CAREER: Linking systemic stem cell activation to vertebrate limb regeneration

Harvard University, Cambridge MA

Investigators

Abstract

This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117- 2). Salamanders can completely regenerate their legs following amputation throughout life. How they regenerate is still mysterious despite centuries of research. Humans and other mammals have very limited natural limb regeneration abilities, restricted to just the very tips of the fingers and toes. Meanwhile, some invertebrates, like planarians, can regrow almost every body part. The huge variation in regenerative ability is also not well understood. This project aims to fill in missing information about how salamanders regenerate limbs. It will also address how similar these processes in salamanders are to regenerative processes in other super-regenerators and to injury responses in mammals. A systemic response to amputation was recently discovered in axolotl salamanders in which some cells throughout the body are activated to make new cells. The identity and function of these cells will be addressed. Recent data also indicates the nervous system transmits signals that distant cells use to sense an amputation. This project will test several molecules detected in nerves to determine if they are required for the body-wide injury response. It will also address the role of molecules that travel in the blood in transmitting amputation injury signals in axolotls. Finally, the project will examine molecules important for body-wide injury responses in planarians and mice in the axolotl context. Together, these experiments will build understanding of how systemic responses are linked to local regeneration of a body part. They will also fill in gaps in knowledge about evolution of regenerative processes, and they will inform future regenerative medicine approaches. This project will also train many undergraduate students in a new course and through a laboratory exchange program. Finally, it will reach out to the public with a weekly web documentary that interviews scientists from diverse backgrounds. With the recent application of a variety of molecular genetic tools in axolotl, amputation responses and subsequent limb regeneration can finally be understood at a mechanistic level. This project seeks to define the connections between systemic progenitor cell activation, defined as cell-cycle re-entry, and limb regeneration in axolotl. The fate and identity of systemically-activated cells will be investigated using integrative retroviruses in vivo as well as transgenic axolotls and single-cell RNA-seq. The specific roles of the nervous and circulatory systems in systemic activation will also be examined experimentally, using both gain-of-function and loss-of-function approaches, and building on RNA-seq data from peripheral nerves and responding tissues as well as plasma proteomics. The project will also test for possible conserved roles of previously identified factors in injury responses in both planarians and mice, such as EGR1, Wnts, and HGF, in the axolotl system. Together, these experiments will inform a developing model in which systemic injury responses may represent ancestral processes that have been differentially elaborated upon at the local injury site during evolutionary time, manifesting in a large range of regenerative outcomes in extant species. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.