Long-term survival of retinal ganglion cells following optic nerve section in adult bcl-2 transgenic mice

BAX activation in mouse retinal ganglion cells occurs in two temporally and mechanistically distinct steps

BACKGROUND

Pro-apoptotic BAX is a central mediator of retinal ganglion cell (RGC) death after optic nerve damage. BAX activation occurs in two stages including translocation of latent BAX to the mitochondrial outer membrane (MOM) and then permeabilization of the MOM to facilitate the release of apoptotic signaling molecules. As a critical component of RGC death, BAX is an attractive target for neuroprotective therapies and an understanding of the kinetics of BAX activation and the mechanisms controlling the two stages of this process in RGCs is potentially valuable in informing the development of a neuroprotective strategy.

METHODS

The kinetics of BAX translocation were assessed by both static and live-cell imaging of a GFP-BAX fusion protein introduced into RGCs using AAV2-mediated gene transfer in mice. Activation of BAX was achieved using an acute optic nerve crush (ONC) protocol. Live-cell imaging of GFP-BAX was achieved using explants of mouse retina harvested 7 days after ONC. Kinetics of translocation in RGCs were compared to GFP-BAX translocation in 661W tissue culture cells. Permeabilization of GFP-BAX was assessed by staining with the 6A7 monoclonal antibody, which recognizes a conformational change in this protein after MOM insertion. Assessment of individual kinases associated with both stages of activation was made using small molecule inhibitors injected into the vitreous either independently or in concert with ONC surgery. The contribution of the Dual Leucine Zipper-JUN-N-Terminal Kinase cascade was evaluated using mice with a double conditional knock-out of both Mkk4 and Mkk7.

RESULTS

ONC induces the translocation of GFP-BAX in RGCs at a slower rate and with less intracellular synchronicity than 661W cells, but exhibits less variability among mitochondrial foci within a single cell. GFP-BAX was also found to translocate in all compartments of an RGC including the dendritic arbor and axon. Approximately 6% of translocating RGCs exhibited retrotranslocation of BAX immediately following translocation. Unlike tissue culture cells, which exhibit simultaneous translocation and permeabilization, RGCs exhibited a significant delay between these two stages, similar to detached cells undergoing anoikis. Translocation, with minimal permeabilization could be induced in a subset of RGCs using an inhibitor of Focal Adhesion Kinase (PF573228). Permeabilization after ONC, in a majority of RGCs, could be inhibited with a broad spectrum kinase inhibitor (sunitinib) or a selective inhibitor for p38/MAPK14 (SB203580). Intervention of DLK-JNK axis signaling abrogated GFP-BAX translocation after ONC.

CONCLUSIONS

A comparison between BAX activation kinetics in tissue culture cells and in cells of a complex tissue environment shows distinct differences indicating that caution should be used when translating findings from one condition to the other. RGCs exhibit both a delay between translocation and permeabilization and the ability for translocated BAX to be retrotranslocated, suggesting several stages at which intervention of the activation process could be exploited in the design of a therapeutic strategy.

BAX activation in mouse retinal ganglion cells occurs in two temporally and mechanistically distinct steps

Background Pro-apoptotic BAX is a central mediator of retinal ganglion cell (RGC) death after optic nerve damage. BAX activation occurs in two stages including translocation of latent BAX to the mitochondrial outer membrane (MOM) and then permeabilization of the MOM to facilitate the release of apoptotic signaling molecules. As a critical component of RGC death, BAX is an attractive target for neuroprotective therapies and an understanding of the kinetics of BAX activation and the mechanisms controlling the two stages of this process in RGCs is potentially valuable in informing the development of a neuroprotective strategy. Methods The kinetics of BAX translocation were assessed by both static and live-cell imaging of a GFP-BAX fusion protein introduced into RGCs using AAV2-mediated gene transfer in mice. Activation of BAX was achieved using an acute optic nerve crush (ONC) protocol. Live-cell imaging of GFP-BAX was achieved using explants of mouse retina harvested 7 days after ONC. Kinetics of translocation in RGCs were compared to GFP-BAX translocation in 661W tissue culture cells. Permeabilization of GFP-BAX was assessed by staining with the 6A7 monoclonal antibody, which recognizes a conformational change in this protein after MOM insertion. Assessment of individual kinases associated with both stages of activation was made using small molecule inhibitors injected into the vitreous either independently or in concert with ONC surgery. The contribution of the Dual Leucine Zipper-JUN-N-Terminal Kinase cascade was evaluated using mice with a double conditional knock-out of both Mkk4 and Mkk7 . Results ONC induces the translocation of GFP-BAX in RGCs at a slower rate and with less intracellular synchronicity than 661W cells, but exhibits less variability among mitochondrial foci within a single cell. GFP-BAX was also found to translocate in all compartments of an RGC including the dendritic arbor and axon. Approximately 6% of translocating RGCs exhibited retrotranslocation of BAX immediately following translocation. Unlike tissue culture cells, which exhibit simultaneous translocation and permeabilization, RGCs exhibited a significant delay between these two stages, similar to detached cells undergoing anoikis. Translocation, with minimal permeabilization could be induced in a subset of RGCs using an inhibitor of Focal Adhesion Kinase (PF573228). Permeabilization after ONC, in a majority of RGCs, could be inhibited with a broad spectrum kinase inhibitor (sunitinib) or a selective inhibitor for p38/MAPK14 (SB203580). Intervention of DLK-JNK axis signaling abrogated GFP-BAX translocation after ONC. Conclusions A comparison between BAX activation kinetics in tissue culture cells and in cells of a complex tissue environment shows distinct differences indicating that caution should be used when translating findings from one condition to the other. RGCs exhibit both a delay between translocation and permeabilization and the ability for translocated BAX to be retrotranslocated, suggesting several stages at which intervention of the activation process could be exploited in the design of a therapeutic strategy.

Autophagy in major human diseases

Autophagy is a core molecular pathway for the preservation of cellular and organismal homeostasis. Pharmacological and genetic interventions impairing autophagy responses promote or aggravate disease in a plethora of experimental models. Consistently, mutations in autophagy-related processes cause severe human pathologies. Here, we review and discuss preclinical data linking autophagy dysfunction to the pathogenesis of major human disorders including cancer as well as cardiovascular, neurodegenerative, metabolic, pulmonary, renal, infectious, musculoskeletal, and ocular disorders.

Cell Replacement Therapy for Retinal and Optic Nerve Diseases: Cell Sources, Clinical Trials and Challenges

The aim of this review was to provide an update on the potential of cell therapies to restore or replace damaged and/or lost cells in retinal degenerative and optic nerve diseases, describing the available cell sources and the challenges involved in such treatments when these techniques are applied in real clinical practice. Sources include human fetal retinal stem cells, allogenic cadaveric human cells, adult hippocampal neural stem cells, human CNS stem cells, ciliary pigmented epithelial cells, limbal stem cells, retinal progenitor cells (RPCs), human pluripotent stem cells (PSCs) (including both human embryonic stem cells (ESCs) and human induced pluripotent stem cells (iPSCs)) and mesenchymal stem cells (MSCs). Of these, RPCs, PSCs and MSCs have already entered early-stage clinical trials since they can all differentiate into RPE, photoreceptors or ganglion cells, and have demonstrated safety, while showing some indicators of efficacy. Stem/progenitor cell therapies for retinal diseases still have some drawbacks, such as the inhibition of proliferation and/or differentiation in vitro (with the exception of RPE) and the limited long-term survival and functioning of grafts in vivo. Some other issues remain to be solved concerning the clinical translation of cell-based therapy, including (1) the ability to enrich for specific retinal subtypes; (2) cell survival; (3) cell delivery, which may need to incorporate a scaffold to induce correct cell polarization, which increases the size of the retinotomy in surgery and, therefore, the chance of severe complications; (4) the need to induce a localized retinal detachment to perform the subretinal placement of the transplanted cell; (5) the evaluation of the risk of tumor formation caused by the undifferentiated stem cells and prolific progenitor cells. Despite these challenges, stem/progenitor cells represent the most promising strategy for retinal and optic nerve disease treatment in the near future, and therapeutics assisted by gene techniques, neuroprotective compounds and artificial devices can be applied to fulfil clinical needs.

Too much death can kill you: inhibiting intrinsic apoptosis to treat disease

Apoptotic cell death is implicated in both physiological and pathological processes. Since many types of cancerous cells intrinsically evade apoptotic elimination, induction of apoptosis has become an attractive and often necessary cancer therapeutic approach. Conversely, some cells are extremely sensitive to apoptotic stimuli leading to neurodegenerative disease and immune pathologies. However, due to several challenges, pharmacological inhibition of apoptosis is still only a recently emerging strategy to combat pathological cell loss. Here, we describe several key steps in the intrinsic (mitochondrial) apoptosis pathway that represent potential targets for inhibitors in disease contexts. We also discuss the mechanisms of action, advantages and limitations of small-molecule and peptide-based inhibitors that have been developed to date. These inhibitors serve as important research tools to dissect apoptotic signalling and may foster new treatments to reduce unwanted cell loss.

Optic nerve regeneration: A long view

The optic nerve conveys information about the outside world from the retina to multiple subcortical relay centers. Until recently, the optic nerve was widely believed to be incapable of re-growing if injured, with dire consequences for victims of traumatic, ischemic, or neurodegenerative diseases of this pathway. Over the past 10-20 years, research from our lab and others has made considerable progress in defining factors that normally suppress axon regeneration and the ability of retinal ganglion cells, the projection neurons of the retina, to survive after nerve injury. Here we describe research from our lab on the role of inflammation-derived growth factors, suppression of inter-cellular signals among diverse retinal cell types, and combinatorial therapies, along with related studies from other labs, that enable animals with optic nerve injury to regenerate damaged retinal axons back to the brain. These studies raise the possibility that vision might one day be restored to people with optic nerve damage.

Axon injury signaling and compartmentalized injury response in glaucoma

Axonal degeneration is an active, highly controlled process that contributes to beneficial processes, such as developmental pruning, but also to neurodegeneration. In glaucoma, ocular hypertension leads to vision loss by killing the output neurons of the retina, the retinal ganglion cells (RGCs). Multiple processes have been proposed to contribute to and/or mediate axonal injury in glaucoma, including: neuroinflammation, loss of neurotrophic factors, dysregulation of the neurovascular unit, and disruption of the axonal cytoskeleton. While the inciting injury to RGCs in glaucoma is complex and potentially heterogeneous, axonal injury is ultimately thought to be the key insult that drives glaucomatous neurodegeneration. Glaucomatous neurodegeneration is a complex process, with multiple molecular signals contributing to RGC somal loss and axonal degeneration. Furthermore, the propagation of the axonal injury signal is complex, with injury triggering programs of degeneration in both the somal and axonal compartment. Further complicating this process is the involvement of multiple cell types that are known to participate in the process of axonal and neuronal degeneration after glaucomatous injury. Here, we review the axonal signaling that occurs after injury and the molecular signaling programs currently known to be important for somal and axonal degeneration after glaucoma-relevant axonal injuries.

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.

Glaucoma-inducing Procedure in an In Vivo Rat Model and Whole-mount Retina Preparation

Glaucoma is a disease of the central nervous system affecting retinal ganglion cells (RGCs). RGC axons making up the optic nerve carry visual input to the brain for visual perception. Damage to RGCs and their axons leads to vision loss and/or blindness. Although the specific cause of glaucoma is unknown, the primary risk factor for the disease is an elevated intraocular pressure. Glaucoma-inducing procedures in animal models are a valuable tool to researchers studying the mechanism of RGC death. Such information can lead to the development of effective neuroprotective treatments that could aid in the prevention of vision loss. The protocol in this paper describes a method of inducing glaucoma - like conditions in an in vivo rat model where 50 µl of 2 M hypertonic saline is injected into the episcleral venous plexus. Blanching of the vessels indicates successful injection. This procedure causes loss of RGCs to simulate glaucoma. One month following injection, animals are sacrificed and eyes are removed. Next, the cornea, lens, and vitreous are removed to make an eyecup. The retina is then peeled from the back of the eye and pinned onto sylgard dishes using cactus needles. At this point, neurons in the retina can be stained for analysis. Results from this lab show that approximately 25% of RGCs are lost within one month of the procedure when compared to internal controls. This procedure allows for quantitative analysis of retinal ganglion cell death in an in vivo rat glaucoma model.

Functional architecture of the retina: development and disease

Structure and function are highly correlated in the vertebrate retina, a sensory tissue that is organized into cell layers with microcircuits working in parallel and together to encode visual information. All vertebrate retinas share a fundamental plan, comprising five major neuronal cell classes with cell body distributions and connectivity arranged in stereotypic patterns. Conserved features in retinal design have enabled detailed analysis and comparisons of structure, connectivity and function across species. Each species, however, can adopt structural and/or functional retinal specializations, implementing variations to the basic design in order to satisfy unique requirements in visual function. Recent advances in molecular tools, imaging and electrophysiological approaches have greatly facilitated identification of the cellular and molecular mechanisms that establish the fundamental organization of the retina and the specializations of its microcircuits during development. Here, we review advances in our understanding of how these mechanisms act to shape structure and function at the single cell level, to coordinate the assembly of cell populations, and to define their specific circuitry. We also highlight how structure is rearranged and function is disrupted in disease, and discuss current approaches to re-establish the intricate functional architecture of the retina.

Pou4f1 and pou4f2 are dispensable for the long-term survival of adult retinal ganglion cells in mice

PURPOSE

To investigate the role of Pou4f1 and Pou4f2 in the survival of adult retinal ganglion cells (RGCs).

METHODS

Conditional alleles of Pou4f1 and Pou4f2 were generated (Pou4f1loxP and Pou4f2loxP respectively) for the removal of Pou4f1 and Pou4f2 in adult retinas. A tamoxifen-inducible Cre was used to delete Pou4f1 and Pou4f2 in adult mice and retinal sections and flat mounts were subjected to immunohistochemistry to confirm the deletion of both alleles and to quantify the changes in the number of RGCs and other retinal neurons. To determine the effect of loss of Pou4f1 and Pou4f2 on RGC survival after axonal injury, controlled optic nerve crush (CONC) was performed and RGC death was assessed.

RESULTS

Pou4f1 and Pou4f2 were ablated two weeks after tamoxifen treatment. Retinal interneurons and Müller glial cells are not affected by the ablation of Pou4f1 or Pou4f2 or both. Although the deletion of both Pou4f1 and Pou4f2 slightly delays the death of RGCs at 3 days post-CONC in adult mice, it does not affect the cell death progress afterwards. Moreoever, deletion of Pou4f1 or Pou4f2 or both has no impact on the long-term viability of RGCs at up to 6 months post-tamoxifen treatment.

CONCLUSION

Pou4f1 and Pou4f2 are involved in the acute response to damage to RGCs but are dispensable for the long-term survival of adult RGC in mice.

Sigma receptor 1 modulates ER stress and Bcl2 in murine retina

Sigma receptor 1 (σR1), a non-opiate transmembrane protein located on endoplasmic reticulum (ER) and mitochondrial membranes, is considered to be a molecular chaperone. Marked protection against cell death has been observed when ligands for σR1 have been used in in vitro and in vivo models of retinal cell death. Mice lacking σR1 (σR1(-/-)) manifest late-onset loss of retinal ganglion cells and retinal electrophysiological changes (after many months). The role of σR1 in the retina and the mechanisms by which its ligands afford neuroprotection are unclear. We therefore used σR1(-/-) mice to investigate the expression of ER stress genes (BiP/GRP78, Atf6, Atf4, Ire1α) and proteins involved in apoptosis (BCL2, BAX) and to examine the retinal transcriptome at young ages. Whereas no significant changes occurred in the expression of major ER stress genes (over a period of a year) in neural retina, marked changes were observed in these genes, especially Atf6, in isolated retinal Müller glial cells. BCL2 levels decreased in σR1(-/-) retina concomitantly with decreases in NFkB and pERK1/2. We postulate that σR1 regulates ER stress in retinal Müller cells and that the role of σR1 in retinal neuroprotection probably involves BCL2 and some of the proteins that modify its expression (such as ERK, NFκB). Data from the analysis of the retinal transcriptome of σR1 null mice provide new insights into the role of σR1 in retinal neuroprotection.

Melanopsin ganglion cells are the most resistant retinal ganglion cell type to axonal injury in the rat retina

We report that the most common retinal ganglion cell type that remains after optic nerve transection is the M1 melanopsin ganglion cell. M1 ganglion cells are members of the intrinsically photosensitive retinal ganglion cell population that mediates non-image-forming vision, comprising ∼2.5% of all ganglion cells in the rat retina. In the present study, M1 ganglion cells comprised 1.7±1%, 28±14%, 55±13% and 82±8% of the surviving ganglion cells 7, 14, 21 and 60 days after optic nerve transection, respectively. Average M1 ganglion cell somal diameter and overall morphological appearance remained unchanged in non-injured and injured retinas, suggesting a lack of injury-induced degeneration. Average M1 dendritic field size increased at 7 and 60 days following optic nerve transection, while average dendritic field size remained similar in non-injured retinas and in retinas at 14 and 21 days after optic nerve transection. These findings demonstrate that M1 ganglion cells are more resistant to injury than other ganglion cell types following optic nerve injury, and provide an opportunity to develop pharmacological or genetic therapeutic approaches to mitigate ganglion cell death and save vision following optic nerve injury.

BCL2L1 (BCL-X) promotes survival of adult and developing retinal ganglion cells

The Bcl-2 family is responsible for regulating cell death pathways in neurons during development, after injury and in disease. The activation of the pro-death family member BAX is often the final step before cell death in neurons. Pro-survival family members such as BCL-X (BCL2L1) act to inhibit BAX activation. Overexpression studies have suggested that BCL-X could play an important physiological role in mediating neuronal viability. Loss-of-function studies performed in vivo have implicated BCL-X as a mediator of neuronal survival during the early stages of neurodevelopment. To assess whether BCL-X is needed to promote the survival of neurons in the central nervous system throughout life, Bcl-x was conditionally removed from the optic cup or throughout the adult mouse. During development BCL-X was required for the survival of differentiating retinal ganglion cells (RGCs) leading up to their normal window of developmental death. Despite its expression in adult RGCs, BCL-X was not required for maintaining RGC viability in adult retinas. However, the loss of BCL-X in adult RGCs did significantly increase the rate of death of RGCs after axonal injury. Thus, in developing and injured RGCs there appears to be an active cell survival program preventing neuronal death.

Full-length axon regeneration in the adult mouse optic nerve and partial recovery of simple visual behaviors

The mature optic nerve cannot regenerate when injured, leaving victims of traumatic nerve damage or diseases such as glaucoma with irreversible visual losses. Recent studies have identified ways to stimulate retinal ganglion cells to regenerate axons part-way through the optic nerve, but it remains unknown whether mature axons can reenter the brain, navigate to appropriate target areas, or restore vision. We show here that with adequate stimulation, retinal ganglion cells are able to regenerate axons the full length of the visual pathway and on into the lateral geniculate nucleus, superior colliculus, and other visual centers. Regeneration partially restores the optomotor response, depth perception, and circadian photoentrainment, demonstrating the feasibility of reconstructing central circuitry for vision after optic nerve damage in mature mammals.

Four steps to optic nerve regeneration

The failure of the optic nerve to regenerate after injury or in neurodegenerative disease remains a major clinical and scientific problem. Retinal ganglion cell (RGC) axons course through the optic nerve and carry all the visual information to the brain, but after injury, they fail to regrow through the optic nerve and RGC cell bodies typically die, leading to permanent loss of vision. There are at least 4 hurdles to overcome in preserving RGCs and regenerating their axons: 1) increase RGC survival, 2) overcome the inhibitory environment of the optic nerve, 3) enhance RGC intrinsic axon growth potential, and 4) optimize the mapping of RGC connections back into their targets in the brain.

WITHDRAWN: Reprint of: Variations in the rheostat model of apoptosis: What studies of retinal ganglion cell death tell us about the functions of the Bcl2 family proteins

The Publisher regrets that this article is an accidental duplication of an article that has already been published, doi:10.1016/j.exer.2010.03.004. The duplicate article has therefore been withdrawn.

WITHDRAWN: Reprint of: Mechanisms of retinal ganglion cell injury and defense in glaucoma

The Publisher regrets that this article is an accidental duplication of an article that has already been published, doi:10.1016/j.exer.2010.04.002. The duplicate article has therefore been withdrawn.

Bone marrow mononuclear cells increase retinal ganglion cell survival and axon regeneration in the adult rat

The central nervous system (CNS) of adult mammals generally does not regenerate, and many studies have attempted to identify factors that could increase neuroprotection and/or axonal outgrowth after CNS lesions. Using the optic nerve crush of rats as a model for CNS injury, we investigated the effect of intravitreal transplantation of syngeneic bone-marrow mononuclear cells (BMMCs) on the survival of retinal ganglion cells (RGC) and on the regeneration of optic axons. Control animals received intravitreal saline injections after lesion. Injections of BMMCs resulted in a 1.6-fold increase in the number of RGCs surviving 14 days after injury. The BMMC-treated animals also had increased numbers of axons, which grew up to 1.5 mm from the crush site, and also had reduced Müller glia activation. Analysis of mRNAs in all conditions revealed an increase in levels of fibroblast growth factor 2 (FGF-2) mRNA in treated animals 14 days after injury. To investigate whether the regenerated axons could reach the brain, we retrograde labeled the RGCs by injecting a lipophilic tracer into the superior colliculus. We also analyzed the expression of NGFI-A in the superficial layers of the superior colliculus as a possible marker of synaptic input from RGC axons. We found evidence that more RGCs were able to reach the brain after treatment and we showed that NGFI-A expression was higher in the treated animals 60 days after injury. These results demonstrate that transplant of BMMCs can increase neuroprotection and neuroregeneration after injury in a model of optic nerve crush, and these effects could be mediated by FGF-2.

Mechanisms of retinal ganglion cell injury and defense in glaucoma

Glaucoma is a disease in which retinal ganglion cells (RGCs) die leading ultimately to blindness. Over the past decade and a half, information has begun to emerge regarding specific molecular responses of the retina to conditions of elevated intraocular pressure (IOP). It is now clear that the state of the RGC in glaucoma depends on a balance of pro-survival and pro-death pathways in the retina and details of these responses are still being worked out. In this review, we will discuss the evidence supporting the involvement of specific apoptotic cascades as well as the insults that trigger RGC apoptosis. In addition, we will present evidence supporting the existence of endogenous protective mechanisms as well as exogenous neuroprotective strategies.

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.

Variations in the rheostat model of apoptosis: what studies of retinal ganglion cell death tell us about the functions of the Bcl2 family proteins

Studies of the functions of members of the Bcl2 gene family suggested that apoptosis was controlled by a rheostat in which anti-apoptotic proteins like BCL2 bound and sequestered pro-apoptotic proteins like BAX. Our current understanding of these proteins suggests that this is a simplistic model. The new rheostat model predicts that BH3-only peptides act as neutralizing ligands for the anti-apoptotic proteins, thus allowing molecules like BAX to become activated and initiate mitochondrial dysfunction - a critical step in the intrinsic apoptotic program. Studies of retinal ganglion cell apoptosis indicate that a threshold of BAX expression is required for its successful activation, which is independent of the overall concentration of anti-apoptotic proteins in these cells.

Neuroprotective effect of transcorneal electrical stimulation on light-induced photoreceptor degeneration

Direct electrical stimulation of neural tissues is a strategic approach to treat injured axons by accelerating their outgrowth [Al-Majed, A.A., Neumann, C.M., Brushart, T.M., Gordon, T., 2000. Brief electrical stimulation promotes the speed and accuracy of motor axonal regeneration. J. Neurosci. 20, 2602-2608] and promoting their regeneration [Geremia, N.M., Gordon, T., Brushart, T.M., Al-Majed, A.A., Verge, V.M.K., 2007. Electrical stimulation promotes sensory neuron regeneration and growth-associated gene expression. Exp. Neurol. 205, 347-359]. Recently, transcorneal electrical stimulation (TCES), a novel less invasive method, has been shown to rescue axotomized and damaged retinal ganglion cells [Morimoto, T., Miyoshi, T., Matsuda, S., Tano, Y., Fujikado, T., Fukuda, Y., 2005. Transcorneal electrical stimulation rescues axotomized retinal ganglion cells by activating endogenous retinal IGF-1 system. Invest. Ophthalmol. Vis. Sci. 46(6), 2147-2155]. Here, we investigated the neuroprotection of TCES on light-induced photoreceptor degeneration and the underlying mechanism. Adult male Sprague-Dawley (SD) rats received TCES before (pre-TCES) or after (post-TCES) intense light exposure. After fourteen days of light exposure, retinal histology and electroretinography were performed to evaluate the neuroprotective effect of TCES. The mRNA and protein levels of apoptotic-associated genes including Bcl-2, Bax, Caspase-3 as well as ciliary neurotrophic factor (CNTF) and brain-derived neurotrophic factor (BDNF) in the retinas were determined by real-time PCR and Western blot analysis. The localization of these gene products in the retinas was examined by immunohistochemistry. Both pre- and post-TCES ameliorated the progressive photoreceptor degeneration. The degree of rescue depended on the strength of the electric charge. Post-TCES showed a relatively better and longer-term protective effect than pre-TCES. Real-time PCR and Western blot analysis revealed an upregulation of Bcl-2, CNTF, and BDNF and a downregulation of Bax in the retinas after TCES. Immunohistochemical studies showed that Bcl-2 and CNTF were selectively upregulated in Müller cells. These findings provide a new therapeutic method to prevent or delay photoreceptor degeneration through activating the intrinsic survival system.

Tyrosine hydroxylase-positive amacrine interneurons in the mouse retina are resistant against the application of various parkinsonian toxins

Toxins such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), 6-hydroxydopamine (6-OHDA), or rotenone have been used to induce degeneration of dopaminergic (DA) neurons in the nigrostriatal pathway and to reproduce pathological characteristics of Parkinson's disease (PD). DA neurons are also present in the retina, and visual impairments in PD patients have been reported. We examined the vulnerability of TH-positive (TH(+)) amacrine interneurons in the retina against MPTP, 6-OHDA, or rotenone-induced cell death. We intraperitoneally (i.p.) injected mice with MPTP, which induced degeneration of DA neurons in the midbrain. However, no death of TH(+) amacrine cells was detectable in the same mice. HPLC analysis revealed a 9 times lower level of the toxic metabolite of MPTP, MPP(+), in the eye compared with the striatum. Another membrane-permeable compound (Tat-Hsp70) could be delivered into the retina after i.p. application, suggesting that the blood-retina barrier (BRB) could be overcome after systemic application. Possible reason for the survival of retinal amacrine cells after systemic MPTP application was a less efficient conversion into toxic MPP(+) in the retina or a general higher resistance against toxic insults of retinal DA neurons compared with DA neurons in the substantia nigra pars compacta (SNpc). Therefore, we directly injected high doses of MPP(+), 6-OHDA, or rotenone into the eye. No loss of TH(+) amacrine cells in the retina was observed, suggesting different properties and less vulnerability of amacrine neurons compared with DA neurons in the midbrain.

Impediments to eye transplantation: ocular viability following optic-nerve transection or enucleation

Maintenance of ocular viability is one of the major impediments to successful whole-eye transplantation. This review provides a comprehensive understanding of the current literature to help guide future studies in order to overcome this hurdle. A systematic multistage review of published literature was performed. Three specific questions were addressed: (1) Is recovery of visual function following eye transplantation greater in cold-blooded vertebrates when compared with mammals? (2) Is outer retina function following enucleation and reperfusion improved compared with enucleation alone? (3) Following optic-nerve transection, is there a correlation between retinal ganglion cell (RGC) survival and either time after transection or proximity of the transection to the globe? In a majority of the studies performed in the literature, recovery of visual function can occur after whole-eye transplantation in cold-blooded vertebrates. Following enucleation (and reperfusion), outer retinal function is maintained from 4 to 9 h. RGC survival following optic-nerve transection is inversely related to both the time since transection and the proximity of transection to the globe. Lastly, neurotrophins can increase RGC survival following optic-nerve transection. This review of the literature suggests that the use of a donor eye is feasible for whole-eye transplantation.

Retinal ganglion cell population in adult albino and pigmented mice: a computerized analysis of the entire population and its spatial distribution

In adult Swiss albino and C57 pigmented mice, RGCs were identified with a retrogradely transported neuronal tracer applied to both optic nerves (ON) or superior colliculi (SCi). After histological processing, the retinas were prepared as whole-mounts, examined and photographed under a fluorescence microscope equipped with a motorized stage controlled by a commercial computer image analysis system: Image-Pro Plus((R)) (IPP). Retinas were imaged as a stack of 24-bit color images (140 frames per retina) using IPP with the Scope-Pro plug-in 5.0 and the images montaged to create a high-resolution composite of the retinal whole-mount when required. Single images were also processed by specific macros written in IPP that apply a sequence of filters and transformations in order to separate individual cells for automatic counting. Cell counts were later transferred to a spreadsheet for statistical analysis and used to generate a RGC density map for each retina.

RESULTS

The mean total numbers of RGCs labeled from the ON, in Swiss (49,493+/-3936; n=18) or C57 mice (42,658+/-1540; n=10) were slightly higher than the mean numbers of RGCs labeled from the SCi, in Swiss (48,733+/-3954; n=43) or C57 mice (41,192+/-2821; n=42), respectively. RGCs were distributed throughout the retina and density maps revealed a horizontal region in the superior retina near the optic disk with highest RGC densities. In conclusion, the population of mice RGCs may be counted automatically with a level of confidence comparable to manual counts. The distribution of RGCs adopts a form of regional specialization that resembles a horizontal visual streak.

Mouse models of retinal ganglion cell death and glaucoma

Once considered too difficult to use for glaucoma studies, mice are now becoming a powerful tool in the research of the molecular and pathological events associated with this disease. Often adapting technologies first developed in rats, ganglion cell death in mice can be induced using acute models and chronic models of experimental glaucoma. Similarly, elevated IOP has been reported in transgenic animals carrying defects in targeted genes. Also, one group of mice, from the DBA/2 line of inbred animals, develops a spontaneous optic neuropathy with many features of human glaucoma that is associated with IOP elevation caused by an anterior chamber pigmentary disease. The advent of mice for glaucoma research is already having a significant impact on our understanding of this disease, principally because of the access to genetic manipulation technology and genetics already well established for these animals.

Axons of retinal ganglion cells are insulted in the optic nerve early in DBA/2J glaucoma

Here, we use a mouse model (DBA/2J) to readdress the location of insult(s) to retinal ganglion cells (RGCs) in glaucoma. We localize an early sign of axon damage to an astrocyte-rich region of the optic nerve just posterior to the retina, analogous to the lamina cribrosa. In this region, a network of astrocytes associates intimately with RGC axons. Using BAX-deficient DBA/2J mice, which retain all of their RGCs, we provide experimental evidence for an insult within or very close to the lamina in the optic nerve. We show that proximal axon segments attached to their cell bodies survive to the proximity of the lamina. In contrast, axon segments in the lamina and behind the eye degenerate. Finally, the Wld(s) allele, which is known to protect against insults to axons, strongly protects against DBA/2J glaucoma and preserves RGC activity as measured by pattern electroretinography. These experiments provide strong evidence for a local insult to axons in the optic nerve.

Functional study in NSE-Hu-Bcl-2 transgenic mice: a model for retinal diseases starting in Müller cells

In NSE-Hu-Bcl-2 transgenic mice, line 71, retina undergoes early postnatal degeneration linked to the prior death of Müller cells. The purpose of this study was to complete the characterization of this retinal dysfunction by using electroretinographic (ERG) recordings in both scotopic and photopic conditions. Here, we showed that both rod and cone systems were profoundly affected in NSE-Hu-Bcl-2 transgenic mice as soon as 15 postnatal days in accordance with histological study performed previously.

Overexpression of Bcl-2 or Bcl-xL prevents spiral ganglion neuron death and inhibits neurite growth

Spiral ganglion neurons (SGNs) provide afferent innervation to the cochlea and rely on contact with hair cells (HCs) for their survival. Following deafferentation due to hair cell loss, SGNs gradually die. In a rat culture model, we explored the ability of prosurvival members of the Bcl-2 family of proteins to support the survival and neurite outgrowth of SGNs. We found that overexpression of either Bcl-2 or Bcl-xL significantly increases SGN survival in the absence of neurotrophic factors, establishing that the Bcl-2 pathway is sufficient for SGN cell survival and that SGN deprived of trophic support die by an apoptotic mechanism. However, in contrast to observations in central neurons and PC12 cells where Bcl-2 appears to promote neurite growth, both Bcl-2 and Bcl-xL overexpression dramatically inhibit neurite outgrowth in SGNs. This inhibition of neurite growth by Bcl-2 occurs in nearly all SGNs even in the presence of multiple neurotrophic factors implying that Bcl-2 directly inhibits neurite growth rather than simply rescuing a subpopulation of neurons incapable of extending neurites without additional stimuli. Thus, although overexpression of prosurvival members of the Bcl-2 family prevents SGN loss following trophic factor deprivation, the inhibition of neurite growth by these molecules may limit their efficacy for support of auditory nerve maintenance or regeneration following hair cell loss.

The mouse pattern electroretinogram

Mouse models of optic nerve disease such as glaucoma, optic neuritis, ischemic optic neuropathy, and mitochondrial optic neuropathy are being developed at increasing rate to investigate specific pathophysiological mechanisms and the effect of neuroprotective treatments. The use of these models may be greatly enhanced by the availability of non-invasive methods able to monitor retinal ganglion cell (RGC) function longitudinally such as the Pattern Electroretinogram (PERG). While the use of the PERG as a tool to probe inner retina function in mammals is known since 25 years, relatively less information is available for the mouse. Here, the PERG technique and the main applications in the mouse are reviewed.

Dominant inheritance of retinal ganglion cell resistance to optic nerve crush in mice

Background

Several neurodegenerative diseases are influenced by complex genetics that affect an individual's susceptibility, disease severity, and rate of progression. One such disease is glaucoma, a chronic neurodegenerative condition of the eye that targets and stimulates apoptosis of CNS neurons called retinal ganglion cells. Since ganglion cell death is intrinsic, it is reasonable that the genes that control this process may contribute to the complex genetics that affect ganglion cell susceptibility to disease. To determine if genetic background influences susceptibility to optic nerve damage, leading to ganglion cell death, we performed optic nerve crush on 15 different inbred lines of mice and measured ganglion cell loss. Resistant and susceptible strains were used in a reciprocal breeding strategy to examine the inheritance pattern of the resistance phenotype. Because earlier studies had implicated Bax as a susceptibility allele for ganglion cell death in the chronic neurodegenerative disease glaucoma, we conducted allelic segregation analysis and mRNA quantification to assess this gene as a candidate for the cell death phenotype.

Results

Inbred lines showed varying levels of susceptibility to optic nerve crush. DBA/2J mice were most resistant and BALB/cByJ mice were most susceptible. F1 mice from these lines inherited the DBA/2J phenotype, while N2 backcross mice exhibited the BALB/cByJ phenotype. F2 mice exhibited an intermediate phenotype. A Wright Formula calculation suggested as few as 2 dominant loci were linked to the resistance phenotype, which was corroborated by a Punnett Square analysis of the distribution of the mean phenotype in each cross. The levels of latent Bax mRNA were the same in both lines, and Bax alleles did not segregate with phenotype in N2 and F2 mice.

Conclusion

Inbred mice show different levels of resistance to optic nerve crush. The resistance phenotype is heritable in a dominant fashion involving relatively few loci. Bax was excluded as a candidate gene for this phenotype.

Attempts to restore visual function after optic nerve damage in adult mammals

Retinal ganglion cells (RGCs) and their axons, i.e., optic nerve (ON) fibers, provide a good experimental model for research on damaged CNS neurons and their functional ecovery. After the ON transection most RGCs undergo retrograde and anterograde degeneration but they can be rescued and regenerated by transplantation of a piece of peripheral nerve (PN). When the nerve graft was bridged to the visual center, regenerating RGC axons can restore the central visual projection. Behavioral recovery of relatively simple visual function has been proved in such PN-grafted rodents. Intravitreal injections of various neurotrophic factors and cytokines to activate intracellular signaling mechanism of RGCs and electrical stimulation to the cut end of ON have promoting effects on their survival and axonal regeneration. Axotomized RGCs in adult cats are also shown to survive and regenerate their axons through the PN graft. Among the cat RGC types, Y cells, which function as visual motion detector, tend to survive and regenerate axons better than others. X cells, which are essential for acute vision, suffer from rapid death after ON transection but they can be rescued by intravitreal application of neurotrophins accompanied with elevation of cAMP. To restore visual function in adult mammals with damaged optic pathway, the comprehensive and integrative strategies of multiple approaches will be needed, taking care of functional diversity of RGC types.

Cooperative effects ofbcl-2and AAV-mediated expression of CNTF on retinal ganglion cell survival and axonal regeneration in adult transgenic mice

We used a gene therapy approach in transgenic mice to assess the cooperative effects of combining anti-apoptotic and growth-promoting stimuli on adult retinal ganglion cell (RGC) survival and axonal regeneration following intraorbital optic nerve injury. Bi-cistronic adeno-associated viral vectors encoding a secretable form of ciliary neurotrophic factor and green fluorescent protein (AAV-CNTF-GFP) were injected into eyes of mice that had been engineered to over-express the anti-apoptotic protein bcl-2. For comparison this vector was also injected into wildtype (wt) mice, and both mouse strains were injected with control AAV encoding GFP. Five weeks after optic nerve injury we confirmed that bcl-2 over-expression by itself promoted the survival of axotomized RGCs, but in contrast to previous reports we also saw regeneration of some mature RGC axons beyond the optic nerve crush. AAV-mediated expression of CNTF in adult retinas significantly increased the survival and axonal regeneration of RGCs following axotomy in wt and bcl-2 transgenic mice; however, the effects were greatest in the transgenic strain. Compared with AAV-GFP-injected bcl-2 mice, RGC viability was increased by about 50% (mean, 36 738 RGCs per retina), and over 1000 axons per optic nerve regenerated 1-1.5 mm beyond the crush. These findings exemplify the importance of using a multifactorial therapeutic approach that enhances both neuroprotection and regeneration after central nervous system injury.

ADAPTIVE ROLES OF PROGRAMMED CELL DEATH DURING NERVOUS SYSTEM DEVELOPMENT

The programmed cell death (PCD) of developing cells is considered an essential adaptive process that evolved to serve diverse roles. We review the putative adaptive functions of PCD in the animal kingdom with a major focus on PCD in the developing nervous system. Considerable evidence is consistent with the role of PCD in events ranging from neurulation and synaptogenesis to the elimination of adult-generated CNS cells. The remarkable recent progress in our understanding of the genetic regulation of PCD has made it possible to perturb (inhibit) PCD and determine the possible repercussions for nervous system development and function. Although still in their infancy, these studies have so far revealed few striking behavioral or functional phenotypes.

Actions of neurotrophic factors and their signaling pathways in neuronal survival and axonal regeneration

Adult axons in the mammalian central nervous system do not elicit spontaneous regeneration after injury, although many affected neurons have survived the neurotrauma. However, axonal regeneration does occur under certain conditions. These conditions include: (a) modification of regrowth environment, such as supply of peripheral nerve bridges and transplantation of Schwann cells or olfactory ensheathing glia to the injury site; (b) application of neurotrophic factors at the cell soma and axon tips; (c) blockade of growth-inhibitory molecules such as Nogo-A, myelin-associated glycoprotein, and oligodendrocyte-myelin glycoprotein; (d) prevention of chondroitin-sulfate-proteoglycans-related scar tissue formation at the injury site using chondroitinase ABC; and (e) elevation of intrinsic growth potential of injured neurons via increasing intracellular cyclic adenosine monophosphate level. A large body of evidence suggests that these conditions achieve enhanced neuronal survival and axonal regeneration through sometimes overlapping and sometimes distinct signal transduction mechanisms, depending on the targeted neuronal populations and intervention circumstances. This article reviews the available information on signal transduction pathways underlying neurotrophic-factor-mediated neuronal survival and neurite outgrowth/axonal regeneration. Better understanding of signaling transduction is important in helping us develop practical therapeutic approaches for encouraging neuronal survival and axonal regeneration after traumatic injury in clinical context.

Long-term in vivo inhibition of CNS neurodegeneration by Bcl-XL gene transfer

The inherently low regenerative capacity of the CNS demands effective strategies to inhibit neurodegeneration in acute lesions but also in slowly progressive neurological disorders. Therefore, therapeutic targets that can interact with the degeneration cascade to block, not just postpone, neuronal degeneration need to be defined. Bcl-X(L), a protein protecting the integrity of the mitochondrial membrane potential, was investigated for its neuroprotective properties in a long-term in vivo model of neuronal cell death. An AAV-2-based vector was used to express both Bcl-X(L) and EGFP in retinal ganglion cells (RGCs) of the adult rat retina. Transection of the optic nerve results in degeneration of RGCs in control retinae, while Bcl-X(L)-overexpressing ganglion cells were protected from degeneration. At 2 weeks after axotomy, 94% of the transduced RGCs survived the lesion (15% in controls). For the first time, we investigated RGC survival up to 8 weeks after axotomy and detected that 46% of the Bcl-X(L)-overexpressing RGCs still survived, representing significantly increased neuroprotection compared to neurotrophin-based approaches. We could also show that the axons of AAV-Bcl-X(L)-transduced RGCs remained morphologically intact after the lesion, thus providing the basis for regeneration-inducing attempts.

Axotomized mouse retinal ganglion cells containing melanopsin show enhanced survival, but not enhanced axon regrowth into a peripheral nerve graft

Melanopsin is found in only approximately 2% of mouse retinal ganglion cells (RGCs), making these RGCs uniquely and directly photosensitive. Given that the majority of RGCs die after axotomy and that grafting of a peripheral nerve to the eye provides a permissive environment for axon regrowth, the present study examined the survival and axonal regrowth of melanopsin-containing RGCs in mice. One month after optic nerve transection and grafting, RGCs with regrown axons were labeled from the grafts and retinae were processed to visualize melanopsin and TUJ1. Melanopsin-positive and negative RGCs were counted and compared to axotomized RGCs from ungrafted eyes and uninjured RGCs. Melanopsin-positive RGCs showed a 3-fold increase in survival rate compared to non-melanopsin RGCs. Despite this enhanced survival, melanopsin-containing RGCs did not show increased axon regrowth into nerve grafts.

Bcl-2 overexpression does not promote axonal regeneration of the entorhino-hippocampal connections in vitro after axotomy

CNS lesions trigger cell death in injured neurons and glia. Genes of the bcl-2 family play crucial roles in the control of apoptosis and cell survival in the CNS. Recently, it has been suggested that overexpression of bcl-2 induces axonal elongation and regeneration in vitro and in vivo. Here, we analyze the regenerative potential of bcl-2 overexpression in the axotomized entorhino-hippocampal connection in organotypic slice cocultures. Our results show that in slice cocultures from bcl-2-overexpressing mice, there is a decrease in the number of dead neurons in the entorhinal cortex. In addition, axonal regeneration is not enhanced after axotomy. Thus, in the entorhino-hippocampal formation in vitro, bcl-2 overexpression rescues neurons from axotomy-induced cell death but fails to enhance the regeneration of the entorhino-hippocampal connection.

The molecular biology of retinal ganglion cell death: caveats and controversies

Understanding the molecular pathways activated in dying retinal ganglion cells may lead to the development of therapies aimed at blocking the cell death process. As we learn more about ganglion cell death, it is becoming clear that several new hurdles must be overcome before preventing this process can be a realistic therapy. This review details three caveats about retinal ganglion cell death that should be considered. The first caveat centers on a critical step in the cell death pathway involving mitochondria. Blocking biochemical events after mitochondrial dysfunction, such as the caspase cascade, may provide only a transient effect on survival, since the cell has already sustained lethal damage. The second caveat is that blocking one cell death pathway may be ineffective because alternate pathways can become active. This caveat seems to be particularly relevant in neurons exposed to excitotoxic insults. The third caveat is that although it is possible to block cell death, this does not guarantee that the cell will be able to function normally. Consequently, it may be important to provide additional treatment to restore normal cell function in conjunction with therapies aimed at preventing their death.

Excess Bcl-XL increases the intrinsic growth potential of adult CNS neurons in vitro

The regenerative potential of adult mammalian CNS neurons is limited. Recent data suggest that inactivation of major growth inhibitors may not suffice to induce robust regeneration from mature neurons unless the intrinsic growth state is modulated. To investigate a possible role of Bcl-XL for axon regeneration in the adult mammalian CNS, Bcl-XL was adenovirally overexpressed in severed rat RGCs. Bcl-XL overexpression in mature axotomized RGCs in vivo increased both numbers [3.10-fold (+/-0.20)] and cumulative length [6.72-fold (+/-0.47)] of neurites regenerated from retinal explants, and this effect was further pronounced in the central retina where specific and dense axoplasmatic transduction occurs. Similarly, delayed Bcl-XL gene transfer to explanted retinae 12-13 days after lesion increased the numbers and length of emanating neurites by a factor of 5.22 (+/-0.41) and 8.29 (+/-0.69), respectively. In vivo, intraretinal sprouting of unmyelinated RGC axons into the nerve fiber layer was increased. However, fiber ingrowth into the optic nerve remained sparse, likely due to myelin inhibitors and scar components. Therefore, Bcl-XL overexpression may enhance, but may not be sufficient to, restitute functional regeneration in the adult CNS. As assessed by cell quantification analysis, Bcl-XL overexpression rescued a higher proportion of RGCs in vivo than in vitro. Therefore, Bcl-XL is capable to induce both neuronal survival and axon regeneration, but these two processes appear to be differentially modified by distinct pathways in vivo.

Bcl-2 family regulation of neuronal development and neurodegeneration

Neuronal cell death is a key feature of both normal nervous system development and neuropathological conditions. The Bcl-2 family, via its regulation of both caspase-dependent and caspase-independent cell death pathways, is uniquely positioned to critically control neuronal cell survival. Targeted gene disruptions of specific bcl-2 family members and the generation of transgenic mice overexpressing anti- or pro-apoptotic Bcl-2 family members have confirmed the importance of the Bcl-2 family in the nervous system. Data from studies of human brain tissue and experimental animal models of neuropathological conditions support the hypothesis that the Bcl-2 family regulates cell death in the mature nervous system and suggest that pharmacological manipulation of Bcl-2 family action could prove beneficial in the treatment of human neurological conditions such as stroke and neurodegenerative diseases.