Embryonic nutritional hyperglycemia decreases cell proliferation in the zebrafish retina

Diabetic retinopathy (DR) is one of the leading causes of blindness in the world. While there is a major focus on the study of juvenile/adult DR, the effects of hyperglycemia during early retinal development are less well studied. Recent studies in embryonic zebrafish models of nutritional hyperglycemia (high-glucose exposure) have revealed that hyperglycemia leads to decreased cell numbers of mature retinal cell types, which has been related to a modest increase in apoptotic cell death and altered cell differentiation. However, how embryonic hyperglycemia impacts cell proliferation in developing retinas still remains unknown. Here, we exposed zebrafish embryos to 50 mM glucose from 10 h postfertilization (hpf) to 5 days postfertilization (dpf). First, we confirmed that hyperglycemia increases apoptotic death and decreases the rod and Müller glia population in the retina of 5-dpf zebrafish. Interestingly, the increase in cell death was mainly observed in the ciliary marginal zone (CMZ), where most of the proliferating cells are located. To analyze the impact of hyperglycemia in cell proliferation, mitotic activity was first quantified using pH3 immunolabeling, which revealed a significant decrease in mitotic cells in the retina (mainly in the CMZ) at 5 dpf. A significant decrease in cell proliferation in the outer nuclear and ganglion cell layers of the central retina in hyperglycemic animals was also detected using the proliferation marker PCNA. Overall, our results show that nutritional hyperglycemia decreases cellular proliferation in the developing retina, which could significantly contribute to the decline in the number of mature retinal cells.

Characteristics of Whale Müller Glia in Primary and Immortalized Cultures

Müller cells are the principal glial cells in the retina and they assume many of the functions carried out by astrocytes, oligodendrocytes and ependymal cells in other regions of the central nervous system. Müller cells express growth factors, neurotransmitter transporters and antioxidant agents that could fulfill important roles in preventing excitotoxic damage to retinal neurons. Vertebrate Müller cells are well-defined cells, characterized by a common set of features throughout the phylum. Nevertheless, several major differences have been observed among the Müller cells in distinct vertebrates, such as neurogenesis, the capacity to reprogram fish Müller glia to neurons. Here, the Müller glia of the largest adult mammal in the world, the whale, have been analyzed, and given the difficulties in obtaining cetacean cells for study, these whale glia were analyzed both in primary cultures and as immortalized whale Müller cells. After isolating the retina from the eye of a beached sei whale (Balaenoptera borealis), primary Müller cell cultures were established and once the cultures reached confluence, half of the cultures were immortalized with the simian virus 40 (SV40) large T-antigen commonly used to immortalize human cell lines. The primary cell cultures were grown until cells reached senescence. Expression of the principal molecular markers of Müller cells (GFAP, Vimentin and Glutamine synthetase) was studied in both primary and immortalized cells at each culture passage. Proliferation kinetics of the cells were analyzed by time-lapse microscopy: the time between divisions, the time that cells take to divide, and the proportion of dividing cells in the same field. The karyotypes of the primary and immortalized whale Müller cells were also characterized. Our results shown that W21M proliferate more rapidly and they have a stable karyotype. W21M cells display a heterogeneous cell morphology, less motility and a distinctive expression of some typical molecular markers of Müller cells, with an increase in dedifferentiation markers like α-SMA and β-III tubulin, while they preserve their GS expression depending on the culture passage. Here we also discuss the possible influence of the animal's age and size on these cells, and on their senescence.

TNFα activates MAPK and Jak-Stat pathways to promote mouse Müller cell proliferation

Mouse Müller cells, considered as dormant retinal progenitors, often respond to retinal injury by undergoing reactive gliosis rather than displaying neural regenerative responses. Tumor necrosis factor alpha (TNFα) is a key cytokines induced after injury and implicated in mediating inflammatory and neural regenerative responses in zebrafish. To investigate the involvement of TNFα in mouse retinal injury, adult C57BL/6J mice were subjected to light damage for 14 consecutive days. TNFα was elevated in the retina of mice exposed to light damage, which induced Müller cell proliferation in vitro. Affymetrix microarray showed that, in Müller cells, TNFα induces up-regulation of inflammatory and proliferation-related genes, including NFKB2, leukemia inhibitory factor, interleukin-6, janus kinase (Jak) 1, Jak2, signal transducer and activator of transcription (Stat) 1, Stat2, mitogen-activated protein kinase (MAPK) 7, and MAP4K4 but down-regulation of neuroprogenitor genes, including Sox9, Ascl1, Wnt2 and Hes1. Blocking the Jak/Stat and MAPK pathways attenuated TNFα-induced Müller cell proliferation. These results suggest that TNFα may drive the proliferation and inflammatory response, rather than the neural regenerative potential, of mouse Müller cells.

Optimization of a Method to Isolate and Culture Adult Porcine, Rats and Mice Müller Glia in Order to Study Retinal Diseases

Müller cells are the predominant glial elements in the retina, extending vertically across this structure, and they fulfill a wealth support roles that are critical for neurons. Alterations to the behavior and phenotype of Müller glia are often seen in animal models of retinal degeneration and in retinal tissue from patients with a variety of retinal disorders. Thus, elucidating the mechanisms underlying the development of retinal diseases would help better understand the cellular processes involved in such pathological changes. Studies into Müller cell activity in vitro have been hindered by the difficulty in obtaining pure cell populations and the tendency of these cells to rapidly differentiate in culture. Most protocols currently used to isolate Müller glia use neonatal or embryonic tissue but here, we report an optimized protocol that facilitates the reliable and straightforward isolation and culture of Müller cells from adult pigs, rats and mice. The protocol described here provides an efficient method for the rapid isolation of adult mammalian Müller cells, which represents a reliable platform to study therapeutic targets and to test the effects of drugs that might combat retinal diseases.

Opposing Actions of Fgf8a on Notch Signaling Distinguish Two Muller Glial Cell Populations that Contribute to Retina Growth and Regeneration

The teleost retina grows throughout life and exhibits a robust regenerative response following injury. Critical to both these events are Muller glia (or, Muller glial cells; MGs), which produce progenitors for retinal growth and repair. We report that Fgf8a may be an MG niche factor that acts through Notch signaling to regulate spontaneous and injury-dependent MG proliferation. Remarkably, forced Fgf8a expression inhibits Notch signaling and stimulates MG proliferation in young tissue but increases Notch signaling and suppresses MG proliferation in older tissue. Furthermore, cessation of Fgf8a signaling enhances MG proliferation in both young and old retinal tissue. Our study suggests that multiple MG populations contribute to retinal growth and regeneration, and it reveals a previously unappreciated role for Fgf8a and Notch signaling in regulating MG quiescence, activation, and proliferation.

Light-induced degeneration and microglial response in the retina of an epibenthonic pigmented teleost: age-dependent photoreceptor susceptibility to cell death

Constant intense light causes apoptosis of photoreceptors in the retina of albino fish. However, very few studies have been performed on pigmented species. Tench (Tinca tinca) is a teleost inhabiting dimly lit environments that has a predominance of rods within the photoreceptor layer. To test the hypothesis that constant high intensity light can result in retinal damage in such pigmented epibenthonic teleost species, photodegeneration of the retina was investigated in the larvae and in juveniles of tench to assess whether any damage may also be dependent on fish age. We exposed both groups of animals to 5 days of constant darkness, followed by 4 days of constant 20,000 lx light, and then by 6 days of recovery in a 14 h light:10 h dark cycle. The results showed that the retina of the larvae group exhibited abundant photoreceptor cell apoptosis during the time of exposition to intense light, whereas that of juveniles was indifferent to it. Damaged retinas showed a strong TUNEL signal in photoreceptor nuclei, and occasionally a weak cytoplasmic TUNEL signal in Müller glia. Specific labelling of microglial cells with Griffonia simplicifolia lectin (GSL) histochemistry revealed that photoreceptor cell death alerts microglia in the degenerating retina, leading to local proliferation, migration towards the injured outer nuclear layer (ONL), and enhanced phagocytosis of photoreceptor debris. During the first days of intense light treatment, Müller cells phagocytosed dead photoreceptor cells but, once microglial cells became activated, there was a progressive increase in the phagocytic capacity of the microglia.

Retinal histogenesis and cell differentiation in an elasmobranch species, the small-spotted catshark Scyliorhinus canicula

Here we present a detailed study of the major events in the retinal histogenesis in a slow-developing elasmobranch species, the small-spotted catshark, during embryonic, postnatal and adult stages using classical histological and immunohistological methods, providing a complete neurochemical characterization of retinal cells. We found that the retina of the small-spotted catshark was fully differentiated prior to birth. The major developmental events in retinal cell differentiation occurred during the second third of the embryonic period. Maturational features described in the present study were first detected in the central retina and, as development progressed, they spread to the rest of the retina following a central-to-peripheral gradient. While the formation of both plexiform layers occurs simultaneously in the retina of the most common fish models, in the small-spotted catshark retina the emergence of the outer plexiform layer was delayed with respect to the inner plexiform layer. According to the expression of the markers used, retinal cell differentiation followed a vitreal-to-scleral gradient, with the exception of Müller cells that were the last cell type generated during retinogenesis. This vitreal-to-scleral progression of neural differentiation seems to be specific to slow-developing fish species.

Macrophage and microglia ontogeny in the mouse visual system can be traced by the expression of Cathepsins B and D

Here, we show a detailed chronotopographical analysis of cathepsin B and D expression during development of the mouse visual system. Both proteases were detected in large rounded/ameboid cells usually located in close relationship with prominent sites of extensive physiological cell death. In concordance with their morphological features and topographical distribution, we demonstrate that expressing cells corresponded with macrophages and microglial precursors. We found that as microglial precursors differentiated the expression of both cathepsins was down-regulated. Of interest, cathepsin B and D transcripts were never observed in degenerating cells. Our findings point to a role for cathepsin D and B in cell debris degradation after apoptotic processes rather than promoting cell death, as proposed for other developmental models. Additionally their pattern of expression suggests a role in the maturation of the microglial precursors.

Astroglial structures in the zebrafish brain

To understand components shaping the neuronal environment we studied the astroglial cells in the zebrafish brain using immunocytochemistry for structural and junctional markers, electron microscopy including freeze fracturing, and probed for the water channel protein aquaporin-4. Glial fibrillary acidic protein (GFAP) and glutamine synthetase (GS) showed largely overlapping immunoreactivity: GFAP in the main glial processes and GS in main processes and smaller branches. Claudin-3 immunoreactivity was spread in astroglial cells along their major processes. The ventricular lining was immunoreactive for the tight-junction associated protein ZO-1, in the telencephalon located on the dorsal, lateral, and medial surface due to the everting morphogenesis. In the tectum, subpial glial endfeet were also positive for ZO-1. Correspondingly, electron microscopy revealed junctional complexes between subpial glial endfeet. However, in freeze-fracture analysis tight junctional strands were not found between astroglial membranes, either in the optic tectum or in the telencephalon. Occurrence of aquaporin-4, the major astrocytic water channel in mammals, was demonstrated by polymerase chain reaction (PCR) analysis and immunocytochemistry in tectum and telencephalon. Localization of aquaporin-4 was not polarized but distributed along the entire radial extent of the cell. Interestingly, their membranes were devoid of the orthogonal arrays of particles formed by aquaporin-4 in mammals. Finally, we investigated astroglial cells in proliferative areas. Brain lipid basic protein, a marker of early glial differentiation but not GS, were present in some proliferation zones, whereas cells lining the ventricle were positive for both markers. Thus, astroglial cells in the zebrafish differ in many aspects from mammalian astrocytes.

Cellular signaling and factors involved in Müller cell gliosis: neuroprotective and detrimental effects

Müller cells are active players in normal retinal function and in virtually all forms of retinal injury and disease. Reactive Müller cells protect the tissue from further damage and preserve tissue function by the release of antioxidants and neurotrophic factors, and may contribute to retinal regeneration by the generation of neural progenitor/stem cells. However, Müller cell gliosis can also contribute to neurodegeneration and impedes regenerative processes in the retinal tissue by the formation of glial scars. This article provides an overview of the neuroprotective and detrimental effects of Müller cell gliosis, with accounts on the cellular signal transduction mechanisms and factors which are implicated in Müller cell-mediated neuroprotection, immunomodulation, regulation of Müller cell proliferation, upregulation of intermediate filaments, glial scar formation, and the generation of neural progenitor/stem cells. A proper understanding of the signaling mechanisms implicated in gliotic alterations of Müller cells is essential for the development of efficient therapeutic strategies that increase the supportive/protective and decrease the destructive roles of gliosis.

Evidence for a columnar organization of cones, Müller cells, and neurons in the retina of a cichlid fish

In the retina of many lower vertebrates, the arrangement of cells, in particular of cone photoreceptors, is highly regular. The data presented in this report show that in the retina of a cichlid fish (Astatotilapia burtoni) the regular arrangement is not restricted to cone photoreceptors and their synaptic terminals but can be found in elements of the inner retina as well. A variety of immunocytochemical and other markers was used in combination with confocal microscopy on whole-mount preparations and tangential sections. Nearest neighbor analysis was performed and density recovery profiles as auto- and cross-correlograms were generated. Cells displaying a regular arrangement of their synaptic processes in matching radial register to each other were identified for each major retinal neuronal cell type except ganglion cells (i.e. photoreceptors, horizontal cells, bipolar cells, and amacrine cells). The precise location of some of the corresponding cell bodies was not as regular but still non-random, however there was no spatial cross-correlation between cell bodies of different types. The radial processes of Müller glial cells displayed a distribution correlating to the arrangement of photoreceptors and neurons. Thus, for one Müller glial cell I found two PKC-positive cone bipolar cells, a spatially corresponding grid of parvalbumin-positive amacrine cell processes, one H1 horizontal cell, and two pairs of double cones. There was no evidence among ganglion cells matching this pattern, possibly due to the lack of suitable markers. Although many other cell types do not follow this matching regular mosaic arrangement, a basic columnar building block can be postulated for the retina at least in cichlid fish. This suggests a functional radial unit from photoreceptors to the inner plexiform layer.

An ultramicroscopic study on the distribution of Müller cell processes in the outer retinal layers of the zebrafish

The zebrafish, Brachydanio rerio, is a good model for studying the development of various organs. We have assayed the distribution pattern of Müller cell processes in zebrafish retinas by electron microscopy. In the outer nuclear (ON) layer, multiple layers of Müller cell processes were present along both sides of the large pyramidal endings of the synaptic terminals. We found that the inner segments (ISs) of the zebrafish photoreceptors (PRs), including the cones, double cones and rods, were arranged in different planes, and that the Müller cell processes formed multilayered sheaths around virtually all PR compartments except their outer segments. Thus, Müller cell processes beyond the outer limiting membrane (OLM) are more easily observed in zebrafish retina than in the retinas of other species. To our knowledge, this is the first study to show the exact ultrastructural distribution of Müller cell processes around the OLM and the PR layer in zebrafish retina.

Investigation of the migration path for new rod photoreceptors in the adult cichlid fish retina

In the retina of adult teleost, precursor cells divide in the outer nuclear layer and give rise to new rod photoreceptors. These new rods migrate from the outer limiting membrane to the inner edge of the outer nuclear layer (ONL) before differentiating. In order to understand which cues these cells use during migration and insertion at the appropriate location we combined cell-specific stains in the retina of the cichlid fish Haplochromis burtoni, viewed with confocal laserscan microscopy: Dividing cells were labeled with bromodeoxyuridine (BrdU), Müller glial cells, cone photoreceptors, and horizontal cells were detected by specific antibodies. During the migration phase (24 to 48 h after BrdU uptake) up to 46% of BrdU-labeled cells were spindle shaped and radially oriented. Most of them were in direct proximity to Müller cell processes. Four days after BrdU-uptake, most labeled cells (91%) were found in the inner portion of the ONL and displayed a spherical shape. This marks the end of the movement of the new rods. At this stage, the labeled cells showed no preference to lie near glial fibers but were often found close to the pedicles of double cones. The leading edge of migrating cells reached into the outer plexiform layer (OPL) but not further than processes of horizontal cells. This is beyond the location of mature rods. We hypothesize that the cells are repelled in the OPL and insert back in the ONL to differentiate as rods.

The degenerative and regenerative processes after the elimination of the proliferative peripheral retina of fish

We have analyzed the modifications in the tench (Tinca tinca) retina after the complete cryo-elimination of the proliferative growing zone (PGZ), which participates in the continuous growth of the retina throughout the life of the fish. By using immunohistochemistry and electron microscopy we demonstrated that, after the lesion, degenerative and regenerative processes take place in the PGZ, in the ciliary zone, and in the transition zone located between the PGZ and the central retina. After 120 days postlesion, the PGZ was completely regenerated and its composition was similar to that of the control animals. Numerous proliferative PCNA-positive cells reappeared and new ganglion cells were formed. In the transition zone and the central retina numerous proliferative PCNA-positive cells also appeared. These are arranged, on occasion, as columnar units from the inner to the outer nuclear layer where the rod precursors and the progenitor cells, respectively, were located. The Müller cells, closely associated with these columnar units, appeared to use them as guides to migration during the regenerative process. Notably, modifications occurred in the ciliary zone, whose cells acquired similar characteristics to the PGZ cells. The ciliary zone cells, the Müller cells, the rod precursors, and the proliferative cells located in the inner nuclear layer appear to participate actively in the regeneration of the PGZ.

The glial design of a teleost optic nerve head supporting continuous growth

This study demonstrates the peculiarities of the glial organization of the optic nerve head (ONH) of a fish, the tench (Tinca tinca), by using immunohistochemistry and electron microscopy. We employed antibodies specific for the macroglial cells: glutamine synthetase (GS), glial fibrillary acidic protein (GFAP), and S100. We also used the N518 antibody to label the new ganglion cells' axons, which are continuously added to the fish retina, and the anti-proliferating cell nuclear antigen (PCNA) antibody to specifically locate dividing cells. We demonstrate a specific regional adaptation of the GS-S100-positive Müller cells' vitreal processes around the optic disc, strongly labeled with the anti-GFAP antibody. In direct contact with these Müller cells' vitreal processes, there are S100-positive astrocytes and S100-negative cells ultrastructurally identified as microglial cells. Moreover, a population of PCNA-positive cells, characterized as glioblasts, forms the limit between the retina and the optic nerve in a region homologous to the Kuhnt intermediary tissue of mammals. Finally, in the intraocular portion of the optic nerve there are differentiating oligodendrocytes arranged in rows. Both the glioblasts and the rows of developing cells could serve as a pool of glial elements for the continuous growth of the visual system.

Mitotic activation of proliferative cells in the inner nuclear layer of the mature fish retina: regulatory signals and molecular markers

New neurons continuously differentiate within the otherwise mature retina of teleost fish, both under normal conditions and in response to injury. We investigated the effects of surgical injury and intraocular injection of neurotrophic factors on the mitotic rate of proliferative inner nuclear layer cells (PINC). PINC are continually born in the inner nuclear layer and then migrate to the outer nuclear layer (ONL). Surgical excision of a part of a retina activates PINC mitotic activity near and far from the lesion. In the injured eye, up-regulation of PINC cells is largest in the dorsonasal sector of the retina, regardless of the site of lesion. Up-regulation extends even to the unlesioned contralateral eye, where it occurs in the same dorsonasal sector. Intraocular injection of ciliary neurotrophic factor mimics the effect of injury on PINC in the treated eye but not on the untreated contralateral retina. We searched for the expression in PINC of Pax6, a transcription factor linked to retinal progenitor cells and found that less than 0.5% of all PINC cells express it. Importantly, the number of Pax6-expressing PINC does not change significantly in the retinas subjected to any of the experimental manipulations tested. Under normal conditions, the default fate of PINC cells is to migrate to the ONL and, likely, replenish the rod progenitor pool. PINC respond to injury (both surgical and light-dependent) by increasing their mitotic rate; this increase is long lived, but there are no changes in the expression level of Pax6. PINC probably are a heterogenous cell population that can be specified for ultimate, different purposes: creating rod precursors, creating founder cells, creating cone precursors. Several fates are recognized now, but others may also be possible.

Energy metabolism of fish brain

This review focuses on recent research on the metabolic function of fish brain. Fish brain is isolated from the systemic circulation by a blood-brain barrier that allows the transport of glucose, monocarboxylates and amino acids. The limited information available in fishes suggests that oxidation of exogenous glucose and oxidative phosphorylation provide most of the ATP required for brain function in teleosts, whereas oxidation of ketones and amino acids occurs preferentially in elasmobranchs. In several agnathans and benthic teleosts brain glycogen levels rather than exogenous glucose may be the proximate glucose source for oxidation. In situations when glucose is in limited supply, teleost brains utilize other fuels such as lactate or ketones. Information on use of lipids and amino acids as fuels in fish brain is scarce. The main pathways of brain energy metabolism are changed by several effectors. Thus, several parameters of brain energy metabolism have been demonstrated to change post-prandially in teleostean fishes. The absence of food in teleosts elicits profound changes in brain energy metabolism (increased glycogenolysis and use of ketones) in a way similar to that demonstrated in mammals though delayed in time. Environmental factors induce changes in brain energy parameters in teleosts such as the enhancement of glycogenolysis elicited by pollutants, increased capacity for anaerobic glycolysis under hypoxia/anoxia or changes in substrate utilization elicited by adaptation to cold. Furthermore, several studies demonstrate effects of melatonin, insulin, glucagon, GLP-1, cortisol or catecholamines on energy parameters of teleost brain, although in most cases the results are quite preliminary being difficult to relate the effects of those hormones to physiological situations. The few studies performed with the different cell types available in the nervous system of fish allow us to hypothesize few functional relationships among those cells. Future research perspectives are also outlined.

Temperature induces variations in the retinal cell proliferation rate in a cyprinid

We quantitatively evaluate the changes of the proliferative cell populations in the adult tench retinas maintained at 6 degrees C and 20 degrees C by both PCNA antigen detection and flow cytometry-based DNA measurements. Both the overall percentage of S-phase cells in the whole retinas and the number of PCNA-positive cells in each of the retinal layers were significantly lower in the tench kept at 6 degrees C, indicating that temperature affects the retinal germinal cell proliferation.

Zebrafish vimentin: molecular characterization, assembly properties and developmental expression

To provide a basis for the investigation of the intermediate filament (IF) protein vimentin in one of the most promising experimental vertebrate systems, the zebrafish (Danio rerio), we have isolated a cDNA clone of high sequence identity to and with the characteristic features of human vimentin. Using this clone we produced recombinant zebrafish vimentin and studied its assembly behaviour. Unlike other vimentins, zebrafish vimentin formed unusually thick filaments when assembled at temperatures below 21 degrees C. At 37 degrees C few filaments were observed, which often also terminated in aggregated masses, indicating that its assembly was severely disturbed at this temperature. Between 21 and 34 degrees C apparently normal IFs were generated. By viscometry, the temperature optimum of assembly was determined to be around 28 degrees C. At this temperature, zebrafish vimentin partially rescued, in mixing experiments, the temperature-dependent assembly defect of trout vimentin. Therefore it is apparently able to "instruct" the misorganized trout vimentin such that it can enter normal IFs. This feature, that assembly is best at the normal body temperature of various species, puts more weight on the assumption that vimentin is vital for some aspects of generating functional adult tissues. Remarkably, like in most other vertebrates, zebrafish vimentin appears to be an abundant factor in the lens and the retina as well as transiently, during development, in various parts of the central and peripheral nervous system. Therefore, promising cell biological investigations may now be performed with cells involved in the generation of the vertebrate eye and brain, and, in particular, the retina. Moreover, the power of genetics of the zebrafish system may be employed to investigate functional properties of vimentin in vivo.