Protein interactions in Xenopus germ plasm RNP particles

Interplay of RNA-binding proteins controls germ cell development in zebrafish

The specification of germ cells in zebrafish mostly relies on an inherited mechanism by which localized maternal determinants, called germ plasm, confer germline fate in the early embryo. Extensive studies have partially allowed the identification of key regulators governing germ plasm formation and subsequent germ cell development. RNA-binding proteins, acting in concert with other germ plasm components, play essential roles in the organization of the germ plasm and the specification, migration, maintenance, and differentiation of primordial germ cells. The loss of their functions impairs germ cell formation and causes sterility or sexual conversion. Evidence is emerging that they instruct germline development through differential regulation of mRNA fates in somatic and germ cells. However, the challenge remains to decipher the complex interplay of maternal germ plasm components in germ plasm compartmentalization and germ cell specification. Because failure to control the developmental outcome of germ cells disrupts the formation of gametes, it is important to gain a complete picture of regulatory mechanisms operating in the germ cell lineage. This review sheds light on the contributions of RNA-binding proteins to germ cell development in zebrafish and highlights intriguing questions that remain open for future investigation.

Solubility phase transition of maternal RNAs during vertebrate oocyte-to-embryo transition

The oocyte-to-embryo transition (OET) is regulated by maternal products stored in the oocyte cytoplasm, independent of transcription. How maternal products are precisely remodeled to dictate the OET remains largely unclear. In this work, we discover the dynamic solubility phase transition of maternal RNAs during Xenopus OET. We have identified 863 maternal transcripts that transition from a soluble state to a detergent-insoluble one after oocyte maturation. These RNAs are enriched in the animal hemisphere, and many of them encode key cell cycle regulators. In contrast, 165 transcripts, including nearly all Xenopus germline RNAs and some vegetally localized somatic RNAs, undergo an insoluble-to-soluble phase transition. This phenomenon is conserved in zebrafish. Our results demonstrate that the phase transition of germline RNAs influences their susceptibility to RNA degradation machinery and is mediated by the remodeling of germ plasm. This work thus identifies important remodeling mechanisms that act on RNAs to control vertebrate OET.

Phase transition of maternal RNAs during vertebrate oocyte-to-embryo transition

The oocyte-to-embryo transition (OET) is regulated by maternal products stored in the oocyte cytoplasm, independent of transcription. How maternal products are precisely remodeled to dictate the OET remains an open question. In this work, we discover the dynamic phase transition of maternal RNAs during Xenopus OET. We have identified 863 maternal transcripts that transition from a soluble state to a detergent-insoluble one after oocyte maturation. These RNAs are enriched in the animal hemisphere and many of them encode key cell cycle regulators. In contrast, 165 transcripts, including nearly all Xenopus germline RNAs and some vegetally localized somatic RNAs, undergo an insoluble-to-soluble phase transition. This phenomenon is conserved in zebrafish. Our results demonstrate that the phase transition of germline RNAs influences their susceptibility to RNA degradation machinery and is mediated by the remodeling of germ plasm. This work thus uncovers novel remodeling mechanisms that act on RNAs to regulate vertebrate OET.

RBPMS2 Is a Myocardial-Enriched Splicing Regulator Required for Cardiac Function

BACKGROUND

RBPs (RNA-binding proteins) perform indispensable functions in the post-transcriptional regulation of gene expression. Numerous RBPs have been implicated in cardiac development or physiology based on gene knockout studies and the identification of pathogenic RBP gene mutations in monogenic heart disorders. The discovery and characterization of additional RBPs performing indispensable functions in the heart will advance basic and translational cardiovascular research.

METHODS

We performed a differential expression screen in zebrafish embryos to identify genes enriched in nkx2.5-positive cardiomyocytes or cardiopharyngeal progenitors compared to nkx2.5-negative cells from the same embryos. We investigated the myocardial-enriched gene RNA-binding protein with multiple splicing (variants) 2 [RBPMS2)] by generating and characterizing rbpms2 knockout zebrafish and human cardiomyocytes derived from RBPMS2-deficient induced pluripotent stem cells.

RESULTS

We identified 1848 genes enriched in the nkx2.5-positive population. Among the most highly enriched genes, most with well-established functions in the heart, we discovered the ohnologs rbpms2a and rbpms2b, which encode an evolutionarily conserved RBP. Rbpms2 localizes selectively to cardiomyocytes during zebrafish heart development and strong cardiomyocyte expression persists into adulthood. Rbpms2-deficient embryos suffer from early cardiac dysfunction characterized by reduced ejection fraction. The functional deficit is accompanied by myofibril disarray, altered calcium handling, and differential alternative splicing events in mutant cardiomyocytes. These phenotypes are also observed in RBPMS2-deficient human cardiomyocytes, indicative of conserved molecular and cellular function. RNA-sequencing and comparative analysis of genes mis-spliced in RBPMS2-deficient zebrafish and human cardiomyocytes uncovered a conserved network of 29 ortholog pairs that require RBPMS2 for alternative splicing regulation, including RBFOX2, SLC8A1, and MYBPC3.

CONCLUSIONS

Our study identifies RBPMS2 as a conserved regulator of alternative splicing, myofibrillar organization, and calcium handling in zebrafish and human cardiomyocytes.

Primordial Germ Cell Specification in Vertebrate Embryos: Phylogenetic Distribution and Conserved Molecular Features of Preformation and Induction

The differentiation of primordial germ cells (PGCs) occurs during early embryonic development and is critical for the survival and fitness of sexually reproducing species. Here, we review the two main mechanisms of PGC specification, induction, and preformation, in the context of four model vertebrate species: mouse, axolotl, Xenopus frogs, and zebrafish. We additionally discuss some notable molecular characteristics shared across PGC specification pathways, including the shared expression of products from three conserved germline gene families, DAZ (Deleted in Azoospermia) genes, nanos-related genes, and DEAD-box RNA helicases. Then, we summarize the current state of knowledge of the distribution of germ cell determination systems across kingdom Animalia, with particular attention to vertebrate species, but include several categories of invertebrates - ranging from the "proto-vertebrate" cephalochordates to arthropods, cnidarians, and ctenophores. We also briefly highlight ongoing investigations and potential lines of inquiry that aim to understand the evolutionary relationships between these modes of specification.

Raising Antibodies for Use in Xenopus

For work in Xenopus, frog-specific antibodies must usually be raised, although a few antibodies against mammalian proteins cross-react. To produce an immunogen for antibody production, human embryonic kidney (HEK) expression systems can be used as described here. For most laboratories, the actual method of raising the antibody is determined by local ethical regulations controlling the adjuvant and injection protocols used. Because these steps are often outsourced, they are not included in this protocol.

Exploring the Activities of RBPMS Proteins in Myocardial Biology

Numerous RNA-binding proteins (RBPs) are expressed in the heart, and mutations in several RBPs have been implicated in cardiovascular disease through genetic associations, animal modeling, and mechanistic studies. However, the functions of many more cardiac RBPs, and their relevance to disease states, remain to be elucidated. Recently, we have initiated studies to characterize the functions of the RBPs RBPMS and RBPMS2 in regulating myocardial biology in zebrafish and higher vertebrate species. These studies began when we learned, using an unbiased gene discovery approach, that rbpms2a and rbpms2b in zebrafish are robust markers of embryonic myocardium. This observation, which is consistent with published data, suggests that the encoded proteins are likely to be performing critical functions in regulating one or more aspects of cardiomyocyte differentiation, proliferation, survival, and/or contractility. This notion is supported by recent reports demonstrating that zebrafish embryos with disrupted Rbpms2 function exhibit gross signs of cardiac distress. Interestingly, a 20-year-old study determined that myocardial tissue from the frog, chick, and mouse also express high levels of Rbpms and/or Rbpms2, which is suggestive of evolutionary conservation of function. In this review, we will provide a historical account of how RBPMS and RBPMS2 genes were discovered, attempt to clarify some potentially confusing nomenclature, and summarize published observations that inform our ongoing studies.

RNA Localization in the Vertebrate Oocyte: Establishment of Oocyte Polarity and Localized mRNA Assemblages

RNA localization is a fundamental mechanism for controlling cell structure and function. Early development in fish and amphibians requires the localization of specific mRNAs to establish the initial differences in cell fates prior to the onset of zygotic genome activation. RNA localization in these oocytes (e.g., Xenopus and zebrafish) requires that animal-vegetal polarity be established early in oogenesis, mediated by formation of the Balbiani body/mitochondrial cloud. This structure serves as a platform for assembly and transport of germline determinants to the future vegetal pole and also sets up the machinery for the localization of non-germline transcripts later in oogenesis. Understanding these polarization and localization mechanisms is critical for understanding the basis for early embryonic development in these organisms and also for understanding the role of RNA compartmentalization in animal gametogenesis. Here we outline recent advances in elucidating the molecular basis for the establishment of oocyte polarity at the level of Balbiani body assembly as well as the formation of RNP assemblies for early and late pathway mRNA localization in the oocyte.

Functional equivalence of germ plasm organizers

The proteins Oskar (Osk) in Drosophila and Bucky ball (Buc) in zebrafish act as germ plasm organizers. Both proteins recapitulate germ plasm activities but seem to be unique to their animal groups. Here, we discover that Osk and Buc show similar activities during germ cell specification. Drosophila Osk induces additional PGCs in zebrafish. Surprisingly, Osk and Buc do not show homologous protein motifs that would explain their related function. Nonetheless, we detect that both proteins contain stretches of intrinsically disordered regions (IDRs), which seem to be involved in protein aggregation. IDRs are known to rapidly change their sequence during evolution, which might obscure biochemical interaction motifs. Indeed, we show that Buc binds to the known Oskar interactors Vasa protein and nanos mRNA indicating conserved biochemical activities. These data provide a molecular framework for two proteins with unrelated sequence but with equivalent function to assemble a conserved core-complex nucleating germ plasm.

Multicellular organisms use gametes for their propagation. Gametes are formed from germ cells, which are specified during embryogenesis in some animals by the inheritance of RNP granules known as germ plasm. Transplantation of germ plasm induces extra germ cells, whereas germ plasm ablation leads to the loss of gametes and sterility. Therefore, germ plasm is key for germ cell formation and reproduction. However, the molecular mechanisms of germ cell specification by germ plasm in the vertebrate embryo remain an unsolved question. Proteins, which assemble the germ plasm, are known as germ plasm organizers. Here, we show that the two germ plasm organizers Oskar from the fly and Bucky ball from the fish show similar functions by using a cross species approach. Both are intrinsically disordered proteins, which rapidly changed their sequence during evolution. Moreover, both proteins still interact with conserved components of the germ cell specification pathway. These data might provide a first example of two proteins with the same biological role, but distinct sequence.

rbpms2 functions in Balbiani body architecture and ovary fate

The most prominent developmental regulators in oocytes are RNA-binding proteins (RNAbps) that assemble their targets into ribonucleoprotein granules where they are stored, transported and translationally regulated. RNA-binding protein of multiple splice forms 2, or Rbpms2, interacts with molecules that are essential to reproduction and egg patterning, including bucky ball, a key factor for Bb formation. Rbpms2 is localized to germ granules in primordial germ cells (PGCs) and to the Balbiani body (Bb) of oocytes, although the mechanisms regulating Rbpms2 localization to these structures are unknown. Using mutant Rbpms2 proteins, we show that Rbpms2 requires distinct protein domains to localize within germ cells and somatic cells. Accumulation and localization to subcellular compartments in the germline requires an intact RNA binding domain. Whereas in zebrafish somatic blastula cells, the conserved C-terminal domain promotes localization to the bipolar centrosomes/spindle. To investigate Rbpms2 functions, we mutated the duplicated and functionally redundant zebrafish rbpms2 genes. The gonads of rbpms2a;2b (rbpms2) mutants initially contain early oocytes, however definitive oogenesis ultimately fails during sexual differentiation and, rbpms2 mutants develop as fertile males. Unlike other genes that promote oogenesis, failure to maintain oocytes in rbpms2 mutants was not suppressed by mutation of Tp53. These findings reveal a novel and essential role for rbpms2 in oogenesis. Ultrastructural and immunohistochemical analyses revealed that rbpms2 is not required for the asymmetric accumulation of mitochondria and Buc protein in oocytes, however its absence resulted in formation of abnormal Buc aggregates and atypical electron-dense cytoplasmic inclusions. Our findings reveal novel and essential roles for rbpms2 in Buc organization and oocyte differentiation.

Oocyte development relies on posttranscriptional regulation by RNA binding proteins (RNAbps). RNAbps form large multi-molecular structures called RNPs (ribonucleoproteins) that further aggregate into regulatory granules within germ cells. In zebrafish primary oocytes, a large transient RNP aggregate called the Balbiani body (Bb) is essential for localizing patterning molecules and germline determinants within oocytes. RNA-binding protein of multiple splice forms 2, or Rbpms2, localizes to germ granules and the Bb, and interacts with bucky ball, a key factor for Bb formation. We show that Rbpms2 requires RNA binding for localization within germ cells, and that the C-term and RRM contribute to Rbpms2 subcellular localization in distinct somatic cell types. To investigate Rbpms2 functions we mutated the duplicated zebrafish rbpms2 genes. Consistent with redundant functions, rbpms2a and rbpms2b gene expression overlaps, and single mutants have no discernible phenotypes. Although rbpms2a;2b double mutants have cardiac phenotypes, those that reach adulthood are exclusively fertile males. Genetic analysis shows that rbpms2 mutant oocytes are not maintained even when Tp53, a regulator of cell death is absent. Initial oocyte polarity is established in rbpms2 mutants based on asymmetric distribution of Buc protein and mitochondria; however, abnormal Buc structures and atypical cytoplasmic inclusions form. This work reveals independent Rbpms2 functions in promoting Bb integrity, and as a novel regulator of ovary fate.

Molecular characterization, tissue distribution, localization and mRNA expression of the bucky ball gene in the Dabry's sturgeon (Acipenser dabryanus) during oogenesis

In many organisms, germ cells are specified during embryogenesis by the inheritance of maternally deposited RNAs and proteins termed germ plasm. In vertebrates, the bucky ball (buc) gene plays an essential role in the germ plasm aggregation. In this study, the full-length cDNA of buc homologue in Dabry's sturgeon, Adbuc, was isolated and characterized. Multiple sequence alignments showed that the BUVE domain of Buc was highly conserved in vertebrates, despite exhibiting low identities with each other across the whole protein. By quantitative real-time PCR analysis, we found that Adbuc RNAs were only detected in the gonad with a high level in the ovary and a very low level in the testis. During embryogenesis, these RNAs were highly expressed from the unfertilized eggs to blastula, declined dramatically from the gastrula stage, and hardly found after the neurula stage. Moreover, with the development of ovary, the expression level of Adbuc was increasing. By in situ hybridization, the signal of Adbuc was not found in the oogonia, increased slightly in the stage I oocytes, and extremely strong in the stage II oocytes, suggesting that the signal became much stronger with increasing size of oocytes. Additionally, Adbuc co-localized with the mitochondrial cloud. Thus, we conclude that Dabry's sturgeon buc gene might also function in germplasm formation.

Convergent evolution of germ granule nucleators: A hypothesis

Germ cells have been considered "the ultimate stem cell" because they alone, during normal development of sexually reproducing organisms, are able to give rise to all organismal cell types. Morphological descriptions of a specialized cytoplasm termed 'germ plasm' and associated electron dense ribonucleoprotein (RNP) structures called 'germ granules' within germ cells date back as early as the 1800s. Both germ plasm and germ granules are implicated in germ line specification across metazoans. However, at a molecular level, little is currently understood about the molecular mechanisms that assemble these entities in germ cells. The discovery that in some animals, the gene products of a small number of lineage-specific genes initiate the assembly (also termed nucleation) of germ granules and/or germ plasm is the first step towards facilitating a better understanding of these complex biological processes. Here, we draw on research spanning over 100years that supports the hypothesis that these nucleator genes may have evolved convergently, allowing them to perform analogous roles across animal lineages.

Coordination of cellular differentiation, polarity, mitosis and meiosis - New findings from early vertebrate oogenesis

A mechanistic dissection of early oocyte differentiation in vertebrates is key to advancing our knowledge of germline development, reproductive biology, the regulation of meiosis, and all of their associated disorders. Recent advances in the field include breakthroughs in the identification of germline stem cells in Medaka, in the cellular architecture of the germline cyst in mice, in a mechanistic dissection of chromosomal pairing and bouquet formation in meiosis in mice, in tracing oocyte symmetry breaking to the chromosomal bouquet of meiosis in zebrafish, and in the biology of the Balbiani body, a universal oocyte granule. Many of the major events in early oogenesis are universally conserved, and some are co-opted for species-specific needs. The chromosomal events of meiosis are of tremendous consequence to gamete formation and have been extensively studied. New light is now being shed on other aspects of early oocyte differentiation, which were traditionally considered outside the scope of meiosis, and their coordination with meiotic events. The emerging theme is of meiosis as a common groundwork for coordinating multifaceted processes of oocyte differentiation. In an accompanying manuscript we describe methods that allowed for investigations in the zebrafish ovary to contribute to these breakthroughs. Here, we review these advances mostly from the zebrafish and mouse. We discuss oogenesis concepts across established model organisms, and construct an inclusive paradigm for early oocyte differentiation in vertebrates.

Mitochondrial matters: Mitochondrial bottlenecks, self-assembling structures, and entrapment in the female germline

Mitochondrial replacement therapy, a procedure to generate embryos with the nuclear genome of a donor mother and the healthy mitochondria of a recipient egg, has recently emerged as a promising strategy to prevent transmission of devastating mitochondrial DNA diseases and infertility. The procedure may produce an embryo that is free of diseased mitochondria. A recent study addresses important fundamental questions about the mechanisms underlying maternal inheritance and translational questions regarding the transgenerational effectiveness of this promising therapeutic strategy. This review considers recent advances in our understanding of maternal inheritance of mitochondria, implications for fertility and mitochondrial disease, and potential roles for the Balbiani body, an ancient oocyte structure, in mitochondrial selection in oocytes, with emphasis on therapies to remedy mitochondrial disorders.

Mitochondria as a Signaling Hub and Target for Phenoptosis Shutdown

Mitochondria have long been studied as the main energy source and one of the most important generators of reactive oxygen species in the eukaryotic cell. Yet, new data suggest mitochondria serve as a powerful cellular regulator, pathway trigger, and signal hub. Some of these crucial mitochondrial functions appear to be associated with RNP-granules. Deep and versatile involvement of mitochondria in general cellular regulation may be the legacy of parasitic behavior of the ancestors of mitochondria in the host cells. In this regard, we also discuss here the perspectives of using mitochondria-targeted compounds for systemic correction of phenoptotic shifts.

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.

Multiplicity of Buc copies in Atlantic salmon contrasts with loss of the germ cell determinant in primates, rodents and axolotl

BACKGROUND

The primordial germ cells (PGCs) giving rise to gametes are determined by two different mechanisms in vertebrates. While the germ cell fate in mammals and salamanders is induced by zygotic signals, maternally delivered germ cell determinants specify the PGCs in birds, frogs and teleost fish. Assembly of the germ plasm in the oocyte is organized by the single Buc in zebrafish, named Velo1 in Xenopus, and by Oskar in Drosophila. Secondary loss of oskar in several insect lineages coincides with changes in germline determination strategies, while the presence of buc in mammals suggests functions not associated with germline formation.

RESULTS

To clarify the evolutionary history of buc we searched for the gene in genomes available from various chordates. No buc sequence was found in lamprey and chordate invertebrates, while the gene was identified in a conserved syntenic region in elephant shark, spotted gar, teleosts, Comoran coelacanth and most tetrapods. Rodents have probably lost the buc gene, while a premature translation stop was found in primates and in Mexican axolotl lacking germ plasm. In contrast, several buc and buc-like (bucL) paralogs were identified in the teleosts examined, including zebrafish, and the tetraploid genome of Atlantic salmon harbors seven buc and bucL genes. Maternal salmon buc1a, buc2a and buc2b mRNAs were abundant in unfertilized eggs together with dnd and vasa mRNAs. Immunostained salmon Buc1a was restricted to cleavage furrows in 4-cell stage embryos similar to a fluorescent zebrafish Buc construct injected in salmon embryos. Salmon Buc1a and Buc2a localized together with DnD, Vasa and Dazl within the Balbiani body of early oocytes.

CONCLUSIONS

Buc probably originated more than 400 million years ago and might have played an ancestral role in assembling germ plasm. Functional redundancy or subfunctionalization of salmon Buc paralogs in germline formation is suggested by the maternally inherited mRNAs of three salmon buc genes, the localized Buc1a in the cleavage furrows and the distribution of Buc1a and Buc2a in the Balbiani body during oogenesis. The extra-ovarian expression of salmon buc genes and the presence of a second zebrafish bucL gene suggest additional functions not related to germ cell specification.

Hermes (Rbpms) is a Critical Component of RNP Complexes that Sequester Germline RNAs during Oogenesis

The germ cell lineage in Xenopus is specified by the inheritance of germ plasm that assembles within the mitochondrial cloud or Balbiani body in stage I oocytes. Specific RNAs, such as nanos1, localize to the germ plasm. nanos1 has the essential germline function of blocking somatic gene expression and thus preventing Primordial Germ Cell (PGC) loss and sterility. Hermes/Rbpms protein and nanos RNA co-localize within germinal granules, diagnostic electron dense particles found within the germ plasm. Previous work indicates that nanos accumulates within the germ plasm through a diffusion/entrapment mechanism. Here we show that Hermes/Rbpms interacts with nanos through sequence specific RNA localization signals found in the nanos-3'UTR. Importantly, Hermes/Rbpms specifically binds nanos, but not Vg1 RNA in the nucleus of stage I oocytes. In vitro binding data show that Hermes/Rbpms requires additional factors that are present in stage I oocytes in order to bind nanos1. One such factor may be hnRNP I, identified in a yeast-2-hybrid screen as directly interacting with Hermes/Rbpms. We suggest that Hermes/Rbpms functions as part of a RNP complex in the nucleus that facilitates selection of germline RNAs for germ plasm localization. We propose that Hermes/Rbpms is required for nanos RNA to form within the germinal granules and in this way, participates in the germline specific translational repression and sequestration of nanos RNA.

The Birth of Animal Development: Multicellularity and the Germline

The evolution of multicellular animals has been attributed to many kinds of selective advantage; here I suggest that the evolution of somatic cells to feed and protect the germline was central to the appearance of animals. This would have been driven by selection for extreme anisogamy--the evolution of sperm and egg. Evidence is adduced from the germline stem cells of simple animals (defining germline as any cell that normally produces the next generation via the sexual process) and from the control circuitry ubiquitous in animal germlines. With the soma and its elaboration came animal development, as we understand it.

Kinesin-1 interacts with Bucky ball to form germ cells and is required to pattern the zebrafish body axis

In animals, specification of the primordial germ cells (PGCs), the stem cells of the germ line, is required to transmit genetic information from one generation to the next. Bucky ball (Buc) is essential for germ plasm (GP) assembly in oocytes, and its overexpression results in excess PGCs in zebrafish embryos. However, the mechanistic basis for the excess PGCs in response to Buc overexpression, and whether endogenous Buc functions during embryogenesis, are unknown. Here, we show that endogenous Buc, like GP and overexpressed Buc-GFP, accumulates at embryonic cleavage furrows. Furthermore, we show that the maternally expressed zebrafish Kinesin-1 Kif5Ba is a binding partner of Buc and that maternal kif5Ba (Mkif5Ba) plays an essential role in germline specification in vivo. Specifically, Mkif5Ba is required to recruit GP to cleavage furrows and thereby specifies PGCs. Moreover, Mkif5Ba is required to enrich Buc at cleavage furrows and for the ability of Buc to promote excess PGCs, providing mechanistic insight into how Buc functions to assemble embryonic GP. In addition, we show that Mkif5Ba is also essential for dorsoventral (DV) patterning. Specifically, Mkif5Ba promotes formation of the parallel vegetal microtubule array required to asymmetrically position dorsal determinants (DDs) towards the prospective dorsal side. Interestingly, whereas Syntabulin and wnt8a translocation depend on kif5Ba, grip2a translocation does not, providing evidence for two distinct mechanisms by which DDs might be asymmetrically distributed. These studies identify essential roles for maternal Kif5Ba in PGC specification and DV patterning, and provide mechanistic insight into Buc functions during early embryogenesis.

A functional Bucky ball-GFP transgene visualizes germ plasm in living zebrafish

In many animals, the germline is specified by maternal RNA-granules termed germ plasm. The correct localization of germ plasm during embryogenesis is therefore crucial for the specification of germ cells. In zebrafish, we previously identified Bucky ball (Buc) as a key regulator of germ plasm formation. Here, we used a Buc antibody to describe its continuous germ plasm localization. Moreover, we generated a transgenic Buc-GFP line for live imaging, which visualizes germ plasm from its assembly during oogenesis up to the larval stages. Live imaging of Buc-GFP generated stunning movies, as they highlighted the dynamic details of germ plasm movements. Moreover, we discovered that Buc was still detected in primordial germ cells 2 days after fertilization. Interestingly, the transgene rescued buc mutants demonstrating genetically that the Buc-GFP fusion protein is functional. These results show that Buc-GFP exerts all biochemical interactions essential for germline development and highlight the potential of this line to analyze the molecular regulation of germ plasm formation.

Regulation of RNA granule dynamics by phosphorylation of serine-rich, intrinsically disordered proteins in C. elegans

RNA granules have been likened to liquid droplets whose dynamics depend on the controlled dissolution and condensation of internal components. The molecules and reactions that drive these dynamics in vivo are not well understood. In this study, we present evidence that a group of intrinsically disordered, serine-rich proteins regulate the dynamics of P granules in C. elegans embryos. The MEG (maternal-effect germline defective) proteins are germ plasm components that are required redundantly for fertility. We demonstrate that MEG-1 and MEG-3 are substrates of the kinase MBK-2/DYRK and the phosphatase PP2A(PPTR-½). Phosphorylation of the MEGs promotes granule disassembly and dephosphorylation promotes granule assembly. Using lattice light sheet microscopy on live embryos, we show that GFP-tagged MEG-3 localizes to a dynamic domain that surrounds and penetrates each granule. We conclude that, despite their liquid-like behavior, P granules are non-homogeneous structures whose assembly in embryos is regulated by phosphorylation.

Assemblages: functional units formed by cellular phase separation

The partitioning of intracellular space beyond membrane-bound organelles can be achieved with collections of proteins that are multivalent or contain low-complexity, intrinsically disordered regions. These proteins can undergo a physical phase change to form functional granules or other entities within the cytoplasm or nucleoplasm that collectively we term “assemblage.” Intrinsically disordered proteins (IDPs) play an important role in forming a subset of cellular assemblages by promoting phase separation. Recent work points to an involvement of assemblages in disease states, indicating that intrinsic disorder and phase transitions should be considered in the development of therapeutics.