Shedding of neurexin 3ß ectodomain by ADAM10 releases a soluble fragment that affects the development of newborn neurons.

Neurexins are transmembrane synaptic cell adhesion molecules involved in the development and maturation of neuronal synapses. In the present study, we report that Nrxn3ß is processed by the metalloproteases ADAM10, ADAM17, and by the intramembrane-cleaving protease γ-secretase, producing secreted neurexin3ß (sNrxn3ß) and a single intracellular domain (Nrxn3ß-ICD). We further completed the full characterization of the sites at which Nrxn3ß is processed by these proteases. Supporting the physiological relevance of the Nrxn3ß processing, we demonstrate in vivo a significant effect of the secreted shedding product sNrxn3ß on the morphological development of adult newborn neurons in the mouse hippocampus. We show that sNrxn3ß produced by the cells of the dentate gyrus increases the spine density of newborn neurons whereas sNrxn3ß produced by the newborn neuron itself affects the number of its mossy fiber terminal extensions. These results support a pivotal role of sNrxn3ß in plasticity and network remodeling during neuronal development.

Activation of α7 nicotinic receptors by orthosteric and allosteric agonists: influence on single-channel kinetics and conductance.

Nicotinic acetylcholine receptors (nAChRs) are oligomeric transmembrane proteins in which five subunits coassemble to form a central ion channel pore. Conventional agonists, such as acetylcholine (ACh), bind to an orthosteric site, located at subunit interfaces in the extracellular domain. More recently, it has been demonstrated that nAChRs can also be activated by ligands binding to an allosteric transmembrane site. In the case of α7 nAChRs, ACh causes rapid activation and almost complete desensitization. In contrast, allosteric agonists such as 4-(4-bromophenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c] quin oline-8-sulfonamide (4BP-TQS) activate α7 nAChRs more slowly and cause only low levels of apparent desensitization. In the present study, single-channel patch-clamp recording has been used to investigate differences in the mechanism of activation of α7 nAChRs by ACh and 4BP-TQS. The most striking difference between activation by ACh and 4BP-TQS is in single-channel kinetics. In comparison with activation by ACh, single-channel open times and burst lengths are substantially longer (~160-800-fold, respectively), and shut times are shorter (~8-fold) when activated by 4BP-TQS. In addition, coapplication of ACh and 4BP-TQS results in a further increase in single-channel burst lengths. Mean burst lengths seen when the two agonists are coapplied (3099 ± 754 ms) are ~2.5-fold longer than with 4BP-TQS alone and ∼370-fold longer than with ACh alone. Intriguingly, the main single-channel conductance of α7 nAChRs, was significantly larger when activated by 4BP-TQS (100.3 ± 2.4 pS) than when activated by ACh (90.0 ± 2.7 pS), providing evidence that activation by allosteric and orthosteric agonists results in different α7 nAChRs open-channel conformations.