Glucose and glycerol transport in adipocytes from a structural perspective

Abstract: Adipocytes are crucial energy reservoirs to maintain metabolic homeostasis of glucose and lipids in the human body.Glucose transporters (GLUTs) and aquaporins (AQPs) play an important role in metabolic regulation of glucose andlipids in human adipocytes. Specifically, glucose transporter 4 (GLUT4) and aquaporin 7 (AQP7) are the centralplayers for glucose transport and glycerol efflux in adipocytes. In addition, GLUT family members are overexpressedin a vast majority of cancer cells to satisfy their increased energy demand, thus, inhibitors targeting GLUTs arebecoming relevant therapeutics for cancers treatment. To control the uptake/release of nutrients, GLUTs and AQPshave been suggested to be controlled by trafficking mechanisms. TUG (tether containing UBX domain for GLUT4in mouse) and PLIN1 (human perilipin 1) have previously been suggested to bind with GLUT4 and AQP7intracellularly and release them upon hormonal stimulation. Here, the mRNA expression levels of GLUTs and AQPsin adipose tissue has been investigated, and detailed characterization of the interaction between GLUT4 and ASPL(human TUG homolog) and AQP7 and PLIN1 in vitro have been executed. In addition, a new series of glucosetransporter 1 (GLUT1) inhibitors was structurally and functionally investigated. Finally, the AQP7 structure waselucidated by single particle cryo-EM.In this thesis I suggest that GLUT4 interacts with ASPL using its intracellular helical domain to bind to the Cterminusof ASPL. Rat GLUT4 was expressed in Pichia pastoris, and purified, and showing even single-particledistribution in negative staining, providing insight on further structural study of GLUT4 by single particle cryo-EM.Docking models of the complex of GLUT4 and ASPL were generated and suggest that ASPL forms complex withGLUT4 by multiple domains including both ubiquitin-like domain (UBL2) and ubiquitin domain (UBX). In addition,PGL13 and PGL14, as new series of GLUT1 inhibitors were suggested to utilize two sites of GLUT1, thetransmembrane domain and intracellular helical domain. Moreover, in human adipocytes AQP7 gene showedmarkedly higher-level expression than other aquaglyceroporins. The C-terminal domain of PLIN1 was suggestedto be central for the complex formation with AQP7 and AQP3. The AQP7 structure was determined at theresolution of 2.55 Å by cryo-EM, adopting the formation of dimer of tetramers. Two tetramers were dimerized byextracellular protruding C loops with a rotation of approximate 11° around central axis. The central pore is formedby four monomers and restricted by two leucine filters. Moreover, well-defined densities were discovered in thecentral pores showing decent fitting with some small metabolic products.