Notch signalling coordinates tissue growth and wing fate specification in Drosophila

During the development of a given organ, tissue growth and fate specification are simultaneously controlled by the activity of a discrete number of signalling molecules. Here, we report that these two processes are extraordinarily coordinated in the Drosophila wing primordium, which extensively proliferates during larval development to give rise to the dorsal thoracic body wall and the adult wing. The developmental decision between wing and body wall is defined by the opposing activities of two secreted signalling molecules, Wingless and the EGF receptor ligand Vein. Notch signalling is involved in the determination of a variety of cell fates, including growth and cell survival. We present evidence that growth of the wing primordium mediated by the activity of Notch is required for wing fate specification. Our data indicate that tissue size modulates the activity range of the signalling molecules Wingless and Vein. These results highlight a crucial role of Notch in linking proliferation and fate specification in the developing wing primordium.     

An early requirement for FGF signalling in the acquisition of neural cell fate in the chick embryo

BACKGROUND: In Xenopus embryos, fibroblast growth factors (FGFs) and secreted inhibitors of bone morphogenetic protein (BMP)-mediated signalling have been implicated in neural induction. The precise roles, if any, that these factors play in neural induction in amniotes remains to be established. RESULTS: To monitor the initial steps of neural induction in the chick embryo, we developed an in vitro assay of neural differentiation in epiblast cells. Using this assay, we found evidence that neural cell fate is specified in utero, before the generation of the primitive streak or Hensen's node. Early epiblast cells expressed both Bmp4 and Bmp7, but the expression of both genes was downregulated as cells acquired neural fate. During prestreak and gastrula stages, exposure of epiblast cells to BMP4 activity in vitro was sufficient to block the acquisition of neural fate and to promote the generation of epidermal cells. Fgf3 was also found to be expressed in the early epiblast, and ongoing FGF signalling in epiblast cells was required for acquisition of neural fate and for the suppression of Bmp4 and Bmp7 expression. CONCLUSIONS: The onset of neural differentiation in the chick embryo occurs in utero, before the generation of Hensen's node. Fgf3, Bmp4 and Bmp7 are each expressed in prospective neural cells, and FGF signalling appears to be required for the repression of Bmp expression and for the acquisition of neural fate. Subsequent exposure of epiblast cells to BMPs, however, can prevent the generation of neural tissue and induce cells of epidermal character.     

Modifiers of muscle and heart cell fate specification identified by gain-of-function screen in Drosophila

The homeobox genes ladybird in Drosophila and their vertebrate counterparts Lbx1 genes display restricted expression patterns in a subset of muscle precursors and are both implicated in diversification of muscle cell fates. In order to gain new insights into mechanisms controlling conserved aspects of cell fate specification, we have performed a gain-of-function (GOF) screen for modifiers of the mesodermal expression of ladybird genes using a collection of EP element carrying Drosophila lines. Amongst the identified genes, several have been previously implicated in cell fate specification processes, thus validating the strategy of our screen. Observed GOF phenotypes have led us to identification of an important number of candidate genes, whose myogenic and/or cardiogenic functions remain to be investigated. Amongst them, the EP insertions close to rhomboid, yan and rac2 suggest new roles for these genes in diversification of muscle and/or heart cell lineages. The analysis of loss and GOF of rhomboid and yan reveals their new roles in specification of ladybird-expressing precursors of adult muscles (LaPs) and ladybird/tinman-positive pericardial cells. Observed phenotypes strongly suggest that rhomboid and yan act at the level of progenitor and founder cells and contribute to the diversification of mesodermal fates. Our analysis of rac2 phenotypes clearly demonstrates that the altered mesodermal level of Rho-GTPase Rac2 can influence specification of a number of cardiac and muscular cell types including those expressing ladybird. Finding that in rac2 mutants ladybird and even skipped-positive muscle founders are overproduced, indicate a new early function for this gene during segregation of muscle progenitors and/or specification of founder cells. Intriguingly, rhomboid, yan and rac2 act as conserved components of Receptor Tyrosine Kinases (RTKs) signalling pathways, suggesting that RTK signalling constitutes a part of a conserved regulatory network governing diversification of muscle and heart cell types.     

Multiple signalling pathways establish cell fate and cell number in Drosophila malpighian tubules

A unique cell, the tip mother cell, arises in the primordium of each Drosophila Malpighian tubule by lateral inhibition within a cluster of achaete-expressing cells. This cell maintains achaete expression and divides to produce daughters of equivalent potential, of which only one, the tip cell, adopts the primary fate and continues to express achaete, while in the other, the sibling cell, achaete expression is lost (M. Hoch et al., 1994, Development 120, 3439-3450). In this paper we chart the mechanisms by which achaete expression is differentially maintained in the tip cell lineage to stabilise cell fate. First, wingless is required to maintain the expression of achaete in the tubule primordium so that wingless mutants lack tip cells. Conversely, increasing wingless expression results in the persistence of achaete expression in the cell cluster. Second, Notch signalling is restricted by the asymmetric segregation of Numb, as the tip mother cell divides, so that achaete expression is maintained only in the tip cell. In embryos mutant for Notch tip cells segregate at the expense of sibling cells, whereas in numb neither daughter cell adopts the tip cell fate resulting in tubules with two sibling cells. Conversely, when numb is overexpressed two tip cells segregate and tubules have no sibling cells. Analysis of cell proliferation in the developing tubules of embryos lacking Wingless after the critical period for tip cell allocation reveals an additional requirement for wingless for the promotion of cell division. In contrast, alteration in the expression of numb has no effect on the final tubule cell number.     

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.     

Control of cell fate choices by lateral signaling in the adult peripheral nervous system of Drosophila melanogaster

The thoracic integument of the adult fruit fly is a relatively simple but highly patterned structure. It is composed of sensory organ cells distributed within a monolayer of epidermal cells. Both cell types are easily detected at the cuticular surface, as each external sense organ forms a sensory bristle and each epidermal cell secretes a small nonsensory hair. Inhibitory cell—cell interactions play a key role in regulating the distribution as well as the formation of the sense organs. This review focuses on the role of these cell—cell interactions in the adoption of alternative cell fates. We also show that Notch, Hairless, and Suppressor of Hairless, three components of this intercellular signaling pathway, exhibit dose-dependent genetic interactions. Finally we address how this intercellular signaling mechanism may be modulated to result in highly reproducible outcomes. © 1996 Wiley-Liss, Inc.     

Olig bHLH proteins interact with homeodomain proteins to regulate cell fate acquisition in progenitors of the ventral neural tube

BACKGROUND: Organizing signals such as Sonic hedgehog are thought to specify neuronal subtype identity by regulating the expression of homeodomain proteins in progenitors of the embryonic neural tube. One of these, Nkx2.2, is necessary and sufficient for the development of V3 interneurons. RESULTS: We report that Olig genes, encoding basic helix-loop-helix (bHLH) proteins, are expressed in a subset of Nkx2.2 progenitors before the establishment of interneurons and oligodendroglial precursors. Gain-of-function analysis in transgenic mouse embryos indicates that Olig genes specifically inhibit the establishment of Sim1-expressing V3 interneurons. Moreover, coexpression of Olig2 with Nkx2.2 in the chick neural tube generated cells expressing Sox10, a marker of oligodendroglial precursors. Colocalization of Olig and Nkx2.2 proteins at the dorsal extent of the Nkx2.2 expression domain is consistent with regulatory interactions that define the potential of progenitor cells in the border region. CONCLUSIONS: Interactions between homeodomain and Olig bHLH proteins evidently regulate neural cell fate acquisition and diversification in the ventral neural tube. In particular, interactions between Olig and Nkx2.2 proteins inhibit V3 interneuron development and promote the formation of alternate cell types, including those expressing Sox10.     

Transformation of peripheral inputs by the first-order lateral line brainstem nucleus

Extracellular recording techniques were used to record the responses of medial nucleus cells and posterior lateral line nerve fibers in mottled sculpin, Cottus bairdi, and goldfish, Carassius auratus, to a 50-Hz dipole source (vibrating sphere). Responses were characterized in terms of (1) receptive fields that relate responsiveness (spike rate and phase-locking) to the location of the source along the length of the fish, (2) input-output functions that relate responsiveness to vibration amplitude for a fixed source location, and (3) peri-stimulus time histograms that relate responsiveness to time during a sustained period of vibration. Relative to posterior lateral line nerve fibers, medial nucleus cells in both species were similar in showing (1) lower spontaneous and evoked rates of spike activity, (2) greater degrees of adaptation, (3) greater heterogeneity in all response characteristics, and (4) evidence for inhibitory/excitatory interactions. Whereas receptive fields of nerve fibers in both species faithfully reflect both pressure gradient amplitudes (with rate changes) and directions (with phase-angle changes) in the stimulus field, receptive fields of medial nucleus were more difficult to relate to the stimulus field. Some, but not all, receptive fields could be modeled with excitatory center/inhibitory surround and inhibitory center/excitatory surround organizations. Accepted: 26 November 1997     

Control of Drosophila eye specification by Wingless signalling

Organ formation requires early specification of the groups of cells that will give rise to specific structures. The Wingless protein plays an important part in this regional specification of imaginal structures in Drosophila, including defining the region of the eye-antennal disc that will become retina. We show that Wingless signalling establishes the border between the retina and adjacent head structures by inhibiting the expression of the eye specification genes eyes absent, sine oculis and dachshund. Ectopic Wingless signalling leads to the repression of these genes and the loss of eyes, whereas loss of Wingless signalling has the opposite effects. Wingless expression in the anterior of wild-type discs is complementary to that of these eye specification genes. Contrary to previous reports, we find that under conditions of excess Wingless signalling, eye tissue is transformed not only into head cuticle but also into a variety of inappropriate structures.     

Milton controls the early acquisition of mitochondria by Drosophila oocytes

Mitochondria in many species enter the young oocyte en mass from interconnected germ cells to generate the large aggregate known as the Balbiani body. Organelles and germ plasm components frequently associate with this structure. Balbiani body mitochondria are thought to populate the germ line, ensuring that their genomes will be inherited preferentially. We find that milton, a gene whose product was previously shown to associate with Kinesin and to mediate axonal transport of mitochondria, is needed to form a normal Balbiani body. In addition, germ cells mutant for some milton or Kinesin heavy chain (Khc) alleles transport mitochondria to the oocyte prematurely and excessively, without disturbing Balbiani body-associated components. Our observations show that the oocyte acquires the majority of its mitochondria by competitive bidirectional transport along microtubules mediated by the Milton adaptor. These experiments provide a molecular explanation for Balbiani body formation and, surprisingly, show that viable fertile offspring can be obtained from eggs in which the normal program of mitochondrial acquisition has been severely perturbed.     

The acquisition of neural fate in the chick

Neural development in the chick embryo is now understood in great detail on a cellular and a molecular level. It begins already before gastrulation, when a separation of neural and epidermal cell fates occurs under the control of FGF and BMP/Wnt signalling, respectively. This early specification becomes further refined around the tip of the primitive streak, until finally the anterior-posterior level of the neuroectoderm becomes established through progressive caudalization. In this review we focus on processes in the chick embryo and put classical and more recent molecular data into a coherent scenario.