A polarized epithelium organized by beta- and alpha-catenin predates cadherin and metazoan origins

Genetic analyses reveal wildfire particulates as environmental pollutants rather than nutrient sources for corals

Heterotrophic nutrients are crucial for coral growth and recovery from bleaching events. Although wildfire emissions are a potential source of these nutrients, their impact on corals was minimally investigated. In this microcosm experiment, Acropora formosa corals exhibited rapid tissue detachment upon exposure to wildfire fine particulate matter (PM2.5). Physiological and genetic analyses revealed mechanisms associated with oxidation-reduction homeostasis and nutrient metabolism. Excessive hydrogen peroxide was generated as corals activated cytochrome P450 enzymes and the respiratory burst in phagocytic cells to metabolize PM2.5, leading to oxidative damage, mitochondrial dysfunction, and cell apoptosis due to reduced superoxide dismutase activity and compromised glutathione antioxidant function. Subsequently, corals upregulated the transcriptions of genes which are related to tyrosine receptor proteins to regulate multicellular development for self-repair, increasing energy consumption. However, Symbiodiniaceae upregulated their metabolism and retained photosynthates, reducing nutrient supply to the coral host. Therefore, the host temporarily utilized lipid reserves via the glyoxylate cycle, but excessive consumption disrupted lipid and carbohydrate metabolism, ultimately weakening cell adhesion and causing coral tissue detachment. Additionally, the downregulation of HSP70 expression, potentially linked to decreased sacsin and mitochondrial apoptosis, accelerated coral heat bleaching. This study elucidates the mechanisms by which wildfire PM2.5 at environmental concentrations poses risks to corals, particularly in a warming climate.

β-Catenin localization in the ctenophore Mnemiopsis leidyi suggests an ancestral role in cell adhesion and nuclear function

BACKGROUND

The emergence of multicellularity in animals marks a pivotal evolutionary event, which was likely enabled by molecular innovations in the way cells adhere and communicate with one another. β-Catenin is significant to this transition due to its dual role as both a structural component in the cadherin-catenin complex and as a transcriptional coactivator involved in the Wnt/β-catenin signaling pathway. However, our knowledge of how this protein functions in ctenophores, one of the earliest diverging metazoans, is limited.

RESULTS

To study β-catenin function in the ctenophore Mnemiopsis leidyi, we generated affinity-purified polyclonal antibodies targeting Mlβ-catenin. We then used this tool to observe β-catenin protein localization in developing Mnemiopsis embryos. In this article, we provide evidence of consistent β-catenin protein enrichment at cell-cell interfaces in Mnemiopsis embryos. Additionally, we found β-catenin enrichment in some nuclei, particularly restricted to the oral pole around the time of gastrulation. The Mlβ-catenin affinity-purified antibodies now provide us with a powerful reagent to study the ancestral functions of β-catenin in cell adhesion and transcriptional regulation.

CONCLUSIONS

The localization pattern of embryonic Mlβ-catenin suggests that this protein had an ancestral role in cell adhesion and may have a nuclear function as well.

The origin and evolution of Wnt signalling

The Wnt signal transduction pathway has essential roles in the formation of the primary body axis during development, cellular differentiation and tissue homeostasis. This animal-specific pathway has been studied extensively in contexts ranging from developmental biology to medicine for more than 40 years. Despite its physiological importance, an understanding of the evolutionary origin and primary function of Wnt signalling has begun to emerge only recently. Recent studies on very basal metazoan species have shown high levels of conservation of components of both canonical and non-canonical Wnt signalling pathways. Furthermore, some pathway proteins have been described also in non-animal species, suggesting that recruitment and functional adaptation of these factors has occurred in metazoans. In this Review, we summarize the current state of research regarding the evolutionary origin of Wnt signalling, its ancestral function and the characteristics of the primal Wnt ligand, with emphasis on the importance of genomic studies in various pre-metazoan and basal metazoan species.

Bill Weis (1959-2023): Pioneering structural biologist and biochemist who revolutionized our understanding of cell adhesion and Wnt signaling

In October 2023, cell biology lost one of its brightest stars, Bill Weis, gone too soon at the age of 64. Bill was a masterful biochemist and structural biologist who made landmark contributions to a remarkable number of fields, most notably cell–cell adhesion, Wnt signaling, and signaling by G-protein coupled receptors.

Immunohistochemical expression of beta-catenin, BMP4 and TGF-beta in odontomas

Changes in the expression of nuclear β-catenin are responsible for tumorigenesis. Beta-catenin acts synergistically with the TGF-β/BMPs pathway. This interaction leads to greater dentin deposition and may explain the differences between distinct tooth morphologies and hamartomas. The aim of this study was to investigate the role of β-catenin, BMP4 and TGF-β in the development of odontomas. This cross-sectional, retrospective, immunohistochemical study evaluated 30 compound odontomas, 30 complex odontomas and 17 tooth germs. The results showed that BMP4 and TGF-β were more immunoexpressed in the ectomesenchyme of complex odontomas (median = 33.7, p < 0.001; median = 76.4, p = 0.002, respectively). Higher immunoexpression of BMP4 and TGF-β was also observed in the epithelium of tooth germs (median = 2.0, p < 0.001; median = 120.3, p < 0.001, respectively). TGF-β and BMP4 showed a positive and significant correlation (p < 0.001). Both TGF-β and BMP4 were positively correlated with nuclear β-catenin in ectomesenchyme (p = 0.047 and p = 0.023, respectively). Developing teeth exhibited higher concentrations of the proteins studied in odontogenic epithelium, especially during the bud and cap stages. Higher immunoexpression in odontomas occurred mainly in the ectomesenchyme. We therefore suggest that changes in the ectomesenchyme can lead to the development of odontomas.

Origin and development of primary animal epithelia

Epithelia are the first organized tissues that appear during development. In many animal embryos, early divisions give rise to a polarized monolayer, the primary epithelium, rather than a random aggregate of cells. Here, we review the mechanisms by which cells organize into primary epithelia in various developmental contexts. We discuss how cells acquire polarity while undergoing early divisions. We describe cases where oriented divisions constrain cell arrangement to monolayers including organization on top of yolk surfaces. We finally discuss how epithelia emerge in embryos from animals that branched early during evolution and provide examples of epithelia-like arrangements encountered in single-celled eukaryotes. Although divergent and context-dependent mechanisms give rise to primary epithelia, here we trace the unifying principles underlying their formation.

Sculpting with stiffness: rigidity as a regulator of morphogenesis

From a physical perspective, morphogenesis of tissues results from interplay between their material properties and the mechanical forces exerted on them. The importance of mechanical forces in influencing cell behaviour is widely recognised, whereas the importance of tissue material properties in vivo, like stiffness, has only begun to receive attention in recent years. In this mini-review, we highlight key themes and concepts that have emerged related to how tissue stiffness, a fundamental material property, guides various morphogenetic processes in living organisms.

Embryonic β-Catenin Is Required for Priming of the Uterus to Implantation

Repeated implantation failure is a major cause of infertility among healthy women. Uterine β-catenin (CTNNB1) plays a critical role in implantation. However, the role of embryonic CTNNB1 during implantation remains unclear. We addressed this topic by analyzing mice carrying Ctnnb1-deficient (Ctnnb1^Δ/Δ) embryos. Ctnnb1^Δ/Δ embryos were produced by intercrossing mice bearing Ctnnb1-deficient eggs and sperms. We found that Ctnnb1^Δ/Δ embryos developed to the blastocyst stage; thereafter, they were resorbed, leaving empty decidual capsules. Moreover, leukemia inhibitory factor, a uterine factor essential for implantation, was undetectable in Ctnnb1^Δ/Δ blastocysts. Furthermore, CDX2, a transcription factor that determines the fate of trophectoderm cells, was not observed in Ctnnb1^Δ/Δ blastocysts. Intrauterine injection with uterine fluids (from control mice) and recombinant mouse leukemia inhibitory factor proteins rescued the uterine response to Ctnnb1^Δ/Δ blastocysts. These results suggest that embryonic CTNNB1 is required for the secretion of blastocyst-derived factor(s) that open the implantation window, indicating that the uterine response to implantation can be induced using supplemental materials. Therefore, our results may contribute to the discovery of a similar mechanism in humans, leading to a better understanding of the pathogenesis of repeated implantation failure.

The nematode α-catenin ortholog, HMP1, has an extended α-helix when bound to actin filaments

The regulation of cell-cell junctions during epidermal morphogenesis ensures tissue integrity, a process regulated by α-catenin. This cytoskeletal protein connects the cadherin complex to filamentous actin at cell-cell junctions. The cadherin-catenin complex plays key roles in cell physiology, organism development, and disease. While mutagenesis of Caenorhabditis elegans cadherin and catenin shows that these proteins are key for embryonic morphogenesis, we know surprisingly little about their structure and attachment to the cytoskeleton. In contrast to mammalian α-catenin that functions as a dimer or monomer, the α-catenin ortholog from C. elegans, HMP1 for humpback, is a monomer. Our cryogenic electron microscopy (cryoEM) structure of HMP1/α-catenin reveals that the amino- and carboxy-terminal domains of HMP1/α-catenin are disordered and not in contact with the remaining HMP1/α-catenin middle domain. Since the carboxy-terminal HMP1/α-catenin domain is the F-actin-binding domain (FABD), this interdomain constellation suggests that HMP1/α-catenin is constitutively active, which we confirm biochemically. Our perhaps most surprising finding, given the high sequence similarity between the mammalian and nematode proteins, is our cryoEM structure of HMP1/α-catenin bound to F-actin. Unlike the structure of mammalian α-catenin bound to F-actin, binding to F-actin seems to allosterically convert a loop region of the HMP1/α-catenin FABD to extend an HMP1/α-catenin FABD α-helix. We use cryoEM and bundling assays to show for the first time how the FABD of HMP1/α-catenin bundles actin in the absence of force. Collectively, our data advance our understanding of α-catenin regulation of cell-cell contacts and additionally aid our understanding of the evolution of multicellularity in metazoans.

Cellularization across eukaryotes: Conserved mechanisms and novel strategies

Many eukaryotes form multinucleated cells during their development. Some cells persist as such during their lifetime, others choose to cleave each nucleus individually using a specialized cytokinetic process known as cellularization. What is cellularization and how is it achieved across the eukaryotic tree of life? Are there common pathways among all species supporting a shared ancestry, or are there key differences, suggesting independent evolutionary paths? In this review, we discuss common strategies and key mechanistic differences in how cellularization is executed across vastly divergent eukaryotic species. We present a number of novel methods and non-model organisms that may provide important insight into the evolutionary origins of cellularization.

Crosstalk Between β-CATENIN-Mediated Cell Adhesion and the WNT Signaling Pathway

Cell adhesion and stable signaling regulation are fundamental ways of maintaining homeostasis. Among them, the Wnt/β-CATENIN signaling plays a key role in embryonic development and maintenance of body dynamic homeostasis. At the same time, the key signaling molecule β-CATENIN in the Wnt signaling can also function as a cytoskeletal linker protein to regulate tissue barriers, cell migration, and morphogenesis. Dysregulation of the balance between Wnt signaling and adherens junctions can lead to disease. How β-CATENIN maintains the independence of these two functions, or mediates the interaction and balance of these two functions, has been explored and debated for a long time. In this study, we will focus on five aspects of β-CATENIN chaperone molecules, phosphorylation of β-CATENIN and related proteins, epithelial mesenchymal transition, β-CATENIN homolog protein γ-CATENIN and disease, thus deepening the understanding of the Wnt/β-CATENIN signaling and the homeostasis between cell adhesion and further addressing related disease problems.

Structure and evolution of neuronal wiring receptors and ligands

One of the fundamental properties of a neuronal circuit is the map of its connections. The cellular and developmental processes that allow for the growth of axons and dendrites, selection of synaptic targets, and formation of functional synapses use neuronal surface receptors and their interactions with other surface receptors, secreted ligands, and matrix molecules. Spatiotemporal regulation of the expression of these receptors and cues allows for specificity in the developmental pathways that wire stereotyped circuits. The families of molecules controlling axon guidance and synapse formation are generally conserved across animals, with some important exceptions, which have consequences for neuronal connectivity. Here, we summarize the distribution of such molecules across multiple taxa, with a focus on model organisms, evolutionary processes that led to the multitude of such molecules, and functional consequences for the diversification or loss of these receptors.

Cell polarity in the protist-to-animal transition

A signature feature of the animal kingdom is the presence of epithelia: sheets of polarized cells that both insulate the organism from its environment and mediate interactions with it. Epithelial cells display a marked apico-basal polarity, which is highly conserved across the animal kingdom, both in terms of morphology and of molecular regulators. How did this architecture first evolve? Although the last eukaryotic common ancestor almost certainly possessed a simple form of apico-basal polarity (marked by the presence of one or several flagella at a single cellular pole), comparative genomics and evolutionary cell biology reveal that the polarity regulators of animal epithelial cells have a surprisingly complex and stepwise evolutionary history. Here, we retrace their evolutionary assembly. We suggest that the "polarity network" that polarized animal epithelial cells evolved by integration of initially independent cellular modules that evolved at distinct steps of our evolutionary ancestry. The first module dates back to the last common ancestor of animals and amoebozoans and involved Par1, extracellular matrix proteins, and the integrin-mediated adhesion complex. Other regulators, such as Cdc42, Dlg, Par6 and cadherins evolved in ancient unicellular opisthokonts, and might have first been involved in F-actin remodeling and filopodial dynamics. Finally, the bulk of "polarity proteins" as well as specialized adhesion complexes evolved in the metazoan stem-line, in concert with the newly evolved intercellular junctional belts. Thus, the polarized architecture of epithelia can be understood as a palimpsest of components of distinct histories and ancestral functions, which have become tightly integrated in animal tissues.

The cellular slime mold Fonticula alba forms a dynamic, multicellular collective while feeding on bacteria

Multicellularity evolved in fungi and animals, or the opisthokonts, from their common amoeboflagellate ancestor but resulted in strikingly distinct cellular organizations. The origins of this multicellularity divergence are not known. The stark mechanistic differences that underlie the two groups and the lack of information about ancestral cellular organizations limits progress in this field. We discovered a new type of invasive multicellular behavior in Fonticula alba, a unique species in the opisthokont tree, which has a simple, bacteria-feeding sorocarpic amoeba lifestyle. This invasive multicellularity follows germination dependent on the bacterial culture state, after which amoebae coalesce to form dynamic collectives that invade virgin bacterial resources. This bacteria-dependent social behavior emerges from amoeba density and allows for rapid and directed invasion. The motile collectives have animal-like properties but also hyphal-like search and invasive behavior. These surprising findings enrich the diverse multicellularities present within the opisthokont lineage and offer a new perspective on fungal origins.

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Unexpected bacterial-state-dependent culture conditions for Fonticula alba

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A multicellular invasion of bacterial food resources that is distinct from fruiting

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A leader-led invasive collectivity that is an emergent property

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Insights into the origins of invasive hyphal and fruiting multicellularity in dikarya

Born to Run? Diverse Modes of Epithelial Migration

Bundled with various kinds of adhesion molecules and anchored to the basement membrane, the epithelium has historically been considered as an immotile tissue and, to migrate, it first needs to undergo epithelial-mesenchymal transition (EMT). Since its initial description more than half a century ago, the EMT process has fascinated generations of developmental biologists and, more recently, cancer biologists as it is believed to be essential for not only embryonic development, organ formation, but cancer metastasis. However, recent progress shows that epithelium is much more motile than previously realized. Here, we examine the emerging themes in epithelial collective migration and how this has impacted our understanding of EMT.

The Evolutionary Assembly of Neuronal Machinery

Neurons are highly specialized cells equipped with a sophisticated molecular machinery for the reception, integration, conduction and distribution of information. The evolutionary origin of neurons remains unsolved. How did novel and pre-existing proteins assemble into the complex machinery of the synapse and of the apparatus conducting current along the neuron? In this review, the step-wise assembly of functional modules in neuron evolution serves as a paradigm for the emergence and modification of molecular machinery in the evolution of cell types in multicellular organisms. The pre-synaptic machinery emerged through modification of calcium-regulated large vesicle release, while the postsynaptic machinery has different origins: the glutamatergic postsynapse originated through the fusion of a sensory signaling module and a module for filopodial outgrowth, while the GABAergic postsynapse incorporated an ancient actin regulatory module. The synaptic junction, in turn, is built around two adhesion modules controlled by phosphorylation, which resemble septate and adherens junctions. Finally, neuronal action potentials emerged via a series of duplications and modifications of voltage-gated ion channels. Based on these origins, key molecular innovations are identified that led to the birth of the first neuron in animal evolution.

The origin of animals: an ancestral reconstruction of the unicellular-to-multicellular transition

How animals evolved from a single-celled ancestor, transitioning from a unicellular lifestyle to a coordinated multicellular entity, remains a fascinating question. Key events in this transition involved the emergence of processes related to cell adhesion, cell–cell communication and gene regulation. To understand how these capacities evolved, we need to reconstruct the features of both the last common multicellular ancestor of animals and the last unicellular ancestor of animals. In this review, we summarize recent advances in the characterization of these ancestors, inferred by comparative genomic analyses between the earliest branching animals and those radiating later, and between animals and their closest unicellular relatives. We also provide an updated hypothesis regarding the transition to animal multicellularity, which was likely gradual and involved the use of gene regulatory mechanisms in the emergence of early developmental and morphogenetic plans. Finally, we discuss some new avenues of research that will complement these studies in the coming years.

Cell-extracellular matrix interactions in the fluidic phase direct the topology and polarity of self-organized epithelial structures

Introduction

In vivo, cells are surrounded by extracellular matrix (ECM). To build organs from single cells, it is generally believed that ECM serves as scaffolds to coordinate cell positioning and differentiation. Nevertheless, how cells utilize cell‐ECM interactions for the spatiotemporal coordination to different ECM at the tissue scale is not fully understood.

Methods

Here, using in vitro assay with engineered MDCK cells expressing H2B‐mCherry (nucleus) and gp135/Podocalyxin‐GFP (apical marker), we show in multi‐dimensions that such coordination for epithelial morphogenesis can be determined by cell‐soluble ECM interaction in the fluidic phase.

Results

The coordination depends on the native topology of ECM components such as sheet‐like basement membrane (BM) and type I collagen (COL) fibres: scaffold formed by BM (COL) facilitates a close‐ended (open‐ended) coordination that leads to the formation of lobular (tubular) epithelium. Further, cells form apicobasal polarity throughout the entire lobule/tubule without a complete coverage of ECM at the basal side, and time‐lapse two‐photon scanning imaging reveals the polarization occurring early and maintained through the lobular expansion. During polarization, gp135‐GFP was converged to the apical surface collectively in the lobular/tubular structures, suggesting possible intercellular communications. Under suspension culture, the polarization was impaired with multi‐lumen formation in the tubules, implying the importance of ECM biomechanical microenvironment.

Conclusion

Our results suggest a biophysical mechanism for cells to form polarity and coordinate positioning at tissue scale, and in engineering epithelium through cell‐soluble ECM interaction and self‐assembly.

Oxygen-sensing mechanisms across eukaryotic kingdoms and their roles in complex multicellularity

Oxygen-sensing mechanisms of eukaryotic multicellular organisms coordinate hypoxic cellular responses in a spatiotemporal manner. Although this capacity partly allows animals and plants to acutely adapt to oxygen deprivation, its functional and historical roots in hypoxia emphasize a broader evolutionary role. For multicellular life-forms that persist in settings with variable oxygen concentrations, the capacity to perceive and modulate responses in and between cells is pivotal. Animals and higher plants represent the most complex life-forms that ever diversified on Earth, and their oxygen-sensing mechanisms demonstrate convergent evolution from a functional perspective. Exploring oxygen-sensing mechanisms across eukaryotic kingdoms can inform us on biological innovations to harness ever-changing oxygen availability at the dawn of complex life and its utilization for their organismal development.

Walking the tight wire between cell adhesion and WNT signalling: a balancing act for β-catenin

CTNNB1 (catenin β-1, also known as β-catenin) plays a dual role in the cell. It is the key effector of WNT/CTNNB1 signalling, acting as a transcriptional co-activator of TCF/LEF target genes. It is also crucial for cell adhesion and a critical component of cadherin-based adherens junctions. Two functional pools of CTNNB1, a transcriptionally active and an adhesive pool, can therefore be distinguished. Whether cells merely balance the distribution of available CTNNB1 between these functional pools or whether interplay occurs between them has long been studied and debated. While interplay has been indicated upon artificial modulation of cadherin expression levels and during epithelial-mesenchymal transition, it is unclear to what extent CTNNB1 exchange occurs under physiological conditions and in response to WNT stimulation. Here, we review the available evidence for both of these models, discuss how CTNNB1 binding to its many interaction partners is controlled and propose avenues for future studies.

The WNT/β‐catenin dependent transcription: A tissue‐specific business

β‐catenin‐mediated Wnt signaling is an ancient cell‐communication pathway in which β‐catenin drives the expression of certain genes as a consequence of the trigger given by extracellular WNT molecules. The events occurring from signal to transcription are evolutionarily conserved, and their final output orchestrates countless processes during embryonic development and tissue homeostasis. Importantly, a dysfunctional Wnt/β‐catenin pathway causes developmental malformations, and its aberrant activation is the root of several types of cancer. A rich literature describes the multitude of nuclear players that cooperate with β‐catenin to generate a transcriptional program. However, a unified theory of how β‐catenin drives target gene expression is still missing. We will discuss two types of β‐catenin interactors: transcription factors that allow β‐catenin to localize at target regions on the DNA, and transcriptional co‐factors that ultimately activate gene expression. In contrast to the presumed universality of β‐catenin's action, the ensemble of available evidence suggests a view in which β‐catenin drives a complex system of responses in different cells and tissues. A malleable armamentarium of players might interact with β‐catenin in order to activate the right “canonical” targets in each tissue, developmental stage, or disease context. Discovering the mechanism by which each tissue‐specific β‐catenin response is executed will be crucial to comprehend how a seemingly universal pathway fosters a wide spectrum of processes during development and homeostasis. Perhaps more importantly, this could ultimately inform us about which are the tumor‐specific components that need to be targeted to dampen the activity of oncogenic β‐catenin.

This article is categorized under:

Cancer > Molecular and Cellular Physiology

Cancer > Genetics/Genomics/Epigenetics

Cancer > Stem Cells and Development

Tracing the Evolutionary Origin of Desmosomes

Cadherin-based cell-cell junctions help metazoans form polarized sheets of cells, which are necessary for the development of organs and the compartmentalization of functions. The components of the protein complexes that generate cadherin-based junctions have ancient origins, with conserved elements shared between animals as diverse as sponges and vertebrates. In invertebrates, the formation and function of epithelial sheets depends on classical cadherin-containing adherens junctions, which link actin to the plasma membrane through α-, β- and p120 catenins. Vertebrates also have a new type of cadherin-based intercellular junction called the desmosome, which allowed for the creation of more complex and effective tissue barriers against environmental stress. While desmosomes have a molecular blueprint that is similar to that of adherens junctions, desmosomal cadherins - called desmogleins and desmocollins - link intermediate filaments (IFs) rather than actin to the plasma membrane through protein complexes comprising relatives of β-catenin (plakoglobin) and p120 catenin (plakophilins). In turn, desmosomal catenins interact with members of the IF-binding plakin family to create the desmosome-IF linking complex. In this Minireview, we discuss when and how desmosomal components evolved, and how their ability to anchor the highly elastic and tough IF cytoskeleton endowed vertebrates with robust tissues capable of not only resisting but also properly responding to environmental stress.

Genetic transformation of Spizellomyces punctatus, a resource for studying chytrid biology and evolutionary cell biology

Chytrids are early-diverging fungi that share features with animals that have been lost in most other fungi. They hold promise as a system to study fungal and animal evolution, but we lack genetic tools for hypothesis testing. Here, we generated transgenic lines of the chytrid Spizellomyces punctatus, and used fluorescence microscopy to explore chytrid cell biology and development during its life cycle. We show that the chytrid undergoes multiple rounds of synchronous nuclear division, followed by cellularization, to create and release many daughter ‘zoospores’. The zoospores, akin to animal cells, crawl using actin-mediated cell migration. After forming a cell wall, polymerized actin reorganizes into fungal-like cortical patches and cables that extend into hyphal-like structures. Actin perinuclear shells form each cell cycle and polygonal territories emerge during cellularization. This work makes Spizellomyces a genetically tractable model for comparative cell biology and understanding the evolution of fungi and early eukaryotes.

Insights into the origin of metazoan multicellularity from predatory unicellular relatives of animals

BACKGROUND

The origin of animals from their unicellular ancestor was one of the most important events in evolutionary history, but the nature and the order of events leading up to the emergence of multicellular animals are still highly uncertain. The diversity and biology of unicellular relatives of animals have strongly informed our understanding of the transition from single-celled organisms to the multicellular Metazoa. Here, we analyze the cellular structures and complex life cycles of the novel unicellular holozoans Pigoraptor and Syssomonas (Opisthokonta), and their implications for the origin of animals.

RESULTS

Syssomonas and Pigoraptor are characterized by complex life cycles with a variety of cell types including flagellates, amoeboflagellates, amoeboid non-flagellar cells, and spherical cysts. The life cycles also include the formation of multicellular aggregations and syncytium-like structures, and an unusual diet for single-celled opisthokonts (partial cell fusion and joint sucking of a large eukaryotic prey), all of which provide new insights into the origin of multicellularity in Metazoa. Several existing models explaining the origin of multicellular animals have been put forward, but these data are interestingly consistent with one, the "synzoospore hypothesis."

CONCLUSIONS

The feeding modes of the ancestral metazoan may have been more complex than previously thought, including not only bacterial prey, but also larger eukaryotic cells and organic structures. The ability to feed on large eukaryotic prey could have been a powerful trigger in the formation and development of both aggregative (e.g., joint feeding, which also implies signaling) and clonal (e.g., hypertrophic growth followed by palintomy) multicellular stages that played important roles in the emergence of multicellular animals.

Diverse cell junctions with unique molecular composition in tissues of a sponge (Porifera)

The integrity and organization of animal tissues depend upon specialized protein complexes that mediate adhesion between cells with each other (cadherin-based adherens junctions), and with the extracellular matrix (integrin-based focal adhesions). Reconstructing how and when these cell junctions evolved is central to understanding early tissue evolution in animals. We examined focal adhesion protein homologs in tissues of the freshwater sponge, Ephydatia muelleri (phylum Porifera; class Demospongiae). Our principal findings are that (1) sponge focal adhesion homologs (integrin, talin, focal adhesion kinase, etc.) co-precipitate as a complex, separate from adherens junction proteins; (2) that actin-based structures resembling focal adhesions form at the cell–substrate interface, and their abundance is dynamically regulated in response to fluid shear; (3) focal adhesion proteins localize to both cell–cell and cell–extracellular matrix adhesions, and; (4) the adherens junction protein β-catenin is co-distributed with focal adhesion proteins at cell–cell junctions everywhere except the choanoderm, and at novel junctions between cells with spicules, and between cells with environmental bacteria. These results clarify the diversity, distribution and molecular composition of cell junctions in tissues of E. muelleri, but raise new questions about their functional properties and ancestry.

A unicellular relative of animals generates a layer of polarized cells by actomyosin-dependent cellularization

In animals, cellularization of a coenocyte is a specialized form of cytokinesis that results in the formation of a polarized epithelium during early embryonic development. It is characterized by coordinated assembly of an actomyosin network, which drives inward membrane invaginations. However, whether coordinated cellularization driven by membrane invagination exists outside animals is not known. To that end, we investigate cellularization in the ichthyosporean Sphaeroforma arctica, a close unicellular relative of animals. We show that the process of cellularization involves coordinated inward plasma membrane invaginations dependent on an actomyosin network and reveal the temporal order of its assembly. This leads to the formation of a polarized layer of cells resembling an epithelium. We show that this stage is associated with tightly regulated transcriptional activation of genes involved in cell adhesion. Hereby we demonstrate the presence of a self-organized, clonally-generated, polarized layer of cells in a unicellular relative of animals.

A Polarizing Issue: Diversity in the Mechanisms Underlying Apico-Basolateral Polarization In Vivo

Polarization along an apico-basolateral axis is a hallmark of epithelial cells and is essential for their selective barrier and transporter functions, as well as for their ability to provide mechanical resiliency to organs. Loss of polarity along this axis perturbs development and is associated with a wide number of diseases. We describe three steps involved in polarization: symmetry breaking, polarity establishment, and polarity maintenance. While the proteins involved in these processes are highly conserved among epithelial tissues and species, the execution of these steps varies widely and is context dependent. We review both theoretical principles underlying these steps and recent work demonstrating how apico-basolateral polarity is established in vivo in different tissues, highlighting how developmental and physiological contexts play major roles in the execution of the epithelial polarity program.

Inherent forms and the evolution of evolution

John Bonner presented a provocative conjecture that the means by which organisms evolve has itself evolved. The elements of his postulated nonuniformitarianism in the essay under discussion-the emergence of sex, the enhanced selection pressures on larger multicellular forms-center on a presumed close mapping of genotypic to phenotypic change. A different view emerges from delving into earlier work of Bonner's in which he proposed the concept of "neutral phenotypes" and "neutral morphologies" allied to D'Arcy Thompson's analysis of physical determinants of form and studied the conditional elicitation of intrinsic organizational properties of cell aggregates in social amoebae. By comparing the shared and disparate mechanistic bases of morphogenesis and developmental outcomes in the embryos of metazoans (animals), closely related nonmetazoan holozoans, more distantly related dictyostelids, and very distantly related volvocine algae, I conclude, in agreement with Bonner's earlier proposals, that understanding the evolution of multicellular evolution requires knowledge of the inherent forms of diversifying lineages, and that the relevant causative factors extend beyond genes and adaptation to the physics of materials.

Somatic multicellularity as a satisficing solution to the prediction-error minimization problem

Adaptive success in the biosphere requires the dynamic ability to adjust physiological, transcriptional, and behavioral responses to environmental conditions. From chemical networks to organisms to whole communities, biological entities at all levels of organization seek to optimize their predictive power. Here, we argue that this fundamental drive provides a novel perspective on the origin of multicellularity. One way for unicellular organisms to minimize surprise with respect to external inputs is to be surrounded by reproductively-disabled, i.e. somatic copies of themselves - highly predictable agents which in effect reduce uncertainty in their microenvironments. We show that the transition to multicellularity can be modeled as a phase transition driven by environmental threats. We present modeling results showing how multicellular bodies can arise if non-reproductive somatic cells protect their reproductive parents from environmental lethality. We discuss how a somatic body can be interpreted as a Markov blanket around one or more reproductive cells, and how the transition to somatic multicellularity can be represented as a transition from exposure of reproductive cells to a high-uncertainty environment to their protection from environmental uncertainty by this Markov blanket. This is, effectively, a transition by the Markov blanket from transparency to opacity for the variational free energy of the environment. We suggest that the ability to arrest the cell cycle of daughter cells and redirect their resource utilization from division to environmental threat amelioration is the key innovation of obligate multicellular eukaryotes, that the nervous system evolved to exercise this control over long distances, and that cancer is an escape by somatic cells from the control of reproductive cells. Our quantitative model illustrates the evolutionary dynamics of this system, provides a novel hypothesis for the origin of multicellular animal bodies, and suggests a fundamental link between the architectures of complex organisms and information processing in proto-cognitive cellular agents.

Evidence of Signaling and Adhesion Roles for β-Catenin in the Sponge Ephydatia muelleri

β-Catenin acts as a transcriptional coactivator in the Wnt/β-catenin signaling pathway and a cytoplasmic effector in cadherin-based cell adhesion. These functions are ancient within animals, but the earliest steps in β-catenin evolution remain unresolved due to limited data from key lineages-sponges, ctenophores, and placozoans. Previous studies in sponges have characterized β-catenin expression dynamics and used GSK3B antagonists to ectopically activate the Wnt/β-catenin pathway; both approaches rely upon untested assumptions about the conservation of β-catenin function and regulation in sponges. Here, we test these assumptions using an antibody raised against β-catenin from the sponge Ephydatia muelleri. We find that cadherin-complex genes coprecipitate with endogenous Em β-catenin from cell lysates, but that Wnt pathway components do not. However, through immunostaining we detect both cell boundary and nuclear populations, and we find evidence that Em β-catenin is a conserved substrate of GSK3B. Collectively, these data support conserved roles for Em β-catenin in both cell adhesion and Wnt signaling. Additionally, we find evidence for an Em β-catenin population associated with the distal ends of F-actin stress fibers in apparent cell-substrate adhesion structures that resemble focal adhesions. This finding suggests a fundamental difference in the adhesion properties of sponge tissues relative to other animals, in which the adhesion functions of β-catenin are typically restricted to cell-cell adhesions.

The cadherin-catenin complex is necessary for cell adhesion and embryogenesis in Nematostella vectensis

The cadherin-catenin complex is a conserved, calcium-dependent cell-cell adhesion module that is necessary for normal development and the maintenance of tissue integrity in bilaterian animals. Despite longstanding evidence of a deep ancestry of calcium-dependent cell adhesion in animals, the requirement of the cadherin-catenin complex to coordinate cell-cell adhesion has not been tested directly in a non-bilaterian organism. Here, we provide the first analysis of classical cadherins and catenins in the Starlet Sea Anemone, Nematostella vectensis. Gene expression, protein localization, siRNA-mediated knockdown of α-catenin, and calcium-dependent cell aggregation assays provide evidence that a bonafide cadherin-catenin complex is present in the early embryo, and that α-catenin is required for normal embryonic development and the formation of cell-cell adhesions between cells dissociated from whole embryos. Together these results support the hypothesis that the cadherin-catenin complex was likely a complete and functional cell-cell adhesion module in the last common cnidarian-bilaterian ancestor. SUMMARY STATEMENT: Embryonic manipulations and ex vivo adhesion assays in the sea anemone, Nematostella vectensis, indicate that the necessity of the cadherin-catenin complex for mediating cell-cell adhesion is deeply conserved in animal evolution.

Effects of equibiaxial mechanical stretch on extracellular matrix-related gene expression in human calvarial osteoblasts

Mechanical stretch commonly promotes craniofacial suture remodeling during interceptive orthodontics. The mechanical responses of osteoblasts in craniofacial sutures play a role in suture remodeling. Moreover, the extracellular matrix (ECM) produced by osteoblasts is crucial for the transduction of mechanical signals that promote cell differentiation. Therefore, we aimed to investigate the effect of mechanical stretch on cell viability and ECM-related gene-expression changes in human osteoblasts. Human calvarial osteoblasts (HCObs) were subjected to 2% deformation. Caspase activity, MTT, and cell viability assays were used to estimate osteoblast apoptosis, proliferation, and viability, respectively. Real-time RT-PCR (RT² -PCR) arrays were used to assess expression of cytoskeletal-, apoptosis-, osteogenesis-, and ECM-related genes. We found that mechanical stretch significantly increased osteoblast viability and cell proliferation, and decreased the activities of caspases 3 and 7. Moreover, the expression of 18 genes related to osteoblast differentiation, apoptosis, and ECM remodeling changed by more than two-fold in a time-dependent manner. Therefore, mechanical stretch promotes HCOb viability and alters expression of genes that are closely related to suture remodeling under mechanical stretch.

Adherens Junctions and Desmosomes Coordinate Mechanics and Signaling to Orchestrate Tissue Morphogenesis and Function: An Evolutionary Perspective

Cadherin-based adherens junctions (AJs) and desmosomes are crucial to couple intercellular adhesion to the actin or intermediate filament cytoskeletons, respectively. As such, these intercellular junctions are essential to provide not only integrity to epithelia and other tissues but also the mechanical machinery necessary to execute complex morphogenetic and homeostatic intercellular rearrangements. Moreover, these spatially defined junctions serve as signaling hubs that integrate mechanical and chemical pathways to coordinate tissue architecture with behavior. This review takes an evolutionary perspective on how the emergence of these two essential intercellular junctions at key points during the evolution of multicellular animals afforded metazoans with new opportunities to integrate adhesion, cytoskeletal dynamics, and signaling. We discuss known literature on cross-talk between the two junctions and, using the skin epidermis as an example, provide a model for how these two junctions function in concert to orchestrate tissue organization and function.

Analysis of a vinculin homolog in a sponge (phylum Porifera) reveals that vertebrate-like cell adhesions emerged early in animal evolution

The evolution of cell-adhesion mechanisms in animals facilitated the assembly of organized multicellular tissues. Studies in traditional animal models have revealed two predominant adhesion structures, the adherens junction (AJ) and focal adhesions (FAs), which are involved in the attachment of neighboring cells to each other and to the secreted extracellular matrix (ECM), respectively. The AJ (containing cadherins and catenins) and FAs (comprising integrins, talin, and paxillin) differ in protein composition, but both junctions contain the actin-binding protein vinculin. The near ubiquity of these structures in animals suggests that AJ and FAs evolved early, possibly coincident with multicellularity. However, a challenge to this perspective is that previous studies of sponges-a divergent animal lineage-indicate that their tissues are organized primarily by an alternative, sponge-specific cell-adhesion mechanism called "aggregation factor." In this study, we examined the structure, biochemical properties, and tissue localization of a vinculin ortholog in the sponge Oscarella pearsei (Op). Our results indicate that Op vinculin localizes to both cell-cell and cell-ECM contacts and has biochemical and structural properties similar to those of vertebrate vinculin. We propose that Op vinculin played a role in cell adhesion and tissue organization in the last common ancestor of sponges and other animals. These findings provide compelling evidence that sponge tissues are indeed organized like epithelia in other animals and support the notion that AJ- and FA-like structures extend to the earliest periods of animal evolution.

Activation of β-catenin causes defects in embryonic development during maternal-to-zygotic transition in mice

The early stage of embryogenesis is an important and complex cell-remodeling event in reproductive biology. To develop into a normal zygote, maternal-to-zygotic transition (MZT) is especially important for both zygotic genome activation (ZGA) and degradation of maternal products during the early stage of embryonic development. β-Catenin has been identified as an important regulator of embryonic development and adult stem cell division via the canonical Wnt/β-catenin signalling pathway. However, the role of activated β-catenin during MZT remains elusive. In the present study, we found that β-catenin is mainly expressed during embryogenesis in the cell membrane from the zygote- to morula-stage embryos but not in MII oocytes. To analyze the function of activated β-catenin during MZT, we conducted a β-catenin activation assay during embryogenesis. Our results indicated that development beyond the two-cell stage was inhibited in zygotes with β-catenin activation. Further analysis showed that activated form of β-catenin protein was increased and the phosphorylated form of β-catenin protein was decreased in culture embryos. Taken together, our study reveals that activation of β-catenin may play a vital role in zygotic development, determining the developmental potential of mouse embryos.

Catenins Steer Cell Migration via Stabilization of Front-Rear Polarity

Cell migration plays a pivotal role in morphogenetic and pathogenetic processes. To achieve directional migration, cells must establish a front-to-rear axis of polarity. Here we show that components of the cadherin-catenin complex function to stabilize this front-rear polarity. Neural crest and glioblastoma cells undergo directional migration in vivo or in vitro. During this process, αE-catenin accumulated at lamellipodial membranes and then moved toward the rear with the support of a tyrosine-phosphorylated β-catenin. This relocating αE-catenin bound to p115RhoGEF, leading to gathering of active RhoA in front of the nucleus where myosin-IIB arcs assemble. When catenins or p115RhoGEF were removed, cells lost the polarized myosin-IIB assembly, as well as the capability for directional movement. These results suggest that, apart from its well-known function in cell adhesion, the β-catenin/αE-catenin complex regulates directional cell migration by restricting active RhoA to perinuclear regions and controlling myosin-IIB dynamics at these sites.

β-catenin-mediated adhesion is required for successful preimplantation mouse embryo development

β-catenin (CTNNB1) is integral to cell adhesion and to the canonical Wnt signaling pathway. The effects of maternal and zygotic CTNNB1 on embryogenesis have each been separately assessed, whereas the effect of its total absence has not. As the 'traditional' conditional Ctnnb1 knockout alleles give rise to truncated CTNNB1 fragments, we designed a new knockout allele incapable of CTNNB1 production. Mouse embryos lacking intact maternal/zygotic CTNNB1 from two knockout strains were examined in detail. Preimplantation embryos are formed, yet abnormalities in their size and shape were found throughout pre- and early postimplantation development. In the absence of the zona pellucida, embryos lacking CTNNB1 undergo fission and these separated blastomeres can become small trophoblastic vesicles, which in turn induce decidual reactions. Comparing the severity of this defective adhesion phenotype in embryos bearing the null allele with those carrying the 'traditional' knockout allele suggests a hypomorphic effect of the truncated CTNNB1 protein fragment, an important observation with possible impact on previous and future studies.

Cell-cell adhesion in metazoans relies on evolutionarily conserved features of the α-catenin·β-catenin-binding interface

Stable tissue integrity during embryonic development relies on the function of the cadherin·catenin complex (CCC). The Caenorhabditis elegans CCC is a useful paradigm for analyzing in vivo requirements for specific interactions among the core components of the CCC, and it provides a unique opportunity to examine evolutionarily conserved mechanisms that govern the interaction between α- and β-catenin. HMP-1, unlike its mammalian homolog α-catenin, is constitutively monomeric, and its binding affinity for HMP-2/β-catenin is higher than that of α-catenin for β-catenin. A crystal structure shows that the HMP-1·HMP-2 complex forms a five-helical bundle structure distinct from the structure of the mammalian α-catenin·β-catenin complex. Deletion analysis based on the crystal structure shows that the first helix of HMP-1 is necessary for binding HMP-2 avidly in vitro and for efficient recruitment of HMP-1 to adherens junctions in embryos. HMP-2 Ser-47 and Tyr-69 flank its binding interface with HMP-1, and we show that phosphomimetic mutations at these two sites decrease binding affinity of HMP-1 to HMP-2 by 40-100-fold in vitro. In vivo experiments using HMP-2 S47E and Y69E mutants showed that they are unable to rescue hmp-2(zu364) mutants, suggesting that phosphorylation of HMP-2 on Ser-47 and Tyr-69 could be important for regulating CCC formation in C. elegans Our data provide novel insights into how cadherin-dependent cell-cell adhesion is modulated in metazoans by conserved elements as well as features unique to specific organisms.

Generating Cellular Diversity and Spatial Form: Wnt Signaling and the Evolution of Multicellular Animals

There were multiple prerequisites to the evolution of multicellular animal life, including the generation of multiple cell fates ("cellular diversity") and their patterned spatial arrangement ("spatial form"). Wnt proteins operate as primordial symmetry-breaking signals. By virtue of their short-range nature and their capacity to activate both lineage-specifying and cell-polarizing intracellular signaling cascades, Wnts can polarize cells at their site of contact, orienting the axis of cell division while simultaneously programming daughter cells to adopt diverging fates in a spatially stereotyped way. By coupling cell fate to position, symmetry-breaking Wnt signals were pivotal in constructing the metazoan body by generating cellular diversity and spatial form.

Regulation of nucleosome positioning by a CHD Type III chromatin remodeler and its relationship to developmental gene expression in Dictyostelium

Nucleosome placement and repositioning can direct transcription of individual genes; however, the precise interactions of these events are complex and largely unresolved at the whole-genome level. The Chromodomain-Helicase-DNA binding (CHD) Type III proteins are a subfamily of SWI2/SNF2 proteins that control nucleosome positioning and are associated with several complex human disorders, including CHARGE syndrome and autism. Type III CHDs are required for multicellular development of animals and Dictyostelium but are absent in plants and yeast. These CHDs can mediate nucleosome translocation in vitro, but their in vivo mechanism is unknown. Here, we use genome-wide analysis of nucleosome positioning and transcription profiling to investigate the in vivo relationship between nucleosome positioning and gene expression during development of wild-type (WT) Dictyostelium and mutant cells lacking ChdC, a Type III CHD protein ortholog. We demonstrate major nucleosome positional changes associated with developmental gene regulation in WT. Loss of chdC caused an increase of intragenic nucleosome spacing and misregulation of gene expression, affecting ∼50% of the genes that are repositioned during WT development. These analyses demonstrate active nucleosome repositioning during Dictyostelium multicellular development, establish an in vivo function of CHD Type III chromatin remodeling proteins in this process, and reveal the detailed relationship between nucleosome positioning and gene regulation, as cells transition between developmental states.

The role of apical cell-cell junctions and associated cytoskeleton in mechanotransduction

Tissues of multicellular organisms are characterised by several types of specialised cell-cell junctions. In vertebrate epithelia and endothelia, tight and adherens junctions (AJ) play critical roles in barrier and adhesion functions, and are connected to the actin and microtubule cytoskeletons. The interaction between junctions and the cytoskeleton is crucial for tissue development and physiology, and is involved in the molecular mechanisms governing cell shape, motility, growth and signalling. The machineries which functionally connect tight and AJ to the cytoskeleton comprise proteins which either bind directly to cytoskeletal filaments, or function as adaptors for regulators of the assembly and function of the cytoskeleton. In the last two decades, specific cytoskeleton-associated junctional molecules have been implicated in mechanotransduction, revealing the existence of multimolecular complexes that can sense mechanical cues and translate them into adaptation to tensile forces and biochemical signals. Here, we summarise the current knowledge about the machineries that link tight and AJ to actin filaments and microtubules, and the molecular basis for mechanotransduction at epithelial and endothelial AJ.

Evolution and diversity of cadherins and catenins

Cadherin genes encode a superfamily of conserved transmembrane proteins that share an adhesive ectodomain composed of tandem cadherin repeats. More than 100 human cadherin superfamily members have been identified, which can be classified into three families: major cadherins, protocadherins and cadherin-related proteins. These superfamily members are involved in diverse fundamental cellular processes including cell-cell adhesion, morphogenesis, cell recognition and signaling. Epithelial cadherin (E-cadherin) is the founding cadherin family member. Its cytoplasmic tail interacts with the armadillo catenins, p120 and β-catenin. Further, α-catenin links the cadherin/armadillo catenin complex to the actin filament network. Even genomes of ancestral metazoan species such as cnidarians and placozoans encode a limited number of distinct cadherins and catenins, emphasizing the conservation and functional importance of these gene families. Moreover, a large expansion of the cadherin and catenin families coincides with the emergence of vertebrates and reflects a major functional diversification in higher metazoans. Here, we revisit and review the functions, phylogenetic classifications and co-evolution of the cadherin and catenin protein families.

Phylogenetic evidence for the modular evolution of metazoan signalling pathways

Communication among cells was paramount to the evolutionary increase in cell type diversity and, ultimately, the origin of large body size. Across the diversity of Metazoa, there are only few conserved cell signalling pathways known to orchestrate the complex cell and tissue interactions regulating development; thus, modification to these few pathways has been responsible for generating diversity during the evolution of animals. Here, we summarize evidence for the origin and putative function of the intracellular, membrane-bound and secreted components of seven metazoan cell signalling pathways with a special focus on early branching metazoans (ctenophores, poriferans, placozoans and cnidarians) and basal unikonts (amoebozoans, fungi, filastereans and choanoflagellates). We highlight the modular incorporation of intra- and extracellular components in each signalling pathway and suggest that increases in the complexity of the extracellular matrix may have further promoted the modulation of cell signalling during metazoan evolution. Most importantly, this updated view of metazoan signalling pathways highlights the need for explicit study of canonical signalling pathway components in taxa that do not operate a complete signalling pathway. Studies like these are critical for developing a deeper understanding of the evolution of cell signalling.This article is part of the themed issue 'Evo-devo in the genomics era, and the origins of morphological diversity'.

Metazoan evolution of the armadillo repeat superfamily

The superfamily of armadillo repeat proteins is a fascinating archetype of modular-binding proteins involved in various fundamental cellular processes, including cell-cell adhesion, cytoskeletal organization, nuclear import, and molecular signaling. Despite their diverse functions, they all share tandem armadillo (ARM) repeats, which stack together to form a conserved three-dimensional structure. This superhelical armadillo structure enables them to interact with distinct partners by wrapping around them. Despite the important functional roles of this superfamily, a comprehensive analysis of the composition, classification, and phylogeny of this protein superfamily has not been reported. Furthermore, relatively little is known about a subset of ARM proteins, and some of the current annotations of armadillo repeats are incomplete or incorrect, often due to high similarity with HEAT repeats. We identified the entire armadillo repeat superfamily repertoire in the human genome, annotated each armadillo repeat, and performed an extensive evolutionary analysis of the armadillo repeat proteins in both metazoan and premetazoan species. Phylogenetic analyses of the superfamily classified them into several discrete branches with members showing significant sequence homology, and often also related functions. Interestingly, the phylogenetic structure of the superfamily revealed that about 30 % of the members predate metazoans and represent an ancient subset, which is gradually evolving to acquire complex and highly diverse functions.

Rabs set the stage for polarity

Cell polarity refers to the asymmetric localization of cellular components that allows cells to carry out their specialized functions, be they epithelial barrier function, transmission of action potentials in nerve cells, or modulation of the immune response. The establishment and maintenance of cell polarity requires the directed trafficking of membrane proteins and lipids - essential processes that are mediated by Rab GTPases. Interestingly, several of the Rabs that impact polarity are present in the earliest eukaryotes, and the Rab polarity repertoire has expanded as cells have become more complex. There is a substantial conservation of Rab function across diverse cell types. Rabs act through an assortment of effector proteins that include scaffolding proteins, cytoskeletal motors, and other small GTPases. In this review we highlight the similarities and differences in Rab function for the instruction of polarity in diverse cell types.

Adherens Junctions Modulate Diffusion between Epithelial Cells in Trichoplax adhaerens

Trichoplax adhaerens is the sole named member of Placozoa, an ancient metazoan phylum. This coin-shaped animal glides on ventral cilia to find and digest algae on the substrate. It has only six cell types, all but two of which are incorporated into the epithelium that encloses it. The upper epithelium is thin, composed of a pavement of relatively large polygonal disks, each bearing a cilium. The lower epithelium is thick and composed primarily of narrow ciliated cells that power locomotion. Interspersed among these cells are two different secretory cells: one containing large lipophilic granules that, when released, lyse algae under the animal; the other, less abundant, is replete with smaller secretory granules containing neuropeptides. All cells within both epithelia are joined by adherens junctions that are stabilized by apical actin networks. Cells are held in place during shape changes or under osmotic stress, but dissociate in low calcium. Neither tight, septate, nor gap junctions are evident, leaving only the adherens junction to control the permeability of the epithelium. Small (<4 kDa) fluorescent dextrans introduced into artificial seawater readily penetrate into the animal between the cells. Larger dextrans enter slowly, except in animals treated with reduced calcium, indicating that the adherens junctions form a circumferential belt around each cell that impedes diffusion into the animal. During feeding, the limited permeability of the adherens junctions helps to confine material released from lysed algae within the narrow space under the animal, where it is absorbed by endocytosis.

Synthetic Morphogenesis

Throughout biology, function is intimately linked with form. Across scales ranging from subcellular to multiorganismal, the identity and organization of a biological structure's subunits dictate its properties. The field of molecular morphogenesis has traditionally been concerned with describing these links, decoding the molecular mechanisms that give rise to the shape and structure of cells, tissues, organs, and organisms. Recent advances in synthetic biology promise unprecedented control over these molecular mechanisms; this opens the path to not just probing morphogenesis but directing it. This review explores several frontiers in the nascent field of synthetic morphogenesis, including programmable tissues and organs, synthetic biomaterials and programmable matter, and engineering complex morphogenic systems de novo. We will discuss each frontier's objectives, current approaches, constraints and challenges, and future potential.

Characterization of the Cadherin-Catenin Complex of the Sea Anemone Nematostella vectensis and Implications for the Evolution of Metazoan Cell-Cell Adhesion

The cadherin-catenin complex (CCC) mediates cell-cell adhesion in bilaterian animals by linking extracellular cadherin-based adhesions to the actin cytoskeleton. However, it is unknown whether the basic organization of the complex is conserved across all metazoans. We tested whether protein interactions and actin-binding properties of the CCC are conserved in a nonbilaterian animal, the sea anemone Nematostella vectensis We demonstrated that N. vectensis has a complete repertoire of cadherin-catenin proteins, including two classical cadherins, one α-catenin, and one β-catenin. Using size-exclusion chromatography and multi-angle light scattering, we showed that α-catenin and β-catenin formed a heterodimer that bound N. vectensis Cadherin-1 and -2. Nematostella vectensis α-catenin bound F-actin with equivalent affinity as either a monomer or an α/β-catenin heterodimer, and its affinity for F-actin was, in part, regulated by a novel insert between the N- and C-terminal domains. Nematostella vectensis α-catenin inhibited Arp2/3 complex-mediated nucleation of actin filaments, a regulatory property previously thought to be unique to mammalian αE-catenin. Thus, despite significant differences in sequence, the key interactions of the CCC are conserved between bilaterians and cnidarians, indicating that the core function of the CCC as a link between cell adhesions and the actin cytoskeleton is ancestral in the eumetazoans.