Characterization of Lipid Binding Properties of Plasmodium falciparum Acyl-Coenzyme A Binding Proteins and Their Competitive Inhibition by Mefloquine

Synthesis of New Enantiopure Aminoalcohol Fluorenes as Promising Antimalarial Compounds

Herein, we report the design, synthesis, and characterisation of a new library of enantiopure aminoalcohol fluorenes, as well as their in vitro evaluation for biological properties, including activity against two strains of P. falciparum (3D7 and W2) and cytotoxicity on the HepG2 cell line. All tested compounds exhibited good to excellent antimalarial potency with IC50 values ranging from 0.7 to 70.2 nM whatever the strain. Interestingly, most compounds showed equal or better antimalarial activity compared to the reference drugs lumefantrine, mefloquine and chloroquine. Despite moderate cytotoxicity in the micromolar range, all aminoalcohol fluorenes displayed an excellent selectivity index higher than 100 due to strong antimalarial activity. Furthermore, we report in silico analyses of physicochemical and pharmacokinetic properties for all compounds, highlighting the drug-likeness of compound 10 and its promising potential for further studies.

Uncovering the Mechanism of Action of Antiprotozoal Agents: A Survey on Photoaffinity Labeling Strategy

Plasmodium, Leishmania, and Trypanosoma parasites are responsible for infectious diseases threatening millions of people worldwide. Despite more recent efforts devoted to the search for new antiprotozoal agents, efficacy, safety, and resistance issues still hinder the development of suited therapeutic options. The lack of robustly validated targets and the complexity of parasite's diseases have made phenotypic screening a preferential drug discovery strategy for the identification of new chemical entities. However, via this approach, no information on biological target(s) and mechanisms of action of compounds are provided. Among the target deconvolution strategies useful to fill this gap, photoaffinity labeling (PAL) has emerged as one of most suited to enable investigation in a complex cellular environment. More recently, PAL has been exploited to unravel the molecular basis of bioactive compounds' function in live parasites, allowing elucidation of the mechanism of action of both approved drugs and new chemical entities. Besides highlighting new potential drug targets, PAL can provide valuable information on efficacy and liabilities of small molecules at the molecular level, which could be exploited to greatly facilitate the rational optimization of compounds in terms of potency and safety. In this review, we will report the most recent studies that have leveraged PAL to disclose the biological targets and mechanism of action of phenotypically active compounds targeting kinetoplastid diseases (i.e., human African trypanosomiasis, leishmaniasis, and Chagas disease) and malaria. Moreover, we will comment on potential perspectives that this innovative approach can provide in aiding the discovery and development of new antiprotozoal drugs.

Mapping of Nuclear Localization Signal in Secreted Liver-Specific Protein 2 of Plasmodium falciparum

The secretory proteome of Plasmodium exhibits differential spatial and functional activity within host cells. Plasmodium secretes proteins that translocate into the human host cell nucleus. Liver-specific protein 2 of Plasmodium falciparum (Pf-LISP2) shows nuclear accumulation in human hepatocytes during the late liver stage of malaria parasite development. However, the nuclear translocation mechanism for Pf-LISP2 remains largely uncharacterized. Here, we identified a classical bipartite nuclear localization signal (NLS) located in the C-terminal region of Pf-LISP2. Phylogenetic analysis revealed that this NLS is unique to Plasmodium falciparum and its close relative Plasmodium reichenowi, suggesting an evolutionary adaptation linked to their shared primate hosts. Functional assays confirmed the NLS's nuclear import activity, as fusion constructs of the Pf-LISP2 NLS with Pf-aldolase (Pf-aldolase-NLS-EGFP) localized exclusively to the nucleus of HepG2 cells. Mutation analysis of key lysine and arginine residues in the bipartite NLS demonstrated that the basic amino acid clusters are essential for nuclear localization. Importin-α/β interaction was found to be crucial for Pf-LISP2 nuclear transport, as coexpression of the NLS constructs with the importin-α/β inhibitor mCherry-Bimax2 significantly blocked nuclear translocation. Specific interactions between the lysine and arginine residues of Pf-LISP2's NLS and the conserved tryptophan and asparagine residues of human importin-α1 facilitate the cytosol-to-nuclear translocation of Pf-LISP2. Additionally, LISP2 lacks any nuclear export signal. These results provide new insights into the mechanisms of nuclear transport in Plasmodium falciparum, potentially contributing to the understanding of its pathogenicity and host-cell interactions during liver-stage infection.

Evaluation of In Vitro Inhibition of β-Hematin Formation: A Step Towards a Comprehensive Understanding of the Mechanism of Action of New Arylamino Alcohols

Currently, artemisinin-based combination therapy is recommended as first-line treatment of uncomplicated falciparum malaria. Arylamino alcohols (AAAs) such as mefloquine (MQ) are the preferred partner drugs due to their longer half-life, reliable absorption and strong antimalarial activity. However, the mode of action of MQ remains poorly understood and its neurotoxicity limits its use. Furthermore, the emergence of drug-resistant parasites requires development of new antimalarial drugs. The aim of this study was to evaluate the β-hematin inhibition capacity of three pairs of enantiopure AAAs 1-3 (a/S and b/R) derived from MQ or enpiroline (ENP), a pyridine-based MQ analog with strong antimalarial activity. Inhibition of β-hematin-the synthetic counterpart of hemozoin formation-was determined for each compound. Antimalarial activity against W2 and 3D7 Plasmodium falciparum strains as well as percentages of inhibition of β-hematin formation were compared to those of reference molecules, i.e., chloroquine (CQ), MQ and ENP. Furthermore, a cytotoxicity study on the human-derived hepatocarcinoma cell line HepG2 was performed. With high antimalarial activity, stronger ability to inhibit β-hematin formation and low cytotoxicity, AAAs 1a-b and 2a are the most promising. These findings provide a better understanding of their potential mechanisms of action and may pave the way toward developing new lead compounds.

Conformational plasticity links structural instability of NAA10F128I and NAA10F128L mutants to their catalytic deregulation

The acetylation of proteins' N-terminal amino groups by the N-acetyltransferase complexes plays a crucial role in modulating the spatial stability and functional activities of diverse human proteins. Mutations disrupting the stability and function of NAA10 result in X-linked rare genetic disorders. In this study, we conducted a global analysis of the impact of fifteen disease-associated missense mutations in NAA10. The analyses revealed that mutations in specific residues, such as Y43, V107, V111, and F128, predictably disrupted interactions essential for NAA10 stability, while most mutations (except R79C, A111W, Q129P, and N178K) expectedly led to structural destabilization. Mutations in many conserved residues within short linear motifs and post-translational modification sites were predicted to affect NAA10 functionality and regulation. All mutations were classified as pathogenic, with F128I and F128L identified as the most destabilizing mutations. The findings show that the F128L and F128I mutations employ different mechanisms for the loss of catalytic activities of NAA10F128L and NAA10F128I due to their structural instability. These two mutations induce distinct folding energy states that differentially modulate the structures of different regions of NAA10F128L and NAA10F128I. Specifically, the predicted instability caused by the F128I mutation results in decreased flexibility within the substrate-binding region, impairing the substrate peptide binding ability of NAA10F128I. Conversely, F128L is predicted to reduce the flexibility of the region containing the acetyl-CoA binding residues in NAA10F128L. Our study provides insights into the mechanism of catalytic inactivation of mutants of NAA10, particularly elucidating the mechanistic features of the structural and functional pathogenicity of the F128L and F128I mutations.

The artemisinin-induced dormant stages of Plasmodium falciparum exhibit hallmarks of cellular quiescence/senescence and drug resilience

Recrudescent infections with the human malaria parasite, Plasmodium falciparum, presented traditionally the major setback of artemisinin-based monotherapies. Although the introduction of artemisinin combination therapies (ACT) largely solved the problem, the ability of artemisinin to induce dormant parasites still poses an obstacle for current as well as future malaria chemotherapeutics. Here, we use a laboratory model for induction of dormant P. falciparum parasites and characterize their transcriptome, drug sensitivity profile, and cellular ultrastructure. We show that P. falciparum dormancy requires a ~ 5-day maturation process during which the genome-wide gene expression pattern gradually transitions from the ring-like state to a unique form. The transcriptome of the mature dormant stage carries hallmarks of both cellular quiescence and senescence, with downregulation of most cellular functions associated with growth and development and upregulation of selected metabolic functions and DNA repair. Moreover, the P. falciparum dormant stage is considerably more resistant to antimalaria drugs compared to the fast-growing asexual stages. Finally, the irregular cellular ultrastructure further suggests unique properties of this developmental stage of the P. falciparum life cycle that should be taken into consideration by malaria control strategies.

Identification and characterization of nuclear localization signals in the circumsporozoite protein of Plasmodium falciparum

Secretory proteins of Plasmodium exhibit differential spatial and functional activity within the host cell nucleus. However, the nuclear localization signals (NLSs) for these proteins remain largely uncharacterized. In this study, we have identified and characterized two NLSs in the circumsporozoite protein of Plasmodium falciparum (Pf-CSP). Both NLSs in the Pf-CSP contain clusters of lysine and arginine residues essential for specific interactions with the conserved tryptophan and asparagine residues of importin-α, facilitating nuclear translocation of Pf-CSP. While the two NLSs of Pf-CSP function independently and are both crucial for nuclear localization, a single NLS of Pf-CSP leads to weak nuclear localization. These findings shed light on the mechanism of nuclear penetrability of secretory proteins of Plasmodium proteins.

An In Silico and In Vitro Assessment of the Neurotoxicity of Mefloquine

Mefloquine (MQ) is a quinoline-based anti-malarial drug used for chemoprophylaxis or as a treatment in combination with artesunate. Although MQ has clear anti-Plasmodium falciparum properties, it can induce neurotoxicity and undesired neuropsychiatric side effects in humans. Hence, this study aimed to characterize the neurotoxicity of MQ using human neuroblastoma SH-SY5Y cells. The effects of MQ on neuronal toxicity and cell viability were investigated over a concentration range of 1-100 µM using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and lactate dehydrogenase (LDH) assays. The influence of MQ on cellular bioenergetics was examined by measuring cellular ATP levels and from the induction of reactive oxygen species (ROS). An in silico approach was used to assess the potential neurotoxicity of MQ mediated via binding to the active sites of acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) and then experimentally validated via in vitro enzymatic assays. MQ was cytotoxic to neuronal cells in a concentration and exposure duration dependent manner and induced a significant reduction in viability at concentrations of ≥25 µM after a 24 h exposure. MQ adversely impacted cellular bioenergetics and significantly depleted ATP production at concentrations of ≥1 µM after 24 h. MQ-induced cellular ROS production, which was correlated with the induction of apoptosis, as revealed by flow cytometry. In silico studies suggested that MQ was a dual cholinesterase inhibitor and one with remarkably potent binding to BuChE. Modelling data were supported by in vitro studies which showed that MQ inhibited both human AChE and BuChE enzymes. In summary, MQ is an antimalarial drug that may induce neurotoxicity by impacting cellular bioenergetics and perturbing the activity of cholinesterases at exposure concentrations relevant to human dosage.

Conformational Tuning Shapes the Balance between Functional Promiscuity and Specialization in Paralogous Plasmodium Acyl-CoA Binding Proteins

Paralogous proteins confer enhanced fitness to organisms via complex sequence-conformation codes that shape functional divergence, specialization, or promiscuity. Here, we dissect the underlying mechanism of promiscuous binding versus partial subfunctionalization in paralogues by studying structurally identical acyl-CoA binding proteins (ACBPs) from Plasmodium falciparum that serve as promising drug targets due to their high expression during the protozoan proliferative phase. Combining spectroscopic measurements, solution NMR, SPR, and simulations on two of the paralogues, A16 and A749, we show that minor sequence differences shape nearly every local and global conformational feature. A749 displays a broader and heterogeneous native ensemble, weaker thermodynamic coupling and cooperativity, enhanced fluctuations, and a larger binding pocket volume compared to A16. Site-specific tryptophan probes signal a graded reduction in the sampling of substates in the holo form, which is particularly apparent in A749. The paralogues exhibit a spectrum of binding affinities to different acyl-CoAs with A749, the more promiscuous and hence the likely ancestor, binding 1000-fold stronger to lauroyl-CoA under physiological conditions. We thus demonstrate how minor sequence changes modulate the extent of long-range interactions and dynamics, effectively contributing to the molecular evolution of contrasting functional repertoires in paralogues.

Acyl-CoA binding protein regulates nutrient-dependent autophagy

BACKGROUND

Homeostasis of autophagy under normal conditions and nutrient stress is maintained by adaptive activation of regulatory proteins. However, the protein-lipid crosstalk that modulates the switch from suppression to activation of autophagy initiation is largely unknown.

RESULTS

Here, we show that human diazepam-binding inhibitor (DBI), also known as acyl-CoA binding protein (ACBP), binds to phosphatidylethanolamine of the phagophore membrane under nutrient-rich growth conditions, leading to inhibition of LC3 lipidation and suppression of autophagy initiation. Specific residues, including the conserved tyrosine residues of DBI, interact with phosphatidylethanolamine to stabilize the later molecule in the acyl-CoA binding cavity of the protein. Under starvation, phosphorylation of serine-21 of DBI mediated by the AMP-activated protein kinase results in a drastic reduction in the affinity of the protein for phosphatidylethanolamine. The release of serine-21 phosphorylated DBI from the phagophore upon nutrient starvation restores the high LC3 lipidation flux and maturation of the phagophore to autophagosome.

CONCLUSION

DBI acts as a strategic barrier against overactivation of phagophore maturation under nutrient-rich conditions, while triggering autophagy under nutrient-deficient conditions.

The flexibility of Apicomplexa parasites in lipid metabolism

Apicomplexa are obligate intracellular parasites responsible for major human infectious diseases such as toxoplasmosis and malaria, which pose social and economic burdens around the world. To survive and propagate, these parasites need to acquire a significant number of essential biomolecules from their hosts. Among these biomolecules, lipids are a key metabolite required for parasite membrane biogenesis, signaling events, and energy storage. Parasites can either scavenge lipids from their host or synthesize them de novo in a relict plastid, the apicoplast. During their complex life cycle (sexual/asexual/dormant), Apicomplexa infect a large variety of cells and their metabolic flexibility allows them to adapt to different host environments such as low/high fat content or low/high sugar levels. In this review, we discuss the role of lipids in Apicomplexa parasites and summarize recent findings on the metabolic mechanisms in host nutrient adaptation.

HYPK coordinates degradation of polyneddylated proteins by autophagy

Selective degradation of protein aggregates by macroautophagy/autophagy is an essential homeostatic process of safeguarding cells from the effects of proteotoxicity. Among the ubiquitin-like proteins, NEDD8 conjugation to misfolded proteins is prominent in stress-induced protein aggregates, albeit the function of neddylation in autophagy is unclear. Here, we report that polyneddylation functions as a post-translational modification for autophagic degradation of proteotoxic-stress induced protein aggregates. We also show that HYPK functions as an autophagy receptor in the polyneddylation-dependent aggrephagy. The scaffolding function of HYPK is facilitated by its C-terminal ubiquitin-associated domain and N-terminal tyrosine-type LC3-interacting region which bind to NEDD8 and LC3 respectively. Both NEDD8 and HYPK are positive modulators of basal and proteotoxicity-induced autophagy, leading to protection of cells from protein aggregates, such as aggregates of mutant HTT exon 1. Thus, we propose an indispensable and additive role of neddylation and HYPK in clearance of protein aggregates by autophagy, resulting in cytoprotective effect during proteotoxic stress.Abbreviations: ATG5, autophagy related 5; ATG12, autophagy related 12; ATG14, autophagy related 14; BECN1, beclin 1; CBL, casitas B-lineage lymphoma; CBLB, Cbl proto-oncogene B; GABARAP, GABA type A receptor-associated protein; GABARAPL1, GABA type A receptor associated protein like 1; GABARAPL2, GABA type A receptor associated protein like 2; GFP, green fluorescent protein; HTT, huntingtin; HTT97Q exon 1, huntingtin 97-glutamine exon 1; HUWE1, HECT, UBA and WWE domain containing E3 ubiquitin protein ligase 1; HYPK, huntingtin interacting protein K; IgG, immunoglobulin G; IMR-32, Institute for Medical Research-32; KD, knockdown; K_d, dissociation constant; LAMP1, lysosomal associated membrane protein 1; LIR, LC3 interacting region; MAP1LC3/LC3, microtubule associated protein 1 light chain 3; MAP1LC3A/LC3A, microtubule associated protein 1 light chain 3 alpha; MAP1LC3B/LC3B, microtubule associated protein 1 light chain 3 beta; MARK1, microtubule affinity regulating kinase 1; MARK2, microtubule affinity regulating kinase 2; MARK3, microtubule affinity regulating kinase 3; MARK4, microtubule affinity regulating kinase 4; MCF7, Michigan Cancer Foundation-7; MTOR, mechanistic target of rapamycin kinase; NAE1, NEDD8 activating enzyme E1 subunit 1; NBR1, NBR1 autophagy cargo receptor; NEDD8, NEDD8 ubiquitin like modifier; Ni-NTA, nickel-nitrilotriacetic acid; NUB1, negative regulator of ubiquitin like proteins 1; PIK3C3, phosphatidylinositol 3-kinase catalytic subunit type 3; PolyQ, poly-glutamine; PSMD8, proteasome 26S subunit, non-ATPase 8; RAD23A, RAD23 homolog A, nucleotide excision repair protein; RAD23B, RAD23 homolog B, nucleotide excision repair protein; RFP, red fluorescent protein; RPS27A, ribosomal protein S27a; RSC1A1, regulator of solute carriers 1; SNCA, synuclein alpha; SIK1, salt inducible kinase 1; siRNA, small interfering ribonucleic acid; SOD1, superoxide dismutase 1; SPR, surface plasmon resonance; SQSTM1, sequestosome 1; SUMO1, small ubiquitin like modifier 1; TAX1BP1, Tax1 binding protein 1; TDRD3, tudor domain containing 3; TNRC6C, trinucleotide repeat containing adaptor 6C; TOLLIP, toll interacting protein; TUBA, tubulin alpha; TUBB, tubulin beta class I; UBA, ubiquitin-associated; UBA1, ubiquitin like modifier activating enzyme 1; UBA5, ubiquitin like modifier activating enzyme 5; UBAC1, UBA domain containing 1; UBAC2, UBA domain containing 2; UBAP1, ubiquitin associated protein 1; UBAP2, ubiquitin associated protein 2; UBASH3B, ubiquitin associated and SH3 domain containing B; UBD/FAT10, ubiquitin D; UBE2K, ubiquitin conjugating enzyme E2 K; UBLs, ubiquitin-like proteins; UBL7, ubiquitin like 7; UBQLN1, ubiquilin 1; UBQLN2, ubiquilin 2; UBQLN3, ubiquilin 3; UBQLN4, ubiquilin 4; UBXN1, UBX domain protein 1; ULK1, unc-51 like autophagy activating kinase 1; URM1, ubiquitin related modifier 1; USP5, ubiquitin specific peptidase 5; USP13, ubiquitin specific peptidase 13; VPS13D, vacuolar protein sorting 13 homolog D.

Design, synthesis, and characterization of novel aminoalcohol quinolines with strong in vitro antimalarial activity

Malaria is the fifth most lethal parasitic infections in the world. Herein, five new series of aminoalcohol quinolines including fifty-two compounds were designed, synthesized and evaluated in vitro against Pf3D7 and PfW2 strains. Among them, fourteen displayed IC₅₀ values below or near of 50.0 nM whatever the strain with selectivity index often superior to 100.17b was found as a promising antimalarial candidate with IC₅₀ values of 14.9 nM and 11.0 nM against respectively Pf3D7 and PfW2 and a selectivity index higher than 770 whatever the cell line is. Further experiments were achieved to confirm the safety and to establish the preliminary ADMET profile of compound 17b before the in vivo study performed on a mouse model of P. berghei ANKA infection. The overall data of this study allowed to establish new structure-activity relationships and the development of novel agents with improved pharmacokinetic properties.

Cellular targets of mefloquine

Mefloquine is a quinoline-based compound widely used as an antimalarial drug, particularly in chemoprophylaxis. Although decades of research have identified various aspects of mefloquine's anti-Plasmodium properties, toxic effects offset its robust use in humans. Mefloquine exerts harmful effects in several types of human cells by targeting many of the cellular lipids, proteins, and complexes, thereby blocking a number of downstream signaling cascades. In general, mefloquine modulates several cellular phenomena, such as alteration of membrane potential, induction of oxidative stress, imbalance of ion homeostasis, disruption of metabolism, failure of organelle function, etc., leading to cell cycle arrest and programmed cell death. This review aims to summarize the information on functional and mechanistic findings related to the cytotoxic effects of mefloquine.

Antimalarial drug discovery: from quinine to the most recent promising clinical drug candidates

Malaria is a tropical threatening disease caused by Plasmodium parasites, resulting in 409,000 deaths in 2019. The delay of mortality and morbidity has been compounded by the widespread of drug resistant parasites from Southeast Asia since two decades. The emergence of artemisinin-resistant Plasmodium in Africa, where most cases are accounted, highlights the urgent need for new medicines. In this effort, the World Health Organization and Medicines for Malaria Venture joined to define clear goals for novel therapies and characterized the target candidate profile. This ongoing search for new treatments is based on imperative labor in medicinal chemistry which is summarized here with particular attention to hit-to-lead optimizations, key properties, and modes of action of these novel antimalarial drugs. This review, after presenting the current antimalarial chemotherapy, from quinine to the latest marketed drugs, focuses in particular on recent advances of the most promising antimalarial candidates in clinical and preclinical phases.

Anti-parasitic drugs modulate the non-selective channels formed by connexins or pannexins

The proteins connexins, innexins, and pannexins are the subunits of non-selective channels present in the cell membrane in vertebrates (connexins and pannexins) and invertebrates (innexins). These channels allow the transfer of ions and molecules across the cell membrane or, and in many cases, between the cytoplasm of neighboring cells. These channels participate in various physiological processes, particularly under pathophysiological conditions, such as bacterial, viral, and parasitic infections. Interestingly, some anti-parasitic drugs also block connexin- or pannexin-formed channels. Their effects on host channels permeable to molecules that favor parasitic infection can further explain the anti-parasitic effects of some of these compounds. In this review, the effects of drugs with known anti-parasitic activity that modulate non-selective channels formed by connexins or pannexins are discussed. Previous studies that have reported the presence of these proteins in worms, ectoparasites, and protozoa that cause parasitic infections have also been reviewed.

Differential Stabilities of Mefloquine-Bound Human and Plasmodium falciparum Acyl-CoA-Binding Proteins

Toxic effects of pharmacological drugs restrict their robust application against human diseases. Although used as a drug in the combinatorial therapy to treat malaria, the use of mefloquine is not highly recommended because of its adverse effects in humans. Mefloquine inhibits the binding of acyl-CoAs to acyl-CoA-binding proteins of Plasmodium falciparum (PfACBPs) and human (hACBP). In this study, we have used molecular dynamics simulation and other computational approaches to investigate the differences of stabilities of mefloquine-PfACBP749 and mefloquine-hACBP complexes. The stability of mefloquine in the binding cavity of PfACBP749 is less than its stability in the binding pocket of hACBP. Although the essential tyrosine residues (tyrosine-30 and tyrosine-33 of PfACBP749 and tyrosine-29 and tyrosine-32 of hACBP) mediate the initial binding of mefloquine to the proteins by π-stacking interactions, additional temporally longer interactions between mefloquine and aspartate-22 and methionine-25 of hACBP result in stronger binding of mefloquine to hACBP. The higher fluctuation of mefloquine-binding residues of PfACBP749 contributes to the instability of mefloquine in the binding cavity of the protein. On the contrary, in the mefloquine-bound state, the stability of hACBP protein is less than the stability of PfACBP749. The helix-to-coil transition of the N-terminal hydrophobic region of hACBP has a destabilizing effect upon the protein's structure. This causes the induction of aggregation properties in the hACBP in the mefloquine-bound state. Taken together, we describe the mechanistic features that affect the differential dynamic stabilities of mefloquine-bound PfACBP749 and hACBP proteins.

Mefloquine binding to human acyl-CoA binding protein leads to redox stress-mediated apoptotic death of human neuroblastoma cells

Malaria is an infectious disease that is caused by different species of Plasmodium. Several antimalarial drugs are used to counter the spread and infectivity of Plasmodium species. However, humans are also vulnerable to many of the antimalarial drugs, including the quinoline-based drugs. In particular, the antimalarial mefloquine has been reported to show adverse neuropsychiatric effects in humans. Though mefloquine is known to be neurotoxic, the molecular mechanisms associated with this phenomenon are still obscure. In this study, we show that mefloquine binds to and inactivates the human acyl-CoA binding protein (hACBP), potentially inducing redox stress in human neuroblastoma cells (IMR-32). Mefloquine occupies the acyl-CoA binding pocket of hACBP by interacting with several of the critical acyl-CoA binding amino acids. This leads to the competitive inhibition of acyl-CoA(s) binding to hACBP and to the accumulation of lipid droplets inside the IMR-32 cells. The accumulation of cytosolic lipid globules and oxidative stress finally correlates with the apoptotic death of cells. Taken together, our study deciphers a mechanistic detail of how mefloquine leads to the death of human cells by perturbing the activity of hACBP and lipid homeostasis.

T54R mutation destabilizes the dimer of superoxide dismutase 1T54R by inducing steric clashes at the dimer interface

Mutations cause abnormalities in protein structure, function and oligomerization. Different mutations in the superoxide dismutase 1 (SOD1) protein cause its misfolding, loss of dimerization and aggravate its aggregation in the amyotrophic lateral sclerosis disease. In this study, we report the mechanistic details of how a threonine-to-arginine mutation at the 54^th position (T54R) of SOD1 results in destabilization of the dimer interface of SOD1^T54R. Using computational and experimental methods, we show that the T54R mutation increases fluctuation of the mutation-harboring loop (R54-loop) of SOD1^T54R. Fluctuation of this loop causes steric clashes that involve arginine-54 (R54) and other residues of SOD1^T54R, resulting in loss of inter-subunit contacts at the dimer interface. Since the T54 residue-containing loop is necessary for the dimerization of wild-type SOD1, fluctuation of the R54-loop, steric clashes involving R54 and loss of inter-subunit contacts give rise to the loss of SOD1^T54R dimer stability. This correlates to energetically unfavorable tethering of the monomers of SOD1^T54R. The outcome is gradual splitting of SOD1^T54R dimers into monomers, thereby exposing the previously buried hydrophobic interface residues to the aqueous environment. This event finally leads to aggregation of SOD1^T54R. T54R mutation has no effect in altering the relative positions of copper and zinc ion binding residues of SOD1^T54R. The native SOD1 structure is stable, and there is no destabilizing effect at its dimer interface. Overall, our study reveals the intricate mechanism of T54R mutation-associated destabilization of the dimer of the SOD1^T54R protein.

T54R mutation destabilizes the dimer of SOD1^T54R.

An IRES-dependent translation of HYPK mRNA generates a truncated isoform of the protein that lacks the nuclear localization and functional ability

Different mechanisms of translation initiation process exist to start the protein synthesis from various viral and eukaryotic mRNA. The cap-independent and tertiary structure directed translation initiation of mRNAs forms the basis of internal ribosome entry site (IRES) mediated translation initiation that helps in cellular protein production in different conditions. HYPK protein sequesters different aggregation-prone proteins to help in the cellular proteostasis. HYPK mRNA is differentially translated from an internal start/initiation codon to generate an amino terminal-truncated isoform (HSPC136) of HYPK protein. In this study, we report that an IRES-dependent translation initiation of HYPK mRNA results in the formation of the HSPC136/HYPK-ΔN isoform of HYPK protein. The IRES-driven translation product, HYPK-ΔN, lacks the N-terminal tri-arginine motif that acts as the nuclear localization signal (NLS) in the full-length HYPK protein. While the full-length HYPK protein translocates to the nucleus and prevents the aggregation of the mutant p53 (p53-R248Q) protein, the HYPK-ΔN lacks this activity. The NLS of HYPK is not evolutionarily conserved and its exclusive presence in the HYPK of higher eukaryotic animals imparts additional advantage to the HYPK protein in tackling the cytosolic as well as nuclear protein aggregates. The presence of the NLS in full-length HYPK also allows this protein to modulate the cell cycle. These results provide a mechanistic detail of HYPK mRNA's translation initiation control by an IRES that dictates the formation of HYPC136/HYPK-ΔN which lacks the nuclear localization and functional ability.