Implications of Transforming Growth Factor Beta Activating Kinase I (TAK1) Induced Phosphorylation on Organelle Remodeling in Human Pathologies

Abstract

Transforming growth factor β, (TGF-β) signaling is a broadly studied field and affects every aspect of mammalian physiology starting from embryogenesis to adult homeostasis. Most TGF-β effector are believed to be transcriptional in nature, however, their direct effects on cellular functions remain to be understood. This dissertation reports my work on the identification of novel partners for TAK1, the TGF-β activating kinase I, exploring the mechanistic implications of TAK1’s targeted phosphorylation on organelle remodeling, with possible implications in cancer and metabolic diseases. In summary, this work reports that TAK1, a kinase responsive to numerous growth factors and cytokines including TGF-β and TNF-α, triggers ER tubulation by activating αTAT1, a MT-acetylating enzyme that enhances ER-sliding. We show that this TAK1/αTAT1-dependent ER remodeling promotes cell survival by actively downregulating BOK, an ER membrane-associated proapoptotic effector. While BOK is normally protected from degradation when complexed with IP3R, it is rapidly degraded upon their dissociation during the ER sheets-to-tubules conversion. These findings demonstrate a distinct mechanism of ligand-induced ER remodeling and suggest that the TAK1/αTAT1 pathway may be a key target in ER stress and dysfunction. Additionally, my work reports TAK1 as a direct regulator of mitochondrial fusion, as its activation induces rapid fragmentation through Mfn2 (Mitofusion 2) inactivation. TAK1 phosphorylates Mfn2 at Ser249, which inhibits the binding of GTP required for Mfn trans-dimerization and mitochondrial membrane fusion. Accordingly, expression of Mfn2-S249 phosphomimetics (Mfn2-E/D) constitutively promote fission whereas alanine mutant (Mfn2-A) yields hyperfused mitochondria and increased bioenergetics in cells. In mice, Mfn2-E knock-in yields embryonic lethality in homozygotes whereas heterozygotes are viable but exhibit increased visceral fat accumulation despite normal body weight and cognitive/motor functions compared to wildtype and Mfn2-A mice. Mature white adipocytes isolated from mutant mice reveal cell-autonomous TAK1-related effects on mitochondrial remodeling and lipid metabolism. These results identify Mfn2-S249 as a dynamic phosphoregulatory switch of mitochondrial fusion during development and energy homeostasis.