The Role of Insulin Regulated Aminopeptidase in Endocytic Trafficking and Receptor Signaling in Immune Cells

Abstract

Insulin regulated aminopeptidase (IRAP) is a type II transmembrane protein with broad tissue distribution initially identified as a major component of Glut4 storage vesicles (GSV) in adipocytes. Despite its almost ubiquitous expression, IRAP had been extensively studied mainly in insulin responsive cells, such as adipocytes and muscle cells. In these cells, the enzyme displays a complex intracellular trafficking pattern regulated by insulin. Early studies using fusion proteins joining the IRAP cytosolic domain to various reporter proteins, such as GFP or the transferrin receptor (TfR), showed that the complex and regulated trafficking of the protein depends on its cytosolic domain. This domain contains several motifs involved in IRAP trafficking, as demonstrated by mutagenesis studies. Also, proteomic studies and yeast two-hybrid experiments showed that the IRAP cytosolic domain engages in multiple protein interactions with cytoskeleton components and vesicular trafficking adaptors. These findings led to the hypothesis that IRAP is not only a cargo of GSV but might be a part of the sorting machinery that controls GSV dynamics. Recent work in adipocytes, immune cells, and neurons confirmed this hypothesis and demonstrated that IRAP has a dual function. Its carboxy-terminal domain located inside endosomes is responsible for the aminopeptidase activity of the enzyme, while its amino-terminal domain located in the cytosol functions as an endosomal trafficking adaptor. In this review, we recapitulate the published protein interactions of IRAP and summarize the increasing body of evidence indicating that IRAP plays a role in intracellular trafficking of several proteins. We describe the impact of IRAP deletion or depletion on endocytic trafficking and the consequences on immune cell functions. These include the ability of dendritic cells to cross-present antigens and prime adaptive immune responses, as well as the control of innate and adaptive immune receptor signaling and modulation of inflammatory responses.

To Be or Not to Be: The Case of Endoplasmic Reticulum Aminopeptidase 2

“…M1 aminopeptidases (12 enzymes in humans) require Zn 2+ for hydrolysis ( 10 ). Endoplasmic reticulum aminopeptidases 1 and 2 (ERAP1 and ERAP2, respectively) belong to the oxytocinase subfamily of M1 aminopeptidases together with insulin-responsive aminopeptidase (IRAP) 1 , the latter being present in intracellular vesicles including endosomes and engaged in cross-presentation, i.e., unconventional presentation of extracellular antigens by HLA-I molecules, and several other functions ( 11 – 13 ). These proteins are composed of four domains.…”

Mast cell-mediated inflammation relies on insulin-regulated aminopeptidase controlling cytokine export from the Golgi

“…From here, IRAP and Glut4 are re-internalized into sorting endosomes, where they have been shown to interact with the retromer complex that promotes their deviation from the degradative late endosomal/lysosomal pathway and retrieves them for retrograde TGN (Pan et al, 2017), the GSV assembly and budding site. Of note, in contrast to the relatively restricted Glut4 expression patterns to insulinresponsive tissues, IRAP-containing endosomes are widely expressed amongst cell types and tissues, where they are mobilized by cell-specific surface receptor signaling and employed for various cell type-specific functions (Kandror and Pilch, 2011;Bogan, 2012;Descamps et al, 2020). In this line, with regards to immune cells, the trafficking of IRAP endosomes has recently been recognized to intersect with and regulate phagosome maturation and MHC-I cross-presentation in dendritic cells (Saveanu et al, 2009;Weimershaus et al, 2012Weimershaus et al, , 2018Weimershaus et al, , 2020, activation of TLR9 (Babdor et al, 2017), as well as endo-and exocytic trafficking in T cells for the supply of TCR signaling components and optimal TCR signaling (Evnouchidou et al, 2020).…”

“…Of note, in contrast to the relatively restricted Glut4 expression patterns to insulin-responsive tissues, IRAP-containing endosomes are widely expressed amongst cell types and tissues, where they are mobilized by cell-specific surface receptor signaling and employed for various cell type-specific functions 37–39 . In this line, with regards to immune cells, the trafficking of IRAP endosomes has recently been recognized to intersect with and regulate phagosome maturation and MHC-I cross-presentation in dendritic cells 40–43 , activation of TLR9 44 , as well as endo-and exocytic trafficking in T cells for the supply of TCR signaling components and optimal TCR signaling 45 .…”

A short ERAP2 that binds IRAP is expressed in macrophages independently from gene variation

“…IRAP is a multifaceted protein: it has been shown to be involved in cross-presentation in the endosomes of dendritic cells (DC) and, when in the cell membrane, to catalyze the final step of the angiotensinogen to angiotensin IV (AT4) conversion being itself a receptor for AT4 (5,6). In addition, it has been shown to be involved in several other functions, ranging from the insulin metabolic pathway to vesicular trafficking and even in cognitive processes (7,8). Furthermore, the three genes have been found associated with autoimmune and inflammatory conditions, hypertension, and cancer (2,(9)(10)(11)(12)(13)(14)(15).…”