Inflammation is an obligatory attempt of the immune system to protect the host from infections. However, unregulated synthesis of pro-inflammatory products can have detrimental effects. Although mechanisms that lead to inflammation are well appreciated, those that restrain it are not adequately understood. Creating macrophage-specific L13a-knockout (KO) mice here we report that depletion of ribosomal protein L13a abrogates the endogenous translation control of several chemokines in macrophages. Upon LPS-induced endotoxemia these animals displayed symptoms of severe inflammation caused by widespread infiltration of macrophages in major organs causing tissue injury and reduced survival rates. Macrophages from these KO animals show unregulated expression of several chemokines e.g. CXCL13, CCL22, CCL8 and CCR3. These macrophages failed to show L13a-dependent RNA binding complex formation on target mRNAs. In addition, increased polyribosomal abundance of these mRNAs shows a defect in translation control in the macrophages. Thus, our studies provide the first evidence of an essential extra-ribosomal function of ribosomal protein L13a in resolving physiological inflammation in a mammalian host.

Mammalian Toll-like receptors (TLR) recognize microbial products and elicit transient immune responses that protect the infected host from disease. TLR4-which signals from both plasma and endosomal membranes-is activated by bacterial lipopolysaccharides (LPS) and induces many cytokine genes, the prolonged expression of which causes septic shock in mice. We report here that the expression of some TLR4-induced genes in myeloid cells requires the protein kinase activity of the epidermal growth factor receptor (EGFR). EGFR inhibition affects TLR4-induced responses differently depending on the target gene. The induction of interferon-b (IFN-b) and IFN-inducible genes is strongly inhibited, whereas TNF-a induction is enhanced. Inhibition is specific to the IFN-regulatory factor (IRF)-driven genes because EGFR is required for IRF activation downstream of TLR-as is IRF co-activator b-catenin-through the PI3 kinase/AKT pathway. Administration of an EGFR inhibitor to mice protects them from LPS-induced septic shock and death by selectively blocking the IFN branch of TLR4 signaling. These results demonstrate a selective regulation of TLR4 signaling by EGFR and highlight the potential use of EGFR inhibitors to treat septic shock syndrome.

Objective Unresolved inflammatory response of macrophages plays a pivotal role in the pathogenesis of atherosclerosis. Previously we showed that ribosomal protein L13a-dependent translational silencing suppresses the synthesis of a cohort of inflammatory proteins in monocytes and macrophages. We also found that genetic abrogation of L13a expression in macrophages significantly compromised the resolution of inflammation in a mouse model of LPS-induced endotoxemia. However, its function in the pathogenesis of atherosclerosis is not known. Here, we examine whether L13a in macrophage has a protective role against high fat diet-induced atherosclerosis. Approach and Results We bred the macrophage-specific L13a knockout mice L13a Flox+/+ Cre+/+ onto apoE−/− background and generated the experimental double knockout (KO) mice L13a Flox+/+ Cre+/+ apoE−/−. L13a Flox+/+ Cre−/− mice on apoE−/− background were used as controls. Control and KO mice were subjected to high-fat diet for 10 weeks. Evaluation of aortic sinus sections and entire aorta by en face showed significantly higher atherosclerosis in the KO mice. Severity of atherosclerosis in KO mice was accompanied by thinning of the smooth muscle cell (SMC) layer in the media, larger macrophage area in the intimal plaque region and higher plasma levels of inflammatory cytokines. In addition, macrophages isolated from KO mice had higher polyribosomal abundance of several target mRNAs, thus showing defect in translation control. Conclusion Our data demonstrate that loss of L13a in macrophages increases susceptibility to atherosclerosis in apoE−/− mice, revealing an important role of L13a-dependent translational control as an endogenous protection mechanism against atherosclerosis.

We report a novel extraribosomal innate immune function of mammalian ribosomal protein L13a, whereby it acts as an antiviral agent. We found that L13a is released from the 60S ribosomal subunit in response to infection by respiratory syncytial virus (RSV), an RNA virus of the Pneumovirus genus and a serious lung pathogen. Unexpectedly, the growth of RSV was highly enhanced in L13a-knocked-down cells of various lineages as well as in L13a knockout macrophages from mice. In all L13a-deficient cells tested, translation of RSV matrix (M) protein was specifically stimulated, as judged by a greater abundance of M protein and greater association of the M mRNA with polyribosomes, while general translation was unaffected. In silico RNA folding analysis and translational reporter assays revealed a putative hairpin in the 3=untranslated region (UTR) of M mRNA with significant structural similarity to the cellular GAIT (gamma-activated inhibitor of translation) RNA hairpin, previously shown to be responsible for assembling a large, L13a-containing ribonucleoprotein complex that promoted translational silencing in gamma interferon (IFN-␥)-activated myeloid cells. However, RNA-protein interaction studies revealed that this complex, which we named VAIT (respiratory syncytial virus-activated inhibitor of translation) is functionally different from the GAIT complex. VAIT is the first report of an extraribosomal L13a-mediated, IFN-␥-independent innate antiviral complex triggered in response to virus infection. We provide a model in which the VAIT complex strongly hinders RSV replication by inhibiting the translation of the rate-limiting viral M protein, which is a new paradigm in antiviral defense. IMPORTANCEThe innate immune mechanisms of host cells are diverse in nature and act as a broad-spectrum cellular defense against viruses. Here, we report a novel innate immune mechanism functioning against respiratory syncytial virus (RSV), in which the cellular ribosomal protein L13a is released from the large ribosomal subunit soon after infection and inhibits the translation of a specific viral mRNA, namely, that of the matrix protein M. Regarding its mechanism, we show that the recognition of a specific secondary structure in the 3= untranslated region of the M mRNA leads to translational arrest of the mRNA. We also show that the level of M protein in the infected cell is rate limiting for viral morphogenesis, providing a rationale for L13a to target the M mRNA for suppression of RSV growth. Translational silencing of a viral mRNA by a deployed ribosomal protein is a new paradigm in innate immunity.

bIn contrast to prokaryotes, the precise mechanism of incorporation of ribosomal proteins into ribosomes in eukaryotes is not well understood. For the majority of eukaryotic ribosomal proteins, residues critical for rRNA binding, a key step in the hierarchical assembly of ribosomes, have not been well defined. In this study, we used the mammalian ribosomal protein L13a as a model to investigate the mechanism(s) underlying eukaryotic ribosomal protein incorporation into ribosomes. This work identified the arginine residue at position 68 of L13a as being essential for L13a binding to rRNA and incorporation into ribosomes. We also demonstrated that incorporation of L13a takes place during maturation of the 90S preribosome in the nucleolus, but that translocation of L13a into the nucleolus is not sufficient for its incorporation into ribosomes. Incorporation of L13a into the 90S preribosome was required for rRNA methylation within the 90S complex. However, mutations abolishing ribosomal incorporation of L13a did not affect its ability to be phosphorylated or its extraribosomal function in GAIT element-mediated translational silencing. These results provide new insights into the mechanism of ribosomal incorporation of L13a and will be useful in guiding future studies aimed at fully deciphering mammalian ribosome biogenesis. Ribosome biogenesis is an essential and highly complex process in all living organisms. Ribosomes are composed of numerous precisely assembled proteins and rRNA molecules; thus, they present a paradigm for RNA-protein recognition/binding and ribonucleoprotein (RNP) folding. In prokaryotes, the highly ordered process of ribosome assembly is relatively well defined and has been shown to involve orchestrated changes in rRNA conformation and protein binding steps (1). In contrast, very little is known about the mechanism of eukaryotic ribosome assembly. Despite the general similarity of eukaryotic and prokaryotic ribosomes, they differ substantially in their size, structure, and subunit compositions (including the numbers and sequences of proteins and rRNAs). Another key difference is that the process of maturation and assembly of eukaryotic ribosomes is a highly compartmentalized process which starts in the nucleolus and finishes in the cytoplasm (2). Factors that are not ribosome components but are involved in the ribosome maturation and assembly process also may differ between prokaryotes and eukaryotes. Production of eukaryotic ribosomes (including export from the nucleolus into the cytoplasm) requires highly synchronized synthesis of four rRNAs, about 80 ribosomal proteins, 150 proteins that serve as assembly factors, and 70 small nucleolar RNAs (snoRNAs) that direct modifications to rRNAs (3, 4). Advances in affinity purification methods using epitope-tagged proteins and mass spectrometry have allowed molecular characterization of several distinct RNP complexes corresponding to nucleolar preribosomes at different stages of assembly (5-7). However, this information was primarily derived from stud...