Ideal strategies to ameliorate CNS damage should promote both neuronal survival and axon regeneration. The receptor Neogenin promotes neuronal apoptosis. Its ligand prevents death, but the resulting repulsive guidance molecule a (RGMa)-Neogenin interaction also inhibits axonal growth, countering any prosurvival benefits. Here, we explore strategies to inhibit Neogenin, thus simultaneously enhancing survival and regeneration. We show that bone morphogenetic protein (BMP) and RGMa-dependent recruitment of Neogenin into lipid rafts requires an interaction between RGMa and Neogenin subdomains. RGMa or Neogenin peptides that prevent this interaction, BMP inhibition by Noggin, or reduction of membrane cholesterol all block Neogenin raft localization, promote axon outgrowth, and prevent neuronal apoptosis. Blocking Neogenin raft association influences axonal pathfinding, enhances survival in the developing CNS, and promotes survival and regeneration in the injured adult optic nerve and spinal cord. Moreover, lowering cholesterol disrupts rafts and restores locomotor function after spinal cord injury. These data reveal a unified strategy to promote both survival and regeneration in the CNS.
BM88 is a cell-cycle exit and neuronal differentiation protein that has been used as a marker of surviving retinal ganglion cells (RGCs) after optic nerve injury. Thy1.1 has also been used as a marker for RGC loss, but after optic nerve crush (ONC) a decrease in Thy1.1 expression precedes the loss of RGCs. The purpose of this study was to determine if BM88 expression was correlated with RGC loss after ONC and optic nerve transection (ONT) injuries.
Retinal ganglion cells (RGCs) are neurons that relay visual signals from the retina to the brain. The RGC cell bodies reside in the retina and their fibers form the optic nerve. Full transection (axotomy) of the optic nerve is an extra-retinal injury model of RGC degeneration. Optic nerve transection permits time-kinetic studies of neurodegenerative mechanisms in neurons and resident glia of the retina, the early events of which are reported here. One day after injury, and before atrophy of RGC cell bodies was apparent, glia had increased levels of phospho-Akt, phospho-S6, and phospho-ERK1/2; however, these signals were not detected in injured RGCs. Three days after injury there were increased levels of phospho-Rb and cyclin A proteins detected in RGCs, whereas these signals were not detected in glia. DNA hyperploidy was also detected in RGCs, indicative of cell cycle re-entry by these post-mitotic neurons. These events culminated in RGC death, which is delayed by pharmacological inhibition of the MAPK/ERK pathway. Our data show that a remote injury to RGC axons rapidly conveys a signal that activates retinal glia, followed by RGC cell cycle re-entry, DNA hyperploidy, and neuronal death that is delayed by preventing glial MAPK/ERK activation. These results demonstrate that complex and variable neuro-glia interactions regulate healthy and injured states in the adult mammalian retina.
Recent studies have identified anti-apoptotic functions for vascular endothelial growth factor (VEGF) in the central nervous system (CNS). However, VEGF therapy has been hampered by a tendency to promote vascular permeability, edema, and inflammation. Recently, engineered zinc finger proteins (ZFPs) that upregulate multiple forms of VEGF in their natural biological ratios, have been developed to overcome these negative side effects. We used retinal trauma and ischemia models, and a cortical pial strip ischemia model to determine if VEGF upregulating ZFPs are neuroprotective in the adult CNS. Optic nerve transection and ophthalmic artery ligation lead to the apoptotic degeneration of retinal ganglion cells (RGCs) and are, respectively, two highly reproducible models for CNS trauma or ischemia. Adeno-associated vectors (AAV) vectors encoding VEGF-ZFPs (AAV-VEGF-ZFP) significantly increased RGC survival by ?twofold at 14 days after optic nerve transection or ophthalmic artery ligation. Furthermore, AAV-VEGF-ZFP enhanced recovery of the pupillary light reflex. RECA-1 immunostaining demonstrated no appreciable differences between retinas treated with AAV-VEGF-ZFP and controls, suggesting that AAV-VEGF-ZFP treatment did not affect retinal vasculature. Following pial strip of the forelimb motor cortex, brains treated with an adenovirus encoding VEGF ZFPs (AdV-ZFP) showed higher neuronal survival, accelerated wound contraction, and reduced lesion volume between 1 and 6 weeks after injury. Behavioral testing using the cylinder test for vertical exploration showed that AdV-VEGF-ZFP treatment enhanced contralateral forelimb function within the first 2 weeks after injury. Our results indicate that VEGF ZFP therapy is neuroprotective following traumatic injury or stroke in the adult mammalian CNS.
To promote functional recovery after CNS injuries, it is crucial to develop strategies that enhance both neuronal survival and regeneration. Here, we report that caspase-6 is upregulated in injured retinal ganglion cells and that its inhibition promotes both survival and regeneration in these adult CNS neurons. Treatment of rat retinal whole mounts with Z-VEID-FMK, a selective inhibitor of caspase-6, enhanced ganglion cell survival. Moreover, retinal explants treated with this drug extended neurites on myelin. We also show that caspase-6 inhibition resulted in improved ganglion cell survival and robust axonal regeneration following optic nerve injury in adult rats. The effects of Z-VEID-FMK were similar to other caspase inhibitory peptides including Z-LEHD-FMK and Z-VAD-FMK. In searching for downstream effectors for caspase-6, we identified caspase-8, whose expression pattern resembled that of caspase-6 in the injured eye. We then showed that caspase-8 is activated downstream of caspase-6 in the injured adult retina. Furthermore, we investigated the role of caspase-8 in RGC apoptosis and regenerative failure both in vitro and in vivo. We observed that caspase-8 inhibition by Z-IETD-FMK promoted survival and regeneration to an extent similar to that obtained with caspase-6 inhibition. Our results indicate that caspase-6 and caspase-8 are components of a cellular pathway that prevents neuronal survival and regeneration in the adult mammalian CNS.
Retinal ganglion cells (RGCs) are central nervous system (CNS) neurons that transmit visual information from the retina to the brain. Apoptotic RGC degeneration causes visual impairment that can be modeled by optic nerve crush. Neuronal apoptosis is also a salient feature of CNS trauma, ischemia (stroke), and diseases of the CNS such as Alzheimers, Parkinsons, multiple sclerosis, and amyotrophic lateral sclerosis. Optic nerve crush induces the apoptotic cell death of ? 70% of RGCs within the first 14 days after injury. This model is particularly attractive for studying adult neuron apoptosis because the time-course of RGC death is well established and axon regeneration within the myelinated optic nerve can be concurrently evaluated. Here, we performed a large scale iTRAQ proteomic study to identify and quantify proteins of the rat retina at 1, 3, 4, 7, 14, and 21 days after optic nerve crush. In total, 337 proteins were identified, and 110 were differentially regulated after injury. Of these, 58 proteins were upregulated (>1.3 ×), 46 were downregulated (<0.7 ×), and 6 showed both positive and negative regulation over 21 days, relative to normal retinas. Among the differentially expressed proteins, Thymosin-?4 showed an early upregulation at 3 days, the time-point that immediately precedes the induction of RGC apoptosis after injury. We examined the effect of exogenous Thymosin-?4 administration on RGC death after optic nerve injury. Intraocular injections of Thymosin-?4 significantly increased RGC survival by ? 3-fold compared to controls and enhanced axon regeneration after crush, demonstrating therapeutic potential for CNS insults. Overall, our study identified numerous proteins that are differentially regulated at key time-points after optic nerve crush, and how the temporal profiles of their expression parallel RGC death. This data will aid in the future development of novel therapeutics to promote neuronal survival and regeneration in the adult CNS.
Retinal ganglion cells (RGCs) are CNS neurons that output visual information from the retina to the brain, via the optic nerve. The optic nerve can be accessed within the orbit of the eye and completely transected (axotomized), cutting the axons of the entire RGC population. Optic nerve transection is a reproducible model of apoptotic neuronal cell death in the adult CNS (1-4). This model is particularly attractive because the vitreous chamber of the eye acts as a capsule for drug delivery to the retina, permitting experimental manipulations via intraocular injections. The diffusion of chemicals through the vitreous fluid ensures that they act upon the entire RGC population. Viral vectors, plasmids or short interfering RNAs (siRNAs) can also be delivered to the vitreous chamber in order to infect or transfect retinal cells (5-12). The high tropism of Adeno-Associated Virus (AAV) vectors is beneficial to target RGCs, with an infection rate approaching 90% of cells near the injection site (6, 7, 13-15). Moreover, RGCs can be selectively transfected by applying siRNAs, plasmids, or viral vectors to the cut end of the optic nerve (16-19) or injecting vectors into their target the superior colliculus (10). This allows researchers to study apoptotic mechanisms in the injured neuronal population without confounding effects on other bystander neurons or surrounding glia. RGC apoptosis has a characteristic time-course whereby cell death is delayed 3-4 days postaxotomy, after which the cells rapidly degenerate. This provides a window for experimental manipulations directed against pathways involved in apoptosis. Manipulations that directly target RGCs from the transected optic nerve stump are performed at the time of axotomy, immediately after cutting the nerve. In contrast, when substances are delivered via an intraocular route, they can be injected prior to surgery or within the first 3 days after surgery, preceding the initiation of apoptosis in axotomized RGCs. In the present article, we demonstrate several methods for experimental manipulations after optic nerve transection.
Retinal ganglion cells (RGCs) are CNS neurons that output visual information from the retina to the brain, via the optic nerve. The optic nerve can be accessed within the orbit of the eye and completely transected (axotomized), cutting the axons of the entire RGC population. Optic nerve transection is a reproducible model of apoptotic neuronal cell death in the adult CNS (1-4). This model is particularly attractive because the vitreous chamber of the eye acts as a capsule for drug delivery to the retina, permitting experimental manipulations via intraocular injections. The diffusion of chemicals through the vitreous fluid ensures that they act upon the entire RGC population. Moreover, RGCs can be selectively transfected by applying short interfering RNAs (siRNAs), plasmids, or viral vectors to the cut end of the optic nerve (5-7) or injecting vectors into their target, the superior colliculus (8). This allows researchers to study apoptotic mechanisms in the desired neuronal population without confounding effects on other bystander neurons or surrounding glia. An additional benefit is the ease and accuracy with which cell survival can be quantified after injury. The retina is a flat, layered tissue and RGCs are localized in the innermost layer, the ganglion cell layer. The survival of RGCs can be tracked over time by applying a fluorescent tracer (3% Fluorogold) to the cut end of the optic nerve at the time of axotomy, or by injecting the tracer into the superior colliculus (RGC target) one week prior to axotomy. The tracer is retrogradely transported, labeling the entire RGC population. Because the ganglion cell layer is a monolayer (one cell thick), RGC densities can be quantified in flat-mounted tissue, without the need for stereology. Optic nerve transection leads to the apoptotic death of 90% of injured RGCs within 14 days postaxotomy (9-11). RGC apoptosis has a characteristic time-course whereby cell death is delayed 3-4 days postaxotomy, after which the cells rapidly degenerate. This provides a time window for experimental manipulations directed against pathways involved in apoptosis.
Retinal ganglion cell (RGC) degeneration is an important cause of visual impairment, and results in part from microglia-mediated inflammation. Numerous experimental studies have focused on identifying drug targets to rescue these neurons. We recently showed that K(V)1.1 and K(V)1.3 channels are expressed in adult rat RGCs and that siRNA-mediated knockdown of either channel reduces RGC death after optic nerve transection. Earlier we found that K(V)1.3 channels also contribute to microglial activation and neurotoxicity; raising the possibility that these channels contribute to neurodegeneration through direct roles in RGCs and through inflammatory mechanisms. Here, RGC survival was increased by combined siRNA-mediated knockdown of K(V)1.1 and K(V)1.3 in RGCs, but survival was much greater when knockdown of either channel was combined with intraocular injection of a K(V)1.3 channel blocker (agitoxin-2 or margatoxin). After axotomy, increased expression of several inflammation-related molecules preceded RGC loss and, consistent with a dual mechanism, their expression was differentially affected when channel knockdown in RGCs was combined with K(V)1.3 blocker injection. K(V)1.3 blockers reduced activation of retinal microglia and their tight apposition along RGC axon fascicles after axotomy, but did not prevent their migration from the inner plexiform to the damaged ganglion cell layer. Expression of several growth factors increased after axotomy; and again, there were differences following blocker injection compared with RGC-selective channel knockdown. These results provide evidence that K(V)1.3 channels play important roles in apoptotic degeneration of adult RGCs through cell-autonomous mechanisms mediated by channels in the neurons, and nonautonomous mechanisms mediated by microglia and inflammation.
Retinal ischemia is a very useful model to study the impact of various cell death pathways, such as apoptosis and necrosis, in the ischemic retina. However, it is important to note that the retina is formed as an outpouching of the diencephalon and is part of the central nervous system. As such, the cell death pathways initiated in response to ischemic damage in the retina reflect those found in other areas of the central nervous system undergoing similar trauma. The retina is also more accessible than other areas of the central nervous system, thus making it a simpler model to work with and study. By utilizing the retinal model, we can greatly increase our knowledge of the cell death processes initiated by ischemia which lead to degeneration in the central nervous system. This paper examines work that has been done so far to characterize various aspects of cell death in the retinal ischemia model, such as various pathways which are activated, and the role neurotrophic factors, and discusses how these are relevant to the treatment of ischemic damage in both the retina and the greater central nervous system.
Retinal ganglion cell (RGC) degeneration is an important cause of visual impairment and can be modeled by optic nerve transection, which causes the death of 90% of RGCs within 14 days postaxotomy. We performed a proteomic study to identify and quantify proteins in the rat retina after optic nerve transection. Our goal was to isolate potential targets for therapeutic intervention to prevent RGC degeneration.
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