Loss of β-PIX inhibits focal adhesion disassembly and promotes keratinocyte motility via myosin light chain activation

Abstract

During healing of the skin, the cytoskeleton of keratinocytes and their matrix adhesions, including focal adhesions (FAs), undergo reorganization. These changes are coordinated by small GTPases and their regulators, including the guanine nucleotide exchange factor β-PIX (also known as ARHGEF7). In fibroblasts, β-PIX activates small GTPases, thereby enhancing migration. In keratinocytes in vitro, β-PIX localizes to FAs. To study β-PIX functions, we generated β-PIX knockdown keratinocytes. During wound closure of β-PIX knockdown cell monolayers, disassembly of FAs is impaired, and their number and size are increased. In addition, in the β-PIX knockdown cells, phosphorylated myosin light chain (MLC; also known as MYL2) is present not only in the leading edge of cells at the wound front, but also in the cells following the front, while p21-activated kinase 2 (PAK2), a regulator of MLC kinase (MYLK), is mislocalized. Inhibition or depletion of MYLK restores FA distribution in β-PIX knockdown cells. Traction forces generated by β-PIX knockdown cells are increased relative to those in control cells, a result consistent with an unexpected enhancement in the migration of single β-PIX knockdown cells and monolayers of such cells. We propose that targeting β-PIX might be a means of promoting epithelialization of wounds in vivo.