Cone cell-specific genes expressed in retinoblastoma

Retinal organoid chip: engineering a physiomimetic oxygen gradient for optimizing long term culture of human retinal organoids

An oxygen gradient across the retina plays a crucial role in its development and function. The inner retina resides in a hypoxic environment (2% O2) adjacent to the vitreous cavity. Oxygenation levels rapidly increase towards the outer retina (18% O2) at the choroid. In addition to retinal stratification, oxygen levels are critical for the health of retinal ganglion cells (RGCs), which relay visual information from the retina to the brain. Human stem cell derived retinal organoids are being engineered to mimic the structure and function of human retina for applications such as disease modeling, development of therapeutics, and cell replacement therapies. However, rapid degeneration of the retinal ganglion cell layers are a common limitation of human retinal organoid platforms. We report the design of a novel retinal organoid chip (ROC) that maintains a physiologically relevant oxygen gradient and allows the maturation of inner and outer retinal cell phenotypes for human retinal organoids. Our PDMS-free ROC holds 55 individual retinal organoids that were manually seeded, cultured for extended periods (over 150 days), imaged in situ, and retrieved. ROC was designed from first principles of liquid and gas mass transport, and fabricated from biologically- and chemically inert materials using rapid prototyping techniques such as micromachining, laser cutting, 3D printing and bonding. After computational and experimental validation of oxygen gradients, human induced pluripotent stem cell derived retinal organoids were transferred into the ROC, differentiated, cultured and imaged within the chip. ROCs that maintained active perfusion and stable oxygen gradients were successful in inducing higher viability of RGCs within retinal organoids than static controls, or ROC without oxygen gradients. Our physiologically relevant and higher-throughput retinal organoid culture system is well suited for applications in studying developmental perturbations to primate retinogenesis, including those driven by inherited traits, fetal environmental exposure to toxic agents, or acquired by genetic mutations, such as retinoblastoma.

Risk Factors for Extraocular Relapse in Retinoblastoma

BACKGROUND

Metastatic retinoblastoma remains a significant challenge in pediatric oncology, with stark disparities in survival outcomes between high-income countries (HICs) and low-income countries (LICs). Delayed diagnosis and treatment, driven by socioeconomic factors and limitations in healthcare systems, contribute to poorer outcomes in LICs. Histopathological characteristics, including high-risk pathology factors (HRPFs) and the extent of ocular tumor invasion, are critical for predicting metastatic risk and guiding treatment strategies.

METHODS

This review examines the role of clinical, histopathological, and molecular characteristics in assessing metastatic risk in retinoblastoma. Literature on HRPFs, tumor invasion, and molecular subtypes was analyzed to understand their impact on risk stratification and therapy optimization, particularly in resource-limited settings.

RESULTS

Retinoblastoma is increasingly recognized as a heterogeneous disease with at least two distinct molecular subtypes. High-risk cases frequently exhibit genetic alterations that underscore the need to incorporate molecular profiling into risk assessment. Current adjuvant therapy approaches, however, vary widely, and debates persist regarding their necessity based on tumor characteristics. Integrated strategies that combine clinical, histopathological, and molecular data show promise in improving management and survival outcomes.

CONCLUSIONS

Addressing the disparities in metastatic retinoblastoma outcomes requires a multifaceted approach. By integrating clinical, histopathological, and molecular insights, management strategies can be optimized to improve survival, particularly in resource-limited settings where challenges are most pronounced.

Blastocyst-like Structures in the Peripheral Retina of Young Adult Beagles

In this immunohistological study on the peripheral retina of 3-year-old beagle dogs, excised retina specimens were immunostained with antibodies against nestin, Oct4, Nanog, Sox2, CDX2, cytokeratin 18 (CK 18), RPE65, and YAP1, as well as hematoxylin and DAPI, two nuclear stains. Our findings revealed solitary cysts of various sizes in the inner retina. Intriguingly, a mass of small round cells with scant cytoplasms was observed in the cavity of small cysts, while many disorganized cells partially occupied the cavity of the large cysts. The small cysts were strongly positive for nestin, Oct4, Nanog, Sox2, CDX2, CK18, and YAP1. RPE65-positive cells were exclusively observed in the tissue surrounding the cysts. Since RPE65 is a specific marker of retinal pigment epithelial (RPE) cells, the surrounding cells of the peripheral cysts were presumably derived from RPE cells that migrated intraretinally. In the small cysts, intense positive staining for nestin, a marker of retinal stem cells, seemed to indicate that they were derived from retinal stem cells. The morphology and positive staining for markers of blastocyst and RPE cells indicated that the small cysts may have formed structures resembling the blastocyst, possibly caused by the interaction between retinal stem cells and migrated RPE cells.

Retinal organoids provide unique insights into molecular signatures of inherited retinal disease throughout retinogenesis

The demand for induced pluripotent stem cells (iPSC)-derived retinal organoid and retinal pigment epithelium (RPE) models for the modelling of inherited retinopathies has increased significantly in the last decade. These models are comparable with foetal retinas up until the later stages of retinogenesis, expressing all of the key neuronal markers necessary for retinal function. These models have proven to be invaluable in the understanding of retinogenesis, particular in the context of patient-specific diseases. Inherited retinopathies are infamously described as clinically and phenotypically heterogeneous, such that developing gene/mutation-specific animal models in each instance of retinal disease is not financially or ethically feasible. Further to this, many animal models are insufficient in the study of disease pathogenesis due to anatomical differences and failure to recapitulate human disease phenotypes. In contrast, iPSC-derived retinal models provide a high throughput platform which is physiologically relevant for studying human health and disease. They also serve as a platform for drug screening, gene therapy approaches and in vitro toxicology of novel therapeutics in pre-clinical studies. One unique characteristic of stem cell-derived retinal models is the ability to mimic in vivo retinogenesis, providing unparalleled insights into the effects of pathogenic mutations in cells of the developing retina, in a highly accessible way. This review aims to give the reader an overview of iPSC-derived retinal organoids and/or RPE in the context of disease modelling of several inherited retinopathies including Retinitis Pigmentosa, Stargardt disease and Retinoblastoma. We describe the ability of each model to recapitulate in vivo disease phenotypes, validate previous findings from animal models and identify novel pathomechanisms that underpin individual IRDs.

Second hit impels oncogenesis of retinoblastoma in patient-induced pluripotent stem cell-derived retinal organoids: direct evidence for Knudson's theory

Retinoblastoma (Rb) is a type of malignant tumor due to abnormal retinogenesis with biallelic mutations of the RB1 gene. Its pathogenesis has been proposed as a "two-mutation hypothesis" by Knudson since 1971; however, there remain some debates on disease onset sufficiency of the biallelic RB1 mutations. To obtain straightforward evidence for this hypothesis, we investigated whether two-hit mutations of the RB1 gene drive tumorigenesis in patient-induced pluripotent stem cell (hiPSC)-derived human retinal organoids (hROs) and whether single allelic mutation hiPSC-derived hROs exhibit molecular and cellular defects. We generated hiPSCs with a heterozygous germline mutation (RB1^{m1/ wt} ) from a Rb patient. A second-allele RB1 gene mutation was knocked in to produce compound heterozygous mutations (RB1^m1/m2 ) in the hiPSCs. These two hiPSC lines were independently developed into hROs through a stepwise differentiation. The hiPSC-RB1^m1/m2 derived organoids demonstrated tumorigenesis in dishes, consistent with Rb profiles in spatiotemporal transcriptomes, in which developmentally photoreceptor fate-determining markers, CRX and OTX2, were highly expressed in hiPSC-RB1^m1/m2 derived hROs. Additionally, ARR3⁺ maturing cone precursors were co-labeled with proliferative markers Ki67 or PCNA, in agreement with the consensus that human Rb is originated from maturing cone precursors. Finally, we demonstrated that retinal cells of hROs with monoallelic RB1 mutation were abnormal in molecular aspects due to its haploinsufficiency. In conclusion, this study provides straightforward supporting evidence in a way of reverse genetics for "two-hit hypothesis" in the Rb tumorigenesis and opens new avenues for development of early intervention and treatment of Rb.

A high-risk retinoblastoma subtype with stemness features, dedifferentiated cone states and neuronal/ganglion cell gene expression

Retinoblastoma is the most frequent intraocular malignancy in children, originating from a maturing cone precursor in the developing retina. Little is known on the molecular basis underlying the biological and clinical behavior of this cancer. Here, using multi-omics data, we demonstrate the existence of two retinoblastoma subtypes. Subtype 1, of earlier onset, includes most of the heritable forms. It harbors few genetic alterations other than the initiating RB1 inactivation and corresponds to differentiated tumors expressing mature cone markers. By contrast, subtype 2 tumors harbor frequent recurrent genetic alterations including MYCN-amplification. They express markers of less differentiated cone together with neuronal/ganglion cell markers with marked inter- and intra-tumor heterogeneity. The cone dedifferentiation in subtype 2 is associated with stemness features including low immune and interferon response, E2F and MYC/MYCN activation and a higher propensity for metastasis. The recognition of these two subtypes, one maintaining a cone-differentiated state, and the other, more aggressive, associated with cone dedifferentiation and expression of neuronal markers, opens up important biological and clinical perspectives for retinoblastomas.

New In Vitro Cellular Model for Molecular Studies of Retinitis Pigmentosa

Retinitis pigmentosa (RP) is an inherited form of retinal degeneration characterized by primary rod photoreceptor cell death followed by cone loss. Mutations in several genes linked to the disease cause increased levels of cyclic guanosine monophosphate (cGMP) and calcium ion influxes. The purpose of this project was to develop a new in vitro photoreceptor degeneration model for molecular studies of RP. 661W cells were genetically modified to stably express the neural retina leucine zipper (NRL) transcription factor. One clone (661W-A11) was selected based on the expression of Nrl target genes. 661W-A11 showed a significant increase in expression of rod-specific genes but not of cone-specific genes, compared with 661W cells. Zaprinast was used to inhibit phosphodiesterase 6 (PDE6) activity to mimic photoreceptor degeneration in vitro. The activation of cell death pathways resulting from PDE6 inhibition was confirmed by detection of decreased viability and increased intracellular cGMP and calcium, as well as activation of protein kinase G (PKG) and calpains. In this new in vitro system, we validated the effects of previously published neuroprotective drugs. The 661W-A11 cells may serve as a new model for molecular studies of RP and for high-throughput drug screening.

Molecular Genetics of Intraocular Tumors

PURPOSE

To explore the value of molecular technologies in the pathologic evaluation, diagnosis, and treatment of retinoblastoma and uveal melanoma.

METHODS

Review of the peer-reviewed literature on the molecular pathology of primary intraocular tumors.

CONCLUSION

Molecular tests are playing an increasingly important role in the diagnosis of intraocular tumors. They provide information valuable for diagnosis, prognosis, screening regimens, genetic counselling, and treatment. These technologies are becoming easier, faster, and with higher sensitivity and accuracy.

Conservative management of retinoblastoma: Challenging orthodoxy without compromising the state of metastatic grace. "Alive, with good vision and no comorbidity"

Retinoblastoma is lethal by metastasis if left untreated, so the primary goal of therapy is to preserve life, with ocular survival, visual preservation and quality of life as secondary aims. Historically, enucleation was the first successful therapeutic approach to decrease mortality, followed over 100 years ago by the first eye salvage attempts with radiotherapy. This led to the empiric delineation of a window for conservative management subject to a "state of metastatic grace" never to be violated. Over the last two decades, conservative management of retinoblastoma witnessed an impressive acceleration of improvements, culminating in two major paradigm shifts in therapeutic strategy. Firstly, the introduction of systemic chemotherapy and focal treatments in the late 1990s enabled radiotherapy to be progressively abandoned. Around 10 years later, the advent of chemotherapy in situ, with the capitalization of new routes of targeted drug delivery, namely intra-arterial, intravitreal and now intracameral injections, allowed significant increase in eye preservation rate, definitive eradication of radiotherapy and reduction of systemic chemotherapy. Here we intend to review the relevant knowledge susceptible to improve the conservative management of retinoblastoma in compliance with the "state of metastatic grace", with particular attention to (i) reviewing how new imaging modalities impact the frontiers of conservative management, (ii) dissecting retinoblastoma genesis, growth patterns, and intraocular routes of tumor propagation, (iii) assessing major therapeutic changes and trends, (iv) proposing a classification of relapsing retinoblastoma, (v) examining treatable/preventable disease-related or treatment-induced complications, and (vi) appraising new therapeutic targets and concepts, as well as liquid biopsy potentiality.

Developmental stage-specific proliferation and retinoblastoma genesis in RB-deficient human but not mouse cone precursors

Most retinoblastomas initiate in response to the inactivation of the RB1 gene and loss of functional RB protein. The tumors may form with few additional genomic changes and develop after a premalignant retinoma phase. Despite this seemingly straightforward etiology, mouse models have not recapitulated the genetic, cellular, and stage-specific features of human retinoblastoma genesis. For example, whereas human retinoblastomas appear to derive from cone photoreceptor precursors, current mouse models develop tumors that derive from other retinal cell types. To investigate the basis of the human cone-specific oncogenesis, we compared developmental stage-specific cone precursor responses to RB loss in human and murine retina cultures and in cone-specific Rb1-knockout mice. We report that RB-depleted maturing (ARR3⁺) but not immature (ARR3^-) human cone precursors enter the cell cycle, proliferate, and form retinoblastoma-like lesions with Flexner-Wintersteiner rosettes, then form low or nonproliferative premalignant retinoma-like lesions with fleurettes and p16^INK4A and p130 expression, and finally form highly proliferative retinoblastoma-like masses. In contrast, in murine retina, only RB-depleted immature (Arr3^-) cone precursors entered the cell cycle, and they failed to progress from S to M phase. Moreover, whereas intrinsically highly expressed MDM2 and MYCN contribute to RB-depleted maturing (ARR3⁺) human cone precursor proliferation, ectopic MDM2 and Mycn promoted only immature (Arr3^-) murine cone precursor cell-cycle entry. These findings demonstrate that developmental stage-specific as well as species- and cell type-specific features sensitize to RB1 inactivation and reveal the human cone precursors' capacity to model retinoblastoma initiation, proliferation, premalignant arrest, and tumor growth.

Hybrid three-dimensional (3D) bioprinting of retina equivalent for ocular research

In this article, a hybrid retina construct was created via three-dimensional (3D) bioprinting technology. The construct was composed of a PCL ultrathin membrane, ARPE-19 cell monolayer and Y79 cell-laden alginate/pluronic bioink. 3D bioprinting technology was applied herein to deliver the ARPE-19 cells and Y79 cell-laden bioink to ensure homogeneous ARPE-19 cell seeding; subsequently, two distinctive Y79 cell-seeding patterns were bioprinted on top of the ARPE-19 cell monolayer. The bioprinted ARPE-19 cells were evaluated by prestoblue assay, F-actin, and hematoxylin/eosin (HE) staining, and then the cells were observed under laser scanning and invert microscopy for 14 days. The Y79 cells in alginate/pluronic bioink after bioprinting had been closely monitored for 7 days. Live/dead assay and scanning electrical microscopy (SEM) were employed to investigate Y79 cell viability and morphology. Both the ARPE-19 and Y79 cells were in excellent condition, and the successfully bioprinted retina model could be utilized in drug delivery, disease mechanism and treatment method discoveries.

Spatiotemporal Patterns of Tumor Occurrence in Children with Intraocular Retinoblastoma

PURPOSE

To accurately map the retinal area covered by tumor in a prospectively enrolled cohort of children diagnosed with retinoblastoma.

METHODS

Orbital MRI in 106 consecutive retinoblastoma patients (44 bilateral) was analyzed. For MRI-visible tumors, the polar angle and angle of eccentricity of points defining tumor perimeter on the retina were determined by triangulation from images in three orthogonal planes. The centroid of the mapped area was calculated to approximate tumor origin, and the location and cumulative tumor burden were analyzed in relation to mutation type (germline vs. somatic), tumor area, and patient age at diagnosis. Location of small tumors undetected by MRI was approximated with fundoscopic images.

RESULTS

Mapping was successful for 129 tumors in 91 eyes from 67 patients (39 bilateral, 43 germline mutation). Cumulative tumor burden was highest within the macula and posterior pole and was asymmetrically higher within the inferonasal periphery. Tumor incidence was lowest in the superotemporal periphery. Tumor location varied with age at diagnosis in a complex pattern. Tumor location was concentrated in the macula and superonasal periphery in patients <5.6 months, in the inferotemporal quadrant of the posterior pole in patients 5.6-8.8 months, in the inferonasal quadrant in patients 8.8-13.2 months, and in the nasal and superotemporal periphery in patients >13.2 months. The distribution of MRI-invisible tumors was consistent with the asymmetry of mapped tumors.

CONCLUSIONS

MRI-based mapping revealed a previously unrecognized pattern of retinoblastoma localization that evolves with age at diagnosis. The structured spatiotemporal distribution of tumors may provide valuable clues about cellular or molecular events associated with tumorigenesis in the developing retina.

Cell specific post-translational processing of pikachurin, a protein involved in retinal synaptogenesis

Pikachurin is a recently identified, highly conserved, extracellular matrix-like protein. Murine pikachurin has 1,017 amino acids (~110 kDa), can bind to α-dystroglycan, and has been found to localize mainly in the synaptic cleft of photoreceptor ribbon synapses. Its knockout selectively disrupts synaptogenesis between photoreceptor and bipolar cells. To further characterize this synaptic protein, we used an antibody raised against the N-terminal of murine pikachurin on Western blots of mammalian and amphibian retinas and found, unexpectedly, that a low weight ~60-kDa band was the predominant signal for endogenous pikachurin. This band was predicted to be an N-terminal product of post-translational cleavage of pikachurin. A similar sized protein was also detected in human Y79 retinoblastoma cells, a cell line with characteristics of photoreceptor cells. In Y79 cells, endogenous pikachurin immunofluorescence was found on the cell surface of living cells. The expression of the N-fragment was not significantly affected by dystroglycan overexpression in spite of the biochemical evidence for pikachurin-α-dystroglycan binding. The presence of a corresponding endogenous C-fragment was not determined because of the lack of a suitable antibody. However, a protein of ~65 kDa was detected in Y79 cells expressing recombinant pikachurin with a C-terminal tag. In contrast, in QBI-HEK 293A cells, whose endogenous pikachurin protein level is negligible, recombinant pikachurin did not appear to be cleaved. Instead pikachurin was found either intact or as dimers. Finally, whole and N- and C-fragments of recombinant pikachurin were present in the conditioned media of Y79 cells indicating the secretion of pikachurin. The site of cleavage, however, was not conclusively determined. Our data suggest the existence of post-translational cleavage of pikachurin protein as well as the extracellular localization of cleaved protein specifically by retinal cells. The functions of the pikachurin N- and C-fragments in the photoreceptor ribbon synapse are unknown.

Retinal horizontal cells lacking Rb1 sustain persistent DNA damage and survive as polyploid giant cells

The retinoblastoma tumor susceptibility gene, Rb1, is a key regulator of the cell cycle, and mutations in this gene have been found in many human cancers. Prior studies showed that retina-specific knockout of Rb1 in the mouse results in the formation of abnormally large horizontal cells, but the development, fate, and genomic status of these cells remain unknown. In this study, we conditionally inactivate Rb1 in early retinal progenitors and show that the loss of Rb1 leads to the rapid degeneration of most retinal cells except horizontal cells, which persist as giant cells with aberrant centrosome content, DNA damage, and polyploidy/aneuploidy. We observed inappropriate cell cycle entry of Rb1-deficient horizontal cells during the first postnatal weeks, which dropped off abruptly by P30. Despite extensive DNA damage in Rb1-deficient horizontal cells, these cells can still enter mitosis. Adult Rb1-deficient horizontal cells display elevated DNA content (5N-34N) that varied continuously, suggesting the presence of aneuploidy. We also found evidence of supernumerary and disoriented centrosomes in a rare population of mitotic cells in the mutant retinas. Overall our data demonstrate that horizontal cells are a remarkably robust cell type and can survive for months despite extensive DNA damage and elevated genome content.

Retinoblastoma has properties of a cone precursor tumor and depends upon cone-specific MDM2 signaling

Retinoblastomas result from the inactivation of the RB1 gene and the loss of Rb protein, yet the cell type in which Rb suppresses retinoblastoma and the circuitry that underlies the need for Rb are undefined. Here, we show that retinoblastoma cells express markers of postmitotic cone precursors but not markers of other retinal cell types. We also demonstrate that human cone precursors prominently express MDM2 and N-Myc, that retinoblastoma cells require both of these proteins for proliferation and survival, and that MDM2 is needed to suppress ARF-induced apoptosis in cultured retinoblastoma cells. Interestingly, retinoblastoma cell MDM2 expression was regulated by the cone-specific RXRgamma transcription factor and a human-specific RXRgamma consensus binding site, and proliferation required RXRgamma, as well as the cone-specific thyroid hormone receptor-beta2. These findings provide support for a cone precursor origin of retinoblastoma and suggest that human cone-specific signaling circuitry sensitizes to the oncogenic effects of RB1 mutations.

Insights from mouse models into human retinoblastoma

Novel murine models of retinoblastoma based on Rb gene deletion in concert with inactivation of Rb family members have recently been developed. These new Rb knockout models of retinoblastoma provide excellent tools for pre-clinical studies and for the exploration of the genetics of tumorigenesis driven by RB inactivation. This review focuses on the developmental consequences of Rb deletion in the retina and the genetic interactions between Rb and the two other members of the pocket protein family, p107 (Rbl1) and p130 (Rbl2). There is increasing appreciation that homozygous RB mutations are insufficient for human retinoblastoma. Identifying and understanding secondary gene alterations that cooperate with RB inactivation in tumorigenesis may be facilitated by mouse models. Recent investigation of the p53 pathway in retinoblastoma, and evidence of spatial topology to early murine retinoblastoma are also discussed in this review.

Identification of tumorigenic retinal stem-like cells in human solid retinoblastomas

Retinoblastoma (RB) is the most common malignant tumor of the retina in human children. Although it has been hypothesized for a long time that RB derives from multipotent retinal stem cells (RSCs) or retinoblasts, the direct evidence that the presence of tumorigenic RSCs in RB tumors is still lacking. Some studies indicate that malignant tumors contain tumor stem cells similar to their normal tissue stem cell counterparts. With in vitro culture and differentiation method we demonstrate that tumorigenic retinal stem-like cells (RSLCs) indeed exist in RB lesions and that RB tumor-derived cultures encompass undifferentiated cells capable of extensive proliferation as clonal nonadherent neurospheres and can differentiate into different retinal cells in vitro. Interestingly, cultured cells expressed retinal development related genes including nestin, CD133, pax6, chx10 and Rx, and overexpressed Bmi-1, a gene required for self-renewal and proliferation of stem cells. Significantly, when these cultured cells were intraocularly transplanted into SCID mice, they gave rise to new tumors with histomorphological features and immunophenotypes similar to their parental primary RBs. The results show that RBs contain tumorigenic RSLCs that contribute to tumorigenesis. This study provides a new insight to investigate the histogenesis of RBs and establishes a model for other RB research.

Nonsynaptic localization of the excitatory amino acid transporter 4 in photoreceptors

Excitatory amino acid transporters (EAATs) are involved in regulating extracellular glutamate levels at synaptic regions in the CNS. EAAT1, 2, 3, and 5 have been found in the mammalian retina, but the presence of EAAT4 has remained controversial. Recently, we found a high level of EAAT4 mRNA in the human retina, and this observation lead us to examine whether EAAT4 was expressed in the mammalian retina. Immunoblotting studies showed the presence of EAAT4-immunoreactive proteins in human and mouse retinas, corresponding to EAAT4 monomers and dimers. Immunohistochemistry revealed that EAAT4 was localized in rod and cone photoreceptor outer segments in the human retina, and in the outer and inner segments of mouse and ground squirrel retinas. In no case was EAAT4 found in the outer plexiform layer or in any other layer in the retina. EAAT4 expression by photoreceptors was confirmed by immunoblotting a purified rod outer segment preparation, which showed the presence of a 50-kDa EAAT4-immunoreactive protein. In addition, the EAAT4-associated protein, GTRAP41, was found in the human, mouse, and squirrel retinas as well as in the rod outer segment preparation. Further immunocytochemical and co-immunoprecipitation experiments demonstrated that GTRAP41 was colocalized and interacted in vivo with EAAT4. Importantly, glutamate uptake and drug inhibition experiments showed that an EAAT4-like glutamate uptake system is present in the rod outer segments. Finally, we examined whether glutamate signaling mediated by EAAT4 can modulate rod outer segment phagocytosis by the retinal pigment epithelium. Results of the present study show that EAAT4 is present in the outer segments, a nonsynaptic region of photoreceptors, where it might provide a feedback mechanism for sensing extracellular glutamate or serve as an outer barrier to prevent glutamate from escaping from the retina.

Tissue-specific tumor suppressor activity of retinoblastoma gene homologs p107 and p130

The retinoblastoma gene family consists of three genes: RB, p107, and p130. While loss of pRB causes retinoblastoma in humans and pituitary gland tumors in mice, tumorigenesis in other tissues may be suppressed by p107 and p130. To test this hypothesis, we have generated chimeric mice from embryonic stem cells carrying compound loss-of-function mutations in the Rb gene family. We found that Rb/p107- and Rb/p130-deficient mice were highly cancer prone. We conclude that in a variety of tissues tumor development by loss of pRB is suppressed by its homologs p107 and p130. The redundancy of the retinoblastoma proteins in vivo is reflected by the behavior of Rb-family-defective mouse embryonic fibroblasts in vitro.

Cell type-specific effects of Rb deletion in the murine retina

Certain cells of the human retina are extremely sensitive to loss of function of the retinoblastoma tumor suppressor gene RB. Retinoblastomas develop early in life and at high frequency in individuals heterozygous for a germ-line RB mutation, and sporadic retinoblastomas invariably have somatic mutation in the RB gene. In contrast, retinoblastomas do not develop in Rb+/- mice. Although retinoblastoma is thought to have developmental origins, the function of Rb in retinal development has not been fully characterized. Here we studied the role of Rb in normal retinal development and in retinoblastoma using conditional Rb mutations in the mouse. In late embryogenesis, Rb-deficient retinas exhibited ectopic S-phase and high levels of p53-independent apoptosis, particularly in the differentiating retinal ganglion cell layer. During postnatal retinal development, loss of Rb led to more widespread retinal apoptosis, and adults showed loss of photoreceptors and bipolar cells. Conditional Rb mutation in the retina did not result in retinoblastoma formation even in a p53-mutant background. However, on a p107- or p130-deficient background, Rb mutation in the retina caused retinal dysplasia or retinoblastoma.

Expression of cone-photoreceptor-specific antigens in a cell line derived from retinal tumors in transgenic mice

PURPOSE

To examine an immortalized mouse retinal cell line (661W) for markers characteristic of photoreceptor cells and validate its photoreceptor origin.

METHODS

The 661W cells were cloned from retinal tumors of a transgenic mouse line that expresses the simian virus (SV) 40 T antigen under control of the human interphotoreceptor retinol-binding protein (IRBP) promoter. Morphologic, immunocytochemical, and immunoblot analyses were performed to characterize these cells. Total cellular protein was used for immunoblot analysis of various photoreceptor-specific proteins.

RESULTS

661W cells grew as a monolayer and exhibited processes characteristic of neuronal cells. Immunoblot analysis showed that 661W cells expressed SV40 T antigen, blue and green cone pigments, transducin, and cone arrestin. Immunocytochemical detection of blue and green opsins showed distribution throughout the cell, the nucleus included. However, these cells did not express rod-specific antigens, such as opsin and arrestin or rod- and cone-specific proteins such as phosducin, peripherin/rds, and ROM1. Furthermore, the cells did not express RPE65, a cone- and RPE-cell-specific protein.

CONCLUSIONS

661W cells demonstrate cellular and biochemical characteristics exhibited by cone photoreceptor cells. These cells also resemble neuronal cells with their spindlelike processes and should serve as a useful alternative in vitro model for the study of cone photoreceptor cell biology and associated diseases.

Tumor formation in mice with somatic inactivation of the retinoblastoma gene in interphotoreceptor retinol binding protein-expressing cells

The retinoblastoma suppressor gene product Rb has been assigned a critical role in cell cycle regulation, the induction of differentiation, and inhibition of oncogenic transformation. Inheritance of a mutant RB allele in humans is responsible for bilateral retinoblastoma, a malignant tumor of the retina. Trilateral retinoblastoma (TRB) is a rare variant of familial retinoblastoma in which, in addition to retinal tumors, tumors develop from the pineal gland, an organ ontologically related to the retina. Germline inactivation of Rb in mice leads to mid-gestational lethality with defects in erythropoeisis and neurogenesis. This embryonic lethality prohibits the analysis of Rb function in selected cell types at later stages of development or in the adult. Here, we describe the Cre-LoxP mediated somatic inactivation of Rb in a subset of neuroendocrine cells, including photoreceptor cells. We observed neuroendocrine tumors of the pineal and pituitary gland. These tumors invariably showed inactivation of Rb and Trp53. Remarkably, loss of Rb in photoreceptor cells does not lead to retinoblastoma or any phenotypic changes, not even when photoreceptor cells are made deficient in Rb, p107 and Trp53. Our results highlight the important differences that exist in tumor susceptibility between mice and man (e.g pineal gland) and question the photoreceptor cell origin of human retinoblastoma.

Reproducible high efficiency gene transfer into Y79 retinoblastoma cells using adenofection

Several photoreceptor-specific genes are actively transcribed in Y79 retinoblastoma (Rb) cells, making this cell line potentially useful for the study of photoreceptor metabolism. The utility of these cells is limited because commonly used methods of gene transfer into Y79 cells are inefficient and lack reproducibility. In contrast, we found that adenovirus transduction yields high efficiency gene transfer, however, generation of recombinant adenovirus is lengthy and time consuming. Here, we show that adenofection, a method of coupling adenovirus to plasmid DNA for improved gene transfer, is efficient for gene delivery into Y79 cells. Recombinant adenovirus expressing bacterial lacZ was noncovalently complexed to GFP or luciferase reporter plasmids with polyethylenimine. Efficiency of plasmid gene delivery was determined by monitoring GFP fluorescence. For comparison, calcium phosphate-mediated or cationic lipid transfection was performed in Y79 and HEK293 cells using standard protocols. The adenofection protocol yielded significantly higher efficiencies in Y79 cells than that obtained in these cells with calcium phosphate or cationic lipids. This method will facilitate any experiment requiring reproducible high-level gene transfer. Here, we show that adenofection can be used to analyze activity of the rod photoreceptor PDE6A gene promoter.

Ionotropic glutamate receptors expressed in human retinoblastoma Y79 cells

Mammalian retinal photoreceptors and pinealocytes have common characteristic that they secrete melatonin and L-glutamate as chemical transmitters. Although pinealocytes express glutamate receptors and receive glutamate signals, whether or not photoreceptors express glutamate receptors is unknown. Here, we investigated the expression of the glutamate receptors in cultured Y79 clonal human retinoblastoma cells, as model systems of photoreceptors. Reverse transcription-polymerase chain reaction (RT-PCR) analysis indicated that GluR1, GluR5, GluR7, EAA2, NR1, NR2A and NR2D mRNAs were present in the cultured cells. Northern analysis confirmed the presence of GluR7, EAA2, NR1, NR2A and NR2D mRNAs, while other mRNAs were under the detection limit. Addition of (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid or kainate increases intracellular (Ca(2+)) in Fura-2 loaded cells, which is blocked by 6-cyano-7-nitroquinoxaline-2,3-dione. N-methyl-D-aspartate also increases intracellular (Ca(2+)). These results demonstrated the presence of functional ionotropic receptors in Y79 cells.

Retinoblastoma: the disease, gene and protein provide critical leads to understand cancer

Retinoblastoma has contributed much to the understanding of cancer. The protein product of the RB gene, pRB, is a multifaceted regulator of transcription which controls the cell cycle, differentiation and apoptosis in normal development of specific tissues. Elucidating the mechanisms in which pRB plays a critical role will enable novel therapies and strategies for prevention, not only for retinoblastoma, but for cancer in general.

Retinoblastoma in transgenic mice: models of hereditary retinoblastoma

Retinoblastoma, the most common intraocular malignancy ill childhood, has served as a paradigm for the study of genetic mechanisms of oncogenesis. The retinoblastoma susceptibility gene RB1 was the first tumor suppressor gene to be cloned, and genetic and molecular biologic studies of this tumor have greatly expanded the understanding of the mechanics of tumorigenesis. Human retinoblastoma has essentially no naturally occuring animal counterpart. The development of transgenic murine models of retinoblastoma have created an experimental tool for manipulation of a tumor gene system in vivo. These models have also enabled studies of new therapeutic modalities. This review outlines the development of the transgenic murine models of retinoblastoma, together with the genetic mechanisms of retinoblastoma origin. Current therapeutic innovations developed by means of the transgenic models are described.

Trilateral retinoblastoma: insights into histogenesis and management

Trilateral retinoblastoma (TRb) is a syndrome involving midline intracranial malignancies in children with the heritable form of retinoblastoma. All cases of TRb reported from 1971 to 1997 were reviewed. The histopathologic findings, clinical features, treatment modalities, and survival rates from 80 cases were evaluated. Histopathologic findings from intracranial malignancies demonstrated primitive neuroectodermal tumors in 61.5% of cases. Various degrees of neuronal or photoreceptor differentiation were seen in the other 38.5% of cases. Autopsy, histopathologic, and radiologic examinations did not show a more definitive site of origin of these intracranial tumors, although "pinealoblastoma" was often the diagnosis reported. These findings, together with analysis of the histopathologic similarities among human primitive neuroectodermal tumors, pinealoblastoma, retinoblastoma, and ependymoblastoma, suggest that TRb more likely arises from a germinal layer of predisposed primitive subependymal neuroblasts that are not necessarily destined for pineal or photoreceptor differentiation. Trilateral tumors have also been found in transgenic mice expressing the simian virus 40 T-antigen. Transgenic murine intracranial tumors are primitive neuroectodermal tumors arising from the subependymal layer. Transgenic mice with the murine interphotoreceptor cell binding protein promoter and simian virus 40 T-antigen also develop pineal tumors. Trilateral retinoblastoma is usually fatal, with an average survival time of 11.2 months. Therapies include radiation, systemic chemotherapy, intrathecal chemotherapy, and surgical resection/craniotomy in combination with radiation and/or chemotherapy. Survival may be prolonged with combination chemotherapy (24.6 months) and if neuroradiologic screening identifies TRb before symptoms are present (23.5 months). Recent success with platinum-based chemoreduction of intraocular retinoblastoma may indicate a similar role for platinum-based chemotherapy in the treatment of TRb. Routine central nervous system imaging should be considered in the management of TRb.

A 221-bp fragment of the mouse opsin promoter directs expression specifically to the rod photoreceptors of transgenic mice

Mutations in the human rod opsin gene have been shown to segregate with autosomal dominant retinitis pigmentosa (ADRP) and photoreceptor degeneration in transgenic mice. While these degenerations are characterized by the primary degeneration of rods, cones eventually die as well. To determine whether this subsequent cone degeneration is the result of expression of mutant rod opsin in the cones, the retinal cell-type specificity of a 221-bp fragment of the mouse rod opsin promoter was evaluated. Two transgenic mouse lines generated by injecting a fusion gene comprised of a 221-bp fragment of the mouse rod opsin promoter and the simian virus 40 large tumor antigen gene (Tag) were examined. The expression of Tag causes photoreceptor cell degeneration in members of both transgenic lines. However, the two lines differed with respect to the level of Tag expression and the rate and extent of photoreceptor cell degeneration. Immunocytochemical localization of opsin and Tag in surviving photoreceptor cells was determined and the results were confirmed by reverse transcriptase polymerase chain reaction (RT-PCR). Rod- and cone-mediated function was evaluated by electroretinography (ERG). In the higher Tag-expressing transgenic line only one row of nuclei remained in the outer nuclear layer at postnatal day (P) 150. While these nuclei showed no antigenicity for rod opsin or Tag, they did stain with an antibody that reacts with both rod and cone S-antigens (arrestins), indicating that these cells were surviving photoreceptor nuclei. Positive staining with peanut agglutinin, which uniquely decorates matrix domains surrounding cones in the normal retina, confirmed that the surviving photoreceptor nuclei were of cone origin. RT-PCR substantiated the results from immunostaining; amplification product was obtained using blue cone opsin transcripts but not from either Tag or rod opsin transcripts. The second transgenic mouse line exhibited a much slower photoreceptor cell death that was associated with low levels of Tag transgene transcript. At P120, approximately 50% of photoreceptors remained and an approximately 45% reduction in the rod ERG a-wave was observed. Cone-mediated ERGs, however, were normal. The results demonstrate the rod-specific expression of Tag as directed by the 221-bp fragment of the mouse rod opsin promoter and suggest that the cone degeneration in ADRP or transgenic mice associated with mutations in the rod opsin gene is a secondary effect of rod degeneration.

The bZIP transcription factor Nrl stimulates rhodopsin promoter activity in primary retinal cell cultures

In vitro DNA binding assays and transient transfection analysis with monkey kidney cells have implicated Nrl, a member of the Maf-Nrl subfamily of bZIP transcription factors, and the Nrl response element (NRE) in the regulation of rhodopsin expression. We have now further explored the role of the NRE and surrounding promoter elements. Using the yeast one-hybrid screen with integrated NRE and flanking DNA as bait, the predominant clone obtained was bovine Nrl. Recovery of truncated clones in the screen demonstrated that the carboxyl-terminal half of Nrl, which contains the basic and leucine zipper domains, is sufficient for DNA binding. To functionally dissect the rhodopsin promoter, transient expression studies with primary chick retinal cell cultures were performed. Deletion and mutation analyses identified two positive regulatory sequences: one between -40 and -84 base pairs (bp) and another between -84 and -130 bp. Activity of the -40 to -84 region was shown to be largely due to the NRE. On co-transfection with an NRL expression vector, there were 3-5-fold increases in the activity of rhodopsin promoter constructs containing an intact NRE but little or no effect with rhodopsin promoters containing a mutated or deleted NRE. Nrl was more effective than the related bZIP proteins, c-Fos and c-Jun, in stimulating rhodopsin promoter activity. The -84- to -130-bp region acted synergistically with the NRE to enhance both the level of basal expression and the degree of Nrl-mediated trans-activation. These studies support Nrl as a regulator of rhodopsin expression in vivo, identify an additional regulatory region just upstream of the NRE, and demonstrate the utility of primary retinal cell cultures for characterizing both the cis-acting response elements and trans-acting factors that regulate photoreceptor gene expression.

Ret 4, a positive acting rhodopsin regulatory element identified using a bovine retina in vitro transcription system

Previous transgenic mouse studies demonstrated that the bovine rhodopsin sequence between -222 and +70 base pairs (bp) contains a minimal promoter, which is sufficient to direct photoreceptor cell-specific expression of a lacZ reporter gene. To more fully define the DNA regulatory elements and protein factors involved in regulating rhodopsin transcription, we have developed an in vitro transcription system derived from bovine retinal nuclear extracts. Retinal extracts, as compared to liver, HeLa, and Drosophila embryonic cell extracts, demonstrated preferential activity for the rhodopsin promoter. A template spanning the bovine rhodopsin upstream region from -590 to +15 bp showed significant activation relative to the basal activity seen with a TATA box containing -38 to +15 bp template. Deletion analysis indicated that the region between -85 and -38 bp contained significant positive regulatory activity. This activity was not observed with HeLa extracts, suggesting that it might be retina-specific. Systematic site-directed mutagenesis of the subregion from -64 to -38 bp indicated that sequences between -60 and -58 bp and between -48 and -40 bp harbor critical elements. The former sequence is part of the binding site for the retina-specific transcription factor Nrl, which has been implicated in rhodopsin regulation. Electrophoretic mobility shift assays showed that the latter sequence (-48 to -40 bp), and flanking DNA, designated Ret 4, is bound by both retina-specific and ubiquitously expressed protein factors. Shift assays with mutant oligomers further defined the putative recognition sequences for these protein factors. Together, our results suggest that multiple promoter elements and transcriptional factors are involved in regulating photoreceptor-specific rhodopsin transcription.

Hydroxyindole-O-methyltransferase in Y-79 cells: regulation by serum

Hydroxyindole-O-methyltransferase (HIOMT) catalyzes the last step in the synthesis of melatonin. In the present study, the regulation of HIOMT expression was examined in the human Y-79 retinoblastoma cell line. Cells were grown in suspension culture using medium supplemented with 10% fetal calf serum (FCS). HIOMT activity and mRNA were strongly reduced when FCS was substituted with 0.1% bovine serum albumin (BSA), and were restored by addition of FCS. The effect of FCS on HIOMT expression was relatively selective, because the abundance of mRNA encoding actin, G3PDH or interphotoreceptor retinoid-binding protein did not change following serum deprivation. However, S-antigen (arrestin) mRNA was regulated by serum coordinately with HIOMT mRNA, suggesting that S-antigen expression is also controlled by a serum factor. The effect of serum on HIOMT expression was not duplicated by treatment with a series of known differentiating factors, nor was it reduced by dialysis or stripping procedures which remove steroids, growth factors and thyroid hormones.

Rod photoreceptor-specific gene expression in human retinoblastoma cells

Retinoblastoma cells in culture have previously been shown to express cone-specific genes but not their rod counterparts. We have detected the messages for the rod alpha, beta, and gamma subunits of cGMP phosphodiesterase (PDE), the rod alpha subunit of transducin, rod opsin, and the cone alpha' subunit of PDE in RNA of human Y-79 retinoblastoma cells by reverse transcription-PCR. Quantitative analysis of the mRNAs for the rod alpha and cone alpha' PDE subunits revealed that they were expressed at comparable levels; however, the transcript encoding the rod beta PDE subunit was 10 times more abundant in these cells. Northern hybridization analysis of Y-79 cell RNA confirmed the presence of the transcripts for rod and cone PDE catalytic subunits. To test whether the transcriptional machinery required for the expression of rod-specific genes was endogenous in Y-79 retinoblastoma cells, cultures were transfected with a construct containing the promoter region of the rod beta PDE subunit gene attached to the firefly luciferase reporter vector. Significant levels of reporter enzyme activity were observed in the cell lysates. Our results demonstrate that the Y-79 retinoblastoma cell line is a good model system for the study of transcriptional regulation of rod-specific genes.

Glial-, neuronal- and photoreceptor-specific cell markers in rosettes of retinoblastoma and retinal dysplasia

Previous studies have shown that a rosette formation represents an attempt to form embryonic retinal tissue, primarily rods and cones. To test the theories as to the origin and characteristics of retinoblastoma cells, we compared the characteristics of tumor rosettes with those of dysplastic rosettes seen in retinal dysplasia using the glial, neuronal and photoreceptor markers. Forty-four retinoblastoma and one retinal dysplasia specimens were analyzed by indirect immunohistochemistry, using specific antibodies against glial fibrillary acidic protein, S-100 protein, myelin basic protein, neuron-specific enolase, neurofilament, retinal S-antigen and retinal pigment epithelial antigen. In human retinoblastoma, all the glial, neuronal, retinal pigment epithelial, and photoreceptor cell markers, except for the neurofilament, were present in parts of rosette-forming tumor cells. However, their localization was different for each antigen and it was not clear whether each tumor cell possesses several antigens. These immuno-positive tumor cells were cytologically indistinguishable from other rosette-forming cells at the light microscopic level. In retinal dysplasia, neuron specific enolase and retinal S-antigen were diffusely expressed in the dysplastic rosettes, however, other antigen were not seen in those rosettes. The staining pattern by immunocytochemistry is totally different in tumor rosettes from dysplastic ones. We found varying localizations of different immunoreactivities within tumor rosettes. These results led us to suggest that tumor cells in the rosettes of retinoblastoma may have the ability to differentiate into neural and glial cells. To prove the theory that retinoblastoma cells may have originated from a primitive neuroectodermal cell capable of multipotentiality, further investigation is needed.

Radial asymmetry in the topography of retinoblastoma. Clues to the cell of origin

Retinoblastoma is a malignancy of the human developing retina. In situ as well as in vitro studies have attributed tumoral histogenesis either to a primitive retinoblast with neuronal and glial differentiation potentials, or to a photosensory progenitor cell. Here it is shown in vivo that the retinal topography of 457 retinoblastoma and retinoma foci is radially asymmetrical. Tumor density appears to mimic the horizontal visual streak characteristic of red/green cone cell distribution. Such a non-random distribution seems to invalidate the hypothesis of a primitive multipotential neuroblast as the unique source of retinoblastoma and may support the view that retinoblastoma evolves along the cone cell lineage.

Amplification of the gene encoding the alpha-subunit of the mitochondrial ATP synthase complex in a human retinoblastoma cell line

A cDNA clone encoding the precursor of the alpha-subunit of the human mitochondrial ATP synthase (F1-ATPS) complex was isolated from a library prepared from the poly(A)+ RNA present in a retinoblastoma (RB) cell line. Northern blot analysis of RNAs derived from a variety of transformed cell lines as well as from normal human fetal tissues indicated that RNA expression was significantly higher in two of the four RB cell lines analysed, Y79 (10- to 30-fold) and RB522A (3- to 8-fold), than in other cell lines or tissues. The increased mRNA level was apparently the result of gene amplification in Y79, but not in RB522A.

Bilateral retinal and brain tumors in transgenic mice expressing simian virus 40 large T antigen under control of the human interphotoreceptor retinoid-binding protein promoter

We have previously shown that postnatal expression of the viral oncoprotein SV40 T antigen in rod photoreceptors (transgene MOT1), at a time when retinal cells have withdrawn from the mitotic cycle, leads to photoreceptor cell death (Al-Ubaidi et al., 1992. Proc. Natl. Acad. Sci. USA. 89:1194-1198). To study the effect of the specificity of the promoter, we replaced the mouse opsin promoter in MOT1 by a 1.3-kb promoter fragment of the human IRBP gene which is expressed in both rod and cone photoreceptors during embryonic development. The resulting construct, termed HIT1, was injected into mouse embryos and five transgenic mice lines were established. Mice heterozygous for HIT1 exhibited early bilateral retinal and brain tumors with varying degrees of incidence. Histopathological examination of the brain and eyes of three of the families showed typical primitive neuroectodermal tumors. In some of the bilateral retinal tumors, peculiar rosettes were observed, which were different from the Flexner-Wintersteiner rosettes typically associated with human retinoblastomas. The ocular and cerebral tumors, however, contained Homer-Wright rosettes, and showed varying degrees of immunoreactivity to antibodies against the neuronal specific antigens, synaptophysin and Leu7, but not to antibodies against photoreceptor specific proteins. Taken together, the results indicate that the specificity of the promoter used for T antigen and/or the time of onset of transgene expression determines the fate of photoreceptor cells expressing T antigen.

Bi-modal differentiation pattern in a new human neuroblastoma cell line in vitro

We have isolated a human neuroblastoma (NB) cell line, HTLA230, from the bone-marrow aspirate of a patient with stage-IV disease. Subcutaneous tumors after inoculation of HTLA230 cells into nude mice were composed of primitive neuroblasts which rarely contained neuro-secretory granules. Cytogenetic studies of the cell line demonstrated 2 distinct populations of cells with common chromosomal markers. Stable sub-clones with a differentiated or undifferentiated cell morphology were isolated, demonstrating phenotypical heterogeneity of the HTLA230 parental cell line. Treatment with retinoic acid (RA) induced extensive neurite outgrowth in the parental cell line and in phenotypically differentiated sub-clones, but rarely in undifferentiated ones. Long-term treatment with RA was not associated with down-modulation of mycN-gene expression, which could be achieved only in cultures treated additionally with aphidicolin, a DNA-synthesis inhibitor, thus eliminating growing NB cells. A RA resistant subclone (CI-5) was isolated from parental HTLA230 cells grown at clonal cell density. Cells originally showed a homogeneously differentiated morphology; however, flat cells (F-cells) appeared with time and were subsequently separately propagated. Transdifferentiation of isolated F-cells into cells with neuron-like (N-cell) morphology was observed. Immunohistochemical analysis demonstrated that F-cells had lost the expression of neuronal markers, including HNK-I and A2B5, and expressed the intermediate filament, vimentin. Furthermore, F-cells showed high incorporation of [methyl-3H] thymidine (3H-TdR) by autoradiography but no mycN protein could be detected, although present in the parental cell line. These results then suggest that the isolated NB cell line and the RA-resistant variant line represent an excellent in vitro model with which the bi-modal differentiation pathway of NB can be analyzed on a molecular biological level.

Retinoblastoma: messenger RNA for interphotoreceptor retinoid binding protein

Surgically excised retinoblastomas from 14 patients (age range nine months to two years) were assessed by immunocytochemistry for the expression of photoreceptor-specific proteins and neuronal and glial cell markers. Adjacent tissues were examined for messenger RNA expression of interphotoreceptor retinoid-binding protein (IRBP) using Northern blots. For immunocytochemical stains (ABC method), monoclonal and polyclonal antibodies included S-Ag, rhodopsin, neuron specific enolase (NSE), glial fibrillary acidic protein (GFAP), IRBP, neural adhesion molecule (N-CAM), and rod and cone specific transducin (TR alpha and TC alpha). Histopathology revealed mostly poorly differentiated tumors with necrosis and lack of Flexner-Wintersteiner rosettes. Immunocytochemical staining showed focal IRBP expression in one of the tumors and S-antigen in two cases. Immunoreactivity with rhodopsin was negative. N-CAM, a neural adhesive protein which appears to be involved in the regulation of adhesive interaction during neuronal differentiation, was positive except in two cases. All tumors showed immunoreactivity with NSE, whereas GFAP staining was limited to the perivascular glial tissue confirming the essential neuronal nature of retinoblastoma cells. TC alpha was detected in all tumors and TR alpha in one case. Messenger RNA for IRBP was detected in tumors in which IRBP immunoreactivity could not be detected.

Photoreceptor degeneration induced by the expression of simian virus 40 large tumor antigen in the retina of transgenic mice

Expression of the viral oncogene encoding the simian virus 40 (SV40) large tumor antigen (T antigen) typically promotes tumorigenesis in mammalian cells. To generate transgenic mice that express T antigen in rod photoreceptors, a chimeric construct consisting of a mouse opsin promoter fragment fused to the coding region of SV40 T antigen was generated. Expression of T antigen in the transgenic retina began at early stages of postnatal development concomitant with expression of endogenous opsin. Instead of inducing hyperplasia or tumor formation, T-antigen expression caused a rapidly progressing photoreceptor degeneration. The degeneration was accompanied by sustained DNA synthesis in photoreceptor cells, as evidenced by incorporation of [3H]thymidine and by the appearance of mitotic figures at postnatal day 10, a stage when nontransgenic photoreceptor cells are postmitotic and quiescent. Although transgenic photoreceptor cells undergo S phase and enter mitosis, the consequences of T-antigen expression are not proliferation and tumorigenesis but proliferation and cell death.

Heme oxygenase: expression in human retina and modulation by stress agents in a human retinoblastoma cell model system

PCR and Southern blot analyses demonstrate that mRNA for heme oxygenase (HO), a well known "stress protein" in a number of tissues, is present in human retina. Western and northern blots show that the protein and mRNA are also expressed in human Y-79 retinoblastoma cells in culture and that the HO enzyme is rapidly induced by its substrate, heme. Moreover, HO is also induced by two chemicals, sodium arsenite and menadione, that act as agents of oxidative stress. HO is the regulatory enzyme in the heme degradative pathway and an increase in its activity could lead to the accumulation of bilirubin, an antioxidant, in the cell at the expense of heme, a prooxidant. The HO pathway may thus be of importance in protecting the retina against oxidative stress in vivo. Moreover, the Y-79 culture system should provide an excellent model for use in examining stress mechanisms in retinal cells at a molecular level.

Molecular cloning and sequence analysis of cDNA and genomic DNA for the human cone transducin alpha subunit

A novel GTP binding protein (G protein) alpha subunit cDNA was isolated from a T cell leukemia cell line, Jurkat, utilizing polymerase chain reaction (PCR). The predicted amino acid sequence of this G protein alpha subunit showed the highest identity (96.6%) to bovine cone cell-specific transducin (Tc alpha). The organization of the coding region of this G protein alpha subunit gene was composed of 8 exons and 7 introns. Northern hybridization revealed the presence of this G protein message in a retinoblastoma cell line, Y79. In Jurkat, however, the message was detectable only by reverse transcription/PCR. Taken together, this novel G protein alpha subunit must be human Tc.

N-acetyltransferase and protein synthesis modulate melatonin production by Y79 human retinoblastoma cells

Melatonin is synthesized by the vertebrate pineal gland in a circadian fashion and is involved in numerous circadian and seasonal processes in the organism. The vertebrate retina also produces melatonin rhythmically to regulate rhythmic physiological processes in the eye. In both organs, melatonin is synthesized from serotonin by the sequential action of the enzymes, N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT), and can be stimulated by increases in cyclic AMP through a mechanism requiring protein synthesis. The regulation of ocular melatonin biosynthesis in mammals and particularly humans, has not been well studied. Recently, we have shown that Y79 human retinoblastoma cells produce melatonin and that cAMP can stimulate melatonin production. Y79 cells, therefore, provide a model system in which to study melatonin synthesis in human tissue. We report that cAMP stimulates NAT, but not HIOMT activity in Y79 cells, and that stimulation of NAT activity is linearly related to melatonin release. In addition, the stimulation of NAT and melatonin requires protein synthesis. The turnover of NAT is rather rapid, with a half-life of about 20 min. These results suggest that the regulation of melatonin in Y79 retinoblastoma cells is similar to that found in the retina and pineal of other vertebrates.

In vitro differentiation of human retinoblastoma cells into neuronal phenotypes

In order to characterize the cell type(s) of origin of human retinoblastoma cells by immunophenotyping, primary cells from seven retinoblastomas and of the corresponding cell lines (RBL lines), as well as four retinoblastoma (RB) lines established by other groups, were compared with rat and human retina cells, and with the adenovirus E1A-transformed human retinoblast cell line HER-Xho1-CC2. Analyses using monoclonal antibodies (Mabs) RB13-2 and RB21-7, originally raised against prenatal rat brain cells and recognizing neural cell surface antigens expressed in a developmental-stage-dependent manner, and three cell-type-specific Mabs (Q211, M501, Mab directed against vimentin) developed by other groups, gave the following results: (i) Retinoblastomas consist of cells expressing differentiated neuronal phenotypes during cultivation in vitro; (ii) All of the newly established RBL lines express neuronal phenotypes; and (iii) Cell lines such as Y79, which have been propagated in vitro for extended periods, do not express antigens specific for the neuronal pathway and cannot, therefore, be considered phenotypically representative of retinoblastoma cells.

Neuronal and photoreceptor differentiation of retinoblastoma in culture

This study describes an in vitro system, which favors attachment and differentiation of primary retinoblastoma cells. Indeed, tumor cells from primary retinoblastomas were successfully grown as long-term monolayer cultures, using collagen type-1 coated flasks, preincubation of tumor cells in fetal calf serum and a medium richly supplemented with serum. This reliable system allowed growth and attachment and induced differentiation towards neuronal and photoreceptor morphology. Each of the nine tumors studied showed substantial cell proliferation and differentiation. Cells with glial characteristics were observed in lower numbers. Furthermore, in five cell-lines, some cell clusters showed pigmentation characteristic of pigment epithelial cells (PE). By electron microscopy, a significant proportion of cells showed conventional neuronal differentiation; others formed typical Flexner-Wintersteiner rosettes with individual photoreceptor cells showing a more highly differentiated pattern quite similar to fleurette inner segments. Immunohistochemical studies did not reveal evidence of transition between undifferentiated retinoblastoma cells and glial cells, although foci of multipolar cells were found resembling glial elements and containing glial fibrillary acidic protein. Thus no definite sign of glial differentiation was observed. These results show that most primary retinoblastoma cells can grow in attachment culture and differentiate into at least two distinct morphological cell types. This study also offers a potentially rewarding system to analyse the factors controlling growth and differentiation of human tumor cells in vitro.

Expression of the functional cone phototransduction cascade in retinoblastoma

Retinoblastoma is a malignant intraocular tumor that primarily affects small children. These tumors are primitive neuroectodermal malignancies, however some of them show morphologic evidence of differentiation into photoreceptors. Phototransduction cascades are a series of biochemical reactions that convert a photon of light into a neural impulse in rods and cones. The components of these cascades are uniquely expressed in photoreceptors and, although functionally similar, distinct components of these cascades are expressed in rods and cones. Using HPLC anion exchange chromatography, Western blot analysis, and specific monoclonal and polyclonal antibodies, we found that the cone but not the rod cGMP phosphodiesterase is functionally expressed in all six primary retinoblastomas examined and in three continuous retinoblastoma cell lines. Morphologic evidence of differentiation did not correlate with the expression of the enzyme. Furthermore, GTP analogues could activate the phosphodiesterase activity suggesting that an intact phototransduction cascade is present in the tumors. The presence of the cone phototransduction cascade in retinoblastoma confirms that this tumor has biochemically differentiated along the cone cell lineage.