A distinct effect of transient and sustained upregulation of cellular factor XIII in the goldfish retina and optic nerve on optic nerve regeneration

Unlike in mammals, fish retinal ganglion cells (RGCs) have a capacity to repair their axons even after optic nerve transection. In our previous study, we isolated a tissue type transglutaminase (TG) from axotomized goldfish retina. The levels of retinal TG (TG(R)) mRNA increased in RGCs 1-6weeks after nerve injury to promote optic nerve regeneration both in vitro and in vivo. In the present study, we screened other types of TG using specific FITC-labeled substrate peptides to elucidate the implications for optic nerve regeneration. This screening showed that the activity of only cellular coagulation factor XIII (cFXIII) was increased in goldfish optic nerves just after nerve injury. We therefore cloned a full-length cDNA clone of FXIII A subunit (FXIII-A) and studied temporal changes of FXIII-A expression in goldfish optic nerve and retina during regeneration. FXIII-A mRNA was initially detected at the crush site of the optic nerve 1h after injury; it was further observed in the optic nerve and achieved sustained long-term expression (1-40days after nerve injury). The cells producing FXIII-A were astrocytes/microglial cells in the optic nerve. By contrast, the expression of FXIII-A mRNA and protein was upregulated in RGCs for a shorter time (3-10days after nerve injury). Overexpression of FXIII-A in RGCs achieved by lipofection induced significant neurite outgrowth from unprimed retina, but not from primed retina with pretreatment of nerve injury. Addition of extracts of optic nerves with injury induced significant neurite outgrowth from primed retina, but not from unprimed retina without pretreatment of nerve injury. The transient increase of cFXIII in RGCs promotes neurite sprouting from injured RGCs, whereas the sustained increase of cFXIII in optic nerves facilitates neurite elongation from regrowing axons.     

Upregulation of IGF-I in the goldfish retinal ganglion cells during the early stage of optic nerve regeneration

Goldfish retinal ganglion cells (RGCs) can regrow their axons after optic nerve injury. However, the reason why goldfish RGCs can regenerate after nerve injury is largely unknown at the molecular level. To investigate regenerative properties of goldfish RGCs, we divided the RGC regeneration process into two components: (1) RGC survival, and (2) axonal elongation processes. To characterize the RGC survival signaling pathway after optic nerve injury, we investigated cell survival/death signals such as Bcl-2 family members in the goldfish retina. Amounts of phospho-Akt (p-Akt) and phospho-Bad (p-Bad) in the goldfish retina rapidly increased four- to five-fold at the protein level by 3-5 days after nerve injury. Subsequently, Bcl-2 levels increased 1.7-fold, accompanied by a slight reduction in caspase-3 activity 10-20 days after injury. Furthermore, level of insulin-like growth factor-I (IGF-I), which activates the phosphatidyl inositol-3-kinase (PI3K)/Akt system, increased 2-3 days earlier than that of p-Akt in the goldfish retina. The cellular localization of these molecular changes was limited to RGCs. IGF-I treatment significantly induced phosphorylation of Akt, and strikingly induced neurite outgrowth in the goldfish retina in vitro. On the contrary, addition of the PI3K inhibitor wortmannin, and IGF-I antibody inhibited Akt phosphorylation and neurite outgrowth in an explant culture. Thus, we demonstrated, for the first time, the signal cascade for early upregulation of IGF-I, leading to RGC survival and axonal regeneration in adult goldfish retinas through PI3K/Akt system after optic nerve injury. The present data strongly indicate that IGF-I is one of the most important molecules for controlling regeneration of RGCs after optic nerve injury.     

Wortmannin Blocks Goldfish Retinal Phosphatidylinositol 3-Kinase and Neurite Outgrowth

The goldfish retina has been used extensively for the study of nerve regeneration. A role for phosphatidylinositol 3-kinase (PI3K) in neurite outgrowth from goldfish retinal explants has been examined by means of wortmannin (WT), a selective inhibitor of the enzyme. The presence of PI3K in retinal extracts was determined by means of immunoprecipitation as well as by an in vitro assay system for catalytic activity. The relative amount of the p85 subunit of PI3K detected by western blot in the retina following optic nerve crush was unchanged. WT inhibited goldfish brain PI3K activity at concentrations as low as 10^–9 M, approximating that reported for inhibition of mammalian PI3K's. Daily addition of 10^–8 M WT to retinal explants, activated by prior crush of the optic nerve, significantly inhibited neurite outgrowth during a 7 day in vitro culture period, while a single addition of WT to freshly explanted retina had no effect on neurite outgrowth. These results suggest that a PI3K-mediated process may be critical for nerve regrowth.     

Nitric oxide-cGMP signaling regulates axonal elongation during optic nerve regeneration in the goldfish in vitro and in vivo

Nitric oxide (NO) signaling results in both neurotoxic and neuroprotective effects in CNS and PNS neurons, respectively, after nerve lesioning. We investigated the role of NO signaling on optic nerve regeneration in the goldfish ( Carassius auratus ). NADPH diaphorase staining revealed that nitric oxide synthase (NOS) activity was up-regulated primarily in the retinal ganglion cells (RGCs) 5–40 days after axotomy. Levels of neuronal NOS (nNOS) mRNA and protein also increased in the RGCs alone during this period. This period (5–40 days) overlapped with the process of axonal elongation during regeneration of the goldfish optic nerve. Therefore, we evaluated the effect of NO signaling molecules upon neurite outgrowth from adult goldfish axotomized RGCs in culture. NO donors and dibutyryl cGMP increased neurite outgrowth dose-dependently. In contrast, a nNOS inhibitor and small interfering RNA, specific for the nNOS gene, suppressed neurite outgrowth from the injured RGCs. Intra-ocular dibutyryl cGMP promoted the axonal regeneration from injured RGCs in vivo . None of these molecules had an effect on cell death/survival in this culture system. This is the first report showing that NO-cGMP signaling pathway through nNOS activation is involved in neuroregeneration in fish CNS neurons after nerve lesioning.     

Neuronal Intermediate Filament Expression During Neurite Outgrowth from Explanted Goldfish Retina: Effect of Retinoic Acid

Regulation of the goldfish neuronal intermediate filament proteins ON1 and ON2 was investigated in a retinal explant system. The synthesis of these proteins in explanted retina decreased with increasing time in culture, despite continuing neurite outgrowth. Thus, ON1/ON2 neurofilament expression is regulated independently from neurite outgrowth. During regeneration of the goldfish optic nerve in vivo, the expression of these proteins increased during the later phase of the process, when growing axons make contact with the optic tectum. The declining synthesis of ON1 and ON2 during neurite outgrowth in culture suggests that factors extrinsic to the retina are necessary to support synthesis of these proteins. Treating retinal explants with retinoic acid stimulated the synthesis of the ON1/ON2 proteins in a dose-dependent manner. This stimulation was effective during a period of declining synthesis of the ON1/ON2 proteins, restoring their synthesis towards initial levels of expression. These results show that retinoic acid serves as a modulator of neurofilament expression in this in vitro model of nerve regeneration.     

Molecular cloning of gefiltin (ON1): serial expression of two new neurofilament mRNAs during optic nerve regeneration.

The goldfish visual pathway displays a remarkable capacity for continued development and plasticity. The intermediate filament proteins of this pathway do not match the intermediate filament protein composition of adult higher vertebrate neurons, which lack the capacity for growth and development. Using a goldfish retina lambda gt10 library we isolated cDNA clones representing the predominant goldfish optic nerve neurofilament protein, ON1. The mRNA for this protein is abundant in retinal ganglion cells, and its level increases slowly during optic nerve regeneration. The rate of ON1 mRNA accumulation after optic nerve crush was compared with that of plasticin, a previously described novel type III neurofilament from goldfish retinal ganglion cells. Plasticin mRNA is normally expressed at low steady state levels, but accumulates dramatically and rapidly, preceding gefiltin mRNA, in response to optic nerve crush. The predicted amino acid sequence for ON1 indicates that it is a novel intermediate filament protein. We have named it gefiltin, for goldfish eye intermediate filament protein. The serial expression of plasticin and gefiltin is discussed with respect to the diversity of neurofilament proteins during neurogenesis.     

Protein Kinase Inhibitors Block Neurite Outgrowth from Explants of Goldfish Retina

A role for protein phosphorylation in the process of neurite outgrowth has been inferred from many studies of the effects of protein kinase inhibitors and activators on cultured neurotumor cells and primary neuronal cells from developing brain or ganglia. Here we re-examine this issue, using a culture system derived from a fully differentiated neuronal system undergoing axonal regeneration—the explanted goldfish retina following optic nerve crush. Of the relatively non-selective protein kinase inhibitors employed, H7, staurosporine and K_252a were found to block neurite outgrowth, whereas HA1004 had no effect, a result which appears to rule out a critical role for protein kinase A. The more selective protein kinase C inhibitors, sphingosine, calphostin C and Ro-31-8220 were all inhibitory, as was prolonged treatment with phorbol ester and the protein phosphatase inhibitor okadaic acid. These results are in support of a role for protein kinase C in axonal regrowth.     

Early downregulation of IGF-I decides the fate of rat retinal ganglion cells after optic nerve injury

Retinal ganglion cells (RGCs) die by apoptosis after optic nerve injury. A number of reports have separately shown changes in pro-apoptotic proteins such as the Bcl-2 family members following optic nerve injury. However, induction time of these apoptotic signals has not been identified due to different treatments of the optic nerve, and insufficient time intervals for measurements. Therefore, the stream of cell death signals is not well understood. In the present study, we systematically reinvestigated a detailed time course of these cell death/survival signals in the rat retina after optic nerve crush, to determine the signal cascade leading to RGC apoptosis. The most conspicuous changes detected in the retina were the rapid inactivation of phospho-Akt and phospho-Bad proteins 2-3 days after optic nerve damage, and the subsequent gradual activation of Bax protein and caspase-3 activity accompanied by cell loss of RGCs 6 days after nerve injury. Cellular localization of these molecular changes was limited to RGCs. Furthermore, amount of insulin-like growth factor-I (IGF-I), an activator of the phosphatidyl inositol-3-kinase (PI3K)/Akt system, was initially decreased from RGCs 1-2 days just prior to the inactivation of phospho-Akt by optic nerve crush. Conversely, supplementation with IGF-I into the rat retina induced upregulation of phospho-Akt expression and cell survival of RGCs both in vitro and in vivo. Thus, injury to the optic nerve might induce early changes in cellular homeostasis with a plausible loss of trophic support for injured RGCs. Actually, IGF-I drastically enhanced neurite outgrowth from adult rat RGCs via a wortmannin-dependent mechanism in a retinal explant culture. Our data strongly indicate that IGF-I is a key molecule that induces RGC apoptosis or RGC survival and regeneration in the retina during the early stage of optic nerve injury.     

Changes in α-Tubulin and Actin Gene Expression During Optic Nerve Regeneration in Frog Retina

The optic nerve of the bullfrog was transected and the regeneration process was investigated. We previously reported that alpha-tubulin mRNA in the retina increased to a maximum 1-2 h after optic nerve transection with no specific change in actin mRNA. In the present investigation, we examined the long-term effect of optic nerve transection. Northern blot analysis revealed that alpha-tubulin mRNA increased again gradually after the rapid and transient increase and actin mRNA increased to a maximum at 7 days (more than twofold compared to the control retinas). The period during which actin mRNA reaches a maximal increase almost corresponds to the time lag between the axotomy and the initiation of axonal outgrowth. The main cytoskeletons of neuronal growth cones have been shown to consist of actin-containing microfilaments. Therefore, the transient increase of actin mRNA may have a relationship to the initial outgrowth of axons. On the other hand, the rapid and transient increase of alpha-tubulin mRNA observed in our previous studies is probably one of the initial responses of retinal ganglion cells to the axotomy, and the gradual increase in alpha-tubulin mRNA observed in this study can probably be interpreted as provision of the structural materials necessary for axonal elongation.     

Myosin light chain kinase: expression in neurons and upregulation during axon regeneration

Myosin light-chain kinase (MLCK) regulates actin-myosin II interactions in nonskeletal muscle cells, and the use of specific pharmacological inhibitors has implicated MLCK in retinal growth cone motility and neurite outgrowth. To further establish the existence and functions of MLCK in neurons, we isolated cDNAs encoding two forms of goldfish MLCK that were differentially expressed in the brain and gut and we sequenced the form most abundantly expressed in the brain (GFMLCK1). In situ hybridization with a cRNA probe specific to GFMLCK1 revealed widespread expression in CNS neurons, including tectal periventricular neurons and cerebellar and medullary neurons. After optic nerve crush, expression was markedly increased in the retinal ganglion cells. Expression peaked during the phase of axonal outgrowth, which, when taken together with our previous pharmacological studies, further supports a role for MLCK in growth cone motility. © 1996 John Wiley & Sons, Inc.     

Taurine trophic modulation of goldfish retinal outgrowth and its interaction with the optic tectum

Summary. Goldfish retinal explant outgrowth in the presence of fetal calf serum is stimulated by taurine. In the absence of it, but with glucose in the medium, length of neurites is still elevated by the amino acid. Using the medium in the presence of glucose, but in the absence of fetal calf serum, we explored the effect of optic tectum medium from cultures of them coming from goldfish without crush of the optic nerve or 3, 5, 10, 14 and 20 days after crush. Retinal explants, intact or from goldfish with crush of the optic nerve 10 days prior to starting the culture, were employed in order to measure the possible effect of optic tectum media and the inter action with taurine. In other type of experiments the optic nerve was crushed 1, 2, 4, 7 and 10 days before dissection of the optic tectum, and then co-cultured with intact or 10 days post-crush retinal explants. Optic tectum media produced a time-dependent effect on outgrowth in lesioned retinas with a maximum effect around 5 days after the lesion for the corresponding optic tectum. Taurine, 4 mM, did not further affect the outgrowth in the presence of optic tectum media, but did significantly increase length of neurites either in intact or in post-lesion retinas. Co-culture of optic tectum at different days post-lesion and retinas at 10 days post-lesion increased the outgrowth around 4 days post-lesion, in a preparation resulting in mutual effects of both types of tissues. The addition of taurine in these conditions did not further increase outgrowth, rather inhibited it according to the time after lesion of optic nerve corresponding to the co-cultured optic tectum. The effect of taurine was concentration-dependent, since 0.2 mM was more effective than 2 or 4 mM in the presence of optic tectum with lesion of 2 days. These results demonstrate the time-course of the regeneration processes in the visual system of goldfish, indicating the crucial periods after crush in which the tectum could produce stimulation and later decrease or no effect on outgrowth from the retina. In addition, they are evidences of the interaction between taurine and optic tectum production of time-produced specific agents. The mechanisms underlying these effects are closely related to calcium, as it was demonstrated by the addition of extracellular or intracellular chelators to the medium, which inhibited the effects of the optic tectum and the trophic properties of taurine in this system. The inhibitor of taurine transport, guanidoethylsulfonate, also decreased the stimulatory effects of the optic tectum and of taurine, indicating an interaction of substances produced by the tectum with taurine, and an effect of taurine mediated through its entrance to the cells. Overall, retinal explants outgrowth in the absence of fetal calf serum, the interaction of agents of the optic tectum and taurine modulates outgrowth from the retina, and these effects are mediated by calcium levels and by the levels of intracellular taurine.     

Neurolin, a cell surface glycoprotein on growing retinal axons in the goldfish visual system, is reexpressed during retinal axonal regeneration.

The mAb E 21 recognizes a cell surface glycoprotein selectively associated with fish retinal ganglion cell axons that are in a state of growth. All retinal axons and ganglion cells in goldfish embryos stained for E 21. In adult fish, however, E 21 immunoreactivity exhibited a patterned distribution in ganglion cells in the marginal growth zone of the continuously enlarging fish retina and the new axons emerging from these cells in the retina, optic nerve, and optic tract. The E 21 antigen was absent from older axons, except the terminal arbor layer in the tectum, the Stratum fibrosum et griseum superficiale where it was uniformly distributed. Upon optic nerve transection, the previously unlabeled axons reacquired E 21 positivity as they regenerated throughout their path to the tectum. Several months after ONS, however, E 21 staining disappeared from the regenerated axons over most of their lengths but reappeared as in normal fish in the terminal arbor layer. The immunoaffinity-purified E 21 antigen, called Neurolin, has an apparent molecular mass of 86 kD and contains the HNK1/L2 carbohydrate moiety, like several members of the class of cell adhesion molecules of the Ig superfamily. The NH2-terminal amino acid sequence has homologies to the cell adhesion molecule DM-Grasp recently described in the chicken. Thus, retinal ganglion cell axons express Neurolin during their development and are able to reexpress this candidate cell adhesion molecule during axonal regeneration, suggesting that Neurolin is functionally important for fish retinal axon growth.     

Uptake of Retrograde Tracers by Intact Optic Nerve Axons: A New Way to Label Retinal Ganglion Cells

Retrograde labelling of retinal ganglion cells with optic nerve transection often leads to degeneration of ganglion cells in prolonged experiments. Here we report that an intact optic nerve could uptake retrograde tracers applied onto the surface of the nerve, leading to high efficiency labelling of ganglion cells in the retina with long-term survival of cells. This method labelled a similar number of ganglion cells (2289±174 at 2 days) as the retrograde labeling technique from the superior colliculus (2250±94) or optic nerve stump (2279±114) after transection. This finding provides an alternative way to label retinal ganglion cells without damaging the optic tract. This will facilitate anatomical studies in identifying the morphology and connectivity of retinal ganglion cells, allowing secondary or triple labelling manipulations for long-term investigations.     

Expression and localization of ionotropic glutamate receptor subunits in the goldfish retina—an in situ hybridization and immunocytochemical study

The expression and distribution of AMPA, kainate and NMDA glutamate receptor subunits was studied in the goldfish retina. For the immunocytochemical localization of the AMPA receptor antisera against GluR2, GluR2/3 and GluR4 were used, and for in situ hybridization rat specific probes for GluR1 and GluR2 and goldfish specific probes for GluR3 and GluR4 were used. The localization of the low affinity kainate receptor and NMDA receptor was studied using antisera against GluR5-7 and NR1. All AMPA receptor subtypes were demonstrated to be present in the goldfish retina both by immunocytochemistry and in situ hybridization. In situ hybridization revealed expression of all AMPA receptors subunit at the inner border of the INL. Only GluR3 was also strongly expressed in the outer border of the INL. Some of the ganglion cells displayed a strong signal for GluR1, GluR3 and GluR4. GluR1-immunoreactivity was present in subsets of bipolar, amacrine, and ganglion cells. GluR2 and GluR2/3-immunoreactivity was mainly localized in the outer plexiform layer. GluR2 and GluR2/3-immunoreactivity are associated with the photoreceptor synaptic terminals. GluR4-immunoreactivity is present on Müller cells in the inner retina and on dendrites of bipolar cells in the OPL, whereas GluR5-7-immunoreactivity was prominently present on horizontal cell axon terminals. Finally, NR1-immunoreactivity was confined to amacrine cells, the inner plexiform layer and ganglion cells. This study shows that there is a strong heterogeneity of glutamate receptor subunit expression in the various layers of the retina. Of the AMPA receptor subunits GluR3 seems to be expressed the most widely in all layers with strong glutamatergic synaptic interactions whereas all the other subunits seem to have a more restricted expressed pattern.     

The Na,K-ATPase alpha4 gene (Atp1a4) encodes a ouabain-resistant alpha subunit and is tightly linked to the alpha2 gene (Atp1a2) on mouse chromosome 1.

We have isolated and characterized cDNA clones encoding the murine homologue of a putative fourth Na,K-ATPase alpha subunit isoform (alpha4). The predicted polypeptide is 1032 amino acids in length and exhibits 75% amino acid sequence identity to the rat alpha1, alpha2, and alpha3 subunits. Within the first extracellular loop, the alpha4 subunit is highly divergent from other Na,K-ATPase alpha subunits. Because this region of Na,K-ATPase is a major determinant of ouabain sensitivity, we tested the ability of the rodent alpha4 subunit to transfer ouabain resistance in a transfection protocol. We find that a cDNA containing the complete rodent alpha4 ORF is capable of conferring low levels of ouabain resistance upon HEK 293 cells, an indication that the alpha4 subunit can substitute for the endogenous ouabain-sensitive alpha subunit of human cells. Nucleotide sequences specific for the murine alpha4 subunit were used to identify the chromosomal position of the alpha4 subunit gene. By hybridizing an alpha4 probe with a series of BACs, we localized the alpha4 subunit gene (Atp1a4) to the distal portion of mouse chromosome 1, in very close proximity to the murine Na,K-ATPase alpha2 subunit gene. In adult mouse tissues, we detected expression of the alpha4 subunit gene almost exclusively in testis, with low levels of expression in epididymis. The close similarities in the organization and expression pattern of the murine and human alpha4 subunit genes suggest that these two genes are orthologous. Together, our studies indicate that the alpha4 subunit represents a functional Na,K-ATPase alpha subunit isoform.