Editors' ChoiceDependence Receptors

PP2A, the Key to Life and Death

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Sci. Signal.  04 Jan 2011:
Vol. 4, Issue 154, pp. ec6
DOI: 10.1126/scisignal.4154ec6

The netrin-1 receptor UNC5H2 (also known as UNC5HB), which functions in neuronal navigation, patterning of epithelial tissues, and developmental angiogenesis, is a "dependence receptor"—a class of receptor that promotes cell death in the absence of ligand (see Bagri and Ashkenazi). Guenebeaud et al. identified PR65β, a scaffold protein that couples the protein phosphatase 2A (PP2A) catalytic subunit (PP2Ac) with regulatory B subunits or with the PP2A inhibitor CIP2A, in a small interfering RNA screen for genes implicated in cell death after loss of netrin-1 activity. PR65β knockdown promoted cell survival, decreased PP2A activity, and blocked the increase in caspase activation associated with netrin-1 knockdown. Transfection of UNC5H2 into human embryonic kidney (HEK293) cells causes apoptosis, which is mediated by activation of death-associated protein kinase (DAPK) the activity of which is inhibited by autophosphorylation. Guenebeaud et al. found that overexpression of UNC5H2 in HEK293 cells was associated with increased PP2A activity and that knockdown of PR65β in this system reduced PP2A activity and prevented cell death. In the absence of netrin-1, the proteins UNC5H2, PR65β, PP2Ac, and DAPK formed a complex that was detected by coimmunoprecipitation (in transfected HEK293 cells and in mouse brain), colocalization by immunofluorescence (transfected cells), and size-exclusion chromatography (transfected cells). The addition of netrin-1 to the cells selectively disrupted the interaction of UNC5H2 with PP2A and PR65β but not with DAPK. Netrin-1 promoted the association of PP2A with the inhibitor CIP2A, and overexpression of CIP2A in the HEK293-UNC5H2 system reduced the number of apoptotic cells. Overexpression of PP2A or knockdown of CIP2A reduced phosphorylation of DAPK at the inhibitory site, and knockdown of PR65β or addition of a PP2A inhibitor increased DAPK phosphorylation at the inhibitory site. Thus, UNC5H2 appears to recruit different PP2A regulators to produce either a survival or death signal. The importance of PR65β, and thus PP2A, in survival signaling in vivo was confirmed by knocking down PR65β in zebrafish embryos and showing that this rescued the vascular defects associated with netrin-1 deficiency.

C. Guenebeaud, D. Goldschneider, M. Castets, C. Guix, G. Chazot, C. Delloye-Bourgeois, A. Eisenberg-Lerner, G. Shohat, M. Zhang, V. Laudet, A. Kimchi, A. Bernet, P. Mehlen, The dependence receptor UNC5H2/B triggers apoptosis via PP2A-mediated dephosphorylation of DAP kinase. Mol. Cell 40, 863–876 (2010). [PubMed]

A. Bagri, A. Ashkenazi, UNCovering the molecular machinery of dependence receptor signaling. Mol. Cell 40, 851–853 (2010). [PubMed]