Oral administration of bovine milk-derived extracellular vesicles induces senescence in the primary tumor but accelerates cancer metastasis

The concept that extracellular vesicles (EVs) from the diet can be absorbed by the intestinal tract of the consuming organism, be bioavailable in various organs, and in-turn exert phenotypic changes is highly debatable. Here, we isolate EVs from both raw and commercial bovine milk and characterize them by electron microscopy, nanoparticle tracking analysis, western blotting, quantitative proteomics and small RNA sequencing analysis. Orally administered bovine milk-derived EVs survive the harsh degrading conditions of the gut, in mice, and is subsequently detected in multiple organs. Milk-derived EVs orally administered to mice implanted with colorectal and breast cancer cells reduce the primary tumor burden. Intriguingly, despite the reduction in primary tumor growth, milk-derived EVs accelerate metastasis in breast and pancreatic cancer mouse models. Proteomic and biochemical analysis reveal the induction of senescence and epithelial-to-mesenchymal transition in cancer cells upon treatment with milk-derived EVs. Timing of EV administration is critical as oral administration after resection of the primary tumor reverses the pro-metastatic effects of milk-derived EVs in breast cancer models. Taken together, our study provides context-based and opposing roles of milk-derived EVs as metastasis inducers and suppressors.

Fn14: a new player in cancer-induced cachexia

PURPOSE OF REVIEW

Although cancer cachexia is a very significant condition that is present in up to 80% of cancer cases, the cause of the condition has remained a puzzle. Cancer cachexia is a condition which is mainly characterised by muscle wasting, mobilization of fat reserves, and overall metabolic disturbance. This review aims to highlight some of the recent findings in cancer cachexia research.

RECENT RESEARCH

It has been recently demonstrated that the expression of a single receptor, fibroblast growth factor-inducible 14, on a tumour can initiate cachexia and that this can be completely ablated by treatment with an antibody against this receptor. Also recently described was the role of parathyroid hormone receptor-binding proteins in causing cachexia through a mechanism where white adipose tissue is replaced with brown adipose tissue. In parallel, work done in drosophila suggests that the impaired insulin signalling is a direct cause of cancer cachexia through the release of an insulin growth factor binding protein that inhibits insulin and insulin-like growth factor 1 signalling.

SUMMARY

Successful therapies are urgently needed to combat cancer cachexia in the clinic. Recent research is making progress toward discovering the underlying molecular causes of the condition, which could lead to new therapeutic approaches and in the future contribute to more positive clinical outcomes for cancer sufferers.

Targeting of Fn14 Prevents Cancer-Induced Cachexia and Prolongs Survival

The cytokine TWEAK and its cognate receptor Fn14 are members of the TNF/TNFR superfamily and are upregulated in tumors. We found that Fn14, when expressed in tumors, causes cachexia and that antibodies against Fn14 dramatically extended lifespan by inhibiting tumor-induced weight loss although having only moderate inhibitory effects on tumor growth. Anti-Fn14 antibodies prevented tumor-induced inflammation and loss of fat and muscle mass. Fn14 signaling in the tumor, rather than host, is responsible for inducing this cachexia because tumors in Fn14- and TWEAK-deficient hosts developed cachexia that was comparable to that of wild-type mice. These results extend the role of Fn14 in wound repair and muscle development to involvement in the etiology of cachexia and indicate that Fn14 antibodies may be a promising approach to treat cachexia, thereby extending lifespan and improving quality of life for cancer patients.

Mitochondrial-nuclear communications

Mitochondria cannot be made de novo but replicate by a mechanism of recruitment of new proteins, which are added to preexisting subcompartments. Although mitochondria have their own DNA, more than 98% of the total protein complement of the organelle is encoded by the nuclear genome. Mitochondrial biogenesis requires a coordination of expression of two genomes and therefore cross talk between the nucleus and mitochondria. In mammals, regulation of mitochondrial biogenesis and proliferation is influenced by external factors, such as nutrients, hormones, temperature, exercise, hypoxia, and aging. This complexity points to the existence of a coordinated and tightly regulated network connecting different pathways. Communications are also required for eliciting mitochondrial responses to specific stress pathways. This review covers the mechanisms of mitochondrial biogenesis and the way cells respond to external signals to maintain mitochondrial function and cellular homeostasis.

Discovery of genes activated by the mitochondrial unfolded protein response (mtUPR) and cognate promoter elements

In an accompanying paper, we show that the mitochondrial Unfolded Protein Response or mtUPR is initiated by the activation of transcription of chop through an AP-1 element in the chop promoter. Further, we show that the c/ebpβ gene is similarly activated and CHOP and C/EBPβ subsequently hetero-dimerise to activate transcription of mtUPR responsive genes. Here, we report the discovery of six additional mtUPR responsive genes. We found that these genes encoding mitochondrial proteases YME1L1 and MPPβ, import component Tim17A and enzymes NDUFB2, endonuclease G and thioredoxin 2, all contain a CHOP element in their promoters. In contrast, genes encoding mitochondrial proteins Afg3L2, Paraplegin, Lon and SAM 50, which do not have a CHOP element, were not up-regulated. Conversely, genes with CHOP elements encoding cytosolic proteins were not induced by the accumulation of unfolded proteins in mitochondria. These results indicate that mtUPR responsive genes appear to share a requirement for a CHOP element, but that this is not sufficient for the regulation of the mtUPR. A more detailed analysis of promoters of mtUPR responsive genes revealed at least two additional highly conserved, putative regulatory sites either side of the CHOP element, one a motif of 12 bp which lies 14 bp upstream of the CHOP site and another 9 bp element, 2 bp downstream of the CHOP site. Both of these additional elements are conserved in the promoters of 9 of the ten mtUPR responsive genes we have identified so far, the exception being the Cpn60/10 bidirectional promoter. Mutation of each of these elements substantially reduced the mtUPR responsiveness of the promoters suggesting that these elements coordinately regulate mtUPR.

The chop gene contains an element for the positive regulation of the mitochondrial unfolded protein response

We have previously reported on the discovery of a mitochondrial specific unfolded protein response (mtUPR) in mammalian cells, in which the accumulation of unfolded protein within the mitochondrial matrix results in the transcriptional activation of nuclear genes encoding mitochondrial stress proteins such as chaperonin 60, chaperonin 10, mtDnaJ, and ClpP, but not those encoding stress proteins of the endoplasmic reticulum (ER) or the cytosol. Analysis of the chaperonin 60/10 bidirectional promoter showed that the CHOP element was required for the mtUPR and that the transcription of the chop gene is activated by mtUPR. In order to investigate the role of CHOP in the mtUPR, we carried out a deletion analysis of the chop promoter. This revealed that the transcriptional activation of the chop gene by mtUPR is through an AP-1 (activator protein-1) element. This site lies alongside an ERSE element through which chop transcription is activated in response to the ER stress response (erUPR). Thus CHOP can be induced separately in response to 2 different stress response pathways. We also discuss the potential signal pathway between mitochondria and the nucleus for the mtUPR.

Mitochondrial morphology and distribution in mammalian cells

It is now appreciated that mitochondria form tubular networks that adapt to the requirements of the cell by undergoing changes in their shape through fission and fusion. Proper mitochondrial distribution also appears to be required for ATP delivery and calcium regulation, and, in some cases, for cell development. While we now realise the great importance of mitochondria for the cell, we are only beginning to work out how these organelles undergo the drastic morphological changes that are essential for cellular function. Of the few known components involved in shaping mitochondria, some have been found to be essential to life and their gene mutations are linked to neurological disorders, while others appear to be recruited in the activation of cell death pathways. Here we review our current understanding of the functions of the main players involved in mitochondrial fission, fusion and distribution in mammalian cells.

Characterisation of the protein composition of peripheral blood mononuclear cell microsomes by SDS-PAGE and mass spectrometry

Approximately 340 leucocyte plasma membrane proteins have been characterised by the eight Human Leucocyte Differentiation Antigen workshops held between 1982 and 2004, based primarily on their reactivity with monoclonal antibodies. The human genome is predicted to encode approximately 34,000 cDNA transcripts, of which between 15% and 20% are predicted to contain one or more transmembrane helices. We have used SDS-PAGE separation coupled with mass spectrometry-based peptide mass tag identification to identify novel plasma membrane proteins in microsome preparations prepared from mononuclear cells obtained from human peripheral blood. A total of 361 distinct proteins were identified in a single preparation, including 37 known leucocyte plasma membrane proteins, 27 potential novel plasma membrane proteins whose expression on PBMC is poorly characterised, and 51 other proteins for which the subcellular location could not be determined. Expression analysis using cDNA panels indicates that several of these novel plasma membrane proteins are differentially expressed in lymphocyte subsets. These results show that previously unidentified lymphocyte plasma membrane proteins can be identified using this approach.

Dissection of the mitochondrial import and assembly pathway for human Tom40

Tom40 is the channel-forming subunit of the translocase of the mitochondrial outer membrane (TOM complex), essential for protein import into mitochondria. Tom40 is synthesized in the cytosol and contains information for its mitochondrial targeting and assembly. A number of stable import intermediates have been identified for Tom40 precursors in fungi, the first being an association with the sorting and assembly machinery (SAM) of the outer membrane. By examining the import pathway of human Tom40, we have been able to elucidate additional features in its import. We identify that Hsp90 is involved in delivery of the Tom40 precursor to mitochondria in an ATP-dependent manner. The precursor then forms its first stable intermediate with the outer face of the TOM complex before its membrane integration and assembly. Deletion of an evolutionary conserved region within Tom40 disrupts the TOM complex intermediate and causes it to stall at a new complex in the intermembrane space that we identify to be the mammalian SAM. Unlike its fungal counterparts, the human Tom40 precursor is not found stably arrested at a SAM intermediate. Nevertheless, we show that Tom40 assembly is reduced in mitochondria depleted of human Sam50. These findings are discussed in context with current models from fungal studies.

In silico evaluation of two mass spectrometry-based approaches for the identification of novel human leukocyte cell-surface proteins

The identification and quantitation of cell-surface proteins expressed by leukocytes currently use the wide availability of monoclonal antibodies (mAb) in immunohistochemical and flow cytometric assays. Presently, approximately 400 such proteins have been characterized; however, analysis of the completed human genome sequence indicates that it may contain several thousand as-yet unidentified molecules, which may be expressed on the leukocyte cell surface. Recent advances in protein isolation and analysis using mass spectrometry illustrate that it is now feasible to identify the protein composition of a complex sample such as a plasma membrane extract. Such an approach may be useful for the identification of the cell-surface proteins that have not been identified using mAb techniques. Here, we detail the results of an in silico evaluation of the peptides isolated using two methods used to label plasma membrane proteins to determine whether these methods are suitable for the identification of known leukocyte cell-surface proteins by mass spectrometry. The labeling of cell-surface proteins before isolation and characterization is a valuable means of differentiating between plasma membrane and internal membrane proteins The results indicate that although the majority of cell-surface proteins can be identified using either of the approaches, others known to be important diagnostically and/or therapeutically would not be identified using either approach. The implication of this for the use of these techniques in the discovery of new leukocyte cell-surface proteins is discussed.

Isolation and characterization of an IgNAR variable domain specific for the human mitochondrial translocase receptor Tom70

The new antigen receptor (IgNAR) from sharks is a disulphide bonded dimer of two protein chains, each containing one variable and five constant domains, and functions as an antibody. In order to assess the antigen-binding capabilities of isolated IgNAR variable domains (VNAR), we have constructed an in vitro library incorporating synthetic CDR3 regions of 15-18 residues in length. Screening of this library against the 60 kDa cytosolic domain of the 70 kDa outer membrane translocase receptor from human mitochondria (Tom70) resulted in one dominant antigen-specific clone (VNAR 12F-11) after four rounds of in vitro selection. VNAR 12F-11 was expressed into the Escherichia coli periplasm and purified by anti-FLAG affinity chromatography at yields of 3 mg x L(-1). Purified protein eluted from gel filtration columns as a single monomeric protein and CD spectrum analysis indicated correct folding into the expected beta-sheet conformation. Specific binding to Tom70 was demonstrated by ELISA and BIAcore (Kd = 2.2 +/- 0.31 x 10(-9) m-1) indicating that these VNAR domains can be efficiently displayed as bacteriophage libraries, and selected against target antigens with an affinity and stability equivalent to that obtained for other single domain antibodies. As an initial step in producing 'intrabody' variants of 12F-11, the impact of modifying or removing the conserved immunoglobulin intradomain disulphide bond was assessed. High affinity binding was only retained in the wild-type protein, which combined with our inability to affinity mature 12F-11, suggests that this particular VNAR is critically dependent upon precise CDR loop conformations for its binding affinity.

Molecular chaperones Hsp90 and Hsp70 deliver preproteins to the mitochondrial import receptor Tom70

The role of cytosolic factors in protein targeting to mitochondria is poorly understood. Here, we show that in mammals, the cytosolic chaperones Hsp90 and Hsp70 dock onto a specialized TPR domain in the import receptor Tom70 at the outer mitochondrial membrane. This interaction serves to deliver a set of preproteins to the receptor for subsequent membrane translocation dependent on the Hsp90 ATPase. Disruption of the chaperone/Tom70 recognition inhibits the import of these preproteins into mitochondria. In yeast, Hsp70 rather than Hsp90 is used in import, and Hsp70 docking is required for the formation of a productive preprotein/Tom70 complex. We outline a novel mechanism in which chaperones are recruited for a specific targeting event by a membrane-bound receptor.

Import of nuclear-encoded proteins into mitochondria

The majority of mitochondrial proteins are encoded by nuclear genes, synthesized in the cytosol and subsequently imported into mitochondria through protein translocation machineries of the outer and inner membranes. In this review, we discuss the arrangement of the various translocation complexes and the function of individual import components. We also outline the various targeting pathways that preproteins can take in order to reach their appropriate sub-mitochondrial compartment.

Insertion and assembly of human tom7 into the preprotein translocase complex of the outer mitochondrial membrane

Tom7 is a component of the translocase of the outer mitochondrial membrane (TOM) and assembles into a general import pore complex that translocates preproteins into mitochondria. We have identified the human Tom7 homolog and characterized its import and assembly into the mammalian TOM complex. Tom7 is imported into mitochondria in a nucleotide-independent manner and is anchored to the outer membrane with its C terminus facing the intermembrane space. Unlike studies in fungi, we found that human Tom7 assembles into an approximately 120-kDa import intermediate in HeLa cell mitochondria. To detect subunits within this complex, we employed a novel supershift analysis whereby mitochondria containing newly imported Tom7 were incubated with antibodies specific for individual TOM components prior to separation by blue native electrophoresis. We found that the 120-kDa complex contains Tom40 and lacks receptor components. This intermediate can be chased to the stable approximately 380-kDa mammalian TOM complex that additionally contains Tom22. Overexpression of Tom22 in HeLa cells results in the rapid assembly of Tom7 into the 380-kDa complex indicating that Tom22 is rate-limiting for TOM complex formation. These results indicate that the levels of Tom22 within mitochondria dictate the assembly of TOM complexes and hence may regulate its biogenesis.

A mitochondrial specific stress response in mammalian cells

Cells respond to a wide variety of stresses through the transcriptional activation of genes that harbour stress elements within their promoters. While many of these elements are shared by genes encoding proteins representative of all subcellular compartments, cells can also respond to stresses that are specific to individual organelles, such as the endoplasmic reticulum un folded protein response. Here we report on the discovery and characterization of a mitochondrial stress response in mammalian cells. We find that the accumulation of unfolded protein within the mitochondrial matrix results in the transcriptional upregulation of nuclear genes encoding mitochondrial stress proteins such as chaperonin 60, chaperonin 10, mtDnaJ and ClpP, but not those encoding stress proteins of the endoplasmic reticulum. Analysis of the chaperonin 60/10 bidirectional promoter identified a CHOP element as the mitochondrial stress response element. Dominant-negative mutant forms of CHOP and overexpression of CHOP revealed that this transcription factor, in association with C/EBPbeta, regulates expression of mitochondrial stress genes in response to the accumulation of unfolded proteins.

Import and assembly of proteins into mitochondria of mammalian cells

Most of our knowledge regarding the process of protein import into mitochondria has come from research employing fungal systems. This review outlines recent advances in our understanding of this process in mammalian cells. In particular, we focus on the characterisation of cytosolic molecular chaperones that are involved in binding to mitochondrial-targeted preproteins, as well as the identification of both conserved and novel subunits of the import machineries of the outer and inner mitochondrial membranes. We also discuss diseases associated with defects in import and assembly of mitochondrial proteins and what is currently known about the regulation of import in mammals.

A (1-->4)-beta-mannan-specific monoclonal antibody and its use in the immunocytochemical location of galactomannans

Galactomannan was coupled to a protein carrier for the preparation of monoclonal antibodies. The monoclonal antibodies generated bound to galactomannans from different sources as well as to glucomannan and galactoglucomannan. One monoclonal antibody, BGM C6, was characterised and found to be specific for (1-->4)-beta-linked mannopyranosyl residues; it had a binding affinity estimated at 1x10(-6) M for the (1-->4)-beta-linked mannohexaose. BGM C6 was used in immunogold labelling studies to locate galactomannans in the endosperm walls of normal coconuts (Cocos nucifera L.) and those of the mutant makapuno at two different developmental stages. The pattern and intensity of antibody labelling varied for each type of coconut at the mature and immature stages, indicating differences in the galactomannan composition of the endosperm walls.

Translocation of proteins into mitochondria

The translocase of the outer mitochondrial membrane (TOM) is composed of receptors, a channel protein, and its modulators that function together to import proteins into mitochondria. Although the import pathway of proteins directed to the mitochondrial matrix has been well characterized, recent studies into the import pathway taken by proteins into the other submitochondrial compartments have broadened our understanding into the way the TOM machinery recognizes, interacts, and translocates proteins.

Functional analysis of human metaxin in mitochondrial protein import in cultured cells and its relationship with the Tom complex

Metaxin is an outer membrane protein of mammalian mitochondria which is suggested to be involved in protein import into the organelle. RNA blot analysis showed that distribution of metaxin mRNA in human tissues differs from that of mRNA for the translocase component Tom20. Effect of overexpression of human metaxin on mitochondrial preprotein import and processing in COS-7 cells was studied. Overexpression of metaxin resulted in impaired mitochondrial import of natural and chimeric preproteins and in their accumulation. We previously reported that overexpression of Tom20 in cultured cells causes inhibition of import of mitochondrial preprotein. Coexpression of metaxin with Tom20 had no further effect on the preprotein import. Overexpression of the cytosolic domain of metaxin also caused inhibition of preprotein import, although less strongly than the full-length metaxin. In blue native PAGE, Tom40, Tom22, and a portion of Tom20 migrated as a complex of approximately 400 kDa, and the other portion of Tom20 migrated in smaller forms of approximately 100 and approximately 40 kDa. On the other hand, metaxin migrated at a position of approximately 50 kDa. These results confirm earlier in vitro results that metaxin participates in preprotein import into mammalian mitochondria, and indicates that it does not associate with the Tom complex.

Characterisation of several Hsp70 interacting proteins from mammalian organelles

Since both the spectrum and characteristics of in vivo substrates with affinity for Hsp70 members are largely unknown, we have investigated the range and type of mammalian organellar proteins which selectively interact with immobilised Escherichia coli Hsp70 (DnaK). Amongst a subset of organellar proteins selectively retained on DnaK, the major constituents represent unstable proteins and subunits of oligomeric proteins. The interactions with DnaK were diminished in the presence of mt-Hsp70 and BiP, while the complexes formed with DnaK were dissociated in the presence of K+ and GrpE-like co-chaperones, suggesting that these organellar proteins constitute general Hsp70 substrates. Protein sequence analysis identified the major DnaK interacting constituents as the mitochondrial transcription factor A, the alpha- (but not the beta-) subunit of succinyl CoA synthetase, mitochondrial 2,4-dienoyl CoA reductase, endoplasmic reticulum cyclophilin-B, peroxisomal multifunctional enzyme and a previously undescribed peroxisomal protein suspected to represent an isoform of 2,4-dienoyl CoA reductase. The selective retention of these fully synthesised proteins on Hsp70 most likely reflects the function of this molecular chaperone in protein biogenesis, but additionally, could extend the known functions of Hsp70 to include modulating the activities of certain proteins or enzymes which are important in cellular homeostasis.

Characterization of the novel mitochondrial protein import component, Tom34, in mammalian cells

Tom34 is a newly-found component of the mitochondrial protein import machinery in mammalian cells with no apparent counterpart in fungi. RNA blot and immunoblot analyses showed that the expression of Tom34 varies among tissues and differs from that of the core translocase component Tom20. In contrast to a previous report [Nuttal, S.D. et al. (1997) DNA Cell Biol. 16, 1067-1074], the present study using a newly-prepared anti-Tom34 antibody with a high titer showed that Tom34 is present largely in the cytosolic fraction and partly in the mitochondrial and membrane fractions after fractionation of tissues and cells, and that the membrane-associated form is largely extractable with 0.1 M sodium carbonate. The in vitro import of preproteins into isolated rat mitochondria was strongly inhibited by DeltahTom34 which lacks the NH2-terminal hydrophobic region of human Tom34 (hTom34). Import was also strongly inhibited by anti-hTom34. In pulse-chase experiments using COS-7 cells, pre-ornithine transcarbamylase (pOTC) was rapidly processed to the mature form. Coexpression of hTom34 resulted in a stimulation of pOTC processing, whereas the coexpression of hTom34 antisense RNA caused inhibition. The results confirm that Tom34 plays a role in mitochondrial protein import in mammals, and suggest it to be an ancillary component of the translocation machinery in mammalian cells.

Evidence for the existence of distinct mammalian cytosolic, microsomal, and two mitochondrial GrpE-like proteins, the Co-chaperones of specific Hsp70 members

We previously reported the cDNA cloning and characterization of a mammalian mitochondrial GrpE protein ( approximately 21 kDa, mt-GrpE#1) and now provide evidence for the presence of distinct cytosolic ( approximately 40 kDa), microsomal ( approximately 50 kDa), and additional mitochondrial ( approximately 22 kDa, mt-GrpE#2) GrpE-like members. While a cytosolic GrpE-like protein has recently been identified, the demonstration of both a microsomal and a second mitochondrial GrpE-like member represents the first in any biological system. Investigation of the microsomal and two mitochondrial GrpE-like proteins revealed that they bound specifically to Escherichia coli DnaK, and the complexes formed were not disrupted in the presence of 0.5 M salt but were readily dissociated in the presence of 5 mM ATP. The functional integrity of mt-GrpE#1 and #2 was verified by their ability to specifically interact with and stimulate the ATPase activity of mammalian mitochondrial Hsp70 (mt-Hsp70). Analysis of the cDNA sequences encoding the two mammalian mitochondrial GrpE-like proteins revealed approximately 47% positional identity at the amino acid level, the presence of a highly conserved mitochondrial leader sequence, and putative destabilization elements within the 3'-untranslated region of the mt-GrpE#2 transcript which are not present in the mt-GrpE#1 transcript. A constitutive expression of both mitochondrial GrpE-like transcripts in 22 distinct mouse tissues was observed but possible different post-transcriptional regulation of the mt-GrpE#1 and #2 transcripts may confer a different expression pattern of the encoded proteins.

Isolation and purification of immunoglobulins from chicken eggs using thiophilic interaction chromatography

We report on the use of thiophilic interaction chromatography for the purification of IgY from egg yolk. This procedure permits the purification to homogeneity of IgY in a single chromatographic step after ammonium sulfate fractication. This study also compares the use of an improved T-gel which has a higher capacity for immunoglobulin than the original T-gel, having a capacity in excess of 25 mg IgY/ml resin. The recovery from this procedure is close to 100%, providing a simple and efficient means for purifying IgY from egg yolk. We also determined that the amount of specific antibody present in egg yolk from an immunised chicken is around 1% of total IgY.

Substratum adhesion and gliding in a diatom are mediated by extracellular proteoglycans

Diatoms are unicellular microalgae encased in a siliceous cell wall, or frustule. Pennate diatoms, which possess bilateral symmetry, attach to the substratum at a slit in the frustule called the raphe. These diatoms not only adhere, but glide across surfaces whilst maintaining their attachment, secreting a sticky mucilage that forms a trail behind the gliding cells. We have raised monoclonal antibodies to the major cell surface proteoglycans of the marine raphid diatom Stauroneis decipiens Hustedt. The antibody StF.H4 binds to the cell surface, in the raphe and to adhesive trails and inhibits the ability of living diatoms to adhere to the substratum and to glide. Moreover, StF.H4 binds to a periodate-insensitive epitope on four frustule-associated proteoglycans (relative molecular masses 87, 112, and > 200 kDa). Another monoclonal antibody, StF.D5, binds to a carbohydrate epitope on the same set of proteoglycans, although the antibody binds only to the outer surface of the frustule and does not inhibit cell motility and adhesion.

The genes encoding mammalian chaperonin 60 and chaperonin 10 are linked head-to-head and share a bidirectional promoter

Chaperonins are a class of stress-inducible molecular chaperones involved in protein folding. We report the cloning, sequencing and characterisation of the rat mitochondrial chaperonin 60 and chaperonin 10 genes. The two genes are arranged in a head-to-head configuration and together comprise 14 kb and contain 14 introns. The genes are linked together by a region of approximately 280 bp, which constitutes a bidirectional promoter and includes a common heat-shock element. Insertion of the shared promoter region between two reporter genes is sufficient to drive their expression under both constitutive and heat-shock conditions. The arrangement of the mammalian chaperonin genes suggests the potential to provide the coordinated regulation of their products in a manner that is mechanistically distinct from, yet conceptually similar to, that employed by the bacterial chaperonin (groE) operon.

hTom34: a novel translocase for the import of proteins into human mitochondria

Most mitochondrial proteins are nuclear encoded, synthesized on cytosolic ribosomes, and imported into the mitochondria. We have identified and characterized a 309 amino acid human protein with a molecular weight of 34 kDa that functions as a subunit of the translocase for the import of such proteins. hTom34 (34-kDa Translocase of the Outer Mitochondrial Membrane) is displayed on the surface of mitochondria and is resistant to extraction under alkaline conditions. Antibodies raised against hTom34 specifically inhibit in vitro import of the mitochondrial precursor protein preornithine transcarbamylase into mitochondria isolated from rat liver. Based on trypsin digestion experiments, the receptor has a large (27 kDa) C-terminal domain exposed to the cytosol. This novel component of the protein import machinery possesses a 62 residue motif conserved with the Tom70 family of mitochondrial receptors but otherwise appears to have no counterpart so far characterized in the mitochondria of any other species.

The role of molecular chaperones in mitochondrial protein import and folding

Molecular chaperones play a critical role in many cellular processes. This review concentrates on their role in targeting of proteins to the mitochondria and the subsequent folding of the imported protein. It also reviews the role of molecular chaperons in protein degradation, a process that not only regulates the turnover of proteins but also eliminates proteins that have folded incorrectly or have aggregated as a result of cell stress. Finally, the role of molecular chaperones, in particular to mitochondrial chaperonins, in disease is reviewed. In support of the endosymbiont theory on the origin of mitochondria, the chaperones of the mitochondrial compartment show a high degree of similarity to bacterial molecular chaperones. Thus, studies of protein folding in bacteria such as Escherichia coli have proved to be instructive in understanding the process in the eukaryotic cell. As in bacteria, the molecular chaperone genes of eukaryotes are activated by a variety of stresses. The regulation of stress genes involved in mitochondrial chaperone function is reviewed and major unsolved questions regarding the regulation, function, and involvement in disease of the molecular chaperones are identified.

Isolation and characterisation of a cDNA encoding rat mitochondrial GrpE, a stress-inducible nucleotide-exchange factor of ubiquitous appearance in mammalian organs

In contrast to the E. coli chaperones DnaK, GroEL and GroES, cDNAs encoding mitochondrial homologues of DnaJ and GrpE from higher eukaryotes have yet to be reported. Based on peptide sequences, we have isolated a cDNA encoding a 217 residue nuclear encoded precursor of rat mitochondrial GrpE (mt-GrpE) including a typical mitochondrial presequence of 27 residues. Western blotting revealed that the 21 kDa GrpE homologue is present exclusively in the mitochondrial fraction where it comprises only approximately 0.03% of the total soluble protein, while Northern blotting showed that the mt-GrpE transcript is present in most if not all organs. By contrast to other mitochondrial chaperones, the levels of mt-GrpE and its transcript in cultured cells are only marginally increased in response to the proline analog L-azetidine 2-carboxylic acid but not by heat shock. Furthermore, members of the GrpE family exhibit a much lower degree of sequence identity than do the well studied members of the Hsp70, Hsp60 and Hsp10 families.

Selective induction of mitochondrial chaperones in response to loss of the mitochondrial genome

Molecular chaperones are known to play key roles in the synthesis, transport and folding of nuclear-encoded mitochondrial proteins and of proteins encoded by mitochondrial DNA. Although the regulation of heat-shock genes has been the subject of considerable investigation, regulation of the genes encoding mitochondrial chaperones is not well defined. We have found that stress applied specifically to the mitochondria of mammalian cells is capable of eliciting an organelle-specific, molecular chaperone response. Using the loss of mitochondrial DNA as a means of producing a specific mitochondrial stress, we show by Western-blot analysis that mtDNA-less (rho 0) rat hepatoma cells show an increase in the steady-state levels of chaperonin 60 (cpn 60) and chaperonin 10 (cpn 10). Nuclear transcription assays show that the upregulation of these chaperones is due to transcriptional activation. There was no effect on the inducible cytosolic Hsp 70, Hsp 72, nor on mtHsp 70 in rho 0 cells, leading us to concluded that stress applied selectively to mitochondria elicits a specific molecular chaperone response. Heat stress was able to provide an additional induction of cpn 60 and cpn 10 above that obtained for the rho 0 state alone, indicating that these genes have separate regulatory elements for the specific mitochondrial and general stress responses. Since the mitochondrial-specific chaperones are encoded by nuclear DNA, there must be a mechanism for molecular communication between the mitochondrion and nucleus and this system can address how stress is communicated between these organelles.

Affinity purification, overexpression, and characterization of chaperonin 10 homologues synthesized with and without N-terminal acetylation

Utilizing the ability of bacterial chaperonin 60 (GroEL) to functionally interact with chaperonin 10 (Cpn10) homologues in an ATP-dependent fashion, we have purified substantial amounts of mammalian, chloroplast, and thermophilic Cpn10 homologues from their natural host. In addition, large amounts of recombinant rat Cpn10 were produced in Escherichia coli and found to be identical to its authentic counterpart except for the lack of N-terminal acetylation. By comparing these two forms of Cpn10, it was found that acetylation does not influence the oligomeric structure of Cpn10 and is not essential for chaperone activity or mitochondrial import in vitro. In contrast, N-terminal acetylation proved crucial in the protection of Cpn10 against degradation by N-ethylmaleimide-sensitive proteases derived from organellar preparations of rat liver. The availability of large amounts of both affinity-purified and recombinant Cpn10 will facilitate not only further characterization of the eukaryotic folding machinery but also further scrutiny of the reported function of Cpn10 as early pregnancy factor.

Affinity-purification and identification of GrpE homologues from mammalian mitochondria

We used affinity chromatography on DnaK columns to identify a mitochondrial GrpE homologue from bovine, porcine and rat liver mitochondria. The 24 kDa GrpE homologue bound specifically to the DnaK column and was not eluted with 1 M KCl but readily with 5 mM ATP. Sequence analysis of the bovine homologue (85 residues) revealed 42% positional identity to mitochondrial GrpEp from S. cerevisiae and about 30% identity to the bacterial counterparts. Thus, GrpE homologues from higher and lower eukaryotes are highly conserved.

Role of chaperones in the biogenesis and maintenance of the mitochondrion

All cells depend on correctly folded proteins for optimal function. A central question in cellular biology is how such folded structures are formed and maintained, a process that is now recognized to rely heavily on a group of proteins called molecular chaperones. Molecular chaperones constitute distinct families of proteins that are ubiquitous and highly conserved from bacteria to humans. They appear to bind nonnative conformations of most, if not all, proteins, thereby preventing their aggregation and subsequent inactivation. The chaperones not only protect newly synthesized proteins during transport and folding, but also serve to maintain the cell in a healthy state during exposure to a multitude of stress conditions. Accordingly, chaperones are expressed constitutively, but their synthesis is further enhanced during stress conditions. Detailed insights into the role of molecular chaperones have come from studies of mitochondrial protein biogenesis, a process in which chaperones act as unfoldases, pulling devices, and foldases. In this review we summarize these developments and further discuss the potential role of chaperones in mitochondrial DNA metabolism and human mitochondrial disease states.

Solution structure of the acetylated and noncleavable mitochondrial targeting signal of rat chaperonin 10

Chaperonin 10 (Cpn10) is one of only a few mitochondrial matrix proteins synthesized without a cleavable targeting signal. Using a truncated form of Cpn10 and synthetic peptides in mitochondrial import assays, we show that the N-terminal region is both necessary and sufficient for organellar targeting in vitro. To elucidate the structural features of this topogenic signal, peptides representing residues 1-25 of rat Cpn10 were synthesized with and without the naturally occurring N-terminal acetylation. 1H NMR spectroscopy in 20% CF3CH2OH,H2O showed that both peptides assume a stable helix-turn-helix motif and are highly amphiphilic in nature. Chemical shift and coupling constant data revealed that the N-terminal helix is stabilized by N-acetylation, whereas NOE and exchange studies were used to derive a three dimensional structure for the acetylated peptide. These findings are discussed with respect to a recent model predicting that targeting sequences forming a continuous alpha-helix of more than 11 residues cannot adopt a conformation necessary for proteolysis by the matrix located signal peptidases (Hammen, P. K., Gorenstein, D. G., and Weiner, H. (1994) Biochemistry 33, 8610-8617).

Comparison of native and mutant proteins provides a sequence-specific assignment of the cysteinyl ligand proton NMR resonances in the 2[Fe4S4] ferredoxin from Clostridium pasteurianum

A sequence-specific assignment is presented for the eight low-field paramagnetically shifted cysteinyl ligand proton NMR resonances in the 2[Fe4S4] ferredoxin from Clostridium pasteurianum. The assignment is based upon comparison of chemical shifts in 1D and 2D NMR spectra of native oxidized protein and those of three mutants. The mutant proteins G12A and G41A were designed to produce minor local structural changes (hence small chemical shift perturbations) in either cluster I (glycine 12 to alanine) or in cluster II (glycine 41 to alanine). Observed chemical shift changes in spectra of the double mutant G12,41A support the interpretation. The comparison is aided by structural models derived from the crystal structure of the related ferredoxin from Peptococcus aerogenes. Each of the eight low-field resonances is assigned to a beta-proton from a different cysteinyl ligand, and so connectivities established from previous TOCSY and HMQC data allow assignment of all 24 cysteinyl ligand protons.

cDNA cloning and efficient mitochondrial import of pre-mtHSP70 from rat liver

Members of the 70-kD heat shock protein family have been found in all free-living organisms investigated and in major compartments of eukaryotic cells where they are essential to a wide range of functions, including protein folding and targeting. We have isolated a mitochondrial homolog (mtHSP70) from rat liver using ATP agarose affinity chromatography. Its identity was confirmed on the basis of immunological analysis and Ca(2+)-dependent autophosphorylation. Using protein sequence obtained from the amino termius and nine endo Lys-C peptide fragments, we have employed oligonucleotides to isolate a full-length cDNA clone. The open reading frame encodes a protein of 679 amino acids and calculated M(r) 73,913 daltons. The sequence has a high degree of identity with other members of the HSP70 family, including Escherichia coli DnaK (51%), Saccharomyces cerevisiae SSC1p (65%), the constitutive cytosolic HSP70 from rat, HSC70 (46%), and the rat endoplasmic reticulum isoform, BiP, (49%). The cDNA encodes a precursor protein with a 46-amino-acid signal peptide that is absent from the protein isolated from rat liver. The protein also shows a high degree of identity (98%) with a protein isolated from mouse and human tissues (PBP74, Domanico et al., 1993; mortalin, Wadhwa et al., 1993a; CSA, Michikawa et al., 1993a); however, the intracellular localization of these proteins is uncertain. We show that the precursor of mtHSP70 is efficiently imported into isolated mitochondria from rat liver and processed from 74 kD to the mature 69-kD protein.

Generation of a stable folding intermediate which can be rescued by the chaperonins GroEL and GroES

Pig heart mitochondrial malate dehydrogenase was chemically denatured in guanidine HCl. Upon 50-fold dilution of the denaturant spontaneous refolding could be observed in the temperature range 12-32 degrees C. At 36 degrees C spontaneous refolding was not observed but a stable folding intermediate that is fairly resistant to aggregation was formed. This intermediate is readily refolded by the chaperonins GroEL and GroES and may prove useful in future attempts to describe several aspects of chaperonin action at physiological temperatures.

Isolation of a cDNA clone specifying rat chaperonin 10, a stress-inducible mitochondrial matrix protein synthesised without a cleavable presequence

We have isolated a cDNA clone encoding chaperonin 10 from rat liver. The cDNA specifies a protein of 102 amino acids which, when transcribed and translated in vitro, yields a single basic product (pI > 9) that co-migrates exactly with the heat shock inducible cpn10 of rat hepatoma cells during 2D gel-electrophoresis. It is concluded that cpn10, unlike the majority of nuclear-encoded proteins of the mitochondrial matrix, is synthesised without a cleavable targeting signal and that, following removal of the initiating methionine, it becomes acetylated prior to mitochondrial import. Incubation of 3H- or 35S-labelled cpn10 with mitochondria confirms these conclusions and shows that cpn10 is imported into mitochondria in an energy-dependent process which is inhibited by the presence of 2,4-dinitrophenol.

A (1-->3,1-->4)-beta-glucan-specific monoclonal antibody and its use in the quantitation and immunocytochemical location of (1-->3,1-->4)-beta-glucans

Monoclonal antibodies were raised against a (1-->3,1-->4)-beta-glucan-bovine serum albumin (BSA) conjugate. One antibody (BG1) selected for further characterization, was specific for (1-->3,1-->4)-beta-glucan, displaying no binding activity against a (1-->3)-beta-glucan-BSA conjugate and minimal binding against a cellopentaose-BSA conjugate. A range of oligosaccharides was prepared by enzymatic digestion of (1-->3,1-->4)-beta-glucan, purified by size exclusion chromatography and characterized by 1H-NMR and anion exchange chromatography. These (1-->3,1-->4)-beta-oligoglucosides, together with (1-->3)-beta- and (1-->4)-beta-oligoglucosides were used to characterize the binding site of the monoclonal antibody (BG1) by competitive inhibition. The monoclonal antibody showed maximal binding to a heptasaccharide with the structure Glc(1-->3) Glc(1-->4) Glc(1-->4) Glc(1-->3) Glc(1-->4) Glc(1-->4) Glc and was determined to have an affinity constant of 3.8 x 10(4) M-1 for this oligoglucoside. The monoclonal antibody (BG1) has been used to develop a sensitive sandwich ELISA for the specific quantitation of (1-->3,1-->4)-beta-glucans. The assay operates in the range 1-10 ng ml-1 and shows no significant cross-reaction with tamarind xyloglucan, wheat endosperm arabinoxylan or carboxymethylpachyman ((1-->3)-beta-glucan). When used with a second-stage, rabbit anti-mouse gold conjugate and viewed under the electron microscope, the monoclonal antibody probe was found to bind strongly to the walls of the aleurone in thin sections of immature wheat (Triticum aestivum) cv. Millewa grains but not to the middle lamella region.(ABSTRACT TRUNCATED AT 250 WORDS)

A constitutive form of heat-shock protein 70 is located in the outer membranes of mitochondria from rat liver

HSP73, the constitutive form of heat-shock protein 70, has been implicated in the translocation of preproteins across the mitochondrial membranes, being required for maintaining mitochondrial preproteins in an import competent conformation. Here we report that highly purified mitochondrial outer membranes contain a protein indistinguishable from HSP73 as a tightly associated peripheral component of the membrane. This membrane form of HSP73 was photolabelled with [alpha-32P]ATP and could be released from the outer membrane with sodium carbonate, but not after incubation of the membranes with salt or with ATP. A sensitive immunoassay with an anti-HSP73 monoclonal antibody, revealed the association of HSP73 with mitochondrial outer membrane vesicles at a level similar to that of preprotein import receptors.

Do cytosolic factors prevent promiscuity at the membrane surface?

From the point of view of a preprotein, escaping from the cytosol into a specific organelle must seem an arduous, almost impossible task. How is it that preproteins resist the temptation to fold prematurely, to avoid the multiple membrane surfaces in the cell, and manage instead to enter only the translocation apparatus of a single organelle?

The complete primary structure of rat chaperonin 10 reveals a putative beta alpha beta nucleotide-binding domain with homology to p21ras

The first complete amino-acid sequence of a mitochondrial chaperonin 10 is reported. The amino-terminal alanine residue is acetylated, a modification that may be required for the interaction with heptameric chaperonin 60. Part of the sequence constitutes a potential dinucleotide binding motif and is identical with 7 out of 10 residues in the GTP-binding site of p21ras. This similarity may be the structural basis for the recently discovered complex between p21ras and chaperonin 60 in intact cells (Ikawa, S. and Weinberg, R.A. (1992) Proc. Natl. Acad. Sci. USA 89, 2012-2016).

Substoichiometric amounts of the molecular chaperones GroEL and GroES prevent thermal denaturation and aggregation of mammalian mitochondrial malate dehydrogenase in vitro

The molecular chaperones GroEL and GroES were produced at very high levels in Escherichia coli, purified, and shown to protect pig mitochondrial malate dehydrogenase (MDH) against thermal inactivation in vitro. The apparent rate of MDH inactivation at 37 degrees C was reduced by a factor of at least 5 in a process which required only GroEL, GroES, and ATP. GroEL alone did not protect MDH against thermal inactivation but kept the denatured protein soluble and thereby prevented its aggregation. Reactivation of this soluble and inactive form of MDH could be achieved by addition of GroES even after 120 days of storage at -20 degrees C. Protection could be extended for more than 24 hr at 37 degrees C and was observed at molar ratios of chaperones to MDH as low as 1:4, suggesting that GroEL and GroES perform multiple turnovers in the absence of auxiliary chaperones. The availability of these chaperones in large quantities combined with the apparent promiscuity of GroEL binding shows great potential for stabilization of many proteins for which thermostable variants are not available. We speculate that GroEL and GroES perform similar protective roles in vivo and thereby increase the half-life of proteins which otherwise might aggregate under physiological conditions.

Prechaperonin 60 and preornithine transcarbamylase share components of the import apparatus but have distinct maturation pathways in rat liver mitochondria

Mitochondrial preornithine transcarbamylase (p-OTC) and premalate dehydrogenase (p-MDH) are the only two matrix-located preproteins so far identified for which the proteolytic processing in vitro requires the formation of genuine processing intermediates, i-OTC and i-MDH, respectively. To establish the processing of other preproteins during import with respect to the two-step processing of p-OTC and p-MDH, the chelators EDTA and 1,10-phenanthroline were used to study the import and processing of rat prechaperonin 60 (p-cpn60) and p-OTC by mitochondria from four cpn60-containing organs. We found no evidence for a secondary processing step in the maturation of p-cpn60, but a clear requirement for two-step processing of p-OTC, even in three organs which do not contain ornithine transcarbamylase. The metal-ion requirement of the p-OTC processing activities in the organelle is consistent with the proposition that the mitochondrial processing protease (MPP) and mitochondrial intermediate peptidase (MIP) activities defined in vitro [Kalousek, F., Hendrick, J.P. & Rosenberg, L. E. (1988) Proc. Natl Acad. Sci. USA 85, 7536-7540] are responsible for precursor processing in vivo. The authenticity of two-step processing in vivo was, furthermore, established by demonstrating that i-OTC accumulates to high levels in Spodoptora frugiperda insect cells supplemented with MnCl2. The inability of the insect cells to process p-OTC fully is not a characteristic of cells grown in culture since cultured rat hepatoma cells process p-OTC to the fully processed m-OTC. Finally, we find that the import and processing of p-cpn60 and p-OTC is inhibited in an identical fashion by presequence-bovine-serum-albumin conjugates. The differences in proteolytic maturation between p-cpn60 and p-OTC are therefore not likely to result from different import pathways as the two precursors compete for common components of the import apparatus.

Post-translational processing of barley beta-glucan endohydrolases in the baculovirus-insect cell expression system

Two cDNAs encoding barley (1-->3,1-->4)-beta-glucanase (EC 3.2.1.73) isoenzymes EI and EII have been expressed in Spodoptera frugiperda (Sf9) cell cultures using the baculovirus AcNPV vector. Modifications to both the 5' and 3' ends of the cDNAs were required before satisfactory levels of expression were obtained. The modified cDNAs directed high levels of (1-->3,1-->4)-beta-glucanase expression in the Sf9 insect cell cultures, with yields of approximately 10 mg/liter of isoenzyme EI (expEI) and 15 mg/liter of isoenzyme EII (expEII). Amino acid sequence analyses showed that the expressed enzymes were processed correctly at their amino termini. However, affinity chromatography of the isoenzyme expEII on concanavalin-A (conA)-Sepharose indicated that, although the enzyme is glycosylated, the structures of the carbohydrate chains differ from those of the native enzyme. When a cDNA encoding the homologous barley (1-->3)-beta-glucanase (EC 3.2.1.39) isoenzyme GII was expressed in insect cells, aberrant amino-terminal processing of the nascent polypeptide was sometimes observed. The forms with incompletely removed signal peptides retained their substrate specificity, but exhibited slightly reduced catalytic efficiency, altered chromatographic behavior, and reduced stability at elevated temperatures. The results show that high levels of expression of recombinant plant proteins can be obtained in insect cells, but they emphasize the need to characterize thoroughly the products that are expressed in the heterologous insect cell system before comparisons are made with the native enzyme or with engineered enzyme mutants.

Heat shock proteins of barley mitochondria and chloroplasts. Identification of organellar hsp 10 and 12: putative chaperonin 10 homologues

Tissue slices from barley seedlings were subjected to heat shock and metabolically labelled with [35S]methionine and [35S]cysteine. Mitochondria and chloroplasts were isolated and shown to contain two novel heat shock proteins of 10 and 12 kDa, respectively. The possibility that these proteins, like a mitochondrial 10 kDa stress protein recently isolated from rat hepatoma cells [(1992) Proc. Natl. Acad. Sci. 89, in press] represent eukaryotic chaperonin 10 homologues is discussed.

Identification of a mammalian 10-kDa heat shock protein, a mitochondrial chaperonin 10 homologue essential for assisted folding of trimeric ornithine transcarbamoylase in vitro

We have identified a 10-kDa stress-inducible mitochondrial protein. The protein is synthesized at elevated rates in cultured rat hepatoma cells challenged with heat shock or amino acid analogues and, therefore, designated heat shock protein 10 (Hsp10). Hsp10 was purified to homogeneity from rat liver and found to exhibit a native molecular mass of 65 kDa, as opposed to a monomeric molecular mass of 10,813.4 +/- 0.41 Da. The amino acid sequence of rat Hsp10 disclosed extensive sequence similarity with bacterial chaperonin (Cpn) 10. Rat Hsp10 and Escherichia coli Cpn60 were used to reconstitute functional trimeric rat ornithine transcarbamoylase from a chemically denatured state with high efficiency. This process depended completely upon rat Hsp10 and was abolished in the presence of a nonhydrolyzable ATP analogue. We conclude that Hsp10 is a eukaryotic Cpn10 homologue and, therefore, together with Cpn60 essential for mitochondrial protein biogenesis. The Cpn-mediated protein-folding apparatus, thus, exhibits a high degree of conservation between prokaryotes and mitochondria of higher eukaryotes.