PTEN: Bridging Endocytosis and Signaling

Abstract

The transduction of signals in the PTEN/PI3-kinase (PI3K) pathway is built around a phosphoinositide (PIP) lipid messenger, phosphatidylinositol trisphosphate, PI(3,4,5)P₃ or PIP₃ Another, more ancient role of this family of messengers is the control of endocytosis, where a handful of separate PIPs act like postal codes. Prominent among them is PI(3)P, which helps to ensure that endocytic vesicles, their cargo, and membranes themselves reach their correct destinations. Traditionally, the cancer and the endocytic functions of the PI3K signaling pathway have been studied by cancer and membrane biologists, respectively, with some notable but overall minimal overlap. Modern microscopy has enabled monitoring of the PTEN/PI3K pathway in action. Here, we explore the flurry of groundbreaking concepts emerging from those efforts. The discovery that PTEN contains an autonomous PI(3)P reader domain, fused to the catalytic PIP₃ eraser domain has prompted us to explore the relationship between PI3K signaling and endocytosis. This revealed how PTEN can achieve signal termination in a precisely controlled fashion, because endocytosis can package the PIP₃ signal into discrete units that PTEN will erase. We explore how PTEN can bridge the worlds of endocytosis and PI3K signaling and discuss progress on how PI3K/AKT signaling can be acting from internal membranes. We discuss how the PTEN/PI3K system for growth control may have emerged from principles of endocytosis, and how this development could have affected the evolution of multicellular organisms.

Figure 1.

Phosphoinositide (PIP) species and their role in cell compartmentalization. (A) The different PIP species, their phosphate patterns, and the enzymes that govern their conversions. (B) Enrichment of different PIP species on select membranes defines membrane biology and function. PI(4,5)P₂ marks the plasma membrane, PI(3)P defines the endocytosed vesicles (V), early endosome (EE) and propagates endocytic internalization, PI(4)P is deposited on recycling endosomes (REs) and promotes exocytosis, PI(3,5)P₂ is found on multivesicular bodies (MVBs), and all PIP species are found at the lysosome (Ly).

Figure 2.

The reader–eraser model for PTEN function on endosomes. (A) Superresolution microscopy image of cytoplasmic PTEN arrangement along microtubules (MTs) in fibroblast cells. This pattern is expected for proteins that bind PI(3)P vesicles because vesicles depend on MTs for trafficking through the cytoplasm. Scale bar, 500 nm. (B) Crystal structure of the PTEN phosphatase superimposed onto a vesicle membrane through C2 domain interaction with PI(3)P. This allows for the PTEN catalytic pocket (density shown in yellow) in the phosphatase domain to target PIP₃. Below, the linear domain structure of PTEN, the PI(3)P-binding motif (bold red line), and ubiquitination sites critical for nuclear import are shown.

Figure 3.

Models of PTEN and AKT signaling from endomembranes. (Left side) The journey of PTEN through the cell. Clathrin-mediated endocytosis (CME) produces internalized vesicles that are initially RAB5/APPL1/AKT-positive (not shown). Microtubule minus-end-directed transport and maturation through the endosomal system shifts membrane identity toward PI(3)P/EEA1-positive vesicles/early endosomes. Such PI(3)P-positive internal membranes can recruit PTEN, thus favoring AKT displacement. NDFIP1/NEDD4-1 association with late endosomes (Gorla et al. 2019; Murray et al. 2019) provides a PTEN release mechanism from endosomes, either through monoubiquitination and subsequent IPO11-mediated nuclear import (Trotman et al. 2007; Wang et al. 2007; Chen et al. 2017), or through polyubiquitination and degradation by the proteasome. (Right side) AKT localization and position of downstream substrates during CME. CME captures and concentrates activated RTK-ligand complexes into clathrin-coated pits (CCPs) at the cell surface. PIP₃ generated via the class I PI3-kinases (RTK-ligand arrows) recruits AKT to CCPs. SHIP2, a 5′-phosphatase prevents spillage of PIP₃ from CCPs onto the unlimited plasma membrane (PM) and produces PI(3,4)P₂, thus recruiting AKT and aiding CCP maturation. Internalization via endocytosis leads to the production of a naked vesicle in the cytosol that recruits RAB5 and APPL1. APPL1 interaction with AKT favors AKT phosphorylation of its downstream substrate GSK3β, but not TSC1/2. However, dynamic positioning of the lysosome through kinesin-/dynein-dependent active transport in response to nutrient availability can promote TSC1/2 phosphorylation by AKT.