Yellow fluorescent protein-tagged and cyan fluorescent protein-tagged imaging analysis of glucocorticoid receptor and importins in single living cells

The Biologist's Guide to the Glucocorticoid Receptor's Structure

The glucocorticoid receptor α (GRα) is a member of the nuclear receptor superfamily and functions as a glucocorticoid (GC)-responsive transcription factor. GR can halt inflammation and kill off cancer cells, thus explaining the widespread use of glucocorticoids in the clinic. However, side effects and therapy resistance limit GR's therapeutic potential, emphasizing the importance of resolving all of GR's context-specific action mechanisms. Fortunately, the understanding of GR structure, conformation, and stoichiometry in the different GR-controlled biological pathways is now gradually increasing. This information will be crucial to close knowledge gaps on GR function. In this review, we focus on the various domains and mechanisms of action of GR, all from a structural perspective.

Fluorescence resonance energy transfer in revealing protein-protein interactions in living cells

Genes are expressed to proteins for a wide variety of fundamental biological processes at the cellular and organismal levels. However, a protein rarely functions alone, but rather acts through interactions with other proteins to maintain normal cellular and organismal functions. Therefore, it is important to analyze the protein-protein interactions to determine functional mechanisms of proteins, which can also guide to develop therapeutic targets for treatment of diseases caused by altered protein-protein interactions leading to cellular/organismal dysfunctions. There is a large number of methodologies to study protein interactions in vitro, in vivo and in silico, which led to the development of many protein interaction databases, and thus, have enriched our knowledge about protein-protein interactions and functions. However, many of these interactions were identified in vitro, but need to be verified/validated in living cells. Furthermore, it is unclear whether these interactions are direct or mediated via other proteins. Moreover, these interactions are representative of cell- and time-average, but not a single cell in real time. Therefore, it is crucial to detect direct protein-protein interactions in a single cell during biological processes in vivo, towards understanding the functional mechanisms of proteins in living cells. Importantly, a fluorescence resonance energy transfer (FRET)-based methodology has emerged as a powerful technique to decipher direct protein-protein interactions at a single cell resolution in living cells, which is briefly described in a limited available space in this mini-review.

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.

Glucocorticoid therapy and ocular hypertension

The projected number of people who will develop age-related macular degeneration in estimated at 2020 is 196 million and is expected to reach 288 million in 2040. Also, the number of people with Diabetic retinopathy will grow from 126.6 million in 2010 to 191.0 million by 2030. In addition, it is estimated that there are 2.3 million people suffering from uveitis worldwide. Because of the anti-inflammatory properties of glucocorticoids (GCs), they are often used topically and/or intravitreally to treat ocular inflammation conditions or edema associated with macular degeneration and diabetic retinopathy. Unfortunately, ocular GC therapy can lead to severe side effects. Serious and sometimes irreversible eye damage can occur as a result of the development of GC-induced ocular hypertension causing secondary open-angle glaucoma. According to the world health organization, glaucoma is the second leading cause of blindness in the world and it is estimated that 80 million will suffer from glaucoma by 2020. In the current review, mechanisms of GC-induced damage in ocular tissue, GC-resistance, and enhancing GC therapy will be discussed.

Brain Corticosteroid Receptor Dynamics and Trafficking: Implications from Live Cell Imaging

Adrenal corticosteroids (cortisol in humans or corticosterone in rodents) exert numerous effects in the central nervous system that regulate the stress response, mood, learning and memory, and various neuroendocrine functions. Corticosterone actions in the brain are mediated by two corticosteroid receptors, glucocorticoid receptor (GR) and mineralocorticoid receptor (MR), and they show a high degree of colocalization in the hippocampal region. These receptors predominantly reside in the cytoplasm without ligand and are translocated into the nucleus upon ligand binding to act as transcriptional factors. Thus, their subcellualr localizations are an important component of their biological activity. Given the differential action of MR and GR in the central nervous system, it is important to elucidate how the trafficking of these receptors between the cytoplasm and the nucleus and their interactions are regulated by ligand or other molecules to exert transcriptional activity. In this review, the authors focus on the nucleocytoplasmic and subnuclear trafficking of GR and MR in neural cells and nonneural cells and discuss various factors affecting the dynamics of these receptors.

Structural Biology and Regulation of Protein Import into the Nucleus

Proteins are translated in the cytoplasm, but many need to access the nucleus to perform their functions. Understanding how these nuclear proteins are transported through the nuclear envelope and how the import processes are regulated is therefore an important aspect of understanding cell function. Structural biology has played a key role in understanding the molecular events during the transport processes and their regulation, including the recognition of nuclear targeting signals by the corresponding receptors. Here, we review the structural basis of the principal nuclear import pathways and the molecular basis of their regulation. The pathways involve transport factors that are members of the β-karyopherin family, which can bind cargo directly (e.g., importin-β, transportin-1, transportin-3, importin-13) or through adaptor proteins (e.g., importin-α, snurportin-1, symportin-1), as well as unrelated transport factors such as Hikeshi, involved in the transport of heat-shock proteins, and NTF2, involved in the transport of RanGDP. Solenoid proteins feature prominently in these pathways. Nuclear transport factors recognize nuclear targeting signals on the cargo proteins, including the classical nuclear localization signals, recognized by the adaptor importin-α, and the PY nuclear localization signals, recognized by transportin-1. Post-translational modifications, particularly phosphorylation, constitute key regulatory mechanisms operating in these pathways.

The effect of dexamethasone on lentiviral vector infection is associated with importin α

Importin α (Imα) plays an important role during the shuttling of the HIV-1 preintegration complex (PIC) from the cytoplasm to the nucleus. Imα may bind to the glucocorticoid receptor (GR), which is localized to nucleus following hormone binding. However, it remains unclear whether the binding of dexamethasone (Dex) to GR affects the Imα redistribution and, thus, alters PIC import. In our study, 293T cells were transfected with the lentiviral vector (LV) carrying the luciferase (Luci) gene following Dex or RU486 pretreatment. The Luci activity (LucA) in the Dex or RU486 group was significantly higher compared to that in the control group (P≤0.01). The effects of Dex and RU486 were inhibited by the Imα inhibitor Bimax1 (P≤0.01), although the inhibitory effect of Bimax1 was alleviated by increasing the Dex dose. Furthermore, it was observed that the LucA in the 30-min Dex treatment group was lower compared to that in the 30-min Dex pretreatment group (P≤0.01). These results suggested that Dex may improve PIC import via increasing the cytoplasmic Imα levels. Kunming mice were transfected in vivo with the LV, either 30 min or 15 h following an intraperitoneal injection of Dex. The LucA in the liver of the 30-min group mice was significantly lower compared to that of the 15-h group mice (P≤0.01), suggesting that the effect of Dex on LV infection depends mainly on the suppression of immune and inflammatory responses in vivo. Taken together, our data indicated that the effect of Dex on LV infection may be associated with Imα, constituting a novel signaling pathway mediating the effects of Dex on HIV-1 infection.

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.

Dynamics of corticosteroid receptors: lessons from live cell imaging

Adrenal corticosteroids (cortisol in humans or corticosterone in rodents) exert numerous effects on the central nervous system that regulates the stress response, mood, learning and memory, and various neuroendocrine functions. Corticosterone (CORT) actions in the brain are mediated via two receptor systems: the glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR). It has been shown that GR and MR are highly colocalized in the hippocampus. These receptors are mainly distributed in the cytoplasm without hormones and translocated into the nucleus after treatment with hormones to act as transcriptional factors. Thus the subcellular dynamics of both receptors are one of the most important issues. Given the differential action of MR and GR in the central nervous system, it is of great consequence to clarify how these receptors are trafficked between cytoplasm and nucleus and their interactions are regulated by hormones and/or other molecules to exert their transcriptional activity. In this review, we focus on the nucleocytoplasmic and subnuclear trafficking of GR and MR in neural cells and non-neural cells analyzed by using molecular imaging techniques with green fluorescent protein (GFP) including fluorescence recovery after photobleaching (FRAP) and fluorescence resonance energy transfer (FRET), and discuss various factors affecting the dynamics of these receptors. Furthermore, we discuss the future directions of in vivo molecular imaging of corticosteroid receptors at the whole brain level.

Hormone binding and co-regulator binding to the glucocorticoid receptor are allosterically coupled

The glucocorticoid receptor initiates the cellular response to glucocorticoid steroid hormones in vertebrates. Co-regulator proteins dock to the receptor in response to hormone binding and potentiate the transcriptional activity of the receptor by modifying DNA and recruiting essential transcription factors like RNA polymerase II. Hormones and co-regulators bind at distinct sites in the ligand binding domain yet function cooperatively to mediate transcriptional control. This study reveals and quantifies energetic coupling between two binding sites using purified components. Using a library of peptides taken from co-regulator proteins, we determine the pattern of co-regulator binding to the glucocorticoid receptor ligand binding domain. We show that peptides from co-regulators differ in their effects on hormone binding and kinetics. Peptides from DAX1 and SRC1 bind with similar affinity, but DAX1 binding is coupled to hormone binding, and SRC1 is not. Mechanistic details of co-regulator binding and coupling to the hormone binding pocket are uncovered by analysis of properties endowed by mutation of a key residue in the allosteric network connecting the sites.

Imaging of transcription factor trafficking in living cells: lessons from corticosteroid receptor dynamics

Adrenal corticosteroids (cortisol in humans/corticosterone in rodents) readily enter the brain and exert markedly diverse effects, such as the stress response of target neural cells. These effects are regulated via two receptor systems, the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR), both are ligand-inducible transcription factors. GR and MR predominantly reside in the cytoplasm in the absence of corticosterone (CORT), but are quickly translocated into the nucleus upon binding CORT. Then these receptors form dimers to bind hormone responsive elements and regulate the expression of target genes. Given the different actions of MR and GR in the central nervous system, it is important to elucidate how the trafficking of these receptors between the cytoplasm and nucleus and their interaction are regulated by ligands or other molecules to exert transcriptional activity. However, these processes have still not been completely clarified. To address these issues, we have tried to observe more dynamic subcellular trafficking processes in living cells by employing a green fluorescent protein (GFP). In this chapter, we describe our recent studies of corticosteroid receptor dynamics in living cells focusing on three points: (1) time-lapse imaging of GFP-labeled corticosteroid receptors; (2) intranuclear dynamics of GFP-labeled corticosteroid receptors using the fluorescence recovery after photobleaching (FRAP) technique; and (3) the possibility of heterodimers formation using the fluorescence resonance energy transfer (FRET) technique. These studies demonstrate that GR and MR were quickly translocated from the cytoplasm to nucleus after CORT treatment. The time course of the nuclear translocation of GR and MR differed depending on the concentration of CORT. The FRAP study showed that liganded GR and MR in the nucleus were highly mobile, and not trapped by specific organelles. We detected GR-MR heterodimers, which were affected by changes in CORT concentrations in response to various hormonal milieu such as circadian rhythm and stress. Our findings may provide new insights into the dynamic status of corticosteroid receptors in living cells and the molecular basis of the regulation of stress by these receptors.

Nuclear import of the glucocorticoid receptor-hsp90 complex through the nuclear pore complex is mediated by its interaction with Nup62 and importin beta

Glucocorticoid receptor (GR) is cytoplasmic in the absence of ligand and localizes to the nucleus after steroid binding. Previous evidence demonstrated that the hsp90-based heterocomplex bound to GR is required for the efficient retrotransport of the receptor to the nuclear compartment. We examined the putative association of GR and its associated chaperone heterocomplex with structures of the nuclear pore. We found that importin beta and the integral nuclear pore glycoprotein Nup62 interact with hsp90, hsp70, p23, and the TPR domain proteins FKBP52 and PP5. Nup62 and GR were able to interact in a more efficient manner when chaperoned by the hsp90-based heterocomplex. Interestingly, the binding of hsp70 and p23 to Nup62 does not require the presence of hsp90, whereas the association of FKBP52 and PP5 is hsp90 dependent, as indicated by the results of experiments where the hsp90 function was disrupted with radicicol. The ability of both FKBP52 and PP5 to interact with Nup62 was abrogated in cells overexpressing the TPR peptide. Importantly, GR cross-linked to the hsp90 heterocomplex was able to translocate to the nucleus in digitonin-permeabilized cells treated with steroid, suggesting that GR could pass through the pore in its untransformed state.

Regional distribution of importin subtype mRNA expression in the nervous system: study of early postnatal and adult mouse

Importin-alpha and beta1 mediate the translocation of macromolecules bearing nuclear localization signals across the nuclear pore complex. Five importin-alpha isoforms have been identified in mice and six in human. Some of these importins play an important role in neural activity such as long term potentiation, but the functional differences of each isoform in the CNS are still unclear. We performed in situ hybridization (ISH) using non-isotopic probes to clarify the expression patterns of importin-alpha subtypes (alpha5, alpha7, alpha1, alpha4, alpha3) and importin-beta1 in the mouse CNS of adult and early postnatal stages. The mRNAs of the importin-alpha subtypes and importin beta1 were expressed throughout the CNS with specific patterns; importin-alpha5, alpha7, alpha3, and beta1 showed moderate to high expression levels throughout the brain and spinal cord; importin-alpha4 showed a lack of expression in limited regions; and importin-alpha1 showed a low expression level throughout the brain and spinal cord but with a moderate expression level in the olfactory bulb and reticular system. We also demonstrated that importin-alphas and beta1 mRNAs were predominantly expressed in neurons in the adult mouse brain by using double-labeling fluorescence ISH and immunohistochemistry. Moreover, importin-alphas and beta1 mRNAs were detected throughout the CNS of postnatal mice and were highly expressed in the external granule layer of the cerebellar cortex on postnatal days 0, 4, and 10. This is the first report of importin-alphas and beta1 expression throughout the CNS of adult mice, as well as in the developing brain, including cell type specific localization.

Steroid receptor signalling in the brain--lessons learned from molecular imaging

Studies with green fluorescent protein (GFP) have revealed the subcellular distribution of many steroid hormone receptors to be much more dynamic than previously thought. Fluorescence resonance energy transfer (FRET) and fluorescence recovery after photobleaching (FRAP) are powerful techniques with which to examine protein-protein interaction and the mobility of tagged proteins, respectively. FRET analysis revealed that steroid treatment (with corticosterone or testosterone) induces direct interaction of the glucocorticoid receptor (GR) and importin alpha in the cytoplasm and that, shortly after nuclear entry, the GR detaches from importin alpha. The mineralocorticoid receptor (MR) and androgen receptor (AR) show the same trafficking. Upon oestradiol treatment, ERalpha and ERbeta in the same cell are relocalised to form a discrete pattern and are localised in the same discrete cluster (subnuclear foci). FRAP analysis showed that nuclear ERalpha and ERbeta are most dynamic and mobile in the absence of the ligand, and that mobility decreases slightly after ligand treatment. Genomic as well as non-genomic actions of steroid hormones influence the cellular function of target tissues spacio-temporally.

The glucocorticoid responses are shaped by molecular chaperones

The glucocorticoid receptor is a known regulator of a variety of physiological processes. Its mode of action is well defined: upon hormone binding, it undergoes a conformational change, translocates to the nucleus and modulates the transcription of target genes. Molecular chaperones have a widely recognized role in the folding of newly made proteins, but their participation in further maturation of folded proteins to their active states and beyond tends to be underestimated. This review presents the current knowledge on how the Hsp70 and Hsp90 chaperone machines help to shape the responses to glucocorticoids. We discuss the contributions of these molecular chaperones to folding, activation, intracellular transport, transcriptional regulation, and decay of the glucocorticoid receptor.

Dynamics of glucocorticoid receptor and mineralocorticoid receptor: implications from live cell imaging studies

Adrenal corticosteroids (cortisol in humans/corticosterone in rodents) readily enter the brain and exert markedly diverse effects, such as the stress response of target neural cells. These effects are regulated via two receptor systems, the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR), both of which are ligand-inducible transcription factors. It is generally accepted that GR and MR predominantly reside in the cytoplasm in the absence of corticosterone (CORT), and are quickly translocated into the nucleus upon binding CORT. Then these receptors form dimers to bind hormone-responsive elements and regulate the expression of target genes. Given the different actions of MR and GR in the central nervous system, it is important to elucidate how the trafficking of these receptors between the cytoplasm and nucleus and their interaction are regulated by ligands or other molecules to exert transcriptional activity. However, the precise mechanisms of these processes are still not completely clarified. To address these issues, we have tried to observe more dynamic subcellular trafficking processes in living cells by employing a green fluorescent protein. In this review, we describe our recent studies of corticosteroid receptor dynamics in living cells focusing on three points: (1) the effects of a ligand, corticosteroid; (2) the carrier molecules involved in active nuclear transport, importins, and (3) the possibility of heterodimer formation. These studies demonstrate that GR and MR were quickly translocated from the cytoplasm to the nucleus after CORT treatment by associating with importin molecules. GR and MR differed in their response to the concentration of CORT in neural cells and non-neural cells. In the nuclear region, we detected GR-MR heterodimers, which were affected by changes in CORT concentrations in response to various hormonal milieus such as circadian rhythm and stress.

Importin-mediated nuclear transport in neurons

The polarized morphology of neurons poses a particular challenge to intracellular signal transduction. Local signals generated at distal sites must be retrogradely transported to the nucleus to produce persistent changes in neuronal function. Such communication of signals between distal neuronal compartments and the nucleus occurs during axon guidance, synapse formation, synaptic plasticity and following neuronal injury. Recent studies have begun to delineate a role for the active nuclear import pathway in transporting signals from axons and dendrites to the nucleus. In this pathway, soluble cargo proteins are recognized by nuclear transport carriers, called importins, which mediate their translocation from the cytoplasm into the nucleus. In neurons, importins might serve an additional function by carrying signals from distal sites to the soma.

Effect of initial subcellular localization of progesterone receptor on import kinetics and transcriptional activity

Progesterone receptors (PR) are ligand-activated transcription factors that modulate transcription by activating genes. There are two isoforms of PR, PRA and PRB. In most cell contexts, the PRA isoform is a repressor of the PRB isoform. Without hormone induction, PRA is mostly located in the nucleus whereas PRB distributes both in the nucleus and in the cytoplasm. In this paper, a new model system has been used to study the impact of initial subcellular localization, and import rate of progesterone receptor on transcriptional activity. This new model system involves using a mutant version of PRB which is found only in the cytoplasmic compartment of cells in the unliganded state, making the distribution of the receptor more homogeneous to start with compared with the previous model, wild type (wt) PRB, which has a more heterogeneous distribution (nuclear and cytoplasmic even without ligand). Import kinetics has been shown to be one of the major means by which to regulate PR transcriptional activity. Fluorescence microscopy was used to measure green fluorescent protein tagged PRB import rate into the nucleus. Luciferase reporter gene assay was used to measure transcriptional activity of PRB. In addition, a two-hybrid assay was performed to measure the interaction between PRB and importin alpha. Mutant versions of PRA and PRB with the constitutively active nuclear localization signal removed were created (PRA-NLSc mutant and PRB-NLSc mutant). These PR mutants were found to localize mainly in the cytoplasm in the absence of hormone. With addition of hormone, PR mutants translocated to the nucleus, although at a slower rate compared to wt PRB. Our results show that the activation of reporter gene transcription is proportional to the nuclear import rate of PRB-NLSc mutant, and the difference in import kinetics between wt PRB and the PRB-NLSc mutant is due to a stronger interaction of wt PRB with importin alpha. We also show that the hormone inducible NLS in PR, NLSh, is a weak nuclear localization signal even without hormone and can act as a weak hormone dependent nuclear localization signal when combined with the ligand binding domain of PR. In addition, by changing the initial subcellular localization of PRA from the nucleus to the cytoplasm, this diminished PRA's ability to act as an inhibitor of PRB.

Nuclear localization and dynamic properties of the Marek's disease virus oncogene products Meq and Meq/vIL8

Marek's disease virus (MDV) is an avian herpesvirus that causes T-cell lymphomas and immune suppression in susceptible chickens. At least one gene product, MDV Eco Q-encoded protein (Meq), is essential for the oncogenicity of MDV. Alternative splicing permits the meq gene to give rise to two major transcripts encoding proteins designated Meq and Meq/vIL8. Meq is a basic leucine zipper protein capable of modulating transcription. The Meq/vIL8 protein retains a modified leucine zipper, along with the mature receptor-binding portion of vIL8, but lacks the domain of Meq responsible for transcriptional modulation. In this report, we describe studies using fusions between either Meq or Meq/vIL8 and fluorescent proteins to characterize the distribution and properties of these products in chicken embryo fibroblasts (CEFs). Meq and Meq/vIL8 both localized to the nucleoplasm, nucleoli, and Cajal bodies of transfected cells. Similar distributions were found for fluorescent fusion proteins and native Meq or Meq/vIL8. Fluorescence recovery after photobleaching and photoactivatable green fluorescent protein revealed that Meq exhibited mobility properties similar to those of other transcription factors, while Meq/vIL8 was far less mobile. In addition, fluorescence resonance energy transfer studies indicated the formation of Meq/vIL8 homodimers in CEFs. Time lapse studies revealed the coordinated elimination of a portion of Meq and Meq/vIL8 from the nucleus. Our data provide new insight regarding the dynamic cellular properties of two forms of a herpesvirus-encoded oncoprotein and suggest that these forms may have fundamentally different functions in MDV-infected cells.

Intracellular localization and nucleocytoplasmic trafficking of steroid receptors: an overview

Subcellular compartmentalization and dynamic movements of steroid receptors are major steps in executing their transcription regulatory function. Though significant progress has been made in understanding the mechanisms underlying nuclear import of NLS-bearing proteins, our general and mechanistic understanding about the nuclear export processes has begun to emerge only recently. The discovery of most commonly utilized CRM1/exportin1 dependent nuclear export pathway is attributed to a potent nuclear export inhibitor leptomycin B that helped dissecting this and other nuclear export pathways. Simultaneously, utilization of green fluorescent protein (GFP)-tagged intracellular steroid receptors has contributed to not only resolving controversial issue of subcellular localization of unliganded hormone receptors but also provided further insight into finer details of receptor dynamics in living cells. With judicious use of leptomycin B and expression of GFP-tagged receptors in living cells, existence of exportin1/CRM1 independent pathway(s), nuclear export signals and receptors for bi-directional translocation that are unique to steroid receptor trafficking have been specified. Currently, we appear to be arriving at a consensus that steroid/nuclear receptors follow dynamic nucleocytoplasmic processes that deviate from the ones commonly utilized by majority of other proteins.

Chaperoning of glucocorticoid receptors

A multiprotein hsp90/hsp70-based chaperone machinery functions as a 'cradle-to-grave' system for regulating the steroid binding, trafficking and turnover of the glucocorticoid receptor (GR). In an ATP-dependent process where hsp70 and hsp90 act as essential chaperones and Hop, hsp40, and p23 act as nonessential co-chaperones, the machinery assembles complexes between the ligand binding domain of the GR and hsp90. During GR-hsp90 heterocomplex assembly, the hydrophobic ligand-binding cleft is opened to access by steroid, and subsequent binding of steroid within the cleft triggers a transformation of the receptor such that it engages in more dynamic cycles of assembly/disassembly with hsp90 that are required for rapid dynein-dependent translocation to the nucleus. Within the nucleus, the hsp90 chaperone machinery plays a critical role both in GR movement to transcription regulatory sites and in the disassembly of regulatory complexes as the hormone level declines. The chaperone machinery also plays a critical role in stabilization of the GR to ubiquitylation and proteasomal degradation. The initial GR interaction with hsp70 appears to be critical for the triage between hsp90 heterocomplex assembly and preservation of receptor function vs CHIP-dependent ubiquitylation and proteasomal degradation. The hsp90 chaperone machinery is ubiquitous and functionally conserved among eukaryotes, and it is possible that all physiologically significant actions of hsp90 require the hsp70-dependent assembly of client protein-hsp90 heterocomplexes.

Importin 4 Is Responsible for Ligand-independent Nuclear Translocation of Vitamin D Receptor

Vitamin D receptor (VDR) is localized in nuclei and acts as a ligand-dependent transcription factor. To clarify the molecular mechanisms underlying the nuclear translocation of VDR, we utilized an in vitro nuclear transport assay using digitonin-permeabilized semi-intact cells. In this assay, recombinant whole VDR-(4-427) and a truncated mutant VDR-(4-232) lacking the carboxyl terminus of VDR were imported to nuclei even in the absence of ligand. In contrast, VDR-(91-427) lacking the amino-terminal DNA-binding domain was not imported to nuclei in the absence of ligand, and was efficiently imported in its liganded form. These results suggested that there are two distinct mechanisms underlying the nuclear transport of VDR; ligand-dependent and -independent pathways, and that the different regions of VDR are responsible for these processes. Therefore, we performed the yeast two-hybrid screening using VDR-(4-232) as the bait to explore the molecules responsible for ligand-independent nuclear translocation of VDR, and have identified importin 4 as an interacting protein. In the reconstruction experiments where transport factors were applied as recombinant proteins, recombinant importin 4 facilitated nuclear translocation of VDR regardless of its ligand, whereas importin beta failed in transporting VDR even in the presence of ligand. In conclusion, importin 4, not importin beta, is responsible for the ligand-independent nuclear translocation of VDR.

Role of hsp90 and the hsp90-binding immunophilins in signalling protein movement

The ubiquitous protein chaperone hsp90 has been shown to regulate more than 100 proteins involved in cellular signalling. These proteins are called 'client proteins' for hsp90, and a multiprotein hsp90/hsp70-based chaperone machinery forms client protein.hsp90 heterocomplexes in the cytoplasm and the nucleus. In the case of signalling proteins that act as transcription factors, the client protein.hsp90 complexes also contain one of several TPR domain immunophilins or immunophilin homologs that bind to a TPR domain binding site on hsp90. Using several intracellular receptors and the tumor suppressor p53 as examples, we review evidence that dynamic assembly of heterocomplexes with hsp90 is required for rapid movement through the cytoplasm to the nucleus along microtubular tracks. The role of the immunophilin in this system is to connect the client protein.hsp90 complex to cytoplasmic dynein, the motor protein for retrograde movement toward the nucleus. Upon arrival at the nuclear pores, the receptor.hsp90.immunophilin complexes are transferred to the nuclear interior by importin-dependent facilitated diffusion. The unliganded receptors then distribute within the nucleus to diffuse patches from which they proceed in a ligand-dependent manner to discrete nuclear foci where chromatin binding occurs. We review evidence that dynamic assembly of heterocomplexes with hsp90 is required for movement to these foci and for the dynamic exchange of transcription factors between chromatin and the nucleoplasm.

Imaging analysis of mineralocorticoid receptor and importins in single living cells by using GFP color variants

Mineralocorticoid receptor (MR) is a ligand-dependent transcription factor involved in gene regulation in association with another corticosteroid receptor, glucocorticoid receptor. In the absence of ligand, MR resides both in the cytoplasm and in the nucleus. Agonists increase the number of MRs residing in the nucleus. Importins are docking proteins for karyopherin-mediated binding of substrate in a nuclear import pathway. To investigate the interactions between MR and importins, we analyzed the subcellular distribution of MR and importins in response to ligand in living COS-1 cells, which do not express endogenous MR, by using fusion proteins labeled with different spectral variants of green fluorescent protein. In the cells coexpressing fluorescent protein-tagged (FP)-MR and FP-importin alpha, the proteins simultaneously moved into the nucleus from the cytoplasm upon activation with ligand treatment. In the cells coexpressing FP-MR and FP-importin beta, FP-MR moved into the nucleus from the cytoplasm, but the distribution of FP-importin beta was little changed upon ligand treatment. Analysis of a mutant of MR, in which nuclear localization signal (NLS) is inactivated, demonstrated that the intact NLS is necessary for the trafficking of MR related to importin alpha. This is the first visual evidence of the nuclear import of MR in association with importin alpha in single living cells.

Visualization of glucocorticoid receptor and mineralocorticoid receptor interactions in living cells with GFP-based fluorescence resonance energy transfer

Adrenal corticosteroids readily enter the brain and exert markedly diverse effects, including stress responses in the target neural cells via two receptor systems, the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR). It has been shown that the GR and MR are highly colocalized in the hippocampus. Given the differential action of the MR and GR in the hippocampal region, it is important to elucidate how these receptors interact with each other in response to corticosteroids. We investigated the heterodimerization of the MR and GR with green fluorescent protein-based fluorescence resonance energy transfer (FRET) microscopy in living cells with spatiotemporal manner. FRET was evaluated in three ways: (1) ratio imaging; (2) emission spectra; and (3) acceptor photobleaching. FRET analysis demonstrated that cyan fluorescent protein-GR and yellow fluorescent protein-MR form heterodimers after corticosterone (CORT) treatment both in the nucleus of cultured hippocampal neurons and COS-1 cells, whereas they do not form heterodimers in the cytoplasm. The content of the GR-MR heterodimer was higher at 10(-6) m CORT than at 10(-9) m CORT and reached a maximum level after 60 min of CORT treatment in both cultured hippocampal neurons and COS-1 cells. The distribution pattern of heterodimers in the nucleus of cultured hippocampal neurons was more restricted than that in COS-1 cells. The present study using mutant fusion proteins in nuclear localization signal showed that these corticosteroid receptors are not translocated into the nucleus in the form of heterodimers even after treatment with ligand and thus allow no heterodimerization to take place in the cytoplasm. These results obtained with FRET analyses give new insights into the sites, time course, and effects of ligand concentration on heterodimersization of the GR and MR.

Shuttling components of nuclear import machinery involved in nuclear translocation of steroid receptors exit nucleus via exportin-1/CRM-1* independent pathway

The nucleocytoplasmic transport processes are mediated by soluble transport factors constantly navigating between nuclear and cytoplasmic compartments. Our understanding about nuclear export of general 'nuclear import factors' that deliver the cargo to the nucleus is still fragmentary. Utilizing green fluorescent protein tagged glucocorticoid receptor (GR) and relA as our working model and with judicious use of LMB, we show in living cells that all the soluble components of the nuclear import machinery exit nucleus via exportin1/CRM1 independent pathway(s).