The Willner Family Center for Vascular Biology

Nava Dekel, Director
The Philip M. Klutznick Professorial Chair of Developmental Biology

The Willner Family Center for Vascular Biology was officially inaugurated on November 3rd, 1999. The Center was designed to focus on the regulation of angiogenic processes and on the identification of signaling pathways and the mechanisms involved in the transduction of these signals in biological systems. Since de-regulation of such processes are a cause for many human diseases (e.g., cancer, heart failure and stroke), an effort is made to use our results to develop tools for early diagnosis of these ailments, and for the design of new drugs for pharmacological intervention.

The long-range goals of the Center are: (i) to support innovative ideas, while still in their seeding stage, when it is not yet possible to obtain financial support from conventional funding agencies; (ii) to nurture budding research of young outstanding investigators before their reputation is established; (iii) to finance research that requires an inter-disciplinary effort; (iv) to encourage collaboration with hospitals and with other centers of excellence in Israel and abroad; (v) to train doctoral and post-doctoral students in bioregulation and vascular biology.

The Center supported this year the scientific work of the following groups:

Dr. Lilach Gilboa "Gonad morphogenesis and establishment of germ line stem cells in Drosophila melanogaster": Many organs rely on stem cells for normal development, function and regeneration. The adult ovary of Drosophila employs germ line stem cells (GSCs) for the continual production of eggs throughout the lifetime of the animal. The known location of GSCs and the genetic tools available for Drosophila research have made the adult ovary a leading system in understanding the principles of stem cell biology. Despite the wealth of information regarding adult GSCs, less is known about how, during larval development, the adult niche forms and how it affects GSC establishment of from primordial germ cells (PGCs). Our lab studies two aspects of larval ovary development: A. How the somatic cells of the ovary control proliferation of PGCs, thereby determining the number of stem cells the adult ovary will contain, and B. How the somatic niches for GSCs form, and how niche formation contributes to the establishment of GSCs from PGCs. PGCs in the larval ovary reside in close proximity to Somatic Intermingled Cells (ICs). PGCs and ICs communicate via an Epidermal Growth Factor Receptor (EGFR)-dependent feedback loop (Gilboa and Lehmann, 2006). PGCs produce Spitz, which is required for IC survival and for the production of an unknown substance that represses PGC proliferation. To reveal the identity of this unknown substance we used microarray analysis, comparing wild-type ovaries to ovaries that express an activated form of EGFR. The microarray, together with other lines of evidence suggested one ligand emanating from ICs, is repressing PGC proliferation. Indeed, reducing the amount of this ligand in ovaries results in over-proliferation of PGCs. We are currently investigating the molecular mechanisms that underlie the repression of PGC proliferation by this ligand. We are also investigating the signals that positively control PGC proliferation. Our preliminary results suggest that the ligand Decapentaplegic (Dpp) is required for proliferation of PGCs. In the past year our lab conducted a genetic screen to identify regulators of niche formation and stem cell maintenance. The screen was based on direct observation of precociously differentiating PGCs in larval ovaries. The novel detection mode allowed us to uncover novel genes that are important for both niche formation and stem cell maintenance. We are now studying some of these regulators. Combined, our studies will lead to a better understanding of the complex relationship between stem cells and the organs they reside in. Cross talk between stem cell and niches determine the number of stem cells an organ contains, their division rate, maintenance and differentiation. Better understanding of the biological principles underlying such complex relationships is required for our understanding of normal development, disease and, possibly, its treatment.

Dr. Atan Gross - "BID: a master regulator of cell life and death decisions": Apoptosis is essential for both the development and maintenance of tissue homeostasis in multicellular organisms. Thus, defects in apoptosis contribute to a variety of diseases including cancer, AIDS, neurodegenerative diseases, stroke and autoimmune disorders. Proteins in the BCL-2 family are critical regulators of the commitment to apoptosis, yet their cell death regulatory function remains a mystery. We have picked to focus our studies on BID, a pro-apoptotic member of this family. Mitochondria are a major site of action for BID, yet a detailed understanding of its actions at this organelle is lacking. In the first line of research, we are exploring the activities of BID at the mitochondria by studying its interaction with a novel and uncharacterized protein named mitochondrial carrier homolog 2 (MTCH2)/Met-induced mitochondrial protein (MIMP). We have recently revealed that MTCH2/MIMP acts as a mitochondrial receptor for BID and plays a critical role in liver apoptosis. We have also found that MTCH2/MIMP is involved in mitochondria metabolism, and our future goals are to determine its exact function at the mitochondria and how it may connect apoptosis and metabolism. In a second line of research, we are exploring the activities of BID in the response of cells to DNA damage. We have previously found that DNA damage induces the phosphorylation of BID by the ataxia-telangiectasia mutated (ATM) kinase, and that this phosphorylation is important for cell cycle arrest at the S phase and for inhibition of apoptosis. More recently we have revealed that phosphorylated BID plays a critical role in protecting bone marrow cells from DNA damage, and our future goals are to determine the mechanistic details of BID's activities in the hematopoietic lineage. A better understanding of BID's function at the mitochondria and in the nucleus will most likely yield critical insights for manipulating the apoptotic and DNA repair processes in the treatment of cancer and other diseases.

Dr. Eldad Tzahor "For the past few years, our lab has been focusing on the identification of candidate signaling molecules and tissue-specific transcription factors that regulate head muscle development during early vertebrate embryogenesis."

Lineage plasticity of the cranial paraxial mesoderm:
The developing heart is a specialized muscular vessel that serves as a pump for both the systemic and pulmonary circuits. This extremely complicated organ is highly sensitive to genetic perturbations, which are reflected in the numerous congenital heart defects that affect ~1% of all live births. The multiplicity of cardiac progenitor populations in various vertebrate species is an emerging area of intense focus in many laboratories, due to the enormous therapeutic potential of these avenues for treating heart disease. During early embryogenesis, heart and skeletal muscle progenitor cells are thought to derive from distinct regions of the mesoderm (i.e., lateral plate mesoderm and paraxial mesoderm, respectively). We recently employed both in vitro and in vivo experimental systems in the avian embryo to explore how mesoderm progenitors in the head differentiate into both heart and skeletal muscles. Utilizing fate mapping studies, gene expression analyses, and manipulations of signaling pathways in the chick embryo, we demonstrated that cells from the cranial paraxial mesoderm contribute to both myocardial and endocardial cell populations within the cardiac outflow tract. We further showed that bone morphogenic protein (BMP) signaling affects the specification of mesoderm cells in the head: application of BMP4 to chick embryos, both in vitro and in vivo, induces cardiac differentiation in the cranial paraxial mesoderm, and blocks the differentiation of skeletal muscle precursors in these cells. Our results demonstrate that cells within the cranial paraxial mesoderm play a vital role in cardiogenesis, as a new source of cardiac progenitors that populate the cardiac outflow tract in vivo.

Craniofacial muscle patterning:
Craniofacial development requires the orchestrated integration of multiple interactions among progenitor cells derived from both the cranial paraxial mesoderm and the cranial neural crest (CNC). In the vertebrate head, mesoderm-derived cells fuse together to form a myofiber, which is attached to specific CNC-derived skeletal elements in a highly coordinated manner. Although it has long been suggested that the CNC plays an indirect role in the formation of the head musculature, the precise molecular underpinnings of this exquisitely tuned process, and the significance of the CNC's contribution to it, are far less clear. In a recent study we analyzed head skeletal muscle patterning and differentiation in vivo, in three mouse models involving genetic perturbations of CNC development, as well as in CNC-ablated chick embryos. Our results demonstrate that although early specification of the skeletal muscle lineage is CNC-independent, CNC cells play an important role at later developmental stages, regulating the expression patterns of myogenic genes, the migration and axial registration of the mesoderm cells, and the subsequent differentiation of myoblasts in the branchial arches. This study supports a model in which CNC cells control craniofacial development and patterning by regulating positional interactions with mesoderm-derived muscle progenitors that together shape the cranial musculoskeletal architecture during vertebrate embryogenesis.

The contribution of Islet1-expressing splanchnic mesoderm cells to distinct jaw muscles reveals significant heterogeneity in head muscle development:
Heart development takes place in close apposition to the developing head. The term "cardio-craniofacial morphogenetic field" reflects the intimate developmental relationship between the head, face, and heart, which is also reflected in numerous cardiac and craniofacial birth defects. Nathan et al, have characterized the nature of the cardio-craniofacial mesoderm in both chick and mouse embryos, using several lineage tracing and gene expression techniques. At both the cellular and molecular levels, the cardio-craniofacial mesoderm can be divided into two compartments, the cranial paraxial mesoderm, and splanchnic mesoderm (SpM), part of which comprises the anterior heart field (AHF). We have found that each of these compartments contributes to the developing heart in a temporally regulated manner. Following linear heart tube stages, we have found that Isl1+/SpM cells contribute to the distal part of the pharyngeal (branchial) mesoderm, as well as to the cardiac outflow tract. Molecular analyses of the head muscles demonstrated distinct molecular and developmental programs for CPM and Isl1+/SpM-derived branchiomeric muscles. Furthermore, we have provided evidence that the Wnt/β-catenin pathway regulates Isl1 and Nkx2.5 gene expression, presumably by fine-tuning boundary formation within the cardio-craniofacial mesoderm.