This project has two parts, one investigating the communication between parasympathetic nerve development and SMG epithelial morphogenesis, and the second identifying embryonic mouse salivary gland stem/progenitor cell populations within the gland. The maintenance of a progenitor cell population as a reservoir of undifferentiated cells is required for organ development and regeneration. Parasympathetic nerves are a vital component of the progenitor cell niche during development, maintaining a pool of progenitors for organogenesis. Injured adult organs do not regenerate after parasympathectomy, and there are few treatments to improve organ regeneration, particularly after damage by therapeutic irradiation. We recently found that restoring parasympathetic function with the neurotrophic factor neurturin after irradiation increases epithelial organ regeneration. We used SMG explant culture and injured the tissue with irradiation. The progenitors survived, parasympathetic function was diminished, and epithelial apoptosis reduced expression of neurturin, which consequently increased neuronal apoptosis. Treatment with neurturin reduced neuronal apoptosis, restored parasympathetic function, and increased epithelial regeneration. Furthermore biopsies of adult human salivary glands damaged by irradiation also had reduced parasympathetic innervation. We proposed that neurturin will protect the parasympathetic nerves from damage and improve organ regeneration. This concept may be broadly applicable to other organs where parasympathetic innervation influences their function. We are using viral vectors that express neurotrophic factors to infect salivary glands in vivo and ex vivo to improve the function of surviving nerves in repairing the gland. In the second part of this project, we are studying the cell surface receptor Kit, which has been used to isolate stem/progenitor cells from adult SMGs. These Kit+ progenitor/stem cells can be transplanted into irradiated adult SMGs to regenerate the damaged tissue and restore secretory function. The mechanisms of Kit+ function and Kit+ progenitor cell-mediated regeneration are not yet understood. We are investigating how Kit signaling influences progenitor cell expansion. We have discovered that combined Kit and Fgfr2b signaling specifically regulates distal progenitor expansion, which coordinates the branching organ architecture. Fgfr2b signaling upregulates the epithelial Kit pathway so that combined Fgfr2b/Kit signaling, via separate Akt and MAPK pathways, amplifies Fgfr2b-dependent transcription and increases proliferation in the end bud. Combined Kit/Fgfr2b signaling selectively expands the Kit+K14+Sox10+ distal progenitors. We have analyzed SMGs from mice with mutations in the Kit receptor. The SMGs are smaller and have reduced number of Kit+ distal progenitor cells. However, the genetic loss of Kit signaling unexpectedly depletes the K5+ proximal progenitors. This occurs because the distal progenitors produce neurotrophic factors, which maintain proximal progenitors via communication with the neuronal niche. Our findings provide a framework to direct regeneration and tissue engineering of epithelial branched organs. Understanding the cell lineage of progenitor cells within the salivary glands will be important from the clinical perspective where progenitor cells of specific lineages may be more appropriate than pluripotent stem cells for clinical transplantation to regenerate irradiation-damaged salivary glands.
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