Neurons of the peripheral nervous system have long been known to require survival factors to prevent their death during development. But why they selectively become dependent on secretory molecules has remained a mystery, as is the observation that in the central nervous system, most neurons do not show this dependency. Using engineered embryonic stem cells, we show here that the neurotrophin receptors TrkA and TrkC (tropomyosin receptor kinase A and C, also known as Ntrk1 and Ntrk3, respectively) instruct developing neurons to die, both in vitro and in vivo. By contrast, TrkB (also known as Ntrk2), a closely related receptor primarily expressed in the central nervous system, does not. These results indicate that TrkA and TrkC behave as dependence receptors, explaining why developing sympathetic and sensory neurons become trophic-factor-dependent for survival. We suggest that the expansion of the Trk gene family that accompanied the segregation of the peripheral from the central nervous system generated a novel mechanism of cell number control.

ATP-dependent chromatin remodeling activities function to manipulate chromatin structure during gene regulation. One of the ways in which they do this is by altering the positions of nucleosomes along DNA. Here we provide support for the ability of these complexes to move nucleosomes into positions in which DNA is unraveled from one edge. This is expected to result in the loss of histone-DNA contacts that are important for retention of one H2A/H2B dimer within the nucleosome. Consistent with this we find that several chromatin remodeling complexes are capable of catalyzing the exchange of H2A/H2B dimers between chromatin fragments in an ATP-dependent reaction. This provides eukaryotes with additional means by which they may manipulate chromatin structure.

Kapinos et al. show that nuclear pore complex permeability and cargo release functionalities are concomitantly regulated by karyopherin occupancy and turnover in a systematic continuum. This highlights increasingly important roles for the soluble nucleocytoplasmic transport machinery that depart from established views of the nuclear pore complex selectivity mechanism.

Previous studies have identified sin mutations that alleviate the requirement for the yeast SWI/SNF chromatin remodelling complex, which include point changes in the yeast genes encoding core histones. Here we characterise the biochemical properties of nucleosomes bearing these mutations. We find that sin mutant nucleosomes have a high inherent thermal mobility. As the SWI/SNF complex can alter nucleosome positioning, the higher mobility of sin mutant nucleosomes provides a means by which sin mutations may substitute for SWI/SNF function. The location of sin mutations also provides a new opportunity for insights into the mechanism for nucleosome mobilisation. We find that both mutations altering histone DNA contacts at the nucleosome dyad and mutations in the dimertetramer interface influence nucleosome mobility. Furthermore, incorporation of H2A.Z into nucleosomes, which also alters dimer-tetramer interactions, affects nucleosome mobility. Thus, variation of histone sequence or subtype provides a means by which eukaryotes may regulate access to chromatin through alterations to nucleosome mobility. The EMBO Journal (2004) IntroductionNucleosomes are the universal molecular packaging state of DNA in nuclei. They are responsible for compacting eukaryotic genomes to allow them to fit into the limited volume of the cell nucleus. The nucleosome core particle and an additional variable length of unbound linker DNA together comprise the fundamental repeating unit of chromatin (Kornberg, 1974). The nucleosome core particle consists of a core of eight polypeptides, two copies each of the four highly conserved histone proteins H2A, H2B, H3 and H4, around which 147 bp of DNA are wrapped in 1.7 superhelical turns (Luger et al, 1997). H3 and H4 associate to form histone fold dimers as do histones H2A and H2B. Each histone fold dimer associates to form an octameric spiral with dyad symmetry.The H2AÀH2B histone fold dimer units are less stably associated within the octamer than the two H3ÀH4 histone fold dimers (Eickbush and Moudrianakis, 1978). A number of extra protein elements decorate the regular spiral of histone fold dimers, including unstructured 'tails' that extend outside the DNA superhelix, the additional H3 aN helix that organises the most exterior turn of bound DNA, a structured H2A C-terminal extension that passes over the top face of the histone octamer, and an H2B aC helix lying above the histone dimer (Luger and Richmond, 1998).A consequence of the organisation of DNA into nucleosomes is that all genetic processes must contend with a chromatin substrate. In the case of gene regulation, wrapping into nucleosomes makes DNA sequences differentially accessible to transcription factors (Owen-Hughes and Workman, 1994). Accessibility of a particular site depends on the absolute 'position' of the nucleosome (Polach and Widom, 1995), and many cases have been recognised in which nucleosome positioning affects genomic accessibility (Simpson, 1990;Lomvardas and Thanos, 2002;Miller and Widom, 2003). In this way, nucleoso...

Nuclear pore complexes (NPCs) discriminate nonspecific macromolecules from importin and exportin receptors, collectively termed “karyopherins” (Kaps), that mediate nucleocytoplasmic transport. This selective barrier function is attributed to the behavior of intrinsically disordered phenylalanine-glycine nucleoporins (FG Nups) that guard the NPC channel. However, NPCs in vivo are typically enriched with different Kaps, and how they impact the NPC barrier remains unknown. Here, we show that two major Kaps, importinβ1/karyopherinβ1 (Kapβ1) and exportin 1/chromosomal maintenance 1 (CRM1), are required to fortify NPC barrier function in vivo. Their enrichment at the NPC is sustained by promiscuous binding interactions with the FG Nups, which enable CRM1 to compensate for the loss of Kapβ1 as a means to maintain NPC barrier function. However, such a compensatory mechanism is constrained by the cellular abundances and different binding kinetics for each respective Kap, as evidenced for importin-5. Consequently, we find that NPC malfunction and nucleocytoplasmic leakage result from poor Kap enrichment.