| PTEN, (phosphatase and tensin homolog deleted in chromosome ten) is mutated in most advanced human cancers, and it is one of the best-characterized tumor suppressor genes. In zebrafish, two loci have been reported, which encode two isoforms of the human homolog, Ptena and Ptenb. None of these isoforms are required for development of zebrafish embryos. PTEN is a negative regulator of the PI3K-Akt signaling pathway and essential for mammalian embryogenesis, since mice lacking PTEN die at around gastrulation. To address PTEN function in early development and during gastrulation, I cloned, characterized and conducted a gain-of-function analysis using the longest of the two zebrafish Pten isoforms, Ptena454, in 1-2 cell embryos. Overexpression of Ptena454 altered phospho-Akt levels and impaired cell movements associated with gastrulation. Heat shocking embryos elevated phospho-Akt levels, which lowered endogenous phospho-Ptena levels thereby elevating Ptena activity. Inhibiting protein kinase casein kinase 2 activity, a negative regulator of PTEN, also reduced phospho-Pten levels and augmented the effects due to Ptena overexpression. Low phospho-Akt and corresponding low phospho-Gsk-3 and high phospho-Pten levels accompanied wortmannin and LY294002 treatment, which inhibited PI3K activity. Collectively, these data show that Pten regulation is dependent on phospho-Akt levels. I propose a model by which homeostasis in rapidly dividing and migrating embryonic cells depends on a counterbalance between pro-survival signaling employing CK2 and Gsk-3 and the pro-apoptotic activity of Pten.;Key words. PTEN, Akt, zebrafish, gastrulation, cell movements. |
