Converting Repulsion to Attraction with CICR

Abstract

Growth cones guide neurites to their targets by monitoring chemoattractive and chemorepellant cues. Many cues elicit localized increases in cytosolic free calcium concentration ([Ca 2+ ] c ), but, curiously, both attractive and repulsive diffusible cues can increase [Ca 2+ ] c on the side of the growth cone facing the source of the gradient so that the growth cone turns toward (attraction) or away from (repulsion) the side with greater [Ca 2+ ] c . Ooashi et al . used focal laser-induced photolysis (FLIP) of caged Ca 2+ to transiently increase [Ca 2+ ] c in the growth cones of dorsal root ganglion (DRG) neurons grown on different substrata. Neurons grown on L1 or N-cadherin turned toward, whereas neurons grown on laminin turned away from, the side on which [Ca 2+ ] c was greater. Neurons grown on L1 and N-cadherin had more adenosine 3′,5′-monophosphate (cAMP) and showed increased cAMP binding to the regulatory subunits of cAMP-dependent protein kinase (PKA) than did neurons grown on laminin. Inhibition of cAMP signaling converted Ca 2+ -mediated attraction to repulsion, whereas pharmacological activation of PKA converted Ca 2+ -mediated repulsion to attraction. PKA phosphorylates ryanodine receptors (RyRs), facilitating calcium-induced calcium release (CICR), and pharmacological analysis combined with calcium imaging in conjunction with FLIP showed PKA-dependent activation of RyR-mediated CICR in growth cones on L1 or N-cadherin. Analysis of calcium signals and of the turning behavior of DRG neurons from mice lacking the type 3 RyR isoform (RyR3) implicated RyR3 in PKA-dependent CICR and attractive turning. Experiments with calcium buffers or with different amounts of the Ca 2+ -caging compound indicated that the source of the cytosolic Ca 2+ signal--rather than its amplitude--determined turning behavior. Thus, the authors conclude that a Ca 2+ signal that triggers CICR stimulates attractive turning, whereas a Ca 2+ signal without CICR elicits repulsion. N. Ooashi, A. Futatsugi, F. Yoshihara, K. Mikoshiba, H. Kamiguchi, Cell adhesion molecules regulate Ca 2+ -mediated steering of growth cones via cyclic AMP and ryanodine receptor type 3. J. Cell Biol. 170 , 1159-1167 (2005). [Abstract] [Full Text]