Cytoprotective peptide humanin binds and inhibits proapoptotic Bcl-2/Bax family protein BimEL

Humanin (HN) is a recently identified endogenous peptide that protects cells against cytotoxicity induced by various stimuli. Recently, we showed that HN binds to and inhibits Bax, a proapoptotic Bcl-2 family protein, suggesting a mechanism for HN action. In this study, we identified Bim, a Bcl-2 homology 3-only member of the Bcl-2/Bax family, as an additional HN target protein. Using in vitro protein binding, immunoprecipitation, and coimmunolocalization assays, we demonstrated that HN binds directly to the extra long isoform of Bim (BimEL) but not the long (BimL) or short (BimS) isoforms. HN also protects cells against apoptosis induced by BimEL but not BimL and BimS in gene transfection studies. In contrast, mutants of HN which failed to bind BimEL failed to protect from BimEL-induced cell death. Moreover, HN inhibited BimEL-induced release of SMAC and cytochrome c from mitochondria isolated from bax-/-cells, indicating that HN can suppress BimEL independently of its effect on Bax. Finally, we demonstrate that HN prevents BimEL-induced oligomerization of Bak using isolated mitochondria. Taken together, our results indicate that the inhibition of BimEL may contribute to the antiapoptotic properties of the HN peptide.     

Caspase-3 inhibits the growth of breast cancer cells independent of protease activity

In this study, the MCF-7 breast cancer cells that lack caspase-3 were transfected with a wild type (WT) or mutant caspase-3 cDNA. Expression of the WT, but not of the mutant, caspase-3 was associated with increased caspase activity and susceptibility to staurosporine (STS)-induced apoptosis. Both derivatives displayed inhibition of cell growth compared with vector control cells. Growth inhibition was associated with increased expression of the cyclin dependent kinase (CDK) inhibitor p27Kip1 in the WT, but not in the mutant caspase-3 expressing cells. Cyclin D1 expression level was not affected by caspase-3 expression. Phosphorylation of the Akt protein was decreased in both WT and mutant caspase transfected cells, although Akt expression level remained unchanged. These results suggest that caspase-3 might have biological functions independent of its protease activity and that its loss might contribute to tumor development by increasing the growth potential of cancer cells.     

Aberrant expression of the autoantigen heterogeneous nuclear ribonucleoprotein-A2 (RA33) and spontaneous formation of rheumatoid arthritis-associated anti-RA33 autoantibodies in TNF-alpha transgenic mice

Human TNF-alpha transgenic (hTNFtg) mice develop erosive arthritis closely resembling rheumatoid arthritis (RA). To investigate mechanisms leading to pathological autoimmune reactions in RA, we examined hTNFtg animals for the presence of RA-associated autoantibodies including Abs to citrullinated epitopes (anti-cyclic citrullinated peptide), heterogeneous nuclear ribonucleoprotein (hnRNP)-A2 (anti-RA33), and heat shock proteins (hsp) (anti-hsp). Although IgM anti-hsp Abs were detected in 40% of hTNFtg and control mice, IgG anti-hsp Abs were rarely seen, and anti-cyclic citrullinated peptide Abs were not seen at all. In contrast, >50% of hTNFtg mice showed IgG anti-RA33 autoantibodies, which became detectable shortly after the onset of arthritis. These Abs were predominantly directed to a short epitope, which was identical with an epitope previously described in MRL/lpr mice. Incidence of anti-RA33 was significantly decreased in mice treated with the osteoclast inhibitor osteoprotegerin and also in c-fos-deficient mice lacking osteoclasts. Pronounced expression of hnRNP-A2 and a smaller splice variant was seen in joints of hTNFtg mice, whereas expression was low in control animals. Although the closely related hnRNP-A1 was also overexpressed, autoantibodies to this protein were infrequently detected. Because expression of hnRNP-A2 in thymus, spleen, brain, and lung was similar in hTNFtg and control mice, aberrant expression appeared to be restricted to the inflamed joint. Finally, immunization of hTNFtg mice with recombinant hnRNP-A2 or a peptide harboring the major B cell epitope aggravated arthritis. These findings suggest that overproduction of TNF-alpha leads to aberrant expression of hnRNP-A2 in the rheumatoid joint and subsequently to autoimmune reactions, which may enhance the inflammatory and destructive process.     

CXCR5-dependent seeding of follicular niches by B and Th cells augments antiviral B cell responses

The chemokine receptor CXCR5 and its ligand CXCL13 define the structure of B cell follicles within secondary lymphoid organs. Here, we examined the impact of CXCR5 on antiviral B cell responses in vivo. CXCR5-/- mice showed a normal production of IgM and IgG acutely after infection with vesicular stomatitis virus (VSV) and developed VSV-specific germinal centers. However, impaired Ig class switch and Ab production were observed under conditions of limited availability of Ag (i.e., after immunization with nonreplicating viral particles or soluble Ag). Adoptive transfer of CXCR5-deficient, VSV-specific B and Th cells demonstrated that CXCR5 expression on both B and Th cells is required for an efficient Ig class switch. These experiments revealed that CXCR5 is critical for the coordinated interaction of antiviral T and B cells through its impact on initial B cell expansion and the recruitment of Ag-specific B and Th cells to germinal centers.     

The protein-tyrosine phosphatase SHP-1 regulates the phosphorylation of alpha-actinin

Platelet activation triggers integrin alpha(IIb)beta(3)-dependent signals and the induction of tyrosine phosphorylation of the cytoskeletal protein alpha-actinin. We have previously reported that alpha-actinin is phosphorylated by the focal adhesion kinase (FAK). In this study, a phosphatase of 68 kDa that dephosphorylated alpha-actinin in vitro was isolated from platelet lysates by three sequential chromatography steps. The phosphatase was identified as SHP-1 by electrospray tandem mass spectrometry. alpha-Actinin was dephosphorylated in vitro by recombinant SHP-1 and by SHP-1 immunoprecipitated from unstimulated or thrombin-stimulated platelet lysates. SHP-1 immunoprecipitated from lysates of platelets adherent to fibrinogen, however, failed to dephosphorylate alpha-actinin. In contrast, the activity of SHP-1 against a synthetic substrate was not affected by the mode of platelet activation. The robust and sustained phosphorylation of alpha-actinin detected in platelets adherent to fibrinogen thus correlates with a decrease in the activity of SHP-1 toward it. Tyrosine phosphorylation of alpha-actinin is seen in vanadate-treated COS-7 cells that are co-transfected with alpha-actinin and wild type FAK. Triple transfection of the cells with cDNAs encoding for alpha-actinin, FAK, and wild type SHP-1 abolished the phosphorylation of alpha-actinin. The phosphorylation of FAK, however, was barely affected by the expression of wild type SHP-1. Both alpha-actinin and FAK were phosphorylated in cells co-expressing alpha-actinin, FAK, and a catalytic domain mutant (C453S) of SHP-1. These findings establish that SHP-1 can dephosphorylate alpha-actinin in vitro and in vivo and suggest that SHP-1 may regulate the tethering of receptors to the cytoskeleton and/or the extent of cross-linking of actin filaments in cells such as platelets.     

Extended molecular dynamics simulation of the carbon monoxide migration in sperm whale myoglobin

We report the results of an extended molecular dynamics simulation on the migration of photodissociated carbon monoxide in wild-type sperm whale myoglobin. Our results allow following one possible ligand migration dynamics from the distal pocket to the Xe1 cavity via a path involving the other xenon binding cavities and momentarily two additional packing defects along the pathway. Comparison with recent time resolved structural data obtained by Laue crystallography with subnanosecond to millisecond resolution shows a more than satisfactory agreement. In fact, according to time resolved crystallography, CO, after photolysis, can occupy the Xe1 and Xe4 cavities. However, no information on the trajectory of the ligand from the distal pocket to the Xe1 is available. Our results clearly show one possible path within the protein. In addition, although our data refer to a single trajectory, the local dynamics of the ligand in each cavity is sufficiently equilibrated to obtain local structural and thermodynamic information not accessible to crystallography. In particular, we show that the CO motion and the protein fluctuations are strictly correlated: free energy calculations of the migration between adjacent cavities show that the migration is not a simple diffusion but is kinetically or thermodynamically driven by the collective motions of the protein; conversely, the protein fluctuations are influenced by the ligand in such a way that the opening/closure of the passage between adjacent cavities is strictly correlated to the presence of CO in its proximity. The compatibility between time resolved crystallographic experiments and molecular dynamics simulations paves the way to a deeper understanding of the role of internal dynamics and packing defects in the control of ligand binding in heme proteins.     

Roles for holes: are cavities in proteins mere packing defects?

Atomic packing in proteins is not optimized, most structures containing internal cavities, which have been identified by molecular modelling and characterized experimentally. Cavities seem to play a role in assisting conformational changes between domains or subunit interfaces. Comparison between homologous proteins from thermophiles and mesophiles indicates that optimizing packing enhances stabilization at the expense of flexibility. For proteins which interact with small ligands or substrates, cavities seem to play a role in controlling binding and catalysis, rather than being mere "packing defects". We believe that a more complete analysis on the localization, conservation and role of cavities in protein structures (by modelling and site-directed mutagenesis), will reveal that rather than being randomly distributed, they are located in key positions to allow structural dynamics and thereby functional control.     

Further investigation of simian immunodeficiency virus Vif function in human cells

Primate lentivirus Vif proteins function by suppressing the antiviral activity of the cell-encoded apolipoprotein B mRNA-editing enzyme-catalytic polypeptide-like (APOBEC) proteins APOBEC3G and APOBEC3F. It has been hypothesized that species-specific susceptibilities of APOBEC proteins to Vif proteins may help govern the transmission of primate lentiviruses to new host species. Consistent with this view and with previous results, we report that the Vif proteins of several diverse simian immunodeficiency viruses (SIVs) that are not known to infect humans are not effective inhibitors of human APOBEC3G or APOBEC3F when assessed in transient-transfection experiments. Unexpectedly, this lack of SIV Vif function did not prevent the replication of two vif-deficient SIVs (SIVtan and SIVmnd1; isolated from tantalus monkeys and mandrills, respectively) in a human T-cell line, HUT78, that expresses both APOBEC 3G and APOBEC3F, a finding which demonstrates that some SIVs are partially resistant to the antiretroviral effects of these enzymes irrespective of Vif function. Additional virus replication studies also revealed that the Vif protein of SIVtan is, in fact, active in human T cells, as it substantially enhanced the replication of its cognate virus and human immunodeficiency virus type 1. In sum, we now consider it improbable that species-specific restrictions to SIV Vif function can explain the lack of human infection with certain SIVs. Instead, our data reveal that the species-specific modulation of Vif function is more complex than previously envisioned and that additional (as-yet-unidentified) viral or host factors may be involved in regulating this dynamic interaction between host and pathogen.     

New simian immunodeficiency virus infecting De Brazza's monkeys (Cercopithecus neglectus): evidence for a cercopithecus monkey virus clade

Nearly complete sequences of simian immunodeficiency viruses (SIVs) infecting 18 different nonhuman primate species in sub-Saharan Africa have now been reported; yet, our understanding of the origins, evolutionary history, and geographic distribution of these viruses still remains fragmentary. Here, we report the molecular characterization of a lentivirus (SIVdeb) naturally infecting De Brazza's monkeys (Cercopithecus neglectus). Complete SIVdeb genomes (9,158 and 9227 bp in length) were amplified from uncultured blood mononuclear cell DNA of two wild-caught De Brazza's monkeys from Cameroon. In addition, partial pol sequences (650 bp) were amplified from four offspring of De Brazza's monkeys originally caught in the wild in Uganda. Full-length (9068 bp) and partial pol (650 bp) SIVsyk sequences were also amplified from Sykes's monkeys (Cercopithecus albogularis) from Kenya. Analysis of these sequences identified a new SIV clade (SIVdeb), which differed from previously characterized SIVs at 40 to 50% of sites in Pol protein sequences. The viruses most closely related to SIVdeb were SIVsyk and members of the SIVgsn/SIVmus/SIVmon group of viruses infecting greater spot-nosed monkeys (Cercopithecus nictitans), mustached monkeys (Cercopithecus cephus), and mona monkeys (Cercopithecus mona), respectively. In phylogenetic trees of concatenated protein sequences, SIVdeb, SIVsyk, and SIVgsn/SIVmus/SIVmon clustered together, and this relationship was highly significant in all major coding regions. Members of this virus group also shared the same number of cysteine residues in their extracellular envelope glycoprotein and a high-affinity AIP1 binding site (YPD/SL) in their p6 Gag protein, as well as a unique transactivation response element in their viral long terminal repeat; however, SIVdeb and SIVsyk, unlike SIVgsn, SIVmon, and SIVmus, did not encode a vpu gene. These data indicate that De Brazza's monkeys are naturally infected with SIVdeb, that this infection is prevalent in different areas of the species' habitat, and that geographically diverse SIVdeb strains cluster in a single virus group. The consistent clustering of SIVdeb with SIVsyk and the SIVmon/SIVmus/SIVgsn group also suggests that these viruses have evolved from a common ancestor that likely infected a Cercopithecus host in the distant past. The vpu gene appears to have been acquired by a subset of these Cercopithecus viruses after the divergence of SIVdeb and SIVsyk.