Cre-mediated germline mosaicism: a new transgenic mouse for the selective removal of residual markers from tri-lox conditional alleles

PLEKHS1 drives PI3Ks and remodels pathway homeostasis in PTEN-null prostate

The PIP3/PI3K network is a central regulator of metabolism and is frequently activated in cancer, commonly by loss of the PIP3/PI(3,4)P2 phosphatase, PTEN. Despite huge research investment, the drivers of the PI3K network in normal tissues and how they adapt to overactivation are unclear. We find that in healthy mouse prostate PI3K activity is driven by RTK/IRS signaling and constrained by pathway feedback. In the absence of PTEN, the network is dramatically remodeled. A poorly understood YXXM- and PIP3/PI(3,4)P2-binding PH domain-containing adaptor, PLEKHS1, became the dominant activator and was required to sustain PIP3, AKT phosphorylation, and growth in PTEN-null prostate. This was because PLEKHS1 evaded pathway-feedback and experienced enhanced PI3K- and Src-family kinase-dependent phosphorylation of Y258XXM, eliciting PI3K activation. hPLEKHS1 mRNA and activating Y419 phosphorylation of hSrc correlated with PI3K pathway activity in human prostate cancers. We propose that in PTEN-null cells receptor-independent, Src-dependent tyrosine phosphorylation of PLEKHS1 creates positive feedback that escapes homeostasis, drives PIP3 signaling, and supports tumor progression.

Stepwise generation of AID knock-in and conditional knockout mice from a single gene-targeting event

Activation-induced cytidine deaminase (AID) encoded by the Aicda gene initiates class-switch recombination and somatic hypermutation of immunoglobulin genes. In addition to this function, AID is also implicated in the epigenetic regulation in pluripotent stem cells and in the oncogenesis of lymphoid and non-lymphoid origins. To examine AID's role in specific cell types, we developed mouse strains of conditional knockout (Aicda-FL) and knock-in with a red fluorescent protein gene (RFP) inserted into the Aicda locus (Aicda-RFP). These two strains were obtained from a single targeting event in embryonic stem cells by a three-loxP or tri-lox strategy. Partial and complete recombination among the three loxP sites in the Aicda-RFP locus gave rise to Aicda-FL and AID-deficient loci (Aicda-KO), respectively, after mating Aicda-RFP mice with Cre-expressing mice driven by tissue-non-specific alkaline phosphate promoter. We confirmed RFP expression in B cells of germinal centers of intestine-associated lymphoid tissue. Mice homozygous for each allele were obtained and were checked for AID activity by class-switch and hypermutation assays. AID activity was normal for Aicda-FL but partially and completely absent for Aicda-RFP and Aicda-KO, respectively. Aicda-FL and Aicda-RFP mice would be useful for studying AID function in subpopulations of B cells and in non-lymphoid cells.

The tight junction protein cingulin regulates the vascular response to burn injury in a mouse model

Edema formation due to the collapse of physiological barriers and the associated delayed healing process is still a central problem in the treatment of burn injuries. In healthy individuals, tight junctions form a barrier to fluid and small molecules. Cingulin is a cytoplasmic component of tight junctions and is involved in the regulation of the paracellular barrier. Endothelial specific cingulin knock-out mice provide new insight into the influence of tight junction proteins on edema formation and angiogenesis during wound healing. Knock-out mice lacking the head domain of cingulin in endothelial cells (Cgn^ΔEC) were created by breeding Cgn^fl/fl mice with Tie1-cre mice. Using a no-touch hot air jet a burn trauma was induced on the ear of the mouse. Over a period of 12 days microcirculatory parameters such as edema formation, angiogenesis and leukocyte-endothelial interactions were visualized using intravital fluorescence microscopy. At baseline, Cgn^ΔEC mice surprisingly showed significantly less tracer extravasation compared to Cgn^fl/fl littermates, whereas, after burn injury, edema was consistently higher in Cgn^ΔEC mice. Non-perfused area after wounding was increased, but there was no difference in vessel diameters, contraction or dilation of arteries in Cgn^ΔEC mice. Moreover, cingulin knock-out did not cause a difference in leukocyte adhesion after burn injury. In summary, cingulin limits non-perfused area after burn injury and maintains the paracellular barrier of blood vessels. Since edema formation with serious systemic effects is a central problem of burn wounds, understanding the importance of tight junction proteins might help to find new treatment strategies for burn wounds.

Muc5b-deficient mice develop early histological lung abnormalities

Gel-forming mucins are the main organic component responsible for physical properties of the mucus hydrogels. While numerous biological functions of these mucins are well documented, specific physiological functions of each mucin are largely unknown. To investigate in vivo functions of the gel-forming mucin Muc5b, which is one of the major secreted airway mucins, along with Muc5ac, we generated mice in which Muc5b was disrupted and maintained in the absence of environmental stress. Adult Muc5b-deficient mice displayed bronchial hyperplasia and metaplasia, interstitial thickening, alveolar collapse, immune cell infiltrates, fragmented and disorganized elastin fibers and collagen deposits that were, for approximately one-fifth of the mice, associated with altered pulmonary function leading to respiratory failure. These lung abnormalities start early in life, as demonstrated in one-quarter of 2-day-old Muc5b-deficient pups. Thus, the mouse mucin Muc5b is essential for maintaining normal lung function.

A modifier in the 129S2/SvPasCrl genome is responsible for the viability of Notch1[12f/12f] mice

Background

Mouse NOTCH1 carries a highly conserved O-fucose glycan at Thr466 in epidermal growth factor-like repeat 12 (EGF12) of the extracellular domain. O-Fucose at this site has been shown by X-ray crystallography to be recognized by both DLL4 and JAG1 Notch ligands. We previously showed that a Notch1 Thr466Ala mutant exhibits very little ligand-induced NOTCH1 signaling in a reporter assay, whereas a Thr466Ser mutation enables the transfer of O-fucose and reverts the NOTCH1 signaling defect. We subsequently generated a mutant mouse with the Thr466Ala mutation termed Notch1[12f](Notch1^tm2Pst). Surprisingly, homozygous Notch1[12f/12f] mutants on a mixed background were viable and fertile.

Results

We now report that after backcrossing to C57BL/6 J mice for 11–15 generations, few homozygous Notch1[12f/12f] embryos were born. Timed mating showed that embryonic lethality occurred by embryonic day (E) ~E11.5, somewhat delayed compared to mice lacking Notch1 or Pofut1 (the O-fucosyltransferase that adds O-fucose to Notch receptors), which die at ~E9.5. The phenotype of C57BL/6 J Notch1[12f/12f] embryos was milder than mutants affected by loss of a canonical Notch pathway member, but disorganized vasculogenesis in the yolk sac, delayed somitogenesis and development were characteristic. In situ hybridization of Notch target genes Uncx4.1 and Dll3 or western blot analysis of NOTCH1 cleavage did not reveal significant differences at E9.5. However, qRT-PCR of head cDNA showed increased expression of Dll3, Uncx4.1 and Notch1 in E9.5 Notch1[12f/12f] embryos. Sequencing of cDNA from Notch1[12f/12f] embryo heads and Southern analysis showed that the Notch1[12f] locus was intact following backcrossing. We therefore looked for evidence of modifying gene(s) by crossing C57BL/6 J Notch1 [12f/+] mice to 129S2/SvPasCrl mice. Intercrosses of the F1 progeny gave viable F2 Notch1[12f/12f] mice.

Conclusion

We conclude that the 129S2/SvPasCrl genome contains a dominant modifying gene that rescues the functions of NOTCH1[12f] in signaling. Identification of the modifying gene has the potential to illuminate novel factor(s) that promote Notch signaling when an O-fucose glycan is absent from EGF12 of NOTCH1.

Acetylcholine Modulates the Hormones of the Growth Hormone/Insulinlike Growth Factor-1 Axis During Development in Mice

Pituitary growth hormone (GH) and insulinlike growth factor (IGF)-1 are anabolic hormones whose physiological roles are particularly important during development. The activity of the GH/IGF-1 axis is controlled by complex neuroendocrine systems including two hypothalamic neuropeptides, GH-releasing hormone (GHRH) and somatostatin (SRIF), and a gastrointestinal hormone, ghrelin. The neurotransmitter acetylcholine (ACh) is involved in tuning GH secretion, and its GH-stimulatory action has mainly been shown in adults but is not clearly documented during development. ACh, together with these hormones and their receptors, is expressed before birth, and somatotroph cells are already responsive to GHRH, SRIF, and ghrelin. We thus hypothesized that ACh could contribute to the modulation of the main components of the somatotropic axis during development. In this study, we generated a choline acetyltransferase knockout mouse line and showed that heterozygous mice display a transient deficit in ACh from embryonic day 18.5 to postnatal day 10, and they recover normal ACh levels from the second postnatal week. This developmental ACh deficiency had no major impact on weight gain and cardiorespiratory status of newborn mice. Using this mouse model, we found that endogenous ACh levels determined the concentrations of circulating GH and IGF-1 at embryonic and postnatal stages. In particular, serum GH level was correlated with brain ACh content. ACh also modulated the levels of GHRH and SRIF in the hypothalamus and ghrelin in the stomach, and it affected the levels of these hormones in the circulation. This study identifies ACh as a potential regulator of the somatotropic axis during the developmental period.

BRAF activates PAX3 to control muscle precursor cell migration during forelimb muscle development

Migration of skeletal muscle precursor cells is a key step during limb muscle development and depends on the activity of PAX3 and MET. Here, we demonstrate that BRAF serves a crucial function in formation of limb skeletal muscles during mouse embryogenesis downstream of MET and acts as a potent inducer of myoblast cell migration. We found that a fraction of BRAF accumulates in the nucleus after activation and endosomal transport to a perinuclear position. Mass spectrometry based screening for potential interaction partners revealed that BRAF interacts and phosphorylates PAX3. Mutation of BRAF dependent phosphorylation sites in PAX3 impaired the ability of PAX3 to promote migration of C2C12 myoblasts indicating that BRAF directly activates PAX3. Since PAX3 stimulates transcription of the Met gene we propose that MET signaling via BRAF fuels a positive feedback loop, which maintains high levels of PAX3 and MET activity required for limb muscle precursor cell migration.

DOI:http://dx.doi.org/10.7554/eLife.18351.001

Sequestosome 1/p62 Protein Is Associated with Autophagic Removal of Excess Hepatic Endoplasmic Reticulum in Mice

Xenobiotics exposure increases endoplasmic reticulum (ER) proliferation and cytochrome P-450 (CYP) induction to sustain metabolic requirements. Whether autophagy is essential for the removal of excess ER and CYP and whether an autophagy receptor is involved in this process in mammals remains elusive. In this study, we show that autophagy is induced in mouse livers after withdrawal of the hepatic mitogen 1,4-bis[2-(3,5-dichloropyridyloxy)] benzene (TCPOBOP). Although isolated autophagosomes, autolysosomes, and lysosomes from mouse livers after withdrawal of TCPOBOP contained ER proteins, those in control mouse livers did not. Liver-specific Atg5 knockout mice had higher basal hepatic ER content that was further increased and sustained after withdrawal of TCPOBOP compared with wild-type mice. In addition to regulating ER degradation, our results also suggest that autophagy plays a role in regulating the homeostasis of hepatic CYP because blocking autophagy led to increased CYP2B10 accumulation either at the basal level or following TCPOBOP withdrawal. Furthermore, we found that the autophagy receptor protein sequestosome 1 (SQSTM1)/p62 is associated with the ER. After withdrawal of TCPOBOP, p62 knockout mice had increased ER content in the liver compared with wild-type mice. These results suggest that p62 may act as an autophagy receptor for the autophagic removal of excess ER in the mouse liver. Taken together, our results indicate that autophagy is important for the removal of excess ER and hepatic CYP enzymes in mouse livers, a process associated with the autophagy receptor protein p62.

p62/Sequestosome-1 Is Indispensable for Maturation and Stabilization of Mallory-Denk Bodies

Mallory-Denk bodies (MDBs) are hepatocytic protein aggregates found in steatohepatitis and several other chronic liver diseases as well as hepatocellular carcinoma. MDBs are mainly composed of phosphorylated keratins and stress protein p62/Sequestosome-1 (p62), which is a common component of cytoplasmic aggregates in a variety of protein aggregation diseases. In contrast to the well-established role of keratins, the role of p62 in MDB pathogenesis is still elusive. We have generated total and hepatocyte-specific p62 knockout mice, fed them with 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) to induce MDBs and allowed the mice to recover from DDC intoxication on a standard diet to investigate the role of p62 in MDB formation and elimination. In the absence of p62, smaller, granular and less distinct MDBs appeared, which failed to mature to larger and compact inclusions. Moreover, p62 deficiency impaired the binding of other proteins such as NBR1 and Hsp25 to MDBs and altered the cellular defense mechanism by downregulation of Nrf2 target genes. Upon recovery from DDC intoxication on a standard diet, there was an enhanced reduction of p62-deficient MDBs, which was accompanied by a pronounced decrease in ubiquitinated proteins. Our data provide strong evidence that keratin aggregation is the initial step in MDB formation in steatohepatitis-related mouse models. Interaction of p62 with keratin aggregates then leads to maturation i.e., enlargement and stabilization of the MDBs as well as recruitment of other MDB-associated proteins.

Myeloid cell-specific inositol polyphosphate-4-phosphatase type I knockout mice impair bacteria clearance in a murine peritonitis model

Phosphatidylinositol 3-kinase (PI3K)/Akt signaling has been implicated in the anti-inflammatory response in a mouse model of endotoxemia and sepsis. The present study focused on the role of inositol polyphosphate-4-phosphatase type I (Inpp4a), which dephosphorylates PtdIns(3,4)P2 to PtdIns(3)P, in bacterial infections. We prepared myeloid cell-specific Inpp4a-conditional knockout mice. Macrophages from these mice showed increased Akt phosphorylation and reduced production of inflammatory cytokines in response to LPS or Escherichia coli in vitro. The Inpp4a knockout mice survived for a shorter time than wild type mice after i.p. infection with E. coli, with less production of inflammatory cytokines. Additionally, E. coli clearance from blood and lung was significantly impaired in the knockout mice. A likely mechanism is that the Inpp4a-catalyzed dephosphorylation of PtdIns(3,4)P2 down-regulates Akt pathways, which, in turn, increases the production of inflammatory mediators. This mechanism at least fits the decreased E. coli clearance and short survival in the Inpp4a knockout mice.

INPP4B Is a PtdIns(3,4,5)P3 Phosphatase That Can Act as a Tumor Suppressor

Inositol polyphosphate 4-phosphatase B (INPP4B) has been identified as a tumor suppressor mutated in human breast, ovary, and prostate cancers. The molecular mechanism underlying INPP4B's tumor-suppressive role is currently unknown. Here, we demonstrate that INPP4B restrains tumor development by dephosphorylating the PtdIns(3,4,5)P3 that accumulates in situations of PTEN deficiency. In vitro, INPP4B directly dephosphorylates PtdIns(3,4,5)P3. In vivo, neither inactivation of Inpp4b (Inpp4b(Δ/Δ)) nor heterozygous deletion of Pten (Pten(+/-)) in mice causes thyroid abnormalities, but a combination of these mutations induces malignant thyroid cancers with lung metastases. At the molecular level, simultaneous deletion of Inpp4b and Pten synergistically increases PtdIns(3,4,5)P3 levels and activates AKT downstream signaling proteins in thyroid cells. We propose that the PtdIns(3,4,5)P3 phosphatase activity of INPP4B can function as a "back-up" mechanism when PTEN is deficient, making INPP4B a potential novel therapeutic target for PTEN-deficient or PIK3CA-activated cancers.

SIGNIFICANCE

Although INPP4B expression is reduced in several types of human cancers, our work on Inpp4B-deficient mice provides the first evidence that INPP4B is a bona fide tumor suppressor whose function is particularly important in situations of PTEN deficiency. Our biochemical data demonstrate that INPP4B directly dephosphorylates PtdIns(3,4,5)P3.

Lymphotoxin beta receptor signaling limits mucosal damage through driving IL-23 production by epithelial cells

The immune mechanisms regulating epithelial cell repair after injury remain poorly defined. We demonstrate here that lymphotoxin beta receptor (LTβR) signaling in intestinal epithelial cells promotes self-repair after mucosal damage. Using a conditional gene-targeted approach, we demonstrate that LTβR signaling in intestinal epithelial cells is essential for epithelial interleukin-23 (IL-23) production and protection against epithelial injury. We further show that epithelial-derived IL-23 promotes mucosal wound healing by inducing the IL-22-mediated proliferation and survival of epithelial cells and mucus production. Additionally, we identified CD4(-)CCR6(+)T-bet(-) RAR-related orphan receptor gamma t (RORγt)(+) lymphoid tissue inducer cells as the main producers of protective IL-22 after epithelial damage. Thus, our results reveal a novel role for LTβR signaling in epithelial cells in the regulation of intestinal epithelial cell homeostasis to limit mucosal damage.

In vitro generation of colonic epithelium from primary cells guided by microstructures

The proliferative compartment of the colonic epithelium in vivo is located in the basal crypt where colonic stem cells and transit-amplifying cells reside and fuel the rapid renewal of non-proliferative epithelial cells as they migrate toward the gut lumen. To mimic this tissue polarity, microstructures composed of polydimethylsiloxane (PDMS) microwells and Matrigel micropockets were used to guide a combined 2-dimensional (2D) and 3-dimensional (3D) hybrid culture of primary crypts isolated from the murine colon. The 2D and 3D culture of crypts on a planar PDMS surface was first investigated in terms of cell proliferation and stem cell activity. 3D culture of crypts with overlaid Matrigel generated enclosed, but highly proliferative spheroids (termed colonoids). 2D culture of crypts produced a spreading monolayer of cells, which were non-proliferative. A combined 2D/3D hybrid culture was generated in a PDMS microwell platform on which crypts were loaded by centrifugation into microwells (diameter = 150 μm, depth = 150 μm) followed by addition of Matrigel that formed micropockets locking the crypts within the microwells. Embedded crypts first underwent 3D expansion inside the wells. After the cells filled the microwells, they migrated onto the surrounding surface forming a 2D monolayer in the array regions without Matrigel. This unique 2D/3D hybrid culture generated a continuous, millimeter-scale colonic epithelial tissue in vitro, which resembled the polarized architecture (i.e. distinct proliferative and non-proliferative zones) and geometry of the colonic epithelium in vivo. This work initiates the construction of a "colon-on-a-chip" using primary cells/tissues with the ultimate goal of producing the physiologic structure and organ-level function of the colon.

Selective disruption of acetylcholine synthesis in subsets of motor neurons: a new model of late-onset motor neuron disease

Motor neuron diseases are characterized by the selective chronic dysfunction of a subset of motor neurons and the subsequent impairment of neuromuscular function. To reproduce in the mouse these hallmarks of diseases affecting motor neurons, we generated a mouse line in which ~40% of motor neurons in the spinal cord and the brainstem become unable to sustain neuromuscular transmission. These mice were obtained by conditional knockout of the gene encoding choline acetyltransferase (ChAT), the biosynthetic enzyme for acetylcholine. The mutant mice are viable and spontaneously display abnormal phenotypes that worsen with age including hunched back, reduced lifespan, weight loss, as well as striking deficits in muscle strength and motor function. This slowly progressive neuromuscular dysfunction is accompanied by muscle fiber histopathological features characteristic of neurogenic diseases. Unexpectedly, most changes appeared with a 6-month delay relative to the onset of reduction in ChAT levels, suggesting that compensatory mechanisms preserve muscular function for several months and then are overwhelmed. Deterioration of mouse phenotype after ChAT gene disruption is a specific aging process reminiscent of human pathological situations, particularly among survivors of paralytic poliomyelitis. These mutant mice may represent an invaluable tool to determine the sequence of events that follow the loss of function of a motor neuron subset as the disease progresses, and to evaluate therapeutic strategies. They also offer the opportunity to explore fundamental issues of motor neuron biology.

Transmembrane protein ESDN promotes endothelial VEGF signaling and regulates angiogenesis

Aberrant blood vessel formation contributes to a wide variety of pathologies, and factors that regulate angiogenesis are attractive therapeutic targets. Endothelial and smooth muscle cell-derived neuropilin-like protein (ESDN) is a neuropilin-related transmembrane protein expressed in ECs; however, its potential effect on VEGF responses remains undefined. Here, we generated global and EC-specific Esdn knockout mice and demonstrated that ESDN promotes VEGF-induced human and murine EC proliferation and migration. Deletion of Esdn in the mouse interfered with adult and developmental angiogenesis, and knockdown of the Esdn homolog (dcbld2) in zebrafish impaired normal vascular development. Loss of ESDN in ECs blunted VEGF responses in vivo and attenuated VEGF-induced VEGFR-2 signaling without altering VEGF receptor or neuropilin expression. Finally, we found that ESDN associates with VEGFR-2 and regulates its complex formation with negative regulators of VEGF signaling, protein tyrosine phosphatases PTP1B and TC-PTP, and VE-cadherin. These findings establish ESDN as a regulator of VEGF responses in ECs that acts through a mechanism distinct from neuropilins. As such, ESDN may serve as a therapeutic target for angiogenesis regulation.

Efficient site-specific transgenesis and enhancer activity tests in medaka using PhiC31 integrase

Established transgenesis methods for fish model systems allow efficient genomic integration of transgenes. However, thus far a way of controlling copy number and integration sites has not been available, leading to variable transgene expression caused by position effects. The integration of transgenes at predefined genomic positions enables the direct comparison of different transgenes, thereby improving time and cost efficiency. Here, we report an efficient PhiC31-based site-specific transgenesis system for medaka. This system includes features that allow the pre-selection of successfully targeted integrations early on in the injected generation. Pre-selected embryos transmit the correctly integrated transgene through the germline with high efficiency. The landing site design enables a variety of applications, such as reporter and enhancer switch, in addition to the integration of any insert. Importantly, this allows assaying of enhancer activity in a site-specific manner without requiring germline transmission, thus speeding up large-scale analyses of regulatory elements.

Targeted mutagenesis tools for modelling psychiatric disorders

In the 1980s, the basic principles of gene targeting were discovered and forged into sharp tools for efficient and precise engineering of the mouse genome. Since then, genetic mouse models have substantially contributed to our understanding of major neurobiological concepts and are of utmost importance for our comprehension of neuropsychiatric disorders. The "domestication" of site-specific recombinases and the continuous creative technological developments involving the implementation of previously identified biological principles such as transcriptional and posttranslational control now enable conditional mutagenesis with high spatial and temporal resolution. The initiation and successful accomplishment of large-scale efforts to annotate functionally the entire mouse genome and to build strategic resources for the research community have significantly accelerated the rapid proliferation and broad propagation of mouse genetic tools. Addressing neurobiological processes with the assistance of genetic mouse models is a routine procedure in psychiatric research and will be further extended in order to improve our understanding of disease mechanisms. In light of the highly complex nature of psychiatric disorders and the current lack of strong causal genetic variants, a major future challenge is to model of psychiatric disorders more appropriately. Humanized mice, and the recently developed toolbox of site-specific nucleases for more efficient and simplified tailoring of the genome, offer the perspective of significantly improved models. Ultimately, these tools will push the limits of gene targeting beyond the mouse to allow genome engineering in any model organism of interest.

Mitochondrial respiration without ubiquinone biosynthesis

Ubiquinone (UQ), a.k.a. coenzyme Q, is a redox-active lipid that participates in several cellular processes, in particular mitochondrial electron transport. Primary UQ deficiency is a rare but severely debilitating condition. Mclk1 (a.k.a. Coq7) encodes a conserved mitochondrial enzyme that is necessary for UQ biosynthesis. We engineered conditional Mclk1 knockout models to study pathogenic effects of UQ deficiency and to assess potential therapeutic agents for the treatment of UQ deficiencies. We found that Mclk1 knockout cells are viable in the total absence of UQ. The UQ biosynthetic precursor DMQ9 accumulates in these cells and can sustain mitochondrial respiration, albeit inefficiently. We demonstrated that efficient rescue of the respiratory deficiency in UQ-deficient cells by UQ analogues is side chain length dependent, and that classical UQ analogues with alkyl side chains such as idebenone and decylUQ are inefficient in comparison with analogues with isoprenoid side chains. Furthermore, Vitamin K2, which has an isoprenoid side chain, and has been proposed to be a mitochondrial electron carrier, had no efficacy on UQ-deficient mouse cells. In our model with liver-specific loss of Mclk1, a large depletion of UQ in hepatocytes caused only a mild impairment of respiratory chain function and no gross abnormalities. In conjunction with previous findings, this surprisingly small effect of UQ depletion indicates a nonlinear dependence of mitochondrial respiratory capacity on UQ content. With this model, we also showed that diet-derived UQ10 is able to functionally rescue the electron transport deficit due to severe endogenous UQ deficiency in the liver, an organ capable of absorbing exogenous UQ.

Mouse nuclear myosin I knock-out shows interchangeability and redundancy of myosin isoforms in the cell nucleus

BACKGROUND

Nuclear myosin I (NM1) is a nuclear isoform of the well-known "cytoplasmic" Myosin 1c protein (Myo1c). Located on the 11(th) chromosome in mice, NM1 results from an alternative start of transcription of the Myo1c gene adding an extra 16 amino acids at the N-terminus. Previous studies revealed its roles in RNA Polymerase I and RNA Polymerase II transcription, chromatin remodeling, and chromosomal movements. Its nuclear localization signal is localized in the middle of the molecule and therefore directs both Myosin 1c isoforms to the nucleus.

METHODOLOGY/PRINCIPAL FINDINGS

In order to trace specific functions of the NM1 isoform, we generated mice lacking the NM1 start codon without affecting the cytoplasmic Myo1c protein. Mutant mice were analyzed in a comprehensive phenotypic screen in cooperation with the German Mouse Clinic. Strikingly, no obvious phenotype related to previously described functions has been observed. However, we found minor changes in bone mineral density and the number and size of red blood cells in knock-out mice, which are most probably not related to previously described functions of NM1 in the nucleus. In Myo1c/NM1 depleted U2OS cells, the level of Pol I transcription was restored by overexpression of shRNA-resistant mouse Myo1c. Moreover, we found Myo1c interacting with Pol II. The ratio between Myo1c and NM1 proteins were similar in the nucleus and deletion of NM1 did not cause any compensatory overexpression of Myo1c protein.

CONCLUSION/SIGNIFICANCE

We observed that Myo1c can replace NM1 in its nuclear functions. Amount of both proteins is nearly equal and NM1 knock-out does not cause any compensatory overexpression of Myo1c. We therefore suggest that both isoforms can substitute each other in nuclear processes.

Strategies for designing transgenic DNA constructs

Generation and characterization of transgenic mice are important elements of biomedical research. In recent years, transgenic technology has become more versatile and sophisticated, mainly because of the incorporation of recombinase-mediated conditional expression and targeted insertion, site-specific endonuclease-mediated genome editing, siRNA-mediated gene knockdown, various inducible gene expression systems, and fluorescent protein marking and tracking techniques. Site-specific recombinases (such as PhiC31) and engineered endonucleases (such as ZFN and Talen) have significantly enhanced our ability to target transgenes into specific genomic loci, but currently a great majority of transgenic mouse lines are continuingly being created using the conventional random insertion method. A major challenge for using this conventional method is that the genomic environment at the integration site has a substantial influence on the expression of the transgene. Although our understanding of such chromosomal position effects and our means to combat them are still primitive, adhering to some general guidelines can significantly increase the odds of successful transgene expression. This chapter first discusses the major problems associated with transgene expression, and then describes some of the principles for using plasmid and bacterial artificial chromosomes (BACs) for generating transgenic constructs. Finally, the strategies for conducting each of the major types of transgenic research are discussed, including gene overexpression, promoter characterization, cell-lineage tracing, mutant complementation, expression of double or multiple transgenes, siRNA knockdown, and conditional and inducible systems.

Depressed levels of prostaglandin F2α in mice lacking Akr1b7 increase basal adiposity and predispose to diet-induced obesity

Negative regulators of white adipose tissue (WAT) expansion are poorly documented in vivo. Prostaglandin F(2α) (PGF(2α)) is a potent antiadipogenic factor in cultured preadipocytes, but evidence for its involvement in physiological context is lacking. We previously reported that Akr1b7, an aldo-keto reductase enriched in adipose stromal vascular fraction but absent from mature adipocytes, has antiadipogenic properties possibly supported by PGF(2α) synthase activity. To test whether lack of Akr1b7 could influence WAT homeostasis in vivo, we generated Akr1b7(-/-) mice in 129/Sv background. Akr1b7(-/-) mice displayed excessive basal adiposity resulting from adipocyte hyperplasia/hypertrophy and exhibited greater sensitivity to diet-induced obesity. Following adipose enlargement and irrespective of the diet, they developed liver steatosis and progressive insulin resistance. Akr1b7 loss was associated with decreased PGF(2α) WAT contents. Cloprostenol (PGF(2α) agonist) administration to Akr1b7(-/-) mice normalized WAT expansion by affecting both de novo adipocyte differentiation and size. Treatment of 3T3-L1 adipocytes and Akr1b7(-/-) mice with cloprostenol suggested that decreased adipocyte size resulted from inhibition of lipogenic gene expression. Hence, Akr1b7 is a major regulator of WAT development through at least two PGF(2α)-dependent mechanisms: inhibition of adipogenesis and lipogenesis. These findings provide molecular rationale to explore the status of aldo-keto reductases in dysregulations of adipose tissue homeostasis.

Cingulin is dispensable for epithelial barrier function and tight junction structure, and plays a role in the control of claudin-2 expression and response to duodenal mucosa injury

Cingulin (CGN) is a 140 kDa protein, which is localized to the cytoplasmic region of vertebrate tight junctions (TJ), and regulates gene expression and RhoA signaling in cultured cells. To investigate the function of CGN at the organism level, we generated CGN knockout (CGN(-/-)) mice by homologous recombination. CGN(-/-) mice are viable and fertile, and are born at the expected mendelian ratios. Immunohistochemistry, immunofluorescence, electron microscopy and permeability assays of epithelial tissues of CGN(-/-) mice show no cingulin labeling at junctions, a normal localization of TJ proteins, and normal TJ structure and barrier function. Microarray analysis of intestinal cells does not show significant changes in gene expression between CGN(-/-) and CGN(+/+) mice, whereas immunoblotting analysis shows a twofold increase in the levels of claudin-2 protein in the duodenum and the kidney of CGN(-/-) mice, compared to CGN(+/+) littermates. Furthermore, CGN(-/-) mice show an exacerbated response to the ulcerogenic action of cysteamine, whereas acute injury of the colon by dextran sodium sulfate elicits undistinguishable responses in CGN(-/-) and CGN(+/+) mice. We conclude that at the organism level cingulin is dispensable for the structure and barrier function of TJ, and is embedded in signaling networks that control the expression of claudin-2, and the mucosal response to acute injury in the duodenum.

Generation and characterization of a humanized PPARδ mouse model

BACKGROUND AND PURPOSE

Humanized mice for the nuclear receptor peroxisome proliferator-activated receptor δ (PPARδ), termed PPARδ knock-in (PPARδ KI) mice, were generated for the investigation of functional differences between mouse and human PPARδ and as tools for early drug efficacy assessment.

EXPERIMENTAL APPROACH

Human PPARδ function in lipid metabolism was assessed at baseline, after fasting or when challenged with the GW0742 compound in mice fed a chow diet or high-fat diet (HFD).

KEY RESULTS

Analysis of PPARδ mRNA levels revealed a hypomorph expression of human PPARδ in liver, macrophages, small intestine and heart, but not in soleus and quadriceps muscles, white adipose tissue and skin. PPARδ KI mice displayed a small decrease of high-density lipoprotein-cholesterol whereas other lipid parameters were unaltered. Plasma metabolic parameters were similar in wild-type and PPARδ KI mice when fed chow or HFD, and following physiological (fasting) and pharmacological (GW0742 compound) activation of PPARδ. Gene expression profiling in liver, soleus muscle and macrophages showed similar gene patterns regulated by mouse and human PPARδ. The anti-inflammatory potential of human PPARδ was also similar to mouse PPARδ in liver and isolated macrophages.

CONCLUSIONS AND IMPLICATIONS

These data indicate that human PPARδ can compensate for mouse PPARδ in the regulation of lipid metabolism and inflammation. Overall, this novel PPARδ KI mouse model shows full responsiveness to pharmacological challenge and represents a useful tool for the preclinical assessment of PPARδ activators with species-specific activity.

Specific nuclear localizing sequence directs two myosin isoforms to the cell nucleus in calmodulin-sensitive manner

Background

Nuclear myosin I (NM1) was the first molecular motor identified in the cell nucleus. Together with nuclear actin, they participate in crucial nuclear events such as transcription, chromatin movements, and chromatin remodeling. NM1 is an isoform of myosin 1c (Myo1c) that was identified earlier and is known to act in the cytoplasm. NM1 differs from the “cytoplasmic” myosin 1c only by additional 16 amino acids at the N-terminus of the molecule. This amino acid stretch was therefore suggested to direct NM1 into the nucleus.

Methodology/Principal Findings

We investigated the mechanism of nuclear import of NM1 in detail. Using over-expressed GFP chimeras encoding for truncated NM1 mutants, we identified a specific sequence that is necessary for its import to the nucleus. This novel nuclear localization sequence is placed within calmodulin-binding motif of NM1, thus it is present also in the Myo1c. We confirmed the presence of both isoforms in the nucleus by transfection of tagged NM1 and Myo1c constructs into cultured cells, and also by showing the presence of the endogenous Myo1c in purified nuclei of cells derived from knock-out mice lacking NM1. Using pull-down and co-immunoprecipitation assays we identified importin beta, importin 5 and importin 7 as nuclear transport receptors that bind NM1. Since the NLS sequence of NM1 lies within the region that also binds calmodulin we tested the influence of calmodulin on the localization of NM1. The presence of elevated levels of calmodulin interfered with nuclear localization of tagged NM1.

Conclusions/Significance

We have shown that the novel specific NLS brings to the cell nucleus not only the “nuclear” isoform of myosin I (NM1 protein) but also its “cytoplasmic” isoform (Myo1c protein). This opens a new field for exploring functions of this molecular motor in nuclear processes, and for exploring the signals between cytoplasm and the nucleus.

Targeted mutation of SLC4A5 induces arterial hypertension and renal metabolic acidosis

The human SLC4A5 gene has been identified as a hypertension susceptibility gene based on the association of single nucleotide polymorphisms with blood pressure (BP) levels and hypertension status. The biochemical basis of this association is unknown particularly since no single gene variant was linked to hypertension in humans. SLC4A5 (NBCe2, NBC4) is expressed in the collecting duct of the kidney and acts as an electrogenic ion-transporter that transports sodium and bicarbonate with a 1:2 or 1:3 stoichiometry allowing bicarbonate reabsorption with relatively minor concurrent sodium uptake. We have mutated the Slc4a5 gene in mice, which caused a persistent increase in systolic and diastolic BP. Slc4a5 mutant mice also displayed a compensated metabolic acidosis and hyporeninemic hypoaldosteronism. Analysis of kidney physiology revealed elevated fluid intake and urine excretion and increased glomerular filtration rate. Transcriptome analysis uncovers possible compensatory mechanisms induced by SLC4A5 mutation, including upregulation of SLC4A7 and pendrin as well as molecular mechanisms associated with hypertension. Induction of metabolic alkalosis eliminated the BP difference between wild-type and Slc4a5 mutant mice. We conclude that the impairment of the function of SLC4A5 favors development of a hypertensive state. We reason that the loss of sodium-sparing bicarbonate reabsorption by SLC4A5 initiates a regulatory cascade consisting of compensatory bicarbonate reabsorption via other sodium-bicarbonate transporters (e.g. SLC4A7) at the expense of an increased sodium uptake. This will ultimately raise BP and cause hypoaldosteronism, thus providing a mechanistic explanation for the linkage of the SLC4A5 locus to hypertension in humans.

Protein 4.1B contributes to the organization of peripheral myelinated axons

Neurons are characterized by extremely long axons. This exceptional cell shape is likely to depend on multiple factors including interactions between the cytoskeleton and membrane proteins. In many cell types, members of the protein 4.1 family play an important role in tethering the cortical actin-spectrin cytoskeleton to the plasma membrane. Protein 4.1B is localized in myelinated axons, enriched in paranodal and juxtaparanodal regions, and also all along the internodes, but not at nodes of Ranvier where are localized the voltage-dependent sodium channels responsible for action potential propagation. To shed light on the role of protein 4.1B in the general organization of myelinated peripheral axons, we studied 4.1B knockout mice. These mice displayed a mildly impaired gait and motility. Whereas nodes were unaffected, the distribution of Caspr/paranodin, which anchors 4.1B to the membrane, was disorganized in paranodal regions and its levels were decreased. In juxtaparanodes, the enrichment of Caspr2, which also interacts with 4.1B, and of the associated TAG-1 and Kv1.1, was absent in mutant mice, whereas their levels were unaltered. Ultrastructural abnormalities were observed both at paranodes and juxtaparanodes. Axon calibers were slightly diminished in phrenic nerves and preterminal motor axons were dysmorphic in skeletal muscle. βII spectrin enrichment was decreased along the axolemma. Electrophysiological recordings at 3 post-natal weeks showed the occurrence of spontaneous and evoked repetitive activity indicating neuronal hyperexcitability, without change in conduction velocity. Thus, our results show that in myelinated axons 4.1B contributes to the stabilization of membrane proteins at paranodes, to the clustering of juxtaparanodal proteins, and to the regulation of the internodal axon caliber.

Disruption of serine/threonine protein phosphatase 5 (PP5:PPP5c) in mice reveals a novel role for PP5 in the regulation of ultraviolet light-induced phosphorylation of serine/threonine protein kinase Chk1 (CHEK1)

PP5 is a ubiquitously expressed Ser/Thr protein phosphatase. High levels of PP5 have been observed in human cancers, and constitutive PP5 overexpression aids tumor progression in mouse models of tumor development. However, PP5 is highly conserved among species, and the roles of PP5 in normal tissues are not clear. Here, to help evaluate the biological actions of PP5, a Cre/loxP-conditional mouse line was generated. In marked contrast to the early embryonic lethality associated with the genetic disruption of other PPP family phosphatases (e.g. PP2A and PP4), intercrosses with mouse lines that ubiquitously express Cre recombinase starting early in development (e.g. MeuCre40 and ACTB-Cre) produced viable and fertile PP5-deficient mice. Phenotypic differences caused by the total disruption of PP5 were minor, suggesting that small molecule inhibitors of PP5 will not have widespread systemic toxicity. Examination of roles for PP5 in fibroblasts generated from PP5-deficient embryos (PP5(-/-) mouse embryonic fibroblasts) confirmed some known roles and identified new actions for PP5. PP5(-/-) mouse embryonic fibroblasts demonstrated increased sensitivity to UV light, hydroxyurea, and camptothecin, which are known activators of ATR (ataxia-telangiectasia and Rad3-related) kinase. Further study revealed a previously unrecognized role for PP5 downstream of ATR activation in a UV light-induced response. The genetic disruption of PP5 is associated with enhanced and prolonged phosphorylation of a single serine (Ser-345) on Chk1, increased phosphorylation of the p53 tumor suppressor protein (p53) at serine 18, and increased p53 protein levels. A comparable role for PP5 in the regulation of Chk1 phosphorylation was also observed in human cells.

Essential roles of ECAT15-2/Dppa2 in functional lung development

Many transcription factors and DNA binding proteins play essential roles in the development of organs in which they are highly and/or specifically expressed. Embryonic stem cell (ESC)-associated transcript 15-1 (ECAT15-1) and ECAT15-2, also known as developmental pluripotency-associated 4 (Dppa4) and Dppa2, respectively, are enriched in mouse ESCs and preimplantation embryos, and their genes encode homologous proteins with a common DNA binding domain known as the SAP motif. Previously, ECAT15-1 was shown to be important in lung development, while it is dispensable in early development. In this study, we generated ECAT15-2 single and ECAT15-1 ECAT15-2 double knockout (double KO) mice and found that almost all mutants, like ECAT15-1 mutants, died around birth with respiratory defects. Paradoxically, the expression of neither ECAT15-1 nor ECAT15-2 was detected in lung organogenesis. Several genes, such as Nkx2-5, Gata4, and Pitx2, were downregulated in the ECAT15-2-null lung. On the other hand, genomic DNA of these genes showed inactive chromatin statuses in ECAT15-2-null ESCs, but not in wild-type ESCs. The chromatin immunoprecipitation (ChIP) assay revealed that ECAT15-2 binds to the regulatory region of Nkx2-5 in ESCs. These data suggest that ECAT15-2 has important roles in lung development, where it is no longer expressed, by leaving epigenetic marks from earlier developmental stages.

Controlled somatic and germline copy number variation in the mouse model

Changes in the number of chromosomes, but also variations in the copy number of chromosomal regions have been described in various pathological conditions, such as cancer and aneuploidy, but also in normal physiological condition. Our classical view of DNA replication and mitotic preservation of the chromosomal integrity is now challenged as new technologies allow us to observe such mosaic somatic changes in copy number affecting regions of chromosomes with various sizes. In order to go further in the understanding of copy number influence in normal condition we could take advantage of the novel strategy called Targeted Asymmetric Sister Chromatin Event of Recombination (TASCER) to induce recombination during the G2 phase so that we can generate deletions and duplications of regions of interest prior to mitosis. Using this approach in the mouse we could address the effects of copy number variation and segmental aneuploidy in daughter cells and allow us to explore somatic mosaics for large region of interest in the mouse.

TIP110/p110nrb/SART3/p110 regulation of hematopoiesis through CMYC

Intracellular factors are involved in and essential for hematopoiesis. HIV-1 Tat-interacting protein of 110 kDa (TIP110; p110(nrb)/SART3/p110) is an RNA-binding nuclear protein implicated in the regulation of HIV-1 gene and host gene transcription, pre-mRNA splicing, and cancer immunology. In the present study, we demonstrate a role for TIP110 in the regulation of hematopoiesis. TIP110 was expressed in human CD34(+) cells and decreased with differentiation of CD34(+) cells. TIP110 mRNA was also expressed in phenotyped mouse marrow hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs). Using TIP110 transgenic (TIP110(TG)) and haploinsufficient (TIP110(+/-)) mice, we found that increased TIP110 expression enhanced HPC numbers, survival, and cell cycling, whereas decreased TIP110 expression had the opposite effects. Moreover, TIP110(+/-) bone marrow HPCs responded more effectively, and TIP110(TG) HPCs less effectively, than those of wild-type control mice to recovery from the cell-cycle-active drug 5-fluorouracil (5-FU). Unexplained sex differences were noted in HSC competitive repopulating ability, but not HPC numbers, in TIP110(TG) mice. Intracellularly, TIP110 regulated CMYC and GATA2 expression at the transcriptional level, and TIP110 and CMYC reciprocally regulated the expression of each other. These results demonstrate a role for TIP110 in the regulation of hematopoiesis, effects that are likely linked to TIP110 regulation of CMYC.

Specificity protein 2 (Sp2) is essential for mouse development and autonomous proliferation of mouse embryonic fibroblasts

Background

The zinc finger protein Sp2 (specificity protein 2) is a member of the glutamine-rich Sp family of transcription factors. Despite its close similarity to Sp1, Sp3 and Sp4, Sp2 does not bind to DNA or activate transcription when expressed in mammalian cell lines. The expression pattern and the biological relevance of Sp2 in the mouse are unknown.

Methodology/Principal Findings

Whole-mount in situ hybridization of mouse embryos between E7.5 and E9.5 revealed abundant expression in most embryonic and extra-embryonic tissues. In order to unravel the biological relevance of Sp2, we have targeted the Sp2 gene by a tri-loxP strategy. Constitutive Sp2null and conditional Sp2cko knockout alleles were obtained by crossings with appropriate Cre recombinase expressing mice. Constitutive disruption of the mouse Sp2 gene (Sp2null) resulted in severe growth retardation and lethality before E9.5. Mouse embryonic fibroblasts (MEFs) derived from Sp2null embryos at E9.5 failed to grow. Cre-mediated ablation of Sp2 in Sp2cko/cko MEFs obtained from E13.5 strongly impaired cell proliferation.

Conclusions/Significance

Our results demonstrate that Sp2 is essential for early mouse development and autonomous proliferation of MEFs in culture. Comparison of the Sp2 knockout phenotype with the phenotypes of Sp1, Sp3 and Sp4 knockout strains shows that, despite their structural similarity and evolutionary relationship, all four glutamine-rich members of the Sp family of transcription factors have distinct non-redundant functions in vivo.

Generation of an allele to inactivate Wnt4 gene function conditionally in the mouse

The Wnt family member Wnt4 is critical for the development of several organs, including the kidney, gonad, and adrenal, mammary, and pituitary glands. To study its potential postnatal functions, we generated a floxed Wnt4 allele. A single loxP site was targeted to the second intron, while a loxP-Neo-loxP cassette was placed 3' from Exon 5. The floxed Neo cassette was subsequently removed by crossing with MeuCre40 transgenic mice. The Wnt4 gene was specifically inactivated with CAGCre and another Wnt4 allele, Wnt4(EGFPCre), in which the Cre is driven by the endogenous Wnt4 promoter. Deletion of Wnt4 gene function with CAGCre impaired kidney development, as is the case with the conventional knockout. Similarly, the Wnt4(EGFPCre)-mediated inactivation of Wnt4 function considerably reduced the amount of Wnt4 transcripts, led to a severe defect in kidney development, and caused the female embryos to undergo partial sex reversal to males. All in all, the floxed Wnt4 allele serves as a useful tool for studying the roles of Wnt4 signaling during the life cycle.

Ablation of arginylation in the mouse N-end rule pathway: loss of fat, higher metabolic rate, damaged spermatogenesis, and neurological perturbations

In the N-end rule pathway of protein degradation, the destabilizing activity of N-terminal Asp, Glu or (oxidized) Cys residues requires their conjugation to Arg, which is recognized directly by pathway's ubiquitin ligases. N-terminal arginylation is mediated by the Ate1 arginyltransferase, whose physiological substrates include the Rgs4, Rgs5 and Rgs16 regulators of G proteins. Here, we employed the Cre-lox technique to uncover new physiological functions of N-terminal arginylation in adult mice. We show that postnatal deletion of mouse Ate1 (its unconditional deletion is embryonic lethal) causes a rapid decrease of body weight and results in early death of approximately 15% of Ate1-deficient mice. Despite being hyperphagic, the surviving Ate1-deficient mice contain little visceral fat. They also exhibit an increased metabolic rate, ectopic induction of the Ucp1 uncoupling protein in white fat, and are resistant to diet-induced obesity. In addition, Ate1-deficient mice have enlarged brains, an enhanced startle response, are strikingly hyperkinetic, and are prone to seizures and kyphosis. Ate1-deficient males are also infertile, owing to defects in Ate1(-/-) spermatocytes. The remarkably broad range of specific biological processes that are shown here to be perturbed by the loss of N-terminal arginylation will make possible the dissection of regulatory circuits that involve Ate1 and either its known substrates, such as Rgs4, Rgs5 and Rgs16, or those currently unknown.

Tissue-specific conditional CCM2 knockout mice establish the essential role of endothelial CCM2 in angiogenesis: implications for human cerebral cavernous malformations

Cerebral cavernous malformations (CCM) are vascular malformations of the brain that lead to cerebral hemorrhages. In 20% of CCM patients, this results from an autosomal dominant condition caused by loss-of-function mutations in one of the three CCM genes. High expression levels of the CCM genes in the neuroepithelium indicate that CCM lesions might be caused by a loss of function of these genes in neural cells rather than in vascular cells. However, their in vivo function, particularly during cerebral angiogenesis, is totally unknown. We developed mice with constitutive and tissue-specific CCM2 deletions to investigate CCM2 function in vivo. Constitutive deletion of CCM2 leads to early embryonic death. Deletion of CCM2 from neuroglial precursor cells does not lead to cerebrovascular defects, whereas CCM2 is required in endothelial cells for proper vascular development. Deletion of CCM2 from endothelial cells severely affects angiogenesis, leading to morphogenic defects in the major arterial and venous blood vessels and in the heart, and results in embryonic lethality at mid-gestation. These findings establish the essential role of endothelial CCM2 for proper vascular development and strongly suggest that the endothelial cell is the primary target in the cascade of events leading from CCM2 mutations to CCM cerebrovascular lesions.

Tissue-selective knockouts of steroid receptors: a novel paradigm in the study of steroid action

The use of tissue-selective rather than ubiquitous knockouts of steroid receptors allows a more refined study of the mechanism of steroid action in defined target tissues and circumvents problems such as early lethality or major developmental defects precluding studies in affected organs. In this chapter, we describe the main steps involved in the development of tissue-selective steroid receptor knockouts by Cre/loxP technology. Problems in the development of a mouse strain with a floxed receptor allele, the selection of a suitable Cre expressing mouse strain, the generation of cell-selective knockouts by crossbreeding of the mentioned mouse strains, and the control of appropriate receptor inactivation are discussed taking the generation of mice with a Sertoli cell-selective ablation of the androgen receptor as an example.

Lactaturia and loss of sodium-dependent lactate uptake in the colon of SLC5A8-deficient mice

SLC5A8 is a member of the sodium/glucose cotransporter family. It has been proposed that SLC5A8 might act as an apical iodide transporter in the thyroid follicular cells or as a transporter of short chain monocarboxylates. We have directly addressed the functional role of SLC5A8 in vivo by generation of SLC5A8 mutant mice. We found that SLC5A8 is responsible for the re-absorption of lactate at the apical membrane of the kidney proximal tubules and of serous salivary gland ducts. In addition, SLC5A8 mediated the uptake of lactate into colonocytes under physiological conditions. We did not find any evidence of SLC5A8 being essential for the apical iodide transport in the thyroid gland, even if the ion-cotransporter SLC26A4, causing the human Pendred syndrome, is missing. Because SLC5A8 is transcriptionally silenced in many tumors, it has been proposed that SLC5A8-mediated transport of butyrate suppresses tumor formation. Treatment of Slc5a8(-/-) mice with carcinogens and breeding to the Apc(min) mouse line did not reveal a higher incidence of tumor formation. We conclude that SLC5A8 is instrumental in preventing lactaturia and loss of sodium-dependent uptake of lactate in the colon but does not have any apparent role in the prevention of tumor formation and growth.

In vivo consequences of deleting EGF repeats 8-12 including the ligand binding domain of mouse Notch1

BACKGROUND

Notch signaling is highly conserved in the metazoa and is critical for many cell fate decisions. Notch activation occurs following ligand binding to Notch extracellular domain. In vitro binding assays have identified epidermal growth factor (EGF) repeats 11 and 12 as the ligand binding domain of Drosophila Notch. Here we show that an internal deletion in mouse Notch1 of EGF repeats 8-12, including the putative ligand binding domain (lbd), is an inactivating mutation in vivo. We also show that maternal and zygotic Notch1(lbd/lbd) mutant embryos develop through gastrulation to mid-gestation.

RESULTS

Notch1(lbd/lbd) embryos died at mid-gestation with a phenotype indistinguishable from Notch1 null mutants. In embryonic stem (ES) cells, Notch1(lbd) was expressed on the cell surface at levels equivalent to wild type Notch1, but Delta1 binding was reduced to the same level as in Notch1 null cells. In an ES cell co-culture assay, Notch signaling induced by Jagged1 or Delta1 was reduced to a similar level in Notch1(lbd) and Notch1 null cells. However, the Notch1(lbd/lbd) allele was expressed similarly to wild type Notch1 in Notch1(lbd/lbd) ES cells and embryos at E8.75, indicating that Notch1 signaling is not essential for the Notch1 gene to be expressed. In addition, maternal and zygotic Notch1 mutant blastocysts developed through gastrulation.

CONCLUSION

Mouse Notch1 lacking the ligand binding domain is expressed at the cell surface but does not signal in response to the canonical Notch ligands Delta1 and Jagged1. Homozygous Notch1(lbd/lbd) mutant embryos die at approximately E10 similar to Notch1 null embryos. While Notch1 is expressed in oocytes and blastocysts, Notch1 signaling via canonical ligands is dispensable during oogenesis, blastogenesis, implantation and gastrulation.

A missense mutation in podocin leads to early and severe renal disease in mice

Mutations in the NPHS2 gene, encoding podocin, are responsible for familial autosomal recessive and sporadic cases of steroid-resistant nephrotic syndrome. We have successfully generated a mouse model in which the common p.R138Q mutation found in nephrotic patients is expressed in the kidney. Homozygous mice express the mutant protein, which is mislocated to the cytoplasm, along with a portion of the nephrin pool. These mice die within the first month of life, but their survival depends on the genetic background. Albuminuria manifests early and leads to progressive renal insufficiency, characterized histologically by diffuse mesangiolysis and mesangial sclerosis, endothelial lesions along with podocyte abnormalities such as widespread foot process effacement. Gene expression profiling revealed marked differences between these and the podocin-null mice, including significant perturbations of podocyte-expressed genes such as Cd2ap, Vegfa and the transcription factors Lmx1b and Zhx2. Upregulation of Serpine1 and Tgfb1 implicates these as potential mediators of disease progression in these mice. This mouse model of nephrotic syndrome may serve as a valuable tool in studies of in vivo intracellular protein trafficking of podocyte proteins, as well as testing therapeutic modalities aimed at correcting the targeting of mutant proteins.

The O-fucose glycan in the ligand-binding domain of Notch1 regulates embryogenesis and T cell development

Mechanisms by which the extracellular domain of Notch1 controls Notch1 signaling are not well defined. Here, we show that the O-fucose glycan in the Notch1 ligand-binding domain regulates the strength of Notch1 signaling during embryogenesis, postweaning growth, and T cell development in the mouse. Heterozygotes carrying a Notch1(12f) allele and an inactive Notch1 allele die at approximately embryonic day (E)12 with a typical Notch1 null phenotype. Homozygous Notch1(12f/12f) mice are viable and fertile but grow somewhat more slowly than littermates after weaning. Notch1(12f/12f) thymocytes bind less Delta1 and exhibit reduced Notch1 signaling. The number of double-positive (DP) and single-positive (SP) T cells are decreased in Notch1(12f/12f) thymus, and DP T cells are more apoptotic. By contrast, proportionately more SP cells have matured, and SP-to-DP ratios are increased in mutant thymus. Thus, the O-fucose glycan in EGF12 of mouse Notch1 is required for optimal Notch1 signaling and T cell development in mammals.

Stabilization of beta-catenin affects mouse embryonic liver growth and hepatoblast fate

During hepatogenesis, after the liver has budded out of the endoderm, the hepatoblasts quickly expand and differentiate into either hepatocytes or biliary cells, the latter of which arise only within the ductal plate surrounding the portal vein. Because the Wnt/beta-catenin pathway is involved in liver homeostasis and regeneration and in liver carcinogenesis, we investigated here a role for Wnt/beta-catenin signaling in the embryonic liver. A cyclization recombination (Cre)/locus of X-over P1 (loxP) strategy was chosen to perform adenomatous polyposis coli (Apc) invalidation in order to activate ectopic beta-catenin signaling in hepatoblasts; an appropriate transgenic model expressing the Cre recombinase was used. Phenotypic and immunolocalization studies, together with messenger RNA analyses, by microarray and real-time quantitative polymerase chain reaction approaches were performed on this model during normal hepatogenesis. The loss of Apc allowed beta-catenin activation in the hepatoblasts after the formation of the liver bud and led to embryonic lethality. In this model, the liver became hypoplastic, and hepatocyte differentiation failed, whereas beta-catenin-activated ducts developed and gave rise to fully differentiated bile ducts when transplanted into adult recipient livers. Microarray analyses suggested that beta-catenin plays a role in repressing the hepatocyte genetic program and remodeling the ductal plate. According to these data, in normal embryonic livers, beta-catenin was transiently activated in the nascent bile ducts.

CONCLUSION

We demonstrated a key role for the Wnt/beta-catenin pathway in liver embryonic growth and in controlling the fate of hepatoblasts, preventing them from differentiating toward the hepatocyte lineage, and guiding them to biliary ductal morphogenesis.

Generation of a floxed allele of the mouse Endoglin gene

Endoglin is an auxiliary receptor for TGFbeta signalling. Heterozygous germline Endoglin mutations have been identified in patients with the vascular abnormality, Hereditary Haemorrhagic Telangiectasia. Endoglin is upregulated in endothelial cells during angiogenesis and loss of Endoglin in the mouse results in embryonic lethality at mid-gestation. This phenotype points to an important role of Endoglin in new blood vessel formation but precludes analysis at later stages in development and in postnatal life. To bypass this limitation and allow further investigations of the function of Endoglin we have generated a floxed Endoglin allele in which loxP sites flank exons 5 and 6. Mice homozygous for this allele are normal and in the presence of appropriate Cre lines will allow time and cell specific Endoglin deletion for in vivo analysis of function in cardiovascular development and disease.

Selective Role of NFATc3 in Positive Selection of Thymocytes

The four Ca(2+)-dependent NFATc proteins are both signal transducers and transcription factors that reside in the cytoplasm until dephosphorylation by calcineurin. Dephosphorylation exposes nuclear import sequences and sends NFATc proteins into the nucleus where they assemble with nuclear partners into NFAT transcription complexes. Recent genetic studies have indicated that calcineurin-NFAT signaling is a major determinant of vertebrate morphogenesis and development. Mice lacking calcineurin activity show a complete block in positive selection of CD4 and CD8 double-positive thymocytes, yet the role of the NFATc proteins in T cell development has been controversial. In this study, we address the requirement for NFATc3 in T cell development by generating NFATc3 conditional knockout mice. We show that specific deletion of NFATc3 in thymocytes causes a partial block at the double-negative stage 3 and also a partial block in positive selection. Furthermore, the defect does not become more pronounced when NFATc2 is also absent, consistent with the fact that NFATc2-null mice do not have a T cell developmental defect. Expression of a nuclear (and constitutively active) NFATc1 even at subphysiological levels can rescue the transition of double-negative to double-positive thymocytes in RAG-null mice, but is unable to rescue development of CD4 and CD8 single-positive cells. In addition to NFATc3, this suggests a role for NFATc1 in T cell development. Our studies indicate that the signals that direct positive selection likely use both NFATc1 and NFATc3 downstream of calcineurin.

A mutant alphaII-spectrin designed to resist calpain and caspase cleavage questions the functional importance of this process in vivo

alpha- and beta-spectrins are components of molecular scaffolds located under the lipid bilayer and named membrane skeletons. Disruption of these scaffolds through mutations in spectrins demonstrated that they are involved in the membrane localization or the maintenance of proteins associated with them. The ubiquitous alphaII-spectrin chain bears in its central region a unique domain that is sensitive to several proteases such as calpains or caspases. The conservation of this region in vertebrates suggests that the proteolysis of alphaII-spectrin by these enzymes could be involved in important functions. To assess the role of alphaII-spectrin cleavage in vivo, we generated a murine model in which the exons encoding the region defining this cleavage sensitivity were disrupted by gene targeting. Surprisingly, homozygous mice expressing this mutant alphaII-spectrin appeared healthy, bred normally, and had no histological anomaly. Remarkably, the mutant alphaII-spectrin assembles correctly into the membrane skeleton, thus challenging the notion that this region is required for the stable biogenesis of the membrane skeleton in nonerythroid cells. Our finding also argues against a critical role of this particular alphaII-spectrin cleavage in either major cellular functions or in normal development.

Conditional somatic mutagenesis in the mouse using site-specific recombinases

In the last decade, site-specific recombinases (SSRs), such as Cre and Flp, have emerged as indispensable tools for the precise in vivo manipulation of the mouse genome. It is now feasible to control, in space and time, the onset of gene knockouts in almost any tissue of the mouse, thus greatly facilitating the creation of sophisticated animal models for human disease and drug development. This review describes the basic principles and current status of the SSR technology, with a focus on strategies for conditional somatic mutagenesis using the Cre/lox system and ligand-activated Cre recombinases. Practical hints for generating and analysing conditional mouse mutants will be given and exciting novel applications of the SSR technology will be discussed, such as cell fate mapping and the combined use of Cre, Flp and other biotechnological tools. It will be shown how genetic manipulation of the mouse by site-specific recombination can provide new solutions to old problems in the analysis of human physiology and pathophysiology and how it can be employed for drug discovery and development.

Cre/loxP-mediated chromosome engineering of the mouse genome

Together with numerous other genome modifications, chromosome engineering offers a very powerful tool to accelerate the functional analysis of the mammalian genome. The technology, based on the Cre/loxP system, is used more and more in the scientific community in order to generate new chromosomes carrying deletions, duplications, inversions and translocations in targeted regions of interest. In this review, we will present the basic principle of the technique either in vivo or in vitro and we will briefly describe some applications to provide highly valuable genetic tools, to decipher the mammalian genome organisation and to analyze human diseases in the mouse.

Modeling chromosomes in mouse to explore the function of genes, genomic disorders, and chromosomal organization

One of the challenges of genomic research after the completion of the human genome project is to assign a function to all the genes and to understand their interactions and organizations. Among the various techniques, the emergence of chromosome engineering tools with the aim to manipulate large genomic regions in the mouse model offers a powerful way to accelerate the discovery of gene functions and provides more mouse models to study normal and pathological developmental processes associated with aneuploidy. The combination of gene targeting in ES cells, recombinase technology, and other techniques makes it possible to generate new chromosomes carrying specific and defined deletions, duplications, inversions, and translocations that are accelerating functional analysis. This review presents the current status of chromosome engineering techniques and discusses the different applications as well as the implication of these new techniques in future research to better understand the function of chromosomal organization and structures.

Hypertension, kidney, and transgenics: a fresh perspective

In this review, we outline the application and contribution of transgenic technology to establishing the genetic basis of blood pressure regulation and its dysfunction. Apart from a small number of examples where high blood pressure is the result of single gene mutation, essential hypertension is the sum of interactions between multiple environmental and genetic factors. Candidate genes can be identified by a variety of means including linkage analysis, quantitative trait locus analysis, association studies, and genome-wide scans. To test the validity of candidate genes, it is valuable to model hypertension in laboratory animals. Animal models generated through selective breeding strategies are often complex, and the underlying mechanism of hypertension is not clear. A complementary strategy has been the use of transgenic technology. Here one gene can be selectively, tissue specifically, or developmentally overexpressed, knocked down, or knocked out. Although resulting phenotypes may still be complicated, the underlying genetic perturbation is a starting point for identifying interactions that lead to hypertension. We recognize that the development and maintenance of hypertension may involve many systems including the vascular, cardiac, and central nervous systems. However, given the central role of the kidney in normal and abnormal blood pressure regulation, we intend to limit our review to models with a broadly renal perspective.

Branching and nucleokinesis defects in migrating interneurons derived from doublecortin knockout mice

Type I lissencephaly results from mutations in the doublecortin (DCX) and LIS1 genes. We generated Dcx knockout mice to further understand the pathophysiological mechanisms associated with this cortical malformation. Dcx is expressed in migrating interneurons in developing human and mouse brains. Video microscopy analyses of such tangentially migrating neuron populations derived from the medial ganglionic eminence show defects in migratory dynamics. Specifically, the formation and division of growth cones, leading to the production of new branches, are more frequent in knockout cells, although branches are less stable. Dcx-deficient cells thus migrate in a disorganized manner, extending and retracting short branches and making less long-distant movements of the nucleus. Despite these differences, migratory speeds and distances remain similar to wild-type cells. These novel data thus highlight a role for Dcx, a microtubule-associated protein enriched at the leading edge in the branching and nucleokinesis of migrating interneurons.

Spontaneous ectopic recombination in cell-type-specific Cre mice removes loxP-flanked marker cassettes in vivo

Conditional gene targeting using the Cre/loxP technology generally includes integration of a selection marker cassette flanked by loxP recognition sites (floxed) in the target gene locus. Subsequent marker removal avoids possible impairment of gene expression or mosaicism due to partial and total deletions after Cre-mediated recombination in vivo. The use of deleter Cre mice for in vivo marker removal in floxed connexin43 mice revealed considerable mosaicism, but no selective marker removal. In addition, we noted that several Cre transgenic lines displayed spontaneous ectopic activity, reminiscent of deleter Cre mice, and required the confirmation of cell type-specific deletion in every individual mouse. When we used myosin heavy chain promoter Cre (alphaMyHC-Cre) mice for cardiomyocyte specific deletion, we observed, in addition to cardiomyocyte-restricted or complete excision, selective marker removal in a subgroup of mice as well. Thus, selective marker removal can be achieved as a byproduct of cell-type restricted deletion.

Colorectal cancers in a new mouse model of familial adenomatous polyposis: influence of genetic and environmental modifiers

Murine models of familial adenomatous polyposis harbor a germinal heterozygous mutation on Apc tumor suppressor gene. They are valuable tools for studying intestinal carcinogenesis, as most human sporadic cancers contain inactivating mutations of APC. However, Apc(+/-) mice, such as the well-characterized Apc(Min/+) model, develop cancers principally in the small intestine, while humans develop mainly colorectal cancers. We used a Cre-loxP strategy to achieve a new model of germline Apc invalidation in which exon 14 is deleted. We compared the phenotype of these Apc(Delta14/+) mice to that of the classical Apc(Min/+). The main phenotypic difference is the shift of the tumors in the distal colon and rectum, often associated with a rectal prolapse. Thus, the severity of the colorectal phenotype is partly due to the particular mutation Delta14, but also to environmental parameters, as mice raised in conventional conditions developed more colon cancers than those raised in pathogen-free conditions. All lesions, including early lesions, revealed Apc LOH and loss of Apc gene expression. They accumulated beta-catenin, overexpressed the beta-catenin target genes cyclin D1 and c-Myc, and the distribution pattern of glutamine synthetase, a beta-catenin target gene recently identified in the liver, was mosaic in intestinal adenomas. The Apc(Delta14/+) model is thus a useful new tool for studies on the molecular mechanisms of colorectal tumorigenesis.