Research ArticleBiochemistry

Phosphorylation of the phosphatase PTPROt at Tyr399 is a molecular switch that controls osteoclast activity and bone mass in vivo

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Science Signaling  08 Jan 2019:
Vol. 12, Issue 563, eaau0240
DOI: 10.1126/scisignal.aau0240

A phosphoswitch for a phosphatase

The receptor-type tyrosine phosphatase PTPROt either stimulates or inhibits the kinase Src in different contexts. In osteoclasts, PTPROt stimulates Src activity, which promotes bone resorption. Roth et al. described mice lacking PTPROt entirely and mice lacking a putative phosphorylation site at the C terminus of PTPROt, Tyr399. Phenotypic analyses of the mice, combined with experiments in osteoclasts derived from them and in cultured cells, demonstrated that Tyr399 is a phosphoswitch that controls PTPROt activity toward Src. When Tyr399 was not phosphorylated, PTPROt dephosphorylated Src at an activating site, thus inhibiting Src activity. When PTPROt Tyr399 was phosphorylated, PTPROt recruited Src through the adaptor protein Grb2 and dephosphorylated an inhibitory site in Src, thereby activating the kinase. These findings explain how PTPROt both positively and negatively influences Src activity and suggest a potential switch function for C-terminal tyrosine residues in other tyrosine phosphatases.

Abstract

Bone resorption by osteoclasts is essential for bone homeostasis. The kinase Src promotes osteoclast activity and is activated in osteoclasts by the receptor-type tyrosine phosphatase PTPROt. In other contexts, however, PTPROt can inhibit Src activity. Through in vivo and in vitro experiments, we show that PTPROt is bifunctional and can dephosphorylate Src both at its inhibitory residue Tyr527 and its activating residue Tyr416. Whereas wild-type and PTPROt knockout mice exhibited similar bone masses, mice in which a putative C-terminal phosphorylation site, Tyr399, in endogenous PTPROt was replaced with phenylalanine had increased bone mass and reduced osteoclast activity. Osteoclasts from the knock-in mice also showed reduced Src activity. Experiments in cultured cells and in osteoclasts derived from both mouse strains demonstrated that the absence of phosphorylation at Tyr399 caused PTPROt to dephosphorylate Src at the activating site pTyr416. In contrast, phosphorylation of PTPROt at Tyr399 enabled PTPROt to recruit Src through Grb2 and to dephosphorylate Src at the inhibitory site Tyr527, thus stimulating Src activity. We conclude that reversible phosphorylation of PTPROt at Tyr399 is a molecular switch that selects between its opposing activities toward Src and maintains a coherent signaling output, and that blocking this phosphorylation event can induce physiological effects in vivo. Because most receptor-type tyrosine phosphatases contain potential phosphorylation sites at their C termini, we propose that preventing phosphorylation at these sites or its consequences may offer an alternative to inhibiting their catalytic activity to achieve therapeutic benefit.

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