Delayed skin wound repair in proline-rich protein tyrosine kinase 2 knockout mice.

Proline-rich protein tyrosine kinase 2 (Pyk2) is a member of the focal adhesion kinase family. We used Pyk2 knockout (Pyk2-KO) mice to study the role of Pyk2 in cutaneous wound repair. We report that the rate of wound closure was delayed in Pyk2-KO compared with control mice. To examine whether impaired wound healing of Pyk2-KO mice was caused by a keratinocyte cell-autonomous defect, the capacities of primary keratinocytes from Pyk2-KO and wild-type (WT) littermates to heal scratch wounds in vitro were compared. The rate of scratch wound repair was decreased in Pyk2-KO keratinocytes compared with WT cells. Moreover, cultured human epidermal keratinocytes overexpressing the dominant-negative mutant of Pyk2 failed to heal scratch wounds. Conversely, stimulation of Pyk2-dependent signaling via WT Pyk2 overexpression induced accelerated scratch wound closure and was associated with increased expression of matrix metalloproteinase (MMP)-1, MMP-9, and MMP-10. The Pyk2-stimulated increase in the rate of scratch wound repair was abolished by coexpression of the dominant-negative mutant of PKCδ and by GM-6001, a broad-spectrum inhibitor of MMP activity. These results suggest that Pyk2 is essential for skin wound reepithelialization in vivo and in vitro and that it regulates epidermal keratinocyte migration via a pathway that requires PKCδ and MMP functions.

Serum response factor controls transcriptional network regulating epidermal function and hair follicle morphogenesis.

Serum response factor (SRF) is a transcription factor that regulates the expression of growth-related immediate-early, cytoskeletal, and muscle-specific genes to control growth, differentiation, and cytoskeletal integrity in different cell types. To investigate the role for SRF in epidermal development and homeostasis, we conditionally knocked out SRF in epidermal keratinocytes. We report that SRF deletion disrupted epidermal barrier function leading to early postnatal lethality. Mice lacking SRF in epidermis displayed morphogenetic defects, including an eye-open-at-birth phenotype and lack of whiskers. SRF-null skin exhibited abnormal morphology, hyperplasia, aberrant expression of differentiation markers and transcriptional regulators, anomalous actin organization, enhanced inflammation, and retarded hair follicle (HF) development. Transcriptional profiling experiments uncovered profound molecular changes in SRF-null E17.5 epidermis and revealed that many previously identified SRF target CArG box-containing genes were markedly upregulated in SRF-null epidermis, indicating that SRF may function to repress transcription of a subset of its target genes in epidermis. Remarkably, when transplanted onto nude mice, engrafted SRF-null skin lacked hair but displayed normal epidermal architecture with proper expression of differentiation markers, suggesting that although keratinocyte SRF is essential for HF development, a cross-talk between SRF-null keratinocytes and the surrounding microenvironment is likely responsible for the barrier-deficient mutant epidermal phenotype.

p38delta Mitogen-activated protein kinase is essential for skin tumor development in mice.

Activating Ras mutations occur in a large portion of human tumors. Yet, the signaling pathways involved in Ras-induced tumor formation remain incompletely understood. The mitogen-activated protein kinase pathways are among the best studied Ras effector pathways. The p38 mitogen-activated protein kinase isoforms are important regulators of key biological processes including cell proliferation, differentiation, survival, inflammation, senescence, and tumorigenesis. However, the specific in vivo contribution of individual p38 isoforms to skin tumor development has not been elucidated. Recent studies have shown that p38delta, a p38 family member, functions as an important regulator of epidermal keratinocyte differentiation and survival. In the present study, we have assessed the effect of p38delta deficiency on skin tumor development in vivo by subjecting p38delta knockout mice to a two-stage 7,12-dimethylbenz(a)anthracene/12-O-tetradecanoylphorbol-13-acetate chemical skin carcinogenesis protocol. We report that mice lacking p38delta gene exhibited a marked resistance to development of 7,12-dimethylbenz(a)anthracene/12-O-tetradecanoylphorbol-13-acetate-induced skin papillomas, with increased latency and greatly reduced incidence, multiplicity, and size of tumors compared with wild-type mice. Our data suggest that the underlying mechanism for reduced susceptibility to skin carcinogenesis in p38delta-null mice involves a defect in proliferative response associated with aberrant signaling through the two major transformation-promoting pathways: extracellular signal-regulated kinase 1/2-activator protein 1 and signal transducer and activator of transcription 3. These findings strongly suggest an in vivo role for p38delta in promoting cell proliferation and tumor development in epidermis and may have therapeutic implication for skin cancer.

Investigation of Frizzled-5 during embryonic neural development in mouse.

Recent studies revealed that the Wnt receptor Frizzled-5 (Fzd5) is required for eye and retina development in zebrafish and Xenopus, however, its role during mammalian eye development is unknown. In the mouse embryo, Fzd5 is prominently expressed in the pituitary, distal optic vesicle, and optic stalk, then later in the progenitor zone of the developing retina. To elucidate the role of Fzd5 during eye development, we analyzed embryos with a germline disruption of the Fzd5 gene at E10.25, just before embryos die due to defects in yolk sac angiogenesis. We observed severe defects in optic cup morphogenesis and lens development. However, in embryos with conditional inactivation of Fzd5 using Six3-Cre, we observed no obvious early eye defects. Analysis of Axin2 mRNA expression and TCF/LEF-responsive reporter activation demonstrate that Fzd5 does not regulate the Wnt/beta-catenin pathway in the eye. Thus, the function of Fzd5 during eye development appears to be species-dependent.

Xath5 regulates neurogenesis in the Xenopus olfactory placode.

Helix-loop-helix (HLH) genes function as important regulators of neurogenesis in both the peripheral and central nervous systems. The olfactory system is an ideal tissue in which to study the role of these genes in regulating the acquisition of neuronal cell fate, particularly that of the olfactory receptor neuron (ORN). Here we describe the expression of several basic HLH (bHLH) and repeat HLH (rHLH) factors during olfactory placode development in Xenopus laevis. Our work reveals that a combination of both bHLH and rHLH genes are sequentially expressed within the nascent olfactory placode during normal development. Moreover, overexpression of the bHLH factor, Xenopus atonal homologue 5 (Xath5), promotes olfactory neural fate independent of cellular proliferation within a restricted domain at the anterior of the embryo. Collectively, our data argue that HLH genes are expressed in a cascade during olfactory placode development and that the activity of an atonal homologue, Xath5, can promote ORN fate but only in the appropriate developmental context.