Spatially and temporally coordinated changes in gene expression are crucial to orderly progression of embryogenesis. We combine mouse genetics with experimental manipulation of signalling to analyze the kinetics by which the SHH morphogen and the BMP antagonist gremlin 1 (GREM1) control gene expression in the digit-forming mesenchyme of mouse limb buds. Although most mesenchymal cells respond rapidly to SHH signalling, the transcriptional upregulation of specific SHH target signals in the mesenchyme occurs with differential temporal kinetics and in a spatially restricted fashion. In particular, the expression of the BMP antagonist Grem1 is always upregulated in mesenchymal cells located distal to the SHH source and acts upstream of FGF signalling by the apical ectodermal ridge. GREM1/FGF-mediated feedback signalling is, in turn, required to propagate SHH and establish the presumptive digit expression domains of the Notch ligand jagged 1 (Jag1) and 5ЈHoxd genes in the distal limb bud mesenchyme. Their establishment is significantly delayed in Grem1-deficient limb buds and cannot be rescued by specific restoration of SHH signalling in mutant limb buds. This shows that GREM1/FGF feedback signalling is required for regulation of the temporal kinetics of the mesenchymal response to SHH signalling. Finally, inhibition of SHH signal transduction at distinct time points reveals the differential temporal dependence of Grem1, Jag1 and 5ЈHoxd gene expression on SHH signalling. In particular, the expression of Hoxd13 depends on SHH signal transduction significantly longer than does Hoxd11 expression, revealing that the reverse co-linear establishment, but not maintenance of their presumptive digit expression domains, depends on SHH signalling.

No abstract

The influenza A virus M2 ion channel protein has the longest cytoplasmic tail (CT) among the three viral envelope proteins and is well conserved between different viral strains. It is accessible to the host cellular machinery after fusion with the endosomal membrane and during the trafficking, assembly, and budding processes. We hypothesized that identification of host cellular interactants of M2 CT could help us to better understand the molecular mechanisms regulating the M2-dependent stages of the virus life cycle. Using yeast two-hybrid screening with M2 CT as bait, a novel interaction with the human annexin A6 (AnxA6) protein was identified, and their physical interaction was confirmed by coimmunoprecipitation assay and a colocalization study of virus-infected human cells. We found that small interfering RNA (siRNA)-mediated knockdown of AnxA6 expression significantly increased virus production, while its overexpression could reduce the titer of virus progeny, suggesting a negative regulatory role for AnxA6 during influenza A virus infection. Further characterization revealed that AnxA6 depletion or overexpression had no effect on the early stages of the virus life cycle or on viral RNA replication but impaired the release of progeny virus, as suggested by delayed or defective budding events observed at the plasma membrane of virus-infected cells by transmission electron microscopy. Collectively, this work identifies AnxA6 as a novel cellular regulator that targets and impairs the virus budding and release stages of the influenza A virus life cycle. Influenza A virus (IAV) is an enveloped virus with a segmented negative-sense RNA genome. The eight RNA segments encode 11 proteins (44). Influenza virus is pleomorphic (23), forming spherical virions that are ϳ100 nm in diameter as well as filamentous virions that are ϳ100 nm in diameter and over 20 m in length. The viral lipid envelope, derived by budding from the apical plasma membrane of the host cell, contains two major envelope glycoproteins, the receptor-binding/membrane fusion protein hemagglutinin (HA) and the enzyme neuraminidase (NA). A third minor (ϳ16 to 20 molecules/virion) integral membrane protein, M2, is a proton-selective ion channel that allows virion interior acidification for efficient uncoating after fusion in endosomes. M2 contains 97 amino acid residues and assembles into a homotetramer. This small type III integral membrane protein contains a single 19-residue transmembrane domain that forms the pore of the ion channel, a short (24-residue) amino-terminal ectodomain, and a long (54-residue) carboxy-terminal cytoplasmic domain (27,34,45). This cytoplasmic tail (CT) is highly conserved among viral strains (57).In the early stages of the replication cycle, after virus internalization by endocytosis, endosomal acidification activates M2 ion channel activity, causing acidification of the virus interior and leading to dissociation of the matrix protein M1 from the viral ribonucleoprotein (vRNP) complex (reviewed in reference 47). M2 is the targ...

H5pp provide a reliable and safe alternative for sero-diagnosis and sero-epidemiology of H5N1 infections in a BSL-2 setting.