Coimmunostaining for ACF7 (green), microtubules (red), F-Actin (blue) reveals that in wild type cells, ACF7 localizes to the tips of microtubules that are in close contact with F-actin cables.

Image courtesy of Dr Elaine Fuchs, Rockefeller University, NY, USA.

The interaction between microtubules and filamentous (F)-actin regulates cellular functions as diverse as intracellular transport, cell shape and cell migration. Reporting in Cell, Elaine Fuchs and colleagues now show that the spectraplakin ACF7 guides microtubule growth along F-actin towards focal adhesions (FAs) and that microtubule targeting to FAs is required for FA dynamics and cell migration.

Spectraplakins are a broadly expressed class of proteins able to directly bind to and crosslink microtubule and F-actin networks. As loss of ACF7 is lethal, the authors created conditional knockout mice that lack ACF7 only in the skin epidermis. Although no major defects in skin or coat hair were observed, mutant mice had delayed wound healing response. Slow wound repair was attributed to impaired epidermal cell migration, which was due to a defect in extracellular matrix adhesion. In contrast to wild-type cells — which undergo continual cycles of FA formation, maturation and disassembly — mutant keratinocytes had impaired FA dynamics, including a reduction in assembly and disassembly rates, and enlarged FAs at the cell periphery.

Signalling pathways that involve Rho GTPases and focal adhesion kinase (FAK) are known to control FA turnover, but were unaffected following ACF7 knockdown. So how does ACF7 affect FA dynamics? Given the increasing evidence that microtubule targeting is involved in FA turnover, the authors investigated whether impaired cytoskeletal dynamics might underlie the observed FA defects. Treatment of keratinocytes with microtubule-depolymerizing drugs reduced FA dynamics in a manner similar to that seen in ACF7 knockdown. Fluorescence microscopy showed aberrant microtubule organization in the absence of ACF7; microtubules failed to co-align with actin cables and plus-ends no longer converged at peripheral FAs. The authors found that ACF7 bound to microtubule-plus-end proteins including EB1 and CLASPs. Although these proteins still localized at the plus-end tip in the absence of ACF7, their localization was random with respect to F-actin and FAs. Together, these data suggest that ACF7 mediates the interaction between microtubule plus-ends and actin-FA networks.

Interestingly, the capacity of ACF7 to crosslink microtubules and F-actin was not sufficient to rescue ACF7 knockdown. Truncated proteins able to bind to both cytoskeletal networks failed to rescue both the coordinated movement between microtubules and actin cables, and actin-guided tracking to peripheral FAs. The authors showed that cell movement and cytoskeletal coordination also required a conserved motif that had actin-enhanced ATPase activity. Thus, ACF7 crosslinking function, together with its ATPase activity mediate the growth of microtubules along actin cables. However, further studies will be required to understand how ACF7's ATPase activity might coordinate microtubule and actin dynamics.

Author: Kim Baumann