The role of peroxisomes in ageing

Group members: Adam Kawalek, Sanjeev Kumar, Sophie Lefevre, Selva Manivannan, Christian Scheckhuber

All living cells continuously have to cope with factors that affect their viability. Important threats of aerobic eukaryotic cells are reactive oxygen species (ROS), which may damage important macromolecules such as proteins, lipids and DNA . Intracellular a ccumulation of components that are damaged by ROS may contribute to ageing, a process that can be defined as the deterioration of cells in time, accompanied by a loss of viability. ROS also can act as second messengers and trigger cell death, but may have positive effects as well by activating pathways aimed at saving the cell from demise.

Several mechanisms exist that prevent or reduce the negative effects of ROS (also designated oxidative stress) and thus enhance cell viability. These include antioxidants and antioxidant enzymes and molecular mechanisms that repair or eliminate components that have been damaged by ROS. Knowledge on these mechanisms is highly relevant for medicine, e.g. to combat ageing and ageing related diseases.

Eukaryotic cells contain two types of organelles that produce ROS as byproduct of oxidative metabolism, namely mitochondria and peroxisomes. It is generally accepted that the mitochondrion is a key player in cell viability, both as ROS producer and – in mammalian cells – in mediating programmed cell death.

The role of peroxisomes in cell viability is as yet largely underexposed, but recent data indicated that dysfunctional peroxisomes significantly contribute to oxidative stress and reduce viability. We recently showed that the absence of the peroxisomal antioxidant enzyme Pmp20 (a peroxiredoxin) resulted in enhanced ROS formation and necrotic cell death in the yeast Hansenula polymorpha (Bener Aksam et al., 2008).

We also showed that the peroxisomal Lon protease is important for cell viability.

We focus on three major topics:

- what is the role of intact peroxisomes in ageing?

- what is the role of peroxisomal anti-oxidant enzymes in ageing?

- what is the role of peroxisomal quality control processes, such as the peroxisomal Lon protease and peroxisome degradation by autophagy (pexophagy) in ageing.

Peroxisome housekeeping processes.

In eukaryotic cells various mechanisms exist that are responsible for the removal of non-functional proteins. We studied how non functional peroxisomal proteins or peroxisomes are removed from the cells. Our data indicated that H. polymorpha peroxisomes contain a Lon protease, Pln, which degrades unfolded and non-assembled peroxisomal matrix proteins. We also observed that whole peroxisomes are constitutively degraded by pexophagy during normal vegetative growth of WT cells.

Deletion of both H. polymorpha PLN and ATG1(in pln.atg1 cells) resulted in a significant decrease in cell viability relative to WT. These data indicate that removal of non-functional peroxisomal proteins/peroxisomes is physiologically very important for the cells.

Because peroxisomes are important organelles in the production of reactive oxygen species (ROS), we analyzed whether defects in peroxisomal housekeeping may lead to ROS accumulation. Peroxisomes harbor hydrogen peroxide producing oxidases together with catalase that is responsible for decomposition of hydrogen peroxide. An imbalance in production and degradation of hydrogen peroxide may result in release of ROS from peroxisomes into the cytosol.

In pln.atg1 cells, but also in cells of pln and atg1 single deletion strains, the intracellular levels of reactive oxygen species was increased relative to WT controls. Most likely this is due to inactivation of peroxisomal catalase, because cytochemical staining experiments revealed that, despite the fact that catalase protein and enzyme activities were normal, not all peroxisomes in pln, atg1 and pln.atg1 cells contained enzymatically active catalase.

In the absence of HpPln or pexophagy unfolded and damaged peroxisomal matrix proteins may accumulate, leading to the formation of sticky protein aggregates that may affect the balance between hydrogen peroxide production and degradation.