Changes in retinal ganglion cell axons after optic nerve crush: neurofilament expression is not the sole determinant of calibre

Interleukin-4 protects retinal ganglion cells and promotes axon regeneration

BACKGROUND

The preservation of retinal ganglion cells (RGCs) and the facilitation of axon regeneration are crucial considerations in the management of various vision-threatening disorders. Therefore, we investigate the efficacy of interleukin-4 (IL-4), a potential therapeutic agent, in promoting neuroprotection and axon regeneration of retinal ganglion cells (RGCs) as identified through whole transcriptome sequencing in an in vitro axon growth model.

METHODS

A low concentration of staurosporine (STS) was employed to induce in vitro axon growth. Whole transcriptome sequencing was utilized to identify key target factors involved in the molecular mechanism underlying axon growth. The efficacy of recombinant IL-4 protein on promoting RGC axon growth was validated through in vitro experiments. The protective effect of recombinant IL-4 protein on somas of RGCs was assessed using RBPMS-specific immunofluorescent staining in mouse models with optic nerve crush (ONC) and N-methyl-D-aspartic acid (NMDA) injury. The protective effect on RGC axons was evaluated by anterograde labeling of cholera toxin subunit B (CTB), while the promotion of RGC axon regeneration was assessed through both anterograde labeling of CTB and immunofluorescent staining for growth associated protein-43 (GAP43).

RESULTS

Whole-transcriptome sequencing of staurosporine-treated 661 W cells revealed a significant upregulation in intracellular IL-4 transcription levels during the process of axon regeneration. In vitro experiments demonstrated that recombinant IL-4 protein effectively stimulated axon outgrowth. Subsequent immunostaining with RBPMS revealed a significantly higher survival rate of RGCs in the rIL-4 group compared to the vehicle group in both NMDA and ONC injury models. Axonal tracing with CTB confirmed that recombinant IL-4 protein preserved long-distance projection of RGC axons, and there was a notably higher number of surviving axons in the rIL-4 group compared to the vehicle group following NMDA-induced injury. Moreover, intravitreal delivery of recombinant IL-4 protein substantially facilitated RGC axon regeneration after ONC injury.

CONCLUSION

The recombinant IL-4 protein exhibits the potential to enhance the survival rate of RGCs, protect RGC axons against NMDA-induced injury, and facilitate axon regeneration following ONC. This study provides an experimental foundation for further investigation and development of therapeutic agents aimed at protecting the optic nerve and promoting axon regeneration.

Progression of optic atrophy in traumatic optic neuropathy: retrograde neuronal degeneration in humans

OBJECTIVE

We used optical coherence tomography (OCT) to document the time course of retrograde neuronal degeneration following indirect optic nerve injury.

METHODS

We retrospectively studied patients diagnosed with unilateral indirect traumatic optic neuropathy (TON). Patients with total or near-total optic atrophy were included. All patients underwent complete ophthalmological examinations, including OCT imaging, within 1 day and at 1, 2, 3, 4, 6, 8, 12, 24, and 48 weeks after trauma.

RESULTS

The mean thicknesses of the circumpapillary retinal nerve fiber layer (cpRNFL) and macular retinal ganglion cell-inner plexiform layer (mGCIPL) decreased significantly at 2 weeks after trauma (p = 0.027 and p = 0.043). Changes in mGCIPL thickness preceded changes in cpRNFL thickness. The rates of reduction in mGCIPL and cpRNFL thicknesses were greatest between 2 to 4 weeks and 4 to 6 weeks after trauma. The reduction in mGCIPL thickness then slowed, and stabilized at 12 weeks after trauma. The proportions of cpRNFL and mGCIPL losses at 2, 4, 6, 8, and 12 weeks compared to 24 weeks were 17.1, 33.7, 59.8, 77.9, and 87.9% and 30.0, 73.3, 76.1, 88.3, and 97.9%, respectively.

CONCLUSIONS

OCT revealed optic atrophy progression 2 weeks after trauma, which was most rapid from 2 to 6 weeks, and then gradually stabilized. Loss of retinal ganglion cell bodies and dendrites seemed to precede the axonal degeneration. Observations of morphological changes in retinal layers using OCT in TON patients improve our understanding of retrograde neuronal degeneration of the central nervous system.

Longitudinal Morphological and Functional Assessment of RGC Neurodegeneration After Optic Nerve Crush in Mouse

The mouse optic nerve crush (ONC) model has been widely used to study optic neuropathies and central nervous system (CNS) axon injury and repair. Previous histological studies of retinal ganglion cell (RGC) somata in retina and axons in ON demonstrate significant neurodegeneration after ONC, but longitudinal morphological and functional assessment of RGCs in living animals is lacking. It is essential to establish these assays to provide more clinically relevant information for early detection and monitoring the progression of CNS neurodegeneration. Here, we present in vivo data gathered by scanning laser ophthalmoscopy (SLO), optical coherence tomography (OCT), and pattern electroretinogram (PERG) at different time points after ONC in mouse eyes and corresponding histological quantification of the RGC somata and axons. Not surprisingly, direct visualization of RGCs by SLO fundus imaging correlated best with histological quantification of RGC somata and axons. Unexpectedly, OCT did not detect obvious retinal thinning until late time points (14 and 28-days post ONC) and instead detected significant retinal swelling at early time points (1–5 days post-ONC), indicating a characteristic initial retinal response to ON injury. PERG also demonstrated an early RGC functional deficit in response to ONC, before significant RGC death, suggesting that it is highly sensitive to ONC. However, the limited progression of PERG deficits diminished its usefulness as a reliable indicator of RGC degeneration.

Inducible rodent models of glaucoma

Glaucoma is one of the leading causes of vision impairment worldwide. In order to further understand the molecular pathobiology of this disease and to develop better therapies, clinically relevant animal models are necessary. In recent years, both the rat and mouse have become popular models in glaucoma research. Key reasons are: many important biological similarities shared among rodent eyes and the human eye; development of improved methods to induce glaucoma and to evaluate glaucomatous damage; availability of genetic tools in the mouse; as well as the relatively low cost of rodent studies. Commonly studied rat and mouse glaucoma models include intraocular pressure (IOP)-dependent and pressure-independent models. The pressure-dependent models address the most important risk factor of elevated IOP, whereas the pressure-independent models assess "normal tension" glaucoma and other "non-IOP" related factors associated with glaucomatous damage. The current article provides descriptions of these models, their characterizations, specific techniques to induce glaucoma, mechanisms of injury, advantages, and limitations.

Aquaporins in the eye: expression, function, and roles in ocular disease

BACKGROUND

All thirteen known mammalian aquaporins have been detected in the eye. Moreover, aquaporins have been identified as playing essential roles in ocular functions ranging from maintenance of lens and corneal transparency to production of aqueous humor to maintenance of cellular homeostasis and regulation of signal transduction in the retina.

SCOPE OF REVIEW

This review summarizes the expression and known functions of ocular aquaporins and discusses their known and potential roles in ocular diseases.

MAJOR CONCLUSIONS

Aquaporins play essential roles in all ocular tissues. Remarkably, not all aquaporin function as a water permeable channel and the functions of many aquaporins in ocular tissues remain unknown. Given their vital roles in maintaining ocular function and their roles in disease, aquaporins represent potential targets for future therapeutic development.

GENERAL SIGNIFICANCE

Since aquaporins play key roles in ocular physiology, an understanding of these functions is important to improving ocular health and treating diseases of the eye. It is likely that future therapies for ocular diseases will rely on modulation of aquaporin expression and/or function. This article is part of a Special Issue entitled Aquaporins.

Relative course of retinal nerve fiber layer birefringence and thickness and retinal function changes after optic nerve transection

PURPOSE

To test the hypothesis that alterations of RNFL birefringence precede changes in RNFL thickness in an experimental model of RGC injury and, secondarily, to determine the time course of RGC functional abnormalities relative to RNFL birefringence and thickness changes.

METHODS

RNFL birefringence was measured by scanning laser polarimetry (GDx VCC; Carl Zeiss Meditec, Inc., Dublin, CA). RNFL thickness was measured by spectral domain optical coherence tomography (SD-OCT, Spectralis HRA+OCT; Heidelberg Engineering, GmbH, Heidelberg, Germany). Retinal function was assessed by three forms of electroretinography (ERG): slow-sequence multifocal (mf)ERG (VERIS; EDI, San Mateo, CA); pattern-reversal (P)ERG (Utas-E3000; LKC Technologies, Inc. Gaithersburg, MD); and photopic full-field flash (ff)ERG (Utas-E3000; LKC Technologies). All measurements were obtained in both eyes of four adult rhesus macaque monkeys (Macaca mulatta) during two baseline sessions, and again 1 week and 2 weeks after unilateral optic nerve transection (ONT).

RESULTS

ONT was successfully completed in three subjects. RNFL birefringence declined by 15% 1 week after ONT (P = 0.043), whereas there was no significant change in RNFL thickness (+1%, P = 0.42). Two weeks after ONT, RNFL retardance had declined by 39% (P = 0.018), whereas RNFL thickness had declined by only 15% (P = 0.025). RGC functional abnormalities were present 1 week after ONT, including decreased amplitudes relative to baseline of the mfERG high-frequency components (-65%, P = 0.018), the PERG N95 component (-70%, P = 0.007), and the photopic negative response of the ffERG (-44%, P = 0.005).

CONCLUSIONS

RNFL birefringence declined before and faster than RNFL thickness after ONT. RGC functional abnormalities were present 1 week after ONT, when RNFL thickness had not yet begun to change. RNFL birefringence changes after acute RGC injury are associated with RGC dysfunction. Together, they reflect RGC abnormalities that precede axonal caliber changes and loss.

Rodent models for glaucoma retinopathy and optic neuropathy

Animal models are useful to elucidate the etiology and pathology of glaucoma and to develop novel and more effective therapies for the disease. Because of the substantial similarities between the rodent and primate eyes, and the advances of relevant study techniques, rat and mouse models of glaucoma have recently become popular as research tools. This review surveys research techniques used in the measurement of rodent intraocular pressure, and also the evaluation of pertinent morphologic, biochemical, and functional changes in the retina, optic nerve head, and optic nerve. This review further describes in detail the individual rodent models, some of which serve as surrogate models and do not entail ocular hypertension, whereas others involve transient or chronic increases of intraocular pressure. The technical considerations and theoretical concerns of these models, their advantages, and limitations, are also discussed.

Rat retinal ganglion cell loss caused by kainate, NMDA and ischemia correlates with a reduction in mRNA and protein of Thy-1 and neurofilament light

Quantification of retinal ganglion cell (RGC) loss/survival following a defined insult to the retina is a prerequisite in order to allow a comparison to be made between the effectiveness of potential neuroprotective drugs. The purpose of the present study was to extend the characterisation of our previously published semiquantitative RT-PCR assay to assess RGC loss/survival. Comparisons were made between the total mRNA levels of the ganglion cell-specific markers Thy-1 and neurofilament light (NF-L) in the retina at specific times after an intravitreal injection of N-methyl-D-aspartate (NMDA) or kainate or after 45 min of ischemia/reperfusion and also between the levels of NF-L mRNA and protein at various times after NMDA injection. Changes in Thy-1 and NF-L immunoreactivities were also observed. NMDA, kainate and ischemia/reperfusion all caused a reduction in the retinal content of Thy-1 and NF-L mRNAs and immunoreactivities. An excellent correlation was observed between the levels of the two mRNAs after these treatments. After NMDA, loss of NF-L mRNA was shown to precede loss of NF-L protein but total loss of each marker was similar after 7 days. The results of the study demonstrate that injury and subsequent death of RGCs, which occurs after ischemia/reperfusion and after intraocular injection of NMDA or kainate, can be followed by measurement of total retinal levels of Thy-1 and NF-L mRNAs and NF-L protein. The assays provides accurate, practical and complementary methods for assessing the potential benefits of neuroprotective drugs on RGCs which have been injured by a variety of experimental modalities.

Tubulin expression and axonal transport in injured and regenerating neurons in the adult mammalian central nervous system

Microtubules are essential components of the cytoskeleton required for axonal growth. To investigate how changes in tubulin transport and expression may affect axon regeneration, injury in the adult mammalian central nervous system was studied. Axotomized retinal ganglion cells (RGCs) that do not regenerate were compared with RGCs that regenerate their axons when the optic nerve is replaced with a peripheral nerve graft. When RGC axons regenerated through peripheral nerve grafts, the rate of slow transport increased but decreased when no regrowth occurred. To investigate the molecular mechanisms that mediate these responses, alterations in tubulin mRNA levels after injury were examined. Total tubulin mRNA levels fell after injury in the optic nerve but increased in those RGCs that regenerated their axons into a peripheral nerve graft. Further, the expression of four separate beta-tubulin isotypes in injured rat RGCs was characterized. mRNA levels for all four isotypes decreased in RGCs after injury in the optic nerve. How the autoregulation of tubulin expression may contribute to the changes in beta-tubulin isotype expression after injury is discussed.