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. Here we show that restoring parasympathetic function with the neurotrophic factor neurturin increases epithelial organ regeneration after damage. We use mouse salivary gland explant culture containing fluorescently-labeled progenitors, and injure the tissue with irradiation. The progenitors survive, parasympathetic function is diminished, and epithelial apoptosis reduces expression of neurturin, which increases neuronal apoptosis. Treatment with neurturin reduces neuronal apoptosis, restores parasympathetic function, and increases epithelial regeneration. Furthermore adult human salivary glands damaged by irradiation also have reduced parasympathetic innervation. We propose that neurturin will protect the parasympathetic nerves from damage and improve organ regeneration. This concept may be applicable for other organs where parasympathetic innervation influences their function.

SummaryOrgan formation and regeneration require epithelial progenitor expansion to engineer, maintain, and repair the branched tissue architecture. Identifying the mechanisms that control progenitor expansion will inform therapeutic organ (re)generation. Here, we discover that combined KIT and fibroblast growth factor receptor 2b (FGFR2b) signaling specifically increases distal progenitor expansion during salivary gland organogenesis. FGFR2b signaling upregulates the epithelial KIT pathway so that combined KIT/FGFR2b signaling, via separate AKT and mitogen-activated protein kinase (MAPK) pathways, amplifies FGFR2b-dependent transcription. Combined KIT/FGFR2b signaling selectively expands the number of KIT+K14+SOX10+ distal progenitors, and a genetic loss of KIT signaling depletes the distal progenitors but also unexpectedly depletes the K5+ proximal progenitors. This occurs because the distal progenitors produce neurotrophic factors that support gland innervation, which maintains the proximal progenitors. Furthermore, a rare population of KIT+FGFR2b+ cells is present in adult glands, in which KIT signaling also regulates epithelial-neuronal communication during homeostasis. Our findings provide a framework to direct regeneration of branched epithelial organs.

Enzymes of the AID/APOBEC family, characterized by the targeted deamination of cytosine to generate uracil within DNA, mediate numerous critical immune responses. One family member, activation-induced cytidine deaminase (AID), selectively introduces uracil into antibody variable and switch regions, promoting antibody diversity through somatic hypermutation or class switching. Other family members, including APOBEC3F and APOBEC3G, play an important role in retroviral defense by acting on viral reverse transcripts. These enzymes are distinguished from one another by targeting cytosine within different DNA sequence contexts; however, the reason for these differences is not known. Here, we report the identification of a recognition loop of 9 -11 amino acids that contributes significantly to the distinct sequence motifs of individual family members. When this recognition loop is grafted from the donor APOBEC3F or 3G proteins into the acceptor scaffold of AID, the mutational signature of AID changes toward that of the donor proteins. These loop-graft mutants of AID provide useful tools for dissecting the biological impact of DNA sequence preferences upon generation of antibody diversity, and the results have implications for the evolution and specialization of the AID/APOBEC family.The polynucleotide cytosine deaminases have been identified as key contributors to both the adaptive and innate immune responses to pathogens. This enzyme family includes activation-induced cytidine deaminase (AID), 2 which initiates antibody diversification, and the APOBEC3 enzymes, which inhibit retroviral infection (1, 2). Although the enzymes share a common chemical mechanism for catalyzing the deamination of cytosine to generate mutagenic uracil within DNA, they have distinct biological functions based on differences in their expression, localization, and DNA sequence specificity (2, 3).AID is a B-cell-specific enzyme whose catalytic action on mammalian antibody genes initiates somatic hypermutation and class switch recombination (4, 5). The uracil deamination product is processed by familiar DNA repair proteins, including uracil DNA N-glycosylase (UNG) and mismatch repair enzymes. However, the cellular environment within activated B-cells promotes mutagenic processing by these pathways, resulting in point mutations that ultimately enhance antibody affinity in somatic hypermutation or promote clustered double-strand breaks that alter antibody isotype in class switch recombination (6, 7).The activity of AID is restricted to relatively small 3-kb regions within the immunoglobulin locus around rearranged variable genes and heavy chain switch regions. Within these regions, the targeting of AID is partially dictated by its DNA sequence preferences. For example, it has been noted that the mutable complementarity determining regions from variable gene segments have an abundance of AGC codons as compared with adjacent framework regions that function in stabilizing antibody structure (8). In vivo, somatic hypermutation is focused on WRCW (W ϭ A...