The immunophilin FKBP52 specifically binds to tubulin and prevents microtubule formation

Management of cytoskeleton architecture by molecular chaperones and immunophilins

Cytoskeletal structure is continually remodeled to accommodate normal cell growth and to respond to pathophysiological cues. As a consequence, several cytoskeleton-interacting proteins become involved in a variety of cellular processes such as cell growth and division, cell movement, vesicle transportation, cellular organelle location and function, localization and distribution of membrane receptors, and cell-cell communication. Molecular chaperones and immunophilins are counted among the most important proteins that interact closely with the cytoskeleton network, in particular with microtubules and microtubule-associated factors. In several situations, heat-shock proteins and immunophilins work together as a functionally active heterocomplex, although both types of proteins also show independent actions. In circumstances where homeostasis is affected by environmental stresses or due to genetic alterations, chaperone proteins help to stabilize the system. Molecular chaperones facilitate the assembly, disassembly and/or folding/refolding of cytoskeletal proteins, so they prevent aberrant protein aggregation. Nonetheless, the roles of heat-shock proteins and immunophilins are not only limited to solve abnormal situations, but they also have an active participation during the normal differentiation process of the cell and are key factors for many structural and functional rearrangements during this course of action. Cytoskeleton modifications leading to altered localization of nuclear factors may result in loss- or gain-of-function of such factors, which affects the cell cycle and cell development. Therefore, cytoskeletal components are attractive therapeutic targets, particularly microtubules, to prevent pathological situations such as rapidly dividing tumor cells or to favor the process of cell differentiation in other cases. In this review we will address some classical and novel aspects of key regulatory functions of heat-shock proteins and immunophilins as housekeeping factors of the cytoskeletal network.

Parvulin 17 promotes microtubule assembly by its peptidyl-prolyl cis/trans isomerase activity

The parvulin-type peptidyl-prolyl cis/trans isomerases (PPIases) have been shown to be involved in tumor progression and the pathogenesis of Alzheimer's disease and were therefore a subject of intense research. Here, we describe a role for parvulin 17 in microtubule assembly. Co-precipitation experiments and sedimentation assays demonstrated that parvulin 17 interacts with tubulin in a GTP-dependent manner and thereby promotes the formation of microtubules, as shown by transmission electron microscopy and a microtubule polymerization assay. The microtubule-assembly-promoting properties of parvulin 17 seem to depend on its PPIase activity. Thus, catalytic deficient variants of parvulin 17 were not able to promote microtubule formation. Accordingly, inhibitors of parvulin 17 activity also prevent parvulin-catalyzed tubulin polymerization. The analysis of tubulin interaction sites on parvulin using peptide microarrays revealed that tubulin interacts with the substrate binding pocket of parvulin. Additionally, β-tubulin peptide scan on microarrays demonstrates interaction of parvulin 17 with an Arg-Pro-Asp motif corresponding to proline residue 87 of β-tubulin. Confocal laser scanning microscopy points to a function of parvulin 17 in microtubule dynamics as well. Parvulin 17 is predominantly found in the cytosol and colocalizes with microtubules.

Bending tau into shape: the emerging role of peptidyl-prolyl isomerases in tauopathies

The Hsp90-associated cis-trans peptidyl-prolyl isomerase--FK506 binding protein 51 (FKBP51)--was recently found to co-localize with the microtubule (MT)-associated protein tau in neurons and physically interact with tau in brain tissues from humans who died from Alzheimer's disease (AD). Tau pathologically aggregates in neurons, a process that is closely linked with cognitive deficits in AD. Tau typically functions to stabilize and bundle MTs. Cellular events like calcium influx destabilize MTs, disengaging tau. This excess tau should be degraded, but sometimes it is stabilized and forms higher-order aggregates, a pathogenic hallmark of tauopathies. FKBP51 was also found to increase in forebrain neurons with age, further supporting a novel role for FKBP51 in tau processing. This, combined with compelling evidence that the prolyl isomerase Pin1 regulates tau stability and phosphorylation dynamics, suggests an emerging role for isomerization in tau pathogenesis.

Organization and function of the FKBP52 and FKBP51 genes

Best established as components of steroid hormone receptor complexes, it is now clear that the large molecular weight immunophilins, FKBP52 and FKBP51, play important regulatory roles elsewhere in the cell. This review outlines what is known about the organization of the genes, FKBP4 and FKBP5, respectively, encoding these proteins and describes their diverse actions in the nervous system, reproduction, and cancer. The organization of FKBP4 and FKBP5 is very similar among the chordates, and gene expression is influenced by both genetic and epigenetic mechanisms. Recent studies identifying roles of FKBP52 and FKBP51 in the regulation of the microtubule-associated protein tau and microtubule assembly are discussed, as is their interaction with and influence on the transient receptor potential canonical (TRPC) subfamily of ion channel proteins.

Subcellular rearrangement of hsp90-binding immunophilins accompanies neuronal differentiation and neurite outgrowth

FKBP51 and FKBP52 (FK506-binding protein 51 and 52) are tetratricopeptide repeat-domain immunophilins belonging to the tetratricopeptide-protein•hsp90•hsp70•p23 heterocomplex bound to steroid receptors. Immunophilins are related to receptor folding, subcellular localization, and hormone-dependent transcription. Also, they bind the immunosuppressant macrolide FK506, which shows neuroregenerative and neuroprotective actions by a still unknown mechanism. In this study, we demonstrate that in both, undifferentiated neuroblastoma cells and embryonic hippocampal neurons, the FKBP52•hsp90•p23 heterocomplex concentrates in a perinuclear structure. Upon cell stimulation with FK506, this structure disassembles and this perinuclear area becomes transcriptionally active. The acquisition of a neuronal phenotype is accompanied by increased expression of βIII-tubulin, Map-2, Tau-1, but also hsp90, hsp70, p23, and FKBP52. During the early differentiation steps, the perinuclear heterocomplex redistributes along the cytoplasm and nascent neurites, p23 binds to intermediate filaments and microtubules acquired higher filamentary organization. While FKBP52 moves towards neurites and concentrates in arborization bodies and terminal axons, FKBP51, whose expression remains constant, replaces FKBP52 in the perinuclear structure. Importantly, neurite outgrowth is favored by FKBP52 over-expression or FKBP51 knock-down, and is impaired by FKBP52 knock-down or FKBP51 over-expression, indicating that the balance between these FK506-binding proteins plays a key role during the early mechanism of neuronal differentiation.

Microarray analysis of spaceflown murine thymus tissue reveals changes in gene expression regulating stress and glucocorticoid receptors

The detrimental effects of spaceflight and simulated microgravity on the immune system have been extensively documented. We report here microarray gene expression analysis, in concert with quantitative RT-PCR, in young adult C57BL/6NTac mice at 8 weeks of age after exposure to spaceflight aboard the space shuttle (STS-118) for a period of 13 days. Upon conclusion of the mission, thymus lobes were extracted from space flown mice (FLT) as well as age- and sex-matched ground control mice similarly housed in animal enclosure modules (AEM). mRNA was extracted and an automated array analysis for gene expression was performed. Examination of the microarray data revealed 970 individual probes that had a 1.5-fold or greater change. When these data were averaged (n = 4), we identified 12 genes that were significantly up- or down-regulated by at least 1.5-fold after spaceflight (P < or = 0.05). The genes that significantly differed from the AEM controls and that were also confirmed via QRT-PCR were as follows: Rbm3 (up-regulated) and Hsph110, Hsp90aa1, Cxcl10, Stip1, Fkbp4 (down-regulated). QRT-PCR confirmed the microarray results and demonstrated additional gene expression alteration in other T cell related genes, including: Ctla-4, IFN-alpha2a (up-regulated) and CD44 (down-regulated). Together, these data demonstrate that spaceflight induces significant changes in the thymic mRNA expression of genes that regulate stress, glucocorticoid receptor metabolism, and T cell signaling activity. These data explain, in part, the reported systemic compromise of the immune system after exposure to the microgravity of space.

The RET51/FKBP52 complex and its involvement in Parkinson disease

The tyrosine kinase receptor RET51 is expressed in distinct families of neurons where it promotes different functions. FKBP52 is an immunophilin with neuroprotective effects on different kinds of neurons. In this paper, we demonstrate that RET51 activation by both glial cell line-derived neurotrophic factor (GDNF) and NGF triggers the formation of RET51/FKBP52 complex. The substitution of the tyrosine 905 of RET51, a key residue phosphorylated by both GDNF and NGF, disrupts the RET51/FKBP52 complex. NGF and GDNF have a functional role in dopaminergic (DA) neurons where RET51 and FKBP52 are expressed with a yet undefined function. To clarify if RET51/FKBP52 complex should exert its function in DA neurons, we used an indirect approach by screening the genes encoding for RET51 and FKBP52 in a group of 30 Parkinson's disease patients. The degeneration of DA neurons is the main feature of PD, which is associated to a complex multifactorial aetiology combining environmental, age-related and genetic factors. We found a compound heterozygous carrying two mutations in RET and FKBP52 that are sufficient to disrupt the RET51/FKBP52 complex, indicating its potential role in PD.

The Hsp90 cochaperone, FKBP51, increases Tau stability and polymerizes microtubules

Imbalanced protein load within cells is a critical aspect for most diseases of aging. In particular, the accumulation of proteins into neurotoxic aggregates is a common thread for a host of neurodegenerative diseases. Our previous work demonstrated that age-related changes to the cellular chaperone repertoire contributes to abnormal buildup of the microtubule-associated protein tau that accumulates in a group of diseases termed tauopathies, the most common being Alzheimer's disease. Here, we show that the Hsp90 cochaperone, FK506-binding protein 51 (FKBP51), which possesses both an Hsp90-interacting tetratricopeptide domain and a peptidyl-prolyl cis-trans isomerase (PPIase) domain, prevents tau clearance and regulates its phosphorylation status. Regulation of the latter is dependent on the PPIase activity of FKBP51. FKB51 enhances the association of tau with Hsp90, but the FKBP51/tau interaction is not dependent on Hsp90. In vitro FKBP51 stabilizes microtubules with tau in a reaction depending on the PPIase activity of FKBP51. Based on these new findings, we propose that FKBP51 can use the Hsp90 complex to isomerize tau, altering its phosphorylation pattern and stabilizing microtubules.

A role for FKBP52 in Tau protein function

Tau is a microtubule-associated protein, which is widely expressed in the central nervous system, predominantly in neurons, where it regulates microtubule dynamics, axonal transport, and neurite outgrowth. The aberrant assembly of Tau is the hallmark of several human neurodegenerative diseases, collectively known as tauopathies. They include Alzheimer's disease, Pick's disease, progressive supranuclear palsy, and frontotemporal dementia and parkinsonism linked to chromosome 17. Several abnormalities in Tau, such as hyperphosphorylation and aggregation, alter its function and are central to the pathogenic process. Here, we describe biochemical and functional interactions between FKBP52 and Tau. FKBP52 is a member of the FKBP (FK506-binding protein) family that comprises intracellular protein effectors of immunosuppressive drugs (such as FK506 and rapamycin). We found that FKBP52, which is abundant in brain, binds directly and specifically to Tau, especially in its hyperphosphorylated form. The relevance of this observation was confirmed by the colocalization of both proteins in the distal part of the axons of cortical neurons and by the antagonistic effect of FKBP52 on the ability of Tau to promote microtubule assembly. Overexpression of FKBP52 in differentiated PC12 cells prevented the accumulation of Tau and resulted in reduced neurite length. Taken together, these findings indicate a role for FKBP52 in Tau function and may help to decipher and modulate the events involved in Tau-induced neurodegeneration.

Molecular chaperones, essential partners of steroid hormone receptors for activity and mobility

Steroid hormone receptors (SHRs) are notorious intracellular travellers, transiting among different cellular compartments as they mature, are subjected to regulation and exert their biological functions. Understanding the processes governing the intracellular traffic of SHRs is important, since their unbalanced or erroneous localization could lead to the development of diseases. In this review, we not only explore the functions of the heat-shock protein 90 (Hsp90) molecular chaperone machine for the intracellular transport of SHRs, but also for the regulation of their nuclear mobility, for their recycling and for the regulation of their transcriptional output.

Natural product scaffolds as leads to drugs

Numerous ‘scaffolds’ that have been identified in natural product structures have led to very significant numbers of approved drugs and drug candidates for a multiplicity of diseases over the years. In this mini-review, we discuss the base scaffolds (chemical skeletons) that we feel have produced very significant numbers of agents as drugs or drug leads and, in a number of cases, compounds that can be used as chemical synthons or that present activities in biological areas that were not obvious from their earlier history.

Targeted ablation reveals a novel role of FKBP52 in gene-specific regulation of glucocorticoid receptor transcriptional activity

FKBP52 is a tetratricopeptide repeat (TPR) protein with peptidyl-prolyl isomerase activity and is found in steroid receptor complexes, including glucocorticoid receptor (GR). It is generally accepted that FKBP52 has a stimulatory effect on GR transcriptional activity. However, the mechanism by which FKBP52 controls GR is not yet clear, with reports showing effects on GR hormone-binding affinity and/or hormone-induced nuclear translocation. To address this issue, we have generated mice with targeted ablation of the FKBP52 gene. To date, no overt defects of GR-regulated physiology have been found in these animals, demonstrating that FKBP52 is not an essential regulator of global GR activity. To better assess the impact of FKBP52 on GR, mouse embryonic fibroblasts (MEFs) were generated from wild-type (WT) and FKBP52-deficient (KO) animals. Analysis of GR activity at reporter genes showed an approximate 70% reduction of activity in 52KO MEF cells, with no effect of FKBP52 loss on thyroid receptor. Interestingly, GR activity at endogenous genes was not globally affected in 52KO cells, with reduced activity at GILZ and FKBP51, but not at SGK and p21. Thus, FKBP52 appears to be a gene-specific modulator of GR. To investigate the mechanism of this action, analyses of GR heterocomplex composition, hormone-binding affinity, and ability to undergo hormone-induced nuclear translocation and DNA-binding were performed. Interestingly, no effect of FKBP52 loss was found for any of these GR properties, suggesting that the main function of FKBP52 is a heretofore-unknown ability to control GR activity at target genes. Lastly, loss of FKBP52 did not affect the ability of GR to undergo hormone-induced autologous down-regulation, showing that FKBP52 does not contribute to all branches of GR signaling. The implications of these results to the potential actions of FKBP52 on GR activity in vivo are discussed.

Identification of all FK506-binding proteins from Neurospora crassa

Immunophilins are intracellular receptors of immunosuppressive drugs, carrying peptidyl-prolyl cis-trans isomerase activity, with a general role in protein folding but also involved in specific regulatory mechanisms. Four immunophilins of the FKBP-type (FK506-binding proteins) were identified in the genome of Neurospora crassa. Previously, FKBP22 has been located in the endoplasmic reticulum as part of chaperone/folding complexes and FKBP13 has been found to have a dual location in the cytoplasm and mitochondria. FKBP11 is apparently located exclusively in the cytoplasm. It is not expressed during vegetative development of the fungus although its expression can be induced with calcium and during sexual development. Overexpression of the respective gene appears to confer a growth advantage to the fungus in media containing some divalent ions. FKBP50 is a nuclear protein and its genetic inactivation leads to a temperature-sensitive phenotype. None of these proteins is, alone or in combination, essential for N. crassa, as demonstrated by the isolation of a mutant strain lacking all four FKBPs.

Automated SPR-LC-MS/MS system for protein interaction analysis

We have developed a novel automated system to analyze protein complexes by integrating a surface plasmon resonance (SPR) biosensor with highly sensitive nanoflow liquid chromatography-tandem mass spectrometry (LC-MS/MS). A His-tagged protein, which is also tagged with FLAG and biotinylated sequences, was expressed in mammalian cells. After purification by using the His tag from the cell lysate, the sample protein mixture was applied to an SPR biosensor and the protein complex was captured on the sensor chip. The automated SPR-LC-MS/MS was then performed: (1) two-step on-chip purification of the protein complex by using the FLAG and the biotinylated tags, (2) on-chip protease digestion of the complex, and (3) online nanoflow LC-MS/MS analysis of the resulting peptide fragments for protein identification. All of these processes could be monitored in real-time by the SPR biosensor. We validated the performance of the system using either FK506-binding protein 52 kDa (FKBP52) or ribosomal protein S19 (rpS19) as bait. Thus, the fully automated SPR-LC-MS/MS system appeared to be a powerful tool for functional proteomics studies, particularly for snapshot analysis of functional cellular complexes and machines.

Noncatalytic role of the FKBP52 peptidyl-prolyl isomerase domain in the regulation of steroid hormone signaling

Hormone-dependent transactivation by several of the steroid hormone receptors is potentiated by the Hsp90-associated cochaperone FKBP52, although not by the closely related FKBP51. Here we analyze the mechanisms of potentiation and the functional differences between FKBP51 and FKBP52. While both have peptidyl-prolyl isomerase activity, this is not required for potentiation, as mutations abolishing isomerase activity did not affect potentiation. Genetic selection in Saccharomyces cerevisiae for gain of potentiation activity in a library of randomly mutated FKBP51 genes identified a single residue at position 119 in the N-terminal FK1 domain as being a critical difference between these two proteins. In both the yeast model and mammalian cells, the FKBP51 mutation L119P, which is located in a hairpin loop overhanging the catalytic pocket and introduces the proline found in FKBP52, conferred significant potentiation activity, whereas the converse P119L mutation in FKBP52 decreased potentiation. A second residue in this loop, A116, also influences potentiation levels; in fact, the FKBP51-A116V L119P double mutant potentiated hormone signaling as well as wild-type FKBP52 did. These results suggest that the FK1 domain, and in particular the loop overhanging the catalytic pocket, is critically involved in receptor interactions and receptor activity.