Smad2 role in mesoderm formation, left-right patterning and craniofacial development

Signalling by the transforming growth factor-beta (TGF-beta) superfamily of proteins depends on the phosphorylation and activation of SMAD proteins by heteromeric complexes of ligand-specific type I and type II receptors with serine/threonine-kinase activity. The vertebrate SMAD family includes at least nine members, of which Smad2 has been shown to mediate signalling by activin and TGF-beta. In Xenopus, Smad2 can induce dorsal mesoderm, mimicking Vg-1, activin and nodal. Here we investigate the function of Smad2 in mammalian development by generating two independent Smad2 mutant alleles in mice by gene targeting. We show that homozygous mutant embryos fail to form an organized egg cylinder and lack mesoderm, like mutant mice lacking nodal or ActRIB, the gene encoding the activin type-I receptor. About 20 per cent of Smad2 heterozygous embryos have severe gastrulation defects and lack mandibles or eyes, indicating that the gene dosage of Smad2 is critical for signalling. Mice trans-heterozygous for both Smad2 and nodal mutations display a range of phenotypes, including gastrulation defects, complex craniofacial abnormalities such as cyclopia, and defects in left-right patterning, indicating that Smad2 may mediate nodal signalling in these developmental processes. Our results show that Smad2 function is essential for early development and for several patterning processes in mice.     

The Eph family in the patterning of neural development

The Eph family represents the largest subfamily of receptor tyrosine kinases. Its members are predominantly expressed in the developing and adult nervous system. Besides playing an important role in the contact-mediated repulsion of axons, they have recently also been implicated in the control of cell migration. Characteristics of the Eph family are extended promiscuity in the interaction between receptors and ligands, the necessity of membrane attachment of the ligands to exert their function, the lack of induction of mitogenic responses, and the bi-directional signalling of receptors and ligands.     

Hrs, a tyrosine kinase substrate with a conserved double zinc finger domain, is localized to the cytoplasmic surface of early endosomes

Hrs is a 115-kDa double zinc finger protein that is rapidly tyrosine phosphorylated in growth factor-stimulated cells. However, its function remains unknown. Here we show that Hrs is localized to early endosomes. Intracellular localization of endogenous Hrs and exogenously expressed Hrs tagged with the hemagglutinin epitope was examined by immunofluorescence staining using anti-Hrs and anti-hemagglutinin epitope antibodies, respectively. Hrs was detected in vesicular structures and was colocalized with the transferrin receptor, a marker for early endosomes, but only partially with CD63, a marker for late endosomes. A zinc finger domain deletion mutant of Hrs was also colocalized with the transferrin receptor, suggesting that the zinc finger domain is not required for its correct localization. Immunoelectron microscopy showed that Hrs was localized to the cytoplasmic surface of these structures. By subcellular fractionation, Hrs was recovered both in the cytoplasmic and membrane fractions. The membrane-associated Hrs was extracted from the membrane by alkali treatment, suggesting that it is peripherally associated with early endosomes. These results, together with our finding that Hrs is homologous to Vps27p, a protein essential for protein traffic through a prevacuolar compartment in yeast, suggest that Hrs is involved in vesicular transport through early endosomes.     

Effects of excess vitamin A on the cranial neural crest in the chick embryo

Experiments on fertile hens' eggs have shown that an excess of vitamin A has a highly specific effect on the migration of cranial neural crest cells in the early stages of chick embryogenesis. Migration is disrupted and retarded. This is consistent with the causal mechanism proposed by Poswillo for the Treacher Collins syndrome.     

The expression and regulation of chick EphA7 suggests roles in limb patterning and innervation

Eph receptors and their ligands, the ephrins, have been implicated in early patterning and axon guidance in vertebrate embryos. Members of these families play pivotal roles in the formation of topographic maps in the central nervous system, the formation of brain commissures, and in the guidance of neural crest cells and motor axons through the anterior half of the somites. Here, we report a highly dynamic expression pattern of the chick EphA7 gene in the developing limb. Expression is detected in discrete domains of the dorsal mesenchyme from 3 days of incubation. The expressing cells are adjacent to the routes where axons grow to innervate the limb at several key points: the region of plexus formation, the bifurcation between dorsal and ventral fascicles, and the pathway followed by axons innervating the dorsal muscle mass. These results suggested a role for EphA7 in cell-cell contact-mediated signalling in dorsal limb patterning and/or axon guidance. We carried out experimental manipulations in the chick embryo wing bud to alter the dorsoventral patterning of the limb. The analyses of EphA7 expression and innervation in the operated wings indicate that a signal emanating from the dorsal ectoderm regulates EphA7 in such a way that, in its absence, the wing bud lacks EphA7 expression and shows innervation defects at the regions where the gene was downregulated. EphA7 downregulation in the dorsal mesenchyme after dorsal ectoderm removal is more rapid than that of Lmx-1, the gene known to mediate dorsalisation in response to the ectodermal signal. These results add a new gene to the dorsalisation signalling pathway in the limb. Moreover, they implicate the Eph receptor family in the patterning and innervation of the developing limb, extending its role in axon pathfinding to the distal periphery.     

Amplification and over-expression of OZF, a gene encoding a zinc finger protein, in human pancreatic carcinomas

The OZF gene encodes a protein consisting of 10 zinc finger motifs and is located on chromosome 19q3.1. We report here the amplification and over-expression of the OZF gene in pancreatic carcinomas. Increased gene copy number was detected in 3 of 12 tumour cell lines and 2 of 12 primary pancreatic carcinomas. Expression was detected in all cell lines, and the gene was over-expressed in cell lines with OZF gene amplification. Five of 8 tumours, including 2 primary tumours with OZF gene amplification, displayed high levels of OZF protein, whereas normal pancreas expressed low levels. Immuno-histochemical analysis showed that expression was restricted to tumour cells. Thus, high-level expression of OZF is frequent in pancreatic carcinomas and may contribute to the development or progression of this tumour.     

msh may play a conserved role in dorsoventral patterning of the neuroectoderm and mesoderm

Many of the mechanisms that govern the patterning of the Drosophila neuroectoderm and mesoderm are still unknown. Here we report the sequence, expression, and regulation of the homeobox gene msh, which is likely to play an important role in the early patterning events of these two tissue primordia. msh expression is first observed in late blastoderm embryos and occurs in longitudinal bands of cells that are fated to become lateral neuroectoderm. This expression is under the control of dorsoventral axis-determination genes and depends on dpp-mediated repression in the dorsal half of the embryo and on fib-(EGF-) mediated repression ventrally. The bands of msh expression define the cells that will form the lateral columns of proneural gene expression and give rise to the lateral row of SI neuroblasts. This suggests that msh may be one of the upstream regulators of the achaete-scute (AS-C) genes and may play a role that is analogous to that of the homeobox gene vnd/NK2 in the medial sector of the neuroectoderm. During neuroblast segregation, msh expression is maintained in a subset of neuroblasts, indicating that msh, like vnd/NK2, could function in both dorsoventral patterning of the neuroectoderm and neuroblast specification. The later phase of msh expression that occurs after the first wave of neuroblast segregation in defined ectodermal and mesodermal clusters of cells points to similar roles of msh in patterning and cell fate specification of the peripheral nervous system, dorsal musculature, and the fat body. A comparison of the expression patterns of the vertebrate homologs of msh, vnd/NK2, and AS-C genes reveals striking similarities in dorsoventral patterning of the Drosophila and vertebrate neuroectoderm and indicates that genetic circuitries in neural patterning are evolutionarily conserved.     

A major, transformation-sensitive PKC-binding protein is also a PKC substrate involved in cytoskeletal remodeling

Protein kinase C (PKC) plays a major role in regulating cell growth, transformation, and gene expression; however, identifying phosphorylation events that mediate these responses has been difficult. We expression-cloned a group of PKC-binding proteins and identified a high molecular weight, heat-soluble protein as the major PKC-binding protein in REF52 fibroblasts (Chapline, C., Mousseau, B., Ramsay, K., Duddy, S., Li, Y., Kiley, S. C., and Jaken, S. (1996) J. Biol. Chem. 271, 6417-6422). In this study, we demonstrate that this PKC-binding protein, clone 72, is also a PKC substrate in vitro and in vivo. Using a combination of phosphopeptide mapping, Edman degradation, and electrospray mass spectrometry, serine residues 283, 300, 507, and 515 were identified as the major in vitro PKC phosphorylation sites in clone 72. Phosphorylation state-selective antibodies were raised against phosphopeptides encompassing each of the four phosphorylation sites. These antibodies were used to determine that phorbol esters stimulate phosphorylation of serines 283, 300, 507, and 515 in cultured cells, indicating that clone 72 is directly phosphorylated by PKC in living cells. Phosphorylated clone 72 preferentially accumulates in membrane protrusions and ruffles, indicating that PKC activation and clone 72 phosphorylation are involved in membrane-cytoskeleton remodeling. These data lend further evidence to the model that PKCs directly interact with, phosphorylate, and modify the functions of a group of substrate proteins, STICKs (substrates that interact with C-kinase).     

Cloning and early dorsal axial expression of Flik, a chick follistatin-related gene: evidence for involvement in dorsalization/neural induction

We have cloned and sequenced a chick gene, Flik (follistatin-like) that appears to be the homolog of the mammalian TSC36. The ORF encodes a secreted protein of approx 38 kDa, containing a single cysteine-rich domain that shows a strong relationship with the second of the four from which Follistatin is constructed. The remainder of the Flik protein shows no strong family affinities. We describe here the normal expression pattern of the gene during primitive streak and neurula stages. We also give revised data for early neurectodermal expression of chick follistatin (Connolly et al., 1995, Developmental Genetics 17(1), 65-77) and follow the new ectopic expression of both genes during the induction of a second neural axis, after grafting of Hensen's node into a peripheral position in a host blastoderm. Both genes mark the organizer (node and/or mesodermal head process) and early neural plate, and could thus be involved in intercellular signaling during mesodermal dorsalization and neural induction. Flik expression appears earlier than that of follistatin however, and unlike that of follistatin, it is maintained strongly in the dorsal midline with intensity smoothly declining into presumptive lateral regions. We show that both genes are upregulated in host tissue in the neighborhood of node grafts, but whereas follistatin is transcribed after 8-10 hr in host epiblast that has formed new neural plate, Flik is expressed within 4 hr in this region, sometimes detectable before the first structural changes (columnarization) of neuralization. Thus, although ectodermal Flik expression is later confined within neural plate, and mesodermal expression concentrated in dorsal axial tissue, its early distribution is consistent with the idea that the encoded protein may first be involved in generating a graded system that positions the boundaries of both neural and dorsal axial mesodermal territories. The results are discussed in relation to this hypothesis and to other recent findings regarding control of vertebrate dorsoventral patterning.     

The delayed entry of thoracic neural crest cells into the dorsolateral path is a consequence of the late emigration of melanogenic neural crest cells from the neural tube

Recent studies have shown that changes in dendritic architecture are an important component of functional plasticity in the adult central nervous system. In the present study, we determined whether gonadectomy induces changes in dendritic architecture in the arcuate nucleus, a target tissue for gonadal hormones. A combination of retrograde labeling with systemically injected Fluoro-Gold and intracellular injection of neurons in a fixed-slice preparation was used to examine the morphology of neuroendocrine neurons in the rat arcuate nucleus. Intracellullary filled arcuate neuroendocrine neurons (8-21 neurons per brain) from intact (n = 5) and orchidectomized (n = 5) animals were reconstructed with the aid of a computer microscope. A quantitative analysis revealed that orchidectomy had no effect on the number and distribution of Fluoro-Gold-labeled neuroendocrine neurons in the rat arcuate nucleus. The morphology of arcuate neuroendocrine neurons in intact animals was relatively simple, with the majority of neurons (79%) having only two primary dendrites and few dendritic spines. Compared with intact controls, arcuate neuroendocrine neurons in the orchidectomized group had significantly larger somatic profile areas and exhibited significant increases in dendrite length, dendrite volume, terminal branch number, and spines per unit length of dendrite. The increase in terminal branch number in orchidectomized animals was due primarily to the appearance of short branches that gave a striking, claw-like appearance to many of the distal dendrites. These results provide evidence for hormonal regulation of dendritic morphology of arcuate neuroendocrine neurons in adult mammals.     

GBP, an inhibitor of GSK-3, is implicated in Xenopus development and oncogenesis

Dorsal accumulation of beta-catenin in early Xenopus embryos is required for body axis formation. Recent evidence indicates that beta-catenin is dorsally stabilized by the localized inhibition of the kinase Xgsk-3, utilizing a novel Wnt ligand-independent mechanism. Using a two-hybrid screen, we identified GBP, a maternal Xgsk-3-binding protein that is homologous to a T cell protooncogene in three well-conserved domains. GBP inhibits in vivo phosphorylation by Xgsk-3, and ectopic GBP expression induces an axis by stabilizing beta-catenin within Xenopus embryos. Importantly, antisense oligonucleotide depletion of the maternal GBP mRNA demonstrates that GBP is required for the establishment of the dorsal-ventral axis in Xenopus embryos. Our results define a family of GSK-3-binding proteins with roles in development and cell proliferation.