Musashi-1, an RNA-binding protein, is indispensable for survival of photoreceptors

Possible retinotoxicity of long-term vardenafil treatment

Phosphodiesterase (PDE) inhibitors - such as vardenafil - are used primarily for treating erectile dysfunction via increasing cyclic guanosine monophosphate (cGMP) levels. Recent studies have also demonstrated their significant cardioprotective effects in several diseases, including diabetes, upon long-term, continuous application. However, PDE inhibitors are not specific for PDE5 and also inhibit the retinal isoform. A sustained rise in cGMP in photoreceptors is known to be toxic; therefore, we hypothesized that long-term vardenafil treatment might result in retinotoxicity. The hypothesis was tested in a clinically relevant animal model of type 2 diabetes mellitus. Histological experiments were performed on lean and diabetic Zucker Diabetic Fatty rats. Half of the animals were treated with vardenafil for six months, and the retinal effects were evaluated. Vardenafil treatment alleviated rod outer segment degeneration but decreased rod numbers in some positions and induced changes in the interphotoreceptor matrix, even in control animals. Vardenafil treatment decreased total retinal thickness in the control and diabetic groups and reduced the number of nuclei in the outer nuclear layer. Müller cell activation was detectable even in the vardenafil-treated control animals, and vardenafil did not improve gliosis in the diabetic group. Vardenafil-treated animals showed complex retinal alterations with improvements in some parameters while deterioration in others. Our results point towards the retinotoxicity of vardenafil, even without diabetes, which raises doubts about the retinal safety of long-term continuous vardenafil administration. This effect needs to be considered when approving PDE inhibitors for alternative indications.

Unique transcriptomes of sensory and non-sensory neurons: insights from Splicing Regulatory States

Alternative Splicing (AS) programs serve as instructive signals of cell type specificity, particularly within the brain, which comprises dozens of molecularly and functionally distinct cell types. Among them, retinal photoreceptors stand out due to their unique transcriptome, making them a particularly well-suited system for studying how AS shapes cell type-specific molecular functions. Here, we use the Splicing Regulatory State (SRS) as a novel framework to discuss the splicing factors governing the unique AS pattern of photoreceptors, and how this pattern may aid in the specification of their highly specialized sensory cilia. In addition, we discuss how other sensory cells with ciliated structures, for which data is much scarcer, also rely on specific SRSs to implement a proteome specialized in the detection of sensory stimuli. By reviewing the general rules of cell type- and tissue-specific AS programs, firstly in the brain and subsequently in specialized sensory neurons, we propose a novel paradigm on how SRSs are established and how they can diversify. Finally, we illustrate how SRSs shape the outcome of mutations in splicing factors to produce cell type-specific phenotypes that can lead to various human diseases.

Short C-terminal Musashi-1 proteins regulate pluripotency states in embryonic stem cells

The RNA-binding protein Musashi-1 (MSI1) regulates the proliferation and differentiation of adult stem cells. However, its role in embryonic stem cells (ESCs) and early embryonic development remains poorly understood. Here, we report the presence of short C-terminal MSI1 (MSI1-C) proteins in early mouse embryos and mouse ESCs, but not in human ESCs, under conventional culture conditions. In mouse embryos and mESCs, deletion of MSI1-C together with full-length MSI1 causes early embryonic developmental arrest and pluripotency dissolution. MSI1-C is induced upon naive induction and facilitates hESC naive pluripotency acquisition, elevating the pluripotency of primed hESCs toward a formative-like state. MSI1-C proteins are nuclear localized and bind to RNAs involved in DNA-damage repair (including MLH1, BRCA1, and MSH2), conferring on hESCs better survival in human-mouse interspecies cell competition and prolonged ability to form blastoids. This study identifies MSI1-C as an essential regulator in ESC pluripotency states and early embryonic development.

RNA-binding proteins and post-transcriptional regulation in lens biology and cataract: Mediating spatiotemporal expression of key factors that control the cell cycle, transcription, cytoskeleton and transparency

Development of the ocular lens - a transparent tissue capable of sustaining frequent shape changes for optimal focusing power - pushes the boundaries of what cells can achieve using the molecular toolkit encoded by their genomes. The mammalian lens contains broadly two types of cells, the anteriorly located monolayer of epithelial cells which, at the equatorial region of the lens, initiate differentiation into fiber cells that contribute to the bulk of the tissue. This differentiation program involves massive upregulation of select fiber cell-expressed RNAs and their subsequent translation into high amounts of proteins, such as crystallins. But intriguingly, fiber cells achieve this while also simultaneously undergoing significant morphological changes such as elongation - involving about 1000-fold length-wise increase - and migration, which requires modulation of cytoskeletal and cell adhesion factors. Adding further to the challenges, these molecular and cellular events have to be coordinated as fiber cells progress toward loss of their nuclei and organelles, which irreversibly compromises their potential for harnessing genetically hardwired information. A long-standing question is how processes downstream of signaling and transcription, which may also participate in feedback regulation, contribute toward orchestrating these cellular differentiation events in the lens. It is now becoming clear from findings over the past decade that post-transcriptional gene expression regulatory mechanisms are critical in controlling cellular proteomes and coordinating key processes in lens development and fiber cell differentiation. Indeed, RNA-binding proteins (RBPs) such as Caprin2, Celf1, Rbm24 and Tdrd7 have now been described in mediating post-transcriptional control over key factors (e.g. Actn2, Cdkn1a (p21Cip1), Cdkn1b (p27Kip1), various crystallins, Dnase2b, Hspb1, Pax6, Prox1, Sox2) that are variously involved in cell cycle, transcription, cytoskeleton maintenance and differentiation in the lens. Furthermore, deficiencies of these RBPs have been shown to result in various eye and lens defects and/or cataract. Because fiber cell differentiation in the lens occurs throughout life, the underlying regulatory mechanisms operational in development are expected to also be recruited for the maintenance of transparency in aged lenses. Indeed, in support of this, TDRD7 and CAPRIN2 loci have been linked to age-related cataract in humans. Here, I will review the role of key RBPs in the lens and their importance in understanding the pathology of lens defects. I will discuss advances in RBP-based gene expression control, in general, and the important challenges that need to be addressed in the lens to define the mechanisms that determine the epithelial and fiber cell proteome. Finally, I will also discuss in detail several key future directions including the application of bioinformatics approaches such as iSyTE to study RBP-based post-transcriptional gene expression control in the aging lens and in the context of age-related cataract.

Mouse Models of Inherited Retinal Degeneration with Photoreceptor Cell Loss

Inherited retinal degeneration (RD) leads to the impairment or loss of vision in millions of individuals worldwide, most frequently due to the loss of photoreceptor (PR) cells. Animal models, particularly the laboratory mouse, have been used to understand the pathogenic mechanisms that underlie PR cell loss and to explore therapies that may prevent, delay, or reverse RD. Here, we reviewed entries in the Mouse Genome Informatics and PubMed databases to compile a comprehensive list of monogenic mouse models in which PR cell loss is demonstrated. The progression of PR cell loss with postnatal age was documented in mutant alleles of genes grouped by biological function. As anticipated, a wide range in the onset and rate of cell loss was observed among the reported models. The analysis underscored relationships between RD genes and ciliary function, transcription-coupled DNA damage repair, and cellular chloride homeostasis. Comparing the mouse gene list to human RD genes identified in the RetNet database revealed that mouse models are available for 40% of the known human diseases, suggesting opportunities for future research. This work may provide insight into the molecular players and pathways through which PR degenerative disease occurs and may be useful for planning translational studies.

Regulation of cell proliferation in the retinal pigment epithelium: Differential regulation of the death-associated protein like-1 DAPL1 by alternative MITF splice forms

Vertebrate eye development and homoeostasis critically depend on the regulation of proliferation of cells forming the retinal pigment epithelium (RPE). Previous results indicated that the death-associated protein like-1 DAPL1 cell autonomously suppresses RPE proliferation in vivo and in vitro. Here, we show in human RPE cell lines that the pigment cell transcription factor MITF regulates RPE cell proliferation by upregulating DAPL1 expression. DAPL1 regulation by MITF is, however, mediated predominantly by (-) MITF, one of two alternative splice isoforms of MITF that lacks six residues located upstream of the DNA-binding basic domain. Furthermore, we find that the regulation of DAPL1 by MITF is indirect in that (-) MITF stimulates the transcription of Musashi homolog-2 (MSI2), which negatively regulates the processing of the anti-DAPL1 microRNA miR-7. Our results provide molecular insights into the regulation of RPE cell proliferation and quiescence and may help us understand the mechanisms of normal RPE maintenance and of eye diseases associated with either RPE hyperproliferation or the lack of regenerative proliferation.

Inherited Retinal Disease Therapies Targeting Precursor Messenger Ribonucleic Acid

Inherited retinal diseases are an extremely diverse group of genetically and phenotypically heterogeneous conditions characterized by variable maturation of retinal development, impairment of photoreceptor cell function and gradual loss of photoreceptor cells and vision. Significant progress has been made over the last two decades in identifying the many genes implicated in inherited retinal diseases and developing novel therapies to address the underlying genetic defects. Approximately one-quarter of exonic mutations related to human inherited diseases are likely to induce aberrant splicing products, providing opportunities for the development of novel therapeutics that target splicing processes. The feasibility of antisense oligomer mediated splice intervention to treat inherited diseases has been demonstrated in vitro, in vivo and in clinical trials. In this review, we will discuss therapeutic approaches to treat inherited retinal disease, including strategies to correct splicing and modify exon selection at the level of pre-mRNA. The challenges of clinical translation of this class of emerging therapeutics will also be discussed.

Histological Evaluation of Diabetic Neurodegeneration in the Retina of Zucker Diabetic Fatty (ZDF) Rats

In diabetes, retinal dysfunctions exist prior to clinically detectable vasculopathy, however the pathology behind these functional deficits is still not fully established. Previously, our group published a detailed study on the retinal histopathology of type 1 diabetic (T1D) rat model, where specific alterations were detected. Although the majority of human diabetic patients have type 2 diabetes (T2D), similar studies on T2D models are practically absent. To fill this gap, we examined Zucker Diabetic Fatty (ZDF) rats - a model for T2D - by immunohistochemistry at the age of 32 weeks. Glial reactivity was observed in all diabetic specimens, accompanied by an increase in the number of microglia cells. Prominent outer segment degeneration was detectable with changes in cone opsin expression pattern, without a decrease in the number of labelled elements. The immunoreactivity of AII amacrine cells was markedly decreased and changes were detectable in the number and staining of some other amacrine cell subtypes, while most other cells examined did not show any major alterations. Overall, the retinal histology of ZDF rats shows a surprising similarity to T1D rats indicating that despite the different evolution of the disease, the neuroretinal cells affected are the same in both subtypes of diabetes.

Neurodevelopmental protein Musashi-1 interacts with the Zika genome and promotes viral replication

A recent outbreak of Zika virus in Brazil has led to a simultaneous increase in reports of neonatal microcephaly. Zika targets cerebral neural precursors, a cell population essential for cortical development, but the cause of this neurotropism remains obscure. Here we report that the neural RNA-binding protein Musashi-1 (MSI1) interacts with the Zika genome and enables viral replication. Zika infection disrupts the binding of MSI1 to its endogenous targets, thereby deregulating expression of factors implicated in neural stem cell function. We further show that MSI1 is highly expressed in neural progenitors of the human embryonic brain and is mutated in individuals with autosomal recessive primary microcephaly. Selective MSI1 expression in neural precursors could therefore explain the exceptional vulnerability of these cells to Zika infection.

Study of retinal neurodegeneration and maculopathy in diabetic Meriones shawi: A particular animal model with human-like macula

The purpose of this work was to evaluate a potentially useful animal model, Meriones shawi (M.sh)-developing metabolic X syndrome, diabetes and possessing a visual streak similar to human macula-in the study of diabetic retinopathy and diabetic macular edema (DME). Type 2 diabetes (T2D) was induced by high fat diet administration in M.sh. Body weights, blood glucose levels were monitored throughout the study. Diabetic retinal histopathology was evaluated 3 and 7 months after diabetes induction. Retinal thickness was measured, retinal cell types were labeled by immunohistochemistry and the number of stained elements were quantified. Apoptosis was determined with TUNEL assay. T2D induced progressive changes in retinal histology. A significant decrease of retinal thickness and glial reactivity was observed without an increase in apoptosis rate. Photoreceptor outer segment degeneration was evident, with a significant decrease in the number of all cones and M-cone subtype, but-surprisingly-an increase in S-cones. Damage of the pigment epithelium was also confirmed. A decrease in the number and labeling intensity of parvalbumin- and calretinin-positive amacrine cells and a loss of ganglion cells was detected. Other cell types showed no evident alterations. No DME-like condition was noticed even after 7 months. M.sh could be a useful model to study the evolution of diabetic retinal pathology and to identify the role of hypertension and dyslipidemia in the development of the reported alterations. Longer follow up would be needed to evaluate the potential use of the visual streak in modeling human macular diseases.

Musashi RNA-Binding Proteins as Cancer Drivers and Novel Therapeutic Targets

Aberrant gene expression that drives human cancer can arise from epigenetic dysregulation. Although much attention has focused on altered activity of transcription factors and chromatin-modulating proteins, proteins that act posttranscriptionally can potently affect expression of oncogenic signaling proteins. The RNA-binding proteins (RBP) Musashi-1 (MSI1) and Musashi-2 (MSI2) are emerging as regulators of multiple critical biological processes relevant to cancer initiation, progression, and drug resistance. Following identification of Musashi as a regulator of progenitor cell identity in Drosophila, the human Musashi proteins were initially linked to control of maintenance of hematopoietic stem cells, then stem cell compartments for additional cell types. More recently, the Musashi proteins were found to be overexpressed and prognostic of outcome in numerous cancer types, including colorectal, lung, and pancreatic cancers; glioblastoma; and several leukemias. MSI1 and MSI2 bind and regulate the mRNA stability and translation of proteins operating in essential oncogenic signaling pathways, including NUMB/Notch, PTEN/mTOR, TGFβ/SMAD3, MYC, cMET, and others. On the basis of these activities, MSI proteins maintain cancer stem cell populations and regulate cancer invasion, metastasis, and development of more aggressive cancer phenotypes, including drug resistance. Although RBPs are viewed as difficult therapeutic targets, initial efforts to develop MSI-specific inhibitors are promising, and RNA interference-based approaches to inhibiting these proteins have had promising outcomes in preclinical studies. In the interim, understanding the function of these translational regulators may yield insight into the relationship between mRNA expression and protein expression in tumors, guiding tumor-profiling analysis. This review provides a current overview of Musashi as a cancer driver and novel therapeutic target. Clin Cancer Res; 23(9); 2143-53. ©2017 AACR.

The Musashi 1 Controls the Splicing of Photoreceptor-Specific Exons in the Vertebrate Retina

Alternative pre-mRNA splicing expands the coding capacity of eukaryotic genomes, potentially enabling a limited number of genes to govern the development of complex anatomical structures. Alternative splicing is particularly prevalent in the vertebrate nervous system, where it is required for neuronal development and function. Here, we show that photoreceptor cells, a type of sensory neuron, express a characteristic splicing program that affects a broad set of transcripts and is initiated prior to the development of the light sensing outer segments. Surprisingly, photoreceptors lack prototypical neuronal splicing factors and their splicing profile is driven to a significant degree by the Musashi 1 (MSI1) protein. A striking feature of the photoreceptor splicing program are exons that display a "switch-like" pattern of high inclusion levels in photoreceptors and near complete exclusion outside of the retina. Several ubiquitously expressed genes that are involved in the biogenesis and function of primary cilia produce highly photoreceptor specific isoforms through use of such "switch-like" exons. Our results suggest a potential role for alternative splicing in the development of photoreceptors and the conversion of their primary cilia to the light sensing outer segments.

Conditional rod photoreceptor ablation reveals Sall1 as a microglial marker and regulator of microglial morphology in the retina

Neurodegeneration has been shown to induce microglial activation and the infiltration of monocyte-derived macrophages into the CNS, resulting in the coexistence of these two populations within the same lesion, though their distinct features remain elusive. To investigate the impact of rod photoreceptor degeneration on microglial activation, we generated a toxin-mediated genetic model of rod degeneration. Rod injury induced microglial proliferation and migration toward the photoreceptors. Bone marrow transplantation revealed the invasion of monocyte-derived macrophages into the retina, with microglia and the infiltrating macrophages showing distinct distribution patterns in the retina. By comparing the gene expression profiles of the activated microglia and infiltrating macrophages, we identified microglia-specific genes, including Ak1, Ctsf, Sall1, Phlda3, and Spns2. An analysis of Sall1gfp knock-in mice showed GFP expression in the microglia of developing and mature healthy retinas. DTA injury induced the expansion of Sall1gfp(+) microglia, whereas Ly6C(+) monocyte-derived macrophages were mostly Sall1gfp(-) , supporting the idea that Sall1 is exclusively expressed in microglia within the retinal phagocyte pool. We evaluated the contribution of microglia to the phagocyte pool in rd1 mutant retinas and found that Sall1gfp(+) microglia constituted the majority of phagocytes. A Sall1 deficiency did not affect microglial colonization of the retina and the cortex, but it did change their morphology from a ramified to a more amoeboid appearance. The morphological defects observed in Sall1-deficient microglia were not rescued by the presence of wild-type non-microglial cells, suggesting that Sall1 functions cell-autonomously in microglia. Taken together, our data indicate that Sall1 regulates microglial morphology during development. GLIA 2016;64:2005-2024.

RNA-binding proteins in eye development and disease: implication of conserved RNA granule components

The molecular biology of metazoan eye development is an area of intense investigation. These efforts have led to the surprising recognition that although insect and vertebrate eyes have dramatically different structures, the orthologs or family members of several conserved transcription and signaling regulators such as Pax6, Six3, Prox1, and Bmp4 are commonly required for their development. In contrast, our understanding of posttranscriptional regulation in eye development and disease, particularly regarding the function of RNA-binding proteins (RBPs), is limited. We examine the present knowledge of RBPs in eye development in the insect model Drosophila as well as several vertebrate models such as fish, frog, chicken, and mouse. Interestingly, of the 42 RBPs that have been investigated for their expression or function in vertebrate eye development, 24 (~60%) are recognized in eukaryotic cells as components of RNA granules such as processing bodies, stress granules, or other specialized ribonucleoprotein (RNP) complexes. We discuss the distinct developmental and cellular events that may necessitate potential RBP/RNA granule-associated RNA regulon models to facilitate posttranscriptional control of gene expression in eye morphogenesis. In support of these hypotheses, three RBPs and RNP/RNA granule components Tdrd7, Caprin2, and Stau2 are linked to ocular developmental defects such as congenital cataract, Peters anomaly, and microphthalmia in human patients or animal models. We conclude by discussing the utility of interdisciplinary approaches such as the bioinformatics tool iSyTE (integrated Systems Tool for Eye gene discovery) to prioritize RBPs for deriving posttranscriptional regulatory networks in eye development and disease. WIREs RNA 2016, 7:527-557. doi: 10.1002/wrna.1355 For further resources related to this article, please visit the WIREs website.

The Musashi family RNA-binding proteins in stem cells

The Musashi family is an evolutionarily conserved group of RNA-binding proteins. In mammal, two members of the group, Msi1 and Msi2, have been identified to date. Msi1 is considered to play roles in maintaining the stem cell status (stemness) of neural stem/progenitor cells in adults and in the development of central nervous system through translational regulation of its target mRNAs, which encode regulators of signal transduction and the cell cycle. Recently, strong expression of Msi1 in various somatic stem/progenitor cells of adult tissues, such as eye, gut, stomach, breast, and hair follicle, has been reported. The protein is also expressed in various cancer cells, and ectopically emerging cells have been found in neural tissues of patients with diseases involving neural disorder, including epilepsy. Many novel target mRNAs and regulatory pathways of Msi1 have been reported in recent years. Here, we present a review of the functions and action mechanisms of Msi1 protein and discuss possible directions for further study.

Downregulation of Msi1 suppresses the growth of human colon cancer by targeting p21cip1

Musashi1 (Msi1), a member of the RNA-binding protein (RBP) family, is highly expressed in neural progenitor or stem cells for the maintenance of stemness as well as in various cancers. Emerging studies have demonstrated that it regulates cell processes by translational activation or suppresses specifically bound mRNA. In the present study, we initially reported remarkably increased expression of Msi1 in colon cancer tissues compared with adjacent non-tumor tissues. Knockdown of Msi1 significantly suppressed the proliferation, colony formation, tumorsphere formation and the progression of implanted colon cancers, and induced cell cycle attest at G0/G1 phase, along with the upregulated expression of p21(cip1). Reporter assays using a chimeric mRNA that combined luciferase and the 3'-UTR of p21(cip1) revealed that Msi1 decreased the reporter activity through the specific motif. Thus, the current results suggested that downregulation of Msi1 could inhibit the growth of colon cancers and Msi1 may be a promising therapeutic target molecule for human colon cancers.

A transcriptome for the study of early processes of retinal regeneration in the adult newt, Cynops pyrrhogaster

Retinal regeneration in the adult newt is a useful system to uncover essential mechanisms underlying the regeneration of body parts of this animal as well as to find clues to treat retinal disorders such as proliferative vitreoretinopathy. Here, to facilitate the study of early processes of retinal regeneration, we provide a de novo assembly transcriptome and inferred proteome of the Japanese fire bellied newt (Cynops pyrrhogaster), which was obtained from eyeball samples of day 0-14 after surgical removal of the lens and neural retina. This transcriptome (237,120 in silico transcripts) contains most information of cDNAs/ESTs which has been reported in newts (C. pyrrhogaster, Pleurodeles waltl and Notophthalmus viridescence) thus far. On the other hand, de novo assembly transcriptomes reported lately for N. viridescence only covered 16-31% of this transcriptome, suggesting that most constituents of this transcriptome are specific to the regenerating eye tissues of C. pyrrhogaster. A total of 87,102 in silico transcripts of this transcriptome were functionally annotated. Coding sequence prediction in combination with functional annotation revealed that 76,968 in silico transcripts encode protein/peptides recorded in public databases so far, whereas 17,316 might be unique. qPCR and Sanger sequencing demonstrated that this transcriptome contains much information pertaining to genes that are regulated in association with cell reprogramming, cell-cycle re-entry/proliferation, and tissue patterning in an early phase of retinal regeneration. This data also provides important insight for further investigations addressing cellular mechanisms and molecular networks underlying retinal regeneration as well as differences between retinal regeneration and disorders. This transcriptome can be applied to ensuing comprehensive gene screening steps, providing candidate genes, regardless of whether annotated or unique, to uncover essential mechanisms underlying early processes of retinal regeneration.

Acquisition of germ plasm accelerates vertebrate evolution

Primordial germ cell (PGC) specification occurs either by induction from pluripotent cells (epigenesis) or by a cell-autonomous mechanism mediated by germ plasm (preformation). Among vertebrates, epigenesis is basal, whereas germ plasm has evolved convergently across lineages and is associated with greater speciation. We compared protein-coding sequences of vertebrate species that employ preformation with their sister taxa that use epigenesis and demonstrate that genes evolve more rapidly in species containing germ plasm. Furthermore, differences in rates of evolution appear to cause phylogenetic incongruence in protein-coding sequence comparisons between vertebrate taxa. Our results support the hypothesis that germ plasm liberates constraints on somatic development and that enhanced evolvability drives the evolution of germ plasm.

Correlation of changes of (non)exfoliated endometrial organelles and expressions of Musashi-1 and β-catenin with endometriosis in menstrual period

This study aims to investigate the correlation of structural changes of endometrial organelles and expressions of Musashi-1 (Msi-1) and β-catenin with the endometriosis (EMs) in the menstrual period. The structural changes of exfoliated and nonexfoliated endometrial organelles in the experimental group and the control group were observed by the transmission electron microscopy (TEM) on the first and fifth day of menstruation. (1) TEM: compared with the control group, the exfoliated endometrial organelles in the experimental group on the first day were rich, with irregular nucleus, the bi-nucleolus could be seen, with rich chromatin; while the shapes of epithelial secretory cells in the nonexfoliated endometrial gland were irregular, with abundant organelles, the basal film varied in width, with abnormal curvature, and a lot of intercellular collagen fibers could be seen. (2) The expressions of Msi-1 and β-catenin in the exfoliated and nonexfoliated endometrium of the experimental group were higher than those of the control group and exhibited positively correlation, while no correlation could be found within the control group. (1) The organelles' structural changes might cause the changes of endometrial cellular functions. (2) Msi-1 might participate in the formation of EMs through activating the Wnt/β-catenin signaling pathway.

Spatial distribution of prominin-1 (CD133)-positive cells within germinative zones of the vertebrate brain

BACKGROUND

In mammals, embryonic neural progenitors as well as adult neural stem cells can be prospectively isolated based on the cell surface expression of prominin-1 (CD133), a plasma membrane glycoprotein. In contrast, characterization of neural progenitors in non-mammalian vertebrates endowed with significant constitutive neurogenesis and inherent self-repair ability is hampered by the lack of suitable cell surface markers. Here, we have investigated whether prominin-1-orthologues of the major non-mammalian vertebrate model organisms show any degree of conservation as for their association with neurogenic geminative zones within the central nervous system (CNS) as they do in mammals or associated with activated neural progenitors during provoked neurogenesis in the regenerating CNS.

METHODS

We have recently identified prominin-1 orthologues from zebrafish, axolotl and chicken. The spatial distribution of prominin-1-positive cells--in comparison to those of mice--was mapped in the intact brain in these organisms by non-radioactive in situ hybridization combined with detection of proliferating neural progenitors, marked either by proliferating cell nuclear antigen or 5-bromo-deoxyuridine. Furthermore, distribution of prominin-1 transcripts was investigated in the regenerating spinal cord of injured axolotl.

RESULTS

Remarkably, a conserved association of prominin-1 with germinative zones of the CNS was uncovered as manifested in a significant co-localization with cell proliferation markers during normal constitutive neurogenesis in all species investigated. Moreover, an enhanced expression of prominin-1 became evident associated with provoked, compensatory neurogenesis during the epimorphic regeneration of the axolotl spinal cord. Interestingly, significant prominin-1-expressing cell populations were also detected at distinct extraventricular (parenchymal) locations in the CNS of all vertebrate species being suggestive of further, non-neurogenic neural function(s).

CONCLUSION/INTERPRETATION

Collectively, our work provides the first data set describing a comparative analysis of prominin-1-positive progenitor cells across species establishing a framework for further functional characterization in the context of regeneration.

Musashi1-CreER(T2) : a new cre line for conditional mutagenesis in neural stem cells

The RNA-binding protein Musashi1 (Msi1) is one of two mammalian homologues of Drosophila Musashi, which is required for the asymmetric cell division of sensory organ precursor cells. In the mouse central nervous system (CNS), Msi1 is preferentially expressed in mitotically active progenitor cells in the ventricular zone (VZ) of the neural tube during embryonic development and in the subventricular zone (SVZ) of the postnatal brain. Previous studies showed that cells in the SVZ can contribute to long-term neurogenesis in the olfactory bulb (OB), but it remains unclear whether Msi1-expressing cells have self-renewing potential and can contribute to neurogenesis in the adult. Here, we describe the generation of Msi1-CreER(T2) knock-in mice and show by cell lineage tracing that Msi1-CreER(T2) -expressing cells mark neural stem cells (NSCs) in both the embryonic and adult brain. Msi1-CreER(T2) mice thus represent a new tool in our arsenal for genetically manipulating NSCs, which will be essential for understanding the molecular mechanisms underlying neural development.

Lens and retina regeneration: new perspectives from model organisms

Comparative studies of lens and retina regeneration have been conducted within a wide variety of animals over the last 100 years. Although amphibians, fish, birds and mammals have all been noted to possess lens- or retina-regenerative properties at specific developmental stages, lens or retina regeneration in adult animals is limited to lower vertebrates. The present review covers the newest perspectives on lens and retina regeneration from these different model organisms with a focus on future trends in regeneration research.

Quantitative iTRAQ analysis of retinal ganglion cell degeneration after optic nerve crush

Retinal ganglion cells (RGCs) are central nervous system (CNS) neurons that transmit visual information from the retina to the brain. Apoptotic RGC degeneration causes visual impairment that can be modeled by optic nerve crush. Neuronal apoptosis is also a salient feature of CNS trauma, ischemia (stroke), and diseases of the CNS such as Alzheimer's, Parkinson's, multiple sclerosis, and amyotrophic lateral sclerosis. Optic nerve crush induces the apoptotic cell death of ∼ 70% of RGCs within the first 14 days after injury. This model is particularly attractive for studying adult neuron apoptosis because the time-course of RGC death is well established and axon regeneration within the myelinated optic nerve can be concurrently evaluated. Here, we performed a large scale iTRAQ proteomic study to identify and quantify proteins of the rat retina at 1, 3, 4, 7, 14, and 21 days after optic nerve crush. In total, 337 proteins were identified, and 110 were differentially regulated after injury. Of these, 58 proteins were upregulated (>1.3 ×), 46 were downregulated (<0.7 ×), and 6 showed both positive and negative regulation over 21 days, relative to normal retinas. Among the differentially expressed proteins, Thymosin-β4 showed an early upregulation at 3 days, the time-point that immediately precedes the induction of RGC apoptosis after injury. We examined the effect of exogenous Thymosin-β4 administration on RGC death after optic nerve injury. Intraocular injections of Thymosin-β4 significantly increased RGC survival by ∼ 3-fold compared to controls and enhanced axon regeneration after crush, demonstrating therapeutic potential for CNS insults. Overall, our study identified numerous proteins that are differentially regulated at key time-points after optic nerve crush, and how the temporal profiles of their expression parallel RGC death. This data will aid in the future development of novel therapeutics to promote neuronal survival and regeneration in the adult CNS.

Distinct and conserved prominin-1/CD133-positive retinal cell populations identified across species

Besides being a marker of various somatic stem cells in mammals, prominin-1 (CD133) plays a role in maintaining the photoreceptor integrity since mutations in the PROM1 gene are linked with retinal degeneration. In spite of that, little information is available regarding its distribution in eyes of non-mammalian vertebrates endowed with high regenerative abilities. To address this subject, prominin-1 cognates were isolated from axolotl, zebrafish and chicken, and their retinal compartmentalization was investigated and compared to that of their mammalian orthologue. Interestingly, prominin-1 transcripts--except for the axolotl--were not strictly restricted to the outer nuclear layer (i.e., photoreceptor cells), but they also marked distinct subdivisions of the inner nuclear layer (INL). In zebrafish, where the prominin-1 gene is duplicated (i.e., prominin-1a and prominin-1b), a differential expression was noted for both paralogues within the INL being localized either to its vitreal or scleral subdivision, respectively. Interestingly, expression of prominin-1a within the former domain coincided with Pax-6-positive cells that are known to act as progenitors upon injury-induced retino-neurogenesis. A similar, but minute population of prominin-1-positive cells located at the vitreal side of the INL was also detected in developing and adult mice. In chicken, however, prominin-1-positive cells appeared to be aligned along the scleral side of the INL reminiscent of zebrafish prominin-1b. Taken together our data indicate that in addition to conserved expression of prominin-1 in photoreceptors, significant prominin-1-expressing non-photoreceptor retinal cell populations are present in the vertebrate eye that might represent potential sources of stem/progenitor cells for regenerative therapies.

Disruption of the chaperonin containing TCP-1 function affects protein networks essential for rod outer segment morphogenesis and survival

Type II Chaperonin Containing TCP-1 (CCT, also known as TCP-1 Ring Complex, TRiC) is a multi-subunit molecular machine thought to assist in the folding of ∼ 10% of newly translated cytosolic proteins in eukaryotes. A number of proteins folded by CCT have been identified in yeast and cultured mammalian cells, however, the function of this chaperonin in vivo has never been addressed. Here we demonstrate that suppressing the CCT activity in mouse photoreceptors by transgenic expression of a dominant-negative mutant of the CCT cofactor, phosducin-like protein (PhLP), results in the malformation of the outer segment, a cellular compartment responsible for light detection, and triggers rapid retinal degeneration. Investigation of the underlying causes by quantitative proteomics identified distinct protein networks, encompassing ∼ 200 proteins, which were significantly affected by the chaperonin deficiency. Notably among those were several essential proteins crucially engaged in structural support and visual signaling of the outer segment such as peripherin 2, Rom1, rhodopsin, transducin, and PDE6. These data for the first time demonstrate that normal CCT function is ultimately required for the morphogenesis and survival of sensory neurons of the retina, and suggest the chaperonin CCT deficiency as a potential, yet unexplored, cause of neurodegenerative diseases.