The chicken limb deformity gene encodes nuclear proteins expressed in specific cell types during morphogenesis

Formins: emerging players in the dynamic plant cell cortex

Formins (FH2 proteins) are an evolutionarily conserved family of eukaryotic proteins, sharing the common FH2 domain. While they have been, until recently, understood mainly as actin nucleators, formins are also engaged in various additional aspects of cytoskeletal organization and signaling, including, but not limited to, the crosstalk between the actin and microtubule networks. A surprising diversity of domain organizations has been discovered among the FH2 proteins, and specific domain setups have been found in plants. Seed plants have two clades of formins, one of them (Class I) containing mostly transmembrane proteins, while members of the other one (Class II) may be anchored to membranes via a putative membrane-binding domain related to the PTEN antioncogene. Thus, plant formins present good candidates for possible mediators of coordination of the cortical actin and microtubule cytoskeletons, as well as their attachment to the plasma membrane, that is, aspects of cell cortex organization likely to be important for cell and tissue morphogenesis. Although experimental studies of plant formin function are hampered by the large number of formin genes and their functional redundancy, recent experimental work has already resulted in some remarkable insights into the function of FH2 proteins in plants.

Effect of tropomyosin on formin-bound actin filaments

Formins are conservative proteins with important roles in the regulation of the microfilament system in eukaryotic cells. Previous studies showed that the binding of formins to actin made the structure of actin filaments more flexible. Here, the effects of tropomyosin on formin-induced changes in actin filaments were investigated using fluorescence spectroscopic methods. The temperature dependence of the Förster-type resonance energy transfer showed that the formin-induced increase of flexibility of actin filaments was diminished by the binding of tropomyosin to actin. Fluorescence anisotropy decay measurements also revealed that the structure of flexible formin-bound actin filaments was stabilized by the binding of tropomyosin. The stabilizing effect reached its maximum when all binding sites on actin were occupied by tropomyosin. The effect of tropomyosin on actin filaments was independent of ionic strength, but became stronger as the magnesium concentration increased. Based on these observations, we propose that in cells there is a molecular mechanism in which tropomyosin binding to actin plays an important role in forming mechanically stable actin filaments, even in the case of formin-induced rapid filament assembly.

Global control regions and regulatory landscapes in vertebrate development and evolution

During the course of evolution, many genes that control the development of metazoan body plans were co-opted to exert novel functions, along with the emergence or modification of structures. Gene amplification and/or changes in the cis-regulatory modules responsible for the transcriptional activity of these genes have certainly contributed in a major way to evolution of gene functions. In some cases, these processes led to the formation of groups of adjacent genes that appear to be controlled by both global and shared mechanisms.

c-Myc controls the balance between hematopoietic stem cell self-renewal and differentiation

The activity of adult stem cells is essential to replenish mature cells constantly lost due to normal tissue turnover. By a poorly understood mechanism, stem cells are maintained through self-renewal while concomitantly producing differentiated progeny. Here, we provide genetic evidence for an unexpected function of the c-Myc protein in the homeostasis of hematopoietic stem cells (HSCs). Conditional elimination of c-Myc activity in the bone marrow (BM) results in severe cytopenia and accumulation of HSCs in situ. Mutant HSCs self-renew and accumulate due to their failure to initiate normal stem cell differentiation. Impaired differentiation of c-Myc-deficient HSCs is linked to their localization in the differentiation preventative BM niche environment, and correlates with up-regulation of N-cadherin and a number of adhesion receptors, suggesting that release of HSCs from the stem cell niche requires c-Myc activity. Accordingly, enforced c-Myc expression in HSCs represses N-cadherin and integrins leading to loss of self-renewal activity at the expense of differentiation. Endogenous c-Myc is differentially expressed and induced upon differentiation of long-term HSCs. Collectively, our data indicate that c-Myc controls the balance between stem cell self-renewal and differentiation, presumably by regulating the interaction between HSCs and their niche.

Formin homology 2 domains occur in multiple contexts in angiosperms

Background

Involvement of conservative molecular modules and cellular mechanisms in the widely diversified processes of eukaryotic cell morphogenesis leads to the intriguing question: how do similar proteins contribute to dissimilar morphogenetic outputs. Formins (FH2 proteins) play a central part in the control of actin organization and dynamics, providing a good example of evolutionarily versatile use of a conserved protein domain in the context of a variety of lineage-specific structural and signalling interactions.

Results

In order to identify possible plant-specific sequence features within the FH2 protein family, we performed a detailed analysis of angiosperm formin-related sequences available in public databases, with particular focus on the complete Arabidopsis genome and the nearly finished rice genome sequence. This has led to revision of the current annotation of half of the 22 Arabidopsis formin-related genes. Comparative analysis of the two plant genomes revealed a good conservation of the previously described two subfamilies of plant formins (Class I and Class II), as well as several subfamilies within them that appear to predate the separation of monocot and dicot plants. Moreover, a number of plant Class II formins share an additional conserved domain, related to the protein phosphatase/tensin/auxilin fold. However, considerable inter-species variability sets limits to generalization of any functional conclusions reached on a single species such as Arabidopsis.

Conclusions

The plant-specific domain context of the conserved FH2 domain, as well as plant-specific features of the domain itself, may reflect distinct functional requirements in plant cells. The variability of formin structures found in plants far exceeds that known from both fungi and metazoans, suggesting a possible contribution of FH2 proteins in the evolution of the plant type of multicellularity.

The formins: active scaffolds that remodel the cytoskeleton

Evolutionarily conserved in eukaryotes, formin homology (FH) proteins, or formins, exert their effects on the actin and microtubule (MT) networks during meiosis, mitosis, the maintenance of cell polarity, vesicular trafficking, signaling to the nucleus and embryonic development. Once thought to be only molecular scaffolds that indirectly affected cellular functions through the binding of other proteins, recent in vitro studies have illustrated that they can function as actin nucleators in the formation of new filaments. The connection between formins and MTs is less well understood. In yeast, the MT effects appear to be dependent on the ability of formins to generate polarized actin cables whereas, in mammalian cells, formin signals that cause MT stabilization and polarization might be more direct. A subclass of formins, the Diaphanous-related formins (Drfs), can act as effectors for Rho small GTPases, yet it is not clear what GTPase binding contributes to formin function.

The formin-homology-domain-containing protein FHOD1 enhances cell migration

Formin-homology-domain-containing proteins interact with Rho-family GTPases and regulate actin cytoskeleton organization and gene transcription. FHOD1 is a member of this family, interacts with Rac1 and induces transcription from the serum response element. In this study, we examined the effects of FHOD1 expression on cytoskeletal organization and function in mammalian cells. FHOD1 proteins were stably expressed in WM35 melanoma cells and NIH-3T3 fibroblasts. Cells expressing full-length FHOD1 demonstrated an elongated phenotype compared with vector-transfected cells and cells expressing a truncated FHOD1 (1-421) that lacks the conserved FH1 and FH2 domains. Full-length FHOD1 co-localized with filamentous actin at cell peripheries. Cells transiently expressing a C-terminal FHOD1 truncation mutant (DeltaC, residues 1-1010), which lacks an autoinhibitory protein-protein interaction domain, displayed prominent stress fibers. FHOD1 (1-421) did not induce stress fibers but localized to membrane ruffles in a manner similar to the full-length protein, indicating that the FH1 and FH2 domains are required for stress fiber appearance. FHOD1 DeltaC (1-1010)-dependent stress fibers were sensitive to dominant-negative RacN17 and the RhoA and ROCK inhibitors, C3 transferase and Y-27632. Stable overexpression of full-length FHOD1 enhanced the migration of WM35 and NIH-3T3 cells to type-I collagen and fibronectin, respectively. Cells expressing FHOD1 (1-421) migrated similar to control cells. Integrin expression and activation were not affected by FHOD1 expression. Moreover, FHOD1 overexpression did not alter integrin usage during adhesion or migration. These data demonstrate that FHOD1 interacts with and regulates the structure of the cytoskeleton and stimulates cell migration in an integrin-independent manner.

c-Myc regulates mammalian body size by controlling cell number but not cell size

Overexpression of the proto-oncogene c-myc has been implicated in the genesis of diverse human tumours. c-Myc seems to regulate diverse biological processes, but its role in tumorigenesis and normal physiology remains enigmatic. Here we report the generation of an allelic series of mice in which c-myc expression is incrementally reduced to zero. Fibroblasts from these mice show reduced proliferation and after complete loss of c-Myc function they exit the cell cycle. We show that Myc activity is not needed for cellular growth but does determine the percentage of activated T cells that re-enter the cell cycle. In vivo, reduction of c-Myc levels results in reduced body mass owing to multiorgan hypoplasia, in contrast to Drosophila c-myc mutants, which are smaller as a result of hypotrophy. We find that c-myc substitutes for c-myc in fibroblasts, indicating they have similar biological activities. This suggests there may be fundamental differences in the mechanisms by which mammals and insects control body size. We propose that in mammals c-Myc controls the decision to divide or not to divide and thereby functions as a crucial mediator of signals that determine organ and body size.

Formin-2, a novel formin homology protein of the cappuccino subfamily, is highly expressed in the developing and adult central nervous system

Formin-1 is the founding member of a family of genes of emerging biological and medical importance that share specific domains of homology, allowing them to be classified together as the formin homology proteins. Although deficiency mutations in formin-1 lead to profound developmental defects in limb and kidney formation, similar deficiency mutations in more distantly related members of this family (diaphanous and cappuccino in Drosophila and BNI1 in yeast) have ostensibly unrelated phenotypes. Here we describe murine and human formin-2 (Fmn2), a gene which bears a high degree of similarity to formin-1 and cappuccino. The mouse gene, which encodes a putative 1567-amino-acid open reading frame and maps to mouse Chromosome 1, is expressed almost exclusively in the developing and mature central nervous system. Expression begins at embryonic day 9. 5 in the developing spinal cord and brain structures and continues in neonatal and adult brain structures including the olfactory bulb, cortex, thalamus, hypothalamus, hippocampus and cerebellum. Human formin-2 has a similar expression pattern.

Formin family proteins in cytoskeletal control

Functions of the cell cortex, including motility, adhesion, and cytokinesis, are mediated by the reorganization of the actin cytoskeleton. The assembly of the cytoskeletal components at cortical sites is regulated dynamically in a temporal and spatial manner. Recent evidence indicates that the formin family proteins play a crucial role in the reorganization of the cytoskeleton. In this review, recent advance in the understanding of the functions of formin family proteins is discussed.

Are plant formins integral membrane proteins?

BACKGROUND

The formin family of proteins has been implicated in signaling pathways of cellular morphogenesis in both animals and fungi; in the latter case, at least, they participate in communication between the actin cytoskeleton and the cell surface. Nevertheless, they appear to be cytoplasmic or nuclear proteins, and it is not clear whether they communicate with the plasma membrane, and if so, how. Because nothing is known about formin function in plants, I performed a systematic search for putative Arabidopsis thaliana formin homologs.

RESULTS

I found eight putative formin-coding genes in the publicly available part of the Arabidopsis genome sequence and analyzed their predicted protein sequences. Surprisingly, some of them lack parts of the conserved formin-homology 2 (FH2) domain and the majority of them seem to have signal sequences and putative transmembrane segments that are not found in yeast or animals formins.

CONCLUSIONS

Plant formins define a distinct subfamily. The presence in most Arabidopsis formins of sequence motifs typical or transmembrane proteins suggests a mechanism of membrane attachment that may be specific to plant formins, and indicates an unexpected evolutionary flexibility of the conserved formin domain.

Signal relay by BMP antagonism controls the SHH/FGF4 feedback loop in vertebrate limb buds

Outgrowth and patterning of the vertebrate limb are controlled by reciprocal interactions between the posterior mesenchyme (polarizing region) and a specialized ectodermal structure, the apical ectodermal ridge (AER). Sonic hedgehog (SHH) signalling by the polarizing region modulates fibroblast growth factor (FGF)4 signalling by the posterior AER, which in turn maintains the polarizing region (SHH/FGF4 feedback loop). Here we report that the secreted bone-morphogenetic-protein (BMP) antagonist Gremlin relays the SHH signal from the polarizing region to the AER. Mesenchymal Gremlin expression is lost in limb buds of mouse embryos homozygous for the limb deformity (Id) mutation, which disrupts establishment of the SHH/FGF4 feedback loop. Grafting Gremlin-expressing cells into ld mutant limb buds rescues Fgf4 expression and restores the SHH/FGF4 feedback loop. Analysis of Shh-null mutant embryos reveals that SHH signalling is required for maintenance of Gremlin and Formin (the gene disrupted by the ld mutations). In contrast, Formin, Gremlin and Fgf4 activation are independent of SHH signalling. This study uncovers the cascade by which the SHH signal is relayed from the posterior mesenchyme to the AER and establishes that Formin-dependent activation of the BMP antagonist Gremlin is sufficient to induce Fgf4 and establish the SHH/FGF4 feedback loop.

Embryonic renal epithelia: induction, nephrogenesis, and cell differentiation

Embryonic metanephroi, differentiating into the adult kidney, have come to be a generally accepted model system for organogenesis. Nephrogenesis implies a highly controlled series of morphogenetic and differentiation events that starts with reciprocal inductive interactions between two different primordial tissues and leads, in one of two mainstream processes, to the formation of mesenchymal condensations and aggregates. These go through the intricate process of mesenchyme-to-epithelium transition by which epithelial cell polarization is initiated, and they continue to differentiate into the highly specialized epithelial cell populations of the nephron. Each step along the developmental metanephrogenic pathway is initiated and organized by signaling molecules that are locally secreted polypeptides encoded by different gene families and regulated by transcription factors. Nephrogenesis proceeds from the deep to the outer cortex, and it is directed by a second, entirely different developmental process, the ductal branching of the ureteric bud-derived collecting tubule. Both systems, the nephrogenic (mesenchymal) and the ductogenic (ureteric), undergo a repeat series of inductive signaling that serves to organize the architecture and differentiated cell functions in a cascade of developmental gene programs. The aim of this review is to present a coherent picture of principles and mechanisms in embryonic renal epithelia.

Identification and characterization of a protein containing formin homology (FH1/FH2) domains

A novel member of the Formin/Diaphanous family of proteins was cloned and characterized. A 4kB mRNA is ubiquitously expressed but is found in abundance in the spleen. FHOS (Formin Homologue Overexpressed in Spleen) contains a 3414bp open reading frame and encodes for an approximately 128kDa protein. FHOS has sequence homology to Diaphanous and Formin proteins within the Formin Homology (FH)1 and FH2 domains. FHOS also contains a coiled-coil, a collagen-like domain, two nuclear localization signals, and several potential PKC and PKA phosphorylation sites. FHOS-specific antiserum was generated and used to determine that FHOS is a predominantly cytoplasmic protein and is expressed in a variety of human cell lines. FHOS was mapped to chromosome 16q22 between framework markers WI-5594 and WI-9392.

Cdc42: An essential Rho-type GTPase controlling eukaryotic cell polarity

Cdc42p is an essential GTPase that belongs to the Rho/Rac subfamily of Ras-like GTPases. These proteins act as molecular switches by responding to exogenous and/or endogenous signals and relaying those signals to activate downstream components of a biological pathway. The 11 current members of the Cdc42p family display between 75 and 100% amino acid identity and are functional as well as structural homologs. Cdc42p transduces signals to the actin cytoskeleton to initiate and maintain polarized gorwth and to mitogen-activated protein morphogenesis. In the budding yeast Saccharomyces cerevisiae, Cdc42p plays an important role in multiple actin-dependent morphogenetic events such as bud emergence, mating-projection formation, and pseudohyphal growth. In mammalian cells, Cdc42p regulates a variety of actin-dependent events and induces the JNK/SAPK protein kinase cascade, which leads to the activation of transcription factors within the nucleus. Cdc42p mediates these processes through interactions with a myriad of downstream effectors, whose number and regulation we are just starting to understand. In addition, Cdc42p has been implicated in a number of human diseases through interactions with its regulators and downstream effectors. While much is known about Cdc42p structure and functional interactions, little is known about the mechanism(s) by which it transduces signals within the cell. Future research should focus on this question as well as on the detailed analysis of the interactions of Cdc42p with its regulators and downstream effectors.

Gli3 (Xt) and formin (ld) participate in the positioning of the polarising region and control of posterior limb-bud identity

During initiation of limb-bud outgrowth in vertebrate embryos, the polarising region (limb-bud organizer) is established upon activation of the Sonic Hedgehog (SHH) signaling molecule at the posterior limb-bud margin. Another hallmark of establishing anteroposterior limb-bud identities is the colinear activation of HoxD genes located at the 5′ end of the cluster (5′HoxD genes). The unique and shared functions of Gli3 and formin in these determinative events were genetically analyzed using single and double homozygous Extra-toes (Xt; disrupting Gli3) and limb deformity (ld; disrupting formin) mouse embryos. Analysis of the limb skeletal phenotypes reveals genetic interaction of the two genes. In addition to loss of digit identity and varying degrees of polydactyly, proximal skeletal elements are severely shortened in Xt;ld double homozygous limbs. The underlying molecular defects affect both establishment of the polarising region and posterior limb-bud identity. In particular, the synergism between Gli3- and formin-mediated mesenchyme-AER interactions positions the SHH signaling center at the posterior limb-bud margin. The present study shows that establishment and positioning of the polarising region is regulated both by restriction of Shh through Gli3 and its positive feedback regulation through formin. Concurrently, Gli3 functions independently of formin during initial posterior nesting of 5′HoxD domains, whereas their subsequent distal restriction and anterior expansion depends on genetic interaction of Gli3 and formin.

Role of the NF-ATc transcription factor in morphogenesis of cardiac valves and septum

In lymphocytes, the expression of early immune response genes is regulated by NF-AT transcription factors which translocate to the nucleus after dephosphorylation by the Ca2+-dependent phosphatase, calcineurin. We report here that mice bearing a disruption in the NF-ATc gene fail to develop normal cardiac valves and septa and die of circulatory failure before day 14.5 of development. NF-ATc is first expressed in the heart at day 7.5, and is restricted to the endocardium, a specialized endothelium that gives rise to the valves and septum. Within the endocardium, specific inductive events appear to activate NF-ATc: it is localized to the nucleus only in endocardial cells that are adjacent to the interface with the cardiac jelly and myocardium, which are thought to give the inductive stimulus to the valve primordia. Treatment of wild-type embryos with FK506, a specific calcineurin inhibitor, prevents nuclear localization of NF-ATc. These data indicate that the Ca2+/calcineurin/NF-ATc signalling pathway is essential for normal cardiac valve and septum morphogenesis; hence, NF-ATc and its regulatory pathways are candidates for genetic defects underlying congenital human heart disease.

FH proteins as cytoskeletal organizers

Regulation of cell shape is a poorly understood yet central issue in cell biology. Recent experiments indicate that FH proteins link cellular signalling pathways to changes in cell shape. Members of the FH protein family play essential roles in cytokinesis and in driving alterations in cell polarity. This review discusses the structure and function of these proteins and examines the evidence that they interact specifically with Rho GTPases and profilin to organize the actin-based cytoskeleton.

Diverse aspects of metanephric development

Mammalian nephrogenesis constitutes a series of complex developmental processes in which there is a differentiation and rapid proliferation of pluripotent cells leading to the formation of a defined sculpted tissue mass, and this is followed by a continuum of cell replication and terminal differentiation. Metanephrogenesis ensues with the intercalation of epithelial ureteric bud into loosely organized metanephric mesenchyme. Such an interaction is reciprocal, such that the intercalating ureteric bud induces the conversion of metanephric mesenchyme into an epithelial phenotype, while the mesenchyme stimulates the iterations of the ureteric bud. The induced mesenchyme then undergoes a series of developmental stages to form a mature glomerulus and tubular segments of the kidney. Coincidental with the formation of these nephric elements, the developing kidney is vascularized by the process of vasculogenesis and angiogenesis. Thus, the process of metanephric development is quite complex, and it involves a diverse group of molecules who's biological activities are inter-linked with one another and they regulate, in a concerted manner, the differentiation and maturation of the mammalian kidney. This diverse group of molecules include extracellular matrix (ECM) proteins and their receptors, ECM-degrading enzymes and their inhibitors, growth factors and their receptors, proto-oncogenes and transcription factors. A large body of literature data are available, which suggest a critical role of these molecules in metanephric development, and this review summarizes the recent developments that relate to metanephrogenesis.

CA150, a nuclear protein associated with the RNA polymerase II holoenzyme, is involved in Tat-activated human immunodeficiency virus type 1 transcription

Maximal human immunodeficiency virus type 1 (HIV-1) gene expression requires specific cellular factors in addition to the virus-encoded trans-activator protein Tat and the RNA element TAR. We developed a functional assay, based on transcriptional activation in vitro, to identify these cellular factors. Here, we describe the purification and molecular cloning of CA150, a nuclear protein that is associated with the human RNA polymerase II holoenzyme and is involved in Tat-dependent HIV-1 transcriptional activation. The sequence of CA150 contains an extensive glutamine- and alanine-rich repeat that is found in transcriptional modulators such as GAL11 and SSN6 in Saccharomyces cerevisiae and Zeste in Drosophila melanogaster. Immunodepletion of CA150 abolished Tat trans activation in vitro. Moreover, overexpression of a mutant CA150 protein specifically and dramatically decreased Tat-mediated activation of the HIV-1 promoter in vivo, strongly suggesting a role for CA150 in HIV-1 gene regulation. Immunoprecipitation experiments demonstrated that both CA150 and Tat associate with the RNA polymerase II holoenzyme. Furthermore, we found that functional Tat associates with the holoenzyme whereas activation-deficient Tat mutants do not. Thus, we propose that Tat action is transduced via an RNA polymerase II holoenzyme that contains CA150.

Model systems for the study of kidney development: use of the pronephros in the analysis of organ induction and patterning

Most vertebrate organs, once formed, continue to perform the function for which they were generated until the death of the organism. The kidney is a notable exception to this rule. Vertebrates, even those that do not undergo metamorphosis, utilize a progression of more complex kidneys as they grow and develop. This is presumably due to the changing conditions to which the organism must respond to retain what Homer Smith referred to as our physiological freedom. To quote, "Recognizing that we have the kind of blood we have because we have the kind of kidneys we have, we must acknowledge that our kidneys constitute the major foundation of our physiological freedom. Only because they work the way they do has it become possible for us to have bones, muscles, glands, and brains. Superficially, it might be said that the function of the kidneys is to make urine; but in a more considered view one can say that the kidneys make the stuff of philosophy itself" ("From Fish to Philosopher," Little, Brown and Co., Boston, 1953). Different kidneys are used to make the stuff of philosophy at different stages of development depending on the age and needs of the organism, rather than the usual approach of simply making embryonic organs larger as the animal grows. Although evolution has provided the higher vertebrates with complex adult kidneys, they continue to utilize simple kidneys in embryogenesis. In lower vertebrates with simple adult kidneys, even more simple versions are used during early developmental stages. In this review the anatomy, development, and gene expression patterns of the embryonic kidney, the pronephros, will be described and compared to the more complex kidney forms. Despite some differences in anatomy, similar developmental pathways seem to be responsible for the induction and the response to induction in both evanescent and permanent kidney forms. Gene expression patterns can, therefore, be added to the morphological and functional data indicating that all forms of the kidney are closely related structures. Given the similarities between the development of simple and complex kidneys, the embryonic kidneys may be an ideal model system in which to investigate the genesis of multicomponent organ systems.

Actin cytoskeleton: are FH proteins local organizers?

The FH proteins, defined by the presence of 'formin homology' regions, are important for a number of actin-dependent processes, including polarized cell growth and cytokinesis. They are large, probably multi-domain, proteins and their function may be in part mediated by an interaction with profilin.

Dorso-ventral limb polarity and origin of the ridge: on the fringe of independence?

Molecular and developmental studies of limb pattern formation have recently gained widespread attention. The fact that vertebrate limbs are amenable to both genetic and embryological manipulations has established this model system as a valuable paradigm for studying vertebrate development. Limb buds are polarised along all three major axes and the establishment of the dorso-ventral (DV) polarity is dependent upon cues localised in the trunk, where a DV ectodermal interface is produced by confrontation of dorsal and ventral identities. By analogy to Drosophila imaginal disc development, this interface has been proposed to determine and position an ectodermal organising centre, the Apical Ectodermal Ridge (AER), controlling limb bud outgrowth. Recent fate mapping studies and studies of genes regulating DV limb polarity, AER formation and differentiation suggest, however, that DV patterning and AER induction, though coordinately regulated during limb bud outgrowth, may early on be more dissociated than expected.

FBP WW domains and the Abl SH3 domain bind to a specific class of proline-rich ligands

WW domains are conserved protein motifs of 38-40 amino acids found in a broad spectrum of proteins. They mediate protein-protein interactions by binding proline-rich modules in ligands. A 10 amino acid proline-rich portion of the morphogenic protein, formin, is bound in vitro by both the WW domain of the formin-binding protein 11 (FBP11) and the SH3 domain of Abl. To explore whether the FBP11 WW domain and Abl SH3 domain bind to similar ligands, we screened a mouse limb bud expression library for putative ligands of the FBP11 WW domain. In so doing, we identified eight ligands (WBP3 through WBP10), each of which contains a proline-rich region or regions. Peptide sequence comparisons of the ligands revealed a conserved motif of 10 amino acids that acts as a modular sequence binding the FBP11 WW domain, but not the WW domain of the putative signal transducing factor, hYAP65. Interestingly, the consensus ligand for the FBP11 WW domain contains residues that are also required for binding by the Abl SH3 domain. These findings support the notion that the FBP11 WW domain and the Abl SH3 domain can compete for the same proline-rich ligands and suggest that at least two subclasses of WW domains exist, namely those that bind a PPLP motif, and those that bind a PPXY motif.

cdc12p, a protein required for cytokinesis in fission yeast, is a component of the cell division ring and interacts with profilin

As in many other eukaryotic cells, cell division in fission yeast depends on the assembly of an actin ring that circumscribes the middle of the cell. Schizosaccharomyces pombe cdc12 is an essential gene necessary for actin ring assembly and septum formation. Here we show that cdc12p is a member of a family of proteins including Drosophila diaphanous, Saccharomyces cerevisiae BNI1, and S. pombe fus1, which are involved in cytokinesis or other actin-mediated processes. Using indirect immunofluorescence, we show that cdc12p is located in the cell division ring and not in other actin structures. When overexpressed, cdc12p is located at a medial spot in interphase that anticipates the future ring site. cdc12p localization is altered in actin ring mutants. cdc8 (tropomyosin homologue), cdc3 (profilin homologue), and cdc15 mutants exhibit no specific cdc12p staining during mitosis. cdc4 mutant cells exhibit a medial cortical cdc12p spot in place of a ring. mid1 mutant cells generally exhibit a cdc12p spot with a single cdc12p strand extending in a random direction. Based on these patterns, we present a model in which ring assembly originates from a single point on the cortex and in which a molecular pathway for the functions of cytokinesis proteins is suggested. Finally, we found that cdc12 and cdc3 mutants show a synthetic-lethal genetic interaction, and a proline-rich domain of cdc12p binds directly to profilin cdc3p in vitro, suggesting that one function of cdc12p in ring assembly is to bind profilin.

Conservation of the Notch signalling pathway in mammalian neurogenesis

The Notch pathway functions in multiple cell fate determination processes in invertebrate embryos, including the decision between the neuroblast and epidermoblast lineages in Drosophila. In the mouse, targeted mutation of the Notch pathway genes Notch1 and RBP-Jk has demonstrated a role for these genes in somite segmentation, but a function in neurogenesis and in cell fate decisions has not been shown. Here we show that these mutations lead to altered expression of the Notch signalling pathway homologues Hes-5, Mash-1 and Dll1, resulting in enhanced neurogenesis. Precocious neuronal differentiation is indicated by the expanded expression domains of Math4A, neuroD and NSCL-1. The RBP-Jk mutation has stronger effects on expression of these genes than does the Notch1 mutation, consistent with functional redundancy of Notch genes in neurogenesis. Our results demonstrate conservation of the Notch pathway and its regulatory mechanisms from fly to mouse, and support a role for the murine Notch signalling pathway in the regulation of neural stem cell differentiation.

The mouse formin (Fmn) gene: genomic structure, novel exons, and genetic mapping

Mutations in the mouse formin (Fmn) gene, formerly known as the limb deformity (ld) gene, give rise to recessively inherited limb deformities and renal malformations or aplasia. The Fmn gene encodes many differentially processed transcripts that are expressed in both adult and embryonic tissues. To study the genomic organization of the Fmn locus, we have used Fmn probes to isolate and characterize genomic clones spanning 500 kb. Our analysis of these clones shows that the Fmn gene is composed of at least 24 exons and spans 400 kb. We have identified two novel exons that are expressed in the developing embryonic limb bud as well as adult tissues such as brain and kidney. We have also used a microsatellite polymorphism from within the Fmn gene to map it genetically to a 2.2-cM interval between D2Mit58 and D2Mit103.

Limb deformity proteins: role in mesodermal induction of the apical ectodermal ridge

During early limb development, distal tip ectoderm is induced by the underlying mesenchyme to form the apical ectodermal ridge. Subsequent limb growth and patterning depend on reciprocal signaling between the mesenchyme and ridge. Mice that are homozygous for mutations at the limb deformity (ld) locus do not form a proper ridge and the anteroposterior axis of the limb is shortened. Skeletal analyses reveal shortened limbs that involve loss and fusion of distal bones and digits, defects in both anteroposterior and proximodistal patterning. Using molecular markers and mouse-chick chimeras we examined the ridge-mesenchymal interactions to determine the origin of the ld patterning defects. In the ld ridge, fibroblast growth factor 8 (Fgf8) RNA is decreased and Fgf4 RNA is not detected. In the ld mesenchyme, Sonic hedgehog (Shh), Evx1 and Wnt5a expression is decreased. In chimeras between ld ectoderm and wild-type mesenchyme, a ridge of normal morphology and function is restored, Fgf8 and Shh are expressed normally, Fgf4 is induced and a normal skeletal pattern arises. These results suggest that the ld mesenchyme is unable to induce the formation of a completely functional ridge. This primary defect causes a disruption of ridge function and subsequently leads to the patterning defects observed in ld limbs. We propose a model in which ridge induction requires at least two phases: an early competence phase, which includes induction of Fgf8 expression, and a later differentiation phase in which Fgf4 is induced and a morphological ridge is formed. Ld proteins appear to act during the differentiation phase.

Molecular interaction between limb deformity proteins (formins) and Src family kinases

Ld proteins (formins) are encoded by the limb deformity (ld) gene and define a family of related gene products regulating establishment of embryonic polarity. In this study we establish that chicken and murine Ld proteins interact directly with Src family kinases (c-Src and c-Fyn). Specific binding is mediated by the proline-rich domain present in Ld proteins and the ligand binding surface of the Src SH3 domain. Co-immunoprecipitation of Ld and c-Src proteins from transfected cells shows that these proteins associate in vivo. Immunolocalization and biochemical fractionation of fibroblasts confirms the predominant nuclear localization of Ld proteins, but unexpectedly identifies a population of Ld proteins associated to cellular membranes. This population of Ld proteins co-localizes with membrane-associated c-Src proteins at both plasma and perinuclear membranes. These studies indicate that the morphoregulatory Ld proteins interact with signal transduction cascades by association to membrane-bound Src family kinases.

Positional cloning of the Fanconi anaemia group A gene

Fanconi anaemia (FA) is an autosomal recessive disorder associated with progressive bone-marrow failure, a variety of congenital abnormalities, and predisposition to acute myeloid leukaemia. Cells from FA patients show increased sensitivity to bifunctional DNA crosslinking agents such as diepoxybutane and mitomycin C, with characteristic chromosome breakage. FA is genetically heterogeneous, at least five different complementation groups (FA-A to FA-E) having been described. The gene for group C (FAC) was cloned by functional complementation and mapped to chromosome 9q22.3 (refs 3, 5), but the genes for the other complementation groups have not yet been identified. The group A gene (FAA) has recently been mapped to chromosome 16q24.3 by linkage analysis, and accounts for 60-65% of FA cases. We narrowed the candidate region by linkage and allelic association analysis, and have isolated a gene that is mutated in FA-A patients. The gene encodes a protein of 1,455 amino acids that has no significant homology to any other known proteins, and may therefore represent a new class of genes associated with the prevention or repair of DNA damage.

Genetic evidence that formins function within the nucleus

The murine limb deformity (ld) locus encodes a set of proteins, termed formins, that are required for embryonic limb and kidney development. Previous studies had indicated that these proteins are located in the nucleus and cytoplasm and have biochemical properties consistent with an action within the nucleus. To test the notion that nuclear localization is crucial for formin function, we carried out molecular and biochemical studies on three ld alleles. We show that two transgene-induced alleles, ldTgHd and ldTgBri, generate similar COOH-truncated formins that lack the terminal 110 amino acids, while a third allele, ldIn2, generates a less extensively truncated formin that lacks the terminal 42 amino acids. Using subcellular fractionation analysis, we find that wild-type formin is detected in both nuclear and cytosolic fractions; in contrast, the truncated formins encoded by ldTgHd and ldTgBri are strictly cytosolic. The less extensively truncated ldIn2 formin shows a similar, but less complete, localization defect. Consistent with this weaker cellular phenotype, hind limbs from ldIn2 mice have milder skeletal defects than those of ldTgBri mice. These observations define a small region in the carboxyl terminus that is required for nuclear localization and suggest that nuclear localization plays a role in formin action.

The tumor suppressor gene Brca1 is required for embryonic cellular proliferation in the mouse

Mutations of the BRCA1 gone in humans are associated with predisposition to breast and ovarian cancers. We show here that Brca1+/- mice are normal and fertile and lack tumors by age eleven months. Homozygous Brca1(5-6) mutant mice die before day 7.5 of embryogenesis. Mutant embryos are poorly developed, with no evidence of mesoderm formation. The extraembryonic region is abnormal, but aggregation with wild-type tetraploid embryos does not rescue the lethality. In vivo, mutant embryos do not exhibit increased apoptosis but show reduced cell proliferation accompanied by decreased expression of cyclin E and mdm-2, a regulator of p53 activity. The expression of cyclin-dependent kinase inhibitor p21 is dramatically increased in the mutant embryos. Buttressing these in vivo observations is the fact that mutant blastocyst growth is grossly impaired in vitro. Thus, the death of Brca1(5-6) mutant embryos prior to gastrulation may be due to a failure of the proliferative burst required for the development of the different germ layers.

Polydactyly in the Strong's luxoid mouse is suppressed by limb deformity alleles

The study of limb development has provided insight into pattern formation during vertebrate embryogenesis. Genetic approaches offer powerful ways to identify the critical molecules and their pathways of action required to execute a complex morphogenetic program. We have applied genetic analysis to the process of limb development by studying two mouse mutants, limb deformity (ld) and Strong's luxoid (lst). These mutations confer contrasting phenotypic alterations to the anteroposterior limb pattern. The six mutant ld alleles are fully recessive and result in oligosyndactyly of all four limbs. By contrast, the two mutant lst alleles result in a mirror-image polydactylous limb phenotype inherited in a semidominant fashion. Morphological and molecular analysis of embryonic limbs has shown that the ld and lst alleles affect the extent and distribution of two key signaling centers differentially: the apical ectodermal ridge and the zone of polarizing activity. Molecular characterization of the ld gene has defined a new family of evolutionarily conserved proteins termed the formins. The underlying molecular defect in the lst mutation has not been identified; however, both loci are tightly linked on mouse chromosome 2, suggesting the possibility that they may be allelic. In this study, we have used genetic analysis to examine the epistatic and allelic relationships of ld and lst. We observed that in + ld/lst + double heterozygotes, a single mutant ld allele is able to suppress the semi-dominant polydactylous lst limb phenotype. By segregating the lst and ld loci in a backcross, we observed that these loci recombine and are separated by a genetic distance of approximately 6 cM. Therefore, while our observations demonstrate a genetic interaction between ld and lst, it is probable that ld and lst are not allelic. Instead, lst and ld may be operating either in a linear or in a parallel (bypass) genetic pathway to affect the limb signaling centers.

Cappuccino, a Drosophila maternal effect gene required for polarity of the egg and embryo, is related to the vertebrate limb deformity locus

We report the molecular isolation of cappuccino (capu), a gene required for localization of molecular determinants within the developing Drosophila oocyte. The carboxy-terminal half of the capu protein is closely related to that of the vertebrate limb deformity locus, which is known to function in polarity determination in the developing vertebrate limb. In addition, capu shares both a proline-rich region and a 70-amino-acid domain with a number of other genes, two of which also function in pattern formation, the Saccharomyes cerevisiae BNI1 gene and the Aspergillus FigA gene. We also show that capu mutant oocytes have abnormal microtubule distributions and premature microtubule-based cytoplasmic streaming within the oocyte, but that neither the speed nor the timing of the cytoplasmic streaming correlates with the strength of the mutant allele. This suggests that the premature cytoplasmic streaming in capu mutant oocytes does not suffice to explain the patterning defects. By inducing cytoplasmic streaming in wild-type oocytes during mid-oogenesis, we show that premature cytoplasmic streaming can displace staufen protein from the posterior pole, but not gurken mRNA from around the oocyte nucleus.

Formin isoforms are differentially expressed in the mouse embryo and are required for normal expression of fgf-4 and shh in the limb bud

Mice homozygous for the recessive limb deformity (ld) mutation display both limb and renal defects. The limb defects, oligodactyly and syndactyly, have been traced to improper differentiation of the apical ectodermal ridge (AER) and shortening of the anteroposterior limb axis. The renal defects, usually aplasia, are thought to result from failure of ureteric bud outgrowth. Since the ld locus gives rise to multiple RNA isoforms encoding several different proteins (termed formins), we wished to understand their role in the formation of these organs. Therefore, we first examined the embryonic expression patterns of the four major ld mRNA isoforms. Isoforms I, II and III (all containing a basic amino terminus) are expressed in dorsal root ganglia, cranial ganglia and the developing kidney including the ureteric bud. Isoform IV (containing an acidic amino terminus) is expressed in the notochord, the somites, the apical ectodermal ridge (AER) of the limb bud and the developing kidney including the ureteric bud. Using a lacZ reporter assay in transgenic mice, we show that this differential expression of isoform IV results from distinct regulatory sequences upstream of its first exon. These expression patterns suggest that all four isoforms may be involved in ureteric bud outgrowth, while isoform IV may be involved in AER differentiation. To define further the developmental consequences of the ld limb defect, we analyzed the expression of a number of genes thought to play a role in limb development. Most significantly, we find that although the AERs of ld limb buds express several AER markers, they do not express detectable levels of fibroblast growth factor 4 (fgf-4), which has been proposed to be the AER signal to the mesoderm. Thus we conclude that one or more formins are necessary to initiate and/or maintain fgf-4 production in the distal limb. Since ld limbs form distal structures such as digits, we further conclude that while fgf-4 is capable of supporting distal limb outgrowth in manipulated limbs, it is not essential for distal outgrowth in normal limb development. In addition, ld limbs show a severe decrease in the expression of several mesodermal markers, including sonic hedgehog (shh), a marker for the polarizing region and Hoxd-12, a marker for posterior mesoderm. We propose that incomplete differentiation of the AER in ld limb buds leads to reduction of polarizing activity and defects along the anteroposterior axis.

Limb deformity proteins during avian neurulation and sense organ development

The nuclear Limb deformity (Ld) proteins (formins) are expressed during the avian primitive streak stages. Initially, they are detected predominantly in cells of the forming notochord, scattered mesodermal precursors and the induced neural plate. No expression is detected in endodermal cells. The subsequent graded distribution of Ld positive cells along the anterior-posterior axis of the neural tube follows the antero-posterior progression of its differentiation. The Ld proteins are also differentially expressed during induction and development of both the inner ear and eye. An unequal distribution of Ld proteins along the dorso-ventral axis of the otic vesicle is observed during its initial patterning. In the eye, the Ld proteins are expressed by the optic vesicle during secondary induction of the lens placode. Following induction, the proteins are also expressed by the newly formed lens placode, a process which is reminiscent of homeogenetic induction. During differentiation of the retina and lens, the Ld domains seem to demarcate territories, giving rise to specific eye structures. A comparative analysis of the Ld distribution and BrdU incorporation in the two sense organs indicates that the proteins are predominantly expressed by committed and/or differentiating (post-mitotic) cells. In general, expression of Ld proteins is induced during determination and remains during differentiation of particular cell-types. This study implies that the nuclear Ld proteins are involved in morphogenesis of both neuro-ectodermal and mesodermal structures.

Dorsoventral patterning of the avian mesencephalon/metencephalon: role of the notochord and floor plate in suppressing Engrailed-2

Transcription factors that are spatially and temporally restricted within the embryo may be used for dorsoventral and rostrocaudal positional information during development. The Engrailed-2 (En-2) gene is expressed across the mesencephalon/metencephalon (mes/met) boundary in the cerebellar primordium with strong dorsolateral expression and limited expression in the floor plate. In a previous experiment we demonstrated that, after removal of Hensen's node, embryos lacked a notochord in the head and the pattern of En-2 expression was normal rostrocaudally, but it was expanded into the ventral midline of the neural tube. This suggested that the notochord suppresses En-2 in the ventral neural tube during normal development. To test further the ability of the notochord (and floor plate) to suppress En-2, we transplanted ventral midline tissues from HH 5-9 quail embryos beneath the rostral neural plate of HH 4-6 chick embryos. After 24 hours in culture, 90% of the embryos with quail notochord or floor plate near the mes/met of the host lacked En-2 expression adjacent to the graft, and suppression was distance dependent. Enzymatically isolated notochords also suppressed En-2 (71%), but the results from isolated floor plates were inconclusive. Other grafts served as controls and included tissues from the trunk ventral midline, mes/met level dorsolateral neural plate, and trunk dorsolateral neural plate/somite. Collectively, the results suggest that during normal development the notochord and possibly the floor plate are important regulators of normal En-2 expression.

Diaphanous is required for cytokinesis in Drosophila and shares domains of similarity with the products of the limb deformity gene

We show that the Drosophila gene diaphanous is required for cytokinesis. Males homozygous for the dia1 mutation are sterile due to a defect in cytokinesis in the germline. Females trans-heterozygous for dia1 and a deficiency are sterile and lay eggs with defective eggshells; failure of cytokinesis is observed in the follicle cell layer. Null alleles are lethal. Death occurs at the onset of pupation due to the absence of imaginal discs. Mitotic figures in larval neuroblasts were found to be polyploid, apparently due to a defect in cytokinesis. The predicted 123 × 10(3) M(r) protein contains two domains shared by the formin proteins, encoded by the limb deformity gene in the mouse. These formin homology domains, which we have termed FH1 and FH2, are also found in Bni1p, the product of a Saccharomyces cerevisiae gene required for normal cytokinesis in diploid yeast cells.

Towards a genetic basis for kidney development

Although it is not easy to investigate the regulatory basis of developmental processes in most mammalian tissues, the mouse kidney has several distinct advantages as a model system. Its development involves a wide variety of developmental processes that include induction, stem-cell regulation, a mesenchyme-to-epithelium transition, epithelial morphogenesis and pattern formation. Further, there are several genetic disorders associated with its development, much of nephrogenesis will take place in vitro and a significant start has been made in elucidating the regulatory molecules involved in its ontogeny. Here, we summarise current knowledge on how the various aspects of kidney development are controlled at the genetic level. For this, we have compiled a table showing when and where the more than forty regulatory genes thus far identified are expressed during nephrogenesis (this table being a subset of a database also containing information on structural and functional proteins expressed during nephrogenesis). The data on the regulatory genes demonstrate, in particular, the importance of the Wilms' tumour gene, WT1, in nephrogenesis, the growth-stimulating interaction between the hepatocyte growth factor and its receptor, c-met, and the differences between uninduced and induced metanephric mesenchyme. In an attempt to highlight those stable developmental pathways which underpin the formation of the kidney and to facilitate future work, we have identified possible checkpoints occurring during nephrogenesis (stages at which a positive signal is needed for development to continue). The data to hand suggest that such checkpoints occur when metanephric mesenchyme is established in the intermediate mesoderm, when induction takes place, when stem cells are activated and before mesenchyme aggregates to form nephrogenic condensations.(ABSTRACT TRUNCATED AT 250 WORDS)

Programmed cell death is affected in the novel mouse mutant Fused toes (Ft)

We have identified a novel dominant mouse mutant that is characterised by fused toes on the fore limbs and a thymic hyperplasia, in heterozygous animals. Homozygosity of the mutation leads to malformation of the developing brain, lost of the genetic control of left-right asymmetry and to death around day 10 of development. Analysis of both limb development and induction of apoptosis in immature thymocytes in vitro suggest that programmed cell death is affected by the mutation. Since the mutation was caused via a transgene insertion we were able to map it to the D region on mouse chromosome 8. So far, no mutation that affects programmed cell death has been mapped to this chromosome. Thus, this mutation will allow the identification of a novel gene involved in programmed cell death during mammalian development.

Deficient outgrowth of the ureteric bud underlies the renal agenesis phenotype in mice manifesting the limb deformity (ld) mutation

Mice which are homozygous for the limb deformity (ld) mutation also manifest an incompletely penetrant unilateral or bilateral renal agenesis phenotype. Intercross experiments suggest that the differences in penetrance of the renal agenesis phenotype between homozygous mice with different ld alleles are due to intrinsic differences in the strength of the mutant alleles or to one or more closely linked modifying loci, and not to generalized differences in genetic background. Analysis of ld/ld embryos between embryonic days 11-13 reveals delayed outgrowth or complete absence of the ureteric bud, the inducer of metanephric mesenchyme. Since explants of ld/ld metanephric mesenchyme differentiate in culture when apposed to embryonic spinal cord, we conclude that deficient ureteric bud outgrowth is the morphologic basis for renal agenesis in ld/ld mice. However, since ld transcripts can be detected in both metanephric mesenchyme and ureteric bud, the molecular basis for the deficiency in ureteric bud outgrowth could reside in either component.

Sequential expression of HNF-3 beta and HNF-3 alpha by embryonic organizing centers: the dorsal lip/node, notochord and floor plate

Axial patterning in the nervous system of vertebrate embryos depends on inductive signals that derive from the organizer region (the dorsal lip in amphibians and the node in birds and mammals) and leter from the notochord and floor plate. Previous studies have shown that Pintallavis, a member of the HNF-3/fork head transcription factor family, is expressed selectively by these cell groups in frog embryos and may be involved in regulating neural development. We report here that in early rat and mouse embryos, the embryonic endoderm, the node, the notochord and the floor plate express two related transcription factors, HNF-3 alpha and HNF-3 beta, which also function in the control of liver cell differentiation. Early embryonic tissues express variant forms of HNF-3 beta which derive from the use of 5' alternative exons. Within the organizer region and notochord, HNF-3 beta and HNF-3 alpha have distinct temporal patterns of expression and appear in partially overlapping domains. The early expression pattern of mammalian HNF-3 beta in the node, notochord and midline neural plate cells is similar to that of Pintallavis in frog embryos. There does not appear to be a Pintallavis homologue in mice. This prompted us to isolate and analyze the expression of the frog HNF-3 beta gene. In frog embryos, HNF-3 beta is expressed in the dorsal lip, pharyngeal endoderm and floor plate. In contrast to mammalian HNF-3 beta, the onset of frog HNF-3 beta expression in neural tissue occurs after neural tube closure. Thus, the combined expression patterns of Pintallavis and HNF-3 beta in frogs is equivalent to that of HNF-3 beta in rats and mice. Within neural tissue, the onset of expression of these regulatory genes define successive stages in the differentiation of floor plate cells. The results reported here show that closely related members of the HNF-3/fork head gene family are expressed by axial midline cell groups involved in neural induction and patterning and suggest the involvement of these genes in the development of the vertebrate neuraxis.

Ectopic expression of genes during chicken limb pattern formation using replication defective retroviral vectors

A gene transfer method to ectopically express genes during chicken limb pattern formation using replication defective retroviral vectors has been established. Spherical non-proliferating (mitomycin C treated) aggregates of clonal retrovirus producing cells were grafted directly into developing chicken wing buds. The cell aggregates had to be placed in direct contact with the highly proliferative cells of the wing bud to promote efficient in vivo infection of embryonic cells by the released retroviral particles. The majority of grafts resulted in widespread expression of a reporter gene (encoding bacterial beta-galactosidase) during limb pattern formation and early limb bud outgrowth without affecting morphogenesis. This method provides a novel approach to study the effects of ectopic gene expression on limb pattern formation. Possible future applications to study other developmental processes are discussed.

FGF-4 regulates expression of Evx-1 in the developing mouse limb

We describe here the temporal and spatial pattern of expression of Evx-1, a murine homolog of the Drosophila even-skipped gene, in the developing limb bud. Evx-1 RNA is first detected in distal limb (progress zone) mesenchyme shortly after the formation of the apical ectodermal ridge. The level of Evx-1 RNA increases during the next 24 hours of development, and then decreases in the subsequent 24 hours, such that by the time the ridge regresses Evx-1 RNA is undetectable. At all these stages, Evx-1 RNA is localized primarily to the posterior distal mesenchyme, in the region immediately underlying that portion of the ridge in which the Fgf-4 gene is expressed. Using an in vitro culture system, we show that the ridge is required for both the induction and maintenance of Evx-1 expression in the distal mesenchyme. We also demonstrate that in the absence of the ridge, FGF-4, as well as other FGF proteins, can induce Evx-1 expression in the limb bud. However, this effect appears to be indirect, since it can be blocked by an inhibitor of protein synthesis. Additional studies demonstrate that the effect of FGF-4 on Evx-1 expression is modulated by BMP-2. These data serve to identify Evx-1 as a downstream gene in the FGF signal transduction pathway in the limb.

Formins: phosphoprotein isoforms encoded by the mouse limb deformity locus

Mutations at the mouse limb deformity (ld) locus result in defects of growth and patterning of the limb and kidney during embryonic development. The gene responsible for this phenotype is large and complex, with the capacity to generate a number of alternatively spliced messenger RNA transcripts encoding nuclear protein isoforms called "formins." We have made polyclonal antibodies to specific formin peptides and have confirmed the authenticity of the antibodies' reactivity, using cell lines derived from mice with molecularly defined mutations at the ld locus. In addition, we have used these antibodies to detect and characterize polypeptides encoded by both wild-type and mutant ld alleles. In so doing, we show that a formin isoform (i) is modified by posttranslational phosphorylation at serine and threonine residues and (ii) when present in a crude nuclear extract, is retained by DNA-cellulose.

Defects of embryonic organogenesis resulting from targeted disruption of the N-myc gene in the mouse

The highest expression of the N-myc gene occurs during embryonic organogenesis in the mouse ontogeny, with the peak of expression around embryonic day 9.5. Homozygous N-myc-deficient mice, produced by germline transmission of a disrupted allele in ES cells, developed normally to day 10.5, indicating dispensability of N-myc expression in the earlier period, but later accumulated organogenic abnormalities and died around day 11.5. The most notable abnormalities were found in the limb bud, visceral organs (lung, stomach, liver and heart) and the central/peripheral nervous systems, and were highly correlated with the site of N-myc expression. The limb buds and the lungs excised from N-myc-deficient mutant embryos were placed in culture to allow their development to stages beyond the point of death of the embryos. Analyses indicated that the mutant limbs failed to develop distal structures and the development of bronchi from the trachea was defective in the lungs. The latter defect was largely corrected by addition of fetal calf serum to the culture medium, suggesting that an activity missing in the mutant lung was replenished by a component of the serum. The phenotype of N-myc-deficient mutant embryos indicated requirement of the N-myc function in many instances of tissue interactions in organogenesis and also in cell-autonomous regulation of tissue maturation.