Credit: Neil Smith

Epidermal homeostasis is maintained by stem cell renewal and differentiation and is known to involve signals from the extracellular matrix (ECM). Watt and colleagues now show that these signals are transduced by a cascade involving actin and the transcription factor serum response factor (SRF).

actin is the primary sensor for epidermal stem cell differentiation.

To examine signals from the ECM that are transduced to epidermal stem cells, the authors developed micropatterned surfaces composed of type I collagen on which they seeded single cells. They observed more differentiated cells (measured by expression of the differentiation marker involucrin) on small surfaces (20 μm diameter) than large surfaces (50 μm diameter), indicating that a reduced adhesive area promotes epidermal stem cell differentiation. This was not related to the ECM present on the surface but was affected by cell shape, as cells with a large adhesive area (spread cells) had lower expression of involucrin than cells with a small adhesive area.

Cell shape is regulated by the actin cytoskeleton, so the authors assessed differences in actin between cells growing on small and large surfaces. Cells on 20 μm surfaces had higher levels of filamentous actin (F-actin) and lower levels of globular actin (G-actin) than cells on 50 μm surfaces. Interestingly, inhibiting actin polymerization in cells on 20 μm surfaces blocked differentiation, and treatment of cells on 50 μm surfaces with an F-actin-stabilizing agent promoted differentiation; these findings reveal an integral role for actin in epidermal stem cell differentiation.

So how does actin influence differentiation? G-actin is known to bind megakaryocytic acute leukaemia (MAL; also known as MKL1) in the cytoplasm and to prevent it from binding SRF, thereby inhibiting transcription. Consistent with this, MAL localized to the perinuclear cell body in cells on 20 μm surfaces (which show enhanced differentiation and high levels of F-actin) and to lamellipodia in cells on 50 μm surfaces (which have more G-actin). Furthermore, depletion of MAL or SRF using small interfering RNAs (siRNAs) significantly decreased differentiation of cells on 20 μm surfaces, and overexpression of MAL increased SRF activity and involucrin expression. These findings suggest that, in the absence of G-actin, MAL-mediated SRF activation promotes epidermal stem cell differentiation. This could be mediated by effects on FOS and JUNB, two SRF target genes that have been implicated in this process. Indeed, depletion of FOS or JUNB using siRNAs significantly inhibited involucrin expression by cells on 20 μm surfaces. Although both are SRF target genes and both control differentiation, the authors found that JUNB expression is regulated by G-actin whereas FOS expression is stimulated by serum.

Together, these findings show that actin is the primary sensor for epidermal stem cell differentiation. G-actin (which is predominant in spread cells) binds MAL and inhibits transcription by preventing it from binding to SRF, whereas when spreading is restricted G-actin levels decrease and inhibition is relieved, promoting differentiation.