A global double-fluorescent Cre reporter mouse

Expression of the BMP receptor Alk3 in the second heart field is essential for development of the dorsal mesenchymal protrusion and atrioventricular septation

RATIONALE

The dorsal mesenchymal protrusion (DMP) is a prong of mesenchyme derived from the second heart field (SHF) located at the venous pole of the developing heart. Recent studies have shown that perturbation of its development is associated with the pathogenesis of atrioventricular (AV) septal defect. Although the importance of the DMP to AV septation is now established, the molecular and cellular mechanisms underlying its development are far from fully understood. Prior studies have demonstrated that bone morphogenetic protein (BMP) signaling is essential for proper formation of the AV endocardial cushions and the cardiac outflow tract. A role for BMP signaling in regulation of DMP development remained to be elucidated.

OBJECTIVE

To determine the role of BMP signaling in DMP development.

METHODS AND RESULTS

Conditional deletion of the BMP receptor Alk3 from venous pole SHF cells leads to impaired formation of the DMP and a completely penetrant phenotype of ostium primum defect, a hallmark feature of AV septal defects. Analysis of mutants revealed decreased proliferative index of SHF cells and, consequently, reduced number of SHF cells at the cardiac venous pole. In contrast, volume and expression of markers associated with proliferation and active BMP/transforming growth factor β signaling were not significantly altered in the AV cushions of SHF-Alk3 mutants.

CONCLUSIONS

BMP signaling is required for expansion of the SHF-derived DMP progenitor population at the cardiac venous pole. Perturbation of Alk3-mediated BMP signaling from the SHF results in impaired development of the DMP and ostium primum defects.

Pathfinding of corticothalamic axons relies on a rendezvous with thalamic projections

Major outputs of the neocortex are conveyed by corticothalamic axons (CTAs), which form reciprocal connections with thalamocortical axons, and corticosubcerebral axons (CSAs) headed to more caudal parts of the nervous system. Previous findings establish that transcriptional programs define cortical neuron identity and suggest that CTAs and thalamic axons may guide each other, but the mechanisms governing CTA versus CSA pathfinding remain elusive. Here, we show that thalamocortical axons are required to guide pioneer CTAs away from a default CSA-like trajectory. This process relies on a hold in the progression of cortical axons, or waiting period, during which thalamic projections navigate toward cortical axons. At the molecular level, Sema3E/PlexinD1 signaling in pioneer cortical neurons mediates a "waiting signal" required to orchestrate the mandatory meeting with reciprocal thalamic axons. Our study reveals that temporal control of axonal progression contributes to spatial pathfinding of cortical projections and opens perspectives on brain wiring.

Endothelial SRF/MRTF ablation causes vascular disease phenotypes in murine retinae

Retinal vessel homeostasis ensures normal ocular functions. Consequently, retinal hypovascularization and neovascularization, causing a lack and an excess of vessels, respectively, are hallmarks of human retinal pathology. We provide evidence that EC-specific genetic ablation of either the transcription factor SRF or its cofactors MRTF-A and MRTF-B, but not the SRF cofactors ELK1 or ELK4, cause retinal hypovascularization in the postnatal mouse eye. Inducible, EC-specific deficiency of SRF or MRTF-A/MRTF-B during postnatal angiogenesis impaired endothelial tip cell filopodia protrusion, resulting in incomplete formation of the retinal primary vascular plexus, absence of the deep plexi, and persistence of hyaloid vessels. All of these features are typical of human hypovascularization-related vitreoretinopathies, such as familial exudative vitreoretinopathies including Norrie disease. In contrast, conditional EC deletion of Srf in adult murine vessels elicited intraretinal neovascularization that was reminiscent of the age-related human pathologies retinal angiomatous proliferation and macular telangiectasia. These results indicate that angiogenic homeostasis is ensured by differential stage-specific functions of SRF target gene products in the developing versus the mature retinal vasculature and suggest that the actin-directed MRTF-SRF signaling axis could serve as a therapeutic target in the treatment of human vascular retinal diseases.

IRF7-dependent IFN-β production in response to RANKL promotes medullary thymic epithelial cell development

The contributions of IFN regulatory factor (IRF) 3/7 and the type I IFNs IFN-α/β to the innate host defense have been extensively investigated; however, their role in thymic development is less clear. In this study, we show that mice lacking the type I IFN receptor IFN-α/β receptor (IFNAR) or the downstream transcription factor STAT1 harbor a significant reduction in self-Ag-presenting, autoimmune regulator (AIRE)(+) medullary thymic epithelial cells (mTECs). Constitutive IFNAR signaling occurs in the thymic medulla in the absence of infection or inflammation. Receptor activator for NF-κB (RANK) ligand stimulation results in IFN-β upregulation, which in turn inhibits RANK signaling and facilitates AIRE expression in mTECs. Finally, we find that IRF7 is required for thymic IFN-β induction, maintenance of thymic architecture, and mTEC differentiation. We conclude that spatially and temporally coordinated cross talks between the RANK ligand/RANK and IRF7/IFN-β/IFNAR/STAT1 pathways are essential for differentiation of AIRE(+) mTECs.

Development, composition, and structural arrangements of the ciliary zonule of the mouse

PURPOSE

Here, we examined the development, composition, and structural organization of the ciliary zonule of the mouse. Fibrillin 1, a large glycoprotein enriched in force-bearing tissues, is a prominent constituent of the mouse zonule. In humans, mutations in the gene for fibrillin 1 (FBN1) underlie Marfan syndrome (MS), a disorder characterized by lens dislocation and other ocular symptoms.

METHODS

Fibrillin expression was analyzed by in situ hybridization. The organization of the zonule was visualized using antibodies to Fbn1, Fbn2, and microfibril-associated glycoprotein-1 (Magp1) in conjunction with 5-ethynyl-2'-deoxyuridine (EdU), an S-phase marker.

RESULTS

Microfibrils, enriched in Fbn2 and Magp1, were prominent components of the temporary vascular tunic of the embryonic lens. Fbn2 expression by nonpigmented ciliary epithelial cells diminished postnatally and there was a concomitant increase in Fbn1 expression, especially in cells located in valleys between the ciliary folds. Zonular fibers projected from the posterior pars plicata to the lens in anterior, equatorial, and posterior groupings. The attachment point of the posterior zonular fibers consisted of a dense meshwork of radially oriented microfibrils that we termed the fibrillar girdle. The fibrillar girdle was located directly above the transition zone, a region of the lens epithelium in which cells commit to terminal differentiation.

CONCLUSIONS

The development and arrangement of the murine ciliary zonule are similar to those of humans, and consequently the mouse eye may be a useful model in which to study ocular complications of MS.

Degeneration and impaired regeneration of gray matter oligodendrocytes in amyotrophic lateral sclerosis

Oligodendrocytes associate with axons to establish myelin and provide metabolic support to neurons. In the spinal cord of ALS mice, oligodendrocytes downregulate transporters that transfer glycolytic substrates to neurons and oligodendrocyte progenitors (NG2⁺ cells) exhibit enhanced proliferation and differentiation, although the cause of these changes in oligodendroglia is unknown. Here we report that there is extensive degeneration of gray matter oligodendrocytes in the spinal cord of ALS mice before disease onset. Although new oligodendrocytes were formed, they failed to mature, resulting in progressive demyelination. Oligodendrocyte dysfunction also is prevalent in human ALS, as gray matter demyelination and reactive changes in NG2⁺ cells were observed in motor cortex and spinal cord of ALS patients. Selective removal of mutant SOD1 from oligodendroglia substantially delayed disease onset and prolonged survival in ALS mice, suggesting that ALS-linked genes enhance the vulnerability of motor neurons and accelerate disease by directly impairing the function of oligodendrocytes.

Incomplete cre-mediated excision leads to phenotypic differences between Stra8-iCre; Mov10l1(lox/lox) and Stra8-iCre; Mov10l1(lox/Δ) mice

In the Cre-loxp system, expression level and activity of Cre recombinase in a Cre deleter line are critical because these determine not only the cell specificity of gene knockout (KO), but also the efficiency of Cre-mediated excision in a specific cell lineage. Although the spatiotemporal expression pattern of a Cre transgene is usually defined upon the generation of the mouse line, the Cre excision efficiency in a specific targeted cell lineage is rarely evaluated and often assumed to be 100%. Incomplete excision can lead to highly variable phenotypes owing to mosaicism (i.e., coexistence of cells with the flox or the recombined flox allele) and this problem has long been overlooked. Here, we report that Stra8-codon-improved Cre recombinase (iCre), a transgenic allele expressing iCre under the control of the male germ cell-specific Stra8 promoter, could efficiently delete one Mov10l1 flox allele in spermatogenic cells, whereas the excision was incomplete when two Mov10l1 flox alleles were present. The incomplete Cre-mediated excision led to a testicular phenotype that was much less severe than that in the true conditional KO (inactivation, 100%) mice. Our findings suggest that it is essential to determine the efficiency of Cre excision when Cre-loxp system is used for deleting genes in a specific cell lineage and the Cre; gene(lox) (/)(Δ) genotype should be used to evaluate phenotypes instead of Cre; gene(lox/lox) owing to the fact that the latter usually bears incomplete deletion of the flox allele(s).

Non-parallel recombination limits Cre-LoxP-based reporters as precise indicators of conditional genetic manipulation

Cre/LoxP-mediated recombination allows for conditional gene activation or inactivation. When combined with an independent lineage-tracing reporter allele, this technique traces the lineage of presumptive genetically modified Cre-expressing cells. Several studies have suggested that floxed alleles have differential sensitivities to Cre-mediated recombination, which raises concerns regarding utilization of Cre-reporters to monitor recombination of other floxed loci of interest. Here, we directly investigate the recombination correlation, at cellular resolution, between several floxed alleles induced by Cre-expressing mouse lines. The recombination correlation between different reporter alleles varied greatly in otherwise genetically identical cell types. The chromosomal location of floxed alleles, distance between LoxP sites, sequences flanking the LoxP sites, and the level of Cre activity per cell all likely contribute to observed variations in recombination correlation. These findings directly demonstrate that, due to non-parallel recombination events, commonly available Cre reporter mice cannot be reliably utilized, in all cases, to trace cells that have DNA recombination in independent-target floxed alleles, and that careful validation of recombination correlations are required for proper interpretation of studies designed to trace the lineage of genetically modified populations, especially in mosaic situations.

Supporting conditional mouse mutagenesis with a comprehensive cre characterization resource

Full realization of the value of the loxP-flanked alleles generated by the International Knockout Mouse Consortium will require a large set of well-characterized cre-driver lines. However, many cre driver lines display excision activity beyond the intended tissue or cell type, and these data are frequently unavailable to the potential user. Here we describe a high-throughput pipeline to extend characterization of cre driver lines to document excision activity in a wide range of tissues at multiple time points and disseminate these data to the scientific community. Our results show that the majority of cre strains exhibit some degree of unreported recombinase activity. In addition, we observe frequent mosaicism, inconsistent activity and parent-of-origin effects. Together, these results highlight the importance of deep characterization of cre strains, and provide the scientific community with a critical resource for cre strain information.

The cre-loxP system is widely used for the generation of conditional gene knockouts. Here Heffner et al. systematically characterize cre recombinase activity in tissues of embryonic and adult cre-driver mouse strains and provide an online resource for scientists.

Analysis of endogenous Oct4 activation during induced pluripotent stem cell reprogramming using an inducible Oct4 lineage label

The activation of endogenous Oct4 transcription is a key step in the reprogramming of somatic cells into induced pluripotent stem (iPS) cells but until now it has been difficult to analyze this critical event in the reprogramming process. We have generated a transgenic mouse that expresses the tamoxifen-inducible Cre recombinase MerCreMer under the control of the endogenous Oct4 locus, enabling lineage tracing of Oct4 expression in cells in vivo or in vitro, during either reprogramming or differentiation. Using this novel resource, we have determined the timing and outcome of endogenous Oct4 induction during fibroblast reprogramming. We show that both the initiation of this key reprogramming step and the ability of cells activating endogenous Oct4 expression to complete reprogramming are not influenced by the presence of exogenous c-Myc, although the overall efficiency of the process is increased by c-Myc. Oct4 lineage tracing reveals that new reprogramming events continue to initiate over a period of 3 weeks. Furthermore, the analysis of mixed colonies, where only a subset of daughter cells induce endogenous Oct4 expression, indicates the role of unknown, stochastic events in the progression of reprogramming from the initial events to a pluripotent state. Our transgenic mouse model and cells derived from it provide powerful and precise new tools for the study of iPS cell reprogramming mechanisms and have wider implications for the investigation of the role of Oct4 during development.

Iron regulatory proteins control a mucosal block to intestinal iron absorption

Mammalian iron metabolism is regulated systemically by the hormone hepcidin and cellularly by iron regulatory proteins (IRPs) that orchestrate a posttranscriptional regulatory network. Through ligand-inducible genetic ablation of both IRPs in the gut epithelium of adult mice, we demonstrate that IRP deficiency impairs iron absorption and promotes mucosal iron retention via a ferritin-mediated "mucosal block." We show that IRP deficiency does not interfere with intestinal sensing of body iron loading and erythropoietic iron need, but rather alters the basal expression of the iron-absorption machinery. IRPs thus secure sufficient iron transport across absorptive enterocytes by restricting the ferritin "mucosal block" and define a basal set point for iron absorption upon which IRP-independent systemic regulatory inputs are overlaid.

Wntless is required for peripheral lung differentiation and pulmonary vascular development

Wntless (Wls), a gene highly conserved across the animal kingdom, encodes for a transmembrane protein that mediates Wnt ligand secretion. Wls is expressed in developing lung, wherein Wnt signaling is necessary for pulmonary morphogenesis. We hypothesize that Wls plays a critical role in modulating Wnt signaling during lung development and therefore affects processes critical for pulmonary morphogenesis. We generated conditional Wls mutant mice utilizing Shh-Cre and Dermo1-Cre mice to delete Wls in the embryonic respiratory epithelium and mesenchyme, respectively. Epithelial deletion of Wls disrupted lung branching morphogenesis, peripheral lung development and pulmonary endothelial differentiation. Epithelial Wls mutant mice died at birth due to respiratory failure caused by lung hypoplasia and pulmonary hemorrhage. In the lungs of these mice, VEGF and Tie2-angiopoietin signaling pathways, which mediate vascular development, were downregulated from early stages of development. In contrast, deletion of Wls in mesenchymal cells of the developing lung did not alter branching morphogenesis or early mesenchymal differentiation. In vitro assays support the concept that Wls acts in part via Wnt5a to regulate pulmonary vascular development. We conclude that epithelial Wls modulates Wnt ligand activities critical for pulmonary vascular differentiation and peripheral lung morphogenesis. These studies provide a new framework for understanding the molecular mechanisms underlying normal pulmonary vasculature formation and the dysmorphic pulmonary vasculature development associated with congenital lung disease.

Selective requirement of PI3K/PDK1 signaling for Kras oncogene-driven pancreatic cell plasticity and cancer

Oncogenic Kras activates a plethora of signaling pathways, but our understanding of critical Ras effectors is still very limited. We show that cell-autonomous phosphoinositide 3-kinase (PI3K) and 3-phosphoinositide-dependent protein kinase 1 (PDK1), but not Craf, are key effectors of oncogenic Kras in the pancreas, mediating cell plasticity, acinar-to-ductal metaplasia (ADM), and pancreatic ductal adenocarcinoma (PDAC) formation. This contrasts with Kras-driven non-small cell lung cancer, where signaling via Craf, but not PDK1, is an essential tumor-initiating event. These in vivo genetic studies together with pharmacologic treatment studies in models of human ADM and PDAC demonstrate tissue-specific differences of oncogenic Kras signaling and define PI3K/PDK1 as a suitable target for therapeutic intervention specifically in PDAC.

Vps34 deficiency reveals the importance of endocytosis for podocyte homeostasis

The molecular mechanisms that maintain podocytes and consequently, the integrity of the glomerular filtration barrier are incompletely understood. Here, we show that the class III phosphoinositide 3-kinase vacuolar protein sorting 34 (Vps34) plays a central role in modulating endocytic pathways, maintaining podocyte homeostasis. In mice, podocyte-specific conditional knockout of Vps34 led to early proteinuria, glomerular scarring, and death within 3-9 weeks of age. Vps34-deficient podocytes exhibited substantial vacuolization and foot process effacement. Although the formation of autophagosomes and autophagic flux were impaired, comparisons between podocyte-specific Vps34-deficient mice, autophagy-deficient mice, and doubly deficient mice suggested that defective autophagy was not primarily responsible for the severe phenotype caused by the loss of Vps34. In fact, Rab5-positive endosomal compartments, endocytosis, and fluid-phase uptake were severely disrupted in Vps34-deficient podocytes. Vps34 deficiency in nephrocytes, the podocyte-like cells of Drosophila melanogaster, resulted in a block between Rab5- and Rab7-positive endosomal compartments. In summary, these data identify Vps34 as a major regulator of endolysosomal pathways in podocytes and underline the fundamental roles of endocytosis and fluid-phase uptake for the maintenance of the glomerular filtration barrier.

Enpp2/Autotaxin in dermal papilla precursors is dispensable for hair follicle morphogenesis

Systematic ablation of previously identified dermal papilla (DP) signature genes in embryonic DP precursors will reveal their functional roles during hair follicle morphogenesis. In this study, we validate Enpp2/Autotaxin as one of the highest expressed signature genes in postnatal DP, and demonstrate specific expression of this lysophosphatidic acid (LPA)-generating enzyme in embryonic dermal condensates. We further identify dermal and epidermal expression of several LPA receptors, suggesting that LPA signaling could contribute to follicle morphogenesis in both mesenchymal and epithelial compartments. We then use the recently characterized Cre-expressing Tbx18 knock-in line to conditionally ablate Enpp2 in embryonic DP precursors. Despite efficient gene knockout in embryonic day 14.5 (E14.5) dermal condensates, morphogenesis proceeds regularly with normal numbers, lengths, and sizes of all hair follicle types, suggesting that Enpp2 is not required for hair follicle formation. To interrogate DP signature gene expression, we finally isolate control and Enpp2-null DP precursors and identify the expression and upregulation of LIPH, an alternative LPA-producing enzyme, suggesting that this gene could functionally compensate for the absence of Enpp2. We conclude that future coablation of both LPA-producing enzymes or of several LPA receptors may reveal the functional role of LPA signaling during hair follicle morphogenesis.

Hopx expression defines a subset of multipotent hair follicle stem cells and a progenitor population primed to give rise to K6+ niche cells

The mammalian hair follicle relies on adult resident stem cells and their progeny to fuel and maintain hair growth throughout the life of an organism. The cyclical and initially synchronous nature of hair growth makes the hair follicle an ideal system with which to define homeostatic mechanisms of an adult stem cell population. Recently, we demonstrated that Hopx is a specific marker of intestinal stem cells. Here, we show that Hopx specifically labels long-lived hair follicle stem cells residing in the telogen basal bulge. Hopx(+) cells contribute to all lineages of the mature hair follicle and to the interfollicular epidermis upon epidermal wounding. Unexpectedly, our analysis identifies a previously unappreciated progenitor population that resides in the lower hair bulb of anagen-phase follicles and expresses Hopx. These cells co-express Lgr5, do not express Shh and escape catagen-induced apoptosis. They ultimately differentiate into the cytokeratin 6-positive (K6) inner bulge cells in telogen, which regulate the quiescence of adjacent hair follicle stem cells. Although previous studies have suggested that K6(+) cells arise from Lgr5-expressing lower outer root sheath cells in anagen, our studies indicate an alternative origin, and a novel role for Hopx-expressing lower hair bulb progenitor cells in contributing to stem cell homeostasis.

N-wasp is required for stabilization of podocyte foot processes

Alteration of cortical actin structures is the common final pathway leading to podocyte foot process effacement and proteinuria. The molecular mechanisms that safeguard podocyte foot process architecture and maintain the three-dimensional actin network remain elusive. Here, we demonstrate that neuronal Wiskott-Aldrich syndrome protein (N-WASP), which promotes actin nucleation, is required to stabilize podocyte foot processes. Mice lacking N-WASP specifically in podocytes were born with normal kidney function but developed significant proteinuria 3 weeks after birth, suggesting an important role for N-WASP in maintaining foot processes. In addition, inducing deletion of N-WASP in adult mice resulted in severe proteinuria and kidney failure. Electron microscopy showed an accumulation of electron-dense patches of actin and strikingly altered morphology of podocyte foot processes. Although basic actin-based processes such as cell migration were not affected, primary cultures of N-WASP-deficient podocytes revealed significant impairment of dynamic actin reorganization events, including the formation of circular dorsal ruffles. Taken together, our findings suggest that N-WASP-mediated actin nucleation of branched microfilament networks is specifically required for the maintenance of foot processes, presumably sustaining the mechanical resistance of the filtration barrier.

Mastermind-like transcriptional co-activator-mediated Notch signaling is indispensable for maintaining conjunctival epithelial identity

Conjunctival goblet cells primarily synthesize mucins to lubricate the ocular surface, which is essential for normal vision. Notch signaling has been known to associate with goblet cell differentiation in intestinal and respiratory tracts, but its function in ocular surface has yet to be fully characterized. Herein, we demonstrate that conditional inhibition of canonical Notch signaling by expressing dominant negative mastermind-like 1 (dnMaml1) in ocular surface epithelia resulted in complete suppression of goblet cell differentiation during and subsequent to development. When compared with the ocular surface of wild-type mice (OS(Wt)), expression of dnMaml1 at the ocular surface (OS(dnMaml1)) caused conjunctival epithelial hyperplasia, aberrant desquamation, failure of Mucin 5ac (Muc5ac) synthesis, subconjunctival inflammation and epidermal metaplasia in cornea. In addition, conditional deletion of Notch1 from the ocular surface epithelia partially recapitulated OS(dnMaml1) phenotypes. We have demonstrated that N1-ICD (Notch1 intracellular domain) transactivated the mouse Krüppel-like factor 4 (Klf) promoter and that Klf4 directly bound to and significantly potentiated the Muc5ac promoter. By contrast, OS(dnMaml1) dampened Klf4 and Klf5 expression, and diminished Muc5ac synthesis. Collectively, these findings indicated that Maml-mediated Notch signaling plays a pivotal role in the initiation and maintenance of goblet cell differentiation for normal ocular surface morphogenesis and homeostasis through regulation of Klf4 and Klf5.

Characterization of the adipocyte cellular lineage in vivo

Mature adipocytes are generated through the proliferation and differentiation of precursor cells. Our prior studies identified adipocyte progenitors in white adipose tissue (WAT) as Lin−:CD29+:CD34+:Sca-1+:CD24+ (CD24+) cells that are capable of generating functional WAT¹. Here, we employ several Cre recombinase mouse models to identify the adipocyte cellular lineage in vivo. While it has been proposed that white adipocytes are derived from endothelial² and hematopoietic^{3, 4} lineages, we find that neither of these lineages label white adipocytes. However, platelet-derived growth factor receptor α (PdgfRα)-Cre trace labels all white adipocytes. Analysis of WAT from PdgfRα-Cre reporter mice identifies CD24+ and Lin−:CD29+:CD34+:Sca-1+:CD24− (CD24−) cells as adipocyte precursors. We show that CD24+ cells generate the CD24− population in vivo and the CD24− cells express late markers of adipogenesis. From these data we propose a model where the CD24+ adipocyte progenitors become further committed to the adipocyte lineage as CD24 expression is lost, generating CD24− preadipocytes. This characterization of the adipocyte cellular lineage will facilitate study of the mechanisms that regulate WAT formation in vivo and WAT mass expansion in obesity.

Jun is required in Isl1-expressing progenitor cells for cardiovascular development

Jun is a highly conserved member of the multimeric activator protein 1 transcription factor complex and plays an important role in human cancer where it is known to be critical for proliferation, cell cycle regulation, differentiation, and cell death. All of these biological functions are also crucial for embryonic development. Although all Jun null mouse embryos die at mid-gestation with persistent truncus arteriosus, a severe cardiac outflow tract defect also seen in human congenital heart disease, the developmental mechanisms are poorly understood. Here we show that murine Jun is expressed in a restricted pattern in several cell populations important for cardiovascular development, including the second heart field, pharyngeal endoderm, outflow tract and atrioventricular endocardial cushions and post-migratory neural crest derivatives. Several genes, including Isl1, molecularly mark the second heart field. Isl1 lineages include myocardium, smooth muscle, neural crest, endocardium, and endothelium. We demonstrate that conditional knockout mouse embryos lacking Jun in Isl1-expressing progenitors display ventricular septal defects, double outlet right ventricle, semilunar valve hyperplasia and aortic arch artery patterning defects. In contrast, we show that conditional deletion of Jun in Tie2-expressing endothelial and endocardial precursors does not result in aortic arch artery patterning defects or embryonic death, but does result in ventricular septal defects and a low incidence of semilunar valve defects, atrioventricular valve defects and double outlet right ventricle. Our results demonstrate that Jun is required in Isl1-expressing progenitors and, to a lesser extent, in endothelial cells and endothelial-derived endocardium for cardiovascular development but is dispensable in both cell types for embryonic survival. These data provide a cellular framework for understanding the role of Jun in the pathogenesis of congenital heart disease.

Remyelination reporter reveals prolonged refinement of spontaneously regenerated myelin

Neurological diseases and trauma often cause demyelination, resulting in the disruption of axonal function and integrity. Endogenous remyelination promotes recovery, but the process is not well understood because no method exists to definitively distinguish regenerated from preexisting myelin. To date, remyelinated segments have been defined as anything abnormally short and thin, without empirical data to corroborate these morphological assumptions. To definitively identify regenerated myelin, we used a transgenic mouse with an inducible membrane-bound reporter and targeted Cre recombinase expression to a subset of glial progenitor cells after spinal cord injury, yielding remarkably clear visualization of spontaneously regenerated myelin in vivo. Early after injury, the mean length of sheaths regenerated by Schwann cells and oligodendrocytes (OLs) was significantly shorter than control, uninjured myelin, confirming past assumptions. However, OL-regenerated sheaths elongated progressively over 6 mo to approach control values. Moreover, OL-regenerated myelin thickness was not significantly different from control myelin at most time points after injury. Thus, many newly formed OL sheaths were neither thinner nor shorter than control myelin, vitiating accepted dogmas of what constitutes regenerated myelin. We conclude that remyelination, once thought to be static, is dynamic and elongates independently of axonal growth, in contrast to stretch-based mechanisms proposed in development. Further, without clear identification, past assessments have underestimated the extent and quality of regenerated myelin.

Generation of islet-like cells from mouse gall bladder by direct ex vivo reprogramming

Cell replacement is an emerging therapy for type 1 diabetes. Pluripotent stem cells have received a lot of attention as a potential source of transplantable β-cells, but their ability to form teratomas poses significant risks. Here, we evaluated the potential of primary mouse gall bladder epithelial cells (GBCs) as targets for ex vivo genetic reprogramming to the β-cell fate. Conditions for robust expansion and genetic transduction of primary GBCs by adenoviral vectors were developed. Using a GFP reporter for insulin, conditions for reprogramming were then optimized. Global expression analysis by RNA-sequencing was used to quantitatively compare reprogrammed GBCs (rGBCs) to true β-cells, revealing both similarities and differences. Adenoviral-mediated expression of NEUROG3, Pdx1, and MafA in GBCs resulted in robust induction of pancreatic endocrine genes, including Ins1, Ins2, Neurod1, Nkx2-2 and Isl1. Furthermore, expression of GBC-specific genes was repressed, including Sox17 and Hes1. Reprogramming was also enhanced by addition of retinoic acid and inhibition of Notch signaling. Importantly, rGBCs were able to engraft long term in vivo and remained insulin-positive for 15weeks. We conclude that GBCs are a viable source for autologous cell replacement in diabetes, but that complete reprogramming will require further manipulations.

Genetic identification of C fibres that detect massage-like stroking of hairy skin in vivo

Stroking of the skin produces pleasant sensations that can occur during social interactions with conspecifics, such as grooming. Despite numerous physiological studies (reviewed in ref. 2), molecularly defined sensory neurons that detect pleasant stroking of hairy skin in vivo have not been reported. Previously, we identified a rare population of unmyelinated sensory neurons in mice that express the G-protein-coupled receptor MRGPRB4 (refs 5, 6). These neurons exclusively innervate hairy skin with large terminal arborizations that resemble the receptive fields of C-tactile (CT) afferents in humans. Unlike other molecularly defined mechanosensory C-fibre subtypes, MRGPRB4(+) neurons could not be detectably activated by sensory stimulation of the skin ex vivo. Therefore, we developed a preparation for calcium imaging in the spinal projections of these neurons during stimulation of the periphery in intact mice. Here we show that MRGPRB4(+) neurons are activated by massage-like stroking of hairy skin, but not by noxious punctate mechanical stimulation. By contrast, a different population of C fibres expressing MRGPRD was activated by pinching but not by stroking, consistent with previous physiological and behavioural data. Pharmacogenetic activation of Mrgprb4-expressing neurons in freely behaving mice promoted conditioned place preference, indicating that such activation is positively reinforcing and/or anxiolytic. These data open the way to understanding the function of MRGPRB4 neurons during natural behaviours, and provide a general approach to the functional characterization of genetically identified subsets of somatosensory neurons in vivo.

Modeling clear cell sarcomagenesis in the mouse: cell of origin differentiation state impacts tumor characteristics

Clear cell sarcoma (CCS) of tendons and aponeuroses is a deadly soft-tissue malignancy resembling melanoma, with a predilection for young adults. EWS-ATF1, the fusion product of a balanced chromosomal translocation between chromosomes 22 and 12, is considered the definitional feature of the tumor. Conditional expression of the EWS-ATF1 human cDNA in the mouse generates CCS-like tumors with 100% penetrance. Tumors, developed through varied means of initiating expression of the fusion oncogene, model human CCS morphologically, immunohistochemically, and by genome-wide expression profiling. We also demonstrate that although fusion oncogene expression in later stages of differentiation can transform mesenchymal progenitor cells and generate tumors resembling CCS generally, expression in cells retaining stem cell markers permits the full melanoma-related phenotype.

An inducible knockout mouse to model the cell-autonomous role of PTEN in initiating endometrial, prostate and thyroid neoplasias

PTEN is one of the most frequently mutated tumor suppressor genes in human cancers. The role of PTEN in carcinogenesis has been validated by knockout mouse models. PTEN heterozygous mice develop neoplasms in multiple organs. Unfortunately, the embryonic lethality of biallelic excision of PTEN has inhibited the study of complete PTEN deletion in the development and progression of cancer. By crossing PTEN conditional knockout mice with transgenic mice expressing a tamoxifen-inducible Cre-ER^T under the control of a chicken actin promoter, we have generated a tamoxifen-inducible mouse model that allows temporal control of PTEN deletion. Interestingly, administration of a single dose of tamoxifen resulted in PTEN deletion mainly in epithelial cells, but not in stromal, mesenchymal or hematopoietic cells. Using the mT/mG double-fluorescent Cre reporter mice, we demonstrate that epithelial-specific PTEN excision was caused by differential Cre activity among tissues and cells types. Tamoxifen-induced deletion of PTEN resulted in extremely rapid and consistent formation of endometrial in situ adenocarcinoma, prostate intraepithelial neoplasia and thyroid hyperplasia. We also analyzed the role of PTEN ablation in other epithelial cells, such as the tubular cells of the kidney, hepatocytes, colonic epithelial cells or bronchiolar epithelium, but those tissues did not exhibit neoplastic growth. Finally, to validate this model as a tool to assay the efficacy of anti-tumor drugs in PTEN deficiency, we administered the mTOR inhibitor everolimus to mice with induced PTEN deletion. Everolimus dramatically reduced the progression of endometrial proliferations and significantly reduced thyroid hyperplasia. This model could be a valuable tool to study the cell-autonomous mechanisms involved in PTEN-loss-induced carcinogenesis and provides a good platform to study the effect of anti-neoplastic drugs on PTEN-negative tumors.

Genetic Cre-loxP assessment of epicardial cell fate using Wt1-driven Cre alleles

RATIONALE

Wt1-Cre-based tools are important reagents for studying epicardial cell fate and gene function.

OBJECTIVE

To better describe the properties of Wt1-Cre-based tools to enhance their use in Cre-loxP-based experiments.

METHODS AND RESULTS

In contrast to recently reported results, we show that constitutive Wt1(GFPCre) in combination with certain Cre-activated reporters can be used to trace (pro) epicardial cell fate. Wt1(CreERT2) can be efficiently induced by tamoxifen administration. We show substantial labeling of coronary endothelial cells when induction is performed at late but not early stages of heart development.

CONCLUSIONS

Wt1-based Cre alleles are useful tools for genetic lineage tracing of epicardial cells and mesothelium of other organs. Using these tools with proper understanding of their properties and limitations enables genetic labeling of epicardial cells and their derivatives.

Dual role for mammalian DNA polymerase ζ in maintaining genome stability and proliferative responses

DNA polymerase ζ (polζ) is critical for bypass of DNA damage and the associated mutagenesis, but also has unique functions in mammals. It is required for embryonic development and for viability of hematopoietic cells, but, paradoxically, skin epithelia appear to survive polζ deletion. We wished to determine whether polζ functions in a tissue-specific manner and how polζ status influences skin tumorigenesis. Mice were produced in which Rev3L (the catalytic subunit of polζ) was deleted in tissues expressing keratin 5. Efficient epidermal deletion of Rev3L was tolerated but led to skin and hair abnormalities, accompanied by evidence of DNA breaks. Unchallenged mice developed tumors in keratin 5-expressing tissues with age, consistent with the chromosomal instability accompanying a polζ defect. Unexpectedly, mice with the Rev3L deletion were much more sensitive to UVB radiation than mice defective in other DNA repair genes. Following irradiation, polζ-defective mice failed to mount skin-regenerative responses and responded to stress by mobilizing melanocytes to the epidermis. However, they did not develop skin tumors after chronic UVB irradiation. To determine the proliferative potential of polζ-deficient skin epithelia, keratinocytes were isolated and examined. These keratinocytes harbored chromosomal gaps and breaks and exhibited a striking proliferation defect. These results can be unified by a model in which slowly dividing cells accumulate replication-associated DNA breaks but otherwise survive Rev3L deletion, but functional polζ is essential for responses requiring rapid proliferation, both in cell culture and in vivo. The results reveal a biological role for mammalian polζ in tolerating DNA damage and enabling proliferative responses in vivo.

Constitutive activation of Gli2 impairs bone formation in postnatal growing mice

Indian hedgehog (Ihh) signaling is indispensable for osteoblast differentiation during endochondral bone development in the mouse embryo. We have previously shown that the Gli2 transcription activator critically mediates Ihh function in osteoblastogenesis. To explore the possibility that activation of Hedgehog (Hh) signaling may enhance bone formation, we generated mice that expressed a constitutively active form of Gli2 in the Osx-lineage cells. Unexpectedly, these mice exhibited severe osteopenia due to a marked decrease in osteoblast number and function, although bone resorption was not affected. Quantitative analyses of the molecular markers indicated that osteoblast differentiation was impaired in the mutant mouse. However, the osteoblast-lineage cells isolated from these mice exhibited more robust osteoblast differentiation than normal in vitro. Similarly, pharmacological stimulation of Hh signaling enhanced osteoblast differentiation from Osx-expressing cells isolated from the wild-type mouse. Thus, even though Hh signaling directly promotes osteoblast differentiation in vitro, constitutive activation of this pathway impairs bone formation in vivo, perhaps through an indirect mechanism.

Conditional loss of kidney microRNAs results in congenital anomalies of the kidney and urinary tract (CAKUT)

MicroRNAs have emerged as essential regulators of gene expression and may play important roles in a variety of human disorders. To understand the role of microRNA-mediated gene regulation in the kidney, we deleted the microRNA-processing enzyme Dicer in developing renal tubules and parts of the ureteric bud in mice. Genetic deletion of Dicer resulted in renal failure and death of the animals at 4-6 weeks of age. Interestingly, the kidneys of microRNA-deficient animals were small due to a reduced number of nephrons and showed massive hydronephrosis due to ureteropelvic junction obstruction. This phenotype is reminiscent of congenital anomalies of the kidney and urinary tract (CAKUT), an important group of human disorders characterized by a combination of renal hypoplasia with congenital abnormalities of the urinary tract. We used metanephric kidney cultures to examine the developmental defects underlying these pathologies. Dicer knockout kidneys showed a significant reduction of tubular branching explaining renal hypoplasia. Moreover, the ureters of these kidneys showed an altered morphology and impaired motility. These functional changes went along with altered expression of smooth muscle actin implying a defect in the differentiation of ureteric smooth muscle cells. In addition, we show the polycystic kidney disease gene Pkd1 to be a target of miR-20 implying that this interaction may contribute to the molecular basis for the cystogenesis in our model. In conclusion, these data demonstrate an essential role for microRNA-dependent gene regulation in mammalian kidney development and suggest that deregulation of microRNAs may underlie CAKUT, the most important group of renal disorders in humans.

Mesothelial cells give rise to hepatic stellate cells and myofibroblasts via mesothelial-mesenchymal transition in liver injury

In many organs, myofibroblasts play a major role in the scarring process in response to injury. In liver fibrogenesis, hepatic stellate cells (HSCs) are thought to transdifferentiate into myofibroblasts, but the origins of both HSCs and myofibroblasts remain elusive. In the developing liver, lung, and intestine, mesothelial cells (MCs) differentiate into specific mesenchymal cell types; however, the contribution of this differentiation to organ injury is unknown. In the present study, using mouse models, conditional cell lineage analysis has demonstrated that MCs expressing Wilms tumor 1 give rise to HSCs and myofibroblasts during liver fibrogenesis. Primary MCs, isolated from adult mouse liver using antibodies against glycoprotein M6a, undergo myofibroblastic transdifferentiation. Antagonism of TGF-β signaling suppresses transition of MCs to mesenchymal cells both in vitro and in vivo. These results indicate that MCs undergo mesothelial-mesenchymal transition and participate in liver injury via differentiation to HSCs and myofibroblasts.

The "replacement hypothesis": corneal stem cell origin epithelia are replaced by limbal stem cell origin epithelia in mouse cornea during maturation

PURPOSE

Previous studies have shown that the stem cells of corneal epithelia are located at the limbal basal layer. Limbal stem cells are believed to be the source of corneal epithelial cell proliferation and differentiation. This study tested the replacement hypothesis, which suggests that corneal stem cell origin epithelia may be replaced by limbal stem cell origin epithelia after 2 weeks of age in mice.

METHODS

The cytokeratin 12 expression pattern in the cornea was examined using K12 IRES-Cre and Cre reporter mice.

RESULTS

Before 2 weeks of age, K12 expression in corneal epithelia showed a mosaic pattern. After 2 weeks of age, centripetal K12 IRES-Cre expression gradually elongated from the limbal area. Around 12 weeks of age, the mosaic expression pattern disappeared from the center of the cornea. Temporal and spatial observations of K12 IRES-Cre expression patterns suggested that the mosaic pattern cells proliferated and amassed at the same position from day 15.5 of the embryonic stage at the latest.

CONCLUSIONS

Therefore, these cells were considered corneal stem cell origin epithelia. In contrast, centripetal pattern cell populations were considered limbal stem cell origin epithelia because they originated from the limbal area and moved to the center of the cornea. These observations suggest that corneal stem cell origin epithelia are replaced by limbal stem cell origin epithelia after 2 weeks of age in mice.

Radial glial neural progenitors regulate nascent brain vascular network stabilization via inhibition of Wnt signaling

Radial glial cells, which are neural stem cells well known for their role in neurogenesis, also play an unexpected role in stabilizing nascent blood vessels in the brain.

The cerebral cortex performs complex cognitive functions at the expense of tremendous energy consumption. Blood vessels in the brain are known to form stereotypic patterns that facilitate efficient oxygen and nutrient delivery. Yet little is known about how vessel development in the brain is normally regulated. Radial glial neural progenitors are well known for their central role in orchestrating brain neurogenesis. Here we show that, in the late embryonic cortex, radial glial neural progenitors also play a key role in brain angiogenesis, by interacting with nascent blood vessels and regulating vessel stabilization via modulation of canonical Wnt signaling. We find that ablation of radial glia results in vessel regression, concomitant with ectopic activation of Wnt signaling in endothelial cells. Direct activation of Wnt signaling also results in similar vessel regression, while attenuation of Wnt signaling substantially suppresses regression. Radial glial ablation and ectopic Wnt pathway activation leads to elevated endothelial expression of matrix metalloproteinases, while inhibition of metalloproteinase activity significantly suppresses vessel regression. These results thus reveal a previously unrecognized role of radial glial progenitors in stabilizing nascent brain vascular network and provide novel insights into the molecular cascades through which target neural tissues regulate vessel stabilization and patterning during development and throughout life.

The brain is an energy-intensive organ that consumes about 10 times as much energy per unit volume as the rest of the body. It therefore requires a highly efficient vascular network for oxygen and nutrient delivery, and as a result compromises in blood vessel networks influence a wide array of brain diseases. Our current understanding is that brain-specific neural cell types are involved in shaping its vascular network, but unfortunately little is known about the cellular or molecular mechanisms involved. Using a mouse genetic model, we have found that radial glial cells, a stem cell type well known for its fundamental role in neural circuit formation, also play an unexpected role in brain vessel development. We find that radial glial cells are essential for the stabilization of newly formed blood vessels in the late embryonic brain, and do so in large part through down-regulating canonical Wnt signaling in endothelial cells (which line the interior surface of blood vessels). These findings provide new insight into how new vessels in the brain are normally stabilized and how this process may be compromised and contribute to diseases.

Quantitative analysis of polarity in 3D reveals local cell coordination in the embryonic mouse heart

Anisotropies that underlie organ morphogenesis have been quantified in 2D, taking advantage of a reference axis. However, morphogenesis is a 3D process and it remains a challenge to analyze cell polarities in 3D. Here, we have designed a novel procedure that integrates multidisciplinary tools, including image segmentation, statistical analyses, axial clustering and correlation analysis. The result is a sensitive and unbiased assessment of the significant alignment of cell orientations in 3D, compared with a random axial distribution. Taking the mouse heart as a model, we validate the procedure at the fetal stage, when cardiomyocytes are known to be aligned. At the embryonic stage, our study reveals that ventricular cells are already coordinated locally. The centrosome-nucleus axes and the cell division axes are biased in a plane parallel to the outer surface of the heart, with a minor transmural component. We show further alignment of these axes locally in the plane of the heart surface. Our method is generally applicable to other sets of vectors or axes in 3D tissues to map the regions where they show significant alignment.

β-catenin in the alveolar epithelium protects from lung fibrosis after intratracheal bleomycin

RATIONALE

Alveolar epithelial cells (AECs) play central roles in the response to lung injury and the pathogenesis of pulmonary fibrosis.

OBJECTIVES

We aimed to determine the role of β-catenin in alveolar epithelium during bleomycin-induced lung fibrosis.

METHODS

Genetically modified mice were developed to selectively delete β-catenin in AECs and were crossed to cell fate reporter mice that express β-galactosidase (βgal) in cells of AEC lineage. Mice were given intratracheal bleomycin (0.04 units) and assessed for AEC death, inflammation, lung injury, and fibrotic remodeling. Mouse lung epithelial cells (MLE12) with small interfering RNA knockdown of β-catenin underwent evaluation for wound closure, proliferation, and bleomycin-induced cytotoxicity.

MEASUREMENTS AND MAIN RESULTS

Increased β-catenin expression was noted in lung parenchyma after bleomycin. Mice with selective deletion of β-catenin in AECs had greater AEC death at 1 week after bleomycin, followed by increased numbers of fibroblasts and enhanced lung fibrosis as determined by semiquantitative histological scoring and total collagen content. However, no differences in lung inflammation or protein levels in bronchoalveolar lavage were noted. In vitro, β-catenin-deficient AECs showed increased bleomycin-induced cytotoxicity as well as reduced proliferation and impaired wound closure. Consistent with these findings, mice with AEC β-catenin deficiency showed delayed recovery after bleomycin.

CONCLUSIONS

β-Catenin in the alveolar epithelium protects against bleomycin-induced fibrosis. Our studies suggest that AEC survival and wound healing are enhanced through β-catenin-dependent mechanisms. Activation of the developmentally important β-catenin pathway in AECs appears to contribute to epithelial repair after epithelial injury.

Conditionally ablated Pten in prostate basal cells promotes basal-to-luminal differentiation and causes invasive prostate cancer in mice

Prostate glands comprise two major epithelial cell types: luminal and basal. Luminal cells have long been considered the cellular origin of prostate cancer (CaP). However, recent evidence from a prostate regeneration assay suggests that prostate basal cells can also give rise to CaP. Here, we characterize Pten-deficient prostate lesions arising from keratin 5-expressing basal cells in a temporally controlled system in mice. Pten-deficient prostate lesions arising from basal cells exhibited luminal phenotypes with higher invasiveness, and the cell fate of Pten-deficient basal cells was traced to neoplastic luminal cells. After temporally ablating Pten in keratin 8-expressing luminal cells, luminal-derived Pten-deficient prostate tumors exhibited slower disease progression, compared with basal-derived tumors, within 13 weeks after Pten ablation. Cellular proliferation was significantly increased in basal-derived versus luminal-derived Pten-deficient prostate lesions. Increased tumor invasion into the smooth muscle layer and aberrantly regulated aggressive signatures (Smad4 and Spp1) were identified exclusively in basal-derived Pten-deficient lesions. Interestingly, p63-expressing cells, which represent basal stem and progenitor cells of basal-derived Pten-deficient prostate lesions, were significantly increased, relative to cells of the luminal-derived prostate lesion. Furthermore, castration did not suppress cellular proliferation of either basal-derived or luminal-derived Pten-deficient prostate tumors. Taken together, our data suggest that, although prostate malignancy can originate from both basal and luminal populations, these two populations differ in aggressive potential.

Visualizing leukocyte trafficking in the living brain with 2-photon intravital microscopy

Intravital imaging of the superficial brain tissue in mice represents a powerful tool for the dissection of the cellular and molecular cues underlying inflammatory and infectious central nervous system (CNS) diseases. We present here a step-by-step protocol that will enable a non-specialist to set up a two-photon brain-imaging model. The protocol offers a two-part approach that is specifically optimized for imaging leukocytes but can be easily adapted to answer varied CNS-related biological questions. The protocol enables simultaneous visualization of fluorescently labeled immune cells, the pial microvasculature and extracellular structures such as collagen fibers at high spatial and temporal resolution. Intracranial structures are exposed through a cranial window, and physiologic conditions are maintained during extended imaging sessions via continuous superfusion of the brain surface with artificial cerebrospinal fluid (aCSF). Experiments typically require 1-2 h of preparation, which is followed by variable periods of immune cell tracking. Our methodology converges the experience of two laboratories over the past 10 years in diseased animal models such as cerebral ischemia, lupus, cerebral malaria, and toxoplasmosis. We exemplify the utility of this protocol by tracking leukocytes in transgenic mice in the pial vessels under steady-state conditions.

Conditional deletion of the Pten gene in the mouse prostate induces prostatic intraepithelial neoplasms at early ages but a slow progression to prostate tumors

The PTEN tumor suppressor gene is frequently inactivated in human prostate cancer. Using Osr1 (odd skipped related 1)-Cre mice, we generated a novel conditional Pten knockout mouse strain, Pten^LoxP:Osr1-Cre. Conditional biallelic and monoallelic Pten knockout mice were viable. Deletion of Pten expression was detected in the prostate of Pten^LoxP/LoxP:Osr1-Cre mice as early as 2 weeks of age. Intriguingly, Pten^LoxP/LoxP:Osr1-Cre mice develop high-grade prostatic intraepithelial neoplasms (PINs) with high penetrance as early as one-month of age, and locally invasive prostatic tumors after 12-months of age. Pten^LoxP/+:Osr1-Cre mice show only mild oncogenic changes after 8-weeks of age. Castration of Pten^LoxP/LoxP:Osr1-Cre mice shows no significant regression of prostate tumors, although a shift of androgen receptor (AR) staining from the nuclei to cytoplasm is observed in Pten null tumor cells of castrated mice. Enhanced Akt activity is observed in Pten null tumor cells of castrated Pten^LoxP/LoxP:Osr1-Cre. This study provides a novel mouse model that can be used to investigate a primary role of Pten in initiating oncogenic transformation in the prostate and to examine other genetic and epigenetic changes that are required for tumor progression in the mouse prostate.

Ric-8a, a guanine nucleotide exchange factor for heterotrimeric G proteins, regulates bergmann glia-basement membrane adhesion during cerebellar foliation

The cerebellum consists of an intricate array of lobules that arises during the process of foliation. Foliation not only increases surface area, but may also facilitate organization of cerebellar neural circuitry. Defects in cerebellar foliation are associated with a number of diseases. Yet, little is known about how foliation, a process involving large-scale and simultaneous movement of several different cell types, is coordinated by cell-cell signaling at the molecular level. Here we show that Ric-8a, a guanine nucleotide exchange factor in the G-protein-coupled receptor pathway, is specifically required in Bergmann glia during cerebellar foliation. We find that ric-8a mutation in mice results in disorganized Bergmann glial scaffolding, defective granule cell migration, and disrupted Purkinje cell positioning. These abnormalities result from primary defects in Bergmann glia since mutations in granule cells do not show similar effects. They first arise during late embryogenesis, at the onset of foliation, when ric-8a mutant Bergmann glia fail to maintain adhesion to the basement membrane specifically at emerging fissures. This suggests that Ric-8a is essential for the enhanced Bergmann glia-basement membrane adhesion required for fissure formation. Indeed, we find that ric-8a-deficient cerebellar glia show decreased affinity for basement membrane components. We also find that weakening Bergmann glia-basement membrane interaction by β1 integrin deletion results in a similar phenotype. These results thus reveal a novel role of Ric-8a in modulating Bergmann glia-basement membrane adhesion during foliation, and provide new insights into the signaling pathways that coordinate cellular movement during cerebellar morphogenesis.

Colony-stimulating factor 1 receptor (CSF1R) signaling in injured neurons facilitates protection and survival

Colony-stimulating factor 1 and IL-34 protect against and partially reverse neurodegeneration in mice in part via promoting CREB signaling.

Colony-stimulating factor 1 (CSF1) and interleukin-34 (IL-34) are functional ligands of the CSF1 receptor (CSF1R) and thus are key regulators of the monocyte/macrophage lineage. We discovered that systemic administration of human recombinant CSF1 ameliorates memory deficits in a transgenic mouse model of Alzheimer’s disease. CSF1 and IL-34 strongly reduced excitotoxin-induced neuronal cell loss and gliosis in wild-type mice when administered systemically before or up to 6 h after injury. These effects were accompanied by maintenance of cAMP responsive element–binding protein (CREB) signaling in neurons rather than in microglia. Using lineage-tracing experiments, we discovered that a small number of neurons in the hippocampus and cortex express CSF1R under physiological conditions and that kainic acid–induced excitotoxic injury results in a profound increase in neuronal receptor expression. Selective deletion of CSF1R in forebrain neurons in mice exacerbated excitotoxin-induced death and neurodegeneration. We conclude that CSF1 and IL-34 provide powerful neuroprotective and survival signals in brain injury and neurodegeneration involving CSF1R expression on neurons.

Developmental stage-specific contribution of LGR5(+) cells to basal and luminal epithelial lineages in the postnatal mammary gland

The leucine-rich repeat-containing heterotrimeric guanine nucleotide-binding protein-coupled receptor 5 (LGR5) has been identified as a marker of cycling stem cells in several epithelial tissues, including small intestine, colon, stomach and hair follicle. To investigate whether LGR5 also marks mammary epithelial stem cells, we performed in situ lineage-tracing studies and mammary gland reconstitutions with LGR5-expressing mammary epithelial cells. Interestingly, the LGR5 progeny population in mammary epithelium switches from the luminal to the myoepithelial compartment during the first 12 days of postnatal development, likely reflecting local changes in Wnt signalling. Together, our findings point to a stage-specific contribution of LGR5-expressing cells to luminal and basal epithelial lineages during postnatal mammary gland development.

Novel ROSA26 Cre-reporter knock-in C57BL/6N mice exhibiting green emission before and red emission after Cre-mediated recombination

The Cre/loxP system is a strategy for controlling temporal and/or spatial gene expression through genome alteration in mice. As successful Cre/loxP genome alteration depends on Cre-driver mice, Cre-reporter mice are essential for validation of Cre gene expression in vivo. In most Cre-reporter mouse strains, although the presence of reporter product indicates the expression of Cre recombinase, it has remained unclear whether a lack of reporter signal indicates either no Cre recombinase expression or insufficient reporter gene promoter activity. We produced a novel ROSA26 knock-in Cre-reporter C57BL/6N strain exhibiting green emission before and red after Cre-mediated recombination, designated as strain R26GRR. Ubiquitous green fluorescence and no red fluorescence were observed in R26GRR mice. To investigate the activation of tdsRed, EGFP-excised R26GRR, R26RR, mice were produced through the crossing of C57BL/6N mice with R26GRR/Ayu1-Cre F1 mice. R26RR mice showed extraordinarily strong red fluorescence in almost all tissues examined, suggesting ubiquitous activation of the second reporter in all tissues after Cre/loxP recombination. Moreover, endothelial cell lineage and pancreatic islet-specific expression of red fluorescence were detected in R26GRR/Tie2-Cre F1 mice and R26GRR /Ins1-Cre F1 mice, respectively. These results indicated that R26GRR mice are a useful novel Cre-reporter mouse strain. In addition, R26GRR mice with a pure C57BL/6N background represent a valuable source of green-to-red photoconvertible cells following Cre/loxP recombination for application in transplantation studies. The R26GRR mouse strain will be available from RIKEN BioResource Center (http://www.brc.riken.jp/lab/animal/en/).

FGF2-induced Ras/Erk MAPK signalling maintains lymphatic endothelial cell identity by up-regulating endothelial cell-specific gene expression and suppressing TGFβ signalling via Smad2

The lymphatic endothelial cell (LEC) fate decision program during development has been revealed. However, the mechanism underlying the maintenance of differentiated LEC identity remains largely unknown. Here, we show that fibroblast growth factor 2 (FGF2) plays a fundamental role in maintaining a differentiated LEC trait. In addition to demonstrating the appearance of alpha-smooth muscle actin (αSMA) expressing LECs in mouse lymphedematous skin in vivo, we found that mouse-immortalized LECs lose their characteristics and undergo endothelial-to-mesenchymal transition (EndMT) when cultured in FGF2-depleted medium. FGF2 depletion acted synergistically with transforming growth factor (TGF) β to induce EndMT. We also found that H-Ras-overexpressing LECs were resistant to EndMT. Ras activation not only upregulated FGF2-induced Erk MAPK activation, but also suppressed TGFβ-induced activation of Smad2 by modulating Smad2 phosphorylation via Erk MAPKs. These results suggest that FGF2 may regulate LEC-specific gene expression and suppress TGFβ signalling in LECs via Smad2 in a Ras/Erk MAP kinase-dependent manner. Taken together, our findings provide a new insight into the FGF2/Ras/Erk MAPK-dependent mechanism that maintains and modulates the LEC trait.

Computer-assisted annotation of murine Sertoli cell small RNA transcriptome

Mammalian genomes encode a large number of small noncoding RNAs (sncRNAs) that play regulatory roles during development and adulthood by affecting gene expression. Several sncRNA species, including microRNAs (miRNAs), piwi-interacting RNAs (piRNAs), endogenous small interfering RNAs (endo-siRNAs), and small nucleolar RNAs (snoRNAs), are abundantly expressed in the testis and required for normal testicular development and spermatogenesis. To evaluate global changes in sncRNA expression, the next-generation sequencing (NGS)-based sncRNA transcriptomic analysis has become routine, because it allows rapid determination of the small RNA transcriptome of a particular testicular cell type. However, annotation of small RNA NGS reads can be challenging due to the volume of reads obtained, which is usually in the millions. Therefore, we developed a computer-assisted sncRNA annotation protocol that could identify not only known sncRNAs but also previously uncharacterized ones. Using this protocol, we annotated NGS reads of a Sertoli cell sncRNA library, and we report to our knowledge the first comprehensive annotation of the sncRNA transcriptome of immature murine Sertoli cells. Moreover, the computer-assisted sncRNA annotation pipeline that we report is applicable for annotating NGS reads derived from other cell types and/or sequencing platforms.

Preparation and delivery of 4-hydroxy-tamoxifen for clonal and polyclonal labeling of cells of the surface ectoderm, skin, and hair follicle

To study the cell behavior during morphogenesis of mouse surface ectoderm, skin, and hair follicles, we (1-3) have developed a new method to temporally induce clones that is based on a tamoxifen-dependent Cre recombinase. The classical protocol consisting in dissolving 4-hydroxy-tamoxifen or tamoxifen in corn oil to perform intraperitoneal (ip) injections (4) is not optimal to control the pharmacokinetic parameters of the induction as it leads to experimental variability in terms of timing and level of induction. We have developed a new protocol that consists in solubilizing 4-OHT or tamoxifen in an aqueous solvent using Cremophor(®) EL (5). This allows for intravenous (iv) and intraperitoneal injections.

Retinol dehydrogenase 10 is indispensible for spermatogenesis in juvenile males

Retinoic acid (RA), an active vitamin A derivative, is essential for mammalian spermatogenesis. Genetic studies have revealed that oxidation of vitamin A to retinal by retinol dehydrogenase 10 (RDH10) is critical for embryonic RA biosynthesis. However, physiological roles of RDH10 in postnatal RA synthesis remain unclear, given that Rdh10 loss-of-function mutations lead to early embryonic lethality. We conducted in vivo genetic studies of Rdh10 in postnatal mouse testes and found that an RDH10 deficiency in Sertoli cells, but not in germ cells, results in a mild germ cell depletion phenotype. A deficiency of RDH10 in both Sertoli and germ cells in juvenile mice results in a blockage of spermatogonial differentiation, similar to that seen in vitamin A-deficient animals. This defect in spermatogenesis arises from a complete deficiency in juvenile testicular RA synthesis and can be rescued by retinoid administration. Thus, in juvenile mice, the primary, but not exclusive, source of RA in the testes is Sertoli cells. In contrast, adult Rdh10-deficient mice exhibit phenotypically normal spermatogenesis, indicating that during development a change occurs in either the cellular source of RA or the retinaldehyde dehydrogenase involved in RA synthesis.

Generation and characterization of a tamoxifen inducible Msx1(CreERT2) knock-in allele

Msx1, a member of the Msx gene family, encodes a homeodomain transcription factor and plays critical roles during mouse development in numerous organs. By homologous recombination, we generated a new Msx1 allele (Msx1(CreERT2) ) in which the CreERT2 fusion protein is produced in place of the endogenous Msx1 protein. Using different reporter mouse strains and appropriate tamoxifen treatments, we show that, in mice bearing the Msx1(CrERT2) allele, CreERT2 is capable to induce loxP genomic recombination specifically in Msx1-expressing cells and that this can be obtained during embryonic development as well as after birth. These results show that this new mouse line can be used for lineage tracing of Msx1-expressing cells and their descendants and, combined with Cre-inducible Msx null alleles, for the analysis of Msx1 and/or Msx2 functions in the Msx1-expressing organs, in a time-dependant manner.

Alveolar type II cells possess the capability of initiating lung tumor development

Identifying cells of tumor origin is a fundamental question in tumor biology. Answers to this central question will not only advance our understanding of tumor initiation and progression but also have important therapeutic implications. In this study, we aimed to uncover the cells of origin of lung adenocarcinoma, a major subtype of non-small cell lung cancer. To this end, we developed new mouse models of lung adenocarcinoma that enabled selective manipulation of gene activity in surfactant associated protein C (SPC)-expressing cells, including alveolar type II cells and bronchioalveolar stem cells (BASCs) that reside at the bronchioalveolar duct junction (BADJ). Our findings showed that activation of oncogenic Kras alone or in combination with the removal of the tumor suppressor p53 in SPC⁺ cells resulted in development of alveolar tumors. Similarly, sustained EGF signaling in SPC⁺ cells led to alveolar tumors. By contrast, BASCs failed to proliferate or produce tumors under these conditions. Importantly, in a mouse strain in which Kras/p53 activity was selectively altered in type II cells but not BASCs, alveolar tumors developed while BADJs retained normal architecture. These results confirm and extend previous findings and support a model in which lung adenocarcinoma can initiate in alveolar type II cells. Our results establish the foundation for elucidating the molecular mechanisms by which lung cancer initiates and progresses in a specific lung cell type.

Protective role of vascular smooth muscle cell PPARγ in angiotensin II-induced vascular disease

AIMS

Vascular peroxisome proliferator-activated receptor γ (PPARγ) activation improves vascular remodelling and endothelial function in hypertensive rodents. The goal of this study was to determine that vascular smooth muscle cell (VSMC) PPARγ exerts a vascular protective role beyond its metabolic effects.

METHODS AND RESULTS

We generated a model of adult inducible VSMC-specific Pparγ inactivation to test the hypothesis that PPARγ counteracts angiotensin (Ang) II-induced vascular remodelling and endothelial dysfunction. Inducible VSMC Pparγ knockout mice were generated by crossing Pparγ floxed mice with mice expressing a tamoxifen-inducible Cre recombinase Smooth muscle (Sm) myosin heavy chain promoter control. Eight-to-ten-week-old SmPparγ(-/-) and control mice were infused with a nonpressor dose of Ang II for 7 days. Blood pressure was unaffected. Mesenteric arteries showed eutrophic remodelling in Ang II-infused control mice and hypertrophic remodelling in Ang II-infused SmPparγ(-/-) mice. Endothelium-dependent relaxation to acetylcholine was reduced in SmPparγ(-/-) mice and further impaired by Ang II infusion, and was unaffected by an inhibitor of NO synthase, suggesting a defect of NO-mediated relaxation. SmPparγ deletion increased the sensitivity to Ang II-induced contraction. SmPparγ(-/-) mice exhibited enhanced Ang II-induced vascular NADPH oxidase activity and adhesion molecule ICAM-1 and chemokine monocyte chemotactic protein-1 expression. The antioxidant Superoxide dismutase 3 expression was decreased by SmPparγ deletion. Ang II infusion increased the expression of CD3 T-cell co-receptor chain δ and decreased Adiponectin in perivascular adipose tissue of SmPparγ(-/-) mice.

CONCLUSION

Inducible Pparγ inactivation in VSMCs exacerbated Ang II-induced vascular remodelling and endothelial dysfunction via enhanced vascular oxidative stress and inflammation, revealing the protective role of VSMC PPARγ in angiotensin II-induced vascular injury.

Light microscopic visualization of podocyte ultrastructure demonstrates oscillating glomerular contractions

Podocytes, the visceral epithelial cells of the kidney glomerulus, elaborate primary and interdigitating secondary extensions to enwrap the glomerular capillaries. A hallmark of podocyte injury is the loss of unique ultrastructure and simplification of the cell shape, called foot process effacement, which is a classic feature of proteinuric kidney disease. Although several key pathways have been identified that control cytoskeletal regulation, actin dynamics, and polarity signaling, studies into the dynamic regulation of the podocyte structure have been hampered by the fact that ultrastructural analyses require electron microscopic imaging of fixed tissue. We developed a new technique that allows for visualization of podocyte foot processes using confocal laser scanning microscopy. The combination of inducible and mosaic expression of membrane-tagged fluorescent proteins in a small subset of podocytes enabled us to acquire light microscopic images of podocyte foot processes in unprecedented detail, even in living podocytes of freshly isolated glomeruli. Moreover, this technique visualized oscillatory glomerular contractions and confirmed the morphometric evaluations obtained in static electron microscopic images of podocyte processes. These data suggest that the new technique will provide an extremely powerful tool for studying the dynamics of podocyte ultrastructure.