MEX-5 and MEX-6 function to establish soma/germline asymmetry in early C. elegans embryos

How germ granules promote germ cell fate

Germ cells are the only cells in the body capable of giving rise to a new organism, and this totipotency hinges on their ability to assemble membraneless germ granules. These specialized RNA and protein complexes are hallmarks of germ cells throughout their life cycle: as embryonic germ granules in late oocytes and zygotes, Balbiani bodies in immature oocytes, and nuage in maturing gametes. Decades of developmental, genetic and biochemical studies have identified protein and RNA constituents unique to germ granules and have implicated these in germ cell identity, genome integrity and gamete differentiation. Now, emerging research is defining germ granules as biomolecular condensates that achieve high molecular concentrations by phase separation, and it is assigning distinct roles to germ granules during different stages of germline development. This organization of the germ cell cytoplasm into cellular subcompartments seems to be critical not only for the flawless continuity through the germline life cycle within the developing organism but also for the success of the next generation.

EDC-3 and EDC-4 regulate embryonic mRNA clearance and biomolecular condensate specialization

Animal development is dictated by the selective and timely decay of mRNAs in developmental transitions, but the impact of mRNA decapping scaffold proteins in development is unclear. This study unveils the roles and interactions of the DCAP-2 decapping scaffolds EDC-3 and EDC-4 in the embryonic development of C. elegans. EDC-3 facilitates the timely removal of specific embryonic mRNAs, including cgh-1, car-1, and ifet-1 by reducing their expression and preventing excessive accumulation of DCAP-2 condensates in somatic cells. We further uncover a role for EDC-3 in defining the boundaries between P bodies, germ granules, and stress granules. Finally, we show that EDC-4 counteracts EDC-3 and engenders the assembly of DCAP-2 with the GID (CTLH) complex, a ubiquitin ligase involved in maternal-to-zygotic transition (MZT). Our findings support a model where multiple RNA decay mechanisms temporally clear maternal and zygotic mRNAs throughout embryonic development.

PLK-1 regulates MEX-1 polarization in the C. elegans zygote

The one-cell C. elegans embryo undergoes an asymmetric cell division during which germline factors such as the RNA-binding proteins POS-1 and MEX-1 segregate to the posterior cytoplasm, leading to their asymmetric inheritance to the posterior germline daughter cell. Previous studies found that the RNA-binding protein MEX-5 recruits polo-like kinase PLK-1 to the anterior cytoplasm where PLK-1 inhibits the retention of its substrate POS-1, leading to POS-1 segregation to the posterior. In this study, we tested whether PLK-1 similarly regulates MEX-1 polarization. We find that both the retention of MEX-1 in the anterior and the segregation of MEX-1 to the posterior depend on PLK kinase activity and on the interaction between MEX-5 and PLK-1. Human PLK1 directly phosphorylates recombinant MEX-1 on 9 predicted PLK-1 sites in vitro, four of which were identified in previous phosphoproteomic analysis of C. elegans embryos. The introduction of alanine substitutions at these four PLK-1 phosphorylation sites (MEX-1(4A)) significantly weakened the inhibition of MEX-1 retention in the anterior, thereby weakening MEX-1 segregation to the posterior. In contrast, mutation of a predicted CDK1 phosphorylation site had no effect on MEX-1 retention or on MEX-1 segregation. MEX-1(4A) mutants are viable and fertile but display significant sterility and fecundity defects at elevated temperatures. Taken together with our previous findings, these findings suggest PLK-1 phosphorylation drives both MEX-1 and POS-1 polarization during the asymmetric division of the zygote.

The mex-3 3' untranslated region is essential for reproduction during temperature stress

Organisms must sense temperature and modify their physiology to ensure survival during environmental stress. Elevated temperature leads to reduced fertility in most sexually reproducing organisms. Maternally supplied mRNAs are required for embryogenesis. They encode proteins that govern early events in embryonic patterning. RNA-binding proteins (RBPs) are major effectors of maternal mRNA regulation. MEX-3 is a conserved RBP essential for anterior patterning of Caenorhabditis elegans embryos. We previously demonstrated that the mex-3 3' untranslated region (3'UTR) represses MEX-3 abundance in the germline yet is dispensable for fertility. Here, we show that the 3'UTR becomes essential during thermal stress. Deletion of the 3'UTR causes a highly penetrant temperature sensitive embryonic lethality phenotype distinct from a mex-3 null. Loss of the 3'UTR decreases MEX-3 abundance specifically in maturing oocytes and early embryos experiencing temperature stress, suggesting a mechanism that regulates MEX-3 abundance at the oocyte-to-embryo transition is sensitive to temperature. We propose that a primary role of the mex-3 3'UTR is to buffer MEX-3 expression to ensure viability during fluctuating temperature. We hypothesize that a major role of maternally supplied mRNAs is to ensure robust expression of key cell fate determinants in uncertain conditions.

Order from chaos: cellular asymmetries explained with modelling

Molecules inside cells are subject to physical forces and undergo biochemical interactions, continuously changing their physical properties and dynamics. Despite this, cells achieve highly ordered molecular patterns that are crucial to regulate various cellular functions and to specify cell fate. In the Caenorhabditis elegans one-cell embryo, protein asymmetries are established in the narrow time window of a cell division. What are the mechanisms that allow molecules to establish asymmetries, defying the randomness imposed by Brownian motion? Mathematical and computational models have paved the way to the understanding of protein dynamics up to the 'single-molecule level' when resolution represents an issue for precise experimental measurements. Here we review the models that interpret cortical and cytoplasmic asymmetries in the one-cell C. elegans embryo.

A spatiotemporally resolved atlas of mRNA decay in the C. elegans embryo reveals differential regulation of mRNA stability across stages and cell types

During embryonic development, cells undergo dynamic changes in gene expression that are required for appropriate cell fate specification. Although both transcription and mRNA degradation contribute to gene expression dynamics, patterns of mRNA decay are less well-understood. Here we directly measured spatiotemporally resolved mRNA decay rates transcriptome-wide throughout C. elegans embryogenesis by transcription inhibition followed by bulk and single-cell RNA-sequencing. This allowed us to calculate mRNA half-lives within specific cell types and developmental stages and identify differentially regulated mRNA decay throughout embryonic development. We identified transcript features that are correlated with mRNA stability and found that mRNA decay rates are associated with distinct peaks in gene expression over time. Moreover, we provide evidence that, on average, mRNA is more stable in the germline compared to in the soma and in later embryonic stages compared to in earlier stages. This work suggests that differential mRNA decay across cell states and time helps to shape developmental gene expression, and it provides a valuable resource for studies of mRNA turnover regulatory mechanisms.

Mechanisms of lineage specification in Caenorhabditis elegans

The studies of cell fate and lineage specification are fundamental to our understanding of the development of multicellular organisms. Caenorhabditis elegans has been one of the premiere systems for studying cell fate specification mechanisms at single cell resolution, due to its transparent nature, the invariant cell lineage, and fixed number of somatic cells. We discuss the general themes and regulatory mechanisms that have emerged from these studies, with a focus on somatic lineages and cell fates. We next review the key factors and pathways that regulate the specification of discrete cells and lineages during embryogenesis and postembryonic development; we focus on transcription factors and include numerous lineage diagrams that depict the expression of key factors that specify embryonic founder cells and postembryonic blast cells, and the diverse somatic cell fates they generate. We end by discussing some future perspectives in cell and lineage specification.

The role of RNA-binding proteins in orchestrating germline development in Caenorhabditis elegans

RNA passed from parents to progeny controls several aspects of early development. The germline of the free-living nematode Caenorhabditis elegans contains many families of evolutionarily conserved RNA-binding proteins (RBPs) that target the untranslated regions of mRNA transcripts to regulate their translation and stability. In this review, we summarize what is known about the binding specificity of C. elegans germline RNA-binding proteins and the mechanisms of mRNA regulation that contribute to their function. We examine the emerging role of miRNAs in translational regulation of germline and embryo development. We also provide an overview of current technology that can be used to address the gaps in our understanding of RBP regulation of mRNAs. Finally, we present a hypothetical model wherein multiple 3'UTR-mediated regulatory processes contribute to pattern formation in the germline to ensure the proper and timely localization of germline proteins and thus a functional reproductive system.

Structural and functional organization of germ plasm condensates

Reproductive success of metazoans relies on germ cells. These cells develop early during embryogenesis, divide and undergo meiosis in the adult to make sperm and oocytes. Unlike somatic cells, germ cells are immortal and transfer their genetic material to new generations. They are also totipotent, as they differentiate into different somatic cell types. The maintenance of immortality and totipotency of germ cells depends on extensive post-transcriptional and post-translational regulation coupled with epigenetic remodeling, processes that begin with the onset of embryogenesis [1, 2]. At the heart of this regulation lie germ granules, membraneless ribonucleoprotein condensates that are specific to the germline cytoplasm called the germ plasm. They are a hallmark of all germ cells and contain several proteins and RNAs that are conserved across species. Interestingly, germ granules are often structured and tend to change through development. In this review, we describe how the structure of germ granules becomes established and discuss possible functional outcomes these structures have during development.

Delineating the mechanisms and design principles of Caenorhabditis elegans embryogenesis using in toto high-resolution imaging data and computational modeling

The nematode (roundworm) Caenorhabditis elegans is one of the most popular animal models for the study of developmental biology, as its invariant development and transparent body enable in toto cellular-resolution fluorescence microscopy imaging of developmental processes at 1-min intervals. This has led to the development of various computational tools for the systematic and automated analysis of imaging data to delineate the molecular and cellular processes throughout the embryogenesis of C. elegans, such as those associated with cell lineage, cell migration, cell morphology, and gene activity. In this review, we first introduce C. elegans embryogenesis and the development of techniques for tracking cell lineage and reconstructing cell morphology during this process. We then contrast the developmental modes of C. elegans and the customized technologies used for studying them with the ones of other animal models, highlighting its advantage for studying embryogenesis with exceptional spatial and temporal resolution. This is followed by an examination of the physical models that have been devised-based on accurate determinations of developmental processes afforded by analyses of imaging data-to interpret the early embryonic development of C. elegans from subcellular to intercellular levels of multiple cells, which focus on two key processes: cell polarization and morphogenesis. We subsequently discuss how quantitative data-based theoretical modeling has improved our understanding of the mechanisms of C. elegans embryogenesis. We conclude by summarizing the challenges associated with the acquisition of C. elegans embryogenesis data, the construction of algorithms to analyze them, and the theoretical interpretation.

The conserved helicase ZNFX-1 memorializes silenced RNAs in perinuclear condensates

RNA-mediated interference (RNAi) is a conserved mechanism that uses small RNAs (sRNAs) to silence gene expression. In the Caenorhabditis elegans germline, transcripts targeted by sRNAs are used as templates for sRNA amplification to propagate silencing into the next generation. Here we show that RNAi leads to heritable changes in the distribution of nascent and mature transcripts that correlate with two parallel sRNA amplification loops. The first loop, dependent on the nuclear Argonaute HRDE-1, targets nascent transcripts and reduces but does not eliminate productive transcription at the locus. The second loop, dependent on the conserved helicase ZNFX-1, targets mature transcripts and concentrates them in perinuclear condensates. ZNFX-1 interacts with sRNA-targeted transcripts that have acquired poly(UG) tails and is required to sustain pUGylation and robust sRNA amplification in the inheriting generation. By maintaining a pool of transcripts for amplification, ZNFX-1 prevents premature extinction of the RNAi response and extends silencing into the next generation.

Modeling protein dynamics in Caenorhabditis elegans embryos reveals that the PLK-1 gradient relies on weakly coupled reaction-diffusion mechanisms

Intracellular gradients have essential roles in cell and developmental biology, but their formation is not fully understood. We have developed a computational approach facilitating interpretation of protein dynamics and gradient formation. We have combined this computational approach with experiments to understand how Polo-Like Kinase 1 (PLK-1) forms a cytoplasmic gradient in Caenorhabditis elegans embryos. Although the PLK-1 gradient depends on the Muscle EXcess-5/6 (MEX-5/6) proteins, we reveal differences in PLK-1 and MEX-5 gradient formation that can be explained by a model with two components, PLK-1 bound to MEX-5 and unbound PLK-1. Our combined approach suggests that a weak coupling between PLK-1 and MEX-5 reaction–diffusion mechanisms dictates the dynamic exchange of PLK-1 with the cytoplasm, explaining PLK-1 high diffusivity and smooth gradient.

Protein gradients have fundamental roles in cell and developmental biology. In the one-cell Caenorhabditis elegans embryo, the mitotic Polo-Like Kinase 1 (PLK-1) forms an anterior-rich cytoplasmic gradient, which is crucial for asymmetric cell division and embryonic development. The PLK-1 gradient depends on the RNA-binding Muscle-EXcess-5 protein (MEX-5), whose slow-diffusing complexes accumulate in the anterior via a reaction–diffusion mechanism. Here, we combine experiments and a computational approach to investigate the dynamics of PLK-1 gradient formation. We find that the gradient of PLK-1 initiates later, is less steep, and forms with slower dynamics than does the MEX-5 gradient. The data show that PLK-1 diffuses faster than MEX-5 in both anterior and posterior cytoplasmic regions. Our simulations suggest that binding to slow-diffusing MEX-5 is required for PLK-1 gradient formation, but that a significant fraction of unbound PLK-1 is necessary to justify the different gradient dynamics. We provide a computational tool able to predict gradient establishment prior to cell division and show that a two-component, bound and unbound, model of PLK-1 dynamics recapitulates the experimental observations.

The great small organisms of developmental genetics: Caenorhabditis elegans and Drosophila melanogaster

Experimental embryologists working at the turn of the 19th century suggested fundamental mechanisms of development, such as localized cytoplasmic determinants and tissue induction. However, the molecular basis underlying these processes proved intractable for a long time, despite concerted efforts in many developmental systems to isolate factors with a biological role. That road block was overcome by combining developmental biology with genetics. This powerful approach used unbiased genome-wide screens to isolate mutants with developmental defects and to thereby identify genes encoding key determinants and regulatory pathways that govern development. Two small invertebrates were the pioneers: the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans. Their modes of development differ in many ways, but the two together led the way to unraveling the molecular mechanisms of many fundamental developmental processes. The discovery of the grand homologies between key players in development throughout the animal kingdom underscored the usefulness of studying these small invertebrate models for animal development and even human disease. We describe developmental genetics in Drosophila and C. elegans up to the rise of genomics at the beginning of the 21st Century. Finally, we discuss themes that emerge from the histories of such distinct organisms and prospects of this approach for the future.

Regulation of biomolecular condensates by interfacial protein clusters

Biomolecular condensates are cellular compartments that can form by phase separation in the absence of limiting membranes. Studying the P granules of Caenorhabditis elegans, we find that condensate dynamics are regulated by protein clusters that adsorb to the condensate interface. Using in vitro reconstitution, live observations, and theory, we demonstrate that localized assembly of P granules is controlled by MEG-3, an intrinsically disordered protein that forms low dynamic assemblies on P granules. Following classic Pickering emulsion theory, MEG-3 clusters lower surface tension and slow down coarsening. During zygote polarization, MEG-3 recruits the DYRK family kinase MBK-2 to accelerate spatially regulated growth of the P granule emulsion. By tuning condensate-cytoplasm exchange, interfacial clusters regulate the structural integrity of biomolecular condensates, reminiscent of the role of lipid bilayers in membrane-bound organelles.

A 4D single-cell protein atlas of transcription factors delineates spatiotemporal patterning during embryogenesis

Complex biological processes such as embryogenesis require precise coordination of cell differentiation programs across both space and time. Using protein-fusion fluorescent reporters and four-dimensional live imaging, we present a protein expression atlas of transcription factors (TFs) mapped onto developmental cell lineages during Caenorhabditis elegans embryogenesis, at single-cell resolution. This atlas reveals a spatiotemporal combinatorial code of TF expression, and a cascade of lineage-specific, tissue-specific and time-specific TFs that specify developmental states. The atlas uncovers regulators of embryogenesis, including an unexpected role of a skin specifier in neurogenesis and the critical function of an uncharacterized TF in convergent muscle differentiation. At the systems level, the atlas provides an opportunity to model cell state-fate relationships, revealing a lineage-dependent state diversity within functionally related cells and a winding trajectory of developmental state progression. Collectively, this single-cell protein atlas represents a valuable resource for elucidating metazoan embryogenesis at the molecular and systems levels.

A balance between antagonizing PAR proteins specifies the pattern of asymmetric and symmetric divisions in C. elegans embryogenesis

Coordination between cell differentiation and proliferation during development requires the balance between asymmetric and symmetric modes of cell division. However, the cellular intrinsic cue underlying the choice between these two division modes remains elusive. Here, we show evidence in Caenorhabditis elegans that the invariable lineage of the division modes is specified by the balance between antagonizing complexes of partitioning-defective (PAR) proteins. By uncoupling unequal inheritance of PAR proteins from that of fate determinants during cell division, we demonstrate that changes in the balance between PAR-2 and PAR-6 can be sufficient to re-program the division modes from symmetric to asymmetric and vice versa in two daughter cells. The division mode adopted occurs independently of asymmetry in cytoplasmic fate determinants, cell-size asymmetry, and cell-cycle asynchrony between sister cells. We propose that the balance between PAR proteins represents an intrinsic self-organizing cue for the specification of the two division modes during development.

The coordination of spindle-positioning forces during the asymmetric division of the Caenorhabditis elegans zygote

In Caenorhabditis elegans zygote, astral microtubules generate forces essential to position the mitotic spindle, by pushing against and pulling from the cortex. Measuring microtubule dynamics there, we revealed the presence of two populations, corresponding to pulling and pushing events. It offers a unique opportunity to study, under physiological conditions, the variations of both spindle-positioning forces along space and time. We propose a threefold control of pulling force, by polarity, spindle position and mitotic progression. We showed that the sole anteroposterior asymmetry in dynein on-rate, encoding pulling force imbalance, is sufficient to cause posterior spindle displacement. The positional regulation, reflecting the number of microtubule contacts in the posterior-most region, reinforces this imbalance only in late anaphase. Furthermore, we exhibited the first direct proof that dynein processivity increases along mitosis. It reflects the temporal control of pulling forces, which strengthens at anaphase onset following mitotic progression and independently from chromatid separation. In contrast, the pushing force remains constant and symmetric and contributes to maintaining the spindle at the cell centre during metaphase.

Physically asymmetric division of the C. elegans zygote ensures invariably successful embryogenesis

Asymmetric divisions that yield daughter cells of different sizes are frequent during early embryogenesis, but the importance of such a physical difference for successful development remains poorly understood. Here, we investigated this question using the first division of Caenorhabditis elegans embryos, which yields a large AB cell and a small P1 cell. We equalized AB and P1 sizes using acute genetic inactivation or optogenetic manipulation of the spindle positioning protein LIN-5. We uncovered that only some embryos tolerated equalization, and that there was a size asymmetry threshold for viability. Cell lineage analysis of equalized embryos revealed an array of defects, including faster cell cycle progression in P1 descendants, as well as defects in cell positioning, division orientation, and cell fate. Moreover, equalized embryos were more susceptible to external compression. Overall, we conclude that unequal first cleavage is essential for invariably successful embryonic development of C. elegans.

The Role of Cytoplasmic MEX-5/6 Polarity in Asymmetric Cell Division

In the process of asymmetric cell division, the mother cell induces polarity in both the membrane and the cytosol by distributing substrates and components asymmetrically. Such polarity formation results from the harmonization of the upstream and downstream polarities between the cell membrane and the cytosol. MEX-5/6 is a well-investigated downstream cytoplasmic protein, which is deeply involved in the membrane polarity of the upstream transmembrane protein PAR in the Caenorhabditis elegans embryo. In contrast to the extensive exploration of membrane PAR polarity, cytoplasmic polarity is poorly understood, and the precise contribution of cytoplasmic polarity to the membrane PAR polarity remains largely unknown. In this study, we explored the interplay between the cytoplasmic MEX-5/6 polarity and the membrane PAR polarity by developing a mathematical model that integrates the dynamics of PAR and MEX-5/6 and reflects the cell geometry. Our investigations show that the downstream cytoplasmic protein MEX-5/6 plays an indispensable role in causing a robust upstream PAR polarity, and the integrated understanding of their interplay, including the effect of the cell geometry, is essential for the study of polarity formation in asymmetric cell division.

The first steps in the life of a worm: Themes and variations in asymmetric division in C. elegans and other nematodes

Starting with Boveri in the 1870s, microscopic investigation of early embryogenesis in a broad swath of nematode species revealed the central role of asymmetric cell division in embryonic axis specification, blastomere positioning, and cell fate specification. Notably, across the class Chromadorea, a conserved theme emerges-asymmetry is first established in the zygote and specifies its asymmetric division, giving rise to an anterior somatic daughter cell and a posterior germline daughter cell. Beginning in the 1980s, the emergence of Caenorhabditis elegans as a model organism saw the advent of genetic tools that enabled rapid progress in our understanding of the molecular mechanisms underlying asymmetric division, in many cases defining key paradigms that turn out to regulate asymmetric division in a wide range of systems. Yet, the consequence of this focus on C. elegans came at the expense of exploring the extant diversity of developmental variation exhibited across nematode species. Given the resurgent interest in evolutionary studies facilitated in part by new tools, here we revisit the diversity in this asymmetric first division, juxtaposing molecular insight into mechanisms of symmetry-breaking, spindle positioning and fate specification, with a consideration of plasticity and variability within and between species. In the process, we hope to highlight questions of evolutionary forces and molecular variation that may have shaped the extant diversity of developmental mechanisms observed across Nematoda.

Germ cell ribonucleoprotein granules in different clades of life: From insects to mammals

Ribonucleoprotein (RNP) granules are no newcomers in biology. Found in all life forms, ranging across taxa, these membrane-less "organelles" have been classified into different categories based on their composition, structure, behavior, function, and localization. Broadly, they can be listed as stress granules (SGs), processing bodies (PBs), neuronal granules (NGs), and germ cell granules (GCGs). Keeping in line with the topic of this review, RNP granules present in the germ cells have been implicated in a wide range of cellular functions including cellular specification, differentiation, proliferation, and so forth. The mechanisms used by them can be diverse and many of them remain partly obscure and active areas of research. GCGs can be of different types in different organisms and at different stages of development, with multiple types coexisting in the same cell. In this review, the different known subcategories of GCGs have been studied with respect to five distinct model organisms, namely, Drosophila, Caenorhabditis elegans, Xenopus, Zebrafish, and mammals. Of them, the cytoplasmic polar granules in Drosophila, P granules in C. elegans, balbiani body in Xenopus and Zebrafish, and chromatoid bodies in mammals have been specifically emphasized upon. A descriptive account of the same has been provided along with insights into our current understanding of their functional significance with respect to cellular events relating to different developmental and reproductive processes. This article is categorized under: RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes RNA Export and Localization > RNA Localization RNA in Disease and Development > RNA in Disease.

PLK-1 Regulation of Asymmetric Cell Division in the Early C. elegans Embryo

PLK1 is a conserved mitotic kinase that is essential for the entry into and progression through mitosis. In addition to its canonical mitotic functions, recent studies have characterized a critical role for PLK-1 in regulating the polarization and asymmetric division of the one-cell C. elegans embryo. Prior to cell division, PLK-1 regulates both the polarization of the PAR proteins at the cell cortex and the segregation of cell fate determinants in the cytoplasm. Following cell division, PLK-1 is preferentially inherited to one daughter cell where it acts to regulate the timing of centrosome separation and cell division. PLK1 also regulates cell polarity in asymmetrically dividing Drosophila neuroblasts and during mammalian planar cell polarity, suggesting it may act broadly to connect cell polarity and cell cycle mechanisms.

The conserved molting/circadian rhythm regulator NHR-23/NR1F1 serves as an essential co-regulator of C. elegans spermatogenesis

In sexually reproducing metazoans, spermatogenesis is the process by which uncommitted germ cells give rise to haploid sperm. Work in model systems has revealed mechanisms controlling commitment to the sperm fate, but how this fate is subsequently executed remains less clear. While studying the well-established role of the conserved nuclear hormone receptor transcription factor, NHR-23/NR1F1, in regulating C. elegans molting, we discovered that NHR-23/NR1F1 is also constitutively expressed in developing primary spermatocytes and is a critical regulator of spermatogenesis. In this novel role, NHR-23/NR1F1 functions downstream of the canonical sex-determination pathway. Degron-mediated depletion of NHR-23/NR1F1 within hermaphrodite or male germlines causes sterility due to an absence of functional sperm, as depleted animals produce arrested primary spermatocytes rather than haploid sperm. These spermatocytes arrest in prometaphase I and fail to either progress to anaphase or attempt spermatid-residual body partitioning. They make sperm-specific membranous organelles but fail to assemble their major sperm protein into fibrous bodies. NHR-23/NR1F1 appears to function independently of the known SPE-44 gene regulatory network, revealing the existence of an NHR-23/NR1F1-mediated module that regulates the spermatogenesis program.

Going with the flow: insights from Caenorhabditis elegans zygote polarization

Cell polarity is the asymmetric distribution of cellular components along a defined axis. Polarity relies on complex signalling networks between conserved patterning proteins, including the PAR (partitioning defective) proteins, which become segregated in response to upstream symmetry breaking cues. Although the mechanisms that drive the asymmetric localization of these proteins are dependent upon cell type and context, in many cases the regulation of actomyosin cytoskeleton dynamics is central to the transport, recruitment and/or stabilization of these polarity effectors into defined subcellular domains. The transport or advection of PAR proteins by an actomyosin flow was first observed in the Caenorhabditis elegans zygote more than a decade ago. Since then a multifaceted approach, using molecular methods, high-throughput screens, and biophysical and computational models, has revealed further aspects of this flow and how polarity regulators respond to and modulate it. Here, we review recent findings on the interplay between actomyosin flow and the PAR patterning networks in the polarization of the C. elegans zygote. We also discuss how these discoveries and developed methods are shaping our understanding of other flow-dependent polarizing systems. This article is part of a discussion meeting issue 'Contemporary morphogenesis'.

Phase Separation in Germ Cells and Development

The animal germline is an immortal cell lineage that gives rise to eggs and/or sperm each generation. Fusion of an egg and sperm, or fertilization, sets off a cascade of developmental events capable of producing an array of different cell types and body plans. How germ cells develop, function, and eventually give rise to entirely new organisms is an important question in biology. A growing body of evidence suggests that phase separation events likely play a significant and multifaceted role in germ cells and development. Here, we discuss the organization, dynamics, and potential functions of phase-separated compartments in germ cells and examine the various ways in which phase separation might contribute to the development of multicellular organisms.

Centrosome Aurora A gradient ensures single polarity axis in C. elegans embryos

Cellular asymmetries are vital for generating cell fate diversity during development and in stem cells. In the newly fertilized Caenorhabditis elegans embryo, centrosomes are responsible for polarity establishment, i.e. anterior-posterior body axis formation. The signal for polarity originates from the centrosomes and is transmitted to the cell cortex, where it disassembles the actomyosin network. This event leads to symmetry breaking and the establishment of distinct domains of evolutionarily conserved PAR proteins. However, the identity of an essential component that localizes to the centrosomes and promotes symmetry breaking was unknown. Recent work has uncovered that the loss of Aurora A kinase (AIR-1 in C. elegans and hereafter referred to as Aurora A) in the one-cell embryo disrupts stereotypical actomyosin-based cortical flows that occur at the time of polarity establishment. This misregulation of actomyosin flow dynamics results in the occurrence of two polarity axes. Notably, the role of Aurora A in ensuring a single polarity axis is independent of its well-established function in centrosome maturation. The mechanism by which Aurora A directs symmetry breaking is likely through direct regulation of Rho-dependent contractility. In this mini-review, we will discuss the unconventional role of Aurora A kinase in polarity establishment in C. elegans embryos and propose a refined model of centrosome-dependent symmetry breaking.

A Disorder-to-Order Transition Mediates RNA Binding of the Caenorhabditis elegans Protein MEX-5

CCCH-type tandem zinc finger (TZF) domains are found in many RNA-binding proteins (RBPs) that regulate the essential processes of post-transcriptional gene expression and splicing through direct protein-RNA interactions. In Caenorhabditis elegans, RBPs control the translation, stability, or localization of maternal messenger RNAs required for patterning decisions before zygotic gene activation. MEX-5 (Muscle EXcess) is a C. elegans protein that leads a cascade of RBP localization events that is essential for axis polarization and germline differentiation after fertilization. Here, we report that at room temperature, the CCCH-type TZF domain of MEX-5 contains an unstructured zinc finger that folds upon binding of its RNA target. We have characterized the structure and dynamics of the TZF domain of MEX-5 and designed a variant MEX-5 in which both fingers are fully folded in the absence of RNA. Within the thermal range experienced by C. elegans, the population of the unfolded state of the TZF domain of MEX-5 varies. We observe that the TZF domain becomes less disordered at lower temperatures and more disordered at higher temperatures. However, in the temperature range in which C. elegans is fertile, when MEX-5 needs to be functional, only one of the two zinc fingers is folded.

Asymmetric cell division from a cell to cells: Shape, length, and location of polarity domain

Asymmetric cell division is one of the most elegant biological systems by which cells create daughter cells with different functions and increase cell diversity. In particular, PAR polarity in the cell membrane plays a critical role in regulating the whole process of asymmetric cell division. Numerous studies have been conducted to determine the underlying mechanism of PAR polarity formation using both experimental and theoretical approaches in the last 10 years. However, they have mostly focused on answering the fundamental question of how this exclusive polarity is established but the precise dynamics of polarity domain have been little notified. In this review, I focused on studies on the shape, length, and location of PAR polarity from a theoretical perspective that may be important for an integrated understanding of the entire process of asymmetric cell division.

Developmental Plasticity and Cellular Reprogramming in Caenorhabditis elegans

While Caenorhabditis elegans was originally regarded as a model for investigating determinate developmental programs, landmark studies have subsequently shown that the largely invariant pattern of development in the animal does not reflect irreversibility in rigidly fixed cell fates. Rather, cells at all stages of development, in both the soma and germline, have been shown to be capable of changing their fates through mutation or forced expression of fate-determining factors, as well as during the normal course of development. In this chapter, we review the basis for natural and induced cellular plasticity in C. elegans We describe the events that progressively restrict cellular differentiation during embryogenesis, starting with the multipotency-to-commitment transition (MCT) and subsequently through postembryonic development of the animal, and consider the range of molecular processes, including transcriptional and translational control systems, that contribute to cellular plasticity. These findings in the worm are discussed in the context of both classical and recent studies of cellular plasticity in vertebrate systems.

Cell polarity-dependent centrosome separation in the C. elegans embryo

Bondaz et al. show that in C. elegans embryos, the microtubule depolymerase KLP-7/MCAK is required for efficient centrosome separation in the somatic AB cell, but not the germline P1 cell. This difference in spindle assembly depends on cell polarity via the mitotic kinase PLK1.

In animal cells, faithful chromosome segregation depends on the assembly of a bipolar spindle driven by the timely separation of the two centrosomes. Here we took advantage of the highly stereotypical cell divisions in Caenorhabditis elegans embryos to identify new regulators of centrosome separation. We find that at the two-cell stage, the somatic AB cell initiates centrosome separation later than the germline P1 cell. This difference is strongly exacerbated by the depletion of the kinesin-13 KLP-7/MCAK, resulting in incomplete centrosome separation at NEBD in AB but not P1. Our genetic and cell biology data indicate that this phenotype depends on cell polarity via the enrichment in AB of the mitotic kinase PLK-1, which itself limits the cortical localization of the dynein-binding NuMA orthologue LIN-5. We postulate that the timely separation of centrosomes is regulated in a cell type–dependent manner.

P Granules Protect RNA Interference Genes from Silencing by piRNAs

P granules are perinuclear condensates in C. elegans germ cells proposed to serve as hubs for self/non-self RNA discrimination by Argonautes. We report that a mutant (meg-3 meg-4) that does not assemble P granules in primordial germ cells loses competence for RNA-interference over several generations and accumulates silencing small RNAs against hundreds of endogenous genes, including the RNA-interference genes rde-11 and sid-1. In wild type, rde-11 and sid-1 transcripts are heavily targeted by piRNAs and accumulate in P granules but maintain expression. In the primordial germ cells of meg-3 meg-4 mutants, rde-11 and sid-1 transcripts disperse in the cytoplasm with the small RNA biogenesis machinery, become hyper-targeted by secondary sRNAs, and are eventually silenced. Silencing requires the PIWI-class Argonaute PRG-1 and the nuclear Argonaute HRDE-1 that maintains trans-generational silencing of piRNA targets. These observations support a "safe harbor" model for P granules in protecting germline transcripts from piRNA-initiated silencing.

Gene Cascade Finder: A tool for identification of gene cascades and its application in Caenorhabditis elegans

Obtaining a comprehensive understanding of the gene regulatory networks, or gene cascades, involved in cell fate determination and cell lineage segregation in Caenorhabditis elegans is a long-standing challenge. Although RNA-sequencing (RNA-Seq) is a promising technique to resolve these questions, the bioinformatics tools to identify associated gene cascades from RNA-Seq data remain inadequate. To overcome these limitations, we developed Gene Cascade Finder (GCF) as a novel tool for building gene cascades by comparison of mutant and wild-type RNA-Seq data along with integrated information of protein-protein interactions, expression timing, and domains. Application of GCF to RNA-Seq data confirmed that SPN-4 and MEX-3 regulate the canonical Wnt pathway during embryonic development. Moreover, lin-35, hsp-3, and gpa-12 were found to be involved in MEX-1-dependent neurogenesis, and MEX-3 was found to control the gene cascade promoting neurogenesis through lin-35 and apl-1. Thus, GCF could be a useful tool for building gene cascades from RNA-Seq data.

The maternal-to-zygotic transition revisited

The development of animal embryos is initially directed by maternal gene products. Then, during the maternal-to-zygotic transition (MZT), developmental control is handed to the zygotic genome. Extensive research in both vertebrate and invertebrate model organisms has revealed that the MZT can be subdivided into two phases, during which very different modes of gene regulation are implemented: initially, regulation is exclusively post-transcriptional and post-translational, following which gradual activation of the zygotic genome leads to predominance of transcriptional regulation. These changes in the gene expression program of embryos are precisely controlled and highly interconnected. Here, we review current understanding of the mechanisms that underlie handover of developmental control during the MZT.

A high-content imaging approach to profile C. elegans embryonic development

The Caenorhabditis elegans embryo is an important model for analyzing mechanisms of cell fate specification and tissue morphogenesis. Sophisticated lineage-tracing approaches for analyzing embryogenesis have been developed but are labor intensive and do not naturally integrate morphogenetic readouts. To enable the rapid classification of developmental phenotypes, we developed a high-content method that employs two custom strains: a Germ Layer strain that expresses nuclear markers in the ectoderm, mesoderm and endoderm/pharynx; and a Morphogenesis strain that expresses markers labeling epidermal cell junctions and the neuronal cell surface. We describe a procedure that allows simultaneous live imaging of development in 80-100 embryos and provide a custom program that generates cropped, oriented image stacks of individual embryos to facilitate analysis. We demonstrate the utility of our method by perturbing 40 previously characterized developmental genes in variants of the two strains containing RNAi-sensitizing mutations. The resulting datasets yielded distinct, reproducible signature phenotypes for a broad spectrum of genes that are involved in cell fate specification and morphogenesis. In addition, our analysis provides new in vivo evidence for MBK-2 function in mesoderm fate specification and LET-381 function in elongation.

Spatial regulation of the polarity kinase PAR-1 by parallel inhibitory mechanisms

The MARK/PAR-1 family of kinases are conserved regulators of cell polarity that share a conserved C-terminal kinase-associated domain (KA1). Localization of MARK/PAR-1 kinases to specific regions of the cell cortex is a hallmark of polarized cells. In Caenorhabditiselegans zygotes, PAR-1 localizes to the posterior cortex under the influence of another polarity kinase, aPKC/PKC-3. Here, we report that asymmetric localization of PAR-1 protein is not essential, and that PAR-1 kinase activity is regulated spatially. We find that, as in human MARK1, the PAR-1 KA1 domain is an auto-inhibitory domain that suppresses kinase activity. Auto-inhibition by the KA1 domain functions in parallel with phosphorylation by PKC-3 to suppress PAR-1 activity in the anterior cytoplasm. The KA1 domain also plays an additional role that is essential for germ plasm maintenance and fertility. Our findings suggest that modular regulation of kinase activity by redundant inhibitory inputs contributes to robust symmetry breaking by MARK/PAR-1 kinases in diverse cell types.

Aurora A depletion reveals centrosome-independent polarization mechanism in Caenorhabditis elegans

How living systems break symmetry in an organized manner is a fundamental question in biology. In wild-type Caenorhabditis elegans zygotes, symmetry breaking during anterior-posterior axis specification is guided by centrosomes, resulting in anterior-directed cortical flows and a single posterior PAR-2 domain. We uncover that C. elegans zygotes depleted of the Aurora A kinase AIR-1 or lacking centrosomes entirely usually establish two posterior PAR-2 domains, one at each pole. We demonstrate that AIR-1 prevents symmetry breaking early in the cell cycle, whereas centrosomal AIR-1 instructs polarity initiation thereafter. Using triangular microfabricated chambers, we establish that bipolarity of air-1(RNAi) embryos occurs effectively in a cell-shape and curvature-dependent manner. Furthermore, we develop an integrated physical description of symmetry breaking, wherein local PAR-2-dependent weakening of the actin cortex, together with mutual inhibition of anterior and posterior PAR proteins, provides a mechanism for spontaneous symmetry breaking without centrosomes.

Single-molecule dynamics of the P granule scaffold MEG-3 in the Caenorhabditis elegans zygote

During the asymmetric division of the Caenorhabditis elegans zygote, germ (P) granules are disassembled in the anterior cytoplasm and stabilized/assembled in the posterior cytoplasm, leading to their inheritance by the germline daughter cell. P granule segregation depends on MEG (maternal-effect germline defective)-3 and MEG-4, which are enriched in P granules and in the posterior cytoplasm surrounding P granules. Here we use single-molecule imaging and tracking to characterize the reaction/diffusion mechanisms that result in MEG-3::Halo segregation. We find that the anteriorly enriched RNA-binding proteins MEX (muscle excess)-5 and MEX-6 suppress the retention of MEG-3 in the anterior cytoplasm, leading to MEG-3 enrichment in the posterior. We provide evidence that MEX-5/6 may work in conjunction with PLK-1 kinase to suppress MEG-3 retention in the anterior. Surprisingly, we find that the retention of MEG-3::Halo in the posterior cytoplasm surrounding P granules does not appear to contribute significantly to the maintenance of P granule asymmetry. Rather, our findings suggest that the formation of the MEG-3 concentration gradient and the segregation of P granules are two parallel manifestations of MEG-3′s response to upstream polarity cues.

PIE-1 Translation in the Germline Lineage Contributes to PIE-1 Asymmetry in the Early Caenorhabditis elegans Embryo

In the C. elegans embryo, the germline lineage is established through successive asymmetric cell divisions that each generate a somatic and a germline daughter cell. PIE-1 is an essential maternal factor that is enriched in embryonic germline cells and is required for germline specification. We estimated the absolute concentration of PIE-1::GFP in germline cells and find that PIE-1::GFP concentration increases by roughly 4.5 fold, from 92 nM to 424 nM, between the 1 and 4-cell stages. Previous studies have shown that the preferential inheritance of PIE-1 by germline daughter cells and the degradation of PIE-1 in somatic cells are important for PIE-1 enrichment in germline cells. In this study, we provide evidence that the preferential translation of maternal PIE-1::GFP transcripts in the germline also contributes to PIE-1::GFP enrichment. Through an RNAi screen, we identified Y14 and MAG-1 (Drosophila tsunagi and mago nashi) as regulators of embryonic PIE-1::GFP levels. We show that Y14 and MAG-1 do not regulate PIE-1 degradation, segregation or synthesis in the early embryo, but do regulate the concentration of maternally-deposited PIE-1::GFP. Taken together, or findings point to an important role for translational control in the regulation of PIE-1 levels in the germline lineage.

The kinases PIG-1 and PAR-1 act in redundant pathways to regulate asymmetric division in the EMS blastomere of C. elegans

The PAR-1 kinase of C. elegans is localized to the posterior of the one-cell embryo and its mutations affect asymmetric spindle placement and partitioning of cytoplasmic components in the first cell cycle. However, par-1 mutations do not cause failure to restrict the anterior PAR polarity complex to the same extent as mutations in the posteriorly localized PAR-2 protein. Further, it has been difficult to examine the role of PAR-1 in subsequent divisions due to the early defects in par-1 mutant embryos. Here we show that the PIG-1 kinase acts redundantly with PAR-1 to restrict the anterior PAR-3 protein for normal polarity in the one-cell embryo. By using a temperature sensitive allele of par-1, which exhibits enhanced lethality when combined with a pig-1 mutation, we have further explored roles for these genes in subsequent divisions. We find that both PIG-1 and PAR-1 regulate spindle orientation in the EMS blastomere of the four-cell stage embryo to ensure that it undergoes an asymmetric division. In this cell, PIG-1 and PAR-1 act in parallel pathways for spindle positioning, PIG-1 in the MES-1/SRC-1 pathway and PAR-1 in the Wnt pathway.

A continuum of mRNP complexes in embryonic microRNA-mediated silencing

MicroRNAs (miRNAs) impinge on the translation and stability of their target mRNAs, and play key roles in development, homeostasis and disease. The gene regulation mechanisms they instigate are largely mediated through the CCR4–NOT deadenylase complex, but the molecular events that occur on target mRNAs are poorly resolved. We observed a broad convergence of interactions of germ granule and P body mRNP components on AIN-1/GW182 and NTL-1/CNOT1 in Caenorhabditis elegans embryos. We show that the miRISC progressively matures on the target mRNA from a scanning form into an effector mRNP particle by sequentially recruiting the CCR4–NOT complex, decapping and decay, or germ granule proteins. Finally, we implicate intrinsically disordered proteins, key components in mRNP architectures, in the embryonic function of lsy-6 miRNA. Our findings define dynamic steps of effector mRNP assembly in miRNA-mediated silencing, and identify a functional continuum between germ granules and P bodies in the C. elegans embryo.

Establishment of the PAR-1 cortical gradient by the aPKC-PRBH circuit

Cell polarity is the asymmetric compartmentalization of cellular components. An opposing gradient of partitioning-defective protein kinases, atypical protein kinase C (aPKC) and PAR-1, at the cell cortex guides diverse asymmetries in the structure of metazoan cells, but the mechanism underlying their spatial patterning remains poorly understood. Here, we show in Caenorhabditis elegans zygotes that the cortical PAR-1 gradient is patterned as a consequence of dual mechanisms: stabilization of cortical dynamics and protection from aPKC-mediated cortical exclusion. Dual control of cortical PAR-1 depends on a physical interaction with the PRBH-domain protein PAR-2. Using a reconstitution approach in heterologous cells, we demonstrate that PAR-1, PAR-2, and polarized Cdc42-PAR-6-aPKC comprise the minimal network sufficient for the establishment of an opposing cortical gradient. Our findings delineate the mechanism governing cortical polarity, in which a circuit consisting of aPKC and the PRBH-domain protein ensures the local recruitment of PAR-1 to a well-defined cortical compartment.

The P Granules of C. elegans: A Genetic Model for the Study of RNA-Protein Condensates

P granules are RNA/protein condensates in the germline of Caenorhabditis elegans. Genetic analyses have begun to identify the proteins that regulate P granule assembly in the cytoplasm of zygotes. Among them, the RGG-domain protein PGL-3, the intrinsically disordered protein MEG-3, and the RNA helicase LAF-1 all bind and phase separate with RNA in vitro. We discuss how RNA-induced phase separation, competition with other RNA-binding proteins, and reversible phosphorylation contribute to the asymmetric localization of P granules in the cytoplasm of newly fertilized embryos. P granules contain RNA silencing complexes that monitor the germline transcriptome and may provide an RNA memory of germline gene expression across generations.

Rapid diffusion-state switching underlies stable cytoplasmic gradients in the Caenorhabditis elegans zygote

Protein concentration gradients organize cells and tissues and commonly form through diffusion away from a local source of protein. Interestingly, during the asymmetric division of the Caenorhabditis elegans zygote, the RNA-binding proteins MEX-5 and PIE-1 form opposing concentration gradients in the absence of a local source. In this study, we use near-total internal reflection fluorescence (TIRF) imaging and single-particle tracking to characterize the reaction/diffusion dynamics that maintain the MEX-5 and PIE-1 gradients. Our findings suggest that both proteins interconvert between fast-diffusing and slow-diffusing states on timescales that are much shorter (seconds) than the timescale of gradient formation (minutes). The kinetics of diffusion-state switching are strongly polarized along the anterior/posterior (A/P) axis by the PAR polarity system such that fast-diffusing MEX-5 and PIE-1 particles are approximately symmetrically distributed, whereas slow-diffusing particles are highly enriched in the anterior and posterior cytoplasm, respectively. Using mathematical modeling, we show that local differences in the kinetics of diffusion-state switching can rapidly generate stable concentration gradients over a broad range of spatial and temporal scales.

Genome-wide surveys reveal polarity and cytoskeletal regulators mediate LKB1-associated germline stem cell quiescence

Background

Caenorhabditis elegans can endure long periods of environmental stress by altering their development to execute a quiescent state called “dauer”. Previous work has implicated LKB1 - the causative gene in the autosomal dominant, cancer pre-disposing disease called Peutz-Jeghers Syndrome (PJS), and its downstream target AMPK, in the establishment of germline stem cell (GSC) quiescence during the dauer stage. Loss of function mutations in both LKB1/par-4 and AMPK/aak(0) result in untimely GSC proliferation during the onset of the dauer stage, although the molecular mechanism through which these factors regulate quiescence remains unclear. Curiously, the hyperplasia observed in par-4 mutants is more severe than AMPK-compromised dauer larvae, suggesting that par-4 has alternative downstream targets in addition to AMPK to regulate germline quiescence.

Results

We conducted three genome-wide RNAi screens to identify potential downstream targets of the protein kinases PAR-4 and AMPK that mediate dauer-dependent GSC quiescence. First, we screened to identify genes that phenocopy the par-4-dependent hyperplasia when compromised by RNAi. Two additional RNAi screens were performed to identify genes that suppressed the germline hyperplasia in par-4 and aak(0) dauer larvae, respectively. Interestingly, a subset of the candidates we identified are involved in the regulation of cell polarity and cytoskeletal function downstream of par-4, in an AMPK-independent manner. Moreover, we show that par-4 temporally regulates actin cytoskeletal organization within the dauer germ line at the rachis-adjacent membrane, in an AMPK-independent manner.

Conclusion

Our data suggest that the regulation of the cytoskeleton and cell polarity may contribute significantly to the tumour suppressor function of LKB1/par-4.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4847-y) contains supplementary material, which is available to authorized users.

Post-translational regulation of the maternal-to-zygotic transition

The maternal-to-zygotic transition (MZT) is essential for the developmental control handed from maternal products to newly synthesized zygotic genome in the earliest stages of embryogenesis, including maternal component (mRNAs and proteins) degradation and zygotic genome activation (ZGA). Various protein post-translational modifications have been identified during the MZT, such as phosphorylation, methylation and ubiquitination. Precise post-translational regulation mechanisms are essential for the timely transition of early embryonic development. In this review, we summarize recent progress regarding the molecular mechanisms underlying post-translational regulation of maternal component degradation and ZGA during the MZT and discuss some important issues in the field.

Spindle assembly checkpoint strength is linked to cell fate in the Caenorhabditis elegans embryo

The spindle assembly checkpoint (SAC) is a conserved mitotic regulator that preserves genome stability by monitoring kinetochore-microtubule attachments and blocking anaphase onset until chromosome biorientation is achieved. Despite its central role in maintaining mitotic fidelity, the ability of the SAC to delay mitotic exit in the presence of kinetochore-microtubule attachment defects (SAC "strength") appears to vary widely. How different cellular aspects drive this variation remains largely unknown. Here we show that SAC strength is correlated with cell fate during development of Caenorhabditis elegans embryos, with germline-fated cells experiencing longer mitotic delays upon spindle perturbation than somatic cells. These differences are entirely dependent on an intact checkpoint and only partially attributable to differences in cell size. In two-cell embryos, cell size accounts for half of the difference in SAC strength between the larger somatic AB and the smaller germline P₁ blastomeres. The remaining difference requires asymmetric cytoplasmic partitioning downstream of PAR polarity proteins, suggesting that checkpoint-regulating factors are distributed asymmetrically during early germ cell divisions. Our results indicate that SAC activity is linked to cell fate and reveal a hitherto unknown interaction between asymmetric cell division and the SAC.

Polo-like Kinase Couples Cytoplasmic Protein Gradients in the C. elegans Zygote

Intracellular protein gradients underlie essential cellular and developmental processes, but the mechanisms by which they are established are incompletely understood. During the asymmetric division of the C. elegans zygote, the RNA-binding protein MEX-5 forms an anterior-rich cytoplasmic gradient that causes the RNA-binding protein POS-1 to form an opposing, posterior-rich gradient. We demonstrate that the polo-like kinase PLK-1 mediates the repulsive coupling between MEX-5 and POS-1 by increasing the mobility of POS-1 in the anterior. PLK-1 is enriched in the anterior cytoplasm and phosphorylates POS-1, which is both necessary and sufficient to increase POS-1 mobility. Regulation of POS-1 mobility depends on both the interaction between PLK-1 and MEX-5 and between MEX-5 and RNA, suggesting that MEX-5 may recruit PLK-1 to RNA in the anterior. The low concentration of MEX-5/PLK-1 in the posterior cytoplasm provides a permissive environment for the retention of POS-1, which depends on POS-1 RNA binding. Our findings describe a novel reaction/diffusion mechanism in which the asymmetric distribution of cytoplasmic PLK-1 couples two RNA-binding protein gradients, thereby partitioning the cytoplasm.

A Single-Cell Biochemistry Approach Reveals PAR Complex Dynamics during Cell Polarization

Regulated protein-protein interactions are critical for cell signaling, differentiation, and development. For the study of dynamic regulation of protein interactions in vivo, there is a need for techniques that can yield time-resolved information and probe multiple protein binding partners simultaneously, using small amounts of starting material. Here we describe a single-cell protein interaction assay. Single-cell lysates are generated at defined time points and analyzed using single-molecule pull-down, yielding information about dynamic protein complex regulation in vivo. We established the utility of this approach by studying PAR polarity proteins, which mediate polarization of many animal cell types. We uncovered striking regulation of PAR complex composition and stoichiometry during Caenorhabditis elegans zygote polarization, which takes place in less than 20 min. PAR complex dynamics are linked to the cell cycle by Polo-like kinase 1 and govern the movement of PAR proteins to establish polarity. Our results demonstrate an approach to study dynamic biochemical events in vivo.