Eugene Tkachenko

Fibroblast growth factor-specific modulation of cellular response by syndecan-4.

Proteoglycans participate in growth factor interaction with the cell surface through their heparan sulfate chains (HS), but it is not known if they are otherwise involved in growth factor signaling. It appears now that the syndecan-4 core protein, a transmembrane proteoglycan shown previously to bind phosphatidylinositol 4,5-bisphosphate (PIP2) and activate PKCα, participates in mediating the effects of fibroblast growth factor (FGF)2 on cell function. Mutations in the cytoplasmic tail of syndecan-4 that either reduced its affinity to PIP2 (PIP2 −) or disrupted its postsynaptic density 95, disk large, zona occludens-1 (PDZ)-dependent binding (PDZ−) produced a FGF2-specific dominant negative phenotype in endothelial cells as evidenced by the marked decline of their migration and proliferation rates and the impairment of their capacity to form tubes. In both cases, the molecular mechanism was determined to consist of a decrease in the syndecan-4–dependent activation of PKCα. This decrease was caused either by inhibition of FGF2-induced syndecan-4 dephosphorylation in the case of the PDZ− mutation or by disruption of basolateral targeting of syndecan-4 and its associated PDZ-dependent complex in the case of the PIP2 − mutation. These results suggest that PKCα activation and PDZ-mediated formation of a serine/threonine phosphatase-containing complex by syndecan-4 are downstream events of FGF2 signaling.

Fibroblast growth factor 2 endocytosis in endothelial cells proceed via syndecan-4-dependent activation of Rac1 and a Cdc42-dependent macropinocytic pathway

Full activity of fibroblast growth factors (FGFs) requires their internalization in addition to the interaction with cell surface receptors. Recent studies have suggested that the transmembrane proteoglycan syndecan-4 functions as a FGF2 receptor. In this study we investigated the molecular basis of syndecan endocytosis and its role in FGF2 internalization in endothelial cells. We found that syndecan-4 uptake, induced either by treatment with FGF2 or by antibody clustering, requires the integrity of plasma membrane lipid rafts for its initiation, occurs in a non-clathrin-, non-dynamin-dependent manner and involves Rac1, which is activated by syndecan-4 clustering. FGF2 was internalized in a complex with syndecan-4 in 70 kDa dextran-containing endocytic vesicles. FGF2 and syndecan-4 but not dextran endocytosis were blocked by the dominant negative Rac1 while amiloride and the dominant-negative Cdc42 blocked internalization of dextran in addition to FGF2 and syndecan-4. Taken together, these results demonstrate that FGF2 endocytosis requires syndecan-4 clustering-dependent activation of Rac1 and the intact CDC42-dependent macropinocytic pathway.

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.

An easy to assemble microfluidic perfusion device with a magnetic clamp

We have built and characterized a magnetic clamp for reversible sealing of PDMS microfluidic chips against cover glasses with cell cultures and a microfluidic chip for experiments on shear stress response of endothelial cells. The magnetic clamp exerts a reproducible uniform pressure on the microfluidic chip, achieving fast and reliable sealing for liquid pressures up to 40 kPa inside the chip with <10% deformations of microchannels and minimal variations of the substrate shear stress in perfusion flow. The microfluidic chip has 8 test regions with the substrate shear stress varying by a factor of 2 between each region, thus covering a 128-fold range from low venous to arterial. The perfusion is driven by differential pressure, which makes it possible to create pulsatile flows mimicking pulsing in the vasculature. The setup is tested by 15-40 hours perfusions over endothelial monolayers with shear stress in the range of 0.07-9 dyn/cm(2). Excellent cell viability at all shear stresses and alignment of cells along the flow at high shear stresses are repeatedly observed. A scratch wound healing assay under a shear flow is demonstrated and cell migration velocities are measured. Transfection of cells with a fluorescent protein is performed, and migrating fluorescent cells are imaged at a high resolution under shear flow in real time. The magnetic clamp can be closed with minimal mechanical perturbation to cells on the substrate and used with a variety of microfluidic chips for experiments with adherent and non-adherent cells.

Endomitotic Megakaryocytes that Form a Bipolar Spindle Exhibit Cleavage Furrow Ingression Followed by Furrow Regression

Megakaryocyte (MK) differentiation is marked by the development of progressive polyploidy, due to repeated incomplete cell cycles in which mitosis is aborted during anaphase, a process termed endomitosis. We have postulated that anaphase in endomitotic MKs diverges from diploid mitosis at a point distal to the assembly of the midzone, possibly involving impaired cleavage furrow progression. To define the extent of furrow initiation and ingression in endomitosis, we performed time-lapse imaging of MKs expressing yellow fluorescent protein (YFP)-tubulin and monitored shape change as they progressed through anaphase. We found that in early endomitotic cells that have a bipolar spindle, cleavage furrows form that can undergo significant ingression, but furrows regress to produce polyploid cells. Compared to cells that divide, cells that exhibit furrow regression have a slower rate of furrow ingression and do not furrow as deeply. More highly polyploid MKs undergoing additional endomitotic cycles also show measurable furrowing that is followed by regression, but the magnitude of the shape change is less than seen in the early MKs. This suggests that in the earliest endomitotic cycles when there is formation of a bipolar spindle, the failure of cytokinesis occurs late, following assembly and initial constriction of the actin/myosin ring, whereas in endomitotic MKs that are already polyploid there is secondary inhibition of furrow progression. This behavior of furrow ingression followed by regression may explain why midbody remnants are occasionally observed in polyploid MKs. This finding has important implications for the potential mechanisms for cytokinesis failure in endomitosis.

CD98hc (SLC3A2) participates in fibronectin matrix assembly by mediating integrin signaling

Integrin-dependent assembly of the fibronectin (Fn) matrix plays a central role in vertebrate development. We identify CD98hc, a membrane protein, as an important component of the matrix assembly machinery both in vitro and in vivo. CD98hc was not required for biosynthesis of cellular Fn or the maintenance of the repertoire or affinity of cellular Fn binding integrins, which are important contributors to Fn assembly. Instead, CD98hc was involved in the cells ability to exert force on the matrix and did so by dint of its capacity to interact with integrins to support downstream signals that lead to activation of RhoA small GTPase. Thus, we identify CD98hc as a membrane protein that enables matrix assembly and establish that it functions by interacting with integrins to support RhoA-driven contractility. CD98hc expression can vary widely; our data show that these variations in CD98hc expression can control the capacity of cells to assemble an Fn matrix, a process important in development, wound healing, and tumorigenesis.

Syndecan-4 clustering induces cell migration in a PDZ-dependent manner

Cell migration is a dynamic process involving formation of a leading edge in the direction of migration and adhesion points from which tension is generated to move the cell body forward. At the same time, disassembly of adhesion points occurs at the back of the cell, a region known as the trailing edge. Syndecan-4 (S4) is a transmembrane proteoglycan thought to be involved in the formation of focal adhesions. Recent studies have shown that its cytoplasmic domain can engage in signal transduction, making S4 a bona fide receptor. Here, we show that ligand clustering of cell surface S4 on endothelial cells initiates a signaling cascade that results in activation of Rac1, induction of cell polarization, and stimulation of cell migration that depends on S4 interaction with its PDZ-binding partner. Expression of an S4 mutant lacking its PDZ-binding region (S4-PDZ−) leads to decreased cell motility and a failure to form a trailing edge. On clustering S4, but not S4-PDZ−, targets activated Rac1 to the leading edge of live cells. Cells lacking synectin, a PDZ domain containing protein that interacts with S4, fail to migrate in response to S4 clustering. Both S4-PDZ−–expressing and synectin−/− endothelial cells exhibit elevated basal levels of Rac1. Thus, our data suggest that S4 promotes endothelial cell migration in response to ligand binding by activating Rac1 and localizing it to the leading edge, and that these processes are dependent on its PDZ-binding domain interaction with synectin.

Syndecan 4 Regulates FGFR1 Signaling in Endothelial Cells by Directing Macropinocytosis

Macropinocytosis controls the kinetics of endothelial signaling initiated by a fibroblast growth factor receptor. Limiting the Signal Through Macropinocytosis Fibroblast growth factor 2 (FGF2) triggers migration and proliferation of endothelial cells by binding to fibroblast growth factor receptor 1 (FGFR1) and the co-receptor syndecan 4 (S4). Activation of FGFR1 initiates signaling through mitogen-activated protein kinases (MAPKs). Elfenbein et al. found that S4 decreased the internalization of FGFR1 through a process called macropinocytosis. Furthermore, S4-mediated macropinocytosis of FGFR1 decreased the amplitude and increased the deactivation kinetics of MAPK signaling. Thus, these results indicate that S4 controls the duration of MAPK activation in response to binding of FGF2 to FGFR1. Fibroblast growth factor 2 (FGF2) induces endothelial cell migration and angiogenesis through two classes of receptors: receptor tyrosine kinases, such as FGF receptor 1 (FGFR1), and heparan sulfate proteoglycans, such as syndecan 4 (S4). We examined the distinct contributions of FGFR1 and S4 in shaping the endothelial response to FGF2. S4 determined the kinetics and magnitude of FGF2-induced mitogen-activated protein kinase (MAPK) signaling by promoting the macropinocytosis of the FGFR1-S4-FGF2 signaling complex. Internalization of the S4 receptor complex was independent of clathrin and dynamin, proceeded from lipid raft–enriched membranes, and required activation of the guanosine triphosphatases RhoG and Rab5. Genetic knockout of S4, disruption of S4 function, or inhibition of Rab5 led to increased endocytosis and MAPK signaling. These data define the mechanism by which FGFR1 and S4 coordinate downstream signaling upon FGF2 stimulation: FGFR1 initiates MAPK signaling, whereas S4-dependent FGFR1 macropinocytosis modulates the kinetics of MAPK activation. Our studies identify S4 as a regulator of MAPK signaling and address the question of how distinct classes of FGFRs individually contribute to signal transduction in endothelial cells.

Localized α4 Integrin Phosphorylation Directs Shear Stress–Induced Endothelial Cell Alignment

Vascular endothelial cells respond to laminar shear stress by aligning in the direction of flow, a process which may contribute to atheroprotection. Here we report that localized &agr;4 integrin phosphorylation is a mechanism for establishing the directionality of shear stress–induced alignment in microvascular endothelial cells. Within 5 minutes of exposure to a physiological level of shear stress, endothelial &agr;4 integrins became phosphorylated on Ser988. In wounded monolayers, phosphorylation was enhanced at the downstream edges of cells relative to the source of flow. The shear-induced &agr;4 integrin phosphorylation was blocked by inhibitors of cAMP-dependent protein kinase A (PKA), an enzyme involved in the alignment of endothelial cells under prolonged shear. Moreover, shear-induced localized activation of the small GTPase Rac1, which specifies the directionality of endothelial alignment, was similarly blocked by PKA inhibitors. Furthermore, endothelial cells bearing a nonphosphorylatable &agr;4(S988A) mutation failed to align in response to shear stress, thus establishing &agr;4 as a relevant PKA substrate. We thereby show that shear-induced PKA-dependent &agr;4 integrin phosphorylation at the downstream edge of endothelial cells promotes localized Rac1 activation, which in turn directs cytoskeletal alignment in response to shear stress.

Integrin αV is necessary for gastrulation movements that regulate vertebrate body asymmetry

Integrin αV can form heterodimers with several β subunits to mediate cell-cell and cell-extracellular matrix interactions. During zebrafish gastrulation, αV is expressed maternally and zygotically. Here, we used a morpholino-mediated αV knockdown strategy to study αV function. Although αV morphants displayed vascular defects, they also exhibited left-right body asymmetry defects affecting multiple visceral organs. This was preceded by mislocalization of dorsal forerunner cells (DFCs) and malformation of the Kupffers vesicle (KV) laterality organ. These defects were rescued with morpholino-resistant αV mRNA. Like αV, integrin β1b was expressed in DFCs, and β1b knockdown largely recapitulated the laterality phenotype of αV morphants. When tracked in real-time, individual DFCs of both morphants showed defects in DFC migration, preventing them from organizing into a KV of normal shape and size. Thus, we propose that αVβ1b mediates cellular interactions that are necessary for DFC clustering and movements necessary for Kupffers vesicle formation, uncovering an early contribution of integrins to the regulation of vertebrate laterality.