A remarkable feature of limb regeneration is the ability to regenerate normal limb morphology and anatomical patterning. Although it is thought that regeneration uses similar mechanisms to those employed during development, it is not well understood how this is achieved in the context of varying blastema size. In a new study, Akira Satoh, Yoshihiro Morishita and colleagues investigate the allometric scaling of blastema size and pattern expressions of key genes relative to the size of the limb stump in axolotls. To find out more about the work, we caught up with first author Saya Furukawa, and corresponding authors Akira Satoh, professor at Okayama University, and Yoshihiro Morishita, Principal Investigator at RIKEN, Japan.
Akira Satoh (left), Saya Furukawa (centre) and Yoshihiro Morishita (right)
Akira and Yoshihiro, can you give us your scientific biography and the questions your lab is trying to answer?
AS: I obtained my PhD at Tohoku University, Japan. Afterward, I conducted postdoctoral research at the University of California, Irvine, USA, for three years under the supervision of Dr D. M. Gardiner and Dr S. V. Bryant. Since 2009, I established my own laboratory in 2009 at Okayama University, Japan. My initial goal was to identify the molecules responsible for inducing limb regeneration in amphibians. Fortunately, I succeeded in identifying the regeneration-inductive molecules (FGF and BMP) and published these findings in 2014 (Makanae et al., 2014). Our next goal is to transfer these insights to mammalian systems.
YM: I received my PhD from the University of Tokyo Graduate School of Frontier Sciences in 2005. After serving as an assistant professor for six and a half years at a theoretical biology lab at Kyushu University, Japan, I have been a Principal Investigator at RIKEN since 2012, where I lead an interdisciplinary lab with researchers from various fields, aiming to uncover the principles of organ morphogenesis. In particular, my research aims to uncover the rules on cell/tissue dynamics underlying morphogenesis through detailed measurements and data analysis, and, if possible, to derive their mathematical representation. This includes understanding both the universal mechanisms of morphogenesis shared across species and organs, as well as the mechanisms that give rise to species-specific or unique forms.
Saya, how did you come to work in the lab and what drives your research today?
SF: I've been deeply fascinated by the regulation of animal body systems since my teenage years, and this interest gradually evolved into a passion for developmental and regenerative biology. Dr Akira Satoh's research on axolotl limb regeneration aligned with my interest in uncovering the regeneration mechanism from a fundamental biological perspective, rather than an applied one. In addition to this, his openness to discussing scientific questions has reassured me that this lab is the ideal environment for my PhD training. He also gave me an opportunity to collaborate with Dr Yoshihiro Morishita, which made it possible to use mathematical concepts effectively for the analysis in our recent research article, and also broadened my perspective. What drives my research today is a strong desire to understand how the unique morphogenetic mechanisms of urodele amphibians relate to their exceptional regenerative abilities.
Tell us about the background of the field that inspired your work
AS, YM & SF: It has been considered that limb regeneration is carried out by a mechanism similar to limb development in most parts. However, what sets limb regeneration apart from the developmental process is that limb regeneration occurs in much larger and various sizes of fields. Thus, we sought to reveal underlying scale-invariant mechanisms that make consistent limb morphogenesis possible from varying blastema sizes.
We sought to reveal underlying scale-invariant mechanisms that make consistent limb morphogenesis possible from varying blastema sizes
Can you give us the key results of the paper in a paragraph?
AS, YM & SF: We examined the allometric relationship between blastema size and expression patterns of key genes relative to limb stump size. Our results showed that all factors scaled allometrically rather than isometrically; specifically, their relative sizes decreased with an increase in limb stump size. However, the ratio of the Shh/Fgf8 signalling dominant region was nearly constant, independent of blastema/body size. Furthermore, the relative spatial patterns of cell density and proliferation activity, as well as the relative position of first digit formation, were scale invariant in the summed Shh/Fgf8 crosstalk region. Our results suggest that the scale invariance of patterning is one of the key mechanisms enabling regeneration from blastemas of different sizes to achieve normal morphology.
Saya, when doing the research, did you have any particular result or eureka moment that has stuck with you?
SF: Coming up with the idea of defining the region between the posterior end of Shh expression and the anterior end of Fgf8 expression as the ΩSF and normalizing the gene expression patterns by the size of the ΩSF was a eureka moment and a pivotal turning point in this research. I felt as though the dots finally connected when we discovered that gene expressions, cell distribution patterns, and the position of the first emerging digit were all scale invariant within the ΩSF.
Despite variations in limb and blastema sizes, axolotls can regenerate their limbs completely. Scale bar: 500 µm.
And what about the flipside: any moments of frustration or despair?
SF: Like many other research projects, there were some challenging moments. Since this research was a collaborative project with Dr Yoshihiro Morishita, who specializes in mathematical biology, the analytical methods we used were not commonly used in our lab. As a result, I had to constantly seek out new approaches and ideas, undergoing frequent iterations of trial and error to refine the methods. At times, the process was frustrating. However, with the kind support of my supervisors and lab members, I made steady progress. I believe that every challenge I faced has contributed to my growth as a PhD student.
Why did you choose to submit this paper to Development?
AS, YM & SF: In Japan (perhaps even in other countries), Development has been known as one of the leading and sophisticated journals. Publishing a paper in Development has been thought to be a sort of a signature as an established developmental biologist. This is our first one (AS and SF) and we are so proud of this.
Saya, what is next for you after this paper?
SF: I am now beginning to look for a postdoctoral position where I can further explore limb morphogenesis. With about a year and a half left before completing my PhD, I am also continuing my research on axolotl limb regeneration and attempting to publish my next work before graduation.
Akira, what are the implications of your findings to understand organ development?
AS: The ability of organisms of the same species to develop organs of identical shapes but of different sizes has not been studied much. However, this is very important when considering regeneration in the human body. Embryonic developmental systems consist of much smaller organs and tissues than those that are present in a fully developed system, but it is unknown whether regenerative systems, which recapitulate embryonic systems in many aspects, can produce consistently shaped tissues of different sizes. This study provides great insight into how to overcome this ‘size problem’ when considering adult organ regeneration.
Yoshihiro, where will this story take your lab next?
YM: In our previous research (Morishita et al., 2023), we demonstrated that the rescaled tissue deformation dynamics during limb development (i.e. the temporal changes in the position of each cell relative to the whole limb tissue) are highly conserved across species with different sizes, particularly in the early stages of development. This finding suggests that morphogenesis and size determination are governed by independent mechanisms. While the scaling of chemical patterning demonstrated in this study represents one way to achieve consistent morphogenesis regardless of size, it remains unknown whether cells are ‘aware’ of the absolute size of the embryo or organ to which they belong, and if so, how this recognition occurs and how cells regulate their responses based on the current size. I would like to address these questions using limb development and regeneration as a model system.
Finally, let's move outside the lab – what do you like to do in your spare time?
AS: I enjoy farmer-like activities and doing sports.
YM: I enjoy learning mathematical concepts and new data analysis methods that I don't know yet. I also like watching YouTube, dramas, and, recently, chatting with AI.
SF: In my spare time, I like to watch movies and dramas. I also enjoy singing, dancing, and swimming, which contribute to relieving my stress and maintaining my mental health.
S.F. & A.S.: Graduate School of Environmental, Life, Nature Science and Technology, Okayama University, Okayama 700-8530, Japan.
Y.M.: Laboratory for Developmental Morphogeometry, RIKEN Center for Biosystems Dynamics Research, Kobe 650-0047, Japan.
A.S.: Research Core for Interdisciplinary Sciences (RCIS), Okayama University, Okayama 700-8530, Japan.
E-mail: [email protected]; [email protected]
