Discovery of Arginine Ethyl Ester as Polyglutamine Aggregation Inhibitor: Conformational Transitioning of Huntingtin N-Terminus Augments Aggregation Suppression

Polyglutamine disorders: Pathogenesis and potential drug interventions

Polyglutamine/poly(Q) diseases are a group nine hereditary neurodegenerative disorders caused due to abnormally expanded stretches of CAG trinucleotide in functionally distinct genes. All human poly(Q) diseases are characterized by the formation of microscopically discernable poly(Q) positive aggregates, the inclusion bodies. These toxic inclusion bodies are responsible for the impairment of several cellular pathways such as autophagy, transcription, cell death, etc., that culminate in disease manifestation. Although, these diseases remain largely without treatment, extensive research has generated mounting evidences that various events of poly(Q) pathogenesis can be developed as potential drug targets. The present review article briefly discusses the key events of disease pathogenesis, model system-based investigations that support the development of effective therapeutic interventions against pathogenesis of human poly(Q) disorders, and a comprehensive list of pharmacological and bioactive compounds that have been experimentally shown to alleviate poly(Q)-mediated neurotoxicity. Interestingly, due to the common cause of pathogenesis, all poly(Q) diseases share etiology, thus, findings from one disease can be potentially extrapolated to other poly(Q) diseases as well.

Alleviation of albumin glycation-induced diabetic cardiomyopathy by L-Arginine: Insights into Nrf-2 signaling

In hyperglycemia, accelerated glycation and oxidative stress give rise to many diabetic complications, such as diabetic cardiomyopathy (DCM). Glycated human serum albumin (GHSA) has disturbed structural integrity and hampered functional capabilities. When GHSA accumulates around cardiac cells, Nrf-2 is dysregulated, aiding oxidative stress. L-Arginine (L-Arg) is prescribed to patients with diabetes and cardiovascular diseases. This research contributes to the mechanistic insights on antiglycation and antioxidant potential of L-Arg in alleviating DCM. HSA was glycated with methylglyoxal in the presence of L-Arg (20-640 mM). Structural and functional modifications of HSA were studied. L-Arg and HSA, GHSA interactions, and thermodynamics were determined by steady-state fluorescence. H9c2 cardiomyocytes were given treatments of GHSA-L-Arg along with the inhibitor of the receptor of AGEs. Cellular antioxidant levels, detoxification enzyme activities were measured. Gene, protein expressions, and immunofluorescence data examined the activation and nuclear translocation of Nrf-2 during glycation and oxidative stress. L-Arg protected HSA from glycation-induced structural and functional modifications. The binding affinity of L-Arg was more towards HSA (104 M-1). L-Arg, specifically at lower concentration (20 mM), upregulated Nrf-2 gene, protein expressions and facilitated its nuclear translocation by activating Nrf-2 signaling. The study concluded that L-Arg can be of therapeutic advantage in glycation-induced DCM and associated oxidative stress.

Elevated SLC7A2 expression is associated with an abnormal neuroinflammatory response and nitrosative stress in Huntington's disease

We previously identified solute carrier family 7 member 2 (SLC7A2) as one of the top upregulated genes when normal Huntingtin was deleted. SLC7A2 has a high affinity for L-arginine. Arginine is implicated in inflammatory responses, and SLC7A2 is an important regulator of innate and adaptive immunity in macrophages. Although neuroinflammation is clearly demonstrated in animal models and patients with Huntington's disease (HD), the question of whether neuroinflammation actively participates in HD pathogenesis is a topic of ongoing research and debate. Here, we studied the role of SLC7A2 in mediating the neuroinflammatory stress response in HD cells. RNA sequencing (RNA-seq), quantitative RT-PCR and data mining of publicly available RNA-seq datasets of human patients were performed to assess the levels of SLC7A2 mRNA in different HD cellular models and patients. Biochemical studies were then conducted on cell lines and primary mouse astrocytes to investigate arginine metabolism and nitrosative stress in response to neuroinflammation. The CRISPR-Cas9 system was used to knock out SLC7A2 in STHdhQ7 and Q111 cells to investigate its role in mediating the neuroinflammatory response. Live-cell imaging was used to measure mitochondrial dynamics. Finally, exploratory studies were performed using the Enroll-HD periodic human patient dataset to analyze the effect of arginine supplements on HD progression. We found that SLC7A2 is selectively upregulated in HD cellular models and patients. HD cells exhibit an overactive response to neuroinflammatory challenges, as demonstrated by abnormally high iNOS induction and NO production, leading to increased protein nitrosylation. Depleting extracellular Arg or knocking out SLC7A2 blocked iNOS induction and NO production in STHdhQ111 cells. We further examined the functional impact of protein nitrosylation on a well-documented protein target, DRP-1, and found that more mitochondria were fragmented in challenged STHdhQ111 cells. Last, analysis of Enroll-HD datasets suggested that HD patients taking arginine supplements progressed more rapidly than others. Our data suggest a novel pathway that links arginine uptake to nitrosative stress via upregulation of SLC7A2 in the pathogenesis and progression of HD. This further implies that arginine supplements may potentially pose a greater risk to HD patients.

Advances in nitric oxide regulators for the treatment of ischemic stroke

Ischemic stroke (IS) is a life-threatening disease worldwide. Nitric oxide (NO) derived from l-arginine catalyzed by NO synthase (NOS) is closely associated with IS. Three isomers of NOS (nNOS, eNOS and iNOS) produce different concentrations of NO, resulting in quite unlike effects during IS. Of them, n/iNOSs generate high levels of NO, detrimental to brain by causing nerve cell apoptosis and/or necrosis, whereas eNOS releases small amounts of NO, beneficial to the brain via increasing cerebral blood flow and improving nerve function. As a result, a large variety of NO regulators (NO donors or n/iNOS inhibitors) have been developed for fighting IS. Regrettably, up to now, no review systematically introduces the progresses in this area. This article first outlines dynamic variation rule of NOS/NO in IS, subsequently highlights advances in NO regulators against IS, and finally presents perspectives based on concentration-, site- and timing-effects of NO production to promote this field forward.

Neurobiochemical characteristics of arginine-rich peptides explain their potential therapeutic efficacy in neurodegenerative diseases

Neurodegenerative diseases, including Alzheimer̕ s disease (AD), Parkinson̕ s disease (PD), Huntington̕ s disease (HD), and Amyotrophic Lateral Sclerosis (ALS) require special attention to find new potential treatment methods. This review aims to summarize the current knowledge of the relationship between the biochemical properties of arginine-rich peptides (ARPs) and their neuroprotective effects to deal with the harmful effects of risk factors. It seems that ARPs have portrayed a promising and fantastic landscape for treating neurodegeneration-associated disorders. With multimodal mechanisms of action, ARPs play various unprecedented roles, including as the novel delivery platforms for entering the central nervous system (CNS), the potent antagonists for calcium influx, the invader molecules for targeting mitochondria, and the protein stabilizers. Interestingly, these peptides inhibit the proteolytic enzymes and block protein aggregation to induce pro-survival signaling pathways. ARPs also serve as the scavengers of toxic molecules and the reducers of oxidative stress agents. They also have anti-inflammatory, antimicrobial, and anti-cancer properties. Moreover, by providing an efficient nucleic acid delivery system, ARPs can play an essential role in developing various fields, including gene vaccines, gene therapy, gene editing, and imaging. ARP agents and ARP/cargo therapeutics can be raised as an emergent class of neurotherapeutics for neurodegeneration. Part of the aim of this review is to present recent advances in treating neurodegenerative diseases using ARPs as an emerging and powerful therapeutic tool. The applications and progress of ARPs-based nucleic acid delivery systems have also been discussed to highlight their usefulness as a broad-acting class of drugs.

The Emerging Landscape of Small-Molecule Therapeutics for the Treatment of Huntington's Disease

Huntington's disease (HD) is a progressive neurodegenerative disorder caused by a CAG repeat expansion in the huntingtin gene (HTT). The new insights into HD's cellular and molecular pathways have led to the identification of numerous potent small-molecule therapeutics for HD therapy. The field of HD-targeting small-molecule therapeutics is accelerating, and the approval of these therapeutics to combat HD may be expected in the near future. For instance, preclinical candidates such as naphthyridine-azaquinolone, AN1, AN2, CHDI-00484077, PRE084, EVP4593, and LOC14 have shown promise for further optimization to enter into HD clinical trials. This perspective aims to summarize the advent of small-molecule therapeutics at various stages of clinical development for HD therapy, emphasizing their structure and design, therapeutic effects, and specific mechanisms of action. Further, we have highlighted the key drivers involved in HD pathogenesis to provide insights into the basic principle for designing promising anti-HD therapeutic leads.

Arginine and Arginine-Rich Peptides as Modulators of Protein Aggregation and Cytotoxicity Associated With Alzheimer's Disease

A substantial body of evidence indicates cationic, arginine-rich peptides (CARPs) are effective therapeutic compounds for a range of neurodegenerative pathologies, with beneficial effects including the reduction of excitotoxic cell death and mitochondrial dysfunction. CARPs, therefore, represent an emergent class of promising neurotherapeutics with multimodal mechanisms of action. Arginine itself is a known chaotrope, able to prevent misfolding and aggregation of proteins. The putative role of proteopathies in chronic neurodegenerative diseases such as Alzheimer's disease (AD) warrants investigation into whether CARPs could also prevent the aggregation and cytotoxicity of amyloidogenic proteins, particularly amyloid-beta and tau. While monomeric arginine is well-established as an inhibitor of protein aggregation in solution, no studies have comprehensively discussed the anti-aggregatory properties of arginine and CARPs on proteins associated with neurodegenerative disease. Here, we review the structural, physicochemical, and self-associative properties of arginine and the guanidinium moiety, to explore the mechanisms underlying the modulation of protein aggregation by monomeric and multimeric arginine molecules. Arginine-rich peptide-based inhibitors of amyloid-beta and tau aggregation are discussed, as well as further modulatory roles which could reduce proteopathic cytotoxicity, in the context of therapeutic development for AD.

TAR RNA Mediated Folding of a Single-Arginine-Mutant HIV-1 Tat Protein within HeLa Cells Experiencing Intracellular Crowding

The various effects of native protein folding on the stability and folding rate of intrinsically disordered proteins (IDPs) in crowded intracellular environments are important in biomedicine. Although most studies on protein folding have been conducted in vitro, providing valuable insights, studies on protein folding in crowded intracellular environments are scarce. This study aimed to explore the effects of intracellular molecular crowding on the folding of mutant transactivator HIV-1 Tat based on intracellular interactions, including TAR RNA, as proof of the previously reported chaperna-RNA concept. Considering that the Tat-TAR RNA motif binds RNA, we assessed the po tential function of TAR RNA as a chaperna for the refolding of R52Tat, a mutant in which the argi nine (R) residues at R52 have been replaced with alanine (A) by site-directed mutagenesis. We mon itored Tat-EGFP and Tat folding in HeLa cells via time-lapse fluorescence microscopy and biolayer interferometry using EGFP fusion as an indicator for folding status. These results show that the refolding of R52A Tat was stimulated well at a 0.3 μM TAR RNA concentration; wild-type Tat refolding was essentially abolished because of a reduction in the affinity for TAR RNA at that con centration. The folding and refolding of R52Tat were mainly promoted upon stimulation with TAR RNA. Our findings provide novel insights into the therapeutic potential of chaperna-mediated fold ing through the examination of as-yet-unexplored RNA-mediated protein folding as well as viral genetic variants that modulate viral evolutionary linkages for viral diseases inside a crowded intra cellular environment.

A New Chemoenzymatic Semisynthetic Approach Provides Insight into the Role of Phosphorylation beyond Exon1 of Huntingtin and Reveals N-Terminal Fragment Length-Dependent Distinct Mechanisms of Aggregation

Huntington's disease is a neurodegenerative disorder caused by the expansion of a polyglutamine repeat (>36Q) in the N-terminal domain of the huntingtin protein (Htt), which renders the protein or fragments thereof more prone to aggregate and form inclusions. Although several Htt N-terminal fragments of different lengths have been identified within Htt inclusions, most studies on the mechanisms, sequence, and structural determinants of Htt aggregation have focused on the Httexon1 (Httex1). Herein, we investigated the aggregation properties of mutant N-terminal Htt fragments of various lengths (Htt171, Htt140, and Htt104) in comparison to mutant Httex1 (mHttex1). We also present a new chemoenzymatic semisynthetic strategy that enables site-specific phosphorylation of Htt beyond Httex1. These advances yielded insights into how post-translational modifications (PTMs) and structured domains beyond Httex1 influence aggregation mechanisms, kinetics, and fibril morphology of longer N-terminal Htt fragments. We demonstrate that phosphorylation at T107 significantly slows the aggregation of mHtt171, whereas phosphorylation at T107 and S116 accelerates the aggregation, underscoring the importance of crosstalk between different PTMs. The mHtt171 proteins aggregate via a different mechanism and form oligomers and fibrillar aggregates with morphological properties that are distinct from that of mHttex1. These observations suggest that different N-terminal fragments could have distinct aggregation mechanisms and that a single polyQ-targeting antiaggregation strategy may not effectively inhibit the aggregation of all N-terminal Htt fragments. Finally, our results underscore the need for further studies to investigate the aggregation mechanisms of Htt fragments and how the various fragments interact with each other and influence Htt toxicity and disease progression.

Protein Aggregation Inhibitors as Disease-Modifying Therapies for Polyglutamine Diseases

The polyglutamine (polyQ) diseases are a group of inherited neurodegenerative diseases caused by the abnormal expansion of a CAG trinucleotide repeat that are translated into an expanded polyQ stretch in the disease-causative proteins. The expanded polyQ stretch itself plays a critical disease-causative role in the pathomechanisms underlying polyQ diseases. Notably, the expanded polyQ stretch undergoes a conformational transition from the native monomer into the β-sheet-rich monomer, followed by the formation of soluble oligomers and then insoluble aggregates with amyloid fibrillar structures. The intermediate soluble species including the β-sheet-rich monomer and oligomers exhibit substantial neurotoxicity. Therefore, protein conformation stabilization and aggregation inhibition that target the upstream of the insoluble aggregate formation would be a promising approach toward the development of disease-modifying therapies for polyQ diseases. PolyQ aggregation inhibitors of different chemical categories, such as intrabodies, peptides, and small chemical compounds, have been identified through intensive screening methods. Among them, recent advances in the brain delivery methods of several peptides and the screening of small chemical compounds have brought them closer to clinical utility. Notably, the recent discovery of arginine as a potent conformation stabilizer and aggregation inhibitor of polyQ proteins both in vitro and in vivo have paved way to the clinical trial for the patients with polyQ diseases. Meanwhile, expression reduction of expanded polyQ proteins per se would be another promising approach toward disease modification of polyQ diseases. Gene silencing, especially by antisense oligonucleotides (ASOs), have succeeded in reducing the expression of polyQ proteins in the animal models of various polyQ diseases by targeting the aberrant mRNA with expanded CAG repeats. Of note, some of these ASOs have recently been translated into clinical trials. Here we overview and discuss these recent advances toward the development of disease modifying therapies for polyQ diseases. We envision that combination therapies using aggregation inhibitors and gene silencing would meet the needs of the patients with polyQ diseases and their caregivers in the near future to delay or prevent the onset and progression of these currently intractable diseases.