Igf signaling couples retina growth with body growth by modulating progenitor cell division

Inhibition of IGF-1Rα affects the differentiation fate of rat optic cup-derived retinal stem cells to retinal ganglion cells in vitro

AIM

To explore the impact of insulin-like growth factor-1 receptor α (IGF-1Rα) on the differentiation fate of optic-cup-derived retinal stem cells (OC-RSCs) into retinal ganglion cells (RGCs) in vitro.

METHODS

OC-RSCs were isolated from optic cups of rats on embryonic day 12.5, and high-purity OC-RSCs were obtained by conditioned culture and passage. Differentiation of OC-RSCs into RGCs under different serum concentrations was examined using flow cytometry, and the serum concentration with high interference with differentiation ratio was selected. Furthermore, the effect of blocking IGF-1Rα on the differentiation of OC-RSCs into RGCs was analyzed through immunocytochemistry and Western blotting.

RESULTS

Immunohistochemical analysis revealed IGF-1Rα was highly expressed in rat embryos at day 12.5. OC-RSCs were isolated and purified, and high-purity OC-RSCs were obtained. When 2.5% serum was administered, the ratio of differentiated RGCs (Thy-1.1 positive) decreased significantly, and the results of immunoblotting also confirmed the blockade of IGF-1Rα reduced Thy-1.1 protein expression.

CONCLUSION

IGF-1Rα blocking can reduce the differentiation of OC-RSCs into RGCs.

Regulation and function of insulin and insulin-like growth factor receptor signalling

Receptors of insulin and insulin-like growth factors (IGFs) are receptor tyrosine kinases whose signalling controls multiple aspects of animal physiology throughout life. In addition to regulating metabolism and growth, insulin-IGF receptor signalling has recently been linked to a variety of new, cell type-specific functions. In the last century, key questions have focused on how structural differences of insulin and IGFs affect receptor activation, and how insulin-IGF receptor signalling translates into pleiotropic biological functions. Technological advances such as cryo-electron microscopy have provided a detailed understanding of how native and engineered ligands activate insulin-IGF receptors. In this Review, we highlight recent structural and functional insights into the activation of insulin-IGF receptors, and summarize new agonists and antagonists developed for intervening in the activation of insulin-IGF receptor signalling. Furthermore, we discuss recently identified regulatory mechanisms beyond ligand-receptor interactions and functions of insulin-IGF receptor signalling in diseases.

Müller glia in short-term dark adaptation of the Austrolebias charrua retina: Cell proliferation and cytoarchitecture

Fish with unique life cycles offer valuable insights into retinal plasticity, revealing mechanisms of environmental adaptation, cell proliferation, and thus, potentially regeneration. The variability of the environmental factors to which Austrolebias annual fishes are exposed has acted as a strong selective pressure shaping traits such as nervous system plasticity. This has contributed to adaptation to their extreme conditions including the decreased luminosity as ponds dry out. In particular, the retina of A. charrua has been shown to respond to 30 days of decreased luminosity by exacerbating cell proliferation Now, we aimed to determine the cellular component of the retina involved in shorter-term responses. To this end, we performed 5-bromo-2'-deoxyuridine (BrdU) experiments, exposing adult fish to a short period (11 days) of constant darkness. Strikingly, in control conditions, neurogenesis in the inner nuclear and ganglion cell layer in the differentiated retina was detected. In constant darkness, we observed an effect on inner nuclear layer cell proliferation and changes in retinal cytoarchitecture of the retina with cell clusters located in the inner plexiform layer. Additionally, increased BLBP (brain lipid-binding protein) presence was detected in darkness, which has been previously associated with immature and reactivated Müller glia. Thus, our results suggest that the A. charrua retina can respond to environmental changes via rapid activation of progenitor cells in the INL, namely the Müller glia This leads us to hypothesize, that cell proliferation and neurogenesis might contribute to the responses to the functional needs of organisms, potentially playing an adaptive role.

Visualisation of gene expression within the context of tissues using an X-ray computed tomography-based multimodal approach

The development of an organism is orchestrated by the spatial and temporal expression of genes. Accurate visualisation of gene expression patterns in the context of the surrounding tissues offers a glimpse into the mechanisms that drive morphogenesis. We developed correlative light-sheet fluorescence microscopy and X-ray computed tomography approach to map gene expression patterns to the whole organism`s 3D anatomy. We show that this multimodal approach is applicable to gene expression visualized by protein-specific antibodies and fluorescence RNA in situ hybridisation offering a detailed understanding of individual phenotypic variations in model organisms. Furthermore, the approach offers a unique possibility to identify tissues together with their 3D cellular and molecular composition in anatomically less-defined in vitro models, such as organoids. We anticipate that the visual and quantitative insights into the 3D distribution of gene expression within tissue architecture, by multimodal approach developed here, will be equally valuable for reference atlases of model organisms development, as well as for comprehensive screens, and morphogenesis studies of in vitro models.

Molecular mechanisms controlling vertebrate retinal patterning, neurogenesis, and cell fate specification

This review covers recent advances in understanding the molecular mechanisms controlling neurogenesis and specification of the developing retina, with a focus on insights obtained from comparative single cell multiomic analysis. We discuss recent advances in understanding the mechanisms by which extrinsic factors trigger transcriptional changes that spatially pattern the optic cup (OC) and control the initiation and progression of retinal neurogenesis. We also discuss progress in unraveling the core evolutionarily conserved gene regulatory networks (GRNs) that specify early- and late-state retinal progenitor cells (RPCs) and neurogenic progenitors and that control the final steps in determining cell identity. Finally, we discuss findings that provide insight into regulation of species-specific aspects of retinal patterning and neurogenesis, including consideration of key outstanding questions in the field.

Unravelling the link between embryogenesis and adult stem cell potential in the ciliary marginal zone: A comparative study between mammals and teleost fish

The discovery and study of adult stem cells have revolutionized regenerative medicine by offering new opportunities for treating various medical conditions. Anamniote stem cells, which retain their full proliferative capacity and full differentiation range throughout their lifetime, harbour a greater potential compared to mammalian adult stem cells, which only exhibit limited stem cell potential. Therefore, understanding the mechanisms underlying these differences is of significant interest. In this review, we examine the similarities and differences of adult retinal stem cells in anamniotes and mammals, from their embryonic stages in the optic vesicle to their residence in the postembryonic retinal stem cell niche, the ciliary marginal zone located in the retinal periphery. In anamniotes, developing precursors of retinal stem cells are exposed to various environmental cues during their migration in the complex morphogenetic remodelling of the optic vesicle to the optic cup. In contrast, their mammalian counterparts in the retinal periphery are primarily instructed by neighbouring tissues once they are in place. We explore the distinct modes of optic cup morphogenesis in mammals and teleost fish and highlight molecular mechanisms governing morphogenesis and stem cells instruction. The review concludes with the molecular mechanisms of ciliary marginal zone formation and offers a perspective on the impact of comparative single cell transcriptomic studies to reveal the evolutionary similarities and differences.

Insulin-like growth factor-1 stimulates retinal cell proliferation via activation of multiple signaling pathways

Insulin-like growth factor-1 (IGF-1) plays critical roles in the development of the central nervous system (CNS), including the retina, regulating cell proliferation, differentiation, and survival. Here, we investigated the role of IGF-1 on retinal cell proliferation using primary cultures from rat neural retina. Our data show that IGF-1 stimulates retinal cell proliferation and regulates the expression of neurotrophic factors, such as interleukin-4 and brain-derived neurotrophic factor. In addition, our results indicates that IGF-1-induced retinal cell proliferation requires activation of multiple signaling pathways, including phosphatidylinositol 3-kinase, protein kinase Src, phospholipase-C, protein kinase C delta, and mitogen-activated protein kinase pathways. We further show that activation of matrix metalloproteinases and epidermal growth factor receptor is also necessary for IGF-1 enhancing retinal cell proliferation. Overall, these results unveil potential mechanisms by which IGF-1 ensures retinal cell proliferation and support the notion that manipulation of IGF-1 signaling may be beneficial in CNS disorders associated with abnormal cell proliferation.

Cell Sources for Retinal Regeneration: Implication for Data Translation in Biomedicine of the Eye

The main degenerative diseases of the retina include macular degeneration, proliferative vitreoretinopathy, retinitis pigmentosa, and glaucoma. Novel approaches for treating retinal diseases are based on cell replacement therapy using a variety of exogenous stem cells. An alternative and complementary approach is the potential use of retinal regeneration cell sources (RRCSs) containing retinal pigment epithelium, ciliary body, Müller glia, and retinal ciliary region. RRCSs in lower vertebrates in vivo and in mammals mostly in vitro are able to proliferate and exhibit gene expression and epigenetic characteristics typical for neural/retinal cell progenitors. Here, we review research on the factors controlling the RRCSs' properties, such as the cell microenvironment, growth factors, cytokines, hormones, etc., that determine the regenerative responses and alterations underlying the RRCS-associated pathologies. We also discuss how the current data on molecular features and regulatory mechanisms of RRCSs could be translated in retinal biomedicine with a special focus on (1) attempts to obtain retinal neurons de novo both in vivo and in vitro to replace damaged retinal cells; and (2) investigations of the key molecular networks stimulating regenerative responses and preventing RRCS-related pathologies.

Precise in vivo functional analysis of DNA variants with base editing using ACEofBASEs target prediction

Single nucleotide variants (SNVs) are prevalent genetic factors shaping individual trait profiles and disease susceptibility. The recent development and optimizations of base editors, rubber and pencil genome editing tools now promise to enable direct functional assessment of SNVs in model organisms. However, the lack of bioinformatic tools aiding target prediction limits the application of base editing in vivo. Here, we provide a framework for adenine and cytosine base editing in medaka (Oryzias latipes) and zebrafish (Danio rerio), ideal for scalable validation studies. We developed an online base editing tool ACEofBASEs (a careful evaluation of base-edits), to facilitate decision-making by streamlining sgRNA design and performing off-target evaluation. We used state-of-the-art adenine (ABE) and cytosine base editors (CBE) in medaka and zebrafish to edit eye pigmentation genes and transgenic GFP function with high efficiencies. Base editing in the genes encoding troponin T and the potassium channel ERG faithfully recreated known cardiac phenotypes. Deep-sequencing of alleles revealed the abundance of intended edits in comparison to low levels of insertion or deletion (indel) events for ABE8e and evoBE4max. We finally validated missense mutations in novel candidate genes of congenital heart disease (CHD) dapk3, ube2b, usp44, and ptpn11 in F0 and F1 for a subset of these target genes with genotype-phenotype correlation. This base editing framework applies to a wide range of SNV-susceptible traits accessible in fish, facilitating straight-forward candidate validation and prioritization for detailed mechanistic downstream studies.

Comparative Study in Zebrafish and Medaka Unravels the Mechanisms of Tissue Regeneration

Tissue regeneration has been in the spotlight of research for its fascinating nature and potential applications in human diseases. The trait of regenerative capacity occurs diversely across species and tissue contexts, while it seems to decline over evolution. Organisms with variable regenerative capacity are usually distinct in phylogeny, anatomy, and physiology. This phenomenon hinders the feasibility of studying tissue regeneration by directly comparing regenerative with non-regenerative animals, such as zebrafish (Danio rerio) and mice (Mus musculus). Medaka (Oryzias latipes) is a fish model with a complete reference genome and shares a common ancestor with zebrafish approximately 110–200 million years ago (compared to 650 million years with mice). Medaka shares similar features with zebrafish, including size, diet, organ system, gross anatomy, and living environment. However, while zebrafish regenerate almost every organ upon experimental injury, medaka shows uneven regenerative capacity. Their common and distinct biological features make them a unique platform for reciprocal analyses to understand the mechanisms of tissue regeneration. Here we summarize current knowledge about tissue regeneration in these fish models in terms of injured tissues, repairing mechanisms, available materials, and established technologies. We further highlight the concept of inter-species and inter-organ comparisons, which may reveal mechanistic insights and hint at therapeutic strategies for human diseases.

Retinal Stem Cell 'Retirement Plans': Growth, Regulation and Species Adaptations in the Retinal Ciliary Marginal Zone

The vertebrate retina develops from a specified group of precursor cells that adopt distinct identities and generate lineages of either the neural retina, retinal pigmented epithelium, or ciliary body. In some species, including teleost fish and amphibians, proliferative cells with stem-cell-like properties capable of continuously supplying new retinal cells post-embryonically have been characterized and extensively studied. This region, termed the ciliary or circumferential marginal zone (CMZ), possibly represents a conserved retinal stem cell niche. In this review, we highlight the research characterizing similar CMZ-like regions, or stem-like cells located at the peripheral margin, across multiple different species. We discuss the proliferative parameters, multipotency and growth mechanisms of these cells to understand how they behave in vivo and how different molecular factors and signalling networks converge at the CMZ niche to regulate their activity. The evidence suggests that the mature retina may have a conserved propensity for homeostatic growth and plasticity and that dysfunction in the regulation of CMZ activity may partially account for dystrophic eye growth diseases such as myopia and hyperopia. A better understanding of the properties of CMZ cells will enable important insight into how an endogenous generative tissue compartment can adapt to altered retinal physiology and potentially even restore vision loss caused by retinal degenerative conditions.