Vinculin mediated axon growth requires interaction with actin but not talin

Why This is Cool – Axon growth, migration, and guidance are necessary developmental processes to form our neural circuits and the misregulation of these processes is linked to multiple neural disorders. This study looks at a missing piece of the basic cell biology behind this process using genetic structure/function mutants, in vitro cultures, and embryonic/neonatal mouse models. Their first result starts with a traditional knockout approach where they demonstrate that vinculin is necessary for axon growth and migration. Vinculin is another adaptor protein functioning in both cell-cell and cell-extracellular matrix (ECM) adhesion, very similarly and cooperatively with my love, talin. The interaction between vinculin and talin has been investigated recently and shown to be specifically important for mechanosensing (1). The authors then go several steps further by investigating the impact of numerous functional mutants on axon growth and migration. These are summarized in the table below. Subjectively, the most interesting part of these results is that disrupting the talin binding site in vinculin (A50I mutant) had no effect on these processes. This seems contrary to a lot of studies that demonstrate how these proteins work together and contrary to a previously established role of talin in axon regeneration (2). Another interesting part of this study is the results using the vinculin autoinhibition mutant (Vcl-T12). Vinculin, like talin, can exist in a closed conformation, where the head binds the tail, masking important binding sites and hyperactivation caused by disrupting this interaction is detrimental to morphogenesis (3). Here they show that hyperactive vinculin increases axon growth and migration, providing an opposite result to many of their other mutants. Lastly, the authors explore the tail only mutants in more detail, showing that they may increase actin binding too far, as an actin binding mutant can rescue many of the effects.

Mutant	Potential Function	Axon Growth	Migration	Branching
Vcl-FL	Full Length	Same	Same	N/A
Vcl-258	Lacks Tail	Decreased	Same	N/A
Vcl-851	Lacks Tail	Decreased	Same	N/A
Vcl-T	Tail Only	Decreased*	Decreased	Increased
Vcl-T12	Autoinhibition Loss	Increased*	Increased/Same	N/A
Vcl-LD	PIP2 Binding Loss	Decreased	Same	N/A
Vcl-FL (A50I)	Talin Binding Loss	Same	Same	N/A
Vcl-FL (I997A)	Actin Binding Loss	Decreased	Same/Rescue	Same/Rescue

Table 1: Summary of results for each point mutation studied. * confirmed by in vivo model

 

Why I Selected It – I came for the talin mention in the title, but stayed for the structure/function analysis and super resolution microscopy. I have never worked on cultured or in vivo neurons, but it’s a consistently fascinating field for actin regulation and cell migration. I also personally think these kinds of structure/function mutant survey papers describing proteins with a known multitude of functions do a lot of heavy lifting in their respective fields. The authors provide a great resource for others not only in the results they described, but in the detailed descriptions of mutant constructs as well.

Open Questions –

1. How do you reconcile your talin binding negative results with the previous results that indicate a role (2)?

Related References –

1. Vinculin & Talin
   • Yao M., et al. Mechanical activation of vinculin binding to talin locks talin in an unfolded confirmation. Sci Reports. 4:4610 (2015).
2. Talin in Axon Regeneration
   • Tan C.L., et al. Full length talin stimulates integrin activation and axon regeneration. Mol Cell Neurosci. 68:1-8. (2015)
3. Vinculin Autoinhibition
   • Bakolitsa C., et al. Structural basis for vinculin activation at sites of cell adhesion. Nature. 430(6999):583-586. (2004).
   • Maartens A.P., et al. Drosophila vinculin is more harmful when hyperactive than absent, and can circumvent integrin to form adhesion complexes. J Cell Sci. 129:4354-4365. (2016).

 

doi: https://doi.org/10.1242/prelights.23722

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