Cell signaling by receptor-tyrosine kinases

The transdifferentiation of epithelial cells into motile mesenchymal cells, a process known as epithelial-mesenchymal transition (EMT), is integral in development, wound healing and stem cell behaviour, and contributes pathologically to fibrosis and cancer progression. This switch in cell differentiation and behaviour is mediated by key transcription factors, including SNAIL, zinc-finger E-box-binding (ZEB) and basic helix-loop-helix transcription factors, the functions of which are finely regulated at the transcriptional, translational and post-translational levels. The reprogramming of gene expression during EMT, as well as non-transcriptional changes, are initiated and controlled by signalling pathways that respond to extracellular cues. Among these, transforming growth factor-β (TGFβ) family signalling has a predominant role; however, the convergence of signalling pathways is essential for EMT.

Molecular mechanisms of epithelial–mesenchymal transition

https://mootha.med.harvard.edu/PubPDFs/Wu1999.pdf

Mechanisms Controlling Mitochondrial Biogenesis and Respiration through the Thermogenic Coactivator PGC-1

https://www.medicinalgenomics.com/wp-content/uploads/2013/01/Resveratrol-rescues-mito-function-through-sirtuin-and-PGC-1.pdf

Resveratrol improves mitochondrial function and protects against metabolic disease by activating SIRT1 and PGC-1alpha.

Over the past 10 years, mitochondrial defects have been implicated in a wide variety of degenerative diseases, aging, and cancer. Studies on patients with these diseases have revealed much about the complexities of mitochondrial genetics, which involves an interplay between mutations in the mitochondrial and nuclear genomes. However, the pathophysiology of mitochondrial diseases has remained perplexing. The essential role of mitochondrial oxidative phosphorylation in cellular energy production, the generation of reactive oxygen species, and the initiation of apoptosis has suggested a number of novel mechanisms for mitochondrial pathology. The importance and interrelationship of these functions are now being studied in mouse models of mitochondrial disease.

/pdf/mitochondrial-diseases-in-man-and-mouse-43jzv1iswb.pdf

Mitochondrial diseases in man and mouse.

The regulation of mitochondrial DNA (mtDNA) expression is crucial for mitochondrial biogenesis during development and differentiation. We have disrupted the mouse gene for mitochondrial transcription factor A (Tfam; formerly known as m-mtTFA) by gene targetting of loxP-sites followed by cre-mediated excision in vivo. Heterozygous knockout mice exhibit reduced mtDNA copy number and respiratory chain deficiency in heart. Homozygous knockout embryos exhibit a severe mtDNA depletion with abolished oxidative phosphorylation. Mutant embryos proceed through implantation and gastrulation, but die prior to embryonic day (E)10.5. Thus, Tfam is the first mammalian protein demonstrated to regulate mtDNA copy number in vivo and is essential for mitochondrial biogenesis and embryonic development.

Mitochondrial transcription factor A is necessary for mtDNA maintenance and embryogenesis in mice

We describe a strategy for generating an allelic series of mutations at a given locus that requires the production of only one targetted mouse line. The ‘allelogenic’ mouse line we produced carries a hypomorphic allele of Fgf8, which can be converted to a null allele by mating to ere transgenic animals. The hypomorphic allele can also be reverted to wild-type by mating the allelogenic mice to flp transgenic animals, thereby generating a mouse line suitable for Cre-induced tissue-specific knockout experiments. Analysis of embryos carrying different combinations of these alleles revealed requirements for Fgf8 gene function during gastrulation, as well as cardiac, craniofacial, forebrain, midbrain and cerebellar development.

An Fgf8 mutant allelic series generated by Cre- and Flp-mediated recombination.

One of the most powerful tools that the molecular biology revolution has given us is the ability to turn genes on and off at our discretion. In the mouse, this has been accomplished by using binary systems in which gene expression is dependent on the interaction of two components, resulting in either transcriptional transactivation or DNA recombination. During recent years, these systems have been used to analyse complex and multi-staged biological processes, such as embryogenesis and cancer, with unprecedented precision. Here, I review these systems and discuss certain studies that exemplify the advantages and limitations of each system.

Conditional control of gene expression in the mouse

Fgf8 and Fgf4 encode FGF family members that are coexpressed in the primitive streak of the gastrulating mouse embryo. We have analyzed the phenotype of Fgf8(-/-) embryos and discovered that they fail to express Fgf4 in the streak. In the absence of both FGF8 and FGF4, epiblast cells move into the streak and undergo an epithelial-to-mesenchymal transition, but most cells then fail to move away from the streak. As a consequence, no embryonic mesoderm- or endoderm-derived tissues develop, although extraembryonic tissues form. Patterning of the prospective neuroectoderm is greatly perturbed in the mutant embryos. Anterior neuroectoderm markers are widely expressed, at least in part because the anterior visceral endoderm, which provides signals that regulate their expression, is not displaced proximally in the absence of definitive endoderm. Posterior neuroectoderm markers are not expressed, presumably because there is neither mesendoderm underlying the prospective neuroectoderm nor a morphologically normal node to provide the inductive signals necessary for their expression. This study identifies Fgf8 as a gene essential for gastrulation and shows that signaling via FGF8 and/or FGF4 is required for cell migration away from the primitive streak.

/pdf/targeted-disruption-of-fgf8-causes-failure-of-cell-migration-4eq6ku30de.pdf

Targeted disruption of Fgf8 causes failure of cell migration in the gastrulating mouse embryo

Vertebrate limb development depends on signals from the apical ectodermal ridge (AER), which rims the distal tip of the limb bud1. Removal of the AER in chick results in limbs lacking distal skeletal elements2,3. Fibroblast growth factor (FGF) proteins can substitute for the AER (refs 4–7), suggesting that FGF signalling mediates AER activity. Of the four mouse Fgf genes (Fgf4 , Fgf8, Fgf9, Fgf17) known to display AER-specific expression domains within the limb bud (AER-Fgfs), only Fgf8 is expressed throughout the AER. Moreover, Fgf8 expression precedes that of other AER-Fgfs (refs 8–13), suggesting that Fgf8 may perform unique functions early in limb development6,7. In mice, loss of function of Fgf4 (refs 13,14), Fgf9 (D. Ornitz, pers. comm.) or Fgf17 (ref. 15) has no effect on limb formation. We report here that inactivating Fgf8 in early limb ectoderm causes a substantial reduction in limb-bud size, a delay in Shh expression, misregulation of Fgf4 expression, and hypoplasia or aplasia of specific skeletal elements. Our data identify Fgf8 as the only known AER-Fgf individually necessary for normal limb development, and provide insight into the function of Fgf signalling from the AER in the normal outgrowth and patterning of the limb.

Mark Lewandoski

Papers

Mitochondrial transcription factor A is necessary for mtDNA maintenance and embryogenesis in mice

An Fgf8 mutant allelic series generated by Cre- and Flp-mediated recombination.

Conditional control of gene expression in the mouse

Targeted disruption of Fgf8 causes failure of cell migration in the gastrulating mouse embryo

Fgf8 signalling from the AER is essential for normal limb development.