Rapid accumulation of nucleostemin in nucleolus during newt regeneration

GNL3 is an evolutionarily conserved stem cell gene influencing cell proliferation, animal growth and regeneration in the hydrozoan Hydractinia

Nucleostemin (NS) is a vertebrate gene preferentially expressed in stem and cancer cells, which acts to regulate cell cycle progression, genome stability and ribosome biogenesis. NS and its paralogous gene, GNL3-like (GNL3L), arose in the vertebrate clade after a duplication event from their orthologous gene, G protein Nucleolar 3 (GNL3). Research on invertebrate GNL3, however, has been limited. To gain a greater understanding of the evolution and functions of the GNL3 gene, we have performed studies in the hydrozoan cnidarian Hydractinia symbiolongicarpus, a colonial hydroid that continuously generates pluripotent stem cells throughout its life cycle and presents impressive regenerative abilities. We show that Hydractinia GNL3 is expressed in stem and germline cells. The knockdown of GNL3 reduces the number of mitotic and S-phase cells in Hydractinia larvae of different ages. Genome editing of Hydractinia GNL3 via CRISPR/Cas9 resulted in colonies with reduced growth rates, polyps with impaired regeneration capabilities, gonadal morphological defects, and low sperm motility. Collectively, our study shows that GNL3 is an evolutionarily conserved stem cell and germline gene involved in cell proliferation, animal growth, regeneration and sexual reproduction in Hydractinia, and sheds new light into the evolution of GNL3 and of stem cell systems.

Pigment Epithelia of the Eye: Cell-Type Conversion in Regeneration and Disease

Pigment epithelial cells (PECs) of the retina (RPE), ciliary body, and iris (IPE) are capable of altering their phenotype. The main pathway of phenotypic switching of eye PECs in vertebrates and humans in vivo and/or in vitro is neural/retinal. Besides, cells of amphibian IPE give rise to the lens and its derivatives, while mammalian and human RPE can be converted along the mesenchymal pathway. The PECs’ capability of conversion in vivo underlies the lens and retinal regeneration in lower vertebrates and retinal diseases such as proliferative vitreoretinopathy and fibrosis in mammals and humans. The present review considers these processes studied in vitro and in vivo in animal models and in humans. The molecular basis of conversion strategies in PECs is elucidated. Being predetermined onto- and phylogenetically, it includes a species-specific molecular context, differential expression of transcription factors, signaling pathways, and epigenomic changes. The accumulated knowledge regarding the mechanisms of PECs phenotypic switching allows the development of approaches to specified conversion for many purposes: obtaining cells for transplantation, creating conditions to stimulate natural regeneration of the retina and the lens, blocking undesirable conversions associated with eye pathology, and finding molecular markers of pathology to be targets of therapy.

Cellular and molecular profiles of anterior nervous system regeneration in Diopatra claparedii Grube, 1878 (Annelida, Polychaeta)

The polychaete Diopatra claparedii Grube, 1878 is among those organisms successfully carrying out full body regeneration, including the whole nervous system. Thus, D. claparedii potentially can be regarded for the nervous system regeneration (NSR) study. However, data on the property of its nervous system and the NSR profile are still lacking. In this study, we investigated the morphology of D. claparedii anterior nervous system (ANS) and examined the cellular and molecular profiles on its early anterior NSR. The nervous system of D. claparedii consists of a symmetry brain with nerves branching off, circumpharyngeal connectives that connect the brain and nerve cord as well as obvious segmental ganglia. Moreover, we identified changes in the cellular condition of the ganglionic cells in the regenerating tissue, such as the accumulation of lysosomes and lipofuscins, elongated mitochondria and multiple nucleoli. Furthermore, mRNA of tissues at two regenerating stages, as well as intact tissue (non-regenerating), were sequenced with Illumina sequencer. We identified from these tissues 37,248 sequences, 18 differential expressed proteins of which upregulated were involved in NSR with noelin-like isoform X2 turned up to be the highest being expressed. Our results highlight the cellular and molecular changes during early phase of NSR, thus providing essential insights on regeneration within Annelida and understanding the neurodegenerative diseases.

Study of Natural Longlife Juvenility and Tissue Regeneration in Caudate Amphibians and Potential Application of Resulting Data in Biomedicine

The review considers the molecular, cellular, organismal, and ontogenetic properties of Urodela that exhibit the highest regenerative abilities among tetrapods. The genome specifics and the expression of genes associated with cell plasticity are analyzed. The simplification of tissue structure is shown using the examples of the sensory retina and brain in mature Urodela. Cells of these and some other tissues are ready to initiate proliferation and manifest the plasticity of their phenotype as well as the correct integration into the pre-existing or de novo forming tissue structure. Without excluding other factors that determine regeneration, the pedomorphosis and juvenile properties, identified on different levels of Urodele amphibians, are assumed to be the main explanation for their high regenerative abilities. These properties, being fundamental for tissue regeneration, have been lost by amniotes. Experiments aimed at mammalian cell rejuvenation currently use various approaches. They include, in particular, methods that use secretomes from regenerating tissues of caudate amphibians and fish for inducing regenerative responses of cells. Such an approach, along with those developed on the basis of knowledge about the molecular and genetic nature and age dependence of regeneration, may become one more step in the development of regenerative medicine.

A comprehensive reference transcriptome resource for the Iberian ribbed newt Pleurodeles waltl, an emerging model for developmental and regeneration biology

Urodele newts have unique biological properties, notably including prominent regeneration ability. The Iberian ribbed newt, Pleurodeles waltl, is a promising model amphibian distinguished by ease of breeding and efficient transgenic and genome editing methods. However, limited genetic information is available for P. waltl. We conducted an intensive transcriptome analysis of P. waltl using RNA-sequencing to build and annotate gene models. We generated 1.2 billion Illumina reads from a wide variety of samples across 12 different tissues/organs, unfertilized egg, and embryos at eight different developmental stages. These reads were assembled into 1,395,387 contigs, from which 202,788 non-redundant ORF models were constructed. The set is expected to cover a large fraction of P. waltl protein-coding genes, as confirmed by BUSCO analysis, where 98% of universal single-copy orthologs were identified. Ortholog analyses revealed the gene repertoire evolution of urodele amphibians. Using the gene set as a reference, gene network analysis identified regeneration-, developmental-stage-, and tissue-specific co-expressed gene modules. Our transcriptome resource is expected to enhance future research employing this emerging model animal for regeneration research as well as for investigations in other areas including developmental biology, stem cell biology, and cancer research. These data are available via our portal website, iNewt (http://www.nibb.ac.jp/imori/main/).

Lens regeneration: a historical perspective

The idea of regenerating injured body parts has captivated human imagination for centuries, and the topic still remains an area of extensive scientific research. This review focuses on the process of lens regeneration: its history, our current knowledge, and the questions that remain unanswered. By highlighting some of the milestones that have shaped our understanding of this phenomenon and the contributions of scientists who have dedicated their lives to investigating these questions, we explore how regeneration enquiry evolved into the science it is today, and how technological advances accelerated our understanding of these remarkable processes.

Behavior of Stem-Like Cells, Precursors for Tissue Regeneration in Urodela, Under Conditions of Microgravity

We summarize data from our experiments on stem-like cell-dependent regeneration in amphibians in microgravity. Considering its deleterious effect on many tissues, we asked whether microgravity is compatible with reparative processes, specifically activation and proliferation of source cells. Experiments were conducted using tailed amphibians, which combine profound regenerative capabilities with high robustness, allowing an in vivo study of lens, retina, limb, and tail regeneration in challenging settings of spaceflight. Microgravity promoted stem-like cell proliferation to a varying extent (up to 2-fold), and it seemed to speed up source cell dedifferentiation, as well as sequential differentiation in retina, lens, and limb, leading to formation of bigger and more developed regenerates than in 1g controls. It also promoted proliferation and hypertrophy of Müller glial cells, eliciting a response similar to reactive gliosis. A significant increase in stem-like cell proliferation was mostly beneficial for regeneration and only in rare cases caused moderate tissue growth abnormalities. It is important that microgravity yielded a lasting effect even if applied before operations. We hypothesize on the potential mechanisms of gravity-dependent changes in stem-like cell behavior, including fibroblast growth factor 2 signaling pathway and heat shock proteins, which were affected in our experimental settings. Taken together, our data indicate that microgravity does not disturb the natural regenerative potential of newt stem-like cells, and, depending on the system, even stimulates their dedifferentiation, proliferation, and differentiation. We discuss these data along with publications on mammalian stem cell behavior in vitro and invertebrate regeneration in vivo in microgravity. In vivo data are very scarce and require further research using contemporary methods of cell behavior analysis to elucidate mechanisms of stem cell response to altered gravity. They are relevant for both practical applications, such as managing human reparative responses in spaceflight, and fundamental understanding of stem cell biology.

Knockdown of Nucleostemin in an ovarian cancer SKOV-3 cell line and its effects on cell malignancy

Nucleostemin plays an essential role in the proliferation of some stem cells and cancer cells, but no research has been conducted to assess the link between Nucleostemin and ovarian cancer. To investigate the role of Nucleostemin in ovarian cancer tumorigenesis, we generated short hairpin (sh)RNA to knockdown the expression of the Nucleostemin gene in an ovarian cancer SKOV-3 cell line. We found that knockdown of this gene led to cell-cycle arrest, as well as to an increase in apoptosis. In addition, migration and invasion demonstrated significant inhibition in Nucleostemin-deficient cells. Furthermore, the knockdown of Nucleostemin dramatically suppressed xenograft progression in BALB/c nude mice. Our findings suggest that Nucleostemin is associated with malignancy in an ovarian cancer SKOV-3 cell line.

Cellular and Molecular Preconditions for Retinal Pigment Epithelium (RPE) Natural Reprogramming during Retinal Regeneration in Urodela

Many regeneration processes in animals are based on the phenomenon of cell reprogramming followed by proliferation and differentiation in a different specialization direction. An insight into what makes natural (in vivo) cell reprogramming possible can help to solve a number of biomedical problems. In particular, the first problem is to reveal the intrinsic properties of the cells that are necessary and sufficient for reprogramming; the second, to evaluate these properties and, on this basis, to reveal potential endogenous sources for cell substitution in damaged tissues; and the third, to use the acquired data for developing approaches to in vitro cell reprogramming in order to obtain a cell reserve for damaged tissue repair. Normal cells of the retinal pigment epithelium (RPE) in newts (Urodela) can change their specialization and transform into retinal neurons and ganglion cells (i.e., actualize their retinogenic potential). Therefore, they can serve as a model that provides the possibility to identify factors of the initial competence of vertebrate cells for reprogramming in vivo. This review deals mainly with the endogenous properties of native newt RPE cells themselves and, to a lesser extent, with exogenous mechanisms regulating the process of reprogramming, which are actively discussed.

Nucleostemin Knockdown Sensitizes Hepatocellular Carcinoma Cells to Ultraviolet and Serum Starvation-Induced Apoptosis

Nucleostemin (NS) is a GTP-binding protein that is predominantly expressed in embryonic and adult stem cells but not in terminally differentiated cells. NS plays an essential role in maintaining the continuous proliferation of stem cells and some types of cancer cells. However, the role of NS in hepatocellular carcinoma (HCC) remains unclear. Therefore, this study aimed to clarify the role of NS in HCC. First, we demonstrated high expression of NS in most HCC cell lines and liver cancer tissues. NS knockdown induced a severe decline in cell viability of MHCC97H cells as detected by MTT and cell proliferation assays. Next, we used ultraviolet (UV) and serum starvation-induced apoptosis models to investigate whether NS suppression or up-regulation affects HCC cell apoptosis. After UV treatment or serum starvation, apoptosis was strongly enhanced in MHCC97H and Bel7402 cells transfected with small interfering RNA against NS, whereas NS overexpression inhibited UV- and serum-induced apoptosis of HCC cells. Furthermore, after UV irradiation, inhibition of NS increased the expression of pro-apoptosis protein caspase 3 and decreased the expression of anti-apoptosis protein Bcl-2. A caspase 3 inhibitor could obviously prevent NS knockdown-induced apoptosis. In conclusion, our study demonstrated overexpression of NS in most HCC tissues compared with their matched surrounding tissues, and silencing NS promoted UV- and serum starvation-induced apoptosis of MHCC97H and Bel7402 cells. Therefore, the NS gene might be a potential therapeutic target of HCC.

Newt lens transdifferentiation: from lentectomy to immuno-FISH

Newt lens regeneration is achieved by a unique cellular regulation of transdifferentiation where the dorsal iris pigmented epithelial cells (PECs) dedifferentiate and redifferentiate into lens cells. Recent studies have shown that nuclear architecture of PECs is dynamically changed and unique epigenetic regulation in somatic nucleus is crucial in the lens transdifferentiation. Immuno-FISH, detection of protein and gene loci in nucleus, is one of the effective tools to analyze nuclear architecture of PECs. In this chapter a whole process from lentectomy to immuno-FISH is described.

Turning a new page on nucleostemin and self-renewal

A quintessential trait of stem cells is embedded in their ability to self-renew without incurring DNA damage as a result of genome replication. One key self-renewal factor is the nucleolar GTP-binding protein nucleostemin (also known as guanine-nucleotide-binding protein-like 3, GNL3, in invertebrate species). Several studies have recently pointed to an unexpected role of nucleostemin in safeguarding the genome integrity of stem and cancer cells. Since its discovery, the predominant presence of nucleostemin in the nucleolus has led to the notion that it might function in the card-carrying event of the nucleolus--the biogenesis of ribosomes. As tantalizing as this might be, a ribosomal role of nucleostemin is refuted by evidence from recent studies, which argues that nucleostemin depletion triggers a primary event of DNA damage in S phase cells that then leads to ribosomal perturbation. Furthermore, there have been conflicting reports regarding the p53 dependency of nucleostemin activity and the cell cycle arrest profile of nucleostemin-depleted cells. In this Commentary, I propose a model that explains how the many contradictory observations surrounding nucleostemin can be reconciled and suggest that this protein might not be as multi-tasking as has been previously perceived. The story of nucleostemin highlights the complexity of the underlying molecular events associated with the appearance of any cell biological phenotype and also signifies a new understanding of the genome maintenance program in stem cells.

Deer antler--a novel model for studying organ regeneration in mammals

Deer antler is the only mammalian organ that can fully grow back once lost from its pedicle - the base from which it grows. Therefore, antlers probably offer the most pertinent model for studying organ regeneration in mammals. This paper reviews our current understanding of the mechanisms underlying regeneration of antlers, and provides insights into the possible use for human regenerative medicine. Based on the definition, antler renewal belongs to a special type of regeneration termed epimorphic. However, histological examination failed to detect dedifferentiation of any cell type on the pedicle stump and the formation of a blastema, which are hallmark features of classic epimorphic regeneration. Instead, antler regeneration is achieved through the recruitment, proliferation and differentiation of the single cell type in the pedicle periosteum (PP). The PP cells are the direct derivatives of cells resident in the antlerogenic periosteum (AP), a tissue that exists in prepubertal deer calves and can induce ectopic antler formation when transplanted elsewhere on the deer body. Both the AP and PP cells express key embryonic stem cell markers and can be induced to differentiate into multiple cell lineages in vitro and, therefore, they are termed antler stem cells, and antler regeneration is a stem cell-based epimorphic regeneration. Comparisons between the healing process on the stumps from an amputated mouse limb and early regeneration of antlers suggest that the stump of a mouse limb cannot regenerate because of the limited potential of periosteal cells in long bones to proliferate. If we can impart a greater potential of these periosteal cells to proliferate, we might at least be able to partially regenerate limbs lost from humans. Taken together, a greater understanding of the mechanisms that regulate the regeneration of antlers may provide a valuable insight to aid the field of regenerative medicine. This article is part of a Directed Issue entitled: Regenerative Medicine: the challenge of translation.

Reprogramming of the chick retinal pigmented epithelium after retinal injury

Background

One of the promises in regenerative medicine is to regenerate or replace damaged tissues. The embryonic chick can regenerate its retina by transdifferentiation of the retinal pigmented epithelium (RPE) and by activation of stem/progenitor cells present in the ciliary margin. These two ways of regeneration occur concomitantly when an external source of fibroblast growth factor 2 (FGF2) is present after injury (retinectomy). During the process of transdifferentiation, the RPE loses its pigmentation and is reprogrammed to become neuroepithelium, which differentiates to reconstitute the different cell types of the neural retina. Somatic mammalian cells can be reprogrammed to become induced pluripotent stem cells by ectopic expression of pluripotency-inducing factors such as Oct4, Sox2, Klf4, c-Myc and in some cases Nanog and Lin-28. However, there is limited information concerning the expression of these factors during natural regenerative processes. Organisms that are able to regenerate their organs could share similar mechanisms and factors with the reprogramming process of somatic cells. Herein, we investigate the expression of pluripotency-inducing factors in the RPE after retinectomy (injury) and during transdifferentiation in the presence of FGF2.

Results

We present evidence that upon injury, the quiescent (p27^Kip1+/BrdU-) RPE cells transiently dedifferentiate and express sox2, c-myc and klf4 along with eye field transcriptional factors and display a differential up-regulation of alternative splice variants of pax6. However, this transient process of dedifferentiation is not sustained unless FGF2 is present. We have identified lin-28 as a downstream target of FGF2 during the process of retina regeneration. Moreover, we show that overexpression of lin-28 after retinectomy was sufficient to induce transdifferentiation of the RPE in the absence of FGF2.

Conclusion

These findings delineate in detail the molecular changes that take place in the RPE during the process of transdifferentiation in the embryonic chick, and specifically identify Lin-28 as an important factor in this process. We propose a novel model in which injury signals initiate RPE dedifferentiation, while FGF2 up-regulates Lin-28, allowing for RPE transdifferentiation to proceed.

A novel role of nucleostemin in maintaining the genome integrity of dividing hepatocytes during mouse liver development and regeneration

During liver development and regeneration, hepatocytes undergo rapid cell division and face an increased risk of DNA damage associated with active DNA replication. The mechanism that protects proliferating hepatocytes from replication-induced DNA damage remains unclear. Nucleostemin (NS) is known to be up-regulated during liver regeneration, and loss of NS is associated with increased DNA damage in cancer cells. To determine whether NS is involved in protecting the genome integrity of proliferating hepatocytes, we created an albumin promoter-driven NS conditional-null (albNS(cko) ) mouse model. Livers of albNS(cko) mice begin to show loss of NS in developing hepatocytes from the first postnatal week and increased DNA damage and hepatocellular injury at 1-2 weeks of age. At 3-4 weeks, albNS(cko) livers develop bile duct hyperplasia and show increased apoptotic cells, necrosis, regenerative nodules, and evidence suggestive of hepatic stem/progenitor cell activation. CCl4 treatment enhances degeneration and DNA damage in NS-deleted hepatocytes and increases biliary hyperplasia and A6(+) cells in albNS(cko) livers. After 70% partial hepatectomy, albNS(cko) livers show increased DNA damage in parallel with a blunted and prolonged regenerative response. The DNA damage in NS-depleted hepatocytes is explained by the impaired recruitment of a core DNA repair enzyme, RAD51, to replication-induced DNA damage foci.

CONCLUSION

This work reveals a novel genome-protective role of NS in developing and regenerating hepatocytes.

A comparative study of nucleostemin family members in zebrafish reveals specific roles in ribosome biogenesis

Nucleostemin (NS) is an essential protein for the growth and viability of developmental stem cells. Its functions are multi-faceted, including important roles in ribosome biogenesis and in the p53-induced apoptosis pathway. While NS has been well studied, the functions of its family members GNL2 and GNL3-like (GNL3L) remain relatively obscure despite a high degree of sequence and domain homology. Here, we use zebrafish lines carrying mutations in the ns family to compare and contrast their functions in vertebrates. We find the loss of zebrafish ns or gnl2 has a major impact on 60S large ribosomal subunit formation and/or function due to cleavage impairments at distinct sites of pre-rRNA transcript. In both cases this leads to a reduction of total protein synthesis. In contrast, gnl3l loss shows relatively minor rRNA processing delays that ultimately have no appreciable effects on ribosome biogenesis or protein synthesis. However, the loss of gnl3l still results in p53 stabilization, apoptosis, and lethality similarly to ns and gnl2 loss. The depletion of p53 in all three of the mutants led to partial rescues of the morphological phenotypes and surprisingly, a rescue of the 60S subunit collapse in the ns mutants. We show that this rescue is due to an unexpected effect of p53 loss that even in wild type embryos results in an increase of 60S subunits. Our study presents an in-depth description of the mechanisms through which ns and gnl2 function in vertebrate ribosome biogenesis and shows that despite the high degree of sequence and domain homology, gnl3l has critical functions in development that are unrelated to the ribosome.

Epigenetics and regeneration

During newt lens regeneration a unique transdifferentiation event occurs. In this process, dorsal iris pigmented epithelial cells transdifferentiate into lens cells. This system should provide a new insight into cellular plasticity in basic and applied research. Recently, a series of approaches to study epigenetic reprogramming during transdifferentiation have been performed. In this review, we introduce the regulation of dynamic regulation of core-histone modifications and the emergence of an oocyte-type linker histone during transdifferentiation. Finally, we show supporting evidence that there are common strategies of reprogramming between newt somatic cell in transdifferentiation and oocytes after somatic cell nuclear transfer.

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.

Regeneration and reprogramming

Recent reprogramming studies indicate that mammalian, somatic cells have the potential to achieve pluripotent states and undergo cell type switching. Such cellular traits are observed under natural conditions in animals that regenerate complex organs. A number of invertebrates display the amazing trait of whole body regeneration. Underlying this trait is the maintenance of pluripotent cells in somatic tissue, and molecular studies indicate the use of common players associated with pluripotency and germ cell properties between these invertebrates and mammalian pluripotent cells. In regenerative vertebrates, heart regeneration, lens regeneration, and retinal regeneration provide good examples of dedifferentiation and transdifferentiation. The molecular factors associated with these phenomena are discussed.

Deer antler regeneration: a stem cell-based epimorphic process

Full regeneration of deer antlers, a bona fide epimorphic process in mammals, is in defiance of the general rule of nature. Revealing the mechanism underlying this unique exception would place us in a better position to promote organ regeneration in humans. Antler regeneration takes place in yearly cycles from its pedicle, a permanent protuberance on the frontal bone. Both growing antlers and pedicles consist of internal (cartilage and bone) and external components (skin, blood vessels, and nerves). Recent studies have demonstrated that the regeneration of both internal and external components relies on the presence of pedicle periosteum (PP). PP cells express key embryonic stem cell markers (Oct4, Nanog, and SOX2) and are multipotent, so are termed antler stem cells. Now it is clear that proliferation and differentiation of PP cells directly forms internal antler components; however, how PP initiates and maintains the regeneration of external antler components is thus far not known. Based on the direct as well as indirect evidence that is presented in this review, I put forward the following hypothesis to address this issue. The full regenerative ability of external antler tissue components is achieved through PP-derived chemical induction and PP-derived mechanical stimulation: the former triggers the regeneration of these external components, whereas the latter drives their rapid elongation. Eventual identification of the putative PP-derived chemical factors would open up a new avenue for devising effective therapies for lesions involving each of these tissue components, be they traumatic, degenerative, or linked to developmental (genetic) anomalies.

Nucleostemin inhibits TRF1 dimerization and shortens its dynamic association with the telomere

TRF1 is a key component of the telomere-capping complex and binds double-strand telomeric DNA as homodimers. So far, it is not clear whether TRF1 dimerization coincides with its telomere binding or is actively controlled before it binds the telomere, and in the latter case, how this event might affect its telomere association. We previously found that TRF1 dimerization and its telomere binding can be increased by GNL3L, which is the vertebrate paralogue of nucleostemin (NS). Here, we show that NS and GNL3L bind TRF1 directly but competitively through two separate domains of TRF1. In contrast to GNL3L, NS prevents TRF1 dimerization through a mechanism not determined by its ability to displace TRF1-bound GNL3L. Furthermore, NS is capable of shortening the dynamic association of TRF1 with the telomere in normal and TRF2(ΔBΔM)-induced telomere-damaged cells without affecting the amount of telomere-bound TRF1 proteins in vivo. Importantly, NS displays a protective function against the formation of telomere-dysfunction-induced foci. This work demonstrates that TRF1 dimerization is actively and oppositely regulated by NS and GNL3L extrachromosomally. Changing the relative amount of TRF1 monomers versus dimers in the nucleoplasm might affect the dynamic association of TRF1 with the telomere and the repair of damaged telomeres.

Nucleolar stress is an early response to myocardial damage involving nucleolar proteins nucleostemin and nucleophosmin

Nucleolar stress, characterized by loss of nucleolar integrity, has not been described in the cardiac context. In addition to ribosome biogenesis, nucleoli are critical for control of cell proliferation and stress responses. Our group previously demonstrated induction of the nucleolar protein nucleostemin (NS) in response to cardiac pathological insult. NS interacts with nucleophosmin (NPM), a marker of nucleolar stress with cytoprotective properties. The dynamic behavior of NS and NPM reveal that nucleolar disruption is an early event associated with stress response in cardiac cells. Rapid translocation of NS and NPM to the nucleoplasm and suppression of new preribosomal RNA synthesis occurs in both neonatal rat cardiomyocytes (NRCM) and cardiac progenitor cells (CPC) upon exposure to doxorubicin or actinomycin D. Silencing of NS significantly increases cell death resulting from doxorubicin treatment in CPC, whereas NPM knockdown alone induces cell death. Overexpression of either NS or NPM significantly decreases caspase 8 activity in cultured cardiomyocytes challenged with doxorubicin. The presence of altered nucleolar structures resulting from myocardial infarction in mice supports the model of nucleolar stress as a general response to pathological injury. Collectively, these findings serve as the initial description of myocardial nucleolar stress and establish the postulate that nucleoli acts as sensors of stress, regulating the cellular response to pathological insults.

GNL3L depletion destabilizes MDM2 and induces p53-dependent G2/M arrest

Guanine nucleotide binding protein-like 3-like (GNL3L) is a nucleolar protein and the vertebrate paralogue of nucleostemin (NS). We previously reported that nucleoplasmic mobilization of NS stabilizes MDM2 (mouse double minute 2). Here, we investigated the role of GNL3L as a novel MDM2 regulator. We found that GNL3L binds MDM2 in vivo and displays the same function as NS in stabilizing MDM2 protein and preventing its ubiquitylation. The interaction between GNL3L and MDM2 also takes place in the nucleoplasm. However, the MDM2 regulatory activity of GNL3L occurs constitutively and does not so much depend on the nucleolar release mechanism as NS does. GNL3L depletion triggers G2/M arrest in the p53-wild-type HCT116 cells more than in the p53-null cells, and upregulates specific p53 targets (that is, Bax, 14-3-3σ and p21) without affecting the ubiquitylation or stability of p53 proteins. The inhibitory activity of GNL3L on p53-mediated transcription correlates with the increased expression of GNL3L and reduced expression of 14-3-3σ and p21 in human gastrointestinal tumors. This work shows that in contrast to most nucleolar proteins that negatively control MDM2, GNL3L and NS are the only two that are designed to stabilize MDM2 protein under basal or induced condition, respectively, and may act as tumor-promoting genes.

Molecular and cellular aspects of amphibian lens regeneration

Lens regeneration among vertebrates is basically restricted to some amphibians. The most notable cases are the ones that occur in premetamorphic frogs and in adult newts. Frogs and newts regenerate their lens in very different ways. In frogs the lens is regenerated by transdifferentiation of the cornea and is limited only to a time before metamorphosis. On the other hand, regeneration in newts is mediated by transdifferentiation of the pigment epithelial cells of the dorsal iris and is possible in adult animals as well. Thus, the study of both systems could provide important information about the process. Molecular tools have been developed in frogs and recently also in newts. Thus, the process has been studied at the molecular and cellular levels. A synthesis describing both systems was long due. In this review we describe the process in both Xenopus and the newt. The known molecular mechanisms are described and compared.

Oocyte-type linker histone B4 is required for transdifferentiation of somatic cells in vivo

The ability to reprogram in vivo a somatic cell after differentiation is quite limited. One of the most impressive examples of such a process is transdifferentiation of pigmented epithelial cells (PECs) to lens cells during lens regeneration in newts. However, very little is known of the molecular events that allow newt cells to transdifferentiate. Histone B4 is an oocyte-type linker histone that replaces the somatic-type linker histone H1 during reprogramming mediated by somatic cell nuclear transfer (SCNT). We found that B4 is expressed and required during transdifferentiation of PECs. Knocking down of B4 decreased proliferation and increased apoptosis, which resulted in considerable smaller lens. Furthermore, B4 knockdown altered gene expression of key genes of lens differentiation and nearly abolished expression of gamma-crystallin. These data are the first to show expression of oocyte-type linker histone in somatic cells and its requirement in newt lens transdifferentiation and suggest that transdifferentiation in newts might share common strategies with reprogramming after SCNT.

Vertebrates that regenerate as models for guiding stem cels

There are several animal model organisms that have the ability to regenerate severe injuries by stimulating local cells to restore damaged and lost organs and appendages. In this chapter, we will describe how various vertebrate animals regenerate different structures (central nervous system, heart and appendages) as well as detail specific cellular and molecular features concerning the regeneration of these structures.

Post-mitotic role of nucleostemin as a promoter of skeletal muscle cell differentiation

Nucleostemin (NS) is a nucleolar protein abundantly expressed in a variety of proliferating cells and undifferentiated cells. Its known functions include cell cycle regulation and the control of pre-rRNA processing. It also has been proposed that NS has an additional role in undifferentiated cells due to its downregulation during stem cell differentiation and its upregulation during tissue regeneration. Here, however, we demonstrate that skeletal muscle cell differentiation has a unique expression profile of NS in that it is continuously expressed during differentiation. NS was expressed at similar levels in non-proliferating muscle stem cells (satellite cells), rapidly proliferating precursor cells (myoblasts) and post-mitotic terminally differentiated cells (myotubes and myofibers). The sustained expression of NS during terminal differentiation is necessary to support increased protein synthesis during this process. Downregulation of NS inhibited differentiation of myoblasts to myotubes, accompanied by striking downregulation of key myogenic transcription factors, such as myogenin and MyoD. In contrast, upregulation of NS inhibited proliferation and promoted muscle differentiation in a p53-dependent manner. Our findings provide evidence that NS has an unexpected role in post-mitotic terminal differentiation. Importantly, these findings also indicate that, contrary to suggestions in the literature, the expression of NS cannot always be used as a reliable indicator for undifferentiated cells or proliferating cells.

Identification of tumor-initiating cells in a highly aggressive brain tumor using promoter activity of nucleostemin

Controversy remains over whether the cancer stem cell (CSC) theory applies to all tumors. To determine whether cells within a highly aggressive solid tumor are stochastically or hierarchically organized, we combined a reporter system where the nucleostemin (NS) promoter drives GFP expression (termed NS-GFP) with a mouse brain tumor model induced by retroviral Ras expression on a p16(Ink4a)/p19(Arf)-deficient background. The NS-GFP system allowed us to monitor the differentiation process of normal neural stem/precursor cells by analyzing GFP fluorescence intensity. In tumor-bearing mice, despite the very high frequency of tumorigenic cells, we successfully identified the NS-GFP(+) cells as tumor-initiating cells (T-ICs). The clonal studies conclusively established that phenotypical heterogeneity can exist among the cells comprising a genetically homogeneous tumor, suggesting that this aggressive brain tumor follows the CSC model. Detailed analyses of the NS-GFP(+) brain tumor cells revealed that T-ICs showed activation of the receptor tyrosine kinase c-Met, which functions in tumor invasiveness. Thus, the NS-GFP system provides a powerful tool to elucidate stem cell biology in normal and malignant tissues.

Expression of stem cell pluripotency factors during regeneration in newts

In this study, we present data indicating that mammalian stem cell pluripotency-inducing factors are expressed during lens and limb regeneration in newts. The apparent expression even in intact tissues and the ensued regulation during regeneration raises the possibility that these factors might regulate tissue-specific reprogramming and regeneration. Furthermore, these factors should enable us to understand the similarities and differences between animal regeneration in the newt and stem cell strategies in mammals. Developmental Dynamics 238:1613-1616, 2009. (c) 2009 Wiley-Liss, Inc.

Nucleostemin: a latecomer with new tricks

Nucleostemin was first identified in neural stem cells and has become a focus of research in cell cycle control, tumorigenesis and cellular senescence. As the biology of nucleostemin begins to be unveiled in multiple species, an ensuing task is to resolve the apparent differences between the functions of mammalian and invertebrate nucleostemin and its homologues, an issue of pressing interest given the role of nucleostemin in stem cell self-renewal and tissue regeneration. A genome-wide search reveals that nucleostemin and its closest homologue, GNL3L, only emerge as separate genes in vertebrates and possess conserved protein sequences as evolution proceeded to the Mammalia. The invertebrate orthologue of nucleostemin and GNL3L resembles GNL3L more than it does nucleostemin in function, raising the idea that nucleostemin acquires new properties while GNL3L inherits an evolutionarily fixed role, and that the birth of nucleostemin may signify the appearance of new functional features in the vertebrate lineage.

Identifying neural progenitor cells in the adult brain

There is incontrovertible evidence that neural progenitor cells (NPC) are found in the adult brain. The ability to identify and track NPC in the adult brain is of considerable importance if the properties of these cells are to be harnessed as potential therapies for degenerative brain disorders. The most commonly used approach of identifying these NPC in experimental studies, bromodeoxyuridine (BrdU) labelling, is outlined in this chapter. Immunohistochemical protocols for detecting endogenous and exogenous (introduced via transplantation) NPC in fresh-frozen and paraffin wax embedded brain tissue are described. Advice on how to label these NPC is also offered and multi-label fluorescence immunochemical staining approaches to determine the differentiation fate of NPC are described.

Comparison of molecular and cellular events during lower jaw regeneration of newt (Cynops pyrrhogaster) and West African clawed frog (Xenopus tropicalis)

When mammals, including humans, lose a major part of their lower jaw, they are unable to regenerate the lost structures. Urodele amphibians, especially newts, can regenerate their lower jaw after amputation, whereas most anuran amphibians, including the West African clawed frog, can not. In the present study, we investigated the difference between newts and frogs during lower jaw regeneration. One difference was the distribution of myosin heavy chain (MHC) mRNA after lower jaw amputation: MHC mRNA was immediately expressed at the tip of the amputated lower jaw in newts but not in frogs. Moreover, there were proliferating cells that expressed Pax7 in newts but not in frogs, although proliferating cells were present in both animals. These results suggest that the difference of the jaw-regenerating abilities between newts and frogs depends on the expression of MHC mRNA at the tip of the amputated jaw and the contribution of Pax7-positive cells.

Myocardial induction of nucleostemin in response to postnatal growth and pathological challenge

Stem cell-specific proteins and regulatory pathways that determine self-renewal and differentiation have become of fundamental importance in understanding regenerative and reparative processes in the myocardium. One such regulatory protein, named nucleostemin, has been studied in the context of stem cells and several cancer cell lines, where expression is associated with proliferation and maintenance of a primitive cellular phenotype. We find nucleostemin is present in young myocardium and is also induced following cardiomyopathic injury. Nucleostemin expression in cardiomyocytes is induced by fibroblast growth factor-2 and accumulates in response to Pim-1 kinase activity. Cardiac stem cells also express nucleostemin that is diminished in response to commitment to a differentiated phenotype. Overexpression of nucleostemin in cultured cardiac stem cells increases proliferation while preserving telomere length, providing a mechanistic basis for potential actions of nucleostemin in promotion of cell survival and proliferation as seen in other cell types.

Slicing across kingdoms: regeneration in plants and animals

Multicellular organisms possessing relatively long life spans are subjected to diverse, constant, and often intense intrinsic and extrinsic challenges to their survival. Animal and plant tissues wear out as part of normal physiological functions and can be lost to predators, disease, and injury. Both kingdoms survive this wide variety of insults by strategies that include the maintenance of adult stem cells or the induction of stem cell potential in differentiated cells. Repatterning mechanisms often deploy embryonic genes, but the question remains in both plants and animals whether regeneration invokes embryogenesis, generic patterning mechanisms, or unique circuitry comprised of well-established patterning genes.