Conditional Creation and Rescue of Nipbl-Deficiency in Mice Reveals Multiple Determinants of Risk for Congenital Heart Defects

Elucidating the causes of congenital heart defects is made difficult by the complex morphogenesis of the mammalian heart, which takes place early in development, involves contributions from multiple germ layers, and is controlled by many genes. Here, we use a conditional/invertible genetic strategy to identify the cell lineage(s) responsible for the development of heart defects in a Nipbl-deficient mouse model of Cornelia de Lange Syndrome, in which global yet subtle transcriptional dysregulation leads to development of atrial septal defects (ASDs) at high frequency. Using an approach that allows for recombinase-mediated creation or rescue of Nipbl deficiency in different lineages, we uncover complex interactions between the cardiac mesoderm, endoderm, and the rest of the embryo, whereby the risk conferred by genetic abnormality in any one lineage is modified, in a surprisingly non-additive way, by the status of others. We argue that these results are best understood in the context of a model in which the risk of heart defects is associated with the adequacy of early progenitor cell populations relative to the sizes of the structures they must eventually form

Multiple Organ System Defects and Transcriptional Dysregulation in the Nipbl+/− Mouse, a Model of Cornelia de Lange Syndrome

Cornelia de Lange Syndrome (CdLS) is a multi-organ system birth defects disorder linked, in at least half of cases, to heterozygous mutations in the NIPBL gene. In animals and fungi, orthologs of NIPBL regulate cohesin, a complex of proteins that is essential for chromosome cohesion and is also implicated in DNA repair and transcriptional regulation. Mice heterozygous for a gene-trap mutation in Nipbl were produced and exhibited defects characteristic of CdLS, including small size, craniofacial anomalies, microbrachycephaly, heart defects, hearing abnormalities, delayed bone maturation, reduced body fat, behavioral disturbances, and high mortality (75–80%) during the first weeks of life. These phenotypes arose despite a decrease in Nipbl transcript levels of only ∼30%, implying extreme sensitivity of development to small changes in Nipbl activity. Gene expression profiling demonstrated that Nipbl deficiency leads to modest but significant transcriptional dysregulation of many genes. Expression changes at the protocadherin beta (Pcdhb) locus, as well as at other loci, support the view that NIPBL influences long-range chromosomal regulatory interactions. In addition, evidence is presented that reduced expression of genes involved in adipogenic differentiation may underlie the low amounts of body fat observed both in Nipbl+/− mice and in individuals with CdLS

Restoration of retinal development in Vsx2 deficient mice by reduction of Gdf11 levels.

The visual system homeobox 2 (Vsx2/Chx10) gene is required for proper eye development in vertebrates. In mouse, mutations in Vsx2 cause micro-ophthalmia, optic nerve aplasia, and failed retinal development, as well as aberrant retinal lamination. GDF11, a member of the TGF-β superfamily of signaling molecules known to function as an autocrine negative regulator of sensory neuron neurogenesis, has also been shown to have dramatic effects on retinal development: Absence of Gdf11 results in an increase in numbers of retinal ganglion cells, resulting from changes in the fates of retinal stem/progenitor cells in Gdf11 null retinas. In the present study, we performed genetic manipulations of the GDF11 signaling system to determine whether alterations in Gdf11 activity levels can improve retinal development in Vsx2-null mice. We found that removal of Gdf11 alleles in Vsx2 orJ/orJ mice can rescue retinal development and thickness to a substantial extent, and partially restores the expression of genes important for the development of specific types of retinal neurons. The molecular mechanism(s) by which reduction of Gdf11 activity enables rescue of retinal development in Vsx2 mutant animals is currently being investigated, and should provide important insights for potential treatment of retinal dystrophies. © 2012 Springer Science+Business Media, LLC

Foxg1 promotes olfactory neurogenesis by antagonizing Gdf11.

Foxg1, a winged-helix transcription factor, promotes the development of anterior neural structures; in mice lacking Foxg1, development of the cerebral hemispheres and olfactory epithelium (OE) is severely reduced. It has been suggested that Foxg1 acts by positively regulating the expression of growth factors, such as Fgf8, which support neurogenesis. However, Foxg1 also binds Smad transcriptional complexes, allowing it to negatively regulate the effects of TGFbeta family ligands. Here, we provide evidence that this latter effect explains much of the ability of Foxg1 to drive neurogenesis in the OE. We show that Foxg1 is expressed in developing OE at the same time as the gene encoding growth differentiation factor 11 (Gdf11), a TGFbeta family member that mediates negative-feedback control of OE neurogenesis. Mutations in Gdf11 rescue, to a considerable degree, the major defects in Foxg1(-/-) OE, including the early, severe loss of neural precursors and olfactory receptor neurons, and the subsequent collapse of both neurogenesis and nasal cavity formation. Rescue is gene-dosage dependent, with loss of even one allele of Gdf11 restoring substantial neurogenesis. Notably, we find no evidence for a disruption of Fgf8 expression in Foxg1(-/-) OE. However, we do observe both a failure of expression of follistatin (Fst), which encodes a secreted Gdf11 antagonist normally expressed in and around OE, and an increase in the expression of Gdf11 itself within the remaining OE in these mutants. Fst expression is rescued in Foxg1(-/-);Gdf11(-/-) and Foxg1(-/-);Gdf11(+/-) mice. These data suggest that the influence of Foxg1 on Gdf11-mediated negative feedback of neurogenesis may be both direct and indirect. In addition, defects in development of the cerebral hemispheres in Foxg1(-/-) mice are not rescued by mutations in Gdf11, nor is Gdf11 expressed at high levels within these structures. Thus, the pro-neurogenic effects of Foxg1 are likely to be mediated through different signaling pathways in different parts of the nervous system

Genetic enhancement of limb defects in a mouse model of Cornelia de Lange syndrome.

Cornelia de Lange Syndrome (CdLS) is characterized by a wide variety of structural and functional abnormalities in almost every organ system of the body. CdLS is now known to be caused by mutations that disrupt the function of the cohesin complex or its regulators, and studies of animal models and cell lines tell us that the effect of these mutations is to produce subtle yet pervasive dysregulation of gene expression. With many hundreds of mostly small gene expression changes occurring in every cell type and tissue, identifying the etiology of any particular birth defect is very challenging. Here we focus on limb abnormalities, which are commonly seen in CdLS. In the limb buds of the Nipbl-haploinsufficient mouse (Nipbl(+/-) mouse), a model for the most common form of CdLS, modest gene expression changes are observed in several candidate pathways whose disruption is known to cause limb abnormalities, yet the limbs of Nipbl(+/-) mice develop relatively normally. We hypothesized that further impairment of candidate pathways might produce limb defects similar to those seen in CdLS, and performed genetic experiments to test this. Focusing on Sonic hedgehog (Shh), Bone morphogenetic protein (Bmp), and Hox gene pathways, we show that decreasing Bmp or Hox function (but not Shh function) enhances polydactyly in Nipbl(+/-) mice, and in some cases produces novel skeletal phenotypes. However, frank limb reductions, as are seen in a subset of individuals with CdLS, do not occur, suggesting that additional signaling and/or gene regulatory pathways are involved in producing such dramatic changes. © 2016 Wiley Periodicals, Inc

Foxg1 promotes olfactory neurogenesis by antagonizing Gdf11

Foxg1, a winged-helix transcription factor, promotes the development of anterior neural structures; in mice lacking Foxg1, development of the cerebral hemispheres and olfactory epithelium (OE) is severely reduced. It has been suggested that Foxg1 acts by positively regulating the expression of growth factors, such as Fgf8, which support neurogenesis. However, Foxg1 also binds Smad transcriptional complexes, allowing it to negatively regulate the effects of TGFβ family ligands. Here, we provide evidence that this latter effect explains much of the ability of Foxg1 to drive neurogenesis in the OE. We show that Foxg1 is expressed in developing OE at the same time as the gene encoding growth differentiation factor 11 (Gdf11), a TGFβ family member that mediates negative-feedback control of OE neurogenesis. Mutations in Gdf11 rescue, to a considerable degree, the major defects in Foxg1-/- OE, including the early, severe loss of neural precursors and olfactory receptor neurons, and the subsequent collapse of both neurogenesis and nasal cavity formation. Rescue is gene-dosage dependent, with loss of even one allele of Gdf11 restoring substantial neurogenesis. Notably, we find no evidence for a disruption of Fgf8 expression in Foxg1-/- OE. However, we do observe both a failure of expression of follistatin (Fst), which encodes a secreted Gdf11 antagonist normally expressed in and around OE, and an increase in the expression of Gdf11 itself within the remaining OE in these mutants. Fst expression is rescued in Foxg1-/-;Gdf11-/- and Foxg1-/-;Gdf11+/- mice. These data suggest that the influence of Foxg1 on Gdf11-mediated negative feedback of neurogenesis may be both direct and indirect. In addition, defects in development of the cerebral hemispheres in Foxg1-/- mice are not rescued by mutations in Gdf11, nor is Gdf11 expressed at high levels within these structures. Thus, the pro-neurogenic effects of Foxg1 are likely to be mediated through different signaling pathways in different parts of the nervous system

The role of foxg1 in the development of neural stem cells of the olfactory epithelium.

The olfactory epithelium (OE) of the mouse is an excellent model system for studying principles of neural stem cell biology because of its well-defined neuronal lineage and its ability to regenerate throughout life. To approach the molecular mechanisms of stem cell regulation in the OE, we have focused on Foxg1, also known as brain factor 1, which is a member of the Forkhead transcription factor family. Foxg1(-/-) mice show major defects in the OE at birth, suggesting that Foxg1 plays an important role in OE development. We find that Foxg1 is expressed in cells within the basal compartment of the OE, the location where OE stem and progenitor cells are known to reside. Since FoxG1 is known to regulate proliferation of neuronal progenitor cells during telencephalon development, we performed bromodeoxyuridine pulse-chase labeling of Sox2-expressing neural stem cells during primary OE neurogenesis. We found the percentage of Sox2-expressing cells that retained bromodeoxyuridine was twice as high in Foxg1(-/-) OE cells as in the wild type, suggesting that these cells are delayed and/or halted in their development in the absence of Foxg1. Our findings suggest that the proliferation and/or subsequent differentiation of Sox2-expressing neural stem cells in the OE is regulated by Foxg1