Computer-assisted analysis of particulate axoplasmic flow in organized CNS tissue cultures

Calcium dependent modulation of fast axonal transport

The highly differentiated structure of the neuron poses special problems for the intracellular movement of molecules throughout the cell. Molecular transport distances from the synthesizing neuron cell body along the axon (which has no substantial synthetic capabilities) to the axon terminal are very great. The transported substances, transport support structures, translocator motors, and control elements are currently the focus of intense research. Interruption of this flow of molecules could have disastrous effects upon the cell and ultimately the organism resulting in neuropathological conditions. Calcium plays a critical role in modulating fast-axonal transport (FAT) speeds. Before discussing the effect of calcium on FAT, we summarize our broad perspective on the role of axonal transport in neurologic disease.

Fast axonal transport alterations in amyotrophic lateral sclerosis (ALS) and in parathyroid hormone (PTH)-treated axons

Video-enhanced contrast techniques have been used to study fast axonal transport of organelles in diseased and normal human axons. A broad perspective on the importance of axonal transport in the pathogenesis of human neurological disorders is presented and problems in dealing with human nerve summarized. Results from analysis of organelle traffic in axons from motor nerve in patients with amyotrophic lateral sclerosis (ALS) show: 1) higher mean speed of anterograde organelles, 2) lower mean speed of retrograde organelles, and 3) lower retrograde organelle traffic density. Hyperparathyroidism, another human clinical syndrome, can mimic ALS. The effect of parathyroid hormone (PTH) on axons in vitro is to increase the mean speed of both anterograde and retrograde organelle traffic. The dose response curve and time course of the PTH effect are delineated. Dihydropyridine calcium channel antagonists block the PTH effect, implicating extracellular calcium in the alteration of organelle traffic speed. The results are discussed in relation to neuronal function and the regulation of fast axonal transport.

Simulation of axoplasmic transport

We have analysed a kinetic model of axonal transport by simulating experimental tracer profiles. The existence of three phases of axoplasmic transport is assumed: fast anterograde, slow anterograde and retrograde. Each phase has its characteristic velocity. Transported materials are postulated to shift between these phases. Also catabolism and sequestration of material is allowed for in our model. Thus, we have set up equations which contain axonal transport, diffusion and cross-over terms. The rate constants of material shifts were determined by computer fitting to experimental data. Best-fitted values of the rate constants for transfer of material between the fast and slow phases were both 2 X 10(-5) sec-1, while the rate constants for transfer between the fast and retrograde phases were both 1 X 10(-5) sec-1. The rate constant of material loss from the slow phase to the extracellular space was 1 X 10(-6) sec-1. The material shift between the slow and retrograde phases was negligibly small. These data show that there is exchange of material between the fast and slow phases and between the fast and retrograde phases. However, there is no significant exchange between the slow and retrograde phases. Diffusion was found to have only a minor effect on the profiles. The velocity of the fast anterograde track in cold-blooded animals was predicted to be around 200 mm/day, or, in other words, to be close to experimentally observed values of the fast anterograde component of axonal transport.

Effects of disruption of microtubules on translocation of particles and morphology in tissue cultured neurites

The role of microtubules in axoplasmic transport was studied by using colchicine and vinblastine in tissue cultured dorsal root ganglion cells dissected from chick embryos. After 3-4 days in culture, the effects of the agents on particle movement were examined under Nomarski optics. The movements of the particles were analyzed with high-contrast video systems. The number of transported particles passing through one region decreased after exposure to colchicine or vinblastine in concentrations greater than 1 X 10(-7) M, but these agents did not affect the velocity of the transported particles. After treatment with the agents swelling occurred in regions where particles sometimes stopped or slowed down in normal conditions. The intervals between swollen regions were 5-25 micron and transported particles finally stopped in the swollen regions. After 1 h of treatment with 1 X 10(-5) M colchicine, electron micrographs showed many organelles but no microtubules in the swollen regions. It is thought from these results that the disruption of microtubules in neurites might occur at the ends of the microtubules, followed by swellings in the neurites and particle transport blockade, but the morphological change itself might not have an effect on the transport of particles.

Oscillatory motion of intra-axonal organelles of Xenopus laevis following inhibition of their rapid transport

The motion of optically detected organelles in myelinated axons of Xenopus laevis was studied in axons bathed in a potassium glutamate based medium and in axons in a similar medium to which various inhibitors of axonal transport were added. Organelles in the potassium glutamate medium had a motion which was indistinguishable from that previously described for organelles in axons bathed in a conventional physiological saline. Colchicine, dimethylsulphoxide, 2,4-dinitrophenol, hyperosmotic solutions, raised concentrations of intracellular calcium ions, and media made up in deuterium oxide caused either a complete or partial inhibition of both anterograde and retrograde organelle transport. When directioned transport had been inhibited by any of the agents used, organelles displayed a longitudinally oriented oscillatory motion. An analysis of the variable, or oscillatory, component of organelle motion in axons not treated with inhibitors and in axons in which transport was partially or completely inhibited produced evidence that the dominant frequency components were similar in each case. A maximal estimate of the dominant frequency of oscillatory motion was 0.18 +/- 0.02 Hz. The distribution of instantaneous velocities about the mean velocity was not substantially altered as the transport of organelles was inhibited. The evidence suggested that the variable component of motion of organelles which show directioned transport is related to the oscillatory motion of organelles whose transport is inhibited.

Distribution of peroxidase activity at nodes of Ranvier after intramuscular administration of horseradish peroxidase in the cat

The occurrence of intra-axonal peroxidase activity in large myelinated nerve fibres to the plantaris longus muscle of the cat was investigated histochemically 10-12 h after intramuscular administration of horseradish peroxidase. Internodal horseradish peroxidase activity was low. Its demonstration required electron microscopical analysis of serial sections. In contrast to this, many nodal regions did, even in the light-microscope, disclose a high and characteristically distributed horseradish peroxidase activity. At such nodes the distal part of the constricted axon segment was crowded with horseradish peroxidase-positive bodies of which several were of the so-called multivesicular type. The proximal part of the constricted axon segment was almost free of horseradish peroxidase-positive bodies and contained a densely-packed vesiculotubular membranous system. This striking polarization of the nodal axon region is discussed and tentatively interpreted as the consequence of a physiological damming-up of both somatofugally- and somatopetally-transported materials.

A study of the motion of organelles which undergo retrograde and anterograde rapid axonal transport in Xenopus

1. Axonally transported organelles were detected optically in myelinated axons from Xenopus laevis at room temperature (21-23 degrees C). Details of the motion of organelles which were transported in the retrograde and anterograde directions were studied using filmed records.2. A group of 133 organelles with a mean retrograde velocity of 0.91 mum/sec was compared with a group of thirty-nine organelles with a mean anterograde velocity of 0.93 mum/sec.3. Averaged power spectra of the positional deviations about the mean positional change through time were constructed for organelles which travelled in the retrograde and anterograde directions. Most of the power in the two spectra was at frequencies below 0.2 Hz and each contained a single peak at 0.02-0.04 Hz. The power spectrum for retrograde organelle motion had a magnitude about twice that for anterograde organelle motion.4. Estimates of the instantaneous velocity of organelles which travelled in either direction varied smoothly with time. Instantaneous velocity was not a smooth function of organelle position, (i.e. was ;saltatory').5. Histograms of the estimates for the groups of organelles whose major motion was retrograde or anterograde were broad, covering a range of about 3 mum/sec, were unimodal, and passed through zero to include a small group of values which indicated motion in the opposite (minor) direction.6. Organelles spent, on average, more time moving in the minor direction the lower their mean velocity.7. The variation in instantaneous velocity was greater for organelles which travelled in the retrograde direction than for those which travelled in the anterograde direction. No correlation was found between the variation of instantaneous velocity and the mean velocity of the organelles.8. Images of organelles occasionally appeared to rotate while the organelle continued to move in the major direction of travel.9. Evidence is presented that spatially related properties of the axon influence organelle velocity and that this influence is common to organelles which travel in the two major directions.10. A hypothesis is presented to account for the findings. This supposes that each organelle travels through a stationary axoplasm and is propelled by the resultant of two opposing driving forces whose relative magnitude fluctuates with time. Spatially dependent properties of the axoplasm modify the postulated time-related cycle of motion.

Morphology of axonal transport abnormalities in primate eyes

The ultrastructure of the retina and optic nerve head was studied in primate eyes after central retinal artery occlusion. Within 2 hours of the vascular occlusion the inner retinal layers undergo watery (isosmotic) swelling. This watery swelling of axons and astroglia extends into the nerve head as far back as the anterior boundary of the scleral lamina cribrosa. The swelling is increased 4 hours after the occlusion, and by 24 hours disintegration has occurred. At the optic nerve head mitochondria and vesicles of smooth endoplasmic reticulum begin to accumulate within 2 hours. The accumulation increases at 4 hours and persists to 24 hours. The watery swelling seems characteristic of ischaemic axons. Membranous organelles accumulate at the boundary of an ischaemic zone when material carried by axonal transport is brought via the healthy axon segment to the boundary, but they cannot proceed further into the ischaemic zone. Such accumulation is typical of locations where rapid orthograde axonal transport or retrograde axonal transport is blocked. In contrast, when slow axonal flow is impaired, the swelling is characterised by an excess of cytoplasmic gel without a marked accumulation of organelles. Rapid orthograde transport and retrograde transport seem to be closely related to one another, while slow axoplasmic flow seems fundamentally different. From morphological findings we suspect that, in experimental glaucoma, intraocular pressure first affects the intracellular physiological process of rapid orthograde and retrograde axonal transport. Watery swelling may not occur unless the ischaemic injury to cell metabolism is more advanced. In contrast, in experimental papilloedema, the swelling results predominantly from impaired slow axoplasmic flow.

Axonal transport of the mitochondria-specific lipid, diphosphatidylglycerol, in the rat visual system

Rats 24 d old were injected intraocularly with [2-3H]glycerol and [35S]methionine and killed 1 h-60 d later. 35S label in protein and 3H label in total phospholipid and a mitochondria-specific lipid, diphosphatidylglycerol(DPG), were determined in optic pathway structures (retinas, optic nerves, optic tracts, lateral geniculate bodies, and superior colliculi). Incorporation of label into retinal protein and phospholipid was nearly maximal 1 h postinjection, after which the label appeared in successive optic pathway structures. Based on the time difference between the arrival of label in the optic tract and superior colliculus, it was calculated that protein and phospholipid were transported at a rate of about 400 mm/d, and DPG at about half this rate. Transported labeled phospholipid and DPG, which initially comprised 3-5% of the lipid label, continued to accumulate in the visual structures for 6-8 d postinjection. The distribution of transported material among the optic pathway structures as a function of time differed markedly for different labeled macromolecules. Rapidly transported proteins distributed preferentially to the nerve endings (superior colliculus and lateral geniculate). Total phospholipid quickly established a pattern of comparable labeling of axon (optic nerve and tract) and nerve endings. In contrast, the distribution of transported labeled DPG gradually shifted toward the nerve ending and stabilized by 2-4 d. A model is proposed in which apparent "transport" of mitochondria is actually the result of random bidirectional saltatory movements of individual mitochondria which equilibrate them among cell body, axon, and nerve ending pools.

The movement of membranous organelles in axons. Electron microscopic identification of anterogradely and retrogradely transported organelles

To identify the structures to be rapidly transported through the axons, we developed a new method to permit local cooling of mouse saphenous nerves in situ without exposing them. By this method, both anterograde and retrograde transport were successfully interrupted, while the structural integrity of the nerves was well preserved. Using radioactive tracers, anterogradely transported proteins were shown to accumulate just proximal to the cooled site, and retrogradely transported proteins just distal to the cooled site. Where the anterogradely transported proteins accumulated, the vesiculotubular membranous structures increased in amount inside both myelinated and unmyelinated axons. Such accumulated membranous structures showed a relatively uniform diameter of 50--80 nm, and some of them seemed to be continuous with the axonal smooth endoplasmic reticulum (SER). Thick sections of nerves selectively stained for the axonal membranous structures revealed that the network of the axonal SER was also packed inside axons proximal to the cooled site. In contrast, large membranous bodies of varying sizes accumulated inside axons just distal to the cooled site, where the retrogradely transported proteins accumulated. These bodies were composed mainly of multivesicular bodies and lamellated membranous structures. When horseradish peroxidase was administered in the distal end of the nerve, membranous bodies showing this activity accumulated, together with unstained membranous bodies. Hence, we are led to propose that, besides mitochondria, the membranous components in the axon can be classified into two systems from the viewpoint of axonal transport: "axonal SER and vesiculotubular structures" in the anterograde direction and "large membranous bodies" in the retrograde direction.

Effect of age on axoplasmic transport of cholinesterase in rat sciatic nerves

Senile muscle atrophy has been attributed to an impaired ability of old nerves to transport trophic factors. To evaluate the effect of age on axoplasmic transport, we measured the accumulation of cholinesterase activity above a ligature around sciatic nerves of young (7--8 months), middle-aged (19--20 months), and old (31--32 months) male rats. Protein content and cholinesterase activity per mm of nerve were higher in middle-aged and old than in young nerves. However, accumulation of cholinesterase activity was significantly lower by middle age and was strikingly reduced by old age. This large reduction in axoplasmic transport appeared to result from factors other than axonal loss. A model in which old nerves have an increased number of temporary focal blockages of particle movement in axoplasmic channels is proposed to explain the decreased transport.