Identification and characterization of pancreatic eukaryotic initiation factor 2 alpha-subunit kinase, PEK, involved in translational control

FAD-linked presenilin-1 mutants impede translation regulation under ER stress

FAD mutations in presenilin-1 (PS1) cause attenuation of the induction of the endoplasmic reticulum (ER)-resident chaperone GRP78/BiP under ER stress, due to disturbed function of IRE1, the sensor for accumulation of unfolded protein in the ER lumen. PERK, an ER-resident transmembrane protein kinase, is also a sensor for the unfolded protein response (UPR), causing phosphorylation of eukaryotic initiation factor 2alpha (eIF2alpha) to inhibit translation initiation. Here, we report that the FAD mutant PS1 disturbs the UPR by attenuating both the activation of PERK and the phosphorylation of eIF2alpha. Consistent with the results of a disturbed UPR, inhibition of protein synthesis under ER stress was impaired in cells expressing PS1 mutants. These results suggest that mutant PS1 impedes general translational attenuation regulated by PERK and eIF2alpha, resulting in an increased load of newly synthesized proteins into the ER and subsequently increasing vulnerability to ER stress.

Activation of GCN2 in UV-irradiated cells inhibits translation

BACKGROUND

Mammalian cells subjected to ultraviolet (UV) irradiation actively repress DNA replication, transcription, and mRNA translation. While the effects of UV irradiation on DNA replication and transcription have been extensively studied, the mechanism(s) responsible for translational repression are poorly understood.

RESULTS

Here, we demonstrate that UV irradiation elicits phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF2alpha) by activating the kinase GCN2 in a manner that does not require SAPK/JNK or p38 MAP kinase. GCN2-/- cells, and cells expressing nonphosphorylatable eIF2alpha as their only source of eIF2alpha protein, fail to repress translation in response to UV irradiation.

CONCLUSIONS

These results provide a mechanism for translation inhibition by UV irradiation and identify a hitherto unrecognized role for mammalian GCN2 as a mediator of the cellular response to UV stress.

Loss of kinase activity in a patient with Wolcott-Rallison syndrome caused by a novel mutation in the EIF2AK3 gene

Wolcott-Rallison syndrome (WRS) is an autosomal recessive disorder characterized by neonatal or early infancy type 1 diabetes, epiphyseal dysplasia, and growth retardation. Mutations in the EIF2AK3 gene, encoding the eukaryotic initiation factor 2alpha-kinase 3 (EIF2AK3), have been found in WRS patients. Here we describe a girl who came to our attention at 2 months of age with severe hypertonic dehydration and diabetic ketoacidosis. A diagnosis of type 1 diabetes was made and insulin treatment initiated. Growth retardation and microcephaly were also present. Anti-islet cell autoantibodies were negative, and mitochondrial diabetes was excluded. Imaging revealed a hypoplastic pancreas and typical signs of spondylo-epiphyseal dysplasia. The diagnosis of WRS was therefore made at age 5 years. Sequencing analysis of her EIF2AK3 gene revealed the presence of a homozygous T to C exchange in exon 13 leading to the missense serine 877 proline mutation. The mutated kinase, although it partly retains the ability of autophosphorylation, is unable to phosphorylate its natural substrate, eukaryotic initiation factor 2alpha (eIF2alpha). This is the first case in which the pathophysiological role of EIF2AK3 deficiency in WRS is confirmed at the molecular level. Our data demonstrate that EIF2AK3 kinase activity is essential for pancreas islet function and bone development in humans, and we suggest EIF2AK3 as a possible target for therapeutic intervention in diabetes.

The PERK eukaryotic initiation factor 2 alpha kinase is required for the development of the skeletal system, postnatal growth, and the function and viability of the pancreas

Phosphorylation of eukaryotic initiation factor 2 alpha (eIF-2 alpha) is typically associated with stress responses and causes a reduction in protein synthesis. However, we found high phosphorylated eIF-2 alpha (eIF-2 alpha[P]) levels in nonstressed pancreata of mice. Administration of glucose stimulated a rapid dephosphorylation of eIF-2 alpha. Among the four eIF-2 alpha kinases present in mammals, PERK is most highly expressed in the pancreas, suggesting that it may be responsible for the high eIF-2 alpha[P] levels found therein. We describe a Perk knockout mutation in mice. Pancreata of Perk(-/-) mice are morphologically and functionally normal at birth, but the islets of Langerhans progressively degenerate, resulting in loss of insulin-secreting beta cells and development of diabetes mellitus, followed later by loss of glucagon-secreting alpha cells. The exocrine pancreas exhibits a reduction in the synthesis of several major digestive enzymes and succumbs to massive apoptosis after the fourth postnatal week. Perk(-/-) mice also exhibit skeletal dysplasias at birth and postnatal growth retardation. Skeletal defects include deficient mineralization, osteoporosis, and abnormal compact bone development. The skeletal and pancreatic defects are associated with defects in the rough endoplasmic reticulum of the major secretory cells that comprise the skeletal system and pancreas. The skeletal, pancreatic, and growth defects are similar to those seen in human Wolcott-Rallison syndrome.

The unfolded protein response in nutrient sensing and differentiation

Eukaryotic cells coordinate protein-folding reactions in the endoplasmic reticulum with gene expression in the nucleus and messenger RNA translation in the cytoplasm. As the rate of protein synthesis increases, protein folding can be compromised, so cells have evolved signal-transduction pathways that control transcription and translation -- the 'unfolded protein response'. Recent studies indicate that these pathways also coordinate rates of protein synthesis with nutrient and energy stores, and regulate cell differentiation to survive nutrient-limiting conditions or to produce large amounts of secreted products such as hormones, antibodies or growth factors.

Dimerization and release of molecular chaperone inhibition facilitate activation of eukaryotic initiation factor-2 kinase in response to endoplasmic reticulum stress

Phosphorylation of eukaryotic initiation factor-2 (eIF2) by pancreatic eIF2 kinase (PEK), induces a program of translational expression in response to accumulation of malfolded protein in the endoplasmic reticulum (ER). This study addresses the mechanisms activating PEK, also designated PERK or EIF2AK3. We describe the characterization of two regions in the ER luminal portion of the transmembrane PEK that carry out distinct functions in the regulation of this eIF2 kinase. The first region mediates oligomerization between PEK polypeptides, and deletion of this portion of PEK blocked induction of eIF2 kinase activity. The second characterized region of PEK facilitates interaction with ER chaperones. In the absence of stress, PEK associates with ER chaperones GRP78 (BiP) and GRP94, and this binding is released in response to ER stress. ER luminal sequences flanking the transmembrane domain are required for GRP78 interaction, and deletion of this portion of PEK led to its activation even in the absence of ER stress. These results suggest that this ER chaperone serves as a repressor of PEK activity, and release of ER chaperones from PEK when misfolded proteins accumulate in the ER induces gene expression required to enhance the protein folding capacity of the ER.

The protein kinase/endoribonuclease IRE1alpha that signals the unfolded protein response has a luminal N-terminal ligand-independent dimerization domain

In response to accumulation of unfolded proteins in the endoplasmic reticulum (ER), cells activate an intracellular signal transduction pathway called the unfolded protein response (UPR). IRE and PERK are the two type-I ER transmembrane protein kinase receptors that signal the UPR. The N-terminal luminal domains (NLDs) of IRE1 and PERK sense ER stress conditions by a common mechanism and transmit the signal to regulate the cytoplasmic domains of these receptors. To provide an experimental system amenable to detailed biochemical and structural analysis to elucidate the mechanism of ER-transmembrane signaling mechanism mediated by the NLD, we overexpressed the soluble luminal domain of human IRE1alpha in COS-1 cells by transient DNA transfection. Here we report the expression, purification, and characterization of the soluble NLD. The biological function of the NLD was confirmed by its ability to associate with itself and to interact with both the membrane-bound full-length IRE1alpha receptor and the ER chaperone BiP. Functional and spectral studies suggested that the highly conserved N-linked glycosylation site is not required for proper protein folding and self-association. Interestingly, we demonstrated that the NLD forms stable dimers linked by intermolecular disulfide bridges. Our data support that the luminal domain represents a novel ligand-independent dimerization domain.

Ultraviolet light inhibits translation through activation of the unfolded protein response kinase PERK in the lumen of the endoplasmic reticulum

Exposure to ultraviolet light can cause inflammation, premature skin aging, and cancer. UV irradiation alters the expression of multiple genes that encode functions to repair DNA damage, arrest cell growth, and induce apoptosis. In addition, UV irradiation inhibits protein synthesis, although the mechanism is not known. In this report, we show that UV irradiation induces phosphorylation of eukaryotic translation initiation factor 2 on the alpha-subunit (eIF2alpha) and inhibits protein synthesis in a dosage- and time-dependent manner. The UV-induced phosphorylation of eIF2alpha was prevented by the overexpression of a non-phosphorylatable mutant of eIF2alpha (S51A). PERK is an eIF2alpha protein kinase localized to the endoplasmic reticulum that is activated by the accumulation of unfolded proteins in the endoplasmic reticulum. Expression of trans-dominant-negative mutants of PERK also prevented eIF2alpha phosphorylation upon UV treatment and protected from the associated translation attenuation. The luminal domain of dominant-negative mutant PERK formed heterodimers with endogenous PERK to inhibit the PERK signaling pathway. In contrast, eIF2alpha phosphorylation was not inhibited by overexpression of a trans-dominant-negative mutant kinase, PKR, supporting the theory that UV-induced eIF2alpha phosphorylation is specifically mediated by PERK. These results support a novel mechanism by which UV irradiation regulates translation via an endoplasmic reticulum-stress signaling pathway.

Two distinct stress signaling pathways converge upon the CHOP promoter during the mammalian unfolded protein response

CHOP is a non-ER localized transcription factor that is induced by a variety of adverse physiological conditions including ER stress. Accumulation of unfolded proteins in the ER activates an unfolded protein response pathway that targets both ER resident chaperones (e.g. BiP) and CHOP. Hence, it is unclear if CHOP induction during ER stress occurs through the ER stress response element that is conserved in both CHOP and ER chaperone promoters, or through a separate regulatory pathway conserved among different CHOP inducing cellular stress conditions. We identified a bona fide ER stress element in the hamster CHOP promoter and found that similar transcription complexes containing NF-Y bound to both the CHOP and BiP ER stress response elements. In addition, we demonstrated for the first time the importance of the C/EBP-ATF composite site for CHOP regulation during ER stress. Activation of the ER transmembrane eIF2alpha kinase, PERK, induced ATF4 protein expression, direct binding to the composite site in CHOP promoter, and as a consequence, CHOP protein induction. We propose that this eIF2alpha-kinase/ATF4/C/EBP-ATF composite site pathway is conserved for CHOP regulation during various cellular stress conditions including ER stress. Our data indicate that both the ERSE and the PERK-ATF4 pathways converge on the CHOP promoter during ER stress and provide insights into the similarities and differences between CHOP and ER chaperone expression during normal and stress conditions.

Essential role of calcineurin in response to endoplasmic reticulum stress

Depletion of calcium ions (Ca2+) from the endoplasmic reticulum (ER) of yeast cells resulted in the activation of the unfolded protein response (UPR) signaling pathway involving Ire1p and Hac1p. The depleted ER also stimulated Ca2+ influx at the plasma membrane through the Cch1p-Mid1p Ca2+ channel and another system. Surprisingly, both Ca2+ influx systems were stimulated upon accumulation of misfolded proteins in the ER even in the presence of Ca2+. The ability of misfolded ER proteins to stimulate Ca2+ influx at the plasma membrane did not require Ire1p or Hac1p, and Ca2+ influx and signaling factors were not required for initial UPR signaling. However, activation of the Ca2+ channel, calmodulin, calcineurin and other factors was necessary for long-term survival of cells undergoing ER stress. A similar calcium cell survival (CCS) pathway operates in the pathogenic fungi and promotes resistance to azole antifungal drugs. These findings reveal an unanticipated new regulatory mechanism that couples ER stress to Ca2+ influx and signaling pathways, which help to prevent cell death and promote resistance to an important class of fungistatic drugs.

Japanese encephalitis virus infection initiates endoplasmic reticulum stress and an unfolded protein response

The malfunctioning of the endoplasmic reticulum (ER) of cells in hosts ranging from yeast to mammals can trigger an unfolded protein response (UPR). Such malfunctioning can result from a variety of ER stresses, including the inhibition of protein glycosylation and calcium imbalance. To cope with ER stresses, cells may rely on the UPR to send a signal(s) from the ER to the nucleus to stimulate appropriate cellular responses, including induction of chaperone expression. During Japanese encephalitis virus (JEV) infection, the lumen of the ER rapidly accumulates substantial amounts of viral proteins for virus progeny production. In the present study, we demonstrate that as evidenced by certain chaperone inductions, JEV infection triggers the UPR in fibroblast BHK-21 cells and in neuronal N18 and NT-2 cells, in which JEV results in apoptotic cell death. By contrast, no UPR was observed in apoptosis-resistant K562 cells infected by JEV. JEV infection also activates expression of CHOP/GADD153, a distinctive transcription factor often induced by the UPR, and appears to trigger activation of p38 mitogen-activated protein kinase, a posttranslational activator of CHOP. Ectopic enforcement of CHOP expression enhanced JEV-induced apoptosis, whereas treatment with a p38-specific inhibitor, SB203580, partially blocked JEV-induced apoptosis. Interestingly, bcl-2 overexpression and treatment with a pancaspase inhibitor, z-VAD-fmk, inhibited CHOP induction and diminished JEV-induced apoptosis, suggesting that Bcl-2 and caspases could be the upstream regulators of CHOP. Our results thus suggest that virus-induced ER stress may participate, via p38-dependent and CHOP-mediated pathways, in the apoptotic process triggered by JEV infection.

The mRNA of the translationally controlled tumor protein P23/TCTP is a highly structured RNA, which activates the dsRNA-dependent protein kinase PKR

The dsRNA-activated protein kinase PKR is involved in signal transduction pathways that mediate cellular processes as diverse as cell growth and differentiation, the stress response, and apoptosis. PKR was originally described as an interferon-inducible elF2alpha kinase involved in the antiviral defense mechanism of the cell. The interaction of the kinase with specific viral RNAs has been studied in much detail, but information about cellular mRNAs, which are able to bind and activate PKR, is scarce. In search for such cellular mRNAs, we developed a cloning strategy to identify individual mRNA species from the dsRNA-rich fraction of Daudi cell poly(A)+ RNA. Two out of five cDNA clones we obtained contained sequences derived from the mRNA of the translationally controlled tumor protein P23/TCTP, indicating that this mRNA is present in the dsRNA-rich fraction. Secondary structure predictions and gel electrophoretic mobility investigations on P23/TCTP transcripts confirmed the potential of this mRNA to form extensive secondary structure. A full-length P23 transcript, but not a truncated version thereof, was able to bind to PKR in vitro and in vivo. Transient transfection experiments in human 293 cells showed that coexpression of full-length P23 mRNA leads to partial inhibition of the expression of a beta-galactosidase reporter gene in trans. Additional coexpression of a dominant negative mutant of PKR or of adenovirus VA1 RNA suppressed this inhibition, indicating that it is mediated by PKR. Studies on P23/TCTP expression in cells from PKR-knockout mice suggest that P23/TCTP mRNA translation is regulated by PKR. Hence, our results demonstrate that the mRNA of P23/TCTP may both activate PKR and be subject to translational regulation by this kinase.

The mammalian endoplasmic reticulum as a sensor for cellular stress

The recent elucidation of the mammalian unfolded protein response pathway has revealed a unique and transcriptionally complex signal transduction pathway that protects cells from a variety of physical and biochemical stresses that can occur during normal development and in disease states. Although the stress conditions are monitored in the endoplasmic reticulum, the beneficial effects of this pathway are extended to other cellular organelles and to the organism itself.

Initiation factor eIF2 alpha phosphorylation in stress responses and apoptosis

The alpha subunit of polypeptide chain initiation factor eIF2 can be phosphorylated by a number of related protein kinases which are activated in response to cellular stresses. Physiological conditions which result in eIF2 alpha phosphorylation include virus infection, heat shock, iron deficiency, nutrient deprivation, changes in intracellular calcium, accumulation of unfolded or denatured proteins and the induction of apoptosis. Phosphorylated eIF2 acts as a dominant inhibitor of the guanine nucleotide exchange factor eIF2B and prevents the recycling of eIF2 between successive rounds of protein synthesis. Extensive phosphorylation of eIF2 alpha and strong inhibition of eIF2B activity can result in the downregulation of the overall rate of protein synthesis; less marked changes may lead to alterations in the selective translation of alternative open reading frames in polycistronic mRNAs, as demonstrated in yeast. These mechanisms can provide a signal transduction pathway linking eukaryotic cellular stress responses to alterations in the control of gene expression at the translational level.

Gene-specific regulation by general translation factors

Protein synthesis is the ultimate step of gene expression and a key control point for regulation. In particular, it enables cells to rapidly manipulate protein production without new mRNA synthesis, processing, or export. Recent studies have enhanced our understanding of the translation initiation process and helped elucidate how modifications of the general translational machinery regulate gene-specific protein production.

IRE1-mediated unconventional mRNA splicing and S2P-mediated ATF6 cleavage merge to regulate XBP1 in signaling the unfolded protein response

All eukaryotic cells respond to the accumulation of unfolded proteins in the endoplasmic reticulum (ER) by signaling an adaptive pathway termed the unfolded protein response (UPR). In yeast, a type-I ER transmembrane protein kinase, Ire1p, is the proximal sensor of unfolded proteins in the ER lumen that initiates an unconventional splicing reaction on HAC1 mRNA. Hac1p is a transcription factor required for induction of UPR genes. In higher eukaryotic cells, the UPR also induces site-2 protease (S2P)-mediated cleavage of ER-localized ATF6 to generate an N-terminal fragment that activates transcription of UPR genes. To elucidate the requirements for IRE1alpha and ATF6 for signaling the mammalian UPR, we identified a UPR reporter gene that was defective for induction in IRE1alpha-null mouse embryonic fibroblasts and S2P-deficient Chinese hamster ovary (CHO) cells. We show that the endoribonuclease activity of IRE1alpha is required to splice XBP1 (X-box binding protein) mRNA to generate a new C terminus, thereby converting it into a potent UPR transcriptional activator. IRE1alpha was not required for ATF6 cleavage, nuclear translocation, or transcriptional activation. However, ATF6 cleavage was required for IRE1alpha-dependent induction of UPR transcription. We propose that nuclear-localized IRE1alpha and cytoplasmic-localized ATF6 signaling pathways merge through regulation of XBP1 activity to induce downstream gene expression. Whereas ATF6 increases the amount of XBP1 mRNA, IRE1alpha removes an unconventional 26-nucleotide intron that increases XBP1 transactivation potential. Both processing of ATF6 and IRE1alpha-mediated splicing of XBP1 mRNA are required for full activation of the UPR.

Effect of CCK and intracellular calcium to regulate eIF2B and protein synthesis in rat pancreatic acinar cells

Pancreatic secretagogues enhance acinar protein synthesis at physiological concentrations and inhibit protein synthesis at high concentrations. We investigated the potential role in this process of the eukaryotic translation initiation factor (eIF)2B. Cholecystokinin (CCK) at 10-100 pM did not significantly affect eIF2B activity, which averaged 35.4 nmol guanosine 5'-diphosphate exchanged per minute per milligram protein under control conditions; higher CCK concentrations reduced eIF2B activity to 38.2% of control. Carbamylcholine chloride (Carbachol, CCh), A-23187, and thapsigargin also inhibited eIF2B and protein synthesis, whereas bombesin and the CCK analog JMV-180 were without effect. Previous studies have shown that eIF2B can be negatively regulated by glycogen synthase kinase-3 (GSK-3). However, GSK-3 activity, as assessed by phosphorylation state, was inhibited at high concentrations of CCK, an effect that should have stimulated, rather than repressed, eIF2B activity. An alternative mechanism for regulating eIF2B is through phosphorylation of the alpha-subunit of eIF2, which converts it into an inhibitor of eIF2B. CCK, CCh, A-23187, and thapsigargin all enhanced eIF2alpha phosphorylation, suggesting that eIF2B activity is regulated by eIF2alpha phosphorylation under these conditions. Removal of Ca(2+) from the medium enhanced the inhibitory action of CCK on both protein synthesis and eIF2B activity as well as further increasing eIF2alpha phosphorylation. Although it is likely that other mechanisms account for the stimulation of acinar protein synthesis, these results suggest that the inhibition of acinar protein synthesis by CCK occurs as a result of depletion of Ca(2+) from the endoplasmic reticulum lumen leading to phosphorylation of eIF2alpha and inhibition of eIF2B.

Molecular pathways of protein synthesis inhibition during brain reperfusion: implications for neuronal survival or death

Protein synthesis inhibition occurs in neurons immediately on reperfusion after ischemia and involves at least alterations in eukaryotic initiation factors 2 (eIF2) and 4 (eIF4). Phosphorylation of the alpha subunit of eIF2 [eIF2(alphaP)] by the endoplasmic reticulum transmembrane eIF2alpha kinase PERK occurs immediately on reperfusion and inhibits translation initiation. PERK activation, along with depletion of endoplasmic reticulum Ca2+ and inhibition of the endoplasmic reticulum Ca2+ -ATPase, SERCA2b, indicate that an endoplasmic reticulum unfolded protein response occurs as a consequence of brain ischemia and reperfusion. In mammals, the upstream unfolded protein response components PERK, IRE1, and ATF6 activate prosurvivial mechanisms (e.g., transcription of GRP78, PDI, SERCA2b ) and proapoptotic mechanisms (i.e., activation of Jun N-terminal kinases, caspase-12, and CHOP transcription). Sustained eIF2(alphaP) is proapoptotic by inducing the synthesis of ATF4, the CHOP transcription factor, through "bypass scanning" of 5' upstream open-reading frames in ATF4 messenger RNA; these upstream open-reading frames normally inhibit access to the ATF4 coding sequence. Brain ischemia and reperfusion also induce mu-calpain-mediated or caspase-3-mediated proteolysis of eIF4G, which shifts message selection to m 7 G-cap-independent translation initiation of messenger RNAs containing internal ribosome entry sites. This internal ribosome entry site-mediated translation initiation (i.e., for apoptosis-activating factor-1 and death-associated protein-5) can also promote apoptosis. Thus, alterations in eIF2 and eIF4 have major implications for which messenger RNAs are translated by residual protein synthesis in neurons during brain reperfusion, in turn constraining protein expression of changes in gene transcription induced by ischemia and reperfusion. Therefore, our current understanding shifts the focus from protein synthesis inhibition to the molecular pathways that underlie this inhibition, and the role that these pathways play in prosurvival and proapoptotic processes that may be differentially expressed in vulnerable and resistant regions of the reperfused brain.

The action of molecular chaperones in the early secretory pathway

The endoplasmic reticulum (ER) serves as a way-station during the biogenesis of nearly all secreted proteins, and associated with or housed within the ER are factors required to catalyze their import into the ER and facilitate their folding. To ensure that only properly folded proteins are secreted and to temper the effects of cellular stress, the ER can target aberrant proteins for degradation and/or adapt to the accumulation of misfolded proteins. Molecular chaperones play critical roles in each of these phenomena.

Regulation of starvation- and virus-induced autophagy by the eIF2alpha kinase signaling pathway

The eIF2alpha kinases are a family of evolutionarily conserved serine/threonine kinases that regulate stress-induced translational arrest. Here, we demonstrate that the yeast eIF2alpha kinase, GCN2, the target phosphorylation site of Gcn2p, Ser-51 of eIF2alpha, and the eIF2alpha-regulated transcriptional transactivator, GCN4, are essential for another fundamental stress response, starvation-induced autophagy. The mammalian IFN-inducible eIF2alpha kinase, PKR, rescues starvation-induced autophagy in GCN2-disrupted yeast, and pkr null and Ser-51 nonphosphorylatable mutant eIF2alpha murine embryonic fibroblasts are defective in autophagy triggered by herpes simplex virus infection. Furthermore, PKR and eIF2alpha Ser-51-dependent autophagy is antagonized by the herpes simplex virus neurovirulence protein, ICP34.5. Thus, autophagy is a novel evolutionarily conserved function of the eIF2alpha kinase pathway that is targeted by viral virulence gene products.

Increased expression of presenilin 2 inhibits protein synthesis

Mutations in the presenilin genes PS1 and PS2 are a major cause of early onset familial Alzheimer's disease (AD). Previous studies have suggested that presenilins have several functions, including gamma-secretase activity. It was also shown that presenilin expression is increased in the brains of some AD patients and ischemic rodents. The present study examines the effect of increased presenilin expression on protein synthesis. We show here that overexpression of wild-type PS2 (PS2wt) or PS2 mutant containing the FAD mutation N141I (PS2mut) in various cell lines inhibits the synthesis of coexpressed reporter and endogenous proteins. Furthermore, endogenous PS2 seems to be needed for translation inhibition since PS2 null fibroblasts were translationally more active than PS2(+/+) fibroblasts under conditions known to inhibit translation. Overexpression of PS1 also appeared to cause inhibition of protein synthesis, but its effect was much weaker than that of PS2. Taken together, the results suggest that increased expression of PS2 and possibly also of PS1 inhibits translation and that presenilins may function as regulators of protein synthesis.

Complementary signaling pathways regulate the unfolded protein response and are required for C. elegans development

The unfolded protein response (UPR) is a transcriptional and translational intracellular signaling pathway activated by the accumulation of unfolded proteins in the lumen of the endoplasmic reticulum (ER). We have used C. elegans as a genetic model system to dissect UPR signaling in a multicellular organism. C. elegans requires ire-1-mediated splicing of xbp-1 mRNA for UPR gene transcription and survival upon ER stress. In addition, ire-1/xbp-1 acts with pek-1, a protein kinase that mediates translation attenuation, in complementary pathways that are essential for worm development and survival. We propose that UPR transcriptional activation by ire-1 as well as translational attenuation by pek-1 maintain ER homeostasis. The results demonstrate that the UPR and ER homeostasis are essential for metazoan development.

Sarco/endoplasmic reticulum calcium ATPase-2 expression is regulated by ATF6 during the endoplasmic reticulum stress response: intracellular signaling of calcium stress in a cardiac myocyte model system

The recently described transcription factor, ATF6, mediates the expression of proteins that compensate for potentially stressful changes in the endoplasmic reticulum (ER), such as reduced ER calcium. In cardiac myocytes the maintenance of optimal calcium levels in the sarcoplasmic reticulum (SR), a specialized form of the ER, is required for proper contractility. The present study investigated the hypothesis that ATF6 serves as a regulator of the expression of sarco/endoplasmic reticulum calcium ATPase-2 (SERCA2), a protein that transports calcium into the SR from the cytoplasm. Depletion of SR calcium in cultured cardiac myocytes fostered the translocation of ATF6 from the ER to the nucleus, activated the promoter for rat SERCA2, and led to increased levels of SERCA2 protein. SERCA2 promoter induction by calcium depletion was partially blocked by dominant-negative ATF6, whereas constitutively activated ATF6 led to SERCA2 promoter activation. Mutation analyses identified a promoter-proximal ER stress-response element in the rat SERCA2 gene that was required for maximal induction by ATF6 and calcium depletion. Although this element was shown to be responsible for all of the effects of ATF6 on SERCA2 promoter activation, it was responsible for only a portion of the effects of calcium depletion. Thus, SERCA2 induction in response to calcium depletion appears to be a potentially physiologically important compensatory response to this stress that involves intracellular signaling pathways that are both dependent and independent of ATF6.

Heme-regulated eIF2alpha kinase (HRI) is required for translational regulation and survival of erythroid precursors in iron deficiency

Although the physiological role of tissue-specific translational control of gene expression in mammals has long been suspected on the basis of biochemical studies, direct evidence has been lacking. Here, we report on the targeted disruption of the gene encoding the heme-regulated eIF2alpha kinase (HRI) in mice. We establish that HRI, which is expressed predominantly in erythroid cells, regulates the synthesis of both alpha- and beta-globins in red blood cell (RBC) precursors by inhibiting the general translation initiation factor eIF2. This inhibition occurs when the intracellular concentration of heme declines, thereby preventing the synthesis of globin peptides in excess of heme. In iron-deficient HRI(-/-) mice, globins devoid of heme aggregated within the RBC and its precursors, resulting in a hyperchromic, normocytic anemia with decreased RBC counts, compensatory erythroid hyperplasia and accelerated apoptosis in bone marrow and spleen. Thus, HRI is a physiological regulator of gene expression and cell survival in the erythroid lineage.

Translation initiation control by heme-regulated eukaryotic initiation factor 2alpha kinase in erythroid cells under cytoplasmic stresses

Cytoplasmic stresses, including heat shock, osmotic stress, and oxidative stress, cause rapid inhibition of protein synthesis in cells through phosphorylation of eukaryotic initiation factor 2alpha (eIF2alpha) by eIF2alpha kinases. We have investigated the role of heme-regulated inhibitor (HRI), a heme-regulated eIF2alpha kinase, in stress responses of erythroid cells. We have demonstrated that HRI in reticulocytes and fetal liver nucleated erythroid progenitors is activated by oxidative stress induced by arsenite, heat shock, and osmotic stress but not by endoplasmic reticulum stress or nutrient starvation. While autophosphorylation is essential for the activation of HRI, the phosphorylation status of HRI activated by different stresses is different. The contributions of HRI in various stress responses were assessed with the aid of HRI-null reticulocytes and fetal liver erythroid cells. HRI is the only eIF2alpha kinase activated by arsenite in erythroid cells, since HRI-null cells do not induce eIF2alpha phosphorylation upon arsenite treatment. HRI is also the major eIF2alpha kinase responsible for the increased eIF2alpha phosphorylation upon heat shock in erythroid cells. Activation of HRI by these stresses is independent of heme and requires the presence of intact cells. Both hsp90 and hsc70 are necessary for all stress-induced HRI activation. However, reactive oxygen species are involved only in HRI activation by arsenite. Our results provide evidence for a novel function of HRI in stress responses other than heme deficiency.

Endoplasmic reticulum stress-induced cysteine protease activation in cortical neurons: effect of an Alzheimer's disease-linked presenilin-1 knock-in mutation

Endoplasmic reticulum (ER) stress elicits protective responses of chaperone induction and translational suppression and, when unimpeded, leads to caspase-mediated apoptosis. Alzheimer's disease-linked mutations in presenilin-1 (PS-1) reportedly impair ER stress-mediated protective responses and enhance vulnerability to degeneration. We used cleavage site-specific antibodies to characterize the cysteine protease activation responses of primary mouse cortical neurons to ER stress and evaluate the influence of a PS-1 knock-in mutation on these and other stress responses. Two different ER stressors lead to processing of the ER-resident protease procaspase-12, activation of calpain, caspase-3, and caspase-6, and degradation of ER and non-ER protein substrates. Immunocytochemical localization of activated caspase-3 and a cleaved substrate of caspase-6 confirms that caspase activation extends into the cytosol and nucleus. ER stress-induced proteolysis is unchanged in cortical neurons derived from the PS-1 P264L knock-in mouse. Furthermore, the PS-1 genotype does not influence stress-induced increases in chaperones Grp78/BiP and Grp94 or apoptotic neurodegeneration. A similar lack of effect of the PS-1 P264L mutation on the activation of caspases and induction of chaperones is observed in fibroblasts. Finally, the PS-1 knock-in mutation does not alter activation of the protein kinase PKR-like ER kinase (PERK), a trigger for stress-induced translational suppression. These data demonstrate that ER stress in cortical neurons leads to activation of several cysteine proteases within diverse neuronal compartments and indicate that Alzheimer's disease-linked PS-1 mutations do not invariably alter the proteolytic, chaperone induction, translational suppression, and apoptotic responses to ER stress.

Differences in HAC1 mRNA processing and translation between yeast and mammalian cells indicate divergence of the eukaryotic ER stress response

Perturbation of normal endoplasmic reticulum (ER) function induces a stress response found throughout eukaryotes, sometimes termed the unfolded protein response (UPR). In yeast, auxotrophic mutants have identified two genes, IRE1 and HAC1, whose products are key components. Normally HAC1 mRNA is not translated owing to a 252-nt "intron." Disruption of ER function activates Ire1p to remove this intron through endogenous endoribonuclease activity. Together with tRNA ligase, cleavage and splicing produces a translatable HAC1 mRNA to give Hac1p, a transcription factor that upregulates the expression of genes responsive to ER stress. No Hac1p homologue has been identified in mammalian cells, but Ire1p homologues exist with endoribonuclease activity required for a fully functional UPR, raising the possibility that the key features of the yeast UPR might be conserved in higher eukaryotic cells. To address this, we expressed yeast HAC1 in HeLa and HEK 293T human cell lines, both on its own and as fusions with yellow fluorescent protein (YFP) to investigate its processing and translation. HAC1 mRNA was not processed, but efficiently translated irrespective of whether the cells were subjected to ER stress. Expression of exogenous HAC1 mRNA constructs in yeast showed UPR-induced splicing required the presence of its 3' UTR. These results suggest that the mammalian ER stress response has diverged from the yeast UPR.

Regulated expression of GRP78 during vasopressin-induced hypertrophy of heart-derived myocytes

Although the development of cellular hypertrophy is widely believed to involve Ca(2+) signaling, potential supporting roles for sequestered Ca(2+) in this process have not been explored. H9c2 cardiomyocytes respond to arginine vasopressin with an initial mobilization of Ca(2+) stores and reduced rates of mRNA translation followed by repletion of Ca(2+) stores, up-regulation of translation beyond initial rates, and the development of hypertrophy. Rates of synthesis of the endoplasmic reticulum (ER) chaperones, GRP78 and GRP94, were found to increase preferentially at early times of vasopressin treatment. Total GRP78 content increased 2- to 3-fold within 8 h after which the chaperone was subject to post-translational modification. Preferential synthesis of GRP78 and the increase in chaperone content both occurred at pM vasopressin concentrations and were abolished at supraphysiologic Ca(2+) concentrations. Co-treatment with phorbol myristate acetate decreased vasopressin-dependent Ca(2+) mobilization and slowed appearance of new GRP78 molecules in response to the hormone, whereas 24 h pretreatment with phorbol ester prolonged vasopressin-dependent Ca(2+) mobilization and further increased rates of GRP78 synthesis in response to the hormone. Findings did not support a role for newly synthesized GRP78 in translational up-regulation by vasopressin. However up-regulation, which does not depend on Ca(2+) sequestration, appeared to expedite chaperone expression. This report provides the first evidence that a Ca(2+)-mobilizing hormone at physiologic concentrations signals increased expression of GRP78. Translational tolerance to depletion of ER Ca(2+) stores, typifying a robust ER stress response, did not accompany vasopressin-induced hypertrophy.

Translational control is required for the unfolded protein response and in vivo glucose homeostasis

The accumulation of unfolded protein in the endoplasmic reticulum (ER) attenuates protein synthesis initiation through phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF2alpha) at Ser51. Subsequently, transcription of genes encoding adaptive functions including the glucose-regulated proteins is induced. We show that eIF2alpha phosphorylation is required for translation attenuation, transcriptional induction, and survival in response to ER stress. Mice with a homozygous mutation at the eIF2alpha phosphorylation site (Ser51Ala) died within 18 hr after birth due to hypoglycemia associated with defective gluconeogenesis. In addition, homozygous mutant embryos and neonates displayed a deficiency in pancreatic beta cells. The results demonstrate that regulation of translation through eIF2alpha phosphorylation is essential for the ER stress response and in vivo glucose homeostasis.

Diabetes mellitus and exocrine pancreatic dysfunction in perk-/- mice reveals a role for translational control in secretory cell survival

The protein kinase PERK couples protein folding in the endoplasmic reticulum (ER) to polypeptide biosynthesis by phosphorylating the alpha subunit of eukaryotic translation initiation factor 2 (eIF2alpha), attenuating translation initiation in response to ER stress. PERK is highly expressed in mouse pancreas, an organ active in protein secretion. Under physiological conditions, PERK was partially activated, accounting for much of the phosphorylated eIF2alpha in the pancreas. The exocrine and endocrine pancreas developed normally in Perk-/- mice. Postnatally, ER distention and activation of the ER stress transducer IRE1alpha accompanied increased cell death and led to progressive diabetes mellitus and exocrine pancreatic insufficiency. These findings suggest a special role for translational control in protecting secretory cells from ER stress.

Intracellular signaling from the endoplasmic reticulum to the nucleus: the unfolded protein response in yeast and mammals

Cellular survival of endoplasmic reticulum stress requires the unfolded protein response (UPR), a stress response first elucidated genetically in yeast. While we continue to refine our knowledge of the yeast system, especially the breadth and significance of the transcriptional response, conservation of the system's elements has allowed identification of corresponding and additional components of the mammalian UPR. Recent results reveal that the output of the mammalian UPR reaches beyond transcriptional regulation of secretory pathway components to control of general translation, the cell cycle and programmed cell death.

Brain ischemia and reperfusion activates the eukaryotic initiation factor 2alpha kinase, PERK

Reperfusion after global brain ischemia results initially in a widespread suppression of protein synthesis in neurons, which persists in vulnerable neurons, that is caused by the inhibition of translation initiation as a result of the phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 (eIF2alpha). To identify kinases responsible for eIF2alpha phosphorylation [eIF2alpha(P)] during brain reperfusion, we induced ischemia by bilateral carotid artery occlusion followed by post-ischemic assessment of brain eIF2alpha(P) in mice with homozygous functional knockouts in the genes encoding the heme-regulated eIF2alpha kinase (HRI), or the amino acid-regulated eIF2alpha kinase (GCN2). A 10-fold increase in eIF2alpha(P) was observed in reperfused wild-type mice and in the HRI-/- or GCN2-/- mice. However, in all reperfused groups, the RNA-dependent protein kinase (PKR)-like endoplasmic reticulum eIF2alpha kinase (PERK) exhibited an isoform mobility shift on SDS-PAGE, consistent with the activation of the kinase. These data indicate that neither HRI nor GCN2 are required for the large increase in post-ischemic brain eIF2alpha(P), and in conjunction with our previous report that eIF2alpha(P) is produced in the brain of reperfused PKR-/- mice, provides evidence that PERK is the kinase responsible for eIF2alpha phosphorylation in the early post-ischemic brain.

The unfolded protein response and Alzheimer's disease

Disruption of calcium homeostasis, inhibition of protein glycosylation, and reduction of disulfide bonds provoke accumulation of unfolded protein in the endoplasmic reticulum (ER), and are therefore a type of 'ER stress'. Normal cells respond to ER stress by increasing transcription of genes encoding ER-resident chaperones such as GRP78/BiP, GRP94 and protein disulfide isomerase to facilitate protein folding. This induction system is termed the unfolded protein response. Familial Alzheimer's disease-linked presenilin-1 (PS1) mutation downregulates the unfolded protein response and leads to vulnerability to ER stress. The mechanisms by which mutant PS1 affects the ER stress response are attributed to the inhibited activation of ER stress transducers such as IRE1, PERK and ATF6.

Minimum requirements for the function of eukaryotic translation initiation factor 2

Eukaryotic translation initiation factor 2 (eIF2) is a G protein heterotrimer required for GTP-dependent delivery of initiator tRNA to the ribosome. eIF2B, the nucleotide exchange factor for eIF2, is a heteropentamer that, in yeast, is encoded by four essential genes and one nonessential gene. We found that increased levels of wild-type eIF2, in the presence of sufficient levels of initiator tRNA, overcome the requirement for eIF2B in vivo. Consistent with bypassing eIF2B, these conditions also suppress the lethal effect of overexpressing the mammalian tumor suppressor PKR, an eIF2alpha kinase. The effects described are further enhanced in the presence of a mutation in the G protein (gamma) subunit of eIF2, gcd11-K250R, which mimics the function of eIF2B in vitro. Interestingly, the same conditions that bypass eIF2B also overcome the requirement for the normally essential eIF2alpha structural gene (SUI2). Our results suggest that the eIF2betagamma complex is capable of carrying out the essential function(s) of eIF2 in the absence of eIF2alpha and eIF2B and are consistent with the idea that the latter function primarily to regulate the level of eIF2.GTP.Met-tRNA(i)(Met) ternary complexes in vivo.

Budding Yeast GCN1 Binds the GI Domain to Activate the eIF2α Kinase GCN2

When starved for a single amino acid, the budding yeast Saccharomyces cerevisiae activates the eukaryotic initiation factor 2alpha (eIF2alpha) kinase GCN2 in a GCN1-dependent manner. Phosphorylated eIF2alpha inhibits general translation but selectively derepresses the synthesis of the transcription factor GCN4, which leads to coordinated induction of genes involved in biosynthesis of various amino acids, a phenomenon called general control response. We recently demonstrated that this response requires binding of GCN1 to the GI domain occurring at the N terminus of GCN2 (Kubota, H., Sakaki, Y., and Ito, T. (2000) J. Biol. Chem. 275, 20243-20246). Here we provide the first evidence for the involvement of GCN1-GCN2 interaction in activation of GCN2 per se. We identified a C-terminal segment of GCN1 sufficient to bind the GI domain and used a novel dual bait two-hybrid method to identify mutations rendering GCN1 incapable of interacting with GCN2. The yeast bearing such an allele, gcn1-F2291L, fails to display derepression of GCN4 translation and hence general control response, as does a GI domain mutant, gcn2-Y74A, defective in association with GCN1. Furthermore, we demonstrated that phosphorylation of eIF2alpha is impaired in both mutants. Since GCN2 is the sole eIF2alpha kinase in yeast, these findings indicate a critical role of GCN1-GCN2 interaction in activation of the kinase in vivo.

Ischemia-induced inhibition of the initiation factor 2alpha phosphatase activity in the rat brain

Rats were subjected to the standard four-vessel occlusion model of brain transient ischemia for 30 min. Following different recirculation periods, the level of phosphorylation of the initiation factor 2 subunit alpha (eIF2alpha) and the eIF2alpha kinase/s and phosphatase/s activity were determined. eIF2alpha phosphorylation significantly increased very early during reperfusion (10-30 min), recovering at 4 h of reperfusion. Activation of any eIF2alpha kinases studied during ischemia or reperfusion was not noted. Conversely, eIF2alpha phosphatase activity significantly decreased at 10-15 min of reperfusion, reaching values even higher than in controls at 2-4 h of reperfusion. Our results support the hypothesis that the reperfusion-induced phosphorylated eIF2alpha changes are at least a result of the transiently eIF2alpha phosphatase inhibition.

Screening from a subtracted embryonic chick hindbrain cDNA library: identification of genes expressed during hindbrain, midbrain and cranial neural crest development

The vertebrate hindbrain is segmented into a series of transient structures called rhombomeres. Despite knowing several factors that are responsible for the segmentation and maintenance of the rhombomeres, there are still large gaps in understanding the genetic pathways that govern their development. To find previously unknown genes that are expressed within the embryonic hindbrain, a subtracted chick hindbrain cDNA library has been made and 445 randomly picked clones from this library have been analysed using whole mount in situ hybridisation. Thirty-six of these clones (8%) display restricted expression patterns within the hindbrain, midbrain or cranial neural crest and of these, twenty-two are novel and eleven encode peptides that correspond to or are highly related to proteins with previously uncharacterised roles during early neural development. The large proportion of genes with restricted expression patterns and previously unknown functions in the embryonic brain identified during this screen provides insights into the different types of molecules that have spatially regulated expression patterns in cranial neural tissue.

Carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) induces initiation factor 2 alpha phosphorylation and translation inhibition in PC12 cells

We have investigated the effect of the mitochondrial uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) on protein synthesis rate and initiation factor 2 (eIF2) phosphorylation in PC12 cells differentiated with nerve growth factor. FCCP treatment induced a very rapid 2-fold increase in intracellular Ca(2+) concentration that was accompanied by a strong protein synthesis rate inhibition (68%). The translation inhibition correlated with an increased phosphorylation of the alpha subunit of eIF2 (eIF2 alpha) (25% vs. 7%, for FCCP-treated and control cells, respectively) and a 1.7-fold increase in the double-stranded RNA-dependent protein kinase activity. No changes in the PKR endoplasmic reticulum-related kinase or eIF2 alpha phosphatase were found. Translational regulation may play a significant role in the process triggered by mitochondrial calcium mobilization.

Characterization of the initiation factor eIF2B and its regulation in Drosophila melanogaster

Eukaryotic initiation factor (eIF) 2B catalyzes a key regulatory step in the initiation of mRNA translation. eIF2B is well characterized in mammals and in yeast, although little is known about it in other eukaryotes. eIF2B is a hetropentamer which mediates the exchange of GDP for GTP on eIF2. In mammals and yeast, its activity is regulated by phosphorylation of eIF2alpha. Here we have cloned Drosophila melanogaster cDNAs encoding polypeptides showing substantial similarity to eIF2B subunits from yeast and mammals. They also exhibit the other conserved features of these proteins. D. melanogaster eIF2Balpha confers regulation of eIF2B function in yeast, while eIF2Bepsilon shows guanine nucleotide exchange activity. In common with mammalian eIF2Bepsilon, D. melanogaster eIF2Bepsilon is phosphorylated by glycogen synthase kinase-3 and casein kinase II. Phosphorylation of partially purified D. melanogaster eIF2B by glycogen synthase kinase-3 inhibits its activity. Extracts of D. melanogaster S2 Schneider cells display eIF2B activity, which is inhibited by phosphorylation of eIF2alpha, showing the insect factor is regulated similarly to eIF2B from other species. In S2 cells, serum starvation increases eIF2alpha phosphorylation, which correlates with inhibition of eIF2B, and both effects are reversed by serum treatment. This shows that eIF2alpha phosphorylation and eIF2B activity are under dynamic regulation by serum. eIF2alpha phosphorylation is also increased by endoplasmic reticulum stress in S2 cells. These are the first data concerning the structure, function or control of eIF2B from D. melanogaster.

Inhibitory sequences in the N-terminus of the double-stranded-RNA-dependent protein kinase, PKR, are important for regulating phosphorylation of eukaryotic initiation factor 2alpha (eIF2alpha)

During viral infection, phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF2alpha) by the interferon-induced RNA-dependent protein kinase, PKR, leads to inhibition of translation initiation and viral proliferation. Activation of PKR is mediated by association of virally encoded double-stranded RNAs (dsRNAs) with two dsRNA binding domains (dsRBDs) located in the N-terminus of PKR. To better understand the molecular mechanisms regulating PKR, we characterized the activities of wild-type and mutant versions of human PKR expressed and purified from yeast. The catalytic rate of eIF2alpha phosphorylation by our purified PKR was increased in response to dsRNA, but not single-stranded RNA or DNA, consistent with the properties previously described for PKR purified from mammalian sources. While both dsRBD1 and dsRBD2 were required for activation of PKR by dsRNA, only deletion of dsRBD1 severely reduced the basal eIF2alpha kinase activity. Removal of as few as 25 residues at the C-terminal junction of dsRBD2 dramatically increased eIF2alpha kinase activity and characterization of larger deletions that included dsRBD1 demonstrated that removal of these negative-acting sequences could bypass the dsRBD1 requirement for in vitro phosphorylation of eIF2alpha. Heparin, a known in vitro activator of PKR, enhanced eIF2alpha phosphorylation by PKR mutants lacking their entire N-terminal sequences, including the dsRBDs. The results indicate that induction of PKR activity is mediated by multiple mechanisms, one of which involves release of inhibition by negative-acting sequences in PKR.

Translational control by the ER transmembrane kinase/ribonuclease IRE1 under ER stress

Under conditions of endoplasmic reticulum (ER) stress, mammalian cells induce both translational repression and the unfolded protein response that transcriptionally activates genes encoding ER-resident molecular chaperones. To date, the only known pathway for translational repression in response to ER stress has been the phosphorylation of eIF-2alpha by the double-stranded RNA-activated protein kinase (PKR) or the transmembrane PKR-like ER kinase (PERK). Here we report another pathway in which the ER transmembrane kinase/ribonuclease IRE1beta induces translational repression through 28S ribosomal RNA cleavage in response to ER stress. The evidence suggests that both pathways are important for efficient translational repression during the ER stress response.

The double-stranded RNA-activated protein kinase PKR is dispensable for regulation of translation initiation in response to either calcium mobilization from the endoplasmic reticulum or essential amino acid starvation

The alpha-subunit of eukaryotic initiation factor eIF2 is a preferred substrate for the double-stranded RNA-activated protein kinase, PKR. Phosphorylation of eIF2alpha converts the factor from a substrate into a competitive inhibitor of the guanine nucleotide exchange factor, eIF2B, leading to a decline in mRNA translation. Early studies provided evidence implicating PKR as the kinase that phosphorylates eIF2alpha under conditions of cell stress such as the accumulation of misfolded proteins in the lumen of the endoplasmic reticulum, i.e., the unfolded protein response (UPR). However, the recent identification of a trans-microsomal membrane eIF2alpha kinase, termed PEK or PERK, suggests that this kinase, and not PKR, might be the kinase that is activated by misfolded protein accumulation. Similarly, genetic studies in yeast provide compelling evidence that a kinase termed GCN2 phosphorylates eIF2alpha in response to amino acid deprivation. However, no direct evidence showing activation of the mammalian homologue of GCN2 by amino acid deprivation has been reported. In the present study, we find that in fibroblasts treated with agents that promote the UPR, protein synthesis is inhibited as a result of a decrease in eIF2B activity. Furthermore, the reduction in eIF2B activity is associated with enhanced phosphorylation of eIF2alpha. Importantly, the magnitude of the change in each parameter is identical in wildtype cells and in fibroblasts containing a chromosomal deletion in the PKR gene (PKR-KO cells). In a similar manner, we find that during amino acid deprivation the inhibition of protein synthesis and extent of increase in eIF2alpha phosphorylation are identical in wildtype and PKR-KO cells. Overall, the results show that PKR is not required for increased eIF2alpha phosphorylation or inhibition of protein synthesis under conditions promoting the UPR or in response to amino acid deprivation.

Characterization of phosphopeptides from protein digests using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and nanoelectrospray quadrupole time-of-flight mass spectrometry

A two-step mass spectrometric method for characterization of phosphopeptides from peptide mixtures is presented. In the first step, phosphopeptide candidates were identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) based on their higher relative intensities in negative ion MALDI spectra than in positive ion MALDI spectra. The detection limit for this step was found to be 18 femtomoles or lower in the case of unfractionated in-solution digests of a model phosphoprotein, beta-casein. In the second step, nanoelectrospray tandem mass (nES-MS/MS) spectra of doubly or triply charged precursor ions of these candidate phosphopeptides were obtained using a quadrupole time-of-flight (Q-TOF) mass spectrometer. This step provided information about the phosphorylated residues, and ruled out nonphosphorylated candidates, for these peptides. After [(32)P] labeling and reverse-phase high-performance liquid chromatography (RP-HPLC) to simplify the mixtures and to monitor the efficiency of phosphopeptide identification, we used this method to identify multiple autophosphorylation sites on the PKR-like endoplasmic reticulum kinase (PERK), a recently discovered mammalian stress-response protein.

Ischemia-induced phosphorylation of initiation factor 2 in differentiated PC12 cells: role for initiation factor 2 phosphatase

An in vitro model of ischemia was obtained by subjecting PC12 cells differentiated with nerve growth factor to a combination of glucose deprivation plus anoxia. Immediately after the ischemic period, the protein synthesis rate was significantly inhibited (80%) and western blots of cell extracts revealed a significant accumulation of phosphorylated eukaryotic initiation factor 2, alpha subunit, eIF2(alphaP) (42%). Upon recovery, eIF2(alphaP) levels returned to control values after 30 min, whereas protein synthesis was still partially inhibited (33%) and reached almost control values within 2 h. The activities of the mammalian eIF2alpha kinases, double-stranded RNA-activated protein kinase, mammalian GCN2 homologue, and endoplasmic reticulum-resident kinase, were determined. None of the eIF2alpha kinases studied showed increased activity in ischemic cells as compared with controls. Exposure of cells to cell-permeable inhibitors of protein phosphatases 1 and 2A, calyculin A or tautomycin, induced dose- and time-dependent accumulation of eIF2(alphaP), mimicking an ischemic effect. Protein phosphatase activity, as measured with [(32)P]phosphorylase a as a substrate, diminished during ischemia and returned to control levels upon 30-min recovery. In addition, the rate of eIF2(alphaP) dephosphorylation was significantly lower in ischemic cells, paralleling both the greatest translational inhibition and the highest eIF2(alphaP) levels. The endogenous phosphatase activity from control and ischemic extracts showed different sensitivity to inhibitor 2 and fostriecin in in vitro assays, inhibitor-2 effect in ischemic cells being lower than in control cells. Together these results indicate that an eIF2alpha phosphatase, probably protein phosphatase 1, is implicated in the ischemia-induced eIF2(alphaP) accumulation in PC12 cells.

PACT, a stress-modulated cellular activator of interferon-induced double-stranded RNA-activated protein kinase, PKR

The interferon (IFN)-induced, double-stranded (ds)RNA-activated serine-threonine protein kinase, PKR, is a key mediator of the antiviral activities of IFNs. In addition, PKR activity is also involved in regulation of cell proliferation, apoptosis, and signal transduction. In virally infected cells, dsRNA has been shown to bind and activate PKR kinase function. Implication of PKR activity in normal cellular processes has invoked activators other than dsRNA because RNAs with perfectly duplexed regions of sufficient length that are able to activate PKR are absent in cellular RNAs. We have recently reported cloning of PACT, a novel protein activator of PKR. PACT heterodimerizes with PKR and activates it by direct protein-protein interaction. Overexpression of PACT in mammalian cells leads to phosphorylation of the alpha subunit of the eukaryotic initiation factor 2 (eIF2alpha), the cellular substrate for PKR, and leads to inhibition of protein synthesis. Here, we present evidence that endogenous PACT acts as a protein activator of PKR in response to diverse stress signals such as serum starvation, and peroxide or arsenite treatment. Following exposure of cells to these stress agents, PACT is phosphorylated and associates with PKR with increased affinity. PACT-mediated activation of PKR leads to enhanced eIF2alpha phosphorylation followed by apoptosis. Based on the results presented here, we propose that PACT is a novel stress-modulated physiological activator of PKR.

Upregulation of BiP and CHOP by the unfolded-protein response is independent of presenilin expression

Presenilin 1 (PS1), a polytopic membrane protein, has a critical role in the trafficking and proteolysis of a selected set of transmembrane proteins. The vast majority of individuals affected with early onset familial Alzheimer's disease (FAD) carry missense mutations in PS1. Two studies have suggested that loss of PS1 function, or expression of FAD-linked PS1 variants, compromises the mammalian unfolded-protein response (UPR), and we sought to evaluate the potential role of PS1 in the mammalian UPR. Here we show that that neither the endoplasmic reticulum (ER) stress-induced accumulation of BiP and CHOP messenger RNA, nor the activation of ER stress kinases IRE1alpha and PERK, is compromised in cells lacking both PS1 and PS2 or in cells expressing FAD-linked PS1 variants. We also show that the levels of BiP are not significantly different in the brains of individuals with sporadic Alzheimer's disease or PS1-mediated FAD to levels in control brains. Our findings provide evidence that neither loss of PS1 and PS2 function, nor expression of PS1 variants, has a discernable impact on ER stress-mediated induction of the several established 'readouts' of the UPR pathway.

Separate domains in GCN1 for binding protein kinase GCN2 and ribosomes are required for GCN2 activation in amino acid-starved cells

GCN2 stimulates GCN4 translation in amino acid-starved cells by phosphorylating the alpha-subunit of translation initiation factor 2. GCN2 function in vivo requires the GCN1/GCN20 complex, which binds to the N-terminal domain of GCN2. A C-terminal segment of GCN1 (residues 2052-2428) was found to be necessary and sufficient for binding GCN2 in vivo and in vitro. Overexpression of this fragment in wild-type cells impaired association of GCN2 with native GCN1 and had a dominant Gcn(-) phenotype, dependent on Arg2259 in the GCN1 fragment. Substitution of Arg2259 with Ala in full-length GCN1 abolished complex formation with native GCN2 and destroyed GCN1 regulatory function. Consistently, the Gcn(-) phenotype of gcn1-R2259A, but not that of gcn1Delta, was suppressed by overexpressing GCN2. These findings prove that GCN2 binding to the C-terminal domain of GCN1, dependent on Arg2259, is required for high level GCN2 function in vivo. GCN1 expression conferred sensitivity to paromomycin in a manner dependent on its ribosome binding domain, supporting the idea that GCN1 binds near the ribosomal acceptor site to promote GCN2 activation by uncharged tRNA.

PERK mediates cell-cycle exit during the mammalian unfolded protein response

The accumulation of unfolded proteins in the endoplasmic reticulum (ER) triggers the unfolded protein response (UPR)-signaling pathway. The UPR coordinates the induction of ER chaperones with decreased protein synthesis and growth arrest in the G(1) phase of the cell cycle. Three ER transmembrane protein kinases (Ire1alpha, Ire1beta, and PERK) have been implicated as proximal effectors of the mammalian UPR. We now demonstrate that activation of PERK signals the loss of cyclin D1 during the UPR, culminating in cell-cycle arrest. Overexpression of wild-type PERK inhibited cyclin D1 synthesis in the absence of ER stress, thereby inducing a G(1) phase arrest. PERK expression was associated with increased phosphorylation of the translation elongation initiation factor 2alpha (eIF2alpha), an event previously shown to block cyclin D1 translation. Conversely, a truncated form of PERK lacking its kinase domain acted as a dominant negative when overexpressed in cells, attenuating both cyclin D1 loss and cell-cycle arrest during the UPR without compromising induction of ER chaperones. These data demonstrate that PERK serves as a critical effector of UPR-induced growth arrest, linking stress in the ER to control of cell-cycle progression.