Turning Away from Calcium

Abstract

Robles et al . investigated calcium (Ca 2+ )-dependent regulation of neurite outgrowth and discovered that the Ca 2+ -activated protease calpain can regulate growth cone guidance in response to integrin-mediated cues. Proper development of the nervous system depends on the ability of filopodia, fingerlike projections that extend from neuronal growth cones, to recognize and respond to environmental cues. Transient localized changes in filopodial intracellular calcium ([Ca 2+ ] i ) occur from numerous stimuli that affect neurite outgrowth. These changes have been linked to various growth cone behaviors; the downstream effectors linking [Ca 2+ ] i signals to guidance mechanisms, however, have remained unclear. Robles et al . investigated the role of calpain in mediating filopodial guidance in Xenopus spinal neurons grown on different substrata. Calpain inhibition promoted the outgrowth of neurons grown on tenascin, an integrin-binding extracellular matrix protein that stimulates high-frequency filopodial Ca 2+ transients, by increasing filopodial motility. Calpain inhibition had little effect on neurites grown on substrata that either did not stimulate high-frequency Ca 2+ transients or did not bind integrins. Both substratum-dependent Ca 2+ -signaling and high-frequency [Ca 2+ ] i transients were produced by photolysis of caged Ca 2+ -activated calpain, as determined with a fluorescent calpain substrate. Localized photolysis of caged Ca 2+ promoted growth cone turning, which depended on calpain activity. Immunofluorescence analysis indicated that Ca 2+ - and substratum-dependent calpain activity decreased phosphotyrosine levels and Src tyrosine kinase autophosphorylation. Consistent with this, pharmacological inhibition of Src family kinases promoted filopodial stability and inhibited neurite outgrowth on a substratum that did not promote calpain signaling, but had little effect on neurite outgrowth on tenascin. Thus, calpain-dependent regulation of phosphotyrosine levels appears to play a role in mediating Ca 2+ -dependent guidance in response to inhibitory integrin-dependent signals. E. Robles, A. Huttenlocher, T. M. Gomez, Filopodial calcium transients regulate growth cone motility and guidance through local activation of calpain. Neuron 38 , 597-609 (2003). [Online Journal]