Pekka Katajisto, Julia Döhla, Christine Chaffer, Nalle Pentinmikko, Nemanja Marjanovic, Sharif Iqbal, Roberto Zoncu, Walter Chen, Robert A. Weinberg, David M. Sabatini
The amount of mitochondrial content in a stem cell is thought to influence its tendency to differentiate. In this study, the authors use stemlike cells (SLCs), expressing photoactivatable green fluorescent protein (paGFP), to investigate the effect of protein aging. PaGFP only fluoresces after exposure to UV light. After UV exposure, and allowing the cells to age, old proteins fluoresce whereas new proteins do not. The authors tag a particular mitochondrial protein (Omp25) with paGFP and find that stem cells apportion the >10 hour old proteins asymmetrically between daughters, by a factor of ~5.6. This effect was not found for membrane proteins of other organelles, and not found at all in non-stem cells.
The authors probe this further, by using mitochondrial proteins fused to a Snap-tag. This method allows precise temporal labelling: young proteins appear green and proteins which are 10h old appear red. Interestingly, they find that young proteins are mainly found in the periphery of the cell. Whilst old mitochondria were asymetrically partitioned, young mitochondria were more uniformly distributed between daughters. They found that this asymmetry indicated the formation of two lineages amongst the daughters: daughters with mainly young mitochondria were more stem-like whereas those with old mitochondria tended to differentiate.
Finally, the authors sought to determine the cause of this asymmetry. Membrane potential was investigated, as this is known to correlate with stemness properties. Although they did indeed find that more stem-like cells tended to have higher ΔΨm, alterations of ΔΨm with an uncoupler had no effect on age-selective segregation, so this appears to be an effect rather than a cause of the age asymmetry. However, upon inhibition of Parkin (a pro-mitophagy protein) or Drp1 (pro-fission), daughter cells tended to inherit old and young mitochondria more symmetrically. Thus the authors suggest that perturbations which challenge mitochondrial quality control tend to remove this age-asymmetric partitioning, which maintains stemness properties.