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Fusion of protein aggregates facilitates asymmetric damage segregation.

Coelho M, Lade SJ, Alberti S, Gross T, Tolić IM - PLoS Biol. (2014)

Bottom Line: Our model predicts that, after stress, the increased number of aggregates fuse into a single large unit, which is inherited asymmetrically by one daughter cell, whereas the other one is born clean.We experimentally confirmed that fusion increases segregation asymmetry, for a range of stresses, and identified Hsp16 as a fusion factor.Our work shows that fusion of protein aggregates promotes the formation of damage-free cells.

View Article: PubMed Central - PubMed

Affiliation: Max Planck Institute of Molecular Cell Biology and Genetics, Dresden, Germany; FAS Center for Systems Biology, Harvard University, Cambridge, Massachusetts, United States of America.

ABSTRACT
Asymmetric segregation of damaged proteins at cell division generates a cell that retains damage and a clean cell that supports population survival. In cells that divide asymmetrically, such as Saccharomyces cerevisiae, segregation of damaged proteins is achieved by retention and active transport. We have previously shown that in the symmetrically dividing Schizosaccharomyces pombe there is a transition between symmetric and asymmetric segregation of damaged proteins. Yet how this transition and generation of damage-free cells are achieved remained unknown. Here, by combining in vivo imaging of Hsp104-associated aggregates, a form of damage, with mathematical modeling, we find that fusion of protein aggregates facilitates asymmetric segregation. Our model predicts that, after stress, the increased number of aggregates fuse into a single large unit, which is inherited asymmetrically by one daughter cell, whereas the other one is born clean. We experimentally confirmed that fusion increases segregation asymmetry, for a range of stresses, and identified Hsp16 as a fusion factor. Our work shows that fusion of protein aggregates promotes the formation of damage-free cells. Fusion of cellular factors may represent a general mechanism for their asymmetric segregation at division.

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Hsp16 is required for efficient aggregate fusion, which facilitates asymmetric segregation and generation of cells clean of aggregates.(A) Quantification of the percentage of contact events between two aggregates which resulted in fusion. Nucleation (B), fusion (C), and fission (D) events per cell cycle in the wild-type strain and in the strains in which Hsp16, Hsp40, or Hsp70 was deleted (statistical difference between wild type and mutants: **p<0.01). (E) Time-lapse of aggregate movement in the wild type and the strain where Hsp16, Hsp40, or Hsp70 was deleted (arrows mark aggregates, magenta indicates contact between aggregates, green corresponds to fusion). In the kymographs, fusion events are visible as the merging of two aggregates (two traces merge into one thicker trace; magenta lines on top correspond to time interval depicted in panels). In the Δhsp16 mutant, a contact event does not give rise to fusion, and the aggregates remain separated. (F) Aggregate segregation in wild-type and hsp16 deleted strains, under control and heat stress conditions. (G) Aggregate amount of asymmetry in wild-type and a Δhsp16 strain (in which fusion was reduced; see labels and regions 1–3 depicted in Figure 3C). Error bars on the control data are not visible because they are smaller than the circles representing the data. (H) Fraction of cells born without aggregates at the first three divisions after stress (see labels). The data in (A–D), (G), and (H) are mean ± SEM; number of aggregate contact events or cell cycles from >3 independent experiments are given in the graphs. In (E) and (F) thin lines encircle cells; scale bars, 1 µm. (I) Summary: stress increases nucleation of aggregates, leading to an increased number of aggregates per cell. Fusion decreases the total aggregate number to a single large aggregate, forcing its asymmetric segregation. This results in the birth of a clean cell. See also Figure S4.
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pbio-1001886-g004: Hsp16 is required for efficient aggregate fusion, which facilitates asymmetric segregation and generation of cells clean of aggregates.(A) Quantification of the percentage of contact events between two aggregates which resulted in fusion. Nucleation (B), fusion (C), and fission (D) events per cell cycle in the wild-type strain and in the strains in which Hsp16, Hsp40, or Hsp70 was deleted (statistical difference between wild type and mutants: **p<0.01). (E) Time-lapse of aggregate movement in the wild type and the strain where Hsp16, Hsp40, or Hsp70 was deleted (arrows mark aggregates, magenta indicates contact between aggregates, green corresponds to fusion). In the kymographs, fusion events are visible as the merging of two aggregates (two traces merge into one thicker trace; magenta lines on top correspond to time interval depicted in panels). In the Δhsp16 mutant, a contact event does not give rise to fusion, and the aggregates remain separated. (F) Aggregate segregation in wild-type and hsp16 deleted strains, under control and heat stress conditions. (G) Aggregate amount of asymmetry in wild-type and a Δhsp16 strain (in which fusion was reduced; see labels and regions 1–3 depicted in Figure 3C). Error bars on the control data are not visible because they are smaller than the circles representing the data. (H) Fraction of cells born without aggregates at the first three divisions after stress (see labels). The data in (A–D), (G), and (H) are mean ± SEM; number of aggregate contact events or cell cycles from >3 independent experiments are given in the graphs. In (E) and (F) thin lines encircle cells; scale bars, 1 µm. (I) Summary: stress increases nucleation of aggregates, leading to an increased number of aggregates per cell. Fusion decreases the total aggregate number to a single large aggregate, forcing its asymmetric segregation. This results in the birth of a clean cell. See also Figure S4.

Mentions: The model predicts that fusion facilitates asymmetric segregation in response to different levels of stress, where high asymmetry is optimal (Figure 4C, filled circles). This behavior was also observed experimentally for a range of stresses (Figure 4C, filled squares). We observed that in divisions 2 and 3 after stress, the percentage of cells born without aggregates was higher for stress conditions that originated in a higher aggregate amount (Figure S4J). This phenomenon can be explained by the higher number of fusion events observed for high stress levels (e.g., heat stress as opposed to oxidative stress; Figure 3B), which can result in the faster generation of a single large aggregate. Once large aggregates are formed, nucleation of aggregates decreases in favor of the growth of the large aggregates: as observed for unstressed cells (Figure S4C), the nonvisible aggregates have a higher probability to fuse with large preexisting aggregates. We conclude that in response to increased aggregate amount, an increase in fusion leads to fewer aggregates and thus more asymmetric segregation, which promotes the formation of aggregate-free cells.


Fusion of protein aggregates facilitates asymmetric damage segregation.

Coelho M, Lade SJ, Alberti S, Gross T, Tolić IM - PLoS Biol. (2014)

Hsp16 is required for efficient aggregate fusion, which facilitates asymmetric segregation and generation of cells clean of aggregates.(A) Quantification of the percentage of contact events between two aggregates which resulted in fusion. Nucleation (B), fusion (C), and fission (D) events per cell cycle in the wild-type strain and in the strains in which Hsp16, Hsp40, or Hsp70 was deleted (statistical difference between wild type and mutants: **p<0.01). (E) Time-lapse of aggregate movement in the wild type and the strain where Hsp16, Hsp40, or Hsp70 was deleted (arrows mark aggregates, magenta indicates contact between aggregates, green corresponds to fusion). In the kymographs, fusion events are visible as the merging of two aggregates (two traces merge into one thicker trace; magenta lines on top correspond to time interval depicted in panels). In the Δhsp16 mutant, a contact event does not give rise to fusion, and the aggregates remain separated. (F) Aggregate segregation in wild-type and hsp16 deleted strains, under control and heat stress conditions. (G) Aggregate amount of asymmetry in wild-type and a Δhsp16 strain (in which fusion was reduced; see labels and regions 1–3 depicted in Figure 3C). Error bars on the control data are not visible because they are smaller than the circles representing the data. (H) Fraction of cells born without aggregates at the first three divisions after stress (see labels). The data in (A–D), (G), and (H) are mean ± SEM; number of aggregate contact events or cell cycles from >3 independent experiments are given in the graphs. In (E) and (F) thin lines encircle cells; scale bars, 1 µm. (I) Summary: stress increases nucleation of aggregates, leading to an increased number of aggregates per cell. Fusion decreases the total aggregate number to a single large aggregate, forcing its asymmetric segregation. This results in the birth of a clean cell. See also Figure S4.
© Copyright Policy
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC4061010&req=5

pbio-1001886-g004: Hsp16 is required for efficient aggregate fusion, which facilitates asymmetric segregation and generation of cells clean of aggregates.(A) Quantification of the percentage of contact events between two aggregates which resulted in fusion. Nucleation (B), fusion (C), and fission (D) events per cell cycle in the wild-type strain and in the strains in which Hsp16, Hsp40, or Hsp70 was deleted (statistical difference between wild type and mutants: **p<0.01). (E) Time-lapse of aggregate movement in the wild type and the strain where Hsp16, Hsp40, or Hsp70 was deleted (arrows mark aggregates, magenta indicates contact between aggregates, green corresponds to fusion). In the kymographs, fusion events are visible as the merging of two aggregates (two traces merge into one thicker trace; magenta lines on top correspond to time interval depicted in panels). In the Δhsp16 mutant, a contact event does not give rise to fusion, and the aggregates remain separated. (F) Aggregate segregation in wild-type and hsp16 deleted strains, under control and heat stress conditions. (G) Aggregate amount of asymmetry in wild-type and a Δhsp16 strain (in which fusion was reduced; see labels and regions 1–3 depicted in Figure 3C). Error bars on the control data are not visible because they are smaller than the circles representing the data. (H) Fraction of cells born without aggregates at the first three divisions after stress (see labels). The data in (A–D), (G), and (H) are mean ± SEM; number of aggregate contact events or cell cycles from >3 independent experiments are given in the graphs. In (E) and (F) thin lines encircle cells; scale bars, 1 µm. (I) Summary: stress increases nucleation of aggregates, leading to an increased number of aggregates per cell. Fusion decreases the total aggregate number to a single large aggregate, forcing its asymmetric segregation. This results in the birth of a clean cell. See also Figure S4.
Mentions: The model predicts that fusion facilitates asymmetric segregation in response to different levels of stress, where high asymmetry is optimal (Figure 4C, filled circles). This behavior was also observed experimentally for a range of stresses (Figure 4C, filled squares). We observed that in divisions 2 and 3 after stress, the percentage of cells born without aggregates was higher for stress conditions that originated in a higher aggregate amount (Figure S4J). This phenomenon can be explained by the higher number of fusion events observed for high stress levels (e.g., heat stress as opposed to oxidative stress; Figure 3B), which can result in the faster generation of a single large aggregate. Once large aggregates are formed, nucleation of aggregates decreases in favor of the growth of the large aggregates: as observed for unstressed cells (Figure S4C), the nonvisible aggregates have a higher probability to fuse with large preexisting aggregates. We conclude that in response to increased aggregate amount, an increase in fusion leads to fewer aggregates and thus more asymmetric segregation, which promotes the formation of aggregate-free cells.

Bottom Line: Our model predicts that, after stress, the increased number of aggregates fuse into a single large unit, which is inherited asymmetrically by one daughter cell, whereas the other one is born clean.We experimentally confirmed that fusion increases segregation asymmetry, for a range of stresses, and identified Hsp16 as a fusion factor.Our work shows that fusion of protein aggregates promotes the formation of damage-free cells.

View Article: PubMed Central - PubMed

Affiliation: Max Planck Institute of Molecular Cell Biology and Genetics, Dresden, Germany; FAS Center for Systems Biology, Harvard University, Cambridge, Massachusetts, United States of America.

ABSTRACT
Asymmetric segregation of damaged proteins at cell division generates a cell that retains damage and a clean cell that supports population survival. In cells that divide asymmetrically, such as Saccharomyces cerevisiae, segregation of damaged proteins is achieved by retention and active transport. We have previously shown that in the symmetrically dividing Schizosaccharomyces pombe there is a transition between symmetric and asymmetric segregation of damaged proteins. Yet how this transition and generation of damage-free cells are achieved remained unknown. Here, by combining in vivo imaging of Hsp104-associated aggregates, a form of damage, with mathematical modeling, we find that fusion of protein aggregates facilitates asymmetric segregation. Our model predicts that, after stress, the increased number of aggregates fuse into a single large unit, which is inherited asymmetrically by one daughter cell, whereas the other one is born clean. We experimentally confirmed that fusion increases segregation asymmetry, for a range of stresses, and identified Hsp16 as a fusion factor. Our work shows that fusion of protein aggregates promotes the formation of damage-free cells. Fusion of cellular factors may represent a general mechanism for their asymmetric segregation at division.

Show MeSH
Related in: MedlinePlus