Dendritic cells (DCs) are antigen-presenting cells of the mammalian immune system that exist in either an immature, unactivated state or a mature, activated state. Immature DCs (iDCs) patrol peripheral tissues for foreign and/or pathogenic antigens (i.e., antigen sampling), localizing to sites of inflammation. Once there, iDCs find and internalize antigens by phagocytosis, macropinocytosis, or cell surface receptor-mediated endocytosis1. During these activities, iDCs fluctuate between fast and slow motility, respectively, presumably to provide an optimal speed for efficient antigen sampling and capture2. Concomitant with antigen capture and processing, iDCs undergo maturation, including changes in F-actin and myosin II functional localization which underlies the shift to primarily fast motility. Degraded antigens are presented on the mature dendritic cell surface as major histocompatibility complex (MHC)-II-peptide complexes. Activated DCs migrate chemotactically via lymphatic vessels to naïve T cells in lymphoid organs (e.g., lymph nodes) where the captured antigens are presented to T cells (i.e., immunological synapse), thereby activating them and the adaptive immune system response1 (Fig. 1).
This newsletter focuses on the dynamic actin changes associated with DC migration and the regulation by actin binding proteins and Ras superfamily GTPases.
In in vivo (i.e., interstitial spaces of tissues) or in vitro 3D environments, DCs migrate by protrusion of the F-actin-enriched leading edge independent of myosin II-stimulated actomyosin contractility in an integrin/adhesion-independent manner, engaging in “amoeboid” movements. When 3D migration (in vivo or in vitro) involves movement through a confining environment (e.g., narrow gaps between cells), myosin II-dependent actomyosin contractions at the trailing edge of the cell are additionally engaged2-4. Myosin II has a functionally-dependent gradient in DCs5,6. During antigen capture and processing, myosin II is enriched at the front of iDCs where it regulates antigen uptake and degradation, processes correlated with slow motility6. Conversely, myosin II is enriched at the rear of the cell during fast migration. Myosin II's gradient and role in increased iDC migration speed and persistence requires IP3 receptor 1 (IP3R1)-mediated calcium release from the endoplasmic reticulum. Myosin II is activated by calcium-mediated phosphorylation of myosin II regulatory light chain (MLC)5. Like myosin II, F-actin has a gradient correlated with DC motility. During fast migration in iDCs and mature DCs, the tail end of each has an enriched pool of F-actin. Conversely, during slow migration in both types of DCs, the enrichment is at the front of the cell, although typically, only iDCs migrate slowly7.
CD74 (aka Ii) is a MHC class II-associated invariant chain that interacts with myosin II and transiently reduces DC velocity via regulation of myosin II localization and reduction of phosphorylated MLC2 in vivo and in in vitro confined environments. Indeed, the localization of myosin II to the front of slowly migrating iDCs requires CD746.
The signaling adaptor protein Eps8 is also an actin capping protein, essential for DC polarization and formation of elongated, migratory, dense F-actin-containing protrusions in DCs. In Eps8-deficient DCs, in vitro deficits in chemotactic migration and slower in vivo motility to lymph nodes occurs in response to inflammation. Eps8’s effect are migratory-specific8.
Arp2/3-mediated actin nucleation and branched filament formation regulate migratory speed and front-localized F-actin distribution within iDCs, while rear-localized F-actin in mature DCs depends upon Mammalian Diaphanous-related formin isoform 1 (mDia1) activity. Arp2/3-regulated F-actin enrichment correlates with decreased migratory speed and increased antigen sampling and uptake7. Arp2/3-mediated actin nucleation is also essential for DC migration through narrow, confined spaces as it creates a perinuclear, dynamic actin network that disrupts the nuclear lamina and allows the typically rigid nucleus to deform and fit through such spaces9.
Ras superfamily GTPases: Rho and Rap1
Actin polymerization is regulated by Rho GTPase effectors, and Rho activity itself is regulated. The Rho effector mDia1 is critical for fast migration speed and persistence as mDia1 regulates actin nucleation and polymerization at the rear of both types of DCs. However, mDia1-mediated functions predominate in mature DCs where it is also important for DC adhesion with and entry into the extracellular matrix (ECM)and then chemotactic migration to lymph nodes10. Deactivation of Rho is by GTPase activating proteins (GAPs), including myosin IXb (Myo9b). As a RhoGAP, Myo9b controls cofilin activation which is essential for dynamic re-arrangement of the F-actin cytoskeleton and also controls myosin II-stimulated actomyosin contractility, pathway alterations correlated with impaired chemotactic migration in vivo and in vitro11.
The Ras superfamily GTPase Rap1 also regulates DC motility via re-arrangement of the actin cytoskeleton. During DC migration, ATP is degraded in regulatory T cells by the constitutively active CD39 and CD73 ectonucleotidases, resulting in adenosine release. Adenosine activates a cAMP-mediated pathway which results in the sequential activation of Epac1 and Rap1. Rap1 localizes to the subcortical actin cytoskeleton where it mediates remodeling of the actin cytoskeleton, resulting in increased DC migration to regulatory T cells12.
Understanding the role of actin and actin binding proteins in DC migration is of clinical importance. Loss of the Wiskott-Aldrich syndrome protein (WASP) causes Wiskott-Aldrich Syndrome, a X-linked primary immunodeficiency. WASP regulates actin nucleation in partnership with Arp2/3. WASP deficiency is correlated with decreased chemokine-mediated DC migration in vitro, and in WASP-deficient mice, DCs have slower motility to and mislocalization within lymph nodes13-15.
DCs are antigen presenting cells in the mammalian immune system, that are essential for activation of the adaptive immune response (Fig. 1). All aspects of iDC and mature DC function and localization rely upon dynamic re-arrangment of the actin cytoskeleton which is regulated by various actin binding proteins and signaling pathways. It is important from both a basic sciences and clinical perspective to fully understand this regulation. To this end, Cytoskeleton, Inc. offers a variety of live cell actin imaging reagents, purified actin and actin binding proteins, antibodies, phalloidin stains, and functional assay kits.
Related Products & Resources
Actin Live Cell Imaging and Acti-Stain™ Phalloidins
Acti-stain 488 Phalloidin (Cat. # PHDG1-A)
Rhodamine Phalloidin (Cat. # PHDR1)
Acti-stain 555 Phalloidin (Cat. # PHDH1-A)
Acti-stain 670 Phalloidin (Cat. # PHDN1-A)
Spirochrome SiR700-Actin Kit (Cat. # CY-SC013)
Spirochrome SiR-Actin Kit (Cat. # CY-SC001)
Actin Biochem Kits
G-Actin/F-actin In Vivo Assay Biochem Kit (Cat. # BK037)
Actin Binding Protein Spin-Down Assay Biochem Kit: rabbit skeletal muscle actin (Cat. # BK001)
Actin Binding Protein Spin-Down Assay Biochem Kit: human platelet actin (Cat. # BK013)
Actin Polymerization Biochem Kit (fluorescence format): rabbit skeletal muscle actin (Cat. # BK003)
Rac1,2,3 G-LISA Activation Assay (Colorimetric format) 96 assays (Cat. # BK125)
Rac1 G-LISA Activation Assay Kit (Colorimetric Based) 24 assays (Cat. # BK128-S)
Rac1 G-LISA Activation Assay Kit (Colorimetric Based) 96 assays (Cat. # BK128)
RhoA G-LISA Activation Assay Kit (Colorimetric format) 24 assays (Cat. # BK124-S)
RhoA G-LISA Activation Assay Kit (Colorimetric format) 96 assays (Cat. # BK124)