TUBULAR ARRAYS DERIVED FROM MYELIN BREAKDOWN DURING WALLERIAN DEGENERATION OF PERIPHERAL NERVE

Die in pieces: How Drosophila sheds light on neurite degeneration and clearance

Dendrites and axons are delicate neuronal membrane extensions that undergo degeneration after physical injuries. In neurodegenerative diseases, they often degenerate prior to neuronal death. Understanding the mechanisms of neurite degeneration has been an intense focus of neurobiology research in the last two decades. As a result, many discoveries have been made in the molecular pathways that lead to neurite degeneration and the cell-cell interactions responsible for the subsequent clearance of neuronal debris. Drosophila melanogaster has served as a prime in vivo model system for identifying and characterizing the key molecular players in neurite degeneration, thanks to its genetic tractability and easy access to its nervous system. The knowledge learned in the fly provided targets and fuel for studies in other model systems that have further enhanced our understanding of neurodegeneration. In this review, we will introduce the experimental systems developed in Drosophila to investigate injury-induced neurite degeneration, and then discuss the biological pathways that drive degeneration. We will also cover what is known about the mechanisms of how phagocytes recognize and clear degenerating neurites, and how recent findings in this area enhance our understanding of neurodegenerative disease pathology.

Sculpting neural circuits by axon and dendrite pruning

The assembly of functional neural circuits requires the combined action of progressive and regressive events. Regressive events encompass a variety of inhibitory developmental processes, including axon and dendrite pruning, which facilitate the removal of exuberant neuronal connections. Most axon pruning involves the removal of axons that had already made synaptic connections; thus, axon pruning is tightly associated with synapse elimination. In many instances, these developmental processes are regulated by the interplay between neurons and glial cells that act instructively during neural remodeling. Owing to the importance of axon and dendritic pruning, these remodeling events require precise spatial and temporal control, and this is achieved by a range of distinct molecular mechanisms. Disruption of these mechanisms results in abnormal pruning, which has been linked to brain dysfunction. Therefore, understanding the mechanisms of axon and dendritic pruning will be instrumental in advancing our knowledge of neural disease and mental disorders.

The effect of nanofiber-guided cell alignment on the preferential differentiation of neural stem cells

Stem cells display sensitivity to substrate presentation of topographical cues via changes in cell morphology. These biomechanical responses may be transmitted to the nucleus through cytoskeletal-linked signaling pathways. Here we investigate the influence of aligned substratum topography on the cell morphology and subsequently, the neuronal differentiation capabilities of adult neural stem cells (ANSCs). ANSCs that were cultured on aligned fibers elongated along the major fiber axis. Upon induction of differentiation with retinoic acid, a higher fraction of cells on aligned fibers exhibited markers of neuronal differentiation as compared with cells on random fiber or unpatterned surfaces. This effect was in part due to substrate selectivity, whereby aligned fiber substrates were less receptive to the attachment and continued survival of oligodendrocytes than random fiber or unpatterned substrates. Substrate-induced elongation alone was also effective in upregulating canonical Wnt signaling in ANSCs, which was further potentiated by retinoic acid treatment. These findings suggest a mechanism by which morphological control of stem cells operates in concert with biochemical cues for cell fate determination.

Mechanisms of Disease: what factors limit the success of peripheral nerve regeneration in humans?

Functional recovery after repair of peripheral nerve injury in humans is often suboptimal. Over the past quarter of a century, there have been significant advances in human nerve repair, but most of the developments have been in the optimization of surgical techniques. Despite extensive research, there are no current therapies directed at the molecular mechanisms of nerve regeneration. Multiple interventions have been shown to improve nerve regeneration in small animal models, but have not yet translated into clinical therapies for human nerve injuries. In many rodent models, regeneration occurs over relatively short distances, so the duration of denervation is short. By contrast, in humans, nerves often have to regrow over long distances, and the distal portion of the nerve progressively loses its ability to support regeneration during this process. This can be largely attributed to atrophy of Schwann cells and loss of a Schwann cell basal lamina tube, which results in an extracellular environment that is inhibitory to nerve regeneration. To develop successful molecular therapies for nerve regeneration, we need to generate animal models that can be used to address the following issues: improving the intrinsic ability of neurons to regenerate to increase the speed of axonal outgrowth; preventing loss of basal lamina and chronic denervation changes in the denervated Schwann cells; and overcoming inhibitory cues in the extracellular matrix.

Lysophosphatidylcholine-induced incipient demyelination: involvement of a new tubular structure

Demyelination was induced in the rat sciatic and tibial nerves by microinjection with lysophosphatidylcholine (LPC). Accompanying early myelin lysis (1-24 h) was the formation of vesicles and tubular structures. The tubules which are novel structures have a diameter range of 24-27 nm, a centre-to-centre spacing 30-50 nm and may extend for 3 microns in length. In this form they are arranged as a monolayer in the periaxonal space. As demyelination progressed and the periaxonal space widened the tubules increased in number and became more irregularly arranged. The tubules are apparently derived from the myelin lamellae/Schwann cell plasma membrane, while the axolemma remains intact.

Acute inflammatory demyelinating polyneuropathy in a diabetic patient: predominance of vesicular disruption in myelin sheaths

A diabetic woman underwent an incision of the right big toe for an abscess and developed a typical Guillain-Barré syndrome 48 h later. A biopsy of a peripheral nerve, performed 10 days later, showed modifications usually seen in diabetic patients, as well as the characteristic ultrastructural modifications of the Guillain-Barré syndrome (GBS). Moreover, 22% of myelinated fibers exhibited vesicular disruption of the myelin sheaths. This lesion is rarely encountered on the biopsies of peripheral nerve in GBS and concerns only a few myelinated fibers. Such a prominence of myelinic vesicular disruption and its occurrence in a diabetic patient are discussed.

Polyarteritis nodosa and peripheral neuropathy. Ultrastructural study of 13 cases

Peripheral nerves from 13 patients suffering from polyarteritis nodosa with multiple mononeuropathy were studied by light and electron microscopy. In the majority of cases, the vascular lesions were associated with Wallerian-like degeneration. Myelinated fibers presenting a normal axon with a disproportionately thin myelin sheath were less numerous. Unusual abnormalities consisted of swollen axons with an accumulation of organelles. Unmyelinated fibers were also damaged. A quantitative estimation of myelinated fibers loss did not show any selective vulnerability of either the large or the small diameter group.

Demyelination produced by experimental allergic neuritis serum and anti-galactocerebroside antiserum in CNS cultures. An ultrastructural study

Cultures of mouse cerebellum were exposed to sera from rabbits with experimental allergic neuritis induced by whole peripheral nerve immunization (WN-EAN) and to rabbit anti-galactocerebroside (GC) antisera, and were studied by electron microscopy. Both antisera produced almost identical demyelinative patterns. These consisted of large intramyelinic splittings, "smudged" changes of myelin, degeneration of oligodendrocytes, and phagocytosis of myelin by astrocytes, changes similar to those described after application of whole spinal cord-induced experimental allergic encephalomyelitis (WM-EAE) sera. In addition, patterns which have been considered more characteristic of in vivo demyelinative lesions have been found, susch as vesicular disruption of myelin lamellae and peeling off and phagocytosis of myelin by phagocytic mononuclear cells with electron dense cytoplasm. The morphologic similarities between demyelinative patterns in central nervous system (CNS) cultures induced by anti-GC antiserum and WN-EAN serum and WM-EAE serum, and the fact that elevated antibody titers to GC are found in sera from rabbits with WN-EAN and WM-EAE (Saida, et al., 1977), support the concept that anti-GC antibody is the major factor in the production of CNS demyelination in vitro by sera from rabbits with WN-EAN and WN-EAE.

The fine structure of the cns in multiple sclerosis. II. Vesilcular demyelination in an acute case

Tubular vesicular and net-like dissolution of myelin sheaths associated with complete demyelination and preservation of axons, is described in the brain, obtained within 4 h of death, from a patient who died with acute multiple sclerosis (MS). It was rare, being found in only three out of twenty-three blocks examined, and was seen mainly in the partially demyelinated margin of an active plaque in the white matter. The possibility that post-mortem autolytic changes or fixation artefact might have accounted for the appearances is considered, but is thought unlikely. The findings, though not specific would be expected if current theories invoking myelin-directed 'autoimmune' mechanisms in MS pathogenesis are correct. However, other myelin-directed mechanisms cannot be excluded on the basis of this evidence alone.

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.

Ultrastructural sequence of myelin degradation. II. Wallerian degeneration in the rat femoral nerve

Myelin degradation in Wallerian degeneration of rat femoral nerves has been studied with the electron microscope. In the initial stages, a decrease of myelin periodicity from 115 A to 88 A was noted, followed by the transformation of the myelin structure into uniformly layered lipid inclusions. 10--14 days after nerve section, most of the inclusions found represented unstructured lipid droplets or crystals. In the later stages of degeneration, numerous pleomorphic lamellar inclusions were found, some of them resembling the structure of pi and micron-granules. Lysosomal enzyme activity was found especially in pleomorphic inclusions during the late stages of myelin degradation. Normal myelin sheaths, as well as unstructured lipid droplets and crystals, were devoid of enzyme activity. The results are compared with alterations found in Wallerian degeneration of the central nervous system.

The early stages of Wallerian degeneration in the severed optic nerve of the newt (Triturus viridescens)

The initiation of Wallerian degeneration in the severed optic nerve of the newt (Triturus viridescens) was very rapid and intense. Significant degeneration of nonmyelinated axons was observed as early as six hours after lesion (h.a.l.) and was almost complete by 48 h.a.l. Initial degeneration of non-myelinated axons began in "extracellular digestion chambers" formed between burgeoning ependymoglial processes. The remaining fragments and debris were later phagocytized by surrounding ependymoglial processes. Many axons of myelinated fibers have degenerated as early as 6 h.a.l. However, the overall population of myelinated axons degenerates at a much slower rate than nonmyelinated ones, for many of them appear intact as late as 48 h.a.l. Some myelin sheaths show significant signs of degeneration by 6 h.a.l. Indeed, by this time a number of myelinated fibers have completely degenerated leaving only large vacuolated spaces in the nerve parenchyma. Swelling and vacuolization of the sheath are among the earliest signs of myelin degeneration. The ependymoglial cell response to optic nerve lesion is manyfold and dramatic. By 6 h.a.l. there are signs of burgeoning ependymoglial processes which begin to resemble scar formation (gliosis) by 48 h.a.l. The morphological evidence is consistent with the concept of an important phagocytic role of ependymoglial cells during the early stages of optic nerve degeneration.

Recurrent experimental allergic neuritis. An electron microscope study

Experimental allergic neuritis has been induced in 52 guinea pigs by the inoculation of rabbit peripheral nerve in Freund's adjuvant. The majority of the animals developed an acute monophasic illness after a mean interval of 16 days and, if they survived, recovered fully after an average period of 52 days. Two animals displayed a more chronic course and 1 animal relapsed spontaneously after clinical recovery had occurred. Twenty-three animals that recovered were re-inoculated when recovery was complete and in 4 a relapse was induced. In the remainder, no clinical response was observed, even after repeated reinoculations. The ultrastructural appearances in the animals with a monophasic illness were similar to those previously reported. The appearances differed markedly in those animals that were induced to relapse and were similar to those in the animal that relapsed spontaneously. The findings indicated persistent demyelination and remyelination with striking hypertrophic changes (onion bulb neuropathy). The possible reasons for the differences between the two groups are discussed.

Ultrastructure of the prawn nerve sheaths. Role of fixative and osmotic pressure in vesiculation of thin cytoplasmic laminae

The sheaths from freshly teased nerve fibers of the prawn exhibit a positive radial birefringence, consistent with their EM appearance as highly organized laminated structures composed of numerous thin cytoplasmic sheets or laminae bordered by unit membranes and arranged concentrically around the axon. The closely apposed membranes in these sheaths are fragile and often break down into rows of vesicles during fixation. Desmosome-like attachment zones occur in many regions of the sheath. The membranes within these zones resist vesiculation and thereby provide a "control" region for relating the type of vesicles formed in the fragile portions of the sheaths to the specific fixation conditions. It is proposed that during fixation the production of artifactual vesicles is governed by an interplay of three factors: (a) direct chemical action of the fixative on the polar strata of adjacent unit membranes, (b) osmotic forces applied to membranes during fixation, and (c) the pre-existing natural relations between adjacent membranes. It is found that permanganate best preserves the continuity of the membranes but will still produce vesicles if the fixative exerts severe osmotic forces. These results support other reports (19) of the importance of comparing tissues fixed by complementary procedures so that systematic artifacts will not be described as characteristic of the natural state.

Lysomes in the rat sciatic nerve following crush

Peripheral nerves undergoing degeneration are favorable material for studying the types, origins, and functions of lysosomes. The following lysosomes are described: (a) Autophagic vacuoles in altered Schwann cells. Within these vacuoles the myelin and much of the axoplasm which it encloses in the normal nerve are degraded (Wallerian degeneration). The delimiting membranes of the vacuoles apparently form from myelin lamellae. Considered as possible sources of their acid phosphatase are Golgi vesicles (primary lysosomes), lysosomes of the dense body type, and the endoplasmic reticulum which lies close to the vacuoles. (b) Membranous bodies that accumulate focally in myelinated fibers in a zone extending 2 to 3 mm distal to the crush. These appear to arise from the endoplasmic reticulum in which demonstrable acid phosphatase activity increases markedly within 2 hours after the nerve is crushed. (c) Autophagic vacuoles in the axoplasm of fibers proximal to the crush. The breakdown of organelles within these vacuoles may have significance for the reorganization of the axoplasm preparatory to regeneration. (d) Phagocytic vacuoles of altered Schwann cells. As myelin degeneration begins, some axoplasm is exposed. This is apparently engulfed by the filopodia of the Schwann cells, and degraded within the phagocytic vacuoles thus formed. (e) Multivesicular bodies in the axoplasm of myelinated fibers. These are generally seen near the nodes of Ranvier.