Regulation of Axonal Extension and Growth Cone Motility by Calmodulin-Dependent Protein Kinase I

Abstract

Calcium and calmodulin (CaM) are important signaling molecules that regulate axonal or dendritic extension and branching. The Ca 2ϩ -dependent stimulation of neurite elongation has generally been assumed to be mediated by CaM-kinase II (CaMKII), although other members of the CaMK family are highly expressed in developing neurons. We have examined this assumption using a combination of dominant-negative CaMKs (dnCaMKs) and other specific CaMK inhibitors. Here we report that inhibition of cytosolic CaMKI, but not CaMKII or nuclear CaMKIV, dramatically decreases axonal outgrowth and branching in cultured neonatal hippocampal and postnatal cerebellar granule neurons. CaMKI is found throughout the cell cytosol, including the growth cone. Growth cones of neurons expressing dnCaMI or dnCaMKK, the upstream activator of CaMKI, exhibit collapsed morphology with a prominent reduction in lamellipodia. Live-cell imaging confirms that these morphological changes are associated with a dramatic decrease in growth cone motility. Treatment of neurons with 1,8-naphthoylene benzimidazole-3-carboxylic acid (STO-609), an inhibitor of CaMKK, causes a similar change in morphology and reduction in growth cone motility, and this inhibition can be rescued by transfection with an STO-609-insensitive mutant of CaMKK or by transfection with constitutively active CaMKI. These results identify CaMKI as a positive transducer of growth cone motility and axon outgrowth and provide a new physiological role for the CaMKK-CaMKI pathway.

GABA Regulates Dendritic Growth by Stabilizing Lamellipodia in Newly Generated Interneurons of the Olfactory Bulb

“…Lamellipodia are known to play a crucial role in growth cone motility (Dickson, 2002). Thus, it has been proposed that lamellipodia might help to stabilize growth cones and promote neurite elongation (Wayman et al, 2004). Consistent with this idea, we found that GABA promotes stabilization of newly formed dendritic segments, thereby inducing a net elongation of dendrites.…”

Differential Outgrowth of Axons and their Branches Is Regulated by Localized Calcium Transients

“…It seems likely that differential activation of calcium frequency-dependent effectors such as CaMKII and calcineurin would occur in each axonal process. Positive effectors such as CaMKI and CaMKII, which have been shown to promote neurite outgrowth (Borodinsky et al, 2003;Wayman et al, 2004Wayman et al, , 2006 are preferentially activated in axonal processes with higher frequencies of Ca 2ϩ transients (Tang and Kalil, 2005). CaMKII␤ has been shown to bind directly to actin filaments and increase filopodial dynamics (Fink et al, 2003), consistent with results indicating that direct elevation of Ca 2ϩ activity by photolysis of caged calcium can rapidly evoke protrusion of new filopodia through polymerization of actin filaments (Lau et al, 1999).…”

Convergent lines of evidence support CAMKK2 as a schizophrenia susceptibility gene

“…One study also showed the enzyme encoded by CAMKK2 can phosphorylate AMP-activated protein kinase. 68 The CAMKK2 gene has the strongest expression in brain 31 and it influences signaling cascades that are involved with learning and memory, 32,69 neuronal differentiation and migration, 70 neurite outgrowth 34,35,70 and synapse formation.…”

“…CAMKK2 can promote neurite outgrowth during brain development and inhibition of CAMKK2 inhibits neurite outgrowth and causes axonal growth cone collapse. 34,35 In addition, transgenic studies performed in mice have shown that CAMKK2 is required for late long-term potentiation (LTP) in the hippocampus and spatial memory formation. 32 Furthermore, previous studies have revealed that CAMKK2 is required for the activation of hippocampal cyclic AMPresponsive element-binding protein (CREB) by spatial training, 32 a key step in long-term memory formation.…”