Full length talin stimulates integrin activation and axon regeneration

Abstract

Integrin function is regulated by activation involving conformational changes that modulate ligand-binding affinity and downstream signaling. Activation is regulated through inside-out signaling which is controlled by many signaling pathways via a final common pathway through kindlin and talin, which bind to the intracellular tail of beta integrins. Previous studies have shown that the axon growth inhibitory molecules NogoA and chondroitin sulfate proteoglycans (CSPGs) inactivate integrins. Overexpressing kindlin-1 in dorsal root ganglion (DRG) neurons activates integrins, enabling their axons to overcome inhibitory molecules in the environment, and promoting regeneration in vivo following dorsal root crush. Other studies have indicated that expression of the talin head alone or with kindlin can enhance integrin activation. Here, using adult rat DRG neurons, we investigate the effects of overexpressing various forms of talin on axon growth and integrin signaling. We found that overexpression of the talin head activated axonal integrins but inhibited downstream signaling via FAK, and did not promote axon growth. Similarly, co-expression of the talin head and kindlin-1 prevented the growth-promoting effect of kindlin-1, suggesting that the talin head acts as a form of dominant negative for integrin function. Using full-length talin constructs in PC12 cells we observed that neurite growth was enhanced by the expression of wild-type talin and more so by two ‘activated’ forms of talin produced by point mutation (on laminin and aggrecan–laminin substrates). Nevertheless, co-expression of full-length talin with kindlin did not promote neurite growth more than either molecule alone. In vivo, we find that talin is present in PNS axons (sciatic nerve), and also in CNS axons of the corticospinal tract.

Integrins promote axonal regeneration after injury of the nervous system

“…Talin isoforms 1 and 2 are expressed in the nervous system (Monkley, Pritchard & Critchley, ; Senetar, Moncman & McCann, ; Debrand et al ., ; Tan et al ., ). In nerve growth factor‐stimulated PC12 cells, overexpression of the full‐length and constitutively activated isoforms of talin has been shown to promote neurite outgrowth in the presence of the repulsive extracellular matrix protein aggrecan (Tan et al ., ). Dingyu et al .…”

“…However, the large size of the full‐length protein presents a challenge for talin expression in neurons. In studies using primary cultures of DRG neurons, only the talin head domain has been overexpressed (Tan et al ., ). The talin‐head domain is required to interact with the cytoplasmic tail of the β integrin subunit and to activate the heterodimeric receptor (García‐Alvarez et al ., ; Tadokoro et al ., ; Wegener et al ., ).…”

“…The talin‐head domain is required to interact with the cytoplasmic tail of the β integrin subunit and to activate the heterodimeric receptor (García‐Alvarez et al ., ; Tadokoro et al ., ; Wegener et al ., ). However, the talin head domain alone acted as a dominant negative for endogenous talin, and DRG neurite outgrowth on laminin and on aggrecan–laminin substrates was reduced (Tan et al ., ). Based on these results, the talin head domain alone is not suitable to promote integrin signalling.…”

“…Integrin activation is ubiquitous throughout the body, but the exact mechanism of the ‘inside‐out’ activation by kindlin and talin is subject to intense debate (reviewed in Meves et al ., ; Moser et al ., ; Shattil, Kim & Ginsberg, ; Campbell & Humphries, ; Calderwood et al ., ; Eva & Fawcett, ). Despite the limited number of studies investigating the role of these molecules in the nervous system, they have been utilised to enhance integrin‐ligand binding and axonal outgrowth of neurons (Tan et al ., , ; Dingyu et al ., ; Cheah et al ., ) as discussed below.…”

The C. elegans BRCA2-ALP/Enigma Complex Regulates Axon Regeneration via a Rho GTPase-ROCK-MLC Phosphorylation Pathway

“…Growth cone behav-iors are driven by the dynamic reorganization of actin, which in turn is modulated by a number of actin-binding proteins that affect how actin networks are assembled, organized, and remodeled. In particular, actin-binding proteins act as important regulators of axon regeneration by stabilizing filamentous (F) actin (Cho et al, 2015;Tan et al, 2015).…”