Bazooka regulates microtubule organization and spatial restriction of germ plasm assembly in the Drosophila oocyte

The Balbiani body is formed by microtubule-controlled molecular condensation of Buc in early oogenesis

Vertebrate oocyte polarity has been observed for two centuries and is essential for embryonic axis formation and germline specification, yet its underlying mechanisms remain unknown. In oocyte polarization, critical RNA-protein (RNP) granules delivered to the oocyte's vegetal pole are stored by the Balbiani body (Bb), a membraneless organelle conserved across species from insects to humans. However, the mechanisms of Bb formation are still unclear. Here, we elucidate mechanisms of Bb formation in zebrafish through developmental biomolecular condensation. Using super-resolution microscopy, live imaging, biochemical, and genetic analyses in vivo, we demonstrate that Bb formation is driven by molecular condensation through phase separation of the essential intrinsically disordered protein Bucky ball (Buc). Live imaging, molecular analyses, and fluorescence recovery after photobleaching (FRAP) experiments in vivo reveal Buc-dependent changes in the Bb condensate's dynamics and apparent material properties, transitioning from liquid-like condensates to a solid-like stable compartment. Furthermore, we identify a multistep regulation by microtubules that controls Bb condensation: first through dynein-mediated trafficking of early condensing Buc granules, then by scaffolding condensed granules, likely through molecular crowding, and finally by caging the mature Bb to prevent overgrowth and maintain shape. These regulatory steps ensure the formation of a single intact Bb, which is considered essential for oocyte polarization and embryonic development. Our work offers insight into the long-standing question of the origins of embryonic polarity in non-mammalian vertebrates, supports a paradigm of cellular control over molecular condensation by microtubules, and highlights biomolecular condensation as a key process in female reproduction.

Polarity Events in the Drosophila melanogaster Oocyte

Cell polarity is a pre-requirement for many fundamental processes in animal cells, such as asymmetric cell division, axon specification, morphogenesis and epithelial tissue formation. For all these different processes, polarization is established by the same set of proteins, called partitioning defective (Par) proteins. During development in Drosophila melanogaster, decision making on the cellular and organism level is achieved with temporally controlled cell polarization events. The initial polarization of Par proteins occurs as early as in the germline cyst, when one of the 16 cells becomes the oocyte. Another marked event occurs when the anterior–posterior axis of the future organism is defined by Par redistribution in the oocyte, requiring external signaling from somatic cells. Here, we review the current literature on cell polarity events that constitute the oogenesis from the stem cell to the mature egg.

Restriction of subapical proteins during cellularization depends on the onset of zygotic transcription and the formin Dia

Cortical domains are characterized by spatially restricted polarity proteins. The pattern of cortical domains is dynamic and changes during cell differentiation and development. Although there is a good understanding for how the cortical pattern is maintained, e. g. by mutual antagonism, less is known about how the initial pattern is established, and its dynamics coordinated with developmental progression. Here we investigate the initial restriction of subapical marker proteins during the syncytial-cellular transition in Drosophila embryos. The subapical markers Canoe/Afadin, the complex ELMO-Sponge, Baz and Arm become initially restricted between apical and lateral domains during cellularization. We define the role of zygotic genome activation as a timer for subapical domain formation. Subapical markers remained widely spread in embryos treated with α-amanitin and became precociously restricted in mutant embryos with premature zygotic transcription. In contrast, remodeling of the nuclear division cycle without cytokinesis to a full cell cycle is not a prerequisite for subapical domain formation, since we observed timely subapical restriction in embryos undergoing an extra nuclear cycle. We provide evidence that earliest subapical markers ELMO-Sponge and Canoe are required for subapical accumulation of Baz. Supporting an important role of cortical F-actin in subapical restriction, we found that the formin Dia was required for Baz restriction, and its distribution depended on the onset of zygotic gene expression. In summary, we define zygotic transcription as a timer, to which subapical markers respond in a dia-dependent mechanism.

Signaling between somatic follicle cells and the germline patterns the egg and embryo of Drosophila

In Drosophila, specification of the embryonic body axes requires signaling between the germline and the somatic follicle cells. These signaling events are necessary to properly localize embryonic patterning determinants in the egg or eggshell during oogenesis. There are three maternal patterning systems that specify the anterior-posterior axis, and one that establishes the dorsal-ventral axis. We will first review oogenesis, focusing on the establishment of the oocyte and nurse cells and patterning of the follicle cells into different subpopulations. We then describe how two coordinated signaling events between the oocyte and follicle cells establish polarity of the oocyte and localize the anterior determinant bicoid, the posterior determinant oskar, and Gurken/epidermal growth factor (EGF), which breaks symmetry to initiate dorsal-ventral axis establishment. Next, we review how dorsal-ventral asymmetry of the follicle cells is transmitted to the embryo. This process also involves Gurken-EGF receptor (EGFR) signaling between the oocyte and follicle cells, leading to ventrally-restricted expression of the sulfotransferase Pipe. These events promote the ventral processing of Spaetzle, a ligand for Toll, which ultimately sets up the embryonic dorsal-ventral axis. We then describe the activation of the terminal patterning system by specialized polar follicle cells. Finally, we present open questions regarding soma-germline signaling during Drosophila oogenesis required for cell identity and embryonic axis formation.

The CPEB translational regulator, Orb, functions together with Par proteins to polarize the Drosophila oocyte

orb is a founding member of the CPEB family of translational regulators and is required at multiple steps during Drosophila oogenesis. Previous studies showed that orb is required during mid-oogenesis for the translation of the posterior/germline determinant oskar mRNA and the dorsal-ventral determinant gurken mRNA. Here, we report that orb also functions upstream of these axes determinants in the polarization of the microtubule network (MT). Prior to oskar and gurken translational activation, the oocyte MT network is repolarized. The MT organizing center at the oocyte posterior is disassembled, and a new MT network is established at the oocyte anterior. Repolarization depends upon cross-regulatory interactions between anterior (apical) and posterior (basal) Par proteins. We show that repolarization of the oocyte also requires orb and that orb is needed for the proper functioning of the Par proteins. orb interacts genetically with aPKC and cdc42 and in egg chambers compromised for orb activity, Par-1 and aPKC protein and aPKC mRNA are mislocalized. Moreover, like cdc42-, the defects in Par protein localization appear to be connected to abnormalities in the cortical actin cytoskeleton. These abnormalities also disrupt the localization of the spectraplakin Shot and the microtubule minus-end binding protein Patronin. These two proteins play a critical role in the repolarization of the MT network.

Collaborative Control of Cell Cycle Progression by the RNA Exonuclease Dis3 and Ras Is Conserved Across Species

Dis3 encodes a conserved RNase that degrades or processes all RNA species via an N-terminal PilT N terminus (PIN) domain and C-terminal RNB domain that harbor, respectively, endonuclease activity and 3'-5' exonuclease activity. In Schizosaccharomyces pombe, dis3 mutations cause chromosome missegregation and failure in mitosis, suggesting dis3 promotes cell division. In humans, apparently hypomorphic dis3 mutations are found recurrently in multiple myeloma, suggesting dis3 opposes cell division. Except for the observation that RNAi-mediated depletion of dis3 function drives larval arrest and reduces tissue growth in Drosophila, the role of dis3 has not been rigorously explored in higher eukaryotic systems. Using the Drosophila system and newly generated dis3 null alleles, we find that absence of dis3 activity inhibits cell division. We uncover a conserved CDK1 phosphorylation site that when phosphorylated inhibits Dis3's exonuclease, but not endonuclease, activity. Leveraging this information, we show that Dis3's exonuclease function is required for mitotic cell division: in its absence, cells are delayed in mitosis and exhibit aneuploidy and overcondensed chromosomes. In contrast, we find that modest reduction of dis3 function enhances cell proliferation in the presence of elevated Ras activity, apparently by accelerating cells through G2/M even though each insult by itself delays G2/M. Additionally, we find that dis3 and ras genetically interact in worms and that dis3 can enhance cell proliferation under growth stimulatory conditions in murine B cells. Thus, reduction, but not absence, of dis3 activity can enhance cell proliferation in higher organisms.

Multiple Roles for Egalitarian in Polarization of the Drosophila Egg Chamber

The Drosophila egg chamber provides a useful model for examining mechanisms by which cell fates are specified and maintained in the context of a complex tissue. The egg chamber is also an excellent model for understanding the mechanism by which cytoskeletal filaments are organized and the critical interplay between cytoskeletal organization, polarity establishment, and cell fate specification. Previous work has shown that Egalitarian (Egl) is required for specification and maintenance of oocyte fate. Mutants in egl either completely fail to specify an oocyte, or if specified, the oocyte eventually reverts back to nurse cell fate. Due to this very early role for Egl in egg chamber maturation, it is unclear whether later stages of egg chamber development also require Egl function. In this report, we have depleted Egl at specific stages of egg chamber development. We demonstrate that in early-stage egg chambers, Egl has an additional role in organization of oocyte microtubules. In the absence of Egl function, oocyte microtubules completely fail to reorganize. As such, the localization of microtubule motors and their cargo is disrupted. In addition, Egl also appears to function in regulating the translation of critical polarity determining messenger RNAs (mRNAs). Finally, we demonstrate that in midstage egg chambers, Egl does not appear to be required for microtubule organization, but rather for the correct spatial localization of oskar, bicoid, and gurken mRNAs.

Establishing and maintaining cell polarity with mRNA localization in Drosophila

How cell polarity is established and maintained is an important question in diverse biological contexts. Molecular mechanisms used to localize polarity proteins to distinct domains are likely context-dependent and provide a feedback loop in order to maintain polarity. One such mechanism is the localized translation of mRNAs encoding polarity proteins, which will be the focus of this review and may play a more important role in the establishment and maintenance of polarity than is currently known. Localized translation of mRNAs encoding polarity proteins can be used to establish polarity in response to an external signal, and to maintain polarity by local production of polarity determinants. The importance of this mechanism is illustrated by recent findings, including orb2-dependent localized translation of aPKC mRNA at the apical end of elongating spermatid tails in the Drosophila testis, and the apical localization of stardust A mRNA in Drosophila follicle and embryonic epithelia.

Bazooka/PAR3 is dispensable for polarity in Drosophila follicular epithelial cells

Apico-basal polarity is the defining characteristic of epithelial cells. In Drosophila, apical membrane identity is established and regulated through interactions between the highly conserved Par complex (Bazooka/Par3, atypical protein kinase C and Par6), and the Crumbs complex (Crumbs, Stardust and PATJ). It has been proposed that Bazooka operates at the top of a genetic hierarchy in the establishment and maintenance of apico-basal polarity. However, there is still ambiguity over the correct sequence of events and cross-talk with other pathways during this process. In this study, we reassess this issue by comparing the phenotypes of the commonly used baz(4) and baz(815-8) alleles with those of the so far uncharacterized baz(XR11) and baz(EH747) null alleles in different Drosophila epithelia. While all these baz alleles display identical phenotypes during embryonic epithelial development, we observe strong discrepancies in the severity and penetrance of polarity defects in the follicular epithelium: polarity is mostly normal in baz(EH747) and baz(XR11) while baz(4) and baz(815) (-8) show loss of polarity, severe multilayering and loss of epithelial integrity throughout the clones. Further analysis reveals that the chromosomes carrying the baz(4) and baz(815-8) alleles may contain additional mutations that enhance the true baz loss-of-function phenotype in the follicular epithelium. This study clearly shows that Baz is dispensable for the regulation of polarity in the follicular epithelium, and that the requirement for key regulators of cell polarity is highly dependent on developmental context and cell type.

RNA in development: how ribonucleoprotein granules regulate the life cycles of pathogenic protozoa

Ribonucleoprotein (RNP) granules are important posttranscriptional regulators of messenger RNA (mRNA) fate. Several types of RNP granules specifically regulate gene expression during development of multicellular organisms and are commonly referred to as germ granules. The function of germ granules is not entirely understood and probably diverse, but it is generally agreed that one main function is posttranscriptional regulation of gene expression during early development, when transcription is silent. One example is the translational repression of maternally derived mRNAs in oocytes. Here, I hope to show that the need for regulation of gene expression by RNP granules is not restricted to animal development, but plays an equally important role during the development of pathogenic protozoa. Apicomplexa and Trypanosomatidae have complex life cycles with frequent host changes. The need to quickly adapt gene expression to a new environment as well as the ability to suppress translation to survive latencies is critical for successful completion of life cycles. Posttranscriptional gene regulation is not necessarily simpler in protozoa. Apicomplexa surprise with the presence of micro RNA (miRNAs) and upstream open reading frames (µORFs). Trypanosomes have an unusually large repertoire of different RNP granule types. A better understanding of RNP granules in protozoa may help to gain insight into the evolutionary origin of RNP granules: Trypanosomes for example have two types of granules with interesting similarities to animal germ granules.

Nuclear genes with sex bias in Ruditapes philippinarum (Bivalvia, veneridae): Mitochondrial inheritance and sex determination in DUI species

Mitochondria are inherited maternally in most metazoans, but in bivalves with Doubly Uniparental Inheritance (DUI) a mitochondrial lineage is transmitted through eggs (F-type), and another through sperm (M-type). In DUI species, a sex-ratio distortion of the progeny was observed: some females produce a female-biased offspring (female-biased family), others a male-biased progeny (male-biased family), and others a 50:50 sex-ratio. A peculiar segregation pattern of M-type mitochondria in DUI organisms appears to be correlated with the sex bias of these families. According to a proposed model for the inheritance of M-type mitochondria in DUI, the transmission of sperm mitochondria is controlled by three nuclear genes, named W, X, and Z. An additional S gene with different dosage effect would be involved in sex determination. In this study, we analyzed structure and localization of three transcripts (psa, birc, and anubl1) with specific sex and family biases in the Manila clam Ruditapes philippinarum. In situ hybridization confirmed the localization of these transcripts in gametogenic cells. In other animals, homologs of these genes are involved in reproduction and ubiquitination. We hypothesized that these genes may have a role in sex determination and could also be responsible for the maintenance/degradation of spermatozoon mitochondria during embryo development of the DUI species R. philippinarum, so that we propose them as candidate factors of the W/X/Z/S system.

Regulation of microtubule stability and organization by mammalian Par3 in specifying neuronal polarity

Polarization of mammalian neurons with a specified axon requires precise regulation of microtubule and actin dynamics in the developing neurites. Here we show that mammalian partition defective 3 (mPar3), a key component of the Par polarity complex that regulates the polarization of many cell types including neurons, directly regulates microtubule stability and organization. The N-terminal portion of mPar3 exhibits strong microtubule binding, bundling, and stabilization activity, which can be suppressed by its C-terminal portion via an intramolecular interaction. Interestingly, the intermolecular oligomerization of mPar3 is able to relieve the intramolecular interaction and thereby promote microtubule bundling and stabilization. Furthermore, disruption of this microtubule regulatory activity of mPar3 impairs its function in axon specification. Together, these results demonstrate a role for mPar3 in directly regulating microtubule organization that is crucial for neuronal polarization.

Next generation organelles: structure and role of germ granules in the germline

Germ cells belong to a unique class of stem cells that gives rise to eggs and sperm, and ultimately to an entire organism after gamete fusion. In many organisms, germ cells contain electron-dense structures that are also known as nuage or germ granules. Although germ granules were discovered more than 100 years ago, their composition, structure, assembly, and function are not fully understood. Germ granules contain non-coding RNAs, mRNAs, and proteins required for germline development. Here we review recent studies that highlight the importance of several protein families in germ granule assembly and function, including germ granule inducers, which initiate the granule formation, and downstream components, such as RNA helicases and Tudor domain-Piwi protein-piRNA complexes. Assembly of these components into one granule is likely to result in a highly efficient molecular machine that ensures translational control and protects germline DNA from mutations caused by mobile genetic elements. Furthermore, recent studies have shown that different somatic cells, including stem cells and neurons, produce germ granule components that play a crucial role in stem cell maintenance and memory formation, indicating a much more diverse functional repertoire for these organelles than previously thought.

Doubly uniparental inheritance of mitochondria as a model system for studying germ line formation

BACKGROUND

Doubly Uniparental Inheritance (DUI) of mitochondria occurs when both mothers and fathers are capable of transmitting mitochondria to their offspring, in contrast to the typical Strictly Maternal Inheritance (SMI). DUI was found in some bivalve molluscs, in which two mitochondrial genomes are inherited, one through eggs, the other through sperm. During male embryo development, spermatozoon mitochondria aggregate in proximity of the first cleavage furrow and end up in the primordial germ cells, while they are dispersed in female embryos.

METHODOLOGY/PRINCIPAL FINDINGS

We used MitoTracker, microtubule staining and transmission electron microscopy to examine the mechanisms of this unusual distribution of sperm mitochondria in the DUI species Ruditapes philippinarum. Our results suggest that in male embryos the midbody deriving from the mitotic spindle of the first division concurs in positioning the aggregate of sperm mitochondria. Furthermore, an immunocytochemical analysis showed that the germ line determinant Vasa segregates close to the first cleavage furrow.

CONCLUSIONS/SIGNIFICANCE

In DUI male embryos, spermatozoon mitochondria aggregate in a stable area on the animal-vegetal axis: in organisms with spiral segmentation this zone is not involved in cleavage, so the aggregation is maintained. Moreover, sperm mitochondria reach the same embryonic area in which also germ plasm is transferred. In 2-blastomere embryos, the segregation of sperm mitochondria in the same region with Vasa suggests their contribution in male germ line formation. In DUI male embryos, M-type mitochondria must be recognized by egg factors to be actively transferred in the germ line, where they become dominant replacing the Balbiani body mitochondria. The typical features of germ line assembly point to a common biological mechanism shared by DUI and SMI organisms. Although the molecular dynamics of the segregation of sperm mitochondria in DUI species are unknown, they could be a variation of the mechanism regulating the mitochondrial bottleneck in all metazoans.

A late phase of germ plasm accumulation during Drosophila oogenesis requires lost and rumpelstiltskin

Asymmetric mRNA localization is an effective mechanism for establishing cellular and developmental polarity. Posterior localization of oskar in the Drosophila oocyte targets the synthesis of Oskar to the posterior, where Oskar initiates the assembly of the germ plasm. In addition to harboring germline determinants, the germ plasm is required for localization and translation of the abdominal determinant nanos. Consequently, failure of oskar localization during oogenesis results in embryos lacking germ cells and abdominal segments. oskar accumulates at the oocyte posterior during mid-oogenesis through a well-studied process involving kinesin-mediated transport. Through live imaging of oskar mRNA, we have uncovered a second, mechanistically distinct phase of oskar localization that occurs during late oogenesis and results in amplification of the germ plasm. Analysis of two newly identified oskar localization factors, Rumpelstiltskin and Lost, that are required specifically for this late phase of oskar localization shows that germ plasm amplification ensures robust abdomen and germ cell formation during embryogenesis. In addition, our results indicate the importance of mechanisms for adapting mRNAs to utilize multiple localization pathways as necessitated by the dramatic changes in ovarian physiology that occur during oogenesis.

A PAR-1-dependent orientation gradient of dynamic microtubules directs posterior cargo transport in the Drosophila oocyte

Cytoskeletal organization is central to establishing cell polarity in various cellular contexts, including during messenger ribonucleic acid sorting in Drosophila melanogaster oocytes by microtubule (MT)-dependent molecular motors. However, MT organization and dynamics remain controversial in the oocyte. In this paper, we use rapid multichannel live-cell imaging with novel image analysis, tracking, and visualization tools to characterize MT polarity and dynamics while imaging posterior cargo transport. We found that all MTs in the oocyte were highly dynamic and were organized with a biased random polarity that increased toward the posterior. This organization originated through MT nucleation at the oocyte nucleus and cortex, except at the posterior end of the oocyte, where PAR-1 suppressed nucleation. Our findings explain the biased random posterior cargo movements in the oocyte that establish the germline and posterior.

Elaborating polarity: PAR proteins and the cytoskeleton

Cell polarity is essential for cells to divide asymmetrically, form spatially restricted subcellular structures and participate in three-dimensional multicellular organization. PAR proteins are conserved polarity regulators that function by generating cortical landmarks that establish dynamic asymmetries in the distribution of effector proteins. Here, we review recent findings on the role of PAR proteins in cell polarity in C. elegans and Drosophila, and emphasize the links that exist between PAR networks and cytoskeletal proteins that both regulate PAR protein localization and act as downstream effectors to elaborate polarity within the cell.

Cell polarity in eggs and epithelia: parallels and diversity

Cell polarity, the generation of cellular asymmetries, is necessary for diverse processes in animal cells, such as cell migration, asymmetric cell division, epithelial barrier function, and morphogenesis. Common mechanisms generate and transduce cell polarity in different cells, but cell type-specific processes are equally important. In this review, we highlight the similarities and differences between the polarity mechanisms in eggs and epithelia. We also highlight the prospects for future studies on how cortical polarity interfaces with other cellular processes, such as morphogenesis, exocytosis, and lipid signaling, and how defects in polarity contribute to tumor formation.