Dissection of Merkel cell formation in hairy and glabrous skin reveals a common requirement for FGFR2-mediated signalling

Merkel cells are mechanosensory cells involved in tactile discrimination. Merkel cells have been primarily studied in the murine back skin, where they are found in specialized structures called touch domes located around primary hair follicles. Yet, little is known about the morphogenesis of Merkel cells in areas of the skin devoid of hair, such as the glabrous paw skin. Here, we describe Merkel cell formation in the glabrous paw skin during embryogenesis. We first found in the glabrous paw skin that Merkel cells were specified at E15.5, 24 hours later, compared to in the back skin. Additionally, by performing lineage-tracing experiments, we found that unlike in the back skin, SOX9(+) cells do not give rise to Merkel cells in the glabrous paw skin. Finally, we compared the transcriptomes of Merkel cells in the back and the glabrous paw skin and showed that they are similar. Genetic and transcriptome studies showed that the formation of Merkel cells in both regions was controlled by similar regulators. Among them was FGFR2, an upstream factor of MAPK signalling that was reported to have a critical function in Merkel cell formation in the back skin. Here, we showed that FGFR2 is also required for Merkel cell development in the glabrous paw skin. Taken together, our results demonstrate that Merkel cells in the murine back skin and glabrous paw skin are similar, and even though their formation is controlled by a common genetic programme, their precursor cells might differ.

RNA-dependent chromatin targeting of TET2 for endogenous retrovirus control in pluripotent stem cells

Ten-eleven translocation (TET) proteins play key roles in the regulation of DNA-methylation status by oxidizing 5-methylcytosine (5mC) to generate 5-hydroxymethylcytosine (5hmC), which can both serve as a stable epigenetic mark and participate in active demethylation. Unlike the other members of the TET family, TET2 does not contain a DNA-binding domain, and it remains unclear how it is recruited to chromatin. Here we show that TET2 is recruited by the RNA-binding protein Paraspeckle component 1 (PSPC1) through transcriptionally active loci, including endogenous retroviruses (ERVs) whose long terminal repeats (LTRs) have been co-opted by mammalian genomes as stage- and tissue-specific transcriptional regulatory modules. We found that PSPC1 and TET2 contribute to ERVL and ERVL-associated gene regulation by both transcriptional repression via histone deacetylases and post-transcriptional destabilization of RNAs through 5hmC modification. Our findings provide evidence for a functional role of transcriptionally active ERVs as specific docking sites for RNA epigenetic modulation and gene regulation.

Zfp281 Coordinates Opposing Functions of Tet1 and Tet2 in Pluripotent States

Pluripotency is increasingly recognized as a spectrum of cell states defined by their growth conditions. Although naive and primed pluripotency states have been characterized molecularly, our understanding of events regulating state acquisition is wanting. Here, we performed comparative RNA sequencing of mouse embryonic stem cells (ESCs) and defined a pluripotent cell fate (PCF) gene signature associated with acquisition of naive and primed pluripotency. We identify Zfp281 as a key transcriptional regulator for primed pluripotency that also functions as a barrier toward achieving naive pluripotency in both mouse and human ESCs. Mechanistically, Zfp281 interacts with Tet1, but not Tet2, and its direct transcriptional target, miR-302/367, to negatively regulate Tet2 expression to establish and maintain primed pluripotency. Conversely, ectopic Tet2 alone, but not Tet1, efficiently reprograms primed cells toward naive pluripotency. Our study reveals a molecular circuitry in which opposing functions of Tet1 and Tet2 control acquisition of alternative pluripotent states.

The mesenchymal transcription factor SNAI-1 instructs human liver specification

Epithelial-mesenchymal transition (EMT) and the mesenchymal-epithelial transition (MET) are processes required for embryo organogenesis. Liver develops from the epithelial foregut endoderm from which the liver progenitors, hepatoblasts, are specified. The migrating hepatoblasts acquire a mesenchymal phenotype to form the liver bud. In mid-gestation, hepatoblasts mature into epithelial structures: the hepatocyte cords and biliary ducts. While EMT has been associated with liver bud formation, nothing is known about its contribution to hepatic specification. We previously established an efficient protocol from human embryonic stem cells (hESC) to generate hepatic cells (Hep cells) resembling the hepatoblasts expressing alpha-fetoprotein (AFP) and albumin (ALB). Here we show that Hep cells express both epithelial (EpCAM and E-cadherin) and mesenchymal (vimentin and SNAI-1) markers. Similar epithelial and mesenchymal hepatoblasts were identified in human and mouse fetal livers, suggesting a conserved interspecies phenotype. Knock-down experiments demonstrated the importance of SNAI-1 in Hep cell hepatic specification. Moreover, ChIP assays revealed direct binding of SNAI-1 in the promoters of AFP and ALB genes consistent with its transcriptional activator function in hepatic specification. Altogether, our hESC-derived Hep cell cultures reveal the dual mesenchymal and epithelial phenotype of hepatoblast-like cells and support the unexpected transcriptional activator role of SNAI-1 in hepatic specification.

Silencing structural and nonstructural genes in baculovirus by RNA interference

We review several aspects of RNAi and gene silencing with baculovirus. We show that the potency of RNAi in Spodoptera frugiperda (Sf21) insect cells correlates well with the efficiency of transfection of the siRNA. Using a fluorescein-labeled siRNA we found that the siRNA localized in areas surrounding the endoplasmic reticulum (ER). Both long (700 nucleotides long) and small ( approximately 25 nucleotides long) interfering RNAs were equally effective in initiating RNA interference (RNAi), and the duration of the interfering effect was indistinguishable. Even though RNAi in Sf21 cells is very effective, in vitro experiments show that these cells fragment the long dsRNA into siRNA poorly, when compared to HEK cells. Finally, we show that in vivo inhibition of baculovirus infection with dsRNA homologous to genes that are essential for baculovirus infectivity depends strongly on the amount of dsRNA used in the assays. Five hundred nanogram of dsRNA directly injected into the haemolymph of insects prevent animal death to over 95%. In control experiments, over 96% of insects not injected with dsRNA or injected with an irrelevant dsRNA died within a week. These results demonstrate the efficiency of dsRNA for in vivo prevention of a viral infection by virus that is very cytotoxic and lytic in animals.

Using double-stranded RNA to prevent in vitro and in vivo viral infections by recombinant baculovirus

Introduction of double-stranded RNA (dsRNA) into a wide variety of cells and organisms results in post-transcriptional depletion of the homologue endogenous mRNA. This well-preserved phenomenon known as RNA interference (RNAi) is present in evolutionarily diverse organisms such as plants, fungi, insects, metazoans, and mammals. Because the identification of the targeted mRNA by the RNAi machinery depends upon Watson-Crick base-pairing interactions, RNAi can be exquisitely specific. We took advantage of this powerful and flexible technique to demonstrate that selective silencing of genes essential for viral propagation prevents in vitro and in vivo viral infection. Using the baculovirus Autographa californica, a rapidly replicating and highly cytolytic double-stranded DNA virus that infects many different insect species, we show for the first time that introduction of dsRNA from gp64 and ie1, two genes essential for baculovirus propagation, results in prevention of viral infection in vitro and in vivo. This is the first report demonstrating the use of RNAi to inhibit a viral infection in animals. This inhibition was specific, because dsRNA from the polyhedrin promoter (used as control) or unrelated dsRNAs did not affect the time course of viral infection. The most relevant consequences from the present study are: 1) RNAi offers a rapid and efficient way to interfere with viral genes to assess the role of specific proteins in viral function and 2) using RNAi to interfere with viral genes essential for cell infection may provide a powerful therapeutic tool for the treatment of viral infections.