Identification of BiP as a CB 1 receptor-interacting protein that fine-tunes cannabinoid signaling in the mouse brain

Neuronal nitric-oxide synthase, unlike its endothelial and inducible counterparts, displays a PDZ (PSD-95/Dlg/ZO-1) domain located at its N terminus involved in subcellular targeting. The C termini of various cellular proteins insert within the binding groove of this PDZ domain and determine the subcellular distribution of neuronal NOS (nNOS). The molecular mechanisms underlying these interactions are poorly understood because the PDZ domain of nNOS can apparently exhibit class I, class II, and class III binding specificity. In addition, it has been recently suggested that the PDZ domain of nNOS binds with very low affinity to the C termini of target proteins, and a necessary simultaneous lateral interaction must take place for binding to occur. We describe herein that the PDZ domain of nNOS can behave as a bona fide class III PDZ domain and bind to C-terminal sequences with acidic residues at the P ؊2 position with low micromolar binding constants. Binding to C-terminal sequences with a hydrophobic residue at the P ؊2 position plus an acidic residue at the P ؊3 position (class II) can also occur, although interactions involving residues extending up to the P ؊7 position mediate this type of binding. This promiscuous behavior also extends to its association to class I sequences, which must display a Glu residue at P ؊3 and a Thr residue at P ؊2 . By means of site-directed mutagenesis and NMR spectroscopy, we have been able to identify the residues involved in each specific type of binding and rationalize the mechanisms used to recognize binding partners. Finally, we have analyzed the high affinity association of the PDZ domain of nNOS to claudin-3 and claudin-14, two tight junction tetraspan membrane proteins that are essential components of the paracellular barrier. Neuronal NOS (nNOS)2 is expressed constitutively in specific neurons of the brain and in the spinal cord, peripheral nitrergic nerves, epithelial cells of various organs, pancreatic islet cells, and vascular smooth muscle (1). nNOS differs from the two other mammalian isoforms, endothelial NOS and inducible NOS by an additional ϳ300-residue N-terminal extension mostly involved in specific subcellular targeting. The N terminus of nNOS contains a PDZ domain (first found in the proteins PSD-95, Dlg, and ZO-1) (2), a ␤-hairpin module that associates to ␣1-syntrophin and PSD-95 (3), a DYNLL1 binding site (4), and a stretch known to bind to repeats R16/R17 of dystrophin (5). Brain nNOS is found in particulate and soluble forms in cells, and the differential subcellular localization of nNOS in various tissues may contribute to its diverse functions. Reinforcement of the idea that the subcellular targeting is exquisitely governed by this N-terminal extension came from the observation that an N-terminal deletion mutant of nNOS is an active, mislocalized enzyme (6). In the nervous system, the PDZ domain of PSD-95 is known to couple NMDA receptors to nNOS so that ⅐ NO release becomes reversibly regulated by Ca 2ϩ /calmodulin binding (1). PDZ domains are modula...

High-grade gliomas constitute the most frequent and aggressive form of primary brain cancer in adults. These tumors express cannabinoid CB 1 and CB 2 receptors, as well as other elements of the endocannabinoid system. Accruing preclinical evidence supports that pharmacological activation of cannabinoid receptors located on glioma cells exerts overt anti-tumoral effects by modulating key intracellular signaling pathways. The mechanism of this cannabinoid receptor-evoked anti-tumoral activity in experimental models of glioma is intricate and may involve an inhibition not only of cancer cell survival/proliferation, but also of invasiveness, angiogenesis, and the stem cell-like properties of cancer cells, thereby affecting the complex tumor microenvironment. However, the precise biological role of the endocannabinoid system in the generation and progression of glioma seems very context-dependent and remains largely unknown. Increasing our basic knowledge on how (endo)cannabinoids act on glioma cells could help to optimize experimental cannabinoid-based anti-tumoral therapies, as well as the preliminary clinical testing that is currently underway.