Involvement of a novel Tnf receptor homologue in hair follicle induction.

Although inductive interactions are known to be essential for specification of cell fate in many vertebrate tissues, the signals and receptors responsible for transmitting this information remain largely unidentified. Mice with mutations in the downless (dl) gene have defects in hair follicle induction, lack sweat glands and have malformed teeth. These structures originate as ectodermal placodes, which invaginate into the underlying mesenchyme and differentiate to form specific organs. Positional cloning of the dl gene began with identification of the transgenic family OVE1. One branch of the family, dl(OVE1B), carries an approximately 600-kb deletion at the dl locus caused by transgene integration. The mutated locus has been physically mapped in this family, and a 200-kb mouse YAC clone, YAC D9, has been identified and shown to rescue the dl phenotype in the spontaneous dl(Jackson) (dl(J), recessive) and Dl(sleek) (Dl(slk), dominant negative) mutants. Here we report the positional cloning of the dl gene, which encodes a novel member of the tumour necrosis factor (Tnf) receptor (Tnfr) family. The mutant phenotype and dl expression pattern suggests that this gene encodes a receptor that specifies hair follicle fate. Its ligand is likely to be the product of the tabby (Ta) gene, as Ta mutants have a phenotype identical to that of dl mutants and Ta encodes a Tnf-like protein.

Mutations in the human homologue of mouse dl cause autosomal recessive and dominant hypohidrotic ectodermal dysplasia.

X-linked hypohidrotic ectodermal dysplasia results in abnormal morphogenesis of teeth, hair and eccrine sweat glands. The gene (ED1) responsible for the disorder has been identified, as well as the analogous X-linked gene (Ta) in the mouse. Autosomal recessive disorders, phenotypically indistinguishable from the X-linked forms, exist in humans and at two separate loci (crinkled, cr, and downless, dl) in mice. Dominant disorders, possibly allelic to the recessive loci, are seen in both species (ED3, Dlslk). A candidate gene has recently been identified at the dl locus that is mutated in both dl and Dlslk mutant alleles. We isolated and characterized its human DL homologue, and identified mutations in three families displaying recessive inheritance and two with dominant inheritance. The disorder does not map to the candidate gene locus in all autosomal recessive families, implying the existence of at least one additional human locus. The putative protein is predicted to have a single transmembrane domain, and shows similarity to two separate domains of the tumour necrosis factor receptor (TNFR) family.

A transgenic insertion upstream of sox9 is associated with dominant XX sex reversal in the mouse.

In most mammals, male development is triggered by the transient expression of the Y-chromosome gene, Sry, which initiates a cascade of gene interactions ultimately leading to the formation of a testis from the indifferent fetal gonad. Several genes, in particular Sox9, have a crucial role in this pathway. Despite this, the direct downstream targets of Sry and the nature of the pathway itself remain to be clearly established. We report here a new dominant insertional mutation, Odsex (Ods), in which XX mice carrying a 150-kb deletion (approximately 1 Mb upstream of Sox9) develop as sterile XX males lacking Sry. During embryogenesis, wild-type XX fetal gonads downregulate Sox9 expression, whereas XY and XX Ods/+ fetal gonads upregulate and maintain its expression. We propose that Ods has removed a long-range, gonad-specific regulatory element that mediates the repression of Sox9 expression in XX fetal gonads. This repression would normally be antagonized by Sry protein in XY embryos. Our data are consistent with Sox9 being a direct downstream target of Sry and provide genetic evidence to support a general repressor model of sex determination in mammals.

Inversin, a novel gene in the vertebrate left-right axis pathway, is partially deleted in the inv mouse.

Visceral left-right asymmetry occurs in all vertebrates, but the inversion of embryo turning (inv) mouse, which resulted following a random transgene insertion, is the only model in which these asymmetries are consistently reversed. We report positional cloning of the gene underlying this recessive phenotype. Although transgene insertion was accompanied by neighbouring deletion and duplication events, our YAC phenotype rescue studies indicate that the mutant phenotype results from the deletion. After extensively characterizing the 47-kb deleted region and flanking sequences from the wild-type mouse genome, we found evidence for only one gene sequence in the deleted region. We determined the full-length 5.5-kb cDNA sequence and identified 16 exons, of which exons 3-11 were eliminated by the deletion, causing a frameshift. The novel gene specifies a 1062-aa product with tandem ankyrin-like repeat sequences. Characterization of complementing and non-complementing YAC transgenic families revealed that correction of the inv mutant phenotype was concordant with integration and intact expression of this novel gene, which we have named inversin (Invs).

X-linked anhidrotic ectodermal dysplasia with immunodeficiency is caused by impaired NF-kappaB signaling.

The molecular basis of X-linked recessive anhidrotic ectodermal dysplasia with immunodeficiency (EDA-ID) has remained elusive. Here we report hypomorphic mutations in the gene IKBKG in 12 males with EDA-ID from 8 kindreds, and 2 patients with a related and hitherto unrecognized syndrome of EDA-ID with osteopetrosis and lymphoedema (OL-EDA-ID). Mutations in the coding region of IKBKG are associated with EDA-ID, and stop codon mutations, with OL-EDA-ID. IKBKG encodes NEMO, the regulatory subunit of the IKK (IkappaB kinase) complex, which is essential for NF-kappaB signaling. Germline loss-of-function mutations in IKBKG are lethal in male fetuses. We show that IKBKG mutations causing OL-EDA-ID and EDA-ID impair but do not abolish NF-kappaB signaling. We also show that the ectodysplasin receptor, DL, triggers NF-kappaB through the NEMO protein, indicating that EDA results from impaired NF-kappaB signaling. Finally, we show that abnormal immunity in OL-EDA-ID patients results from impaired cell responses to lipopolysaccharide, interleukin (IL)-1beta, IL-18, TNFalpha and CD154. We thus report for the first time that impaired but not abolished NF-kappaB signaling in humans results in two related syndromes that associate specific developmental and immunological defects.

TAK1 is activated in the myocardium after pressure overload and is sufficient to provoke heart failure in transgenic mice.

The transforming-growth-factor-beta-activated kinase TAK1 is a member of the mitogen-activated protein kinase kinase kinase family, which couples extracellular stimuli to gene transcription. The in vivo function of TAK1 is not understood. Here, we investigated the potential involvement of TAK1 in cardiac hypertrophy. In adult mouse myocardium, TAK1 kinase activity was upregulated 7 days after aortic banding, a mechanical load that induces hypertrophy and expression of transforming growth factor beta. An activating mutation of TAK1 expressed in myocardium of transgenic mice was sufficient to produce p38 mitogen-activated protein kinase phosphorylation in vivo, cardiac hypertrophy, interstitial fibrosis, severe myocardial dysfunction, 'fetal' gene induction, apoptosis and early lethality. Thus, TAK1 activity is induced as a delayed response to mechanical stress, and can suffice to elicit myocardial hypertrophy and fulminant heart failure.

Leukemia inhibitory factor improves survival of retroviral vector-infected hematopoietic stem cells in vitro, allowing efficient long-term expression of vector-encoded human adenosine deaminase in vivo.

Low recovery and poor retroviral vector infection efficiency of hematopoietic stem cells has hindered application of gene therapy for disease affecting blood-forming tissues. Developmental restriction (or death) of stem cells during ex vivo infection has contributed to these difficulties. In these studies we report that the cytokine leukemia inhibitory factor (LIF) directly or indirectly supported the survival of hematopoietic stem cells during culture of bone marrow with vector-producing fibroblasts, resulting in efficient recovery of stem cells able to compete for engraftment in irradiated recipient animals. The infection efficiency of hematopoietic stem cells recovered from these cultures was approximately 80%; and all recipients (20/20) of the LIF-treated marrow were stably engrafted with the progeny of provirus-bearing stem cells. Expression of vector-encoded human adenosine deaminase (hADA) was detected in all recipients at levels averaging 15-50% of endogenous murine ADA in all their hematolymphoid tissues. Survival of stem cells in untreated cultures was approximately 10% of that observed from LIF-treated cultures, resulting in poor engraftment of recipient animals with transplanted cells. The infection efficiency of the few stem cells recovered from untreated cultures, however, was high (approximately 80%), suggesting that LIF did not have an effect on infection efficiency per se, but acted at the level of stem cell survival. Consistent with the poor engraftment observed in the control animals, expression of vector-encoded ADA was only approximately 4-20% of the endogenous levels. These results support the postulated role of LIF as a regulator of hematopoiesis and suggest that cytokine stimulation can positively affect inefficient retroviral vector transduction in hematopoietic stem cells.

Bilateral retinal and brain tumors in transgenic mice expressing simian virus 40 large T antigen under control of the human interphotoreceptor retinoid-binding protein promoter.

We have previously shown that postnatal expression of the viral oncoprotein SV40 T antigen in rod photoreceptors (transgene MOT1), at a time when retinal cells have withdrawn from the mitotic cycle, leads to photoreceptor cell death (Al-Ubaidi et al., 1992. Proc. Natl. Acad. Sci. USA. 89:1194-1198). To study the effect of the specificity of the promoter, we replaced the mouse opsin promoter in MOT1 by a 1.3-kb promoter fragment of the human IRBP gene which is expressed in both rod and cone photoreceptors during embryonic development. The resulting construct, termed HIT1, was injected into mouse embryos and five transgenic mice lines were established. Mice heterozygous for HIT1 exhibited early bilateral retinal and brain tumors with varying degrees of incidence. Histopathological examination of the brain and eyes of three of the families showed typical primitive neuroectodermal tumors. In some of the bilateral retinal tumors, peculiar rosettes were observed, which were different from the Flexner-Wintersteiner rosettes typically associated with human retinoblastomas. The ocular and cerebral tumors, however, contained Homer-Wright rosettes, and showed varying degrees of immunoreactivity to antibodies against the neuronal specific antigens, synaptophysin and Leu7, but not to antibodies against photoreceptor specific proteins. Taken together, the results indicate that the specificity of the promoter used for T antigen and/or the time of onset of transgene expression determines the fate of photoreceptor cells expressing T antigen.

Tissue-specific expression in transgenic mice of a fused gene containing RSV terminal sequences.

Transgenic mice were generated with pRSV-CAT, a chimeric gene construct containing the long terminal repeat of Rous sarcoma virus (RSV) linked to the bacterial gene encoding chloramphenicol acetyltransferase (CAT). CAT expression, detected in adult animals of five independent strains, was preferentially directed to organs rich in tendon, bone, and muscle. This pattern reflects the disease specificity of the intact virus and suggests that the tissue tropism of RSV is determined at least in part by the presence of endogenous tissue-specific factors that can promote expression of genetic information linked to the long terminal repeat. In two of the mouse strains, insertion of the pRSV-CAT DNA resulted in developmental abnormalities. One of these strains was characterized by a dominant trait of embryonic lethality, the other by a recessive trait of fused toes in all four feet.

Reversal of left-right asymmetry: a situs inversus mutation.

A recessive mutation was identified in a family of transgenic mice that resulted in a reversal of left-right polarity (situs inversus) in 100 percent of the homozygous transgenic mice tested. Sequences that flanked the transgenic integration site were cloned and mapped to mouse chromosome 4, between the Tsha and Hxb loci. During early embryonic development, the direction of postimplantation turning, one of the earliest manifestations of left-right asymmetry, was reversed in homozygous transgenic embryos. This insertional mutation identifies a gene that controls embryonic turning and visceral left-right polarity.

Oncogenesis of the lens in transgenic mice.

Neoplastic tumors of the ocular lens of vertebrates do not naturally occur. Transgenic mice carrying a hybrid gene comprising the murine alpha A-crystallin promoter (-366 to +46) fused to the coding sequence of the SV40 T antigens developed lens tumors, which obliterated the eye cavity and even invaded neighboring tissue, thus establishing that the lens is not refractive to oncogenesis. Large-T antigen was detected early in lens development; it elicited morphological changes and specifically interfered with differentiation of lens fiber cells. Both alpha- and beta-crystallins persisted in many of the lens tumor cells, while gamma-crystallin was selectively reduced. Accessibility, characteristic morphology, and defined protein markers make this transparent epithelial eye tissue a potentially useful system for testing tumorigenicity of oncogenes and for studying malignant transformation from its inception until death of the animal.

The homeobox gene NKX3.2 is a target of left-right signalling and is expressed on opposite sides in chick and mouse embryos.

Vertebrate internal organs display invariant left-right (L-R) asymmetry. A signalling cascade that sets up L-R asymmetry has recently been identified (reviewed in [1]). On the right side of Hensen's node, activin represses Sonic hedgehog (Shh) expression and induces expression of the genes for the activin receptor (ActRIIa) and fibroblast growth factor-8 (FGF8) [2] [3]. On the left side, Shh induces nodal expression in lateral plate mesoderm (LPM); nodal in turn upregulates left-sided expression of the bicoid-like homeobox gene Pitx2 [4] [5] [6]. Here, we found that the homeobox gene NKX3.2 is asymmetrically expressed in the anterior left LPM and in head mesoderm in the chick embryo. Misexpression of the normally left-sided signals Nodal, Lefty2 and Shh on the right side, or ectopic application of retinoic acid (RA), resulted in upregulation of NKX3.2 contralateral to its normal expression in left LPM. Ectopic application of FGF8 on the left side blocked NKX3.2 expression, whereas the FGF receptor-1 (FGFR-1) antagonist SU5402, implanted on the right side, resulted in bilateral NKX3.2 expression in the LPM, suggesting that FGF8 is an important negative determinant of asymmetric NKX3.2 expression. NKX3.2 expression was also found to be asymmetric in the mouse LPM but, unlike in the chick, it was expressed in the right LPM. In the inversion of embryonic turning (inv) mouse mutant, which has aberrant L-R development, NKX3.2 was expressed predominantly on the left side. Thus, NKX3.2 transcripts accumulate on opposite sides of mouse and chick embryos although, in both the mouse and chick, NKX3.2 expression is controlled by the L-R signalling pathways.

Lens-specific expression of transforming growth factor beta1 in transgenic mice causes anterior subcapsular cataracts.

Transgenic mice were generated by microinjection of a construct containing a self-activating human TGF-beta1 cDNA driven by the lens-specific alphaA-crystallin promoter. Seven transgenic founder mice were generated, and four transgenic lines expressing TGF-beta1 were characterized. By postnatal day 21, mice from the four families exhibited anterior subcapsular cataracts. The lenses in these mice developed focal plaques of spindle-shaped cells that expressed alpha-smooth muscle actin, and that resembled the plaques seen in human anterior subcapsular cataracts. Transgenic mice showed additional ocular defects, including corneal opacification and structural changes in the iris and ciliary body. The corneal opacities were associated with increased exfoliation of the squamous layer of the corneal epithelium and increased DNA replication in the basal epithelium.

Gene recombination in postmitotic cells. Targeted expression of Cre recombinase provokes cardiac-restricted, site-specific rearrangement in adult ventricular muscle in vivo.

Mouse models of human disease can be generated by homologous recombination for germline loss-of-function mutations. However, embryonic-lethal phenotypes and systemic, indirect dysfunction can confound the use of knock-outs to elucidate adult pathophysiology. Site-specific recombination using Cre recombinase can circumvent these pitfalls, in principle, enabling temporal and spatial control of gene recombination. However, direct evidence is lacking for the feasibility of Cre-mediated recombination in postmitotic cells. Here, we exploited transgenic mouse technology plus adenoviral gene transfer to achieve Cre-mediated recombination in cardiac muscle. In vitro, Cre driven by cardiac-specific alpha-myosin heavy chain (alphaMyHC) sequences elicited recombination selectively at loxP sites in purified cardiac myocytes, but not cardiac fibroblasts. In vivo, this alphaMyHC-Cre transgene elicited recombination in cardiac muscle, but not other organs, as ascertained by PCR analysis and localization of a recombination-dependent reporter protein. Adenoviral delivery of Cre in vivo provoked recombination in postmitotic, adult ventricular myocytes. Recombination between loxP sites was not detected in the absence of Cre. These studies demonstrate the feasibility of using Cre-mediated recombination to regulate gene expression in myocardium, with efficient induction of recombination even in terminally differentiated, postmitotic muscle cells. Moreover, delivery of Cre by viral infection provides a simple strategy to control the timing of recombination in myocardium.

CaM kinase signaling induces cardiac hypertrophy and activates the MEF2 transcription factor in vivo.

Hypertrophic growth is an adaptive response of the heart to diverse pathological stimuli and is characterized by cardiomyocyte enlargement, sarcomere assembly, and activation of a fetal program of cardiac gene expression. A variety of Ca(2+)-dependent signal transduction pathways have been implicated in cardiac hypertrophy, but whether these pathways are independent or interdependent and whether there is specificity among them are unclear. Previously, we showed that activation of the Ca(2+)/calmodulin-dependent protein phosphatase calcineurin or its target transcription factor NFAT3 was sufficient to evoke myocardial hypertrophy in vivo. Here, we show that activated Ca(2+)/calmodulin-dependent protein kinases-I and -IV (CaMKI and CaMKIV) also induce hypertrophic responses in cardiomyocytes in vitro and that CaMKIV overexpressing mice develop cardiac hypertrophy with increased left ventricular end-diastolic diameter and decreased fractional shortening. Crossing this transgenic line with mice expressing a constitutively activated form of NFAT3 revealed synergy between these signaling pathways. We further show that CaMKIV activates the transcription factor MEF2 through a posttranslational mechanism in the hypertrophic heart in vivo. Activated calcineurin is a less efficient activator of MEF2-dependent transcription, suggesting that the calcineurin/NFAT and CaMK/MEF2 pathways act in parallel. These findings identify MEF2 as a downstream target for CaMK signaling in the hypertrophic heart and suggest that the CaMK and calcineurin pathways preferentially target different transcription factors to induce cardiac hypertrophy.

Sperm-specific expression of angiotensin-converting enzyme (ACE) is mediated by a 91-base-pair promoter containing a CRE-like element.

The gene encoding the testis isozyme of angiotensin-converting enzyme (testis ACE) is one example of the many genes expressed uniquely during spermatogenesis. This protein is expressed by developing germ cells late in their development and results from the activation of a sperm-specific promoter that is located within intron 12 of the gene encoding the somatic isozyme of ACE. In vitro transcription, DNase footprinting, gel shift assays, and transgenic mouse studies have been used to define the minimal testes ACE promoter and to characterize DNA-protein interactions mediating germ cell-specific expression. These studies show that proper cell- and stage-specific expression of testis ACE requires only a small portion of the immediate upstream sequence extending to -91. A critical motif within this core promoter is a cyclic AMP-responsive element sequence that interacts with a testis-specific transactivating factor. Since this putative cyclic AMP-responsive element has been conserved within the testis ACE promoters of different species and is found at the same site in other genes that are expressed specifically in the testis, it may provide a common mechanism for the recognition of sperm-specific promoters.

The retinoblastoma protein-binding region of simian virus 40 large T antigen alters cell cycle regulation in lenses of transgenic mice.

Regulation of the cell cycle is a critical aspect of cellular proliferation, differentiation, and transformation. In many cell types, the differentiation process is accompanied by a loss of proliferative capability, so that terminally differentiated cells become postmitotic and no longer progress through the cell cycle. In the experiments described here, the ocular lens has been used as a system to examine the role of the retinoblastoma protein (pRb) family in regulation of the cell cycle during differentiation. The ocular lens is an ideal system for such studies, since it is composed of just two cell types: epithelial cells, which are capable of proliferation, and fiber cells, which are postmitotic. In order to inactivate pRb in viable mice, genes encoding either a truncated version of simian virus 40 large T antigen or the E7 protein of human papillomavirus were expressed in a lens-specific fashion in transgenic mice. Lens fiber cells in the transgenic mice were found to incorporate bromodeoxyuridine, implying inappropriate entry into the cell cycle. Surprisingly, the lens fiber cells did not proliferate as tumor cells but instead underwent programmed cell death, resulting in lens ablation and microphthalmia. Analogous lens alterations did not occur in mice expressing a modified version of the truncated T antigen that was mutated in the binding domain for the pRb family. These experimental results indicate that the retinoblastoma protein family plays a crucial role in blocking cell cycle progression and maintaining terminal differentiation in lens fiber cells. Apoptotic cell death ensues when fiber cells are induced to remain in or reenter the cell cycle.

Regulation of cyclin and cyclin-dependent kinase gene expression during lens differentiation requires the retinoblastoma protein.

The retinoblastoma protein (pRb) functions as a negative regulator of the cell cycle and is essential to maintain certain cell types in a post-mitotic state during terminal differentiation. In the ocular lens, inactivation of this protein is sufficient to cause lens fiber cells, which are normally post-mitotic, to enter the cell cycle. The current studies address whether regulation of the cell cycle during lens fiber differentiation in normal lenses or in lenses in which pRB has been inactivated is accompanied by changes in expression of cyclin and cyclin-dependent kinase genes. In the normal lens, our experiments using in-situ hybridization reveal that the expression of cyclin A, cyclin B1, cdc2 and cdk2 is restricted to the proliferative epithelial cells, with no expression in the differentiating fiber cells. Cyclins D1 and D2 and cdk4 show a less restrictive pattern and are expressed in some of the post-mitotic cells. Lenses from RB-deficient embryos, in contrast, show inappropriate expression in the fiber cells of cyclins A, B1 and E, as well as cdc2 and cdk2. The lens fiber cells in these embryos express protein markers for differentiation, such as beta- and gamma-crystallins, even though the cells do not withdraw from the cell cycle. These results indicate that the regulated expression of multiple cell cycle regulatory genes during lens fiber cell differentiation requires the presence of pRb.

Characterization of five members of the actin gene family in the sea urchin.

Hybridization of an actin cDNA clone (pSA38) to restriction enzyme digests of Strongylocentrotus purpuratus DNA indicates that the sea urchin genome contains at least five different actin genes. A sea urchin genomic clone library was screened for recombinants which hydridize to pSA38 and four genomic clones were isolated. Restriction maps were generated which indicate that three of these recombinants contain different actin genes, and that the fourth may be an allele to one of these. The restriction maps suggest that one clone contains two linked actin genes. This fact, which was confirmed by heteroduplex analysis, indicates that the actin gene family may be clustered. The linked genes are oriented in the same direction and spaced about 8.0 kilobases apart. In heteroduplexes between genomic clones two intervening sequences were seen. Significant homology is confined to the actin coding region and does not include any flanking sequence. Southern blot analysis reveals that repetitive DNA sequences are found in the region of the actin genes.