Inhibition of Mutant αB Crystallin-Induced Protein Aggregation by a Molecular Tweezer

Role of the Alpha-B-Crystallin Protein in Cardiomyopathic Disease

Alpha-B-crystallin, a member of the small heat shock family of proteins, has been implicated in a variety of cardiomyopathies and in normal cardiac homeostasis. It is known to function as a molecular chaperone, particularly for desmin, but also interacts with a wide variety of additional proteins. The molecular chaperone function is also enhanced by signal-dependent phosphorylation at specific residues under stress conditions. Naturally occurring mutations in CRYAB, the gene that encodes alpha-B-crystallin, have been suggested to alter ionic intermolecular interactions that affect dimerization and chaperone function. These mutations have been associated with myofibrillar myopathy, restrictive cardiomyopathy, and hypertrophic cardiomyopathy and promote pathological hypertrophy through different mechanisms such as desmin aggregation, increased reductive stress, or activation of calcineurin-NFAT signaling. This review will discuss the known mechanisms by which alpha-B-crystallin functions in cardiac homeostasis and the pathogenesis of cardiomyopathies and provide insight into potential future areas of exploration.

Small molecules as modulators of the proteostasis machinery: Implication in cardiovascular diseases

With the escalating prevalence of cardiovascular diseases, the substantial socioeconomic burden on healthcare systems is intensifying. Accumulating empirical evidence underscores the pivotal role of the proteostasis network in regulating cardiac homeostasis and function. Disruptions in proteostasis may contribute to the loss of protein function or the acquisition of toxic functions, which are intricately linked to the development of cardiovascular ailments such as atrial fibrillation, heart failure, atherosclerosis, and cardiac aging. It is widely acknowledged that the proteostasis network encompasses molecular chaperones, autophagy, and the ubiquitin proteasome system (UPS). Consequently, the proteostasis network emerges as an appealing target for therapeutic interventions in cardiovascular diseases. Numerous small molecules, acting as modulators of the proteostasis machinery, have exhibited therapeutic efficacy in managing cardiovascular diseases. This review centers on elucidating the role of the proteostasis network in various cardiovascular diseases and explores the potential of small molecules as therapeutic agents.

Molecular Tweezers: Supramolecular Hosts with Broad-Spectrum Biological Applications

Lysine-selective molecular tweezers (MTs) are supramolecular host molecules displaying a remarkably broad spectrum of biologic activities. MTs act as inhibitors of the self-assembly and toxicity of amyloidogenic proteins using a unique mechanism. They destroy viral membranes and inhibit infection by enveloped viruses, such as HIV-1 and SARS-CoV-2, by mechanisms unrelated to their action on protein self-assembly. They also disrupt biofilm of Gram-positive bacteria. The efficacy and safety of MTs have been demonstrated in vitro, in cell culture, and in vivo, suggesting that these versatile compounds are attractive therapeutic candidates for various diseases, infections, and injuries. A lead compound called CLR01 has been shown to inhibit the aggregation of various amyloidogenic proteins, facilitate their clearance in vivo, prevent infection by multiple viruses, display potent anti-biofilm activity, and have a high safety margin in animal models. The inhibitory effect of CLR01 against amyloidogenic proteins is highly specific to abnormal self-assembly of amyloidogenic proteins with no disruption of normal mammalian biologic processes at the doses needed for inhibition. Therapeutic effects of CLR01 have been demonstrated in animal models of proteinopathies, lysosomal-storage diseases, and spinal-cord injury. Here we review the activity and mechanisms of action of these intriguing compounds and discuss future research directions. SIGNIFICANCE STATEMENT: Molecular tweezers are supramolecular host molecules with broad biological applications, including inhibition of abnormal protein aggregation, facilitation of lysosomal clearance of toxic aggregates, disruption of viral membranes, and interference of biofilm formation by Gram-positive bacteria. This review discusses the molecular and cellular mechanisms of action of the molecular tweezers, including the discovery of distinct mechanisms acting in vitro and in vivo, and the application of these compounds in multiple preclinical disease models.

Lysine-selective molecular tweezers are cell penetrant and concentrate in lysosomes

Lysine-selective molecular tweezers are promising drug candidates against proteinopathies, viral infection, and bacterial biofilm. Despite demonstration of their efficacy in multiple cellular and animal models, important questions regarding their mechanism of action, including cell penetrance and intracellular distribution, have not been answered to date. The main impediment to answering these questions has been the low intrinsic fluorescence of the main compound tested to date, called CLR01. Here, we address these questions using new fluorescently labeled molecular tweezers derivatives. We show that these compounds are internalized in neurons and astrocytes, at least partially through dynamin-dependent endocytosis. In addition, we demonstrate that the molecular tweezers concentrate rapidly in acidic compartments, primarily lysosomes. Accumulation of molecular tweezers in lysosomes may occur both through the endosomal-lysosomal pathway and via the autophagy-lysosome pathway. Moreover, by visualizing colocalization of molecular tweezers, lysosomes, and tau aggregates we show that lysosomes likely are the main site for the intracellular anti-amyloid activity of molecular tweezers. These findings have important implications for the mechanism of action of molecular tweezers in vivo, explaining how administration of low doses of the compounds achieves high effective concentrations where they are needed, and supporting the development of these compounds as drugs for currently cureless proteinopathies.

Inhibition of Staphylococcus aureus biofilm-forming functional amyloid by molecular tweezers

Biofilms are rigid and largely impenetrable three-dimensional matrices constituting virulence determinants of various pathogenic bacteria. Here, we demonstrate that molecular tweezers, unique supramolecular artificial receptors, modulate biofilm formation of Staphylococcus aureus. In particular, the tweezers affect the structural and assembly properties of phenol-soluble modulin α1 (PSMα1), a biofilm-scaffolding functional amyloid peptide secreted by S. aureus. The data reveal that CLR01, a diphosphate tweezer, exhibits significant S. aureus biofilm inhibition and disrupts PSMα1 self-assembly and fibrillation, likely through inclusion of lysine side chains of the peptide. In comparison, different peptide binding occurs in the case of CLR05, a tweezer containing methylenecarboxylate units, which exhibits lower affinity for the lysine residues yet disrupts S. aureus biofilm more strongly than CLR01. Our study points to a possible role for molecular tweezers as potent biofilm inhibitors and antibacterial agents, particularly against untreatable biofilm-forming and PSM-producing bacteria, such as methicillin-resistant S. aureus.

The molecular tweezer CLR01 improves behavioral deficits and reduces tau pathology in P301S-tau transgenic mice

BACKGROUND

Molecular tweezers (MTs) are broad-spectrum inhibitors of abnormal protein aggregation. A lead MT, called CLR01, has been demonstrated to inhibit the aggregation and toxicity of multiple amyloidogenic proteins in vitro and in vivo. Previously, we evaluated the effect of CLR01 in the 3 × Tg mouse model of Alzheimer's disease, which overexpresses mutant human presenilin 1, amyloid β-protein precursor, and tau and found that subcutaneous administration of the compound for 1 month led to a robust reduction of amyloid plaques, neurofibrillary tangles, and microgliosis. CLR01 also has been demonstrated to inhibit tau aggregation in vitro and tau seeding in cell culture, yet because in Alzheimer's disease (AD) and in the 3 × Tg model, tau hyperphosphorylation and aggregation are thought to be downstream of Aβ insults, the study in this model left open the question whether CLR01 affected tau in vivo directly or indirectly.

METHODS

To determine if CLR01 could ameliorate tau pathology directly in vivo, we tested the compound similarly using the P301S-tau (line PS19) mouse model. Mice were administered 0.3 or 1.0 mg/kg per day CLR01 and tested for muscle strength and behavioral deficits, including anxiety- and disinhibition-like behavior. Their brains then were analyzed by immunohistochemical and biochemical assays for pathological forms of tau, neurodegeneration, and glial pathology.

RESULTS

CLR01 treatment ameliorated muscle-strength deterioration, anxiety-, and disinhibition-like behavior. Improved phenotype was associated with decreased levels of pathologic tau forms, suggesting that CLR01 exerts a direct effect on tau in vivo. Limitations of the study included a relatively short treatment period of the mice at an age in which full pathology is not yet developed. In addition, high variability in this model lowered the statistical significance of the findings of some outcome measures.

CONCLUSIONS

The findings suggest that CLR01 is a particularly attractive candidate for the treatment of AD because it targets simultaneously the two major pathogenic proteins instigating and propagating the disease, amyloid β-protein (Aβ), and tau, respectively. In addition, our study suggests that CLR01 can be used for the treatment of other tauopathies in the absence of amyloid pathology.

Reductive Stress-Induced Mitochondrial Dysfunction and Cardiomyopathy

The goal of this review was to summarize reported studies focusing on cellular reductive stress-induced mitochondrial dysfunction, cardiomyopathy, dithiothreitol- (DTT-) induced reductive stress, and reductive stress-related free radical reactions published in the past five years. Reductive stress is considered to be a double-edged sword in terms of antioxidation and disease induction. As many underlying mechanisms are still unclear, further investigations are obviously warranted. Nonetheless, reductive stress is thought to be caused by elevated levels of cellular reducing power such as NADH, glutathione, and NADPH; and this area of research has attracted increasing attention lately. Albeit, we think there is a need to conduct further studies in identifying more indicators of the risk assessment and prevention of developing heart damage as well as exploring more targets for cardiomyopathy treatment. Hence, it is expected that further investigation of underlying mechanisms of reductive stress-induced mitochondrial dysfunction will provide novel insights into therapeutic approaches for ameliorating reductive stress-induced cardiomyopathy.

Induction of Proteasome Subunit Low Molecular Weight Protein (LMP)-2 Is Required to Induce Active Remodeling in Adult Rat Ventricular Cardiomyocytes

Isolated adult rat ventricular cardiomyocytes (ARVC) adapt to the two-dimensional surface of culture dishes once they are isolated from the three-dimensional heart tissue. This process mimics aspects of cardiac adaptation to pressure overload and requires an initial breakdown of sarcomeric structures. The present study therefore aimed to identify key steps in this remodeling process. ARVC were cultured under serum-free or serum-supplemented conditions and their sizes and shapes were analyzed as well as apoptosis and the ability to disintegrate their sarcomeres. ARVC require serum-factors in order to adapt to cell culture conditions. More ARVC survived if they were able to breakdown their sarcomeres and mononucleated ARVC, which were smaller than binucleated ARVC, had a better chance to adapt. During the early phase of adaptation, proteasome subunit low molecular weight protein (LMP)-2 was induced. Inhibition of LMP-2 up-regulation by siRNA attenuated the process of successful adaptation. In vivo, LMP-2 was induced in the left ventricle of spontaneously hypertensive rats during the early phase of adaptation to pressure overload. In conclusion, the data suggest that breakdown of pre-existing sarcomeres is optimized by induction of LMP-2 and that it is required for cardiac remodeling processes, for example, occurring during pressure overload.

Ube2v1 Positively Regulates Protein Aggregation by Modulating Ubiquitin Proteasome System Performance Partially Through K63 Ubiquitination

RATIONALE

Compromised protein quality control can result in proteotoxic intracellular protein aggregates in the heart, leading to cardiac disease and heart failure. Defining the participants and understanding the underlying mechanisms of cardiac protein aggregation is critical for seeking therapeutic targets. We identified Ube2v1 (ubiquitin-conjugating enzyme E2 variant 1) in a genome-wide screen designed to identify novel effectors of the aggregation process. However, its role in the cardiomyocyte is undefined.

OBJECTIVE

To assess whether Ube2v1 regulates the protein aggregation caused by cardiomyocyte expression of a mutant αB crystallin (CryAB^R120G) and identify how Ube2v1 exerts its effect.

METHODS AND RESULTS

Neonatal rat ventricular cardiomyocytes were infected with adenoviruses expressing either wild-type CryAB (CryAB^WT) or CryAB^R120G. Subsequently, loss- and gain-of-function experiments were performed. Ube2v1 knockdown decreased aggregate accumulation caused by CryAB^R120G expression. Overexpressing Ube2v1 promoted aggregate formation in CryAB^WT and CryAB^R120G-expressing neonatal rat ventricular cardiomyocytes. Ubiquitin proteasome system performance was analyzed using a ubiquitin proteasome system reporter protein. Ube2v1 knockdown improved ubiquitin proteasome system performance and promoted the degradation of insoluble ubiquitinated proteins in CryAB^R120G cardiomyocytes but did not alter autophagic flux. Lys (K) 63-linked ubiquitination modulated by Ube2v1 expression enhanced protein aggregation and contributed to Ube2v1's function in regulating protein aggregate formation. Knocking out Ube2v1 exclusively in cardiomyocytes by using AAV9 (adeno-associated virus 9) to deliver multiplexed single guide RNAs against Ube2v1 in cardiac-specific Cas9 mice alleviated CryAB^R120G-induced protein aggregation, improved cardiac function, and prolonged lifespan in vivo.

CONCLUSIONS

Ube2v1 plays an important role in protein aggregate formation, partially by enhancing K63 ubiquitination during a proteotoxic stimulus. Inhibition of Ube2v1 decreases CryAB^R120G-induced aggregate formation through enhanced ubiquitin proteasome system performance rather than autophagy and may provide a novel therapeutic target to treat cardiac proteinopathies.

Cardiac Fibrosis in Proteotoxic Cardiac Disease is Dependent Upon Myofibroblast TGF -β Signaling

Background

Transforming growth factor beta (TGF‐β) is an important cytokine in mediating the cardiac fibrosis that often accompanies pathogenic cardiac remodeling. Cardiomyocyte‐specific expression of a mutant αB‐crystallin (CryAB^R120G), which causes human desmin‐related cardiomyopathy, results in significant cardiac fibrosis. During onset of fibrosis, fibroblasts are activated to the so‐called myofibroblast state and TGF‐β binding mediates an essential signaling pathway underlying this process. Here, we test the hypothesis that fibroblast‐based TGF‐β signaling can result in significant cardiac fibrosis in a disease model of cardiac proteotoxicity that has an exclusive cardiomyocyte‐based etiology.

Methods and Results

Against the background of cardiomyocyte‐restricted expression of CryAB^R120G, we have partially ablated TGF‐β signaling in cardiac myofibroblasts to observe whether cardiac fibrosis is reduced despite the ongoing pathogenic stimulus of CryAB^R120G production. Transgenic CryAB^R120G mice were crossed with mice containing a floxed allele of TGF‐β receptor 2 (Tgfbr2^f/f). The double transgenic animals were subsequently crossed to another transgenic line in which Cre expression was driven from the periostin locus (Postn) so that Tgfbr2 would be ablated with myofibroblast conversion. Structural and functional assays were then used to determine whether general fibrosis was affected and cardiac function rescued in CryAB^R120G mice lacking Tgfbr2 in the myofibroblasts. Ablation of myofibroblast specific TGF‐β signaling led to decreased morbidity in a proteotoxic disease resulting from cardiomyocyte autonomous expression of CryAB^R120G. Cardiac fibrosis was decreased and hypertrophy was also significantly attenuated, with a significant improvement in survival probability over time, even though the primary proteotoxic insult continued.

Conclusions

Myofibroblast‐targeted knockdown of Tgfbr2 signaling resulted in reduced fibrosis and improved cardiac function, leading to improved probability of survival.

Molecular Lysine Tweezers Counteract Aberrant Protein Aggregation

Molecular tweezers (MTs) are supramolecular host molecules equipped with two aromatic pincers linked together by a spacer (Gakh, 2018). They are endowed with fascinating properties originating from their ability to hold guests between their aromatic pincers (Chen and Whitlock, 1978; Zimmerman, 1991; Harmata, 2004). MTs are finding an increasing number of medicinal applications, e.g., as bis-intercalators for DNA such as the anticancer drug Ditercalinium (Gao et al., 1991), drug activity reverters such as the bisglycoluril tweezers Calabadion 1 (Ma et al., 2012) as well as radioimmuno detectors such as Venus flytrap clusters (Paxton et al., 1991). We recently embarked on a program to create water-soluble tweezers which selectively bind the side chains of lysine and arginine inside their cavity. This unique recognition mode is enabled by a torus-shaped, polycyclic framework, which is equipped with two hydrophilic phosphate groups. Cationic amino acid residues are bound by the synergistic effect of disperse, hydrophobic, and electrostatic interactions in a kinetically fast reversible process. Interactions of the same kind play a key role in numerous protein-protein interactions, as well as in pathologic protein aggregation. Therefore, these particular MTs show a high potential to disrupt such events, and indeed inhibit misfolding and self-assembly of amyloidogenic polypeptides without toxic side effects. The mini-review provides insight into the unique binding mode of MTs both toward peptides and aggregating proteins. It presents the synthesis of the lead compound CLR01 and its control, CLR03. Different biophysical experiments are explained which elucidate and help to better understand their mechanism of action. Specifically, we show how toxic aggregates of oligomeric and fibrillar protein species are dissolved and redirected to form amorphous, benign assemblies. Importantly, these new chemical tools are shown to be essentially non-toxic in vivo. Due to their reversible moderately tight binding, these agents are not protein-, but rather process-specific, which suggests a broad range of applications in protein misfolding events. Thus, MTs are highly promising candidates for disease-modifying therapy in early stages of neurodegenerative diseases. This is an outstanding example in the evolution of supramolecular concepts toward biological application.

Highly Dynamic Changes in the Activity and Regulation of Macroautophagy in Hearts Subjected to Increased Proteotoxic Stress

Macroautophagy (referred to as autophagy hereafter) plays an important role in the quality control of cellular proteins and organelles. Transcription Factor EB (TFEB) globally activates the expression of genes in the autophagic-lysosomal pathway (ALP) to replenish lysosomes and ALP machineries. We previously reported that myocardial TFEB signaling was impaired in advanced cardiac proteinopathy; however, myocardial ALP status and TFEB activity at earlier stages of cardiac proteinopathy remain uncharacterized. Here a stable line of CryAB^R120G transgenic (R120G) and non-transgenic (NTG) littermate mice with cardiomyocyte-restricted overexpression of CryAB^R120G were used at 1, 3, and 6 months of age. At 1 month when no cardiac phenotypes other than aberrant protein aggregation are discernible, R120G mice displayed a 5-fold increase in myocardial LC3-II flux. Interestingly, the LC3-II flux increase co-existed with increases in mTOR complex 1 (mTORC1) activities as well as cytoplasmic, but not nuclear, TFEB proteins. This increase in cytoplasmic TFEB proteins occurred without any discernible alteration in TFEB activity as reflected by unchanged mRNA levels of representative TFEB target genes (Mcoln1, M6pr, Sqstm1, Vps18, and Uvrag). At 3 months of age when hypertrophy and diastolic malfunction start to develop, the LC3-II flux remained significantly increased but to a lesser degree (2-fold) than at 1 month. The LC3-II flux increase was associated with decreased mTORC1 activities and with increased nuclear TFEB proteins and TFEB activities. At 6 months of age when congestive heart failure is apparent in R120G mice, both LC3-II flux and TFEB activities were severely suppressed, while mTORC1 activity increased. We conclude that changes in both autophagy and TFEB signaling are highly dynamic during the progression of cardiac proteinopathy. Increases in autophagy occur before increases in TFEB activities but both increase in the compensatory stage of cardiac proteinopathy. Once congestive heart failure develops, both autophagy and TFEB signaling become impaired. Our results suggest that TFEB signaling is regulated by both mTORC1-dependent and -independent mechanisms in hearts subjected to increased proteotoxic stress. For therapeutic exploration, it will be important to test the effect of TFEB stimulation at the early, intermediate, and late stages of cardiac proteinopathy.

Pharmaceutical Applications of Molecular Tweezers, Clefts and Clips

Synthetic acyclic receptors, composed of two arms connected with a spacer enabling molecular recognition, have been intensively explored in host-guest chemistry in the past decades. They fall into the categories of molecular tweezers, clefts and clips, depending on the geometry allowing the recognition of various guests. The advances in synthesis and mechanistic studies have pushed them forward to pharmaceutical applications, such as neurodegenerative disorders, infectious diseases, cancer, cardiovascular disease, diabetes, etc. In this review, we provide a summary of the synthetic molecular tweezers, clefts and clips that have been reported for pharmaceutical applications. Their structures, mechanism of action as well as in vitro and in vivo results are described. Such receptors were found to selectively bind biological guests, namely, nucleic acids, sugars, amino acids and proteins enabling their use as biosensors or therapeutics. Particularly interesting are dynamic molecular tweezers which are capable of controlled motion in response to an external stimulus. They proved their utility as imaging agents or in the design of controlled release systems. Despite some issues, such as stability, cytotoxicity or biocompatibility that still need to be addressed, it is obvious that molecular tweezers, clefts and clips are promising candidates for several incurable diseases as therapeutic agents, diagnostic or delivery tools.

The molecular tweezer CLR01 inhibits aberrant superoxide dismutase 1 (SOD1) self-assembly in vitro and in the G93A-SOD1 mouse model of ALS

Mutations in superoxide dismutase 1 (SOD1) cause 15-20% of familial amyotrophic lateral sclerosis (fALS) cases. The resulting amino acid substitutions destabilize SOD1's protein structure, leading to its self-assembly into neurotoxic oligomers and aggregates, a process hypothesized to cause the characteristic motor-neuron degeneration in affected individuals. Currently, effective disease-modifying therapy is not available for ALS. Molecular tweezers prevent formation of toxic protein assemblies, yet their protective action has not been tested previously on SOD1 or in the context of ALS. Here, we tested the molecular tweezer CLR01-a broad-spectrum inhibitor of the self-assembly and toxicity of amyloid proteins-as a potential therapeutic agent for ALS. Using recombinant WT and mutant SOD1, we found that CLR01 inhibited the aggregation of all tested SOD1 forms in vitro Next, we examined whether CLR01 could prevent the formation of misfolded SOD1 in the G93A-SOD1 mouse model of ALS and whether such inhibition would have a beneficial therapeutic effect. CLR01 treatment decreased misfolded SOD1 in the spinal cord significantly. However, these histological findings did not correlate with improvement of the disease phenotype. A small, dose-dependent decrease in disease duration was found in CLR01-treated mice, relative to vehicle-treated animals, yet motor function did not improve in any of the treatment groups. These results demonstrate that CLR01 can inhibit SOD1 misfolding and aggregation both in vitro and in vivo, but raise the question whether such inhibition is sufficient for achieving a therapeutic effect. Additional studies in other less aggressive ALS models may be needed to determine the therapeutic potential of this approach.

The molecular tweezer CLR01 inhibits Ebola and Zika virus infection

Ebola (EBOV) and Zika viruses (ZIKV) are responsible for recent global health threats. As no preventive vaccines or antiviral drugs against these two re-emerging pathogens are available, we evaluated whether the molecular tweezer CLR01 may inhibit EBOV and ZIKV infection. This small molecule has previously been shown to inactivate HIV-1 and herpes viruses through a selective interaction with lipid-raft-rich regions in the viral envelope, which results in membrane disruption and loss of infectivity. We found that CLR01 indeed blocked infection of EBOV and ZIKV in a dose-dependent manner. The tweezer inhibited infection of epidemic ZIKV strains in cells derived from the anogenital tract and the central nervous system, and remained antivirally active in the presence of semen, saliva, urine and cerebrospinal fluid. Our findings show that CLR01 is a broad-spectrum inhibitor of enveloped viruses with prospects as a preventative microbicide or antiviral agent.