The establishment of polarity is an essential process in early neuronal development. Although a number of molecules controlling neuronal polarity have been identified, genetic evidence about their physiological roles in this process is mostly lacking. We analyzed the consequences of loss of Cdc42, a central regulator of polarity in multiple systems, on the polarization of mammalian neurons. Genetic ablation of Cdc42 in the brain led to multiple abnormalities, including striking defects in the formation of axonal tracts. Neurons from the Cdc42 null animals sprouted neurites but had a strongly suppressed ability to form axons bothin vivoand in culture. This was accompanied by disrupted cytoskeletal organization, enlargement of the growth cones, and inhibition of filopodial dynamics. Axon formation in the knock-out neurons was rescued by manipulation of the actin cytoskeleton, indicating that the effects of Cdc42 ablation are exerted through modulation of actin dynamics. In addition, the knock-outs showed a specific increase in the phosphorylation (inactivation) of the Cdc42 effector cofilin. Furthermore, the active, nonphosphorylated form of cofilin was enriched in the axonal growth cones of wild-type, but not of mutant, neurons. Importantly, cofilin knockdown resulted in polarity defects quantitatively analogous to the ones seen after Cdc42 ablation. We conclude that Cdc42 is a key regulator of axon specification, and that cofilin is a physiological downstream effector of Cdc42 in this process.

Squamous cell carcinoma (SCC) of the skin is a common complication of immunosuppressive treatment with calcineurin inhibitors in graft-recipient patients. Here it is shown that the intact calcineurin/NFAT signalling pathway is important for suppressing SCC development, with a key role for increased expression of the ATF3 transcription factor in tumorigenesis. Calcineurin inhibitors are the mainstay of immunosuppressive treatment for organ transplant recipients. However, treatment with these drugs commonly leads to squamous cell carcinoma (SCC) of the skin. It is shown here that an intact calcineurin/NFAT signalling pathway is important for suppressing SCC development. Inhibition of this pathway leads to increased expression of the ATF3 protein, which has a key role in tumorigenesis. Calcineurin inhibitors such as cyclosporin A (CsA) are the mainstay of immunosuppressive treatment for organ transplant recipients. Squamous cell carcinoma (SCC) of the skin is a major complication of treatment with these drugs, with a 65 to 100-fold higher risk than in the normal population1. By contrast, the incidence of basal cell carcinoma (BCC), the other major keratinocyte-derived tumour of the skin, of melanoma and of internal malignancies increases to a significantly lesser extent1. Here we report that genetic and pharmacological suppression of calcineurin/nuclear factor of activated T cells (NFAT) function promotes tumour formation in mouse skin and in xenografts, in immune compromised mice, of H-rasV12 (also known as Hras1)-expressing primary human keratinocytes or keratinocyte-derived SCC cells. Calcineurin/NFAT inhibition counteracts p53 (also known as TRP53)-dependent cancer cell senescence, thereby increasing tumorigenic potential. ATF3, a member of the ‘enlarged’ AP-1 family, is selectively induced by calcineurin/NFAT inhibition, both under experimental conditions and in clinically occurring tumours, and increased ATF3 expression accounts for suppression of p53-dependent senescence and enhanced tumorigenic potential. Thus, intact calcineurin/NFAT signalling is critically required for p53 and senescence-associated mechanisms that protect against skin squamous cancer development.

During peripheral nervous system (PNS) myelination, Schwann cells must interpret extracellular cues to sense their environment and regulate their intrinsic developmental program accordingly. The pathways and mechanisms involved in this process are only partially understood. We use tissue-specific conditional gene targeting to show that members of the Rho GTPases, cdc42 and rac1, have different and essential roles in axon sorting by Schwann cells. Our results indicate that although cdc42 is required for normal Schwann cell proliferation, rac1 regulates Schwann cell process extension and stabilization, allowing efficient radial sorting of axon bundles.

Differentiation of skin stem cells into hair follicles (HFs) requires the inhibition of β-catenin degradation, which is controlled by a complex containing axin and the protein kinase GSK3β. Using conditional gene targeting in mice, we show now that the small GTPase Cdc42 is crucial for differentiation of skin progenitor cells into HF lineage and that it regulates the turnover of β-catenin. In the absence of Cdc42, degradation of β-catenin was increased corresponding to a decreased phosphorylation of GSK3β at Ser 9 and an increased phosphorylation of axin, which is known to be required for binding of β-catenin to the degradation machinery. Cdc42-mediated regulation of β-catenin turnover was completely dependent on PKCζ, which associated with Cdc42, Par6, and Par3. These data suggest that Cdc42 regulation of β-catenin turnover is important for terminal differentiation of HF progenitor cells in vivo.