Decreased expression of pro-apoptotic Bcl-2 family members during retinal development and differential sensitivity to cell death

Integration of Theory and Practice in Medical Morphology Curriculum in Postgraduate Training: A Flipped Classroom and Case-based Learning Exercise

OBJECTIVE

The integration of training in theory and practice across the medical education spectrum is being encouraged to increase student understanding and skills in the sciences. This study aimed to determine the deciding factors that drive students' perceived advantages in class to improve precision education and the teaching model.

METHODS

A mixed strategy of an existing flipped classroom (FC) and a case-based learning (CBL) model was conducted in a medical morphology curriculum for 575 postgraduate students. The subjective learning evaluation of the individuals (learning time, engagement, study interest and concentration, and professional integration) was collected and analyzed after FC-CBL model learning.

RESULTS

The results from the general evaluation showed promising results of the medical morphology in the FC-CBL model. Students felt more engaged by instructors in person and benefited in terms of time-saving, flexible arrangements, and professional improvement. Our study contributed to the FC-CBL model in Research Design in postgraduate training in 4 categories: 1) advancing a guideline of precision teaching according to individual characteristics; 2) revealing whether a learning background is needed for a Research Design course to guide setting up a preliminary course; 3) understanding the perceived advantages and their interfaces; and 4) barriers and/or improvement to implement the FC-CBL model in the Research Design class, such as a richer description of e-learning and hands-on practice.

CONCLUSION

Undertaking a FC-CBL combined model could be a useful addition to pedagogy for medical morphology learning in postgraduate training.

MicroRNAs in the Mouse Developing Retina

The retina is among the highest organized tissues of the central nervous system. To achieve such organization, a finely tuned regulation of developmental processes is required to form the retinal layers that contain the specialized neurons and supporting glial cells to allow precise phototransduction. MicroRNAs are a class of small RNAs with undoubtful roles in fundamental biological processes, including neurodevelopment of the brain and the retina. This review provides a short overview of the most important findings regarding microRNAs in the regulation of retinal development, from the developmental-dependent rearrangement of the microRNA expression program to the key roles of particular microRNAs in the differentiation and maintenance of retinal cell subtypes.

Nuclear NAD+-biosynthetic enzyme NMNAT1 facilitates development and early survival of retinal neurons

Despite mounting evidence that the mammalian retina is exceptionally reliant on proper NAD+ homeostasis for health and function, the specific roles of subcellular NAD+ pools in retinal development, maintenance, and disease remain obscure. Here, we show that deletion of the nuclear-localized NAD+ synthase nicotinamide mononucleotide adenylyltransferase-1 (NMNAT1) in the developing murine retina causes early and severe degeneration of photoreceptors and select inner retinal neurons via multiple distinct cell death pathways. This severe phenotype is associated with disruptions to retinal central carbon metabolism, purine nucleotide synthesis, and amino acid pathways. Furthermore, transcriptomic and immunostaining approaches reveal dysregulation of a collection of photoreceptor and synapse-specific genes in NMNAT1 knockout retinas prior to detectable morphological or metabolic alterations. Collectively, our study reveals previously unrecognized complexity in NMNAT1-associated retinal degeneration and suggests a yet-undescribed role for NMNAT1 in gene regulation during photoreceptor terminal differentiation.

Rutin hydrate ameliorates cadmium chloride-induced spatial memory loss and neural apoptosis in rats by enhancing levels of acetylcholine, inhibiting JNK and ERK1/2 activation and activating mTOR signalling

This study aimed at studying the potential neuroprotective effect of Rutin hydrate (RH) alone or in conjugation with α-tocopherol against cadmium chloride (CdCl₂)-induced neurotoxicity and cognitive impairment in rats and to investigate the mechanisms of action. Rats intoxicated with CdCl₂ were treated with the vehicle, RH, α-tocopherol or combined treatment were examined, and compared to control rats received vehicle or individual doses of either drug. Data confirmed that RH improves spatial memory function by increasing acetylcholine availability, boosting endogenous antioxidant potential, activating cell survival and inhibiting apoptotic pathways, an effect that is more effective when RH was conjugated with α-tocopherol. Mechanism of RH action includes activation of PP2A mediated inhibiting of ERK1/2 and JNK apoptotic pathways and inhibition of PTEN mediated activation of mTOR survival pathway. In conclusion, RH affords a potent neuroprotection against CdCl₂-induced brain damage and memory dysfunction and co-administration of α-tocopherol enhances its activity.

Bigh3 silencing increases retinoblastoma tumor growth in the murine SV40-TAg-Rb model

BIGH3, a secreted protein of the extracellular matrix interacts with collagen and integrins on the cell surface. BIGH3 can have opposing functions in cancer, acting either as tumor suppressor or promoter by enhancing tumor progression and angiogenesis. In the eye, BIGH3 is expressed in the cornea and the retinal pigment epithelium and could impact on the development of retinoblastoma, the most common paediatric intraocular neoplasm. Retinoblastoma initiation requires the inactivation of both alleles of the RB1 tumor suppressor gene in the developing retina and tumor progression involves additional genomic changes. To determine whether BIGH3 affects retinoblastoma development, we generated a retinoblastoma mouse model with disruption of the Bigh3 genomic locus. Bigh3 silencing in these mice resulted in enhanced tumor development in the retina. A decrease in apoptosis is involved in the initial events of tumorigenesis, followed by an increased activity of the pro-survival ERK pathway as well as an upregulation of cyclin-dependent kinases (CDKs). Taken together, these data suggest that BIGH3 acts as a tumor suppressor in the retina.

BAX to basics: How the BCL2 gene family controls the death of retinal ganglion cells

Retinal ganglion cell (RGC) death is the principal consequence of injury to the optic nerve. For several decades, we have understood that the RGC death process was executed by apoptosis, suggesting that there may be ways to therapeutically intervene in this cell death program and provide a more direct treatment to the cells and tissues affected in diseases like glaucoma. A major part of this endeavor has been to elucidate the molecular biological pathways active in RGCs from the point of axonal injury to the point of irreversible cell death. A major component of this process is the complex interaction of members of the BCL2 gene family. Three distinct family members of proteins orchestrate the most critical junction in the apoptotic program of RGCs, culminating in the activation of pro-apoptotic BAX. Once active, BAX causes irreparable damage to mitochondria, while precipitating downstream events that finish off a dying ganglion cell. This review is divided into two major parts. First, we summarize the extent of knowledge of how BCL2 gene family proteins interact to facilitate the activation and function of BAX. This area of investigation has rapidly changed over the last few years and has yielded a dramatically different mechanistic understanding of how the intrinsic apoptotic program is run in mammalian cells. Second, we provided a comprehensive analysis of nearly two decades of investigation of the role of BAX in the process of RGC death, much of which has provided many important insights into the overall pathophysiology of diseases like glaucoma.

Neuroprotective Effect of Magnesium Acetyltaurate Against NMDA-Induced Excitotoxicity in Rat Retina

Glutamate excitotoxicity plays a major role in the loss of retinal ganglion cells (RGCs) in glaucoma. The toxic effects of glutamate on RGCs are mediated by the overstimulation of N-methyl-D-aspartate (NMDA) receptors. Accordingly, NMDA receptor antagonists have been suggested to inhibit excitotoxicity in RGCs and delay the progression and visual loss in glaucoma patients. The purpose of the present study was to examine the potential neuroprotective effect of Mg acetyltaurate (MgAT) on RGC death induced by NMDA. MgAT was proposed mainly due to the combination of magnesium (Mg) and taurine which may provide neuroprotection by dual mechanisms of action, i.e., inhibition of NMDA receptors and antioxidant effects. Rats were divided into 5 groups and were given intravitreal injections. Group 1 (PBS group) was injected with vehicle; group 2 (NMDA group) was injected with NMDA while groups 3 (pre-), 4 (co-), and 5 (post-) treatments were injected with MgAT, 24 h before, in combination or 24 h after NMDA injection respectively. NMDA and MgAT were injected in PBS at doses 160 and 320 nmol, respectively. Seven days after intravitreal injection, the histological changes in the retina were evaluated using hematoxylin & eosin (H&E) staining. Optic nerves were dissected and stained in Toluidine blue for grading on morphological neurodegenerative changes. The extent of apoptosis in retinal tissue was assessed by TUNEL assay and caspase-3 immunohistochemistry staining. The estimation of neurotrophic factor, oxidative stress, pro/anti-apoptotic factors and caspase-3 activity in retina was done using enzyme-linked immunosorbent assay (ELISA) technique. The retinal morphometry showed reduced thickness of ganglion cell layer (GCL) and reduction in the number of retinal cells in GCL in NMDA group compared to the MgAT-treated groups. TUNEL and caspase-3 staining showed increased number of apoptotic cells in inner retina. The results were further corroborated by the estimation of neurotrophic factor, oxidative stress, pro/anti-apoptotic factors, and caspase-3 activity in retina. In conclusion, current study revealed that intravitreal MgAT prevents retinal and optic nerve damage induced by NMDA. Overall, our data demonstrated that the pretreatment with MgAT was more effective than co- and posttreatment. This protective effect of MgAT against NMDA-induced retinal cell apoptosis could be attributed to the reduction of retinal oxidative stress and activation of BDNF-related neuroprotective mechanisms.

Altered astrocyte morphology and vascular development in dystrophin-Dp71-null mice

Understanding retinal vascular development is crucial because many retinal vascular diseases such as diabetic retinopathy (in adults) or retinopathy of prematurity (in children) are among the leading causes of blindness. Given the localization of the protein Dp71 around the retinal vessels in adult mice and its role in maintaining retinal homeostasis, the aim of this study was to determine if Dp71 was involved in astrocyte and vascular development regulation. An experimental study in mouse retinas was conducted. Using a dual immunolabeling with antibodies to Dp71 and anti-GFAP for astrocytes on retinal sections and isolated astrocytes, it was found that Dp71 was expressed in wild-type (WT) mouse astrocytes from early developmental stages to adult stage. In Dp71-null mice, a reduction in GFAP-immunopositive astrocytes was observed as early as postnatal day 6 (P6) compared with WT mice. Using real-time PCR, it was showed that Dp71 mRNA was stable between P1 and P6, in parallel with post-natal vascular development. Regarding morphology in Dp71-null and WT mice, a significant decrease in overall astrocyte process number in Dp71-null retinas at P6 to adult age was found. Using fluorescence-conjugated isolectin Griffonia simplicifolia on whole mount retinas, subsequent delay of developing vascular network at the same age in Dp71-null mice was found. An evidence that the Dystrophin Dp71, a membrane-associated cytoskeletal protein and one of the smaller Duchenne muscular dystrophy gene products, regulates astrocyte morphology and density and is associated with subsequent normal blood vessel development was provided.

Tgfbi/Bigh3 silencing activates ERK in mouse retina

BIGH3 is a secreted protein, part of the extracellular matrix where it interacts with collagen and integrins on the cell surface. BIGH3 can play opposing roles in cancer, acting as either tumor suppressor or promoter, and its mutations lead to different forms of corneal dystrophy. Although many studies have been carried out, little is known about the physiological role of BIGH3. Using the cre-loxP system, we generated a mouse model with disruption of the Bigh3 genomic locus. Bigh3 silencing did not result in any apparent phenotype modifications, the mice remained viable and fertile. We were able to determine the presence of BIGH3 in the retinal pigment epithelium (RPE). In the absence of BIGH3, a transient decrease in the apoptotic process involved in retina maturation was observed, leading to a transient increase in the INL thickness at P15. This phenomenon was accompanied by an increased activity of the pro-survival ERK pathway.

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.

Zinc oxide nanoparticles inhibit murine photoreceptor-derived cell proliferation and migration via reducing TGF-β and MMP-9 expression in vitro

OBJECTIVES

To investigate behaviour and expression of transforming growth factor-β (TGF-β) and matrix metalloproteinases (MMP-9) in murine photoreceptor-derived cells (661W) after incubation with zinc oxide (ZnO) nanoparticles.

MATERIALS AND METHODS

We explored effects of ZnO nanoparticles on 661W cells using a real-time cell electronic sensing system, flow cytometry, multiple function microplate reading, real-time quantitative PCR detection system and enzyme-linked immunosorbent assay respectively.

RESULTS

Our results indicate that ZnO nanoparticles induced overload of calcium and reactive oxygen species within cells, causing formation of apoptotic bodies, disruption of cell cycle distribution, and reduction in expression of TGF-β and MMP-9, to suppress cell proliferation and migration. Our findings show that disruption of intracellular calcium homoeostasis and overproduction of reactive oxygen species were closely associated with reduction of TGF-β and MMP-9 in 661W cells under ZnO nanoparticle treatment.

CONCLUSIONS

Results of our study indicate that ZnO nanoparticles suppressed cell proliferation and migration, and reduced production of TGF-β and MMP-9 at both gene and protein levels. Our findings contribute to the understanding of the molecular mechanisms that reduced TGF-β and MMP-9 levels inhibit cell proliferation and migration under ZnO nanoparticle influence.

Dynamic molecular monitoring of retina inflammation by in vivo Raman spectroscopy coupled with multivariate analysis

Retinal tissue is damaged during inflammation in Multiple Sclerosis. We assessed molecular changes in inflamed murine retinal cultures by Raman spectroscopy. Partial Least Squares-Discriminant analysis (PLS-DA) was able to classify retina cultures as inflamed with high accuracy. Using Multivariate Curve Resolution (MCR) analysis, we deconvolved 6 molecular components suffering dynamic changes along inflammatory process. Those include the increase of immune mediators (Lipoxygenase, iNOS and TNFα), changes in molecules involved in energy production (Cytochrome C, phenylalanine and NADH/NAD+) and decrease of Phosphatidylcholine. Raman spectroscopy combined with multivariate analysis allows monitoring the evolution of retina inflammation.

Urocortin 2 is associated with abdominal aortic aneurysm and mediates anti-proliferative effects on vascular smooth muscle cells via corticotrophin releasing factor receptor 2

There are no current effective drug therapies for abdominal aortic aneurysm, an important cause of death in older adults. Our study suggests that urocortin 2 participates in the disease process and may serve as a putative therapeutic target.

The Bcl-2 family member BIM has multiple glaucoma-relevant functions in DBA/2J mice

Axonal insult induces retinal ganglion cell (RGC) death through a BAX-dependent process. The pro-apoptotic Bcl-2 family member BIM is known to induce BAX activation. BIM expression increased in RGCs after axonal injury and its induction was dependent on JUN. Partial and complete Bim deficiency delayed RGC death after mechanical optic nerve injury. However, in a mouse model of glaucoma, DBA/2J mice, Bim deficiency did not prevent RGC death in eyes with severe optic nerve degeneration. In a subset of DBA/2J mice, Bim deficiency altered disease progression resulting in less severe nerve damage. Bim deficient mice exhibited altered optic nerve head morphology and significantly lessened intraocular pressure elevation. Thus, a decrease in axonal degeneration in Bim deficient DBA/2J mice may not be caused by a direct role of Bim in RGCs. These data suggest that BIM has multiple roles in glaucoma pathophysiology, potentially affecting susceptibility to glaucoma through several mechanisms.

ER stress in retinal degeneration in S334ter Rho rats

The S334ter rhodopsin (Rho) rat (line 4) bears the rhodopsin gene with an early termination codon at residue 334 that is a model for several such mutations found in human patients with autosomal dominant retinitis pigmentosa (ADRP). The Unfolded Protein Response (UPR) is implicated in the pathophysiology of several retinal disorders including ADRP in P23H Rho rats. The aim of this study was to examine the onset of UPR gene expression in S334ter Rho retinas to determine if UPR is activated in ADRP animal models and to investigate how the activation of UPR molecules leads to the final demise of S334ter Rho photoreceptors. RT-PCR was performed to evaluate the gene expression profiles for the P10, P12, P15, and P21 stages of the development and progression of ADRP in S334ter Rho photoreceptors. We determined that during the P12–P15 period, ER stress-related genes are strongly upregulated in transgenic retinas, resulting in the activation of the UPR that was confirmed using western blot analysis and RT-PCR. The activation of UPR was associated with the increased expression of JNK, Bik, Bim, Bid, Noxa, and Puma genes and cleavage of caspase-12 that together with activated calpains presumably compromise the integrity of the mitochondrial MPTP, leading to the release of pro-apoptotic AIF1 into the cytosol of S334ter Rho photoreceptor cells. Therefore, two major cross-talking pathways, the UPR and mitochondrial MPTP occur in S334ter-4 Rho retina concomitantly and eventually promote the death of the photoreceptor cells.

Preventing retinal apoptosis--is there a common therapeutic theme?

There is an urgent need for therapies for retinal diseases; retinitis pigmentosa sufferers have no treatment options available and those targeted at other retinopathies have shown limited effectiveness. The process of programmed cell death or apoptosis although complex, remains a possible target for the treatment of retinal diseases. Having identified apoptosis in the vertebrate retina in populations of immature neurons as an essential part of development it was proposed that re-activation of these developmental cell death pathways might provide insight into the death mechanisms operating in retinal diseases. However, the discovery that numerous factors initiate and mediate the apoptotic cascade in mature photoreceptors has resulted in a relatively untargeted approach to examining and arresting apoptosis in the retina. In the last 5 years, mouse models have been treated with a diverse range of drugs or factors including anti-oxidants, growth factors, steroid hormones, calcium/calpain inhibitors and tetracycline antibiotics. Therefore to draw a unifying theme from these broad research areas is challenging. However, this review focusses on two targets which are currently under investigation, reactive oxygen species and mammalian target of rapamycin, drawing together the common themes of these research areas.

Early apoptosis of rod photoreceptors in Rpe65(-/-) mice is associated with the upregulated expression of lysosomal-mediated autophagic genes

RPE65-related Leber's congenital amaurosis (LCA) is a rod-cone dystrophy whose clinical outcome is mainly attributed to the loss of rod photoreceptors followed by cone degeneration. Pathogenesis in Rpe65(-/-) mice is characterized by a slow and progressive degeneration of rods dependent on the constitutive activation of unliganded opsin. We previously reported that this opsin-mediated apoptosis of rods was dependent on Bcl-2-apoptotic pathway and Bax-induced pro-death activity. In this study, we report early initial apoptosis in the newly differentiated retina of Rpe65(-/-) mice. Apoptotic photoreceptors were identified as rods and resulted from pathological phototransduction signaling. This wave of early apoptosis triggered Bcl-2-related pathway and Bax apoptotic activity, while activation of the caspases was not induced. Following cellular stress, multiple signaling pathways are initiated which either commit cells to death or trigger pro-survival responses including autophagy. We report that Bcl-2-related early rod apoptosis was associated with the upregulation of autophagy markers including chaperone-mediated autophagy (CMA) substrate receptor LAMP-2 and lysosomal hydrolases Cathepsin S and Lysozyme. This suggests that lysosomal-mediated autophagy may be triggered in response to early rod apoptosis in Rpe65-LCA disease. These results highlight that Rpe65-related primary stress induces early signaling events, which trigger Bax-induced-apoptotic pathway and autophagy-mediated cellular response. These events may determine retinal cell fate, progression and severity of the disease.

BBC3 (PUMA) regulates developmental apoptosis but not axonal injury induced death in the retina

BACKGROUND

Naturally occurring apoptosis is a developmental process that shapes the retina by eliminating overproduced neurons. In the absence of the proapoptotic Bcl-2 family member BAX, developmental apoptosis in the retina is disrupted and extra neurons survive. It is unknown how BAX is activated or if this regulation varies between neuronal types and subtypes. Since the Bcl-2 family members BIM, BID, and BBC3 (PUMA) are powerful direct activators of BAX, we used mice deficient for each of these genes to investigate their importance in developmental apoptosis.

RESULTS

Bax deficient mice have an increase in retinal ganglion cells (RGCs), bipolar cells and dopaminergic amacrine cells, but not photoreceptors, horizontal cells or cholinergic amacrine cells. The retinas of adult Bim and Bid deficient mice appeared to have no increase in any retinal cell type. Bbc3 deficient mice, either homozygous or heterozygous for a null allele of Bbc3, had an increase in the same cell types as Bax deficient mice. An analogous result may occur in the brain where, similar to Bax deficient mice, Bbc3 deficient mice have a larger gross brain weight compared to wild type mice. In contrast to its developmental role, BBC3 did not appear to be a primary factor in BAX-dependent axonal injury induced neurodegeneration in adult RGCs.

CONCLUSION

The regulation of BAX activation in the retina appears to be complex, dependent on the developmental stage of the animal, the nature of the insult and even the type of neuron.

The genomic, biochemical, and cellular responses of the retina in inherited photoreceptor degenerations and prospects for the treatment of these disorders

The association of more than 140 genes with human photoreceptor degenerations, together with studies of animal models of these monogenic diseases, has provided great insight into their pathogenesis. Here we review the responses of the retina to photoreceptor mutations, including mechanisms of photoreceptor death. We discuss the roles of oxidative metabolism, mitochondrial reactive oxygen species, metabolic stress, protein misfolding, and defects in ciliary proteins, as well as the responses of Müller glia, microglia, and the retinal vasculature. Finally, we report on potential pharmacologic and biologic therapies, the critical role of histopathology as a prerequisite to treatment, and the exciting promise of gene therapy in animal models and in phase 1 trials in humans.

Role of the immune modulator programmed cell death-1 during development and apoptosis of mouse retinal ganglion cells

PURPOSE

Mammalian programmed cell death (PD)-1 is a membrane-associated receptor regulating the balance between T-cell activation, tolerance, and immunopathology; however, its role in neurons has not yet been defined. The hypothesis that PD-1 signaling actively promotes retinal ganglion cell (RGC) death within the developing mouse retina was investigated.

METHODS

Mature retinal cell types expressing PD-1 were identified by immunofluorescence staining of vertical retina sections; developmental expression was localized by immunostaining and quantified by Western blot analysis. PD-1 involvement in developmental RGC survival was assessed in vitro using retinal explants and in vivo using PD-1 knockout mice. PD-1 ligand gene expression was detected by RT-PCR.

RESULTS

PD-1 is expressed in most adult RGCs and undergoes dynamic upregulation during the early postnatal window of retinal cell maturation and physiological programmed cell death (PCD). In vitro blockade of PD-1 signaling during this time selectively increases the survival of RGCs. Furthermore, PD-1-deficient mice show a selective increase in RGC number in the neonatal retina at the peak of developmental RGC death. Lastly, gene expression of the immune PD-1 ligand genes Pdcd1lg1 and Pdcd1lg2 was found throughout postnatal retina maturation.

CONCLUSIONS

These findings collectively support a novel role for a PD-1-mediated signaling pathway in developmental PCD during postnatal RGC maturation.

Histone deacetylase activity in conjunction with E2F-1 and p53 regulates Apaf-1 expression in 661W cells and the retina

Apaf-1 and the cysteine proteases known as caspases are genes central to the intrinsic apoptotic pathway in the retina. Previously, we have shown that histone deacetylase (HDAC) activity regulates Apaf-1 expression in the retina. In this study, we unravel the detailed molecular mechanism of HDAC-mediated regulation of Apaf-1 initially by use of a cell line (661W), which expresses some cone-specific genes and then by means of an ex vivo retinal explant system. Inhibition of HDAC activity by trichostatin A (TSA) up-regulates Apaf-1 expression, which precedes the induction of apoptosis. Furthermore, by a bioinformatics approach, we identify E2F-1 and p53 binding sites on the mouse Apaf-1 promoter and show by chromatin immunoprecipitation assays that these sites are occupied in vitro and that treatment with TSA results in increased binding of E2F-1 and p53 to the Apaf-1 promoter. By performing siRNA to these transcription factors, we illustrate that they govern Apaf-1 expression levels in vitro. Finally, in a retinal explant system, we show that similar to our 661W results, E2F-1 and p53 are up-regulated after inhibition of HDAC activity in the retina. This correlates with our previous observation in the explant system that Apaf-1 expression increases significantly and leads to an induction of apoptosis after inhibition of HDAC activity. Overall, we propose a role for HDAC activity, E2F-1, and p53 in the regulation of Apaf-1 expression in 661W cells; initial data also indicate a regulatory role in the retina.

Rosiglitazone acts as a neuroprotectant in retinal cells via up-regulation of sestrin-1 and SOD-2

Rosiglitazone is a member of the thiazolidinedione family of synthetic peroxisome proliferator-activated receptor (PPAR) agonists. It is a selective ligand of the PPARgamma subtype and functions by regulating the transcription of insulin-responsive genes. A screen of FDA-approved compounds identified rosiglitazone as a novel anti-apoptotic agent in retinal cells both in vitro and in vivo, functioning as a neuroprotectant in response to oxidative and calcium stress. We have found that the likely mechanism of action is via increased protein expression of the antioxidant enzymes superoxide dismutase 2 (SOD-2) and sestrin-1, boosting antioxidant defences. Transcription of both genes appears to be mediated by PPARgamma as their up-regulation is reversed by the PPARgamma antagonist GW9662 and proliferator hormone response elements were found in the putative promoter regions of mouse SOD-2 and sestrin-1. However, further investigation revealed that p53 expression was also induced in response to rosiglitazone and chromatin immunoprecipitation assays confirm that it is a bona fide target of PPARgamma. Furthermore, inhibition of p53 partially blocks the observed increase in SOD-2 and sestrin-1 expression indicating that p53 expression is upstream of both antioxidants. We conclude that rosiglitazone may increase cell survival in retinal diseases and potentially other neuronal diseases in which oxidative stress is a key factor.

Pathological apoptosis in the developing brain

More than half of the initially-formed neurons are deleted in certain brain regions during normal development. This process, whereby cells are discretely removed without interfering with the further development of remaining cells, is called programmed cell death (PCD). The term apoptosis is used to describe certain morphological manifestations of PCD. Many of the effectors of this developmental cell death program are highly expressed in the developing brain, making it more susceptible to accidental activation of the death machinery, e.g. following hypoxia-ischemia or irradiation. Recent evidence suggests, however, that activation and regulation of cell death mechanisms under pathological conditions do not exactly mirror physiological, developmentally regulated PCD. It may be argued that the conditions after e.g. ischemia are not even compatible with the execution of PCD as we know it. Under pathological conditions cells are exposed to various stressors, including energy failure, oxidative stress and unbalanced ion fluxes. This results in parallel triggering and potential overshooting of several different cell death pathways, which then interact with one another and result in complex patterns of biochemical manifestations and cellular morphological features. These types of cell death are here called "pathological apoptosis," where classical hallmarks of PCD, like pyknosis, nuclear condensation and caspase-3 activation, are combined with non-PCD features of cell death. Here we review our current knowledge of the mechanisms involved, with special focus on the potential for therapeutic intervention tailored to the needs of the developing brain.

Developmental silencing and independency from E2F of apoptotic gene expression in postmitotic tissues

The involvement of caspases in postmitotic cell death is controversial. Here we report that adult brain and heart are devoid of many key pro-apoptotic proteins due to a progressive postnatal silencing event involving a reduction of their transcript levels. E2F has been shown to control cell cycle progression and to be transcriptional activator of apoptotic genes. However, our data demonstrate that apoptotic gene expression in heart, brain and liver, as well as cardiac and neuronal apoptotic gene silencing during development, are E2F-independent events. Therefore, the genes regulating caspase-dependent cell death are expressed in embryonic organs in an E2F-independent manner and a developmental-related silencing event represses these genes in postmitotic adult tissues.

Bim expression indicates the pathway to retinal cell death in development and degeneration

Programmed cell death (PCD) during development of the mouse retina involves activation of the mitochondrial pathway. Previous work has shown that the multidomain Bcl-2 family proteins Bax and Bak are fundamentally involved in this process. To induce mitochondrial membrane permeabilization, Bax and Bak require that prosurvival members of the family be inactivated by binding of "BH3-only" members. We showed previously that the BH3-only protein BimEL is highly expressed during postnatal retinal development but decreases dramatically thereafter. The purpose of this study was to investigate a possible role for Bim, in retinal development and degeneration, upstream of Bax and Bak. Bim-/- mice analyzed for defective retinal development exhibit an increase in retinal thickness and a delay in PCD, thereby confirming a role for Bim. We also demonstrate that in response to certain death stimuli, bim+/+ retinal explants upregulate BimEL leading to caspase activation and cell death, whereas bim-/- explants are resistant to apoptosis. Finally, we analyzed Bim expression in the retinal degeneration (rd) mouse, an in vivo model of retinal degeneration. Bim isoforms, which decrease during development, are not reexpressed during retinal degeneration and ultimately photoreceptor cells die by a caspase-independent mechanism. Thus, we conclude that in cases in which BimEL is reexpressed during pathological cell death, developmental cell death pathways are reactivated. However, the absence of BimEL expression correlates with caspase-independent death in the rd model.

A Critical role for Bim in retinal ganglion cell death

Optic nerve transection results in the death of retinal ganglion cells (RGCs) by apoptosis. Apoptosis is regulated by the Bcl-2 family of proteins, of which the Bcl-2 homology (BH3) -only proteins forms a subset. As BH3-only proteins have been shown to play a significant role in regulating cell death in the central nervous system, we wished to investigate the role of Bcl-2 interacting mediator of cell death (Bim), a prominent member of this protein family in the regulation of cell death in the RGC layer using in vitro retinal explants. In this study, we use an innovative retinal shaving procedure to isolate the cells of the ganglion cell layer to use for western blotting. Members of the BH3-only protein family are down-regulated during retinal development and are not normally expressed in the adult retina. Using this procedure, we demonstrate that Bim is re-expressed and its expression is increased over time following axotomy. Expression of Bad and Bik decreases over the same time course, whereas there is no indication that Bid and Puma are re-expressed. We show that explants from Bim knockout mice are resistant to axotomy-induced death when compared with their wild-type counterparts. Genetic deletion of Bim also prevents caspase 3 cleavage. The activity of Bim can be negatively regulated by phosphorylation. We show that the decrease of Bim phosphorylation correlates with a decrease in expression of survival kinases such as pAkt and pERK over the same time course. These results implicate Bim re-expression as being essential for axotomy-induced death of RGCs and that phosphorylation of Bim negatively regulates its activity in RGCs.

Induction of BIM(EL) following growth factor withdrawal is a key event in caspase-dependent apoptosis of 661W photoreceptor cells

Apoptosis of photoreceptor cells in the early postnatal period is a normal feature of mammalian retinal development. The role of mitochondria and caspases in the process has been well established; however, the identification of key apoptotic mediators still remains elusive. Here we report that BIM(EL), a pro-apoptotic BCL-2 family member, may be one such molecule. Following growth factor deprivation, BIM(EL) was up-regulated in mouse 661W cone photoreceptors. This event correlated with the release of mitochondrial apoptogenic factors into the cytosol, the activation of caspases and apoptosis. Moreover, a similar behaviour was observed in response to UV radiation, ionomycin or H(2)O(2) treatments. We identified the PI3K-Akt-FKHRL1 signalling cascade as the main regulatory pathway of BIM(EL) expression in these cells. Finally, using RNA interference, we were able to silence BIM(EL) expression and subsequently suppress caspase-3 activation. In conclusion, we propose BIM(EL) as a critical factor in mitochondria-dependent apoptosis of 661W photoreceptors.

Growth hormone and cell survival in the neural retina: caspase dependence and independence

Growth hormone has recently been shown to be expressed in the retinal ganglion cells of embryonic chicks, in which it induces cell survival during neurogenesis. The mechanism of this action has been examined in neural retina explants from 6-day-old and 8-day-old embryos that were incubated for 48 h in 10 M growth hormone, to reduce the number of spontaneous apoptotic cells. This anti-apoptotic action was accompanied by a reduction in caspase-3 expression and, at embryonic day 8, by reduced expression of apoptosis inducing factor-1, which is caspase independent. These actions were specific, as other genes involved in apoptotic signaling (bcl-2, bcl-x, bid and inhibitor of apoptosis protein-1) were unaffected. These results therefore demonstrate caspase-dependent and caspase-independent pathways in growth hormone-induced retinal cell survival.

Analysis of apoptotic and survival mediators in the early post-natal and mature retina

Apoptosis, a cellular process critical to retinal neurogenesis, has been implicated in several neurodegenerative disorders. As the retina matures the suppression of apoptosis occurs and the emphasis shifts towards survival. To identify the cellular changes that bring about this critical shift in the balance, we performed an expression analysis of pro- and anti-apoptotic mediators in the immature, post-natal day 6 (P6) and the post-mitotic adult P60 mouse retina. Laser capture microdissection (LCM) of the P6 and the P60 retina, followed by reverse transcriptase-polymerase chain reaction (RT-PCR) was employed to elucidate changes in the mRNA expression of Apaf-1, caspase-3 and caspase-9 in the individual retinal layers in the young and mature tissue. RT-PCR and Western blotting of whole P6 and P60 retinal preparations was carried out to determine changes in other caspases and key survival mediators at the mRNA and protein level, respectively. Our results demonstrate that each neuronal cell layer in the adult retina down-regulates the gene expression of Apaf-1 and caspase-3, and to a lesser extent, caspase-9. The protein expression levels of other executioner and initiator caspases are also reduced in the adult tissue. Interestingly, XIAP, a potent caspase inhibitor, increases in expression in the adult retina. Additionally, we demonstrate age-dependent increased expression and activation status of the components of the MAPK transduction cascade. Conversely, we observe decreased PI3-K and AKT expression and decreased activity of AKT (pAKT) in the adult retina. Furthermore, results from RNAi experiments demonstrate an additional mechanism of PI3-K regulation in photoreceptor cells. Our findings suggest that a survival strategy adopted by the post-mitotic retina involves a down-regulation of key pro-apoptotic factors concomitant with changes in expression and activation status of certain pro-survival mediators.

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.