Design of novel fluorescent mitochondria-targeted peptides with iron-selective sensing activity

Chelating mitochondrial iron and copper: Recipes, pitfalls and promise

Iron and copper chelation therapy plays a crucial role in treating conditions associated with metal overload, such as hemochromatosis or Wilson's disease. However, conventional chelators face challenges in reaching the core of iron and copper metabolism - the mitochondria. Mitochondria-targeted chelators can specifically target and remove metal ions from mitochondria, showing promise in treating diseases linked to mitochondrial dysfunction, including neurodegenerative diseases and cancer. Additionally, they serve as specific mitochondrial metal sensors. However, designing these new molecules presents its own set of challenges. Depending on the chelator's intended use to prevent or to promote redox cycling of the metals, the chelating moiety must possess different donor atoms and an optimal value of the electrode potential of the chelator-metal complex. Various targeting moieties can be employed for selective delivery into the mitochondria. This review also provides an overview of the current progress in the design of mitochondria-targeted chelators and their biological activity investigation.

Interactions and Transport of a Bioconjugated Peptide Targeting the Mitomembrane

The Szeto-Schiller (SS) peptides are a subclass of cell-penetrating peptides that can specifically target mitochondria and mediate conditions caused by mitochondrial dysfunction. In this work, we constructed an iron-chelating SS peptide and studied its interaction with a mitochondrial-mimicking membrane using atomistic molecular dynamics (MD) simulations. We report that the peptide/membrane interaction is thermodynamically favorable, and the localization of the peptide to the membrane is driven by electrostatic interactions between the cationic residues and the anionic phospholipid headgroups. The insertion of the peptide into the membrane is driven by hydrophobic interactions between the aromatic side chains in the peptide and the lipid acyl tails. We also probed the translocation of the peptide across the membrane by applying nonequilibrium steered MD simulations and resolved the translocation pathway, free energy profile, and metastable states. We explored four distinct orientations of the peptide along the translocation pathway and found that one orientation was energetically more favorable than the other orientations. We tested a significantly slower pulling velocity on the most thermodynamically favorable system and compared metastable states during peptide translocation. We found that the peptide can optimize hydrophobic interactions with the membrane by having aromatic side chains interacting with the lipid acyl tails instead of forming π-π interactions with each other. The mechanistic insights emerging from our work will potentially facilitate improved peptide design with enhanced activity.

The synthesis and properties of mitochondrial targeted iron chelators

Iron levels in mitochondria are critically important for the normal functioning of the organelle. Abnormal levels of iron and the associated formation of toxic oxygen radicals have been linked to a wide range of diseases and consequently it is important to be able to both monitor and control levels of the mitochondrial labile iron pool. To this end a series of iron chelators which are targeted to mitochondria have been designed. This overview describes the synthesis of some of these molecules and their application in monitoring mitochondrial labile iron pools and in selectively removing excess iron from mitochondria.

Epigenomic regulation by labile iron

Iron is an essential micronutrient metal for cellular functions but can generate highly reactive oxygen species resulting in oxidative damage. For these reasons its uptake and metabolism is highly regulated. A small but dynamic fraction of ferrous iron inside the cell, termed intracellular labile iron, is redox-reactive and ready to participate multiples reactions of intracellular enzymes. Due to its nature its determination and precise quantification has been a roadblock. However, recent progress in the development of intracellular labile iron probes are allowing the reevaluation of our current understanding and unmasking new functions. The role of intracellular labile iron in regulating the epigenome was recently discovered. This chapter examine how intracellular labile iron can modulate histone and DNA demethylation and how its pool can mediate a signaling pathway from cAMP serving as a sensor of the metabolic needs of the cells.

The role of mitochondrial labile iron in Friedreich's ataxia skin fibroblasts sensitivity to ultraviolet A

Mitochondrial labile iron (LI) is a major contributor to the susceptibility of skin fibroblasts to ultraviolet A (UVA)-induced oxidative damage leading to necrotic cell death via ATP depletion. Mitochondria iron overload is a key feature of the neurodegenerative disease Friedreich's ataxia (FRDA). Here we show that cultured primary skin fibroblasts from FRDA patients are 4 to 10-fold more sensitive to UVA-induced death than their healthy counterparts. We demonstrate that FRDA cells display higher levels of mitochondrial LI (up to 6-fold on average compared to healthy counterparts) and show higher increase in mitochondrial reactive oxygen species (ROS) generation after UVA irradiation (up to 2-fold on average), consistent with their differential sensitivity to UVA. Pre-treatment of the FRDA cells with a bespoke mitochondrial iron chelator fully abrogates the UVA-mediated cell death and reduces UVA-induced damage to mitochondrial membrane and the resulting ATP depletion by a factor of 2. Our results reveal a link between FRDA as a disease of mitochondrial iron overload and sensitivity to UVA of skin fibroblasts. Our findings suggest that the high levels of mitochondrial LI in FRDA cells which contribute to high levels of mitochondrial ROS production after UVA irradiation are likely to play a crucial role in the marked sensitivity of these cells to UVA-induced oxidative damage. This study may have implications not only for FRDA but also for other diseases of mitochondrial iron overload, with the view to develop topical mitochondria-targeted iron chelators as skin photoprotective agents.

A ratiometric iron probe enables investigation of iron distribution within tumour spheroids

Iron dysregulation is implicated in numerous diseases, and iron homeostasis is profoundly influenced by the labile iron pool (LIP). Tools to easily observe changes in the LIP are limited, with calcein AM-based assays most widely used. We describe here FlCFe1, a ratiometric analogue of calcein AM, which also provides the capacity for imaging iron in 3D cell models.

The significance, trafficking and determination of labile iron in cytosol, mitochondria and lysosomes

The labile iron pool (LIP) is a pool of chelatable and redox-active iron, not only essential for a wide variety of metabolic process, but also as a catalyst in the Fenton reaction, causing the release of hazardous reactive oxygen species (ROS) with potential for inducing oxidative stress and cell damage. The cellular LIP represents the entirety of every heterogenous sub-pool of iron, not only present in the cytosol, but also in mitochondria, lysosomes and the nucleus, which have all been detected and characterized by various fluorescent methods. Accumulated evidence indicates that alterations in the intracellular LIP can substantially contribute to a variety of injurious processes and initiate pathological development. Herein, we present our understanding of the role of the cellular LIP. To fully review the LIP, firstly, the significance of cellular labile iron in different subcellular compartments is presented. And then, the trafficking processes of cellular labile iron between/in cytosol, mitochondria and lysosomes are discussed in detail. Then, the recent progress in uncovering and assessing the cellular LIP by fluorescent methods have been noted. Overall, this summary may help to comprehensively envision the important physiological and pathological roles of the LIP and shed light on profiling the LIP in a real-time and nondestructive manner with fluorescent methods. Undoubtedly, with the advent and development of iron biology, a better understanding of iron, especially the LIP, may also enhance treatments for iron-related diseases.

Mitochondrial Iron in Human Health and Disease

Mitochondria are an iconic distinguishing feature of eukaryotic cells. Mitochondria encompass an active organellar network that fuses, divides, and directs a myriad of vital biological functions, including energy metabolism, cell death regulation, and innate immune signaling in different tissues. Another crucial and often underappreciated function of these dynamic organelles is their central role in the metabolism of the most abundant and biologically versatile transition metals in mammalian cells, iron. In recent years, cellular and animal models of mitochondrial iron dysfunction have provided vital information in identifying new proteins that have elucidated the pathways involved in mitochondrial homeostasis and iron metabolism. Specific signatures of mitochondrial iron dysregulation that are associated with disease pathogenesis and/or progression are becoming increasingly important. Understanding the molecular mechanisms regulating mitochondrial iron pathways will help better define the role of this important metal in mitochondrial function and in human health and disease.

A Powerful Mitochondria-Targeted Iron Chelator Affords High Photoprotection against Solar Ultraviolet A Radiation

Mitochondria are the principal destination for labile iron, making these organelles particularly susceptible to oxidative damage on exposure to ultraviolet A (UVA, 320–400 nm), the oxidizing component of sunlight. The labile iron-mediated oxidative damage caused by UVA to mitochondria leads to necrotic cell death via adenosine triphosphate depletion. Therefore, targeted removal of mitochondrial labile iron via highly specific tools from these organelles may be an effective approach to protect the skin cells against the harmful effects of UVA. In this work, we designed a mitochondria-targeted hexadentate (tricatechol-based) iron chelator linked to mitochondria-homing SS-like peptides. The photoprotective potential of this compound against UVA-induced oxidative damage and cell death was evaluated in cultured primary skin fibroblasts. Our results show that this compound provides unprecedented protection against UVA-induced mitochondrial damage, adenosine triphosphate depletion, and the ensuing necrotic cell death in skin fibroblasts, and this effect is fully related to its potent iron-chelating property in the organelle. This mitochondria-targeted iron chelator has therefore promising potential for skin photoprotection against the deleterious effects of the UVA component of sunlight.

Dual selective iron chelating probes with a potential to monitor mitochondrial labile iron pools

Mitochondria-targeted peptides incorporating dual fluorescent and selective iron chelators have been designed as novel biosensors for the mitochondrial labile iron pool. The probes were demonstrated to specifically co-localize with mitochondria and their fluorescence emission was found to be sensitive to the presence of iron.