Volume 37, Issue 6, Pages 493-506 (June 2016) Actin Rings of Power  Cornelia Schwayer, Mateusz Sikora, Jana Slováková, Roland Kardos, Carl-Philipp Heisenberg  Developmental Cell  Volume 37, Issue 6, Pages 493-506 (June 2016) DOI: 10.1016/j.devcel.2016.05.024 Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 1 Actin Network Organization Cells can form distinct actin network structures depending on their function and localization. The most prominent structures are contractile, branched actomyosin networks built of cross-linked actin fibers coupled to myosin motors. More anisotropic networks are formed when long actin fibers with mixed polarity are linked together and thus aligned with each other by different cross-linker proteins. Parallel actin fibers are generated when cross-linked actin fibers are aligned in the same direction. Regular arrays of parallel actin fibers connected by myosin motors are found in sarcomeres, representing the highest level of actin organization. Actin rings have been associated with most of these structures. Developmental Cell 2016 37, 493-506DOI: (10.1016/j.devcel.2016.05.024) Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 2 Cytokinesis At the onset of anaphase the centralspindlin complex translocates toward the division plane by associating with central spindle MT and astral MT projecting toward the division plane. The centralspindlin complex then recruits and activates the RhoGEF Ect2, resulting in the generation of an active RhoA zone at the division plane. RhoA promotes actomyosin ring assembly by activating the actin nucleator formin and the myosin activator ROCK. Besides the localized assembly of actomyosin fibers, the flow of cortical actomyosin toward the division plane also contributes to cytokinetic ring formation. At late anaphase, actin and myosin (mini)filaments are preferentially aligned along the division plane (Schroeder, 1973) and start to cleave the cell into two parts by circumferential ring constriction. Notably, the degree of such filament alignment can considerably vary depending on the cell type. For the cleavage furrow to form, not only does the cytokinetic ring need to constrict, but the plasma membrane needs also to be tightly attached to the ring through the scaffolding protein anillin and the actin-bundling protein septin. Developmental Cell 2016 37, 493-506DOI: (10.1016/j.devcel.2016.05.024) Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 3 Wound Healing (1) In single-cell and multicellular wound healing, Ca2+-dependent Rho and Cdc42 activity zones regulate the formation and contraction of actomyosin ring-like structures at the wound site. Upon wounding, two zones of Rho-family GTPases are formed at the wound edge, which are separated from each other by Abr (dual GEF-GAP), simultaneously activating Rho and inactivating Cdc42. (2) Flow of cytoskeletal components such as F-actin and myosin promote actomyosin ring formation at the wound edge. In single-cell wound healing, the coupling of the membrane and the actomyosin ring can be mediated via E-cadherin. In multicellular wounds, formation and constriction of actomyosin rings at the wound edge requires anchoring of actomyosin ring segments to the membrane, for example, via AJs or TJs. Developmental Cell 2016 37, 493-506DOI: (10.1016/j.devcel.2016.05.024) Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 4 Drosophila Dorsal Closure At the onset of dorsal closure (dorsal view), the leading edge cells of the lateral epidermis become planar polarized and assemble a supracellular actomyosin cable encompassing a dorsal hole. Constriction of the actomyosin ring promotes dorsal closure by acting in a purse-string-like manner. The ring is composed of a sarcomere-like assembly of F-actin, myosin, and α-actinin and is regulated by its upstream GTPase Rho1, an essential component of ring constriction. As in many other supracellular actomyosin rings, the intracellular ring segments are linked to AJs. Developmental Cell 2016 37, 493-506DOI: (10.1016/j.devcel.2016.05.024) Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 5 Zebrafish Epiboly The spreading of the EVL over the surface of the yolk is driven by constriction of a large actomyosin ring located within the extraembryonic YSL. This actomyosin ring pulls the margin of the EVL toward the vegetal pole by constricting along the circumference of the embryo, thereby acting as a purse-string. In addition, constriction of the ring along its width generates a retrograde flow of actin and myosin within the YSL (black arrows), which, when resisted by friction with adjacent structures, generates a pulling force on the margin of the EVL, promoting EVL epiboly movements. Besides actomyosin, microtubules also flow toward the EVL margin. TJs, connecting the EVL margin to the actomyosin ring within the YSL, potentially ensure proper force transmission between the ring and the EVL. Developmental Cell 2016 37, 493-506DOI: (10.1016/j.devcel.2016.05.024) Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 6 Epithelial Cell Extrusion Cells can extrude either apically (upper panels) or basally (lower panels) depending on the subcellular localization of the contractile actomyosin ring (arrows delineate the direction of cell extrusion). Rho-mediated actin cable constriction can be triggered in response to cell damage (left panels, brown cells) or cellular crowding (right panels, pink cells). Developmental Cell 2016 37, 493-506DOI: (10.1016/j.devcel.2016.05.024) Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 7 Cell-Cell Adhesion Upon cell-cell contact initiation, apposing cell membranes form small cis clusters of cadherin molecules bound to actin, which then engage in trans interactions. In immature contacts with low cortical tension (upper panels) this organization persists and RhoA and actin activity only moderately increases. When cortical tension increases during contact maturation (lower panels), α-catenin unfolds and additional proteins, such as vinculin and Mena/VASP, are recruited to the adhesion clusters. Moreover, ARP2/3-mediated branched actin polymerization at the contact edge and concomitant depletion from the contact center lead to the characteristic actin ring-like appearance at the contact. Developmental Cell 2016 37, 493-506DOI: (10.1016/j.devcel.2016.05.024) Copyright © 2016 Elsevier Inc. Terms and Conditions