What Makes A Cell Grow Old?
A family of proteins has been identified that may limit the number of times any cell can divide, according to research published in the journal Nature Cell Biology this week.
Most cells seem to have an in-built limit to the number of times it can divide. From human cells down to single-celled yeast, they seem to limit themselves to only 20-30 divisions. Cell aging is thought to be related to the build up of toxic agents, but the mechanisms are not well understood.
Now, Rong Li and colleagues in Kansas have identified a group of chemicals that may be responsible for limiting the reproductive lifespan of a cell.Yeast cells multiply by growing and dividing. A mother cell will create more of the components it requires, and then a daughter cell will bud off from the mother.
Each cell can only do this 20-30 times, and so far this has been put down to the build up of damage to DNA.
Multidrug resistance, or MDR, proteins are involved in transporting chemicals across the plasma membrane in yeast, and are inherited from the mother cell. Oddly, this inheritance is asymmetric - rather than a batch of new proteins being produced, then all the proteins split, the newly formed proteins preferentially move into the daughter cell.
Over subsequent divisions, the mother cell will keep hold of the 'old' MDR proteins, which may become damaged and lose function. As these proteins have crucial roles in metabolism and dealing with stress, this will lead to a gradual reduction in fitness of the mother cell, and a limit to how many times it can divide.
Supporting this hypothesis - yeast cells lacking certain MDR genes show a reduced replicative lifespan, while introducing extra MDR genes increases the lifespan.
It's now important to see if membrane transporter proteins show the same asymmetry in stem cells, which could limit the potential of stem cells, and to see if the same proteins play a role in aging in multicellular organisms - including us.