Heat stress enhances phytohemagglutinin synthesis but inhibits its transport out of the endoplasmic reticulum

Endoplasmic Reticulum Stress of Gut Enterocyte and Intestinal Diseases

The endoplasmic reticulum, a vast reticular membranous network from the nuclear envelope to the plasma membrane responsible for the synthesis, maturation, and trafficking of a wide range of proteins, is considerably sensitive to changes in its luminal homeostasis. The loss of ER luminal homeostasis leads to abnormalities referred to as endoplasmic reticulum (ER) stress. Thus, the cell activates an adaptive response known as the unfolded protein response (UPR), a mechanism to stabilize ER homeostasis under severe environmental conditions. ER stress has recently been postulated as a disease research breakthrough due to its significant role in multiple vital cellular functions. This has caused numerous reports that ER stress-induced cell dysfunction has been implicated as an essential contributor to the occurrence and development of many diseases, resulting in them targeting the relief of ER stress. This review aims to outline the multiple molecular mechanisms of ER stress that can elucidate ER as an expansive, membrane-enclosed organelle playing a crucial role in numerous cellular functions with evident changes of several cells encountering ER stress. Alongside, we mainly focused on the therapeutic potential of ER stress inhibition in gastrointestinal diseases such as inflammatory bowel disease (IBD) and colorectal cancer. To conclude, we reviewed advanced research and highlighted future treatment strategies of ER stress-associated conditions.

Induction of protein body formation in plant leaves by elastin-like polypeptide fusions

BACKGROUND

Elastin-like polypeptides are synthetic biopolymers composed of a repeating pentapeptide 'VPGXG' sequence that are valuable for the simple non-chromatographic purification of recombinant proteins. In addition, elastin-like polypeptide fusions have been shown to enhance the accumulation of a range of different recombinant proteins in plants, thus addressing the major limitation of plant-based expression systems, which is a low production yield. This study's main objectives were to determine the general utility of elastin-like polypeptide protein fusions in various intracellular compartments and to elucidate elastin-like polypeptide's mechanism of action for increasing recombinant protein accumulation in the endoplasmic reticulum of plants.

RESULTS

The effect of elastin-like polypeptide fusions on the accumulation of green fluorescent protein targeted to the cytoplasm, chloroplasts, apoplast, and endoplasmic reticulum was evaluated. The endoplasmic reticulum was the only intracellular compartment in which an elastin-like polypeptide tag was shown to significantly enhance recombinant protein accumulation. Interestingly, endoplasmic reticulum-targeted elastin-like polypeptide fusions induced the formation of a novel type of protein body, which may be responsible for elastin-like polypeptide's positive effect on recombinant protein accumulation by excluding the heterologous protein from normal physiological turnover. Although expressed in the leaves of plants, these novel protein bodies appeared similar in size and morphology to the prolamin-based protein bodies naturally found in plant seeds. The elastin-like polypeptide-induced protein bodies were highly mobile organelles, exhibiting various dynamic patterns of movement throughout the cells, which were dependent on intact actin microfilaments and a functional actomyosin motility system.

CONCLUSION

An endoplasmic reticulum-targeted elastin-like polypeptide fusion approach provides an effective strategy for depositing large amounts of concentrated heterologous protein within the limited space of the cell via storage in stable protein bodies. Furthermore, encapsulation of recombinant proteins into physiologically inert organelles can function to insulate the protein from normal cellular mechanisms, thus limiting unnecessary stress to the host cell. Since elastin-like polypeptide is a mammalian-derived protein, this study demonstrates that plant seed-specific factors are not required for the formation of protein bodies in vegetative plant tissues, suggesting that the endoplasmic reticulum possesses an intrinsic ability to form protein body-like accretions in eukaryotic cells when overexpressing particular proteins.

Inducible lectins and plant resistance to pathogens and abiotic stress

Lectin concentration (activity) increases in plant tissues upon infection by pathogens, in response to abiotic stress, as well as during growth and development of tissues. Such a broad range of events accompanied by accumulation of lectins is indicative of their involvement in regulation of integral processes in plant cells. Data concerning the role of lectins in regulation of oxidative stress and stress-induced cytoskeleton rearrangements are presented.

Small heat shock proteins genes are differentially expressed in distinct varieties of common bean

Plants respond to temperature stress by synthesizing a set of heat shock proteins (HSPs), which may be responsible for the acquisition of thermotolerance. In this study, the induction of small HSPs (sHSPs) in eight common bean varieties was evaluated by Northern blot analysis using the W HSP 16.9 cDNA as heterologous probe. Cowpea was used, as a positive control since this plant, as opposed to common bean, is known to grow well under high temperature regimes such as that found in the Brazilian semi-arid region. After the growth period, the plants were submitted to two h of heat shock at 40 ºC. All varieties tested were able to induce sHSP mRNAs that hybridized with W HSP 16.9 probe. However, significant kinetic differences were found when comparing different varieties. SHSP mRNA levels induced after heat shock in cowpea was higher than the levels observed on the bean varieties displaying the highest expression of these proteins. Besides, the sHSP expression was also assessed at the protein accumulation level by Western-blot analysis for cowpea and both IPA 7 and Negro Argel varieties of bean plants. The revealed protein pattern confirmed that sHSPs are differentially expressed in distinct varieties of common bean according their heat stress tolerance.

Influence of KDEL on the fate of trimeric or assembly-defective phaseolin: selective use of an alternative route to vacuoles

The tetrapeptide KDEL is commonly found at the C terminus of soluble proteins of the endoplasmic reticulum (ER), and it contributes to their localization by interacting with a receptor that recycles between the Golgi complex and the ER. We investigated the effects of the addition of KDEL to phaseolin, a protein normally delivered from the ER to storage vacuoles via the Golgi complex. We show that KDEL prevents acquisition of trans-Golgi-specific glycan modifications and causes interactions with the chaperone BiP that are distinct from the ones between BiP and defective proteins. KDEL markedly increases the stability of phaseolin, but a small proportion of phaseolin-KDEL slowly reaches the vacuole without undergoing Golgi-mediated glycan modifications, in a process that can be inhibited by brefeldin A but not monensin. Our results indicate that KDEL can operate with high efficiency before proteins can reach the late Golgi cisternae but allows or promotes delivery to vacuoles via an alternative mechanism. However, addition of KDEL does not alter the destiny of an assembly-defective form of phaseolin, suggesting that the plant ER quality control mechanism is dominant over KDEL effects.

Misfolding and aggregation of vacuolar glycoproteins in plant cells

Phaseolin and lectin-related polypeptides, the abundant oligomeric glycoproteins of bean seeds, are synthesized on the endoplasmic reticulum (ER) and then transported to the storage vacuole via the Golgi apparatus. Glycosylation and folding are among the major modifications these proteins undergo in the ER. Although a recurrent role of N-glycosylation is on protein folding, in previous studies on common bean (Phaseolus vulgaris) seeds we demonstrated that the oligosaccharide side-chains are not required for folding, intracellular transport and activity of storage glycoproteins. We show here that in lima bean (Phaseolus lunatus), incubation of the developing cotyledon with tunicamycin to prevent glycosylation has a dramatic effect on the intracellular transport of the storage glycoproteins. When lacking their glycans, phaseolin and lectin-related polypeptides misfold and are retained in the ER as mixed aggregates to which the chaperone BiP irreversibly associates. The lumen of the ER becomes enlarged to accommodate the aggregated polypeptides. Intracellular transport of legumin, a naturally unglycosylated storage protein, is mostly unaffected by the inhibitor, indicating that the observed phenomenon specifically occurs on glycoproteins. Furthermore, recombinant lima bean phaseolin synthesized in tobacco protoplasts is also correctly folded and matured in the presence of tunicamycin. To our knowledge, this is the first report that describes in detail the block of intracellular transport of vacuolar glycoproteins in plant cells due to aggregation following glycosylation inhibition.

A comparison between virus replication and abiotic stress (heat) as modifiers of host gene expression in pea

Abstract Pea embryonic tissues respond to active replication of pea seed-borne mosaic potyvirus (PSbMV) by the down-regulation of a range of genes and the induction of others. Both of these responses can be seen when tissues are subjected to abiotic stress, particularly heat. We have compared the effects of the two inducers to assess whether the host alterations following virus replication represent generic responses to stress, or more specific effects. Five classes of response were identified: (i) genes induced by both stresses (e.g. heat shock protein 70, hsp70); (ii) genes induced by virus replication but unaffected by heat (e.g. glutathione reductase 2, gor2); (iii) genes induced by heat but unaffected by virus replication (e.g. heat shock factor, hsf); (iv) genes down-regulated by virus replication and unaffected by heat (e.g. vicilin, vic); and (v) genes unaffected by both inducers (e.g. actin, act and beta-tubulin, tub). A change in the appearance and organization of the endoplasmic reticulum (ER) was also seen in cells actively replicating PSbMV RNA. Heat treatment of pea embryonic tissues also produced altered ER, although the changes were different from those seen following virus infection. Collectively, these data show that, while there are some common features of the responses to virus infection and heat, there are also substantial differences. Hence, it appears that the host response to virus replication is not a general stress response.

Protein quality control along the route to the plant vacuole

To acquire information on the relationships between structural maturation of proteins in the endoplasmic reticulum (ER) and their transport along the secretory pathway, we have analyzed the destiny of an assembly-defective form of the trimeric vacuolar storage glycoprotein phaseolin. In leaves of transgenic tobacco, where assembly-competent phaseolin is correctly targeted to the vacuole, defective phaseolin remains located in the ER or a closely related compartment where it represents a major ligand of the chaperone BiP. Defective phaseolin maintained susceptibility to endoglycosidase H and was slowly degraded by a process that is not inhibited by heat shock or brefeldin A, indicating that degradation does not involve transport along the secretory pathway. These results provide evidence for the presence of a quality control mechanism in the ER of plant cells that avoids intracellular trafficking of severely defective proteins and eventually leads to their degradation.

Protein quality control along the route to the plant vacuole

To acquire information on the relationships between structural maturation of proteins in the endoplasmic reticulum (ER) and their transport along the secretory pathway, we have analyzed the destiny of an assembly-defective form of the trimeric vacuolar storage glycoprotein phaseolin. In leaves of transgenic tobacco, where assembly-competent phaseolin is correctly targeted to the vacuole, defective phaseolin remains located in the ER or a closely related compartment where it represents a major ligand of the chaperone BiP. Defective phaseolin maintained susceptibility to endoglycosidase H and was slowly degraded by a process that is not inhibited by heat shock or brefeldin A, indicating that degradation does not involve transport along the secretory pathway. These results provide evidence for the presence of a quality control mechanism in the ER of plant cells that avoids intracellular trafficking of severely defective proteins and eventually leads to their degradation.

Heat shock inhibits release of the signal recognition particle from the endoplasmic reticulum in barley aleurone layers

When barley (Hordeum vulgare) aleurone layers are subjected to heat shock there is a selective degradation of the normally stable mRNAs encoding secreted proteins. Messages for nonsecreted proteins are not degraded. The synthesis of heat shock proteins is not required for this selective message degradation. Our hypothesis explaining this phenomenon is that a component of the early steps in the synthesis of secreted proteins is damaged by heat shock, resulting in a selective halt in translation on secretory mRNAs, which may in turn lead to degradation of those messages. The first committed step in the synthesis of secreted proteins is the binding of the nascent signal sequence to the signal recognition particle. We have obtained cDNA clones and antibodies for the barley 54-kDa subunit of the signal recognition particle. In cell fractionation experiments, more signal recognition particle was bound to the endoplasmic reticulum membranes and less was in the free particle fraction following a heat shock. The results suggest that heat shock inhibits the release of the signal recognition particle from the endoplasmic reticulum. This would, in turn, inhibit the resumption of translation and may be the underlying cause of the secretory message degradation.

Effect of Gibberellin and Heat Shock on the Lipid Composition of Endoplasmic Reticulum in Barley Aleurone Layers

The heat-shock responses of barley (Hordeum vulgare L. cv Hi- malaya) aleurone layers incubated with or without gibberellic acid (GA3) were compared. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that heat shock blocked the synthesis and secretion of secretory proteins from GA3-treated layers but not untreated layers. This suppression of secretory protein synthesis has been correlated with changes in endoplasmic reticulum (ER) membranes (F.C. Belanger, M. R. Brodl, T.-h.D. Ho [1986] Proc Natl Acad Sci USA 83: 1354-1358; L. Sticher, A.K. Biswas, D.S. Bush, R.L. Jones [1990] Plant Physiol 92: 506-513). Our secretion data suggested that the ER membranes of aleurone layers incubated without GA3 may be more heat shock tolerant. To investigate this, the lipid profiles of membrane extracts in aleurone layers labeled with [14C]glycerol were examined. Heat shock markedly increased [14C]glycerol incorporation into phosphatidylcholine (PC), and gas chromatography revealed an increase in the amount of saturated fatty acids associated with thin layer chromatography-purified PC in GA3-treated layers. In contrast, aleurone layers incubated without GA3 at normal temperature contained PC-associated fatty acids with a greater degree of saturation than GA3-treated layers. Heat shock modestly increased the degree of fatty acid saturation in untreated aleurone layers. This same trend was noted in fatty acids isolated from ER membranes purified by continuous sucrose density centrifugation. We propose that increased fatty acid saturation may help sustain ER membrane function in heat-shocked aleurone layers incubated in the absence of GA3.

Expression of the BnmNAP subfamily of napin genes coincides with the induction of Brassica microspore embryogenesis

Brassica napus cv. Topas microspores can be diverted from pollen development toward haploid embryo formation in culture by subjecting them to a heat stress treatment. We show that this switch in developmental pathways is accompanied by the induction of high levels of napin seed storage protein gene expression. Changes in the plant growth or microspore culture conditions were not by themselves sufficient to induce napin gene expression. Specific members of the napin multigene family were cloned from a cDNA library prepared from microspores that had been induced to undergo embryogenesis. The majority of napin clones represented three members (BnmNAP2, BnmNAP3 and BnmNAP4) that, along with a previously isolated napin genomic clone (BngNAP1), constitute the highly conserved BnmNAP subfamily of napin genes. Both RNA gel blot analysis, using a subfamily-specific probe, and histochemical analysis of transgenic plants expressing a BngNAP1 promoter-beta-glucuronidase gene fusion demonstrated that the BnmNAP subfamily is expressed in embryogenic microspores as well as during subsequent stages of microsporic embryo development.

Ubiquitin Pool Modulation and Protein Degradation in Wheat Roots during High Temperature Stress

Ubiquitin, a key component in an ATP-dependent proteolytic pathway, participates in the response of various eucaryotic organisms to high temperature stress. Our objective was to determine if ubiquitin serves a similar capacity for metabolizing altered proteins in higher plants during stress. Degradation of total proteins was measured, and ubiquitin pools (free versus conjugated) were extracted with an improved protocol from wheat (Triticum aestivum L. cv Len) roots treated at 22, 27, 32, 37, and 42 degrees C for 1 hour and assayed by western blots and radioimmunoassays. Heat-shock protein synthesis was detected by in vivo labeling and autoradiography. Mean half-life of total root proteins decreased from 51 hours at 22 degrees C to 23 hours at 40 degrees C. Ubiquitin pools were extracted better and proteolysis was slowed more by the improved protocol than by a conventional procedure for plant proteins. Amounts of high molecular mass conjugates were elevated and levels of low molecular mass conjugates and free ubiquitin were depressed when roots were treated at 37 or 42 degrees C than at lower temperatures; the same high temperatures also induced synthesis of heat-shock proteins. We concluded that high temperatures increase breakdown of root proteins, which are degraded via the ubiquitin proteolytic pathway. A conjugate with an apparent molecular mass of 23 kilodaltons was tentatively identified as an ubiquitinated histone.

Heat Shock Inhibits alpha-Amylase Synthesis in Barley Aleurone without Inhibiting the Activity of Endoplasmic Reticulum Marker Enzymes

The effects of heat shock on the synthesis of alpha-amylase and on the membranes of the endoplasmic reticulum (ER) of barley (Hordeum vulgare) aleurone were studied. Heat shock, imposed by raising the temperature of incubation from 25 degrees C to 40 degrees C for 3 hours, inhibits the accumulation of alpha-amylase and other proteins in the incubation medium of barley aleurone layers treated with gibberellic acid and Ca(2+). When ER is isolated from heat-shocked aleurone layers, less newly synthesized alpha-amylase is found associated with this membrane system. ER membranes, as indicated by the activities of NADH cytochrome c reductase and ATP-dependent Ca(2+) transport, are not destroyed by heat stress, however. Although heat shock did not reduce the activity of ER membrane marker enzymes, it altered the buoyant density of these membranes. Whereas ER from control tissue showed a peak of marker enzyme activity at 27% to 28% sucrose (1.113-1.120 grams per cubic centimeter), ER from heat-shocked tissue peaked at 30% to 32% sucrose (1.127-1.137 grams per cubic centimeter). The synthesis of a group of proteins designated as heat-shock proteins (HSPs) was stimulated by heat shock. These HSPs were localized to different compartments of the aleurone cell. Several proteins ranging from 15 to 30 kilodaltons were found in the ER and the mitochondrial/plasma membrane fractions of heat-shocked cells, but none of the HSPs accumulated in the incubation medium of heat-shocked aleurone layers.

Crosslinking of microsomal proteins identifies P-9000, a protein that is co-transported with phaseolin and phytohemagglutinin in bean cotyledons

Developing cotyledons of the common bean, Phaseolus vulgaris L., transport within their secretory system (endoplasmic reticulum and Golgi apparatus) the abundant vacuolar proteins, phaseolin and phytohemagglutinin. To identify proteins that may play a role in vacuolar targeting, we treated cotyledon microsomal fractions with a bifunctional crosslinking reagent, dithiobis(succinimidyl propionate), isolated protein complexes with antibodies to phaseolin and phytohemagglutinin, and analysed the polypeptides by sodium dodecylsulfate polyacrylamide gel electrophoresis. This allowed us to identify a protein of Mr=9000 (P-9000) that was crosslinked to both phaseolin and phytohemagglutinin. P-900 is abundantly present in the endoplasmic reticulum. The aminoterminus of P-9000 shows extensive sequence identity with the amino-terminus of PA1 (Mr=11 000), a cysteine-rich albumin whose processing products accumulate in the vacuoles of pea (Pisum sativum L.) cotyledons. Like PA1, P-9000 is synthesized as a pre-proprotein that is posttranslationally processed into smaller polypeptides. The possible functions of P-9000 are discussed.

Phosphate Starvation Inducible Metabolism in Lycopersicon esculentum: III. Changes in Protein Secretion under Nutrient Stress

Phosphate starvation increased the secretion of at least six proteins by suspension cultured tomato (Lycopersicon esculentum L. and L. pennellii) cells. Cells exhibited a biphasic response to phosphate (Pi) starvation. The early phase involved enhanced secretion of three proteins in response to transfer to a Pi-depleted media, while biomass accumulation continued at the same rate as in the Pi-sufficient cells. Severe starvation, defined as inhibition of biomass accumulation, induced enhanced secretion of three additional proteins. After sodium dodecyl sulfate-polyacrylamide gel electrophoresis, media proteins were immunoblotted with antibodies reacting specifically to oligosaccharides processed by the Golgi apparatus. Binding patterns showed that the enhancement in secretion during both phases of starvation was Golgi-mediated. Cells undergoing severe starvation had a respiration rate approximately twice that of unstressed cells and secreted 4.4 times more protein into the media per unit biomass. These data suggest overlapping Pi starvation-specific and global stress responses in plant cells. Under these conditions, Golgi-mediated protein secretion is enhanced. We present evidence for phosphate starvation inducible enhancement of Pi uptake. Secreted proteins specific for N and Fe starvation are also identified.

Heath Shock Enhances the Synthesis of a Lectin-Related Protein in Dolichos biflorus Cell Suspension Cultures

Dolichos biflorus cell suspension cultures synthesize at least two lectins or lectin-like proteins that are related to the seed lectin from this plant. The synthesis of one of these proteins, DB57, is greatly enhanced in response to heat shock; this enhancement of synthesis is dependent upon the growth phase of the cells in culture. Early in the growth curve, when the cells are in lag phase, there is no detectable induction of DB57 synthesis above the levels found in control cells. Enhancement of DB57 synthesis becomes apparent during the last half of the exponential phase of growth at which time there is approximately a 10-fold increase in the amount of newly synthesized DB57 at 42 degrees C compared with the amount of DB57 synthesized at the control temperature of 27 degrees C. The implication of this finding is discussed with respect to role of lectins in plants.

Induced k efflux from cultured rose cells : effects of protein-synthesis inhibitors

Inhibitors of translation, cycloheximide, emetine, and puromycin, and inhibitors of transcription, actinomycin D and cordycepin, stimulate a net efflux of K(+) from cultured cells of Rosa damascena. In the case of cycloheximide and emetine, this efflux bears many similarities to the efflux induced by ultraviolet radiation, including a lag period of 0.25 to 2.5 hours and a limited total loss of K(+). The efflux is transient, and continued incubation of cells with cycloheximide and emetine allows the cells to recover the K(+); after this, the cells no longer release K(+) in response to UV or to cycloheximide treatment. This suggests that the efflux process depends on continued protein synthesis. Other treatments such as cerulenin, low temperature (0 degrees C), and high temperature (40 degrees C) also inhibit the UV- and cycloheximide-induced K(+) efflux, suggesting that the induction of efflux may involve the synthesis of new plasma membrane.