Localized surface activity of torso, a receptor tyrosine kinase, specifies terminal body pattern in Drosophila

The subdivision of the Drosophila body into distinct terminal and central domains depends on the torso (tor) protein, a putative receptor tyrosine kinase that is active at both ends of the early embryo. We show that the tor protein is uniformly expressed along the surface membrane of early embryos despite its localized activity at both poles. Further, we present evidence that polarized activity of this protein depends on other terminal gene functions, one of which may be a localized extracellular ligand generated during oogenesis. Finally, using the temperature-sensitive gain-of-function mutation torRL3, we show that different levels of active tor protein can specify distinct portions of the terminal pattern. Thus, we argue (1) that for functions as a ubiquitous surface receptor that is activated by a spatially restricted ligand, and (2) that localized activity of the tor kinase may generate one or more gradients of intracellular signals that control body pattern.

Genetic structure of the abd-A gene of Drosophila

We report the embryonic and adult phenotypes of a number of mutations of the abd-A gene of the bithorax complex. Some of them result in loss of abd-A function in the whole abd-A domain and are usually lethal. These probably eliminate or inactivate abd-A protein products. Other mutations affect only part of the abd-A domain. These are viable, appear to map outside the abd-A transcription unit, and presumably alter the normal spatial regulation of abd-A products. We propose a model of abd-A structure based on a protein-coding region and two cis-regulatory regions. Regulatory region 1, 3′ to the transcription unit, contains positive and negative regulatory elements. Regulatory region 2, 5′ to the transcription unit, establishes the correct level of abd-A activity in the abdominal metameres.

A domain containing leucine-zipper-like motifs mediate novel in vivo interactions between the thyroid hormone and retinoic acid receptors

The thyroid hormones and retinoic acid are potent modulators of differentiation, development, and gene expression. The transcriptional activities of these ligands are mediated by closely related nuclear receptors which bind and activate identical hormone responsive DNA elements. We noticed that a region within the ligand binding or E domain is well conserved between receptors for these hormones. This region contains hydrophobic heptad repeats that are structurally similar to the leucine-zipper dimerization domain. To study the function of this conserved domain, we examined the transcriptional responses of thyroid hormone receptor/c-erbA deletion mutants which lacked the heptad repeats. We previously reported that the chick c-erbA-alpha possesses hormone-independent (constitutive) activity in cells which express endogenous rat thyroid hormone receptor. We now demonstrate that this activity is abolished upon deletion of the conserved heptad repeats. This suggests that the heptad repeats mediate in vivo interactions between chick c-erbA and rat thyroid hormone receptors. To further test this hypothesis deletion mutants of chick c-erbA were constructed which contained all eight heptad repeats but which lacked the zinc-finger DNA binding domain. Although these mutants are transcriptionally inactive, they act in a dominant-negative fashion to block trans-activation by both the chick c-erbA-alpha and the endogenous thyroid hormone and retinoic acid receptors. We suggest that the heptad repeats mediate the formation of inactive mutant/wild-type hetero-dimers. Dimer formation suggests a mechanism to account for the dominant-negative phenotypes displayed by nonhormone binding variants of c-erbA, the proto-oncoprotein v-erbA and patients with the generalized thyroid hormone resistance syndrome.

Characterization of the domain structure of chick c-erbA by deletion mutation: in vitro translation and cell transfection studies

Chicken c-erbA (Ck-c-erbA) cDNA (1250 base pairs), a cellular homologue of the avian erythroblastosis virus v-erbA gene, encodes a 408 amino acid protein which binds L-T3 and its analogs with affinities similar to that of endogenous thyroid hormone nuclear receptors. By analogy with steroid receptors, Ck-c-erbA(Met1-Val408) contains an A and B domain (amino acids 1-50); a putative DNA binding C domain (amino acids 51-118); a hydrophilic D domain (amino acids 119-189); and a putative ligand binding E domain (amino acids 187-408). To further characterize the ligand binding region of Ck-c-erbA, two deletion mutations were constructed: Ck-c-erbA(Met120-Val408) which encodes a 289 amino acid protein lacking regions A, B, and C; and Ck-c-erbA-(Met199-Val408) which encodes a 210 amino acid protein lacking regions A, B, C, D, and the first 12 amino acids of the E region. The in vitro translation products ([35S]methionine) of cDNA transcripts of a human placental c-erbA, Ck-c-erbA (Met1-Val408), and Ck-c-erbA(Met120-Val408) efficiently bind L-[125I]T3, whereas Ck-c-erbA(Met199-Val408) does not bind L-[125I]T3. In frame substitution of the last 14 C-terminal amino acids of Ck-c-erbA(Met1-Val408) for the last 7 C-terminal amino acids of v-erbA reduces but does not eliminate L[125I]T3 binding. These results indicate that a broad region of the E domain is important for ligand binding.(ABSTRACT TRUNCATED AT 250 WORDS)

c-erbA protooncogenes mediate thyroid hormone-dependent and independent regulation of the rat growth hormone and prolactin genes

Regulation of gene expression by the thyroid hormones is thought to be mediated by a nuclear-associated receptor found in a wide variety of cells and tissues. Cellular homologues of the avian erythroblastosis virus oncogene, v-erbA, encode proteins which bind thyroid hormone with similar affinities as thyroid hormone receptors. However, it has not been shown that any of the c-erbA proteins can function as receptor and modulate thyroid hormone responsive genes. In this study, using transient expression of chimeric reporter constructs, we document that the chick fibroblast c-erbA-alpha and the human placental c-erbA-beta modulate cis-acting regulatory sequences of two thyroid hormone responsive genes; rat GH and PRL. From these results we conclude: 1) in a receptor deficient cell line (235-1) both c-erbA subtypes act as hormone-dependent modulators of PRL gene expression and hence function as thyroid hormone receptors, 2) in two different receptor containing cell lines (GH4C1 and GH1), both c-erbA proteins act in a hormone-independent fashion to regulate PRL and GH expression. This suggests that events other than ligand binding can result in formation of a c-erbA protein that modulates transcription of thyroid hormone responsive genes, 3) no qualitative functional differences were detected between alpha- and beta-c-erbA subtypes, and 4) depending on the cell-type, L-T3 acts through its endogenous receptor to stimulate (GH4C1) or suppress (GH1) expression of a chimeric PRL construct. In these cells, c-erbA expression results in the same positive or negative response as the endogenous receptor except that the response occurs in the absence of hormone. These results suggest that the endogenous receptor and the c-erbAs act by augmenting the effect of transcription factors which can positively or negatively control gene expression.

Rat growth hormone gene expression. Both cell-specific and thyroid hormone response elements are required for thyroid hormone regulation

The elements involved in mediating cell-specific and thyroid hormone stimulation of rat growth hormone gene expression have been defined by transfection studies and by nuclease footprinting. 5'-Flanking DNA extending to -104 can mediate cell-specific expression, and this is enhanced 3- to 4-fold with DNA extending to -145. Cell-specific factors, found only in rat growth hormone producing cells, bind within the -137/-107 and -95/-65 regions, and competition studies suggest that the same factor binds to both sites. The sequence A (A or T) TAAAT is found at the center of both footprints at -80 and -122, suggesting that it is a core component of the recognition sequence of the cell-specific factor. Disruption of the spatial and/or distance relationships between the two regions eliminates the enhanced level of cell-specific expression, suggesting a cooperative interaction of the proteins which bind to these elements. Sequences located between -208 and -178 can confer thyroid hormone-regulated expression when linked in either orientation in close proximity to one or both cell-specific elements. The thyroid hormone and cell-specific elements function as an enhancer-like unit and are both required to confer regulated expression to heterologous promoters. We propose that thyroid hormone acts via its receptor to enhance the function of the cell-specific element by forming a more "active" transcription complex which stimulates the level of gene expression.

Analysis of photoaffinity label derivatives to probe thyroid hormone receptor in human fibroblasts, GH1 cells, and soluble receptor preparations

The regulation of growth hormone gene expression by thyroid hormone in cultured GH1 cells is mediated by a chromatin-associated receptor. We have previously described a photoaffinity label derivative of 3,5,3'-triiodo-L-thyronine (L-T3) in which the alanine side chain was modified to form N-2-diazo-3,3,3-trifluoropropionyl-L-T3 (L-[125I]T3-PAL). On exposure to 254 nm UV light, L-[125I]T3-PAL generates a carbene which covalently modifies two thyroid hormone receptor forms in intact GH1 cells; an abundant 47,000 Mr species and a less abundant 57,000 Mr form. We have now synthesized similar photoaffinity label derivatives of 3,5,3',5'-tetraiodo-L-thyronine (L-T4) and 3,3',5'-triiodo-L-thyronine (L-rT3). Both compounds identify the same receptor forms in intact cells and in nuclear extracts in vitro as L-[125I]T3-PAL. Labeling by L-[125I]rT3-PAL was low and consistent with the very low occupancy of receptor by L-rT3. Underivatized L-[125I]T3 and L-[125I]T4 labeled the same receptor forms at 254 nm but at a markedly lower efficiency than their PAL derivatives. In contrast, N-bromoacetyl-L-[125I]T3, a chemical affinity labeling agent, did not derivatize either receptor form in vitro. The relative efficiency of coupling to receptor at 254 nm was L-[125I]T4-PAL greater than L-[125I]T3-PAL greater than L-[125I]T4 greater than L-[125I]T3. Although L-[125I]T4-PAL has a lower affinity for receptor than L-[125I]T3-PAL, its coupling efficiency was 5-10-fold higher. This suggests that the alanine side chain of L-[125I]T4-PAL is positioned in the ligand binding region near a residue which is efficiently modified by photoactivation. With L-[125I]T4-PAL we were able to identify three different molecular weight receptor species in human fibroblast nuclei.

Developmental analysis of a hybrid gene composed of parts of the Ubx and abd-A genes of Drosophila

C1 is a mutation in the bithorax complex (BX-C) of Drosophila resulting from the deletion of parts of the Ubx and abd-A genes. We show that the ;hybrid' gene formed by the fusion of the remaining parts of Ubx and abd-A (5'abd-A/Ubx3') is functional and developmentally active. It specifies parasegment patterns with a mixture of thoracic and abdominal identities. The hybrid gene also has other properties typical of conventional bithorax genes: it can be spatially derepressed in the absence of trans-acting genes like extra Sex combs or Polycomb and in turn represses other homeotics like Sex combs reduced. The comparison of embryos containing exclusively hybrid gene activity with others having no BX-C function indicates that the hybrid gene is active in the body region defined by PS5 to PS14. The expression in PS5 and PS6 suggests that one control region (abx) of Ubx can regulate the transcription of the abd-A promoter.

Aspartoacylase deficiency and N-acetylaspartic aciduria in patients with Canavan disease

An increased amount of N-acetylaspartic acid was found in urine and plasma of three patients, from two families, with the diagnosis of cerebral spongy degeneration (Canavan disease). Aspartoacylase was assayed in cultured skin fibroblasts from one patient of each family and a profound deficiency of this enzyme was found. Although the function of N-acetylaspartic acid is not understood, it is known to occur in high concentration in human brain. The finding of a defect in the metabolism of N-acetylaspartic acid causing progressive spongy degeneration of the brain may lead to a better understanding of the function of this amino acid derivative. The aspartoacylase assay affords a new tool for determining the diagnosis of Canavan disease. Since aspartoacylase activity was present in cultured amniotic cells and chorionic villi, it is likely that the assay for this enzyme can be used for the prenatal diagnosis of Canavan disease.

The effects of maitotoxin on phosphoinositides and calcium metabolism in a primary culture of aortic smooth muscle cells

Maitotoxin, a potent marine toxin isolated from toxic tropical dinoflagellates and poisonous fishes induces contraction of different smooth muscle preparations. Actions of maitotoxin on phosphoinositides and calcium metabolism were studied using a primary culture of aortic smooth muscle cells. Maitotoxin induced a very large increase of cytosolic calcium concentration as evaluated by fura-2 acetoxymethyl ester fluorescence. This increase was concomitant with stimulation of inositol-phosphate accumulation and loss of viability of aortic smooth muscle cells. These responses to maitotoxin were abolished in Ca2+-free medium, and were mimicked by saponin. Calcium ionophores or K+ depolarisation did not induce inositol-phosphate formation. These results suggest that maitotoxin acts by altering smooth muscle cells permeability allowing a sustained calcium influx which is able to activate inositol-phosphate formation and which is lethal for the cells.

Double and triple mutant combinations of bithorax complex of Drosophila

We have constructed double and triple mutant combinations for the Ubx, abd-A and Abd-B genes of the bithorax complex and have examined the homeotic transformations they produce in the larval and adult patterns. Embryos hemizygous for the triple combination exhibit a metameric pattern consisting of parasegments 5-12 being transformed into parasegment 4. In addition, parasegment 13 develops like a mixture of parasegment 3 and 4, and parasegment 14 is abnormal. The same phenotype is displayed by embryos homozygous for DfP9, lacking all the BX-C DNA, >300 kb. This result strongly supports the notion that the BX-C contains only three genes which account for all the developmental functions of the complex. The phenotypes of the different double combinations also support the same view; the Ubx abd-a comthoracic and several abdominal functions. The abd-A Abd-B combination exhibits the same phenotype of DpP10 DfP9, lacking all the abdominal functions except those specific for A1. Our results also indicate that each BX-C gene becomes active autonomously regardless of the presence or functional state of the other BX-C genes.

Trans-regulatory functions in the Abdominal-B gene of the bithorax complex

We have investigated the functional organization of the Abdominal-B gene in the bithorax complex using the expression of the Ultrabithorax gene as an assay for Abdominal-B trans-regulatory functions. Using Polycomb mutants to relax the normal spatial control of Ultrabithorax expression, we have examined the effects of Abdominal-B mutations on the expression of Ultrabithorax protein products in parasegment 14. The results support the hypothesis that the Abdominal-B gene contains two trans-regulatory functions: the m element active in parasegments 10–13 and the r element acting exclusively in parasegment 14.

cis-acting elements of the rat growth hormone gene which mediate basal and regulated expression by thyroid hormone

In GC cells, a growth hormone-producing rat pituitary cell line, 3,5,3'-triiodo-L-thyronine (L-T3) rapidly stimulates the transcription rate of the growth hormone gene which parallels the level of chromatin-associated L-T3-receptor complexes (Yaffe, B. M., and Samuels, H. H. (1984) J. Biol. Chem. 259, 6284-6291). In this study we have functionally mapped the elements of the gene which are involved in mediating basal and hormone-regulated expression. Stable transformation studies indicate that transcriptional regulation of the gene by L-T3 is mediated by sequences in the 5'-flanking region. Transient expression studies were performed using a series of chimeric plasmids in which 5'-flanking DNA was ligated to the chloramphenicol acetyltransferase gene. Transient expression occurred only in cells which expressed the endogenous growth hormone gene. Sequences between -104 and +7 were found to be essential for basal expression. One of the most highly conserved regions (-105 to -145) contains elements which further enhance the level of basal expression but are not necessary for regulated expression by L-T3. DNA between -210 and -181 was found to be essential for stimulation by L-T3 and was shown to function most efficiently with the homologous rat growth hormone promoter (-104 to +7). Sequences from -206 to -198 show about 80% homology with a sequence in the 5'-flanking region of two other rat genes which are regulated by thyroid hormone. Glucocorticoid hormones, which also transcriptionally stimulate the rat growth hormone gene, elicited only minimal effects in both stable and transient expression studies. This suggests that the elements which mediate glucocorticoid regulation of the endogenous gene are found either upstream of the cloned 5'-flanking region (1800 base pairs) or 3' of the cap site.

ATP stimulates inositol phosphates accumulation and calcium mobilization in a primary culture of rat aortic myocytes

The effects of extracellular ATP on phosphoinositide metabolism and intracellular Ca2+ concentration were studied in a primary culture of rat aortic myocytes. ATP increases the level of inositol phosphates, the putative second messenger for Ca2+ mobilization. No saturation of inositol phosphates accumulation is obtained (up to 10(-2) M ATP). Under the same conditions, ATP rapidly mobilizes intracellular Ca2+ in fura-2 loaded myocytes. The mobilization of intracellular Ca2+ is dose-dependent (maximal at 10(-4) M ATP), and is not affected by addition of EGTA. It is concluded that the receptors mediating the cytosolic increase of Ca2+ are of the P2-purinoceptor subtype. The physiological functions of these receptors are not presently known.

Other genes of the Y chromosome

From the moment that the major part of the mammalian Y chromosome ceased to recombine with the X, the action of Muller's ratchet began to whittle away at it to remove all but the essential genes. Consequently, by comparison with their respective X homologues, both human and mouse Y chromosomes are relatively small and probably contain very few genes in a fabric of accumulated junk. Nevertheless, molecular biologists have not been deterred from searching for Y-linked genes and in recent years this has become an increasingly popular pastime. Although hard to find, any Y-linked genes are likely to play important roles in either sex determination or male fertility, a fact which has spurred the search.

How many genes are likely to be present on the chromosome? If we accept the hypothesis that most genes are preceded by an HpaII tiny fragment (HTF) island, we can place an upper limit on the number of genes by considering the frequency with which such islands occur on the chromosome.

Identification and characterization of a parasegment specific regulatory element of the abdominal-B gene of Drosophila

We have characterized mutations of the Abdominal-B gene of the bithorax complex of Drosophila. We conclude that the gene contains two distinct genetic elements: one has a morphogenetic role and acts in parasegments 10, 11, 12, and 13, while the other acts on parasegment 14 and has primarily or exclusively a regulatory function. Evidence indicates that the latter suppresses the activity of the morphogenetic element of Abd-B and of other genes responsible for the development of sclerotic plates. The regulatory element also suppresses those BX-C genes and other homeotics that, in the absence of Polycomb or extra sex combs function, can become active in parasegment 14.

Contrabithorax and the control of spatial expression of the bithorax complex genes of Drosophila

Cbx1 is a dominant mutation of the bithorax complex (BX-C) of Drosophila partially transforming the second thoracic (T2) segment towards the third one (T3). Molecular analysis has shown that Cbx1 arose from a transposition within the BX-C of a DNA fragment of 17 kb containing pbx+ inserted into the Ubx area. In addition to the dominant phenotype, the Cbx1 mutation produces a set of recessive homeotic transformations that we show are characteristic of the Ubx mutations. We present evidence that the dominant and the recessive transformations arise from different mechanisms and suggest the dominant transformation is caused by an alteration of the normal regulatory role of pbx+ resulting in an adventitious expression of some Ubx+ products in T2, while the Ubx phenotype is caused by the breakpoint of the insertion.

5'-Flanking DNA of the rat growth hormone gene mediates regulated expression by thyroid hormone

Thyroid hormone has been shown to rapidly stimulate the rate of rat growth hormone gene transcription which parallels the kinetics of binding of 3,5,3'-triiodo-L-thyronine (L-T3) to its nuclear receptor (Yaffe, B. M., and Samuels, H. H. (1984) J. Biol. Chem. 259, 6284-6291). We have constructed a chimeric gene to explore whether the 5'-flanking region of the rat growth hormone gene contains a DNA element which could mediate thyroid hormone control of growth hormone gene expression. The construct consists of 1.8 kilobase pairs of the 5'-flanking region extending 11 nucleotides downstream from the transcription initiation (cap) site ligated to Escherichia coli DNA containing the structural gene for the enzyme xanthine-guanine phosphoribosyltransferase. GC cells, a growth hormone producing rat pituitary cell line, were transfected with this chimeric gene and stable transformants in which the enzyme is regulated by L-T3 were isolated by positive selection using mycophenolic acid and xanthine. These stable transformants develop with relatively high frequency and show marked L-T3 stimulation of xanthine-guanine phosphoribosyltransferase mRNA which is initiated at the cap site of the growth hormone gene. This study provides the first evidence that the 5'-flanking region of the rat growth hormone gene contains a DNA regulatory element which can mediate control of gene expression by thyroid hormone.