Cutting out circular RNAs: Circular RNAs are widespread, but their functions have been controversial. Piwecka et al. used CRISPR-Cas9 technology to remove the locus encoding the circular RNA Cdr1as from the mouse genome. Single-cell electrophysiological measurements in excitatory neurons revealed an increase in spontaneous vesicle release from the knockout mice and depression in the synaptic response with two consecutive stimuli, indicating that Cdr1as deficiency leads to dysfunction of excitatory synaptic transmission. Small RNA sequencing of several major regions of the brain showed that expression of two microRNAs, miR-7 and miR-671, that bind to Cdr1as decreased and increased, respectively. These results, along with expression analyses, suggest that neuronal Cdr1as stabilizes or transports miR-7, which in turn represses genes that are early responders to different stimuli. Science , this issue p. eaam8526

Genetic analysis in mice has demonstrated a crucial role of the Met tyrosine kinase receptor and its ligand, hepatocyte growth factor/scatter factor (HGF/SF), in development of the liver, muscle, and placenta. Here, we use conditional mutagenesis in mice to analyze the function ofMetduring liver regeneration, using theMx-cretransgene to introduce the mutation in the adult. After partial hepatectomy in mice carrying theMx-cre-inducedMetmutation, regeneration of the liver is impaired. Comparison of signal transduction pathways in control and mutant livers indicates that Met and other signaling receptors cooperate to fully activate particular signaling molecules, for instance, the protein kinase Akt. However, activation of the Erk1/2 kinase during liver regeneration depends exclusively on Met. Signaling crosstalk is thus an important aspect of the regulation of liver regeneration. Analysis of cell cycle progression of hepatocytes in conditionalMetmutant mice indicates a defective exit from quiescence and diminished entry into S phase. Impaired liver regeneration is accompanied by compensatory physiological responses that include prolonged up-regulation of HGF/SF and IL-6 in peripheral blood. Our data demonstrate that the HGF/SF/Met signaling system is essential not only during liver development but also for the regeneration of the organ in the adult.

The ErbB2 ( Her2 ) proto-oncogene encodes a receptor tyrosine kinase, which is frequently amplified and overexpressed in human tumors. ErbB2 provides the target for a novel and effective antibody-based therapy (Trastuzumab/Herceptin) used for the treatment of mammary carcinomas. However, cardiomyopathies develop in a proportion of patients treated with Trastuzumab, and the incidence of such complications is increased by combination with standard chemotherapy. Gene ablation studies have previously demonstrated that the ErbB2 receptor, together with its coreceptor ErbB4 and the ligand Neuregulin-1, are essential for normal development of the heart ventricle. We use here Cre- lox P technology to mutate ErbB2 specifically in ventricular cardiomyocytes. Conditional mutant mice develop a severe dilated cardiomyopathy, with signs of cardiac dysfunction generally appearing by the second postnatal month. We infer that signaling from the ErbB2 receptor, which is enriched in T-tubules in cardiomyocytes, is crucial for adult heart function. Conditional ErbB2 mutant mice provide a model of dilated cardiomyopathy. In particular, they will allow a rigorous assessment of the role of ErbB2 in the heart and provide insight into the molecular mechanisms that underlie the adverse effects of anti-ErbB2 antibodies.

Neuregulins (NDF, heregulin, GGF ARIA, or SMDF) are EGF-like growth and differentiation factors that signal through tyrosine kinase receptors of the ErbB family. Here, we report a novel phenotype in mice with targeted mutations in the erbB2, erbB3, orneuregulin-1 genes. These three mutations cause a severe hypoplasia of the primary sympathetic ganglion chain. We provide evidence that migration of neural crest cells to the mesenchyme lateral of the dorsal aorta, in which they differentiate into sympathetic neurons, depends on neuregulin-1 and its receptors. Neuregulin-1 is expressed at the origin of neural crest cells. Moreover, a tight link between neuregulin-1 expression, the migratory path, and the target site of sympathogenic neural crest cells is observed. Sympathetic ganglia synthesize catecholamines in the embryo and the adult. Accordingly, catecholamine levels in mutant embryos are severely decreased, and we suggest that the lack of catecholamines contributes to the embryonal lethality of the erbB3 mutant mice. Thus,neuregulin-1, erbB2, and erbB3 are required for the formation of the sympathetic nervous system; the block in development observed in mutant mice is caused by a lack of neural crest precursor cells in the anlage of the primary sympathetic ganglion chain. Together with previous observations, these findings establish the neuregulin signaling system as a key regulator in the development of neural crest cells.

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.