The Role of ER-Golgi Membrane Contact Sites and of FAPP1 in Phosphoinositide Homeostasis at the Golgi Complex

Initially considered to be a precursor for highly phosphorylated phosphatidylinositol species, phosphatidylinositol 4-phosphate (PI4P) turned out to be “active” in its own right as a pivotal regulator of multiple cellular functions including membrane trafficking, sphingolipid metabolism, autophagy, and cell migration. PI4P levels at the Golgi in control conditions are the result of the balanced activity of Golgi-localized PI4-kinases (PI4KIIIβ and to a lesser extent PI4KIIα) and a single, highly conserved 4-phosphatase known as SAC1, localized in the ER. To coordinate such a variety of functions, multiple layers of regulation are required to modulate PI4P levels in time and space, by directing both PI4P enzymes localization and their activity. Here I show that the phosphatidylinositol 4-phosphate adaptor protein 1 (FAPP1) acts as a PI4P sensor, binds to PI4P and regulates its levels by interacting with and stimulating SAC1 at the level of ER-TGN membrane contact sites (ERTGoCS). At these ERTGoCS, FAPP1 acts as an adaptor that directs and stabilizes SAC1 towards PI4P-rich domains to promote its activity in trans to dephosphorylate PI4P at the TGN. Consequently, FAPP1 depletion leads to a huge increase in PI4P levels at the Golgi. I found that the FAPP1-regulated PI4P pool controls the post-Golgi trafficking of specific cargoes. One of these is the β-lipoprotein ApoB-100, which is more secreted upon FAPP1 depletion, highlighting a possible physiological role of this pool of PI4P controlled at the level of ER-TGN membrane contact sites. Intriguingly, another affected cargo is the autophagy protein ATG9, whose trafficking from the Golgi is increased in FAPP1 KD cells, thus resulting in a strong induction of autophagy. Besides the regulation of specific trafficking events, I found that the pool of PI4P controlled by FAPP1 recruits and promotes the activation of the PI4P-binding oncogene GOLPH3. When PI4P levels increase in FAPP1-KD cells, GOLPH3 is hyperactivated and promotes uncontrolled cell migration and invasion. My thesis work presents new information on the molecular mechanisms underlying the regulation of PI4P at the Golgi complex, identifying FAPP1 as a negative regulator of PI4P levels. It also sheds light on the need to keep PI4P levels within a certain range since an uncontrolled increase in PI4P can result in oncogenic potential and mis-trafficking of selective cargoes. Overall, this work provides novel insights into the homeostatic control of PI4P with an important contribution in understanding the complex picture of PI4P control and function in cell biology