Integrin activation--the importance of a positive feedback

Mechanochemical mechanism of integrin clustering modulated by nanoscale ligand spacing and rigidity of extracellular substrates

Experimental findings indicate that cell function and behavior such as cell growth, division, migration and differentiation, are subtly regulated via integrin-dependent cell adhesion. Cell adhesion is influenced by nanoscale ligand spacing and rigidity of extracellular substrates, as cell adhesion drops greatly when the ligand spacing is larger than ~60nm, and cell adhesion is stronger on stiff than soft substrates. However, how nanoscale ligand spacing and substrate stiffness jointly affect integrin clustering and hence nascent cell adhesion remains to be elucidated. To quantitatively investigate the phenomena and the underlying mechanochemical mechanism of integrin clustering modulated by ligand spacing and substrate stiffness, we introduced Monte Carlo simulations varying the values of ligand spacing and substrate stiffness. Moreover, the effects of integrin number, integrin binding free energy, integrin association free energy, and local ligand spacing were investigated. The simulation results showed that integrin clustering decreased sharply, when ligand spacing was relatively large such as d_L>60nm in the current simulations, regardless of substrate rigidities, though with close spacing, the clustering increased with the substrate stiffness. The investigation contributes to the goals of understanding and predicting experimental phenomena, directing and optimizing biomaterial design, and manipulating integrin-dependent cell-substrate adhesion in tissue engineering.

Computational modelling of bovine ovarian follicle development

Background

The development of ovarian follicles hinges on the timely exposure to the appropriate combination of hormones. Follicle stimulating hormone (FSH) and luteinizing hormone (LH) are both produced in the pituitary gland and are transported via the blood circulation to the thecal layer surrounding the follicle. From there both hormones are transported into the follicle by diffusion. FSH-receptors are expressed mainly in the granulosa while LH-receptors are expressed in a gradient with highest expression in the theca. How this spatial organization is achieved is not known. Equally it is not understood whether LH and FSH trigger distinct signalling programs or whether the distinct spatial localization of their G-protein coupled receptors is sufficient to convey their distinct biological function.

Results

We have developed a data-based computational model of the spatio-temporal signalling processes within the follicle and (i) predict that FSH and LH form a gradient inside the follicle, (ii) show that the spatial distribution of FSH- and LH-receptors can arise from the well known regulatory interactions, and (iii) find that the differential activity of FSH and LH may well result from the distinct spatial localisation of their receptors, even when both receptors respond with the same intracellular signalling cascade to their ligand.

Conclusion

The model integrates the large amount of published data into a consistent framework that can now be used to better understand how observed defects translate into failed follicle maturation.

A spatial model for integrin clustering as a result of feedback between integrin activation and integrin binding

Integrins are transmembrane adhesion receptors that bind extracellular matrix (ECM) proteins and signal bidirectionally to regulate cell adhesion and migration. In many cell types, integrins cluster at cell-ECM contacts to create the foundation for adhesion complexes that transfer force between the cell and the ECM. Even though the temporal and spatial regulation of these integrin clusters is essential for cell migration, how cells regulate their formation is currently unknown. It has been shown that integrin cluster formation is independent of actin stress fiber formation, but requires active (high-affinity) integrins, phosphoinositol-4,5-bisphosphate (PIP2), talin, and immobile ECM ligand. Based on these observations, we propose a minimal model for initial formation of integrin clusters, facilitated by localized activation and binding of integrins to ECM ligands as a result of biochemical feedback between integrin binding and integrin activation. By employing a diffusion-reaction framework for modeling these reactions, we show how spatial organization of bound integrins into clusters may be achieved by a local source of active integrins, namely protein complexes formed on the cytoplasmic tails of bound integrins. Further, we show how such a mechanism can turn small local increases in the concentration of active talin or active integrin into integrin clusters via positive feedback. Our results suggest that the formation of integrin clusters by the proposed mechanism depends on the relationships between production and diffusion of integrin-activating species, and that changes to the relative rates of these processes may affect the resulting properties of integrin clusters.

Impact of cell shape on cell migration behavior on elastic substrate

Cell shape is known to have profound effects on a number of cell behaviors. In this paper we have studied its role in cell migration through modeling the effect of cell shape on the cell traction force distribution, the traction force dependent stability of cell adhesion and the matrix rigidity dependent traction force formation. To quantify the driving force of cell migration, a new parameter called the motility factor, that takes account of the effect of cell shape, matrix rigidity and dynamic stability of cell adhesion, is proposed. We showed that the motility factor depends on the matrix rigidity in a biphasic manner, which is consistent with the experimental observations of the biphasic dependence of cell migration speed on the matrix rigidity. We showed that the cell shape plays a pivotal role in the cell migration behavior by regulating the traction force at the cell front and rear. The larger the cell polarity, the larger the motility factor is. The keratocyte-like shape has a larger motility factor than the fibroblast-like shape, which explains why keratocyte has a much higher migration speed. The motility factor might be an appropriate parameter for a quantitative description of the driving force of cell migration.

Enhanced cellular sensitivity from partitioning the integrin receptors into multiple clusters

Integrins are essential receptors for the development and functioning of multicellular organisms because they mediate cell adhesion and migration, and regulate cell proliferation and apoptosis. In response to cues in the extracellular matrix, they are observed to organize into many clusters. The number and size of such clusters are observed to vary according to the concentration of and affinity for the extracellular ligand. The realization of a cluster point pattern is governed by a doubly stochastic process, controlling the number of clusters and the number of points per cluster. We construct entropy measures for the separation of two doubly stochastic processes and demonstrate how the self-organization of integrins in multiple clusters contributes to the accuracy in sensing the extracellular environment.

A computational analysis of the dynamic roles of talin, Dok1, and PIPKI for integrin activation

Integrin signaling regulates cell migration and plays a pivotal role in developmental processes and cancer metastasis. Integrin signaling has been studied extensively and much data is available on pathway components and interactions. Yet the data is fragmented and an integrated model is missing. We use a rule-based modeling approach to integrate available data and test biological hypotheses regarding the role of talin, Dok1 and PIPKI in integrin activation. The detailed biochemical characterization of integrin signaling provides us with measured values for most of the kinetics parameters. However, measurements are not fully accurate and the cellular concentrations of signaling proteins are largely unknown and expected to vary substantially across different cellular conditions. By sampling model behaviors over the physiologically realistic parameter range we find that the model exhibits only two different qualitative behaviors and these depend mainly on the relative protein concentrations, which offers a powerful point of control to the cell. Our study highlights the necessity to characterize model behavior not for a single parameter optimum, but to identify parameter sets that characterize different signaling modes.

Integrin activation in the immune system

Modulation of leukocyte adhesiveness is critical to leukocyte function during the immune response. A central paradigm in this phenomenon is represented by integrin activation, which is controlled by inside-out signal transduction mechanisms triggered by selectins, chemoattractants and TcR-bound Ag and facilitated by mechanochemical forces. Integrins are heterodimeric adhesive receptors differently expressed on all leukocyte subtypes. At least two distinct modalities of integrin activation are known, namely conformational changes, leading to increased affinity, and lateral mobility leading to increased valency, both enhancing cell avidity (adhesiveness). Several signal transduction events have been correlated to integrin activation in leukocytes. The complexity of intracellular signaling networks leading to leukocyte integrin activation is likely functional to generate robustness and fine tuning of integrin activation allowing integration of qualitative and quantitative variations of extracellular signals leading to leukocyte-, agonist- and integrin-specific control of adhesion. In this context, the recent modular abstraction proposed for the functional architecture of biological networks may provide a powerful paradigm to understand regulation and specificity of signaling events. Accordingly, pro-adhesive intracellular signaling networks may be organized in regulatory signalosomes, or modules, corresponding to discrete clusters of interacting signaling proteins, with some devoted to context-dependent regulation of specificity and dynamics of integrin activation. The principles and technologies of systems biology, and more specifically of network theory, may help to address this complexity and unveil the inner logic governing leukocyte recruitment during the immune response.

Integrin clustering is driven by mechanical resistance from the glycocalyx and the substrate

Integrins have emerged as key sensory molecules that translate chemical and physical cues from the extracellular matrix (ECM) into biochemical signals that regulate cell behavior. Integrins function by clustering into adhesion plaques, but the molecular mechanisms that drive integrin clustering in response to interaction with the ECM remain unclear. To explore how deformations in the cell-ECM interface influence integrin clustering, we developed a spatial-temporal simulation that integrates the micro-mechanics of the cell, glycocalyx, and ECM with a simple chemical model of integrin activation and ligand interaction. Due to mechanical coupling, we find that integrin-ligand interactions are highly cooperative, and this cooperativity is sufficient to drive integrin clustering even in the absence of cytoskeletal crosslinking or homotypic integrin-integrin interactions. The glycocalyx largely mediates this cooperativity and hence may be a key regulator of integrin function. Remarkably, integrin clustering in the model is naturally responsive to the chemical and physical properties of the ECM, including ligand density, matrix rigidity, and the chemical affinity of ligand for receptor. Consistent with experimental observations, we find that integrin clustering is robust on rigid substrates with high ligand density, but is impaired on substrates that are highly compliant or have low ligand density. We thus demonstrate how integrins themselves could function as sensory molecules that begin sensing matrix properties even before large multi-molecular adhesion complexes are assembled.

Critical cell decisions, including whether to live, proliferate, or assemble into tissue structures, are directed by cues from the extracellular matrix, the external protein scaffold that surrounds cells. Integrin receptors on the cell surface bind to the extracellular matrix and cluster into complexes that translate matrix cues into the set of instructions a cell follows. Using a newly developed model of the cell-matrix interface, in this work we detail a simple yet efficient mechanism by which integrins could “sense” important matrix properties, including chemical composition and mechanical stiffness, and cluster appropriately. This mechanism relies on mechanical resistance to integrin-matrix interaction provided by the glycocalyx, the slimy sugar and protein coating on the cell, as well as the stiffness of the matrix and the cell itself. In general, the resistance alters integrin-ligand reaction rates, such that integrin clustering is favored for many physiologically relevant conditions. Interestingly, the mechanical properties of the cell and ECM are altered in many prevalent diseases, such as cancer, and our work suggests how these mechanical perturbations might adversely influence integrin function.

Kindlins: essential regulators of integrin signalling and cell-matrix adhesion

Integrin-mediated cell-ECM (extracellular matrix) adhesion is a fundamental process that controls cell behaviour. For correct cell-ECM adhesion, both the ligand-binding affinity and the spatial organization of integrins must be precisely controlled; how integrins are regulated, however, is not completely understood. Kindlins constitute a family of evolutionarily conserved cytoplasmic components of cell-ECM adhesions that bind to beta-integrin cytoplasmic tails directly and cooperate with talin in integrin activation. In addition, kindlins interact with many components of cell-ECM adhesions--such as migfilin and integrin-linked kinase--to promote cytoskeletal reorganization. Loss of kindlins causes severe defects in integrin signalling, cell-ECM adhesion and cytoskeletal organization, resulting in early embryonic lethality (kindlin-2), postnatal lethality (kindlin-3) and Kindler syndrome (kindlin-1). It is therefore clear that kindlins, together with several other integrin-proximal proteins, are essential for integrin signalling and cell-ECM adhesion regulation.

Integrin activation

Agonist stimulation of integrin receptors, composed of transmembrane alpha and beta subunits, leads cells to regulate integrin affinity ('activation'), a process that controls cell adhesion and migration, and extracellular matrix assembly. A final step in integrin activation is the binding of talin to integrin beta cytoplasmic domains. We used forward, reverse and synthetic genetics to engineer and order integrin activation pathways of a prototypic integrin, platelet alphaIIbbeta3. PMA activated alphaIIbbeta3 only after expression of both PKCalpha (protein kinase Calpha) and talin at levels approximating those in platelets. Inhibition of Rap1 GTPase reduced alphaIIbbeta3 activation, whereas expression of constitutively active Rap1A(G12V) bypassed the requirement for PKCalpha. Overexpression of a Rap effector, RIAM (Rap1-GTP-interacting adaptor molecule), activated alphaIIbbeta3 and bypassed the requirement for PKCalpha and Rap1. In addition, shRNA (short hairpin RNA)-mediated knockdown of RIAM blocked talin interaction with and activation of integrin alphaIIbbeta3. Rap1 activation caused the formation of an 'activation complex' containing talin and RIAM that redistributed to the plasma membrane and activated alphaIIbbeta3. The central finding was that this Rap1-induced formation of an 'integrin activation complex' leads to the unmasking of the integrin-binding site on talin, resulting in integrin activation.

Platelet integrin alpha(IIb)beta(3): activation mechanisms

Integrin alpha(IIb)beta(3) plays a critical role in platelet aggregation, a central response in hemostasis and thrombosis. This function of alpha(IIb)beta(3) depends upon a transition from a resting to an activated state such that it acquires the capacity to bind soluble ligands. Diverse platelet agonists alter the cytoplasmic domain of alpha(IIb)beta(3) and initiate a conformational change that traverses the transmembrane region and ultimately triggers rearrangements in the extracellular domain to permit ligand binding. The membrane-proximal regions of alpha(IIb) and beta(3) cytoplasmic tails, together with the transmembrane segments of the subunits, contact each other to form a complex which restrains the integrin in the resting state. It is unclasping of this complex that induces integrin activation. This clasping/unclasping process is influenced by multiple cytoplasmic tail binding partners. Among them, talin appears to be a critical trigger of alpha(IIb)beta(3) activation, but other binding partners, which function as activators or suppressors, are likely to act as co-regulators of integrin activation.