Emerging roles of collapsin response mediator proteins (CRMPs) as regulators of voltage-gated calcium channels and synaptic transmission

Abstract

Presynaptic N-type voltage-gated Ca(2+) channels (Cav2.2) form part of an extensive macromolecular complex in the presynaptic terminal. Regulation of Cav2.2 is achieved via protein-protein interactions within the terminal and can directly impact transmitter release which is dependent on Ca(2+) influx via these Cav2.2. We recently identified a novel Cav2.2 interacting partner-the collapsin response mediator protein (CRMP).1 CRMPs are a family of five proteins implicated in signal transduction of neurite outgrowth and axonal guidance. We showed that CRMP-2, a wellstudied member of this family, interacted with Cav2.2 via direct binding to cytoplasmic loops of Cav2.2. Depolarization enhanced the interaction. Further studies revealed that CRMP-2 facilitated an increase in Cav2.2 current density by inserting more Cav2.2 at the cell surface. As a consequence of CRMP-2-mediated increase in Ca(2+) influx, release of the excitatory neurotransmitter glutamate was also increased. CRMP-2 localized to synapses where, surprisingly, its overexpression increased synapse size. We hypothesize that the CRMP-2-calcium channel interaction represents a novel mechanism for modulation of Ca(2+) influx into nerve terminals and, hence, of synaptic strength. In this addendum, we further discuss the significance of this study and the possible implications to the field.

Keywords: CRMP-2; Cav2.2; axonal outgrowth; growth cone; presynaptic calcium channels; surface trafficking; synaptic transmission.

Figure 1

CRMP-2 signaling cascade: a novel role for CRMPs in Ca²⁺ channel regulation and transmitter release. Extracellular signals, such as extracellular matrix, growth factors and guidance cues (semaphorin 3A) activate Neuropilin-1/Plexin A receptors on membranes. A battery of kinases, including RhoK, Cdk5 and GSK-3β phosphorylate CRMPs. Phosphorylated CRMPs have a reduced affinity to tubulin and other interacting molecules and lose their positive effect on axon elongation, thereby causing growth arrest and growth cone collapse. In contrast, non-phosphorylated CRMPs bind strongly to tubulin heterodimers to promote microtubule assembly and Numb-mediated endocytosis thereby promoting axon elongation and branching. In addition to these classically defined roles for CRMPs, our results suggest that CRMPs (assuming both phosphorylated and non-phosphorylated forms) bind to cytoplasmic loops of the Ca²⁺ channel and increase their insertion into the membrane, resulting in an increased current density. This increase culminates into an increase in the release of the excitatory transmitter glutamate. Interestingly, CRMP-2 overexpression increases synapse size not number. This suggests that CRMP-2 regulation of transmitters may occur via a direct effect on Ca_V2.2 or through an effect on changes in synaptic vesicle machinery and release probabilities. Increased synaptic transmission is likely to contribute to synaptic plasticity.

Figure 2

CRMP-4 enhances Ca²⁺ current density in hippocampal neurons. (A) Exemplar current traces obtained from a cell transfected with EGFP and CRMP-4-EGFP evoked by 200-ms steps to +10 mV applied from a holding potential of -80 mV, as shown in the voltage protocol above the traces. Bath solutions contained 1 µM TT X, 10 mM TE A and 1 µM Nifidepine to block Na⁺, K⁺ and L-type voltage-gated Ca²⁺ channels, respectively. (B) Peak current density (pA/pF) measured at +10 mV for EGFP (n = 8) or CRMP-4a-EGFP (n = 8) transfected neurons. *p < 0.05 versus EGFP (One-way ANOVA).