Pattern formation in the absence of cell proliferation: tissue-specific regulation of cell cycle progression by string (stg) during Drosophila eye development.

During Drosophila eye development, the posterior-to-anterior movement of the morphogenetic furrow coordinates cell cycle progression with the early events of pattern formation. The cdc25 phosphatase string (stg) has been proposed to contribute to the synchronization of retinal precursors anterior to the furrow by driving cells in G(2) through mitosis and into a subsequent G(1). Genetic and molecular analysis of Drop (Dr) mutations suggests that they represent novel cis-regulatory alleles of stg that inactivate expression in eye. Retinal precursors anterior to the furrow lacking stg arrest in G(2) and fail to enter mitosis, while cells within the furrow accumulate high levels of cyclins A and B. Although G(2)-arrested cells initiate normal pattern formation, the absence of stg results in retinal patterning defects due to the recruitment of extra photoreceptor cells. These results demonstrate a requirement for stg in cell cycle regulation and cell fate determination during eye development.     

Steroid regulation of programmed cell death during Drosophila development

Steroid hormones play an important role in the regulation of numerous physiological responses, but the mechanisms that enable these systemic signals to trigger specific cell changes remain poorly characterized. Recent studies of Drosophila illustrate several important features of steroid-regulated programmed cell death. A single steroid hormone activates both cell differentiation and cell death in different tissues and at multiple stages during development. While several steroid-regulated genes are required for cell execution, most of these genes function in both cell differentiation and cell death, and require more specific factors to kill cells. Genes that regulate apoptosis during Drosophila embryogenesis are induced by steroids in dying cells later in development. These apoptosis genes likely function downstream of hormone-induced factors to serve a more direct role in the death response. This article reviews the current knowledge of steroid signaling and the regulation of programmed cell death during development of Drosophila.     

Impact of steroid hormone signals on Drosophila cell cycle during development

Metamorphosis of Drosophila involves proliferation, differentiation and death of larval tissues in order to form the adult fly. The major steroid hormone implicated in the larval-pupal transition and adult tissue modelling is ecdysone. Previous reviews have draw together studies connecting ecdysone signaling to the processes of apoptosis and differentiation. Here we discuss those reports connecting the ecdysone pulse to developmentally regulated cell cycle progression.     

Sensillum development in the absence of cell division: the sensillum phenotype of the Drosophila mutant string

We have investigated sensillum development in Drosophila embryos homozygous for mutations in the locus string (stg). In these embryos, cell division is blocked following blastoderm formation. This permits a study of the differentiative fate of undivided precursor cells, in particular those giving rise to the larval sensory organs (sensilla). Of the different cell fates normally represented in the sensilla (i.e., sensory neuron, thecogen cell, trichogen cell, tormogen cell, glia cell), only the phenotype of sensory neurons is expressed morphologically in stg embryos, suggesting that the neuronal fate predominates over the fates of the nonneuronal accessory cells. Consistent with this finding, the P element-lacZ insertion A1-2nd-29, which is a marker for trichogen and tormogen cells in the wild-type embryo, is not expressed in the body wall of the stg embryo. Some sensillum precursor cells appear to express a mixed fate in stg mutants: They express antigens (recognized by the monoclonal antibodies 22C10 and 21A6) which in the wild-type appear in separate cells (sensory neurons and thecogen cell, respectively). The differentiation of undivided cells in stg embryos is not restricted to the peripheral nervous system; in all types of tissues analyzed in this study (e.g., epidermis, intestine, muscle, CNS), precursor cells express characteristics normally exhibited by their progeny.     

Spatial regulation of the gap gene giant during Drosophila development

We describe the regulated expression of the segmentation gene giant (gt) during early embryogenesis. The gt protein is expressed in two broad gradients in precellular embryos, one in anterior regions and the other in posterior regions. Double immunolocalization studies show that the gt patterns overlap with protein gradients specified by the gap genes hunchback (hb) and knirps (kni). Analysis of all known gap mutants, as well as mutations that disrupt each of the maternal organizing centers, indicate that maternal factors are responsible for initiating gt expression, while gap genes participate in the subsequent refinement of the pattern. The maternal morphogen bicoid (bcd) initiates the anterior gt pattern, while nanos (nos) plays a role in the posterior pattern. Gene dosage studies indicate that different thresholds of the bcd gradient might trigger hb and gt expression, resulting in overlapping but noncoincident patterns of expression. We also present evidence that different concentrations of hb protein are instructive in defining the limits of kni and gt expression within the presumptive abdomen. These results suggest that gt is a bona fide gap gene, which acts with hb, Krüppel and kni to initiate striped patterns of gene expression in the early embryo.     

Developmental regulation of the cell cycle.

The control of metazoan cell proliferation, a problem long the domain of cell culture studies, is now being examined in developing animals. Surprisingly, developmental regulation is mediated at a variety of cell-cycle stages. Highly conserved cell-cycle control mechanisms provide a focus for studying the regulatory processes involved.     

Phosphorylation-dependent regulation of septin dynamics during the cell cycle

Septins are GTPases involved in cytokinesis. In yeast, they form a ring at the cleavage site. Using FRAP, we show that septins are mobile within the ring at bud emergence and telophase and are immobile during S, G2, and M phases. Immobilization of the septins is dependent on both Cla4, a PAK-like kinase, and Gin4, a septin-dependent kinase that can phosphorylate the septin Shs1/Sep7. Induction of septin ring dynamics in telophase is triggered by the translocation of Rts1, a kinetochore-associated regulatory subunit of PP2A phosphatase, to the bud neck and correlates with Rts1-dependent dephosphorylation of Shs1. In rts1-Delta cells, the actomyosin ring contracts properly but cytokinesis fails. Together our results implicate septins in a late step of cytokinesis and indicate that proper regulation of septin dynamics, possibly through the control of their phosphorylation state, is required for the completion of cytokinesis.     

The cell cycle and its relation to growth during pattern regulation in wing discs of Drosophila

In this paper we have examined the timing of growth-rate changes, of alterations in the cell cycle, and of the reestablishment of epithelial continuity during the early stages of regulative growth of imaginal wing discs cultured in vivo. Tissue organization was disrupted prior to culture, by dissociation, and centrifugally reaggregated pellets of cells were cultured in adult female flies. Significant increases in cell numbers were detectable during the first day of culture. Flow cytometric analysis of the DNA content of cells in reaggregates during culture indicated that during the first half-day of culture, a significant transient increase in the proportions of S-phase and post-S-phase cells occurred. Scanning electron microscopic examination of dissociated cells during culture confirmed that tissue reorganization began during the first day and was nearly complete by the end of the second day. During the second day of culture, when the growth rate was maximal, the proportions of pre-(G₁) and post-S-phase and mitotic (G₂M) cells resembled those of intact premetamorphic wing discs. In contrast to disrupted tissue, intact or minimally wounded wing discs showed practically no change in cell number during culture. However, with both disrupted and intact tissue, the proportion of G₁ cells increased significantly during the culture period so that by the fourth day of culture G₁ cells predominated. Under our conditions all implants grew extensively, but regeneration by cultured reaggregates formed from pure presumptive notum and pure presumptive distal wing fragments was observed in those implants which grew the most.Our results suggest that both DNA synthesis and transient increased proportions of 4C cells may occur as an early response to injury and that these may precede the onset of cell divisions even under conditions where growth is initiated early. Our observations also suggest that the onset of cell divisions, though it occurs well before the end of the first day of culture, may nonetheless be preceded by cellular contacts and intercellular communication.     

Remodeling of nuclear architecture during the cell cycle in Drosophila embryos

Little is known about what determines the nuclear matrix or how its reorganization is regulated during mitosis. In this study we report on a monoclonal antibody, mAb2A, which identifies a novel nuclear structure in Drosophila embryos which forms a diffuse meshwork at interphase but which undergoes a striking reorganization into a spindle-like structure during pro- and metaphase. Double labelings with α-tubulin and mAb2A antibodies demonstrate that the microtubules of the mitotic apparatus co-localize with this mAb2A labeled structure during metaphase, suggesting it may serve a role in microtubule spindle assembly and/or function during nuclear division. That the mAb2A-labeled nuclear structure is essential for cell division and/or maintenance of nuclear integrity was directly demonstrated by microinjection of mAb2A into early syncytial embryos which resulted in a disintegration of nuclear morphology and perturbation of mitosis. © 1996 Wiley-Liss, Inc.