Characterization of cell type-specific endocannabinoid signaling at biochemical and behavioral level
Final Report Abstract
In the present project, our group investigated how the endocannabinoid system modulates biological processes in different types of neurons focussing on GABAergic, glutamatergic and serotonergic cells. To this end, we have generated genetically modified mice carrying cell-type specific cannabinoid receptor (CB1) mutations and subjected them to behavioural assays to reveal specific changes caused by their mutations. We first investigated how loss of CB1 from forebrain GABAergic or cortical glutamatergic neurons influences the functioning of the endocannabinoid system. We found that in a basal state (without challenging the animals) levels of endocannabinoids as well as expression levels of endocannabinoid synthesising and degrading enzymes are mostly unchanged in juvenile, adult and old mice despite the lack of CB1 in specific cell populations. We also found that in the hippocampi of mice, 70-80% of CB1 is present in GABAergic interneurons, yet, only about 30% of cannabinoid-induced G protein activation could be allocated to these cells. At the same time, glutamatergic cells, which express approximately 25% of hippocampal CB1, are responsible for the majority of cannabinoid-induced G protein activation in this brain region. We discovered that CB1 is part of vast multiprotein complexes of different composition in GABAergic vs. glutamatergic synaptosomes. Mass spectrometric analysis of these protein complexes confirmed already known CB1 interactors in addition to revealing new potentially interacting proteins. Mice lacking CB1 in cortical glutamatergic neurons showed a deficit in spatial learning. This phenotype was most prominent in juvenile mice and got weaker in adult and old animals. At the same time, adult and old mice showed a deficit in habituation to the test context; a phenotype that was not present in juvenile animals. Serotonin is synthesised and released by a small group cells present in distinct brain stem nuclei. These few thousand neurons, however, interact with almost every pertinent portion of the brain. Previously, we showed that every fifth to every twentieth of these neurons express CB1, suggesting that endocannabinoids might influence serotonin-mediated functions of the brain. Indeed, loss of CB1 from serotonergic cells leads to distinctive changes in the behaviour of mice. Specifically, mice lacking CB1 in serotonergic neurons are more anxious and less sociable than their wild-type littermates. Even more strikingly, however, is that these mutants show a significantly changed metabolism and energy balance as compared to control littermates. On a standard diet, they gain less weight than controls and when put on a high-fat diet, they eat less and do not get obese because of their increased energy expenditure. Furthermore, mutants seem to be protected against high caloric diet-caused dysregulation of the glucose homeostasis. CB1 receptor is present in many different neuronal populations, and therefore, is implicated in various physiological and pathological processes. Identifying the underlying molecular mechanisms of how the endocannabinoid system modulates biological processes in different types of neurons might pave the way to new therapeutic approaches as well as may deepen our understanding of how Cannabis compounds may exert their medicinal effects. The results we have gathered during the six years of the DFG-supported research unit FOR 926 will substantially contribute to expanding our knowledge about the cell type specific functioning of the endocannabinoid system.
Publications
- (2010) AAV vector-mediated overexpression of CB1 cannabinoid receptor in pyramidal neurons of the hippocampus protects against seizure-induced excitoxicity. PLoS One 5: e15707
Guggenhuber S, Monory K, Lutz B, Klugmann M
(See online at https://doi.org/10.1371/journal.pone.0015707) - (2010) Alterations in the hippocampal endocannabinoid system in diet-induced obese mice. J Neurosci. 30:6273-6281
Massa F, Mancini G, Schmidt H, Steindel F, Mackie K, Angioni C, Oliet SH, Geisslinger G, Lutz B
(See online at https://doi.org/10.1523/JNEUROSCI.2648-09.2010) - (2011) Role of CB1 cannabinoid receptors on GABAergic neurons in brain aging. Proc Natl Acad Sci USA. 108:11256-11261
Albayram O, Alferink J, Pitsch J, Piyanova A, Neitzert K, Poppensieker K, Mauer D, Michel K, Legler A, Becker A, Monory K, Lutz B, Zimmer A, Bilkei-Gorzo A
(See online at https://doi.org/10.1073/pnas.1016442108) - (2013) Cannabinoid 1 receptors in keratinocytes modulate proinflammatory chemokine secretion and attenuate contact allergic inflammation. J Immunol 190:4929-4936
Gaffal E, Cron M, Glodde N, Bald T, Kuner R, Zimmer A, Lutz B, Tüting T
(See online at https://doi.org/10.4049/jimmunol.1201777) - (2013) Cannabinoid receptor 1 deficiency in a mouse model of Alzheimer's disease leads to enhanced cognitive impairment despite of a reduction in amyloid deposition. Neurobiol Aging 34:2574-2584
Stumm C, Hiebel C, Hanstein R, Purrio M, Nagel H, Conrad A, Lutz B, Behl C, Clement AB
(See online at https://doi.org/10.1016/j.neurobiolaging.2013.05.027) - (2013) Neuron-type specific cannabinoid-mediated G protein signalling in mouse hippocampus. J Neurochem 124:795-807
Steindel F, Lerner R, Häring M, Ruehle S, Marsicano G, Lutz B, Monory K
(See online at https://doi.org/10.1111/jnc.12137) - (2015) Age differences in the role of the cannabinoid type 1 receptor on glutamatergic neurons in habituation and spatial memory acquisition. Life Sci 138:57-63
Legler A, Monory K, Lutz B
(See online at https://doi.org/10.1016/j.lfs.2015.01.034) - (2015) Cannabinoid type-1 receptor signaling in central serotonergic neurons regulates anxiety-like behavior and sociability. Front Behav Neurosci. 9:235
Häring M, Enk V, Aparisi Rey A, Loch S, Ruiz de Azua I, Weber T, Bartsch D, Monory K, Lutz B
(See online at https://doi.org/10.3389/fnbeh.2015.00235) - (2015) Enhanced Functional Activity of the Cannabinoid Type-1 Receptor Mediates Adolescent Behavior. J Neurosci. 35:13975-13988
Schneider M, Kasanetz F, Lynch DL, Friemel CM, Lassalle O, Hurst DP, Steindel F, Monory K, Schäfer C, Miederer I, Leweke FM, Schreckenberger M, Lutz B, Reggio PH, Manzoni OJ, Spanagel R
(See online at https://doi.org/10.1523/JNEUROSCI.1937-15.2015) - (2016) Cell Type-Specific Tandem Affinity Purification of the Mouse Hippocampal CB1 Receptor-Associated Proteome. J Proteome Res 15:3585-3601
Mattheus T, Kukla K, Zimmermann T, Tenzer S, Lutz B
(See online at https://doi.org/10.1021/acs.jproteome.6b00339)
