Expression of beta-amyloid precursor and Bcl-2 proto-oncogene proteins in rat retinas after intravitreal injection of aminoadipic acid

Light, lipids and photoreceptor survival: live or let die?

Due to its constant exposure to light and its high oxygen consumption the retina is highly sensitive to oxidative damage, which is a common factor in inducing the death of photoreceptors after light damage or in inherited retinal degenerations. The high content of docosahexaenoic acid (DHA), the major polyunsaturated fatty acid in the retina, has been suggested to contribute to this sensitivity. DHA is crucial for developing and preserving normal visual function. However, further roles of DHA in the retina are still controversial. Current data support that it can tilt the scale either towards degeneration or survival of retinal cells. DHA peroxidation products can be deleterious to the retina and might lead to retinal degeneration. However, DHA has also been shown to act as, or to be the source of, a survival molecule that protects photoreceptors and retinal pigment epithelium cells from oxidative damage. We have established that DHA protects photoreceptors from oxidative stress-induced apoptosis and promotes their differentiation in vitro. DHA activates the retinoid X receptor (RXR) and the ERK/MAPK pathway, thus regulating the expression of anti and pro-apoptotic proteins. It also orchestrates a diversity of signaling pathways, modulating enzymatic pathways that control the sphingolipid metabolism and activate antioxidant defense mechanisms to promote photoreceptor survival and development. A deeper comprehension of DHA signaling pathways and context-dependent behavior is required to understand its dual functions in retinal physiology.

APP involvement in retinogenesis of mice

Very few studies have examined expression and function of amyloid precursor protein (APP) in the retina. We showed that APP mRNA and protein are expressed according to the different waves of retinal differentiation. Depletion of App led to an absence of amacrine cells, a 50% increase in the number of horizontal cells and alteration of the synapses. The retinas of adult APP(-/-) mice showed only half as many glycinergic amacrine cells as wild-type retinas. We identified Ptf1a, which plays a role in controlling both amacrine and horizontal cell fates, as a downstream effector of APP. The observation of a similar phenotype in sorLA knockout mice, a major regulator of APP processing, suggests that regulation of APP functions via sorLA controls the determination of amacrine and horizontal cell fate. These findings provide novel insights that indicate that APP plays an important role in retinal differentiation.

Overexpression of Bcl-2 in vascular endothelium inhibits the microvascular lesions of diabetic retinopathy

Recent studies on the pathogenesis of diabetic retinopathy have focused on correcting adverse biochemical alterations, but there have been fewer efforts to enhance prosurvival pathways. Bcl-2 is the archetypal member of a group of antiapoptotic proteins. In this study, we investigated the ability of overexpressing Bcl-2 in vascular endothelium to protect against early stages of diabetic retinopathy. Transgenic mice overexpressing Bcl-2 regulated by the pre-proendothelin promoter were generated, resulting in increased endothelial Bcl-2. Diabetes was induced with streptozotocin, and mice were sacrificed at 2 months of study to measure superoxide generation, leukostasis, and immunohistochemistry, and at 7 months to assess retinal histopathology. Diabetes of 2 months duration caused a significant decrease in expression of Bcl-2 in retina, upregulation of Bax in whole retina and isolated retinal microvessels, and increased generation of retinal superoxide and leukostasis. Seven months of diabetes caused a significant increase in the number of degenerate (acellular) capillaries in diabetic animals. Furthermore, overexpression of Bcl-2 in the vascular endothelium inhibited the diabetes-induced degeneration of retinal capillaries and aberrant superoxide generation, but had no effect on Bax expression or leukostasis. Therefore, overexpression of Bcl-2 in endothelial cells inhibits the capillary degeneration that is characteristic of the early stages of diabetic retinopathy, and this effect seems likely to involve inhibition of oxidative stress.

Docosahexaenoic acid prevents apoptosis of retina photoreceptors by activating the ERK/MAPK pathway

Identifying the trophic factors for retina photoreceptors and the intracellular pathways activated to promote cell survival is crucial for treating retina neurodegenerative diseases. Docosahexaenoic acid (DHA), the major retinal polyunsaturated fatty acid, prevents photoreceptor apoptosis during early development in vitro, and upon oxidative stress. However, the signaling mechanisms activated by DHA are still unclear. We investigated whether the extracellular signal regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) or the phosphatidylinositol-3-kinase (PI3K) pathway participated in DHA protection. 1,4-Diamino-2,3-dicyano-1,4-bis(2-aminophynyltio) butadiene (U0126), a specific MEK inhibitor, completely blocked the DHA anti-apoptotic effect. DHA rapidly increased ERK phosphorylation in photoreceptors, whereas U0126 blocked this increase. U0126 hindered DHA prevention of mitochondrial depolarization, and blocked the DHA-induced increase in opsin expression. On the contrary, PI3K inhibitors did not diminish the DHA protective effect. DHA promoted the early expression of Bcl-2, decreased Bax expression and reduced caspase-3 activation in photoreceptors. These results suggest that DHA exclusively activates the ERK/MAPK pathway to promote photoreceptor survival during early development in vitro and upon oxidative stress. This leads to the regulation of Bcl-2 and Bax expression, thus preserving mitochondrial membrane potential and inhibiting caspase activation. Hence, DHA, a lipid trophic factor, promotes photoreceptor survival and differentiation by activating the same signaling pathways triggered by peptidic trophic factors.

Light-induced retinal damage involves tyrosine 33 phosphorylation, mitochondrial and nuclear translocation of WW domain-containing oxidoreductase in vivo

WW domain-containing oxidoreductase WOX1, also named WWOX or FOR, is a known proapoptotic protein and a candidate tumor suppressor. Stress stimuli activate WOX1 via tyrosine 33 (Tyr33) phosphorylation and translocation to the mitochondria and nuclei in vitro. Here, the potential role of WOX1 in light-induced retinal degeneration in vivo was investigated. WOX1 is expressed primarily in the inner retina at perinatal stages, whereas an enhanced expression of WOX1, along with its Tyr33 phosphorylation (p-WOX1), is shown specifically in the retinal ganglion cells in adults. Prolonged exposure of mature rats to constant, low-intensity light (500 lux) for 1-2 months resulted in substantial death of photoreceptors and the presence of activated microglia, astrocytes and Muller glial in the outer retina. However, the inner retina was not or barely affected. In the damaged inner and outer nuclear layers of rat retina, WOX1 and p-WOX1 were overly expressed. Also, WOX1 colocalized with fragments of opsin-positive cones. In rd mice with an inherited retinal deficiency, upregulation of WOX1 and p-WOX1 in degenerated retina was observed with age. By electron microscopy, a large number of immunogold particles of WOX1 and p-WOX1 were found in the damaged mitochondria and condensed nuclei of degenerating photoreceptors, indicating that WOX1 undergoes activation and translocation to these organelles. In contrast, little or no WOX1-positive particles were found in the Golgi apparatus. In conclusion, activated WOX1 is likely to exert apoptosis of neuronal cells in the outer retina during the light-induced injury and in mice with an inherited retinal defect.

The retina as a novel in vivo model for studying the role of molecules of the Bcl-2 family in relation to MPTP neurotoxicity

To determine the roles of different members of the family of B cell lymphoma protooncogene (Bcl-2) in relation to neurotoxin-induced neuronal degeneration, the pattern of the expression of a number of molecules of the Bcl-2 family was studied immunocytochemically in the retinas of C57BL/6J mice after intraperitoneal (IP) injection of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Three days to 12 weeks after MPTP treatment, a detectable reduction of tyrosine hydroxylase immunoreactivity in the amacrine cells was observed, with an increase of Bcl-2 expression in the Müller glial cells, and a de novo expression of Bad and Bax in the retinal ganglion cells, optic nerve fibers and plexiform layers. In contrast, a slight decrease of Bcl-x(L) immunoreactivity in the retinal ganglion cells was observed, whereas Bcl-x(S/L) immunoreactivity was increased slightly in the retinas of MPTP-treated mice compared with that of the controls. In animals that received MPTP injection, an increase in immunostaining of GFAP, glutamine synthetase, and Mac-1 (CD11b) in astrocytes, Müller cells, and microglia was invariably observed, indicating an activation or dysfunction of retinal glial cells. These findings are consistent with the current view that glial dysfunction is important in mediating the cytotoxic effect of a variety of neurotoxic molecules, including MPTP, and that different members of Bcl-2 family may have different roles as far as neuronal degeneration or neuroprotection is concerned.

Ergothioneine treatment protects neurons against N-methyl-D-aspartate excitotoxicity in an in vivo rat retinal model

Injection of the glutamate agonist N-methyl-D-aspartate into the vitreous body of the rat eye resulted in a number of morphological changes in the retina. Most apparent was a dramatic reduction in the density and sizes of neurons accompanied by a decrease in amyloid precursor protein and glial fibrillary acidic protein immunoreactivity. Cell counts revealed that 81% of ganglion cells and 43% of non-ganglion cells were lost as a result of the treatment. However, in animals treated with the antioxidant ergothioneine, these figures dropped to 44 and 31%, respectively. Thus, ergothioneine appears to be neuroprotective in this system and the data suggest that antioxidants may provide a useful means of modulating glutamate-based toxicity.

Amyloid-beta peptide is toxic to neurons in vivo via indirect mechanisms

We have studied the neurotoxicity of amyloid-beta (Abeta) after a single unilateral intravitreal injection. Within the retina apoptotic cells were seen throughout the photoreceptor layer and the inner nuclear layer but not in the ganglion cell layer at 48 h after injection of Abeta(1-42) compared to vehicle control and control peptide. At 5 months, there was a significant reduction in total cell numbers in the ganglion cell layer in Nissl stained retinas. There was glial cell dysfunction with upregulation of glial fibrillary acidic protein and a reduction in the expression of Müller cell associated proteins in the injected retinas. These results suggest an indirect cytotoxic effect of Abeta on retinal neurons and an important role for dysfunction of Müller glia in mediating Abeta neurotoxicity.

Differential expression of Bcl-2 and APP immunoreactivity after intrastriatal injection of MPP+ in the rat

While there is growing evidence that Bcl-2 proto-oncogene and beta-amyloid precursor proteins (APP) are neuroprotective in function, our recent studies have demonstrated that Bcl-2 and APP may be co-expressed and co-regulated in retinal neurons or glia under normal or experimental conditions. Whether Bcl-2 and APP are functionally coupled in other neuronal systems is not clear. This issue was investigated further in the present experiments by examining the expression pattern of two molecules after unilateral intrastriatal injection of 1-methyl-4-phenyl-pyridinium (MPP(+)), a neurotoxic metabolite that selectively damages dopaminergic neurons. One hour to 2 months after MPP(+) injection into rat striatum, a significant increase in Bcl-2 expression was observed in distinct populations of neurons, astrocyte-like and OX-42-positive cells not only in traumatic regions but also in remote areas including the ipsilateral cortex and substantia nigra (SN). No detectable change was observed in the striatum, cortex or SN on the contralateral side of the brain. The immunoreactive pattern and time-dependent APP increase was similar to that of Bcl-2 in the severely injured striatum and cortex. However, an up-regulation of Bcl-2 expression, but not APP, appears in dopaminergic neurons in the ipsilateral SN pars compacta where there was retrograde degeneration. In contrast, APP immunoreactivity was decreased in the hippocampus following intrastriatal injury, whereas, no alteration in Bcl-2 expression was detected. The differential changes in Bcl-2 and APP expression in nigral neurons and some other brain tissues suggest that these proteins may not be co-regulated by a common mechanism, at least in certain neuronal pathways.

Bcl-2 expression during the development and degeneration of RCS rat retinae

In various hereditary retinal degenerations, including that in Royal College of Surgeons (RCS) rats, the photoreceptors ultimately die by apoptosis. Bcl-2 is one of the genes, which regulates apoptosis and is thought to promote survival of cells. This study has investigated the developmental expression of Bcl-2 in RCS rat, which is a well-studied animal model for hereditary retinal degeneration. An antibody against Bcl-2 was used for its immunohistochemical localization in dystrophic RCS rat retinae from postnatal (PN) days 4, 7, 13, 35, 45, 70, 202 and 14 months. Results were compared with Bcl-2 localization in congenic non-dystrophic rats from PN 4, 7, 13, 44, 202 and 14 months. Bcl-2 immunoreactivity in non-dystrophic retinae was already present in PN 4 retinae in the nerve fiber layer (presumably in the endfeet of immature Müller cells) and in the proximal parts of certain radially aligned neuroepithelial cells/immature Müller cell radial processes. With increasing age the immunoreactivity in relatively more mature Müller cell radial processes spread distally towards the outer retina and between PN 13 and 44 it reached the adult distribution. No cell bodies in the ganglion cell layer were found to be immunoreactive. Expression of Bcl-2 immunoreactivity in dystrophic RCS rat retinae closely resembled that of non-dystrophic retinae. No immunoreactivity was seen in photoreceptors or retinal pigment epithelium in dystrophic or non-dystrophic retinae. In conclusion, Bcl-2 expression is not altered, either in terms of its chronology or the cell type expressing it, during retinal degeneration in RCS rats.

Alteration of Bcl-2 expression in the nigrostriatal system after kainate injection with or without melatonin co-treatment

In order to understand further the role of the anti-apoptotic Bcl-2 proto-oncogene protein in excitotoxin-induced brain injury and possible interaction between Bcl-2 and the antioxidant melatonin, the expression of Bcl-2 in various brain parts was studied after intrastriatal injection of kainate (KA, 2.5 nmol) with or without co-treatment of melatonin (10 mg/kg, intraperitoneally (i.p.)). Three days after unilateral injection of KA to the striatum in the rat, a dramatic direct cytotoxic effect was observed, as indicated an expression of Bcl-2 immunoreactivity in TUNEL- and OX-42-positive cells in the KA-injected striatum and traumatized cortical region. A less severe detrimental effect was also observed in the ipsilateral substantia nigra and peritraumatic cortex, as reflected by an upregulation of Bcl-2-immunostained neurons. Surprisingly, a reduction in Bcl-2-immunoreactive neurons that was accompanied by a less severe loss of tyrosine hydroxylase-immunoreactive neurons in the nigrostriatal pathway was observed after co-treatment with melatonin. Western blot analysis confirmed that Bcl-2 expression is elevated in striatum and cortex on the lesioned side, and that its expression was attenuated substantially after systemic administration of melatonin. The results showing an upregulation of Bcl-2 in nigral neurons and reactive microglia after KA lesion are consistent with the view that Bcl-2 is protective in function in the central nervous system.

Beta-amyloid (1-42) affects MTT reduction in astrocytes: implications for vesicular trafficking and cell functionality

Beta-amyloid (Abeta) peptide deposition in the brains of Alzheimer's disease patients results in reactive astrogliosis which may enhance neuronal cell death. Abeta has also been reported to impair important supportive astrocyte functions, such as glutamate uptake in vitro. We studied the effect of amyloid beta-peptide (Abeta) on 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction, cellular ATP content, lactate release, and proliferation using neonatal rat astrocyte cultures. Abeta(1-42) decreased MTT reduction potently in the absence of cell death, but did not affect cellular ATP levels or lactate release. Moreover, the cells displayed increased proliferation after incubation with Abeta(1-42), confirming that the decreased MTT reduction was not deleterious to cell viability. Abeta(1-42) enhanced transfer of MTT dye to the cell surface leading to cessation of MTT reduction and cell death. Bafilomycin A1, but not brefeldin A, prevented the enhanced trafficking of MTT, suggesting that lysosomes, but not Golgi apparatus, are involved. Our results show that the viability of astrocytes themselves is not diminished by beta-amyloid peptide. However, Abeta alters vesicular trafficking in astrocytes, which may disturb the supportive function of astrocytes in the brains of patients with Alzheimer's disease.