Hdac1 and Hdac2 regulate the quiescent state and survival of hair-follicle mesenchymal niche

While cell division is essential for self-renewal and differentiation of stem cells and progenitors, dormancy is required to maintain the structure and function of the stem-cell niche. Here we use the hair follicle to show that during growth, the mesenchymal niche of the hair follicle, the dermal papilla (DP), is maintained quiescent by the activity of Hdac1 and Hdac2 in the DP that suppresses the expression of cell-cycle genes. Furthermore, Hdac1 and Hdac2 in the DP promote the survival of DP cells throughout the hair cycle. While during growth and regression this includes downregulation of p53 activity and the control of p53-independent programs, during quiescence, this predominantly involves p53-independent mechanisms. Remarkably, Hdac1 and Hdac2 in the DP during the growth phase also participate in orchestrating the hair cycle clock by maintaining physiological levels of Wnt signaling in the vicinity of the DP. Our findings not only provide insight into the molecular mechanism that sustains the function of the stem-cell niche in a persistently changing microenvironment, but also unveil that the same mechanism provides a molecular toolbox allowing the DP to affect and fine tune the microenvironment.

Hair-follicle mesenchymal stem-cell activity during homeostasis and wound healing

The mesenchymal components of the hair follicle, the dermal papilla (DP) and dermal sheath (DS), are maintained by hair-follicle dermal stem cells (hfDSCs), but the position of this stem cell population throughout the hair cycle, its contribution to the maintenance of the dermis and the existence of a migratory axis from the DP to the dermis remain unclear. Here we show that during homeostasis DP and DS cells are confined to their compartments, and during the regression phase of the hair cycle, some undergo apoptosis and subsequently are internalized by nearby adipocytes. In contrast, during wound healing, DP/DS cells move towards the wound, but do not directly participate in follicle neogenesis. Furthermore, hfDSCs, driving the cyclic renewal of the DS during the hair cycle, are heterogeneous and housed during the growth phase within the most proximal part of the DS. Our analysis provides insight into the mechanisms of tissue maintenance and unravels a previously-unknown potential function of adipocytes in phagocytosis.

Fgf and Wnt signaling interaction in the mesenchymal niche regulates the murine hair cycle clock

Tissue growth in the adult is an orchestrated process that often requires biological clocks to time stem cell and progenitor activity. Here, we employed the hair follicle, which cycles between growth and regression in a timely-restricted mode, to show that some components of the hair cycle clock reside within the mesenchymal niche of the hair follicle, the dermal papilla (DP), and both Fgf and Wnt signaling pathways interact within the DP to regulate the expression of these components that include Wnt agonists (Rspondins) and antagonists (Dkk2 and Notum). The levels of Wnt agonists and antagonists in the DP are progressively reduced and elevated during the growth phase, respectively. Consequently, Wnt signaling activity in the overlying epithelial progenitor cells decreases, resulting in the induction of the regression phase. Remarkably, DP properties allow Wnt activity in the DP to persist despite the Wnt-inhibiting milieu and consequently synchronize the induction and progression of the regression phase. This study provides insight into the importance of signaling crosstalk in coupling progenitors and their niche to regulate tissue growth.

The underlying mechanisms regulating the mouse hair cycle remain poorly understood. Here, the authors find that Fgf and Wnt signaling pathways interact in the mesenchymal niche of the hair follicle to regulate the molecular clock that dictates the duration of hair growth.

Insights on the trafficking and retro-translocation of glycosphingolipid-binding bacterial toxins

Some bacterial toxins and viruses have evolved the capacity to bind mammalian glycosphingolipids to gain access to the cell interior, where they can co-opt the endogenous mechanisms of cellular trafficking and protein translocation machinery to cause toxicity. Cholera toxin (CT) is one of the best-studied examples, and is the virulence factor responsible for massive secretory diarrhea seen in cholera. CT enters host cells by binding to monosialotetrahexosylganglioside (GM1 gangliosides) at the plasma membrane where it is transported retrograde through the trans-Golgi network (TGN) into the endoplasmic reticulum (ER). In the ER, a portion of CT, the CT-A1 polypeptide, is unfolded and then “retro-translocated” to the cytosol by hijacking components of the ER associated degradation pathway (ERAD) for misfolded proteins. CT-A1 rapidly refolds in the cytosol, thus avoiding degradation by the proteasome and inducing toxicity. Here, we highlight recent advances in our understanding of how the bacterial AB₅ toxins induce disease. We highlight the molecular mechanisms by which these toxins use glycosphingolipid to traffic within cells, with special attention to how the cell senses and sorts the lipid receptors. We also discuss several new studies that address the mechanisms of toxin unfolding in the ER and the mechanisms of CT A1-chain retro-translocation to the cytosol.

Focal adhesion kinase protein regulates Wnt3a gene expression to control cell fate specification in the developing neural plate

FAK is linked to aggressive tumors, but its normal function is not clear. FAK knockdown early in Xenopus development anteriorizes the embryo via a loss of Wnt signaling. Wnt3a expression is FAK dependent in both embryos and human breast cancer cells, suggesting that a FAK–Wnt linkage is highly conserved.

Focal adhesion kinase (FAK) is a cytoplasmic tyrosine kinase protein localized to regions called focal adhesions, which are contact points between cells and the extracellular matrix. FAK protein acts as a scaffold to transfer adhesion-dependent and growth factor signals into the cell. Increased FAK expression is linked to aggressive metastatic and invasive tumors. However, little is known about its normal embryonic function. FAK protein knockdown during early Xenopus laevis development anteriorizes the embryo. Morphant embryos express increased levels of anterior neural markers, with reciprocally reduced posterior neural marker expression. Posterior neural plate folding and convergence-extension is also inhibited. This anteriorized phenotype resembles that of embryos knocked down zygotically for canonical Wnt signaling. FAK and Wnt3a genes are both expressed in the neural plate, and Wnt3a expression is FAK dependent. Ectopic Wnt expression rescues this FAK morphant anteriorized phenotype. Wnt3a thus acts downstream of FAK to balance anterior–posterior cell fate specification in the developing neural plate. Wnt3a gene expression is also FAK dependent in human breast cancer cells, suggesting that this FAK–Wnt linkage is highly conserved. This unique observation connects the FAK- and Wnt-signaling pathways, both of which act to promote cancer when aberrantly activated in mammalian cells.

Sox17 and chordin are required for formation of Kupffer's vesicle and left-right asymmetry determination in zebrafish

Kupffer's vesicle (KV), a ciliated fluid-filled sphere in the zebrafish embryo with a critical role in laterality determination, is derived from a group of superficial cells in the organizer region of the gastrula named the dorsal forerunner cells (DFC). We have examined the role of the expression of sox17 and chordin (chd) in the DFC in KV formation and laterality determination. Whereas sox17 was known to be expressed in DFC, its function in these cells was not studied before. Further, expression of chd in these cells has not been reported previously. Targeted knockdown of Sox17 and Chd in DFC led to aberrant Left-Right (L-R) asymmetry establishment, as visualized by the expression of southpaw and lefty, and heart and pancreas placement in the embryo. These defects correlated with the formation of small KVs with apparently diminished cilia, consistent with the known requirement for ciliary function in the laterality organ for the establishment of L-R asymmetry.

Transcriptome analysis of the zebrafish pineal gland

The zebrafish pineal gland (epiphysis) is a site of melatonin production, contains photoreceptor cells, and functions as a circadian clock pace maker. Here, we have used microarray technology to study the zebrafish pineal transcriptome. Analysis of gene expression at three larval and two adult stages revealed a highly dynamic transcriptional profile, revealing many genes that are highly expressed in the zebrafish pineal gland. Statistical analysis of the data based on Gene Ontology annotation indicates that many transcription factors are highly expressed during larval stages, whereas genes dedicated to phototransduction are preferentially expressed in the adult. Furthermore, several genes were identified that exhibit day/night differences in expression. Among the multiple candidate genes suggested by these data, we note the identification of a tissue-specific form of the unc119 gene with a possible role in pineal development.

Isolation and expression analysis of foxj1 and foxj1.2 in zebrafish embryos

In this report, we present the isolation and identification of a zebrafish homolog of the winged helix\forkhead transcription factor Foxj1. Foxj1 was identified in other species but not in zebrafish. Foxj1 was shown in mice to be required in ciliogenesis and left-right asymmetry establishment. Here we present a spatio-temporal expression pattern of zebrafish foxj1, showing that this gene is expressed in dorsal forerunner cells, Kupffers vesicle, the floor plate, pronephric ducts and kidney. This expression pattern is overlapping but different from that of the foxj1.2, the closest related gene in zebrafish. Foxj1 in zebrafish appears to have similar functions as those reported in other species connected to ciliogenesis and left-right asymmetry.

XenopusMeis3 protein forms a hindbrain-inducing center by activating FGF/MAP kinase and PCP pathways

Knockdown studies in Xenopus demonstrated that the XMeis3gene is required for proper hindbrain formation. An explant assay was developed to distinguish between autonomous and inductive activities of XMeis3 protein. Animal cap explants caudalized by XMeis3 were recombined with explants neuralized by the BMP dominant-negative receptor protein. XMeis3-expressing cells induced convergent extension cell elongations in juxtaposed neuralized explants. Elongated explants expressed hindbrain and primary neuron markers, and anterior neural marker expression was extinguished. Cell elongation was dependent on FGF/MAP-kinase and Wnt-PCP activities. XMeis3 activates FGF/MAP-kinase signaling, which then modulates the PCP pathway. In this manner, XMeis3 protein establishes a hindbrain-inducing center that determines anteroposterior patterning in the brain.

Ras-mediated FGF signaling is required for the formation of posterior but not anterior neural tissue in Xenopus laevis

Fibroblast growth factor (FGF) has been proposed to be involved in the specification and patterning of the developing vertebrate nervous system. There is conflicting evidence, however, concerning the requirement for FGF signaling in these processes. To provide insight into the signaling mechanisms that are important for neural induction and anterior-posterior neural patterning, we have employed the dominant negative Ras mutant, N17Ras, in addition to a truncated FGF receptor (XFD). Both N17Ras and XFD, when expressed in Xenopus laevis animal cap ectoderm, inhibit the ability of FGF to generate neural pattern. They also block induction of posterior neural tissue by XBF2 and XMeis3. However, neither XFD nor N17Ras inhibits noggin, neurogenin, or XBF2 induction of anterior neural markers. MAP kinase activation has been proposed to be necessary for neural induction, yet N17Ras inhibits the phosphorylation of MAP kinase that usually follows explantation of explants. In whole embryos, Ras-mediated FGF signaling is critical for the formation of posterior neural tissues but is dispensable for neural induction.