Ariel Rinon

p53 plays a role in mesenchymal differentiation programs, in a cell fate dependent manner.

Background The tumor suppressor p53 is an important regulator that controls various cellular networks, including cell differentiation. Interestingly, some studies suggest that p53 facilitates cell differentiation, whereas others claim that it suppresses differentiation. Therefore, it is critical to evaluate whether this inconsistency represents an authentic differential p53 activity manifested in the various differentiation programs. Methodology/Principal Findings To clarify this important issue, we conducted a comparative study of several mesenchymal differentiation programs. The effects of p53 knockdown or enhanced activity were analyzed in mouse and human mesenchymal cells, representing various stages of several differentiation programs. We found that p53 down-regulated the expression of master differentiation-inducing transcription factors, thereby inhibiting osteogenic, adipogenic and smooth muscle differentiation of multiple mesenchymal cell types. In contrast, p53 is essential for skeletal muscle differentiation and osteogenic re-programming of skeletal muscle committed cells. Conclusions These comparative studies suggest that, depending on the specific cell type and the specific differentiation program, p53 may exert a positive or a negative effect, and thus can be referred as a “guardian of differentiation” at large.

The contribution of Islet1-expressing splanchnic mesoderm cells to distinct branchiomeric muscles reveals significant heterogeneity in head muscle development

During embryogenesis, paraxial mesoderm cells contribute skeletal muscle progenitors, whereas cardiac progenitors originate in the lateral splanchnic mesoderm (SpM). Here we focus on a subset of the SpM that contributes to the anterior or secondary heart field (AHF/SHF), and lies adjacent to the cranial paraxial mesoderm (CPM), the precursors for the head musculature. Molecular analyses in chick embryos delineated the boundaries between the CPM, undifferentiated SpM progenitors of the AHF/SHF, and differentiating cardiac cells. We then revealed the regionalization of branchial arch mesoderm: CPM cells contribute to the proximal region of the myogenic core, which gives rise to the mandibular adductor muscle. SpM cells contribute to the myogenic cells in the distal region of the branchial arch that later form the intermandibular muscle. Gene expression analyses of these branchiomeric muscles in chick uncovered a distinct molecular signature for both CPM- and SpM-derived muscles. Islet1 (Isl1) is expressed in the SpM/AHF and branchial arch in both chick and mouse embryos. Lineage studies using Isl1-Cre mice revealed the significant contribution of Isl1+ cells to ventral/distal branchiomeric (stylohyoid, mylohyoid and digastric) and laryngeal muscles. By contrast, the Isl1 lineage contributes to mastication muscles (masseter, pterygoid and temporalis) to a lesser extent, with virtually no contribution to intrinsic and extrinsic tongue muscles or extraocular muscles. In addition, in vivo activation of the Wnt/β-catenin pathway in chick embryos resulted in marked inhibition of Isl1, whereas inhibition of this pathway increased Isl1 expression. Our findings demonstrate, for the first time, the contribution of Isl1+ SpM cells to a subset of branchiomeric skeletal muscles.

Mesoderm progenitor cells of common origin contribute to the head musculature and the cardiac outflow tract.

During early embryogenesis, heart and skeletal muscle progenitor cells are thought to derive from distinct regions of the mesoderm (i.e. the lateral plate mesoderm and paraxial mesoderm, respectively). In the present study, we have 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. Using fate-mapping studies, gene expression analyses, and manipulation of signaling pathways in the chick embryo, we demonstrate that cells from the cranial paraxial mesoderm contribute to both myocardial and endocardial cell populations within the cardiac outflow tract. We further show that Bmp signaling affects the specification of mesoderm cells in the head: application of Bmp4, 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. A deeper understanding of mesodermal lineage specification in the vertebrate head is expected to provide insights into the normal, as well as pathological, aspects of heart and craniofacial development.

Cranial neural crest cells regulate head muscle patterning and differentiation during vertebrate embryogenesis

In the vertebrate head, mesoderm cells fuse together to form a myofiber, which is attached to specific cranial neural crest (CNC)-derived skeletal elements in a highly coordinated manner. Although it has long been recognized that CNC plays a role in the formation of the head musculature, the precise molecular underpinnings of this process remain elusive. In the present study we explored the nature of the crosstalk between CNC and mesoderm cells during head muscle development, employing three models for genetic perturbations of CNC development in mice, as well as experimental ablation of CNC in chick embryos. We demonstrate that although early myogenesis is CNC-independent, the migration, patterning and differentiation of muscle precursors are regulated by CNC. In the absence of CNC cells, accumulated myoblasts are kept in a proliferative state, presumably because of an increase of Fgf8 in adjacent tissues, which leads to abnormalities in both differentiation and subsequent myofiber organization in the head. These results have uncovered a surprising degree of complexity and multiple distinct roles for CNC in the patterning and differentiation of muscles during craniofacial development. We suggest that CNC cells control craniofacial development by regulating positional interactions with mesoderm-derived muscle progenitors that together shape the cranial musculoskeletal architecture in vertebrate embryos.

p53 coordinates cranial neural crest cell growth and epithelial-mesenchymal transition/delamination processes.

Neural crest development involves epithelial-mesenchymal transition (EMT), during which epithelial cells are converted into individual migratory cells. Notably, the same signaling pathways regulate EMT function during both development and tumor metastasis. p53 plays multiple roles in the prevention of tumor development; however, its precise roles during embryogenesis are less clear. We have investigated the role of p53 in early cranial neural crest (CNC) development in chick and mouse embryos. In the mouse, p53 knockout embryos displayed broad craniofacial defects in skeletal, neuronal and muscle tissues. In the chick, p53 is expressed in CNC progenitors and its expression decreases with their delamination from the neural tube. Stabilization of p53 protein using a pharmacological inhibitor of its negative regulator, MDM2, resulted in reduced SNAIL2 (SLUG) and ETS1 expression, fewer migrating CNC cells and in craniofacial defects. By contrast, electroporation of a dominant-negative p53 construct increased PAX7+ SOX9+ CNC progenitors and EMT/delamination of CNC from the neural tube, although the migration of these cells to the periphery was impaired. Investigating the underlying molecular mechanisms revealed that p53 coordinates CNC cell growth and EMT/delamination processes by affecting cell cycle gene expression and proliferation at discrete developmental stages; disruption of these processes can lead to craniofacial defects.

LifeMap Discovery™: the embryonic development, stem cells, and regenerative medicine research portal.

LifeMap Discovery™ provides investigators with an integrated database of embryonic development, stem cell biology and regenerative medicine. The hand-curated reconstruction of cell ontology with stem cell biology; including molecular, cellular, anatomical and disease-related information, provides efficient and easy-to-use, searchable research tools. The database collates in vivo and in vitro gene expression and guides translation from in vitro data to the clinical utility, and thus can be utilized as a powerful tool for research and discovery in stem cell biology, developmental biology, disease mechanisms and therapeutic discovery. LifeMap Discovery is freely available to academic nonprofit institutions at http://discovery.lifemapsc.com

Pharyngeal mesoderm regulatory network controls cardiac and head muscle morphogenesis

The search for developmental mechanisms driving vertebrate organogenesis has paved the way toward a deeper understanding of birth defects. During embryogenesis, parts of the heart and craniofacial muscles arise from pharyngeal mesoderm (PM) progenitors. Here, we reveal a hierarchical regulatory network of a set of transcription factors expressed in the PM that initiates heart and craniofacial organogenesis. Genetic perturbation of this network in mice resulted in heart and craniofacial muscle defects, revealing robust cross-regulation between its members. We identified Lhx2 as a previously undescribed player during cardiac and pharyngeal muscle development. Lhx2 and Tcf21 genetically interact with Tbx1, the major determinant in the etiology of DiGeorge/velo-cardio-facial/22q11.2 deletion syndrome. Furthermore, knockout of these genes in the mouse recapitulates specific cardiac features of this syndrome. We suggest that PM-derived cardiogenesis and myogenesis are network properties rather than properties specific to individual PM members. These findings shed new light on the developmental underpinnings of congenital defects.

Seven diverse human embryonic stem cell-derived chondrogenic clonal embryonic progenitor cell lines display site-specific cell fates

AIM The transcriptomes of seven diverse clonal human embryonic progenitor cell lines with chondrogenic potential were compared with that of bone marrow-derived mesenchymal stem cells (MSCs). MATERIALS & METHODS The cell lines 4D20.8, 7PEND24, 7SMOO32, E15, MEL2, SK11 and SM30 were compared with MSCs using immunohistochemical methods, gene expression microarrays and quantitative real-time PCR. RESULTS In the undifferentiated progenitor state, each line displayed unique combinations of site-specific markers, including AJAP1, ALDH1A2, BMP5, BARX1, HAND2, HOXB2, LHX1, LHX8, PITX1, TBX15 and ZIC2, but none of the lines expressed the MSC marker CD74. The lines showed diverse responses when differentiated in the presence of combinations of TGF-β3, BMP2, 4, 6 and 7 and GDF5, with the lines 4D20.8, SK11, SM30 and MEL2 showing osteogenic markers in some differentiation conditions. The line 7PEND24 showed evidence of regenerating articular cartilage and, in some conditions, markers of tendon differentiation. CONCLUSION The scalability of site-specific clonal human embryonic stem cell-derived embryonic progenitor cell lines may provide novel models for the study of differentiation and methods for preparing purified and identified cells types for use in therapy.

Human embryonic stem cell-derived neural crest cells capable of expressing markers of osteochondral or meningeal-choroid plexus differentiation

AIMS The transcriptome and fate potential of three diverse human embryonic stem cell-derived clonal embryonic progenitor cell lines with markers of cephalic neural crest are compared when differentiated in the presence of combinations of TGFβ3, BMP4, SCF and HyStem-C matrices. MATERIALS & METHODS The cell lines E69 and T42 were compared with MEL2, using gene expression microarrays, immunocytochemistry and ELISA. RESULTS In the undifferentiated progenitor state, each line displayed unique markers of cranial neural crest including TFAP2A and CD24; however, none expressed distal HOX genes including HOXA2 or HOXB2, or the mesenchymal stem cell marker CD74. The lines also showed diverse responses when differentiated in the presence of exogenous BMP4, BMP4 and TGFβ3, SCF, and SCF and TGFβ3. The clones E69 and T42 showed a profound capacity for expression of endochondral ossification markers when differentiated in the presence of BMP4 and TGFβ3, choroid plexus markers in the presence of BMP4 alone, and leptomeningeal markers when differentiated in SCF without TGFβ3. CONCLUSION The clones E69 and T42 may represent a scalable source of primitive cranial neural crest cells useful in the study of cranial embryology, and potentially cell-based therapy.