Subcellular fractionation of rapidly transported axonal material in olfactory nerve: evidence for a size- dependent molecule separation during transport

A quantitative study of anterograde and retrograde axonal transport of exogenous proteins in olfactory nerve C-fibers

The pike olfactory nerve which consists of a homogeneous population of C-fibers of 0.25 micron diameter or less was used to study quantitatively both anterograde and retrograde axoplasmic transport of wheat germ agglutinin and horseradish peroxidase. It was found that even in these extremely thin axons anterograde and retrograde transport takes place. Activity distribution profiles (transport profiles) for retrograde transport were established and found to be similar to the typical profiles of anterograde transport as they consisted of a small rapidly moving peak and a saddle region followed by the bulk of the material which moved more slowly. Horseradish peroxidase activity profiles were obtained both after injection into the synaptic region and after injection into the perikaryal region. From these transport profiles a maximal velocity of 25 mm/d (19 degrees C) for the leading peak and of about 7 mm/d for the slower component could be determined. There is no significant difference between the velocities for anterograde and retrograde transport. In the case of wheat germ agglutinin, only injection into the synaptic region resulted in typical transport profiles (retrograde transport) with a peak and saddle region. The maximum velocities of retrograde transport were about the same as for horseradish peroxidase [26 mm/d and 7 mm/d (19 degrees C)]. The electron microscopic analysis of horseradish peroxidase revealed that after injection into the olfactory bulb it was taken up into the neurons where it was found mainly in multivesicular bodies (0.5 micron diameter). In longitudinal sections of the nerve similar but slightly more elongated organelles (diameter 0.25 micron, length 0.4 micron) were found in those segments in which the slowly moving bulk of the peroxidase activity was located. The number of these organelles decreased with distance from the site of injection. The horseradish peroxidase transported within the leading peak could not be assigned to specific structures although several electron microscopic-histochemical methods were applied. It was concluded that anterograde and retrograde transport occur simultaneously in these axons, and that, therefore, even the large organelles, each of which almost fills the axon, must be able to pass each other. This would necessitate that the axons are able to transiently enlarge their diameter considerably.

Theoretical analysis of radioactivity profiles during fast axonal transport: effects of deposition and turnover

In a preceding study [Blum, J.J., and Reed, M.C. (1985): Cell Motil. 5:507-527], factors responsible for the shape and velocity of the leading edge of the radiolabeled organelle profile were analyzed, but processes that might influence the shape of the plateau-like region behind the advancing wave were ignored. It is now shown that deposition of material from the fast transport system into membrane-associated structures, degradation of such deposited material and its return to the soma by the retrograde transport system, or leakage of radiolabeled material from the axon can account for the shape of the plateau. Furthermore, these processes are compatible with the maintenance of such structural inhomogeneities as the nodes of Ranvier.

Rapid axoplasmic transport of free leucine

Axoplasmic transport of free 3H-leucine has been studied in vivo in the pike olfactory nerve following application of labeled leucine to the olfactory mucosa. A considerable amount of free 3H-leucine is transported at constant velocity along the axon in the form of a distinct peak. The maximum transport velocity for free 3H-leucine is the same as for rapidly transported 3H-protein (130 and 135 mm/day, respectively, at 19 degrees C). Microtubule inhibitors block or significantly reduce the amount of free 3H-leucine transported, but do not influence the transport velocity. Disruption of the oxygen supply abolishes free 3H-leucine transport, so that this phenomenon cannot be explained by diffusion. The amount of free leucine in the rapidly moving peak decreases with time and distance along the axon and is not detectable after 5 h or more. The transported 3H-leucine is not derived from the circulation or from proteolysis of rapidly transported proteins. This study may help to resolve the controversy over the axoplasmic transport of free amino acids since it shows that free 3H-leucine is transported rapidly but does not travel by rapid axoplasmic transport to the end of axons longer than about 30 mm.

Molecular diffusion cannot account for spreading of isotope distribution peaks during rapid axoplasmic transport

Diffusion coefficients calculated from spreading peaks of isotope distributions formed during rapid axoplasmic transport are two orders of magnitude larger than coefficients established for proteins in water. For slow transport, however, the coefficients approximate those of diffusion in water. We propose a near-equilibrium partitioning mass transfer process as the major source of peak spreading during rapid transport and suggest the axon can be modeled by a chromatographic column.

General morphology and axonal ultrastructure of the olfactory nerve of the pike, Esox lucius

The olfactory nerve of the European pike (Esox lucius) contains 5.1 X 10(6) axons with an average diameter of 0.20+/-0,04 micron and a length of 5.5 cm in 1 meter long pike. Each axon contains an average of 4 microtubules as well as neurofilaments, smooth endoplasmic reticulum and about 500 mitochondria per centimeter. The number of neurofilaments ranges from zero in 15% of the cross sections to over 10 in 6%. Neurofilaments generally occur in clusters located opposite to microtubule regions. Smooth ER can not be identified in 14% of the cross sections suggesting that this structure may not be continuous. Microtubules often display annular regions (halos) of low electron density ranging in size from 800 to 1300 A. Halos from adjacent tubules usually merge into regional halos. The ratio of axoplasm to glial cytoplasm is 4.4:1, while the ratio of axonal plasma membrane to glial plasma membrane exceeds 7:1. A 4 cm nerve contains 1280 cm2 of axolemma. This nerve represents an extreme in high density axonal packing and is therefore exceptionally well suited for biochemical, biophysical and physiological investigations.