Peter Kijun Kim____Mitochondria and Peroxisomes: A Collaborative Relationship in Redox Homeostasis

The talk will cover unpublished work on the interaction between organelles, particularly mitochondria and peroxisomes, in maintaining cellular function.
The speaker encourages questions and feedback.

The idea that it takes a village of organelles to maintain mitochondria.
Electron micrographs from the 1960s and 70s showed mitochondria in close proximity to the endoplasmic reticulum (ER), lysosomes, and peroxisomes.
These organelles collaborate to maintain mitochondrial function.

Mitochondria are critical for various cellular processes, including metabolism, peptide synthesis, and cellular functions.
They play a role in cell death pathways, autophagy, and innate immunity.
The ER is involved in these processes and couples with mitochondria.
Other organelles, such as lipid droplets and peroxisomes, provide free fatty acids, while endosomes provide amino acids.

Organelles interact through membrane contact sites: close proximity between two bilayers (10-80 nanometers).
Tethering proteins maintain the distance at contact sites, preventing membrane fusion (unlike SNARE proteins).
These sites facilitate the transport of lipids, ions, and amino acids.
Mitochondria form contacts with various organelles.

The speaker's lab is interested in the relationship between peroxisomes and mitochondria, particularly regarding reactive oxygen species (ROS).

ROS are major signaling molecules involved in cell death and differentiation. Maintaining redox homeostasis is important in mitochondrial function.
Excessive ROS can be detrimental to the cell.
Mitochondria produce ROS via the electron transport chain.
Antioxidants regulate ROS levels within mitochondria to maintain redox homeostasis.
Isolated mitochondria can take up ROS; however, it is not certain if it does in vivo.

Peroxisomes contain about 50 different enzymes, mainly involved in lipid metabolism and the synthesis of lipids, and a large quantity of antioxidants, including catalase.
Peroxisomes are often found in close proximity to mitochondria.
Mitochondria are affected in metabolic and genetic diseases where peroxisomes are absent.

A mouse model with a common mutation affecting peroxisome biogenesis was used.
In these mice, catalase is primarily cytosolic instead of being inside peroxisomes.
Mitochondria in these mice exhibit stress.
It was investigated whether peroxisomes regulate mitochondrial ROS.

A method was developed to measure peroxisome-mitochondria contact sites using live-cell imaging.
Peroxisomes that stayed close to mitochondria (within one pixel) for at least 20 seconds were considered to be in contact.
Under normal conditions (glucose as the carbon source), about 15% of peroxisomes are in contact with mitochondria.
When cells were switched to galactose (forcing reliance on oxidative phosphorylation and increasing ROS production), the contact between peroxisomes and mitochondria increased.
When peroxisomal catalase was targeted to mitochondria, the increased contact was reduced, indicating that the contact responds to mitochondrial oxidative stress.

ACBD5 was identified as a tethering protein through BioID crosslinking.
ACBD5 interacts with PTPIP51 (a known mitochondria-ER tethering protein), suggesting that they may act as a tether for each other.

It was demonstrated via immunoprecipitation that ACBD5 interacts with PTPIP51.
Overexpression of PTPIP51 increased mitochondrial-peroxisome contact, but this increase was abolished when ACBD5 was knocked down.
This suggests that both proteins must interact.
Knockdown of ACBD5 also prevented the increase in contact observed with galactose treatment.
ACBD5 knockdown does not affect peroxisome numbers.

A redox-sensitive GFP (RoGFP) was used to measure changes in the redox state of mitochondria and peroxisomes.

In ACBD5 knockout cells, there was a significant increase in mitochondrial oxidation when cells were switched to galactose.
Expressing an artificial tether to force contact between mitochondria and peroxisomes rescued this oxidative stress.

RoGFP was targeted to peroxisomes.
Upon addition of galactose, there was an increase in oxidation within peroxisomes.
When catalase was targeted to mitochondria, this increase in peroxisomal oxidation was not observed, indicating that ROS produced in mitochondria is being transferred to peroxisomes.

What is the mechanism for transferring ROS between mitochondria and peroxisomes?
How is this contact regulated, and does the ER play a role?
Can this process be targeted for therapeutic intervention?