Nuclear import of the homeodomain protein extradenticle in response to Wg and Dpp signalling

Posttranslational Modifications in Conserved Transcription Factors: A Survey of the TALE-Homeodomain Superclass in Human and Mouse

Transcription factors (TFs) guide effector proteins like chromatin-modifying or -remodeling enzymes to distinct sites in the genome and thereby fulfill important early steps in translating the genome’s sequence information into the production of proteins or functional RNAs. TFs of the same family are often highly conserved in evolution, raising the question of how proteins with seemingly similar structure and DNA-binding properties can exert physiologically distinct functions or respond to context-specific extracellular cues. A good example is the TALE superclass of homeodomain-containing proteins. All TALE-homeodomain proteins share a characteristic, 63-amino acid long homeodomain and bind to similar sequence motifs. Yet, they frequently fulfill non-redundant functions even in domains of co-expression and are subject to regulation by different signaling pathways. Here we provide an overview of posttranslational modifications that are associated with murine and human TALE-homeodomain proteins and discuss their possible importance for the biology of these TFs.

Low affinity binding sites in an activating CRM mediate negative autoregulation of the Drosophila Hox gene Ultrabithorax

Specification of cell identity and the proper functioning of a mature cell depend on precise regulation of gene expression. Both binary ON/OFF regulation of transcription, as well as more fine-tuned control of transcription levels in the ON state, are required to define cell types. The Drosophila melanogaster Hox gene, Ultrabithorax (Ubx), exhibits both of these modes of control during development. While ON/OFF regulation is needed to specify the fate of the developing wing (Ubx OFF) and haltere (Ubx ON), the levels of Ubx within the haltere differ between compartments along the proximal-distal axis. Here, we identify and molecularly dissect the novel contribution of a previously identified Ubx cis-regulatory module (CRM), anterobithorax (abx), to a negative auto-regulatory loop that decreases Ubx expression in the proximal compartment of the haltere as compared to the distal compartment. We find that Ubx, in complex with the known Hox cofactors, Homothorax (Hth) and Extradenticle (Exd), acts through low-affinity Ubx-Exd binding sites to reduce the levels of Ubx transcription in the proximal compartment. Importantly, we also reveal that Ubx-Exd-binding site mutations sufficient to result in de-repression of abx activity in a transgenic context are not sufficient to de-repress Ubx expression when mutated at the endogenous locus, suggesting the presence of multiple mechanisms through which Ubx-mediated repression occurs. Our results underscore the complementary nature of CRM analysis through transgenic reporter assays and genome modification of the endogenous locus; but, they also highlight the increasing need to understand gene regulation within the native context to capture the potential input of multiple genomic elements on gene control.

Pbx4 is Required for the Temporal Onset of Zebrafish Myocardial Differentiation

Proper control of the temporal onset of cellular differentiation is critical for regulating cell lineage decisions and morphogenesis during development. Pbx homeodomain transcription factors have emerged as important regulators of cellular differentiation. We previously showed, by using antisense morpholino knockdown, that Pbx factors are needed for the timely activation of myocardial differentiation in zebrafish. In order to gain further insight into the roles of Pbx factors in heart development, we show here that zebrafish pbx4 mutant embryos exhibit delayed onset of myocardial differentiation, such as delayed activation of tnnt2a expression in early cardiomyocytes in the anterior lateral plate mesoderm. We also observe delayed myocardial morphogenesis and dysmorphic patterning of the ventricle and atrium, consistent with our previous Pbx knock-down studies. In addition, we find that pbx4 mutant larvae have aberrant outflow tracts and defective expression of the proepicardial marker tbx18. Finally, we present evidence for Pbx expression in cardiomyocyte precursors as well as heterogeneous Pbx expression among the pan-cytokeratin-expressing proepicardial cells near the developing ventricle. In summary, our data show that Pbx4 is required for the proper temporal activation of myocardial differentiation and establish a basis for studying additional roles of Pbx factors in heart development.

Building and specializing epithelial tubular organs: the Drosophila salivary gland as a model system for revealing how epithelial organs are specified, form and specialize

The past two decades have witnessed incredible progress toward understanding the genetic and cellular mechanisms of organogenesis. Among the organs that have provided key insight into how patterning information is integrated to specify and build functional body parts is the Drosophila salivary gland, a relatively simple epithelial organ specialized for the synthesis and secretion of high levels of protein. Here, we discuss what the past couple of decades of research have revealed about organ specification, development, specialization, and death, and what general principles emerge from these studies.

Specification of regional intestinal stem cell identity during Drosophila metamorphosis

In the adult Drosophila midgut the bone morphogenetic protein (BMP) signaling pathway is required to specify and maintain the acid-secreting region of the midgut known as the copper cell region (CCR). BMP signaling is also involved in the modulation of intestinal stem cell (ISC) proliferation in response to injury. How ISCs are able to respond to the same signaling pathway in a regionally different manner is currently unknown. Here, we show that dual use of the BMP signaling pathway in the midgut is possible because BMP signals are only capable of transforming ISC and enterocyte identity during a defined window of metamorphosis. ISC heterogeneity is established prior to adulthood and then maintained in cooperation with regional signals from surrounding tissue. Our data provide a conceptual framework for how other tissues maintained by regional stem cells might be patterned and establishes the pupal and adult midgut as a novel genetic platform for identifying genes necessary for regional stem cell specification and maintenance.

Gene duplication, lineage-specific expansion, and subfunctionalization in the MADF-BESS family patterns the Drosophila wing hinge

Gene duplication, expansion, and subsequent diversification are features of the evolutionary process. Duplicated genes can be lost, modified, or altered to generate novel functions over evolutionary timescales. These features make gene duplication a powerful engine of evolutionary change. In this study, we explore these features in the MADF-BESS family of transcriptional regulators. In Drosophila melanogaster, the family contains 16 similar members, each containing an N-terminal, DNA-binding MADF domain and a C-terminal, protein-interacting, BESS domain. Phylogenetic analysis shows that members of the MADF-BESS family are expanded in the Drosophila lineage. Three members, which we name hinge1, hinge2, and hinge3 are required for wing development, with a critical role in the wing hinge. hinge1 is a negative regulator of Winglesss expression and interacts with core wing-hinge patterning genes such as teashirt, homothorax, and jing. Double knockdowns along with heterologous rescue experiments are used to demonstrate that members of the MADF-BESS family retain function in the wing hinge, in spite of expansion and diversification for over 40 million years. The wing hinge connects the blade to the thorax and has critical roles in fluttering during flight. MADF-BESS family genes appear to retain redundant functions to shape and form elements of the wing hinge in a robust and fail-safe manner.

Functional dissection of the splice variants of the Drosophila gene homothorax (hth)

Homothorax belongs to the TALE-homeodomain family of transcription factors, together with its vertebrate counterparts, the Meis family of proto-oncogenes. It fulfills many important different functions during embryonic and larval developments in Drosophila, which encompass from subdivision and specification of body parts to assembly of heterochromatin structures. Hth interacts with Extradenticle, another member of the TALE-homeodomain family of conserved transcription factors, to facilitate its entrance to the nucleus. The many different functions described for Hth rely on the complexity of the locus, from which six different isoforms arise. The isoforms can be grouped into full-length and short versions, which contain either one or the two conserved domains of the protein (homeodomain and Exd-interacting domain). We have used molecular and genetic tools to analyze the levels of expression, the distribution and the function of the isoforms during embryonic development. Our results clearly show that the isoforms display distinct levels of expression and are differentially distributed in the embryo. This detailed study also shows that during normal embryonic development not all the Hth isoforms translocate Exd into the nucleus, suggesting that both the proteins can also function separately. We have demonstrated that the full-length Hth protein activates transcription of exd, augmenting the levels of exd mRNA in the cell. The higher levels of Exd protein in those cells facilitate its entrance to the nucleus. Our work demonstrates that hth is a complex gene that should not be considered as a functional unit. The roles of the different isoforms probably rely on their distinct protein domains and conformations and, at the end, on interactions with particular partners.

Dual role for Hox genes and Hox co-factors in conferring leg motoneuron survival and identity in Drosophila

Adult Drosophila walk using six multi-jointed legs, each controlled by ∼50 leg motoneurons (MNs). Although MNs have stereotyped morphologies, little is known about how they are specified. Here, we describe the function of Hox genes and homothorax (hth), which encodes a Hox co-factor, in Drosophila leg MN development. Removing either Hox or Hth function from a single neuroblast (NB) lineage results in MN apoptosis. A single Hox gene, Antennapedia (Antp), is primarily responsible for MN survival in all three thoracic segments. When cell death is blocked, partially penetrant axon branching errors are observed in Hox mutant MNs. When single MNs are mutant, errors in both dendritic and axon arborizations are observed. Our data also suggest that Antp levels in post-mitotic MNs are important for specifying their identities. Thus, in addition to being essential for survival, Hox and hth are required to specify accurate MN morphologies in a level-dependent manner.

Org-1 is required for the diversification of circular visceral muscle founder cells and normal midgut morphogenesis

The T-Box family of transcription factors plays fundamental roles in the generation of appropriate spatial and temporal gene expression profiles during cellular differentiation and organogenesis in animals. In this study we report that the Drosophila Tbx1 orthologue optomotor-blind-related-gene-1 (org-1) exerts a pivotal function in the diversification of circular visceral muscle founder cell identities in Drosophila. In embryos mutant for org-1, the specification of the midgut musculature per se is not affected, but the differentiating midgut fails to form the anterior and central midgut constrictions and lacks the gastric caeca. We demonstrate that this phenotype results from the nearly complete loss of the founder cell specific expression domains of several genes known to regulate midgut morphogenesis, including odd-paired (opa), teashirt (tsh), Ultrabithorax (Ubx), decapentaplegic (dpp) and wingless (wg). To address the mechanisms that mediate the regulatory inputs from org-1 towards Ubx, dpp, and wg in these founder cells we genetically dissected known visceral mesoderm specific cis-regulatory-modules (CRMs) of these genes. The analyses revealed that the activities of the dpp and wg CRMs depend on org-1, the CRMs are bound by Org-1 in vivo and their T-Box binding sites are essential for their activation in the visceral muscle founder cells. We conclude that Org-1 acts within a well-defined signaling and transcriptional network of the trunk visceral mesoderm as a crucial founder cell-specific competence factor, in concert with the general visceral mesodermal factor Biniou. As such, it directly regulates several key genes involved in the establishment of morphogenetic centers along the anteroposterior axis of the visceral mesoderm, which subsequently organize the formation of midgut constrictions and gastric caeca and thereby determine the morphology of the midgut.

Opposing interactions between homothorax and Lobe define the ventral eye margin of Drosophila eye

Patterning in multi-cellular organisms involves progressive restriction of cell fates by generation of boundaries to divide an organ primordium into smaller fields. We have employed the Drosophila eye model to understand the genetic circuitry responsible for defining the boundary between the eye and the head cuticle on the ventral margin. The default state of the early eye is ventral and depends on the function of Lobe (L) and the Notch ligand Serrate (Ser). We identified homothorax (hth) as a strong enhancer of the L mutant phenotype of loss of ventral eye. Hth is a MEIS class gene with a highly conserved Meis-Hth (MH) domain and a homeodomain (HD). Hth is known to bind Extradenticle (Exd) via its MH domain for its nuclear translocation. Loss-of-function of hth, a negative regulator of eye, results in ectopic ventral eye enlargements. This phenotype is complementary to the L mutant phenotype of loss-of-ventral eye. However, if L and hth interact during ventral eye development remains unknown. Here we show that (i) L acts antagonistically to hth, (ii) Hth is upregulated in the L mutant background, and (iii) MH domain of Hth is required for its genetic interaction with L, while its homeodomain is not, (iv) in L mutant background ventral eye suppression function of Hth involves novel MH domain-dependent factor(s), and (v) nuclear localization of Exd is not sufficient to mediate the Hth function in the L mutant background. Further, Exd is not a critical rate-limiting factor for the Hth function. Thus, optimum levels of L and Hth are required to define the boundary between the developing eye and head cuticle on the ventral margin.

Karyopherins in nuclear transport of homeodomain proteins during development

Homeodomain proteins are crucial transcription factors for cell differentiation, cell proliferation and organ development. Interestingly, their homeodomain signature structure is important for both their DNA-binding and their nucleocytoplasmic trafficking. The accurate nucleocytoplasmic distribution of these proteins is essential for their functions. We summarize information on (a) the roles of karyopherins for import and export of homeoproteins, (b) the regulation of their nuclear transport during development, and (c) the corresponding complexity of homeoprotein nucleocytoplasmic transport signals. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.

Hox genes and segmentation of the vertebrate hindbrain

In the vertebrate central nervous system, the hindbrain is an important center for coordinating motor activity, posture, equilibrium, sleep patterns, and essential unconscious functions, such as breathing rhythms and blood circulation. During development, the vertebrate hindbrain depends upon the process of segmentation or compartmentalization to create and organize regional properties essential for orchestrating its highly conserved functional roles. The process of segmentation in the hindbrain differs from that which functions in the paraxial mesoderm to generate somites and the axial skeleton. In the prospective hindbrain, cells in the neural epithelia transiently alter their ability to interact with their neighbors, resulting in the formation of seven lineage-restricted cellular compartments. These different segments or rhombomeres each go on to adopt unique characters in response to environmental signals. The Hox family of transcription factors is coupled to this process. Overlapping or nested patterns of Hox gene expression correlate with segmental domains and provide a combinatorial code and molecular framework for specifying the unique identities of hindbrain segments. The segmental organization and patterns of Hox expression and function are highly conserved among vertebrates and, as a consequence, comparative studies between different species have greatly enhanced our ability to build a picture of the regulatory cascades that control early hindbrain development. The purpose of this chapter is to review what is known about the regulatory mechanisms which establish and maintain Hox gene expression and function in hindbrain development.

Hox specificity unique roles for cofactors and collaborators

Hox proteins are well known for executing highly specific functions in vivo, but our understanding of the molecular mechanisms underlying gene regulation by these fascinating proteins has lagged behind. The premise of this review is that an understanding of gene regulation-by any transcription factor-requires the dissection of the cis-regulatory elements that they act upon. With this goal in mind, we review the concepts and ideas regarding gene regulation by Hox proteins and apply them to a curated list of directly regulated Hox cis-regulatory elements that have been validated in the literature. Our analysis of the Hox-binding sites within these elements suggests several emerging generalizations. We distinguish between Hox cofactors, proteins that bind DNA cooperatively with Hox proteins and thereby help with DNA-binding site selection, and Hox collaborators, proteins that bind in parallel to Hox-targeted cis-regulatory elements and dictate the sign and strength of gene regulation. Finally, we summarize insights that come from examining five X-ray crystal structures of Hox-cofactor-DNA complexes. Together, these analyses reveal an enormous amount of flexibility into how Hox proteins function to regulate gene expression, perhaps providing an explanation for why these factors have been central players in the evolution of morphological diversity in the animal kingdom.

The origins of the Drosophila leg revealed by the cis-regulatory architecture of the Distalless gene

Limb development requires the elaboration of a proximodistal (PD) axis, which forms orthogonally to previously defined dorsoventral (DV) and anteroposterior (AP) axes. In arthropods, the PD axis of the adult leg is subdivided into two broad domains, a proximal coxopodite and a distal telopodite. We show that the progressive subdivision of the PD axis into these two domains occurs during embryogenesis and is reflected in the cis-regulatory architecture of the Distalless (Dll) gene. Early Dll expression, governed by the Dll304 enhancer, is in cells that can give rise to both domains of the leg as well as to the entire dorsal (wing) appendage. A few hours after Dll304 is activated, the activity of this enhancer fades, and two later-acting enhancers assume control over Dll expression. The LT enhancer is expressed in cells that will give rise to the entire telopodite, and only the telopodite. By contrast, cells that activate the DKO enhancer will give rise to a leg-associated larval sensory structure known as the Keilin's organ (KO). Cells that activate neither LT nor DKO, but had activated Dll304, will give rise to the coxopodite. In addition, we describe the trans-acting signals controlling the LT and DKO enhancers, and show, surprisingly, that the coxopodite progenitors begin to proliferate approximately 24 hours earlier than the telopodite progenitors. Together, these findings provide a complete and high-resolution fate map of the Drosophila appendage primordia, linking the primary domains to specific cis-regulatory elements in Dll.

Chromosomal binding sites of the homeotic cofactor Homothorax

The Meis family oncoproteins play a crucial role in leukemogenesis and are highly expressed in other types of cancer as well. The transforming potential of Meis proteins depends on their ability to activate gene expression and therefore, revealing the identity of their target genes is very important. The genome of the fruit fly Drosophila melanogaster contains a single Meis gene, homothorax (hth), which plays multiple roles in embryonic and adult development. Mutations in hth affect the development of numerous embryonic and adult tissues, suggesting that Hth regulates the transcription of a large number of genes. However, it is not known how many genes are regulated directly by Hth and what is the nature of these genes. To address this question, we examined the distribution of the in vivo binding sites of Hth on polytene chromosomes. We found that in the salivary glands (SG) of third instar larvae, Hth binds to approximately 150 chromosomal sites in a very reproducible pattern. More than hundred of these sites were mapped cytologically. Interestingly, Hth accumulates at high levels in some of the most prominent hormone-induced chromosomal puffs, pointing to a possible role of Hth in activation of ecdysone-induced targets. Interfering with the normal transcriptional activity of Hth in larval SGs leads to dramatic reduction in cell size and DNA content implicating Hth in the regulation of cell growth and endoreplication in larval SGs.

A balance between two nuclear localization sequences and a nuclear export sequence governs extradenticle subcellular localization

During animal development, transcription factor activities are modulated by several means, including subcellular localization. The Hox cofactor Extradenticle (Exd) has a dynamic subcellular localization, such that Exd is cytoplasmic by default, but is nuclear when complexed with another homeodomain protein, Homothorax (Hth). These observations raise the question of whether dimerization with Hth simply induces Exd's nuclear localization or, alternatively, if Hth is also necessary for Exd activity. To address this question, we analyzed the nuclear transport signals in Exd, including a divergent nuclear export signal (NES) and two nuclear localization signals (NLSs). We show that, although these signals are weak compared to canonical signals, they balance each other in Exd. We also provide evidence that Exd contains an NLS mask that contributes to its cytoplasmic localization. With these signals characterized, we generated forms of Exd that are nuclear localized in the absence of Hth. Surprisingly, although these Exd forms are functional, they do not phenocopy Hth overexpression. These findings suggest that Hth is required for Exd activity, not simply for inducing its nuclear localization.

Distinct functions of homeodomain-containing and homeodomain-less isoforms encoded by homothorax

The homothorax (hth) gene of Drosophila melanogaster is required for executing Hox functions, for head development, and for forming the proximodistal (PD) axis of the appendages. We show that alternative splicing of hth generates two types of protein isoforms, one that contains a DNA-binding homeodomain (HthFL) and one that does not contain a homeodomain (HDless). Both types of Hth isoforms include the evolutionarily conserved HM domain, which mediates a direct interaction with Extradenticle (Exd), another homeodomain protein. We show that although both HthFL and HDless isoforms of Hth can induce the nuclear localization of Exd, they carry out distinct sets of functions during development. Surprisingly, we find that many of hth's functions, including PD patterning and most Hox-related activities, can be executed by the HDless isoforms. In contrast, antennal development shows an absolute dependency on the HthFL isoform. Thus, alternative splicing of hth results in the generation of multiple transcription factors that execute unique functions in vivo. We further demonstrate that the mouse ortholog of hth, Meis1, also encodes a HDless isoform, suggesting that homeodomain-less variants of this gene family are evolutionarily ancient.

Two-step regulation of Ad4BP/SF-1 gene transcription during fetal adrenal development: initiation by a Hox-Pbx1-Prep1 complex and maintenance via autoregulation by Ad4BP/SF-1

The orphan nuclear receptor Ad4BP/SF-1 (adrenal 4 binding protein/steroidogenic factor 1) is essential for the proper development and function of reproductive and steroidogenic tissues. Although the expression of Ad4BP/SF-1 is specific for those tissues, the mechanisms underlying this tissue-specific expression remain unknown. In this study, we used transgenic mouse assays to examine the regulation of the tissue-specific expression of Ad4BP/SF-1. An investigation of the entire Ad4BP/SF-1 gene locus revealed a fetal adrenal enhancer (FAdE) in intron 4 containing highly conserved binding sites for Pbx-Prep, Pbx-Hox, and Ad4BP/SF-1. Transgenic assays revealed that the Ad4 sites, together with Ad4BP/SF-1, develop an autoregulatory loop and thereby maintain transcription, while the Pbx/Prep and Pbx/Hox sites initiate transcription prior to the establishment of the autoregulatory loop. Indeed, a limited number of Hox family members were found to be expressed in the adrenal primordia. Whether a true fetal-type adrenal cortex is present in mice remained controversial, and this argument was complicated by the postnatal development of the so-called X zone. Using transgenic mice with lacZ driven by the FAdE, we clearly identified a fetal adrenal cortex in mice, and the X zone is the fetal adrenal cells accumulated at the juxtamedullary region after birth.

Transcriptional activation by extradenticle in the Drosophila visceral mesoderm

decapentaplegic (dpp) is a direct target of Ultrabithorax (Ubx) in parasegment 7 (PS7) of the embryonic visceral mesoderm. We demonstrate that extradenticle (exd) and homothorax (hth) are also required for dpp expression in this location, as well as in PS3, at the site of the developing gastric caecae. A 420 bp element from dpp contains EXD binding sites necessary for expressing a reporter gene in both these locations. Using a specificity swap, we demonstrate that EXD directly activates this element in vivo. Activation does not require Ubx, demonstrating that EXD can activate transcription independently of homeotic proteins. Restoration is restricted to the domains of endogenous dpp expression, despite ubiquitous expression of altered specificity EXD. We demonstrate that nuclear EXD is more extensively phosphorylated than the cytoplasmic form, suggesting that EXD is a target of signal transduction by protein kinases.

Patterning function of homothorax/extradenticle in the thorax of Drosophila

In Drosophila, the morphological diversity is generated by the activation of different sets of active developmental regulatory genes in the different body subdomains. Here, we have investigated the role of the homothorax/extradenticle (hth/exd) gene pair in the elaboration of the pattern of the anterior mesothorax (notum). These two genes are active in the same regions and behave as a single functional unit. We find that their original uniform expression in the notum is downregulated during development and becomes restricted to two distinct, α and βsubdomains. This modulation appears to be important for the formation of distinct patterns in the two subdomains. The regulation of hth/exdexpression is achieved by the combined repressing functions of the Pax gene eyegone (eyg) and of the Dpp pathway. hth/exd is repressed in the body regions where eyg is active and that also contain high levels of Dpp activity. We also present evidence for a molecular interaction between the Hth and the Eyg proteins that may be important for the patterning of the α subdomain.

Direct integration of Hox and segmentation gene inputs during Drosophila development

During Drosophila embryogenesis, segments, each with an anterior and posterior compartment, are generated by the segmentation genes while the Hox genes provide each segment with a unique identity. These two processes have been thought to occur independently. Here we show that abdominal Hox proteins work directly with two different segmentation proteins, Sloppy paired and Engrailed, to repress the Hox target gene Distalless in anterior and posterior compartments, respectively. These results suggest that segmentation proteins can function as Hox cofactors and reveal a previously unanticipated use of compartments for gene regulation by Hox proteins. Our results suggest that these two classes of proteins may collaborate to directly control gene expression at many downstream target genes.

Localization of HB9 homeodomain protein and characterization of its nuclear localization signal during chick embryonic skin development

We detected HB9 protein during tarsometatarsal scale skin and late feather development. Immunofluorescent analyses with N-terminal 14 amino acids antiserum revealed that HB9 was strongly expressed in epidermal basal cells of the outer scale face in tarsometatarsal scale skin. Specific expression was also detected in dermal cells at the root region of the feather and around the feather follicle. Furthermore, we observed precise distribution of HB9 protein by immunoelectron microscopy. We detected HB9 protein not only in the nucleus, but also in the cytoplasm in tarsometatarsal scale skin. However, in feather skin HB9 protein was found in the nucleus but not in the cytoplasm. Cytoplasmic localization of HB9 protein in tarsometatarsal scale skin was observed especially in the endoplasmic reticulum and the Golgi apparatus. To address the mechanism of nuclear-cytoplasmic translocation, we determined the nuclear localization signal (NLS) sequences by using eukaryotic green fluorescent protein fusion protein in primary keratinocyte culture. Chick HB9 homeoprotein has two types of the NLS sequences in its homeodomain. One of them is a bipartite type as representatively found in Xenopus nucleoplasmin. The other is very similar to hexapeptide NLS sequences identified in pancreatic duodenum homeobox 1 (PDX1). These sequences functioned not only in keratinocytes but also in dermal fibroblasts. They are conserved in Xenopus, mouse, and human HB9 ortholog. These results indicate that HB9 protein might be involved in chick tarsometatarsal scale and feather development and that nuclear import of HB9 protein might be regulated by these NLS sequences in the homeodomain.

Nucleocytoplasmic shuttling of lgl2 is developmentally regulated in fetal lung

To investigate molecular mechanisms of lung organogenesis, we searched for glucocorticoid-inducible genes in developing lung. We cloned LGL2, a developmentally and hormonally regulated gene in fetal lung (Zhang, C., N. B. Sweezey, S. Gagnon, B. Muskat, D. Koehler, M. Post, and F. Kaplan. 2000. A novel karyopherin-beta homolog is developmentally and hormonally regulated in fetal lung. Am. J. Respir. Cell Mol. Biol. 22:451-459). A comparison of lgl2 protein to sequences in the genome database suggested that lgl2 is a nuclear transport receptor. We report on the functional characterization of lgl2 as an importin beta protein and on the developmental regulation of its nucleocytoplasmic shuttling in fetal lung. We investigated the subcellular localization and Ran-binding properties of lgl2 and its N- and C-terminal regions. We used fluorescence recovery after photobleaching and fluorescence loss in photobleaching to study nucleocytoplasmic shuttling of lgl2. We showed that N-terminal lgl2 supports shuttling at a reduced rate. We showed that the nucleocytoplasmic distribution of lgl2 favors the nucleus in fetal lung and that lgl2 enters the nucleus much more rapidly at fetal Day 18 than at Day 21. Total nuclear recovery of lgl2 was dramatically different at the two time points. Early in development, nuclear import of transcription factors in response to hormones and growth agonists regulates prominent signal transduction pathways that govern lung organogenesis. We speculate that lgl2 may be one important modulator of this process.

The different cardiac expression of the type 2 iodothyronine deiodinase gene between human and rat is related to the differential response of the Dio2 genes to Nkx-2.5 and GATA-4 transcription factors

By producing T3 from T4, type 2 iodothyronine deiodinase (D2) catalyzes the first step in the cascade underlying the effect exerted by thyroid hormone. Type 2 iodothyronine deiodinase mRNA is expressed at high levels in human heart but is barely detectable in the corresponding rodent tissue. Although the heart is a major target of thyroid hormone, the role of cardiac D2 and the factors that regulate its expression are unknown. Here we report that the human Dio2 promoter is very sensitive to the cardiac transcription factors Nkx-2.5 and GATA-4. Nkx-2.5 transactivates a 6.5-kb human (h)Dio2-chloramphenicol acetyltransferase construct, with maximal induction reached with a 633-bp proximal promoter region. Interestingly, despite 73% identity with the corresponding human region, the rat Dio2 promoter is much less responsive to Nkx-2.5 induction. Using EMSA, we found that two sites in the human promoter (C and D) specifically bind Nkx-2.5. In coexpression studies, GATA-4 alone was a poor inducer of the hDio2 promoter; however in synergy with Nkx-2.5, it activated D2 reporter gene expression in the human, but not the rat promoter. Functional analysis showed that both C and D sites are required for the complete Nkx-2.5 response and for the Nkx-2.5/GATA-4 synergistic effect. In neonatal rat primary myocardiocytes, most of the hDio2-chloramphenicol acetyltransferase activity was suppressed by mutation of the Nkx-2.5 binding sites. Finally, a mutant Nkx-2.5 protein (N188K), which causes, in heterozygosity, congenital heart diseases, did not transactivate the Dio2 promoter and interfered with its activity in cardiomyocytes, possibly by titrating endogenous Nkx-2.5 protein away from the promoter. In conclusion, this study shows that Nkx-2.5 and GATA-4 play prime roles in Dio2 gene regulation in the human heart and suggests that it is their synergistic action in humans that causes the differential expression of the cardiac Dio2 gene between humans and rats.

Nonmuscle myosin promotes cytoplasmic localization of PBX

In the absence of MEIS family proteins, two mechanisms are known to restrict the PBX family of homeodomain (HD) transcription factors to the cytoplasm. First, PBX is actively exported from the nucleus via a CRM1-dependent pathway. Second, nuclear localization signals (NLSs) within the PBX HD are masked by intramolecular contacts. In a screen to identify additional proteins directing PBX subcellular localization, we identified a fragment of murine nonmuscle myosin II heavy chain B (NMHCB). The interaction of NMHCB with PBX was verified by coimmunoprecipitation, and immunofluorescence staining revealed colocalization of NMHCB with cytoplasmic PBX in the mouse embryo distal limb bud. The interaction domain in PBX mapped to a conserved PBC-B region harboring a potential coiled-coil structure. In support of the cytoplasmic retention function, the NMHCB fragment competes with MEIS1A to redirect PBX, and the fly PBX homologue EXD, to the cytoplasm of mammalian and insect cells. Interestingly, MEIS1A also localizes to the cytoplasm in the presence of the NMHCB fragment. These activities are largely independent of nuclear export. We show further that the subcellular localization of EXD is deregulated in Drosophila zipper mutants that are depleted of nonmuscle myosin heavy chain. This study reveals a novel and evolutionarily conserved mechanism controlling the subcellular distribution of PBX and EXD proteins.

TGF-beta and the Smad signal transduction pathway

Transforming growth factor beta (TGF-beta) superfamily members are important regulators of many diverse developmental and homeostatic processes and disruption of their activity has been implicated in a variety of human diseases ranging from cancer to chondrodysplasias and pulmonary hypertension. TGF-beta family members signal through transmembrane Ser-Thr kinase receptors that directly regulate the intracellular Smad pathway. Smads are a unique family of signal transduction molecules that can transmit signals directly from the cell surface receptors to the nucleus, where they regulate transcription by interacting with DNA binding partners as well as transcriptional coactivators and corepressors. In addition, more recent evidence indicates that Smads can also function both as substrates and adaptors for ubiquitin protein ligases, which mediate the targeted destruction of intracellular proteins. Smads have thus emerged as multifunctional transmitters of TGF-beta family signals that play critical roles in the development and homeostasis of metazoans.

HOX and non-HOX homeobox genes in leukemic hematopoiesis

Dysregulation of homeobox (HB)-containing genes is becoming increasingly recognized as the underlying basis of many hematologic malignancies. Expression of clustered HB (HOX) genes within the hematopoietic system, and enforced overexpression and knockout studies have provided support for the concept that these homeodomain-containing transcription factors play a significant role in the developmental biology of hematopoietic cells. Diverged HB (non-HOX) genes have recently been identified as either cofactors and/or accelerators of leukemic disease mediated by HOX genes or as bona fide oncogenes. In this review, we examine the evidence that supports a central role for HB genes in normal and malignant hematopoiesis, paying particular attention to the non-HOX class and the possible mechanisms through which they contribute to leukemic transformation.

Requirements for transcriptional repression and activation by Engrailed in Drosophila embryos

Genetic analysis shows that Engrailed (En), a homeodomain-containing transcription factor, has both negative and positive targets. Negative regulation is expected from a factor that has a well-defined repressor domain but activation is harder to comprehend. We used VP16En, a form of En that had its repressor domain replaced by the activation domain of VP16, to show that En activates targets using two parallel routes, by repressing a repressor and by being a bona fide activator. We identified the intermediate repressor activity as being encoded by sloppy paired 1 and 2 and showed that bona fide activation is dramatically enhanced by Wingless signaling. Thus, En is a bifunctional transcription factor and the recruitment of additional cofactors presumably specifies which function prevails on an individual promoter. Extradenticle (Exd) is a cofactor thought to be required for activation by Hox proteins. However, in thoracic segments, Exd is required for repression (as well as activation) by En. This is consistent with in vitro results showing that Exd is involved in recognition of positive and negative targets. Moreover, we provide genetic evidence that, in abdominal segments, Ubx and Abd-A, two homeotic proteins not previously thought to participate in the segmentation cascade, are also involved in the repression of target genes by En. We suggest that, like Exd, Ubx and Abd-A could help En recognize target genes or activate the expression of factors that do so.

Distinct functions of homothorax in leg development in Drosophila

The Drosophila leg is subdivided into two mutually antagonistic proximal and distal domains. The proximal domain is defined by the activity of the homeobox genes homothorax and extradenticle and the distal one by the Dpp/Wg targets Distal-less (Dll) and dachshund (dac). It is known that hth/exd function prevents the activity of Dpp and Wg response genes and that cells deficient for exd activity in the proximal domain differentiate pattern elements corresponding to more distal leg regions. We report new results on the role of hth/exd antagonising the Dpp pathway. In cells expressing hth in the distal leg, there is a debilitation of the Dpp pathway which is reflected in lower levels of Mad phosphorylation and in increased levels of the receptor thick veins. Ectopic hth expression in the distal leg results in JNK-mediated apoptosis, decreased growth and pattern abnormalities. It also causes a general proximalisation of the appendage, which can be explained by interference with the Dpp and Wg pathways. We also report that the repression by hth/exd of the Dpp and Wg target Distal-less is not achieved at the level of transcription but preventing the activation of Dll target genes. We propose that hth/exd function contributes to the normal identity of proximal cells both by limiting the influence of the Dpp and Wg pathways and by activating proximal genes like teashirt (tsh) and aristaless (al).

Prep2: cloning and expression of a new prep family member

We describe Prep2, a new murine homeobox-containing gene closely related to Prep1. The PREP2 protein belongs to the three amino acid loop extension (TALE) superclass of homeodomain-containing proteins and encodes a polypeptide of 462 residues. As for PREP1, PREP2 binds an appropriate site on DNA as a heterodimer with PBX1A. Northern analysis, immunoblotting, immunohistochemistry, and in situ hybridization show widespread Prep2 expression during organogenesis and in the adult. The data suggest that Prep2 functions to varying degrees in a broad array of tissues and developmental processes.

Evidence that Armadillo transduces wingless by mediating nuclear export or cytosolic activation of Pangolin

Secreted proteins of the Wnt family have profound organizing roles during animal development and are transduced via the activities of the Frizzled (Fz) class of transmembrane receptors and the TCF/LEF/Pangolin class of transcription factors. beta-catenins, including Drosophila Armadillo (Arm), link activation of Fz at the cell surface to transcriptional regulation by TCF in the nucleus. The consensus view is that Wnt signaling induces beta-catenin to enter the nucleus and combine with TCF to form a transcription factor complex in which TCF binds DNA and the C-terminal domain of beta-catenin activates transcription. Here, we present findings, which challenge this view and suggest instead that beta-catenin may transduce Wnt signals by exporting TCF from the nucleus or activating it in the cytoplasm.

Wnt-3A enhances bone morphogenetic protein-2-mediated chondrogenesis of murine C3H10T1/2 mesenchymal cells

We have recently reported the chondrogenic effect of bone morphogenetic protein-2 (BMP-2) in high density cultures of the mouse multipotent mesenchymal C3H10T1/2 cell line and have shown the functional requirement of the cell-cell adhesion molecule N-cadherin in BMP-2-induced chondrogenesis in vitro (Denker, A. E., Nicoll, S. B., and Tuan, R. S. (1995) Differentiation 59, 25-34; Haas, A. R., and Tuan, R. S. (1999) Differentiation 64, 77-89). Furthermore, BMP-2 treatment also results in an increased protein level of beta-catenin, a known N-cadherin-associated Wnt signal transducer (Fischer, L., Haas, A., and Tuan, R. S. (2001) Signal Transduction 2, 66-78), suggesting functional cross-talk between the BMP-2 and Wnt signaling pathways. We have observed previously that BMP-2 treatment up-regulates expression of Wnt-3A in high density cultures of C3H10T1/2 cells. To assess the contribution of Wnt-3A to BMP-2-mediated chondrogenesis, we have generated C3H10T1/2 cell lines overexpressing Wnt-3A and various forms of glycogen synthase kinase-3beta (GSK-3beta), an immediate cytosolic component of the Wnt signaling pathway, and examined their response to BMP-2. We show that overexpression of either Wnt-3A or kinase-dead GSK-3beta enhances BMP-2-mediated chondrogenesis. Furthermore, Wnt-3A overexpression results in decreases in both N-cadherin and GSK-3beta protein levels, whereas Wnt-3A as well as kinase-dead GSK-3beta overexpression increase total and nuclear levels of both beta-catenin and LEF-1. Direct cross-talk between Wnts and BMP-2 was also indicated by the up-regulated interaction between beta-catenin and SMAD-4 in response to BMP-2. These results suggest that Wnt-3A acts in a manner opposite to that of other Wnts, such as Wnt-7A, which were previously identified as inhibitory to chondrogenesis, and is the first BMP-2-regulated, chondrogenesis-enhancing member of the Wnt family.

Regulated nuclear trafficking of the homeodomain protein otx1 in cortical neurons

Otx1 is a homeodomain protein required for axon refinement by layer 5 neurons in developing cerebral cortex. Otx1 localizes to the cytoplasm of progenitor cells in the rat ventricular zone, and remains cytoplasmic as neurons migrate and begin to differentiate. Nuclear translocation occurs during the first week of postnatal life, when layer 5 neurons begin pruning their long-distance axonal projections. Deletion analysis reveals that Otx1 is imported actively into cell nuclei, that the N-terminus of Otx1 is necessary for nuclear import, and that a putative nuclear localization sequence within this domain is sufficient to direct nuclear import in a variety of cell lines. In contrast, GFP-Otx1 fusion proteins that contain the N-terminus are retained in the cytoplasm of cortical progenitor cells, mimicking the distribution of Otx1 in vivo. These results suggest that ventricular cells actively sequester Otx1 in the cytoplasm, either by preventing nuclear import or by promoting a balance of export over import signals.

The functions ofpannierduringDrosophilaembryogenesis

The pannier (pnr) gene of Drosophila encodes a zinc-finger transcription factor of the GATA family and is involved in several developmental processes during embryonic and imaginal development. We report some novel aspects of the regulation and function of pnr during embryogenesis. Previous work has shown that pnr is activated by decapentaplegic (dpp) in early development, but we find that after stage 10, the roles are reversed and pnr becomes an upstream regulator of dpp. This function of pnr is necessary for the activation of the Dpp pathway in the epidermal cells implicated in dorsal closure and is not mediated by the JNK pathway, which is also necessary for Dpp activity in these cells. In addition, we show that pnr behaves as a selector-like gene in generating morphological diversity in the dorsoventral body axis. It is responsible for maintaining a subdivision of the dorsal half of the embryo into two distinct, dorsomedial and dorsolateral, regions, and also specifies the identity of the dorsomedial region. These results, together with prior work on its function in adults, suggest that pnr is a major factor in the genetic subdivision of the body of Drosophila.

Developmental modularity and the evolutionary diversification of arthropod limbs

Segmentation is one of the most salient characteristics of arthropods, and differentiation of segments along the body axis is the basis of arthropod diversification. This article evaluates whether the evolution of segmentation involves the differentiation of already independent units, i.e., do segments evolve as modules? Because arthropod segmental differentiation is commonly equated with differential character of appendages, we analyze appendages by comparing similarities and differences in their development. The comparison of arthropod limbs, even between species, is a comparison of serially repeated structures. Arthropod limbs are not only reiterated along the body axis, but limbs themselves can be viewed as being composed of reiterated parts. The interpretation of such reiterated structures from an evolutionary viewpoint is far from obvious. One common view is that serial repetition is evidence of a modular organization, i.e., repeated structures with a common fundamental identity that develop semi-autonomously and are free to diversify independently. In this article, we evaluate arthropod limbs from a developmental perspective and ask: are all arthropod limbs patterned using a similar set of mechanisms which would reflect that they all share a generic coordinate patterning system? Using Drosophila as a basis for comparison, we find that appendage primordia, positioned along the body using segmental patterning coordinates, do indeed have elements of common identity. However, we do not find evidence of a single coordinate system shared either between limbs or among limb branches. Data concerning the other diagnostic of developmental modularity--semi-autonomy of development--are not currently available for sufficient taxa. Nonetheless, some data comparing patterns of morphogenesis provide evidence that limbs cannot always be temporally or spatially decoupled from the development of their neighbors, suggesting that segment modularity is a derived character.

Genetic evidence for the transcriptional-activating function of Homothorax during adult fly development

Homothorax (HTH) is a homeobox-containing protein, which plays multiple roles in the development of the embryo and the adult fly. HTH binds to the homeotic cofactor Extradenticle (EXD) and translocates it to the nucleus. Its function within the nucleus is less clear. It was shown, mainly by in vitro studies, that HTH can bind DNA as a part of ternary HTH/EXD/HOX complexes, but little is known about the transcription regulating function of HTH-containing complexes in the context of the developing fly. Here we present genetic evidence, from in vivo studies, for the transcriptional-activating function of HTH. The HTH protein was forced to act as a transcriptional repressor by fusing it to the Engrailed (EN) repression domain, or as a transcriptional activator, by fusing it to the VP16 activation domain, without perturbing its ability to translocate EXD to the nucleus. Expression of the repressing form of HTH in otherwise wild-type imaginal discs phenocopied hth loss of function. Thus, the repressing form was working as an antimorph, suggesting that normally HTH is required to activate the transcription of downstream target genes. This conclusion was further supported by the observation that the activating form of HTH caused typical hth gain-of-function phenotypes and could rescue hth loss-of-function phenotypes. Similar results were obtained with XMeis3, the Xenopus homologue of HTH, extending the known functional similarity between the two proteins. Competition experiments demonstrated that the repressing forms of HTH or XMeis3 worked as true antimorphs competing with the transcriptional activity of the native form of HTH. We also describe the phenotypic consequences of HTH antimorph activity in derivatives of the wing, labial and genital discs. Some of the described phenotypes, for example, a proboscis-to-leg transformation, were not previously associated with alterations in HTH activity. Observing the ability of HTH antimorphs to interfere with different developmental pathways may direct us to new targets of HTH. The HTH antimorph described in this work presents a new means by which the transcriptional activity of the endogenous HTH protein can be blocked in an inducible fashion in any desired cells or tissues without interfering with nuclear localization of EXD.

TheDrosophilaproboscis is specified by two Hox genes,proboscipediaandSex combs reduced, via repression of leg and antennal appendage genes

The proboscis is one of the most highly modified appendages in Drosophila melanogaster. However, the phenotypes of proboscipedia (pb) mutants, which transform the proboscis into leg or antenna, indicate a basic homology among these limbs. Recent genetic studies have revealed a developmental system for patterning appendages and identified several genes required for limb development. Among these are: extradenticle (exd), homothorax (hth), dachshund (dac), Distal-less (Dll) and spalt (sal). These limb genes have not been well studied in wild-type mouthparts and their role if any in this appendage is not well understood. Here we demonstrate that the homeotic gene products Proboscipedia (Pb) and Sex combs reduced (Scr) regulate the limb genes in the labial disc to give rise to a unique type of appendage, the proboscis. Pb inhibits exd, dac and sal expression and downregulates Dll. This observation explains the ability of Pb to inhibit the effects of ectopically expressed trunk Hox genes in the proboscis, to suppress leg identity in the trunk and to transform antenna to maxillary palp. Scr suppresses sal expression and also downregulates Dll in the labial discs; discs mutant for both pb and Scr give rise to complete antennae, further demonstrating appendage homology. In the labial disc, Pb positively regulates transcription of Scr, whereas in the embryo, Scr positively regulates pb. Additionally, our results suggests a revised fate map of the labial disc. We conclude that the proboscis constitutes a genetically distinct type of appendage whose morphogenesis does not require several important components of leg and/or antennal patterning systems, but retains distal segmental homology with these appendages.

A HOX complex, a repressor element and a 50 bp sequence confer regional specificity to a DPP-responsive enhancer

A central theme during development and homeostasis is the generation of cell type-specific responses to the action of a limited number of extant signaling cascades triggered by extracellular ligands. The molecular mechanisms by which information from such signals are integrated in responding cells in a cell-type specific manner remain poorly understood. We have undertaken a detailed characterization of an enhancer that is regulated by DPP signaling and by the homeotic protein Labial and its partners, Extradenticle and Homothorax. The expression driven by this enhancer (lab550) and numerous deletions and point mutants thereof was studied in wild-type and mutant Drosophila embryos as well as in cultured cells. We find that the lab550 enhancer is composed of two elements, a Homeotic Response Element (HOMRE) and a DPP Response Element (DPPRE) that synergize. None of these two elements can reproduce the expression of lab550, either with regard to expression level or with regard to spatial restriction. The isolated DPPRE of lab550 responds extremely weakly to DPP. Interestingly, we found that the inducibility of this DPPRE is weak because it is tuned down by the action of a repressor element. This repressor element and an additional 50 bp element appear to be crucial for the cooperation of the HOMRE and the DPPRE, and might tightly link the DPP response to the homeotic input. The cooperation between the different elements of the enhancer leads to the segmentally restricted activity of lab550 in the endoderm and provides a mechanism to create specific responses to DPP signaling with the help of a HOX protein complex.

The developmental and molecular biology of genes that subdivide the body of Drosophila

During the past decade, much progress has been made in understanding how the adult fly is built. Some old concepts such as those of compartments and selector genes have been revitalized. In addition, recent work suggests the existence of genes involved in the regionalization of the adult that do not have all the features of selector genes. Nevertheless, they generate morphological distinctions within the body plan. Here we re-examine some of the defining criteria of selector genes and suggest that these newly characterized genes fulfill many, but not all, of these criteria. Further, we propose that these genes can be classified according to the domains in which they function. Finally, we discuss experiments that address the molecular mechanisms by which selector and selector-like gene products function in the fly.

The novel C. elegans gene sop-3 modulates Wnt signaling to regulate Hox gene expression

We describe the properties of a new gene, sop-3, that is required for the regulated expression of a C. elegans Hox gene, egl-5, in a postembryonic neuroectodermal cell lineage. Regulated expression of egl-5 in this cell lineage is necessary for development of the sensory rays of the male tail. sop-3 encodes a predicted novel protein of 1475 amino acids without clear homologs in other organisms. However, the sequence contains motifs consisting of homopolymeric runs of amino acids found in several other transcriptional regulators, some of which also act in Hox gene regulatory pathways. The genetic properties of sop-3 are very similar to those of sop-1, which encodes a component of the transcriptional Mediator complex, and mutations in the two genes are synthetic lethal. This suggests that SOP-3 may act at the level of the Mediator complex in regulating transcription initiation. In a sop-3 loss-of-function background, egl-5 is expressed ectopically in lineage branches that normally do not express this gene. Such expression is dependent on the Hox gene mab-5, as it is in branches where egl-5 is normally expressed. Ectopic egl-5 expression is also dependent on the Wnt pathway. Thus, sop-3 contributes to the combinatorial control of egl-5 by blocking egl-5 activation by MAB-5 and the Wnt pathway in inappropriate lineage branches.

Characterization of homo- and heterodimerization of cardiac Csx/Nkx2.5 homeoprotein

Csx/Nkx2.5 is an evolutionarily conserved homeodomain (HD)-containing transcription factor that is essential for early cardiac development. We found that the HD of Csx/Nkx2.5 binds as a monomer as well as a dimer to its DNA binding sites in the promoter of the atrial natriuretic factor (ANF) gene, an in vivo target gene of Csx/Nkx2.5. Csx/Nkx2.5 physically interacts with each other in vitro as well as in cells, and the HD is critical for homodimerization. Lys(193) and Arg(194), located at the COOH-terminal end of HD, are essential for dimerization. Lys(193) is also required for a specific interaction with the zinc finger transcription factor GATA4. Csx/Nkx2.5 can heterodimerize with other NK2 homeodomain proteins, Nkx2.3 and Nkx2.6/Tix, with different affinities. A single missense mutation, Ile(183) to Pro in the HD of Csx/Nkx2.5, preserved homodimerization function, but totally abolished DNA binding. Ile(183) --> Pro mutant acts in an inhibitory manner on wild type Csx/Nkx2.5 transcriptional activity through the ANF promoter in 10T1/2 cells. However, Ile(183) --> Pro mutant does not inhibit wild type Csx/Nkx2.5 function on the ANF promoter in cultured neonatal cardiac myocytes, possibly due to failure of dimerization in the presence of the target DNA. These results suggest that complex protein-protein interactions of Csx/Nkx2.5 play a role in its transcriptional regulatory function.

How Drosophila appendages develop

Just a glance at the body of the fruit fly Drosophila reveals that it has a main body part--the trunk--and a number of specialized appendages such as legs, wings, halteres and antennae. How do Drosophila appendages develop, what gives each appendage its unique identity, and what can the fruit fly teach us about appendage development in vertebrates?

Immunocytochemical characterization of murine Hex, a homeobox-containing protein

A polyclonal antibody against a glutathione S:-transferase fusion protein containing the 76 COOH-terminal amino acids of Hex, a divergent homeobox gene, was raised in rabbits. Western blot and immunofluorescence reveal that Hex is a 35-37-kD soluble protein present both in the nucleus and cytoplasm of transfected and nontransfected cultured cells as well as in whole mouse embryo. Confocal microscopy of whole mount immunostained mouse embryos at E7. 5 and E8.5 demonstrates that Hex is differentially localized in the cytoplasm and nucleus of definitive endoderm, developing blood islands, and hepatic diverticulum. In particular, in the region of the foregut that gives rise to the liver, Hex expression is nuclear in the endodermal cells of the hepatic diverticulum, whereas expression is primarily cytoplasmic in cells lateral to the liver-forming region. This suggests that nuclear localization of Hex is involved in early hepatic specification and that compartmentalization of Hex protein plays an important role in its function during mouse development.

Cdx1 and cdx2 expression during intestinal development

BACKGROUND & AIMS

The intestine-specific transcription factors Cdx1 and Cdx2 are candidate genes for directing intestinal development, differentiation, and maintenance of the intestinal phenotype. This study focused on the complex patterns of expression of Cdx1 and Cdx2 during mouse gastrointestinal development.

METHODS

Embryonic and postnatal mouse tissues were analyzed by immunohistochemistry to determine protein expression of Cdx1 and Cdx2 in the developing intestinal tract.

RESULTS

Cdx2 protein expression was observed at 9. 5 postcoitum (pc), whereas weak expression of Cdx1 protein was first seen at 12.5 pc in the distal developing intestine (hindgut). Expression of Cdx1 increased from 13.5 to 14.5 pc during the endoderm/epithelial transition with predominately distal expression. In contrast to Cdx1, there was intense expression of Cdx2 in all but the distal portions of the developing intestine. Cdx2 expression remained low in the distal colon throughout postnatal development. A gradient of expression formed in the crypt-villus axis, with Cdx1 primarily in the crypt and Cdx2 primarily in the villus.

CONCLUSIONS

Direct comparison of the patterns of Cdx1 and Cdx2 protein expression during development as performed in this study provides new insights into their potential functional roles. The relative expression of Cdx1 to Cdx2 protein may be important in the anterior to posterior patterning of the intestinal epithelium and in defining patterns of proliferation and differentiation along the crypt-villus axis.

members only encodes a Drosophila nucleoporin required for Rel protein import and immune response activation

Many developmental and physiological responses rely on the selective translocation of transcriptional regulators in and out of the nucleus through the nuclear pores. Here we describe the Drosophila genemembers only (mbo) encoding a nucleoporin homologous to the mammalian Nup88. The phenotypes of mbo mutants andmbo expression during development are cell specific, indicating that the nuclear import capacity of cells is differentially regulated. Using inducible assays for nucleocytoplasmic trafficking we show that mRNA export and classic NLS-mediated protein import are unaffected inmbo mutants. Instead, mbo is selectively required for the nuclear import of the yeast transcription factor GAL4 in a subset of the larval tissues. We have identified the first endogenous targets of the mbo nuclear import pathway in the Rel proteins Dorsal and Dif. In mbo mutants the upstream signaling events leading to the degradation of the IκB homolog Cactus are functional, but Dorsal and Dif remain cytoplasmic and the larval immune response is not activated in response to infection. Our results demonstrate that distinct nuclear import events require different nucleoporins in vivo and suggest a regulatory role for mbo in signal transduction.

Distal-less function during Drosophila appendage and sense organ development

The Drosophila Distal-less (Dll) gene was identified in the early 1980s by means of dominant and recessive mutations that caused both striking antenna-to-leg transformations and leg truncations. The gene initially was named "Bristle on arista" or "Brista" because one aspect of the phenotype is the formation of leg bristles on the antenna (Sato [1984] Drosophila Information Service 60:180-182; Sunkel and Whittle [1987] Wilhelm Roux's. Arch. Dev. Biol. 196:124-132). Subsequent studies have revealed that Dll encodes a homeodomain transcription factor (Cohen et al. [1989] Nature 338:432-434) that is expressed throughout limb development from embryogenesis on (Cohen [1990] Nature 343:173-177; Weigmann and Cohen [1999] Development 126:3823-3830). Dll is required for the elaboration of distal pattern elements in the antenna, the legs, the limb-derived gnathal structures (Cohen and Jurgens [1989] Nature 482-485), and the anal plate (Gorfinkiel et al. [1999] Mech. Dev. 868:113-123) and can initiate proximodistal axis formation when expressed ectopically (Gorfinkiel et al. [1997] Genes Dev. 11:2259-2271). Dll homologs are expressed in developing appendages in at least six coelomate phyla, including chordates (Akimenko et al. [1994] J. Neurosci. 14:3475-3486; Beauchemin and Savard [1992] Dev. Biol. 154:55-65; Bulfone et al. [1993] Mech. Dev. 40:129-140; Dolle et al. [1992] Differentiation 49:93-99; Ferrari et al. [1995] Mech. Dev. 52:257-264; Panganiban et al. [1997] Proc. Natl. Acad. Sci. USA 94:5162-5166; Simeone et al. [1994] Proc. Natl. Acad. Sci. USA 91:2250-2254), consistent with requirements for Dlx function in normal limb development across the animal kingdom. Distal-less also has been implicated in various aspects of vertebrate neurogenesis (see reviews by Kraus and Lufkin [1999] J. Cell. Biochem. 32-33:133-140 and the accompanying review by Beanan and Sargent [2000] Dev. Dyn. 218:000-000). Here, I outline what is known about Dll function and regulation in Drosophila.

Changes in HOXB6 homeodomain protein structure and localization during human epidermal development and differentiation

HOX homeodomain proteins are master developmental regulators, which are now thought to function as transcription factors by forming cooperative DNA binding complexes with PBX or other protein partners. Although PBX proteins exhibit regulated subcellular localization and function in the nucleus in other tissues, little data exists on HOX and PBX protein localization during skin development. We now show that the HOXB6 protein is expressed in the suprabasal layer of the early developing epidermis and throughout the upper layers of late fetal and adult human skin. HOXB6 signal is cytoplasmic throughout fetal epidermal development, but substantially nuclear in normal adult skin. HOXB6 protein is also partially nuclear in hyperproliferative skin conditions, but appears to be cytoplasmic in basal and squamous cell carcinomas. Although all three PBX genes are expressed in fetal epidermis, none of the three PBX proteins exhibit nuclear co-localization with HOXB6 in either fetal or adult epidermis. RNA and protein data suggest that a truncated HOXB6 protein, lacking the homeodomain, is expressed in undifferentiated keratinocytes and that the full-length protein is induced by differentiation. GFP-fusion proteins were used to demonstrate that the full-length HOXB6 protein is localized to the nucleus while the truncated protein is largely cytoplasmic. Taken together, these data suggest that during epidermal development the truncated HOXB6 isoform may function by a mechanism other than as DNA binding protein, and that most of the nuclear, homeodomain-containing HOXB6 protein does not utilize PBX proteins as DNA binding partners in the skin. Published 2000 Wiley-Liss, Inc.

Function and regulation of homothorax in the wing imaginal disc of Drosophila

The gene homothorax (hth) is originally expressed uniformly in the wing imaginal disc but, during development, its activity is restricted to the cells that form the thorax and the hinge, where the wing blade attaches to the thorax, and eliminated in the wing pouch, which forms the wing blade. We show that hth repression in the wing pouch is a prerequisite for wing development; forcing hth expression prevents growth of the wing blade. Both the Dpp and the Wg pathways are involved in hth repression. Cells unable to process the Dpp (lacking thick veins or Mothers against Dpp activity) or the Wg (lacking dishevelled function) signal express hth in the wing pouch. We have identified vestigial (vg) as a Wg and Dpp response factor that is involved in hth control. In contrast to its repressing role in the wing pouch, wg upregulates hth expression in the hinge. We have also identified the gene teashirt (tsh) as a positive regulator of hth in the hinge. tsh plays a role specifying hinge structures, possibly in co-operation with hth.

A dual role for homothorax in inhibiting wing blade development and specifying proximal wing identities in Drosophila

The Drosophila wing imaginal disc gives rise to three body parts along the proximo-distal (P-D) axis: the wing blade, the wing hinge and the mesonotum. Development of the wing blade initiates along part of the dorsal/ventral (D/V) compartment boundary and requires input from both the Notch and wingless (wg) signal transduction pathways. In the wing blade, wg activates the gene vestigial (vg), which is required for the wing blade to grow. wg is also required for hinge development, but wg does not activate vg in the hinge, raising the question of what target genes are activated by wg to generate hinge structures. Here we show that wg activates the gene homothorax (hth) in the hinge and that hth is necessary for hinge development. Further, we demonstrate that hth also limits where along the D/V compartment boundary wing blade development can initiate, thus helping to define the size and position of the wing blade within the disc epithelium. We also show that the gene teashirt (tsh), which is coexpressed with hth throughout most of wing disc development, collaborates with hth to repress vg and block wing blade development. Our results suggest that tsh and hth block wing blade development by repressing some of the activities of the Notch pathway at the D/V compartment boundary.