Molecular cloning of a complementary DNA to rat cyclophilin-like protein mRNA

Nephrotic Syndrome: Oedema Formation and Its Treatment With Diuretics

Oedema is a defining element of the nephrotic syndrome. Its' management varies considerably between clinicians, with no national or international clinical guidelines, and hence variable outcomes. Oedema may have serious sequelae such as immobility, skin breakdown and local or systemic infection. Treatment of nephrotic oedema is often of limited efficacy, with frequent side-effects and interactions with other pharmacotherapy. Here, we describe the current paradigms of oedema in nephrosis, including insights into emerging mechanisms such as the role of the abnormal activation of the epithelial sodium channel in the collecting duct. We then discuss the physiological basis for traditional and novel therapies for the treatment of nephrotic oedema. Despite being the cardinal symptom of nephrosis, few clinical studies guide clinicians to the rational use of therapy. This is reflected in the scarcity of publications in this field; it is time to undertake new clinical trials to direct clinical practice.

Genetics of hypertension: an assessment of progress in the spontaneously hypertensive rat

The application of gene mapping methods to uncover the genetic basis of hypertension in the inbred spontaneously hypertensive rat (SHR) began over 25 yr ago. This animal provides a useful model of genetic high blood pressure, and some of its features are described. In particular, it appears to be a polygenic model of disease, and polygenes participate in human hypertension genetic risk. The SHR hypertension alleles were fixed rapidly by selective breeding in just a few generations and so are presumably common genetic variants present in the outbred Wistar strain from which SHR was created. This review provides a background to the origins and genesis of this rat line. It considers its usefulness as a model organism for a common cardiovascular disease. The progress and obstacles facing mapping are considered in depth, as are the emergence and application of other genome-wide genetic discovery approaches that have been applied to investigate this model. Candidate genes, their identification, and the evidence to support their potential role in blood pressure elevation are considered. The review assesses the progress that has arisen from this work has been limited. Consideration is given to some of the factors that have impeded progress, and prospects for advancing understanding of the genetic basis of hypertension in this model are discussed.

Defining suitable reference genes for RT-qPCR analysis on intestinal epithelial cells

The study of the mammalian intestinal epithelium concerns several aspects of cellular and molecular biology. In fact, most of these studies aim to define molecular components or mechanisms related with the control of stemness and the balance between cell proliferation and differentiation in physiopathological conditions. It is worth mentioning that real-time quantitative reverse transcription-polymerase chain reaction (RT-qPCR) approaches are commonly used, but only a few studies are available regarding suitable reference genes to normalize gene expression data. The present study was designed to validate potential reference genes in freshly isolated proliferating or differentiated epithelial cells from the mouse intestine. We also extended our analysis to the IEC6 intestinal epithelial cells, as a promising model to study intestinal physiopathology in vitro. The stability of six potential reference genes (Hprt1, Ppia, Gapdh, Rplp0, Ppib, and Vil1) has been tested both in epithelial cells isolated from the mouse intestine and in the IEC6 cell line. The software programs-geNorm and Normfinder-were used to obtain an estimation of the expression stability of each gene and, by comparing the results, to identify the most suitable genes for RT-qPCR data normalization. These multiple approaches allowed us to select different suitable reference genes for the correct quantification of mRNAs depending on the differentiated or proliferative nature of the cells.

Analysis of low abundance membrane-associated proteins from rat pancreatic zymogen granules

Zymogen granules (ZG) are specialized storage organelles in the exocrine pancreas that allow the sorting, packaging, and regulated apical secretion of digestive enzymes. As there is a critical need for further understanding of the key processes in regulated secretion to develop new therapeutic options in medicine, we applied a suborganellar proteomics approach to identify peripheral membrane-associated ZG proteins. We focused on the analysis of a "basic" group (pH range 6.2-11) with about 46 spots among which 44 were identified by tandem mass spectrometry. These spots corresponded to 16 unique proteins, including rat mast cell chymase (RMCP-1) and peptidyl-prolyl cis-trans isomerase B (PpiB; cyclophilin B), an ER-resident protein. To confirm that these proteins were specific to zymogen granules and not contaminants of the preparation, we conducted a series of validation experiments. Immunoblotting of ZG subfractions revealed that chymase and PpiB behaved like bona fide peripheral membrane proteins. Their expression in rat pancreas was regulated by feeding behavior. Ultrastructural and immunofluorescence studies confirmed their ZG localization. Furthermore, a chymase-YFP fusion protein was properly targeted to ZG in pancreatic AR42J cells. Interestingly, for both proteins, proteoglycan-binding properties have been reported. The importance of our findings for sorting and packaging during ZG formation is discussed.

Modulation by dietary sodium intake of melanocortin 3 receptor mRNA and protein abundance in the rat kidney

Gamma-melanocyte stimulating hormone (gamma-MSH) is a circulating natriuretic peptide hormone derived from proopiomelanocortin (POMC); its concentration in plasma and pituitary POMC mRNA abundance, increase in rats ingesting a high-sodium diet (HSD, 8% NaCl) compared with a low-sodium diet (LSD, 0.07% NaCl). RT-PCR of rat kidney RNA demonstrated reaction products of the expected size in both cortex and medulla for MC3-R, MC4-R, and MC5-R mRNA; no signal for MC1-R or MC2-R was detected. Relative to beta-actin or cyclophilin, abundance of the three receptor transcripts after 1 wk of the LSD was approximately equal in both cortex and medulla. After 1 wk of the HSD, mRNA abundance of MC4-R and MC5-R was unchanged, whereas that of MC3-R in medulla more than doubled, the ratio of MC3-R/beta-actin signal increasing from 0.38 +/- 0.04 on LSD to 0.84 +/- 0.04 on HSD (P < 0.001). No significant increase occurred in the cortex. The increase in MC3-R expression induced by dietary sodium was observed in inner medullary collecting duct (IMCD) cells isolated from the kidneys of HSD rats, suggesting that these cells were the major site of receptor expression in the medulla. Immunoblots of whole medullary and IMCD cell homogenates detected MC3-R immunoreactive protein; its expression was twice as great in samples from HSD vs. LSD rat kidneys, paralleling the increase in MC3-R mRNA abundance on the HSD. No changes in MC4-R or MC5-R protein expression were observed. Incubation of IMCD cell suspensions with increasing concentrations of gamma2-MSH led to increased cAMP accumulation, with values from rats on the HSD being roughly double the values from LSD rats. Intrarenal infusion of gamma2-MSH (500 fmol/min) increased sodium and cAMP excretion from the infused but not contralateral kidney of HSD rats, while having no effect in LSD rats. These data show that MC3-R is expressed in rat IMCD cells in a manner modulated by dietary sodium intake. Because MC3-R is the receptor with which gamma-MSH interacts, our findings suggest the existence of a sodium-regulating system, activated in response to a HSD, which increases urinary sodium excretion to balance the high-sodium intake.

Cyclophilin B expression in renal proximal tubules of hypertensive rats

Rat cyclophilin-like protein (Cy-LP) is a candidate hypertension gene initially identified by differential hybridization and implicated in renal mechanisms of salt retention and high blood pressure. We report the molecular characterization of rat cyclophilin B (CypB) and demonstrate, through sequence analysis and an allele-specific polymerase chain reaction primer assay, that CypB but not Cy-LP is expressed in rat kidney. CypB is an endoplasmic reticulum-localized prolyl-isomerase that interacts with elongation initiation factor 2-beta, an important regulator of protein translation and a central component of the endoplasmic reticulum stress response to hypoxia or ATP depletion. Active renal transport of sodium is increased in the spontaneously hypertensive rat (SHR), and there is evidence that this coincides with hypoxia and ATP depletion in the renal cortex. In the present studies we have examined expression of CypB in rat proximal tubules, which contributes to the increased renal sodium reabsorption in this model of hypertension. We report that CypB transcript abundance is significantly elevated in proximal convoluted tubules from SHR compared with the control Wistar-Kyoto strain. This upregulation occurs in weanling animals and precedes the development of hypertension, indicating that it is not a simple response to hypertension in SHR. Further, CypB expression is also higher in a proximal tubule cell line derived from SHR compared with a similar line derived from Wistar-Kyoto rats, indicating that this difference is genetically determined. No sequence differences were observed in the CypB cDNA from these 2 strains. These observations suggest that a genetically determined alteration in proximal tubules from SHR occurs that leads to increased expression of CypB. In view of evidence linking CypB to the regulation of elongation initiation factor-2, the upregulation of CypB may result from metabolic stress.

Modeling and analysis of competitive RT-PCR

The present studies demonstrate a theoretical and practical framework for the accurate quantitation of gene expression in RNA extracted from microscopic tissue samples. The approaches are developed around competitive RT-PCR techniques. Assay performance has been examined and validated at both the RT and PCR steps. Our analysis of RT transcription efficiency for a number of native and competitor combinations shows that this property can differ, even for very similar templates. However, this difference is consistent and, once identified and measured, can be removed as an obstacle to accuracy. Using mathematical modeling, we have examined the simulated co-amplification of native and competitor templates in PCR. Useful insights have emerged from such modeling which indicate that differences in initial amplification efficiency and the rate of decay of amplification efficiency during the reaction can rapidly lead to inaccuracy, even while the slope and linearity of log plots of the competitor input and reaction product ratios are close to ideal. Finally, we show here that competitive RT-PCR reactions do not have to remain in the log-linear phase of PCR in order to accomplish accurate and precise quantification. Using appropriate competitors sharing primer binding sites and high internal sequence similarity, identical amplification efficiencies are preserved throughout the reaction. Reaction products, including heteroduplexes formed between native and competitor templates as reactions progress to plateau, can be identified and quantified accurately using the new technique of denaturing HPLC (dHPLC). This analytical technique allows the accuracy of competitive RT-PCR to be preserved beyond the linear phase. The technique has high sensitivity and precision and target abundances as low as 100 copies could be reliably estimated.

A peptidyl-prolyl cis/trans-isomerase (cyclophilin G) in regulated secretory granules

A 27-kDa protein (p27) in horseshoe crab hemocyte that cross-reacts with antiserum against a beta-glucan-sensitive protease zymogen was purified to homogeneity, and its cDNA was cloned. The 1.7-kilobase pair cDNA contains an open reading frame of 660 base pairs, encoding a 23-amino acid signal sequence followed by a mature protein of 197 residues. The sequence of p27 exhibits strong similarity to that of cyclophilin B, a peptidyl-prolyl cis/trans-isomerase. p27 exhibits isomerase activity with a kcat/Km of 0.18 microM-1 s-1 for a peptide substrate; this activity is inhibited by cyclosporin A but is not affected by FK506. Although the p27 precursor possesses an amino-terminal secretory hydrophobic signal sequence, unlike other cyclophilin B molecules, it lacks a conserved carboxyl-terminal endoplasmic reticulum retention signal and it contains a central 8-amino acid insertion. Although p27 is secreted into the culture media of transiently expressed COS cells, it is not detected in horseshoe crab hemolymph plasma but rather is localized to the hemocyte large granules, the regulated secretory granules that are exocytosed upon stimulation. These results indicate that p27 is a new peptidyl-prolyl cis/trans-isomerase in the regulated secretory granules, and is thus designated cyclophilin G. This first report of a cyclophilin homologue in the secretory granule of the horseshoe crab hemocyte suggests that such chaperon-like proteins may constitute a key quality control system for stored proteins in exocytotic granules.

Peptidyl prolyl cis-trans isomerase activity of cyclophilin A in functional homo-oligomeric receptor expression

The functional expression of homo-oligomeric alpha7 neuronal nicotinic and type 3 serotonin receptors is dependent on the activity of a cyclophilin. In this paper we demonstrate that the mechanism of cyclophilin action during functional homo-oligomeric receptor expression in Xenopus oocytes is distinct from the calcineurin-dependent immunosuppressive mechanism by showing that a nonimmunosuppressive analog of cyclosporin A (CsA), SDZ 211-811, reduces functional receptor expression to the same extent as CsA. The cytoplasmic subtype of cyclophilin, cyclophilin A (CyPA), appears to be required for functional receptor expression. This is because overexpression of CyPA and a CyPA mutant that is deficient in CsA binding activity reverses CsA-induced reduction in functional receptor expression. The mechanism of action of CyPA is likely to involve its prolyl isomerase activity because a mutant CyPA with a single amino acid substitution (arginine 55 to alanine) that is predicted to produce a 1000-fold attenuation in isomerase activity fails to reverse the cyclosporin A effect. Our data also suggest that CyPA does not form a stable complex with receptor subunits.

DNA sequence analysis of a cyclophilin gene from maize: developmental expression and regulation by salicylic acid

In plants, such as maize, cyclophilin (Cyp) genes are expressed at a basal level in all tissues. Amounts of Cyp mRNA above the basic level are observed in germinating seedlings, in growing tissues/organs such as roots and leaf meristematic tissue of young maize plants, nodes and embryonic female inflorescences of adult plants and also in non-proliferating tissues such as the internodes of adult plants. Salicylic acid (SA) enhances the transcription of maize Cyp genes. The possible involvement of SA in the pathway leading to defense responses induced by abiotic stresses such as mercuric chloride treatment is discussed. A maize Cyp genomic clone isolated using a maize Cyp cDNA probe contains 737 bp of the 5' upstream and the entire coding region. This Cyp gene is not interrupted by intervening sequences. In the 5' upstream region, characteristic transcription signals as well as putative regulatory sequences were identified. Two TATA boxes are found at positions -56 bp and -66 bp with respect to the transcription start site. Two putative heat shock elements were identified in the promoter region; a metal regulatory element and a third heat shock element were localized in the 5' untranslated leader. Several putative polyadenylation signals and (G)T-rich sequence motifs were identified in the 3' untranslated region.

Expression of active recombinant pallidipin, a novel platelet aggregation inhibitor, in the periplasm of Escherichia coli

The platelet aggregation inhibitor pallidipin is a protein present in the saliva of the blood-sucking triatomine bug Triatoma pallidipennis. Expression of recombinant pallidipin in the periplasm of Escherichia coli was achieved by placing its coding sequence downstream of the alkaline phosphatase (APase) or trc promoter in frame with bacterial leader peptide DNA sequences derived from APase or from the periplasmic form of cyclophilin (Cph). In each case the DNA sequence of mature pallidipin was merged to the leader peptide coding part, either directly, or while introducing additional amino acids, in order to assess their influence on the activity of the leader peptidase and on the biological activity of the recombinant protein. All tested constructs gave rise to abundant periplasmic expression of pallidipin, which was then purified by a combination of cation- and anion-exchange chromatography followed by size-exclusion gel chromatography. Recombinant pallidipin had the expected molecular mass (approximately 19 kDa) and was correctly processed, as demonstrated by SDS/PAGE and N-terminal amino acid sequencing. The highest expression levels were obtained with the three APase-derived expression plasmids. Platelet aggregation tests revealed that E. coli-derived pallidipin was fully active, with an IC50 of 33-89 nM, comparable with that of the native protein, except when an additional N-terminal lysyl-isoleucyl dipeptide was present, which resulted in an IC50 more than ten times higher.

Cyclophilin B trafficking through the secretory pathway is altered by binding of cyclosporin A

Cyclophilin B is targeted to the secretory pathway via an endoplasmic reticulum signal sequence. We analyzed the localization and trafficking of endogenous and transfected cyclophilin B in mammalian cells. Cyclophilin B accumulates both in the endoplasmic reticulum and in complexes on the plasma membrane. The immunosuppressant cyclosporin A specifically mobilizes cyclophilin B from the endoplasmic reticulum, and promotes the secretion of cyclophilin B into the medium. We suggest that cyclosporin A competes with endogenous plasma membrane proteins for association with cyclophilin B in the secretory pathway. These findings argue in favor of a role for cyclophilin B as a chaperone to proteins destined for the plasma membrane, rather than solely as a proline isomerase functioning within the endoplasmic reticulum.

Peptidylproline cis-trans-isomerases: immunophilins

Two sequence-unrelated families of proteins possess peptidylproline cis-trans-isomerase activities (PPIase). PPIases are highly sequence conserved and multifunctional proteins which are present in many types of cells with a considerably divergent phylogenetic distribution. On the cellular level, PPIases occur in every compartment, both as free species and anchored to membranes. Diverse posttranslational modifications such as glycosylation, N-terminal modifications and phosphorylation constitute the additional functional features of PPIases. Folding, assembly and trafficking of proteins in the cellular milieu are regulated by PPIases. These enzymes accelerate the rate of in-vitro protein folding and they have the ability to bind proteins and act as chaperones. Some PPIases are coregulatory subunits of molecular complexes including heat-shock proteins, glucocorticoid receptors and ion channels. Secreted forms of PPIases are inflammatory and chemotactic agents for monocytes, eosinophils and basophils. The potent and clinically useful immunosuppressants CsA, FK506 or rapamycin bind with high affinities to PPIases (immunophilins). The binding criterion allows us to sort the PPIases for the following two superfamilies of proteins: the cyclophilins (CsA-binding proteins) and the FKBP (FK506/rapamycin-binding proteins). Although none of PPIases appeared to be essential for the viability of haploid yeast cells some of the immunophilin/immunosuppressant complexes are toxic both for yeast and mammalian cells. At least seven unlinked genes of cyclophilins and four unlinked genes of FKBP exist in human genomic DNA. Selected immunophilins regulate two different signalling pathways in lymphoid cells, namely the secretion of growth factors by stimulated T-cells and interleukin-2-induced T-cell proliferation. Moreover, selected FKBP mediate the cytotoxic effects of rapamycin in non-lymphoid cells. Accounts of the discovery of PPIases (immunophilins) and their functions are given in this review. A larger spectrum of proteins is analysed in relation to various signal-transduction pathways in lymphoid cells which involve immunophilins or their complexes with the immunosuppressants CsA, FK506 or rapamycin.

Edema of the nephrotic syndrome: the role of the atrial peptide system

The nephrotic syndrome is associated with an expanded interstitial volume and edema due to sodium and water retention. The mechanisms underlying these abnormalities have been only partially clarified. Renal hypoperfusion has been considered the key event that promotes avid sodium and water reabsorption by the kidney. Hypoperfusion results from hypovolemia, a consequence of urinary protein losses and decreased oncotic pressure. However, in some patients plasma volume is normal or even increased, suggesting that in such cases the cause of sodium and water retention might be independent of systemic events and possibly originates in the kidney. Experimental evidence is now available to support this, but the intrarenal mediator(s) that promote the abnormal salt retention are still not fully clear. Atrial natriuretic peptide (ANP), which increases sodium and water excretion, has been suspected to participate in fluid retention. This is consistent with experimental and human data of a markedly blunted natriuretic and diuretic response to systemic infusion of ANP in the nephrotic syndrome. Recent studies of the mechanisms of the blunted natriuretic and diuretic response to ANP documented an increased activity of renal sympathetic nerves, but the results are controversial. The altered response to ANP also may be related to a defect in the number and affinity of receptor-binding sites for the peptide. Evidence also is available of a possible defect at the level of intracellular cyclic guanosine monophosphate, the second messenger of ANP. The gene encoding for a cyclophilin-like protein, which is increased in sodium-retaining conditions, is upregulated in the kidneys of nephrotic rats, and the infusion of ANP further increases cyclophilin-like protein mRNA. Thus, multiple factors probably act in concert to induce edema formation in the nephrotic syndrome. In this review we specifically address the tubular insensitivity to the natriuretic and diuretic action of ANP, which could be an important initiating event and could possibly contribute to sustaining the edema.

Mitochondrial cyclophilins

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Purification and N-terminal sequencing of peptidyl-prolyl cis-trans-isomerase from rat liver mitochondrial matrix reveals the existence of a distinct mitochondrial cyclophilin

1. Rat liver mitochondrial matrix peptidyl-prolyl cis-trans-isomerase (PPIase) has been purified. The major form of the enzyme has a molecular mass of 18.6 kDa, with a minor active component of 17.6 kDa. 2. The second-order rate constant for cyclosporin A binding to the enzyme was determined from the time-dependence of the inhibition of PPIase by low concentrations of cyclosporin A and found to be 0.9 microM-1.s-1 at 10 degrees C. 3. The Ki for cyclosporin A inhibition of the enzyme was 3.6 nM, and the half-life for dissociation of the enzyme-inhibitor complex was 3.6 min. 4. From the specific activity of the pure enzyme it can be calculated that isolated liver mitochondria contain approx. 45 pmol of enzyme per mg of total mitochondrial protein. Higher values estimated previously [Halestrap & Davidson (1990) Biochem. J. 268, 153-160] are explained by the use of a short (30 s) preincubation period of the enzyme with cyclosporin, which is insufficient to allow full equilibration of the binding of the inhibitor to the PPIase. 5. N-Terminal sequencing of the 18.6 and 17.5 kDa forms of PPIase show the presence of mitochondrial presequences of 13 and three amino acids respectively, with the remaining sequence having a strong sequence similarity to other cyclophilins. 6. Parallel purification and N-terminal sequencing of rat cytosolic PPIase showed the two proteins to have significant differences, implying that they are probably products of separate genes.

Isolation and partial characterization of membrane-associated cyclophilin and a related 22-kDa glycoprotein

The presence of membrane-associated proteins which stereospecifically bind cyclosporin A and react with anti-cyclophilin antibodies has been documented in rat tissues. Extraction of membranes with 6 M urea or 0.5% Chaps releases cyclosporin-binding activity that is 5-12% of that found in cytosol. Cyclosporin-A-binding proteins are present in most subcellular organelles of liver, but microsomes contain the greatest activity. These proteins can be purified by adsorption onto a cyclosporin-A affinity column and elution with cyclosporin A. Two major fractions are resolved on SDS/PAGE: an 18-kDa fraction is comprised of two isoforms that are similar if not identical to the two major cytosolic isoforms of cyclophilin. In addition, in microsomes an approximately equal quantity of a 22-kDa glycoprotein was detected. Based on partial sequencing (five peptides, 89 amino acids) this protein is similar but not identical to human cyclophilin B. This 22-kDa isoform is poorly recognized by affinity-purified anti-cyclophilin antibodies and comprises several predominant isoforms (pI approximately 9.3-9.6). Selective binding of membrane 22-kDa cyclophilin to peanut lectin suggests the oligosaccharides contain a terminal galactosyl-N-galactosamine residue.

The mechanism of action of cyclosporin A and FK506

CsA and FK506 are powerful suppressors of the immune system, most notably of T cells. They act at a point in activation that lies between receptor ligation and the transcription of early genes. Here, Stuart Schreiber and Gerald Crabtree review recent findings that indicate CsA and FK506 operate as prodrugs: they bind endogenous intracellular receptors, the immunophilins, and the resulting complex targets the protein phosphatase, calcineurin, to exert the immunosuppressive effect.

Presence of autoantibodies to peptidyl-prolyl cis-trans isomerase (cyclosporin A-binding protein) in systemic lupus erythematosus

Several autoantibodies against cytoplasmic or nuclear components of cells have been reported in autoimmune diseases. We report here a previously unrecognized autoantibody to peptidyl-prolyl cis-trans isomerase (PPIase) in patients with systemic lupus erythematosus (SLE). PPIase, which catalyzes the cis-trans isomerization of proline imidic peptide bonds in oligopeptides, has recently been found to be identical to cyclophilin, a specific binding protein of a potent immunosuppressant, cyclosporin A. IgG and IgM anti-PPIase antibodies were detected in 40 and 20% of unselected patients with SLE, respectively, by ELISA. The reactivity of these sera was confirmed by immunoblotting experiments. Sera from rheumatoid arthritis patients showed no reactivity and 1 of 8 sera from systemic sclerosis patients and 1 of 25 sera from normal controls showed only weak reactivity. Unexpectedly, the anti-PPIase antibody was unable to inhibit PPIase activity, indicating that the autoantibody recognizes an epitope of PPIase which is different from the active site of PPIase. The levels of the anti-PPIase antibody in SLE patients correlated with remissions and flares of the disease. The anti-PPIase antibody was higher in patients with active SLE than those with inactive disease. The prevalence of the active stage of the disease was significantly higher in IgG anti-PPIase antibody-positive SLE patients as compared to antibody-negative SLE patients. These data define the presence of a new autoantibody against PPIase and its association with the activity and certain clinical manifestations in SLE.

Regulation of the expression of the rat angiotensin II receptor mRNA

Regulation of the expression levels of the rat angiotensin II receptor mRNA in the adrenal, aorta, kidney, and brain was assessed by the competitive polymerase chain reaction method. The bilateral nephrectomy or the administration of Dup753 markedly reduced the expression levels of this receptor mRNA in the adrenal and brain stem, but not in the kidney nor aorta. A continuous infusion of angiotensin II increased the expression level of this receptor mRNA in the adrenal but not in the other tissues. It is suggested that the expression level of this receptor mRNA in the adrenal is dependent on the renin angiotensin aldosterone system.

s-cyclophilin is retained intracellularly via a unique COOH-terminal sequence and colocalizes with the calcium storage protein calreticulin

Cyclophilins (cyclosporin A-binding proteins) are conserved, ubiquitous, and abundant proteins that accelerate the isomerization of XaaPro peptide bonds and the refolding of proteins in vitro. s-Cyclophilin is a member of the cyclophilin family with unique NH2- and COOH-terminal extensions, and with a signal sequence. We now report that s-cyclophilin is retained in the cell, and that the conserved s-cyclophilin-specific COOH-terminal extension VEKPFAIAKE is sufficient to direct a secretory protein to s-cyclophilin containing structures. Antibodies to s-cyclophilin-specific peptides were produced and the location of the protein was determined by an immunocytochemical study at the light microscopic level. s-Cyclophilin colocalized with the Ca(2+)-binding protein calreticulin and, to a lesser extent, with the microsomal Ca(2+)-ATPase in the myogenic cell line L6, and with the Ca(2+)-binding protein calsequestrin in skeletal muscle. In activated platelets, s-cyclophilin immunoreactivity was detected in a ring-like structure that might correspond to the Ca(2+)-storing and -releasing dense tubular network. In spreading cells, s-cyclophilin containing vesicular structures accumulated at actin-rich protrusion sites. While s-cyclophilin consistently codistributed with Ca2+ storage site markers, the distribution of s-cyclophilin immunoreactivity was not identical to that of ER markers. To determine whether the COOH-terminal extension of s-cyclophilin was involved in its intracellular transport we added this sequence to the COOH-terminus of the secretory protein glia-derived nexin. Appropriate constructs were expressed transiently in cultured cells and proteins were detected with specific antibodies. We found that glia-derived nexin with the COOH-terminal sequence VEKPFAIAKE (but not with the control sequence GLVVMNIT) colocalized with endogenous s-cyclophilin, indicating that the sequence contained retention information. These results indicate that s-cyclophilin is a retained component of an intracellular organelle and that it may accumulate in specialized portions of the ER, and possibly in calciosomes. Because of its conserved structure, widespread distribution, and abundance s-cyclophilin may be a useful marker to study the biogenesis and distribution of ER subcompartments.

Murine cyclophilin-S1: a variant peptidyl-prolyl isomerase with a putative signal sequence expressed in differentiating F9 cells

Fractionation of differentiating murine teratocarcinoma F9 cells and extraction of the nuclear/microsomal pellets with ethidium bromide led to the purification and microsequencing of the protein mCyP-S1, a novel cyclosporin A-sensitive peptidyl-prolyl cis-trans isomerase (PPIase). mCyP-S1 is a new member of the cyclophilin class of proteins. Cloning and sequencing of the mCyP-S1 cDNA revealed extended coding capacity for a putative N-terminal signal sequence, suggesting processing of mCyP-S1 during intracellular translocation across the membrane of the endoplasmic reticulum. mCyP-S1 is abundantly expressed in a variety of mouse organ tissues and its mRNA levels increase during F9 cell differentiation. Specific subcellular localization of PPIases is postulated to contribute to functional specificities of this class of enzymes.

Basic fibroblast growth factor induces 3T3 fibroblasts to synthesize and secrete a cyclophilin-like protein and beta 2-microglobulin

When stimulated by fibroblast growth factor (FGF) BALB/c 3T3 cells synthesize and secrete elevated amounts of five proteins called the 'superinducible proteins', or SIPs. The expression of these proteins is greatly enhanced if the cells are treated with cycloheximide during induction. The 24 kDa protein (SIP24) has been purified and antiserum raised against it. This protein is N-glycosylated and probably structurally constrained by one or more intramolecular disulfide bonds. The amino acid sequences of three of four peptides show significant identity with cyclophilin, an abundant cytoplasmic protein believed to mediate the immunosuppressive effects of cyclosporin A. Several members of the cyclophilin family have been identified, and cDNA clones of two cyclophilin-like proteins with signal sequences have been reported. Here we show that at least one cyclophilin-like protein is secreted and that its expression is regulated by growth factors. The 12.5 kDa protein (SIP12.5) was found to be immunoprecipitated by an antiserum raised to human beta 2-microglobulin. This protein is strongly induced by interferon, which is a characteristic of the beta 2-microglobulin gene. Thus, FGF stimulates mouse embryo 3T3 cells to produce two proteins related to immune regulatory molecules. This may reflect an interaction between immune cells and nonimmune cells that occurs in vivo during processes such as wound healing when growth factors are released locally.

An endoplasmic reticulum-specific cyclophilin

Cyclophilin is a ubiquitously expressed cytosolic peptidyl-prolyl cis-trans isomerase that is inhibited by the immunosuppressive drug cyclosporin A. A degenerate oligonucleotide based on a conserved cyclophilin sequence was used to isolate cDNA clones representing a ubiquitously expressed mRNA from mice and humans. This mRNA encodes a novel 20-kDa protein, CPH2, that shares 64% sequence identity with cyclophilin. Bacterially expressed CPH2 binds cyclosporin A and is a cyclosporin A-inhibitable peptidyl-prolyl cis-trans isomerase. Cell fractionation of rat liver followed by Western blot (immunoblot) analysis indicated that CPH2 is not cytosolic but rather is located exclusively in the endoplasmic reticulum. These results suggest that cyclosporin A mediates its effect on cells through more than one cyclophilin and that cyclosporin A-induced misfolding of T-cell membrane proteins normally mediated by CPH2 plays a role in immunosuppression.

An endoplasmic reticulum-specific cyclophilin

Cyclophilin is a ubiquitously expressed cytosolic peptidyl-prolyl cis-trans isomerase that is inhibited by the immunosuppressive drug cyclosporin A. A degenerate oligonucleotide based on a conserved cyclophilin sequence was used to isolate cDNA clones representing a ubiquitously expressed mRNA from mice and humans. This mRNA encodes a novel 20-kDa protein, CPH2, that shares 64% sequence identity with cyclophilin. Bacterially expressed CPH2 binds cyclosporin A and is a cyclosporin A-inhibitable peptidyl-prolyl cis-trans isomerase. Cell fractionation of rat liver followed by Western blot (immunoblot) analysis indicated that CPH2 is not cytosolic but rather is located exclusively in the endoplasmic reticulum. These results suggest that cyclosporin A mediates its effect on cells through more than one cyclophilin and that cyclosporin A-induced misfolding of T-cell membrane proteins normally mediated by CPH2 plays a role in immunosuppression.

Human cyclophilin B: a second cyclophilin gene encodes a peptidyl-prolyl isomerase with a signal sequence

We report the cloning and characterization of a cDNA encoding a second human cyclosporin A-binding protein (hCyPB). Homology analyses reveal that hCyPB is a member of the cyclophilin B (CyPB) family, which includes yeast CyPB, Drosophila nina A, and rat cyclophilin-like protein. This family is distinguished from the cyclophilin A (CyPA) family by the presence of endoplasmic reticulum (ER)-directed signal sequences. hCyPB has a hydrophobic leader sequence not found in hCyPA, and its first 25 amino acids are removed upon expression in Escherichia coli. Moreover, we show that hCyPB is a peptidyl-prolyl cis-trans isomerase which can be inhibited by cyclosporin A. These observations suggest that other members of the CyPB family will have similar enzymatic properties. Sequence comparisons of the CyPB proteins show a central, 165-amino acid peptidyl-prolyl isomerase and cyclosporin A-binding domain, flanked by variable N-terminal and C-terminal domains. These two variable regions may impart compartmental specificity and regulation to this family of cyclophilin proteins containing the conserved core domain. Northern blot analyses show that hCyPB mRNA is expressed in the Jurkat T-cell line, consistent with its possible target role in cyclosporin A-mediated immunosuppression.