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Oligonucleotide-based fluorescent in situ hybridization (FISH) coupled with high-resolution high-sensitivity microscopy allows the visualization of single RNA molecules within fixed cells and tissues as distinct foci. We show here that combinatorial labeling of RNA molecules with several fluorescent dyes extends the number of genes that can be targeted simultaneously beyond the number of fluorophores used. This approach also inherently validates the identification of transcripts reducing false positive counts. We have used combinatorial FISH and image analysis to measure the transcript densities of six genes using three fluorophores. This has allowed us to visualize the endothelial maturation of lateral mesoderm in an in vitro ES differentiation assay from a single snapshot of molecular identities. Our observations show that, under these specific conditions, endothelial maturation follows a homogeneous course with a gradual increase in expression of Cdh5 and a concomitant loss of early transcription factors, arguing that maturation is governed in a generally deterministic manner. This methodology is limited by the number of fluorophores that can be used and by the available microscopic resolution, but currently available equipment should allow the visualization of transcripts from 10 or more genes simultaneously. [ABSTRACT FROM AUTHOR]

From a list of protein kinases (PKs) that are newly induced upon differentiation of mouse embryonic stem cells to mesendoderm, we identified a previously uncharacterized kinase, Vlk (vertebrate lonesome kinase), that is well conserved in vertebrates but has no homologs outside of the vertebrate lineage. Its kinase domain cannot be classified into any of the previously defined kinase groups or families. Although Vlk is first expressed in E-cadherin-positive anterior visceral endoderm and mesendoderm, its expression is later confined to E-cadherin-negative mesenchyme. Vlk is enriched in the Golgi apparatus and blocks VSVG transport from the Golgi to the plasma membrane. Targeted disruption of Vlk leads to a defect in lung development and to delayed ossification of endochondral bone. Vlk-/- mice display neonatal lethality due to respiratory failure, with a suckling defect arising from a cleft palate. Our results demonstrate that Vlk is a novel vertebrate-specific PK that is involved in the regulation of the rate of protein export from the Golgi, thereby playing an important role in the formation of functional stroma by mesenchymal cells. [ABSTRACT FROM AUTHOR]