Roles of GR Isoforms and Hsp90-binding Immunophilins in the Modulation of Glucocorticoid Biological Responses

Glucocorticoid steroids play cardinal roles during the life span of an individual, modulating almost all aspects of the physiology, including the metabolism of carbohydrates, lipids and amino acids, as well as the immune response, neurological biology, stress adaptation, apoptosis, cell division, cell fate, inflammatory responses, etc. Glucocorticoids exert their biological effects by activation of the glucocorticoid receptor (GR), a bona fide ligand-activated transcription factor belonging to the nuclear receptor superfamily. The GR is expressed in virtually all cells of the human body showing isoformic versions and also transcription variants. GR forms oligomeric heterocomplexes that include the 90-kDa heat-shock protein (Hsp90) as an essential hub of the chaperone oligomer. The nature of chaperones associated with this heterocomplex is responsible for the modulation of the subcellular localization of the GR and its biological actions in a given tissue or cell type. In this sense, the discovery that immunophilins containing tetratricopeptide repeats (TPR) domains are responsible for the GR cytoplasmic transport mechanism and the nuclear retention half-time of the receptor opened new trends in our understanding of its complex mechanism of action. Because the properties of GR ligands influence these protein-protein interactions, specific steroid•receptor complexes may confer the GR different features providing new therapeutic opportunities to manage the disease. In this article, we analyze multiple aspects of the GR mechanism of action, some properties of the GR isoforms, and the latest findings revealing the roles of Hsp90-binding immunophilins to manage the glucocorticoid biological response.

The structure of an Hsp90-immunophilin complex reveals cochaperone recognition of the client maturation state

The Hsp90 chaperone promotes folding and activation of hundreds of client proteins in the cell through an ATP-dependent conformational cycle guided by distinct cochaperone regulators. The FKBP51 immunophilin binds Hsp90 with its tetratricopeptide repeat (TPR) domain and catalyzes peptidyl-prolyl isomerase (PPIase) activity during folding of kinases, nuclear receptors, and tau. Here we determined the cryoelectron microscopy (cryo-EM) structure of the human Hsp90:FKBP51:p23 complex to 3.3 Å, which, together with mutagenesis and crosslinking analyses, reveals the basis for cochaperone binding to Hsp90 during client maturation. A helix extension in the TPR functions as a key recognition element, interacting across the Hsp90 C-terminal dimer interface presented in the closed, ATP conformation. The PPIase domain is positioned along the middle domain, adjacent to Hsp90 client binding sites, whereas a single p23 makes stabilizing interactions with the N-terminal dimer. With this architecture, FKBP51 is positioned to act on specific client residues presented during Hsp90-catalyzed remodeling.

Plant Cyclophilins: Multifaceted Proteins With Versatile Roles

Cyclophilins constitute a family of ubiquitous proteins that bind cyclosporin A (CsA), an immunosuppressant drug. Several of these proteins possess peptidyl-prolyl cis-trans isomerase (PPIase) activity that catalyzes the cis-trans isomerization of the peptide bond preceding a proline residue, essential for correct folding of the proteins. Compared to prokaryotes and other eukaryotes studied until now, the cyclophilin gene families in plants exhibit considerable expansion. With few exceptions, the role of the majority of these proteins in plants is still a matter of conjecture. However, recent studies suggest that cyclophilins are highly versatile proteins with multiple functionalities, and regulate a plethora of growth and development processes in plants, ranging from hormone signaling to the stress response. The present review discusses the implications of cyclophilins in different facets of cellular processes, particularly in the context of plants, and provides a glimpse into the molecular mechanisms by which these proteins fine-tune the diverse physiological 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.

Cardiac ryanodine receptor distribution is dynamic and changed by auxiliary proteins and post-translational modification

The effects of the immunophilins, FKBP12 and FKBP12.6, and phosphorylation on type II ryanodine receptor (RyR2) arrangement and function were examined using correlation microscopy (line scan confocal imaging of Ca²⁺ sparks and dual-tilt electron tomography) and dSTORM imaging of permeabilized Wistar rat ventricular myocytes. Saturating concentrations (10 µmol/L) of either FKBP12 or 12.6 significantly reduced the frequency, spread, amplitude and Ca²⁺ spark mass relative to control, while the tomograms revealed both proteins shifted the tetramers into a largely side-by-side configuration. Phosphorylation of immunophilin-saturated RyR2 resulted in structural and functional changes largely comparable to phosphorylation alone. dSTORM images of myocyte surfaces demonstrated that both FKBP12 and 12.6 significantly reduced RyR2 cluster sizes, while phosphorylation, even of immunophilin-saturated RyR2, increased them. We conclude that both RyR2 cluster size and the arrangement of tetramers within clusters is dynamic and respond to changes in the cellular environment. Further, these changes affect Ca²⁺ spark formation.

Molecular determinants of Drosophila immunophilin FKBP39 nuclear localization

FK506-binding proteins (FKBPs) belong to a distinct class of immunophilins that interact with immunosuppressants. They use their peptidyl-prolyl isomerase (PPIase) activity to catalyze the cis-trans conversion of prolyl bonds in proteins during protein-folding events. FKBPs also act as a unique group of chaperones. The Drosophila melanogaster peptidyl-prolyl cis-trans isomerase FK506-binding protein of 39 kDa (FKBP39) is thought to act as a transcriptional modulator of gene expression in 20-hydroxyecdysone and juvenile hormone signal transduction. The aim of this study was to analyze the molecular determinants responsible for the subcellular distribution of an FKBP39-yellow fluorescent protein (YFP) fusion construct (YFP-FKBP39). We found that YFP-FKBP39 was predominantly nucleolar. To identify the nuclear localization signal (NLS), a series of YFP-tagged FKBP39 deletion mutants were prepared and examined in vivo. The identified NLS signal is located in a basic domain. Detailed mutagenesis studies revealed that residues K188 and K191 are crucial for the nuclear targeting of FKBP39 and its nucleoplasmin-like (NPL) domain contains the sequence that controls the nucleolar-specific translocation of the protein. These results show that FKBP39 possesses a specific NLS in close proximity to a putative helix-turn-helix (HTH) motif and FKBP39 may bind DNA in vivo and in vitro.

The spliceosomal proteins PPIH and PRPF4 exhibit bi-partite binding

Pre-mRNA splicing is a dynamic, multistep process that is catalyzed by the RNA (ribonucleic acid)-protein complex called the spliceosome. The spliceosome contains a core set of RNAs and proteins that are conserved in all organisms that perform splicing. In higher organisms, peptidyl-prolyl isomerase H (PPIH) directly interacts with the core protein pre-mRNA processing factor 4 (PRPF4) and both integrate into the pre-catalytic spliceosome as part of the tri-snRNP (small nuclear RNA-protein complex) subcomplex. As a first step to understand the protein interactions that dictate PPIH and PRPF4 function, we expressed and purified soluble forms of each protein and formed a complex between them. We found two sites of interaction between PPIH and the N-terminus of PRPF4, an unexpected result. The N-terminus of PRPF4 is an intrinsically disordered region and does not adopt secondary structure in the presence of PPIH. In the absence of an atomic resolution structure, we used mutational analysis to identify point mutations that uncouple these two binding sites and find that mutations in both sites are necessary to break up the complex. A discussion of how this bipartite interaction between PPIH and PRPF4 may modulate spliceosomal function is included.

Immunophilin ligands in peripheral nerve regeneration

Immunophilins are a family of receptors for immunosuppressive drugs like cyclo-sporin A, FK506, rapamycin and their non-immunosuppressive analogues, which are collectively referred to as immunophilin ligands. The present study aimed to review neuroprotective and neuroregenerative actions of immunophilin ligands on peripheral nerve injuries. The special emphasis was given to well-known immunosuppressive drug FK506. Its historical background, administration routes, dosages, neuroregenerative, neuroprotective actions, therapeutic time window in administration, action mechanism and side effects of FK506 were reviewed.

The Role of Crk Adaptor Proteins in T-Cell Adhesion and Migration

Crk adaptor proteins are key players in signal transduction from a variety of cell surface receptors. They are involved in early steps of lymphocyte activation through their SH2-mediated transient interaction with signal transducing effector molecules, such as Cbl, ZAP-70, CasL, and STAT5. In addition, they constitutively associate, via their SH3 domain, with effector molecules, such as C3G, that mediate cell adhesion and regulate lymphocyte extravasation and recruitment to sites of inflammation. Recent studies demonstrated that the conformation and function of CrkII is subjected to a regulation by immunophilins, which also affect CrkII-dependent T-cell adhesion to fibronectin and migration toward chemokines. This article addresses mechanisms that regulate CrkII conformation and function, in general, and emphasizes the role of Crk proteins in receptor-coupled signaling pathways that control T-lymphocyte adhesion and migration to inflammatory sites.

Role of molecular chaperones and TPR-domain proteins in the cytoplasmic transport of steroid receptors and their passage through the nuclear pore

In the absence of hormone, corticosteroid receptors such as GR (glucocorticoid receptor) and (mineralocorticoid receptor) are primarily located in the cytoplasm. Upon steroid-binding, they rapidly accumulate in the nucleus. Regardless of their primary location, these receptors and many other nuclear factors undergo a constant and dynamic nucleocytoplasmic shuttling. All members of the steroid receptor family are known to form large oligomeric structures with the heat-shock proteins of 90-kDa (hsp90) and 70-kDa (hsp70), the small acidic protein p23, and a tetratricopeptide repeat (TPR) -domain protein such as FK506-binding proteins (FKBPs), cyclophilins (CyPs) or the serine/threonine protein phosphatase 5 (PP5). It has always been stated that the dissociation of the chaperone heterocomplex (a process normally referred to as receptor "transformation") is the first step that permits the nuclear import of steroid receptors. However the experimental evidence is consistent with a model where the chaperone machinery is required for the retrotransport of the receptor through the cytoplasm and also facilitates the passage through the nuclear pore. Recent evidence indicates that the hsp90-based chaperone system also interacts with structures of the nuclear pore such as importin β and the integral nuclear pore glycoprotein Nup62 facilitating the passage of the untransformed receptor through the nuclear pore.

Chaperoning the chaperone: a role for the co-chaperone Cpr7 in modulating Hsp90 function in Saccharomyces cerevisiae

Heat-shock protein 90 (Hsp90) of Saccharomyces cerevisiae is an abundant essential eukaryotic molecular chaperone involved in the activation and stabilization of client proteins, including several transcription factors and oncogenic kinases. Hsp90 undergoes a complex series of conformational changes and interacts with partner co-chaperones such as Sba1, Cpr6, Cpr7, and Cns1 as it binds and hydrolyzes ATP. In the absence of nucleotide, Hsp90 is dimerized only at the carboxy-terminus. In the presence of ATP, Hsp90 also dimerizes at the amino-terminus, creating a binding site for Sba1. Truncation of a charged linker region of yeast Hsp90 (Hsp82Δlinker) was known to disrupt the ability of Hsp82 to undergo amino-terminal dimerization and bind Sba1. We found that yeast expressing Hsp82Δlinker constructs exhibited a specific synthetic lethal phenotype in cells lacking CPR7. The isolated tetratricopeptide repeat domain of Cpr7 was both necessary and sufficient for growth in those strains. Cpr6 and Cpr7 stably bound the carboxy-terminus of wild-type Hsp82 only in the presence of nonhydrolyzable ATP and formed an Hsp82-Cpr6-Cpr7 ternary complex. However, in cells expressing Hsp82Δlinker or lacking CPR7, Cpr6 was able to bind Hsp82 in the presence or absence of nucleotide. Overexpression of CNS1, but not of other co-chaperones, in cpr7 cells restored nucleotide-dependent Hsp82-Cpr6 interaction. Together, our results suggest that the in vivo functions of Cpr7 include modulating Hsp90 conformational changes, mediating proper signaling of the nucleotide-bound state to the carboxy-terminus of Hsp82, or regulating Hsp82-Cpr6 interaction.

Chaperones and foldases in endoplasmic reticulum stress signaling in plants

Molecular chaperones and foldases are a diverse group of proteins that in vivo bind to misfolded or unfolded proteins (non-native or unstable proteins) and play important role in their proper folding. Stress conditions compel altered and heightened chaperone and foldase expression activity in the endoplasmic reticulum (ER), which highlights the role of these proteins, due to which several of the proteins under these classes were identified as heat shock proteins. Different chaperones and foldases are active in different cellular compartment performing specific tasks. The review will discuss the role of the ER chaperones and foldases under stress conditions to maintain proper protein folding dynamics in the plant cells and recent advances in the field. The ER chaperones and foldases, which are described in article, are binding protein (BiP), glucose regulated protein (GRP94), protein-disulfide isomerase (PDI), peptidyl-prolyl isomerases (PPI), immunophilins, calnexin and calreticulin.

Altered expression and function of ryanodine receptors and FKBP12.6 after subarachnoid hemorrhage: more than meets the eye

Subarachnoid hemorrhage decreases the expression of ryanodine receptors and increases the expression of the immunophilin, FK506-binding protein (FKBP)12.6 in cerebrovascular smooth muscle cells, reducing the occurrence of Ca(2+) sparks, limiting negative feedback regulation of cerebrovascular tone and promoting vasospasm. However, given the large number of binding partners with which FKBP12.6 and related proteins interact, it appears likely that the altered expression of this immunophilin will have a broader impact on cerebrovascular smooth muscle function. In addition, the mechanisms responsible for the measured changes in expression of ryanodine receptors and FKBP12.6 yet remain unknown. These topics should prove to be fertile ground for future studies.