Amino acid changes in Drosophila alphaPS2betaPS integrins that affect ligand affinity

Actin-driven nanotopography promotes stable integrin adhesion formation in developing tissue

Morphogenesis requires building stable macromolecular structures from highly dynamic proteins. Muscles are anchored by long-lasting integrin adhesions to resist contractile force. However, the mechanisms governing integrin diffusion, immobilization, and activation within developing tissues remain elusive. Here, we show that actin polymerization-driven membrane protrusions form nanotopographies that enable strong adhesion at Drosophila muscle attachment sites (MASs). Super-resolution microscopy reveals that integrins assemble adhesive belts around Arp2/3-dependent actin protrusions, forming invadosome-like structures with membrane nanotopographies. Single protein tracking shows that, during MAS development, integrins become immobile and confined within diffusion traps formed by the membrane nanotopographies. Actin filaments also display restricted motion and confinement, indicating strong mechanical connection with integrins. Using isolated muscle cells, we show that substrate nanotopography, rather than rigidity, drives adhesion maturation by regulating actin protrusion, integrin diffusion and immobilization. These results thus demonstrate that actin-polymerization-driven membrane protrusions are essential for the formation of strong integrin adhesions sites in the developing embryo, and highlight the important contribution of geometry to morphogenesis.

β1D integrin splice variant stabilizes integrin dynamics and reduces integrin signaling by limiting paxillin recruitment

Heterodimeric integrin receptors control cell adhesion, migration and extracellular matrix assembly. While the α integrin subunit determines extracellular ligand specificity, the β integrin chain binds to an acidic residue of the ligand, and cytoplasmic adapter protein families such as talins, kindlins and paxillin, to form mechanosensing cell matrix adhesions. Alternative splicing of the β1 integrin cytoplasmic tail creates ubiquitously expressed β1A, and the heart and skeletal muscle-specific β1D form. To study the physiological difference between these forms, we developed fluorescent β1 integrins and analyzed their dynamics, localization, and cytoplasmic adapter recruitment and effects on cell proliferation. On fibronectin, GFP-tagged β1A integrin showed dynamic exchange in peripheral focal adhesions, and long, central fibrillar adhesions. In contrast, GFP-β1D integrins exchanged slowly, forming immobile and short central adhesions. While adhesion recruitment of GFP-β1A integrin was sensitive to C-terminal tail mutagenesis, GFP-β1D integrin was recruited independently of the distal NPXY motif. In addition, a P786A mutation in the proximal, talin-binding NPXY⁷⁸³ motif switched β1D to a highly dynamic integrin. In contrast, the inverse A786P mutation in β1A integrin interfered with paxillin recruitment and proliferation. Thus, differential β1 integrin splicing controls integrin-dependent adhesion signaling, to adapt to the specific physiological needs of differentiated muscle cells.

The role of a conserved membrane proximal cysteine in altering αPS2CβPS integrin diffusion

Cysteine residues (Cys) in the membrane proximal region are common post-translational modification (PTM) sites in transmembrane proteins. Herein, the effects of a highly conserved membrane proximal α-subunit Cys¹³⁶⁸ on the diffusion properties of αPS2CβPS integrins are reported. Sequence alignment shows that this cysteine is palmitoylated in human α3 and α6 integrin subunits. Replacing Cys¹³⁶⁸ in wild-type integrins with valine (Val¹³⁶⁸) putatively blocks a PTM site and alters integrins' ligand binding and diffusion characteristics. Both fluorescence recovery after photobleaching (FRAP) and single particle tracking (SPT) diffusion measurements show Val¹³⁶⁸ integrins are more mobile compared to wild-type integrins. Approximately 33% and 8% more Val¹³⁶⁸ integrins are mobile as measured by FRAP and SPT, respectively. The mobile Val¹³⁶⁸ integrins also exhibit less time-dependent diffusion, as measured by FRAP. Tandem mass spectrometry data suggest that Cys¹³⁶⁸ contains a redox or palmitoylation PTM in αPS2CβPS integrins. This membrane proximal Cys may play an important role in the diffusion of other alpha subunits that contain this conserved residue.

Single particle tracking with sterol modulation reveals the cholesterol-mediated diffusion properties of integrin receptors

A combination of sterol modulation with cyclodextrins plus fluorescence microscopy revealed a biophysical mechanism behind cholesterol's influence on the diffusion of a ubiquitous class of receptors called integrins. The heterogeneous diffusion of integrins bound to ligand-coated quantum dots was measured using single particle tracking (SPT), and the ensemble changes in integrin diffusion were measured by fluorescence recovery after photobleaching (FRAP). A 25 ± 1% reduction of membrane cholesterol resulted in three significant changes to the diffusion of ligand-bound αPS2CβPS integrins as measured by SPT. There was a 23% increase in ligand-bound mobile integrins; there was a statistically significant increase in the average diffusion coefficient inside zones of confined diffusion, and histograms of confined integrin trajectories showed an increased frequency in the range of 0.1-1 μm(2) s(-1) and a decreased frequency in the 0.001-0.1 μm(2) s(-1) range. No statistical change was measured in the duration of confinement nor the size of confined zones. Restoring the cholesterol-depleted cells with exogenous cholesterol or exogenous epicholesterol resulted in similar diffusion properties. Epicholesterol differs from cholesterol in the orientation of a single hydroxyl group. The ability of epicholesterol to substitute for cholesterol suggests a biophysical mechanism for cholesterol's effect on integrin diffusion. Influences of bilayer thickness, viscosity and organization are discussed as possible explanations for the measured changes in integrin diffusion when the membrane cholesterol concentration is reduced.

Identification of integrin beta subunit mutations that alter affinity for extracellular matrix ligand

We examined over 50 mutations in the Drosophila βPS integrin subunit that alter integrin function in situ for their ability to bind a soluble monovalent ligand, TWOW-1. Surprisingly, very few of the mutations, which were selected for conditional lethality in the fly, reduce the ligand binding ability of the integrin. The most prevalent class of mutations activates the integrin heterodimer. These findings emphasize the importance of integrin affinity regulation and point out how molecular interactions throughout the integrin molecule are important in keeping the integrin in a low affinity state. Mutations strongly support the controversial deadbolt hypothesis, where the CD loop in the β tail domain acts to restrain the I domain in the inactive, bent conformation. Site-directed mutations in the cytoplasmic domains of βPS and αPS2C reveal different effects on ligand binding from those observed for αIIbβ3 integrins and identify for the first time a cytoplasmic cysteine residue, conserved in three human integrins, as being important in affinity regulation. In the fly, we find that genetic interactions of the βPS mutations with reduction in talin function are consistent with the integrin affinity differences measured in cells. Additionally, these genetic interactions report on increased and decreased integrin functions that do not result in affinity changes in the PS2C integrin measured in cultured cells.

Talin and signaling through integrins

Integrin adhesion receptors are essential for the development and functioning of multicellular animals. Integrins mediate cell adhesion to the extracellular matrix and to counter-receptors on adjacent cells, and the ability of integrins to bind extracellular ligands is regulated in response to intracellular signals that act on the short cytoplasmic tails of integrin subunits. Integrin activation, the rapid conversion of integrin receptors from low to high affinity, requires binding of talin to integrin β tails and, once bound, talin provides a connection from activated integrins to the actin cytoskeleton. A wide range of experimental approaches have contributed to the current understanding of the importance of talin in integrin signaling. Here, we describe two methods that have been central to our investigations of talin; a biochemical assay that has allowed characterization of interactions between integrin cytoplasmic tails and talin, and a fluorescent-activated cell-sorting procedure to assess integrin activation in cultured cells expressing talin domains, mutants, dominant negative constructs, or shRNA.

Noninvasive measurements of integrin microclustering under altered membrane cholesterol levels

Reported herein is a method that can be used to study the role of cholesterol in the microclustering of a ubiquitous class of membrane receptors, termed integrins. Integrin microclustering was measured using a fluorescence resonance energy transfer assay that does not require direct attachment of fluorescent donors or acceptors onto the integrins, and thus minimizes unwanted perturbations to integrin clustering. Membrane cholesterol levels were reduced using methyl-beta-cyclodextrin (mbetaCD), as confirmed by Amplex Red assays of total cellular lipid or plasma membrane lipid extract. Subsequent changes in integrin microclustering were measured in cells expressing wild-type (WT) or mutant integrins. Although less integrin microclustering was measured after 27% membrane cholesterol depletion in a cell line expressing WT integrins, there was no statistically significant change for cells expressing alpha-cytoplasmic integrin mutants after a 45% reduction in plasma membrane cholesterol, and a significant increase in clustering for cells expressing ligand-binding domain integrin mutants after a 57% decrease in membrane cholesterol. These results are explained by differences in WT and mutant integrin partitioning into lipid nanodomains. Restoration of original cholesterol levels was used to confirm that the measured changes in membrane properties were cholesterol-dependent. No correlations between lipid diffusion and integrin microclustering were measured by means of fluorescence recovery after photobleaching using a fluorescent lipid mimetic. Similar lipid diffusion coefficients were measured after cholesterol depletion, irrespective of the integrins being expressed.

Integrin alphaIIbbeta3 activation in Chinese hamster ovary cells and platelets increases clustering rather than affinity

Integrin alphaIIbbeta3 affinity regulation by talin binding to the cytoplasmic tail of beta3 is a generally accepted model for explaining activation of this integrin in Chinese hamster ovary cells and human platelets. Most of the evidence for this model comes from the use of multivalent ligands. This raises the possibility that the activation being measured is that of increased clustering of the integrin rather than affinity. Using a newly developed assay that probes integrins on the surface of cells with only monovalent ligands prior to fixation, I do not find increases in affinity of alphaIIbbeta3 integrins by talin head fragments in Chinese hamster ovary cells, nor do I observe affinity increases in human platelets stimulated with thrombin. Binding to a multivalent ligand does increase in both of these cases. This assay does report affinity increases induced by either Mn(2+), a cytoplasmic domain mutant (D723R) in the cytoplasmic domain of beta3, or preincubation with a peptide ligand. These results reconcile the previously observed differences between talin effects on integrin activation in Drosophila and vertebrate systems and suggest new models for talin regulation of integrin activity in human platelets.

Differences in regulation of Drosophila and vertebrate integrin affinity by talin

Integrin-mediated cell adhesion is essential for development of multicellular organisms. In worms, flies, and vertebrates, talin forms a physical link between integrin cytoplasmic domains and the actin cytoskeleton. Loss of either integrins or talin leads to similar phenotypes. In vertebrates, talin is also a key regulator of integrin affinity. We used a ligand-mimetic Fab fragment, TWOW-1, to assess talin's role in regulating Drosophila alphaPS2 betaPS affinity. Depletion of cellular metabolic energy reduced TWOW-1 binding, suggesting alphaPS2 betaPS affinity is an active process as it is for vertebrate integrins. In contrast to vertebrate integrins, neither talin knockdown by RNA interference nor talin head overexpression had a significant effect on TWOW-1 binding. Furthermore, replacement of the transmembrane or talin-binding cytoplasmic domains of alphaPS2 betaPS with those of human alphaIIb beta3 failed to enable talin regulation of TWOW-1 binding. However, substitution of the extracellular and transmembrane domains of alphaPS2 betaPS with those of alphaIIb beta3 resulted in a constitutively active integrin whose affinity was reduced by talin knockdown. Furthermore, wild-type alphaIIb beta3 was activated by overexpression of Drosophila talin head domain. Thus, despite evolutionary conservation of talin's integrin/cytoskeleton linkage function, talin is not sufficient to regulate Drosophila alphaPS2 betaPS affinity because of structural features inherent in the alphaPS2 betaPS extracellular and/or transmembrane domains.

Modulation of ligand binding by alternative splicing of the alphaPS2 integrin subunit

The Drosophila alphaPS2 integrin subunit is found in two isoforms. alphaPS2C contains 25 residues not found in alphaPS2m8, encoded by the alternative eighth exon. Previously, it was shown that cells expressing alphaPS2C spread more effectively than alphaPS2m8 cells on fragments of the ECM protein Tiggrin, and that alphaPS2C-containing integrins are relatively insensitive to depletion of Ca(2+). Using a ligand mimetic probe for Tiggrin affinity (TWOW-1), we show that the affinity of alphaPS2CbetaPS for this ligand is much higher than that of alphaPS2m8betaPS. However, the two isoforms become more similar in the presence of activating levels of Mn(2+). Modeling indicates that the exon 8-encoded residues replace the third beta strand of the third blade of the alpha subunit beta-propeller structure, and generate an exaggerated loop between this and the fourth strand. alphaPS2 subunits with the extra loop structure but with an m8-like third strand, or subunits with a C-like strand but an m8-like short loop, both fail to show alphaPS2C-like affinity for TWOW-1. Surprisingly, a single C > m8-like change at the third strand-loop transition point is sufficient to make alphaPS2C require Ca(2+) for function, despite the absence of any known cation binding site in this region. These data indicate that alternative splicing in integrin alpha subunit extracellular domains may affect ligand affinity via relatively subtle alterations in integrin conformation. These results may have relevance for vertebrate alpha6 and alpha7, which are alternatively spliced at the same site.

Mutations in the Drosophila alphaPS2 integrin subunit uncover new features of adhesion site assembly

The Drosophila alphaPS2betaPS integrin is required for diverse development events, including muscle attachment. We characterized six unusual mutations in the alphaPS2 gene that cause a subset of the null phenotype. One mutation changes a residue in alphaPS2 that is equivalent to the residue in alphaV that contacts the arginine of RGD. This change severely reduced alphaPS2betaPS affinity for soluble ligand, abolished the ability of the integrin to recruit laminin to muscle attachment sites in the embryo and caused detachment of integrins and talin from the ECM. Three mutations that alter different parts of the alphaPS2 beta-propeller, plus a fourth that eliminated a late phase of alphaPS2 expression, all led to a strong decrease in alphaPS2betaPS at muscle ends, but, surprisingly, normal levels of talin were recruited. Thus, although talin recruitment requires alphaPS2betaPS, talin levels are not simply specified by the amount of integrin at the adhesive junction. These mutations caused detachment of talin and actin from integrins, suggesting that the integrin-talin link is weaker than the ECM-integrin link.

General in vivo assay for the study of integrin cell membrane receptor microclustering

A method for measuring the microclustering of a class of cell surface receptors called integrins is reported. Integrins are proteins involved in bidirectional signaling across the cell membrane and are important in cell adhesion, growth, and survival. Their activity is regulated by changes in protein conformation and protein clustering. The developed in vivo clustering assay uses fluorescence resonance energy transfer (FRET) and has the benefit of requiring a single cloning step to generate FRET donors and acceptors that can be used to measure the clustering of a series of integrin mutants. The FRET reporters contain extracellular donor or acceptor fluorescent protein attached to native integrin cytoplasmic and transmembrane domains, and these are expressed along with wild-type or mutant integrins. Expression of the FRET reporters has no affect on the ligand binding properties of coexpressed integrins. FRET values are calculated for cell lines spreading on ligand coated surfaces, and these values are independent of fluorescent protein expression. No FRET is observed in cell lines expressing the reporters in the absence of integrins. Integrin-dependent FRET values increase approximately 2-3-fold when the integrins contain mutations that result in increased ligand binding affinities.

Expression of Plxdc2/TEM7R in the developing nervous system of the mouse

Plexin-domain containing 2 (Plxdc2) is a relatively uncharacterised transmembrane protein with an area of nidogen homology and a plexin repeat (PSI domain) in its extracellular region. Here, we describe Plxdc2 expression in the embryonic mouse, with particular emphasis on the developing central nervous system. Using light microscopy and optical projection tomography (OPT), we analyse RNA in situ hybridization patterns and expression of two reporter genes, beta-geo (a fusion of beta-galactosidase to neomycin phosphotransferase) and placental alkaline phosphatase (PLAP) in a Plxdc2 gene trap mouse line (KST37; [Leighton, P.A., Mitchell, K.J., Goodrich, L.V., Lu, X., Pinson, K., Scherz, P., Skarnes, W.C., Tessier-Lavigne, M., 2001. Defining brain wiring patterns and mechanisms through gene trapping in mice. Nature 410, 174-179]). At mid-embryonic stages (E9.5-E11.5) Plxdc2-betageo expression is prominent in a number of patterning centres of the brain, including the cortical hem, midbrain-hindbrain boundary and the midbrain floorplate. Plxdc2 is expressed in other tissues, most notably the limbs, lung buds and developing heart, as well as the spinal cord and dorsal root ganglia. At E15.5, expression is apparent in a large number of discrete nuclei and structures throughout the brain, including the glial wedge and derivatives of the cortical hem. Plxdc2-betageo expression is particularly strong in the developing Purkinje cell layer, especially in the posterior half of the cerebellum. The PLAP marker is expressed in a number of axonal tracts, including the posterior commissure, mammillotegmental tract and cerebellar peduncle. We compare Plxdc2-betageo expression in the embryonic brain with the much more restricted expression of the related gene Plxdc1 and with members of the Wnt family (Wnt3a, Wnt5a and Wnt8b) that show a striking overlap with Plxdc2 expression in certain areas.