Chungho Kim

Regulation of Integrin Activation

Regulation of cell-cell and cell-matrix interaction is essential for the normal physiology of metazoans and is important in many diseases. Integrin adhesion receptors can rapidly increase their affinity (integrin activation) in response to intracellular signaling events in a process termed inside-out signaling. The transmembrane domains of integrins and their interactions with the membrane are important in inside-out signaling. Moreover, integrin activation is tightly regulated by a complex network of signaling pathways. Here, we review recent progress in understanding how the membrane environment can, in cooperation with integrin-binding proteins, regulate integrin activation.

The structure of the integrin αIIbβ3 transmembrane complex explains integrin transmembrane signalling

Heterodimeric integrin adhesion receptors regulate cell migration, survival and differentiation in metazoa by communicating signals bi‐directionally across the plasma membrane. Protein engineering and mutagenesis studies have suggested that the dissociation of a complex formed by the single‐pass transmembrane (TM) segments of the α and β subunits is central to these signalling events. Here, we report the structure of the integrin αIIbβ3 TM complex, structure‐based site‐directed mutagenesis and lipid embedding estimates to reveal the structural event that underlies the transition from associated to dissociated states, that is, TM signalling. The complex is stabilized by glycine‐packing mediated TM helix crossing within the extracellular membrane leaflet, and by unique hydrophobic and electrostatic bridges in the intracellular leaflet that mediate an unusual, asymmetric association of the 24‐ and 29‐residue αIIb and β3 TM helices. The structurally unique, highly conserved integrin αIIbβ3 TM complex rationalizes bi‐directional signalling and represents the first structure of a heterodimeric TM receptor complex.

The structure of an integrin/talin complex reveals the basis of inside‐out signal transduction

Fundamental to cell adhesion and migration, integrins are large heterodimeric membrane proteins that uniquely mediate inside‐out signal transduction, whereby adhesion to the extracellular matrix is activated from within the cell by direct binding of talin to the cytoplasmic tail of the β integrin subunit. Here, we report the first structure of talin bound to an authentic full‐length β integrin tail. Using biophysical and whole cell measurements, we show that a specific ionic interaction between the talin F3 domain and the membrane–proximal helix of the β tail disrupts an integrin α/β salt bridge that helps maintain the integrin inactive state. Second, we identify a positively charged surface on the talin F2 domain that precisely orients talin to disrupt the heterodimeric integrin transmembrane (TM) complex. These results show key structural features that explain the ability of talin to mediate inside‐out TM signalling.

Protein Kinase A Governs a RhoA-RhoGDI Protrusion-Retraction Pacemaker in Migrating Cells

The cyclical protrusion and retraction of the leading edge is a hallmark of many migrating cells involved in processes such as development, inflammation and tumorigenesis. The molecular identity of the signalling mechanisms that control these cycles has remained unknown. Here, we used live-cell imaging of biosensors to monitor spontaneous morphodynamic and signalling activities, and employed correlative image analysis to examine the role of cyclic-AMP-activated protein kinase A (PKA) in protrusion regulation. PKA activity at the leading edge is closely synchronized with rapid protrusion and with the activity of RhoA. Ensuing PKA phosphorylation of RhoA and the resulting increased interaction between RhoA and RhoGDI (Rho GDP-dissociation inhibitor) establish a negative feedback mechanism that controls the cycling of RhoA activity at the leading edge. Thus, cooperation between PKA, RhoA and RhoGDI forms a pacemaker that governs the morphodynamic behaviour of migrating cells.

Basic amino-acid side chains regulate transmembrane integrin signalling

Side chains of Lys/Arg near transmembrane domain (TMD) membrane–water interfaces can ‘snorkel’, placing their positive charge near negatively charged phospholipid head groups; however, snorkelling’s functional effects are obscure. Integrin β TMDs have such conserved basic amino acids. Here we use NMR spectroscopy to show that integrin β3(Lys 716) helps determine β3 TMD topography. The αΙΙbβ3 TMD structure indicates that precise β3 TMD crossing angles enable the assembly of outer and inner membrane ‘clasps’ that hold the αβ TMD together to limit transmembrane signalling. Mutation of β3(Lys 716) caused dissociation of αΙΙbβ3 TMDs and integrin activation. To confirm that altered topography of β3(Lys 716) mutants activated αΙΙbβ3, we used directed evolution of β3(K716A) to identify substitutions restoring default state. Introduction of Pro(711) at the midpoint of β3 TMD (A711P) increased αΙΙbβ3 TMD association and inactivated integrin αΙΙbβ3(A711P,K716A). β3(Pro 711) introduced a TMD kink of 30 ± 1° precisely at the border of the outer and inner membrane clasps, thereby decoupling the tilt between these segments. Thus, widely occurring snorkelling residues in TMDs can help maintain TMD topography and membrane-embedding, thereby regulating transmembrane signalling.

Talin activates integrins by altering the topology of the β transmembrane domain.

Talin binding to the integrin β tail alters the β transmembrane domain’s topology, resulting in integrin activation.

Molecular mechanism of inside‐out integrin regulation

Summary.  Integrins are cell surface adhesion and signaling receptors important for cell adhesion, survival and migration. Integrins are known to be regulated by signals from inside the cells. Such inside‐out regulation modulates affinities of integrins for their extracellular matrix ligand and is critical for thrombosis, haemostasis and immune response. Talin and kindlin, two integrin binding proteins, have been shown to be important regulators of integrin function. In this review, we will focus on the molecular mechanism of integrin regulation that has emerged from recent structural, biochemical and genetic studies.

Reconstruction of integrin activation

Integrins are integral membrane proteins that mediate cell-matrix and cell-cell adhesion. They are important for vascular development and hematopoiesis, immune and inflammatory responses, and hemostasis. Integrins are also signaling receptors that can transmit information bidirectionally across plasma membranes. Research in the past 2 decades has made progress in unraveling the mechanisms of integrin signaling and brings the field to the moment of attempting synthetic reconstruction of the signaling pathways in vitro. Reconstruction of biologic processes provides stringent tests of our understanding of the process, as evidenced by studies of other biologic machines, such as ATP synthase, lactose permease, and G-protein-coupled receptors. Here, we review recent progress in reconstructing integrin signaling and the insights that we have gained through these experiments.

CD98hc (SLC3A2) Interaction with the Integrin β Subunit Cytoplasmic Domain Mediates Adhesive Signaling

In mammals, β1 integrin adhesion receptors generate signals that mediate cell spreading, migration, proliferation, and survival. CD98, a heterodimeric transmembrane protein, physically associates with certain integrin β subunit cytoplasmic domains (tails) via its heavy chain, CD98hc (SLC3A2), and loss of CD98hc impairs integrin signaling. Here we have used the lack of CD98hc interaction with the Drosophila integrin βPS tail for a homology scanning analysis that implicated the C-terminal 8 residues of β3 (Thr755-Thr802) in CD98hc binding. We then identified point mutations in the β3 C terminus (T755K and T758M) that abolish CD98hc association and a double mutation in the corresponding residues in the βPS tail (K839T,M842T), which resulted in gain of CD98hc interaction. Furthermore, the loss of function β3(T755K) mutation or the gain of function β3/βPS(K839T,M842T) led to a loss or gain of integrin-mediated cell spreading, respectively. Thus, we have identified critical integrin residues required for CD98hc interaction and in doing so have shown that CD98c interaction with the integrin β tail is required for its ability to mediate integrin signaling. These studies also provide new insights into how CD98hc may cooperate with other cytoplasmic domain binding proteins to modulate integrin functions and into the evolution of integrin signaling.

Amelioration of sepsis by TIE2 activation–induced vascular protection

Vascular protection through TIE2 activation is a potential treatment strategy to ameliorate sepsis. Antibody TIEs sepsis up in knots Sepsis, or severe systemic inflammation caused by infection, has a high mortality despite the availability of antibiotic treatment, and more specific therapies are urgently needed. One of the difficult-to-treat and potentially life-threatening components of sepsis is vascular disintegration and leakage. Han et al. have discovered an antibody, called ABTAA, which binds to a ligand called angiopoietin 2 (ANG2) in the vasculature, but then activates it instead of blocking its activity like standard antibodies. When ABTAA binds to ANG2, it causes clustering of ANG2 and subsequently its receptor TIE2 at the site, and the resulting signaling cascade protects the vascular walls and blunts the damaging effects of sepsis, greatly increasing survival in mouse models of the disease. Protection of endothelial integrity has been recognized as a frontline approach to alleviating sepsis progression, yet no effective agent for preserving endothelial integrity is available. Using an unusual anti–angiopoietin 2 (ANG2) antibody, ABTAA (ANG2-binding and TIE2-activating antibody), we show that activation of the endothelial receptor TIE2 protects the vasculature from septic damage and provides survival benefit in three sepsis mouse models. Upon binding to ANG2, ABTAA triggers clustering of ANG2, assembling an ABTAA/ANG2 complex that can subsequently bind and activate TIE2. Compared with a conventional ANG2-blocking antibody, ABTAA was highly effective in augmenting survival from sepsis by strengthening the endothelial glycocalyx, reducing cytokine storms, vascular leakage, and rarefaction, and mitigating organ damage. Together, our data advance the role of TIE2 activation in ameliorating sepsis progression and open a potential therapeutic avenue for sepsis to address the lack of sepsis-specific treatment.