Tomer Sivron

The enigma of myelin-associated growth inhibitors in spontaneously regenerating nervous systems

Recent results shed new light on how some nervous systems can regenerate after injury while others cannot. Until recently, it was widely believed that the main difference between systems that regenerate and those that do not lies in the normal state of their permissiveness to the regenerating axons. Thus, while nonregenerative systems, such as the rat optic nerve, were shown to contain myelin-associated growth inhibitors, regenerative systems, such as the fish optic nerve, were thought to have no such inhibitors. However, it has now been demonstrated that spontaneously regenerating systems do contain growth inhibitors, though their levels seem to be lower than in nonregenerative systems. The main difference, however, appears to reside in the systems response to injury. This article discusses the involvement of myelin-associated growth inhibitors in the spontaneously regenerating nervous system of fish, traces the apparent discrepancy, and shows how it has been resolved recently.

Oligodendrocyte cytotoxic factor associated with fish optic nerve regeneration: implications for mammalian CNS regeneration.

The limited capacity for regenerative axonal growth by adult mammalian central neurons has been attributed, at least in part, to the presence of mature oligodendrocytes, which are non-permissive for axonal growth. These cells do not interfere with growth during development, as developmental growth is largely completed before the maturation of the oligodendrocytes. Unlike mammals, fish central nervous system is endowed with a high regenerative capability. When soluble substances derived from regenerating fish optic nerves are applied to injured adult rabbit optic nerves, regenerative axonal growth is permitted. Therefore, in the present study, we tested whether the fish optic nerve, after injury, is endowed with a mechanism by which it avoids the possible inhibitory effect of the process-bearing mature oligodendrocytes. Specifically, we looked for the possible presence of soluble substances that can regulate the number of process-bearing mature oligodendrocytes. We found that soluble substances derived from regenerating fish optic nerve, when added to cultures of oligodendrocytes derived from newborn or injured adult rat optic nerves, caused a decrease in the number of process-bearing mature oligodendrocytes. Soluble substances derived from normal noninjured fish optic nerves, had a significantly lower effect. The observed decrease in the number of mature oligodendrocytes could not be mimicked by the addition of platelet-derived growth factor (PDGF), a known mitogen of oligodendrocyte progenitors which transiently inhibits their maturation. This study suggests a role to oligodendrocyte inhibitory/cytotoxic factor(s) in regeneration.

Chapter 27 Cytokines and cytokine-related substances regulating glial cell response to injury of the central nervous system

Publisher Summary The ability or inability of nerves to regenerate their injured axons depends on the cellular milieu surrounding the axons and its response to axonal injury. Recent research has shed more light on the nature of these cells and their associated soluble and insoluble substances in the mature nerve in the resting state and after injury. Several studies have helped to elucidate key cellular processes and substances in regeneration, and have demonstrated the involvement of regeneration-related cross-talk between the immune and the nervous systems. These findings are based on two independent lines of research, one involving studies of the fish optic nerve, and the other involving studies of mammalian sciatic nerves. Both systems regenerate readily after injury. These results suggest that macrophages and/or their products (cytokines) at the site of the injury affect local glial cells in a way that benefits regeneration. Such effects presumably include elimination of oligodendrocytes and proper activation of astrocytes. Taken together, the observations that inflammation is beneficial for regeneration and that anti-inflammatory agents promote posttraumatic rescue of fibers from secondary degeneration lead to propose that inflammation has dissimilar effects on axonal rescue and regeneration.

Astrocytes play a major role in the control of neuronal proliferation in vitro

The elements that control neuronal proliferation are largely unknown. Proliferating neurons in cultures of goldfish brain were studied in an attempt to identify the cell types involved. Neuronal proliferation was found to occur only when the neuronal stem cells were in direct contact with astrocytes, and never directly on the substrate. The regulation of neuronal proliferation thus appears to be mediated, at least in part, by contact with astrocytes. In addition, neurite extension was inhibited by medium conditioned by fish astrocytes. Since neurite extension and neuronal proliferation are mutually exclusive processes, inhibition of neurite extension by soluble substances derived from the astrocytes is probably one of the mechanisms controlling neuronal proliferation. The complex reciprocal relationship between neurons and astrocytes is also demonstrated by an observed inhibition of astrocytic proliferation by medium conditioned by differentiating fish neurons. This inhibition of astrocytic proliferation might be part of a mechanism through which interference with neuronal differentiation by astrocytes is avoided. The results of this study thus suggest that astrocytes, in addition to their known roles in controlling neuronal migration, neuronal differentiation and neurite elongation, may also play a role in the control of neuronal proliferation.

Nonpermissive Nature of Fish Optic Nerves to Axonal Growth Is Due to Presence of Myelin-Associated Growth Inhibitors

Fish optic nerve sections were recently shown to be nonpermissive to growth of adult retinal axons. In addition, fish optic nerve myelin was found to inhibit growth of adult retinal axons and this inhibition was neutralized by IN-1 antibodies (known to block rat myelin-associated inhibitors). In this study we examined whether the growth nonpermissiveness of fish optic nerves which had not been injured prior to their excision results, at least in part, from the presence of myelin-associated growth inhibitors. It was found that preincubation of the sections with IN-1 antibodies, known to recognize and neutralize the myelin-associated growth inhibitors of the rat central nervous system, increases sixfold the number of axons that grow on these sections. This demonstrates that fish myelin-associated growth inhibitors, which are similar to those of rat, are at least partly responsible for the growth nonpermissiveness of normal fish optic nerves.

Intermediate filaments reminiscent of immature cells expressed by goldfish (Carassius auratus) astrocytes and oligodendrocytes in vitro

The expression of intermediate filaments is developmentally regulated. In the mammalian embryo keratins are the first to appear, followed by vimentin, while the principal intermediate filament of the adult brain is glial fibrillary acidic protein. The intermediate filaments expressed by a cell thus reflect its state of differentiation. The differentiation state of cells, and especially of glial cells, in turn determines their ability to support axonal growth. In this study we used three new antibodies directed against three fish intermediate filaments (glial fibrillary acidic protein, keratin 8 and vimentin), in order to determine the identity and level of expression of intermediate filaments present in fish glial cells in culture. We found that fish astrocytes and oligodendrocytes are both able to express keratin 8 and vimentin. We further demonstrate that under proliferative conditions astrocytes express high keratin 8 levels and most oligodendrocytes also express keratin 8, whereas under nonproliferative conditions the astrocytes express only low keratin 8 levels and most oligodendrocytes do not express keratin 8 at all. These results suggest that the fish glial cells retain characteristics of immature cells. The findings are also discussed in relation to the fish glial lineage.