Roles of fibroblast growth factors in tissue homeostasis and regeneration after injury

Abstract

In living organisms, tissue injury and repair processes evolved to protect organisms from various tissue damage and insults that arise from both extrinsic and intrinsic factors. Unlike many non-vertebrates and some vertebrates such as salamanders, tissue repair capabilities in mammals are relatively limited. In humans, understanding how appropriate tissue regeneration processes occur without organ function impairment and systemic adverse effects has important clinical implications in the prognosis and treatment of disease. Tissue regeneration processes in vertebrates require stem cells which are maintained and regulated through complex interactions between the stem cell and cellular and humoral components of the surrounding milieu. Here, I investigated the role of paracrine fibroblast growth factors (FGFs) in tissue injury and regeneration, specifically in the intestine and muscle. Ionizing radiation has been used in various model organisms as an experimental paradigm for acute tissue injury and repair processes. In my studies, I assessed the gain of function effects of FGF2 in gastrointestinal tract regeneration after radiation injury. In another setting, aging is known to act as a modifier of regenerative processes in skeletal muscle tissue. Clinically, age-dependent decline in muscle regenerative function has been suggested to contribute to sarcopenia, a decrease in muscle mass and function, in older people. To explore a potential role for FGFs in muscle regeneration, I investigated the loss-of-function effects of FGF1, which has recently been shown to be a regulator of adipose tissue homeostasis, in skeletal muscle regeneration after chemical injury. I assessed the mitigation effect of FGF2 in gastrointestinal tract injury after whole body irradiation. In IEC-6 cell based screening, FGF2 showed possible mitigative effects by reducing apoptosis among FGF1, FGF2, FGF19. For in vivo experiments, 12 Gy of whole body irradiation to maximize acute gastrointestinal syndrome in mice. After 12 Gy whole body irradiation, adjuvant FGF2 with bone marrow transplantation (BMT) showed significantly increased 2-weeks survival rates, over other experimental groups. Treating mice with FGF2 after BMT augmented proliferation, decreased apoptosis and improved crypt regeneration, while there was no significant improvement with FGF2 alone or BMT alone. Furthermore, intestinal hyperpermeability, a key measure of radiation-induced gastroinstestinal damage, was absent by combined treatment of FGF2 and BMT, accompanied by improved cytokine profiles. In addition, several genes associated with intestinal barrier integrity and stem cell markers were significantly up-regulated with adjuvant FGF2 combined with BMT. These results suggested that FGF2 following BMT could mitigate intestinal irradiation damage through stem cell regeneration, maintaining barrier integrity maintenance, and cytokine modulation. In contrast to the gain-of-function role of extrinsically administered FGF2 in gastrointestinal tract as a model to study tissue repair, I investigated the intrinsic role and loss-of-function phenotypes of FGF1, another paracrine FGF, in skeletal muscle regeneration. In FGF1 muscle specific knock-out (mKO) mice, muscle regeneration was impaired after chemical injury, and cross-section area of regenerating muscle fiber was smaller in FGF1 mKO compared to the wild type (WT) mice. Disorganized regenerating fiber and adipocyte infiltration in the regenerating area was also observed in FGF1 mKO mice. To probe the functional mechanism of FGF1 in myogenesis, I examined the effect of FGF1 signal blockade in a C2C12 myoblast differentiation model. Among FGF receptors (FGFRs), FGFR4 specific small molecule inhibitor potently inhibited myotube formation. Similarly, treatment of FGFR4 specific inhibitor led to formation of disorganized muscle fibers and fatty infiltration in regenerating muscle after chemical injury. On the other hand, injured muscle regenerated normally with FGFR1 specific blockade. In C2C12 myoblast differentiation model, I identified FGFR4-Wnt pathway as a possible downstream mediator and PPAR$\delta$ as a possible upstream regulator of FGF1 during myogenesis. Taken together, I show that FGF2 has beneficial effects on irradiation induced gastrointestinal injury, and that FGF1 is required for appropriate skeletal muscle regeneration, supporting the importance of paracrine FGFs in tissue repair process after injury.