Hsp90 interacts with inducible NO synthase client protein in its heme-free state and then drives heme insertion by an ATP-dependent process

Design patterns of biological cells

Design patterns are generalized solutions to frequently recurring problems. They were initially developed by architects and computer scientists to create a higher level of abstraction for their designs. Here, we extend these concepts to cell biology to lend a new perspective on the evolved designs of cells' underlying reaction networks. We present a catalog of 21 design patterns divided into three categories: creational patterns describe processes that build the cell, structural patterns describe the layouts of reaction networks, and behavioral patterns describe reaction network function. Applying this pattern language to the E. coli central metabolic reaction network, the yeast pheromone response signaling network, and other examples lends new insights into these systems.

Functional maturation of cytochromes P450 3A4 and 2D6 relies on GAPDH- and Hsp90-Dependent heme allocation

Cytochrome P450 3A4 and 2D6 (EC 1.14.13.97 and 1.14.14.1; CYP3A4 and 2D6) are heme-containing enzymes that catalyze the oxidation of a wide number of xenobiotic and drug substrates and thus broadly impact human biology and pharmacologic therapies. Although their activities are directly proportional to their heme contents, little is known about the cellular heme delivery and insertion processes that enable their maturation to functional form. We investigated the potential involvement of GAPDH and chaperone Hsp90, based on our previous studies linking these proteins to intracellular heme allocation. We studied heme delivery and insertion into CYP3A4 and 2D6 after they were transiently expressed in HEK293T and GlyA CHO cells or when naturally expressed in HEPG2 cells in response to rifampicin, and also investigated their associations with GAPDH and Hsp90 in cells. The results indicate that GAPDH and its heme binding function is involved in delivery of mitochondria-generated heme to apo-CYP3A4 and 2D6, and that cell chaperone Hsp90 is additionally involved in driving their heme insertions. Uncovering how cells allocate heme to CYP3A4 and 2D6 provides new insight on their maturation processes and how this may help to regulate their functions in health and disease.

Heat shock protein 90: biological functions, diseases, and therapeutic targets

Heat shock protein 90 (Hsp90) is a predominant member among Heat shock proteins (HSPs), playing a central role in cellular protection and maintenance by aiding in the folding, stabilization, and modification of diverse protein substrates. It collaborates with various co-chaperones to manage ATPase-driven conformational changes in its dimer during client protein processing. Hsp90 is critical in cellular function, supporting the proper operation of numerous proteins, many of which are linked to diseases such as cancer, Alzheimer's, neurodegenerative conditions, and infectious diseases. Recognizing the significance of these client proteins across diverse diseases, there is a growing interest in targeting Hsp90 and its co-chaperones for potential therapeutic strategies. This review described biological background of HSPs and the structural characteristics of HSP90. Additionally, it discusses the regulatory role of heat shock factor-1 (HSF-1) in modulating HSP90 and sheds light on the dynamic chaperone cycle of HSP90. Furthermore, the review discusses the specific contributions of HSP90 in various disease contexts, especially in cancer. It also summarizes HSP90 inhibitors for cancer treatment, offering a thoughtful analysis of their strengths and limitations. These advancements in research expand our understanding of HSP90 and open up new avenues for considering HSP90 as a promising target for therapeutic intervention in a range of diseases.

Hemoglobin resident in the lung epithelium is protective for smooth muscle soluble guanylate cyclase function

Hemoglobin (Hb) present in the lung epithelium is of unknown significance. However Hb being an nitric oxide (NO) scavenger can bind to NO and reduce its deleterious effects. Hence we postulated an NO scavenging role for this lung Hb. Doing transwell co-culture with bronchial epithelial cells, A549/16-HBE (apical) and human airway smooth muscle cells (HASMCs as basal), we found that Hb can protect the smooth muscle soluble guanylyl cyclase (sGC) from excess NO. Inducing the apical A549/16-HBE cells with cytokines to trigger iNOS expression and NO generation caused a time dependent increase in SNO-sGC and this was accompanied with a concomitant drop in sGC-α1β1 heterodimerization. Silencing Hbαβ in the apical cells further increased the SNO on sGC with a faster drop in the sGC heterodimer and these effects were additive along with further silencing of thioredoxin 1 (Trx1). Since heme of Hb is critical for NO scavenging we determined the Hb heme in a mouse model of allergic asthma (OVA) and found that Hb in the inflammed OVA lungs was low in heme or heme-free relative to those of naïve lungs. Further we established a direct correlation between the status of the sGC heterodimer and the Hb heme from lung samples of human asthma, iPAH, COPD and cystic fibrosis. These findings present a new mechanism of protection of lung sGC by the epithelial Hb, and suggests that this protection maybe lost in asthma or COPD where lung Hb is unable to scavenge the NO due to it being heme-deprived.

Dynamic cycling with a unique Hsp90/Hsp70-dependent chaperone machinery and GAPDH is needed for heme insertion and activation of neuronal NO synthase

Heat shock protein 90 (Hsp90) is known to mediate heme insertion and activation of heme-deficient neuronal nitric oxide (NO) synthase (apo-nNOS) in cells by a highly dynamic interaction that has been extremely difficult to study mechanistically with the use of subcellular systems. In that the heme content of many critical hemeproteins is regulated by Hsp90 and the heme chaperone GAPDH, the development of an in vitro system for the study of this chaperone-mediated heme regulation would be extremely useful. Here, we show that use of an antibody-immobilized apo-nNOS led not only to successful assembly of chaperone complexes but the ability to show a clear dependence on Hsp90 and GAPDH for heme-mediated activation of apo-nNOS. The kinetics of binding for Hsp70 and Hsp90, the ATP and K⁺ dependence, and the absolute requirement for Hsp70 in assembly of Hsp90•apo-nNOS heterocomplexes all point to a similar chaperone machinery to the well-established canonical machine regulating steroid hormone receptors. However, unlike steroid receptors, the use of a purified protein system containing Hsp90, Hsp70, Hsp40, Hop, and p23 is unable to activate apo-nNOS. Thus, heme insertion requires a unique Hsp90-chaperone complex. With this newly developed in vitro system, which recapitulates the cellular process requiring GAPDH as well as Hsp90, further mechanistic studies are now possible to better understand the components of the Hsp90-based chaperone system as well as how this heterocomplex works with GAPDH to regulate nNOS and possibly other hemeproteins.

HSP90 mediates the connection of multiple programmed cell death in diseases

Heat shock protein (HSP) 90, an important component of the molecular chaperone network, is closely concerned with cellular signaling pathways and stress response by participating in the process of maturation and activation of client proteins, playing a crucial role both in the normal and abnormal operation of the organism. In functionally defective tissues, programmed cell death (PCD) is one of the regulable fundamental mechanisms mediated by HSP90, including apoptosis, autophagy, necroptosis, ferroptosis, and others. Here, we show the complex relationship between HSP90 and different types of PCD in various diseases, and discuss the possibility of HSP90 as the common regulatory nodal in multiple PCD, which would provide a new perspective for the therapeutic approaches in disease.

Low levels of nitric oxide promotes heme maturation into several hemeproteins and is also therapeutic

Nitric oxide (NO) is a signal molecule and plays a critical role in the regulation of vascular tone, displays anti-platelet and anti-inflammatory properties. While our earlier and current studies found that low NO doses trigger a rapid heme insertion into immature heme-free soluble guanylyl cyclase β subunit (apo-sGCβ), resulting in a mature sGC-αβ heterodimer, more recent evidence suggests that low NO doses can also trigger heme-maturation of hemoglobin and myoglobin. This low NO phenomena was not only limited to sGC and the globins, but was also found to occur in all three nitric oxide synthases (iNOS, nNOS and eNOS) and Myeloperoxidase (MPO). Interestingly high NO doses were inhibitory to heme-insertion for these hemeproteins, suggesting that NO has a dose-dependent dual effect as it can act both ways to induce or inhibit heme-maturation of key hemeproteins. While low NO stimulated heme-insertion of globins required the presence of the NO-sGC-cGMP signal pathway, iNOS heme-maturation also required the presence of an active sGC. These effects of low NO were significantly diminished in the tissues of double (n/eNOS^−/−) and triple (n/i/eNOS^−/−) NOS knock out mice where lung sGC was found be heme-free and the myoglobin or hemoglobin from the heart/lungs were found be low in heme, suggesting that loss of endogenous NO globally impacts the whole animal and that this impact of low NO is both essential and physiologically relevant for hemeprotein maturation. Effects of low NO were also found to be protective against ischemia reperfusion injury on an ex vivo lung perfusion (EVLP) system prior to lung transplant, which further suggests that low NO levels are also therapeutic.

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Low levels of NO enable heme-maturation of the globins by a process that required an NO triggered heme-insertion into sGCβ.

•This effect of low NO was also found to occur for all three nitric oxide synthases (NOSs) and Myeloperoxidase (MPO).

•Tissues from n/eNOS^−/− and n/i/eNOS^−/− knock out mice had low heme levels in the globins, while sGC was largely heme-free.

•Low NO at ppm levels also manifests itself as a therapy during ischemic reperfusion injury of lungs on the EVLP.

New roles for GAPDH, Hsp90, and NO in regulating heme allocation and hemeprotein function in mammals

The intracellular trafficking of mitochondrial heme presents a fundamental challenge to animal cells. This article provides some background on heme allocation, discusses some of the concepts, and then reviews research done over the last decade, much in the author's laboratory, that is uncovering unexpected and important roles for glyceraldehyde 3-phosphate dehydrogenase (GAPDH), heat shock protein 90 (hsp90), and nitric oxide (NO) in enabling and regulating the allocation of mitochondrial heme to hemeproteins that mature and function outside of the mitochondria. A model for how hemeprotein functions can be regulated in cells through the coordinate participation of GAPDH, hsp90, and NO in allocating cellular heme is presented.

Nitric Oxide Trickle Drives Heme into Hemoglobin and Muscle Myoglobin

Ever since the days of NO being proclaimed as the “molecule of the year”, the molecular effects of this miracle gas on the globins have remained elusive. While its vasodilatory role in the cardiopulmonary system and the vasculature is well recognized, the molecular underpinnings of the NO–globin axis are incompletely understood. We show, by transwell co-culture of nitric oxide (NO) generating, HEK eNOS/nNOS cells, and K562 erythroid or C2C12 muscle myoblasts, that low doses of NO can effectively insert heme into hemoglobin (Hb) and myoglobin (Mb), making NO not only a vasodilator, but also a globin heme trigger. We found this process to be dependent on the NO flux, occurring at low NO doses and fading at higher doses. This NO-triggered heme insertion occurred into Hb in just 30 min in K562 cells and into muscle Mb in C2C12 myoblasts between 30 min and 1 h, suggesting that the classical effect of NO on upregulation of globin (Hb or Mb) is just not transcriptional, but may involve sufficient translational events where NO can cause heme-downloading into the apo-globins (Hb/Mb). This effect of NO is unexpected and highlights its significance in maintaining globins in its heme-containing holo-form, where such heme insertions might be required in the circulating blood or in the muscle cells to perform spontaneous functions.

Hsp90 in Human Diseases: Molecular Mechanisms to Therapeutic Approaches

The maturation of hemeprotein dictates that they incorporate heme and become active, but knowledge of this essential cellular process remains incomplete. Studies on chaperon Hsp90 has revealed that it drives functional heme maturation of inducible nitric oxide synthase (iNOS), soluble guanylate cyclase (sGC) hemoglobin (Hb) and myoglobin (Mb) along with other proteins including GAPDH, while globin heme maturations also need an active sGC. In all these cases, Hsp90 interacts with the heme-free or apo-protein and then drives the heme maturation by an ATP dependent process before dissociating from the heme-replete proteins, suggesting that it is a key player in such heme-insertion processes. As the studies on globin maturation also need an active sGC, it connects the globin maturation to the NO-sGC (Nitric oxide-sGC) signal pathway, thereby constituting a novel NO-sGC-Globin axis. Since many aggressive cancer cells make Hbβ/Mb to survive, the dependence of the globin maturation of cancer cells places the NO-sGC signal pathway in a new light for therapeutic intervention. Given the ATPase function of Hsp90 in heme-maturation of client hemeproteins, Hsp90 inhibitors often cause serious side effects and this can encourage the alternate use of sGC activators/stimulators in combination with specific Hsp90 inhibitors for better therapeutic intervention.

Indoleamine dioxygenase and tryptophan dioxygenase activities are regulated through GAPDH- and Hsp90-dependent control of their heme levels

Indoleamine-2, 3-dioxygenase (IDO1) and Tryptophan-2, 3-dioxygense (TDO) are heme-containing dioxygenases that catalyze the conversion of tryptophan to N-formyl-kynurenine and thus enable generation of l-kynurenine and related metabolites that govern the immune response and broadly impact human biology. Given that TDO and IDO1 activities are directly proportional to their heme contents, it is important to understand their heme delivery and insertion processes. Early studies established that TDO and IDO1 heme levels are sub-saturating in vivo and subject to change but did not identify the cellular mechanisms that provide their heme or enable dynamic changes in their heme contents. We investigated the potential involvement of GAPDH and chaperone Hsp90, based on our previous studies linking these proteins to intracellular heme allocation. We studied heme delivery and insertion into IDO1 and TDO expressed in both normal and heme-deficient HEK293T cells and into IDO1 naturally expressed in HeLa cells in response to IFN-γ, and also investigated the interactions of TDO and IDO1 with GAPDH and Hsp90 in cells and among their purified forms. We found that GAPDH delivered both mitochondrially-generated and exogenous heme to apo-IDO1 and apo-TDO in cells, potentially through a direct interaction with either enzyme. In contrast, we found Hsp90 interacted with apo-IDO1 but not with apo-TDO, and was only needed to drive heme insertion into apo-IDO1. By uncovering the cellular processes that allocate heme to IDO1 and TDO, our study provides new insight on how their activities and l-kynurenine production may be controlled in health and disease.

NO rapidly mobilizes cellular heme to trigger assembly of its own receptor

Nitric oxide (NO) performs many biological functions, but how it operates at the molecular and cellular levels is not fully understood. We discovered that cell NO generation at physiologic levels triggers a rapid redeployment of intracellular heme, an iron-containing cofactor, and we show that this drives the assembly of the natural NO receptor protein, soluble guanylyl cyclase, which is needed for NO to perform its biological signaling functions. Our study uncovers a way that NO can shape biological signaling processes and a way that cells may use NO to control their hemeprotein activities through deployment of the heme cofactor. These concepts broaden our understanding of NO function in biology and medicine.

Nitric oxide (NO) signaling in biology relies on its activating cyclic guanosine monophosphate (cGMP) production by the NO receptor soluble guanylyl cyclase (sGC). sGC must obtain heme and form a heterodimer to become functional, but paradoxically often exists as an immature heme-free form in cells and tissues. Based on our previous finding that NO can drive sGC maturation, we investigated its basis by utilizing a fluorescent sGC construct whose heme level can be monitored in living cells. We found that NO generated at physiologic levels quickly triggered cells to mobilize heme to immature sGC. This occurred when NO was generated within cells or by neighboring cells, began within seconds of NO exposure, and led cells to construct sGC heterodimers and thus increase their active sGC level by several-fold. The NO-triggered heme deployment involved cellular glyceraldehyde-3-phosphate dehydrogenase (GAPDH)–heme complexes and required the chaperone hsp90, and the newly formed sGC heterodimers remained functional long after NO generation had ceased. We conclude that NO at physiologic levels triggers assembly of its own receptor by causing a rapid deployment of cellular heme. Redirecting cellular heme in response to NO is a way for cells and tissues to modulate their cGMP signaling and to more generally tune their hemeprotein activities wherever NO biosynthesis takes place.

GAPDH is involved in the heme-maturation of myoglobin and hemoglobin

GAPDH, a heme chaperone, has been previously implicated in the incorporation of heme into iNOS and soluble guanylyl cyclase (sGC). Since sGC is critical for myoglobin (Mb) heme-maturation, we investigated the role of GAPDH in the maturation of this globin, as well as hemoglobins α, β, and γ. Utilizing cell culture systems, we found that overexpression of wild-type GAPDH increased, whereas GAPDH mutants H53A and K227A decreased, the heme content of Mb and Hbα and Hbβ. Overexpression of wild-type GAPDH fully recovered the heme-maturation inhibition observed with the GAPDH mutants. Partial rescue was observed by overexpression of sGCβ1 but not by overexpression of a sGCΔβ1 deletion mutant, which is unable to bind the sGCα1 subunit required to form the active sGCα1β1 complex. Wild type and mutant GAPDH was found to be associated in a complex with each of the globins and Hsp90. GAPDH at endogenous levels was found to be associated with Mb in differentiating C2C12 myoblasts, and with Hbγ or Hbα in differentiating HiDEP-1 erythroid progenitor cells. Knockdown of GAPDH in C2C12 cells suppressed Mb heme-maturation. GAPDH knockdown in K562 erythroleukemia cells suppressed Hbα and Hbγ heme-maturation as well as Hb dimerization. Globin heme incorporation was not only dependent on elevated sGCα1β1 heterodimer formation, but also influenced by iron provision and magnitude of expression of GAPDH, d-aminolevulinic acid, and FLVCR1b. Together, our data support an important role for GAPDH in the maturation of myoglobin and γ, β, and α hemoglobins.

Hemin accumulation and identification of a heme-binding protein clan in K562 cells by proteomic and computational analysis

Heme (iron protoporphyrin IX) is an essential regulator conserved in all known organisms. We investigated the kinetics of intracellular accumulation of hemin (oxidized form) in human transformed proerythroid K562 cells using [¹⁴ C]-hemin and observed that it is time and temperature-dependent, affected by the presence of serum proteins, as well as the amphipathic/hydrophobic properties of hemin. Hemin-uptake exhibited saturation kinetics as a function of the concentration added, suggesting the involvement of a carrier-cell surface receptor-mediated process. The majority of intracellular hemin accumulated in the cytoplasm, while a substantial portion entered the nucleus. Cytosolic proteins isolated by hemin-agarose affinity column chromatography (HACC) were found to form stable complexes with [⁵⁹ Fe]-hemin. The HACC fractionation and Liquid chromatography-mass spectrometry analysis of cytosolic, mitochondrial, and nuclear protein isolates from K562 cell extracts revealed the presence of a large number of hemin-binding proteins (HeBPs) of diverse ontologies, including heat shock proteins, cytoskeletal proteins, enzymes, and signaling proteins such as actinin a4, mitogen-activated protein kinase 1 as well as several others. The subsequent computational analysis of the identified HeBPs using HemoQuest confirmed the presence of various hemin/heme-binding motifs [C(X)nC, H, Y] in their primary structures and conformations. The possibility that these HeBPs contribute to a heme intracellular trafficking protein network involved in the homeostatic regulation of the pool and overall functions of heme is discussed.

Nitric Oxide Synthase Inhibitors into the Clinic at Last

The 1998 Nobel Prize in Medicine and Physiology for the discovery of nitric oxide, a nitrogen containing reactive oxygen species (also termed reactive nitrogen or reactive nitrogen/oxygen species) stirred great hopes. Clinical applications, however, have so far pertained exclusively to the downstream signaling of cGMP enhancing drugs such as phosphodiesterase inhibitors and soluble guanylate cyclase stimulators. All clinical attempts, so far, to inhibit NOS have failed even though preclinical models were strikingly positive and clinical biomarkers correlated perfectly. This rather casts doubt on our current way of target identification in drug discovery in general and our way of patient stratification based on correlating but not causal biomarkers or symptoms. The opposite, NO donors, nitrite and enhancing NO synthesis by eNOS/NOS3 recoupling in situations of NO deficiency, are rapidly declining in clinical relevance or hold promise but need yet to enter formal therapeutic guidelines, respectively. Nevertheless, NOS inhibition in situations of NO overproduction often jointly with enhanced superoxide (or hydrogen peroxide production) still holds promise, but most likely only in acute conditions such as neurotrauma (Stover et al., J Neurotrauma 31(19):1599-1606, 2014) and stroke (Kleinschnitz et al., J Cereb Blood Flow Metab 1508-1512, 2016; Casas et al., Proc Natl Acad Sci U S A 116(14):7129-7136, 2019). Conversely, in chronic conditions, long-term inhibition of NOS might be too risky because of off-target effects on eNOS/NOS3 in particular for patients with cardiovascular risks or metabolic and renal diseases. Nitric oxide synthases (NOS) and their role in health (green) and disease (red). Only neuronal/type 1 NOS (NOS1) has a high degree of clinical validation and is in late stage development for traumatic brain injury, followed by a phase II safety/efficacy trial in ischemic stroke. The pathophysiology of NOS1 (Kleinschnitz et al., J Cereb Blood Flow Metab 1508-1512, 2016) is likely to be related to parallel superoxide or hydrogen peroxide formation (Kleinschnitz et al., J Cereb Blood Flow Metab 1508-1512, 2016; Casas et al., Proc Natl Acad Sci U S A 114(46):12315-12320, 2017; Casas et al., Proc Natl Acad Sci U S A 116(14):7129-7136, 2019) leading to peroxynitrite and protein nitration, etc. Endothelial/type 3 NOS (NOS3) is considered protective only and its inhibition should be avoided. The preclinical evidence for a role of high-output inducible/type 2 NOS (NOS2) isoform in sepsis, asthma, rheumatic arthritis, etc. was high, but all clinical development trials in these indications were neutral despite target engagement being validated. This casts doubt on the role of NOS2 in humans in health and disease (hence the neutral, black coloring).

Maturation, inactivation, and recovery mechanisms of soluble guanylyl cyclase

Soluble guanylate cyclase (sGC) is a heme-containing heterodimeric enzyme that generates many molecules of cGMP in response to its ligand nitric oxide (NO); sGC thereby acts as an amplifier in NO-driven biological signaling cascades. Because sGC helps regulate the cardiovascular, neuronal, and gastrointestinal systems through its cGMP production, boosting sGC activity and preventing or reversing sGC inactivation are important therapeutic and pharmacologic goals. Work over the last two decades is uncovering the processes by which sGC matures to become functional, how sGC is inactivated, and how sGC is rescued from damage. A diverse group of small molecules and proteins have been implicated in these processes, including NO itself, reactive oxygen species, cellular heme, cell chaperone Hsp90, and various redox enzymes as well as pharmacologic sGC agonists. This review highlights their participation and provides an update on the processes that enable sGC maturation, drive its inactivation, or assist in its recovery in various settings within the cell, in hopes of reaching a better understanding of how sGC function is regulated in health and disease.

Heat shock protein 90α increases superoxide generation from neuronal nitric oxide synthases

Neuronal nitric oxide synthase (nNOS) generates superoxide, particularly at sub-optimal l-arginine (l-Arg) substrate concentrations. Heat shock protein 90 (Hsp90) was reported to inhibit superoxide generation from nNOS protein. However, commercially available Hsp90 product from bovine brain tissues with unspecified Hsp90α and Hsp90β contents and an undefined Hsp90 protein oligomeric state was utilized. These two Hsp90s can have opposite effect on superoxide production by NOS. Importantly, emerging evidence indicates that nNOS splice variants are involved in different biological functions by functioning distinctly in redox signaling. In the present work, purified recombinant human Hsp90α, in its native dimeric state, was used in electron paramagnetic resonance (EPR) spin trapping experiments to study the effects of Hsp90α on superoxide generation from nNOS splice variants nNOSμ and nNOSα. Human Hsp90α was found to significantly increase superoxide generation from nNOSμ and nNOSα proteins under l-Arg-depleted conditions and Hsp90α influenced superoxide production by nNOSμ and nNOSα at varying degrees. Imidazole suppressed the spin adduct signal, indicating that superoxide was produced at the heme site of nNOS in the presence of Hsp90α, whereas l-Arg repletion diminished superoxide production by the nNOS-Hsp90α. Moreover, NADPH consumption rate values exhibited a similar trend/difference as a function of Hsp90α and l-Arg. Together, these EPR spin trapping and NADPH oxidation kinetics results demonstrated noticeable Hsp90α-induced increases in superoxide production by nNOS and a distinguishable effect of Hsp90α on nNOSμ and nNOSα proteins.

Heat Shock Proteins Accelerate the Maturation of Brain Endothelial Cell Glucocorticoid Receptor in Focal Human Drug-Resistant Epilepsy

Pharmacoresistance in epilepsy is a major challenge to successful clinical therapy. Glucocorticoid receptor (GR) dysregulation can affect the underlying disease pathogenesis. We recently reported that local drug biotransformation at the blood-brain barrier is upregulated by GR, which controls drug-metabolizing enzymes (e.g., cytochrome P450s, CYPs) and efflux drug transporters (MDR1) in human epileptic brain endothelial cells (EPI-ECs). Here, we establish that this mechanism is influenced upstream by GR and its association with heat shock proteins/co-chaperones (Hsps) during maturation, which differentially affect human epileptic (EPI) tissue and brain endothelial cells. Overexpressed GR, Hsp90, Hsp70, and Hsp40 were found in EPI vs. NON-EPI brain regions. Elevated neurovascular GR expression and co-localization with Hsps was evident in the EPI regions with cortical dysplasia, predominantly in the brain micro-capillaries and neurons. A corresponding increase in ATPase activity (*p < 0.05) was found in the EPI regions. The GR-Hsp90/Hsp70 binding patterns indicated a faster chaperone-promoted maturation of GR, leading to its overactivation in both the tissue and EPI-ECs derived from EPI/focal regions and GR silencing in EPI-ECs slowed such GR-Hsp interactions. Significantly accelerated GR nuclear translocation was determined in EPI-ECs following treatment with GR modulators/ligands dexamethasone, rifampicin, or phenytoin. Our findings reveal that overexpressed GR co-localizes with Hsps in the neurovasculature of EPI brain, increased GR maturation by Hsps accelerates EPI GR machinery, and furthermore this change in EPI and NON-EPI GR-Hsp interaction alters with the age of seizure onset in epileptic patients, together affecting the pathophysiology and drug regulation in the epileptic brain endothelium.

Dynamic regulation of NADPH oxidase 5 by intracellular heme levels and cellular chaperones

NADPH oxidase 5 (NOX5) is a transmembrane signaling enzyme that produces superoxide in response to elevated cytosolic calcium. In addition to its association with numerous human diseases, NOX5 has recently been discovered to play crucial roles in the immune response and cardiovascular system. Details of NOX5 maturation, and specifically its response to changes in intracellular heme levels have remained unclear. Here we establish an experimental system in mammalian cells that allows us to probe the influence of heme availability on ROS production by NOX5. We identified a mode of dynamic regulatory control over NOX5 activity through modulation of its heme saturation and oligomeric state by intracellular heme levels and Hsp90 binding. This regulatory mechanism allows for fine-tuning and reversible modulation of NOX5 activity in response to stimuli.

Unbiased proteomics identifies plasminogen activator inhibitor-1 as a negative regulator of endothelial nitric oxide synthase

Nitric oxide (NO) produced by endothelial nitric oxide synthase (eNOS) is a critical mediator of vascular function. eNOS is tightly regulated at various levels, including transcription, co- and posttranslational modifications, and by various protein-protein interactions. Using stable isotope labeling with amino acids in cell culture (SILAC) and mass spectrometry (MS), we identified several eNOS interactors, including the protein plasminogen activator inhibitor-1 (PAI-1). In cultured human umbilical vein endothelial cells (HUVECs), PAI-1 and eNOS colocalize and proximity ligation assays demonstrate a protein-protein interaction between PAI-1 and eNOS. Knockdown of PAI-1 or eNOS eliminates the proximity ligation assay (PLA) signal in endothelial cells. Overexpression of eNOS and HA-tagged PAI-1 in COS7 cells confirmed the colocalization observations in HUVECs. Furthermore, the source of intracellular PAI-1 interacting with eNOS was shown to be endocytosis derived. The interaction between PAI-1 and eNOS is a direct interaction as supported in experiments with purified proteins. Moreover, PAI-1 directly inhibits eNOS activity, reducing NO synthesis, and the knockdown or antagonism of PAI-1 increases NO bioavailability. Taken together, these findings place PAI-1 as a negative regulator of eNOS and disruptions in eNOS-PAI-1 binding promote increases in NO production and enhance vasodilation in vivo.

Structure, Function, and Regulation of the Hsp90 Machinery

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

Myoglobin maturation is driven by the hsp90 chaperone machinery and by soluble guanylyl cyclase

Myoglobin (Mb) maturation involves heme incorporation as a final step. We investigated a role for heat shock protein (hsp) 90 in Mb maturation in C2C12 skeletal muscle myoblasts and cell lines. We found the following: 1) Hsp90 directly interacts preferentially with heme-free Mb both in purified form and in cells. 2) Hsp90 drives heme insertion into apoprotein-Mb in an ATP-dependent process. 3) During differentiation of C2C12 myoblasts into myotubes, the apo-Mb-hsp90 complex associates with 5 cell cochaperons, Hsp70, activator of hsp90 ATPase protein 1 (Aha1), alanyl-tRNA synthetase domain containing 1 (Aarsd1), cell division cycle 37 (Cdc37), and stress induced phosphoprotein 1 (STIP1) in a pattern that is consistent with their enabling Mb maturation. 4) Mb heme insertion was significantly increased in cells that had a functional soluble guanylyl cyclase (sGC)-cGMP signaling pathway and was diminished upon small interfering RNA knockdown of sGCβ1 or upon overexpression of a phosphodiesterase to prevent cGMP buildup. Together, our findings suggest that hsp90 works in concert with cochaperons (Hsp70, Aha1, Aarsd1, STIP1, and Cdc37) and an active sGC-cGMP signaling pathway to promote heme insertion into immature apo-Mb, and thus generate functional Mb during muscle myotube formation. This fills gaps in our understanding and suggests new ways to potentially control these processes.-Ghosh, A., Dai, Y., Biswas, P., Stuehr, D. J. Myoglobin maturation is driven by the hsp90 chaperone machinery and by soluble guanylyl cyclase.

Use of Computational Biochemistry for Elucidating Molecular Mechanisms of Nitric Oxide Synthase

Nitric oxide (NO) is an essential signaling molecule in the regulation of multiple cellular processes. It is endogenously synthesized by NO synthase (NOS) as the product of L-arginine oxidation to L-citrulline, requiring NADPH, molecular oxygen, and a pterin cofactor. Two NOS isoforms are constitutively present in cells, nNOS and eNOS, and a third is inducible (iNOS). Despite their biological relevance, the details of their complex structural features and reactivity mechanisms are still unclear. In this review, we summarized the contribution of computational biochemistry to research on NOS molecular mechanisms. We described in detail its use in studying aspects of structure, dynamics and reactivity. We also focus on the numerous outstanding questions in the field that could benefit from more extensive computational investigations.

Nitric oxide synthase enzymology in the 20 years after the Nobel Prize

This review briefly summarizes what was known about NOS enzymology at the time of the Nobel Prize award in 1998 and then discusses from the author's perspective some of the advances in NOS enzymology over the subsequent 20 years, focused on five aspects: the maturation process of NOS enzymes and its regulation; the mechanism of NO synthesis; the redox roles played by the 6R-tetrahydrobiopterin cofactor; the role of protein conformational behaviour in enabling NOS electron transfer and its regulation by NOS structural elements and calmodulin, and the catalytic cycling pathways of NOS enzymes and their influence on NOS activity. LINKED ARTICLES: This article is part of a themed section on Nitric Oxide 20 Years from the 1998 Nobel Prize. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.2/issuetoc.

Identification of HSP90 as a direct target of artemisinin for its anti-inflammatory activity via quantitative chemical proteomics

Global profiling of the target proteins of ART for its anti-inflammatory activity via ABPP combined with quantitative chemical proteomics.

The Hsp70-Hsp90 Chaperone Cascade in Protein Folding

Conserved families of molecular chaperones assist protein folding in the cell. Here we review the conceptual advances on three major folding routes: (i) spontaneous, chaperone-independent folding; (ii) folding assisted by repetitive Hsp70 cycles; and (iii) folding by the Hsp70-Hsp90 cascades. These chaperones prepare their protein clients for folding on their own, without altering their folding path. A particularly interesting role is reserved for Hsp90. The function of Hsp90 in folding is its ancient function downstream of Hsp70, free of cochaperone regulation and present in all kingdoms of life. Eukaryotic signalling networks, however, embrace Hsp90 by a plethora of cochaperones, transforming the profolding machinery to a folding-on-demand factor. We discuss implications for biology and molecular medicine.

Inhibition of ferrochelatase impairs vascular eNOS/NO and sGC/cGMP signaling

Ferrochelatase (FECH) is an enzyme necessary for heme synthesis, which is essential for maintaining normal functions of endothelial nitric oxide synthase (eNOS) and soluble guanylyl cyclase (sGC). We tested the hypothesis that inhibition of vascular FECH to attenuate heme synthesis downregulates eNOS and sGC expression, resulting in impaired NO/cGMP-dependent relaxation. To this end, isolated bovine coronary arteries (BCAs) were in vitro incubated without (as controls) or with N-methyl protoporphyrin (NMPP; 10⁻⁵–10^-7M; a selective FECH antagonist) for 24 and 72 hours respectively. Tissue FECH activity, heme, nitrite/NO and superoxide levels were sequentially measured. Protein expression of FECH, eNOS and sGC was detected by western blot analysis. Vascular responses to various vasoactive agents were evaluated via isometric tension studies. Treatment of BCAs with NMPP initiated a time- and dose-dependent attenuation of FECH activity without changes in its protein expression, followed by significant reduction in the heme level. Moreover, ACh-induced relaxation and ACh-stimulated release of NO were significant reduced, associated with suppression of eNOS protein expression in NMPP-treated groups. Decreased relaxation to NO donor spermine-NONOate reached the statistical significance in BCAs incubated with NMPP for 72 hours, concomitantly with downregulation of sGCβ1 expression that was independent of heat shock protein 90 (HSP90), nor did it significantly affect BCA relaxation caused by BAY 58–2667 that activates sGC in the heme-deficiency. Neither vascular responses to non-NO/sGC-mediators nor production of superoxide was affected by NMPP-treatment. In conclusion, deletion of vascular heme production via inhibiting FECH elicits downregulation of eNOS and sGC expression, leading to an impaired NO-mediated relaxation in an oxidative stress-independent manner.

Hsp90 chaperones hemoglobin maturation in erythroid and nonerythroid cells

Maturation of adult (α2β2) and fetal hemoglobin (α2γ2) tetramers requires that heme be incorporated into each globin. While hemoglobin alpha (Hb-α) relies on a specific erythroid chaperone (alpha Hb-stabilizing protein, AHSP), the other chaperones that may help mature the partner globins (Hb-γ or Hb-β) in erythroid cells, or may enable nonerythroid cells to express mature Hb, are unknown. We investigated a role for heat-shock protein 90 (hsp90) in Hb maturation in erythroid precursor cells that naturally express Hb-α with either Hb-γ (K562 and HiDEP-1 cells) or Hb-β (HUDEP-2) and in nonerythroid cell lines that either endogenously express Hb-αβ (RAW and A549) or that we transfected to express the globins. We found the following: (i) AHSP and hsp90 associate with distinct globin partners in their immature heme-free states (AHSP with apo-Hbα, and hsp90 with apo-Hbβ or Hb-γ) and that hsp90 does not associate with mature Hb. (ii) Hsp90 stabilizes the apo-globins and helps to drive their heme insertion reactions, as judged by pharmacologic hsp90 inhibition or by coexpression of an ATP-ase defective hsp90. (iii) In nonerythroid cells, heme insertion into all globins became hsp90-dependent, which may explain how mixed Hb tetramers can mature in cells that do not express AHSP. Together, our findings uncover a process in which hsp90 first binds to immature, heme-free Hb-γ or Hb-β, drives their heme insertion process, and then dissociates to allow their heterotetramer formation with Hb-α. Thus, in driving heme insertion, hsp90 works in concert with AHSP to generate functional Hb tetramers during erythropoiesis.

Evidence for Hsp90 Co-chaperones in Regulating Hsp90 Function and Promoting Client Protein Folding

Molecular chaperones are a diverse group of highly conserved proteins that transiently interact with partially folded polypeptide chains during normal cellular processes such as protein translation, translocation, and disassembly of protein complexes. Prior to folding or after denaturation, hydrophobic residues that are normally sequestered within a folded protein are exposed to the aqueous environment and are prone to aggregation or misfolding. Multiple classes of molecular chaperones, such as Hsp70s and Hsp40s, recognize and transiently bind polypeptides with exposed hydrophobic stretches in order to prevent misfolding. Other types of chaperones, such as Hsp90, have more specialized functions in that they appear to interact with only a subset of cellular proteins. This chapter focuses on the role of Hsp90 and partner co-chaperones in promoting the folding and activation of a diverse group of proteins with critical roles in cellular signaling and function.

The Influence of Hyperoxia On Heat Shock Proteins Expression and Nitric Oxide Synthase Activity – the Review

Any stay in an environment with an increased oxygen content (a higher oxygen partial pressure, pO2) and an increased pressure (hyperbaric conditions) leads to an intensification of oxidative stress. Reactive oxygen species (ROS) damage the molecules of proteins, nucleic acids, cause lipid oxidation and are engaged in the development of numerous diseases, including diseases of the circulatory system, neurodegenerative diseases, etc. There are certain mechanisms of protection against unfavourable effects of oxidative stress. Enzymatic and non-enzymatic systems belong to them. The latter include, among others, heat shock proteins (HSP). Their precise role and mechanism of action have been a subject of intensive research conducted in recent years. Hyperoxia and hyperbaria also have an effect on the expression and activity of nitrogen oxide synthase (NOS). Its product - nitrogen oxide (NO) can react with reactive oxygen species and contribute to the development of nitrosative stress. NOS occurs as isoforms in various tissues and exhibit different reactions to the discussed factors. The authors have prepared a brief review of research determining the effect of hyperoxia and hyperbaria on HSP expression and NOS activity.

Regulation of sGC via hsp90, Cellular Heme, sGC Agonists, and NO: New Pathways and Clinical Perspectives

SIGNIFICANCE

Soluble guanylate cyclase (sGC) is an intracellular enzyme that plays a primary role in sensing nitric oxide (NO) and transducing its multiple signaling effects in mammals. Recent Advances: The chaperone heat shock protein 90 (hsp90) associates with signaling proteins in cells, including sGC, where it helps to drive heme insertion into the sGC-β1 subunit. This allows sGC-β1 to associate with a partner sGC-α1 subunit and mature into an NO-responsive active form.

CRITICAL ISSUES

In this article, we review evidence to date regarding the mechanisms that modulate sGC activity by a pathway where binding of hsp90 or sGC agonist to heme-free sGC dictates the assembly and fate of an active sGC heterodimer, both by NO and heme-dependent or heme-independent pathways.

FUTURE DIRECTIONS

We discuss some therapeutic implications of the NO-sGC-hsp90 nexus and its potential as a marker of inflammatory disease. Antioxid. Redox Signal. 26, 182-190.

17-DMAG, an HSP90 Inhibitor, Ameliorates Multiple Organ Dysfunction Syndrome via Induction of HSP70 in Endotoxemic Rats

Sepsis is a systemic inflammatory disorder, accompanied with elevated oxidative stress, leading to multiple organ dysfunction syndrome (MODS), and disseminated intravascular coagulation. 17-Dimethylaminoethylamino- 17-demethoxygeldanamycin (17-DMAG), a heat shock protein (HSP) 90 inhibitor, has been reported to possess anti-inflammatory effects. In this study, the beneficial effects of 17-DMAG on lipopolysaccharide (LPS) induced MODS and DIC was evaluated in anesthetized rats. 17-DMAG (5 mg/kg, i.p.) was significantly increased survival rate, and prevented hypotension in LPS (30 mg/kg i.v. infused for 4 h) induced endotoxemia. The elevated levels of alanine aminotransferase (ALT), creatine phosphokinase (CPK), lactate dehydrogenase, creatinine, nitric oxide (NO) metabolites, IL-6, and TNF-α in LPS-exposed rat plasma were significantly reduced by 17-DMAG. Moreover, 17-DMAG suppressed LPS-induced superoxide anion production and caspase 3 activation in heart tissues. LPS induced the prolongation of prothrombin time, and a pronounced decrease in platelet count, which were improved by 17-DMAG. 17-DMAG markedly induced HSP70 and heme oxygenase (HO)-1, and suppressed inducible nitric oxide synthase (iNOS) and phosphorylated NF-κB p65 protein expression in organs 6 h after LPS initiation. Pretreatment with high dose of quercetin (300 mg/kg, i.p.), as an HSP70 inhibitor, reversed the beneficial effects of 17-DMAG on survival rate, plasma levels of ALT, CPK, creatinine, IL-6, and NO metabolites, iNOS induction, and caspase-3 activation in LPS-treated rats. In conclusion, 17-DMAG possesses the anti-inflammatory and antioxidant effects that were proved through LPS-induced acute inflammation, which is associated with induction of HSP70 and HO-1, leading to prevent MODS in sepsis.

Targeted proteomics identify metabolism-dependent interactors of yeast cytochrome c peroxidase: implications in stress response and heme trafficking

Recently we discovered that cytochrome c peroxidase (Ccp1) functions primarily as a mitochondrial H2O2 sensor and heme donor in yeast cells. When cells switch their metabolism from fermentation to respiration mitochondrial H2O2 levels spike, and overoxidation of its polypeptide labilizes Ccp1's heme. A large pool of heme-free Ccp1 exits the mitochondria and enters the nucleus and vacuole. To gain greater insight into the mechanisms of Ccp1's H2O2-sensing and heme-donor functions during the cell's different metabolic states, here we use glutathione-S-transferase (GST) pulldown assays, combined with 1D gel electrophoresis and mass spectrometry to probe for interactors of apo- and holoCcp1 in extracts from 1 d fermenting and 7 d stationary-phase respiring yeast. We identified Ccp1's peroxidase cosubstrate Cyc1 and 28 novel interactors of GST-apoCcp1 and GST-holoCcp1 including mitochondrial superoxide dismutase 2 (Sod2) and cytosolic Sod1, the mitochondrial transporter Pet9, the three yeast isoforms of glyceraldehyde-3-phosphate dehydrogenase (Tdh3/2/1), heat shock proteins including Hsp90 and Hsp70, and the main peroxiredoxin in yeast (Tsa1) as well as its cosubstrate, thioreoxin (Trx1). These new interactors expand the scope of Ccp1's possible roles in stress response and in heme trafficking and suggest several new lines of investigation. Furthermore, our targeted proteomics analysis underscores the limitations of large-scale interactome studies that found only 4 of the 30 Ccp1 interactors isolated here.

Nox5 stability and superoxide production is regulated by C-terminal binding of Hsp90 and CO-chaperones

Heat shock protein 90 (Hsp90) is a molecular chaperone that orchestrates the folding and stability of proteins that regulate cellular signaling, proliferation and inflammation. We have previously shown that Hsp90 controls the production of reactive oxygen species by modulating the activity of Noxes1-3 and 5, but not Nox4. The goal of the current study was to define the regions on Nox5 that bind Hsp90 and determine how Hsp90 regulates enzyme activity. In isolated enzyme activity assays, we found that Hsp90 inhibitors selectively decrease superoxide, but not hydrogen peroxide, production. The addition of Hsp90 alone only modestly increases Nox5 enzyme activity but in combination with the co-chaperones, Hsp70, HOP, Hsp40, and p23 it robustly stimulated superoxide, but not hydrogen peroxide, production. Proximity ligation assays reveal that Nox5 and Hsp90 interact in intact cells. In cell lysates using a co-IP approach, Hsp90 binds to Nox5 but not Nox4, and the degree of binding can be influenced by calcium-dependent stimuli. Inhibition of Hsp90 induced the degradation of full length, catalytically inactive and a C-terminal fragment (aa398-719) of Nox5. In contrast, inhibition of Hsp90 did not affect the expression levels of N-terminal fragments (aa1-550) suggesting that Hsp90 binding maintains the stability of C-terminal regions. In Co-IP assays, Hsp90 was bound only to the C-terminal region of Nox5. Further refinement using deletion analysis revealed that the region between aa490-550 mediates Hsp90 binding. Converse mapping experiments show that the C-terminal region of Nox5 bound to the M domain of Hsp90 (aa310-529). In addition to Hsp90, Nox5 bound other components of the foldosome including co-chaperones Hsp70, HOP, p23 and Hsp40. Silencing of HOP, Hsp40 and p23 reduced Nox5-dependent superoxide. In contrast, increased expression of Hsp70 decreased Nox5 activity whereas a mutant of Hsp70 failed to do so. Inhibition of Hsp90 results in the loss of higher molecular weight complexes of Nox5 and decreased interaction between monomers. Collectively these results show that the C-terminal region of Nox5 binds to the M domain of Hsp90 and that the binding of Hsp90 and select co-chaperones facilitate oligomerization and the efficient production of superoxide.

Stromal cell-derived factor 2 is critical for Hsp90-dependent eNOS activation

Endothelial nitric oxide synthase (eNOS) catalyzes the conversion of l-arginine and molecular oxygen into l-citrulline and nitric oxide (NO), a gaseous second messenger that influences cardiovascular physiology and disease. Several mechanisms regulate eNOS activity and function, including phosphorylation at Ser and Thr residues and protein-protein interactions. Combining a tandem affinity purification approach and mass spectrometry, we identified stromal cell-derived factor 2 (SDF2) as a component of the eNOS macromolecular complex in endothelial cells. SDF2 knockdown impaired agonist-stimulated NO synthesis and decreased the phosphorylation of eNOS at Ser(1177), a key event required for maximal activation of eNOS. Conversely, SDF2 overexpression dose-dependently increased NO synthesis through a mechanism involving Akt and calcium (induced with ionomycin), which increased the phosphorylation of Ser(1177) in eNOS. NO synthesis by iNOS (inducible NOS) and nNOS (neuronal NOS) was also enhanced upon SDF2 overexpression. We found that SDF2 was a client protein of the chaperone protein Hsp90, interacting preferentially with the M domain of Hsp90, which is the same domain that binds to eNOS. In endothelial cells exposed to vascular endothelial growth factor (VEGF), SDF2 was required for the binding of Hsp90 and calmodulin to eNOS, resulting in eNOS phosphorylation and activation. Thus, our data describe a function for SDF2 as a component of the Hsp90-eNOS complex that is critical for signal transduction in endothelial cells.

Heat Shock Protein 90 Associates with the Per-Arnt-Sim Domain of Heme-free Soluble Guanylate Cyclase: IMplications for Enzyme Maturation

Heat shock protein 90 (hsp90) drives heme insertion into the β1 subunit of soluble guanylate cyclase (sGC) β1, which enables it to associate with a partner sGCα1 subunit and mature into a nitric oxide (NO)-responsive active form. We utilized fluorescence polarization measurements and hydrogen-deuterium exchange mass spectrometry to define molecular interactions between the specific human isoforms hsp90β and apo-sGCβ1. hsp90β and its isolated M domain, but not its isolated N and C domains, bind with low micromolar affinity to a heme-free, truncated version of sGCβ1 (sGCβ1(1-359)-H105F). Surprisingly, hsp90β and its M domain bound to the Per-Arnt-Sim (PAS) domain of apo-sGC-β1(1-359), which lies adjacent to its heme-binding (H-NOX) domain. The interaction specifically involved solvent-exposed regions in the hsp90β M domain that are largely distinct from sites utilized by other hsp90 clients. The interaction strongly protected two regions of the sGCβ1 PAS domain and caused local structural relaxation in other regions, including a PAS dimerization interface and a segment in the H-NOX domain. Our results suggest a means by which the hsp90β interaction could prevent apo-sGCβ1 from associating with its partner sGCα1 subunit while enabling structural changes to assist heme insertion into the H-NOX domain. This mechanism would parallel that in other clients like the aryl hydrocarbon receptor and HIF1α, which also interact with hsp90 through their PAS domains to control protein partner and small ligand binding interactions.

A model in which heat shock protein 90 targets protein-folding clefts: rationale for a new approach to neuroprotective treatment of protein folding diseases

In an EBM Minireview published in 2010, we proposed that the heat shock protein (Hsp)90/Hsp70-based chaperone machinery played a major role in determining the selection of proteins that have undergone oxidative or other toxic damage for ubiquitination and proteasomal degradation. The proposal was based on a model in which the Hsp90 chaperone machinery regulates signaling by modulating ligand-binding clefts. The model provides a framework for thinking about the development of neuroprotective therapies for protein-folding diseases like Alzheimer's disease (AD), Parkinson's disease (PD), and the polyglutamine expansion disorders, such as Huntington's disease (HD) and spinal and bulbar muscular atrophy (SBMA). Major aberrant proteins that misfold and accumulate in these diseases are "client" proteins of the abundant and ubiquitous stress chaperone Hsp90. These Hsp90 client proteins include tau (AD), α-synuclein (PD), huntingtin (HD), and the expanded glutamine androgen receptor (polyQ AR) (SBMA). In this Minireview, we update our model in which Hsp90 acts on protein-folding clefts and show how it forms a rational basis for developing drugs that promote the targeted elimination of these aberrant proteins.

Nitric oxide and heat shock protein 90 activate soluble guanylate cyclase by driving rapid change in its subunit interactions and heme content

The chaperone heat shock protein 90 (hsp90) associates with signaling proteins in cells including soluble guanylate cyclase (sGC). hsp90 associates with the heme-free (apo) sGC-β1 subunit and helps to drive heme insertion during maturation of sGC to its NO-responsive active form. Here, we found that NO caused apo-sGC-β1 to rapidly and transiently dissociate from hsp90 and associate with sGC-α1 in cells. This NO response (i) required that hsp90 be active and that cellular heme be available and be capable of inserting into apo-sGC-β1; (ii) was associated with an increase in sGC-β1 heme content; (iii) could be mimicked by the heme-independent sGC activator BAY 60-2770; and (iv) was followed by desensitization of sGC toward NO, sGC-α1 disassociation, and reassociation with hsp90. Thus, NO promoted a rapid, transient, and hsp90-dependent heme insertion into the apo-sGC-β1 subpopulation in cells, which enabled it to combine with the sGC-α1 subunit to form the mature enzyme. The driving mechanism likely involves conformational changes near the heme site in sGC-β1 that can be mimicked by the pharmacologic sGC activator. Such dynamic interplay between hsp90, apo-sGC-β1, and sGC-α1 in response to NO is unprecedented and represent new steps by which cells can modulate the heme content and activity of sGC for signaling cascades.

Dissecting regulation mechanism of the FMN to heme interdomain electron transfer in nitric oxide synthases

Nitric oxide synthase (NOS), a flavo-hemoprotein, is responsible for biosynthesis of nitric oxide (NO) in mammals. Three NOS isoforms, iNOS, eNOS and nNOS (inducible, endothelial, and neuronal NOS), achieve their biological functions by tight control of interdomain electron transfer (IET) process through interdomain interactions. In particular, the FMN-heme IET is essential in coupling electron transfer in the reductase domain with NO synthesis in the heme domain by delivery of electrons required for O2 activation at the catalytic heme site. Emerging evidence indicates that calmodulin (CaM) activates NO synthesis in eNOS and nNOS by a conformational change of the FMN domain from its shielded electron-accepting (input) state to a new electron-donating (output) state, and that CaM is also required for proper alignment of the FMN and heme domains in the three NOS isoforms. In the absence of a structure of full-length NOS, an integrated approach of spectroscopic, rapid kinetic and mutagenesis methods is required to unravel regulation mechanism of the FMN-heme IET process. This is to investigate the roles of the FMN domain motions and the docking between the primary functional FMN and heme domains in regulating NOS activity. The recent developments in this area that are driven by the combined approach are the focuses of this review. A better understanding of the roles of interdomain FMN/heme interactions and CaM binding may serve as a basis for the rational design of new selective modulators of the NOS enzymes.

Heat-shock protein 70 is involved in hyperbaric oxygen preconditioning on decompression sickness in rats

Decompression sickness (DCS) is a major concern in diving and space walk. Hyperbaric oxygen (HBO) preconditioning has been proved to enhance tolerance to DCS via nitric oxide. Heat-shock protein (HSP) 70 was also found to have protective effects against DCS. We hypothesized that the beneficial effects of HBO preconditioning on DCS was related to levels of elevated HSP70. HSPs (70, 27 and 90) expressed in tissues of spinal cord and lung in rats was detected at different time points following HBO exposure by Western blot. HSP27 and HSP90 showed a slight but not significant increase after HBO. HSP70 increased and reached highest at 18 h following exposure before decreasing. Then rats were exposed to HBO and subjected to simulated air dive and rapid decompression to induce DCS 18 h after HBO. The severity of DCS, along with levels of HSP70 expression, as well as the extent of oxidative and apoptotic parameters in the lung and spinal cord were compared among different groups of rats pretreated with HBO, HBO plus NG-nitro-l-arginine-methyl ester (l-NAME), HBO plus quercetin or normobaric air. HBO preconditioning significantly reduced the morbidity of DCS (from 66.7% to 36.7%), reduced levels of oxidation (malondialdehyde, 8-hydroxyguanine and hydrogen peroxide) and apoptosis (caspase-3 and -9 activities and the number of apoptotic cells). l-NAME or quercetin eliminated most of the beneficial effects of HBO on DCS, and counteracted the stimulation of HSP70 by HBO. Bubbles in pulmonary artery were detected using ultrasound imaging to observe the possible effect of HBO preconditioning on DCS bubble formation. The amounts of bubbles in rats pretreated with HBO or air showed no difference. These results suggest that HSP70 was involved in the beneficial effects of HBO on DCS in rats, suspected be by the antioxidation and antiapoptosis effects.

Proteomic analysis of the NOS2 interactome in human airway epithelial cells

The cytokine-inducible isoform of nitric oxide synthase (NOS2) is constitutively expressed in human respiratory epithelia and is upregulated in inflammatory lung disease. Here, we sought to better define the protein interactions that may be important for NOS2 activity and stability, as well as to identify potential targets of NOS2-derived NO, in the respiratory epithelium. We overexpressed Flag-tagged, catalytically-inactive NOS2 in A549 cells and used mass spectrometry to qualitatively identify NOS2 co-immunoprecipitating proteins. Stable isotope labeling of amino acids in cell culture (SILAC) was used to quantify the coordinate effects of cytokine stimulation on NOS2-protein interactions. Multi-protein networks dominated the NOS2 interactome, and cytokine-inducible interactions with allosteric activators and with the ubiquitin-proteasome system were correlated with cytokine-dependent increases in NO metabolites and in NOS2 ubiquitination. The ubiquitin ligase scaffolding protein, FBXO45, was identified as a novel, direct NOS2 interactor. Similar to the SPRY domain-containing SOCS box (SPSB) proteins, FBXO45 requires Asn27 in the (23)DINNN(27) motif of NOS2 for its interaction. However, FBXO45 is unique from the SPSBs in that it recruits a distinct E3 ligase complex containing MYCBP2 and SKP1. Collectively, these findings demonstrate the general utility of interaction proteomics for defining new aspects of NOS2 physiology.

Heme transport and erythropoiesis

In humans, systemic heme homeostasis is achieved via coordinated regulation of heme synthesis, transport and degradation. Although the heme biosynthesis and degradation pathways have been well characterized, the pathways for heme trafficking and incorporation into hemoproteins remain poorly understood. In the past few years, researchers have exploited genetic, cellular and biochemical tools, to identify heme transporters and, in the process, reveal unexpected functions for this elusive group of proteins. However, given the complexity of heme trafficking pathways, current knowledge of heme transporters is fragmented and sometimes contradictory. This review seeks to focus on recent studies on heme transporters with specific emphasis on their functions during erythropoiesis.

Quantitative analysis of HSP90-client interactions reveals principles of substrate recognition

HSP90 is a molecular chaperone that associates with numerous substrate proteins called clients. It plays many important roles in human biology and medicine, but determinants of client recognition by HSP90 have remained frustratingly elusive. We systematically and quantitatively surveyed most human kinases, transcription factors, and E3 ligases for interaction with HSP90 and its cochaperone CDC37. Unexpectedly, many more kinases than transcription factors bound HSP90. CDC37 interacted with kinases, but not with transcription factors or E3 ligases. HSP90::kinase interactions varied continuously over a 100-fold range and provided a platform to study client protein recognition. In wild-type clients, HSP90 did not bind particular sequence motifs, but rather associated with intrinsically unstable kinases. Stabilization of the kinase in either its active or inactive conformation with diverse small molecules decreased HSP90 association. Our results establish HSP90 client recognition as a combinatorial process: CDC37 provides recognition of the kinase family, whereas thermodynamic parameters determine client binding within the family.

Soluble guanylyl cyclase requires heat shock protein 90 for heme insertion during maturation of the NO-active enzyme

Heme insertion is key during maturation of soluble guanylyl cyclase (sGC) because it enables sGC to recognize NO and transduce its multiple biological effects. Although sGC is often associated with the 90-kDa heat shock protein (hsp90) in cells, the implications are unclear. The present study reveals that hsp90 is required to drive heme insertion into sGC and complete its maturation. We used a mammalian cell culture approach and followed heme insertion into transiently and endogenously expressed heme-free sGC. We used pharmacological hsp90 inhibitors, an ATP-ase inactive hsp90 mutant, and heme-dependent or heme-independent sGC activators as tools to decipher the role of hsp90. Our findings suggest that hsp90 complexes with apo-sGC, drives heme insertion through its inherent ATPase activity, and then dissociates from the mature, heme-replete sGC. Together, this improves our understanding of sGC maturation and reveals a unique means to control sGC activity in cells, and it has important implications for hsp90 inhibitor-based cancer therapy.