The metalloproteinase Papp-aa functions as a molecular switch linking IGF signaling to adaptive epithelial growth

Abstract

Human patients carrying inactivating mutations in the pregnancy-associated plasma protein-a2 (PAPP-A2) gene display short status and lower bone mineral density. The underlying mechanisms are not well understood. Using a zebrafish model, here we report a [Ca 2+ ]dependent mechanism by which Papp-aa regulates bone calcification via promoting Ca 2+transporting epithelial cell (ionocyte) reactivation. Ionocyte, normally quiescent, re-enter the cell cycle in response to low [Ca 2+ ] stress. Deletion of Papp-aa abolished ionocyte reactivation and resulted in a complete lack of calcified bone. Re-expression of Papp-aa, but not its active site mutant, rescued ionocyte reactivation. Inhibition of Papp-aa activity pharmacologically or by overexpressing STC1 or STC2 impaired ionocyte reactivation. Loss of Papp-aa expression or activity resulted in diminished IGF1 receptor-mediated Akt-Tor signaling activity in ionocytes and expression of a constitutively active Akt rescued ionocyte reactivation. Biochemically, Pappaa cleaved Igfbp5a, a high-affinity IGF binding protein specifically expressed in ionocytes. Under normal [Ca2+] conditions, the Papp-aa-mediated Igfbp5a proteolysis was suppressed and IGFs sequestered in the IGF/Igfbp5a complex. Forced release of IGFs from the complex was sufficient to activate the IGF-Akt-Tor signaling and promote ionocyte reactivation. These findings suggest that Papp-aa functions as a [Ca 2+ ]-regulated molecular switch linking IGF signaling to adaptive epithelial growth and bone calcification.

Insulin-Like Growth Factor Binding Protein-5 in Physiology and Disease

“…Genetic deletion of Papp-aa abolished low [Ca 2+ ]-induced ionocyte proliferation, while it had little effect on ionocyte proliferation when fish were kept in normal conditions (47). Loss of Papp-aa expression or activity resulted in diminished IGF1 receptor-mediated Akt-Tor signaling in ionocytes in response to low [Ca 2+ ] stress (47). This phenotype was similar to the igfbp5a −/− mutant fish (88).…”

“…Biochemically, Papp-aa cleaved IGFBP-5a. Re-introduction of wild-type Papp-aa rescued cell proliferation and IGF signaling, while a protease deficient Pappaa mutant could not rescue the ionocyte proliferative response (47) (Figure 4). This idea was supported by the fact that treatment of fish with NBI-31772, an aptamer that can displace and release IGF from the IGF/IGFBP complex (103) promoted ionocyte proliferation under normal [Ca 2+ ].…”

“…The deletion of PAPP-A2 in osteoblast cells in mice led to a significant reduction in growth as measured by both body mass and tail length (46). The proteolytic regulation of IGFBP-5 by PAPP-A and PAPP-A2 is conserved in zebrafish (47). In addition to PAPP-A and PAPP-A2, a number of other proteases have been reported to degrade IGFBP-5.…”

“…How can these seemingly opposite effects be explained? A recent study by Liu et al (47) has elucidated that while IGFBP-5 inhibits IGF signaling in zebrafish Ca 2+ transporting ionocytes under normal conditions, it potentiates IGF signaling under low [Ca 2+ ] stress. In addition to IGFBP-5a, these ionocytes highly express Papp-aa, a zebrafish homolog of the IGFBP protease PAPP-A.…”

“…Treatment of fish with ZnCl 2 or batimastat, two metalloproteinase inhibitors (102), inhibited the low [Ca 2+ ] stress-induced ionocyte proliferation, suggesting that Papp-aa protease activity is critical. Genetic deletion of Papp-aa abolished low [Ca 2+ ]-induced ionocyte proliferation, while it had little effect on ionocyte proliferation when fish were kept in normal conditions (47). Loss of Papp-aa expression or activity resulted in diminished IGF1 receptor-mediated Akt-Tor signaling in ionocytes in response to low [Ca 2+ ] stress (47).…”