Tyrosine Phosphorylation of CD13 Regulates Inflammatory Cell–Cell Adhesion and Monocyte Trafficking

Abstract

CD13 is a large cell surface peptidase expressed on the monocytes and activated endothelial cells important for homing to and resolving the damaged tissue at sites of injury. We have previously shown that crosslinking of human monocytic CD13 with activating antibodies induces strong adhesion to endothelial cells in a tyrosine kinase- and microtubule-dependent manner. In the current study we examined the molecular mechanisms underlying these observations in vitro and in vivo. We found that crosslinking of CD13 on U937 monocytic cells induced phosphorylation of a number of proteins, including Src, FAK and ERK and inhibition of these abrogated CD13-dependent adhesion. We found that CD13 itself was phosphorylated in a Src dependent manner, an unexpected finding as its 7 amino acid cytoplasmic tail was assumed to be inert. Furthermore, CD13 was constitutively associated with the scaffolding protein IQGAP1 and CD13 crosslinkinginduced complex formation with the actin-binding protein α-actinin, linking membrane-bound CD13 to the cytoskeleton, further supporting CD13 as an inflammatory adhesion molecule. Mechanistically, mutation of the conserved CD13 cytoplasmic tyrosine to phenylalanine abrogated adhesion, Src, FAK and ERK phosphorylation and cytoskeletal alterations upon antibody crosslinking. Finally, CD13 was phosphorylated in isolated murine inflammatory peritoneal exudate cells and adoptive transfer of monocytic cell lines engineered to express the mutant CD13 were severely impaired in their ability to migrate into the inflamed peritoneum, confirming that CD13 phosphorylation is relevant to inflammatory cell trafficking in vivo. Therefore, this study identifies CD13 as a novel, direct activator of intracellular signaling pathways in pathophysiological conditions.

CD13 Restricts TLR4 Endocytic Signal Transduction in Inflammation

“…As an extracellular peptidase, CD13 cleaves single amino acid residues from small peptides, and therefore, its substrates vary depending on the tissue where it is expressed. However, our observations have clearly shown that CD13 is a surprisingly multifunctional molecule that often acts independently of its enzymatic activity to regulate diverse processes such as angiogenesis, inflammatory trafficking, Ag presentation, cell-cell adhesion, coronavirus infection, and maintenance of the stem cell niche (9)(10)(11)(12)(13)(14). Although it is difficult to reconcile how a single molecule can regulate such apparently disparate functions, the fact that CD13 participates in the endocytosis of various receptors may underlie its wide range of activities.…”

“…We have identified CD13 as an inflammatory adhesion molecule that is required for optimal monocyte trafficking to sites of ischemic injury in numerous tissues (10,(12)(13)(14)21). In particular, in a model of ischemic peripheral muscle injury, the ratios of infiltrating monocytes in ischemic tissues from animals lacking CD13 were markedly altered, resulting in a distinctly "prohealing" cytokine milieu (13,14,22). However, healing of damaged tissue and muscle regeneration was unambiguously impaired in these mice despite the favorable inflammatory environment.…”

“…The CD13/aminopeptidase N membrane metallopeptidase is highly expressed on myeloid cells and activated endothelial cells and participates in various innate and adaptive immune mechanisms (9)(10)(11)(12)(13)(14). Recently, we found that CD13 negatively regulates receptor-mediated, dynamin-dependent Ag uptake by dendritic cells (DCs) to control the adaptive immune response (9).…”

IQGAP1: Insights into the function of a molecular puppeteer

“…In immune cells, IQGAP1 was found to be constitutively associated with CD13, a homing molecule and a cell surface peptidase expressed on monocytes and activated endothelial cells (Subramani et al , 2013). Further, CXCR2, a G-protein-coupled receptor (GPCR) involved in immune cell chemotaxis, directly binds IQGAP1 at the leading edge of human neutrophils (Neel et al , 2011).…”

Bone marrow transcriptome and epigenome profiles of equine common variable immunodeficiency patients unveil block of B lymphocyte differentiation

“…Monocyte markers PU.1/SPI1, MAFB, CSF1R, ANPEP, and M-CSF/CSF1 were expressed at comparable levels (Figure 3F) [66, 67]. The mature monocyte lineage marker CD14 was significantly up-regulated in the bone marrow of all equine CVID patients (FDR ≤ 0.033).…”