Phosphodiesterases as targets for modulating T-cell responses

The cAMP-protein kinase A (PKA) signaling pathway is strongly involved in the regulation and modulation of immune responses, and cAMP is the most potent and acute inhibitor of T-cell activation. Thus, cAMP levels in the cell must be tightly regulated. Cyclic AMP-specific phosphodiesterases (PDEs) provide the only mechanism for degrading cAMP in cells, thereby functioning as key regulators of signaling. To obtain a complete immune response with optimal cytokine production and T-cell proliferation, ligation of both the T-cell receptor (TCR) and the CD28 receptor is required. However, engagement of the TCR in primary T cells is followed by rapid cAMP production in lipid rafts and activation of the cAMP- PKA-Csk pathway inhibiting proximal T-cell signaling. In contrast, TCR/CD28 costimulation leads to the recruitment of a PDE4/β-arrestin complex to rafts in a phosphatidylinositol 3-kinase (PI3K)-dependent manner, resulting in the downregulation of cAMP levels. Thus, the activities of both PKA and PDE4 seem to be important for regulation of TCR-induced signaling and T-cell function. The use of selective inhibitors has revealed that PDEs are important drug targets in several diseases with an inflammatory component where immune function is important such as asthma, chronic obstructive pulmonary disease (COPD), cardiovascular diseases, and neurological disorders. PDEs are also interesting drug targets in immunosuppression following transplantation and for modulation of immune responses.

Quantitative proteome analysis of detergent-resistant membranes identifies the differential regulation of protein kinase C isoforms in apoptotic T cells

Several lines of evidence suggest that detergent-resistant membranes (DRMs) (also known as lipid rafts and glycosphingolipid-enriched microdomains) may have a role in signaling pathways of apoptosis. Here, we developed a method that combines DRMs isolation and methanol/chloroform extraction with stable isotope labeling with amino acids in cell culture-based quantitative proteome analysis of DRMs from control and cisplatin-induced apoptotic Jurkat T cells. This approach enabled us to enrich proteins with a pivotal role in cell signaling of which several were found with increased or decreased amounts in DRMs upon induction of apoptosis. Specifically, we show that three isoforms of protein kinase C (PKC) are regulated differently upon apoptosis. Although PKC alpha which belongs to the group of conventional PKCs is highly up-regulated in DRMs, the levels of two novel PKCs, PKC eta and PKC theta, are significantly reduced. These alterations/differences in PKC regulation are verified by immunoblotting and confocal microscopy. In addition, a specific enrichment of PKC alpha in apoptotic blebs and buds is shown. Furthermore, we observe an increased expression of ecto-PKC alpha as a result of exposure to cisplatin using flow cytometry. Our results demonstrate that in-depth proteomic analysis of DRMs provides a tool to study differential localization and regulation of signaling molecules important in health and disease.

Novel mechanism of signaling by CD28

Ligation of both the T cell receptor (TCR) and the CD28 receptor is required for full T cell activation to occur. Engagement of the TCR in primary T cells is followed by rapid cAMP production in lipid rafts and activation of the cAMP-protein kinase A (PKA)-Csk pathway inhibiting proximal T cell signaling. However, CD28 stimulation leads to recruitment of a beta-arrestin/phosphodiesterase-4 (PDE4) complex to rafts, resulting in down-regulation of cAMP levels. Thus, the activities of both PKA and PDE4 seem to be important for regulation of TCR-induced signaling and T cell function. This review will focus on the novel mechanism whereby CD28 through PI3K regulates recruitment of a PKB/beta-arrestin/PDE4 complex thereby allowing a complete T cell activation to proceed.

Role of cAMP phosphodiesterase 4 in regulation of T-cell function

Ligation of both the T-cell receptor (TCR) and the CD28 receptor is required for full T-cell activation to occur. Engagement of the TCR in primary T cells is followed by rapid cAMP production in lipid rafts resulting in raft-associated protein kinase A (PKA) activation and inhibition of proximal T-cell signaling. However, upon TCR and CD28 cross-ligation, beta-arrestin in complex with cAMP-specific phosphodiesterase 4 (PDE4) is recruited to lipid rafts, thus downregulating cAMP levels. Consequently, the activities of both PKA and PDE4 seem to be important for the regulation of TCR-induced signaling and T-cell function. We, therefore, propose a novel role for TCR and CD28 co-stimulation in the downmodulation of TCR-induced cAMP-mediated inhibitory signals through the recruitment of beta-arrestin and PDE4 to lipid rafts, thus allowing a full T-cell response to occur.

Cellular localisation and nuclear export of the human bZIP transcription factor TCF11

TCF11 is a ubiquitous transcription factor of the CNC-bZIP family. The activity of this vital protein is strictly regulated and we have previously published that the two major translated protein forms show a clearly different transactivation ability in transient transfections. Only the full-length form is active in a variety of mammalian cells [J. Biol. Chem. 276 (2001) 17641]. Here we further investigate the complex regulation of TCF11, studying the cellular localisation of some of the different protein isoforms. The full-length form is located both in the cytoplasm and the nucleus, while the internally initiated shorter protein form is restricted to nuclear localisation. A nuclear export signal (NES) localised in the N-terminus of TCF11 is responsible for the active nuclear export of the protein. This export is highly sensitive to leptomycin B (LMB) and is largely blocked by mutating three of the leucine residues in the signal region. These results indicate that export occurs through the Crm1-mediated pathway. Due to alternative splicing within the tcf11 gene, different isoforms of the longer protein form are produced. Some of these isoforms, one identical to Nrf1, lack the NES and are thereby restricted to nuclear localisation.

Deamidations in recombinant human phenylalanine hydroxylase. Identification of labile asparagine residues and functional characterization of Asn --> Asp mutant forms

Recombinant human phenylalanine hydroxylase (hPAH) expressed in Escherichia coli for 24 h at 28 degrees C has been found by two-dimensional electrophoresis to exist as a mixture of four to five molecular forms as a result of nonenzymatic deamidation of labile Asn residues. The multiple deamidations alter the functional properties of the enzyme including its affinity for l-phenylalanine and tetrahydrobiopterin, catalytic efficiency, and substrate inhibition and also result in enzyme forms more susceptible to limited tryptic proteolysis. Asn(32) in the regulatory domain deamidates very rapidly because of its nearest neighbor amino acid Gly(33) (Solstad, T., Carvalho, R. N., Andersen, O. A., Waidelich, D., and Flatmark, T. (2003) Eur. J. Biochem., in press). Matrix-assisted laser desorption/ionization time of flight-mass spectrometry of the tryptic peptides in the catalytic domain of a 24-h (28 degrees C) expressed enzyme has shown Asn(376) and Asn(133) to be labile residues. Site-directed mutagenesis of nine Asn residues revealed that the deamidations of Asn(32) and Asn(376) are the main determinants for the functional and regulatory differences observed between the 2- and 24-h-induced wild-type (wt) enzyme. The Asn(32) --> Asp, Asn(376) --> Asp, and the double mutant forms expressed for 2 h at 28 degrees C revealed qualitatively similar regulatory properties as the highly deamidated 24-h expressed wt-hPAH. Moreover, deamidation of Asn(32) in the wt-hPAH (24 h expression at 28 degrees C) and the Asn(32) --> Asp mutation both increase the initial rate of phosphorylation of Ser(16) by cAMP-dependent protein kinase (p < 0.005). By contrast, the substitution of Gly(33) with Ala or Val, both preventing the deamidation of Asn(32), resulted in enzyme forms that were phosphorylated at a similar rate as nondeamidated wt-hPAH, even on 24-h expression. The other Asn --> Asp substitutions (in the catalytic domain) revealed that Asn(207) and Asn(223) have an important stabilizing structural function. Finally, two recently reported phenylketonuria mutations at Asn residues in the catalytic domain were studied, i.e. Asn(167) --> Ile and Asn(207) --> Asp, and their phenotypes were characterized.

A comparison of kinetic and regulatory properties of the tetrameric and dimeric forms of wild-type and Thr427-->Pro mutant human phenylalanine hydroxylase: contribution of the flexible hinge region Asp425-Gln429 to the tetramerization and cooperative substrate binding

Recombinant human phenylalanine hydroxylase (hPAH, phenylalanine 4-monooxygenase EC 1.14.16.1) is catalytically active both as a tetramer and a dimer [Knappskog, P.M., Flatmark, T., Aarden, J.M., Haavik, J. and Martínez, A. (1996) Eur. J. Biochem. 242, 813-821]. In the present study we have further characterized the differences in kinetic and regulatory properties of the two oligomeric forms when expressed in Escherichia coli. The positive cooperativity of L-Phe binding to the tetrameric form both in enzyme kinetic studies (h = 1.6) and intrinsic tryptophan fluorescence measurements (h = 2.3) was abolished in the dimer, which also revealed a catalytic efficiency (Vmax/[S]0.5) of only 35% of the tetramer. Whereas the catalytic activity of the tetramer was activated fivefold to sixfold by preincubation with L-Phe, the dimer revealed only a 1.6-fold activation. The crystal structure has identified a five-residue flexible hinge region (Asp425-Gln429) that links the beta-strand Tbeta2 (Ile421-Leu424) and the 24 residue amphipathic alpha-helix Talpha1 (Gln428-Lys452) at the C-terminus which forms an antiparallel coiled-coil structure in the center of the tetramer [Fusetti, F., Erlandsen, H., Flatmark, T. & Stevens, R.C. (1998) J. Biol. Chem. 273, 16962-16967]. The potential role of this flexible hinge in the tetramerization and the conformational transition of wt-hPAH on the cooperative binding of L-Phe was examined by site-specific mutagenesis. Substitution of Thr427 by a Pro (as in tyrosine hydroxylase) resulted in a mutant protein which was isolated mainly (about 95%) as a dimer. The isolated tetramer of T427P revealed no kinetic cooperativity of L-Phe binding, the catalytic efficiency (Vmax/[S]0.5) was decreased to about 39% of the wild-type tetramer and it was not activated by L-Phe preincubation. The dimeric forms of T427P and wt-hPAH revealed rather similar kinetic properties. The lack of kinetic cooperativity of the T427P tetramer was associated with a corresponding change in the binding isotherm for L-Phe as studied by intrinsic tryptophan fluorescence measurements. Protein stability was also reduced both for the E. coli expressed and the in vitro synthesized mutant enzyme. Collectively, these results indicate that the positive cooperativity of L-Phe binding to wt-hPAH requires a tetrameric enzyme with a C-terminal flexible hinge region (Asp425-Gln429) which has a structural role in the formation of the enzyme tetramer. Furthermore, this hinge region represents a motif in the PAH structure that is involved in the conformational change transmitted through the protein on the cooperative binding of L-Phe to tetrameric wt-hPAH. This conclusion is further supported by studies on two disease (phenylketonuria)-associated mutant forms.

Crystal structure and site-specific mutagenesis of pterin-bound human phenylalanine hydroxylase

The crystal structure of the dimeric catalytic domain (residues 118-424) of human PheOH (hPheOH), cocrystallized with the oxidized form of the cofactor (7,8-dihydro-L-biopterin, BH(2)), has been determined at 2.0 A resolution. The pterin binds in the second coordination sphere of the catalytic iron (the C4a atom is 6.1 A away), and interacts through several hydrogen bonds to two water molecules coordinated to the iron, as well as to the main chain carbonyl oxygens of Ala322, Gly247, and Leu249 and the main chain amide of Leu249. Some important conformational changes are seen in the active site upon pterin binding. The loop between residues 245 and 250 moves in the direction of the iron, and thus allows for several important hydrogen bonds to the pterin ring to be formed. The pterin cofactor is in an ideal orientation for dioxygen to bind in a bridging position between the iron and the pterin. The pterin ring forms an aromatic pi-stacking interaction with Phe254, and Tyr325 contributes to the positioning of the pterin ring and its dihydroxypropyl side chain by hydrophobic interactions. Of particular interest in the hPheOH x BH(2) binary complex structure is the finding that Glu286 hydrogen bonds to one of the water molecules coordinated to the iron as well as to a water molecule which hydrogen bonds to N3 of the pterin ring. Site-specific mutations of Glu286 (E286A and E286Q), Phe254 (F254A and F254L), and Tyr325 (Y325F) have confirmed the important contribution of Glu286 and Phe254 to the normal positioning of the pterin cofactor and catalytic activity of hPheOH. Tyr325 also contributes to the correct positioning of the pterin, but has no direct function in the catalytic reaction, in agreement with the results obtained with rat TyrOH [Daubner, S. C., and Fitzpatrick, P. F. (1998) Biochemistry 37, 16440-16444]. Superposition of the binary hPheOH.BH(2) complex onto the crystal structure of the ligand-free rat PheOH (which contains the regulatory and catalytic domains) [Kobe, B., Jennings, I. G., House, C. M., Michell, B. J., Goodwill, K. E., Santarsiero, B. D., Stevens, R. C., Cotton, R. G. H., and Kemp, B. E. (1999) Nat. Struct. Biol. 6, 442-448] reveals that the C2'-hydroxyl group of BH(2) is sufficiently close to form hydrogen bonds to Ser23 in the regulatory domain. Similar interactions are seen with the hPheOH.adrenaline complex and Ser23. These interactions suggest a structural explanation for the specific regulatory properties of the dihydroxypropyl side chain of BH(4) (negative effector) in the full-length enzyme in terms of phosphorylation of Ser16 and activation by L-Phe.

Partial characterization and three-dimensional-structural localization of eight mutations in exon 7 of the human phenylalanine hydroxylase gene associated with phenylketonuria

The molecular basis for the metabolic defect in patients with phenylketonuria has been characterized for seven missense point mutations (R252G/Q, L255V/S, A259V/T and R270S) and a termination mutation (G272X) in an evolutionarily conserved motif of exon 7 in the catalytic domain of the human phenylalanine hydroxylase (hPAH) gene. The mutations were expressed in three heterologous in vitro systems. When expressed as fusion proteins with maltose-binding protein in Escherichia coli five of the mutant proteins demonstrated a defect in the normal ability of hPAH to fold and assemble as homotetramer/dimer, and they were mostly recovered as inactive aggregated forms. Only for the R252Q and L255V mutants were catalytically active tetramer and dimer recovered and for R252G some dimer, i.e. 20% (R252Q, tetramer), 44% (L255V, tetramer) and 4.4% (R252G, dimer) of the activity for the respective wild-type (wt) forms. When expressed by a coupled in vitro transcription-translation system, all the mutant enzymes were recovered as a mixture of non-phosphorylated and phosphorylated forms with a low homospecific activity (i.e. maximum 11% of wt-hPAH for the L255V mutant). When transiently expressed in human embryonic kidney (A293) cells a very low level of immunoreactive PAH protein was recovered in spite of normal PAH mRNA levels. All these mutations resulted in variant hPAH proteins which revealed a defect in oligomerization, an increased sensitivity to limited proteolysis in vitro, reduced cellular stability and a variable reduction in their catalytic activity. All these effects seem to result from structural perturbations of the monomer, and based on the crystal structure of the catalytic domain of hPAH, an explanation is provided for the impact of the mutations on the folding and oligomerization of the monomers.