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The immunosuppressive drugs cyclosporin A (CsA) and FK506 both interfere with a Ca(2+)-sensitive T-cell signal transduction pathway, thereby preventing the activation of specific transcription factors (such as NF-AT and NF-IL2A) involved in lymphokine gene expression. CsA and FK506 seem to act by interaction with their cognate intracellular receptors, cyclophilin and FKBP, respectively (see ref. 11 for review). The Ca2+/calmodulin-regulated phosphatase calcineurin is a major target of drug-isomerase complexes in vitro. We have therefore tested the hypothesis that this interaction is responsible for the in vivo effects of CsA/FK506. We report here that overexpression of calcineurin in Jurkat cells renders them more resistant to the effects of CsA and FK506 and augments both NFAT- and NFIL2A-dependent transcription. These results identify calcineurin as a key enzyme in the T-cell signal transduction cascade and provide biological evidence to support the notion that the interaction of drug-isomerase complexes with calcineurin underlies the molecular basis of CsA/FK506-mediated immunosuppression.

New methods for the genome-wide analysis of chromatin are providing insight into its roles in development and their underlying mechanisms. Current studies indicate that chromatin is dynamic, with its structure and its histone modifications undergoing global changes during transitions in development and in response to extracellular cues. In addition to DNA methylation and histone modification, ATP-dependent enzymes that remodel chromatin are important controllers of chromatin structure and assembly, and are major contributors to the dynamic nature of chromatin. Evidence is emerging that these chromatin-remodelling enzymes have instructive and programmatic roles during development. Particularly intriguing are the findings that specialized assemblies of ATP-dependent remodellers are essential for establishing and maintaining pluripotent and multipotent states in cells.An essential aspect of building a mammalian cell is packing 1.7 metres of DNA into a 5-micrometre nucleus in a form that allows it to be replicated and transcribed in stable, tissuespecific patterns. The basic unit of chromatin assembly is the nucleosome 1 , which compacts DNA about sevenfold. However, because the overall level of compaction of the vertebrate genome is several thousand fold, relatively little of the DNA in vertebrates is present on simple nucleosomal templates in vivo. Instead, most chromatin is present in undefined, highly compacted structures that remain available for the induction of developmental programs that specify cell fate and morphogenesis.At least three processes control the assembly and regulation of chromatin: DNA methylation (see ref. 2 for a review); histone modifications (see ref. 3 for a review); and ATP-dependent chromatin remodelling, which is the focus of this Review. ATP-dependent remodelling seems to be crucial for both the assembly of chromatin structures and their dissolution. About 30 genes encode the ATPase subunits of these complexes in mammals. With few exceptions, these ATPases seem to be genetically non-redundant, with mutation of the encoding genes often having severe effects on the early embryo or giving rise to maternaleffect phenotypes (in which the phenotype of the embryo reflects the genotype of the mother). Indeed, in many cases, the genes encoding the ATPases or their subunits are haploinsufficient (that is, one copy is insufficient for development), indicating that their role in specific processes is rate limiting. Despite their genetic non-redundancy, the various ATPases seem to have similar activities when studied in vitro: they all increase nucleosome mobility 4 . Therefore, it is clear that better in vitro assays are needed to tease apart their biological functions.Correspondence should be addressed to G.R.C. (crabtree@stanford.edu). Author Information Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests. NIH Public Access Author ManuscriptNature. Author manuscript; available in PMC 2011 March 18. NIH-PA Au...

Mammalian neural stem cells (NSCs) have the capacity to both self-renew and to generate all the neuronal and glial cell-types of the adult nervous system. Global chromatin changes accompany the transition from proliferating NSCs to committed neuronal lineages, but the mechanisms involved have been unclear. Using a proteomics approach, we show that a switch in subunit composition of neural, ATP-dependent SWI/SNF-like chromatin remodeling complexes accompanies this developmental transition. Proliferating neural stem and progenitor cells express complexes in which BAF45a, a Krüppel/PHD domain protein and the actin-related protein BAF53a are quantitatively associated with the SWI2/SNF2-like ATPases, Brg and Brm. As neural progenitors exit the cell cycle, these subunits are replaced by the homologous BAF45b, BAF45c, and BAF53b. BAF45a/53a subunits are necessary and sufficient for neural progenitor proliferation. Preventing the subunit switch impairs neuronal differentiation, indicating that this molecular event is essential for the transition from neural stem/progenitors to postmitotic neurons. More broadly, these studies suggest that SWI/SNF-like complexes in vertebrates achieve biological specificity by combinatorial assembly of their subunits.