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Some animals can regenerate even complex organs. Salamanders can grow back the exact missing part of an arm and fully restore its function. This is possible as the cells that remain after the injury remember their original position within the limb. How this positional memory is encoded was a long-standing question in biology. A team of scientists led by Maximina Yun has now identified Tig1 as a protein determining cell position within the salamander’s limb. They show that Tig1 plays a central role in the salamander’s ability to regenerate correctly. The results were published in the journal Nature Communications.

Humans have a limited ability to heal their bodies after injury. However, several animal species can regrow their tissues, organs, and even whole body parts. Among them, salamanders are known to have remarkable regeneration abilities. They can grow back fully functioning limbs in a matter of weeks. Regardless of what part of an arm they lose, salamanders are always able to grow back the exact part that is missing.

“Growing back the missing part of the organ sounds natural, yet it represents a significant biological challenge. The organism needs to first identify which exact part is missing. This means that the cells that remain in the stump need to know where they are in the context of the full structure. This positional information is key to regenerating the missing part,” says Maximina Yun, research group leader at the Center for Regenerative Therapies Dresden (CRTD) and Max Planck Institute of Molecular Cell Biology and Genetics (MPI-CBG), who led the study.

The Memory Factor

Scientists have long speculated that the information about a cell’s position within the organ or body part has to be somehow hard-wired into that cell. “We expected that this kind of positional memory, or identity, would ideally be present in a form of a gradient. For example, in the case of an arm, cells belonging to the shoulder would contain high amounts of such factor, yet its concentration within the cells would gradually decrease towards the fingers,” adds Yun. “In such a way, the amount of the factor would determine how far away from the core of the body a cell actually is.”

Dr. Yun worked together with the DRESDEN-concept Genome Center, Andras Simon’s group (Karolinska Institute, Sweden), and Tobias Gerber (EMBL Heidelberg). The team took advantage of the modern single-cell RNA (scRNA) sequencing technology to look for factors that could store the memory of the cell’s position. They analyzed the contents of single cells all along the salamander’s arm in the search for molecules that change in concentration and therefore could encode the positional information within the cell.

One protein stood out within the collected data. “We found that a protein known as Tig1 is actually expressed in a gradient in the salamander’s arm,” says Catarina Oliveira, main author of this study. “The further down the arm, the less Tig1 the cells have, making it the perfect candidate for a factor that could store the memory of cell’s position within the limb.”

More than an Indicator

Tig1 is a protein present on the surface of the cells. Being projected on the outside of the cell, the scientists thought that it is only storing the information about the cell position. “We were truly surprised to see what happened when we modified the levels of Tig1 in cells. When we tuned up the level of Tig1 in cells destined to become part of the hand, which normally have very little of it, these cells reprogramed their molecular identity to that of cells that will become part of the forearm,” says Yun. When the levels of Tig1 were changed, so were the genes that were expressed. The pattern of gene expression was now matching the cells that would normally be present higher up in the salamander limb. “This shows that Tig1 is not only indicating the position of a cell within the limb, but it is likely one of the factors that actually determine it,” adds Yun.

The team has also shown that Tig1 is crucial for salamanders to regenerate correctly. Indeed, changing the levels of the protein in cells of a regrowing arm leads to growth defects. This further highlights the importance of Tig1 in the regeneration process and specifically, in deciding just how much to regenerate.

A Significant Milestone

This work answers a long-standing question in the fields of regeneration and developmental biology. So far, only a handful of other molecules have been associated with the positional identity of the cells. This is also the first example of a protein present on the surface of the cell which can reprogram the pattern of gene expression within the cell towards a “proximal” positional identity, i.e., a forearm identity.

Although the results come from salamanders, Tig1 may play a similar role in other animals. “Tig1 is highly conserved across evolution. This means that it is present in a rather unchanged form in many animals. It remains to be seen if Tig1 determines positional identity in other species as well,” explains Yun.

The researchers would like to build on these findings and explore the mechanism by which Tig1 can determine the positional identity of cells. The team is already looking for Tig1 protein partners, both at the cell surface as well as inside the cells. “One way Tig1 can influence gene expression inside the cells is through impacting the mechanical properties of the cells,” says Yun. “We are happy that our group is now also associated to the Cluster of Excellence Physics of Life (PoL) at the TU Dresden, allowing us to partner with excellent biophysicists to investigate the mechanism of Tig1 function in detail.”