Ionotropic and Metabotropic Mechanisms of Allosteric Modulation of α7 Nicotinic Receptor Intracellular Calcium

The pharmacological targeting of the alpha 7 nicotinic acetylcholine receptor (alpha 7) is a promising strategy in the development of new drugs for neurologic diseases. Because alpha 7 receptors regulate cellular calcium, we investigated how the prototypical type II-positive allosteric modulator PNU120596 affects alpha 7-mediated calcium signaling. Live imaging experiments show that PNU120596 augments ryanodine receptor-driven calcium-induced calcium release (CICR), inositol-induced calcium release (IICR), and phospholipase C activation by the alpha 7 receptor. Both influx of calcium through the alpha 7 nicotinic acetylcholine receptor (nAChR) channel as well as the binding of intracellular G proteins were involved in the effect of PNU120596 on intracellular calcium. This is evidenced by the findings that chelation of extracellular calcium, expression of alpha 7(D44A) or alpha 7(345-348A) mutant subunits, or blockade of calcium store release compromised the ability of PNU120596 to increase intracellular calcium transients generated by alpha 7 ligand activation. Spatiotemporal stochastic modeling of calcium transient responses corroborates these results and indicates that alpha 7 receptor activation enables calcium microdomains locally and to lesser extent in the distant cytosol. From the model, allosteric modulation of the receptor activates CICR locally via ryanodine receptors and augments IICR through enhanced calcium influx due to prolonged alpha 7 nAChR opening. These findings provide a new mechanistic framework for understanding the effect of alpha 7 receptor allosteric modulation on both local and global calcium dynamics.