Growth from regenerating goldfish retinal cultures in the absence of serum or hormonal supplements: tissue extract effects

Optic nerve glia secrete a low-molecular-weight factor that stimulates retinal ganglion cells to regenerate axons in goldfish

The ability of lower vertebrates to regenerate an injured optic nerve has been widely studied as a model for understanding neural development and plasticity. We have recently shown that, in goldfish, the optic nerve contains two molecules that stimulate retinal ganglion cells to regenerate their axons in culture: a low-molecular-weight factor that is active even at low concentrations (axogenesis factor-1) and a somewhat less active polypeptide of molecular weight 10,000-15,000 (axogenesis factor-2). Both are distinct from other molecules described previously in this system. The present study pursues the biological source and functional significance of axogenesis factor-1. Earlier studies have shown that cultured goldfish glia provide a highly favorable environment for fish or rat retinal ganglion cells to extend axons. We report that the glia in these cultures secrete high levels of a factor that is identical to axogenesis factor-1 in its chromatographic properties and biological activity, along with a larger molecule that may coincide with axogenesis factor-2. Axogenesis factor-1 derived from either goldfish glial cultures or optic nerve fragments is a hydrophilic molecule with an estimated molecular weight of 700-800. Prior studies have reported that goldfish retinal fragments, when explanted in organ culture, only extend axons if the ganglion cells had been "primed" to begin regenerating in vivo for one to two weeks. However, axogenesis factor-1 caused the same degree of outgrowth irrespective of whether ganglion cells had been induced to regenerate new axons in vivo. Moreover, ganglion cells primed to begin regenerating in vivo continued to extend axons in culture only when axogenesis factor-1 was present. In summary, this study shows that glial cells of the goldfish optic nerve secrete a low-molecular-weight factor that initiates axonal regeneration from retinal ganglion cells.

Goldfish retinal axons respond to position-specific properties of tectal cell membranes in vitro

Using a special in vitro assay, we tested whether retinal ganglion cell axons in an adult vertebrate, the goldfish (which can regenerate a retinotopic projection after optic nerve section), recognize position-specific differences in cell surface membranes of their target, the tectum opticum. On a surface consisting of alternating stripes of membranes from rostral and caudal tectum, temporal axons accumulate on membranes derived from their retinotopically related rostral tectal half. Nasal axons grow randomly over both types of membranes. Nasal and temporal axons can elongate on both rostral and caudal membranes. A quantitative growth test, however, revealed that caudal membranes are less permissive substrates for the outgrowth of temporal axons than rostral membranes, and than rostral or caudal membranes for nasal axons.

The interaction of substrate and taurine modulates the outgrowth from regenerating goldfish retinal explants

Retinal growth from explants attached to different substrates was evaluated in the absence and the presence of various concentrations of taurine. Fiber thickness was higher in poly-L-lysine, collagen and laminin than in the other substrates tested. Explants on wheatgerm, soybean, and red kidney bean lectins did not show emission of neurites at 5 days in culture; growth on laminin was very poor at this period of time. By day 10 nerve growth index was possible to evaluate on soybean and laminin. Outgrowth from retinal explants on wheatgerm lectin was only observed in the presence of taurine. The percentage of growing explants on laminin was increased by the amino acid. Emission of neurites from explants on concanavalin A and lentil lectin was constant from 5 to 10 days in culture and was not accelerated or increased by taurine. Growth on poly-L-lysine and collagen was stimulated by the addition of taurine in a concentration-dependent manner. The amino acid stimulates outgrowth from explants attached to peanut lectin in a bell-shaped concentration dependency similar to that observed on poly-L-lysine. The effect of taurine on retinal explants is substrate-dependent, as an indication of the interaction of soluble molecular components plus charge and structure of the extracellular matrix in order to modulate the outgrowth from regenerating tissue.

A neurotrophic factor derived from goldfish brain: characterization and purification

Previous studies carried out in our laboratory have demonstrated that goldfish brain contains substances that promote neurite extension from regenerating retinae in culture. Fractionation of the brain extract by molecular sieving chromatography revealed the presence of several molecular species, including two peaks that have neurotrophic activity, representing low-molecular-weight substances. One peak was eluted (P-a) with an apparent molecular weight of about 13 kDa and was designated substratum neurite extension factor (SNEF) because it retained its neurotrophic activity when adsorbed onto the substratum. This recovered Sephadex fraction (P-a) when applied in vivo intraocularly caused an earlier capacity of the corresponding retinae to sprout in vitro. Thus, at 3 and 5 days after injury the neuritic growth indices from the factor-treated retinae were of 0.9 +/- 0.2 and 2.8 +/- 0.5, respectively, as compared with indices of 0.3 +/- 0.1 and 0.9 +/- 0.2, respectively, in retinae of injured but nontreated nerves. The factor was further purified by two steps of HPLC (ion exchange followed by reversed phase). The results showed that it is an acidic glycoprotein with an apparent molecular weight of 10 kDa.

Selective retinal reinnervation of a surgically created tectal island in goldfish. II. Electron microscopic analysis

In the preceding study (Edwards et al., '85), we showed that regenerating optic axons reestablish a topographically restricted projection to a caudal tectal island created by surgical removal of a 1-mm-wide strip of caudal tectum in goldfish. In the present ultrastructural study, we evaluated the dependence of this axonal outgrowth on the presence of tectal target tissue caudal to the gap. Axon counts in the lesion zone were compared between cases with complete caudal tectal ablation and cases with ablation sparing a caudal tectal island (with and without optic nerve crush). During the postoperative interval of 20-50 days (early period), up to about 1,000 unmyelinated axons with features characteristic of optic axons were present in numerous small subpial bundles in both preparations. In the subsequent interval of 50-110 days (middle period), less than 200 axons were counted caudal to simple half-tecta, whereas 4,000-14,000 myelinated and unmyelinated axons were present in a few large bundles which crossed the lesion zone of tectal island cases. In this period, optic terminals could be demonstrated in the tectal island using the anterograde horseradish peroxidase method. At 170-300 days after surgery (late period), bridging bundles contained between 2,000 and 6,000 largely myelinated axons. We conclude that caudal tectal tissue is not necessary for the initial outgrowth of a small number of axons beyond a rostral half-tectum. The target is essential, however, for the maintenance of these axon fascicles and for the subsequent massive outgrowth of axons to the island. The contributions of glial guidance, diffuse exploratory outgrowth, and target-produced trophic factors to the formation of an initially exuberant projection to the island are discussed. A process of selective axon collateral withdrawal is proposed to account for the decrease in axon numbers within bridging bundles in the late period and for the late restriction in the retinal origin of the island projection indicated by results in the preceding study (Edwards et al., '85).

Neurotrophic stimulation of fetal rat retinal explant neurite outgrowth and cell survival: age-dependent relationships

Serum-free tissue culture conditions have been defined where stimulation of neurite outgrowth from fetal rat retinal explants occurred only in the presence of an active fraction (BE) prepared from a pig brain extract purification procedure. Under these conditions, 18-20-day fetal retinal explants survived and continued to extend long radial neurites for at least 3 weeks in the presence of BE. However, if fibronectin was not equilibrated onto the basic collagen/poly-L-lysine substrate the neurite outgrowth was restricted to a short halo about the circumference of the explant. In addition, a dose-response relationship was demonstrated in the presence of increasing concentrations of BE with respect to the neurite growth index. The half-maximal response for BE was estimated to be between 5 and 10 micrograms/ml. In addition a number of important age-dependent relationships were observed with respect to BE stimulation of retinal neurite outgrowth and cell survival. An inverse relationship was demonstrated between increased developmental age and responsiveness to BE. After 1 week in culture, there was a 3-fold reduction in retinal neurite length measured from the 2-day neonatal explant when compared to that of the 18-day fetus. There was also a significant inverse relationship demonstrated between the length of time before BE was added to the culture medium and the ability of 20-day fetal explants to extend neurites onto the culture substrate. If BE was added as late as 2 weeks after initial explant culture, the various neurite outgrowth indices were significantly lower than in those situations where BE was added at the time of culture or 1 week later. These results imply that BE not only is required for stimulating neurite outgrowth from fetal rat retinal explants, but may be important in survival and maturation of developing retinal neurons. This hypothesis was confirmed when morphometric analysis was performed on 16- and 20-day explants cultured for a week in the presence or absence of BE. The number of necrotic cells in the developing retinal ganglion plexiform-cell layer of 20-day fetal explants was significantly lower when treated with BE. Conversely, the density of identifiable differentiating retinal ganglion-like cells was significantly greater in response to BE treatment in both 16- and 20-day retinal explants.

Target dependent and independent stages in regeneration

Goldfish retinae deprived of their target by tectal ablation showed characteristics of regeneration, such as sprouting activity in culture and changes in protein synthesis. These features resemble changes occurring in regenerating retinae of optic nerves which were subjected only to crush injury. Retinae of tectal ablated nerves maintained their ability to sprout in culture longer than retinae of crushed-injured optic nerves. This may indicate a possible absence of the machinery which regulates growth termination as reflected in this case, by the enduring growth activity in culture. Among the changes in protein synthesis which occurred in retinae following either tectal ablation or crush injury, the most pronounced were in tubulin and in some other polypeptides of the following molecular weights: 46-49, 65 and 74 kdalton. Results are discussed with respect to the possible role of the target in initiating regeneration upon disconnection of the nerve, and in terminating growth upon reconnection.