Vasa and the germ line lineage in a colonial urochordate

In vitro detection of marine invertebrate stem cells: utilizing molecular and cellular biology techniques and exploring markers

Marine invertebrate stem cells (MISCs) represent a distinct category of pluripotent and totipotent cells with remarkable abilities for self-renewal and differentiation into multiple germ layers, akin to their vertebrate counterparts. These unique cells persist throughout an organism's adult life and have been observed in various adult marine invertebrate phyla. MISCs play crucial roles in numerous biological processes, including developmental biology phenomena specific to marine invertebrates, such as senescence, delayed senescence, whole-body regeneration, and asexual reproduction. Furthermore, they serve as valuable models for studying stem cell biology. Despite their significance, information about MISCs remains scarce and scattered in the scientific literature. In this review, we have carefully collected and summarized valuable information about MISC detection by perusing the articles that study and detect MISCs in various marine invertebrate organisms. The review begins by defining MISCs and highlighting their unique features compared to vertebrates. It then discusses the common markers for MISC detection and in vitro techniques employed in invertebrate and vertebrates investigation. This comprehensive review provides researchers and scientists with a cohesive and succinct overview of MISC characteristics, detection methods, and associated biological phenomena in marine invertebrate organisms. We aim to offer a valuable resource to researchers and scientists interested in marine invertebrate stem cells, fostering a better understanding of their broader implications in biology. With ongoing advancements in scientific techniques and the continued exploration of marine invertebrate species, we anticipate that further discoveries will expand our knowledge of MISCs and their broader implications in biology.

Germ Line/Multipotency Genes Show Differential Expression during Embryonic Development of the Annelid Enchytraeus coronatus

Germ line development and the origin of the primordial germ cells (PGCs) are very variable and may occur across a range of developmental stages and in several developmental contexts. In establishing and maintaining germ line, a conserved set of genes is involved. On the other hand, these genes are expressed in multipotent/pluripotent cells that may give rise to both somatic and germline cells. To begin elucidating mechanisms by which the germ line is specified in Enchytraeus coronatus embryos, we identified twenty germline/multipotency genes, homologs of Vasa, PL10, Piwi, Nanos, Myc, Pumilio, Tudor, Boule, and Bruno, using transcriptome analysis and gene cloning, and characterized their expression by whole-mount in situ hybridization. To answer the question of the possible origin of PGCs in this annelid, we carried out an additional description of the early embryogenesis. Our results suggest that PGCs derive from small cells originating at the first two divisions of the mesoteloblasts. PGCs form two cell clusters, undergo limited proliferation, and migrate to the developing gonadal segments. In embryos and juvenile E. coronatus, homologs of the germline/multipotency genes are differentially expressed in both germline and somatic tissue including the presumptive germ cell precursors, posterior growth zone, developing foregut, and nervous system.

Vasa, Piwi, and Pl10 Expression during Sexual Maturation and Asexual Reproduction in the Annelid Pristina longiseta

Naidids are tiny, transparent freshwater oligochaetes, which are well known for their ability to propagate asexually. Despite the fact that sexually mature individuals and cocoons with embryos are sometimes found in nature, in long-period laboratory cultures, worms reproduce agametically only. In this paper, we showed, for the first time, the expression of Vasa, Piwi, and Pl10 homologs in mature Pristina longiseta worms with well-developed reproductive system structures and germ cells. Although the animals have been propagated asexually by paratomic fission for over 20 years in our lab, some individuals become sexualized under standard conditions for our laboratory culture and demonstrate various stages of maturation. The fully matured animals developed a complete set of sexual apparatus including spermatheca, atrium, seminal vesicles, and ovisac. They also had a clitellum and were able to form cocoons. The cues for the initiation of sexual maturation are still unknown for P. longiseta; nevertheless, our data suggest that the laboratory strain of P. longiseta maintains the ability to become fully sexually mature and to establish germline products even after a long period of agametic reproduction. On the other hand, many of the sexualized worms formed a fission zone and continued to reproduce asexually. Thus, in this species, the processes of asexual reproduction and sexual maturation do not preclude each other, and Vasa, Piwi, and Pl10 homologs are expressed in both somatic and germline tissue including the posterior growth zone, fission zone, nervous system, germline cells, and gametes.

Improved Media Formulations for Primary Cell Cultures Derived from a Colonial Urochordate

The cultivation of marine invertebrate cells in vitro has garnered significant attention due to the availability of diverse cell types and cellular potentialities in comparison to vertebrates and particularly in response to the demand for a multitude of applications. While cells in the colonial urochordate Botryllus schlosseri have a very high potential for omnipotent differentiation, no proliferating cell line has been established in Botryllus, with results indicating that cell divisions cease 24-72 h post initiation. This research assessed how various Botryllus blood cell types respond to in vitro conditions by utilizing five different refinements of cell culture media (TGM1-TGM5). During the initial week of culture, there was a noticeable medium-dependent increase in the proliferation and viability of distinct blood cell types. Within less than one month from initiation, we developed medium-specific primary cultures, a discovery that supports larger efforts to develop cell type-specific cultures. Specific cell types were easily distinguished and classified based on their natural fluorescence properties using confocal microscopy. These results are in agreement with recent advances in marine invertebrate cell cultures, demonstrating the significance of optimized nutrient media for cell culture development and for cell selection.

The novel Orshina Rhythm in a colonial urochordate signifies the display of recurrent aging/rejuvenation sequels

When it comes to aging, some colonial invertebrates present disparate patterns from the customary aging phenomenon in unitary organisms, where a single senescence phenomenon along ontogeny culminates in their inevitable deaths. Here we studied aging processes in 81 colonies of the marine urochordate Botryllus schlosseri each followed from birth to death (over 720 days). The colonies were divided between three life history strategies, each distinct from the others based on the presence/absence of colonial fission: NF (no fission), FA (fission develops after the colony reaches maximal size), and FB (fission develops before the colony reaches maximal size). The study revealed recurring patterns in sexual reproductive statuses (hermaphroditism and male-only settings), colonial vigor, and size. These recurring patterns, collectively referred to as an Orshina, with one or more 'astogenic segments' on the genotype level. The combination of these segments forms the Orshina rhythm. Each Orshina segment lasts about three months (equivalent to 13 blastogenic cycles), and concludes with either the colonial death or rejuvenation, and is manipulated by absence/existing of fission events in NF/FA/FB strategies. These findings indicate that reproduction, life span, death, rejuvenation and fission events are important scheduled biological components in the constructed Orshina rhythm, a novel aging phenomenon.

Transcriptome landscapes that signify Botrylloides leachi (Ascidiacea) torpor states

Harsh environments enforce the expression of behavioural, morphological, physiological, and reproductive rejoinders, including torpor. Here we study the morphological, cellular, and molecular alterations in torpor architype in the colonial urochordate Botrylloides aff. leachii by employing whole organism Transmission electron (TEM) and light microscope observations, RNA sequencing, real-time polymerase chain reaction (qPCR) quantification of selected genes, and immunolocalization of WNT, SMAD and SOX2 gene expressions. On the morphological level, torpor starts with gradual regression of all zooids and buds which leaves the colony surviving as condensed vasculature remnants that may be 'aroused' to regenerate fully functional colonies upon changes in the environment. Simultaneously, we observed altered distributions of hemolymph cell types. Phagocytes doubled in number, while the number of morula cells declined by half. In addition, two new circulating cell types were observed, multi-nucleated and bacteria-bearing cells. RNA sequencing technology revealed marked differences in gene expression between different organism compartments and states: active zooids and ampullae, and between mid-torpor and naive colonies, or naive and torpid colonies. Gene Ontology term enrichment analyses further showed disparate biological processes. In torpid colonies, we observed overall 233 up regulated genes. These genes included NR4A2, EGR1, MUC5AC, HMCN2 and. Also, 27 transcription factors were upregulated in torpid colonies including ELK1, HDAC3, RBMX, MAZ, STAT1, STAT4 and STAT6. Interestingly, genes involved in developmental processes such as SPIRE1, RHOA, SOX11, WNT5A and SNX18 were also upregulated in torpid colonies. We further validated the dysregulation of 22 genes during torpor by utilizing qPCR. Immunohistochemistry of representative genes from three signaling pathways revealed high expression of these genes in circulated cells along torpor. WNT agonist administration resulted in early arousal from torpor in 80% of the torpid colonies while in active colonies WNT agonist triggered the torpor state. Abovementioned results thus connote unique transcriptome landscapes associated with Botrylloides leachii torpor.

The functional diversity of the POUV-class proteins across vertebrates

POUV is a relatively newly emerged class of POU transcription factors present in jawed vertebrates (Gnathostomata). The function of POUV-class proteins is inextricably linked to zygotic genome activation (ZGA). A large body of evidence now extends the role of these proteins to subsequent developmental stages. While some functions resemble those of other POU-class proteins and are related to neuroectoderm development, others have emerged de novo. The most notable of the latter functions is pluripotency control by Oct4 in mammals. In this review, we focus on these de novo functions in the best-studied species harbouring POUV proteins-zebrafish, Xenopus (anamniotes) and mammals (amniotes). Despite the broad diversity of their biological functions in vertebrates, POUV proteins exert a common feature related to their role in safeguarding the undifferentiated state of cells. Here we summarize numerous pieces of evidence for these specific functions of the POUV-class proteins and recap available loss-of-function data.

A pan-metazoan concept for adult stem cells: the wobbling Penrose landscape

Adult stem cells (ASCs) in vertebrates and model invertebrates (e.g. Drosophila melanogaster) are typically long-lived, lineage-restricted, clonogenic and quiescent cells with somatic descendants and tissue/organ-restricted activities. Such ASCs are mostly rare, morphologically undifferentiated, and undergo asymmetric cell division. Characterized by 'stemness' gene expression, they can regulate tissue/organ homeostasis, repair and regeneration. By contrast, analysis of other animal phyla shows that ASCs emerge at different life stages, present both differentiated and undifferentiated phenotypes, and may possess amoeboid movement. Usually pluri/totipotent, they may express germ-cell markers, but often lack germ-line sequestering, and typically do not reside in discrete niches. ASCs may constitute up to 40% of animal cells, and participate in a range of biological phenomena, from whole-body regeneration, dormancy, and agametic asexual reproduction, to indeterminate growth. They are considered legitimate units of selection. Conceptualizing this divergence, we present an alternative stemness metaphor to the Waddington landscape: the 'wobbling Penrose' landscape. Here, totipotent ASCs adopt ascending/descending courses of an 'Escherian stairwell', in a lifelong totipotency pathway. ASCs may also travel along lower stemness echelons to reach fully differentiated states. However, from any starting state, cells can change their stemness status, underscoring their dynamic cellular potencies. Thus, vertebrate ASCs may reflect just one metazoan ASC archetype.

Articulating the "stem cell niche" paradigm through the lens of non-model aquatic invertebrates

Stem cells (SCs) in vertebrates typically reside in "stem cell niches" (SCNs), morphologically restricted tissue microenvironments that are important for SC survival and proliferation. SCNs are broadly defined by properties including physical location, but in contrast to vertebrates and other "model" organisms, aquatic invertebrate SCs do not have clearly documented niche outlines or properties. Life strategies such as regeneration or asexual reproduction may have conditioned the niche architectural variability in aquatic or marine animal groups. By both establishing the invertebrates SCNs as independent types, yet allowing inclusiveness among them, the comparative analysis will allow the future functional characterization of SCNs.

Studying Tunicata WBR Using Botrylloides anceps

Tunicates are marine filter-feeding invertebrates that can be found worldwide and which are the closest phylogenetic group to the vertebrates (Craniata). Of particular interest, colonial tunicates are the only known chordates that can undergo Whole-Body Regeneration (WBR) via vascular budding. In Botrylloides anceps, a fully functional adult regenerates from a fragment of the vascular system in around 2 weeks after amputation. In this chapter, we present protocols to collect B. anceps colonies, confirm their species, breed them in the lab, monitor WBR and perform histological staining on cryosections.

Stem cells of aquatic invertebrates as an advanced tool for assessing ecotoxicological impacts

Environmental stressors are assessed through methods that quantify their impacts on a wide range of metrics including species density, growth rates, reproduction, behaviour and physiology, as on host-pathogen interactions and immunocompetence. Environmental stress may induce additional sublethal effects, like mutations and epigenetic signatures affecting offspring via germline mediated transgenerational inheritance, shaping phenotypic plasticity, increasing disease susceptibility, tissue pathologies, changes in social behaviour and biological invasions. The growing diversity of pollutants released into aquatic environments requires the development of a reliable, standardised and 3R (replacement, reduction and refinement of animals in research) compliant in vitro toolbox. The tools have to be in line with REACH regulation 1907/2006/EC, aiming to improve strategies for potential ecotoxicological risks assessment and monitoring of chemicals threatening human health and aquatic environments. Aquatic invertebrates' adult stem cells (ASCs) are numerous and can be pluripotent, as illustrated by high regeneration ability documented in many of these taxa. This is of further importance as in many aquatic invertebrate taxa, ASCs are able to differentiate into germ cells. Here we propose that ASCs from key aquatic invertebrates may be harnessed for applicable and standardised new tests in ecotoxicology. As part of this approach, a battery of modern techniques and endpoints are proposed to be tested for their ability to correctly identify environmental stresses posed by emerging contaminants in aquatic environments. Consequently, we briefly describe the current status of the available toxicity testing and biota-based monitoring strategies in aquatic environmental ecotoxicology and highlight some of the associated open issues such as replicability, consistency and reliability in the outcomes, for understanding and assessing the impacts of various chemicals on organisms and on the entire aquatic environment. Following this, we describe the benefits of aquatic invertebrate ASC-based tools for better addressing ecotoxicological questions, along with the current obstacles and possible overhaul approaches.

Sexual and asexual development: two distinct programs producing the same tunicate

Colonial tunicates are the only chordate that possess two distinct developmental pathways to produce an adult body: either sexually through embryogenesis or asexually through a stem cell-mediated renewal termed blastogenesis. Using the colonial tunicate Botryllus schlosseri, we combine transcriptomics and microscopy to build an atlas of the molecular and morphological signatures at each developmental stage for both pathways. The general molecular profiles of these processes are largely distinct. However, the relative timing of organogenesis and ordering of tissue-specific gene expression are conserved. By comparing the developmental pathways of B. schlosseri with other chordates, we identify hundreds of putative transcription factors with conserved temporal expression. Our findings demonstrate that convergent morphology need not imply convergent molecular mechanisms but that it showcases the importance that tissue-specific stem cells and transcription factors play in producing the same mature body through different pathways.

What is the role of PIWI family proteins in adult pluripotent stem cells? Insights from asexually reproducing animals, planarians

Planarians have a remarkable regenerative ability owing to their adult pluripotent stem cells (aPSCs), which are called "neoblasts." Planarians maintain a considerable number of neoblasts throughout their adulthood to supply differentiated cells for the maintenance of tissue homeostasis and asexual reproduction (fission followed by regeneration). Thus, planarians serve as a good model to study the regulatory mechanisms of in vivo aPSCs. In asexually reproducing invertebrates, such as sponge, Hydra, and planaria, piwi family genes are the markers most commonly expressed in aPSCs. While piwi family genes are known as guardians against transposable elements in the germline cells of animals that only sexually propagate, their functions in the aPSC system have remained elusive. In this review, we introduce recent knowledge on the PIWI family proteins in the aPSC system in planarians and other organisms and discuss how PIWI family proteins contribute to the regulation of the aPSC system.

UV-B radiation bearings on ephemeral soma in the shallow water tunicate Botryllus schlosseri

Sedentary shallow water marine organisms acquire numerous protective mechanisms to mitigate the detrimental effects of UV radiation (UV-R). Here we investigated morphological and gene expression outcomes in colonies of the cosmopolitan ascidian Botryllus schlosseri, up to 15-days post UV-B irradiation. Astogeny in Botryllus is characterized by weekly repeating sets of asexual budding, coinciding with apoptotic elimination of functional zooids (blastogenesis). Ten UV-B doses were administered to three clusters: sublethal, enhanced-mortality, lethal (LD₅₀ = 6.048 kJ/m²) which differed in mortality rates, yet reflected similar distorted morphotypes, and arrested blastogenesis, all intensified in the enhanced-mortality/lethal clusters. Even the sub-lethal doses inflicted expression modifications in 8 stress proteins (HSP 90/70 families and NIMA) as well as morphological blastogenesis. The morphological/gene-expression impacts in surviving colonies lasted for 15 days post irradiation (two blastogenic-cycles), where all damaged and arrested zooids/buds were absorbed, after which the colonies returned to their normal blastogenic-cycles and gene expression profiles, and initiated new buds. The above reflects a novel colonial maintenance strategy associated with the disposable-soma tenet, where the ephemeral soma in Botryllus is eliminated without engaging with the costs of repair, whereas other colonial components, primarily the pool of totipotent stem cells, are sustained under yet unknown colonial-level regulatory cues.

Immunohistochemical Detection of Vasa Antigen and Apoptosis-Related DNA Fragmentation in Ovaries of Sheep Fetuses Prenatally Exposed to Vitamin D Deficiency

The primordial germ cells (PGCs) in female animals are comprised of diplotene oocytes arrested in the first meiotic prophase. Expression of Vasa is one of the key factors required for subsequent resumption of development and recruitment of PGCs into the growing follicle class. Since vitamin D regulates recruitment of PGCs and developmental competence of ovarian follicles, this study was designed to investigate the expression of Vasa and rate of apoptosis in foetal ovaries prenatally restricted from dietary vitamin D. Nineteen sexually mature Welsh mountain ewes were randomly assigned to vitamin D deficient (VDD) and vitamin D control (VDC) diets from 17d before mating, up to 125d of gestation, when fetal ovaries were collected and fixed in formalin for immunohistochemistry and TUNEL assay. VDD ovaries had fewer healthy oocytes that could stain positive for Vasa as well as a lower integrated density value for DAB staining intensity. Conversely, TUNNEL staining in VDD animals showed a higher integrated density value and percentage of affected area (P<0.05). The present findings indicate that Vasa expression is decreased, while the rate of apoptosis increased in VDD fetal ovaries, and this may adversely affect resumption of growth and development of PGCs reserve.

IAP genes partake weighty roles in the astogeny and whole body regeneration in the colonial urochordate Botryllus schlosseri

Inhibitors of Apoptosis Protein (IAP) genes participate in processes like apoptosis, proliferation, innate immunity, inflammation, cell motility, differentiation and in malignancies. Here we reveal 25 IAP genes in the tunicate Botryllus schlosseri's genome and their functions in two developmental biology phenomena, a new mode of whole body regeneration (WBR) induced by budectomy, and blastogenesis, the four-staged cycles of botryllid ascidian astogeny. IAP genes that were specifically upregulated during these developmental phenomena were identified, and protein expression patterns of one of these genes, IAP28, were followed. Most of the IAP genes upregulation recorded at blastogenetic stages C/D was in concert with the upregulation at 100 μM H₂O₂ apoptotic-induced treatment and in parallel to expressions of AIF1, Bax, Mcl1, caspase 2 and two orthologues of caspase 7. Wnt agonist altered the takeover duration along with reduced IAP expressions, and displacement of IAP28⁺ phagocytes. WBR was initiated solely at blastogenetic stage D, where zooidal absorption was attenuated and regeneration centers were formed either from remains of partially absorbed zooids or from deformed ampullae. Subsequently, bud-bearing zooids developed, in concert with a massive IAP28-dependent phagocytic wave that eliminated the old zooids, then proceeded with the establishment of morphologically normal-looking colonies. IAP4, IAP14 and IAP28 were also involved in WBR, in conjunction with the expression of the pro-survival PI3K-Akt pathway. IAPs function deregulation by Smac mimetics resulted in severe morphological damages, attenuation in bud growth and differentiation, and in destabilization of colonial coordination. Longtime knockdown of IAP functions prior to the budectomy, resulted in colonial death.

Characterization and localization of primordial germ cells in Totoaba macdonaldi

The totoaba, Totoaba macdonaldi, is an endangered fish of the Gulf of California with high economic and ecological potential. Therefore, our purpose was to characterize the Primordial Germ Cells (PGCs) of this Sciaenid with two objectives: (1) to provide the basis for PGCs cryopreservation to preserve the genetic resources and (2) to take the first step to know the gonadal genesis and sex differentiation of totoaba. Immunofluorescence analysis performed from 2-cell stage to 8-day after hatch (DAH) shows that Vasa protein is specific for PGCs. These cells were first observed in the peripheral and dorsal regions of the blastodisc at approximately the 50%-epiboly stage and migrated to both sides of embryo body during the development. Finally, at 7 DAH the PGCs of the hatching embryo reached the place where the gonad will be developed. Histology analysis of larvae showed a genital ridge with enclosed PGCs on the dorsal side of the peritoneum at 9 DAH, gonadal primordium growth was observed at 11 DAH as a result of the interaction between PGCs and somatic cells derived from the peritoneum. Results of qPCR showed that vasa expression was restricted to the embryonic and early larval development, highest values were observed in 2-cell and mid-blastula stage suggesting the maternal inheritance of vasa mRNA. These findings support the hypothesis of preformation in T. macdonaldi PGCs. The migration pattern of PGCs allow us to recommend the isolation and subsequent cryopreservation of these cells before 7 DAH when the embryonic and larval development is given at 21 °C.

Effects of temperature and salinity stress on DNA methylation in a highly invasive marine invertebrate, the colonial ascidian Didemnum vexillum

Environmentally induced epigenetic changes may contribute to phenotypic plasticity, increase adaptive potential in changing environments, and play a key role in the establishment and spread of invasive species in new habitats. In this study, we used methylation-sensitive amplified polymorphism (MSAP) to assess environmentally induced DNA methylation changes in a globally invasive clonal ascidian, Didemnum vexillum. We tested the effect of increasing temperature (19, 25 and 27 °C) and decreasing salinity (34, 32, 30, 28 and 26 practical salinity units (PSU)) on global DNA methylation, growth and survival rates. Exposure to 27 °C resulted in significant changes in DNA methylation over time. Growth also decreased in colonies exposed to high temperatures, suggesting they were under thermal stress. In contrast, no differences in growth nor DNA methylation patterns were observed in colonies exposed to a decreasing salinity gradient, potentially due to prior adaptation. The results of this study show that environmental stress can induce significant global DNA methylation changes in an invasive marine invertebrate on very rapid timescales, and that this response varies depending on the type, magnitude, and duration of the stressor. Changes in genomic DNA methylation and the rate of growth may act to ‘buy survival time’ under stressful conditions, expanding the distribution limits of this globally invasive species.

Whole-Body Regeneration in the Colonial Tunicate Botrylloides leachii

The colonial marine invertebrate Botrylloides leachii belongs to the Tunicata subphylum, the closest invertebrate relatives to the vertebrate group and the only known class of chordates that can undergo whole-body regeneration (WBR). This dramatic developmental process allows a minute isolated fragment of B. leachii's vascular system, or a colony excised of all adults, to restore a functional animal in as little as 10 days. In addition to this exceptional regenerative capacity, B. leachii can reproduce both sexually, through a tadpole larval stage, and asexually, through palleal budding. Thus, three alternative developmental strategies lead to the establishment of filter-feeding adults. Consequently, B. leachii is particularly well suited for comparative studies on regeneration and should provide novel insights into regenerative processes in chordates.Here, after a short introduction on regeneration, we overview the biology of B. leachii as well as the current state of knowledge on WBR in this species and in related species of tunicates. Finally, we highlight the possible future directions that research might take in the study of WBR, including thoughts on technological approaches that appear most promising in this context. Overall, we provide a synthesis of the current knowledge on WBR in B. leachii to support research in this chordate species.

Gonad development and hermaphroditism in the ascidian Botryllus schlosseri

The colonial ascidian Botryllus schlosseri is an ideal model organism for studying gonad development and hermaphroditism. B. schlosseri has been reared in laboratories for over half a century, and its unique biology allows investigators to probe the processes of germ cell migration and gonad formation, resorption, and regeneration. Following metamorphosis, colonies of B. schlosseri show a synchronized and sequential fertility program that, under standard laboratory conditions, begins with a juvenile stage with no visible gonads and subsequently develops testes at 9 weeks followed later by the production of oocytes-thus resulting in hermaphroditic individuals. The timing of oocyte production varies according to the season, and adult B. schlosseri colonies can cycle among infertile and both male and hermaphrodite fertile states in response to changing environmental conditions. Thus, these acidians are amenable to studying the molecular mechanisms controlling fertility, and recent genomic and transcriptomic databases are providing insight to the key genes involved. Here, we review the techniques and approaches developed to study germ cell migration and gonad formation in B. schlosseri, and include novel videos showing processes related to oocyte ovulation and sperm discharge. In the future, this valuable invertebrate model system may help understand the mechanisms of gonad development and regeneration in a chordate. Mol. Reprod. Dev. 84: 158-170, 2017. © 2016 Wiley Periodicals, Inc.

Secrets from immortal worms: What can we learn about biological ageing from the planarian model system?

Understanding how some animals are immortal and avoid the ageing process is important. We currently know very little about how they achieve this. Research with genetic model systems has revealed the existence of conserved genetic pathways and molecular processes that affect longevity. Most of these established model organisms have relatively short lifespans. Here we consider the use of planarians, with an immortal life-history that is able to entirely avoid the ageing process. These animals are capable of profound feats of regeneration fueled by a population of adult stem cells called neoblasts. These cells are capable of indefinite self-renewal that has underpinned the evolution of animals that reproduce only by fission, having disposed of the germline, and must therefore be somatically immortal and avoid the ageing process. How they do this is only now starting to be understood. Here we suggest that the evidence so far supports the hypothesis that the lack of ageing is an emergent property of both being highly regenerative and the evolution of highly effective mechanisms for ensuring genome stability in the neoblast stem cell population. The details of these mechanisms could prove to be very informative in understanding how the causes of ageing can be avoided, slowed or even reversed.

Mechanisms of Vertebrate Germ Cell Determination

Two unique characteristics of the germ line are the ability to persist from generation to generation and to retain full developmental potential while differentiating into gametes. How the germ line is specified that allows it to retain these characteristics within the context of a developing embryo remains unknown and is one focus of current research. Germ cell specification proceeds through one of two basic mechanisms: cell autonomous or inductive. Here, we discuss how germ plasm driven germ cell specification (cell autonomous) occurs in both zebrafish and the frog Xenopus. We describe the segregation of germ cells during embryonic development of solitary and colonial ascidians to provide an evolutionary context to both mechanisms. We conclude with a discussion of the inductive mechanism as exemplified by both the mouse and axolotl model systems. Regardless of mechanism, several general themes can be recognized including the essential role of repression and posttranscriptional regulation of gene expression.

Expression Pattern of Mouse Vasa Homologue (MVH) in the Ovaries of C57BL/6 Female Mice

Background

Vasa (a DEAD-box helicase, also known as Ddx4) is an ATP-dependent RNA helicase highly conserved among all animals. Research on the presence and function of DDX4 in female mammals is limited. To gain greater insight into its distribution and role in female mice, we detected the expression of DDX4 protein in the ovaries and analyzed its expression pattern.

Material/Methods

MVH was detected in the cytoplasm of oocytes in all non-apoptotic follicles.

Results

In the present study, we found that higher expression levels of ~55–60 kDa MVH isoform in the ovaries were followed by the accumulations of preovulatory follicles.

Conclusions

Higher levels of MVH protein in the ovaries might prepare oocytes for the competence to resume meiosis.

Population genetics features for persistent, but transient, Botryllus schlosseri (Urochordata) congregations in a central Californian marina

The colonial tunicate Botryllus schlosseri is a globally distributed, invasive ascidian that has colonized the Californian coasts of the USA during the mid-late 1940s and has, since the late 1980s, spread north to Washington. This study analyzes the population genetic characteristics of transient populations residing at the Elkhorn Yacht-Club (EYC), in central California (seven sessions, 1996-2008), which suffered periodic catastrophes caused by episodic fresh-water floods and a single sampling session (in the year 2001) of five West-Coast populations using the mtDNA COI gene and five microsatellite markers. EYC microsatellite results were further compared with the closely situated but persistent population of the Santa Cruz Harbor (SCH) to understand the impact on EYC population regeneration processes after the 2005-flood catastrophe. All microsatellites were highly polymorphic, revealing a large number of unique alleles at different sampling dates. Whereas pairwise θ did not reveal significant differences between the EYC time-series samplings, the overall θ was significant, as it was between all the 2001 West Coast populations. The most likely cluster number was 3 for the EYC samples whereas two K values were obtained (2 and 5) for the 2001 samples. Tajima's D and Fu's/Fs tests did not reject the null hypothesis for COI neutral evolution, except for in the EYC-2000, 2007 and two 2001 samplings. The wide geographical range of the analyses has indicated that following the EYC 2005-flood catastrophe, newcomers could have originated from neighboring populations, from deep-water colonies that may have escaped the 2005 low salinity event, or less expectedly, from far away West-Coast populations, while revealing that the SCH population is the most probable source for the EYC population.

Temporally and spatially dynamic germ cell niches in Botryllus schlosseri revealed by expression of a TGF-beta family ligand and vasa

Background

Germ cells are specified during early development and are responsible for generating gametes in the adult. After germ cells are specified, they typically migrate to a particular niche in the organism where they reside for the remainder of its lifetime. For some model organisms, the specification and migration of germ cells have been extensively studied, but how these events occur in animals that reproduce both sexually and asexually is not well understood.

Results

We have identified a novel TGF-β family member in Botryllus schlosseri, tgfβ-f, and found that it is expressed by follicle cell progenitors and the differentiated follicle and support cells surrounding the maturing gametes. Using the expression of tgfβ-f and the germ cell marker vasa, we have found that nearly all germ cells in Botryllus are associated with tgfβ-f-expressing follicle progenitors in clusters consisting solely of those two cell types. These clusters were mostly small, consisting of ten or fewer cells, and generally contained between a 2:1 and 1:1 ratio of follicle progenitors to germ cells. Clusters of germ and follicle progenitor cells were primarily localized to niches in the primary and secondary buds, but could also be found in other locations including the vasculature. We analyzed the location of germ cell clusters throughout the asexual life cycle of Botryllus and found that at the stage when germ cells are first detected in the secondary bud niche, a dramatic change in the size and location of germ/follicle cell clusters also occurred.

Conclusions

Our findings suggest that germ/follicle cell clusters have predictable migratory patterns during the weekly asexual developmental cycle in Botryllus. An increased number of small clusters and the presence of clusters in the vasculature coinciding with the appearance of clusters in the secondary bud suggest that fragmentation of clusters and the migration of smaller clusters through the vasculature may be an important aspect of Botryllus reproductive biology, ensuring the transmission of the germline to subsequent asexual generations.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-016-0047-5) contains supplementary material, which is available to authorized users.

Simultaneous exposure to estrogen and androgen resulted in feminization and endocrine disruption

Estrogen, which is synthesized earlier in females than androgen in males, is critical for sex determination in non-mammalian vertebrates. However, it remains unknown that what would happen to the gonadal phenotype if estrogen and androgen were administrated simultaneously. In this study, XY and XX tilapia fry were treated with the same dose of 17α-methyltestosterone (MT) and 17β-estradiol (E2) alone and in combination from 0 to 30 days after hatching. Treatment of XY fish with E2 resulted in male to female sex reversal, while treatment of XX fish with MT resulted in female to male sex reversal. In contrast, simultaneous treatment of XX and XY fish with MT and E2 resulted in female, but with cyp11b2 and cyp19a1a co-expressed in the ovary. Serum 11-ketotestosteron level of the MT and E2 simultaneously treated XX and XY female was similar to that of the XY control, while serum E2 level of these two groups was similar to that of the XX control. Transcriptomic cluster analysis revealed that the MT and E2 treated XX and XY gonads clustered into the same branch with the XX control. However a small fraction of genes, which showed disordered expression, may be associated with stress response. These results demonstrated that estrogen could maintain the female phenotype of XX fish and feminize XY fish even in the presence of androgen. Simultaneous treatment with estrogen and androgen up-regulated the endogenous estrogen and androgen synthesis, and resulted in disordered gene expression and endocrine disruption in tilapia.

A piece of the pi(e): The diverse roles of animal piRNAs and their PIWI partners

Small non-coding RNAs are indispensable to many biological processes. A class of endogenous small RNAs, termed PIWI-interacting RNAs (piRNAs) because of their association with PIWI proteins, has known roles in safeguarding the genome against inordinate transposon mobilization, embryonic development, and stem cell regulation, among others. This review discusses the biogenesis of animal piRNAs and their diverse functions together with their PIWI protein partners, both in the germline and in somatic cells, and highlights the evolutionarily conserved aspects of these molecular players in animal biology.

Distal Regeneration Involves the Age Dependent Activity of Branchial Sac Stem Cells in the Ascidian Ciona intestinalis

Tunicates have high capacities for regeneration but the underlying mechanisms and their relationship to life cycle progression are not well understood. Here we investigate the regeneration of distal structures in the ascidian tunicate Ciona intestinalis. Analysis of regenerative potential along the proximal−distal body axis indicated that distal organs, such as the siphons, their pigmented sensory organs, and the neural complex, could only be replaced from body fragments containing the branchial sac. Distal regeneration involves the formation of a blastema composed of cells that undergo cell proliferation prior to differentiation and cells that differentiate without cell proliferation. Both cell types originate in the branchial sac and appear in the blastema at different times after distal injury. Whereas the branchial sac stem cells are present in young animals, they are depleted in old animals that have lost their regeneration capacity. Thus Ciona adults contain a population of age‐related stem cells located in the branchial sac that are a source of precursors for distal body regeneration.

Whole-mount fluorescent in situ hybridization staining of the colonial tunicate Botryllus schlosseri

Botryllus schlosseri is a colonial ascidian with characteristics that make it an attractive model for studying immunology, stem cell biology, evolutionary biology, and regeneration. Transcriptome sequencing and the recent completion of a draft genome sequence for B. schlosseri have revealed a large number of genes, both with and without vertebrate homologs, but analyzing the spatial and temporal expression of these genes in situ has remained a challenge. Here, we report a robust protocol for in situ hybridization that enables the simultaneous detection of multiple transcripts in whole adult B. schlosseri using Tyramide Signal Amplification in conjunction with digoxigenin- and dinitrophenol-labeled RNA probes. Using this protocol, we have identified a number of genes that can serve as markers for developing and mature structures in B. schlosseri, permitting analysis of phenotypes induced in loss-of-function experiments.

Ontology for the asexual development and anatomy of the colonial chordate Botryllus schlosseri

Ontologies provide an important resource to integrate information. For developmental biology and comparative anatomy studies, ontologies of a species are used to formalize and annotate data that are related to anatomical structures, their lineage and timing of development. Here, we have constructed the first ontology for anatomy and asexual development (blastogenesis) of a bilaterian, the colonial tunicate Botryllus schlosseri. Tunicates, like Botryllus schlosseri, are non-vertebrates and the only chordate taxon species that reproduce both sexually and asexually. Their tadpole larval stage possesses structures characteristic of all chordates, i.e. a notochord, a dorsal neural tube, and gill slits. Larvae settle and metamorphose into individuals that are either solitary or colonial. The latter reproduce both sexually and asexually and these two reproductive modes lead to essentially the same adult body plan. The Botryllus schlosseri Ontology of Development and Anatomy (BODA) will facilitate the comparison between both types of development. BODA uses the rules defined by the Open Biomedical Ontologies Foundry. It is based on studies that investigate the anatomy, blastogenesis and regeneration of this organism. BODA features allow the users to easily search and identify anatomical structures in the colony, to define the developmental stage, and to follow the morphogenetic events of a tissue and/or organ of interest throughout asexual development. We invite the scientific community to use this resource as a reference for the anatomy and developmental ontology of B. schlosseri and encourage recommendations for updates and improvements.

Germ lineage properties in the urochordate Botryllus schlosseri - from markers to temporal niches

The primordial germ cells (PGCs) in the colonial urochordate Botryllus schlosseri are sequestered in late embryonic stage. PGC-like populations, located at any blastogenic stage in specific niches, inside modules with curtailed lifespan, survive throughout the life of the colony by repeated weekly migration to newly formed buds. This cyclical migration and the lack of specific markers for PGC-like populations are obstacles to the study on PGCs. For that purpose, we isolated the Botryllus DDX1 (BS-DDX1) and characterized it by normal expression patterns and by specific siRNA knockdown experiments. Expression of BS-DDX1 concurrent with BS-Vasa, γ-H2AX, BS-cadherin and phospho-Smad1/5/8, demarcate PGC cells from soma cells and from more differentiated germ cells lineages, which enabled the detection of additional putative transient niches in zooids. Employing BS-cadherin siRNA knockdown, retinoic acid (RA) administration or β-estradiol administration affirmed the BS-Vasa(+)BS-DDX1(+)BS-cadherin(+)γ-H2AX(+)phospho-Smad1/5/8(+) population as the B. schlosseri PGC-like cells. By striving to understand the PGC-like cells trafficking between transient niches along blastogenic cycles, CM-DiI-stained PGC-like enriched populations from late blastogenic stage D zooids were injected into genetically matched colonial ramets at blastogenic stages A or C and their fates were observed for 9 days. Based on the accumulated data, we conceived a novel network of several transient and short lived 'germ line niches' that preserve PGCs homeostasis, protecting these cells from the weekly astogenic senescence processes, thus enabling the survival of the PGCs throughout the organism's life.

The DEAD-box helicase Vasa: evidence for a multiplicity of functions in RNA processes and developmental biology

DEAD-box helicases related to the Drosophila protein Vasa (also known as Ddx4) are found throughout the animal kingdom. They have been linked to numerous processes in gametogenesis, germ cell specification, and stem cell biology, and alterations in Vasa expression are associated with malignancy of tumor cells and with some human male infertility syndromes. Experimental results indicating how Vasa contributes to all these different cellular and developmental processes are discussed, using examples from planarians, Caenorhabditis elegans, Drosophila, sea urchin, zebrafish, Xenopus, mouse, and human. Molecular, cellular, and developmental functions of Vasa and its orthologs are reviewed in this article. Evidence linking Vasa to translational regulation, to biogenesis of small RNAs, and to chromosome condensation is examined. Finally, potential overlapping functions between Vasa and related DEAD-box helicases (Belle, or Ddx3, and DEADSouth, or Ddx25) are explored. This article is part of a Special Issue entitled: The biology of RNA helicases - Modulation for life.

Ovarian Stem Cells-the Pros and Cons

The potential for postnatal de novo oogenesis in mammals and in humans has become very controversial in the fields of reproductive science and biology. Historically, it has been thought that females of most mammalian species lose the ability to produce oocytes at birth. A contemporary understanding of stem cell biology together with novel experimental methods has challenged the model of a prenatal fixed ovarian primordial follicle pool that declines with age. Researchers have suggested replenishment of post-natal oocytes by germ-line stem cells (GSCs). According to this theory, GSCs produce oocytes and primordial follicles throughout the lifetime of the adult female. This review describes recent approaches supporting the revolutionary idea of de novo oogenesis in mammals and humans of reproductive-age and provides counter arguments from opponents of this novel and innovative concept.

The "Stars and Stripes" Metaphor for Animal Regeneration-Elucidating Two Fundamental Strategies along a Continuum

A number of challenges have hindered the development of a unified theory for metazoan regeneration. To describe the full range of complex regeneration phenomena in Animalia, we suggest that metazoans that regenerate missing body parts exhibit biological attributes that are tailored along a morpho-spatial regeneration continuum, illustrated in its polar scenarios by the USA “stars and stripes” flag. Type 1 organisms (“T1, ‘stars’”) are typical colonial organisms (but contain unitary taxa) that are able to regenerate “whole new stars”, namely, whole bodies and colonial modules, through systemic induction and sometimes multiple regeneration foci (hollow regeneration spheres, resembling the blastula) that compete for dominance. They regenerate soma and germ constituents with pluripotent adult stem cells and exhibit somatic-embryogenesis mode of ontogeny. Type 2 organisms (“T2, ‘stripes’”) are capable of limited regeneration of somatic constituents via fate-restricted stem cells, and regenerate through centralized inductions that lead to a single regeneration front. T2 organisms are unitary and use preformistic mode of ontogeny. T1 and T2 organisms also differ in interpretation of what constitutes positional information. T2 organisms also execute alternative, less effective, regeneration designs (i.e., scar formation). We assigned 15 characteristics that distinguish between T1/T2 strategies: those involving specific regeneration features and those operating on biological features at the whole-organism level. Two model organisms are discussed, representing the two strategies of T1/T2 along the regeneration continuum, the Botrylloides whole body regeneration (T1) and the mouse digit-tip regeneration (T2) phenomena. The above working hypothesis also postulates that regeneration is a primeval attribute of metazoans. As specified, the “stars and stripes” paradigm allows various combinations of the biological features assigned to T1 and T2 regeneration strategies. It does not consider any concentration gradient or thresholds and does not refer to the “epimorphosis” and “morphallaxis” terms, regeneration types across phyla or across body plans. The “stars and stripes” paradigm also ignores, at this stage of analysis, cases of regeneration loss that may obscure biological trajectories. The main advantage of the “stars and stripes” paradigm is that it allows us to compare T1/T2 regeneration, as well as other modes of regeneration, through critical determining characteristics.

Repeated, long-term cycling of putative stem cells between niches in a basal chordate

The mechanisms that sustain stem cells are fundamental to tissue maintenance. Here, we identify "cell islands" (CIs) as a niche for putative germ and somatic stem cells in Botryllus schlosseri, a colonial chordate that undergoes weekly cycles of death and regeneration. Cells within CIs express markers associated with germ and somatic stem cells and gene products that implicate CIs as signaling centers for stem cells. Transplantation of CIs induced long-term germline and somatic chimerism, demonstrating self-renewal and pluripotency of CI cells. Cell labeling and in vivo time-lapse imaging of CI cells reveal waves of migrations from degrading CIs into developing buds, contributing to soma and germline development. Knockdown of cadherin, which is highly expressed within CIs, elicited the migration of CI cells to circulation. Piwi knockdown resulted in regeneration arrest. We suggest that repeated trafficking of stem cells allows them to escape constraints imposed by the niche, enabling self-preservation throughout life.

Autonomy in specification of primordial germ cells and their passive translocation in the sea urchin

The process of germ line determination involves many conserved genes, yet is highly variable. Echinoderms are positioned at the base of Deuterostomia and are crucial to understanding these evolutionary transitions, yet the mechanism of germ line specification is not known in any member of the phyla. Here we demonstrate that small micromeres (SMics), which are formed at the fifth cell division of the sea urchin embryo, illustrate many typical features of primordial germ cell (PGC) specification. SMics autonomously express germ line genes in isolated culture, including selective Vasa protein accumulation and transcriptional activation of nanos; their descendants are passively displaced towards the animal pole by secondary mesenchyme cells and the elongating archenteron during gastrulation; Cadherin (G form) has an important role in their development and clustering phenotype; and a left/right integration into the future adult anlagen appears to be controlled by a late developmental mechanism. These results suggest that sea urchin SMics share many more characteristics typical of PGCs than previously thought, and imply a more widely conserved system of germ line development among metazoans.

Role of Vasa, Piwi, and Myc-expressing coelomic cells in gonad regeneration of the colonial tunicate, Botryllus primigenus

In the colonial tunicate, Botryllus primigenus Oka, gonads consist of indifferent germline precursor cells, the primordial testis and ovary, and mature gonads, of which the immature gonad components can be reconstructed de novo in vascular buds that arise from the common vascular system, although the mechanism is uncertain. In this study, we investigated how and what kinds of cells regenerated the gonad components. We found that few Vasa-positive cells in the hemocoel entered the growing vascular bud, where their number increased, and finally developed exclusively into female germ cells. Simultaneously, small cell aggregates consisting of Vasa(-) and Vasa(±) cells appeared de novo in the lateral body cavity of developing vascular buds. Double fluorescent in situ hybridization showed that these cell aggregates were both Piwi- and Myc-positive. They could form germline precursor cells and a primordial testis and ovary that strongly expressed Vasa. Myc knockdown by RNA interference conspicuously lowered Piwi expression and resulted in the loss of germline precursor cells without affecting Vasa(+) oocyte formation. Myc may contribute to gonad tissue formation via Piwi maintenance. When human recombinant BMP 4 was injected in the test vessel, coelomic Piwi(+) cells were induced to express Vasa in the blood. We conclude, therefore, that in vascular buds of B. primigenus, female germ cells can develop from homing Vasa(+) cells in the blood, and that other gonad components can arise from coelomic Vasa(-)/Piwi(+)/Myc(+) cells.

Induced stem cell neoplasia in a cnidarian by ectopic expression of a POU domain transcription factor

The evolutionary origin of stem cell pluripotency is an unresolved question. In mammals, pluripotency is limited to early embryos and is induced and maintained by a small number of key transcription factors, of which the POU domain protein Oct4 is considered central. Clonal invertebrates, by contrast, possess pluripotent stem cells throughout their life, but the molecular mechanisms that control their pluripotency are poorly defined. To address this problem, we analyzed the expression pattern and function of Polynem (Pln), a POU domain gene from the marine cnidarian Hydractinia echinata. We show that Pln is expressed in the embryo and adult stem cells of the animal and that ectopic expression in epithelial cells induces stem cell neoplasms and loss of epithelial tissue. Neoplasm cells downregulated the transgene but expressed the endogenous Pln gene and also Nanos, Vasa, Piwi and Myc, which are all known cnidarian stem cell markers. Retinoic acid treatment caused downregulation of Pln and the differentiation of neoplasm cells to neurosensory and epithelial cells. Pln downregulation by RNAi led to differentiation. Collectively, our results suggest an ancient role of POU proteins as key regulators of animal stem cells.

The multiple hats of Vasa: its functions in the germline and in cell cycle progression

Vasa, an ATP-dependent RNA helicase, is broadly conserved among various organisms from cnidarians to mammals. It has a rich history of utility as a germline marker, and is believed to function as a positive translational regulator in the determination and maintenance of germline cells. Studies in non-model organisms, however, revealed that Vasa is also present in somatic cells of many tissues. In many cases these cells are multipotent, are non-germline associated, and give rise to a variety of different tissue types. Recent work now also demonstrates that Vasa functions in the regulation of the cell cycle. Here, we discuss this newly described function of Vasa in mitotic and meiotic cell cycles, and we address the conundrum created within these observations, that is, that most cells are mitotically independent of Vasa, yet when Vasa is present in a cell, it appears to be essential for cell cycle progression.

Asexual propagation and regeneration in colonial ascidians

Regeneration is widely distributed among the metazoans. However, clear differences exist as to the degree of regenerative capacity: some phyla can only replace missing body parts, whereas others can generate entirely new individuals. Ascidians are animals that possess a remarkable regenerative plasticity and exhibit a great diversity of mechanisms for asexual propagation and survival. They are marine invertebrate members of the subphylum Tunicata and represent modern-day descendants of the chordate ancestor; in their tadpole stage they exhibit a chordate body plan that is resorbed during metamorphosis. Solitary species grow into an adult that can reach several centimeters in length, whereas colonial species grow by asexual propagation, creating a colony of genetically identical individuals. In this review, we present an overview of the biology of colonial ascidians as a paradigm for study in stem cell and regenerative biology. Focusing on botryllid ascidians, we introduce the potential roles played by multipotent epithelia and multipotent/pluripotent stem cells as source of asexual propagation and regenerative plasticity in the different budding mechanisms, and consider the putative mechanism of body repatterning in a non-embryonic scenario. We also discuss the involvement of intra-colony homeostatic processes in regulating budding potential, and the functional link between allorecognition, chimerism, and regenerative potential.

Cell cultures from marine invertebrates: new insights for capturing endless stemness

Despite several decades of extensive research efforts, there is yet no single permanent cell line available from marine invertebrates as these cells stop dividing in vitro within 24-72 h after their isolation, starting cellular quiescence. This ubiquitous quiescent state should be modified in a way that at least some of the quiescent cells will become pluripotent, so they will have the ability to divide and become immortal. Following the above need, this essay introduces the rationale that the discipline of marine invertebrates' cell culture should gain from applying of two research routes, relevant to mammalian systems but less explored in the marine arena. The first is the use of adult stem cells (ASC) from marine organisms. Many marine invertebrate taxa maintain large pools of ASC in adulthood. Ample evidence attests that these cells from sponges, cnidarians, flatworms, crustaceans, mollusks, echinoderms, and ascidians play important roles in maintenance, regeneration, and asexual cloning, actively proliferating in vivo, resembling the vertebrates' cancer stem cells features. The second route is to target resting somatic cell constituents, manipulating them in the same way as has recently been performed on mammalian induced pluripotent stem (iPS) cells. While "iPS cells" are the outcome of an experimental manipulation, ASC are natural and rather frequent in a number of marine invertebrates. Above two cell categories reveal that there are more than a few types of seeds (cells) waiting to be sowed in the right soil (in vitro environmental conditions) for acquiring stemness and immortality. This rationale carries the potential to revolutionize the discipline of marine invertebrate cell cultures. When cultured "correctly," ASC and "iPS cells" from marine invertebrates may stay in their primitive stage and proliferate without differentiating into cells lineages, harnessing the stem cell's inherent abilities of self-replication versus differentiated progenies, toward the development of immortal cell lines.