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Asymmetric cell division generates cell diversity across all kingdoms of life. We use the budding yeast S. cerevisiae as a model system to understand the mechanisms and functions of asymmetric cell division.

The asymmetric division of budding yeast results in ageing. Indeed, yeast mother cells have a limited division potential. Damages such as protein aggregates accumulate in the mother cell. How they are asymmetrically inherited to allow rejuvenation of daughter cells is still poorly understood. Beyond identifying the mechanisms of asymmetric cell division, we follow the idea that ageing allow cells to keep memories of their past adaptations to cope better with future stress.

1. Prion-like proteins can store cellular memory

We discovered a new type of epigenetic memory. Haploid yeast cells come into two

sexual types, MATa and MATalpha. When a MATa cell meets a MATalpha cell, they

both arrest their cell cycle in the G1 phase and grow towards each other by forming

a shmoo. These cells communicate by secreting a mating pheromone (a-factor for 

MATa cells and alpha-factor for MATalpha cells). Cells respond to pheromone by

arresting their cell division cycle and preparing for mating. When they reach each

other, they fuse and become a diploid cell. 

                                                                                                                                                                                                           Two successful mating

                                                                                                                                                                                                                events highlighted by

                                                                                                                                                                                                                arrows (1 frame = 10 minutes).

                                                           However, life is not always that simple. When cells are exposed to pheromone only,

                                                           without a mating partner in reach, they first shmoo and then can escape the

                                                           pheromone-induced arrest, re-enter the cell cycle and produce daughter cells.

                                                           Mother cells maintain this pheromone refractory state for the remainder of their life

                                                           span. In contrast, daughter cells are born naïve and respond to pheromone.

      Cells were exposed to alpha-factor

      (7nM). The mother cell is at first

        shmooing and then only produces

        daughter cells, which shmoo. Time

        is in hours and minutes.

The pheromone refractory state depends on the inactivation of Whi3.

Whi3 inhibits translation of mRNA encoding Cln3, the G1 cyclin required

for the G1 to S phase transition. Whi3 contains prion-like domains (PrD)

that promote an inactivating conformational change upon pheromone

treatment. We termed this type of protein mnemon. A mnemon is a

protein that change of fold and super-assembly can encode a cellular

memory. It is specifically induced by a stimulus and inherited

asymmetrically during cell division. In contrast, prions are symmetrically

inherited during cell division. PrD containing proteins are very common

in most living organisms and many of these molecules cause proteopathies

such as neurodegenerative diseases or Creutzfeldt-Jakob disease. We test

the idea that many of them behave as mnemons and we aim at

understanding their biology in physiological and pathological contexts.

                                                                            Prions are common epigenetic elements in yeast and prion-like elements                                                                                such as mnemons are probably very common in all organisms. They all                                                                                  allow a great phenotypic diversity based on the very same genome. The                                                                                  red and white cells illustrate this diversity well. The white ones are the                                                                                    same as the red ones genetically, but the white ones have a prion [PSI+].

We aim at discovering new mnemons, how they work, how their functions are regulated and how widespread they are in the yeast proteome and also in other organisms, including vertebrates.

2. Fate of prion-like proteins during ageing.

Ageing impacts most living organisms with an array of physiological

functions that decline. Cellular ageing is accompanied by the 

accumulation of protein deposit, or proteins clumping together. In

budding yeast, supposedly many proteins form a single deposit

during ageing. This deposit is nearly always inherited by the mother

cell, while its daughter is rejuvenated. One of our aim is to understand

how this deposit is inherited during cell division. We also explore what

are the properties of old cell that induce the accumulation of these

aggregates. We particularly focus on prion-like proteins, as we have             The left panel shows a movie of an old yeast cell 

discovered that the mnemon Whi3 does aggregate in old cells. To                 expressing Hsp104-GFP. The age-induced protein

achieve this, we use microfluidic chips to follow the full life span of               deposit is seen as a dot that remains in the mother

single yeast cells.                                                                                                       cell. The right panel shows a young yeast cell (top) 

                                                                                                                                      expressing the mnemon Whi3-GFP. The signal is           

                                                                                                                                                                            diffuse, while in an old yeast cell (bottom) it forms

                                                                                                                                 aggregates.

Therefore, we are advancing our understanding of the role of prion-like proteins in cellular memory and how their biophysical properties lead these same proteins to aggregate and become detrimental during ageing.

We aim at proposing an integrative model of their biology, first using the power of yeast genetics and then extending it to other organisms.

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