An absence of Twist1 results in aberrant cardiac neural crest morphogenesis

Periostin: biology and function in cancer

Periostin (POSTN), a member of the matricellular protein family, is a secreted adhesion-related protein produced in the periosteum and periodontal ligaments. Matricellular proteins are a nonstructural family of extracellular matrix (ECM) proteins that regulate a wide range of biological processes in both normal and pathological conditions. Recent studies have demonstrated the key roles of these ECM proteins in the tumor microenvironment. Furthermore, periostin is an essential regulator of bone and tooth formation and maintenance, as well as cardiac development. Also, periostin interacts with multiple cell-surface receptors, especially integrins, and triggers signals that promote tumor growth. According to recent studies, these signals are implicated in cancer cell survival, epithelial-mesenchymal transition (EMT), invasion, and metastasis. In this review, we will summarize the most current data regarding periostin, its structure and isoforms, expressions, functions, and regulation in normal and cancerous tissues. Emphasis is placed on its association with cancer progression, and also future potential for periostin-targeted therapeutic approaches will be explored.

Single Cell Sequencing Reveals Mechanisms of Persistent Truncus Arteriosus Formation after PDGFRα and PDGFRβ Double Knockout in Cardiac Neural Crest Cells

Persistent truncus arteriosus (PTA) is an uncommon and complex congenital cardiac malformation accounting for about 1.2% of all congenital heart diseases (CHDs), which is caused by a deficiency in the embryonic heart outflow tract’s (OFT) septation and remodeling. PDGFRα and PDGFRβ double knockout (DKO) in cardiac neural crest cells (CNCCs) has been reported to cause PTA, but the underlying mechanisms remain unclear. Here, we constructed a PTA mouse model with PDGFRα and PDGFRβ double knockout in Pax3+ CNCCs and described the condensation failure into OFT septum of CNCC-derived cells due to disturbance of cell polarity in the DKO group. In addition, we further explored the mechanism with single-cell RNA sequencing. We found that two main cell differentiation trajectories into vascular smooth muscle cells (VSMCs) from cardiomyocytes (CMs) and mesenchymal cells (MSs), respectively, were interrupted in the DKO group. The process of CM differentiation into VSMC stagnated in a transitional CM I-like state, which contributed to the failure of OFT remodeling and muscular septum formation. On the other hand, a Penk+ transitional MS II cluster closely related to cell condensation into the OFT septum disappeared, which led to the OFT’s septation absence directly. In conclusion, the disturbance of CNCC-derived cells caused by PDGFRα and PDGFRβ knockout can lead to the OFT septation disorder and the occurrence of PTA.

Epithelial to mesenchymal transition during mammalian neural crest cell delamination

Epithelial to mesenchymal transition (EMT) is a well-defined cellular process that was discovered in chicken embryos and described as "epithelial to mesenchymal transformation" [1]. During EMT, epithelial cells lose their epithelial features and acquire mesenchymal character with migratory potential. EMT has subsequently been shown to be essential for both developmental and pathological processes including embryo morphogenesis, wound healing, tissue fibrosis and cancer [2]. During the past 5 years, interest and study of EMT especially in cancer biology have increased exponentially due to the implied role of EMT in multiple aspects of malignancy such as cell invasion, survival, stemness, metastasis, therapeutic resistance and tumor heterogeneity [3]. Since the process of EMT in embryogenesis and cancer progression shares similar phenotypic changes, core transcription factors and molecular mechanisms, it has been proposed that the initiation and development of carcinoma could be attributed to abnormal activation of EMT factors usually required for normal embryo development. Therefore, developmental EMT mechanisms, whose timing, location, and tissue origin are strictly regulated, could prove useful for uncovering new insights into the phenotypic changes and corresponding gene regulatory control of EMT under pathological conditions. In this review, we initially provide an overview of the phenotypic and molecular mechanisms involved in EMT and discuss the newly emerging concept of epithelial to mesenchymal plasticity (EMP). Then we focus on our current knowledge of a classic developmental EMT event, neural crest cell (NCC) delamination, highlighting key differences in our understanding of NCC EMT between mammalian and non-mammalian species. Lastly, we highlight available tools and future directions to advance our understanding of mammalian NCC EMT.

TWIST1 transcriptionally upregulates complement 3 in glomerular mesangial cells from spontaneously hypertensive rats

We have shown that complement 3 (C3) is upregulated in cardiovascular and renal organs, which induces the synthetic phenotype and exaggerates the growth of mesenchymal cells from spontaneously hypertensive rats (SHRs). However, the mechanisms of the upregulation of C3 have remained unclear. In the present study, we investigated the role of TWIST1, a transcription factor that regulates mesodermal embryogenesis, in the upregulation of C3 in glomerular mesangial cells (GMCs) from SHRs and Wistar-Kyoto (WKY) rats. Immunocytochemical staining and western blot analysis showed that the expression of TWIST1 in GMCs from SHRs was higher than that in GMCs from WKY rats in vivo and in vitro. Real-time PCR analysis showed increases in the expression of Twist1 mRNA with attenuated expression of miR-151-3p in GMCs from SHRs compared to that in cells from WKY rats. Chromatin immunoprecipitation assays showed increases in TWIST1 binding to the C3 promoter in GMCs from SHRs compared to that in cells from WKY rats. Transfection of Twist1 cDNA by a lentiviral vector increased the expression of C3 mRNA in GMCs from WKY rats. TWIST1 siRNA significantly decreased the mRNA expression of C3 and osteopontin in GMCs from SHRs. These results indicate that the increases in TWIST1 expression, attenuation of miR-151-3p, and strong binding of TWIST1 upregulate C3 gene expression in GMCs from SHRs. The enhanced TWIST1-C3 system induces the synthetic phenotype of mesenchymal tissue that may be associated with cardiovascular and renal remodeling in hypertension.

The transcription factor Sox7 modulates endocardiac cushion formation contributed to atrioventricular septal defect through Wnt4/Bmp2 signaling

Cardiac septum malformations account for the largest proportion in congenital heart defects. The transcription factor Sox7 has critical functions in the vascular development and angiogenesis. It is unclear whether Sox7 also contributes to cardiac septation development. We identified a de novo 8p23.1 deletion with Sox7 haploinsufficiency in an atrioventricular septal defect (AVSD) patient using whole exome sequencing in 100 AVSD patients. Then, multiple Sox7 conditional loss-of-function mice models were generated to explore the role of Sox7 in atrioventricular cushion development. Sox7 deficiency mice embryos exhibited partial AVSD and impaired endothelial to mesenchymal transition (EndMT). Transcriptome analysis revealed BMP signaling pathway was significantly downregulated in Sox7 deficiency atrioventricular cushions. Mechanistically, Sox7 deficiency reduced the expressions of Bmp2 in atrioventricular canal myocardium and Wnt4 in endocardium, and Sox7 binds to Wnt4 and Bmp2 directly. Furthermore, WNT4 or BMP2 protein could partially rescue the impaired EndMT process caused by Sox7 deficiency, and inhibition of BMP2 by Noggin could attenuate the effect of WNT4 protein. In summary, our findings identify Sox7 as a novel AVSD pathogenic candidate gene, and it can regulate the EndMT involved in atrioventricular cushion morphogenesis through Wnt4-Bmp2 signaling. This study contributes new strategies to the diagnosis and treatment of congenital heart defects.

TWIST1 and chromatin regulatory proteins interact to guide neural crest cell differentiation

Protein interaction is critical molecular regulatory activity underlining cellular functions and precise cell fate choices. Using TWIST1 BioID-proximity-labeling and network propagation analyses, we discovered and characterized a TWIST-chromatin regulatory module (TWIST1-CRM) in the neural crest cells (NCC). Combinatorial perturbation of core members of TWIST1-CRM: TWIST1, CHD7, CHD8, and WHSC1 in cell models and mouse embryos revealed that loss of the function of the regulatory module resulted in abnormal differentiation of NCCs and compromised craniofacial tissue patterning. Following NCC delamination, low level of TWIST1-CRM activity is instrumental to stabilize the early NCC signatures and migratory potential by repressing the neural stem cell programs. High level of TWIST1 module activity at later phases commits the cells to the ectomesenchyme. Our study further revealed the functional interdependency of TWIST1 and potential neurocristopathy factors in NCC development.

Shaping the head and face during development relies on a complex ballet of molecular signals that orchestrates the movement and specialization of various groups of cells. In animals with a backbone for example, neural crest cells (NCCs for short) can march long distances from the developing spine to become some of the tissues that form the skull and cartilage but also the pigment cells and nervous system.

NCCs mature into specific cell types thanks to a complex array of factors which trigger a precise sequence of binary fate decisions at the right time and place. Amongst these factors, the protein TWIST1 can set up a cascade of genetic events that control how NCCs will ultimately form tissues in the head. To do so, the TWIST1 protein interacts with many other molecular actors, many of which are still unknown.

To find some of these partners, Fan et al. studied TWIST1 in the NCCs of mice and cells grown in the lab. The experiments showed that TWIST1 interacted with CHD7, CHD8 and WHSC1, three proteins that help to switch genes on and off, and which contribute to NCCs moving across the head during development. Further work by Fan et al. then revealed that together, these molecular actors are critical for NCCs to form cells that will form facial bones and cartilage, as opposed to becoming neurons. This result helps to show that there is a trade-off between NCCs forming the face or being part of the nervous system.

One in three babies born with a birth defect shows anomalies of the head and face: understanding the exact mechanisms by which NCCs contribute to these structures may help to better predict risks for parents, or to develop new approaches for treatment.

The heart of the neural crest: cardiac neural crest cells in development and regeneration

Cardiac neural crest cells (cNCCs) are a migratory cell population that stem from the cranial portion of the neural tube. They undergo epithelial-to-mesenchymal transition and migrate through the developing embryo to give rise to portions of the outflow tract, the valves and the arteries of the heart. Recent lineage-tracing experiments in chick and zebrafish embryos have shown that cNCCs can also give rise to mature cardiomyocytes. These cNCC-derived cardiomyocytes appear to be required for the successful repair and regeneration of injured zebrafish hearts. In addition, recent work examining the response to cardiac injury in the mammalian heart has suggested that cNCC-derived cardiomyocytes are involved in the repair/regeneration mechanism. However, the molecular signature of the adult cardiomyocytes involved in this repair is unclear. In this Review, we examine the origin, migration and fates of cNCCs. We also review the contribution of cNCCs to mature cardiomyocytes in fish, chick and mice, as well as their role in the regeneration of the adult heart.

Mis-Expression of a Cranial Neural Crest Cell-Specific Gene Program in Cardiac Neural Crest Cells Modulates HAND Factor Expression, Causing Cardiac Outflow Tract Phenotypes

Congenital heart defects (CHDs) occur with such a frequency that they constitute a significant cause of morbidity and mortality in both children and adults. A significant portion of CHDs can be attributed to aberrant development of the cardiac outflow tract (OFT), and of one of its cellular progenitors known as the cardiac neural crest cells (NCCs). The gene regulatory networks that identify cardiac NCCs as a distinct NCC population are not completely understood. Heart and neural crest derivatives (HAND) bHLH transcription factors play essential roles in NCC morphogenesis. The Hand1^PA/OFT enhancer is dependent upon bone morphogenic protein (BMP) signaling in both cranial and cardiac NCCs. The Hand1^PA/OFT enhancer is directly repressed by the endothelin-induced transcription factors DLX5 and DLX6 in cranial but not cardiac NCCs. This transcriptional distinction offers the unique opportunity to interrogate NCC specification, and to understand why, despite similarities, cranial NCC fate determination is so diverse. We generated a conditionally active transgene that can ectopically express DLX5 within the developing mouse embryo in a Cre-recombinase-dependent manner. Ectopic DLX5 expression represses cranial NCC Hand1^PA/OFT-lacZ reporter expression more effectively than cardiac NCC reporter expression. Ectopic DLX5 expression induces broad domains of NCC cell death within the cranial pharyngeal arches, but minimal cell death in cardiac NCC populations. This study shows that transcription control of NCC gene regulatory programs is influenced by their initial specification at the dorsal neural tube.

MAPK and PI3K signaling: At the crossroads of neural crest development

Receptor tyrosine kinase-mediated growth factor signaling is essential for proper formation and development of the neural crest. The many ligands and receptors implicated in these processes signal through relatively few downstream pathways, frequently converging on the MAPK and PI3K pathways. Despite decades of study, there is still considerable uncertainty about where and when these signaling pathways are required and how they elicit particular responses. This review summarizes our current understanding of growth factor-induced MAPK and PI3K signaling in the neural crest.

Evolution and development of the fish jaw skeleton

The evolution of the jaw represents a key innovation in driving the diversification of vertebrate body plans and behavior. The pharyngeal apparatus originated as gill bars separated by slits in chordate ancestors to vertebrates. Later, with the acquisition of neural crest, pharyngeal arches gave rise to branchial basket cartilages in jawless vertebrates (agnathans), and later bone and cartilage of the jaw, jaw support, and gills of jawed vertebrates (gnathostomes). Major events in the evolution of jaw structure from agnathans to gnathostomes include axial regionalization of pharyngeal elements and formation of a jaw joint. Hox genes specify the anterior-posterior identity of arches, and edn1, dlx, hand2, Jag1b-Notch2 signaling, and Nr2f factors specify dorsal-ventral identity. The formation of a jaw joint, an important step in the transition from an un-jointed pharynx in agnathans to a hinged jaw in gnathostomes involves interaction between nkx3.2, hand2, and barx1 factors. Major events in jaw patterning between fishes and reptiles include changes to elements of the second pharyngeal arch, including a loss of opercular and branchiostegal ray bones and transformation of the hyomandibula into the stapes. Further changes occurred between reptiles and mammals, including the transformation of the articular and quadrate elements of the jaw joint into the malleus and incus of the middle ear. Fossils of transitional jaw phenotypes can be analyzed from a developmental perspective, and there exists potential to use genetic manipulation techniques in extant taxa to test hypotheses about the evolution of jaw patterning in ancient vertebrates. This article is categorized under: Comparative Development and Evolution > Evolutionary Novelties Early Embryonic Development > Development to the Basic Body Plan Comparative Development and Evolution > Body Plan Evolution.

Hand Factors in Cardiac Development

Congenital heart defects account for 1% of infant mortality and 10% of in utero deaths. As the vertebrate embryo develops, multiple tissue types develop in tandem to morphologically pattern the functional heart. Underlying cardiac development is a network of transcription factors known to tightly control these morphological events. Members of the Twist family of basic helix-loop-helix transcription factors, Hand1 and Hand2, are essential to this process. The expression patterns and functional role of Hand factors in neural crest cells, endocardium, myocardium, and epicardium is indicative of their importance during cardiogenesis; however, to date, an extensive understanding of the transcriptional targets of Hand proteins and their overall mechanism of action remain unclear. In this review, we summarize the recent findings that further outline the crucial functions of Hand factors during heart development and in post-natal heart function. Anat Rec, 302:101-107, 2019. © 2018 Wiley Periodicals, Inc.

From Neural Crest Development to Cancer and Vice Versa: How p75NTR and (Pro)neurotrophins Could Act on Cell Migration and Invasion?

The p75 neurotrophin receptor (p75^NTR), also known as low-affinity nerve growth factor, belongs to the tumor necrosis factor family of receptors. p75^NTR is widely expressed in the nervous system during the development, as well as, in the neural crest population, since p75^NTR has been described as ubiquitously expressed and considered as a neural crest marker. Neural crest cells (NCCs) constitute an transient population accurately migrating and invading, with precision, defined sites of the embryo. During migration, NCCs are guided along distinct migratory pathways by specialized molecules present in the extracellular matrix or on the surfaces of those cells. Two main processes direct NCC migration during the development: (1) an epithelial-to-mesenchymal transition and (2) a process known as contact inhibition of locomotion. In adults, p75^NTR remains expressed by NCCs and has been identified in an increasing number of cancer cells. Nonetheless, the regulation of the expression of p75^NTR and the underlying mechanisms in stem cell biology or cancer cells have not yet been sufficiently addressed. The main objective of this review is therefore to analyze elements of our actual knowledge regarding p75^NTR roles during the development (mainly focusing on neural crest development) and see how we can transpose that information from development to cancer (and vice versa) to better understand the link between p75^NTR and cell migration and invasion. In this review, we successively analyzed the molecular mechanisms of p75^NTR when it interacts with several coreceptors and/or effectors. We then analyzed which signaling pathways are the most activated or linked to NCC migration during the development. Regarding cancer, we analyzed the described molecular pathways underlying cancer cell migration when p75^NTR was correlated to cancer cell migration and invasion. From those diverse sources of information, we finally summarized potential molecular mechanisms underlying p75^NTR activation in cell migration and invasion that could lead to new research areas to develop new therapeutic protocols.

The HAND1 frameshift A126FS mutation does not cause hypoplastic left heart syndrome in mice

Aims

To test if a human Hand1 frame shift mutation identified in human samples is causative of hypoplastic left heart syndrome (HLHS).

Methods and results

HLHS is a poorly understood single ventricle congenital heart defect that affects two to three infants in every 10 000 live births. The aetiologies of HLHS are largely unknown. The basic helix-loop-helix transcription factor HAND1 is required for normal heart development. Interrogation of HAND1 sequence from fixed HLHS tissues identified a somatic frame-shift mutation at Alanine 126 (NP_004812.1 p.Ala126Profs13X defined as Hand1A126fs). Hand1A126fs creates a truncated HAND1 protein that predictively functions as dominant negative. To determine if this mutation is causative of HLHS, we engineered a conditional Hand1A126fs mouse allele. Activation of this allele with Nkx2.5Cre results in E14.5 lethality accompanied by cardiac outflow tract and intraventricular septum abnormalities. Using αMHC-Cre or Mef2CAHF-Cre to activate Hand1A126fs results in reduced phenotype and limited viability. Left ventricles of Hand1A126FS mutant mice are not hypoplastic.

Conclusions

Somatically acquired Hand1A126FS mutation is not causative of HLHS. Hand1A126FS mutation does exhibit embryonic lethal cardiac defects that reflect a dominant negative function supporting the critical role of Hand1 in cardiogenesis.

Extraocular Muscle Repair and Regeneration

PURPOSE OF REVIEW

The goal of this review is to summarize the unique regenerative milieu within mature mammalian extraocular muscles (EOMs). This will aid in understanding disease propensity for and sparing of EOMs in skeletal muscle diseases as well as the recalcitrance of the EOM to injury.

RECENT FINDINGS

The EOMs continually remodel throughout life and contain an extremely enriched number of myogenic precursor cells that differ in number and functional characteristics from those in limb skeletal muscle. The EOMs also contain a large population of Pitx2-positive myogenic precursor cells that provide the EOMs with many of their unusual biological characteristics, such as myofiber remodeling and skeletal muscle disease sparing. This environment provides for rapid and efficient remodeling and regeneration after various types of injury. In addition, the EOMs show a remarkable ability to respond to perturbations of single muscles with coordinated changes in the other EOMs that move in the same plane.

SUMMARY

These data will inform Ophthalmologists as they work toward developing new treatments for eye movement disorders, new approaches for repair after nerve or direct EOMs injury, as well as suggest potential explanations for the unusual disease propensity and disease sparing characteristics of human EOM.

Congenital diaphragmatic hernias: from genes to mechanisms to therapies

Congenital diaphragmatic hernias (CDHs) and structural anomalies of the diaphragm are a common class of congenital birth defects that are associated with significant morbidity and mortality due to associated pulmonary hypoplasia, pulmonary hypertension and heart failure. In ∼30% of CDH patients, genomic analyses have identified a range of genetic defects, including chromosomal anomalies, copy number variants and sequence variants. The affected genes identified in CDH patients include transcription factors, such as GATA4, ZFPM2, NR2F2 and WT1, and signaling pathway components, including members of the retinoic acid pathway. Mutations in these genes affect diaphragm development and can have pleiotropic effects on pulmonary and cardiac development. New therapies, including fetal endoscopic tracheal occlusion and prenatal transplacental fetal treatments, aim to normalize lung development and pulmonary vascular tone to prevent and treat lung hypoplasia and pulmonary hypertension, respectively. Studies of the association between particular genetic mutations and clinical outcomes should allow us to better understand the origin of this birth defect and to improve our ability to predict and identify patients most likely to benefit from specialized treatment strategies.

Defective Hand1 phosphoregulation uncovers essential roles for Hand1 in limb morphogenesis

The morphogenesis of the vertebrate limbs is a complex process in which cell signaling and transcriptional regulation coordinate diverse structural adaptations in diverse species. In this study, we examine the consequences of altering Hand1 dimer choice regulation within developing vertebrate limbs. Although Hand1 deletion via the limb-specific Prrx1-Cre reveals a non-essential role for Hand1 in mouse limb morphogenesis, altering Hand1 phosphoregulation, and consequently Hand1 dimerization affinities, results in a severe truncation of proximal-anterior limb elements. Molecular analysis reveals a non-cell-autonomous mechanism that causes widespread cell death within the embryonic limb bud. In addition, we observe changes in proximal-anterior gene regulation, including a reduction in the expression of Irx3, Irx5, Gli3 and Alx4, all of which are upregulated in Hand2 limb conditional knockouts. A reduction of Hand2 and Shh gene dosage improves the integrity of anterior limb structures, validating the importance of the Twist-family bHLH dimer pool in limb morphogenesis.

Summary: Altering Hand1 phosphoregulation, and consequently Hand1 dimerization affinities, results in a severe truncation of anterior-proximal limb elements in mice.

Differential expression pattern of Twist1 in mouse preimplantation embryos suggests its multiple roles during early development

PURPOSE

The purpose of the present study is to understand Twist-related protein 1 (Twist1) spatiotemporal expression patterns and functions during early embryo development.

METHODS

We performed whole-mount double immunofluorescence staining and reverse transcription (RT)-PCR analysis of the Twist1 protein and gene throughout the preimplantation development in mice.

RESULTS

We determined that after compaction, the expression of Twist1 becomes developmentally differentiated and targeted in the inner cells of embryos. In blastocysts at E4.5, uniform staining of the inner cell mass was apparent, and it had been gradually translocated to the nucleus of hatched embryonic cells at E4.75. Furthermore, the effect of potential regulators of Twist on its expression level during blastocyst development was also sought. Accordingly, Twist1 expression appeared to be upregulated in both mRNA and protein level following culture of embryos in the presence of high glucose.

CONCLUSIONS

Our study revealed the dynamic Twist localization within the early stage of embryo. The results are discussed in terms of potential roles of Twist1 in the processes of lineage segregation, hatching, and implantation in post-compaction embryos and in blastocysts.

Exclusion of Dlx5/6 expression from the distal-most mandibular arches enables BMP-mediated specification of the distal cap

Cranial neural crest cells (crNCCs) migrate from the neural tube to the pharyngeal arches (PAs) of the developing embryo and, subsequently, differentiate into bone and connective tissue to form the mandible. Within the PAs, crNCCs respond to local signaling cues to partition into the proximo-distally oriented subdomains that convey positional information to these developing tissues. Here, we show that the distal-most of these subdomains, the distal cap, is marked by expression of the transcription factor Hand1 (H1) and gives rise to the ectomesenchymal derivatives of the lower incisors. We uncover a H1 enhancer sufficient to drive reporter gene expression within the crNCCs of the distal cap. We show that bone morphogenic protein (BMP) signaling and the transcription factor HAND2 (H2) synergistically regulate H1 distal cap expression. Furthermore, the homeodomain proteins distal-less homeobox 5 (DLX5) and DLX6 reciprocally inhibit BMP/H2-mediated H1 enhancer regulation. These findings provide insights into how multiple signaling pathways direct transcriptional outcomes that pattern the developing jaw.

MESP1 Mutations in Patients with Congenital Heart Defects

Identifying the genetic etiology of congenital heart disease (CHD) has been challenging despite being one of the most common congenital malformations in humans. We previously identified a microdeletion in a patient with a ventricular septal defect containing over 40 genes including MESP1 (mesoderm posterior basic helix-loop-helix transcription factor 1). Because of the importance of MESP1 as an early regulator of cardiac development in both in vivo and in vitro studies, we tested for MESP1 mutations in 647 patients with congenital conotruncal and related heart defects. We identified six rare, nonsynonymous variants not seen in ethnically matched controls and one likely race-specific nonsynonymous variant. Functional analyses revealed that three of these variants altered activation of transcription by MESP1. Two of the deleterious variants are located within the conserved HLH domain and thus impair the protein-protein interaction of MESP1 and E47. The third deleterious variant was a loss-of-function frameshift mutation. Our results suggest that pathologic variants in MESP1 may contribute to the development of CHD and that additional protein partners and downstream targets could likewise contribute to the wide range of causes for CHD.

Functional Analysis of Two Novel Mutations in TWIST1 Protein Motifs Found in Ventricular Septal Defect Patients

The aim of this study was to investigate the possible genetic effect of sequence variations in TWIST1 on the pathogenesis of ventricular septal defect in humans. We examined the coding region of TWIST1 in a cohort of 196 Chinese people with non-syndromic ventricular septal defect patients and 200 healthy individuals as the controls. We identified two novel potential disease-associated mutations, NM_000474.3:c.247G>A (G83S) and NM_000474.3:c.283A>G (S95G). Both of them were identified for the first time and were not observed in the 200 controls without congenital heart disease. Using a dual-luciferase reporter assay, we showed that both of the mutations significantly down-regulated the repressive effect of TWIST1 on the E-cadherin promoter. Furthermore, a mammalian two-hybrid assay showed that both of the mutations significantly affected the interaction between TWIST1 and KAT2B. New mutations in the transcription factor TWIST1 that affect protein function were identified in 1.0 % (2/196) of Chinese patients with ventricular septal defect. Our data show, for the first time, that TWIST1 has a potential causative effect on the development of ventricular septal defect.

SOX9 modulates the expression of key transcription factors required for heart valve development

Heart valve formation initiates when endothelial cells of the heart transform into mesenchyme and populate the cardiac cushions. The transcription factor SOX9 is highly expressed in the cardiac cushion mesenchyme, and is essential for heart valve development. Loss of Sox9 in mouse cardiac cushion mesenchyme alters cell proliferation, embryonic survival, and valve formation. Despite this important role, little is known about how SOX9 regulates heart valve formation or its transcriptional targets. Therefore, we mapped putative SOX9 binding sites by ChIP-Seq in E12.5 heart valves, a stage at which the valve mesenchyme is actively proliferating and initiating differentiation. Embryonic heart valves have been shown to express a high number of genes that are associated with chondrogenesis, including several extracellular matrix proteins and transcription factors that regulate chondrogenesis. Therefore, we compared regions of putative SOX9 DNA binding between E12.5 heart valves and E12.5 limb buds. We identified context-dependent and context-independent SOX9-interacting regions throughout the genome. Analysis of context-independent SOX9 binding suggests an extensive role for SOX9 across tissues in regulating proliferation-associated genes including key components of the AP-1 complex. Integrative analysis of tissue-specific SOX9-interacting regions and gene expression profiles on Sox9-deficient heart valves demonstrated that SOX9 controls the expression of several transcription factors with previously identified roles in heart valve development, including Twist1, Sox4, Mecom and Pitx2. Together, our data identify SOX9-coordinated transcriptional hierarchies that control cell proliferation and differentiation during valve formation.

FGF8 signaling sustains progenitor status and multipotency of cranial neural crest-derived mesenchymal cells in vivo and in vitro

The cranial neural crest (CNC) cells play a vital role in craniofacial development and regeneration. They are multi-potent progenitors, being able to differentiate into various types of tissues. Both pre-migratory and post-migratory CNC cells are plastic, taking on diverse fates by responding to different inductive signals. However, what sustains the multipotency of CNC cells and derivatives remains largely unknown. In this study, we present evidence that FGF8 signaling is able to sustain progenitor status and multipotency of CNC-derived mesenchymal cells both in vivo and in vitro. We show that augmented FGF8 signaling in pre-migratory CNC cells prevents cell differentiation and organogenesis in the craniofacial region by maintaining their progenitor status. CNC-derived mesenchymal cells with Fgf8 overexpression or control cells in the presence of exogenous FGF8 exhibit prolonged survival, proliferation, and multi-potent differentiation capability in cell cultures. Remarkably, exogenous FGF8 also sustains the capability of CNC-derived mesenchymal cells to participate in organogenesis such as odontogenesis. Furthermore, FGF8-mediated signaling strongly promotes adipogenesis but inhibits osteogenesis of CNC-derived mesenchymal cells in vitro. Our results reveal a specific role for FGF8 in the maintenance of progenitor status and in fate determination of CNC cells, implicating a potential application in expansion and fate manipulation of CNC-derived cells in stem cell-based craniofacial regeneration.

Decreased mRNA and protein expression of TWIST1 in myocardial tissue of fetuses with ventricular septal defects

Ventricular septal defect (VSD) is the most common type of congenital heart disease (CHD). The single gene mutations or absences that contribute to VSD development are well established; however, the aim of the present study was to measure gene expression variation between VSDs and normal fetal myocardial tissue. TWIST1, an important tumor biomarker, is a basic helix-loop-helix transcription factor that regulates cell proliferation, migration and differentiation in embryonic development and transformed tumor cells. Although growing evidence demonstrates that TWIST1 participates in a variety of human neoplastic diseases, the role of TWIST1 in VSD has remained elusive. Twenty-six VSD fetal myocardial tissue samples and 12 normal samples at matched gestational weeks (22-28 weeks) were included in the present study. Using reverse transcription quantitative polymerase chain reaction (PCR) and real-time PCR, it was demonstrated that TWIST1 mRNA was reduced by almost two-fold in the VSD samples compared with the normal samples. Western blot analysis also revealed that TWIST1 expression was decreased by ~three-fold (P=0.001) in the VSD samples compared with that in the normal samples. Of note, five complete ventricular (also called functionally univentricular or single ventricular) septal ageneses were identified among the specimens. For the five complete ventricular septal agenesis samples, similar results to those for other VSD fetal myocardial tissues were obtained. In conclusion, the results of the present study showed that TWIST1 mRNA and protein levels were reduced in VSDs. The present study was the first, to the best of our knowledge, to report that TWIST1 is not only a tumor biomarker, but may also be involved in the pathogenesis of VSD.

Hand2 is an essential regulator for two Notch-dependent functions within the embryonic endocardium

The basic-helix-loop-helix (bHLH) transcription factor Hand2 plays critical roles during cardiac morphogenesis via expression and function within myocardial, neural crest, and epicardial cell populations. Here, we show that Hand2 plays two essential Notch-dependent roles within the endocardium. Endocardial ablation of Hand2 results in failure to develop a patent tricuspid valve, intraventricular septum defects, and hypotrabeculated ventricles, which collectively resemble the human congenital defect tricuspid atresia. We show endocardial Hand2 to be an integral downstream component of a Notch endocardium-to-myocardium signaling pathway and a direct transcriptional regulator of Neuregulin1. Additionally, Hand2 participates in endocardium-to-endocardium-based cell signaling, with Hand2 mutant hearts displaying an increased density of coronary lumens. Molecular analyses further reveal dysregulation of several crucial components of Vegf signaling, including VegfA, VegfR2, Nrp1, and VegfR3. Thus, Hand2 functions as a crucial downstream transcriptional effector of endocardial Notch signaling during both cardiogenesis and coronary vasculogenesis.

MyoR modulates cardiac conduction by repressing Gata4

The cardiac conduction system coordinates electrical activation through a series of interconnected structures, including the atrioventricular node (AVN), the central connection point that delays impulse propagation to optimize cardiac performance. Although recent studies have uncovered important molecular details of AVN formation, relatively little is known about the transcriptional mechanisms that regulate AV delay, the primary function of the mature AVN. We identify here MyoR as a novel transcription factor expressed in Cx30.2(+) cells of the AVN. We show that MyoR specifically inhibits a Cx30.2 enhancer required for AVN-specific gene expression. Furthermore, we demonstrate that MyoR interacts directly with Gata4 to mediate transcriptional repression. Our studies reveal that MyoR contains two nonequivalent repression domains. While the MyoR C-terminal repression domain inhibits transcription in a context-dependent manner, the N-terminal repression domain can function in a heterologous context to convert the Hand2 activator into a repressor. In addition, we show that genetic deletion of MyoR in mice increases Cx30.2 expression by 50% and prolongs AV delay by 13%. Taken together, we conclude that MyoR modulates a Gata4-dependent regulatory circuit that establishes proper AV delay, and these findings may have wider implications for the variability of cardiac rhythm observed in the general population.

Hand1 phosphoregulation within the distal arch neural crest is essential for craniofacial morphogenesis

In this study we examine the consequences of altering Hand1 phosphoregulation in the developing neural crest cells (NCCs) of mice. Whereas Hand1 deletion in NCCs reveals a nonessential role for Hand1 in craniofacial development and embryonic survival, altering Hand1 phosphoregulation, and consequently Hand1 dimerization affinities, in NCCs results in severe mid-facial clefting and neonatal death. Hand1 phosphorylation mutants exhibit a non-cell-autonomous increase in pharyngeal arch cell death accompanied by alterations in Fgf8 and Shh pathway expression. Together, our data indicate that the extreme distal pharyngeal arch expression domain of Hand1 defines a novel bHLH-dependent activity, and that disruption of established Hand1 dimer phosphoregulation within this domain disrupts normal craniofacial patterning.

Id1 Expression Level Determines the Differentiation of Human Dental Pulp Stem Cells

TWIST1 plays a crucial role in dentinogenesis, and its activity depends on both a dimerization partner selection and phosphorylation. Other factors, like Id proteins, can affect the availability of dimerization partners for TWIST1, subsequently leading to diverse biological outcomes. The purpose of this study was to evaluate an impact of Id1 expression on differentiation of dental pulp stem cells (DPSCs). The altered expression of Id1 was achieved by transfection of human DPSCs with lentiviral vectors either driving an entire sequence of Id1, hence leading to Id1 overexpression, or carrying the Id1 silencing sequence. We observed that both overexpression and silencing of Id1 modulated human DPSC differentiation. Id1 overexpression resulted in a prevailing formation of TWIST1 homodimer and increased expression of genes encoding dentin sialophosphoprotein and dentin matrix protein 1, which confirm an enhanced odontogenic differentiation of DPSCs. Concurrently, Id1 silencing produced an opposite effect, slowing DPSC differentiation. These results highlight Id1 as an important modulator of molecular events during DPSC commitment and differentiation, which should be considered in dental research on tissue engineering. Moreover, we assume that the balance between TWIST1 dimerization forms in DPSCs might function in a cell-type-specific manner.

Long distance multiple-site directed plasmid mutagenesis by one-step PCR using non-overlapped primers

Site-directed mutagenesis is a very important technique in molecular biological researches. We have developed a new method for long distance multiple-site plasmid mutation by one-step PCR using non-overlap primers. These primers were carefully designed and contained desired mutations in the middle of the primers flanked with 18-25 bp of correct sequence. One pair of the primers was able to generate a short megaprimer. Decreases in the concentrations of these primers increased efficiency of the multiple-site plasmid mutation. All of the mutant PCRs were performed at a common annealing temperature at 55 °C. This method could be widely used in all multiple-site plasmid mutations.

Loss of Hand2 in a population of Periostin lineage cells results in pronounced bradycardia and neonatal death

The Periostin Cre (Postn-Cre) lineage includes endocardial and neural crest derived mesenchymal cells of the cardiac cushions, neural crest-derived components of the sympathetic and enteric nervous systems, and cardiac fibroblasts. In this study, we use the Postn-Cre transgenic allele to conditionally ablate Hand2 (H2CKO). We find that Postn-Cre H2CKOs die shortly after birth despite a lack of obvious cardiac structural defects. To ascertain the cause of death, we performed a detailed comparison of the Postn-Cre lineage and Hand2 expression at mid and late stages of embryonic development. Gene expression analyses demonstrate that Postn-Cre ablates Hand2 from the adrenal medulla as well as the sphenopalatine ganglia of the head. In both cases, Hand2 loss-of-function dramatically reduces expression of Dopamine Beta Hydroxylase (Dbh), a gene encoding a crucial catecholaminergic biosynthetic enzyme. Expression of the genes Tyrosine Hydroxylase (Th) and Phenylethanolamine N-methyltransferase (Pnmt), which also encode essential catecholaminergic enzymes, were severely reduced in postnatal adrenal glands. Electrocardiograms demonstrate that 3-day postnatal Postn-Cre H2CKO pups exhibit sinus bradycardia. In conjunction with the aforementioned gene expression analyses, these results strongly suggest that the observed postnatal lethality occurs due to a catecholamine deficiency and subsequent heart failure.

The roles of HLH transcription factors in epithelial mesenchymal transition and multiple molecular mechanisms

Epithelial-to-mesenchymal transition (EMT) is presently recognized as an important event and the initiating stage for tumor invasion and metastasis. Several EMT inducers have been identified, among which the big family of helix-loop-helix (HLH) transcription factors are rising as a novel and promising family of proteins in EMT mediation, such as Twist1, Twist2, E47, and HIFs, etc. Due to the variety and complexities of HLH members, the pathways and mechanisms they employ to promote EMT are also complex and characteristic. In this review, we will discuss the roles of various HLH proteins in the regulation and sustenance of the EMT and multiple cellular mechanisms, attempting to provide a novel and broadened view towards the link between HLH proteins and EMT.

Riding the crest of the wave: parallels between the neural crest and cancer in epithelial-to-mesenchymal transition and migration

The neural crest (NC) is first induced as an epithelial population of cells at the neural plate border requiring complex signaling between bone morphogenetic protein, Wnt, and fibroblast growth factors to differentiate the neural and NC fate from the epidermis. Remarkably, following induction, these cells undergo an epithelial-to-mesenchymal transition (EMT), delaminate from the neural tube, and migrate through various tissue types and microenvironments before reaching their final destination where they undergo terminal differentiation. This process is mirrored in cancer metastasis, where a primary tumor will undergo an EMT before migrating and invading other cell populations to create a secondary tumor site. In recent years, as our understanding of NC EMT and migration has deepened, important new insights into tumorigenesis and metastasis have also been achieved. These discoveries have been driven by the observation that many cancers misregulate developmental genes to reacquire proliferative and migratory states. In this review, we examine how the NC provides an excellent model for studying EMT and migration. These data are discussed from the perspective of the gene regulatory networks that control both NC and cancer cell EMT and migration. Deciphering these processes in a comparative manner will expand our knowledge of the underlying etiology and pathogenesis of cancer and promote the development of novel targeted therapeutic strategies for cancer patients. © 2013 Wiley Periodicals, Inc.

Twist1 controls a cell-specification switch governing cell fate decisions within the cardiac neural crest

Neural crest cells are multipotent progenitor cells that can generate both ectodermal cell types, such as neurons, and mesodermal cell types, such as smooth muscle. The mechanisms controlling this cell fate choice are not known. The basic Helix-loop-Helix (bHLH) transcription factor Twist1 is expressed throughout the migratory and post-migratory cardiac neural crest. Twist1 ablation or mutation of the Twist-box causes differentiation of ectopic neuronal cells, which molecularly resemble sympathetic ganglia, in the cardiac outflow tract. Twist1 interacts with the pro-neural factor Sox10 via its Twist-box domain and binds to the Phox2b promoter to repress transcriptional activity. Mesodermal cardiac neural crest trans-differentiation into ectodermal sympathetic ganglia-like neurons is dependent upon Phox2b function. Ectopic Twist1 expression in neural crest precursors disrupts sympathetic neurogenesis. These data demonstrate that Twist1 functions in post-migratory neural crest cells to repress pro-neural factors and thereby regulate cell fate determination between ectodermal and mesodermal lineages.

During vertebrate development, a unique population of cells, termed neural crest cells, migrates throughout the developing embryo, generating various cell types, for example, the smooth muscle that divides the aorta and pulmonary artery where they connect to the heart, and the autonomic neurons, which coordinate organ function. The distinctions between neural crest cells that will form smooth muscle and those that will become neurons are thought to occur prior to migration. Here, we show that, in mice with mutations of the transcription factor Twist1, a subpopulation of presumptive smooth muscle cells, following migration to the heart, instead mis-specify to resemble autonomic neurons. Twist1 represses transcription of the pro-neural factor Phox2b both through antagonism of its upstream effector, Sox10, and through direct binding to its promoter. Phox2b is absolutely required for autonomic neuron development, and indeed, the aberrant neurons in Twist1 mutants disappear when Phox2b is also mutated. Ectopic Twist1 expression within all neural crest cells disrupts the specification of normal autonomic neurons. Collectively, these data reveal that neural crest cells can alter their cell fate from mesoderm to ectoderm after they have migrated and that Twist1 functions to maintain neural crest cell potency during embryonic development.

Identification of a rudimentary neural crest in a non-vertebrate chordate

Neural crest arises at the neural plate border, expresses a core set of regulatory genes, and produces a diverse array of cell types including ectomesenchyme derivatives that elaborate the vertebrate head^1,2. The evolution of neural crest has been postulated as a key event leading to the appearance of new cell types that fostered the transition from filter feeding to active predation in ancestral vertebrates³. However, the origin of neural crest remains controversial, since homologous cell types have not been unambiguously identified in non-vertebrate chordates^1,4. Here we show that the tunicate Ciona intestinalis possesses a cephalic melanocyte lineage (a9.49) similar to neural crest that can be reprogrammed into migrating ectomesenchyme by the targeted misexpression of Twist. Our results suggest that the neural crest melanocyte regulatory network predated the divergence of tunicates and vertebrates. We propose that the co-option of mesenchyme determinants, such as Twist, into the neural plate ectoderm was crucial for the emergence of the vertebrate “new head”³.

Twist1 transcriptional targets in the developing atrio-ventricular canal of the mouse

Malformations of the cardiovascular system are the most common type of birth defect in humans, frequently affecting the formation of valves and septa. During heart valve and septa formation, cells from the atrio-ventricular canal (AVC) and outflow tract (OFT) regions of the heart undergo an epithelial-to-mesenchymal transformation (EMT) and invade the underlying extracellular matrix to give rise to endocardial cushions. Subsequent maturation of newly formed mesenchyme cells leads to thin stress-resistant leaflets. TWIST1 is a basic helix-loop-helix transcription factor expressed in newly formed mesenchyme cells of the AVC and OFT that has been shown to play roles in cell survival, cell proliferation and differentiation. However, the downstream targets of TWIST1 during heart valve formation remain unclear. To identify genes important for heart valve development downstream of TWIST1, we performed global gene expression profiling of AVC, OFT, atria and ventricles of the embryonic day 10.5 mouse heart by tag-sequencing (Tag-seq). Using this resource we identified a novel set of 939 genes, including 123 regulators of transcription, enriched in the valve forming regions of the heart. We compared these genes to a Tag-seq library from the Twist1 null developing valves revealing significant gene expression changes. These changes were consistent with a role of TWIST1 in controlling differentiation of mesenchymal cells following their transformation from endothelium in the mouse. To study the role of TWIST1 at the DNA level we performed chromatin immunoprecipitation and identified novel direct targets of TWIST1 in the developing heart valves. Our findings support a role for TWIST1 in the differentiation of AVC mesenchyme post-EMT in the mouse, and suggest that TWIST1 can exert its function by direct DNA binding to activate valve specific gene expression.

Twist factor regulation of non-cardiomyocyte cell lineages in the developing heart

The heart is a complex organ that is composed of numerous cell types, which must integrate their programs for proper specification, differentiation and cardiac morphogenesis. During cardiogenesis members of the Twist-family of basic helix-loop-helix (bHLH) transcription factors play distinct roles within cardiac lineages such as the endocardium and extra-cardiac lineages such as the cardiac neural crest (cNCC) and epicardium. While the study of these cell populations is often eclipsed by that of cardiomyocytes, the contributions of non-cardiomyocytes to development and disease are increasingly being appreciated as both dynamic and essential. This review summarizes what is known regarding Twist-family bHLH function in extra-cardiac cell populations and the endocardium, with a focus on regulatory mechanisms, downstream targets, and expression profiles. Improving our understanding of the molecular pathways that Twist-family bHLH factors mediate in these lineages will be necessary to ascertain how their dysfunction leads to congenital disease and adult pathologies such as myocardial infarctions and cardiac fibroblast induced fibrosis. Indeed, this knowledge will prove to be critical to clinicians seeking to improve current treatments.

Specific inactivation of Twist1 in the mandibular arch neural crest cells affects the development of the ramus and reveals interactions with hand2

BACKGROUND

The basic helix-loop-helix (bHLH) transcription factor Twist1 fulfills an essential function in neural crest cell formation, migration, and survival and is associated with the craniosynostic Saethre-Chotzen syndrome in humans. However, its functions during mandibular development, when it may interact with other bHLH transcription factors like Hand2, are unknown because mice homozygous for the Twist1 null mutation die in early embryogenesis. To determine the role of Twist1 during mandibular development, we used the Hand2-Cre transgene to conditionally inactivate the gene in the neural crest cells populating the mandibular pharyngeal arch.

RESULTS

The mutant mice exhibited a spectrum of craniofacial anomalies, including mandibular hypoplasia, altered middle ear development, and cleft palate. It appears that Twist1 is essential for the survival of the neural crest cells involved in the development of the mandibular ramal elements. Twist1 plays a role in molar development and cusp formation by participating in the reciprocal signaling needed for the formation of the enamel knot. This gene is also needed to control the ossification of the mandible, a redundant role shared with Hand2.

CONCLUSION

Twist1, along with Hand2, is essential for the proximodistal patterning and development of the mandible and ossification.

Analysis of neural crest cell fate during cardiovascular development using Cre-activated lacZ/β-galactosidase staining

It is important to identify the mechanisms regulating cardiovascular development. However, complex genetic tools are often required, including transgenic animals that express the lacZ transgene encoding the β-galactosidase enzyme under the control of a specific promoter or following recombination with the Cre recombinase. The latter can be useful for identifying specific cell populations of the developing cardiovascular system, including neural crest cells. The tracking of these cells can help clarify their fate in mutant embryos and elucidate the etiology of some congenital cardiovascular birth defects. This chapter highlights the methods used to stain embryonic tissues in whole mount or sections to detect the expression of the lacZ transgene with a focus on tracking cardiac neural crest cells using the Wnt1-Cre and R26R mouse lines. We also provide a protocol using fluorescence-activated cell sorting for collecting neural crest cells for further analysis. These protocols can be used with any embryos expressing Cre and lacZ.

Twist1 directly regulates genes that promote cell proliferation and migration in developing heart valves

Twist1, a basic helix-loop-helix transcription factor, is expressed in mesenchymal precursor populations during embryogenesis and in metastatic cancer cells. In the developing heart, Twist1 is highly expressed in endocardial cushion (ECC) valve mesenchymal cells and is down regulated during valve differentiation and remodeling. Previous studies demonstrated that Twist1 promotes cell proliferation, migration, and expression of primitive extracellular matrix (ECM) molecules in ECC mesenchymal cells. Furthermore, Twist1 expression is induced in human pediatric and adult diseased heart valves. However, the Twist1 downstream target genes that mediate increased cell proliferation and migration during early heart valve development remain largely unknown. Candidate gene and global gene profiling approaches were used to identify transcriptional targets of Twist1 during heart valve development. Candidate target genes were analyzed for evolutionarily conserved regions (ECRs) containing E-box consensus sequences that are potential Twist1 binding sites. ECRs containing conserved E-box sequences were identified for Twist1 responsive genes Tbx20, Cdh11, Sema3C, Rab39b, and Gadd45a. Twist1 binding to these sequences in vivo was determined by chromatin immunoprecipitation (ChIP) assays, and binding was detected in ECCs but not late stage remodeling valves. In addition identified Twist1 target genes are highly expressed in ECCs and have reduced expression during heart valve remodeling in vivo, which is consistent with the expression pattern of Twist1. Together these analyses identify multiple new genes involved in cell proliferation and migration that are differentially expressed in the developing heart valves, are responsive to Twist1 transcriptional function, and contain Twist1-responsive regulatory sequences.

Hand factors as regulators of cardiac morphogenesis and implications for congenital heart defects

Almost 15 years of careful study have established the related basic Helix-Loop-Helix (bHLH) transcription factors Hand1 and Hand2 as critical for heart development across evolution. Hand factors make broad contributions, revealed through animal models, to the development of multiple cellular lineages that ultimately contribute to the heart. They perform critical roles in ventricular cardiomyocyte growth, differentiation, morphogenesis, and conduction. They are also important for the proper development of the cardiac outflow tract, epicardium, and endocardium. Molecularly, they function both through DNA binding and through protein-protein interactions, which are regulated transcriptionally, posttranscriptionally by microRNAs, and posttranslationally through phosphoregulation. Although direct Hand factor transcriptional targets are progressively being identified, confirmed direct targets of Hand factor transcriptional activity in the heart are limited. Identification of these targets will be critical to model the mechanisms by which Hand factor bHLH interactions affect developmental pathways. Improved understanding of Hand factor-mediated transcriptional cascades will be necessary to determine how Hand factor dysregulation translates to human disease phenotypes. This review summarizes the insight that animal models have provided into the regulation and function of these factors during heart development, in addition to the recent findings that suggest roles for HAND1 and HAND2 in human congenital heart disease.

Phosphorylation of the Twist1-family basic helix-loop-helix transcription factors is involved in pathological cardiac remodeling

Background

The Twist1-family basic helix-loop-helix (bHLH) transcription factors including Twist1, Hand1 and Hand2, play an essential role in heart development and are implicated in pathological heart remodeling. Previously, it was reported that these bHLH transcription factors can be regulated by phosphorylation within the basic-helix I domain, which is involved in developmental processes such as limb formation and trophoblast differentiation. However, how phosphorylation of Twist1 family functions in post-natal heart is elusive.

Principal Findings

Here, we generated transgenic mice with over-expression of Hand1 and Twist1 mutants (to mimic or to abolish phosphorylation) in cardiomyocytes and found pathological cardiac remodeling leading to heart failure and sudden death. Gene expression profile analysis revealed up-regulation of growth-promoting genes and down-regulation of metabolic genes. It is well known that aberrant activation of Akt signaling causes pathological cardiac remodeling and results in heart failure. The basic-helix I domain of Twist1 family members contain Akt substrate consensus motif and may be downstream targets of Akt signaling. Using biochemical analysis, we demonstrated that Hand1 and Twist1 were phosphorylated by Akt in the basic-helix I domain. Phosphorylation of Hand1 regulated its transcriptional activation of luciferase reporter genes and DNA binding ability.

Conclusions

This study provides novel insights into the regulation of Twist1 family in cardiac remodeling and suggests that the Twist1 family can be regulated by Akt signaling.

Analysis of a Hand1 hypomorphic allele reveals a critical threshold for embryonic viability

Loss-of-function analysis of the basic helix-loop-helix (bHLH) transcription factor Hand1 indicates critical roles in development. In an effort to generate a Hand1 cDNA knock-in reporter mouse, we generated two hypomorphic alleles, which extend embryonic survival to between embryonic day (E) 10.5 and E12.5. Heart morphogenesis appears largely normal; however, hypomorphic mice display thin left ventricular myocardium and reduction in pharyngeal mesoderm. Caudal defects, large allantois, and thickened yolk sac are observed and consistent with systemic Hand1 gene deletion. Hand1 mRNA is expressed at 30% of wild-type littermates and known Hand1-dependent genes show intermediate expression compared with wild-type and Hand1 null mice. Interestingly, putative bHLH partners, Hand2 and Twist1, show altered expression in both Hand1 null and hypomorphic backgrounds and intercrossing the Hand1 hypomorphic mice onto the Hand2 systemic null background exacerbates the cardiac and lateral mesoderm phenotypes. Together, these data define a critical threshold of Hand1 expression that is necessary for embryonic survival.

Hand2 loss-of-function in Hand1-expressing cells reveals distinct roles in epicardial and coronary vessel development

RATIONALE

The basic helix-loop-helix (bHLH) transcription factors Hand1 and Hand2 are essential for embryonic development. Given their requirement for cardiogenesis, it is imperative to determine their impact on cardiovascular function.

OBJECTIVE

To deduce the role of Hand2 within the epicardium.

METHOD AND RESULTS

We engineered a Hand1 allele expressing Cre recombinase. Cardiac Hand1 expression is largely limited to cells of the primary heart field, overlapping little with Hand2 expression. Hand1 is expressed within the septum transversum, and the Hand1 lineage marks the proepicardial organ and epicardium. To examine Hand factor functional overlap, we conditionally deleted Hand2 from Hand1-expressing cells. Hand2 mutants display defective epicardialization and fail to form coronary arteries, coincident with altered extracellular matrix deposition and Pdgfr expression.

CONCLUSIONS

These data demonstrate a hierarchal relationship whereby transient Hand1 septum transversum expression defines epicardial precursors that are subsequently dependent on Hand2 function.

Analysis of the Hand1 cell lineage reveals novel contributions to cardiovascular, neural crest, extra-embryonic, and lateral mesoderm derivatives

The basic Helix-Loop-Helix (bHLH) transcription factors Hand1 and Hand2 play critical roles in the development of multiple organ systems during embryogenesis. The dynamic expression patterns of these two factors within developing tissues obfuscate their respective unique and redundant organogenic functions. To define cell lineages potentially dependent upon Hand gene expression, we generated a mutant allele in which the coding region of Hand1 is replaced by Cre recombinase. Subsequent Cre-mediated activation of β-galactosidase or eYFP reporter alleles enabled lineage trace analyses that clearly define the fate of Hand1-expressing cells. Hand1-driven Cre marks specific lineages within the extra embryonic tissues, placenta, sympathetic nervous system, limbs, jaw, and several cell types within the cardiovascular system. Comparisons between Hand1 expression and Hand1-lineage greatly refine our understanding of its dynamic spatial-temporal expression domains and raise the possibility of novel Hand1 functions in structures not thought to be Hand1-dependent.

Preaxial polydactyly: interactions among ETV, TWIST1 and HAND2 control anterior-posterior patterning of the limb

Preaxial polydactyly (PPD) is a common limb-associated birth defect characterized by extra digit(s) in the anterior autopod. It often results from ectopic sonic hedgehog (Shh) expression in the anterior limb bud. Although several transcription factors are known to restrict Shh expression to the posterior limb bud, how they function together remains unclear. Here we provide evidence from mouse conditional knockout limb buds that the bHLH family transcription factor gene Twist1 is required to inhibit Shh expression in the anterior limb bud mesenchyme. More importantly, we uncovered genetic synergism between Twist1 and the ETS family transcription factor genes Etv4 and Etv5 (collectively Etv), which also inhibit Shh expression. Biochemical data suggest that this genetic interaction is a result of direct association between TWIST1 and ETV proteins. Previous studies have shown that TWIST1 functions by forming homodimers or heterodimers with other bHLH factors including HAND2, a key positive regulator of Shh expression. We found that the PPD phenotype observed in Etv mutants is suppressed by a mutation in Hand2, indicative of genetic antagonism. Furthermore, overexpression of ETV proteins influences the dimerization of these bHLH factors. Together, our data suggest that through biochemical interactions, the Shh expression regulators ETV, TWIST1 and HAND2 attain a precise balance to establish anterior-posterior patterning of the limb.

Diversity in the molecular and cellular strategies of epithelium-to-mesenchyme transitions: Insights from the neural crest

Although epithelial to mesenchymal transitions (EMT) are often viewed as a unique event, they are characterized by a great diversity of cellular processes resulting in strikingly different outcomes. They may be complete or partial, massive or progressive, and lead to the complete disruption of the epithelium or leave it intact. Although the molecular and cellular mechanisms of EMT are being elucidated owing chiefly from studies on transformed epithelial cell lines cultured in vitro or from cancer cells, the basis of the diversity of EMT processes remains poorly understood. Clues can be collected from EMT occuring during embryonic development and which affect equally tissues of ectodermal, endodermal or mesodermal origins. Here, based on our current knowledge of the diversity of processes underlying EMT of neural crest cells in the vertebrate embryo, we propose that the time course and extent of EMT do not depend merely on the identity of the EMT transcriptional regulators and their cellular effectors but rather on the combination of molecular players recruited and on the possible coordination of EMT with other cellular processes.

Hand2 regulates chondrogenesis in vitro and in vivo

Hand2 is a transcription factor of the basic helix-loop-helix family that plays essential roles during development. Hand2 determines the anterior-posterior axis during limb development and there is also substantial evidence that Hand2 regulates limb skeletogenesis. However, little is known about how Hand2 might regulate skeletogenesis. Here we show that, in a limb bud micromass culture system, over-expression of Hand2 represses chondrogenesis and the expression of chondrocytic genes, Sox9, type II collagen and aggrecan. Furthermore, we show that Hand2 is induced by the activation of canonical Wnt signaling, which strongly represses chondrogenesis. Surprisingly, Hand2 repressed chondrogenesis in a DNA binding- and dimer formation-independent manner. To examine the in vivo role of Hand2 in mice, we targeted the expression of Hand2 to the cartilage using regulatory elements from the collagen II gene. The resulting transgenic mice displayed a dwarf phenotype, with axial, appendicular and craniofacial skeletal deformities. Hand2 strongly inhibited chondrogenesis in the axial and cranial base skeleton. In the sternum, Hand2 inhibited endochondral ossification by slowing chondrocyte maturation. These data support a model of Hand2 regulating endochondral ossification via at least two steps: (1) determination of the site of chondrogenesis by outlining the region of the future cartilage template and (2) regulation of the rate of chondrocyte maturation.

Transcriptional regulation of heart valve progenitor cells

The development and normal function of the heart valves requires complex interactions among signaling molecules, transcription factors and structural proteins that are tightly regulated in time and space. Here we review the roles of critical transcription factors that are required for specific aspects of normal valve development. The early progenitors of the heart valves are localized in endocardial cushions that express transcription factors characteristic of mesenchyme, including Twist1, Tbx20, Msx1 and Msx2. As the valve leaflets mature, they are composed of complex stratified extracellular matrix proteins that are regulated by the transcriptional functions of NFATc1, Sox9, and Scleraxis. Each of these factors has analogous functions in differentiation of related connective tissue lineages. Together, the precise timing and localized functions of specific transcription factors control cell proliferation, differentiation, elongation, and remodeling processes that are necessary for normal valve structure and function. In addition, there is increasing evidence that these same transcription factors contribute to congenital, as well as degenerative, valve disease.