Identification of Novel Regulators of Mitochondrial Protein Quality Control

Abstract

Chronic pesticide exposure has been linked to several human diseases including cancer, Parkinson’s disease, Alzheimer’s disease, multiple sclerosis, cardiovascular disease, and kidney disease. Due to the potential risks to human health, the usage of pesticides has been a controversial topic. Paraquat (PQ), a commonly used pesticide worldwide, has toxic effects due to its generation of reactive oxygen species (ROS) in mitochondria and other cellular compartments. Mitochondria are an essential organelle in eukaryotic cells responsible for energy production, and thus it is intuitive that there exists a dedicated stress response pathway to protect their function. This study focuses on the mitochondria-associated degradation (MAD) pathway, which promotes mitochondrial protein quality control through identification, retrotranslocation, ubiquitination, and degradation of damaged, dysfunctional mitochondrial proteins. We have identified two novel proteins, Y and Z, in MAD, potentially as E3 ubiquitin ligases.

Popular Abstract

Protecting Mitochondria - a Matter of Life and Parkinson’s Disease

As we age, our cells accumulate oxidative damage, especially in the mitochondria. Failure to remove damaged mitochondria has been connected to severe neurodegenerative diseases such as Parkinson’s disease. Our study has revealed two proteins involved in the cellular mechanisms dedicated to protect mitochondria and maintain their fitness and health.

Parkinson’s disease affects 1 out of 100 persons over the age of 60, and in most cases there is no specific known cause. However, as modern research advances, scientists find more clues to what factors may cause Parkinson’s. Recently, defects in mitochondria have emerged as a potential cause for disease.

The mitochondrion is a very important part of our cells. It works as the cell’s power plant by producing energy, and it plays a central role in metabolism and programmed cell death. Due to its many important roles it is crucial for the cell to maintain the health and fitness of the mitochondria. The mechanisms of mitochondrial quality control are not well known, but are becoming increasingly interesting to scientists. One reason is that failure to remove damaged mitochondria have been connected to severe neurodegenerative diseases such as Parkinson’s disease.

What causes this detrimental damage to mitochondria? During energy-production, highly reactive oxygen molecules are produced as a natural side-effect in the mitochondria. These molecules immediately react with other structures, such as proteins, fats, and DNA, causing molecular damage known as oxidative damage. Over time there is an accumulation of oxidative damage in the cell, which is the reason why many scientists argue that oxidative damage is the cause of aging. This is also the reason why food containing anti-oxidants are claimed to have anti-aging effects.

As mitochondria are highly important organelles, but also the major source of reactive oxygen molecules, there are several mechanisms dedicated to reduce damage to the mitochondria. One of these is the mitochondria-associated degradation pathway (MAD). The purpose of MAD is removal of mitochondrial proteins that have been damaged, in many cases through oxidation by reactive oxygen species. In MAD, the damaged proteins are recognized and transported to the mitochondrial surface where a “degradation-flag” is attached to the damaged protein. The flag is made up of at least four ubiquitins, a small signaling protein. Due to the attached ubiquitin-flag, the damaged protein is sent to the proteasome, the protein recycling center, where it is degraded into its building blocks; amino acids.

Although we have a good idea of the general principle of how MAD works, the molecular players involved are not yet identified. To study MAD we used common baker’s yeast, as yeast cells are easier to work with than human cells, but still similar enough to draw interesting conclusions from. MAD is however not particularly active during normal conditions. To increase the level of oxidative damage, and to mimic conditions of aging, we added the common pesticide Paraquat to the growth medium. Paraquat is toxic because it causes damage similar to exceptionally rapid aging. It is taken up by cells and transported to the mitochondria, where it causes the formation of massive amounts of reactive oxygen molecules. Interestingly, chronic exposure to Paraquat has also been linked to Parkinson’s disease.

In our studies we knocked out yeast genes that had previously been identified as potential players in MAD. We then studied the relative health and oxidation levels of mitochondria, to see whether the gene was important to maintain general mitochondrial fitness. We also studied the amount of protein that carried the ubiquitin degradation-flag, to see whether the gene was involved in the MAD pathway. Our studies identified two proteins involved in MAD, likely acting to attach ubiquitin to the damaged proteins.

Further research on these two proteins, MAD and other mitochondrial quality control mechanisms may in the future lead to identification of new therapeutic targets for treatments to prevent development of neurodegenerative diseases such as Parkinson’s.

• author: Bratbakken Lundvall, My
• supervisor: Johan Svensson Bonde
• organization: Pure and Applied Biochemistry
• course: KBK820 20151
• year: 2015
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering; Chemistry; Biology and Life Sciences
• keywords: Mitochondrial Quality Control, Oxidative Stress, Ubiquitin ligases, Ubiquitin, Yeast, Parkinson's Disease, Mitochondria, Paraquat, Oxidative Damage, Superoxides, tillämpad biokemi, applied biochemistry
• language: English
• id: 7991366
• date added to LUP: 2016-01-13 15:28:12
• date last changed: 2016-01-13 15:28:12