Proteinase activity of peripheral nerves during Wallerian degeneration

Transient, focal accumulation of axonal mitochondria during the early stages of wallerian degeneration

Wallerian degeneration was produced in guinea pig sciatic nerves by a crush injury. At intervals of 2, 12, 24, 36, 48, 72, and 96 hours after the crush, the nerves were fixed in osmium tetroxide, and blocks from the distal, degenerating segment identified topographically prior to embedding in Araldite or Epon. Phase and electron microscopic study of serial cross- and longitudinal sections reveals a striking, localized accumulation of axonal mitochondria which precedes or accompanies the swelling and fragmentation previously reported by others. These focal accumulations of mitochondria are transient and are most frequently observed in the paranodal axoplasm of large myelinated fibers 24 to 36 hours after crush injury, but are also occasionally identified in small myelinated fibers and unmyelinated axons. Migration and proliferation of axonal mitochondria are considered as possible explanations of these observations.

Freeze-fracture analysis of myelin membrane changes in Wallerian degeneration

Transection of mouse sciatic nerves produced microscopic changes in the myelin sheaths distal to the transection. Studied with freeze-fracture, these microscopic changes were correlated with alterations in the macromolecular organization of nerve membranes. In control mice, sciatic nerve myelin contained randomly distributed intramembranous particles. In the early stages of myelin breakdown the lamellae split and large areas of myelin membrane lacked intramembranous particles. The remaining particles clustered with a greater than normal density. Degenerating myelin was found within Schwann cells which still had an outer mesaxon and a normal distribution of intramembranous particles on the cell outer membrane. As the degeneration proceeded, myelin ovoids formed which completely lacked intramembranous particles. The findings suggest that during Wallerian degeneration there is a progression of myelin changes leading to the eventual loss of myelin intramembranous particles. These observations are morphological evidence that Schwann cells remove components from selective portions of their membrane during Wallerian degeneration.

Synthesis of myelin, particulate, and soluble protein subfractions of rat sciatic nerve during the early stage of Wallerian degeneration: a comparison of metabolic studies using double and single isotope methods and recovery

The recovery, electrophoretic composition and synthesis of the myelin, particulate protein and soluble protein subfractions of rat sciatic nerve were compared in normal, sham-operated, and degenerating rat sciatic nerve at one, three and five days after neurotomy. Both single and double isotope methods were used to measure changes in synthesis in vitro and double isotope methods were used in vivo. The wet weights of nerves undergoing Wallerian degeneration for 5 days increased by 40 percent compared to normal and sham-operated nerves. The recovery, specific radioactivity, and synthesis of the myelin was reduced. The effect on myelin protein synthesis was similar in vitro and in vivo. The myelin loss was relatively constant in amount (30-40 microgram) regardless of differences in nerve sizes of young and old rats, consequently the percentage of myelin loss was inversely proportional to nerve size. The recovery of particulate protein increased, its rate of synthesis remained unchanged, and accordingly the specific radioactivity was decreased. The recovery, specific radioactivity, and the rate of synthesis of the soluble protein fraction were all elevated. The protein composition of the three fractions, as analyzed qualitatively by polyacrylamide disc gel electrophoresis, remained essentially unchanged through five days of degeneration. With regard to comparisons of the single and double isotope methods, results shows that the latter are more ideally suited to measuring changes in synthesis during the non-steady state conditions that are characteristics of rapid degeneration.

Lipid metabolism and oxygen consumption in a hereditary demyelinating neuropathy, the trembler mouse: an in vitro study

Sciatic nerves from 15-day-old trembler and control mice were maintained in vitro up to 53 h and the metabolism of myelin lipids and the oxygen consumption were investigated [35S]Sulfate was incorporated into sulfatides at a higher rate and turned over more rapidly in trembler nerves than in controls. [14C]Galactose was incorporated into cerebrosides of trembler nerves at a lower rate and turned over like the controls. In contrast, synthesis of sulfatides labeled with [14]galactose was increased in mutants and no significant turnover was observed for both trembler and control nerves during the whole incubation period. Similar results were obtained using [3H]serine as a precursor and no significant differences were observed in the turnover rates of sphingomyelin and phosphatidylcholine between trembler and control nerves. These data suggest the presence of two different pools of cerebrosides, a small one formed by the fast recycling of sulfatides and which does not mix with a second, larger one. The rate of oxygen consumption did not change significantly during the incubation period and was 2-3-fold higher in trembler nerves than in controls, reflecting, at least partly, the increased sulfatide metabolism.

Pattern of myelin breakdown during sciatic nerve Wallerian degeneration: reversal of the order of assembly

Myelin sheaths of rapidly growing rats were sequentially labeled with the 3H and 14C isotopes of leucine as precursors of protein synthesis. The two injections were separated by time intervals ranging from 2 to 12 d. Wallerian degeneration was initiated by sciatic nerve neurotomy at 2 or 10 d after the second injection of radioactivity. After 5 d of degeneration, myelin was purified and the ratio of isotopes was determined in the delipidated protein. Regardless of the order in which the two isotopes were administered, the relative recovery of radioactivity resultant from the second injection was greatly reduced in degenerating nerves compared with sham-operated controls. Radioactivity incorporated from the first injection was also reduced, but to a lesser extent. Consequently, the isotope ratio corresponding to the first/second injection was greater in degenerating nerves than in controls, and the ratio increased in proportion to the time interval separating the two injections. The magnitude of the effect of degeneration was only slightly greater when degeneration was initiated 2 d after the second injection than when initiated 10 d after the last injection. Consequently, myelin disintegration rather than diminished incorporation of radioactivity accounts for the losses of radioactivity. Furthermore, the pattern of myelin degeneration preferentially involves the last myelin to be formed.

Chronic relapsing experimental allergic encephalomyelitis: morphological sequence of myelin degradation

Myelin degradation in chronic relapsing experimental allergic encephalomyelitis was studied using light microscopic histochemistry and electron microscopy. In the earliest stages, a large number of myelin stripping macrophages were found. The Luxol fast blue positive degradation products were then gradually transformed into PAS positive material. No sudanophilic stage of myelin degradation was found in this model. In electron microscopy, the Luxol fast blue positive material was identified as uniformly layered lipid inclusions with a periodicity of 4.0--4.5 nm. During further digestion, this material was transformed into polymorph structured material, consisting of lamellar leaflets with a diameter of 7--10 nm, curved cylindrical profiles and granular osmiophilic material.

Ultrastructural sequence of myelin degradation. I. Wallerian degeneration in the rat optic nerve

The ultrastructural events in myelin degradation in the rat optic nerve following transection have been studied. Myelin debris was found in cells similar to multipotential glia cells (Vaughn and Peters, 1968) as well as in astrocytes and in few oligodendrocytes. The different types of inclusions found during myelin degradation were described in their quantitative relations. Similarities to inclusions described in adrenoleukodystrophy adn multiple sclerosis are discussed. By comparison of the ultrastructural findings with histochemical and biochemical data available a hypothetical model of myelin degradation is presented. The process starts with the degradation of digestible proteins resulting in uniformly layered lipid inclusions. Lipid degradation leads to the formation of unstructured lipid droplets and crystals. During the late stages of Wallerian degeneration numerous polymorph inclusion typed can be found, probably representing poorly digestible lipids or lipoproteins.

Time course of proteolytic enzyme alterations in the motor end-plates after stimulation

Authors studied the proteolytic enzyme activity of motor end plates in the diaphragms and M. flexor digitorum brevis of rats after administration of cholinesterase inhibitors and after supramaximal electric stimulation, using the colour film digestion technique. Enhanced enzyme activity characterizing stimulated motor end plates persisted up to 12 h after stimulation. No proteolytic activity could be shown any more 24 h after stimulation, indicating restoration of the resting state. The role of metabolites (oligopeptides and amino-acids) released by the function-dependent axoterminal proteolysis is discussed in relation to neurochemical transmission processes.