Senescent Cells Key to Axolotl Limb Regeneration

Senescent cells have been implicated in a variety of processes typically connected with deterioration and aging. Recent studies suggest that the short-term presence of senescent cells can actually be beneficial. A new comprehensive study by the Yun group shows that cellular senescence plays a critical role during axolotl limb regeneration. Senescent cells in the regenerating structure use the Wnt pathway to signal to surrounding cells and create a microenvironment that favors regeneration and growth. The results were published in the journal Developmental Cell.

Senescent cells, often referred to as “zombie cells”, are no longer dividing but also not dying. Their buildup is considered one of the hallmarks of aging however recent studies suggest that they also play a role in positive processes such as wound healing and tumor suppression.

‘‘Our understanding of the role that senescent cells play in regenerative processes is very limited. Since most of the current knowledge relies on in-vitro studies, it was clear to us that to get new insights we need to find a way to study senescent cells in-vivo, i.e., analyze them within the animal during the process of regeneration,” says Dr. Maximina Yun, research group leader at the Center for Regenerative Therapies Dresden (CRTD) and the Cluster of Excellence Physics of Life at TUD Dresden University of Technology as well as the Max Planck Institute of Molecular Cell Biology and Genetics (MPI-CBG) in Dresden.

Studying Regeneration in Axolotl
To study the regeneration process, the Yun group turned to axolotls. These Mexican salamanders are one of a handful of species that can regenerate complex body parts such as limbs, spinal cord, and parts of their eyes, heart, and brain. Upon injury, axolotls close their wound and form a blastema - a special cell mass that contains progenitor cells. Over time, these cells differentiate and rebuild the repertoire of tissues needed for a functional body part.

‘‘After injury, senescent cells are transiently present in the blastema. We found a clever way to target only these cells and poison them to turn them off. In such a way, we could observe what happens to the regeneration process when the senescent cells are virtually gone,” explains Dr. Yun.

Limit Damage and Turn On Repair
The team found that without the senescent cells, fewer cells were dividing, effectively putting the regeneration on hold. It was as if the senescent cells in blastema were sending signals to their neighboring cells to divide. “A key feature of senescent cells is indeed their capacity to modulate their immediate environment,” adds Dr. Yun.

Based on the results, the team was able to show that these cells secrete molecules that belong to the so-called Wnt pathway to signal to neighboring cells. Their subsequent experiments showed that the signaling works two-fold. First, it prevents the progenitor cells from becoming senescent. Second, it promotes the proliferation of progenitor cells, and therefore the regrowth of the missing tissue.

A Mix of Cutting-Edge Approaches
In collaboration with the DRESDEN-concept Genome Center and the labs of CRTD alumni Prof. Volker Busskamp and Dr. Sumeet Pal Singh, the team used single-cell RNA transcriptomics to analyze different characteristics of the senescent cells in the blastema. This allowed them to identify the Wnt signaling as the pathway used by the senescent cells to influence their environment. The RNA analysis also showed the origin of the senescent cells in the blastema.

“We found that the senescent cells originated mostly from the connective tissue and macrophages, special immune cells that exist within the tissue. We were also able to show that their senescence is likely brought about by disruption of ribosomal biogenesis. Their protein production factories are dysregulated and this can lead to a stop of the cell cycle, resulting in a state of cell senescence,” explains Dr. Yun.

To directly influence the senescent cells in the tissue and turn them off, the group worked together with Prof. Ramón Martínez-Mañez to develop specially coated nanoparticles that can travel through the tissue and exclusively bind to senescent cells.

“Regeneration is a complex phenomenon. The more we know about it, the more we see how much it relies on an interplay of different tissues. It is therefore crucial for us to look at the regeneration process as a whole and in its native context of an animal,“ explains Dr. Yun, “It also necessitates an interdisciplinary approach to move our understanding of regenerative processes forward.“

Original Publication
Qinghao Yu, Hannah E. Walters, Giovanni Pasquini, Sumeet Pal Singh, Martina Lachnit, Catarina R. Oliveira, Daniel León-Periñán, Andreas Petzold, Preethi Kesavan, Cristina Subiran Adrados, Ines Garteizgogeascoa, Dunja Knapp, Anne Wagner, Andrea Bernardos, María Alfonso, Gayathri Nadar, Alwin M. Graf, Konstantin E. Troyanovskiy, Andreas Dahl, Volker Busskamp, Ramón Martínez-Máñez, Maximina H. Yun: Cellular senescence promotes progenitor cell expansion during axolotl limb regeneration. Developmental Cell (October 2023)
Link: https://doi.org/10.1016/j.devcel.2023.09.009

Scientific Contact:
Dr. Maximina H. Yun
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