Pericardin,a Drosophila type IV collagen-like protein is involved in the morphogenesis and maintenance of the heart epithelium during dorsal ectoderm closure

The steps that lead to the formation of a single primitive heart tube are highly conserved in vertebrate and invertebrate embryos. Concerted migration of the two lateral cardiogenic regions of the mesoderm and endoderm (or ectoderm in invertebrates) is required for their fusion at the midline of the embryo. Morphogenetic signals are involved in this process and the extracellular matrix has been proposed to serve as a link between the two layers of cells. Pericardin (Prc), a novel Drosophila extracellular matrix protein is a good candidate to participate in heart tube formation. The protein has the hallmarks of a type IV collagen alpha-chain and is mainly expressed in the pericardial cells at the onset of dorsal closure. As dorsal closure progresses, Pericardin expression becomes concentrated at the basal surface of the cardioblasts and around the pericardial cells, in close proximity to the dorsal ectoderm. Pericardin is absent from the lumen of the dorsal vessel.Genetic evidence suggests that Prc promotes the proper migration and alignment of heart cells. Df(3)vin6 embryos, as well as embryos in which prc has been silenced via RNAi, exhibit similar and significant defects in the formation of the heart epithelium. In these embryos, the heart epithelium appears disorganized during its migration to the dorsal midline. By the end of embryonic development, cardial and pericardial cells are misaligned such that small clusters of both cell types appear in the heart; these clusters of cells are associated with holes in the walls of the heart. A prc transgene can partially rescue each of these phenotypes, suggesting that prc regulates these events. Our results support, for the first time, the function of a collagen-like protein in the coordinated migration of dorsal ectoderm and heart cells.     

Spire, an actin nucleation factor, regulates cell division during Drosophila heart development

The Drosophila dorsal vessel is a beneficial model system for studying the regulation of early heart development. Spire (Spir), an actin-nucleation factor, regulates actin dynamics in many developmental processes, such as cell shape determination, intracellular transport, and locomotion. Through protein expression pattern analysis, we demonstrate that the absence of spir function affects cell division in Myocyte enhancer factor 2-, Tinman (Tin)-, Even-skipped- and Seven up (Svp)-positive heart cells. In addition, genetic interaction analysis shows that spir functionally interacts with Dorsocross, tin, and pannier to properly specify the cardiac fate. Furthermore, through visualization of double heterozygous embryos, we determines that spir cooperates with CycA for heart cell specification and division. Finally, when comparing the spir mutant phenotype with that of a CycA mutant, the results suggest that most Svp-positive progenitors in spir mutant embryos cannot undergo full cell division at cell cycle 15, and that Tin-positive progenitors are arrested at cell cycle 16 as double-nucleated cells. We conclude that Spir plays a crucial role in controlling dorsal vessel formation and has a function in cell division during heart tube morphogenesis.     

Dual functions of the heartless fibroblast growth factor receptor in development of the Drosophila embryonic mesoderm

The Drosophila embryonic mesoderm forms by invagination of the ventral-most blastoderm cells. Subsequent development of this germ layer involves the dorsolateral migration of the internalized cells and expansion by cell division, followed by the specification of particular cell fates through the coordinate actions of both intrinsic and extrinsic regulatory mechanisms. The latter include several intercellular signals that function across germ layers. These processes combine to generate a diversity of mesodermal subtypes, including the cardial and pericardial cells of the heart or dorsal vessel, a complete set of somatic muscle founders each with its unique identity, a population of cells that form the visceral musculature, and other cells that develop into hemocytes and the fat body. Here, we review recent evidence for the involvement of a fibroblast growth factor receptor (FGFR) encoded by the heartless (htl) gene in early directional migration of the Drosophila mesoderm. In addition, we provide new data that 1) demonstrate a second role for Htl in promoting the specification of the precursors to certain cardiac and somatic muscle cells in the Drosophila embryo, independent of its cell migration function, 2) suggest that Ras and at least one other signal transduction pathway act downstream of Htl, and 3) establish a functional relationship between the Ras pathway and Tinman (Tin), a homeodomain factor that is essential for specifying some of the same dorsal mesodermal cells that are dependent on Htl. Finally, parallels between requirements for FGFR signaling in Drosophila and vertebrate mesoderm development are considered. Dev. Genet. 22:212–229, 1998. © 1998 Wiley-Liss, Inc.     

The PDZ protein Canoe regulates the asymmetric division of Drosophila neuroblasts and muscle progenitors

Asymmetric cell division is a conserved mechanism to generate cellular diversity during animal development and a key process in cancer and stem cell biology. Despite the increasing number of proteins characterized, the complex network of proteins interactions established during asymmetric cell division is still poorly understood. This suggests that additional components must be contributing to orchestrate all the events underlying this tightly modulated process. The PDZ protein Canoe (Cno) and its mammalian counterparts AF-6 and Afadin are critical to regulate intracellular signaling and to organize cell junctions throughout development. Here, we show that Cno functions as a new effector of the apical proteins Inscuteable (Insc)-Partner of Inscuteable (Pins)-Galphai during the asymmetric division of Drosophila neuroblasts (NBs). Cno localizes apically in metaphase NBs and coimmunoprecipitates with Pins in vivo. Furthermore, Cno functionally interacts with the apical proteins Insc, Galphai, and Mushroom body defect (Mud) to generate correct neuronal lineages. Failures in muscle and heart lineages are also detected in cno mutant embryos. Our results strongly support a new function for Cno regulating key processes during asymmetric NB division: the localization of cell-fate determinants, the orientation of the mitotic spindle, and the generation of unequal-sized daughter cells.     

svp基因在果蝇副心肌细胞生长中的作用

果蝇心脏位于身体背部,是一个体节性重复的线性管状结构。在hedgehog（hh）基因的信号诱导下,seven-up（svp）基因调控果蝇的心脏发育,在每个体节的两个心肌细胞和两个副心肌细胞中表达。结果表明,在svp纯合突变体中,报告基因lacZ在心肌细胞中的表达图式正常,但在副心肌细胞中的表达图型明显异常,而且部分EPC细胞生长尺寸增加。某些体节的DA1肌肉祖细胞缺失,晚期突变体胚胎体壁肌肉细胞也呈现异常,表明基因svp的活性对果蝇副心肌细胞、DA1肌肉祖细胞和体壁肌肉细胞的分化是必须的,并且可能与EPC副心肌细胞的尺寸生长有关。     

Uncovering the link between malfunctions in Drosophila neuroblast asymmetric cell division and tumorigenesis

Asymmetric cell division is a developmental process utilized by several organisms. On the most basic level, an asymmetric division produces two daughter cells, each possessing a different identity or fate. Drosophila melanogaster progenitor cells, referred to as neuroblasts, undergo asymmetric division to produce a daughter neuroblast and another cell known as a ganglion mother cell (GMC). There are several features of asymmetric division in Drosophila that make it a very complex process, and these aspects will be discussed at length. The cell fate determinants that play a role in specifying daughter cell fate, as well as the mechanisms behind setting up cortical polarity within neuroblasts, have proved to be essential to ensuring that neurogenesis occurs properly. The role that mitotic spindle orientation plays in coordinating asymmetric division, as well as how cell cycle regulators influence asymmetric division machinery, will also be addressed. Most significantly, malfunctions during asymmetric cell division have shown to be causally linked with neoplastic growth and tumor formation. Therefore, it is imperative that the developmental repercussions as a result of asymmetric cell division gone awry be understood.     

Second heart field and the development of the outflow tract in human embryonic heart

The second heart field (SHF) is indicated to contribute to the embryonic heart development. However, less knowledge is available about SHF development of human embryo due to the difficulty of collecting embryos. In this study, serial sections of human embryos from Carnegie stage 10 (CS10) to CS16 were stained with antibodies against Islet‐1 (Isl‐1), Nkx2.5, GATA4, myosin heavy chain (MHC) and α‐smooth muscle actin (α‐SMA) to observe spatiotemporal distribution of SHF and its contribution to the development of the arterial pole of cardiac tube. Our findings suggest that during CS10 to CS12, SHF of the human embryo is composed of the bilateral pharyngeal mesenchyme, the central mesenchyme of the branchial arch and splanchnic mesoderm of the pericardial cavity dorsal wall. With development, SHF translocates and consists of ventral pharyngeal mesenchyme and dorsal wall of the pericardial cavity. Hence, the SHF of human embryo shows a dynamic spatiotemporal distribution pattern. The formation of the Isl‐1 positive condense cell prongs provides an explanation for the saddle structure formation at the distal pole of the outflow tract. In human embryo, the Isl‐1 positive cells of SHF may contribute to the formation of myocardial outflow tract (OFT) and the septum during different development stages.     

Homeotic genes autonomously specify the anteroposterior subdivision of the Drosophila dorsal vessel into aorta and heart

The embryonic dorsal vessel in Drosophila possesses anteroposterior polarity and is subdivided into two chamber-like portions, the aorta in the anterior and the heart in the posterior. The heart portion features a wider bore as compared with the aorta and develops inflow valves (ostia) that allow the pumping of hemolymph from posterior toward the anterior. Here, we demonstrate that homeotic selector genes provide positional information that determines the anteroposterior subdivision of the dorsal vessel. Antennapedia (Antp), Ultrabithorax (Ubx), abdominal-A (abd-A), and Abdominal-B (Abd-B) are expressed in distinct domains along the anteroposterior axis within the dorsal vessel, and, in particular, the domain of abd-A expression in cardioblasts and pericardial cells coincides with the heart portion. We provide evidence that loss of abd-A function causes a transformation of the heart into aorta, whereas ectopic expression of abd-A in more anterior cardioblasts causes the aorta to assume heart-like features. These observations suggest that the spatially restricted expression and activity of abd-A determine heart identities in cells of the posterior portion of the dorsal vessel. We also show that Abd-B, which at earlier stages is expressed posteriorly to the cardiogenic mesoderm, represses cardiogenesis. In light of the developmental and morphological similarities between the Drosophila dorsal vessel and the primitive heart tube in early vertebrate embryos, these data suggest that Hox genes may also provide important anteroposterior cues during chamber specification in the developing vertebrate heart.     

Localization-dependent and -independent roles of numb contribute to cell-fate specification in Drosophila

During asymmetric cell division, protein determinants are segregated into one of the two daughter cells. The Numb protein acts as a segregating determinant during both mouse and Drosophila development. In flies, Numb localizes asymmetrically and is required for cell-fate specification in the central and peripheral nervous systems, as well as during muscle and heart development. Whether its asymmetric segregation is important to the performance of these functions is not firmly established. Here, we demonstrate that Numb acts both in a localization-dependent and in a localization-independent manner. We have generated numb mutants that affect only the asymmetric localization of the protein during mitosis. We demonstrate that asymmetric segregation of Numb into one of the two daughter cells is absolutely essential for cell-fate specification in the Drosophila peripheral nervous system. Numb localization is also essential in MP2 neuroblasts in the central nervous system and during muscle development. Surprisingly, in dividing ganglion mother cells or during heart development, Numb function is independent of its ability to segregate asymmetrically in mitosis. Our results suggest that two classes of asymmetric cell division exist, each with different requirements for asymmetric inheritance of cell-fate determinants.     

Distribution of CD133 reveals glioma stem cells self-renew through symmetric and asymmetric cell divisions

Malignant gliomas contain a population of self-renewing tumorigenic stem-like cells; however, it remains unclear how these glioma stem cells (GSCs) self-renew or generate cellular diversity at the single-cell level. Asymmetric cell division is a proposed mechanism to maintain cancer stem cells, yet the modes of cell division that GSCs utilize remain undetermined. Here, we used single-cell analyses to evaluate the cell division behavior of GSCs. Lineage-tracing analysis revealed that the majority of GSCs were generated through expansive symmetric cell division and not through asymmetric cell division. The majority of differentiated progeny was generated through symmetric pro-commitment divisions under expansion conditions and in the absence of growth factors, occurred mainly through asymmetric cell divisions. Mitotic pair analysis detected asymmetric CD133 segregation and not any other GSC marker in a fraction of mitoses, some of which were associated with Numb asymmetry. Under growth factor withdrawal conditions, the proportion of asymmetric CD133 divisions increased, congruent with the increase in asymmetric cell divisions observed in the lineage-tracing studies. Using single-cell-based observation, we provide definitive evidence that GSCs are capable of different modes of cell division and that the generation of cellular diversity occurs mainly through symmetric cell division, not through asymmetric cell division.     

When timing is everything: role of cell cycle regulation in asymmetric division

Asymmetric division is a fundamental mechanism of generating cell diversity during development. One of its hallmarks is asymmetric localization during mitosis of proteins that specify daughter cell fate. Studies in Drosophila show that subcellular localization of many proteins required for asymmetric division of neuronal progenitors correlates with progression through mitosis. Yet, how cell cycle and asymmetric division machineries cooperate remains unclear. Recent data show that (1) key cell cycle regulators are required for asymmetric localization of cell fate determinants and for cell fate determination and (2) molecules that mediate asymmetric division can also act to modulate proliferation potential of progenitor cells.     

Asymmetric cell division in the morphogenesis of Drosophila melanogaster macrochaetae

Asymmetric cell division (ACD) is the basic process which creates diversity in the cells of multicellular organisms. As a result of asymmetric cell division, daughter cells acquire the ability to differentiate and specialize in a given direction, which is different from that of their parent cells and from each other. This type of division is observed in a wide range of living organisms from bacteria to vertebrates. It has been shown that the molecular-genetic control mechanism of ACD is evolutionally conservative. The proteins involved in the process of ACD in different kinds of animals have a high degree of homology. Sensory organs--setae (macrochaetae)--of Drosophila are widely used as a model system for studying the genetic control mechanisms of asymmetric division. Setae located in an orderly manner on the head and body of the fly play the role of mechanoreceptors. Each of them consists of four specialized cells--offspring of the only sensory organ precursor cell (SOPC), which differentiates from the imaginal wing disc at the larval stage of the late third age. The basic differentiation and further specialization of the daughter cells of SOPC is an asymmetric division process. In this summary, experimental data on genes and their products controlling asymmetric division of SOPC and daughter cells, and also the specialization of the latter, have been systemized. The basic mechanisms which determine the time cells enter into asymmetric mitosis and which provides the structural characteristics of the asymmetric division process--the polar distribution of protein determinants Numb and Neuralized--the orientation of the mitotic spindle in relation to these determinants, and the uneven segregation of the determinants into the daughter cells that determines the direction of their development have been discussed.