MicroRNAs couple cell fate and developmental timing in retina

Exploring Early-Stage Retinal Neurodegeneration in Murine Pigmentary Glaucoma: Insights From Gene Networks and miRNA Regulation Analyses

Purpose

Glaucoma is a group of heterogeneous optic neuropathies characterized by the progressive degeneration of retinal ganglion cells. However, the underlying mechanisms have not been understood completely. We aimed to elucidate the genetic network associated with the development of pigmentary glaucoma with DBA/2J (D2) mouse model of glaucoma and corresponding genetic control D2-Gpnmb (D2G) mice carrying the wild type (WT) Gpnmb allele.

Methods

Retinas isolated from 13 D2 and 12 D2G mice were subdivided into 2 age groups: pre-onset (1-6 months: samples were collected at approximately 1-2, 2-4, and 5-6 months) and post-onset (7-15 months: samples were collected at approximately 7-9, 10-12, and 13-15 months) glaucoma were compared. Differential gene expression (DEG) analysis and gene-set enrichment analyses were performed. To identify micro-RNAs (miRNAs) that target Gpnmb, miRNA expression levels were correlated with time point matched mRNA expression levels. A weighted gene co-expression network analysis (WGCNA) was performed using the reference BXD mouse population. Quantitative real-time PCR (qRT-PCR) was used to validate Gpnmb and miRNA expression levels.

Results

A total of 314 and 86 DEGs were identified in the pre-onset and post-onset glaucoma groups, respectively. DEGs in the pre-onset glaucoma group were associated with the crystallin gene family, whereas those in the post-onset group were related to innate immune system response. Of 1329 miRNAs predicted to target Gpnmb, 3 miRNAs (miR-125a-3p, miR-3076-5p, and miR-214-5p) were selected. A total of 47 genes demonstrated overlapping with the identified DEGs between D2 and D2G, segregated into their time-relevant stages. Gpnmb was significantly downregulated, whereas 2 out of 3 miRNAs were significantly upregulated (P < 0.05) in D2 mice at both 3-and 10-month time points.

Conclusions

These findings suggest distinct gene-sets involved in pre-and post-glaucoma in the D2 mouse. We identified three miRNAs regulating Gpnmb in the development of murine pigmentary glaucoma.

Jarid2 promotes temporal progression of retinal progenitors via repression of Foxp1

Transitions in competence underlie the ability of CNS progenitors to generate a diversity of neurons and glia. Retinal progenitor cells in mouse generate early-born cell types embryonically and late-born cell types largely postnatally. We find that the transition from early to late progenitor competence is regulated by Jarid2. Loss of Jarid2 results in extended production of early cell types and extended expression of early progenitor genes. Jarid2 can regulate histone modifications, and we find reduction of repressive mark H3K27me3 on a subset of early progenitor genes with loss of Jarid2, most notably Foxp1. We show that Foxp1 regulates the competence to generate early-born retinal cell types, promotes early and represses late progenitor gene expression, and is required for extending early retinal cell production after loss of Jarid2. We conclude that Jarid2 facilitates progression of retinal progenitor temporal identity by repressing Foxp1, which is a primary regulator of early temporal patterning.

Ikaros family proteins redundantly regulate temporal patterning in the developing mouse retina

Temporal identity factors regulate competence of neural progenitors to generate specific cell types in a time-dependent manner, but how they operate remains poorly defined. In the developing mouse retina, the Ikaros zinc-finger transcription factor Ikzf1 regulates production of early-born cell types, except cone photoreceptors. In this study we show that, during early stages of retinal development, another Ikaros family protein, Ikzf4, functions redundantly with Ikzf1 to regulate cone photoreceptor production. Using CUT&RUN and functional assays, we show that Ikzf4 binds and represses genes involved in late-born rod photoreceptor specification, hence favoring cone production. At late stages, when Ikzf1 is no longer expressed in progenitors, we show that Ikzf4 re-localizes to target genes involved in gliogenesis and is required for Müller glia production. We report that Ikzf4 regulates Notch signaling genes and is sufficient to activate the Hes1 promoter through two Ikzf GGAA-binding motifs, suggesting a mechanism by which Ikzf4 may influence gliogenesis. These results uncover a combinatorial role for Ikaros family members during nervous system development and provide mechanistic insights on how they temporally regulate cell fate output.

Retina regeneration: lessons from vertebrates

Unlike mammals, vertebrates such as fishes and frogs exhibit remarkable tissue regeneration including the central nervous system. Retina being part of the central nervous system has attracted the interest of several research groups to explore its regenerative ability in different vertebrate models including mice. Fishes and frogs completely restore the size, shape and tissue structure of an injured retina. Several studies have unraveled molecular mechanisms underlying retina regeneration. In teleosts, soon after injury, the Müller glial cells of the retina reprogram to form a proliferating population of Müller glia-derived progenitor cells capable of differentiating into various neural cell types and Müller glia. In amphibians, the transdifferentiation of retinal pigment epithelium and differentiation of ciliary marginal zone cells contribute to retina regeneration. In chicks and mice, supplementation with external growth factors or genetic modifications cause a partial regenerative response in the damaged retina. The initiation of retina regeneration is achieved through sequential orchestration of gene expression through controlled modulations in the genetic and epigenetic landscape of the progenitor cells. Several developmental biology pathways are turned on during the Müller glia reprogramming, retinal pigment epithelium transdifferentiation and ciliary marginal zone differentiation. Further, several tumorigenic pathways and gene expression events also contribute to the complete regeneration cascade of events. In this review, we address the various retinal injury paradigms and subsequent gene expression events governed in different vertebrate species. Further, we compared how vertebrates such as teleost fishes and amphibians can achieve excellent regenerative responses in the retina compared with their mammalian counterparts.

Regulatory role of miR-146a in corneal epithelial wound healing via its inflammatory targets in human diabetic cornea

PURPOSE

MiR-146a upregulated in limbus vs. central cornea and in diabetic vs. non-diabetic limbus has emerged as an important immune and inflammatory signaling mediator in corneal epithelial wound healing. Our aim was to investigate the potential inflammation-related miR-146a target genes and their roles in normal and impaired diabetic corneal epithelial wound healing.

METHODS

Our previous data from RNA-seq combined with quantitative proteomics of limbal epithelial cells (LECs) transfected with miR-146a mimic vs. mimic control were analyzed. Western blot and immunostaining were used to confirm the expression of miR-146a inflammatory target proteins in LECs and organ-cultured corneas. Luminex assay was performed on conditioned media at 6- and 20-h post-wounding in miR-146a mimic/inhibitor transfected normal and diabetic cultured LECs.

RESULTS

Overexpression of miR-146a decreased the expression of pro-inflammatory TRAF6 and IRAK1 and downstream target NF-κB after challenge with lipopolysaccharide (LPS) or wounding. Additionally, miR-146a overexpression suppressed the production of downstream inflammatory mediators including secreted cytokines IL-1α, IL-1β, IL-6 and IL-8, and chemokines CXCL1, CXCL2 and CXCL5. These cytokines and chemokines were upregulated in normal but not in diabetic LEC during wounding. Furthermore, we achieved normalized levels of altered secreted cytokines and chemokines in diabetic wounded LEC via specific inhibition of miR-146a.

CONCLUSION

Our study documented significant impact of miR-146a on the expression of inflammatory mediators at the mRNA and protein levels during acute inflammatory responses and wound healing, providing insights into the regulatory role of miR-146a in corneal epithelial homeostasis in normal and diabetic conditions.

Oscillatory Behaviors of microRNA Networks: Emerging Roles in Retinal Development

A broad repertoire of transcription factors and other genes display oscillatory patterns of expression, typically ranging from 30 min to 24 h. These oscillations are associated with a variety of biological processes, including the circadian cycle, somite segmentation, cell cycle, and metabolism. These rhythmic behaviors are often prompted by transcriptional feedback loops in which transcriptional activities are inhibited by their corresponding gene target products. Oscillatory transcriptional patterns have been proposed as a mechanism to drive biological clocks, the molecular machinery that transforms temporal information into accurate spatial patterning during development. Notably, several microRNAs (miRNAs) -small non-coding RNA molecules-have been recently shown to both exhibit rhythmic expression patterns and regulate oscillatory activities. Here, we discuss some of these new findings in the context of the developing retina. We propose that miRNA oscillations are a powerful mechanism to coordinate signaling pathways and gene expression, and that addressing the dynamic interplay between miRNA expression and their target genes could be key for a more complete understanding of many developmental processes.

MicroRNA-214 in Health and Disease

MicroRNAs (miRNAs) are endogenously expressed, non-coding RNA molecules that mediate the post-transcriptional repression and degradation of mRNAs by targeting their 3′ untranslated region (3′-UTR). Thousands of miRNAs have been identified since their first discovery in 1993, and miR-214 was first reported to promote apoptosis in HeLa cells. Presently, miR-214 is implicated in an extensive range of conditions such as cardiovascular diseases, cancers, bone formation and cell differentiation. MiR-214 has shown pleiotropic roles in contributing to the progression of diseases such as gastric and lung cancers but may also confer cardioprotection against excessive fibrosis and oxidative damage. These contrasting functions are achieved through the diverse cast of miR-214 targets. Through silencing or overexpressing miR-214, the detrimental effects can be attenuated, and the beneficial effects promoted in order to improve health outcomes. Therefore, discovering novel miR-214 targets and understanding how miR-214 is dysregulated in human diseases may eventually lead to miRNA-based therapies. MiR-214 has also shown promise as a diagnostic biomarker in identifying breast cancer and coronary artery disease. This review provides an up-to-date discussion of miR-214 literature by describing relevant roles in health and disease, areas of disagreement, and the future direction of the field.

Transcriptional and epigenetic regulation of temporal patterning in neural progenitors

During development, neural progenitors undergo temporal patterning as they age to sequentially generate differently fated progeny. Temporal patterning of neural progenitors is relatively well-studied in Drosophila. Temporal cascades of transcription factors or opposing temporal gradients of RNA-binding proteins are expressed in neural progenitors as they age to control the fates of the progeny. The temporal progression is mostly driven by intrinsic mechanisms including cross-regulations between temporal genes, but environmental cues also play important roles in certain transitions. Vertebrate neural progenitors demonstrate greater plasticity in response to extrinsic cues. Recent studies suggest that vertebrate neural progenitors are also temporally patterned by a combination of transcriptional and post-transcriptional mechanisms in response to extracellular signaling to regulate neural fate specification. In this review, we summarize recent advances in the study of temporal patterning of neural progenitors in Drosophila and vertebrates. We also discuss the involvement of epigenetic mechanisms, specifically the Polycomb group complexes and ATP-dependent chromatin remodeling complexes, in the temporal patterning of neural progenitors.

Therapeutic effects of mesenchymal stem cells-derived extracellular vesicles' miRNAs on retinal regeneration: a review

Extracellular vesicles (EVs), which consist of microvesicles and exosomes, are secreted from all cells to transform vital information in the form of lipids, proteins, mRNAs and small RNAs such as microRNAs (miRNAs). Many studies demonstrated that EVs' miRNAs have effects on target cells. Numerous people suffer from the blindness caused by retinal degenerations. The death of retinal neurons is irreversible and creates permanent damage to the retina. In the absence of acceptable cures for retinal degenerative diseases, stem cells and their paracrine agents including EVs have become a promising therapeutic approach. Several studies showed that the therapeutic effects of stem cells are due to the miRNAs of their EVs. Considering the effects of microRNAs in retinal cells development and function and studies which provide the possible roles of mesenchymal stem cells-derived EVs miRNA content on retinal diseases, we focused on the similarities between these two groups of miRNAs that could be helpful for promoting new therapeutic techniques for retinal degenerative diseases.

The potential of mechanistic information organised within the AOP framework to increase regulatory uptake of the developmental neurotoxicity (DNT) in vitro battery of assays

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Current in vivo DNT testing for regulatory purposes is not effective.

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In vitro assays anchored to key neurodevelopmental processes are available.

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Development of Adverse Outcome Pathways is required to increase mechanistic understanding of DNT effects.

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DNT Integrated Approaches to Testing and Assessment for various regulatory purposes should be developed.

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The OECD Guidance Document on use of in vitro DNT battery of assays is currently under development.

A major challenge in regulatory developmental neurotoxicity (DNT) assessment is lack of toxicological information for many compounds. Therefore, the Test Guidelines programme of the Organisation for Economic Cooperation and Development (OECD) took the initiative to coordinate an international collaboration between diverse stakeholders to consider integration of alternative approaches towards improving the current chemical DNT testing. During the past few years, a series of workshops was organized during which a consensus was reached that incorporation of a DNT testing battery that relies on in vitro assays anchored to key neurodevelopmental processes should be developed. These key developmental processes include neural progenitor cell proliferation, neuronal and oligodendrocyte differentiation, neural cell migration, neurite outgrowth, synaptogenesis and neuronal network formation, as well key events identified in the existing Adverse Outcome Pathways (AOPs). AOPs deliver mechanistic information on the causal links between molecular initiating event, intermediate key events and an adverse outcome of regulatory concern, providing the biological context to facilitate development of Integrated Approaches to Testing and Assessment (IATA) for various regulatory purposes. Developing IATA case studies, using mechanistic information derived from AOPs, is expected to increase scientific confidence for the use of in vitro methods within an IATA, thereby facilitating regulatory uptake. This manuscript summarizes the current state of international efforts to enhance DNT testing by using an in vitro battery of assays focusing on the role of AOPs in informing the development of IATA for different regulatory purposes, aiming to deliver an OECD guidance document on use of in vitro DNT battery of assays that include in vitro data interpretation.

Transcriptome Analysis Reveals Altered Inflammatory Pathway in an Inducible Glial Cell Model of Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1), the most frequent inherited muscular dystrophy in adults, is caused by the CTG repeat expansion in the 3′UTR of the DMPK gene. Mutant DMPK RNA accumulates in nuclear foci altering diverse cellular functions including alternative splicing regulation. DM1 is a multisystemic condition, with debilitating central nervous system alterations. Although a defective neuroglia communication has been described as a contributor of the brain pathology in DM1, the specific cellular and molecular events potentially affected in glia cells have not been totally recognized. Thus, to study the effects of DM1 mutation on glial physiology, in this work, we have established an inducible DM1 model derived from the MIO-M1 cell line expressing 648 CUG repeats. This new model recreated the molecular hallmarks of DM1 elicited by a toxic RNA gain-of-function mechanism: accumulation of RNA foci colocalized with MBNL proteins and dysregulation of alternative splicing. By applying a microarray whole-transcriptome approach, we identified several gene changes associated with DM1 mutation in MIO-M1 cells, including the immune mediators CXCL10, CCL5, CXCL8, TNFAIP3, and TNFRSF9, as well as the microRNAs miR-222, miR-448, among others, as potential regulators. A gene ontology enrichment analyses revealed that inflammation and immune response emerged as major cellular deregulated processes in the MIO-M1 DM1 cells. Our findings indicate the involvement of an altered immune response in glia cells, opening new windows for the study of glia as potential contributor of the CNS symptoms in DM1.

The Impact of miRNAs in Health and Disease of Retinal Pigment Epithelium

MicroRNAs (miRNAs), a class of non-coding RNAs, are essential key players in the control of biological processes in both physiological and pathological conditions. miRNAs play important roles in fine tuning the expression of many genes, which often have roles in common molecular networks. miRNA dysregulation thus renders cells vulnerable to aberrant fluctuations in genes, resulting in degenerative diseases. The retinal pigment epithelium (RPE) is a monolayer of polarized pigmented epithelial cells that resides between the light-sensitive photoreceptors (PR) and the choriocapillaris. The demanding physiological functions of RPE cells require precise gene regulation for the maintenance of retinal homeostasis under stress conditions and the preservation of vision. Thus far, our understanding of how miRNAs function in the homeostasis and maintenance of the RPE has been poorly addressed, and advancing our knowledge is central to harnessing their potential as therapeutic agents to counteract visual impairment. This review focuses on the emerging roles of miRNAs in the function and health of the RPE and on the future exploration of miRNA-based therapeutic approaches to counteract blinding diseases.

In Situ Hybridization and Immunostaining of Xenopus Brain

The dynamic expression pattern analysis provides the primary information of gene function. Differences of the RNA and/or protein location will provide valuable information for gene expression regulation. Generally, in situ hybridization (ISH) and immunohistochemistry (IHC) are two main techniques to visualize the locations of gene transcripts and protein products in situ, respectively. Here we describe the protocol for the whole brain dissection, the in situ hybridization, and the immunostaining of the developing Xenopus brain sections. Additionally, we point out the modification of in situ hybridization for microRNA expression detection.

On the Generation and Regeneration of Retinal Ganglion Cells

Retinal development follows a conserved neurogenic program in vertebrates to orchestrate the generation of specific cell types from multipotent progenitors in sequential but overlapping waves. In this program, retinal ganglion cells (RGCs) are the first cell type generated. RGCs are the final output neurons of the retina and are essential for vision and circadian rhythm. Key molecular steps have been defined in multiple vertebrate species to regulate competence, specification, and terminal differentiation of this cell type. This involves neuronal-specific transcription factor networks, regulators of chromatin dynamics and miRNAs. In mammals, RGCs and their optic nerve axons undergo neurodegeneration and loss in glaucoma and other optic neuropathies, resulting in irreversible vision loss. The incapacity of RGCs and axons to regenerate reinforces the need for the design of efficient RGC replacement strategies. Here we describe the essential molecular pathways for the differentiation of RGCs in vertebrates, as well as experimental manipulations that extend the competence window for generation of this early cell type from late progenitors. We discuss recent advances in regeneration of retinal neurons in vivo in both mouse and zebrafish and discuss possible strategies and barriers to achieving RGC regeneration as a therapeutic approach for vision restoration in blinding diseases such as glaucoma.

Integrated Transcriptome and Proteome Analyses Reveal the Regulatory Role of miR-146a in Human Limbal Epithelium via Notch Signaling

MiR-146a is upregulated in the stem cell-enriched limbal region vs. central human cornea and can mediate corneal epithelial wound healing. The aim of this study was to identify miR-146a targets in human primary limbal epithelial cells (LECs) using genomic and proteomic analyses. RNA-seq combined with quantitative proteomics based on multiplexed isobaric tandem mass tag labeling was performed in LECs transfected with miR-146a mimic vs. mimic control. Western blot and immunostaining were used to confirm the expression of some targeted genes/proteins. A total of 251 differentially expressed mRNAs and 163 proteins were identified. We found that miR-146a regulates the expression of multiple genes in different pathways, such as the Notch system. In LECs and organ-cultured corneas, miR-146a increased Notch-1 expression possibly by downregulating its inhibitor Numb, but decreased Notch-2. Integrated transcriptome and proteome analyses revealed the regulatory role of miR-146a in several other processes, including anchoring junctions, TNF-α, Hedgehog signaling, adherens junctions, TGF-β, mTORC2, and epidermal growth factor receptor (EGFR) signaling, which mediate wound healing, inflammation, and stem cell maintenance and differentiation. Our results provide insights into the regulatory network of miR-146a and its role in fine-tuning of Notch-1 and Notch-2 expressions in limbal epithelium, which could be a balancing factor in stem cell maintenance and differentiation.

Identification of key miRNAs and genes for mouse retinal development using a linear model

MicroRNAs (miRNAs) are upstream regulators of gene expression and are involved in several biological processes. The purpose of the present study was to obtain a detailed spatiotemporal miRNA expression profile in mouse retina, to identify one or more miRNAs that are key to mouse retinal development and to investigate the roles and mechanisms of these miRNAs. The miRNA expression pattern of the developing mouse retina was acquired from Locked Nucleic Acid microarrays. Data were processed to identify differentially expressed miRNAs (DE‑miRNAs) using the linear model in Python 3.6. Following bioinformatics analysis and reverse transcription‑quantitative polymerase chain reaction validation, 8 miRNAs (miR‑9‑5p, miR‑130a‑3p, miR‑92a‑3p, miR‑20a‑5p, miR‑93‑5p, miR‑9‑3p, miR‑709 and miR‑124) were identified as key DE‑miRNAs with low variability during mouse retinal development. Gene Ontology analysis revealed that the target genes of the DE‑miRNAs were enriched in cellular metabolic processes. Kyoto Encyclopedia of Genes and Genomes analysis demonstrated that the target genes of the DE‑miRNAs were significantly enriched in PI3K/AKT/mTOR, class O of forkhead box transcription factors, mitogen‑activated protein kinase (MAPK), neurotrophin and transforming growth factor (TGF)‑β signaling, as well as focal adhesion and the axon guidance pathway. PI3K, AKT, PTEN, MAPK1, Son of Sevenless, sphingosine‑1‑phosphate receptor 1, BCL‑2L11, TGF‑β receptor type 1/2 and integrin α (ITGA)/ITGAB, which are key components of the aforementioned pathways and were revealed to be target genes of several of the DE‑miRNAs. The present study used a linear model to identify several DE‑miRNAs, as well as their target genes and associated pathways, which may serve crucial roles in mouse retinal development. Therefore, the results obtained in the present study may provide the groundwork for further experiments.

MicroRNAs in Vascular Eye Diseases

Since the discovery of the first microRNA (miRNA) decades ago, studies of miRNA biology have expanded in many biomedical research fields, including eye research. The critical roles of miRNAs in normal development and diseases have made miRNAs useful biomarkers or molecular targets for potential therapeutics. In the eye, ocular neovascularization (NV) is a leading cause of blindness in multiple vascular eye diseases. Current anti-angiogenic therapies, such as anti-vascular endothelial growth factor (VEGF) treatment, have their limitations, indicating the need for investigating new targets. Recent studies established the roles of various miRNAs in the regulation of pathological ocular NV, suggesting miRNAs as both biomarkers and therapeutic targets in vascular eye diseases. This review summarizes the biogenesis of miRNAs, and their functions in the normal development and diseases of the eye, with a focus on clinical and experimental retinopathies in both human and animal models. Discovery of novel targets involving miRNAs in vascular eye diseases will provide insights for developing new treatments to counter ocular NV.

HMGA Genes and Proteins in Development and Evolution

HMGA (high mobility group A) (HMGA1 and HMGA2) are small non-histone proteins that can bind DNA and modify chromatin state, thus modulating the accessibility of regulatory factors to the DNA and contributing to the overall panorama of gene expression tuning. In general, they are abundantly expressed during embryogenesis, but are downregulated in the adult differentiated tissues. In the present review, we summarize some aspects of their role during development, also dealing with relevant studies that have shed light on their functioning in cell biology and with emerging possible involvement of HMGA1 and HMGA2 in evolutionary biology.

Gene polymorphism and plasma levels of miR-155 in diabetic retinopathy

Circulating microRNA-155 (miR-155) is associated with type 2 diabetes mellitus (T2DM) and the rs767649 polymorphism in the pre-MIR155 gene is associated with miR-155 expression. However, their relationship with diabetic retinopathy (DR) is still unknown. Therefore, the aim of this case-control study was to test the hypothesis that the rs767649 polymorphism in the pre-MIR155 gene is associated with DR in South Brazilians with T2DM. We also evaluated the association of plasma levels of miR-155 with DR and the rs767649 polymorphism in a subgroup of subjects. The rs767649 polymorphism was genotyped in 139 blood donors and 546 T2DM patients (244 had no DR, 161 had non-proliferative DR and 141 had proliferative DR). miR-155 expression was quantified in 20 blood donors and 60 T2DM patients (20 from each group). Among T2DM patients, the carriership of the A allele and the A allele were more frequent in subjects with DR than in those without it (P < 0.05), and the A allele was independently associated with an increased risk of DR (adjusted OR = 2.12, 95% CI = 1.12-4.01). The plasma levels of miR-155 were lower in T2DM patients than in blood donors (P < 0.001). However, the miR-155 levels did not differ according to the presence and severity of DR or according to rs767649 genotypes among T2DM patients. These findings support that the rs767649 polymorphism in the pre-MIR155 gene is associated with DR in T2DM and that the miR-155 plasma levels might be associated with T2DM. Additional studies are needed to further investigate their clinical significance in DR and T2DM.

In Vivo Assessment of Neural Precursor Cell Cycle Kinetics in the Amphibian Retina

Cell cycle progression is intimately linked to cell fate commitment during development. In addition, adult stem cells show specific proliferative behaviors compared to progenitors. Exploring cell cycle dynamics and regulation is therefore of utmost importance, but constitutes a great challenge in vivo. Here we provide a protocol for evaluating in vivo the length of all cell cycle phases of neural stem and progenitor cells in the post-embryonic Xenopus retina. These cells are localized in the ciliary marginal zone (CMZ), a peripheral region of the retina that sustains continuous neurogenesis throughout the animal's life. The CMZ bears two tremendous advantages for cell cycle kinetics analyses. First, this region, where proliferative cells are sequestered, can be easily delineated. Second, the spatial organization of the CMZ mirrors the temporal sequence of retinal development, allowing for topological distinction between retinal stem cells (residing in the most peripheral margin), and amplifying progenitors (located more centrally). We describe herein how to determine CMZ cell cycle parameters using a combination of (i) a cumulative labeling assay, (ii) the percentage of labeled mitosis calculation, and (iii) the mitotic index measurement. Taken together, these techniques allow us to estimate total cell cycle length (T_C) as well as the duration of all cell cycle phases (T_S/G2/M/G1). Although the method presented here was adapted to the particular system of the CMZ, it should be applicable to other tissues and developmental stages as well.

Temporal Requirement of Mab21l2 During Eye Development in Chick Reveals Stage-Dependent Functions for Retinogenesis

Purpose

Different missense mutations in the single exon gene Mab21l2 have been identified in unrelated families with various bilateral eye malformations, including microphthalmia, anophthalmia, and coloboma, but the molecular function of Mab21l2 during eye development still remains largely unknown.

Methods

We have established an in vivo Mab21l2-deficient eye development model in chick, by using a Mab21l2 RNA interference construct that we electroporated in ovo in prospective retinal cells. In addition, we designed a Mab21l2 gain-of-function electroporation vector. Mab21l2-modulated retinas were analyzed on consecutive sections in terms of morphology, and molecular markers for apoptosis, cell proliferation, and retinogenesis.

Results

Our Mab21l2-deficient chick model mimics human ocular phenotypes. When Mab21l2 is downregulated prior to optic vesicle formation, the embryos develop anophthalmia, and Mab21l2 inhibition by optic cup stages results in a microphthalmic colobomatous phenotype. Our results show that inhibition of Mab21l2 affects cell proliferation, cell cycle exit, and the expression of Atoh7/Ath5, NeuroD4/Ath3, Isl1, Pax6, AP-2α, and Prox1. In addition, Mab21l2 overexpression hampers cell cycle exit and differentiation of retinal progenitor cells (RPCs).

Conclusions

Our results highlight the importance of a regulated temporal expression of Mab21l2 during eye development: At early stages, Mab21l2 is required to maintain RPC proliferation and expansion of cell number; before retinogenesis, a decrease in Mab21l2 expression in proliferating RPCs is required for cell cycle exit and differentiation; during retinogenesis, Mab21l2 is chronologically upregulated in RGCs, followed by differentiated horizontal and amacrine cells and cone photoreceptor cells.

MicroRNA-129 modulates neuronal migration by targeting Fmr1 in the developing mouse cortex

During cortical development, neuronal migration is one of the most important steps for normal cortical formation and function, and defects in this process cause many brain diseases. However, the molecular mechanisms underlying this process remain largely unknown. In this study, we found that miR-129-5p and miR-129-3p were expressed in both neural progenitor cells and cortical neurons in the developing murine cortex. Moreover, abnormal miR-129 expression could block radial migration of both the deeper layer and upper layer neurons, and impair the multipolar to bipolar transition. However, antagomir-mediated inhibition resulted in overmigration of neurons. In addition, we showed that Fragile X Mental Retardation gene 1 (Fmr1), which is mutated in the autism spectrum disorder fragile X syndrome, is an important regulatory target for miR-129-5p. Furthermore, Fmr1 loss-of-function and gain-of-function experiments showed opposite effects on miR-129 regulation of neuronal migration, and restoring Fmr1 expression could counteract the deleterious effect of miR-129 on neuronal migration. Taken together, our results suggest that miR-129-5p could modulate the expression of fragile X mental retardation 1 protein (FMRP) to ensure normal neuron positioning in the developing cerebral cortex.

miR-29c regulates neurogliogenesis in the mammalian retina through REST

In the developing central nervous system, including its simple and accessible model retina, neurogenesis is followed by gliogenesis. However, the mechanism underlying the neurogliogenic switch remains poorly understood despite the identification of several regulatory genes, associated with the lineage identity and transition. The mechanism may involve cross talks between regulatory genes, facilitated through microRNAs. Here, we posit miR-29c as one of the regulatory miRNAs that may influence neuronal versus glial differentiation. We observed that the temporal patterns of miR-29c expression corresponded with late retinal histogenesis, the stage in the developing retina when neurogliogenic decision predominantly occurs. Examination of the effects of miR-29c on neurogliogenesis by the perturbation of function approach revealed that miR-29c preferentially facilitated differentiation of late RPCs into rod photoreceptors and bipolar cells, the late-born neurons, at the expense of Müller glia, the sole glia generated by retinal progenitor cells. We further observed that miR-29c facilitated neurogenesis and inhibited gliogenesis by regulating the expression of RE-1 silencing transcription factor (REST), which encodes a transcriptional repressor of cell cycle regulators and neuronal genes. Thus, miR-29c may influence neurogliogenic decision in the developing retina by regulating the instructive out put of a molecular axis helmed by REST.

Mapping the landscape of tandem repeat variability by targeted long read single molecule sequencing in familial X-linked intellectual disability

BACKGROUND

The etiology of more than half of all patients with X-linked intellectual disability remains elusive, despite array-based comparative genomic hybridization, whole exome or genome sequencing. Since short read massive parallel sequencing approaches do not allow the detection of larger tandem repeat expansions, we hypothesized that such expansions could be a hidden cause of X-linked intellectual disability.

METHODS

We selectively captured over 1800 tandem repeats on the X chromosome and characterized them by long read single molecule sequencing in 3 families with idiopathic X-linked intellectual disability.

RESULTS

In male DNA samples, full tandem repeat length sequences were obtained for 88-93% of the targets and up to 99.6% of the repeats with a moderate guanine-cytosine content. Read length and analysis pipeline allow to detect cases of > 900 bp tandem repeat expansion. In one family, one repeat expansion co-occurs with down-regulation of the neighboring MIR222 gene. This gene has previously been implicated in intellectual disability and is apparently linked to FMR1 and NEFH overexpression associated with neurological disorders.

CONCLUSIONS

This study demonstrates the power of single molecule sequencing to measure tandem repeat lengths and detect expansions, and suggests that tandem repeat mutations may be a hidden cause of X-linked intellectual disability.

MicroRNA-129-5p is regulated by choline availability and controls EGF receptor synthesis and neurogenesis in the cerebral cortex

Choline availability modulates neurogenesis and cerebral cortex development through the regulation of neural progenitor cell (NPC) proliferative and differentiation capacity. In this study, we demonstrated that cortical NPC self-renewal is controlled by choline via the expression of a microRNA (miR-129-5p), whose role in the developing brain has not been examined, and which, in turn, inhibits synthesis of the epidermal growth factor receptor (EGFR) protein. Specifically, we found that low choline (LC) availability led to the upregulation of miR-129-5p expression in cortical NPCs in vitro and in vivo, causing the downregulation of EGFR and thereby disrupting NPC self-renewal and cortical neurogenesis. Furthermore, in response to LC availability, methylation potential (the S-adenosylmethionine: S-adenosylhomocysteine ratio) in the developing brain was reduced. Restoring methylation potential in LC cortical NPCs led to the re-establishment of normal miR-129-5p expression. We concluded that inhibiting miR-129-5p function and restoring EGFR protein levels in vivo is sufficient to reverse LC-induced defects in cortical NPC self-renewal. For the first time, to our knowledge, we have identified the molecular links that explain how a change in the availability of the diet metabolite choline impacts the essential cellular processes underlying brain development.-Trujillo-Gonzalez, I., Wang, Y., Friday, W. B., Vickers, K. C., Toth, C. L., Molina-Torres, L., Surzenko, N., Zeisel, S. H. MicroRNA-129-5p is regulated by choline availability and controls EGF receptor synthesis and neurogenesis in the cerebral cortex.

MiR-214 is an important regulator of the musculoskeletal metabolism and disease

MiR-214 belongs to a family of microRNA (small, highly conserved noncoding RNA molecules) precursors that play a pivotal role in biological functions, such as cellular function, tissue development, tissue homeostasis, and pathogenesis of diseases. Recently, miR-214 emerged as a critical regulator of musculoskeletal metabolism. Specifically, miR-214 can mediate skeletal muscle myogenesis and vascular smooth muscle cell proliferation, migration, and differentiation. MiR-214 also modulates osteoblast function by targeting specific molecular pathways and the expression of various osteoblast-related genes; promotes osteoclast activity by targeting phosphatase and tensin homolog (Pten); and mediates osteoclast-osteoblast intercellular crosstalk via an exosomal miRNA paracrine mechanism. Importantly, dysregulation in miR-214 expression is associated with pathological bone conditions such as osteoporosis, osteosarcoma, multiple myeloma, and osteolytic bone metastasis of breast cancer. This review discusses the cellular targets of miR-214 in bone, the molecular mechanisms governing the activities of miR-214 in the musculoskeletal system, and the putative role of miR-214 in skeletal diseases. Understanding the biology of miR-214 could potentially lead to the development of miR-214 as a possible biomarker and a therapeutic target for musculoskeletal diseases.

MicroRNAs in Retinal Development

The small RNA regulatory molecules called microRNAs (miRNAs) play key roles in the development of most organisms. The expression of many different miRNAs has been described in the developing and mature vertebrate retina. The ability of miRNAs to regulate a diversity of messenger RNA targets allows them to have effects on many different developmental processes, but the functions of only a few miRNAs have been documented to date. Developmental transitions between cell states appear to be particularly sensitive to miRNA loss of function, as evidenced by specific miRNA knockdowns or from global perturbations in miRNA levels (e.g., Dicer deletion). However, we are still in only the very early stages of understanding the range of cellular functions miRNAs control during development.

Evidences for a New Role of miR-214 in Chondrogenesis

miR-214 is known to play a role in mammalian skeletal development through inhibition of osteogenesis and stimulation of osteoclastogenesis, but data regarding other vertebrates, as well as a possible role in chondrogenesis, remain unknown. Here, we show that miR-214 expression is detected in bone and cartilage of zebrafish skeleton, and is downregulated during murine ATDC5 chondrocyte differentiation. Additionally, we observed a conservation of the transcriptional regulation of miR-214 primary transcript Dnm3os in vertebrates, being regulated by Ets1 in ATDC5 chondrogenic cells. Moreover, overexpression of miR-214 in vitro and in vivo mitigated chondrocyte differentiation probably by targeting activating transcription factor 4 (Atf4). Indeed, miR-214 overexpression in vivo hampered cranial cartilage formation of zebrafish and coincided with downregulation of atf4 and of the key chondrogenic players sox9 and col2a1. We show that miR-214 overexpression exerts a negative role in chondrogenesis by impacting on chondrocyte differentiation possibly through conserved mechanisms.

miR Cluster 143/145 Directly Targets Nrl and Regulates Rod Photoreceptor Development

Retinal histogenesis requires coordinated and temporal functioning of factors by which different cell types are generated from multipotent progenitors. Development of rod photoreceptors is regulated by multiple transcription factors, and Nrl is one of the major factors involved in their fate specification. Presence or absence of Nrl at the postnatal stages decides the generation of cone photoreceptors or other later retinal cells. This suggests the need for regulated expression of Nrl in order to accelerate the generation of other cell types during retinal development. We found that miR cluster 143/145, comprising miR-143 and miR-145, targets and imparts a posttranscriptional inhibition of Nrl. Expression of both miRNAs was differentially regulated during retinal development and showed least expression at PN1 stage in which most of the rod photoreceptors are generated. Downregulation of rod photoreceptor regulators and markers upon miR cluster 143/145 overexpression demonstrated that this cluster indeed negatively regulates rod photoreceptors. Further, we prove that Nrl positively regulates miR cluster 143/145, thus establishing a feedback loop regulatory mechanism. This may be one possible mechanism by which Nrl is posttranscriptionally regulated to facilitate the generation of other cell types in retina.

Astrocytes at the Hub of the Stress Response: Potential Modulation of Neurogenesis by miRNAs in Astrocyte-Derived Exosomes

Repetitive stress negatively affects several brain functions and neuronal networks. Moreover, adult neurogenesis is consistently impaired in chronic stress models and in associated human diseases such as unipolar depression and bipolar disorder, while it is restored by effective antidepressant treatments. The adult neurogenic niche contains neural progenitor cells in addition to amplifying progenitors, neuroblasts, immature and mature neurons, pericytes, astrocytes, and microglial cells. Because of their particular and crucial position, with their end feet enwrapping endothelial cells and their close communication with the cells of the niche, astrocytes might constitute a nodal point to bridge or transduce systemic stress signals from peripheral blood, such as glucocorticoids, to the cells involved in the neurogenic process. It has been proposed that communication between astrocytes and niche cells depends on direct cell-cell contacts and soluble mediators. In addition, new evidence suggests that this communication might be mediated by extracellular vesicles such as exosomes, and in particular, by their miRNA cargo. Here, we address some of the latest findings regarding the impact of stress in the biology of the neurogenic niche, and postulate how astrocytic exosomes (and miRNAs) may play a fundamental role in such phenomenon.

MicroRNA-214 modulates neural progenitor cell differentiation by targeting Quaking during cerebral cortex development

The accurate generation of an appropriate number of different neuronal and glial subtypes is fundamental to normal brain functions and requires tightly orchestrated spatial and temporal developmental programmes to maintain the balance between the proliferation and the differentiation of neural progenitor cells. However, the molecular mechanism governing this process has not been fully elucidated. Here, we found that miR-214-3p was highly expressed in neural progenitor cells and dynamically regulated during neocortical development. Moreover, our in vivo and in vitro studies showed that miR-214 inhibited self-renewal of neural progenitor cells and promoted neurogenesis. In addition, after target screening, we identified miR-214 targets including Quaking (Qki) by binding the 3'- untranslated region (3'-UTR) of the Qki mRNA, which was specifically expressed in the progenitor cells of the proliferative ventricular zone as 3 Qki isoforms. Furthermore, overexpression and knockdown of Qki showed that the different isoforms of Qki had different functions in the regulation of neural progenitor cells differentiation. Moreover, overexpression of Qki could counteract the function of miR-214 in neurogenesis. Our results revealed that miR-214 maintains the balance between neural progenitor/stem cell proliferation and differentiation together with Quaking, its target gene.

Genome-wide analysis suggests a differential microRNA signature associated with normal and diabetic human corneal limbus

Small non-coding RNAs, in particular microRNAs (miRNAs), regulate fine-tuning of gene expression and can impact a wide range of biological processes. However, their roles in normal and diseased limbal epithelial stem cells (LESC) remain unknown. Using deep sequencing analysis, we investigated miRNA expression profiles in central and limbal regions of normal and diabetic human corneas. We identified differentially expressed miRNAs in limbus vs. central cornea in normal and diabetic (DM) corneas including both type 1 (T1DM/IDDM) and type 2 (T2DM/NIDDM) diabetes. Some miRNAs such as miR-10b that was upregulated in limbus vs. central cornea and in diabetic vs. normal limbus also showed significant increase in T1DM vs. T2DM limbus. Overexpression of miR-10b increased Ki-67 staining in human organ-cultured corneas and proliferation rate in cultured corneal epithelial cells. MiR-10b transfected human organ-cultured corneas showed downregulation of PAX6 and DKK1 and upregulation of keratin 17 protein expression levels. In summary, we report for the first time differential miRNA signatures of T1DM and T2DM corneal limbus harboring LESC and show that miR-10b could be involved in the LESC maintenance and/or their early differentiation. Furthermore, miR-10b upregulation may be an important mechanism of corneal diabetic alterations especially in the T1DM patients.

A microRNA-129-5p/Rbfox crosstalk coordinates homeostatic downscaling of excitatory synapses

Synaptic downscaling is a homeostatic mechanism that allows neurons to reduce firing rates during chronically elevated network activity. Although synaptic downscaling is important in neural circuit development and epilepsy, the underlying mechanisms are poorly described. We performed small RNA profiling in picrotoxin (PTX)-treated hippocampal neurons, a model of synaptic downscaling. Thereby, we identified eight microRNAs (miRNAs) that were increased in response to PTX, including miR-129-5p, whose inhibition blocked synaptic downscaling in vitro and reduced epileptic seizure severity in vivo Using transcriptome, proteome, and bioinformatic analysis, we identified the calcium pump Atp2b4 and doublecortin (Dcx) as miR-129-5p targets. Restoring Atp2b4 and Dcx expression was sufficient to prevent synaptic downscaling in PTX-treated neurons. Furthermore, we characterized a functional crosstalk between miR-129-5p and the RNA-binding protein (RBP) Rbfox1. In the absence of PTX, Rbfox1 promoted the expression of Atp2b4 and Dcx. Upon PTX treatment, Rbfox1 expression was downregulated by miR-129-5p, thereby allowing the repression of Atp2b4 and Dcx. We therefore identified a novel activity-dependent miRNA/RBP crosstalk during synaptic scaling, with potential implications for neural network homeostasis and epileptogenesis.

Maturation arrest in early postnatal sensory receptors by deletion of the miR-183/96/182 cluster in mouse

The polycistronic miR-183/96/182 cluster is preferentially and abundantly expressed in terminally differentiating sensory epithelia. To clarify its roles in the terminal differentiation of sensory receptors in vivo, we deleted the entire gene cluster in mouse germline through homologous recombination. The miR-183/96/182 null mice display impairment of the visual, auditory, vestibular, and olfactory systems, attributable to profound defects in sensory receptor terminal differentiation. Maturation of sensory receptor precursors is delayed, and they never attain a fully differentiated state. In the retina, delay in up-regulation of key photoreceptor genes underlies delayed outer segment elongation and possibly mispositioning of cone nuclei in the retina. Incomplete maturation of photoreceptors is followed shortly afterward by early-onset degeneration. Cell biologic and transcriptome analyses implicate dysregulation of ciliogenesis, nuclear translocation, and an epigenetic mechanism that may control timing of terminal differentiation in developing photoreceptors. In both the organ of Corti and the vestibular organ, impaired terminal differentiation manifests as immature stereocilia and kinocilia on the apical surface of hair cells. Our study thus establishes a dedicated role of the miR-183/96/182 cluster in driving the terminal differentiation of multiple sensory receptor cells.

Electroacupuncture enhances rehabilitation through miR-181b targeting PirB after ischemic stroke

Recent studies have demonstrated microRNAs (miRNAs) and proteins are beneficial to axon regeneration, which may be involved in Electroacupuncture (EA) therapy against stroke. In this study, we aimed to determine the pivotal role of PirB in EA-produced rehabilitation against ischemic stroke; and to screen and investigate the potential miRNAs directly regulating PirB expression. The results showed EA treatment enhanced axon regeneration and new projections from the corticospinal tract at 28 d after cerebral ischemic reperfusion injury of rats. Then, we found EA decreased pirb mRNA and PirB protein expression in the penumbra within 28 days after reperfusion. The reduction of PirB expression facilitated neurite outgrowth after oxygen-glucose deprivation injury. The miRNA microarray showed the level of twenty kinds of miRNAs changed in the penumbra after EA administration. The bioinformatics study and luciferase assay verified miR-181b directly regulated pirb mRNA expression. EA increased miR-181b levels in the penumbras, and improved neurobehavioral function rehabilitation through miR-181b direct targeting of pirb mRNA to regulate the expression of PirB, RhoA and GAP43. In conclusion, we provide the first evidence that EA enhances rehabilitation against stroke by regulating epigenetic changes to directly act on its targets, such as the miR-181b/PirB/RhoA/GAP43 axis, which is a novel mechanism of EA therapy.

Large-Scale microRNA Expression Profiling Identifies Putative Retinal miRNA-mRNA Signaling Pathways Underlying Form-Deprivation Myopia in Mice

Development of myopia is associated with large-scale changes in ocular tissue gene expression. Although differential expression of coding genes underlying development of myopia has been a subject of intense investigation, the role of non-coding genes such as microRNAs in the development of myopia is largely unknown. In this study, we explored myopia-associated miRNA expression profiles in the retina and sclera of C57Bl/6J mice with experimentally induced myopia using microarray technology. We found a total of 53 differentially expressed miRNAs in the retina and no differences in miRNA expression in the sclera of C57BL/6J mice after 10 days of visual form deprivation, which induced -6.93 ± 2.44 D (p < 0.000001, n = 12) of myopia. We also identified their putative mRNA targets among mRNAs found to be differentially expressed in myopic retina and potential signaling pathways involved in the development of form-deprivation myopia using miRNA-mRNA interaction network analysis. Analysis of myopia-associated signaling pathways revealed that myopic response to visual form deprivation in the retina is regulated by a small number of highly integrated signaling pathways. Our findings highlighted that changes in microRNA expression are involved in the regulation of refractive eye development and predicted how they may be involved in the development of myopia by regulating retinal gene expression.

MicroRNAs and ectodermal specification I. Identification of miRs and miR-targeted mRNAs in early anterior neural and epidermal ectoderm

The establishment of cell lineages occurs via a dynamic progression of gene regulatory networks (GRNs) that underlie developmental commitment and differentiation. To investigate how microRNAs (miRs) function in this process, we compared miRs and miR targets at the initiation of the two major ectodermal lineages in Xenopus. We used next-generation sequencing to identify over 170 miRs expressed in midgastrula ectoderm expressing either noggin or a constitutively active BMP receptor, reflecting anterior neural or epidermal ectoderm, respectively; 125 had not previously been identified in Xenopus. We identified the locations of the pre-miR sequences in the X. laevis genome. Neural and epidermal ectoderm express broadly similar sets of miRs. To identify targets of miR-dependent translational control, we co-immunoprecipitated Argonaute-Ribonucleoprotein (Ago-RNP) complexes from early neural and epidermal ectoderm and sequenced the associated RNA. The Ago-RNP RNAs from these tissues represent overlapping, yet distinct, subsets of genes. Moreover, the profile of Ago-RNP associated genes differs substantially from the profile of total RNAs in these tissues. We generated target predictions for the "high confidence" Ago-RNP RNAs using the identified ectodermal miRs; These RNAs generally had target sites for multiple miRs. Oct4 orthologues, as well as many of their previously identified transcriptional targets, are represented in the Ago-RNP pool in both tissues, suggesting that miR-dependent regulation contributes to the downregulation of the oct4 gene regulatory network and the reduction in ectodermal pluripotency.

Risk factors and biomarkers of age-related macular degeneration

A biomarker can be a substance or structure measured in body parts, fluids or products that can affect or predict disease incidence. As age-related macular degeneration (AMD) is the leading cause of blindness in the developed world, much research and effort has been invested in the identification of different biomarkers to predict disease incidence, identify at risk individuals, elucidate causative pathophysiological etiologies, guide screening, monitoring and treatment parameters, and predict disease outcomes. To date, a host of genetic, environmental, proteomic, and cellular targets have been identified as both risk factors and potential biomarkers for AMD. Despite this, their use has been confined to research settings and has not yet crossed into the clinical arena. A greater understanding of these factors and their use as potential biomarkers for AMD can guide future research and clinical practice. This article will discuss known risk factors and novel, potential biomarkers of AMD in addition to their application in both academic and clinical settings.

Pilocarpine-induced seizures trigger differential regulation of microRNA-stability related genes in rat hippocampal neurons

Epileptogenesis in the temporal lobe elicits regulation of gene expression and protein translation, leading to reorganization of neuronal networks. In this process, miRNAs were described as being regulated in a cell-specific manner, although mechanistics of miRNAs activity are poorly understood. The specificity of miRNAs on their target genes depends on their intracellular concentration, reflecting the balance of biosynthesis and degradation. Herein, we confirmed that pilocarpine application promptly (<30 min) induces status epilepticus (SE) as revealed by changes in rat electrocorticogram particularly in fast-beta range (21–30 Hz). SE simultaneously upregulated XRN2 and downregulated PAPD4 gene expression in the hippocampus, two genes related to miRNA degradation and stability, respectively. Moreover, SE decreased the number of XRN2-positive cells in the hilus, while reduced the number of PAPD4-positive cells in CA1. XRN2 and PAPD4 levels did not change in calretinin- and CamKII-positive cells, although it was possible to determine that PAPD4, but not XRN2, was upregulated in parvalbumin-positive cells, revealing that SE induction unbalances the accumulation of these functional-opposed proteins in inhibitory interneurons that directly innervate distinct domains of pyramidal cells. Therefore, we were able to disclose a possible mechanism underlying the differential regulation of miRNAs in specific neurons during epileptogenesis.

MicroRNA changes through Müller glia dedifferentiation and early/late rod photoreceptor differentiation

Cell-type determination is a complex process driven by the combinatorial effect of extrinsic signals and the expression of transcription factors and regulatory genes. MicroRNAs (miRNAs) are non-coding RNAs that, generally, inhibit the expression of target genes and have been involved, among other processes, in cell identity acquisition. To search for candidate miRNAs putatively involved in mice rod photoreceptor and Müller glia (MG) identity, we compared miRNA expression profiles between late-stage retinal progenitor cells (RPCs), CD73-immunopositive (CD73+) rods and postnatal MG. We found a close similarity between RPCs and CD73+ miRNA expression profiles but a divergence between CD73+ and MG miRNA signatures. We validated preferentially expressed miRNAs in the CD73+ subpopulation (miR-182, 183, 124a, 9(∗), 181c and 301b(∗)) or MG (miR-143, 145, 214, 199a-5p, 199b(∗), and 29a). Taking advantage of the unique capacity of MG to dedifferentiate into progenitor-like cells that can be differentiated to a rod phenotype in response to external cues, we evaluated changes of selected miRNAs in MG-derived progenitors (MGDP) during neuronal differentiation. We found decreased levels of miR-143 and 145, but increased levels of miR-29a in MGDP. In MGDPs committed to early neuronal lineages we found increased levels of miR-124a and upregulation of miR-124a, 9(∗) and 181c during MGDP acquisition of rod phenotypes. Furthermore, we demonstrated that ectopic miR-124 expression is sufficient to enhance early neuronal commitment of MGDP. Our data reveal a dynamic regulation of miRNAs in MGDP through early and late neuronal commitment and miRNAs that could be potential targets to exploit the silent neuronal differentiation capacity of MG in mammals.

Mechanisms of temporal identity regulation in mouse retinal progenitor cells

While much progress has been made in recent years toward elucidating the transcription factor codes controlling how neural progenitor cells generate the various glial and neuronal cell types in a particular spatial domain, much less is known about how these progenitors alter their output over time. In the past years, work in the developing mouse retina has provided evidence that a transcriptional cascade similar to the one used in Drosophila neuroblasts might control progenitor temporal identity in vertebrates. The zinc finger transcription factor Ikzf1 (Ikaros), an ortholog of Drosophila hunchback, was reported to confer early temporal identity in retinal progenitors and, more recently, the ortholog of Drosophila castor, Casz1, was found to function as a mid/late temporal identity factor that is negatively regulated by Ikzf1. The molecular mechanisms by which these temporal identity factors function in retinal progenitors, however, remain unknown. Here we briefly review previous work on the vertebrate temporal identity factors in the retina, and propose a model by which they might operate.

miR-430 regulates oriented cell division during neural tube development in zebrafish

MicroRNAs have emerged as critical regulators of gene expression. Originally shown to regulate developmental timing, microRNAs have since been implicated in a wide range of cellular functions including cell identity, migration and signaling. miRNA-430, the earliest expressed microRNA during zebrafish embryogenesis, is required to undergo morphogenesis and has previously been shown to regulate maternal mRNA clearance, Nodal signaling, and germ cell migration. The functions of miR-430 in brain morphogenesis, however, remain unclear. Herein we find that miR-430 instructs oriented cell divisions in the neural rod required for neural midline formation. Loss of miR-430 function results in mitotic spindle misorientation in the neural rod, failed neuroepithelial integration after cell division, and ectopic cell accumulation in the dorsal neural tube. We propose that miR-430, independently of canonical apicobasal and planar cell polarity (PCP) pathways, coordinates the stereotypical cell divisions that instruct neural tube morphogenesis.

MicroRNAs: Not "Fine-Tuners" but Key Regulators of Neuronal Development and Function

MicroRNAs (miRNAs) are a class of short non-coding RNAs that operate as prominent post-transcriptional regulators of eukaryotic gene expression. miRNAs are abundantly expressed in the brain of most animals and exert diverse roles. The anatomical and functional complexity of the brain requires the precise coordination of multilayered gene regulatory networks. The flexibility, speed, and reversibility of miRNA function provide precise temporal and spatial gene regulatory capabilities that are crucial for the correct functioning of the brain. Studies have shown that the underlying molecular mechanisms controlled by miRNAs in the nervous systems of invertebrate and vertebrate models are remarkably conserved in humans. We endeavor to provide insight into the roles of miRNAs in the nervous systems of these model organisms and discuss how such information may be used to inform regarding diseases of the human brain.

MicroRNA-20a is essential for normal embryogenesis by targeting vsx1 mRNA in fish

MicroRNAs are major post-transcriptional regulators of gene expression and have essential roles in diverse developmental processes. In vertebrates, some regulatory genes play different roles at different developmental stages. These genes are initially transcribed in a wide embryonic region but restricted within distinct cell types at subsequent stages during development. Therefore, post-transcriptional regulation is required for the transition from one developmental stage to the next and the establishment of different cell identities. However, the regulation of many multiple functional genes at post-transcription level during development remains unknown. Here we show that miR-20a can target the mRNA of vsx1, a multiple functional gene, at the 3'-UTR and inhibit protein expression in both goldfish and zebrafish. The expression of miR-20a is initiated ubiquitously at late gastrula stage and exhibits a tissue-specific pattern in the developing retina. Inhibition of vsx1 3'-UTR mediated protein expression occurs when and where miR-20a is expressed. Decoying miR-20a resulted in severely impaired head, eye and trunk formation in association with excessive generation of vsx1 marked neurons in the spinal cord and defects of somites in the mesoderm region. These results demonstrate that miR-20a is essential for normal embryogenesis by restricting Vsx1 expression in goldfish and zebrafish, and that post-transcriptional regulation is an essential mechanism for Vsx1 playing different roles in diverse developmental processes.