et al.
(See online at https://doi.org/10.1016/j.euroneuro.2013.10.008) - (2016). Cannabinoid Type 2 Receptors Mediate a Cell Type-Specific Plasticity in the Hippocampus. Neuron 90: 795–809
Stempel AV, Stumpf A, Zhang H-Y, Özdoğan T, Pannasch U, Theis A-K, et al.
(See online at https://doi.org/10.1016/j.neuron.2016.03.034)
