A hypomorphic allele of dab1 reveals regional differences in reelin-Dab1 signaling during brain development

The disabled 1 (Dab1) p80 protein is essential for reelin signaling during brain development. p80 has an N-terminal domain for association with reelin receptors, followed by reelin-dependent tyrosine phosphorylation sites and about 310 C-terminal residues of unknown function. We have generated mutant mice that express only a natural splice form of Dab1, p45, that lacks the C-terminal region of p80. The normal development of these mice implies that the receptor-binding region and tyrosine phosphorylation sites of p80 are sufficient for reelin signaling. However, a single copy of the truncated gene does not support normal development of the neocortex and hippocampus. The CA1 region of the hippocampus is split into two well-organized layers, while the marginal zone of the neocortex is invaded by late-born cortical plate neurons. The haploinsufficiency of the p45 allele of Dab1 implies that the C terminus of p80 affects the strength of reelin-Dab1 signaling, yet there is no apparent change in reelin-dependent tyrosine phosphorylation of p45 relative to p80. Therefore, we suggest that the C-terminal region of Dab1 p80 is involved in signaling to downstream effector molecules. Furthermore, the presence of late-born cortical plate neurons in the marginal zone reveals a requirement for reelin-Dab1 signaling in late-born cortical plate neurons, and helps distinguish models for the cortical inversion in the reeler mutant mouse.     

Tail regression in Ciona intestinalis (Prochordate) involves a Caspase-dependent apoptosis event associated with ERK activation

Two apoptotic events take place during embryonic development of Ciona intestinalis. The first concerns extra-embryonic cells and precedes hatching. The second controls tail regression at metamorphosis, occurs through a polarized wave originating from tail extremity, and is caspase dependent. This was shown by: (1) in vivo incorporation of a fluorescent marker of caspase activation in different cell types of the tail; (2) detection of an activated form of caspase 3-like protein by western blotting; and (3) failure of 30% of larvae to undergo metamorphosis after treatment of fertilized eggs with a pan-caspase inhibitor. In addition, Ciona embryos express a single ERK protein, specifically phosphorylated at metamorphosis. ERK activation was shown to be located in cells of the tail. Addition of MEK inhibitor in the culture medium prevented ERK activation and metamorphosis. In silico analysis of Ciona genome pointed to 15 caspases with high homology with humans, and a single ERK gene with high homology to both mammalian ERK1 and ERK2. It is concluded that the sequence of events leading to metamorphosis includes ERK phosphorylation followed by caspase-dependent apoptosis and tail regression.     

Lateral flagella of Aeromonas species are essential for epithelial cell adherence and biofilm formation

Mesophilic Aeromonas strains express a single polar flagellum in all culture conditions and produce lateral flagella on solid media. Such hyperflagellated cells demonstrate increased adherence. Nine lateral flagella genes, lafA-U for Aeromonas hydrophila, and four Aeromonas caviae genes, lafA1, lafA2, lafB and fliU, were isolated. Mutant characterization, nucleotide and N-terminal sequencing demonstrated that the A. hydrophila and A. caviae lateral flagellins were almost identical, but were distinct from their polar flagellum counterparts. The aeromonad lateral flagellins exhibited higher molecular masses on SDS-PAGE, and this aberrant migration was thought to result from post-translational modification through glycosylation. Mutation of the Aeromonas lafB, lafS or both A. caviae lateral flagellins caused the loss of lateral flagella and a reduction in adherence and biofilm formation. Mutations in lafA1, lafA2, fliU or lafT resulted in strains that expressed lateral flagella, but had reduced adherence levels. Mutation of the lateral flagella loci did not affect polar flagellum synthesis, but the polarity of the transposon insertions on the A. hydrophila lafTlU genes resulted in non-motility. However, mutations that abolished polar flagellum production also inhibited lateral flagella expression. We conclude that Aeromonas lateral flagella: (i) play a role in adherence and biofilm formation; (ii) are distinct from the polar flagellum; (iii) synthesis is dependent upon the presence of a polar flagellum filament; and (iv) that the motor proteins of the polar and lateral flagella systems appear to be shared.     

Distamycin A affects the stability of NF-kappaB p50-DNA complexes in a sequence-dependent manner

The effect of two different DNA minor groove binding molecules, Hoechst 33258 and distamycin A, on the binding kinetics of NF-kappaB p50 to three different specific DNA sequences was studied at various salt concentrations. Distamycin A was shown to significantly increase the dissociation rate constant of p50 from the sequences PRDII (5'-GGGAAATTCC-3') and Ig-kappa B (5'-GGGACTTTCC-3') but had a negligible effect on the dissociation from the palindromic target-kappaB binding site (5'-GGGAATTCCC-3'). By comparison, the effect of Hoechst 33258 on binding of p50 to each sequence was found to be minimal. The dissociation rates for the protein--DNA complexes increased at higher potassium chloride concentrations for the PRDII and Ig-kappaB binding motifs and this effect was magnified by distamycin A. In contrast, p50 bound to the palindromic target-kappaB site with a much higher intrinsic affinity and exhibited a significantly reduced salt dependence of binding over the ionic strength range studied, retaining a K(D) of less than 10 pM at 150 mM KCl. Our results demonstrate that the DNA binding kinetics of p50 and their salt dependence is strongly sequence-dependent and, in addition, that the binding of p50 to DNA can be influenced by the addition of minor groove-binding drugs in a sequence-dependent manner.     

Helicase structure and mechanism

Structural information on helicase proteins has expanded recently beyond the DNA helicases Rep and PcrA, and the hepatitis C virus RNA helicase to include UvrB, members of the DEA(D/H)-box RNA helicase family, examples of DnaB-related helicases and RuvB. The expanding database of structures has clarified the structural 'theme and variations' that relate the different helicase families. Furthermore, information is emerging on the functions of the conserved helicase motifs and their participation in the mechanisms by which these proteins catalyze the remodeling of DNA and RNA in ATP-dependent activities.     

An approximately 400 kDa membrane-associated complex that contains one molecule of the resistance protein Cf-4

Despite sharing more than 91% sequence identity, the tomato Cf-4 and Cf-9 proteins discriminate between two Cladosporium-encoded avirulence determinants, Avr4 and Avr9. Comparative studies between Cf-4 and Cf-9 are thus of particular interest. To investigate Cf-4 protein function in initiating defence signalling, we established transgenic tobacco lines and derived cell suspension cultures expressing c-myc-tagged Cf-4. Cf-4:myc encodes a membrane-localized glycoprotein of approximately 145 kDa, which confers recognition of Avr4. Elicitation of Cf-4:myc and Cf-9:myc tobacco cell cultures with Avr4 and Avr9, respectively, triggered the synthesis of active oxygen species and MAP kinase activation. Additionally, an Agrobacterium-mediated transient assay was used to express Cf-4:myc and a newly engineered fusion protein Cf-4:TAP. Both transiently expressed proteins were found to be functional in an in vivo assay, conferring a hypersensitive response (HR) to Avr4. Consistent with previous observations that Cf-9 is present in a protein complex, gel filtration analysis of microsomal fractions solubilized with octylglucoside revealed that epitope-tagged Cf-4 proteins migrated at a molecular mass of 350-475 kDa. Using blue native gel electrophoresis, the molecular size was confirmed to be approximately 400 kDa. Significantly, this complex appeared to contain only one Cf-4 molecule, supporting the idea that, as previously described for Cf-9, additional glycoprotein partners participate with Cf-4 in the perception of the Avr4 protein. Intriguingly, Cf proteins and Clavata2 (CLV2) of Arabidopsis are highly similar in structure, and the molecular mass of Cf-4 and CLV complexes is also very similar (400 and 450 kDa, respectively). However, extensive characterization of the Cf-4 complex revealed essentially identical characteristics to the Cf-9 complex and significant differences from the CLV2 complex.     

Myosin-X is an unconventional myosin that undergoes intrafilopodial motility

Filopodia are thin cellular protrusions that are important in cell motility and neuronal growth cone guidance. The actin filaments that make up the core of a filopodium undergo continuous retrograde flow towards the cell body. Surface receptors or particles can couple to this retrograde flow and can also move forward to the tips of filopodia, although the molecular basis of forward transport is unknown. We report here that myosin-X (Myo10 or M10), the founding member of a novel class of myosins, localizes to the tips of filopodia and undergoes striking forward and rearward movements within filopodia, which we term intrafilopodial motility. The movements of the GFP-M10 puncta correspond to forward and rearward movements of phase-dense granules along the filopodia. Finally, overexpressing full-length M10 (but not truncated forms of M10) causes an increase in the number and length of filopodia, indicating that M10 or its cargo may function in filopodial dynamics. The localization and movements of M10 strongly suggest that it functions as a motor for intrafilopodial motility.     

The p53 tumour suppressor inhibits glucocorticoid-induced proliferation of erythroid progenitors

Hypoxia encountered at high altitude, blood loss and erythroleukemia instigate stress erythropoiesis, which involves glucocorticoid-induced proliferation of erythroid progenitors (ebls). The tumour suppressor p53 stimulates hematopoietic cell maturation and antagonizes glucocorticoid receptor (GR) activity in hypoxia, suggesting that it may inhibit stress erythropoiesis. We report that mouse fetal liver ebls that lack p53 proliferate better than wild-type cells in the presence of the GR agonist dexamethasone. An important mediator of GR-induced ebl self-renewal, the c-myb gene, is induced to higher levels in p53^–/– ebls by dexamethasone. The stress response to anemia is faster in the spleens of p53^–/– mice, as shown by the higher levels of colony forming units erythroids and the increase in the CD34/c-kit double positive population. Our results show that p53 antagonizes GR-mediated ebl expansion and demonstrate for the first time that p53–GR cross-talk is important in a physiological process in vivo: stress erythropoiesis.