A conserved signaling pathway: the Drosophila toll-dorsal pathway

Groucho/TLE family proteins and transcriptional repression

The Drosophila Groucho (Gro) protein is the prototype for a large family of corepressors, examples of which are found in most metazoans. This family includes the human transducin-like Enhancer of split (TLE) proteins. As corepressors, Gro/TLE family proteins do not bind to DNA directly, but rather are recruited to the template by DNA-bound repressor proteins. Gro/TLE family proteins are required for many developmental processes, including lateral inhibition, segmentation, sex determination, dorsal/ventral pattern formation, terminal pattern formation, and eye development. These proteins are characterized by a conserved N-terminal glutamine-rich domain and a conserved C-terminal WD-repeat domain. The primary role of the glutamine-rich domain is apparently to mediate tetramerization, while the WD-repeat domain may mediate interactions with DNA-bound repressors. The glutamine rich and WD-repeat domains are separated by a less conserved region containing domains that have been implicated in transcriptional repression and nuclear localization. In addition to encoding full-length Gro/TLE family proteins, most metazoan genomes encode truncated family members that contain the N-terminal oligomerization domain, but lack the C-terminal WD-repeat domain. These truncated proteins may negatively regulate full-length Gro/TLE proteins, perhaps by sequestering them in non-productive complexes. Gro/TLE family proteins probably repress transcription by multiple mechanisms. For example, a glycine/proline-rich domain in the central variable region functions to recruit the histone deacetylase Rpd3 to the template. This histone deacetylase then presumably silences transcription by altering local chromatin structure. Other repression domains in Gro may function in a histone deacetylase-independent manner. Many aspects of Gro/TLE protein function remain to be explored, including the possible post-translational regulation of Gro/TLE activity as well as the mechanisms by which Gro/TLE proteins direct repression at a distance.

The Drosophila tumor necrosis factor receptor-associated factor-1 (DTRAF1) interacts with Pelle and regulates NFkappaB activity

A member of the tumor necrosis factor (TNF) receptor-associated factor (TRAF) family was identified in Drosophila. DTRAF1 contains 7 zinc finger domains followed by a TRAF domain, similar to mammalian TRAFs and other members of the family identified in data bases from Caenorhabditis elegans, Arabidopsis, and Dictyostelium. Analysis of DTRAF1 binding to different members of the human TNF receptor family showed that this protein can interact through its TRAF domain with the p75 neurotrophin receptor and weakly with the lymphotoxin-beta receptor. DTRAF1 can also self-associate and binds to human TRAF1, TRAF2, and TRAF4. Interestingly, DTRAF1 interacts with human cIAP-1 and cIAP-2 but not with Drosophila DIAP-1 and -2. By itself, DTRAF1 did not induce significant NFkappaB activation when overexpressed in mammalian cells, although it specifically increased NFkappaB induction by TRAF6. In contrast, TRAF2-mediated NFkappaB induction was partially inhibited by DTRAF1. Mutants of DTRAF1 lacking the N-terminal region inhibited NFkappaB induction by either TRAF2 or TRAF6. DTRAF1 specifically associated with the regulatory N-terminal domain of Pelle, a Drosophila homolog of the human kinase interleukin-1 receptor-associated kinase (IRAK). Interestingly, though Pelle and DTRAF1 individually were unable to induce NFkappaB in a human cell line, co-expression of Pelle and DTRAF1 resulted in significant NFkappaB activity. Interactions of DTRAF1 with human TRAF-, TNF receptor-, and IAP-family proteins imply strong evolutionary conservation of TRAF protein structure and function throughout Metazoan evolution.

Bacterial lipopolysaccharide activates NF-kappaB through toll-like receptor 4 (TLR-4) in cultured human dermal endothelial cells. Differential expression of TLR-4 and TLR-2 in endothelial cells

A missense mutation in the cytoplasmic domain of the Toll-like receptor-4 (TLR-4) has been identified as the defect responsible for lipopolysaccharide (LPS) hyporesponsiveness in C3H/HeJ mice. TLR-4 and TLR-2 have recently been implicated in LPS signaling in studies where these receptors were overexpressed in LPS non-responsive 293 human embryonic kidney cells. However, the signaling role of TLR-4 or TLR-2 in human cells with natural LPS response remains largely undefined. Here we show that human dermal microvessel endothelial cells (HMEC) and human umbilical vein endothelial cells express predominantly TLR-4 but very weak TLR-2 and respond vigorously to LPS but not to Mycobacterium tuberculosis 19-kDa lipoprotein. Transient transfection of non-signaling mutant forms of TLR-4 and anti-TLR-4 monoclonal antibody inhibited LPS-induced NF-kappaB activation in HMEC, while a monoclonal antibody against TLR-2 was ineffective. In contrast to LPS responsiveness, the ability of HMEC to respond to 19-kDa lipoprotein correlated with the expression of TLR-2. Transfection of TLR-2 into HMEC conferred responsiveness to 19-kDa lipoprotein. These data indicate that TLR-4 is the LPS signaling receptor in HMEC and that human endothelial cells (EC) express predominantly TLR-4 and weak TLR-2, which may explain why they do not respond to 19-kDa lipoprotein. The differential expression of TLRs on human EC may have important implications in the participation of vascular EC in innate immune defense mechanisms against various infectious pathogens, which may use different TLRs to signal.

Two attacin antibacterial genes of Drosophila melanogaster

Insects express a battery of potent antimicrobial proteins in response to injury and infection. Recent work from several laboratories has demonstrated that this response is neither stereotypic nor completely nonspecific, and that different pathways are responsible for inducing the expression of antifungal and antibacterial peptides. Here we report the cloning of two closely linked attacin genes from Drosophila melanogaster. We compare their protein coding sequences and find the amino acid sequences to be more highly conserved than the nucleotide sequences, suggesting that both genes are expressed. Like other antimicrobial peptides, attacin expression is strongly induced in infected and injured flies. Unlike others, attacin transcription is uniquely sensitive to mutations in the 18-Wheeler receptor protein, and thus may be regulated by a distinct signaling pathway. The number and organization of binding sites for kappaB and other transcription factors in the promoter regions of both attacin genes are consistent with strong and rapid immune induction. We demonstrate that these promoter regions are sufficient to direct beta-galactosidase expression in transformed Drosophila third-instar larval fat body in a bacterially inducible manner. We present a comparison of the promoter regions of the two attacin genes to those cloned from other antimicrobial peptide genes to assist a better understanding of how antimicrobial genes are differentially regulated.

The homeobox gene mirror links EGF signalling to embryonic dorso-ventral axis formation through notch activation

Recent studies in vertebrates and Drosophila melanogaster have revealed that Fringe-mediated activation of the Notch pathway has a role in patterning cell layers during organogenesis. In these processes, a homeobox-containing transcription factor is responsible for spatially regulating fringe (fng) expression and thus directing activation of the Notch pathway along the fng expression border. Here we show that this may be a general mechanism for patterning epithelial cell layers. At three stages in Drosophila oogenesis, mirror (mirr) and fng have complementary expression patterns in the follicle-cell epithelial layer, and at all three stages loss of mirr enlarges, and ectopic expression of mirr restricts, fng expression, with consequences for follicle-cell patterning. These morphological changes are similar to those caused by Notch mutations. Ectopic expression of mirr in the posterior follicle cells induces a stripe of rhomboid (rho) expression and represses pipe (pip), a gene with a role in the establishment of the dorsal-ventral axis, at a distance. Ectopic Notch activation has a similar long-range effect on pip. Our results suggest that Mirror and Notch induce secretion of diffusible morphogens and we have identified TGF-beta (encoded by dpp) as such a molecule in germarium. We also found that mirr expression in dorsal follicle cells is induced by the EGF-receptor (EGFR) pathway and that mirr then represses pip expression in all but the ventral follicle cells, connecting EGFR activation in the dorsal follicle cells to repression of pip in the dorsal and lateral follicle cells. Our results suggest that the differentiation of ventral follicle cells is not a direct consequence of germline signalling, but depends on long-range signals from dorsal follicle cells, and provide a link between early and late events in Drosophila embryonic dorsal-ventral axis formation.

Kupffer cell activation by lipopolysaccharide in rats: role for lipopolysaccharide binding protein and toll-like receptor 4

Lipopolysaccharide (LPS) binding protein (LBP) is a key serum factor that mediates LPS activation of mononuclear cells. In the presence of LBP, 1/1,000 the concentration of LPS is sufficient to activate peripheral blood monocytes. Previous studies with Kupffer cells have shown a variable effect of serum on LPS activation of these cells and led to the conclusion that, unlike extrahepatic mononuclear cells, Kupffer cells do not respond to LPS in an LBP-dependent fashion. Because there are multiple components in serum other than LBP that might affect LPS activation, these reports with serum are difficult to interpret. To investigate the specific role of LBP in LPS activation of Kupffer cells, we produced a functional recombinant rat LBP using a baculovirus expression system, which we used to selectively examine the role of LBP's on Kupffer-cell function. Isolated Kupffer cells exposed to increasing concentrations of LPS (0, 1, 10 ng/mL) showed a dose-dependent increase in TNF-alpha production, which was augmented and accelerated by the presence of LBP. The effects of LBP on Kupffer cell activation by LPS are dependent on a functional Toll-like receptor 4 (Tlr 4) because Kupffer cells from C3H/HeJ mice failed to respond to LPS in the presence of LBP. LBP plays an important role in mediating Kupffer cell activation by LPS, and these effects are dependent on the presence of functioning Tlr 4.

Drosophila larval neuromuscular junction: molecular components and mechanisms underlying synaptic plasticity

Understanding the mechanisms that mediate synaptic plasticity is a primary goal of molecular neuroscience. The Drosophila larval neuromuscular junction provides a particularly useful model for investigating the roles of synaptic components in both structural and functional plasticity. The powerful molecular genetics of this system makes it possible to uncover new synaptic components and signaling molecules, as well as their function in the intact organism. Together with the mouse hippocampus and Aplysia dissociated cell culture, the Drosophila larval neuromuscular junction has been among the most valuable model systems for examining the molecular and cellular basis of neuronal plasticity.

Comparative genomics of the eukaryotes

A comparative analysis of the genomes of Drosophila melanogaster, Caenorhabditis elegans, and Saccharomyces cerevisiae-and the proteins they are predicted to encode-was undertaken in the context of cellular, developmental, and evolutionary processes. The nonredundant protein sets of flies and worms are similar in size and are only twice that of yeast, but different gene families are expanded in each genome, and the multidomain proteins and signaling pathways of the fly and worm are far more complex than those of yeast. The fly has orthologs to 177 of the 289 human disease genes examined and provides the foundation for rapid analysis of some of the basic processes involved in human disease.

Innate recognition systems in insect immunity and development: new approaches in Drosophila

Phagocytosis is important in immune defense and development in vertebrates and invertebrates. It leads to swift ingestion of microorganisms and dying cells by phagocytes. How particles are recognized is not well understood. Studies in insects, and Drosophila in particular, argue that these are powerful model systems to dissect this process.

Interleukin-1 and cutaneous inflammation: a crucial link between innate and acquired immunity

As our primary interface with the environment, the skin is constantly subjected to injury and invasion by pathogens. The fundamental force driving the evolution of the immune system has been the need to protect the host against overwhelming infection. The ability of T and B cells to recombine antigen receptor genes during development provides an efficient, flexible, and powerful immune system with nearly unlimited specificity for antigen. The capacity to expand subsets of antigen-specific lymphocytes that become activated by environmental antigens (memory response) is termed "acquired" immunity. Immunologic memory, although a fundamental aspect of mammalian biology, is a relatively recent evolutionary event that permits organisms to live for years to decades. "Innate" immunity, mediated by genes that remain in germ line conformation and encode for proteins that recognize conserved structural patterns on microorganisms, is a much more ancient system of host defense. Defensins and other antimicrobial peptides, complement and opsonins, and endocytic receptors are all considered components of the innate immune system. None of these, however, are signal-transducing receptors. Most recently, a large family of cell surface receptors that mediate signaling through the NF-kappaB transcription factor has been identified. This family of proteins shares striking homology with plant and Drosophila genes that mediate innate immunity. In mammals, this family includes the type I interleukin-1 receptor, the interleukin-18 receptor, and a growing family of Toll-like receptors, two of which were recently identified as signal-transducing receptors for bacterial endotoxin. In this review, we discuss how interleukin-1 links the innate and acquired immune systems to provide synergistic host defense activities in skin.

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.

Gene expressions of lipopolysaccharide receptors, toll-like receptors 2 and 4, are differently regulated in mouse T lymphocytes

Toll-like receptors (TLRs) are a family of mammalian proteins homologous to Drosophila Toll. Human TLR2 was shown to mediate the responsiveness to lipopolysaccharide (LPS). On the other hand, gene mutations of mouse TLR4 (mTLR4) in LPS-hyporesponsive strains have suggested that mTLR4 is essential for LPS-signaling in mice, but the role of mTLR2 has not been explored. This report describes molecular cloning of the mTLR2 cDNA. Overexpression of mTLR2 and mouse CD14 conferred LPS-inducibility of c-Jun N-terminal kinase phosphorylation and nuclear factor-κB activation to COS7 cells, suggesting that mTLR2 is a signaling receptor for LPS. BothmTLR2 and mTLR4 genes were expressed in T cells. Treatment with anti-CD3ɛ, PMA plus ionomycin, or interleukin-2 (IL-2)/IL-15 increased mTLR2 but not mTLR4 messenger RNA (mRNA) in some T cell lines. Specific inhibitors of mitogen-activated extracellular signal-regulated kinase and fusion protein 38 (p38) kinase inhibited mTLR2 mRNA up-regulation by PMA plus ionomycin. This suggests that extracellular signal-regulated kinase and p38 kinase pathways were involved. Additionally, LPS treatment of EL-4 cell line decreasedIL-4 gene expression. Our results indicate that both mTLR2 and mTLR4 are involved in LPS signaling, but their expressions are regulated differently in T cells, and that LPS may directly affect T-cell functions by binding to TLRs.

A functional interaction between dorsal and components of the Smt3 conjugation machinery

To identify proteins that regulate the function of Dorsal, a Drosophila Rel family transcription factor, we employed a yeast two-hybrid screen to search for genes encoding Dorsal-interacting proteins. Six genes were identified, including two that encode previously known Dorsal-interacting proteins (Twist and Cactus), three that encode novel proteins, and one that encodes Drosophila Ubc9 (DmUbc9), a protein thought to conjugate the ubiquitin-like polypeptide Smt3 to protein substrates. We have found that DmUbc9 binds and conjugates Drosophila Smt3 (DmSmt3) to Dorsal. In cultured cells, DmUbc9 was found to relieve inhibition of Dorsal nuclear uptake by Cactus, allowing Dorsal to enter the nucleus and activate transcription. The effect of DmUbc9 on Dorsal activity was potentiated by the overexpression of DmSmt3. We have also identified a DmSmt3-activating enzyme, DmSAE1/DmSAE2 and found that it further potentiates Dorsal-mediated activation.

Activation of the heterodimeric IkappaB kinase alpha (IKKalpha)-IKKbeta complex is directional: IKKalpha regulates IKKbeta under both basal and stimulated conditions

Signal-induced nuclear expression of the eukaryotic NF-kappaB transcription factor involves the stimulatory action of select mitogen-activated protein kinase kinase kinases on the IkappaB kinases (IKKalpha and IKKbeta) which reside in a macromolecular signaling complex termed the signalsome. While genetic studies indicate that IKKbeta is the principal kinase involved in proinflammatory cytokine-induced IkappaB phosphorylation, the function of the equivalently expressed IKKalpha is less clear. Here we demonstrate that assembly of IKKalpha with IKKbeta in the heterodimeric signalsome serves two important functions: (i) in unstimulated cells, IKKalpha inhibits the constitutive IkappaB kinase activity of IKKbeta; (ii) in activated cells, IKKalpha kinase activity is required for the induction of IKKbeta. The introduction of kinase-inactive IKKalpha, activation loop mutants of IKKalpha, or IKKalpha antisense RNA into 293 or HeLa cells blocks NIK (NF-kappaB-inducing kinase)-induced phosphorylation of the IKKbeta activation loop occurring in functional signalsomes. In contrast, catalytically inactive mutants of IKKbeta do not block NIK-mediated phosphorylation of IKKalpha in these macromolecular signaling complexes. This requirement for kinase-proficient IKKalpha to activate IKKbeta in heterodimeric IKK signalsomes is also observed with other NF-kappaB inducers, including tumor necrosis factor alpha, human T-cell leukemia virus type 1 Tax, Cot, and MEKK1. Conversely, the theta isoform of protein kinase C, which also induces NF-kappaB/Rel, directly targets IKKbeta for phosphorylation and activation, possibly acting through homodimeric IKKbeta complexes. Together, our findings indicate that activation of the heterodimeric IKK complex by a variety of different inducers proceeds in a directional manner and is dependent on the kinase activity of IKKalpha to activate IKKbeta.

Tlr4: central component of the sole mammalian LPS sensor

Mutations of the mouse Lps locus abolish responses to lipopolysaccharide (LPS). Positional cloning work has revealed that Lps encodes the Toll-like receptor 4 (Tlr4), which functions as the transmembrane component of the LPS receptor complex, an unduplicated pathway for the detection of endotoxin. The structurally related protein Tlr2 makes no contribution to LPS signal transduction.

Toll signaling pathways in the innate immune response

The Toll signaling pathway, which is required for the establishment of the dorsal-ventral axis in Drosophila embryos, plays an important role in the response of larval and adult Drosophila to microbial infections. Recent genetic evidence has shown that a mammalian Toll-like receptor, mouse Tlr4, is the signal transducing receptor activated by bacterial lipopolysaccharide. Thus, Toll-like receptors appear to detect a variety of microbial components and to trigger a defensive reaction in both Drosophila and mammals. Genetic data from both Drosophila and mice have defined components required for activation of Toll-like receptors and for the downstream pathways activated by the Toll-like receptors.

HMG boxes of DSP1 protein interact with the rel homology domain of transcription factors

Formation of the dorsoventral axis in Drosophila melanogaster is mediated through control of the expression of several genes by the morphogen Dorsal. In the ventral part of the embryo Dorsal activates twist and represses zen amongst others. Recently, several proteins have been shown to assist Dorsal in the repression of zen, one of which is DSP1, a HMG box protein that was isolated as a putative co-repressor of Dorsal. In this report we used a DSP1 null mutant to ascertain in vivo the involvement of DSP1 in Dorsal-mediated repression of zen but not in the activation of twist. We show that Dorsal has the ability to interact with DSP1 in vitro as well as with rat HMG1. Using truncated versions of the proteins we located the domains of interaction as being the HMG boxes for DSP1 and HMG1 and the Rel domain for Dorsal. Finally, studies of the zen DNA binding properties of Dorsal and another related Rel protein (Gambif1 from Anopheles gambiae) revealed that their DNA binding affinities were increased in the presence of DSP1 and HMG1.

Presence and roles of calcium gradients along the dorsal-ventral axis in Drosophila embryos

Dorsal-ventral specification of the Drosophila embryo is mediated by signaling pathways which have been very well described in genetic terms. However, little is known about the physiology of Drosophila development. By imaging patterns of free cytosolic calcium in Drosophila embryos, we found that several calcium gradients are generated along the dorsal-ventral axis. The most pronounced gradient is formed during stage 5, in which calcium levels are high dorsally. Manipulation of the stage 5 calcium gradient affects specification of the amnioserosa, the dorsal-most region of the embryo. We further show that this calcium gradient is inhibited in pipe, Toll, and dorsal mutants, but is unaltered in decapentaplegic (dpp) or punt mutants, suggesting that the stage 5 calcium gradient is formed by a suppression of ventral calcium concentrations. We conclude that calcium plays a role in specification of the dorsal embryonic region.

Distinct Activities of p52/NF-κB Required for Proper Secondary Lymphoid Organ Microarchitecture: Functions Enhanced by Bcl-3

Mice rendered deficient in p52, a subunit of NF-κB, or in Bcl-3, an IκB-related regulator that associates with p52 homodimers, share defects in the microarchitecture of secondary lymphoid organs. The mutant mice are impaired in formation of B cell follicles and are unable to form proper follicular dendritic cell (FDC) networks upon antigenic challenge. The defects in formation of B cell follicles may be attributed, at least in part, to impaired production of the B lymphocyte chemoattractant (BLC) chemokine, possibly a result of defective FDCs. The p52- and Bcl-3-deficient mice exhibit additional defects within the splenic marginal zone, including reduced numbers of metallophilic macrophages, reduced deposition of the laminin-β2 chain and impaired expression of a mucosal addressin marker on sinus-lining cells. Whereas p52-deficient mice are severely defective in all of these aspects, Bcl-3-deficient mice are only partially defective. We determined that FDCs or other non-hemopoietic cells that underlie FDCs are intrinsically impaired in p52-deficient mice. Adoptive transfers of wild-type bone marrow into p52-deficient mice failed to restore FDC networks or follicles. The transfers did restore metallophilic macrophages to the marginal zone, however. Together, the results suggest that p52 carries out functions essential for a proper splenic microarchitecture in both hemopoietic and non-hemopoietic cells and that Bcl-3 is important in enhancing these essential activities of p52.

How NF-kappaB is activated: the role of the IkappaB kinase (IKK) complex

Rel/NF-kappaB transcription factors are primarily regulated by association with inhibitor IkappaB proteins. Thus, in most cells NF-kappaB exists in the cytoplasm in an inactive complex bound to IkappaB. Most agents that activate NF-kappaB do so through a common pathway based on phosphorylation-induced, proteasome-mediated degradation of IkappaB. The key regulatory step in this pathway involves activation of a high molecular weight IkappaB kinase (IKK) complex, whose catalysis is generally carried out by a heterodimeric kinase consisting of IKKalpha and IKKbeta subunits. This review describes the identification of proteins in the IKK complex, and the regulation and physiological functions of IKK.

Control of development and immunity by rel transcription factors in Drosophila

The Drosophila Rel/NF-kappaB transcription factors - Dorsal, Dif, and Relish - control several biological processes, including embryonic pattern formation, muscle development, immunity, and hematopoiesis. Molecular-genetic analysis of 12 mutations that cause embryonic dorsal/ventral patterning defects has defined the steps that control the formation of this axis. Regulated activation of the Toll receptor leads to the establishment of a gradient of nuclear Dorsal protein, which in turn governs the subdivision of the axis and specification of ventral, lateral and dorsal fates. Phenotypic analysis of dorsal-ventral embryonic mutants and the characterization of the two other fly Rel proteins, Dif and Relish, have shown that the intracellular portion of the Toll to Cactus pathway also controls the innate immune response in Drosophila. Innate immunity and hematopoiesis are regulated by analogous Rel/NF-kappaB-family pathways in mammals. The elucidation of the complex regulation and diverse functions of Drosophila Rel proteins underscores the relevance of basic studies in Drosophila.

Toll-like receptor 2 functions as a pattern recognition receptor for diverse bacterial products

Toll-like receptors (TLRs) 2 and 4 are signal transducers for lipopolysaccharide, the major proinflammatory constituent in the outer membrane of Gram-negative bacteria. We observed that membrane lipoproteins/lipopeptides from Borrelia burgdorferi, Treponema pallidum, and Mycoplasma fermentans activated cells heterologously expressing TLR2 but not those expressing TLR1 or TLR4. These TLR2-expressing cells were also stimulated by living motile B. burgdorferi, suggesting that TLR2 recognition of lipoproteins is relevant to natural Borrelia infection. Importantly, a TLR2 antibody inhibited bacterial lipoprotein/lipopeptide-induced tumor necrosis factor release from human peripheral blood mononuclear cells, and TLR2-null Chinese hamster macrophages were insensitive to lipoprotein/lipopeptide challenge. The data suggest a role for the native protein in cellular activation by these ligands. In addition, TLR2-dependent responses were seen using whole Mycobacterium avium and Staphylococcus aureus, demonstrating that this receptor can function as a signal transducer for a wide spectrum of bacterial products. We conclude that diverse pathogens activate cells through TLR2 and propose that this molecule is a central pattern recognition receptor in host immune responses to microbial invasion.

Control of mucin transcription by diverse injury-induced signaling pathways

Mucin production is an evolutionarily ancient defense mechanism that is retained in mammals and operates at all mucosal surfaces to protect the host against pathogens and irritants. As in lower organisms, the mammalian mucosa (epithelium) produces mucin in response to diverse insults. Our studies aim to understand the intracellular signaling and gene regulation mechanisms mediating mucin production in response to clinically important insults. To date, we find that the signaling pathway triggered by each type of insult is distinct. Relatively common, however, is the involvement of the protein tyrosine kinase c-Src, the MAP kinase kinase MEK 1/2, and the transcription factor NF-kappaB. Basbaum C, Lemjabbar H, Longphre M, Li D, Gensch E, McNamara N. Control of mucin transcription by diverse injury-induced signaling pathways.

The NF-kappa B activation pathway: a paradigm in information transfer from membrane to nucleus

Nuclear factor kappa B (NF-kappaB)/Rel proteins are dimeric, sequence-specific transcription factors involved in the activation of an exceptionally large number of genes in response to inflammation, viral and bacterial infections, and other stressful situations requiring rapid reprogramming of gene expression. In unstimulated cells, NF-kappaB is sequestered in an inactive form in the cytoplasm bound to inhibitory IkappaB proteins. Stimulation leads to the rapid phosphorylation, ubiquitinylation, and ultimately proteolytic degradation of IkappaB, which frees NF-kappaB to translocate to the nucleus and activate the transcription of its target genes. The multisubunit IkappaB kinase (IKK) responsible for the inducible phosphorylation of IkappaB appears to be the initial point of convergence for most stimuli that activate NF-kappaB. IKK contains two catalytic subunits, IKKalpha and IKKbeta, both of which phosphorylate IkappaB at sites phosphorylated in vivo. Gene knockout studies indicate that IKKbeta is primarily responsible for the activation of NF-kappaB in response to proinflammatory stimuli, whereas IKKalpha is essential for keratinocyte differentiation. The activity of IKK is regulated by phosphorylation. IKK contains a regulatory subunit, IKKgamma, which is critical for activation of IKK and is postulated to serve as a recognition site for upstream activators. When phosphorylated, the IKK recognition site on IkappaBalpha serves as a specific recognition site for the kappa-TrCP-like component of a Skp1-Cullin-F-box-type E3 ubiquitin-protein ligase. A variety of other signaling events, including phosphorylation of NF-kappaB, phosphorylation of IKK, new synthesis of IkappaBs, and the processing of NF-kappaB precursors provide mechanisms of modulating the amount and duration of NF-kappaB activity.

Mouse receptor interacting protein 3 does not contain a caspase-recruiting or a death domain but induces apoptosis and activates NF-kappaB

The death domain-containing receptor superfamily and their respective downstream mediators control whether or not cells initiate apoptosis or activate NF-kappaB, events critical for proper immune system function. A screen for upstream activators of NF-kappaB identified a novel serine-threonine kinase capable of activating NF-kappaB and inducing apoptosis. Based upon domain organization and sequence similarity, this novel kinase, named mRIP3 (mouse receptor interacting protein 3), appears to be a new RIP family member. RIP, RIP2, and mRIP3 contain an N-terminal kinase domain that share 30 to 40% homology. In contrast to the C-terminal death domain found in RIP or the C-terminal caspase-recruiting domain found in RIP2, the C-terminal tail of mRIP3 contains neither motif and is unique. Despite this feature, overexpression of the mRIP3 C terminus is sufficient to induce apoptosis, suggesting that mRIP3 uses a novel mechanism to induce death. mRIP3 also induced NF-kappaB activity which was inhibited by overexpression of either dominant-negative NIK or dominant-negative TRAF2. In vitro kinase assays demonstrate that mRIP3 is catalytically active and has autophosphorylation site(s) in the C-terminal domain, but the mRIP3 catalytic activity is not required for mRIP3 induced apoptosis and NF-kappaB activation. Unlike RIP and RIP2, mRIP3 mRNA is expressed in a subset of adult tissues and is thus likely to be a tissue-specific regulator of apoptosis and NF-kappaB activity. While the lack of a dominant-negative mutant precludes linking mRIP3 to a known upstream regulator, characterizing the expression pattern and the in vitro functions of mRIP3 provides insight into the mechanism(s) by which cells modulate the balance between survival and death in a cell-type-specific manner.

Differential roles of TLR2 and TLR4 in recognition of gram-negative and gram-positive bacterial cell wall components

Toll-like receptor (TLR) 2 and TLR4 are implicated in the recognition of various bacterial cell wall components, such as lipopolysaccharide (LPS). To investigate in vivo roles of TLR2, we generated TLR2-deficient mice. In contrast to LPS unresponsiveness in TLR4-deficient mice, TLR2-deficient mice responded to LPS to the same extent as wild-type mice. TLR2-deficient macrophages were hyporesponsive to several Gram-positive bacterial cell walls as well as Staphylococcus aureus peptidoglycan. TLR4-deficient macrophages lacked the response to Gram-positive lipoteichoic acids. These results demonstrate that TLR2 and TLR4 recognize different bacterial cell wall components in vivo and TLR2 plays a major role in Gram-positive bacterial recognition.

New insights into the roles of ReL/NF-kappa B transcription factors in immune function, hemopoiesis and human disease

In mammals, Rel/NF-kappa B proteins are a small family of transcription factors which serve as pivotal regulators of immune, inflammatory and acute phase responses. Pathways leading to the activation of Rel/NF-kappa B have recently been dissected in some detail and shown to converge on a unique high molecular weight cytoplasmic complex that includes several kinases and regulatory molecules. Moreover, gene targeting experiments have identified novel roles for Rel/NF-kappa B proteins in the development and maturation of hemopoietic precursors as well as in the function of mature cells in the immune system. These include regulating the cell cycle, controlling cell survival and providing a link between the innate and adaptive immune systems. Since the dysregulation of Rel/NF-kappa B function is associated with various pathologies including inflammatory and neoplastic disease, new insights into the role of Rel/NF-kappa B in human disease may provide a basis for therapeutic strategies in the treatment of chronic inflammatory diseases and certain malignancies.

Constitutive activation of toll-mediated antifungal defense in serpin-deficient Drosophila

The antifungal defense of Drosophila is controlled by the spaetzle/Toll/cactus gene cassette. Here, a loss-of-function mutation in the gene encoding a blood serine protease inhibitor, Spn43Ac, was shown to lead to constitutive expression of the antifungal peptide drosomycin, and this effect was mediated by the spaetzle and Toll gene products. Spaetzle was cleaved by proteolytic enzymes to its active ligand form shortly after immune challenge, and cleaved Spaetzle was constitutively present in Spn43Ac-deficient flies. Hence, Spn43Ac negatively regulates the Toll signaling pathway, and Toll does not function as a pattern recognition receptor in the Drosophila host defense.

Motorneurons, reactive oxygen, and life span in Drosophila

Aging and life span are widely recognized, but poorly understood, aspects of basic biology. Fortunately, genetic approaches to understanding the mechanisms governing these processes are beginning to bear fruit. One line of investigation has established that incompletely reduced forms of oxygen, arising as by-products of respiration and cellular catabolism, play an important, and perhaps universal, role in aging and life span determination. An important refinement of this model of aging, suggested by recent experiments in our laboratory, is that the critical nexus of the relationship between reactive oxygen species and life span is highly localized and, in fact, may reside principally in the motorneuron. Here we analyze the strengths and weaknesses of the reactive oxygen species/motorneuron model of aging by comparing the studies on which it is based, which used the approach of targeted transgene expression in Drosophila, with studies from other laboratories using different genetic approaches, principally mutation and selection. The results encourage the view that an understanding of the mechanisms that underlie this widely recognized aspect of basic biology is within our grasp.

Revisiting catalysis by chymotrypsin family serine proteases using peptide substrates and inhibitors with unnatural main chains

Chymotrypsin family serine proteases play essential roles in key biological and pathological processes and are frequently targets of drug discovery efforts. This large enzyme family is also among the most advanced model systems for detailed studies of enzyme mechanism and structure/function relationships. Productive interactions between these enzymes and their substrates are widely believed to mimic the "canonical" interactions between serine proteases and "standard" inhibitors observed in numerous protease-inhibitor complexes. To test this central hypothesis we have synthesized and characterized a series of peptide analogs, based on model substrates and inhibitors of trypsin, that contain unnatural main chains. These results call into question a long accepted theory regarding the interaction of chymotrypsin family serine proteases with substrates and suggest that the canonical interactions observed between these enzymes and standard inhibitors may represent nonproductive rather than productive, substrate-like interactions.

ECSIT is an evolutionarily conserved intermediate in the Toll/IL-1 signal transduction pathway

Activation of NF-kappaB as a consequence of signaling through the Toll and IL-1 receptors is a major element of innate immune responses. We report the identification and characterization of a novel intermediate in these signaling pathways that bridges TRAF6 to MEKK-1. This adapter protein, which we have named ECSIT (evolutionarily conserved signaling intermediate in Toll pathways), is specific for the Toll/IL-1 pathways and is a regulator of MEKK-1 processing. Expression of wild-type ECSIT accelerates processing of MEKK-1, whereas a dominant-negative fragment of ECSIT blocks MEKK-1 processing and activation of NF-kappaB. These results indicate an important role for ECSIT in signaling to NF-kappaB and suggest that processing of MEKK-1 is required for its function in the Toll/IL-1 pathway.

Host resistance to infection: genetic control of lipopolysaccharide responsiveness by TOLL-like receptor genes

Gram-negative bacterial lipopolysaccharide evokes a protective inflammatory response in the normal host. Through genetic analysis of mutant mice, the gene encoding Toll-like receptor 4 (Tlr4) was recently identified as a critical component of this host defense mechanism. Tlr4 is a member of an ancient gene family that regulates antimicrobial host defense in plants, invertebrates and mammals.

Interaction and specificity of Rel-related proteins in regulating Drosophila immunity gene expression

NF-kappaB/Rel family proteins regulate genes that are critical for many cellular processes including apoptosis, inflammation, immune response, and development. NF-kappaB/Rel proteins function as homodimers or heterodimers, which recognize specific DNA sequences within target promoters. We examined the activity of different Drosophila Rel-related proteins in modulating Drosophila immunity genes by expressing the Rel proteins in stably transfected cell lines. We also compared how different combinations of these transcriptional regulators control the activity of various immunity genes. The results show that Rel proteins are directly involved in regulating the Drosophila antimicrobial response. Furthermore, the drosomycin and defensin expression is best induced by the Relish/Dif and the Relish/Dorsal heterodimers, respectively, whereas the attacin activity can be efficiently up-regulated by the Relish homodimer and heterodimers. These results illustrate how the formation of Rel protein dimers differentially regulate target gene expression.

Structure of the specificity domain of the Dorsal homologue Gambif1 bound to DNA

BACKGROUND

NF-kappa B/Rel transcription factors play important roles in immunity and development in mammals and insects. Their activity is regulated by their cellular localization, homo- and heterodimerization and association with other factors on their target gene promoters. Gambif1 from Anopheles gambiae is a member of the Rel family and a close homologue of the morphogen Dorsal, which establishes dorsoventral polarity in the Drosophila embryo.

RESULTS

We present the crystal structure of the N-terminal specificity domain of Gambif1 bound to DNA. This first structure of an insect Rel protein-DNA complex shows that Gambif1 binds a GGG half-site element using a stack of three arginine sidechains. Differences in affinity to Dorsal binding sites in target gene promoters are predicted to arise from base changes in these GGG elements. An arginine that is conserved in class II Rel proteins (members of which contain a transcription activation domain) contacts the outermost guanines of the DNA site. This previously unseen specific contact contributes strongly to the DNA-binding affinity and might be responsible for differences in specificity between Rel proteins of class I and II.

CONCLUSIONS

The Gambif1-DNA complex structure illustrates how differences in Dorsal affinity to binding sites in developmental gene promoters are achieved. Comparison with other Rel-DNA complex structures leads to a general model for DNA recognition by Rel proteins.

IRAK-M is a novel member of the Pelle/interleukin-1 receptor-associated kinase (IRAK) family

The interleukin-1 receptor-associated kinase (IRAK) was first described as a signal transducer for interleukin-1 (IL-1) and has later been implicated in signal transduction of other members of the Toll/IL-1 receptor family. We now report the identification and characterization of a novel IRAK-like molecule. In contrast to the ubiquitously expressed IRAK and IRAK-2, this new IRAK-like molecule is found mainly in cells of monomyeloic origin and is, therefore, designated IRAK-M. Although IRAK-M and IRAK-2 exhibit only a negligible autophosphorylation activity, they can reconstitute the IL-1 response in a 293 mutant cell line lacking IRAK. In addition, we show for the first time that members of the IRAK family are indispensable elements of lipopolysaccharide signal transduction. The discovery of IRAK-M adds another level of complexity to our understanding of signaling by members of the Toll/IL-1 receptor family.

Cutting Edge: Recognition of Gram-Positive Bacterial Cell Wall Components by the Innate Immune System Occurs Via Toll-Like Receptor 2

Invasive infection with Gram-positive and Gram-negative bacteria often results in septic shock and death. The basis for the earliest steps in innate immune response to Gram-positive bacterial infection is poorly understood. The LPS component of the Gram-negative bacterial cell wall appears to activate cells via CD14 and Toll-like receptor (TLR) 2 and TLR4. We hypothesized that Gram-positive bacteria might also be recognized by TLRs. Heterologous expression of human TLR2, but not TLR4, in fibroblasts conferred responsiveness to Staphylococcus aureus and Streptococcus pneumoniae as evidenced by inducible translocation of NF-κB. CD14 coexpression synergistically enhanced TLR2-mediated activation. To determine which components of Gram-positive cell walls activate Toll proteins, we tested a soluble preparation of peptidoglycan prepared from S. aureus. Soluble peptidoglycan substituted for whole organisms. These data suggest that the similarity of clinical response to invasive infection by Gram-positive and Gram-negative bacteria is due to bacterial recognition via similar TLRs.

Unresponsiveness of MyD88-deficient mice to endotoxin

MyD88 is a general adaptor protein that plays an important role in the Toll/IL-1 receptor family signalings. Recently, Toll-like receptors 2 and 4 (TLR2 and TLR4) have been suggested to be the signaling receptors for lipopolysaccharide (LPS). In this study, we demonstrate that MyD88 knockout mice lack the ability to respond to LPS as measured by shock response, B cell proliferative response, and secretion of cytokines by macrophages and embryonic fibroblasts. However, activation of neither NF-kappaB nor the mitogen-activated protein (MAP) kinase family is abolished in MyD88 knockout mice. These findings demonstrate that signaling via MyD88 is essential for LPS response, but the inability of MyD88 knockout mice to induce LPS-dependent gene expression cannot simply be attributed to lack of the activation of MAP kinases and NF-kappaB.

Oyster IKK-like protein shares structural and functional properties with its mammalian homologues

In our search for genes involved in oyster immunity we isolated a cDNA encoding a polypeptide closely related to the mammalian IkappaB kinase (IKK) family. IKK proteins play a central role in cell signaling by regulating nuclear factor-kappaB (NF-kappaB) activation. We report here the cloning of an oyster IKK-like protein (oIKK) which possesses the characteristic organization of the mammalian IKK proteins, namely an amino-terminal kinase domain followed by a leucine zipper region and a carboxyl-terminal helix-loop-helix motif. When transfected into human cell lines, oIKK activated the expression of NF-kappaB-controlled reporter gene, whereas transfections with mutants of oIKK deleted within the kinase domain or within the helix-loop-helix motif respectively abolished and greatly reduced reporter gene activation. These results indicate that oIKK can replace the hIKK-alpha in catalyzing NF-kappaB nuclear translocation, and in triggering gene expression. Our results sustain the concept of an evolutionarily conserved signaling machinery in which IKK plays a major role.

Cutting Edge: Cells That Carry A Null Allele for Toll-Like Receptor 2 Are Capable of Responding to Endotoxin

Toll-like receptor (TLR) 2 and TLR4 have been implicated in the responses of cells to LPS (endotoxin). CD14-transfected Chinese hamster ovary (CHO)-K1 fibroblasts (CHO/CD14) are exquisitely sensitive to endotoxin. Sequence analysis of CHO-TLR2, compared with human and mouse TLR2, revealed a single base pair deletion. This frameshift mutation resulted in an alternative stop codon, encoding a protein devoid of transmembrane and intracellular domains. CHO-TLR2 cDNA failed to enable LPS signaling upon transient transfection into human epithelial kidney 293 cells. Site-directed mutagenesis of CHO-TLR2 enabled expression of a presumed full-length hamster TLR2 that conferred LPS responsiveness in human epithelial kidney 293 cells. Genomic TLR2 DNA from primary hamster macrophages also contained the frameshift mutation found in CHO fibroblasts. Nevertheless, hamster peritoneal macrophages were found to respond normally to LPS, as evidenced by the induction of cytokines. These results imply that expression of TLR2 is sufficient but not essential for mammalian responses to endotoxin.

A mosaic analysis in Drosophila fat body cells of the control of antimicrobial peptide genes by the Rel proteins Dorsal and DIF

Expression of the gene encoding the antifungal peptide Drosomycin in Drosophila adults is controlled by the Toll signaling pathway. The Rel proteins Dorsal and DIF (Dorsal-related immunity factor) are possible candidates for the transactivating protein in the Toll pathway that directly regulates the drosomycin gene. We have examined the requirement of Dorsal and DIF for drosomycin expression in larval fat body cells, the predominant immune-responsive tissue, using the yeast site-specific flp/FRT recombination system to generate cell clones homozygous for a deficiency uncovering both the dorsal and the dif genes. Here we show that in the absence of both genes, the immune-inducibility of drosomycin is lost but can be rescued by overexpression of either dorsal or dif under the control of a heat-shock promoter. This result suggests a functional redundancy between both Rel proteins in the control of drosomycin gene expression in the larvae of Drosophila. Interestingly, the gene encoding the antibacterial peptide Diptericin remains fully inducible in the absence of the dorsal and dif genes. Finally, we have used fat body cell clones homozygous for various mutations to show that a linear activation cascade Spaetzle--> Toll-->Cactus-->Dorsal/DIF leads to the induction of the drosomycin gene in larval fat body cells.

Lipopolysaccharide recognition, CD14, and lipopolysaccharide receptors

The ability of a host to sense invasion by a pathogenic organism, and to respond appropriately to control infection, is paramount to survival. To that end, an array of receptors and binding proteins has evolved as part of the innate immune system to detect Gram-negative bacteria. This article reviews the role of CD14, other LPS binding proteins, and the Toll family of receptors in the innate recognition of bacterial lipopolysaccharide.

Mesoderm-determining transcription in Drosophila is alleviated by mutations in TAF(II)60 and TAF(II)110

In Drosophila, a coordinate interplay between the Rel transcription factor Dorsal and the basic Helix-Loop-Helix transcription factor Twist initiates mesoderm formation by activating the zygotic expression of mesoderm-determining genes. Here, we show that TBP-associated-factors (TAF(II)s) within the basal transcription factor TFIID mediate transcriptional activation by Dorsal and Twist. Dorsal interacts with TAF(II)110 and TAF(II)60, while Twist contacts TAF(II)110. The TAF(II):activator interactions mediate simple and synergistic transactivation by Dorsal and Twist in vitro. Mutations in TAF(II)60 or TAF(II)110 alleviate the transcription of Dorsal and Twist target genes. Gene dosage assays imply that an interplay of Dorsal and Twist with TAF(II)110 is critically required for the activation of mesoderm-determining gene expression in the Drosophila embryo. The results provide evidence that TAF(II)-subunits within the TFIID complex play an important role during the molecular events leading to initiation of mesoderm formation in Drosophila.

Induction and regulation of antimicrobial peptides in Drosophila

Activation of the innate immune response involves recognition of the infectious agent and the subsequent activation of cellular and humoral reactions. In insects, a number of immunity genes are activated at the level of transcription leading to the synthesis of antimicrobial peptides. Genetic analyses in Drosophila have identified several signal transduction pathways that promote activation of these immunity genes. Recent data suggest that the insect immune system is able to discriminate between a bacterial and a fungal infection, and responds by higher levels of activation of the appropriate peptides to repel the infection. These and other recent data on transcription factors and regulation of antimicrobial genes are integrated into a model to suggest how differential activation of antifungal and antibacterial peptides can occur in response to fungal and bacterial infection.

Serine proteinase inhibitors in arthropod immunity

Arthropod hemolymph contains proteins with serine proteinase inhibitory activity. These inhibitors may exist in plasma or in hemocyte granules. Serine proteinase inhibitors from the Kazal, Kunitz, alpha-macroglobulin, and serpin families have been identified in arthropod hemolymph and have been characterized biochemically. Two new families of low molecular weight serine proteinase inhibitors have recently been discovered: one in silkworms (the Bombyx family) and another in locusts and a crayfish. The serine proteinase inhibitors in arthropod hemolymph are likely to function in protecting their hosts from infection by pathogens or parasites. Some may inhibit fungal or bacterial proteinases. Others probably have roles in regulating endogenous proteinases involved in coagulation, prophenol oxidase activation, or cytokine activation.

Phylogenetic perspectives in innate immunity

The concept of innate immunity refers to the first-line host defense that serves to limit infection in the early hours after exposure to microorganisms. Recent data have highlighted similarities between pathogen recognition, signaling pathways, and effector mechanisms of innate immunity in Drosophila and mammals, pointing to a common ancestry of these defenses. In addition to its role in the early phase of defense, innate immunity in mammals appears to play a key role in stimulating the subsequent, clonal response of adaptive immunity.

Modulation of tumor necrosis factor and interleukin-1-dependent NF-kappaB activity by mPLK/IRAK

The innate immune response is an important defense against pathogenic agents. A component of this response is the NF-kappaB-dependent activation of genes encoding inflammatory cytokines such as interleukin-8 (IL-8) and cell adhesion molecules like E-selectin. Members of the serine/threonine innate immune kinase family of proteins have been proposed to mediate the innate immune response. One serine/threonine innate immune kinase family member, the mouse Pelle-like kinase/human interleukin-1 receptor-associated kinase (mPLK/IRAK), has been proposed to play an obligate role in promoting IL-1-mediated inflammation. However, it is currently unknown whether mPLK/IRAK catalytic activity is required for IL-1-dependent NF-kappaB activation. The present study demonstrates that mPLK/IRAK catalytic activity is not required for IL-1-mediated activation of an NF-kappaB-dependent signal. Intriguingly, catalytically inactive mPLK/IRAK inhibits type 1 tumor necrosis factor (TNF) receptor-dependent NF-kappaB activation. The pathway through which mPLK/IRAK mediates this TNF response is TRADD- and TRAF2-independent. Our data suggest that in addition to its role in IL-1 signaling, mPLK/IRAK is a component of a novel signal transduction pathway through which TNF R1 activates NF-kappaB-dependent gene expression.