Chapter One - Neurotrophic Factors and the Regeneration of Adult Retinal Ganglion Cell Axons
Introduction
The adult central nervous system (CNS) normally has only a limited ability to regenerate axons after injury. There appear to be many reasons for this, including a lack of sufficient trophic support and presence of extrinsic inhibitory factors that limit plasticity, as well as intrinsic changes in neuronal responsiveness that negatively impact regenerative potential. Although direct injury to the optic nerve (ON) in humans is not common, over the past 30 years or so the mammalian retinal ganglion cell (RGC) has been a consistent target of experimenters attempting to understand how best to promote the viability and regenerative capacity of adult neurons after CNS trauma (Benowitz and Yin, 2008, Berry et al., 2008, Chierzi and Fawcett, 2001, Harvey et al., 2006). The retina and ON are embryologically part of the CNS, and RGCs are useful model neurons because they can be directly targeted by injections into the vitreal chamber and the ON is a white matter tract containing oligodendroglia that is accessible and relatively easy to manipulate, either by crush, stretch, or full or partial transection injury. Further, RGC survival and axonal regrowth can readily be quantified, and the connectivity and topographic order of regenerating RGC axons can be assessed using anatomical, physiological, and behavioral methods. RGC axons can also be targeted more centrally, at sites further away from the eye, by performing intracranial ON injuries or by lesioning the optic tract, usually between the thalamus and superior colliculus (SC). In this way, regenerative potential of RGCs can be compared in vivo after proximal versus distal axonal injuries.
The brief given to us for this review was a focus on neurotrophic factors and axonal regeneration. We will discuss a range of diffusible peptides with trophic actions on RGCs, including the classic neurotrophins such as brain-derived neurotrophic factor (BDNF) and neurotrophin-4/5 (NT-4/5), as well as other factors such as ciliary neurotrophic factor (CNTF), leukemia inhibitory factor (LIF), glial cell-derived neurotrophic factor (GDNF), insulin-like growth factor-1 (IGF-1), the fibroblast growth factors (FGFs), and hepatocyte growth factor (HGF). It is, of course, axiomatic that the regrowth of injured axons requires viable parent neurons; thus, the impact of these factors on RGC survival also needs to be considered. In this context, studies aimed at developing therapies to enhance RGC responsiveness after ON trauma have direct ophthalmological relevance because RGC loss is seen in clinical conditions such as glaucoma, retinal ischemia, retinitis pigmentosa, diabetic retinopathy, optic neuritis, multiple sclerosis, and Alzheimer's disease (e.g., Harvey et al., 2006, Johnson et al., 2011, Kern and Barber, 2008 for reviews). Many studies have examined the impact of neurotrophic factors on RGC survival in animal models of these ophthalmic conditions, but while acknowledging their seminal importance, here we focus on how these various factors influence the RGC regenerative process per se. We also briefly comment on signaling pathways recruited by these factors and how they (i) overcome the loss of intrinsic regenerative capacity in adult RGCs and (ii) interact with signals from the extracellular environment to ameliorate growth-inhibitory components of the injured mature CNS.
Section snippets
Trophic Dependence of RGCs During Development
The commonly held view is that RGCs compete for neurotrophic support; those that in some way fail to obtain sufficient support die, resulting in activation of proapoptotic pathways and the loss of over half the RGC population during the developmental period. Thus, early postnatal removal of a central target such as the SC during the period of naturally occurring cell death causes a rapid and massive increase in RGC death, while addition of factors such as BDNF or NT-4/5 results in increased RGC
Axonal Regeneration: Optic Nerve Injuries
The likely sensitivity of maturing RGCs to multiple trophic factors is relevant to adult RGC responses to axotomy because many of these factors are already present within the eye. Even in neonatal rats there is evidence that intraretinally derived neurotrophic factors play a role in maintaining RGC viability (de Araujo and Linden, 1993, Seki et al., 2003, Spalding et al., 2004). In the adult, trophic factors including CNTF, BDNF, basic FGF and GDNF are produced by cells within the retina or ON
Receptors and Miscellaneous Signaling Pathways that Influence Axonal Regeneration
Although this review is focused on neurotrophic factors, the reader should keep in mind that any axogenic effects elicited in injured adult RGCs by these factors should be integrated into what is known about the intracellular signaling pathways that need to be either activated or inactivated to overcome intrinsic (Moore et al., 2011, Sun and He, 2010) or extrinsic (Berry et al., 2008) restrictions on axonal growth. Any impact of endogenous, retinally derived trophic factors is likely to be
Exogenous Neurotrophic Factors and RGC Axonal Regeneration
Neurotrophic factors influence RGC viability and regenerative potential signal via different receptor complexes and signaling cascades, although there is some convergence of these intracellular pathways. These pathways have been reviewed in detail many times, but a brief summary is pertinent here because these cascades relate to other mechanistic studies alluded to in previous paragraphs.
BDNF and NT-4/5 activation of MAPK–ERK and PI3K–AKT pathways results in process outgrowth, cellular
Mode of Delivery: Recombinant Factors
Neurotrophic factors can be delivered to injured RGCs in various ways, either into the vitreal chamber of the eye itself or by application to the injured ON or to central target regions for subsequent retrograde transport by viable projecting neurons. Factors can be injected as a recombinant protein, via nonviral or viral vector-based delivery of an appropriate gene, or by transplantation of cells expressing an appropriate neurotrophic factor.
In general, mature RGCs are less responsive to
Neurotrophic Support and Autologous PN Grafts
As alluded to above, autologous PN grafts containing Schwann cells provide a bridging environment more conducive to the long-distance regeneration of adult RGC axons (Berry et al., 1988, Bray and Aguayo, 1989, Harvey et al., 2006, Heiduschka and Thanos, 2000, Watanabe, 2010). Using this model, it is possible not only to count the number of regrowing RGC axons but also retrogradely label regenerating neurons for morphological characterization and determination of the proportion of viable RGCs
Nonviral Delivery Systems for Neurotrophic Factors
As discussed earlier, bolus injections of relatively high, almost certainly nonphysiological, concentrations of neurotrophic factors into the eye may not necessarily be the optimal approach for eliciting maximal regeneration of RGC axons because of compensatory responses in RGCs and other retinal cells. In addition, it is expensive to give repeat injections of recombinant growth factors, and the injections themselves will initiate inflammatory and other changes in the eye (Cao et al., 2001).
Cellular Delivery of Neurotrophic Factors
An alternate strategy for delivering neurotrophic factors to RGCs over a prolonged period, without the need for repeated intraocular injections, is to implant cells—either as free suspensions or encapsulated in a semipermeable framework—into the vitreal chamber (Zanin et al., 2012). Such an approach has been used in studies targeting photoreceptors (e.g., Emerich and Thanos, 2008, Lawrence et al., 2004, Lund et al., 2003, Sieving et al., 2006). Suspension injections require attention to
Viral Vector Delivery of Neurotrophic Factors
Viral vectors are modified, replication-deficient viruses in which the viral genome is replaced by a therapeutic gene, providing an in vivo method for long-term, targeted supply of a trophic factor to injured neurons, including RGCs (Hellström & Harvey, 2011). Initial studies used a modified adenovirus (AdV) to deliver neurotrophic factors for protective RGC therapy. AdV vectors have been trialed that encode CNTF (van Adel et al., 2005, Weise et al., 2000), BDNF (Di Polo et al., 1998, Isenmann
Combined Gene Therapy and Pharmacotherapy
Previous studies using rCNTF showed that coinjection with the cyclic AMP analogue CPT-cAMP significantly increased the proportion of surviving RGCs capable of regenerating an axon into an autologous PN graft (Cui et al., 2003). This enhancement is mediated by a number of kinase systems including PKA, PI3K/AKT, and MAPK/ERK (Park et al., 2004), and also by moderation of a CNTF-induced increase in SOCS3 expression (Park et al., 2009). Interestingly, CPT-cAMP does not augment RGC regenerative
Administration of Neurotrophic Factors to the Axonal Growth Environment
Provision of neurotrophic support to the somata of injured RGCs clearly enhances their survival and regenerative capacity, the extent of each depending on the factor that is introduced into the eye. Elsewhere in the CNS, some studies have used vectors or pumps to supply neurotrophic factors to the cell bodies of injured neurons, but the majority, especially, for example, those examining therapies for spinal cord injury, apply factors primarily in and around the lesion site. To link this type of
Axonal Regeneration and Optic Tract Injury Studies
Mention was made earlier regarding the location of axotomy relative to the parent cell body, and how this has a profound influence on the ability of an injured adult neuron to regenerate an axon. Most modern-day visual system regeneration studies use the ON, and almost all crush or transect the nerve within 1–1.5 mm of the eye. After injury at greater distances from the eye, including after intracranial ON transection, there may be greater RGC survival but very few if any RGCs regenerate an axon
Indirect Actions of Neurotrophic Factors
Up to this point, we have mostly focused on the actions of exogenously applied neurotrophic factors that are mediated by cognate receptors expressed by the injured RGCs themselves. However, this is an oversimplification: there is considerable evidence that intraocular trophic factor injections activate other cellular constituents in the eye which in turn produce factors that may indirectly contribute to RGC survival and axonal regeneration (e.g., Berry et al., 2008, Cui et al., 2009). For
Conclusions
Exogenous neurotrophic factors have a beneficial effect on the regeneration of adult RGC axons, not only by potentiating the effects of intrinsic growth-promoting programs but also by counteracting at least some of the inhibitory signaling within, as well as extrinsic to, the neuron. Some factors such as CNTF elicit long-distance regeneration and can be applied at the soma or in the vicinity of the regrowing axons themselves, other factors such as BDNF or NT-4/5 elicit mostly terminal sprouting
References (174)
- et al.
ROCK inhibition promotes adult retinal ganglion cell neurite outgrowth only in the presence of growth promoting factors
Molecular and Cellular Neuroscience
(2009) - et al.
Chronic and acute models of retinal neurodegeneration TrkA activity are neuroprotective whereas p75NTR activity is neurotoxic through a paracrine mechanism
The Journal of Biological Chemistry
(2010) - et al.
Rewiring the injured CNS: Lessons from the optic nerve
Experimental Neurology
(2008) - et al.
Optic axons regenerate into sciatic nerve isografts only in the presence of Schwann cells
Brain Research Bulletin
(1988) - et al.
Effects of adeno-associated virus-vectored ciliary neurotrophic factor on retinal structure and function in mice with a P216L rds/peripherin mutation
Experimental Eye Research
(2002) - et al.
Brain-derived neurotrophic factor is an anterograde survival factor in the rat visual system
Current Biology
(2000) - et al.
Development of normal and injury-induced gene expression of aFGF, bFGF, CNTF, BDNF, GFAP and IGF-I in the rat retina
Experimental Eye Research
(2001) - et al.
Brain-derived neurotrophic factor reduces TrkB protein and mRNA in the normal retina and following optic nerve crush in adult rats
Brain Research
(2004) - et al.
Upregulation of ciliary neurotrophic factor in reactive Müller cells in the rat retina following optic nerve transection
Brain Research
(2000) - et al.
The role of macrophages in optic nerve regeneration
Neuroscience
(2009)
Intraocular elevation of cyclic AMP potentiates ciliary neurotrophic factor-induced regeneration of adult rat retinal ganglion cell axons
Molecular and Cellular Neuroscience
The life, death and regenerative ability of immature and mature rat retinal ganglion cells are influenced by their birthdate
Experimental Neurology
Expression of the growth-associated protein GAP-43 in adult rat retinal ganglion cells following axon injury
Neuron
BDNF/trkB signaling in the developmental sculpting of visual connections
Progress in Brain Research
Combined effect of brain-derived neurotrophic factor and LINGO-1 fusion protein on long-term survival of retinal ganglion cells in chronic glaucoma
Neuroscience
Gene therapy and transplantation in CNS repair: The visual system
Progress in Retinal and Eye Research
Restoration of the retinofugal pathway
Progress in Retinal and Eye Research
Negative impact of rAAV2 mediated expression of SOCS3 on the regeneration of adult retinal ganglion cell axons
Molecular and Cellular Neuroscience
Kidins220/ARMS modulates the activity of microtubule-regulating proteins and controls neuronal polarity and development
The Journal of Biological Chemistry
The importance of transgene and cell type on the regeneration of adult retinal ganglion cell axons within reconstituted bridging grafts
Experimental Neurology
Interactive effects of C3, cyclic AMP and ciliary neurotrophic factor on adult retinal ganglion cell survival and axonal regeneration
Molecular and Cellular Neuroscience
Lentiviral-mediated transfer of CNTF to Schwann cells within reconstructed peripheral nerve grafts enhances adult retinal ganglion cell survival and axonal regeneration
Molecular Therapy
Ciliary neurotrophic factor is an axogenesis factor for retinal ganglion cells
Neuroscience
Neurotrophin signal transduction in the nervous system
Current Opinion in Neurobiology
BDNF increases the number of axotomized rat retinal ganglion cells expressing GAP-43, L1, and TAG-1 mRNA—A supportive role for nitric oxide?
Neurobiology of Disease
Neurturin enhances the survival of axotomized retinal ganglion cells in vivo: Combined effects with glial cell line-derived neurotrophic factor and brain-derived neurotrophic factor
Neuroscience
Inhibition of p75(NTR) in glia potentiates TrkA-mediated survival of injured retinal ganglion cells
Molecular and Cellular Neuroscience
Long-term protection of retinal structure but not function using rAAV.CNTF in animal models of retinitis pigmentosa
Molecular Therapy
GDNF, Ret, GFRalpha1 and 2 in the adult rat retino-tectal system after optic nerve transection
Experimental Neurology
The regrowth of axons within tissue defects in the CNS is promoted by implanted hydrogel matrices that contain BDNF and CNTF producing fibroblasts
Experimental Neurology
Schwann cell-derived factor-induced modulation of the NgR/p75NTR/EGFR axis disinhibits axon growth through CNS myelin in vivo and in vitro
Brain
GSK3 beta regulates myelin-dependent axon outgrowth inhibition through CRMP4
The Journal of Neuroscience
Ocular gene therapy: A review of nonviral strategies
Molecular Vision
In glaucoma the upregulated truncated TrkC.T1 receptor isoform in glia causes increased TNF-alpha production, leading to retinal ganglion cell death
Investigative Ophthalmology & Visual Science
Inflammation and axon regeneration
Current Opinion in Neurology
Epidermal growth factor receptor antagonists and CNS regeneration: Mechanisms and controversies
Brain Research Bulletin
Regeneration of axons in the visual system
Restorative Neurology and Neuroscience
Gene therapy for central nervous system repair
Current Opinion in Molecular Therapeutics
Peripheral nerve explants grafted into the vitreous body of the eye promote the regeneration of retinal ganglion cell axons severed in the optic nerve
Journal of Neurocytology
Application of Rho antagonist to neuronal cell bodies promotes neurite growth in compartmented cultures and regeneration of retinal ganglion cell axons in the optic nerve of adult rats
The Journal of Neuroscience
Exploring the capacity of CNS neurons to survive injury, regrow axons and form new synapses in adult mammals
Spontaneous axonal regeneration after optic nerve injury in adult rat
Neuroreport
Engrafted chicken neural tube-derived stem cells support the innate propensity for axonal regeneration within the rat optic nerve
Investigative Ophthalmology & Visual Science
TrkB gene transfer protects retinal ganglion cells from axotomy-induced death in vivo
The Journal of Neuroscience
Regeneration in the mammalian optic nerve
Restorative Neurology and Neuroscience
Sprouting of axon-like processes from axotomized retinal ganglion cells is influenced by the distance of axotomy from the cell body and the mode of transplantation of the peripheral nerve
Restorative Neurology and Neuroscience
Neurotrophic regulation of retinal ganglion cell synaptic connectivity: From axons and dendrites to synapses
The International Journal of Developmental Biology
Synergistic effect of Nogo-neutralizing antibody IN-1 and ciliary neurotrophic factor on axonal regeneration in adult rodent visual systems
Journal of Neurotrauma
NT-4/5 reduces naturally occurring retinal ganglion cell death in neonatal rats
Neuroreport
At least two mechanisms are involved in the death of retinal ganglion cells following target ablation in neonatal rats
The Journal of Neuroscience
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