Plk2 Raps up Ras to subdue synapses

Abstract

We recently identified the activity-inducible protein kinase Plk2 as a novel overseer of the balance between Ras and Rap small GTPases. Plk2 achieves a profound level of regulatory control by interacting with and phosphorylating at least four Ras and Rap guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs). Combined, these actions result in synergistic suppression of Ras and hyperstimulation of Rap signaling. Perturbation of Plk2 function abolished homeostatic adaptation of synapses to enhanced activity and impaired behavioral adaptation in various learning tasks, indicating that this regulation was critical for maintaining appropriate Ras/Rap levels. These studies provide insights into the highly cooperative nature of Ras and Rap regulation in neurons. However, different GEF and GAP substrates of Plk2 also controlled specific aspects of dendritic spine morphology, illustrating the ability of individual GAPs/GEFs to assemble microdomains of Ras and Rap signaling that respond to different stimuli and couple to distinct output pathways.

Figure 1

Coordinated regulation of Ras/Rap GAPs and GEFs by Plk2. Schematic model for Plk2 function in Ras/Rap signaling. Strong synaptic activation (lightning bolts) induces commensurate levels of Plk2 gene expression. Plk2 targets multiple GAPs and GEFs that regulate Ras/Rap cycling between active, GTP bound (awake and glowing, light bulb on) and inactive, GDP bound (asleep and dull, light bulb off) states. RasGRF1/CDC25Mm is a neuron-specific Ras GEF involved in long-term memory consolidation., PDZGEF1 (also called RapGEF2, nRapGEP, CNrasGEF or RA-GEF) is a Rap GEF found at synapses., SynGAP is a Ras GAP that is an abundant component of the postsynaptic density., SPAR is a Rap GAP that is highly enriched in dendritic spines in association with actin cytoskeleton. Plk2 antagonizes SPAR and RasGRF1, while promoting PDZFEG1 and SynGAP activity, leading to synergistic inactivation of Ras and hyperactivation of Rap. As a result, AMPA receptors are removed from the neuronal surface and dendritic spines undergo shrinkage or elimination, helping to counteract the initial overexcitation.

Figure 2

Microdomains of Ras/Rap signaling mediated by GAPs/GEFs. A variety of synaptically localized GAPs (shown in teal) and GEFs (periwinkle) maintain Ras and Rap in the OFF and ON states, respectively. RasGRF1 interacts with the NR2B subunit of NMDA receptors (NMDARs). PDZGEF1 associates with S-SCAM, a postsynaptic scaffolding protein. SynGAP binds to the PDZ domains of scaffold protein PSD-95., SPAR is also associated with PSD-95 via a guanylate kinase domain interaction. Microdomains of Ras and Rap scaffolded by different GAPs/GEFs mediate specific aspects of dendritic spine morphogenesis. Under appropriate stimuli, homeostatic plasticity driven by Plk2 (dashed yellow lines) coordinately regulate GAPs/GEFs. Acute plasticity mechanisms (pink) may engage similar sets of proteins as homeostatic ones but differ in time course as well as type of stimulus. Other GAPs/GEFs not studied here may form additional microdomains that control different facets of plasticity and respond to distinct patterns of synaptic activity.