Microtrabecular structure of the axoplasmic matrix: visualization of cross-linking structures and their distribution.
Mark H. Ellisman,K R Porter +1 more
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This lattice or microtrabecular matrix of axoplasm was found to consist of an organized system of cross-bridges between microtubules, neurofilaments, cisternae of the SER, and the plasma membrane, and it is proposed that formation and deformation of this system are involved in rapid axonal transport.Abstract:
Axoplasmic transport is a dramatic example of cytoplasmic motility. Constituents of axoplasm migrate as far as 400 mm/d or at approximately 5 micron/s. Thin-section studies have identified the major morphological elements within the axoplasm as being microtubules, neurofilaments (100-A filaments), an interconnected and elongated varicose component of smooth endoplasmic reticulum (SER), more dilated and vesicular organelles resembling portions of SER, multivesicular bodies, mitochondria, and, finally, a matrix of ground substance in which the tubules, filaments, and vesicles are suspended. In the ordinary thin-section image, the ground substance is comprised of wispy fragments which, in not being noticeably tied together, do not give the impression of representing more than a condensation of what might be a homogeneous solution of proteins. With the high-voltage microscope on thick (0.5-micron) sections, we have noticed, however, that the so-called wispy fragments are part of a three-dimensional lattice. We contend that this lattice is not an artifact of aldehyde fixation, and our contention is supported by its visability after rapid-freezing and freeze-substitution. This lattice or microtrabecular matrix of axoplasm was found to consist of an organized system of cross-bridges between microtubules, neurofilaments, cisternae of the SER, and the plasma membrane. We propose that formation and deformation of this system are involved in rapid axonal transport. To facilitate electron microscope visualization of the trabecular connections between elements of axoplasm, the following three techniques were used: first, the addition of tannic acid to the primary fixative, OsO4 postfixation, then en bloc staining in uranyl acetate for conventional transmission electron microscope (TEM); second, embedding tissue in polyethylene glycol for thin sectioning, dissolving out the embedding medium from the sections and blocks, critical-point-drying (J. J. Wolosewick, 1980, J. Cell Biol., 86:675-681.), and then observing the matrix-free sections with TEM or the blocks with a scanning electron microscope; and third, rapid freezing of fixed tissue followed by freeze-etching and rotary-shadowing with replicas observed by TEM. All of these procedures yielded images of cross-linking elements between neurofilaments and organelles of the axoplasm. These improvements in visualization should enable us to examine the distribution of trabecular links on motile axonal organelles.read more
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Organization of mammalian neurofilament polypeptides within the neuronal cytoskeleton
TL;DR: It is concluded that the 195-Kd polypeptide participates in a cross-linking function, and that this function is, at least in certain neurons, most prevalent in the mature axon.
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Control of axonal caliber by neurofilament transport.
TL;DR: The regenerating neuron was used as a model system for testing the hypotheses that neurofilaments are intrinsic determinants of axonal caliber, and that neuro Filaments content is controlled by the axonal transport of neurofilament content.
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Dynamic behavior of endoplasmic reticulum in living cells
Christopher Lee,Lan Bo Chen +1 more
TL;DR: It is proposed that microtubules act as the mechanism for constructing the polygonal lattice of interconnected membrane tubules that constitutes ER and that as such they play a role in generating ER motility and in constructing and maintaining the ER network in living cells.
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Multiple phosphorylation sites in mammalian neurofilament polypeptides.
TL;DR: The high phosphate content of the 200,000 dalton polypeptide was reflected by the extensive shifts both in its isoelectric point and apparent molecular weight after treatment of neurofilaments with alkaline phosphatase.
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JNK1 is required for maintenance of neuronal microtubules and controls phosphorylation of microtubule-associated proteins.
TL;DR: It is found that a member of the c-Jun NH(2)-terminal protein kinase (JNK) subgroup of MAP kinases, JNK1, is involved in regulation of MT dynamics in neuronal cells.
References
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Microtrabecular lattice of the cytoplasmic ground substance. Artifact or reality.
J J Wolosewick,K R Porter +1 more
TL;DR: Observations indicate that the microtrabecular lattice seen in electron micrographs closely represents the nonrandom structure of the cytoplasmic ground substance of living cultured cells.
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