Mammalian aPKC/Par polarity complex mediated regulation of epithelial division orientation and cell fate

Oriented cell division is a key regulator of tissue architecture and crucial for morphogenesis and homeostasis. Balanced regulation of proliferation and differentiation is an essential property of tissues not only to drive morphogenesis but also to maintain and restore homeostasis. In many tissues orientation of cell division is coupled to the regulation of differentiation producing daughters with similar (symmetric cell division, SCD) or differential fate (asymmetric cell division, ACD). This allows the organism to generate cell lineage diversity from a small pool of stem and progenitor cells. Division orientation and/or the ratio of ACD/SCD need to be tightly controlled. Loss of orientation or an altered ratio can promote overgrowth, alter tissue architecture and induce aberrant differentiation, and have been linked to morphogenetic diseases, cancer and aging. A key requirement for oriented division is the presence of a polarity axis, which can be established through cell intrinsic and/or extrinsic signals. Polarity proteins translate such internal and external cues to drive polarization. In this review we will focus on the role of the polarity complex aPKC/Par3/Par6 in the regulation of division orientation and cell fate in different mammalian epithelia. We will compare the conserved function of this complex in mitotic spindle orientation and distribution of cell fate determinants and highlight common and differential mechanisms in which this complex is used by tissues to adapt division orientation and cell fate to the specific properties of the epithelium.     

Analysis of cell movements and fate mapping during early embryogenesis in Drosophila melanogaster

Four spatially differentiated surface regions, called aeropyle crown, flat, stripe, and micropyle, are found on the mature eggshell (chorion). Specializations of the apical surfaces of the secretory follicular epithelial cells are implicated in the formation of regional patterns on the chorion. Some of these specializations are restricted to cells overlying certain regions; others are shared by more than one region. Differences between regions are more apparent on the surface than within the bulk of the chorion. Evidence is presented that distinct cell populations, corresponding to the regions, are present long before the start of choriogenesis. One hundred eighty-six chorion-specific polypeptides have been resolved by two-dimensional gel electrophoresis. Fifteen of these are found entirely or predominantly in the aeropyle crown and stripe regions, while eight others are restricted to the aeropyle crown region. Certain of the spatially restricted components are quite unusual in their amino acid compositions when compared with previously analyzed chorion components. Others are closely related, although clearly distinct.     

Mechanical modulation of osteochondroprogenitor cell fate

Mesenchymal cells are natural tissue builders. They exhibit an extraordinary capacity to metamorphize into differentiated cells, using extrinsic spatial and temporal inputs and intrinsic algorithms, as well as to build and adapt their own habitat. In addition to providing a habitat for osteoprogenitor cells, tissues of the skeletal system provide mechanical support and protection for the multiple organs of vertebrate organisms. This review examines the role of mechanics on determination of cell fate during pre-, peri- and postnatal development of the skeleton as well as during tissue genesis and repair in postnatal life. The role of cell mechanics is examined and brought into context of intrinsic cues during mesenchymal condensation. Remarkable new insights regarding structure function relationships in mesenchymal stem cells, and their influence on determination of cell fate are integrated in the context of de novo tissue generation and postnatal repair. Key differences in the formation of osteogenic and chondrogenic condensations are discussed in relation to direct intramembranous and indirect endochondral ossification. New approaches are discussed to elucidate and exploit extrinsic cues to generate tissues in the laboratory and in the clinic.     

Essential role of Bmp signaling and its positive feedback loop in the early cell fate evolution of chordates

In chordates, early separation of cell fate domains occurs prior to the final specification of ectoderm to neural and non-neural as well as mesoderm to dorsal and ventral during development. Maintaining such division with the establishment of an exact border between the domains is required for the formation of highly differentiated structures such as neural tube and notochord. We hypothesized that the key condition for efficient cell fate separation in a chordate embryo is the presence of a positive feedback loop for Bmp signaling within the gene regulatory network (GRN), underlying early axial patterning. Here, we therefore investigated the role of Bmp signaling in axial cell fate determination in amphioxus, the basal chordate possessing a centralized nervous system. Pharmacological inhibition of Bmp signaling induces dorsalization of amphioxus embryos and expansion of neural plate markers, which is consistent with an ancestral role of Bmp signaling in chordate axial patterning and neural plate formation. Furthermore, we provided evidence for the presence of the positive feedback loop within the Bmp signaling network of amphioxus. Using mRNA microinjections we found that, in contrast to vertebrate Vent genes, which promote the expression of Bmp4, amphioxus Vent1 is likely not responsible for activation of cephalochordate ortholog Bmp2/4. Cis-regulatory analysis of amphioxus Bmp2/4, Admp and Chordin promoters in medaka embryos revealed remarkable conservation of the gene regulatory information between vertebrates and basal chordates. Our data suggest that emergence of a positive feedback loop within the Bmp signaling network may represent a key molecular event in the evolutionary history of the chordate cell fate determination.     

The WAVE/SCAR complex promotes polarized cell movements and actin enrichment in epithelia during C. elegans embryogenesis

The WAVE/SCAR complex promotes actin nucleation through the Arp2/3 complex, in response to Rac signaling. We show that loss of WVE-1/GEX-1, the only C. elegans WAVE/SCAR homolog, by genetic mutation or by RNAi, has the same phenotype as loss of GEX-2/Sra1/p140/PIR121, GEX-3/NAP1/HEM2/KETTE, or ABI-1/ABI, the three other components of the C. elegans WAVE/SCAR complex. We find that the entire WAVE/SCAR complex promotes actin-dependent events at different times and in different tissues during development. During C. elegans embryogenesis loss of CED-10/Rac1, WAVE/SCAR complex components, or Arp2/3 blocks epidermal cell migrations despite correct epidermal cell differentiation. 4D movies show that this failure occurs due to decreased membrane dynamics in specific epidermal cells. Unlike myoblasts in Drosophila, epidermal cell fusions in C. elegans can occur in the absence of WAVE/SCAR or Arp2/3. Instead we find that subcellular enrichment of F-actin in epithelial tissues requires the Rac-WAVE/SCAR-Arp2/3 pathway. Intriguingly, we find that at the same stage of development both F-actin and WAVE/SCAR proteins are enriched apically in one epithelial tissue and basolaterally in another. We propose that temporally and spatially regulated actin nucleation by the Rac-WAVE/SCAR-Arp2/3 pathway is required for epithelial cell organization and movements during morphogenesis.     

Quantitative analysis of cellular differentiation during early embryogenesis ofPlatynereis dumerilii

Summary As in many spiralian embryos with unequal cleavage, cleavage inPlatynereis follows an invariant pattern. Preceding each cleavage the cytoplasm is reorganized, allowing the spiral cleavage mode to produce cells with different cytoplasmic composition. The fertilized egg undergoes a dramatic ooplasmic segregation after the completion of the cortical reaction. As a consequence, a plug of clear cytoplasm becomes located at the animal pole. Once the four quadrants of the embryo have been established, the cleavage sequence of the D quadrant differs clearly from that of the other three quadrants. The results presented here suggest that differential distribution of the clear cytoplasm governs this sequence. The first quartet of micromeres, which will form the ectoderm and the cerebral ganglia of the head, is clearly bilaterally symmetrical from the onset of the third cleavage. Dorsoventral polarity and bilateral symmetry in the ectoderm of the trunk is expressed most markedly by the dorsal location of the large 2d cell, whose rapid proliferation is bilaterally symmetrical with respect to the median plane. As a result of this proliferation it comes to fill most of the posttrochal region (ectoderm, three pairs of anlagen for the setal sacs, and the ventral plate which forms the nerve cord). The other micromeres contribute only a minor portion of the ventral ectoderm and are involved in the formation of the stomodaeum. The mesentoblast, 4d, i.e. the stem cell of the primary mesoderm, forms at the sixth cleavage, also in a position on the dorsal mid-line. The daughter cells, which arise from 4d by strictly bilaterally symmetrical cleavage, form the mesodermal germ bands, which lie beneath the ectoderm. The trochoblasts are formed by asynchronously cleaving founder cells, but further cleavages in these cells are synchronous. This suggests that cell-cell interaction is involved in the development of this alleged mosaic embryo.     

Drosophila aPKC regulates cell polarity and cell proliferation in neuroblasts and epithelia

Cell polarity is essential for generating cell diversity and for the proper function of most differentiated cell types. In many organisms, cell polarity is regulated by the atypical protein kinase C (aPKC), Bazooka (Baz/Par3), and Par6 proteins. Here, we show that Drosophila aPKC zygotic null mutants survive to mid-larval stages, where they exhibit defects in neuroblast and epithelial cell polarity. Mutant neuroblasts lack apical localization of Par6 and Lgl, and fail to exclude Miranda from the apical cortex; yet, they show normal apical crescents of Baz/Par3, Pins, Inscuteable, and Discs large and normal spindle orientation. Mutant imaginal disc epithelia have defects in apical/basal cell polarity and tissue morphology. In addition, we show that aPKC mutants show reduced cell proliferation in both neuroblasts and epithelia, the opposite of the lethal giant larvae (lgl) tumor suppressor phenotype, and that reduced aPKC levels strongly suppress most lgl cell polarity and overproliferation phenotypes.     

Integrating cellular dimensions with cell differentiation during early development

Early embryo development is characterized by alteration of cellular dimensions and fating of blastomeres. An emerging concept is that cell size and shape drive cellular differentiation during early embryogenesis in a variety of model organisms. In this review, we summarize recent advances that elucidate the contribution of the physical dimensions of a cell to major embryonic transitions and cell fate specification in vivo. We also highlight techniques and newly evolving methods for manipulating the sizes and shapes of cells and whole embryos in situ and ex vivo. Finally, we provide an outlook for addressing fundamental questions in the field and more broadly uncovering how changes to cell size control decision making in a variety of biological contexts.     

Lineage and fate in Drosophila: role of the gene tramtrack in sense organ development

 The tactile bristles of the fly comprise four cells that originate from a single precursor cell through a fixed lineage. The gene tramtrack (ttk) plays a crucial role in defining the fates of these cells. Here we analyse the normal pattern of expression of ttk, as well as the effect of ttk overexpression at different steps of the lineage. We show that ttk is never expressed in cells having a neural potential, and that in cells where ttk is expressed, there is a delay between division and the onset of expression. The ectopic expression of ttk before some stage of the cell cycle can block further cell division. Furthermore, this expression transforms neural into non-neural cells, suggesting that ttk acts as a repressor of neural fate at each step of the lineage. Our results suggest that ttk is probably not involved in setting up the mechanism that creates an asymmetry between sister cells, but rather in the implementation of that choice. Received: 10 October 1996 / Accepted: 11 February 1997     

Evolution of early embryogenesis in rhabditid nematodes

The cell-biological events that guide early-embryonic development occur with great precision within species but can be quite diverse across species. How these cellular processes evolve and which molecular components underlie evolutionary changes is poorly understood. To begin to address these questions, we systematically investigated early embryogenesis, from the one- to the four-cell embryo, in 34 nematode species related to C. elegans. We found 40 cell-biological characters that captured the phenotypic differences between these species. By tracing the evolutionary changes on a molecular phylogeny, we found that these characters evolved multiple times and independently of one another. Strikingly, all these phenotypes are mimicked by single-gene RNAi experiments in C. elegans. We use these comparisons to hypothesize the molecular mechanisms underlying the evolutionary changes. For example, we predict that a cell polarity module was altered during the evolution of the Protorhabditis group and show that PAR-1, a kinase localized asymmetrically in C. elegans early embryos, is symmetrically localized in the one-cell stage of Protorhabditis group species. Our genome-wide approach identifies candidate molecules—and thereby modules—associated with evolutionary changes in cell-biological phenotypes.     

JAK／STAT signaling regulates tissue outgrowth and male germline stem cell fate in Drosophila

In multicellular organisms, biological activities are regulated by cell signaling. The various signal transduction pathways regulate cell fate, proliferation, migration, and polarity. Miscoordination of the communicative signals will lead to disasters like cancer and other fatal diseases. The JAK/STAT signal transduction pathway is one of the pathways, which was first identified in vertebrates and is highly conserved throughout evolution. Studying the JAK/STAT signal transduction pathway in Drosophila provides an excellent opportunity to understand the molecular mechanism of the cell regulation during development and tumor formation. In this review, we discuss the general overview of JAK/STAT signaling in Drosophila with respect to its functions in the eye development and stem cell fate determination.