gli, a zinc finger transcription factor and oncogene, is expressed during normal mouse development

Prostate development requires Sonic hedgehog expressed by the urogenital sinus epithelium

The prostate gland develops from the urogenital sinus by a testosterone-dependent process of ductal morphogenesis. Sonic hedgehog (Shh) is expressed in the urogenital sinus epithelium and the time course of expression coincides with the formation of the main prostatic ducts. Expression is most abundant in the lumen of the urogenital sinus and in the contiguous proximal duct segments. The initial upregulation of Shh expression in the male urogenital sinus depends on the presence of testosterone. The function of Shh was examined in the male urogenital sinus which was transplanted under the renal capsule of an adult male host mouse. Blockade of Shh function by a neutralizing antibody interferes with Shh signaling and abrogates growth and ductal morphogenesis in the transplanted tissue. These observations show that testosterone-dependent Shh expression in the urogenital sinus is necessary for the initiation of prostate development.

Essential role for Sonic hedgehog during hair follicle morphogenesis

The hair follicle is a source of epithelial stem cells and site of origin for several types of skin tumors. Although it is clear that follicles arise by way of a series of inductive tissue interactions, identification of the signaling molecules driving this process remains a major challenge in skin biology. In this study we report an obligatory role for the secreted morphogen Sonic hedgehog (Shh) during hair follicle development. Hair germs comprising epidermal placodes and associated dermal condensates were detected in both control and Shh -/- embryos, but progression through subsequent stages of follicle development was blocked in mutant skin. The expression of Gli1 and Ptc1 was reduced in Shh -/- dermal condensates and they failed to evolve into hair follicle papillae, suggesting that the adjacent mesenchyme is a critical target for placode-derived Shh. Despite the profound inhibition of hair follicle morphogenesis, late-stage follicle differentiation markers were detected in Shh -/- skin grafts, as well as cultured vibrissa explants treated with cyclopamine to block Shh signaling. Our findings reveal an essential role for Shh during hair follicle morphogenesis, where it is required for normal advancement beyond the hair germ stage of development.

Expression of lymphomagenic oncogenes in T-cell lymphomas of HPV 16 transgenic mice

We have previously established that a dimer repeat of the complete HPV 16 genome is sufficient to cause multiple organ malignancies, either carcinomas or T-cell lymphomas, in transgenic mice. Here, we report the expression of oncogenes supporting the notion that these tumors arose via multiple oncogenic pathways. In these mice, the transgenic HPV 16 genome cosegregated with the tumor phenotype. E6/E7 expression was observed in both carcinomas and T-cell lymphomas, while E2 expression was observed only in T-cell lymphomas. Some of the T-cell lymphomas revealed E2 expression alone, implying that oncogenic pathways of HPV other than the one involving E6/E7 existed in these transgenic mice. To establish that this is the case, expression of genes downstream from E6/E7 and oncogenes involved in T-cell lymphoma formation were analyzed. p53 mutations were observed in two of five tumors that lacked E6 expression. High levels of c-myc gene expression were observed in five of six tumors with E7 expression, suggesting that a pathway involving E7, inactivation of Rb, and activation of c-myc is important in tumorigenesis of HPV 16 in these transgenic animals. High levels of expression of the c-Pim gene were also noted in two of three c-myc-expressing T-cell lymphomas, suggesting cooperation between these two proto-oncogenes. Activation of Hox-11, Tal2/SCL-2, and Rbtn1/Ttg1 expression, which are highly associated with human T-cell acute lymphoblastic leukemia (T-ALL), was observed in three of three T-cell lymphomas with E2 expression but not E6/E7 expression, showing that pathways to tumor formation not involving E6/E7 exist in these transgenic animals. At least two oncogenic pathways to tumors in HPV 16 transgenic mice exist, one involving E6/E7 and c-myc and the other involving E2 and lymphomagenic oncogenes.

Diminished Sonic hedgehog signaling and lack of floor plate differentiation in Gli2 mutant mice

Floor plate cells at the midline of the neural tube are specified by high-level activity of Sonic hedgehog (Shh) secreted by notochord, whereas motor neurons are thought to be specified by a lower level activity of Shh secreted in turn by floor plate cells. In Drosophila, the Gli zinc finger protein Cubitus interruptus functions as a transcription factor activating Hedgehog-responsive genes. We report that the expression of known Shh-responsive genes such as Ptc and Gli1 is downregulated in mutant mice lacking Gli2 function. Gli2 mutants fail to develop a floor plate yet still develop motor neurons, which occupy the ventral midline of the neural tube. Our results imply that Gli2 is required to mediate high level but not low level Shh activity and show that the development of motor neurons can occur in the absence of floor plate induction.

Signalling interactions during facial development

The development of the vertebrate face is a dynamic multi-step process which starts with the formation of neural crest cells in the developing brain and their subsequent migration to form, together with mesodermal cells, the facial primordia. Signalling interactions co-ordinate the outgrowth of the facial primordia from buds of undifferentiated mesenchyme into the intricate series of bones and cartilage structures that, together with muscle and other tissues, form the adult face. Some of the molecules that are thought to be involved have been identified through the use of mouse mutants, data from human craniofacial syndromes and by expression studies of signalling molecules during facial development. However, the way that these molecules control the epithelial-mesenchymal interactions which mediate facial outgrowth and morphogenesis is unclear. The role of neural crest cells in these processes has also not yet been well defined. In this review we discuss the complex interaction of all these processes during face development and describe the candidate signalling molecules and their possible target genes.

Chromosomal and Gene Amplification in Diffuse Large B-Cell Lymphoma

Chromosomal translocations leading to deregulation of specific oncogenes characterize approximately 50% of cases of diffuse large B-cell lymphomas (DLBL). To characterize additional genetic features that may be of value in delineating the clinical characteristics of DLBL, we studied a panel of 96 cases at diagnosis consecutively ascertained at the Memorial Sloan-Kettering Cancer Center (MSKCC) for incidence of gene amplification, a genetic abnormality previously shown to be associated with tumor progression and clinical outcome. A subset of 20 cases was subjected to comparative genomic hybridization (CGH) analysis, which identified nine sites of chromosomal amplification (1q21-23, 2p12-16, 8q24, 9q34, 12q12-14, 13q32, 16p12, 18q21-22, and 22q12). Candidate amplified genes mapped to these sites were selected for further analysis based on their known roles in lymphoid cell and lymphoma development, and/or history of amplification in tumors. Probes for six genes, which fulfilled these criteria,REL (2p12-16), MYC (8q24), BCL2 (18q21),GLI, CDK4, and MDM2 (12q13-14), were used in a quantitative Southern blotting analysis of the 96 DLBL DNAs. Each of these genes was amplified (four or more copies) with incidence ranging from 11% to 23%. This analysis is consistent with our previous finding that REL amplification is associated with extranodal presentation. In addition, BCL2 rearrangement and/or REL, MYC, BCL2, GLI,CDK4, and MDM2 amplification was associated with advanced stage disease. These data show, for the first time, that amplification of chromosomal regions and genes is a frequent phenomenon in DLBL and demonstrates their potential significance in lymphomagenesis.

Combinatorial Gli gene function in floor plate and neuronal inductions by Sonic hedgehog

Within the developing vertebrate nervous system, it is not known how progenitor cells interpret the positional information provided by inducing signals or how the domains in which distinct groups of neural cells differentiate are defined. Gli proteins may be involved in these processes. In the frog neural plate, we have previously shown that the zinc finger transcription factor Gli1 is expressed in midline cells and mediates the effects of Shh inducing floor plate differentiation. In contrast, Gli2 and Gli3 are expressed throughout the neural plate except for the midline. Here, it is shown that Gli3 and Shh repress each other whereas Gli2, like Gli1, is a target of Shh signaling. However, only Gli1 can induce the differentiation of floor plate cells. In addition, Gli2 and Gli3 repress the ectopic induction of floor plate cells by Gli1 in co-injection assays and inhibit endogenous floor plate differentiation. The definition of the floor plate domain, therefore, appears to be defined by the antagonizing activities of Gli2 and Gli3 on Gli1 function. Because both Gli1 and Gli2 are induced by Shh, these results establish a regulatory feedback loop triggered by Shh that restricts floor plate cells to the midline. We have also previously shown that the Gli genes induce neuronal differentiation and here it is shown that there is specificity to the types of neurons the Gli proteins induce. Only Gli1 induces Nkx2.1/TTF-1(+) ventral forebrain neurons. Moreover, Gli2 and Gli3 inhibit their differentiation. In contrast, the differentiation of spinal motor neurons can be induced by the two ventrally expressed Gli genes, Gli1 and Gli2, suggesting that Gli2 directly mediates induction of motor neurons by Shh. In addition, Gli3 inhibits motor neuron differentiation by Gli2. Thus, combinatorial Gli function may pattern the neural tube, integrating positional information and cell type differentiation.

Characterization of the promoter region and genomic organization of GLI, a member of the Sonic hedgehog-Patched signaling pathway

GLI is the prototype for the Gli-Kruppel gene family characterized by a consensus C2-H2 zinc finger domain and is believed to function as a transcription activator in the vertebrate Sonic hedgehog-Patched signal transduction pathway. Understanding GLI gene regulation may be of importance to understanding causes of human birth defects and cancer. To begin to understand the regulation of this developmentally important gene we have cloned the human GLI gene and functionally characterized its 5' flanking region. The GLI gene is composed of 12 exons and 11 introns and in the zinc finger coding region shares a highly conserved splicing pattern with several other Gli family members in both vertebrates and C. elegans. A major transcription initiation site was identified upstream of the GLI translation start site along with three minor transcription initiation sites. The region surrounding the transcription initiation sites lacks TATA and CCAAT consensus sequences, has a high GC content, includes a CpG island, and contains several GC boxes. A 487bp segment surrounding the transcription initiation sites increased expression of a luciferase reporter gene 15-fold in Tera-1 cells and was defined as the core promoter region of human GLI. In transgenic mice this region directed beta-galactosidase expression to the central nervous system on embryonic days 10.5-12.5 and to sites of endochondral ossification on embryonic days 12.5 and 13.5 in a pattern comparable to the endogenous expression pattern of mouse gli within these tissues. The previously identified gastrointestinal expression of gli was not driven by this region and may require elements outside of the core promoter. Sequence analysis of the 5' flanking region of the mouse gli gene and the full-length mouse gli cDNA demonstrated high homology with human GLI, suggesting conservation of GLI regulation and function.

GLI activates transcription through a herpes simplex viral protein 16-like activation domain

Three proteins have been identified in mammals, GLI, GLI2, and GLI3, which share a highly conserved zinc finger domain with Drosophila Cubitus interruptus and are believed to function as transcription factors in the vertebrate Sonic hedgehog-Patched signaling pathway. To understand the role GLI plays in the Sonic hedgehog-Patched pathway and mechanisms of GLI-induced transcriptional regulation, we have characterized its transcriptional regulatory properties and contributions of specific domains to transcriptional regulation. We have demonstrated that GLI activates expression of reporter constructs in HeLa cells in a concentration-dependent manner through the GLI consensus binding motif and that a GAL4 binding domain-GLI fusion protein activates reporter expression through the GAL4 DNA binding site. GLI-induced transcriptional activation requires the carboxyl-terminal amino acids 1020-1091, which includes an 18-amino acid region highly similar to the alpha-helical herpes simplex viral protein 16 activation domain, including the consensus recognition element for the human TFIID TATA box-binding protein-associated factor TAFII31 and conservation of all three amino acid residues believed to contact directly chemically complementary residues in TAFII31. The presence of this region in the GLI activation domain provides a mechanism for GLI-induced transcriptional regulation.

Activation of the transcription factor Gli1 and the Sonic hedgehog signalling pathway in skin tumours

Sporadic basal cell carcinoma (BCC) is the most common type of malignant cancer in fair-skinned adults. Familial BCCs and a fraction of sporadic BCCs have lost the function of Patched (Ptc), a Sonic hedgehog (Shh) receptor that acts negatively on this signalling pathway. Overexpression of Shh can induce BCCs in mice. Here we show that ectopic expression of the zinc-finger transcription factor Gli1 in the embryonic frog epidermis results in the development of tumours that express endogenous Gli1. We also show that Shh and the Gli genes are normally expressed in hair follicles, and that human sporadic BCCs consistently express Gli1 but not Shh or Gli3. Because Gli1, but not Gli3, acts as a target and mediator of Shh signalling, our results suggest that expression of Gli1 in basal cells induces BCC formation. Moreover, loss of Ptc or overexpression of Shh cannot be the sole causes of Gli1 induction and sporadic BCC formation, as they do not occur consistently. Thus any mutations leading to the expression of Gli1 in basal cells are predicted to induce BCC formation.

Conservation of the C.elegans tra-2 3'UTR translational control

The Caenorhabditis elegans sex-determination gene, tra-2, is translationally regulated by two 28 nt elements (DREs) located in the 3'UTR that bind a factor called DRF. This regulation requires the laf-1 gene activity. We demonstrate that the nematode Caenorhabditis briggsae tra-2 gene and the human oncogene GLI are translationally regulated by elements that are functionally equivalent to DREs. Here, we rename the DREs to TGEs (tra-2 and GLI elements). Similarly to the C.elegans tra-2 TGEs, the C.briggsae tra-2 and GLI TGEs repress translation of a reporter transgene in a laf-1 dependent manner. Furthermore, they regulate poly(A) tail length and bind DRF. We also find that the C.elegans TGEs control translation and poly(A) tail length in C.briggsae and rodent cells. Moreover, these same organisms contain a factor that specifically associates with the C.elegans TGEs. These findings are consistent with the TGE control being present in C.briggsae and rodent cells. Three lines of evidence indicate that C.briggsae tra-2 and GLI are translationally controlled in vivo by TGEs. First, like C.elegans tra-2 TGEs, the C.briggsae tra-2 and GLI TGEs control translation and poly(A) tail lengths in C.briggsae and rodent cells, respectively. Second, the same factor in C.briggsae and mammalian cells that binds to the C.elegans tra-2 TGEs binds the C.briggsae tra-2 and GLI TGEs. Third, deletion of the GLI TGE increases GLI's ability to transform cells. These findings suggest that TGE control is conserved and regulates the expression of other mRNAs.

Evidence for the involvement of the Gli gene family in embryonic mouse lung development

Murine Gli, Gli2, and Gli3 are zinc finger genes related to Drosophila cubitus interuptus, a component of the hedgehog signal transduction pathway. In the embryonic lung, all three Gli genes are strongly expressed at the pseudoglandular stage, in distinct but overlapping domains of the mesoderm. Expression of Gli and Gli3, but not of Gli2, is subsequently downregulated at the canalicular stage, coincident with a decline in the expression of sonic hedgehog (Shh) and the hedgehog receptor gene, patched (Ptc). Overexpression of Shh in the lung results in increased levels of Ptc mRNA. Gli, but not Gli2, is also upregulated, suggesting a differential involvement of the Gli genes in the regulation of Ptc by SHH during lung development. Gli3 is not upregulated by Shh overexpression. However, its importance for lung development is shown by the finding that Gli3XtJ embryos, homozygous for a mutation involving a deletion of the Gli3 gene, have a stereotypic pattern of abnormalities in lung morphogenesis. The pulmonary defects in these embryos, consisting of localized shape changes and size reductions, correlate with normal Gli3 expression. Thus, our data indicate that one of the Gli genes, Gli3, is essential for normal lung development, and that another, Gli, can be placed downstream of Shh signaling in the lung.

Gli1 is a target of Sonic hedgehog that induces ventral neural tube development

The vertebrate zinc finger genes of the Gli family are homologs of the Drosophila gene cubitus interruptus. In frog embryos, Gli1 is expressed transiently in the prospective floor plate during gastrulation and in cells lateral to the midline during late gastrula and neurula stages. In contrast, Gli2 and Gli3 are absent from the neural plate midline with Gli2 expressed widely and Gli3 in a graded fashion with highest levels in lateral regions. In mouse embryos, the three Gli genes show a similar pattern of expression in the neural tube but are coexpressed throughout the early neural plate. Because Gli1 is the only Gli gene expressed in prospective floor plate cells of frog embryos, we have investigated a possible involvement of this gene in ventral neural tube development. Here we show that Shh signaling activates Gli1 transcription and that widespread expression of endogenous frog or human glioma Gli1, but not Gli3, in developing frog embryos results in the ectopic differentiation of floor plate cells and ventral neurons within the neural tube. Floor-plate-inducing ability is retained when cytoplasmic Gli1 proteins are forced into the nucleus or are fused to the VP16 transactivating domain. Thus, our results identify Gli1 as a midline target of Shh and suggest that it mediates the induction of floor plate cells and ventral neurons by Shh acting as a transcriptional regulator.

Cloning of a mouse smoothened cDNA and expression patterns of hedgehog signalling molecules during chondrogenesis and cartilage differentiation in clonal mouse EC cells, ATDC5

Hedgehog (hh) family proteins appear to use the conserved targets in their signalling pathway including Patched (Ptc), Smoothened (Smo), and Gli. Although Indian hedgehog (Ihh) plays an important role in endochondral bone formation, the involvement of hh signalling molecules in skeletogenesis is unknown. We cloned a mouse (m) Smo cDNA and studied the expression patterns of Ihh, Ptc, Smo, and Gli mRNAs in mouse chondrogenic EC cells, ATDC5. The deduced amino acid sequence of mSmo consisted of 793 amino acids and was 98 and 93% homologous to the rat (r) Smo and human (h) Smo, respectively. In ATDC5 cells, the expression of Ihh mRNA paralleled that of type X collagen mRNA. Smo, Ptc, and Gli mRNAs were constitutively expressed throughout chondrogenesis and the subsequent cartilage differentiation processes except for the transient decrease in Ptc mRNA at the cellular condensation stage. Our data suggest that hh signalling molecules may be involved in chondrogenesis and cartilage differentiation in ATDC5 cells.

A role for Xenopus Gli-type zinc finger proteins in the early embryonic patterning of mesoderm and neuroectoderm

Gli-type zinc finger proteins play important regulatory roles in vertebrate and invertebrate embryogenesis. In Xenopus, the Gli-type proteins XGli-3 and XGli-4 are first expressed in earliest stages of mesoderm and neural development. Transient transfection assays reveal that XGli-3 and XGli-4 can function as transcription repressors. Counteracting the Gli-protein repressor activity by ectopic expression of a fusion protein that contains the Gli-zinc finger cluster connected to the E1A activator domain in Xenopus embryos results in specific morphological alterations in the developing somites and in the central nervous system. Altered expression characteristics for a broad set of molecular markers highlighting specific aspects of mesodermal and neural differentiation demonstrate an important role for Gli-type zinc finger proteins in the early mesodermal and neural patterning of Xenopus embryos.

A binding site for Gli proteins is essential for HNF-3beta floor plate enhancer activity in transgenics and can respond to Shh in vitro

The floor plate plays important roles in ventral pattern formation and axonal guidance within the neural tube of vertebrate embryos. A critical event for floor plate development is the induction of a winged helix transcription factor, Hepatocyte Nuclear Factor-3beta (HNF-3beta). The enhancer for floor plate expression of HNF-3beta is located 3′ of the transcription unit and consists of multiple elements. HNF-3beta induction depends on the notochord-derived signal, Sonic hedgehog (Shh). Genetic analysis in Drosophila has led to the identification of genes involved in the Hh signalling pathway, and cubitus interruptus (ci), encoding a protein with five zinc finger motifs, was placed downstream. In the present work, we test the involvement of Gli proteins, the mouse homologues of Ci, in activation of the floor plate enhancer of HNF-3beta. Transgenic analysis shows that a Gli-binding site is required for the activity of the minimal floor plate enhancer of HNF-3beta in vivo. Three Gli genes are differentially expressed in the developing neural tube. Gli expression is restricted to the ventral part, while Gli2 and Gli3 are expressed throughout the neural tube and dorsally, respectively. Strong Gli and Gli2, and weak Gli3 expressions transiently overlap with HNF-3beta at the time of its induction. Consistent with ventrally localized expression, Gli expression can be up-regulated by Shh in a cell line. Finally, the Gli-binding site acts as a Shh responsive element, and human GLI, but not GLI3, can activate this binding site in tissue culture. Taken together, these findings suggest that Gli, and probably also Gli2, are good candidates for transcriptional activators of the HNF-3beta floor plate enhancer, and the binding site for Gli proteins is a key element for response to Shh signalling. These results also support the idea that Gli/Ci are evolutionary conserved transcription factors in the Hedgehog signalling pathway.

Expression of the mouse Gli and Ptc genes is adjacent to embryonic sources of hedgehog signals suggesting a conservation of pathways between flies and mice

The three mouse Gli genes are putative transcription factors which are the homologs of cubitus interruptus (ci) in Drosophila. Along with the gene patched (Ptc), ci has been implicated in the hedgehog (Hh) signal transduction pathway. To assess the role of Gli in embryogenesis, we compared its expression with that of Ptc and Hh family members in mouse. We found that Gli and Ptc are expressed in similar domains in diverse regions of the developing mouse embryo and these regions are adjacent to Hh signals. We also show that Gli is expressed ectopically along with Ptc and Shh in Strong's luxoid mutant mice. These results are consistent with conservation of the Hh signal transduction pathway in mice with Gli potentially mediating Hh signaling in multiple regions of the developing embryo.

Specific and redundant functions of Gli2 and Gli3 zinc finger genes in skeletal patterning and development

The correct patterning of vertebrate skeletal elements is controlled by inductive interactions. Two vertebrate hedgehog proteins, Sonic hedgehog and Indian hedgehog, have been implicated in skeletal development. During somite differentiation and limb development, Sonic hedgehog functions as an inductive signal from the notochord, floor plate and zone of polarizing activity. Later in skeletogenesis, Indian hedgehog functions as a regulator of chondrogenesis during endochondral ossification. The vertebrate Gli zinc finger proteins are putative transcription factors that respond to Hedgehog signaling. In Drosophila, the Gli homolog cubitus interruptus is required for the activation of hedgehog targets and also functions as a repressor of hedgehog expression. We show here that Gli2 mutant mice exhibit severe skeletal abnormalities including cleft palate, tooth defects, absence of vertebral body and intervertebral discs, and shortened limbs and sternum. Interestingly, Gli2 and Gli3 (C.-c. Hui and A. L. Joyner (1993). Nature Genet. 3, 241–246) mutant mice exhibit different subsets of skeletal defects indicating that they implement specific functions in the development of the neural crest, somite and lateral plate mesoderm derivatives. Although Gli2 and Gli3 are not functionally equivalent, double mutant analysis indicates that, in addition to their specific roles, they also serve redundant functions during skeletal development. The role of Gli2 and Gli3 in Hedgehog signaling during skeletal development is discussed.

A novel profile of expressed sequence tags for zinc finger encoding genes from the poorly differentiated exocrine pancreatic cell line AR4IP

Genes encoding for C2H2 zinc finger proteins are known to regulate normal cell proliferation and differentiation and have often been found to be mutated in different forms of cancer. We are interested in understanding the role of these genes as regulators of cell proliferation and differentiation in the exocrine pancreas. Therefore, we have generated expressed sequence tags (ESTs) encoding pancreas-enriched zinc finger peptides using the polymerase chain reaction and hybridization techniques [Adams, M.D. et al. (1991) Science, 252, 1651-1656]. Here we report the primary structure and expression pattern of 18 different zinc finger-encoding cDNAs (DZF-1-18) from the azaserine-derived tumoral cell line AR4IP which displays a poorly differentiated phenotype. Sequence analysis shows that all of these clones encode peptides which share the consensus DNA-binding motif with the Drosophila zinc finger transcription factor krüppel. High stringency Northern blot analysis shows that eight different zinc finger transcripts are expressed at high levels in normal adult rat pancreas and therefore constitute good candidates to play a role as transcription factors in exocrine pancreatic cells.

Identification and characterization of zinc finger encoding genes from the tumoral exocrine pancreatic cell line AR42J

Zinc finger transcription factors are DNA-binding proteins that are known to determine the identity of cells by regulating cell-specific gene expression. In addition, mutations in some members of this family have been found to be associated with several neoplasias, including Wilms' tumor and leukemias. Because the mechanisms that regulate normal, as well as neoplastic, pancreatic cell differentiation are poorly understood, we are searching for pancreas-enriched transcription factors that may determine the identity of pancreatic cells. Towards this end, we have used the polymerase chain reaction and degenerate primers against the highly conserved (Cys2-His2) zinc finger domain to amplify novel transcription factor encoding cDNAs from the well-characterized pancreatic acinar cell line AR42J. Using this approach, we have identified 17 different zinc finger encoding cDNAs (AZF-1 to -17). Sequence analysis shows that all of these clones encode for different zinc finger peptides which share the consensus DNA binding motif with the Drosophila transcription factor krüppel. As a first step in the characterization of these genes, the purified PCR products were used to determine their spatial pattern of expression by northern blot analysis. Using these techniques, we find that numerous zinc finger encoding genes are expressed in AR42J acinar cells as well as in normal adult rat pancreas and suggest that they may play a role as transcription factors in exocrine pancreatic cells.

Tripartite signaling of pattern: interactions between Hedgehogs, BMPs and Wnts in the control of vertebrate development

A central issue in embryonic development is the resolution of how groups of equivalent cells are transformed into orderly and patterned arrays of distinct cell types. Recent studies suggest the involvement of the Hedgehog, Wnt and bone morphogenetic protein families in the patterning of different tissue types in vertebrate embryos. The integrated actions of members of these three families of signaling proteins appear to have been recruited in the patterning of neural tissue in addition to several different tissues. Over the past year, a clearer picture of the diverse roles of these signaling proteins in embryonic development has begun to emerge.

Genetic aspects of embryonic eye development in vertebrates

The vertebrate eye comprises tissues from different embryonic origins, e.g., iris and ciliary body are derived from the wall of the diencephalon via optic vesicle and optic cup. Lens and cornea, on the other hand, come from the overlying surface ectoderm. The timely action of transcription factors and inductive signals ensure the correct development of the different eye components. Establishing the genetic basis of eye defects has been an important tool for the detailed analysis of this complex process. One of the main control genes for eye development was discovered by the analysis of the allelic series of the Small eye mouse mutants and characterized as Pax6. It is involved in the interaction between the optic cup and the overlaying ectoderm. The central role for Pax6 in eye development is conserved throughout the animal kingdom as the murine Pax6 gene induces ectopic eyes in transgenic Drosophila despite the obvious diverse organization of the eye in the fruit fly compared to vertebrates. In human, mutations in the PAX6 gene are responsible for aniridia and Peter's anomaly. In addition to Pax6, other mutations affecting the interaction of the optic cup and the lens placode have been documented in the mouse. For the differentiation of the retina from the optic cup several genes are responsible: Mi leads to microphthalmia, if mutated, and encodes for a transcription factor, which is expressed in the melanocytes of the pigmented layer of the retina. In addition, further genes are implicated in the correct development of the retina, e.g., Chx10, Dlx1, GH6, Msx1 and -2, Otx1 and -2, or Wnt7b. Mutations within the retinoblastoma gene (RB1) are responsible for retinal tumors. Knock-out mutants of RB1 exhibit a block of lens differentiation prior to the retinal defect. Besides the influence of Rb1, the lens differentiates under the influence of growth factors (e.g., FGF, IGF, PDGF, TGF), and specific genes become activated encoding cytoskeletal proteins (e.g., filensin, phakinin, vimentin), structural proteins (e.g., crystallins) or membrane proteins (e.g., Mip). The optic nerve originates from the neural retina; ganglion cells grow to the optic stalk, forming the optic nerve. Its retrograde walk to the brain through the rudiment of the optic stalk depends on the correct Pax2 expression.

Common molecular pathways for patterning of the body axis, limbs, central nervous system, and face during embryonic development

Many congenital anomalies affecting the face are known to appear as syndromes or associations, in combination with other defects. Often, these involve the limbs, eyes, central nervous system, and body axis. A general, and understandable, tendency among clinical researchers has been to seek a single cell type or definable embryologic process on which to ascribe the etiologic basis for such associations. The possibility of a gene, or group of genes, under coordinate control has not received much attention until recently. With the advent of recombinant DNA technology and the current explosion in basic research on the molecular bases of embryonic development, however, several possibilities are beginning to emerge. Here, I will list a few genes whose expression during development suggests that the molecules they encode are used as part of a coordinate molecular pathway, and that they play a role in the development of systems that often appear together in congenital associations or syndromes.

Disruption of the mouse MRF4 gene identifies multiple waves of myogenesis in the myotome

MRF4 (herculin/Myf-6) is one of the four member MyoD family of transcription factors identified by their ability to enforce skeletal muscle differentiation upon a wide variety of nonmuscle cell types. In this study the mouse germline MRF4 gene was disrupted by targeted recombination. Animals homozygous for the MRF4bh1 allele, a deletion of the functionally essential bHLH domain, displayed defective axial myogenesis and rib pattern formation, and they died at birth. Differences in somitogenesis between homozygous MRF4bh1 embryos and their wild-type littermates provided evidence for three distinct myogenic regulatory programs (My1-My3) in the somite, which correlate temporally and spatially with three waves of cellular recruitment to the expanding myotome. The first program (My1), marked initially by Myf-5 expression and followed by myogenin, began on schedule in the MRF4bh1/bh1 embryos at day 8 post coitum (E8). A second program (My2) was highly deficient in homozygous mutant MRF4 embryos, and normal expansion of the myotome failed. Moreover, expression of downstream muscle-specific genes, including FGF-6, which is a candidate regulator of inductive interactions, did not occur normally. The onset of MyoD expression around E10.5 in wild-type embryos marks a third myotomal program (My3), the execution of which was somewhat delayed in MRF4 mutant embryos but ultimately led to extensive myogenesis in the trunk. By E15 it appeared to have largely compensated for the defective My2 program in MRF4 mutants. Homozygous MRF4bh1 animals also showed improper rib pattern formation perhaps due to the absence of signals from cells expressing the My2 program. Finally, a later and relatively mild phenotype was detected in intercostal muscles of newborn animals.

Analysis of cubitus interruptus regulation in Drosophila embryos and imaginal disks

The cubitus interruptus (ci) gene of Drosophila is expressed in all anterior compartment cells in both embryos and imaginal disks where it encodes a putative zinc-finger protein related to the vertebrate Gli and C. elegans Tra-1 proteins. Using ci/lacZ fusions, we located regulatory sequences responsible for the normal pattern of ci expression, and obtained evidence that separate elements regulate its expression in embryos and imaginal disks. Mutants that delete a portion of this regulatory region express ci ectopically in the posterior compartments of their wing imaginal disks and have wings with malformed posterior compartments. Similar deletions of ci/lacZ fusion constructs also result in ectopic posterior compartment expression. Evidence that the engrailed protein normally represses ci in posterior compartments includes the expansion of ci expression into posterior compartment cells that lack engrailed function, diminution of ci expression upon overexpression of engrailed protein in anterior compartment cells, and the ability of engrailed protein to bind to the ci regulatory region in vivo and in vitro. We suggest that engrailed protein directly represses ci expression in posterior compartment cells.

Use of representational difference analysis for the identification of mdm2 oncogene amplification in diethylstilbestrol-induced murine uterine adenocarcinomas

Exposure in utero to the synthetic estrogen diethylstilbestrol (DES) is associated with the subsequent development of reproductive-tract malignancies in female offspring. To search for the genetic targets of DES, representational difference analysis was used to compare genomic DNA from DES-associated mouse uterine adenocarcinoma cells with genomic DNA from normal CD-1 mouse tissue. Several difference clones were obtained, all of which recognized rearranged and amplified sequences in tumor compared with normal DNA. One of these difference fragments mapped to a region of mouse chromosome 10 that includes the mdm2 oncogene. Amplification and overexpression of mdm2 was found in all three early-passage cell lines established from independent DES-associated cancers. These findings demonstrate the potential power of representational difference analysis in cancer research and suggest a genetic mechanism for DES-induced carcinogenesis.