Without epithelial–mesenchymal transitions, in which polarized epithelial cells are converted into motile cells, multicellular organisms would be incapable of getting past the blastula stage of embryonic development. However, this important developmental programme has a more sinister role in tumour progression. Epithelial–mesenchymal transition provides a new basis for understanding the progression of carcinoma towards dedifferentiated and more malignant states.

Epithelial–mesenchymal transitions in tumour progression

https://www.cell.com/developmental-cell/pdf/S1534-5807(08)00209-8.pdf

Epithelial-Mesenchymal Transition: At the Crossroads of Development and Tumor Metastasis

Hepatocyte growth factor/scatter factor and its receptor, the tyrosine kinase Met, arose late in evolution and are unique to vertebrates In spite of this, Met uses molecules such as Gab1 — homologues of which are present in Caenorhabditis elegans and Drosophila melanogaster — for downstream signalling Pivotal roles for Met in development and cancer have been established: Met controls cell migration and growth in embryogenesis; it also controls growth, invasion and metastasis in cancer cells; and activating Met mutations predispose to human cancer

Met, metastasis, motility and more

Cell-adhesion molecules, once believed to function primarily in tethering cells to extracellular ligands, are now recognized as having broader functions in cellular signalling cascades. The CD44 transmembrane glycoprotein family adds new aspects to these roles by participating in signal-transduction processes — not only by establishing specific transmembrane complexes, but also by organizing signalling cascades through association with the actin cytoskeleton. CD44 and its associated partner proteins monitor changes in the extracellular matrix that influence cell growth, survival and differentiation.

CD44: From adhesion molecules to signalling regulators

Adult skeletal muscle has a remarkable ability to regenerate following myotrauma. Because adult myofibers are terminally differentiated, the regeneration of skeletal muscle is largely dependent on a small population of resident cells termed satellite cells. Although this population of cells was identified 40 years ago, little is known regarding the molecular phenotype or regulation of the satellite cell. The use of cell culture techniques and transgenic animal models has improved our understanding of this unique cell population; however, the capacity and potential of these cells remain ill-defined. This review will highlight the origin and unique markers of the satellite cell population, the regulation by growth factors, and the response to physiological and pathological stimuli. We conclude by highlighting the potential therapeutic uses of satellite cells and identifying future research goals for the study of satellite cell biology.

Myogenic satellite cells: physiology to molecular biology.

Polypeptide growth factors are important effectors of cell growth and differentiation in vitro and are thought to be critical for processes such as specification of cell fate, tissue growth and organogenesis in vivo. Scatter factor/hepatocyte growth factor (SF/HGF) is the prototype of an emerging family of growth factors that resemble in their domain structure and mechanism of activation the blood proteinase plasminogen. The cellular responses of SF/HGF are mediated by the c-Met tyrosine kinase receptor. Here we report that mice lacking SF/HGF fail to complete development and die in utero. The mutation affects the embryonic liver, which is reduced in size and shows extensive loss of parenchymal cells. In addition, development of the placenta, particularly of trophoblast cells, is impaired. Thus, SF/HGF is essential for the development of several epithelial organs.

Scatter factor/hepatocyte growth factor is essential for liver development

Limb muscles develop from cells that migrate from the somites. The signal that induces migration of myogenic precursor cells to the limb emanates from the mesenchyme of the limb bud. Here we report that the c-met-encoded receptor tyrosine kinase is essential for migration of myogenic precursor cells into the limb anlage and for migration into diaphragm and tip of tongue. In c-met homozygous mutant (-/-) mouse embryos, the limb bud and diaphragm are not colonized by myogenic precursor cells and, as a consequence, skeletal muscles of the limb and diaphragm do not form. In contrast, development of the axial skeletal muscles proceeds in the absence of c-met signalling. The specific ligand of the c-met protein, the motility and growth factor scatter factor/hepatocyte growth factor, is expressed in limb mesenchyme and can thus provide the signal for migration which is received by c-met. We have therefore identified a paracrine signalling system that regulates migration of myogenic precursor cells.

Essential role for the c-met receptor in the migration of myogenic precursor cells into the limb bud.

Hepatocyte growth factor/scatter factor is an axonal chemoattractant and a neurotrophic factor for spinal motor neurons

Hypaxial skeletal muscles develop from migratory and non-migratory precursor cells that are generated by the lateral lip of the dermomyotome. Previous work shows that the formation of migratory precursors requires the c-Met and SF/HGF genes. We show here that in mice lacking c-Met or SF/HGF, the initial development of the dermomyotome proceeds appropriately and growth and survival of cells in the dermomyotome are not affected. Migratory precursors are also correctly specified, as monitored by the expression of Lbx1. However, these cells remain aggregated and fail to take up long range migration. We conclude that parallel but independent cues converge on the migratory hypaxial precursors in the dermomyotomal lip after they are laid down: a signal given by SF/HGF that controls the emigration of the precursors, and an as yet unidentified signal that controls Lbx1. SF/HGF and c-Met act in a paracrine manner to control emigration, and migratory cells only dissociate from somites located close to SF/HGF-expressing cells. During long range migration, prolonged receptor-ligand-interaction appears to be required, as SF/HGF is expressed both along the routes and at the target sites of migratory myogenic progenitors. Mice that lack c-Met die during the second part of gestation due to a placental defect. Rescue of the placental defect by aggregation of tetraploid (wild type) and diploid (c-Met-/-) morulae allows development of c-Met mutant animals to term. They lack muscle groups that derive from migratory precursor cells, but display otherwise normal skeletal musculature.

The role of SF/HGF and c-Met in the development of skeletal muscle.

During development, cranial motor neurons extend their axons along distinct pathways into the periphery. For example, branchiomotor axons extend dorsally to leave the hindbrain via large dorsal exit points. They then grow in association with sensory ganglia, to their targets, the muscles of the branchial arches. We have investigated the possibility that pathway tissues might secrete diffusible chemorepellents or chemoattractants that guide cranial motor axons, using co-cultures in collagen gels. We found that explants of dorsal neural tube or hindbrain roof plate chemorepelled cranial motor axons, while explants of cranial sensory ganglia were weakly chemoattractive. Explants of branchial arch mesenchyme were strongly growth-promoting and chemoattractive for cranial motor axons. Enhanced and oriented axon outgrowth was also elicited by beads loaded with Hepatocyte Growth Factor (HGF); antibodies to this protein largely blocked the outgrowth and orientation effects of the branchial arch on motor axons. HGF was expressed in the branchial arches, whilst Met, which encodes an HGF receptor, was expressed by subpopulations of cranial motor neurons. Mice with targetted disruptions of HGF or Met showed defects in the navigation of hypoglossal motor axons into the branchial region. Branchial arch tissue may thus act as a target-derived factor that guides motor axons during development. This influence is likely to be mediated partly by Hepatocyte Growth Factor, although a component of branchial arch-mediated growth promotion and chemoattraction was not blocked by anti-HGF antibodies.

Friedhelm Bladt

Papers

Scatter factor/hepatocyte growth factor is essential for liver development

Essential role for the c-met receptor in the migration of myogenic precursor cells into the limb bud.

Hepatocyte growth factor/scatter factor is an axonal chemoattractant and a neurotrophic factor for spinal motor neurons

The role of SF/HGF and c-Met in the development of skeletal muscle.

The branchial arches and HGF are growth-promoting and chemoattractant for cranial motor axons