Downstream Effector Of Wnt Signaling Research Articles

Phosphorylation of Shrimp Tcf by a Viral Protein Kinase WSV083 Suppresses Its Antiviral Effect.

Nuclear DNA-binding TCF proteins, which act as the main downstream effectors of Wnt signaling, are essential for the regulation of cell fate and innate immunity. However, their role during viral infection in shrimp remains unknown. Herein, we demonstrated that Litopenaeus vannamei TCF (LvTcf) acts independently of Lvβ-catenin to promote interferon-like protein LvVago1 production, thus mounting the response to WSSV infection. Further, we observed that WSV083, a WSSV serine/threonine protein kinase, bound to LvTcf and phosphorylated it. Phosphorylated LvTcf was then recognized and degraded via the ubiquitin-proteasome pathway. Moreover, mass spectrometry analyses indicated that the T39 and T104 residues of LvTcf were target sites phosphorylated by WSV083. Point mutation analyses suggested that additional sites of LvTcf may undergo phosphorylation via WSV083. Taken together, the current work provides valuable insights into host immunity and viral pathogenesis. LvTcf is not only a modulator of shrimp innate immunity but is also an important target for WSSV immune evasion. Thus, the current findings will help improve disease control in shrimps.

Cooperative antiproliferative signaling by aspirin and indole-3-carbinol targets microphthalmia-associated transcription factor gene expression and promoter activity in human melanoma cells

Antiproliferative signaling of combinations of the nonsteroidal anti-inflammatory drug acetylsalicylic acid (aspirin) and indole-3-carbinol (I3C), a natural indolecarbinol compound derived from cruciferous vegetables, was investigated in human melanoma cells. Melanoma cell lines with distinct mutational profiles were sensitive to different extents to the antiproliferative response of aspirin, with oncogenic BRAF-expressing G361 cells and wild-type BRAF-expressing SK-MEL-30 cells being the most responsive. I3C triggered a strong proliferative arrest of G361 melanoma cells and caused only a modest decrease in the proliferation of SK-MEL-30 cells. In both cell lines, combinations of aspirin and I3C cooperatively arrested cell proliferation and induced a G1 cell cycle arrest, and nearly ablated protein and transcript levels of the melanocyte master regulator microphthalmia-associated transcription factor isoform M (MITF-M). In melanoma cells transfected with a -333/+120-bp MITF-M promoter-luciferase reporter plasmid, treatment with aspirin and I3C cooperatively disrupted MITF-M promoter activity, which accounted for the loss of MITF-M gene products. Mutational analysis revealed that the aspirin required the LEF1 binding site, whereas I3C required the BRN2 binding site to mediate their combined and individual effects on MITF-M promoter activity. Consistent with LEF1 being a downstream effector of Wnt signaling, aspirin, but not I3C, downregulated protein levels of the Wnt co-receptor LDL receptor-related protein-6 and β-catenin and upregulated the β-catenin destruction complex component Axin. Taken together, our results demonstrate that aspirin-regulated Wnt signaling and I3C-targeted signaling pathways converge at distinct DNA elements in the MITF-M promoter to cooperatively disrupt MITF-M expression and melanoma cell proliferation.

Axud1 Integrates Wnt Signaling and Transcriptional Inputs to Drive Neural Crest Formation

Neural crest cells are induced at the neural plate border by the combined action of transcription factors and signaling molecules. Here, we show that Axud1, a downstream effector of Wnt signaling, represents a critical missing link that integrates signaling and transcriptional cues to mediate neural crest formation. Axud1 is a transcription factor expressed in neural crest progenitors in a Wnt1/β-catenin-dependent manner. Axud1 loss leads to downregulation of multiple genes involved in neural crest specification, similar to the effects of Wnt1 knockdown. Importantly, Axud1 is sufficient to rescue neural crest formation after disruption of Wnt signaling. Furthermore, it physically interacts with neural plate border genes Pax7 and Msx1 in vivo to directly activate transcription of stem cell factor FoxD3, initiating the neural crest program. Thus, Axud1 integrates Wnt signaling with transcriptional inputs to endow the neural crest with its unique molecular signature.

Abstract 4250: Nitric oxide synthase activity and its modulation in the treatment of colorectal cancer

Abstract The American Cancer Society estimates more than 141,000 new cases of and about 50,000 deaths from colorectal cancer every year. Treatment options include surgery, radiation therapy and targeted therapies such as anti-angiogenics. However, no therapies address the key driving factor of colorectal cancer: inflammation. It is well known that chronic inflammatory conditions such as Crohn's Disease, ulcerative colitis, diabetes, obesity and cigarette smoking all elevate the risk of developing colorectal cancer. One of the hallmarks of chronic inflammation is the elevated levels of reactive oxygen/nitrogen species (ROS/RNS). A primary source of these ROS/RNS is uncoupled Nitric Oxide Synthase. Under non-inflammatory conditions NOS generates Nitric Oxide. However, in an inflammatory environment, such as the oxidative tumor microenvironment, NOS's cofactor tetrahydrobiopterin (BH4) is oxidized to dihydrobiopterin (BH2). NOS bound to BH2 is said to be uncoupled and produces superoxide O2- and peroxyinitrite (ONOO-). Previous work in our and other's labs have shown that increased production of ROS/RNS leads to the activation of pro-inflammatory/proliferative molecules such as NFκB, Stat3, β-Catenin and Akt. NOS can be re-coupled by supplementing cells and animals with BH4 or its precursor Sepiapterin (SP). Herein Wwe have shownshow that by recoupling NOS with SP in HCT116, Caco-2 and HT29 cells, decreased tumor cell proliferation, increased β-Catenin degradation and decreased Akt activity. We have also seen increased tumor cell death measured via by in vitro clonogenic assays, as well decreased metabolic uptake in AOM/DSS induced colorectal cancer in vivo measured via by F18Deoxy-Glucose PET imaging. We believe by recoupling NOS both in vivo and in vitro we are modulatingion Wnt signaling via Akt and GSK-3β. Further studies are needed to elucidate the mechanism of decreased Akt activity and the downstream effectors of Wnt signaling which are affected by NOS recoupling. Lastly, we wish to combine radiation therapy with Sepiapterin SP to determine if the combination of these therapies can increase tumor cell deathincrease tumor control. Citation Format: Asim Alam, Chris Rabender, Li Wang, Sundaresan Gobalakrishnan, Jamal Zweit. Nitric oxide synthase activity and its modulation in the treatment of colorectal cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4250. doi:10.1158/1538-7445.AM2014-4250

Analysis of a Jup hypomorphic allele reveals a critical threshold for postnatal viability

Mutations in the human Jup gene cause arrhythmogenic right ventricular cardiomyopathy (ARVC), a heart muscle disease that often leads to sudden cardiac death. Inactivation of the murine Jup gene (also known as plakoglobin) results in embryonic lethality due to cardiac rupture. In an effort to generate a conditional knockout allele, a neomycin cassette was introduced into the murine plakoglobin (PG) gene. This allele (PG F(N)) functions as a hypomorph when combined with a null allele (PG Δ). About half of the PG F(N)/Δ animals were smaller than their littermates and died before weaning age, whereas the remaining PG F(N)/Δ animals survived. Despite the reduced levels of PG in the heart, there were no signs of cardiomyopathy or cardiac dysfunction as determined by echocardiography. Importantly, the PG homolog, β-catenin (CTNNB1), was increased in the PG F(N)/Δ hearts. In addition to its structural role as part of the N-cadherin/catenin adhesion complex, β-catenin is a downstream effector of Wnt signaling. However, no change in β-catenin/TCF reporter activity was observed in PG F(N)/Δ embryos suggesting that excess β-catenin was not likely causing increased transcription of Wnt/β-catenin target genes. These data suggest novel function(s) for PG beyond the heart and define a critical threshold of PG expression that is necessary for postnatal survival.

Cilia, Wnt signaling, and the cytoskeleton

Primary cilia have recently been highlighted as key regulators in development and disease. This review focuses on current work demonstrating the broad role of cilia-related proteins in developmental signaling systems. Of particular consideration is the importance of the basal body region, located at the base of the cilium, in its role as a focal point for many signaling pathways and as a microtubule organizing center. As the cilium is effectively a microtubular extension of the cytoskeleton, investigating connections between the cilium and the cytoskeleton provides greater insight into signaling and cell function. Of the many signaling pathways associated with primary cilia, the most extensively studied in association with the cytoskeleton and cytoskeletal rearrangements are both canonical and non-canonical Wnt pathways. One of the key concepts currently emerging is a possible additional role for the traditionally 'cilia-related' proteins in other aspects of cellular processes. In many cases, disruption of such processes manifests at the level of the cilium. While the involvement of cilia and cilia-related proteins in signaling pathways is currently being unraveled, there is a growing body of evidence to support the notion that ciliary proteins are required not only for regulation of Wnt signaling, but also as downstream effectors of Wnt signaling. This review summarizes recent advances in our understanding of the involvement of cilia and basal body proteins in Wnt signaling pathways.

Combinatorial Human Progenitor Cell Transplantation Optimizes Islet Regeneration Through Secretion of Paracrine Factors

Transplanted human bone marrow (BM) and umbilical cord blood (UCB) progenitor cells activate islet-regenerative or revascularization programs depending on the progenitor subtypes administered. Using purification of multiple progenitor subtypes based on a conserved stem cell function, high aldehyde dehydrogenase (ALDH) activity (ALDH(hi)), we have recently shown that transplantation of BM-derived ALDH(hi) progenitors improved systemic hyperglycemia and augmented insulin secretion by increasing islet-associated proliferation and vascularization, without increasing islet number. Conversely, transplantation of culture-expanded multipotent-stromal cells (MSCs) derived from BM ALDH(hi) cells augmented total beta cell mass via formation of beta cell clusters associated with the ductal epithelium, without sustained islet vascularization. To identify paracrine effectors produced by islet-regenerative MSCs, culture-expanded BM ALDH(hi) MSCs were transplanted into streptozotocin-treated nonobese diabetic/severe combine immune deficient (SCID) mice and segregated into islet-regenerative versus nonregenerative cohorts based on hyperglycemia reduction, and subsequently compared for differential production of mRNA and secreted proteins. Regenerative MSCs showed increased expression of matrix metalloproteases, epidermal growth factor receptor (EGFR)-activating ligands, and downstream effectors of Wnt signaling. Regenerative MSC supernatant also contained increased levels of pro-angiogenic versus pro-inflammatory cytokines, and augmented the expansion of ductal epithelial but not beta cells in vitro. Conversely, co-culture with UCB ALDH(hi) cells induced beta cell but not ductal epithelial cell proliferation. Sequential transplantation of MSCs followed by UCB ALDH(hi) cells improved hyperglycemia and glucose tolerance by increasing beta cell mass associated with the ductal epithelium and by augmenting intra-islet capillary densities. Thus, combinatorial human progenitor cell transplantation stimulated both islet-regenerative and revascularization programs. Understanding the progenitor-specific pathways that modulate islet-regenerative and revascularization processes may provide new approaches for diabetes therapy.

Abstract 3280: Loss of the tumor suppressor APC in cells expressing lysozyme M results in murine endometrial hyperplasia

Abstract The Adenomatous polyposis coli (APC1) tumor suppressor is a multi-function protein that is involved in both the Wnt signaling cascade and cytoskeletal remodeling. Mutations or loss of APC1 can result in defects in cell migration, proliferation, and differentiation. APC frequently collaborates with the canonical Diaphanous-related formin, mDia1 (DIAPH1), which also functions to regulate cytoskeletal dynamics. APC1 and DIAPH1 are genetically linked on Chromosome 5q in a region frequently disrupted in myeloproliferative neoplasms (MPNs). Mice lacking mDia1 (Drf1) and the Apc min mice develop myelodysplastic phenotypes. The physical and functional linkage of APC and mDia1 as 5q tumor suppressors and in cytoskeletal remodeling led us to explore the role of APC in tumor suppression in myeloid-derived cell lineages. We created a mouse which lacks APC in myeloid lineage cells by breeding Apc-flox mice with mice expressing Cre recombinase under the control of lysozyme M (LyzsM) gene promoter. Lysozyme M is expressed in myeloid lineage cells including monocytes, macrophages and granulocytes. Surprisingly, the female mice develop cystic endometrial hyperplasia and are unable to sustain pregnancy. Lysozyme M expression appears to be limited to tissue resident macrophages within the uterus and was not detected in uterine epithelial cells. However, there is a small subset of uterine epithelial cells with increased β-catenin expression, suggesting that these cells lack APC. This subset of cells has elevated expression of cyclin D1, a regulator of cell cycle progression and a downstream effector of Wnt signaling, resulting in increased proliferation. Interestingly, the uterine epithelial cells with increased β-catenin expression also have increased estrogen receptor ≤ (ERα) expression. Estrogen stimulates uterine growth and unopposed estrogen signaling is a hallmark of Type I endometrial cancer. It is unclear if the endometrial hyperplasia seen in these mice is due to lysozyme M expression early on in uterine epithelial cell development or perhaps in a progenitor cell population. A second alternative is that loss of APC in tissue resident myeloid cells residing in the uterine stroma affects epithelial morphology. Regardless, these data suggest that APC plays a role in uterine tissue homeostasis, perhaps even regulating ERα expression. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3280. doi:1538-7445.AM2012-3280

TCF3 inhibits F9 embryonal carcinoma growth by the down-regulation of Oct4.

T-cell factor 3 (TCF3), a downstream effector of Wnt signaling in embryonic stem (ES) cells, plays an important role in pluripotent self-renewal and proliferation. Loss of TCF3 delays the differentiation of mouse ES cells. The purpose of this study was to investigate the effect of TCF3 on embryonal carcinoma (EC). The mouse F9 EC cell line and a tumor-bearing mouse model were used to evaluate the anti-EC tumor effects of TCF3 invitro and invivo, respectively. The overexpression of TCF3 significantly inhibited proliferation, colony-forming and migration in F9 EC cells by approximately 30, 45 and 30%, respectively. The invivo mouse model showed that the overexpression of TCF3 significantly reduced tumor volume (36.4%) and tumor weight (34.8%), malignancy progression and local infiltration and prolonged the life span of tumor-bearing mice. Overexpression of TCF3 significantly down-regulated Oct4 expression in the F9 EC cells. The results indicate that TCF3 is an inhibitor of the malignant phenotypes of embryonal carcinoma through the regulation of Oct4 expression.

Matrix Metalloproteinase (MMP)-9 Induced by Wnt Signaling Increases the Proliferation and Migration of Embryonic Neural Stem Cells at Low O2 Levels

Recent studies have shown that various neural and embryonic stem cells cultured in 1-8% oxygen (O(2)), levels lower than those typically used in cell culture (20.9%), displayed increased rates of proliferation; however, the molecular mechanisms underlying these changes are largely undefined. In this study, using rigorously controlled O(2) levels, we found that neural stem cells (NSCs) from embryonic day 15 rat cortex increased their rate of proliferation and migration in 1% O(2) relative to 20% O(2) without changes in viability. We sought to identify molecular changes in NSCs grown in 1% O(2) that might account for these increases. In 1% O(2), levels of the hypoxia-inducible transcription factor HIF-1α were transiently increased. Reduced adherence of NSCs in 1% O(2) to basement membrane-coated plates was observed, and quantitative RT-PCR analysis confirmed that the levels of mRNA for an assortment of cell adhesion and extracellular matrix molecules were altered. Most notable was a 5-fold increase in matrix metalloproteinase (MMP)-9 mRNA. Specific inhibition of MMP-9 activity, verified using a fluorescent substrate assay, prevented the increase in proliferation and migration in 1% O(2). The canonical Wnt pathway was recently shown to be activated in stem cells in low O(2) via HIF-1α. Inhibition of Wnt signaling by DKK-1 also prevented the increase in proliferation, migration, and MMP-9 expression. Thus, MMP-9 is a key molecular effector, downstream of HIF-1α and Wnt activation, responsible for increased rates of NSC proliferation and migration in 1% O(2).

New G-CSF-Dependent Signaling Pathway and Its Role In Patients with Severe Congenital Neutropenia and Acute Myeloid Leukemia

AbstractAbstract 385We demonstrated that stimulation of hematopoietic CD34+ cells with granulocyte-colony stimulating factor (G-CSF) leads to phosphorylation of hematopoietic cell-specific Lyn substrate 1 (HCLS1) protein and HCLS1 interaction with G-CSF receptor-associated tyrosine kinases Lyn and Syk. Activated HCLS1 binds to lymphoid-enhancer factor 1 (LEF-1) protein, inducing LEF-1-dependent autoregulation of the LEF-1 gene promoter. Transcription factor LEF-1 is a downstream effector of Wnt signaling and a master regulator of myeloid differentiation. In patients with severe congenital neutropenia (CN), inherited mutations in the HCLS1-associated protein × 1 (HAX1) lead to profound defects in the G-CSF-triggered expression and phosphorylation of HCLS1 protein and subsequently, to reduced expression of LEF-1 and abrogated granulopoiesis. Using immunoprecipitation assay, we identified endogenous HAX1 protein in the complex with LEF-1 and HCLS1 proteins. HAX1 is known as a mitochondrial protein, however using cell fractionation experiments of different myeloid cell lines (NB4, HL60, THP-1) and of primary CD34+ cells we identified endogenous HAX1 protein in different intracellular compartments. Thus, HAX1 protein was expressed in mitochondria, cytoplasm, nucleus, cytosol and in membranous fraction. Expression levels of HAX1 protein varied between different cell lines and between maturation stage of the cells. In vitro inhibition of HCLS1 or HAX1 by shRNA severely disrupted granulocytic differentiation of CD34+ cells, due to a lack of LEF-1 and C/EBPa protein expression and subsequently abolished activation of LEF-1 and C/EBPa gene promoters. Intriguingly, LEF-1 in turn induced HCLS1 mRNA and protein expression by direct binding to the HCLS1 gene promoter and inhibition of LEF-1 in CD34+ cells leads to severe downregulation of HCLS1 protein levels. These findings demonstrated a reciprocal feed-back regulation between LEF-1 and HCLS1 proteins. Moreover, HCLS1-/- mice are neutropenic due to a lack of LEF-1 protein expression and defective in vitro G-CSF-triggered F-actin rearrangement in bone marrow myeloid progenitor cells, as compared to WT mice. At the same time, in bone marrow biopsies of 88.46 % of tested patients (46 out of 52 patients) with acute myeloid leukemia (AML), we found significantly elevated levels of HCLS1 protein. Inhibition of HCLS1 in the AML cell lines and primary blasts of AML patients resulted in reduced proliferation and increased apoptosis. Taken together, we described a novel G-CSFR downstream signaling pathway, with dose-dependent effects on myelopoiesis: diminished expression of HCLS1 and LEF-1 resulted in a “maturation arrest” of granulopoiesis and severe neutropenia, but hyperactivation of one of these proteins led to a hyperproliferation of myeloid progenitors and AML. Disclosures:No relevant conflicts of interest to declare.

AV-65, a Novel Inhibitor of the Wnt/β-Catenin Signaling Pathway, Inhibits the Proliferation of Myeloma Cells.

Abstract 2866Poster Board II-842Although new molecular targeting agents against multiple myeloma (MM) have been developed, MM still remains an incurable disease. It is important to continue to investigate new therapeutic agents based on the biology of MM cells. β-catenin is the downstream effector of Wnt signaling and it regulates genes implicated in malignant progression. We have demonstrated that blockade of Wnt/β-catenin signaling pathway inhibits the progression of MM by using RNA interference methods with an in vivo mouse model (Ashihara E, et al. Clin Cancer Res 15:2731, 2009.). In this study, we investigated the effects of AV-65, a novel inhibitor of the Wnt/β-catenin signaling pathway, on MM cells.The system to identify a series of small molecule compounds using a biomarker driven approach has been established. A gene expression biomarker signature reporting on the inhibition of Wnt/β-catenin signaling was generated upon treatment of a colon cancer cell line with β-catenin siRNA. This gene expression signatiure was used to screen a small molecule compound library to identify compounds which mimic knockdown of β-catenin and thus potentially inhibit the Wnt/β-catenin signaling pathway. One compound series, LC-363, was discovered from this screen and validated as novel Wnt/β-catenin signaling inhibitors (Strovel JW, et al. ASH meeting, 2007.). We investigated the inhibitory effects of AV-65, one of LC-363 compounds, on MM cell proliferation. AV-65 inhibited the proliferation of MM cells in a time- and a dose-dependent manner and the values of IC50 at 72 hrs were ranging from 11.7 to 82.1 nM. AV-65 also showed an inhibitory effect on the proliferation of RPMI8226/LR-5 melphalan-resistant MM cells (provided from Dr. William S. Dalton). In flow cytometric analysis, apoptotic cells were increased by AV-65 treatment in a time- and a dose-dependent manner. Western blotting analysis showed that β-catenin was ubiquitinated and that the expression of nuclear β-catenin diminished (Figure 1). Moreover, AV-65 suppressed T-cell factor transcriptional activities, resulting in the decrease of c-myc expression. Taken together, AV-65 promotes the degradation of β-catenin, resulting in the induction of apoptosis of MM cells.We next investigated the in vivo effects of AV-65 using an orthotopic MM-bearing mouse model. AV-65 inhibits the growth of MM cells and significantly prolongs the survival rates (Figure 2). In conclusion, AV-65 inhibited the proliferation of MM cells via inhibition of the Wnt/β-catenin signaling pathway. AV-65 is a promising therapeutic agent for treatment of MM. [Display omitted] Disclosures:No relevant conflicts of interest to declare.

Ovol2 Suppresses Cell Cycling and Terminal Differentiation of Keratinocytes by Directly Repressing c-Myc and Notch1

Ovol2 belongs to the Ovo family of evolutionarily conserved zinc finger transcription factors that act downstream of key developmental signaling pathways including Wg/Wnt and BMP/TGF-beta. We previously reported Ovol2 expression in the basal layer of epidermis, where epidermal stem/progenitor cells reside. In this work, we use HaCaT human keratinocytes to investigate the cellular and molecular functions of Ovol2. We show that depletion of Ovol2 leads to transient cell expansion but a loss of cells with long term proliferation potential. Mathematical modeling and experimental findings suggest that both faster cycling and precocious withdrawal from the cell cycle underlie this phenotype. Ovol2 depletion also accelerates extracellular signal-induced terminal differentiation in two- and three-dimensional culture models. By chromatin immunoprecipitation, luciferase reporter, and functional rescue assays, we demonstrate that Ovol2 directly represses two critical downstream targets, c-Myc and Notch1, thereby suppressing keratinocyte transient proliferation and terminal differentiation, respectively. These findings shed light on how an epidermal cell maintains a proliferation-competent and differentiation-resistant state.

Identification of genes expressed preferentially in the developing peripheral margin of the optic cup.

Specification of the peripheral optic cup by Wnt signaling is critical for formation of the ciliary body/iris. Identification of marker genes for this region during development provides a starting point for functional analyses. During transcriptional profiling of single cells from the developing eye, two cells were identified that expressed genes not found in most other single cell profiles. In situ hybridizations demonstrated that many of these genes were expressed in the peripheral optic cup in both early mouse and chicken development, and in the ciliary body/iris at subsequent developmental stages. These analyses indicate that the two cells probably originated from the developing ciliary body/iris. Changes in expression of these genes were assayed in embryonic chicken retinas when canonical Wnt signaling was ectopically activated by CA-beta-catenin. Twelve ciliary body/iris genes were identified as upregulated following induction, suggesting they are excellent candidates for downstream effectors of Wnt signaling in the optic cup.

Active β-Catenin Signaling Is an Inhibitory Pathway for Human Immunodeficiency Virus Replication in Peripheral Blood Mononuclear Cells

The Wnt/beta-catenin pathway is involved in cell functions governing development and disease. In modeling postentry restriction of human immunodeficiency virus (HIV) replication in astrocytes, we reported that part of this natural resistance to productive replication of HIV in astrocytes involved expression of proteins of the Wnt/beta-catenin signaling pathway. We determined here whether induction of beta-catenin signaling in peripheral blood mononuclear cells (PBMCs) can modulate HIV replication. Given that lithium is an inducer of beta-catenin signaling, we used it as a tool to determine the impact of beta-catenin signaling on HIV replication in PBMCs. We demonstrated that lithium inhibited the replication of T-tropic and primary isolates of HIV by >90% and did so in noncytotoxic/noncytostatic concentrations and in a beta-catenin-dependent manner. Specifically, inhibiting beta-catenin signaling by transfection of dominant-negative mutant constructs to either T-cell factor 4, the downstream effector of Wnt signaling, or beta-catenin, the central mediator of this pathway, abrogated the ability of lithium to inhibit HIV replication. Moreover, when Wnt/beta-catenin signaling was inhibited, the level of HIV replication was enhanced by fourfold. To confirm the in vivo relevance of the beta-catenin pathway in repressing HIV replication, we evaluated HIV-positive antiretroviral therapy-naive patients who were on lithium therapy. These patients demonstrated a reduction in viral load, which increased as the dose of lithium was reduced. Collectively, these data indicate that beta-catenin signaling is an intrinsic molecular pathway restricting HIV replication in PBMCs.

Human Immunodeficiency Virus-Restricted Replication in Astrocytes and the Ability of Gamma Interferon To Modulate This Restriction Are Regulated by a Downstream Effector of the Wnt Signaling Pathway

Astrocyte dysregulation correlates with the severity and the rate of human immunodeficiency virus (HIV)-associated dementia (HAD) progression, highlighting a pivotal role for astrocytes in HIV neuropathogenesis. Yet, astrocytes limit HIV, indicating that they possess an intrinsic molecular mechanism to restrict HIV replication. We previously established that this restriction can be partly overcome by priming astrocytes with gamma interferon (IFN-gamma), which is elevated in the cerebral spinal fluid of HAD patients. We evaluated the mechanism of restrictive HIV replication in astrocytes and how IFN-gamma priming modulates this restriction. We demonstrate that the downstream effector of Wnt signaling, T-cell factor 4 (TCF-4), is part of a transcriptional complex that is immunoprecipitated with HIV TAR-containing region in untreated astrocytes but not in IFN-gamma-treated cells. Blocking TCF-4 activity with a dominant-negative mutant enhanced HIV replication by threefold in both the astrocytoma cell line U87MG and primary fetal astrocytes. Using a TCF-4 reporter plasmid, we directly demonstrate that Wnt signaling is active in human astrocytes and is markedly reduced by IFN-gamma treatment. Collectively, these data implicate TCF-4 in repressing HIV replication and the ability of IFN-gamma to regulate this restriction by inhibiting TCF-4. Given that TCF-4 is the downstream effector of Wnt signaling, harnessing Wnt signaling as an intrinsic molecular mechanism to limit HIV replication may emerge as a powerful tool to regulate HIV replication within and outside of the brain.

Cloning and Characterization of ThreeXenopus Slug Promoters Reveal Direct Regulation by Lef/β-Catenin Signaling

In amphibians and birds, one of the first steps of neural crest cell (NCC) determination is expression of the transcription factor Slug. This marker has been used to demonstrate that BMP and Wnt molecules play a major role in NCC induction. However, it is unknown whether Slug expression is directly or indirectly regulated by these signals. We report here the cloning and characterization of three Xenopus Slug promoters: that of the Xenopus tropicalis slug gene and those of two Xenopus laevis Slug pseudoalleles. Although the three genes encode proteins with almost identical amino acid sequences and are expressed with similar spatiotemporal patterns, their 5'-flanking regions are quite different. A striking difference is a deletion in the X. tropicalis gene located precisely at the transcription initiation site that results in the X. tropicalis promoter being inefficient in X. laevis. Additionally, we identified two regions common to the three promoters that are necessary and sufficient to drive specific expression in NCCs. Interestingly, one of the common regulatory regions presents a functional Lef/beta-catenin-binding site necessary for specific expression. As the Lef.beta-catenin complex is a downstream effector of Wnt signaling, these results suggest that Xenopus Slug is a direct target of NCC determination signals.

Organizer Induction Determines Left–Right Asymmetry inXenopus

Vertebrates appear bilaterally symmetrical but have considerable left–right (LR) asymmetry in the anatomy and placement of internal organs such as the heart. Although a number of asymmetrically expressed genes are known to affect LR patterning, both the initial source of asymmetry and the mechanism that correctly orients the LR axis remain controversial. In this study, we show that the induction of dorsal organizing centers in the embryo can orient LR asymmetry. Ectopic organizing centers were induced by microinjection of mRNA encoding a variety of body axis duplicating proteins, including members of the Wnt signal transduction pathway. The ectopic and primary body axes form side-by-side conjoined twins, with the secondary axis developing as either the left or right sibling. In all cases, correct LR asymmetry was observed in the left twin, regardless of whether it was derived from the primary axis or inducedde novoby injection ofXwnt-8, β-catenin,orSiamoismRNA. In contrast, the right twin was generally unbiased, regardless of the origin of the left body axis, as seen in many instances of experimentally induced and spontaneous conjoined twins. An unanticipated exception was that right twins induced by β-catenin and Siamois, two downstream effectors of Wnt signaling, exhibited predominately normal heart looping, even when they formed the right twin. Taken together, these results indicate that LR asymmetry is locally oriented as a consequence of Wnt signaling through β-catenin and Siamois. We discuss the possibility that signals upstream of β-catenin and Siamois might be required in order for a right sibling to be randomized.