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The FGF receptor Heartless (HTL) is required for mesodermal cell migration in the Drosophila gastrula. We show that mesoderm cells undergo different phases of specific cell shape changes during mesoderm migration. During the migratory phase, the cells adhere to the basal surface of the ectoderm and exhibit extensive protrusive activity. HTL is required for the protrusive activity of the mesoderm cells. Moreover, the early phenotype of htl mutants suggests that HTL is required for the adhesion of mesoderm cells to the ectoderm. In a genetic screen we identified pebble (pbl) as a novel gene required for mesoderm migration. pbl encodes a guanyl nucleotide exchange factor (GEF) for RHO1 and is known as an essential regulator of cytokinesis. We show that the function of PBL in cell migration is independent of the function of PBL in cytokinesis. Although RHO1 acts as a substrate for PBL in cytokinesis, compromising RHO1 function in the mesoderm does not block cell migration. These data suggest that the function of PBL in cell migration might be mediated through a pathway distinct from RHO1. This idea is supported by allele-specific differences in the expressivity of the cytokinesis and cell migration phenotypes of different pbl mutants. We show that PBL is autonomously required in the mesoderm for cell migration. Like HTL, PBL is required for early cell shape changes during mesoderm migration. Expression of a constitutively active form of HTL is unable to rescue the early cellular defects in pbl mutants, suggesting that PBL is required for the ability of HTL to trigger these cell shape changes. These results provide evidence for a novel function of the Rho-GEF PBL in HTL-dependent mesodermal cell migration.

Mutations in the tumor suppressor gene PTEN (MMAC1/TEP1) are associated with a large number of human cancers and several autosomal-dominant disorders. Mice mutant for PTEN die at early embryonic stages and the mutant embryonic fibroblasts display decreased sensitivity to cell death. Overexpression of PTEN in different mammalian tissue culture cells affects various processes including cell proliferation, cell death and cell migration. We have characterized the Drosophila PTEN gene and present evidence that both inactivation and overexpression of PTEN affect cell size, while overexpression of PTEN also inhibits cell cycle progression at early mitosis and promotes cell death during eye development in a context-dependent manner. Furthermore, we have shown that PTEN acts in the insulin signaling pathway and all signals from the insulin receptor can be antagonized by either Drosophila or human PTEN, suggesting a potential means for alleviating symptoms associated with altered insulin signaling.

In Paramecium the development of cell shape and surface pattern during division depends on a precise spatial and temporal pattern of duplication of the ciliary basal bodies which are the organizers of the cortical cytoskeleton. According to their localization, basal bodies will duplicate once, more than once or not all and this duplication is coupled with cell division, as is centrosomal duplication in metazoan cells. We describe here a monogenic nuclear recessive mutation, crochu1 (cro1), resulting in abnormal cell shape and cortical pattern and hypersensitivity to nocodazole. The cytological analysis, by immunofluorescence and electron microscopy, demonstrates that the mutation causes hyper duplication of basal bodies and releases both spatial and temporal control of duplication as basal bodies continue to proliferate in interphase and do so at ectopic locations, beneath the surface and in cortical territories where no duplication occurs in the wild type. However, the abnormal surface organization of cro1 cells does not affect the program of basal body duplication during division. By genetic analysis, no interaction was detected with the sm19 mutation which impairs basal body duplication. In contrast, the cro1 mutation suppresses the nocodazole resistance conferred by nocr1, a mutation in a beta-tubulin gene. This interaction suggests that the primary effect of the mutation bears on microtubule dynamics, whose instability, normally increased during division, would persist throughout the interphase and provide a signal for constitutive basal body duplication.

During Drosophila gastrulation, mesodermal precursors are brought into the interior of the embryo by formation of the ventral furrow. The first steps of ventral furrow formation involve a flattening of the apical surface of the presumptive mesodermal cells and a constriction of their apical diameters. In embryos mutant for folded gastrulation (fog), these cell shape changes occur but the timing and synchrony of the constrictions are abnormal. A similar phenotype is seen in a maternal effect mutant, concertina (cta). fog encodes a putative secreted protein whereas cta encodes an (alpha)-subunit of a heterotrimeric G protein. We have proposed that localized expression of the fog signaling protein induces apical constriction by interacting with a receptor whose downstream cellular effects are mediated by the cta G(alpha)protein.

In order to test this model, we have ectopically expressed fog at the blastoderm stage using an inducible promoter. In addition, we have examined the constitutive activation of cta protein by blocking GTP hydrolysis using both in vitro synthesized mutant alleles and cholera toxin treatment. Activation of the fog/cta pathway by any of these procedures results in ectopic cell shape changes in the gastrula. Uniform fog expression rescues the gastrulation defects of fog null embryos but not cta mutant embryos, arguing that cta functions downstream of fog expression. The normal location of the ventral furrow in embryos with uniformly expressed fog suggests the existence of a fog-independent pathway determining mesoderm-specific cell behaviors and invagination. Epistasis experiments indicate that this pathway requires snail but not twist expression.

Mutations in the HYDRA1 (HYD1) gene of Arabidopsis thaliana can prevent normal morphological development of embryos and seedlings. Three allelic mutants (hydra 1-1, hydra1-2 and hydra1-3) have been identified, and in each the seedling is characterized by having a variable number of cotyledons, a short and wide hypocotyl and a much reduced root system. hydra1 embryos appear to develop normally to the octant stage, but fail to establish a distinct protoderm and lack bilateral symmetry, developing multiple cotyledonary primordia of irregular size and shape. Cells of the embryo proper, but not the suspensor, exhibit abnormalities in size and shape. The hydra1 embryo fails to develop an embryonic root, but embryos and seedlings express molecular markers of apical-basal polarity. Mutant seedlings produce leaves to form a small cabbage-like habit and may occasionally produce sterile flowers, though the mutation is commonly seedling-lethal. hydra1 seedlings exhibit abnormal radial patterning, but nevertheless express at least one molecular marker of vascular cell differentiation. A model is proposed in which the HYDRA1 protein functions as an essential component of the cell expansion system.

Inactivation of the lethal(2)giant larvae (l(2)gl) gene results in malignant transformation of imaginal disc cells and neuroblasts of the larval brain in Drosophila. Subcellular localization of the l(2)gl gene product, P127, and its biochemical characterization have indicated that it participates in the formation of the cytoskeletal network. In this paper, genetic and phenotypic analyses of a temperature-sensitive mutation (l(2)glts3) that behaves as a hypomorphic allele at restrictive temperature are presented. In experimentally overaged larvae obtained by using mutants in the production of ecdysone, the l(2)glts3 mutation displays a tumorous potential. This temperature-sensitive allele of the l(2)gl gene has been used to describe the primary function of the gene before tumor progression. A reduced contribution of both maternal and zygotic activities in l(2)glts3 homozygous mutant embryos blocks embryogenesis at the end of germ-band retraction. The mutant embryos are consequently affected in dorsal closure and head involution and show a hypertrophy of the midgut. These phenotypes are accompanied by an arrest of the cell shape changes normally occurring in lateral epidermis and in epithelial midgut cells. l(2)gl activity is also necessary for larval fife and the critical period falls within the third instar larval stage. Finally, l(2)gl activity is required during oogenesis and mutations in the gene disorganize egg chambers and cause abnormalities in the shape of follicle cells, which are eventually internalized within the egg chamber. These results together with the tumoral phenotype of epithelial imaginal disc cells strongly suggest that the l(2)gl product is required in vivo in different types of epithelial cells to control their shape during development.

A genetic analysis of root development in Arabidopsis thaliana has identified mutants that have abnormal morphogenesis. Four of these root morphogenesis mutants show dramatic alterations in post-embryonic root development. The short-root mutation results in a change from indeterminate to determinate root growth and the loss of internal root cell layers. The cobra and lion's tail mutations cause abnormal root cell expansion which is conditional upon the rate of root growth. Expansion is greatest in the epidermal cells in cobra and in the stele cells in lion's tail. The sabre mutation causes abnormal cell expansion that is greatest in the root cortex cell layer and is independent of the root growth rate. The tissue-specific effects of these mutations were characterized with monoclonal antibodies and a transgenic marker line. Genetic combinations of the four mutants have provided insight into the regulation of growth and cell shape during Arabidopsis root development.