Mitochondrial dynamics in yeast

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

Mitochondria are essential organelles performing important cellular functions ranging from bioenergetics and metabolism to apoptotic signaling and immune responses. They are highly dynamic at different structural and functional levels. Mitochondria have been shown to constantly undergo fusion and fission processes and dynamically interact with other organelles such as the endoplasmic reticulum, peroxisomes, and lipid droplets. The field of mitochondrial dynamics has evolved hand in hand with technological achievements including advanced fluorescence super-resolution nanoscopy. Dynamic remodeling of the cristae membrane within individual mitochondria, discovered very recently, opens up a further exciting layer of mitochondrial dynamics. In this review, we discuss mitochondrial dynamics at the following levels: (a) within an individual mitochondrion, (b) among mitochondria, and (c) between mitochondria and other organelles. Although the three tiers of mitochondrial dynamics have in the past been classified in a hierarchical manner, they are functionally connected and must act in a coordinated manner to maintain cellular functions and thus prevent various human diseases.

Is mitochondrial morphology important for cellular physiology?

Mitochondria are double membrane-bound organelles the network morphology of which in cells is shaped by opposing events of fusion and fission executed by dynamin-like GTPases. Mutations in these genes can perturb the form and functions of mitochondria in cell and animal models of mitochondrial diseases. An expanding array of chemical, mechanical, and genetic stressors can converge on mitochondrial-shaping proteins and disrupt mitochondrial morphology. In recent years, studies aimed at disentangling the multiple roles of mitochondrial-shaping proteins beyond fission or fusion have provided insights into the homeostatic relevance of mitochondrial morphology. Here, I review the pleiotropy of mitochondrial fusion and fission proteins with the aim of understanding whether mitochondrial morphology is important for cell and tissue physiology.

A detailed review of pharmacology of MFN1 (mitofusion-1)-mediated mitochondrial dynamics: Implications for cellular health and diseases

The mitochondria are responsible for the production of cellular ATP, the regulation of cytosolic calcium levels, and the organization of numerous apoptotic proteins through the release of cofactors necessary for the activation of caspases. This level of functional adaptability can only be attained by sophisticated structural alignment. The morphology of the mitochondria does not remain unchanged throughout time; rather, it undergoes change as a result of processes known as fusion and fission. Fzo in flies, Fzo1 in yeast, and mitofusins in mammals are responsible for managing the outer mitochondrial membrane fusion process, whereas Mgm1 in yeast and optic atrophy 1 in mammals are responsible for managing the inner mitochondrial membrane fusion process. The fusion process is composed of two phases. MFN1, a GTPase that is located on the outer membrane of the mitochondria, is involved in the process of linking nearby mitochondria, maintaining the potential of the mitochondrial membrane, and apoptosis. This article offers specific information regarding the functions of MFN1 in a variety of cells and organs found in living creatures. According to the findings of the literature review, MFN1 plays an important part in a number of diseases and organ systems; nevertheless, the protein's function in other disease models and cell types has to be investigated in the near future so that it can be chosen as a promising marker for the therapeutic and diagnostic potentials it possesses. Overall, the major findings of this review highlight the pivotal role of mitofusin (MFN1) in regulating mitochondrial dynamics and its implications across various diseases, including neurodegenerative disorders, cardiovascular diseases, and metabolic syndromes. Our review identifies novel therapeutic targets within the MFN1 signaling pathways and underscores the potential of MFN1 modulation as a promising strategy for treating mitochondrial-related diseases. Additionally, the review calls for further research into MFN1's molecular mechanisms to unlock new avenues for clinical interventions, emphasizing the need for targeted therapies that address MFN1 dysfunction.

Contribution of YPRO15C Overexpression to the Resistance of Saccharomyces cerevisiae BY4742 Strain to Furfural Inhibitor

Lignocellulosic biomass is still considered a feasible source of bioethanol production. Saccharomyces cerevisiae can adapt to detoxify lignocellulose-derived inhibitors, including furfural. Tolerance of strain performance has been measured by the extent of the lag phase for cell proliferation following the furfural inhibitor challenge. The purpose of this work was to obtain a tolerant yeast strain against furfural through overexpression of YPR015C using the in vivo homologous recombination method. The physiological observation of the overexpressing yeast strain showed that it was more resistant to furfural than its parental strain. Fluorescence microscopy revealed improved enzyme reductase activity and accumulation of oxygen reactive species due to the harmful effects of furfural inhibitor in contrast to its parental strain. Comparative transcriptomic analysis revealed 79 genes potentially involved in amino acid biosynthesis, oxidative stress, cell wall response, heat shock protein, and mitochondrial-associated protein for the YPR015C overexpressing strain associated with stress responses to furfural at the late stage of lag phase growth. Both up- and down-regulated genes involved in diversified functional categories were accountable for tolerance in yeast to survive and adapt to the furfural stress in a time course study during the lag phase growth. This study enlarges our perceptions comprehensively about the physiological and molecular mechanisms implicated in the YPR015C overexpressing strain's tolerance under furfural stress. Construction illustration of the recombinant plasmid. a) pUG6-TEF1p-YPR015C, b) integration diagram of the recombinant plasmid pUG6-TEF1p-YPR into the chromosomal DNA of Saccharomyces cerevisiae.

Yeast Chromatin Mutants Reveal Altered mtDNA Copy Number and Impaired Mitochondrial Membrane Potential

Mitochondria are multifunctional, dynamic organelles important for stress response, cell longevity, ageing and death. Although the mitochondrion has its genome, nuclear-encoded proteins are essential in regulating mitochondria biogenesis, morphology, dynamics and function. Moreover, chromatin structure and epigenetic mechanisms govern the accessibility to DNA and control gene transcription, indirectly influencing nucleo-mitochondrial communications. Thus, they exert crucial functions in maintaining proper chromatin structure, cell morphology, gene expression, stress resistance and ageing. Here, we present our studies on the mtDNA copy number in Saccharomyces cerevisiae chromatin mutants and investigate the mitochondrial membrane potential throughout their lifespan. The mutants are arp4 (with a point mutation in the ARP4 gene, coding for actin-related protein 4-Arp4p), hho1Δ (lacking the HHO1 gene, coding for the linker histone H1), and the double mutant arp4 hho1Δ cells with the two mutations. Our findings showed that the three chromatin mutants acquired strain-specific changes in the mtDNA copy number. Furthermore, we detected the disrupted mitochondrial membrane potential in their chronological lifespan. In addition, the expression of nuclear genes responsible for regulating mitochondria biogenesis and turnover was changed. The most pronounced were the alterations found in the double mutant arp4 hho1Δ strain, which appeared as the only petite colony-forming mutant, unable to grow on respiratory substrates and with partial depletion of the mitochondrial genome. The results suggest that in the studied chromatin mutants, hho1Δ, arp4 and arp4 hho1Δ, the nucleus-mitochondria communication was disrupted, leading to impaired mitochondrial function and premature ageing phenotype in these mutants, especially in the double mutant.

Reduced OPA1, Mitochondrial Fragmentation and Increased Susceptibility to Apoptosis in Granular Corneal Dystrophy Type 2 Corneal Fibroblasts

The progressive degeneration of granular corneal dystrophy type 2 (GCD2) corneal fibroblasts is associated with altered mitochondrial function, but the underlying mechanisms are incompletely understood. We investigated whether an imbalance of mitochondrial dynamics contributes to mitochondrial dysfunction of GCD2 corneal fibroblasts. Transmission electron microscopy revealed several small, structurally abnormal mitochondria with altered cristae morphology in GCD2 corneal fibroblasts. Confocal microscopy showed enhanced mitochondrial fission and fragmented mitochondrial tubular networks. Western blotting revealed higher levels of MFN1, MFN2, and pDRP1 and decreased levels of OPA1 and FIS1 in GCD2. OPA1 reduction by short hairpin RNA (shRNA) resulted in fragmented mitochondrial tubular networks and increased susceptibility to mitochondrial stress-induced apoptosis. A decrease in the mitochondrial biogenesis-related transcription factors NRF1 and PGC1α was observed, while there was an increase in the mitochondrial membrane proteins TOM20 and TIM23. Additionally, reduced levels of mitochondrial DNA (mtDNA) were exhibited in GCD2 corneal fibroblasts. These observations suggest that altered mitochondrial fission/fusion and biogenesis are the critical molecular mechanisms that cause mitochondrial dysfunction contributing to the degeneration of GCD2 corneal fibroblasts.

The status and trends of mitochondrial dynamics research: A global bibliometric and visualized analysis

BACKGROUND

Mitochondria are remarkably dynamic organelles encapsulated by bilayer membranes. The dynamic properties of mitochondria are critical for energy production.

AIMS

The aim of our study is to investigate the global status and trends of mitochondrial dynamics research and predict popular topics and directions in the field.

METHODS

Publications related to the studies of mitochondrial dynamics from 2002 to 2021 were retrieved from Web of Science database. A total of 4,576 publications were included. Bibliometric analysis was conducted by visualization of similarities viewer and GraphPadPrism 5 software.

RESULTS

There is an increasing trend of mitochondrial dynamics research during the last 20 years. The cumulative number of publications about mitochondrial dynamics research followed the logistic growth model [Formula: see text]. The USA made the highest contributions to the global research. The journal Biochimica et Biophysica Acta (BBA)-Molecular Cell Research had the largest publication numbers. Case Western Reserve University is the most contributive institution. The main research orientation and funding agency were cell biology and HHS. All keywords related studies could be divided into three clusters: "Related disease research", "Mechanism research" and "Cell metabolism research".

CONCLUSIONS

Attention should be drawn to the latest popular research and more efforts will be put into mechanistic research, which may inspire new clinical treatments for the associated diseases.

Evidence for Persistent Heteroplasmy and Ancient Recombination in the Mitochondrial Genomes of the Edible Yellow Chanterelles From Southwestern China and Europe

Mitochondrial genes and genomes have patterns of inheritance that are distinctly different from those of nuclear genes and genomes. In nature, the mitochondrial genomes in eukaryotes are generally considered non-recombining and homoplasmic. If heteroplasmy and recombination exist, they are typically very limited in both space and time. Here we show that mitochondrial heteroplasmy and recombination may not be limited to a specific population nor exit only transiently in the basidiomycete Cantharellus cibarius and related species. These edible yellow chanterelles are an ecologically very important group of fungi and among the most prominent wild edible mushrooms in the Northern Hemisphere. At present, very little is known about the genetics and population biology of these fungia cross large geographical distances. Our study here analyzed a total of 363 specimens of edible yellow chanterelles from 24 geographic locations in Yunnan in southwestern China and six geographic locations in five countries in Europe. For each mushroom sample, we obtained the DNA sequences at two genes, one in the nuclear genome and one in the mitochondrial genome. Our analyses of the nuclear gene, translation elongation factor 1-alpha (tef-1) and the DNA barcode of C. cibarius and related species, suggested these samples belong to four known species and five potential new species. Interestingly, analyses of the mitochondrial ATP synthase subunit 6 (atp6) gene fragment revealed evidence of heteroplasmy in two geographic samples in Yunnan and recombination within the two new putative species in Yunnan. Specifically, all four possible haplotypes at two polymorphic nucleotide sites within the mitochondrial atp6 gene were found distributed across several geographic locations in Yunnan. Furthermore, these four haplotypes were broadly distributed across multiple phylogenetic clades constructed based on nuclear tef-1 sequences. Our results suggest that heteroplasmy and mitochondrial recombination might have happened repeatedly during the evolution of the yellow chanterelles. Together, our results suggest that the edible yellow chanterelles represent an excellent system from which to study the evolution of mitochondrial-nuclear genome relationships.

Mitochondrial dynamics, positioning and function mediated by cytoskeletal interactions

The ability of a mitochondrion to undergo fission and fusion, and to be transported and localized within a cell are central not just to proper functioning of mitochondria, but also to that of the cell. The cytoskeletal filaments, namely microtubules, F-actin and intermediate filaments, have emerged as prime movers in these dynamic mitochondrial shape and position transitions. In this review, we explore the complex relationship between the cytoskeleton and the mitochondrion, by delving into: (i) how the cytoskeleton helps shape mitochondria via fission and fusion events, (ii) how the cytoskeleton facilitates the translocation and anchoring of mitochondria with the activity of motor proteins, and (iii) how these changes in form and position of mitochondria translate into functioning of the cell.

The Functional Impact of Mitochondrial Structure Across Subcellular Scales

Mitochondria are key determinants of cellular health. However, the functional role of mitochondria varies from cell to cell depending on the relative demands for energy distribution, metabolite biosynthesis, and/or signaling. In order to support the specific needs of different cell types, mitochondrial functional capacity can be optimized in part by modulating mitochondrial structure across several different spatial scales. Here we discuss the functional implications of altering mitochondrial structure with an emphasis on the physiological trade-offs associated with different mitochondrial configurations. Within a mitochondrion, increasing the amount of cristae in the inner membrane improves capacity for energy conversion and free radical-mediated signaling but may come at the expense of matrix space where enzymes critical for metabolite biosynthesis and signaling reside. Electrically isolating individual cristae could provide a protective mechanism to limit the spread of dysfunction within a mitochondrion but may also slow the response time to an increase in cellular energy demand. For individual mitochondria, those with relatively greater surface areas can facilitate interactions with the cytosol or other organelles but may be more costly to remove through mitophagy due to the need for larger phagophore membranes. At the network scale, a large, stable mitochondrial reticulum can provide a structural pathway for energy distribution and communication across long distances yet also enable rapid spreading of localized dysfunction. Highly dynamic mitochondrial networks allow for frequent content mixing and communication but require constant cellular remodeling to accommodate the movement of mitochondria. The formation of contact sites between mitochondria and several other organelles provides a mechanism for specialized communication and direct content transfer between organelles. However, increasing the number of contact sites between mitochondria and any given organelle reduces the mitochondrial surface area available for contact sites with other organelles as well as for metabolite exchange with cytosol. Though the precise mechanisms guiding the coordinated multi-scale mitochondrial configurations observed in different cell types have yet to be elucidated, it is clear that mitochondrial structure is tailored at every level to optimize mitochondrial function to meet specific cellular demands.

Miro (Mitochondrial Rho GTPase), a key player of mitochondrial axonal transport and mitochondrial dynamics in neurodegenerative diseases

Miro (mitochondrial Rho GTPases) a mitochondrial outer membrane protein, plays a vital role in the microtubule-based mitochondrial axonal transport, mitochondrial dynamics (fusion and fission) and Mito-Ca²⁺ homeostasis. It forms a major protein complex with Milton (an adaptor protein), kinesin and dynein (motor proteins), and facilitates bidirectional mitochondrial axonal transport such as anterograde and retrograde transport. By forming this protein complex, Miro facilitates the mitochondrial axonal transport and fulfills the neuronal energy demand, maintain the mitochondrial homeostasis and neuronal survival. It has been demonstrated that altered mitochondrial biogenesis, improper mitochondrial axonal transport, and mitochondrial dynamics are the early pathologies associated with most of the neurodegenerative diseases (NDs). Being the sole mitochondrial outer membrane protein associated with mitochondrial axonal transport-related processes, Miro proteins can be one of the key players in various NDs such as Alzheimer's disease (AD), Parkinson's disease (PD), Amyotrophic lateral sclerosis (ALS) and Huntington's disease (HD). Thus, in the current review, we have discussed the evolutionarily conserved Miro proteins and its role in the pathogenesis of the various NDs. From this, we indicated that Miro proteins may act as a potential target for a novel therapeutic intervention for the treatment of various NDs.

Unbiased yeast screens identify cellular pathways affected in Niemann-Pick disease type C

Niemann-Pick disease type C (NPC) is a rare lysosomal storage disease caused by mutations in either the NPC1 or NPC2 genes. Mutations in the NPC1 gene lead to the majority of clinical cases (95%); however, the function of NPC1 remains unknown. To gain further insights into the biology of NPC1, we took advantage of the homology between the human NPC1 protein and its yeast orthologue, Niemann-Pick C-related protein 1 (Ncr1). We recreated the NCR1 mutant in yeast and performed screens to identify compensatory or redundant pathways that may be involved in NPC pathology, as well as proteins that were mislocalized in NCR1-deficient yeast. We also identified binding partners of the yeast Ncr1 orthologue. These screens identified several processes and pathways that may contribute to NPC pathogenesis. These included alterations in mitochondrial function, cytoskeleton organization, metal ion homeostasis, lipid trafficking, calcium signalling, and nutrient sensing. The mitochondrial and cytoskeletal abnormalities were validated in patient cells carrying mutations in NPC1, confirming their dysfunction in NPC disease.

Mitochondrial Quality Control: Role in Cardiac Models of Lethal Ischemia-Reperfusion Injury

The current standard of care for acute myocardial infarction or 'heart attack' is timely restoration of blood flow to the ischemic region of the heart. While reperfusion is essential for the salvage of ischemic myocardium, re-introduction of blood flow paradoxically kills (rather than rescues) a population of previously ischemic cardiomyocytes-a phenomenon referred to as 'lethal myocardial ischemia-reperfusion (IR) injury'. There is long-standing and exhaustive evidence that mitochondria are at the nexus of lethal IR injury. However, during the past decade, the paradigm of mitochondria as mediators of IR-induced cardiomyocyte death has been expanded to include the highly orchestrated process of mitochondrial quality control. Our aims in this review are to: (1) briefly summarize the current understanding of the pathogenesis of IR injury, and (2) incorporating landmark data from a broad spectrum of models (including immortalized cells, primary cardiomyocytes and intact hearts), provide a critical discussion of the emerging concept that mitochondrial dynamics and mitophagy (the components of mitochondrial quality control) may contribute to the pathogenesis of cardiomyocyte death in the setting of ischemia-reperfusion.

Functional Interplay between Cristae Biogenesis, Mitochondrial Dynamics and Mitochondrial DNA Integrity

Mitochondria are vital cellular organelles involved in a plethora of cellular processes such as energy conversion, calcium homeostasis, heme biogenesis, regulation of apoptosis and ROS reactive oxygen species (ROS) production. Although they are frequently depicted as static bean-shaped structures, our view has markedly changed over the past few decades as many studies have revealed a remarkable dynamicity of mitochondrial shapes and sizes both at the cellular and intra-mitochondrial levels. Aberrant changes in mitochondrial dynamics and cristae structure are associated with ageing and numerous human diseases (e.g., cancer, diabetes, various neurodegenerative diseases, types of neuro- and myopathies). Another unique feature of mitochondria is that they harbor their own genome, the mitochondrial DNA (mtDNA). MtDNA exists in several hundreds to thousands of copies per cell and is arranged and packaged in the mitochondrial matrix in structures termed mt-nucleoids. Many human diseases are mechanistically linked to mitochondrial dysfunction and alteration of the number and/or the integrity of mtDNA. In particular, several recent studies identified remarkable and partly unexpected links between mitochondrial structure, fusion and fission dynamics, and mtDNA. In this review, we will provide an overview about these recent insights and aim to clarify how mitochondrial dynamics, cristae ultrastructure and mtDNA structure influence each other and determine mitochondrial functions.

Drp1 Phosphorylation Is Indispensable for Steroidogenesis in Leydig Cells

The initial steps of steroidogenesis occur in the mitochondria. Dynamic changes in the mitochondria are associated with their fission and fusion. Therefore, understanding the cellular and molecular relationships between steroidogenesis and mitochondrial dynamics is important. The hypothesis of the current study is that mitochondrial fission and fusion are closely associated with steroid hormone synthesis in testicular Leydig cells. Steroid hormone production, induced by dibutyryl cAMP (dbcAMP) in Leydig cells, was accompanied by increased mitochondrial mass. Mitochondrial elongation increased during the dbcAMP-induced steroid production, whereas mitochondrial fragmentation was reduced. Among the mitochondrial-shaping proteins, the level of dynamin-associated protein 1 (Drp1) was altered in response to dbcAMP stimulation. The increase in Drp1 Ser 637 phosphorylation correlated with steroid hormone production in the MA-10 Leydig cells as well as in the primary adult rat Leydig cells. Drp1 was differentially expressed in the Leydig cells during testicular development. Finally, gonadotropin administration altered the status of Drp1 phosphorylation in the Leydig cells of immature rat testes. Overall, mitochondrial dynamics is directly linked to steroidogenesis, and Drp1 plays an important regulatory role during steroidogenesis. This study shows that Drp1 level is regulated by cAMP and that its phosphorylation via protein kinase A (PKA) activation plays a decisive role in mitochondrial shaping by offering an optimal environment for steroid hormone biosynthesis in Leydig cells. Therefore, it is suggested that PKA-mediated Drp1 Ser 637 phosphorylation is indispensable for steroidogenesis in the Leydig cells, and this phosphorylation results in mitochondrial elongation via the relative attenuation of mitochondrial fission during steroidogenesis.

The hypoxic transcription factor KlMga2 mediates the response to oxidative stress and influences longevity in the yeast Kluyveromyces lactis

Hypoxia is defined as the decline of oxygen availability, depending on environmental supply and cellular consumption rate. The decrease in O2 results in reduction of available energy in facultative aerobes. The response and/or adaptation to hypoxia and other changing environmental conditions can influence the properties and functions of membranes by modifying lipid composition. In the yeast Kluyveromyces lactis, the KlMga2 gene is a hypoxic regulatory factor for lipid biosynthesis—fatty acids and sterols—and is also involved in glucose signaling, glucose catabolism and is generally important for cellular fitness.

In this work we show that, in addition to the above defects, the absence of the KlMGA2 gene caused increased resistance to oxidative stress and extended lifespan of the yeast, associated with increased expression levels of catalase and SOD genes. We propose that KlMga2 might also act as a mediator of the oxidative stress response/adaptation, thus revealing connections among hypoxia, glucose signaling, fatty acid biosynthesis and ROS metabolism in K. lactis.

Dynamin-related protein 1: A critical protein in the pathogenesis of neural system dysfunctions and neurodegenerative diseases

Mitochondria play a key role in the maintenance of neuronal function by continuously providing energy. Here, we will give a detailed review about the recent developments in regards to dynamin-related protein 1 (Drp1) induced unbalanced mitochondrial dynamics, excessive mitochondrial division, and neuronal injury in neural system dysfunctions and neurodegenerative diseases, including the Drp1 knockout induced mice embryonic death, the dysfunction of the Drp1-dependent mitochondrial division induced neuronal cell apoptosis and impaired neuronal axonal transportation, the abnormal interaction between Drp1 and amyloid β (Aβ) in Alzheimer's disease (AD), the mutant Huntingtin (Htt) in Huntington's disease (HD), and the Drp1-associated pathogenesis of other neurodegenerative diseases such as Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS). Drp1 is required for mitochondrial division determining the size, shape, distribution, and remodeling as well as maintaining of mitochondrial integrity in mammalian cells. In addition, increasing reports indicate that the Drp1 is involved in some cellular events of neuronal cells causing some neural system dysfunctions and neurodegenerative diseases, including impaired mitochondrial dynamics, apoptosis, and several posttranslational modification induced increased mitochondrial divisions. Recent studies also revealed that the Drp1 can interact with Aβ, phosphorylated τ, and mutant Htt affecting the mitochondrial shape, size, distribution, axonal transportation, and energy production in the AD and HD neuronal cells. These changes can affect the health of mitochondria and the function of synapses causing neuronal injury and eventually leading to the dysfunction of memory, cognitive impairment, resting tremor, posture instability, involuntary movements, and progressive muscle atrophy and paralysis in patients.

A Targeted Mutation Identified through pKa Measurements Indicates a Postrecruitment Role for Fis1 in Yeast Mitochondrial Fission

The tail-anchored protein Fis1 is implicated as a passive tether in yeast mitochondrial fission. We probed the functional role of Fis1 Glu-78, whose elevated side chain pKa suggests participation in protein interactions. Fis1 binds partners Mdv1 or Dnm1 tightly, but mutation E78A weakens Fis1 interaction with Mdv1, alters mitochondrial morphology, and abolishes fission in a growth assay. In fis1Δ rescue experiments, Fis1-E78A causes a novel localization pattern in which Dnm1 uniformly coats the mitochondria. By contrast, Fis1-E78A at lower expression levels recruits Dnm1 into mitochondrial punctate structures but fails to support normal fission. Thus, Fis1 makes multiple interactions that support Dnm1 puncta formation and may be essential after this step, supporting a revised model for assembly of the mitochondrial fission machinery. The insights gained by mutating a residue with a perturbed pKa suggest that side chain pKa values inferred from routine NMR sample pH optimization could provide useful leads for functional investigations.

Time representation of mitochondrial morphology and function after acute spinal cord injury

Changes in mitochondrial morphology and function play an important role in secondary damage after acute spinal cord injury. We recorded the time representation of mitochondrial morphology and function in rats with acute spinal cord injury. Results showed that mitochondria had an irregular shape, and increased in size. Mitochondrial cristae were disordered and mitochondrial membrane rupture was visible at 2-24 hours after injury. Fusion protein mitofusin 1 expression gradually increased, peaked at 8 hours after injury, and then decreased to its lowest level at 24 hours. Expression of dynamin-related protein 1, amitochondrial fission protein, showed the opposite kinetics. At 2-24 hours after acute spinal cord injury, malondialdehyde content, cytochrome c levels and caspase-3 expression were increased, but glutathione content, adenosine triphosphate content, Na(+)-K(+)-ATPase activity and mitochondrial membrane potential were gradually reduced. Furthermore, mitochondrial morphology altered during the acute stage of spinal cord injury. Fusion was important within the first 8 hours, but fission played a key role at 24 hours. Oxidative stress was inhibited, biological productivity was diminished, and mitochondrial membrane potential and permeability were reduced in the acute stage of injury. In summary, mitochondrial apoptosis is activated when the time of spinal cord injury is prolonged.

Phenotypic characterization and comparative transcriptomics of evolved Saccharomyces cerevisiae strains with improved tolerance to lignocellulosic derived inhibitors

BACKGROUND

Lignocellulosic biomass continues to be investigated as a viable source for bioethanol production. However, the pretreatment process generates inhibitory compounds that impair the growth and fermentation performance of microorganisms such as Saccharomyces cerevisiae. Pinewood specifically has been shown to be challenging in obtaining industrially relevant ethanol titers. An industrial S. cerevisiae strain was subjected to directed evolution and adaptation in pretreated pine biomass and resultant strains, GHP1 and GHP4, exhibited improved growth and fermentative ability on pretreated pine in the presence of related inhibitory compounds. A comparative transcriptomic approach was applied to identify and characterize differences in phenotypic stability of evolved strains.

RESULTS

Evolved strains displayed different fermentative capabilities with pretreated pine that appear to be influenced by the addition or absence of 13 inhibitory compounds during pre-culturing. GHP4 performance was consistent independent of culturing conditions, while GHP1 performance was dependent on culturing with inhibitors. Comparative transcriptomics revealed 52 genes potentially associated with stress responses to multiple inhibitors simultaneously. Fluorescence microscopy revealed improved cellular integrity of both strains with mitochondria exhibiting resistance to the damaging effects of inhibitors in contrast to the parent.

CONCLUSIONS

Multiple potentially novel genetic targets have been discovered for understanding stress tolerance through the characterization of our evolved strains. This study specifically examines the synergistic effects of multiple inhibitors and identified targets will guide future studies in remediating effects of inhibitors and further development of robust yeast strains for multiple industrial applications.

Proteasome Impairment Induces Recovery of Mitochondrial Membrane Potential and an Alternative Pathway of Mitochondrial Fusion

Mitochondria are vital and highly dynamic organelles that continuously fuse and divide to maintain mitochondrial quality. Mitochondrial dysfunction impairs cellular integrity and is known to be associated with various human diseases. However, the mechanism by which the quality of mitochondria is maintained remains largely unexplored. Here we show that impaired proteasome function recovers the growth of yeast cells lacking Fzo1, a pivotal protein for mitochondrial fusion. Decreased proteasome activity increased the mitochondrial oxidoreductase protein Mia40 and the ratio of the short isoform of mitochondrial intermembrane protein Mgm1 (s-Mgm1) to the long isoform (l-Mgm1). The increase in Mia40 restored mitochondrial membrane potential, while the increase in the s-Mgm1/l-Mgm1 ratio promoted mitochondrial fusion in an Fzo1-independent manner. Our findings demonstrate a new pathway for mitochondrial quality control that is induced by proteasome impairment.

Absorption of monochromatic and narrow band radiation in the visible and near IR by both mitochondrial and non-mitochondrial photoacceptors results in photobiomodulation

In addition to the major functions performed by in the cell, mitochondria play a major role in cell-light interaction. Accordingly it is generally accepted that mitochondria are crucial in cell photobiomodulation; however a variety of biomolecules themselves proved to be targets of light irradiation. We describe whether and how mitochondria can interact with monochromatic and narrow band radiation in the red and near IR optical regions with dissection of both structural and functional effects likely leading to photobiostimulation. Moreover we also report that a variety of biomolecules localized in mitochondria and/or in other cell compartments including cytochrome c oxidase, some proteins, nucleic acids and adenine nucleotides are light sensitive with major modifications in their biochemistry. All together the reported investigations show that the elucidation of the mechanism of the light interaction with biological targets still remains to be completed, this needing further research, however the light sensitivity of a variety of molecules strongly suggests that photobiomodulation could be used in both in photomedicine and in biotechnology.

Perspectives of mitochondrial medicine

Mitochondrial medicine was established more than 50 years ago after discovery of the very first pathology caused by impaired mitochondria. Since then, more than 100 mitochondrial pathologies have been discovered. However, the number may be significantly higher if we interpret the term "mitochondrial medicine" more widely and include in these pathologies not only those determined by the genetic apparatus of the nucleus and mitochondria, but also acquired mitochondrial defects of non-genetic nature. Now the main problems of mitochondriology arise from methodology, this being due to studies of mitochondrial activities under different models and conditions that are far from the functioning of mitochondria in a cell, organ, or organism. Controversial behavior of mitochondria ("friends and foes") to some extent might be explained by their bacterial origin with possible preservation of "egoistic" features peculiar to bacteria. Apparently, for normal mitochondrial functioning it is essential to maintain homeostasis of a number of mitochondrial elements such as mitochondrial DNA structure, membrane potential, and the system of mitochondrial quality control. Abrogation of these elements can cause a number of pathologies that have become subjects of mitochondrial medicine. Some approaches to therapy of mitochondrial pathologies are discussed.

A functional genomic analysis of Arabidopsis thaliana PP2C clade D

In the reference dicot plant Arabidopsis thaliana, the PP2C family of P-protein phosphatases includes the products of 80 genes that have been separated into ten multi-protein clades plus six singletons. Clade D includes the products of nine genes distributed among three chromosomes (APD1, At3g12620; APD2, At3g17090; APD3, At3g51370; APD4, At3g55050; APD5, At4g33920; APD6, At4g38520; APD7, At5g02760; APD8, At5g06750; and APD9, At5g66080). As part of a functional genomics analysis of protein phosphorylation, we retrieved expression data from public databases and determined the subcellular protein localization of the members of clade D. While the nine proteins have been grouped together based upon primary sequence alignments, we observed no obvious common patterns in expression or localization. We found chimera with the GFP associated with the nucleus, plasma membrane, the endomembrane system, and mitochondria in transgenic plants.

Maintenance of mitochondrial morphology by autophagy and its role in high glucose effects on chronological lifespan of Saccharomyces cerevisiae

In Saccharomyces cerevisiae, mitochondrial morphology changes when cells are shifted between nonfermentative and fermentative carbon sources. Here, we show that cells of S. cerevisiae grown in different glucose concentrations display different mitochondrial morphologies. The morphology of mitochondria in the cells growing in 0.5% glucose was similar to that of mitochondria in respiring cells. However, the mitochondria of cells growing in higher glucose concentrations (2% and 4%) became fragmented after growth in these media, due to the production of acetic acid; however, the fragmentation was not due to intracellular acidification. From a screen of mutants involved in sensing and utilizing nutrients, cells lacking TOR1 had reduced mitochondrial fragmentation, and autophagy was found to be essential for this reduction. Mitochondrial fragmentation in cells grown in high glucose was reversible by transferring them into conditioned medium from a culture grown on 0.5% glucose. Similarly, the chronological lifespan of cells grown in high glucose medium was reduced, and this phenotype could be reversed when cells were transferred to low glucose conditioned medium. These data indicate that chronological lifespan seems correlated with mitochondrial morphology of yeast cells and that both phenotypes can be influenced by factors from conditioned medium of cultures grown in low glucose medium.

A uvs-5 strain is deficient for a mitofusin gene homologue, fzo1, involved in maintenance of long life span in Neurospora crassa

Mitochondria are highly dynamic organelles that continuously fuse and divide. To maintain mitochondria, cells establish an equilibrium of fusion and fission events, which are mediated by dynamin-like GTPases. We previously showed that an mus-10 strain, a mutagen-sensitive strain of the filamentous fungus Neurospora crassa, is defective in an F-box protein that is essential for the maintenance of mitochondrial DNA (mtDNA), long life span, and mitochondrial morphology. Similarly, a uvs-5 mutant accumulates deletions within its mtDNA, has a shortened life span, and harbors fragmented mitochondria, the latter of which is indicative of an imbalance between mitochondrial fission and fusion. Since the uvs-5 mutation maps very close to the locus of fzo1, encoding a mitofusin homologue thought to mediate mitochondrial outer membrane fusion, we determined the sequence of the fzo1 gene in the uvs-5 mutant. A single amino acid substitution (Q368R) was found in the GTPase domain of the FZO1 protein. Expression of wild-type FZO1 in the uvs-5 strain rescued the mutant phenotypes, while expression of a mutant FZO1 protein did not. Moreover, when knock-in of the Q368R mutation was performed on a wild-type strain, the resulting mutant displayed phenotypes identical to those of the uvs-5 mutant. Therefore, we concluded that the previously unidentified uvs-5 gene is fzo1. Furthermore, we used immunoprecipitation analysis to show that the FZO1 protein interacts with MUS-10, which suggests that these two proteins may function together to maintain mitochondrial morphology.

Vital mitochondrial functions show profound changes during yeast culture ageing

During a 10-day culture ageing, cells of the wild-type Saccharomyces cerevisiae strain JC 482 retain their viability, while mitochondrial function and morphology change. Cell routine and uncoupled respiration rates increase to a maximum on day 4 and then decline to near zero. The decline, which occurs also in mitochondria isolated from cells of different age, is not due to increasing proportion of petites. Rhodamine 123 fluorescence intensity reporting on mitochondrial membrane potential appears to drop slightly for 4 days and then more sharply at the time when respiration rate also decreases. The MitoTracker Green fluorescent signal related to the mitochondrial content per cell also decreases. The branched tubular mitochondrial network of 1-day-old cells dissolves into short fragments; during the first 4 days, this fragmentation is associated with increasing function of mitochondria, while later on, it accompanies functional decline, which is also indicated by the decreasing ratio of Rhodamine 123 fluorescence to MitoTracker Green fluorescence. As shown by cell counting, microscopy and flow cytometry, the cell size distribution in the population broadens, and the population thus becomes more heterogeneous. The changes in respiration rate, mitochondrial membrane potential, mass and structure point to changes in the mitochondrial status during ageing.

Defective mitochondrial fusion, altered respiratory function, and distorted cristae structure in skin fibroblasts with heterozygous OPA1 mutations

Deleterious consequences of heterozygous OPA1 mutations responsible for autosomal dominant optic atrophy remain a matter of debate. Primary skin fibroblasts derived from patients have shown diverse mitochondrial alterations that were however difficult to resolve in a unifying scheme. To address the potential use of these cells as disease model, we undertook parallel and quantitative analyses of the diverse reported alterations in four fibroblast lines harboring different OPA1 mutations, nonsense or missense, in the guanosine triphosphatase or the C-terminal coiled-coil domains. We tackled several factors potentially underlying discordant reports and showed that fibroblasts with heterozygous OPA1 mutations present with several mitochondrial alterations. These included defective mitochondrial fusion during pharmacological challenge with the protonophore carbonyl cyanide m-chlorophenyl hydrazone, significant mitochondrial elongation with decreased OPA1 and DRP1 proteins, and abnormal mitochondrial fragmentation during glycolysis shortage or exogenous oxidative stress. Respiratory complex IV activity and subunits steady-state were decreased without alteration of the mitochondrial deoxyribonucleic acid size, amount or transcription. Physical link between OPA1 protein and oxidative phosphorylation was shown by reciprocal immunoprecipitation. Altered cristae structure coexisted with normal response to pro-apoptotic stimuli and expression of Bax or Bcl2 proteins. Skin fibroblasts with heterozygous OPA1 mutations thus share significant mitochondrial remodeling, and may therefore be useful for analyzing disease pathophysiology. Identifying whether the observed alterations are also present in ganglion retinal cells, and which of them underlies their degeneration process remains however an essential goal for therapeutic strategy.

A designed point mutant in Fis1 disrupts dimerization and mitochondrial fission

Mitochondrial and peroxisomal fission are essential processes with defects resulting in cardiomyopathy and neonatal lethality. Central to organelle fission is Fis1, a monomeric tetratricopeptide repeat (TPR)-like protein whose role in assembly of the fission machinery remains obscure. Two nonfunctional, Saccharomyces cerevisiae Fis1 mutants (L80P or E78D/I85T/Y88H) were previously identified in genetic screens. Here, we find that these two variants in the cytosolic domain of Fis1 (Fis1ΔTM) are unexpectedly dimeric. A truncation variant of Fis1ΔTM that lacks an N-terminal regulatory domain is also found to be dimeric. The ability to dimerize is a property innate to the native Fis1ΔTM amino acid sequence as we find this domain is dimeric after transient exposure to elevated temperature or chemical denaturants and is kinetically trapped at room temperature. This is the first demonstration of a specific self-association in solution for the Fis1 cytoplasmic domain. We propose a three-dimensional domain-swapped model for dimerization that is validated by a designed mutation, A72P, which potently disrupts dimerization of wild-type Fis1. A72P also disrupts dimerization of nonfunctional variants, indicating a common structural basis for dimerization. The obligate monomer variant A72P, like the dimer-promoting variants, is nonfunctional in fission, consistent with a model in which Fis1 activity depends on its ability to interconvert between monomer and dimer species. These studies suggest a new functionally important manner in which TPR-containing proteins may reversibly self-associate.

Comprehensive application of an mtDsRed2-Tg mouse strain for mitochondrial imaging

Mitochondria are essential for many cellular functions such as oxidative phosphorylation and calcium homeostasis; consequently, mitochondrial dysfunction could cause many diseases, including neurological disorders. Recently, mitochondrial dynamics, such as fusion, fission, and transportation, have been visualized in living cells by using time-lapse imaging systems. The changes in mitochondrial morphology could be an indicator for estimating the activity of mitochondrial biological function. Here, we report a transgenic mouse strain, mtDsRed2-Tg, which expresses a red fluorescent protein, DsRed2, exclusively in mitochondria. Mitochondrial morphology could be clearly observed in various tissues of this strain under confocal microscope. Recently, many transgenic mouse strains in which enhanced green fluorescent protein (EGFP)-tagged proteins of interest are expressed have been established for physiological analysis in vivo. After mating these strains with mtDsRed2-Tg mice, red-colored mitochondria and green-colored proteins were detected simultaneously using fluorescent imaging systems, and the interactions between mitochondria and those proteins could be morphologically analyzed in cells and tissues of the F(1) hybrids. Thus, mtDsRed2-Tg mice can be a powerful tool for bioimaging studies on mitochondrial functions.

Selection for mitonuclear co-adaptation could favour the evolution of two sexes

Mitochondria are descended from free-living bacteria that were engulfed by another cell between one and a half to two billion years ago. A redistribution of DNA led to most genetic information being lost or transferred to a large central genome in the nucleus, leaving a residual genome in each mitochondrion. Oxidative phosphorylation, the most critical function of mitochondria, depends on the functional compatibility of proteins encoded by both the nucleus and mitochondria. We investigate whether selection for adaptation between the nuclear and mitochondrial genomes (mitonuclear co-adaptation) could, in principle, have promoted uniparental inheritance of mitochondria and thereby the evolution of two mating types or sexes. Using a mathematical model, we explore the importance of the radical differences in ploidy levels, sexual and asexual modes of inheritance, and mutation rates of the nucleus and mitochondria. We show that the major features of mitochondrial inheritance, notably uniparental inheritance and bottlenecking, enhance the co-adaptation of mitochondrial and nuclear genes and therefore improve fitness. We conclude that, under a wide range of conditions, selection for mitonuclear co-adaptation favours the evolution of two distinct mating types or sexes in sexual species.

Actin in Mung Bean Mitochondria and Implications for Its Function

Here, a large fraction of plant mitochondrial actin was found to be resistant to protease and high-salt treatments, suggesting it was protected by mitochondrial membranes. A portion of this actin became sensitive to protease or high-salt treatment after removal of the mitochondrial outer membrane, indicating that some actin is located inside the mitochondrial outer membrane. The import of an actin–green fluorescent protein (GFP) fusion protein into the mitochondria in a transgenic plant, actin:GFP, was visualized in living cells and demonstrated by flow cytometry and immunoblot analyses. Polymerized actin was found in mitochondria of actin:GFP plants and in mung bean (Vigna radiata). Notably, actin associated with mitochondria purified from early-developing cotyledons during seed germination was sensitive to high-salt and protease treatments. With cotyledon ageing, mitochondrial actin became more resistant to both treatments. The progressive import of actin into cotyledon mitochondria appeared to occur in concert with the conversion of quiescent mitochondria into active forms during seed germination. The binding of actin to mitochondrial DNA (mtDNA) was demonstrated by liquid chromatography–tandem mass spectrometry analysis. Porin and ADP/ATP carrier proteins were also found in mtDNA-protein complexes. Treatment with an actin depolymerization reagent reduced the mitochondrial membrane potential and triggered the release of cytochrome C. The potential function of mitochondrial actin and a possible actin import pathway are discussed.

Mitonuclear match: optimizing fitness and fertility over generations drives ageing within generations

Many conserved eukaryotic traits, including apoptosis, two sexes, speciation and ageing, can be causally linked to a bioenergetic requirement for mitochondrial genes. Mitochondrial genes encode proteins involved in cell respiration, which interact closely with proteins encoded by nuclear genes. Functional respiration requires the coadaptation of mitochondrial and nuclear genes, despite divergent tempi and modes of evolution. Free-radical signals emerge directly from the biophysics of mosaic respiratory chains encoded by two genomes prone to mismatch, with apoptosis being the default penalty for compromised respiration. Selection for genomic matching is facilitated by two sexes, and optimizes fitness, adaptability and fertility in youth. Mismatches cause infertility, low fitness, hybrid breakdown, and potentially speciation. The dynamics of selection for mitonuclear function optimize fitness over generations, but the same selective processes also operate within generations, driving ageing and age-related diseases. This coherent view of eukaryotic energetics offers striking insights into infertility and age-related diseases.

Fusing a lasting relationship between ER tubules

Atlastin is an integral membrane GTPase localized to the endoplasmic reticulum (ER). In vitro and in vivo analyses indicate that atlastin is a membrane fusogen capable of driving membrane fusion, suggesting a role in ER structure and maintenance. Interestingly, mutations in the human atlastin-1 gene, SPG3A, cause a form of autosomal dominant hereditary spastic paraplegia (HSP). The etiology of HSP is unclear, but two predominant forms of the disorder are caused by mutant proteins that affect ER structure, formation and maintenance in motor neurons. In this review, we describe the current knowledge about the molecular mechanism of atlastin function and its potential role in HSP. Greater understanding of the function of atlastin and associated proteins should provide important insight into normal ER biogenesis and maintenance, as well as the pathology of disease.

Peroxisome biogenesis: something old, something new, something borrowed

Eukaryotic cells are characterized by their varied complement of organelles. One set of membrane-bound, usually spherical compartments are commonly grouped together under the term peroxisomes. Peroxisomes function in regulating the synthesis and availability of many diverse lipids by harnessing the power of oxidative reactions and contribute to a number of metabolic processes essential for cellular differentiation and organismal development.

Loss of mitochondrial DNA in the yeast cardiolipin synthase crd1 mutant leads to up-regulation of the protein kinase Swe1p that regulates the G2/M transition

The anionic phospholipid cardiolipin and its precursor phosphatidylglycerol are synthesized and localized in the mitochondrial inner membrane of eukaryotes. They are required for structural integrity and optimal activities of a large number of mitochondrial proteins and complexes. Previous studies showed that loss of anionic phospholipids leads to cell inviability in the absence of mitochondrial DNA. However, the mechanism linking loss of anionic phospholipids to petite lethality was unclear. To elucidate the mechanism, we constructed a crd1Deltarho degrees mutant, which is viable and mimics phenotypes of pgs1Delta in the petite background. We found that loss of cardiolipin in rho degrees cells leads to elevated expression of Swe1p, a morphogenesis checkpoint protein. Moreover, the retrograde pathway is activated in crd1Deltarho degrees cells, most likely due to the exacerbation of mitochondrial dysfunction. Interestingly, the expression of SWE1 is dependent on retrograde regulation as elevated expression of SWE1 is suppressed by deletion of RTG2 or RTG3. Taken together, these findings indicate that activation of the retrograde pathway leads to up-regulation of SWE1 in crd1Deltarho degrees cells. These results suggest that anionic phospholipids are required for processes that are essential for normal cell division in rho degrees cells.

Mitochondrial dynamics

Mitochondrial dynamics is a key feature for the interaction of mitochondria with other organelles within a cell and also for the maintenance of their own integrity. Four types of mitochondrial dynamics are discussed: Movement within a cell and interactions with the cytoskeleton, fusion and fission events which establish coherence within the chondriome, the dynamic behavior of cristae and their components, and finally, formation and disintegration of mitochondria (mitophagy). Due to these essential functions, disturbed mitochondrial dynamics are inevitably connected to a variety of diseases. Localized ATP gradients, local control of calcium-based messaging, production of reactive oxygen species, and involvement of other metabolic chains, that is, lipid and steroid synthesis, underline that physiology not only results from biochemical reactions but, in addition, resides on the appropriate morphology and topography. These events and their molecular basis have been established recently and are the topic of this review.

Ccm1p/Ygr150cp, a pentatricopeptide repeat protein, is essential to remove the fourth intron of both COB and COX1 pre-mRNAs in Saccharomyces cerevisiae

YGR150C gene product (Ygr150cp) is one of the three mitochondrially located Saccharomyces cerevisiae proteins with pentatricopeptide repeat (PPR) motifs. Ygr150cp is essential for mitochondrial functionality but its molecular targets are still unknown. This study was undertaken to define the role of Ygr150cp in mitochondria biogenesis. Repression of Ygr150cp expression in complemented mutants prevented their use of glycerol or lactate, but allowed limited growth on ethanol-containing medium. RNA hybridization studies showed that Deltaygr150c meiotic segregants produced COB and COX1 transcripts but failed to process them into the mature forms. Detailed RT-PCR assays revealed that Deltaygr150c specifically failed to remove the fourth intron of both COB and COX1 pre-mRNAs while all other group I introns were excised. Expression of Ygr150cp mutants without any of the PPR motifs did not complement the growth phenotype. Accordingly, we designate YGR150C as CCM1 (COB and COX1 mRNA maturation). This report provides the first evidence of PPR protein involvement in the specific removal of group I introns in mitochondria of S. cerevisiae.

Genome-wide fitness and expression profiling implicate Mga2 in adaptation to hydrogen peroxide

Caloric restriction extends lifespan, an effect once thought to involve attenuation of reactive oxygen species (ROS) generated by aerobic metabolism. However, recent evidence suggests that caloric restriction may in fact raise ROS levels, which in turn provides protection from acute doses of oxidant through a process called adaptation. To shed light on the molecular mechanisms of adaptation, we designed a series of genome-wide deletion fitness and mRNA expression screens to identify genes involved in adaptation to hydrogen peroxide. Combined with known transcriptional interactions, the integrated data implicate Yap1 and Skn7 as central transcription factors of both the adaptive and acute oxidative responses. They also identify the transcription factors Mga2 and Rox1 as active exclusively in the adaptive response and show that Mga2 is essential for adaptation. These findings are striking because Mga2 and Rox1 have been thought to control the response to hypoxic, not oxidative, conditions. Expression profiling of mga2Δ and rox1Δ knockouts shows that these factors most strongly regulate targets in ergosterol, fatty-acid, and zinc metabolic pathways. Direct quantitation of ergosterol reveals that its basal concentration indeed depends on Mga2, but that Mga2 is not required for the decrease in ergosterol observed during adaptation.

Reactive oxygen species (ROS) damage a variety of structures within the cell, resulting in disease and aging. In a seemingly paradoxical effect termed adaptation, it is possible to prevent damage caused by ROS by pre-treating the cell with a small amount of oxidant. We studied this process in order to identify the mechanisms that provide this protection. Our study identified a number of genes and processes with previously unappreciated roles in adaptation. The mechanisms we identified are remarkable because they are distinct from those previously known to protect the cell from ROS. Although this study is conducted in yeast, the wide conservation of adaptation among many organisms suggests that the results from this study may be widely applicable.

Mitochondrial fragmentation is involved in methamphetamine-induced cell death in rat hippocampal neural progenitor cells

Methamphetamine (METH) induces neurodegeneration through damage and apoptosis of dopaminergic nerve terminals and striatal cells, presumably via cross-talk between the endoplasmic reticulum and mitochondria-dependent death cascades. However, the effects of METH on neural progenitor cells (NPC), an important reservoir for replacing neurons and glia during development and injury, remain elusive. Using a rat hippocampal NPC (rhNPC) culture, we characterized the METH-induced mitochondrial fragmentation, apoptosis, and its related signaling mechanism through immunocytochemistry, flow cytometry, and Western blotting. We observed that METH induced rhNPC mitochondrial fragmentation, apoptosis, and inhibited cell proliferation. The mitochondrial fission protein dynamin-related protein 1 (Drp1) and reactive oxygen species (ROS), but not calcium (Ca2+) influx, were involved in the regulation of METH-induced mitochondrial fragmentation. Furthermore, our results indicated that dysregulation of ROS contributed to the oligomerization and translocation of Drp1, resulting in mitochondrial fragmentation in rhNPC. Taken together, our data demonstrate that METH-mediated ROS generation results in the dysregulation of Drp1, which leads to mitochondrial fragmentation and subsequent apoptosis in rhNPC. This provides a potential mechanism for METH-related neurodegenerative disorders, and also provides insight into therapeutic strategies for the neurodegenerative effects of METH.

The cell-type specificity of mitochondrial dynamics

Recent advances in mitochondrial imaging have revealed that in many cells mitochondria can be highly dynamic. They can undergo fission/fusion processes modulated by various mitochondria-associated proteins and also by conformational transitions in the inner mitochondrial membrane. Moreover, precise mitochondrial distribution can be achieved by their movement along the cytoskeleton, recruiting various connector and motor proteins. Such movement is evident in various cell types ranging from yeast to mammalian cells and serves to direct mitochondria to cellular regions of high ATP demand or to transport mitochondria destined for elimination. Existing data also demonstrate that many aspects of mitochondrial dynamics, morphology, regulation and intracellular organization can be cell type-/tissue-specific. In many cells like neurons, pancreatic cells, HL-1 cells, etc., complex dynamics of mitochondria include fission, fusion, small oscillatory movements of mitochondria, larger movements like filament extension, retraction, fast branching in the mitochondrial network and rapid long-distance intracellular translocation of single mitochondria. Alternatively, mitochondria can be rather fixed in other cells and tissues like adult cardiomyocytes or skeletal muscles with a very regular organelle organization between myofibrils, providing the bioenergetic basis for contraction. Adult cardiac cells show no displacement of mitochondria with only very small-amplitude rapid vibrations, demonstrating remarkable, cell type-dependent differences in the dynamics and spatial arrangement of mitochondria. These variations and the cell-type specificity of mitochondrial dynamics could be related to specific cellular functions and demands, also indicating a significant role of integrations of mitochondria with other intracellular systems like the cytoskeleton, nucleus and endoplasmic reticulum (ER).

The mitochondrial cycle of Arabidopsis shoot apical meristem and leaf primordium meristematic cells is defined by a perinuclear tentaculate/cage-like mitochondrion

Plant cells exhibit a high rate of mitochondrial DNA (mtDNA) recombination. This implies that before cytokinesis, the different mitochondrial compartments must fuse to allow for mtDNA intermixing. When and how the conditions for mtDNA intermixing are established are largely unknown. We have investigated the cell cycle-dependent changes in mitochondrial architecture in different Arabidopsis (Arabidopsis thaliana) cell types using confocal microscopy, conventional, and three-dimensional electron microscopy techniques. Whereas mitochondria of cells from most plant organs are always small and dispersed, shoot apical and leaf primordial meristematic cells contain small, discrete mitochondria in the cell periphery and one large, mitochondrial mass in the perinuclear region. Serial thin-section reconstructions of high-pressure-frozen shoot apical meristem cells demonstrate that during G1 through S phase, the large, central mitochondrion has a tentaculate morphology and wraps around one nuclear pole. In G2, both types of mitochondria double their volume, and the large mitochondrion extends around the nucleus to establish a second sheet-like domain at the opposite nuclear pole. During mitosis, approximately 60% of the smaller mitochondria fuse with the large mitochondrion, whose volume increases to 80% of the total mitochondrial volume, and reorganizes into a cage-like structure encompassing first the mitotic spindle and then the entire cytokinetic apparatus. During cytokinesis, the cage-like mitochondrion divides into two independent tentacular mitochondria from which new, small mitochondria arise by fission. These cell cycle-dependent changes in mitochondrial architecture explain how these meristematic cells can achieve a high rate of mtDNA recombination and ensure the even partitioning of mitochondria between daughter cells.

Quantitative 3-D imaging of eukaryotic cells using soft X-ray tomography

Imaging has long been one of the principal techniques used in biological and biomedical research. Indeed, the field of cell biology grew out of the first electron microscopy images of organelles in a cell. Since this landmark event, much work has been carried out to image and classify the organelles in eukaryotic cells using electron microscopy. Fluorescently labeled organelles can now be tracked in live cells, and recently, powerful light microscope techniques have pushed the limit of optical resolution to image single molecules. In this paper, we describe the use of soft X-ray tomography, a new tool for quantitative imaging of organelle structure and distribution in whole, fully hydrated eukaryotic Schizosaccharomyces pombe cells. In addition to imaging intact cells, soft X-ray tomography has the advantage of not requiring the use of any staining or fixation protocols--cells are simply transferred from their growth environment to a sample holder and immediately cryofixed. In this way the cells can be imaged in a near native state. Soft X-ray tomography is also capable of imaging relatively large numbers of cells in a short period of time, and is therefore a technique that has the potential to produce information on organelle morphology from statistically significant numbers of cells.

The class V myosin motor protein, Myo2, plays a major role in mitochondrial motility in Saccharomyces cerevisiae

The actin cytoskeleton is essential for polarized, bud-directed movement of cellular membranes in Saccharomyces cerevisiae and thus ensures accurate inheritance of organelles during cell division. Also, mitochondrial distribution and inheritance depend on the actin cytoskeleton, though the precise molecular mechanisms are unknown. Here, we establish the class V myosin motor protein, Myo2, as an important mediator of mitochondrial motility in budding yeast. We found that mutants with abnormal expression levels of Myo2 or its associated light chain, Mlc1, exhibit aberrant mitochondrial morphology and loss of mitochondrial DNA. Specific mutations in the globular tail of Myo2 lead to aggregation of mitochondria in the mother cell. Isolated mitochondria lacking functional Myo2 are severely impaired in their capacity to bind to actin filaments in vitro. Time-resolved fluorescence microscopy revealed a block of bud-directed anterograde mitochondrial movement in cargo binding–defective myo2 mutant cells. We conclude that Myo2 plays an important and direct role for mitochondrial motility and inheritance in budding yeast.

Dissection of the carboxyl-terminal domain of the proteasomal subunit Rpn11 in maintenance of mitochondrial structure and function

We have previously demonstrated that the C-terminal part of Rpn11, a deubiquitinating enzyme in the lid of the proteasome, is essential for maintaining a correct cell cycle and normal mitochondrial morphology and function. The two roles are apparently unlinked as the mitochondrial role is mapped to the Carboxy-terminus, whereas the catalytic deubiquitinating activity is found within the N-terminal region. The mitochondrial defects are observed in rpn11-m1 (originally termed mpr1-1), a mutation that generates Rpn11 lacking the last 31 amino acids. No mitochondrial phenotypes are recorded for mutations in the MPN+/JAMM motif. In the present study, we investigated the participation of the last 31 amino acids of the Rpn11 protein by analysis of intragenic revertants and site-specific mutants. We identified a putative alpha-helix necessary for the maintenance of a correct cell cycle and determined that a very short region at the C-terminus of Rpn11 is essential for the maintenance of tubular mitochondrial morphology. Furthermore, we show that expression of the C-terminal part of Rpn11 is able to complement in trans all of the rpn11-m1 mitochondrial phenotypes. Finally, we investigate the mechanisms by which Rpn11 controls the mitochondrial shape and show that Rpn11 may regulate the mitochondrial fission and tubulation processes.