Signaling to the Nucleus in Excitatory and Inhibitory Neurons

Abstract

Activity-regulated CREB phosphorylation and gene expression are important for cellular plasticity, survival, addiction, learning, and memory. Local signaling at CaV1 voltage-gated Ca²⁺ channels activates a CaM Kinase dependent pathway of communication between the cell surface and nuclear CREB. In excitatory neurons, γCaMKII acts as a molecular shuttle to transport CaM to the nucleus. γCaMKII nuclear translocation depends on phosphorylation of γCaMKII at Thr287 to trap Ca²⁺/CaM, and dephosphorylation of γCaMKII at Ser334 to unveil a nuclear localization signal and send γCaMKII/CaM to the nucleus. Upon arrival within the nucleus, Ca²⁺/CaM activates CaMKK and its substrate CaMKIV, the CREB kinase. In vivo, γCaMKII signaling is a critical component of training-induced gene expression and hippocampal-dependent spatial learning.

Properly operating CNS circuits depend not only on excitatory neurons, but also on inhibitory interneurons that in turn rely upon activity-dependent gene expression for morphological development, connectivity and functional E:I coordination. Despite its importance, such excitation-transcription coupling is poorly understood in inhibitory interneurons. We focused on parvalbumin-positive (PV+) interneurons, which comprise ~40% of cortical GABAergic interneurons in vivo and fire action potentials much faster than excitatory neurons. In the face of these much faster firing frequencies, how do PV+ cells faithfully link Ca²⁺ influx to transcription?

We report that PV+ interneurons employ a novel CaM Kinase-dependent pathway to trigger CREB phosphorylation and gene expression. As in excitatory neurons, Ca²⁺ influx through CaV1 channels triggers CaM translocation to the nucleus, initiating a nuclear CaMK cascade and phosphorylation of CREB by CaMKIV. However, this pathway differs from the CaMK pathway in excitatory neurons in two ways. First, nuclear signaling in PV+ cells is mediated by γCaMK I rather than γCaMKII; depolarization triggers γCaMKI redistribution to the nucleus and thereby supports CREB phosphorylation. Second, the kinetics of CREB phosphorylation are slower and more prominently sigmoid in PV+ interneurons and are rate-limited by low levels of the nuclear CREB kinase CaMKIV. This opposes saturation of phospho-CREB in PV+ cells in the face of intense firing.

Our findings resolve a long-standing question about how specificity is conferred in CaM Kinase-dependent signaling to the nucleus. In doing so, we demonstrate the generality across multiple cell types of CaM shuttling to drive nuclear CaMK activity. These findings are important for understanding disease pathophysiology, as both PV+ interneuron dysfunction and the signaling molecules underpinning E-T coupling in PV+ cells are linked to neuropsychiatric disease.