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

The role of heart and neural crest derivatives-expressed protein factors in pregnancy

Heart and neural crest derivatives-expressed protein 1 (HAND1) and Heart and neural crest derivatives-expressed protein 2 (HAND2), members of the Twist-family of basic Helix-Loop-Helix (bHLH) proteins, act as critical transcription factors that play a key role in various developmental processes, including placental development and fetal growth during pregnancy. This review aims to explore the current understanding of HAND1 and HAND2 in pregnant maintenance and their potential implications for maternal and fetal health. We will summarize the mechanisms of action of HAND1 and HAND2 in pregnancy, their expression regulation and association with pregnancy complications such as preterm birth and preeclampsia. Furthermore, we will discuss the potential therapeutic implications of targeting HAND1 and HAND2 in pregnancy-related disorders. This review highlights the importance of HAND1 and HAND2 in pregnancy and their potential as targets for future research and therapeutic interventions of gestational disorders.

Temporally resolved early bone morphogenetic protein-driven transcriptional cascade during human amnion specification

Amniogenesis, a process critical for continuation of healthy pregnancy, is triggered in a collection of pluripotent epiblast cells as the human embryo implants. Previous studies have established that bone morphogenetic protein (BMP) signaling is a major driver of this lineage specifying process, but the downstream BMP-dependent transcriptional networks that lead to successful amniogenesis remain to be identified. This is, in part, due to the current lack of a robust and reproducible model system that enables mechanistic investigations exclusively into amniogenesis. Here, we developed an improved model of early amnion specification, using a human pluripotent stem cell-based platform in which the activation of BMP signaling is controlled and synchronous. Uniform amniogenesis is seen within 48 hr after BMP activation, and the resulting cells share transcriptomic characteristics with amnion cells of a gastrulating human embryo. Using detailed time-course transcriptomic analyses, we established a previously uncharacterized BMP-dependent amniotic transcriptional cascade, and identified markers that represent five distinct stages of amnion fate specification; the expression of selected markers was validated in early post-implantation macaque embryos. Moreover, a cohort of factors that could potentially control specific stages of amniogenesis was identified, including the transcription factor TFAP2A. Functionally, we determined that, once amniogenesis is triggered by the BMP pathway, TFAP2A controls the progression of amniogenesis. This work presents a temporally resolved transcriptomic resource for several previously uncharacterized amniogenesis states and demonstrates a critical intermediate role for TFAP2A during amnion fate specification.

Temporally resolved early BMP-driven transcriptional cascade during human amnion specification

Amniogenesis, a process critical for continuation of healthy pregnancy, is triggered in a collection of pluripotent epiblast cells as the human embryo implants. Previous studies have established that BMP signaling is a major driver of this lineage specifying process, but the downstream BMP-dependent transcriptional networks that lead to successful amniogenesis remain to be identified. This is, in part, due to the current lack of a robust and reproducible model system that enables mechanistic investigations exclusively into amniogenesis. Here, we developed an improved model of early amnion specification, using a human pluripotent stem cell-based platform in which the activation of BMP signaling is controlled and synchronous. Uniform amniogenesis is seen within 48 hours after BMP activation, and the resulting cells share transcriptomic characteristics with amnion cells of a gastrulating human embryo. Using detailed time-course transcriptomic analyses, we established a previously uncharacterized BMP-dependent amniotic transcriptional cascade, and identified markers that represent five distinct stages of amnion fate specification; the expression of selected markers was validated in early post-implantation macaque embryos. Moreover, a cohort of factors that could potentially control specific stages of amniogenesis was identified, including the transcription factor TFAP2A. Functionally, we determined that, once amniogenesis is triggered by the BMP pathway, TFAP2A controls the progression of amniogenesis. This work presents a temporally resolved transcriptomic resource for several previously uncharacterized amniogenesis states and demonstrates a critical intermediate role for TFAP2A during amnion fate specification.

In vitro induction of patterned branchial arch-like aggregate from human pluripotent stem cells

Early patterning of neural crest cells (NCCs) in the craniofacial primordium is important for subsequent development of proper craniofacial structures. However, because of the complexity of the environment of developing tissues, surveying the early specification and patterning of NCCs is difficult. In this study, we develop a simplified in vitro 3D model using human pluripotent stem cells to analyze the early stages of facial development. In this model, cranial NCC-like cells spontaneously differentiate from neural plate border-like cells into maxillary arch-like mesenchyme after a long-term culture. Upon the addition of EDN1 and BMP4, these aggregates are converted into a mandibular arch-like state. Furthermore, temporary treatment with EDN1 and BMP4 induces the formation of spatially separated domains expressing mandibular and maxillary arch markers within a single aggregate. These results suggest that this in vitro model is useful for determining the mechanisms underlying cell fate specification and patterning during early facial development.

Loss of Neogenin alters branchial arch development and leads to craniofacial skeletal defects

The formation of complex structures, such as the craniofacial skeleton, requires precise and intricate two-way signalling between populations of cells of different embryonic origins. For example, the lower jaw, or mandible, arises from cranial neural crest cells (CNCCs) in the mandibular portion of the first branchial arch (mdBA1) of the embryo, and its development is regulated by signals from the ectoderm and cranial mesoderm (CM) within this structure. The molecular mechanisms underlying CM cell influence on CNCC development in the mdBA1 remain poorly defined. Herein we identified the receptor Neogenin as a key regulator of craniofacial development. We found that ablation of Neogenin expression via gene-targeting resulted in several craniofacial skeletal defects, including reduced size of the CNCC-derived mandible. Loss of Neogenin did not affect the formation of the mdBA1 CM core but resulted in altered Bmp4 and Fgf8 expression, increased apoptosis, and reduced osteoblast differentiation in the mdBA1 mesenchyme. Reduced BMP signalling in the mdBA1 of Neogenin mutant embryos was associated with alterations in the gene regulatory network, including decreased expression of transcription factors of the Hand, Msx, and Alx families, which play key roles in the patterning and outgrowth of the mdBA1. Tissue-specific Neogenin loss-of-function studies revealed that Neogenin expression in mesodermal cells contributes to mandible formation. Thus, our results identify Neogenin as a novel regulator of craniofacial skeletal formation and demonstrates it impinges on CNCC development via a non-cell autonomous mechanism.

Single cell evaluation of endocardial Hand2 gene regulatory networks reveals HAND2-dependent pathways that impact cardiac morphogenesis

The transcription factor HAND2 plays essential roles during cardiogenesis. Hand2 endocardial deletion (H2CKO) results in tricuspid atresia or double inlet left ventricle with accompanying intraventricular septum defects, hypo-trabeculated ventricles and an increased density of coronary lumens. To understand the regulatory mechanisms of these phenotypes, single cell transcriptome analysis of mouse E11.5 H2CKO hearts was performed revealing a number of disrupted endocardial regulatory pathways. Using HAND2 DNA occupancy data, we identify several HAND2-dependent enhancers, including two endothelial enhancers for the shear-stress master regulator KLF2. A 1.8 kb enhancer located 50 kb upstream of the Klf2 TSS imparts specific endothelial/endocardial expression within the vasculature and endocardium. This enhancer is HAND2-dependent for ventricular endocardium expression but HAND2-independent for Klf2 vascular and valve expression. Deletion of this Klf2 enhancer results in reduced Klf2 expression within ventricular endocardium. These data reveal that HAND2 functions within endocardial gene regulatory networks including shear-stress response.

TGF-β signaling and Creb5 cooperatively regulate Fgf18 to control pharyngeal muscle development

The communication between myogenic cells and their surrounding connective tissues is indispensable for muscle morphogenesis. During late embryonic development in mice, myogenic progenitors migrate to discrete sites to form individual muscles. The detailed mechanism of this process remains unclear. Using mouse levator veli palatini (LVP) development as a model, we systematically investigated how a distinct connective tissue subpopulation, perimysial fibroblasts, communicates with myogenic cells to regulate mouse pharyngeal myogenesis. Using single-cell RNAseq data analysis, we identified that TGF-β signaling is a key regulator for the perimysial fibroblasts. Loss of TGF-β signaling in the neural crest-derived palatal mesenchyme leads to defects in perimysial fibroblasts and muscle malformation in the soft palate in Osr2Cre;Tgfbr1fl/fl mice. In particular, Creb5, a transcription factor expressed in the perimysial fibroblasts, cooperates with TGF-β signaling to activate expression of Fgf18. Moreover, Fgf18 supports pharyngeal muscle development in vivo and exogenous Fgf18 can partially rescue myogenic cell numbers in Osr2Cre;Tgfbr1fl/fl samples, illustrating that TGF-β-regulated Fgf18 signaling is required for LVP development. Collectively, our findings reveal the mechanism by which TGF-β signaling achieves its functional specificity in defining the perimysial-to-myogenic signals for pharyngeal myogenesis.

Bmp Signaling Regulates Hand1 in a Dose-Dependent Manner during Heart Development

The bone morphogenetic protein (Bmp) signaling pathway and the basic helix–loop–helix (bHLH) transcription factor Hand1 are known key regulators of cardiac development. In this study, we investigated the Bmp signaling regulation of Hand1 during cardiac outflow tract (OFT) development. In Bmp2 and Bmp4loss-of-function embryos with varying levels of Bmp in the heart, Hand1 is sensitively decreased in response to the dose of Bmp expression. In contrast, Hand1 in the heart is dramatically increased in Bmp4 gain-of-function embryos. We further identified and characterized the Bmp/Smad regulatory elements in Hand1. Combined transfection assays and chromatin immunoprecipitation (ChIP) experiments indicated that Hand1 is directly activated and bound by Smads. In addition, we found that upon the treatment of Bmp2 and Bmp4, P19 cells induced Hand1 expression and favored cardiac differentiation. Together, our data indicated that the Bmp signaling pathway directly regulates Hand1 expression in a dose-dependent manner during heart development.

GATA3 is essential for separating patterning domains during facial morphogenesis

Neural crest cells (NCCs) within the mandibular and maxillary prominences of the first pharyngeal arch are initially competent to respond to signals from either region. However, mechanisms that are only partially understood establish developmental tissue boundaries to ensure spatially correct patterning. In the 'hinge and caps' model of facial development, signals from both ventral prominences (the caps) pattern the adjacent tissues whereas the intervening region, referred to as the maxillomandibular junction (the hinge), maintains separation of the mandibular and maxillary domains. One cap signal is GATA3, a member of the GATA family of zinc-finger transcription factors with a distinct expression pattern in the ventral-most part of the mandibular and maxillary portions of the first arch. Here, we show that disruption of Gata3 in mouse embryos leads to craniofacial microsomia and syngnathia (bony fusion of the upper and lower jaws) that results from changes in BMP4 and FGF8 gene regulatory networks within NCCs near the maxillomandibular junction. GATA3 is thus a crucial component in establishing the network of factors that functionally separate the upper and lower jaws during development.

The Mandibular and Hyoid Arches-From Molecular Patterning to Shaping Bone and Cartilage

The mandibular and hyoid arches collectively make up the facial skeleton, also known as the viscerocranium. Although all three germ layers come together to assemble the pharyngeal arches, the majority of tissue within viscerocranial skeletal components differentiates from the neural crest. Since nearly one third of all birth defects in humans affect the craniofacial region, it is important to understand how signalling pathways and transcription factors govern the embryogenesis and skeletogenesis of the viscerocranium. This review focuses on mouse and zebrafish models of craniofacial development. We highlight gene regulatory networks directing the patterning and osteochondrogenesis of the mandibular and hyoid arches that are actually conserved among all gnathostomes. The first part of this review describes the anatomy and development of mandibular and hyoid arches in both species. The second part analyses cell signalling and transcription factors that ensure the specificity of individual structures along the anatomical axes. The third part discusses the genes and molecules that control the formation of bone and cartilage within mandibular and hyoid arches and how dysregulation of molecular signalling influences the development of skeletal components of the viscerocranium. In conclusion, we notice that mandibular malformations in humans and mice often co-occur with hyoid malformations and pinpoint the similar molecular machinery controlling the development of mandibular and hyoid arches.

Deletion of a Hand1 lncRNA-Containing Septum Transversum Enhancer Alters lncRNA Expression but Is Not Required for Hand1 Expression

We have previously identified a Hand1 transcriptional enhancer that drives expression within the septum transversum, the origin of the cells that contribute to the epicardium. This enhancer directly overlaps a common exon of a predicted family of long non-coding RNAs (lncRNA) that are specific to mice. To interrogate the necessity of this Hand1 enhancer, as well as the importance of these novel lncRNAs, we deleted the enhancer sequences, including the common exon shared by these lncRNAs, using genome editing. Resultant homozygous Hand1 enhancer mutants (Hand1^ΔST/ΔST) present with no observable phenotype. Assessment of lncRNA expression reveals that Hand1^ΔST/ΔST mutants effectively eliminate detectable lncRNA expression. Expression analysis within Hand1^ΔST/ΔST mutant hearts indicates higher levels of Hand1 than in controls. The generation of Hand1 compound heterozygous mutants with the Hand1^LacZ null allele (Hand1^ΔST/LacZ) also did not reveal any observable phenotypes. Together these data indicate that deletion of this Hand1 enhancer and by consequence a family of murine-specific lncRNAs does not impact embryonic development in observable ways.

HAND transcription factors cooperatively specify the aorta and pulmonary trunk

Congenital heart defects (CHDs) affecting the cardiac outflow tract (OFT) constitute a significant cause of morbidity and mortality. The OFT develops from migratory cell populations which include the cardiac neural crest cells (cNCCs) and secondary heart field (SHF) derived myocardium and endocardium. The related transcription factors HAND1 and HAND2 have been implicated in human CHDs involving the OFT. Although Hand1 is expressed within the OFT, Hand1 NCC-specific conditional knockout mice (H1CKOs) are viable. Here we show that these H1CKOs present a low penetrance of OFT phenotypes, whereas SHF-specific Hand1 ablation does not reveal any cardiac phenotypes. Further, HAND1 and HAND2 appear functionally redundant within the cNCCs, as a reduction/ablation of Hand2 on an NCC-specific H1CKO background causes pronounced OFT defects. Double conditional Hand1 and Hand2 NCC knockouts exhibit persistent truncus arteriosus (PTA) with 100% penetrance. NCC lineage-tracing and Sema3c in situ mRNA expression reveal that Sema3c-expressing cells are mis-localized, resulting in a malformed septal bridge within the OFTs of H1CKO;H2CKO embryos. Interestingly, Hand1 and Hand2 also genetically interact within the SHF, as SHF H1CKOs on a heterozygous Hand2 background exhibit Ventricular Septal Defects (VSDs) with incomplete penetrance. Previously, we identified a BMP, HAND2, and GATA-dependent Hand1 OFT enhancer sufficient to drive reporter gene expression within the nascent OFT and aorta. Using these transcription inputs as a probe, we identify a novel Hand2 OFT enhancer, suggesting that a conserved BMP-GATA dependent mechanism transcriptionally regulates both HAND factors. These findings support the hypothesis that HAND factors interpret BMP signaling within the cNCCs to cooperatively coordinate OFT morphogenesis.

The key roles of the lysine acetyltransferases KAT6A and KAT6B in physiology and pathology

Histone modifications and more specifically ε-lysine acylations are key epigenetic regulators that control chromatin structure and gene transcription, thereby impacting on various important cellular processes and phenotypes. Furthermore, lysine acetylation of many non-histone proteins is involved in key cellular processes including transcription, DNA damage repair, metabolism, cellular proliferation, mitosis, signal transduction, protein folding, and autophagy. Acetylation affects protein functions through multiple mechanisms including regulation of protein stability, enzymatic activity, subcellular localization, crosstalk with other post-translational modifications as well as regulation of protein-protein and protein-DNA interactions. The paralogous lysine acetyltransferases KAT6A and KAT6B which belong to the MYST family of acetyltransferases, were first discovered approximately 25 years ago. KAT6 acetyltransferases acylate both histone H3 and non-histone proteins. In this respect, KAT6 acetyltransferases play key roles in regulation of transcription, various developmental processes, maintenance of hematopoietic and neural stem cells, regulation of hematopoietic cell differentiation, cell cycle progression as well as mitosis. In the current review, we discuss the physiological functions of the acetyltransferases KAT6A and KAT6B as well as their functions under pathological conditions of aberrant expression, leading to several developmental syndromes and cancer. Importantly, both upregulation and downregulation of KAT6 proteins was shown to play a role in cancer formation, progression, and therapy resistance, suggesting that they can act as oncogenes or tumor suppressors. We also describe reciprocal regulation of expression between KAT6 proteins and several microRNAs as well as their involvement in cancer formation, progression and resistance to therapy.

Neural crest cells require Meis2 for patterning the mandibular arch via the Sonic hedgehog pathway

Cranial neural crest cells (cNCCs) originate in the anterior neural tube and populate pharyngeal arches in which they contribute to formation of bone and cartilage. This cell population also provides molecular signals for the development of tissues of non-neural crest origin, such as the tongue muscles, teeth enamel or gland epithelium. Here we show that the transcription factor Meis2 is expressed in the oral region of the first pharyngeal arch (PA1) and later in the tongue primordium. Conditional inactivation of Meis2 in cNCCs resulted in loss of Sonic hedgehog signalling in the oropharyngeal epithelium and impaired patterning of PA1 along the lateral-medial and oral-aboral axis. Failure of molecular specification of PA1, illustrated by altered expression of Hand1/2, Dlx5, Barx1, Gsc and other markers, led to hypoplastic tongue and ectopic ossification of the mandible. Meis2-mutant mice thus display craniofacial defects that are reminiscent of several human syndromes and patients with mutations in the Meis2 gene.

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.

Partially Penetrant Cardiac Neural Crest Defects in Hand1 Phosphomutant Mice: Dimer Choice That Is Not So Critical

Hand1 is a basic Helix-loop-Helix transcription factor that exhibits post-translationally regulated dimer partner choice that allows for a diverse set of Hand1 transcriptional complexes. Indeed, when Hand1 phosphoregulation is altered, conditionally activated hypophorylation (Hand1^PO4-) and phosphorylation mimic (Hand1^PO4+) Hand1 alleles disrupt both craniofacial and limb morphogenesis with 100% penetrance. Interestingly, activation of conditional Hand1 Phosphomutant alleles within post-migratory neural crest cells produce heart defects that include ventricular septal defects, double-outlet right ventricle, persistent truncus arteriosus with partial penetrance. Single versus double-lobed thymus is a distinguishing feature between Wnt1-Cre;Hand1^PO4-/+ and Wnt1-Cre;Hand1^PO4+/+ mice. These data show that although Hand1 dimer regulation plays critical and consistent roles in disrupting craniofacial and limb morphogenesis, Hand1 dimer regulation during cardiac outflow track formation is less critical for normal morphogenesis. This review will present the OFT phenotypes observed in Hand1 Phosphomutant mice, and discuss possible mechanisms of how penetrance differences within the same tissues within the same embryos could be variable.

Hedgehog signaling patterns the oral-aboral axis of the mandibular arch

Development of vertebrate jaws involves patterning neural crest-derived mesenchyme cells into distinct subpopulations along the proximal-distal and oral-aboral axes. Although the molecular mechanisms patterning the proximal-distal axis have been well studied, little is known regarding the mechanisms patterning the oral-aboral axis. Using unbiased single-cell RNA-seq analysis followed by in situ analysis of gene expression profiles, we show that Shh and Bmp4 signaling pathways are activated in a complementary pattern along the oral-aboral axis in mouse embryonic mandibular arch. Tissue-specific inactivation of hedgehog signaling in neural crest-derived mandibular mesenchyme led to expansion of BMP signaling activity to throughout the oral-aboral axis of the distal mandibular arch and subsequently duplication of dentary bone in the oral side of the mandible at the expense of tongue formation. Further studies indicate that hedgehog signaling acts through the Foxf1/2 transcription factors to specify the oral fate and pattern the oral-aboral axis of the mandibular mesenchyme.

Transcriptomics reveals complex kinetics of dorsal-ventral patterning gene expression in the mandibular arch

The mandibular or first pharyngeal arch forms the upper and lower jaws in all gnathostomes. A gene regulatory network that defines ventral, intermediate, and dorsal domains along the dorsal-ventral (D-V) axis of the arch has emerged from studies in zebrafish and mice, but the temporal dynamics of this process remain unclear. To define cell fate trajectories in the arches we have performed quantitative gene expression analyses of D-V patterning genes in pharyngeal arch primordia in zebrafish and mice. Using NanoString technology to measure transcript numbers per cell directly we show that, in many cases, genes expressed in similar D-V domains and induced by similar signals vary dramatically in their temporal profiles. This suggests that cellular responses to D-V patterning signals are likely shaped by the baseline kinetics of target gene expression. Furthermore, similarities in the temporal dynamics of genes that occupy distinct pathways suggest novel shared modes of regulation. Incorporating these gene expression kinetics into our computational models for the mandibular arch improves the accuracy of patterning, and facilitates temporal comparisons between species. These data suggest that the magnitude and timing of target gene expression help diversify responses to patterning signals during craniofacial development.

Modeling craniofacial development reveals spatiotemporal constraints on robust patterning of the mandibular arch

How does pattern formation occur accurately when confronted with tissue growth and stochastic fluctuations (noise) in gene expression? Dorso-ventral (D-V) patterning of the mandibular arch specifies upper versus lower jaw skeletal elements through a combination of Bone morphogenetic protein (Bmp), Endothelin-1 (Edn1), and Notch signaling, and this system is highly robust. We combine NanoString experiments of early D-V gene expression with live imaging of arch development in zebrafish to construct a computational model of the D-V mandibular patterning network. The model recapitulates published genetic perturbations in arch development. Patterning is most sensitive to changes in Bmp signaling, and the temporal order of gene expression modulates the response of the patterning network to noise. Thus, our integrated systems biology approach reveals non-intuitive features of the complex signaling system crucial for craniofacial development, including novel insights into roles of gene expression timing and stochasticity in signaling and gene regulation.

Probing the origin of matching functional jaws: roles of Dlx5/6 in cranial neural crest cells

Gnathostome jaws derive from the first pharyngeal arch (PA1), a complex structure constituted by Neural Crest Cells (NCCs), mesodermal, ectodermal and endodermal cells. Here, to determine the regionalized morphogenetic impact of Dlx5/6 expression, we specifically target their inactivation or overexpression to NCCs. NCC-specific Dlx5/6 inactivation (NCC^∆Dlx5/6) generates severely hypomorphic lower jaws that present typical maxillary traits. Therefore, differently from Dlx5/6 null-embryos, the upper and the lower jaws of NCC^∆Dlx5/6 mice present a different size. Reciprocally, forced Dlx5 expression in maxillary NCCs provokes the appearance of distinct mandibular characters in the upper jaw. We conclude that: (1) Dlx5/6 activation in NCCs invariably determines lower jaw identity; (2) the morphogenetic processes that generate functional matching jaws depend on the harmonization of Dlx5/6 expression in NCCs and in distinct ectodermal territories. The co-evolution of synergistic opposing jaws requires the coordination of distinct regulatory pathways involving the same transcription factors in distant embryonic territories.

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.

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.

Hand factor ablation causes defective left ventricular chamber development and compromised adult cardiac function

Coordinated cardiomyocyte growth, differentiation, and morphogenesis are essential for heart formation. We demonstrate that the bHLH transcription factors Hand1 and Hand2 play critical regulatory roles for left ventricle (LV) cardiomyocyte proliferation and morphogenesis. Using an LV-specific Cre allele (Hand1^LV-Cre), we ablate Hand1-lineage cardiomyocytes, revealing that DTA-mediated cardiomyocyte death results in a hypoplastic LV by E10.5. Once Hand1-linage cells are removed from the LV, and Hand1 expression is switched off, embryonic hearts recover by E16.5. In contrast, conditional LV loss-of-function of both Hand1 and Hand2 results in aberrant trabeculation and thickened compact zone myocardium resulting from enhanced proliferation and a breakdown of compact zone/trabecular/ventricular septal identity. Surviving Hand1;Hand2 mutants display diminished cardiac function that is rescued by concurrent ablation of Hand-null cardiomyocytes. Collectively, we conclude that, within a mixed cardiomyocyte population, removal of defective myocardium and replacement with healthy endogenous cardiomyocytes may provide an effective strategy for cardiac repair.

The left ventricle of the heart drives blood flow throughout the body. Impaired left ventricle function, associated either with heart failure or with certain, severe cardiac birth defects, constitutes a significant cause of mortality. Understanding how heart muscle grows is vital to developing improved treatments for these diseases. Unfortunately, genetic tools necessary to study the left ventricle have been lacking. Here we engineer the first mouse line to enable specific genetic study of the left ventricle. We show that, unlike in the adult heart, the embryonic left ventricle is remarkably tolerant of cell death, as remaining cells have the capacity to proliferate and to restore heart function. Conversely, disruption of two related genes, Hand1 and Hand2, within the left ventricle causes cells to assume the wrong identity, and to consequently overgrow and impair cardiac function. Ablation of these mutant cells rescues heart function. We conclude that selective removal of defective heart muscle and replacement with healthy cells may provide an effective therapy to treat heart failure.

Characterization of the distal-less homologue gene, PfDlx, involved in regulating the expression of Pif in the pearl oyster, Pinctada fucata

Distal-less (Dlx) homeobox transcription factors play an important role in regulating various aspects of vertebrate biology. In vertebrates and invertebrates, distal-less is a highly conserved and well-studied transcription factor. In pearl oyster, we have identified a homologue of this gene, Dlx, and cloned the full-length cDNA. Genomic structure analysis revealed that PfDlx genomic DNA contained three exons and two introns. Their deduced amino acid sequences all showed the highest identity with homologues in Crassostrea gigas. Analyses of PfDlx mRNA in tissues and developmental stages showed high expressions in gonad, polar body stage, 2-4 cells and 32 cells. After shell notching, the changes in expression of Dlx shows that it reached a maximum at 24h. In co-transfection experiments, PfDlx significantly activates reporter constructs containing a Pif promoter. Through using RNAi techniques, we demonstrated that down-regulation of Dlx in P. fucata did not significantly disrupt the development of the nacreous layer in scanning electron microscopy, but it significantly down-regulated the expression of Pif gene. Thus, our work suggests that PfDlx might participate in regulating the expression of the Pif gene in the pearl oyster.

Negative regulation of endothelin signaling by SIX1 is required for proper maxillary development

Jaw morphogenesis is a complex event mediated by inductive signals that establish and maintain the distinct developmental domains required for formation of hinged jaws, the defining feature of gnathostomes. The mandibular portion of pharyngeal arch 1 is patterned dorsally by Jagged-Notch signaling and ventrally by endothelin receptor A (EDNRA) signaling. Loss of EDNRA signaling disrupts normal ventral gene expression, the result of which is homeotic transformation of the mandible into a maxilla-like structure. However, loss of Jagged-Notch signaling does not result in significant changes in maxillary development. Here we show in mouse that the transcription factor SIX1 regulates dorsal arch development not only by inducing dorsal Jag1 expression but also by inhibiting endothelin 1 (Edn1) expression in the pharyngeal endoderm of the dorsal arch, thus preventing dorsal EDNRA signaling. In the absence of SIX1, but not JAG1, aberrant EDNRA signaling in the dorsal domain results in partial duplication of the mandible. Together, our results illustrate that SIX1 is the central mediator of dorsal mandibular arch identity, thus ensuring separation of bone development between the upper and lower jaws.