Biochemical and Spectroscopic Characterization of the Human Mitochondrial Amidoxime Reducing Components hmARC-1 and hmARC-2 Suggests the Existence of a New Molybdenum Enzyme Family in Eukaryotes

The mitochondrial amidoxime reducing component mARC is a newly discovered molybdenum enzyme that is presumed to form the catalytical part of a three-component enzyme system, consisting of mARC, heme/cytochrome b₅, and NADH/FAD-dependent cytochrome b₅ reductase. mARC proteins share a significant degree of homology to the molybdenum cofactor-binding domain of eukaryotic molybdenum cofactor sulfurase proteins, the latter catalyzing the post-translational activation of aldehyde oxidase and xanthine oxidoreductase. The human genome harbors two mARC genes, referred to as hmARC-1/MOSC-1 and hmARC-2/MOSC-2, which are organized in a tandem arrangement on chromosome 1. Recombinant expression of hmARC-1 and hmARC-2 proteins in Escherichia coli reveals that both proteins are monomeric in their active forms, which is in contrast to all other eukaryotic molybdenum enzymes that act as homo- or heterodimers. Both hmARC-1 and hmARC-2 catalyze the N-reduction of a variety of N-hydroxylated substrates such as N-hydroxy-cytosine, albeit with different specificities. Reconstitution of active molybdenum cofactor onto recombinant hmARC-1 and hmARC-2 proteins in the absence of sulfur indicates that mARC proteins do not belong to the xanthine oxidase family of molybdenum enzymes. Moreover, they also appear to be different from the sulfite oxidase family, because no cysteine residue could be identified as a putative ligand of the molybdenum atom. This suggests that the hmARC proteins and sulfurase represent members of a new family of molybdenum enzymes.     

Differential in vivo binding dynamics of somatic and oocyte-specific linker histones in oocytes and during ES cell nuclear transfer

The embryonic genome is formed by fusion of a maternal and a paternal genome. To accommodate the resulting diploid genome in the fertilized oocyte dramatic global genome reorganizations must occur. The higher order structure of chromatin in vivo is critically dependent on architectural chromatin proteins, with the family of linker histone proteins among the most critical structural determinants. Although somatic cells contain numerous linker histone variants, only one, H1FOO, is present in mouse oocytes. Upon fertilization H1FOO rapidly populates the introduced paternal genome and replaces sperm-specific histone-like proteins. The same dynamic replacement occurs upon introduction of a nucleus during somatic cell nuclear transfer. To understand the molecular basis of this dynamic histone replacement process, we compared the localization and binding dynamics of somatic H1 and oocyte-specific H1FOO and identified the molecular determinants of binding to either oocyte or somatic chromatin in living cells. We find that although both histones associate readily with chromatin in nuclei of somatic cells, only H1FOO is capable of correct chromatin association in the germinal vesicle stage oocyte nuclei. This specificity is generated by the N-terminal and globular domains of H1FOO. Measurement of in vivo binding properties of the H1 variants suggest that H1FOO binds chromatin more tightly than somatic linker histones. We provide evidence that both the binding properties of linker histones as well as additional, active processes contribute to the replacement of somatic histones with H1FOO during nuclear transfer. These results provide the first mechanistic insights into the crucial step of linker histone replacement as it occurs during fertilization and somatic cell nuclear transfer.     

Histidine 129 in the 75-kDa subunit of mitochondrial complex I from Yarrowia lipolytica is not a ligand for [Fe4S4] cluster N5 but is required for catalytic activity

Respiratory chain complex I contains 8-9 iron-sulfur clusters. In several cases, the assignment of these clusters to subunits and binding motifs is still ambiguous. To test the proposed ligation of the tetranuclear iron-sulfur cluster N5 of respiratory chain complex I, we replaced the conserved histidine 129 in the 75-kDa subunit from Yarrowia lipolytica with alanine. In the mutant strain, reduced amounts of fully assembled but destabilized complex I could be detected. Deamino-NADH: ubiquinone oxidoreductase activity was abolished completely by the mutation. However, EPR spectroscopic analysis of mutant complex I exhibited an unchanged cluster N5 signal, excluding histidine 129 as a cluster N5 ligand.     

Inhibition of nuclear import of LIMK2 in endothelial cells by protein kinase C-dependent phosphorylation at Ser-283

LIM kinases (LIMKs) are mainly in the cytoplasm and regulate actin dynamics through cofilin phosphorylation. Recently, it has been reported that nuclear localization of LIMKs can mediate suppression of cyclin D1 expression. Using immunofluorescence monitoring of enhanced green fluorescent protein-tagged LIMK2 in combination with photobleaching techniques and leptomycin B treatment, we demonstrate that LIMK2 shuttles between the cytoplasm and the nucleus in endothelial cells. Sequence analysis predicted two PKC phosphorylation sites in LIMK2 but not in LIMK1. One site at Ser-283 is present between the PDZ and the kinase domain, and the other site at Thr-494 is within the kinase domain. Activation of PKC by phorbol ester treatment of endothelial cells stimulated LIMK2 phosphorylation at Ser-283 and inhibited nuclear import of LIMK2 and the PDZ kinase construct of LIMK2 (amino acids 142-638) but not of LIMK1. The PKC-delta isoform phosphorylated LIMK2 at Ser-283 in vitro. Mutational analysis indicated that LIMK2 phosphorylation at Ser-283 but not Thr-494 was functional. Serum stimulation of endothelial cells also inhibited nuclear import of PDZK-LIMK2 by protein kinase C-dependent phosphorylation of Ser-283. Our study shows that phorbol ester and serum stimulation of endothelial cells inhibit nuclear import of LIMK2 but not LIMK1. This effect was dependent on PKC-delta-mediated phosphorylation of Ser-283. Since phorbol ester enhanced cyclin D1 expression and subsequent G1-to-S-phase transition of endothelial cells, we suggest that the PKC-mediated exclusion of LIMK2 from the nucleus might be a mechanism to relieve suppression of cyclin D1 expression by LIMK2.     

Role of the programmed death-1 pathway in regulation of alloimmune responses in vivo

Programmed death-1 (PD-1), an inhibitory receptor up-regulated on activated T cells, has been shown to play a critical immunoregulatory role in peripheral tolerance, but its role in alloimmune responses is poorly understood. Using a novel alloreactive TCR-transgenic model system, we examined the functions of this pathway in the regulation of alloreactive CD4+ T cell responses in vivo. PD-L1, but not PD-1 or PD-L2, blockade accelerated MHC class II-mismatched skin graft (bm12 (I-Abm12) into B6 (I-Ab)) rejection in a similar manner to CTLA-4 blockade. In an adoptive transfer model system using the recently described anti-bm12 (ABM) TCR-transgenic mice directly reactive to I-Abm12, PD-1 and PD-L1 blockade enhanced T cell proliferation early in the immune response. In contrast, at a later time point preceding accelerated allograft rejection, only PD-L1 blockade enhanced T cell proliferation. In addition, PD-L1 blockade enhanced alloreactive Th1 cell differentiation. Apoptosis of alloantigen-specific T cells was inhibited significantly by PD-L1 but not PD-1 blockade, indicating that PD-1 may not be the receptor for the apoptotic effect of the PD-L1-signaling pathway. Interestingly, the effect of PD-L1 blockade was dependent on the presence of CD4+ CD25+ regulatory T cells in vivo. These data demonstrate a critical role for the PD-1 pathway, particularly PD-1/PD-L1 interactions, in the regulation of alloimmune responses in vivo.     

Cutting edge: neutrophil granulocyte serves as a vector for Leishmania entry into macrophages

Macrophages (MF) are the final host cells for multiplication of the intracellular parasite Leishmania major (L. major). However, polymorphonuclear neutrophil granulocytes (PMN), not MF, are the first leukocytes that migrate to the site of infection and encounter the parasites. Our previous studies indicated that PMN phagocytose but do not kill L. major. Upon infection with Leishmania, apoptosis of human PMN is delayed and takes 2 days to occur. Infected PMN were found to secrete high levels of the chemokine MIP-1beta, which attracts MF. In this study, we investigated whether MF can ingest parasite-infected PMN. We observed that MF readily phagocytosed infected apoptotic PMN. Leishmania internalized by this indirect way survived and multiplied in MF. Moreover, ingestion of apoptotic infected PMN resulted in release of the anti-inflammatory cytokine TGF-beta by MF. These data indicate that Leishmania can misuse granulocytes as a "Trojan horse" to enter their final host cells "silently" and unrecognized.     

TIRC7 deficiency causes in vitro and in vivo augmentation of T and B cell activation and cytokine response

The membrane protein T cell immune response cDNA 7 (TIRC7) was recently identified and was shown to play an important role in T cell activation. To characterize the function of TIRC7 in more detail, we generated TIRC7-deficient mice by gene targeting. We observed disturbed T and B cell function both in vitro and in vivo in TIRC7(-/-) mice. Histologically, primary and secondary lymphoid organs showed a mixture of hypo-, hyper-, and dysplastic changes of multiple lymphohemopoietic compartments. T cells from TIRC7(-/-) mice exhibited significantly increased proliferation and expression of IL-2, IFN-gamma, and IL-4 in response to different stimuli. Resting T cells from TIRC7(-/-) mice exhibited decreased CD62L, but increased CD11a and CD44 expression, suggesting an in vivo expansion of memory/effector T cells. Remarkably, activated T cells from TIRC7(-/-) mice expressed lower levels of CTLA-4 in comparison with wild-type cells. B cells from TIRC7-deficient mice exhibited significantly higher in vitro proliferation following stimulation with anti-CD40 Ab or LPS plus IL-4. B cell hyperreactivity was reflected in vivo by elevated serum levels of various Ig classes and higher CD86 expression on B cells. Furthermore, TIRC7 deficiency resulted in an augmented delayed-type hypersensitivity response that was also reflected in increased mononuclear infiltration in the skin obtained from TIRC7-deficient mice food pads. In summary, the data strongly support an important role for TIRC7 in regulating both T and B cell responses.