PIKFYVE-dependent regulation of MTORC1 and TFEB

Abstract

TFEB (transcription factor EB) is essential for the upregulation of gene expression required for macroautophagy/autophagy and lysosomal function. Under nutrient-rich conditions, TFEB is inactivated by MTOR (mechanistic target of rapamycin kinase) complex 1 (MTORC1)-dependent phosphorylation on the lysosome. This suppresses TFEB activity via preventing its translocation to the nucleus. Conversely, under starvation conditions and low MTORC1 activity, MTORC1 sites on TFEB are dephosphorylated, and TFEB translocates to the nucleus, where it activates its transcriptional program. We recently found that the inhibition of PIKFYVE, which produces phosphatidylinositol-3,5-bisphosphate on lysosomes, leads to the dephosphorylation and translocation of TFEB to the nucleus in a manner dependent on PPP2/PP2A (protein phosphatase 2) even under nutrient-rich conditions. Importantly, interaction of TFEB with MTORC1 but not with PPP2 is disrupted when PIKFYVE is inhibited. This suggests that PIKFYVE inhibition results in a loss of contact between MTORC1 and TFEB, which allows PPP2-dependent dephosphorylation of TFEB to be dominant. Interestingly, PIKFYVE-dependent localization of MTORC1 to lysosomes and TFEB phosphorylation requires the RRAG small GTPases. Thus, PIKFYVE may play a critical role in enhancing the activity of RRAG small GTPases, and thereby act as an essential suppressor of TFEB. Therefore, in addition to its direct roles in endosome homeostasis, PIKFYVE may also exert key functions in the regulation of the TFEB-dependent transcriptional program by interacting with the lysosomal nutrient-sensing machinery.

Keywords: MTORC1; PIKFYVE; TFEB; lysosomes; phosphoinositide lipid; protein phosphatase 2A.

Figure 1.

Phosphoinositide turnover controls the MTORC1 signaling on lysosomes. (A) Under nutrient-rich conditions, PIKFYVE induces the interaction between RRAG GTPases and MTORC1 on the surface of lysosomes. Whereas lysosomal MTORC1 phosphorylates TFEB in a manner independent of RHEB, other substrates such as RPS6KB1, EIF4EBP1, and ULK1 require RHEB for MTORC1-dependent phosphorylation. (B) Under PIKFYVE-inhibited conditions, lysosomal MTORC1 recruitment by RRAG small GTPases is decreased, resulting in a significant reduction of MTORC1-dependent TFEB phosphorylation. Despite the reduction of lysosomal MTORC1, PIKFYVE inhibition does not significantly affect RHEB-MTORC1-induced RPS6KB1, EIF4EBP1, or ULK1 phosphorylation, likely due to the enhancement of RHEB activity. PIKFYVE inhibition reduces the interaction between MTORC1 and TFEB, leading to PPP2 phosphatase activity to become the dominant mechanism of TFEB regulation.