A study of proteins from ganglion cells of the goldfish retina

Human brain lectin: a soluble lectin that binds actin

A biotinylated probe was used for detection of endogenous ligands of a human brain lectin on blotted human brain soluble proteins. Of the various proteins from brain extract resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, five reacted with the biotinylated probe. After elimination of saccharidic moieties by periodic treatment of the same extract, a single band with Mr approximately 43,000 was recognized by the lectin. This band was identified as actin using an anti-actin antibody. These results were confirmed by binding of biotinylated lectin to purified actin.

Molecular and cellular aspects of nerve regeneration

Injury of an axon leads to at least four independent events, summarized in Figure 1: first, deprivation of the nerve cell body from target-derived or mediated substances, which leads to a derepressed or a permissive state; second, disruption of anterograde transport, with a resultant accumulation of anterogradely transported molecules; third, environmental response with possible consequent changes in constituents of the extracellular matrix and substances secreted from the surrounding cells; and fourth, appearance of growth inhibitors and modified protease activity. It seems that the first three of these events are obligatory, but not sufficient, i.e., they lead to a growth state only if the cell body is able to respond to the injury-induced signals from the environment (a and b). The regenerative state is characterized by alterations in protein synthesis and axonal transport and by sprouting activity. The subsequent elongation of the growing fibers depends on a continuous supply of appropriate growth factors. These factors are presumably anchored to the appropriate extracellular matrix that serves as a substratum for elongating fibers. It should be mentioned that the proliferating nonneuronal cells have a conducive effect on regeneration by forming a scaffold for the growing fibers. Accordingly, the lack of regeneration may stem from a deficiency in the ability of glial cells to provide the appropriate soluble components or from insufficient formation of extracellular matrix. In this respect, one may consider regeneration of an injured axon as a process which involves regeneration of both the nonneuronal cells and the supported axons. The regeneration of glial cells may fulfill the rules which are applied to regeneration of any other proliferating tissue. Furthermore, the processes of regeneration in the axon and the glial cells are mutually dependent. Perhaps the triggering factors provided by the nonneuronal cells affect the nonneuronal cells themselves by modulating their postlesion gliosis and thereby inducing their appropriate activation. In such a case, regeneration of nonneuronal cells may resemble an autocrine type of regulation that exists also during ontogeny. The growth regulation is shifted back to the paracrine type upon neuronal maturation or cessation of axonal growth. When the elongating fibers reach the vicinity of the target organ, they are under the influence of the target-derived factors, which guide the fibers and eventually cease their elongation.

The response of optic tract glia during regeneration of the goldfish visual system. II. Tectal factors stimulate optic tract glia

After transection, retinal ganglion cell axons of the goldfish will regenerate by growing into a primary target tissue, the optic tectum. To determine what role the target tissue may play in regulating glial cell growth, we measured biosynthetic activity of optic tract glia following excision of the optic tectum and compared it to activity of glia found in the regenerating visual system. Ablation of the tectum reduced glial incorporation of both [3H]thymidine and [35S]methionine. Tectal ablation also led to nearly 80% reduction of amino acids incorporated by oligodendroglia as well as a decrease in the amount of newly synthesized protein found within multipotential glia and within cytoplasmic projections of astroglia. Since the tectal influence upon optic tract glia was detected at a time when tract and tectum are physically separated, we sought to determine if the optic tectum contained soluble glia-promoting factors. A soluble fraction recovered from tecta of the regenerating visual system increased amino acid incorporation within optic tract glia at 2-3-fold above preparations incubated with fractions from control, intact tecta. Comparisons of radiolabeled proteins separated by sodium dodecyl polyacrylamide gel electrophoresis from regenerating and factor-stimulated optic tract were similar and indicated that a soluble tectal fraction promoted biosynthesis of specific glial proteins. Our findings suggest that during regeneration of the goldfish visual system glia are influenced by humoral factor(s) released from the synaptic target site.

The response of optic tract glia during regeneration of the goldfish visual system. I. Biosynthetic activity within different glial populations after transection of retinal ganglion cell axons

We monitored biosynthetic activity of optic tract glia during regeneration of retinal ganglion cell axons in the goldfish and found that the greatest level of incorporated [3H]thymidine and [3H]leucine occurred in glia by 10-15 days after axotomy. During this period there was a marked increase in the number of oligodendroglia and multipotential glia near the site of injury with no change occurring in the astroglial population. Electron microscopic autoradiography showed that oligodendroglia and multipotential cells incorporated 5-7-fold more thymidine than did cells of intact control preparations. Though all glial cell types incorporated more [3H]leucine during axonal regeneration, oligodendroglia and multipotential cells together accounted for more than 90% of measured radioactivity. In order to characterize glial-stimulating events specific to axonal regeneration, we produced axonal degeneration in the optic tract by removal of the retina. Optic tract glia during axonal degeneration incorporated less amino acid when compared to glia associated with regenerating axons. The degenerating optic tract also had less 2',3'-cyclic nucleotide 3'-phosphohydrolase, an enzyme produced by oligodendroglia, than that found in the regenerating visual system. Our results suggest that in response to ganglion cell axotomy oligodendroglia and multipotential glia of the goldfish optic tract proliferate. Moreover, regenerating axons provide one type of stimulant for glial protein biosynthesis.

Target regulation of protein biosynthesis in retinal ganglion cells during regeneration of the goldfish visual system

Transected ganglion cells of the goldfish retina demonstrate a marked increase in protein biosynthesis as their axons regrow into a primary target tissue, the optic tectum. In order to examine what role the target tissue may play in regulating the pattern of neuronal protein biosynthesis, the tectum is removed at the time of axotomy. By 10 days after surgery, the production of specific polypeptides including the structural proteins, tubulin and actin, are not affected by tectal ablation. However, during a later phase of regeneration when axons would normally begin to re-connect with the tectum, the appearance of a radiolabeled polypeptide of 300 kDaltons is blocked by tectal ablation. These data suggest that ganglion cells are regulated by target tissues late in the regenerative process perhaps by contact between axons and cells of the tectum.

Retrograde transport of goldfish optic nerve proteins labeled by N-succinimidyl [3H]propionate

Following injection of the acylating reagent N-succinimidyl [3H]propionate into the optic nerve of goldfish, labeled protein appeared in the ipsilateral retina and contralateral tectum in a time-dependent manner. Autoradiography indicated the presence of the labeled material in the neuroplasm of the retinal ganglion cells and their projections. While most of the recoverable injected radioactivity was confined to the injection site even after 1 week, labeled proteins arriving in the retina by retrograde flow or in the tectum by anterograde flow had distinctly different patterns, a result suggesting specific transport processes rather than diffusion. In contrast to reported studies with the labeling agent in other species, a prominent 68,000 molecular weight component was not seen. The results are discussed in relation to the role of retrograde transport in regeneration.