Imaging of genomic loci in Trypanosoma brucei using an optimised LacO-LacI system

Visualisation of genomic loci by microscopy is essential for understanding nuclear organisation, particularly at the single cell level. One powerful technique for studying the positioning of genomic loci is through the Lac Operator-Lac Repressor (LacO-LacI) system, in which LacO repeats introduced into a specific genomic locus can be visualised through expression of a LacI-protein fused to a fluorescent tag. First utilised in Trypanosoma brucei over 20 years ago, we have now optimised this system with short, stabilised LacO repeats of less than 2 kb paired with a constitutively expressed mNeongreen::LacI fusion protein to facilitate visualisation of genomic loci. We demonstrate the compatibility of this system with super-resolution microscopy and propose its suitability for multiplexing with inducible RNAi or protein over expression which will allow analysis of nuclear organisation after perturbation of gene expression.

Pedal to the Metal: Nuclear Splicing Bodies Turbo-Charge VSG mRNA Production in African Trypanosomes

The African trypanosome Trypanosoma brucei is a parasite of the mammalian bloodstream and tissues, where an antigenically variable Variant Surface Glycoprotein (VSG) coat protects it from immune attack. This dense layer comprised of ∼107 VSG proteins, makes VSG by far the most abundant mRNA (7-10% total) and protein (∼10% total) in the bloodstream form trypanosome. How can such prodigious amounts of VSG be produced from a single VSG gene? Extremely high levels of RNA polymerase I (Pol I) transcription of the active VSG provide part of the explanation. However, recent discoveries highlight the role of pre-mRNA processing, both in maintaining high levels of VSG transcription, as well as its monoallelic expression. Trypanosome mRNAs are matured through trans-splicing a spliced leader (SL) RNA to the 5' end of precursor transcripts, meaning abundant SL RNA is required throughout the nucleus. However, requirement for SL RNA in the vicinity of the active VSG gene is so intense, that the cell reconfigures its chromatin architecture to facilitate interaction between the SL RNA genes and the active VSG. This presumably ensures that sufficient localised SL RNA is available, and not limiting for VSG mRNA expression. Recently, novel nuclear splicing bodies which appear to provide essential trans-splicing components, have been identified associating with the active VSG. These observations highlight the underappreciated role of pre-mRNA processing in modulating gene expression in trypanosomes. Dissecting the function of these nuclear RNA processing bodies should help us elucidate the mechanisms of both VSG expression and monoallelic exclusion in T. brucei.

Identification of serum glycoprotein ligands for the immunomodulatory receptor blood dendritic cell antigen 2

Blood dendritic cell antigen 2 (BDCA-2) is a C-type lectin found on the surface of plasmacytoid dendritic cells. It functions as a glycan-binding receptor that downregulates the production of type I interferons and thus plays a role in oligosaccharide-mediated immunomodulation. The carbohydrate recognition domain in BDCA-2 binds selectively to galactose-terminated bi-antennary glycans. Because the plasmacytoid dendritic cells function in a plasma environment rich in glycoproteins, experiments have been undertaken to identify endogenous ligands for blood dendritic cell antigen 2. A combination of blotting, affinity chromatography and proteomic analysis reveals that serum glycoprotein ligands for BDCA-2 include IgG, IgA and IgM. Compared to binding of IgG, which was previously described, IgA and IgM bind with higher affinity. The association constants for the different subclasses of immunoglobulins are below and roughly proportional to the serum concentrations of these glycoprotein ligands. Binding to the other main serum glycoprotein ligand, α2-macroglobulin, is independent of whether this protease inhibitor is activated. Binding to all of these glycoprotein ligands is mediated predominantly by bi-antennary glycans in which each branch bears a terminal galactose residue. The different affinities of the glycoprotein ligands reflect the different numbers of these galactose-terminated glycans and their degree of exposure on the native glycoproteins. The results suggest that normal serum levels of immunoglobulins could downmodulate interferon stimulation of further antibody production.

Repo-Man/PP1 regulates heterochromatin formation in interphase

Repo-Man is a protein phosphatase 1 (PP1) targeting subunit that regulates mitotic progression and chromatin remodelling. After mitosis, Repo-Man/PP1 remains associated with chromatin but its function in interphase is not known. Here we show that Repo-Man, via Nup153, is enriched on condensed chromatin at the nuclear periphery and at the edge of the nucleopore basket. Repo-Man/PP1 regulates the formation of heterochromatin, dephosphorylates H3S28 and it is necessary and sufficient for heterochromatin protein 1 binding and H3K27me3 recruitment. Using a novel proteogenomic approach, we show that Repo-Man is enriched at subtelomeric regions together with H2AZ and H3.3 and that depletion of Repo-Man alters the peripheral localization of a subset of these regions and alleviates repression of some polycomb telomeric genes. This study shows a role for a mitotic phosphatase in the regulation of the epigenetic landscape and gene expression in interphase.

Repo-Man is a chromosome-binding subunit of protein phosphatase 1 to regulate mitosis. Here, de Castro and colleagues show that Repo-Man also regulates heterochromatin during interphase, and regulates gene repression and chromatin organization.