A simple yet previously unanswered question prompted researchers at University Medical Center Groningen to observe living yeast cells for hours under a confocal microscope: where do nanodiamond particles go after a cell divides? The question has now been answered, and the result was far from what researchers expected. The study, published in Nanomaterials in March 2020, involved drg. Aryan Morita, M.Sc., Ph.D., a researcher from the Department of Biomedical Sciences, Faculty of Dentistry, Universitas Gadjah Mada, who was one of the study's leading authors alongside an international team led by Prof. Romana Schirhagl in Groningen, the Netherlands.
As Small as a Virus, as Bright as a Star
Fluorescent nanodiamonds (FNDs) are diamond particles measuring only a few nanometers that emit bright red light when illuminated with a green laser. At just 70 nanometers in diameter—approximately one-thousandth the diameter of a human hair—their potential as biological markers is remarkable. Unlike conventional fluorescent dyes, which fade within minutes, FNDs do not photobleach. This property has attracted scientists interested in using them to track processes inside living cells over extended periods.
FNDs are also known for their excellent biocompatibility. Previous studies have shown that these particles do not damage mammalian cells. However, what happens to FNDs when cells divide, particularly in model organisms such as the yeast Saccharomyces cerevisiae, had never been systematically investigated. This study represents the Groningen team's original contribution to the field.
Yeast was chosen for good reason. This single-celled organism reproduces asymmetrically, with a mother cell producing a smaller daughter cell. More importantly, yeast has a diffusion-barrier mechanism in its membrane that naturally prevents harmful substances, including aging-related factors, from being transferred to daughter cells. The question was whether FNDs would be treated in the same way as these “harmful substances.”
Daughter Cells Receive More Particles
The research team introduced FNDs into yeast cells by temporarily removing the cell wall through a process called spheroplasting, then allowing the cell wall to regenerate after the particles had entered. The FND-containing cells were subsequently monitored for six hours using a custom-built confocal microscope equipped with a 532-nm laser.
The results were surprising. Rather than remaining in the mother cell as might have been expected based on yeast's diffusion-barrier mechanism, the nanodiamond particles were found more frequently in daughter cells. Quantitative data from 100 observed cells showed that 21% of uncoated FNDs and 28.4% of lipid-coated FNDs were transferred to daughter cells, while only 14.9% and 21.6%, respectively, remained in the mother cells.
“These findings differ from what occurs with heat-induced protein aggregates, in which the particles tend to remain in the mother cell. This suggests that FNDs may not be recognized by the active control mechanisms of yeast cells.”
According to the researchers, the most plausible explanation is particle size. With a diameter of 70 nm, FNDs are substantially smaller than protein aggregates, which can reach 600 nm. Smaller particles can pass more easily through the bud neck, the narrow connection between mother and daughter cells during cell division. In addition, FNDs may lack molecular markers recognized by the cell's active transport system, allowing them to pass through without being actively “returned” to the mother cell.
A Second Surprise: Most Particles Are Expelled from the Cell
An equally surprising finding was that most of the particles were actually expelled from the cells. A comparison of particle numbers before and after cell division showed a significant decrease. The researchers estimated that 98.35% of uncoated FNDs and 98.38% of lipid-coated FNDs were excreted from the cells.
This phenomenon has not previously been reported in mammalian cells and appears to represent a unique response of yeast to an imbalanced condition, such as when cells are transferred into a medium with a different composition. The researchers also tracked particle movement inside the cells using mean square displacement (MSD) analysis. Both uncoated and lipid-coated FNDs exhibited confined diffusion, meaning that the particles moved randomly within a limited space rather than moving freely in and out of the cell like molecules in solution. There was no significant difference in displacement or diffusion coefficients between the two types of FNDs. The only statistically significant difference was the alpha value, a measure of the particles' freedom of movement. Lipid-coated FNDs showed a lower alpha value because of their tendency to form aggregates inside the cells.
From Yeast to Broader Applications
This research is more than an academic exercise. Understanding the fate of nanoparticles inside dividing cells is important for developing nanoscale temperature sensors and magnetometers in living cells—technologies being explored to understand cellular aging and stress.
drg. Aryan Morita, M.Sc., Ph.D., who completed his doctoral studies in Groningen with an LPDP scholarship from Indonesia's Ministry of Finance, now brings this expertise to FKG UGM. Research of this kind could pave the way for innovations in dental nanobiomaterials, including the potential use of nanoparticles as diagnostic markers or more precise drug-delivery systems in the future.
The discovery that daughter yeast cells receive more nanodiamond particles than their mother cells—contrary to the logic of a well-known cellular protection mechanism—is a reminder that cell biology always contains surprises. And sometimes, the most interesting answers come from the simplest organisms.
Authors: Achmad Zam Zam Aghasy, DDS, M.Kes.; Hazra Alifia Muharam
Photo: Pexels
Sumber DOI: https://doi.org/10.3390/nano10030516