The metalloproteinase Papp-aa controls epithelial cell quiescence-proliferation transition

• Main Authors: Chengdong Liu, Shuang Li, Pernille Rimmer Noer, Kasper Kjaer-Sorensen, Anna Karina Juhl, Allison Goldstein, Caihuan Ke, Claus Oxvig, Cunming Duan
• Format: Article
• Language: English
• Published: eLife Sciences Publications Ltd 2020-04-01
• Series: eLife
• Subjects: pappalysin; STC; IGFBP5; IGF signaling; zebrafish; Ca2+ deficiency
• Online Access: https://elifesciences.org/articles/52322

Human patients carrying PAPP‐A2 inactivating mutations have low bone mineral density. The underlying mechanisms for this reduced calcification are poorly understood. Using a zebrafish model, we report that Papp-aa regulates bone calcification by promoting Ca2+-transporting epithelial cell (ionocyte) quiescence-proliferation transition. Ionocytes, which are normally quiescent, re-enter the cell cycle under low [Ca2+] stress. Genetic deletion of Papp-aa, but not the closely related Papp-ab, abolished ionocyte proliferation and reduced calcified bone mass. Loss of Papp-aa expression or activity resulted in diminished IGF1 receptor-Akt-Tor signaling in ionocytes. Under low Ca2+ stress, Papp-aa cleaved Igfbp5a. Under normal conditions, however, Papp-aa proteinase activity was suppressed and IGFs were sequestered in the IGF/Igfbp complex. Pharmacological disruption of the IGF/Igfbp complex or adding free IGF1 activated IGF signaling and promoted ionocyte proliferation. These findings suggest that Papp-aa-mediated local Igfbp5a cleavage functions as a [Ca2+]-regulated molecular switch linking IGF signaling to bone calcification by stimulating epithelial cell quiescence-proliferation transition under low Ca2+ stress.