The NK-2 homeobox gene and the early development of the central nervous system of Drosophila

The alternative protein isoform NK2B, encoded by the vnd/NK-2 proneural gene, directly activates transcription and is expressed following the start of cells differentiation

NK-2 is a homeodomain protein essential for the development of the central nervous system in the Drosophila embryo. Here, we show that the vnd/NK-2 gene encodes an additional protein isoform (NK-2B) that differs from the known one (NK-2A) in its N-terminal domain. While NK-2A is a transcription repressor, NK-2B directly activates transcription from promoters containing NK-2 binding sites, with its N-terminal domain possessing a strong transcription activation potency. The transcription of NK-2B starts at the onset of metamorphosis. Its expression is observed in precursors of differentiating photoreceptors and in photoreceptors of the adult eye. Both NK-2B and NK-2A are expressed in the lamina. However, the expression of NK-2A is mostly associated with the undifferentiated state of nervous cells.

Kinetic analysis of Drosophila Vnd protein containing homeodomain with its target sequence

Homeodomain (HD) is a highly conserved DNA-binding domain composed of helix-turn-helix motif. Drosophila Vnd (Ventral nervous system defective) containing HD acts as a regulator to either enhance or suppress gene expression upon binding to its target sequence. In this study, kinetic analysis of Vnd binding to DNA was performed. The result demonstrates that DNA-binding affinity of the recombinant protein containing HD and NK2-specific domain (NK2-SD) was higher than that of the full-length Vnd. To access whether phosphorylation sites within HD and NK2-SD affect the interaction of the protein with the target sequence, alanine substitutions were introduced. The result shows that S631A mutation within NK2-SD does not contribute significantly to the DNA-binding affinity. However, S571A and T600A mutations within HD showed lower affinity for DNA binding. In addition, DNA-binding analysis using embryonic nuclear protein also demonstrates that Vnd interacts with other nuclear proteins, suggesting the existence of Vnd as a complex.

Homeobox genes in vertebrate forebrain development and disease

Homeobox genes are an evolutionarily conserved class of transcription factors that are key regulators of developmental processes such as regional specification, patterning, migration and differentiation. In both mouse and humans, the developing forebrain is marked by distinct boundaries of homeobox gene expression at different developmental time points. These genes regulate the patterning of the forebrain along the dorsal/ventral and rostral/caudal axes and are also essential for the differentiation of specific neuronal subtypes. Inhibitory interneurons that arise from the ganglionic eminences and migrate tangentially to the neocortex and hippocampus are dramatically affected by mutations in several homeobox genes. In this review, we discuss the identification, expression patterns, loss- and/or gain-of-function models, and confirmed transcriptional targets for a set of homeobox genes required for the correct development of the forebrain in the mouse. In humans, mutations of homeobox genes expressed in the forebrain have been shown to result in mental retardation, epilepsy or movement disorders. The number of homeobox genes currently linked to human nervous system disease is surprisingly low, perhaps reflecting the essential functions of these genes throughout embryogenesis or the degree of functional redundancy during central nervous system development.

Linking pattern formation to cell-type specification: Dichaete and Ind directly repress achaete gene expression in the Drosophila CNS

Mechanisms regulating CNS pattern formation and neural precursor formation are remarkably conserved between Drosophila and vertebrates. However, to date, few direct connections have been made between genes that pattern the early CNS and those that trigger neural precursor formation. Here, we use Drosophila to link directly the function of two evolutionarily conserved regulators of CNS pattern along the dorsoventral axis, the homeodomain protein Ind and the Sox-domain protein Dichaete, to the spatial regulation of the proneural gene achaete (ac) in the embryonic CNS. We identify a minimal achaete regulatory region that recapitulates half of the wild-type ac expression pattern in the CNS and find multiple putative Dichaete-, Ind-, and Vnd-binding sites within this region. Consensus Dichaete sites are often found adjacent to those for Vnd and Ind, suggesting that Dichaete associates with Ind or Vnd on target promoters. Consistent with this finding, we observe that Dichaete can physically interact with Ind and Vnd. Finally, we demonstrate the in vivo requirement of adjacent Dichaete and Ind sites in the repression of ac gene expression in the CNS. Our data identify a direct link between the molecules that pattern the CNS and those that specify distinct cell-types.

Specification and development of the pars intercerebralis and pars lateralis, neuroendocrine command centers in the Drosophila brain

The central neuroendocrine system in the Drosophila brain includes two centers, the pars intercerebralis (PI) and pars lateralis (PL). The PI and PL contain neurosecretory cells (NSCs) which project their axons to the ring gland, a complex of peripheral endocrine glands flanking the aorta. We present here a developmental and genetic study of the PI and PL. The PI and PL are derived from adjacent neurectodermal placodes in the dorso-medial head. The placodes invaginate during late embryogenesis and become attached to the brain primordium. The PI placode and its derivatives express the homeobox gene Dchx1 and can be followed until the late pupal stage. NSCs labeled by the expression of Drosophila insulin-like peptide (Dilp), FMRF, and myomodulin form part of the Dchx1 expressing PI domain. NSCs of the PL can be followed throughout development by their expression of the adhesion molecule FasII. Decapentaplegic (Dpp), secreted along the dorsal midline of the early embryo, inhibits the formation of the PI and PL placodes; loss of the signal results in an unpaired, enlarged placodeal ectoderm. The other early activated signaling pathway, EGFR, is positively required for the maintenance of the PI placode. Of the dorso-medially expressed head gap genes, only tailless (tll) is required for the specification of the PI. Absence of the corpora cardiaca, the endocrine gland innervated by neurosecretory cells of the PI and PL, does not affect the formation of the PI/PL, indicating that inductive stimuli from their target tissue are not essential for early PI/PL development.

Identification and analysis of vnd/NK-2 homeodomain binding sites in genomic DNA

Vnd/NK-2 homeodomain affinity column chromatography was used to purify Drosophila DNA fragments bound by the vnd/NK-2 homeodomain. Sequencing the selected genomic DNA fragments led to the identification of 77 Drosophila DNA fragments that were grouped into 42 vnd/NK-2 homeodomain-binding loci. Most loci were within upstream or intronic regions, especially first introns. Nineteen of the Drosophila DNA fragments cloned correspond to one locus, termed Clone A, which is 312 bp in length and contains five vnd/NK-2 homeodomain core consensus binding sites, 5'-AAGTG, and is part of the first intron of the Beadex gene. We further analyzed the interactions between Clone A and vnd/NK-2 homeodomain protein by mobility-shift assay, DNase I footprinting, methylation interference, and ethylation interference. The DNase I footprinting analysis of Clone A with vnd/NK-2 homeodomain protein revealed three strong binding sites and one weak binding site between 15 and 130 bp of Clone A. We also analyzed binding of the vnd/NK-2 homeodomain to the 5'-flanking sequence of vnd/NK-2 genomic DNA. The DNase I footprinting result showed that there are two strong binding sites and five weak binding sites in the fragment between -385 and -675 bp from the transcription start site of the vnd/NK-2 gene.

Contextual interactions determine whether the Drosophila homeodomain protein, Vnd, acts as a repressor or activator

At the molecular level, members of the NKx2.2 family of transcription factors establish neural compartment boundaries by repressing the expression of homeobox genes specific for adjacent domains [Muhr et al. (2001) Cell, 104, 861-873; Weiss et al. (1998) Genes Dev., 12, 3591-3602]. The Drosophila homologue, vnd, interacts genetically with the high-mobility group protein, Dichaete, in a manner suggesting co-operative activation [Zhao and Skeath (2002) Development, 129, 1165-1174]. However, evidence for direct interactions and transcriptional activation is lacking. Here, we present molecular evidence for the interaction of Vnd and Dichaete that leads to the activation of target gene expression. Two-hybrid interaction assays indicate that Dichaete binds the Vnd homeodomain, and additional Vnd sequences stabilize this interaction. In addition, Vnd has two activation domains that are typically masked in the intact protein. Whether vnd can activate or repress transcription is context-dependent. Full-length Vnd, when expressed as a Gal4 fusion protein, acts as a repressor containing multiple repression domains. A divergent domain in the N-terminus, not found in vertebrate Vnd-like proteins, causes the strongest repression. The co-repressor, Groucho, enhances Vnd repression, and these two proteins physically interact. The data presented indicate that the activation and repression domains of Vnd are complex, and whether Vnd functions as a transcriptional repressor or activator depends on both intra- and inter-molecular interactions.

Mapping activation and repression domains of the vnd/NK-2 homeodomain protein

A transient transfection assay using Drosophila S2 tissue culture cells and WT and mutant Drosophila vnd/NK-2 homeobox cDNAs was used to localize repression and activation domains of vnd/NK-2 homeodomain protein. A repression domain was identified near the N terminus of vnd/NK-2 homeodomain protein (amino acid residues 154-193), which contains many hydrophobic amino acid residues. The major determinants of the repression domain were shown to be amino acid residues F155, W158, I161, L162, L163, and W166. Truncated protein consisting of the N-terminal repression domain and the DNA-binding homeodomain repressed transcription as efficiently as WT vnd/NK-2 protein. An activation domain was identified between the tinman domain and the homeodomain (amino acid residues 277-543), which consists of a glutamine-rich subdomain and two acidic subdomains. No effect was detected of the tinman domain or the NK-2-specific domain on either activation or repression of a beta-galactosidase reporter gene.

Mutations that affect the ability of the vnd/NK-2 homeoprotein to regulate gene expression: transgenic alterations and tertiary structure

The importance in downstream target regulation of tertiary structure and DNA binding specificity of the protein encoded by the vndNK-2 homeobox gene is analyzed. The ectopic expression patterns of WT and four mutant vndNK-2 genes are analyzed together with expression of two downstream target genes, ind and msh, which are down-regulated by vndNK-2. Three mutants are deletions of conserved regions (i.e., tinman motif, acidic motif, and NK-2 box), and the fourth, Y54M vndNK-2, corresponds to a single amino acid residue replacement in the homeodomain. Of the four ectopically expressed mutant genes examined, only the Y54M mutation inactivates the ability of the vndNK-2 homeodomain protein to repress ind and msh. The acidic motif deletion mutant slightly reduced the ability of the protein to repress ind and msh. By contrast, both tinman and NK-2 box deletion mutants behaved as functional vndNK-2 genes in their ability to repress ind and msh. The NMR-determined tertiary structures of the Y54M vndNK-2 homeodomain, both free and bound to DNA, are compared with the WT analog. The only structural difference observed for the mutant homeodomain is in the complex with DNA and involved closer interaction of the methionine-54 with A2, rather than with C3 of the (-) strand of the DNA. This subtle change in the homeodomain-DNA complex resulted in modifications of binding affinities to DNA. These changes resulting from a single amino acid residue replacement constitute the molecular basis for the phenotypic alterations observed on ectopic expression of the Y54M vndNK-2 gene during embryogenesis.

Sequence-specific DNA binding by the vnd/NK-2 homeodomain of Drosophila

The ventral nervous system defective (vnd)/NK-2 homeodomain and some flanking amino acid residues were expressed in Escherichia coli, purified to homogeneity, and the protein was covalently coupled to Sepharose. Oligodeoxynucleotides that contained 16-bp random sequences were purified by vnd/NK-2 affinity column chromatography, cloned, and sequenced. The consensus nucleotide sequence of the vnd/NK-2 homeodomain binding site was shown to be T(T/C)AAGTG(G/C). The apparent equilibrium dissociation constant (K(D)) of the vnd/NK-2 homeodomain for the consensus sequence is 1.9 x 10(-10) M. In addition, results of competition between oligodeoxynucleotides for binding to the vnd/NK-2 homeodomain and determination of the apparent K(D) values of oligodeoxynucleotides that differ from the consensus sequence by only a single base pair demonstrate that the four central nucleotides, AAGT, in this sequence play a major role in determining the affinity of binding.

Regulatory DNA required for vnd/NK-2 homeobox gene expression pattern in neuroblasts

Vnd/NK-2 protein was detected in 11 neuroblasts per hemisegment in Drosophila embryos, 9 medial and 2 intermediate neuroblasts. Fragments of DNA from the 5'-flanking region of the vnd/NK-2 gene were inserted upstream of an enhancerless betagalactosidase gene in a P-element and used to generate transgenic fly lines. Antibodies directed against Vnd/NK-2 and beta-galactosidase proteins then were used in double-label experiments to correlate the expression of beta-galactosidase and Vnd/NK-2 proteins in identified neuroblasts. DNA region A, which corresponds to the -4.0 to -2.8-kb fragment of DNA from the 5'-flanking region of the vnd/NK-2 gene was shown to contain one or more strong enhancers required for expression of the vnd/NK-2 gene in ten neuroblasts. DNA region B (-5.3 to -4.0 kb) contains moderately strong enhancers for vnd/NK-2 gene expression in four neuroblasts. Hypothesized DNA region C, whose location was not identified, contains one or more enhancers that activate vnd/NK-2 gene expression only in one neuroblast. These results show that nucleotide sequences in at least three regions of DNA regulate the expression of the vnd/NK-2 gene, that the vnd/NK-2 gene can be activated in different ways in different neuroblasts, and that the pattern of vnd/NK-2 gene expression in neuroblasts of the ventral nerve cord is the sum of partial patterns.

Analysis of three Ptx2 splice variants on transcriptional activity and differential expression pattern in the brain

Three different transcripts of the homeodomain gene termed pituitary homeobox (Ptx) 2 (Pitx2/Brx/Rieg/Solurshin/Arp) were cloned from different species encoding proteins belonging to the paired-like family of homeodomain proteins. Ptx2a (324 amino acids), Ptx2b (271 amino acids), and Ptx2c (318 amino acids) share the C terminus, including the homeodomain, and have different N termini. Here we report the comparative analysis of all three different Ptx2 splice variants for their transcriptional activity and their expression pattern in the adult rat brain. Ptx2 is able to trans-activate via different model promoters in different cell lines. A mild difference in trans-activating potential is observed among the splice variants, but the underlying mechanism is at present unknown. It is surprising that all Ptx2 transcripts displayed an identical expression pattern in the brain. This markedly restricted pattern is limited to the following brain areas: the anterior and intermediate lobes of the pituitary gland, the subthalamic nucleus, the posterior hypothalamic nucleus, the mammillary bodies, the red nucleus, and the deep gray layer of the superior colliculus. The data presented suggest that all variants of Ptx2 are involved in the development and regulation of distinct neuronal cell groups and the pituitary gland.

Smoluchowski dynamics of the vnd/NK-2 homeodomain from Drosophila melanogaster: second-order maximum correlation approximation

The mode coupling diffusion theory is applied to the derivation of local dynamics in proteins in solution. The rotational dynamics of the bonds along the protein sequence are calculated and compared to the experimentally measured nmr (15)N spin-lattice relaxation time T(1), at 36.5, 60.8, and 81.1 MHz of the vnd/NK-2 homeodomain from Drosophila melanogaster. The starting point for the calculations is the experimental three-dimensional solution structure of the homeodomain determined by multidimensional nmr spectroscopy. The higher order mode-coupling computations are compared also with the recently published first-order approximation calculations. The more accurate calculations improve substantially the first-order ORZLD calculations and show that the role of the strength of the hydrodynamic interactions becomes crucial to fix the order of magnitude of the rotational dynanics for these very compact molecules characterized by partial screening of the internal atoms to water. However, the relative mobility of the bonds along the sequence and the differential fluctuations depend only weakly on the hydrodynamic strength but strongly on the geometry of the three-dimensional structure and on the statistics incorporated into the theory. Both rigid and fluctuating dynamic models are examined, with fluctuations evaluated using molecular dynamics simulations. The comparison with nmr data shows that mode coupling diffusion accounts for the T(1) relaxation pattern at low frequency where the rotational tumbling dominates. An important contribution of internal motions in the nanosecond time scale is seen at high frequencies and is discussed in terms of diffusive concepts.

Cngsc, a homologue of goosecoid, participates in the patterning of the head, and is expressed in the organizer region of Hydra

We have isolated Cngsc, a hydra homologue of goosecoid gene. The homeodomain of Cngsc is identical to the vertebrate (65-72%) and Drosophila (70%) orthologues. When injected into the ventral side of an early Xenopus embryo, Cngsc induces a partial secondary axis. During head formation, Cngsc expression appears prior to, and directly above, the zone where the tentacles will emerge, but is not observed nearby when the single apical tentacle is formed. This observation indicates that the expression of the gene is not necessary for the formation of a tentacle per se. Rather, it may be involved in defining the border between the hypostome and the tentacle zone. When Cngsc(+) tip of an early bud is grafted into the body column, it induces a secondary axis, while the adjacent Cngsc(−) region has much weaker inductive capacities. Thus, Cngsc is expressed in a tissue that acts as an organizer. Cngsc is also expressed in the sensory neurons of the tip of the hypostome and in the epithelial endodermal cells of the upper part of the body column. The plausible roles of Cngsc in organizer function, head formation and anterior neuron differentiation are similar to roles goosecoid plays in vertebrates and Drosophila. It suggests widespread evolutionary conservation of the function of the gene.

The homeodomain transcription factor NK-4 acts as either a transcriptional activator or repressor and interacts with the p300 coactivator and the Groucho corepressor

NK-4 (tinman) encodes an NK-2 class homeodomain transcription factor that is required for development of the Drosophila dorsal mesoderm, including heart. Genetic evidence suggests its important role in mesoderm subdivision, yet the properties of NK-4 as a transcriptional regulator and the mechanism of gene transcription by NK-4 are not completely understood. Here, we describe its properties as a transcription factor and its interaction with the p300 coactivator and the Groucho corepressor. We demonstrate that NK-4 can activate or repress target genes in cultured cells, depending on functional domains that are conserved between Drosophila melanogaster and Drosophila virilis NK-4 genes. Using GAL4-NK-4 fusion constructs, we have mapped a transcriptional activation domain (amino acids 1-110) and repression domains (amino acids 111-188 and the homeodomain) and found an inhibitory function for the homeodomain in transactivation by NK-4. Furthermore, we demonstrate that NK-4-dependent transactivation is augmented by the p300 coactivator and show that NK-4 physically interacts with p300 via the activation domain. In addition, cotransfection experiments indicate that the repressor activity of NK-4 is strongly enhanced by the Groucho corepressor. Using immunoprecipitation and in vitro pull-down assays, we show that NK-4 directly interacts with the Groucho corepressor, for which the homeodomain is required. Together, our results indicate that NK-4 can act as either a transcriptional activator or repressor and provide the first evidence of NK-4 interactions with the p300 coactivator and the Groucho corepressor.

Smoluchowski dynamics of the vnd/NK-2 homeodomain from Drosophila melanogaster: first-order mode-coupling approximation

This work is the first in a series devoted to applying mode coupling diffusion theory to the derivation of local dynamics properties of proteins in solution. The first-order mode-coupling approximation, or optimized Rouse-Zimm local dynamics (ORZLD), is applied here to derive the rotational dynamics of the bonds and compare the calculated with the experimental nmr 15N spin-lattice relaxation time behavior of the vnd/NK-2 homeodomain from Drosophila melanogaster. The starting point for the calculations is the experimental three-dimensional structure of the homeodomain determined by multidimensional nmr spectroscopy. The results of the computations are compared with experimentally measured 15N spin-lattice relaxation times T1, at 34.5 and 60.8 MHz, to check the first-order approximation. To estimate the relative importance of internal and overall rotation, both rigid and fluctuating dynamic models are examined, with fluctuations evaluated using molecular dynamics (MD) simulations. The correlation times for the fundamental bond vector time correlation function and for the second-order bond orientational TCF are obtained as a function of the residue number for vnd/NK-2. The stability of the corresponding local dynamics pattern for the fluctuating structure as a function of the length of the MD trajectory is presented. Diffusive dynamics, which is essentially free of model parameters even at first order in the mode-coupling diffusion approach, confirm that local dynamics of proteins can be described in terms of rotational diffusion of a fluctuating quasi-rigid structure. The comparison with the nmr data shows that the first-order mode coupling diffusion approximation accounts for the correct order of magnitude of the results and of important qualitative aspects of the data sensitive to conformational changes. Indications are obtained from this study to efficiently extend the theory to higher order in the mode-coupling expansion. These results demonstrate the promise of the mode-coupling approach, where the local dynamics of proteins is described in terms of rotational diffusion of a fluctuating quasi-rigid structure, to analyze nmr spin-lattice relaxation behavior.

Neuroblast pattern formation: regulatory DNA that confers the vnd/NK-2 homeobox gene pattern on a reporter gene in transgenic lines of Drosophila

DNA fragments -0.57, -2.2, -2.9, -5.3, and -8.4 kb in length from the upstream regulatory region of the vnd/NK-2 gene were cloned in the 5'-flanking region of a beta-galactosidase (beta-gal) reporter gene in the P-element pCaSpeR-AUG-beta-gal, and the effects of the DNA on the pattern and time of expression of beta-gal were determined in transgenic embryos. Embryos from 11 lines transformed with -8.4 kb of vnd/NK-2 regulatory DNA expressed beta-gal patterns that closely resemble those of vnd/NK-2. In embryos from four lines transformed with -5.3 kb of vnd/NK-2 DNA, beta-gal was found in the normal vnd/NK-2 pattern in the nerve cord but not in part of the cephalic region. beta-Gal patterns in embryos from transgenic lines containing -0.57, -2.2, or -2.9 kb of vnd/NK-2 DNA did not resemble vnd/NK-2. Null vnd/NK-2 mutant embryos containing the homozygous P-element p[-8.4 to +0.34 beta-gal] expressed little beta-gal in contrast to siblings with a wild-type vnd/NK-2 gene. We conclude that (i) the 8.4-kb DNA fragment from the vnd/NK-2 gene contains the nucleotide sequences required to generate the normal pattern of vnd/NK-2 gene expression, sequences that may be involved in the switch between neuroblast vs. epidermoblast pathways of development, (ii) the 5'-flanking region of the vnd/NK-2 gene between -5.3 and -8. 4 kb is required for vnd/NK-2 gene expression in the most dorsoanterior part of the cephalic region, and (iii) vnd/NK-2 protein is required, directly or indirectly, for maintenance of vnd/NK-2 gene expression.

Structural basis of an embryonically lethal single Ala --> Thr mutation in the vnd/NK-2 homeodomain

The structural and DNA binding behavior is described for an analog of the vnd/NK-2 homeodomain, which contains a single amino acid residue alanine to threonine replacement in position 35 of the homeodomain. Multidimensional nuclear magnetic resonance, circular dichroism, and electrophoretic gel retardation assays were carried out on recombinant 80-aa residue proteins that encompass the wild-type and mutant homeodomains. The mutant A35T vnd/NK-2 homeodomain is unable to adopt a folded conformation free in solution at temperatures down to -5 degreesC in contrast to the behavior of the corresponding wild-type vnd/NK-2 homeodomain, which is folded into a functional three-dimensional structure below 25 degreesC. The A35T vnd/NK-2 binds specifically to the vnd/NK-2 target DNA sequence, but with an affinity that is 50-fold lower than that of the wild-type homeodomain. Although the three-dimensional structure of the mutant A35T vnd/NK-2 in the DNA bound state shows characteristic helix-turn-helix behavior similar to that of the wild-type homeodomain, a notable structural deviation in the mutant A35T analog is observed for the amide proton of leucine-40. The wild-type homeodomain forms an unusual i,i-5 hydrogen bond with the backbone amide oxygen of residue 35. In the A35T mutant this amide proton resonance is shifted upfield by 1.27 ppm relative to the resonance frequency for the wild-type analog, thereby indicating a significant alteration of this i,i-5 hydrogen bond.

Site-directed mutations in the vnd/NK-2 homeodomain. Basis of variations in structure and sequence-specific DNA binding

Secondary structures, DNA binding properties, and thermal denaturation behavior of six site-directed mutant homeodomains encoded by the vnd/NK-2 gene from Drosophila melanogaster are described. Three single site H52R, Y54M, and T56W mutations, two double site H52R/T56W and Y54M/T56W mutations, and one triple site H52R/Y54M/T56W mutation were investigated. These positions were chosen based on their variability across homeodomains displaying differences in secondary structure and DNA binding specificity. Multidimensional NMR, electrophoretic mobility shift assays, and circular dichroism spectropolarimetry studies were carried out on recombinant 80-amino acid residue proteins containing the homeodomain. Position 56, but more importantly position 56 in combination with position 52, plays an important role in determining the length of the recognition helix. The H52R mutation alone does not affect the length of this helix but does increase the thermal stability. Introduction of site mutations at positions 52 and 56 in vnd/NK-2 does not modify their high affinity binding to the 18-base pair DNA fragment containing the vnd/NK-2 consensus binding sequence, CAAGTG. Site mutations involving position 54 (Y54M, Y54M/T56W, and H52R/Y54M/T56W) all show a decrease of 1 order of magnitude in their binding affinity. The roles in structure and sequence specificity of individual atom-atom interactions are described.

Mesoderm is required for the formation of a segmented endodermal cell layer in the leech Helobdella

The homeobox gene Lox3 is expressed in a segmentally iterated pattern within the endoderm of the leech Helobdella. We use that expression here to study endoderm differentiation following experimental ablations of mesoderm. Lox3 RNA was first detected by in situ hybridization at the stage when a definitive cellular endoderm is formed from its syncytial precursor and was never observed in derivatives of other germ layers. Expression is initially distributed throughout the endoderm, but rapidly disappears from specific regions of the nascent gut wall so as to produce a pattern of segmental stripes. The stripe pattern differs markedly between midgut organs, with thin stripes of Lox3 expression in the intercaecal constrictions of the crop and wide stripes of Lox3 expression marking the caecal bulges of the intestine. Lox3 expression in the rectum is not obviously segmental. Ablation of segmental mesoderm in the early Helobdella embryo prevents the formation of definitive endoderm and the expression of Lox3 RNA and leads to abnormalities in the morphogenesis of the gut tube. These endodermal deficits are precisely coextensive with the zone of mesodermal deficiency, suggesting that the mesoderm normally acts to promote the formation of the endodermal cell layer via local cell interactions. The segmental pattern of Lox3 expression is largely unaffected in portions of the endoderm surrounding such deficits, suggesting that endodermal segmentation is not established by lateral interactions within that tissue layer. Rather, we propose that the segmental organization of the endoderm is imprinted by vertical interactions with the segmental mesoderm.

Twist-mediated activation of the NK-4 homeobox gene in the visceral mesoderm of Drosophila requires two distinct clusters of E-box regulatory elements

NK-4, also called msh2 and tinman, encodes a homeodomain transcription factor that is required for the development of the dorsal mesoderm and its derivatives in the Drosophila embryo. Genetic analyses indicate that NK-4 resides downstream of the mesodermal determinant twist, which encodes a basic helix-loop-helix-type transcription factor. However, the regulation of NK-4 by twist remains poorly understood. Using expression assays in cultured cells and transgenic flies, we show that two distinct clusters of E-box regulatory sequences, present upstream of the NK-4 gene, mediate NK-4 expression in the visceral mesoderm. These elements are conserved between the Drosophila melanogaster and Drosophila virilis NK-4 genes and serve as binding sites for Twist (E1 cluster) and NK-4 (E2 cluster) proteins. In cultured cells, Twist and NK-4 binding results in activation of NK-4 gene expression. In transgenic animals, the E1 and E2 clusters are functionally connected, and both elements are required for NK-4 activation in cells of the visceral mesoderm and also for NK-4 repression in cells of the somatic musculature. These results demonstrate that NK-4 is a direct transcriptional target for Twist and its own gene product in visceral mesodermal cells, supporting the idea that twist and NK-4 function in the subdivision of the mesoderm during Drosophila embryogenesis.

Expression of a developmentally regulated gene, Mng10, in identified neurosecretory cells in the CNS of Manduca sexta

We are interested in the molecular events underlying the development of the nervous system of Manduca sexta during the final 24 h of the pupal molt. In this article we describe a gene, Mng10, that is expressed in the abdominal nervous system of M. sexta and is developmentally regulated over this 24-h period. In situ hybridization analysis shows that the transcript is localized predominantly to a single pair of uniquely identifiable neurosecretory neurons, the NS-L1 cells in the abdominal ganglia. Mng10 is a single copy gene encoding a 229 amino acid protein with a predicted molecular mass of 26 kDa. At the amino acid level the protein shows 34% identity to the yeast transcription unit, Yer082. Northern blot analysis shows that the transcript of Mng10 is very rare, comprising about 0.001% of the poly (A)+ RNA from the CNS and is detectable at 4 h but not 24 h prior to pupal ecdysis. One of the physiological events that develops over the final 24 h of the pupal molt is the ability of the nervous system to respond to the neuropeptide eclosion hormone. In this context, it is interesting to note that the NS-L1 cells are members of the group of 50 neurons that show increased cGMP immunoreactivity when the nervous system is exposed to the neuropeptide eclosion hormone.