Recycling of albumin in kidney cells and macrophages

J Louber1, I Mahmoud1, J Chia2, I Glauser2, C Dumont1, J Mintern1, S Dower2, A Verhagen2 and P Gleeson1

  1. Department of Biochemistry and Molecular Biology, Bio21 Institute, University of Melbourne
  2. CSL Limited, Research, Bio21 Institute

Albumin and immunoglobulin G (IgG) are the two most abundant blood proteins, with long serum half-lives due to an efficient recycling system mediated by the neonatal Fc receptor, FcRn. FcRn-mediated protection of albumin and IgG has guided engineering of many novel therapeutics. FcRn is expressed in multiple tissues, but endothelial and hematopoietic cells appear to be the major sites of FcRn-mediated IgG recycling. Despite this, the role of FcRn in albumin homeostasis within these cells is not known. A better definition of FcRn and albumin trafficking within different cell types is required to optimise the serum lifespan of new recombinant protein therapeutics. Here we studied albumin trafficking in human embryonic kidney (HEK) derived cells that stably express FcRn. After FcRn-mediated internalisation, albumin traffics to early endosomes, and then recycling endosomes before disappearing from the cells. Moreover, albumin does not co-localise with markers of the late endosomes/lysosomes. Using an ELISA-based assay, we demonstrate that albumin is recycled into the culture medium in an FcRn-dependent manner. We also addressed the recycling of albumin in primary hematopoietic cells by using bone marrow derived macrophages (BMDM) isolated from mice expressing human FcRn, but lacking mouse FcRn. In BMDM, albumin is mainly internalised by macropinocytosis and traffics to structures positive for early endosomes markers but is not transported to recycling endosomes and lysosomes. Contrary to its FcRn-dependent recycling in HEK cells, internalised albumin is recycled independently of FcRn within BMDM. These findings highlight the importance of investigating a range of different cell types to understand the mechanisms of albumin recycling.