Wnt signaling positively regulates endothelial cell fate specification in the Fli1a-positive progenitor population via Lef1

Preliminary analysis of pathways and their implications during salinity stress in abalone

Transcriptome sequencing has offered immense opportunities to study non-model organisms. Abalone is an important marine mollusk that encounters harsh environmental conditions in its natural habitat and under aquaculture conditions; hence, research that increases molecular information to understand abalone physiology and stress response is noteworthy. Accordingly, the study used transcriptome sequencing of the gill tissues of abalone exposed to low salinity stress. The aim is to explore some enriched pathways during salinity stress and the crosstalk and functions of the genes involved in the candidate biological processes for future further analysis of their expression patterns. The data suggest that abalone genes such as YAP/TAZ, Myc, Nkd, and Axin (involved in the Hippo signaling pathway) and PI3K/Akt, SHC, and RTK (involved in the Ras signaling pathways) might mediate growth and development. Thus, deregulation of the Hippo and Ras pathways by salinity stress could be a possible mechanism by which unfavorable salinities influence growth in abalone. Furthermore, PEPCK, GYS, and PLC genes (mediating the Glucagon signaling pathway) might be necessary for glucose homeostasis, reproduction, and abalone meat sensory qualities; hence, a need to investigate how they might be influenced by environmental stress. Genes such as MYD88, IRAK1/4, JNK, AP-1, and TRAF6 (mediating the MAPK signaling pathway) could be useful in understanding abalone's innate immune response to environmental stresses. Finally, the aminoacyl-tRNA biosynthesis pathway hints at the mechanism by which new raw materials for protein biosynthesis are mobilized for physiological processes and how abalone might respond to this process during salinity stress. Low salinity clearly regulated genes in these pathways in a time-dependent manner, as hinted by the heat maps. In the future, qRT-PCR verification and in-depth study of the various genes and proteins discussed would provide enormous molecular information resources for the abalone biology.

The role of Wnt signaling in mesenchymal stromal cell-driven angiogenesis

Development, growth, and remodeling of blood vessels occur through an intricate process involving cell differentiation, proliferation, and rearrangement by cell migration under the direction of various signaling pathways. Recent reports highlight that resident and exogenous mesenchymal stromal cells (MSCs) have the potential to regulate the neovascularization process through paracrine secretion of proangiogenic factors. Recent research has established that the vasculogenic potential of MSCs is regulated by several signaling pathways, including the Wnt signaling pathway, and their interplay. These findings emphasize the complex nature of the vasculogenic process and underscore the importance of understanding the underlying molecular mechanisms for the development of effective cell-based therapies in regenerative medicine. This review provides an updated briefing on the canonical and non-canonical Wnt signaling pathways and summarizes the recent reports of both in vitro and in vivo studies with the involvement of MSCs of various sources in the vasculogenic process mediated by Wnt signaling pathways. Here we outline the current understanding of the plausible role of the Wnt signaling pathway, specifically in MSC-regulated angiogenesis.

Dermomyotome-derived endothelial cells migrate to the dorsal aorta to support hematopoietic stem cell emergence

Development of the dorsal aorta is a key step in the establishment of the adult blood-forming system, since hematopoietic stem and progenitor cells (HSPCs) arise from ventral aortic endothelium in all vertebrate animals studied. Work in zebrafish has demonstrated that arterial and venous endothelial precursors arise from distinct subsets of lateral plate mesoderm. Here, we profile the transcriptome of the earliest detectable endothelial cells (ECs) during zebrafish embryogenesis to demonstrate that tissue-specific EC programs initiate much earlier than previously appreciated, by the end of gastrulation. Classic studies in the chick embryo showed that paraxial mesoderm generates a subset of somite-derived endothelial cells (SDECs) that incorporate into the dorsal aorta to replace HSPCs as they exit the aorta and enter circulation. We describe a conserved program in the zebrafish, where a rare population of endothelial precursors delaminates from the dermomyotome to incorporate exclusively into the developing dorsal aorta. Although SDECs lack hematopoietic potential, they act as a local niche to support the emergence of HSPCs from neighboring hemogenic endothelium. Thus, at least three subsets of ECs contribute to the developing dorsal aorta: vascular ECs, hemogenic ECs, and SDECs. Taken together, our findings indicate that the distinct spatial origins of endothelial precursors dictate different cellular potentials within the developing dorsal aorta.

Endoderm-derived islet1-expressing cells differentiate into endothelial cells to function as the vascular HSPC niche in zebrafish

Endothelial cells (ECs) line blood vessels and serve as a niche for hematopoietic stem and progenitor cells (HSPCs). Recent data point to tissue-specific EC specialization as well as heterogeneity; however, it remains unclear how ECs acquire these properties. Here, by combining live-imaging-based lineage-tracing and single-cell transcriptomics in zebrafish embryos, we identify an unexpected origin for part of the vascular HSPC niche. We find that islet1 (isl1)-expressing cells are the progenitors of the venous ECs that constitute the majority of the HSPC niche. These isl1-expressing cells surprisingly originate from the endoderm and differentiate into ECs in a process dependent on Bmp-Smad signaling and subsequently requiring npas4l (cloche) function. Single-cell RNA sequencing analyses show that isl1-derived ECs express a set of genes that reflect their distinct origin. This study demonstrates that endothelial specialization in the HSPC niche is determined at least in part by the origin of the ECs.

Single-cell transcriptomic analysis of vascular endothelial cells in zebrafish embryos

Vascular endothelial cells exhibit substantial phenotypic and transcriptional heterogeneity which is established during early embryogenesis. However, the molecular mechanisms involved in establishing endothelial cell diversity are still not well understood. Zebrafish has emerged as an advantageous model to study vascular development. Despite its importance, the single-cell transcriptomic profile of vascular endothelial cells during zebrafish development is still missing. To address this, we applied single-cell RNA-sequencing (scRNA-seq) of vascular endothelial cells isolated from zebrafish embryos at the 24 hpf stage. Six distinct clusters or subclusters related to vascular endothelial cells were identified which include arterial, two venous, cranial, endocardial and endothelial progenitor cell subtypes. Furthermore, we validated our findings by characterizing novel markers for arterial, venous, and endocardial cells. We experimentally confirmed the presence of two transcriptionally different venous cell subtypes, demonstrating heterogeneity among venous endothelial cells at this early developmental stage. This dataset will be a valuable resource for future functional characterization of vascular endothelial cells and interrogation of molecular mechanisms involved in the establishment of their heterogeneity and cell-fate decisions.

Updated perspectives on vascular cell specification and pluripotent stem cell-derived vascular organoids for studying vasculopathies

Vasculopathy is a pathological process occurring in the blood vessel wall, which could affect the haemostasis and physiological functions of all the vital tissues/organs and is one of the main underlying causes for a variety of human diseases including cardiovascular diseases. Current pharmacological interventions aiming to either delay or stop progression of vasculopathies are suboptimal, thus searching novel, targeted, risk-reducing therapeutic agents, or vascular grafts with full regenerative potential for patients with vascular abnormalities are urgently needed. Since first reported, pluripotent stem cells (PSCs), particularly human-induced PSCs, have open new avenue in all research disciplines including cardiovascular regenerative medicine and disease remodelling. Assisting with recent technological breakthroughs in tissue engineering, in vitro construction of tissue organoid made a tremendous stride in the past decade. In this review, we provide an update of the main signal pathways involved in vascular cell differentiation from human PSCs and an extensive overview of PSC-derived tissue organoids, highlighting the most recent discoveries in the field of blood vessel organoids as well as vascularization of other complex tissue organoids, with the aim of discussing the key cellular and molecular players in generating vascular organoids.

Wnt/β-catenin signaling acts cell-autonomously to promote cardiomyocyte regeneration in the zebrafish heart

Zebrafish can achieve scar-free healing of heart injuries, and robustly replace all cardiomyocytes lost to injury via dedifferentiation and proliferation of mature cardiomyocytes. Previous studies suggested that Wnt/β-catenin signaling is active in the injured zebrafish heart, where it induces fibrosis and prevents cardiomyocyte cell cycling. Here, via targeting the destruction complex of the Wnt/β-catenin pathway with pharmacological and genetic tools, we demonstrate that Wnt/β-catenin activity is required for cardiomyocyte proliferation and dedifferentiation, as well as for maturation of the scar during regeneration. Using cardiomyocyte-specific conditional inhibition of the pathway, we show that Wnt/β-catenin signaling acts cell-autonomously to promote cardiomyocyte proliferation. Our results stand in contrast to previous reports and rather support a model in which Wnt/β-catenin signaling plays a positive role during heart regeneration in zebrafish.

Mapping Human Pluripotent Stem Cell-derived Erythroid Differentiation by Single-cell Transcriptome Analysis

There is an imbalance between the supply and demand of functional red blood cells (RBCs) in clinical applications. This imbalance can be addressed by regenerating RBCs using several in vitro methods. Induced pluripotent stem cells (iPSCs) can handle the low supply of cord blood and the ethical issues in embryonic stem cell research and provide a promising strategy to eliminate immune rejection. However, no complete single-cell level differentiation pathway exists for the iPSC-derived RBC differentiation system. In this study, we used iPSC line BC1 to establish a RBCs regeneration system. The 10× genomics single-cell transcriptome platform was used to map the cell lineage and differentiation trajectories on day 14 of the regeneration system. We observed that iPSCs differentiation was not synchronized during embryoid body (EB) culture. The cells (day 14) mainly consisted of mesodermal and various blood cells, similar to the yolk sac hematopoiesis. We identified six cell classifications and characterized the regulatory transcription factors (TFs) networks and cell-cell contacts underlying the system. iPSCs undergo two transformations during the differentiation trajectory, accompanied by the dynamic expression of cell adhesion molecules and estrogen-responsive genes. We identified different stages of erythroid cells, such as burst-forming unit erythroid (BFU-E) and orthochromatic erythroblasts (ortho-E), and found that the regulation of TFs (e.g., TFDP1 and FOXO3) is erythroid-stage specific. Immune erythroid cells were identified in our system. This study provides systematic theoretical guidance for optimizing the iPSCs-derived RBCs differentiation system, and this system is a useful model for simulating in vivo hematopoietic development and differentiation.

Progenitor/Stem Cells in Vascular Remodeling during Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is characterized by an important occlusive vascular remodeling with the production of new endothelial cells, smooth muscle cells, myofibroblasts, and fibroblasts. Identifying the cellular processes leading to vascular proliferation and dysfunction is a major goal in order to decipher the mechanisms leading to PAH development. In addition to in situ proliferation of vascular cells, studies from the past 20 years have unveiled the role of circulating and resident vascular in pulmonary vascular remodeling. This review aims at summarizing the current knowledge on the different progenitor and stem cells that have been shown to participate in pulmonary vascular lesions and on the pathways regulating their recruitment during PAH. Finally, this review also addresses the therapeutic potential of circulating endothelial progenitor cells and mesenchymal stem cells.

Transglutaminase Type 2 regulates the Wnt/β-catenin pathway in vertebrates

TG2 is a multifunctional enzyme involved in several cellular processes and has emerging as a potential regulator of gene expression. In this regard, we have recently shown that TG2 is able to activate HSF1, the master transcriptional regulator of the stress-responsive genes; however, its effect on the overall gene expression remains unclear. To address this point, we analyzed, by RNA-seq, the effect of TG2 on the overall transcriptome as well as we characterized the TG2 interactome in the nucleus. The data obtained from these omics approaches reveal that TG2 markedly influences the overall cellular transcriptome profile and specifically the Wnt and HSF1 pathways. In particular, its ablation leads to a drastic downregulation of many key members of these pathways. Interestingly, we found that key components of the Wnt/β-catenin pathway are also downregulated in cells lacking HSF1, thus confirming that TG2 regulates the HSF1 and this axis controls the Wnt signaling. Mechanistic studies revealed that TG2 can regulate the Wnt pathway by physically interacts with β-catenin and its nuclear interactome includes several proteins known to be involved in the regulation of the Wnt signaling. In order to verify whether this effect is playing a role in vivo, we ablated TG2 in Danio rerio. Our data show that the zebrafish lacking TG2 cannot complete the development and their death is associated with an evident downregulation of the Wnt pathway and a defective heat-shock response. Our findings show for the first time that TG2 is essential for the correct embryonal development of lower vertebrates, and its action is mediated by the Wnt/HSF1 axis.

Dynamic Expression Profiles of β-Catenin during Murine Cardiac Valve Development

β-catenin has been widely studied in many animal and organ systems across evolution, and gain or loss of function has been linked to a number of human diseases. Yet fundamental knowledge regarding its protein expression and localization remains poorly described. Thus, we sought to define whether there was a temporal and cell-specific regulation of β-catenin activities that correlate with distinct cardiac morphological events. Our findings indicate that activated nuclear β-catenin is primarily evident early in gestation. As development proceeds, nuclear β-catenin is down-regulated and becomes restricted to the membrane in a subset of cardiac progenitor cells. After birth, little β-catenin is detected in the heart. The co-expression of β-catenin with its main transcriptional co-factor, Lef1, revealed that Lef1 and β-catenin expression domains do not extensively overlap in the cardiac valves. These data indicate mutually exclusive roles for Lef1 and β-catenin in most cardiac cell types during development. Additionally, these data indicate diverse functions for β-catenin within the nucleus and membrane depending on cell type and gestational timing. Cardiovascular studies should take into careful consideration both nuclear and membrane β-catenin functions and their potential contributions to cardiac development and disease.

Advantages and Challenges of Cardiovascular and Lymphatic Studies in Zebrafish Research

Since its introduction, the zebrafish has provided an important reference system to model and study cardiovascular development as well as lymphangiogenesis in vertebrates. A scientific workshop, held at the 2018 European Zebrafish Principal Investigators Meeting in Trento (Italy) and chaired by Massimo Santoro, focused on the most recent methods and studies on cardiac, vascular and lymphatic development. Daniela Panáková and Natascia Tiso described new molecular mechanisms and signaling pathways involved in cardiac differentiation and disease. Arndt Siekmann and Wiebke Herzog discussed novel roles for Wnt and VEGF signaling in brain angiogenesis. In addition, Brant Weinstein's lab presented data concerning the discovery of endothelium-derived macrophage-like perivascular cells in the zebrafish brain, while Monica Beltrame's studies refined the role of Sox transcription factors in vascular and lymphatic development. In this article, we will summarize the details of these recent discoveries in support of the overall value of the zebrafish model system not only to study normal development, but also associated disease states.

Development of the brain vasculature and the blood-brain barrier in zebrafish

To ensure tissue homeostasis the brain needs to be protected from blood-derived fluctuations or pathogens that could affect its function. Therefore, the brain capillaries develop tissue-specific properties to form a selective blood-brain barrier (BBB), allowing the passage of essential molecules to the brain and blocking the penetration of potentially harmful compounds or cells. Previous studies reported the presence of this barrier in zebrafish. The intrinsic features of the zebrafish embryos and larvae in combination with optical techniques, make them suitable for the study of barrier establishment and maturation. In this review, we discuss the most recent contributions to the development and formation of a functional zebrafish BBB. Moreover, we compare the molecular and cellular characteristic of the zebrafish and the mammalian BBB.

Recent insights into vascular development from studies in zebrafish

PURPOSE OF REVIEW

Zebrafish has provided a powerful platform to study vascular biology over the past 25 years, owing to their distinct advantages for imaging and genetic manipulation. In this review, we summarize recent progress in vascular biology with particular emphasis on vascular development in zebrafish.

RECENT FINDINGS

The advent of transcription activator-like effector nuclease and clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 genome-editing technologies has dramatically facilitated reverse genetic approaches in zebrafish, as in other models. Here, we highlight recent studies on vascular development in zebrafish which mainly employed forward or reverse genetics combined with high-resolution imaging. These studies have advanced our understanding of diverse areas in vascular biology, including transcriptional regulation of endothelial cell differentiation, endothelial cell signaling during angiogenesis and lymphangiogenesis, vascular bed-specific developmental mechanisms, and perivascular cell recruitment.

SUMMARY

The unique attributes of the zebrafish model have allowed critical cellular and molecular insights into fundamental mechanisms of vascular development. Knowledge acquired through recent zebrafish work further advances our understanding of basic mechanisms underlying vascular morphogenesis, maintenance, and homeostasis. Ultimately, insights provided by the zebrafish model will help to understand the genetic, cellular, and molecular underpinnings of human vascular malformations and diseases.

Wnt/β-catenin signaling regulates VE-cadherin-mediated anastomosis of brain capillaries by counteracting S1pr1 signaling

Canonical Wnt signaling is crucial for vascularization of the central nervous system and blood-brain barrier (BBB) formation. BBB formation and modulation are not only important for development, but also relevant for vascular and neurodegenerative diseases. However, there is little understanding of how Wnt signaling contributes to brain angiogenesis and BBB formation. Here we show, using high resolution in vivo imaging and temporal and spatial manipulation of Wnt signaling, different requirements for Wnt signaling during brain angiogenesis and BBB formation. In the absence of Wnt signaling, premature Sphingosine-1-phosphate receptor (S1pr) signaling reduces VE-cadherin and Esama at cell-cell junctions. We suggest that Wnt signaling suppresses S1pr signaling during angiogenesis to enable the dynamic junction formation during anastomosis, whereas later S1pr signaling regulates BBB maturation and VE-cadherin stabilization. Our data provides a link between brain angiogenesis and BBB formation and identifies Wnt signaling as coordinator of the timing and as regulator of anastomosis.

Gfi1aa and Gfi1b set the pace for primitive erythroblast differentiation from hemangioblasts in the zebrafish embryo

The transcriptional repressors Gfi1(a) and Gfi1b are epigenetic regulators with unique and overlapping roles in hematopoiesis. In different contexts, Gfi1 and Gfi1b restrict or promote cell proliferation, prevent apoptosis, influence cell fate decisions, and are essential for terminal differentiation. Here, we show in primitive red blood cells (prRBCs) that they can also set the pace for cellular differentiation. In zebrafish, prRBCs express 2 of 3 zebrafish Gfi1/1b paralogs, Gfi1aa and Gfi1b. The recently identified zebrafish gfi1aa gene trap allele qmc551 drives erythroid green fluorescent protein (GFP) instead of Gfi1aa expression, yet homozygous carriers have normal prRBCs. prRBCs display a maturation defect only after splice morpholino-mediated knockdown of Gfi1b in gfi1aa qmc551 homozygous embryos. To study the transcriptome of the Gfi1aa/1b double-depleted cells, we performed an RNA-Seq experiment on GFP-positive prRBCs sorted from 20-hour-old embryos that were heterozygous or homozygous for gfi1aa qmc551 , as well as wt or morphant for gfi1b We subsequently confirmed and extended these data in whole-mount in situ hybridization experiments on newly generated single- and double-mutant embryos. Combined, the data showed that in the absence of Gfi1aa, the synchronously developing prRBCs were delayed in activating late erythroid differentiation, as they struggled to suppress early erythroid and endothelial transcription programs. The latter highlighted the bipotent nature of the progenitors from which prRBCs arise. In the absence of Gfi1aa, Gfi1b promoted erythroid differentiation as stepwise loss of wt gfi1b copies progressively delayed Gfi1aa-depleted prRBCs even further, showing that Gfi1aa and Gfi1b together set the pace for prRBC differentiation from hemangioblasts.