From soap bubbles to multicellular organisms: Unraveling the role of cell adhesion and physical constraints in tile pattern formation and tissue morphogenesis.

Tile patterns, in which numerous cells are arranged in a regular pattern, are found in a variety of multicellular organisms and play important functional roles. Such regular arrangements of cells are regulated by various cell adhesion molecules. On the other hand, cell shape is also known to be regulated by physical constraints similar to those of soap bubbles. In particular, circumference minimization plays an important role, and cell adhesion negatively affects this process, thereby regulating tissue morphogenesis based on physical properties. Here, we focus on the Drosophila compound eye and the mouse auditory epithelium, and summarize the mechanisms of tile pattern formation by cell adhesion molecules such as cadherins, Irre Cell Recognition Modules (IRMs), and nectins. Phenomena that cannot be explained by physical stability based on cortical tension alone have been reported in the tile pattern formation in the compound eye, suggesting that previously unexplored forces such as cellular concentric expansion force may play an important role. We would like to summarize perspectives for future research on the mechanisms of tissue morphogenesis.

MOB1 regulates thymocyte egress and T-cell survival in mice in a YAP1-independent manner.

Mammalian STE20-like protein kinase 1/2 (MST1/2) and nuclear Dbf2-related kinase 1/2 (NDR1/2) are core components of Hippo signaling that are also known to be important regulators of lymphocyte trafficking. However, little is understood about the roles of other Hippo pathway molecules in these cells. Here, we present the first analysis of the function of Mps one binder kinase activator-1 (MOB1) in T lymphocytes in vivo. T-cell-specific double knockout (DKO) of MOB1A/B in mice [tMob1 DKO mice] reduces the number of naïve T cells in both the circulation and secondary lymphoid organs, but leads to an accumulation of CD4+ CD8- and CD4- CD8+ single-positive (SP) cells in the thymus. In vitro, naïve MOB1A/B-deficient T cells show increased apoptosis and display impaired trafficking capacity in response to the chemokine CCL19. These defects are linked to suppression of the activation of MST and NDR kinases, but are independent of the downstream transcriptional co-activator Yes-associated protein 1 (YAP1). Thus, MOB1 proteins play an important role in thymic egress and T-cell survival that is mediated by a pathway other than conventional Hippo-YAP1 signaling.

Hippo pathway controls cell adhesion and context-dependent cell competition to influence skin engraftment efficiency.

Cell competition is involved in mammalian embryogenesis and tumor elimination and progression. It was previously shown that, whereas NIH3T3 mouse fibroblasts expressing high levels of the yes-associated protein 1(YAP1) target TEA domain family (TEAD) transcription factors become "winners" in cell competitions, Madin-Darby canine kidney cells expressing activated YAP1 become "losers" and are eliminated from culture monolayers. Thus, YAP1's role in cell competitions is clearly context dependent. Here, we show that keratinocytes overexpressing a constitutively activated YAP1 mutant lose in in vitro competitions with control cells conducted in standard tissue culture dishes and undergo apical extrusion. Similarly, cells in which endogenous YAP1 is activated by NF2 knockdown become losers. The YAP1-overexpressing cells exhibit a decrease in cell-matrix adhesion because of defective expression of adhesion molecules such as fibronectin-1. Cell adhesion-mediated proliferation is also impaired. However, because of intrinsic factors, YAP1-expressing cells proliferate faster than control cells when cocultured in dishes impeding cell adhesion. In vivo, Mob1a/b-deficient (YAP1-activated) epidermis, which shows decreased expression of type XVII collagen, cannot be engrafted successfully onto donor mice. YAP1-activated skin grafts shrink away from surrounding control skin, and the epidermis peels off the basement membrane. Our data show that YAP1 activation controls cell competition in part by decreasing cell adhesion.-Nishio, M., Miyachi, Y., Otani, J., Tane, S., Omori, H., Ueda, F., Togashi, H., Sasaki, T., Mak, T. W., Nakao, K., Fujita, Y., Nishina, H., Maehama, T., Suzuki, A. Hippo pathway controls cell adhesion and context-dependent cell competition to influence skin engraftment efficiency.

A population dynamics model of cell-cell adhesion incorporating population pressure and density saturation.

We discuss several continuum cell-cell adhesion models based on the underlying microscopic assumptions. We propose an improvement on these models leading to sharp fronts and intermingling invasion fronts between different cell type populations. The model is based on basic principles of localized repulsion and nonlocal attraction due to adhesion forces at the microscopic level. The new model is able to capture both qualitatively and quantitatively experiments by Katsunuma et al. (2016). We also review some of the applications of these models in other areas of tissue growth in developmental biology. We finally explore the resulting qualitative behavior due to cell-cell repulsion.

Hippo vs. Crab: tissue-specific functions of the mammalian Hippo pathway.

The Hippo signaling pathway is a vital suppressor of tumorigenesis that is often inactivated in human cancers. In normal cells, the Hippo pathway is triggered by external forces such as cell crowding, or changes to the extracellular matrix or cell polarity. Once activated, Hippo signaling down-regulates transcription supported by the paralogous cofactors YAP1 and TAZ. The Hippo pathway's functions in normal and cancer biology have been dissected by studies of mutant mice with null or conditional tissue-specific mutations of Hippo signaling elements. In this review, we attempt to systematically summarize results that have been gleaned from detailed in vivo characterizations of these mutants. Our goal is to describe the physiological roles of Hippo signaling in several normal organ systems, as well as to emphasize how disruption of the Hippo pathway, and particularly hyperactivation of YAP1/TAZ, can be oncogenic.

Cellular recognition and patterning in sensory systems.

Cells dissociated from various tissues of vertebrate embryos preferentially reaggregate with cells from the same tissue when they are mixed together. This tissue-specific recognition process in vertebrates is mainly mediated by a family of cell adhesion molecules because of their specific binding properties. Recent studies have revealed that two families of adhesion molecules, nectins and cadherins, are associated with each other, and these associations provide cells with the differential adhesive affinities required for cellular recognition and complex cellular pattern formations during development. This review provides an overview of recent findings regarding the cooperative functions of nectins and cadherins, as well as a discussion of the molecular basis underlying these functions.

Aberrant cochlear hair cell attachments caused by Nectin-3 deficiency result in hair bundle abnormalities.

The organ of Corti consists of sensory hair cells (HCs) interdigitated with nonsensory supporting cells (SCs) to form a checkerboard-like cellular pattern. HCs are equipped with hair bundles on their apical surfaces. We previously reported that cell-adhesive nectins regulate the checkerboard-like cellular patterning of HCs and SCs in the mouse auditory epithelium. Nectin-1 and -3 are differentially expressed in normal HCs and SCs, respectively, and in Nectin-3-deficient mice a number of HCs are aberrantly attached to each other. We show here that these aberrantly attached HCs in Nectin-3-deficient mice, but not unattached ones, show disturbances of the orientation and morphology of the hair bundles and the positioning of the kinocilium, with additional abnormal localisation of cadherin-catenin complexes and the apical-basal polarity proteins Pals1 and Par-3. These results indicate that, owing to the loss of Nectin-3, hair cells contact each other inappropriately and form abnormal junctions, ultimately resulting in abnormal hair bundle orientation and morphology.

Continuous models for cell-cell adhesion.

Cell adhesion is the binding of a cell to another cell or to an extracellular matrix component. This process is essential in organ formation during embryonic development and in maintaining multicellular structure. Armstrong et al. (2006) [J. Theor. Biol. 243, pp. 98-113] proposed a nonlocal advection-diffusion system as a possible continuous mathematical model for cell-cell adhesion. Although the system is attractive and challenging, it gives biologically unrealistic numerical solutions under certain situations. We identify the problems and change underlying idea of cell movement from "cells move randomly" to "cells move from high to low pressure regions". Then we provide a modified continuous model for cell-cell adhesion. Numerical experiments illustrate that the modified model is able to replicate not only Steinberg׳s cell sorting experiments but also some phenomena which cannot be captured at all by Armstrong-Painter-Sherratt model.

Periderm cells covering palatal shelves have tight junctions and their desquamation reduces the polarity of palatal shelf epithelial cells in palatogenesis.

In palatogenesis, bilateral palatal shelves grow and fuse with each other to establish mesenchyme continuity across the horizontal palate. The palatal shelves are covered with the medial edge epithelium (MEE) in which most apical cells are periderm cells. We investigated localization and roles of tight junction (TJ) and adherens junction (AJ) components and an apical membrane marker in the MEE in palatogenesis. Immunofluorescence and immunoelectron microscopy analyses revealed that TJs were located at the boundary between neighboring periderm cells, whereas AJ components were localized at the boundary between all epithelial cells in the MEE. Specifically, typical AJs were observed at the boundaries between neighboring periderm cells and between periderm cells and underlying epithelial cells where the signal for nectin-1 was observed. The TGF-ß-induced desquamation of periderm cells reduced the polarity of remaining epithelial cells as estimated by changes of epithelial cell morphology and the staining of the polarity marker and the AJ components. These less polarized epithelial cells then intermingled and finally disappeared at least partly by apoptosis. These results indicate that periderm cells covering growing palatal shelves have bona fide TJs and their desquamation reduces the polarity of palatal shelf epithelial cells in palatogenesis.

Impacts of manipulating cell sorting on in vitro hair follicle regeneration.

Hair follicle morphogenesis is triggered by epithelial-mesenchymal interactions. Several approaches have been developed for preparing hair follicle organoids using epithelial and mesenchymal cells; however, the current understanding of the relevance of in vitro spontaneous organization processes to hair regeneration is limited. In the present study, we used Y27632, a rho-associated kinase inhibitor, to investigate the effects of manipulation of cell sorting on hair regeneration in vitro. Dissociated hair follicle-inducible epithelial and mesenchymal cells were cultured in Y27632-containing media in 96-well plates or polydimethylsiloxane microarray plates. We found that Y27632 supplementation modulated the spatial distribution of epithelial and mesenchymal cells from a dumbbell shape to a core-shell configuration via a spontaneous organization process. New hair follicles with typical morphological features emerged in the Y27632-treated core-shell-shaped aggregates, and hair shafts sprouted with approximately 100% efficiency in vitro. Gene chip analysis and pathway-inhibition experiments revealed that the phosphatidylinositol-3-kinase/protein kinase B- and Ras-signaling pathways were involved in hair-like sprouting in the Y27632-treated hair follicle organoids. Our findings enhance the understanding of hair follicle organogenesis and the development of hair follicle organoids.

A numerical algorithm for modeling cellular rearrangements in tissue morphogenesis.

Among morphological phenomena, cellular patterns in developing sensory epithelia have gained attention in recent years. Although physical models for cellular rearrangements are well-established thanks to a large bulk of experimental work, their computational implementation lacks solid mathematical background and involves experimentally unreachable parameters. Here we introduce a level set-based computational framework as a tool to rigorously investigate evolving cellular patterns, and study its mathematical and computational properties. We illustrate that a compelling feature of the method is its ability to correctly handle complex topology changes, including frequent cell intercalations. Combining this accurate numerical scheme with an established mathematical model, we show that the proposed framework features minimum possible number of parameters and is capable of reproducing a wide range of tissue morphological phenomena, such as cell sorting, engulfment or internalization. In particular, thanks to precise mathematical treatment of cellular intercalations, this method succeeds in simulating experimentally observed development of cellular mosaic patterns in sensory epithelia.

Hearing loss in mice with disruption of auditory epithelial patterning in the cochlea.

In the cochlear auditory epithelia, sensory hair and supporting cells are arranged in a checkerboard-like mosaic pattern, which is conserved across a wide range of species. The cell adhesion molecules nectin-1 and nectin-3 are required for this pattern formation. The checkerboard-like pattern is thought to be necessary for auditory function, but has never been examined. Here, we showed the significance of checkerboard-like cellular pattern in the survival and function of sensory hair cells in the cochlear auditory epithelia of nectin-3 knockout (KO) mice. Nectin-3 KO mice showed progressive hearing loss associated with degeneration of aberrantly attached hair cells via apoptosis. Apoptotic hair cell death was due to the disorganization of tight junctions between the hair cells. Our study revealed that the checkerboard-like cellular pattern in the auditory epithelium provides a structural basis for ensuring the survival of cochlear hair cells and hearing function.

Interneurite affinity is regulated by heterophilic nectin interactions in concert with the cadherin machinery.

Neurites recognize their specific partners during the formation of interneuronal connections. In hippocampal pyramidal neurons, axons attach to dendrites for their synaptogenesis, but the dendrites do not form stable contacts with each other, suggesting the presence of a mechanism to allow their selective associations. Nectin-1 (N1), an immunoglobulin domain adhesive protein, is preferentially localized in axons, and its heterophilic partner, N3, is present in both axons and dendrites; we tested their potential roles in interneurite recognition. The overexpression of N1, causing its mislocalization to dendrites, induced atypical dendrodendritic as well as excessive axodendritic associations. On the contrary, the genetic deletion of N1 loosened the contacts between axons and dendritic spines. Those actions of nectins required cadherin-catenin activities, but the overexpression of cadherin itself could not accelerate neurite attachment. These results suggest that the axon-biased localization of N1 and its trans-interaction with N3 in cooperation with the cadherin machinery is critical for the ordered association of axons and dendrites.

Involvement of afadin in the formation and remodeling of synapses in the hippocampus.

In the hippocampus, synapses are formed between mossy fiber terminals and CA3 pyramidal cell dendrites and comprise highly developed synaptic junctions (SJs) and puncta adherentia junctions (PAJs). Dynamic remodeling of synapses in the hippocampus is implicated in learning and memory. Components of both the nectin-afadin and cadherin-catenin cell adhesion systems exclusively accumulate at PAJs. We investigated the role of afadin at synapses in mice in which the afadin gene was conditionally inactivated in hippocampal neurons. In these mutant mice, the signals for not only nectins, but also N-cadherin and beta-catenin, were hardly detected in the CA3 area, in addition to loss of the signal for afadin, resulting in disruption of PAJs. Ultrastructural analysis revealed an increase in the number of perforated synapses, suggesting the instability of SJs. These results indicate that afadin is involved not only in the assembly of nectins and cadherins at synapses, but also in synaptic remodeling.

Cadherin regulates dendritic spine morphogenesis.

Synaptic remodeling has been postulated as a mechanism underlying synaptic plasticity, and cadherin adhesion molecules are thought to be a regulator of such a process. We examined the effects of cadherin blockage on synaptogenesis in cultured hippocampal neurons. This blockade resulted in alterations of dendritic spine morphology, such as filopodia-like elongation of the spine and bifurcation of its head structure, along with concomitant disruption of the distribution of postsynaptic proteins. The accumulation of synapsin at presynaptic sites and synaptic vesicle recycling were also perturbed, although these synaptic responses to the cadherin blockade became less evident upon the maturation of the synapses. These findings suggest that cadherin regulates dendritic spine morphogenesis and related synaptic functions, presumably cooperating with cadherin-independent adhesive mechanisms to maintain spine-axon contacts.

Targeting the Hippo signalling pathway for cancer treatment.

The Hippo signalling pathway monitors cell-cell contact and external factors that shape tissue structure. In mice, tumourigenesis and developmental abnormalities are common consequences of dysregulated Hippo signalling. Expression of Hippo pathway components is also frequently altered in human tumours and correlates with poor prognosis and reduced patient survival. Thus, the Hippo pathway is an attractive anti-cancer target. Here, we provide an overview of the function and regulation of Hippo signalling components and summarize progress to date on the development of agents able to regulate Hippo signalling for cancer therapy.

Differential and Cooperative Cell Adhesion Regulates Cellular Pattern in Sensory Epithelia.

Animal tissues are composed of multiple cell types arranged in complex and elaborate patterns. In sensory epithelia, including the auditory epithelium and olfactory epithelium, different types of cells are arranged in unique mosaic patterns. These mosaic patterns are evolutionarily conserved, and are thought to be important for hearing and olfaction. Recent progress has provided accumulating evidence that the cellular pattern formation in epithelia involves cell rearrangements, movements, and shape changes. These morphogenetic processes are largely mediated by intercellular adhesion systems. Differential adhesion and cortical tension have been proposed to promote cell rearrangements. Many different types of cells in tissues express various types of cell adhesion molecules. Although cooperative mechanisms between multiple adhesive systems are likely to contribute to the production of complex cell patterns, our current understanding of the cooperative roles between multiple adhesion systems is insufficient to entirely explain the complex mechanisms underlying cellular patterning. Recent studies have revealed that nectins, in cooperation with cadherins, are crucial for the mosaic cellular patterning in sensory organs. The nectin and cadherin systems are interacted with one another, and these interactions provide cells with differential adhesive affinities for complex cellular pattern formations in sensory epithelia, which cannot be achieved by a single mechanism.

Synergistic action of nectins and cadherins generates the mosaic cellular pattern of the olfactory epithelium.

In the olfactory epithelium (OE), olfactory cells (OCs) and supporting cells (SCs), which express different cadherins, are arranged in a characteristic mosaic pattern in which OCs are enclosed by SCs. However, the mechanism underlying this cellular patterning is unclear. Here, we show that the cellular pattern of the OE is established by cellular rearrangements during development. In the OE, OCs express nectin-2 and N-cadherin, and SCs express nectin-2, nectin-3, E-cadherin, and N-cadherin. Heterophilic trans-interaction between nectin-2 on OCs and nectin-3 on SCs preferentially recruits cadherin via α-catenin to heterotypic junctions, and the differential distributions of cadherins between junctions promote cellular intercalations, resulting in the formation of the mosaic pattern. These observations are confirmed by model cell systems, and various cellular patterns are generated by the combinatorial expression of nectins and cadherins. Collectively, the synergistic action of nectins and cadherins generates mosaic pattern, which cannot be achieved by a single mechanism.

Afadin regulates puncta adherentia junction formation and presynaptic differentiation in hippocampal neurons.

The formation and remodeling of mossy fiber-CA3 pyramidal cell synapses in the stratum lucidum of the hippocampus are implicated in the cellular basis of learning and memory. Afadin and its binding cell adhesion molecules, nectin-1 and nectin-3, together with N-cadherin, are concentrated at puncta adherentia junctions (PAJs) in these synapses. Here, we investigated the roles of afadin in PAJ formation and presynaptic differentiation in mossy fiber-CA3 pyramidal cell synapses. At these synapses in the mice in which the afadin gene was conditionally inactivated before synaptogenesis by using nestin-Cre mice, the immunofluorescence signals for the PAJ components, nectin-1, nectin-3 and N-cadherin, disappeared almost completely, while those for the presynaptic components, VGLUT1 and bassoon, were markedly decreased. In addition, these signals were significantly decreased in cultured afadin-deficient hippocampal neurons. Furthermore, the interevent interval of miniature excitatory postsynaptic currents was prolonged in the cultured afadin-deficient hippocampal neurons compared with control neurons, indicating that presynaptic functions were suppressed or a number of synapse was reduced in the afadin-deficient neurons. Analyses of presynaptic vesicle recycling and paired recordings revealed that the cultured afadin-deficient neurons showed impaired presynaptic functions. These results indicate that afadin regulates both PAJ formation and presynaptic differentiation in most mossy fiber-CA3 pyramidal cell synapses, while in a considerable population of these neurons, afadin regulates only PAJ formation but not presynaptic differentiation.

Regulation of dendritic filopodial interactions by ZO-1 and implications for dendrite morphogenesis.

Neuronal dendrites dynamically protrude many fine filopodia in the early stages of neuronal development and gradually establish complex structures. The importance of the dendritic filopodia in the formation of axo-dendritic connections is established, but their role in dendrite morphogenesis remains unknown. Using time-lapse imaging of cultured rat hippocampal neurons, we revealed here that many filopodia dynamically protruded from dendrites and transiently interacted with each other to form dendritic filopodia-filopodia contacts in the early stages of neuronal development. The MAGUK family member, Zonula Occludens-1 (ZO-1), which is known to be associated with the nectin and cadherin cell adhesion systems, was concentrated at these dendritic filopodia-filopodia contact sites and also at the tips of free dendritic filopodia. Overexpression of ZO-1 increased the formation of dendritic filopodia and their interactions, and induced abnormal dendrite morphology. Conversely, knockdown of ZO-1 decreased the formation of dendritic filopodia and their interactions, and induced abnormal dendrite morphology which was different from that induced by the overexpression of ZO-1. The components of the nectin and cadherin systems were co-localized with ZO-1 at the dendritic filopodia-filopodia contact sites, but not at the tips of free dendritic filopodia. Overexpression of ZO-1 increased the accumulation of these cell adhesive components at the dendritic filopodia-filopodia contact sites and stabilized their interactions, whereas knockdown of ZO-1 reduced their accumulation at the dendritic filopodia-filopodia contact sites. These results indicate that ZO-1 regulates dendritic filopodial dynamics, which is implicated in dendrite morphogenesis cooperatively with the nectin and cadherin systems in cultured neurons.