Functional analysis of the Hsp90-associated human peptidyl prolyl cis/trans isomerases FKBP51, FKBP52 and Cyp40

Secretome Analyses Identify FKBP4 as a GBA1-Associated Protein in CSF and iPS Cells from Parkinson's Disease Patients with GBA1 Mutations

Mutations in the GBA1 gene increase the risk of developing Parkinson's disease (PD). However, most carriers of GBA1 mutations do not develop PD throughout their lives. The mechanisms of how GBA1 mutations contribute to PD pathogenesis remain unclear. Cerebrospinal fluid (CSF) is used for detecting pathological conditions of diseases, providing insights into the molecular mechanisms underlying neurodegenerative disorders. In this study, we utilized the proximity extension assay to examine the levels of metabolism-linked protein in the CSF from 17 PD patients carrying GBA1 mutations (GBA1-PD) and 17 idiopathic PD (iPD). The analysis of CSF secretome in GBA1-PD identified 11 significantly altered proteins, namely FKBP4, THOP1, GLRX, TXNDC5, GAL, SEMA3F, CRKL, APLP1, LRP11, CD164, and NPTXR. To investigate GBA1-associated CSF changes attributed to specific neuronal subtypes responsible for PD, we analyzed the cell culture supernatant from GBA1-PD-induced pluripotent stem cell (iPSC)-derived midbrain dopaminergic (mDA) neurons. The secretome analysis of GBA1-PD iPSC-derived mDA neurons revealed that five differently regulated proteins overlapped with those identified in the CSF analysis: FKBP4, THOP1, GLRX, GAL, and CRKL. Reduced intracellular level of the top hit, FKPB4, was confirmed via Western Blot. In conclusion, our findings identify significantly altered CSF GBA1-PD-associated proteins with FKPB4 being firmly attributed to mDA neurons.

Cryo-EM reveals how Hsp90 and FKBP immunophilins co-regulate the glucocorticoid receptor

Hsp90 is an essential molecular chaperone responsible for the folding and activation of hundreds of 'client' proteins, including the glucocorticoid receptor (GR). Previously, we revealed that Hsp70 and Hsp90 remodel the conformation of GR to regulate ligand binding, aided by co-chaperones. In vivo, the co-chaperones FKBP51 and FKBP52 antagonistically regulate GR activity, but a molecular understanding is lacking. Here we present a 3.01 Å cryogenic electron microscopy structure of the human GR:Hsp90:FKBP52 complex, revealing how FKBP52 integrates into the GR chaperone cycle and directly binds to the active client, potentiating GR activity in vitro and in vivo. We also present a 3.23 Å cryogenic electron microscopy structure of the human GR:Hsp90:FKBP51 complex, revealing how FKBP51 competes with FKBP52 for GR:Hsp90 binding and demonstrating how FKBP51 can act as a potent antagonist to FKBP52. Altogether, we demonstrate how FKBP51 and FKBP52 integrate into the GR chaperone cycle to advance GR to the next stage of maturation.

CyclosporinA Derivative as Therapeutic Candidate for Alport Syndrome by Inducing Mutant Type IV Collagen Secretion

Key Points

Screening of natural product extracts to find candidate compounds that increase mutant type IV collagen α 3,4,5 (α 345(IV)) trimer secretion in Alport syndrome (AS). Cyclosporin A (CsA) and alisporivir (ALV) increase mutant α 345(IV) trimer secretion in AS. PPIF/cyclophilin D mediates the effect of CsA and ALV on mutant trimer secretion.

Background

Type IV collagen α 3,4,5 (α 345(IV)) is an obligate trimer that is secreted to form a collagen network, which is the structural foundation of basement membrane. Mutation in one of the genes (COL4A3 , A4 , A5 ) encoding these proteins underlies the progressive genetic nephropathy Alport syndrome (AS) due to deficiency in trimerization and/or secretion of the α 345(IV) trimer. Thus, improving mutant α 345(IV) trimerization and secretion could be a good therapeutic approach for AS.

Methods

Using the nanoluciferase-based platform that we previously developed to detect α 345(IV) formation and secretion in HEK293T cells, we screened libraries of natural product extracts and compounds to find a candidate compound capable of increasing mutant α 345(IV) secretion.

Results

The screening of >13,000 extracts and >600 compounds revealed that cyclosporin A (CsA) increased the secretion of mutant α 345(IV)-G1244D. To elucidate the mechanism of the effect of CsA, we evaluated CsA derivatives with different ability to bind to calcineurin (Cn) and cyclophilin (Cyp). Alisporivir (ALV), which binds to Cyp but not to Cn, increased the trimer secretion of mutant α 345(IV). Knockdown studies on Cyps showed that PPIF/cyclophilin D was involved in the trimer secretion-enhancing activity of CsA and ALV. We confirmed that other α 345(IV) mutants are also responsive to CsA and ALV.

Conclusions

CsA was previously reported to improve proteinuria in patients with AS, but owing to its nephrotoxic effect, CsA is not recommended for treatment in patients with AS. Our data raise the possibility that ALV could be a safer option than CsA. This study provides a novel therapeutic candidate for AS with an innovative mechanism of action and reveals an aspect of the intracellular regulatory mechanism of α 345(IV) that was previously unexplored.

Role of FKBP5 and its genetic mutations in stress-induced psychiatric disorders: an opportunity for drug discovery

Stress-induced mental health disorders are affecting many people around the world. However, effective drug therapy for curing psychiatric diseases does not occur sufficiently. Many neurotransmitters, hormones, and mechanisms are essential in regulating the body's stress response. One of the most critical components of the stress response system is the hypothalamus-pituitary-adrenal (HPA) axis. The FKBP prolyl isomerase 51 (FKBP51) protein is one of the main negative regulators of the HPA axis. FKBP51 negatively regulates the cortisol effects (the end product of the HPA axis) by inhibiting the interaction between glucocorticoid receptors (GRs) and cortisol, causing reduced transcription of downstream cortisol molecules. By regulating cortisol effects, the FKBP51 protein can indirectly regulate the sensitivity of the HPA axis to stressors. Previous studies have indicated the influence of FKBP5 gene mutations and epigenetic changes in different psychiatric diseases and drug responses and recommended the FKBP51 protein as a drug target and a biomarker for psychological disorders. In this review, we attempted to discuss the effects of the FKBP5 gene, its mutations on different psychiatric diseases, and drugs affecting the FKBP5 gene.

The assembly of the MCM2-7 hetero-hexamer and its significance in DNA replication

The mini-chromosome maintenance proteins 2-7 (MCM2-7) hexamer is a protein complex that is key for eukaryotic DNA replication, which occurs only once per cell cycle. To achieve DNA replication, eukaryotic cells developed multiple mechanisms that control the timing of the loading of the hexamer onto chromatin and its activation as the replicative helicase. MCM2-7 is highly abundant in proliferating cells, which confers resistance to replication stress. Thus, the presence of an excess of MCM2-7 is important for maintaining genome integrity. However, the mechanism via which high MCM2-7 levels are achieved, other than the transcriptional upregulation of the MCM genes in the G1 phase, remained unknown. Recently, we and others reported that the MCM-binding protein (MCMBP) plays a role in the maintenance of high MCM2-7 levels and hypothesized that MCMBP functions as a chaperone in the assembly of the MCM2-7 hexamer. In this review, we discuss the roles of MCMBP in the control of MCM proteins and propose a model of the assembly of the MCM2-7 hexamer. Furthermore, we discuss a potential mechanism of the licensing checkpoint, which arrests the cells in the G1 phase when the levels of chromatin-bound MCM2-7 are reduced, and the possibility of targeting MCMBP as a chemotherapy for cancer.

Structure and function of the TPR-domain immunophilins FKBP51 and FKBP52 in normal physiology and disease

Coordinated cochaperone interactions with Hsp90 and associated client proteins are crucial for a multitude of signaling pathways in normal physiology, as well as in disease settings. Research on the molecular mechanisms regulated by the Hsp90 multiprotein complexes has demonstrated increasingly diverse roles for cochaperones throughout Hsp90-regulated signaling pathways. Thus, the Hsp90-associated cochaperones have emerged as attractive therapeutic targets in a wide variety of disease settings. The tetratricopeptide repeat (TPR)-domain immunophilins FKBP51 and FKBP52 are of special interest among the Hsp90-associated cochaperones given their Hsp90 client protein specificity, ubiquitous expression across tissues, and their increasingly important roles in neuronal signaling, intracellular calcium release, peptide bond isomerization, viral replication, steroid hormone receptor function, and cell proliferation to name a few. This review summarizes the current knowledge of the structure and molecular functions of TPR-domain immunophilins FKBP51 and FKBP52, recent findings implicating these immunophilins in disease, and the therapeutic potential of targeting FKBP51 and FKBP52 for the treatment of disease.

FPR1 is essential for rapamycin-induced lifespan extension in Saccharomyces cerevisiae

FK506-sensitive proline rotamase 1 protein (Fpr1p), which is a homologue of the mammalian prolyl isomerase FK506-binding protein of 12 kDa (FKBP12), is known to play important roles in protein folding and prevention of protein aggregation. Although rapamycin is known to bind to Fpr1p to inhibit Tor1p mediated-mechanistic Target Of Rapamycin (mTOR) activity, the physiological functions of Fpr1p on lifespan remain unclear. In this study, we used the eukaryotic model Saccharomyces cerevisiae to demonstrate that deletion of FPR1 reduced yeast chronological lifespan (CLS), and there was no benefit on lifespan upon rapamycin treatment, indicating that lifespan extension mechanism of rapamycin in yeast is exclusively dependent on FPR1. Furthermore, there was a significant increase in CLS of fpr1Δ cells during caloric restriction (CR), suggesting that rapamycin affects lifespan in a different way compared to CR. This study highlights the importance of FPR1 for rapamycin-induced lifespan extension.

Cryo-EM reveals how Hsp90 and FKBP immunophilins co-regulate the Glucocorticoid Receptor

Hsp90 is an essential molecular chaperone responsible for the folding and activation of hundreds of 'client' proteins, including the glucocorticoid receptor (GR)1-3. Previously, we revealed that GR ligand binding activity is inhibited by Hsp70 and restored by Hsp90, aided by co-chaperones4. We then presented cryo-EM structures mechanistically detailing how Hsp70 and Hsp90 remodel the conformation of GR to regulate ligand binding5,6. In vivo, GR-chaperone complexes are found associated with numerous Hsp90 co-chaperones, but the most enigmatic have been the immunophilins FKBP51 and FKBP52, which further regulate the activity of GR and other steroid receptors7-9. A molecular understanding of how FKBP51 and FKBP52 integrate with the GR chaperone cycle to differentially regulate GR activation in vivo is lacking due to difficulties reconstituting these interactions. Here, we present a 3.01 Å cryo-EM structure of the GR:Hsp90:FKBP52 complex, revealing , for the first time, that FKBP52 directly binds to the folded, ligand-bound GR using three novel interfaces, each of which we demonstrate are critical for FKBP52-dependent potentiation of GR activity in vivo. In addition, we present a 3.23 Å cryo-EM structure of the GR:Hsp90:FKBP51 complex, which, surprisingly, largely mimics the GR:Hsp90:FKBP52 structure. In both structures, FKBP51 and FKBP52 directly engage the folded GR and unexpectedly facilitate release of p23 through an allosteric mechanism. We also reveal that FKBP52, but not FKBP51, potentiates GR ligand binding in vitro, in a manner dependent on FKBP52-specific interactions. Altogether, we reveal how FKBP51 and FKBP52 integrate into the GR chaperone cycle to advance GR to the next stage of maturation and how FKBP51 and FKBP52 compete for GR:Hsp90 binding, leading to functional antagonism.

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.

Genome-wide characterization of FK506-binding proteins, parvulins and phospho-tyrosyl phosphatase activators in wheat and their regulation by heat stress

Peptidyl-prolyl cis-trans isomerases (PPIases) are ubiquitous proteins which are essential for cis-trans isomerisation of peptide bonds preceding the proline residue. PPIases are categorized into four sub-families viz., cyclophilins, FK506-binding proteins (FKBPs), parvulins and protein phosphatase 2A phosphatase activators (PTPAs). Apart from catalysing the cis-trans isomerization, these proteins have also been implicated in diverse cellular functions. Though PPIases have been identified in several important crop plants, information on these proteins, except cyclophilins, is scanty in wheat. In order to understand the role of these genes in wheat, we carried out genome-wide identification using computational approaches. The present study resulted in identification of 71 FKBP (TaFKBP) 12 parvulin (TaPar) and 3 PTPA (TaPTPA) genes in hexaploid wheat genome, which are distributed on different chromosomes with uneven gene densities. The TaFKBP and TaPar proteins, besides PPIase domain, also contain additional domains, indicating functional diversification. In silico prediction also revealed that TaFKBPs are localized to ER, nucleus, chloroplast and cytoplasm, while the TaPars are confined to cytoplasm and nucleus. The TaPTPAs, on the contrary, appear to be present only in the cytoplasm. Evolutionary studies predicted that most of the TaFKBP, TaPar and TaPTPA genes in hexaploid wheat have been derived from their progenitor species, with some events of loss or gain. Syntenic analysis revealed the presence of many collinear blocks of TaFKBP genes in wheat and its sub-genome donors. qRT-PCR analysis demonstrated that expression of TaFKBP and TaPar genes is regulated differentially by heat stress, suggesting their likely involvement in thermotolerance. The findings of this study will provide basis for further functional characterization of these genes and their likely applications in crop improvement.

FK506 binding protein 51: Its role in the adipose organ and beyond

There is a great body of evidence that the adipose organ plays a central role in the control not only of energy balance, but importantly, in the maintenance of metabolic homeostasis. Interest in the study of different aspects of its physiology grew in the last decades due to the pandemic of obesity and the consequences of metabolic syndrome. It was not until recently that the first evidence for the role of the high molecular weight immunophilin FK506 binding protein (FKBP) 51 in the process of adipocyte differentiation have been described. Since then, many new facets have been discovered of this stress-responsive FKBP51 as a central node for precise coordination of many cell functions, as shown for nuclear steroid receptors, autophagy, signaling pathways as Akt, p38 MAPK, and GSK3, as well as for insulin signaling and the control of glucose homeostasis. Thus, the aim of this review is to integrate and discuss the recent advances in the understanding of the many roles of FKBP51 in the adipose organ.

Chaperoning activity of the cyclophilin family prevents tau aggregation

Tauopathies, such as Alzheimer's disease, are characterized by the misfolding and progressive accumulation of the microtubule associated protein tau. Chaperones, tasked with maintaining protein homeostasis, can become imbalanced with age and contribute to the progression of neurodegenerative disease. Cyclophilins are a promising pool of underinvestigated chaperones with peptidyl-prolyl isomerase activity that may play protective roles in regulating tau aggregation. Using a Thioflavin T fluorescence-based assay to monitor in vitro tau aggregation, all eight cyclophilins, which include PPIA to PPIH prevent tau aggregation, with PPIB, PPIC, PPID, and PPIH showing the greatest inhibition. The low thermal stability of PPID and the strong heparin binding of PPIB undermines the simplistic interpretation of reduced tau aggregation. In a cellular model of tau accumulation, all cyclophilins, except PPID and PPIH, reduce insoluble tau. PPIB, PPIC, PPIE, and PPIF also reduce soluble tau levels with PPIC exclusively protecting cells from tau seeding. Overall, this study demonstrates cyclophilins prevent tau fibril formation and many reduce cellular insoluble tau accumulation with PPIC having the greatest potential as a molecular tool to mitigate tau seeding and accumulation.

Low-Dose Tacrolimus Promotes the Migration and Invasion and Nitric Oxide Production in the Human-Derived First Trimester Extravillous Trophoblast Cells In Vitro

Placentation is one of the most important determinants for a successful pregnancy, and this is dependent on the process of trophoblast migration and invasion. Progesterone receptors (PGR) are critical effectors of progesterone (P4) signaling that is required for trophoblast migration and invasion conducive to a successful gestation. In immune complicated pregnancies, evidence has shown that abnormal placentation occurs because of aberrant expression of PGR. Therapeutic intervention with tacrolimus (FK506) was able to restore PGR expression and improve pregnancy outcomes in immune-complicated gestations; however, the exact mode of action of tacrolimus in assisting placentation is not clear. Here, we attempt to uncover the mode of action of tacrolimus by examining its effects on trophoblast invasion and migration in the human-derived extravillous trophoblast (EVT) cell line, the HTR-8/SVneo cells. Using a variety of functional assays, we demonstrated that low-dose tacrolimus (10 ng/mL) was sufficient to significantly (p < 0.001) stimulate the migration and invasion of the HTR-8/SVneo cells, inducing their cytosolic/nuclear progesterone receptor expression and activation, and modulating their Nitric Oxide (NO) production. Moreover, tacrolimus abrogated the suppressive effect of the NOS inhibitor Nω- Nitro-L-Arginine Methyl Ester (L-NAME) on these vital processes critically involved in the establishment of human pregnancy. Collectively, our data suggest an immune-independent mode of action of tacrolimus in positively influencing placentation in complicated gestations, at least in part, through promoting the migration and invasion of the first trimester extravillous trophoblast cells by modulating their NO production and activating their cytosolic/nuclear progesterone-receptors. To our knowledge, this is the first report to show that the mode of action of tacrolimus as a monotherapy for implantation failure is plausibly PGR-dependent.

Hsp90 and Associated Co-Chaperones of the Malaria Parasite

Heat shock protein 90 (Hsp90) is one of the major guardians of cellular protein homeostasis, through its specialized molecular chaperone properties. While Hsp90 has been extensively studied in many prokaryotic and higher eukaryotic model organisms, its structural, functional, and biological properties in parasitic protozoans are less well defined. Hsp90 collaborates with a wide range of co-chaperones that fine-tune its protein folding pathway. Co-chaperones play many roles in the regulation of Hsp90, including selective targeting of client proteins, and the modulation of its ATPase activity, conformational changes, and post-translational modifications. Plasmodium falciparum is responsible for the most lethal form of human malaria. The survival of the malaria parasite inside the host and the vector depends on the action of molecular chaperones. The major cytosolic P. falciparum Hsp90 (PfHsp90) is known to play an essential role in the development of the parasite, particularly during the intra-erythrocytic stage in the human host. Although PfHsp90 shares significant sequence and structural similarity with human Hsp90, it has several major structural and functional differences. Furthermore, its co-chaperone network appears to be substantially different to that of the human host, with the potential absence of a key homolog. Indeed, PfHsp90 and its interface with co-chaperones represent potential drug targets for antimalarial drug discovery. In this review, we critically summarize the current understanding of the properties of Hsp90, and the associated co-chaperones of the malaria parasite.

Single-cell RNA Sequencing Analysis Reveals New Immune Disorder Complexities in Hypersplenism

Hypersplenism (HS) is a concomitant symptom of liver or blood disease. Not only does the treatment of HS face challenges, but the transcriptome of individual cells is also unknown. Here, the transcriptional profiles of 43,037 cells from four HS tissues and one control tissue were generated by the single-cell RNA sequencing and nine major cell types, including T-cells, B-cells, NK cells, hematopoietic stem cells, neutrophil cells, mast cells, endothelial cells, erythrocytes, and dendritic cells were identified. Strikingly, the main features were the lack of CCL5+ B-cells in HS and the presence of SESN1+ B cells in HS with hepatocellular carcinoma (HS-HCC). In cell-cell interaction analysis, CD74-COPA and CD94-HLA-E in HS were found to be up-regulated. We further explored HS-specifically enriched genes (such as FKBP5, ADAR, and RPS4Y1) and found that FKBP5 was highly expressed in HCC-HS, leading to immunosuppression. Taken together, this research provides new insights into the genetic characteristics of HS via comprehensive single-cell transcriptome analysis.

MCMBP promotes the assembly of the MCM2-7 hetero-hexamer to ensure robust DNA replication in human cells

The MCM2–7 hetero-hexamer is the replicative DNA helicase that plays a central role in eukaryotic DNA replication. In proliferating cells, the expression level of the MCM2–7 hexamer is kept high, which safeguards the integrity of the genome. However, how the MCM2–7 hexamer is assembled in living cells remains unknown. Here, we revealed that the MCM-binding protein (MCMBP) plays a critical role in the assembly of this hexamer in human cells. MCMBP associates with MCM3 which is essential for maintaining the level of the MCM2–7 hexamer. Acute depletion of MCMBP demonstrated that it contributes to MCM2–7 assembly using nascent MCM3. Cells depleted of MCMBP gradually ceased to proliferate because of reduced replication licensing. Under this condition, p53-positive cells exhibited arrest in the G1 phase, whereas p53-null cells entered the S phase and lost their viability because of the accumulation of DNA damage, suggesting that MCMBP is a potential target for killing p53-deficient cancers.

The switch from client holding to folding in the Hsp70/Hsp90 chaperone machineries is regulated by a direct interplay between co-chaperones

Folding of stringent clients requires transfer from Hsp70 to Hsp90. The co-chaperone Hop physically connects the chaperone machineries. Here, we define its role from the remodeling of Hsp70/40-client complexes to the mechanism of client transfer and the conformational switching from stalled to active client-processing states of Hsp90. We show that Hsp70 together with Hsp40 completely unfold a stringent client, the glucocorticoid receptor ligand-binding domain (GR-LBD) in large assemblies. Hop remodels these for efficient transfer onto Hsp90. As p23 enters, Hsp70 leaves the complex via switching between binding sites in Hop. Current concepts assume that to proceed to client folding, Hop dissociates and the co-chaperone p23 stabilizes the Hsp90 closed state. In contrast, we show that p23 functionally interacts with Hop, relieves the stalling Hsp90-Hop interaction, and closes Hsp90. This reaction allows folding of the client and is thus the key regulatory step for the progression of the chaperone cycle.

Impact of Fkbp5 × early life adversity × sex in humanised mice on multidimensional stress responses and circadian rhythmicity

The cumulative load of genetic predisposition, early life adversity (ELA) and lifestyle shapes the prevalence of psychiatric disorders. Single nucleotide polymorphisms (SNPs) in the human FKBP5 gene were shown to modulate disease risk. To enable investigation of disease-related SNPs in behaviourally relevant context, we generated humanised mouse lines carrying either the risk (AT) or the resiliency (CG) allele of the rs1360780 locus and exposed litters of these mice to maternal separation. Behavioural and physiological aspects of their adult stress responsiveness displayed interactions of genotype, early life condition, and sex. In humanised females carrying the CG- but not the AT-allele, ELA led to altered HPA axis functioning, exploratory behaviour, and sociability. These changes correlated with differential expression of genes in the hypothalamus, where synaptic transmission, metabolism, and circadian entrainment pathways were deregulated. Our data suggest an integrative role of FKBP5 in shaping the sex-specific outcome of ELA in adulthood.

The Potential of Hsp90 in Targeting Pathological Pathways in Cardiac Diseases

Heat shock protein 90 (Hsp90) is a molecular chaperone that interacts with up to 10% of the proteome. The extensive involvement in protein folding and regulation of protein stability within cells makes Hsp90 an attractive therapeutic target to correct multiple dysfunctions. Many of the clients of Hsp90 are found in pathways known to be pathogenic in the heart, ranging from transforming growth factor β (TGF-β) and mitogen activated kinase (MAPK) signaling to tumor necrosis factor α (TNFα), G_s and G_q g-protein coupled receptor (GPCR) and calcium (Ca²⁺) signaling. These pathways can therefore be targeted through modulation of Hsp90 activity. The activity of Hsp90 can be targeted through small-molecule inhibition. Small-molecule inhibitors of Hsp90 have been found to be cardiotoxic in some cases however. In this regard, specific targeting of Hsp90 by modulation of post-translational modifications (PTMs) emerges as an attractive strategy. In this review, we aim to address how Hsp90 functions, where Hsp90 interacts within pathological pathways, and current knowledge of small molecules and PTMs known to modulate Hsp90 activity and their potential as therapeutics in cardiac diseases.

Saccharomyces cerevisiae Fpr1 functions as a chaperone to inhibit protein aggregation

Peptidyl prolyl isomerases (PPIases) accelerate the rate limiting step of protein folding by catalyzing cis/trans isomerization of peptidyl prolyl bonds. The larger PPIases have been shown to be multi-domain proteins, with functions other than isomerization of the proline-containing peptide bond. Recently, a few smaller PPIases have also been described for their ability to stabilize folding intermediates. The yeast Fpr1 (FK506-sensitive proline rotamase) is a homologue of the mammalian prolyl isomerase FKBP12 (FK506-binding protein of 12 kDa). Its ability to stabilize stressed cellular proteins has not been reported yet. We had earlier reported upregulation of Fpr1 in yeast cells exposed to proteotoxic stress conditions. In this work, we show that yeast Fpr1 exhibits characteristics typical of a general chaperone of the proteostasis network. Aggregation of mutant huntingtin fragment was higher in Fpr1-deleted as compared to parental yeast cells. Overexpression of Fpr1 led to reduced protein aggregation by decreasing the amount of oligomers and diverting the aggregation pathway towards the formation of detergent-soluble species. This correlated well with higher survival of these cells. Purified and enzymatically active yeast Fpr1 was able to inhibit aggregation of mutant huntingtin fragment and luciferase in vitro in a concentration-dependent manner; suggesting a direct action for aggregation inhibitory action of Fpr1. Overexpression of yeast Fpr1 was able to protect E. coli cells against thermal shock. This work establishes the role of Fpr1 in the protein folding network and will be used for the identification of novel pharmacological leads in disease conditions.

Systematic investigation and comparison of US FDA-approved immunosuppressive drugs FK506, cyclosporine and rapamycin for neuromuscular regeneration following chronic nerve compression injury

Aim: To compare therapeutic benefits of different immunophilin ligands for treating nerve injuries. Materials & methods: Cyclosporine, FK506 and rapamycin, were evaluated first in vitro on a serum-free culture of embryonic dorsal root ganglia followed by a new in vivo model of chronic nerve compression. Results: Outcomes of the in vitro study have shown a potent effect of cyclosporine and FK506, on dorsal root ganglia axonal outgrowth, comparable to the effect of nerve growth factor. Rapamycin exhibited only a moderate effect. The in vivo study revealed the beneficial effects of cyclosporine, FK506 and rapamycin for neuromuscular regeneration. Cyclosporine showed the better maintenance of the tissues and function. Conclusion: Cyclosporine, FK506 and rapamycin drugs showed potential for treating peripheral nerve chronic compression injuries.

Discovery of pentapeptide-inhibitor hits targeting FKBP51 by combining computational modeling and X-ray crystallography

FKBP51 is well-known as a cochaperone of Hsp90 machinery and implicated in many human diseases including stress-related diseases, tau-mediated neurodegeneration and cancers, which makes FKBP51 an attractive drug target for the therapy of FKBP51-associated diseases. However, it has been reported that only nature product rapamycin, cyclosporine A, FK506 and its derivatives exhibit good binding affinities when bound to FKBP51 by now. Given the advantages of peptide-inhibitors, we designed and obtained 20 peptide-inhibitor hits through structure-based drug design. We further characterized the interaction modes of the peptide-inhibitor hits on the FK1 domain of FKBP51 by biochemical and structural biology methods. Structural analysis revealed that peptide-inhibitor hits form U-shaped conformations and occupy the FK506 binding pocket and share similar interaction modes with FK506. Using molecular dynamics simulations, we delved into the interaction dynamics and found that hits are anchored to the FK506 binding pocket in a quite stable conformation. Meanwhile, it was shown that interactions between FK1 and peptide-inhibitor hits are mainly attributed to the hydrogen bond networks comprising I87 and Y113 and FPF cores of peptide-inhibitors involved extensive hydrophobic interactions. We presumed that the peptide design strategy based on the small molecule structure probably shed new lights on the peptide-inhibitor discovery of other targets. The findings presented here could also serve as a structural basis and starting point facilitating the optimization and generation of FKBP51 peptide-inhibitors with better bio-activities.

FKBP4 integrates FKBP4/Hsp90/IKK with FKBP4/Hsp70/RelA complex to promote lung adenocarcinoma progression via IKK/NF-κB signaling

FKBP4 belongs to the family of immunophilins, which serve as a regulator for steroid receptor activity. Thus, FKBP4 has been recognized to play a critical role in several hormone-dependent cancers, including breast and prostate cancer. However, there is still no research to address the role of FKBP4 on lung adenocarcinoma (LUAD) progression. We found that FKBP4 expression was elevated in LUAD samples and predicted significantly shorter overall survival based on TCGA and our cohort of LUAD patients. Furthermore, FKBP4 robustly increased the proliferation, metastasis, and invasion of LUAD in vitro and vivo. Mechanistic studies revealed the interaction between FKBP4 and IKK kinase complex. We found that FKBP4 potentiated IKK kinase activity by interacting with Hsp90 and IKK subunits and promoting Hsp90/IKK association. Also, FKBP4 promotes the binding of IKKγ to IKKβ, which supported the facilitation role in IKK complex assembly. We further identified that FKBP4 TPR domains are essential for FKBP4/IKK interaction since its association with Hsp90 is required. In addition, FKBP4 PPIase domains are involved in FKBP4/IKKγ interaction. Interestingly, the association between FKBP4 and Hsp70/RelA favors the transport of RelA toward the nucleus. Collectively, FKBP4 integrates FKBP4/Hsp90/IKK with FKBP4/Hsp70/RelA complex to potentiate the transcriptional activity and nuclear translocation of NF-κB, thereby promoting LUAD progression. Our findings suggest that FKBP4 may function as a prognostic biomarker of LUAD and provide a newly mechanistic insight into modulating IKK/NF-κB signaling.

Aha1 Exhibits Distinctive Dynamics Behavior and Chaperone-Like Activity

Aha1 is the only co-chaperone known to strongly stimulate the ATPase activity of Hsp90. Meanwhile, besides the well-studied co-chaperone function, human Aha1 has also been demonstrated to exhibit chaperoning activity against stress-denatured proteins. To provide structural insights for a better understanding of Aha1's co-chaperone and chaperone-like activities, nuclear magnetic resonance (NMR) techniques were used to reveal the unique structure and internal dynamics features of full-length human Aha1. We then found that, in solution, both the two domains of Aha1 presented distinctive thermal stabilities and dynamics behaviors defined by their primary sequences and three-dimensional structures. The low thermal stability (melting temperature of Aha1^28-162: 54.45 °C) and the internal dynamics featured with slow motions on the µs-ms time scale were detected for Aha1's N-terminal domain (Aha1N). The aforementioned experimental results suggest that Aha1N is in an energy-unfavorable state, which would therefore thermostatically favor the interaction of Aha1N with its partner proteins such as Hsp90's middle domain. Differently from Aha1N, Aha1C (Aha1's C-terminal domain) exhibited enhanced thermal stability (melting temperature of Aha1^204-335: 72.41 °C) and the internal dynamics featured with intermediate motions on the ps-ns time scale. Aha1C's thermal and structural stabilities make it competent for the stabilization of the exposed hydrophobic groove of dimerized Hsp90's N-terminal domain. Of note, according to the NMR data and the thermal shift results, although the very N-terminal region (M1-W27) and the C-terminal relaxin-like factor (RLF) motif showed no tight contacts with the remaining parts of human Aha1, they were identified to play important roles in the recognition of intrinsically disordered pathological α-synuclein.

General Structural and Functional Features of Molecular Chaperones

Molecular chaperones are a group of structurally diverse and highly conserved ubiquitous proteins. They play crucial roles in facilitating the correct folding of proteins in vivo by preventing protein aggregation or facilitating the appropriate folding and assembly of proteins. Heat shock proteins form the major class of molecular chaperones that are responsible for protein folding events in the cell. This is achieved by ATP-dependent (folding machines) or ATP-independent mechanisms (holders). Heat shock proteins are induced by a variety of stresses, besides heat shock. The large and varied heat shock protein class is categorised into several subfamilies based on their sizes in kDa namely, small Hsps (HSPB), J domain proteins (Hsp40/DNAJ), Hsp60 (HSPD/E; Chaperonins), Hsp70 (HSPA), Hsp90 (HSPC), and Hsp100. Heat shock proteins are localised to different compartments in the cell to carry out tasks specific to their environment. Most heat shock proteins form large oligomeric structures, and their functions are usually regulated by a variety of cochaperones and cofactors. Heat shock proteins do not function in isolation but are rather part of the chaperone network in the cell. The general structural and functional features of the major heat shock protein families are discussed, including their roles in human disease. Their function is particularly important in disease due to increased stress in the cell. Vector-borne parasites affecting human health encounter stress during transmission between invertebrate vectors and mammalian hosts. Members of the main classes of heat shock proteins are all represented in Plasmodium falciparum, the causative agent of cerebral malaria, and they play specific functions in differentiation, cytoprotection, signal transduction, and virulence.

FKBP5 polymorphisms induce differential glucocorticoid responsiveness in primary CNS cells - First insights from novel humanized mice

The brain is a central hub for integration of internal and external conditions and, thus, a regulator of the stress response. Glucocorticoids are the essential communicators of this response. Aberrations in glucocorticoid signaling are a common symptom in patients with psychiatric disorders. The gene FKBP5 encodes a chaperone protein that functionally inhibits glucocorticoid signaling and, thus, contributes to the regulation of stress. In the context of childhood trauma, differential expression of FKBP5 has been found in psychiatric patients compared to controls. These variations in expression levels of FKBP5 were reported to be associated with differences in stress responsiveness in human carriers of the single nucleotide polymorphism (SNP) rs1360780. Understanding the mechanisms underlying FKBP5 polymorphism‐associated glucocorticoid responsiveness in the CNS will lead to a better understanding of stress regulation or associated pathology. To study these mechanisms, two novel humanized mouse lines were generated. The lines carried either the risk (A/T) allele or the resilient (C/G) allele of rs1360780. Primary cells from CNS (astrocytes, microglia, and neurons) were analyzed for their basal expression levels of FKBP5 and their responsiveness to glucocorticoids. Differential expression of FKBP5 was found for these cell types and negatively correlated with the cellular glucocorticoid responsiveness. Astrocytes revealed the strongest transcriptional response, followed by microglia and neurons. Furthermore, the risk allele (A/T) was associated with greater induction of FKBP5 than the resilience allele. Novel FKBP5‐humanized mice display differential glucocorticoid responsiveness due to a single intronic SNP. The vulnerability to stress signaling in the shape of glucocorticoids in the brain correlated with FKBP5 expression levels. The strong responsiveness of astrocytes to glucocorticoids implies astrocytes play a prominent role in the stress response, and in FKBP5‐related differences in glucocorticoid signaling. The novel humanized mouse lines will allow for further study of the role that FKBP5 SNPs have in risk and resilience to stress pathology.

Solution structure of the Hop TPR2A domain and investigation of target druggability by NMR, biochemical and in silico approaches

Heat shock protein 90 (Hsp90) is a molecular chaperone that plays an important role in tumour biology by promoting the stabilisation and activity of oncogenic 'client' proteins. Inhibition of Hsp90 by small-molecule drugs, acting via its ATP hydrolysis site, has shown promise as a molecularly targeted cancer therapy. Owing to the importance of Hop and other tetratricopeptide repeat (TPR)-containing cochaperones in regulating Hsp90 activity, the Hsp90-TPR domain interface is an alternative site for inhibitors, which could result in effects distinct from ATP site binders. The TPR binding site of Hsp90 cochaperones includes a shallow, positively charged groove that poses a significant challenge for druggability. Herein, we report the apo, solution-state structure of Hop TPR2A which enables this target for NMR-based screening approaches. We have designed prototype TPR ligands that mimic key native 'carboxylate clamp' interactions between Hsp90 and its TPR cochaperones and show that they block binding between Hop TPR2A and the Hsp90 C-terminal MEEVD peptide. We confirm direct TPR-binding of these ligands by mapping 1H-15N HSQC chemical shift perturbations to our new NMR structure. Our work provides a novel structure, a thorough assessment of druggability and robust screening approaches that may offer a potential route, albeit difficult, to address the chemically challenging nature of the Hop TPR2A target, with relevance to other TPR domain interactors.

Post-translational modifications and stress adaptation: the paradigm of FKBP51

Adaptation to stress is a fundamental requirement to cope with changing environmental conditions that pose a threat to the homeostasis of cells and organisms. Post-translational modifications (PTMs) of proteins represent a possibility to quickly produce proteins with new features demanding relatively little cellular resources. FK506 binding protein (FKBP) 51 is a pivotal stress protein that is involved in the regulation of several executers of PTMs. This mini-review discusses the role of FKBP51 in the function of proteins responsible for setting the phosphorylation, ubiquitination and lipidation of other proteins. Examples include the kinases Akt1, CDK5 and GSK3β, the phosphatases calcineurin, PP2A and PHLPP, and the ubiquitin E3-ligase SKP2. The impact of FKBP51 on PTMs of signal transduction proteins significantly extends the functional versatility of this protein. As a stress-induced protein, FKBP51 uses re-setting of PTMs to relay the effect of stress on various signaling pathways.

Regulation of FKBP51 and FKBP52 functions by post-translational modifications

FKBP51 and FKBP52 are two iconic members of the family of peptidyl-prolyl-(cis/trans)-isomerases (EC: 5.2.1.8), which comprises proteins that catalyze the cis/trans isomerization of peptidyl-prolyl peptide bonds in unfolded and partially folded polypeptide chains and native state proteins. Originally, both proteins have been studied as molecular chaperones belonging to the steroid receptor heterocomplex, where they were first discovered. In addition to their expected role in receptor folding and chaperoning, FKBP51 and FKBP52 are also involved in many biological processes, such as signal transduction, transcriptional regulation, protein transport, cancer development, and cell differentiation, just to mention a few examples. Recent studies have revealed that both proteins are subject of post-translational modifications such as phosphorylation, SUMOlyation, and acetylation. In this work, we summarize recent advances in the study of these immunophilins portraying them as scaffolding proteins capable to organize protein heterocomplexes, describing some of their antagonistic properties in the physiology of the cell, and the putative regulation of their properties by those post-translational modifications.

SUMO conjugation as regulator of the glucocorticoid receptor-FKBP51 cellular response to stress

In order to adequately respond to stressful stimuli, glucocorticoids (GCs) target almost every tissue of the body. By exerting a negative feedback loop in the hypothalamic-pituitary-adrenal (HPA) axis GCs inhibit their own synthesis and restore homeostasis. GCs actions are mostly mediated by the GC receptor (GR), a member of the nuclear receptor superfamily. Alterations of the GR activity have been associatedto different diseases including mood disorders and can lead to severe complication. Therefore, understanding the molecular complexity of GR modulation is mandatory for the development of new and effective drugs for treating GR-associated disorders. FKBP51 is a GR chaperone that has gained much attention because it is a strong inhibitor of GR activity and has a crucial role in psychiatric diseases. Both GR and FKBP51 activity are regulated by SUMOylation, a posttranslational (PTM). In this review, we focus on the impact of SUMO-conjugation as a regulator of this pathway.

Focus on FKBP51: A molecular link between stress and metabolic disorders

BACKGROUND

Obesity, Type 2 diabetes (T2D) as well as stress-related disorders are rising public health threats and major burdens for modern society. Chronic stress and depression are highly associated with symptoms of the metabolic syndrome, but the molecular link is still not fully understood. Furthermore, therapies tackling these biological disorders are still lacking. The identification of shared molecular targets underlying both pathophysiologies may lead to the development of new treatments. The FK506 binding protein 51 (FKBP51) has recently been identified as a promising therapeutic target for stress-related psychiatric disorders and obesity-related metabolic outcomes.

SCOPE OF THE REVIEW

The aim of this review is to summarize current evidence of in vitro, preclinical, and human studies on the stress responsive protein FKBP51, focusing on its newly discovered role in metabolism. Also, we highlight the therapeutic potential of FKBP51 as a new treatment target for symptoms of the metabolic syndrome.

MAJOR CONCLUSIONS

We conclude the review by emphasizing missing knowledge gaps that remain and future research opportunities needed to implement FKBP51 as a drug target for stress-related obesity or T2D.

Structure, Function, and Regulation of the Hsp90 Machinery

Heat shock protein 90 (Hsp90) is a molecular chaperone involved in the maturation of a plethora of substrates ("clients"), including protein kinases, transcription factors, and E3 ubiquitin ligases, positioning Hsp90 as a central regulator of cellular proteostasis. Hsp90 undergoes large conformational changes during its ATPase cycle. The processing of clients by cytosolic Hsp90 is assisted by a cohort of cochaperones that affect client recruitment, Hsp90 ATPase function or conformational rearrangements in Hsp90. Because of the importance of Hsp90 in regulating central cellular pathways, strategies for the pharmacological inhibition of the Hsp90 machinery in diseases such as cancer and neurodegeneration are being developed. In this review, we summarize recent structural and mechanistic progress in defining the function of organelle-specific and cytosolic Hsp90, including the impact of individual cochaperones on the maturation of specific clients and complexes with clients as well as ways of exploiting Hsp90 as a drug target.

Penicisulfuranol A, a novel C-terminal inhibitor disrupting molecular chaperone function of Hsp90 independent of ATP binding domain

The goal of this study is to explore the mechanism of a heat shock protein 90 (Hsp90) C-terminal inhibitor, Penicisulfuranol A (PEN-A), for cancer therapy. PEN-A was produced by a mangrove endophytic fungus Penicillium janthinellum and had a new structure with a rare 3H-spiro [benzofuran-2, 2'-piperazine] ring system. PEN-A caused depletion of multiple Hsp90 client proteins without induction of heat shock protein 70 (Hsp70). Subsequently, it induced apoptosis and inhibited xerograph tumor growth of HCT116 cells in vitro and in vivo. Mechanism studies showed that PEN-A was bound to C-terminus of Hsp90 at the binding site different from ATP binding domain. Therefore, it inhibited dimerization of Hsp90 C-terminus, depolymerization of ADH protein by C-terminus of Hsp90, and interaction of co-chaperones with Hsp90. These inhibitory effects of PEN-A were similar to those of novobiocin, an inhibitor binding to interaction site for ATP of C-terminus of Hsp90. Furthermore, our study revealed that disulfide bond was essential moiety for inhibition activity of PEN-A on Hsp90. This suggested that PEN-A may be bound to cysteine residues near amino acid region which was responsible for dimerization of Hsp90. All results indicate that PEN-A is a novel C-terminal inhibitor of Hsp90 and worthy for further study in the future not only for drug development but also for unraveling the bioactivities of Hsp90.

Biological Actions of the Hsp90-binding Immunophilins FKBP51 and FKBP52

Immunophilins are a family of proteins whose signature domain is the peptidylprolyl-isomerase domain. High molecular weight immunophilins are characterized by the additional presence of tetratricopeptide-repeats (TPR) through which they bind to the 90-kDa heat-shock protein (Hsp90), and via this chaperone, immunophilins contribute to the regulation of the biological functions of several client-proteins. Among these Hsp90-binding immunophilins, there are two highly homologous members named FKBP51 and FKBP52 (FK506-binding protein of 51-kDa and 52-kDa, respectively) that were first characterized as components of the Hsp90-based heterocomplex associated to steroid receptors. Afterwards, they emerged as likely contributors to a variety of other hormone-dependent diseases, stress-related pathologies, psychiatric disorders, cancer, and other syndromes characterized by misfolded proteins. The differential biological actions of these immunophilins have been assigned to the structurally similar, but functionally divergent enzymatic domain. Nonetheless, they also require the complementary input of the TPR domain, most likely due to their dependence with the association to Hsp90 as a functional unit. FKBP51 and FKBP52 regulate a variety of biological processes such as steroid receptor action, transcriptional activity, protein conformation, protein trafficking, cell differentiation, apoptosis, cancer progression, telomerase activity, cytoskeleton architecture, etc. In this article we discuss the biology of these events and some mechanistic aspects.

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.

Structure and pro-toxic mechanism of the human Hsp90/PPIase/Tau complex

The molecular chaperone Hsp90 is critical for the maintenance of cellular homeostasis and represents a promising drug target. Despite increasing knowledge on the structure of Hsp90, the molecular basis of substrate recognition and pro-folding by Hsp90/co-chaperone complexes remains unknown. Here, we report the solution structures of human full-length Hsp90 in complex with the PPIase FKBP51, as well as the 280 kDa Hsp90/FKBP51 complex bound to the Alzheimer’s disease-related protein Tau. We reveal that the FKBP51/Hsp90 complex, which synergizes to promote toxic Tau oligomers in vivo, is highly dynamic and stabilizes the extended conformation of the Hsp90 dimer resulting in decreased Hsp90 ATPase activity. Within the ternary Hsp90/FKBP51/Tau complex, Hsp90 serves as a scaffold that traps the PPIase and nucleates multiple conformations of Tau’s proline-rich region next to the PPIase catalytic pocket in a phosphorylation-dependent manner. Our study defines a conceptual model for dynamic Hsp90/co-chaperone/client recognition.

The chaperone Hsp90 plays a key role in maintaining cellular homeostasis. Here the authors provide structural insights into substrate recognition and the pro-folding mechanism of Hsp90/co-chaperone complexes by studying the complex of Hsp90 with its co-chaperone FKBP51 and the substrate Tau bound Hsp90/FKBP51 ternary complex using a NMR based integrative approach.

Hsp90 and FKBP51: complex regulators of psychiatric diseases

Mood disorders affect nearly a quarter of the world's population. Therefore, understanding the molecular mechanisms underlying these conditions is of great importance. FK-506 binding protein 5 (FKBP5) encodes the FKBP51 protein, a heat shock protein 90 kDa (Hsp90) co-chaperone, and is a risk factor for several affective disorders. FKBP51, in coordination with Hsp90, regulates glucocorticoid receptor (GR) activity via a short negative feedback loop. This signalling pathway rapidly restores homeostasis in the hypothalamic-pituitary-adrenal (HPA) axis following stress. Expression of FKBP5 increases with age through reduced DNA methylation. High levels of FKBP51 are linked to GR resistance and reduced stress coping behaviour. Moreover, common allelic variants in the FKBP5 gene are associated with increased risk of developing affective disorders like anxiety, depression and post-traumatic stress disorder (PTSD). This review highlights the current understanding of the Hsp90 co-chaperone, FKBP5, in disease from both human and animal studies. In addition, FKBP5 genetic implications in the clinic involving life stress exposure, gender differences and treatment outcomes are discussed.This article is part of the theme issue 'Heat shock proteins as modulators and therapeutic targets of chronic disease: an integrated perspective'.

Hypothalamic-pituitary-adrenal axis variants and childhood trauma influence anxiety sensitivity in South African adolescents

Anxiety sensitivity (AS) is characterised by the fear of anxiety-related symptoms and is a risk factor for the development of anxiety-related disorders. We examined whether genetic variation in three stress response genes, CRHR1, NR3C1, and FKBP5, interact with childhood trauma (CT) to predict AS in South African adolescents. Xhosa (n = 634) and Coloured (n = 317) students completed self-report measures of AS and CT, and a total of eighteen polymorphisms within CRHR1, NR3C1, and FKBP5 were genotyped. Differences in AS based on genetic variation and CT were analysed within population and gender groups using multiple linear regression. Associations were found between AS and FKBP5 rs9296158 (p = 0.025) and rs737054 (p = 0.045) in Coloured males. Analysis of gene x CT interactions indicated that NR3C1 rs190488 CC-genotype, NR3C1 rs10482605 G-allele addition, and FKBP5 rs3800373 C-allele addition protect against AS with increasing CT in Xhosa females (p = 0.009), Xhosa males (p = 0.036) and Coloured males (p = 0.049), respectively. We identified two different protective single nucleotide polymorphism (SNP) combinations in a four-SNP CRHR1 haplotype in Coloured males. An analysis of the interaction between CT and a six-SNP FKBP5 haplotype in Coloured males revealed both protective and risk allelic combinations. Our results provide evidence for the influence of both genetic variation in CRHR1, NR3C1 and FKBP5, as well as CT x SNP interactions, on AS in South African adolescents. This study reinforces the importance of examining the influence of gene-environment (G X E) interactions within gender and population groups.

The integrity and organization of the human AIPL1 functional domains is critical for its role as a HSP90-dependent co-chaperone for rod PDE6

Biallelic mutations in the photoreceptor-expressed aryl hydrocarbon receptor interacting protein-like 1 (AIPL1) are associated with autosomal recessive Leber congenital amaurosis (LCA), the most severe form of inherited retinopathy in early childhood. AIPL1 functions as a photoreceptor-specific co-chaperone that interacts with the molecular chaperone HSP90 to facilitate the stable assembly of the retinal cyclic GMP (cGMP) phosphodiesterase (PDE6) holoenzyme. In this study, we characterized the functional deficits of AIPL1 variations, some of which induce aberrant pre-mRNA AIPL1 splicing leading to the production of alternative AIPL1 isoforms. We investigated the ability of the AIPL1 variants to mediate an interaction with HSP90 and modulate the rod cGMP PDE6 stability and activity. Our data revealed that both the FK506 binding protein (FKBP)-like domain and the tetratricopeptide repeat (TPR) domain of AIPL1 are required for interaction with HSP90. We further demonstrate that AIPL1 significantly modulates the catalytic activity of heterologously expressed rod PDE6. Although the N-terminal FKBP-like domain of AIPL1 binds the farnesylated PDE6α subunit through direct interaction with the farnesyl moiety, mutations compromising the integrity of the C-terminal TPR domain of AIPL1 also failed to modulate PDE6 activity efficiently. These AIPL1 variants moreover failed to promote the HSP90-dependent stabilization of the PDE6α subunit in the cytosol. In summary, we have successfully validated the disease-causing status of the AIPL1 variations in vitro. Our findings provide insight into the mechanism underlying the co-chaperone role of AIPL1 and will be critical for ensuring an early and effective diagnosis of AIPL1 LCA patients.

The FKBP51 Glucocorticoid Receptor Co-Chaperone: Regulation, Function, and Implications in Health and Disease

Among the chaperones and co-chaperones regulating the glucocorticoid receptor (GR), FK506 binding protein (FKBP) 51 is the most intensely investigated across different disciplines. This review provides an update on the role of the different co-chaperones of Hsp70 and Hsp90 in the regulation of GR function. The development leading to the focus on FKBP51 is outlined. Further, a survey of the vast literature on the mechanism and function of FKBP51 is provided. This includes its structure and biochemical function, its regulation on different levels—transcription, post-transcription, and post-translation—and its function in signaling pathways. The evidence portraying FKBP51 as a scaffolding protein organizing protein complexes rather than a chaperone contributing to the folding of individual proteins is collated. Finally, FKBP51’s involvement in physiology and disease is outlined, and the promising efforts in developing drugs targeting FKBP51 are discussed.

Molecular mechanism and targeted therapy of Hsp90 involved in lung cancer: New discoveries and developments (Review)

The exploration of the molecular mechanisms and signaling pathways on lung cancer is very important for developing new strategies of diagnosis and treatment to this disease, such as finding valuable lung cancer markers and molecularly targeted therapies. Previously, a number of studies disclose that heat shock protein 90 (Hsp90) is upregulated in cancer cells, tissues and serum of lung cancer patients, and its upregulation intimately correlates with the occurrence, development and outcome of lung cancer. On the contrary, inhibition of Hsp90 can suppress cell proliferation, motility and metastasis of lung cancer and promote apoptosis of lung cancer cells via complex signaling pathways. In addition, a series of Hsp90 inhibitors have been investigated as effective molecular targeted therapy tactics fighting against lung cancer. This review, systematically summarizes the role of Hsp90 in lung cancer, the molecular mechanisms and development of anti-Hsp90 treatment in lung cancer.

Elevated FKBP52 expression indicates a poor outcome in patients with breast cancer

The 52-kDa FK506-binding protein (FKBP52), a regulator of steroid hormone receptor signaling, is potentially involved in a variety of hormone-dependent cancer types. The present study investigated the expression and clinical implications of FKBP52 in breast cancer. Immunohistochemistry was performed on samples from 145 breast cancer patients and on 66 unmatched breast non-cancerous tissues (as controls) to determine the expression level of FKBP52. Publicly available microarray and RNA-seq datasets used in the present study were downloaded from the European Bioinformatics Institute ArrayExpress. Kaplan-Meier survival analysis was also performed. FKBP52 expression was moderately higher in the tumors than that in the non-cancerous tissues, but this difference was not statistically significant (P=0.176). However, available microarray datasets exhibited a significant difference in FKBP52 mRNA levels between breast tumors and controls. In the 145 breast cancer patients, elevated FKBP52 expression was significantly associated with advanced Tumor-Node-Metastasis (TNM) stage (P=0.015), lymph node metastasis (P=0.015) and tumors with poor histological differentiation (P=0.047). FKBP52 expression was negatively associated with estrogen receptor expression (P=0.033), but positively associated with human epidermal growth factor receptor 2 expression (P=0.033). However, there was no association between FKBP52 and progesterone receptor expression. Survival analyses demonstrated that FKBP52 was indicative of a poor overall survival rate (P=0.026), which was consistent with the result of Kaplan-Meier analysis, exhibiting a negative association between the mRNA of FKBP52 and overall survival (OS) (P=0.044). Other than for FKBP52 [hazard ratio (HR), 2.315; 95% confidence interval (CI), 1.077-4.975; P=0.032], univariate analysis revealed that clinical stage exhibited a significant influence on the prognosis of the breast cancer patients (HR, 2.148; 95% CI, 1.011-4.566; P=0.047). However, multivariate analysis revealed that only clinical stage, not FKBP52, was an independent prognostic factor (HR, 2.721; 95% CI, 1.169-6.335; P=0.020). Patients were further classified according to their OS. Compared with the controls (3.94±2.992), FKBP52 expression in breast cancer patients with OS of ≤3 years (5.39±3.409; P=0.042) or OS of ≤5 years (5.88±3.473; P=0.005) was significantly increased, respectively. However, no significant difference in FKBP52 expression was observed between controls and individuals with an OS time of >3 years (4.84±3.769; P=0.109) or >5 years (5.32±3.372; P=0.090). Elevated FKBP52 expression may be involved in tumor progression and invasion, given its positive association with TNM stage and lymph node metastasis. Although it is not an independent predictor, FKBP52 has promise as a biological marker for estimating the progression of breast cancer.

The Plasticity of the Hsp90 Co-chaperone System

The Hsp90 system in the eukaryotic cytosol is characterized by a cohort of co-chaperones that bind to Hsp90 and affect its function. Although progress has been made regarding the underlying biochemical mechanisms, how co-chaperones influence Hsp90 client proteins in vivo has remained elusive. By investigating the effect of 12 Hsp90 co-chaperones on the activity of different client proteins in yeast, we find that deletion of co-chaperones can have a neutral or negative effect on client activity but can also lead to more active clients. Only a few co-chaperones are active on all clients studied. Closely related clients and even point mutants can depend on different co-chaperones. These effects are direct because differences in client-co-chaperone interactions can be reconstituted in vitro. Interestingly, some co-chaperones affect client conformation in vivo. Thus, co-chaperones adapt the Hsp90 cycle to the requirements of the client proteins, ensuring optimal activation.

Hsp70 facilitates trans-membrane transport of bacterial ADP-ribosylating toxins into the cytosol of mammalian cells

Binary enterotoxins Clostridium (C.) botulinum C2 toxin, C. perfringens iota toxin and C. difficile toxin CDT are composed of a transport (B) and a separate non-linked enzyme (A) component. Their B-components mediate endocytic uptake into mammalian cells and subsequently transport of the A-components from acidic endosomes into the cytosol, where the latter ADP-ribosylate G-actin resulting in cell rounding and cell death causing clinical symptoms. Protein folding enzymes, including Hsp90 and peptidyl-prolyl cis/trans isomerases facilitate transport of the A-components across endosomal membranes. Here, we identified Hsp70 as a novel host cell factor specifically interacting with A-components of C2, iota and CDT toxins to facilitate their transport into the cell cytosol. Pharmacological Hsp70-inhibition specifically prevented pH-dependent trans-membrane transport of A-components into the cytosol thereby protecting living cells and stem cell-derived human miniguts from intoxication. Thus, Hsp70-inhibition might lead to development of novel therapeutic strategies to treat diseases associated with bacterial ADP-ribosylating toxins.

A FKBP5 mutation is associated with Paget's disease of bone and enhances osteoclastogenesis

Paget's disease of bone (PDB) is a common metabolic bone disease that is characterized by aberrant focal bone remodeling, which is caused by excessive osteoclastic bone resorption followed by disorganized osteoblastic bone formation. Genetic factors are a critical determinant of PDB pathogenesis, and several susceptibility genes and loci have been reported, including SQSTM1, TNFSF11A, TNFRSF11B, VCP, OPTN, CSF1 and DCSTAMP. Herein, we report a case of Chinese familial PDB without mutations in known genes and identify a novel c.163G>C (p.Val55Leu) mutation in FKBP5 (encodes FK506-binding protein 51, FKBP51) associated with PDB using whole-exome sequencing. Mutant FKBP51 enhanced the Akt phosphorylation and kinase activity in cells. A study of osteoclast function using FKBP51^V55L KI transgenic mice proved that osteoclast precursors from FKBP51^V55L mice were hyperresponsive to RANKL, and osteoclasts derived from FKBP51^V55L mice displayed more intensive bone resorbing activity than did FKBP51^WT controls. The osteoclast-specific molecules tartrate-resistant acid phosphatase, osteoclast-associated receptor and transcription factor NFATC1 were increased in bone marrow-derived monocyte/macrophage cells (BMMs) from FKBP51^V55L mice during osteoclast differentiation. However, c-fos expression showed no significant difference in the wild-type and mutant groups. Akt phosphorylation in FKBP51^V55L BMMs was elevated in response to RANKL. In contrast, IκB degradation, ERK phosphorylation and LC3II expression showed no difference in wild-type and mutant BMMs. Micro-CT analysis revealed an intensive trabecular bone resorption pattern in FKBP51^V55L mice, and suspicious osteolytic bone lesions were noted in three-dimensional reconstruction of distal femurs from mutant mice. These results demonstrate that the mutant FKBP51^V55L promotes osteoclastogenesis and function, which could subsequently participate in PDB development.