Number Of Microtubules In Axons Research Articles

Neuronal atrophy and synaptic alteration in a mouse model of dentatorubral-pallidoluysian atrophy

Dentatorubral-pallidoluysian atrophy (DRPLA) is a hereditary spinocerebellar degeneration caused by expansion of a CAG repeat in the disease protein. Despite the restricted and stable brain lesions, DRPLA patients show a variety of clinical symptoms and the brain exhibits generalized atrophy. In previous studies of DRPLA, we proposed that intranuclear diffuse accumulation of the mutant protein is a significant pathological feature of neurons, and that the variable prevalence of this pathology may be relevant to the variation of symptoms observed in patients with different repeat sizes. In this study, to elucidate the pathogenesis of the brain atrophy in DRPLA, we conducted morphological and statistical analyses of neurons affected by the polyglutamine pathology in DRPLA transgenic (Tg) mice with 129 polyglutamine stretches. Golgi-impregnated pyramidal neurons in cerebral cortical layer V of 15-week-old Tg mice showed significant atrophy of the perikarya and dendrites. Dendritic spines were decreased in number and size, and showed a change in morphology resulting in dominance of stubby spines. Interestingly, dendritic arborization was preserved. Electron microscopy revealed that axons in the pyramis and corpus callosum were also atrophic. The number of axonal microtubules was preserved; however, the inter-microtubule spacing was significantly decreased. In the neuropil of cerebral cortical layers II and III, atrophy of the pre-synaptic areas and lengths of the post-synaptic density was detected, but synaptic vesicle diameter was preserved. These results suggest that neuronal atrophy is an essential feature of the cell pathology in DRPLA and that this is closely related to polyglutamine pathogenesis and development of the clinical phenotype.

Gene replacement in mice reveals that the heavily phosphorylated tail of neurofilament heavy subunit does not affect axonal caliber or the transit of cargoes in slow axonal transport.

The COOH-terminal tail of mammalian neurofilament heavy subunit (NF-H), the largest neurofilament subunit, contains 44-51 lysine–serine–proline repeats that are nearly stoichiometrically phosphorylated after assembly into neurofilaments in axons. Phosphorylation of these repeats has been implicated in promotion of radial growth of axons, control of nearest neighbor distances between neurofilaments or from neurofilaments to other structural components in axons, and as a determinant of slow axonal transport. These roles have now been tested through analysis of mice in which the NF-H gene was replaced by one deleted in the NF-H tail. Loss of the NF-H tail and all of its phosphorylation sites does not affect the number of neurofilaments, alter the ratios of the three neurofilament subunits, or affect the number of microtubules in axons. Additionally, it does not reduce interfilament spacing of most neurofilaments, the speed of action potential propagation, or mature cross-sectional areas of large motor or sensory axons, although its absence slows the speed of acquisition of normal diameters. Most surprisingly, at least in optic nerve axons, loss of the NF-H tail does not affect the rate of transport of neurofilament subunits.

The effects of trifluoperazine on fast and slow axonal transport in the rabbit vagus nerve

The effects of trifluoperazine (TFP) on fast and slow axonal transport (AXT) of labeled proteins were examined in the rabbit vagus nerve. Cuffs soaked in a 10 mM, but not 0.1 mM or 1 mM, concentration of TFP applied locally around the vagus nerve in vivo blocked both fast and slow AXT, as measured by the accumulation of 3H-labeled proteins. In vitro, fast AXT was affected by 0.1 mM TFP. The TFP cuff treatment caused a reduction in the number of axonal microtubules (MT) whereas cuffs soaked in saline had no effect. The levels of ATP, ADP, and AMP were not significantly lowered by the TFP treatment. The results suggest that both fast and slow AXT are sensitive to TFP treatment, and that the axonal MT-system may be the main target of the drug.

The cochlear nerve in the cat: topography, cochleotopy, and fiber spectrum.

The topographical and cytological features of the cochlear nerve in normal adult cats were studied by microdissection, light microscopy, transmission and scanning electron microscopy. The 2-mm long cochlear nerve trunk is situated within the internal acoustic meatus. The nerve is wrapped like a roll with the more basal fibers situated peripheral to the more apical ones. The fibers are fasciculated according to their target in the cochlea and follow a helical course with the sharpest spiralling in the distal portion of the fiber. Apical and basal coil fibers are of approximately the same length measured from the spiral lamina to the rostral border of the anteroventral cochlear nucleus. The nerve contains about 90% thick, myelinated fibers, about 10% unmyelinated axons, and a few thin, but heavily myelinated axons. The first group shows a unimodal distribution of axonal diameters, with a gradual increase in average diameter from basal to apical coil fibers. The number of axonal microtubules and neurofilaments have a high linear correlation to axonal circumference without principal cochleotopic differences. The myelin sheaths are thicker and the internodal segments shorter than expected from similarly sized peripheral fibers in rodents. The possible nature and origin of the three fiber categories are discussed.

The effects of low temperature on myelin formation in optic nerves of Xenopus tadpoles

To test the effect of cold on CNS myelin formation, optic nerves of stages 52–55 Xenopus tadpoles were examined electron microscopically after maintenance at 15, 10, 7 and 4 °C for 1–7 days. Nerves from tadpoles maintained at 15 °C resembled 22 °C (room temperature) controls. After 3 days at 10, 7, or 4 °C, tongue processes and perikarya of many myelin forming oligodendrocytes were swollen and filled with vesicular membrane profiles. The number of axonal microtubules was decreased in affected fibers but the lamellar structure of their myelin sheaths remained normal. Astrocytes were hypertrophic and contained large aggregates of filaments. Longer exposure to 10 or 7 °C increased the number of affected fibers but the changes were not more severe or associated with degeneration. The delayed onset, lack of progression and reversibility of the changes indicated that cold has a direct metabolic effect on myelin forming oligodendrocytes. Alterations produced by nerve transection or exposure to mitotic inhibitors differed, suggesting that cold induced changes were not due primarily to either axonal degeneration or reduced axonal transport.

Effects of lidocaine on axonal morphology, microtubules, and rapid transport in rabbit vagus nerve in vitro.

AbstractTreatment with lidocaine, at exposures which completely block impulse conduction and rapid axonal transport, resulted in a sequence of morphological effects on rabbit vagus never in vitro. Low exposures (0.3% for 90 min, 0.4% for 45 min, 0.6% for 25 min) caused a reorientation and proliferation of smooth endoplasmic reticulum (SER); the number of axonal microtubules either remained normal or was increased. Intermediate exposures (0.4% for 90 min, 0.6% for 45 min) Produced a similar effect on the SER, as well as causing a 50% reduction in microtubule number and some swelling of axons and Schwann cells. The highest exposure (0.6% for 75 min) caused over 90% reduction in microtubule number, a partial loss of neurofilaments, and severe swelling of axons, Schwann call, and mitochondria. Reversibility of these effects was tested in lidocaine‐treated nerves that had been washed with fresh culture medium. There was good recovery of nerve exposures. After intermediate exposures, there was only partial return of rapid transport, even though imposures, there was morphological, and no recovery occured throughout the excised vagus nerve in efferent and afferent axons as well as Schwann cells, and these events did not coincide with changes in the functional state of rapid axonal transport.

Axoplasmic transport in the crayfish nerve cord. The role of fibrillar constituents of neurons.

(a) Axoplasmic transport of tritium-labeled proteins in crayfish nerve cord was confirmed at a slow rate of 1 mm/day. A second proteinaceous component which moves at a rate of 10 mm/day was also detected. Radioautography and biochemical analysis indicate that proteins migrating at these velocities have a perikaryal origin and move caudad within axons as sharply defined peaks. (b) Evidence is presented for the blockage of the slow and the fast movement of proteins by intraganglionic injection of the anti-mitotic agent vinblastine sulfate (0.1 mM). (c) Electron microscope observations of vinblastine-treated ganglia revealed a reduction in the number of axonal microtubules and the formation of intracellular aggregates presumably composed of microtubular protein. (d) These findings would be compatible with the involvement of microtubules in both slow and fast axoplasmic transport. However, the block induced by vinblastine was detected in regions of the cord (up to 10 mm away from the injection site) where the number and morphology of microtubules appeared unaltered. In addition, axons showing effects of vinblastine occasionally contained mitochondria with remarkably dense and thickened membranes. (e) In association with the surfaces of axonal microtubules are lateral filamentous elements (40-80 A in diameter) which also showed vinblastine-induced alterations. Our observations indicate that such filiform structures, associated with microtubules, may be a necessary component in the transport mechanism(s).

Studies on the translocation of noradrenaline-containing vesicles in post-ganglionic sympathetic neurones in vitro. Inhibition of movement by colchicine and vinblastine and evidence for the involvement of axonal microtubules.

1. Methods are presented for studying axonal transport mechanisms in preparations of constricted hypogastric nerve/inferior mesenteric ganglia maintained in vitro for periods up to 48 hr. Under these conditions the ultrastructure of the tissue is excellently preserved.2. The proximo-distal movement of noradrenaline and noradrenaline storage vesicles in the non-myelinated axons of these preparations is inhibited by both colchicine (10 mug/ml.) and vinblastine sulphate (1 mug/ml.) whilst the movement of mitochondria appears to be unaffected.3. Neither colchicine nor vinblastine sulphate depletes accumulated dense-cored vesicles of their stores of noradrenaline.4. These drugs reduce the accumulation of noradrenaline and densecored vesicles against a constriction when they are in contact with the nerve trunks only, and are denied direct access to the nerve cell bodies in the ganglion.5. The only other morphological change that can be attributed to the action of colchicine and vinblastine is a marked reduction in the number of axonal microtubules.6. The experiments provide strong support for the view that the axonal system of microtubules is closely involved in the proximo-distal movement of noradrenaline storage vesicles within noradrenergic neurones. The microtubular system does not appear to be involved in mitochondrial movement.