The Talin Head Domain Reinforces Integrin-Mediated Adhesion by Promoting Adhesion Complex Stability and Clustering

Abstract

Talin serves an essential function during integrin-mediated adhesion in linking integrins to actin via the intracellular adhesion complex. In addition, the N-terminal head domain of talin regulates the affinity of integrins for their ECM-ligands, a process known as inside-out activation. We previously showed that in Drosophila, mutating the integrin binding site in the talin head domain resulted in weakened adhesion to the ECM. Intriguingly, subsequent studies showed that canonical inside-out activation of integrin might not take place in flies. Consistent with this, a mutation in talin that specifically blocks its ability to activate mammalian integrins does not significantly impinge on talin function during fly development. Here, we describe results suggesting that the talin head domain reinforces and stabilizes the integrin adhesion complex by promoting integrin clustering distinct from its ability to support inside-out activation. Specifically, we show that an allele of talin containing a mutation that disrupts intramolecular interactions within the talin head attenuates the assembly and reinforcement of the integrin adhesion complex. Importantly, we provide evidence that this mutation blocks integrin clustering in vivo. We propose that the talin head domain is essential for regulating integrin avidity in Drosophila and that this is crucial for integrin-mediated adhesion during animal development.

IPP Complex Reinforces Adhesion by Relaying Tension-Dependent Signals to Inhibit Integrin Turnover

“…In mammalian cells, IPP complex modulates integrin extracellular adhesion for ECM ligands in a talin-or kindlindependent manner (Honda et al, 2009(Honda et al, , 2013Huet-Calderwood et al, 2014). In Drosophila, talin strengthens integrin adhesion by facilitating integrin clustering rather than increasing integrin affinity (Brown et al, 2002;Bunch, 2010;Ellis et al, 2014;Helsten et al, 2008). We tested whether IPP complex facilitates talindependent integrin clustering (Calderwood et al, 2013).…”

In vivo regulation of integrin turnover by outside-in activation

“…Specifically, whereas divalent cations were able to induce changes in integrin affinity to its ligands, Talin-mediated inside-out signaling failed to do so (Helsten et al, 2008). Further work has shown that the introduction of a point mutation in Talin that specifically blocked the ability of its head domain to induce inside-out activation but did not impinge on its ability to bind to the integrin cytoplasmic tail did not give rise to a detectable phenotype (Ellis et al, 2014). Taken together, these studies point to a model whereby outside-in integrin activation but not Talin-mediated inside-out integrin activation is present in flies.…”

“…Intriguing evidence from Drosophila, using a sensor for integrin activation, has suggested that Mn 2+ might be able to activate integrins in vivo, in larval imaginal discs (Helsten et al, 2008). Moreover, reverse and forward genetic approaches in the fly have identified point mutations in Talin and the β-integrin subunit (known as βPS integrin) that have been proposed to modulate activation (Ellis et al, 2014(Ellis et al, , 2011Jannuzi et al, 2002Jannuzi et al, , 2004Pines et al, 2012Pines et al, , 2011Tanentzapf and Brown, 2006).…”

Talin – the master of integrin adhesions

“…The impairment of integrin activation by the L325R talin mutation was confirmed by the defects in hematopoietic cells (Stefanini et al, 2014;Yago et al, 2015). In the fly, the R367A talin mutation caused lethality but with a much weaker phenotype than a null allele, whereas no defects were detected with L334R (Tanentzapf and Brown, 2006a;Ellis et al, 2014). Thus, for integrin-mediated adhesion to the ECM within intact tissues it is difficult to make a firm conclusion from current evidence regarding the importance of inside-out activation in general, and the contribution of talin in particular.…”