Two-Photon Autofluorescence Imaging Reveals Cellular Structures Throughout the Retina of the Living Primate Eye

PURPOSE

Although extrinsic fluorophores can be introduced to label specific cell types in the retina, endogenous fluorophores, such as NAD(P)H, FAD, collagen, and others, are present in all retinal layers. These molecules are a potential source of optical contrast and can enable noninvasive visualization of all cellular layers. We used a two-photon fluorescence adaptive optics scanning light ophthalmoscope (TPF-AOSLO) to explore the native autofluorescence of various cell classes spanning several layers in the unlabeled retina of a living primate eye.

METHODS

Three macaques were imaged on separate occasions using a custom TPF-AOSLO. Two-photon fluorescence was evoked by pulsed light at 730 and 920 nm excitation wavelengths, while fluorescence emission was collected in the visible range from several retinal layers and different locations. Backscattered light was recorded simultaneously in confocal modality and images were postprocessed to remove eye motion.

RESULTS

All retinal layers yielded two-photon signals and the heterogeneous distribution of fluorophores provided optical contrast. Several structural features were observed, such as autofluorescence from vessel walls, Müller cell processes in the nerve fibers, mosaics of cells in the ganglion cell and other nuclear layers of the inner retina, as well as photoreceptor and RPE layers in the outer retina.

CONCLUSIONS

This in vivo survey of two-photon autofluorescence throughout the primate retina demonstrates a wider variety of structural detail in the living eye than is available through conventional imaging methods, and broadens the use of two-photon imaging of normal and diseased eyes.

Downregulation of the Canonical WNT Signaling Pathway by TGFβ1 Inhibits Photoreceptor Differentiation of Adult Human Müller Glia with Stem Cell Characteristics

Müller glia are responsible for the retina regeneration observed in zebrafish. Although the human retina harbors Müller glia with stem cell characteristics, there is no evidence that they regenerate the retina after disease or injury. Transforming growth factor-β (TGFβ) and Wnt signaling regulate retinal neurogenesis and inflammation, but their roles in the neural differentiation of human Müller stem cells (hMSC) are not known. We examined hMSC lines in vitro for the expression of various Wnt signaling components and for their modulation by TGFβ1, as well as the effect of this cytokine on the photoreceptor differentiation of these cells. Culture of hMSC with a combination of factors that induce photoreceptor differentiation of hMSC (FGF2, taurine, retinoic acid, and insulin-like growth factor type1; FTRI), markedly upregulated the expression of components of the canonical Wnt signaling pathway, including WNT2B, DKK1, and active β-CATENIN. Although FTRI did not modify mRNA expression of WNT5B, a component of the noncanonical/planar cell polarity Wnt pathway, it upregulated its secretion. Furthermore, TGFβ1 not only decreased WNT2B expression, but also inhibited FTRI-induced photoreceptor differentiation of hMSC, as determined by expression of the photoreceptor markers NR2E3, RHODOPSIN, and RECOVERIN. Inhibition of TGFβ1 signaling by an ALK5 inhibitor prevented TGFβ1-induced changes in the expression of the two Wnt ligands examined. More importantly, inhibition of the canonical WNT signaling by XAV-939 prevented FTRI-induced photoreceptor differentiation. These observations suggest that TGFβ may play a key role in preventing neural differentiation of hMSC and may constitute a potential target for induction of endogenous regeneration of the human retina.

Heparin-binding EGF-like growth factor (HB-EGF) stimulates the proliferation of Müller glia-derived progenitor cells in avian and murine retinas

Müller glia can be stimulated to de-differentiate, proliferate and form Müller glia-derived progenitor cells (MGPCs) that regenerate retinal neurons. In the zebrafish retina, heparin-binding EGF-like growth factor (HB-EGF) may be one of the key factors that stimulate the formation of proliferating MGPCs. Currently nothing is known about the influence of HB-EGF on the proliferative potential of Müller glia in retinas of birds and rodents. In the chick retina, we found that levels of both hb-egf and egf-receptor are rapidly and transiently up-regulated following NMDA-induced damage. Although intraocular injections of HB-EGF failed to stimulate cell-signaling or proliferation of Müller glia in normal retinas, HB-EGF stimulated proliferation of MGPCs in damaged retinas. By comparison, inhibition of the EGF-receptor (EGFR) decreased the proliferation of MGPCs in damaged retinas. HB-EGF failed to act synergistically with FGF2 to stimulate the formation of MGPCs in the undamaged retina and inhibition of EGF-receptor did not suppress FGF2-mediated formation of MGPCs. In the mouse retina, HB-EGF stimulated the proliferation of Müller glia following NMDA-induced damage. Furthermore, HB-EGF not only stimulated MAPK-signaling in Müller glia/MGPCs, but also activated mTor- and Jak/Stat-signaling. We propose that levels of expression of EGFR are rate-limiting to the responses of Müller glia to HB-EGF and the expression of EGFR can be induced by retinal damage, but not by FGF2-treatment. We conclude that HB-EGF is mitogenic to Müller glia in both chick and mouse retinas, and HB-EGF is an important player in the formation of MGPCs in damaged retinas.

Subset of early radial glial progenitors that contribute to the development of callosal neurons is absent from avian brain

The classical view of mammalian cortical development suggests that pyramidal neurons are generated in a temporal sequence, with all radial glial cells (RGCs) contributing to both lower and upper neocortical layers. A recent opposing proposal suggests there is a subgroup of fate-restricted RGCs in the early neocortex, which generates only upper-layer neurons. Little is known about the existence of fate restriction of homologous progenitors in other vertebrate species. We investigated the lineage of selected Emx2+ [vertebrate homeobox gene related to Drosophila empty spiracles (ems)] RGCs in mouse neocortex and chick forebrain and found evidence for both sequential and fate-restricted programs only in mouse, indicating that these complementary populations coexist in the developing mammalian but not avian brain. Among a large population of sequentially programmed RGCs in the mouse brain, a subset of self-renewing progenitors lack neurogenic potential during the earliest phase of corticogenesis. After a considerable delay, these progenitors generate callosal upper-layer neurons and glia. On the other hand, we found no homologous delayed population in any sectors of the chick forebrain. This finding suggests that neurogenic delay of selected RGCs may be unique to mammals and possibly associated with the evolution of the corpus callosum.

Cellular and subcellular imaging of motor protein-based behavior in embryonic rat brain

Development of the cerebral cortex is a very dynamic process, involving a series of complex morphogenetic events. Following division of progenitor cells in the ventricular zone, neurons undergo a series of morphological changes and migrate outward toward the cortical plate, where they differentiate and integrate into functional circuits. Errors at several of stages during neurogenesis and migration cause a variety of severe cortical malformations. A number of disease genes encode factors associated with the cytoskeleton, which plays a crucial role throughout cortical development. Methods for regulating gene expression coupled with imaging of subcellular structures have provided important insight into the mechanisms governing normal and abnormal brain development. We describe here a series of protocols for imaging motor protein-dependent processes in real time in the developing rat brain.

Sodium salicylate reduced insulin resistance in the retina of a type 2 diabetic rat model

Sodium salicylate has been reported to reduce markers of diabetic retinopathy in a type 1 rat model. Because rates of type 2 diabetes are on the rise, we wanted to determine whether salicylate could improve insulin resistance in a type 2 rat model, as well as improve retinal function. We treated lean and obese BBZDR/Wor type 2 diabetic rats with salicylate in their chow for 2 months. Prior to salicylate treatment, rats underwent an electroretinogram to measure retinal function. After 2 months of treatment, rats underwent an additional electroretinogram prior to sacrifice. In addition to the animal model, we also treated retinal endothelial cells (REC) and rat Müller cells with salicylate and performed the same analyses as done for the rat retinal lysates. To investigate the role of salicylate in insulin signaling, we measured TNFα and caspase 3 levels by ELISA, as well as performed Western blotting for insulin receptor substrate 1, insulin receptor, SOCS3, and pro- and anti-apoptotic markers. Data demonstrated that salicylate significantly improved retinal function, as well as reduced TNFα and SOCS3-induced insulin resistance in all samples. Overall, results suggest that salicylate is effective in reducing insulin resistance in the retina of type 2 diabetic rat models.

Human pluripotent stem cell-derived radial glia recapitulate developmental events and provide real-time access to cortical neurons and astrocytes

Studies of human cerebral cortex development are limited by difficulties in accessing and manipulating human neural tissue at specific development stages. We have derived human radial glia (hRG), which are responsible for most cerebral cortex neurogenesis, from human pluripotent stem cells. These hRG display the hallmark morphological, cellular, and molecular features of radial glia in vitro. They can be passaged and generate layer-specific subtypes of cortical neurons in a temporal and passage-dependent fashion. In later passages, they adopt a distinct progenitor phenotype that gives rise to cortical astrocytes and GABAergic interneurons. These hRG are also capable of following developmental cues to engraft, differentiate, migrate, and integrate into the embryonic mouse cortex when injected into E14 lateral ventricles. Moreover, hRG-derived cells can be cryopreserved at specific stages and retain their stage-specific phenotypes and competence when revived. Our study demonstrates that cultured hRG maintain a cell-intrinsic clock that regulates the progressive generation of stage-specific neuronal and glial subtypes. It also describes an easily accessible cell source for studying hRG lineage specification and progression and an on-demand supply of specific cortical neuron subtypes and astrocytes.

Morphological changes of radial glial cells during mouse embryonic development

During brain development, the radial glial cell acts as a scaffold to support radial migration of postmitotic neurons. However, the morphological changes of radial glial cells during embryo development are poorly understood. We used in utero electroporation and immunohistochemistry to study the dynamics of radial glial cells accompanied by cortical development in mice from embryonic day 14 to postnatal day 0. We found that different segments of radial glial cells changed by the growth of different layers of cortex, such as marginal zone, cortical plate, intermediate zone and ventricular zone. Moreover, the length, angle and number of branches of the radial glial cell changed significantly at the late stage of neurogenesis. All these changes were consistent with the distinct phases of locomotion. Thus, we speculated that morphological changes of the radial glial cell were associated with the neuronal migration and dendritic development.

Runx1t1 regulates the neuronal differentiation of radial glial cells from the rat hippocampus

The brain has the highest Runx1t1 level relative to the levels in other organs. Runx1t1 might have a regulatory function as a transcriptional corepressor in the differentiation/development of the nervous system. Neurogenesis requires factors that regulate the proliferation of progenitors and activate the neuronal differentiation process. However, the precise role of Runx1t1 in hippocampal neurogenesis is unclear. We knocked down Runx1t1 in hippocampal radial glial cells (RGCs) with Runx1t1-RNA interference using lentiviral vectors. We also used LV-Runx1t1 to induce Runx1t1 overexpression in vitro. We have provided experimental evidence that decreased Runx1t1 expression reduced the neuronal differentiation of RGCs, and increased Runx1t1 expression caused a greater number of RGCs to differentiate into neurons. We have concluded that Runx1t1 could be involved in the process through which RGCs differentiate into neurons.

Unidirectional photoreceptor-to-Müller glia coupling and unique K+ channel expression in Caiman retina

Background

Müller cells, the principal glial cells of the vertebrate retina, are fundamental for the maintenance and function of neuronal cells. In most vertebrates, including humans, Müller cells abundantly express Kir4.1 inwardly rectifying potassium channels responsible for hyperpolarized membrane potential and for various vital functions such as potassium buffering and glutamate clearance; inter-species differences in Kir4.1 expression were, however, observed. Localization and function of potassium channels in Müller cells from the retina of crocodiles remain, hitherto, unknown.

Methods

We studied retinae of the Spectacled caiman (Caiman crocodilus fuscus), endowed with both diurnal and nocturnal vision, by (i) immunohistochemistry, (ii) whole-cell voltage-clamp, and (iii) fluorescent dye tracing to investigate K⁺ channel distribution and glia-to-neuron communications.

Results

Immunohistochemistry revealed that caiman Müller cells, similarly to other vertebrates, express vimentin, GFAP, S100β, and glutamine synthetase. In contrast, Kir4.1 channel protein was not found in Müller cells but was localized in photoreceptor cells. Instead, 2P-domain TASK-1 channels were expressed in Müller cells. Electrophysiological properties of enzymatically dissociated Müller cells without photoreceptors and isolated Müller cells with adhering photoreceptors were significantly different. This suggests ion coupling between Müller cells and photoreceptors in the caiman retina. Sulforhodamine-B injected into cones permeated to adhering Müller cells thus revealing a uni-directional dye coupling.

Conclusion

Our data indicate that caiman Müller glial cells are unique among vertebrates studied so far by predominantly expressing TASK-1 rather than Kir4.1 K⁺ channels and by bi-directional ion and uni-directional dye coupling to photoreceptor cells. This coupling may play an important role in specific glia-neuron signaling pathways and in a new type of K⁺ buffering.

NFκB signaling is essential for the lipopolysaccharide-induced increase of type 2 deiodinase in tanycytes

The enzyme type 2 deiodinase (D2) is a major determinant of T₃ production in the central nervous system. It is highly expressed in tanycytes, a specialized cell type lining the wall of the third ventricle. During acute inflammation, the expression of D2 in tanycytes is up-regulated by a mechanism that is poorly understood at present, but we hypothesized that cJun N-terminal kinase 1 (JNK1) and v-rel avian reticuloendotheliosis viral oncogene homolog A (RelA) (the 65 kD subunit of NFκB) inflammatory signal transduction pathways are involved. In a mouse model for acute inflammation, we studied the effects of lipopolysaccharide (LPS) on mRNA expression of D2, JNK1, and RelA in the periventricular area (PE) and the arcuate nucleus-median eminence of the hypothalamus. We next investigated LPS-induced D2 expression in primary tanycyte cell cultures. In the PE, the expression of D2 was increased by LPS. In the arcuate nucleus, but not in the PE, we found increased RelA mRNA expression. Likewise, LPS increased D2 and RelA mRNA expression in primary tanycyte cell cultures, whereas JNK1 mRNA expression did not change. Phosphorylation of RelA and JNK1 was increased in tanycyte cell cultures 15-60 minutes after LPS stimulation, confirming activation of these pathways. Finally, inhibition of RelA with the chemical inhibitors sulfasalazine and 4-Methyl-N¹-(3-phenylpropyl)benzene-1,2-diamine (JSH-23) in tanycyte cell cultures prevented the LPS-induced D2 increase. We conclude that NFκB signaling is essential for the up-regulation of D2 in tanycytes during inflammation.

Transplantation of photoreceptors derived from human Muller glia restore rod function in the P23H rat

Müller glia possess stem cell characteristics that have been recognized to be responsible for the regeneration of injured retina in fish and amphibians. Although these cells are present in the adult human eye, they are not known to regenerate human retina in vivo. Human Müller glia with stem cell characteristics (hMSCs) can acquire phenotypic and genotypic characteristics of rod photoreceptors in vitro, suggesting that they may have potential for use in transplantation strategies to treat human photoreceptor degenerations. Much work has been undertaken in rodents using various sources of allogeneic stem cells to restore photoreceptor function, but the effect of human Müller glia-derived photoreceptors in the restoration of rod photoreceptor function has not been investigated. This study aimed to differentiate hMSCs into photoreceptor cells by stimulation with growth and differentiation factors in vitro to upregulate gene and protein expression of CRX, NR2E3, and rhodopsin and various phototransduction markers associated with rod photoreceptor development and function and to examine the effect of subretinal transplantation of these cells into the P23H rat, a model of primary photoreceptor degeneration. Following transplantation, hMSC-derived photoreceptor cells migrated and integrated into the outer nuclear layer of the degenerated retinas and led to significant improvement in rod photoreceptor function as shown by an increase in a-wave amplitude and slope using scotopic flash electroretinography. These observations suggest that hMSCs can be regarded as a cell source for development of cell-replacement therapies to treat human photoreceptor degenerations and may also offer potential for the development of autologous transplantation.

[Three-dimensional visualization of the brain ependyma and tanycytes]

The aim of this investigation was to develop an integrated approach to spatial reconstruction of the cells lining the ventricles of the brain using confocal laser microscopy and immunocytochemical reaction to vimentin. The work was performed on paraffin sections of rat brain of different thickness (5 and 10 microm). To visualize the immunocytochemical reaction the fluorescent dyes in the visible range were selected: SYTOX Green selectively staining the nucleus and indocarbocyanin (Cy-3) conjugated with streptavidin. As a result of testing of various processing conditions, the protocol which allows to receive an intensive staining of the structures was developed. The set of fluorochromes proposed in confocal laser microscopy allows to separate easily the channels, to study the structures independently, if needed, and does not require the use of an expensive ultraviolet laser.

Elevated cell proliferation and VEGF production by high-glucose conditions in Müller cells involve XIAP

PURPOSE

Müller cells have important roles in the pathogenesis of diabetic retinopathy by promoting cell proliferation and inducing the production of vascular endothelial growth factor (VEGF) under hyperglycemic conditions. The objective of this study was to determine the potential mechanism of Müller cell proliferation and VEGF production due to high-glucose conditions.

METHODS

Primary cultured rat Müller cells were incubated with medium containing variable concentrations of glucose and/or embelin, a specific inhibitor of X-linked inhibitor of apoptosis protein (XIAP), for 72 h. The proliferation of Müller cells was assessed by the MTT assay. The expression and/or phosphorylation of 146 proteins were assessed using protein pathway array.

RESULTS

High concentrations of glucose-induced Müller cell proliferation and altered expression and/or phosphorylation of 47 proteins that have been identified to have key roles in several important signaling pathways (XIAP, VEGF, HIF1α, NFκB, etc) and are involved in the regulation of cell survival, proliferation, or apoptosis. However, Müller cell alterations induced by high-glucose conditions were counteracted by the XIAP inhibitor embelin, and 26 proteins/phosphorylations (out of 47) were restored to their normal levels. Nine proteins, including NFκB p65, p-p38, tumor necrosis factor-α, urokinase-type plasminogen activator, CREB, IL-1β, HCAM, estrogen receptor-α, and p-Stat3, were involved in regulatory networks between XIAP and VEGF.

CONCLUSIONS

The current study suggests that XIAP may be a potential regulator that can mediate a series of pathological changes induced by high-glucose conditions in Müller cells. Therefore, embelin could be a potential agent for the prevention and treatment of diabetic retinopathy.

Acquisition of RGC phenotype in human Müller glia with stem cell characteristics is accompanied by upregulation of functional nicotinic acetylcholine receptors

PURPOSE

Human Müller glia with stem cell characteristics (hMGSCs) can be induced to express genes and proteins of retinal ganglion cells (RGCs) upon in vitro inhibition of Notch-1 activity. However, it is not known whether expression of these markers is accompanied by acquisition of RGC function. This study investigated whether hMGSCs that express RGC markers also display neural functionality, as measured by their intracellular calcium concentration ([Ca(2+)]i) responsiveness following neurotransmitter stimulation in vitro.

METHODS

Changes in mRNA expression of RGC markers and neurotransmitter receptors were assessed either by conventional or quantitative reverse transcription PCR (RT-PCR), while changes in protein levels were confirmed by immunocytochemistry. The [Ca(2+)]i levels were estimated by fluorescence microscopy.

RESULTS

We showed that while undifferentiated hMGSCs displayed a profound elevation of [Ca(2+)]i after stimulation with N-methyl-D-aspartate (NMDA), this was lost following Notch-1 inhibition. Conversely, untreated hMGSCs did not respond to muscarinic receptor stimulation, whereas [Ca(2+)]i was increased in differentiated hMGSCs that expressed RGC precursor markers. Differentiated hMGSC-derived RGCs, but not undifferentiated hMGSCs, responded to stimulation by nicotine with a substantial rise in [Ca(2+)]i, which was inhibited by the α4β2 and α6β2 nicotinic receptor antagonist methyllycaconitine. Notch-1 attenuation not only caused a decrease in the gene expression of the Notch effector HES1 and increased expression of RGC markers, but also an increase in the gene and protein expression of α4 and α6 nicotinic receptor subunits.

CONCLUSIONS

These observations suggest that in response to Notch-1 inhibition, hMGSCs differentiate into a population of RGCs that exhibit some of the functionality observed in differentiated RGCs.

New functions of Müller cells

Müller cells, the major type of glial cells in the retina, are responsible for the homeostatic and metabolic support of retinal neurons. By mediating transcellular ion, water, and bicarbonate transport, Müller cells control the composition of the extracellular space fluid. Müller cells provide trophic and anti-oxidative support of photoreceptors and neurons and regulate the tightness of the blood-retinal barrier. By the uptake of glutamate, Müller cells are more directly involved in the regulation of the synaptic activity in the inner retina. This review gives a survey of recently discoved new functions of Müller cells. Müller cells are living optical fibers that guide light through the inner retinal tissue. Thereby they enhance the signal/noise ratio by minimizing intraretinal light scattering and conserve the spatial distribution of light patterns in the propagating image. Müller cells act as soft, compliant embedding for neurons, protecting them in case of mechanical trauma, and also as soft substrate required for neurite growth and neuronal plasticity. Müller cells release neuroactive signaling molecules which modulate neuronal activity, are implicated in the mediation of neurovascular coupling, and mediate the homeostasis of the extracellular space volume under hypoosmotic conditions which are a characteristic of intense neuronal activity. Under pathological conditions, a subset of Müller cells may differentiate to neural progenitor/stem cells which regenerate lost photoreceptors and neurons. Increasing knowledge of Müller cell function and responses in the normal and diseased retina will have great impact for the development of new therapeutic approaches for retinal diseases.