Effects of neurotrophins on the survival and regrowth of injured retinal neurons

Neuroprotection in Glaucoma

Glaucoma is a degenerative optic neuropathy characterized by retinal ganglion cell (RGC) loss and visual field defects. It is known that in some glaucoma patients, death of RGCs continues despite intraocular pressure (IOP) reduction. Neuroprotection in the field of glaucoma is defined as any treatment, independent of IOP reduction, which prevents RGC death. Glutamate antagonists, ginkgo biloba extract, neurotrophic factors, antioxidants, calcium channel blockers, brimonidine, glaucoma medications with blood regulatory effect and nitric oxide synthase inhibitors are among compounds with possible neuroprotective activity in preclinical studies. A few agents (such as brimonidine or memantine) with neuroprotective effects in experimental studies have advanced to clinical trials; however the results of clinical trials for these agents have not been conclusive. Nevertheless, lack of compelling clinical evidence has not prevented the off-label use of some of these compounds in glaucoma practice. Stem cell transplantation has been reported to halt experimental neurodegenerative disease processes in the absence of cell replacement. It has been hypothesized that transplantation of some types of stem cells activates multiple neuroprotective pathways via secretion of various factors. The advantage of this approach is a prolonged and targeted effect. Important concerns in this field include the secretion of unwanted harmful mediators, graft survival issues and tumorigenesis. Neuroprotection in glaucoma, pharmacologically or by stem cell transplantation, is an interesting subject waiting for broad and multidisciplinary collaborative studies to better clarify its role in clinical practice.

Charcot-Marie-Tooth 2b associated Rab7 mutations cause axon growth and guidance defects during vertebrate sensory neuron development

Background

Charcot-Marie-Tooth2b (CMT2b) is an axonal form of a human neurodegenerative disease that preferentially affects sensory neurons. CMT2b is dominantly inherited and is characterized by unusually early onset, presenting in the second or third decade of life. Five missense mutations in the gene encoding Rab7 GTPase have been identified as causative in human CMT2b disease. Although several studies have modeled CMT2b disease in cultured neurons and in Drosophila, the mechanisms by which defective Rab7 leads to disease remain poorly understood.

Results

We used zebrafish to investigate the effects of CMT2b-associated Rab7 mutations in a vertebrate model. We generated transgenic animals expressing the CMT2b-associated mutant forms of Rab7 in sensory neurons, and show that these Rab7 variants cause neurodevelopmental defects, including defects in sensory axon growth, branching and pathfinding at early developmental stages. We also find reduced axon growth and branching in neurons expressing a constitutively active form of Rab7, suggesting these defects may be caused by Rab7 gain-of-function. Further, we use high-speed, high-resolution imaging of endosome transport in vivo and find that CMT2b-associated Rab7 variants cause reduced vesicle speeds, suggesting altered transport may underlie axon development defects.

Conclusions

Our data provide new insight into how disease-associated alterations in Rab7 protein disrupt cellular function in vertebrate sensory neurons. Moreover, our findings suggest that defects in axon development may be a previously unrecognized component of CMT2b disease.

BDNF treatment and extended recovery from optic nerve trauma in the cat

PURPOSE

We examined the treatment period necessary to restore retinal and visual stability following trauma to the optic nerve.

METHODS

Cats received unilateral optic nerve crush and no treatment (NT), treatment of the injured eye with brain-derived neurotrophic factor (BDNF), or treatment of the injured eye combined with treatment of visual cortex for 2 or 4 weeks. After 1-, 2-, 4-, or 6-week survival periods, pattern electroretinograms (PERGs) were obtained and retinal ganglion cell (RGC) survival determined.

RESULTS

In the peripheral retina, RGC survival for NT, eye only, and eye + cortex animals was 55%, 78%, and 92%, respectively, at 1 week, and 31%, 60%, and 93%, respectively, at 2 weeks. PERGs showed a similar pattern of improvement. After 4 weeks, RGC survival was 7%, 29%, and 53% in each group, with PERGs in the dual-treated animals similar to the 1- to 2-week animals. For area centralis (AC), the NT, eye only, and eye + cortex animals showed 47%, 78%, and 82% survival, respectively, at 2 weeks, and 13%, 54%, and 81% survival, respectively, at 4 weeks. Removing the pumps at 2 weeks resulted in ganglion cell survival levels of 76% and 74% in the AC at 4 and 6 weeks postcrush, respectively. The PERGs from 2-week treated, but 4- and 6-week survival animals were comparable to those of the 2-week animals.

CONCLUSIONS

Treating the entire central visual pathway is important following optic nerve trauma. Long-term preservation of central vision may be achieved with as little as 2 weeks of treatment using this approach.

Retinal p75 and bax overexpression is associated with retinal ganglion cells apoptosis in a rat model of glaucoma

PURPOSE

The administration of neurotrophins has been clearly demonstrated to support survival of retina cells during a variety of insults. Increased levels of neurotrophins, such as the nerve growth factor (NGF), have been found in experimental models of glaucoma. Nevertheless, loss of retinal cells does occur in the course of ocular hypertension. Therefore, this study sought to address whether timely changes in NGF and its receptors, trkA(NGFR) and p75(NTR), might explain the progression of retinal damage during experimental glaucoma.

METHODS

A well-characterized technique to induce glaucoma in rats was utilized. The animals were sacrificed after 10, 20 and 35 days from induction of glaucoma. Retinal ganglion cell (RGC) apoptosis, retinal expression of NGF protein as well as Bcl-2, Bax, trkA(NGFR) and p75(NTR) transcript expression were detected. The balance between trkA(NGFR) and p75(NTR) was examined, considering their anti- and pro-apoptotic role in cell death, respectively.

RESULTS

We demonstrated that in our model of experimental glaucoma, the loss of retinal ganglion cells (RGCs) is accompanied by a timely increase of retinal NGF. Moreover, we found that the trkA(NGFR)/p75(NTR) mRNA ratio and the Bcl-2/Bax mRNA were both decreased, indicating a p75(NGFR) and Bax over-expression.

CONCLUSIONS

Retinal NGF is over-expressed in experimental glaucoma, but this NGF increase is not sufficient to support survival of RGCs. The failure of NGF trophic support might be associated with the progressive up-regulation of p75(NTR) in relation to trkA(NGFR).

EphB3: an endogenous mediator of adult axonal plasticity and regrowth after CNS injury

Endogenous mechanisms underlying the remodeling of neuronal circuitry after mammalian CNS injury or disease remain primarily unknown. Here, we investigated axonal plasticity after optic nerve injury and found that macrophages recruited into the injury site and adult retinal ganglion cell (RGC) axons, which undergo injury-induced sprouting and terminal remodeling, were linked by their respective expression of a ligand and receptor pair active in axon guidance. Recruited macrophages specifically upregulated mRNA encoding the guidance molecule EphB3 and expressed EphB proteins capable of binding Ephrin B molecules in vivo and in vitro. Injured adult RGC axons in turn expressed EphrinB3, a known receptor for EphB3, and RGC axons bound recombinant EphB3 protein injected into the optic nerve. In vitro, EphB3 supported adult RGC axon outgrowth, and axons turned toward a source of this guidance molecule. In vivo, both reduction of EphB3 function in adult heterozygous animals and loss of function in homozygous animals greatly decreased RGC axon re-extension or sprouting after optic nerve injury. Comparisons of axon re-extension in EphB3 null and wild-type littermates showed that this loss of axonal plasticity was not attributable to a difference in intrinsic axon growth potential. Rather, the results indicated an essential role for local optic nerve-derived EphB3 in regulating adult RGC axon plasticity after optic nerve injury. Of note, the loss of EphB3 did not affect the ability of injured RGC axons to elaborate complex terminal branching, suggesting that additional EphB3-independent mechanisms governed adult axon branching triggered by CNS damage.

TrkB gene transfer protects retinal ganglion cells from axotomy-induced death in vivo

Injury-induced downregulation of neurotrophin receptors may limit the response of neurons to trophic factors, compromising their ability to survive. We tested this hypothesis in a model of CNS injury: retinal ganglion cell (RGC) death after transection of the adult rat optic nerve. TrkB mRNA rapidly decreased in axotomized RGCs to approximately 50% of the level in intact retinas. TrkB gene transfer into RGCs combined with exogenous BDNF administration markedly increased neuronal survival: 76% of RGCs remained alive at 2 weeks after axotomy, a time when >90% of these neurons are lost without treatment. Activation of mitogen-activated protein kinase, but not phosphatidylinositol-3 kinase, was required for TrkB-induced survival. These data provide proof-of-principle that enhancing the capacity of injured neurons to respond to trophic factors can be an effective neuroprotective strategy in the adult CNS.

Axon damage and repair in multiple sclerosis

It is well known that within long-standing multiple sclerosis (MS) lesions there is axonal loss but whether it is an early or late event has been more difficult to establish. The use of immunocytochemical methods that reveal axonal end-bulbs is a valuable approach to investigating acute axonal injury in human pathological material. The application of these techniques to multiple sclerosis tissue reveals evidence of axonal injury in acute lesions; the distribution of the end-bulbs in acute and active-chronic lesions is associated with regions of maximal density of infiltrating macrophages. Axon injury within the MS lesion will result in both Wallerian degeneration of the axon and also retrograde degeneration of the cell body. The functional consequences of the axon injury will depend upon numbers of axons injured and the topographical organization of the fibres coursing through the lesion. The molecular mechanisms by which the recruited leucocytes damage or transect the axons are not known. However, investigations in the Wld mutant mouse with very slow Wallerian degeneration demonstrate that axon degeneration is not simply a passive disintegration of the axon but has clear parallels with the active processes of programmed cell death. The presence of early axon injury and the consequences of an ever increasing load of neuronal damage has important implications not only for when therapy should be initiated in MS but also the therapeutic target.