Transition from non-motile behaviour to directed migration during early PGC development in zebrafish

Znf706 regulates germ plasm assembly and primordial germ cell development in zebrafish

The cell fate of primordial germ cell (PGC) in zebrafish is pre-determined by maternally deposited germ plasm, which is packaged into ribonucleoprotein complex in oocytes and inherited into PGC-fated cells in embryos. However, the maternal factors regulating the assembly of germ plasm and PGC development remain poorly understood. In this study, we report that the maternal transcription factor Znf706 regulates the assembly of germ plasm factors into a granule-like structure localized perinuclearly in PGC during migration. Maternal and zygotic mutants of znf706 exhibit deficient germ plasm scattering at the early embryonic stage, decreased PGC numbers with some mislocation during PGC migration, and a lower female ratio in adulthood. Notably, the implementation of Znf706 CUT&Tag and RNA-seq on immature oocytes uncovers that Znf706 in stage I oocytes may promote transcription of several mitochondrial genes in addition to other functions. Hence, we propose that Znf706 is implicated in germ plasm assembly and PGC development in zebrafish.

Evolving Lessons on Metazoan Primordial Germ Cells in Diversity and Development

Germ cells are pivotal for the continuation of biological species. The metazoan germline develops from primordial germ cells (PGCs) that undergo multiple rounds of mitotic divisions. The PGCs are specified by either maternal inheritance of asymmetrically polarized cytoplasmic mRNAs/proteins (found in roundworms, flies, fishes, frogs, and fowl) or via direct induction of epiblast cells from adjacent extraembryonic ectoderm in mammals. In all vertebrates, PGCs remain uncommitted to meiosis and migrate to colonize the developing gonadal ridge before sex determination. Multiple RNA-binding proteins (e.g., Vasa, Dnd, Dazl, etc.) play crucial roles in PGC identity, expansion, survival, and migration. Postsex determination in mouse embryos, Gata4, expressing nascent gonads, induces Dazl expression in newly arriving germ cells that supports retinoic acid-mediated induction of meiotic onset. This article briefly discusses the developmental events regulating the PGC specification and commitment in metazoans. We also highlight the recent progress towards the in vitro generation of functional PGC-like cells in rodents and humans.

The mechanical journey of primordial germ cells

Primordial germ cells (PGCs) are the earliest progenitors of germline cells of the gonads in animals. The tissues that arise from primordial germ cells give rise to male and female gametes and are thus responsible for transmitting genetic information to subsequent generations. The development of gonads, from single cells to fully formed organs, is of great interest to the reproductive biology community. In most higher animals, PGCs are initially specified at a site away from the gonads. They then migrate across multiple tissue microenvironments to reach a mesodermal mass of cells called the genital ridge, where they associate with somatic cells to form sex-specific reproductive organs. Their migratory behavior has been studied extensively to identify which tissues they interact with and how this might affect gonad development. A crucial point overlooked by classical studies has been the physical environment experienced by PGCs as they migrate and the mechanical challenges they might encounter along the way. It has long been understood that migrating cells can sense and adapt to physical forces around them via a variety of mechanisms, and studies have shown that these mechanical signals can guide stem cell fate. In this review, we summarize the mechanical microenvironment of migrating PGCs in different organisms. We describe how cells can adapt to this environment and how this adaptation can influence cell fate. Finally, we propose that mechanical signals play a crucial role in the normal development of the germline and shed light on this unexplored area of developmental biology.

Traceability of primordial germ cells in three neotropical fish species aiming genetic conservation actions

Primordial germ cells (PGCs) are embryonic pluripotent cells that can differentiate into spermatogonia and oogonia, and therefore, PGCs are a genetic source for germplasm conservation through cryobanking and the generation of germline chimeras. The knowledge of PGC migration routes is essential for transplantation studies. In this work, the mRNA synthesized from the ddx4 3'UTR sequence of Pseudopimelodus mangurus, in fusion with gfp or dsred, was microinjected into zygotes of three neotropical species (P. mangurus, Astyanax altiparanae, and Prochilodus lineatus) for PGC labeling. Visualization of labeled PGCs was achieved by fluorescence microscopy during embryonic development. In addition, ddx4 and dnd1 expressions were evaluated during embryonic development, larvae, and adult tissues of P. mangurus, to validate their use as a PGC marker. As a result, the effective identification of presumptive PGCs was obtained. DsRed-positive PGC of P. mangurus was observed in the hatching stage, GFP-positive PGC of A. altiparanae in the gastrula stage, and GFP-positive PGCs from P. lineatus were identified at the segmentation stage, with representative labeling percentages of 29% and 16% in A. altiparanae and P. lineatus, respectively. The expression of ddx4 and dnd1 of P. mangurus confirmed the specificity of these genes in germ cells. These results point to the functionality of the P. mangurus ddx4 3'UTR sequence as a PGC marker, demonstrating that PGC labeling was more efficient in A. altiparanae and P. lineatus. The procedures used to identify PGCs in P. mangurus consolidate the first step for generating germinal chimeras as a conservation action of P. mangurus.

Characteristics of the Vasa Gene in Silurus asotus and Its Expression Response to Letrozole Treatment

The identification and expression of germ cells are important for studying sex-related mechanisms in fish. The vasa gene, encoding an ATP-dependent RNA helicase, is recognized as a molecular marker of germ cells and plays a crucial role in germ cell development. Silurus asotus, an important freshwater economic fish species in China, shows significant sex dimorphism with the female growing faster than the male. However, the molecular mechanisms underlying these sex differences especially involving in the vasa gene in this fish remain poorly understood. In this work, the vasa gene sequence of S. asotus (named as Savasa) was obtained through RT-PCR and rapid amplification of cDNA end (RACE), and its expression in embryos and tissues was analyzed using qRT-PCR and an in situ hybridization method. Letrozole (LT) treatment on the larvae fish was also conducted to investigate its influence on the gene. The results revealed that the open reading frame (ORF) of Savasa was 1989 bp, encoding 662 amino acids. The SaVasa protein contains 10 conserved domains unique to the DEAD-box protein family, showing the highest sequence identity of 95.92% with that of Silurus meridionalis. In embryos, Savasa is highly expressed from the two-cell stage to the blastula stage in early embryos, with a gradually decreasing trend from the gastrula stage to the heart-beating stage. Furthermore, Savasa was initially detected at the end of the cleavage furrow during the two-cell stage, later condensing into four symmetrical cell clusters with embryonic development. At the gastrula stage, Savasa-positive cells increased and began to migrate towards the dorsal side of the embryo. In tissues, Savasa is predominantly expressed in the ovaries, with almost no or lower expression in other detected tissues. Moreover, Savasa was expressed in phase I-V oocytes in the ovaries, as well as in spermatogonia and spermatocytes in the testis, implying a specific expression pattern of germ cells. In addition, LT significantly upregulated the expression of Savasa in a concentration-dependent manner during the key gonadal differentiation period of the fish. Notably, at 120 dph after LT treatment, Savasa expression was the lowest in the testis and ovary of the high concentration group. Collectively, findings from gene structure, protein sequence, phylogenetic analysis, RNA expression patterns, and response to LT suggest that Savasa is maternally inherited with conserved features, serving as a potential marker gene for germ cells in S.asotus, and might participate in LT-induced early embryonic development and gonadal development processes of the fish. This would provide a basis for further research on the application of germ cell markers and the molecular mechanisms of sex differences in S. asotus.

The journey of a generation: advances and promises in the study of primordial germ cell migration

The germline provides the genetic and non-genetic information that passes from one generation to the next. Given this important role in species propagation, egg and sperm precursors, called primordial germ cells (PGCs), are one of the first cell types specified during embryogenesis. In fact, PGCs form well before the bipotential somatic gonad is specified. This common feature of germline development necessitates that PGCs migrate through many tissues to reach the somatic gonad. During their journey, PGCs must respond to select environmental cues while ignoring others in a dynamically developing embryo. The complex multi-tissue, combinatorial nature of PGC migration is an excellent model for understanding how cells navigate complex environments in vivo. Here, we discuss recent findings on the migratory path, the somatic cells that shepherd PGCs, the guidance cues somatic cells provide, and the PGC response to these cues to reach the gonad and establish the germline pool for future generations. We end by discussing the fate of wayward PGCs that fail to reach the gonad in diverse species. Collectively, this field is poised to yield important insights into emerging reproductive technologies.

A Novel, Efficient Method to Isolate Chicken Primordial Germ Cells from Embryonic Blood Using Cell Culture Inserts

Primordial germ cells (PGCs) play a crucial role in preserving poultry genetic resources and conducting transgenic research. A system for the rapid isolation of PGCs from single chicken embryonic blood was established in this paper. We found that PGCs can migrate to the lower layer of chicken embryonic fibroblasts (CEFs) through pores smaller than their diameter, while blood cells cannot, when co-cultured with CEFs of passages two to three. Based on the characteristics of PGCs, we developed a new PGC isolation method (cell culture insert/CEF adhesion method) that utilizes a 3 μm cell culture insert and CEFs of passages two to three. Using this method, approximately 700 PGCs can be isolated from the blood of a single chicken embryo at Hamburger and Hamilton (H&H) stage 17 of development. The separation rate achieved was 87.5%, with a separation purity of 95%. The separation rate of this method was 41.4% higher than the common Percoll density gradient centrifugation method and 33.6% higher than lysis with ACK buffer. PGCs isolated from embryonic blood could proliferate 37-fold within 2 weeks when cultured in a feeder-free culture system. They also continued to express the SSEA-1 and DAZL proteins and retained the ability to migrate in vivo. Overall, PGCs separated using cell culture inserts/CEF adhesion method retain their stem cell characteristics and migration ability. PGCs also exhibit good proliferation efficiency, making them suitable for subsequent transgenic experiments or genetic resource preservation.

Effect of environmental level of methomyl on hatching, morphology, immunity and development related genes expression in zebrafish (Danio rerio) embryo

The extensive use of carbamate pesticides has led to a range of environmental and health problems, such as surface and groundwater contamination, and endocrine disorders in organisms. In this study, we focused on examining the effects of toxic exposure to the carbamate pesticide methomyl on the hatching, morphology, immunity and developmental gene expression levels in zebrafish embryos. Four concentrations of methomyl (0, 2, 20, and 200 μg/L) were administered to zebrafish embryos for a period of 96 h. The study found that exposure to methomyl accelerated the hatching process of zebrafish embryos, with the strongest effect recorded at the concentration of 2 μg/L. Methomyl exposure also trigged significantly reductions in heart rate and caused abnormalities in larvae morphology, and it also stimulated the synthesis and release of several inflammatory factors such as IL-1β, IL-6, TNF-α and INF-α, lowered the IgM contents, ultimately enhancing inflammatory response and interfering with immune function. All of these showed the significant effects on exposure time, concentration and their interaction (Time × Concentration). Furthermore, the body length of zebrafish exposed to methomyl for 96 h was significantly shorter, particularly at higher concentrations (200 μg/L). Methomyl also affected the expression levels of genes associated with development (down-regulated igf1, bmp2b, vasa, dazl and piwi genes), demonstrating strong developmental toxicity and disruption of the endocrine system, with the most observed at the concentration of 200 μg/L and 96 h exposure to methomyl. The results of this study provide valuable reference information on the potential damage of methomyl concentrations in the environment on fish embryo development, while also supplementing present research on the immunotoxicity of methomyl.

The mitochondrial protein Sod2 is important for the migration, maintenance, and fitness of germ cells

To maintain a range of cellular functions and to ensure cell survival, cells must control their levels of reactive oxygen species (ROS). The main source of these molecules is the mitochondrial respiration machinery, and the first line of defense against these toxic substances is the mitochondrial enzyme superoxide dismutase 2 (Sod2). Thus, investigating early expression patterns and functions of this protein is critical for understanding how an organism develops ways to protect itself against ROS and enhance tissue fitness. Here, we report on expression pattern and function of zebrafish Sod2, focusing on the role of the protein in migration and maintenance of primordial germ cells during early embryonic development. We provide evidence that Sod2 is involved in purifying selection of vertebrate germ cells, which can contribute to the fitness of the organism in the following generations.

Spatial organization and function of RNA molecules within phase-separated condensates in zebrafish are controlled by Dnd1

Germ granules, condensates of phase-separated RNA and protein, are organelles that are essential for germline development in different organisms. The patterning of the granules and their relevance for germ cell fate are not fully understood. Combining three-dimensional in vivo structural and functional analyses, we study the dynamic spatial organization of molecules within zebrafish germ granules. We find that the localization of RNA molecules to the periphery of the granules, where ribosomes are localized, depends on translational activity at this location. In addition, we find that the vertebrate-specific Dead end (Dnd1) protein is essential for nanos3 RNA localization at the condensates' periphery. Accordingly, in the absence of Dnd1, or when translation is inhibited, nanos3 RNA translocates into the granule interior, away from the ribosomes, a process that is correlated with the loss of germ cell fate. These findings highlight the relevance of sub-granule compartmentalization for post-transcriptional control and its importance for preserving germ cell totipotency.

Spatial organization and function of RNA molecules within phase-separated condensates are controlled by Dnd1

Germ granules, condensates of phase-separated RNA and protein, are organelles essential for germline development in different organisms The patterning of the granules and its relevance for germ cell fate are not fully understood. Combining three-dimensional in vivo structural and functional analyses, we study the dynamic spatial organization of molecules within zebrafish germ granules. We find that localization of RNA molecules to the periphery of the granules, where ribosomes are localized depends on translational activity at this location. In addition, we find that the vertebrate-specific Dead end (Dnd1) protein is essential for nanos3 RNA localization at the condensates' periphery. Accordingly, in the absence of Dnd1, or when translation is inhibited, nanos3 RNA translocates into the granule interior, away from the ribosomes, a process that is correlated with loss of germ cell fate. These findings highlight the relevance of sub-granule compartmentalization for posttranscriptional control, and its importance for preserving germ cell totipotency.

MACC1-induced migration in tumors: Current state and perspective

Malignant tumors are still a global, heavy health burden. Many tumor types cannot be treated curatively, underlining the need for new treatment targets. In recent years, metastasis associated in colon cancer 1 (MACC1) was identified as a promising biomarker and drug target, as it is promoting tumor migration, initiation, proliferation, and others in a multitude of solid cancers. Here, we will summarize the current knowledge about MACC1-induced tumor cell migration with a special focus on the cytoskeletal and adhesive systems. In addition, a brief overview of several in vitro models used for the analysis of cell migration is given. In this context, we will point to issues with the currently most prevalent models used to study MACC1-dependent migration. Lastly, open questions about MACC1-dependent effects on tumor cell migration will be addressed.

Two Novel lncRNAs Regulate Primordial Germ Cell Development in Zebrafish

Long noncoding RNAs (lncRNAs) are regulatory transcripts in various biological processes. However, the role of lncRNAs in germline development remains poorly understood, especially for fish primordial germ cell (PGC) development. In this study, the lncRNA profile of zebrafish PGC was revealed by single cell RNA-sequencing and bioinformatic prediction. We established the regulation network of lncRNA-mRNA associated with PGC development, from which we identified three novel lncRNAs-lnc172, lnc196, and lnc304-highly expressing in PGCs and gonads. Fluorescent in situ hybridization indicated germline-specific localization of lnc196 and lnc304 in the cytoplasm and nucleus of spermatogonia, spermatocyte, and occyte, and they were co-localized with vasa in the cytoplasm of the spermatogonia. By contrast, lnc172 was localized in the cytoplasm of male germline, myoid cells and ovarian somatic cells. Loss- and gain-of-function experiments demonstrated that knockdown and PGC-specific overexpression of lnc304 as well as universal overexpression of lnc172 significantly disrupted PGC development. In summary, the present study revealed the lncRNA profile of zebrafish PGC and identified two novel lncRNAs associated with PGC development, providing new insights for understanding the regulatory mechanism of PGC development.

Primordial germ cells adjust their protrusion type while migrating in different tissue contexts in vivo

In both physiological processes and disease contexts, migrating cells have the ability to adapt to conditions in their environment. As an in vivo model for this process, we use zebrafish primordial germ cells that migrate throughout the developing embryo. When migrating within an ectodermal environment, the germ cells form fewer and smaller blebs when compared with their behavior within mesodermal environment. We find that cortical tension of neighboring cells is a parameter that affects blebbing frequency. Interestingly, the change in blebbing activity is accompanied by the formation of more actin-rich protrusions. These alterations in cell behavior that correlate with changes in RhoA activity could allow the cells to maintain dynamic motility parameters, such as migration speed and track straightness, in different settings. In addition, we find that the polarity of the cells can be affected by stiff structures positioned in their migration path This article has an associated 'The people behind the papers' interview.

How germline genes promote malignancy in cancer cells

Cancers often express hundreds of genes otherwise specific to germ cells, the germline/cancer (GC) genes. Here, we present and discuss the hypothesis that activation of a "germline program" promotes cancer cell malignancy. We do so by proposing four hallmark processes of the germline: meiosis, epigenetic plasticity, migration, and metabolic plasticity. Together, these hallmarks enable replicative immortality of germ cells as well as cancer cells. Especially meiotic genes are frequently expressed in cancer, implying that genes unique to meiosis may play a role in oncogenesis. Because GC genes are not expressed in healthy somatic tissues, they form an appealing source of specific treatment targets with limited side effects besides infertility. Although it is still unclear why germ cell specific genes are so abundantly expressed in cancer, from our hypothesis it follows that the germline's reproductive program is intrinsic to cancer development.

A self-generated Toddler gradient guides mesodermal cell migration

The sculpting of germ layers during gastrulation relies on the coordinated migration of progenitor cells, yet the cues controlling these long-range directed movements remain largely unknown. While directional migration often relies on a chemokine gradient generated from a localized source, we find that zebrafish ventrolateral mesoderm is guided by a self-generated gradient of the initially uniformly expressed and secreted protein Toddler/ELABELA/Apela. We show that the Apelin receptor, which is specifically expressed in mesodermal cells, has a dual role during gastrulation, acting as a scavenger receptor to generate a Toddler gradient, and as a chemokine receptor to sense this guidance cue. Thus, we uncover a single receptor-based self-generated gradient as the enigmatic guidance cue that can robustly steer the directional migration of mesoderm through the complex and continuously changing environment of the gastrulating embryo.

Hand2 delineates mesothelium progenitors and is reactivated in mesothelioma

The mesothelium lines body cavities and surrounds internal organs, widely contributing to homeostasis and regeneration. Mesothelium disruptions cause visceral anomalies and mesothelioma tumors. Nonetheless, the embryonic emergence of mesothelia remains incompletely understood. Here, we track mesothelial origins in the lateral plate mesoderm (LPM) using zebrafish. Single-cell transcriptomics uncovers a post-gastrulation gene expression signature centered on hand2 in distinct LPM progenitor cells. We map mesothelial progenitors to lateral-most, hand2-expressing LPM and confirm conservation in mouse. Time-lapse imaging of zebrafish hand2 reporter embryos captures mesothelium formation including pericardium, visceral, and parietal peritoneum. We find primordial germ cells migrate with the forming mesothelium as ventral migration boundary. Functionally, hand2 loss disrupts mesothelium formation with reduced progenitor cells and perturbed migration. In mouse and human mesothelioma, we document expression of LPM-associated transcription factors including Hand2, suggesting re-initiation of a developmental program. Our data connects mesothelium development to Hand2, expanding our understanding of mesothelial pathologies.

Transcriptional dynamics of the circulating chicken primordial germ cells revealing key genes in cell adhesion and proliferation prior to gonad colonization

Primordial germ cells (PGCs), precursors to sperms and oocytes, are responsible for the transfer of genetic information to the next generation. The PGCs arise far away from the developing gonad and thus have to migrate across the embryo to reach their site of function. The migration of PGCs from extraembryonic regions to the genital ridges is accomplished through distinct routes among different species. In particular, the birds PGCs utilized the developing circulation system to travel long distance before settling within the gonad. This study screened the transcriptome profile of chicken PGCs isolated from the bloodstream and the genital ridges to identify the cell intrinsic signals that could guide the unique migration path through circulation. We found cell adhesion and extracellular matrix (ECM) associated pathways were highly enriched in the PGCs from blood but not gonads. The platelet-derived growth factor receptors (PDGFRA and PDGFRB) were downregulated during gonad colonization and knockdown of either PDGFRA or PDGFRB inhibit the proliferation of blood PGCs. Furthermore, the migration of blood PGCs was impaired by the suppression of PDGFRA but not PDGFRB. Hence, the chicken PGCs show dynamic transcriptional remodeling during the blood-to-gonad migration and colonization. The free-floating PGCs in the circulation already express genes associated with cell-cell and cell-ECM interactions and therefore prepare for gonadal colonization.

Proper Balance of Small GTPase rab10 Is Critical for PGC Migration in Zebrafish

MicroRNAs (miRNAs) play important roles in post-transcriptional repression in nearly every biological process including germ cell development. Previously, we have identified a zebrafish germ plasm-specific miRNA miR-202-5p, which regulates PGC migration through targeting cdc42se1 to protect cdc42 expression. However, knockdown of cdc42se1 could not significantly rescue PGC migration in maternal miR-202 mutant (MmiR-202) embryos, indicating that there are other target genes of miR-202-5p required for the regulation of PGC migration. Herein, we revealed the transcriptional profiles of wild type and MmiR-202 PGCs and obtained 129 differentially expressed genes (DEGs), of which 42 DEGs were enriched cell migration-related signaling pathways. From these DEGs, we identified two novel miR-202-5p target genes prdm12b and rab10. Furthermore, we found that disruption of rab10 expression led to significantly migratory defects of PGC by overexpression of rab10 siRNA, or WT, inactive as well as active forms of rab10 mRNA, and WT rab10 overexpression mediated migratory defects could be partially but significantly rescued by overexpression of miR-202-5p, demonstrating that rab10 is an important factor involved miR-202-5p mediated regulation of PGC migration. Taken together, the present results provide significant information for understanding the molecular mechanism by which miR-202-5p regulates PGC migration in zebrafish.

Conserved germ plasm characteristics across the Danio and Devario lineages

In many animal species, germ cell specification requires the inheritance of germ plasm, a biomolecular condensate containing maternally derived RNAs and proteins. Most studies of germ plasm composition and function have been performed in widely evolutionarily divergent model organisms, such as Caenorhabditis elegans, Drosophila, Xenopus laevis, and Danio rerio (zebrafish). In zebrafish, 12 RNAs localize to germ plasm at the furrows of the early embryo. Here, we tested for the presence of these RNAs in three additional species within the Danionin clade: Danio kyathit, Danio albolineatus, and Devario aequipinnatus. By visualizing nanos RNA, we find that germ plasm segregation patterns during early embryogenesis are conserved across these species. Ten additional germ plasm RNAs exhibit localization at the furrows of early embryos in all three non-zebrafish Danionin species, consistent with germ plasm localization. One component of zebrafish germ plasm, ca15b, lacked specific localization in embryos of the more distantly related D. aequipinnatus. Our findings show that within a subset of closely related Danionin species, the vast majority of germ plasm RNA components are conserved. At the same time, the lack of ca15b localization in D. aequipinnatus germ plasm highlights the potential for the divergence of germ plasm composition across a restricted phylogenetic space.

Zebrafish Primordial Germ Cell Migration

Similar to many other organisms, zebrafish primordial germ cells (PGCs) are specified at a location distinct from that of gonadal somatic cells. Guided by chemotactic cues, PGCs migrate through embryonic tissues toward the region where the gonad develops. In this process, PGCs employ a bleb-driven amoeboid migration mode, characterized by low adhesion and high actomyosin contractility, a strategy used by other migrating cells, such as leukocytes and certain types of cancer cells. The mechanisms underlying the motility and the directed migration of PGCs should be robust to ensure arrival at the target, thereby contributing to the fertility of the organism. These features make PGCs an excellent model for studying guided single-cell migration in vivo. In this review, we present recent findings regarding the establishment and maintenance of cell polarity that are essential for motility and discuss the mechanisms by which cell polarization and directed migration are controlled by chemical and physical cues.

Manipulating and Visualizing the Germline with Transgenic Lines

Transgenic zebrafish in which the germline is specifically labeled with enhanced green fluorescent protein (eGFP) can be used for continuous observation of germline development during the lifetime, from the primordial germ cells (PGCs) in the early embryo to the gametes in the mature gonad. In this chapter, we describe a procedure for the generation of transgenic fish Tg(piwil1:egfp-UTRnanos3), the sample preparation for live imaging of PGCs, for high-resolution imaging of germ cells in developing gonads, and quantifying PGC numbers. The methods described in this chapter are not only applicable to the study of germ cells, but also provide general advices for researchers who are willing to generate transgenic zebrafish and do observation on live embryos as well as on fixed tissues.

Visualization of Transcriptional Activity in Early Zebrafish Primordial Germ Cells

Here, we describe a fast and straightforward methodology to in vivo detect transcriptional activity in the early zebrafish germ line. We report how fluorescently labeled morpholinos, targeted to nascent early transcripts, can be used to track the onset of transcriptional events during early embryogenesis. This method could be applied to any tagged cell line in a developing early zebrafish embryo as long as the gene of interest is expressed at high enough level for morpholino detection and is expressed at the first and main wave of genome activation, for which the protocol has been verified. The protocol, in combination with genetic manipulation, allows studies of mechanisms driving zygotic genome activation (ZGA) in individual cells. The reported procedures apply to a broad range of purposes for zebrafish embryo manipulation in view of imaging nuclear molecules in specific cell types.

Germ cell differentiation requires Tdrd7-dependent chromatin and transcriptome reprogramming marked by germ plasm relocalization

In many animal models, primordial germ cell (PGC) development depends on maternally deposited germ plasm, which prevents somatic cell fate. Here, we show that PGCs respond to regulatory information from the germ plasm in two distinct phases using two distinct mechanisms in zebrafish. We demonstrate that PGCs commence zygotic genome activation together with the somatic blastocysts with no demonstrable differences in transcriptional and chromatin opening. Unexpectedly, both PGC and somatic blastocysts activate germ-cell-specific genes, which are only stabilized in PGCs by cytoplasmic germ plasm determinants. Disaggregated perinuclear relocalization of germ plasm during PGC migration is regulated by the germ plasm determinant Tdrd7 and is coupled to dramatic divergence between PGC and somatic transcriptomes. This transcriptional divergence relies on PGC-specific cis-regulatory elements characterized by promoter-proximal distribution. We show that Tdrd7-dependent reconfiguration of chromatin accessibility is required for elaboration of PGC fate but not for PGC migration.

•

No evidence for transcriptional activation delay in zebrafish PGCs

•

Germ-plasm-associated post-transcriptional divergence during ZGA

•

Epigenetic reprogramming marks onset of PGC migration

•

Epigenetic reprogramming in PGCs relies on Tdrd7, coupled to germ plasm relocalization

The Role of DNA Methylation Reprogramming during Sex Determination and Transition in Zebrafish

DNA methylation is a prevalent epigenetic modification in vertebrates, and it has been shown to be involved the regulation of gene expression and embryo development. However, it remains unclear how DNA methylation regulates sexual development, especially in species without sex chromosomes. To determine this, we utilized zebrafish to investigate DNA methylation reprogramming during juvenile germ cell development and adult female-to-male sex transition. We reveal that primordial germ cells (PGCs) undergo significant DNA methylation reprogramming during germ cell development, and the methylome of PGCs is reset to an oocyte/ovary-like pattern at 9 days post fertilization (9 dpf). When DNA methyltransferase (DNMT) activity in juveniles was blocked after 9 dpf, the zebrafish developed into females. We also show that Tet3 is involved in PGC development. Notably, we find that DNA methylome reprogramming during adult zebrafish sex transition is similar to the reprogramming during the sex differentiation from 9 dpf PGCs to sperm. Furthermore, inhibiting DNMT activity can prevent the female-to-male sex transition, suggesting that methylation reprogramming is required for zebrafish sex transition. In summary, DNA methylation plays important roles in zebrafish germ cell development and sexual plasticity.

The Central Role of Cadherins in Gonad Development, Reproduction, and Fertility

Cadherins are a group of membrane proteins responsible for cell adhesion. They are crucial for cell sorting and recognition during the morphogenesis, but they also play many other roles such as assuring tissue integrity and resistance to stretching, mechanotransduction, cell signaling, regulation of cell proliferation, apoptosis, survival, carcinogenesis, etc. Within the cadherin superfamily, E- and N-cadherin have been especially well studied. They are involved in many aspects of sexual development and reproduction, such as germline development and gametogenesis, gonad development and functioning, and fertilization. E-cadherin is expressed in the primordial germ cells (PGCs) and also participates in PGC migration to the developing gonads where they become enclosed by the N-cadherin-expressing somatic cells. The differential expression of cadherins is also responsible for the establishment of the testis or ovary structure. In the adult testes, N-cadherin is responsible for the integrity of the seminiferous epithelium, regulation of sperm production, and the establishment of the blood-testis barrier. Sex hormones regulate the expression and turnover of N-cadherin influencing the course of spermatogenesis. In the adult ovaries, E- and N-cadherin assure the integrity of ovarian follicles and the formation of corpora lutea. Cadherins are expressed in the mature gametes and facilitate the capacitation of sperm in the female reproductive tract and gamete contact during fertilization. The germ cells and accompanying somatic cells express a series of different cadherins; however, their role in gonads and reproduction is still unknown. In this review, we show what is known and unknown about the role of cadherins in the germline and gonad development, and we suggest topics for future research.

E-cadherin focuses protrusion formation at the front of migrating cells by impeding actin flow

The migration of many cell types relies on the formation of actomyosin-dependent protrusions called blebs, but the mechanisms responsible for focusing this kind of protrusive activity to the cell front are largely unknown. Here, we employ zebrafish primordial germ cells (PGCs) as a model to study the role of cell-cell adhesion in bleb-driven single-cell migration in vivo. Utilizing a range of genetic, reverse genetic and mathematical tools, we define a previously unknown role for E-cadherin in confining bleb-type protrusions to the leading edge of the cell. We show that E-cadherin-mediated frictional forces impede the backwards flow of actomyosin-rich structures that define the domain where protrusions are preferentially generated. In this way, E-cadherin confines the bleb-forming region to a restricted area at the cell front and reinforces the front-rear axis of migrating cells. Accordingly, when E-cadherin activity is reduced, the bleb-forming area expands, thus compromising the directional persistence of the cells.

The arrival of migratory cells at their targets relies on following precise routes within tissues. Here the authors demonstrate that the cell adhesion molecule E-cadherin can control the path of cell migration by confining the site where bleb-type protrusions form within the cell front.

Cell-intrinsic Fgf signaling contributes to primordial germ cell homing in zebrafish

Primordial germ cells (PGCs) are specified before gastrulation and migrate toward the developing gonads. Previous in vitro studies have demonstrated a cell-intrinsic requirement of fibroblast growth factors (FGFs) by PGCs; however, no evidence suggests FGFs signal directly to PGCs in vivo. Here, using zebrafish as the animal model, we identified the mRNA expressions of Fgf receptors (Fgfrs) and determined the roles of Fgf signaling in migrating PGCs. To clarify the functions of Fgf signaling, we manipulated Fgf signaling specifically in PGCs using dominant-negative (dn) and constitutively-active (ca) Fgfrs and revealed a requirement of a basal Fgf signaling level for the robust arrival of PGCs. Repression of Fgf signaling in PGCs swayed the marginal positioning of PGCs as early as 6 h post-fertilization (6 hpf) and disrupted their arrival at the gonadal ridge at 24 hpf. On the other hand, the ectopic PGC phenotypes caused by the dn-Fgfrs could be alleviated by constitutive activation of Fgf signaling. In addition, we carefully ruled out the somatic effects in mosaic embryos by injecting RNA materials into one blastomere of the four- or eight-cell stage embryos. Injection of dn-Fgfrs into one of eight blastomeres hampered the arrival of only the treated PGCs, while the other PGCs remained unaffected. Furthermore, mosaic treatment of ca-Fgfrs rescued the ectopic rates of dn-Fgfr treated PGCs, while the other PGCs remained more ectopic within the same embryos. Interestingly, PGC-specific repression of Fgf signaling did not compromise the PGC number. To our knowledge, this is the first in vivo evidence to show that Fgf signaling plays a cell-intrinsic role in the migration of vertebrate PGCs.

Collectively stabilizing and orienting posterior migratory forces disperses cell clusters in vivo

Individual cells detach from cohesive ensembles during development and can inappropriately separate in disease. Although much is known about how cells separate from epithelia, it remains unclear how cells disperse from clusters lacking apical-basal polarity, a hallmark of advanced epithelial cancers. Here, using live imaging of the developmental migration program of Drosophila primordial germ cells (PGCs), we show that cluster dispersal is accomplished by stabilizing and orienting migratory forces. PGCs utilize a G protein coupled receptor (GPCR), Tre1, to guide front-back migratory polarity radially from the cluster toward the endoderm. Posteriorly positioned myosin-dependent contractile forces pull on cell-cell contacts until cells release. Tre1 mutant cells migrate randomly with transient enrichment of the force machinery but fail to separate, indicating a temporal contractile force threshold for detachment. E-cadherin is retained on the cell surface during cell separation and augmenting cell-cell adhesion does not impede detachment. Notably, coordinated migration improves cluster dispersal efficiency by stabilizing cell-cell interfaces and facilitating symmetric pulling. We demonstrate that guidance of inherent migratory forces is sufficient to disperse cell clusters under physiological settings and present a paradigm for how such events could occur across development and disease.

The role of the cytoskeleton in germ plasm aggregation and compaction in the zebrafish embryo

The transmission of genetic information from one generation to another is crucial for survival of animal species. This is accomplished by the induction of primordial germ cells (PGCs) that will eventually establish the germline. In some animals the germline is induced by signals in gastrula, whereas in others it is specified by inheritance of maternal determinants, known as germ plasm. In zebrafish, aggregation and compaction of maternally derived germ plasm during the first several embryonic cell cycles is essential for generation of PGCs. These processes are controlled by cellular functions associated with the cellular division apparatus. Ribonucleoparticles containing germ plasm components are bound to both the ends of astral microtubules and a dynamic F-actin network through a mechanism integrated with that which drives the cell division program. In this chapter we discuss the role that modifications of the cell division apparatus, including the cytoskeleton and cytoskeleton-associated proteins, play in the regulation of zebrafish germ plasm assembly.

The Paf1 complex and P-TEFb have reciprocal and antagonist roles in maintaining multipotent neural crest progenitors

Multipotent progenitor populations are necessary for generating diverse tissue types during embryogenesis. We show the RNA polymerase-associated factor 1 complex (Paf1C) is required to maintain multipotent progenitors of the neural crest (NC) lineage in zebrafish. Mutations affecting each Paf1C component result in near-identical NC phenotypes; alyron mutant embryos carrying a null mutation in paf1 were analyzed in detail. In the absence of zygotic paf1 function, definitive premigratory NC progenitors arise but fail to maintain expression of the sox10 specification gene. The mutant NC progenitors migrate aberrantly and fail to differentiate appropriately. Blood and germ cell progenitor development is affected similarly. Development of mutant NC could be rescued by additional loss of positive transcription elongation factor b (P-TEFb) activity, a key factor in promoting transcription elongation. Consistent with the interpretation that inhibiting/delaying expression of some genes is essential for maintaining progenitors, mutant embryos lacking the CDK9 kinase component of P-TEFb exhibit a surfeit of NC progenitors and their derivatives. We propose Paf1C and P-TEFb act antagonistically to regulate the timing of the expression of genes needed for NC development.

Hmgcr promotes a long-range signal to attract Drosophila germ cells independently of Hedgehog

During development, many cell types migrate along stereotyped routes determined through deployment of cell surface or secreted guidance molecules. Although we know the identity of many of these molecules, the distances over which they natively operate can be difficult to determine. Here, we have quantified the range of an attractive signal for the migration of Drosophila germ cells. Their migration is guided by an attractive signal generated by the expression of genes in the 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (Hmgcr) pathway, and by a repulsive signal generated by the expression of Wunens. We demonstrate that the attractive signal downstream of Hmgcr is cell-contact independent and acts at long range, the extent of which depends on Hmgcr levels. This range would be sufficient to reach all of the germ cells for their entire migration. Furthermore, Hmgcr-mediated attraction does not require Wunens but can operate simultaneously with Wunen-mediated repulsion. Finally, several papers posit Hedgehog (Hh) as being the germ cell attractant downstream of H mgcr Here, we provide evidence that this is not the case.

Dorsal convergence of gastrula cells requires Vangl2 and an adhesion protein-dependent change in protrusive activity

Lateral zebrafish hypoblast cells initiate dorsal convergence near mid-gastrulation and exhibit non-polarized morphologies, limited cell-cell contact and indirect migration trajectories. By late gastrulation, mesodermal cells become packed as they engage in planar cell polarity (PCP)-dependent movement. Here, we aimed to understand this transition in cell behavior by examining the relationship between protrusion dynamics and establishment of PCP and directed migration. We found that wild-type cells undergo a reduction in bleb protrusions near late gastrulation accompanied by a VANGL planar cell polarity protein 2 (Vangl2)-regulated increase in filopodia number and polarization. Manipulation of blebs is sufficient to interfere with PCP and directed migration. We show that Vangl2, fibronectin and cadherin 2 function to suppress blebbing. Vangl2 maintains ezrin b (Ezrb) protein levels and higher Ezrb activation rescues defective mediolateral cell alignment and migration paths in vangl2 mutant embryos. Transplantation experiments show that loss of vangl2 disrupts protrusion formation cell-autonomously while fibronectin acts non-autonomously. We propose that dorsal convergence requires the coordinated action of Vangl2, Ezrb and cell-adhesion proteins to inhibit blebs and promote polarized actin-rich protrusive activity and PCP.

Transcriptomic profile of early zebrafish PGCs by single cell sequencing

Single cell RNA-seq is a powerful and sensitive way to capture the genome-wide gene expression. Here, single cell RNA-seq was utilized to study the transcriptomic profile of early zebrafish PGCs (primordial germ cells) at three different developmental stages. The three stages were 6, 11 and 24 hpf (hours post fertilization). For each developmental stage, three zebrafish PGCs from one embryo were collected, and 9 samples in total were used in this experiment. Single cell RNA-seq results showed that 5099–7376 genes were detected among the 9 samples, and the number of expressed genes decreased as development progressed. Based on the gene expression pattern, samples from 6 and 11 hpf clustered closely, while samples from 24 hpf were more dispersed. By WGCNA (weighted gene co-expression network analysis), the two biggest modules that had inverse gene expression patterns were found to be related to PGC formation or migration. Functional enrichment analysis for these two modules showed that PGCs mainly conducted migration and cell division in early development (6/11 hpf) and translation activity became active in late development (24 hpf). Differentially expressed gene analyses showed that more genes were downregulated than upregulated between two adjacent stages, and genes related to PGC formation or migration reported by previous studies decreased significantly from 11 to 24 hpf. Our results provide base knowledge about zebrafish PGC development at the single cell level and can be further studied by other researchers interested in biological development.

Retention of paternal DNA methylome in the developing zebrafish germline

Two waves of DNA methylation reprogramming occur during mammalian embryogenesis; during preimplantation development and during primordial germ cell (PGC) formation. However, it is currently unclear how evolutionarily conserved these processes are. Here we characterise the DNA methylomes of zebrafish PGCs at four developmental stages and identify retention of paternal epigenetic memory, in stark contrast to the findings in mammals. Gene expression profiling of zebrafish PGCs at the same developmental stages revealed that the embryonic germline is defined by a small number of markers that display strong developmental stage-specificity and that are independent of DNA methylation-mediated regulation. We identified promoters that are specifically targeted by DNA methylation in somatic and germline tissues during vertebrate embryogenesis and that are frequently misregulated in human cancers. Together, these detailed methylome and transcriptome maps of the zebrafish germline provide insight into vertebrate DNA methylation reprogramming and enhance our understanding of the relationships between germline fate acquisition and oncogenesis.

Heterogeneity of primordial germ cells

Primordial germ cells (PGCs) must complete a complex and dynamic developmental program during embryogenesis to establish the germline. This process is highly conserved and involves a diverse array of tasks required of PGCs, including migration, survival, sex differentiation, and extensive epigenetic reprogramming. A common theme across many organisms is that PGC success is heterogeneous: only a portion of all PGCs complete all these steps while many other PGCs are eliminated from further germline contribution. The differences that distinguish successful PGCs as a population are not well understood. Here, we examine variation that exists in PGCs as they navigate the many stages of this developmental journey. We explore potential sources of PGC heterogeneity and their potential implications in affecting germ cell behaviors. Lastly, we discuss the potential for PGC development to function as a multistage selection process that assesses heterogeneity in PGCs to refine germline quality.

The Cytoskeleton-A Complex Interacting Meshwork

The cytoskeleton of animal cells is one of the most complicated and functionally versatile structures, involved in processes such as endocytosis, cell division, intra-cellular transport, motility, force transmission, reaction to external forces, adhesion and preservation, and adaptation of cell shape. These functions are mediated by three classical cytoskeletal filament types, as follows: Actin, microtubules, and intermediate filaments. The named filaments form a network that is highly structured and dynamic, responding to external and internal cues with a quick reorganization that is orchestrated on the time scale of minutes and has to be tightly regulated. Especially in brain tumors, the cytoskeleton plays an important role in spreading and migration of tumor cells. As the cytoskeletal organization and regulation is complex and many-faceted, this review aims to summarize the findings about cytoskeletal filament types, including substructures formed by them, such as lamellipodia, stress fibers, and interactions between intermediate filaments, microtubules and actin. Additionally, crucial regulatory aspects of the cytoskeletal filaments and the formed substructures are discussed and integrated into the concepts of cell motility. Even though little is known about the impact of cytoskeletal alterations on the progress of glioma, a final point discussed will be the impact of established cytoskeletal alterations in the cellular behavior and invasion of glioma.

Abundance of Early Embryonic Primordial Germ Cells Promotes Zebrafish Female Differentiation as Revealed by Lifetime Labeling of Germline

Teleost sex differentiation largely depends on the number of undifferentiated germ cells. Here, we describe the generation and characterization of a novel transgenic zebrafish line, Tg(piwil1:egfp-UTRnanos3)^ihb327Tg, which specifically labels the whole lifetime of germ cells, i.e., from primordial germ cells (PGCs) at shield stage to the oogonia and early stage of oocytes in the ovary and to the early stage of spermatogonia, spermatocyte, and spermatid in the testis. By using this transgenic line, we carefully observed the numbers of PGCs from early embryonic stage to juvenile stage and the differentiation process of ovary and testis. The numbers of PGCs became variable at as early as 1 day post-fertilization (dpf). Interestingly, the embryos with a high amount of PGCs mainly developed into females and the ones with a low amount of PGCs mainly developed into males. By using transient overexpression and transgenic induction of PGC-specific bucky ball (buc), we further proved that induction of abundant PGCs at embryonic stage promoted later ovary differentiation and female development. Taken together, we generate an ideal transgenic line Tg(piwil1:egfp-UTRnanos3)^ihb327Tg which can visualize zebrafish germline for a lifetime, and we have utilized this line to study germ cell development and gonad differentiation of teleost and to demonstrate that the increase of PGC number at embryonic stage promotes female differentiation.

Aggregation, segregation, and dispersal of homotypic germ plasm RNPs in the early zebrafish embryo

BACKGROUND

In zebrafish and many other organisms, specification of primordial germ cells (PGCs) requires the transmission of maternally-derived germ plasm. Zebrafish germ plasm ribonucleoparticles (RNPs) aggregate along the cleavage furrows during the first several cell cycles, segregate asymmetrically during the cleavage stages, and undergo cytoplasmic dispersal in the late blastula.

RESULTS

For all tested germ plasm RNAs [carbonic anhydrase 15b (ca15b), deleted in azoospermia-like (dazl), dead end (dnd), nanos 3 (nos3), regulator of G-protein signaling14a (rgs14a), and vasa/DEAD box polypeptide 4 (vasa/ddx4)], RNPs are homotypic (containing a single RNA type), with RNPs packing tightly yet remaining distinct within germ plasm aggregates. Homotypic clustering of RNAs within RNPs is observed before aggregation in the cortex and is maintained through germ plasm recruitment, asymmetric segregation and RNP dispersal. We also identify a step of germ plasm fragmentation during the cleavage stages that precedes RNP dispersal.

CONCLUSIONS

Our findings suggest that germ plasm aggregates act as subcellular compartments that temporarily collect and carry single RNA-type RNPs from fertilization until their cytoplasmic dispersal in PGCs at the end of the blastula period, and describe a previously unknown fragmentation step that allows for an increase in the pool of germ plasm-carrying cells, presumably PGCs. Developmental Dynamics 248:306-318, 2019. © 2019 Wiley Periodicals, Inc.

Fluid dynamics during bleb formation in migrating cells in vivo

Blebs are cellular protrusions observed in migrating cells and in cells undergoing spreading, cytokinesis, and apoptosis. Here we investigate the flow of cytoplasm during bleb formation and the concurrent changes in cell volume using zebrafish primordial germ cells (PGCs) as an in vivo model. We show that bleb inflation occurs concomitantly with cytoplasmic inflow into it and that during this process the total cell volume does not change. We thus show that bleb formation in primordial germ cells results primarily from redistribution of material within the cell rather than being driven by flow of water from an external source.

"A narrow bridge home": The dorsal mesentery in primordial germ cell migration

Specification of primordial germ cells (PGCs) in all vertebrates takes place in extragonadal sites. This requires migration of PGCs through embryonic tissues towards the genital ridges by both passive and active types of migration. Commonly, colonization in the genital ridges follows migration of the PGCs along the thin tissue of the dorsal mesentery. Here we review the anatomy of the dorsal mesentery, the role it plays in migration of PGCs, and the interactions of PGCs with different cell types, extracellular matrix and signaling pathways that are all essential for attraction and orientation of PGCs along the dorsal mesentery towards the gonad anlage.

Tdrd6a Regulates the Aggregation of Buc into Functional Subcellular Compartments that Drive Germ Cell Specification

Phase separation represents an important form of subcellular compartmentalization. However, relatively little is known about how the formation or disassembly of such compartments is regulated. In zebrafish, the Balbiani body (Bb) and the germ plasm (Gp) are intimately linked phase-separated structures essential for germ cell specification and home to many germ cell-specific mRNAs and proteins. Throughout development, these structures occur as a single large aggregate (Bb), which disperses throughout oogenesis and upon fertilization accumulates again into relatively large assemblies (Gp). Formation of the Bb requires Bucky ball (Buc), a protein with prion-like properties. We found that the multi-tudor domain-containing protein Tdrd6a interacts with Buc, affecting its mobility and aggregation properties. Importantly, lack of this regulatory interaction leads to significant defects in germ cell development. Our work presents insights into how prion-like protein aggregations can be regulated and highlights the biological relevance of such regulatory events.

•

Tdrd6a is required for Bucky ball mobility within aggregates, and for PGC formation

•

Maternal Tdrd6a coordinates transcript deposition into future PGCs

•

A dimethylated tri-RG motif in Bucky ball mediates interaction with Tdrd6a

•

The tri-RG motif is essential for Balbiani body and germ cell formation

The Air Sac Primordium of Drosophila: A Model for Invasive Development

The acquisition of invasive properties preceding tumor metastasis is critical for cancer progression. This phenomenon may result from mutagenic disruption of typical cell function, but recent evidence suggests that cancer cells frequently co-opt normal developmental programs to facilitate invasion as well. The signaling cascades that have been implicated present an obstacle to identifying effective therapeutic targets because of their complex nature and modulatory capacity through crosstalk with other pathways. Substantial efforts have been made to study invasive behavior during organogenesis in several organisms, but another model found in Drosophilamelanogaster has not been thoroughly explored. The air sac primordium (ASP) appears to be a suitable candidate for investigating the genes and morphogens required for invasion due to the distinct overlap in the events that occur during its normal growth and the development of metastatic tumor cells. Among these events are the conversion of larval cells in the trachea into a population of mitotically active cells, reduced cell–cell contact along the leading edge of the ASP, and remodeling of the extracellular matrix (ECM) that surrounds the structure. Here, we summarize the development of ASPs and invasive behavior observed therein.

Cell motility in cancer invasion and metastasis: insights from simple model organisms

Metastasis remains the greatest challenge in the clinical management of cancer. Cell motility is a fundamental and ancient cellular behaviour that contributes to metastasis and is conserved in simple organisms. In this Review, we evaluate insights relevant to human cancer that are derived from the study of cell motility in non-mammalian model organisms. Dictyostelium discoideum, Caenorhabditis elegans, Drosophila melanogaster and Danio rerio permit direct observation of cells moving in complex native environments and lend themselves to large-scale genetic and pharmacological screening. We highlight insights derived from each of these organisms, including the detailed signalling network that governs chemotaxis towards chemokines; a novel mechanism of basement membrane invasion; the positive role of E-cadherin in collective direction-sensing; the identification and optimization of kinase inhibitors for metastatic thyroid cancer on the basis of work in flies; and the value of zebrafish for live imaging, especially of vascular remodelling and interactions between tumour cells and host tissues. While the motility of tumour cells and certain host cells promotes metastatic spread, the motility of tumour-reactive T cells likely increases their antitumour effects. Therefore, it is important to elucidate the mechanisms underlying all types of cell motility, with the ultimate goal of identifying combination therapies that will increase the motility of beneficial cells and block the spread of harmful cells.

Rapid progression through the cell cycle ensures efficient migration of primordial germ cells - The role of Hsp90

Zebrafish primordial germ cells (PGCs) constitute a useful in vivo model to study cell migration and to elucidate the role of specific proteins in this process. Here we report on the role of the heat shock protein Hsp90aa1.2, a protein whose RNA level is elevated in the PGCs during their migration. Reducing Hsp90aa1.2 activity slows down the progression through the cell cycle and leads to defects in the control over the MTOC number in the migrating cells. These defects result in a slower migration rate and compromise the arrival of PGCs at their target, the region where the gonad develops. Our results emphasize the importance of ensuring rapid progression through the cell cycle during single-cell migration and highlight the role of heat shock proteins in the process.

Bleb Expansion in Migrating Cells Depends on Supply of Membrane from Cell Surface Invaginations

Cell migration is essential for morphogenesis, organ formation, and homeostasis, with relevance for clinical conditions. The migration of primordial germ cells (PGCs) is a useful model for studying this process in the context of the developing embryo. Zebrafish PGC migration depends on the formation of cellular protrusions in form of blebs, a type of protrusion found in various cell types. Here we report on the mechanisms allowing the inflation of the membrane during bleb formation. We show that the rapid expansion of the protrusion depends on membrane invaginations that are localized preferentially at the cell front. The formation of these invaginations requires the function of Cdc42, and their unfolding allows bleb inflation and dynamic cell-shape changes performed by migrating cells. Inhibiting the formation and release of the invaginations strongly interfered with bleb formation, cell motility, and the ability of the cells to reach their target.

Dissection of Larval Zebrafish Gonadal Tissue

Although wild zebrafish possess a ZZ/ZW sex-determination system, domesticated zebrafish have lost the sex chromosome. They utilize a polygenic sex determination system, where several genes distributed throughout the genome collectively determine the sex identities of individual fish. Currently, the genes involved in regulating gonad development and how they work remain elusive. Normally, isolating gonadal tissue is the first step to examine the sex developmental processes. Here, we present a procedure to isolate gonadal tissue from 17 dpf (days post fertilization) and 25 dpf zebrafish larvae. The isolated gonadal tissue may be subsequently examined by morphology and gene expression profiling.