A fundamental impediment to understanding the brain is the availability of inexpensive and robust methods for targeting and manipulating specific neuronal populations. The need to overcome this barrier is pressing because there are considerable anatomical, physiological, cognitive, and behavioral differences between mice and higher mammalian species in which it is difficult to specifically target and manipulate genetically defined functional cell-types. In particular, it is unclear the degree to which insights from mouse models can shed light on the neural mechanisms that mediate cognitive functions in higher species including humans. Here we describe a novel recombinant adeno-associated virus (rAAV) that restricts gene expression to GABAergic interneurons within the telencephalon. We demonstrate that the viral expression is specific and robust, allowing for morphological visualization, activity monitoring and functional manipulation of interneurons in both mice and non-genetically tractable species, thus opening the possibility to study GABA-ergic function in virtually any vertebrate species.

PR-Set7/Set8/KMT5A is the sole enzyme known to catalyze monomethylation of histone H4 lysine 20 (H4K20) and is present only in multicellular organisms that compact a large fraction of their DNA. We found that mouse embryos that are homozygous null mutants for the gene PR-Set7 display early embryonic lethality prior to the eight-cell stage. Death was due to the absence of PR-Set7 catalytic activity, since microinjection of the wild type, but not a catalytically inactive version, into two-cell embryos rescued the phenotype. A lack of PR-Set7 activity resulted not only in depletion of H4K20me1 but also in reduced levels of the H4K20me2/3 marks catalyzed by the Suv4-20h1/h2 enzymes, implying that H4K20me1 may be essential for the function of these enzymes to ensure the dimethylated and trimethylated states. Embryonic stem cells that were inducibly deleted for PR-Set7 passed through an initial G 2 /M phase, but the progeny were defective at the subsequent S and G 2 /M phases, exhibiting a delay in their cell cycle, accumulation at G 2 /M, massive DNA damage, and improper mitotic chromosome condensation. Cell cycle analysis after synchronization indicated that the defects were a consequence of decreased H4K20me1 due to the absence of PR-Set7. Most importantly, the lack of H4K20me1 also resulted in defects in chromosome condensation in interphase nuclei. These results demonstrate the critical role of H4K20 monomethylation in mammals in a developmental context.Posttranslational modifications (PTMs) on histones influence intra-and internucleosomal interactions and thereby contribute to the diversity in nucleosome and chromatin structure that impacts distinct genomic processes. Among these modifications, histone methylation had been considered to be relatively stable, but recent studies demonstrated that, similar to the other PTMs, it too is subject to regulation. Many methylating and demethylating enzymes that target different lysine and arginine residues have been identified. These enzymes have distinct specificities with respect to the methylation status (monomethyl, dimethyl, and trimethyl) of each residue. Furthermore, increasing numbers of proteins harboring the motifs that specifically recognize various methylated residues have been identified. These proteins, or effectors, mediate/regulate elaborate chromatin-based processes, such as gene expression, which are dictated by the presence of the PTMs. Thus, the catalysis/removal of PTMs and their recognition by effectors constitute an intricately designed system that is key to genomic integrity and function.One of the residues of histone H4 that can be monomethylated, dimethylated, or trimethylated is lysine 20. Recent comprehensive analysis of H4 modifications with top-down mass spectrometry in human and in Drosophila melanogaster cells revealed that the dimethyl group is deposited on the majority of total H4K20, indicative of the wide distribution of H4K20me2 on chromatin (18, 34). On the other hand, H4K20me1 and H4K20me3 are relatively few in abundance. The study a...

Several membrane-associated proteins are known to modulate the activity and range of potent morphogenetic signals during development. In particular,members of the EGF-CFC family encode glycosyl-phosphatidylinositol(GPI)-linked proteins that are essential for activity of the transforming growth factor β (TGFβ) ligand Nodal, a factor that plays a central role in establishing the vertebrate body plan. Genetic and biochemical studies have indicated that EGF-CFC proteins function as cell-autonomous co-receptors for Nodal; by contrast, cell culture data have suggested that the mammalian EGF-CFC protein Cripto can act as a secreted signaling factor. Here we show that Cripto acts non-cell-autonomously during axial mesendoderm formation in the mouse embryo and may possess intercellular signaling activity in vivo. Phenotypic analysis of hypomorphic mutants demonstrates that Criptois essential for formation of the notochordal plate, prechordal mesoderm and foregut endoderm during gastrulation. Remarkably, Cripto null mutant cells readily contribute to these tissues in chimeras, indicating non-cell-autonomy. Consistent with these loss-of-function analyses,gain-of-function experiments in chick embryos show that exposure of node/head process mesoderm to soluble Cripto protein results in alterations in cell fates toward anterior mesendoderm, in a manner that is dependent on Nodal signaling. Taken together, our findings support a model in which Cripto can function in trans as an intercellular mediator of Nodal signaling activity.