Hey2 enhancer activity defines unipotent progenitors for left ventricular cardiomyocytes in juxta-cardiac field of early mouse embryo.

The cardiac crescent is the first structure of the heart and contains progenitor cells of the first heart field, which primarily differentiate into left ventricular cardiomyocytes. The interface between the forming cardiac crescent and extraembryonic tissue is known as the juxta-cardiac field (JCF), and progenitor cells in this heart field contribute to the myocardium of the left ventricle and atrioventricular canal as well as the epicardium. However, it is unclear whether there are progenitor cells that differentiate specifically into left ventricular cardiomyocytes. We have previously demonstrated that an enhancer of the gene encoding the Hey2 bHLH transcriptional repressor is activated in the ventricular myocardium during mouse embryonic development. In this study, we aimed to investigate the characteristics of cardiomyocyte progenitor cells and their cell lineages by analyzing Hey2 enhancer activity at the earliest stages of heart formation. We found that the Hey2 enhancer initiated its activity prior to cardiomyocyte differentiation within the JCF. Hey2 enhancer-active cells were present rostrally to the Tbx5-expressing region at the early phase of cardiac crescent formation and differentiated exclusively into left ventricular cardiomyocytes in a lineage distinct from the Tbx5-positive lineage. By the late phase of cardiac crescent formation, Hey2 enhancer activity became significantly overlapped with Tbx5 expression in cells that contribute to the left ventricular myocardium. Our study reveals that a population of unipotent progenitor cells for left ventricular cardiomyocytes emerge in the JCF, providing further insight into the mode of cell type diversification during early cardiac development.

Loss of myocardial Hey2/Hrt2 function disrupts rightward shift of atrioventricular cushion tissue and causes tricuspid atresia.

BACKGROUND: Endocardial cushion tissue is primordia of the valves and septa of the adult heart, and its malformation causes various congenital heart diseases (CHDs). Tricuspid atresia (TA) is defined as congenital absence or agenesis of the tricuspid valve caused by endocardial cushion defects. However, little is known about what type of endocardial cushion defect causes TA. RESULTS: Using three-dimensional volume rendering image analysis, we demonstrated morphological changes of endocardial cushion tissue in developing Hey2/Hrt2 KO mouse embryos that showed malformation of the tricuspid valve, which resembled human TA at neonatal period. In control embryos, atrioventricular (AV) endocardial cushion tissues showed rightward shift to form a tricuspid valve. However, the rightward shift of endocardial cushion tissue was disrupted in Hey2/Hrt2 KO embryos, leading to the misalignment of AV cushions. We also found that muscular tissue filled up the space between the right atrium and ventricle, resulting in the absence of the tricuspid valve. Moreover, analysis using tissue-specific conditional KO mice showed that HEY2/HRT2-expressing myocardium may physically regulate the AV shift. CONCLUSION: Disruption of rightward cushion movement is an initial cue of TA phenotype, and myocardial HEY2/HRT2 is necessary for the regulation of proper alignment of AV endocardial cushion tissue.

Human Genetics of Tricuspid Atresia and Univentricular Heart.

Tricuspid atresia (TA) is a rare congenital heart condition that presents with a complete absence of the right atrioventricular valve. Because of the rarity of familial and/or isolated cases of TA, little is known about the potential genetic abnormalities contributing to this condition. Potential responsible chromosomal abnormalities were identified in exploratory studies and include deletions in 22q11, 4q31, 8p23, and 3p as well as trisomies 13 and 18. In parallel, potential culprit genes include the ZFPM2, HEY2, NFATC1, NKX2-5, MYH6, and KLF13 genes. The aim of this chapter is to expose the genetic components that are potentially involved in the pathogenesis of TA in humans. The large variability in phenotypes and genotypes among cases of TA suggests a genetic network that involves many components yet to be unraveled.

Molecular Pathways and Animal Models of Tricuspid Atresia and Univentricular Heart.

The process of valve formation is a complex process that involves intricate interplay between various pathways at precise times. Although we have not completely elucidated the molecular pathways that lead to normal valve formation, we have identified a few major players in this process. We are now able to implicate TGF-ß, BMP, and NOTCH as suspects in tricuspid atresia (TA), as well as their downstream targets: NKX2-5, TBX5, NFATC1, GATA4, and SOX9. We know that the TGF-ß and the BMP pathways converge on the SMAD4 molecule, and we believe that this molecule plays a very important role to tie both pathways to TA. Similarly, we look at the NOTCH pathway and identify the HEY2 as a potential link between this pathway and TA. Another transcription factor that has been implicated in TA is NFATC1. While several mouse models exist that include part of the TA abnormality as their phenotype, no true mouse model can be said to represent TA. Bridging this gap will surely shed light on this complex molecular pathway and allow for better understanding of the disease process.

Human Genetics of Ventricular Septal Defect.

Ventricular septal defects (VSDs) are recognized as one of the commonest congenital heart diseases (CHD), accounting for up to 40% of all cardiac malformations, and occur as isolated CHDs as well as together with other cardiac and extracardiac congenital malformations in individual patients and families. The genetic etiology of VSD is complex and extraordinarily heterogeneous. Chromosomal abnormalities such as aneuploidy and structural variations as well as rare point mutations in various genes have been reported to be associated with this cardiac defect. This includes both well-defined syndromes with known genetic cause (e.g., DiGeorge syndrome and Holt-Oram syndrome) and so far undefined syndromic forms characterized by unspecific symptoms. Mutations in genes encoding cardiac transcription factors (e.g., NKX2-5 and GATA4) and signaling molecules (e.g., CFC1) have been most frequently found in VSD cases. Moreover, new high-resolution methods such as comparative genomic hybridization enabled the discovery of a high number of different copy number variations, leading to gain or loss of chromosomal regions often containing multiple genes, in patients with VSD. In this chapter, we will describe the broad genetic heterogeneity observed in VSD patients considering recent advances in this field.

The Gridlock transcriptional repressor impedes vertebrate heart regeneration by restricting expression of lysine methyltransferase.

Teleost zebrafish and neonatal mammalian hearts exhibit the remarkable capacity to regenerate through dedifferentiation and proliferation of pre-existing cardiomyocytes (CMs). Although many mitogenic signals that stimulate zebrafish heart regeneration have been identified, transcriptional programs that restrain injury-induced CM renewal are incompletely understood. Here, we report that mutations in gridlock (grl; also known as hey2), encoding a Hairy-related basic helix-loop-helix transcriptional repressor, enhance CM proliferation and reduce fibrosis following damage. In contrast, myocardial grl induction blunts CM dedifferentiation and regenerative responses to heart injury. RNA sequencing analyses uncover Smyd2 lysine methyltransferase (KMT) as a key transcriptional target repressed by Grl. Reduction in Grl protein levels triggered by injury induces smyd2 expression at the wound myocardium, enhancing CM proliferation. We show that Smyd2 functions as a methyltransferase and modulates the Stat3 methylation and phosphorylation activity. Inhibition of the KMT activity of Smyd2 reduces phosphorylated Stat3 at cardiac wounds, suppressing the elevated CM proliferation in injured grl mutant hearts. Our findings establish an injury-specific transcriptional repression program in governing CM renewal during heart regeneration, providing a potential strategy whereby silencing Grl repression at local regions might empower regeneration capacity to the injured mammalian heart.

Genetic lineage tracing in skin reveals predominant expression of HEY2 in dermal papilla during telogen and that HEY2+ cells contribute to the regeneration of dermal cells during wound healing.

Dermal papilla (DP) cells are specialized mesenchymal cells that play a crucial role in regulating hair morphology, colour and growth through the secretion of specific factors. It is still unclear what the source of progenitor cells is for dermal cell regeneration during wound healing, and whether DP cells are involved in this process. We analyzed the gene expression profile of various skin cell populations using existing datasets and found that the Hey2 gene was predominantly expressed in DP cells. We introduced Hey2-CreERT2 knockin mice and crossed them with Rosa26-ZsGreen reporter mice. After induction in the double transgenic mice by administration of tamoxifen, the reporter ZsGreen was found to be predominantly expressed in DP cells both at anagen and telogen phases, and broadly expressed in some other dermal cells at anagen. We also created a wound after tamoxifen induction, and found there were abundant ZsGreen+ cells in the regenerated dermis. We conclude that the HEY2+ DP cells and dermal cells exhibit some stemness properties and can contribute to the dermal cell regeneration during wound healing.

Expression of Hey2 transcription factor in the early embryonic ventricles is controlled through a distal enhancer by Tbx20 and Gata transcription factors.

Development of multi-chambered heart is associated with spatio-temporal regulation of gene expression. A basic helix-loop-helix transcription factor Hey2 is specifically expressed in the embryonic mouse ventricles and is indispensable for ventricular myocyte differentiation, compartment identity and morphogenesis of the heart. However, how Hey2 transcription is precisely regulated in the heart remains unclear. In this study, we identified a distal Hey2 enhancer conserved in the mouse and human to possess specific transcriptional activity in ventricular free wall myocytes at the looping stage of cardiac development. Deletion of the enhancer significantly decreased endogenous Hey2 expression in the ventricular myocardium but not in other tissues of mouse embryos. Mutation/deletion of the conserved binding sites for T-box and Gata proteins, but not NK-2 proteins, abolished the enhancer activity, and Tbx20 null mice completely lost the enhancer activity in the embryonic ventricles. Luciferase reporter analysis suggested that the ventricular enhancer activity was controlled by Tbx20 through its DNA binding and cooperative function with cardiac Gata proteins. These results delineate a regulatory mechanism of ventricular Hey2 expression and help fully understand molecular cascades in myocardial cell differentiation and cardiac morphogenesis during embryonic development.

Hey2 regulates the size of the cardiac progenitor pool during vertebrate heart development.

A key event in heart development is the timely addition of cardiac progenitor cells, defects in which can lead to congenital heart defects. However, how the balance and proportion of progenitor proliferation versus addition to the heart is regulated remains poorly understood. Here, we demonstrate that Hey2 functions to regulate the dynamics of cardiac progenitor addition to the zebrafish heart. We found that the previously noted increase in myocardial cell number found in the absence of Hey2 function was due to a pronounced expansion in the size of the cardiac progenitor pool. Expression analysis and lineage tracing of hey2-expressing cells showed that hey2 is active in cardiac progenitors. Hey2 acted to limit proliferation of cardiac progenitors, prior to heart tube formation. Use of a transplantation approach demonstrated a likely cell-autonomous (in cardiac progenitors) function for Hey2. Taken together, our data suggest a previously unappreciated role for Hey2 in controlling the proliferative capacity of cardiac progenitors, affecting the subsequent contribution of late-differentiating cardiac progenitors to the developing vertebrate heart.

Germline variants in HEY2 functional domains lead to congenital heart defects and thoracic aortic aneurysms.

PURPOSE: In this study we aimed to establish the genetic cause of a myriad of cardiovascular defects prevalent in individuals from a genetically isolated population, who were found to share a common ancestor in 1728. METHODS: Trio genome sequencing was carried out in an index patient with critical congenital heart disease (CHD); family members had either exome or Sanger sequencing. To confirm enrichment, we performed a gene-based association test and meta-analysis in two independent validation cohorts: one with 2685 CHD cases versus 4370 . These controls were also ancestry-matched (same as FTAA controls), and the other with 326 cases with familial thoracic aortic aneurysms (FTAA) and dissections versus 570 ancestry-matched controls. Functional consequences of identified variants were evaluated using expression studies. RESULTS: We identified a loss-of-function variant in the Notch target transcription factor-encoding gene HEY2. The homozygous state (n = 3) causes life-threatening congenital heart defects, while 80% of heterozygous carriers (n = 20) had cardiovascular defects, mainly CHD and FTAA of the ascending aorta. We confirm enrichment of rare risk variants in HEY2 functional domains after meta-analysis (MetaSKAT p = 0.018). Furthermore, we show that several identified variants lead to dysregulation of repression by HEY2. CONCLUSION: A homozygous germline loss-of-function variant in HEY2 leads to critical CHD. The majority of heterozygotes show a myriad of cardiovascular defects.

A role of Hey2 transcription factor for right ventricle development through regulation of Tbx2-Mycn pathway during cardiac morphogenesis.

A basic helix-loop-helix transcription factor Hey2 is expressed in the ventricular myocardium and endocardium of mouse embryos, and Hey2 null mice die perinatally showing ventricular septal defect, dysplastic tricuspid valve and hypoplastic right ventricle. In order to understand region-specific roles of Hey2 during cardiac morphogenesis, we generated Hey2 conditional knockout (cKO) mice using Mef2c-AHF-Cre, which was active in the anterior part of the second heart field and the right ventricle and outflow tract of the heart. Hey2 cKO neonates reproduced three anomalies commonly observed in Hey2 null mice. An earliest morphological defect was the lack of right ventricular extension along the apico-basal axis at midgestational stages. Underdevelopment of the right ventricle was present in all cKO neonates including those without apparent atresia of right-sided atrioventricular connection. RNA sequencing analysis of cKO embryos identified that the gene expression of a non-chamber T-box factor Tbx2 was ectopically induced in the chamber myocardium of the right ventricle. Consistently, mRNA expression of the Mycn transcription factor, which was a cell cycle regulator transcriptionally repressed by Tbx2, was down regulated, and the number of S-phase cells was significantly decreased in the right ventricle of cKO heart. These results suggest that Hey2 plays an important role in right ventricle development during cardiac morphogenesis, at least in part, through mitigating Tbx2-dependent inhibition of Mycn expression.

microRNA-599 promotes apoptosis and represses proliferation and epithelial-mesenchymal transition of papillary thyroid carcinoma cells via downregulation of Hey2-depentent Notch signaling pathway.

Although papillary thyroid carcinoma (PTC) has a favorable prognosis after surgical or medical treatment, its survival rate is still very low. Therefore, finding more reliable therapy methods to limit PTC is a necessity. Compelling evidence has implicated the role of microRNAs (miRNAs or miRs) in PTC. This study aims at investigating the possible effect of microRNA-599 (miR-599) on proliferation, apoptosis, and epithelial-mesenchymal transition (EMT) of PTC cells by targeting Hey2 gene. Differentially expressed genes/miRNAs associated with PTC were screened based on microarray analysis. Then, the expression of the candidate gene, as well as, the regulatory miRNA were detected in PTC cells, the related signaling pathway was verified. Afterward, the relationship between the miR and the candidate gene was verified by dual-luciferase reporter gene assay. Subsequently, the effects of overexpressed miR and silenced candidate gene on cell proliferation, cell apoptosis, EMT, migration, and invasion were detected. In PTC tissues and cells, miR-599 was downregulated while Hey2 expressed highly. Hey2 is a target gene of miR-559. In addition, the expression of Bax and E-cadherin was elevated while that of Hey2, Notch1, Delta-like1, Hes1, N1ICD, Jagged1, Snail, Slug, N-cadherin and Vimentin, and Bcl-2 was reduced in cells treated with upregulated miR-599 or downregulated Hey2. Moreover, miR-599 overexpression or Hey2 silencing inhibited cell proliferation, migration, invasion, along with EMT but promoted apoptosis. This study verified that miR-599 promotes apoptosis and represses proliferation, EMT of PTC cells through inactivating the Notch signaling pathway by downregulating Hey2, which has great clinical significance for PTC treatment.

Notch-HEY2 signaling pathway contributes to the differentiation of CD34+ hematopoietic-like stem cells from adult peripheral blood insulin-producing cells after the treatment with platelet-derived mitochondria.

Previous works characterized a novel cell population from adult human peripheral blood, designated peripheral blood insulin-producing cells (PB-IPC). PB-IPC displayed the pluripotent potential of differentiations after the treatment with platelet-derived mitochondria and gave rise to three germ layer-derived cells such as the mitochondrion-induced CD34+ hematopoietic stem cells (HSC)-like cells (miCD34+ HSC). To determine the molecular mechanism underlying the differentiation of miCD34+ cells, mechanistic studies established that MitoTracker Deep Red-labeled mitochondria could enter into the PB-IPC in a dose-dependent manner. Blocking Notch signaling pathway with a γ-secretase inhibitor, DAPT, markedly inhibited the proliferation of PB-IPC and improved the differentiation of miCD34+ HSC. Additionally, treatment with platelet-derived mitochondria can reprogram the differentiation of PB-IPC into miCD34+ HSC through inhibition of the Notch/HEY2 signaling pathway, as demonstrated by blocking experiments with HEY2 small interfering RNA (siRNA). The data indicated that Notch signaling pathway contributes to the miCD34+ HSC differentiation, thus advancing our understanding of the mitochondrial reprogramming and the potential treatment of human hematopoietic disease.

microRNA-146a and Hey2 form a mutual negative feedback loop to regulate the inflammatory response in chronic apical periodontitis.

Chronic apical periodontitis (CAP) is defined as chronic inflammation of the dental pulp and root canal system. Porphyromonas endodontalis lipopolysaccharide ( P. endodontalis LPS) plays an important role in inducing an inflammatory response in CAP. microRNA-146a (miR-146a) is a key regulator of inflammation and is induced by LPS. Hairy and enhancer-of-split related with YRPW motif 2 (Hey2) has been confirmed to be induced by the Notch signaling pathway, which is involved in tooth development, pulp regeneration, and repair after injury. Our study aimed to investigate the functional role of miR-146a via the targeting of Hey2 in CAP as well as the underlying mechanism. Compared with 13 healthy controls, miR-146a and Hey2 expressions were significantly higher in 20 patients with CAP. In addition, miR-146a, Hey2, interleukin (IL)-6, IL-1ß, and tumor necrosis factor (TNF)-α expressions were significantly increased in MC3T3-E1 cells stimulated with different concentrations (0-20 µg/mL) of P. endodontalis LPS for different amounts of time (0-48 hours). Moreover, miR-146a, which acts as an anti-inflammatory mediator, negatively regulated the expression of IL-6, IL-1ß, and TNF-α, and Hey2 was confirmed as a target gene of miR-146a by a luciferase reporter assay. Hey2 also negatively regulated miR-146a, IL-6, IL-1ß, and TNF-α expressions, and P. endodontalis LPS strongly induced Hey2 recruitment to the IL-6 promoter (-400 ~ -200 bp). These findings suggest that miR-146a and Hey2 form a mutual negative feedback regulatory loop, demonstrating a novel mechanism that regulates inflammatory responses in CAP.

Attenuation of PRRX2 and HEY2 enables efficient conversion of adult human skin fibroblasts to neurons.

The direct conversion of accessible cells such as human fibroblasts to inaccessible cells, particularly neurons, opens up many opportunities for using the human model system to study diseases and discover therapies. Previous studies have indicated that the neuronal conversion of adult human skin fibroblasts is much harder than that for human lung fibroblasts, which are used in many experiments. Here we formally report this differential plasticity of human skin versus lung fibroblasts in their transdifferentiation to induced neurons. Using RNAseq of isogenic and non-isogenic pairs of human skin and lung fibroblasts at different days in their conversion to neurons, we found that several master regulators (TWIST1, TWIST2, PRRX1 and PRRX2) in the fibroblast Gene Regulatory Network were significantly downregulated in lung fibroblasts, but not in skin fibroblasts. By knocking down each of these genes and other genes that suppress the neural fate, such as REST, HES1 and HEY2, we found that the combined attenuation of HEY2 and PRRX2 significantly enhanced the transdifferentiation of human skin fibroblasts induced by ASCL1 and p53 shRNA. The new method, which overexpressed ASCL1 and knocked down p53, HEY2 and PRRX2 (ApH2P2), enabled the efficient transdifferentiation of adult human skin fibroblasts to MAP2+ neurons in 14 days. It would be useful for a variety of applications that require the efficient and speedy derivation of patient-specific neurons from skin fibroblasts.

Transcriptional and Epigenetic Regulation of Cardiac Electrophysiology.

Spatiotemporal gene expression during cardiac development is a highly regulated process. Activation of key signaling pathways involved in electrophysiological programming, such as Notch and Wnt signaling, occurs in early cardiovascular development and these pathways are reactivated during pathologic remodeling. Direct targets of these signaling pathways have also been associated with inherited arrhythmias such as Brugada syndrome and arrhythmogenic cardiomyopathy. In addition, evidence is emerging from animal models that reactivation of Notch and Wnt signaling during cardiac pathology may predispose to acquired arrhythmias, underscoring the importance of elucidating the transcriptional and epigenetic effects on cardiac gene regulation. Here, we highlight specific examples where gene expression dictates electrophysiological properties in both normal and diseased hearts.

A Novel Somatic Variant in HEY2 Unveils an Alternative Splicing Isoform Linked to Ventricular Septal Defect.

Congenital heart defects (CHDs) are the leading cause of death in infants under 1 year of age. Aberrations in the expression and function of cardiac transcription factors (TFs) are a major contributor to CHDs. Despite the numerous studies undertaken to functionally characterize these TFs, their exact role in different stages of cardiogenesis is still not fully elucidated. Here we focused on HEY2, a basic helix loop helix transcriptional repressor, and its potential role in human ventricular septal defects. Genetic analysis was performed based on sequencing of DNA and cDNA obtained from post-operational cardiac tissues and blood of 17 Lebanese patients with various CHDs. The screen covered the entire coding regions of the GATA4, NKX2.5, TBX5, TBX20 and HEY2 genes. Our results revealed two novel somatic mutations, namely p.Ala229Thr and p.161_190 del, affecting HEY2 in the diseased cardiac tissues of two patients with VSD. These results suggest a potential role of HEY2 in regulating ventricular septation in humans.

Hey1 and Hey2 control the spatial and temporal pattern of mammalian auditory hair cell differentiation downstream of Hedgehog signaling.

Mechano-sensory hair cells (HCs), housed in the inner ear cochlea, are critical for the perception of sound. In the mammalian cochlea, differentiation of HCs occurs in a striking basal-to-apical and medial-to-lateral gradient, which is thought to ensure correct patterning and proper function of the auditory sensory epithelium. Recent studies have revealed that Hedgehog signaling opposes HC differentiation and is critical for the establishment of the graded pattern of auditory HC differentiation. However, how Hedgehog signaling interferes with HC differentiation is unknown. Here, we provide evidence that in the murine cochlea, Hey1 and Hey2 control the spatiotemporal pattern of HC differentiation downstream of Hedgehog signaling. It has been recently shown that HEY1 and HEY2, two highly redundant HES-related transcriptional repressors, are highly expressed in supporting cell (SC) and HC progenitors (prosensory cells), but their prosensory function remained untested. Using a conditional double knock-out strategy, we demonstrate that prosensory cells form and proliferate properly in the absence of Hey1 and Hey2 but differentiate prematurely because of precocious upregulation of the pro-HC factor Atoh1. Moreover, we demonstrate that prosensory-specific expression of Hey1 and Hey2 and its subsequent graded downregulation is controlled by Hedgehog signaling in a largely FGFR-dependent manner. In summary, our study reveals a critical role for Hey1 and Hey2 in prosensory cell maintenance and identifies Hedgehog signaling as a novel upstream regulator of their prosensory function in the mammalian cochlea. The regulatory mechanism described here might be a broadly applied mechanism for controlling progenitor behavior in the central and peripheral nervous system.

Duplication of HEY2 in cardiac and neurologic development.

HEY2 is a basic helix-loop-helix (bHLH) transcription factor that plays an important role in the developing mammalian heart and brain. In humans, nonsynonymous mutations in HEY2 have been described in patients with atrial ventricular septal defects, and a subset of individuals with chromosomal deletions involving HEY2 have cardiac defects and cognitive impairment. Less is known about the potential effects of HEY2 overexpression. Here, we describe a female child with tetralogy of Fallot who developed severe right ventricular outflow tract obstruction due to a combination of infundibular and valvular pulmonary stenosis. She was also noted to have hypotonia, lower extremity weakness, fine motor delay and speech delay. A copy number variation (CNV) detection analysis followed by real-time quantitative PCR analysis revealed a single gene duplication of HEY2. This is the only duplication involving HEY2 identified in our database of over 70,000 individuals referred for CNV analysis. In the developing heart, overexpression of HEY2 is predicted to cause decreased expression of the cardiac transcription factor GATA4 which, in turn, has been shown to cause tetralogy of Fallot. In mice, misexpression of Hey2 in the developing brain leads to inhibition of neurogenesis and promotion of gliogenesis. Hence, duplication of HEY2 may be a contributing factor to both the congenital heart defects and the neurodevelopmental problems evident in our patient. These results suggest that individuals with HEY2 duplications should be screened for congenital heart defects and monitored closely for evidence of developmental delay and/or cognitive impairment.

Hairy and Enhancer of Split-related with YRPW motif (HEY)2 regulates bone remodeling in mice.

Notch induces Hairy and Enhancer of Split-related with YRPW motif (Hey)1, Hey2, and HeyL expression in osteoblasts, but the contributions of these genes to the skeletal effects of Notch are not fully understood. HEY1 misexpression has limited skeletal impact, female HeyL null mice display increased bone mass, and Hey2 inactivation is developmentally lethal. To inactivate Hey2 in immature or mature osteoblasts, Hey2(loxP/loxP) mice were crossed with transgenics expressing CRE under the control of the osterix (Osx-Cre) or osteocalcin (Oc-Cre) promoters to generate Osx-Cre(+/-);Hey2(Δ/Δ) or Oc-Cre(+/-);Hey2(Δ/Δ) mice. Trabecular bone volume increased in 3-month-old Osx-Cre(+/-);Hey2(Δ/Δ) and Oc-Cre(+/-);Hey2(Δ/Δ) male mice and in 1-month-old Oc-Cre(+/-);Hey2(Δ/Δ) female mice, although 3-month-old Oc-Cre(+/-);Hey2(Δ/Δ) females developed osteopenia. Alkaline phosphatase liver/bone/kidney (ALPL) expression and activity were suppressed in osteoblasts from Oc-Cre(+/-);Hey2(Δ/Δ) mice of both sexes. To overexpress HEY2 in osteoblasts, transgenic mice where a 3.6-kb fragment of the rat collagen type-I α1 promoter directs HEY2 expression were created. Three-month-old Hey2 transgenic males exhibited decreased osteoblast activity and increased bone resorption and developed osteopenia at 6 months of age. Hey2 transgenic females exhibited reduced osteoblast number and function, but no changes in bone resorption. HEY2 overexpression in osteoblasts from mice of both sexes inhibited ALPL expression and activity and suppressed osteocalcin transcripts in cells from male mice only. HEY2 overexpression in osteoblasts from male mice enhanced bone resorption by co-cultured splenocytes and induced interleukin-6, a molecule that promotes osteoclastogenesis. In conclusion, HEY2 decreases skeletal mass and regulates bone remodeling in male mice.