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Chakrabarty K, Nayak D, Debnath J, Das D, Shetty R, Ghosh A. Retinal organoids in disease modeling and drug discovery: Opportunities and challenges. Surv Ophthalmol 2023:S0039-6257(23)00127-3. [PMID: 37778668 DOI: 10.1016/j.survophthal.2023.09.003] [Citation(s) in RCA: 4] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/24/2023] [Revised: 09/25/2023] [Accepted: 09/25/2023] [Indexed: 10/03/2023]
Abstract
Diseases leading to retinal cell loss can cause severe visual impairment and blindness. The lack of effective therapies to address retinal cell loss and the absence of intrinsic regeneration in the human retina leads to an irreversible pathological condition. Progress in recent years in the generation of human three-dimensional retinal organoids from pluripotent stem cells makes it possible to recreate the cytoarchitecture and associated cell-cell interactions of the human retina in remarkable detail. These human three-dimensional retinal organoid systems made of distinct retinal cell types and possessing contextual physiological responses allow the study of human retina development and retinal disease pathology in a way animal model and two-dimensional cell cultures were unable to achieve. We describe the derivation of retinal organoids from human pluripotent stem cells and their application for modeling retinal disease pathologies, while outlining the opportunities and challenges for its application in academia and industry.
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Affiliation(s)
- Koushik Chakrabarty
- GROW Research Laboratory, Narayana Nethralaya Foundation, Bangalore, Karnataka, India.
| | - Divyani Nayak
- GROW Research Laboratory, Narayana Nethralaya Foundation, Bangalore, Karnataka, India
| | - Jayasree Debnath
- GROW Research Laboratory, Narayana Nethralaya Foundation, Bangalore, Karnataka, India
| | - Debashish Das
- Stem Cell Research Lab, GROW Lab, Narayana Nethralaya Foundation, Narayana Nethralaya, Bangalore, Karnataka, India
| | - Rohit Shetty
- Department of Cornea and Refractive Surgery, Narayana Nethralaya Eye Hospital, Bangalore, Karnataka, India
| | - Arkasubhra Ghosh
- GROW Research Laboratory, Narayana Nethralaya Foundation, Bangalore, Karnataka, India
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Wu HY, Rong Y, Bansal PK, Wei P, Guo H, Morgan JI. TTLL1 and TTLL4 polyglutamylases are required for the neurodegenerative phenotypes in pcd mice. PLoS Genet 2022; 18:e1010144. [PMID: 35404950 PMCID: PMC9022812 DOI: 10.1371/journal.pgen.1010144] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/17/2021] [Revised: 04/21/2022] [Accepted: 03/14/2022] [Indexed: 02/01/2023] Open
Abstract
Polyglutamylation is a dynamic posttranslational modification where glutamate residues are added to substrate proteins by 8 tubulin tyrosine ligase-like (TTLL) family members (writers) and removed by the 6 member Nna1/CCP family of carboxypeptidases (erasers). Genetic disruption of polyglutamylation leading to hyperglutamylation causes neurodegenerative phenotypes in humans and animal models; the best characterized being the Purkinje cell degeneration (pcd) mouse, a mutant of the gene encoding Nna1/CCP1, the prototypic eraser. Emphasizing the functional importance of the balance between glutamate addition and elimination, loss of TTLL1 prevents Purkinje cell degeneration in pcd. However, whether Ttll1 loss protects other vulnerable neurons in pcd, or if elimination of other TTLLs provides protection is largely unknown. Here using a mouse genetic rescue strategy, we characterized the contribution of Ttll1, 4, 5, 7, or 11 to the degenerative phenotypes in cerebellum, olfactory bulb and retinae of pcd mutants. Ttll1 deficiency attenuates Purkinje cell loss and function and reduces olfactory bulb mitral cell death and retinal photoreceptor degeneration. Moreover, degeneration of photoreceptors in pcd is preceded by impaired rhodopsin trafficking to the rod outer segment and likely represents the causal defect leading to degeneration as this too is rescued by elimination of TTLL1. Although TTLLs have similar catalytic properties on model substrates and several are highly expressed in Purkinje cells (e.g. TTLL5 and 7), besides TTLL1 only TTLL4 deficiency attenuated degeneration of Purkinje and mitral cells in pcd. Additionally, TTLL4 loss partially rescued photoreceptor degeneration and impaired rhodopsin trafficking. Despite their common properties, the polyglutamylation profile changes promoted by TTLL1 and TTLL4 deficiencies in pcd mice are very different. We also report that loss of anabolic TTLL5 synergizes with loss of catabolic Nna1/CCP1 to promote photoreceptor degeneration. Finally, male infertility in pcd is not rescued by loss of any Ttll. These data provide insight into the complexity of polyglutamate homeostasis and function in vivo and potential routes to ameliorate disorders caused by disrupted polyglutamylation.
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Affiliation(s)
- Hui-Yuan Wu
- Department of Developmental Neurobiology, St. Jude Children’s Research Hospital, Memphis, Tennessee, United States of America
- School of Pharmaceutical Science and Technology, Tianjin University, Tianjin, China
| | - Yongqi Rong
- Department of Developmental Neurobiology, St. Jude Children’s Research Hospital, Memphis, Tennessee, United States of America
| | - Parmil K. Bansal
- Department of Developmental Neurobiology, St. Jude Children’s Research Hospital, Memphis, Tennessee, United States of America
| | - Peng Wei
- Department of Developmental Neurobiology, St. Jude Children’s Research Hospital, Memphis, Tennessee, United States of America
| | - Hong Guo
- Department of Developmental Neurobiology, St. Jude Children’s Research Hospital, Memphis, Tennessee, United States of America
| | - James I. Morgan
- Department of Developmental Neurobiology, St. Jude Children’s Research Hospital, Memphis, Tennessee, United States of America
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Hydrogel-based milliwell arrays for standardized and scalable retinal organoid cultures. Sci Rep 2020; 10:10275. [PMID: 32581233 PMCID: PMC7314858 DOI: 10.1038/s41598-020-67012-7] [Citation(s) in RCA: 33] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/08/2019] [Accepted: 06/01/2020] [Indexed: 12/31/2022] Open
Abstract
The development of improved methods to culture retinal organoids is relevant for the investigation of mechanisms of retinal development under pathophysiological conditions, for screening of neuroprotective compounds, and for providing a cellular source for clinical transplantation. We report a tissue-engineering approach to accelerate and standardize the production of retinal organoids by culturing mouse embryonic stem cells (mESC) in optimal physico-chemical microenvironments. Arrayed round-bottom milliwells composed of biomimetic hydrogels, combined with an optimized medium formulation, promoted the rapid generation of retina-like tissue from mESC aggregates in a highly efficient and stereotypical manner: ∼93% of the aggregates contained retinal organoid structures. 26 day-old retinal organoids were composed of ∼80% of photoreceptors, of which ∼22% are GNAT2-positive cones, an important and rare sensory cell type that is difficult to study in rodent models. The compartmentalization of retinal organoids into predefined locations on a two-dimensional array not only allowed us to derive almost all aggregates into retinal organoids, but also to reliably capture the dynamics of individual organoids, an advantageous requirement for high-throughput experimentation. Our improved retinal organoid culture system should be useful for applications that require scalability and single-organoid traceability.
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Abstract
The use of retinal organoids requires efficient differentiation from induced pluripotent stem cells (iPSCs). In this issue of Cell Reports, Wang et al. (2018) examine how the chromatin landscape after iPSC programming predicts their ability to differentiate into retinal tissue.
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Wang L, Hiler D, Xu B, AlDiri I, Chen X, Zhou X, Griffiths L, Valentine M, Shirinifard A, Sablauer A, Thiagarajan S, Barabas ME, Zhang J, Johnson D, Frase S, Dyer MA. Retinal Cell Type DNA Methylation and Histone Modifications Predict Reprogramming Efficiency and Retinogenesis in 3D Organoid Cultures. Cell Rep 2019. [PMID: 29514090 PMCID: PMC5872828 DOI: 10.1016/j.celrep.2018.01.075] [Citation(s) in RCA: 44] [Impact Index Per Article: 8.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022] Open
Abstract
Diverse cell types can be reprogrammed into pluripotent stem cells by ectopic expression of Oct4 (Pou5f1), Klf4, Sox3, and Myc. Many of these induced pluripotent stem cells (iPSCs) retain memory, in terms of DNA methylation and histone modifications (epigenetic memory), of their cellular origins, and this may bias subsequent differentiation. Neurons are difficult to reprogram, and there has not been a systematic side-by-side characterization of reprogramming efficiency or epigenetic memory across different neuronal subtypes. Here, we compare reprogramming efficiency of five different retinal cell types at two different stages of development. Retinal differentiation from each iPSC line was measured using a quantitative standardized scoring system called STEM-RET and compared to the epigenetic memory. Neurons with the lowest reprogramming efficiency produced iPSC lines with the best retinal differentiation and were more likely to retain epigenetic memory of their cellular origins. In addition, we identified biomarkers of iPSCs that are predictive of retinal differentiation.
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Affiliation(s)
- Lu Wang
- Department of Developmental Neurobiology, St. Jude Children's Research Hospital, Memphis, TN 38105, USA
| | - Daniel Hiler
- Department of Developmental Neurobiology, St. Jude Children's Research Hospital, Memphis, TN 38105, USA
| | - Beisi Xu
- Department of Computational Biology, St. Jude Children's Research Hospital, Memphis, TN 38105, USA
| | - Issam AlDiri
- Department of Developmental Neurobiology, St. Jude Children's Research Hospital, Memphis, TN 38105, USA
| | - Xiang Chen
- Department of Computational Biology, St. Jude Children's Research Hospital, Memphis, TN 38105, USA
| | - Xin Zhou
- Department of Computational Biology, St. Jude Children's Research Hospital, Memphis, TN 38105, USA
| | - Lyra Griffiths
- Department of Developmental Neurobiology, St. Jude Children's Research Hospital, Memphis, TN 38105, USA
| | - Marc Valentine
- Cytogenetics Shared Resource, St. Jude Children's Research Hospital, Memphis, TN 38105, USA
| | - Abbas Shirinifard
- Department of Developmental Neurobiology, St. Jude Children's Research Hospital, Memphis, TN 38105, USA
| | - András Sablauer
- Department of Diagnostic Imaging, St. Jude Children's Research Hospital, Memphis, TN 38105, USA
| | - Suresh Thiagarajan
- Department of Diagnostic Imaging, St. Jude Children's Research Hospital, Memphis, TN 38105, USA
| | - Marie-Elizabeth Barabas
- Department of Developmental Neurobiology, St. Jude Children's Research Hospital, Memphis, TN 38105, USA
| | - Jiakun Zhang
- Department of Developmental Neurobiology, St. Jude Children's Research Hospital, Memphis, TN 38105, USA
| | - Dianna Johnson
- Department of Ophthalmology, University of Tennessee Health Science Center, Memphis, TN 38163, USA
| | - Sharon Frase
- Cell and Tissue Imaging Shared Resource, St. Jude Children's Research Hospital, Memphis, TN 38105, USA
| | - Michael A Dyer
- Department of Developmental Neurobiology, St. Jude Children's Research Hospital, Memphis, TN 38105, USA; Department of Ophthalmology, University of Tennessee Health Science Center, Memphis, TN 38163, USA; Howard Hughes Medical Institute, Chevy Chase, MD 20815, USA.
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6
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Yanai A, McNab P, Gregory-Evans K. Retinal therapy with induced pluripotent stem cells; leading the way to human clinical trials. EXPERT REVIEW OF OPHTHALMOLOGY 2019. [DOI: 10.1080/17469899.2019.1568872] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Affiliation(s)
- Anat Yanai
- Department of Ophthalmology and Visual Sciences, Faculty of Medicine, University of British Columbia, Vancouver, BC, Canada
| | - Pia McNab
- Department of Ophthalmology and Visual Sciences, Faculty of Medicine, University of British Columbia, Vancouver, BC, Canada
| | - Kevin Gregory-Evans
- Department of Ophthalmology and Visual Sciences, Faculty of Medicine, University of British Columbia, Vancouver, BC, Canada
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7
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Murali A, Ramlogan-Steel CA, Andrzejewski S, Steel JC, Layton CJ. Retinal explant culture: A platform to investigate human neuro-retina. Clin Exp Ophthalmol 2018; 47:274-285. [PMID: 30378239 DOI: 10.1111/ceo.13434] [Citation(s) in RCA: 31] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/16/2018] [Revised: 10/01/2018] [Accepted: 10/22/2018] [Indexed: 01/09/2023]
Abstract
The retina is the tissue responsible for light detection, in which retinal neurons convert light energy into electrical signals to be transported towards the visual cortex. Damage of retinal neurons leads to neuronal cell death and retinal pathologies, compromising visual acuity and eventually leading to irreversible blindness. Models of retinal neurodegeneration include 2D systems like cell lines, disassociated cultures and co-cultures, and 3D models like organoids, organotypic retinal cultures and animal models. Of these, ex vivo human retinal cultures are arguably the most suitable models for translational research as they retain complex inter-cellular interactions of the retina and precisely mimic in-situ responses. In this review, we summarize the distinguishing features of the human retina which are important to preserve in experimental culture, the historical development of human retinal culture systems, the factors affecting ex vivo human retinal culture and the applications and challenges associated with current methods of human retinal explant culture.
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Affiliation(s)
- Aparna Murali
- LVF Ophthalmology Research Centre, Translational Research Institute, Woolloongabba, Queensland, Australia.,Faculty of Medicine, The University of Queensland, Herston, Queensland, Australia
| | - Charmaine A Ramlogan-Steel
- Faculty of Medicine, The University of Queensland, Herston, Queensland, Australia.,School of Health, Medical and Applied Sciences, CQUniversity, North Rockhampton, Queensland, Australia
| | - Slawomir Andrzejewski
- LVF Ophthalmology Research Centre, Translational Research Institute, Woolloongabba, Queensland, Australia.,Faculty of Medicine, The University of Queensland, Herston, Queensland, Australia
| | - Jason C Steel
- School of Health, Medical and Applied Sciences, CQUniversity, North Rockhampton, Queensland, Australia
| | - Christopher J Layton
- LVF Ophthalmology Research Centre, Translational Research Institute, Woolloongabba, Queensland, Australia.,Faculty of Medicine, The University of Queensland, Greenslopes Hospital, Brisbane, Queensland, Australia
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Gasparini SJ, Llonch S, Borsch O, Ader M. Transplantation of photoreceptors into the degenerative retina: Current state and future perspectives. Prog Retin Eye Res 2018; 69:1-37. [PMID: 30445193 DOI: 10.1016/j.preteyeres.2018.11.001] [Citation(s) in RCA: 118] [Impact Index Per Article: 19.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/05/2018] [Revised: 10/29/2018] [Accepted: 11/06/2018] [Indexed: 12/12/2022]
Abstract
The mammalian retina displays no intrinsic regenerative capacities, therefore retinal degenerative diseases such as age-related macular degeneration (AMD) or retinitis pigmentosa (RP) result in a permanent loss of the light-sensing photoreceptor cells. The degeneration of photoreceptors leads to vision impairment and, in later stages, complete blindness. Several therapeutic strategies have been developed to slow down or prevent further retinal degeneration, however a definitive cure i.e. replacement of the lost photoreceptors, has not yet been established. Cell-based treatment approaches, by means of photoreceptor transplantation, have been studied in pre-clinical animal models over the last three decades. The introduction of pluripotent stem cell-derived retinal organoids represents, in principle, an unlimited source for the generation of transplantable human photoreceptors. However, safety, immunological and reproducibility-related issues regarding the use of such cells still need to be solved. Moreover, the recent finding of cytoplasmic material transfer between donor and host photoreceptors demands reinterpretation of several former transplantation studies. At the same time, material transfer between healthy donor and dysfunctional patient photoreceptors also offers a potential alternative strategy for therapeutic intervention. In this review we discuss the history and current state of photoreceptor transplantation, the techniques used to assess rescue of visual function, the prerequisites for effective transplantation as well as the main roadblocks, including safety and immune response to the graft, that need to be overcome for successful clinical translation of photoreceptor transplantation approaches.
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Affiliation(s)
- Sylvia J Gasparini
- CRTD/Center for Regenerative Therapies Dresden, Center for Molecular and Cellular Bioengineering (CMCB), Technische Universität Dresden, Fetscherstraße 105, 01307, Dresden, Germany
| | - Sílvia Llonch
- CRTD/Center for Regenerative Therapies Dresden, Center for Molecular and Cellular Bioengineering (CMCB), Technische Universität Dresden, Fetscherstraße 105, 01307, Dresden, Germany
| | - Oliver Borsch
- CRTD/Center for Regenerative Therapies Dresden, Center for Molecular and Cellular Bioengineering (CMCB), Technische Universität Dresden, Fetscherstraße 105, 01307, Dresden, Germany
| | - Marius Ader
- CRTD/Center for Regenerative Therapies Dresden, Center for Molecular and Cellular Bioengineering (CMCB), Technische Universität Dresden, Fetscherstraße 105, 01307, Dresden, Germany.
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9
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Stem cells and genome editing: approaches to tissue regeneration and regenerative medicine. J Hum Genet 2017; 63:165-178. [PMID: 29192237 DOI: 10.1038/s10038-017-0348-0] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/30/2017] [Revised: 08/20/2017] [Accepted: 08/21/2017] [Indexed: 12/20/2022]
Abstract
Understanding the basis of regeneration of each tissue and organ, and incorporating this knowledge into clinical treatments for degenerative tissues and organs in patients, are major goals for researchers in regenerative biology. Here we provide an overview of current work, from high-regeneration animal models, to stem cell-based culture models, transplantation technologies, large-animal chimeric models, and programmable nuclease-based genome-editing technologies. Three-dimensional culture generating organoids, which represents intact tissue/organ identity including cell fate and morphology are getting more general approaches in the fields by taking advantage of embryonic stem cells, induced pluripotent stem cells and adult stem cells. The organoid culture system potentially has profound impact on the field of regenerative medicine. We also emphasize that the large animal model, in particular pig model would be a hope to manufacture humanized organs in in vivo empty (vacant) niche, which now potentially allows not only appropriate cell fate identity but nearly the same property as human organs in size. Therefore, integrative and collaborative researches across different fields might be critical to the aims needed in clinical trial.
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10
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Aldiri I, Xu B, Wang L, Chen X, Hiler D, Griffiths L, Valentine M, Shirinifard A, Thiagarajan S, Sablauer A, Barabas ME, Zhang J, Johnson D, Frase S, Zhou X, Easton J, Zhang J, Mardis ER, Wilson RK, Downing JR, Dyer MA. The Dynamic Epigenetic Landscape of the Retina During Development, Reprogramming, and Tumorigenesis. Neuron 2017; 94:550-568.e10. [PMID: 28472656 PMCID: PMC5508517 DOI: 10.1016/j.neuron.2017.04.022] [Citation(s) in RCA: 173] [Impact Index Per Article: 24.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/05/2016] [Revised: 02/15/2017] [Accepted: 04/14/2017] [Indexed: 12/21/2022]
Abstract
In the developing retina, multipotent neural progenitors undergo unidirectional differentiation in a precise spatiotemporal order. Here we profile the epigenetic and transcriptional changes that occur during retinogenesis in mice and humans. Although some progenitor genes and cell cycle genes were epigenetically silenced during retinogenesis, the most dramatic change was derepression of cell-type-specific differentiation programs. We identified developmental-stage-specific super-enhancers and showed that most epigenetic changes are conserved in humans and mice. To determine how the epigenome changes during tumorigenesis and reprogramming, we performed integrated epigenetic analysis of murine and human retinoblastomas and induced pluripotent stem cells (iPSCs) derived from murine rod photoreceptors. The retinoblastoma epigenome mapped to the developmental stage when retinal progenitors switch from neurogenic to terminal patterns of cell division. The epigenome of retinoblastomas was more similar to that of the normal retina than that of retina-derived iPSCs, and we identified retina-specific epigenetic memory.
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Affiliation(s)
- Issam Aldiri
- Department of Developmental Neurobiology, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA
| | - Beisi Xu
- Department of Computational Biology, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA
| | - Lu Wang
- Department of Developmental Neurobiology, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA
| | - Xiang Chen
- Department of Computational Biology, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA
| | - Daniel Hiler
- Department of Developmental Neurobiology, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA
| | - Lyra Griffiths
- Department of Developmental Neurobiology, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA
| | - Marc Valentine
- Cytogenetics Shared Resource, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA
| | - Abbas Shirinifard
- Department of Diagnostic Imaging, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA
| | - Suresh Thiagarajan
- Department of Diagnostic Imaging, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA
| | - Andras Sablauer
- Department of Diagnostic Imaging, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA
| | - Marie-Elizabeth Barabas
- Department of Developmental Neurobiology, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA
| | - Jiakun Zhang
- Department of Developmental Neurobiology, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA
| | - Dianna Johnson
- Department of Ophthalmology, University of Tennessee Health Science Center, Memphis, Tennessee 38163, USA
| | - Sharon Frase
- Cell and Tissue Imaging Shared Resource, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA
| | - Xin Zhou
- Department of Computational Biology, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA
| | - John Easton
- Department of Computational Biology, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA
| | - Jinghui Zhang
- Department of Computational Biology, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA
| | - Elaine R Mardis
- Department of Genetics, The Genome Institute, Washington University School of Medicine in St. Louis, St. Louis, Missouri 63108, USA; Department of Medicine, The Genome Institute, Washington University School of Medicine in St. Louis, St. Louis, Missouri 63108, USA
| | - Richard K Wilson
- Department of Genetics, The Genome Institute, Washington University School of Medicine in St. Louis, St. Louis, Missouri 63108, USA; Siteman Cancer Center, The Genome Institute, Washington University School of Medicine in St. Louis, St. Louis, Missouri 63108, USA
| | - James R Downing
- Department of Pathology, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA
| | - Michael A Dyer
- Department of Developmental Neurobiology, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA; Department of Ophthalmology, University of Tennessee Health Science Center, Memphis, Tennessee 38163, USA; Howard Hughes Medical Institute, Chevy Chase, Maryland 20815, USA.
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