The cannabinoid receptor interacting protein CRIP1 binds to glutamate receptors in the CNS: Molecular mechanisms and functional implications

• Applicant: Professor Dr. Ralf Enz
• Subject Area: Molecular Biology and Physiology of Neurons and Glial Cells
• Term: from 2013 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 240914721

Glutamate is the most important excitatory neurotransmitter in the central nervous system (CNS) and activates G-protein coupled (metabotropic) glutamate receptors (mGluR1-mGluR8). Also endocannabinoids regulate G-protein coupled receptors (CB1 and CB2) that regulate neuronal activity and are responsible for psychotic effects of marijuana. Photoreceptors of the mammalian retina co-express pre-synaptically mGluR8 and CB1 that are both part of negative feedback loops regulating glutamate release from photoreceptor terminals. In this way, mGluR8 and CB1 participate in adaptive mechanisms that regulate the sensitivity of the retina over a large range of illumination.Crosstalk between different neurotransmitter receptor classes represents a key mechanism to coordinate signal pathways in neurons. Functional interactions between CB1 and mGluR types were reported. In addition, receptors for adenosine and serotonin form heterodimers with CB1 or mGluR2, and the formation of heterotrimeric receptor complexes composed of adenosine, dopamine, glutamate and endocannabinoid receptors in different combinations was demonstrated. In addition, intracellular scaffold proteins can physically link functionally connected receptor types.Recently, we identified the cannabinoid receptor interacting protein 1a CRIP1a as a new binding partner of the mGluR8a isoform. Therefore, we suggest a functional/physical interaction between mGluR8a and CB1 via CRIP1. This hypothesis is based on (i) the identification of CRIP1a as a mGluR8a interactor, (ii) the co-expression of CRIP1a, CB1 and mGluR8a in pre-synaptic photoreceptor terminals, (iii) similar physiological functions of CB1 and mGluR8a (inhibition of the pre-synaptic glutamate release), (iv) a high homology between the mapped 9 amino acid long CRIP1a binding sequence in the CB1 C-terminus and a linear stretch of 6 amino acids in the mGluR8a C-terminus and (v) finally on the known biological principle that receptor crosstalk can occur via intracellular proteins.We plan to characterise molecular mechanisms that mediate the identified mGluR8a/CRIP1a interaction, as well as to elucidate a suggested physical/functional crosstalk between mGluR8a and CB1 in the CNS. Our working programme will first analyse molecular mechanisms of the binding between mGluR8a, CB1 and CRIP1a, thereby providing molecular tools and mechanistic knowledge for successive functional studies. These data will be complemented by anatomical co-localization experiments. Thereafter, we plan to investigate functional consequences of the newly identified protein interactions, first using recombinantly expressed proteins in HEK-293 cells and thereafter in photoreceptors.Our studies are designed to describe new principles of neurotransmitter receptor function in the CNS and will specifically elucidate molecular mechanisms that regulate signalling at the first visual synapse in the mammalian retina.

• DFG Programme: Research Grants