GCUNC45 is the first Hsp90 co-chaperone to show alpha/beta isoform specificity

The Hsp90β Isoform: An Attractive Target for Drug Development

The beta isoform of 90 kDa heat shock protein (Hsp90β) plays a critical role in maintaining cellular proteostasis by assisting in the folding and refolding of proteins, which is essential for both normal cellular function and stress response. It is constitutively expressed in mammalian cells, differentiating it from the inducible Hsp90α isoform. Hsp90β's involvement in diverse cellular processes, such as signal transduction, cell cycle control, and apoptosis, underscores its significant role in various diseases, including cancer and neurodegenerative disorders. The isoform-specific functions of Hsp90β and its interaction with unique client proteins make it a promising target for therapeutic intervention, particularly in the development of selective inhibitors that avoid the adverse effects observed with pan-Hsp90 inhibitors. This review delves into the structural and functional intricacies of Hsp90β, its role in disease, and the potential for selective drug development.

Advances in the structures, mechanisms and targeting of molecular chaperones

Molecular chaperones, a class of complex client regulatory systems, play significant roles in the prevention of protein misfolding and abnormal aggregation, the modulation of protein homeostasis, and the protection of cells from damage under constantly changing environmental conditions. As the understanding of the biological mechanisms of molecular chaperones has increased, their link with the occurrence and progression of disease has suggested that these proteins are promising targets for therapeutic intervention, drawing intensive interest. Here, we review recent advances in determining the structures of molecular chaperones and heat shock protein 90 (HSP90) chaperone system complexes. We also describe the features of molecular chaperones and shed light on the complicated regulatory mechanism that operates through interactions with various co-chaperones in molecular chaperone cycles. In addition, how molecular chaperones affect diseases by regulating pathogenic proteins has been thoroughly analyzed. Furthermore, we focus on molecular chaperones to systematically discuss recent clinical advances and various drug design strategies in the preclinical stage. Recent studies have identified a variety of novel regulatory strategies targeting molecular chaperone systems with compounds that act through different mechanisms from those of traditional inhibitors. Therefore, as more novel design strategies are developed, targeting molecular chaperones will significantly contribute to the discovery of new potential drugs.

Despite their structural similarities, the cytosolic isoforms of human Hsp90 show different behaviour in thermal unfolding due to their conformation: An FTIR study

Heat shock proteins 90 (Hsp90) are chaperones that promote the proper folding of other proteins under high temperature stress situations. Hsp90s are highly conserved and ubiquitous proteins, and in mammalian cells, they are localized in the cytoplasm, endoplasmic reticulum, and mitochondria. Cytoplasmic Hsp90 are named Hsp90α and Hsp90β and differ mainly in their expression pattern: Hsp90α is expressed under stress conditions, while Hsp90β is a constitutive protein. Structurally, both share the same characteristics by presenting three well-conserved domains, one of which, the N-terminal domain, has a binding site for ATP to which various drugs targeting this protein, including radicicol, can bind. The protein is mainly found in dimeric form and adopts different conformations depending on the presence of ligands, co-chaperones and client proteins. In this study, some aspects of structure and thermal unfolding of cytoplasmic human Hsp90 were analysed by infrared spectroscopy. The effect on Hsp90β of binding with a non-hydrolysable ATP analogue and radicicol was also examined. The results obtained showed that despite the high similarity in secondary structure the two isoforms exhibit substantial differences in their behaviour during thermal unfolding, as Hsp90α exhibits higher thermal stability, slower denaturation process and different event sequence during unfolding. Ligand binding strongly stabilizes Hsp90β and slightly modifies the secondary structure of the protein as well. Most likely, these structural and thermostability characteristics are closely related to the conformational cycling of the chaperone and its propensity to exist in monomer or dimer form.

Beyond Chaperoning: UCS Proteins Emerge as Regulators of Myosin-Mediated Cellular Processes

The UCS (UNC-45/CRO1/She4p) family of proteins has emerged as chaperones specific for the folding, assembly, and function of myosin. UCS proteins participate in various myosin-dependent cellular processes including myofibril organization and muscle functions, cell differentiation, striated muscle development, cytokinesis, and endocytosis. Mutations in the genes that code for UCS proteins cause serious defects in myosin-dependent cellular processes. UCS proteins that contain an N-terminal tetratricopeptide repeat (TPR) domain are called UNC-45. Vertebrates usually possess two variants of UNC-45, the ubiquitous general-cell UNC-45 (UNC-45A) and the striated muscle UNC-45 (UNC-45B), which is exclusively expressed in skeletal and cardiac muscles. Except for the TPR domain in UNC-45, UCS proteins comprise of several irregular armadillo (ARM) repeats that are organized into a central domain, a neck region, and the canonical C-terminal UCS domain that functions as the chaperoning module. With or without TPR, UCS proteins form linear oligomers that serve as scaffolds that mediate myosin folding, organization into myofibrils, repair, and motility. This chapter reviews emerging functions of these proteins with a focus on UNC-45 as a dedicated chaperone for folding, assembly, and function of myosin at protein and potentially gene levels. Recent experimental evidences strongly support UNC-45 as an absolute regulator of myosin, with each domain of the chaperone playing different but complementary roles during the folding, assembly, and function of myosin, as well as recruiting Hsp90 as a co-chaperone to optimize key steps. It is becoming increasingly clear that UNC-45 also regulates the transcription of several genes involved in myosin-dependent cellular processes.

Mutations in Hsp90 Cochaperones Result in a Wide Variety of Human Disorders

The Hsp90 molecular chaperone, along with a set of approximately 50 cochaperones, mediates the folding and activation of hundreds of cellular proteins in an ATP-dependent cycle. Cochaperones differ in how they interact with Hsp90 and their ability to modulate ATPase activity of Hsp90. Cochaperones often compete for the same binding site on Hsp90, and changes in levels of cochaperone expression that occur during neurodegeneration, cancer, or aging may result in altered Hsp90-cochaperone complexes and client activity. This review summarizes information about loss-of-function mutations of individual cochaperones and discusses the overall association of cochaperone alterations with a broad range of diseases. Cochaperone mutations result in ciliary or muscle defects, neurological development or degeneration disorders, and other disorders. In many cases, diseases were linked to defects in established cochaperone-client interactions. A better understanding of the functional consequences of defective cochaperones will provide new insights into their functions and may lead to specialized approaches to modulate Hsp90 functions and treat some of these human disorders.

Human Hsp90 cochaperones: perspectives on tissue-specific expression and identification of cochaperones with similar in vivo functions

The Hsp90 molecular chaperone is required for the function of hundreds of different cellular proteins. Hsp90 and a cohort of interacting proteins called cochaperones interact with clients in an ATP-dependent cycle. Cochaperone functions include targeting clients to Hsp90, regulating Hsp90 ATPase activity, and/or promoting Hsp90 conformational changes as it progresses through the cycle. Over the last 20 years, the list of cochaperones identified in human cells has grown from the initial six identified in complex with steroid hormone receptors and protein kinases to about fifty different cochaperones found in Hsp90-client complexes. These cochaperones may be placed into three groups based on shared Hsp90 interaction domains. Available evidence indicates that cochaperones vary in client specificity, abundance, and tissue distribution. Many of the cochaperones have critical roles in regulation of cancer and neurodegeneration. A more limited set of cochaperones have cellular functions that may be limited to tissues such as muscle and testis. It is likely that a small set of cochaperones are part of the core Hsp90 machinery required for the folding of a wide range of clients. The presence of more selective cochaperones may allow greater control of Hsp90 activities across different tissues or during development.

Structures of Hsp90α and Hsp90β bound to a purine-scaffold inhibitor reveal an exploitable residue for drug selectivity

Hsp90α and Hsp90β are implicated in a number of cancers and neurodegenerative disorders but the lack of selective pharmacological probes confounds efforts to identify their individual roles. Here, we analyzed the binding of an Hsp90α-selective PU compound, PU-11-trans, to the two cytosolic paralogs. We determined the co-crystal structures of Hsp90α and Hsp90β bound to PU-11-trans, as well as the structure of the apo Hsp90β NTD. The two inhibitor-bound structures reveal that Ser52, a nonconserved residue in the ATP binding pocket in Hsp90α, provides additional stability to PU-11-trans through a water-mediated hydrogen-bonding network. Mutation of Ser52 to alanine, as found in Hsp90β, alters the dissociation constant of Hsp90α for PU-11-trans to match that of Hsp90β. Our results provide a structural explanation for the binding preference of PU inhibitors for Hsp90α and demonstrate that the single nonconserved residue in the ATP-binding pocket may be exploited for α/β selectivity.

The co-chaperone UNC45A is essential for the expression of mitotic kinase NEK7 and tumorigenesis

Cumulative evidence suggests that the heat shock protein 90 (Hsp90) co-chaperone UNC-45 myosin chaperone A (UNC45A) contributes to tumorigenesis and that its expression in cancer cells correlates with proliferation and metastasis of solid tumors. However, the molecular mechanism by which UNC45A regulates cancer cell proliferation remains largely unknown. Here, using siRNA-mediated gene silencing and various human cells, we report that UNC45A is essential for breast cancer cell growth, but is dispensable for normal cell proliferation. Immunofluorescence microscopy, along with gene microarray and RT-quantitative PCR analyses, revealed that UNC45A localizes to the cancer cell nucleus, where it up-regulates the transcriptional activity of the glucocorticoid receptor and thereby promotes expression of the mitotic kinase NIMA-related kinase 7 (NEK7). We observed that UNC45A-deficient cancer cells exhibit extensive pericentrosomal material disorganization, as well as defects in centrosomal separation and mitotic chromosome alignment. Consequently, these cells stalled in metaphase and cytokinesis and ultimately underwent mitotic catastrophe, phenotypes that were rescued by heterologous NEK7 expression. Our results identify a key role for the co-chaperone UNC45A in cell proliferation and provide insight into the regulatory mechanism. We propose that UNC45A represents a promising new therapeutic target to inhibit cancer cell growth in solid tumor types.

Interaction of the middle domains stabilizes Hsp90α dimer in a closed conformation with high affinity for p23

The human genome encodes two highly similar cytosolic Hsp90 proteins called isoforms Hsp90α and Hsp90β. Of the 300 client proteins for Hsp90 identified so far only a handful interact specifically with one Hsp90 isoform. Here we report for the first time that Hsp90 cochaperone p23 binds preferentially to Hsp90α and that this interaction is mediated by the middle domain of Hsp90α. Based on the homology modeling, we infer that the middle domains in the Hsp90α dimer bind stronger with each other than in the Hsp90β dimer. Therefore, compared to Hsp90β, Hsp90α may adopt closed conformation more easily. Hsp90 interacts with p23 in the closed conformation. Hsp90α binds human recombinant p23 about three times stronger than Hsp90β but with significantly smaller exothermic enthalpy as determined by isothermal titration calorimetry of direct binding between the purified proteins. As p23 binds to Hsp90 in a closed conformation, stabilization of the Hsp90α dimer in the closed conformation by its middle domains explains preference of p23 to this Hsp90 isoform.

Chaperone Activity and Dimerization Properties of Hsp90α and Hsp90β in Glucocorticoid Receptor Activation by the Multiprotein Hsp90/Hsp70-Dependent Chaperone Machinery

Several hundred proteins cycle into heterocomplexes with a dimer of the chaperone heat shock protein 90 (Hsp90), regulating their activity and turnover. There are two isoforms of Hsp90, Hsp90α and Hsp90β, and their relative chaperone activities and composition in these client protein•Hsp90 heterocomplexes has not been determined. Here, we examined the activity of human Hsp90α and Hsp90β in a purified five-protein chaperone machinery that assembles glucocorticoid receptor (GR)•Hsp90 heterocomplexes to generate high-affinity steroid-binding activity. We found that human Hsp90α and Hsp90β have equivalent chaperone activities, and when mixed together in this assay, they formed only GR•Hsp90αα and GR•Hsp90ββ homodimers and no GR•Hsp90αβ heterodimers. In contrast, GR•Hsp90 heterocomplexes formed in human embryonic kidney (HEK) cells also contain GR•Hsp90αβ heterodimers. The formation of GR•Hsp90αβ heterodimers in HEK cells probably reflects the longer time permitted for exchange to form Hsp90αβ heterodimers in the cell versus in the cell-free assembly conditions. This purified GR-activating chaperone machinery can be used to determine how modifications of Hsp90 affect its chaperone activity. To that effect, we have tested whether the unique phosphorylation of Hsp90α at threonines 5 and 7 that occurs during DNA damage repair affects its chaperone activity. We showed that the phosphomimetic mutant Hsp90α T5/7D has the same intrinsic chaperone activity as wild-type human Hsp90α in activation of GR steroid-binding activity by the five-protein machinery, supporting the conclusion that T5/7 phosphorylation does not affect Hsp90α chaperone activity.

UCS protein function is partially restored in the Saccharomyces cerevisiae she4 mutant with expression of the human UNC45-GC, but not UNC45-SM

A dedicated UNC45, Cro1, She4 (UCS) domain-containing protein assists in the Hsp90-mediated folding of the myosin head. Only weak sequence conservation exists between the single UCS protein of simple eukaryotes (She4 in budding yeast) and the two UCS proteins of higher organisms (the general cell and striated muscle UNC45s; UNC45-GC and UNC45-SM, respectively). In vertebrates, UNC45-GC facilitates cytoskeletal functions, whereas the 55% identical UNC45-SM assists assembly of the contractile apparatus of cardiac and skeletal muscles. A Saccharomyces cerevisiae she4Δ mutant, totally lacking any UCS protein, was engineered to express as its sole Hsp90 either the Hsp90α or the Hsp90β isoforms of human cytosolic Hsp90. A transient induction of the human UNC45-GC, but not UNC45-SM, could rescue the defective endocytosis in these she4Δ cells at 39 °C, irrespective of whether they possessed Hsp90α or Hsp90β. UNC45-GC-mediated rescue of the localisation of a Myo5-green fluorescent protein (GFP) fusion to cortical patches at 39 °C was more efficient in the yeast containing Hsp90α, though this may relate to more efficient functioning of Hsp90α as compared to Hsp90β in these strains. Furthermore, inducible expression of UNC45-GC, but not UNC45-SM, could partially rescue survival at a more extreme temperature (45 °C) that normally causes she4Δ mutant yeast cells to lyse. The results indicate that UCS protein function has been most conserved-yeast to man-in the UNC45-GC, not UNC45-SM. This may reflect UNC45-GC being the vertebrate UCS protein that assists formation of the actomyosin complexes needed for cytokinesis, cell morphological change, and organelle trafficking-events also facilitated by the myosins in yeast.

UNC-45a promotes myosin folding and stress fiber assembly

Contractile actomyosin bundles, stress fibers, are crucial for adhesion, morphogenesis, and mechanosensing in nonmuscle cells. However, the mechanisms by which nonmuscle myosin II (NM-II) is recruited to those structures and assembled into functional bipolar filaments have remained elusive. We report that UNC-45a is a dynamic component of actin stress fibers and functions as a myosin chaperone in vivo. UNC-45a knockout cells display severe defects in stress fiber assembly and consequent abnormalities in cell morphogenesis, polarity, and migration. Experiments combining structured-illumination microscopy, gradient centrifugation, and proteasome inhibition approaches revealed that a large fraction of NM-II and myosin-1c molecules fail to fold in the absence of UNC-45a. The remaining properly folded NM-II molecules display defects in forming functional bipolar filaments. The C-terminal UNC-45/Cro1/She4p domain of UNC-45a is critical for NM-II folding, whereas the N-terminal tetratricopeptide repeat domain contributes to the assembly of functional stress fibers. Thus, UNC-45a promotes generation of contractile actomyosin bundles through synchronized NM-II folding and filament-assembly activities.

Substrate specificity in the context of molecular chaperones

Molecular chaperones are one of the key players in protein biology and as such their structure and mechanism of action have been extensively studied. However the substrate specificity of molecular chaperones has not been well investigated. This review aims to summarize what is known about the substrate specificity and substrate recognition motifs of chaperones so as to better understand what substrate specificity means in the context of molecular chaperones. Available literature shows that the majority of chaperones have broad substrate range and recognize non-native conformations of proteins depending on recognition of hydrophobic and/or charged patches. Based on these recognition motifs chaperones can select for early, mid or late folding intermediates. Another major contributor to chaperone specificity are the co-chaperones they interact with as well as the sub-cellular location they are expressed in and the inducability of their expression. Some chaperones which have only one or a few known substrates are reported. In their case the mode of recognition seems to be specific structural complementarity between chaperone and substrate. It can be concluded that the vast majority of chaperones do not show a high degree of specificity but recognize elements that signal non-native protein conformation and their substrate range is modulated by the context they function in. However a few chaperones are known that display exquisite specificity of their substrate e.g. mammalian heat shock protein 47 collagen interaction. © 2017 IUBMB Life, 69(9):647-659, 2017.

Mutation of the Ser18 phosphorylation site on the sole Saccharomyces cerevisiae UCS protein, She4, can compromise high-temperature survival

Folding of the myosin head often requires the joint actions of Hsp90 and a dedicated UNC45, Cro1, She4 (UCS) domain-containing cochaperone protein. Relatively weak sequence conservation exists between the single UCS protein of simple eukaryotes (She4 in budding yeast) and the two UCS proteins of higher organisms (the general cell and smooth muscle UNC45s; UNC45-GC and UNC45-SM respectively). In vertebrates, UNC45-GC facilitates cytoskeletal function whereas the 55% identical UNC45-SM assists in the assembly of the contractile apparatus of cardiac and skeletal muscles. UNC45-SM, unlike UNC45-GC, shares with yeast She4 an IDSL sequence motif known to be a site of in vivo serine phosphorylation in yeast. Investigating this further, we found that both a non-phosphorylatable (S18A) and a phosphomimetic (S18E) mutant form of She4 could rescue the type 1 myosin localisation and endocytosis defects of the yeast she4Δ mutant at 39 °C. Nevertheless, at higher temperature (45 °C), only She4 (S18A), not She4(S18E), could substantially rescue the cell lysis defect of she4Δ mutant cells. In the yeast two-hybrid system, the non-phosphorylatable S18A and S251A mutant forms of She4 and UNC45-SM still displayed the stress-enhanced in vivo interaction with Hsp90 seen with the wild-type She4 and UNC45-SM. Such high-temperature enforcement to interaction was though lost with the phosphomimetic mutant forms (She4(S18E) and UNC45-SM (S251E)), an indication that phosphorylation might suppress these increases in She4/Hsp90 and UNC45-SM/Hsp90 interaction with stress.

A Remodeled Hsp90 Molecular Chaperone Ensemble with the Novel Cochaperone Aarsd1 Is Required for Muscle Differentiation

Hsp90 is the ATP-consuming core component of a very abundant molecular chaperone machine that handles a substantial portion of the cytosolic proteome. Rather than one machine, it is in fact an ensemble of molecular machines, since most mammalian cells express two cytosolic isoforms of Hsp90 and a subset of up to 40 to 50 cochaperones and regulate their interactions and functions by a variety of posttranslational modifications. We demonstrate that the Hsp90 ensemble is fundamentally remodeled during muscle differentiation and that this remodeling is not just a consequence of muscle differentiation but possibly one of the drivers to accompany and to match the vast proteomic changes associated with this process. As myoblasts differentiate into myotubes, Hsp90α disappears and only Hsp90β remains, which is the only isoform capable of interacting with the novel muscle-specific Hsp90 cochaperone Aarsd1L. Artificially maintaining Hsp90α or knocking down Aarsd1L expression interferes with the differentiation of C2C12 myotubes. During muscle differentiation, Aarsd1L replaces the more ubiquitous cochaperone p23 and in doing so dampens the activity of the glucocorticoid receptor, one of the Hsp90 clients relevant to muscle functions. This cochaperone switch protects muscle cells against the inhibitory effects of glucocorticoids and may contribute to preventing muscle wasting induced by excess glucocorticoids.

The first report of direct inhibitors that target the C-terminal MEEVD region on heat shock protein 90

Sixteen linear and cyclic peptides were designed de novo to target the C-terminus of heat shock protein 90 (Hsp90). Protein binding data indicates that three compounds directly block co-chaperone access to Hsp90's C-terminus and luciferase renaturation assays confirm Hsp90-mediated protein folding is disrupted. This is the first report of an inhibitor that binds directly to the C-terminal MEEVD region of Hsp90.

UCS proteins: chaperones for myosin and co-chaperones for Hsp90

The UCS (UNC-45/CRO1/She4p) family of proteins has emerged as chaperones that are specific for the folding, assembly and function of myosin. These proteins participate in various important myosin-dependent cellular processes that include myofibril organization and muscle functions, cell differentiation, cardiac and skeletal muscle development, cytokinesis and endocytosis. Mutations in the genes that code for UCS proteins cause serious defects in these actomyosin-based processes. Homologs of UCS proteins can be broadly divided into (1) animal UCS proteins, generally known as UNC-45 proteins, which contain an N-terminal tetratricopeptide repeat (TPR) domain in addition to the canonical UCS domain, and (2) fungal UCS proteins, which lack the TPR domain. Structurally, except for TPR domain, both sub-classes of UCS proteins comprise of several irregular armadillo (ARM) repeats that are divided into two-domain architecture: a combined central-neck domain and a C-terminal UCS domain. Structural analyses suggest that UNC-45 proteins form elongated oligomers that serve as scaffolds to recruit Hsp90 and/or Hsp70 to form a multi-protein chaperoning complex that assists myosin heads to fold and simultaneously organize them into myofibrils. Similarly, fungal UCS proteins may dimerize to promote folding of non-muscle myosins as well as determine their step size along actin filaments. These findings confirm UCS proteins as a new class of myosin-specific chaperones and co-chaperones for Hsp90. This chapter reviews the implications of the outcome of studies on these proteins in cellular processes such as muscle formation, and disease states such as myopathies and cancer.

Middle domain of human Hsp90 isoforms differentially binds Aha1 in human cells and alters Hsp90 activity in yeast

Hsp90 is an essential chaperone for more than 200 client proteins in eukaryotic cells. The human genome encodes two highly similar cytosolic Hsp90 proteins called Hsp90α and Hsp90β. Most of the client proteins can interact with either Hsp90 protein; however, only a handful client proteins and one co-chaperone that interact specifically with one of the Hsp90 isoforms were identified. Structural differences underlying these isoform-specific interactions were not studied. Here we report for the first time that the Hsp90 co-chaperone Aha1 interacts preferentially with Hsp90α. The distinction depends on the middle domain of Hsp90. The middle domain of Hsp90α is also responsible for the slow growth phenotype of yeasts that express this isoform as a sole source of Hsp90. These results suggest that differences in the middle domain of Hsp90α and Hsp90β may be responsible for the isoform-specific interactions with selected proteins. Also shown here within, we determine that preferential chaperoning of cIAP1 by Hsp90β is mediated by the N-terminal domain of this isoform.

Correlation between HSP90α expression and invasion and metastasis of esophageal cancer cells

AIM: To investigate the correlation between heat shock protein 90α (HSP90α) expression and invasion and metastasis of esophageal cell carcinoma.

METHODS: The differential expression of HSP90α mRNA and protein was tested by RT-PCR and Western blot, respectively, in esophageal cell carcinoma cell lines with different invasive ability (CE81T-0 and CE81T-4). An siRNA targeting HSP90α was transfected into CE81T-4 cells, and cells transfected with a negative control and non-transfected cells were used as controls. The expression of HSP90α mRNA and protein was tested by RT-PCR and Western blot in these three groups of cells. The viability of cells was examined by MTT assay; cell cycle and apoptosis were analyzed by flow cytometry; and the changes in migration and invasion of CE81T-4 cells after transfection were tested by scratch assay and Transwell assay.

RESULTS: The expression levels of HSP90α mRNA and protein in CE81T-4 cells were significantly higher than those in CE81T-0 cells (P < 0.05). After HSP90α siRNA transfection in EC81T-4 cells, RT-PCR and Western blot analyses showed that the levels of HSP90α mRNA and protein expression were significantly lower in the experimental than in the blank control and negative control group (P < 0.05). MTT assay showed that cell proliferation ability in the experimental group was significantly lower compared with the other two groups (P < 0.05). Cell apoptosis rate was significantly higher in the experimental group compared with other two groups (P < 0.05). HSP90α siRNA transfection resulted in cell cycle arrest in G₀/G₁ phase. Transwell invasion assay indicated that number of cells penetrating the polycarbonate membrane was significantly lower in the experimental group than in the other two groups (P < 0.05). In addition, the migration ability of cells in the experimental group was significantly lower compared with the other two groups.

CONCLUSION: Down-regulation of expression of HSP90α may reduce invasion and metastasis of esophageal cell carcinoma cells.

A quantitative chaperone interaction network reveals the architecture of cellular protein homeostasis pathways

Chaperones are abundant cellular proteins that promote the folding and function of their substrate proteins (clients). In vivo, chaperones also associate with a large and diverse set of cofactors (cochaperones) that regulate their specificity and function. However, how these cochaperones regulate protein folding and whether they have chaperone-independent biological functions is largely unknown. We combined mass spectrometry and quantitative high-throughput LUMIER assays to systematically characterize the chaperone-cochaperone-client interaction network in human cells. We uncover hundreds of chaperone clients, delineate their participation in specific cochaperone complexes, and establish a surprisingly distinct network of protein-protein interactions for cochaperones. As a salient example of the power of such analysis, we establish that NUDC family cochaperones specifically associate with structurally related but evolutionarily distinct β-propeller folds. We provide a framework for deciphering the proteostasis network and its regulation in development and disease and expand the use of chaperones as sensors for drug-target engagement.

The UNC-45 myosin chaperone: from worms to flies to vertebrates

UNC-45 (uncoordinated mutant number 45) is a UCS (UNC-45, CRO1, She4p) domain protein that is critical for myosin stability and function. It likely aides in folding myosin during cellular differentiation and maintenance, and protects myosin from denaturation during stress. Invertebrates have a single unc-45 gene that is expressed in both muscle and nonmuscle tissues. Vertebrates possess one gene expressed in striated muscle (unc-45b) and another that is more generally expressed (unc-45a). Structurally, UNC-45 is composed of a series of α-helices connected by loops. It has an N-terminal tetratricopeptide repeat domain that binds to Hsp90 and a central domain composed of armadillo repeats. Its C-terminal UCS domain, which is also comprised of helical armadillo repeats, interacts with myosin. In this chapter, we present biochemical, structural, and genetic analyses of UNC-45 in Caenorhabditis elegans, Drosophila melanogaster, and various vertebrates. Further, we provide insights into UNC-45 functions, its potential mechanism of action, and its roles in human disease.

Intra-renal transfection of heat shock protein 90 alpha or beta (Hsp90α or Hsp90β) protects against ischemia/reperfusion injury

BACKGROUND

We previously reported that radicicol (Hsp90 inhibitor) induced a reduction in the renal blood flow and glomerular filtration rate, in part due to a reduction in urinary NO2/NO3 excretion, suggesting that Hsp90 regulates renal vascular tone in physiological conditions. However, there is a lack of information concerning Hsp90α or Hsp90β role on eNOS activity and their association with acute kidney injury (AKI) characterized by an inadequate NO production. This study evaluated the effects of Hsp90α or Hsp90β intra-renal transfection under ischemia/reperfusion (IR) injury.

METHODS

Uninephrectomized (Nx) rats were intra-renally transfected through injections with Hsp90α or Hsp90β cloned into pcDNA3.1(+) or empty vector (EV) at 48 h before inducing IR, as indicated in the following groups: (i) Nx+sham, (ii) Nx+IR, (iii) Nx+IR+EV, (iv) Nx+IR+Hsp90α and (v) Nx+IR+Hsp90β. After 24 h, physiological, histopathological, biochemical and molecular studies were performed.

RESULTS

IR-induced renal dysfunction, structural injury, tubular proliferation, the elevation of urinary Hsp72 and the reduction of urinary NO2/NO3 excretion. These alterations were associated with reduced eNOS-Hsp90 coupling and changes in the eNOS phosphorylation state mediated through a reduction in PKCα and increased Rho kinase expression. In contrast, intra-renal transfection of Hsp90α or Hsp90β prevented IR injury that was associated with the restoration of eNOS-Hsp90 coupling, eNOS activating phosphorylation and PKCα and Rho kinase levels.

CONCLUSIONS

Here we showed that eNOS-Hsp90 uncoupling plays a critical role in promoting NO reduction during IR. This effect was effectively reversed through Hsp90α or Hsp90β intra-renal transfection, suggesting their implication in regulating NO/eNOS pathway and the renal vascular tone.

Lack of developmental redundancy between Unc45 proteins in zebrafish muscle development

Since the majority of protein-coding genes in vertebrates have intra-genomic homologues, it has been difficult to eliminate the potential of functional redundancy from analyses of mutant phenotypes, whether produced by genetic lesion or transient knockdown. Further complicating these analyses, not all gene products have activities that can be assayed in vitro, where the efficiency of the various family members can be compared against constant substrates. Two vertebrate UNC-45 homologues, unc45a and unc45b, affect distinct stages of muscle differentiation when knocked down in cell culture and are functionally redundant in vitro. UNC-45 proteins are members of the UCS (UNC-45/CRO1/She4p) protein family that has been shown to regulate myosin-dependent functions from fungi to vertebrates through direct interaction with the myosin motor domain. To test whether the same functional relationship exists between these unc45 paralogs in vivo, we examined the developmental phenotypes of doubly homozygous unc45b^−/−; unc45a^−/− mutant zebrafish embryos. We focused specifically on the combined effects on morphology and gene expression resulting from the zygotic lack of both paralogs. We found that unc45b^−/− and unc45b^−/−; unc45a^−/− embryos were phenotypically indistinguishable with both mutants displaying identical cardiac, skeletal muscle, and jaw defects. We also found no evidence to support a role for zygotic Unc45a function in myoblast differentiation. In contrast to previous in vitro work, this rules out a model of functional redundancy between Unc45a and Unc45b in vivo. Instead, our phylogenetic and phenotypic analyses provide evidence for the role of functional divergence in the evolution of the UCS protein family.

The hERG channel is dependent upon the Hsp90α isoform for maturation and trafficking

Heat shock protein 90 (Hsp90) has emerged as a promising therapeutic target for the treatment of cancer. Several Hsp90 inhibitors have entered clinical trials. However, some toxicological detriments have arisen, such as cardiotoxicity resulting from hERG inhibition following the administration of Hsp90 inhibitors. We sought to investigate this toxicity as hERG has been previously reported as a client protein that depends upon Hsp90 for its maturation and functional trafficking. In this study we show that hERG depends upon a single Hsp90 isoform. hERG preferentially co-immunoprecipitated with Hsp90α, and genetic knockdown of Hsp90α, but not Hsp90β, resulted in a trafficking-defective hERG channel. This study demonstrates the importance of delineating the isoform dependence of Hsp90 client proteins and provides rationale for the design of isoform-selective Hsp90 inhibitors that avoid detrimental effects.

Approaches for defining the Hsp90-dependent proteome

Hsp90 is the target of ongoing drug discovery studies seeking new compounds to treat cancer, neurodegenerative diseases, and protein folding disorders. To better understand Hsp90's roles in cellular pathologies and in normal cells, numerous studies have utilized proteomics assays and related high-throughput tools to characterize its physical and functional protein partnerships. This review surveys these studies, and summarizes the strengths and limitations of the individual attacks. We also include downloadable spreadsheets compiling all of the Hsp90-interacting proteins identified in more than 23 studies. These tools include cross-references among gene aliases, human homologues of yeast Hsp90-interacting proteins, hyperlinks to database entries, summaries of canonical pathways that are enriched in the Hsp90 interactome, and additional bioinformatic annotations. In addition to summarizing Hsp90 proteomics studies performed to date and the insights they have provided, we identify gaps in our current understanding of Hsp90-mediated proteostasis. This article is part of a Special Issue entitled: Heat Shock Protein 90 (HSP90).

Differential turnover of myosin chaperone UNC-45A isoforms increases in metastatic human breast cancer

UNC-45A is a molecular chaperone targeted to non-muscle myosins and is essential for cell division. Here, we show that UNC-45A mRNA and protein expression was elevated in human breast carcinomas and cell lines derived from breast carcinoma metastases. Moreover, small hairpin RNA knockdowns of endogenously overexpressed UNC-45A in the most metastatic cell line led to significant decreases in the rates of cell proliferation and invasion, concomitant with reduction in the interaction of myosin II with actin filaments. Exploring the mechanism of these findings further, we found that UNC-45A is alternatively expressed at the mRNA and protein levels as two isoforms. The two isoforms differ only by a proline-rich 15-amino-acid sequence near the amino-terminus. In the increased expression with metastatic activity, the ratio of the isoform mRNAs remained constant, but the 929-amino-acid protein isoform showed increases up to about 3-fold in comparison to the 944-amino-acid isoform. The differential accumulation was explained by cellular labeling experiments that showed that the 944 isoform is degraded at a 5-fold greater rate than the 929 isoform and that this degradation required the ubiquitin-proteasome system.

Hsp90 regulates NADPH oxidase activity and is necessary for superoxide but not hydrogen peroxide production

The goal of this study was to identify whether heat-shock protein 90 (Hsp90) regulates the production of superoxide and other reactive oxygen species from the NADPH oxidases (Nox). We found that pharmacological and genetic inhibition of Hsp90 directly reduced Nox5-derived superoxide without secondarily modifying signaling events. Coimmunoprecipitation and bioluminescence resonance energy transfer studies suggest that the C-terminus of Nox5 binds to Hsp90. Long-term Hsp90 inhibition reduced Nox5 expression and provides further evidence that Nox5 is an Hsp90 client protein. Inhibitors of Hsp90 also reduced superoxide from Nox1, Nox2 (neutrophils), and Nox3. However, Nox4, which emits only hydrogen peroxide, was unaffected by Hsp90 inhibitors. Hydrogen peroxide production from the other Nox enzymes was not affected by short-term inhibition of Hsp90, but long-term inhibition reduced production of all reactive oxygen species coincident with loss of enzyme expression. Expression of chimeric Nox enzymes consisting of N-terminal Nox1 or Nox3 and C-terminal Nox4 resulted in only hydrogen peroxide formation that was insensitive to Hsp90 inhibitors. We conclude that Hsp90 binds to the C-terminus of Noxes1-3 and 5 and is necessary for enzyme stability and superoxide production. Hsp90 does not bind to the C-terminus of Nox4 and is not required for hydrogen peroxide formation.

Understanding of the Hsp90 molecular chaperone reaches new heights

Heat shock protein 90 (Hsp90) was the focus of a recent meeting in the Swiss Alps, where the Hsp90 community met to discuss the operation and functions of this ubiquitous and essential molecular chaperone.

Targeting heat shock proteins in cancer

Heat shock proteins (HSPs) HSP27, HSP70 and HSP90 are powerful chaperones. Their expression is induced in response to a wide variety of physiological and environmental insults including anti-cancer chemotherapy, thus allowing the cell to survive to lethal conditions. Different functions of HSPs have been described to account for their cytoprotective function, including their role as molecular chaperones as they play a central role in the correct folding of misfolded proteins, but also their anti-apoptotic properties. HSPs are often overexpressed in cancer cells and this constitutive expression is necessary for cancer cells' survival. HSPs may have oncogene-like functions and likewise mediate "non-oncogene addiction" of stressed tumor cells that must adapt to a hostile microenvironment, thereby becoming dependent for their survival on HSPs. HSP-targeting drugs have therefore emerged as potential anti-cancer agents. This review describes the different molecules and approaches being used or proposed in cancer therapy based on the in inhibition of HSP90, HSP70 and HSP27.

The regulatory mechanism of Hsp90alpha secretion from endothelial cells and its role in angiogenesis during wound healing

Heat shock protein 90alpha (Hsp90alpha) is a ubiquitously expressed molecular chaperone, which is essential for the maintenance of eukaryote homeostasis. Hsp90alpha can also be secreted extracellularly and is associated with several physiological and pathological processes including wound healing, cancer, infectious diseases and diabetes. Angiogenesis, defined as the sprouting of new blood vessels from pre-existing capillaries via endothelial cell proliferation and migration, commonly occurs in and contributes to the above mentioned processes. However, the secretion of Hsp90alpha from endothelial cells and also its function in angiogenesis are still unclear. Here we investigated the role of extracellular Hsp90alpha in angiogenesis using dermal endothelial cells in vitro and a wound healing model in vivo. We find that the secretion of Hsp90alpha but not Hsp90beta is increased in activated endothelial cells with the induction of angiogenic factors and matrix proteins. Secreted Hsp90alpha localizes on the leading edge of endothelial cells and promotes their angiogenic activities, whereas Hsp90alpha neutralizing antibodies reverse the effect. Furthermore, using a mouse skin wound healing model in vivo, we demonstrate that extracellular Hsp90alpha localizes on blood vessels in granulation tissues of wounded skin and promotes angiogenesis during wound healing. Taken together, our study reveals that Hsp90alpha can be secreted by activated endothelial cells and is a positive regulator of angiogenesis, suggesting the potential application of Hsp90alpha as a stimulator for wound repair.

Mutations that increase both Hsp90 ATPase activity in vitro and Hsp90 drug resistance in vivo

Hsp90 inhibitors are currently tested in clinical trials as anticancer agents. We investigated whether inhibitor resistance may arise as a result of a point mutation in Hsp90. We used yeast cells that expressed human Hsp90beta to select inhibitor-resistant mutants from the randomly mutagenized library. Single amino acid substitution, I123T, in a selected mutant was sufficient to confer inhibitor resistance. Transfection of human cells with the HSP90beta I123T and the corresponding HSP90alpha I128T yielded cell lines resistant to inhibitors of the Hsp90 ATPase. Unexpectedly, mutations did not result in diminished inhibitor binding in vitro. Similarly resistant cells were obtained after transfection with previously described A116N and T31I mutants of HSP90beta that cause increase in ATPase activity in vitro. Inhibitor-resistant phenotypes of the I123T and A116N mutants depended on their increased affinity for Aha1, whereas T31I mutation did not result in increased Aha1 binding. These results show possible scenario by which resistance may arise in patients treated with Hsp90 inhibitors. Additionally, our results show that each isoform of Hsp90 can alone sustain cellular functions.

Novel Hsp90 partners discovered using complementary proteomic approaches

Hsp90 is an essential eukaryotic molecular chaperone that stabilizes a large set of client proteins, many of which are involved in various cellular signaling pathways. The current list of Hsp90 interactors comprises about 200 proteins and this number is growing steadily. In this paper, we report on the application of three complementary proteomic approaches directed towards identification of novel proteins that interact with Hsp90. These methods are coimmunoprecipitation, pull down with biotinylated geldanamycin, and immobilization of Hsp90beta on sepharose. In all, this study led to the identification of 42 proteins, including 18 proteins that had not been previously characterized as Hsp90 interactors. These novel Hsp90 partners not only represent abundant protein species, but several proteins were identified at low levels, among which signaling kinase Cdk3 and putative transcription factor tripartite motif-containing protein 29. Identification of tetratricopeptide-repeat-containing mitochondrial import receptor protein Tom34 suggests the involvement of Hsp90 in the early steps of translocation of mitochondrial preproteins. Taken together, our data expand the knowledge of the Hsp90 interactome and provide a further step in our understanding of the Hsp90 chaperone system.

UNC45A confers resistance to histone deacetylase inhibitors and retinoic acid

To identify potential biomarkers of therapy response, we have previously done a large-scale gain-of-function genetic screen to identify genes whose expression confers resistance to histone deacetylase inhibitors (HDACI). This genetic screen identified two genes with a role in retinoic acid signaling, suggesting that HDACIs target retinoic acid signaling as part of their anticancer effect. We study here a third gene identified in this genetic screen, UNC45A, and assess its role in retinoic acid signaling and responses to HDACIs using cell-based proliferation and differentiation assays and transcriptional reporter gene assays. The vertebrate Unc45 genes are known for their roles in muscle development and the assembly and cochaperoning of the muscle motor protein myosin. Here, we report that human UNC45A (GCUNC45) can render transformed cells resistant to treatment with HDACIs. We show that UNC45A also inhibits signaling through the retinoic acid receptor alpha. Expression of UNC45A inhibits retinoic acid-induced proliferation arrest and differentiation of human neuroblastoma cells and inhibits the induction of endogenous retinoic acid receptor target genes. These data establish an unexpected role for UNC45A in causing resistance to both HDACI drugs and retinoic acid. Moreover, our data lend further support to the notion that HDACIs exert their anticancer effect, at least in part, through an effect on retinoic acid signaling.

The levels of retinoic acid-inducible gene I are regulated by heat shock protein 90-alpha

Retinoic acid-inducible gene I (RIG-I) is an intracellular pattern recognition receptor that plays important roles during innate immune responses to viral dsRNAs. The mechanisms and signaling molecules that participate in the downstream events that follow activation of RIG-I are incompletely characterized. In addition, the factors that define intracellular availability of RIG-I and determine its steady-state levels are only partially understood but are likely to play a major role during innate immune responses. It was recently reported that the antiviral activity of RIG-I is negatively regulated by specific E3 ubiquitin ligases, suggesting participation of the proteasome in the regulation of RIG-I levels. In this study, we used immunoprecipitation combined with mass spectrometry to identify RIG-I-interacting proteins and found that RIG-I forms part of a protein complex that includes heat shock protein 90-alpha (HSP90-alpha), a molecular chaperone. Biochemical studies using purified systems demonstrated that the association between RIG-I and HSP90-alpha is direct but does not involve participation of the CARD domain. Inhibition of HSP90 activity leads to the dissociation of the RIG-I-HSP90 complex, followed by ubiquitination and proteasomal degradation of RIG-I. In contrast, the levels of RIG-I mRNA are unaffected. Our studies also show that the ability of RIG-I to respond to stimulation with polyinosinic:polycytidylic acid is abolished when its interaction with HSP90 is inhibited. These novel findings point to HSP90-alpha as a chaperone that shields RIG-I from proteasomal degradation and modulates its activity. These studies identify a new mechanism whose dysregulation may seriously compromise innate antiviral responses in mammals.

The heat shock protein 90 inhibitor IPI-504 induces apoptosis of AKT-dependent diffuse large B-cell lymphomas

Heat shock protein 90 (HSP90) is a molecular chaperone that stabilizes critical client proteins in multiple cancers. Gene expression profiling was utilized to characterize HSP90 isoform expression in primary human diffuse large B-cell lymphomas (DLBCLs). HSP90 alpha and beta isoforms were differentially expressed in subsets of tumours defined by their transcriptional profiles. Thereafter, we assessed the activity of the HSP90 inhibitor, IPI-504, in an extensive panel of DLBCL cell lines. IPI-504, which interacts with the conserved ATP-binding site in both HSP90 isoforms, inhibited proliferation and induced apoptosis in the majority of DLBCL cell lines at low micromolar concentrations. IPI-504-sensitive cell lines expressed high levels of the HSP90 client protein, pAKT, and exhibited dose-dependent decreases in pAKT levels following IPI-504 treatment and significantly reduced proliferation following AKT RNAi. Furthermore, the combination of low-dose (<1 micromol/l) IPI-504 and the AKT/Pi3K pathway inhibitor, LY24009, was synergistic in IPI-504-sensitive DLBCL cell lines. Low-dose IPI-504 was also synergistic with the chemotherapeutic agent, doxorubicin. The HSP90 inhibitor IPI-504 warrants further investigation in DLBCL alone and in combination with identified client protein inhibitors and active chemotherapeutic agents.

Myosin assembly, maintenance and degradation in muscle: Role of the chaperone UNC-45 in myosin thick filament dynamics

Myofibrillogenesis in striated muscle cells requires a precise ordered pathway to assemble different proteins into a linear array of sarcomeres. The sarcomere relies on interdigitated thick and thin filaments to ensure muscle contraction, as well as properly folded and catalytically active myosin head. Achieving this organization requires a series of protein folding and assembly steps. The folding of the myosin head domain requires chaperone activity to attain its functional conformation. Folded or unfolded myosin can spontaneously assemble into short myosin filaments, but further assembly requires the short and incomplete myosin filaments to assemble into the developing thick filament. These longer filaments are then incorporated into the developing sarcomere of the muscle. Both myosin folding and assembly require factors to coordinate the formation of the thick filament in the sarcomere and these factors include chaperone molecules. Myosin folding and sarcomeric assembly requires association of classical chaperones as well as folding cofactors such as UNC-45. Recent research has suggested that UNC-45 is required beyond initial myosin head folding and may be directly or indirectly involved in different stages of myosin thick filament assembly, maintenance and degradation.