Mice with defects in HB-EGF ectodomain shedding show severe developmental abnormalities

Heparin-binding EGF-like growth factor (HB-EGF) is first synthesized as a membrane-anchored form (proHB-EGF), and its soluble form (sHB-EGF) is released by ectodomain shedding from proHB-EGF. To examine the significance of proHB-EGF processing in vivo, we generated mutant mice by targeted gene replacement, expressing either an uncleavable form (HBuc) or a transmembrane domain-truncated form (HBdeltatm) of the molecule. HB(uc/uc) mice developed severe heart failure and enlarged heart valves, phenotypes similar to those in proHB-EGF null mice. On the other hand, mice carrying HBdeltatm exhibited severe hyperplasia in both skin and heart. These results indicate that ectodomain shedding of proHB-EGF is essential for HB-EGF function in vivo, and that this process requires strict control.     

Neutralization of toxic heme by Plasmodium falciparum histidine-rich protein 2

Plasmodium falciparum histidine-rich protein 2 (PfHRP2) has been suggested to be an initiator of the polymerization of heme, which is produced as by-product on the digestion of hemoglobin, and a promoter of the H(2)O(2)-induced degradation of heme in food vacuoles of the malarial parasite. In this work, we have designed PfHRP2 model peptides, R18 and R27 (18 and 27 residues, respectively), and used them for optical and electron spin resonance spectroscopic measurements to confirm that the axial ligands of the heme-PfHRP2 complex are the nitrogenous donors derived from the imidazole moieties of histidine residues of PfHRP2. In addition, we revealed that the affinities of R18 and R27 for heme (K(d) = 2.21 x 10(-6) M and 0.71 x 10(-6) M, respectively) might be as high as that of PfHRP2 (K(d) = 0.94 x 10(-6) M). The R27 peptide can remove heme from membrane-intercalated heme and inhibit heme-induced hemolysis. Therefore, we suggest another function of PfHRP2: it may play an important role in the neutralization of toxic heme in the parasite cytoplasm and infected erythrocytes by removing heme from heme-bound membranes or reducing heme-induced hemolysis.     

Molecular cloning and characterization of mouse Tspan-3, a novel member of the tetraspanin superfamily, expressed on resting dendritic cells

Dendritic cells (DCs) are the most potent antigen-presenting cells and play an essential role for triggering T-cell-mediated immune responses. In search for novel cell surface molecules expressed on DCs involved in T cell priming by representational differential analysis, we identified a mouse homologue of Tspan-3 (mTspan-3), a novel member of the tetraspanin superfamily. The mTspan-3 consists of four hydrophobic, putative transmembrane regions, forming a small and a large extracellular loop, with short intracellular amino and carboxil tails. Although the mTspan-3 is expressed on a variety of immune cell types including resting DCs, its expression on DCs is downregulated during activation induced by cross-linking CD40 with anti-CD40 monoclonal antibody. These results suggest that mTspan-3 may be involved in the function of DCs in association with T cell stimulation.     

The Kinetochore Protein Kis1/Eic1/Mis19 Ensures the Integrity of Mitotic Spindles through Maintenance of Kinetochore Factors Mis6/CENP-I and CENP-A

Microtubules play multiple roles in a wide range of cellular phenomena, including cell polarity establishment and chromosome segregation. A number of microtubule regulators have been identified, including microtubule-associated proteins and kinases, and knowledge of these factors has contributed to our molecular understanding of microtubule regulation of each relevant cellular process. The known regulators, however, are insufficient to explain how those processes are linked to one another, underscoring the need to identify additional regulators. To find such novel mechanisms and microtubule regulators, we performed a screen that combined genetics and microscopy for fission yeast mutants defective in microtubule organization. We isolated approximately 900 mutants showing defects in either microtubule organization or the nuclear envelope, and these mutants were classified into 12 categories. We particularly focused on one mutant, kis1, which displayed spindle defects in early mitosis. The kis1 mutant frequently failed to assemble a normal bipolar spindle. The responsible gene encoded a kinetochore protein, Mis19 (also known as Eic1), which localized to the interface of kinetochores and spindle poles. We also found that the inner kinetochore proteins Mis6/CENP-I and Cnp1/CENP-A were delocalized from kinetochores in the kis1 cells and that kinetochore-microtubule attachment was defective. Another mutant, mis6, also displayed similar spindle defects. We conclude that Kis1 is required for inner kinetochore organization, through which Kis1 ensures kinetochore-microtubule attachment and spindle integrity. Thus, we propose an unexpected relationship between inner kinetochore organization and spindle integrity.     

Molecular Evolution of Hemagglutinin (H) Gene in Measles Virus Genotypes D3, D5, D9, and H1

We studied the molecular evolution of H gene in four prevalent Asian genotypes (D3, D5, D9, and H1) of measles virus (MeV). We estimated the evolutionary time scale of the gene by the Bayesian Markov Chain Monte Carlo (MCMC) method. In addition, we predicted the changes in structure of H protein due to selective pressures. The phylogenetic tree showed that the first division of these genotypes occurred around 1931, and further division of each type in the 1960–1970s resulted in four genotypes. The rate of molecular evolution was relatively slow (5.57×10⁻⁴ substitutions per site per year). Only two positively selected sites (F476L and Q575K) were identified in H protein, although these substitutions might not have imparted significant changes to the structure of the protein or the epitopes for phylactic antibodies. The results suggested that the prevalent Asian MeV genotypes were generated over approximately 30–40 years and H protein was well conserved.     

Initiation of Cyclin B Degradation by the 26S Proteasome upon Egg Activation

Immediately before the transition from metaphase to anaphase, the protein kinase activity of maturation or M-phase promoting factor (MPF) is inactivated by a mechanism that involves the degradation of its regulatory subunit, cyclin B. The availability of biologically active goldfish cyclin B produced in Escherichia coli and purified goldfish proteasomes (a nonlysosomal large protease) has allowed the role of proteasomes in the regulation of cyclin degradation to be examined for the first time. The 26S, but not the 20S proteasome, digested recombinant 49-kD cyclin B at lysine 57 (K57), producing a 42-kD truncated form. The 42-kD cyclin was also produced by the digestion of native cyclin B forming a complex with cdc2, a catalytic subunit of MPF, and a fragment transiently appeared during cyclin degradation when eggs were released from metaphase II arrest by egg activation. Mutant cyclin at K57 was resistant to both digestion by the 26S proteasome and degradation at metaphase/anaphase transition in Xenopus egg extracts. The results of this study indicate that the destruction of cyclin B is initiated by the ATP-dependent and ubiquitin-independent proteolytic activity of 26S proteasome through the first cutting in the NH₂ terminus of cyclin (at K57 in the case of goldfish cyclin B). We also surmise that this cut allows the cyclin to be ubiquitinated for further destruction by ubiquitin-dependent activity of the 26S proteasome that leads to MPF inactivation.     

Proteasome inhibition suppresses DNA-dependent protein kinase activation caused by camptothecin

The ubiquitin–proteasome pathway plays an important role in DNA damage signaling and repair by facilitating the recruitment and activation of DNA repair factors and signaling proteins at sites of damaged chromatin. Proteasome activity is generally not thought to be required for activation of apical signaling kinases including the PI3K-related kinases (PIKKs) ATM, ATR, and DNA-PK that orchestrate downstream signaling cascades in response to diverse genotoxic stimuli. In a previous work, we showed that inhibition of the proteasome by MG-132 suppressed 53BP1 (p53 binding protein1) phosphorylation as well as RPA2 (replication protein A2) phosphorylation in response to the topoisomerase I (TopI) poison camptothecin (CPT). To address the mechanism of proteasome-dependent RPA2 phosphorylation, we investigated the effects of proteasome inhibitors on the upstream PIKKs. MG-132 sharply suppressed CPT-induced DNA-PKcs autophosphorylation, a marker of the activation, whereas the phosphorylation of ATM and ATR substrates was only slightly suppressed by MG-132, suggesting that DNA-PK among the PIKKs is specifically regulated by the proteasome in response to CPT. On the other hand, MG-132 did not suppress DNA-PK activation in response to UV or IR. MG-132 blocked the interaction between DNA-PKcs and Ku heterodimer enhanced by CPT, and hydroxyurea pre-treatment completely abolished CPT-induced DNA-PKcs autophosphorylation, indicating a requirement for ongoing DNA replication. CPT-induced TopI degradation occurred independent of DNA-PK activation, suggesting that DNA-PK activation does not require degradation of trapped TopI complexes. The combined results suggest that CPT-dependent replication fork collapse activates DNA-PK signaling through a proteasome dependent, TopI degradation-independent pathway. The implications of DNA-PK activation in the context of TopI poison-based therapies are discussed.     

Does conditioned taste aversion learning in the pond snail Lymnaea stagnalis produce conditioned fear?

In conditioned taste aversion (CTA) training performed on the pond snail Lymnaea stagnalis, a stimulus (the conditional stimulus, CS; e.g., sucrose) that elicits a feeding response is paired with an aversive stimulus (the unconditional stimulus, US) that elicits the whole-body withdrawal response and inhibits feeding. After CTA training and memory formation, the CS no longer elicits feeding. We hypothesize that one reason for this result is that after CTA training the CS now elicits a fear response. Consistent with this hypothesis, we predict the CS will cause (1) the heart to skip a beat and (2) a significant change in the heart rate. Such changes are seen in mammalian preparations exposed to fearful stimuli. We found that in snails exhibiting long-term memory for one-trial CTA (i.e., good learners) the CS significantly increased the probability of a skipped heartbeat, but did not significantly change the heart rate. The probability of a skipped heartbeat was unaltered in control snails given backward conditioning (US followed by CS) or in snails that did not acquire associative learning (i.e., poor learners) after the one-trial CTA training. These results suggest that as a consequence of acquiring CTA, the CS evokes conditioned fear in the conditioned snails, as evidenced by a change in the nervous system control of cardiac activity.