Integrins Control Vesicular Trafficking; New Tricks for Old Dogs

Abstract

Integrins are transmembrane receptors that transduce biochemical and mechanical signals across the plasma membrane and promote cell adhesion and migration. In addition, integrin adhesion complexes are functionally and structurally linked to components of the intracellular trafficking machinery and accumulating data now reveal that they are key regulators of endocytosis and exocytosis in a variety of cell types. Here, we highlight recent insights into integrin control of intracellular trafficking in processes such as degranulation, mechanotransduction, cell-cell communication, antibody production, virus entry, Toll-like receptor signaling, autophagy, and phagocytosis, as well as the release and uptake of extracellular vesicles. We discuss the underlying molecular mechanisms and the implications for a range of pathophysiological contexts, including hemostasis, immunity, tissue repair, cancer, and viral infection.

Keywords: clathrin; endocytosis; exocytosis; immunity; integrins; viral infection.

Figure 1

Key Figure. Integrins Promote Exocytosis of Biosynthetic and Secretory Vesicles. Integrins connect to the actin cytoskeleton via talin and remodel actin filaments via Rho GTPases, formins, and other proteins. Integrins also connect to microtubules via a complex of proteins, called the cortical microtubule stabilizing complex (CMSC) [22]. In this way, integrin adhesion sites are linked to the exocytic machinery, consisting of Rab GTPases, their effectors, and motor proteins. This machinery ensures the outward traffic of Golgi-derived vesicles carrying newly synthesized proteins (biosynthetic pathway), as well as secretory vesicles, which store proteins that are released in response to a specific cue (regulated secretion). Localized exocytosis of newly synthesized proteins occurs near integrin-controlled adhesion complexes [17,18,28] and is directed from the Golgi by guanine nucleotide exchange factor (GEF)-H1, an activator of RhoA that is associated with microtubules [29]. Abbreviations: CLASP, cytoplasmic linker associated protein; EB1, end-binding protein 1; ECM, extracellular matrix; ELKS, protein rich in amino acids E,L,K, and S; KANK, KN motif, and ankyrin repeat domain-containing; KIF21A, kinesin family member 21A.

Figure 2

Integrins Control Leukocyte and Platelet Degranulation, Important for Immunity and Hemostasis. (A) Upon antigen recognition on target cells by cytotoxic T cells, a tight interaction is established through αLβ2 and intercellular adhesion molecule (ICAM), which enables local release of lytic granules to kill the target cell, thereby preventing collateral damage to other cells [37., 38., 39.]. Integrin-dependent granule convergence is achieved by a number of regulatory protein networks downstream of αLβ2 [40] and directed granule trafficking through Rab27. (B) In platelets, activation of the integrin αIIbβ3 is essential for platelet spreading and aggregation, as well as clot formation and retraction. Ligation and signaling from β1 integrins are required for the release of fibronectin and fibrinogen from platelet α-granules, which stimulates further platelet spreading and aggregation [42]. Moreover, Rho-mediated clot-retraction and Rac-mediated platelet spreading are dependent on αIIbβ3 endocytic trafficking, which is regulated by ADP-ribosylation factor 6 (Arf6) and vacuolar protein sorting-associated protein 33B (Vps33B) [44,46].

Figure 3

Integrin Crosstalk with the Clathrin Machinery Regulates Adhesion Turnover, Mechanotransduction, and Endocytosis. Microtubules deliver clathrin adaptors that trigger disassembly of focal adhesions (FAs) and recruit the integrins and their ligands into clathrin-coated pits [47,48]. Binding of clathrin adaptors to integrins is also involved in the formation of the more static flat clathrin lattices (FCLs), which develop when αvβ5 forms stable interactions with immobilized vitronectin; this prevents the formation of pits and thereby ‘frustrates’ the endocytic process [52,55,59]. These structures depend on high rigidity of extracellular matrix (ECM) ligands, but are associated with low intracellular tension, as they do not depend on myosin II-activity [60]. Yet, increases in cytoskeletal tension, for example, by guanine nucleotide exchange factor (GEF)-H1/RhoA-mediated actomyosin contractility in response to microtubule depolymerization, stimulate FA assembly and integrin translocation from the FCLs to FAs [59,65]. Abbreviations: AP, adaptor protein; ARH, autosomal recessive hypercholesterolemia; Dab2, Disabled-2; EPS15, epidermal growth factor receptor pathway substrate 15.

Figure 4

Integrins Control Binding and Uptake of Extracellular Vesicles and Viruses. Extracellular vesicles (EVs) are either derived from multivesicular bodies (exosomes), or bud from the plasma membrane (microvesicles). Integrins and/or integrin ligands on the surface of EVs can mediate EV binding to target cells. Following binding, EVs can induce signaling into the target cell (i) and/or transfer their content into the cell (ii). Enveloped viruses can use envelope proteins to bind integrins on target cells prior to viral entry. Various naked viruses can be released by infected cells at the prelytic stage via packaging into host EVs, which may allow integrin-mediated entry of target cells (iii). Abbreviations: CMV, Cytomegalovirus; CoV, coronavirus; EBV, Epstein-Barr virus; FAK, focal adhesion kinase; FN, fibronectin; HAV, hepatitis A virus; HEV, hepatitis E virus; HIV, human immunodeficiency virus; ICAM, intercellular adhesion molecule; SARS, severe acute respiratory syndrome; VCAM-1, vascular cell adhesion molecule-1.

Figure 5

Integrin Regulation of Vesicular Trafficking and Autophagy Pathways Controls Receptor Signaling. Integrins regulate endocytic recycling of receptor tyrosine kinases, such as vascular endothelial growth factor receptor 2 (VEGFR2), under the control of growth factors and proteins that associate with their cytoplasmic tails [87,89,90]. Moreover, internalized integrins cosignal with these receptors from endosomes toward the extracellular signal-regulated kinase (ERK) and AKT pathways, to synergistically drive cell proliferation and survival [91,92]. Some integrins mediate phagocytosis of (complement-opsonized) pathogens, such as herpes simplex virus and Listeria monocytogenes [99,100]. Following internalization, these integrins can induce the recruitment of microtubule-associated protein 1A/1B light chains 3B (LC3) to form autophagosomes, which fuse with lysosomes to degrade the pathogens [94,95]. Integrin αvβ3 can directly interact with certain Toll-like receptors (TLRs) and limit their signaling, possibly through recruitment of Src and Syk, thereby decreasing cellular activation [95]. Moreover, αMβ2 signaling through Src and Syk can also inhibit TLR signaling toward NF-κB and interferon-regulatory factor 3 pathways [101., 102., 103., 104., 105.]. Several integrins may therefore function as a ‘sensor’ that regulates TLR signaling by stimulating necessary responses against pathogens but preventing excessive responses leading to autoimmunity. Abbreviations: FAK, focal adhesion kinase; IL, interleukin; IFN, interferon; IRF, interferon-regulatory factor; MyD88; myeloid differentiation primary response 88; NF-κB, nuclear factor-κ of activated B cells; Syk, spleen tyrosine kinase; TNF, tumor necrosis factor; TRIF, TIR-domain-containing adapter-inducing interferon-β; VEGF, vascular endothelial growth factor.