Similarities and differences between primary and secondary degeneration of the optic nerve and the effect of minocycline

Secondary Degeneration Impairs Myelin Ultrastructural Development in Adulthood following Adolescent Neurotrauma in the Rat Optic Nerve

Adolescence is a critical period of postnatal development characterized by social, emotional, and cognitive changes. These changes are increasingly understood to depend on white matter development. White matter is highly vulnerable to the effects of injury, including secondary degeneration in regions adjacent to the primary injury site which alters the myelin ultrastructure. However, the impact of such alterations on adolescent white matter maturation is yet to be investigated. To address this, female piebald-virol-glaxo rats underwent partial transection of the optic nerve during early adolescence (postnatal day (PND) 56) with tissue collection two weeks (PND 70) or three months later (PND 140). Axons and myelin in the transmission electron micrographs of tissue adjacent to the injury were classified and measured based on the appearance of the myelin laminae. Injury in adolescence impaired the myelin structure in adulthood, resulting in a lower percentage of axons with compact myelin and a higher percentage of axons with severe myelin decompaction. Myelin thickness did not increase as expected into adulthood after injury and the relationship between the axon diameter and myelin thickness in adulthood was altered. Notably, dysmyelination was not observed 2 weeks postinjury. In conclusion, injury in adolescence altered the developmental trajectory, resulting in impaired myelin maturation when assessed at the ultrastructural level in adulthood.

Gadd45 in Neuronal Development, Function, and Injury

The growth arrest and DNA damage-inducible (Gadd) 45 proteins have been associated with numerous cellular mechanisms including cell cycle control, DNA damage sensation and repair, genotoxic stress, neoplasia, and molecular epigenetics. The genes were originally identified in in vitro screens of irradiation- and interleukin-induced transcription and have since been implicated in a host of normal and aberrant central nervous system processes. These include early and postnatal development, injury, cancer, memory, aging, and neurodegenerative and psychiatric disease states. The proteins act through a variety of molecular signaling cascades including the MAPK cascade, cell cycle control mechanisms, histone regulation, and epigenetic DNA demethylation. In this review, we provide a comprehensive discussion of the literature implicating each of the three members of the Gadd45 family in these processes.

Insulin Signaling as a Therapeutic Target in Glaucomatous Neurodegeneration

Glaucoma is a multifactorial disease that is conventionally managed with treatments to lower intraocular pressure (IOP). Despite these efforts, many patients continue to lose their vision. The degeneration of retinal ganglion cells (RGCs) and their axons in the optic tract that characterizes glaucoma is similar to neurodegeneration in other age-related disorders of the central nervous system (CNS). Identifying the different molecular signaling pathways that contribute to early neuronal dysfunction can be utilized for neuroprotective strategies that prevent degeneration. The discovery of insulin and its receptor in the CNS and retina led to exploration of the role of insulin signaling in the CNS. Historically, insulin was considered a peripherally secreted hormone that regulated glucose homeostasis, with no obvious roles in the CNS. However, a growing number of pre-clinical and clinical studies have demonstrated the potential of modulating insulin signaling in the treatment of neurodegenerative diseases. This review will highlight the role that insulin signaling plays in RGC neurodegeneration. We will focus on how this pathway can be therapeutically targeted to promote RGC axon survival and preserve vision.

Dimethyl Fumarate Promotes the Survival of Retinal Ganglion Cells after Optic Nerve Injury, Possibly through the Nrf2/HO-1 Pathway

This study aimed to verify whether dimethyl fumarate (DMF) promotes the survival of retinal ganglion cells (RGCs) after optic nerve crush (ONC) accompanied by activation of the NF-E2-related factor 2 (Nrf2)/heme oxygenase-1 (HO-1) pathway. We examined changes in the densities of tubulin β3 (TUBB3)-positive RGCs and the amplitudes of the positive scotopic threshold response (pSTR), reflecting the functional activity of RGCs, recorded on an electroretinogram, with daily administration of DMF, on day 7 after ONC. Furthermore, immunohistochemical and immunoblotting analyses were performed to study the activation of the Nrf2/HO-1 pathway using retinas treated with daily administration of DMF. Daily administration of DMF increasedthe density of TUBB3-positive RGCs in a dose-dependent fashion and significantly increased the amplitude of the pSTR. Immunohistochemical analysis showed that DMF administration increased the immunoreactivity for Nrf2 and HO-1, a potent antioxidant enzyme, in RGCs immunolabeled with RNA-binding protein with multiple splicing (RBPMS). Immunoblotting analysis revealed an increase in the nuclear expression of Nrf2 and marked upregulation of HO-1 after DMF administration. These results suggest that DMF has survival-promoting effects in RGC after ONC, possibly via the Nrf2/HO-1 pathway.

Quantitative profiling of regional protein expression in rat retina after partial optic nerve transection using fluorescence difference two‑dimensional gel electrophoresis

To examine the difference between primary and secondary retinal ganglion cell (RGC) degeneration, the protein expression at four regions of retina including superior, temporal, inferior and nasal quadrant in a rat model of partial optic nerve transection (pONT) using 2-D Fluorescence Difference Gel Electrophoresis (DIGE) were investigated. Unilateral pONT was performed on the temporal side of optic nerves of adult Wistar rats to separate primary and secondary RGC loss. Topographical quantification of RGCs labeled by Rbpms antibody and analysis of axonal injury by grading of optic nerve damage at 1 week (n=8) and 8 weeks (n=15) after pONT demonstrated early RGC loss at temporal region, which is considered as primary RGC degeneration and progressing RGC loss at nasal region, which is considered as secondary RGC degeneration. Early protein expression in each retinal quadrant (n=4) at 2 weeks after pONT was compared with the corresponding quadrant in the contralateral control eye by DIGE. For all comparisons, 24 differentially expressed proteins (>1.2-fold; P<0.05; ≥3 non-duplicated peptide matches) were identified by mass spectrometry (MS). Interestingly, in the nasal retina, serum albumin and members of crystallin family, including αA, αB, βA2, βA3, βB2 and γS indicating stress response were upregulated. By contrast, only αB and βA2 crystallin proteins were altered in temporal quadrant. In the superior and inferior quadrants, βB2 crystallin, keratin type I, S-arrestin and lamin-B1 were upregulated, while heat shock cognate 71 kDa protein and heterogeneous nuclear ribonucleoproteins A2/B1 were downregulated. In summary, the use of DIGE followed by MS is useful to detect early regional protein regulation in the retina after localized optic nerve injury.

Lycium barbarum (Wolfberry) Increases Retinal Ganglion Cell Survival and Affects both Microglia/Macrophage Polarization and Autophagy after Rat Partial Optic Nerve Transection

The rat partial optic nerve transection (PONT) model has been used for studying secondary degeneration of retinal ganglion cells (RGCs) in recent years. In this study, we carried out PONT of the temporal side of rat optic nerves, whereas PONT was carried out of the superior side in the previous publication. We found that this surgery is better and easier than the previous method and can produce a repeatable and reliable model. We detected significant changes in the polarization of microglia/macrophages and the level of autophagy in optic nerves after PONT. We also used this model to detect the effects of the polysaccharides extracted from Lycium barbarum (LBP) on the survival of RGCs and the changes in the polarization of microglia/macrophages and the level of autophagy after PONT. We find that LBP can delay secondary degeneration of RGCs after temporal injury of optic nerves, promote the M2 polarization of microglia/macrophages, and down-regulate the level of autophagy after PONT. In conclusion, we find that the polarization of microglia/macrophages and the autophagy level change after PONT; LBP treatment delays secondary degeneration of RGCs; and the polarization of microglia/macrophages and the level of autophagy are also altered after LBP treatment.

Retinal genes are differentially expressed in areas of primary versus secondary degeneration following partial optic nerve injury

BACKGROUND

Partial transection (PT) of the optic nerve is an established experimental model of secondary degeneration in the central nervous system. After a dorsal transection, retinal ganglion cells (RGCs) with axons in ventral optic nerve are intact but vulnerable to secondary degeneration, whereas RGCs in dorsal retina with dorsal axons are affected by primary and secondary injuries. Using microarray, we quantified gene expression changes in dorsal and ventral retina at 1 and 7 days post PT, to characterize pathogenic pathways linked to primary and secondary degeneration.

RESULTS

In comparison to uninjured retina Cryba1, Cryba2 and Crygs, were significantly downregulated in injured dorsal retina at days 1 and 7. While Ecel1, Timp1, Mt2A and CD74, which are associated with reducing excitotoxicity, oxidative stress and inflammation, were significantly upregulated. Genes associated with oxygen binding pathways, immune responses, cytokine receptor activity and apoptosis were enriched in dorsal retina at day 1 after PT. Oxygen binding and apoptosis remained enriched at day 7, as were pathways involved in extracellular matrix modification. Fewer changes were observed in ventral retina at day 1 after PT, most associated with the regulation of protein homodimerization activity. By day 7, apoptosis, matrix organization and signal transduction pathways were enriched. Discriminant analysis was also performed for specific functional gene groups to compare expression intensities at each time point. Altered expression of selected genes (ATF3, GFAP, Ecel1, TIMP1, Tp53) and proteins (GFAP, ECEL1 and ATF3) were semi-quantitatively assessed by qRT-PCR and immunohistochemistry respectively.

CONCLUSION

There was an acute and complex primary injury response in dorsal retina indicative of a dynamic interaction between neuroprotective and neurodegenerative events; ventral retina vulnerable to secondary degeneration showed a delayed injury response. Both primary and secondary injury resulted in the upregulation of numerous genes linked to RGC death, but differences in the nature of these changes strongly suggest that death occurred via different molecular mechanisms.

Neuroinflammation in glaucoma: A new opportunity

Mounting evidence suggests neuroinflammation is a key process in glaucoma, yet the precise roles are not known. Understanding these complex processes, which may also be a key in other common neurodegenerations such as Alzheimer's disease, will lead to targeted therapeutics for a disease that affects as many as 80 million people worldwide. Here, we define neuroinflammation as any immune-relevant response by a variety of cell types including astrocytes, microglia, and peripherally derived cells occurring in the optic nerve head and/or retina. In this review article, we first discuss clinical evidence for neuroinflammation in glaucoma and define neuroinflammation in glaucoma. We then review the inflammatory pathways that have been associated with glaucoma. Finally, we set out key research directions that we believe will greatly advance our understanding of the role of neuroinflammation in glaucoma. This review arose from a discussion of neuroinflammation in glaucoma at the 2015 meeting of The Lasker/IRRF Initiative for Innovation in Vision Science. This manuscript sets out to summarize one of these sessions; "Inflammation and Glaucomatous Neurodegeneration", as well as to review the current state of the literature surrounding neuroinflammation in glaucoma.

BAX to basics: How the BCL2 gene family controls the death of retinal ganglion cells

Retinal ganglion cell (RGC) death is the principal consequence of injury to the optic nerve. For several decades, we have understood that the RGC death process was executed by apoptosis, suggesting that there may be ways to therapeutically intervene in this cell death program and provide a more direct treatment to the cells and tissues affected in diseases like glaucoma. A major part of this endeavor has been to elucidate the molecular biological pathways active in RGCs from the point of axonal injury to the point of irreversible cell death. A major component of this process is the complex interaction of members of the BCL2 gene family. Three distinct family members of proteins orchestrate the most critical junction in the apoptotic program of RGCs, culminating in the activation of pro-apoptotic BAX. Once active, BAX causes irreparable damage to mitochondria, while precipitating downstream events that finish off a dying ganglion cell. This review is divided into two major parts. First, we summarize the extent of knowledge of how BCL2 gene family proteins interact to facilitate the activation and function of BAX. This area of investigation has rapidly changed over the last few years and has yielded a dramatically different mechanistic understanding of how the intrinsic apoptotic program is run in mammalian cells. Second, we provided a comprehensive analysis of nearly two decades of investigation of the role of BAX in the process of RGC death, much of which has provided many important insights into the overall pathophysiology of diseases like glaucoma.

Changes in parvalbumin immunoreactive retinal ganglion cells and amacrine cells after optic nerve injury

Parvalbumin (PARV) is a Ca(2+)-binding protein that may offer resistance to cell death as it primarily functions to maintain Ca(2+) homeostasis. The purpose of this study was to investigate whether PARV expressing retinal ganglion cells (RGCs) would be more resistant to cell death than RGCs that do not express PARV. RGCs in Sprague-Dawley rats were retrogradely labeled with Fluorogold (FG). After 2-28 days following an optic nerve crush (ONC) injury immunohistochemistry was performed on the sections using antibodies against PARV and markers of RGCs. The proportion of retinal ganglion cell layer cells labeled with PARV colocalized with FG or Brn3a and labeled only with PARV (displaced amacrine cells; dACs) were analyzed. PARV staining intensity was measured in ACs, dACs, and RGCs. Double labeling studies revealed that 49% of RGCs and 22% of dACs expressed PARV. There was an immediate reduction in RGC PARV staining after ONC but the overall rate of cell death after 28 days was similar in PARV and non-PARV expressing RGCs. There was no change in PARV AC or dAC number or staining intensity. Although this study suggests that there is no selective survival of the subpopulation of RGCs that contain PARV, there is down-regulation of PARV expression by these RGCs. This suggests that down-regulation of PARV may contribute to RGC death due to a compromised Ca(2+) buffering capacity. Maintaining PARV expression after injury could be an important neuroprotective strategy to improve RGC survival after injury.

Direct optic nerve sheath (DONS) application of Schwann cells prolongs retinal ganglion cell survival in vivo

Cell-based therapies are increasingly recognized as a potential strategy to treat retinal neurodegenerative disease. Their administration, however, is normally indirect and complex, often with an inability to assess in real time their effects on cell death and their migration/integration into the host retina. In the present study, using a partial optic nerve transection (pONT) rat model, we describe a new method of Schwann cell (SC) delivery (direct application to injured optic nerve sheath, SC/DONS), which was compared with intravitreal SC delivery (SC/IVT). Both SC/DONS and SC/IVT were able to be assessed in vivo using imaging to visualize retinal ganglion cell (RGC) apoptosis and SC retinal integration. RGC death in the pONT model was best fitted to the one-phase exponential decay model. Although both SC/DONS and SC/IVT altered the temporal course of RGC degeneration in pONT, SC/DONS resulted in delayed but long-lasting effects on RGC protection, compared with SC/IVT treatment. In addition, their effects on primary and secondary degeneration, and axonal regeneration, were also investigated, by histology, whole retinal counting, and modelling of RGC loss. SC/DONS was found to significantly reduce RGC apoptosis in vivo and significantly increase RGC survival by targeting secondary rather than primary degeneration. Both SC/DONS and SC/IVT were found to promote RGC axonal regrowth after optic nerve injury, with evidence of GAP-43 expression in RGC somas and axons. SC/DONS may have the potential in the treatment of optic neuropathies, such as glaucoma. We show that SC transplantation can be monitored in real time and that the protective effects of SCs are associated with targeting secondary degeneration, with implications for translating cell-based therapies to the clinic.

Mechanisms of secondary degeneration after partial optic nerve transection

Secondary degeneration occurs commonly in the central nervous system after traumatic injuries and following acute and chronic diseases, including glaucoma. A constellation of mechanisms have been shown to be associated with secondary degeneration including apoptosis, necrosis, autophagy, oxidative stress, excitotoxicity, derangements in ionic homeostasis and calcium influx. Glial cells, such as microglia, astrocytes and oligodendrocytes, have also been demonstrated to take part in the process of secondary injury. Partial optic nerve transection is a useful model which was established about 13 years ago. The merit of this model compared with other optic nerve injury models used for glaucoma study, including complete optic nerve transection model and optic nerve crush model, is the possibility to separate primary degeneration from secondary degeneration in location. Therefore, it provides a good tool for the study of secondary degeneration. This review will focus on the research progress of the mechanisms of secondary degeneration using partial optic nerve transection model.

Minocycline upregulates pro-survival genes and downregulates pro-apoptotic genes in experimental glaucoma

PURPOSE

Minocycline, a second-generation tetracycline with anti-inflammatory and anti-apoptotic properties, was reported to be neuroprotective in experimental glaucoma and optic nerve transection as well as in other neurodegenerative diseases. The purpose of this study was to investigate the mechanism underlying that neuroprotective effect in murine glaucoma.

METHODS

Elevated intraocular pressure was induced in 159 rats by the translimbal photocoagulation laser model. Minocycline 22 mg/kg or saline was injected intraperitoneally starting 3 days before the induction of glaucoma, and continued daily until the animals were sacrificed. The effect of minocycline on gene expression was evaluated using a quantitative polymerase chain reaction (PCR) array for apoptosis. The involvement of selected pro-apoptotic, pro-survival, and inflammatory genes was further analyzed by quantitative real-time PCR at multiple time points. Immunohistochemistry was used to study the effect of minocycline on microglial activation and to localize Bcl-2 changes.

RESULTS

Minocycline significantly increased the anti-apoptotic gene Bcl-2 expression at day 8 and day 14 after the induction of glaucoma (p = 0.04 and p = 0.03 respectively), and decreased IL-18 expression in the retina at day 14 and day 30 (p = 0.04 and p < 0.001 respectively). PCR arrays suggested that additional genes were affected by minocycline, including Tp53bp2, TRAF4, osteoprotegerin, caspase 1 and 4, and members of the tumor necrosis factor superfamily. Additionally, minocycline decreased the amount of activated microglia in glaucomatous eyes.

CONCLUSIONS

These results suggest that minocycline upregulates pro-survival genes and downregulates apoptotic genes, thus shifting the balance toward the anti-apoptotic side in experimental glaucoma.

Minocycline mechanism of neuroprotection involves the Bcl-2 gene family in optic nerve transection

The second-generation tetracycline, minocycline, has been shown to exhibit neuroprotective therapeutic benefits in many neurodegenerative diseases including experimental glaucoma and optic nerve transection (ONT). This study investigated the mechanism underlying minocycline neuroprotection in a model of ONT. ONT was applied unilaterally in 36 Wistar rat eyes. The rats were randomly divided into a minocycline (22 mg/kg/d) treatment group and a saline treatment group (control). Treatment (minocycline or saline) was given by intraperitoneal injections initiated 3 d before ONT and continued daily until the end of the experiment. The involvement of pro-apoptotic, pro-survival and inflammatory pathways was analyzed by quantitative Real-Time Polymerase Chain Reaction at 4 h and 3 d after the transection in both treatment groups. The involvement of Bcl-2 protein was evaluated by immunohistochemistry. We found that Minocycline significantly increased the expression of the antiapoptotic gene bcl-2 4 h after transection (n = 8, p = 0.008) and decreased the expression of Bax at the same time point (n = 8, p = 0.03). Tumor Necrosis Factor α (TNFα), Inhibitor of Apoptosis Protein (IAP1) and Gadd45α were significantly upregulated in the retinas of eyes with ONTs compared to control (n = 10 for each gene, p = 0.02, p = 0.03, p = 0.04, respectively) but this effect was unaffected by minocycline. This study further support that the mechanism underlying minocycline neuroprotection involves the Bcl-2 gene family, suggesting that minocycline has antiapoptotic properties that support its value as a promising neuroprotective drug.

Lycium barbarum (wolfberry) reduces secondary degeneration and oxidative stress, and inhibits JNK pathway in retina after partial optic nerve transection

Our group has shown that the polysaccharides extracted from Lycium barbarum (LBP) are neuroprotective for retinal ganglion cells (RGCs) in different animal models. Protecting RGCs from secondary degeneration is a promising direction for therapy in glaucoma management. The complete optic nerve transection (CONT) model can be used to study primary degeneration of RGCs, while the partial optic nerve transection (PONT) model can be used to study secondary degeneration of RGCs because primary degeneration of RGCs and secondary degeneration can be separated in location in the same retina in this model; in other situations, these types of degeneration can be difficult to distinguish. In order to examine which kind of degeneration LBP could delay, both CONT and PONT models were used in this study. Rats were fed with LBP or vehicle daily from 7 days before surgery until sacrifice at different time-points and the surviving numbers of RGCs were evaluated. The expression of several proteins related to inflammation, oxidative stress, and the c-jun N-terminal kinase (JNK) pathways were detected with Western-blot analysis. LBP did not delay primary degeneration of RGCs after either CONT or PONT, but it did delay secondary degeneration of RGCs after PONT. We found that LBP appeared to exert these protective effects by inhibiting oxidative stress and the JNK/c-jun pathway and by transiently increasing production of insulin-like growth factor-1 (IGF-1). This study suggests that LBP can delay secondary degeneration of RGCs and this effect may be linked to inhibition of oxidative stress and the JNK/c-jun pathway in the retina.

Structural and functional effects of hemiretinal endodiathermy axotomy in cynomolgus macaques

PURPOSE

Outer retinal injury has been well described in glaucoma. To better understand the source of this injury, we wanted to develop a reliable model of partial retinal ganglion cell (RGC) axotomy.

METHODS

Endodiathermy spots were placed along the inferior 180° adjacent to the optic nerve margin in the right eyes of four cynomolgus monkeys. Fluorescein angiography, spectral domain optical coherence tomography (SD-OCT), and multifocal electroretinography (mfERG) were performed at various intervals. Two animals were sacrificed at 3 months. Two animals were sacrificed at 4 months, at which time they underwent an injection of fluorescent microspheres to measure regional choroidal blood flow. Retinal immunohistochemistry for glial fibrillary acidic protein (GFAP), rhodopsin, S-cone opsin, and M/L-cone opsin were performed, as were axon counts of the optic nerves.

RESULTS

At 3 months, there was marked thinning of the inferior nerve fiber layer on SD-OCT. The mfERG waveforms were consistent with inner but not outer retinal injury. Greater than 95% reduction in axons was seen in the inferior optic nerves but no secondary degeneration superiorly. There was marked thinning of the nerve fiber and ganglion cell layers in the inferior retinas. However, the photoreceptor histology was similar in the axotomized and nonaxotomized areas. Regional choroidal blood flow was not affected by the axotomy.

CONCLUSIONS

Unlike experimental glaucoma, hemiretinal endodiathermy axotomy (HEA) of the RGCs produces no apparent anatomic, functional, or blood flow effects on the outer retina and choroid.

Regulation of cell death and survival pathways in secondary degeneration of the optic nerve - a long-term study

PURPOSE

To investigate cell death and survival pathways in secondary degeneration of the optic nerve (ON) and retina over a period of 6 months.

METHODS

A partial transection model of the ON that morphologically separates primary and secondary degeneration was applied unilaterally in 89 Wistar rat eyes. The upper third of the retinas were analyzed for primary degeneration, while the lower third of the retinas were analyzed for secondary degeneration. The involvement of members of the mitogen-activated protein (MAP) kinase pathway and the PI-3-kinase/Akt pathway were evaluated in primary and secondary degeneration in multiple time points over a period of 6 months using immunohistochemistry and western blotting. Results were compared to corresponding areas from control fellow eyes.

RESULTS

All investigated members of the MAP kinase pathway were significantly activated in primary degeneration, secondary degeneration or both. P-SAPK/JNK and P-ERK were activated in primary degeneration without a concomitant activation in secondary degeneration. The prosurvival protein p-Akt, a member of the PI-3-kinase survival pathway, was significantly activated in secondary but not in primary degeneration. P-c-jun and p-ATF were significantly activated in both primary and secondary degeneration. The time-dependent pattern of activation was different for each protein and in secondary degeneration the activation of these proteins was usually short termed.

CONCLUSIONS

The significant involvement of the MAP kinase pathway and the PI-3-kinase survival pathway in secondary degeneration of the ON and retina is short termed despite continuous retinal ganglion cells (RGCs)apoptosis for at least 6 months.