Timelapse showing the nuclear envelope exhibiting less mechanical stability after PLK-2 recruitment ("+Auxin", compared to a normal nucleus "-Auxin").
Lehigh scientist develops a new molecular tool to answer a longstanding question in the cell biology of reproduction.
The biological factory that produces reproductive cells has a robust quality control system to identify and eliminate eggs with chromosomal abnormalities. For years, cell biologists have sought to uncover the mechanisms behind this system, relying on traditional genetic tools.
Now, a novel molecular tool developed by Chenshu Liu, assistant professor of biological sciences, is enabling scientists to precisely manipulate protein interactions inside a cell, offering new insights into how reproductive cells detect errors and make life-or-death decisions during the making of eggs.
The findings, recently published in the journal Science, stem from studies using C. elegans, a nematode worm commonly used in genetic research. The research deepens our understanding of the quality control processes that ensure the production of healthy eggs and could provide new perspectives in addressing infertility and congenital conditions such as Down Syndrome.
“This quality control system has been challenging to study using only existing genetic tools,” said Liu, who collaborated with Abby F. Dernburg of the University of California, Berkeley. “The new tool we developed provides the ability to directly test what had only been hinted at before.”
Reproductive cells—sperm and eggs—contain half the number of chromosomes found in other body cells, a reduction critical for reproduction. When a sperm meets an egg, the resulting offspring now has a complete set of chromosomes. This halving occurs through meiosis, a specialized type of cell division.
During meiosis, one progenitor cell duplicates its DNA and goes through two sequential rounds of cell division to make four cells, each with half the original number of chromosomes.
As a key step in this process, in an oocyte—the precursor of an egg—every pair of chromosomes inherited from both parents (e.g. Chromosome-21 from the father and Chromosome-21 from the mother) must find each other, align side-by-side, exchange genetic materials and then separate. When chromosomes don’t pair up side-by-side precisely, failed separation can result, leading to abnormal chromosome numbers in an egg and potential infertility, pregnancy loss or conditions such as Down Syndrome.
“To prevent that from happening, oocytes normally pass through a quality control checkpoint of sorts, to eliminate those with errors,” Liu explained.
This checkpoint detects defects, relays signals, and triggers apoptosis, or programmed cell death, in abnormal cells. Prior research suggested that a region at the end of each chromosome is important to this checkpoint. These chromosome ends interact with the nuclear envelope—the membrane surrounding the nucleus. However, the mechanisms by which they carry out the quality control mission remained unclear.
To understand these mechanisms, the researchers zeroed in on a protein called PLK-2, which prior research had indicated may play a key role in the checkpoint function. The protein had been observed as staying at chromosome ends and the nuclear envelope in abnormal oocytes.
However, the previous genetic studies were unable to determine whether PLK-2’s presence at these locations triggered the checkpoint or was a byproduct of it.
Microscopic image of C. elegans oocytes, with chromosomes illuminated in purple and the nuclear envelope in green.
To solve this puzzle, the researchers developed a new molecular tool in C. elegans called “chemically induced proximity” (CIP), which uses a plant hormone called auxin as a molecular “glue” to tether one protein to another, changing its location inside a living cell.
Using CIP, the team could manipulate where PLK-2 goes and acts.
“The CIP system is like calling a rideshare service to get a worker to a specific job site. In this case, the proteins are the workers, scattered throughout the cell. With the CIP, we can send a chemical linker that pairs the protein with a driver already heading to the job site,” Liu said. “This approach allows us to relocate proteins with precision, uncovering how their presence at specific locations affects cellular functions."
The researchers found that directing PLK-2 to chromosome ends at the nuclear envelope caused chemical modifications to the nuclear envelope, making it more pliable and less mechanically stable. This destabilization led to apoptosis, in a process dependent on the mechanosensitive ion channel called Piezo1/PEZO-1.
The connection to Piezo1 channels was unexpected. Piezo channels are better known for sensing mechanical forces at the cell’s outer membrane in tissues like skin and blood vessels, a discovery that was jointly awarded the 2021 Nobel Prize in Physiology or Medicine.
“This is the first time Piezo channels at the nuclear membrane have been linked to quality control during reproduction,” Liu said. “It shows that Piezo can also respond to events originating in the cell nucleus and this opens up a whole new area of research.”
Although this research was conducted in tiny worms, similar quality control mechanisms could operate in mammals, including humans. Because errors associated with meiotic chromosome separation are a major cause of age-related decline in egg quality, this research on meiotic quality control could potentially benefit human reproductive health.
In addition, the CIP system developed in this study is a powerful tool that scientists can use to manipulate protein dynamics across various biological systems, with the potential to make significant impacts far beyond the field of reproductive health.
“Oocytes have a finite lifespan,” Liu said. “Those waking up later in life may have waited dormant for as long as 50 years! We hope this research provides new perspectives in understanding age-related oocyte quality decline, and one day can hopefully help extend the quality lifespan of these amazing, life-bearing cells.”
Story by Dan Armstrong
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