Optic Nerve Crush in Mice to Study Retinal Ganglion Cell Survival and Regeneration

Small extracellular vesicles derived from human induced pluripotent stem cell-differentiated neural progenitor cells mitigate retinal ganglion cell degeneration in a mouse model of optic nerve injury

JOURNAL/nrgr/04.03/01300535-202502000-00034/figure1/v/2024-05-28T214302Z/r/image-tiff Several studies have found that transplantation of neural progenitor cells (NPCs) promotes the survival of injured neurons. However, a poor integration rate and high risk of tumorigenicity after cell transplantation limits their clinical application. Small extracellular vesicles (sEVs) contain bioactive molecules for neuronal protection and regeneration. Previous studies have shown that stem/progenitor cell-derived sEVs can promote neuronal survival and recovery of neurological function in neurodegenerative eye diseases and other eye diseases. In this study, we intravitreally transplanted sEVs derived from human induced pluripotent stem cells (hiPSCs) and hiPSCs-differentiated NPCs (hiPSC-NPC) in a mouse model of optic nerve crush. Our results show that these intravitreally injected sEVs were ingested by retinal cells, especially those localized in the ganglion cell layer. Treatment with hiPSC-NPC-derived sEVs mitigated optic nerve crush-induced retinal ganglion cell degeneration, and regulated the retinal microenvironment by inhibiting excessive activation of microglia. Component analysis further revealed that hiPSC-NPC derived sEVs transported neuroprotective and anti-inflammatory miRNA cargos to target cells, which had protective effects on RGCs after optic nerve injury. These findings suggest that sEVs derived from hiPSC-NPC are a promising cell-free therapeutic strategy for optic neuropathy.

Comparative analysis of Krüppel-like factors expression in the retinas of zebrafish and mice during development and after injury

The Krüppel-like factors (KLFs) have emerged as important transcriptional regulators of various cellular processes, including neural development. Some of them have been described as intrinsic factors involved in axon regeneration in the central nervous system (CNS) of vertebrates. Zebrafish are known for their ability to regenerate several tissues in adulthood, including the CNS, a capability lost during vertebrate evolution and absent in adult mammals. The role that KLFs could play in this differential ability remains unknown. Therefore, in this study, we analyzed the endogenous response of certain KLFs implicated in axon regeneration (KLFs 6, 7, 9, and 13) during retina development and after axon injury. The results showed that the expression of Klfs 6, 7, and 13 decreases in the developing retina of mice but not in zebrafish, while the mRNA levels of Klf9 strongly increase in both species. The response to injury was further analyzed using optic nerve crush (ONC) as a model of lesion. Our analysis during the acute phase (hours) demonstrated an induction of Klfs 6 and 7 expression exclusively in the zebrafish retina, while Klfs 9 and 13 mRNA levels increased in both species. Further analysis of the chronic response (days) showed that mRNA levels of Klf6 transiently increase in the retinas of both zebrafish and mice, whereas those of Klf7 decrease later after optic nerve injury. In addition, the analysis revealed that the expression of Klf9 decreases, while that of Klf13 increases in the retinas of zebrafish in response to optic nerve injury but remains unaltered in mice. Altogether, these findings support the hypothesis that KLFs may play a role in the differential axon regeneration abilities exhibited by fish and mice.

Inhibition of CRMP2 Phosphorylation Suppresses Microglia Activation in the Retina and Optic Nerve and Promotes Optic Nerve Regeneration After Optic Nerve Injury

As the primary connection between the eye and brain, the optic nerve plays a pivotal role in visual information transmission. Injuries to the optic nerve can occur for various reasons, including trauma, glaucoma, and neurodegenerative diseases. Retinal ganglion cells (RGCs), a type of neurons that extend axons through the optic nerve, can rapidly respond to injury and initiate cell death. Additionally, following optic nerve injury microglia, which serve as markers of neuroinflammation, transition from a resting state to an activated state. The phosphorylation of collapsin response mediator protein2 (CRMP2) in the semaphorin 3A (Sema3A) signalling pathway affects several processes, including axon guidance and neuron regeneration. In this study, we used an optic nerve crush (ONC) mouse model to investigate the effects of suppressing CRMP2 phosphorylation on microglia activation. We found that CRMP2 phosphorylation inhibitor suppressed RGCs loss and promoted neuronal regeneration following ONC. In addition, CRMP2 S522A mutant (CRMP2 KI) mice exhibited decreased microglial activation in both the retina and optic nerve following ONC. These results suggest that inhibiting the phosphorylation of CRMP2 can alleviate the loss of RGCs and microglial activation after optic nerve injury, providing insight into the development of treatments for optical neuropathies and neurodegenerative diseases.

5' Transgenes drive leaky expression of 3' transgenes in Cre-inducible bi-cistronic vectors

Molecular cloning techniques enabling contemporaneous expression of two or more protein-coding sequences provide an invaluable tool for understanding the molecular regulation of cellular functions. The Cre-lox system is used for inducing the expression of recombinant proteins encoded within a bi-/poly-cistronic cassette. However, leak expression of transgenes is often observed in the absence of Cre recombinase activity, compromising the utility of this approach. To investigate the mechanism of leak expression, we generated Cre-inducible bi-cistronic vectors to monitor the expression of transgenes positioned either 5' or 3' of a 2A peptide or internal ribosomal entry site (IRES) sequence. Cells transfected with these bi-cistronic vectors exhibited Cre-independent leak expression specifically of transgenes positioned 3' of the 2A peptide or IRES sequence. Similarly, AAV-FLEX vectors encoding bi-cistronic cassettes or fusion proteins revealed the selective Cre-independent leak expression of transgenes positioned at the 3' end of the open reading frame. Our data demonstrate that 5' transgenes confer promoter-like activity that drives the expression of 3' transgenes. An additional lox-STOP-lox cassette between the 2A sequence and 3' transgene dramatically decreased Cre-independent transgene expression. Our findings highlight the need for appropriate experimental controls when using Cre-inducible bi-/poly-cistronic constructs and inform improved design of vectors for more tightly regulated inducible transgene expression.

Reversal of injury-associated retinal ganglion cell gene expression by a phosphodiesterase anchoring disruptor peptide

Loss of retinal ganglion cells (RGCs) is central to the pathogenesis of optic neuropathies such as glaucoma. Increased RGC cAMP signaling is neuroprotective. We have shown that displacement of the cAMP-specific phosphodiesterase PDE4D3 from an RGC perinuclear compartment by expression of the modified PDE4D3 N-terminal peptide 4D3(E) increases perinuclear cAMP and protein kinase A activity in cultured neurons and in vivo RGC survival after optic nerve crush (ONC) injury. To explore mechanisms by which PDE4D3 displacement promotes neuroprotection, in this study mice intravitreally injected with an adeno-associated virus to express an mCherry-tagged 4D3(E) peptide were subjected to ONC injury and analyzed by single cell RNA-sequencing (scRNA-seq). 4D3(E)-mCherry expression was associated with an attenuation of injury-induced changes in gene expression, thereby supporting the hypothesis that enhanced perinuclear PKA signaling promotes neuroprotective RGC gene expression.

A combination of topical and systemic administration of brimonidine is neuroprotective in the murine optic nerve crush model

Glaucoma is a multifactorial optic neuropathy that primarily affecting retinal ganglion cells (RGC). Brimonidine is an intraocular pressure-lowering drug with reported neuroprotective properties. This study aimed to compare the neuroprotective effects of topical and intraperitoneal (IP) brimonidine on RGCs from different retinal segments in a murine optic nerve crush (ONC) model.

METHODS

forty-one Balb/c mice underwent unilateral ONC and were divided into three study groups: fifteen animals received saline drops twice per day and two additional IP injections of saline; fourteen mice received brimonidine drops twice per day; and 12 mice received brimonidine eye drops twice per day and two additional IP brimonidine injections. Animals were sacrificed seven days post-ONC, and immunohistochemical staining of retinal whole mounts was performed using neuronal NeuN and GFAP staining. Microscopic pictures of the central, middle, and peripheral regions of the retina were taken. The density of the retinal cells was assessed.

RESULTS

The total RGC density after ONC and RGC densities in all retinal eccentricities were significantly higher in the brimonidine eye drop and IP combination treatment group than in the saline drop + saline IP, and brimonidine drop treatment groups.

CONCLUSIONS

brimonidine eye drops supplemented with IP brimonidine injections improved RGC survival in a preclinical model of ONC.

NF1 mutation-driven neuronal hyperexcitability sets a threshold for tumorigenesis and therapeutic targeting of murine optic glioma

BACKGROUND

With the recognition that noncancerous cells function as critical regulators of brain tumor growth, we recently demonstrated that neurons drive low-grade glioma initiation and progression. Using mouse models of neurofibromatosis type 1 (NF1)-associated optic pathway glioma (OPG), we showed that Nf1 mutation induces neuronal hyperexcitability and midkine expression, which activates an immune axis to support tumor growth, such that high-dose lamotrigine treatment reduces Nf1-OPG proliferation. Herein, we execute a series of complementary experiments to address several key knowledge gaps relevant to future clinical translation.

METHODS

We leverage a collection of Nf1-mutant mice that spontaneously develop OPGs to alter both germline and retinal neuron-specific midkine expression. Nf1-mutant mice harboring several different NF1 patient-derived germline mutations were employed to evaluate neuronal excitability and midkine expression. Two distinct Nf1-OPG preclinical mouse models were used to assess lamotrigine effects on tumor progression and growth in vivo.

RESULTS

We establish that neuronal midkine is both necessary and sufficient for Nf1-OPG growth, demonstrating an obligate relationship between germline Nf1 mutation, neuronal excitability, midkine production, and Nf1-OPG proliferation. We show anti-epileptic drug (lamotrigine) specificity in suppressing neuronal midkine production. Relevant to clinical translation, lamotrigine prevents Nf1-OPG progression and suppresses the growth of existing tumors for months following drug cessation. Importantly, lamotrigine abrogates tumor growth in two Nf1-OPG strains using pediatric epilepsy clinical dosing.

CONCLUSIONS

Together, these findings establish midkine and neuronal hyperexcitability as targetable drivers of Nf1-OPG growth and support the use of lamotrigine as a potential chemoprevention or chemotherapy agent for children with NF1-OPG.

Assessment of Brain-Derived Neurotrophic Factor on Retinal Structure and Visual Function in Rodent Models of Optic Nerve Crush

The effects of brain-derived neurotrophic factor (BDNF) on retinal ganglion cell (RGC) survival and visual function were assessed in rat and mouse models of optic nerve (ON) crush. ONs were crushed on Day 1, followed by intravitreal injections of a vehicle or BDNF on Days 1 and 8. The spatial frequency threshold was measured using optokinetic tracking on Days 7 and 14. On Day 15, ganglion cell complex (GCC) thickness was quantified using optical coherence tomography. Furthermore, all eyes were enucleated for immunohistochemical analysis of the surviving RGC somas and axons. BDNF significantly reduced the RGC soma in mice and increased GCC thickness in intact eyes, with apparent axonal swelling in both species. It displayed significantly greater RGC soma survival in eyes with ON injury, with moderately thicker axonal bundles in both species and a thicker GCC in rats. Visual function was significantly reduced in all ON-crushed animals, regardless of BDNF treatment. Thus, we obtained a comprehensive analysis of the structural and functional impact of BDNF in intact and ON-crushed eyes in two rodent models. Our results provide a foundation for further BDNF evaluation and the design of preclinical studies on neuroprotectants using BDNF as a reference positive control.

Cytoprotective Small Compound M109S Attenuated Retinal Ganglion Cell Degeneration Induced by Optic Nerve Crush in Mice

BAX plays an essential role in retinal ganglion cell (RGC) death induced by optic nerve injury. Recently, we developed M109S, an orally bioactive and cytoprotective small compound (CPSC) that inhibits BAX-mediated cell death. We examined whether M109S can protect RGC from optic nerve crush (ONC)-induced apoptosis. M109S was administered starting 5 h after ONC for 7 days. M109S was orally administered in two groups (5 mg/kg twice a day or 7.5 mg/kg once a day). The retina was stained with anti-BRN3A and cleaved Caspase-3 (active Caspase-3) that are the markers of RGC and apoptotic cells, respectively. ONC decreased the number of BRN3A-positive RGC and increased the number of active Caspase-3-expressing apoptotic cells. In ONC-treated retina, there were cells that were double stained with anti-BRN3A and ant-cleaved Caspase-3, indicating that apoptosis in BRN3A-positive RGCs occurred. M109S inhibited the decrease of BRN3A-positive cells whereas it inhibited the increase of active Caspase-3-positive cells in the retina of ONC-treated mice, suggesting that M109S inhibited apoptosis in RGCs. M109S did not induce detectable histological damage to the lungs or kidneys in mice, suggesting that M109S did not show toxicities in the lung or kidneys when the therapeutic dose was used. The present study suggests that M109S is effective in rescuing damaged RGCs. Since M109S is an orally bioactive small compound, M109S may become the basis for a portable patient-friendly medicine that can be used to prevent blindness by rescuing damaged optic nerve cells from death.

Modeling complex age-related eye disease

Modeling complex eye diseases like age-related macular degeneration (AMD) and glaucoma poses significant challenges, since these conditions depend highly on age-related changes that occur over several decades, with many contributing factors remaining unknown. Although both diseases exhibit a relatively high heritability of >50%, a large proportion of individuals carrying AMD- or glaucoma-associated genetic risk variants will never develop these diseases. Furthermore, several environmental and lifestyle factors contribute to and modulate the pathogenesis and progression of AMD and glaucoma. Several strategies replicate the impact of genetic risk variants, pathobiological pathways and environmental and lifestyle factors in AMD and glaucoma in mice and other species. In this review we will primarily discuss the most commonly available mouse models, which have and will likely continue to improve our understanding of the pathobiology of age-related eye diseases. Uncertainties persist whether small animal models can truly recapitulate disease progression and vision loss in patients, raising doubts regarding their usefulness when testing novel gene or drug therapies. We will elaborate on concerns that relate to shorter lifespan, body size and allometries, lack of macula and a true lamina cribrosa, as well as absence and sequence disparities of certain genes and differences in their chromosomal location in mice. Since biological, rather than chronological, age likely predisposes an organism for both glaucoma and AMD, more rapidly aging organisms like small rodents may open up possibilities that will make research of these diseases more timely and financially feasible. On the other hand, due to the above-mentioned anatomical and physiological features, as well as pharmacokinetic and -dynamic differences small animal models are not ideal to study the natural progression of vision loss or the efficacy and safety of novel therapies. In this context, we will also discuss the advantages and pitfalls of alternative models that include larger species, such as non-human primates and rabbits, patient-derived retinal organoids, and human organ donor eyes.

CD38 Deficiency Protects Mouse Retinal Ganglion Cells Through Activating the NAD+/Sirt1 Pathway in Ischemia-Reperfusion and Optic Nerve Crush Models

Purpose

The purpose of this study was to investigate the protective effect of CD38 deletion on retinal ganglion cells (RGCs) in a mouse retinal ischemia/reperfusion (I/R) model and an optic nerve crush (ONC) model, and to elucidate the underlying molecular mechanisms.

Methods

Retinal I/R and ONC models were constructed in mice. PCR was used to identify the deletion of CD38 gene in mice, hematoxylin and eosin (H&E) staining was used to evaluate the changes in retinal morphology, and electroretinogram (ERG) was used to evaluate the changes in retinal function. The survival of RGCs and activation of retinal macroglia were evaluated by immunofluorescence staining. The expression of Sirt1, CD38, Ac-p65, Ac-p53, TNF-α, IL-1β, and Caspase3 proteins in the retina was further evaluated by protein imprinting.

Results

In retinal I/R and ONC models, CD38 deficiency reduced the loss of RGCs and activation of macroglia and protected the retinal function. CD38 deficiency increased the concentration of NAD+, reduced the degree of acetylation of NF-κB p65 and p53, and reduced expression of the downstream inflammatory cytokines TNFα, IL-1β, and apoptotic protein Caspase3 in the retina in the ONC model. Intraperitoneal injection of the Sirt1 inhibitor EX-527 partially counteracted the effects of CD38 deficiency, suggesting that CD38 deficiency acts at least in part through the NAD+/Sirt1 pathway.

Conclusions

CD38 plays an important role in the pathogenesis of retinal I/R and ONC injury. CD38 deletion protects RGCs by attenuating inflammatory responses and apoptosis through the NAD+/Sirt1 pathway.

NFATc4 Knockout Promotes Neuroprotection and Retinal Ganglion Cell Regeneration After Optic Nerve Injury

Retinal ganglion cells (RGCs), neurons transmitting visual information via the optic nerve, fail to regenerate their axons after injury. The progressive loss of RGC function underlies the pathophysiology of glaucoma and other optic neuropathies, often leading to irreversible blindness. Therefore, there is an urgent need to identify the regulators of RGC survival and the regenerative program. In this study, we investigated the role of the family of transcription factors known as nuclear factor of activated T cells (NFAT), which are expressed in the retina; however, their role in RGC survival after injury is unknown. Using the optic nerve crush (ONC) model, widely employed to study optic neuropathies and central nervous system axon injury, we found that NFATc4 is specifically but transiently up-regulated in response to mechanical injury. In the injured retina, NFATc4 immunolocalized primarily to the ganglionic cell layer. Utilizing NFATc4-/- and NFATc3-/- mice, we demonstrated that NFATc4, but not NFATc3, knockout increased RGC survival, improved retina function, and delayed axonal degeneration. Microarray screening data, along with decreased immunostaining of cleaved caspase-3, revealed that NFATc4 knockout was protective against ONC-induced degeneration by suppressing pro-apoptotic signaling. Finally, we used lentiviral-mediated NFATc4 delivery to the retina of NFATc4-/- mice and reversed the pro-survival effect of NFATc4 knockout, conclusively linking the enhanced survival of injured RGCs to NFATc4-dependent mechanisms. In summary, this study is the first to demonstrate that NFATc4 knockout may confer transient RGC neuroprotection and decelerate axonal degeneration after injury, providing a potent therapeutic strategy for optic neuropathies.

Longitudinal imaging of vitreal hyperreflective foci in mice with acute optic nerve damage using visible-light optical coherence tomography

Hyperreflective foci (HRFs) appear in optical coherence tomography (OCT) images of the retina and vitreous of patients with various ocular diseases. HRFs are hypothesized to be immune cells that appear in response to ischemia or tissue damage. To accurately identify HRFs and establish their clinical significance, it is necessary to replicate the detection of similar patterns in vivo in a small animal model. We combined visible-light OCT with temporal speckle averaging (TSA) to visualize and track vitreal HRFs (VHRFs) densities for three days after an optic nerve crush (ONC) injury. Resulting vis-OCT images revealed that VHRF density significantly increased approximately 10-fold at 12 h after ONC and returned to baseline three days after ONC. Additional immunohistochemistry results confirmed these VHRFs as inflammatory cells induced from optic nerve damage.

Ca2+/Calmodulin-Dependent Protein Kinase II Enhances Retinal Ganglion Cell Survival But Suppresses Axon Regeneration after Optic Nerve Injury

Neuroprotection after injury or in neurodegenerative disease remains a major goal for basic and translational neuroscience. Retinal ganglion cells (RGCs), the projection neurons of the eye, degenerate in optic neuropathies after axon injury, and there are no clinical therapies to prevent their loss or restore their connectivity to targets in the brain. Here we demonstrate a profound neuroprotective effect of the exogenous expression of various Ca2+/calmodulin-dependent protein kinase II (CaMKII) isoforms in mice. A dramatic increase in RGC survival following the optic nerve trauma was elicited by the expression of constitutively active variants of multiple CaMKII isoforms in RGCs using adeno-associated viral (AAV) vectors across a 100-fold range of AAV dosing in vivo. Despite this neuroprotection, however, short-distance RGC axon sprouting was suppressed by CaMKII, and long-distance axon regeneration elicited by several pro-axon growth treatments was likewise inhibited even as CaMKII further enhanced RGC survival. Notably, in a dose-escalation study, AAV-expressed CaMKII was more potent for axon growth suppression than the promotion of survival. That diffuse overexpression of constitutively active CaMKII strongly promotes RGC survival after axon injury may be clinically valuable for neuroprotection per se. However, the associated strong suppression of the optic nerve axon regeneration demonstrates the need for understanding the intracellular domain- and target-specific CaMKII activities to the development of CaMKII signaling pathway-directed strategies for the treatment of optic neuropathies.

Brain injury drives optic glioma formation through neuron-glia signaling

Tissue injury and tumorigenesis share many cellular and molecular features, including immune cell (T cells, monocytes) infiltration and inflammatory factor (cytokines, chemokines) elaboration. Their common pathobiology raises the intriguing possibility that brain injury could create a tissue microenvironment permissive for tumor formation. Leveraging several murine models of the Neurofibromatosis type 1 (NF1) cancer predisposition syndrome and two experimental methods of brain injury, we demonstrate that both optic nerve crush and diffuse traumatic brain injury induce optic glioma (OPG) formation in mice harboring Nf1-deficient preneoplastic progenitors. We further elucidate the underlying molecular and cellular mechanisms, whereby glutamate released from damaged neurons stimulates IL-1β release by oligodendrocytes to induce microglia expression of Ccl5, a growth factor critical for Nf1-OPG formation. Interruption of this cellular circuit using glutamate receptor, IL-1β or Ccl5 inhibitors abrogates injury-induced glioma progression, thus establishing a causative relationship between injury and tumorigenesis.

A molecular switch for neuroprotective astrocyte reactivity

The intrinsic mechanisms that regulate neurotoxic versus neuroprotective astrocyte phenotypes and their effects on central nervous system degeneration and repair remain poorly understood. Here we show that injured white matter astrocytes differentiate into two distinct C3-positive and C3-negative reactive populations, previously simplified as neurotoxic (A1) and neuroprotective (A2)1,2, which can be further subdivided into unique subpopulations defined by proliferation and differential gene expression signatures. We find the balance of neurotoxic versus neuroprotective astrocytes is regulated by discrete pools of compartmented cyclic adenosine monophosphate derived from soluble adenylyl cyclase and show that proliferating neuroprotective astrocytes inhibit microglial activation and downstream neurotoxic astrocyte differentiation to promote retinal ganglion cell survival. Finally, we report a new, therapeutically tractable viral vector to specifically target optic nerve head astrocytes and show that raising nuclear or depleting cytoplasmic cyclic AMP in reactive astrocytes inhibits deleterious microglial or macrophage cell activation and promotes retinal ganglion cell survival after optic nerve injury. Thus, soluble adenylyl cyclase and compartmented, nuclear- and cytoplasmic-localized cyclic adenosine monophosphate in reactive astrocytes act as a molecular switch for neuroprotective astrocyte reactivity that can be targeted to inhibit microglial activation and neurotoxic astrocyte differentiation to therapeutic effect. These data expand on and define new reactive astrocyte subtypes and represent a step towards the development of gliotherapeutics for the treatment of glaucoma and other optic neuropathies.

Glaucoma Animal Models beyond Chronic IOP Increase

Glaucoma is a complex and multifactorial disease defined as the loss of retinal ganglion cells (RGCs) and their axons. Besides an elevated intraocular pressure (IOP), other mechanisms play a pivotal role in glaucoma onset and progression. For example, it is known that excitotoxicity, immunological alterations, ischemia, and oxidative stress contribute to the neurodegeneration in glaucoma disease. To study these effects and to discover novel therapeutic approaches, appropriate animal models are needed. In this review, we focus on various glaucoma animal models beyond an elevated IOP. We introduce genetically modified mice, e.g., the optineurin E50K knock-in or the glutamate aspartate transporter (GLAST)-deficient mouse. Excitotoxicity can be mimicked by injecting the glutamate analogue N-methyl-D-aspartate intravitreally, which leads to rapid RGC degeneration. To explore the contribution of the immune system, the experimental autoimmune glaucoma model can serve as a useful tool. Here, immunization with antigens led to glaucoma-like damage. The ischemic mechanism can be mimicked by inducing a high IOP for a certain amount of time in rodents, followed by reperfusion. Thereby, damage to the retina and the optic nerve occurs rapidly after ischemia/reperfusion. Lastly, we discuss the importance of optic nerve crush models as model systems for normal-tension glaucoma. In summary, various glaucoma models beyond IOP increase can be utilized.

The concept of gene therapy for glaucoma: the dream that has not come true yet

Gene therapies, despite of being a relatively new therapeutic approach, have a potential to become an important alternative to current treatment strategies in glaucoma. Since glaucoma is not considered a single gene disease, the identified goals of gene therapy would be rather to provide neuroprotection of retinal ganglion cells, especially, in intraocular-pressure-independent manner. The most commonly reported type of vector for gene delivery in glaucoma studies is adeno-associated virus serotype 2 that has a high tropism to retinal ganglion cells, resulting in long-term expression and low immunogenic profile. The gene therapy studies recruit inducible and genetic animal models of optic neuropathy, like DBA/2J mice model of high-tension glaucoma and the optic nerve crush-model. Reported gene therapy-based neuroprotection of retinal ganglion cells is targeting specific genes translating to growth factors (i.e., brain derived neurotrophic factor, and its receptor TrkB), regulation of apoptosis and neurodegeneration (i.e., Bcl-xl, Xiap, FAS system, nicotinamide mononucleotide adenylyl transferase 2, Digit3 and Sarm1), immunomodulation (i.e., Crry, C3 complement), modulation of neuroinflammation (i.e., erythropoietin), reduction of excitotoxicity (i.e., CamKIIα) and transcription regulation (i.e., Max, Nrf2). On the other hand, some of gene therapy studies focus on lowering intraocular pressure, by impacting genes involved in both, decreasing aqueous humor production (i.e., aquaporin 1), and increasing outflow facility (i.e., COX2, prostaglandin F2α receptor, RhoA/RhoA kinase signaling pathway, MMP1, Myocilin). The goal of this review is to summarize the current state-of-art and the direction of development of gene therapy strategies for glaucomatous neuropathy.

Total recall: the role of PIDDosome components in neurodegeneration

The PIDDosome is a multiprotein complex that includes p53-induced protein with a death domain 1 (PIDD1), receptor-interacting protein-associated ICH-1/CED-3 homologous protein with a death domain (RAIDD), and caspase-2, the activation of which is driven by PIDDosome assembly. In addition to the key role of the PIDDosome in the regulation of cell differentiation, tissue homeostasis, and organogenesis and regeneration, caspase-2, RAIDD and PIDD1 engagement in neuronal development was shown. Here, we focus on the involvement of PIDDosome components in neurodegenerative disorders, including retinal neuropathies, different types of brain damage, and Alzheimer's disease (AD), Huntington's disease (HD), and Lewy body disease. We also discuss pathogenic variants of PIDD1, RAIDD, and caspase-2 that are associated with intellectual, behavioral, and psychological abnormalities, together with prospective PIDDosome inhibition strategies and their potential clinical application.

Phosphorylated S6K1 and 4E-BP1 play different roles in constitutively active Rheb-mediated retinal ganglion cell survival and axon regeneration after optic nerve injury

Ras homolog enriched in brain (Rheb) is a small GTPase that activates mammalian target of rapamycin complex 1 (mTORC1). Previous studies have shown that constitutively active Rheb can enhance the regeneration of sensory axons after spinal cord injury by activating downstream effectors of mTOR. S6K1 and 4E-BP1 are important downstream effectors of mTORC1. In this study, we investigated the role of Rheb/mTOR and its downstream effectors S6K1 and 4E-BP1 in the protection of retinal ganglion cells. We transfected an optic nerve crush mouse model with adeno-associated viral 2-mediated constitutively active Rheb and observed the effects on retinal ganglion cell survival and axon regeneration. We found that overexpression of constitutively active Rheb promoted survival of retinal ganglion cells in the acute (14 days) and chronic (21 and 42 days) stages of injury. We also found that either co-expression of the dominant-negative S6K1 mutant or the constitutively active 4E-BP1 mutant together with constitutively active Rheb markedly inhibited axon regeneration of retinal ganglion cells. This suggests that mTORC1-mediated S6K1 activation and 4E-BP1 inhibition were necessary components for constitutively active Rheb-induced axon regeneration. However, only S6K1 activation, but not 4E-BP1 knockdown, induced axon regeneration when applied alone. Furthermore, S6K1 activation promoted the survival of retinal ganglion cells at 14 days post-injury, whereas 4E-BP1 knockdown unexpectedly slightly decreased the survival of retinal ganglion cells at 14 days post-injury. Overexpression of constitutively active 4E-BP1 increased the survival of retinal ganglion cells at 14 days post-injury. Likewise, co-expressing constitutively active Rheb and constitutively active 4E-BP1 markedly increased the survival of retinal ganglion cells compared with overexpression of constitutively active Rheb alone at 14 days post-injury. These findings indicate that functional 4E-BP1 and S6K1 are neuroprotective and that 4E-BP1 may exert protective effects through a pathway at least partially independent of Rheb/mTOR. Together, our results show that constitutively active Rheb promotes the survival of retinal ganglion cells and axon regeneration through modulating S6K1 and 4E-BP1 activity. Phosphorylated S6K1 and 4E-BP1 promote axon regeneration but play an antagonistic role in the survival of retinal ganglion cells.

Glycerophospholipid Analysis of Optic Nerve Regeneration Models Indicate Potential Membrane Order Changes Associated with the Lipidomic Shifts

Purpose: Optic nerve (ON) injury causes irreversible degeneration, leading to vision loss that cannot be restored with available therapeutics. Current therapies slow further degeneration but do not promote regeneration. New regenerative factors have been discovered that are successful in vivo. However, the mechanisms of efficient long-distance regeneration are still unknown. Membrane expansion by lipid insertion is an essential regenerative process, so lipid profiles for regenerating axons can provide insight into growth mechanisms. This article's analysis aims to add to the increasingly available ON regeneration lipid profiles and relate it to membrane order/properties. Methods: In this study, we present an analysis of glycerophospholipids, one of the largest axonal lipid groups, from three mammalian ON regeneration lipid profiles: Wnt3a, Zymosan + CPT-cAMP, and Phosphatase/Tensin homolog knockout (PTENKO) at 7 and 14 days post crush (dpc). Significant lipid classes, species, and ontological properties were crossreferenced between treatments and analyzed using Metaboanalyst 5.0 and Lipid Ontology (LION). Membrane order changes associated with significant lipid classes were evaluated by C-Laurdan dye and exogenous lipids provided to a neuroblastoma cell line. Results and Conclusions: At 7 dpc, ONs show increased lysoglycerophospholipids and decreased phosphatidylethanolamines (PEs)/negative intrinsic curvature lipids. At 14 dpc, regenerative treatments show divergence: Wnt3a displays higher lysoglycerophospholipid content, while Zymosan and PTENKO decrease lysoglycerophospholipids and increase phosphatidylcholine (PC)-related species. Membrane order imaging indicates lysoglycerophospholipids decreases membrane order while PE and PC had no significant membrane order effects. Understanding these changes will allow therapeutic development targeting lipid metabolic pathways that can be used for vision loss treatments.

Electrical stimulation alters DNA methylation and promotes neurite outgrowth

Electrical stimulation (ES) influences neural regeneration and functionality. We here investigate whether ES regulates DNA demethylation, a critical epigenetic event known to influence nerve regeneration. Retinal ganglion cells (RGCs) have long served as a standard model for central nervous system neurons, whose growth and disease development are reportedly affected by DNA methylation. The current study focuses on the ability of ES to rescue RGCs and preserve vision by modulating DNA demethylation. To evaluate DNA demethylation pattern during development, RGCs from mice at different stages of development, were analyzed using qPCR for ten-eleven translocation (TETs) and immunostained for 5 hydroxymethylcytosine (5hmc) and 5 methylcytosine (5mc). To understand the effect of ES on neurite outgrowth and DNA demethylation, cells were subjected to ES at 75 µAmp biphasic ramp for 20 min and cultured for 5 days. ES increased TETs mediated neurite outgrowth, DNA demethylation, TET1 and growth associated protein 43 levels significantly. Immunostaining of PC12 cells following ES for histone 3 lysine 9 trimethylation showed cells attained an antiheterochromatin configuration. Cultured mouse and human retinal explants stained with β-III tubulin exhibited increased neurite growth following ES. Finally, mice subjected to optic nerve crush injury followed by ES exhibited improved RGCs function and phenotype as validated using electroretinogram and immunohistochemistry. Our results point to a possible therapeutic regulation of DNA demethylation by ES in neurons.

Retinal ganglion cell repopulation for vision restoration in optic neuropathy: a roadmap from the RReSTORe Consortium

Retinal ganglion cell (RGC) death in glaucoma and other optic neuropathies results in irreversible vision loss due to the mammalian central nervous system's limited regenerative capacity. RGC repopulation is a promising therapeutic approach to reverse vision loss from optic neuropathies if the newly introduced neurons can reestablish functional retinal and thalamic circuits. In theory, RGCs might be repopulated through the transplantation of stem cell-derived neurons or via the induction of endogenous transdifferentiation. The RGC Repopulation, Stem Cell Transplantation, and Optic Nerve Regeneration (RReSTORe) Consortium was established to address the challenges associated with the therapeutic repair of the visual pathway in optic neuropathy. In 2022, the RReSTORe Consortium initiated ongoing international collaborative discussions to advance the RGC repopulation field and has identified five critical areas of focus: (1) RGC development and differentiation, (2) Transplantation methods and models, (3) RGC survival, maturation, and host interactions, (4) Inner retinal wiring, and (5) Eye-to-brain connectivity. Here, we discuss the most pertinent questions and challenges that exist on the path to clinical translation and suggest experimental directions to propel this work going forward. Using these five subtopic discussion groups (SDGs) as a framework, we suggest multidisciplinary approaches to restore the diseased visual pathway by leveraging groundbreaking insights from developmental neuroscience, stem cell biology, molecular biology, optical imaging, animal models of optic neuropathy, immunology & immunotolerance, neuropathology & neuroprotection, materials science & biomedical engineering, and regenerative neuroscience. While significant hurdles remain, the RReSTORe Consortium's efforts provide a comprehensive roadmap for advancing the RGC repopulation field and hold potential for transformative progress in restoring vision in patients suffering from optic neuropathies.

The use of Nicotinamide and Nicotinamide riboside as an adjunct therapy in the treatment of glaucoma

Glaucoma is an optic neuropathy characterized by death of retinal ganglion cells (RGCs), which leads to progressive visual field loss and may result in blindness. Currently, the only available treatment to avoid or delay progression in glaucoma patients is to decrease intraocular pressure (IOP). However, despite adequate IOP control, approximately 25% of the patients continue to progress. To delay or prevent optic nerve damage in glaucoma, two forms of vitamin B3, nicotinamide (NAM) and nicotinamide riboside (NR) are emerging as viable adjuvant therapies. These compounds are nicotinamide adenine dinucleotide (NAD) precursors. NAD is essential for proper cell functioning and is involved in several metabolic activities, including protection against reactive oxygen species, contribution to the performance of various enzymes, and maintenance of mitochondrial function. Due to its beneficial effects and to the evidence of the reduction of NAD bioavailability with aging, researchers are seeking ways to replenish the cellular NAD pool, by administrating its precursors (NAM and NR), believing that it will reduce the RGC vulnerability to external stressors, such as increased IOP. This article attempts to analyze the current knowledge regarding the use of NAM and NR for the prevention and/or treatment of glaucoma.

Intravitreal injection of Huperzine A promotes retinal ganglion cells survival and axonal regeneration after optic nerve crush

Traumatic optic neuropathy (TON) is a condition that causes massive loss of retinal ganglion cells (RGCs) and their axonal fibers, leading to visual insufficiency. Several intrinsic and external factors can limit the regenerative ability of RGC after TON, subsequently resulting in RGC death. Hence, it is important to investigate a potential drug that can protect RGC after TON and enhance its regenerative capacity. Herein, we investigated whether Huperzine A (HupA), extracted from a Chinese herb, has neuroprotective effects and may enhance neuronal regeneration following the optic nerve crush (ONC) model. We compared the three modes of drug delivery and found that intravitreal injection of HupA could promote RGC survival and axonal regeneration after ONC. Mechanistically, HupA exerted its neuroprotective and axonal regenerative effects through the mTOR pathway; these effects could be blocked by rapamycin. To sum up, our findings suggest a promising application of HupA in the clinical treatment of traumatic optic nerve.

Aldose reductase inhibition decelerates optic nerve degeneration by alleviating retinal microglia activation

As part of the central nervous system (CNS), retinal ganglion cells (RGCs) and their axons are the only neurons in the retina that transmit visual signals from the eye to the brain via the optic nerve (ON). Unfortunately, they do not regenerate upon injury in mammals. In ON trauma, retinal microglia (RMG) become activated, inducing inflammatory responses and resulting in axon degeneration and RGC loss. Since aldose reductase (AR) is an inflammatory response mediator highly expressed in RMG, we investigated if pharmacological inhibition of AR can attenuate ocular inflammation and thereby promote RGC survival and axon regeneration after ON crush (ONC). In vitro, we discovered that Sorbinil, an AR inhibitor, attenuates BV2 microglia activation and migration in the lipopolysaccharide (LPS) and monocyte chemoattractant protein-1 (MCP-1) treatments. In vivo, Sorbinil suppressed ONC-induced Iba1 + microglia/macrophage infiltration in the retina and ON and promoted RGC survival. Moreover, Sorbinil restored RGC function and delayed axon degeneration one week after ONC. RNA sequencing data revealed that Sorbinil protects the retina from ONC-induced degeneration by suppressing inflammatory signaling. In summary, we report the first study demonstrating that AR inhibition transiently protects RGC and axon from degeneration, providing a potential therapeutic strategy for optic neuropathies.

Defining Selective Neuronal Resilience and Identifying Targets for Neuroprotection and Axon Regeneration Using Single-Cell RNA Sequencing: Experimental Approaches

A prevalent feature among neurodegenerative conditions, including axonal injury, is that certain neuronal types are disproportionately affected, while others are more resilient. Identifying molecular features that separate resilient from susceptible populations could reveal potential targets for neuroprotection and axon regeneration. A powerful approach to resolve molecular differences across cell types is single-cell RNA-sequencing (scRNA-seq). scRNA-seq is a robustly scalable approach that enables the parallel sampling of gene expression across many individual cells. Here we present a systematic framework to apply scRNA-seq to track neuronal survival and gene expression changes following axonal injury. Our methods utilize the mouse retina because it is an experimentally accessible central nervous system tissue and its cell types have been comprehensively characterized by scRNA-seq. This chapter will focus on preparing retinal ganglion cells (RGCs) for scRNA-seq and pre-processing of sequencing results.

Assessment of outer retinal thickness and function in mice after experimental optic nerve trauma

BACKGROUND

Optic nerve trauma caused by crush injury is frequently used for investigating experimental treatments that protect retinal ganglion cells (RGCs) and induce axonal regrowth. Retaining outer retinal light responses is essential for therapeutic rescue of RGCs after injury. However, whether optic nerve crush also damages the structure or function of photoreceptors has not been systematically investigated. In this study, we investigated whether outer retinal thickness and visual function are altered by optic nerve crush in the mouse.

METHODS

Wildtype mice underwent optic nerve crush and intravitreal injection of a control solution in one eye with the fellow eye remaining uninjured. Two weeks after injury, the thickness of the ganglion cell region (GCL to IPL) and photoreceptor layer (bottom of the OPL to top of the RPE) were measured using OCT. Retinal function was assessed using flash ERGs. Immunodetection of RGCs was performed on retinal cryosections and RGCs and ONL nuclei rows were counted. Multiple comparison analyses were conducted using Analysis of Variance (ANOVA) with Tukey's post hoc test and P values less than 0.05 were considered statistically significant.

RESULTS

Optic nerve crush injury induced RGC death as expected, demonstrated by thinning of the ganglion cell region and RGC loss. In contrast, outer retinal thickness, photopic and scotopic a-wave and b-wave amplitudes and photoreceptor nuclei counts, were equivalent between injured and uninjured eyes.

CONCLUSIONS

Secondary degeneration of the outer retina was not detected after optic nerve injury in the presence of significant RGC death, suggesting that the retina has the capacity to compartmentalize damage. These findings also indicate that experimental treatments to preserve the GCL and rescue vision using this optic nerve injury model would not require additional strategies to preserve the ONL.

Influences of Glaucoma on the Structure and Function of Synapses in the Visual System

Significance: Glaucoma is an age-related neurodegenerative disorder of the visual system associated with sensitivity to intraocular pressure (IOP). It is the leading irreversible cause of vision loss worldwide, and vision loss results from damage and dysfunction of the retinal output neurons known as retinal ganglion cells (RGCs). Recent Advances: Elevated IOP and optic nerve injury triggers pruning of RGC dendrites, altered morphology of excitatory inputs from presynaptic bipolar cells, and disrupted RGC synaptic function. Less is known about RGC outputs, although evidence to date indicates that glaucoma is associated with altered mitochondrial and synaptic structure and function in RGC-projection targets in the brain. These early functional changes likely contribute to vision loss and might be a window into early diagnosis and treatment. Critical Issues: Glaucoma affects different RGC populations to varying extents and along distinct time courses. The influence of glaucoma on RGC synaptic function as well as the mechanisms underlying these effects remain to be determined. Since RGCs are an especially energetically demanding population of neurons, altered intracellular axon transport of mitochondria and mitochondrial function might contribute to RGC synaptic dysfunction in the retina and brain as well as RGC vulnerability in glaucoma. Future Directions: The mechanisms underlying differential RGC vulnerability remain to be determined. Moreover, the timing and mechanisms of RGCs synaptic dysfunction and degeneration will provide valuable insight into the disease process in glaucoma. Future work will be able to capitalize on these findings to better design diagnostic and therapeutic approaches to detect disease and prevent vision loss. Antioxid. Redox Signal. 37, 842-861.

Surgical Menopause Impairs Retinal Conductivity and Worsens Prognosis in an Acute Model of Rat Optic Neuropathy

Deficiency of estradiol during the menopausal period is an important risk factor for neurodegenerative diseases, including various optic neuropathies. The aim of this study was to evaluate the impact of surgical menopause on the function and survival ratio of RGCs in the rat model of ONC (optic nerve crush). We used eight-week-old female Long Evans rats, divided into two main groups depending on the time between ovariectomy procedure (OVA) and euthanasia (two weeks vs. seven weeks), and subgroups—OVA, OVA + ONC, or ONC. Retinal function was assessed with electroretinography (ERG). RGC loss ratio was evaluated using immunolabelling and counting of RGCs. Seven weeks after OVA, the menopause morphologically affected interneurons but not RGC; however, when the ONC procedure was applied, RGCs appeared to be more susceptible to damage in case of deprivation of estrogens. In our analysis, PhNR (photopic negative responses) were severely diminished in the OVA + ONC group. A deprivation of estrogens in menopause results in accelerated retinal neurodegeneration that firstly involves retinal interneurons. The lack of estrogens increases the susceptibility of RGCs to insults.

Optic nerve injury models under varying forces

PURPOSE

To explore the pathological changes in optic nerve injury models under varying forces.

METHODS

The rats were classified into 4 groups: sham operation (SH), 0.1, 0.3, and 0.5 N. Modeling was performed using the lateral optic nerve pulling method. Seven days after modeling, Brn3a immunofluorescence was used to detect retinal ganglion cell (RGC) number, terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining was used to detect RGC apoptosis, and flash visual evoked potential (FVEP) was used to detect the optic nerve function on days 1, 3, and 7 after modeling. In addition, LC3 II and P62 expression levels in retinal tissues were detected by western blotting to observe the changes in autophagy levels.

RESULTS

RGC number decreased 7 d after modeling, and it showed a downward trend with increasing damaging force. The number of apoptotic RGCs in ganglion cell layer in the 0.3 and 0.5 N groups was increased and was higher than that in the 0.1 N group. The difference in FVEP of rats in each group was mainly reflected in the P2 peak latency. LC3 II and P62 expression levels in retinal tissue of 0.3 and 0.5 N groups were higher than those of the SH and 0.1 groups; however, the difference between the 0.1 N and SH groups was not statistically significant.

CONCLUSION

Precisely controlling the force of the optic nerve clamping injury model is necessary because different forces acting on the optic nerve will lead to differences in the loss of optic neurons, the conduction function of the optic nerve, and autophagy level in retinal tissues.

Dark Rearing in the Visual Critical Period Causes Structural Changes in Myelinated Axons in the Adult Mouse Visual Pathway

An appropriate sensory experience during the early developmental period is important for brain maturation. Dark rearing during the visual critical period delays the maturation of neuronal circuits in the visual cortex. Although the formation and structural plasticity of the myelin sheaths on retinal ganglion cell axons modulate the visual function, the effects of dark rearing during the visual critical period on the structure of the retinal ganglion cell axons and their myelin sheaths are still unclear. To address this question, mice were reared in a dark box during the visual critical period and then normally reared to adulthood. We found that myelin sheaths on the retinal ganglion cell axons of dark-reared mice were thicker than those of normally reared mice in both the optic chiasm and optic nerve. Furthermore, whole-mount immunostaining with fluorescent axonal labeling and tissue clearing revealed that the myelin internodal length in dark-reared mice was shorter than that in normally reared mice in both the optic chiasm and optic nerve. These findings demonstrate that dark rearing during the visual critical period affects the morphology of myelin sheaths, shortens and thickens myelin sheaths in the visual pathway, despite the mice being reared in normal light/dark conditions after the dark rearing.

Quantitative BONCAT Allows Identification of Newly Synthesized Proteins after Optic Nerve Injury

Retinal ganglion cells (RGCs) die after optic nerve trauma or in degenerative disease. However, acute changes in protein expression that may regulate RGC response to injury are not fully understood, and detailed methods to quantify new protein synthesis have not been tested. Here, we develop and apply a new in vivo quantitative measure of newly synthesized proteins to examine changes occurring in the retina after optic nerve injury. Azidohomoalanine, a noncanonical amino acid, was injected intravitreally into the eyes of rodents of either sex with or without optic nerve injury. Isotope variants of biotin-alkyne were used for quantitative BONCAT (QBONCAT) mass spectrometry, allowing identification of protein synthesis and transport rate changes in more than 1000 proteins at 1 or 5 d after optic nerve injury. In vitro screening showed several newly synthesized proteins regulate axon outgrowth in primary neurons in vitro This novel approach to targeted quantification of newly synthesized proteins after injury uncovers a dynamic translational response within broader proteostasis regulation and enhances our understanding of the cellular response to injury.SIGNIFICANCE STATEMENT Optic nerve injury results in death and degeneration of retinal ganglion cells and their axons. The specific cellular response to injury, including changes in new protein synthesis, is obscured by existing proteins and protein degradation. In this study, we introduce QBONCAT to isolate and quantify acute protein synthesis and subsequent transport between cellular compartments. We identify novel candidate protein effectors of the regenerative response and uncover their regulation of axon growth in vitro, validating the utility of QBONCAT for the discovery of novel regulatory and therapeutic candidates after optic nerve injury.

Intraocular Delivery of a Collagen Mimetic Peptide Repairs Retinal Ganglion Cell Axons in Chronic and Acute Injury Models

Vision loss through the degeneration of retinal ganglion cell (RGC) axons occurs in both chronic and acute conditions that target the optic nerve. These include glaucoma, in which sensitivity to intraocular pressure (IOP) causes early RGC axonal dysfunction, and optic nerve trauma, which causes rapid axon degeneration from the site of injury. In each case, degeneration is irreversible, necessitating new therapeutics that protect, repair, and regenerate RGC axons. Recently, we demonstrated the reparative capacity of using collagen mimetic peptides (CMPs) to heal fragmented collagen in the neuronal extracellular milieu. This was an important step in the development of neuronal-based therapies since neurodegeneration involves matrix metalloproteinase (MMP)-mediated remodeling of the collagen-rich environment in which neurons and their axons exist. We found that intraocular delivery of a CMP comprising single-strand fractions of triple helix human type I collagen prevented early RGC axon dysfunction in an inducible glaucoma model. Additionally, CMPs also promoted neurite outgrowth from dorsal root ganglia, challenged in vitro by partial digestion of collagen. Here, we compared the ability of a CMP sequence to protect RGC axons in both inducible glaucoma and optic nerve crush. A three-week +40% elevation in IOP caused a 67% degradation in anterograde transport to the superior colliculus, the primary retinal projection target in rodents. We found that a single intravitreal injection of CMP during the period of IOP elevation significantly reduced this degradation. The same CMP delivered shortly after optic nerve crush promoted significant axonal recovery during the two-week period following injury. Together, these findings support a novel protective and reparative role for the use of CMPs in both chronic and acute conditions affecting the survival of RGC axons in the optic projection to the brain.

Neuroinflammation, Microglia and Implications for Retinal Ganglion Cell Survival and Axon Regeneration in Traumatic Optic Neuropathy

Traumatic optic neuropathy (TON) refers to a pathological condition caused by a direct or indirect insult to the optic nerves, which often leads to a partial or permanent vision deficit due to the massive loss of retinal ganglion cells (RGCs) and their axonal fibers. Retinal microglia are immune-competent cells residing in the retina. In rodent models of optic nerve crush (ONC) injury, resident retinal microglia gradually become activated, form end-to-end alignments in the vicinity of degenerating RGC axons, and actively internalized them. Some activated microglia adopt an amoeboid morphology that engulf dying RGCs after ONC. In the injured optic nerve, the activated microglia contribute to the myelin debris clearance at the lesion site. However, phagocytic capacity of resident retinal microglia is extremely poor and therefore the clearance of cellular and myelin debris is largely ineffective. The presence of growth-inhibitory myelin debris and glial scar formed by reactive astrocytes inhibit the regeneration of RGC axons, which accounts for the poor visual function recovery in patients with TON. In this Review, we summarize the current understanding of resident retinal microglia in RGC survival and axon regeneration after ONC. Resident retinal microglia play a key role in facilitating Wallerian degeneration and the subsequent axon regeneration after ONC. However, they are also responsible for producing pro-inflammatory cytokines, chemokines, and reactive oxygen species that possess neurotoxic effects on RGCs. Intraocular inflammation triggers a massive influx of blood-borne myeloid cells which produce oncomodulin to promote RGC survival and axon regeneration. However, intraocular inflammation induces chronic neuroinflammation which exacerbates secondary tissue damages and limits visual function recovery after ONC. Activated retinal microglia is required for the proliferation of oligodendrocyte precursor cells (OPCs); however, sustained activation of retinal microglia suppress the differentiation of OPCs into mature oligodendrocytes for remyelination after injury. Collectively, controlled activation of retinal microglia and infiltrating myeloid cells facilitate axon regeneration and nerve repair. Recent advance in single-cell RNA-sequencing and identification of microglia-specific markers could improve our understanding on microglial biology and to facilitate the development of novel therapeutic strategies aiming to switch resident retinal microglia’s phenotype to foster neuroprotection.

An optimized procedure to record visual evoked potential in mice

Visual evoked potential (VEP) is commonly used to evaluate visual acuity in both clinical and basic studies. Subdermal needle electrodes or skull pre-implanted screw electrodes are usually used to record VEP in rodents. However, the VEP amplitudes recorded by the former are small while the latter may damage the brain. In this study, we established a new invasive procedure for VEP recording, and made a series of comparisons of VEP parameters recorded from different electrode locations, different times of day (day and night) and bilateral eyes, to evaluate the influence of these factors on VEP in mice. Our data reveal that our invasive method is reliable and can record VEP with good waveforms and large amplitudes. The comparison data show that VEP is greatly influenced by active electrode locations and difference between day and night. In C57 or CD1 ONC (optic nerve crush) models and Brn3b^AP/AP mice, which are featured by loss of retinal ganglion cells (RGCs), amplitudes of VEP N1 and P1 waves are drastically reduced. The newly established VEP procedure is very reliable and stable, and is particularly useful for detecting losses of RGC quantities, functions or connections to the brain. Our analyses of various recording conditions also provide useful references for future studies.

Effects of Hydrostatic Pressure on Electrical Retinal Activity in a Multielectrode Array-Based ex vivo Glaucoma Acute Model

Glaucoma is a heterogeneous eye disease causing atrophy of the optic nerve head (ONH). The optic nerve is formed by the axons of the retinal ganglion cells (RGCs) that transmit visual input to the brain. The progressive RGC loss during glaucoma leads to irreversible vision loss. An elevated intraocular pressure (IOP) is described as main risk factor in glaucoma. In this study, a multielectrode array (MEA)-based ex vivo glaucoma acute model was established and the effects of hydrostatic pressure (10, 30, 60, and 90 mmHg) on the functionality and survival of adult male and female wild-type mouse (C57BL/6) retinae were investigated. Spontaneous activity, response rate to electrical and light stimulation, and bursting behavior of RGCs was analyzed prior, during, and after pressure stress. No pressure related effects on spontaneous firing and on the response rate of the RGCs were observed. Even a high pressure level (90 mmHg for 2 h) did not disturb the RGC functionality. However, the cells’ bursting behavior significantly changed under 90 mmHg. The number of spikes in bursts doubled during pressure application and stayed on a high level after pressure stress. Addition of the amino sulfonic acid taurine (1 mM) showed a counteracting effect. OFF ganglion cells did not reveal an increase in bursts under pressure stress. Live/dead staining after pressure application showed no significant changes in RGC survival. The findings of our ex vivo model suggest that RGCs are tolerant toward high, short-time pressure stress.

Retinal Ganglion Cell Axon Regeneration Requires Complement and Myeloid Cell Activity within the Optic Nerve

Axon regenerative failure in the mature CNS contributes to functional deficits following many traumatic injuries, ischemic injuries, and neurodegenerative diseases. The complement cascade of the innate immune system responds to pathogen threat through inflammatory cell activation, pathogen opsonization, and pathogen lysis, and complement is also involved in CNS development, neuroplasticity, injury, and disease. Here, we investigated the involvement of the classical complement cascade and microglia/monocytes in CNS repair using the mouse optic nerve injury (ONI) model, in which axons arising from retinal ganglion cells (RGCs) are disrupted. We report that central complement C3 protein and mRNA, classical complement C1q protein and mRNA, and microglia/monocyte phagocytic complement receptor CR3 all increase in response to ONI, especially within the optic nerve itself. Importantly, genetic deletion of C1q, C3, or CR3 attenuates RGC axon regeneration induced by several distinct methods, with minimal effects on RGC survival. Local injections of C1q function-blocking antibody revealed that complement acts primarily within the optic nerve, not retina, to support regeneration. Moreover, C1q opsonizes and CR3+ microglia/monocytes phagocytose growth-inhibitory myelin debris after ONI, a likely mechanism through which complement and myeloid cells support axon regeneration. Collectively, these results indicate that local optic nerve complement-myeloid phagocytic signaling is required for CNS axon regrowth, emphasizing the axonal compartment and highlighting a beneficial neuroimmune role for complement and microglia/monocytes in CNS repair.SIGNIFICANCE STATEMENT Despite the importance of achieving axon regeneration after CNS injury and the inevitability of inflammation after such injury, the contributions of complement and microglia to CNS axon regeneration are largely unknown. Whereas inflammation is commonly thought to exacerbate the effects of CNS injury, we find that complement proteins C1q and C3 and microglia/monocyte phagocytic complement receptor CR3 are each required for retinal ganglion cell axon regeneration through the injured mouse optic nerve. Also, whereas studies of optic nerve regeneration generally focus on the retina, we show that the regeneration-relevant role of complement and microglia/monocytes likely involves myelin phagocytosis within the optic nerve. Thus, our results point to the importance of the innate immune response for CNS repair.

Posttranslational Modification of Sox11 Regulates RGC Survival and Axon Regeneration

The failure of adult CNS neurons to survive and regenerate their axons after injury or in neurodegenerative disease remains a major target for basic and clinical neuroscience. Recent data demonstrated in the adult mouse that exogenous expression of Sry-related high-mobility-box 11 (Sox11) promotes optic nerve regeneration after optic nerve injury but exacerbates the death of a subset of retinal ganglion cells (RGCs), α-RGCs. During development, Sox11 is required for RGC differentiation from retinal progenitor cells (RPCs), and we found that mutation of a single residue to prevent SUMOylation at lysine 91 (K91) increased Sox11 nuclear localization and RGC differentiation in vitro. Here, we explored whether this Sox11 manipulation similarly has stronger effects on RGC survival and optic nerve regeneration. In vitro, we found that non-SUMOylatable Sox11^K91A leads to RGC death and suppresses axon outgrowth in primary neurons. We furthermore found that Sox11^K91A more strongly promotes axon regeneration but also increases RGC death after optic nerve injury in vivo in the adult mouse. RNA sequence (RNA-seq) data showed that Sox11 and Sox11^K91A increase the expression of key signaling pathway genes associated with axon growth and regeneration but downregulated Spp1 and Opn4 expression in RGC cultures, consistent with negatively regulating the survival of α-RGCs and ipRGCs. Thus, Sox11 and its SUMOylation site at K91 regulate gene expression, survival and axon growth in RGCs, and may be explored further as potential regenerative therapies for optic neuropathy.

MEF2 transcription factors differentially contribute to retinal ganglion cell loss after optic nerve injury

Loss of retinal ganglion cells (RGCs) in optic neuropathies results in permanent partial or complete blindness. Myocyte enhancer factor 2 (MEF2) transcription factors have been shown to play a pivotal role in neuronal systems, and in particular MEF2A knockout was shown to enhance RGC survival after optic nerve crush injury. Here we expanded these prior data to study bi-allelic, tri-allelic and heterozygous allele deletion. We observed that deletion of all MEF2A, MEF2C, and MEF2D alleles had no effect on RGC survival during development. Our extended experiments suggest that the majority of the neuroprotective effect was conferred by complete deletion of MEF2A but that MEF2D knockout, although not sufficient to increase RGC survival on its own, increased the positive effect of MEF2A knockout. Conversely, MEF2A over-expression in wildtype mice worsened RGC survival after optic nerve crush. Interestingly, MEF2 transcription factors are regulated by post-translational modification, including by calcineurin-catalyzed dephosphorylation of MEF2A Ser-408 known to increase MEF2A-dependent transactivation in neurons. However, neither phospho-mimetic nor phospho-ablative mutation of MEF2A Ser-408 affected the ability of MEF2A to promote RGC death in vivo after optic nerve injury. Together these findings demonstrate that MEF2 gene expression opposes RGC survival following axon injury in a complex hierarchy, and further support the hypothesis that loss of or interference with MEF2A expression might be beneficial for RGC neuroprotection in diseases such as glaucoma and other optic neuropathies.