Functional analysis of a tripartite stability element within the CD40 ligand 3' untranslated region

A Posttranscriptional Pathway of CD40 Ligand mRNA Stability Is Required for the Development of an Optimal Humoral Immune Response

CD40 ligand (CD40L) mRNA stability is dependent on an activation-induced pathway that is mediated by the binding complexes containing the multifunctional RNA-binding protein, polypyrimidine tract-binding protein 1 (PTBP1) to a 3' untranslated region of the transcript. To understand the relationship between regulated CD40L and the requirement for variegated expression during a T-dependent response, we engineered a mouse lacking the CD40L stability element (CD40LΔ5) and asked how this mutation altered multiple aspects of the humoral immunity. We found that CD40LΔ5 mice expressed CD40L at 60% wildtype levels, and lowered expression corresponded to significantly decreased levels of T-dependent Abs, loss of germinal center (GC) B cells and a disorganized GC structure. Gene expression analysis of B cells from CD40LΔ5 mice revealed that genes associated with cell cycle and DNA replication were significantly downregulated and genes linked to apoptosis upregulated. Importantly, somatic hypermutation was relatively unaffected although the number of cells expressing high-affinity Abs was greatly reduced. Additionally, a significant loss of plasmablasts and early memory B cell precursors as a percentage of total GL7⁺ B cells was observed, indicating that differentiation cues leading to the development of post-GC subsets was highly dependent on a threshold level of CD40L. Thus, regulated mRNA stability plays an integral role in the optimization of humoral immunity by allowing for a dynamic level of CD40L expression on CD4 T cells that results in the proliferation and differentiation of pre-GC and GC B cells into functional subsets.

The RNA-binding protein RBM47 inhibits non-small cell lung carcinoma metastasis through modulation of AXIN1 mRNA stability and Wnt/β-catentin signaling

BACKGROUND

Non-small-cell lung cancer (NSCLC) remains a highly prevalent and deadly form of cancer, with efforts to better understand the molecular basis of the progression of this disease being essential to its effective treatment. Several recent studies have highlighted the ability of RNA-binding proteins (RBPs) to regulate a wide range of cellular processes in both healthy and pathogenic contexts. Among these RBPs, RNA binding motif protein 47 (RBM47) has recently been identified as a tumor suppressor in both breast and colon cancers, whereas its role in NSCLC is poorly understood.

METHODS

RBM47 expression in NSCLC samples was evaluated by RT-PCR, western blotting and immunohistochemistry analysis. Molecular and cellular techniques including lentiviral vector-mediated knockdown were used to elucidate the functions and mechanisms of RBM47.

RESULTS

This study sought to analyze the expression and role of RBM47 in NSCLC. In the present study, we observed reduced levels of RBM47 expression in NSCLC, with these reductions corresponding to a poorer prognosis and more advanced disease including a higher TNM stage (p = 0.022), a higher likelihood of tumor thrombus (p = 0.001), and pleural invasion (p = 0.033). Through functional analyses in vitro and in vivo, we further demonstrated that these RBP was able to disrupt the proliferation, migration, and invasion of NSCLC cells. At a molecular level, we determined that RBM47 was able to bind the AXIN1 mRNA, stabilizing it and thereby enhancing the consequent suppression of Wnt/β-catentin signaling.

CONCLUSION

Together our findings reveal that RBM47 targets AXIN1 in order to disrupt Wnt/β-catenin signaling in NSCLC and thereby disrupting tumor progression. These results thus offer new insights into the molecular biology of NSCLC, and suggest that RBM47 may also have value as a prognostic biomarker and/or therapeutic target in NSCLC patients.

The RNA binding protein neuro-oncological ventral antigen 1 (NOVA1) regulates IL-6 mRNA stability to enhance JAK2-STAT3 signaling in CRC

The molecular mechanisms governing the metastasis of colorectal cancer (CRC) are incompletely understood. In the present study, we found NOVA1 to be expressed at higher levels in CRC cell lines and tissue samples, and this upregulation was positively correlated with TNM stage (p = 0.034), poor differentiation (p = 0.001), and lymph node metastasis (p = 0.008). Both overall survival (OS) and relapse-free survival (RFS) were both significantly decreased in patients with high NOVA1 expression relative to those with low expression. Through a multivariate analysis, we determined that NOVA1 independently predicted poor outcomes in those with CRC. In further functional studies, we found that NOVA1 expression controlled the proliferation and invasive characteristics of CRC cells via a mechanism wherein NOVA1 bound and stabilized the IL6 mRNA, enhancing IL-6/JAK2/STAT3 signaling to in turn upregulate matrix metalloproteinases (MMPs) 2, 7, and 9. NOVA1 therefore plays key functional roles in regulating CRC progression, and our results further indicate that it serve as a valuable prognostic biomarker and potentially a target for therapeutic treatment in individuals with CRC.

The RNA-Binding Protein, Polypyrimidine Tract-Binding Protein 1 (PTBP1) Is a Key Regulator of CD4 T Cell Activation

We have previously shown that the RNA binding protein, polypyrimidine tract-binding protein (PTBP1) plays a critical role in regulating the expression of CD40L in activated CD4 T cells. This is achieved mechanistically through message stabilization at late times of activation as well as by altered distribution of CD40L mRNA within distinct cellular compartments. PTBP1 has been implicated in many different processes, however whether PTBP1 plays a broader role in CD4 T cell activation is not known. To examine this question, experiments were designed to introduce shRNA into primary human CD4 T cells to achieve decreased, but not complete ablation of PTBP1 expression. Analyses of shPTB-expressing CD4 T cells revealed multiple processes including cell proliferation, activation-induced cell death and expression of activation markers and cytokines that were regulated in part by PTBP1 expression. Although there was an overall decrease in the steady-state level of several activation genes, only IL-2 and CD40L appeared to be regulated by PTBP1 at the level of RNA decay suggesting that PTBP1 is critical at different regulatory steps of expression that is gene-specific. Importantly, even though the IL-2 protein levels were reduced in cells with lowered PTBP1, the steady-state level of IL-2 mRNA was significantly higher in these cells suggesting a block at the translational level. Evaluation of T cell activation in shPTB-expressing T cells revealed that PTBP1 was linked primarily to the activation of the PLCγ1/ERK1/2 and the NF-κB pathways. Overall, our results reveal the importance of this critical RNA binding protein in multiple steps of T cell activation.

Novel and recurrent AID mutations underlie prevalent autosomal recessive form of HIGM in consanguineous patients

Immunoglobulin class switch recombination deficiencies (Ig-CSR-D) are characterized by normal or elevated serum IgM level and absence of IgG, IgA, and IgE. Most reported cases are due to X-linked CD40L deficiency. Activation-induced cytidine deaminase deficiency is the most frequent autosomal recessive form, whereas CD40 deficiency is more rare. Herein, we present the first North African study on hyper IgM (HIGM) syndrome including 16 Tunisian patients. Phenotypic and genetic studies allowed us to determine their molecular basis. Three CD40LG mutations have been identified including two novels (c.348_351dup and c.782_*2del) and one already reported mutation (g.6182G>A). No mutation has been found in another patient despite the lack of CD40L expression. Interestingly, three AICDA mutations have been identified in 11 patients. Two mutations were novel (c.91T>C and c.389A>C found in one and five patients respectively), and one previously reported splicing mutation (c.156+1T>G) was found in five patients. Only one CD40-deficient patient, bearing a novel mutation (c.109T>G), has been identified. Thus, unlike previous reports, AID deficiency is the most frequent underlying molecular basis (68%) of Ig-CSR-D in Tunisian patients. This finding and the presence of specific recurrent mutations are probably due to the critical role played by inbreeding in North African populations.

Polypyrimidine tract-binding protein is critical for the turnover and subcellular distribution of CD40 ligand mRNA in CD4+ T cells

CD40L (CD154) is regulated at the posttranscriptional level by an activation-induced process that results in a highly stable transcript at extended times of T cell activation. Transcript stability is mediated by polypyrimidine tract-binding protein (PTB)-containing complexes (complex I and II) that bind to three adjacent CU-rich sequences within the 3' untranslated region. To assess the role of PTB in the expression and distribution of CD40L mRNA, PTB was targeted using short hairpin RNA in both primary T cells and a T cell line that recapitulates the stability phase of regulated CD40L mRNA decay. PTB knockdown resulted in a marked decrease in the mRNA stability that resulted in lowered CD40L surface expression. PTB was also critical for appropriate distribution of CD40L mRNA between the nucleus and cytoplasm and in the cytoplasm between the cytosol and the translating polysomes. The activation-induced formation of PTB-specific ribonucleoprotein complexes was observed only with cytoplasmic and not nuclear PTB indicating functional differences in the protein defined by cellular localization. Finally, we observed that cytoplasmic and nuclear PTB isoforms were differentially modified relative to each other and that the changes in cytoplasmic PTB were consistent with activation-induced phosphorylation. Together this work suggests that differentially modified PTB regulates CD40L expression at multiple steps by 1) retaining CD40L mRNA in the nucleus, 2) directly regulating mRNA stability at late times of activation, and 3) forming a ribonuclear complex that preferentially associates with translating ribosomes thus leading to an enhanced level of CD40L protein.

In vivo post-transcriptional regulation of CD154 in mouse CD4+ T cells

Interactions between CD40 and its ligand CD154 are involved in the progression of both cell mediated and innate immunity. These interactions are brought about by the transient expression of CD154 on activated CD4(+) T cells, which is regulated, in part, at the level of mRNA turnover. Here we have focused on analyzing the pattern of post-transcriptional regulation in mouse CD4(+) T cells in response to activation. Initial experiments identify a region of the murine CD154 mRNA that binds a polypyrimidine tract-binding protein-containing complex (mComplex I), which is activation-dependent and binds to a single CU-rich site within the 3' uTR Subsequent findings demonstrate that in vivo polyclonal activation of T cells leads to a pattern of differential CD154 mRNA stability that is directly dependent on extent of activation. Furthermore, in vitro activation of antigen-primed T cells shows that the CD154 mRNA half-life increases relative to that of unprimed cells. Importantly, this is the first report demonstrating that the regulation of CD154 in vivo is connected to an activation-induced program of mRNA decay and thus provides strong evidence for post-transcriptional mechanisms having a physiological role in regulating CD154 expression during an ongoing immune response.

Post-transcriptional regulation in lymphocytes: the case of CD154

The control of mRNA decay is emerging as an important control point and a major contributor to gene expression in both immune and non-immune cells. The identification of protein factors and cis-acting elements responsible for transcript degradation has illuminated a comprehensive picture of precisely orchestrated events required to both regulate and establish the decay process. One gene that is highly regulated at the post-transcriptional level is CD40 ligand (CD154 or CD40L). CD154 on CD4(+) T cells is tightly controlled by an interacting network of transcriptional and post-transcriptional processes that result in precise surface levels of protein throughout an extended time course of antigen stimulation. The activation-induced stabilization of the CD154 transcript by a polypyrimidine tract-binding protein (PTB)-complex is a key event that corresponds to the temporal expression of CD154. In this review, we discuss known and potential roles of major mRNA decay pathways in lymphocytes and focus on the unique post-transcriptional mechanisms leading to CD154 expression by activated CD4(+) T cells.