Cloning, chromosomal organization and expression analysis of Neurl, the mouse homolog of Drosophila melanogaster neuralized gene

The Drosophila neuralized (neur) gene belongs to the neurogenic group of genes involved in regulating cell-cell interactions required for neural precursor development. neur mutant phenotypes include strong overcommitment to neural fates at the expense of epidermal fates. The human neuralized homolog (NEURL) has been recently determined and found to map to chromosome 10q25.1 within the region frequently deleted in malignant astrocytomas. Because of its potential importance in developmental processes, we analyzed the structure of the mouse homolog, Neurl, and its expression pattern in embryonic tissues. Neurl activity is detected from early developmental stages in several tissues and organs including neural tissues, limbs, the skeletal system, sense organs and internal organs undergoing epithelial-mesenchymal interactions. Neurl encodes a polypeptide associated with the plasma membrane but also detected in the cytoplasm. Similarly to the Drosophila gene, mammalian neuralized may code for an important regulatory factor.     

The function of the neurogenic genes during epithelial development in the Drosophila embryo.

The complex embryonic phenotype of the six neurogenic mutations Notch, mastermind, big brain, Delta, Enhancer of split and neuralized was analyzed by using different antibodies and PlacZ markers, which allowed us to label most of the known embryonic tissues. Our results demonstrate that all of the neurogenic mutants show abnormalities in many different organs derived from all three germ layers. Defects caused by the neurogenic mutations in ectodermally derived tissues fell into two categories. First, all cell types that delaminate from the ectoderm (neuroblasts, sensory neurons, peripheral glia cells and oenocytes) are increased in number. Secondly, ectodermal tissues that in the wild type form epithelial structures lose their epithelial phenotype and dissociate (optic lobe, stomatogastric nervous system) or show significant differentiative abnormalities (trachea, Malpighian tubules and salivary gland). Abnormalities in tissues derived from the mesoderm were observed in all six neurogenic mutations. Most importantly, somatic myoblasts do not fuse and/or form an aberrant muscle pattern. Cardioblasts (which form the embryonic heart) are increased in number and show differentiative abnormalities; other mesodermal cell types (fat body, pericardial cells) are significantly decreased. The development of the endoderm (midgut rudiments) is disrupted in most of the neurogenic mutations (Notch, Delta, Enhancer of split and neuralized) during at least two stages. Defects occur as early as during gastrulation when the invaginating midgut rudiments prematurely lose their epithelial characteristics. Later, the transition of the midgut rudiments to form the midgut epithelium does not occur. In addition, the number of adult midgut precursor cells that segregate from the midgut rudiments is strongly increased. We propose that, at least in the ectodermally and endodermally derived tissues, neurogenic gene function is primarily involved in interactions among cells that need to acquire or to maintain an epithelial phenotype.     

Transmission Problem in Primary Induction

It has been suggested that during the neuralization step of primary induction in the amphibian embryo the inductive signals are mediated from mesodermal cells to the responding competent neuroepithelium by means other than cell contacts. This idea corroborated by experiments in which the interacting tissues were separated by a Nuclepore filter with pores of 0.05 μm (series 1) or by a dialyzing membrane with pores of only 12,000 daltons (series 2). After 18–22 h exposure to mesoderm followed by 8–10 days' culture in isolation the ectodermal explants were neuralized in both series with about 80% differentiating into archencephalic structures. These results exclude the possibility of cell contact as a mediating mechanism in this step.
In a third series similar experiments were made using a special Nuclepore filter with dense pores of 0.6 μm and the exposure time was prolonged to 24 h. During subsequent culture in isolation the ectoderm was neuralized in every case, except forebrain, the ectoderm also differentiated in 25% to hindbrain and less frequently to spinal cord, myotomes, and in some cases even to notochord. The result is interpreted to mean that during the prolonged exposure the tissues have had time to age to an early neurula stage, and the ectodermal cells, after being neuralized, have had time to form cell contacts by cytoplasmic bridges through the pores resulting in the segregation of the preneuralized ectoderm into more caudal structures than the forebrain.     

Erasure and reestablishment of random allelic expression imbalance after epigenetic reprogramming

Clonal level random allelic expression imbalance and random monoallelic expression provides cellular heterogeneity within tissues by modulating allelic dosage. Although such expression patterns have been observed in multiple cell types, little is known about when in development these stochastic allelic choices are made. We examine allelic expression patterns in human neural progenitor cells before and after epigenetic reprogramming to induced pluripotency, observing that loci previously characterized by random allelic expression imbalance (0.63% of expressed genes) are generally reset to a biallelic state in induced pluripotent stem cells (iPSCs). We subsequently neuralized the iPSCs and profiled isolated clonal neural stem cells, observing that significant random allelic expression imbalance is reestablished at 0.65% of expressed genes, including novel loci not found to show allelic expression imbalance in the original parental neural progenitor cells. Allelic expression imbalance was associated with altered DNA methylation across promoter regulatory regions, with clones characterized by skewed allelic expression being hypermethylated compared to their biallelic sister clones. Our results suggest that random allelic expression imbalance is established during lineage commitment and is associated with increased DNA methylation at the gene promoter.     

In vivo footprinting of a low molecular weight glutenin gene (LMWG-1D1) in wheat endosperm.

The quality of the wheat grain is determined by the quantity and composition of storage proteins (prolamins) which are synthesized exclusively in endosperm tissue. We are investigating the mechanisms underlying the regulation of expression of a prolamin gene, the low molecular weight glutenin gene LMWG-1D1. The LMWG-1D1 promoter contains the endosperm box, a sequence motif highly conserved in the promoter region of a large number of storage protein genes, which is thought to confer endosperm-specific expression of prolamin genes. Here we show by in vivo DMS footprinting of wheat endosperm tissue that the endosperm box becomes occupied by putative trans-acting factors during grain ripening. During early stages of development the endosperm motif within the 5' half of the endosperm box becomes occupied first, followed by binding of a second activity to a GCN4/jun-like motif in the 3' half just prior to the stage of maximum gene expression. Occupancy of the endosperm box is highly tissue-specific: no protection was observed in husk and leaf tissues. Several binding activities were identified in vitro from nuclear protein extracts of wheat endosperm which bind specifically to the endosperm and GCN4/jun motifs identified by in vivo footprinting.     

The BMP/CHORDIN antagonism controls sensory pigment cell specification and differentiation in the ascidian embryo

We have investigated the role of the bone morphogenetic protein (BMP) pathway during neural tissue formation in the ascidian embryo. The orthologue of the BMP antagonist, chordin, was isolated from the ascidian Halocynthia roretzi. While both the expression pattern and the phenotype observed by overexpressing chordin or BMPb (the dpp-subclass BMP) do not suggest a role for these factors in neural induction, BMP/CHORDIN antagonism was found to affect neural patterning. Overexpression of BMPb induced ectopic sensory pigment cells in the brain lineages that do not normally form pigment cells and suppressed pressure organ formation within the brain. Reciprocally, overexpressing chordin suppressed pigment cell formation and induced ectopic pressure organ. We show that pigment cell formation occurs in three steps. (1) During cleavage stages ectodermal cells are neuralized by a vegetal signal that can be substituted by bFGF. (2) At the early gastrula stage, BMPb secreted from the lateral nerve cord blastomeres induces those neuralized blastomeres in close proximity to adopt a pigment cell fate. (3) At the tailbud stage, among these pigment cell precursors, BMPb induces the differentiation of specifically the anterior type of pigment cell, the otolith; while posteriorly, CHORDIN suppresses BMP activity and allows ocellus differentiation.     

Regulation and cellular localization of ent-kaurene synthesis

The two-step cyclization reaction of ent -kaurene synthesis from geranylgeranyl diphosphate is the first committed step in the biosynthetic pathway of the plant hormone gibberellin. Recent molecular cloning and characterization of the genes encoding the two corresponding enzymes, copalyl diphosphate synthase (CPS) and ent -kau-rene synthase (KS), have demonstrated that ent -kaurene synthesis is localized in the plastids and is highly regulated in specific tissues and cell types during plant development. In addition to occurring in actively growing tissues, ent -kaurene synthesis also takes place in fully expanded leaves. Therefore mature leaves may produce gibberellin intermediates or bioactive gibberellins for transport to responsive tissues. DNA sequence analyses have revealed a conserved aspartate-rich motif, D(I/V)DDTA among CPS and other protonation-initiated terpene cyclases, while KS contains a highly conserved DDXXD motif which was proposed to function as a divalent metal ion-diphos-phate complex binding site in ionization-initiated terpene cyclases and prenyltrans-ferases.     

Bottom-up tissue engineering

Recapitulating the elegant structures formed during development is an extreme synthetic and biological challenge. Great progress has been made in developing materials to support transplanted cells, yet the complexity of tissues is far beyond that found in even the most advanced scaffolds. Self-assembly is a motif used in development and a route for the production of complex materials. Self-assembly of peptides, proteins and other molecules at the nanoscale is promising, but in addition, intriguing ideas are emerging for self-assembly of micron-scale structures. In this brief review, very recent advances in the assembly of micron-scale cell aggregates and microgels will be described and discussed.     

A cell surface differentiation antigen of Drosophila

A monoclonal antibody, generated by immunization with gastrula stage Drosophila melanogaster embryonic cells, recognizes a cell surface antigen which shows tissue and stage specificity. The antigen appears for the first time during cellularization of the blastoderm embryo and is present on all cells until around 12 hr of development. It becomes progressively restricted to specific tissues during the second half of embryogenesis. By the time of hatching, only the nervous system, germ cells, and imaginal cells are positive. During metamorphosis differentiating imaginal tissues become negative so that in the adult only the nervous system and undifferentiated germ cells are positive, with gonadal sheaths showing some staining. A third wave of antigen loss occurs during gametogenesis, resulting in negative staining on the mature sperm and oocyte. All positive tissues appear to contain the same 63-kDa cell surface antigen. The antigen behaves as a general differentiation marker lost by tissues as they approach their terminal differentiated state. The nervous system and possibly gonadal sheaths may be exceptions to this general behavior.     

Increased expression of protein C-mannosylation in the aortic vessels of diabetic Zucker rats

C-Mannosylation is a novel type of glycosylation in proteins. There are several examples of proteins in which the specific motif Trp-X-X-Trp is mannosylated at the first Trp to produce C-mannosylated Trp (CMW). Although C-mannosylation modifies Trp-X-X-Trp, predicted to be a functional motif of various integral proteins such as cytokine receptors, the physiological or pathological relevance of C-mannosylation in the cell is still not known. In this study, to characterize C-mannosylation in biological samples, we generated specific polyclonal antibodies against CMW by using a chemically synthesized CMW as an antigen. Using the antibody, we investigated the effect of hyperglycemic conditions on protein C-mannosylation in cultured cells and diabetic Zucker fatty rats. We found that protein C-mannosylation was increased in macrophage-like RAW264.7 cells under hyperglycemic conditions compared to low-glucose conditions. Furthermore, C-mannosylation was increased in the aortic vessel wall of Zucker fatty rats. Thrombospondin-1 was identified as a protein modified with C-mannosylation, and its expression was also increased in the aortic tissues of Zucker fatty rats. These results indicate that C-mannosylation is increased in specific tissues or cell types under hyperglycemic conditions, suggesting a pathological role for the increased C-mannosylation in the development of diabetic complications.     

Spatial and temporal expression patterns of GDNF family receptor alpha4 in the developing chicken retina

GDNF family receptor alpha (GFRalpha) receptors are involved in the regulation of different aspects of embryonic development such as proliferation, migration, differentiation and survival. To determine the possible role of GFRalpha4 in retinal development, we analysed its expression in the developing chicken retina. We found that GFRalpha4 is temporally co-expressed with c-ret. Both, the temporal and spatial expression of GFRalpha4 is developmentally regulated during retinogenesis and is first detected in cells of the ganglion cell layer at E6. As development of the retina proceeds, the expression of GFRalpha4 extends to cells of the inner half of the inner nuclear layer and to cells of the outermost cell row of the inner nuclear layer. Later on, GFRalpha4 expression is also found in additional cells of the outer half of the inner nuclear layer and in a subpopulation of photoreceptors. A central-to-peripheral gradient of retinal differentiation is evident, as the onset of GFRalpha4 expression is first detectable in the central retina, while it is delayed by two days in its periphery.     

Modulation of neural commitment by changes in target cell contacts in Pleurodeles waltl

In amphibian development, neural structures arise from the presumptive ectoderm at the gastrula stage by an inductive interaction with the chordamesoderm. It has been previously reported that early gastrula presumptive ectoderm can be neuralized when it is dissociated into single cells. A similar result is reported here with regard to Pleurodeles waltl presumptive ectoderm. Using this experimental model system we demonstrate: first, that neuronal and glial lineages can be specified from the presumptive ectoderm without any intervention of the natural inducing tissue; and second, that whereas rupture of cell-cell contacts evoked neural induction, dissociation immediately followed by reaggregation reduces the neuralizing response, pointing toward an active role played by cell-cell contacts of presumptive ectodermal cells in the modulation of neural commitment.     

Regulation and role of PDGF receptor alpha-subunit expression during embryogenesis.

The platelet-derived growth factor receptor alpha-subunit (PDGFR alpha) is the form of the PDGF receptor that is required for binding of PDGF A-chain. Expression of PDGFR alpha within the early embryo is first detected as the mesoderm forms, and remains characteristic of many mesodermal derivatives during later development. By 9.5 days of development, embryos homozygous for the Patch mutation (a deletion of the PDGFR alpha) display obvious growth retardation and deficiencies in mesodermal structures, resulting in the death of more than half of these embryos. Mutant embryos that survive this first critical period are viable until a new set of defects become apparent in most connective tissues. For example, the skin is missing the dermis and connective tissue components are reduced in many organs. By this stage, expression of PDGFR alpha mRNA is also found in neural crest-derived mesenchyme, and late embryonic defects are associated with both mesodermal and neural crest derivatives. Except for the neural crest, the lens and choroid plexus, PDGFR alpha mRNA is not detected in ectodermal derivatives until late in development in the central nervous system. Expression is not detected in any embryonic endodermal derivative at any stage of development. These results demonstrate that PDGFR alpha is differentially expressed during development and that this expression is necessary for the development of specific tissues.     

Intramembranous particles in the form of ridges,bracelets or assemblies in arthropod tissues

Non-junctional intramembranous particle arrays in the form of ridges, bracelets or rectilinear assemblies have been found by freeze-fracturingin the cytoplasmic half or P face of the plasma membrane in a variety of arthropod tissues. These tissues include both excitable cells, nerve and muscle, and such other cells as those from the intestinal tract, the tracheal system and the connective tissue. The intramembranous ridges are short rows of fused particles about 10 nm in diameter; comparable particles comprise the bracelets and the rectilinear aggregates, although the former are of lower profile. In cells sending out cytoplasmic projections during migration and development, for example, axons in embryonic, newly hatched or pupal tissues, tracheoles or fibroblasts, the intramembranous ridges are always aligned parallel to the longitudinal axis of the cellular process. The physiological significance of these may be that they play some role in recognition during development, perhaps by contact guidance. The ridges and rectilinear arrays found in the gut could also be involved in recognition and/or adhesion. In muscle, bead-like ridges are intimately associated with the transverse tubular system and may have a receptor function. Irregular and circular low-profile ridges occur in the tissues of the horseshoe crab, Limulus, and ‘bracelet’ forms are found in the inner membrane of insect pupal tracheae. The latter may play a part in the initiation and development of small tracheoles.     

Mobilization of carbohydrates in tissues of female silkworms, Bombyx mori, during metamorphosis

The metabolic activity and mobilization of carbohydrates among tissues of female silkworms were examined during metamorphosis by injecting radioactive ¹⁴C-glucose as a tracer. The isotope injected was incorporated into various tissues with varying degrees and reached a relatively stable state in all tissues tested in about 240 min. The metabolic activities analysed by 4 hr pulse labelling were different for different tissues and ages; in glycogen synthetic activity midgut was highest on the day of the larval-pupal ecdysis, the fat body 2 days later, and ovaries a further 4 days later.When the isotope was injected on the day of larval-pupal ecdysis, it was found predominantly in glycogen first in the midgut, then in the fat body, and finally in the ovaries, proceeding through development. The total radioactivity recovered in the glycogen fractions from these tissues was almost constant throughout development. Ovariectomy caused a rise in synthesis of both glycogen and trehalose in the fat body during the second half of development.From these results it is proposed that the dermand of developing ovaries for carbohydrates exerts a controlling influence over mobilization of glycogen in the fat body.