Physical and functional interaction between SET1/COMPASS complex component CFP-1 and a Sin3S HDAC complex in C. elegans

The CFP1 CXXC zinc finger protein targets the SET1/COMPASS complex to non-methylated CpG rich promoters to implement tri-methylation of histone H3 Lys4 (H3K4me3). Although H3K4me3 is widely associated with gene expression, the effects of CFP1 loss vary, suggesting additional chromatin factors contribute to context dependent effects. Using a proteomics approach, we identified CFP1 associated proteins and an unexpected direct link between Caenorhabditis elegans CFP-1 and an Rpd3/Sin3 small (SIN3S) histone deacetylase complex. Supporting a functional connection, we find that mutants of COMPASS and SIN3 complex components genetically interact and have similar phenotypic defects including misregulation of common genes. CFP-1 directly binds SIN-3 through a region including the conserved PAH1 domain and recruits SIN-3 and the HDA-1/HDAC subunit to H3K4me3 enriched promoters. Our results reveal a novel role for CFP-1 in mediating interaction between SET1/COMPASS and a Sin3S HDAC complex at promoters.

The SET-2/SET1 histone H3K4 methyltransferase maintains pluripotency in the Caenorhabditis elegans germline

Histone H3 Lys 4 methylation (H3K4me) is deposited by the conserved SET1/MLL methyltransferases acting in multiprotein complexes, including Ash2 and Wdr5. Although individual subunits contribute to complex activity, how they influence gene expression in specific tissues remains largely unknown. In Caenorhabditis elegans, SET-2/SET1, WDR-5.1, and ASH-2 are differentially required for germline H3K4 methylation. Using expression profiling on germlines from animals lacking set-2, ash-2, or wdr-5.1, we show that these subunits play unique as well as redundant functions in order to promote expression of germline genes and repress somatic genes. Furthermore, we show that in set-2- and wdr-5.1-deficient germlines, somatic gene misexpression is associated with conversion of germ cells into somatic cells and that nuclear RNAi acts in parallel with SET-2 and WDR-5.1 to maintain germline identity. These findings uncover a unique role for SET-2 and WDR-5.1 in preserving germline pluripotency and underline the complexity of the cellular network regulating this process.

Caenorhabditis elegans Heterochromatin protein 1 (HPL-2) links developmental plasticity, longevity and lipid metabolism

Background

Heterochromatin protein 1 (HP1) family proteins have a well-characterized role in heterochromatin packaging and gene regulation. Their function in organismal development, however, is less well understood. Here we used genome-wide expression profiling to assess novel functions of the Caenorhabditis elegans HP1 homolog HPL-2 at specific developmental stages.

Results

We show that HPL-2 regulates the expression of germline genes, extracellular matrix components and genes involved in lipid metabolism. Comparison of our expression data with HPL-2 ChIP-on-chip profiles reveals that a significant number of genes up- and down-regulated in the absence of HPL-2 are bound by HPL-2. Germline genes are specifically up-regulated in hpl-2 mutants, consistent with the function of HPL-2 as a repressor of ectopic germ cell fate. In addition, microarray results and phenotypic analysis suggest that HPL-2 regulates the dauer developmental decision, a striking example of phenotypic plasticity in which environmental conditions determine developmental fate. HPL-2 acts in dauer at least partly through modulation of daf-2/IIS and TGF-β signaling pathways, major determinants of the dauer program. hpl-2 mutants also show increased longevity and altered lipid metabolism, hallmarks of the long-lived, stress resistant dauers.

Conclusions

Our results suggest that the worm HP1 homologue HPL-2 may coordinately regulate dauer diapause, longevity and lipid metabolism, three processes dependent on developmental input and environmental conditions. Our findings are of general interest as a paradigm of how chromatin factors can both stabilize development by buffering environmental variation, and guide the organism through remodeling events that require plasticity of cell fate regulation.

Regulation of Gephyrin Assembly and Glycine Receptor Synaptic Stability

Gephyrin is required for the formation of clusters of the glycine receptor (GlyR) in the neuronal postsynaptic membrane. It can make trimers and dimers through its N- and C-terminal G and E domains, respectively. Gephyrin oligomerization could thus create a submembrane lattice providing GlyR-binding sites. We investigated the relationships between the stability of cell surface GlyR and the ability of gephyrin splice variants to form oligomers. Using truncated and full-length gephyrins we found that the 13-amino acid sequence (cassette 5) prevents G domain trimerization. Moreover, E domain dimerization is inhibited by the gephyrin central L domain. All of the gephyrin variants bind GlyR beta subunit cytoplasmic loop with high affinity regardless of their cassette composition. Coexpression experiments in COS-7 cells demonstrated that GlyR bound to gephyrin harboring cassette 5 cannot be stabilized at the cell surface. This gephyrin variant was found to deplete synapses from both GlyR and gephyrin in transfected neurons. These data suggest that the relative expression level of cellular variants influence the overall oligomerization pattern of gephyrin and thus the turnover of synaptic GlyR.

The C. elegans HP1 homologue HPL-2 and the LIN-13 zinc finger protein form a complex implicated in vulval development

HP1 proteins are essential components of heterochromatin and contribute to the transcriptional repression of euchromatic genes via the recruitment to specific promoters by corepressor proteins including TIF1 and Rb. The Caenorhabditis elegans HP1 homologue HPL-2 acts in the "synMuv" (synthetic multivulval) pathway, which defines redundant negative regulators of a Ras signaling cascade required for vulval induction. Several synMuv genes encode for chromatin-associated proteins involved in transcriptional regulation, including Rb and components of the Mi-2/NuRD and TIP60/NuA4 chromatin remodeling complexes. Here, we show that HPL-2 physically interacts in vitro and in vivo with the multiple zinc finger protein LIN-13, another member of the synMuv pathway. A variant of the conserved PXVXL motif found in many HP1-interacting proteins mediates LIN-13 binding to the CSD of HPL-2. We further show by in vivo localization studies that LIN-13 is required for HPL-2 recruitment in nuclear foci. Our data suggest that the LIN-13/HPL-2 complex may physically link a subset of the Rb related synMuv proteins to chromatin.

Deciphering the structural framework of glycine receptor anchoring by gephyrin

Glycine is the major inhibitory neurotransmitter in the spinal cord and brain stem. Gephyrin is required to achieve a high concentration of glycine receptors (GlyRs) in the postsynaptic membrane, which is crucial for efficient glycinergic signal transduction. The interaction between gephyrin and the GlyR involves the E-domain of gephyrin and a cytoplasmic loop located between transmembrane segments three and four of the GlyR beta subunit. Here, we present crystal structures of the gephyrin E-domain with and without the GlyR beta-loop at 2.4 and 2.7 A resolutions, respectively. The GlyR beta-loop is bound in a symmetric 'key and lock' fashion to each E-domain monomer in a pocket adjacent to the dimer interface. Structure-guided mutagenesis followed by in vitro binding and in vivo colocalization assays demonstrate that a hydrophobic interaction formed by Phe 330 of gephyrin and Phe 398 and Ile 400 of the GlyR beta-loop is crucial for binding.

The existence of a second vesicular glutamate transporter specifies subpopulations of glutamatergic neurons

Before their exocytotic release during stimulation of nerve terminals, nonpeptide neurotransmitters are loaded into synaptic vesicles by specific transporters. Recently, a protein initially identified as brain-specific Na(+)-dependent inorganic phosphate transporter I (BNPI) has been shown to represent a vesicular glutamate transporter (VGLUT1). In this study, we investigated whether a highly homologous "differentiation-associated Na(+)-dependent inorganic phosphate transporter" (DNPI) is involved in glutamatergic transmission. Vesicles isolated from BON cells expressing recombinant DNPI accumulated l-glutamate with bioenergetical and pharmacological characteristics identical to those displayed by VGLUT1 and by brain synaptic vesicles. Moreover, DNPI localized to synaptic vesicles, at synapses exhibiting classical excitatory features. DNPI thus represents a novel vesicular glutamate transporter (VGLUT2). The distributions of each VGLUT transcript in brain were highly complementary, with only a partial regional and cellular overlap. At the protein level, we could only detect either VGLUT1- or VGLUT2-expressing presynaptic boutons. The existence of two VGLUTs thus defines distinct subsets of glutamatergic neurons.

Constitutive phosphorylation of the vesicular inhibitory amino acid transporter in rat central nervous system

gamma-Aminobutyric acid (GABA) and glycine are stored into synaptic vesicles by a recently identified vesicular inhibitory amino acid transporter [VIAAT, also called vesicular GABA transporter (VGAT)]. Immunoblotting analysis revealed that rat brain VIAAT migrated as a doublet during sodium dodecyl sulfate-polyacrylamide gel electrophoresis, with a predominant slower band in all areas examined except olfactory bulb and retina. The slower band corresponded to a phosphorylated form of VIAAT as it was converted to the faster one by treating brain homogenates with alkaline phosphatase or with an endogenous phosphatase identified as type 2A protein-serine/threonine phosphatase using okadaic acid. In contrast, the recombinant protein expressed in COS-7 or PC12 cells co-migrated with the faster band of the brain doublet and was insensitive to alkaline phosphatase. To investigate the influence of VIAAT phosphorylation on vesicular neurotransmitter loading, purified synaptic vesicles were treated with alkaline phosphatase and assayed for amino acid uptake. However, neither GABA nor glycine uptake was affected by VIAAT phosphorylation. These results indicate that VIAAT is constitutively phosphorylated on cytosolic serine or threonine residues in most, but not all, regions of the rat brain. This phosphorylation does not regulate the vesicular loading of GABA or glycine, suggesting that it is involved at other stages of the synaptic vesicle life cycle.

Presence of the vesicular inhibitory amino acid transporter in GABAergic and glycinergic synaptic terminal boutons

The characterization of the Caenorhabditis elegans unc-47 gene recently allowed the identification of a mammalian (gamma)-amino butyric acid (GABA) transporter, presumed to be located in the synaptic vesicle membrane. In situ hybridization data in rat brain suggested that it might also take up glycine and thus represent a general Vesicular Inhibitory Amino Acid Transporter (VIAAT). In the present study, we have investigated the localization of VIAAT in neurons by using a polyclonal antibody raised against the hydrophilic N-terminal domain of the protein. Light microscopy and immunocytochemistry in primary cultures or tissue sections of the rat spinal cord revealed that VIAAT was localized in a subset (63-65%) of synaptophysin-immunoreactive terminal boutons; among the VIAAT-positive terminals around motoneuronal somata, 32.9% of them were also immunoreactive for GAD65, a marker of GABAergic presynaptic endings. Labelling was also found apposed to clusters positive for the glycine receptor or for its associated protein gephyrin. At the ultrastructural level, VIAAT immunoreactivity was restricted to presynaptic boutons exhibiting classical inhibitory features and, within the boutons, concentrated over synaptic vesicle clusters. Pre-embedding detection of VIAAT followed by post-embedding detection of GABA or glycine on serial sections of the spinal cord or cerebellar cortex indicated that VIAAT was present in glycine-, GABA- or GABA- and glycine-containing boutons. Taken together, these data further support the view of a common vesicular transporter for these two inhibitory transmitters, which would be responsible for their costorage in the same synaptic vesicle and subsequent corelease at mixed GABA-and-glycine synapses.

The vesicular monoamine transporter: from chromaffin granule to brain

All characterized monoaminergic cells utilize the same transport system for the vesicular accumulation of monoamines prior to their release. This system operates in neuronal (catecholaminergic, serotoninergic or histaminergic) as well as in endocrine or neuroendocrine cells. For several decades, chromaffin granules from bovine adrenal medulla have been used as a model system, allowing progress in the understanding of the biophysics, the biochemistry and the pharmacology of the monoamine vesicular transporter. The transporters from rat, bovine and man have been cloned. Surprisingly, two genes encode different isoforms of the protein which are differentially expressed in monoaminergic systems. The conjunction of recombinant DNA techniques and expression in secretory or non-secretory cells with the large body of data obtained on the chromaffin granule transporter has allowed rapid progress in the study of the protein. But interestingly enough, this progress has open new possibilities in the study of biological problems, especially in the brain. The transporter is useful for the determination of the relationship between small and large dense core vesicles, for the understanding of the mechanism of the drugs such as 1-methyl-4-phenylpyridinium (MPP+), tetrabenazine or amphetamines, and as a marker in brain development. The possibility of regulations at the vesicular transporter level and of their effect on the quantum size has to be investigated. The vesicular monoamine transporter is also an important target for brain imaging.