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Mitofusins Mfn1 and Mfn2 coordinately regulate mitochondrial fusion and are essential for embryonic development.

Chen H, Detmer SA, Ewald AJ, Griffin EE, Fraser SE, Chan DC - J. Cell Biol. (2003)

Bottom Line: We find that mice deficient in either Mfn1 or Mfn2 die in midgestation.However, whereas Mfn2 mutant embryos have a specific and severe disruption of the placental trophoblast giant cell layer, Mfn1-deficient giant cells are normal.Strikingly, a subset of mitochondria in mutant cells lose membrane potential.

View Article: PubMed Central - PubMed

Affiliation: Division of Biology, Beckman Institute, California Institute of Technology, Pasadena, CA 91125, USA.

ABSTRACT
Mitochondrial morphology is determined by a dynamic equilibrium between organelle fusion and fission, but the significance of these processes in vertebrates is unknown. The mitofusins, Mfn1 and Mfn2, have been shown to affect mitochondrial morphology when overexpressed. We find that mice deficient in either Mfn1 or Mfn2 die in midgestation. However, whereas Mfn2 mutant embryos have a specific and severe disruption of the placental trophoblast giant cell layer, Mfn1-deficient giant cells are normal. Embryonic fibroblasts lacking Mfn1 or Mfn2 display distinct types of fragmented mitochondria, a phenotype we determine to be due to a severe reduction in mitochondrial fusion. Moreover, we find that Mfn1 and Mfn2 form homotypic and heterotypic complexes and show, by rescue of mutant cells, that the homotypic complexes are functional for fusion. We conclude that Mfn1 and Mfn2 have both redundant and distinct functions and act in three separate molecular complexes to promote mitochondrial fusion. Strikingly, a subset of mitochondria in mutant cells lose membrane potential. Therefore, mitochondrial fusion is essential for embryonic development, and by enabling cooperation between mitochondria, has protective effects on the mitochondrial population.

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Defective giant cell layer of mutant placentae. (A–D) DAPI-stained sections of placentae from e10.5 wild-type (A and C) and mutant (B and D) littermate embryos. The boxed areas of A and B are enlarged in C and D. Arrows and arrowheads indicate trophoblast giant cells. Note that the giant cells in D are sparser and have smaller nuclei. (E and F) Hematoxylin-eosin–stained sections from the placentae above. (G and H) PL-I (giant cell marker) RNA in situ analysis of placentae from e9.5 wild-type (G) and mutant (H) littermates.
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fig2: Defective giant cell layer of mutant placentae. (A–D) DAPI-stained sections of placentae from e10.5 wild-type (A and C) and mutant (B and D) littermate embryos. The boxed areas of A and B are enlarged in C and D. Arrows and arrowheads indicate trophoblast giant cells. Note that the giant cells in D are sparser and have smaller nuclei. (E and F) Hematoxylin-eosin–stained sections from the placentae above. (G and H) PL-I (giant cell marker) RNA in situ analysis of placentae from e9.5 wild-type (G) and mutant (H) littermates.

Mentions: We examined placental development in detail because placental insufficiency is one of the most common causes of midgestation lethality (Copp, 1995). Hematoxylin-eosin–stained histological sections of placentae from wild-type and heterozygous embryos showed the typical trilaminar structure composed of a proximal labyrinthine layer, a middle spongiotrophoblast layer, and a distal, circumferential giant cell layer (Fig. 2 E). Trophoblast giant cells are polyploid cells (derived from endoreplication, a process where DNA replication proceeds repeatedly without associated cytokinesis) that lie at the critical interface between fetal and maternal tissues and play important roles in hormone production, recruitment of blood vessels, and invasion of the conceptus into the uterine lining (Cross, 2000). Deficiencies in these cells lead to midgestation lethality (Kraut et al., 1998; Riley et al., 1998; Hesse et al., 2000; Scott et al., 2000). Strikingly, placentae from Mfn2 mutant embryos reproducibly show an impaired giant cell layer with two defects. The giant cells are deficient in quantity, and the few that are observed contain smaller nuclei, implying a reduction in the number of endoreplication cycles (Fig. 2 F). These observations were confirmed with DAPI staining, which highlights the giant cells because of their high DNA content (Fig. 2, A–D). No defects in placental development were detected in Mfn1 mutant embryos (unpublished data) despite expression of Mfn1 in the placenta (Fig. 1 D).


Mitofusins Mfn1 and Mfn2 coordinately regulate mitochondrial fusion and are essential for embryonic development.

Chen H, Detmer SA, Ewald AJ, Griffin EE, Fraser SE, Chan DC - J. Cell Biol. (2003)

Defective giant cell layer of mutant placentae. (A–D) DAPI-stained sections of placentae from e10.5 wild-type (A and C) and mutant (B and D) littermate embryos. The boxed areas of A and B are enlarged in C and D. Arrows and arrowheads indicate trophoblast giant cells. Note that the giant cells in D are sparser and have smaller nuclei. (E and F) Hematoxylin-eosin–stained sections from the placentae above. (G and H) PL-I (giant cell marker) RNA in situ analysis of placentae from e9.5 wild-type (G) and mutant (H) littermates.
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Related In: Results  -  Collection

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fig2: Defective giant cell layer of mutant placentae. (A–D) DAPI-stained sections of placentae from e10.5 wild-type (A and C) and mutant (B and D) littermate embryos. The boxed areas of A and B are enlarged in C and D. Arrows and arrowheads indicate trophoblast giant cells. Note that the giant cells in D are sparser and have smaller nuclei. (E and F) Hematoxylin-eosin–stained sections from the placentae above. (G and H) PL-I (giant cell marker) RNA in situ analysis of placentae from e9.5 wild-type (G) and mutant (H) littermates.
Mentions: We examined placental development in detail because placental insufficiency is one of the most common causes of midgestation lethality (Copp, 1995). Hematoxylin-eosin–stained histological sections of placentae from wild-type and heterozygous embryos showed the typical trilaminar structure composed of a proximal labyrinthine layer, a middle spongiotrophoblast layer, and a distal, circumferential giant cell layer (Fig. 2 E). Trophoblast giant cells are polyploid cells (derived from endoreplication, a process where DNA replication proceeds repeatedly without associated cytokinesis) that lie at the critical interface between fetal and maternal tissues and play important roles in hormone production, recruitment of blood vessels, and invasion of the conceptus into the uterine lining (Cross, 2000). Deficiencies in these cells lead to midgestation lethality (Kraut et al., 1998; Riley et al., 1998; Hesse et al., 2000; Scott et al., 2000). Strikingly, placentae from Mfn2 mutant embryos reproducibly show an impaired giant cell layer with two defects. The giant cells are deficient in quantity, and the few that are observed contain smaller nuclei, implying a reduction in the number of endoreplication cycles (Fig. 2 F). These observations were confirmed with DAPI staining, which highlights the giant cells because of their high DNA content (Fig. 2, A–D). No defects in placental development were detected in Mfn1 mutant embryos (unpublished data) despite expression of Mfn1 in the placenta (Fig. 1 D).

Bottom Line: We find that mice deficient in either Mfn1 or Mfn2 die in midgestation.However, whereas Mfn2 mutant embryos have a specific and severe disruption of the placental trophoblast giant cell layer, Mfn1-deficient giant cells are normal.Strikingly, a subset of mitochondria in mutant cells lose membrane potential.

View Article: PubMed Central - PubMed

Affiliation: Division of Biology, Beckman Institute, California Institute of Technology, Pasadena, CA 91125, USA.

ABSTRACT
Mitochondrial morphology is determined by a dynamic equilibrium between organelle fusion and fission, but the significance of these processes in vertebrates is unknown. The mitofusins, Mfn1 and Mfn2, have been shown to affect mitochondrial morphology when overexpressed. We find that mice deficient in either Mfn1 or Mfn2 die in midgestation. However, whereas Mfn2 mutant embryos have a specific and severe disruption of the placental trophoblast giant cell layer, Mfn1-deficient giant cells are normal. Embryonic fibroblasts lacking Mfn1 or Mfn2 display distinct types of fragmented mitochondria, a phenotype we determine to be due to a severe reduction in mitochondrial fusion. Moreover, we find that Mfn1 and Mfn2 form homotypic and heterotypic complexes and show, by rescue of mutant cells, that the homotypic complexes are functional for fusion. We conclude that Mfn1 and Mfn2 have both redundant and distinct functions and act in three separate molecular complexes to promote mitochondrial fusion. Strikingly, a subset of mitochondria in mutant cells lose membrane potential. Therefore, mitochondrial fusion is essential for embryonic development, and by enabling cooperation between mitochondria, has protective effects on the mitochondrial population.

Show MeSH