Directed evolution of a far-red fluorescent rhodopsin.

Microbial rhodopsins are a diverse group of photoactive transmembrane proteins found in all three domains of life. A member of this protein family, Archaerhodopsin-3 (Arch) of halobacterium Halorubrum sodomense, was recently shown to function as a fluorescent indicator of membrane potential when expressed in mammalian neurons. Arch fluorescence, however, is very dim and is not optimal for applications in live-cell imaging. We used directed evolution to identify mutations that dramatically improve the absolute brightness of Arch, as confirmed biochemically and with live-cell imaging (in Escherichia coli and human embryonic kidney 293 cells). In some fluorescent Arch variants, the pK(a) of the protonated Schiff-base linkage to retinal is near neutral pH, a useful feature for voltage-sensing applications. These bright Arch variants enable labeling of biological membranes in the far-red/infrared and exhibit the furthest red-shifted fluorescence emission thus far reported for a fluorescent protein (maximal excitation/emission at ∼ 620 nm/730 nm).

The novel transmembrane protein Tmem2 is essential for coordination of myocardial and endocardial morphogenesis.

Coordination between adjacent tissues plays a crucial role during the morphogenesis of developing organs. In the embryonic heart, two tissues - the myocardium and the endocardium - are closely juxtaposed throughout their development. Myocardial and endocardial cells originate in neighboring regions of the lateral mesoderm, migrate medially in a synchronized fashion, collaborate to create concentric layers of the heart tube, and communicate during formation of the atrioventricular canal. Here, we identify a novel transmembrane protein, Tmem2, that has important functions during both myocardial and endocardial morphogenesis. We find that the zebrafish mutation frozen ventricle (frv) causes ectopic atrioventricular canal characteristics in the ventricular myocardium and endocardium, indicating a role of frv in the regional restriction of atrioventricular canal differentiation. Furthermore, in maternal-zygotic frv mutants, both myocardial and endocardial cells fail to move to the midline normally, indicating that frv facilitates cardiac fusion. Positional cloning reveals that the frv locus encodes Tmem2, a predicted type II single-pass transmembrane protein. Homologs of Tmem2 are present in all examined vertebrate genomes, but nothing is known about its molecular or cellular function in any context. By employing transgenes to drive tissue-specific expression of tmem2, we find that Tmem2 can function in the endocardium to repress atrioventricular differentiation within the ventricle. Additionally, Tmem2 can function in the myocardium to promote the medial movement of both myocardial and endocardial cells. Together, our data reveal that Tmem2 is an essential mediator of myocardium-endocardium coordination during cardiac morphogenesis.

Vascular morphogenesis in the zebrafish embryo.

During embryonic development, the vertebrate vasculature is undergoing vast growth and remodeling. Blood vessels can be formed by a wide spectrum of different morphogenetic mechanisms, such as budding, cord hollowing, cell hollowing, cell wrapping and intussusception. Here, we describe the vascular morphogenesis that occurs in the early zebrafish embryo. We discuss the diversity of morphogenetic mechanisms that contribute to vessel assembly, angiogenic sprouting and tube formation in different blood vessels and how some of these complex cell behaviors are regulated by molecular pathways.

Complex cell rearrangements during intersegmental vessel sprouting and vessel fusion in the zebrafish embryo.

The formation of intersegmental blood vessels (ISVs) in the zebrafish embryo serves as a paradigm to study angiogenesis in vivo. ISV formation is thought to occur in discrete steps. First, endothelial cells of the dorsal aorta migrate out and align along the dorsoventral axis. The dorsal-most cell, also called tip cell, then joins with its anterior and posterior neighbours, thus establishing a simple vascular network. The vascular lumen is then established via formation of vacuoles, which eventually fuse with those of adjacent endothelial cells to generate a seamless tube with an intracellular lumen. To investigate the cellular architecture and the development of ISVs in detail, we have analysed the arrangement of endothelial cell junctions and have performed single cell live imaging. In contrast to previous reports, we find that endothelial cells are not arranged in a linear head-to-tail configuration but overlap extensively and form a multicellular tube, which contains an extracellular lumen. Our studies demonstrate that a number of cellular behaviours, such as cell divisions, cell rearrangements and dynamic alterations in cell-cell contacts, have to be considered when studying the morphological and molecular processes involved in ISV and endothelial lumen formation in vivo.

Directed Evolution of a Bright Near-Infrared Fluorescent Rhodopsin Using a Synthetic Chromophore.

By engineering a microbial rhodopsin, Archaerhodopsin-3 (Arch), to bind a synthetic chromophore, merocyanine retinal, in place of the natural chromophore all-trans-retinal (ATR), we generated a protein with exceptionally bright and unprecedentedly red-shifted near-infrared (NIR) fluorescence. We show that chromophore substitution generates a fluorescent Arch complex with a 200-nm bathochromic excitation shift relative to ATR-bound wild-type Arch and an emission maximum at 772 nm. Directed evolution of this complex produced variants with pH-sensitive NIR fluorescence and molecular brightness 8.5-fold greater than the brightest ATR-bound Arch variant. The resulting proteins are well suited to bacterial imaging; expression and stability have not been optimized for mammalian cell imaging. By targeting both the protein and its chromophore, we overcome inherent challenges associated with engineering bright NIR fluorescence into Archaerhodopsin. This work demonstrates an efficient strategy for engineering non-natural, tailored properties into microbial opsins, properties relevant for imaging and interrogating biological systems.

Cdh5/VE-cadherin promotes endothelial cell interface elongation via cortical actin polymerization during angiogenic sprouting.

Organ morphogenesis requires the coordination of cell behaviors. Here, we have analyzed dynamic endothelial cell behaviors underlying sprouting angiogenesis in vivo. Two different mechanisms contribute to sprout outgrowth: tip cells show strong migratory behavior, whereas extension of the stalk is dependent upon cell elongation. To investigate the function of Cdh5 in sprout outgrowth, we generated null mutations in the zebrafish cdh5 gene, and we found that junctional remodeling and cell elongation are impaired in mutant embryos. The defects are associated with a disorganization of the actin cytoskeleton and cannot be rescued by expression of a truncated version of Cdh5. Finally, the defects in junctional remodeling can be phenocopied by pharmacological inhibition of actin polymerization, but not by inhibiting actin-myosin contractility. Taken together, our results support a model in which Cdh5 organizes junctional and cortical actin cytoskeletons, as well as provides structural support for polymerizing F-actin cables during endothelial cell elongation.

In vivo analysis reveals a highly stereotypic morphogenetic pathway of vascular anastomosis.

Organ formation and growth requires cells to organize into properly patterned three-dimensional architectures. Network formation within the vertebrate vascular system is driven by fusion events between nascent sprouts or between sprouts and pre-existing blood vessels. Here, we describe the cellular activities that occur during blood vessel anastomosis in the cranial vasculature of the zebrafish embryo. We show that the early steps of the fusion process involve endothelial cell recognition, de novo polarization of endothelial cells, and apical membrane invagination and fusion. These processes generate a unicellular tube, which is then transformed into a multicellular tube via cell rearrangements and cell splitting. This stereotypic series of morphogenetic events is typical for anastomosis in perfused sprouts. Vascular endothelial-cadherin plays an important role early in the anastomosis process and is required for filopodial tip cell interactions and efficient formation of a single contact site.

Blood flow changes coincide with cellular rearrangements during blood vessel pruning in zebrafish embryos.

After the initial formation of a highly branched vascular plexus, blood vessel pruning generates a hierarchically structured network with improved flow characteristics. We report here on the cellular events that occur during the pruning of a defined blood vessel in the eye of developing zebrafish embryos. Time-lapse imaging reveals that the connection of a new blood vessel sprout with a previously perfused multicellular endothelial tube leads to the formation of a branched, Y-shaped structure. Subsequently, endothelial cells in parts of the previously perfused branch rearrange from a multicellular into a unicellular tube, followed by blood vessel detachment. This process is accompanied by endothelial cell death. Finally, we show that differences in blood flow between neighboring vessels are important for the completion of the pruning process. Our data suggest that flow induced changes in tubular architecture ensure proper blood vessel pruning.

The hypothalamic neuropeptide oxytocin is required for formation of the neurovascular interface of the pituitary.

The hypothalamo-neurohypophyseal system (HNS) is the neurovascular structure through which the hypothalamic neuropeptides oxytocin and arginine-vasopressin exit the brain into the bloodstream, where they go on to affect peripheral physiology. Here, we investigate the molecular cues that regulate the neurovascular contact between hypothalamic axons and neurohypophyseal capillaries of the zebrafish. We developed a transgenic system in which both hypothalamic axons and neurohypophyseal vasculature can be analyzed in vivo. We identified the cellular organization of the zebrafish HNS as well as the dynamic processes that contribute to formation of the HNS neurovascular interface. We show that formation of this interface is regulated during development by local release of oxytocin, which affects endothelial morphogenesis. This cell communication process is essential for the establishment of a tight axovasal interface between the neurons and blood vessels of the HNS. We present a unique example of axons affecting endothelial morphogenesis through secretion of a neuropeptide.

Distinct cellular mechanisms of blood vessel fusion in the zebrafish embryo.

Although many of the cellular and molecular mechanisms of angiogenesis have been intensely studied [1], little is known about the processes that underlie vascular anastomosis. We have generated transgenic fish lines expressing an EGFP-tagged version of the junctional protein zona occludens 1 (ZO1) to visualize individual cell behaviors that occur during vessel fusion and lumen formation in vivo. These life observations show that endothelial cells (ECs) use two distinct morphogenetic mechanisms, cell membrane invagination and cord hollowing to generate different types of vascular tubes. During initial steps of anastomosis, cell junctions that have formed at the initial site of cell contacts expand into rings, generating a cellular interface of apical membrane compartments, as defined by the localization of the apical marker podocalyxin-2 (Pdxl2). During the cord hollowing process, these apical membrane compartments are brought together via cell rearrangements and extensive junctional remodeling, resulting in lumen coalescence and formation of a multicellular tube. Vessel fusion by membrane invagination occurs adjacent to a preexisting lumen in a proximal to distal direction and is blood-flow dependent. Here, the invaginating inner cell membrane undergoes concomitant apicobasal polarization and the vascular lumen is formed by the extension of a transcellular lumen through the EC, which forms a unicellular or seamless tube.

Semaphorin-PlexinD1 signaling limits angiogenic potential via the VEGF decoy receptor sFlt1.

Sprouting angiogenesis expands the embryonic vasculature enabling survival and homeostasis. Yet how the angiogenic capacity to form sprouts is allocated among endothelial cells (ECs) to guarantee the reproducible anatomy of stereotypical vascular beds remains unclear. Here we show that Sema-PlxnD1 signaling, previously implicated in sprout guidance, represses angiogenic potential to ensure the proper abundance and stereotypical distribution of the trunk's segmental arteries (SeAs). We find that Sema-PlxnD1 signaling exerts this effect by antagonizing the proangiogenic activity of vascular endothelial growth factor (VEGF). Specifically, Sema-PlxnD1 signaling ensures the proper endothelial abundance of soluble flt1 (sflt1), an alternatively spliced form of the VEGF receptor Flt1 encoding a potent secreted decoy. Hence, Sema-PlxnD1 signaling regulates distinct but related aspects of angiogenesis: the spatial allocation of angiogenic capacity within a primary vessel and sprout guidance.