The small heat shock proteins, HSPB1 and HSPB5, interact differently with lipid membranes

HSPB6: A lipid-dependent molecular chaperone inhibits α-synuclein aggregation

The process of protein misfolding and aggregation is associated with various cytotoxic effects. Understanding how this phenomenon is regulated by the protein homeostasis system, however, is difficult, since it takes place through a complex non-linear network of coupled microscopic steps, including primary nucleation, fibril elongation, and secondary nucleation, which depend on environmental factors. To address this problem, we studied how the aggregation of α-synuclein, a protein associated with Parkinson's disease, is modulated by molecular chaperones and lipid membranes. We focused on small heat shock proteins (sHSPs/HSPBs), which interact with proteins and lipids and are upregulated during aging, a major risk factor for protein misfolding diseases. HSPBs act on different microscopic steps to prevent α-synuclein aggregation, with HSPB6 showing a lipid-dependent chaperone activity. Our findings provide an example of how HSPBs diversified their mechanisms of action to reach an efficient regulation of protein misfolding and aggregation within the complex cellular environment.

Evidence that extracellular HSPB1 contributes to inflammation in alcohol-associated hepatitis

Background and aims

Alcohol-associated hepatitis (AH) is the most life-threatening form of alcohol-associated liver disease (ALD). AH is characterized by severe inflammation attributed to increased levels of ethanol, microbes or microbial components, and damage-associated molecular pattern (DAMP) molecules in the liver. HSPB1 (Heat Shock Protein Family B (Small) Member 1; also known as Hsp25/27) is a DAMP that is rapidly increased in and released from cells experiencing stress, including hepatocytes. The goal of this study was to define the role of HSPB1 in AH pathophysiology.

Methods

Serum HSPB1 was measured in a retrospective study of 184 heathy controls (HC), heavy alcohol consumers (HA), patients with alcohol-associated cirrhosis (AC), and patients with AH recruited from major hospital centers. HSPB1 was also retrospectively evaluated in liver tissue from 10 HC and AH patients and an existing liver RNA-seq dataset. Finally, HSPB1 was investigated in a murine Lieber-DeCarli diet model of early ALD as well as cellular models of ethanol stress in hepatocytes and hepatocyte-macrophage communication during ethanol stress.

Results

Circulating HSPB1 was significantly increased in AH patients and levels positively correlated with disease-severity scores. Likewise, HSPB1 was increased in the liver of patients with severe AH and in the liver of ethanol-fed mice. In vitro , ethanol-stressed hepatocytes released HSPB1, which then triggered TNFα-mediated inflammation in macrophages. Anti-HSPB1 antibody prevented TNFα release from macrophages exposed to media conditioned by ethanol-stressed hepatocytes.

Conclusions

Our findings support investigation of HSPB1 as both a biomarker and therapeutic target in ALD. Furthermore, this work demonstrates that anti-HSPB1 antibody is a rational approach to targeting HSPB1 with the potential to block inflammation and protect hepatocytes, without inactivating host defense.

GRAPHICAL ABSTRACT

HIGHLIGHTS

HSPB1 is significantly increased in serum and liver of patients with alcohol-associated hepatitis.Ethanol consumption leads to early increases in HSPB1 in the mouse liver.Hepatocytes subjected to ethanol stress release HSPB1 into the extracellular environment where it activates TNFα-mediated inflammation in macrophages.Anti-HSPB1 antibody blocks hepatocyte-triggered TNFα in a model of hepatocyte-macrophage communication during ethanol stress.

The role of membrane physiology in sHSP Lo18-lipid interaction and lipochaperone activity

To cope with environmental stresses, organisms, including lactic acid bacteria such as O. oeni, produce stress proteins called HSPs. In wine, O. oeni is constantly confronted by stress affecting its membrane fluidity. To survive through in these deleterious conditions, O. oeni synthesizes Lo18, a unique, small HSP which acts as a molecular chaperone and a lipochaperone. The molecular mechanism underlying its lipochaperone activity, particularly regarding membrane lipid composition, remains poorly understood. In this context, Lo18 lipochaperone activity and the associated modification in protein structure were studied during interaction with different liposomes from O. oeni cultures representing unstressed, stressed and stressed-adapted physiological states. The results showed that the presence of the membrane (whatever its nature) induces a modification of Lo18's structure. Also, the presence of oleic acid and/or phosphatidylglycerol is important to favor Lo18-membrane interaction, allowing lipochaperone activity. This research enhances understanding of sHSP-membrane interactions in bacterial systems.

Reactive astrocytes secrete the chaperone HSPB1 to mediate neuroprotection

Molecular chaperones are protective in neurodegenerative diseases by preventing protein misfolding and aggregation, such as extracellular amyloid plaques and intracellular tau neurofibrillary tangles in Alzheimer's disease (AD). In addition, AD is characterized by an increase in astrocyte reactivity. The chaperone HSPB1 has been proposed as a marker for reactive astrocytes; however, its astrocytic functions in neurodegeneration remain to be elucidated. Here, we identify that HSPB1 is secreted from astrocytes to exert non-cell-autonomous protective functions. We show that in human AD brain, HSPB1 levels increase in astrocytes that cluster around amyloid plaques, as well as in the adjacent extracellular space. Moreover, in conditions that mimic an inflammatory reactive response, astrocytes increase HSPB1 secretion. Concomitantly, astrocytes and neurons can uptake astrocyte-secreted HSPB1, which is accompanied by an attenuation of the inflammatory response in reactive astrocytes and reduced pathological tau inclusions. Our findings highlight a protective mechanism in disease conditions that encompasses the secretion of a chaperone typically regarded as intracellular.

Cholesterol Content Regulates the Interaction of αA-, αB-, and α-Crystallin with the Model of Human Lens-Lipid Membranes

α-Crystallin (αABc) is a major protein comprised of αA-crystallin (αAc) and αB-crystallin (αBc) that is found in the human eye lens and works as a molecular chaperone by preventing the aggregation of proteins and providing tolerance to stress. However, with age and cataract formation, the concentration of αABc in the eye lens cytoplasm decreases, with a corresponding increase in the membrane-bound αABc. This study uses the electron paramagnetic resonance (EPR) spin-labeling method to investigate the role of cholesterol (Chol) and Chol bilayer domains (CBDs) in the binding of αAc, αBc, and αABc to the Chol/model of human lens-lipid (Chol/MHLL) membranes. The maximum percentage of membrane surface occupied (MMSO) by αAc, αBc, and αABc to Chol/MHLL membranes at a mixing ratio of 0 followed the trends: MMSO (αAc) > MMSO (αBc) ≈ MMSO (αABc), indicating that a higher amount of αAc binds to these membranes compared to αBc and αABc. However, with an increase in the Chol concentration in the Chol/MHLL membranes, the MMSO by αAc, αBc, and αABc decreases until it is completely diminished at a mixing ratio of 1.5. The Ka of αAc, αBc, and αABc to Chol/MHLL membranes at a mixing ratio of 0 followed the trend: Ka (αBc) ≈ Ka (αABc) > Ka (αAc), but it was close to zero with the diminished binding at a Chol/MHLL mixing ratio of 1.5. The mobility near the membrane headgroup regions decreased with αAc, αBc, and αABc binding, and the Chol antagonized the capacity of the αAc, αBc, and αABc to decrease mobility near the headgroup regions. No significant change in membrane order near the headgroup regions was observed, with an increase in αAc, αBc, and αABc concentrations. Our results show that αAc, αBc, and αABc bind differently with Chol/MHLL membranes at mixing ratios of 0 and 0.5, decreasing the mobility and increasing hydrophobicity near the membrane headgroup region, likely forming the hydrophobic barrier for the passage of polar and ionic molecules, including antioxidants (glutathione), creating an oxidative environment inside the lens, leading to the development of cataracts. However, all binding was completely diminished at a mixing ratio of 1.5, indicating that high Chol and CBDs inhibit the binding of αAc, αBc, and αABc to membranes, preventing the formation of hydrophobic barriers and likely protecting against cataract formation.

Significant influence of four highly conserved amino-acids in lipochaperon-active sHsps on the structure and functions of the Lo18 protein

To cope with environmental stresses, bacteria have developed different strategies, including the production of small heat shock proteins (sHSP). All sHSPs are described for their role as molecular chaperones. Some of them, like the Lo18 protein synthesized by Oenococcus oeni, also have the particularity of acting as a lipochaperon to maintain membrane fluidity in its optimal state following cellular stresses. Lipochaperon activity is poorly characterized and very little information is available on the domains or amino-acids key to this activity. The aim in this paper is to investigate the importance at the protein structure and function level of four highly conserved residues in sHSP exhibiting lipochaperon activity. Thus, by combining in silico, in vitro and in vivo approaches the importance of three amino-acids present in the core of the protein was shown to maintain both the structure of Lo18 and its functions.

Insight into the biochemical and cell biological function of an intrinsically unstructured heat shock protein, Hsp12 of Ustilago maydis

Small heat shock proteins (sHsps) play diverse roles in the stress response and maintenance of cellular functions. The Ustilago maydis genome codes for few sHsps. Among these, Hsp12 has previously been demonstrated to be involved in the pathogenesis of the fungus by our group. In the present study we further investigated the biological function of the protein in the pathogenic development of U. maydis. Analysis of the primary amino acid sequence of Hsp12 in combination with spectroscopic methods to analyse secondary protein structures revealed an intrinsically disordered nature of the protein. We also carried out detailed analysis on the protein aggregation prevention activity associated with Hsp12. Our data suggest Hsp12 has trehalose-dependent protein aggregation prevention activity. Through assaying the interaction of Hsp12 with lipid membranes in vitro we also showed the ability of U. maydis Hsp12 to induce stability in lipid vesicles. U. maydis hsp12 deletion mutants exhibited defects in the endocytosis process and delayed completion of the pathogenic life cycle. Therefore, U. maydis Hsp12 contributes to the pathogenic development of the fungus through its ability to relieve proteotoxic stress during infection as well as its membrane-stabilizing function.

Functional Diversity of Mammalian Small Heat Shock Proteins: A Review

The small heat shock proteins (sHSPs), whose molecular weight ranges from 12∼43 kDa, are members of the heat shock protein (HSP) family that are widely found in all organisms. As intracellular stress resistance molecules, sHSPs play an important role in maintaining the homeostasis of the intracellular environment under various stressful conditions. A total of 10 sHSPs have been identified in mammals, sharing conserved α-crystal domains combined with variable N-terminal and C-terminal regions. Unlike large-molecular-weight HSP, sHSPs prevent substrate protein aggregation through an ATP-independent mechanism. In addition to chaperone activity, sHSPs were also shown to suppress apoptosis, ferroptosis, and senescence, promote autophagy, regulate cytoskeletal dynamics, maintain membrane stability, control the direction of cellular differentiation, modulate angiogenesis, and spermatogenesis, as well as attenuate the inflammatory response and reduce oxidative damage. Phosphorylation is the most significant post-translational modification of sHSPs and is usually an indicator of their activation. Furthermore, abnormalities in sHSPs often lead to aggregation of substrate proteins and dysfunction of client proteins, resulting in disease. This paper reviews the various biological functions of sHSPs in mammals, emphasizing the roles of different sHSPs in specific cellular activities. In addition, we discuss the effect of phosphorylation on the function of sHSPs and the association between sHSPs and disease.

The HSPB1-p62/SQSTM1 functional complex regulates the unconventional secretion and transcellular spreading of the HD-associated mutant huntingtin protein

Conformational diseases, such as Alzheimer, Parkinson and Huntington diseases, are part of a common class of neurological disorders characterized by the aggregation and progressive accumulation of proteins bearing aberrant conformations. Huntington disease (HD) has autosomal dominant inheritance and is caused by mutations leading to an abnormal expansion in the polyglutamine (polyQ) tract of the huntingtin (HTT) protein, leading to the formation of HTT inclusion bodies in neurons of affected patients. Interestingly, recent experimental evidence is challenging the conventional view by which the disease pathogenesis is solely a consequence of the intracellular accumulation of mutant protein aggregates. These studies reveal that transcellular transfer of mutated huntingtin protein is able to seed oligomers involving even the wild-type (WT) forms of the protein. To date, there is still no successful strategy to treat HD. Here, we describe a novel functional role for the HSPB1-p62/SQSTM1 complex, which acts as a cargo loading platform, allowing the unconventional secretion of mutant HTT by extracellular vesicles. HSPB1 interacts preferentially with polyQ-expanded HTT compared with the WT protein and affects its aggregation. Furthermore, HSPB1 levels correlate with the rate of mutant HTT secretion, which is controlled by the activity of the PI3K/AKT/mTOR signalling pathway. Finally, we show that these HTT-containing vesicular structures are biologically active and able to be internalized by recipient cells, therefore providing an additional mechanism to explain the prion-like spreading properties of mutant HTT. These findings might also have implications for the turn-over of other disease-associated, aggregation-prone proteins.

Is the lipochaperone activity of sHSP a key to the stress response encoded in its primary sequence?

Several strategies have been put in place by organisms to adapt to their environment. One of these strategies is the production of stress proteins such as sHSPs, which have been widely described over the last 30 years for their role as molecular chaperones. Some sHSPs have, in addition, the particularity to exert a lipochaperone role by interacting with membrane lipids to maintain an optimal membrane fluidity. However, the mechanisms involved in this sHSP-lipid interaction remain poorly understood and described rather sporadically in the literature. This review gathers the information concerning the structure and function of these proteins available in the literature in order to highlight the mechanism involved in this interaction. In addition, analysis of primary sequence data of sHSPs available in database shows that sHSPs can interact with lipids via certain amino acid residues present on some β sheets of these proteins. These residues could have a key role in the structure and/or oligomerization dynamics of sHPSs, which is certainly essential for interaction with membrane lipids and consequently for maintaining optimal cell membrane fluidity.

The Small Heat Shock Protein, HSPB1, Interacts with and Modulates the Physical Structure of Membranes

Small heat shock proteins (sHSPs) have been demonstrated to interact with lipids and modulate the physical state of membranes across species. Through these interactions, sHSPs contribute to the maintenance of membrane integrity. HSPB1 is a major sHSP in mammals, but its lipid interaction profile has so far been unexplored. In this study, we characterized the interaction between HSPB1 and phospholipids. HSPB1 not only associated with membranes via membrane-forming lipids, but also showed a strong affinity towards highly fluid membranes. It participated in the modulation of the physical properties of the interacting membranes by altering rotational and lateral lipid mobility. In addition, the in vivo expression of HSPB1 greatly affected the phase behavior of the plasma membrane under membrane fluidizing stress conditions. In light of our current findings, we propose a new function for HSPB1 as a membrane chaperone.

The interaction of heat shock proteins with cellular membranes: a historical perspective

The interaction of heat shock proteins (HSP) with cellular membranes has been an enigmatic process, initially observed by morphological studies, inferred during the purification of HSP70s, and confirmed after the detection of these proteins on the surface of cancer cells and their insertion into artificial lipid bilayers. Today, the association of several HSP with lipid membranes is well established. However, the mechanisms for membrane insertion have been elusive. There is conclusive evidence indicating that HSP70s have a great selectivity for negatively charged phospholipids, whereas other HSP have a broader spectrum of lipid specificity. HSP70 also oligomerizes upon membrane insertion, forming ion conductance channels. The functional role of HSP70 lipid interactions appears related to membrane stabilization that may play a role during cell membrane biogenesis. They could also play a role as membrane chaperones as well as during endocytosis, microautophagy, and signal transduction. Moreover, HSP membrane association is a key component in the extracellular export of these proteins. The presence of HSP70 on the surface of cancer cells and its interaction with lysosome membranes have been envisioned as potential therapeutic targets. Thus, the biology and function of HSP membrane association are reaching a new level of excitement. This review is an attempt to preserve the recollection of the pioneering contributions of many investigators that have participated in this endeavor.

Heat shock proteins and the biogenesis of cellular membranes

The successful function of cells is importantly contributed by lipid membranes that are more than a simple physical barrier. The major components of cellular membranes are lipids, in particular glycerophospholipids, that have the capacity to assemble spontaneously into vesicles containing a lipid bilayer after exposure to an aqueous milieu due to their amphiphilic characteristics. The lipid capacity to form vesicles and encapsulate substrates has been proposed as a fundamental event during the biogenesis of cells. However, the stability of small vesicles is compromised during their expansion into larger and more complex particles. Recent observations by (Cornell et al. Proc Natl Acad Sci U S A 116:17239-17244, 2019) have shown that the insertion of amino acids into rudimentary vesicles could play a stabilizing role that was critical to the formation of early cells. Fatty acids were likely substituted by glycerophospholipids and amino acids replaced by polypeptides during the evolution of protocells. Thus, archaic peptides displaying lipid-binding and membrane-penetrating capacities could have played a key function in the development of current cells. In this regard, heat shock proteins (HSP), particularly the Hsp70 (HSPA) and small HSP (HSPB) families, could have portrayed that role. Indeed, bacterial DnaK is closest in sequence to the earliest members of the Hsp70 family and inserts into lipid membranes spontaneously. Moreover, extensive studies by the Vigh group have shown that, certainly, Hsp70s stabilize membranes. Thus, the ability of ancestral HSP70s and small HSPs to associate with lipids and stabilize membranes could have been a fundamental event in the genesis of cells.

Interaction of HSPA5 (Grp78, BIP) with negatively charged phospholipid membranes via oligomerization involving the N-terminal end domain

Heat shock proteins (HSPs) are ubiquitous polypeptides expressed in all living organisms that participate in several basic cellular processes, including protein folding, from which their denomination as molecular chaperones originated. There are several HSPs, including HSPA5, also known as 78-kDa glucose-regulated protein (GRP78) or binding immunoglobulin protein (BIP) that is an ER resident involved in the folding of polypeptides during their translocation into this compartment prior to the transition to the Golgi network. HSPA5 is detected on the surface of cells or secreted into the extracellular environment. Surface HSPA5 has been proposed to have various roles, such as receptor-mediated signal transduction, a co-receptor for soluble ligands, as well as a participant in tumor survival, proliferation, and resistance. Recently, surface HSPA5 has been reported to be a potential receptor of some viruses, including the novel SARS-CoV-2. In spite of these observations, the association of HSPA5 within the plasma membrane is still unclear. To gain information about this process, we studied the interaction of HSPA5 with liposomes made of different phospholipids. We found that HSPA5 has a high affinity for negatively charged phospholipids, such as palmitoyl-oleoyl phosphoserine (POPS) and cardiolipin (CL). The N-terminal and C-terminal domains of HSPA5 were independently capable of interacting with negatively charged phospholipids, but to a lesser extent than the full-length protein, suggesting that both domains are required for the maximum insertion into membranes. Interestingly, we found that the interaction of HSPA5 with negatively charged liposomes promotes an oligomerization process via intermolecular disulfide bonds in which the N-terminus end of the protein plays a critical role.

Critical insights into the interactions of heat shock protein 70 with phospholipids

Heat shock proteins (Hsps) stabilize the newly synthesized polypeptide chains preventing them from aggregation. They contribute to systemic response under stress and thus behave as signaling molecules. Hsp70 has been detected on the surface of stressed cells. It translocates to the extracellular environment through the plasma membrane without causing cell death. But the interaction of the protein with the membrane leading to the export process remains elusive. Hsp70 has a tendency to generate channels within lipid bilayers, and this has been a driving force for studying protein-lipid interactions. Transport of these proteins across the membrane paves their pathways for performing the desired function. We have attempted to characterize how the interaction of Hsp70 with negatively charged phospholipids affects the structure of lipids. This study will help in explaining the transport mechanism of proteins that are devoid of defined signaling pathways. The interaction of amino acids of Hsp70 with the head and tail group leads to the rearrangement of the hydration layer in contact with the bilayers. Critical analysis of the results obtained from small-angle X-ray scattering along with QCM-D provides valuable insights to analyze the effect of Hsp70 adsorption on an anionic POPS lipid bilayer.

Membrane-Associated Heat Shock Proteins in Oncology: From Basic Research to New Theranostic Targets

Heat shock proteins (HSPs) constitute a large family of conserved proteins acting as molecular chaperones that play a key role in intracellular protein homeostasis, regulation of apoptosis, and protection from various stress factors (including hypoxia, thermal stress, oxidative stress). Apart from their intracellular localization, members of different HSP families such as small HSPs, HSP40, HSP60, HSP70 and HSP90 have been found to be localized on the plasma membrane of malignantly transformed cells. In the current article, the role of membrane-associated molecular chaperones in normal and tumor cells is comprehensively reviewed with implications of these proteins as plausible targets for cancer therapy and diagnostics.

Connecting the Dots in the Neuroglobin-Protein Interaction Network of an Unstressed and Ferroptotic Cell Death Neuroblastoma Model

Neuroglobin is a heme protein of which increased levels provide neuroprotection against amyloid proteinopathy and hemorrhagic damage. These cellular stressors involve the promotion of ferroptosis—an iron-dependent, lipid peroxide-accreting form of cell death. Hence, we questioned whether neuroglobin could oppose ferroptosis initiation. We detected human neuroglobin (hNgb)-EGFP-expressing SH-SY5Y cells to be significantly more resistant to ferroptosis induction, identifying 0.68-fold less cell death. To elucidate the underlying pathways, this study investigated hNgb-protein interactions with a Co-IP-MS/MS approach both under a physiological and a ferroptotic condition. hNgb binds to proteins of the cellular iron metabolism (e.g., RPL15 and PCBP3) in an unstressed condition and shows an elevated binding ratio towards cell death-linked proteins, such as HNRNPA3, FAM120A, and ABRAXAS2, under ferroptotic stress. Our data also reveal a constitutive interaction between hNgb and the longevity-associated heterodimer XRCC5/XRCC6. Disentangling the involvement of hNgb and its binding partners in cellular processes, using Ingenuity Pathway Analysis, resulted in the integration of hNgb in the ubiquitination pathway, mTOR signaling, 14-3-3-mediated signaling, and the glycolysis cascade. We also detected a previously unknown strong link with motor neuropathies. Hence, this study contributes to the elucidation of neuroglobin’s involvement in cellular mechanisms that provide neuroprotection and the upkeep of homeostasis.