Early embryonic polarity establishment and implications for lineage differentiation.

Polarity establishment is one of the key factors affecting early embryonic development. Polarity establishment begins with myosin phosphorylation in the 8-cell embryo, and phosphorylation activates actin leading to its initiation of contractility. Subsequently, actin undergoes reorganization to form an apical domain rich in microvilli on the non-contacting surface of each blastomere, and form the actomyosin ring that marks the maturation of the apical domain in conjunction with polar protein complexes and others. From the process of polarity establishment, it can be seen that the formation of the apical domain is influenced by actin-related proteins and polar protein complexes. Some zygote genome activation (ZGA) and lineage-specific genes also regulate polarity establishment. Polarity establishment underlies the first cell lineage differentiation during early embryonic development. It regulates lineage segregation and morphogenesis by affecting asymmetric cell division, asymmetric localization of lineage differentiation factors, and activity of the Hippo signaling pathway. In this review, we systematically summarize the mechanisms of early embryonic polarity establishment and its impact on lineage differentiation in mammals, and discuss the shortcomings of the currently available studies in terms of regulatory mechanisms and species, thereby providing clues and systematic perspectives for elucidating early embryonic polarity establishment.

Regulation of intrinsic polarity establishment by a differentiation-type MAPK pathway in S. cerevisiae.

All cells establish and maintain an axis of polarity that is critical for cell shape and progression through the cell cycle. A well-studied example of polarity establishment is bud emergence in the yeast Saccharomyces cerevisiae, which is controlled by the Rho GTPase Cdc42p. The prevailing view of bud emergence does not account for regulation by extrinsic cues. Here, we show that the filamentous growth mitogen activated protein kinase (fMAPK) pathway regulates bud emergence under nutrient-limiting conditions. The fMAPK pathway regulated the expression of polarity targets including the gene encoding a direct effector of Cdc42p, Gic2p. The fMAPK pathway also stimulated GTP-Cdc42p levels, which is a critical determinant of polarity establishment. The fMAPK pathway activity was spatially restricted to bud sites and active during the period of the cell cycle leading up to bud emergence. Time-lapse fluorescence microscopy showed that the fMAPK pathway stimulated the rate of bud emergence during filamentous growth. Unregulated activation of the fMAPK pathway induced multiple rounds of symmetry breaking inside the growing bud. Collectively, our findings identify a new regulatory aspect of bud emergence that sensitizes this essential cellular process to external cues.

Centrosome Aurora A regulates RhoGEF ECT-2 localisation and ensures a single PAR-2 polarity axis in C. elegans embryos.

Proper establishment of cell polarity is essential for development. In the one-cell C. elegans embryo, a centrosome-localised signal provides spatial information for polarity establishment. It is hypothesised that this signal causes local inhibition of the cortical actomyosin network, and breaks symmetry to direct partitioning of the PAR proteins. However, the molecular nature of the centrosomal signal that triggers cortical anisotropy in the actomyosin network to promote polarity establishment remains elusive. Here, we discover that depletion of Aurora A kinase (AIR-1 in C. elegans) causes pronounced cortical contractions on the embryo surface, and this creates more than one PAR-2 polarity axis. This function of AIR-1 appears to be independent of its role in microtubule nucleation. Importantly, upon AIR-1 depletion, centrosome positioning becomes dispensable in dictating the PAR-2 axis. Moreover, we uncovered that a Rho GEF, ECT-2, acts downstream of AIR-1 in regulating contractility and PAR-2 localisation, and, notably, AIR-1 depletion influences ECT-2 cortical localisation. Overall, this study provides a novel insight into how an evolutionarily conserved centrosome Aurora A kinase inhibits promiscuous PAR-2 domain formation to ensure singularity in the polarity establishment axis.

Expression characterization and cross-species complementation uncover the functional conservation of YABBY genes for leaf abaxial polarity and carpel polarity establishment in Saccharum spontaneum.

BACKGROUND: Cell polarity establishment and maintenance is indispensable for plant growth and development. In plants, the YABBY transcription factor family has a distinct role in leaf asymmetric polarity establishment and lateral organ initiation. However, for the important sugar crop Saccharum, little information on YABBY genes is available. RESULTS: In this study, a total of 20 sequences for 7 SsYABBY genes were identified in the sugarcane genome, designated as SsYABBY1-7 based on their chromosome locations, and characterized by phylogenetic analysis. We provided a high-resolution map of SsYABBYs' global expression dynamics during vegetative and reproductive organ morphogenesis and revealed that SsYABBY3/4/5 are predominately expressed at the seedling stage of stem and leaf basal zone; SsYABBY2/5/7 are highly expressed in ovules. Besides, cross-species overexpression and/or complementation verified the conserved function of SsYABBY2 in establishing leaf adaxial-abaxial polarity and ovules development. We found that the SsYABBY2 could successfully rescue the leaves curling, carpel dehiscence, and ovule abortion defects in Arabidopsis crc mutant. CONCLUSIONS: Collectively, our study demonstrates that SsYABBY genes retained a conserved function in establishing and preserving leaf adaxial-abaxial polarity and lateral organ development during evolution.

Recent developments in the understanding of PIN polarity.

Polar localization of PIN-FORMED proteins (PINs) at the plasma membrane is essential for plant development as they direct the transport of phytohormone auxin between cells. PIN polar localization to certain sides of a given cell is dynamic, strictly regulated and provides directionality to auxin flow. Signals that act upstream to control subcellular PIN localization modulate auxin distribution, thereby regulating diverse aspects of plant development. Here I summarize the current understanding of mechanisms by which PIN polarity is established, maintained and rearranged to provide a glimpse into the complexity of PIN polarity.

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.

Roles of YABBY transcription factors in the modulation of morphogenesis, development, and phytohormone and stress responses in plants.

The YABBY family is a class of plant-specific transcription factors comprising a typical N-terminal C2C2-type zinc finger domain and a C-terminal helix-loop-helix YABBY domain. YABBY transcription factors play important roles in multiple biological processes, including polarity establishment in plant leaves, the formation and development of reproductive organs, the response to plant hormone signals, resistance to stress, crop breeding and agricultural production. The aim of this review is to summarize our current understanding of the roles, functions and value of the YABBY family in plants, with particular emphasis on new insights into the molecular and physiological mechanisms involved in the YABBY-mediated modulation of polarity establishment, morphogenesis and development, and phytohormone and stress responses in plants. In addition, we propose that this transcription factor family presents great value and potential for research, application and development in crop breeding and agricultural production in the future.

Spatial landmarks regulate a Cdc42-dependent MAPK pathway to control differentiation and the response to positional compromise.

A fundamental problem in cell biology is to understand how spatial information is recognized and integrated into morphogenetic responses. Budding yeast undergoes differentiation to filamentous growth, which involves changes in cell polarity through mechanisms that remain obscure. Here we define a regulatory input where spatial landmarks (bud-site-selection proteins) regulate the MAPK pathway that controls filamentous growth (fMAPK pathway). The bud-site GTPase Rsr1p regulated the fMAPK pathway through Cdc24p, the guanine nucleotide exchange factor for the polarity establishment GTPase Cdc42p. Positional landmarks that direct Rsr1p to bud sites conditionally regulated the fMAPK pathway, corresponding to their roles in regulating bud-site selection. Therefore, cell differentiation is achieved in part by the reorganization of polarity at bud sites. In line with this conclusion, dynamic changes in budding pattern during filamentous growth induced corresponding changes in fMAPK activity. Intrinsic compromise of bud-site selection also impacted fMAPK activity. Therefore, a surveillance mechanism monitors spatial position in response to extrinsic and intrinsic stress and modulates the response through a differentiation MAPK pathway.

A Wiskott-Aldrich syndrome protein is involved in endocytosis in Aspergillus nidulans.

Endocytosis is vital for hyphal tip growth in filamentous fungi and is involved in the tip localization of various membrane proteins. To investigate the function of a Wiskott-Aldrich syndrome protein (WASP) in endocytosis of filamentous fungi, we identified a WASP ortholog-encoding gene, wspA, in Aspergillus nidulans and characterized it. The wspA product, WspA, localized to the tips of germ tubes during germination and actin rings in the subapical regions of mature hyphae. wspA is essential for the growth and functioned in the polarity establishment and maintenance during germination of conidia. We also investigated its function in endocytosis and revealed that endocytosis of SynA, a synaptobrevin ortholog that is known to be endocytosed at the subapical regions of hyphal tips in A. nidulans, did not occur when wspA expression was repressed. These results suggest that WspA plays roles in endocytosis at hyphal tips and polarity establishment during germination.

Advancements in Rice Leaf Development Research.

Rice leaf morphology is a pivotal component of the ideal plant architecture, significantly impacting rice yield. The process of leaf development unfolds through three distinct stages: the initiation of leaf primordia, the establishment and maintenance of polarity, and leaf expansion. Genes regulating leaf morphology encompass transcription factors, hormones, and miRNAs. An in-depth synthesis and categorization of genes associated with leaf development, particularly those successfully cloned, hold paramount importance in unraveling the complexity of rice leaf development. Furthermore, it provides valuable insights into the potential for molecular-level manipulation of rice leaf types. This comprehensive review consolidates the stages of rice leaf development, the genes involved, molecular regulatory pathways, and the influence of plant hormones. Its objective is to establish a foundational understanding of the creation of ideal rice leaf forms and their practical application in molecular breeding.

Context matters: Lessons in epithelial polarity from the Caenorhabditis elegans intestine and other tissues.

Epithelia are tissues with diverse morphologies and functions across metazoans, ranging from vast cell sheets encasing internal organs to internal tubes facilitating nutrient uptake, all of which require establishment of apical-basolateral polarity axes. While all epithelia tend to polarize the same components, how these components are deployed to drive polarization is largely context-dependent and likely shaped by tissue-specific differences in development and ultimate functions of polarizing primordia. The nematode Caenorhabditis elegans (C. elegans) offers exceptional imaging and genetic tools and possesses unique epithelia with well-described origins and roles, making it an excellent model to investigate polarity mechanisms. In this review, we highlight the interplay between epithelial polarization, development, and function by describing symmetry breaking and polarity establishment in a particularly well-characterized epithelium, the C. elegans intestine. We compare intestinal polarization to polarity programs in two other C. elegans epithelia, the pharynx and epidermis, correlating divergent mechanisms with tissue-specific differences in geometry, embryonic environment, and function. Together, we emphasize the importance of investigating polarization mechanisms against the backdrop of tissue-specific contexts, while also underscoring the benefits of cross-tissue comparisons of polarity.

Membrane contact site-dependent cholesterol transport regulates Na+/K+-ATPase polarization and spermiogenesis in Caenorhabditis elegans.

Spermiogenesis in nematodes is a process whereby round and quiescent spermatids differentiate into asymmetric and crawling spermatozoa. The molecular mechanism underlying this symmetry breaking remains uncharacterized. In this study, we revealed that sperm-specific Na+/K+-ATPase (NKA) is evenly distributed on the plasma membrane (PM) of Caenorhabditis elegans spermatids but is translocated to and subsequently enters the invaginated membrane of the spermatozoa cell body during sperm activation. The polarization of NKA depends on the transport of cholesterol from the PM to membranous organelles (MOs) via membrane contact sites (MCSs). The inositol 5-phosphatase CIL-1 and the MO-localized PI4P phosphatase SAC-1 may mediate PI4P metabolism to drive cholesterol countertransport via sterol/lipid transport proteins through MCSs. Furthermore, the NKA function is required for C. elegans sperm motility and reproductive success. Our data imply that the lipid dynamics mediated by MCSs might play crucial roles in the establishment of cell polarity. eGraphical abstract.

Overexpression of the Aspergillus fumigatus Small GTPase, RsrA, Promotes Polarity Establishment during Germination.

Cell polarization comprises highly controlled processes and occurs in most eukaryotic organisms. In yeast, the processes of budding, mating and filamentation require coordinated mechanisms leading to polarized growth. Filamentous fungi, such as Aspergillus fumigatus, are an extreme example of cell polarization, essential for both vegetative and pathogenic growth. A major regulator of polarized growth in yeast is the small GTPase Rsr1, which is essential for bud-site selection. Here, we show that deletion of the putative A. fumigatus ortholog, rsrA, causes only a modest reduction of growth rate and delay in germ tube emergence. In contrast, overexpression of rsrA results in a morphogenesis defect, characterized by a significant delay in polarity establishment followed by the establishment of multiple growth axes. This aberrant phenotype is reversed when rsrA expression levels are decreased, suggesting that correct regulation of RsrA activity is crucial for accurate patterning of polarity establishment. Despite this finding, deletion or overexpression of rsrA resulted in no changes of A. fumigatus virulence attributes in a mouse model of invasive aspergillosis. Additional mutational analyses revealed that RsrA cooperates genetically with the small GTPase, RasA, to support A. fumigatus viability.

Building an apical domain in the early mouse embryo: Lessons, challenges and perspectives.

Cell polarization is critical for lineage segregation and morphogenesis during mammalian embryogenesis. However, the processes and mechanisms that establish cell polarity in the mammalian embryo are not well understood. Recent studies suggest that unique regulatory mechanisms are deployed by the mouse embryo to establish cell polarization. In this review, we discuss the current understanding of cell polarity establishment, focusing on the formation of the apical domain in the mouse embryo. We will also discuss outstanding questions and possible directions for future study.

PIN2 Polarity Establishment in Arabidopsis in the Absence of an Intact Cytoskeleton.

Cell polarity is crucial for the coordinated development of all multicellular organisms. In plants, this is exemplified by the PIN-FORMED (PIN) efflux carriers of the phytohormone auxin: The polar subcellular localization of the PINs is instructive to the directional intercellular auxin transport, and thus to a plethora of auxin-regulated growth and developmental processes. Despite its importance, the regulation of PIN polar subcellular localization remains poorly understood. Here, we have employed advanced live-cell imaging techniques to study the roles of microtubules and actin microfilaments in the establishment of apical polar localization of PIN2 in the epidermis of the Arabidopsis root meristem. We report that apical PIN2 polarity requires neither intact actin microfilaments nor microtubules, suggesting that the primary spatial cue for polar PIN distribution is likely independent of cytoskeleton-guided endomembrane trafficking.

Distinct RopGEFs Successively Drive Polarization and Outgrowth of Root Hairs.

Root hairs are tubular protrusions of the root epidermis that significantly enlarge the exploitable soil volume in the rhizosphere. Trichoblasts, the cell type responsible for root hair formation, switch from cell elongation to tip growth through polarization of the growth machinery to a predefined root hair initiation domain (RHID) at the plasma membrane. The emergence of this polar domain resembles the establishment of cell polarity in other eukaryotic systems [1-3]. Rho-type GTPases of plants (ROPs) are among the first molecular determinants of the RHID [4, 5], and later play a central role in polar growth [6]. Numerous studies have elucidated mechanisms that position the RHID in the cell [7-9] or regulate ROP activity [10-18]. The molecular players that target ROPs to the RHID and initiate outgrowth, however, have not been identified. We dissected the timing of the growth machinery assembly in polarizing hair cells and found that positioning of molecular players and outgrowth are temporally separate processes that are each controlled by specific ROP guanine nucleotide exchange factors (GEFs). A functional analysis of trichoblast-specific GEFs revealed GEF3 to be required for normal ROP polarization and thus efficient root hair emergence, whereas GEF4 predominantly regulates subsequent tip growth. Ectopic expression of GEF3 induced the formation of spatially confined, ROP-recruiting domains in other cell types, demonstrating the role of GEF3 to serve as a membrane landmark during cell polarization.

Actin Polymerization Mediated by AtFH5 Directs the Polarity Establishment and Vesicle Trafficking for Pollen Germination in Arabidopsis.

The process of pollen germination is crucial for flowering plant reproduction, but the mechanisms through which pollen grains establish polarity and select germination sites are not well understood. In this study, we report that a formin family protein, AtFH5, is localized to the vesicles and rotates ahead of Lifeact-mEGFP-labeled actin filaments during pollen germination. The translocation of AtFH5 to the plasma membrane initiates the assembly of a collar-like actin structure at the prospective germination site prior to germination. Genetic and pharmacological evidence further revealed an interdependent relationship between the mobility of AtFH5-labeled vesicles and the polymerization of actin filaments: vesicle-localized AtFH5 promotes actin assembly, and the polymerization and elongation of actin filaments, in turn, is essential for the mobility of AtFH5-labeled vesicles in pollen grains. Taken together, our work revealed a molecular mechanism underlying the polarity establishment and vesicle mobility during pollen germination.

Imaging Polarization in Budding Yeast.

We describe methods for live-cell imaging of yeast cells that we have exploited to image yeast polarity establishment. As a rare event occurring on a fast time-scale, imaging polarization involves a trade-off between spatiotemporal resolution and long-term imaging without excessive phototoxicity. By synchronizing cells in a way that increases resistance to photodamage, we discovered unexpected aspects of polarization including transient intermediates with more than one polarity cluster, oscillatory clustering of polarity factors, and mobile "wandering" polarity sites.

Symmetry breaking and polarity establishment during mouse oocyte maturation.

Mammalian oocyte meiosis encompasses two rounds of asymmetric divisions to generate a totipotent haploid egg and, as by-products, two small polar bodies. Two intracellular events, asymmetric spindle positioning and cortical polarization, are critical to such asymmetric divisions. Actin but not microtubule cytoskeleton has been known to be directly involved in both events. Recent work has revealed a positive feedback loop between chromosome-mediated cortical activation and the Arp2/3-orchestrated cytoplasmic streaming that moves chromosomes. This feedback loop not only maintains meiotic II spindle position during metaphase II arrest, but also brings about symmetry breaking during meiosis I. Prior to an Arp2/3-dependent phase of fast movement, meiotic I spindle experiences a slow and non-directional first phase of migration driven by a pushing force from Fmn2-mediated actin polymerization. In addition to illustrating these molecular mechanisms, mathematical simulations are presented to elucidate mechanical properties of actin-dependent force generation in this system.