A novel approach to studying the inner workings of cells — particularly to track the movement of ions inside the numerous microscopic organelles inside the cell — has been proven in two experiments by researchers at the University of Chicago.
Chemist Junyi Zhou at work in the laboratory of UChicago Prof. Yamuna Krishnan. Two new papers show how to measure potassium and sodium inside cell organelles. Image Credit: Nancy Wong/University of Chicago
In addition to demonstrating that these sensors are functional, their research also proved that organelles actually control ions, answering a biological conundrum that had been up for discussion.
The method can help uncover new details about how cells work, which might improve the understanding of the ability to treat diseases and conditions like Parkinson’s.
‘An Underexplored Area’
Cells require ions such as sodium and potassium to perform essential functions. That is why, during activities like running a marathon, it is crucial to replenish not just water but also electrolytes. These ions play vital roles in processes like signaling and muscle contractions, and cells maintain precise control over their influx and efflux.
However, the function of ions inside the various components of the cell is less well understood. These are the organelles, such as the Golgi bodies, lysosomes, mitochondria, etc., each of which performs a specific task.
This is an underexplored area, because we haven’t had the tools to measure ions inside organelles. But there are good reasons to believe ion concentrations are important inside organelles.
Junyi Zou, Study Co-Author and Chemist, University of Chicago
Zou and colleague Palapuravan, who goes by a single name, are part of UChicago Prof. Yamuna Krishnan's laboratory. Prof. Krishnan specializes in developing minuscule "devices" crafted from DNA to explore cell processes.
These DNA-based devices offer unique advantages. Being biologically compatible and non-toxic, they enable researchers to observe live cells in action, significantly improving conventional methods that cannot be utilized in vivo. Moreover, these devices can endure extreme pH levels that would typically render other sensors ineffective.
To accomplish this goal, the team had to ensure that the sensors targeted a specific organelle within specific cells. To achieve this precision, they utilized a molecule that allowed the sensors to hitch a ride on a protein. This protein naturally moves between the cell membrane and a specific organelle as part of its regular functions.
When the sensors are within the organelle, they interact with surrounding ions and light them up, which can be observed under a microscope.
Zou added, “This allows us to quantify the level of ions by measuring the brightness of the sensor.”
Both Zou and Palapuravan concentrated on distinct ions and organelles.
Palapuravan concentrated on locating potassium in organelles that are involved in the cellular recycling process. Recyclable endosomes are the organelles in charge of sorting and transporting ion channels to and from the cell surface, but nobody had investigated whether or not these ion channels were also actively controlling ion levels elsewhere in the cell.
The outcomes were obvious.
The ion channels are definitely active in organelles.
Palapuravan, Postdoctoral Researcher, University of Chicago
The discovery offers up a new line of research to determine the specific function of ion concentrations.
Palapuravan added, “We know that ion channels are involved in diseases such as Parkinson’s, but pharmaceutical drugs to date generally target ion channels that are only on the plasma membrane, and not in organelles. Their activity inside organelles may turn out to have interesting new roles and new drugs can be developed.”
The Role of Lyosomes
Meanwhile, Zou’s research focused on the monitoring of sodium ions within an organelle known as the lysosome. Lysosomes break down cell detritus and are vital in a variety of disorders, but no one has succeeded in inserting a working sodium sensor.
Using the novel sensors, the researchers discovered evidence that lysosomes were actively controlling cellular sodium, indicating that lysosomes serve a crucial function in assisting cells in managing their sodium levels.
They also discovered evidence to support the idea that the quantity of sodium in the lysosome is critical for an organism's survival in a high-salt environment; worms lacking lysosomal sodium transporting proteins were less likely to survive.
Zhou stated, “It is an interesting connection to the entire organism’s metabolism.”
The sensors should be useful in future research to investigate the role of ions and ion channels within cells, perhaps leading to a new basic knowledge of biology and disease.
I am very excited about the fact our sensors have revealed that sodium and potassium—ions that are so important in health and disease—are actively moving across organelle membranes. Together, both findings change our perception of organelle membranes from being inert bags whose purpose is to simply ferry their contents to a destination, to one where they are fizzing with activity while doing so—thereby letting their insides communicate with the outside.
Yamuna Krishnan, Study Senior Author and Professor, University of Chicago
The studies’ other UChicago authors included Koushambi Mitra, Daphne Oettinger, Joseph Ramirez, Aneesh Tazhe Veetil, Priyanka Dutta Gupta, Jayson Smith, Paschalis Kratsios, and Anand Saminathan, as well as co-authors from Johns Hopkins University School of Medicine, the University of Kentucky College of Medicine, and the University of Illinois College of Medicine.
Journal Reference:
Zou, J., et al. (2023) A DNA nanodevice for mapping sodium at single-organelle resolution. Nature Biotechnology. doi:10.1038/s41587-023-01950-1.