During animal development, cells have to respond appropriately to localized secreted signals. Proper responses to Hedgehog, transforming growth factor-beta, epidermal growth factor and fibroblast growth factor/Ras signals require cognate inducible antagonists such as Patched, Dad, Argos and Sprouty. Wnt signals are crucial in development and neoplasia. Here we show that naked cuticle (nkd), a Drosophila segment-polarity gene, encodes an inducible antagonist for the Wnt signal Wingless (Wg). In fly embryos and imaginal discs nkd transcription is induced by Wg. In embryos, decreased nkd function has an effect similar to excess Wg; at later stages such a decrease appears to have no effect. Conversely, overproduction of Nkd in Drosophila and misexpression of Nkd in the vertebrate Xenopus laevis result in phenotypes resembling those of loss of Wg/Wnt function. nkd encodes a protein with a single EF hand (a calcium-binding motif) that is most similar to the recoverin family of myristoyl switch proteins. Nkd may therefore link ion fluxes to the regulation of the potency, duration or distribution of Wnt signals. Signal-inducible feedback antagonists such as nkd may limit the effects of Wnt proteins in development and disease.

In Drosophila embryos the protein Naked cuticle (Nkd) limits the effects of the Wnt signal Wingless (Wg) during early segmentation. nkd loss of function results in segment polarity defects and embryonic death, but how nkd affects Wnt signaling is unknown. Using ectopic expression, we find that Nkd affects, in a cell-autonomous manner, a transduction step between the Wnt signaling components Dishevelled (Dsh) and Zeste-white 3 kinase (Zw3 Secreted Wnt proteins act as potent mitogens and cellfate regulators in organisms ranging from nematodes to humans. In vertebrates they specify cell fate and control growth in a variety of developmental processes, including brain development, limb formation, axis specification, and gastrulation (for review, see Cadigan and Nusse 1997). In the fruit fly Drosophila, the Wnt protein Wingless (Wg) establishes segment polarity during embryogenesis and is involved in multiple additional patterning events throughout later development (Cadigan and Nusse 1997). wg is first expressed in the developing epidermis in stripes just anterior to cells expressing the engrailed (en) gene and is necessary to maintain en transcription (DiNardo et al. 1988;Martinez Arias et al. 1988). hedgehog (hh) is expressed in the en-expressing cells and positively regulates wg expression in the anterior cells (Ingham et al. 1991;Lee et al. 1992). This positive-feedback loop establishes parasegmental boundaries, the first evidence of the metameric organization of the embryo, between wg-and en/hh-expressing cells. At later stages of embryonic development, a tight balance between Wg and other signaling pathways, such as the Drosophila epidermal growth factor receptor (EGFR), determines whether epidermal cells secrete either naked (smooth) cuticle or hair-like structures called denticles (Dougan and DiNardo 1992;O'Keefe et al. 1997;Szuts et al. 1997). In the absence of wg function, embryos are covered with a lawn of denticles, whereas otherwise wild-type embryos exposed to excess Wg produce a naked cuticle (Martinez Arias et al. 1988;Noordermeer et al. 1992).Genetic and biochemical studies have lead to the identification of the key components of the Wnt/Wg pathway and have uncovered some of the molecular events that are involved in signal transduction (Fig. 1A). Wg binds 7-pass transmembrane receptors of the frizzled family (Fz or Dfz2), which, in turn, activate the cytoplasmic protein Dishevelled (Dsh; Theisen et al. 1994;Bhanot et al. 1996). Dsh antagonizes the activity of a large protein complex that, in the absence of Wg signal, results in Armadillo (Arm)/␤-catenin phosphorylation and subsequent degradation by the ubiquitin-proteasome pathway (Yost et al. 1996;Aberle et al. 1997;Pai et al. 1997). This multiprotein complex includes Zw3/Glycogen synthase kinase 3␤ (Gsk3␤), Adenomatous Polyposis Coli (APC), Axin, and Arm/␤-catenin. Axin constitutes the core of this complex, al-

A wound is a type of injury that damages living tissues. In this review, we will be referring mainly to healing responses in the organs including skin and the lungs. Fibrosis is a process of dysregulated extracellular matrix (ECM) production that leads to a dense and functionally abnormal connective tissue compartment (dermis). In tissues such as the skin, the repair of the dermis after wounding requires not only the fibroblasts that produce the ECM molecules, but also the overlying epithelial layer (keratinocytes), the endothelial cells, and smooth muscle cells of the blood vessel and white blood cells such as neutrophils and macrophages, which together orchestrate the cytokine-mediated signaling and paracrine interactions that are required to regulate the proper extent and timing of the repair process. This review will focus on the importance of extracellular molecules in the microenvironment, primarily the proteoglycans and glycosaminoglycan hyaluronan, and their roles in wound healing. First, we will briefly summarize the physiological, cellular, and biochemical elements of wound healing, including the importance of cytokine cross-talk between cell types. Second, we will discuss the role of proteoglycans and hyaluronan in regulating these processes. Finally, approaches that utilize these concepts as potential therapies for fibrosis are discussed.

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In contrast to adult cutaneous wound repair, early gestational fetal cutaneous wounds heal by a process of regeneration, resulting in little or no scarring. Previous studies indicate that down-regulation of HoxB13, a member of the highly conserved family of Hox transcription factors, occurs during fetal scarless wound healing. No down-regulation was noted in adult wounds. Here, we evaluate healing of adult cutaneous wounds in Hoxb13 knockout (KO) mice, hypothesizing that loss of Hoxb13 in adult skin should result in enhanced wound healing. Tensiometry was used to measure the tensile strength of incisional wounds over a 60-day time course; overall, Hoxb13 KO wounds are significantly stronger than wild-type (WT). Histological evaluation of incisional wounds shows that 7-day-old Hoxb13 KO wounds are significantly smaller and that 60-day-old Hoxb13 KO wounds exhibit a more normal collagen architecture compared with WT wounds. We also find that excisional wounds close at a faster rate in Hoxb13 KO mice. Biochemical and histochemical analyses show that Hoxb13 KO skin contains significantly elevated levels of hyaluronan. Because higher levels of hyaluronan and enhanced wound healing are characteristics of fetal skin, we conclude that loss of Hoxb13 produces a more "fetal-like" state in adult skin.

Wnt/b-catenin signals orchestrate cell fate and behavior throughout the animal kingdom. Aberrant Wnt signaling impacts nearly the entire spectrum of human disease, including birth defects, cancer, and osteoporosis. If Wnt signaling is to be effectively manipulated for therapeutic advantage, we first must understand how Wnt signals are normally controlled. Naked cuticle (Nkd) is a novel and evolutionarily conserved inducible antagonist of Wnt/b-catenin signaling that is crucial for segmentation in the model genetic organism, the fruit fly Drosophila melanogaster. Nkd can bind and inhibit the Wnt signal transducer Dishevelled (Dsh), but the mechanism by which Nkd limits Wnt signaling in the fly embryo is not understood. Here we show that nkd mutants exhibit elevated levels of the b-catenin homolog Armadillo but no alteration in Dsh abundance or distribution. In the fly embryo, Nkd and Dsh are predominantly cytoplasmic, although a recent report suggests that vertebrate Dsh requires nuclear localization for activity in gain-offunction assays. While Dsh-binding regions of Nkd contribute to its activity, we identify a conserved 30-amino-acid motif, separable from Dsh-binding regions, that is essential for Nkd function and nuclear localization. Replacement of the 30-aa motif with a conventional nuclear localization sequence rescued a small fraction of nkd mutant animals to adulthood. Our studies suggest that Nkd targets Dsh-dependent signal transduction steps in both cytoplasmic and nuclear compartments of cells receiving the Wnt signal.

Hyaluronan (HA) is an abundant matrix molecule whose functions in the skin remain to be fully defined. To explore the roles of HA in cutaneous injury responses, double-knockout mice (abbreviated as Has1/3 null) that lack two HA synthase enzymes (Has1 and Has3) but still express functional Has2, were used in two types of experiments: (i) application of 12-O-tetradecanoylphorbol-13-acetate (TPA), and (ii) full-thickness wounding of the skin. Uninjured Has1/3 null mice were phenotypically normal. However, after TPA, the accumulation of HA that normally occurs in wildtype epidermis was blunted in Has1/3 null epidermis. In excisional wound healing experiments, wound closure was significantly faster in Has1/3 null than in wildtype mice. Coincident with this abnormal wound healing, a marked decrease in epidermal and dermal HA and a marked increase in neutrophil efflux from cutaneous blood vessels were observed in Has1/3 null skin relative to wildtype skin. Has1/3 null wounds displayed an earlier onset of myofibroblast differentiation. In summary, selective loss of Has1 and Has3 leads to a pro-inflammatory milieu that favors recruitment of neutrophils and other inflammation-related changes in the dermis.

Background: Hyaluronan (HA), an extracellular glycosaminoglycan, is normally produced by three HA synthase (Has) enzymes. Results: Skin fibroblasts from Has1/Has3 double knock-out mice have higher Has2 expression and HA levels and are resistant to cell death after UVB exposure or serum starvation. Conclusion: HA modulates injury-induced apoptotic responses in fibroblasts. Significance: HA has an important role in cell death responses.