Rewiring vascular patterning through translational control in Arabidopsis

Growth and development of multi-cellular organisms require mechanisms that allow for extensive cell-to-cell communication. In some incidences communication is established by signaling molecules that are directly transported from one cell to the other. These mobile signals in plants have been found in forms of proteins, RNAs, and small molecules. They are transmitted through the vascular system in a long distance (between organs), or through plasmodesmata in a short distance (between cell types). A growing number of studies show that transcription factors contribute as important mobile signals in plants. Transcription factors move in a short or long distance in forms of proteins and RNAs. Such transport activities are very important for patterning and growth of plant organs and tissues. In this chapter, we will comprehensively review transcription factors as mobile signals: factors that have been discovered, mechanisms of their mobility, new tools that will lead to the discovery of mobile transcription factors, and putative mobile transcription factors inferred from cell-type specific RNA profiling data.

FT Protein Acts as a Long-Range Signal in Arabidopsis

![][1] The Spanish National Cancer Institute (CNIO) Meeting on Development and Cancer took place between 4 and 6 February 2008 in Madrid, Spain, and was organized by K. Basler, G. Morata, E. Moreno and M. Torres. ![][2] Developmental biologists rightly argue that genes regulating developmental pathways are reiterated in cancer development. At the recent Spanish National Cancer Institute (CNIO) Meeting on Development and Cancer, novel concepts and processes that are considered to be common to both embryonic and cancer development were discussed. It is worth pointing out that tumour formation should be considered to be a form of aberrant organ development, in which many developmental pathways that control proliferation, differentiation and growth or morphogenesis are affected. This report is structured in three parts, describing the work presented at the meeting according to the stages of tumour development to which it pertains. First, the normal behaviour of cell populations that are the targets of transformation, in particular adult stem cells and the pathways that regulate their behaviour. Second, processes that occur at the early stages of cancer development before morphological malformations are detectable, in particular the process that is best known from studies in flies as ‘cell competition’. Third, developmental processes and pathways that are active in the growing tumour—which we consider to be an extrinsic organ—for example, the recruitment of blood vessels or the role of morphogens in controlling the size and shape of tumours, and the progression from hyperplasia to invasive tumours (Fig 1). Figure 1. Tumour development and genes involved in the different stages. Schematic representation of how stem cells (A) , by acquiring a mutation through cell competition ( B ) and by the selection of additional mutations, can give rise to the ‘tumour organ’ ( C ). The genes involved in these processes are listed on the right‐hand side, and most of them … [1]: /embed/graphic-1.gif [2]: /embed/graphic-2.gif

West, Brown and Enquist's model of allometric scaling again: the same questions remain

Differences in the vascular tree of the femoral trochlear growth cartilage at osteochondrosis-susceptible sites in foals revealed by SWI 3T MRI.

BackgroundDifferent classes of small RNAs (sRNAs) refine the expression of numerous genes in higher eukaryotes by directing protein partners to complementary nucleic acids, where they mediate gene silencing. Plants encode a unique class of sRNAs, called trans-acting small interfering RNAs (tasiRNAs), which post-transcriptionally regulate protein-coding transcripts, as do microRNAs (miRNAs), and both sRNA classes control development through their targets. TasiRNA biogenesis requires multiple components of the siRNA pathway and also miRNAs. But while 21mer siRNAs originating from transgenes can mediate silencing across several cell layers, miRNA action seems spatially restricted to the producing or closely surrounding cells.Principal FindingsWe have previously described the isolation of a genetrap reporter line for TAS3a, the major locus producing AUXIN RESPONS FACTOR (ARF)-regulating tasiRNAs in the Arabidopsis shoot. Its activity is limited to the adaxial (upper) side of leaf primordia, thus spatially isolated from ARF-activities, which are located in the abaxial (lower) side. We show here by in situ hybridization and reporter fusions that the silencing activities of ARF-regulating tasiRNAs are indeed manifested non-cell autonomously to spatially control ARF activities.Conclusions/SignificanceEndogenous tasiRNAs are thus mediators of a mobile developmental signal and might provide effective gene silencing at a distance beyond the reach of most miRNAs.

FT, A Mobile Developmental Signal in Plants

Quantification of microvascular remodeling as a meaningful discovery tool requires mapping and measurement of site-specific changes within vascular trees and networks. Vessel density and other critical vascular parameters are often modulated by molecular regulators as determined by local vascular architecture. For example, enlargement of vessel diameter by vascular endothelial growth factor (VEGF) is restricted to specific generations of vessel branching (Parsons-Wingerter et al., Microvascular Research72: 91, 2006). The averaging of vessel diameter over many successively smaller generations is therefore not particularly useful. The newly automated, user-interactive software VESsel GENeration Analysis (VESGEN) quantifies major vessel parameters within two-dimensional (2D) vascular trees, networks, and tree-network composites. This report reviews application of VESGEN 2D to angiogenic and lymphangiogenic tissues that includes the human and murine retina, embryonic coronary vessels, and avian chorioallantoic membrane. Software output includes colorized image maps with quantification of local vessel diameter, fractal dimension, tortuosity, and avascular spacing. The density of parameters such as vessel area, length, number, and branch point are quantified according to site-specific generational branching within vascular trees. The sole user input requirement is a binary (black/white) vascular image. Future applications of VESGEN will include analysis of 3D vascular architecture and bioinformatic dimensions such as blood flow and receptor localization. Branching analysis by VESGEN has demonstrated that numerous regulators including VEGF(165), basic fibroblast growth factor, transforming growth factor beta-1, angiostatin and the clinical steroid triamcinolone acetonide induce 'fingerprint' or 'signature' changes in vascular patterning that provide unique readouts of dominant molecular signaling.

To date, islet transplantation to treat type 1 diabetes mellitus remains unsuccessful in long-term follow-up, mainly due to failed engraftment and reconstruction of the islet niche. Alternative approaches, such as islet embedding structures (IESs) based on 3D printing have been developed. However, most of them have been implanted subcutaneously and only a few are intended for direct integration into the vascular system through anastomosis. In this study, we 3D printed a proof-of-concept IES using gelatin methacrylate biocompatible ink. This structure consisted of a branched vascular system surrounding both sides of a central cavity dedicated to islets of Langerhans. Furthermore, we designed a bioreactor optimized for these biological structures. This bioreactor allows seeding and perfusion experiments under sterile and physiological conditions. Preliminary experiments aimed to analyze if the vascular channel could successfully be seeded with mature endothelial cells and the central cavity with rat islets. Subsequently, the structures were used for a humanized model seeding human endothelial progenitor cells (huEPC) within the vascular architecture and human islets co-cultured with huEPC within the central cavity. The constructs were tested for hemocompatibility, suture strength, and anastomosability. The 3D printed IES appeared to be hemocompatible and anastomosable using an alternative cuff anastomosis in a simple ex vivo perfusion model. While rat islets alone could not successfully be embedded within the 3D printed structure for 3 days, human islets co-cultivated with huEPC successfully engrafted within the same time. This result emphasizes the importance of co-culture, nursing cells, and islet niche. In conclusion, we constructed a proof-of-concept 3D printed islet embedding device consisting of a vascular channel that is hemocompatible and perspectively anastomosable to clinical scale blood vessels. However, there are numerous limitations in this model that need to be overcome to transfer this technology to the bedside.

We isolated three recessive mutants of Arabidopsis (Arabidopsis thaliana) showing ectopic expression of the xylem-specific marker, pAtxyn3::YFP. Genetic analysis indicated that the phenotypes were caused by mutations in three different genes, designated Abnormal Tracheary Element formation-related gene expression (ate1-3). The ate1 mutants showed a normal DR5::GUS gene expression pattern, and the ate1 mutation did not affect the abnormal vascular pattern formation in the van3 and pin1 mutants, indicating that the ate1 mutation does not affect the vascular pattern organization governed by auxin. The ate mutants showed ectopic lignin deposition, patterned secondary wall thickenings, and cell death, which are characteristic of mature tracheary elements (TEs) in cells ectopically expressing the pAtxyn3::YFP gene. Ectopic TE formation was rapidly induced in parenchymal tissue of the ate mutants in a TE-inducible system with excised hypocotyl. Furthermore, reverse transcription-polymerase chain reaction experiments showed that the expression of TE formation-related genes is up-regulated in the ate mutants. The ate1 mutation also caused ectopic expression of another xylem-specific marker gene, pAt3g62160::YFP. Overall, our results suggest that the ATE genes are responsible for the in situ repression of transdifferentiation into TEs in Arabidopsis and could be participants in the transdifferentiation-masking system.

Myofibroblasts (MFBs) are smooth muscle-like cells that provide contractile force required for tissue repair during wound healing. The leading agonist for MFB differentiation is transforming growth factor β1 (TGFβ1) that induces transcription of genes encoding smooth muscle α-actin (SMαA) and interstitial collagen that are markers for MFB differentiation. TGFβ1 augments activation of Smad transcription factors, pro-survival Akt kinase, and p38 MAP kinase as well as Wingless/int (Wnt) developmental signaling. These actions conspire to activate β-catenin needed for expression of cyclin D, laminin, fibronectin, and metalloproteinases that aid in repairing epithelial cells and their associated basement membranes. Importantly, β-catenin also provides a feed-forward stimulus that amplifies local TGFβ1 autocrine/paracrine signaling causing transition of mesenchymal stromal cells, pericytes, and epithelial cells into contractile MFBs. Complex, mutually interactive mechanisms have evolved that permit several mammalian cell types to activate the SMαA promoter and undergo MFB differentiation. These molecular controls will be reviewed with an emphasis on the dynamic interplay between serum response factor, TGFβ1-activated Smads, Wnt-activated β-catenin, p38/calcium-activated NFAT protein, and the RNA-binding proteins, Purα, Purβ, and YB-1, in governing transcriptional and translational control of the SMαA gene in injury-activated MFBs.

The development and anatomy of root nodules induced in seedlings of Erythrina poeppigiana by a strain of Bradyrhizobium spp (CR 751) is described. Infection apparently occurred via injuries where lateral roots (devoid of root hairs) emerged. The first sign of infection was a group of epidermal cells (fluffy) on the secondary (emerging) root that covered the nodule primordium. Here, thin-walled cells with plastids and in active division were observed but not infection threads. Infection appears to have occurred via protoxylem which was preceded by a dissolution of the vessels' cell wall. A peribacteroid, membrane-generating organelle in infected cells is described. The adjacent noninfected cells showed electron dense depositions (phenolics 2). Developed nodules were spherical 0.5 to 2 cm in diameter. The surface was corky, with lenticels. The peridermis showed several layers of suberized dead cells. The cortex consisted of external, medium and internal layers. Vascular bundles ran in the medium cortex in a centripetal way to join the root vascular system at the base of the nodule. In these vascular bundles, baciliar bacteria were observed which were confirmed to be Bradyrhizobium sp (results of a separate study using immunocitochemistry). Bacteroids densely filled infected cells of the nodule medulla within peribacteroid membranes. Among infected cells non-infected auxiliary cells were seen loaded with starch granules. During later stages of root development few root hairs appeared on tertiary and secondary roots. Infection also occurred via root hairs, resulting in oblong nodules caused by a tumefaction of the root cortex .

Studies of human airways in vitro: A review of the methodology

The regulation of cell specification in plants is particularly important in vascular development. The vascular system is comprised two differentiated tissue types, the xylem and phloem, which form conductive elements for the transport of water, nutrients and signaling molecules. A meristematic layer, the procambium, is located between these two differentiated cell types and divides to initiate vascular growth. We report the identification of a receptor-like kinase (RLK) that is expressed in the vasculature. Histochemical analyses of mutants in this kinase display an aberrant accumulation of highly lignified cells, typical of xylem or fiber cells, within the phloem. In addition, phloem cells are sometimes located adjacent to xylem cells in these mutants. We, therefore, named this RLK XYLEM INTERMIXED WITH PHLOEM 1 (XIP1). Analyses of longitudinal profiles of xip1 mutant stems show malformed cell files, indicating defects in oriented cell divisions or cell morphology. We propose that XIP1 prevents ectopic lignification in phloem cells and is necessary to maintain the organization of cell files or cell morphology in conductive elements.

This paper presents a study of the renal vascular architecture in polycystic kidneys induced in newborn rabbits by a single injection of methylprednisolone acetate. Ink perfusion and polyester resin corrosion casts were utilized, and the vasculature was studied by light and scanning electron microscopy. Vascular alterations were observed in the two main evolutive stages of the renal polycystosis: the stage of tubular cysts and the stage of glomerular cysts. These alterations were compared with the normal vascular pattern of control kidneys. At the tubular cyst stage the sinusoidal vascular cortical plexus persisted for a long period of time and the superficial cortical glomerular tufts were poorly developed. At the glomerular cyst stage, the vascular architecture of the superficial cortex showed numerous alterations. The glomerular tufts were small and their structural pattern was affected. Frequently the hilum was very wide and the afferent and efferent arterioles were on opposite sides of the glomerular tufts. The afferent arterioles were variable in size and length. Sometimes the glomeruli showed several afferent arterioles. There were numerous cases of double efferent arterioles in the superficial glomeruli of the polycystic kidneys. Various types of aglomerular shunts were observed regardless of the cystic stage or age. These data indicate that, in addition to the cysts, the corticoid-induced polycystic kidneys showed alterations of the renal vascular architecture which are interpreted as a malformation. We suggest, on the basis of these morphologic observations, that the pathogenesis of the polycystic kidneys is not related to the vascular alterations.