Ca ²⁺ concentration–dependent premature death of igfbp5a ^−/− fish reveals a critical role of IGF signaling in adaptive epithelial growth

Abstract

The phenotype gap is a challenge for genetically dissecting redundant endocrine signaling pathways, such as the six isoforms in the insulin-like growth factor binding protein (IGFBP) family. Although overexpressed IGFBPs can inhibit or potentiate IGF actions or have IGF-independent actions, mutant mice lacking IGFBP-encoding genes do not exhibit major phenotypes. We found that although zebrafish deficient in did not show overt phenotypes when raised in Ca-rich solutions, they died prematurely in low Ca conditions. A group of epithelial cells expressing take up Ca and proliferate under low Ca conditions because of activation of IGF signaling. Deletion of blunted low Ca stress-induced IGF signaling and impaired adaptive proliferation. Reintroducing zebrafish Igfbp5a, but not its ligand binding-deficient mutant, restored adaptive proliferation. Similarly, adaptive proliferation was restored in zebrafish lacking by expression of human IGFBP5, but not two cancer-associated IGFBP5 mutants. Knockdown of IGFBP5 in human colon carcinoma cells resulted in reduced IGF-stimulated cell proliferation. These results reveal a conserved mechanism by which a locally expressed Igfbp regulates organismal Ca homeostasis and survival by activating IGF signaling in epithelial cells and promoting their proliferation in Ca-deficient states. These findings underscore the importance of physiological context when analyzing loss-of-function phenotypes of endocrine factors.

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

“…Human stanniocalcin (STC) 1 and 2 can directly bind and inhibit PAPP-A proteinase activity Jepsen et al, 2015). When they were each targeted expressed in NaR cells randomly in Tg(igfbp5a:GFP) embryos using the genetic mosaic assay (Liu et al, 2018), the low [Ca 2+ ] stress-induced reactivation was significantly inhibited in TSC1 or STC2 expressing NaR cells ( Fig. 3F-H).…”

“…To determine whether Papp-aa expression is sufficient in inducing NaR cell reactivation, Papp-aa was randomly expressed in NaR cells in pappaa -/-; Tg(igfbp5a:GFP) fish using a Tol2 transposon BAC-mediated genetic mosaic assay (Liu et al, 2018). Re-introduction of Papp-aa restored low [Ca 2+ ] stress-induced NaR cell reactivation ( Fig.…”

“…This is likely an adaptive response, allowing fish to take up adequate Ca 2+ to maintain body Ca 2+ homeostasis and survive under low [Ca 2+ ] conditions. Although igfbp5a -/fish did not show overt phenotypes when raised in high [Ca 2+ ] solutions, they died prematurely in normal and lower [Ca 2+ ] media (Liu et al, 2018), suggesting a [Ca 2+ ]-dependent mechanism(s) exists.…”

“…We have recently found that genetic deletion of the major Papp-a/Papp-a2 substrate IGF binding protein 5a (Igfbp5a) resulted in greatly reduced calcified bone mass even though igfbp5a is not expressed in skeletal tissues in zebrafish (Liu et al, 2018). Zebrafish igfbp5a is specifically expressed in a population Ca 2+ -transporting epithelial cells located in the yolk sac epidermis Liu et al, 2017).…”

“…These cells, normally non-dividing and quiescent, rapidly exit the quiescent state and proliferate in response to low [Ca 2+ ] stress Liu et al, 2017). Genetic deletion of igfbp5a blunted the low [Ca 2+ ] stress-induced IGF signaling and NaR cell proliferation (Liu et al, 2018). This is likely an adaptive response, allowing fish to take up adequate Ca 2+ to maintain body Ca 2+ homeostasis and survive under low [Ca 2+ ] conditions.…”