Brant M. Weinstein

sonic hedgehog and vascular endothelial growth factor Act Upstream of the Notch Pathway during Arterial Endothelial Differentiation

The appearance of molecular differences between arterial and venous endothelial cells before circulation suggests that genetic factors determine these cell types. We find that vascular endothelial growth factor (vegf) acts downstream of sonic hedgehog (shh) and upstream of the Notch pathway to determine arterial cell fate. Loss of Vegf or Shh results in loss of arterial identity, while exogenous expression of these factors causes ectopic expression of arterial markers. Microinjection of vegf mRNA into embryos lacking Shh activity can rescue arterial differentiation. Finally, activation of the Notch pathway in the absence of Vegf signaling can rescue arterial marker gene expression. These studies reveal a complex signaling cascade responsible for establishing arterial cell fate and suggest differential effects of Vegf on developing endothelial cells.

The Control of Vascular Integrity by Endothelial Cell Junctions: Molecular Basis and Pathological Implications

Human pathologies such as vascular malformations, hemorrhagic stroke, and edema have been associated with defects in the organization of endothelial cell junctions. Understanding the molecular basis of these diseases requires different integrated approaches which include basic cell biology, clinical studies, and studies in animal models such as mice and zebrafish. In this review we discuss recent findings derived from these approaches and their possible integration in a common picture.

Cardiac troponin T is essential in sarcomere assembly and cardiac contractility.

Mutations of the gene (TNNT2) encoding the thin-filament contractile protein cardiac troponin T are responsible for 15% of all cases of familial hypertrophic cardiomyopathy, the leading cause of sudden death in young athletes. Mutant proteins are thought to act through a dominant-negative mode that impairs function of heart muscle. TNNT2 mutations can also lead to dilated cardiomyopathy, a leading cause of heart failure. Despite the importance of cardiac troponin T in human disease, its loss-of-function phenotype has not been described. We show that the zebrafish silent heart (sih) mutation affects the gene tnnt2. We characterize two mutated alleles of sih that severely reduce tnnt2 expression: one affects mRNA splicing, and the other affects gene transcription. Tnnt2, together with α-tropomyosin (Tpma) and cardiac troponins C and I (Tnni3), forms a calcium-sensitive regulatory complex within sarcomeres. Unexpectedly, in addition to loss of Tnnt2 expression in sih mutant hearts, we observed a significant reduction in Tpma and Tnni3, and consequently, severe sarcomere defects. This interdependence of thin-filament protein expression led us to postulate that some mutations in tnnt2 may trigger misregulation of thin-filament protein expression, resulting in sarcomere loss and myocyte disarray, the life-threatening hallmarks of TNNT2 mutations in mice and humans.

UNIVERSAL GFP REPORTER FOR THE STUDY OF VASCULAR DEVELOPMENT

Summary: We report the generation and characterization of transgenic mouse and zebrafish expressing green fluorescent protein (GFP) specifically in vascular endothelial cells in a relatively uniform fashion. These reporter lines exhibit fluorescent vessels in developing embryos and throughout adulthood, allowing visualization of the general vascular patterns with single cell resolution. Furthermore, we show the ability to purify endothelial cells from whole embryos and adult organs by a single step fluorescence activated cell sorting. We expect that these transgenic reporters will be useful tools for imaging vascular morphogenesis, global gene expression profile analysis of endothelial cells, and high throughput screening for vascular mutations. genesis 28:75–81, 2000.

Endothelial tubes assemble from intracellular vacuoles in vivo

The formation of epithelial tubes is crucial for the proper development of many different tissues and organs, and occurs by means of a variety of different mechanisms. Morphogenesis of seamless, properly patterned endothelial tubes is essential for the development of a functional vertebrate circulatory system, but the mechanism of vascular lumenization in vivo remains unclear. Evidence dating back more than 100 years has hinted at an important function for endothelial vacuoles in lumen formation. More than 25 years ago, in some of the first endothelial cell culture experiments in vitro, Folkman and Haudenschild described “longitudinal vacuoles” that “appeared to be extruded and connected from one cell to the next”, observations confirmed and extended by later studies in vitro showing that intracellular vacuoles arise from integrin-dependent and cdc42/Rac1-dependent pinocytic events downstream of integrin–extracellular-matrix signalling interactions. Despite compelling data supporting a model for the assembly of endothelial tubes in vitro through the formation and fusion of vacuoles, conclusive evidence in vivo has been lacking, primarily because of difficulties associated with imaging the dynamics of subcellular endothelial vacuoles deep within living animals. Here we use high-resolution time-lapse two-photon imaging of transgenic zebrafish to examine how endothelial tubes assemble in vivo, comparing our results with time-lapse imaging of human endothelial-cell tube formation in three-dimensional collagen matrices in vitro. Our results provide strong support for a model in which the formation and intracellular and intercellular fusion of endothelial vacuoles drives vascular lumen formation.

Angiogenic network formation in the developing vertebrate trunk.

We have used time-lapse multiphoton microscopy of living Tg(fli1:EGFP)y1 zebrafish embryos to examine how a patterned, functional network of angiogenic blood vessels is generated in the early vertebrate trunk. Angiogenic vascular sprouts emerge from the longitudinal trunk axial vessels (the dorsal aorta and posterior cardinal vein) in two spatially and temporally distinct steps. Dorsal aorta-derived sprouts form an initial primary network of vascular segments, followed by emergence of vein-derived secondary vascular sprouts that interact and interconnect dynamically with the primary network to initiate vascular flow. Using transgenic silent heart mutant embryos, we show that the gross anatomical patterning of this network of vessels does not require blood circulation. However, our results suggest that circulatory flow dynamics play an important role in helping to determine the pattern of interconnections between the primary network and secondary sprouts, and thus the final arterial or venous identity of the vessels in the functional network. We discuss a model to explain our results combining genetic programming of overall vascular architecture with hemodynamic determination of circulatory flow patterns.

Live imaging of lymphatic development in the zebrafish.

The lymphatic system has become the subject of great interest in recent years because of its important role in normal and pathological processes. Progress in understanding the origins and early development of this system, however, has been hampered by difficulties in observing lymphatic cells in vivo and in performing defined genetic and experimental manipulation of the lymphatic system in currently available model organisms. Here, we show that the optically clear developing zebrafish provides a useful model for imaging and studying lymphatic development, with a lymphatic system that shares many of the morphological, molecular and functional characteristics of the lymphatic vessels found in other vertebrates. Using two-photon time-lapse imaging of transgenic zebrafish, we trace the migration and lineage of individual cells incorporating into the lymphatic endothelium. Our results show lymphatic endothelial cells of the thoracic duct arise from primitive veins through a novel and unexpected pathway.

Arterial–Venous Specification During Development

The major arteries and veins of the vertebrate circulatory system are formed early in embryonic development, before the onset of circulation, following de novo aggregation of “angioblast” progenitors in a process called vasculogenesis. Initial embryonic determination of artery or vein identity is regulated by variety of genetic factors that work in concert to specify endothelial cell fate, giving rise to 2 distinct components of the circulatory loop possessing unique structural characteristics. Work in multiple in vivo animal model systems has led to a detailed examination of the interacting partners that determine arterial and venous specification. We discuss the hierarchical arrangement of many signaling molecules, including Hedgehog (Hh), vascular endothelial growth factor (VEGF), Notch, and chicken ovalbumin upstream-transcription factor II (COUP-TFII) that promote or inhibit divergent pathways of endothelial cell fate. Elucidation of the functional role of these genetic determinants of blood vessel specification together with the epigenetic factors involved in subsequent modification of arterial–venous identity will allow for potential new therapeutic targets for vascular disorders.

Arteries and veins: making a difference with zebrafish

Arteries and veins are structurally different and have long been functionally defined by the direction of blood flow that they carry. However, a growing body of evidence indicates that the identity of the endothelial cells that line these vessels is determined in the developing embryo, before circulation begins. Recent work on the zebrafish has led to the identification of signals that are responsible for arterial and venous differentiation of endothelial cells, and highlights the unique benefits of this model organism in the study of vascular development.

An essential role for Fgfs in endodermal pouch formation influences later craniofacial skeletal patterning

Fibroblast growth factor (Fgf) proteins are important regulators of pharyngeal arch development. Analyses of Fgf8 function in chick and mouse and Fgf3 function in zebrafish have demonstrated a role for Fgfs in the differentiation and survival of postmigratory neural crest cells (NCC) that give rise to the pharyngeal skeleton. Here we describe, in zebrafish, an earlier, essential function for Fgf8 and Fgf3 in regulating the segmentation of the pharyngeal endoderm into pouches. Using time-lapse microscopy, we show that pharyngeal pouches form by the directed lateral migration of discrete clusters of endodermal cells. In animals doubly reduced for Fgf8 and Fgf3, the migration of pharyngeal endodermal cells is disorganized and pouches fail to form. Transplantation and pharmacological experiments show that Fgf8 and Fgf3 are required in the neural keel and cranial mesoderm during early somite stages to promote first pouch formation. In addition, we show that animals doubly reduced for Fgf8 and Fgf3 have severe reductions in hyoid cartilages and the more posterior branchial cartilages. By examining early pouch and later cartilage phenotypes in individual animals hypomorphic for Fgf function, we find that alterations in pouch structure correlate with later cartilage defects. We present a model in which Fgf signaling in the mesoderm and segmented hindbrain organizes the segmentation of the pharyngeal endoderm into pouches. Moreover, we argue that the Fgf-dependent morphogenesis of the pharyngeal endoderm into pouches is critical for the later patterning of pharyngeal cartilages.