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

Interaction of neurotrophin signaling with Bcl-2 localized to the mitochondria and endoplasmic reticulum on spiral ganglion neuron survival and neurite growth

Enhanced spiral ganglion neuron (SGN) survival and regeneration of peripheral axons following deafness will likely enhance the efficacy of cochlear implants. Overexpression of Bcl-2 prevents SGN death but inhibits neurite growth. Here we assessed the consequences of Bcl-2 targeted to either the mitochondria (GFP-Bcl-2-Maob) or the endoplasmic reticulum (ER, GFP-Bcl-2-Cb5) on cultured SGN survival and neurite growth. Transfection of wild-type GFP-Bcl-2, GFP-Bcl-2-Cb5, or GFP-Bcl-2-Maob increased SGN survival, with GFP-Bcl-2-Cb5 providing the most robust response. Paradoxically, expression of GFP-Bcl-2-Maob results in SGN death in the presence of neurotrophin-3 (NT-3) and brain-derived neurotrophic factor (BDNF), neurotrophins that independently promote SGN survival via Trk receptors. This loss of SGNs is associated with cleavage of caspase 3 and appears to be specific for neurotrophin signaling, insofar as coexpression of constitutively active mitogen-activated kinase kinase (MEKDeltaEE) or phosphatidyl inositol-3 kinase (P110), but not other prosurvival stimuli (e.g., membrane depolarization), also results in the loss of SGNs expressing GFP-Bcl-2-Maob. MEKDeltaEE and P110 promote SGN survival, whereas P110 promotes neurite growth to a greater extent than NT-3 or MEKDeltaEE. However, wild-type GFP-Bcl-2, GFP-Bcl-2-Cb5, and GFP-Bcl-2-Maob inhibit neurite growth even in the presence of neurotrophins, MEKDeltaEE, or P110. Historically, Bcl-2 has been thought to act primarily at the mitochondria to prevent neuronal apoptosis. Nevertheless, our data show that Bcl-2 targeted to the ER is more effective at rescuing SGNs in the absence of trophic factors. Additionally, Bcl-2 targeted to the mitochondria results in SGN death in the presence of neurotrophins. (c) 2010 Wiley-Liss, Inc.

SOCS3 deletion promotes optic nerve regeneration in vivo

Axon regeneration failure accounts for permanent functional deficits following CNS injury in adult mammals. However, the underlying mechanisms remain elusive. In analyzing axon regeneration in different mutant mouse lines, we discovered that deletion of suppressor of cytokine signaling 3 (SOCS3) in adult retinal ganglion cells (RGCs) promotes robust regeneration of injured optic nerve axons. This regeneration-promoting effect is efficiently blocked in SOCS3-gp130 double-knockout mice, suggesting that SOCS3 deletion promotes axon regeneration via a gp130-dependent pathway. Consistently, a transient upregulation of ciliary neurotrophic factor (CNTF) was observed within the retina following optic nerve injury. Intravitreal application of CNTF further enhances axon regeneration from SOCS3-deleted RGCs. Together, our results suggest that compromised responsiveness to injury-induced growth factors in mature neurons contributes significantly to regeneration failure. Thus, developing strategies to modulate negative signaling regulators may be an efficient strategy of promoting axon regeneration after CNS injury.

Optic nerve and vitreal inflammation are both RGC neuroprotective but only the latter is RGC axogenic

Intravitreal inflammation, induced by either lens injury, or intravitreal injection of zymosan (IVZ), protects RGC from apoptosis and stimulates axon regeneration after optic nerve transection. Here, we investigate the differential effects of intra-optic nerve zymosan (ONZ) and IVZ injections on RGC neuroprotection and axogenesis. After both IVZ and ONZ injection, zymosan-induced inflammation promoted a similar 4-/5-fold enhancement in RGC survival, compared to optic nerve transected controls, but only IVZ promoted RGC axon regeneration. IVZ was the most effective in activating retinal astrocyte/Müller cells while regulated intramembraneous proteolysis (RIP) of p75(NTR) and inactivation of Rho (key components of the axon growth inhibitory signalling cascade) occurred in both ONZ and IVZ, but only in the latter did RGC axons regenerate. We suggest that neuroprotective factors may be transported to RGC somata by retrograde transport after ONZ and diffuse into the retina after IVZ injection, but an axogenic agent is required to initiate and maintain disinhibited RGC axon regeneration that may be an exclusive property of a Müller cell-derived factor released after IVZ.

Oncomodulin links inflammation to optic nerve regeneration

The inflammatory response that accompanies central nervous system (CNS) injury can affect neurological outcome in both positive and negative ways. In the optic nerve, a CNS pathway that normally fails to regenerate when damaged, intraocular inflammation causes retinal ganglion cells (RGCs) to switch into an active growth state and extend lengthy axons down the nerve. The molecular basis of this phenomenon is uncertain. A prior study showed that oncomodulin (Ocm), a Ca(2+)-binding protein secreted by a macrophage cell line, is a potent axon-promoting factor for RGCs. However, it is not known whether Ocm contributes to the physiological effects of intraocular inflammation in vivo, and there are conflicting reports in the literature regarding its expression and significance. We show here that intraocular inflammation causes infiltrative cells of the innate immune system to secrete high levels of Ocm, and that agents that prevent Ocm from binding to its receptor suppress axon regeneration. These results were verified in different strains, species, and experimental models, and establish Ocm as a potent growth-promoting signal between the innate immune system and neurons in vivo.

Off-target effects of epidermal growth factor receptor antagonists mediate retinal ganglion cell disinhibited axon growth

Inhibition of central nervous system axon growth is reportedly mediated in part by calcium-dependent phosphorylation of axonal epidermal growth factor receptor, with local administration of the epidermal growth factor receptor kinase inhibitors AG1478 and PD168393 to an optic nerve lesion site promoting adult retinal ganglion cell axon regeneration. Here, we show that epidermal growth factor receptor was neither constitutively expressed, nor activated in optic nerve axons in our non-regenerating and regenerating optic nerve injury models, a finding that is inconsistent with phosphorylated epidermal growth factor receptor-dependent intra-axonal signalling of central nervous system myelin-related axon growth inhibitory ligands. However, epidermal growth factor receptor was localized and activated within most glia in the retina and optic nerve post-injury, and thus an indirect glial-dependent mechanism for stimulated retinal ganglion cell axon growth by epidermal growth factor receptor inhibitors seemed plausible. Using primary retinal cultures with added central nervous system myelin extracts, we confirmed previous reports that AG1478/PD168393 blocks epidermal growth factor receptor activation and promotes disinhibited neurite outgrowth. Paradoxically, neurites did not grow in central nervous system myelin extract-containing cultures after short interfering ribonucleic acid-mediated knockdown of epidermal growth factor receptor. However, addition of AG1478 restored neurite outgrowth to short interfering ribonucleic acid-treated cultures, implying that epidermal growth factor receptor does not mediate AG1478-dependent effects. TrkA-/B-/C-Fc fusion proteins and the kinase blocker K252a abrogated the neuritogenic activity in these cultures, correlating with the presence of the neurotrophins brain derived neurotrophic factor, nerve growth factor and neurotrophin-3 in the supernatant and increased intracellular cyclic adenosine monophosphate activity. Neurotrophins released by AG1478 stimulated disinhibited retinal ganglion cell axon growth in central nervous system myelin-treated cultures by the induction of regulated intramembraneous proteolysis of p75(NTR) and Rho inactivation. Retinal astrocytes/Müller cells and retinal ganglion cells were the source of neurotrophins, with neurite outgrowth halved in the presence of glial inhibitors. We attribute AG1478-stimulated neuritogenesis to the induced release of neurotrophins together with raised cyclic adenosine monophosphate levels in treated cultures, leading to axon growth and disinhibition by neurotrophin-induced regulated intramembraneous proteolysis of p75(NTR). These off-target effects of epidermal growth factor receptor kinase inhibition suggest a novel therapeutic approach for designing treatments to promote central nervous system axon regeneration.

Gene therapy targeting glaucoma: where are we?

In a chronic disease such as glaucoma, a therapy that provides a long lasting local effect with minimal systemic side effects, while circumventing the issue of patient compliance, is very attractive. The field of gene therapy is growing rapidly and ocular applications are expanding. Our understanding of the molecular pathogenesis of glaucoma is leading to greater specificity in ocular tissue targeting. Improvements in gene delivery techniques, refinement of vector construction methods, and development of better animal models combine to bring this potential therapy closer to reality.

The role of macrophages in optic nerve regeneration

Following injury to the nervous system, the activation of macrophages, microglia, and T-cells profoundly affects the ability of neurons to survive and to regenerate damaged axons. The primary visual pathway provides a well-defined model system for investigating the interactions between the immune system and the nervous system after neural injury. Following damage to the optic nerve in mice and rats, retinal ganglion cells, the projection neurons of the eye, normally fail to regenerate their axons and soon begin to die. Induction of an inflammatory response in the vitreous strongly enhances the survival of retinal ganglion cells and enables these cells to regenerate lengthy axons beyond the injury site. T cells modulate this response, whereas microglia are thought to contribute to the loss of retinal ganglion cells in this model and in certain ocular diseases. This review discusses the complex and sometimes paradoxical actions of blood-borne macrophages, resident microglia, and T-cells in determining the outcome of injury in the primary visual pathway.

Genetic modulation of apoptotic pathways fails to alter disease course in tripeptidyl-peptidase 1 deficient mice

Late-infantile neuronal ceroid lipofuscinosis (LINCL) is a fatal, incurable neurodegenerative disease of children caused by the loss of the lysosomal protein tripeptidyl-peptidase 1 (TPP1). Previous studies have suggested that Bcl-2-dependent apoptotic pathways are involved in neuronal cell death in LINCL patients and, as a result, anti-apoptotic treatments that increase Bcl-2 activity have been proposed as a potential therapeutic approach. In this study, we have directly investigated whether targeting anti-apoptotic pathways may be of value in LINCL in a mouse model of this disease that lacks TPP1 and which recapitulates many aspect of the human disease, including a greatly shortened life-span. Our approach was to genetically modify apoptotic pathways and determine the effects of these changes on the severe neurodegenerative phenotype of the LINCL mouse. LINCL mice were generated that either lacked the pro-apoptotic p53 or had increased levels of anti-apoptotic Bcl-2, changes that would exacerbate or ameliorate neuronal death, respectively, should pathways involving these proteins be important. Neither modification affected the shortened life-span of the LINCL mouse. These results suggest that either neuronal death in LINCL does not occur via apoptosis or that it occurs via apoptotic pathways not involving p53 or Bcl-2. Alternatively, pathways involving p53 and/or Bcl-2 may be involved in neuronal death under normal circumstances but may not be the only routes to this end. Importantly, our findings suggest that targeting pathways of cell death involving p53 or Bcl-2 do not represent useful directions for developing effective treatment.

Gene therapy and transplantation in the retinofugal pathway

The mature CNS has limited intrinsic capacity for repair after injury; therefore, strategies are needed to enhance the viability and regrowth of damaged neurons. Here we review gene therapy studies in the eye, aimed at improving the survival and regeneration of injured retinal ganglion cells (RGCs). To target RGCs most current methods use recombinant adeno-associated viral vectors (AAV), usually serotype-2 (AAV2), that are injected into the vitreal chamber of the eye. This vector provides long-term transduction of adult RGCs. Strong, constitutive promoters such as CMV and/or beta-actin are commonly used but cell-specific promoters have also been tested. Transgenes encoded by AAV have been selected to limit cell death, enhance growth factor expression, or promote growth cone responsiveness. We have assessed the effects of AAV vectors in adult rodent models (i) after optic nerve (ON) crush and (ii) after transplantation of peripheral nerve (PN) onto the cut ON, a procedure that induces injured RGCs to regenerate axons over longer distances. AAV-CNTF-GFP promotes RGC survival and axonal regrowth in mice after ON crush, and in rats after ON crush or PN transplantation. In rats, intravitreal injection of AAV-BDNF-GFP also increases RGC viability but does not promote regeneration. RGC viability and axonal regrowth is further enhanced when AAV-CNTF-GFP is injected into transgenic mice that over-express bcl-2. Reconstituted PN grafts containing Schwann cells that were transduced ex vivo with lentiviral (LV) vectors encoding a secretable form of CNTF support RGC axonal regrowth, however grafts containing Schwann cells transduced with LV-BDNF or LV-GDNF are less successful. We have also quantified the transduction efficiency and tropism of different AAV vectors injected intravitreally. AAV 2/2 and AAV 2/6 showed highest levels of transduction, AAV 2/8 the lowest, and each serotype displayed different transduction profiles for retinal cells. We are also studying the long-term impact of AAV2-mediated CNTF or BDNF expression on the dendritic morphology of RGCs in normal and PN grafted retinas. Analysis of regenerating RGCs intracellularly injected with lucifer yellow indicates gene-specific changes in dendritic structure that likely impact upon visual function.

Cellular tropism and transduction properties of seven adeno-associated viral vector serotypes in adult retina after intravitreal injection

Recombinant adeno-associated virus (rAAV) vectors are increasingly being used as tools for gene therapy, and clinical trials have begun in patients with genetically linked retinal disorders. Intravitreal injection is optimal for the transduction of retinal ganglion cells (RGCs), although complete selectivity has not been achieved. There may also be advantages in using intravitreal approaches for the transduction of photoreceptors. Here we compared the cellular tropism and transduction efficiency of rAAV2/1, -2/2, -2/3, -2/4, -2/5, -2/6 and -2/8 in adult rat retina after intravitreal injection. Each vector encoded green fluorescent protein (GFP), and the number, laminar distribution and morphology of transduced GFP(+) cells were determined using fluorescent microscopy. Assessment of transduced cell phenotype was based on cell morphology and immunohistochemistry. rAAV2/2 and rAAV2/6 transduced the greatest number of cells, whereas rAAV2/5 and rAAV2/8 were least efficient. Most vectors primarily transduced RGCs; however, rAAV2/6 had a more diverse tropism profile, with 46% identified as amacrine or bipolar cells, 23% as RGCs and 22% as Müller cells. Müller cells were also frequently transduced by rAAV2/4. The highest photoreceptor transduction was seen after intravitreal rAAV2/3 injection. These data facilitate the design and selection of rAAV vectors to target specific retinal cells, potentially leading to an improved gene therapy for various human retinal pathologies.

BAG1 promotes axonal outgrowth and regeneration in vivo via Raf-1 and reduction of ROCK activity

Improved survival of injured neurons and the inhibition of repulsive environmental signalling are prerequisites for functional regeneration. BAG1 (Bcl-2-associated athanogene-1) is an Hsp70/Hsc70-binding protein, which has been shown to suppress apoptosis and enhance neuronal differentiation. We investigated BAG1 as a therapeutic molecule in the lesioned visual system in vivo. Using an adeno-associated viral vector, BAG1 (AAV.BAG1) was expressed in retinal ganglion cells (RGC) and then tested in models of optic nerve axotomy and optic nerve crush. BAG1 significantly increased RGC survival as compared to adeno-associated viral vector enhanced green fluorescent protein (AAV.EGFP) treated controls and this was independently confirmed in transgenic mice over-expressing BAG1 in neurons. The numbers and lengths of regenerating axons after optic nerve crush were also significantly increased in the AAV.BAG1 group. In pRGC cultures, BAG1-over-expression resulted in a approximately 3-fold increase in neurite length and growth cone surface. Interestingly, BAG1 induced an intracellular translocation of Raf-1 and ROCK2 and ROCK activity was decreased in a Raf-1-dependent manner by BAG1-over-expression. In summary, we show that BAG1 acts in a dual role by inhibition of lesion-induced apoptosis and interaction with the inhibitory ROCK signalling cascade. BAG1 is therefore a promising molecule to be further examined as a putative therapeutic tool in neurorestorative strategies.

STAT3 activation protects retinal ganglion cell layer neurons in response to stress

STAT3 is a major signaling molecule for many neurotrophic factors but its direct role in the protection of neurons in response to stress has not been addressed. We have studied the role of STAT3 in protecting retinal neurons from damage induced by ischemia/reperfusion and glutamate excitotoxicity by using adenovirus constructs to introduce active, normal or inactive STAT3 into retinal ganglion cells in culture and cells of the ganglion cell layer in the intact retina. Transient ischemia/reperfusion was induced in adult CD1 mice by elevating the intraocular pressure to the equivalent of 120mmHg for 60min, followed by a return to normal pressure. The levels, activation and distribution of STAT3 protein were evaluated by Western blot and immunocytochemistry. A transient peak of STAT3 activation was seen at 24h post ischemia and a strong increase in STAT3 protein levels 24h later. The increase in levels of STAT3 was detected in both ganglion cell bodies and processes in the plexiform layers by immunocytochemistry. The time course of STAT3 increase was slower than the time course of ganglion cell death as measured by TUNEL assay. Intravitreal injection of NMDA led to peak increases in activated STAT3 and STAT3 at 12 and 24h post insult respectively. Purified RGCs were infected with recombinant wild-type STAT3, constitutively active and dominant negative forms of STAT3 adenoviruses or control empty virus and then treated with glutamate. Surviving infected cells were counted 24 and 48h later. Infection with constitutively active STAT3 gave substantial protection when compared to the other constructs. Similarly, intravitreal injection of constitutively active STAT3 adenovirus one day before ischemia-reperfusion resulted in a decreased neural cell death in the ganglion cell layer compared with GFP adenovirus control. Our results suggest that persistent activation of STAT3 by neurotrophic factors provides strong neuroprotection and will be an effective strategy in a number of chronic retinal diseases.

Combined Therapies in the Treatment of Neurotrauma: Polymers, Bridges and Gene Therapy in Visual System Repair

BACKGROUND

The mature central nervous system (CNS) has limited capacity for self-renewal and repair after injury or neurodegeneration, and therapeutic strategies are needed to promote the viability of damaged neurons and the regrowth of their axons. The retina and optic nerve (ON) are part of the CNS, and the visual system is widely used in experimental studies on injury and repair.

OBJECTIVE

To test various cellular and molecular approaches in attempts to replace retinal ganglion cells (RGCs) in depleted retinas or, more usually, promote the survival of endogenous injured RGCs and stimulate axonal regeneration after ON or intracranial optic tract (OT) injury.

METHODS AND RESULTS

Intraocular injections of brain-derived neurotrophic factor and ciliary neurotrophic factor (CNTF) temporarily increase RGC survival after ON injury. More sustained neuroprotection is obtained using adeno-associated viral vectors to transfect RGCs with brain-derived neurotrophic factor or CNTF genes. After ON crush, intravitreal adeno-associated viral CNTF injections also increase RGC axonal regrowth. Additional protective and growth effects are obtained after intraocular elevation of cAMP and by manipulation of protein kinase signalling pathways in RGCs. Regeneration is increased by transplanting a segment of peripheral nerve onto the cut ON. Schwann cells in peripheral nerve grafts can be genetically modified using lentiviral vectors to over-express CNTF, resulting in increased regrowth of RGC axons. After OT lesions, hydrogels have been used to bridge the injury, sometimes with the incorporation of signalling peptides or cells genetically modified to express neurotrophic factors.

CONCLUSIONS

There is now a general consensus that combinatorial approaches are needed to elicit sustained and effective regenerative responses in injured adult CNS neurons.