αB-crystallin interacts with and prevents stress-activated proteolysis of focal adhesion kinase by calpain in cardiomyocytes

Desmin and its molecular chaperone, the αB-crystallin: How post-translational modifications modulate their functions in heart and skeletal muscles?

Maintenance of the highly organized striated muscle tissue requires a cell-wide dynamic network through protein-protein interactions providing an effective mechanochemical integrator of morphology and function. Through a continuous and complex trans-cytoplasmic network, desmin intermediate filaments ensure this essential role in heart and in skeletal muscle. Besides their role in the maintenance of cell shape and architecture (permitting contractile activity efficiency and conferring resistance towards mechanical stress), desmin intermediate filaments are also key actors of cell and tissue homeostasis. Desmin participates to several cellular processes such as differentiation, apoptosis, intracellular signalisation, mechanotransduction, vesicle trafficking, organelle biogenesis and/or positioning, calcium homeostasis, protein homeostasis, cell adhesion, metabolism and gene expression. Desmin intermediate filaments assembly requires αB-crystallin, a small heat shock protein. Over its chaperone activity, αB-crystallin is involved in several cellular functions such as cell integrity, cytoskeleton stabilization, apoptosis, autophagy, differentiation, mitochondria function or aggresome formation. Importantly, both proteins are known to be strongly associated to the aetiology of several cardiac and skeletal muscles pathologies related to desmin filaments disorganization and a strong disturbance of desmin interactome. Note that these key proteins of cytoskeleton architecture are extensively modified by post-translational modifications that could affect their functional properties. Therefore, we reviewed in the herein paper the impact of post-translational modifications on the modulation of cellular functions of desmin and its molecular chaperone, the αB-crystallin.

Aberrant mechanical loading induces annulus fibrosus cells apoptosis in intervertebral disc degeneration via mechanosensitive ion channel Piezo1

BACKGROUND

Intervertebral disc degeneration (IVDD) is closely associated with the structural damage in the annulus fibrosus (AF). Aberrant mechanical loading is an important inducement of annulus fibrosus cells (AFCs) apoptosis, which contributes to the AF structural damage and aggravates IVDD, but the underlying mechanism is still unclear. This study aims to investigate the mechanism of a mechanosensitive ion channel protein Piezo1 in aberrant mechanical loading-induced AFCs apoptosis and IVDD.

METHODS

Rats were subjected to lumbar instability surgery to induce the unbalanced dynamic and static forces to establish the lumbar instability model. MRI and histological staining were used to evaluate the IVDD degree. A cyclic mechanical stretch (CMS)-stimulated AFCs apoptosis model was established by a Flexcell system in vitro. Tunel staining, mitochondrial membrane potential (MMP) detection, and flow cytometry were used to evaluate the apoptosis level. The activation of Piezo1 was detected using western blot and calcium fluorescent probes. Chemical activator Yoda1, chemical inhibitor GSMTx4, and a lentiviral shRNA-Piezo1 system (Lv-Piezo1) were utilized to regulate the function of Piezo1. High-throughput RNA sequencing (RNA-seq) was used to explore the mechanism of Piezo1-induced AFCs apoptosis. The Calpain activity and the activation of Calpain2/Bax/Caspase3 axis were evaluated by the Calpain activity kit and western blot with the siRNA-mediated Calapin1 or Calpain2 knockdown. Intradiscal administration of Lv-Piezo1 was utilized to evaluate the therapeutic effect of Piezo1 silencing in IVDD rats.

RESULTS

Lumbar instability surgery promoted the expression of Piezo1 in AFCs and stimulated IVDD in rats 4 weeks after surgery. CMS elicited distinct apoptosis of AFCs, with enhanced Piezo1 activation. Yoda1 further promoted CMS-induced apoptosis of AFCs, while GSMTx4 and Lv-Piezo1 exhibited opposite effects. RNA-seq showed that knocking down Piezo1 inhibited the calcium signaling pathway. CMS enhanced Calpain activity and elevated the expression of BAX and cleaved-Caspase3. Calpain2, but not Calpain1 knockdown, inhibited the expression of BAX and cleaved-Caspase3 and alleviated AFCs apoptosis. Lv-Piezo1 significantly alleviated the progress of IVDD in rats after lumbar instability surgery.

CONCLUSIONS

Aberrant mechanical loading induces AFCs apoptosis to promote IVDD by activating Piezo1 and downstream Calpain2/BAX/Caspase3 pathway. Piezo1 is expected to be a potential therapeutic target in treating IVDD.

RIOK3 promotes pancreatic ductal adenocarcinoma cell invasion and metastasis by stabilizing FAK

Pancreatic ductal adenocarcinoma (PDAC) is an extremely aggressive cancer, characterized by a high metastatic burden. RIO Kinase 3 (RIOK3) has been shown to promote invasion and metastasis of PDAC by cytoskeleton remodeling, but the exact mechanism is still unknown. In this study, we analyzed transcriptome sequencing data from RIOK3 stable knockdown PANC-1 cells and TCGA-PDAC data and discovered that RIOK3 was substantially related to focal adhesion signaling in PDAC. Additionally, silencing RIOK3 dramatically decreased Focal Adhesion Kinase (FAK) protein expression and phosphorylation (Tyr397 and Tyr925 sites). Immunoprecipitation assay verified the interaction of RIOK3 and FAK. Furthermore, RIOK3 considerably increased the protein stability of FAK protein but not FAK-Y925F protein. The biological function of RIOK3 in increasing PDAC cell invasion and migration was shown to be dependent on FAK activation. Moreover, we discovered that RIOK3 mutations were mainly characterized by amplification. RIOK3 mRNA was found to be significantly elevated in PDAC tissues and was associated with a poor prognosis. Furthermore, RIOK3 mRNA was significantly upregulated in later T-stage, pre-existing lymph node metastases, and later pathological stage samples. Overall, our study found that RIOK3 promotes PDAC cell invasion and metastasis by stabilizing FAK protein expression and upregulating its phosphorylation. This also provides a new target for therapeutic modalities targeting FAK.

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FAK activation is required for RIOK3 to promote PDAC cell invasion and metastasis.

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RIOK3 binds to and stabilizes the FAK protein.

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RIOK3 is highly expressed in PDAC tissues and associated with poor prognosis.

Direct observation of chaperone-modulated talin mechanics with single-molecule resolution

Talin as a critical focal adhesion mechanosensor exhibits force-dependent folding dynamics and concurrent interactions. Being a cytoplasmic protein, talin also might interact with several cytosolic chaperones; however, the roles of chaperones in talin mechanics remain elusive. To address this question, we investigated the force response of a mechanically stable talin domain with a set of well-known unfoldase (DnaJ, DnaK) and foldase (DnaKJE, DsbA) chaperones, using single-molecule magnetic tweezers. Our findings demonstrate that chaperones could affect adhesion proteins’ stability by changing their folding mechanics; while unfoldases reduce their unfolding force from ~11 pN to ~6 pN, foldase shifts it upto ~15 pN. Since talin is mechanically synced within 2 pN force ranges, these changes are significant in cellular conditions. Furthermore, we determined that chaperones directly reshape the energy landscape of talin: unfoldases decrease the unfolding barrier height from 26.8 to 21.7 k_BT, while foldases increase it to 33.5 k_BT. We reconciled our observations with eukaryotic Hsp70 and Hsp40 and observed their similar function of decreasing the talin unfolding barrier. Quantitative mapping of this chaperone-induced talin folding landscape directly illustrates that chaperones perturb the adhesion protein stability under physiological force, thereby, influencing their force-dependent interactions and adhesion dynamics.

Chakraborty et al. uses single-molecule magnetic tweezers to investigate the chaperone-modulated talin protein mechanics. The results showed that chaperones are involved in the regulation of talin folding/unfolding under mechanical force with some chaperones stabilizing talin and increasing the force, whereas others destabilize it and reduce the force.

Refolding of metacaspase 5 from Trypanosoma cruzi, structural characterization and the influence of c-terminal in protein recombinant production

Metacaspases are known to have a fundamental role in apoptosis-like, a programmed cellular death (PCD) in plants, fungi, and protozoans. The last includes several parasites that cause diseases of great interest to public health, mostly without adequate treatment and included in the neglected tropical diseases category. One of them is Trypanosoma cruzi which causes Chagas disease and has two metacaspases involved in its PCD: TcMCA3 and TcMCA5. Their roles seemed different in PCD, TcMCA5 appears as a proapoptotic protein negatively regulated by its C-terminal sequence, while TcMCA3 is described as a cell cycle regulator. Despite this, the precise role of TcMCA3 and TcMCA5 and their atomic structures remain elusive. Therefore, developing methodologies to allow investigations of those metacaspases is relevant. Herein, we produced full-length and truncated versions of TcMCA5 and applied different strategies for their folded recombinant production from E. coli inclusion bodies. Biophysical assays probed the efficacy of the production method in providing a high yield of folded recombinant TcMCA5. Moreover, we modeled the TcMCA5 protein structure using experimental restraints obtained by XLMS. The experimental design for novel methods and the final protocol provided here can guide studies with other metacaspases. The production of TcMCA5 allows further investigations as protein crystallography, HTS drug discovery to create potential therapeutic in the treatment of Chagas' disease and in the way to clarify how the PCD works in the parasite.

Enhancing Extracellular Adenosine Levels Restores Barrier Function in Acute Lung Injury Through Expression of Focal Adhesion Proteins

Background: Acute respiratory distress syndrome (ARDS) is a clinical presentation of acute lung injury (ALI) with often fatal lung complication. Adenosine, a nucleoside generated following cellular stress provides protective effects in acute injury. The levels of extracellular adenosine can be depleted by equilibrative nucleoside transporters (ENTs). ENT inhibition by pharmaceutical agent dipyridamole promotes extracellular adenosine accumulation and is protective in ARDS. However, the therapeutic potential of dipyridamole in acute lung injury has not yet been evaluated. Methods: Adenosine acts on three adenosine receptors, the adenosine A1 (Adora1), A2a (Adora2a), the A2b (Adora2b) or the adenosine A3 (Adora 3) receptor. Accumulation of adenosine is usually required to stimulate the low-affinity Adora2b receptor. In order to investigate the effect of adenosine accumulation and the contribution of epithelial-specific ENT2 or adora2b expression in experimental ALI, dipyridamole, and epithelial specific ENT2 or Adora2b deficient mice were utilized. MLE12 cells were used to probe downstream Adora2b signaling. Adenosine receptors, transporters, and targets were determined in ARDS lungs. Results: ENT2 is mainly expressed in alveolar epithelial cells and is negatively regulated by hypoxia following tissue injury. Enhancing adenosine levels with ENT1/ENT2 inhibitor dipyridamole at a time when bleomycin-induced ALI was present, reduced further injury. Mice pretreated with the ADORA2B agonist BAY 60-6583 were protected from bleomycin-induced ALI by reducing vascular leakage (558.6 ± 50.4 vs. 379.9 ± 70.4, p < 0.05), total bronchoalveolar lavage fluid cell numbers (17.9 ± 1.8 to 13.4 ± 1.4 e4, p < 0.05), and neutrophil infiltration (6.42 ± 0.25 vs. 3.94 ± 0.29, p < 0.05). While mice lacking Adora2b in AECs were no longer protected by dipyridamole. We also identified occludin and focal adhesion kinase as downstream targets of ADORA2B, thus providing a novel mechanism for adenosine-mediated barrier protection. Similarly, we also observed similar enhanced ADORA2B (3.33 ± 0.67 to 16.12 ± 5.89, p < 0.05) and decreased occludin (81.2 ± 0.3 to 13.3 ± 0.4, p < 0.05) levels in human Acute respiratory distress syndrome lungs. Conclusion: We have highlighted a role of dipyridamole and adenosine signaling in preventing or treating ALI and identified Ent2 and Adora2b as key mediators in important for the resolution of ALI.

Dynamic localization of αB-crystallin at the microtubule cytoskeleton network in beating heart cells

αB-crystallin is highly expressed in the heart and slow skeletal muscle; however, the roles of αB-crystallin in the muscle are obscure. Previously, we showed that αB-crystallin localizes at the sarcomere Z-bands, corresponding to the focal adhesions of cultured cells. In myoblast cells, αB-crystallin completely colocalizes with microtubules and maintains cell shape and adhesion. In this study, we show that in beating cardiomyocytes α-tubulin and αB-crystallin colocalize at the I- and Z-bands of the myocardium, where it may function as a molecular chaperone for tubulin/microtubules. Fluorescence recovery after photobleaching (FRAP) analysis revealed that the striated patterns of GFP-αB-crystallin fluorescence recovered quickly at 37°C. FRAP mobility assay also showed αB-crystallin to be associated with nocodazole-treated free tubulin dimers but not with taxol-treated microtubules. The interaction of αB-crystallin and free tubulin was further confirmed by immunoprecipitation and microtubule sedimentation assay in the presence of 1-100 μM calcium, which destabilizes microtubules. Förster resonance energy transfer analysis showed that αB-crystallin and tubulin were at 1-10 nm apart from each other in the presence of colchicine. These results suggested that αB-crystallin may play an essential role in microtubule dynamics by maintaining free tubulin in striated muscles, such as the soleus or cardiac muscles.

Small heat-shock proteins and their role in mechanical stress

The ability of cells to respond to stress is central to health. Stress can damage folded proteins, which are vulnerable to even minor changes in cellular conditions. To maintain proteostasis, cells have developed an intricate network in which molecular chaperones are key players. The small heat-shock proteins (sHSPs) are a widespread family of molecular chaperones, and some sHSPs are prominent in muscle, where cells and proteins must withstand high levels of applied force. sHSPs have long been thought to act as general interceptors of protein aggregation. However, evidence is accumulating that points to a more specific role for sHSPs in protecting proteins from mechanical stress. Here, we briefly introduce the sHSPs and outline the evidence for their role in responses to mechanical stress. We suggest that sHSPs interact with mechanosensitive proteins to regulate physiological extension and contraction cycles. It is likely that further study of these interactions - enabled by the development of experimental methodologies that allow protein contacts to be studied under the application of mechanical force - will expand our understanding of the activity and functions of sHSPs, and of the roles played by chaperones in general.

Come Together: Protein Assemblies, Aggregates and the Sarcostat at the Heart of Cardiac Myocyte Homeostasis

Homeostasis in vertebrate systems is contingent on normal cardiac function. This, in turn, depends on intricate protein-based cellular machinery, both for contractile function, as well as, durability of cardiac myocytes. The cardiac small heat shock protein (csHsp) chaperone system, highlighted by αB-crystallin (CRYAB), a small heat shock protein (sHsp) that forms ∼3–5% of total cardiac mass, plays critical roles in maintaining proteostatic function via formation of self-assembled multimeric chaperones. In this work, we review these ancient proteins, from the evolutionarily preserved role of homologs in protists, fungi and invertebrate systems, as well as, the role of sHsps and chaperones in maintaining cardiac myocyte structure and function. We propose the concept of the “sarcostat” as a protein quality control mechanism in the sarcomere. The roles of the proteasomal and lysosomal proteostatic network, as well as, the roles of the aggresome, self-assembling protein complexes and protein aggregation are discussed in the context of cardiac myocyte homeostasis. Finally, we will review the potential for targeting the csHsp system as a novel therapeutic approach to prevent and treat cardiomyopathy and heart failure.

Structural and Functional Peculiarities of α-Crystallin

α-Crystallin is the major protein of the eye lens and a member of the family of small heat-shock proteins. Its concentration in the human eye lens is extremely high (about 450 mg/mL). Three-dimensional structure of native α-crystallin is unknown. First of all, this is the result of the highly heterogeneous nature of α-crystallin, which hampers obtaining it in a crystalline form. The modeling based on the electron microscopy (EM) analysis of α-crystallin preparations shows that the main population of the α-crystallin polydisperse complex is represented by oligomeric particles of rounded, slightly ellipsoidal shape with the diameter of about 13.5 nm. These complexes have molecular mass of about 700 kDa. In our opinion, the heterogeneity of the α-crystallin complex makes it impossible to obtain a reliable 3D model. In the literature, there is evidence of an enhanced chaperone function of α-crystallin during its dissociation into smaller components. This may indirectly indicate that the formation of heterogeneous complexes is probably necessary to preserve α-crystallin in a state inactive before stressful conditions. Then, not only the heterogeneity of the α-crystallin complex is an evolutionary adaptation that protects α-crystallin from crystallization but also the enhancement of the function of α-crystallin during its dissociation is also an evolutionary acquisition. An analysis of the literature on the study of α-crystallin in vitro led us to the assumption that, of the two α-crystallin isoforms (αA- and αB-crystallins), it is αA-crystallin that plays the role of a special chaperone for αB-crystallin. In addition, our data on X-ray diffraction analysis of α-crystallin at the sample concentration of about 170-190 mg/mL allowed us to assume that, at a high concentration, the eye lens α-crystallin can be in a gel-like stage. Finally, we conclude that, since all the accumulated data on structural-functional studies of α-crystallin were carried out under conditions far from native, they cannot adequately reflect the features of the functioning of α-crystallin in vivo.

HspB1 phosphorylation regulates its intramolecular dynamics and mechanosensitive molecular chaperone interaction with filamin C

Mechanical force-induced conformational changes in proteins underpin a variety of physiological functions, typified in muscle contractile machinery. Mutations in the actin-binding protein filamin C (FLNC) are linked to musculoskeletal pathologies characterized by altered biomechanical properties and sometimes aggregates. HspB1, an abundant molecular chaperone, is prevalent in striated muscle where it is phosphorylated in response to cues including mechanical stress. We report the interaction and up-regulation of both proteins in three mouse models of biomechanical stress, with HspB1 being phosphorylated and FLNC being localized to load-bearing sites. We show how phosphorylation leads to increased exposure of the residues surrounding the HspB1 phosphosite, facilitating their binding to a compact multidomain region of FLNC proposed to have mechanosensing functions. Steered unfolding of FLNC reveals that its extension trajectory is modulated by the phosphorylated region of HspB1. This may represent a posttranslationally regulated chaperone-client protection mechanism targeting over-extension during mechanical stress.

Peri/epicellular protein disulfide isomerase-A1 acts as an upstream organizer of cytoskeletal mechanoadaptation in vascular smooth muscle cells

Although redox processes closely interplay with mechanoresponses to control vascular remodeling, redox pathways coupling mechanostimulation to cellular cytoskeletal organization remain unclear. The peri/epicellular pool of protein disulfide isomerase-A1 (pecPDIA1) supports postinjury vessel remodeling. Using distinct models, we investigated whether pecPDIA1 could work as a redox-dependent organizer of cytoskeletal mechanoresponses. In vascular smooth muscle cells (VSMCs), pecPDIA1 immunoneutralization impaired stress fiber assembly in response to equibiaxial stretch and, under uniaxial stretch, significantly perturbed cell repositioning perpendicularly to stretch orientation. During cyclic stretch, pecPDIA1 supported thiol oxidation of the known mechanosensor β₁-integrin and promoted polarized compartmentalization of sulfenylated proteins. Using traction force microscopy, we showed that pecPDIA1 organizes intracellular force distribution. The net contractile moment ratio of platelet-derived growth factor-exposed to basal VSMCs decreased from 0.90 ± 0.09 (IgG-exposed controls) to 0.70 ± 0.08 after pecPDI neutralization ( P < 0.05), together with an enhanced coefficient of variation for distribution of force modules, suggesting increased noise. Moreover, in a single cell model, pecPDIA1 neutralization impaired migration persistence without affecting total distance or velocity, whereas siRNA-mediated total PDIA1 silencing disabled all such variables of VSMC migration. Neither expression nor total activity of the master mechanotransmitter/regulator RhoA was affected by pecPDIA1 neutralization. However, cyclic stretch-induced focal distribution of membrane-bound RhoA was disrupted by pecPDI inhibition, which promoted a nonpolarized pattern of RhoA/caveolin-3 cluster colocalization. Accordingly, FRET biosensors showed that pecPDIA1 supports localized RhoA activity at cell protrusions versus perinuclear regions. Thus, pecPDI acts as a thiol redox-dependent organizer and noise reducer mechanism of cytoskeletal repositioning, oxidant generation, and localized RhoA activation during a variety of VSMC mechanoresponses. NEW & NOTEWORTHY Effects of a peri/epicellular pool of protein disulfide isomerase-A1 (pecPDIA1) during mechanoregulation in vascular smooth muscle cells (VSMCs) were highlighted using approaches such as equibiaxial and uniaxial stretch, random single cell migration, and traction force microscopy. pecPDIA1 regulates organization of the cytoskeleton and minimizes the noise of cell alignment, migration directionality, and persistence. pecPDIA1 mechanisms involve redox control of β₁-integrin and localized RhoA activation. pecPDIA1 acts as a novel organizer of mechanoadaptation responses in VSMCs.

EphrinB/EphB signaling contributes to spinal nociceptive processing via calpain‑1 and caspase‑3

Previous studies have indicated that an important subfamily of receptor tyrosine kinases, ephrins and their receptors, are important in pain signaling, particularly in spinal nociceptive processing. In the present study, the role of the ephrin/Eph signaling pathway was confirmed, and it was shown that this signaling was also involved in spinal nociceptive processing through the actions of calpain‑1 and caspase‑3. First, the ephrinB ligands, ephrinB1‑Fc or ephrinB2‑Fc, were introduced into experimental mice via intrathecal injection, and it was found that this injection induced marked time‑ and dose‑dependent mechanical allodynia and thermal hyperalgesia, accompanied by increased levels of calpain‑1 and caspase‑3 in the spinal cord. MDL28170, an inhibitor of calpain‑1, reversed the behavioral effects and ameliorated the increases in calpain‑1 and caspase‑3. Second, it was found that the administration of EphB1 between L5 and L6 in mice inhibited the mechanical allodynia and thermal hyperalgesia induced by chronic constrictive injury. In addition, to demonstrate the cell phenotypes responsible for the increased levels of calpain‑1 and caspase‑3 in the spinal cord following injection with ephrinB2‑Fc, double immunofluorescent labeling was performed, which indicated that calpain‑1 and caspase‑3 were localized in neurons, but not in astrocytes or microglial cells. In conclusion, the present study suggested that ephrinB/EphB signaling contributes to spinal nociceptive processing via the actions of calpain‑1 and caspase‑3.

XPlex: An Effective, Multiplex Cross-Linking Chemistry for Acidic Residues

Cross-linking/Mass spectrometry (XLMS) is a consolidated technique for structural characterization of proteins and protein complexes. Despite its success, the cross-linking chemistry currently used is mostly based on N-hydroxysuccinimide (NHS) esters, which react primarily with lysine residues. One way to expand the current applicability of XLMS into several new areas is to increase the number of cross-links obtainable for a target protein. We introduce a multiplex chemistry (denoted XPlex) that targets Asp, Glu, Lys, and Ser residues. XPlex can generate significantly more cross-links with reactions occurring at lower temperatures and enables targeting proteins that are not possible with NHS ester-based cross-linkers. We demonstrate the effectiveness of our approach in model proteins as well as a target Lys-poor protein, SalBIII. Identification of XPlex spectra requires a search engine capable of simultaneously considering multiple cross-linkers on the same run; to achieve this, we updated the SIM-XL search algorithm with a search mode tailored toward XPlex. In summary, we present a complete chemistry/computational solution for significantly increasing the number of possible distance constraints by mass spectrometry experiments, and thus, we are convinced that XPlex poses as a real complementary approach for structural proteomics studies.

Regulation of the microenvironment for cardiac tissue engineering

The microenvironment of myocardium plays an important role in the fate and function of cardiomyocytes (CMs). Cardiovascular tissue engineering strategies commonly utilize stem cell sources in conjunction with microenvironmental cues that often include biochemical, electrical, spatial and biomechanical factors. Microenvironmental stimulation of CMs, in addition to the incorporation of intercellular interactions from non-CMs, results in the generation of engineered cardiac constructs. Current studies suggest that use of these factors when engineering cardiac constructs improve cardiac function when implanted in vivo. In this review, we summarize the approaches to modulate biochemical, electrical, biomechanical and spatial factors to induce CM differentiation and their subsequent organization for cardiac tissue engineering application.

Small Heat Shock Protein αB-Crystallin Controls Shape and Adhesion of Glioma and Myoblast Cells in the Absence of Stress

Cell shape and adhesion and their proper controls are fundamental for all biological systems. Mesenchymal cells migrate at an average rate of 6 to 60 μm/hr, depending on the extracellular matrix environment and cell signaling. Myotubes, fully differentiated muscle cells, are specialized for power-generation and therefore lose motility. Cell spreading and stabilities of focal adhesion are regulated by the critical protein vinculin from immature myoblast to mature costamere of differentiated myotubes where myofibril Z-band linked to sarcolemma. The Z-band is constituted from microtubules, intermediate filaments, cell adhesion molecules and other adapter proteins that communicate with the outer environment. Mesenchymal cells, including myoblast cells, convert actomyosin contraction forces to tension through mechano-responsive adhesion assembly complexes as Z-band equivalents. There is growing evidence that microtubule dynamics are involved in the generation of contractile forces; however, the roles of microtubules in cell adhesion dynamics are not well determined. Here, we show for the first time that αB-crystallin, a molecular chaperon for tubulin/microtubules, is involved in cell shape determination. Moreover, knockdown of this molecule caused myoblasts and glioma cells to lose their ability for adhesion as they tended to behave like migratory cells. Surprisingly, αB-crystallin knockdown in both C6 glial cells and L6 myoblast permitted cells to migrate more rapidly (2.7 times faster for C6 and 1.3 times faster for L6 cells) than dermal fibroblast. On the other hand, overexpression of αB-crystallin in cells led to an immortal phenotype because of persistent adhesion. Position of matured focal adhesion as visualized by vinculin immuno-staining, stress fiber direction, length, and density were clearly αB-crystallin dependent. These results indicate that the small HSP αB-crystallin has important roles for cell adhesion, and thus microtubule dynamics are necessary for persistent adhesion.

FAK Forms a Complex with MEF2 to Couple Biomechanical Signaling to Transcription in Cardiomyocytes

Focal adhesion kinase (FAK) has emerged as a mediator of mechanotransduction in cardiomyocytes, regulating gene expression during hypertrophic remodeling. However, how FAK signaling is relayed onward to the nucleus is unclear. Here, we show that FAK interacts with and regulates myocyte enhancer factor 2 (MEF2), a master cardiac transcriptional regulator. In cardiomyocytes exposed to biomechanical stimulation, FAK accumulates in the nucleus, binds to and upregulates the transcriptional activity of MEF2 through an interaction with the FAK focal adhesion targeting (FAT) domain. In the crystal structure (2.9 Å resolution), FAT binds to a stably folded groove in the MEF2 dimer, known to interact with regulatory cofactors. FAK cooperates with MEF2 to enhance the expression of Jun in cardiomyocytes, an important component of hypertrophic response to mechanical stress. These findings underscore a connection between the mechanotransduction involving FAK and transcriptional regulation by MEF2, with potential relevance to the pathogenesis of cardiac disease.

αB-crystallin: Portrait of a malignant chaperone as a cancer therapeutic target

αB-crystallin is a widely expressed member of the small heat shock protein family that protects cells from stress by its dual function as a molecular chaperone to preserve proteostasis and as a cell death antagonist that negatively regulates components of the conserved apoptotic cell death machinery. Deregulated expression of αB-crystallin occurs in a broad array of solid tumors and has been linked to tumor progression and poor clinical outcomes. This review will focus on new insights into the molecular mechanisms by which oncogenes, oxidative stress, matrix detachment and other tumor microenvironmental stressors deregulate αB-crystallin expression. We will also review accumulating evidence pointing to an essential role for αB-crystallin in the multi-step metastatic cascade whereby tumor cells colonize distant organs by circumventing a multitude of barriers to cell migration and survival. Finally, we will evaluate emerging strategies to therapeutically target αB-crystallin and/or interacting proteins to selectively activate apoptosis and/or derail the metastatic cascade in an effort to improve outcomes for patients with metastatic disease.

Multidimensional significance of crystallin protein-protein interactions and their implications in various human diseases

BACKGROUND

Crystallins are the important structural and functional proteins in the eye lens responsible for refractive index. Post-translational modifications (PTMs) and mutations are major causative factors that affect crystallin structural conformation and functional characteristics thus playing a vital role in the etiology of cataractogenesis.

SCOPE OF REVIEW

The significance of crystallin protein-protein interactions (PPIs) in the lens and non-lenticular tissues is summarized.

MAJOR CONCLUSIONS

Aberrancy of PPIs between crystallin, its associated protein and metal ions has been accomplished in various human diseases including cataract. A detailed account on multidimensional structural and functional significance of crystallin PPI in humans must be brought into limelight, in order to understand the biochemical and molecular basis augmenting the aberrancies of such interaction. In this scenario, the present review is focused to shed light on studies which will aid to expand our present understanding on disease pathogenesis related to loss of PPI thereby paving the way for putative future therapeutic targets to curb such diseases.

GENERAL SIGNIFICANCE

The interactions with α-crystallins always aid to protect their structural and functional characteristics. The up-regulation of αB-crystallin in the non-lenticular tissues always decodes as biomarker for various stress related disorders. For better understanding and treatment of various diseases, PPI studies provide overall outline about the structural and functional characteristics of the proteins. This information not only helps to find out the route of cataractogenesis but also aid to identify potential molecules to inhibit/prevent the further development of such complicated phenomenon. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Structure and function of α-crystallins: Traversing from in vitro to in vivo

BACKGROUND

The two α-crystallins (αA- and αB-crystallin) are major components of our eye lenses. Their key function there is to preserve lens transparency which is a challenging task as the protein turnover in the lens is low necessitating the stability and longevity of the constituent proteins. α-Crystallins are members of the small heat shock protein family. αB-crystallin is also expressed in other cell types.

SCOPE OF THE REVIEW

The review summarizes the current concepts on the polydisperse structure of the α-crystallin oligomer and its chaperone function with a focus on the inherent complexity and highlighting gaps between in vitro and in vivo studies.

MAJOR CONCLUSIONS

Both α-crystallins protect proteins from irreversible aggregation in a promiscuous manner. In maintaining eye lens transparency, they reduce the formation of light scattering particles and balance the interactions between lens crystallins. Important for these functions is their structural dynamics and heterogeneity as well as the regulation of these processes which we are beginning to understand. However, currently, it still remains elusive to which extent the in vitro observed properties of α-crystallins reflect the highly crowded situation in the lens.

GENERAL SIGNIFICANCE

Since α-crystallins play an important role in preventing cataract in the eye lens and in the development of diverse diseases, understanding their mechanism and substrate spectra is of importance. To bridge the gap between the concepts established in vitro and the in vivo function of α-crystallins, the joining of forces between different scientific disciplines and the combination of diverse techniques in hybrid approaches are necessary. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.