Cobalt amine complexes and Ru265 interact with the DIME region of the mitochondrial calcium uniporter

The mitochondrial calcium uniporter: Balancing tumourigenic and anti-tumourigenic responses

Increased malignancy and poor treatability associated with solid tumour cancers have commonly been attributed to mitochondrial calcium (Ca2+) dysregulation. The mitochondrial Ca2+ uniporter complex (mtCU) is the predominant mode of Ca2+ uptake into the mitochondrial matrix. The main components of mtCU are the pore-forming mitochondrial Ca2+ uniporter (MCU) subunit, MCU dominant-negative beta (MCUb) subunit, essential MCU regulator (EMRE) and the gatekeeping mitochondrial Ca2+ uptake 1 and 2 (MICU1 and MICU2) proteins. In this review, we describe mtCU-mediated mitochondrial Ca2+ dysregulation in solid tumour cancer types, finding enhanced mtCU activity observed in colorectal cancer, breast cancer, oral squamous cell carcinoma, pancreatic cancer, hepatocellular carcinoma and embryonal rhabdomyosarcoma. By contrast, decreased mtCU activity is associated with melanoma, whereas the nature of mtCU dysregulation remains unclear in glioblastoma. Furthermore, we show that numerous polymorphisms associated with cancer may alter phosphorylation sites on the pore forming MCU and MCUb subunits, which cluster at interfaces with EMRE. We highlight downstream/upstream biomolecular modulators of MCU and MCUb that alter mtCU-mediated mitochondrial Ca2+ uptake and may be used as biomarkers or to aid in the development of novel cancer therapeutics. Additionally, we provide an overview of the current small molecule inhibitors of mtCU that interact with the Asp residue of the critical Asp-Ile-Met-Glu motif or through other allosteric regulatory mechanisms to block Ca2+ permeation. Finally, we describe the relationship between MCU- and MCUb-mediating microRNAs and mitochondrial Ca2+ uptake that should be considered in the discovery of new treatment approaches for cancer.

MCU complex: Exploring emerging targets and mechanisms of mitochondrial physiology and pathology

BACKGROUND

Globally, the onset and progression of multiple human diseases are associated with mitochondrial dysfunction and dysregulation of Ca2+ uptake dynamics mediated by the mitochondrial calcium uniporter (MCU) complex, which plays a key role in mitochondrial dysfunction. Despite relevant studies, the underlying pathophysiological mechanisms have not yet been fully elucidated.

AIM OF REVIEW

This article provides an in-depth analysis of the current research status of the MCU complex, focusing on its molecular composition, regulatory mechanisms, and association with diseases. In addition, we conducted an in-depth analysis of the regulatory effects of agonists, inhibitors, and traditional Chinese medicine (TCM) monomers on the MCU complex and their application prospects in disease treatment. From the perspective of medicinal chemistry, we conducted an in-depth analysis of the structure-activity relationship between these small molecules and MCU and deduced potential pharmacophores and binding pockets. Simultaneously, key structural domains of the MCU complex in Homo sapiens were identified. We also studied the functional expression of the MCU complex in Drosophila, Zebrafish, and Caenorhabditis elegans. These analyses provide a basis for exploring potential treatment strategies targeting the MCU complex and provide strong support for the development of future precision medicine and treatments.

KEY SCIENTIFIC CONCEPTS OF REVIEW

The MCU complex exhibits varying behavior across different tissues and plays various roles in metabolic functions. It consists of six MCU subunits, an essential MCU regulator (EMRE), and solute carrier 25A23 (SLC25A23). They regulate processes, such as mitochondrial Ca2+ (mCa2+) uptake, mitochondrial adenosine triphosphate (ATP) production, calcium dynamics, oxidative stress (OS), and cell death. Regulation makes it a potential target for treating diseases, especially cardiovascular diseases, neurodegenerative diseases, inflammatory diseases, metabolic diseases, and tumors.

The mitochondrial calcium uniporter transports Ca 2+ via a ligand-relay mechanism

The mitochondrial calcium uniporter (mtCU) is a multicomponent Ca 2+ -specific channel that imparts mitochondria with the capacity to sense the cytosolic calcium signals. The metazoan mtCU comprises the pore-forming subunit MCU and the essential regulator EMRE, arranged in a tetrameric channel complex, and the Ca 2+ sensing peripheral proteins MICU1-3. The mechanism of mitochondrial Ca 2+ uptake by mtCU and its regulation is poorly understood. Our analysis of MCU structure and sequence conservation, combined with molecular dynamics simulations, mutagenesis, and functional studies, led us to conclude that the Ca 2+ conductance of MCU is driven by a ligand-relay mechanism, which depends on stochastic structural fluctuations in the conserved DxxE sequence. In the tetrameric structure of MCU, the four glutamate side chains of DxxE (the E-ring) chelate Ca 2+ directly in a high-affinity complex (site 1), which blocks the channel. The four glutamates can also switch to a hydrogen bond-mediated interaction with an incoming hydrated Ca 2+ transiently sequestered within the D-ring of DxxE (site 2), thus releasing the Ca 2+ bound at site 1. This process depends critically on the structural flexibility of DxxE imparted by the adjacent invariant Pro residue. Our results suggest that the activity of the uniporter can be regulated through the modulation of local structural dynamics. A preliminary account of this work was presented at the 67 th Annual Meeting of the Biophysical Society in San Diego, CA, February 18-22, 2023.

Structure-Activity Relationships of Metal-Based Inhibitors of the Mitochondrial Calcium Uniporter

The mitochondrial calcium uniporter (MCU) is a transmembrane protein that is responsible for mediating mitochondrial calcium (mCa2+ ) uptake. Given this critical function, the MCU has been implicated as an important target for addressing various human diseases. As such, there has a been growing interest in developing small molecules that can inhibit this protein. To date, metal coordination complexes, particularly multinuclear ruthenium complexes, are the most widely investigated MCU inhibitors due to both their potent inhibitory activities as well as their longstanding use for this application. Recent efforts have expanded the metal-based toolkit for MCU inhibition. This concept paper summarizes the development of new metal-based inhibitors of the MCU and their structure-activity relationships in the context of improving their potential for therapeutic use in managing human diseases related to mCa2+ dysregulation.

Investigation of Cobalt(III) Cage Complexes as Inhibitors of the Mitochondrial Calcium Uniporter

The mitochondrial calcium uniporter (MCU) mediates uptake of calcium ions (Ca2+) into the mitochondria, a process that is vital for maintaining normal cellular function. Inhibitors of the MCU, the most promising of which are dinuclear ruthenium coordination compounds, have found use as both therapeutic agents and tools for studying the importance of this ion channel. In this study, six Co3+ cage compounds with sarcophagine-like ligands were assessed for their abilities to inhibit MCU-mediated mitochondrial Ca2+ uptake. These complexes were synthesized and characterized according to literature procedures and then investigated in cellular systems for their MCU-inhibitory activities. Among these six compounds, [Co(sen)]3+ (3, sen = 5-(4-amino-2-azabutyl)-5-methyl-3,7-diaza-1,9-nonanediamine) was identified to be a potent MCU inhibitor, with IC50 values of inhibition of 160 and 180 nM in permeabilized HeLa and HEK293T cells, respectively. Furthermore, the cellular uptake of compound 3 was determined, revealing moderate accumulation in cells. Most notably, 3 was demonstrated to operate in intact cells as an MCU inhibitor. Collectively, this work presents the viability of using cobalt coordination complexes as MCU inhibitors, providing a new direction for researchers to investigate in future studies.

A Fluorogenic Inhibitor of the Mitochondrial Calcium Uniporter

Inhibitors of the mitochondrial calcium uniporter (MCU) are valuable tools for studying the role of mitochondrial Ca2+ in various pathophysiological conditions. In this study, a new fluorogenic MCU inhibitor, RuOCou, is presented. This compound is an analogue of the known MCU inhibitor Ru265 that contains fluorescent axial coumarin carboxylate ligands. Upon aquation of RuOCou and release of the axial coumarin ligands, a simultaneous increase in its MCU-inhibitory activity and fluorescence intensity is observed. The fluorescence response of this compound enabled its aquation to be monitored in both HeLa cell lysates and live HeLa cells. This fluorogenic prodrug represents a potential theranostic MCU inhibitor that can be leveraged for the treatment of human diseases related to MCU activity.

Dinuclear nitrido-bridged osmium complexes inhibit the mitochondrial calcium uniporter and protect cortical neurons against lethal oxygen-glucose deprivation

Dysregulation of mitochondrial calcium uptake mediated by the mitochondrial calcium uniporter (MCU) is implicated in several pathophysiological conditions. Dinuclear ruthenium complexes are effective inhibitors of the MCU and have been leveraged as both tools to study mitochondrial calcium dynamics and potential therapeutic agents. In this study, we report the synthesis and characterization of Os245 ([Os₂(μ-N)(NH₃)₈Cl₂]³⁺) which is the osmium-containing analogue of our previously reported ruthenium-based inhibitor Ru265. This complex and its aqua-capped analogue Os245' ([Os₂(μ-N)(NH₃)₈(OH₂)₂]⁵⁺) are both effective inhibitors of the MCU in permeabilized and intact cells. In comparison to the ruthenium-based inhibitor Ru265 (k _obs = 4.92 × 10^-3 s^-1), the axial ligand exchange kinetics of Os245 are two orders of magnitude slower (k _obs = 1.63 × 10^-5 s^-1) at 37 °C. The MCU-inhibitory properties of Os245 and Os245' are different (Os245 IC₅₀ for MCU inhibition = 103 nM; Os245' IC₅₀ for MCU inhibition = 2.3 nM), indicating that the axial ligands play an important role in their interactions with this channel. We further show that inhibition of the MCU by these complexes protects primary cortical neurons against lethal oxygen-glucose deprivation. When administered in vivo to mice (10 mg kg^-1), Os245 and Os245' induce seizure-like behaviors in a manner similar to the ruthenium-based inhibitors. However, the onset of these seizures is delayed, a possible consequence of the slower ligand substitution kinetics for these osmium complexes. These findings support previous studies that demonstrate inhibition of the MCU is a promising therapeutic strategy for the treatment of ischemic stroke, but also highlight the need for improved drug delivery strategies to mitigate the pro-convulsant effects of this class of complexes before they can be implemented as therapeutic agents. Furthermore, the slower ligand substitution kinetics of the osmium analogues may afford new strategies for the development and modification of this class of MCU inhibitors.

A ferrocene-containing analogue of the MCU inhibitor Ru265 with increased cell permeability

An analogue of the mitochondrial calcium uniporter (MCU) inhibitor Ru265 containing axial ferrocenecarboxylate ligands is reported. This new complex exhibits enhanced cellular uptake compared to the parent compound Ru265.

Carboxylate-Capped Analogues of Ru265 Are MCU Inhibitor Prodrugs

The mitochondrial calcium uniporter (MCU) is a transmembrane protein that resides on the inner membrane of the mitochondria and mediates calcium uptake into this organelle. Given the critical role of mitochondrial calcium trafficking in cellular function, inhibitors of this channel have arisen as tools for studying the biological relevance of this process and as potential therapeutic agents. In this study, four new analogues of the previously reported Ru-based MCU inhibitor [ClRu(NH3)4(μ-N)Ru(NH3)4Cl]Cl3 (Ru265) are reported. These compounds, which bear axial carboxylate ligands, are of the general formula [(RCO2)Ru(NH3)4(μ-N)Ru(NH3)4(O2CR)]X3, where X = NO3- or CF3SO3- and R = H (1), CH3 (2), CH2CH3 (3), and (CH2)2CH3 (4). These complexes were fully characterized by IR spectroscopy, NMR spectroscopy, and elemental analysis. X-ray crystal structures of 1 and 3 were obtained, revealing the expected presence of both the linear Ru(μ-N)Ru core and axial formate and propionate ligands. The axial carboxylate ligands of complexes 1-4 are displaced by water in buffered aqueous solution to give the aquated compound Ru265'. The kinetics of these processes were measured by 1H NMR spectroscopy, revealing half-lives that span 5.9-9.9 h at 37 °C. Complex 1 with axial formate ligands underwent aquation approximately twice as fast as the other compounds. In vitro cytotoxicity and mitochondrial membrane potential measurements carried out in HeLa and HEK293T cells demonstrated that none of these four complexes negatively affects cell viability or mitochondrial function. The abilities of 1-4 to inhibit mitochondrial calcium uptake in permeabilized HEK293T cells were assessed and compared to that of Ru265. Fresh solutions of 1-4 are approximately 2-fold less potent than Ru265 with IC50 values in the range of 14.7-19.1 nM. Preincubating 1-4 in aqueous buffers for longer time periods to allow for the aquation reactions to proceed increases their potency of mitochondrial uptake inhibition to match that of Ru265. This result indicates that 1-4 are aquation-activated prodrugs of Ru265'. Finally, 1-4 were shown to inhibit mitochondrial calcium uptake in intact, nonpermeabilized cells, revealing their value as tools and potential therapeutic agents for mitochondrial calcium-related disorders.

Mitochondria modulate ameloblast Ca2+ signaling

The role of mitochondria in enamel, the most mineralized tissue in the body, is poorly defined. Enamel is formed by ameloblast cells in two main sequential stages known as secretory and maturation. Defining the physiological features of each stage is essential to understand mineralization. Here, we analyzed functional features of mitochondria in rat primary secretory and maturation-stage ameloblasts focusing on their role in Ca²⁺ signaling. Quantification of the Ca²⁺ stored in the mitochondria by trifluoromethoxy carbonylcyanide phenylhydrazone stimulation was comparable in both stages. The release of endoplasmic reticulum Ca²⁺ pools by adenosine triphosphate in rhod2AM-loaded cells showed similar mitochondrial Ca²⁺ (_m Ca²⁺ ) uptake. However, _m Ca²⁺ extrusion via Na⁺ -Li⁺ -Ca²⁺ exchanger was more prominent in maturation. To address if _m Ca²⁺ uptake via the mitochondrial Ca²⁺ uniporter (MCU) played a role in cytosolic Ca²⁺ (_c Ca²⁺ ) buffering, we stimulated Ca²⁺ influx via the store-operated Ca²⁺ entry (SOCE) and blocked MCU with the inhibitor Ru265. This inhibitor was first tested using the enamel cell line LS8 cells. Ru265 prevented _c Ca²⁺ clearance in permeabilized LS8 cells like ruthenium red, and it did not affect ΔΨm in intact cells. In primary ameloblasts, SOCE stimulation elicited a significantly higher _m Ca²⁺ uptake in maturation ameloblasts. The uptake of Ca²⁺ into the mitochondria was dramatically decreased in the presence of Ru265. Combined, these results suggest an increased mitochondrial Ca²⁺ handling in maturation but only upon stimulation of Ca²⁺ influx via SOCE. These functional studies provide insights not only on the role of mitochondria in ameloblast Ca²⁺ physiology, but also advance the concept that SOCE and _m Ca²⁺ uptake are complementary processes in biological mineralization.

Insight of the role of mitochondrial calcium homeostasis in hepatic insulin resistance

Due to the rapid rise in the prevalence of chronic metabolic disease, more and more clinicians and basic medical researchers focus their eyesight on insulin resistance (IR), an early and central event of metabolic diseases. The occurrence and development of IR are primarily caused by excessive energy intake and reduced energy consumption. Liver is the central organ that controls glucose homeostasis, playing a considerable role in systemic IR. Decreased capacity of oxidative metabolism and mitochondrial dysfunction are being blamed as the direct reason for the development of IR. Mitochondrial Ca²⁺ plays a fundamental role in maintaining proper mitochondrial function and redox stability. The maintaining of mitochondrial Ca²⁺ homeostasis requires the cooperation of ion channels in the inner and outer membrane of mitochondria, such as mitochondrial calcium uniporter complex (MCUC) and voltage-dependent anion channels (VDACs). In addition, the crosstalk between the endoplasmic reticulum (ER), lysosome and plasma membrane with mitochondria is also significant for mitochondrial calcium homeostasis, which is responsible for an efficient network of cellular Ca²⁺ signaling. Here, we review the recent progression in the research about the regulation factors for mitochondrial Ca²⁺ and how the dysregulation of mitochondrial Ca²⁺ homeostasis is involved in the pathogenesis of hepatic IR, providing a new perspective for further exploring the role of ion in the onset and development of IR.