Takaya Ishihara

Dynamics of nucleoid structure regulated by mitochondrial fission contributes to cristae reformation and release of cytochrome c

Mammalian cells typically contain thousands of copies of mitochondrial DNA assembled into hundreds of nucleoids. Here we analyzed the dynamic features of nucleoids in terms of mitochondrial membrane dynamics involving balanced fusion and fission. In mitochondrial fission GTPase dynamin-related protein (Drp1)-deficient cells, nucleoids were enlarged by their clustering within hyperfused mitochondria. In normal cells, mitochondrial fission often occurred adjacent to nucleoids, since localization of Mff and Drp1 is dependent on the nucleoids. Thus, mitochondrial fission adjacent to nucleoids should prevent their clustering by maintaining small and fragmented nucleoids. The enhanced clustering of nucleoids resulted in the formation of highly stacked cristae structures in enlarged bulb-like mitochondria (mito-bulbs). Enclosure of proapoptotic factor cytochrome c, but not of Smac/DIABLO, into the highly stacked cristae suppressed its release from mitochondria under apoptotic stimuli. In the absence of nucleoids, Drp1 deficiency failed to form mito-bulbs and to protect against apoptosis. Thus, mitochondrial dynamics by fission and fusion play a critical role in controlling mitochondrial nucleoid structures, contributing to cristae reformation and the proapoptotic status of mitochondria.

Dynamics of mitochondrial DNA nucleoids regulated by mitochondrial fission is essential for maintenance of homogeneously active mitochondria during neonatal heart development

ABSTRACT Mitochondria are dynamic organelles, and their fusion and fission regulate cellular signaling, development, and mitochondrial homeostasis, including mitochondrial DNA (mtDNA) distribution. Cardiac myocytes have a specialized cytoplasmic structure where large mitochondria are aligned into tightly packed myofibril bundles; however, recent studies have revealed that mitochondrial dynamics also plays an important role in the formation and maintenance of cardiomyocytes. Here, we precisely analyzed the role of mitochondrial fission in vivo. The mitochondrial fission GTPase, Drp1, is highly expressed in the developing neonatal heart, and muscle-specific Drp1 knockout (Drp1-KO) mice showed neonatal lethality due to dilated cardiomyopathy. The Drp1 ablation in heart and primary cultured cardiomyocytes resulted in severe mtDNA nucleoid clustering and led to mosaic deficiency of mitochondrial respiration. The functional and structural alteration of mitochondria also led to immature myofibril assembly and defective cardiomyocyte hypertrophy. Thus, the dynamics of mtDNA nucleoids regulated by mitochondrial fission is required for neonatal cardiomyocyte development by promoting homogeneous distribution of active mitochondria throughout the cardiomyocytes.

Disruption of mitochondrial fission in the liver protects mice from diet-induced obesity and metabolic deterioration

Aim/hypothesisMitochondria and the endoplasmic reticulum (ER) physically interact by close structural juxtaposition, via the mitochondria-associated ER membrane. Inter-organelle communication between the ER and mitochondria has been shown to regulate energy metabolism and to be central to the modulation of various key processes such as ER stress. We aimed to clarify the role of mitochondrial fission in this communication.MethodsWe generated mice lacking the mitochondrial fission protein dynamin-related protein 1 (DRP1) in the liver (Drp1LiKO mice).ResultsDrp1LiKO mice showed decreased fat mass and were protected from high-fat diet (HFD)-induced obesity. Analysis of liver gene expression profiles demonstrated marked elevation of ER stress markers. In addition, we observed increased expression of the fibroblast growth factor 21 (FGF21) gene through induction of activating transcription factor 4, master regulator of the integrated stress response.Conclusions/interpretationDisruption of mitochondrial fission in the liver provoked ER stress, while inducing the expression of FGF21 to increase energy expenditure and protect against HFD-induced obesity.

Mitochondrial Fission Factor Drp1 Maintains Oocyte Quality via Dynamic Rearrangement of Multiple Organelles

Mitochondria are dynamic organelles that change their morphology by active fusion and fission in response to cellular signaling and differentiation. The in vivo role of mitochondrial fission in mammals has been examined by using tissue-specific knockout (KO) mice of the mitochondria fission-regulating GTPase Drp1, as well as analyzing a human patient harboring a point mutation in Drp1, showing that Drp1 is essential for embryonic and neonatal development and neuronal function. During oocyte maturation and aging, structures of various membrane organelles including mitochondria and the endoplasmic reticulum (ER) are changed dynamically, and their organelle aggregation is related to germ cell formation and epigenetic regulation. However, the underlying molecular mechanisms of organelle dynamics during the development and aging of oocytes have not been well understood. Here, we analyzed oocyte-specific mitochondrial fission factor Drp1-deficient mice and found that mitochondrial fission is essential for follicular maturation and ovulation in an age-dependent manner. Mitochondria were highly aggregated with other organelles, such as the ER and secretory vesicles, in KO oocyte, which resulted in impaired Ca(2+) signaling, intercellular communication via secretion, and meiotic resumption. We further found that oocytes from aged mice displayed reduced Drp1-dependent mitochondrial fission and defective organelle morphogenesis, similar to Drp1 KO oocytes. On the basis of these findings, it appears that mitochondrial fission maintains the competency of oocytes via multiorganelle rearrangement.

Physiological roles of mitochondrial fission in cultured cells and mouse development.

Mitochondria, which are double‐membrane organelles thought to have originated through endosymbiosis of bacteria, play important roles in not only energy production, but also cellular signaling, differentiation, and development. The morphology of mitochondria is highly diverse among different tissues, and dynamic changes in mitochondrial morphology have been observed in response to various intracellular signals and stresses. These changes in mitochondrial morphology occur by repeated membrane fusion and fission events, which are regulated by three types of GTPase proteins: OPA1, Mfn1/2, Drp1 in mammalian cells. In recent years, the function and molecular mechanisms of mitochondrial dynamics have been demonstrated in cultured cells; however, the role of mitochondrial fission in maintaining tissue homeostasis remains poorly understood. Here, we review recent advances in research on the physiological role of mitochondrial fission in various differentiated tissue types in mammals.

Distinct types of protease systems are involved in homeostasis regulation of mitochondrial morphology via balanced fusion and fission.

Mitochondrial morphology is dynamically regulated by fusion and fission. Several GTPase proteins control fusion and fission, and posttranslational modifications of these proteins are important for the regulation. However, it has not been clarified how the fusion and fission is balanced. Here, we report the molecular mechanism to regulate mitochondrial morphology in mammalian cells. Ablation of the mitochondrial fission, by repression of Drp1 or Mff, or by over‐expression of MiD49 or MiD51, results in a reduction in the fusion GTPase mitofusins (Mfn1 and Mfn2) in outer membrane and long form of OPA1 (L‐OPA1) in inner membrane. RNAi‐ or CRISPR‐induced ablation of Drp1 in HeLa cells enhanced the degradation of Mfns via the ubiquitin‐proteasome system (UPS). We further found that UPS‐related protein BAT3/BAG6, here we identified as Mfn2‐interacting protein, was implicated in the turnover of Mfns in the absence of mitochondrial fission. Ablation of the mitochondrial fission also enhanced the proteolytic cleavage of L‐OPA1 to soluble S‐OPA1, and the OPA1 processing was reversed by inhibition of the inner membrane protease OMA1 independent on the mitochondrial membrane potential. Our findings showed that the distinct degradation systems of the mitochondrial fusion proteins in different locations are enhanced in response to the mitochondrial morphology.

Relationship between OPA1 and cardiolipin in mitochondrial inner-membrane fusion

Mitochondria are highly dynamic organelles that undergo frequent fusion and fission. The large GTPase optic atrophy 1 (OPA1) is identified as a core component of inner membrane (IM) fusion. OPA1 exists as the membrane-anchored L-OPA1 and the proteolytically cleavage soluble S-OPA1. Recently, we showed that OPA1 and mitochondria-localized lipid cardiolipin (CL) cooperate in heterotypic IM fusion [Ban et al., Nat. Cell Biol. 19 (2017) 856-863]. We reconstituted an in vitro membrane fusion reaction using purified human L-OPA1 and S-OPA1 expressed in silkworm and found that L-OPA1 on one side of the membrane and CL on the other side were sufficient for mitochondrial fusion. L-OPA1 is the major fusion-prone factor in heterotypic fusion. However, the role of S-OPA1 remains unknown as S-OPA1 promoted L-OPA1-dependent heterotypic membrane fusion and homotypic CL-containing membrane fusion, but S-OPA1 alone was not sufficient for heterotypic membrane fusion. L-OPA1- and CL-mediated heterotypic mitochondrial fusion was confirmed in living cells, but tafazzin (Taz1), the causal gene product of Barth syndrome, was not essential for mitochondrial fusion. Taz1-dependent CL maturation might have other roles in the remodeling of mitochondrial DNA nucleoids.

Aclarubicin, an anthracycline anti-cancer drug, fluorescently contrasts mitochondria and reduces the oxygen consumption rate in living human cells

Aclarubicin (Acla), an effective anthracycline chemotherapeutic agent for hematologic cancers and solid tumors, is documented to perturb chromatin function via histone eviction and DNA topoisomerase inhibition in the nucleus, but much less attention has been paid to cytotoxic function in the cytoplasm. Here, we showed that Acla emitted fluorescence and that human cervical cancer HeLa cells exposed to Acla exhibited bright fluorescence signals in the cytoplasm when fluorescence microscopy was performed using the red filter (excitation 530-550nm/emission 575nm). Intriguingly, most of the signals appeared to be partitioned and enriched in entangled tubule-like structures; moreover, these signals merged with the mitochondria-specific MitoTracker signals. Notably, analysis of mitochondrial respiratory activity revealed that the oxygen consumption rate was decreased in Acla-treated cells. These findings suggest that Acla accumulates efficiently in the mitochondria of living human cells and leads to mitochondrial dysfunction, implying a previously overlooked cytotoxicity of Acla in the cytoplasm and adding mechanistic insight of the anti-cancer activity, as well as the side effects, of Acla/anthracycline-based chemotherapy.