Cell Signaling Mechanisms in the Initiation, Propagation, and Termination of Adult Tissue Regeneration

Date of Award

6-2024

Degree Name

Doctor of Philosophy

Department

Biological Sciences

First Advisor

Wendy S. Beane, Ph.D.

Second Advisor

John Spitsbergen, Ph.D.

Third Advisor

Kathryn Docherty, Ph.D.

Fourth Advisor

Adil Akkouch, Ph.D.

Keywords

Apoptosis, planar cell polarity, reactive oxygen species, regeneration, stem cells

Abstract

Wound responses help organisms survive damage from both pathogens and diverse injury types, however it remains a mystery why some wounds result in functional regeneration while others form scars, fail to heal, or cause carcinogenesis. Regenerative abilities vary greatly across phyla; some organisms can undergo whole-body regeneration whereas most vertebrates have either tissue-specific or life stage-limited regenerative capacity. Many genes required for regeneration are highly conserved across embryogenesis, suggesting humans may possess latent molecular tools to regenerate lost or damaged cell types. However, therapies have been largely unsuccessful, partially due to our minimal understanding of how cells communicate across diverse tissue types to orchestrate and terminate new growth from an undefined starting point in situ. This remains a fundamental challenge in regenerative medicine.

Injury activates highly conserved molecular wound responses that regulate both tissue repair (such as wound closure and re-epithelialization) and initiate regeneration through downstream signaling. The propagation of the regenerative response involves the re-establishment of patterning/positional information, which collectively instructs differentiation decisions of proliferating stem cells to replace missing cell types and guide tissue formation. New and existing structures must be properly integrated to maintain overall size and scale, ensuring function. Finally, external growth termination cues must be transduced to internal molecular machinery to repress regenerative responses and return tissues to homeostasis. These data have identified unique signaling mechanisms that regulate the initiation, propagation, and termination of regeneration in the planarian flatworm model system Schmidtea mediterranea. These data show that reactive oxygen species (ROS) accumulation, a highly conserved injury response, functions via a threshold mechanism to signal for wound repair at low levels, while relatively higher levels are required to initiate regeneration-specific transcriptional responses. Through RNA sequencing, general wound-induced genes and regeneration-specific genes whose expression was regulated by ROS levels were identified and their role in wound closure and regeneration was verified via RNA interference (RNAi). Interestingly, ROS are a wellknown regulator of programmed cell death (apoptosis) which occurs early in the regenerative process. Counterintuitively, dying cells can signal to stem cells through mitogen release, which is sometimes called apoptosis-induced proliferation (AiP). Through pharmacological and genetic manipulation of caspases, these data show that apoptosis is required for stem cell proliferation and new tissue growth only in regenerative contexts. Moreover, these analyses suggest extrinsic, rather than intrinsic, apoptosis is required for AiP in planarians, suggesting cell death initiated by specific mechanisms is important for regulating stem cell activities. Finally, planar cell polarity (PCP) signaling (a conserved regulator of tissue morphogenesis) was investigated at the end stages of regeneration; loss of PCP resulted in stem cell hyperproliferation leading to body-wide tissue hyperplasia. Uncontrolled proliferation caused depletion of pluripotent populations and accumulation of progenitor/differentiated cell types. Gene ontology and differential expression analyses identified cell division symmetry as a means by which PCP regulates stem cell proliferation and differentiation; PCP loss resulted in increasing numbers of asymmetric divisions, leading to stem cell depletion and tissue hyperplasia. Collectively, these investigations uncover exciting new potential targets to regulate different phases of adult tissue morphogenesis.

Access Setting

Dissertation-Abstract Only

Restricted to Campus until

6-1-2034

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