Neutrophils promote CXCR3-dependent itch in the development of atopic dermatitis.

Chronic itch remains a highly prevalent disorder with limited treatment options. Most chronic itch diseases are thought to be driven by both the nervous and immune systems, but the fundamental molecular and cellular interactions that trigger the development of itch and the acute-to-chronic itch transition remain unknown. Here, we show that skin-infiltrating neutrophils are key initiators of itch in atopic dermatitis, the most prevalent chronic itch disorder. Neutrophil depletion significantly attenuated itch-evoked scratching in a mouse model of atopic dermatitis. Neutrophils were also required for several key hallmarks of chronic itch, including skin hyperinnervation, enhanced expression of itch signaling molecules, and upregulation of inflammatory cytokines, activity-induced genes, and markers of neuropathic itch. Finally, we demonstrate that neutrophils are required for induction of CXCL10, a ligand of the CXCR3 receptor that promotes itch via activation of sensory neurons, and we find that that CXCR3 antagonism attenuates chronic itch.

Organization of Embryonic Morphogenesis via Mechanical Information.

Embryonic organizers establish gradients of diffusible signaling molecules to pattern the surrounding cells. Here, we elucidate an additional mechanism of embryonic organizers that is a secondary consequence of morphogen signaling. Using pharmacological and localized transgenic perturbations, 4D imaging of the zebrafish embryo, systematic analysis of cell motion, and computational modeling, we find that the vertebrate tail organizer orchestrates morphogenesis over distances beyond the range of morphogen signaling. The organizer regulates the rate and coherence of cell motion in the elongating embryo using mechanical information that is transmitted via relay between neighboring cells. This mechanism is similar to a pressure front in granular media and other jammed systems, but in the embryo the mechanical information emerges from self-propelled cell movement and not force transfer between cells. The propagation likely relies upon local biochemical signaling that affects cell contractility, cell adhesion, and/or cell polarity but is independent of transcription and translation.

Interactions between keratinocytes and somatosensory neurons in itch.

Keratinocytes are epithelial cells that make up the stratified epidermis of the skin. Recent studies suggest that keratinocytes promote chronic itch. Changes in skin morphology that accompany a variety of chronic itch disorders and the multitude of inflammatory mediators secreted by keratinocytes that target both sensory neurons and immune cells highlight the importance of investigating the connection between keratinocytes and chronic itch. This chapter addresses some of the most recent data and models for the role keratinocytes play in the development and maintenance of chronic itch.

Modeling the zebrafish segmentation clock's gene regulatory network constrained by expression data suggests evolutionary transitions between oscillating and nonoscillating transcription.

During segmentation of vertebrate embryos, somites form in accordance with a periodic pattern established by the segmentation clock. In the zebrafish (Danio rerio), the segmentation clock includes six hairy/enhancer of split-related (her/hes) genes, five of which oscillate due to negative autofeedback. The nonoscillating gene hes6 forms the hub of a network of 10 Her/Hes protein dimers, which includes 7 DNA-binding dimers and 4 weak or non-DNA-binding dimers. The balance of dimer species is critical for segmentation clock function, and loss-of-function studies suggest that the her genes have both unique and redundant functions within the clock. However, the precise regulatory interactions underlying the negative feedback loop are unknown. Here, we combine quantitative experimental data, in silico modeling, and a global optimization algorithm to identify a gene regulatory network (GRN) designed to fit measured transcriptional responses to gene knockdown. Surprisingly, we find that hes6, the clock gene that does not oscillate, responds to negative feedback. Consistent with prior in silico analyses, we find that variation in transcription, translation, and degradation rates can mediate the gain and loss of oscillatory behavior for genes regulated by negative feedback. Extending our study, we found that transcription of the nonoscillating Fgf pathway gene sef responds to her/hes perturbation similarly to oscillating her genes. These observations suggest a more extensive underlying regulatory similarity between the zebrafish segmentation clock and the mouse and chick segmentation clocks, which exhibit oscillations of her/hes genes as well as numerous other Notch, Fgf, and Wnt pathway genes.

Segmental assembly of fibronectin matrix requires rap1b and integrin α5.

BACKGROUND: During segmentation of the zebrafish embryo, inside-out signaling activates Integrin α5, which is necessary for somite border morphogenesis. The direct activator of Integrin α5 during this process is unknown. One candidate is Rap1b, a small monomeric GTPase implicated in Integrin activation in the immune system. RESULTS: Knockdown of rap1b, or overexpression of a dominant negative rap1b, causes a mild axis elongation defect in zebrafish. However, disruption of rap1b function in integrin α5(-/-) mutants results in a strong reduction in Fibronectin (FN) matrix assembly in the paraxial mesoderm and a failure in somite border morphogenesis along the entire anterior-posterior axis. Somite patterning appears unaffected, as her1 oscillations are maintained in single and double morphants/mutants, but somite polarity is gradually lost in itgα5(-/-) ; rap1b MO embryos. CONCLUSIONS: In itgα5(-) (/) (-) mutants, rap1b is required for proper somite border morphogenesis in zebrafish. The loss of somite borders is not a result of aberrant segmental patterning. Rather, somite boundary formation initiates but is not completed, due to the failure to assemble FN matrix along the nascent boundary. We propose a model in which Rap1b activates Integrin/Fibronectin receptors as part of an "inside-out" signaling pathway that promotes Integrin binding to FN, FN matrix assembly, and subsequent stabilization of morphological somite boundaries.

The Her7 node modulates the network topology of the zebrafish segmentation clock via sequestration of the Hes6 hub.

Using in vitro and in vivo assays, we define a network of Her/Hes dimers underlying transcriptional negative feedback within the zebrafish segmentation clock. Some of the dimers do not appear to be DNA-binding, whereas those dimers that do interact with DNA have distinct preferences for cis regulatory sequences. Dimerization is specific, with Hes6 serving as the hub of the network. Her1 binds DNA only as a homodimer but will also dimerize with Hes6. Her12 and Her15 bind DNA both as homodimers and as heterodimers with Hes6. Her7 dimerizes strongly with Hes6 and weakly with Her15. This network structure engenders specific network dynamics and imparts greater influence to the Her7 node. Computational analysis supports the hypothesis that Her7 disproportionately influences the availability of Hes6 to heterodimerize with other Her proteins. Genetic experiments suggest that this regulation is important for operation of the network. Her7 therefore has two functions within the zebrafish segmentation clock. Her7 acts directly within the delayed negative feedback as a DNA-binding heterodimer with Hes6. Her7 also has an emergent function, independent of DNA binding, in which it modulates network topology via sequestration of the network hub.