Phosphorylation of Tip60 by GSK-3 determines the induction of PUMA and apoptosis by p53

GSK-3 and Wnt Signaling in Neurogenesis and Bipolar Disorder

The canonical Wnt signaling pathway is critical for development of the mammalian central nervous system and regulates diverse processes throughout adulthood, including adult neurogenesis. Glycogen synthase kinase-3 (GSK-3) antagonizes the canonical Wnt pathway and therefore also plays a central role in neural development and adult neurogenesis. Lithium, the first line of therapy for bipolar disorder, inhibits GSK-3, activates Wnt signaling and stimulates adult neurogenesis, which may be important for its therapeutic effects. GSK-3 also regulates other critical signaling pathways which may contribute to the therapeutic effects of lithium, including growth factor/neurotrophin signaling downstream of Akt. Here we will review the roles of GSK-3 in CNS development and adult neurogenesis, with a focus on the canonical Wnt pathway. We will also discuss the validation of GSK-3 as the relevant target of lithium and the mechanisms downstream of GSK-3 that influence mammalian behavior.

Geranylgeranyl transferase 1 modulates autophagy and apoptosis in human airway smooth muscle

Geranylgeranyl transferase 1 (GGT1) is involved in the posttranslational prenylation of signaling proteins, such as small GTPases. We have shown that blocking the formation of isoprenoids with statins regulates survival of human lung mesenchymal cells; thus, we tested the hypothesis that GGT1 may specifically modulate programmed cell death pathways in these cells. To this end, human airway smooth muscle (HASM) cells were treated with the selective GGT1 inhibitor GGTi-298. Apoptosis was seen using assays for cellular DNA content and caspase activation. Induction of autophagy was observed using transmission electron microscopy, immunoblotting for LC3 lipidation and Atg5-12 complex content, and confocal microscopy to detect formation of lysosome-localized LC3 punctae. Notably, GGT1 inhibition induced expression of p53-dependent proteins, p53 upregulated modulator of apoptosis (Noxa), and damage-regulated autophagy modulator (DRAM), this was inhibited by the p53 transcriptional activation inhibitor cyclic-pifithrin-α. Inhibition of autophagy with bafilomycin-A1 or short-hairpin RNA silencing of Atg7 substantially augmented GGTi-298-induced apoptosis. Overall, we demonstrate for the first time that pharmacological inhibition of GGT1 induces simultaneous p53-dependent apoptosis and autophagy in HASM. Moreover, autophagy regulates apoptosis induction. Thus, our findings identify GGT1 as a key regulator of HASM cell viability.

Proteome-wide identification of ubiquitylation sites by conjugation of engineered lysine-less ubiquitin

Ubiquitin conjugation (ubiquitylation) plays important roles not only in protein degradation but also in many other cellular functions. However, the sites of proteins that are targeted for such modification have remained poorly characterized at the proteomic level. We have now developed a method for the efficient identification of ubiquitylation sites in target proteins with the use of an engineered form of ubiquitin (K0-Ub), in which all seven lysine residues are replaced with arginine. K0-Ub is covalently attached to lysine residues of target proteins via an isopeptide bond, but further formation of a polyubiquitin chain does not occur on K0-Ub. We identified a total of 1392 ubiquitylation sites of 794 proteins from HEK293T cells. Profiling of ubiquitylation sites indicated that the sequences surrounding lysine residues targeted for ubiquitin conjugation do not share a common motif or structural feature. Furthermore, we identified a critical ubiquitylation site of the cyclin-dependent kinase inhibitor p27(Kip1). Mutation of this site thus inhibited ubiquitylation of and stabilized p27(Kip1), suggesting that this lysine residue is the target site of p27(Kip1) for ubiquitin conjugation in vivo. In conclusion, our method based on K0-Ub is a powerful tool for proteome-wide identification of ubiquitylation sites of target proteins.