Active segregation of yeast mitochondria by Myo2 is essential and mediated by Mmr1 and Ypt11

The interactions of subcellular organelles in pulmonary fibrosis induced by carbon black nanoparticles: a comprehensive review

Owing to the widespread use and improper emissions of carbon black nanoparticles (CBNPs), the adverse effects of CBNPs on human health have attracted much attention. In toxicological research, carbon black is frequently utilized as a negative control because of its low toxicity and poor solubility. However, recent studies have indicated that inhalation exposure to CBNPs could be a risk factor for severe and prolonged pulmonary inflammation and fibrosis. At present, the pathogenesis of pulmonary fibrosis induced by CBNPs is still not fully elucidated, but it is known that with small particle size and large surface area, CBNPs are more easily ingested by cells, leading to organelle damage and abnormal interactions between organelles. Damaged organelle and abnormal organelles interactions lead to cell structure and function disorders, which is one of the important factors in the development and occurrence of various diseases, including pulmonary fibrosis. This review offers a comprehensive analysis of organelle structure, function, and interaction mechanisms, while also summarizing the research advancements in organelles and organelle interactions in CBNPs-induced pulmonary fibrosis.

E3 Ligases Regulate Organelle Inheritance in Yeast

Saccharomyces cerevisiae proliferates by budding, which includes the formation of a cytoplasmic protrusion called the 'bud', into which DNA, RNA, proteins, organelles, and other materials are transported. The transport of organelles into the growing bud must be strictly regulated for the proper inheritance of organelles by daughter cells. In yeast, the RING-type E3 ubiquitin ligases, Dma1 and Dma2, are involved in the proper inheritance of mitochondria, vacuoles, and presumably peroxisomes. These organelles are transported along actin filaments toward the tip of the growing bud by the myosin motor protein, Myo2. During organelle transport, organelle-specific adaptor proteins, namely Mmr1, Vac17, and Inp2 for mitochondria, vacuoles, and peroxisomes, respectively, bridge the organelles and myosin. After reaching the bud, the adaptor proteins are ubiquitinated by the E3 ubiquitin ligases and degraded by the proteasome. Targeted degradation of the adaptor proteins is necessary to unload vacuoles, mitochondria, and peroxisomes from the actin-myosin machinery. Impairment of the ubiquitination of adaptor proteins results in the failure of organelle release from myosin, which, in turn, leads to abnormal dynamics, morphology, and function of the inherited organelles, indicating the significance of proper organelle unloading from myosin. Herein, we summarize the role and regulation of E3 ubiquitin ligases during organelle inheritance in yeast.

Mitochondrial protein and organelle quality control-Lessons from budding yeast

Mitochondria are essential for normal cellular function and have emerged as key aging determinants. Indeed, defects in mitochondrial function have been linked to cardiovascular, skeletal muscle and neurodegenerative diseases, premature aging, and age-linked diseases. Here, we describe mechanisms for mitochondrial protein and organelle quality control. These surveillance mechanisms mediate repair or degradation of damaged or mistargeted mitochondrial proteins, segregate mitochondria based on their functional state during asymmetric cell division, and modulate cellular fitness, the response to stress, and lifespan control in yeast and other eukaryotes.

Selective retention of dysfunctional mitochondria during asymmetric cell division in yeast

Decline of mitochondrial function is a hallmark of cellular aging. To counteract this process, some cells inherit mitochondria asymmetrically to rejuvenate daughter cells. The molecular mechanisms that control this process are poorly understood. Here, we made use of matrix-targeted D-amino acid oxidase (Su9-DAO) to selectively trigger oxidative damage in yeast mitochondria. We observed that dysfunctional mitochondria become fusion-incompetent and immotile. Lack of bud-directed movements is caused by defective recruitment of the myosin motor, Myo2. Intriguingly, intact mitochondria that are present in the same cell continue to move into the bud, establishing that quality control occurs directly at the level of the organelle in the mother. The selection of healthy organelles for inheritance no longer works in the absence of the mitochondrial Myo2 adapter protein Mmr1. Together, our data suggest a mechanism in which the combination of blocked fusion and loss of motor protein ensures that damaged mitochondria are retained in the mother cell to ensure rejuvenation of the bud.

The multiple links between actin and mitochondria

Actin plays many well-known roles in cells, and understanding any specific role is often confounded by the overlap of multiple actin-based structures in space and time. Here, we review our rapidly expanding understanding of actin in mitochondrial biology, where actin plays multiple distinct roles, exemplifying the versatility of actin and its functions in cell biology. One well-studied role of actin in mitochondrial biology is its role in mitochondrial fission, where actin polymerization from the endoplasmic reticulum through the formin INF2 has been shown to stimulate two distinct steps. However, roles for actin during other types of mitochondrial fission, dependent on the Arp2/3 complex, have also been described. In addition, actin performs functions independent of mitochondrial fission. During mitochondrial dysfunction, two distinct phases of Arp2/3 complex-mediated actin polymerization can be triggered. First, within 5 min of dysfunction, rapid actin assembly around mitochondria serves to suppress mitochondrial shape changes and to stimulate glycolysis. At a later time point, at more than 1 h post-dysfunction, a second round of actin polymerization prepares mitochondria for mitophagy. Finally, actin can both stimulate and inhibit mitochondrial motility depending on the context. These motility effects can either be through the polymerization of actin itself or through myosin-based processes, with myosin 19 being an important mitochondrially attached myosin. Overall, distinct actin structures assemble in response to diverse stimuli to affect specific changes to mitochondria.

Mitochondria and the cell cycle in budding yeast

As centers for energy production and essential biosynthetic activities, mitochondria are vital for cell growth and proliferation. Accumulating evidence suggests an integrated regulation of these organelles and the nuclear cell cycle in distinct organisms. In budding yeast, a well-established example of this coregulation is the coordinated movement and positional control of mitochondria during the different phases of the cell cycle. The molecular determinants involved in the inheritance of the fittest mitochondria by the bud also seem to be cell cycle-regulated. In turn, loss of mtDNA or defects in mitochondrial structure or inheritance often lead to a cell cycle delay or arrest, indicating that mitochondrial function can also regulate cell cycle progression, possibly through the activation of cell cycle checkpoints. The up-regulation of mitochondrial respiration at G2/M, presumably to fulfil energetic requirements for progression at this phase, also supports a mitochondria-cell cycle interplay. Cell cycle-linked mitochondrial regulation is accomplished at the transcription level and through post-translational modifications, predominantly protein phosphorylation. Here, we address mitochondria-cell cycle interactions in the yeast Saccharomyces cerevisiae and discuss future challenges in the field.

Generation and characterization of conditional yeast mutants affecting each of the 2 essential functions of the scaffolding proteins Boi1/2 and Bem1

Boi1 and Boi2 are closely related yeast scaffolding proteins, either of which can perform an essential function. Previous studies have suggested a role in cell polarity, interacting with lipids, components of the late secretory pathway, and actin nucleators. We report detailed studies of their localization, dynamics, and the generation and characterization of conditional mutants. Boi1/2 are present on the plasma membrane in dynamic patches, then at the bud neck during cytokinesis. These distributions are unaffected by perturbation of the actin cytoskeleton or the secretory pathway. We identify 2 critical aromatic residues, present in both Boi1 and Boi2, in the essential C-terminal Pleckstrin-Homology domain, that cause temperature-sensitive growth resulting in defects in polarized growth leading to cell lysis. The scaffolding protein, Bem1, colocalizes with Boi1 in patches at the growing bud, and at the bud neck, the latter requiring the N-terminal SH3 domain of Boi1p. Loss of function of Boi1-SH3 domain renders Bem1 essential, which can be fully replaced by a fusion of the SH3b and PB1 domains of Bem1. Thus, the 2 essential functions of the Boi1/2/Bem1 proteins can be satisfied by Bem1-SH3b-PB1 and Boi1-Pleckstrin-Homology. Generation and characterization of conditional mutations in the essential function of Bem1 reveal a slow onset of defects in polarized growth, which is difficult to define a specific initial defect. This study provides more details into the functions of Boi1/2 and their relationship with Bem1 and presents the generation of conditional mutants that will be useful for future genetic analysis.

Proteolysis of adaptor protein Mmr1 during budding is necessary for mitochondrial homeostasis in Saccharomyces cerevisiae

In yeast, mitochondria are passed on to daughter cells via the actin cable, motor protein Myo2, and adaptor protein Mmr1. They are released from the actin-myosin machinery after reaching the daughter cells. We report that Mmr1 is rapidly degraded by the ubiquitin-proteasome system in Saccharomyces cerevisiae. Redundant ubiquitin ligases Dma1 and Dma2 are responsible for Mmr1 ubiquitination. Dma1/2-mediated Mmr1 ubiquitination requires phosphorylation, most likely at S414 residue by Ste20 and Cla4. These kinases are mostly localized to the growing bud and nearly absent from mother cells, ensuring phosphorylation and ubiquitination of Mmr1 after the mitochondria enter the growing bud. In dma1Δ dma2Δ cells, transported mitochondria are first stacked at the bud-tip and then pulled back to the bud-neck. Stacked mitochondria in dma1Δ dma2Δ cells exhibit abnormal morphology, elevated respiratory activity, and increased level of reactive oxygen species, along with hypersensitivity to oxidative stresses. Collectively, spatiotemporally-regulated Mmr1 turnover guarantees mitochondrial homeostasis.

A role for cell polarity in lifespan and mitochondrial quality control in the budding yeast Saccharomyces cerevisiae

Babies are born young, largely independent of the age of their mothers. Mother-daughter age asymmetry in yeast is achieved, in part, by inheritance of higher-functioning mitochondria by buds and retention of some high-functioning mitochondria in mother cells. The mitochondrial F box protein, Mfb1p, tethers mitochondria at both poles in a cell cycle-regulated manner: it localizes to and anchors mitochondria at the mother cell tip throughout the cell cycle and at the bud tip before cytokinesis. Here, we report that cell polarity and polarized localization of Mfb1p decline with age in Saccharomyces cerevisiae. Moreover, deletion of genes (BUD1, BUD2, and BUD5) that mediate symmetry breaking during establishment of cell polarity and asymmetric yeast cell division cause depolarized Mfb1p localization and defects in mitochondrial distribution and quality control. Our results support a role for the polarity machinery in lifespan through modulating Mfb1 function in asymmetric inheritance of mitochondria during yeast cell division.

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Budding polarity declines with age

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Polarization of a mitochondrial tether, Mfb1p, within mother cells declines with age

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Defects in budding polarity disrupt Mfb1p polarization and mitochondrial distribution

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Polarity defects affect Mfb1p-mediated mitochondrial quality and lifespan control

Hierarchical integration of mitochondrial and nuclear positioning pathways by the Num1 EF hand

Positioning organelles at the right place and time is critical for their function and inheritance. In budding yeast, mitochondrial and nuclear positioning require the anchoring of mitochondria and dynein to the cell cortex by clusters of Num1. We have previously shown that mitochondria drive the assembly of cortical Num1 clusters, which then serve as anchoring sites for mitochondria and dynein. When mitochondrial inheritance is inhibited, mitochondrial-driven assembly of Num1 in buds is disrupted and defects in dynein-mediated spindle positioning are observed. Using a structure-function approach to dissect the mechanism of mitochondria-dependent dynein anchoring, we found that the EF hand–like motif (EFLM) of Num1 and its ability to bind calcium are required to bias dynein anchoring on mitochondria-associated Num1 clusters. Consistently, when the EFLM is disrupted, we no longer observe defects in dynein activity following inhibition of mitochondrial inheritance. Thus, the Num1 EFLM functions to bias dynein anchoring and activity in nuclear inheritance subsequent to mitochondrial inheritance. We hypothesize that this hierarchical integration of organelle positioning pathways by the Num1 EFLM contributes to the regulated order of organelle inheritance during the cell cycle.

Power to the daughters - mitochondrial and mtDNA transmission during cell division

Mitochondria supply virtually all eukaryotic cells with energy through ATP production by oxidative phosphoryplation (OXPHOS). Accordingly, maintenance of mitochondrial function is fundamentally important to sustain cellular health and various diseases have been linked to mitochondrial dysfunction. Biogenesis of OXPHOS complexes crucially depends on mitochondrial DNA (mtDNA) that encodes essential subunits of the respiratory chain and is distributed in multiple copies throughout the mitochondrial network. During cell division, mitochondria, including mtDNA, need to be accurately apportioned to daughter cells. This process requires an intimate and coordinated interplay between the cell cycle, mitochondrial dynamics and the replication and distribution of mtDNA. Recent years have seen exciting advances in the elucidation of the mechanisms that facilitate these processes and essential key players have been identified. Moreover, segregation of qualitatively distinct mitochondria during asymmetric cell division is emerging as an important quality control step, which secures the maintenance of a healthy cell population.

Roles and regulation of myosin V interaction with cargo

A major question in cell biology is, how are organelles and large macromolecular complexes transported within a cell? Myosin V molecular motors play critical roles in the distribution of organelles, vesicles, and mRNA. Mis-localization of organelles that depend on myosin V motors underlie diseases in the skin, gut, and brain. Thus, the delivery of organelles to their proper destination is important for animal physiology and cellular function. Cargoes attach to myosin V motors via cargo specific adaptor proteins, which transiently bridge motors to their cargoes. Regulation of these adaptor proteins play key roles in the regulation of cargo transport. Emerging studies reveal that cargo adaptors play additional essential roles in the activation of myosin V, and the regulation of actin filaments. Here, we review how motor-adaptor interactions are controlled to regulate the proper loading and unloading of cargoes, as well as roles of adaptor proteins in the regulation of myosin V activity and the dynamics of actin filaments.

Dynamic Regulation of Peroxisomes and Mitochondria during Fungal Development

Peroxisomes and mitochondria are organelles that perform major functions in the cell and whose activity is very closely associated. In fungi, the function of these organelles is critical for many developmental processes. Recent studies have disclosed that, additionally, fungal development comprises a dynamic regulation of the activity of these organelles, which involves a developmental regulation of organelle assembly, as well as a dynamic modulation of the abundance, distribution, and morphology of these organelles. Furthermore, for many of these processes, the dynamics of peroxisomes and mitochondria are governed by common factors. Notably, intense research has revealed that the process that drives the division of mitochondria and peroxisomes contributes to several developmental processes-including the formation of asexual spores, the differentiation of infective structures by pathogenic fungi, and sexual development-and that these processes rely on selective removal of these organelles via autophagy. Furthermore, evidence has been obtained suggesting a coordinated regulation of organelle assembly and dynamics during development and supporting the existence of regulatory systems controlling fungal development in response to mitochondrial activity. Gathered information underscores an important role for mitochondrial and peroxisome dynamics in fungal development and suggests that this process involves the concerted activity of these organelles.

Principles and mechanisms of asymmetric cell division

Asymmetric cell division (ACD) is an evolutionarily conserved mechanism used by prokaryotes and eukaryotes alike to control cell fate and generate cell diversity. A detailed mechanistic understanding of ACD is therefore necessary to understand cell fate decisions in health and disease. ACD can be manifested in the biased segregation of macromolecules, the differential partitioning of cell organelles, or differences in sibling cell size or shape. These events are usually preceded by and influenced by symmetry breaking events and cell polarization. In this Review, we focus predominantly on cell intrinsic mechanisms and their contribution to cell polarization, ACD and binary cell fate decisions. We discuss examples of polarized systems and detail how polarization is established and, whenever possible, how it contributes to ACD. Established and emerging model organisms will be considered alike, illuminating both well-documented and underexplored forms of polarization and ACD.

Asymmetric inheritance of mitochondria in yeast

Mitochondria are essential organelles of virtually all eukaryotic organisms. As they cannot be made de novo, they have to be inherited during cell division. In this review, we provide an overview on mitochondrial inheritance in Saccharomyces cerevisiae, a powerful model organism to study asymmetric cell division. Several processes have to be coordinated during mitochondrial inheritance: mitochondrial transport along the actin cytoskeleton into the emerging bud is powered by a myosin motor protein; cell cortex anchors retain a critical fraction of mitochondria in the mother cell and bud to ensure proper partitioning; and the quantity of mitochondria inherited by the bud is controlled during cell cycle progression. Asymmetric division of yeast cells produces rejuvenated daughter cells and aging mother cells that die after a finite number of cell divisions. We highlight the critical role of mitochondria in this process and discuss how asymmetric mitochondrial partitioning and cellular aging are connected.

The adaptor protein melanophilin regulates dynamic myosin-Va:cargo interaction and dendrite development in melanocytes

The regulation of organelle transport by the cytoskeleton is fundamental for eukaryotic survival. Cytoskeleton motors are typically modular proteins with conserved motor and diverse cargo-binding domains. Motor:cargo interactions are often indirect and mediated by adaptor proteins, for example, Rab GTPases. Rab27a, via effector melanophilin (Mlph), recruits myosin-Va (MyoVa) to melanosomes and thereby disperses them into melanocyte dendrites. To better understand how adaptors regulate motor:cargo interaction, we used single melanosome fluorescence recovery after photobleaching (smFRAP) to characterize the association kinetics among MyoVa, its adaptors, and melanosomes. We found that MyoVa and Mlph rapidly recovered after smFRAP, whereas Rab27a did not, indicating that MyoVa and Mlph dynamically associate with melanosomes and Rab27a does not. This suggests that dynamic Rab27a:effector interaction rather than Rab27a melanosome:cytosol cycling regulates MyoVa:melanosome association. Accordingly, a Mlph-Rab27a fusion protein reduced MyoVa smFRAP, indicating that it stabilized melanosomal MyoVa. Finally, we tested the functional importance of dynamic MyoVa:melanosome interaction. We found that whereas a MyoVa-Rab27a fusion protein dispersed melanosomes in MyoVa-deficient cells, dendrites were significantly less elongated than in wild-type cells. Given that dendrites are the prime sites of melanosome transfer from melanocytes to keratinocytes, we suggest that dynamic MyoVa:melanosome interaction is important for pigmentation in vivo.

The actin motor MYO-5 effect in the intracellular organization of Neurospora crassa

In filamentous fungi, polarized growth is the result of vesicle secretion at the hyphal apex. Motor proteins mediate vesicle transport to target destinations on the plasma membrane via actin and microtubule cytoskeletons. Myosins are motor proteins associated with actin filaments. Specifically, class V myosins are responsible for cargo transport in eukaryotes. We studied the dynamics and localization of myosin V in wild type hyphae of Neurospora crassa and in hyphae that lacked MYO-5. In wild type hyphae, MYO-5-GFP was localized concentrated in the hyphal apex and colocalized with Spitzenkörper. Photobleaching studies showed that MYO-5-GFP was transported to the apex from subapical hyphal regions. The deletion of the class V myosin resulted in a reduced rate of hyphal growth, apical hyperbranching, and intermittent loss of hyphal polarity. MYO-5 did not participate in breaking the symmetrical growth during germination but contributed in the apical organization upon establishment of polarized growth. In the Δmyo-5 mutant, actin was organized into thick cables in the apical and subapical hyphal regions, and the number of endocytic patches was reduced. The microvesicles-chitosomes observed with CHS-1-GFP were distributed as a cloud occupying the apical dome and not in the Spitzenkörper as the WT strain. The mitochondrial movement was not associated with MYO-5, but tubular vacuole position is MYO-5-dependent. These results suggest that MYO-5 plays a role in maintaining apical organization and the integrity of the Spitzenkörper and is required for normal hyphal growth, polarity, septation, conidiation, and proper conidial germination.

Direct membrane binding and self-interaction contribute to Mmr1 function in mitochondrial inheritance

Mitochondrial transport and anchoring mechanisms work in concert to position mitochondria to meet cellular needs. In yeast, Mmr1 functions as a mitochondrial adaptor for Myo2 to facilitate actin-based transport of mitochondria to the bud. Posttransport, Mmr1 is proposed to anchor mitochondria at the bud tip. Although both functions require an interaction between Mmr1 and mitochondria, the molecular basis of the Mmr1-mitochondria interaction is poorly understood. Our in vitro phospholipid binding assays indicate Mmr1 can directly interact with phospholipid membranes. Through structure-function studies we identified an unpredicted membrane-binding domain composed of amino acids 76-195 that is both necessary and sufficient for Mmr1 to interact with mitochondria in vivo and liposomes in vitro. In addition, our structure-function analyses indicate that the coiled-coil domain of Mmr1 is necessary and sufficient for Mmr1 self-interaction and facilitates the polarized localization of the protein. Disrupting either the Mmr1-membrane interaction or Mmr1 self-interaction leads to defects in mitochondrial inheritance. Therefore, direct membrane binding and self-interaction are necessary for Mmr1 function in mitochondrial inheritance and are utilized as a means to spatially and temporally regulate mitochondrial positioning.

Yeast Aim21/Tda2 both regulates free actin by reducing barbed end assembly and forms a complex with Cap1/Cap2 to balance actin assembly between patches and cables

Yeast Aim21 is recruited by the SH3-containing proteins Bbc1 and Abp1 to patches and, with Tda2, reduces barbed end assembly to balance the distribution of actin between patches and cables. Aim21/Tda2 also interacts with Cap1/Cap2, revealing a complex interplay between actin assembly regulators.

How cells balance the incorporation of actin into diverse structures is poorly understood. In budding yeast, a single actin monomer pool is used to build both actin cables involved in polarized growth and actin cortical patches involved in endocytosis. Here we report how Aim21/Tda2 is recruited to the cortical region of actin patches, where it negatively regulates actin assembly to elevate the available actin monomer pool. Aim21 has four polyproline regions and is recruited by two SH3-containing patch proteins, Bbc1 and Abp1. The C-terminal region, which is required for its function, binds Tda2. Cell biological and biochemical data reveal that Aim21/Tda2 is a negative regulator of barbed end filamentous actin (F-actin) assembly, and this activity is necessary for efficient endocytosis and plays a pivotal role in balancing the distribution of actin between cables and patches. Aim21/Tda2 also forms a complex with the F-actin barbed end capping protein Cap1/Cap2, revealing an interplay between regulators and showing the complexity of regulation of barbed end assembly.

Mitochondria-driven assembly of a cortical anchor for mitochondria and dynein

Kraft and Lackner demonstrate that mitochondria drive the assembly of a tether, which serves to stably anchor the organelle itself as well as dynein to the plasma membrane. Thus, mitochondria–plasma membrane tethering influences when and where dynein is anchored, adding to the growing list of interorganelle contact site functions.

Interorganelle contacts facilitate communication between organelles and impact fundamental cellular functions. In this study, we examine the assembly of the MECA (mitochondria–endoplasmic reticulum [ER]–cortex anchor), which tethers mitochondria to the ER and plasma membrane. We find that the assembly of Num1, the core component of MECA, requires mitochondria. Once assembled, Num1 clusters persistently anchor mitochondria to the cell cortex. Num1 clusters also function to anchor dynein to the plasma membrane, where dynein captures and walks along astral microtubules to help orient the mitotic spindle. We find that dynein is anchored by Num1 clusters that have been assembled by mitochondria. When mitochondrial inheritance is inhibited, Num1 clusters are not assembled in the bud, and defects in dynein-mediated spindle positioning are observed. The mitochondria-dependent assembly of a dual-function cortical anchor provides a mechanism to integrate the positioning and inheritance of the two essential organelles and expands the function of organelle contact sites.

Analysis of mitochondrial organization and function in the Drosophila blastoderm embryo

Mitochondria are inherited maternally as globular and immature organelles in metazoan embryos. We have used the Drosophila blastoderm embryo to characterize their morphology, distribution and functions in embryogenesis. We find that mitochondria are relatively small, dispersed and distinctly distributed along the apico-basal axis in proximity to microtubules by motor protein transport. Live imaging, photobleaching and photoactivation analyses of mitochondrially targeted GFP show that they are mobile in the apico-basal axis along microtubules and are immobile in the lateral plane thereby associating with one syncytial cell. Photoactivated mitochondria distribute equally to daughter cells across the division cycles. ATP depletion by pharmacological and genetic inhibition of the mitochondrial electron transport chain (ETC) activates AMPK and decreases syncytial metaphase furrow extension. In summary, we show that small and dispersed mitochondria of the Drosophila blastoderm embryo localize by microtubule transport and provide ATP locally for the fast syncytial division cycles. Our study opens the possibility of use of Drosophila embryogenesis as a model system to study the impact of maternal mutations in mitochondrial morphology and metabolism on embryo patterning and differentiation.

Mitochondrial anchors: Positioning mitochondria and more

The shape and position of mitochondria are intimately connected to both mitochondrial and cellular function. Mitochondrial anchors play a central role in mitochondrial positioning by exerting spatial, temporal, and contextual control over the cellular position of the organelle. Investigations into the molecular mechanisms of mitochondrial anchoring are still in the early stages, and we are beginning to appreciate the number and variety of anchors that exist. From the insight gained thus far, it is clear that mitochondrial anchoring has functional and physiological consequences that extend beyond mitochondrial positioning to other critical cellular processes.

Fusion, fission, and transport control asymmetric inheritance of mitochondria and protein aggregates

Asymmetric inheritance of cell organelles determines the fate of daughter cells. Böckler et al. use yeast as a model to demonstrate that the dynamics of mitochondrial fusion, fission, and transport determine partitioning of mitochondria and cytosolic protein aggregates, which is critical for rejuvenation of daughter cells.

Partitioning of cell organelles and cytoplasmic components determines the fate of daughter cells upon asymmetric division. We studied the role of mitochondria in this process using budding yeast as a model. Anterograde mitochondrial transport is mediated by the myosin motor, Myo2. A genetic screen revealed an unexpected interaction of MYO2 and genes required for mitochondrial fusion. Genetic analyses, live-cell microscopy, and simulations in silico showed that fused mitochondria become critical for inheritance and transport across the bud neck in myo2 mutants. Similarly, fused mitochondria are essential for retention in the mother when bud-directed transport is enforced. Inheritance of a less than critical mitochondrial quantity causes a severe decline of replicative life span of daughter cells. Myo2-dependent mitochondrial distribution also is critical for the capture of heat stress–induced cytosolic protein aggregates and their retention in the mother cell. Together, these data suggest that coordination of mitochondrial transport, fusion, and fission is critical for asymmetric division and rejuvenation of daughter cells.

Organization and dynamics of yeast mitochondrial nucleoids

Mitochondrial DNA (mtDNA) is packaged by association with specific proteins in compact DNA-protein complexes named mitochondrial nucleoids (mt-nucleoids). The budding yeast Saccharomyces cerevisiae is able to grow either aerobically or anaerobically. Due to this characteristic, S. cerevisiae has been extensively used as a model organism to study genetics, morphology and biochemistry of mitochondria for a long time. Mitochondria of S. cerevisiae frequently fuse and divide, and perform dynamic morphological changes depending on the culture conditions and the stage of life cycle of the yeast cells. The mt-nucleoids also dynamically change their morphology, accompanying morphological changes of mitochondria. The mt-nucleoids have been isolated morphologically intact and functional analyses of mt-nucleoid proteins have been extensively performed. These studies have revealed that the functions of mt-nucleoid proteins are essential for maintenance of mtDNA. The aims of this review are to summarize the history on the research of yeast mt-nucleoids as well as recent findings on the organization of the mt-nucleoids and mitochondrial dynamics.

Kinesin-related Smy1 enhances the Rab-dependent association of myosin-V with secretory cargo

Smy1 is a kinesin-related protein that enhances the association of the Myo2 myosin-V motor with its receptor, the Rab Sec4, on secretory vesicles. This function requires Smy1’s head, coiled-coil, and tail domains and is specific for secretory vesicle transport but not for mitochondrial segregation by Myo2, which also uses a Rab protein, Ypt11.

The mechanisms by which molecular motors associate with specific cargo is a central problem in cell organization. The kinesin-like protein Smy1 of budding yeast was originally identified by the ability of elevated levels to suppress a conditional myosin-V mutation (myo2-66), but its function with Myo2 remained mysterious. Subsequently, Myo2 was found to provide an essential role in delivery of secretory vesicles for polarized growth and in the transport of mitochondria for segregation. By isolating and characterizing myo2 smy1 conditional mutants, we uncover the molecular function of Smy1 as a factor that enhances the association of Myo2 with its receptor, the Rab Sec4, on secretory vesicles. The tail of Smy1—which binds Myo2—its central dimerization domain, and its kinesin-like head domain are all necessary for this function. Consistent with this model, overexpression of full-length Smy1 enhances the number of Sec4 receptors and Myo2 motors per transporting secretory vesicle. Rab proteins Sec4 and Ypt11, receptors for essential transport of secretory vesicles and mitochondria, respectively, bind the same region on Myo2, yet Smy1 functions selectively in the transport of secretory vesicles. Thus a kinesin-related protein can function intimately with a myosin-V and its receptor in the transport of a specific cargo.

Dynamics of the mitochondrial network during mitosis

During mitosis, cells undergo massive deformation and reorganization, impacting on all cellular structures. Mitochondria, in particular, are highly dynamic organelles, which constantly undergo events of fission, fusion and cytoskeleton-based transport. This plasticity ensures the proper distribution of the metabolism, and the proper inheritance of functional organelles. During cell cycle, mitochondria undergo dramatic changes in distribution. In this review, we focus on the dynamic events that target mitochondria during mitosis. We describe how the cell-cycle-dependent microtubule-associated protein centromeric protein F (Cenp-F) is recruited to mitochondria by the mitochondrial Rho GTPase (Miro) to promote mitochondrial transport and re-distribution following cell division.

A role for Mfb1p in region-specific anchorage of high-functioning mitochondria and lifespan in Saccharomyces cerevisiae

Previous studies indicate that replicative lifespan in daughter cells of Sacchraromyces cerevisiae depends on the preferential inheritance of young, high-functioning mitochondria. We report here that mitochondria are functionally segregated even within single mother cells in S. cerevisiae. A high-functioning population of mitochondria accumulates at the tip of the mother cell distal to the bud. We find that the mitochondrial F-box protein (Mfb1p) localizes to mitochondria in the mother tip and is required for mitochondrial anchorage at that site, independent of the previously identified anchorage protein Num1p. Deletion of MFB1 results in loss of the mother-tip-localized mitochondrial population, defects in mitochondrial function and premature replicative ageing. Inhibiting mitochondrial inheritance to buds, by deletion of MMR1, in mfb1Δ cells restores mitochondrial distribution, promotes mitochondrial function and extends replicative lifespan. Our results identify a mechanism that retains a reservoir of high-functioning mitochondria in mother cells and thereby preserves maternal reproductive capacity.

Mitochondria are asymmetrically inherited during cell division, a process that can affect cell fate and lifespan. Here the authors describe a mechanism for mitochondrial quality control in yeast that maintains a reservoir of high-functioning mitochondria in mother cells and preserves maternal reproductive capacity.

Mitochondrial anchorage and fusion contribute to mitochondrial inheritance and quality control in the budding yeast Saccharomyces cerevisiae

Fzo1p contributes to mitochondrial inheritance by fusion of mitochondria that enter the bud to mitochondria that are anchored in the bud tip. This promotes retention of mitochondria in the bud tip. However, it also promotes anchorage of lower-functioning mitochondria in the bud tip, which inhibits clearance of those organelles from buds.

Higher-functioning mitochondria that are more reduced and have less ROS are anchored in the yeast bud tip by the Dsl1-family protein Mmr1p. Here we report a role for mitochondrial fusion in bud-tip anchorage of mitochondria. Fluorescence loss in photobleaching (FLIP) and network analysis experiments revealed that mitochondria in large buds are a continuous reticulum that is physically distinct from mitochondria in mother cells. FLIP studies also showed that mitochondria that enter the bud can fuse with mitochondria that are anchored in the bud tip. In addition, loss of fusion and mitochondrial DNA (mtDNA) by deletion of mitochondrial outer or inner membrane fusion proteins (Fzo1p or Mgm1p) leads to decreased accumulation of mitochondria at the bud tip and inheritance of fitter mitochondria by buds compared with cells with no mtDNA. Conversely, increasing the accumulation and anchorage of mitochondria in the bud tip by overexpression of MMR1 results in inheritance of less-fit mitochondria by buds and decreased replicative lifespan and healthspan. Thus quantity and quality of mitochondrial inheritance are ensured by two opposing processes: bud-tip anchorage by mitochondrial fusion and Mmr1p, which favors bulk inheritance; and quality control mechanisms that promote segregation of fitter mitochondria to the bud.

Sharing with your children: Mechanisms of peroxisome inheritance

Organelle inheritance is the process by which eukaryotic cells actively replicate and equitably partition their organelles between mother cell and daughter cell at cytokinesis to maintain the benefits of subcellular compartmentalization. The budding yeast Saccharomyces cerevisiae has proven invaluable in helping to define the factors involved in the inheritance of different organelles and in understanding how these factors act and interact to maintain balance in the organelle populations of actively dividing cells. Inheritance factors can be classified as motors that transport organelles, tethers that retain organelles, and connectors (receptors) that mediate the attachment of organelles to motors and anchors. This article will review how peroxisomes are inherited by cells, with a focus on budding yeast, and will discuss common themes and mechanisms of action that underlie the inheritance of all membrane-enclosed organelles.

Motors, anchors, and connectors: orchestrators of organelle inheritance

Organelle inheritance is a process whereby organelles are actively distributed between dividing cells at cytokinesis. Much valuable insight into the molecular mechanisms of organelle inheritance has come from the analysis of asymmetrically dividing cells, which transport a portion of their organelles to the bud while retaining another portion in the mother cell. Common principles apply to the inheritance of all organelles, although individual organelles use specific factors for their partitioning. Inheritance factors can be classified as motors, which are required for organelle transport; anchors, which immobilize organelles at distinct cell structures; or connectors, which mediate the attachment of organelles to motors and anchors. Here, we provide an overview of recent advances in the field of organelle inheritance and highlight how motor, anchor, and connector molecules choreograph the segregation of a multicopy organelle, the peroxisome. We also discuss the role of organelle population control in the generation of cellular diversity.

Myosin-Powered Membrane Compartment Drives Cytoplasmic Streaming, Cell Expansion and Plant Development

Using genetic approaches, particle image velocimetry and an inert tracer of cytoplasmic streaming, we have made a mechanistic connection between the motor proteins (myosins XI), cargo transported by these motors (distinct endomembrane compartment defined by membrane-anchored MyoB receptors) and the process of cytoplasmic streaming in plant cells. It is shown that the MyoB compartment in Nicotiana benthamiana is highly dynamic moving with the mean velocity of ~3 μm/sec. In contrast, Golgi, mitochondria, peroxisomes, carrier vesicles and a cytosol flow tracer share distinct velocity profile with mean velocities of 0.6-1.5 μm/sec. Dominant negative inhibition of the myosins XI or MyoB receptors using overexpression of the N. benthamiana myosin cargo-binding domain or MyoB myosin-binding domain, respectively, resulted in velocity reduction for not only the MyoB compartment, but also each of the tested organelles, vesicles and cytoplasmic streaming. Furthermore, the extents of this reduction were similar for each of these compartments suggesting that MyoB compartment plays primary role in cytosol dynamics. Using gene knockout analysis in Arabidopsis thaliana, it is demonstrated that inactivation of MyoB1-4 results in reduced velocity of mitochondria implying slower cytoplasmic streaming. It is also shown that myosins XI and MyoB receptors genetically interact to contribute to cell expansion, plant growth, morphogenesis and proper onset of flowering. These results support a model according to which myosin-dependent, MyoB receptor-mediated transport of a specialized membrane compartment that is conserved in all land plants drives cytoplasmic streaming that carries organelles and vesicles and facilitates cell growth and plant development.

The vacuole/lysosome is required for cell-cycle progression

Organelles are distributed to daughter cells, via inheritance pathways. However, it is unclear whether there are mechanisms beyond inheritance, which ensure that organelles are present in all cells. Here we present the unexpected finding that the yeast vacuole plays a positive essential role in initiation of the cell-cycle. When inheritance fails, a new vacuole is generated. We show that this occurs prior to the next cell-cycle, and gain insight into this alternative pathway. Moreover, we find that a combination of a defect in inheritance with an acute block in the vacuole biogenesis results in the loss of a functional vacuole and a specific arrest of cells in early G1 phase. Furthermore, this role for the vacuole in cell-cycle progression requires an intact TORC1-SCH9 pathway that can only signal from a mature vacuole. These mechanisms may serve as a checkpoint for the presence of the vacuole/lysosome.

DOI:http://dx.doi.org/10.7554/eLife.08160.001

Animals, fungi and other eukaryotes have cells that are divided into sub-compartments that are called organelles. Each type of organelle serves a specific purpose that is essential for the life of the cell. Yeast cells have a large organelle called a vacuole; the inside of the vacuole is acidic and contains enzymes that can break down other molecules.

Previous studies have shown that when a budding yeast cell buds to produce a new daughter cell, a process ensures that some of the mother's vacuole is transferred to its daughter. However, yeast mutants that fail to inherit some of their mother's vacuole can still survive. This is because an ‘alternative’ mechanism allows the newly forming daughter to generate its own vacuole from scratch.

Jin and Weisman now unexpectedly show that a new daughter cell cannot become a mother cell until its new vacuole is formed. The experiments made use of yeast mutants that were defective in the ‘inheritance’ mechanism, and double mutants that were defective in both the inheritance and alternative mechanisms. The experiments also revealed that a signal from the vacuole is required before the yeast cell's nucleus can start the cycle of events that lead to the cell dividing. Jin and Weisman suggest that this newly identified communication between the vacuole and the nucleus may help to ensure that critical organelles are present in all cells.

Though it remains unclear why the yeast vacuole is critical for a cell to divide, these findings suggest that the mammalian lysosome (which is similar to the yeast vacuole) may perform a similar critical role in mammals. If this is the case, then understanding how these organelles communicate with the nucleus may provide new insights into how to prevent the uncontrolled growth of tumors and cancer.

DOI:http://dx.doi.org/10.7554/eLife.08160.002

Head-to-tail regulation is critical for the in vivo function of myosin V

Myo2p is regulated by a head-to-tail interaction and defects in this regulatory mechanism lead to a host of cellular problems in vivo, including impaired cargo capture and delayed motor recycling

Cell organization requires regulated cargo transport along cytoskeletal elements. Myosin V motors are among the most conserved organelle motors and have been well characterized in both yeast and mammalian systems. Biochemical data for mammalian myosin V suggest that a head-to-tail autoinhibitory interaction is a primary means of regulation, but the in vivo significance of this interaction has not been studied. Here we generated and characterized mutations in the yeast myosin V Myo2p to reveal that it is regulated by a head-to-tail interaction and that loss of regulation renders the myosin V constitutively active. We show that an unregulated motor is very deleterious for growth, resulting in severe defects in Myo2-mediated transport processes, including secretory vesicle transport, mitochondrial inheritance, and nuclear orientation. All of the defects associated with motor misregulation could be rescued by artificially restoring regulation. Thus, spatial and temporal regulation of myosin V in vivo by a head-to-tail interaction is critical for the normal delivery functions of the motor.

ER-mitochondria contact sites in yeast: beyond the myths of ERMES

A standout feature of eukaryotic cells is the presence of organelles with distinct chemical compositions and physical properties, which aid in the accomplishment of specialized metabolic tasks. This complex topology, however, makes a permanent crosstalk between the organelles a necessity for the coordination of cellular function. While molecule exchange between organelles via the vesicular transport system has been extensively studied, communication via direct connections has only recently become a new matter of interest. These direct connections termed membrane contact sites (MCSs) represent zones of close proximity (10-30nm) between two organelles. Research in the past years has revealed a number of MCSs especially between the ER and almost every other organelle [1(•)]. In particular, the MCSs between the ER and the mitochondria have undergone intense investigation. While the quest for ER-mitochondria MCS components in human cells has led to the revelation of an ever growing number of potential factors, studies in the simpler eukaryote Saccharomyces cerevisiae revealed the actual existence of a molecular tether between the two organelles [2(••)].

Sharing the cell's bounty - organelle inheritance in yeast

Eukaryotic cells replicate and partition their organelles between the mother cell and the daughter cell at cytokinesis. Polarized cells, notably the budding yeast Saccharomyces cerevisiae, are well suited for the study of organelle inheritance, as they facilitate an experimental dissection of organelle transport and retention processes. Much progress has been made in defining the molecular players involved in organelle partitioning in yeast. Each organelle uses a distinct set of factors - motor, anchor and adaptor proteins - that ensures its inheritance by future generations of cells. We propose that all organelles, regardless of origin or copy number, are partitioned by the same fundamental mechanism involving division and segregation. Thus, the mother cell keeps, and the daughter cell receives, their fair and equitable share of organelles. This mechanism of partitioning moreover facilitates the segregation of organelle fragments that are not functionally equivalent. In this Commentary, we describe how this principle of organelle population control affects peroxisomes and other organelles, and outline its implications for yeast life span and rejuvenation.

Role of asymmetric cell division in lifespan control in Saccharomyces cerevisiae

Aging determinants are asymmetrically distributed during cell division in S. cerevisiae, which leads to production of an immaculate, age-free daughter cell. During this process, damaged components are sequestered and retained in the mother cell, and higher functioning organelles and rejuvenating factors are transported to and/or enriched in the bud. Here, we will describe the key quality control mechanisms in budding yeast that contribute to asymmetric cell division of aging determinants including mitochondria, endoplasmic reticulum (ER), vacuoles, extrachromosomal rDNA circles (ERCs), and protein aggregates.

Organelle-based aggregation and retention of damaged proteins in asymmetrically dividing cells

Aggregation of damaged or misfolded proteins is a protective mechanism against proteotoxic stress, abnormalities of which underlie many aging-related diseases. Here, we show that in asymmetrically dividing yeast cells, aggregation of cytosolic misfolded proteins does not occur spontaneously but requires new polypeptide synthesis and is restricted to the surface of ER, which harbors the majority of active translation sites. Protein aggregates formed on ER are frequently also associated with or are later captured by mitochondria, greatly constraining aggregate mobility. During mitosis, aggregates are tethered to well-anchored maternal mitochondria, whereas mitochondria acquired by the bud are largely free of aggregates. Disruption of aggregate-mitochondria association resulted in increased mobility and leakage of mother-accumulated aggregates into the bud. Cells with advanced replicative age exhibit gradual decline of aggregates-mitochondria association, likely contributing to their diminished ability to rejuvenate through asymmetric cell division.

Mitochondrial dynamics and inheritance during cell division, development and disease

During cell division, it is critical to properly partition functional sets of organelles to each daughter cell. The partitioning of mitochondria shares some common features with that of other organelles, particularly in the use of interactions with cytoskeletal elements to facilitate delivery to the daughter cells. However, mitochondria have unique features - including their own genome and a maternal mode of germline transmission - that place additional demands on this process. Consequently, mechanisms have evolved to regulate mitochondrial segregation during cell division, oogenesis, fertilization and tissue development, as well as to ensure the integrity of these organelles and their DNA, including fusion-fission dynamics, organelle transport, mitophagy and genetic selection of functional genomes. Defects in these processes can lead to cell and tissue pathologies.

Shaping the dynamic mitochondrial network

In a majority of cell types, mitochondria form highly dynamic, tubular networks. Maintaining the shape of this complex network is critical for both mitochondrial and cellular function and involves the activities of mitochondrial division, fusion, motility, and tethering. Recent studies have advanced our understanding of the molecular mechanisms underlying these conserved activities and their integration with cellular needs.

Mitochondrial inheritance in yeast

Mitochondria are the site of oxidative phosphorylation, play a key role in cellular energy metabolism, and are critical for cell survival and proliferation. The propagation of mitochondria during cell division depends on replication and partitioning of mitochondrial DNA, cytoskeleton-dependent mitochondrial transport, intracellular positioning of the organelle, and activities coordinating these processes. Budding yeast Saccharomyces cerevisiae has proven to be a valuable model organism to study the mechanisms that drive segregation of the mitochondrial genome and determine mitochondrial partitioning and behavior in an asymmetrically dividing cell. Here, I review past and recent advances that identified key components and cellular pathways contributing to mitochondrial inheritance in yeast. This article is part of a Special Issue entitled: 18th European Bioenergetic Conference. Guest Editors: Manuela Pereira and Miguel Teixeira.