Parallels between wound repair and morphogenesis in the embryo

Critical Areas of Proliferation of Single Cells on Micropatterned Surfaces and Corresponding Cell Type Dependence

Material cues to influence cell proliferation are a fundamental issue in the fields of biomaterials, cell biology, tissue engineering, and regenerative medicine. This paper aims to investigate the proliferation of single mammal cells on micropatterned material surfaces. To this end, we prepared cell-adhesive circular microislands with 20 areas on the nonfouling background and systematically examined adhesion and proliferation behaviors of different kinds of single cells (primary stem and nonstem cells, cancer and normal cell lines) on micropatterns. On the basis of the analysis of experimental data, we found two critical areas about cell proliferation: (1) the critical spreading area of cells from almost no proliferation to confined proliferation, denoted as A_P and (2) the critical spreading area of cells from confined proliferation to almost free proliferation, denoted as A_FP. We further summarized the relative size relationship between these two critical areas and the characteristic areas of cell adhesion on both patterned and nonpatterned surfaces. While proliferation of single primary cells was affected by cell spreading, those cell lines, irrespective of normal and cancer cells, did not exhibit significant cell-spreading effects. As a result, this study reveals that proliferation of single cells is dependent upon spreading area, in particular for primary cells on material surfaces.

Regeneration: the ultimate example of wound healing

The outcome of wound repair in mammals is often characterized by fibrotic scaring. Vertebrates such as zebrafish, frogs, and salamanders not only heal scarlessly, but also can regenerate lost appendages. Decades of study on the process of animal regeneration has produced key insights into the mechanisms of how complex tissue is restored. By examining our current knowledge of regeneration, we can draw parallels with mammalian wound healing to identify the molecular determinants that produce such differing outcomes.

Grainy head and its target genes in epithelial morphogenesis and wound healing

The Grainy head (Grh) family of transcription factors is characterized by a unique DNA-binding domain that binds to a conserved consensus sequence. Nematodes and flies have a single grh gene, whereas mice and humans have evolved three genes encoding Grainy head-like (Grhl) factors. We review the biological function of Grh in different animals and the mechanisms modulating its activity. grh and grhl genes play a remarkably conserved role in epithelial organ development and extracellular barrier repair after tissue damage. Recent studies in flies and vertebrates suggest that Grh factors may be primary determinants of cell adhesion and epithelial tissue formation. Grh proteins can dimerize and act as activators or repressors in different developmental contexts. In flies, tissue-specific, alternative splicing generates different Grh isoforms with different DNA-binding specificities and functions. Grh activity is also modulated by receptor tyrosine kinases: it is phosphorylated by extracellular signal regulated kinase, and this phosphorylation is selectively required for epidermal barrier repair. Two mechanisms have been proposed to explain the repressive function of Grh on target gene transcription. First, Grh can target the Polycomb silencing complex to specific response elements. Second, it can directly compete for DNA binding with transcriptional activators. Understanding the molecular mechanisms of gene regulation by Grh factors is likely to elucidate phylogenetically conserved mechanisms of epithelial cell morphogenesis and regeneration upon tissue damage.

Drosophila cancer models

Cancer is driven by complex genetic and cellular mechanisms. Recently, the Drosophila community has become increasingly interested in exploring cancer issues. The Drosophila field has made seminal contributions to many of the mechanisms that are fundamental to the cancer process; several of these mechanisms have already been validated in vertebrates. Less well known are the Drosophila field's early direct contributions to the cancer field: some of the earliest tumor suppressors were identified in flies. In this review, we identify major contributions that Drosophila studies have made toward dissecting the pathways and mechanisms underlying tumor progression. We also highlight areas, such as drug discovery, where we expect Drosophila studies to make a major scientific impact in the future.

Chick amniogenesis is mediated by an actin cable

This study examined the process of chick amniogenesis to determine whether the actin cable mechanism operating during amnion wound repair is a recapitulation of developmental events. Staining of the developing amnion with fluorescein isothiocyanate-labeled phalloidin indicated the presence of an actin cable in the amniotic head fold, which persisted through to the closure of the amnion. Transmission electron microscopy of the developing amnion revealed linearly arranged actin microfilaments in the elongated cells at the leading edge of the amnion, adjacent to either side of a nodule of numerous cells at the point of midline fusion. A mesh of cytoplasmic actin filaments was seen dispersed throughout the accumulated cells of the nodule. Lamellapodia were absent suggesting that cell crawling is not involved in amniogenesis. Addition of an enzyme inhibitor of Rho, cell-permeable C3 transferase, to the surface of the developing amnion prior to closure appeared to inhibit amniogenesis at the early embryonic stages.

GSK-3β is essential for physiological electric field-directed Golgi polarization and optimal electrotaxis

Endogenous electrical fields (EFs) at corneal and skin wounds send a powerful signal that directs cell migration during wound healing. This signal therefore may serve as a fundamental regulator directing cell polarization and migration. Very little is known of the intracellular and molecular mechanisms that mediate EF-induced cell polarization and migration. Here, we report that Chinese hamster ovary (CHO) cells show robust directional polarization and migration in a physiological EF (0.3–1 V/cm) in both dissociated cell culture and monolayer culture. An EF of 0.6 V/cm completely abolished cell migration into wounds in monolayer culture. An EF of higher strength (≥1 V/cm) is an overriding guidance cue for cell migration. Application of EF induced quick phosphorylation of glycogen synthase kinase 3β (GSK-3β) which reached a peak as early as 3 min in an EF. Inhibition of protein kinase C (PKC) significantly reduced EF-induced directedness of cell migration initially (in 1–2 h). Inhibition of GSK-3β completely abolished EF-induced GA polarization and significantly inhibited the directional cell migration, but at a later time (2–3 h in an EF). Those results suggest that GSK-3β is essential for physiological EF-induced Golgi apparatus (GA) polarization and optimal electrotactic cell migration.

Leading edge-secreted Dpp cooperates with ACK-dependent signaling from the amnioserosa to regulate myosin levels during dorsal closure

Dorsal closure of the Drosophila embryo is an epithelial fusion in which the epidermal flanks migrate to close a hole in the epidermis occupied by the amnioserosa, a process driven in part by myosin-dependent cell shape change. Dpp signaling is required for the morphogenesis of both tissues, where it promotes transcription of myosin from the zipper (zip) gene. Drosophila has two members of the activated Cdc42-associated kinase (ACK) family: DACK and PR2. Overexpression of DACK in embryos deficient in Dpp signaling can restore zip expression and suppress dorsal closure defects, while reducing the levels of DACK and PR2 simultaneously using mutations or amnioserosa-specific knock down by RNAi results in loss of zip expression. ACK function in the amnioserosa may generate a signal cooperating with Dpp secreted from the epidermis in driving zip expression in these two tissues, ensuring that cell shape changes in dorsal closure occur in a coordinated manner.

Micropatterning of single endothelial cell shape reveals a tight coupling between nuclear volume in G1 and proliferation

Shape-dependent local differentials in cell proliferation are considered to be a major driving mechanism of structuring processes in vivo, such as embryogenesis, wound healing, and angiogenesis. However, the specific biophysical signaling by which changes in cell shape contribute to cell cycle regulation remains poorly understood. Here, we describe our study of the roles of nuclear volume and cytoskeletal mechanics in mediating shape control of proliferation in single endothelial cells. Micropatterned adhesive islands were used to independently control cell spreading and elongation. We show that, irrespective of elongation, nuclear volume and apparent chromatin decondensation of cells in G1 systematically increased with cell spreading and highly correlated with DNA synthesis (percent of cells in the S phase). In contrast, cell elongation dramatically affected the organization of the actin cytoskeleton, markedly reduced both cytoskeletal stiffness (measured dorsally with atomic force microscopy) and contractility (measured ventrally with traction microscopy), and increased mechanical anisotropy, without affecting either DNA synthesis or nuclear volume. Our results reveal that the nuclear volume in G1 is predictive of the proliferative status of single endothelial cells within a population, whereas cell stiffness and contractility are not. These findings show that the effects of cell mechanics in shape control of proliferation are far more complex than a linear or straightforward relationship. Our data are consistent with a mechanism by which spreading of cells in G1 partially enhances proliferation by inducing nuclear swelling and decreasing chromatin condensation, thereby rendering DNA more accessible to the replication machinery.

Transforming growth factor: beta signaling is essential for limb regeneration in axolotls

Axolotls (urodele amphibians) have the unique ability, among vertebrates, to perfectly regenerate many parts of their body including limbs, tail, jaw and spinal cord following injury or amputation. The axolotl limb is the most widely used structure as an experimental model to study tissue regeneration. The process is well characterized, requiring multiple cellular and molecular mechanisms. The preparation phase represents the first part of the regeneration process which includes wound healing, cellular migration, dedifferentiation and proliferation. The redevelopment phase represents the second part when dedifferentiated cells stop proliferating and redifferentiate to give rise to all missing structures. In the axolotl, when a limb is amputated, the missing or wounded part is regenerated perfectly without scar formation between the stump and the regenerated structure. Multiple authors have recently highlighted the similarities between the early phases of mammalian wound healing and urodele limb regeneration. In mammals, one very important family of growth factors implicated in the control of almost all aspects of wound healing is the transforming growth factor-beta family (TGF-β). In the present study, the full length sequence of the axolotl TGF-β1 cDNA was isolated. The spatio-temporal expression pattern of TGF-β1 in regenerating limbs shows that this gene is up-regulated during the preparation phase of regeneration. Our results also demonstrate the presence of multiple components of the TGF-β signaling machinery in axolotl cells. By using a specific pharmacological inhibitor of TGF-β type I receptor, SB-431542, we show that TGF-β signaling is required for axolotl limb regeneration. Treatment of regenerating limbs with SB-431542 reveals that cellular proliferation during limb regeneration as well as the expression of genes directly dependent on TGF-β signaling are down-regulated. These data directly implicate TGF-β signaling in the initiation and control of the regeneration process in axolotls.

Development of a Model Bladder Extracellular Matrix Combining Disulfide Cross-Linked Hyaluronan with Decellularized Bladder Tissue

[Image: see text] In this work we investigate the feasibility of modifying porcine-derived BAM to include HA with a view to developing a model, artificial extracellular matrix for the study of bladder cell-matrix interactions. HA-DPTH was incorporated into BAM disks and then cross-linked oxidatively to a disulfide containing hydrogel. Disks were seeded with bladder smooth muscle cells (BSMC) and UEC under three culture configurations and incubated for 3, 7, and 14 d. At each time point, matrix contraction was measured, and media supernatants assayed for cell-secreted gelatinase activity. To evaluate cell adherence and organization, triple immunofluorescent labeling of cell nuclei, actin cytoskeleton, and focal contacts was performed. HA-modified BAM exhibited a significant increase in matrix contraction and induced a higher level of cell-secreted gelatinase activity compared to unmodified BAM. Immunofluorescent labeling demonstrated that BSMCs remained adherent to both scaffold types over time. The distribution and organization of the cytoskeleton and focal contacts did not appear to be altered by the presence of HA. Interestingly, cellular infiltration into modified BAM was evident by 7 d and continued beyond 14 d, while BSMCs seeded onto unmodified BAM remained localized to the surface out to 14 d, with minimal infiltration evident only at day 28. These differences in cell infiltration support the gelatinase activity results. Increases in cell migration and matrix proteolysis in the presence of HA may be contributing factors toward BAM remodeling leading to increased matrix contraction with time. The model ECM developed in this work will be utilized for future studies aimed at elucidating the mechanisms controlling key remodeling events associated with bladder repair. Matrix contraction of cell-seeded BAM scaffolds.

Wound healing ability of Xenopus laevis embryos. I. Rapid wound closure achieved by bisectional half embryos

We examined wound closure in 'half embryos' produced by the transverse bisection of Xenopus laevis embryos at the primary eye vesicle stage. Both the anterior- and posterior-half embryos survived for more than 6 days, and grew into 'half tadpoles'. Histology and videomicroscopy revealed that the open wound in the half embryo was rapidly closed by an epithelial sheet movement in the wound marginal zone. The time-course of wound closure showed a downward convex curve: the wound area decreased to one-fifth of the original area within 30 min, and the wound continued to contract slowly thereafter. The rapidity of closure of the epidermis as well as the absence of inflammatory cells are typical features of an embryonic type of wound healing. There was a dorso-ventral polarity in the motility of the epidermis: the wound was predominantly closed by the ventral and lateral epidermis. The change in the contour of the wound edge with time suggested a complex mechanism involved in the wound closure that could not be explained only by the purse-string theory. The present experimental system would be a unique and useful model for analyses of cellular movements in the embryonic epithelia.

Golgi polarization in a strong electric field

Directional cell migration requires proper cell polarization. The redistribution of the Golgi apparatus is an important event in the polarization and migration of many types of cells, as a polarized Golgi supplies membrane components for leading edge protrusion. Direct current electric fields induce directional cell migration in a wide variety of cells. Here we show that electric fields of 300 mV/mm induce robust Golgi polarization and directional cell migration in CHO cells. Asymmetric Src and PI 3-kinase signalling as well as actin polymerization are essential for electric field-induced Golgi polarization and directional cell migration. The Golgi polarizes at the same time as cells change morphology and migrate directionally in response to an electric field. Golgi polarization in turn significantly reinforces and maintains optimal electrotaxis. It is not known whether electrical signals, when contradicting other directional cues, are still able to polarize cells and direct cell migration. Most strikingly, Golgi polarization and cell migration simply follow the direction of an applied electric field and ignore all other cues generated by wounding a monolayer of CHO cells. Thus, an electric field of 300 mV/mm is the predominant cue to polarize the Golgi and direct cell migration mediated by PI 3-kinase and Src signalling.

Immediate early genes krox-24 and krox-20 are rapidly up-regulated after wounding in the embryonic and adult mouse

Embryos show a remarkable capacity for perfect repair after injury. Wounding of embryonic skin triggers rapid activation of epithelial sweeping and mesenchymal contraction tissue movements that bear striking analogy to several naturally occurring morphogenetic tissue movements, but very little is known about the early molecular signals that might initiate such movements. Here, we describe the rapid and transient up-regulation of two immediate early genes, krox-24 and krox-20, after wounding of the embryonic mouse. Furthermore, we demonstrate that these signals are conserved, but of longer duration, in the neonate and adult wound situation. To further test the roles of these transcription factors in vivo, we performed wound healing studies on embryos lacking either Krox-24 or 20. Despite the dramatic up-regulation of these genes in response to injury, our studies reveal that neither of them on their own is essential for repair.

The genomic response of the Drosophila embryo to JNK signaling

During Drosophila development, the Jun N-terminal kinase signal transduction pathway regulates morphogenetic tissue closure movements that involve cell shape changes and reorganization of the actin cytoskeleton. We analyzed the genome-wide transcriptional response to activation of the JNK pathway in the Drosophila embryo by serial analysis of gene expression (SAGE) and identified loci encoding cell adhesion molecules and cytoskeletal regulators as JNK responsive genes. The role of one of the upregulated genes, chickadee (chic), encoding a Drosophila profilin, in embryogenesis was analyzed genetically. chic-deficient embryos fail to execute the JNK-mediated cytoskeletal rearrangements during dorsal closure. This study demonstrates a transcriptional mechanism of cytoskeletal regulation and establishes SAGE as an advantageous approach for genomic experiments in the fruitfly.

Form of the worm: genetics of epidermal morphogenesis in C. elegans

The development of the epidermis of the nematode worm Caenorhabditis elegans illustrates many common processes of epithelial morphogenesis. In the worm, these morphogenetic movements have been described with single-cell resolution, and the roles of individual cells have been probed in laser killing experiments. Genetic dissection is yielding insights into the molecular mechanisms of these complex morphogenetic processes.