Intrinsic and extrinsic inhibition of oligodendrocyte development by rat retina

Progressive myelination of the retinal nerve fibre layer associated with a lamina cribrosa cleft

Myelination of the retinal nerve fibre layer (MNFL) is generally an incidental asymptomatic finding on fundus exam. While MNFL is thought to be a congenital and stationary finding, there have been cases of acquired and progressive MNFL reported. Here, we discuss a case where a girl in middle childhood presented with reading difficulties and was found to have normal vision and MNFL superior to the left optic disc. Over 5 years, the MNFL was observed to grow, and a new patch appeared inferior to the left disc. A Y-shaped cleft was detected in lamina cribrosa. The shape of the cleft suggested incomplete embryological closure of the optic cup and optic stalk. This case offers support for the concept that defects in lamina cribrosa allow oligodendrocyte precursor cells to enter the retina, where they may later proliferate and differentiate to form MNFL.

Glial Cells in Glaucoma: Friends, Foes, and Potential Therapeutic Targets

Glaucoma is the second leading cause of blindness worldwide, affecting ~80 million people by 2020 (1, 2). The condition is characterized by a progressive loss of retinal ganglion cells (RGCs) and their axons accompanied by visual field loss. The underlying pathophysiology of glaucoma remains elusive. Glaucoma is recognized as a multifactorial disease, and lowering intraocular pressure (IOP) is the only treatment that has been shown to slow the progression of the condition. However, a significant number of glaucoma patients continue to go blind despite intraocular pressure-lowering treatment (2). Thus, the need for alternative treatment strategies is indisputable. Accumulating evidence suggests that glial cells play a significant role in supporting RGC function and that glial dysfunction may contribute to optic nerve disease. Here, we review recent advances in understanding the role of glial cells in the pathophysiology of glaucoma. A particular focus is on the dynamic and essential interactions between glial cells and RGCs and potential therapeutic approaches to glaucoma by targeting glial cells.

Assessment of the Retina of Plp-α-Syn Mice as a Model for Studying Synuclein-Dependent Diseases

Purpose

Synucleinopathies such as multiple system atrophy (MSA) and Parkinson's disease are associated with a variety of visual symptoms. Functional and morphological retinal aberrations are therefore supposed to be valuable biomarkers for these neurodegenerative diseases. This study examined the retinal morphology and functionality resulting from human α-synuclein (α-Syn) overexpression in the transgenic Plp-α-Syn mouse model.

Methods

Immunohistochemistry on retinal sections and whole-mounts was performed on 8- to 11-week-old and 12-month-old Plp-α-Syn mice and C57BL/6N controls. Quantitative RT-PCR experiments were performed to study the expression of endogenous and human α-Syn and tyrosine hydroxylase (TH). We confirmed the presence of human α-Syn in the retina in western blot analyses. Multi-electrode array (MEA) analyses from light-stimulated whole-mounted retinas were used to investigate their functionality.

Results

Biochemical and immunohistochemical analyses showed human α-Syn in the retina of Plp-α-Syn mice. We found distinct staining in different retinal cell layers, most abundantly in rod bipolar cells of the peripheral retina. In the periphery, we also observed a trend toward a decline in the number of retinal ganglion cells. The number of TH+ neurons was unaffected in this human α-Syn overexpression model. MEA recordings showed that Plp-α-Syn retinas were functional but exhibited mild alterations in dim light conditions.

Conclusions

Together, these findings implicate an impairment of retinal neurons in the Plp-α-Syn mouse. The phenotype partly relates to retinal deficits reported in MSA patients. We further propose the suitability of the Plp-α-Syn retina as a biological model to study synuclein-mediated mechanisms.

Oligodendrocyte dysfunction and regeneration failure: A novel hypothesis of delayed encephalopathy after carbon monoxide poisoning

Carbon monoxide (CO) poisoning usually causes brain lesions and delayed encephalopathy, also known as delayed neurological sequelae (DNS). Demyelination of white matter (WM) is one of the most common sites of abnormalities in patients with DNS, but its mechanisms remain unclear. Oligodendrocytes (OLs) are myelinated cells that ensure the rapid conduction of neuronal axon signals and provide the nutritional factors necessary for maintaining nerve integrity in the central nervous system (CNS). OLs readily regenerate and replace damaged myelin membranes around axons in the adult mammalian CNS following demyelination. The ability to regenerate OLs depends on the availability of precursor cells (OPCs) in the CNS of adults. Multiple injury-related signals can induce OPC expansion followed by OL differentiation, axonal contact and myelin regeneration (remyelination). Therefore, OL dysfunction and regeneration failure in the deep WM of the brain are the key pathophysiological mechanisms leading to delayed brain injury after CO poisoning. CO-induced toxicity may interfere with OL function and render OPCs unable to regenerate OLs through some unclear mechanisms, leading to progressive demyelinating damage and resulting in DNS. In the future, combination therapies to reduce OL damage and promote OPC differentiation and remyelination may be important for the prevention and treatmentof DNS after CO poisoning.

The Landscape of Gene Expression and Molecular Regulation Following Spinal Cord Hemisection in Rats

Spinal cord injury (SCI) is a challenging clinical problem worldwide. The cellular state and molecular expression in spinal cord tissue after injury are extremely complex and closely related to functional recovery. However, the spatial and temporal changes of gene expression and regulation in various cell types after SCI are still unclear. Here, we collected the rostral and caudal regions to the lesion at 11 time points over a period of 28 days after rat hemisection SCI. Combining whole-transcriptome sequencing and bioinformatic analysis, we identified differentially expressed genes (DEGs) between spinal cord tissue from injured and sham-operated animals. Significantly altered biological processes were enriched from DEGs in astrocytes, microglia, oligodendrocytes, immune cells, and vascular systems after SCI. We then identified dynamic trends in these processes using the average expression profiles of DEGs. Gene expression and regulatory networks for selected biological processes were also constructed to illustrate the complicate difference between rostral and caudal tissues. Finally, we validated the expressions of some key genes from these networks, including α-synuclein, heme oxygenase 1, bone morphogenetic protein 2, activating transcription factor 3, and leukemia inhibitory factor. Collectively, we provided a comprehensive network of gene expression and regulation to shed light on the molecular characteristics of critical biological processes that occur after SCI, which will broaden the understanding of SCI and facilitate clinical therapeutics for SCI.

Juxtanodin in retinal pigment epithelial cells: Expression and biological activities in regulating cell morphology and actin cytoskeleton organization

Juxtanodin (JN, also known as ermin) was initially identified as an actin cytoskeleton-related oligodendroglial protein in the rat central nervous system. It was subsequently also found in the rat olfactory neuroepithelium, especially at the apical junctional belt of the sustentacular cells. We further examined JN expression and functional roles in the retina using fluorescence histochemistry, confocal microscopy, immuno-electron microscopy, molecular biology, and cell culture. Prominent JN expression was found in the photoreceptor-supporting retinal pigment epithelium (RPE), especially in a zone corresponding to the apices of RPE cells, at the roots of the RPE microvilli, and at the base of RPE cells next to the Bruch's membrane. Partial co-localization of JN immunoreactivity with F-actin (labeled with phalloidin) was observed at the apices and bases of RPE cells. No JN was detected in other cell types of the retina. In cultured human RPE cell line ARPE-19, expression of extrinsic JN up-regulated formation of actin cytoskeleton stress fibers, caused redistribution of more F-actin fibers to the cell periphery, and promoted spreading/enlargement of transfected cells. These findings suggest possible roles of JN in RPE molecular transport, phagocytosis and formation of outer blood-retinal barrier, or possible involvement of JN expression perturbations in pathogenesis of such retinal disorders as proliferative vitreoretinopathy and age-related macular degeneration.

Repeated mild traumatic brain injury in female rats increases lipid peroxidation in neurons

Negative outcomes of mild traumatic brain injury (mTBI) can be exacerbated by repeated insult. Animal models of repeated closed-head mTBI provide the opportunity to define acute pathological mechanisms as the number of mTBI increases. Furthermore, little is known about the effects of mTBI impact site, and how this may affect brain function. We use a closed head, weight drop model of mTBI that allows head movement following impact, in adult female rats to determine the role of the number and location of mTBI on brain pathology and behaviour. Biomechanical assessment of two anatomically well-defined mTBI impact sites were used, anterior (bregma) and posterior (lambda). Location of the impact had no significant effect on impact forces (450 N), and the weight impact locations were on average 5.4 mm from the desired impact site. No between location vertical linear head kinematic differences were observed immediately following impact, however, in the 300 ms post-impact, significantly higher mean vertical head displacement and velocity were observed in the mTBI lambda trials. Breaches of the blood brain barrier were observed with three mTBI over bregma, associated with immunohistochemical indicators of damage. However, an increased incidence of hairline fractures of the skull and macroscopic haemorrhaging made bregma an unsuitable impact location to model repeated mTBI. Repeated mTBI over lambda did not cause skull fractures and were examined more comprehensively, with outcomes following one, two or three mTBI or sham, delivered at 1 day intervals, assessed on days 1-4. We observe a mild behavioural phenotype, with subtle deficits in cognitive function, associated with no identifiable neuroanatomical or inflammatory changes. However, an increase in lipid peroxidation in a subset of cortical neurons following two mTBI indicates increasing oxidative damage with repeated injury in female rats, supported by increased amyloid precursor protein immunoreactivity with three mTBI. This study of acute events following closed head mTBI identifies lipid peroxidation in neurons at the same time as cognitive deficits. Our study adds to existing literature, providing biomechanics data and demonstrating mild cognitive disturbances associated with diffuse injury, predominantly to grey matter, acutely following repeated mTBI.

Role of Myo/Nog Cells in Neuroprotection: Evidence from the Light Damaged Retina

PURPOSE

To identify Myo/Nog cells in the adult retina and test their role in protecting retinal photoreceptors from light damage.

METHODS

Light damage was induced by exposing albino rats raised in dim cyclic light to 1000 lux light for 24 hours. In one group of rats, Myo/Nog cells were purified from rat brain tissue by magnetic cell sorting following binding of the G8 monoclonal antibody (mAb). These cells were injected into the vitreous humour of the eye within 2 hours following bright light exposure. Retinal function was assessed using full-field, flash electroretinogram (ERG) before and after treatment. The numbers of Myo/Nog cells, apoptotic photoreceptors, and the expression of glial fibrillary acidic protein (GFAP) in Muller cells were assessed by immunohistochemistry.

RESULTS

Myo/Nog cells were present in the undamaged retina in low numbers. Light induced damage increased their numbers, particularly in the choroid, ganglion cell layer and outer plexiform layer. Intravitreal injection of G8-positive (G8+) cells harvested from brain mitigated all the effects of light damage examined, i.e. loss of retinal function (ERG), death of photoreceptors and the stress-induced expression of GFAP in Muller cells. Some of the transplanted G8+ cells were integrated into the retina from the vitreous.

CONCLUSIONS

Myo/Nog cells are a subpopulation of cells that are present in the adult retina. They increase in number in response to light induced stress. Intravitreal injection of Myo/Nog cells was protective to the retina, in part, by reducing retinal stress as measured by the Muller cell response. These results suggest that Myo/Nog cells, or the factors they produce, are neuroprotective and may be therapeutic in neurodegenerative retinal diseases.

Hedgehog: A Key Signaling in the Development of the Oligodendrocyte Lineage

The Hedgehog morphogen aroused an enormous interest since it was characterized as an essential signal for ventral patterning of the spinal cord two decades ago. The pathway is notably implicated in the initial appearance of the progenitors of oligodendrocytes (OPCs), the glial cells of the central nervous system which after maturation are responsible for axon myelination. In accordance with the requirement for Hedgehog signaling in ventral patterning, the earliest identifiable cells in the oligodendrocyte lineage are derived from the ventral ventricular zone of the developing spinal cord and brain. Here, we present the current knowledge about the involvement of Hedgehog signaling in the strict spatial and temporal regulation which characterizes the initiation and progression of the oligodendrocyte lineage. We notably describe the ability of the Hedgehog signaling to tightly orchestrate the appearance of specific combinations of genes in concert with other pathways. We document the molecular mechanisms controlling Hedgehog temporal activity during OPC specification. The contribution of the pathway to aspects of OPC development different from their specification is also highlighted especially in the optic nerve. Finally, we report the data demonstrating that Hedgehog signaling-dependency is not a universal situation for oligodendrocyte generation as evidenced in the dorsal spinal cord in contrast to the dorsal forebrain.

Cyclin-dependent kinase 5 mediates adult OPC maturation and myelin repair through modulation of Akt and GsK-3β signaling

Failure of remyelination in diseases, such as multiple sclerosis (MS), leads to permanent axonal damage and irreversible functional loss. The mechanisms controlling remyelination are currently poorly understood. Recent studies implicate the cyclin-dependent kinase 5 (Cdk5) in regulating oligodendrocyte (OL) development and myelination in CNS. In this study, we show that Cdk5 is also an important regulator of remyelination. Pharmacological inhibition of Cdk5 inhibits repair of lysolecithin lesions. This inhibition is a consequence of Cdk5 disruption in neural cells because remyelination in slice cultures is blocked by Cdk5 inhibitors, whereas specific deletion of Cdk5 in OLs inhibits myelin repair. In CNP-Cre;Cdk5(fl/fl) conditional knock-out mouse (Cdk5 cKO), myelin repair was delayed significantly in response to focal demyelinating lesions compared with wild-type animals. The lack of myelin repair was reflected in decreased expression of MBP and proteolipid protein and a reduction in the total number of myelinated axons in the lesion. The number of CC1(+) cells in the lesion sites was significantly reduced in Cdk5 cKO compared with wild-type animals although the total number of oligodendrocyte lineage cells (Olig2(+) cells) was increased, suggesting that Cdk5 loss perturbs the transition of early OL lineage cell into mature OL and subsequent remyelination. The failure of remyelination in Cdk5 cKO animals was associated with a reduction in signaling through the Akt pathway and an enhancement of Gsk-3β signaling pathways. Together, these data suggest that Cdk5 is critical in regulating the transition of adult oligodendrocyte precursor cells to mature OLs that is essential for myelin repair in adult CNS.

Cyclin dependent kinase 5 is required for the normal development of oligodendrocytes and myelin formation

The development of oligodendrocytes, the myelinating cells of the vertebrate CNS, is regulated by a cohort of growth factors and transcription factors. Less is known about the signaling pathways that integrate extracellular signals with intracellular transcriptional regulators to control oligodendrocyte development. Cyclin dependent kinase 5 (Cdk5) and its co-activators play critical roles in the regulation of neuronal differentiation, cortical lamination, neuronal cell migration and axon outgrowth. Here we demonstrate a previously unrecognized function of Cdk5 in regulating oligodendrocyte maturation and myelination. During late embryonic development Cdk5 null animals displayed a reduction in the number of MBP+ cells in the spinal cord, but no difference in the number of OPCs. To determine whether the reduction of oligodendrocytes reflected a cell-intrinsic loss of Cdk5, it was selectively deleted from Olig1+ oligodendrocyte lineage cells. In Olig1(Cre/+); Cdk5(fl/fl) conditional mutants, reduced levels of expression of MBP and PLP mRNA were observed throughout the CNS and ultrastructural analyses demonstrated a significant reduction in the proportion of myelinated axons in the optic nerve and spinal cord. Pharmacological inhibition or RNAi knockdown of Cdk5 in vitro resulted in the reduction in oligodendrocyte maturation, but had no effect on OPC cell proliferation. Conversely, over-expression of Cdk5 promoted oligodendrocyte maturation and enhanced process outgrowth. Consistent with this data, Cdk5(-/-) oligodendrocytes developed significantly fewer primary processes and branches than control cells. Together, these findings suggest that Cdk5 function as a signaling integrator to regulate oligodendrocyte maturation and myelination.

Platelet-derived growth factor-responsive neural precursors give rise to myelinating oligodendrocytes after transplantation into the spinal cords of contused rats and dysmyelinated mice

Spinal cord injury (SCI) results in substantial oligodendrocyte death and subsequent demyelination leading to white-matter defects. Cell replacement strategies to promote remyelination are under intense investigation; however, the optimal cell for transplantation remains to be determined. We previously isolated a platelet-derived growth factor (PDGF)-responsive neural precursor (PRP) from the ventral forebrain of fetal mice that primarily generates oligodendrocytes, but also astrocytes and neurons. Importantly, human PRPs were found to possess a greater capacity for oligodendrogenesis than human epidermal growth factor- and/or fibroblast growth factor-responsive neural stem cells. Therefore, we tested the potential of PRPs isolated from green fluorescent protein (GFP)-expressing transgenic mice to remyelinate axons in the injured rat spinal cord. PRPs were transplanted 1 week after a moderate thoracic (T9) spinal cord contusion in adult male rats. After initial losses, PRP numbers remained stable from 2 weeks posttransplantation onward and those surviving cells integrated into host tissue. Approximately one-third of the surviving cells developed the typical branched phenotype of mature oligodendrocytes, expressing the marker APC-CC1. The close association of GFP cells with myelin basic protein as well as with Kv1.2 and Caspr in the paranodal and juxtaparanodal regions of nodes of Ranvier indicated that the transplanted cells successfully formed mature myelin sheaths. Transplantation of PRPs into dysmyelinated Shiverer mice confirmed the ability of PRP-derived cells to produce compact myelin sheaths with normal periodicity. These findings indicate that PRPs are a novel candidate for CNS myelin repair, although PRP-derived myelinating oligodendrocytes were insufficient to produce behavioral improvements in our model of SCI.

Promoting myelin repair and return of function in multiple sclerosis

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the CNS. Conduction block in demyelinated axons underlies early neurological symptoms, but axonal transection and neuronal loss are believed to be responsible for more permanent chronic deficits. Several therapies are approved for treatment of relapsing-remitting MS, all of which are immunoregulatory and clinically proven to reduce the rate of lesion formation and exacerbation. However, existing approaches are only partially effective in preventing the onset of disability in MS patients, and novel treatments to protect myelin-producing oligodendrocytes and enhance myelin repair may improve long-term outcomes. Studies in vivo in genetically modified mice have assisted in the characterization of mechanisms underlying the generation of neuropathology in MS patients, and have identified potential avenues for oligodendrocyte protection and myelin repair. However, no treatments are yet approved that target these areas directly, and in addition, the relationship between demyelination and axonal transection in the lesions of the disease remains unclear. Here, we review translational research targeting oligodendrocyte protection and myelin repair in models of autoimmune demyelination, and their potential relevance as therapies in MS.

In situ dividing and phagocytosing retinal microglia express nestin, vimentin, and NG2 in vivo

Background

Following injury, microglia become activated with subsets expressing nestin as well as other neural markers. Moreover, cerebral microglia can give rise to neurons in vitro. In a previous study, we analysed the proliferation potential and nestin re-expression of retinal macroglial cells such as astrocytes and Müller cells after optic nerve (ON) lesion. However, we were unable to identify the majority of proliferative nestin⁺ cells. Thus, the present study evaluates expression of nestin and other neural markers in quiescent and proliferating microglia in naïve retina and following ON transection in adult rats in vivo.

Methodology/Principal Findings

For analysis of cell proliferation and cells fates, rats received BrdU injections. Microglia in retinal sections or isolated cells were characterized using immunofluorescence labeling with markers for microglia (e.g., Iba1, CD11b), cell proliferation, and neural cells (e.g., nestin, vimentin, NG2, GFAP, Doublecortin etc.). Cellular analyses were performed using confocal laser scanning microscopy. In the naïve adult rat retina, about 60% of resting ramified microglia expressed nestin. After ON transection, numbers of nestin⁺ microglia peaked to a maximum at 7 days, primarily due to in situ cell proliferation of exclusively nestin⁺ microglia. After 8 weeks, microglia numbers re-attained control levels, but 20% were still BrdU⁺ and nestin⁺, although no further local cell proliferation occurred. In addition, nestin⁺ microglia co-expressed vimentin and NG2, but not GFAP or neuronal markers. Fourteen days after injury and following retrograde labeling of retinal ganglion cells (RGCs) with Fluorogold (FG), nestin⁺NG2⁺ microglia were positive for the dye indicating an active involvement of a proliferating cell population in phagocytosing apoptotic retinal neurons.

Conclusions/Significance

The current study provides evidence that in adult rat retina, a specific resident population of microglia expresses proteins of immature neural cells that are involved in injury-induced cell proliferation and phagocytosis while transdifferentiation was not observed.

PPAR-gamma, Microglial Cells, and Ocular Inflammation: New Venues for Potential Therapeutic Approaches

The last decade has witnessed an increasing interest for the role played by the peroxisome proliferator-activated receptor-γ (PPAR-γ) in controlling inflammation in peripheral organs as well as in the brain. Activation of PPAR-γ has been shown to control the response of microglial cells, the main macrophage population found in brain parenchyma, and limit the inflammation. The anti-inflammatory capacity of PPAR-γ agonists has led to the hypothesis that PPAR-γ might be targeted to modulate degenerative brain diseases in which inflammation has been increasingly recognized as a significant component. Recent experimental evidence suggests that PPAR-γ agonists could be exploited to treat ocular diseases such as diabetic retinopathy, age-related macular degeneration, autoimmune uveitis, and optic neuritis where inflammation has relevant role. Additional PPAR-γ agonist beneficial effects could involve amelioration of retinal microcirculation and inhibition of neovascularization. However, PPAR-γ activation could, in some instances, aggravate the ocular pathology, for example, by increasing the synthesis of vascular endothelial growth factor, a proangiogenic factor that could trigger a vicious circle and further deteriorate retinal perfusion. The development of new in vivo and in vitro models to study ocular inflammation and how to modulate for the eye benefit will be instrumental for the search of effective therapies.

Targeting oligodendrocyte protection and remyelination in multiple sclerosis

Multiple sclerosis is an inflammatory demyelinating disease of the brain and spinal cord with a presumed autoimmune etiology. Conduction block in demyelinated axons underlies early neurological symptoms, whereas axonal transection is believed responsible for more permanent later deficits. Approved treatments for the disease are immunoregulatory and reduce the rate of lesion formation and clinical exacerbation, but are only partially effective in preventing the onset of disability in multiple sclerosis patients. Approaches that directly protect myelin-producing oligodendrocytes and enhance remyelination may improve long-term outcomes and reduce the rate of axonal transection. Studies in genetically modified animals have improved our understanding of mechanisms underlying central nervous system pathology in multiple sclerosis models, and have identified pathways that regulate oligodendrocyte viability and myelin repair. However, although clinical trials are ongoing, many have been unsuccessful, and no treatments are yet approved that target these areas in multiple sclerosis. In this review, we examine avenues for oligodendrocyte protection and endogenous myelin repair in animal models of demyelination and remyelination, and their relevance as therapeutics in human patients.

Interactions between oligodendrocyte precursors control the onset of CNS myelination

The formation of CNS myelin is dependent on the differentiation of oligodendrocyte precursor cells (OPCs) and oligodendrocyte maturation. How the initiation of myelination is regulated is unclear, but it is likely to depend on the development of competence by oligodendrocytes and receptivity by target axons. Here we identify an additional level of control of oligodendrocyte maturation mediated by interactions between the different cellular components of the oligodendrocyte lineage. During development oligodendrocyte precursors mature through a series of stages defined by labeling with monoclonal antibodies A2B5 and O4. Newly differentiated oligodendrocytes begin to express galactocerebroside recognized by O1 antibodies and subsequently mature to myelin basic protein (MBP)-positive cells prior to formation of compact myelin. Using an in vitro brain slice culture system that supports robust myelination, the consequences of ablating cells at different stages of the oligodendrocyte lineage on myelination have been assayed. Elimination of all OPC lineage cells through A2B5+, O4+, and O1+ complement-mediated cell lysis resulted in a delay in development of MBP cells and myelination. Selective elimination of early OPCs (A2B5+) also unexpectedly resulted in delayed MBP expression compared to controls suggesting that early OPCs contribute to the timing of myelination onset. By contrast, elimination of differentiated (O1+) immature oligodendrocytes permanently inhibited the appearance of MBP+ cells suggesting that oligodendrocytes are critical to facilitate the maturation of OPCs. These data illuminate that the presence of intra-lineage feed-forward and feedback cues are important for timely myelination by oligodendrocytes.

Glatiramer acetate-reactive T lymphocytes regulate oligodendrocyte progenitor cell number in vitro: role of IGF-2

Glatiramer acetate (GA) is an immunomodulator approved for therapy of relapsing-remitting multiple sclerosis (RRMS), but recent findings indicate that it may also have additional, neurotrophic effects. Here, we found that supernatants from human GA-reactive T lymphocytes potentiated oligodendrocyte numbers in rodent and human oligodendrocyte progenitor (OPC) cultures. Effects of Th2-polarized lines were stronger than Th1-polarized cells. Microarray and ELISA analyses revealed that neurotrophic factors induced in Th2- and Th1-polarized GA-reactive lines included IGF-2 and BMP-7 respectively, and functional studies confirmed IGF-2 as trophic for OPCs. Our results support the concept that GA therapy may result in supportive effects on oligodendrocytes in RRMS patients.

Heterogeneity of glia in the retina and optic nerve of birds and mammals

We have recently described a novel type of glial cell that is scattered across the inner layers of the avian retina [1]. These cells are stimulated by insulin-like growth factor 1 (IGF1) to proliferate, migrate distally into the retina, and up-regulate the nestin-related intermediate filament transitin. These changes in glial activity correspond with increased susceptibility of neurons to excitotoxic damage. This novel cell-type has been termed the Non-astrocytic Inner Retinal Glia-like (NIRG) cells. The purpose of the study was to investigate whether the retinas of non-avian species contain cells that resemble NIRG cells. We assayed for NIRG cells by probing for the expression of Sox2, Sox9, Nkx2.2, vimentin and nestin. NIRG cells were distinguished from astrocytes by a lack of expression for Glial Fibrilliary Acidic Protein (GFAP). We examined the retinas of adult mice, guinea pigs, dogs and monkeys (Macaca fasicularis). In the mouse retina and optic nerve head, we identified numerous astrocytes that expressed GFAP, S100β, Sox2 and Sox9; however, we found no evidence for NIRG-like cells that were positive for Nkx2.2, nestin, and negative for GFAP. In the guinea pig retina, we did not find astrocytes or NIRG cells in the retina, whereas we identified astrocytes in the optic nerve. In the eyes of dogs and monkeys, we found astrocytes and NIRG-like cells scattered across inner layers of the retina and within the optic nerve. We conclude that NIRG-like cells are present in the retinas of canines and non-human primates, whereas the retinas of mice and guinea pigs do not contain NIRG cells.

Leukemia inhibitory factor regulates the timing of oligodendrocyte development and myelination in the postnatal optic nerve

Leukemia inhibitory factor (LIF) promotes the survival of oligodendrocytes both in vitro and in an animal model of multiple sclerosis, but the possible role of LIF signaling in myelination during normal development has not been investigated. We find that LIF(-/-) mice have a pronounced myelination defect in optic nerve at postnatal day 10. Myelin basic protein (MBP)- and proteolipid protein (PLP)-positive myelin was evident throughout the optic nerve in the wild-type mice, but staining was present only at the chiasmal region in LIF(-/-) mice of the same age. Further experiments suggest that the myelination defect was a consequence of a delay in maturation of oligodendrocyte precursor cell (OPC) population. The number of Olig2-positive cells was dramatically decreased in optic nerve of LIF(-/-) mice, and the distribution of Olig2-positive cells was restricted to the chiasmal region of the nerve in a steep gradient toward the retina. Gene expression profiling and cell culture experiments revealed that OPCs from P10 optic nerve of LIF(-/-) mice remained in a highly proliferative immature stage compared with littermate controls. Interestingly, by postnatal day 14, MBP immunostaining in the LIF(-/-) optic nerve was comparable to that of LIF(+/+) mice. These results suggest that, during normal development of mouse optic nerve, there is a defined developmental time window when LIF is required for correct myelination. Myelination seems to recover by postnatal day 14, so LIF is not necessary for the completion of myelination during postnatal development.

Fibrae medullares in the retina of the RD mouse: a case report

PURPOSE

A rare finding of fibrae medullares in the mouse eye is presented in an animal suffering from retinal degeneration.

MATERIAL AND METHODS

Numerous eyes from different mouse strains were screened for myelinated fibres in the retina. Light and electron microscopy was used to document a single case in a rd mouse.

RESULTS

Myelinated fibres were found in the inner retina but not continuous with the optic nerve.

CONCLUSION

Even in this highly degenerated retina the potency of the optic nerve fibres to become myelinated still remained. The source of the appropriate glia cells is discussed.

Secondary degeneration of the optic nerve following partial transection: the benefits of lomerizine

Secondary degeneration is a form of 'bystander' damage that can affect neural tissue both nearby and remote from an initial injury. Partial optic nerve transection is an excellent model in which to unequivocally differentiate events occurring during secondary degeneration from those resulting from primary CNS injury. We analysed the primary injury site within the optic nerve (ON) and intact areas vulnerable to secondary degeneration. Areas affected by the primary injury showed morphological disruption, loss of beta-III tubulin axonal staining, reduced myelinated axon density, greater proteoglycan expression (phosphacan), increased microglia and macrophage numbers and increased oxidative stress. Similar, but less extreme, changes were seen in areas of the optic nerve undergoing secondary degeneration. The CNS-specific L- and T-type calcium channel blocker lomerizine alleviated some of the changes in areas vulnerable to secondary degeneration. Lomerizine reduced morphological disruption, oxidative stress and phosphacan expression, and limited early increases in macrophage numbers. However, lomerizine failed to prevent progressive de-myelination of ON axons. Within the retina, secondary retinal ganglion cell (RGC) death was significant in areas vulnerable to secondary degeneration. Lomerizine protected RGCs from secondary death at 4 weeks but did not fully restore behavioural function (optokinetic nystagmus). We conclude that blockade of calcium channels is neuroprotective and limits secondary degenerative changes following CNS injury. However such an approach may need to be combined with other treatments to ensure long-term maintenance of full visual function.

Morphological and functional characterization of predifferentiation of myelinating glia-like cells from human bone marrow stromal cells through activation of F3/Notch signaling in mouse retina

Recently, we have demonstrated that F3/contactin and NB-3 are trans-acting extracellular ligands of Notch that promote differentiation of neural stem cells and oligodendrocyte precursor cells into mature oligodendrocytes (OLs). Here, we demonstrate that human bone marrow stromal cells (hBMSCs) can be induced to differentiate into cells with myelinating glial cell characteristics in mouse retina after predifferentiation in vitro. Isolated CD90(+) hBMSCs treated with beta-mercaptoethanol for 1 day and retinoic acid for 3 days in culture changed into myelinating glia-like cells (MGLCs). More cells expressed NG2, an early OL marker, after treatment, but expression of O4, a mature OL marker, was negligible. Subsequently, the population of O4(+) cells was significantly increased after the MGLCs were predifferentiated in culture in the presence of either F3/contactin or multiple factors, including forskolin, basic fibroblast growth factor, platelet-derived growth factor, and heregulin, in vitro for another 3 days. Notably, 2 months after transplantation into mouse retina, the predifferentiated cells changed morphologically into cells resembling mature MGLCs and expressing O4 and myelin basic protein, two mature myelinating glial cell markers. The cells sent out processes to contact and wrap axons, an event that normally occurs during early stages of myelination, in the retina. The results suggest that CD90(+) hBMSCs are capable of morphological and functional differentiation into MGLCs in vivo through predifferentiation by triggering F3/Notch signaling in vitro.

Sonic hedgehog promotes the migration and proliferation of optic nerve oligodendrocyte precursors

Optic nerve (ON) oligodendrocyte precursors (OPCs) are generated under the influence of the Sonic hedgehog (Shh) in the preoptic area from where they migrate to colonise the entire nerve. The molecular events that control this migration are still poorly understood. Recent studies suggested that Shh is often used by the same cell population to control different processes, including cell proliferation and migration, raising the possibility that Shh could contribute to these aspects of OPC development. In support of this idea, we show here that Shh induces the proliferation of OPCs derived from embryonic mouse ON explants and acts as a chemoattractant for their migration. In ovo injections of hybridomas secreting Shh-specific blocking antibody decreases the number of OPCs present in chick ONs, particularly in the retinal portion of the nerve. Altogether these data indicate that Shh contributes to OPC proliferation and distribution along the ON, in addition to their specification.