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Alveologenesis is the culmination of lung development and involves the correct temporal and spatial signals to generate the delicate gas exchange interface required for respiration. Using a Wnt-signaling reporter system, we demonstrate the emergence of a Wnt-responsive alveolar epithelial cell sublineage, which arises during alveologenesis, called the axin2+ alveolar type 2 cell, or AT2Axin2. The number of AT2Axin2 cells increases substantially during late lung development, correlating with a wave of Wnt signaling during alveologenesis. Transcriptome analysis, in vivo clonal analysis, and ex vivo lung organoid assays reveal that AT2sAxin2 promote enhanced AT2 cell growth during generation of the alveolus. Activating Wnt signaling results in the expansion of AT2s, whereas inhibition of Wnt signaling inhibits AT2 cell development and shunts alveolar epithelial development toward the alveolar type 1 cell lineage. These findings reveal a wave of Wnt-dependent AT2 expansion required for lung alveologenesis and maturation.

Postnatal tissue quiescence is thought to be a default state in the absence of a proliferative stimulus such as injury. Although previous studies have demonstrated that certain embryonic developmental programs are reactivated aberrantly in adult organs to drive repair and regeneration, it is not well understood how quiescence is maintained in organs such as the lung, which displays a remarkably low level of cellular turnover. Here we demonstrate that quiescence in the adult lung is an actively maintained state and is regulated by hedgehog signalling. Epithelial-specific deletion of sonic hedgehog (Shh) during postnatal homeostasis in the murine lung results in a proliferative expansion of the adjacent lung mesenchyme. Hedgehog signalling is initially downregulated during the acute phase of epithelial injury as the mesenchyme proliferates in response, but returns to baseline during injury resolution as quiescence is restored. Activation of hedgehog during acute epithelial injury attenuates the proliferative expansion of the lung mesenchyme, whereas inactivation of hedgehog signalling prevents the restoration of quiescence during injury resolution. Finally, we show that hedgehog also regulates epithelial quiescence and regeneration in response to injury via a mesenchymal feedback mechanism. These results demonstrate that epithelial-mesenchymal interactions coordinated by hedgehog actively maintain postnatal tissue homeostasis, and deregulation of hedgehog during injury leads to aberrant repair and regeneration in the lung.

Asthma is one of the most common chronic diseases globally and can be divided into presenting with or without an immune response. Current therapies have little effect on nonimmune disease, and the mechanisms that drive this type of asthma are poorly understood. Here, we have shown that loss of the transcription factors forkhead box P1 (Foxp1) and Foxp4, which are critical for lung epithelial development, in the adult airway epithelium evokes a non-Th2 asthma phenotype that is characterized by airway hyperresponsiveness (AHR) without eosinophilic inflammation. Transcriptome analysis revealed that loss of Foxp1 and Foxp4 expression induces ectopic expression of neuropeptide Y (Npy), which has been reported to be present in the airways of asthma patients, but whose importance in disease pathogenesis remains unclear. Treatment of human lung airway explants with recombinant NPY increased airway contractility. Conversely, loss of Npy in Foxp1- and Foxp4-mutant airway epithelium rescued the AHR phenotype. We determined that NPY promotes AHR through the induction of Rho kinase activity and phosphorylation of myosin light chain, which induces airway smooth muscle contraction. Together, these studies highlight the importance of paracrine signals from the airway epithelium to the underlying smooth muscle to induce AHR and suggest that therapies targeting epithelial induction of this phenotype may prove useful in treatment of noneosinophilic asthma., Introduction The contribution of the immune system and airway smooth muscle to asthma phenotype, including airway hyperresponsiveness (AHR), has been well studied, and most therapies are directed toward inhibiting the [...]

A population of multipotent cardiopulmonary mesoderm progenitors (CPPs) within the posterior pole of the heart expresses Wnt2, Gli1 and Isl1; these CPPs arise from cardiac progenitors before lung development, generate the mesoderm lineages within the cardiac inflow tract and lung, and are regulated by hedgehog expression from the foregut endoderm. Heart and lung linked by shared progenitor The cardiovascular and respiratory systems require precise co-development to form the gas exchange interface for oxygenation of blood, but how do the heart and lung develop together? Using a series of mouse models to perform both cell-fate and loss-of-function analyses, Edward Morrisey and colleagues have now identified a population of multipotent cardiopulmonary progenitors within the posterior pole of the heart that coordinates heart and lung co-development. The authors propose that the heart and lung have evolved an intricate and precise co-developmental process to ensure postnatal viability of terrestrial animals. Co-development of the cardiovascular and pulmonary systems is a recent evolutionary adaption to terrestrial life that couples cardiac output with the gas exchange function of the lung.sup.1. Here we show that the murine pulmonary vasculature develops even in the absence of lung development. We have identified a population of multipotent cardiopulmonary mesoderm progenitors (CPPs) within the posterior pole of the heart that are marked by the expression of Wnt2, Gli1 and Isl1. We show that CPPs arise from cardiac progenitors before lung development. Lineage tracing and clonal analysis demonstrates that CPPs generate the mesoderm lineages within the cardiac inflow tract and lung including cardiomyocytes, pulmonary vascular and airway smooth muscle, proximal vascular endothelium, and pericyte-like cells. CPPs are regulated by hedgehog expression from the foregut endoderm, which is required for connection of the pulmonary vasculature to the heart. Together, these studies identify a novel population of multipotent cardiopulmonary progenitors that coordinates heart and lung co-development that is required for adaptation to terrestrial existence., Author(s): Tien Peng [sup.1] [sup.2] , Ying Tian [sup.1] [sup.3] , Cornelis J. Boogerd [sup.4] , Min Min Lu [sup.3] , Rachel S. Kadzik [sup.5] , Kathleen M. Stewart [sup.1] [...]

Although mutations in Kras are present in 21% of lung tumors, there is a high level of heterogeneity in phenotype and outcome among patients with lung cancer bearing similar mutations, [...]

Ventricular preexcitation, which characterizes Wolff-Parkinson-White syndrome, is caused by the presence of accessory pathways that can rapidly conduct electrical impulses from atria to ventricles, without the intrinsic delay characteristic of the atrioventricular (AV) node. Preexcitation is associated with an increased risk of tachyarrhythmia, palpitations, syncope, and sudden death. Although the pathology and electrophysiology of preexcitation syndromes are well characterized, the developmental mechanisms are poorly understood, and few animal models that faithfully recapitulate the human disorder have been described. Here we show that activation of Notch signaling in the developing myocardium of mice can produce fully penetrant accessory pathways and ventricular preexcitation. Conversely, inhibition of Notch signaling in the developing myocardium resulted in a hypoplastic AV node, with specific loss of slow-conducting cells expressing connexin-30.2 (Cx30.2) and a resulting loss of physiologic AV conduction delay. Taken together, our results suggest that Notch regulates the functional maturation ofAV canal embryonic myocardium during the development of the specialized conduction system. Our results also showthat ventricular preexcitation can arise from inappropriate patterning of the AV canal-derived myocardium., Introduction In the embryonic heart, the atrialmyocardium is separated from ventricular myocardium by a circumferential band of atrioventricular (AV) canal cardiomyocytes that conduct electrical impulses slowly, resulting in AV delay, [...]

Atrial fibrillation is the most common clinical cardiac arrhythmia. It is often initiated by ectopic beats arising from the pulmonary veins and atrium, but the source and mechanism of these beats remains unclear. The melanin synthesis enzyme dopachrome tautomerase (DCT) is involved in intracellular calcium and reactive species regulation in melanocytes. Given that dysregulation of intracellular calcium and reactive species has been described in patients with atrial fibrillation, we investigated the role of DCT in this process. Here, we characterize a unique DCT-expressing cell population within murine and human hearts that populated the pulmonary veins, atria, and atrioventricular canal. Expression profiling demonstrated that this population expressed adrenergic and muscarinic receptors and displayed transcriptional profiles distinct from dermal melanocytes. Adult mice lacking DCT displayed normal cardiac development but an increased susceptibility to atrial arrhythmias. Cultured primary cardiac melanocyte-like cells were excitable, and those lacking DCT displayed prolonged repolarization with early afterdepolarizations. Furthermore, mice with mutations in the tyrosine kinase receptor Kit lacked cardiac melanocyte-like cells and did not develop atrial arrhythmias in the absence of DCT. These data suggest that dysfunction of melanocyte-like cells in the atrium and pulmonary veins may contribute to atrial arrhythmias., Introduction Atrial fibrillation (AF) is the most common cardiac arrhythmia and is associated with significant morbidity (1-4). Most individuals with AF have underlying cardiac disease, but approximately 30%-45% with paroxysmal [...]

Paracrine signaling from lung epithelium to the surrounding mesenchyme is important for lung SMC development and function and is a contributing factor in an array of pulmonary diseases such as bronchopulmonary dysplasia, pulmonary hypertension, and asthma. Wnt7b, which is exclusively expressed in the lung epithelium, is important for lung vascular smooth muscle integrity, but the underlying mechanism by which Wnt signaling regulates lung SMC development is unclear. In this report, we have demonstrated that Wnt7b regulates a program of mesenchymal differentiation in the mouse lung that is essential for SMC development. Genetic loss-of-function studies showed that Wnt7b and [beta]-catenin were required for expression of Pdgfr[alpha] and Pdgfr[beta] and proliferation in pulmonary SMC precursors. In contrast, gain-of-function studies showed that activation of Wnt signaling increased the expression of both Pdgfr[alpha] and Pdgfr[beta] as well as the proliferation of SMC precursors. We further showed that the effect on Pdgfr expression was, in part, mediated by direct transcriptional regulation of the ECM protein tenascin C (Tnc), which was necessary and sufficient for Pdgfr[alpha]/[beta] expression in lung explants. Moreover, this pathway was highly upregulated in a mouse model of asthma and in lung tissue from patients with pulmonary hypertension. Together, these data define a Wnt/Tnc/Pdgfr signaling axis that is critical for smooth muscle development and disease progression in the lung., Introduction The mammalian king is a highly complex organ that develops from the anterior foregut through interactions between the anterior foregut endoderm and the surrounding mesenchyme. Early in development epithelial-mesenchymal [...]

Notch signaling is vital for proper cardiovascular development and function in both humans and animal models. Indeed, mutations in either JAGGED or NOTCH cause congenital heart disease in humans and NOTCH mutations are associated with adult valvular disease. Notch typically functions to mediate developmental interactions between adjacent tissues. Here we show that either absence of the Notch ligand Jagged1 or inhibition of Notch signaling in second heart field tissues results in murine aortic arch artery and cardiac anomalies. In mid-gestation, these mutants displayed decreased Fgf8 and Bmp4 expression. Notch inhibition within the second heart field affected the development of neighboring tissues. For example, faulty migration of cardiac neural crest cells and defective endothelial-mesenchymal transition within the outflow tract endocardial cushions were observed. Furthermore, exogenous Fgf8 was sufficient to rescue the defect in endothelial-mesenchymal transition in explant assays of endocardial cushions following Notch inhibition within second heart field derivatives. These data support a model that relates second heart field, neural crest, and endocardial cushion development and suggests that perturbed Notch-Jagged signaling within second heart field progenitors accounts for some forms of congenital and adult cardiac disease., Introduction Development of the outflow tract (OFT) of the heart is complex, involving input from several embryologically distinct cell populations including cardiac progenitors from the second heart field, cardiac neural [...]

Cerebral cavernous malformations (CCMs) are human vascular malformations caused by mutations in three genes of unknown function: KRIT1, CCM2 and PDCD10. Here we show that the heart of glass (HEG1) receptor, which in zebrafish has been linked to ccm gene function, is selectively expressed in endothelial cells. [Heg1.sup.-/-] mice showed defective integrity of the heart, blood vessels and lymphatic vessels. [Heg1.sup.-/-]; [Ccm2.sup.lacZ/+] and [Ccm2.sup.lacZ/lacZ] mice had more severe cardiovascular defects and died early in development owing to a failure of nascent endothelial cells to associate into patent vessels. This endothelial cell phenotype was shared by zebrafish embryos deficient in heg, krit1 or ccm2 and reproduced in CCM2-deficient human endothelial cells in vitro. Defects in the hearts of zebrafish lacking heg or ccm2, in the aortas of early mouse embryos lacking CCM2 and in the lymphatic vessels of neonatal mice lacking HEG1 were associated with abnormal endothelial cell junctions like those observed in human CCMs. Biochemical and cellular imaging analyses identified a cell-autonomous pathway in which the HEG1 receptor couples to KRIT1 at these cell junctions. This study identifies HEG1-CCM protein signaling as a crucial regulator of heart and vessel formation and integrity., CCMs are a common vascular malformation, with a prevalence of 0.1%-0.5% in the human population (1). CCMs arise primarily in the brain as thin-walled, dilated blood vessels that cause seizures, [...]

Transcription factors regulate tissue patterning and cell fate determination during development; however, expression of early regulators frequently abates upon differentiation, suggesting that they may also play a role in maintaining an undifferentiated phenotype. The transcription factor paired box 3 (Pax3) is expressed by multipotent neural crest precursors and is implicated in neural crest disorders in humans such as Waardenburg syndrome. Pax3 is required for development of multiple neural crest lineages and for activation of lineage-specific programs, yet expression is generally extinguished once neural crest cells migrate from the dorsal neural tube and differentiate. Using a murine Cre-inducible system, we asked whether persistent Pax3 expression in neural crest derivatives would affect development or patterning. We found that persistent expression of Pax3 in cranial neural crest cells resulted in cleft palate, ocular defects, malformation of the sphenoid bone, and perinatal lethality. Furthermore, we demonstrated that Pax3 directly regulates expression of Sostdc1, a soluble inhibitor of bone morphogenetic protein (BMP) signaling. Persistent Pax3 expression renders the cranial crest resistant to BMP-induced osteogenesis. Thus, one mechanism by which Pax3 maintains the undifferentiated state of neural crest mesenchyme may be to block responsiveness to differentiation signals from the environment. These studies provide in vivo evidence for the importance of Pax3 downregulation during differentiation of multipotent neural crest precursors and cranial development., Introduction Among the most intriguing discoveries in developmental biology in the last century has been the identification of powerful developmentally regulated transcription factors that regulate patterning and cell-fate determination during [...]

Myocardin (Myocd) is a potent transcriptional coactivator that has been implicated in cardiovascular development and adaptation of the cardiovascular system to hemodynamic stress. To determine the function of myocardin in the developing cardiovascular system, [Myocd.sup.F/F]/[Wnt1-Cre.sup.+] and [Myocd.sup.F/F]/[Pax3-Cre.sup.+] mice were generated in which the myocardin gene was selectively ablated in neural crest--derived SMCs populating the cardiac outflow tract and great arteries. Both [Myocd.sup.F/F]/[Wnt1-Cre.sup.+] and [Myocd.sup.F/F]/[Pax3-Cre.sup.+] mutant mice survived to birth, but died prior to postnatal day 3 from patent ductus arteriosus (PDA). Neural crest--derived SMCs populating the ductus arteriosus (DA) and great arteries exhibited a cell autonomous block in expression of myocardin-regulated genes encoding SMC-restricted contractile proteins. Moreover, Myocd-deficient vascular SMCs populating the DA exhibited ultrastructural features generally associated with the SMC synthetic, rather than contractile, phenotype. Consistent with these findings, ablation of the Myocd gene in primary aortic SMCs harvested from Myocd conditional mutant mice caused a dramatic decrease in SMC contractile protein expression. Taken together, these data demonstrate that myocardin regulates expression of genes required for the contractile phenotype in neural crest--derived SMCs and provide new insights into the molecular and genetic programs that may underlie PDA., Introduction Congenital heart disease is the most common cause of infantile deaths from birth defects in the United States. One of the most common congenital heart defects, affecting approximately 1 [...]

The anterior heart field (AHF), which contributes to the outflow tract and right ventricle of the heart, is defined in part by expression of the LIM homeobox transcription factor Isl-1. [...]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ydbio.2006.07.048 Byline: Shanru Li (a), Min Min Lu (a), Deying Zhou (a), Stephen R. Hammes (c), Edward E. Morrisey (a)(b) Keywords: Zinc finger; Reproduction; Gonad development; Germ cell; Ovary; Testes Abstract: Mouse gonadal development is regulated by a variety of transcription factors. Here we report the identification and characterization of a novel nuclear zinc finger protein called GATA like protein-1 (GLP-1), which is expressed at high levels in the somatic cells of the developing gonads, including Leydig cells in the testes and granulosa cells in the ovaries. Biochemical analysis of GLP-1 shows that it acts as a transcriptional repressor of GATA factor function. To determine the necessity of GLP-1 in gonadal development, a null allele in mice was generated by replacing all of the coding exons with the bacterial lacZ gene. GLP-1.sup.lacZ null mice are viable with no detectable defects in visceral organ development; however, both males and females are completely infertile. Loss of GLP-1 leads to defective sperm development in males with a marked reduction in mature spermatids observed as early as postnatal week 1. In females, loss of GLP-1 leads to a severe block in germ cell development as early as E17.5. Together, these data identify GLP-1 as a critical nuclear repressor in somatic cells of the gonad that is required for germ cell development, and highlight the importance of somatic-germ cell interactions in the regulation of this critical process. Author Affiliation: (a) Department of Medicine, University of Pennsylvania, Philadelphia, PA 19104, USA (b) Department of Cell and Developmental Biology, University of Pennsylvania, Philadelphia, PA 19104, USA (c) Department of Internal Medicine, Division of Endocrinology and Metabolism, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA Article History: Received 29 March 2006; Revised 7 July 2006; Accepted 28 July 2006

Hop is an unusual homeodomain protein that was first identified in the developing heart where it functions downstream of Nkx2.5 to modulate cardiac gene expression. Hop functions through interactions with histone deacetylase (HDAC) 2 to mediate repression of cardiac-specific genes, and recent studies show that HDAC activity and HDAC2 expression are decreased in people with chronic obstructive pulmonary disease. Here, we show that Hop is expressed in airway epithelium coincident with HDAC2, and expression is induced by the combination of dexamethasone and cAMP in parallel with induction of surfactant protein gene expression. Hop functions in the developing pulmonary airway, acting downstream of Nkx2.1 and GATA6, to negatively regulate surfactant protein expression. Loss of Hop expression in vivo results in defective type 2 pneumocyte development with increased surfactant production and disrupted alveolar formation. Thus Hop represents a novel regulator of pulmonary maturation that is induced by glucocorticoids to mediate functionally important HDAC-dependent negative feedback regulation. homeodomain only protein; histone deacetylase doi:10.1152/ajplung.00385.2005

Branching morphogenesis in the lung serves as a model for the complex patterning that is reiterated in multiple organs throughout development. [beta]-catenin and Wnt signaling mediate critical functions in cell fate specification and differentiation, but specific functions during branching morphogenesis have remained unclear. Here, we show that Wnt/[beta]-catenin signaling regulates proximal distal differentiation of airway epithelium. Inhibition of Wnt/[beta]-catenin signaling, either by expression of Dkk1 or by tissue-specific deletion of [beta]-catenin, results in disruption of distal airway development and expansion of proximal airways. Wnt/[beta]-catenin functions upstream of BMP4, FGF signaling, and Nmyc. Moreover, we show that [beta]-catenin and LEF/TCF activate the promoters of BMP4 and N-myc. Thus, Wnt/[beta]-catenin signaling is a critical upstream regulator of proximal--distal patterning in the lung, in part, through regulation of N-myc, BMP4, and FGF signaling. Keywords: Wnt; [beta]-catenin; Dickkopf-1; BMP; FGF; Lung development

Members of the myocardin-related family of transcription factors play critical roles in regulating vascular smooth muscle and cardiac differentiation. To examine the function of myocardin-related transcription factor (MRTF)-B, mice were generated from ES cells harboring a conditional insertional mutation, or gene trap, of the MRTF-B gene. Expression of the MRTF-B mutant allele results in a fusion protein consisting of the N terminus of MRTF-B fused to [beta]-galactosidase, which is functionally null. Homozygous MRTF-B gene trap mice (MRTF-[B.sup.-/-]) die between embryonic day (E)17.5 and postnatal day 1 from cardiac outflow tract defects. MRTF-B is expressed in the premigratory neural crest, in rhombomeres 3 and 5, and in the neural crest-derived mesenchyme surrounding the aortic arch arteries. Consistent with the pattern of expression, E10.5 and E11.5 MRTF-[B.sup.-/-] mutants exhibit deformation of aortic arch arteries 3, 4, and 6 and severe attenuation of smooth muscle cell differentiation in the arch arteries and the aorticopulmonary septum, despite normal migration and initial patterning of cardiac neural crest cells. Remarkably, the observed pathology was rescued and viable mice generated by intercrossing MRTF-B mutants with mice expressing Cre recombinase under the transcriptional control of the neural crest-restricted Wnt-1 promoter, which results in restoration of normal MRTF-B expression in the neural crest. Taken together, these studies reveal that MRTF-B plays a critical role in regulating differentiation of cardiac neural crest cells into smooth muscle and demonstrate that neural crest-derived smooth muscle differentiation is specifically required for normal cardiovascular morphogenesis. congenital heart disease | myocardin | heart | angiogenesis

Author(s): Deborah Lang [1, 4]; Min Min Lu [1]; Li Huang [1]; Kurt A. Engleka [1]; Maozhen Zhang [1]; Emily Y. Chu [2]; Shari Lipner [3]; Arthur Skoultchi [3]; Sarah [...]

Author(s): Karl-Ludwig Laugwitz [1, 6]; Alessandra Moretti [1, 6]; Jason Lam [1, 6]; Peter Gruber [3]; Yinhong Chen [1]; Sarah Woodard [1]; Li-Zhu Lin [1]; Chen-Leng Cai [1]; Min Min [...]

The bilateral cardiac mesoderm migrates from the lateral region of the embryo to the ventral midline, where it fuses to form the primitive heart tube. It is generally accepted that migration and fusion are essential for subsequent stages of cardiac morphogenesis. We present evidence that, in Foxp4 mutant embryonic mice, each bilateral heart-forming region is capable of developing into a highly differentiated four-chambered mammalian heart in the absence of midline fusion. These data demonstrate that left-right chamber specification, cardiac looping, septation, cardiac myocyte differentiation, and endocardial cushion formation are preprogrammed in the precardiac mesoderm and do not require midline positional identity or heart tube fusion., Although the molecular mechanisms underlying cardiac myocyte differentiation have been extensively examined and initial molecular pathways have been identified, much less is understood about how specified myocardial cells form the [...]

Tbx1 has been implicated as a candidate gene responsible for defective pharyngeal arch remodeling in DiGeorge/Velocardiofacial syndrome. Tbx[1.sup.+/-] mice mimic aspects of the DiGeorge phenotype with variable penetrance, and null mice display severe pharyngeal hypoplasia. Here, we identify enhancer elements in the Tbx1 gene that are conserved through evolution and mediate tissue-specific expression. We describe the generation of transgenic mice that utilize these enhancer elements to direct Cre recombinase expression in endogenous Tbx1 expression domains. We use these Tbx1-Cre mice to fate map Tbx1-expressing precursors and identify broad regions of mesoderm, including early cardiac mesoderm, which are derived from Tbx1-expressing cells. We test the hypothesis that fibroblast growth factor 8 (Fgf8) functions downstream of Tbx1 by performing tissue-specific inactivation of Fgf8 using Tbx1-Cre mice. Resulting newborn mice display DiGeorge-like congenital cardiovascular defects that involve the outflow tract of the heart. Vascular smooth muscle differentiation in the great vessels is disrupted. This data is consistent with a model in which Tbx1 induces Fgf8 expression in the pharyngeal endoderm, which is subsequently required for normal cardiovascular morphogenesis and smooth muscle differentiation in the aorta and pulmonary artery. Keywords: Fgf8; Tbx1-Cre mice; DiGeorge syndrome

The velo-cardio-facial syndrome (VCFS)/DiGeorge syndrome (DGS) is a genetic disorder characterized by phenotypic abnormalities of the derivatives of the pharyngeal arches, including cardiac outflow tract defects. Neural crest cells play a major role in the development of the pharyngeal arches, and defects in these cells are likely responsible for the syndrome. Most patients are hemizygous for a 1.5- to 3.0-Mb region of 22q11, that is suspected to be critical for normal pharyngeal arch development. Mice hemizygous for a 1.5-Mb homologous region of chromosome 16 (Lgdel/+) exhibit conotruncal cardiac defects similar to those seen in affected VCFS/DGS patients. To investigate the role of Lgdel genes in neural crest development, we fate mapped neural crest cells in Lgdel/+ mice and we performed hemizygous neural crest-specific inactivation of Lgdel. Hemizygosity of the Lgdel region does not eliminate cardiac neural crest migration to the forming aortic arches. However, neural crest cells do not differentiate appropriately into smooth muscle in both fourth and sixth aortic arches and the affected aortic arch segments develop abnormally. Tissue-specific hemizygous inactivation of Lgdel genes in neural crest results in normal cardiovascular development. Based on our studies, we propose that Lgdel genes are required for the expression of soluble signals that regulate neural crest cell differentiation. Key Words: Tbx1; pharyngeal arches; aortic arches; smooth muscle; DiGeorge syndrome.

Research has been conducted on the homeodomain protein Hop. Inactivation of Hop in mice via homologous recombination has produced partially penetrant embryonic lethal phenotype with developmental cardiac defects.

The characterization of two distinct classes of hematopoietic stem cells based on CD34 expression and the ability of human bone marrow (BM) cells to differentiate into nonhematopoietic cells introduced new levels of complexity within the stem cell compartment. Here we report the identification and purification of a rare human stem cell population with hematopoietic and hepatic potential based on the expression of a receptor for the complement molecule C1q (C1q[R.sub.p]). We show that Clq[R.sub.p] is a positive marker of all BM-repopulating stem cells because it is expressed on both C[D34.sup.-] and C[D34.sup.+] stem cells from umbilical cord blood and adult BM. In addition, we show that highly purified lineage-negative C[D45.sup.+]C[D38.sup.-]C[D34.sup.+or-]C1q[R.sup.+.sub.p] cells not only have BM-repopulating capacity but also can differentiate into human hepatocytes in vivo. The identification of human hepatocytes in mouse livers indicates that the NOD/SCID (nonobese diabetic/severe combined immunodeficient) mouse model can be a valuable tool to study the differentiation potential of adult human stem cells. These findings may have important scientific and clinical implications in the field of human stem cell biology and transplantation.

Research has been conducted on the velo-cardio-facial/DiGeorge syndrome characterized by the phenotypic features such as cardiovascular defects. The cre-loxP strategy in generating the mice which are hemizygous for the deletion has been used to investigate the etiology of the cardiovascular disorder and the results are presented.