Fusomorphogenesis: cell fusion in organ formation

Syncytia in Utricularia: Origin and Structure

In animals and plants, multinucleate cells (syncytia and coenocytes) are essential in ontogeny and reproduction. Fuso-morphogenesis is the formation of multinucleated syncytia by cell-cell fusion, but coenocytes are formed as a result of mitosis without cytokinesis. However, in plants, coenocytes are more widespread than true syncytia. Except for articulated laticifers, most plant syncytia have a trophic function. Here, we summarize the results of histological, histochemical, and ultrastructural analyses of syncytia in the Utricularia species from the Lentibulariaceae family. Utricularia syncytia, known only from a few species, are heterokaryotic because the syncytium possesses nuclei from two different sources: cells of maternal sporophytic nutritive tissue (placenta) and endosperm haustorium. Thus, syncytium contains both maternal and paternal genetic material. In species from section Utricularia, syncytia are highly active structures (with hypertrophied nuclei, cell wall ingrowths, and extensive cytoskeleton) that exist only during embryo development. They serve as an example of evolutionary unique trophic structures in the plant kingdom.

Nanoscale structural response of biomimetic cell membranes to controlled dehydration

Although cell membranes exist in excess of water under physiological conditions, there are a number of biochemical processes, such as adsorption of biomacromolecules or membrane fusion events, that require partial or even complete transient dehydration of lipid membranes. Even though the dehydration process is crucial for understanding all fusion events, still little is known about the structural adaptation of lipid membranes when their interfacial hydration layer is perturbed. Here, we present the study of the nanoscale structural reorganization of phase-separated, supported lipid bilayers (SLBs) under a wide range of hydration conditions. Model lipid membranes were characterised using a combination of fluorescence microscopy and atomic force microscopy and, crucially, without applying any chemical or physical modifications that have previously been considered essential for maintaining the membrane integrity upon dehydration. We revealed that decreasing the hydration state of the membrane leads to an enhanced mixing of lipids characteristic of the liquid-disordered (Ld) phase with those forming the liquid-ordered (Lo) phase. This is associated with a 2-fold decrease in the hydrophobic mismatch between the Ld and Lo lipid phases and a 3-fold decrease in the line tension for the fully desiccated membrane. Importantly, the observed changes in the hydrophobic mismatch, line tension, and lipid miscibility are fully reversible upon subsequent rehydration of the membrane. These findings provide a deeper insight into the fundamental processes, such as cell-cell fusion, that require partial dehydration at the interface of two membranes.

An Emerging Animal Model for Querying the Role of Whole Genome Duplication in Development, Evolution, and Disease

Whole genome duplication (WGD) or polyploidization can occur at the cellular, tissue, and organismal levels. At the cellular level, tetraploidization has been proposed as a driver of aneuploidy and genome instability and correlates strongly with cancer progression, metastasis, and the development of drug resistance. WGD is also a key developmental strategy for regulating cell size, metabolism, and cellular function. In specific tissues, WGD is involved in normal development (e.g., organogenesis), tissue homeostasis, wound healing, and regeneration. At the organismal level, WGD propels evolutionary processes such as adaptation, speciation, and crop domestication. An essential strategy to further our understanding of the mechanisms promoting WGD and its effects is to compare isogenic strains that differ only in their ploidy. Caenorhabditis elegans (C. elegans) is emerging as an animal model for these comparisons, in part because relatively stable and fertile tetraploid strains can be produced rapidly from nearly any diploid strain. Here, we review the use of Caenorhabditis polyploids as tools to understand important developmental processes (e.g., sex determination, dosage compensation, and allometric relationships) and cellular processes (e.g., cell cycle regulation and chromosome dynamics during meiosis). We also discuss how the unique characteristics of the C. elegans WGD model will enable significant advances in our understanding of the mechanisms of polyploidization and its role in development and disease.

The metalloprotease ADM-4/ADAM17 promotes axonal repair

Axonal fusion is an efficient means of repair following axonal transection, whereby the regenerating axon fuses with its own separated axonal fragment to restore neuronal function. Despite being described over 50 years ago, its molecular mechanisms remain poorly understood. Here, we demonstrate that the Caenorhabditis elegans metalloprotease ADM-4, an ortholog of human ADAM17, is essential for axonal fusion. We reveal that animals lacking ADM-4 cannot repair their axons by fusion, and that ADM-4 has a cell-autonomous function within injured neurons, localizing at the tip of regrowing axon and fusion sites. We demonstrate that ADM-4 overexpression enhances fusion to levels higher than wild type, and that the metalloprotease and phosphatidylserine-binding domains are essential for its function. Last, we show that ADM-4 interacts with and stabilizes the fusogen EFF-1 to allow membranes to merge. Our results uncover a key role for ADM-4 in axonal fusion, exposing a molecular target for axonal repair.

Putting organelles in their place

Experiments in C. elegans reveal new insights into how the ANC-1 protein helps to anchor the nucleus and other organelles in place.

Exploring the nuclear lamina in health and pathology using C. elegans

The eukaryotic genome inside the nucleus is enveloped by two membranes, the Outer Nuclear Membrane (ONM) and the Inner Nuclear Membrane (INM). Tethered to the INM is the nuclear lamina, a fibrillar network composed of lamins-the nuclear intermediate filaments, and membrane associated proteins. The nuclear lamina interacts with several nuclear structures, including chromatin. As most nuclear functions, including regulation of gene expression, chromosome segregation and duplication as well as nuclear structure, are highly conserved in metazoans, the Caenorhabditis elegans nematode serves as a powerful model organism to study nuclear processes and architecture. This translucent organism can easily be observed under a microscope as a live embryo, larvae and even adult. Here we will review the data on nuclear lamina composition and functions gathered from studies using C. elegans model organisms: We will discuss genome spatial organization and its contribution to gene expression. We will review both the interaction between the cytoplasm and the nucleus and mechanotransduction mechanism. Finally, we will discuss disease causing mutation in nuclear lamins, including the use of this animal model in diseases research.

Estimation of C. elegans cell- and tissue volumes

Although C. elegans is one of the best-studied model organisms, an estimate of its cell sizes and tissues is missing. Here we used the Virtual Worm that is based on electron microscopy images to calculate a zeroth-order approximation of cell and tissue sizes of C. elegans. We conclude that the intestine is the largest tissue, followed by the hypodermis, gonads, body wall muscles, pharynx, and neurons. Thus, we provide an approximation of tissue volumes of young adult C. elegans.

Fusogen-mediated neuron-neuron fusion disrupts neural circuit connectivity and alters animal behavior

Ramón y Cajal’s neuron doctrine, which states that neurons are individual cells that do not share any membrane or cytoplasmic continuity between them, has underpinned our view of modern neuroscience. However, there is considerable evidence that fusogens, specialized proteins essential and sufficient for the fusion of cells in other tissues, are expressed in the nervous system of several species in response to viral infection, stress conditions, and neurological disease. By manipulating the expression of fusogens in the chemosensory neurons of Caenorhabditis elegans, our results provide conclusive evidence that deregulation of fusogen expression causes neuronal fusion and can have deleterious effects on neural circuitry and behavioral outputs, revealing a possible novel underlying cause of neurological disorders.

The 100-y-old neuron doctrine from Ramón y Cajal states that neurons are individual cells, rejecting the process of cell−cell fusion in the normal development and function of the nervous system. However, fusogens—specialized molecules essential and sufficient for the fusion of cells—are expressed in the nervous system of different species under conditions of viral infection, stress, or disease. Despite these findings, whether the expression of fusogens in neurons leads to cell−cell fusion, and, if so, whether this affects neuronal fate, function, and animal behavior, has not been explored. Here, using Caenorhabditis elegans chemosensory neurons as a model system, we provide proof-of-principle that aberrant expression of fusogens in neurons results in neuron−neuron fusion and behavioral impairments. We demonstrate that fusion between chemoattractive neurons does not affect the response to odorants, whereas fusion between chemoattractive and chemorepulsive neurons compromises chemosensation. Moreover, we provide evidence that fused neurons are viable and retain their original specific neuronal fate markers. Finally, analysis of calcium transients reveals that fused neurons become electrically coupled, thereby compromising neural circuit connectivity. Thus, we propose that aberrant expression of fusogens in the nervous system disrupts neuronal individuality, which, in turn, leads to a change in neural circuit connectivity and disruption of normal behavior. Our results expose a previously uncharacterized basis of circuit malfunction, and a possible underlying cause of neurological diseases.

Characterization of cell fusion in an experimental mouse model of endometriosis†

Cell fusion is involved in the development of some adult organs, is implicated in the pathogenesis of specific types of cancer, and is known to participate in repair/regeneration processes mediated by bone-marrow-derived cells (BMDCs). Endometriosis is a disease characterized by growth of functional endometrial tissue outside of the uterine cavity. Endometriosis shares some molecular properties with cancer and BMDCs home to endometriosis lesions in a mouse model. Our objective was to determine if cell fusion can occur in endometriosis and establish whether bone-marrow-derived cells participate in cell fusion events in lesions. We employed a Cre-Lox system to identify cell fusion events in a mouse model of endometriosis. Fused cells were detected in endometriotic lesions, albeit at a low frequency (∼1 in 400 cells), localized to the stromal compartment, and displayed restricted proliferation. Using 5-fluorouracil-based nongonadotoxic bone marrow transplantation model, we demonstrate that bone marrow cells represent a principal cell source for fusion events in lesions. Cell fusion progeny uniformly lacked expression of selected markers of hematopoietic, endothelial, and epithelial markers, though they expressed the mesenchymal/stromal markers Sca-1 and CD29. This study is the first to describe the phenomenon of cell fusion in endometriosis and points to a mesenchymal population derived from cell fusion events with limited proliferative activity, properties previously attributed to endometrial stem cells. Their putative role in the pathogenesis of the disease remains to be elucidated.

Convergent evolution of saccate body shapes in nematodes through distinct developmental mechanisms

Background

The vast majority of nematode species have vermiform (worm-shaped) body plans throughout post-embryonic development. However, atypical body shapes have evolved multiple times. The plant-parasitic Tylenchomorpha nematode Heterodera glycines hatches as a vermiform infective juvenile. Following infection and the establishment of a feeding site, H. glycines grows disproportionately greater in width than length, developing into a saccate adult. Body size in Caenorhabditis elegans was previously shown to correlate with post-embryonic divisions of laterally positioned stem cell-like ‘seam’ cells and endoreduplication of seam cell epidermal daughters. To test if a similar mechanism produces the unusual body shape of saccate parasitic nematodes, we compared seam cell development and epidermal ploidy levels of H. glycines to C. elegans. To study the evolution of body shape development, we examined seam cell development of four additional Tylenchomorpha species with vermiform or saccate body shapes.

Results

We confirmed the presence of seam cell homologs and their proliferation in H. glycines. This results in the adult female epidermis having approximately 1800 nuclei compared with the 139 nuclei in the primary epidermal syncytium of C. elegans. Similar to C. elegans, we found a significant correlation between H. glycines body volume and the number and ploidy level of epidermal nuclei. While we found that the seam cells also proliferate in the independently evolved saccate nematode Meloidogyne incognita following infection, the division pattern differed substantially from that seen in H. glycines. Interestingly, the close relative of H. glycines, Rotylenchulus reniformis does not undergo extensive seam cell proliferation during its development into a saccate form.

Conclusions

Our data reveal that seam cell proliferation and epidermal nuclear ploidy correlate with growth in H. glycines. Our finding of distinct seam cell division patterns in the independently evolved saccate species M. incognita and H. glycines is suggestive of parallel evolution of saccate forms. The lack of seam cell proliferation in R. reniformis demonstrates that seam cell proliferation and endoreduplication are not strictly required for increased body volume and atypical body shape. We speculate that R. reniformis may serve as an extant transitional model for the evolution of saccate body shape.

Development of a toolkit for piggyBac-mediated integrative transfection of the human filarial parasite Brugia malayi

BACKGROUND

The human filarial parasites cause diseases that are among the most important causes of morbidity in the developing world. The elimination programs targeting these infections rely on a limited number of drugs, making the identification of new chemotherapeutic agents a high priority. The study of these parasites has lagged due to the lack of reverse genetic methods.

METHODOLOGY/PRINCIPAL FINDINGS

We report a novel co-culture method that results in developmentally competent infective larvae of one of the human filarial parasites (Brugia malayi) and describe a method to efficiently transfect the larval stages of this parasite. We describe the production of constructs that result in integrative transfection using the piggyBac transposon system, and a selectable marker that can be used to identify transgenic parasites. We describe the production and use of dual reporter plasmids containing both a secreted luciferase selectable marker and fluorescent protein reporters that will be useful to study temporal and spatial patterns of gene expression.

CONCLUSIONS/SIGNIFICANCE

The methods and constructs reported here will permit the efficient production of integrated transgenic filarial parasite lines, allowing reverse genetic technologies to be applied to all life cycle stages of the parasite.

Food restriction during pregnancy and female offspring fertility: adverse effects of reprogrammed reproductive lifespan

BACKGROUND

Food restriction during pregnancy can influence the health of the offspring during the adulthood. The aim of the present study was to examine the effect of maternal food restriction (MFR) on the reproductive performance in female rat offspring from the first (FR1) and second (FR2) generations.

METHODS

Adult virgin Wistar female rats were given free access to tap water and were fed ad libitum on standard rodent chow, were mated with virgin adult males, and then were randomly divided into two groups: controls (that was fed ad libitum ) and food-restricted group (FR, that was given only 50% of ad libitum food throughout gestation). Their first (FR1) and the second (FR2) generation of offspring were fed ad libitum and sacrificed before puberty and at adulthood. Their ovaries were removed and their histology evaluated by estimating the number of follicles (total and at various stages of folliculogenesis), and the presence of multi-nuclei oocytes and multi-oocyte follicles.

RESULTS

Total number of ovarian follicles was lower in FR1 females at week 4 in comparison with controls, while it was not different in FR2 females vs.

CONTROLS

The number of the primordial follicle was lower in FR1 and FR2 females vs. controls at both week 4 and at week 8. When compared to the controls, the follicles containing multi-nuclei oocytes were more frequent in ovaries from FR1 and FR2 females at week 4, and higher and lower respectively in ovaries form FR1 and FR2 females at week 8.

CONCLUSION

MFR affects ovarian histology by inducing the development of abnormal follicles in the ovaries in first and second generation offspring. This finding could influence the ovarian function resulting in an early pubertal onset and an early decline in reproductive lifespan.

Auto-fusion and the shaping of neurons and tubes

Cells adopt specific shapes that are necessary for specific functions. For example, some neurons extend elaborate arborized dendrites that can contact multiple targets. Epithelial and endothelial cells can form tiny seamless unicellular tubes with an intracellular lumen. Recent advances showed that cells can auto-fuse to acquire those specific shapes. During auto-fusion, a cell merges two parts of its own plasma membrane. In contrast to cell-cell fusion or macropinocytic fission, which result in the merging or formation of two separate membrane bound compartments, auto-fusion preserves one compartment, but changes its shape. The discovery of auto-fusion in C. elegans was enabled by identification of specific protein fusogens, EFF-1 and AFF-1, that mediate cell-cell fusion. Phenotypic characterization of eff-1 and aff-1 mutants revealed that fusogen-mediated fusion of two parts of the same cell can be used to sculpt dendritic arbors, reconnect two parts of an axon after injury, or form a hollow unicellular tube. Similar auto-fusion events recently were detected in vertebrate cells, suggesting that auto-fusion could be a widely used mechanism for shaping neurons and tubes.

Effects of genetic correction on the differentiation of hair cell-like cells from iPSCs with MYO15A mutation

Deafness or hearing loss is a major issue in human health. Inner ear hair cells are the main sensory receptors responsible for hearing. Defects in hair cells are one of the major causes of deafness. A combination of induced pluripotent stem cell (iPSC) technology with genome-editing technology may provide an attractive cell-based strategy to regenerate hair cells and treat hereditary deafness in humans. Here, we report the generation of iPSCs from members of a Chinese family carrying MYO15A c.4642G>A and c.8374G>A mutations and the induction of hair cell-like cells from those iPSCs. The compound heterozygous MYO15A mutations resulted in abnormal morphology and dysfunction of the derived hair cell-like cells. We used a CRISPR/Cas9 approach to genetically correct the MYO15A mutation in the iPSCs and rescued the morphology and function of the derived hair cell-like cells. Our data demonstrate the feasibility of generating inner ear hair cells from human iPSCs and the functional rescue of gene mutation-based deafness by using genetic correction.

Surface apposition and multiple cell contacts promote myoblast fusion in Drosophila flight muscles

Transmission EM methods reveal that cell–cell fusion of individual myoblasts with growing Drosophila flight muscles is a stepwise process in which the cell adhesion and branched actin machineries mediate tight apposition and formation of multiple contacts and pores between the surfaces of the fusing cells.

Fusion of individual myoblasts to form multinucleated myofibers constitutes a widely conserved program for growth of the somatic musculature. We have used electron microscopy methods to study this key form of cell–cell fusion during development of the indirect flight muscles (IFMs) of Drosophila melanogaster. We find that IFM myoblast–myotube fusion proceeds in a stepwise fashion and is governed by apparent cross talk between transmembrane and cytoskeletal elements. Our analysis suggests that cell adhesion is necessary for bringing myoblasts to within a minimal distance from the myotubes. The branched actin polymerization machinery acts subsequently to promote tight apposition between the surfaces of the two cell types and formation of multiple sites of cell–cell contact, giving rise to nascent fusion pores whose expansion establishes full cytoplasmic continuity. Given the conserved features of IFM myogenesis, this sequence of cell interactions and membrane events and the mechanistic significance of cell adhesion elements and the actin-based cytoskeleton are likely to represent general principles of the myoblast fusion process.

The Formin Diaphanous Regulates Myoblast Fusion through Actin Polymerization and Arp2/3 Regulation

The formation of multinucleated muscle cells through cell-cell fusion is a conserved process from fruit flies to humans. Numerous studies have shown the importance of Arp2/3, its regulators, and branched actin for the formation of an actin structure, the F-actin focus, at the fusion site. This F-actin focus forms the core of an invasive podosome-like structure that is required for myoblast fusion. In this study, we find that the formin Diaphanous (Dia), which nucleates and facilitates the elongation of actin filaments, is essential for Drosophila myoblast fusion. Following cell recognition and adhesion, Dia is enriched at the myoblast fusion site, concomitant with, and having the same dynamics as, the F-actin focus. Through analysis of Dia loss-of-function conditions using mutant alleles but particularly a dominant negative Dia transgene, we demonstrate that reduction in Dia activity in myoblasts leads to a fusion block. Significantly, no actin focus is detected, and neither branched actin regulators, SCAR or WASp, accumulate at the fusion site when Dia levels are reduced. Expression of constitutively active Dia also causes a fusion block that is associated with an increase in highly dynamic filopodia, altered actin turnover rates and F-actin distribution, and mislocalization of SCAR and WASp at the fusion site. Together our data indicate that Dia plays two roles during invasive podosome formation at the fusion site: it dictates the level of linear F-actin polymerization, and it is required for appropriate branched actin polymerization via localization of SCAR and WASp. These studies provide new insight to the mechanisms of cell-cell fusion, the relationship between different regulators of actin polymerization, and invasive podosome formation that occurs in normal development and in disease.

Organisation of the endosperm and endosperm-placenta syncytia in bladderworts (Utricularia, Lentibulariaceae) with emphasis on the microtubule arrangement

Multinucleate cells play an important role in higher plants, especially during reproduction; however, the configurations of their cytoskeletons, which are formed as a result of mitosis without cytokinesis, have mainly been studied in coenocytes. Previous authors have proposed that in spite of their developmental origin (cell fusion or mitosis without cytokinesis), in multinucleate plant cells, radiating microtubules determine the regular spacing of individual nuclei. However, with the exception of specific syncytia induced by parasitic nematodes, there is no information about the microtubular cytoskeleton in plant heterokaryotic syncytia, i.e. when the nuclei of fused cells come from different cell pools. In this paper, we describe the arrangement of microtubules in the endosperm and special endosperm-placenta syncytia in two Utricularia species. These syncytia arise from different progenitor cells, i.e. cells of the maternal sporophytic nutritive tissue and the micropylar endosperm haustorium (both maternal and paternal genetic material). The development of the endosperm in the two species studied was very similar. We describe microtubule configurations in the three functional endosperm domains: the micropylar syncytium, the endosperm proper and the chalazal haustorium. In contrast to plant syncytia that are induced by parasitic nematodes, the syncytia of Utricularia had an extensive microtubular network. Within each syncytium, two giant nuclei, coming from endosperm cells, were surrounded by a three-dimensional cage of microtubules, which formed a huge cytoplasmic domain. At the periphery of the syncytium, where new protoplasts of the nutritive cells join the syncytium, the microtubules formed a network which surrounded small nuclei from nutritive tissue cells and were also distributed through the cytoplasm. Thus, in the Utricularia syncytium, there were different sized cytoplasmic domains, whose architecture depended on the source and size of the nuclei. The endosperm proper was isolated from maternal (ovule) tissues by a cuticle layer, so the syncytium and chalazal haustorium were the only way for nutrients to be transported from the maternal tissue towards the developing embryo.

The Caenorhabditis elegans epidermis as a model skin. I: development, patterning, and growth

The skin of the nematode Caenorhabditis elegans is composed of a simple epidermal epithelium and overlying cuticle. The skin encloses the animal and plays central roles in body morphology and physiology; its simplicity and accessibility make it a tractable genetic model for several aspects of skin biology. Epidermal precursors are specified by a hierarchy of transcriptional regulators. Epidermal cells form on the dorsal surface of the embryo and differentiate to form the epidermal primordium, which then spreads out in a process of epiboly to enclose internal tissues. Subsequent elongation of the embryo into a vermiform larva is driven by cell shape changes and cell fusions in the epidermis. Most epidermal cells fuse in mid-embryogenesis to form a small number of multinucleate syncytia. During mid-embryogenesis the epidermis also becomes intimately associated with underlying muscles, performing a tendon-like role in transmitting muscle force. Post-embryonic development of the epidermis involves growth by addition of new cells to the syncytia from stem cell-like epidermal seam cells and by an increase in cell size driven by endoreplication of the chromosomes in epidermal nuclei.

Isolation and culture of larval cells from C. elegans

Cell culture is an essential tool to study cell function. In C. elegans the ability to isolate and culture cells has been limited to embryonically derived cells. However, cells or blastomeres isolated from mixed stage embryos terminally differentiate within 24 hours of culture, thus precluding post-embryonic stage cell culture. We have developed an efficient and technically simple method for large-scale isolation and primary culture of larval-stage cells. We have optimized the treatment to maximize cell number and minimize cell death for each of the four larval stages. We obtained up to 7.8×10(4) cells per microliter of packed larvae, and up to 97% of adherent cells isolated by this method were viable for at least 16 hours. Cultured larval cells showed stage-specific increases in both cell size and multinuclearity and expressed lineage- and cell type-specific reporters. The majority (81%) of larval cells isolated by our method were muscle cells that exhibited stage-specific phenotypes. L1 muscle cells developed 1 to 2 wide cytoplasmic processes, while L4 muscle cells developed 4 to 14 processes of various thicknesses. L4 muscle cells developed bands of myosin heavy chain A thick filaments at the cell center and spontaneously contracted ex vivo. Neurons constituted less than 10% of the isolated cells and the majority of neurons developed one or more long, microtubule-rich protrusions that terminated in actin-rich growth cones. In addition to cells such as muscle and neuron that are high abundance in vivo, we were also able to isolate M-lineage cells that constitute less than 0.2% of cells in vivo. Our novel method of cell isolation extends C. elegans cell culture to larval developmental stages, and allows use of the wealth of cell culture tools, such as cell sorting, electrophysiology, co-culture, and high-resolution imaging of subcellular dynamics, in investigation of post-embryonic development and physiology.

Syncytia in plants: cell fusion in endosperm-placental syncytium formation in Utricularia (Lentibulariaceae)

The syncytium formed by Utricularia is extremely unusual and perhaps unique among angiosperm syncytia. All typical plant syncytia (articulated laticifers, amoeboid tapetum, the nucellar plasmodium of river weeds) are formed only by fusion of sporophytic cells which possess the same genetic material, unlike Utricularia in which the syncytium possesses nuclei from two different sources: cells of maternal sporophytic nutritive tissue and endosperm haustorium (both maternal and paternal genetic material). How is this kind of syncytium formed and organized and is it similar to other plant syncytial structures? We used light and electron microscopy to reconstruct the step-by-step development of the Utricularia syncytia. The syncytia of Utricularia developed through heterotypic cell fusion involving the digestion of the cell wall, and finally, heterokaryotic multinucleate structures were formed, which possessed different-sized nuclei that were not regularly arranged in the cytoplasm. We showed that these syncytia were characterized by hypertrophy of nuclei, abundant endoplasmic reticulum and organelles, and the occurrence of wall ingrowths. All these characters testify to high activity and may confirm the nutritive and transport functions of the syncytium for the developing embryo. In Utricularia, the formation of the syncytium provides an economical way to redistribute cell components and release nutrients from the digested cell walls, which can now be used for the embryo, and finally to create a large surface for the exchange of nutrients between the placenta and endosperm.

Heterochronic control of AFF-1-mediated cell-to-cell fusion in C. elegans

In normal development cell fusion is essential for organ formation and sexual reproduction. The nematode Caenorhabditis elegans has become an excellent system to study the mechanisms and developmental functions of cell-to-cell fusion. In this review we focus on the heterochronic regulation of cell fusion. Heterochronic genes control the timing of specific developmental events in C. elegans. The first microRNAs discovered were found as mutations that affect heterochronic development and cell-cell fusions. In addition numerous heterochronic transcription factors also control specific cell fusion events in C. elegans. We describe what is known about the heterochronic regulation of cell fusion of the epidermal seam cells. The fusogen AFF-1 was previously shown to mediate the fusion of the lateral epidermal seam cells. Here we provide evidence supporting the model in which LIN-29, the heterochronic Zinc-finger transcription factor that controls the terminal fusion of the seam cells, stimulates AFF-1 expression in the seam cells before they fuse. Therefore, the heterochronic gene LIN-29 controls AFF-1-mediated cell-cell fusion as part of the terminal differentiation program of the epidermal seam cells.

Evolution of programmed cell fusion: common mechanisms and distinct functions

Eukaryotic cells have evolved diverged mechanisms to merge cells. Here, we discuss three types of cell fusion: (1) Non-self-fusion, cells with different genetic contents fuse to start a new organism and fusion between enveloped viruses and host cells; (2) Self-fusion, genetically identical cells fuse to form a multinucleated cell; and (3) Auto-fusion, a single cell fuses with itself by bringing specialized cell membrane domains into contact and transforming itself into a ring-shaped cell. This is a new type of selfish fusion discovered in C. elegans. We divide cell fusion into three stages: (1) Specification of the cell-fusion fate; (2) Cell attraction, attachment, and recognition; (3) Execution of plasma membrane fusion, cytoplasmic mixing and cytoskeletal rearrangements. We analyze cell fusion in diverse biological systems in development and disease emphasizing the mechanistic contributions of C. elegans to the understanding of programmed cell fusion, a genetically encoded pathway to merge specific cells.

Cell fusion in Caenorhabditis elegans

In the nematode Caenorhabditis elegans, 300 of the 959 somatic nuclei present in the adult hermaphrodite are located in syncytia. These syncytia are formed by the fusion of mononucleate cells throughout embryonic and postembryonic development. These cell fusions occur in a well-characterized stereotypical pattern, allowing investigators to study many cell fusion events at the molecular and cellular levels. Using tools that allow visualization of cell membranes, cell junctions, and cell cytoplasm during fusion, genetic screens have identified many C. elegans cell fusion genes, including those that regulate the fusion cell fate decision and two genes that encode components of the cell fusion machinery.

Optical imaging of cell fusion and fusion proteins in Caenorhabditis elegans

Cell fusion is a very dynamic process in which the entire membrane and cellular contents of two or more cells merge into one. Strategies developed to understand the component processes that make up a full fusion event require imaging to be performed over a range of space and time scales. These strategies must cover detection of nanometer-sized pores, monitoring cytoplasmic diffusion and the dynamic localization of proteins that induce fusion competence, and three-dimensional reconstruction of multinucleated cells. Caenorhabditis elegans' small size, predictable development, and transparent body make this organism optimal for microscopic investigations. In this chapter, focus is placed on light microscopy techniques that have been used thus far to study developmental fusion events in C. elegans and the insights that have been gained from them. There is also a general overview of the developmental timing of the cell fusion events. Additionally, several protocols are described for preparing both fixed and live specimens at various developmental stages of C. elegans for examination via optical microscopy.

N-ethylmaleimide sensitive factor is required for fusion of the C. elegans uterine anchor cell

The fusion of the Caenorhabditis elegans uterine anchor cell (AC) with the uterine-seam cell (utse) is an excellent model system for studying cell-cell fusion, which is essential to animal development. We obtained an egg-laying defective (Egl) mutant in which the AC fails to fuse with the utse. This defect is highly specific: other aspects of utse development and other cell fusions appear to occur normally. We find that defect is due to a missense mutation in the nsf-1 gene, which encodes N-ethylmaleimide-sensitive factor (NSF), an intracellular membrane fusion factor. There are two NSF-1 isoforms, which are expressed in distinct tissues through two separate promoters. NSF-1L is expressed in the uterus, including the AC. We find that nsf-1 is required cell-autonomously in the AC for its fusion with the utse. Our results establish AC fusion as a paradigm for studying cell fusion at single cell resolution and demonstrate that the NSF ATPase is a key player in this process.

The Caenorhabditis elegans aristaless orthologue, alr-1, is required for maintaining the functional and structural integrity of the amphid sensory organs

The homeobox-containing aristaless-related protein ARX has been directly linked to the development of a number of human disorders involving mental retardation and epilepsy and clearly plays a critical role in development of the vertebrate central nervous system. In this work, we investigate the role of ALR-1, the Caenorhabditis elegans aristaless orthologue, in amphid sensory function. Our studies indicate that ALR-1 is required for maintenance of the amphid organ structure throughout larval development. Mutant analysis indicates a progressive loss in the amphid neurons' ability to fill with lipophilic dyes as well as a declining chemotactic response. The degeneration in amphid function corresponds with a failure of the glial-like amphid socket cell to maintain its specific cell shape and cell-cell contacts. Consistent with ALR-1 expression within the amphid socket cell, our results indicate a cell autonomous role for ALR-1 in maintaining cell shape. Furthermore, we demonstrate a role for ALR-1 in the proper morphogenesis of the anterior hypodermis. Genetic interaction tests also suggest that ALR-1 may function cooperatively with the cell adhesion processes in maintaining the amphid sensory organs.

Unveiling the mechanisms of cell-cell fusion

Cell-cell fusion is fundamental to the development and physiology of multicellular organisms, but little is known of its mechanistic underpinnings. Recent studies have revealed that many proteins involved in cell-cell fusion are also required for seemingly unrelated cellular processes such as phagocytosis, cell migration, axon growth, and synaptogenesis. We review advances in understanding cell-cell fusion by contrasting it with virus-cell and intracellular vesicle fusion. We also consider how proteins involved in general aspects of membrane dynamics have been co-opted to control fusion of diverse cell types by coupling with specialized proteins involved in cell-cell recognition, adhesion, and signaling.

Lrg1p Is a Rho1 GTPase-activating protein required for efficient cell fusion in yeast

To identify additional cell fusion genes in Saccharomyces cerevisiae, we performed a high-copy suppressor screen of fus2Delta. Higher dosage of three genes, BEM1, LRG1, and FUS1, partially suppressed the fus2Delta cell fusion defect. BEM1 and FUS1 were high-copy suppressors of many cell-fusion-defective mutations, whereas LRG1 suppressed only fus2Delta and rvs161Delta. Lrg1p contains a Rho-GAP homologous region. Complete deletion of LRG1, as well as deletion of the Rho-GAP coding region, caused decreased rates of cell fusion and diploid formation comparable to that of fus2Delta. Furthermore, lrg1Delta caused a more severe mating defect in combination with other cell fusion mutations. Consistent with an involvement in cell fusion, Lrg1p localized to the tip of the mating projection. Lrg1p-GAP domain strongly and specifically stimulated the GTPase activity of Rho1p, a regulator of beta(1-3)-glucan synthase in vitro. beta(1-3)-glucan deposition was increased in lrg1Delta strains and mislocalized to the tip of the mating projection in fus2Delta strains. High-copy LRG1 suppressed the mislocalization of beta(1-3) glucan in fus2Delta strains. We conclude that Lrg1p is a Rho1p-GAP involved in cell fusion and speculate that it acts to locally inhibit cell wall synthesis to aid in the close apposition of the plasma membranes of mating cells.

Nuclear fusions contribute to polyploidization of the gigantic nuclei in the chalazal endosperm of Arabidopsis

Somatic polyploidization is recognized as a means to increase gene expression levels in highly active metabolic cells. The most common mechanisms are endoreplication, endomitosis and cell fusion. In animals and plants the nuclei of multinucleate cells are usually prevented from fusing. Here, we report that the nuclei from the syncytial cyst of the chalazal endosperm of Arabidopsis thaliana (L.) Heynh. are polyploid with some intermediate ploidy levels that cannot be attributed to endoreplication, suggesting nuclear fusion. Analysis of isolated nuclei, together with fluorescent in situ hybridization (FISH), revealed that nuclei from the chalazal endosperm are two or three times bigger than the nuclei from the peripheral endosperm and have a corresponding increase in ploidy. Together with the consistent observation of adjoined nuclei, we propose that nuclear fusion contributes, at least in part, to the process of polyploidization in the chalazal endosperm. Confocal analysis of intact seeds further suggested that free nuclei from the peripheral endosperm get incorporated into the chalazal cyst and likely participate in nuclear fusions.

The Caenorhabditis elegans nuclear receptor gene nhr-25 regulates epidermal cell development

The development of the epidermis of Caenorhabditis elegans involves cell fusion, migration, and differentiation events. To understand the mechanisms underlying these processes, we characterized the roles of NHR-25, a member of the nuclear receptor family of transcription factors. The NHR-25 homologs Ftz-F1 in Drosophila and SF-1 in mammals are involved in various biological processes, including regulation of patterning during development, reproduction, metabolism, metamorphosis, and homeostasis. Impairment of nhr-25 activity leads to severe phenotypes in embryos and many postembryonic tissues. Further analysis has indicated that nhr-25 activity is required for the proper development, including cell-cell fusion, of several epidermal cell types, such as the epidermal syncytial, seam, and Pn.p cells. Our results also suggest that nhr-25 is likely to regulate cell-cell junctions and/or fusion. In a subset of Pn.p cells, called vulval precursor cells, nhr-25 acts collaboratively with the lin-39 Hox gene in regulating vulval cell differentiation. Additionally, our data suggest that nhr-25 may also function with another Hox gene, nob-1, during embryogenesis. Overall, our results indicate that nhr-25 plays an integral role in regulating cellular processes of epidermal cells.

Current concepts in the embryology and genetics of cleft lip and cleft palate

Many mechanisms underlying normal and abnormal craniofacial embryogenesis are well understood. The genetic factors that provoke abnormal development and result in orofacial clefts are not clear, but much progress has occurred in our understanding. Genes or chromosomal rearrangements on many chromosomes can lead to syndromes that include orofacial clefts. This diversity in the mechanisms that can lead to syndromic clefts highlights the fact that the processes leading to the development of the oral cavity and face are complex and sensitive to disturbances at multiple timepoints or within multiple genetic domains. As for nonsyndromic clefting, large-scale family studies are consistent with one or a few loci exerting major effects on phenotypic expression, although no single gene has been identified as a "necessary" locus for development of nonsyndromic clefts. Rather, the emerging consensus is that the genetic etiology of nonsyndromic clefting is complex, with several loci showing significant results in at least some studies. Some of these loci may be genes for susceptibility to environmental factors, some may be modifying loci, and some may be "necessary" loci. Mutations in genes that are now known to control early development are logical candidate genes for future studies of nonsyndromic orofacial clefting. Continued genetic analyses and developmental studies are crucial for eventual understanding of the complex etiology of these common congenital anomalies.

Membrane potential and endocytic activity control disintegration of cell-cell adhesion and cell fusion in vinculin-injected MDBK cells

Cell fusion occurs during fertilization and in the formation of organs such as muscles, placenta, and bones. We have developed an experimental model for epithelial cell fusion which permits analysis of the processes during junction disintegration and formation of polykaryons (Palovuori and Eskelinen [2000] Eur. J. Cell. Biol. 79: 961-974). In the present work, we analyzed the process in detail. Cell fusion was achieved by microinjecting into the cytoplasm of kidney epithelial Madin-Darby bovine kidney (MDBK) cells TAMRA-tagged vinculin, which incorporated into lateral membranes, focal adhesions and nucleus, and, prior fusion, induced internalization of actin, cadherin and plakoglobin to small clusters in cytoplasm. Injected vinculin was still visible at lateral membranes after removal of junctional proteins indicating that it was tightly associated and perturbed the cell-cell contact sites resulting in membrane fragmentation. Injection of active Rac together with vinculin induced accumulation of cadherin to the membranes, but did not affect vinculin-membrane association. However, it hampered cell fusion probably by supporting adherens junctions. In order to stop endocytosis, we lowered intracellular pH of vinculin-injected cells to 5.5 with the aid of nigericin in KCl buffer. In acidified cells, injected vinculin delineated lateral membranes as thick layers, cadherin remained in situ, and cell fusion was completely inhibited. Since this treatment also leads to cell depolarization, we checked the vinculin incorporation in a KCl solution containing nigericin at neutral pH. In these circumstances, both endogenous and injected vinculin delineated lateral membranes as very thin discontinuous layers, but still fusion was hampered most likely due to perturbation in the initial vinculin-membrane association. We suggest that vinculin might function as a sensor of the environment triggering cell fusion during development in circumstances where membrane potential and local and transient pH gradients play a role.

Adult stem cell plasticity: will engineered tissues be rejected?

The dogma that adult tissue-specific stem cells remain committed to supporting only their own tissue has been challenged; a new hypothesis, that adult stem cells demonstrate plasticity in their repertoires, is being tested. This is important because it seems possible that haematopoietic stem cells, for example, could be exploited to generate and perhaps deliver cell-based therapies deep within existing nonhaematopoietic organs. Much of the evidence for plasticity derives from histological studies of tissues from patients or animals that have received grafts of cells or whole organs, from a donor bearing (or lacking) a definitive marker. Detection in the recipient of appropriately differentiated cells bearing the donor marker is indicative of a switch in phenotype of a stem cell or a member of a transit amplifying population or of a differentiated cell. In this review, we discuss evidence for these changes occurring but do not consider the molecular basis of cell commitment. In general, the extent of engraftment is low but may be increased if tissues are damaged. In model systems of liver regeneration, the repeated application of a selection pressure increases levels of engraftment considerably; how this occurs is unclear. Cell fusion plays a part in regeneration and remodelling of the liver, skeletal muscle and even regions of the brain. Genetic disease may be amenable to some forms of cell therapy, yet immune rejection will present challenges. Graft-vs.-host disease will continue to present problems, although this may be avoided if the cells were derived from the recipient or they were tolerized. Despite great expectations for cellular therapies, there are indications that attempts to replace missing proteins could be confounded simply by the development of specific immunity that rejects the new phenotype.

Antibody-targeted cell fusion

Membrane fusion has many potential applications in biotechnology. Here we show that antibody-targeted cell fusion can be achieved by engineering a fusogenic viral membrane glycoprotein complex. Three different single-chain antibodies were displayed at the extracellular C terminus of the measles hemagglutinin (H) protein, and combinations of point mutations were introduced to ablate its ability to trigger fusion through the native viral receptors CD46 and SLAM. When coexpressed with the measles fusion (F) protein, using plasmid cotransfection or bicistronic adenoviral vectors, the retargeted H proteins could mediate antibody-targeted cell fusion of receptor-negative or receptor-positive index cells with receptor-positive target cells. Adenoviral expression vectors mediating human epidermal growth factor receptor (EGFR)-targeted cell fusion were potently cytotoxic against EGFR-positive tumor cell lines and showed superior antitumor potency against EGFR-positive tumor xenografts as compared with control adenoviruses expressing native (untargeted) or CD38-targeted H proteins.

Pristionchus pacificus vulva formation: polarized division, cell migration, cell fusion, and evolution of invagination

Tube formation is a widespread process during organogenesis. Specific cellular behaviors participate in the invagination of epithelial monolayers that form tubes. However, little is known about the evolutionary mechanisms of cell assembly into tubes during development. In Caenorhabditis elegans, the detailed step-to-step process of vulva formation has been studied in wild type and in several mutants. Here we show that cellular processes during vulva development, which involve toroidal cell formation and stacking of rings, are conserved between C. elegans and Pristionchus pacificus, two species of nematodes that diverged approximately 100 million years ago. These cellular behaviors are divided into phases of cell proliferation, short-range migration, and cell fusion that are temporally distinct in C. elegans but not in P. pacificus. Thus, we identify heterochronic changes in the cellular events of vulva development between these two species. We find that alterations in the division axes of two equivalent vulval cells from Left-Right cleavage in C. elegans to Anterior-Posterior division in P. pacificus can cause the formation of an additional eighth ring. Thus, orthogonal changes in cell division axes with alterations in the number and sequence of cell fusion events result in dramatic differences in vulval shape and in the number of rings in the species studied. Our characterization of vulva formation in P. pacificus compared to C. elegans provides an evolutionary-developmental foundation for molecular genetic analyses of organogenesis in different species within the phylum Nematoda.

Molecular networks controlling epithelial cell polarity in development

During embryonic development, polarized epithelial cells are either formed during cleavage or formed from mesenchymal cells. Because the formation of epithelia during embryogenesis has to occur with high fidelity to ensure proper development, embryos allow a functional approach to study epithelial cell polarization in vivo. In particular, genetic model organisms have greatly advanced our understanding of the generation and maintenance of epithelial cell polarity. Many novel and important polarity genes have been identified and characterized in invertebrate systems, like Drosophila melanogaster and Caenorhabditis elegans. With the rapid identification of mammalian homologues of these invertebrate polarity genes, it has become clear that many important protein domains, single proteins and even entire protein complexes are evolutionarily conserved. It is to be expected that the field of epithelial cell polarity is just experiencing the 'top of the iceberg' of a large protein network that is fundamental for the specific adhesive, cell signalling and transport functions of epithelial cells.

The story of cell fusion: big lessons from little worms

The ability of two or more cells to unite to form a new syncytial cell has been utilized in metazoans throughout evolution to form many complex organs, such as muscles, bones and placentae. This requires migration, recognition and adhesion between cells together with fusion of their plasma membranes and rearrangement of their cytoplasmic contents. Until recently, understanding of the mechanisms of cell fusion was restricted to fusion between enveloped viruses and their target cells. The identification of new factors that take part in developmental cell fusion in C. elegans opens the way to understanding how cells fuse and what the functions of this process are. In this review, we describe current knowledge on the mechanisms and putative roles of developmental cell fusion in C. elegans and how cell fusion is regulated, together with other intercellular processes to promote organogenesis.

Control of vulval competence and centering in the nematode Oscheius sp. 1 CEW1

To compare vulva development mechanisms in the nematode Oscheius sp. 1 to those known in Caenorhabditis elegans, we performed a genetic screen for vulva mutants in Oscheius sp. 1 CEW1. Here we present one large category of mutations that we call cov, which affect the specification of the Pn.p ventral epidermal cells along the antero-posterior axis. The Pn.p cells are numbered from 1 to 12 from anterior to posterior. In wild-type Oscheius sp. 1 CEW1, the P(4-8).p cells are competent to form the vulva and the progeny of P(5-7).p actually form the vulva, with the descendants of P6.p adopting a central vulval fate. Among the 17 mutations (defining 13 genes) that we characterize here, group 1 mutations completely or partially abolish P(4-8).p competence, and this correlates with early fusion of the Pn.p cells to the epidermal syncytium. In this group, we found a putative null mutation in the lin-39 HOM-C homolog, the associated phenotype of which could be weakly mimicked by injection of a morpholino against Osp1-lin-39 in the mother's germ line. Using cell ablation in a partially penetrant competence mutant, we show that vulval competence is partially controlled by a gonadal signal. Most other mutants found in the screen display phenotypes unknown in C. elegans. Group 2 mutants show a partial penetrance of Pn.p competence loss and an abnormal centering of the vulva on P5.p, suggesting that these two processes are coregulated by the same pathway in Oscheius sp. 1. Group 3 mutants display an enlarged competence group that includes P3.p, thus demonstrating the existence of a specific mechanism inhibiting P3.p competence. Group 4 mutants display an abnormal centering of the vulval pattern on P7.p and suggest that a specific mechanism centers the vulval pattern on a single Pn.p cell.

The Caenorhabditis elegans sqv genes and functions of proteoglycans in development

In the nematode Caenorhabditis elegans, the vulva is a simple tubular structure linking the gonads with the external cuticle. In this review we summarize knowledge of inter- and intracellular signaling during vulval development and of the genes required for vulval invagination. Mutants of one set of these genes, the sqv genes, have a normal number of vulval precursor cells (VPCs) with an unperturbed cell lineage but the invagination space, normally a tube, is either collapsed or absent. We review evidence that the sqv genes are involved in glycosaminoglycan synthesis and speculate on ways in which defective glycosaminoglycan formation might lead to collapse of the vulval structure.

LIN-39/Hox triggers cell division and represses EFF-1/fusogen-dependent vulval cell fusion

General mechanisms by which Hox genes establish cell fates are known. However, a few Hox effectors mediating cell behaviors have been identified. Here we found the first effector of LIN-39/HoxD4/Dfd in Caenorhabditis elegans. In specific vulval precursor cells (VPCs), LIN-39 represses early and late expression of EFF-1, a membrane protein essential for cell fusion. Repression of eff-1 is also achieved by the activity of CEH-20/Exd/Pbx, a known cofactor of Hox proteins. Unfused VPCs in lin-39(-);eff-1(-) double mutants fail to divide but migrate, executing vulval fates. Thus, lin-39 is essential for inhibition of EFF-1-dependent cell fusion and stimulation of cell proliferation during vulva formation. Supplemental material is available at http://www.genesdev.org.

The zinc finger protein REF-2 functions with the Hox genes to inhibit cell fusion in the ventral epidermis of C. elegans

During larval development in C. elegans, some of the cells of the ventral epidermis, the Pn.p cells, fuse with the growing epidermal syncytium hyp7. The pattern of these cell fusions is regulated in a complex, sexually dimorphic manner. It is essential that some Pn.p cells remain unfused in order for some sex-specific mating structures to be generated. The pattern of Pn.p cell fusion is regulated combinatorially by two genes of the C. elegans Hox gene cluster: lin-39 and mab-5. Some of the complexity in the Pn.p cell fusion pattern arises because these two Hox proteins can regulate each other’s activities. We describe a zinc-finger transcription factor, REF-2, that is required for the Pn.p cells to be generated and to remain unfused. REF-2 functions with the Hox proteins to prevent Pn.p cell fusion. ref-2 may also be a transcriptional target of the Hox proteins.

Cell fusion: an EFFicient sculptor

Mutations in the eff-1 gene of Caenorhabditis elegans, which prevent all cell-cell fusions in the nematode's epidermis, have revealed developmental roles for cell fusion. An extracellular fusogen-like domain in EFF-1 suggests it might direct the fusion of lipid bilayers.

TLP-1 is an asymmetric cell fate determinant that responds to Wnt signals and controls male tail tip morphogenesis in C. elegans

We have isolated mutations defining a new gene, tlp-1, that affect asymmetric cell fates and morphogenesis during the development of the C. elegans tail. tlp-1 mutations cause defects in the specification of asymmetric cell fates in the descendants of the T blast cell, whose polarity is controlled by Wnt signaling and cause abnormal male tail development leading to the formation of a posterior protrusion reminiscent of ‘leptoderan’, or pointy tailed, nematode species. In wild-type C. elegans males, which have a ‘peloderan’ or rounded tail, retraction of the tail tip hypodermis involves a temporally ordered set of cell fusions and changes in cell shape that appear to be heterochronically delayed in tlp-1 males, suggesting that subtle changes in these events can bring about evolutionary changes in morphology. tlp-1 encodes a C2H2 zinc-finger protein that is a member of the Sp family of transcription factors. A TLP-1::GFP fusion protein is expressed in the nuclei of many cells during early embryogenesis and then becomes restricted primarily to posterior cells. At hatching, it is expressed in several head neurons, the posterior intestine cells, tail hypodermal cells, the T cells and specific T-cell descendents in a pattern that suggests TLP-1 may be asymmetrically expressed during the divisions of the T cell lineage. Furthermore, the asymmetry of TLP-1 expression and function appears to be controlled by Wnt signals that control T cell polarity. These results suggest that tlp-1 encodes a transcription factor required for cellular asymmetry that functions downstream of Wnt signals that control cell polarity, as well as in cell fusion and patterning in the C. elegans tail.