Class III beta-tubulin in human retinal pigment epithelial cells in culture and in epiretinal membranes

Immuno-histological detection of resistant columnar units and vulnerable networks in the rat retina after asphyxia-induced transient cardiac arrest

BACKGROUND

Stroke-related loss of vision is one of the residual impairments, restricting the quality of life. However, studies of the ocular manifestations of asphyxia cardiac arrest/resuscitation (ACA/R) have reported very heterogeneous results.

OBJECTIVE

We aimed to evaluate the ACA/R-induced degeneration pattern of the different retinal cell populations in rats using different immuno-histological stainings.

METHODS

The staining pattern of toluidine blue and the ganglion cell markers β-III-tubulin and NeuN; the calcium-binding protein parvalbumin, indicating ganglion, amacrine, and horizontal cells; calretinin D28k, indicating ganglion and amacrine cells; calbindin, indicating horizontal cells; Chx 10, indicating cone bipolar cells; PKCα, indicating ON-type rod bipolar cells; arrestin, indicating cones; and rhodopsin, a marker of rods, as well as the glial cell markers GFAP (indicating astroglia and Müller cells) and IBA1 (indicating microglia), were evaluated after survival times of 7 and 21 days in an ACA/R rat model. Moreover, quantitative morphological analysis of the optic nerve was performed. The ACA/R specimens were compared with those from sham-operated and completely naïve rats.

RESULTS

ACA/R-induced effects were: (i) a significant reduction of retinal thickness after long-term survival; (ii) ganglion cell degeneration, including their fiber network in the inner plexiform layer; (iii) degeneration of amacrine and cone bipolar cells; (iv) degeneration of cone photoreceptors; (v) enhanced resistance to ACA/R by rod photoreceptors, ON-type rod bipolar and horizontal cells, possibly caused by the strong upregulation of the calcium-binding proteins calretinin, parvalbumin, and calbindin, counteracting the detrimental calcium overload; (vi) significant activation of Müller cells as further element of retinal anti-stress self-defense mechanisms; and (vii) morphological alterations of the optic nerve in form of deformed fibers.

CONCLUSIONS

Regardless of the many defects, the surviving neuronal structures seemed to be able to maintain retinal functionality, which can be additionally improved by regenerative processes true to the "use it or lose it" dogma.

Elevated intraocular pressure induces neuron-specific β-III-tubulin expression in non-neuronal vascular cells

PURPOSE

Pathological alterations within optic nerve axons and progressive loss of the parental retinal ganglion cell (RGC) bodies are characteristics of glaucomatous neuropathy. Abnormally elevated intraocular pressure (IOP) is thought to be the major risk factor for most forms of glaucomatous changes, while lowering of the IOP is the mainstream of treatment. However, the pathophysiological mechanisms involved in neurodegenerative changes are poorly understood. It remains still a matter of debate whether elevated IOP harms the neurons directly or indirectly through alterations in the retinal vascularization.

METHODS

We analysed morphological and molecular changes within the retina exposed to elevated IOP in an animal model of glaucoma in vivo, in retinal explants and in cultured dissociated retinal cells each incubated under elevated air pressure in vitro, imitating elevated IOP.

RESULTS

Although ß-III-tubulin expressing RGCs decreased within the course of the disease, total amount of ß-III-tubulin protein within the retina increased, leading to the assumption that other cells than RGCs abnormally express ß-III-tubulin due to elevated IOP. Surprisingly, we found that β-III-tubulin, a marker developmentally regulated and specifically expressed in neurons under normal conditions, was strongly up-regulated in desmin-, PDGFR-β- and α-SMA-positive pericytes as well as in endothelin-1-positive endothelial cells both in vivo under elevated IOP and in vitro under elevated culture atmosphere pressure that simulated IOP elevation. Beta-III-tubulin-driven signalling pathways (ERK 1/2, pERK1/2 and cdc42/Rac) were also regulated.

CONCLUSION

The unprecedented regulation of neuron-specific β-III-tubulin in pericytes and endothelial cells is likely associated with a role of the retinal vasculature in the IOP-induced development and manifestation of glaucomatous degenerative optic nerve response.

Optimization of fixative solution for retinal morphology: a comparison with Davidson's fixative and other fixation solutions

PURPOSE

Numerous fixative solutions are available but many are not amenable to the histomorphological preservation of retinae. The investigators specifically focused on retinal histological studies, which rather than 4% formaldehyde (FA), often use Davidson's fixative. However the latter has its limitations. The purpose of this study was to produce a new fixative which maintains retinae closer to the in vivo conditions.

STUDY DESIGN

Experimental design.

METHODS

Four fixative formulations (4% paraformaldehyde, Davidson's fixative, modified Davidson's fixative and an in-house fixative - TB-Fix) were tested on retinae and the outcomes on histomorphology and immunohistochemical staining for selected antigenic markers was compared.

RESULTS

TB-Fix markedly improved morphological detail following hematoxylin and eosin staining, most importantly eliminating the spongiform appearance in the plexiform layer and the swelling of somata (including Müller cells), when compared to FA, Davidson's fixative and its modified version. Retinal samples fixed with TB-Fix or FA showed comparable results in immunohistological staining for neurons and glia in the retina. Importantly, while the whole eye fixed with FA collapsed in shape and induced artificial retinal detachment, the eye fixed with TB-Fix avoided deformation and detachment. Furthermore, we found that TB-Fix also prevented detachment from the culture plate when used to fix HEK293 cells, which are known to detach from the plate easily.

CONCLUSION

It was demonstrated that TB-Fix provides an overall improvement in the preservation of retinal morphology and chemical composition.

Neuroprotection and neuroregeneration of retinal ganglion cells after intravitreal carbon monoxide release

PURPOSE

Retinal ischemia induces apoptosis leading to neurodegeneration and vision impairment. Carbon monoxide (CO) in gaseous form showed cell-protective and anti-inflammatory effects after retinal ischemia-reperfusion-injury (IRI). These effects were also demonstrated for the intravenously administered CO-releasing molecule (CORM) ALF-186. This article summarizes the results of intravitreally released CO to assess its suitability as a neuroprotective and neuroregenerative agent.

METHODS

Water-soluble CORM ALF-186 (25 μg), PBS, or inactivated ALF (iALF) (all 5 μl) were intravitreally applied into the left eyes of rats directly after retinal IRI for 1 h. Their right eyes remained unaffected and were used for comparison. Retinal tissue was harvested 24 h after intervention to analyze mRNA or protein expression of Caspase-3, pERK1/2, p38, HSP70/90, NF-kappaB, AIF-1 (allograft inflammatory factor), TNF-α, and GAP-43. Densities of fluorogold-prelabeled retinal ganglion cells (RGC) were examined in flat-mounted retinae seven days after IRI and were expressed as mean/mm2. The ability of RGC to regenerate their axon was evaluated two and seven days after IRI using retinal explants in laminin-1-coated cultures. Immunohistochemistry was used to analyze the different cell types growing out of the retinal explants.

RESULTS

Compared to the RGC-density in the contralateral right eyes (2804±214 RGC/mm2; data are mean±SD), IRI+PBS injection resulted in a remarkable loss of RGC (1554±159 RGC/mm2), p<0.001. Intravitreally injected ALF-186 immediately after IRI provided RGC protection and reduced the extent of RGC-damage (IRI+PBS 1554±159 vs. IRI+ALF 2179±286, p<0.001). ALF-186 increased the IRI-mediated phosphorylation of MAP-kinase p38. Anti-apoptotic and anti-inflammatory effects were detectable as Caspase-3, NF-kappaB, TNF-α, and AIF-1 expression were significantly reduced after IRI+ALF in comparison to IRI+PBS or IRI+iALF. Gap-43 expression was significantly increased after IRI+ALF. iALF showed effects similar to PBS. The intrinsic regenerative potential of RGC-axons was induced to nearly identical levels after IRI and ALF or iALF-treatment under growth-permissive conditions, although RGC viability differed significantly in both groups. Intravitreal CO further increased the IRI-induced migration of GFAP-positive cells out of retinal explants and their transdifferentiation, which was detected by re-expression of beta-III tubulin and nestin.

CONCLUSION

Intravitreal CORM ALF-186 protected RGC after IRI and stimulated their axons to regenerate in vitro. ALF conveyed anti-apoptotic, anti-inflammatory, and growth-associated signaling after IRI. CO's role in neuroregeneration and its effect on retinal glial cells needs further investigation.

A Method for the Isolation and Culture of Adult Rat Retinal Pigment Epithelial (RPE) Cells to Study Retinal Diseases

Diseases such as age-related macular degeneration (AMD) affect the retinal pigment epithelium (RPE) and lead to the death of the epithelial cells and ultimately blindness. RPE transplantation is currently a major focus of eye research and clinical trials using human stem cell-derived RPE cells are ongoing. However, it remains to be established to which extent the source of RPE cells for transplantation affects their therapeutic efficacy and this needs to be explored in animal models. Autotransplantation of RPE cells has attractions as a therapy, but existing protocols to isolate adult RPE cells from rodents are technically difficult, time-consuming, have a low yield and are not optimized for long-term cell culturing. Here, we report a newly devised protocol which facilitates reliable and simple isolation and culture of RPE cells from adult rats. Incubation of a whole rat eyeball in 20 U/ml papain solution for 50 min yielded 4 × 10(4) viable RPE cells. These cells were hexagonal and pigmented upon culture. Using immunostaining, we demonstrated that the cells expressed RPE cell-specific marker proteins including cytokeratin 18 and RPE65, similar to RPE cells in vivo. Additionally, the cells were able to produce and secrete Bruch's membrane matrix components similar to in vivo situation. Similarly, the cultured RPE cells adhered to isolated Bruch's membrane as has previously been reported. Therefore, the protocol described in this article provides an efficient method for the rapid and easy isolation of high quantities of adult rat RPE cells. This provides a reliable platform for studying the therapeutic targets, testing the effects of drugs in a preclinical setup and to perform in vitro and in vivo transplantation experiments to study retinal diseases.

Retinal pigment epithelium transplantation: concepts, challenges, and future prospects

The retinal pigment epithelium (RPE) is a single layer of cells that supports the light-sensitive photoreceptor cells that are essential for retinal function. Age-related macular degeneration (AMD) is a leading cause of visual impairment, and the primary pathogenic mechanism is thought to arise in the RPE layer. RPE cell structure and function are well understood, the cells are readily sustainable in laboratory culture and, unlike other cell types within the retina, RPE cells do not require synaptic connections to perform their role. These factors, together with the relative ease of outer retinal imaging, make RPE cells an attractive target for cell transplantation compared with other cell types in the retina or central nervous system. Seminal experiments in rats with an inherited RPE dystrophy have demonstrated that RPE transplantation can prevent photoreceptor loss and maintain visual function. This review provides an update on the progress made so far on RPE transplantation in human eyes, outlines potential sources of donor cells, and describes the technical and surgical challenges faced by the transplanting surgeon. Recent advances in the understanding of pluripotent stem cells, combined with novel surgical instrumentation, hold considerable promise, and support the concept of RPE transplantation as a regenerative strategy in AMD.

Cell models to study regulation of cell transformation in pathologies of retinal pigment epithelium

The retinal pigment epithelium (RPE) plays a key role in the development of many eye diseases leading to visual impairment and even blindness. Cell culture models of pathological changes in the RPE make it possible to study factors responsible for these changes and signaling pathways coordinating cellular and molecular mechanisms of cell interactions under pathological conditions. Moreover, they give an opportunity to reveal target cells and develop effective specific treatment for degenerative and dystrophic diseases of the retina. In this review, data are presented on RPE cell sources for culture models, approaches to RPE cell culturing, phenotypic changes of RPE cells in vitro, the role of signal pathways, and possibilities for their regulation in pathological processes.

The role of O-GlcNAc signaling in the pathogenesis of diabetic retinopathy

Diabetic retinopathy is a leading cause of blindness worldwide. Despite laser and surgical treatments, antiangiogenic and other therapies, and strict metabolic control, many patients progress to visual impairment and blindness. New insights are needed into the pathophysiology of diabetic retinopathy in order to develop new methods to improve the detection and treatment of disease and the prevention of blindness. Hyperglycemia and diabetes result in increased flux through the hexosamine biosynthetic pathway, which, in turn, results in increased PTM of Ser/Thr residues of proteins by O-linked β-N-acetylglucosamine (O-GlcNAc). O-GlcNAcylation is involved in regulation of many nuclear and cytoplasmic proteins in a manner similar to protein phosphorylation. Altered O-GlcNAc signaling has been implicated in the pathogenesis of diabetes and may play an important role in the pathogenesis of diabetic retinopathy. The goal of this review is to summarize the biology of the hexosamine biosynthesis pathway and O-GlcNAc signaling, to present the current evidence for the role of O-GlcNAc signaling in diabetes and diabetic retinopathy, and to discuss future directions for research on O-GlcNAc in the pathogenesis of diabetic retinopathy.

Retinal stem cells and regeneration of vision system

The vertebrate retina is a well-characterized model for studying neurogenesis. Retinal neurons and glia are generated in a conserved order from a pool of mutlipotent progenitor cells. During retinal development, retinal stem/progenitor cells (RPC) change their competency over time under the influence of intrinsic (such as transcriptional factors) and extrinsic factors (such as growth factors). In this review, we summarize the roles of these factors, together with the understanding of the signaling pathways that regulate eye development. The information about the interactions between intrinsic and extrinsic factors for retinal cell fate specification is useful to regenerate specific retinal neurons from RPCs. Recent studies have identified RPCs in the retina, which may have important implications in health and disease. Despite the recent advances in stem cell biology, our understanding of many aspects of RPCs in the eye remains limited. PRCs are present in the developing eye of all vertebrates and remain active in lower vertebrates throughout life. In mammals, however, PRCs are quiescent and exhibit very little activity and thus have low capacity for retinal regeneration. A number of different cellular sources of RPCs have been identified in the vertebrate retina. These include PRCs at the retinal margin, pigmented cells in the ciliary body, iris, and retinal pigment epithelium, and Müller cells within the retina. Because PRCs can be isolated and expanded from immature and mature eyes, it is possible now to study these cells in culture and after transplantation in the degenerated retinal tissue. We also examine current knowledge of intrinsic RPCs, and human embryonic stems and induced pluripotent stem cells as potential sources for cell transplant therapy to regenerate the diseased retina.

Development of human embryonic stem cell therapies for age-related macular degeneration

Age-related macular degeneration (AMD) is the leading cause of vision loss in older adults and ultimately leads to the death of photoreceptor cells in the macular area of the neural retina. Currently, treatments are only available for patients with the wet form of AMD. In this review, we describe recent approaches to develop cell-based therapies for the treatment of AMD. Recent research has focused on replacing the retinal pigment epithelium (RPE), a monolayer of cells vital to photoreceptor cell health. We discuss the various methods used to differentiate and purify RPE from human embryonic stem cells (HESC), and describe the surgical approaches being used to transplant these cells in existing and forthcoming clinical trials.

Communication between corneal epithelial cells and trigeminal neurons is facilitated by purinergic (P2) and glutamatergic receptors

Previously, we demonstrated that nucleotides released upon mechanical injury to corneal epithelium activate purinergic (P2) receptors resulting in mobilization of a Ca²⁺ wave. However, the tissue is extensively innervated and communication between epithelium and neurons is critical and not well understood. Therefore, we developed a co-culture of primary trigeminal neurons and human corneal limbal epithelial cells. We demonstrated that trigeminal neurons expressed a repertoire of P2Yand P2X receptor transcripts and responded to P2 agonists in a concentration-dependent manner. Mechanical injuries to epithelia in the co-cultures elicited a Ca²⁺ wave that mobilized to neurons and was attenuated by Apyrase, an ectonucleotidase. To elucidate the role of factors released from each cell type, epithelial and neuronal cells were cultured, injured, and the wound media from one cell type was collected and added to the other cell type. Epithelial wound media generated a rapid Ca²⁺ mobilization in neuronal cells that was abrogated in the presence of Apyrase, while neuronal wound media elicited a complex response in epithelial cells. The rapid Ca²⁺ mobilization was detected, which was abrogated with Apyrase, but it was followed by Ca²⁺ waves that occurred in cell clusters. When neuronal wound media was preincubated with a cocktail of N-methyl-D-aspartate (NMDA) receptor inhibitors, the secondary response in epithelia was diminished. Glutamate was detected in the neuronal wound media and epithelial expression of NMDA receptor subunit transcripts was demonstrated. Our results indicate that corneal epithelia and neurons communicate via purinergic and NMDA receptors that mediate the wound response in a highly orchestrated manner.

Phenotypic plasticity of retinal pigment epithelial cells from adult human eye in vitro

Phenotypic plasticity of retinal pigment epithelial cells from adult human eye was studied by immunohistochemical methods under different culturing conditions. It was found that retinal pigment epithelium in adult human eye is a heterogeneous population of cells demonstrating different behavior in vitro. Some cells retain epithelial morphology for a long time in culture, while others are rapidly transformed into fibroblast-like cells and synthesize proteins typical of proneural, neural, glial, and photoreceptor cells. However, irrespective of initial morphological features differentiation of retinal pigment cells can be modulated by varying culturing conditions.

TGF-β-stimulated aberrant expression of class III β-tubulin via the ERK signaling pathway in cultured retinal pigment epithelial cells

The class III β-tubulin isotype (β(III)) is expressed exclusively by neurons within the normal human retina and is not present in normal retinal pigment epithelial (RPE) cells in situ or in the early phase of primary cultures. However, aberrant expression of class III β-tubulin has been observed in passaged RPE cells and RPE cells with dedifferentiated morphology in pathologic epiretinal membranes from idiopathic macular pucker, proliferative vitreoretinopathy (PVR) and proliferative diabetic retinopathy (PDR). Transforming growth factor-β (TGF-β) has been implicated in dedifferentiation of RPE cells and has a critical role in the development of proliferative vitreoretinal diseases. Here, we investigated the potential effects of TGF-β on the aberrant expression of class III β-tubulin and the intracellular signaling pathway mediating these changes. TGF-β-induced aberrant expression and O-linked-β-N-acetylglucosamine (O-GlcNac) modification of class III β-tubulin in cultured RPE cells as determined using Western blotting, RT-PCR and immunocytochemistry. TGF-β also stimulated phosphorylation of ERK. TGF-β-induced aberrant expression of class III β-tubulin was significantly reduced by pretreatment with U0126, an inhibitor of ERK phosphorylation. Our findings indicate that TGF-β stimulated aberrant expression of class III β-tubulin via activation of the ERK signaling pathway. These data demonstrate that mature RPE cells have the capacity to express a neuron-associated gene in response to TGF-β stimulation and provide useful information towards understanding the pathogenesis of proliferative vitreoretinal diseases.

The TAg-RB murine retinoblastoma cell of origin has immunohistochemical features of differentiated Muller glia with progenitor properties

PURPOSE

Human retinoblastoma arises from an undefined developing retinal cell after inactivation of RB1. This is emulated in a murine retinoblastoma model by inactivation of pRB by retinal-specific expression of simian virus 40 large T-antigen (TAg-RB). Some mutational events after RB1 loss in humans are recapitulated at the expression level in TAg-RB, supporting preclinical evidence that this model is useful for comparative studies between mouse and human. Here, the characteristics of the TAg-RB cell of origin are defined.

METHODS

TAg-RB mice were killed at ages from embryonic day (E)18 to postnatal day (P)35. Tumors were analyzed by immunostaining, DNA copy number PCR, or real-time quantitative RT-PCR for TAg protein, retinal cell type markers, and retinoblastoma-relevant genes.

RESULTS

TAg expression began at P8 in a row of inner nuclear layer cells that increased in number through P21 to P28, when clusters reminiscent of small tumors emerged from cells that escaped a wave of apoptosis. Early TAg-expressing cells coexpressed the developmental marker Chx10 and glial markers CRALBP, clusterin, and carbonic anhydrase II (Car2), but not TuJ1, an early neuronal marker. Emerging tumors retained expression of only Chx10 and carbonic anhydrase II. As with human retinoblastoma, TAg-RB tumors showed decreased Cdh11 DNA copy number and gain of Kif14 and Mycn. It was confirmed that TAg-RB tumors lose expression of tumor suppressor cadherin-11 and overexpress oncogenes Kif14, Dek, and E2f3.

CONCLUSIONS

TAg-RB tumors displayed molecular similarity to human retinoblastoma and origin in a cell with features of differentiated Müller glia with progenitor properties.

Expression of granulocyte colony stimulating factor and its receptor by retinal pigment epithelial cells: a role in maintaining differentiation-competent state

PURPOSE

Granulocyte colony stimulating factor (GCSF) is a potent hematopoietic factor that stimulates the growth of neutrophil granulocyte precursors, and also regulates the differentiation and survival of neutrophils by inhibiting apoptosis. Incidentally, GCSF is also known to act as an endogenous ligand for brain cells, counteracting acute neuronal degeneration and contributing to long-term plasticity of progenitor cells after cerebral ischemia. Since GCSF was recently reported to be present in retinal ganglions, we examined its expression in retinal pigment epithelial (RPE) cells, which, together with retinal neurons, arise from the same underlying precursor cells.

METHODS

We used reverse transcriptase polymerase chain reaction (PCR) to assay expression of GCSF and GCSF receptor (GCSFR) genes; immunostaining and flow cytometry to assay the presence of GCSFR on cell surfaces; bromodeoxyuridine (BrdU) incorporation measurement to monitor DNA synthesis; and 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay to monitor cell proliferation. The effect of GCSF on differentiation of RPE cells was assessed by immunocytochemistry to detect the presence of various marker proteins.

RESULTS

The D407 RPE cells, as well as RPE derived from cadaver eyes, were found to express both GCSF and GCSFR. Despite the presence of the GCSF receptor, exogenously added GCSF did not result in any proliferation of these cells. We found that GCSF acts like a de-differentiating factor, maintaining RPE cells in the rounded form, and in a transdifferentiation-competent state.

CONCLUSIONS

The expression of GCSF and GCSFR by D407 RPE may be an important factor in RPE cell maintenance.

Negative regulation of RPE cell attachment by carbohydrate-dependent cell surface binding of galectin-3 and inhibition of the ERK-MAPK pathway

Adhesion and spreading of retinal pigment epithelial (RPE) cells on fibronectin-rich extracellular matrices is a crucial event in the pathogenesis of proliferative vitreoretinopathy (PVR). In the present study we explored the capacity of galectin-3, a β-galactoside-binding endogenous lectin, to inhibit early PVR-associated cellular events from a therapeutic perspective. We assessed the relative expression levels of galectin-3 in native RPE and dedifferentiated, cultured RPE. Galectin-3 was constitutively expressed under in vivo and in vitro conditions and was abundant in cultured cells. Treatment of human RPE cells with soluble galectin-3 disclosed no toxicity within control limits up to 250 μg/ml. When added to the medium, galectin-3 dose-dependently inhibited attachment and spreading of the cells on fibronectin by more than 75%. When coated on the plastic surface, galectin-3 alone impaired attachment and spreading of RPE cells, and reduced attachment but not spreading on fibronectin. Galectin-3 bound to the cell surface, and, as determined by the use of the competing sugar β-lactose, galectin-3-mediated effects were dependent on carbohydrate binding. To ascertain the role of the ability of galectin-3 to form pentamers, we proteolytically removed the N-terminal, cross-linking section. The remaining C-terminal carbohydrate-binding domain alone failed to bind to cells and was functionally inactive. These results emphasize the relevance of both properties, i.e., glycan-binding and cross-linking of glycan moieties, for the inhibitory activity of galectin-3. Incubation of mobilized RPE cells with galectin-3 significantly disturbed microfilament assembly and, in correlation with decreased attachment, inhibited ERK phosphorylation. Therefore, galectin-3, acting as a cross-linking lectin on the cell surface, negatively regulates attachment and spreading of RPE cells in vitro. This effect, at least in part, is attributed to an inhibition of the ERK-MAPK pathway, which prevents cytoskeletal rearrangements needed for RPE cell attachment and spreading. Further investigation at this pathway may disclose a promising nouveau perspective for treatment and prophylaxis of early PVR.

Class III beta-tubulin is constitutively coexpressed with glial fibrillary acidic protein and nestin in midgestational human fetal astrocytes: implications for phenotypic identity

Class III beta-tubulin isotype (betaIII-tubulin) is widely regarded as a neuronal marker in developmental neurobiology and stem cell research. To test the specificity of this marker protein, we determined its expression and distribution in primary cultures of glial fibrillary acidic protein (GFAP)-expressing astrocytes isolated from the cerebral hemispheres of 2 human fetuses at 18 to 20 weeks of gestation. Cells were maintained as monolayer cultures for 1 to 21 days without differentiation induction. By immunofluorescence microscopy, coexpression of betaIII-tubulin and GFAP was detected in cells at all time points but in spatially distinct patterns. The numbers of GFAP+ cells gradually decreased from Days 1 to 21 in vitro, whereas betaIII-tubulin immunoreactivity was present in 100% of cells at all time points. beta-III-tubulin mRNA and protein expression were demonstrated in cultured cells by reverse-transcriptase-polymerase chain reaction and immunoblotting, respectively. Glial fibrillary acidic protein+/beta-III-tubulin-positive cells coexpressed nestin and vimentin but lacked neurofilament proteins, CD133, and glutamate-aspartate transporter. Weak cytoplasmic staining was detected with antibodies against microtubule-associated protein 2 isoforms. Confocal microscopy, performed on autopsy brain samples of human fetuses at 16 to 20 gestational weeks, revealed widespread colocalization of GFAP and betaIII-tubulin in cells of the ventricular/subventricular zones and the cortical plate. Our results indicate that in the midgestational human brain, betaIII-tubulin is not neuron specific because it is constitutively expressed in GFAP+/nestin+ presumptive fetal astrocytes.

Retinal pigment epithelium

Retinal pigment epithelium (RPE) arises from neuroectoderm and plays a key role in support of photoreceptor functions. Several degenerative eye diseases, such as macular degeneration or retinitis pigmentosa, are associated with impaired RPE function that may lead to photoreceptor loss and blindness. RPE derived from human embryonic stem (hES) cells can be an important source of this tissue for transplantation to cure such degenerative diseases. This chapter describes differentiation of hES cells to RPE, its subsequent isolation, maintenance in culture, and characterization.

Voltage-dependent ion channels in the mouse RPE: comparison with Norrie disease mice

We studied electrophysiological properties of cultured retinal pigment epithelial (RPE) cells from mouse and a mouse model for Norrie disease. Wild-type RPE cells revealed the expression of ion channels known from other species: delayed-rectifier K(+) channels composed of Kv1.3 subunits, inward rectifier K(+) channels, Ca(V)1.3 L-type Ca(2+) channels and outwardly rectifying Cl(-) channels. Expression pattern and the ion channel characteristics current density, blocker sensitivity, kinetics and voltage-dependence were compared in cells from wild-type and Norrie mice. Although no significant differences were observed, our study provides a base for future studies on ion channel function and dysfunction in transgenic mouse models.

Adult retinal pigment epithelium cells express neural progenitor properties and the neuronal precursor protein doublecortin

The adult mammalian retina is devoid of any detectable neurogenesis. However, different cell types have been suggested to potentially act as neural progenitors in the adult mammalian retina in vitro, such as ciliary body (CB), Muller glia, and retinal pigment epithelium (RPE) cells. In rodents and humans, strong evidence for neural stem or progenitor properties exists only for CB-derived cells, but not for other retinal cell types. Here, we provide a comparative analysis of adult rat CB- and RPE-derived cells suggesting that the two cell types share certain neural progenitor properties in vitro. CB and RPE cells expressed neural progenitor markers such as Nestin, Flk-1, Hes1, and Musashi. They proliferated under adherent and neurosphere conditions and showed limited self-renewal. Moreover, they differentiated into neuronal and glial cells based on the expression of differentiation markers such as the young neuronal marker beta-III tubulin and the glial and progenitor markers GFAP and NG2. Expression of beta-III tubulin was found in cells with neuronal and non-neuronal morphology. A subpopulation of RPE- and CB-derived progenitor cells expressed the neurogenesis-specific protein doublecortin (DCX). Interestingly, DCX expression defined a beta-III tubulin-positive CB and RPE fraction with a distinct neuronal morphology. In summary, the data suggest that RPE cells share with CB cells the potential to de-differentiate into a cell type with neural progenitor-like identity. In addition, DCX expression might define the neuronal-differentiating RPE- and CB-derived progenitor population.

Derivation and comparative assessment of retinal pigment epithelium from human embryonic stem cells using transcriptomics

Human stem-cell derivatives are likely to play an important role in the future of regenerative medicine. Evaluation and comparison to their in vivo counterparts is critical for assessment of their therapeutic potential. Transcriptomics was used to compare a new differentiation derivative of human embryonic stem (hES) cells--retinal pigment epithelium (RPE)--to human fetal RPE. Several hES cell lines were differentiated into putative RPE, which expressed RPEspecific molecular markers and was capable of phagocytosis, an important RPE function. Isolated hES cell-derived RPE was able to transdifferentiate into cells of neuronal lineage and redifferentiate into RPE-like cells through multiple passages (>30 Population doublings). Gene expression profiling demonstrated their higher similarity to primary RPE tissue than of existing human RPE cell lines D407 and ARPE-19, which has been shown to attenuate loss of visual function in animals. This is the first report of the isolation and characterization of putative RPE cells from hES cells, as well as the first application of transcriptomics to assess embryonic stem-cell derivatives and their in vivo counterparts--a "differentiomics" outlook. We describe for the first time, a differentiation system that does not require coculture with animal cells or factors, thus allowing the production of zoonoses-free RPE cells suitable for subretinal transplantation in patients with retinal degenerative diseases. With the further development of therapeutic cloning, or the creation of the banks of homozygous human leucocyte antigen (HLA) hES cells using parthenogenesis, RPE lines could be generated to overcome the problem of immune rejection and could be one of the nearest term applications of stem-cell technology.

The neurogenic competence of progenitors from the postnatal rat retina in vitro

The mammalian retina develops from stem or progenitor cells that are of neuroectodermal origin and derive from bilateral invaginations of the neuroepithelium, the optic vesicles. Shortly after birth, around 12 days postnatal in rats, the retina is fully developed in its cellular parts. Even though different cell types in the adult might be potential sources for retinal stem cells or progenitor cells, the retina is a non-neurogenic region and the diseased retina is devoid of any spontaneous regeneration. In an attempt to link late developmental processes to the adult situation, we analyzed the presence and the neurogenic potential of retinal progenitors during the postnatal period and compared it to adult ciliary body (CB) derived retinal progenitors and subventricular zone (SVZ) derived neural stem cells. Retinal progenitor properties were identified by the capacity to proliferate and by the expression of the progenitor markers Nestin, Flk-1, Chx10, Pax6 and the radial glia marker BLBP. The neurogenic potential was assayed by the expression of the neuronal markers doublecortin, betaIII Tubulin, Map2 and NSE, the glial makers A2B5, NG2, GalC and GFAP, and by incorporation of BrdU. The number of Flk-1 positive cells and concomitantly the number of newly born betaIII Tubulin-positive cells decreased within the first postnatal week in retinal progenitor cultures and no newly generated betaIII Tubulin, but GFAP positive cells were detected thereafter. In contrast to neural stem cells derived from the adult SVZ, postnatal and adult CB derived progenitors had a lower and a restricted proliferation potential and did not generate oligodendrocytes. The work demonstrates, however, that the existence of retinal progenitor cells is not restricted to embryonic development. In the sensory retina the differentiation potential of late retinal progenitors becomes restricted to the glial lineage, whereas neurogenic progenitor cells are still present in the CB. In addition, major differences in growth and differentiation potential of adult neural stem cells and postnatal and adult retinal progenitors are presented.

Adult human retinal pigment epithelial cells capable of differentiating into neurons

We investigated the ability of adult human RPE cells to differentiate into neurons. Two cell lines were evaluated. The cells were cultured in medium with 8% serum, transferred to a neural stem cell maintenance culture, and induced to differentiate with retinoic acid. The cells were immunocytochemically examined at each step. The cells showed epithelial-like morphology with 8% serum and all were immunoreactive for beta-III tubulin. After transfer to the stem cell maintenance culture, they changed morphologically and became immunoreactive for MAP5 and neurofilament200 after inducement with retinoic acid. The ratio of MAP5 positive cells was higher in the young adult RPE cell line. No GFAP or rod-opsin immunoreactive cells were observed. Adult human RPE cells even from old person are capable of differentiating into neurons, although the ratio of mature neurons was greater in the young than in the old cell line in this condition.

Late proliferation of retinal Müller cell progenitors facilitates preferential targeting with retroviral vectors in vitro

During vertebrate neural retina development, the relationship between mitotic activity in progenitor cells and the acquisition of a mature cell phenotype remains an area of controversy. The Müller glial cell has long been recognized as one of the last cell types of the retina to mature, which occurs under the influence of cell-cell interactions. In this report we examine the acquisition of the Müller cell phenotype in relation to mitotic activity. Using immunohistochemical markers, we demonstrate that a gene product characteristic of mature Müller cells, the 2M6 antigen, is expressed in mitotically active cells, even after all the major retina architectural features have been laid down. Furthermore, we show that retroviral infection, a process that requires mitotically active cells, preferentially targets Müller cell progenitors when late embryonic retina is infected in vitro. The two lines of evidence are consistent with a model for Müller cell differentiation that includes a mitotically active progenitor that has already begun to express specific differentiation gene products.

Ectopic expression of Msx2 in chick retinal pigmented epithelium cultures suggests a role in patterning the optic vesicle

During the initial stages of vertebrate retinogenesis, cells of the optic vesicle adopt one of two alternate cell fates. Cells in the distal-most part of the vesicle, immediately beneath the surface ectoderm, undergo neural differentiation; cells in the proximal part differentiate into retinal pigmented epithelial cells. The mechanisms that establish this pattern of differentiation are poorly understood. In the mouse embryo, Msx2, a homeobox-containing transcription factor, is expressed in cells of the optic vesicle that will form the neural retina, whilst the developing retinal pigmented epithelium (RPE) does not express this gene. Msx2 could therefore be involved in patterning the optic vesicle into neural and pigmented domains. To explore this possibility we ectopically expressed mouse Msx2 in cultures of chick RPE cells. Compared with cultures transfected with a control construct, Msx2-transfected cultures contained fewer cells expressing the RPE marker, Mitf, and more cells expressing class III beta-tubulin, a neuronal marker. In addition a small proportion of Msx2-transfected cells acquired a neural-like morphology. These results show that Msx2 can suppress the differentiated state of RPE cells and promote their differentiation into neural cell types. We suggest that Msx2 may pattern the optic vesicle into neural and pigmented domains by affecting the balance between RPE and neural retina differentiation.

Platelet-derived growth factor-A-induced retinal gliosis protects against ischemic retinopathy

Retinal astrocytes are located in the nerve fiber layer and along retinal blood vessels and have been hypothesized to participate in the induction and maintenance of the blood-retinal barrier. Platelet-derived growth factor-A (PDGF-A) is normally produced by retinal ganglion cells and is involved in astrocyte recruitment and proliferation. We used gain-of-function transgenic mice that express PDGF-A in photoreceptors to explore the roles of PDGF-A and astrocytes in the retina. Transgene-positive mice developed glial infiltration of the inner retina and had significantly less oxygen-induced retinal vascular closure and no neovascularization compared with littermate controls, which had prominent vascular closure and neovascularization. The increased survival of endothelial cells in transgenic mice in the face of oxygen-induced down-regulation of vascular endothelial growth factor was accompanied by an increase in astrocyte-derived fibroblast growth factor-2. Therefore, PDGF-A increases retinal astrocytes, which promote the survival of endothelial cells as well as their expression of barrier characteristics.

Immunohistochemical study of rabbit choroidal innervation

Immunocytochemical methods with antibodies to the light (68 kDa), medium (160 kDa), and heavy (200 kDa) chain subunits of the neurofilament triplet have been used to visualize neuronal structures in rabbit choroids. Choroidal nerve fibers were present in the suprachoroid and vascular laminae and absent in the choriocapillary layer. These fibers may be classified as perivascular and intervascular. Perivascular fibers surround all arterial and venous blood vessels and form a network; these fibers were labeled with the three NF antibodies, although they were more easily visualized with anti NF-160 and anti NF-200 than anti NF-68. Intervascular fibers formed two groups. The first group consisted of fibers situated between the blood vessels and parallel to the blood vessel wall surface (paravascular fibers); these fibers were better observed using anti NF-160 and NF-200 than anti NF-68. The second group consisted of fibers which travel the entire length of the choroid until they reach the nerve plexus of the ciliary body (long tract fibers). The plexus was observed with anti NF-68, anti NF-160 and anti NF-200; however, the long tract fibers were more clearly visualized with anti NF-160 and anti NF-200 than with anti NF-68. Two types of choroidal cell were also labeled: ganglion cells and melanocytes. Ganglion cells are small, scarce neurons situated in the peripheral choroid; they were labeled with anti NF-160 and anti NF-200. The melanocytes were only labeled with anti NF-200 and they were the only non neuronal structure visualized using antibodies against neurofilaments.

Multiple forms of tubulin: different gene products and covalent modifications

Tubulin, the subunit protein of microtubules, is an alpha/beta heterodimer. In many organisms, both alpha and beta exist in numerous isotypic forms encoded by different genes. In addition, both alpha and beta undergo a variety of posttranslational covalent modifications, including acetylation, phosphorylation, detyrosylation, polyglutamylation, and polyglycylation. In this review the distribution and possible functional significance of the various forms of tubulin are discussed. In analyzing the differences among tubulin isotypes encoded by different genes, some appear to have no functional significance, some increase the overall adaptability of the organism to environmental challenges, and some appear to perform specific functions including formation of particular organelles and interactions with specific proteins. Purified isotypes also display different properties in vitro. Although the significance of all the covalent modification of tubulin is not fully understood, some of them may influence the stability of modified microtubules in vivo as well as interactions with certain proteins and may help to determine the functional role of microtubules in the cell. The review also discusses isotypes of gamma-tubulin and puts various forms of tubulin in an evolutionary context.

Localisation of vascular endothelial growth factor and its receptors to cells of vascular and avascular epiretinal membranes

AIMS/BACKGROUND

Epiretinal membranes (ERMs) arise from a variety of causes or, in some cases, for unknown reasons. Once established, ERMs tend to progress, becoming more extensive and exerting increasing traction along the inner surface of the retina. One possible cause for their progression is the production of growth factors by cells within ERMs that may provide autocrine or paracrine stimulation. Platelet derived growth factor (PDGF) and its receptors have been localised to cells of ERMs and may play such a role. In this study, comparative data were sought for several other growth factors that have been implicated in ERM formation.

METHODS

Immunohistochemical staining of ERMs was done for PDGF-A, PDGF-B, basic fibroblast growth factor (bFGF), three isoforms of transforming growth factor beta (TGF-beta), and vascular endothelial growth factor (VEGF) and its receptors, flt-1 and flk-1/KDR. Expression of flt-1 and flk-1/KDR was examined in cultured retinal pigmented epithelial (RPE) cells and retinal glia from postmortem eyes by immunohistochemistry and by reverse transcription coupled to polymerase chain reaction (RT-PCR).

RESULTS

Staining was most intense and most frequently observed for VEGF and PDGF-A, both in vascular and avascular ERMs. The majority of cells stained for VEGF in nine of 11 (81.8%) diabetic ERMs and in 14 of 24 (58.3%) proliferative vitreoretinopathy ERMs. The receptors for VEGF, flt-1, and flk-1/KDR were also identified on cells in ERMs and on cultured RPE cells. By RT-PCR, mRNA for flt-1 was identified in RPE cells and retinal glia, and mRNA for flk-1/KDR was identified in RPE cells.

CONCLUSIONS

These data show that VEGF and its receptors are localised to both vascular and avascular ERMs and suggest that VEGF, like PDGF-A, may be an autocrine and paracrine stimulator that may contribute to progression of vascular and avascular ERMs.

Differentiation and transdifferentiation of the retinal pigment epithelium

The retinal pigment epithelium (RPE) lies between the retina and the choroid of the eye and plays a vital role in ocular metabolism. The RPE develops from the same sheet of neuroepithelium as the retina and the two derivatives become distinguished by different expression patterns of a number of transcription factors during embryonic development. As the RPE layer differentiates it expresses a set of unique molecules, many of which are restricted to certain regions of the cell. PRE cells undergo both a loss of polarity and a loss of expression of many of these cell type-specific molecules when placed in monolayer culture. The RPE of many species, including mammals, can be induced to transdifferentiate by growth factors such as basic fibroblast growth factor. Under the influence of such factors the RPE is triggered to alter expression of a wide array of molecules and to take on a retinal epithelium fate, from which differentiated retinal cell types including rod photoreceptors can be produced.

Isoforms of platelet-derived growth factor and its receptors in epiretinal membranes: immunolocalization to retinal pigmented epithelial cells

Epiretinal membranes (ERMs) form on the inner surface of the retina in conjunction with various ocular disease processes, but the factors controlling their development are not understood. The predominant cell types involved are retinal pigmented epithelial (RPE) cells and retinal glia. Cultured RPE cells secrete platelet-derived growth factor (PDGF), which is chemotactic and mitogenic for both RPE cells and retinal glia and, therefore, could be involved in the development of ERMs. In the present study, we performed immunohistochemical staining for PDGF A chain (PDGF-A), PDGF B chain (PDGF-B), and both types of PDGF receptors (PDGFr alpha and PDGFr beta) on ERMs associated with various disease processes. PDGF-A is detected in most ERMs, regardless of the associated disease process, and it appears to be localized predominantly in RPE cells, recognized by the presence of pigment and the immunohistochemical demonstration of some or all of the following RPE-associated epitopes: class III beta-tubulin, keratin, the 65-kDa microsomal protein recognized by the RPE9 antibody, and cellular retinaldehyde-binding protein. PDGF-B is found only in minor subpopulations of cells in about half of the ERMs evaluated and, with only occasional exceptions, appears to be localized almost entirely in blood-borne cells found in and around vessels in vascularized ERMs. Both PDGFr alpha and PDGFr beta are demonstrated in most ERMs with neither isotype consistently predominating: they are found predominantly on RPE cells with many cells expressing both receptor types. ERMs with little or no RPE cell component contain little or no PDGF and PDGF receptor, whereas those in which the RPE cell represents the major cell type, have widespread PDGF and PDGF receptor positivity. These findings show that RPE cells in ERMs produce PDGF-A and PDGF alpha and PDGF beta receptors and suggest that autocrine and paracrine stimulation with PDGF may be involved in ERM pathogenesis.