「IDENTIFICATION AND CHARACTERIZATION OF COLLAPSIN RESPONSE MEDIATOR PROTEIN (CRMP)-62, AN INTRACELLULAR PROTEIN RELATED UNC-33」
五島 良郎（横浜市立大学・医学部・薬理学）
The crux of nervous system development is the accurate extension and synaptic termination of axons. The growth cone at the distal tip of the growing neurite is thought to be responsible fordistinguishing between myriad potential pathways and targets presented during development.Extracellular signals for growth cone guidance include soluble factors, components of the extracellular matrix and cell adhesion molecules. Because a signal filopodial contact can reorient growth cone motility and allow the growth cone to sense shallow gradients of extracellular signals, powerful signal amplification systems are likely to be present in the growth cone membrane. Growth cone membranes can be enriched by a subcellular fractionation method, and contain very high levels of the signal-transducing, heteromeric GTP-binding protein, GO1). Several experiments demonstrate a key role for heteromeric G proteins in regulating growth cone motility.
It has become clear that inhibitory signals which repulse growth cones play a major role in axonal guidance. Collapsin, a growth cone collapsing factor derived from embryonic chick brain membrane extracts, and the related semaphorins, have been characterized at the molecular level2).The cDNA for collapsin encodes an 100 kDa secreted glycoprotein. Because as little as 10 pM collapsin can alter growth cone morphology, there is likely to be a high affinity signal transduction system for collapsin in growth cones.
The molecular mechanisms by which extracellular signals are transduced into growth cone collapse remain incompletely understood. Chick E10 brain membrane extracts containing collapsin cause DRG growth cone collapse in a pertussis toxin(PTX)-sensitive fashion. Mastoparan, a Go/i activator, also induces DRG growth cone collapse by a PTX sensitive mechanism. These data have suggested that at least some repulsive molecules collapse growth cones through a G protein-mediated cascade. Here, we attempt to identify molecules which are involved in growth cone collapse by analysis of X. laevis oocyte expression system. We have isolated a clone (CRMP-62) from a chick DRG cDNA library which mediates an inward currents flux in response to extracellular application of recombinant collapsin3). CRMP-62 is expressed exclusively in the embryonic nervous system. Introduction of an antibody against CRMP-62 into DRG neurons blocks collapsin-induced growth cone collapse.
1) Strittmatter et al., Nature 344, 836-841 (1991)
2) Luo et al., Cell 75, 217-227 (1993)3) Goshima et al., Nature 376, 509-514 (1995)