FAP48, a new protein that forms specific complexes with both immunophilins FKBP59 and FKBP12. Prevention by the immunosuppressant drugs FK506 and rapamycin

Functions of the Hsp90-Binding FKBP Immunophilins

The Hsp90 chaperone is known to interact with a diverse array of client proteins. However, in every case examined, Hsp90 is also accompanied by a single or several co-chaperone proteins. One class of co-chaperone contains a tetratricopeptide repeat (TPR) domain that targets the co-chaperone to the C-terminal region of Hsp90. Within this class are Hsp90-binding peptidylprolyl isomerases, most of which belong to the FK506-binding protein (FKBP) family. Despite the common association of FKBP co-chaperones with Hsp90, it is abundantly clear that the client protein influences, and is often influenced by, the particular FKBP bound to Hsp90. Examples include Xap2 in aryl hydrocarbon receptor complexes and FKBP52 in steroid receptor complexes. In this chapter, we discuss the known functional roles played by FKBP co-chaperones and, where possible, relate distinctive functions to structural differences between FKBP members.

Genetic Basis and Therapies for Vascular Anomalies

Vascular and lymphatic malformations represent a challenge for clinicians. The identification of inherited and somatic mutations in important signaling pathways, including the PI3K (phosphoinositide 3-kinase)/AKT (protein kinase B)/mTOR (mammalian target of rapamycin), RAS (rat sarcoma)/RAF (rapidly accelerated fibrosarcoma)/MEK (mitogen-activated protein kinase kinase)/ERK (extracellular signal-regulated kinases), HGF (hepatocyte growth factor)/c-Met (hepatocyte growth factor receptor), and VEGF (vascular endothelial growth factor) A/VEGFR (vascular endothelial growth factor receptor) 2 cascades has led to the evaluation of tailored strategies with preexisting cancer drugs that interfere with these signaling pathways. The era of theranostics has started for the treatment of vascular anomalies. Registration: URL: https://www.clinicaltrialsregister.eu; Unique identifier: 2015-001703-32.

FK506 binding proteins and inflammation related signalling pathways; basic biology, current status and future prospects for pharmacological intervention

FK506 binding (FKBP) proteins are part of the highly conserved immunophilin family and its members have fundamental roles in the regulation of signalling pathways involved in inflammation, adaptive immune responses, cancer and developmental biology. The original member of this family, FKBP12, is a well-known binding partner for the immunosuppressive drugs tacrolimus (FK506) and sirolimus (rapamycin). FKBP12 and its analog, FKBP12.6, function as cis/trans peptidyl prolyl isomerases (PPIase) and they catalyse the interconversion of cis/trans prolyl conformations. Members of this family uniquely contain a PPIase domain, which may not be functional. The larger FKBPs, such as FKBP51, FKBP52 and FKBPL, contain extra regions, including tetratricopeptide repeat (TPR) domains, which are important for their versatile protein-protein interactions with inflammation-related signalling pathways. In this review we focus on the pivotal role of FKBP proteins in regulating glucocorticoid signalling, canonical and non-canonical NF-κB signalling, mTOR/AKT signalling and TGF-β signalling. We examine the mechanism of action of FKBP based immunosuppressive drugs on these cell signalling pathways and how off target interactions lead to the development of side effects often seen in the clinic. Finally, we discuss the latest advances in the role of FKBPs as therapeutic targets and the development of novel agents for a range of indications of unmet clinical need, including glucocorticoid resistance, obesity, stress-induced inflammation and novel cancer immunotherapy.

Emerging importance of molecular pathogenesis of vascular malformations in clinical practice and classifications

Vascular malformations occur during early vascular development resulting in abnormally formed vessels that can manifest as arterial, venous, capillary or lymphatic lesions, or in combination, and include local tissue overdevelopment. Vascular malformations are largely caused by sporadic somatic gene mutations. This article aims to review and discuss current molecular signaling pathways and therapeutic targets for vascular malformations and to classify vascular malformations according to the molecular pathways involved. A literature review was performed using Embase and Medline. Different MeSH terms were combined for the search strategy, with the aim of encompassing all studies describing the classification, pathogenesis, and treatment of vascular malformations. Major pathways involved in the pathogenesis of vascular malformations are vascular endothelial growth factor (VEGF), Ras/Raf/MEK/ERK, angiopoietin-TIE2, transforming growth factor beta (TGF-β), and PI3K/AKT/mTOR. These pathways are involved in controlling cellular growth, apoptosis, differentiation, and proliferation, and play a central role in endothelial cell signaling and angiogenesis. Many vascular malformations share similar aberrant molecular signaling pathways with cancers and inflammatory disorders. Therefore, selective anticancer agents and immunosuppressants may be beneficial in treating vascular malformations of specific mutations. The current classification systems of vascular malformations, including the International Society of the Study of Vascular Anomalies (ISSVA) classification, are primarily observational and clinical, and are not based on the molecular pathways involved in the pathogenesis of the condition. Several molecular pathways with potential therapeutic targets have been demonstrated to contribute to the development of various vascular anomalies. Classifying vascular malformations based on their molecular pathogenesis may improve treatment by determining the underlying nature of the condition and their potential therapeutic target.

FKBP51 and FKBP12.6-Novel and tight interactors of Glomulin

The protein factor Glomulin (Glmn) is a regulator of the SCF (Skp1-CUL1-F-box protein) E3 ubiquitin-protein ligase complex. Mutations of Glmn lead to glomuvenous malformations. Glmn has been reported to be associated with FK506-binding proteins (FKBP). Here we present in vitro binding analyses of the FKBP—Glmn interaction. Interestingly, the previously described interaction of Glmn and FKBP12 was found to be comparatively weak. Instead, the closely related FKBP12.6 and FKBP51 emerged as novel binding partners. We show different binding affinities of full length and truncated FKBP51 and FKBP52 mutants. Using FKBP51 as a model system, we show that two amino acids lining the FK506-binding site are essential for binding Glmn and that the FKBP51-Glmn interaction is blocked by FKBP ligands. This data suggest FKBP inhibition as a pharmacological approach to regulate Glmn and Glmn-controlled processes.

The Many Faces of FKBP51

The FK506-binding protein 51 (FKBP51) has emerged as a key regulator of endocrine stress responses in mammals and as a potential therapeutic target for stress-related disorders (depression, post-traumatic stress disorder), metabolic disorders (obesity and diabetes) and chronic pain. Recently, FKBP51 has been implicated in several cellular pathways and numerous interacting protein partners have been reported. However, no consensus on the underlying molecular mechanisms has yet emerged. Here, we review the protein interaction partners reported for FKBP51, the proposed pathways involved, their relevance to FKBP51’s physiological function(s), the interplay with other FKBPs, and implications for the development of FKBP51-directed drugs.

Classification and Pathology of Congenital and Perinatal Vascular Anomalies of the Head and Neck

Accurate histopathologic description in correlation with clinical and radiological evaluation is required for treatment of vascular anomalies, both neoplastic and malformative. It is important to examine current clinical, histologic, and immunophenotypical features that distinguish the major types of congenital and perinatal vascular anomalies affecting the head and neck. General discussions of pathogenesis and molecular diagnosis must also be taken into account. This article provides an overview of the features that distinguish the major types of congenital and perinatal vascular anomalies affecting the head and neck, and summarizes the diagnostic histopathologic criteria and nomenclature currently applied to these lesions.

Functions of the Hsp90-binding FKBP immunophilins

Hsp90 functionally interacts with a broad array of client proteins, but in every case examined Hsp90 is accompanied by one or more co-chaperones. One class of co-chaperone contains a tetratricopeptide repeat domain that targets the co-chaperone to the C-terminal region of Hsp90. Within this class are Hsp90-binding peptidylprolyl isomerases, most of which belong to the FK506-binding protein (FKBP) family. Despite the common association of FKBP co-chaperones with Hsp90, it is now clear that the client protein influences, and is influenced by, the particular FKBP bound to Hsp90. Examples include Xap2 in aryl hydrocarbon receptor complexes and FKBP52 in steroid receptor complexes. In this chapter, we discuss the known functional roles played by FKBP co-chaperones and, where possible, relate distinctive functions to structural differences between FKBP members.

Genetics of vascular malformations

Vascular anomalies are developmental defects of the vasculature and encompass a variety of disorders. The majority of these occur sporadically, yet a few are reported to be familial. The identification of genes mutated in the different malformations provides insight into their etiopathogenic mechanisms and the specific roles the associated proteins play in vascular development and maintenance. It is becoming evident that somatic mosaicism plays a major role in the formation of vascular lesions. The importance of utilizing Next-Generating Sequencing (NGS) for high-throughput and "deep" screening of both blood and lesional DNA and RNA is thus emphasized, as the somatic changes are present in low quantities. There are several examples where NGS has already accomplished discovering these changes. The identification of all the causative genes and unraveling of a holistic overview of the pathogenic mechanisms should enable generation of in vitro and in vivo models and lead to development of more effective treatments, not only targeted on symptoms.

Location-dependent coronary artery diffusive and convective mass transport properties of a lipophilic drug surrogate measured using nonlinear microscopy

PURPOSE

Arterial wall mass transport properties dictate local distribution of biomolecules or locally delivered dugs. Knowing how these properties vary between coronary artery locations could provide insight into how therapy efficacy is altered between arterial locations.

METHODS

We introduced an indocarbocyanine drug surrogate to the lumens of left anterior descending and right coronary (LADC; RC) arteries from pigs with or without a pressure gradient. Interstitial fluorescent intensity was measured on live samples with multiphoton microscopy. We also measured binding to porcine coronary SMCs in monoculture.

RESULTS

Diffusive transport constants peaked in the middle sections of the LADC and RC arteries by 2.09 and 2.04 times, respectively, compared to the proximal and distal segments. There was no statistical difference between the average diffusivity value between LADC and RC arteries. The convection coefficients had an upward trend down each artery, with the RC being higher than the LADC by 3.89 times.

CONCLUSIONS

This study demonstrates that the convective and diffusive transport of lipophilic molecules changes between the LADC and the RC arteries as well as along their length. These results may have important implications in optimizing drug delivery for the treatment of coronary artery disease.

Vascular anomalies: from genetics toward models for therapeutic trials

Vascular anomalies are localized abnormalities that occur during vascular development. Several causative genes have been identified not only for inherited but also for some sporadic forms, and the molecular pathways involved are becoming understood. This gives us the opportunity to generate animals carrying the causative genetic defects, which we hope model the phenotype seen in human patients. These models would enable us not only to test known antiangiogenic drugs, but also to develop novel approaches for treatment, directly targeting the mutated protein or molecules implicated in the pathophysiological signaling pathways.

Insights on neoplastic stem cells from gel-based proteomics of childhood germ cell tumors

BACKGROUND

Childhood germ cell tumors (cGCTs), believed to arise from transformed primordial germ cells by an unknown mechanism, provide a unique model system for investigating cell signaling, pluripotency, and the microenvironment of neoplastic stem cells (NSCs) in vivo. This is the first report of proteomics of cGCTs.

PROCEDURE

Four dysgerminomas (DYSs) and four childhood endodermal sinus tumors (cESTs), resembling self-renewing and differentiating NSCs, respectively, were selected. Proteomic studies were performed by 2-DE, SDS-PAGE, and cLC/MS/MS with protein database searching.

RESULTS

2-DE: 9 of 941 spots were differentially regulated with greater than a twofold change in spot volume for at least three of four gels in each group. Two of nine spots had P values for the t-test analysis of comparisons less than 0.001, while the remaining spots had P values from 0.013 to 0.191. Top-ranked proteins were identified in nine of nine spots with 4.0-38% sequence coverage. APOA1, CRK, and PDIA3 were up-regulated in cESTs. TFG, TYMP, VCP, RBBP, FKBP4, and BiP were up-regulated in DYSs. SDS-PAGE: Up-regulation of NF45 and FKBP4 was observed in four of four cESTs and DYSs, respectively. The fold-changes observed correspond with characteristic genetic changes.

CONCLUSION

Differential regulation of FKBP4 and NF45, combined with previous research on immunosuppressant binding, suggests that glucocorticoid receptor signaling merits further investigation in cGCTs and NSCs.

Glomulin: a permissivity factor for vaccinia virus infection

In earlier studies we provided evidence that vaccinia virus (VACV) phosphorylation-activation of host cell signaling effectors is critical for subsequent viral replication. In this report, using mass spectrometry-based proteomics, we have identified 387 host cell proteins that co-immunoprecipitate with VACV in infected, permissive PM1.CCR5 human T cells. Among these, glomulin was distinguishable based on its known interaction with a tyrosine kinase receptor, c-Met, its ability to become tyrosine-phosphorylated, and its association with signaling effectors. siRNA knockdown of glomulin expression in PM1.CCR5 T cells reduces VACV infection. Glomulin interacts with the inactive, nonphosphorylated form of c-MET. We demonstrate that treatment of PM1.CCR5 T cells with a c-Met phosphorylation inhibitor leads to a significant reduction in VACV infectivity. Additionally, inhibition of phosphorylation of c-Met abrogates VACV-inducible phosphorylation of Erk 1/2 and IRS-2, signaling effectors identified as critical for VACV infection. These data identify glomulin as a permissivity factor for VACV infection and as a potential therapeutic target for inhibition of VACV infection.

FKBP family proteins as promising new biomarkers for cancer

FK506-binding proteins (FKBPs) belong to the immunophilin family and bind immunosuppressive drugs, such as FK506 and rapamycin. These proteins, through interactions with steroid hormone receptors, kinases, or other cellular factors, play important roles in various physiological processes and, more interestingly, in pathological processes in mammals. Accumulating evidence has implicated some FKBP members in a variety of processes, such as the cell cycle and survival and apoptotic signaling pathways, particularly in cancers. After the deregulation of their expression was observed in cancer tissues, it became increasingly clear that FKBP members played an important role in tumorigenesis and the response to chemotherapies and radiotherapies and that FKBP members could act as oncogenes or tumor suppressors depending on the tissue type. A wealth of data from in vitro and clinical studies is paving the way for novel, promising roles of FKBPs as diagnostic, prognostic or therapy-monitoring cancer biomarkers.

Pathogenesis of vascular anomalies

Vascular anomalies are localized defects of vascular development. Most of them occur sporadically (ie, there is no familial history of lesions, yet in a few cases clear inheritance is observed). These inherited forms are often characterized by multifocal lesions that are mainly small in size and increase in number with patients' age. The authors review the known (genetic) causes of vascular anomalies and call attention to the concept of Knudson's double-hit mechanism to explain incomplete penetrance and large clinical variation in expressivity observed in inherited vascular anomalies. The authors also discuss the identified pathophysiological pathways involved in vascular anomalies and how it has opened the doors toward a more refined classification of vascular anomalies and the development of animal models that can be tested for specific molecular therapies.

Tetracycline treatment targeting Wolbachia affects expression of an array of proteins in Brugia malayi parasite

Wolbachia is an intracellular endosymbiont of Brugia malayi parasite whose presence is essential for the survival of the parasite. Treatment of B. malayi-infected jirds with tetracycline eliminates Wolbachia, which affects parasite survival and fitness. In the present study we have tried to identify parasite proteins that are affected when Wolbachia is targeted by tetracycline. For this Wolbachia depleted parasites (B. malayi) were obtained by tetracycline treatment of infected Mongolian jirds (Meriones unguiculatus) and their protein profile after 2-DE separation was compared with that of untreated parasites harboring Wolbachia. Approximately 100 protein spots could be visualized followed by CBB staining of 2-D gel and included for comparative analysis. Of these, 54 showed differential expressions, while two new protein spots emerged (of 90.3 and 64.4 kDa). These proteins were subjected to further analysis by MALDI-TOF for their identification using Brugia coding sequence database composed of both genomic and EST sequences. Our study unravels two crucial findings: (i) the parasite or Wolbachia proteins, which disappeared/down-regulated appear be essential for parasite survival and may be used as drug targets and (ii) tetracycline treatment interferes with the regulatory machinery vital for parasites cellular integrity and defense and thus could possibly be a molecular mechanism for the killing of filarial parasite. This is the first proteomic study substantiating the wolbachial genome integrity with its nematode host and providing functional genomic data of human lymphatic filarial parasite B. malayi.

Anchoring of the 26S proteasome to the organellar membrane by FKBP38

FK506-binding protein 38 (FKBP38) is a member of the immunophilin family that resides in the mitochondrial outer membrane and the endoplasmic reticulum (ER) membrane. To investigate the physiological function of FKBP38, we performed a comprehensive search for proteins with which it interacts in human cells by liquid chromatographic and mass spectrometric analysis of FKBP38 immunoprecipitates. Almost all subunits of the 26S proteasome were thus found to interact with FKBP38. In vivo co-immunoprecipitation analyses confirmed that FKBP38 indeed associates with the 26S proteasome via its three tandem tetratricopeptide repeats (TPRs). Binding assays in vitro also revealed that FKBP38 directly interacts with the S4 subunit of the 19S proteasome. Immunofluorescence analysis demonstrated that the subcellular distributions of FKBP38 and the 26S proteasome partially overlapped at mitochondria. Both the abundance and activity of the proteasome in a membrane fraction were markedly reduced for mouse embryonic fibroblasts prepared from Fkbp38(-/-) mice compared with those prepared from wild-type mice. These results suggest that FKBP38 functions to anchor the 26S proteasome at the organellar membrane.

Functional specificity of co-chaperone interactions with Hsp90 client proteins

A wide array of proteins in signal transduction pathways depend on Hsp90 and other chaperone components for functional maturation, regulation, and stability. Among these Hsp90 client proteins are steroid receptors, members from other classes of transcription factors, and representatives of both serine/threonine and tyrosine kinase families. Typically, dynamic complexes form on the client protein, and these consist of Hsp90- plus bound co-chaperones that often have enzymatic activities. In addition to its direct influence on client folding, Hsp90 locally concentrates co-chaperone activity within the client complex, and dynamic exchange of co-chaperones on Hsp90 facilitates sampling of co-chaperone activities that may, or may not, act on the client protein. We are just beginning to understand the nature of biochemical and molecular interactions between co-chaperone and Hsp90-bound client. This review focuses on the differential effects of Hsp90 co-chaperones toward client protein function and on the specificity that allows co-chaperones to discriminate between even closely related clients.

The FK506 binding protein 13 kDa (FKBP13) interacts with the C-chain of complement C1q

Background

The pharmacological action of specific immunosuppressants is mediated by immunophilins. While cyclosporin A binds to cyclophilins, FK506/tacrolimus, rapamycin, and others bind to FK506 binding proteins (FKBPs). Different physiological actions of immunophilins were described but their genuine function, however, remains elusive and is still under investigation. A yeast two-hybrid screen was performed using the FK506 binding protein 13 kDa (FKBP13) as a bait and a fetal liver expression library as a prey.

Results

The C-chain of complement C1q (C1q-C) was detected to interact with FKBP13 in the yeast two-hybrid system and in a protein complementation assay. Neither FKBP12, FKBP25, FKBP52 nor the unrelated immunophilin CypA did react with C1q-C in the yeast system stressing the specificity of the interaction. Binding of C1q-C to FKBP13 could not be prevented in the presence of FK506, demonstrating that possibly other regions than the binding pocket of the drug are responsible for the interaction of the two proteins.

Conclusion

It is concluded that exclusively FKBP13 but no other FKBPs tested so far interact with the C-chain of complement C1q in the two different assays and further work will be initiated to investigate the physiological relevance of the interaction.

Glomulin is predominantly expressed in vascular smooth muscle cells in the embryonic and adult mouse

Mutations in the glomulin gene result in dominantly inherited vascular lesions of the skin known as glomuvenous malformations (GVMs). These lesions are histologically distinguished by their distended vein-like channels containing characteristic 'glomus cells', which appear to be incompletely or improperly differentiated vascular smooth muscle cells (VSMCs). The function of glomulin is currently unknown. We studied glomulin expression during murine development (E9.5 days post-coitum until adulthood) by non-radioactive in situ hybridization. Glomulin was first detected at E10.5 dpc in cardiac outflow tracts. Later, it showed strong expression in VSMCs as well as a limited expression in the perichondrium. At E11.5-14.5 dpc glomulin RNA was most abundant in the walls of the large vessels. At E16.5 dpc expression was also detectable in smaller arteries and veins. The high expression of glomulin in murine vasculature suggests an important role for glomulin in blood vessel development and/or maintenance, which is supported by the vascular phenotype seen in GVM patients with mutations in this gene.

The immunophilin-interacting protein AtFIP37 from Arabidopsis is essential for plant development and is involved in trichome endoreduplication

The FKBP12 (FK506-binding protein 12 kD) immunophilin interacts with several protein partners in mammals and is a physiological regulator of the cell cycle. In Arabidopsis, only one specific partner of AtFKBP12, namely AtFIP37 (FKBP12 interacting protein 37 kD), has been identified but its function in plant development is not known. We present here the functional analysis of AtFIP37 in Arabidopsis. Knockout mutants of AtFIP37 show an embryo-lethal phenotype that is caused by a strong delay in endosperm development and embryo arrest. AtFIP37 promoter::beta-glucuronidase reporter gene constructs show that the gene is expressed during embryogenesis and throughout plant development, in undifferentiating cells such as meristem or embryonic cells as well as highly differentiating cells such as trichomes. A translational fusion with the enhanced yellow fluorescent protein indicates that AtFIP37 is a nuclear protein localized in multiple subnuclear foci that show a speckled distribution pattern. Overexpression of AtFIP37 in transgenic lines induces the formation of large trichome cells with up to six branches. These large trichomes have a DNA content up to 256C, implying that these cells have undergone extra rounds of endoreduplication. Altogether, these data show that AtFIP37 is critical for life in Arabidopsis and implies a role for AtFIP37 in the regulation of the cell cycle as shown for FKBP12 and TOR (target of rapamycin) in mammals.

Promoter activity and gene structure of rabbit FKBP52

A 0.9-kb fragment situated directly upstream of the first ATG of rabbit FKBP52 is rich in acceptor sites for transcription factors, contains a CAAT box at -197 and could represent the proximal promoter of this immunophilin. Transvection analysis of this fragment showed strong promoter activity on the expression of a reporter gene. Deletions at the 5' end of this fragment showed that a basic sequence of 155 base pairs upstream of the CAAT box was sufficient to enhance luciferase expression an average 220-fold compared to the empty vector. This sequence, which contains acceptor sites for transcription factors of the EGR family and heat-shock factors, is closely homologous to 110 base pairs situated directly 5' of FKBP52 exon 1 in human chromosome 12p13.3, suggesting that these transcription factors could be involved in the regulation of the gene in both species. Furthermore, the upstream region of RbFKBP52 contains a large proportion of SINEs (C-repeats, Alu analogs), some of which include the A and B boxes required for transcription of RNA polymerase III, and poly A tracts. RbFKBP52, like HuFKBP52, is made up of 10 exons and 9 introns, a feature shared with other large immunophilins such as FKBP65 and Cyclophilin 40, and which appears widely conserved.

Targeted disruption of p185/Cul7 gene results in abnormal vascular morphogenesis

Cul1, a member of the cullin ubiquitin ligase family, forms a multiprotein complex known as SCF and plays an essential role in numerous cellular and biological activities. A Cul1 homologue, p185 (Cul7), has been isolated as an simian virus 40 large T antigen-binding protein. To understand the physiological role of p185, we generated mice lacking p185. p185-/- embryos are runted and die immediately after birth because of respiratory distress. Dermal and hypodermal hemorrhage is detected in mutant embryos at late gestational stage. p185-/- placentas show defects in the differentiation of the trophoblast lineage with an abnormal vascular structure. We demonstrate that p185 forms an SCF-like complex with Skp1, Rbx1, Fbw6 (Fbx29), and FAP68 (FAP48, glomulin). FAP68 has recently been identified as a gene responsible for familial glomuvenous malformation. These results suggest that p185 forms a multiprotein complex and plays an important role in vascular morphogenesis.

TWISTED DWARF1, a unique plasma membrane-anchored immunophilin-like protein, interacts with Arabidopsis multidrug resistance-like transporters AtPGP1 and AtPGP19

Null-mutations of the Arabidopsis FKBP-like immunophilin TWISTED DWARF1 (TWD1) gene cause a pleiotropic phenotype characterized by reduction of cell elongation and disorientated growth of all plant organs. Heterologously expressed TWD1 does not exhibit cis-trans-peptidylprolyl isomerase (PPIase) activity and does not complement yeast FKBP12 mutants, suggesting that TWD1 acts indirectly via protein-protein interaction. Yeast two-hybrid protein interaction screens with TWD1 identified cDNA sequences that encode the C-terminal domain of Arabidopsis multidrug-resistance-like ABC transporter AtPGP1. This interaction was verified in vitro. Mapping of protein interaction domains shows that AtPGP1 surprisingly binds to the N-terminus of TWD1 harboring the cis-trans peptidyl-prolyl isomerase-like domain and not to the tetratrico-peptide repeat domain, which has been shown to mediate protein-protein interaction. Unlike all other FKBPs, TWD1 is shown to be an integral membrane protein that colocalizes with its interacting partner AtPGP1 on the plasma membrane. TWD1 also interacts with AtPGP19 (AtMDR1), the closest homologue of AtPGP1. The single gene mutation twd1-1 and double atpgp1-1/atpgp19-1 (atmdr1-1) mutants exhibit similar phenotypes including epinastic growth, reduced inflorescence size, and reduced polar auxin transport, suggesting that a functional TWD1-AtPGP1/AtPGP19 complex is required for proper plant development.

Vascular malformations: localized defects in vascular morphogenesis

Vascular anomalies are localized defects of the vasculature, and usually affect a limited number of vessels in a restricted area of the body. They are subdivided into vascular malformations and vascular tumours. Most are sporadic, but Mendelian inheritance is observed in some families. By genetic analysis, several causative genes have been identified during the last 10 years. This has shed light into the pathophysiological pathways involved. Interestingly, in most cases, the primary defect seems to affect the characteristics of endothelial cells. Only mutations in the glomulin gene, responsible for hereditary glomuvenous malformations, are thought to directly affect vascular smooth-muscle cells.

The FKBP-associated protein FAP48 is an antiproliferative molecule and a player in T cell activation that increases IL2 synthesis

FAP48 was identified and cloned thanks to its interaction with FK506-binding proteins (FKBPs) such as FKBP52 and FKBP12, which belong to the large family of immunophilins that bind the macrolide immunosuppressant drugs FK506 and rapamycin. We have previously shown that FAP48-FKBP complexes are dissociated by FK506 and rapamycin, suggesting that FAP48 is an endogenous ligand of FKBP. The present work describes the biochemical consequences of FAP48 overexpression, induced by the tetracycline analogue doxycycline, in an established cell line derived from Jurkat T cells. We report that overexpression of FAP48 results in the inhibition of cellular proliferation as does the exposure of Jurkat T cells to FK506. We also show that the expression levels of argininosuccinate synthetase and the Myc antagonist Mxi1 are modified by overexpression of FAP48, suggesting that these proteins could be good candidates to mediate the antiproliferative effect of FAP48. FAP48 affects neither the calcineurin-dependent nuclear factor of activated T cells (NFAT)1 nor JNKp38-dependent pathways that mediate immunosuppression by FK506. However, contrary to FK506, which blocks IL2 synthesis, we observed that FAP48-FKBP complexes increase IL2 production, thus revealing a previously uncharacterized aspect of the immunosuppressive mechanism of FK506.

The involvement of mammalian and plant FK506-binding proteins (FKBPs) in development

The FK506-binding proteins (FKBPs) are peptidyl prolyl cis/trans isomerases and the information gathered in the last 10 years reveals their involvement in diverse biological systems affecting the function and structure of target proteins. Members of the FKBP family were shown to be growth-regulated and participate in signal transduction. In this review we have chosen to focus on a few examples of the mammalian and plant systems in which members of the FKBP family have been demonstrated to affect the function of proteins or development. The technologies that enable production of knockout mice, Arabidopsis mutants and overexpression in transgenic organisms have revealed the contribution of FKBP to development in higher eukaryotes. It appears that members of the FKBP family have conserved some of their basic functions in the animal and plant kingdom, whereas other functions became unique. Studies that will take advantage of the full genome sequence available for Arabidopsis and the human genome, DNA chip technologies and the use of transgenic complementation system will contribute to the elucidation of the molecular mechanism and biological function of FKBPs.

All of the protein interactions that link steroid receptor.hsp90.immunophilin heterocomplexes to cytoplasmic dynein are common to plant and animal cells

Both plant and animal cells contain high molecular weight immunophilins that bind via tetratricopeptide repeat (TPR) domains to a TPR acceptor site on the ubiquitous and essential protein chaperone hsp90. These hsp90-binding immunophilins possess the signature peptidylprolyl isomerase (PPIase) domain, but no role for their PPIase activity in protein folding has been demonstrated. From the study of glucocorticoid receptor (GR).hsp90.immunophilin complexes in mammalian cells, there is considerable evidence that both hsp90 and the FK506-binding immunophilin FKBP52 play a role in receptor movement from the cytoplasm to the nucleus. The role of FKBP52 is to target the GR.hsp90 complex to the nucleus by binding via its PPIase domain to cytoplasmic dynein, the motor protein responsible for retrograde movement along microtubules. Here, we use rabbit cytoplasmic dynein as a surrogate for the plant homologue to show that two hsp90-binding immunophilins of wheat, wFKBP73 and wFKBP77, bind to dynein. Binding to dynein is blocked by competition with a purified FKBP52 fragment comprising its PPIase domain but is not affected by the immunosuppressant drug FK506, suggesting that the PPIase domain but not PPIase activity is involved in dynein binding. The hsp90/hsp70-based chaperone system of wheat germ lysate assembles complexes between mouse GR and wheat hsp90. These receptor heterocomplexes contain wheat FKBPs, and they bind rabbit cytoplasmic dynein in a PPIase domain-specific manner. Retention by plants of the entire heterocomplex assembly machinery for linking the GR to dynein implies a fundamental role for this process in the biology of the eukaryotic cell.

Mutations in a novel factor, glomulin, are responsible for glomuvenous malformations ("glomangiomas")

Glomuvenous malformations (GVMs) are cutaneous venous lesions characterized by the presence of smooth-muscle--like glomus cells in the media surrounding distended vascular lumens. We have shown that heritable GVMs link to a 4--6-cM region in chromosome 1p21-22. We also identified linkage disequilibrium that allowed a narrowing of this VMGLOM locus to 1.48 Mb. Herein, we report the identification of the mutated gene, glomulin, localized on the basis of the YAC and PAC maps. An incomplete cDNA sequence for glomulin had previously been designated "FAP48," for "FKBP-associated protein of 48 kD." The complete cDNA for glomulin contains an open reading frame of 1,785 nt encoding a predicted protein of 68 kD. The gene consists of 19 exons in which we identified 14 different germline mutations in patients with GVM. In addition, we found a somatic "second hit" mutation in affected tissue of a patient with an inherited genomic deletion. Since all but one of the mutations result in premature stop codons, and since the localized nature of the lesions could be explained by Knudson's two-hit model, GVMs are likely caused by complete loss of function of glomulin. The abnormal phenotype of vascular smooth-muscle cells (VSMCs) in GVMs suggests that glomulin plays an important role in differentiation of these cells--and, thereby, in vascular morphogenesis--especially in cutaneous veins.

Ligand-regulated binding of FAP68 to the hepatocyte growth factor receptor

We have used the yeast two-hybrid system to identify proteins that interact with the intracellular portion of the hepatocyte growth factor (HGF) receptor (Met). We isolated a human cDNA encoding a novel protein of 68 kDa, which we termed FAP68. This protein is homologous to a previously described FK506-binding protein-associated protein, FAP48, which derives from an alternative spliced form of the same cDNA, lacking an 85-nucleotide exon and leading to an early stop codon. Here we show that epithelial cells, in which the HGF receptor is naturally expressed, contain FAP68 and not FAP48 proteins. FAP68 binding to Met requires the last 30 amino acids of the C-terminal tail, which are unique to the HGF receptor. Indeed, FAP68 does not interact with related tyrosine kinases of the Met and insulin receptor families. FAP68 interacts specifically with the inactive form of HGF receptor, such as a kinase-defective receptor or a dephosphorylated wild type receptor. In vivo, endogenous FAP68 can be coimmunoprecipitated with the HGF receptor in the absence of stimuli and not upon HGF stimulation. Thus, FAP68 represents a novel type of effector that interacts with the inactive HGF receptor and is released upon receptor phosphorylation. Free FAP68 exerts a specific stimulatory activity toward the downstream target p70 S6 protein kinase (p70S6K). Significantly, nonphosphorylated HGF receptor prevents FAP68 from stimulating p70S6K. These data suggest a role for FAP68 in coupling HGF receptor signaling to the p70S6K pathway.

FKBPs: at the crossroads of folding and transduction

FK506-binding proteins (FKBPs) belong to the large family of peptidyl-prolyl cis-trans isomerases, which are known to be involved in many cellular processes, such as cell signalling, protein trafficking and transcription. FKBPs associate into protein complexes, although the involvement and precise role of their foldase activity remain to be elucidated. FKBPs represent a large gene family in plants that is involved in growth and development. Disruption of genes encoding FKBPs in plants and animals has underlined the importance of this family of proteins in the regulation of cell division and differentiation.

Evidence that the peptidylprolyl isomerase domain of the hsp90-binding immunophilin FKBP52 is involved in both dynein interaction and glucocorticoid receptor movement to the nucleus

We have previously shown that immunoadsorption of the FKBP52 immunophilin component of steroid receptor.hsp90 heterocomplexes is accompanied by coadsorption of cytoplasmic dynein, a motor protein involved in retrograde transport of vesicles toward the nucleus. Coimmunoadsorption of dynein is competed by an expressed fragment of FKBP52 comprising its peptidylprolyl isomerase (PPIase) domain (Silverstein, A. M., Galigniana, M. D., Kanelakis, K. C., Radanyi, C., Renoir, J.-M., and Pratt, W. B. (1999) J. Biol. Chem. 52, 36980-36986). Here we show that cotransfection of 3T3 cells with the FKBP52 PPIase domain and a green fluorescent protein (GFP) glucocorticoid receptor (GR) chimera inhibits dexamethasone-dependent movement of the GFP-GR from the cytoplasm to the nucleus. Cotransfection with FKBP12 does not affect GFP-GR movement. Inhibition of movement by the FKBP52 PPIase domain is abrogated in cells treated with colcemid to eliminate microtubules prior to steroid addition. After withdrawal of colcemid, microtubules reform, and PPIase inhibition of GFP-GR movement is restored. These observations are consistent with the notion that FKBP52 targets retrograde movement of the GFP-GR along microtubules by linking the receptor to the dynein motor. Here, we also show that native GR.hsp90 heterocomplexes immunoadsorbed from L cell cytosol contain dynein and that GR.hsp90 heterocomplexes assembled in reticulocyte lysate contain cytoplasmic dynein in a manner that is competed by the PPIase domain of FKBP52.

Mutation of FKBP associated protein 48 (FAP48) at proline 219 disrupts the interaction with FKBP12 and FKBP52

Immunophilins are known as intracellular receptors for the immunosuppressant drugs, cyclosporin A, FK506, and rapamycin. They can be divided into two groups, cyclophilins that bind cyclosporin A and FK506 binding proteins (FKBPs) that bind FK506 and rapamycin. Many efforts were made to elucidate the physiological role of the immunophilins. Many of them are involved in intracellular signalling as they bind to calcium channels or to steroid receptor complexes. A yeast two-hybrid screen was used to identify further target proteins that interact with known proteins. Recently, a 48-kDa FKBP associated protein (FAP48) was isolated that binds to FKBP12 and FKBP52. Binding of FAP48 to FKBPs is inhibited by the macrolide FK506 indicating that the binding sites on the immunophilins coincide with the binding site for FK506. A peptidyl-prolyl motif on FAP48 should be responsible for the binding of the protein to FKBPs. We sequentially point mutated proline sites on FAP48 and checked the mutant proteins for interaction with FKBP12 and FKBP52. Mutation of proline 219 to alanine leads to a loss of interaction indicating that a cysteinyl prolyl site might be responsible for the binding of FAP48 to FKBPs. Thus we identified proline 219 being essential for the interaction.

FAP1, a homologue of human transcription factor NF-X1, competes with rapamycin for binding to FKBP12 in yeast

The immunosuppressive drug rapamycin binds to the peptidyl-prolyl cis-trans isomerase FKBP12, and this complex arrests growth of yeast cells and activated T lymphocytes in the G1 phase of the cell cycle. In yeast, loss-of-function mutations in FPR1, the gene encoding FKBP12, or dominant gain-of-function mutations in TOR1 and TOR2, the genes encoding the physical targets of the FKBP12-rapamycin complex, confer rapamycin resistance. Here, we report the cloning and characterization of a novel gene, termed FAP1, which confers resistance to rapamycin by competing with the drug for binding to FKBP12. FAP1 encodes a member of an evolutionarily conserved family of putative transcription factors that includes human NF-X1, Drosophila melanogaster shuttle craft and previously undescribed homologues in Caenorhabditis elegans, Arabidopsis thaliana and Schizosaccharomyces pombe. We provide genetic and biochemical evidence that FAP1 interacts physically with FKBP12 in vivo and in vitro, and that it competes with rapamycin for interaction. Furthermore, mutations in the FKBP12 drug binding/active site or surface residues abolish binding to FAP1. Our results suggest that FAP1 is a physiological ligand for FKBP12 that is highly conserved from yeast to man. Furthermore, prolyl isomerases may commonly bind and regulate transcription factors.

Immunophilins, Refsum disease, and lupus nephritis: the peroxisomal enzyme phytanoyl-COA alpha-hydroxylase is a new FKBP-associated protein

FKBP52 (FKBP59, FKBP4) is a "macro" immunophilin that, although sharing high structural and functional homologies in its amino-terminal domain with FKBP12 (FKBP1), does not have immunosuppressant activity when complexed with FK506, unlike FKBP12. To investigate the physiological function of FKBP52, we used the yeast two-hybrid system as an approach to find its potential protein partners and, from that, its cellular role. This methodology, which already has allowed us to find the FK506-binding protein (FKBP)-associated protein FAP48, also led to the detection of another FKBP-associated protein. Determination of the sequence of this protein permitted its identification as phytanoyl-CoA alpha-hydroxylase (PAHX), a peroxisomal enzyme that so far was unknown as an FKBP-associated protein. Inactivation of this enzyme is responsible for Refsum disease in humans. The protein also corresponds to the mouse protein LN1, which could be involved in the progress of lupus nephritis. We show here that PAHX has the physical capacity to interact with the FKBP12-like domain of FKBP52, but not with FKBP12, suggesting that it is a particular and specific target of FKBP52. Whereas the binding of calcineurin to FKBP12 is potentiated by FK506, the specific association of PAHX and FKBP52 is maintained in the presence of FK506. This observation suggests that PAHX is a serious candidate for studying the cellular signaling pathway(s) involving FKBP52 in the presence of immunosuppressant drugs.

Hmo1p, a high mobility group 1/2 homolog, genetically and physically interacts with the yeast FKBP12 prolyl isomerase

The immunosuppressive drugs FK506 and rapamycin bind to the cellular protein FKBP12, and the resulting FKBP12-drug complexes inhibit signal transduction. FKBP12 is a ubiquitous, highly conserved, abundant enzyme that catalyzes a rate-limiting step in protein folding: peptidyl-prolyl cis-trans isomerization. However, FKBP12 is dispensible for viability in both yeast and mice, and therefore does not play an essential role in protein folding. The functions of FKBP12 may involve interactions with a number of partner proteins, and a few proteins that interact with FKBP12 in the absence of FK506 or rapamycin have been identified, including the ryanodine receptor, aspartokinase, and the type II TGF-beta receptor; however, none of these are conserved from yeast to humans. To identify other targets and functions of FKBP12, we have screened for mutations that are synthetically lethal with an FKBP12 mutation in yeast. We find that mutations in HMO1, which encodes a high mobility group 1/2 homolog, are synthetically lethal with mutations in the yeast FPR1 gene encoding FKBP12. Deltahmo1 and Deltafpr1 mutants share two phenotypes: an increased rate of plasmid loss and slow growth. In addition, Hmo1p and FKBP12 physically interact in FKBP12 affinity chromatography experiments, and two-hybrid experiments suggest that FKBP12 regulates Hmo1p-Hmo1p or Hmo1p-DNA interactions. Because HMG1/2 proteins are conserved from yeast to humans, our findings suggest that FKBP12-HMG1/2 interactions could represent the first conserved function of FKBP12 other than mediating FK506 and rapamycin actions.

The common tetratricopeptide repeat acceptor site for steroid receptor-associated immunophilins and hop is located in the dimerization domain of Hsp90

Structurally related tetratricopeptide repeat motifs in steroid receptor-associated immunophilins and the STI1 homolog, Hop, mediate the interaction with a common cellular target, hsp90. We have identified the binding domain in hsp90 for cyclophilin 40 (CyP40) using a two-hybrid system screen of a mouse cDNA library. All isolated clones encoded the intact carboxyl terminus of hsp90 and overlapped with a common region corresponding to amino acids 558-724 of murine hsp84. The interaction was confirmed in vitro with bacterially expressed CyP40 and deletion mutants of hsp90beta and was delineated further to a 124-residue COOH-terminal segment of hsp90. Deletion of the conserved MEEVD sequence at the extreme carboxyl terminus of hsp90 precludes interaction with CyP40, signifying an important role for this motif in hsp90 function. We show that CyP40 and Hop display similar interaction profiles with hsp90 truncation mutants and present evidence for the direct competition of Hop and FK506-binding protein 52 with CyP40 for binding to the hsp90 COOH-terminal region. Our results are consistent with a common tetratricopeptide repeat interaction site for Hop and steroid receptor-associated immunophilins within a discrete COOH-terminal domain of hsp90. This region of hsp90 mediates ATP-independent chaperone activity, overlaps the hsp90 dimerization domain, and includes structural elements important for steroid receptor interaction.

An Arabidopsis immunophilin, AtFKBP12, binds to AtFIP37 (FKBP interacting protein) in an interaction that is disrupted by FK506

AtFKBP12 is an Arabidopsis cDNA that encodes a protein similar to the mammalian immunophilin, FKBP12. AtFKBP12 was used as 'bait' in a yeast 2-hybrid system to screen for cDNAs in Arabidopsis encoding proteins that bind to FKBP12. Two partial cDNAs were recovered encoding the C-terminus of a protein we have called Arabidopsis thaliana FKBP12 interacting protein 37 (AtFIP37). AtFIP37 is similar to a mammalian protein, FAP48, that also binds to FKBP12. The interaction between AtFKBP12 and AtFIP37 in the 2-hybrid system, as assessed by histidine auxotrophy and beta-galactosidase activity, was disrupted by FK506, but not by cyclosporin A, a drug that binds to cyclophilin A. AtFIP37 was also shown to bind in vitro to AtFKBP12 in GST-fusion protein binding assays. The binding was abolished by prior incubation of AtFKBP12 with FK506. These findings indicate that an Arabidopsis FKBP12 ortholog encodes a protein that binds FK506 and that the interaction between AtFKBP12 and AtFIP37 may involve the FK506 binding site of AtFKBP12. The interaction provides interesting new opportunities for controlling protein:protein interactions in vivo in plants.

Neural actions of immunophilin ligands

Immunophilins, protein receptors for immunosuppressant drugs such as cyclosporin A and FK506, are enriched far more in the brain than in the immune system. Drug-immunophilin complexes bind to calcineurin, inhibiting its phosphatase activity and leading to immunosuppressant effects. The immunophilin FKBP-12 (FK506 binding protein, 12 kDa) forms a complex with the ryanodine and inositol (1,4,5) trisphosphate (IP3) receptors to regulate their physiological release of intracellular Ca2+. Here, Solomon Snyder and colleagues describe how non-immunosuppressant as well as immunosuppressant immunophilin ligands are neurotrophic for numerous classes of damaged neurones, both in culture systems and intact animals. Their ability to stimulate functional regrowth of damaged sciatic, cortical cholinergic, dopamine and 5-HT neurones may have therapeutic relevance.

Phosphorylation of the immunosuppressant FK506-binding protein FKBP52 by casein kinase II: regulation of HSP90-binding activity of FKBP52

FKBP52 (HSP56, p59, HBI) is the 59-kDa immunosuppressant FK506-binding protein and has peptidyl prolyl isomerase as well as a chaperone-like activity in vitro. FKBP52 associates with the heat shock protein HSP90 and is included in the steroid hormone receptor complexes in vivo. FKBP52 possesses a well conserved phosphorylation site for casein kinase II (CK2) that was previously shown to be associated with HSP90. Here we examined whether FKBP52 is phosphorylated by CK2 both in vivo and in vitro. Recombinant rabbit FKBP52 was phosphorylated by purified CK2. We expressed and purified deletion mutants of FKBP52 to determine the site(s) phosphorylated by CK2. Thr-143 in the hinge I region was identified as the major phosphorylation site for CK2. A synthetic peptide corresponding to this region was phosphorylated by CK2, and the peptide competitively inhibited the phosphorylation of other substrates by CK2. The [32P]phosphate labeling of FKBP52-expressing cells revealed that the same site is also phosphorylated in vivo. FK506 binding to FKBP52 did not affect the phosphorylation by CK2 and, conversely, the FK506-binding activity of FKBP52 was not affected by the phosphorylation. Most importantly, CK2-phosphorylated FKBP52 did not bind to HSP90. These results indicate that CK2 phosphorylates FKBP52 both in vitro and in vivo and thus may regulate the protein composition of chaperone-containing complexes such as those of steroid receptors and certain protein kinases.

Effects of the immunosuppressive drugs CsA and FK506 on intracellular signalling and gene regulation

The isolation of Cyclosporin A (CsA) from cultures of the fungus Tolypocladium inflatum and its subsequent elucidation of immunosuppressive properties by Borel et al. (1) was of great clinical consequence. In the early 80s CsA was introduced in the field of organ transplantation resulting in extraordinary improvements of graft survival. CsA has become a first choice drug for patients with allograft organs. The discovery of FK506 by Kino et al. (2) as a novel immuno-suppressant and its introduction into clinics in 1989 (3) extended the available regimen for immunosuppressive therapy. Yet despite their advantages both CsA and FK506 display unwanted side effects and a possible preference of one drug over another remains controversial (4, 5). Although identification of the involvement of the transcription factor NF-AT was an important step forward (6), it has become clear that immunosuppressant action is more complex. CsA and FK506 selectively interact with certain cellular signal transduction pathways. This review briefly describes these effects on signal transduction. We further concentrate on the major known effect of these immunosuppressants, namely the inhibition of the PP2B phosphatase calcineurin. In addition we provide a compilation of effects of CsA and FK506 on gene expression at the level of transcription.