Now, in a study recently published in Nature Communications, Japanese researchers have uncovered the key mechanisms behind physiological cell competition and developmental robustness using an elegant series of experiments in zebrafish.

The research team from Osaka University used zebrafish to visualize the specific cell patterns of spinal cord and muscle tissue. By blocking or inhibiting apoptosis — a type of programmed cell death — they discovered altered cell patterns in these tissues.

“As expected, when we blocked apoptosis during zebrafish development, we observed abnormal patterns in the spinal cord and muscle,” says lead author of the study Kanako Matsumoto. “This finding emphasized the importance of apoptosis for eliminating unfit cells via cell competition, but it also raised a crucial question: how are these unfit cells sensed and removed?”

To uncover the answer, the team focused on a protein known as Sonic hedgehog (Shh), which has a clear activity gradient in developing zebrafish tissue. Through sophisticated imaging techniques, the researchers found that cells with abnormal Shh activity levels for their location displayed high levels of apoptotic markers. Furthermore, when apoptosis was blocked, these unfit cells became more abundant, disrupting the Shh gradient. Together, these results suggested that unfit cells with abnormal Shh activity undergo cell death. This begged another question: how do cells communicate their Shh activity levels to one another?

The team discovered that N-cadherin, a membrane protein, enables neighboring cells to sense and respond to cells with abnormal Shh activity, and that the removal of unfit cells occurs via a specific pathway — the Smad/Foxo3/reactive oxygen species/Bcl2 pathway.

This discovery led to the identification of Foxo3, a protein previously linked to longevity, as a mediator of cell competition. Cell competition can eliminate various unfit cells, including those with abnormal Wnt, Shh, or oncogenic Ras signaling, or those with low pluripotency, abnormal ribosomes, or mitochondrial defects. However, the roles and mechanisms of endogenous cell competition during organogenesis remain unclear. Numerous factors drive cell competition. The researchers therefore wondered: are there any universal machineries regulating the diverse types of cell competition?

“Foxo3 is a common mediator of unfit cell elimination in various types of cell competition in zebrafish and mice, and Foxo3-mediated physiological cell competition is required to eliminate naturally generated unfit cells and for the consequent precise development of embryonic, spinal cord, and muscle tissues,” says Tohru Ishitani, senior author of the study.

Genetic variations in Foxo3 are associated with longevity, whereas low Foxo3 activity is associated with age-related diseases. Foxo3 may also sustain the robustness of tissue homeostasis through cell competition, and reduced Foxo3 activity would thus lead to the accumulation of unfit cells. This poses significant risks not only in disorders of development but also in cancer initiation and aging. Conversely, understanding how cell competition eliminates unfit cells may offer exciting possibilities for new treatments that combat congenital disorders, cancer, age-related diseases, and aging, potentially improving health and longevity.

Interestingly, Foxo3 expression was detected in unfit cells in both zebrafish and mice, pointing to its potential role as a universal marker of cell competition. As co-lead author Yuki Akieda concludes, “This new marker would allow us to precisely detect naturally generated unfit cells, thereby helping us to understand the importance of physiological cell competition and elucidate the causes of ‘abnormality’ of unfit cells.”