Inhibition of glutathione synthesis can enhance cycloheximide-induced protection of developing neurons against axotomy

Silva H, P. de Carvalho R, Ventura ALM, Calaza KC, Silveira MS, Kubrusly RCC, de Melo Reis RA. The Healthy and Diseased Retina Seen through Neuron-Glia Interactions

The retina is the sensory tissue responsible for the first stages of visual processing, with a conserved anatomy and functional architecture among vertebrates. To date, retinal eye diseases, such as diabetic retinopathy, age-related macular degeneration, retinitis pigmentosa, glaucoma, and others, affect nearly 170 million people worldwide, resulting in vision loss and blindness. To tackle retinal disorders, the developing retina has been explored as a versatile model to study intercellular signaling, as it presents a broad neurochemical repertoire that has been approached in the last decades in terms of signaling and diseases. Retina, dissociated and arranged as typical cultures, as mixed or neuron- and glia-enriched, and/or organized as neurospheres and/or as organoids, are valuable to understand both neuronal and glial compartments, which have contributed to revealing roles and mechanisms between transmitter systems as well as antioxidants, trophic factors, and extracellular matrix proteins. Overall, contributions in understanding neurogenesis, tissue development, differentiation, connectivity, plasticity, and cell death are widely described. A complete access to the genome of several vertebrates, as well as the recent transcriptome at the single cell level at different stages of development, also anticipates future advances in providing cues to target blinding diseases or retinal dysfunctions.

Quantification of axotomized ganglion cell death by explant culture of the rat retina

We first demonstrated a temporal profile of retinal ganglion cell (RGC) death after axotomy in situ using a newly developed retinal explant culture system. 1,1'- dioctadecyl- 3,3,3',3'-tetramethylindocarbocyanine perchlorate, a fluorescent tracer, was administered to the superior colliculi of 2 day old Wistar rats to label RGCs retrogradely. Small pieces of retinas were dissected and maintained at the interface between a 5% CO(2) atmosphere and culture media, and temporally observed by fluorescent microscopy. The number of surviving RGCs, identified as fluorescent spots, gradually decreased during the course of experiments for up to 10 days in vitro. We identified apoptotic RGCs by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling. Administration of cycloheximide, actinomycin D, or a caspase-3 inhibitor to media significantly decreased RGC death. This system provides a method of quantifying axotomized RGC death in relation to time-dependent changes in an identical retinal slip.

Ultrastructure of retinal ganglion cell death after axotomy in chick embryos

Axotomy often leads to neuronal death, which occurs after a particularly short delay in immature animals. Tectal lesions were made in embryonic day (E) 12 chick embryos, thereby axotomizing the retinal ganglion cells of the contralateral eye, which then died within 3 days. We here describe the ultrastructural changes in the axotomized ganglion cells. The main changes were nuclear invagination and type 3B (cytoplasmic type) cell death characterized by dilation of the perinuclear space, endoplasmic reticulum, and Golgi apparatus. However, nuclear invagination was never seen in type 3B dying cells. All the axotomy-induced retinal ganglion cell death appears to have been of type 3B; apoptosis was not induced by axotomy, as was confirmed by additional light microscopic experiments showing that it did not increase the frequency of apoptotic markers revealed by terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling (the TUNEL method) labeling and immunoreactivity for activated caspase-3. However, the latter methods did show small numbers of apoptotic cells dying naturally even in control retinas. After the death of the axotomized ganglion cells, they were phagocytosed mainly in Müller processes. The present findings open up the chick tectal lesion model as a system for analyzing type 3B neuronal death in vivo.

Involvement of cyclin-dependent kinases in axotomy-induced retinal ganglion cell death

We have tested the role of cyclin-dependent kinases (CDKs) in the type 3B death of axotomized retinal ganglion cells, by injecting intraocularly olomoucine, roscovitine, or butyrolactone I. Each of these inhibits CDK1, CDK2, and CDK5; CDK1 and CDK2 are involved in cell proliferation, whereas CDK5 is involved in neuronal differentiation. The inhibitors partially protected ganglion cells against the effects of axotomy. These agents may affect the ganglion cells directly, because CDK1, its regulatory subunit cyclin B1, and CDK5 were identified immunohistochemically in the perikarya of ganglion cells, and this was confirmed for CDK1 and CDK5 in Western blots of the ganglion cell layer. These blots showed an axotomy-induced phosphorylation of CDK5 occurring remarkably quickly (within 6 hours of axotomy) but little if any change in the phosphorylation state of CDK1. In addition, we studied the expression of proliferation markers, including proliferating cell nuclear antigen (PCNA) and the synthesis of DNA, by immunohistochemical and autoradiographic methods. Normal or axotomized ganglion cells did not express PCNA and did not synthesize DNA. Although we cannot exclude the possibility that axotomized ganglion cells may leave their quiescent state, our data show that they did not progress beyond the G1 phase of the cell cycle. Finally, in contrast to inhibitors of CDKs, cell cycle blockers with different targets than CDKs did not protect ganglion cells. Globally, our results suggest that axotomy-induced death of ganglion cells involves the activation of CDK1, CDK2, or CDK5 (most probably CDK5) but not the full cell cycle machinery.

Dual role of the NF-kappaB transcription factor in the death of immature neurons

We have previously shown that the extent of axotomy-induced death of retinal ganglion cells is reduced by cycloheximide, an inhibitor of protein synthesis, and that an earlier sublethal oxidative insult induced by buthionine sulfoximine, a glutathione synthesis inhibitor, enhances the protective effects of cycloheximide. Thus, axotomy-induced ganglion cell death seems to involve an interaction between the redox status and genetic expression. The redox-sensitive transcription factor nuclear factor-kappaB (NF-kappaB) is a logical candidate for providing this interaction. In the present study, we injected intraocularly selective inhibitors of NF-kappaB in chick embryos either unlesioned, or after a unilateral tectal lesion, which axotomizes ganglion cells. The number of dying cells in the retina contralateral to the lesion was reduced in embryos receiving NF-kappaB inhibitors as compared with vehicle-injected controls. In contrast, the same NF-kappaB inhibitors administered as pretreatment before intraocular injection of buthionine sulfoximine and cycloheximide drastically raised neuronal death and induced fulgurant degenerative changes in the retina. The most parsimonious interpretation of our results is that in axotomized retinal ganglion cells of chick embryos NF-kappaB may have either death-promoting or death-inhibiting effects. We propose a theoretical model to explain these dual effects assuming the existence of parallel death pathways differently affected by NF-kappaB. These results may have implications for future redox-based therapeutic strategies for neuroprotection.

Neuroprotective effects of a new glutathione peroxidase mimetic on neurons of the chick embryo's retina

During their period of naturally occurring neuronal death, retinal ganglion cells are particularly vulnerable to axotomy. The resulting cell death requires protein synthesis and is redox-regulated, since antioxidants protect axotomized-ganglion cells when given in doses that maintain the redox status near an optimal set-point. Here we report the effects of BXT-51072, a new glutathione peroxidase mimetic, on ganglion cell death induced in various ways in the retinas of chick embryos. The intraocular injection of BXT-51072 protected axotomized neurons at doses in a narrow (tenfold) range. It also reduced the deleterious effects of intraocular tert-butyl hydroperoxide, an inducer of lipid peroxidation, and diminished the excitotoxic degeneration induced by N-methyl-D-aspartate. However, BXT-51072 did not noticeably reduce naturally occurring cell death. Globally, our results show that BXT-51072 has numerous protective effects in the retina. In accordance with published data, the present report indicates that glutathione peroxidase mimetics may have potential applications for neurologic or degenerative diseases.

Inhibition of glutathione depletion by retinoic acid and tocopherol protects cultured neurons from staurosporine-induced oxidative stress and apoptosis

Cellular redox status is an important factor during neuronal apoptosis. In primary cultures of chick embryonic neurons, serum deprivation and treatment with staurosporine (200 nM) for 24 h increased the percentage of apoptotic neurons from 13% in controls to 28%, and 68%, respectively. Both exposure to staurosporine and serum deprivation resulted in a four-fold increase in the mitochondrial reactive oxygen species production 4 h after the onset of the injury. Whereas the intracellular glutathione content remained unchanged by serum deprivation, it was markedly reduced by staurosporine suggesting that an increased reactive oxygen species production was more deleterious at a low intracellular glutathione content. Treatment with L-buthionine-(S,R)-sulfoximine, an inhibitor of the glutathione synthesis, decreased the intracellular glutathione content, but did not significantly alter the percentage of apoptotic neurons. Tocopherol (10 microM) and retinoic acid (0.1 microM) inhibited staurosporine-induced glutathione depletion as well as the increase in the percentage of apoptotic neurons. We conclude that under conditions of an increased reactive oxygen species production a high intracellular glutathione content could protect neurons from apoptotic injury and that drugs inhibiting the glutathione depletion could prevent neurons from oxidative damage.

Inhibitors of mitogen-activated protein kinases protect axotomized developing neurons

Axotomy kills developing neurons by mechanisms dependent on protein synthesis and influenced by the redox status. Amongst the redox-regulated transduction systems regulating gene expression are the mitogen-activated protein kinases (MAPKs). In the chick embryo, inhibitors of two different MAPK pathways, including notably the p38 kinase pathway, reduce the number of dying axotomized retinal ganglion cells. The regulation of the genetic events associated to axotomy-induced death thus seems to involve MAPKs.

Synthesis and cytotoxic evaluation of cycloheximide derivatives as potential inhibitors of FKBP12 with neuroregenerative properties

On the basis of the new finding that the protein synthesis inhibitor cycloheximide (1, 4-[2-(3, 5-dimethyl-2-oxocyclohexyl)-2-hydroxyethyl]-2,6-piperidinedione) is able to competitively inhibit hFKBP12 (K(i) = 3.4 microM) and homologous enzymes, a series of derivatives has been synthesized. The effect of the compounds on the activity of hFKBP12 and their cytotoxicity against eukaryotic cell lines (mouse L-929 fibroblasts, K-562 leukemic cells) were determined. As a result, several less toxic or nontoxic cycloheximide derivatives were identified by N-substitution of the glutarimide moiety and exhibit IC(50) values in the range of 22.0-4.4 microM for inhibition of hFKBP12. Among these compounds cycloheximide-N-(ethyl ethanoate) (10, K(i) = 4.1 microM), which exerted FKBP12 inhibition to an extent comparable to that of cycloheximide (1), was found to cause an approximately 1000-fold weaker inhibitory effect on eukaryotic protein synthesis (IC(50) = 115 microM). Cycloheximide-N-(ethyl ethanoate) (10) was able to significantly speed nerve regeneration in a rat sciatic nerve neurotomy model at dosages of 30 mg/kg.

Protection of axotomized ganglion cells by salicylic acid

Neuronal survival is influenced by the redox environment, and it has been shown that antioxidants protect developing neurons from the effects of axotomy. Here, we show that the intraocular injection of salicylic acid (SA) reduces the number of dying axotomized ganglion cells in the chick embryo. The antioxidant properties of SA are probably responsible for its protective effects, whose U-shaped dose-dependency matches that of several other antioxidants. We conclude that SA protects axotomized neurons by maintaining the redox status near an optimal set-point.

An optimal redox status for the survival of axotomized ganglion cells in the developing retina

The neuronal redox status influences the expression of genes involved in neuronal survival. We previously showed that antioxidants may reduce the number of dying ganglion cells following axotomy in chick embryos. In the present study, we show that various antioxidants, including the new spin trap azulenyl nitrone and 1,3-dimethyl-2-thiourea, protect axotomized ganglion cells, confirming that neuronal death involves an imbalance of the cellular redox status towards oxidation. However, high concentrations of antioxidants did not protect ganglion cells, suggesting that excessive reduction is detrimental for neurons. Simultaneous injections of two different antioxidants gave results only partly supporting this view. Combinations of azulenyl nitrone and N-acetyl cysteine in fact gave greater protection than either antioxidant alone, whereas N-acetyl cysteine lost its neuroprotective effects and diminished those of alpha-phenyl-N-tert-butyl nitrone when the two compounds were injected simultaneously. The results of the combined treatments suggest that azulenyl nitrone and alpha-phenyl-N-tert-butyl nitrone do not have the same chemical effects within the ganglion cells. Moreover, N-acetyl cysteine's own antioxidant properties enhance the spin trapping effects of azulenyl nitrone but potentiate the toxicity of alpha-phenyl-N-tert-butyl nitrone. Our main conclusion is that neuronal survival requires the maintenance of the redox status near an optimal set-point. "Reductive stress" may be as dangerous as oxidative stress.

The heavy metal-responsive transcription factor-1 (MTF-1) is not required for neural differentiation

The zinc finger transcription factor MTF-1 is essential for proper response to heavy metal load and other stress conditions in vertebrates, and also contributes to the maintenance of the cellular redox state. Target genes include metallothioneins (MT-I and MT-II) and gamma-glutamylcysteine synthetase (gamma-GCS), an enzyme involved in glutathione biosynthesis. Although MTF-1 is expressed ubiquitously, the primary defect in null mutant mice is hepatocyte necrosis, which results in embryonic lethality around day E14 and prevents the analysis of delayed effects on other organs. To assess the impact of MTF-1 deficiency on the function of the mature central nervous system, we employed the neural grafting strategy. Neuroectodermal brain tissue obtained from transgenic mouse embryos at gestational day 12.5 was transplanted into the caudoputamen of adult wild-type mice. 33 days later, grafts derived from MTF-1 deficient mice consisted of fully differentiated neuroectodermal tissue and showed no differences to heterozygous control grafts. This indicates that MTF-1 is dispensable for the development and differentiation of the nervous system. Such transplants devoid of MTF-1 may provide a useful tool for the further investigation of the effect of cell stress, including oxidative stress.

Neuronal death in the central nervous system during development

About half the neurons in the brain die at the time when their connections are being formed. This neuronal death is regulated by anterograde and retrograde signals that reflect both electrical activity and the uptake of trophic factors. Our recent data on the isthmo-optic projection indicate that there are in fact two different retrograde signals: a slow-acting survival signal mediated by a neurotrophin, and a fast-acting death signal mediated by calcium entry due to electrical activity in the presynaptic terminals. The developmental roles of the cell death are not well understood, but they appear to include the elimination of aberrant connections. The intracellular mechanisms of the cell death may not always correspond to the apoptotic ones so thoroughly investigated in vitro, because only one of the three morphological types occurring regularly in vivo resembles apoptosis. However, our experiments on retinal ganglion cells indicate that several apoptotic mechanisms apply in this particular in vivo situation: these include an involvement of oxygenated free radicals and glutathione, cell cycle-related events, and probably the synthesis of proteins promoting neuroprotection or cell death.

Cooperation between glutathione depletion and protein synthesis inhibition against naturally occurring neuronal death

It is generally agreed that naturally-occurring neuronal death in developing animals is dependent on the synthesis of proteins. Oxidative stress, as when intracellular concentrations of free radicals are raised or when cell constituents such as membrane lipids or protein thiols are oxidized, is also involved in various types of neuronal death. In the present report, we show that the number of naturally dying retinal cells in the chick embryo can be reduced by intraocular injections of cycloheximide, an inhibitor of protein synthesis. L-buthionine-[S,R]-sulfoximine, an inhibitor of glutathione synthesis, can either enhance or diminish the cell death, depending on the conditions of treatment. Moreover, when the two inhibitors are combined, L-buthionine-[S,R]-sulfoximine potentiates the neuroprotective effects of cycloheximide. Measurements of retinal glutathione concentration and protein synthesis show the specificity of the treatments: buthionine-sulfoximine diminishes glutathione concentrations but not protein synthesis whereas cycloheximide inhibits protein synthesis without decreasing glutathione concentrations. Naturally-occurring neuronal death thus seems to involve the synthesis of proteins, and is also influenced by oxidative phenomena. Our results extend previous data in tectal-lesioned embryos, and suggest that a moderate, non-lethal oxidative stress can enhance the resistance of ganglion cells that might otherwise have died (spontaneously or following axotomy) owing to insufficient retrograde trophic support.