Spindle orientation and epidermal morphogenesis

The dynamics and biophysics of shape formation: Common themes in plant and animal morphogenesis

The emergence of tissue form in multicellular organisms results from the complex interplay between genetics and physics. In both plants and animals, cells must act in concert to pattern their behaviors. Our understanding of the factors sculpting multicellular form has increased dramatically in the past few decades. From this work, common themes have emerged that connect plant and animal morphogenesis-an exciting connection that solidifies our understanding of the developmental basis of multicellular life. In this review, we will discuss the themes and the underlying principles that connect plant and animal morphogenesis, including the coordination of gene expression, signaling, growth, contraction, and mechanical and geometric feedback.

Therapeutic Effectiveness of Anticancer Agents Targeting Different Signaling Molecules Involved in Asymmetric Division of Cancer Stem Cell

Intra-tumoral heterogeneity is maintained by cancer stem cells (CSCs) with dysregulated self-renewal and asymmetric cell division (ACD). According to the cancer stem cell theory, by ACD a CSC can generate two daughter progenies with different fates such as one cancer stem cell and one differentiated cell. Therefore, this type of mitotic division supports vital process of the maintenance of CSC population. But this CSC pool reservation by ACD complicates the treatment of cancer patients, as CSCs give rise to aggressive clones which are prone to metastasis and drug-insensitivity. Hence, identification of therapeutic modalities which can target ACD of cancer stem cell is an intriguing part of cancer research. In this review, other than the discussion about the extrinsic inducers of ACD role of different proteins, miRNAs and lncRNAs in this type of cell division is also mentioned. Other than these, mode of action of the proven and potential drugs targeting ACD of CSC is also discussed here.

Tension at intercellular junctions is necessary for accurate orientation of cell division in the epithelium plane

The direction in which a cell divides is set by the orientation of its mitotic spindle and is important for determining cell fate, controlling tissue shape, and maintaining tissue architecture. Divisions parallel to the epithelial plane sustain tissue expansion. By contrast, divisions perpendicular to the plane promote tissue stratification and lead to the loss of epithelial cells from the tissue-an event that has been suggested to promote metastasis. Much is known about the molecular machinery involved in orienting the spindle, but less is known about the contribution of mechanical factors, such as tissue tension, in ensuring spindle orientation in the plane of the epithelium. This is important as epithelia are continuously subjected to mechanical stresses. To explore this further, we subjected suspended epithelial monolayers devoid of extracellular matrix to varying levels of tissue tension to study the orientation of cell divisions relative to the tissue plane. This analysis revealed that lowering tissue tension by compressing epithelial monolayers or by inhibiting myosin contractility increased the frequency of out-of-plane divisions. Reciprocally, increasing tissue tension by elevating cell contractility or by tissue stretching restored accurate in-plane cell divisions. Moreover, a characterization of the geometry of cells within these epithelia suggested that spindles can sense tissue tension through its impact on tension at subcellular surfaces, independently of their shape. Overall, these data suggest that accurate spindle orientation in the plane of the epithelium relies on a threshold level of tension at intercellular junctions.

Interplay between EGFR, E-cadherin, and PTP1B in epidermal homeostasis

Maintaining epithelial homeostasis is crucial to allow embryo development but also the protective barrier which is ensured by the epidermis. This homeostasis is regulated through the expression of several molecules among which EGFR and E-cadherin which are of major importance. Indeed, defects in the regulation of these proteins lead to abnormalities in cell adhesion, proliferation, differentiation, and migration. Hence, regulation of these two proteins is of the utmost importance as they are involved in numerous skin pathologies and cancers. In the last decades it has been described several pathways of regulation of these two proteins and notably several mechanisms of cross-regulation between these partners. In this review, we aimed to describe the current understanding of the regulation of EGFR and interactions between EGFR and E-cadherin and, in particular, the implication of these cross-regulations in epithelium homeostasis. We pay particular attention to PTP1B, a phosphatase involved in the regulation of EGFR.

Morphogenetic forces planar polarize LGN/Pins in the embryonic head during Drosophila gastrulation

Spindle orientation is often achieved by a complex of Partner of Inscuteable (Pins)/LGN, Mushroom Body Defect (Mud)/Nuclear Mitotic Apparatus (NuMa), Gαi, and Dynein, which interacts with astral microtubules to rotate the spindle. Cortical Pins/LGN recruitment serves as a critical step in this process. Here, we identify Pins-mediated planar cell polarized divisions in several of the mitotic domains of the early Drosophila embryo. We found that neither planar cell polarity pathways nor planar polarized myosin localization determined division orientation; instead, our findings strongly suggest that Pins planar polarity and force generated from mesoderm invagination are important. Disrupting Pins polarity via overexpression of a myristoylated version of Pins caused randomized division angles. We found that disrupting forces through chemical inhibitors, depletion of an adherens junction protein, or blocking mesoderm invagination disrupted Pins planar polarity and spindle orientation. Furthermore, directional ablations that separated mesoderm from mitotic domains disrupted spindle orientation, suggesting that forces transmitted from mesoderm to mitotic domains can polarize Pins and orient division during gastrulation. To our knowledge, this is the first in vivo example where mechanical force has been shown to polarize Pins to mediate division orientation.

KIF18B is a cell type-specific regulator of spindle orientation in the epidermis

Proper spindle orientation is required for asymmetric cell division and the establishment of complex tissue architecture. In the developing epidermis, spindle orientation requires a conserved cortical protein complex of LGN/NuMA/dynein-dynactin. However, how microtubule dynamics are regulated to interact with this machinery and properly position the mitotic spindle is not fully understood. Furthermore, our understanding of the processes that link spindle orientation during asymmetric cell division to cell fate specification in distinct tissue contexts remains incomplete. We report a role for the microtubule catastrophe factor KIF18B in regulating microtubule dynamics to promote spindle orientation in keratinocytes. During mitosis, KIF18B accumulates at the cell cortex, colocalizing with the conserved spindle orientation machinery. In vivo we find that KIF18B is required for oriented cell divisions within the hair placode, the first stage of hair follicle morphogenesis, but is not essential in the interfollicular epidermis. Disrupting spindle orientation in the placode, using mutations in either KIF18B or NuMA, results in aberrant cell fate marker expression of hair follicle progenitor cells. These data functionally link spindle orientation to cell fate decisions during hair follicle morphogenesis. Taken together, our data demonstrate a role for regulated microtubule dynamics in spindle orientation in epidermal cells. This work also highlights the importance of spindle orientation during asymmetric cell division to dictate cell fate specification.

Dermal EZH2 orchestrates dermal differentiation and epidermal proliferation during murine skin development

Skin development and patterning is dependent on factors that regulate the stepwise differentiation of dermal fibroblasts concomitant with dermal-epidermal reciprocal signaling, two processes that are poorly understood. Here we show that dermal EZH2, the methyltransferase enzyme of the epigenetic Polycomb Repressive Complex 2 (PRC2), is a new coordinator of both these processes. Dermal EZH2 activity is present during dermal fibroblast differentiation and is required for spatially restricting Wnt/β-catenin signaling to reinforce dermal fibroblast cell fate. Later in development, dermal EZH2 regulates the expression of reticular dermal markers and initiation of secondary hair follicles. Embryos lacking dermal Ezh2 have elevated epidermal proliferation and differentiation that can be rescued by small molecule inhibition of retinoic acid (RA) signaling. Together, our study reveals that dermal EZH2 is acting like a rheostat to control the levels of Wnt/β-catenin and RA signaling to impact fibroblast differentiation cell autonomously and epidermal keratinocyte development non-cell autonomously, respectively.

Electrically tunable lenses - eliminating mechanical axial movements during high-speed 3D live imaging

The successful investigation of photosensitive and dynamic biological events, such as those in a proliferating tissue or a dividing cell, requires non-intervening high-speed imaging techniques. Electrically tunable lenses (ETLs) are liquid lenses possessing shape-changing capabilities that enable rapid axial shifts of the focal plane, in turn achieving acquisition speeds within the millisecond regime. These human-eye-inspired liquid lenses can enable fast focusing and have been applied in a variety of cell biology studies. Here, we review the history, opportunities and challenges underpinning the use of cost-effective high-speed ETLs. Although other, more expensive solutions for three-dimensional imaging in the millisecond regime are available, ETLs continue to be a powerful, yet inexpensive, contender for live-cell microscopy.

Maintaining the proliferative cell niche in multicellular models of epithelia

The maintenance of the proliferative cell niche is critical to epithelial tissue morphology and function. In this paper we investigate how current modelling methods can result in the erroneous loss of proliferative cells from the proliferative cell niche. Using an established model of the inter-follicular epidermis we find there is a limit to the proliferative cell densities that can be maintained in the basal layer (the niche) if we do not include additional mechanisms to stop the loss of proliferative cells from the niche. We suggest a new methodology that enables maintenance of a desired homeostatic population of proliferative cells in the niche: a rotational force is applied to the two daughter cells during the mitotic phase of division to enforce a particular division direction. We demonstrate that this new methodology achieves this goal. This methodology reflects the regulation of the orientation of cell division.

A biomechanical switch regulates the transition towards homeostasis in oesophageal epithelium

Epithelial cells rapidly adapt their behaviour in response to increasing tissue demands. However, the processes that finely control these cell decisions remain largely unknown. The postnatal period covering the transition between early tissue expansion and the establishment of adult homeostasis provides a convenient model with which to explore this question. Here, we demonstrate that the onset of homeostasis in the epithelium of the mouse oesophagus is guided by the progressive build-up of mechanical strain at the organ level. Single-cell RNA sequencing and whole-organ stretching experiments revealed that the mechanical stress experienced by the growing oesophagus triggers the emergence of a bright Krüppel-like factor 4 (KLF4) committed basal population, which balances cell proliferation and marks the transition towards homeostasis in a yes-associated protein (YAP)-dependent manner. Our results point to a simple mechanism whereby mechanical changes experienced at the whole-tissue level are integrated with those sensed at the cellular level to control epithelial cell fate.

2-O-Octadecylascorbic acid represses RhoGDIβ expression and ameliorates DNA damage-induced abnormal spindle orientations

The appropriate regulation of spindle orientation maintains proper tissue homeostasis and avoids aberrant tissue repair or regeneration. Spindle misorientation due to imbalance or improper functioning leads to a loss of tissue integrity and aberrant growth, such as tissue loss or overgrowth. Pharmacological manipulation to prevent spindle misorientation will enable a better understanding of how spindle orientation is involved in physiological and pathological conditions and will provide therapeutic possibilities to treat patients associated with abnormal tissue function caused by spindle misorientation. N-terminal-deleted Rho guanine nucleotide dissociation inhibitor β (RhoGDIβ/RhoGDI2/LyGDI) produced by caspase-3 activation perturbs spindle orientation in surviving cells following exposure to either ionizing radiation or UVC. Thus, presumably, RhoGDIβ cleaved by caspase-3 activation acts as a determinant of radiation-induced spindle misorientation that promote aberrant tissue repair due to deregulation of directional organization of cell population and therefore becomes a potential target of drugs to prevent such response. The objective of this study was to screen and identify chemicals that suppress RhoGDIβ expression. We focused our attention on ascorbic acid (AA) derivatives because of their impact on the maintenance of skin tissue homeostasis. Here, we screened for AA derivatives that suppress RhoGDIβ expression in HeLa cells and identified a lipophilic derivative, 2-O-octadecylascorbic acid (2-OctadecylAA), as a novel RhoGDIβ inhibitor that ameliorated ionizing radiation-induced abnormal spindle orientations. Among all examined AA derivatives, which were also antioxidative, the inhibition activity was specific to 2-OctadecylAA. Therefore, this activity was not due to simple antioxidant properties. 2-OctadecylAA was previously shown to prevent hepatocellular carcinoma development. Our findings suggest that the anticarcinogenic effects of 2-OctadecylAA are partly due to RhoGDIβ inhibition mechanisms by which spindle orientation perturbations are attenuated. Thus, the molecular targeting features of RhoGDIβ warrant its further development for the treatment or control of spindle orientation abnormalities that affect epithelial homeostasis.

CentTracker: a trainable, machine-learning-based tool for large-scale analyses of Caenorhabditis elegans germline stem cell mitosis

Investigating the complex interactions between stem cells and their native environment requires an efficient means to image them in situ. Caenorhabditis elegans germline stem cells (GSCs) are distinctly accessible for intravital imaging; however, long-term image acquisition and analysis of dividing GSCs can be technically challenging. Here we present a systematic investigation into the technical factors impacting GSC physiology during live imaging and provide an optimized method for monitoring GSC mitosis under minimally disruptive conditions. We describe CentTracker, an automated and generalizable image analysis tool that uses machine learning to pair mitotic centrosomes and that can extract a variety of mitotic parameters rapidly from large-scale data sets. We employ CentTracker to assess a range of mitotic features in a large GSC data set. We observe spatial clustering of mitoses within the germline tissue but no evidence that subpopulations with distinct mitotic profiles exist within the stem cell pool. We further find biases in GSC spindle orientation relative to the germline’s distal–proximal axis and thus the niche. The technical and analytical tools provided herein pave the way for large-scale screening studies of multiple mitotic processes in GSCs dividing in situ, in an intact tissue, in a living animal, under seemingly physiological conditions.

The Aurora-A/TPX2 Axis Directs Spindle Orientation in Adherent Human Cells by Regulating NuMA and Microtubule Stability

Mitotic spindle orientation is a crucial process that defines the axis of cell division, contributing to daughter cell positioning and fate, and hence to tissue morphogenesis and homeostasis.^1,2 The trimeric NuMA/LGN/Gαi complex, the major determinant of spindle orientation, exerts pulling forces on the spindle poles by anchoring astral microtubules (MTs) and dynein motors to the cell cortex.^3,4 Mitotic kinases contribute to correct spindle orientation by regulating nuclear mitotic apparatus protein (NuMA) localization,^5-7 among which the Aurora-A centrosomal kinase regulates NuMA targeting to the cell cortex in metaphase.^8,9 Aurora-A and its activator targeting protein for Xklp2 (TPX2) are frequently overexpressed in cancer,^10-12 raising the question as to whether spindle orientation is among the processes downstream the Aurora-A/TPX2 signaling axis altered under pathological conditions. Here, we investigated the role of TPX2 in the Aurora-A- and NuMA-dependent spindle orientation. We show that, in cultured adherent human cells, the interaction with TPX2 is required for Aurora-A to exert this function. We also show that Aurora-A, TPX2, and NuMA are part of a complex at spindle MTs, where TPX2 acts as a platform for Aurora-A regulation of NuMA. Interestingly, excess TPX2 does not influence NuMA localization but induces a "super-alignment" of the spindle axis with respect to the substrate, although an excess of Aurora-A induces spindle misorientation. These opposite effects are both linked to altered MT stability. Overall, our results highlight the importance of TPX2 for spindle orientation and suggest that spindle orientation is differentially sensitive to unbalanced levels of Aurora-A, TPX2, or the Aurora-A/TPX2 complex.

The dynamics of epidermal stratification during post-larval development in zebrafish

BACKGROUND

The epidermis, as a defensive barrier, is a consistent trait throughout animal evolution. During post-larval development, the zebrafish epidermis thickens by stratification or addition of new cell layers. Epidermal basal stem cells, expressing the transcription factor p63, are known to be involved in this process. Zebrafish post-larval epidermal stratification is a tractable system to study how stem cells participate in organ growth.

METHODS

We used immunohistochemistry, in combination with EdU cell proliferation detection, to study zebrafish epidermal stratification. For this procedure, we selected a window of post-larval stages (5-8 mm of standard length or SL, which normalizes age by size). Simultaneously, we used markers for asymmetric cell division and the Notch signaling pathway.

RESULTS

We found that epidermal stratification is the consequence of several events, including changes in cell shape, active cell proliferation and asymmetrical cell divisions. We identified a subset of highly proliferative epidermal cells with reduced levels of p63, which differed from the basal stem cells with high levels of p63. Additionally, we described different mechanisms that participate in the stratification process, including the phosphorylation of p63, asymmetric cell division regulated by the Par3 and LGN proteins, and expression of Notch genes.

The Reconstructed Human Epidermis in vitro — a Model for Basic and Applied Research of Human Skin

Background. The reconstructed human epidermis (RE) is an in vitro tissue-engineering construct similar to the native epidermis. Objective. To develop a full-layer RE. Describe its structure: determine the presence of all layers of the epidermal component, including basal, spinous and granular layers and stratum corneum of the epidermis; detect the basement membrane, the border between the epidermal and mesenchymal component. Materials and methods. Isolation of keratinocytes and fibroblasts from human donor skin. Cultivation of keratinocytes and fibroblasts in vitro under 2D conditions, cell subculturing and 3D modeling of RE, obtaining cryosections, histological staining, immunohistochemical (IHC) study with antibodies to cytokeratins 14 and 10, Ki67 protein, loricrin, laminin 5 and plectin. Results. A technique was developed for the formation of RE. Histological examination showed that the stratification of keratinocyte layers occurs during the formation of RE. Layers are formed including basal, spinous and granular layers and stratum corneum. The IHC study has shown the proliferative activity of keratinocytes of the basal layer and has detected the presence of marker proteins of keratinocytes at different stages of differentiation. RE basal keratinocytes, like native ones, form hemidesmosomes and synthesize basement membrane proteins. Conclusions. A full-layer human RE was obtained in vitro. RE meets all the characteristics of the native epidermis and it is suitable for basic and practical research in the field of skin biology, dermatology, and cosmetology.

Cell lineage-dependent chiral actomyosin flows drive cellular rearrangements in early Caenorhabditis elegans development

Proper positioning of cells is essential for many aspects of development. Daughter cell positions can be specified via orienting the cell division axis during cytokinesis. Rotatory actomyosin flows during division have been implied in specifying and reorienting the cell division axis, but how general such reorientation events are, and how they are controlled, remains unclear. We followed the first nine divisions of Caenorhabditis elegans embryo development and demonstrate that chiral counter-rotating flows arise systematically in early AB lineage, but not in early P/EMS lineage cell divisions. Combining our experiments with thin film active chiral fluid theory we identify a mechanism by which chiral counter-rotating actomyosin flows arise in the AB lineage only, and show that they drive lineage-specific spindle skew and cell reorientation events. In conclusion, our work sheds light on the physical processes that underlie chiral morphogenesis in early development.

Transgelin-2 and phosphoregulation of the LIC2 subunit of dynein govern mitotic spindle orientation

The molecular motor dynein is essential for mitotic spindle orientation, which defines the axis of cell division. The light intermediate chain subunits, LIC1 and LIC2, define biochemically and functionally distinct vertebrate dynein complexes, with LIC2-dynein playing a crucial role in ensuring spindle orientation. We reveal a novel, mitosis-specific interaction of LIC2-dynein with the cortical actin-bundling protein transgelin-2. Transgelin-2 is required for maintaining proper spindle length, equatorial metaphase chromosome alignment, spindle orientation and timely anaphase onset. We show that transgelin-2 stabilizes the cortical recruitment of LGN-NuMA, which together with dynein is required for spindle orientation. The opposing actions of transgelin-2 and LIC2-dynein maintain optimal cortical levels of LGN-NuMA. In addition, we show that the highly conserved serine 194 phosphorylation of LIC2 is required for proper spindle orientation, by maintaining mitotic centrosome integrity to ensure optimal astral microtubule nucleation. The work reveals two specific mechanisms through which LIC2-dynein regulates mitotic spindle orientation; namely, through a new interactor transgelin-2, which is required for engagement of LGN-NuMA with the actin cortex, and through mitotic phosphoregulation of LIC2 to control microtubule nucleation from the poles.This article has an associated First Person interview with the first author of the paper.

Spindle Orientation-Independent Control of Cell Fate Determination by RGS3 and KIF20A

It was proposed that similar to its role in the invertebrate nervous system, mitotic spindle orientation (or cell cleavage plane orientation) of a dividing neural progenitor cell specifies the fate of daughter cells in the mammalian brain, modulating the production of neurons via symmetric versus asymmetric cell divisions during the course of neurogenesis. Experimental tests of the sufficiency of spindle/cleavage plane orientation in mammalian cell fate determination have yielded conflicting results. On the other hand, the necessity of spindle/cleavage plane orientation in mammalian cell fate determination has not yet been addressed. Here we examined the necessity of spindle/cleavage plane orientation during cortical neurogenesis in mice with loss-of-function of the RGS3-KIF20A interaction axis. We present evidence that while inactivation of RGS3 or KIF20A was linked to a shift in neural progenitor cells from proliferative to differentiative divisions in the developing cortex, these genetic mutations did not lead to anticipated alteration in the orientation of spindle/cleavage plane. Our results indicate that the RGS3-KIF20A axis regulates the balance between proliferation and differentiation in the mammalian cortex employing a mechanism independent of spindle/cleavage plane orientation. These data also caution against using spindle/cleavage plane orientation as the synonym for cell fate determination.

Oriented cell divisions in epithelia: from force generation to force anisotropy by tension, shape and vertices

Mitotic spindle orientation has been linked to asymmetric cell divisions, tissue morphogenesis and homeostasis. The canonical pathway to orient the mitotic spindle is composed of the cortical recruitment factor NuMA and the molecular motor dynein, which exerts pulling forces on astral microtubules to orient the spindle. Recent work has defined a novel role for NuMA as a direct contributor to force generation. In addition, the exploration of geometrical and physical cues combined with the study of classical polarity pathways has led to deeper insights into the upstream regulation of spindle orientation. Here, we focus on how cell shape, junctions and mechanical tension act to orient spindle pulling forces in epithelia, and discuss different roles for spindle orientation in epithelia.

RhoGDIβ affects HeLa cell spindle orientation following UVC irradiation

The molecular signals that regulate mitotic spindle orientation to determine the proper division axis play a critical role in the development and maintenance of tissue homeostasis. However, deregulation of signaling events can result in spindle misorientation, which in turn can trigger developmental defects and cancer progression. Little is known about the cellular signaling pathway involved in the misorientation of proliferating cells that evade apoptosis after DNA damage. In this study, we found that perturbations to spindle orientation were induced in ultraviolet C (UVC)-irradiated surviving cells. N-terminal truncated Rho GDP-dissociation inhibitor β (RhoGDIβ), which is produced by UVC irradiation, distorted the spindle orientation of HeLa cells cultured on Matrigel. The short hairpin RNA-mediated knockdown of RhoGDIβ significantly attenuated UVC-induced misorientation. Subsequent expression of wild-type RhoGDIβ, but not a noncleavable mutant, RhoGDIβ (D19A), again led to a relative increase in spindle misorientation in response to UVC. Our findings revealed that RhoGDIβ impacts spindle orientation in response to DNA damage.

Epithelial geometry regulates spindle orientation and progenitor fate during formation of the mammalian epidermis

The control of cell fate through oriented cell division is imperative for proper organ development. Basal epidermal progenitor cells divide parallel or perpendicular to the basement membrane to self-renew or produce differentiated stratified layers, but the mechanisms regulating the choice between division orientations are unknown. Using time-lapse imaging to follow divisions and fates of basal progenitors, we find that mouse embryos defective for the planar cell polarity (PCP) gene, Vangl2, exhibit increased perpendicular divisions and hyperthickened epidermis. Surprisingly, this is not due to defective Vangl2 function in the epidermis, but to changes in cell geometry and packing that arise from the open neural tube characteristic of PCP mutants. Through regional variations in epidermal deformation and physical manipulations, we show that local tissue architecture, rather than cortical PCP cues, regulates the decision between symmetric and stratifying divisions, allowing flexibility for basal cells to adapt to the needs of the developing tissue.

The ciliary GTPase Arl3 maintains tissue architecture by directing planar spindle orientation during epidermal morphogenesis

Arl/ARF GTPases regulate ciliary trafficking, but their tissue-specific functions are unclear. Here, we demonstrate that ciliary GTPase Arl3 is required for mitotic spindle orientation of mouse basal stem cells during skin development. Arl3 loss diminished cell divisions within the plane of the epithelium, leading to increased perpendicular divisions, expansion of progenitor cells and loss of epithelial integrity. These observations suggest that an Arl3-dependent mechanism maintains cell division polarity along the tissue axis, and disruption of planar spindle orientation has detrimental consequences for epidermal architecture. Defects in planar cell polarity (PCP) can disrupt spindle positioning during tissue morphogenesis. Upon Arl3 loss, the PCP signaling molecules Celsr1 and Vangl2 failed to maintain planar polarized distributions, resulting in defective hair follicle angling, a hallmark of disrupted PCP. In the absence of Celsr1 polarity, frizzled 6 lost its asymmetrical distribution and abnormally segregated to the apical cortex of basal cells. We propose that Arl3 regulates polarized endosomal trafficking of PCP components to compartmentalized membrane domains. Cell-cell communication via ciliary GTPase signaling directs mitotic spindle orientation and PCP signaling, processes that are crucial for the maintenance of epithelial architecture.

Conditional ablation of p130Cas/BCAR1 adaptor protein impairs epidermal homeostasis by altering cell adhesion and differentiation

Background

p130 Crk-associated substrate (p130CAS; also known as BCAR1) is a scaffold protein that modulates many essential cellular processes such as cell adhesion, proliferation, survival, cell migration, and intracellular signaling. p130Cas has been shown to be highly expressed in a variety of human cancers of epithelial origin. However, few data are available regarding the role of p130Cas during normal epithelial development and homeostasis.

Methods

To this end, we have generated a genetically modified mouse in which p130Cas protein was specifically ablated in the epidermal tissue.

Results

By using this murine model, we show that p130Cas loss results in increased cell proliferation and reduction of cell adhesion to extracellular matrix. In addition, epidermal deletion of p130Cas protein leads to premature expression of “late” epidermal differentiation markers, altered membrane E-cadherin/catenin proteins localization and aberrant tyrosine phosphorylation of E-cadherin/catenin complexes. Interestingly, these alterations in adhesive properties in absence of p130Cas correlate with abnormalities in progenitor cells balance resulting in the amplification of a more committed cell population.

Conclusion

Altogether, these results provide evidence that p130Cas is an important regulator of epidermal cell fate and homeostasis.

Electronic supplementary material

The online version of this article (10.1186/s12964-018-0289-z) contains supplementary material, which is available to authorized users.

Spindle rotation in human cells is reliant on a MARK2-mediated equatorial spindle-centering mechanism

Unlike man-made wheels that are centered and rotated via an axle, the mitotic spindle of a human cell is rotated by external cortical pulling mechanisms. Zulkipli et al. identify MARK2’s role in equatorial spindle centering and astral microtubule length, which in turn control spindle rotation.

The plane of cell division is defined by the final position of the mitotic spindle. The spindle is pulled and rotated to the correct position by cortical dynein. However, it is unclear how the spindle’s rotational center is maintained and what the consequences of an equatorially off centered spindle are in human cells. We analyzed spindle movements in 100s of cells exposed to protein depletions or drug treatments and uncovered a novel role for MARK2 in maintaining the spindle at the cell’s geometric center. Following MARK2 depletion, spindles glide along the cell cortex, leading to a failure in identifying the correct division plane. Surprisingly, spindle off centering in MARK2-depleted cells is not caused by excessive pull by dynein. We show that MARK2 modulates mitotic microtubule growth and length and that codepleting mitotic centromere-associated protein (MCAK), a microtubule destabilizer, rescues spindle off centering in MARK2-depleted cells. Thus, we provide the first insight into a spindle-centering mechanism needed for proper spindle rotation and, in turn, the correct division plane in human cells.

BRCA1 controls the cell division axis and governs ploidy and phenotype in human mammary cells

BRCA1 deficiency may perturb the differentiation hierarchy present in the normal mammary gland and is associated with the genesis of breast cancers that are genomically unstable and typically display a basal-like transcriptome. Oriented cell division is a mechanism known to regulate cell fates and to restrict tumor formation. We now show that the cell division axis is altered following shRNA-mediated BRCA1 depletion in immortalized but non-tumorigenic, or freshly isolated normal human mammary cells with graded consequences in progeny cells that include aneuploidy, perturbation of cell polarity in spheroid cultures, and a selective loss of cells with luminal features. BRCA1 depletion stabilizes HMMR abundance and disrupts cortical asymmetry of NUMA-dynein complexes in dividing cells such that polarity cues provided by cell-matrix adhesions were not able to orient division. We also show that immortalized mammary cells carrying a mutant BRCA1 allele (BRCA1 185delAG/+) reproduce many of these effects but in this model, oriented divisions were maintained through cues provided by CDH1⁺ cell-cell junctions. These findings reveal a previously unknown effect of BRCA1 suppression on mechanisms that regulate the cell division axis in proliferating, non-transformed human mammary epithelial cells and consequent downstream effects on the mitotic integrity and phenotype control of their progeny.

Cell-Cell Junctions Organize Structural and Signaling Networks

Cell-cell junctions link cells to each other in tissues, and regulate tissue homeostasis in critical cell processes that include tissue barrier function, cell proliferation, and migration. Defects in cell-cell junctions give rise to a wide range of tissue abnormalities that disrupt homeostasis and are common in genetic abnormalities and cancers. Here, we discuss the organization and function of cell-cell junctions primarily involved in adhesion (tight junction, adherens junction, and desmosomes) in two different epithelial tissues: a simple epithelium (intestine) and a stratified epithelium (epidermis). Studies in these tissues reveal similarities and differences in the organization and functions of different cell-cell junctions that meet the requirements for the specialized functions of each tissue. We discuss cell-cell junction responses to genetic and environmental perturbations that provide further insights into their roles in maintaining tissue homeostasis.

The asymmetric cell division machinery in the spiral-cleaving egg and embryo of the marine annelid Platynereis dumerilii

BACKGROUND

Over one third of all animal phyla utilize a mode of early embryogenesis called 'spiral cleavage' to divide the fertilized egg into embryonic cells with different cell fates. This mode is characterized by a series of invariant, stereotypic, asymmetric cell divisions (ACDs) that generates cells of different size and defined position within the early embryo. Astonishingly, very little is known about the underlying molecular machinery to orchestrate these ACDs in spiral-cleaving embryos. Here we identify, for the first time, cohorts of factors that may contribute to early embryonic ACDs in a spiralian embryo.

RESULTS

To do so we analyzed stage-specific transcriptome data in eggs and early embryos of the spiralian annelid Platynereis dumerilii for the expression of over 50 candidate genes that are involved in (1) establishing cortical domains such as the partitioning defective (par) genes, (2) directing spindle orientation, (3) conveying polarity cues including crumbs and scribble, and (4) maintaining cell-cell adhesion between embryonic cells. In general, each of these cohorts of genes are co-expressed exhibiting high levels of transcripts in the oocyte and fertilized single-celled embryo, with progressively lower levels at later stages. Interestingly, a small number of key factors within each ACD module show different expression profiles with increased early zygotic expression suggesting distinct regulatory functions. In addition, our analysis discovered several highly co-expressed genes that have been associated with specialized neural cell-cell recognition functions in the nervous system. The high maternal contribution of these 'neural' adhesion complexes indicates novel general adhesion functions during early embryogenesis.

CONCLUSIONS

Spiralian embryos are champions of ACD generating embryonic cells of different size with astonishing accuracy. Our results suggest that the molecular machinery for ACD is already stored as maternal transcripts in the oocyte. Thus, the spiralian egg can be viewed as a totipotent yet highly specialized cell that evolved to execute fast and precise ACDs during spiral cleaving stages. Our survey identifies cohorts of factors in P. dumerilii that are candidates for these molecular mechanisms and their regulation, and sets the stage for a functional dissection of ACD in a spiral-cleaving embryo.

UVB exposure of a humanized skin model reveals unexpected dynamic of keratinocyte proliferation and Wnt inhibitor balancing

We developed human dermo-epidermal skin substitutes that are presently applied in phase I and II clinical trials. Here, we used these very same skin equivalents containing melanocytes, named MelSkin, as an experimental skin model. We investigated the effects of ultraviolet B (UVB) irradiation on the skin grafts transplanted on immune-compromised rats. The irradiation induces a strong wound healing response going along with massive proliferation of basal keratinocytes, basically quiescent under nonirradiated, homeostatic conditions. As a consequence of UVB irradiation, the initially clearly defined basal keratinocyte (mono)layer expands into about 3 layers of keratinocytes, all of which still express the basal keratinocyte marker keratin 15. In contrast, epidermal melanocytes remain quiescent under these circumstances. Moreover, the Wnt inhibitors Dickkopf 3 and Wif1 are downregulated upon UVB irradiation in basal keratinocytes, whereas melanocytes continue to express Wnt inhibitors. These findings suggest that there is (a) a suprabasal population, proliferating in the homeostatic state, hence maintaining the integrity of the epidermis, and (b) a basal, usually quiescent keratinocyte population that is induced to massively proliferate upon irradiation. Importantly, the finding that MelSkin responds in a physiological fashion to UVB is of paramount importance in light of the planned clinical application.

mTORC1 loss impairs epidermal adhesion via TGF-β/Rho kinase activation

Despite its central position in oncogenic intracellular signaling networks, the role of mTORC1 in epithelial development has not been studied extensively in vivo. Here, we have used the epidermis as a model system to elucidate the cellular effects and signaling feedback sequelae of mTORC1 loss of function in epithelial tissue. In mice with conditional epidermal loss of the mTORC1 components Rheb or Rptor, mTORC1 loss of function unexpectedly resulted in a profound skin barrier defect with epidermal abrasions, blistering, and early postnatal lethality, due to a thinned epidermis with decreased desmosomal protein expression and incomplete biochemical differentiation. In mice with mTORC1 loss of function, we found that Rho kinase (ROCK) signaling was constitutively activated, resulting in increased cytoskeletal tension and impaired cell-cell adhesion. Inhibition or silencing of ROCK1 was sufficient to rescue keratinocyte adhesion and biochemical differentiation in these mice. mTORC1 loss of function also resulted in marked feedback upregulation of upstream TGF-β signaling, triggering ROCK activity and its downstream effects on desmosomal gene expression. These findings elucidate a role for mTORC1 in the regulation of epithelial barrier formation, cytoskeletal tension, and cell adhesion, underscoring the complexity of signaling feedback following mTORC1 inhibition.

The tumor suppressor CYLD controls epithelial morphogenesis and homeostasis by regulating mitotic spindle behavior and adherens junction assembly

Epithelial morphogenesis and homeostasis are essential for animal development and tissue regeneration, and epithelial disorganization is associated with developmental disorders and tumorigenesis. However, the molecular mechanisms that contribute to the morphogenesis and homeostasis of the epithelium remain elusive. Herein, we report a novel role for the cylindromatosis (CYLD) tumor suppressor in these events. Our results show that CYLD depletion disrupts epithelial organization in both Drosophila egg chambers and mouse skin and intestinal epithelia. Microscopic analysis of proliferating cells in mouse epithelial tissues and cultured organoids reveals that loss of CYLD synergizes with tumor-promoting agents to cause the misorientation of the mitotic spindle. Mechanistic studies show that CYLD accumulates at the cell cortex in epithelial tissues and cultured cells, where it promotes the formation of epithelial adherens junctions through the modulation of microtubule dynamics. These data suggest that CYLD controls epithelial morphogenesis and homeostasis by modulating the assembly of adherens junctions and ensuring proper orientation of the mitotic spindle. Our findings thus provide novel insight into the role of CYLD in development, tissue homeostasis, and tumorigenesis.

Centrosomes in spindle organization and chromosome segregation: a mechanistic view

Centrosomes are complex structures, which are embedded into the opposite poles of the mitotic spindle of most animals, acting as microtubule organizing centres. Surprisingly, in several biological systems, such as flies, chicken, or human cells, centrosomes are not essential for cell division. Nonetheless, they ensure faithful chromosome segregation. Moreover, mis-functioning centrosomes can act in a dominant-negative manner, resulting in erroneous mitotic progression. Here, I review the mechanisms by which centrosomes contribute to proper spindle organization and faithful chromosome segregation under physiological conditions and discuss how errors in centrosome function impair transmission of the genomic material in a pathological setting.

Three-Dimensional Analysis of Cell Division Orientation in Epidermal Basal Layer Using Intravital Two-Photon Microscopy

Epidermal structures are different among body sites, and proliferative keratinocytes in the epidermis play an important role in the maintenance of the epidermal structures. In recent years, intravital skin imaging has been used in mammalian skin research for the investigation of cell behaviors, but most of these experiments were performed with rodent ears. Here, we established a non-invasive intravital imaging approach for dorsal, ear, hind paw, or tail skin using R26H2BEGFP hairless mice. Using four-dimensional (x, y, z, and time) imaging, we successfully visualized mitotic cell division in epidermal basal cells. A comparison of cell division orientation relative to the basement membrane in each body site revealed that most divisions in dorsal and ear epidermis occurred in parallel, whereas the cell divisions in hind paw and tail epidermis occurred both in parallel and oblique orientations. Based on the quantitative analysis of the four-dimensional images, we showed that the epidermal thickness correlated with the basal cell density and the rate of the oblique divisions.

An Atypical MAGUK GK Target Recognition Mode Revealed by the Interaction between DLG and KIF13B

The membrane-associated guanylate kinase (MAGUK) scaffold proteins share a signature guanylate kinase (GK) domain. Despite their diverse functional roles in cell polarity control and synaptic signaling, the currently known mode of action of MAGUK GK is via its binding to phosphorylated short peptides from target proteins. Here, we discover that the GK domain of DLG MAGUK binds to an unphosphorylated and autonomously folded domain within the stalk region (MAGUK binding stalk [MBS] domain) of a kinesin motor KIF13B with high specificity and affinity. The structure of DLG4 GK in complex with KIF13B MBS reveals the molecular mechanism governing this atypical GK/target recognition mode and provides insights into DLG/KIF13B complex-mediated regulation of diverse cellular processes such as asymmetric cell division. We further show that binding to non-phosphorylated targets is another general property of MAGUK GKs, thus expanding the mechanisms of action of the MAGUK family proteins.

Cell adhesion molecule control of planar spindle orientation

Polarized epithelial cells align the mitotic spindle in the plane of the sheet to maintain tissue integrity and to prevent malignant transformation. The orientation of the spindle apparatus is regulated by the immobilization of the astral microtubules at the lateral cortex and depends on the precise localization of the dynein-dynactin motor protein complex which captures microtubule plus ends and generates pulling forces towards the centrosomes. Recent developments indicate that signals derived from intercellular junctions are required for the stable interaction of the dynein-dynactin complex with the cortex. Here, we review the molecular mechanisms that regulate planar spindle orientation in polarized epithelial cells and we illustrate how different cell adhesion molecules through distinct and non-overlapping mechanisms instruct the cells to align the mitotic spindle in the plane of the sheet.

Mitotic Diversity in Homeostatic Human Interfollicular Epidermis

Despite decades of skin research, regulation of proliferation and homeostasis in human epidermis is still insufficiently understood. To address the role of mitoses in tissue regulation, we utilized human long-term skin equivalents and systematically assessed mitoses during early epidermal development and long-term epidermal regeneration. We now demonstrate four different orientations: (1) horizontal, i.e., parallel to the basement membrane (BM) and suggestive of symmetric divisions; (2) oblique with an angle of 45°-70°; or (3) perpendicular, suggestive of asymmetric division. In addition, we demonstrate a fourth substantial fraction of suprabasal mitoses, many of which are committed to differentiation (Keratin K10-positive). As verified also for normal human skin, this spatial mitotic organization is part of the regulatory program of human epidermal tissue homeostasis. As a potential marker for asymmetric division, we investigated for Numb and found that it was evenly spread in almost all undifferentiated keratinocytes, but indeed asymmetrically distributed in some mitoses and particularly frequent under differentiation-repressing low-calcium conditions. Numb deletion (stable knockdown by CRISPR/Cas9), however, did not affect proliferation, neither in a three-day follow up study by life cell imaging nor during a 14-day culture period, suggesting that Numb is not essential for the general control of keratinocyte division.

Epithelial stratification and placode invagination are separable functions in early morphogenesis of the molar tooth

Ectodermal organs, which include teeth, hair follicles, mammary ducts, and glands such as sweat, mucous and sebaceous glands, are initiated in development as placodes, which are epithelial thickenings that invaginate and bud into the underlying mesenchyme. These placodes are stratified into a basal and several suprabasal layers of cells. The mechanisms driving stratification and invagination are poorly understood. Using the mouse molar tooth as a model for ectodermal organ morphogenesis, we show here that vertical, stratifying cell divisions are enriched in the forming placode and that stratification is cell division dependent. Using inhibitor and gain-of-function experiments, we show that FGF signalling is necessary and sufficient for stratification but not invagination as such. We show that, instead, Shh signalling is necessary for, and promotes, invagination once suprabasal tissue is generated. Shh-dependent suprabasal cell shape suggests convergent migration and intercalation, potentially accounting for post-stratification placode invagination to bud stage. We present a model in which FGF generates suprabasal tissue by asymmetric cell division, while Shh triggers cell rearrangement in this tissue to drive invagination all the way to bud formation.

Summary: During tooth development in mice, FGF-dependent vertical cell divisions thicken the tooth placode while Shh drives cell rearrangements that cause invagination.

Epidermal development, growth control, and homeostasis in the face of centrosome amplification

As nucleators of the mitotic spindle and primary cilium, centrosomes play crucial roles in equal segregation of DNA content to daughter cells, coordination of growth and differentiation, and transduction of homeostatic cues. Whereas the majority of mammalian cells carry no more than two centrosomes per cell, exceptions to this rule apply in certain specialized tissues and in select disease states, including cancer. Centrosome amplification, or the condition of having more than two centrosomes per cell, has been suggested to contribute to instability of chromosomes, imbalance in asymmetric divisions, and reorganization of tissue architecture; however, the degree to which these conditions are a direct cause of or simply a consequence of human disease is poorly understood. Here we addressed this issue by generating a mouse model inducing centrosome amplification in a naturally proliferative epithelial tissue by elevating Polo-like kinase 4 (Plk4) expression in the skin epidermis. By altering centrosome numbers, we observed multiciliated cells, spindle orientation errors, and chromosome segregation defects within developing epidermis. None of these defects was sufficient to impart a proliferative advantage within the tissue, however. Rather, impaired mitoses led to p53-mediated cell death and contributed to defective growth and stratification. Despite these abnormalities, mice remained viable and healthy, although epidermal cells with centrosome amplification were still appreciable. Moreover, these abnormalities were insufficient to disrupt homeostasis and initiate or enhance tumorigenesis, underscoring the powerful surveillance mechanisms in the skin.

Modeling of Noisy Spindle Dynamics Reveals Separable Contributions to Achieving Correct Orientation

The mechanisms by which the mammalian mitotic spindle is guided to a predefined orientation through microtubule-cortex interactions have recently received considerable interest, but there has been no dynamic model that describes spindle movements toward the preferred axis in human cells. Here, we develop a dynamic model based on stochastic activity of cues anisotropically positioned around the cortex of the mitotic cell and we show that the mitotic spindle does not reach equilibrium before chromosome segregation. Our model successfully captures the characteristic experimental behavior of noisy spindle rotation dynamics in human epithelial cells, including a weak underlying bias in the direction of rotation, suppression of motion close to the alignment axis, and the effect of the aspect ratio of the interphase cell shape in defining the final alignment axis. We predict that the force exerted per cue has a value that minimizes the deviation of the spindle from the predefined axis. The model has allowed us to systematically explore the parameter space around experimentally relevant configurations, and predict the mechanistic function of a number of established regulators of spindle orientation, highlighting how physical modeling of a noisy system can lead to functional biological understanding. We provide key insights into measurable parameters in live cells that can help distinguish between mechanisms of microtubule and cortical-cue interactions that jointly control the final orientation of the spindle.

LGN Directs Interphase Endothelial Cell Behavior via the Microtubule Network

Angiogenic sprouts require coordination of endothelial cell (EC) behaviors as they extend and branch. Microtubules influence behaviors such as cell migration and cell-cell interactions via regulated growth and shrinkage. Here we investigated the role of the mitotic polarity protein LGN in EC behaviors and sprouting angiogenesis. Surprisingly, reduced levels of LGN did not affect oriented division of EC within a sprout, but knockdown perturbed overall sprouting. At the cell level, LGN knockdown compromised cell-cell adhesion and migration. EC with reduced LGN levels also showed enhanced growth and stabilization of microtubules that correlated with perturbed migration. These results fit a model whereby LGN influences interphase microtubule dynamics in endothelial cells to regulate migration, cell adhesion, and sprout extension, and reveal a novel non-mitotic role for LGN in sprouting angiogenesis.

p600/UBR4 in the central nervous system

A decade ago, the large 600 kDa mammalian protein p600 (also known as UBR4) was discovered as a multifunctional protein with roles in anoikis, viral transformation and protein degradation. Recently, p600 has emerged as a critical protein in the mammalian brain with roles in neurogenesis, neuronal migration, neuronal signaling and survival. How p600 integrates these apparently unrelated functions to maintain tissue homeostasis and murine survival remains unclear. The common molecular basis underlying many of the actions of p600 suggests, however, certain conservation and transposition of these functions across systems. In this review, we summarize the central nervous system functions of p600 and propose new perspectives on its biological complexity in neuronal physiology and neurological diseases.

Hair follicle morphogenesis and epidermal homeostasis in we/we wal/wal mice with postnatal alopecia

Mice with skin and hair follicle (HF) defects are common models of human skin disorders. A mutant strain with the we/we wal/wal genotype develops alopecia. We found the hair shaft structure in the pelage of mutant mice to have significant defects. Although these mice lose their hair at 21 days, a label-retaining cell population persists in HFs until at least day 54. Depilation-induced anagen was accomplished in we/we wal/wal mutants but the resulting hair shafts were short and extremely deformed. Serious abnormalities in epidermis stratification and HF morphogenesis exist in we/we wal/wal homozygous E18.5 embryos. There were significantly fewer HF primordia in this mutant compared with wild type. We discovered specific structures, identified as invalid placodes, positive for ectodysplasin A1 receptor, nuclear β-catenin, and LEF1, which failed to invaginate, produced a double basal-like layer of epidermal cells, and lacked cylindrical keratinocytes. Specification of dermal papillae (DP) was impaired, and the papillary dermis expressed alkaline phosphatase and LEF1. We also detected DP-like groups of intensively stained cells in the absence of visible signs of folliculogenesis in the epidermis. We showed differentiation disturbances in the mutant embryonic E18.5 epidermis and HFs: The cornified layer was absent, the width of the spinous layer was reduced, and HFs lacked LEF1-positive precortex cells. In this study, we used a very interesting and useful mouse model of alopecia. The presence of symptoms of skin disorders in we/we wal/wal murine embryos correlates with the postnatal skin phenotype. This correlation may help to evaluate reasons of alopecia.

Efficient fabrication of epidermal cell sheets using γ-secretase inhibitor

BACKGROUND

Epidermal cell sheets have been utilized for regeneration of skin when skin defects occur and prevention of esophageal stricture after endoscopic submucosal dissection. To reduce the cost of cultivation, a novel culture method to shorten a culture process needs to be developed.

OBJECTIVES

To shorten a culture process of epidermal cell sheets, we developed a novel culture method to accelerate the fabrication of epidermal cell sheets using γ-secretase inhibitor.

METHODS

Normal human epidermal keratinocytes (NHEKs) were cultured using γ-secretase inhibitor, DAPT, during expansion of the cells to confluence and culture without DAPT during stratification. The cell growth, quantitative gene expression of stem/progenitor or differentiation markers, and protein expression of these markers were analyzed to verify the effectiveness of the novel method.

RESULTS

The proliferation of NHEKs on cell-culture inserts was promoted using DAPT. However, NHEKs were not stratified completely in the presence of DAPT. In contrast, NHEKs cultured using DAPT were stratified and differentiated by eliminating the inhibitor after the cells reached confluence. Real-time PCR analyses showed that the gene expressions of putative epithelial stem/progenitor cell markers and epidermis differentiation markers in the cell sheets fabricated using this novel method were significantly higher than those in the cell sheets fabricated without DAPT. Histological and immunofluorescence analyses revealed that it was possible to fabricate well-differentiated epidermal cell sheets efficiently by the novel culture method. The culture period was shortened to 67% of the time required for the control group. In feeder-free conditions, stratified epidermal cell sheets were also fabricated using DAPT.

CONCLUSIONS

The novel culture method using γ-secretase inhibitor, DAPT, was found to be effective for fabricating epidermal cell sheets.

Centrosome duplication: suspending a license by phosphorylating a template

The phosphorylation status of Sfi1, a structural component of the yeast centrosome, governs the centrosome duplication cycle, raising the possibility that licensing of centrosome duplication occurs by modulating Sfi1, which potentially acts as a template for a new centrosome.

Spindle orientation processes in epithelial growth and organisation

This review focuses on the role of orientated cell division (OCD) in two aspects of epithelial growth, namely layer formation and growth in the epithelial plane. Epithelial stratification is invariably associated with fate asymmetric cell divisions. We discuss this through the example of epidermal stratification where cell division plane regulation facilitates concomitant thickening and cell differentiation. Embryonic neuroepithelia are considered as a special case of epithelial stratification. We highlight early ectodermal layer specification, which sets the epidermal versus neuronal fates, as well as later neurogenesis in vertebrates and mammals. We also discuss the heart epicardium as an example of coordinating OCDs with delamination and subsequent differentiation. Epithelial planar growth is examined both in the context of uniform growth, such as in Xenopus epiboly, the Drosophila wing disc and the mammalian intestinal crypt as well as in anisotropic growth, or elongation, such as Drosophila and vertebrate axial elongation and the mouse palate. Coupling between growth perpendicular to and within epithelial planes is recognised, but so are exceptions, as is the often passive role of spindle orientation sometimes hitherto considered to be an active driver of directional growth.