G-protein pathway suppressor 2 (GPS2) interacts with the regulatory factor X4 variant 3 (RFX4_v3) and functions as a transcriptional co-activator

RFX4_v3 (regulatory factor X4 variant 3) is a brain-specific isoform of the transcription factor RFX4. Insertional mutagenesis in mice demonstrates that Rfx4_v3 is crucial for normal brain development. Many genes involved in critical processes during brain morphogenesis are dysregulated in Rfx4_v3 mutant brains. For example, Cx3cl1 is a CX3C-type chemokine that is abundant in brain and is a direct transcriptional target of RFX4_v3 through a specific promoter X-box (X-box 1), the responsive element for RFX proteins. To identify potential interacting partners for RFX4_v3, we performed yeast two-hybrid analysis. Nine candidate interactors were identified, including GPS2 (G-protein pathway suppressor 2). Indirect immunofluorescence demonstrated that GPS2 and RFX4_v3 co-localized to the nucleus. Both GPS2 and RFX4_v3 mRNAs were also present in most portions of the adult mouse brain as well as in brains at different ages, suggesting that the two proteins could bind to each other. Co-immunoprecipitation assays indicated that physical interactions between GPS2 and RFX4_v3 did indeed occur. Furthermore, GPS2 was recruited to the Cx3cl1 promoter by RFX4_v3 and potentiated RFX4_v3 transactivation on this promoter through X-box 1, suggesting that the protein-protein interaction was functionally relevant. GPS2 bound to both the carboxyl-terminal region (amino acids 575-735) and the middle region (amino acids 250-574) of the RFX4_v3 protein. RFX4_v3 amino acids 1-574 stimulated the Cx3cl1 promoter to a similar extent as the full-length RFX4_v3 protein; however, deletion of the carboxyl-terminal region of RFX4_v3 impaired the co-activating abilities of GPS2. Based on these data, we conclude that GPS2 interacts with RFX4_v3 to modulate transactivation of genes involved in brain morphogenesis, including Cx3Cl1.

A unique C-terminal repeat domain maintains the cytosolic localization of the placenta-specific tristetraprolin family member ZFP36L3

Members of the tristetraprolin family of CCCH tandem zinc finger proteins bind to AU-rich elements in certain cellular mRNAs, leading to their deadenylation and destabilization. Studies in knock-out mice demonstrated roles for three of the family members, tristetraprolin, ZFP36L1, and ZFP36L2, in inflammation, chorioallantoic fusion, and early embryonic development, respectively. However, little is known about a recently discovered placenta-specific tristetraprolin family member, ZFP36L3. Tristetraprolin, ZFP36L1, and ZFP36L2 have been shown to shuttle between the nucleus and cytoplasm, using typical hydrophobic amino acid-rich nuclear export sequences, and nuclear localization sequences located within the tandem zinc finger domain. In contrast, we previously showed that green fluorescent protein-labeled ZFP36L3, expressed in HEK 293 cells, remained cytosolic, even in the presence of the nuclear export blocker leptomycin B. We show here that the conserved tandem zinc finger domain contains an active nuclear localization signal. However, the sequence corresponding to the nuclear export signal in the other family members was nonfunctional, and thus did not contribute to the cytosolic localization. The unique C-terminal repeat domain could override the activity of the nuclear localization sequence, preventing the import of ZFP36L3 into the nucleus. Immunostaining of mouse placenta demonstrated that ZFP36L3 was located only in the cytoplasm of trophoblast cells. Thus, in contrast to the other mammalian members of this protein family, ZFP36L3 is a "full-time" cytosolic protein, rather than a nucleocytoplasmic shuttling protein. The significance of this difference in subcellular localization to the physiology of placental trophoblast cells, where ZFP36L3 is selectively expressed, remains to be determined.

Predictors of acquired lipodystrophy in juvenile-onset dermatomyositis and a gradient of severity

We describe the clinical features of 28 patients with juvenile dermatomyositis (JDM) and 1 patient with adult-onset dermatomyositis (DM), all of whom developed lipodystrophy (LD) that could be categorized into 1 of 3 phenotypes, generalized, partial, or focal, based on the pattern of fat loss distribution. LD onset was often delayed, beginning a median of 4.6 years after diagnosis of DM. Calcinosis, muscle atrophy, joint contractures, and facial rash were DM disease features found to be associated with LD. Panniculitis was associated with focal lipoatrophy while the anti-p155 autoantibody, a newly described myositis-associated autoantibody, was more associated with generalized LD. Specific LD features such as acanthosis nigricans, hirsutism, fat redistribution, and steatosis/nonalcoholic steatohepatitis were frequent in patients with LD, in a gradient of frequency and severity among the 3 sub-phenotypes. Metabolic studies frequently revealed insulin resistance and hypertriglyceridemia in patients with generalized and partial LD. Regional fat loss from the thighs, with relative sparing of fat loss from the medial thighs, was more frequent in generalized than in partial LD and absent from DM patients without LD. Cytokine polymorphisms, the C3 nephritic factor, insulin receptor antibodies, and lamin mutations did not appear to play a pathogenic role in the development of LD in our patients. LD is an under-recognized sequela of JDM, and certain DM patients with a severe, prolonged clinical course and a high frequency of calcinosis appear to be at greater risk for the development of this complication. High-risk JDM patients should be screened for metabolic abnormalities, which are common in generalized and partial LD and result in much of the LD-associated morbidity. Further study is warranted to investigate the pathogenesis of acquired LD in patients with DM.

Regulatory factor X4 variant 3: a transcription factor involved in brain development and disease

Regulatory factor X4 variant 3 (RFX4_v3) is a recently identified transcription factor specifically expressed in the brain. Gene disruption in mice demonstrated that interruption of a single allele (heterozygous, +/-) prevented formation of the subcommissural organ (SCO), resulting in congenital hydrocephalus, whereas interruption of two alleles (homozygous, -/-) caused fatal failure of dorsal midline brain structure formation. These mutagenesis studies implicated RFX4_v3 in early brain development as well as the genesis of the SCO. Rfx4_v3 deficiency presumably causes abnormalities in brain by altering the expression levels of many genes that are crucial for brain morphogenesis, such as the signaling components in the Wnt, bone morphogenetic protein, and retinoic acid pathways. RFX4_v3 might affect these critical signaling pathways in brain development. Cx3cl1, a chemokine gene highly expressed in brain, was identified as a direct target for RFX4_v3, indicating that RFX4_v3 possesses trans-acting activity to stimulate gene expression. Rfx4_v3 is highly expressed in the suprachiasmatic nucleus and might be involved in regulating the circadian clock. One haplotype in RFX4_v3 gene is linked to a higher risk of bipolar disorder, suggesting that this protein might contribute to the pathogenesis of the disease. This Mini-Review describes our current knowledge about RFX4_v3, an important protein that appears to be involved in many aspects of brain development and disease.

Phosphorylation site analysis of the anti-inflammatory and mRNA-destabilizing protein tristetraprolin

Tristetraprolin (TTP) is a member of the CCCH zinc finger proteins and is an anti-inflammatory protein. Mice deficient in TTP develop a profound inflammatory syndrome with erosive arthritis, autoimmunity and myeloid hyperplasia. TTP binds to mRNA AU-rich elements with high affinity for UUAUUUAUU nucleotides and causes destabilization of those mRNA molecules. TTP is phosphorylated extensively in vivo and is a substrate for multiple protein kinases in vitro. A number of approaches have been used to identify its phosphorylation sites. This article highlights the recent progress and different approaches utilized for the identification of phosphorylation sites in mammalian TTP. Important but limited results are obtained using traditional methods, including in vivo labeling, site-directed mutagenesis, phosphopeptide mapping and protein sequencing. Mass spectrometry (MS), including MALDI/MS, MALDI/MS/MS, liquid chromatography/MS/MS, immobilized metal ion affinity chromatography (IMAC)/MALDI/MS/MS and multidimensional protein identification technology has led the way in identifying TTP phosphorylation sites. The combination of these approaches has identified multiple phosphorylation sites in mammalian TTP, some of which are predicted by motif scanning to be phosphorylated by several protein kinases. This information should provide the molecular basis for future investigation of TTP's regulatory functions in controlling proinflammatory cytokines.

Diversity in penaeidin antimicrobial peptide form and function

Penaeidins are a diverse family of two-domain antimicrobial peptides expressed in shrimp. Variation in penaeidin sequence results in functional diversity, which was discovered using synthetic reproductions of native penaeidins. An isoform of penaeidin class 3 from Litopenaeus setiferus (Litset Pen3-4) was synthesized using native ligation and compared directly with the synthetic penaeidin class 4 known to be expressed in the same organism. New antimicrobial activity data are included in this review that emphasize differences in effectiveness that are apparent from a direct comparison of two classes. A novel approach to intact penaeidin analysis is presented in the form of Fourier Transform Ion-Cyclotron Resonance Mass Spectrometry, which has implications for the identification of individual penaeidin isoforms without chemical modification or enzymatic cleavage. The new information included in this review helps gather the perspective on relevance of penaeidin diversity to antimicrobial function, the use of synthetic peptides as tools to evaluate specific immune functions and the application of high mass resolution, top-down sequencing methods to the intact analysis of individual penaeidin isoforms.

Characterization of zfs1 as an mRNA-binding and -destabilizing protein in Schizosaccharomyces pombe

Tristetraprolin is a vertebrate CCCH tandem zinc finger protein that can bind to and destabilize certain mRNAs containing AU-rich element binding sites. zfs1 is the single gene in the fission yeast, Schizosaccharomyces pombe, that encodes a protein containing the critical features of the tristetraprolin zinc finger domain. zfs1 has been linked to pheromone signal transduction control and to the coordination of mitosis, but no biological function has been ascribed to the zfs1 protein. Through a functional genomics approach we compared transcript levels in wild-type and zfs1-deficient S. pombe strains; those elevated in the zfs1-deficient strain were examined for the presence of potential tristetraprolin-like binding sites. One such potential target transcript was encoded by arz1, a gene encoding a protein of unknown function that contains armadillo repeats. arz1 mRNA decay was inhibited in the zfs1-deficient strain when it was expressed under the control of a thiamine-repressible promoter. Mutations within one AU-rich element present in the arz1 3'-untranslated region protected this transcript from zfs1-promoted decay, whereas mutating another potential binding site had no effect. Binding assays confirmed a direct interaction between zfs1 and arz1 mRNA-based probes; this interaction was eliminated when key residues were mutated in either zfs1 zinc finger. zfs1 and its targets in S. pombe represent a useful model system for studies of zinc finger protein/AU-rich element interactions that result in mRNA decay.

Tristetraprolin, a negative regulator of mRNA stability, is increased in old B cells and is involved in the degradation of E47 mRNA

We have previously shown that the E2A-encoded transcription factor E47, which regulates class switch in splenic B cells, is down-regulated in old B cells, due to increased E47 mRNA decay. At least part of the decreased stability of E47 mRNA seen in aged B cells is mediated by proteins. We have herein looked at the specific proteins responsible for the degradation of the E47 mRNA and found that tristetraprolin (TTP), a physiological regulator of mRNA expression and stability, is involved in the degradation of the E47 mRNA. Although many studies have characterized TTP expression and function in macrophages, monocytes, mast cells, and T cells, little is known about the expression and function of TTP in primary B cells. We show herein that TTP mRNA and protein expression are induced by LPS in B cells from young and old mice, the levels of TTP in old B cells always being higher than those in young B cells. Although TTP mRNA is degraded at a significantly higher rate in old B cells, TTP mRNA expression is higher in old than in young, likely due to its increased transcription. Like in macrophages, TTP protein expression and function in B cells are dependent upon p38 MAPK. We found that there is less phospho-TTP (inactive form), as well as phospho-p38, in old than in young splenic-activated B cells. This is the first report showing that TTP is involved in the degradation of the E47 mRNA and is up-regulated in old B cells.

Comparative expression of tristetraprolin (TTP) family member transcripts in normal human tissues and cancer cell lines

The tristetraprolin (TTP) family of tandem zinc finger proteins comprises three members in man and most other mammals, with a fourth expressed in rodents. In mice, gene disruption of TTP itself leads to a systemic inflammatory syndrome that is mediated in large part by over-expression of tumor necrosis factor alpha (TNF). This increased expression is secondary to stabilization of the TNF mRNA in the TTP KO mice, a finding that led to the characterization of TTP as an mRNA binding protein that can promote the removal of the poly(A) tail from selected mRNAs and facilitate their nucleolytic destruction. The other human family members behave similarly to TTP in over-expression studies of transfected cells, but gene disruption experiments have implicated them in different physiological processes. In the present study, we developed a real-time PCR assay for all three human family members that allowed for comparative measurements of all three family members in the same tissues and cells. We used this assay to quantitate expression levels of all three transcripts in a variety of normal human tissues, as well as in the ;;NCI 60", a well characterized panel of human tumor cell lines. Although studies in fibroblasts and macrophages derived from knockout mice have failed to demonstrate compensatory expression of the family members in terms of transcript levels, it remains possible that the different family members can function as ;;TTP equivalents" in certain physiological or pathological circumstances.

Regulation of suppressor of cytokine signaling 3 (SOCS3) mRNA stability by TNF-alpha involves activation of the MKK6/p38MAPK/MK2 cascade

The potential of some proinflammatory mediators to inhibit gp130-dependent STAT3 activation by enhancing suppressor of cytokine signaling (SOCS) 3 expression represents an important molecular mechanism admitting the modulation of the cellular response toward gp130-mediated signals. Thus, it is necessary to understand the mechanisms involved in the regulation of SOCS3 expression by proinflammatory mediators. In this study, we investigate SOCS3 expression initiated by the proinflammatory cytokine TNF-alpha. In contrast to IL-6, TNF-alpha increases SOCS3 expression by stabilizing SOCS3 mRNA. Activation of the MAPK kinase 6 (MKK6)/p38(MAPK)-cascade is required for TNF-alpha-mediated stabilization of SOCS3 mRNA and results in enhanced SOCS3 protein expression. In fibroblasts or macrophages deficient for MAPK-activated protein kinase 2 (MK2), a downstream target of the MKK6/p38(MAPK) cascade, basal SOCS3-expression is strongly reduced and TNF-alpha-induced SOCS3-mRNA stabilization is impaired, indicating that MK2 is crucial for the control of SOCS3 expression by p38(MAPK)-dependent signals. As a target for SOCS3 mRNA stability-regulating signals, a region containing three copies of a pentameric AUUUA motif in close proximity to a U-rich region located between positions 2422 and 2541 of the 3' untranslated region of SOCS3 is identified. One factor that could target this region is the zinc finger protein tristetraprolin (TTP), which is shown to be capable of destabilizing SOCS3 mRNA via this region. However, data from TTP-deficient cells suggest that TTP does not play an irreplaceable role in the regulation of SOCS3 mRNA stability by TNF-alpha. In summary, these data indicate that TNF-alpha regulates SOCS3 expression on the level of mRNA stability via activation of the MKK6/p38(MAPK) cascade and that the activation of MK2, a downstream target of p38(MAPK), is important for the regulation of SOCS3 expression.

Substrate dependence of conformational changes in the RNA-binding domain of tristetraprolin assessed by fluorescence spectroscopy of tryptophan mutants

Association of tristetraprolin (TTP) with mRNAs containing selected AU-rich mRNA-destabilizing elements (AREs) initiates rapid cytoplasmic degradation of these transcripts. The RNA-binding activity of TTP is mediated by an internal tandem zinc finger domain that preferentially recognizes U-rich RNA ligands containing adjacent UUAU half-sites and is accompanied by conformational changes within the peptide. Here, we have used analogues of the TTP RNA-binding domain containing specific tryptophan substitutions to probe the Zn2+ and RNA substrate dependence of conformational events within individual zinc fingers. Fluorescence methods demonstrate that the N-terminal, but not C-terminal, zinc finger domain adopts a stably folded conformation in the presence of Zn2+. Denaturant titrations suggest that both the N- and C-terminal zinc fingers exhibit limited structural heterogeneity in the absence of RNA substrates, although this is more pronounced for the C-terminal finger. Binding to a cognate ARE substrate induced significant conformational changes within each zinc finger, which also included increased resistance to chemical denaturation. Studies with mutant ARE ligands revealed that a single UUAU half-site was sufficient to induce structural modulation of the N-terminal finger. However, RNA-dependent folding of the C-terminal zinc finger was only observed in the presence of tandem UUAU half-sites, suggesting that the conformation of this domain is linked not only to RNA substrate recognition but also to the ligand occupancy and/or conformational status of the N-terminal finger. Coupled with previous structural and thermodynamic analyses, these data provide a mechanistic framework for discrimination of RNA substrates involving ligand-dependent conformational adaptation of both zinc fingers within the TTP RNA-binding domain.

RECQL, a member of the RecQ family of DNA helicases, suppresses chromosomal instability

The mouse gene Recql is a member of the RecQ subfamily of DEx-H-containing DNA helicases. Five members of this family have been identified in both humans and mice, and mutations in three of these, BLM, WRN, and RECQL4, are associated with human diseases and a cellular phenotype that includes genomic instability. To date, no human disease has been associated with mutations in RECQL and no cellular phenotype has been associated with its deficiency. To gain insight into the physiological function of RECQL, we disrupted Recql in mice. RECQL-deficient mice did not exhibit any apparent phenotypic differences compared to wild-type mice. Cytogenetic analyses of embryonic fibroblasts from the RECQL-deficient mice revealed aneuploidy, spontaneous chromosomal breakage, and frequent translocation events. In addition, the RECQL-deficient cells were hypersensitive to ionizing radiation, exhibited an increased load of DNA damage, and displayed elevated spontaneous sister chromatid exchanges. These results provide evidence that RECQL has a unique cellular role in the DNA repair processes required for genomic integrity. Genetic background, functional redundancy, and perhaps other factors may protect the unstressed mouse from the types of abnormalities that might be expected from the severe chromosomal aberrations detected at the cellular level.

The Feasibility of Creating a Population-Based National Twin Registry in the United States

Between 4 to 6 million twins exist in the US today who offer scientists a valuable potential resource for conducting behavioral and biomedical research. However, unlike many other countries, there is no national system in the US for identifying twins and eliciting their participation in these important research programs. Therefore, the National Institute of Environmental Health Sciences (NIEHS) is conducting a study to determine the feasibility of creating a national, population-based twin registry in the US. The major goal is to estimate the potential size and characteristics of a national twin registry based on the current twin population in the US, our ability to ascertain and enrol them, and their willingness to participate. Existing US twin cohorts are also being examined in this study as well as alternatives for improving US twin resources should a national twin registry be deemed infeasible. The various options will be compared in terms of possible source populations, generalizability and adequacy for statistically powering various types of etiological studies. Two expert advisory panels have been assembled to assist in the conduct of this study. The Scientific Advisory Panel is charged with providing expertise concerning study goals, design and methodology, and evaluating the study's conclusion. A separate Ethics Advisory Panel is charged with providing expertise on the ethical, legal, and social issues that might be encountered if a national twin registry is ultimately pursued. Having a national population-based twin registry in the US would be advantageous to US scientists and those worldwide. It would provide ample numbers of twin pairs to conduct various types of environmental genomic studies currently not possible with existing US twin resources. It would also allow US scientists to select for characteristics (race, ethnicity, environments, and so on) inherent in our own population. Finally and foremost, it would help to meet the worldwide demand for twin resources which is expected to increase over time, as new genomic and analytical tools become available and new hypotheses emerge concerning the complex interplay between genes, lifestyles and environment.

The feasibility of creating a population-based national twin registry in the United States

Between 4 to 6 million twins exist in the US today who offer scientists a valuable potential resource for conducting behavioral and biomedical research. However, unlike many other countries, there is no national system in the US for identifying twins and eliciting their participation in these important research programs. Therefore, the National Institute of Environmental Health Sciences (NIEHS) is conducting a study to determine the feasibility of creating a national, population-based twin registry in the US. The major goal is to estimate the potential size and characteristics of a national twin registry based on the current twin population in the US, our ability to ascertain and enroll them, and their willingness to participate. Existing US twin cohorts are also being examined in this study as well as alternatives for improving US twin resources should a national twin registry be deemed infeasible. The various options will be compared in terms of possible source populations, generalizability and adequacy for statistically powering various types of etiological studies. Two expert advisory panels have been assembled to assist in the conduct of this study. The Scientific Advisory Panel is charged with providing expertise concerning study goals, design and methodology, and evaluating the study's conclusion. A separate Ethics Advisory Panel is charged with providing expertise on the ethical, legal, and social issues that might be encountered if a national twin registry is ultimately pursued. Having a national population-based twin registry in the US would be advantageous to US scientists and those worldwide. It would provide ample numbers of twin pairs to conduct various types of environmental genomic studies currently not possible with existing US twin resources. It would also allow US scientists to select for characteristics (race, ethnicity, environments, and so on) inherent in our own population. Finally and foremost, it would help to meet the worldwide demand for twin resources which is expected to increase over time, as new genomic and analytical tools become available and new hypotheses emerge concerning the complex interplay between genes, lifestyles and environment.

Novel mRNA targets for tristetraprolin (TTP) identified by global analysis of stabilized transcripts in TTP-deficient fibroblasts

Tristetraprolin (TTP) is a tandem CCCH zinc finger protein that was identified through its rapid induction by mitogens in fibroblasts. Studies of TTP-deficient mice and cells derived from them showed that TTP could bind to certain AU-rich elements in mRNAs, leading to increases in the rates of mRNA deadenylation and destruction. Known physiological target mRNAs for TTP include tumor necrosis factor alpha, granulocyte-macrophage colony-stimulating factor, and interleukin-2beta. Here we used microarray analysis of RNA from wild-type and TTP-deficient fibroblast cell lines to identify transcripts with different decay rates, after serum stimulation and actinomycin D treatment. Of 250 mRNAs apparently stabilized in the absence of TTP, 23 contained two or more conserved TTP binding sites; nine of these appeared to be stabilized on Northern blots. The most dramatically affected transcript encoded the protein Ier3, recently implicated in the physiological control of blood pressure. The Ier3 transcript contained several conserved TTP binding sites that could bind TTP directly and conferred TTP sensitivity to the mRNA in cell transfection studies. These studies have identified several new, physiologically relevant TTP target transcripts in fibroblasts; these target mRNAs encode proteins from a variety of functional classes.

Transmembrane TNF protects mutant mice against intracellular bacterial infections, chronic inflammation and autoimmunity

Using targeted mutagenesis in mice, we have blocked shedding of endogenous murine TNF by deleting its cleavage site. Mutant mice produce physiologically regulated levels of transmembrane TNF (tmTNF), which suffice to support thymocyte proliferation but cannot substitute for the hepatotoxic activities of wild-type TNF following LPS/D-galactosamine challenge in vivo and are not sufficient to support secondary lymphoid organ structure and function. Notably, however, tmTNF is capable of exerting anti-Listerial host defenses while remaining inadequate to mediate arthritogenic functions, as tested in the tristetraprolin-deficient model of TNF-dependent arthritis. Most interestingly, in the EAE model of autoimmune demyelination, tmTNF suppresses disease onset and progression and retains the autoimmune suppressive properties of wild-type TNF. Together, these results indicate that tmTNF preserves a subset of the beneficial activities of TNF while lacking detrimental effects. These data support the hypothesis that selective targeting of soluble TNF may offer several advantages over complete blockade of TNF in the treatment of chronic inflammation and autoimmunity.

Identification of potential target genes for RFX4_v3, a transcription factor critical for brain development

Regulatory factor X4 variant transcript 3 (Rfx4_v3) gene disruption in mice demonstrated that interruption of a single allele (heterozygous, +/-) prevented formation of the subcommissural organ, resulting in congenital hydrocephalus, while interruption of two alleles (homozygous, -/-) caused fatal failure of dorsal midline brain structure formation. To identify potential target genes for RFX4_v3, we used microarray analysis to identify differentially expressed genes in Rfx4_v3-deficient mouse brains at embryonic day 10.5, before gross structural changes were apparent. Of 109 differentially expressed transcripts, 24 were chosen for validation and 22 were confirmed by real-time PCR. Many validated genes encoded critical proteins involved in brain morphogenesis, such as the signaling components in the Wnt, bone morphogenetic protein (BMP) and retinoic acid (RA) pathways. Cx3cl1, a CX3C-type chemokine gene that is highly expressed in brain, was down-regulated in the Rfx4_v3-null mice. Both human and mouse Cx3cl1 proximal promoters contained highly conserved X-boxes, known cis-acting elements for RFX protein binding. Using the Cx3cl1 promoter as an example of a target gene, we demonstrated direct binding of RFX4_v3 to the Cx3cl1 promoter, and trans-acting activity of RFX4_v3 protein to stimulate gene expression. These data suggest that RFX4_v3 may act upstream of critical signaling pathways in the process of brain development.

Myristoylated alanine rich C kinase substrate (MARCKS) heterozygous mutant mice exhibit deficits in hippocampal mossy fiber-CA3 long-term potentiation

The myristoylated alanine-rich C kinase substrate (MARCKS) is a primary protein kinase C (PKC) substrate in brain thought to transduce PKC signaling into alterations in the filamentous (F) actin cytoskeleton. Within the adult hippocampus, MARCKS is highly expressed in the dentate gyrus (DG)-CA3 mossy fiber pathway, but is expressed at low levels in the CA3-CA1 Schaffer collateral-CA1 pathway. We have previously demonstrated that 50% reductions in MARCKS expression in heterozygous Marcks mutant mice produce robust deficits in spatial reversal learning, but not contextual fear conditioning, suggesting that only specific aspects of hippocampal function are impaired by reduction in MARCKS expression. To further elucidate the role of MARCKS in hippocampal synaptic plasticity, in the present study we examined basal synaptic transmission, paired-pulse facilitation, post-tetanic potentiation, and long-term potentiation (LTP) in the hippocampal mossy fiber-CA3 and Schaffer collateral-CA1 pathways of heterozygous Marcks mutant and wild-type mice. We found that LTP is significantly impaired in the mossy fiber-CA3 pathway, but not in the Schaffer collateral-CA1 pathway, in heterozygous Marcks mutant mice, whereas basal synaptic transmission, paired-pulse facilitation, and post-tetanic potentiation are unaffected in both pathways. These findings indicate that a 50% reduction in MARCKS expression impairs processes required for long-term, but not short-term, synaptic plasticity in the mossy fiber-CA3 pathway. The implications of these findings for the role of the mossy fiber-CA3 pathway in hippocampus-dependent learning processes are discussed.

Mitogen-activated protein kinase-activated protein kinase 2 regulates tumor necrosis factor mRNA stability and translation mainly by altering tristetraprolin expression, stability, and binding to adenine/uridine-rich element

The mitogen-activated protein kinase (MAPK) p38/MAPK-activated protein kinase 2 (MK2) signaling pathway plays an important role in the posttranscriptional regulation of tumor necrosis factor (TNF), which is dependent on the adenine/uridine-rich element (ARE) in the 3' untranslated region of TNF mRNA. After lipopolysaccharide (LPS) stimulation, MK2-deficient macrophages show a 90% reduction in TNF production compared to the wild type. Tristetraprolin (TTP), a protein induced by LPS, binds ARE and destabilizes TNF mRNA. Accordingly, macrophages lacking TTP produce large amounts of TNF. Here, we generated MK2/TTP double knockout mice and show that, after LPS stimulation, bone marrow-derived macrophages produce TNF mRNA and protein levels comparable to those of TTP knockout cells, indicating that in the regulation of TNF biosynthesis TTP is genetically downstream of MK2. In addition, we show that MK2 is essential for the stabilization of TTP mRNA, and phosphorylation by MK2 leads to increased TTP protein stability but reduced ARE affinity. These data suggest that MK2 inhibits the mRNA destabilizing activity of TTP and, in parallel, codegradation of TTP together, with the target mRNA resulting in increased cellular levels of TTP.

Cell type-specific upregulation of myristoylated alanine-rich C kinase substrate and protein kinase C-α, -β I, -β II, and -δ in microglia following kainic acid-induced seizures

Myristoylated alanine-rich C kinase substrate (MARCKS) is a widely distributed protein kinase C (PKC) substrate and has been implicated in actin cytoskeletal rearrangement in response to extracellular stimuli. Although MARCKS was extensively examined in various cell culture systems, the physiological function of MARCKS in the central nervous system has not been clearly understood. We investigated alterations of cellular distribution and phosphorylation of MARCKS in the hippocampus following kainic acid (KA)-induced seizures. KA (25 mg/kg, i.p.) was administered to eight to nine week-old C57BL/6 mice. Behavioral seizure activity was observed for 2 h after the onset of seizures and was terminated with diazepam (8 mg/kg, i.p.). The animals were sacrificed and analyzed at various points in time after the initiation of seizure activity. Using double-labeling immunofluorescence analysis, we demonstrated that the expression and phosphorylation of MARCKS was dramatically upregulated specifically in microglial cells after KA-induced seizures, but not in other types of glial cells. PKC alpha, beta I, beta II and delta, from various PKC isoforms examined, also were markedly upregulated, specifically in microglial cells. Moreover, immunoreactivities of phosphorylated MARCKS were co-localized in the activated microglia with those of the above isoforms of PKC. Taken together, our in vivo data suggest that MARCKS is closely linked to microglial activation processes, which are important in pathological conditions, such as neuroinflammation and neurodegeneration.

Identification of the anti-inflammatory protein tristetraprolin as a hyperphosphorylated protein by mass spectrometry and site-directed mutagenesis

Tristetraprolin (TTP) is a zinc-finger protein that binds to AREs (AU-rich elements) within certain mRNAs and causes destabilization of those mRNAs. Mice deficient in TTP develop a profound inflammatory syndrome with erosive arthritis, autoimmunity and myeloid hyperplasia. Previous studies showed that TTP is phosphorylated extensively in intact cells. However, limited information is available about the identities of these phosphorylation sites. We investigated the phosphorylation sites in human TTP from transfected HEK-293 cells by MS and site-directed mutagenesis. A number of phosphorylation sites including Ser66, Ser88, Thr92, Ser169, Ser186, Ser197, Ser218, Ser228, Ser276 and Ser296 were identified by MS analyses using MALDI (matrix-assisted laser-desorption-ionization)-MS, MALDI-tandem MS, LC (liquid chromatography)-tandem MS and multidimensional protein identification technology. Mutations of Ser197, Ser218 and Ser228 to alanine in the human protein significantly increased TTP's gel mobility (likely to be stoichiometric), whereas mutations at the other sites had little effect on its gel mobility. Dephosphorylation and in vivo labelling studies showed that mutant proteins containing multiple mutations were still phosphorylated, and all were able to bind to RNA probes containing AREs. Confocal microscopy showed a similar cytosolic localization of TTP among the various proteins. Ser197, Ser218 and Ser228 are predicted by motif scanning to be potential sites for protein kinase A, glycogen synthase kinase-3 and extracellular-signal-regulated kinase 1 (both Ser218 and Ser228) respectively. The present study has identified multiple phosphorylation sites in the anti-inflammatory protein TTP in mammalian cells and should provide the molecular basis for further studies on the function and regulation of TTP in controlling pro-inflammatory cytokines.

Genetic variations in ZFP36 and their possible relationship to autoimmune diseases

The ZFP36 gene codes for TTP, a regulator of TNF alpha. In mice, TTP deficiency results in a systemic autoimmune inflammatory syndrome with severe arthritis. We hypothesized that genetic variations in ZFP36 are associated with autoimmune disease in humans. The primary objective of this study was to identify human ZFP36 genetic variants in autoimmune disease cases and controls, determine their frequencies in a general clinic population, and construct haplotypes. We resequenced ZFP36 in 316 individuals with autoimmune diseases and identified 28 polymorphisms and determined the frequency of all the known ZFP36 polymorphisms in 484 participants of the Environmental Polymorphism Registry, a regional registry being conducted by the NIEHS. Based on the sequence-verified ZFP36 genotypes, 34 haplotypes were constructed. As a secondary objective, we examined autoimmune disease cases and controls for potential ZFP36 genetic associations. One novel polymorphism, ZFP36*8, a C to T transition in the protein coding domain, was significantly associated with rheumatoid arthritis (RA) in African-Americans (RR=1.23, 95% CI: 1.11-1.36). The data presented here suggest a tentative association between ZFP36 and RA. This finding, as well as the ZFP36 polymorphisms and haplotypes identified here, should form the basis for future association studies in autoimmune diseases.

Interferons limit inflammatory responses by induction of tristetraprolin

Interferons (IFNs) are cytokines with pronounced proinflammatory properties. Here we provide evidence that IFNs also play a key role in decline of inflammation by inducing expression of tristetraprolin (Ttp). TTP is an RNA-binding protein that destabilizes several AU-rich element-containing mRNAs including TNFalpha. By promoting mRNA decay, TTP significantly contributes to cytokine homeostasis. Now we report that IFNs strongly stimulate expression of TTP if a costimulatory stress signal is provided. IFN-induced expression of Ttp depends on the IFN-activated transcription factor STAT1, and the costimulatory stress signal requires p38 MAPK. Within the Ttp promoter we have identified a functional gamma interferon-activated sequence that recruits STAT1. Consistently, STAT1 is required for full expression of Ttp in response to LPS that stimulates both p38 MAPK and, indirectly, interferon signaling. We demonstrate that in macrophages IFN-induced TTP protein limits LPS-stimulated expression of several proinflammatory genes, such as TNFalpha, IL-6, Ccl2, and Ccl3. Thus, our findings establish a link between interferon responses and TTP-mediated mRNA decay during inflammation, and propose a novel immunomodulatory role of IFNs.

Immunogenetic risk and protective factors for the idiopathic inflammatory myopathies: distinct HLA-A, -B, -Cw, -DRB1, and -DQA1 allelic profiles distinguish European American patients with different myositis autoantibodies

The idiopathic inflammatory myopathies (IIM) are systemic connective tissue diseases defined by chronic muscle inflammation and weakness associated with autoimmunity. We have performed low to high resolution molecular typing to assess the genetic variability of major histocompatibility complex loci (HLA-A, -B, -Cw, -DRB1, and -DQA1) in a large population of European American patients with IIM (n = 571) representing the major myositis autoantibody groups. We established that alleles of the 8.1 ancestral haplotype (8.1 AH) are important risk factors for the development of IIM in patients producing anti-synthetase/anti-Jo-1, -La, -PM/Scl, and -Ro autoantibodies. Moreover, a random forests classification analysis suggested that 8.1 AH-associated alleles B*0801 and DRB1*0301 are the principal HLA risk markers. In addition, we have identified several novel HLA susceptibility factors associated distinctively with particular myositis-specific (MSA) and myositis-associated autoantibody (MAA) groups of the IIM. IIM patients with anti-PL-7 (anti-threonyl-tRNA synthetase) autoantibodies have a unique HLA Class I risk allele, Cw*0304 (pcorr = 0.046), and lack the 8.1 AH markers associated with other anti-synthetase autoantibodies (for example, anti-Jo-1 and anti-PL-12). In addition, HLA-B*5001 and DQA1*0104 are novel potential risk factors among anti-signal recognition particle autoantibody-positive IIM patients (pcorr = 0.024 and p = 0.010, respectively). Among those patients with MAA, HLA DRB1*11 and DQA1*06 alleles were identified as risk factors for myositis patients with anti-Ku (pcorr = 0.041) and anti-La (pcorr = 0.023) autoantibodies, respectively. Amino acid sequence analysis of the HLA DRB1 third hypervariable region identified a consensus motif, 70D (hydrophilic)/71R (basic)/74A (hydrophobic), conferring protection among patients producing anti-synthetase/anti-Jo-1 and -PM/Scl autoantibodies. Together, these data demonstrate that HLA signatures, comprising both risk and protective alleles or motifs, distinguish IIM patients with different myositis autoantibodies and may have diagnostic and pathogenic implications. Variations in associated polymorphisms for these immune response genes may reflect divergent pathogenic mechanisms and/or responses to unique environmental triggers in different groups of subjects resulting in the heterogeneous syndromes of the IIM.

HuR as a negative posttranscriptional modulator in inflammation

HuR is an RNA binding protein with an alleged role in the posttranscriptional activation of inflammatory mRNAs bearing AU-rich elements (AREs). Here, we show that the inducible increase of HuR in murine innate compartments suppresses inflammatory responses in vivo. In macrophages, HuR overexpression induced the translational silencing of specific cytokine mRNAs despite positive or nominal effects on their corresponding turnover. By using a model system of ARE dysfunction, we demonstrate that HuR does not alter the accumulation of target mRNAs in the absence of the destabilizing functions of Tristetraprolin but synergizes with the translational silencer TIA-1 to reduce the translation of cytokine mRNAs. Our data suggest that HuR acts in a pleiotropic fashion in inflammation through its functional interactions with specific mRNA subsets and negative posttranscriptional modules.

Effect of myristoylated alanine-rich C kinase substrate (MARCKS) overexpression on hippocampus-dependent learning and hippocampal synaptic plasticity in MARCKS transgenic mice

The myristoylated alanine-rich C kinase substrate (MARCKS) is a primary substrate of protein kinase C (PKC) thought to regulate membrane-filamentous actin cytoskeletal plasticity in response to PKC activity in the regulation of synaptic efficacy. We have recently reported that MARCKS expression is significantly elevated (45%) in the hippocampus of DBA/2J mice, which exhibit impaired hippocampus-dependent learning and hippocampal long-term potentiation (LTP), compared with C57BL/6J mice. The latter finding led us to hypothesize that elevations in MARCKS expression are detrimental to hippocampal plasticity and function. To assess this more directly, we examined hippocampal (CA1) paired-pulse facilitation and LTP, and hippocampus-dependent learning in mice overexpressing MARCKS through the expression of a human MARCKS transgene (Tg+). The human MARCKS protein was confirmed to be expressed in the hippocampus of Tg+ mice but not in Tg- mice. Schaffer collateral paired-pulse facilitation, input-output responses, and LTP did not differ between Tg+ and Tg- mice, indicating that neurotransmitter release, short-term, and long-term synaptic plasticity are not impaired by MARCKS overexpression. In the Morris water maze, Tg+ mice exhibited a mild but significant spatial learning impairment during initial acquisition, and a more severe impairment during reversal training. Tg+ did not exhibit impaired swim speed or visible platform performance relative to Tg- mice, indicating the absence of gross sensorimotor deficits. Fear conditioning to either context or cue was not impaired in Tg+ mice. Behavioral deficits could not be attributed to differences in hippocampal PKC isozyme (alpha beta(II), gamma, epsilon, zeta) or calmodulin expression, or alterations in hippocampal cytoarchitecture or infrapyramidal mossy fiber limb length. Collectively, these results indicate that elevations in MARCKS expression are detrimental to specific aspects of hippocampal function.

Zfp36l3, a Rodent X Chromosome Gene Encoding a Placenta-Specific Member of the Tristetraprolin Family of CCCH Tandem Zinc Finger Proteins

Members of the tristetraprolin (TTP) family of CCCH tandem zinc finger (TZF) proteins can bind directly to AU-rich elements (ARE) in mRNA, causing deadenylation and destabilization of the transcripts to which they bind. We describe here a novel fourth mammalian member of the TTP protein family, designated ZFP36L3, which could also bind directly to ARE-containing RNAs and could promote the deadenylation and degradation of ARE-containing target RNAs. Zfp36l3 transcript expression was detected only in placenta and extraembryonic tissues in the mouse. It was expressed throughout development in the placenta and was particularly highly expressed in the cells of the labyrinthine layer of the trophoblastic placenta. Unlike the other family members, the expression of a ZFP36L3-green fluorescent protein fusion protein was entirely cytoplasmic when expressed in 293 cells, even in the presence of the CRM1-dependent nuclear export inhibitor leptomycin B. Zfp36l3 was located on the mouse X chromosome; a similar predicted gene was present on the rat X chromosome, but there was no evidence for a similar gene in humans. ZFP36L3 may thus be a rodent-specific or even murine-specific member of the TTP protein family. Its presumed role in placental physiology may be unique to rodents or murine rodents, but this role may be subsumed by other family members in nonrodents.

Influence of nonameric AU-rich tristetraprolin-binding sites on mRNA deadenylation and turnover

Tristetraprolin (TTP), a member of the tandem CCCH zinc finger protein family, promotes deadenylation of tumor necrosis factor-alpha and granulocyte-macrophage colony-stimulating factor mRNAs after binding to the AU-rich elements (ARE) in their 3'-untranslated regions. The high affinity TTP-ARE binding occurs between the tandem zinc finger domain and the preferred nonamer UUAUUUAUU. By mutating a well defined core sequence of 24 bases from the tumor necrosis factor-alpha ARE, we compared the influence of four possible nonameric TTP-binding sites in the wild-type ARE with that of a single binding site in the mutated probe on the binding of TTP to the RNA and the subsequent deadenylation of the poly(A) tail. By inserting this 24-base ARE into an otherwise stable transcript, we also attempted to determine the extent of the instability conferred by the presence of one or two TTP-binding sites. These sites were created or modified by mutating the As in the UUAUUUAUU nonamer or by changing the central U in the nonamer, in both cases to C residues. The results suggest that even a single nonamer TTP-binding site can confer at least partial sensitivity to the TTP-mediated mRNA turnover on an otherwise stable mRNA, but that two binding sites make the transcript much more unstable. Even though the central U of the nonamer binding site was predicted by structural studies possibly to permit base substitution, mutation of this U to C greatly inhibited the binding of TTP to the ARE, thus reducing the ability of the TTP to promote deadenylation and instability of the mRNA.

Myristoylated Alanine-rich C Kinase Substrate-mediated Neurotensin Release via Protein Kinase C-δ Downstream of the Rho/ROK Pathway

Myristoylated alanine-rich protein kinase C substrate (MARCKS) is a cellular substrate for protein kinase C (PKC). Recently, we have shown that PKC isoforms-alpha and -delta, as well as the Rho/Rho kinase (ROK) pathway, play a role in phorbol 12-myristate 13-acetate (PMA)-mediated secretion of the gut peptide neurotensin (NT) in the BON human endocrine cell line. Here, we demonstrate that activation of MARCKS protein is important for PMA- and bombesin (BBS)-mediated NT secretion in BON cells. Small interfering RNA (siRNA) to MARCKS significantly inhibited, whereas overexpression of wild-type MARCKS significantly increased PMA-mediated NT secretion. Endogenous MARCKS and green fluorescent protein-tagged wild-type MARCKS were translocated from membrane to cytosol upon PMA treatment, further confirming MARCKS activation. MARCKS phosphorylation was inhibited by PKC-delta siRNA, ROKalpha siRNA, and C3 toxin (a Rho protein inhibitor), suggesting that the PKC-delta and the Rho/ROK pathways are necessary for MARCKS activation. The phosphorylation of PKC-delta was inhibited by C3 toxin, demonstrating that the role of MARCKS in NT secretion was regulated by PKC-delta downstream of the Rho/ROK pathway. BON cell clones stably transfected with the receptor for gastrin releasing peptide, a physiologic stimulant of NT, and treated with BBS, the amphibian equivalent of gastrin releasing peptide, demonstrated a similar MARCKS phosphorylation as noted with PMA. BBS-mediated NT secretion was attenuated by MARCKS siRNA. Collectively, these findings provide evidence for novel signaling pathways, including the sequential regulation of MARCKS activity by Rho/ROK and PKC-delta proteins, in stimulated gut peptide secretion.

Tristetraprolin Down-RegulatesIL-2Gene Expression through AU-Rich Element-Mediated mRNA Decay

Posttranscriptional regulation of IL-2 gene expression at the level of mRNA decay is mediated by an AU-rich element (ARE) found in the 3'-untranslated region. We hypothesized that the ARE-binding protein tristetraprolin (TTP) regulates T lymphocyte IL-2 mRNA decay by interacting with the IL-2 ARE and targeting the transcript for decay. rTTP protein expressed in HeLa cells bound specifically to the IL-2 ARE with high affinity in a gel shift assay. In primary human T lymphocytes, TTP mRNA and protein expression were induced by TCR and CD28 coreceptor stimulation. Using a gel shift assay, we identified a cytoplasmic RNA-binding activity that was induced by TCR and CD28 coreceptor stimulation and bound specifically to the IL-2 ARE sequence. Using anti-TTP Abs, we showed by supershift that this inducible activity contained TTP. We also showed that insertion of the IL-2 ARE sequence into the 3'-untranslated region of a beta-globin reporter construct conferred TTP-dependent mRNA destabilization on the beta-globin reporter. To determine whether TTP also regulates IL-2 gene expression in vivo, we examined IL-2 expression in primary cells from wild-type and TTP knockout mice. Compared with their wild-type counterparts, TCR- and CD28-activated splenocytes and T cells from TTP knockout mice overexpressed IL-2 mRNA and protein. Also, IL-2 mRNA was more stable in activated splenocytes from TTP knockout mice compared with wild-type mice. Taken together, these data suggest that TTP functions to down-regulate IL-2 gene expression through ARE-mediated mRNA decay.

The tandem CCCH zinc finger protein tristetraprolin and its relevance to cytokine mRNA turnover and arthritis

Tristetraprolin (TTP) is the best-studied member of a small family of three proteins in humans that is characterized by a tandem CCCH zinc finger (TZF) domain with highly conserved sequences and spacing. Although initially discovered as a gene that could be induced rapidly and transiently by the stimulation of fibroblasts with growth factors and mitogens, it is now known that TTP can bind to AU-rich elements in mRNA, leading to the removal of the poly(A) tail from that mRNA and increased rates of mRNA turnover. This activity was discovered after TTP-deficient mice were created and found to have a systemic inflammatory syndrome with severe polyarticular arthritis and autoimmunity, as well as medullary and extramedullary myeloid hyperplasia. The syndrome seemed to be due predominantly to excess circulating tumor necrosis factor-α (TNF-α), resulting from the increased stability of the TNF-α mRNA and subsequent higher rates of secretion of the cytokine. The myeloid hyperplasia might be due in part to increased stability of granulocyte–macrophage colony-stimulating factor (GM-CSF). This review highlights briefly the characteristics of the TTP-deficiency syndrome in mice and its possible genetic modifiers, as well as recent data on the characteristics of the TTP-binding site in the TNF-α and GM-CSF mRNAs. Recent structural data on the characteristics of the complex between RNA and one of the TTP-related proteins are reviewed, and used to model the TTP-RNA binding complex. We review the current knowledge of TTP sequence variants in humans and discuss the possible contributions of the TTP-related proteins in mouse physiology and in human monocytes. The TTP pathway of TNF-α and GM-CSF mRNA degradation is a possible novel target for anti-TNF-α therapies for rheumatoid arthritis, and also for other conditions proven to respond to anti-TNF-α therapy.

The CCCH tandem zinc-finger protein Zfp36l2 is crucial for female fertility and early embryonic development

The CCCH tandem zinc finger protein, Zfp36l2, like its better-known relative tristetraprolin (TTP), can decrease the stability of AU-rich element-containing transcripts in cell transfection studies; however, its physiological importance is unknown. We disrupted Zfp36l2 in mice,resulting in decreased expression of a truncated protein in which the N-terminal 29 amino acids had been deleted (ΔN-Zfp36l2). Mice derived from different clones of ES cells exhibited complete female infertility,despite evidence from embryo and ovary transplantation experiments that they could gestate and rear wild-type young. ΔN-Zfp36l2 females apparently cycled and ovulated normally, and their ova could be fertilized; however, the embryos did not progress beyond the two-cell stage of development. These mice represent a specific model of disruption of the earliest stages of embryogenesis, implicating Zfp36l2, a probable mRNA-binding and destabilizing protein, in the physiological control of female fertility at the level of early embryonic development. This newly identified biological role for Zfp36l2 may have implications for maternal mRNA turnover in normal embryogenesis, and conceivably could be involved in some cases of unexplained human female infertility.

Chorioallantoic fusion defects and embryonic lethality resulting from disruption of Zfp36L1, a gene encoding a CCCH tandem zinc finger protein of the Tristetraprolin family

The mouse gene Zfp36L1 encodes zinc finger protein 36-like 1 (Zfp36L1), a member of the tristetraprolin (TTP) family of tandem CCCH finger proteins. TTP can bind to AU-rich elements within the 3'-untranslated regions of the mRNAs encoding tumor necrosis factor (TNF) and granulocyte-macrophage colony-stimulating factor (GM-CSF), leading to accelerated mRNA degradation. TTP knockout mice exhibit an inflammatory phenotype that is largely due to increased TNF secretion. Zfp36L1 has activities similar to those of TTP in cellular RNA destabilization assays and in cell-free RNA binding and deadenylation assays, suggesting that it may play roles similar to those of TTP in mammalian physiology. To address this question we disrupted Zfp36L1 in mice. All knockout embryos died in utero, most by approximately embryonic day 11 (E11). Failure of chorioallantoic fusion occurred in about two-thirds of cases. Even when fusion occurred, by E10.5 the affected placentas exhibited decreased cell division and relative atrophy of the trophoblast layers. Although knockout embryos exhibited neural tube abnormalities and increased apoptosis within the neural tube and also generalized runting, these and other findings may have been due to deficient placental function. Embryonic expression of Zfp36L1 at E8.0 was greatest in the allantois, consistent with a potential role in chorioallantoic fusion. Fibroblasts derived from knockout embryos had apparently normal levels of fully polyadenylated compared to deadenylated GM-CSF mRNA and normal rates of turnover of this mRNA species, both sensitive markers of TTP deficiency in cells. We postulate that lack of Zfp36L1 expression during mid-gestation results in the abnormal stabilization of one or more mRNAs whose encoded proteins lead directly or indirectly to abnormal placentation and fetal death.

RNA sequence elements required for high affinity binding by the zinc finger domain of tristetraprolin: conformational changes coupled to the bipartite nature of Au-rich MRNA-destabilizing motifs

Tristetraprolin (TTP) binds AU-rich elements (AREs) encoded within selected labile mRNAs and targets these transcripts for rapid cytoplasmic decay. RNA binding by TTP is mediated by an approximately 70-amino acid domain containing two tandemly arrayed CCCH zinc fingers. Here we show that a 73-amino acid peptide spanning the TTP zinc finger domain, denoted TTP73, forms a dynamic, equimolar RNA.peptide complex with a 13-nucleotide fragment of the ARE from tumor necrosis factor alpha mRNA, which includes small but significant contributions from ionic interactions. Association of TTP73 with high affinity RNA substrates is accompanied by a large negative change in heat capacity without substantial modification of RNA structure, consistent with conformational changes in the peptide moiety during RNA binding. Analyses using mutant ARE substrates indicate that two adenylate residues located 3-6 bases apart within a uridylate-rich sequence are sufficient for high affinity recognition by TTP73 (K(d) <20 nm), with optimal affinity observed for RNA substrates containing AUUUA or AUUUUA. Linkage of conformational changes and binding affinity to the presence and spacing of these adenylate residues provides a thermodynamic basis for the RNA substrate specificity of TTP.

Immunological characterization of tristetraprolin as a low abundance, inducible, stable cytosolic protein

Tristetraprolin (TTP) is a zinc finger protein that can bind to AU-rich elements within certain mRNAs, resulting in deadenylation and destabilization of those mRNAs. Its physiological targets include the mRNAs encoding the cytokines tumor necrosis factor alpha (TNF) and granulocyte-macrophage colony-stimulating factor. TTP was originally identified on the basis of its massive but transient increase in mRNA levels following mitogen stimulation of fibroblasts. It has been difficult to reconcile this transient mRNA profile with the presumed continuing "need" for TTP protein, for example, to reverse the effects of lipopolysaccharide (LPS)-stimulated TNF secretion. To investigate this and other questions concerning endogenous TTP protein in cells and tissues, we raised a high titer rabbit antiserum against full-length mouse TTP. TTP could be detected on immunoblots of mouse cytosolic tissue extracts; it was most highly expressed in spleen, but its concentration in that tissue was only about 1.5 nm. TTP could be detected readily in splenic macrophages and stromal cells from LPS-injected rats. In both LPS-treated RAW 264.7 macrophages and fetal calf serum-treated mouse embryonic fibroblasts, TTP protein was stable after induction, with minimal degradation occurring for several hours after treatment of the cells with cycloheximide. The biosynthesis of TTP was accompanied by large changes in electrophoretic mobility consistent with progressive phosphorylation. Confocal microscopy revealed that TTP accumulated in a vesicular pattern in the cytosol of the LPS-stimulated RAW 264.7 cells, and was occasionally seen in the cytosol of unstimulated dividing cells. Gel filtration of the endogenous protein suggested that its predominant structure was monomeric. TTP appears to be a low abundance, cytosolic protein in unstimulated cells and tissues, but once induced is relatively stable, in contrast to its very labile mRNA.

Arthritis suppressor genes TIA-1 and TTP dampen the expression of tumor necrosis factor alpha, cyclooxygenase 2, and inflammatory arthritis

TIA-1 and TTP are AU-rich element-binding proteins that prevent the pathological overexpression of tumor necrosis factor alpha (TNF-alpha). TIA-1 inhibits the translation of TNF-alpha transcripts, whereas TTP promotes the degradation of TNF-alpha transcripts. Here we show that TIA-1 and TTP function as arthritis suppressor genes: TIA-1(-/-) mice develop mild arthritis, TTP(-/-) mice develop severe arthritis, and TIA-1(-/-)TTP(-/-) mice develop very severe arthritis. Peritoneal macrophages derived from all three genotypes overexpress cyclooxygenase 2 and TNF-alpha. Surprisingly, lipopolysaccharide-activated TIA-1(-/-)TTP(-/-) macrophages secrete less TNF-alpha protein than either TIA-1(-/-) or TTP(-/-) macrophages. In these mice, arthritogenic cytokine may be produced by neutrophils that accumulate in the bone marrow and peripheral blood. Our results suggest that TIA-1 and TTP are genetic modifiers of inflammatory arthritis that can alter the spectrum of cells that produce arthritogenic cytokines.

Interaction of retroviral Tax oncoproteins with tristetraprolin and regulation of tumor necrosis factor-alpha expression

BACKGROUND

The Tax oncoproteins are transcriptional regulators of viral expression involved in pathogenesis induced by complex leukemogenic retroviruses (or delta-retroviruses, i.e., primate T-cell leukemia viruses and bovine leukemia virus). To better understand the molecular pathways leading to cell transformation, we aimed to identify cellular proteins interacting with Tax.

METHODS

We used a yeast two-hybrid system to identify interacting cellular proteins. Interactions between Tax and candidate interacting cellular proteins were confirmed by glutathione S-transferase (GST) pulldown assays, co-immunoprecipitation, and confocal microscopy. Functional interactions between Tax and one interacting protein, tristetraprolin (TTP), were assessed by analyzing the expression of tumor necrosis factor-alpha (TNF-alpha), which is regulated by TTP, in mammalian cells (HeLa, D17, HEK 293, and RAW 264.7) transiently transfected with combinations of intact and mutant Tax and TTP.

RESULTS

We obtained seven interacting cellular proteins, of which one, TTP, was further characterized. Tax and TTP were found to interact specifically through their respective carboxyl-terminal domains. The proteins colocalized in the cytoplasm in a region surrounding the nucleus of HeLa cells. Furthermore, coexpression of Tax was associated with nuclear accumulation of TTP. TTP is an immediate-early protein that inhibits expression of TNF-alpha at the post-transcriptional level. Expression of Tax reverted this inhibition, both in transient transfection experiments and in stably transfected macrophage cell lines.

CONCLUSION

Tax, through its interactions with the TTP repressor, indirectly increases TNF-alpha expression. This observation is of importance for the cell transformation process induced by leukemogenic retroviruses, because TNF-alpha overexpression plays a central role in pathogenesis.

Graded phenotypic response to partial and complete deficiency of a brain-specific transcript variant of the winged helix transcription factor RFX4

One line of mice harboring a cardiac-specific epoxygenase transgene developed head swelling and rapid neurological decline in young adulthood, and had marked hydrocephalus of the lateral and third ventricles. The transgene was found to be inserted into an intron in the mouse Rfx4 locus. This insertion apparently prevented expression of a novel variant transcript of RFX4 (RFX4_v3), a member of the regulatory factor X family of winged helix transcription factors. Interruption of two alleles resulted in profound failure of dorsal midline brain structure formation and perinatal death, presumably by interfering with expression of downstream genes. Interruption of a single allele prevented formation of the subcommissural organ, a structure important for cerebrospinal fluid flow through the aqueduct of Sylvius, and resulted in congenital hydrocephalus. These data implicate the RFX4_v3 variant transcript as being crucial for early brain development, as well as for the genesis of the subcommissural organ. These findings may be relevant to human congenital hydrocephalus, a birth defect that affects approximately 0.6 per 1000 newborns.

Neuroanatomical development in the absence of PKC phosphorylation of the myristoylated alanine-rich C-kinase substrate (MARCKS) protein

The myristoylated alanine-rich C-kinase substrate protein (MARCKS) is a widely expressed target of protein kinase C (PKC) phosphorylation. Disruption of Marcks in mice leads to a number of developmental defects within the central nervous system that are completely prevented by expression of an epitope-tagged wild-type human MARCKS transgene. In the present study, we investigated whether PKC phosphorylation of MARCKS is necessary for normal central nervous system development and postnatal survival. Expression at approximately twice normal levels of a mutant MARCKS protein in which the four PKC phosphorylatable serines were replaced by asparagines did not allow postnatal survival of Marcks(-/-) pups. Nonetheless, the rescued animals exhibited none of the characteristic anatomical defects seen in the brains and retinas of knockout mice, suggesting that PKC phosphorylation of MARCKS is not required for normal central nervous system development. Expression studies showed that transgene expression was limited to the central nervous system, which has implications for the lack of postnatal survival as well as for the pathogenesis of the neuronal ectopia characteristic of MARCKS deficiency. A novel aspect of the MARCKS-deficient phenotype was also noted, absence of the pontine nuclei; this was also largely reversed in Marcks(-/-) animals expressing the mutant transgene. These data raise the possibility of a role for MARCKS in the netrin-regulated process of pontine nuclei formation.

Tristetraprolin and its family members can promote the cell-free deadenylation of AU-rich element-containing mRNAs by poly(A) ribonuclease

Eukaryotic mRNA stability can be influenced by AU-rich elements (AREs) within mRNA primary sequences. Tristetraprolin (TTP) is a CCCH tandem zinc finger protein that binds to ARE-containing transcripts and destabilizes them, apparently by first promoting the removal of their poly(A) tails. We developed a cell-free system in which TTP and its related proteins stimulated the deadenylation of ARE-containing, polyadenylated transcripts. Transcript deadenylation was not stimulated when a mutant TTP protein was used that was incapable of RNA binding, nor when a mutant ARE was present that did not bind TTP. The ability of TTP to promote transcript deadenylation required Mg(2+), but not ATP or prior capping of the RNA substrate. Cotransfection and additivity studies with the poly(A) RNase (PARN) demonstrated that TTP promoted the ability of this enzyme to deadenylate ARE-containing, polyadenylated transcripts, while having no effect on transcripts lacking an ARE. There was no effect of TTP to act synergistically with enzymatically inactive PARN mutants. We conclude that TTP can promote the deadenylation of ARE-containing, polyadenylated substrates by PARN. This interaction may be responsible for the ability of TTP and its family members to promote the deadenylation of such transcripts in intact cells.

Characteristics of the interaction of a synthetic human tristetraprolin tandem zinc finger peptide with AU-rich element-containing RNA substrates

Tristetraprolin (TTP) and its two known mammalian family members are tandem CCCH zinc finger proteins that can bind to AU-rich elements (AREs) in cellular mRNAs and destabilize those transcripts, apparently by initiating their deadenylation. Previous studies have shown that the approximately 70-amino acid tandem zinc finger domain of TTP is required and sufficient for RNA binding, and that the integrity of both zinc fingers is also required. However, little is known about the kinetics or structure of the peptide-RNA interaction, in part because of difficulties in obtaining soluble recombinant protein or peptides. We characterized the binding of a synthetic 73-amino acid peptide from human TTP to the tumor necrosis factor (TNF) ARE by gel mobility shift analyses and fluorescence anisotropy experiments. Both types of studies yielded a peptide-RNA dissociation constant of approximately 10 nM. Surprisingly, we found that the "footprint" from the TNF ARE required for peptide binding was only approximately 9 bases and that two molecules of peptide could bind to probes containing as little as 19 bases. An identical recombinant peptide exhibited gel shift characteristics similar to those of the synthetic peptide. NMR analysis of the 15N-labeled recombinant peptide suggested that its first zinc finger was structured in solution but that the second was not. The titration of oligonucleotides representing 17, 13, and even 9 bases of the TNF ARE caused an essentially identical, dramatic shift of existing resonances, and the appearance of new resonances in the peptide spectra, so that all amino acids could be assigned. These data suggest that this TTP peptide-RNA complex is structured in solution and might be amenable to NMR structure determination.

Expression and purification of recombinant tristetraprolin that can bind to tumor necrosis factor-alpha mRNA and serve as a substrate for mitogen-activated protein kinases

Tristetraprolin (TTP) is an mRNA-binding protein, but studies of this interaction have been difficult due to problems with the purification of recombinant TTP. In the present study, we expressed human and mouse TTP as glutathione S-transferase and maltose-binding protein (MBP) fusion proteins in Escherichia coli, and purified them by affinity resins and Mono Q chromatography. TTP cleaved from the fusion protein was identified by immunoblotting, MALDI-MS, and protein sequencing, and was further purified to homogeneity by continuous-elution SDS-gel electrophoresis. Purified recombinant TTP bound to the AU-rich element of tumor necrosis factor-alpha (TNFalpha) mRNA and this binding was dependent on Zn(2+). Results from sizing columns suggested that the active species might be in the form of an oligomer of MBP-TTP. Recombinant TTP was phosphorylated by three members of the mitogen-activated protein (MAP) kinase family, p42, p38, and JNK, with half-maximal phosphorylation occurring at approximately 0.5, 0.25, and 0.25 microM protein, respectively. Phosphorylation by these kinases did not appear to affect the ability of TTP to bind to TNFalpha mRNA under the assay conditions. This study describes a procedure for purifying nonfusion protein TTP to homogeneity, demonstrates that TTP's RNA-binding activity is zinc dependent, and that TTP can be phosphorylated by JNK as well as by the other members of the greater MAP kinase family.

Polymorphisms in the genes encoding members of the tristetraprolin family of human tandem CCCH zinc finger proteins

The three known mammalian CCCH tandem zinc finger proteins of the tristetraprolin (TTP) class have recently been demonstrated to be mRNA-binding proteins. The prototype, TTP, functions in normal physiology to promote the instability of the tumor necrosis factor alpha (TNFalpha) and granulocyte-macrophage colony-stimulating factor mRNAs. Conversely, these mRNAs are stabilized in TTP-deficient mice, leading to an inflammatory phenotype characterized by overproduction of these cytokines. To explore sequence variations in TTP and its two related proteins, we sequenced genomic DNA encoding the TTP protein (ZFP36) and those of its two known mammalian relatives, ZFP36L1 and ZFP36L2, from 72 to 92 anonymous human subjects from various geographical and ethnic backgrounds. We also sequenced ZFP36 in genomic DNA from 92 subjects exhibiting evidence of excessive TNFalpha action. The resequencing strategy identified 13 polymorphisms in the protein-coding regions of these three genes, of which six would result in amino acid changes; other putative polymorphisms were identified by EST searches. One mutation in ZFP36L1 was a dinucleotide substitution that would prevent splicing of the single intron. This mutation was identified in only one allele of the original 144 sequenced from an adult female Aka Pygmy from the Central African Republic; a second individual with the same variant allele was found by genotyping 58 additional Aka DNA samples. Analysis of mRNA from one of these subject's lymphoblasts confirmed that ZFP36L1 mRNA levels were approximately 50% of those in a comparable sample without the mutation. The functional significance of this and the other polymorphisms identified remains to be determined by both biochemical and population linkage studies.

Tristetraprolin and other CCCH tandem zinc-finger proteins in the regulation of mRNA turnover

The tristetraprolin (TTP) family of CCCH tandem zinc-finger proteins is composed of three known members in mammals, with a fourth member recently identified in frogs and fish. Although TTP was first cloned more than 10 years ago as a growth factor-induced gene, a physiological function for the protein has been discovered only within the last few years. TTP is now known to bind to so-called class II AU-rich elements within the mRNAs that encode tumour necrosis factor-alpha and granulocyte/macrophage colony-stimulating factor. In both cases, this binding results in destabilization of the mRNA and decreased secretion of the protein. Recent evidence suggests that TTP can accomplish this accelerated mRNA degradation by first promoting removal of the polyadenylated tail from the mRNA (deadenylation). In functional assays in cells, the other family members have similar activities, but are expressed differently in tissues and in response to stimuli, suggesting that they may control the stability of mRNAs under different circumstances from those in which TTP affects mRNA. All of these proteins are phosphoproteins and nucleocytoplasmic shuttling proteins, suggesting that their activities can be regulated in ways other than regulating gene transcription. Together, the TTP family members should be capable of complex regulation of short-lived mRNAs containing this type of AU-rich instability motif.

Load-dependent and -independent regulation of proinflammatory cytokine and cytokine receptor gene expression in the adult mammalian heart

BACKGROUND

Although previous studies have examined the effects of acute hemodynamic pressure overload on proinflammatory cytokine gene expression, the effects of sustained hemodynamic overloading have not been examined.

METHODS AND RESULTS

Sustained hemodynamic pressure overloading was produced in mice by transverse constriction of the aorta. Proinflammatory cytokine and cytokine receptor gene expression were determined by ribonuclease protection assays (RPA) at 6 hours and at 3, 7, 14 and 35 days after banding. M-mode echocardiography was used to assess left ventricular structure and function at identical time points. RPA showed that tumor necrosis factor (TNF), interleukin (IL)-1beta, and IL-6 mRNA levels were maximal at 6 hours and returned to baseline levels within 72 hours. There was a significant increase in IL-1RII and IL-6Ralpha receptor mRNA levels after overloading but no significant increase in TNFR1, TNFR2, IL-1RI, or gp130 mRNA levels. The transient increase in expression of proinflammatory cytokine gene expression was not explained by changes in left ventricular loading conditions, left ventricular wall stress, desensitization of proinflammatory genes, or decreased nuclear factor-kappaB activation. It is interesting that transverse constriction of the aorta provoked an increase in the expression of tristetraprolin, a homeostatic zinc finger protein that is known to destabilize TNF mRNA.

CONCLUSION

Sustained hemodynamic overloading provokes a transient increase in proinflammatory cytokine and cytokine receptor gene expression; however, the decrease in proinflammatory cytokine gene expression occurred in the absence of changes in loading conditions, suggesting that the expression of proinflammatory cytokines in the heart is regulated, at least in part, by load-dependent and load-independent mechanisms.

Members of the tristetraprolin family of tandem CCCH zinc finger proteins exhibit CRM1-dependent nucleocytoplasmic shuttling

Members of the tristetraprolin (TTP) family of CCCH tandem zinc finger proteins can bind directly to certain types of AU-rich elements (AREs) in mRNA. Experiments in TTP-deficient mice have shown that TTP is involved in the physiological destabilization of at least two cytokine mRNAs, those encoding tumor necrosis factor alpha and granulocyte-macrophage colony-stimulating factor. The two other known mammalian members of the TTP family, CMG1 and TIS11D, also contain ARE-binding CCCH tandem zinc finger domains and can also destabilize ARE-containing mRNAs. To investigate the effects of primary sequence on the subcellular localization of these proteins, we constructed green fluorescent protein fusions with TTP, CMG1, and TIS11D; these were predominantly cytoplasmic when expressed in 293 or HeLa cells. Deletion and mutation analyses revealed functional nuclear export signals in the amino terminus of TTP and in the carboxyl termini of CMG1 and TIS11D. This type of leucine-rich nuclear export signal interacts with the nuclear export receptor CRM1; abrogation of CRM1 activity resulted in nuclear accumulation of TTP, CMG1, and TIS11D. These proteins are thus nucleocytoplasmic shuttling proteins and rely on CRM1 for their export from the nucleus. Although TTP, CMG1, and TIS11D lack known nuclear import sequences, mapping experiments revealed that their nuclear accumulation required an intact tandem zinc finger domain but did not require RNA binding ability. These findings suggest possible roles for nuclear import and export in the regulation of cellular TTP, CMG1, and TIS11D activity.

Analysis of the role of RecQ helicases in RNAi in mammals

The identity of mammalian genes involved in RNA interference (RNAi), the targeted sequence-specific mRNA degradation by double-stranded RNA (dsRNA), is poorly defined. Here we report the analysis of mice with null mutations of Wrn, Blm, and RecQ1 genes that are related to Mut-7 and Qde3, two genes essential for RNAi in Caenorhabditis elegans and quelling in Neurospora, respectively. Our results suggest that Wrn, Blm, and RecQ1 are not involved in sequence-specific mRNA degradation in mammals in response to dsRNA, suggesting potential differences in the mammalian RNAi pathway.