WP1: aging

Protein aggregation underlying bacterial aging 

Bacterial aging is known to contribute to the overall variability of the population growth rate. But the underlying molecular mechanisms remain largely to be discovered.

E. coli grows in the form of a rod, which reproduces by dividing in the middle. One new end (or pole) per progeny cell is produced during this division event. Therefore, one of the ends of each cell has just been created during division (termed the new pole), and one is pre-existing from a previous division (termed the old pole). Old poles can exist for many divisions, and if cells are followed over time, an age in divisions can be assigned to each pole, and hence to each cell (the age of a cell being that of its oldest pole). Hence, each division of a single cell yields two daughters: one inherits the "older old pole" (it is called the "old pole cell''), while the other inherits the "younger old pole" (called the "new pole cell").

Measuring in real time the single-bacterial growth rate of every offspring of an initial cell over up to 9 generations, we (F. Taddei's group) have shown that bacteria do age. Namely: the older the cell, the slower it grows and conversely, cells with more consecutive new pole divisions exhibit increasing growth rates (Stewart et al., 2005). Thus, the two offspring are not equal, despite appearances: one is older than the other. Recently, measuring the intracellular localization of the aggregate-binding IbpA small heat shock chaperone protein, we demonstrated that protein aggregates localize and rapidly accumulate (within 3 generations) in the old pole cells, accounting for up to 40% of the bacterial aging (Lindner et al, 2008).

Approaches

Our objective here is to decipher the mechanism underlying protein-aggregation-related aging. The following questions are specifically addressed:

• what is the intracellular mobility pattern of aggregates that accounts for their polar-spatial distribution?
• Is this phenomenon purely passive or active (i.e. requiring ATP-dependant mechanisms, a result of interactions, e.g., with the nucleoids or the cell membranes)?
• What is the molecular link between protein aggregation and growth rate?

Furthermore, by extending our available system to monitor protein aggregation dynamics and cellular aging over unlimited generations, we will provide direct measurements linking protein aggregation, aging, and its natural consequence, i.e. cell death.

To meet with these ends, we develop a synergy between 3D agent-based computer simulations (Berry, 2002) and innovative experimental approaches founded on microfluidics, synthetic biology and time-lapse fluorescence microscopy. This allows us to perform single cell monitoring during large generation numbers and intracellular trajectography of molecular aggregates.