Mitochondrial Dynamics at Different Levels: From Cristae Dynamics to Interorganellar Cross Talk

Update on the correlation between mitochondrial function and osteonecrosis of the femoral head osteocytes

Mitochondrial health is maintained in a steady state through mitochondrial dynamics and autophagy processes. Recent studies have identified healthy mitochondria as crucial regulators of cellular function and survival. This process involves adenosine triphosphate (ATP) synthesis by mitochondrial oxidative phosphorylation (OXPHOS), regulation of calcium metabolism and inflammatory responses, and intracellular oxidative stress management. In the skeletal system, they participate in the regulation of cellular behaviors and the responses of osteoblasts, osteoclasts, chondrocytes, and osteocytes to external stimuli. Indeed, mitochondrial damage or dysfunction occurs in the development of a few bone diseases. For example, mitochondrial damage may lead to an imbalance in osteoblasts and osteoclasts, resulting in osteoporosis, osteomalacia, or poor bone production, and chondrocyte death and inflammatory infiltration in osteoarthritis are the main causes of cartilage degeneration due to mitochondrial damage. However, the opposite exists for osteosarcoma, where overactive mitochondrial metabolism is able to accelerate the proliferation and migration of osteosarcoma cells, which is a major disease feature. Bone is a dynamic organ and osteocytes play a fundamental role in all regions of bone tissue and are involved in regulating bone integrity. This review examines the impact of mitochondrial physiological function on osteocyte health and summarizes the microscopic molecular mechanisms underlying its effects. It highlights that targeted therapies focusing on osteocyte mitochondria may be beneficial for osteocyte survival, providing a new insight for the diagnosis, prevention, and treatment of diseases associated with osteocyte death.

Acetaldehyde dehydrogenase 2 attenuates lipopolysaccharide -induced endothelial barrier damage by inhibiting mitochondrial fission in sepsis-associated encephalopathy

Sepsis-associated encephalopathy (SAE) is a common neurological complication of sepsis, and acetaldehyde dehydrogenase 2 (ALDH2) has been identified as a protective factor for endothelial cells against oxidative stress. In this study, we aimed to investigate the therapeutic potential of ALDH2 and its impact on mitochondrial dynamics using both mouse and brain microvascular endothelial cells (BMECs) injury models induced by lipopolysaccharide (LPS). Our findings demonstrated that ALDH2 attenuated LPS-induced brain endothelial barrier damage, as evidenced by reduced brain water content and Evans blue dye in mice, decreased transepithelial electrical resistance (TEER), and increased fluorescein isothiocyanate-dextran (FITC-Dextran) leakage in bEnd.3 cells. Furthermore, ALDH2 reduced the levels of reactive oxygen species (ROS) and malondialdehyde (MDA), while enhancing the activities of superoxide dismutase (SOD) and catalase (CAT). ALDH2 also decreased 4-HNE content and restored mitochondrial membrane potential and ATP production, promoting a balanced mitochondrial fission and fusion. Notably, our use of the mitochondrial fission inhibitor Mdivi-1 confirmed that ALDH2 alleviated mitochondrial damage by inhibiting dynamin-related protein 1 (Drp1). Consequently, our findings suggest that the effects of ALDH2 on LPS-induced blood-brain barrier (BBB) damage and oxidative stress may alleviate SAE by inhibiting Drp1 to maintain mitochondrial homeostasis.

Molecular machineries shaping the mitochondrial inner membrane

Mitochondria display intricately shaped deep invaginations of the mitochondrial inner membrane (MIM) termed cristae. This peculiar membrane architecture is essential for diverse mitochondrial functions, such as oxidative phosphorylation or the biosynthesis of cellular building blocks. Conserved protein nano-machineries such as F1Fo-ATP synthase oligomers and the mitochondrial contact site and cristae organizing system (MICOS) act as adaptable protein-lipid scaffolds controlling MIM biogenesis and its dynamic remodelling. Signal-dependent rearrangements of cristae architecture and MIM fusion events are governed by the dynamin-like GTPase optic atrophy 1 (OPA1). Recent groundbreaking structural insights into these nano-machineries have considerably advanced our understanding of the functional architecture of mitochondria. In this Review, we discuss how the MIM-shaping machineries cooperate to control cristae and crista junction dynamics, including MIM fusion, in response to cellular signalling pathways. We also explore how mutations affecting MIM-shaping machineries compromise mitochondrial functions.

Mitochondrial and peroxisomal fission in cortical neurogenesis

The human brain is unique in its cellular diversity, intricate cytoarchitecture, function, and complex metabolic and bioenergetic demands, for which mitochondria and peroxisomes are essential. Mitochondria are multifunctional organelles that coordinate various signaling pathways central to neurogenesis. The dynamic morphological changes of the mitochondrial network have been linked to the regulation of bioenergetic and metabolic states. Specific protein machinery is dedicated to mitochondrial fission and fusion, allowing organelle distribution during cell division, organelle repair, and adaptation to environmental stimuli (excellent reviews have been published on these topics [Kondadi and Reichert, 2024; Giacomello et al., 2020; Tilokani et al., 2018; Kraus et al., 2021; Navaratnarajah et al., 2021]). In parallel, peroxisomes contain over 50 different enzymes which regulate metabolic functions that are critical for neurogenesis (Berger et al., 2016; Hulshagen et al., 2008). Peroxisomes share many of the components of their fission machinery with the mitochondria and undergo fission to help meet metabolic demands in response to environmental stimuli (Schrader et al., 2016). This review focuses primarily on the machinery involved in mitochondrial and peroxisomal fission. Mitochondrial fission has been identified as a critical determinant of cell fate decisions (Iwata et al., 2023, 2020; Khacho et al., 2016; King et al., 2021; Prigione and Adjaye, 2010; Vantaggiato et al., 2019; Kraus et al., 2021). The connection between alterations in peroxisomal fission and metabolic changes associated with cellular differentiation remains less clear. Here, we provide an overview of the functional and regulatory aspects of the mitochondrial and peroxisomal fission machinery and provide insight into the current mechanistic understanding by which mitochondrial and peroxisomal fission influence neurogenesis.

Quitting Your Day Job in Response to Stress: Cell Survival and Cell Death Require Secondary Cytoplasmic Roles of Cyclin C and Med13

Following unfavorable environmental cues, cells reprogram pathways that govern transcription, translation, and protein degradation systems. This reprogramming is essential to restore homeostasis or commit to cell death. This review focuses on the secondary roles of two nuclear transcriptional regulators, cyclin C and Med13, which play key roles in this decision process. Both proteins are members of the Mediator kinase module (MKM) of the Mediator complex, which, under normal physiological conditions, positively and negatively regulates a subset of stress response genes. However, cyclin C and Med13 translocate to the cytoplasm following cell death or cell survival cues, interacting with a host of cell death and cell survival proteins, respectively. In the cytoplasm, cyclin C is required for stress-induced mitochondrial hyperfission and promotes regulated cell death pathways. Cytoplasmic Med13 stimulates the stress-induced assembly of processing bodies (P-bodies) and is required for the autophagic degradation of a subset of P-body assembly factors by cargo hitchhiking autophagy. This review focuses on these secondary, a.k.a. "night jobs" of cyclin C and Med13, outlining the importance of these secondary functions in maintaining cellular homeostasis following stress.

Mitochondrial fusion and cristae reorganization facilitate acquisition of cardiomyocyte identity during reprogramming of murine fibroblasts

Cardiomyocytes (CMs) rely on mitochondrial energy produced in highly interconnected mitochondrial networks. Direct reprogramming of cardiac fibroblasts (CFs) into induced CMs (iCMs) shows promise for treating cardiac injury, but little work has investigated mitochondrial energetics and morphology during the conversion of CFs to iCMs. We characterized mitochondria during direct cardiac reprogramming of murine neonatal CFs (mnCFs). Reprogramming increased mitochondrial respiration and interconnectivity but not to the levels of native CMs. We therefore investigated whether perturbations to mitochondrial dynamics impacted reprogramming. Mitochondrial fusion (joining) was essential for iCM generation, while various fission (dividing) genes were reprogramming barriers. In particular, the loss of mitochondrial fission regulator 1 like (Mtfr1l) significantly increased the yield of functionally mature iCMs and induced mitochondrial fusion and respiration. These changes were countered by the concomitant loss of fusion effector optical atrophy protein 1 (Opa1). The present study advances our understanding of mitochondrial barriers to and mechanisms of direct cardiac reprogramming.

A New Perspective on the Role of Alterations in Mitochondrial Proteins Involved in ATP Synthesis and Mobilization in Cardiomyopathies

The heart requires a continuous energy supply to sustain its unceasing contraction-relaxation cycle. Mitochondria, a double-membrane organelle, generate approximately 90% of cellular energy as adenosine triphosphate (ATP) through oxidative phosphorylation, utilizing the electrochemical gradient established by the respiratory chain. Mitochondrial function is compromised by damage to mitochondrial DNA, including point mutations, deletions, duplications, or inversions. Additionally, disruptions to proteins associated with mitochondrial membranes regulating metabolic homeostasis can impair the respiratory chain's efficiency. This results in diminished ATP production and increased generation of reactive oxygen species. This review provides an overview of mutations affecting mitochondrial transporters and proteins involved in mitochondrial energy synthesis, particularly those involved in ATP synthesis and mobilization, and it examines their role in the pathogenesis of specific cardiomyopathies.

Comparative 3D ultrastructure of Plasmodium falciparum gametocytes

Despite the enormous significance of malaria parasites for global health, some basic features of their ultrastructure remain obscure. Here, we apply high-resolution volumetric electron microscopy to examine and compare the ultrastructure of the transmissible male and female sexual blood stages of Plasmodium falciparum as well as the more intensively studied asexual blood stages revisiting previously described phenomena in 3D. In doing so, we challenge the widely accepted notion of a single mitochondrion by demonstrating the presence of multiple mitochondria in gametocytes. We also provide evidence for a gametocyte-specific cytostome, or cell mouth. Furthermore, we generate the first 3D reconstructions of the parasite's endoplasmic reticulum (ER) and Golgi apparatus as well as gametocyte-induced extraparasitic structures in the infected red blood cell. Assessing interconnectivity between organelles, we find frequent structural appositions between the nucleus, mitochondria, and apicoplast. We provide evidence that the ER is a promiscuous interactor with numerous organelles and the trilaminar pellicle of the gametocyte. Public availability of these volumetric electron microscopy resources will facilitate reinterrogation by others with different research questions and expertise. Taken together, we reconstruct the 3D ultrastructure of P. falciparum gametocytes at nanometre scale and shed light on the unique organellar biology of these deadly parasites.

Coordination of cytochrome bc1 complex assembly at MICOS

The boundary and cristae domains of the mitochondrial inner membrane are connected by crista junctions. Most cristae membrane proteins are nuclear-encoded and inserted by the mitochondrial protein import machinery into the inner boundary membrane. Thus, they must overcome the diffusion barrier imposed by crista junctions to reach their final location. Here, we show that respiratory chain complexes and assembly intermediates are physically connected to the mitochondrial contact site and cristae organizing system (MICOS) that is essential for the formation and stability of crista junctions. We identify the inner membrane protein Mar26 (Fmp10) as a determinant in the biogenesis of the cytochrome bc1 complex (complex III). Mar26 couples a Rieske Fe/S protein-containing assembly intermediate to MICOS. Our data indicate that Mar26 maintains an assembly-competent Rip1 pool at crista junctions where complex III maturation likely occurs. MICOS facilitates efficient Rip1 assembly by recruiting complex III assembly intermediates to crista junctions. We propose that MICOS, via interaction with assembly factors such as Mar26, contributes to the spatial and temporal coordination of respiratory chain biogenesis.

SLP2 and MIC13 synergistically coordinate MICOS assembly and crista junction formation

The MICOS complex, essential for cristae organization, comprises MIC10 and MIC60 subcomplexes, with MIC13 as a crucial subunit. MIC13 mutations cause severe mitochondrial hepato-encephalopathy, cristae defects, and MIC10-subcomplex loss. We demonstrate that depletion of the mitochondrial protease YME1L in MIC13 KO stabilizes MIC10-subcomplex, restoring MIC60-MIC10 interaction and crista junction (CJ) defects, indicating MIC13 is crucial for MIC10-subcomplex stabilization rather than MIC60-MIC10 bridging. We identified stomatin-like protein 2 (SLP2) as a key MIC13 interaction partner, essential for cristae morphology and CJ formation. SLP2 serves as an interaction hub for MICOS subunits and stabilizes MIC26 by protecting it from YME1L-mediated degradation. Deleting both SLP2 and MIC13 impairs MIC60-subcomplex assembly and its nanoscale organization. Restoring the MIC10-subcomplex in MIC13-SLP2 double KO cells through YME1L depletion reinstates MIC60-subcomplex assembly and cristae morphology. Overall, we propose SLP2 and the MIC10-subcomplex act as a proteolytically controlled 'seeder' complex, facilitating MICOS-MIB complex assembly and maintaining mitochondrial integrity.

Mitochondrial apolipoprotein MIC26 is a metabolic rheostat regulating central cellular fuel pathways

Mitochondria play central roles in metabolism and metabolic disorders such as type 2 diabetes. MIC26, a mitochondrial contact site and cristae organising system complex subunit, was linked to diabetes and modulation of lipid metabolism. Yet, the functional role of MIC26 in regulating metabolism under hyperglycemia is not understood. We used a multi-omics approach combined with functional assays using WT and MIC26 KO cells cultured in normoglycemia or hyperglycemia, mimicking altered nutrient availability. We show that MIC26 has an inhibitory role in glycolysis and cholesterol/lipid metabolism under normoglycemic conditions. Under hyperglycemia, this inhibitory role is reversed demonstrating that MIC26 is critical for metabolic adaptations. This is partially mediated by alterations of mitochondrial metabolite transporters. Furthermore, MIC26 deletion led to a major metabolic rewiring of glutamine use and oxidative phosphorylation. We propose that MIC26 acts as a metabolic "rheostat," that modulates mitochondrial metabolite exchange via regulating mitochondrial cristae, allowing cells to cope with nutrient overload.

Dynamics of intracellular and intercellular redox communication

Cell and organ metabolism is organized through various signaling mechanisms, including redox, Ca2+, kinase and electrochemical pathways. Redox signaling operates at multiple levels, from interactions between individual molecules in their microenvironment to communication among subcellular organelles, single cells, organs, and the entire organism. Redox communication is a dynamic and ongoing spatiotemporal process. This article focuses on hydrogen peroxide (H2O2), a key second messenger that targets redox-active protein cysteine thiolates. H2O2 gradients across cell membranes are controlled by peroxiporins, specialized aquaporins. Redox-active endosomes, known as redoxosomes, form at the plasma membrane. Cell-to-cell redox communication involves direct contacts, such as per gap junctions that connect cells for transfer of molecules via connexons. Moreover, signaling occurs through the release of redox-active molecules and enzymes into the surrounding space, as well as through various types of extracellular vesicles (EVs) that transport these signals to nearby or distant target cells.

Mitochondrial Quality Control Orchestrates the Symphony of B Cells and Plays Critical Roles in B Cell-Related Diseases

B cells are essential for humoral immune response due to their ability to secrete antibodies. The development of B cells from the bone marrow to the periphery is tightly regulated by a complex set of immune signals, and each subset of B cells has a unique metabolic profile. Mitochondria, which serve as cellular energy powerhouses, play an essential role in regulating cell survival and immune responses. To maintain metabolic homeostasis, mitochondria dynamically adjust their morphology, distribution, and mass via biogenesis, fusion and fission, translocation, and mitophagy. Despite its extreme importance, the role of mitochondrial quality control (MQC) in B cells has not been thoroughly summarized, unlike in T cells. This article aims to review the mechanism of MQC that shapes B cell fate and functions. In addition, we will discuss the physiological and pathological implications of MQC in B cells, providing new insights into potential therapeutic targets for diseases associated with B cell abnormalities.

Mitochondrial dynamics and psychiatric disorders: The missing link

Elucidating the molecular mechanisms of psychopathology is crucial for optimized diagnosis and treatment. Accumulating data have underlined how mitochondrial bioenergetics affect major psychiatric disorders. However, how mitochondrial dynamics, a term addressing mitochondria quality control, including mitochondrial fission, fusion, biogenesis and mitophagy, is implicated in psychopathologies remains elusive. In this review, we summarize the existing literature on mitochondrial dynamics perturbations in psychiatric disorders/neuropsychiatric phenotypes. We include preclinical/clinical literature on mitochondrial dynamics recalibrations in anxiety, depression, post-traumatic stress disorder (PTSD), bipolar disorder and schizophrenia. We discuss alterations in mitochondrial network, morphology and shape, molecular markers of the mitochondrial dynamics machinery and mitochondrial DNA copy number (mtDNAcn) in animal models and human cohorts in brain and peripheral material. By looking for common altered mitochondrial dynamics patterns across diagnoses/phenotypes, we highlight mitophagy and biogenesis as regulators of anxiety and depression pathophysiology, respectively, as well as the fusion mediator dynamin-like 120 kDa protein (Opa1) as a molecular hub contributing to psychopathology. Finally, we comment on limitations and future directions in this novel neuropsychiatry field.

Fundamentals of redox regulation in biology

Oxidation-reduction (redox) reactions are central to the existence of life. Reactive species of oxygen, nitrogen and sulfur mediate redox control of a wide range of essential cellular processes. Yet, excessive levels of oxidants are associated with ageing and many diseases, including cardiological and neurodegenerative diseases, and cancer. Hence, maintaining the fine-tuned steady-state balance of reactive species production and removal is essential. Here, we discuss new insights into the dynamic maintenance of redox homeostasis (that is, redox homeodynamics) and the principles underlying biological redox organization, termed the 'redox code'. We survey how redox changes result in stress responses by hormesis mechanisms, and how the lifelong cumulative exposure to environmental agents, termed the 'exposome', is communicated to cells through redox signals. Better understanding of the molecular and cellular basis of redox biology will guide novel redox medicine approaches aimed at preventing and treating diseases associated with disturbed redox regulation.

USP7 promotes cardiometabolic disorders and mitochondrial homeostasis dysfunction in diabetic mice via stabilizing PGC1β

Diabetic cardiomyopathy (DCM) is a major complication of diabetes and is characterized by left ventricular dysfunction. Currently, there is a lack of effective treatments for DCM. Ubiquitin-specific protease 7 (USP7) plays a key role in various diseases. However, whether USP7 is involved in DCM has not been established. In this study, we demonstrated that USP7 was upregulated in diabetic mouse hearts and NMCMs co-treated with HG+PA or H9c2 cells treated with PA. Abnormalities in diabetic heart morphology and function were reversed by USP7 silencing through conditional gene knockout or chemical inhibition. Proteomic analysis coupled with biochemical validation confirmed that PCG1β was one of the direct protein substrates of USP7 and aggravated myocardial damage through coactivation of the PPARα signaling pathway. USP7 silencing restored the expression of fatty acid metabolism-related proteins and restored mitochondrial homeostasis by inhibiting mitochondrial fission and promoting fusion events. Similar effects were also observed in vitro. Our data demonstrated that USP7 promoted cardiometabolic metabolism disorders and mitochondrial homeostasis dysfunction via stabilizing PCG1β and suggested that silencing USP7 may be a therapeutic strategy for DCM.

Miro-mediated mitochondrial transport: A new dimension for disease-related abnormal cell metabolism?

Mitochondria are versatile and highly dynamic organelles found in eukaryotic cells that play important roles in a variety of cellular processes. The importance of mitochondrial transport in cell metabolism, including variations in mitochondrial distribution within cells and intercellular transfer, has grown in recent years. Several studies have demonstrated that abnormal mitochondrial transport represents an early pathogenic alteration in a variety of illnesses, emphasizing its significance in disease development and progression. Mitochondrial Rho GTPase (Miro) is a protein found on the outer mitochondrial membrane that is required for cytoskeleton-dependent mitochondrial transport, mitochondrial dynamics (fusion and fission), and mitochondrial Ca2+ homeostasis. Miro, as a critical regulator of mitochondrial transport, has yet to be thoroughly investigated in illness. This review focuses on recent developments in recognizing Miro as a crucial molecule in controlling mitochondrial transport and investigates its roles in diverse illnesses. It also intends to shed light on the possibilities of targeting Miro as a therapeutic method for a variety of diseases.