The RNA helicase DDX6 controls early mouse embryogenesis by repressing aberrant inhibition of BMP signaling through miRNA-mediated gene silencing.

The evolutionarily conserved RNA helicase DDX6 is a central player in post-transcriptional regulation, but its role during embryogenesis remains elusive. We here show that DDX6 enables proper cell lineage specification from pluripotent cells by analyzing Ddx6 knockout (KO) mouse embryos and employing an in vitro epiblast-like cell (EpiLC) induction system. Our study unveils that DDX6 is an important BMP signaling regulator. Deletion of Ddx6 causes the aberrant upregulation of the negative regulators of BMP signaling, which is accompanied by enhanced expression of Nodal and related genes. Ddx6 KO pluripotent cells acquire higher pluripotency with a strong inclination toward neural lineage commitment. During gastrulation, abnormally expanded Nodal and Eomes expression in the primitive streak likely promotes endoderm cell fate specification while inhibiting mesoderm differentiation. We also genetically dissected major DDX6 pathways by generating Dgcr8, Dcp2, and Eif4enif1 KO models in addition to Ddx6 KO. We found that the miRNA pathway mutant Dgcr8 KO phenocopies Ddx6 KO, indicating that DDX6 mostly works along with the miRNA pathway during early development, whereas its P-body-related functions are dispensable. Therefore, we conclude that DDX6 prevents aberrant upregulation of BMP signaling inhibitors by participating in miRNA-mediated gene silencing processes. Overall, this study delineates how DDX6 affects the development of the three primary germ layers during early mouse embryogenesis and the underlying mechanism of DDX6 function.

The auxin-inducible degron 2 technology provides sharp degradation control in yeast, mammalian cells, and mice.

Protein knockdown using the auxin-inducible degron (AID) technology is useful to study protein function in living cells because it induces rapid depletion, which makes it possible to observe an immediate phenotype. However, the current AID system has two major drawbacks: leaky degradation and the requirement for a high dose of auxin. These negative features make it difficult to control precisely the expression level of a protein of interest in living cells and to apply this method to mice. Here, we overcome these problems by taking advantage of a bump-and-hole approach to establish the AID version 2 (AID2) system. AID2, which employs an OsTIR1(F74G) mutant and a ligand, 5-Ph-IAA, shows no detectable leaky degradation, requires a 670-times lower ligand concentration, and achieves even quicker degradation than the conventional AID. We demonstrate successful generation of human cell mutants for genes that were previously difficult to deal with, and show that AID2 achieves rapid target depletion not only in yeast and mammalian cells, but also in mice.

Coordinated directional outgrowth and pattern formation by integration of Wnt5a and Fgf signaling in planar cell polarity.

Embryonic morphogenesis of a complex organism requires proper regulation of patterning and directional growth. Planar cell polarity (PCP) signaling is emerging as a crucial evolutionarily conserved mechanism whereby directional information is conveyed. PCP is thought to be established by global cues, and recent studies have revealed an instructive role of a Wnt signaling gradient in epithelial tissues of both invertebrates and vertebrates. However, it remains unclear whether Wnt/PCP signaling is regulated in a coordinated manner with embryonic patterning during morphogenesis. Here, in mouse developing limbs, we find that apical ectoderm ridge-derived Fgfs required for limb patterning regulate PCP along the proximal-distal axis in a Wnt5a-dependent manner. We demonstrate with genetic evidence that the Wnt5a gradient acts as a global cue that is instructive in establishing PCP in the limb mesenchyme, and that Wnt5a also plays a permissive role to allow Fgf signaling to orient PCP. Our results indicate that limb morphogenesis is regulated by coordination of directional growth and patterning through integration of Wnt5a and Fgf signaling.

A new gain-of-function mouse line to study the role of Wnt3a in development and disease.

Wnt/ß-catenin signals are important regulators of embryonic and adult stem cell self-renewal and differentiation and play causative roles in tumorigenesis. Purified recombinant Wnt3a protein, or Wnt3a-conditioned culture medium, has been widely used to study canonical Wnt signaling in vitro or ex vivo. To study the role of Wnt3a in embryogenesis and cancer models, we developed a Cre recombinase activatable Rosa26(Wnt3a) allele, in which a Wnt3a cDNA was inserted into the Rosa26 locus to allow for conditional, spatiotemporally defined expression of Wnt3a ligand for gain-of-function (GOF) studies in mice. To validate this reagent, we ectopically overexpressed Wnt3a in early embryonic progenitors using the T-Cre transgene. This resulted in up-regulated expression of a ß-catenin/Tcf-Lef reporter and of the universal Wnt/ß-catenin pathway target genes, Axin2 and Sp5. Importantly, T-Cre; Rosa26(Wnt3a) mutants have expanded presomitic mesoderm (PSM) and compromised somitogenesis and closely resemble previously studied T-Cre; Ctnnb1(ex3) (ß-catenin(GOF) ) mutants. These data indicate that the exogenously expressed Wnt3a stimulates the Wnt/ß-catenin signaling pathway, as expected. The Rosa26(Wnt3a) mouse line should prove to be an invaluable tool to study the function of Wnt3a in vivo.

Transport of the outer dynein arm complex to cilia requires a cytoplasmic protein Lrrc6.

Lrrc6 encodes a cytoplasmic protein that is expressed specifically in cells with motile cilia including the node, trachea and testes of the mice. A mutation of Lrrc6 has been identified in human patients with primary ciliary dyskinesia (PCD). Mutant mice lacking Lrrc6 show typical PCD defects such as hydrocephalus and laterality defects. We found that in the absence of Lrrc6, the morphology of motile cilia remained normal, but their motility was completely lost. The 9 + 2 arrangement of microtubules remained normal in Lrrc6(-/-) mice, but the outer dynein arms (ODAs), the structures essential for the ciliary beating, were absent from the cilia. In the absence of Lrrc6, ODA proteins such as DNAH5, DNAH9 and IC2, which are assembled in the cytoplasm and transported to the ciliary axoneme, remained in the cytoplasm and were not transported to the ciliary axoneme. The IC2-IC1 interaction, which is the first step of ODA assembly, was normal in Lrrc6(-/-) mice testes. Our results suggest that ODA proteins may be transported from the cytoplasm to the cilia by an Lrrc6-dependent mechanism.

DAAM1 and DAAM2 are co-required for myocardial maturation and sarcomere assembly.

Wnt ligands regulate heart morphogenesis but the underlying mechanisms remain unclear. Two Formin-related proteins, DAAM1 and 2, were previously found to bind the Wnt effector Disheveled. Here, since DAAM1 and 2 nucleate actin and mediate Wnt-induced cytoskeletal changes, a floxed-allele of Daam1 was used to disrupt its function specifically in the myocardium and investigate Wnt-associated pathways. Homozygous Daam1 conditional knockout (CKO) mice were viable but had misshapen hearts and poor cardiac function. The defects in Daam1 CKO mice were observed by mid-gestation and were associated with a loss of protrusions from cardiomyocytes invading the outflow tract. Further, these mice exhibited noncompaction cardiomyopathy (NCM) and deranged cardiomyocyte polarity. Interestingly, Daam1 CKO mice that were also homozygous for an insertion disrupting Daam2 (DKO) had stronger NCM, severely reduced cardiac function, disrupted sarcomere structure, and increased myocardial proliferation, suggesting that DAAM1 and DAAM2 have redundant functions. While RhoA was unaffected in the hearts of Daam1/2 DKO mice, AKT activity was lower than in controls, raising the issue of whether DAAM1/2 are only mediating Wnt signaling. Daam1-floxed mice were thus bred to Wnt5a null mice to identify genetic interactions. The hearts of Daam1 CKO mice that were also heterozygous for the null allele of Wnt5a had stronger NCM and more severe loss of cardiac function than Daam1 CKO mice, consistent with DAAM1 and Wnt5a acting in a common pathway. However, deleting Daam1 further disrupted Wnt5a homozygous-null hearts, suggesting that DAAM1 also has Wnt5a-independent roles in cardiac development.

Non-canonical Wnt5a/Ror2 signaling regulates kidney morphogenesis by controlling intermediate mesoderm extension.

Congenital anomalies of the kidney and urinary tract (CAKUT) affect about 1 in 500 births and are a major cause of morbidity in infants. Duplex collecting systems rank among the most common abnormalities of CAKUT, but the molecular basis for this defect is poorly understood. In mice, conditional deletion of Wnt5a in mesoderm results in bilateral duplex kidney and ureter formation. The ureteric buds (UBs) in mutants emerge as doublets from the intermediate mesoderm (IM)-derived nephric duct (ND) without anterior expansion of the glial cell line-derived neurotrophic factor (Gdnf) expression domain in the surrounding mesenchyme. Wnt5a is normally expressed in a graded manner at the posterior end of the IM, but its expression is down-regulated prior to UB outgrowth at E10.5. Furthermore, ablation of Wnt5a in the mesoderm with an inducible Cre at E7.5 results in duplex UBs, whereas ablation at E8.5 yields normal UB outgrowth, demonstrating that Wnt5a functions in IM development well before the formation of the metanephros. In mutants, the posterior ND is duplicated and surrounding Pax2-positive mesenchymal cells persist in the nephric cord, suggesting that disruption of normal ND patterning prompts the formation of duplex ureters and kidneys. Ror2 homozygous mutants, which infrequently yield duplex collecting systems, show a dramatic increase in incidence with the additional deletion of one copy of Wnt5a, implicating this receptor in non-canonical Wnt5a signaling during IM development. This work provides the first evidence of a role of Wnt5a/Ror2 signaling in IM extension and offers new insights into the etiology of CAKUT and possible involvement of Wnt5a/Ror2 mutations.

Appropriate crypt formation in the uterus for embryo homing and implantation requires Wnt5a-ROR signaling.

Embryo homing and implantation occur within a crypt (implantation chamber) at the antimesometrial (AM) pole along the uterus. The mechanism by which this is achieved is not known. Here, we show that villi-like epithelial projections from the main uterine lumen toward the AM pole at regularly spaced intervals that form crypts for embryo implantation were disrupted in mice with uterine loss or gain of function of Wnt5a, or loss of function of both Ror1 and Ror2. This disruption of Wnt5a-ROR signaling resulted in disorderly epithelial projections, crypt formation, embryo spacing, and impaired implantation. These early disturbances under abnormal Wnt5a-ROR signaling were reflected in adverse late pregnancy events, including defective decidualization and placentation, ultimately leading to compromised pregnancy outcomes. This study presents deeper insight regarding the formation of organized epithelial projections for crypt formation and embryo implantation for pregnancy success.

Regulation of angiogenesis by a non-canonical Wnt-Flt1 pathway in myeloid cells.

Myeloid cells are a feature of most tissues. Here we show that during development, retinal myeloid cells (RMCs) produce Wnt ligands to regulate blood vessel branching. In the mouse retina, where angiogenesis occurs postnatally, somatic deletion in RMCs of the Wnt ligand transporter Wntless results in increased angiogenesis in the deeper layers. We also show that mutation of Wnt5a and Wnt11 results in increased angiogenesis and that these ligands elicit RMC responses via a non-canonical Wnt pathway. Using cultured myeloid-like cells and RMC somatic deletion of Flt1, we show that an effector of Wnt-dependent suppression of angiogenesis by RMCs is Flt1, a naturally occurring inhibitor of vascular endothelial growth factor (VEGF). These findings indicate that resident myeloid cells can use a non-canonical, Wnt-Flt1 pathway to suppress angiogenic branching.

Homozygous loss of BHD causes early embryonic lethality and kidney tumor development with activation of mTORC1 and mTORC2.

Germline mutations in the BHD/FLCN tumor suppressor gene predispose patients to develop renal tumors in the hamartoma syndrome, Birt-Hogg-Dubé (BHD). BHD encodes folliculin, a protein with unknown function that may interact with the energy- and nutrient-sensing AMPK-mTOR signaling pathways. To clarify BHD function in the mouse, we generated a BHD knockout mouse model. BHD homozygous null (BHD(d/d)) mice displayed early embryonic lethality at E5.5-E6.5, showing defects in the visceral endoderm. BHD heterozygous knockout (BHDd(/+)) mice appeared normal at birth but developed kidney cysts and solid tumors as they aged (median kidney-lesion-free survival = 23 months, median tumor-free survival = 25 months). As observed in human BHD kidney tumors, three different histologic types of kidney tumors developed in BHD(d/+) mice including oncocytic hybrid, oncocytoma, and clear cell with concomitant loss of heterozygosity (LOH), supporting a tumor suppressor function for BHD in the mouse. The PI3K-AKT pathway was activated in both human BHD renal tumors and kidney tumors in BHD(d/+) mice. Interestingly, total AKT protein was elevated in kidney tumors compared to normal kidney tissue, but without increased levels of AKT mRNA, suggesting that AKT may be regulated by folliculin through post translational or post-transcriptional modification. Finally, BHD inactivation led to both mTORC1 and mTORC2 activation in kidney tumors from BHD(d/+) mice and human BHD patients. These data support a role for PI3K-AKT pathway activation in kidney tumor formation caused by loss of BHD and suggest that inhibitors of both mTORC1 and mTORC2 may be effective as potential therapeutic agents for BHD-associated kidney cancer.

Deficiency of antiproliferative family protein Ana correlates with development of lung adenocarcinoma.

The abundant in neuroepithelium area (ana) gene was originally identified as a member of the tob/btg family of antiproliferative genes. Like the other family members, Ana inhibits growth of NIH3T3 cells when overexpressed. However, whether or not Ana is involved in tumor progression has been elusive. Here, we show that expression of ana is relatively high in the lung, the expression being restricted in type II alveolar epithelial cells. We further show that ana expression is reduced in 97% of the human lung cancer cell lines examined (61/63) and 86% of clinical samples from lung adenocarcinoma patients (36/42). Long-term observation of ana-deficient (ana−/­) mice reveals that 8% of them develop lung tumors (5/66) by 21 months after birth, while 0% of wild-type mice (0/35) develop the same type of tumors. We also show that exogenously expressed ana gene product suppresses the levels of matrix metalloproteinase-2 (MMP-2) and plasminogen activator inhibitor-1 (PAI-1) expression in lung cancer cells. Taken together, we propose that ana functions as a tumor suppressor and that its product inhibits tumor progression as well by suppressing angiogenesis, invasion, and metastasis.

Deficiency of Myo18B in mice results in embryonic lethality with cardiac myofibrillar aberrations.

Myo18B is an unconventional myosin family protein expressed predominantly in muscle cells. Although conventional myosins are known to be localized on the A-bands and function as a molecular motor for muscle contraction, Myo18B protein was localized on the Z-lines of myofibrils in striated muscles. Like Myo18A, another 18th class of myosin, the N-terminal unique domain of the protein and not the motor domain and the coiled-coil tail is critical for its localization to F-actin in myocytes. Myo18B expression was induced by myogenic differentiation through the binding of myocyte-specific enhancer factor-2 to its promoter. Deficiency of Myo18B caused an embryonic lethality in mice accompanied by disruption of myofibrillar structures in cardiac myocytes at embryonic day 10.5. Thus, Myo18B is a unique unconventional myosin that is predominantly expressed in myocytes and whose expression is essential for the development and/or maintenance of myofibrillar structure.

Osteoporotic bone formation in mice lacking tob2; involvement of Tob2 in RANK ligand expression and osteoclasts differentiation.

Mice lacking tob2, a member of the antiproliferative family genes, had decreased bone mass, and the number of osteoclasts differentiated from bone marrow cells was increased. Overexpression of Tob2 in stromal cells repressed vitamin D(3)-induced osteoclasts formation. Furthermore, expression of RANKL mRNA in stromal cells was increased in the absence of Tob2 and decreased in the presence of Tob2. Tob2 interacted with vitamin D(3) receptor (VDR), which suggests its involvement in vitamin D(3) receptor-mediated regulation of transcription. Because VDR regulates RANKL expression, our data suggest that Tob2 negatively regulates formation of osteoclasts by suppressing RANKL expression through its interaction with VDR.

HOMER2 binds MYO18B and enhances its activity to suppress anchorage independent growth.

MYO18B is a class XVIII myosin, cloned as a tumor suppressor gene candidate. To investigate the mechanisms of MYO18B-dependent tumor suppression, MYO18B-interacting proteins were searched for by a yeast two-hybrid screen. HOMER2, a Homer/Ves1 family protein, was identified as a binding partner of MYO18B. These proteins co-localized in the regions of membrane protrusion and stress fiber, which are known as ones with filamentous actin-rich structures. Expression of HOMER2 enhanced the ability of MYO18B to suppress anchorage-independent growth. These results indicate that HOMER2 and MYO18B cooperate together in tumor suppression.

MYO18B interacts with the proteasomal subunit Sug1 and is degraded by the ubiquitin-proteasome pathway.

MYO18B is a class XVIIIB unconventional myosin encoded by a candidate tumor suppressor gene. To gain insights into the cellular function of this protein, we searched for MYO18B-interacting proteins by a yeast two-hybrid screen. Sug1, a 19S regulator subunit of the 26S proteasome, was identified as a binding partner of the C-terminal tail region of MYO18B. The association of MYO18B with Sug1 was further confirmed by GST pull-down, co-immunoprecipitation, and immunocytochemistry. Furthermore, proteasome dysfunction by a proteasome inhibitor or siRNA-mediated knock-down of Sug1 caused the up-regulation of MYO18B protein and MYO18B was polyubiquitinated in vivo. Collectively, these results suggested that MYO18B is a substrate for proteasomal degradation.

Homozygous deletion and reduced expression of the DOCK8 gene in human lung cancer.

A homozygous deletion of the DOCK8 (dedicator of cytokinesis 8) locus at chromosome 9p24 was found in a lung cancer cell line by array-CGH analysis. Cloning of the full-length DOCK8 cDNA led us to define that the DOCK8 gene encodes a protein consisting of 2,099 amino acids. DOCK8 was expressed in a variety of human organs, including the lungs, and was also expressed in type II alveolar, bronchiolar epithelial and bronchial epithelial cells, which are considered as being progenitors for lung cancer cells. DOCK8 expression was reduced in 62/71 (87%) primary lung cancers compared with normal lung tissue, and the reduction occurred irrespective of the histological type of lung cancer. 5-Aza-2'-deoxy-cytidine and/or Trichostatin A treatments induced DOCK8 expression in lung cancer cell lines with reduced DOCK8 expression. Therefore, epigenetic mechanisms, including DNA methylation and histone deacetylation, were indicated to be involved in DOCK8 down-regulation in lung cancer cells. Further screening revealed homozygous deletions of the DOCK8 gene in a gastric and a breast cancer cell line. DOCK family proteins have been shown to play roles in regulation of migration, morphology, adhesion and growth of cells. Thus, the present results suggest that genetic and epigenetic inactivation of DOCK8 is involved in the development and/or progression of lung and other cancers by disturbing such regulations.

Oligo-astheno-teratozoospermia in mice lacking Cnot7, a regulator of retinoid X receptor beta.

Spermatogenesis is a complex process that involves cooperation of germ cells and testicular somatic cells. Various genetic disorders lead to impaired spermatogenesis, defective sperm function and male infertility. Here we show that Cnot7(-/-) males are sterile owing to oligo-astheno-teratozoospermia, suggesting that Cnot7, a CCR4-associated transcriptional cofactor, is essential for spermatogenesis. Maturation of spermatids is unsynchronized and impaired in seminiferous tubules of Cnot7(-/-) mice. Transplantation of spermatogonial stem cells from male Cnot7(-/-) mice to seminiferous tubules of Kit mutant mice (Kit(W/W-v)) restores spermatogenesis, suggesting that the function of testicular somatic cells is damaged in the Cnot7(-/-) condition. The testicular phenotypes of Cnot7(-/-) mice are similar to those of mice deficient in retinoid X receptor beta (Rxrb). We further show that Cnot7 binds the AF-1 domain of Rxrb and that Rxrb malfunctions in the absence of Cnot7. Therefore, Cnot7 seems to function as a coregulator of Rxrb in testicular somatic cells and is thus involved in spermatogenesis.

Altered gene expression in the adult brain of fyn-deficient mice.

1. Fyn, a member of Src-family tyrosine kinases, is implicated in both brain development and adult brain function. Recent studies have identified some Fyn substrates, however, little is known about the transcriptional targets for Fyn mediated signaling pathways. In the present study, we sought to identify targets downstream of Fyn in vivo. 2. We compared genes expressed in adult hippocampi of wild-type and fyn-deficient mice using gene chips containing more than 12,000 genes. 3. The results showed that 559 transcripts were expressed differentially between these mice. Expression of 20 genes including a substantial number of myelin-associated genes was strongly repressed in fyn-deficient mice. 4. Reduced expression of these myelin-associated genes, such as MBP and MOG, in fyn-deficient mice was also confirmed by real-time PCR and northern blotting, arguing that Fyn is important for function and development of oligodendrocytes. 5. Further analysis of the genes that are differently expressed in fyn-deficient mice may shed light on the molecular mechanism by which Fyn regulates adult neural function.

Phosphorylation of three regulatory serines of Tob by Erk1 and Erk2 is required for Ras-mediated cell proliferation and transformation.

tob is a member of an emerging family of genes with antiproliferative function. Tob is rapidly phosphorylated at Ser 152, Ser 154, and Ser 164 by Erk1 and Erk2 upon growth-factor stimulation. Oncogenic Ras-induced transformation and growth-factor-induced cell proliferation are efficiently suppressed by mutant Tob that carries alanines but not glutamates, mimicking phosphoserines, at these sites. Wild-type Tob inhibits cell growth when the three serine residues are not phosphorylated but is less inhibitory when the serines are phosphorylated. Because growth of Rb-deficient cells was not affected by Tob, Tob appears to function upstream of Rb. Intriguingly, cyclin D1 expression is elevated in serum-starved tob(-/-) cells. Reintroduction of wild-type Tob and mutant Tob with serine-to-alanine but not to glutamate mutations on the Erk phosphorylation sites in these cells restores the suppression of cyclin D1 expression. Finally, the S-phase population was significantly increased in serum-starved tob(-/-) cells as compared with that in wild-type cells. Thus, Tob inhibits cell growth by suppressing cyclin D1 expression, which is canceled by Erk1- and Erk2-mediated Tob phosphorylation. We propose that Tob is critically involved in the control of early G(1) progression.