Role of phosphoinositide 3-kinase regulatory isoforms in development and actin rearrangement

Class Ia phosphoinositide 3-kinases (PI3Ks) are heterodimers of p110 catalytic and p85 regulatory subunits that mediate a variety of cellular responses to growth and differentiation factors. Although embryonic development is not impaired in mice lacking all isoforms of the p85 alpha gene (p85 alpha(-/-) p55 alpha(-/-) p50 alpha(-/-)) or in mice lacking the p85 beta gene (p85 beta(-/-)) (D. A. Fruman, F. Mauvais-Jarvis, D. A. Pollard, C. M. Yballe, D. Brazil, R. T. Bronson, C. R. Kahn, and L. C. Cantley, Nat Genet. 26:379-382, 2000; K. Ueki, C. M. Yballe, S. M. Brachmann, D. Vicent, J. M. Watt, C. R. Kahn, and L. C. Cantley, Proc. Natl. Acad. Sci. USA 99:419-424, 2002), we show here that loss of both genes results in lethality at embryonic day 12.5 (E12.5). The phenotypes of these embryos, including subepidermal blebs flanking the neural tube at E8 and bleeding into the blebs during the turning process, are similar to defects observed in platelet-derived growth factor receptor alpha null (PDGFR alpha(-/-)) mice (P. Soriano, Development 124:2691-2700, 1997), suggesting that PI3K is an essential mediator of PDGFR alpha signaling at this developmental stage. p85 alpha(-/-) p55 alpha(+/+) p50 alpha(+/+) p85 beta(-/-) mice had similar but less severe defects, indicating that p85 alpha and p85 beta have a critical and redundant function in development. Mouse embryo fibroblasts deficient in all p85 alpha and p85 beta gene products (p85 alpha(-/-) p55 alpha(-/-) p50 alpha(-/-) p85 beta(-/-)) are defective in PDGF-induced membrane ruffling. Overexpression of the Rac-specific GDP-GTP exchange factor Vav2 or reintroduction of p85 alpha or p85 beta rescues the membrane ruffling defect. Surprisingly, reintroduction of p50 alpha also restored PDGF-dependent membrane ruffling. These results indicate that class Ia PI3K is critical for PDGF-dependent actin rearrangement but that the SH3 domain and the Rho/Rac/Cdc42-interacting domain of p85, which lacks p50 alpha, are not required for this response.