Lankford K L, Letourneau P C
Department of Cell Biology and Neuroanatomy, University of Minnesota, Twin Cities.
Cell Motil Cytoskeleton. 1991;20(1):7-29. doi: 10.1002/cm.970200103.
In a previous study (J. Cell Biol. 109: 1229-1243, 1989), we reported that conditions which increased growth cone calcium levels and induced neurite retraction in cultured chick DRG neurons also resulted in an apparent loss of actin filaments in the growth cone periphery. We further showed that the actin-stabilizing drug phalloidin could block or reverse calcium-ionophore-induced neurite retraction, indicating that the behavioral changes were mediated, at least in part, by changes in actin filament stability. In this study, we have further characterized the calcium sensitivity of growth cone behavior to identify which features of calcium-induced behavioral effects can be attributed to effects on actin filaments alone, and to assess whether two other second-messenger systems, cAMP and protein kinase C, might influence neurite outgrowth by altering calcium levels or actin stability. The results indicated that growth cone behavior was highly sensitive to small changes in calcium concentrations. Neurite outgrowth was only observed in calcium-permeabilized cells when extracellular calcium concentrations were between 200 and 300 nM, and changes as small as 50 nM commonly produced detectable changes in behavior. Furthermore, low doses of cytochalasins mimicked all of the grossly observable features of growth cone responses to elevation of intracellular calcium, including the apparent preferential destruction of lamellipodial actin filaments and sparing of filopodial actin, suggesting that the behavioral effects of calcium elevation could be explained by loss of actin filaments alone. The effects of cAMP elevation and protein kinase C activation on growth cone behavior, ultrastructure, and fura2-AM-measured calcium levels indicated that the effects of cAMP manipulations could be partially explained by a cAMP-induced lowering of growth cone calcium levels and concomitant increased stabilization of actin filaments, but protein kinase C appeared to act through an independent mechanism.
在之前的一项研究中（《细胞生物学杂志》109: 1229 - 1243, 1989），我们报道称，在培养的鸡背根神经节（DRG）神经元中，那些增加生长锥钙水平并诱导神经突回缩的条件，也导致生长锥周边的肌动蛋白丝明显丢失。我们进一步表明，肌动蛋白稳定药物鬼笔环肽可以阻断或逆转钙离子载体诱导的神经突回缩，这表明行为变化至少部分是由肌动蛋白丝稳定性的变化介导的。在本研究中，我们进一步对生长锥行为的钙敏感性进行了表征，以确定钙诱导的行为效应的哪些特征可仅归因于对肌动蛋白丝的影响，并评估另外两个第二信使系统，即环磷酸腺苷（cAMP）和蛋白激酶C，是否可能通过改变钙水平或肌动蛋白稳定性来影响神经突生长。结果表明，生长锥行为对钙浓度的微小变化高度敏感。仅在细胞外钙浓度介于200至300 nM之间时，才在钙通透细胞中观察到神经突生长，小至50 nM的变化通常会引起行为上可检测到的变化。此外，低剂量的细胞松弛素模拟了生长锥对细胞内钙升高反应的所有可明显观察到的特征，包括片足肌动蛋白丝明显的优先破坏和丝状伪足肌动蛋白的保留，这表明钙升高的行为效应可仅由肌动蛋白丝的丢失来解释。cAMP升高和蛋白激酶C激活对生长锥行为、超微结构以及用fura2 - AM测量的钙水平的影响表明，cAMP操作的效应可部分由cAMP诱导的生长锥钙水平降低以及随之而来的肌动蛋白丝稳定性增加来解释，但蛋白激酶C似乎通过一种独立的机制起作用。
