Toll-like receptor 4 imparts ligand-specific recognition of bacterial lipopolysaccharide

Lipopolysaccharide (LPS) is the main inducer of shock and death in Gram-negative sepsis. Recent evidence suggests that LPS-induced signal transduction begins with CD14-mediated activation of 1 or more Toll-like receptors (TLRs). The lipid A analogues lipid IVa and Rhodobacter sphaeroides lipid A (RSLA) exhibit an uncommon species-specific pharmacology. Both compounds inhibit the effects of LPS in human cells but display LPS-mimetic activity in hamster cells. We transfected human TLR4 or human TLR2 into hamster fibroblasts to determine if either of these LPS signal transducers is responsible for the species-specific pharmacology. RSLA and lipid IVa strongly induced NF-kappaB activity and IL-6 release in Chinese hamster ovary fibroblasts expressing CD14 (CHO/CD14), but these compounds antagonized LPS antagonists in CHO/CD14 fibroblasts that overexpressed human TLR4. No such antagonism occurred in cells overexpressing human TLR2. We cloned TLR4 from hamster macrophages and found that human THP-1 cells expressing the hamster TLR4 responded to lipid IVa as an LPS mimetic, as if they were hamster in origin. Hence, cells heterologously overexpressing TLR4 from different species acquired a pharmacological phenotype with respect to recognition of lipid A substructures that corresponded to the species from which the TLR4 transgene originated. These data suggest that TLR4 is the central lipid A-recognition protein in the LPS receptor complex.

Cell-specific ecdysone-inducible expression of FLP recombinase in mammalian cells

The ability of site-specific recombinases, like FLP and Cre, to catalyze alterations in genomic DNA is well established, whereas their application to genetic engineering strategies has been restricted because of the inability to temporally regulate their expression and subsequent recombination events in specific populations of cells. We describe a regulatory system for ecdysone-controlled expression of FLP recombinase. Furthermore, we demonstrate that ecdysone-induced, FLP-mediated site-specific recombination events can be targeted to specific cells. This system can be applied to cell-lineage studies as well as to the design of gene-therapy strategies, particularly in stem cells.

A composite CMV-IE enhancer/beta-actin promoter is ubiquitously expressed in mouse cutaneous epithelium

In most existing transgenic mouse models developed for the study of specific genes in the skin, the goal has been to target transgene expression to defined populations of cells in the cutaneous epithelium. Keratin promoters have been especially useful for this purpose. In some instances, however, it may be desirable to express a transgene in all the cells of the cutaneous epithelium. Since no ubiquitously expressed promoter sequences had previously been identified, we used lacZ reporter transgenes to test two enhancer/promoter sequences for ubiquitous expression in the skin of adult transgenic mice. We find that a CMV enhancer/CMV promoter is not active in most cell types in the skin, whereas a CMV enhancer/modified beta-actin promoter sequence is active in the suprabasal and basal cells of the epidermis as well as in the epithelial cells of the hair follicles, sebaceous glands, and the dermal papillae.

Elements in the murine c-mos messenger RNA 5'-untranslated region repress translation of downstream coding sequences

Murine c-mos transcripts isolated from testes have 5'-untranslated regions (5'UTRs) of approximately 300 nucleotides with a series of four overlapping open reading frames (ORFs) upstream of the AUG codon that initiates the Mos ORF. Ovarian c-mos transcripts have shorter 5'UTRs (70-80 nucleotides) and contain only 1-2 of the upstream ORFs (uORFs). To test whether these 5'UTRs affect translational efficiency, we have constructed plasmids for the expression of chimeric transcripts with a mos-derived 5'UTR fused to the Escherichia coli beta-galactosidase coding region. Translational efficiency has been evaluated by measuring beta-galactosidase activity NIH3T3 cells transiently transfected with these plasmids and with plasmids where various mutations have been introduced into the 5'UTR. We show that the 5'UTR characteristic of testis-specific c-mos mRNA strongly represses translation relative to the translation of transcripts that contain a 5'UTR derived from beta-globin mRNA, and this is mainly due to the four uORFs. Each of the four upstream AUG triplets can be recognized as a start site for translation, and no single uAUG dominates the repressive effect. The uORFs repress translation by a mechanism that is not affected by the amino acid sequence in the COOH-terminal region of the uORF-encoded peptides. The very short uORF (AUGUGA) present in ovary-specific transcripts does not repress translation. Staining of testis sections from transgenic mice carrying chimeric beta-galactosidase transgene constructs, which contain a mos 5'UTR with or without the uATGs, suggests that the uORFs can dramatically change the pattern of expression in spermatogenic cells.

Developmental silencing of the embryonic zeta-globin gene: concerted action of the promoter and the 3'-flanking region combined with stage-specific silencing by the transcribed segment

Globin gene switching is a well-described model of eucaryotic developmental control. In the case of the human alpha-globin gene cluster, migration of erythropoietic activity from the embryonic yolk sac to the fetal liver is parallaled by the zeta-globin gene silencing and enhanced expression of the alpha-globin genes. To map critical cis determinants of this switch, the human zeta-globin gene, the alpha-globin gene, and chimeric recombinants were introduced into the mouse genome. Consistent with previous studies, expression of the individual alpha- and zeta-globin transgenes was found to be developmentally appropriate. Contrary to current models, however, the alpha- and zeta-globin gene promoters were not sufficient to establish this control. Instead, full silencing of the zeta-globin gene required the combined activities of this promoter, transcribed region, and 3'-flanking sequences. Individually, the silencing activities of the zeta-globin gene promoter and 3'-flanking region were minimal but increased markedly when both regions were present. The zeta-globin transcribed region appeared to contribute to gene silencing by a mechanism specifically activated in definitive erythroblasts in the fetal liver. These data demonstrate that a complex set of controls, requiring at least three determinants and involving at least two independent mechanisms, is necessary for full developmental silencing of the human zeta-globin gene.

The controlling sequence for site-specific chromosome breakage in Tetrahymena

Site-specific chromosome breakage occurs in many ciliated protozoa during nuclear differentiation. We have determined the cis-acting sequence that controls this process in Tetrahymena thermophila. The Tetrahymena ribosomal RNA gene is bounded by two breakage sites. Injection of this gene into developing macronuclei leads to breakage at these sites. Deletion analysis has localized the sequences essential for breakage to a 28 bp region that includes a 15 bp sequence (Cbs) known to be present in other breakage sites. Insertions of Cbs allow breakage to occur at new sites, which is accompanied by elimination of surrounding DNAs and formation of telomeric sequences, as it is at natural sites. Thus, Cbs is the necessary and sufficient sequence signal for chromosome breakage in Tetrahymena.