Publisher Correction: FGF signalling controls the specification of hair placode-derived SOX9 positive progenitors to Merkel cells.

The originally published version of this Article contained an error in Figure 2. In panel e, the blue bar was incorrectly labelled 'KRT8(+)/TOMATO(-)'. Furthermore, during the process of preparing a correction, the publication date of the Article was inadvertently changed to June 20th 2018. Both of these errors have been corrected in the PDF and HTML versions of the Article

FGF signalling controls the specification of hair placode-derived SOX9 positive progenitors to Merkel cells

Merkel cells are innervated mechanosensory cells responsible for light-touch sensations. In murine dorsal skin, Merkel cells are located in touch domes and found in the epidermis around primary hairs. While it has been shown that Merkel cells are skin epithelial cells, the progenitor cell population that gives rise to these cells is unknown. Here, we show that during embryogenesis, SOX9-positive (+) cells inside hair follicles, which were previously known to give rise to hair follicle stem cells (HFSCs) and cells of the hair follicle lineage, can also give rise to Merkel Cells. Interestingly, while SOX9 is critical for HFSC specification, it is dispensable for Merkel cell formation. Conversely, FGFR2 is required for Merkel cell formation but is dispensable for HFSCs. Together, our studies uncover SOX9(+) cells as precursors of Merkel cells and show the requirement for FGFR2-mediated epithelial signalling in Merkel cell specification.Peer reviewe

Revisiting Egyptian Foreign Policy towards Israel under Mubarak: From Cold Peace to Strategic Peace

This article is the first academic study of Egyptian foreign policy towards Israel under Hosni Mubarak (1981–2011). It challenges a deeply entrenched conventional wisdom that Egypt pursued a cold-peace foreign policy towards Israel throughout this period. We demonstrate that Egyptian foreign policy towards Israel was dynamic – comprising cold peace (1981–91), a hybrid foreign policy of cold peace and strategic peace (1991–2003), and a pure strategic peace posture (2003–11). We also use the case of Egyptian foreign policy towards Israel as a heuristic to develop a conception of a new type of peace, strategic peace, as an intermediary analytical category between cold and stable peace

Polycomb-Mediated Repression and Sonic Hedgehog Signaling Interact to Regulate Merkel Cell Specification during Skin Development

An increasing amount of evidence indicates that developmental programs are tightly regulated by the complex interplay between signaling pathways, as well as transcriptional and epigenetic processes. Here, we have uncovered coordination between transcriptional and morphogen cues to specify Merkel cells, poorly understood skin cells that mediate light touch sensations. In murine dorsal skin, Merkel cells are part of touch domes, which are skin structures consisting of specialized keratinocytes, Merkel cells, and afferent neurons, and are located exclusively around primary hair follicles. We show that the developing primary hair follicle functions as a niche required for Merkel cell specification. We find that intraepidermal Sonic hedgehog (Shh) signaling, initiated by the production of Shh ligand in the developing hair follicles, is required for Merkel cell specification. The importance of Shh for Merkel cell formation is further reinforced by the fact that Shh overexpression in embryonic epidermal progenitors leads to ectopic Merkel cells. Interestingly, Shh signaling is common to primary, secondary, and tertiary hair follicles, raising the possibility that there are restrictive mechanisms that regulate Merkel cell specification exclusively around primary hair follicles. Indeed, we find that loss of Polycomb repressive complex 2 (PRC2) in the epidermis results in the formation of ectopic Merkel cells that are associated with all hair types. We show that PRC2 loss expands the field of epidermal cells competent to differentiate into Merkel cells through the upregulation of key Merkel-differentiation genes, which are known PRC2 targets. Importantly, PRC2-mediated repression of the Merkel cell differentiation program requires inductive Shh signaling to form mature Merkel cells. Our study exemplifies how the interplay between epigenetic and morphogen cues regulates the complex patterning and formation of the mammalian skin structures

Publisher Correction: FGF signalling controls the specification of hair placode-derived SOX9 positive progenitors to Merkel cells.

The originally published version of this Article contained an error in Figure 2. In panel e, the blue bar was incorrectly labelled 'KRT8(+)/TOMATO(-)'. Furthermore, during the process of preparing a correction, the publication date of the Article was inadvertently changed to June 20th 2018. Both of these errors have been corrected in the PDF and HTML versions of the Article

Shh signaling activity in the epidermis is required for Merkel cell formation.

<p>(A-C) IF stainings for Merkel cell markers Krt8 (K8) (A,C), Krt20 (K20) (B), Sox2 (B), and Isl1 (C) show a highly significant reduction in the number of Merkel cells in P0 Smoothened epidermis-conditional knockout (Smo cKO) (K14-Cre; Smo^flox/flox) mice when compared to control (ctrl). Quantification of Krt8(+) and Krt20(+) Merkel cells in control and Smo cKO P0 skin (right panel of B) (both p<0.0001). (D) TUNEL staining shows no increase in apoptosis in the skin of P0 Smo cKO mice. Note that cells undergoing cornification are TUNEL(+) as previously reported [<a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1006151#pgen.1006151.ref033" target="_blank">33</a>]. (E) At E15, specification of hair follicles is not affected in P0 Smo cKO mice when compared with control. The number of hair follicles is quantified in the right panel (p = 0.3140). IF staining for Krt14 (K14) labels the epidermis (epi) and Sox2 labels the dermal condensate (dc) of the developing hair follicles in E15 WT and Smo cKO mice. (F) IF staining for Merkel cell markers Krt8 and Sox2 shows a complete absence of Merkel cells in E15 Smo cKO compared to control. The number of Krt8(+) Merkel cells is quantified in the right panel (p = 0.0018). (G) IF stainings for Merkel cell markers Krt8 and <i>in situ</i> hybridization for <i>Gli1</i> RNA in E16 WT skin indicates that <i>Gli1</i> is expressed in about 45% of the Krt14(+) (K14) cells (yellow arrowheads) surrounding the Merkel cells and in about 13% of early Merkel cells present in the skin at this time point, quantified in the right panel (p = 0.0022). (H) <i>In situ</i> hybridization for <i>Gli1</i> RNA in E16 WT back skin indicates that <i>Gli1</i> is not expressed in most Krt14(+) cells of the interfollicular epidermis (IFE). (I,J) IF stainings for Merkel cell markers Krt8 (I,J), Krt20 (I), and Sox2 (J) show that significantly fewer Merkel cells are present in the glabrous paw skin of Smo cKO mice compared to control paws. Quantification of Krt8(+) Merkel cells in paws (right panel of J) (p<0.0001). (K,L) IF stainings for Merkel cell markers Krt8 (K,L), Krt20 (K), and Sox2 (L) show that significantly fewer Merkel cells are present in the glabrous paw skin of Shh cKO mice compared to control paws. Quantification of Krt8(+) Merkel cells in paws (right panel of L) (p<0.0001). Unless otherwise indicated, all epidermis represented is dorsal skin. Scale bars: (A,E): 100μm; (B-D,F-L): 25 μm.</p

Loss of PRC2 results in ectopic formation of Merkel cells around all hair follicle types.

<p>(A) Whole-mount immunofluorescence staining showing Merkel cell-specific marker Krt8 (K8) in P0 Ezh1/2 2KO (K14-Cre; Ezh1^del/del;Ezh2^flox/flox) and EED cKO (K14-Cre; EED^flox/flox) epidermis compared to control (ctrl). Clusters of Merkel cells (≥3 Krt8(+) cells) are quantified, and the number of clusters of Merkel cells per mm² is presented to the right (Kruskal-Wallis test p<0.0001; ctrl vs. Ezh1/2 2KO, *** p<0.0001; ctrl vs. EED cKO, *** p<0.0001). (B-B’) All hair types can have adjacent Merkel cells in EED cKO (B’), while only first wave hair follicles have adjacent Merkel cells in WT (B) epidermis. IF stainings for Sox2 and integrin α8 (α8) are used to label the dermal papillae (dp) or dermal condensate (dc) of different hair follicle types. The dermal papillae of first (left) and second (middle) wave hair follicles are Sox2(+), and the two types of hair follicles can be discriminated by size. The dermal papillae of the third (right) wave hair follicles are Sox2(-)/α8(+), and these hair follicles are very short at P0. IF staining for Sox2 identifies early-specified Merkel Cells (MC) and Krt20 (K20) identifies mature Merkel cells in the epidermis, which is labeled with E-Cadherin (ECad). (C-D”) Early immature Krt8(+) Merkel cells are found around the WT first wave (C) of developing hair follicles, but not the second wave (D) (control: 0% of placodes, 0.81% of hair germs, 19.65% of hair pegs, and 63.89% of bulbous pegs harbor Merkel cells around them; 148 hair follicles analyzed). (C’,C”,D’,D”) Krt8(+) Merkel cells are found around second wave and first wave developing hair follicles in Ezh1/2 2KO (C’,D’) and EED cKO (C”,D”) E16 embryos (Ezh1/2 2KO: 11.26% of placodes, 23.64% of hair germs, 58.33% of hair pegs, and 100% of bulbous pegs harbor Merkel cells around them; 160 hair follicles analyzed. EED cKO: 12.04% of placodes, 25.76% of hair germs, 70.37% of hair pegs, and 100% of bulbous pegs harbor Merkel cells around them; 213 hair follicles analyzed). Krt14 (K14) labels the developing hair follicles, and integrin α8 labels the dermal condensate in the first stages of hair follicle development; this becomes the dermal papilla in the bulbous peg stage. At E16, the first wave hair follicles are in the hair peg or bulbous peg stages (C-C”), while the second wave hair follicles are in the placode and hair germ stages (D-D”). Scale bars (A): 200 μm; (B-D”) 25 μm.</p

Shh overexpression results in increased formation of cells expressing Merkel markers.

<p>(A) Schematic diagram showing transgenic mouse and lentiviral constructs for the Shh overexpression experiment. CD1 female mice were mated with male Rosa26-rtTA mice and <i>in utero</i> lentiviral injections were performed in pregnant females E9. Doxycycline treatment was initiated by gavage at E12, and females were continuously fed Doxycycline until embryo collection at E17. In the presence of Doxycycline, lentivirus-injected mouse embryos express both H2B-RFP and Shh proteins. (B-E) IF stainings showing the altered morphology of epidermis infected with Shh+H2B-RFP (C,C’) and the significant increase in the numbers of Krt8(+) (K8) cells in the infected epidermis (C,C’,E) compared to control, un-infected epidermis (E17 control) (B,B’,D). Quantification of Krt8(+) cells per mm of back skin in E17 control and Shh overexpression (right panel) (p<0.0001). (F,F’) IF staining for Phospho-Histone H3 (PH3) showing a significant increase in proliferation in E17 Shh o/exp epidermis (F’) compared to control (F). Quantification of number of PH3(+) per mm² of Krt14(+) epidermal cells is represented on the right (p<0.0001). (G,G’) TUNEL staining showing no increase in apoptosis in E17 Shh o/exp epidermis (G’) compared to control (G). Note that cells undergoing cornification are TUNEL(+), as previously reported [<a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1006151#pgen.1006151.ref033" target="_blank">33</a>]. (H-I’) IF staining showing an increase in the number of Sox2(+)/Krt8(+) cells in E17 Shh o/exp epidermis (I,I’) compared to control (H,H’). (J-K’) IF stainings showing an increase in the number of Isl1(+)/Krt8(+) cells in E17 Shh o/exp epidermis (K,K’) compared to control (J,J’). Scale bars: (B-C’): 100μm; (D-K’): 25 μm.</p