Fernando Macián

Partners in transcription: NFAT and AP-1.

Combinatorial regulation is a powerful mechanism that enables tight control of gene expression, via integration of multiple signaling pathways that induce different transcription factors required for enhanceosome assembly. The four calcium-regulated transcription factors of the NFAT family act synergistically with AP-1 (Fos/Jun) proteins on composite DNA elements which contain adjacent NFAT and AP-1 binding sites, where they form highly stable ternary complexes to regulate the expression of diverse inducible genes. Concomitant induction of NFAT and AP-1 requires concerted activation of two different signaling pathways: calcium/calcineurin, which promotes NFAT dephosphorylation, nuclear translocation and activation; and protein kinase C (PKC)/Ras, which promotes the synthesis, phosphorylation and activation of members of the Fos and Jun families of transcription factors. A fifth member of the NFAT family, NFAT5, controls the cellular response to osmotic stress, by a mechanism that requires dimer formation and is independent of calcineurin or of interactionwith AP-1. Pharmacological interference with theNFAT:AP-1 interaction may be useful in selective manipulation of the immune response. Balanced activation of NFAT and AP-1 is known to be required for productive immune responses, but the role of NFAT:AP-1 interactions in other cell types and biological processes remains to be understood.

Transcriptional Mechanisms Underlying Lymphocyte Tolerance

In lymphocytes, integration of Ca2+ and other signaling pathways results in productive activation, while unopposed Ca2+ signaling leads to tolerance or anergy. We show that the Ca2+-regulated transcription factor NFAT has an integral role in both aspects of lymphocyte function. Ca2+/calcineurin signaling induces a limited set of anergy-associated genes, distinct from genes induced in the productive immune response; these genes are upregulated in vivo in tolerant T cells and are largely NFAT dependent. T cells lacking NFAT1 are resistant to anergy induction; conversely, NFAT1 induces T cell anergy if prevented from interacting with its transcriptional partner AP-1 (Fos/Jun). Thus, in the absence of AP-1, NFAT imposes a genetic program of lymphocyte anergy that counters the program of productive activation mediated by the cooperative NFAT:AP-1 complex.

T(H) cell differentiation is accompanied by dynamic changes in histone acetylation of cytokine genes.

Naïve T cells differentiate into effector cells upon stimulation with antigen, a process that is accompanied by changes in the chromatin structure of effector cytokine genes. Using histone acetylation to evaluate these changes, we showed that T cell receptor (TCR) stimulation results in early activation of the genes encoding both interleukin 4 and interferon-γ. We found that continued culture in the presence of polarizing cytokines established a selective pattern of histone acetylation on both cytokine genes; this correlated with restricted access of the transcription factor NFAT1 to these gene regulatory regions as well as mutually exclusive gene expression by the differentiated T cells. Our data point to a biphasic process in which cytokine-driven signaling pathways maintain and reinforce chromatin structural changes initiated by the TCR. This process ensures that cytokine genes remain accessible to the relevant transcription factors and promotes functional cooperation of the inducible transcription factor NFAT with lineage-specific transcription factors such as GATA-3 and T-bet.

Gene expression elicited by NFAT in the presence or absence of cooperative recruitment of Fos and Jun

Cooperation between nuclear factor of activated T cells (NFAT) and AP‐1 (Fos–Jun) proteins on composite NFAT–AP‐1 DNA elements constitutes a powerful mechanism for signal integration of the calcium and protein kinase C/Ras pathways in the regulation of gene expression. Here we report that NFAT can induce expression of certain genes in T cells without the need for cooperative recruitment of Fos and Jun. Using NFAT1 mutant proteins that are unable to interact with Fos–Jun dimers but are unaffected in DNA binding or transcriptional activity, we show that expression of interleukin (IL)‐2, granulocyte–macrophage colony‐stimulating factor (GM‐CSF), IL‐3, IL‐4, MIP1α and Fas ligand mRNAs is absolutely dependent on cooperation between NFAT and Fos–Jun; in contrast, NFAT induces tumor necrosis factor α (TNFα) mRNA and IL‐13 promoter activity without any necessity to recruit Fos and Jun. Furthermore, we show that NFAT–Fos–Jun cooperation is also essential to elicit the NFAT‐dependent program of activation‐induced cell death. Our results support the hypothesis that even in a single cell type, NFAT activation can evoke two distinct biological programs of gene expression, dependent or independent of NFAT–AP‐1 cooperation.

The Escherichia coli trmE (mnmE) gene, involved in tRNA modification, codes for an evolutionarily conserved GTPase with unusual biochemical properties

The evolutionarily conserved 50K protein of Escherichia coli, encoded by o454, contains a consensus GTP‐binding motif. Here we show that 50K is a GTPase that differs extensively from regulatory GTPases such as p21. Thus, 50K exhibits a very high intrinsic GTPase hydrolysis rate, rather low affinity for GTP, and extremely low affinity for GDP. Moreover, it can form self‐assemblies. Strikingly, the 17 kDa GTPase domain of 50K conserves the guanine nucleotide‐binding and GTPase activities of the intact 50K molecule. Therefore, the structural requirements for GTP binding and GTP hydrolysis by 50K are without precedent and justify a separate classification in the GTPase superfamily. Immunoelectron microscopy reveals that 50K is a cytoplasmic protein partially associated with the inner membrane. We prove that o454 is allelic with trmE, a gene involved in the biosynthesis of the hypermodified nucleoside 5‐methylaminomethyl‐2‐thiouridine, which is found in the wobble position of some tRNAs. Our results demonstrate that 50K is essential for viability depending on the genetic background. We propose that combination of mutations affecting the decoding process, which separately do not reveal an obvious defect in growth, can give rise to lethal phenotypes, most likely due to synergism.

Reciprocal modulatory interaction between human immunodeficiency virus type 1 Tat and transcription factor NFAT1.

ABSTRACT Human immunodeficiency virus type 1 (HIV-1) gene expression is regulated by interactions between both viral and host factors. These interactions are also responsible for changes in the expression of many host cell genes, including cytokines and other immune regulators, which may account for the state of immunological dysregulation that characterizes HIV-1 infection. We have investigated the role of a host cell protein, the transcription factor NFAT1, in HIV-1 pathogenesis. We show that NFAT1 interacts with Tat and that this interaction, which involves the major transactivation domain of NFAT1 and the amino-terminal region of Tat, results in a reciprocal modulatory interplay between the proteins: whereas Tat enhances NFAT1-driven transcription in Jurkat T cells, NFAT1 represses Tat-mediated transactivation of the HIV-1 long terminal repeat (LTR). Moreover, NFAT1 binds to the κB sites on the viral LTR and negatively regulates NF-κB-mediated activation of HIV-1 transcription, by competing with NF-κB1 for its binding sites on the HIV-1 LTR. Tat-mediated enhancement of NFAT1 transactivation may explain the upregulation of interleukin 2 and other cytokines that occurs during HIV-1 infection. We discuss the potentially opposing roles of NFAT1 and another family member, NFAT2, in regulating gene transcription of HIV-1 and endogenous cytokine genes.

An asymmetric NFAT1 dimer on a pseudo-palindromic κB-like DNA site

The crystal structure of the NFAT1 Rel homology region (RHR) bound to a pseudo-palindromic DNA site reveals an asymmetric dimer interaction between the RHR-C domains, unrelated to the contact seen in Rel dimers such as NFκB. Binding studies with a form of the NFAT1 RHR defective in the dimer contact show loss of cooperativity and demonstrate that the same interaction is present in solution. The structure we have determined may correspond to a functional NFAT binding mode at palindromic sites of genes induced during the anergic response to weak TCR signaling.

An improved vector system for constructing transcriptional lacZ fusions: analysis of regulation of the dnaA, dnaN, recF and gyrB genes of Escherichia coli.

We describe a new vector system for the in vitro construction of transcriptional fusions to the lacZ gene, which is expressed from the translational start signals of galK. The galK ribosome-binding site (RBS) and its natural preceding region ensure a constant efficiency for lacZ translation and, thus, the beta-galactosidase (beta Gal) production of a given fusion is directly proportional to the in vivo transcriptional activity of the inserted DNA fragment. Single-copy lambda prophage versions of multicopy constructs can be made by in vivo recombination. We use this system to compare the transcriptional activities of the promoters present in the dnaA-dnaN-recF-gyrB cluster. The order of strength of these promoters is gyrB > dnaA > recF > dnaN. It is assumed that gyrB belongs to the dnaA-dnaN-recF operon, because the short recF-gyrB intercistronic region does not contain a terminator. By using this new vector system, we have detected strong termination signals within recF that are functional even when recF is translated at its normal rate. The low level of transcription coming to the end of recF, and the highest activity of the gyrB promoter, as well as results obtained with several gyrB::lacZ translational fusions, support the conclusion that gyrB is predominantly expressed from its own promoter under standard growth conditions. Finally, we have found that transcription from the dnaA promoters is constant at different growth rates. This supports the idea that autoregulation of the dnaA gene is responsible for the coupling of the DnaA protein synthesis to cell mass increase, and accumulation of DnaA protein governs the initiation of chromosome replication.

TID1, a mammalian homologue of the drosophila tumor suppressor lethal(2) tumorous imaginal discs, regulates activation-induced cell death in Th2 cells

We previously described two human DnaJ proteins, hTid-1L and hTid-1S, which are derived from alternative splicing of the TID1 gene, the human homologue of the Drosophila tumor suppressor lethal(2) tumorous imaginal discs, and showed that hTid-1L promoted while hTid-1S antagonized apoptosis. There are two subsets of helper T cells, Th1 and Th2, of which Th2 cells are significantly less prone to apoptosis induced by stimulation through the T-cell receptor. This apoptotic process is known as activation-induced cell death (AICD). The molecular basis for the differential susceptibility of Th1 and Th2 cells to AICD is not known. Here we show that the antiapoptotic variant, Tid-1S, is selectively induced in murine Th2 cells following activation. Expression of a dominant-negative mutant of hTid-1S in a Th2 cell line strikingly enhanced activation of caspase 3 in response to CD3 stimulation, and caused the cells to become sensitive to AICD. Hence, the accumulation of Tid-1S in Th2 cells following activation represents a novel mechanism that may contribute to the induction of apoptosis resistance during the activation of Th2 cells.

CHAPTER 288 – The NFAT Family: Structure, Regulation, and Biological Functions

Originally identified as regulators of the immune response in T cells, the NFAT family of transcription factors controls various biological processes in diverse tissues and stages of development. NFAT proteins share a high degree of structural homology with the Rel/NFκB family of transcription factors. Four calcium-regulated family members, NFAT1-4, mediate gene transcription in response to calcium/calcineurin signals, while NFAT5 transduces the hypertonic stress response and is also regulated by signals through an integrin receptor and the T-cell receptor. Once activated, NFAT proteins cooperate with other transcriptional factors and induce specific patterns of gene expression that control distinct biological programs. This chapter reviews recent data on NFAT structure and regulation and summarizes some biological functions controlled by NFAT transcription factors.