A CaMKII/Calcineurin Switch Controls the Direction of Ca2+-Dependent Growth Cone Guidance

Abstract

Axon pathfinding depends on attractive and repulsive turning of growth cones to extracellular cues. Localized cytosolic Ca2+ signals are known to mediate the bidirectional responses, but downstream mechanisms remain elusive. Here, we report that calcium-calmodulin-dependent protein kinase II (CaMKII) and calcineurin (CaN) phosphatase provide a switch-like mechanism to control the direction of Ca(2+)-dependent growth cone turning. A relatively large local Ca2+ elevation preferentially activates CaMKII to induce attraction, while a modest local Ca2+ signal predominantly acts through CaN and phosphatase-1 (PP1) to produce repulsion. The resting level of intracellular Ca2+ concentrations also affects CaMKII/CaN operation: a normal baseline allows distinct turning responses to different local Ca2+ signals, while a low baseline favors CaN-PP1 activation for repulsion. Moreover, the cAMP pathway negatively regulates CaN-PP1 signaling to inhibit repulsion. Finally, CaMKII/CaN-PP1 also mediates netrin-1 guidance. Together, these findings establish a complex Ca2+ mechanism that targets the balance of CaMKII/CaN-PP1 activation to control distinct growth cone responses.

Calcium signaling in axon guidance

“…A recently discovered attraction/repulsion switch Recently a mathematical model has been used to describe the attraction/repulsion switch acting within growth cones [42]. This builds on previous work modeling the switch from long-term potentiation to long-term depression in synaptic plasticity [74], which is controlled by a similar pathway [44,45,47].…”

“…Large calcium increases (steep gradients) with normal extracellular calcium concentrations cause attraction [44]. Conversely, large intracellular calcium increases in the absence of extracellular calcium, or small intracellular calcium increases (shallow gradients), cause repulsion of the growth cone [45].…”

“…Other responses to the calcium elevation propagate the signal along a pathway beginning with Ca 2+ /calmodulin (CaM), and result in either attraction or repulsion of the growth cone along the gradient. Attraction is primarily mediated by CaMKII, whereas repulsion is mediated by CaN in conjunction with protein phosphatase 1 (PP1) [45]. Other crucial components of this pathway include PKA, exchange protein directly activated by cAMP (Epac), inhibitor 1 (I1), and the second messenger cAMP.…”

“…Other crucial components of this pathway include PKA, exchange protein directly activated by cAMP (Epac), inhibitor 1 (I1), and the second messenger cAMP. A recent mathematical model has proposed a mechanism for this molecular switch [42], based on a pathway uncovered by Wen et al [45] and Han et al [47] (Box 1 and Figure 2). Similar turning mechanisms have been proposed to underlie the response of various other cues, in particular morphogens [48,49].…”

“…However, once a critical threshold level is achieved CaMKII affinity for Ca 2+ /CaM increases dramatically [76]. Meanwhile, CaN has a constant affinity for Ca 2+ /CaM at all Ca 2+ concentrations [45]. At normal levels of cAMP, CaN activity predominates at low cytosolic Ca 2+ concentrations and CaMKII activity predominates at high concentrations [42].…”