Fibronectin fibrils can be seen on wild type cells (green, left panels) but are absent on CD98hc-deficient cells (right panels).

Image courtesy of Prof. Mark H. Ginsberg, University of California, San Diego, La Jolla, USA.

Fibronectin (Fn) is a glycoprotein that exists in both a soluble form in blood plasma and an insoluble form in the extracellular matrix (ECM). It regulates cell migration and adhesion, and besides having a key role in mammalian development, it also regulates wound healing and the tumour microenvironment. Most Fn biological functions depend on the insoluble fibrillar form, making the understanding of soluble to insoluble conversion — a process termed Fn matrix assembly — of great significance. In The Journal of Cell Biology, Mark Ginsberg and colleagues report that the membrane protein CD98hc is required for Fn matrix assembly.

The involvement of CD98hc in matrix deposition in vivo was demonstrated in nude mice that develop teratocarcinomas upon subcutaneous injection of embryonic stem (ES) cells. CD98hc-null cells produced small tumours only and the surrounding Fn fibril network was markedly reduced compared to wild-type ES cells. The authors generated conditional CD98hc-null mouse embryonic fibroblasts (MEFs) to assess matrix deposition in vitro. Whereas plated wild-type cells showed abundant Fn fibrils, CD98hc-null cells were devoid of Fn fibrils. Fn synthesis and secretion were unaffected, thus confirming that CD98hc is required for Fn matrix assembly.

The Fn matrix assembles as a consequence of conformational changes in Fn that allow homophilic interactions. Assembly is initiated when activated integrins bind Fn, the integrins then connect to the actin cytoskeleton, and the force generated from cellular contractility is conveyed to Fn, causing its deformation. The ability of CD98hc to capacitate integrin-dependent traction forces onto the ECM was measured by tracing the displacement of fluorescent beads in flexible Fn-coated polyacrylamide substrates seeded on CD98hc-null MEFs; these exerted traction forces that were reduced by three-fold compared with wild-type cells.

The small GTPase RhoA regulates cell-matrix interaction-induced cell contractility. Ginsberg and colleagues observed that matrix-induced RhoA activation was disrupted in CD98hc-null cells. These results show that CD98hc mediates adhesion-dependent RhoA activation, which in turn promotes traction on the ECM and Fn matrix assembly. But how does CD98hc mediate matrix-driven activation of RhoA? The authors had previously found that CD98hc interacts with integrins to mediate integrin signalling, and here they report that only CD98hc chimeric proteins able to interact with integrins can rescue the defects in contractility and matrix assembly displayed by CD98hc-null MEFs.

Together, Ginsberg and colleagues show that the membrane protein CD98hc is required for Fn matrix assembly; this function relies on its interaction with integrins, which mediate matrix-driven RhoA activation, leading to cellular contractility and traction forces. These findings suggest that the changes in CD98hc expression previously seen during wound healing, cell proliferation and tumorigenesis, as well as during other developmental processes, can regulate the deposition of a fibrillar ECM.

Author: Kim Baumann