A novel secreted form of immune suppressor factor with high homology to vacuolar ATPases identified by a forward genetic approach of functional screening based on cell proliferation

A carboxy-terminally truncated human CPSF6 lacking residues encoded by exon 6 inhibits HIV-1 cDNA synthesis and promotes capsid disassembly

Since HIV-1 replication is modulated at multiple stages by host cell factors, identification and characterization of those host cell factors are expected to contribute to the development of novel anti-HIV therapeutics. Previous studies showed that a C-terminally truncated cytosolic form of cleavage and polyadenylation-specific factor 6 (CPSF6-358) inhibits HIV-1 infection through interference with HIV-1 trafficking to the nucleus. Here we identified and characterized a different configuration of C-terminally truncated human CPSF6 (hCPSF6-375) through cDNA expression cloning coupled with ganciclovir-mediated lethal selection. Notably, hCPSF6-375, but not mouse CPSF6-358 (mCPSF6-358) as previously reported, remarkably interfered with viral cDNA synthesis after HIV-1 infection. Moreover, we found that hCPSF6-375 aberrantly accelerated the disassembly of the viral capsid in target cells, while CPSF6-358 did not. Sequence comparison of CPSF6-375 and CPSF6-358 cDNAs showed a lack of exon 6 and additional coding sequence for 54 amino acid residues in the C terminus of hCPSF6-375. Mutational analyses revealed that the residues encoded by exon 6, but not the C-terminal 54 residues in hCPSF6-375, is responsible for impaired viral cDNA synthesis by hCPSF6-375. This is the first report demonstrating a novel mode of HIV-1 inhibition by truncated forms of CPSF6 that involves rapid capsid disassembly and inhibition of viral cDNA synthesis. These findings could facilitate an increased understanding of viral cDNA synthesis in light of the viral capsid disassembly.

Roundabout 4 is expressed on hematopoietic stem cells and potentially involved in the niche-mediated regulation of the side population phenotype

Roundabout (Robo) family proteins are immunoglobulin-type cell surface receptors that are expressed predominantly in the nervous system. The fourth member of this family, Robo4, is distinct from the other family members in that it is expressed specifically in endothelial cells. In this study, we examined the expression of Robo4 in hematopoietic stem cells (HSCs) and its possible role in HSC regulation. Robo4 mRNA was specifically expressed in murine HSCs and the immature progenitor cell fraction but not in lineage-positive cells or differentiated progenitors. Moreover, flow cytometry showed a correlation between higher expression of Robo4 and immature phenotypes of hematopoietic cells. Robo4(high) hematopoietic stem/progenitor cells presented higher clonogenic activity or long-term repopulating activity by colony assays or transplantation assays, respectively. A ligand for Robo4, Slit2, is specifically expressed in bone marrow stromal cells, and its expression was induced in osteoblasts in response to myelosuppressive stress. Interestingly, overexpression of Robo4 or Slit2 in HSCs resulted in their decreased residence in the c-Kit(+)Sca-1(+)Lineage(-)-side population fraction. These results indicate that Robo4 is expressed in HSCs, and Robo4/Slit2 signaling may play a role in HSC homeostasis in the bone marrow niche.

Immune suppressor factor confers stromal cell line with enhanced supporting activity for hematopoietic stem cells

Immune suppressor factor (ISF) is a subunit of the vacuolar ATPase proton pump. We earlier identified a short form of ISF (ShIF) as a stroma-derived factor that supports cytokine-independent growth of mutant Ba/F3 cells. Here, we report that ISF/ShIF supports self-renewal and expansion of primary hematopoietic stem cells (HSCs). Co-culture of murine bone marrow cells with a stromal cell line overexpressing ISF or ShIF (MS10/ISF or MS10/ShIF) not only enhanced their colony-forming activity and the numbers of long-term culture initiating cells, but also maintained the competitive repopulating activity of HSC. This stem cell supporting activity depended on the proton-transfer function of ISF/ShIF. Gene expression analysis of ISF/ShIF-transfected cell lines revealed down-regulation of secreted frizzled-related protein-1 and tissue inhibitor of metalloproteinase-3, and the restoration of their expressions in MS10/ISF cells partially reversed its enhanced LTC-IC supporting activity to a normal level. These results suggest that ISF/ShIF confers stromal cells with enhanced supporting activities for HSCs by modulating Wnt-activity and the extracellular matrix.

Cloning, expression and functional characterization of the putative regeneration and tolerance factor (RTF/TJ6) as a functional vacuolar ATPase proton pump regulatory subunit with a conserved sequence of immunoreceptor tyrosine-based activation motif

In an attempt to identify new immunoreceptor tyrosine-based activation motif (ITAM)-containing human molecules that may regulate hitherto unknown immune cell functions, we BLAST searched the National Center for Biotechnology Information database for ITAM-containing sequences. A human expressed sequence tag showing partial homology to the murine TJ6 (mTJ6) gene and encoding a putative ITAM sequence has been identified and used to clone the human TJ6 (hTJ6) gene from an HL-60-derived cDNA library. hTJ6 was found to encode a protein of 856 residues with a calculated mass of 98 155 Da. Immunolocalization and sequence analysis revealed that hTJ6 is a membrane protein with predicted six transmembrane-spanning regions, typical of ion channels, and a single putative ITAM (residues 452-466) in a juxtamembrane or hydrophobic intramembrane region. hTJ6 is highly homologous to Bos taurus 116-kDa subunit of the vacuolar proton-translocating ATPase. Over-expression of hTJ6 in HEK 293 cells increased H+ uptake into intracellular organelles, an effect that was sensitive to inhibition by bafilomycin, a selective inhibitor of vacuolar H+ pump. Northern blot analysis demonstrated three different hybridizing mRNA transcripts corresponding to 3.2, 5.0 and 7.3 kb, indicating the presence of several splice variants. Significant differences in hTJ6 mRNA levels in human tissues of different origins point to possible tissue-specific function. Although hTJ6 was found to be a poor substrate for tyrosine-phosphorylating enzymes, suggesting that its ITAM sequence is non-functional in protein tyrosine kinase-mediated signaling pathways, its role in organellar H+ pumping suggests that hTJ6 function may participate in protein trafficking/processing.

TWEAK induces NF-kappaB2 p100 processing and long lasting NF-kappaB activation

Tumor necrosis factor (TNF)-like weak inducer of apoptosis (TWEAK) is a member of the TNF superfamily that has been shown to induce angiogenesis, apoptosis in tumor cells, and NF-kappaB activation through binding to its receptor, fibroblast growth factor-inducible 14. We have identified TWEAK as an inducer of constitutive NF-kappaB activation by expression cloning, and we report here sequential regulation by TWEAK of two separate signaling cascades for NF-kappaB activation, the NF-kappaB essential modulator-dependent and -independent signaling pathways. Upon TWEAK stimulation, IkappaBalpha is rapidly phosphorylated, generating NF-kappaB DNA-binding complexes containing p50 and RelA in a manner dependent on the canonical IkappaB kinase complex. Unlike TNF-alpha, TWEAK stimulation results in prolonged NF-kappaB activation with a transition of the DNA-binding NF-kappaB components from RelA- to RelB-containing complexes by 8 h, and the latter remained active in binding at least until 24 h post-stimulation. This long lasting activation is accompanied by the proteasome-mediated processing of NF-kappaB2/p100, which does not depend on the NF-kappaB essential modulator but requires IkappaB kinase 1 and functional NF-kappaB-inducing kinase activity. Finally, we show that fibroblast growth factor-inducible 14 with a mutation at its TNF receptor-associated factor (TRAF)-binding site cannot activate NF-kappaB and that TWEAK fails to induce the p100 processing and IkappaBalpha phosphorylation in cells deficient for TRAF2 and TRAF5. Our results thus identify TWEAK as a novel physiological regulator of the non-canonical pathway for NF-kappaB activation.

Inhibition of human endothelial cell proliferation by ShIF, a vacuolar H(+)-ATPase-like protein

ShIF is a bone marrow stroma cell-derived factor originally identified to support proliferation of bone marrow cells in vitro. This protein shares high sequence homology to the yeast vacuolar H(+)-ATPase subunit, Vph1p, and the 116 kDa proton pump of the rat and bovine synaptic vesicle, Vpp1. We examined the function of ShIF in the proliferation of human umbilical vein endothelial cells (HUVEC). ShIF inhibited HUVEC proliferation in a dose-dependent manner. Recombinant ShIF added at 10 and 20 ng/ml inhibited HUVEC proliferation by 21.6 and 44.3%, respectively and increasing the concentration of ShIF to 100 ng/ml inhibited proliferation by as much as 55.5%. When HUVEC cells were cultured at various concentrations of ShIF in the presence of anti-ShIF antibody, the inhibitory effects of ShIF to HUVEC proliferation were abrogated by 89-91% indicating that the activity of ShIF to HUVEC was specific. HUVEC cultured in the presence of ShIF and bafilomycin, a specific inhibitor of ATPase, resulted to a 90% growth inhibition. Thus, ShIF may act as an antagonist to the ATPase complex by disrupting the production of cellular ATP thereby decreasing the ability of HUVEC to proliferate.

Genetic approach and phenotype-based complementation screening for identification of stroma cell-derived proteins involved in cell proliferation

The functional capacities of stromal cell lines to support stem cell activity are heterogeneous and the mechanism of how they support bone marrow cultures remains unclear. Recently, we reported a strategy of functional analysis in which a genetic approach is combined with phenotype-based complementation screening to search for a novel secreted growth factor from mouse bone marrow stroma called ShIF that supported proliferation of bone marrow cells. To investigate the role of stromal cells in hemopoiesis, we extended this strategy to search for stroma-derived proteins that induce cell proliferation by establishing stroma-dependent Ba/F3 mutants of three stroma cell lines from two mouse tissues. Seven stroma-dependent Ba/F3 mutants were used as responder cells to identify cDNAs from stroma cell lines whose products supported proliferation not only to the mutant cells but also to hemopoietic progenitor cells in vitro.

SF20/IL-25, a novel bone marrow stroma-derived growth factor that binds to mouse thymic shared antigen-1 and supports lymphoid cell proliferation

Using a forward genetic approach and phenotype-based complementation screening to search for factors that stimulate cell proliferation, we have isolated a novel secreted bone marrow stroma-derived growth factor, which we termed SF20/IL-25. This protein signals cells to proliferate via its receptor, which we have identified as mouse thymic shared Ag-1 (TSA-1). Enforced expression of TSA-1 in IL-3-dependent Ba/F3 cells that do not express endogenous TSA-1 rendered cells to proliferate in a dose-dependent manner when stimulated with SF20/IL-25. FDCP2, a factor-dependent hemopoietic cell line that expresses endogenous TSA-1, could also be stimulated to proliferate with SF20/IL-25. Binding of SF20 to TSA-1 was blocked by anti-TSA-1 Ab and SF20-induced proliferation of TSA-1-expressing cells was inhibited by anti-TSA-1. In vitro assay revealed that SF20/IL-25 has no detectable myelopoietic activity but supports proliferation of cells in the lymphoid lineage.