Ribosomal RNA in the Axoplasm of the Squid Giant Axon
Ribosomal RNA in the Axoplasm of the Squid Giant Axon
复制标题
鱿鱼巨轴突轴浆中的核糖体 RNA
DOI：
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发表时间：
1980
影响因子：
4.7
通讯作者：
G. Lazzarini
中科院分区：
文献类型：
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作者：
A. Giuditta;Aroldo Cupellot;G. Lazzarini
鱿鱼分离的巨大轴突能够主动合成轴突蛋白（Fischer and Litvak, 1967; Giuditta et al., 1968），这一证明提供了直接证据，证明轴浆流不能被认为是轴突蛋白的唯一来源，至少在这个系统中是这样。事实上，尽管它的大尺寸提供了实验上的优势，蛋白质流的发生还没有在鱿鱼巨大的轴突中得到证实。在头足类动物中，最近在章鱼的光纤中发现了一种以两种主要速率（大约1℃/ d和1℃/ h）进行的离心运输过程（Cimarra和Giuditta， 1979年）。在轴突发生的蛋白质合成引起的主要问题之一涉及到这一过程的细胞部位。原则上，这可能归因于轴突本身或轴突周围胶质细胞。在后一种情况下，新合成的分子必须从它们的胶质合成部位转移到巨大的轴突上，就像将放射性白蛋白加入外部介质时发生的那样（Giuditta et al., 1971）。这种可能性被用来解释用孤立轴突获得的数据（Lasek et al., 1974; Gainer et al., 1977; Lasek et al., 1977）。也许忽视蛋白质合成轴突系统的可能性的最有力的理由仍然是大量缺乏轴突核糖体的证据，这在成人纤维中很少观察到（Zelena, 1972）。与这些观察结果一致，已确定在Loligo p . e . k的巨型轴突的轴质中，4s RNA是普遍存在的，如果不是唯一的物种（Lasek et al., 1973）。我们自己试图在分离的鱿鱼巨轴突中区分胶质和轴突蛋白质合成位点（Giuditta，未发表的数据），这使我们确信，在区分胶质起源和存在于轴膜或最外层轴浆区域的可能合成位点方面，动力学和追逐实验是不充分的。后一种结构保留在轴质挤压后留下的鞘中的胶质层中，它们对鞘蛋白合成的贡献在生化分析中无法确定。此外，由于轴突和神经胶质膜紧密相连，即使放射自显像研究也可能无法产生鉴别结果，除非在电子显微镜水平上进行。在试图从不同的角度来解决这个问题的过程中，我们考虑到蛋白质合成轴突系统的出现可能意味着与这种活性相关的可溶性因子存在于轴质中。虽然这些因素确实已经被检测到（Giuditta et al., 1977），但要证明轴突系统的蛋白质合成还需要核糖体功能的额外证据。本文报道了从Loligo vulgaris巨轴突中分离的轴浆RNA除了含有主要的4种外，还含有少量的核糖体RNA。在4 s峰上也有尾随的RNA种。
The demonstration that isolated giant axons of the squid are capable of an active synthesis of axoplasmic proteins (Fischer and Litvak, 1967; Giuditta et al., 1968) has provided direct evidence that axoplasmic flow cannot be considered the only source of axonal proteins, at least in this system. Indeed, despite the experimental advantages offered by its large size, the occurrence of a flow of proteins has not been verified in the squid giant axon. In cephalopods, a centrifugal process of transport taking place at two main rates ( 1 c d d a y and 1 c d h , approx.) has been recently shown in octopus optic fibres (Cimarra and Giuditta, 1979). One of the main problems raised by the occurrence of protein synthesis in the axon concerns the cellular site of this process. In principle, this might be ascribed to the axon itself or to the periaxonal glial cells. In the latter case newly synthesized molecules would have to be transferred from their glial synthetic site to the giant axon, as it occurs when radioactive albumin is added to the external medium (Giuditta et al., 1971). This possibility has been proposed to explain data obtained with isolated axons (Lasek et al., 1974; Gainer et al., 1977; Lasek et al., 1977). Perhaps the most cogent reason to disregard the possibility of an axonal system of protein synthesis remains the substantial lack of evidence for axonal ribosomes, which have seldom been observed in adult fibres (Zelena, 1972). In accord with these observations, 4s RNA has been identified as the prevalent if not exclusive species in the axoplasm of giant axons of Loligo p e a k (Lasek et al., 1973). Our own attempts to distinguish between a glial and an axonal site of protein synthesis in isolated squid giant axons (Giuditta, unpublished datu) have convinced us of the inadequacy of kinetic and chase experiments in discriminating a glial origin from a possible synthetic site present in the axolemma or in the outermost axoplasmic regions. The latter structures remain with the glial layer in the sheath which is left after extrusion of the axoplasm and their contribution to the synthesis of sheath proteins cannot be identified in biochemical analyses. In addition, as axonal and glial membranes are closely apposed, even autoradiographic studies might not yield discriminating results unless carried out at the level of the electron microscope. In the attempt to approach the problem from a different perspective we have considered that the occurrence of an axonal system of protein synthesis would have implied the presence in the axoplasm of the soluble factors associated with this activity. .While these factors have indeed been detected (Giuditta et al., 1977), the demonstration of an axonal system of protein synthesis requires the additional evidence of functioning ribosomes. We report in this paper that axoplasmic RNA isolated from giant axons of Loligo vulgaris contains small amounts of ribosomal RNA in addition to the prevailing 4s species. RNA species trailing on the 4 s peak appear to be also present.