During heart development, the onset of heartbeat and blood flow coincides with a ballooning of the cardiac chambers. Here, we have used the zebrafish as a vertebrate model to characterize chamber ballooning morphogenesis of the endocardium, a specialized population of endothelial cells that line the interior of the heart. By combining functional manipulations, fate mapping studies, and high-resolution imaging, we show that endocardial growth occurs without an influx of external cells. Instead, endocardial cell proliferation is regulated, both by blood flow and by Bmp signaling, in a manner independent of vascular endothelial growth factor (VEGF) signaling. Similar to myocardial cells, endocardial cells obtain distinct chamber-specific and inner- versus outer-curvature-specific surface area sizes. We find that the hemodynamic-sensitive transcription factor Klf2a is involved in regulating endocardial cell morphology. These findings establish the endocardium as the flow-sensitive tissue in the heart with a key role in adapting chamber growth in response to the mechanical stimulus of blood flow.

Signaling by Nodal and Bmp is essential for cardiac laterality. How activities of these pathways translate into left-right asymmetric organ morphogenesis is largely unknown. We show that, in zebrafish, Nodal locally reduces Bmp activity on the left side of the cardiac field. This effect is mediated by the extracellular matrix enzyme Hyaluronan synthase 2, expression of which is induced by Nodal. Unilateral reduction of Bmp signaling results in lower expression of nonmuscle myosin II and higher cell motility on the left, driving asymmetric displacement of the entire cardiac field. In silico modeling shows that left-right differences in cell motility are sufficient to induce a robust, directional migration of cardiac tissue. Thus, the mechanism underlying the formation of cardiac left-right asymmetry involves Nodal modulating an antimotogenic Bmp activity.

Rationale The importance for Bmp signaling during embryonic stem cell differentiation into myocardial cells has been recognized. The question when and where Bmp signaling in vivo regulates myocardial differentiation has remained largely unanswered. Objective To identify when and where Bmp signaling regulates cardiogenic differentiation. Methods and Results Here we have observed that in zebrafish embryos, Bmp signaling is active in cardiac progenitor cells prior to their differentiation into cardiomyocytes. Bmp signaling is continuously required during somitogenesis within the anterior lateral plate mesoderm to induce myocardial differentiation. Surprisingly, Bmp signaling is actively repressed in differentiating myocardial cells. We identified the inhibitory Smad6a, which is expressed in the cardiac tissue, to be required to inhibit Bmp signaling and thereby promote expansion of the ventricular myocardium. Conclusion Bmp signaling exerts opposing effects on myocardial differentiation in the embryo by promoting as well as inhibiting cardiac growth.

Abstract. Mammary gland development is controlled by systemic hormones and by growth factors that might complement or mediate hormonal action. Peptides that locally signal growth cessation and stimulate differentiation of the developing epithelium have not been described. Here, we report that recombinant and wild-type forms of mammary-derived growth inhibitor (MDGI) and heart-fatty acid binding protein (FABP), which belong to the FABP family, specifically inhibit growth of normal mouse mammary epithelial cells (MEC), while growth of stromal cells is not suppressed. In mammary gland organ culture, inhibition of ductal growth is associated with the appearance of bulbous alveolar end buds and formation of fully developed lobuloalveolar structures. In parallel, MDGI stimulates its own expression and promotes milk protein synthesis. Selective inhibition of endogenous MDGI expression in MEC by antisense phosphorothioate oligonucleotides suppresses appearance of alveolar end buds and lowers the ~-casein level in organ cultures. Furthermore, MDGI suppresses the mitogenic effects of epidermal growth factor, and epidermal growth factor antagonizes the activities of MDGI. Finally, the regulatory properties of MDGI can be fully mimicked by an l 1-amino acid sequence, represented in the COOH terminus of MDGI and a subfamily of structurally related FABPs. This peptide does not bind fatty acids. To our knowledge, this is the first report about a growth inhibitor promoting mammary gland differentiation.ROWTH development, and differentiation of epithelial tissues are multistage processes that are driven by a combination of paracrine and autocrine signaling factors and interactions of a cell with its extracellular matrix (reviewed in 51, 60, 62). In the mammary gland, these complex interactions are regulated by various steroid and peptide hormones (29,32,46,60). Development of the mouse mammary gland at puberty is characterized by sparsely branching ducts which invade the stroma, followed by the development of lobuloalveolar structures and functional differentiation, i.e., synthesis of milk constituents at pregnancy (1,17,32). By use of endocrine ablation (44), organ culture systems (1,35,54,65) and mammary cells (MEC) ~ growing on a

The arthropod epidermis is an epithelium that deposits the apical cuticle, which is a stratified extracellular matrix (ECM) protecting the animal against pathogens, preventing dehydration and also serving as an exoskeleton. Differentiation of the cuticle conceivably implies coordinated production, secretion and localization of its components. The underlying molecular mechanisms are poorly explored. In this work, we show that the transcription factor Grainy head and the steroid hormone ecdysone drive the production of two partially overlapping sets of cuticle factors. Nevertheless, Grainy head is needed to modulate the expression of ecdysone signalling factors; the significance of this cross-talk is yet unclear. In addition, we found that ecdysone signalling negatively regulates its own impact. In conclusion, our findings suggest that at least two independently triggered pathways have evolved in parallel to cooperatively ensure the stereotypic implementation of the cuticle. As Grainy head is also essential for epithelial differentiation in vertebrates, we speculate that it acts to decode the ancient skin programme common to all animals. Full differentiation of the skin necessitates a second, complementing taxon-specific programme that requires its own decoder, which is represented by ecdysone in arthropods, whereas the vertebrate specific one remains to be identified.

SUMMARYEndodermal organogenesis requires a precise orchestration of cell fate specification and cell movements, collectively coordinating organ size and shape. In Caenorhabditis elegans, uncoordinated-53 (unc-53) encodes a neural guidance molecule that directs axonal growth. One of the vertebrate homologs of unc-53 is neuron navigator 3 (Nav3). Here, we identified a novel vertebrate neuron navigator 3 isoform in zebrafish, nav3a, and we provide genetic evidence in loss-and gain-of-function experiments showing its functional role in endodermal organogenesis during zebrafish embryogenesis. In zebrafish embryos, nav3a expression was initiated at 22 hpf in the gut endoderm and at 40 hpf expanded to the newly formed liver bud. Endodermal nav3a expression was controlled by Wnt2bb signaling and was independent of FGF and BMP signaling. Morpholino-mediated knockdown of nav3a resulted in a significantly reduced liver size, and impaired development of pancreas and swim bladder. In vivo time-lapse imaging of liver development in nav3a morphants revealed a failure of hepatoblast movement out from the gut endoderm during the liver budding stage, with hepatoblasts being retained in the intestinal endoderm. In hepatocytes in vitro, nav3a acts as a positive modulator of actin assembly in lamellipodia and filipodia extensions, allowing cellular movement. Knockdown of nav3a in vitro impeded hepatocyte movement. Endodermal-specific overexpression of nav3a in vivo resulted in additional ectopic endodermal budding beyond the normal liver and pancreatic budding sites. We conclude that nav3a is required for directing endodermal organogenesis involving coordination of endodermal cell behavior.

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The three major isoforms of AMPdeaminase ( M a ) were localized in human skeletal m d e and cultured muscle cells by immunocytochemistry. The M isoform was mainly located in Type II muscle fibers and showed a clear cross-striation.Particularly strong staining was present at the neuromuscular junction. Capillaries were also immunoreactive. The L koform was predomhmtly observed in nerve bundles and to a minor extent in smooth muscle cells and endothelial cells. The E isoform was predominantly present in smooth muscle cells, and to a lesser extent in Type I muscle fibers and nerve bundles. In quadriceps musde of patients with myoadenylate deaminase deficienq, no immMOst?ining for eroduction Adenosine monophosphate deaminase (AMPda; EC 3.5.4.6.) catalyzes the irreversible hydrolytic deamination of AMP to IMP and ammonia. AMPda plays a role in purine nucleotide interconversion and is one of the three enzymes involved in the purine nudeotide cycle (Van den Berghe et al., 1992; Tullson and Terjung, 1991; henstein, 1972, 1990.In muscle, this cycle is involved in (a) the maintenance of the high ATPADP ratio by pulling the myokinase reaction (2 ADP ATP + AMP) towards ATP by AMP removal, (b) limiting the degradation of adenine nucleotides to purine bases, (c) replenishment of citric acid cycle intermediates leading to an enhancement of aerobic energy production, and (d) deamination of amino acids for oxidative metabolism. Two other proposed functions, regulation of phosphofructokinase activity by NH4' concentration and regulation of phosphorylase b activity by the IMP concentration, are less likely (Van den Berghe et al., 1992; Tullson and Terjung, 1991).In human, at least four isofonns of the tetrameric AMPda exist:Correspondence ta T. the M (muscle), the L (liver), and the El and E2 (erythrocyte) isoforrns (Ogasawara et al., 1982). The M, L, and E isoforms differ from each other in kinetic, regulatory, immunological, and chromatographic properties (Ogasawara et al., 1982). In rat these isozymes are named A, B, and C isoforms, respectively (Thompson et al., 1992).AMPda is a multigene family (Morisaki et al., 1990). The M isoform, the predominant form in skeletal muscle, is encoded by the AMPDl gene , the L isoform by the AMPD2 gene (Bausch-Jurken et al., 1992) and the El and E2 isoforms, presumably through altemative splicing, by the AMPD3 gene (Mahnke-Zizelman and Sabina, 1992). Altemative splicing in the AMPDl and AMPD2 genes predicts multiple forms for the M and L isoforms as well (Morisaki et al., 1993;Van den Bergh and Sabina, 1993).Biochemical analysis of AMPda isoforms indicates a restricted distribution in human tissues (Ogasawara et al., 1982). Recently, the distribution of the major isoforms of AMPda has been documented for rat skeletal muscle (Thompson et al., 1992). We report here on the disuibution of AMPda isofonns in human skeletal muscle and cultured muscle cells. We found a salient occurrence of the M isoform in the postsynaptic part of the neuromuscular junction. Materials and MethodsSkeletal Muscle ...