A Knock-In Tristetraprolin (TTP) Zinc Finger Point Mutation in Mice: Comparison with Complete TTP Deficiency

Restraint of inflammasome-driven cytokine responses through the mRNA stability protein TTP

Activation of the NLRP3 inflammasome causes extensive disturbance of cellular homeostasis, with Golgi disruption, mitochondrial dysfunction, and changes in intracellular ion concentration occurring rapidly upon stimulation. Given this, it would seem near certain that these changes might also globally affect cellular signaling pathways, yet few, if any, studies have explored this possibility. Here, we combine genomics and phosphoproteomics to identify inhibition of the ERK1/2 MAP kinase signaling cascade upon inflammasome stimulation. This loss of ERK1/2 activity results in rapid inactivation of the mRNA decay-promoting protein tristetraprolin (TTP), with loss of TTP promoting subsequent increased release of cytokines upon pyroptosis. Further, we observe significantly increased levels of TTP expression in patients with inflammatory bowel disease, a disease for which altered cytokine expression is a key driver of pathogenesis. Inflammasome activation thus rapidly inactivates a pathway designed to suppress cytokine release, potentially exacerbating hyperinflammatory states, including those involved in autoinflammatory disease.

Cytoplasmic mRNA decay controlling inflammatory gene expression is determined by pre-mRNA fate decision

The fidelity of immune responses depends on timely controlled and selective mRNA degradation that is largely driven by RNA-binding proteins (RBPs). It remains unclear whether stochastic or directed processes govern the selection of an individual mRNA molecule for degradation. Using human and mouse cells, we show that tristetraprolin (TTP, also known as ZFP36), an essential anti-inflammatory RBP, destabilizes target mRNAs via a hierarchical molecular assembly. The assembly formation strictly relies on the interaction of TTP with RNA. The TTP homolog ZFP36L1 exhibits similar requirements, indicating a broader relevance of this regulatory program. Unexpectedly, the assembly of the cytoplasmic mRNA-destabilization complex is licensed in the nucleus by TTP binding to pre-mRNA, which we identify as the principal TTP target rather than mRNA. Hence, the fate of an inflammation-induced mRNA is decided concomitantly with its synthesis. This mechanism prevents the translation of excessive and potentially harmful inflammation mediators, irrespective of transcription.

Gene Expression Signatures of Porcine Bone Marrow-Derived Antigen-Presenting Cells Infected with Classical Swine Fever Virus

For a better understanding of classical swine fever (CSF) pathogenesis, a transcriptomic analysis was performed using porcine bone marrow (BM)-derived antigen-presenting cells (APCs) infected ex vivo with two different cDNA-derived classical swine fever virus (CSFV) strains, the low-virulence Pinar de Rio (vPdR-36U) or the lethal vPdR-H30K-5U. The transcriptomic profile of vPdR-36U- or vPdR-H30K-5U-infected versus noninfected cells revealed 946 and 2643 differentially expressed genes (DEGs), respectively. The upregulation of ISG15, CXCL-10, ADAM8, and CSF1 was found after infection with vPdR-36U, which could contribute to the generation of mild CSF forms. In contrast, cells infected with the lethal vPdR-H30K-5U overexpressed the immune checkpoint molecules PD-L1, CD276, and LAG3, which are involved in T-cell exhaustion and could be associated with adaptive immunity impairment. vPdR-H30K-5U also induced increased expression of PPBP, IL-8, IL-6, ECE1, and Rab27b, which are mediators of inflammatory responses that can be involved in cytokine storms. The TNF signaling pathway, which is related to the activation and proliferation of different subsets of immune cells, including CD4+ T cells, was notably upregulated in response to the low-pathogenicity virus. The Th17, Th1, and Th2 differentiation pathways were downregulated by the highly pathogenic virus only, supporting the role of T-cell-mediated immunity in protecting against CSFV.

Tristetraprolin Family Members and Processing Bodies: A Complex Regulatory Network Involved in Fatty Liver Disease, Viral Hepatitis and Hepatocellular Carcinoma

Chronic liver diseases, such as those encountered with obesity, chronic/abusive alcohol consumption or viral infections, represent not only major public health concerns with limited therapeutic options but also important risk factors for the onset of hepatocellular carcinoma (HCC). Deciphering the molecular traits underlying these disorders is of high interest for designing new and effective treatments. The tristetraprolin (TTP) family members are of particular importance given their ability to control the expression of a wide range of genes involved in metabolism, inflammation and carcinogenesis at the post-transcriptional level. This regulation can occur within small cytoplasmic granules, namely, processing bodies (P-bodies), where the mRNA degradation occurs. Increasing evidence indicates that TTP family members and P-bodies are involved in the development of chronic liver diseases and cancers. In this review, we discuss the role of this regulatory mechanism in metabolic-dysfunction-associated steatotic liver disease (MASLD), alcohol-related liver disease (ALD), hepatic viral infections and HCC.

Proceedings of the 2024 Division of Translational Toxicology Satellite Symposium

The 2024 annual Division of Translational Toxicology (DTT) Satellite Symposium, entitled "Pathology Potpourri," was held in Baltimore, Maryland, at the Society of Toxicologic Pathology's 42nd annual meeting. The goal of this symposium was to present and discuss challenging diagnostic pathology and/or nomenclature issues. This article presents summaries of the speakers' talks along with select images that were used by the audience for voting and discussion. Various lesions and topics covered during the symposium included induced nonneoplastic lesions in the mouse kidney, induced and spontaneous neoplastic lesions in the mouse lung, infectious and proliferative lesions in nonhuman primates, an interesting inflammatory lesion in a transgenic mouse strain, and a lesson on artifact recognition.

Synergistic roles of tristetraprolin family members in myeloid cells in the control of inflammation

Members of the tristetraprolin (TTP) family of RNA-binding proteins can bind to and promote the decay of specific transcripts containing AU-rich motifs. ZFP36 (TTP) is best known for regulating pro-inflammatory cytokine expression in myeloid cells; however, its mammalian paralogues ZFP36L1 and ZFP36L2 have not been viewed as important in controlling inflammation. We knocked out these genes in myeloid cells in mice, singly and together. Single-gene myeloid-specific knockouts resulted in almost no spontaneous phenotypes. In contrast, mice with myeloid cell deficiency of all three genes developed severe inflammation, with a median survival of 8 wk. Macrophages from these mice expressed many more stabilized transcripts than cells from myeloid-specific TTP knockout mice; many of these encoded pro-inflammatory cytokines and chemokines. The failure of weight gain, arthritis, and early death could be prevented completely by two normal alleles of any of the three paralogues, and even one normal allele of Zfp36 or Zfp36l2 was enough to prevent the inflammatory phenotype. Our findings emphasize the importance of all three family members, acting in concert, in myeloid cell function.

HUWE1 controls tristetraprolin proteasomal degradation by regulating its phosphorylation

Tristetraprolin (TTP) is a critical negative immune regulator. It binds AU-rich elements in the untranslated-regions of many mRNAs encoding pro-inflammatory mediators, thereby accelerating their decay. A key but poorly understood mechanism of TTP regulation is its timely proteolytic removal: TTP is degraded by the proteasome through yet unidentified phosphorylation-controlled drivers. In this study, we set out to identify factors controlling TTP stability. Cellular assays showed that TTP is strongly lysine-ubiquitinated, which is required for its turnover. A genetic screen identified the ubiquitin E3 ligase HUWE1 as a strong regulator of TTP proteasomal degradation, which we found to control TTP stability indirectly by regulating its phosphorylation. Pharmacological assessment of multiple kinases revealed that HUWE1-regulated TTP phosphorylation and stability was independent of the previously characterized effects of MAPK-mediated S52/S178 phosphorylation. HUWE1 function was dependent on phosphatase and E3 ligase binding sites identified in the TTP C-terminus. Our findings indicate that while phosphorylation of S52/S178 is critical for TTP stabilization at earlier times after pro-inflammatory stimulation, phosphorylation of the TTP C-terminus controls its stability at later stages.

Clinical implications of tristetraprolin (TTP) modulation in the treatment of inflammatory diseases

Abnormal regulation of pro-inflammatory cytokine and chemokine mediators can contribute to the excess inflammation characteristic of many autoimmune diseases, such as rheumatoid arthritis, psoriasis, Crohn's disease, type 1 diabetes, and many others. The tristetraprolin (TTP) family consists of a small group of related RNA-binding proteins that bind to preferred AU-rich binding sites within the 3'-untranslated regions of specific mRNAs to promote mRNA deadenylation and decay. TTP deficient mice develop a severe systemic inflammatory syndrome consisting of arthritis, myeloid hyperplasia, dermatitis, autoimmunity and cachexia, due at least in part to the excess accumulation of proinflammatory chemokine and cytokine mRNAs and their encoded proteins. To investigate the possibility that increased TTP expression or activity might have a beneficial effect on inflammatory diseases, at least two mouse models have been developed that provide proof of principle that increasing TTP activity can promote the decay of pro-inflammatory and other relevant transcripts, and decrease the severity of mouse models of inflammatory disease. Animal studies of this type are summarized here, and we briefly review the prospects for harnessing these insights for the development of TTP-based anti-inflammatory treatments in humans.

A post-transcriptional regulon controlled by TtpA, the single tristetraprolin family member expressed in Dictyostelium discoideum

Post-transcriptional processes mediated by mRNA binding proteins represent important control points in gene expression. In eukaryotes, mRNAs containing specific AU-rich motifs are regulated by binding of tristetraprolin (TTP) family tandem zinc finger proteins, which promote mRNA deadenylation and decay, partly through interaction of a conserved C-terminal CNOT1 binding (CNB) domain with CCR4-NOT protein complexes. The social ameba Dictyostelium discoideum shared a common ancestor with humans more than a billion years ago, and expresses only one TTP family protein, TtpA, in contrast to three members expressed in humans. Evaluation of ttpA null-mutants identified six transcripts that were consistently upregulated compared to WT during growth and early development. The 3'-untranslated regions (3'-UTRs) of all six 'TtpA-target' mRNAs contained multiple TTP binding motifs (UUAUUUAUU), and one 3'-UTR conferred TtpA post-transcriptional stability regulation to a heterologous mRNA that was abrogated by mutations in the core TTP-binding motifs. All six target transcripts were upregulated to similar extents in a C-terminal truncation mutant, in contrast to less severe effects of analogous mutants in mice. All six target transcripts encoded probable membrane proteins. In Dictyostelium, TtpA may control an 'RNA regulon', where a single RNA binding protein, TtpA, post-transcriptionally co-regulates expression of several functionally related proteins.

Conceptual Advances in Control of Inflammation by the RNA-Binding Protein Tristetraprolin

Regulated changes in mRNA stability are critical drivers of gene expression adaptations to immunological cues. mRNA stability is controlled mainly by RNA-binding proteins (RBPs) which can directly cleave mRNA but more often act as adaptors for the recruitment of the RNA-degradation machinery. One of the most prominent RBPs with regulatory roles in the immune system is tristetraprolin (TTP). TTP targets mainly inflammation-associated mRNAs for degradation and is indispensable for the resolution of inflammation as well as the maintenance of immune homeostasis. Recent advances in the transcriptome-wide knowledge of mRNA expression and decay rates together with TTP binding sites in the target mRNAs revealed important limitations in our understanding of molecular mechanisms of TTP action. Such orthogonal analyses lead to the discovery that TTP binding destabilizes some bound mRNAs but not others in the same cell. Moreover, comparisons of various immune cells indicated that an mRNA can be destabilized by TTP in one cell type while it remains stable in a different cell linage despite the presence of TTP. The action of TTP extends from mRNA destabilization to inhibition of translation in a subset of targets. This article will discuss these unexpected context-dependent functions and their implications for the regulation of immune responses. Attention will be also payed to new insights into the role of TTP in physiology and tissue homeostasis.

Tristetraprolin, a Potential Safeguard Against Carcinoma: Role in the Tumor Microenvironment

Tristetraprolin (TTP), a well-known RNA-binding protein, primarily affects the expression of inflammation-related proteins by binding to the targeted AU-rich element in the 3' untranslated region after transcription and subsequently mediates messenger RNA decay. Recent studies have focused on the role of TTP in tumors and their related microenvironments, most of which have referred to TTP as a potential tumor suppressor involved in regulating cell proliferation, apoptosis, and metastasis of various cancers, as well as tumor immunity, inflammation, and metabolism of the microenvironment. Elevated TTP expression levels could aid the diagnosis and treatment of different cancers, improving the prognosis of patients. The aim of this review is to describe the role of TTP as a potential safeguard against carcinoma.

Tristetraprolin targets Nos2 expression in the colonic epithelium

Tristetraprolin (TTP), encoded by the Zfp36 gene, is a zinc-finger protein that regulates RNA stability primarily through association with 3′ untranslated regions (3′ UTRs) of target mRNAs. While TTP is expressed abundantly in the intestines, its function in intestinal epithelial cells (IECs) is unknown. Here we used a cre-lox system to remove Zfp36 in the mouse epithelium to uncover a role for TTP in IECs and to identify target genes in these cells. While TTP was largely dispensable for establishment and maintenance of the colonic epithelium, we found an expansion of the proliferative zone and an increase in goblet cell numbers in the colon crypts of Zfp36^ΔIEC mice. Furthermore, through RNA-sequencing of transcripts isolated from the colons of Zfp36^fl/fl and Zfp36^ΔIEC mice, we found that expression of inducible nitric oxide synthase (iNos or Nos2) was elevated in TTP-knockout IECs. We demonstrate that TTP interacts with AU-rich elements in the Nos2 3′ UTR and suppresses Nos2 expression. In comparison to control Zfp36^fl/fl mice, Zfp36^ΔIEC mice were less susceptible to dextran sodium sulfate (DSS)-induced acute colitis. Together, these results demonstrate that TTP in IECs targets Nos2 expression and aggravates acute colitis.

Importance of the Conserved Carboxyl-Terminal CNOT1 Binding Domain to Tristetraprolin Activity In Vivo

Tristetraprolin (TTP) is an anti-inflammatory protein that modulates the stability of certain cytokine/chemokine mRNAs. After initial high-affinity binding to AU-rich elements in 3' untranslated regions of target mRNAs, mediated through its tandem zinc finger (TZF) domain, TTP promotes the deadenylation and ultimate decay of target transcripts. These transcripts and their encoded proteins accumulate abnormally in TTP knockout (KO) mice, leading to a severe inflammatory syndrome. To assess the importance of the highly conserved C-terminal CNOT1 binding domain (CNBD) of TTP to the TTP deficiency phenotype in mice, we created a mouse model in which TTP lacked its CNBD. CNBD deletion mice exhibited a less severe phenotype than the complete TTP KO mice. In macrophages, the stabilization of target transcripts seen in KO mice was partially normalized in the CNBD deletion mice. In cell-free experiments, recombinant TTP lacking its CNBD could still activate target mRNA deadenylation by purified recombinant Schizosaccharomyces pombe CCR4/NOT complexes, although to a lesser extent than full-length TTP. Thus, TTP lacking its CNBD can still act to promote target mRNA instability in vitro and in vivo These data have implications for TTP family members throughout the eukarya, since species from all four kingdoms contain proteins with linked TZF and CNOT1 binding domains.

The tandem zinc finger RNA binding domain of members of the tristetraprolin protein family

Tristetraprolin (TTP), the prototype member of the protein family of the same name, was originally discovered as the product of a rapidly inducible gene in mouse cells. Development of a knockout (KO) mouse established that absence of the protein led to a severe inflammatory syndrome, due in part to elevated levels of tumor necrosis factor (TNF). TTP was found to bind directly and with high affinity to specific AU-rich sequences in the 3'-untranslated region of the TNF mRNA. This initial binding led to promotion of TNF mRNA decay and inhibition of its translation. Many additional TTP target mRNAs have since been identified, some of which are cytokines and chemokines involved in the inflammatory response. There are three other proteins in the mouse with similar activities and domain structures, but whose KO phenotypes are remarkably different. Moreover, proteins with similar domain structures and activities have been found throughout eukaryotes, demonstrating that this protein family arose from an ancient ancestor. The defining characteristic of this protein family is the tandem zinc finger (TZF) domain, a 64 amino acid sequence with many conserved residues that is responsible for the direct RNA binding. We discuss here many aspects of this protein domain that have been elucidated since the original discovery of TTP, including its sequence conservation throughout eukarya; its apparent continued evolution in some lineages; its functional dependence on many key conserved residues; its "interchangeability" among evolutionarily distant species; and the evidence that RNA binding is required for the physiological functions of the proteins. This article is categorized under: RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes RNA Interactions with Proteins and Other Molecules > Protein-RNA Recognition RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

Inhibiting transcription in cultured metazoan cells with actinomycin D to monitor mRNA turnover

Decay of transcribed mRNA is a key determinant of steady state mRNA levels in cells. Global analysis of mRNA decay in cultured cells has revealed amazing heterogeneity in rates of decay under normal growth conditions, with calculated half-lives ranging from several minutes to many days. The factors that are responsible for this wide range of decay rates are largely unknown, although our knowledge of trans-acting RNA binding proteins and non-coding RNAs that can control decay rates is increasing. Many methods have been used to try to determine mRNA decay rates under various experimental conditions in cultured cells, and transcription inhibitors like actinomycin D have probably the longest history of any technique for this purpose. Despite this long history of use, the actinomycin D method has been criticized as prone to artifacts, and as ineffective for some promoters. With appropriate guidelines and controls, however, it can be a versatile, effective technique for measuring endogenous mRNA decay in cultured mammalian and insect cells, as well as the decay of exogenously-expressed transcripts. It can be used readily on a genome-wide level, and is remarkably cost-effective. In this short review, we will discuss our utilization of this approach in these cells; we hope that these methods will allow more investigators to apply this useful technique to study mRNA decay under the appropriate conditions.