Competence and competition: the challenge of becoming a long-lived plasma cell

Plasma cells provide humoral immunity. They have traditionally been viewed mainly as short-lived end-stage products of B-cell differentiation that deserve little interest. This view is changing, however, because we now know that plasma cells can survive for long periods in the appropriate survival niches and that they are an independent cellular component of immunological memory. Studies of the biology of plasma cells reveal a mechanism of intriguing simplicity and elegance that focuses memory provided by plasma cells on recently encountered pathogens while minimizing the 'fading' of memory for pathogens encountered in the distant past. This mechanism is based on competition for survival niches between newly generated plasmablasts and older plasma cells.     

Nucleosome and chromatin fiber dynamics

Chromosomal DNA is packaged into condensed nucleoprotein suprastructures, yet the DNA can be accessed as needed within this structural context. Recently, progress has been made concerning how the nucleosomal subunits of chromatin fibers are disassembled and reassembled in vitro and in vivo. At the level of the chromatin fiber, the conformational organization of condensed 30 nm secondary structures has been elucidated. A great deal of progress also has been made toward understanding how chromatin architectural proteins, such as MeCP2, MENT, polycomb and HP1alpha, assemble different specific types of secondary and tertiary chromatin structures. The emerging picture is that the inherent dynamics of nucleosomal assemblages at all structural levels are a key link between the condensed domains found in eukaryotic genomes and the functions that take place within them.     

Nucleosome assembly protein 1 exchanges histone H2A-H2B dimers and assists nucleosome sliding

Eukaryotic chromatin is highly dynamic and turns over rapidly even in the absence of DNA replication. Here we show that the acidic histone chaperone nucleosome assembly protein 1 (NAP-1) from yeast reversibly removes and replaces histone protein dimer H2A-H2B or histone variant dimers from assembled nucleosomes, resulting in active histone exchange. Transient removal of H2A-H2B dimers facilitates nucleosome sliding along the DNA to a thermodynamically favorable position. Histone exchange as well as nucleosome sliding is independent of ATP and relies on the presence of the C-terminal acidic domain of yeast NAP-1, even though this region is not required for histone binding and chromatin assembly. Our results suggest a novel role for NAP-1 (and perhaps other acidic histone chaperones) in mediating chromatin fluidity by incorporating histone variants and assisting nucleosome sliding. NAP-1 may function either untargeted (if acting alone) or may be targeted to specific regions within the genome through interactions with additional factors.     

Sequence-specific recognition of DNA in the nucleosome by pyrrole-imidazole polyamides

The ability of DNA-binding proteins to recognize their cognate sites in chromatin is restricted by the structure and dynamics of nucleosomal DNA, and by the translational and rotational positioning of the histone octamer. Here, we use six different pyrrole-imidazole polyamides as sequence-specific molecular probes for DNA accessibility in nucleosomes. We show that sites on nucleosomal DNA facing away from the histone octamer, or even partially facing the histone octamer, are fully accessible and that nucleosomes remain fully folded upon ligand binding. Polyamides only failed to bind where sites are completely blocked by interactions with the histone octamer. Removal of the amino-terminal tails of either histone H3 or histone H4 allowed these polyamides to bind. These results demonstrate that much of the DNA in the nucleosome is freely accessible for molecular recognition in the minor groove, and also support a role for the amino-terminal tails of H3 and H4 in modulating accessibility of nucleosomal DNA.     

H2A.Z alters the nucleosome surface to promote HP1alpha-mediated chromatin fiber folding

Controlling the degree of higher order chromatin folding is a key element in partitioning the metazoan genome into functionally distinct chromosomal domains. However, the mechanism of this fundamental process is poorly understood. Our recent studies suggested that the essential histone variant H2A.Z and the silencing protein HP1alpha may function together to establish a specialized conformation at constitutive heterochromatic domains. We demonstrate here that HP1alpha is a unique chromatin binding protein. It prefers to bind to condensed higher order chromatin structures and alters the chromatin-folding pathway in a novel way to locally compact individual chromatin fibers without crosslinking them. Strikingly, both of these features are enhanced by an altered nucleosomal surface created by H2A.Z (the acidic patch). This shows that the surface of the nucleosome can regulate the formation of distinct higher order chromatin structures mediated by an architectural chromatin binding protein.     

Effect of recombinant human erythropoietin on anterior pituitary function in patients on chronic hemodialysis.

In 7 patients with end stage renal failure, anterior pituitary function was tested by simultaneous application of maximally effective doses of the hypothalamic releasing peptides, corticotropin-releasing hormone, growth hormone-releasing hormone, thyrotropin-releasing hormone and gonadotropin-releasing hormone, and compared to 8 normal controls. In addition to the pituitary hormones, plasma cortisol, thyroxine and testosterone concentrations were measured. To test for possible effects of treatment with recombinant human erythropoietin (rhu-EPO), all patients with chronic renal failure were studied again after partial correction of anemia by treatment with erythropoietin. Before initiation of rhu-EPO treatment, plasma concentrations of follicle-stimulating hormone were significantly elevated and the thyroid-stimulating hormone and prolactin responses to thyrotropin-releasing hormone blunted when compared to normal controls. Treatment with rhu-EPO induced a significant increase in plasma ACTH and follicle-stimulating hormone concentrations. All other pituitary functions remained unchanged. Thus, the general improvement in well-being, working capacity and sexual activity cannot be attributed to hormonal changes.     

The right place at the right time: chaperoning core histone variants

Histone proteins dynamically regulate chromatin structure and epigenetic signaling to maintain cell homeostasis. These processes require controlled spatial and temporal deposition and eviction of histones by their dedicated chaperones. With the evolution of histone variants, a network of functionally specific histone chaperones has emerged. Molecular details of the determinants of chaperone specificity for different histone variants are only slowly being resolved. A complete understanding of these processes is essential to shed light on the genuine biological roles of histone variants, their chaperones, and their impact on chromatin dynamics.