Regulation and function of homeodomain proteins in the embryonic and adult vascular systems

Lineage-Specific Mesenchymal Stromal Cells Derived from Human iPSCs Showed Distinct Patterns in Transcriptomic Profile and Extracellular Vesicle Production

Over the past decades, mesenchymal stromal cells (MSCs) have been extensively investigated as a potential therapeutic cell source for the treatment of various disorders. Differentiation of MSCs from human induced pluripotent stem cells (iMSCs) has provided a scalable approach for the biomanufacturing of MSCs and related biological products. Although iMSCs shared typical MSC markers and functions as primary MSCs (pMSCs), there is a lack of lineage specificity in many iMSC differentiation protocols. Here, a stepwise hiPSC-to-iMSC differentiation method is employed via intermediate cell stages of neural crest and cytotrophoblast to generate lineage-specific MSCs with varying differentiation efficiencies and gene expression. Through a comprehensive comparison between early developmental cell types (hiPSCs, neural crest, and cytotrophoblast), two lineage-specific iMSCs, and six source-specific pMSCs, are able to not only distinguish the transcriptomic differences between MSCs and early developmental cells, but also determine the transcriptomic similarities of iMSC subtypes to postnatal or perinatal pMSCs. Additionally, it is demonstrated that different iMSC subtypes and priming conditions affected EV production, exosomal protein expression, and cytokine cargo.

Tracing PRX1+ cells during molar formation and periodontal ligament reconstruction

Neural crest-derived mesenchymal stem cells (MSCs) are known to play an essential function during tooth and skeletal development. PRX1⁺ cells constitute an important MSC subtype that is implicated in osteogenesis. However, their potential function in tooth development and regeneration remains elusive. In the present study, we first assessed the cell fate of PRX1⁺ cells during molar development and periodontal ligament (PDL) formation in mice. Furthermore, single-cell RNA sequencing analysis was performed to study the distribution of PRX1⁺ cells in PDL cells. The behavior of PRX1⁺ cells during PDL reconstruction was investigated using an allogeneic transplanted tooth model. Although PRX1⁺ cells are spatial specific and can differentiate into almost all types of mesenchymal cells in first molars, their distribution in third molars is highly limited. The PDL formation is associated with a high number of PRX1⁺ cells; during transplanted teeth PDL reconstruction, PRX1⁺ cells from the recipient alveolar bone participate in angiogenesis as pericytes. Overall, PRX1⁺ cells are a key subtype of dental MSCs involved in the formation of mouse molar and PDL and participate in angiogenesis as pericytes during PDL reconstruction after tooth transplantation.

Vascular senescence and ageing: a role for the MEOX proteins in promoting endothelial dysfunction

In the vascular system, ageing is accompanied by the accrual of senescent cells and is associated with an increased risk of vascular disease. Endothelial cell (EC) dysfunction is a hallmark of vascular disease and is characterized by decreased angiogenic potential, reduced nitric oxide bioavailability, impaired vasodilation, increased production of ROS, and enhanced inflammation. In ECs, the major producer of nitric oxide is the endothelial nitric oxide synthase (eNOS) enzyme that is encoded by the NOS3 gene. NOS3/eNOS function is tightly regulated at both the transcriptional and post-transcriptional levels to maintain normal vascular function. A key transcriptional regulator of eNOS expression is p53, which has been shown to play a central role in mediating cellular senescence and thereby vascular dysfunction. Herein, we show that, in ECs, the MEOX homeodomain transcription factors decrease the expression of genes involved in angiogenesis, repress eNOS expression at the mRNA and protein levels, and increase the expression of p53. These findings support a role for the MEOX proteins in promoting endothelial dysfunction.

Homeobox gene TGIF-1 is increased in placental endothelial cells of human fetal growth restriction

Aberrant placental angiogenesis is associated with fetal growth restriction (FGR). In mice, targeted disruption of the homeobox gene, transforming growth β-induced factor (Tgif-1), which is also a transcription factor, causes defective placental vascularisation. Nevertheless, the role of TGIF-1 in human placental angiogenesis is unclear. We have previously reported increased TGIF-1 expression in human FGR placentae and demonstrated localisation of TGIF-1 protein in placental endothelial cells (ECs). However, its functional role remains to be investigated. In this study, we aimed to specifically compare TGIF-1 mRNA expression in placental ECs isolated from human FGR-affected pregnancies with gestation-matched control pregnancies in two independent cohorts from Australia and Canada and to identify the functional role of TGIF-1 in placental angiogenesis using the human umbilical vein endothelial cell-derived cell line, SGHEC-7, and primary human umbilical vein ECs. Real-time PCR revealed that TGIF-1 mRNA expression was significantly increased in ECs isolated from FGR-affected placentae compared with that of controls. The functional roles of TGIF-1 were determined in ECs after TGIF-1 siRNA transfection. TGIF-1 inactivation in ECs significantly reduced TGIF-1 at both the mRNA and protein levels, as well as the proliferative and invasive potential, but significantly increased the angiogenic potential. Using angiogenesis PCR screening arrays, we identified ITGAV, NRP-1, ANPGT-1 and ANPGT-2 as novel downstream targets of TGIF-1, after TGIF-1 inactivation in ECs. Collectively, these results show that TGIF-1 regulates EC function and the expression of angiogenic molecules; and when abnormally expressed, may contribute to the aberrant placental angiogenesis observed in FGR.

Angiocrine functions of organ-specific endothelial cells

Endothelial cells that line capillaries are not just passive conduits for delivering blood. Tissue-specific endothelium establishes specialized vascular niches that deploy sets of growth factors, known as angiocrine factors. These cues participate actively in the induction, specification, patterning and guidance of organ regeneration, as well as in the maintainance of homeostasis and metabolism. When upregulated following injury, they orchestrate self-renewal and differentiation of tissue-specific resident stem and progenitor cells into functional organs. Uncovering the mechanisms by which organotypic endothelium distributes physiological levels of angiocrine factors both spatially and temporally will lay the foundation for clinical trials that promote organ repair without scarring.

Arterial endothelial methylome: differential DNA methylation in athero-susceptible disturbed flow regions in vivo

BACKGROUND

Atherosclerosis is a heterogeneously distributed disease of arteries in which the endothelium plays an important central role. Spatial transcriptome profiling of endothelium in pre-lesional arteries has demonstrated differential phenotypes primed for athero-susceptibility at hemodynamic sites associated with disturbed blood flow. DNA methylation is a powerful epigenetic regulator of endothelial transcription recently associated with flow characteristics. We investigated differential DNA methylation in flow region-specific aortic endothelial cells in vivo in adult domestic male and female swine.

RESULTS

Genome-wide DNA methylation was profiled in endothelial cells (EC) isolated from two robust locations of differing patho-susceptibility:--an athero-susceptible site located at the inner curvature of the aortic arch (AA) and an athero-protected region in the descending thoracic (DT) aorta. Complete methylated DNA immunoprecipitation sequencing (MeDIP-seq) identified over 5500 endothelial differentially methylated regions (DMRs). DMR density was significantly enriched in exons and 5'UTR sequences of annotated genes, 60 of which are linked to cardiovascular disease. The set of DMR-associated genes was enriched in transcriptional regulation, pattern specification HOX loci, oxidative stress and the ER stress adaptive pathway, all categories linked to athero-susceptible endothelium. Examination of the relationship between DMR and mRNA in HOXA genes demonstrated a significant inverse relationship between CpG island promoter methylation and gene expression. Methylation-specific PCR (MSP) confirmed differential CpG methylation of HOXA genes, the ER stress gene ATF4, inflammatory regulator microRNA-10a and ARHGAP25 that encodes a negative regulator of Rho GTPases involved in cytoskeleton remodeling. Gender-specific DMRs associated with ciliogenesis that may be linked to defects in cilia development were also identified in AA DMRs.

CONCLUSIONS

An endothelial methylome analysis identifies epigenetic DMR characteristics associated with transcriptional regulation in regions of atherosusceptibility in swine aorta in vivo. The data represent the first methylome blueprint for spatio-temporal analyses of lesion susceptibility predisposing to endothelial dysfunction in complex flow environments in vivo.

Endothelial epigenetics in biomechanical stress: disturbed flow-mediated epigenomic plasticity in vivo and in vitro

Arterial endothelial phenotype is regulated by local hemodynamic forces that are linked to regional susceptibility to atherogenesis. A complex hierarchy of transcriptional, translational, and post-translational mechanisms is greatly influenced by the characteristics of local arterial shear stress environments. We discuss the emerging role of localized disturbed blood flow on epigenetic mechanisms of endothelial responses to biomechanical stress, including transcriptional regulation by proximal promoter DNA methylation, and post-transcriptional and translational regulation of gene and protein expression by chromatin remodeling and noncoding RNA-based mechanisms. Dynamic responses to flow characteristics in vivo and in vitro include site-specific differentially methylated regions of swine and mouse endothelial methylomes, histone marks regulating chromatin conformation, microRNAs, and long noncoding RNAs. Flow-mediated epigenomic responses intersect with cis and trans factor regulation to maintain endothelial function in a shear-stressed environment and may contribute to localized endothelial dysfunctions that promote atherosusceptibility.

PRRX1- and PRRX2-positive mesenchymal stem/progenitor cells are involved in vasculogenesis during rat embryonic pituitary development

We have recently shown that cells positive for the paired-related homeobox transcription factors PRRX1 and PRRX2 occur in the rat pituitary, and that they are derived from two different origins: pituitary-derived cells positive for stem cell marker SOX2 and extra-pituitary-derived cells negative for SOX2. In this study, we have further characterized the PRRX1- and PRRX2-positive cells that originate from extra-pituitary cells. Immunohistochemical analyses were performed with specific antibodies against PRRX1 and PRRX2 in order to clarify their roles in pituitary vasculogenesis. PRRX1- and PRRX2-positive cells were found in Atwell's recess and at the periphery of the pituitary on embryonic day 15.5 (E15.5). Several PRRX1-positive cells then invaded the anterior lobe, together with a few PRRX2-positive cells, on E16.5. Some PRRX1-positive cells were also positive for mesenchymal stem cell marker NESTIN. Moreover, some PRRX1/NESTIN double-positive cells showed characteristics of vascular endothelial cells with an Isolectin-B4-binding capacity. PRRX1 co-localized with vascular smooth muscle cell/pericyte marker α-smooth muscle actin in the deep area of Atwell's recess. We confirmed the presence of PRRX2/NESTIN double-positive cells at an entry area in Atwell's recess and at the periphery of the pituitary, but PRRX2 did not co-localize with Isolectin B4 or α-smooth muscle actin. These data suggest that PRRX1- and PRRX2-positive mesenchymal stem/progenitor cells are present at the periphery of the embryonic pituitary and at the entry from Atwell's recess and participate in pituitary vasculogenesis by differentiation into vascular endothelial cells and pericytes, whereas the presence of PRRX2 indicates much higher stemness than PRRX1.

Emerging topic: flow-related epigenetic regulation of endothelial phenotype through DNA methylation

Atherosclerosis is a multi-focal disease; it is associated with arterial curvatures, asymmetries and branches/bifurcations where non-uniform arterial geometry generates patterns of blood flow that are considerably more complex than elsewhere, and are collectively referred to as disturbed flow. Such regions are predisposed to atherosclerosis and are the sites of 'athero-susceptible' endothelial cells that express regionally different cell phenotypes than endothelium in nearby athero-protected locations. The regulatory hierarchy of endothelial function includes control at the epigenetic level. MicroRNAs and histone modifications are established epigenetic regulators that respond to disturbed flow. However, very recent reports have linked transcriptional regulation by DNA methylation to endothelial gene expression in disturbed flow in vivo and in vitro. We outline these in the context of site-specific atherosusceptibility mediated by local hemodynamics.

Analysis of homeobox gene action may reveal novel angiogenic pathways in normal placental vasculature and in clinical pregnancy disorders associated with abnormal placental angiogenesis

Homeobox genes are essential for both the development of the blood and lymphatic vascular systems, as well as for their maintenance in the adult. Homeobox genes comprise an important family of transcription factors, which are characterized by a well conserved DNA binding motif; the homeodomain. The specificity of the homeodomain allows the transcription factor to bind to the promoter regions of batteries of target genes and thereby regulates their expression. Target genes identified for homeodomain proteins have been shown to control fundamental cell processes such as proliferation, differentiation, and apoptosis. We and others have reported that homeobox genes are expressed in the placental vasculature, but our knowledge of their downstream target genes is limited. This review highlights the importance of studying the cellular and molecular mechanisms by which homeobox genes and their downstream targets may regulate important vascular cellular processes such as proliferation, migration, and endothelial tube formation, which are essential for placental vasculogenesis and angiogenesis. A better understanding of the molecular targets of homeobox genes may lead to new therapies for aberrant angiogenesis associated with clinically important pregnancy pathologies, including fetal growth restriction and preeclampsia.

Regional expression of HOXA4 along the aorta and its potential role in human abdominal aortic aneurysms

BACKGROUND

The infrarenal abdominal aorta exhibits increased disease susceptibility relative to other aortic regions. Allograft studies exchanging thoracic and abdominal segments showed that regional susceptibility is maintained regardless of location, suggesting substantial roles for embryological origin, tissue composition and site-specific gene expression.

RESULTS

We analyzed gene expression with microarrays in baboon aortas, and found that members of the HOX gene family exhibited spatial expression differences. HOXA4 was chosen for further study, since it had decreased expression in the abdominal compared to the thoracic aorta. Western blot analysis from 24 human aortas demonstrated significantly higher HOXA4 protein levels in thoracic compared to abdominal tissues (P < 0.001). Immunohistochemical staining for HOXA4 showed nuclear and perinuclear staining in endothelial and smooth muscle cells in aorta. The HOXA4 transcript levels were significantly decreased in human abdominal aortic aneurysms (AAAs) compared to age-matched non-aneurysmal controls (P < 0.00004). Cultured human aortic endothelial and smooth muscle cells stimulated with INF-γ (an important inflammatory cytokine in AAA pathogenesis) showed decreased levels of HOXA4 protein (P < 0.0007).

CONCLUSIONS

Our results demonstrated spatial variation in expression of HOXA4 in human aortas that persisted into adulthood and that downregulation of HOXA4 expression was associated with AAAs, an important aortic disease of the ageing population.

The VEGF-regulated transcription factor HLX controls the expression of guidance cues and negatively regulates sprouting of endothelial cells

The HLX gene encoding a diverged homeobox transcription factor has been found to be up-regulated by vascular endothelial growth factor-A (VEGF-A) in endothelial cells. We have now investigated the gene repertoire induced by HLX and its potential biologic function. HLX strongly increased the transcripts for several repulsive cell-guidance proteins including UNC5B, plexin-A1, and semaphorin-3G. In addition, genes for transcriptional repressors such as HES-1 were up-regulated. In line with these findings, adenoviral overexpression of HLX inhibited endothelial cell migration, sprouting, and vessel formation in vitro and in vivo, whereas proliferation was unaffected. This inhibition of sprouting was caused to a significant part by HLX-mediated up-regulation of UNC5B as shown by short hairpin RNA (shRNA)-mediated down-modulation of the respective mRNA. VEGF-A stimulation of endothelial cells induced elevated levels of HLX over longer time periods resulting in especially high up-regulation of UNC5B mRNA as well as an increase in cells displaying UNC5B at their surface. However, induction of HLX was strongly reduced and UNC5B up-regulation completely abrogated when cells were exposed to hypoxic conditions. These data suggest that HLX may function to balance attractive with repulsive vessel guidance by up-regulating UNC5B and to down-modulate sprouting under normoxic conditions.

Environment and vascular bed origin influence differences in endothelial transcriptional profiles of coronary and iliac arteries

Atherosclerotic plaques tend to form in the major arteries at certain predictable locations. As these arteries vary in atherosusceptibility, interarterial differences in endothelial cell biology are of considerable interest. To explore the origin of differences observed between typical atheroprone and atheroresistant arteries, we used DNA microarrays to compare gene expression profiles of harvested porcine coronary (CECs) and iliac artery endothelial cells (IECs) grown in static culture out to passage 4. Fewer differences were observed between the transcriptional profiles of CECs and IECs in culture compared with in vivo, suggesting that most differences observed in vivo were due to distinct environmental cues in the two arteries. One-class significance of microarrays revealed that most in vivo interarterial differences disappeared in culture, as fold differences after passaging were not significant for 85% of genes identified as differentially expressed in vivo at 5% false discovery rate. However, the three homeobox genes, HOXA9, HOXA10, and HOXD3, remained underexpressed in coronary endothelium for all passages by at least nine-, eight-, and twofold, respectively. Continued differential expression, despite removal from the in vivo environment, suggests that primarily heritable or epigenetic mechanism(s) influences transcription of these three genes. Quantitative real-time polymerase chain reaction confirmed expression ratios for seven genes associated with atherogenesis and over- or underexpressed by threefold in CECs relative to IECs. The present study provides evidence that both local environment and vascular bed origin modulate gene expression in arterial endothelium. The transcriptional differences observed here may provide new insights into pathways responsible for coronary artery susceptibility.

In vivo differences between endothelial transcriptional profiles of coronary and iliac arteries revealed by microarray analysis

Endothelial cells (ECs) from different vascular beds display a remarkable heterogeneity in both structure and function. Phenotypic heterogeneity among arterial ECs is particularly relevant to atherosclerosis since the disease occurs predominantly in major arteries, which vary in their atherosusceptibility. To explore EC heterogeneity between typical atheroprone and atheroresistant arteries, we used DNA microarrays to compare gene expression profiles of freshly harvested porcine coronary (CECs) and iliac artery (IECs) ECs. Statistical analysis revealed 51 genes that were differentially expressed in CECs relative to IECs at a false discovery rate of 5%. Seventeen of these genes are known to be involved in atherogenesis. Consistent with coronary arteries being more atherosusceptible, almost all putative atherogenic genes were overexpressed in CECs, whereas all atheroprotective genes were downregulated, relative to IECs. A subset of the identified genes was validated by quantitative polymerase chain reaction (PCR). PCR results suggest that the differences in expression levels between CECs and IECs for the HOXA10 and HOXA9 genes were >100-fold. Gene ontology (GO) and biological pathway analysis revealed a global expression difference between CECs and IECs. Genes in twelve GO categories, including complement immune activation, immunoglobulin-mediated response, and system development, were significantly upregulated in CECs. CECs also overexpressed genes involved in several inflammatory pathways, including the classical pathway of complement activation and the IGF-1-mediated pathway. The in vivo transcriptional differences between CECs and IECs found in this study may provide new insights into the factors responsible for coronary artery atherosusceptibility.

Novel homeobox genes are differentially expressed in placental microvascular endothelial cells compared with macrovascular cells

Angiogenesis is fundamental to normal placental development and aberrant angiogenesis contributes substantially to placental pathologies. The complex process of angiogenesis is regulated by transcription factors leading to the formation of endothelial cells that line the microvasculature. Homeobox genes are important transcription factors that regulate vascular development in embryonic and adult tissues. We have recently shown that placental homeobox genes HLX, DLX3, DLX4, MSX2 and GAX are expressed in placental endothelial cells. Hence, the novel homeobox genes TLX1, TLX2, TGIF, HEX, PHOX1, MEIS2, HOXB7, and LIM6 were detected that have not been reported in endothelial cells previously. Importantly, these homeobox genes have not been previously reported in placental endothelial cells and, with the exception of HEX, PHOX1 and HOXB7, have not been described in any other endothelial cell type. Reverse transcriptase PCR was performed on cDNA from freshly isolated placental microvascular endothelial cells (PLEC), and the human placental microvascular endothelial cell line HPEC. cDNAs prepared from control term placentae, human microvascular endothelial cells (HMVEC) and human umbilical vein macrovascular endothelial cells (HUVEC) were used as controls. PCR analyses showed that all novel homeobox genes tested were expressed by all endothelial cells types. Furthermore, real-time PCR analyses revealed that homeobox genes TLX1, TLX2 and PHOX1 relative mRNA expression levels were significantly decreased in HUVEC compared with microvascular endothelial cells, while the relative mRNA expression levels of MEIS2 and TGIF were significantly increased in macrovascular cells compared with microvascular endothelial cells. Thus we have identified novel homeobox genes in microvascular endothelial cells and have shown that homeobox genes are differentially expressed between micro- and macrovascular endothelial cells.