Control of Cortical Axon Elongation by a GABA-Driven Ca²⁺/Calmodulin-Dependent Protein Kinase Cascade

Abstract

Ca2ϩ signaling plays important roles during both axonal and dendritic growth. Yet whether and how Ca 2ϩ rises may trigger and contribute to the development of long-range cortical connections remains mostly unknown. Here, we demonstrate that two separate limbs of the Ca 2ϩ /calmodulin-dependent protein kinase kinase (CaMKK)-CaMKI cascades, CaMKK-CaMKI␣ and CaMKK-CaMKI␥, critically coordinate axonal and dendritic morphogenesis of cortical neurons, respectively. The axon-specific morphological phenotype required a diffuse cytoplasmic localization and a strikingly ␣-isoform-specific kinase activity of CaMKI. Unexpectedly, treatment with muscimol, a GABA A receptor agonist, selectively stimulated elongation of axons but not of dendrites, and the CaMKK-CaMKI␣ cascade critically mediated this axonogenic effect. Consistent with these findings, during early brain development, in vivo knockdown of CaMKI␣ significantly impaired the terminal axonal extension and thereby perturbed the refinement of the interhemispheric callosal projections into the contralateral cortices. Our findings thus indicate a novel role for the GABA-driven CaMKK-CaMKI␣ cascade as a mechanism critical for accurate cortical axon pathfinding, an essential process that may contribute to fine-tuning the formation of interhemispheric connectivity during the perinatal development of the CNS.

Kinesin-1 sorting in axons controls the differential retraction of arbor terminals

“…Nevertheless, intracellular systems that regulate individual arbor terminals in the axon have not been fully demonstrated. Ca 2+ is a major intracellular signaling molecule that regulates axonal growth (Kater and Mills, 1991;Gomez and Spitzer, 1999;Tang and Kalil, 2005;Ageta-Ishihara et al, 2009). Previous studies have suggested that localized Ca 2+ transients in restricted areas in the arbor enhance axonal growth, accompanied by retraction of other terminals (Hutchins and Kalil, 2008).…”

Ca2+/Calmodulin-dependent Protein Kinase Kinase β Is Regulated by Multisite Phosphorylation

“…Thus, CaMKK/CaMK signaling cascades are regulated at multiple levels by Ca 2ϩ /CaM. CaMKK␣ and CaMKK␤ are expressed most abundantly in the brain (2), where CaMKK/CaMK signaling cascades play important roles in a number of physiologically important neuronal processes, including learning and memory (8,9), neuronal differentiation and migration (10), neurite outgrowth (11)(12)(13), and synaptogenesis (14). In addition, CaMKK␤, but not CaMKK␣, is an upstream activator of the AMP-dependent protein kinase (AMPK) (15)(16)(17).…”

CaMKK–CaMKI Signaling Pathways Differentially Control Axon and Dendrite Elongation in Cortical Neurons: Figure 1.

“…Using an elegant series of genetic studies, including knockdown, knock-out, and overexpression studies, the authors established that CaMKK activation of CaMKI␣ leads to axonal growth, whereas CaMKK activation of CaMKI␥ promotes dendritic outgrowth [Ageta-Ishihara et al (2009), their Fig. 1, Fig.…”

“…This work provides clear evidence for a switch in CaMKKCaMKI signaling to control either dendritic or axonal elongation. Figure 1 provides a simple summary of the CaMKK-CAMKI signaling pathways derived from TakemotoKimura et al (2007) and Ageta-Ishihara et al (2009).…”

“…CaMKI␣ is instead found in the cytoplasm. To address whether this different localization is important for the differential effects on process growth, the authors performed a series of mutagenesis studies [Ageta-Ishihara et al (2009), their Fig. 4] to cause CaMKI␥ to be retained in the cytosol and CaMKI␣ to be targeted to lipid rafts.…”