Differential recruitment of tetratricorpeptide repeat domain immunophilins to the mineralocorticoid receptor influences both heat-shock protein 90-dependent retrotransport and hormone-dependent transcriptional activity

Molecular evidence of altered stress responsivity related to neuroinflammation in the schizophrenia midbrain

Stress and inflammation are risk factors for schizophrenia. Chronic psychosocial stress is associated with subcortical hyperdopaminergia, a core feature of schizophrenia. Hyperdopaminergia arises from midbrain neurons, leading us to hypothesise that changes in stress response pathways may occur in this region. To identify whether transcriptional changes in glucocorticoid and mineralocorticoid receptors (NR3C1/GR, NR3C2/MR) or other stress signalling molecules (FKBP4, FKBP5) exist in schizophrenia midbrain, we measured gene expression in the human brain (N = 56) using qRT-PCR. We assessed whether alterations in these mRNAs were related to previously identified high/low inflammatory status. We investigated relationships between stress-related transcripts themselves, and between FKBP5 mRNA, dopaminergic, and glial cell transcripts in diagnostic and inflammatory subgroups. Though unchanged by diagnosis, GR mRNA levels were reduced in high inflammatory compared to low inflammatory schizophrenia cases (p = 0.026). We found no effect of diagnosis or inflammation on MR mRNA. FKBP4 mRNA was decreased and FKBP5 mRNA was increased in schizophrenia (p < 0.05). FKBP5 changes occurred in high inflammatory (p < 0.001), whereas FKBP4 changes occurred in low inflammatory schizophrenia cases (p < 0.05). The decrease in mRNA encoding the main stress receptor (GR), as well as increased transcript levels of the stress-responsive negative regulator (FKBP5), may combine to blunt the midbrain response to stress in schizophrenia when neuroinflammation is present. Negative correlations between FKBP5 mRNA and dopaminergic transcripts in the low inflammatory subgroup suggest higher levels of FKBP5 mRNA may also attenuate dopaminergic neurotransmission in schizophrenia even when inflammation is absent. We report alterations in GR-mediated stress signalling in the midbrain in schizophrenia.

The Role of FKBPs in Complex Disorders: Neuropsychiatric Diseases, Cancer, and Type 2 Diabetes Mellitus

Stress is a common denominator of complex disorders and the FK-506 binding protein (FKBP)51 plays a central role in stress. Hence, it is not surprising that multiple studies imply the involvement of the FKBP51 protein and/or its coding gene, FKBP5, in complex disorders. This review summarizes such reports concentrating on three disorder clusters-neuropsychiatric, cancer, and type 2 diabetes mellitus (T2DM). We also attempt to point to potential mechanisms suggested to mediate the effect of FKBP5/FKBP51 on these disorders. Neuropsychiatric diseases considered in this paper include (i) Huntington's disease for which increased autophagic cellular clearance mechanisms related to decreased FKBP51 protein levels or activity is discussed, Alzheimer's disease for which increased FKBP51 activity has been shown to induce Tau phosphorylation and aggregation, and Parkinson's disease in the context of which FKBP12 is mentioned; and (ii) mental disorders, for which significant association with the single nucleotide polymorphism (SNP) rs1360780 of FKBP5 intron 7 along with decreased DNA methylation were revealed. Since cancer is a large group of diseases that can start in almost any organ or tissue of the body, FKBP51's role depends on the tissue type and differences among pathways expressed in those tumors. The FKBP51-heat-shock protein-(Hsp)90-p23 super-chaperone complex might function as an oncogene or as a tumor suppressor by downregulating the serine/threonine protein kinase (AKt) pathway. In T2DM, two potential pathways for the involvement of FKBP51 are highlighted as affecting the pathogenesis of the disease-the peroxisome proliferator-activated receptor-γ (PPARγ) and AKt.

FK-binding protein 5: Possible relevance to the pathogenesis of metabolic dysfunction and alcohol-associated liver disease

The FK506-binding protein (FKBP5) plays significant roles in mediating stress responses by interacting with glucocorticoids, participating in adipogenesis, and influencing various cellular pathways throughout the body. In this review, we described the potential role of FKBP5 in the pathogenesis of two common chronic liver diseases, metabolic dysfunction-associated steatotic liver disease (MASLD), and alcohol-associated liver disease (ALD). We provided an overview of the FK-binding protein family and elucidated their roles in cellular stress responses, metabolic diseases, and adipogenesis. We explored how FKBP5 may mechanistically influence the pathogenesis of MASLD and ALD and provided insights for further investigation into the role of FKBP5 in these two diseases.

Heat stress adaptation in cows - Physiological responses and underlying molecular mechanisms

Heat stress is a key abiotic stressor for dairy production in the tropics which is further compounded by the ongoing climate change. Heat stress not only adversely impacts the production and welfare of dairy cows but severely impacts the economics of dairying due to production losses and increased cost of rearing. Over the years, selection has ensured development of high producing breeds, however, the thermotolerance ability of animals has been largely overlooked. In the past decade, the ill effects of climate change have made it pertinent to rethink the selection strategies to opt for climate resilient breeds, to ensure optimum production and reproduction. This has led to renewed interest in evaluation of the impacts of heat stress on cows and the underlying mechanisms that results in their acclimatization and adaptation to varied thermal ambience. The understanding of heat stress and associated responses at various level of animal is crucial to device amelioration strategies to secure optimum production and welfare of cows. With this review, an effort has been made to provide an overview on temperature humidity index as an important indicator of heat stress, general effect of heat stress in dairy cows, and impact of heat stress and subsequent response at physiological, haematological, molecular and genetic level of dairy cows.

The transportosome system as a model for the retrotransport of soluble proteins

The classic model of action of the glucocorticoid receptor (GR) sustains that its associated heat-shock protein of 90-kDa (HSP90) favours the cytoplasmic retention of the unliganded GR, whereas the binding of steroid triggers the dissociation of HSP90 allowing the passive nuclear accumulation of GR. In recent years, it was described a molecular machinery called transportosome that is responsible for the active retrograde transport of GR. The transportosome heterocomplex includes a dimer of HSP90, the stabilizer co-chaperone p23, and FKBP52 (FK506-binding protein of 52-kDa), an immunophilin that binds dynein/dynactin motor proteins. The model shows that upon steroid binding, FKBP52 is recruited to the GR allowing its active retrograde transport on cytoskeletal tracks. Then, the entire GR heterocomplex translocates through the nuclear pore complex. The HSP90-based heterocomplex is released in the nucleoplasm followed by receptor dimerization. Subsequent findings demonstrated that the transportosome is also responsible for the retrotransport of other soluble proteins. Importantly, the disruption of this molecular oligomer leads to several diseases. In this article, we discuss the relevance of this transport machinery in health and disease.

Aldosterone: Renal Action and Physiological Effects

Aldosterone exerts profound effects on renal and cardiovascular physiology. In the kidney, aldosterone acts to preserve electrolyte and acid-base balance in response to changes in dietary sodium (Na+ ) or potassium (K+ ) intake. These physiological actions, principally through activation of mineralocorticoid receptors (MRs), have important effects particularly in patients with renal and cardiovascular disease as demonstrated by multiple clinical trials. Multiple factors, be they genetic, humoral, dietary, or otherwise, can play a role in influencing the rate of aldosterone synthesis and secretion from the adrenal cortex. Normally, aldosterone secretion and action respond to dietary Na+ intake. In the kidney, the distal nephron and collecting duct are the main targets of aldosterone and MR action, which stimulates Na+ absorption in part via the epithelial Na+ channel (ENaC), the principal channel responsible for the fine-tuning of Na+ balance. Our understanding of the regulatory factors that allow aldosterone, via multiple signaling pathways, to function properly clearly implicates this hormone as central to many pathophysiological effects that become dysfunctional in disease states. Numerous pathologies that affect blood pressure (BP), electrolyte balance, and overall cardiovascular health are due to abnormal secretion of aldosterone, mutations in MR, ENaC, or effectors and modulators of their action. Study of the mechanisms of these pathologies has allowed researchers and clinicians to create novel dietary and pharmacological targets to improve human health. This article covers the regulation of aldosterone synthesis and secretion, receptors, effector molecules, and signaling pathways that modulate its action in the kidney. We also consider the role of aldosterone in disease and the benefit of mineralocorticoid antagonists. © 2023 American Physiological Society. Compr Physiol 13:4409-4491, 2023.

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.

Impact of Co-chaperones and Posttranslational Modifications Toward Hsp90 Drug Sensitivity

Posttranslational modifications (PTMs) regulate myriad cellular processes by modulating protein function and protein-protein interaction. Heat shock protein 90 (Hsp90) is an ATP-dependent molecular chaperone whose activity is responsible for the stabilization and maturation of more than 300 client proteins. Hsp90 is a substrate for numerous PTMs, which have diverse effects on Hsp90 function. Interestingly, many Hsp90 clients are enzymes that catalyze PTM, demonstrating one of the several modes of regulation of Hsp90 activity. Approximately 25 co-chaperone regulatory proteins of Hsp90 impact structural rearrangements, ATP hydrolysis, and client interaction, representing a second layer of influence on Hsp90 activity. A growing body of literature has also established that PTM of these co-chaperones fine-tune their activity toward Hsp90; however, many of the identified PTMs remain uncharacterized. Given the critical role of Hsp90 in supporting signaling in cancer, clinical evaluation of Hsp90 inhibitors is an area of great interest. Interestingly, differential PTM and co-chaperone interaction have been shown to impact Hsp90 binding to its inhibitors. Therefore, understanding these layers of Hsp90 regulation will provide a more complete understanding of the chaperone code, facilitating the development of new biomarkers and combination therapies.

Is there a role for HSF1 in viral infections?

Cells undergo numerous processes to adapt to new challenging conditions and stressors. Heat stress is regulated by a family of heat shock factors (HSFs) that initiate a heat shock response by upregulating the expression of heat shock proteins (HSPs) intended to counteract cellular damage elicited by increased environmental temperature. Heat shock factor 1 (HSF1) is known as the master regulator of the heat shock response and upon its activation induces the transcription of genes that encode for molecular chaperones, such as HSP40, HSP70, and HSP90. Importantly, an accumulating body of studies relates HSF1 with viral infections; the induction of fever during viral infection may activate HSF1 and trigger a consequent heat shock response. Here, we review the role of HSF1 in different viral infections and its impact on the health outcome for the host. Studying the relationship between HSF1 and viruses could open new potential therapeutic strategies given the availability of drugs that regulate the activation of this transcription factor.

Brain Mineralocorticoid receptor in health and disease: from molecular signaling to cognitive and emotional function

Brain mineralocorticoid receptors (MR) mediate effects of glucocorticoid hormones in stress adaptation, as well as the effects of aldosterone itself in relation to salt homeostasis. Brain stem MRs respond to aldosterone, whereas forebrain MRs mediate rapid and delayed glucocorticoid effects in conjunction with the glucocorticoid receptor (GR). MR‐mediated effects depend on age, gender, genetic variations, and environmental influences. Disturbed MR activity through chronic stress, certain (endocrine) diseases or during glucocorticoid therapy can cause deleterious effects on affective state, cognitive and behavioural function in susceptible individuals. Considering the important role MR plays in cognition and emotional function in health and disease, MR modulation by pharmacological intervention could relieve stress‐ and endocrine‐related symptoms. Here, we discuss recent pharmacological interventions in the clinic and genetic developments in the molecular underpinnings of MR signalling. Further understanding of MR‐dependent pathways may help to improve psychiatric symptoms in a diversity of settings.

FKBP52 in Neuronal Signaling and Neurodegenerative Diseases: A Microtubule Story

The FK506-binding protein 52 (FKBP52) belongs to a large family of ubiquitously expressed and highly conserved proteins (FKBPs) that share an FKBP domain and possess Peptidyl-Prolyl Isomerase (PPIase) activity. PPIase activity catalyzes the isomerization of Peptidyl-Prolyl bonds and therefore influences target protein folding and function. FKBP52 is particularly abundant in the nervous system and is partially associated with the microtubule network in different cell types suggesting its implication in microtubule function. Various studies have focused on FKBP52, highlighting its importance in several neuronal microtubule-dependent signaling pathways and its possible implication in neurodegenerative diseases such as tauopathies (i.e., Alzheimer disease) and alpha-synucleinopathies (i.e., Parkinson disease). This review summarizes our current understanding of FKBP52 actions in the microtubule environment, its implication in neuronal signaling and function, its interactions with other members of the FKBPs family and its involvement in neurodegenerative disease.

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.

Structural and molecular determinants of mineralocorticoid receptor signalling

During the past decades, the mineralocorticoid receptor (MR) has evolved from a much-overlooked member of the steroid hormone receptor family to an important player, not only in volume and electrolyte homeostasis but also in pathological changes occurring in an increasing number of tissues, especially the renal and cardiovascular systems. Simultaneously, a wealth of information about the structure, interaction partners and chromatin requirements for genomic signalling of steroid hormone receptors became available. However, much of the information for the MR has been deduced from studies of other family members and there is still a lack of knowledge about MR-specific features in ligand binding, chromatin remodelling, co-factor interactions and general MR specificity-conferring mechanisms that can completely explain the differences in pathophysiological function between MR and its closest relative, the glucocorticoid receptor. This review aims to give an overview of the current knowledge of MR structure, signalling and co-factors modulating its activity.

Corticosteroid receptors as a model for the Hsp90•immunophilin-based transport machinery

Steroid receptors form soluble heterocomplexes with the 90-kDa heat-shock protein (Hsp90) and other chaperones and co-chaperones. The assembly and composition of the oligomer is influenced by the presence and nature of the bound steroid. Although these receptors shuttle dynamically in and out of the nucleus, their primary localization in the absence of steroid can be mainly cytoplasmic, mainly nuclear, or partitioned into both cellular compartments. Upon steroid binding, receptors become localized to the nucleus via the transportosome, a retrotransport molecular machinery that comprises Hsp90, a high-molecular-weight immunophilin, and dynein motors. This molecular machinery, first evidenced in steroid receptors, can also be used by other soluble proteins. In this review, we dissect the complete model of this transport machinery system.

Is PTSD-Phenotype Associated with HPA-Axis Sensitivity? Feedback Inhibition and Other Modulating Factors of Glucocorticoid Signaling Dynamics

Previously, we found that basal corticosterone pulsatility significantly impacts the vulnerability for developing post-traumatic stress disorder (PTSD). Rats that exhibited PTSD-phenotype were characterized by blunted basal corticosterone pulsatility amplitude and a blunted corticosterone response to a stressor. This study sought to identify the mechanisms underlining both the loss of pulsatility and differences in downstream responses. Serial blood samples were collected manually via jugular vein cannula at 10-min intervals to evaluate suppression of corticosterone following methylprednisolone administration. The rats were exposed to predator scent stress (PSS) after 24 h, and behavioral responses were assessed 7 days post-exposure for retrospective classification into behavioral response groups. Brains were harvested for measurements of the glucocorticoid receptor, mineralocorticoid receptor, FK506-binding protein-51 and arginine vasopressin in specific brain regions to assess changes in hypothalamus–pituitary–adrenal axis (HPA) regulating factors. Methylprednisolone produced greater suppression of corticosterone in the PTSD-phenotype group. During the suppression, the PTSD-phenotype rats showed a significantly more pronounced pulsatile activity. In addition, the PTSD-phenotype group showed distinct changes in the ventral and dorsal CA1, dentate gyrus as well as in the paraventricular nucleus and supra-optic nucleus. These results demonstrate a pre-trauma vulnerability state that is characterized by an over-reactivity of the HPA and changes in its regulating factors.

The role of PP5 and PP2C in cardiac health and disease

Protein phosphatases are important, for example, as functional antagonists of β-adrenergic stimulation of the mammalian heart. While β-adrenergic stimulations increase the phosphorylation state of regulatory proteins and therefore force of contraction in the heart, these phosphorylations are reversed and thus force is reduced by the activity of protein phosphatases. In this context the role of PP5 and PP2C is starting to unravel. They do not belong to the same family of phosphatases with regard to sequence homology, many similarities with regard to location, activation by lipids and putative substrates have been worked out over the years. We also suggest which pathways for regulation of PP5 and/or PP2C described in other tissues and not yet in the heart might be useful to look for in cardiac tissue. Both phosphatases might play a role in signal transduction of sarcolemmal receptors in the heart. Expression of PP5 and PP2C can be increased by extracellular stimuli in the heart. Because PP5 is overexpressed in failing animal and human hearts, and because overexpression of PP5 or PP2C leads to cardiac hypertrophy and KO of PP5 leads to cardiac hypotrophy, one might argue for a role of PP5 and PP2C in heart failure. Because PP5 and PP2C can reduce, at least in vitro, the phosphorylation state of proteins thought to be relevant for cardiac arrhythmias, a role of these phosphatases for cardiac arrhythmias is also probable. Thus, PP5 and PP2C might be druggable targets to treat important cardiac diseases like heart failure, cardiac hypertrophy and cardiac arrhythmias.

Differential regulation of the glucocorticoid receptor nucleocytoplasmic shuttling by TPR-domain proteins

A dimer of the heat-shock protein of 90-kDa (Hsp90) represents the critical core of the chaperone complex associated to the glucocorticoid receptor (GR) oligomer. The C-terminal end of the Hsp90 dimer shapes a functional acceptor site for co-chaperones carrying tetratricopeptide repeat (TPR) domains, where they bind in a mutually exclusive and competitive manner. They impact on the biological properties of the GR•Hsp90 complex and are major players of the GR transport machinery. Recently, we showed that the overexpression of a chimeric TPR peptide influences the subcellular distribution of GR. In this study, the functional role of endogenous proteins carrying TPR or TPR-like sequences on GR subcellular distribution was characterized. It is demonstrated that, contrarily to the positive influence of FKBP52 on GR nuclear accumulation, FKBP51 and 14-3-3 impaired this property. While SGT1α showed no significant effect, the overexpression of the Ser/Thr phosphatase PP5 resulted in a nearly equal nuclear-cytoplasmic redistribution of GR rather than its typical cytoplasmic localization in the absence of steroid. This observation led to analyse the influence of the phosphorylation status of GR, which resulted not linked to its nucleo-cytoplasmic shuttling mechanism. Nonetheless, it was evidenced that both PP5 and FKBP52 are related to the anchorage of the GR to nucleoskeleton structures. The influence of these TPR domain proteins on the steroid-dependent transcriptional activity of GR was also characterized. It is postulated that the pleiotropic actions of the GR in different cell types may be the consequence of the relative abundance of different TPR-domain interacting co-chaperones.

Corticosteroid-binding-globulin (CBG)-deficient mice show high pY216-GSK3β and phosphorylated-Tau levels in the hippocampus

Corticosteroid-binding globulin (CBG) is the specific carrier of circulating glucocorticoids, but evidence suggests that it also plays an active role in modulating tissue glucocorticoid activity. CBG polymorphisms affecting its expression or affinity for glucocorticoids are associated with chronic pain, chronic fatigue, headaches, depression, hypotension, and obesity with an altered hypothalamic pituitary adrenal axis. CBG has been localized in hippocampus of humans and rodents, a brain area where glucocorticoids have an important regulatory role. However, the specific CBG function in the hippocampus is yet to be established. The aim of this study was to investigate the effect of the absence of CBG on hippocampal glucocorticoid levels and determine whether pathways regulated by glucocorticoids would be altered. We used cbg^-/- mice, which display low total-corticosterone and high free-corticosterone blood levels at the nadir of corticosterone secretion (morning) and at rest to evaluate the hippocampus for total- and free-corticosterone levels; 11β-hydroxysteroid dehydrogenase expression and activity; the expression of key proteins involved in glucocorticoid activity and insulin signaling; microtubule-associated protein tau phosphorylation, and neuronal and synaptic function markers. Our results revealed that at the nadir of corticosterone secretion in the resting state the cbg^-/- mouse hippocampus exhibited slightly elevated levels of free-corticosterone, diminished FK506 binding protein 5 expression, increased corticosterone downstream effectors and altered MAPK and PI3K pathway with increased pY216-GSK3β and phosphorylated tau. Taken together, these results indicate that CBG deficiency triggers metabolic imbalance which could lead to damage and long-term neurological pathologies.

Proof that the high molecular weight immunophilin FKBP52 mediates the in vivo neuroregenerative effect of the macrolide FK506

The immunosuppressant drug FK506 (or tacrolimus) is a macrolide that binds selectively to immunophilins belonging to the FK506-binding protein (FKBP) subfamily, which are abundantly expressed proteins in neurons of the peripheral and central nervous systems. Interestingly, it has been reported that FK506 increases neurite outgrowth in cell cultures, implying a potential impact in putative treatments of neurodegenerative disorders and injuries of the nervous system. Nonetheless, the mechanism of action of this compound is poorly understood and remains to be elucidated, with the only certainty that its neurotrophic effect is independent of its primary immunosuppressant activity. In this study it is demonstrated that FK506 shows efficient neurotrophic action in vitro and profound effects on the recovery of locomotor activity, behavioural features, and erectile function of mice that underwent surgical spinal cord injury. The recovery of the locomotor activity was studied in knock-out mice for either immunophilin, FKBP51 or FKBP52. The experimental evidence demonstrates that the neurotrophic actions of FK506 are the consequence of its binding to FKBP52, whereas FK506 interaction with the close-related partner immunophilin FKBP51 antagonises the function of FKBP52. Importantly, our study also demonstrates that other immunophilins do not replace FKBP52. It is concluded that the final biological response is the resulting outcome of the drug binding to both immunophilins, FKBP51 and FKBP52, the latter being the one that commands the dominant neurotrophic action in vivo.

Nucleocytoplasmic shuttling of the glucocorticoid receptor is influenced by tetratricopeptide repeat-containing proteins

It has been demonstrated that tetratricopeptide-repeat (TPR) domain proteins regulate the subcellular localization of glucocorticoid receptor (GR). This study analyses the influence of the TPR domain of high molecular weight immunophilins in the retrograde transport and nuclear retention of GR. Overexpression of the TPR peptide prevented efficient nuclear accumulation of the GR by disrupting the formation of complexes with the dynein-associated immunophilin FKBP52 (also known as FKBP4), the adaptor transporter importin-β1 (KPNB1), the nuclear pore-associated glycoprotein Nup62 and nuclear matrix-associated structures. We also show that nuclear import of GR was impaired, whereas GR nuclear export was enhanced. Interestingly, the CRM1 (exportin-1) inhibitor leptomycin-B abolished the effects of TPR peptide overexpression, although the drug did not inhibit GR nuclear export itself. This indicates the existence of a TPR-domain-dependent mechanism for the export of nuclear proteins. The expression balance of those TPR domain proteins bound to the GR-Hsp90 complex may determine the subcellular localization and nucleocytoplasmic properties of the receptor, and thereby its pleiotropic biological properties in different tissues and cell types.

Structure and function of the co-chaperone protein phosphatase 5 in cancer

Protein phosphatase 5 (PP5) is a serine/threonine protein phosphatase that regulates many cellular functions including steroid hormone signaling, stress response, proliferation, apoptosis, and DNA repair. PP5 is also a co-chaperone of the heat shock protein 90 molecular chaperone machinery that assists in regulation of cellular signaling pathways essential for cell survival and growth. PP5 plays a significant role in survival and propagation of multiple cancers, which makes it a promising target for cancer therapy. Though there are several naturally occurring PP5 inhibitors, none is specific for PP5. Here, we review the roles of PP5 in cancer progression and survival and discuss the unique features of the PP5 structure that differentiate it from other phosphoprotein phosphatase (PPP) family members and make it an attractive therapeutic target.

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.

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.

Antiviral activity of the mineralocorticoid receptor NR3C2 against Herpes simplex virus Type 1 (HSV-1) infection

Analysis of a genome-scale RNA interference screen of host factors affecting herpes simplex virus type 1 (HSV-1) revealed that the mineralocorticoid receptor (MR) inhibits HSV-1 replication. As a ligand-activated transcription factor the MR regulates sodium transport and blood pressure in the kidney in response to aldosterone, but roles have recently been elucidated for the MR in other cellular processes. Here, we show that the MR and other members of the mineralocorticoid signalling pathway including HSP90 and FKBP4, possess anti-viral activity against HSV-1 independent of their effect on sodium transport, as shown by sodium channel inhibitors. Expression of the MR is upregulated upon infection in an interferon (IFN) and viral transcriptional activator VP16-dependent fashion. Furthermore, the MR and VP16, together with the cellular co-activator Oct-1, transactivate the hormone response element (HRE) present in the MR promoter and those of its transcriptional targets. As the MR induces IFN expression, our data suggests the MR is involved in a positive feedback loop that controls HSV-1 infection.

Corticosteroid Receptors in the Brain: Transcriptional Mechanisms for Specificity and Context-Dependent Effects

Corticosteroid hormones act in the brain to support adaptation to stress via binding to mineralocorticoid and glucocorticoid receptors (MR and GR). These receptors act in large measure as transcription factors. Corticosteroid effects can be highly divergent, depending on the receptor type, but also on brain region, cell type, and physiological context. These differences ultimately depend on differential interactions of MR and GR with other proteins, which determine ligand binding, nuclear translocation, and transcriptional activities. In this review, we discuss established and potential mechanisms that confer receptor and cell type-specific effects of the MR and GR-mediated transcriptional effects in the brain.

Corticosteroids as an adjunct to tuberculosis therapy

INTRODUCTION

Inflammation, or the prolonged resolution of inflammation, contributes to death from tuberculosis. Interest in inflammatory mechanisms and the prospect of beneficial immune modulation as an adjunct to antibacterial therapy has revived and the concept of host directed therapies has been advanced. Such renewed attention has however, overlooked the experience of such therapy with corticosteroids. Areas covered: The authors conducted literature searches and evaluated randomized clinical trials, systematic reviews and current guidelines and summarize these findings. They found evidence of benefit in meningeal and pericardial tuberculosis in HIV-1 uninfected persons, but less so in those HIV-1 coinfected and evidence of harm in the form of opportunist malignancy in those not prescribed antiretroviral therapy. Adjunctive corticosteroids are however of benefit in the treatment and prevention of paradoxical HIV-tuberculosis immune reconstitution inflammatory syndrome. Expert commentary: Further high-quality clinical trials and experimental medicine studies are warranted and analysis of materials arising from such studies could illuminate ways to improve corticosteroid efficacy or identify novel pathways for more specific intervention.

FKBP51 promotes migration and invasion of papillary thyroid carcinoma through NF-κB-dependent epithelial-to-mesenchymal transition

FK506-binding protein 51 (FKBP51) is a member of the immunophilin family, with relevant roles in multiple signaling pathways, tumorigenesis and chemoresistance. However, the function of FKBP51 in papillary thyroid carcinoma (PTC) remains largely unknown. In the present study, increased FKBP51 expression was detected in PTC tissues as compared with adjacent normal tissues, and the expression level was associated with clinical tumor, node and metastasis stage. Using FKBP51-overexpressing K1 cells and FKBP51-knockdown TPC-1 cells, both human PTC cell lines, it was identified that FKBP51 promoted the migration and invasion of PTC, without affecting cell proliferation. Further investigation revealed that FKBP51 activated the NF-κB pathway and epithelial-to-mesenchymal transition (EMT) genes, and EMT was suppressed when NF-κB was inhibited. It was also assessed whether FKBP51 promoted the formation of cytoskeleton to promote migration and invasion of PTC using a tubulin tracker; however, no evidence of such an effect was observed. These results suggested that FKBP51 promotes migration and invasion through NF-κB-dependent EMT.

The Nuclear Receptor Field: A Historical Overview and Future Challenges

In this article we summarize the birth of the field of nuclear receptors, the discovery of untransformed and transformed isoforms of ligand-binding macromolecules, the discovery of the three-domain structure of the receptors, and the properties of the Hsp90-based heterocomplex responsible for the overall structure of the oligomeric receptor and many aspects of the biological effects. The discovery and properties of the subfamily of receptors called orphan receptors is also outlined. Novel molecular aspects of the mechanism of action of nuclear receptors and challenges to resolve in the near future are discussed.

Gene expression regulation by heat-shock proteins: the cardinal roles of HSF1 and Hsp90

The ability to permit gene expression is managed by a set of relatively well known regulatory mechanisms. Nonetheless, this property can also be acquired during a life span as a consequence of environmental stimuli. Interestingly, some acquired information can be passed to the next generation of individuals without modifying gene information, but instead by the manner in which cells read and process such information. Molecular chaperones are classically related to the proper preservation of protein folding and anti-aggregation properties, but one of them, heat-shock protein 90 (Hsp90), is a refined sensor of protein function facilitating the biological activity of properly folded client proteins that already have a preserved tertiary structure. Interestingly, Hsp90 can also function as a critical switch able to regulate biological responses due to its association with key client proteins such as histone deacetylases or DNA methylases. Thus, a growing amount of evidence has connected the action of Hsp90 to post-translational modifications of soluble nuclear factors, DNA, and histones, which epigenetically affect gene expression upon the onset of an unfriendly environment. This response is commanded by the activation of the transcription factor heat-shock factor 1 (HSF1). Even though numerous stresses of diverse nature are known to trigger the stress response by activation of HSF1, it is still unknown whether there are different types of molecular sensors for each type of stimulus. In the present review, we will discuss various aspects of the regulatory action of HSF1 and Hsp90 on transcriptional regulation, and how this regulation may affect genetic assimilation mechanisms and the health of individuals.

Organization of nuclear architecture during adipocyte differentiation

Obesity is a serious health problem worldwide since it is a major risk factor for chronic diseases such as type II diabetes. Obesity is the result of hyperplasia (associated with increased adipogenesis) and hypertrophy (associated with decreased adipogenesis) of the adipose tissue. Therefore, understanding the molecular mechanisms underlying the process of adipocyte differentiation is relevant to delineate new therapeutic strategies for treatment of obesity. As in all differentiation processes, temporal patterns of transcription are exquisitely controlled, allowing the acquisition and maintenance of the adipocyte phenotype. The genome is spatially organized; therefore decoding local features of the chromatin language alone does not suffice to understand how cell type-specific gene expression patterns are generated. Elucidating how nuclear architecture is built during the process of adipogenesis is thus an indispensable step to gain insight in how gene expression is regulated to achieve the adipocyte phenotype. Here we will summarize the recent advances in our understanding of the organization of nuclear architecture as progenitor cells differentiate in adipocytes, and the questions that still remained to be answered.

Mineralocorticoid regulation of cell function: the role of rapid signalling and gene transcription pathways

The mineralocorticoid receptor (MR) and mineralocorticoids regulate epithelial handling of electrolytes, and induces diverse effects on other tissues. Traditionally, the effects of MR were ascribed to ligand-receptor binding and activation of gene transcription. However, the MR also utilises a number of intracellular signalling cascades, often by transactivating unrelated receptors, to change cell function more rapidly. Although aldosterone is the physiological mineralocorticoid, it is not the sole ligand for MR. Tissue-selective and mineralocorticoid-specific effects are conferred through the enzyme 11β-hydroxysteroid dehydrogenase 2, cellular redox status and properties of the MR itself. Furthermore, not all aldosterone effects are mediated via MR, with implication of the involvement of other membrane-bound receptors such as GPER. This review will describe the ligands, receptors and intracellular mechanisms available for mineralocorticoid hormone and receptor signalling and illustrate their complex interactions in physiology and disease.

Hsp90-binding immunophilin FKBP51 forms complexes with hTERT enhancing telomerase activity

FK506-binding proteins are members of the immunophilin family of proteins. Those immunophilins associated to the 90-kDa-heat-shock protein, Hsp90, have been proposed as potential modulators of signalling cascade factors chaperoned by Hsp90. FKBP51 and FKBP52 are the best characterized Hsp90-bound immunophilins first described associated to steroid-receptors. The reverse transcriptase subunit of telomerase, hTERT, is also an Hsp90 client-protein and is highly expressed in cancer cells, where it is required to compensate the loss of telomeric DNA after each successive cell division. Because FKBP51 is also a highly expressed protein in cancer tissues, we analyzed its potential association with hTERT·Hsp90 complexes and its possible biological role. In this study it is demonstrated that both immunophilins, FKBP51 and FKBP52, co-immunoprecipitate with hTERT. The Hsp90 inhibitor radicicol disrupts the heterocomplex and favors the partial cytoplasmic relocalization of hTERT in similar manner as the overexpression of the TPR-domain peptide of the immunophilin. While confocal microscopy images show that FKBP51 is primarily localized in mitochondria and hTERT is totally nuclear, upon the onset of oxidative stress, FKBP51 (but not FKBP52) becomes mostly nuclear colocalizing with hTERT, and longer exposure times to peroxide favors hTERT export to mitochondria. Importantly, telomerase activity of hTERT is significantly enhanced by FKBP51. These observations support the emerging role assigned to FKBP51 as antiapoptotic factor in cancer development and progression, and describe for the first time the potential role of this immunophilin favoring the clonal expansion by enhancing telomerase activity.

Role of Pro-637 and Gln-642 in human glucocorticoid receptors and Ser-843 and Leu-848 in mineralocorticoid receptors in their differential responses to cortisol and aldosterone

Mineralocorticoid receptors (MR) and glucocorticoid receptors (GR) are descended from a common ancestral corticoid receptor. The basis for specificities of human MR for aldosterone and human GR for glucocorticoids, such as cortisol, bearing 17α-hydroxyl-groups, is incompletely understood. Differences in MR at S843 and L848 and GR at the corresponding P637 and Q642 have been proposed as important in their different responses to glucocorticoids with 17α-hydroxyl-groups. We investigated the impact of these residues on binding affinity (Ki) and transcriptional activation (EC50) of mutants MR-S843P, MR-L848Q and MR-S843P/L848Q and mutants GR-P637S, GR-Q642L and GR-P637S/Q642L in the presence of different corticosteroids. Aldosterone, cortisol and corticosterone had similar affinities for wild-type MR and all mutants, while dexamethasone had increased affinity for the three mutants. However, transactivation of MR-S843P and MR-S843P/L848Q by all four steroids was significantly lower than for wild-type MR. In contrast, transactivation of MR-L848Q tended to be 3-fold higher for cortisol and corticosterone and increased 7-fold for dexamethasone, indicating that MR-L848Q has an increased response to glucocorticoids, while retaining a strong response to aldosterone. Compared to wild-type GR, GR-P637S and GR-Q642L had increased affinities and significantly increased transcriptional activity with aldosterone and corticosterone, and GR-P637S had similar transcriptional activity with cortisol and dexamethasone, while GR-Q642L and GR-P637S/Q642L had a significant decrease in transcriptional activity with cortisol and dexamethasone. 3D-models of these MR and GR mutants revealed that dexamethasone and aldosterone, respectively, fit nicely into the steroid-binding pocket, consistent with the affinity of dexamethasone for MR mutants and aldosterone for GR mutants.

Gene-Stress-Epigenetic Regulation of FKBP5: Clinical and Translational Implications

Stress responses and related outcomes vary markedly across individuals. Elucidating the molecular underpinnings of this variability is of great relevance for developing individualized prevention strategies and treatments for stress-related disorders. An important modulator of stress responses is the FK506-binding protein 51 (FKBP5/FKBP51). FKBP5 acts as a co-chaperone that modulates not only glucocorticoid receptor activity in response to stressors but also a multitude of other cellular processes in both the brain and periphery. Notably, the FKBP5 gene is regulated via complex interactions among environmental stressors, FKBP5 genetic variants, and epigenetic modifications of glucocorticoid-responsive genomic sites. These interactions can result in FKBP5 disinhibition that has been shown to contribute to a number of aberrant phenotypes in both rodents and humans. Consequently, FKBP5 blockade may hold promise as treatment intervention for stress-related disorders, and recently developed selective FKBP5 blockers show encouraging results in vitro and in rodent models. Although risk for stress-related disorders is conferred by multiple environmental and genetic factors, the findings related to FKBP5 illustrate how a deeper understanding of the molecular and systemic mechanisms underlying specific gene-environment interactions may provide insights into the pathogenesis of stress-related disorders.

Adipogenesis is under surveillance of Hsp90 and the high molecular weight Immunophilin FKBP51

Adipose tissue plays a central role in the control of energy balance as well as in the maintenance of metabolic homeostasis. It was not until recently that the first evidences of the role of heat shock protein (Hsp) 90 and high molecular weight immunophilin FKBP51 have been described in the process of adipocyte differentiation. Recent reports describe their role in the regulation of PPARγ, a key transcription factor in the control of adipogenesis and the maintenance of the adipocyte phenotype. In addition, novel roles have been uncovered for FKBP51 in the organization of the architecture of the nucleus through its participation in the reorganization of the nuclear lamina. Therefore, the aim of this review is to integrate and discuss the recent advances in the field, with special emphasis on the roles of Hsp90 and FKBP51 in the process of adipocyte differentiation.

Biological relevance of Hsp90-binding immunophilins in cancer development and treatment

Immunophilins are a family of intracellular receptors for immunosuppressive drugs. Those immunophilins that are related to immunosuppression are the smallest proteins of the family, i.e., FKBP12 and CyPA, whereas the other members of the family have higher molecular weight because the show additional domains to the drug-binding site. Among these extra domains, the TPR-domain is perhaps the most relevant because it permits the interaction of high molecular weight immunophilins with the 90-kDa heat-shock protein, Hsp90. This essential molecular chaperone regulates the biological function of several protein-kinases, oncogenes, protein phosphatases, transcription factors and cofactors . Hsp90-binding immunophilins where first characterized due to their association with steroid receptors. They regulate the cytoplasmic transport and the subcellular localization of these and other Hsp90 client proteins, as well as transcriptional activity, cell proliferation, cell differentiation and apoptosis. Hsp90-binding immunophilins are frequently overexpressed in several types of cancers and play a key role in cell survival. In this article we analyze the most important biological actions of the best characterized Hsp90-binding immunophilins in both steroid receptor function and cancer development and discuss the potential use of these immunophilins for therapeutic purposes as potential targets of specific small molecules.

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.

Control of steroid receptor dynamics and function by genomic actions of the cochaperones p23 and Bag-1L

Molecular chaperones encompass a group of unrelated proteins that facilitate the correct assembly and disassembly of other macromolecular structures, which they themselves do not remain a part of. They associate with a large and diverse set of coregulators termed cochaperones that regulate their function and specificity. Amongst others, chaperones and cochaperones regulate the activity of several signaling molecules including steroid receptors, which upon ligand binding interact with discrete nucleotide sequences within the nucleus to control the expression of diverse physiological and developmental genes. Molecular chaperones and cochaperones are typically known to provide the correct conformation for ligand binding by the steroid receptors. While this contribution is widely accepted, recent studies have reported that they further modulate steroid receptor action outside ligand binding. They are thought to contribute to receptor turnover, transport of the receptor to different subcellular localizations, recycling of the receptor on chromatin and even stabilization of the DNA-binding properties of the receptor. In addition to these combined effects with molecular chaperones, cochaperones are reported to have additional functions that are independent of molecular chaperones. Some of these functions also impact on steroid receptor action. Two well-studied examples are the cochaperones p23 and Bag-1L, which have been identified as modulators of steroid receptor activity in nuclei. Understanding details of their regulatory action will provide new therapeutic opportunities of controlling steroid receptor action independent of the widespread effects of molecular chaperones.

NF-κB transcriptional activity is modulated by FK506-binding proteins FKBP51 and FKBP52: a role for peptidyl-prolyl isomerase activity

Hsp90 binding immunophilins FKBP51 and FKBP52 modulate steroid receptor trafficking and hormone-dependent biological responses. With the purpose to expand this model to other nuclear factors that are also subject to nuclear-cytoplasmic shuttling, we analyzed whether these immunophilins modulate NF-κB signaling. It is demonstrated that FKBP51 impairs both the nuclear translocation rate of NF-κB and its transcriptional activity. The inhibitory action of FKBP51 requires neither the peptidylprolyl-isomerase activity of the immunophilin nor its association with Hsp90. The TPR domain of FKBP51 is essential. On the other hand, FKBP52 favors the nuclear retention time of RelA, its association to a DNA consensus binding sequence, and NF-κB transcriptional activity, the latter effect being strongly dependent on the peptidylprolyl-isomerase activity and also on the TPR domain of FKBP52, but its interaction with Hsp90 is not required. In unstimulated cells, FKBP51 forms endogenous complexes with cytoplasmic RelA. Upon cell stimulation with phorbol ester, the NF-κB soluble complex exchanges FKBP51 for FKBP52, and the NF-κB biological effect is triggered. Importantly, FKBP52 is functionally recruited to the promoter region of NF-κB target genes, whereas FKBP51 is released. Competition assays demonstrated that both immunophilins antagonize one another, and binding assays with purified proteins suggest that the association of RelA and immunophilins could be direct. These observations suggest that the biological action of NF-κB in different cell types could be positively regulated by a high FKBP52/FKBP51 expression ratio by favoring NF-κB nuclear retention, recruitment to the promoter regions of target genes, and transcriptional activity.

Steroid receptor coupling becomes nuclear

In this issue of Chemistry & Biology, Grossman et al. report a study on aldosterone-dependent nuclear translocation of the mineralocorticoid receptor (MR). They analyze the dependency of MR retrotransport, DNA-binding, and transcriptional activity on Hsp90 and demonstrate that MR dimerization is a nuclear event.

Rapid mineralocorticoid receptor trafficking

The mineralocorticoid receptor (MR) is a ligand-dependent transcription factor that physiologically regulates water-electrolyte homeostasis and controls blood pressure. The MR can also elicit inflammatory and remodeling processes in the cardiovascular system and the kidneys, which require the presence of additional pathological factors like for example nitrosative stress. However, the underlying molecular mechanism(s) for pathophysiological MR effects remain(s) elusive. The inactive MR is located in the cytosol associated with chaperone molecules including HSP90. After ligand binding, the MR monomer rapidly translocates into the nucleus while still being associated to HSP90 and after dissociation from HSP90 binds to hormone-response-elements called glucocorticoid response elements (GREs) as a dimer. There are indications that rapid MR trafficking is modulated in the presence of high salt, oxidative or nitrosative stress, hypothetically by induction or posttranslational modifications. Additionally, glucocorticoids and the enzyme 11beta hydroxysteroid dehydrogenase may also influence MR activation. Because MR trafficking and its modulation by micro-milieu factors influence MR cellular localization, it is not only relevant for genomic but also for nongenomic MR effects.