Tuning the Lateral Range of L-Type Calcium Channel-Dependent Calcium Signals in Dendrites of Hippocampal Neurons

Abstract

In cultured hippocampal neurons, voltage-gated L-type Ca2+ channel (LTCC) activity is subject to potentiation by protein kinase A (PKA) and to feedback inhibition by STIM1, a sensor of endoplasmic reticulum (ER) [Ca2+]. Feedback by STIM1 is engaged following NMDA receptor-dependent LTCC activation: LTCCs allow influx of Ca2+ that induces Ca2+ release (CICR) from ER stores, and CICR-driven stores depletion activates STIM1, which is able to span the gap between cortical ER and plasma membrane to contact and inhibit LTCCs. PKA potentiation of LTCC activity relies upon anchoring of the kinase to LTCCs by the scaffolding protein AKAP150; AKAP150 co-anchoring of the phosphatase calcineurin (CaN) provides a counterbalance on LTCC activity. Uncaging of MNI-glutamate adjacent to a dendritic spine triggers a rise in [Ca2+]cytosol and a drop in [Ca2+]ER. A substantial component of each of these changes in [Ca2+] is dependent upon LTCC activity, as measured by sensitivity to the LTCC antagonist nimodipine. For a stimulated spine, the nimodipine-sensitive component of both changes in [Ca2+]cytosol and [Ca2+]ER extends in ∼exponential fashion along the adjoining dendrite, with length constants of ∼5 µm. AKAP150 knockdown and replacement with AKAP150 lacking its PKA binding domain reduces the spread of both the cytosolic and ER Ca2+ changes, whereas knockdown and replacement with an AKAP150 mutant lacking the CaN site increases the lateral range of Ca2+ signaling. The effects on [Ca2+]cytosol and [Ca2+]ER of delocalizing PKA or CaN are attributable, respectively, to depotentiation of LTCC Ca2+ influx and hence decreased CICR or to unopposed potentiation and increased CICR. These results suggest that the spatial extent of LTCC-dependent Ca2+ signals in dendrites may be tuned in accord with the local balance of PKA and CaN activity.