Increasing evidence suggests functional compartmentalization of interphase nuclei. This includes preferential interior localization of gene-rich and early replicating chromosome regions versus peripheral localization of gene-poor and late replicating chromosome regions , association of some active genes with nuclear speckles or transcription "factories", and association of transcriptionally repressed genes with heterochromatic regions. Dynamic changes in chromosome compartmentalization imply mechanisms for long-range interphase chromatin movements. However, live cell imaging in mammalian cells has revealed limited chromatin mobility, described as "constrained diffusion". None of these studies, though, have examined a chromosome locus undergoing an inducible repositioning between two different nuclear compartments. Here we demonstrate migration of an interphase chromosome site from the nuclear periphery to the interior 1-2 hr after targeting a transcriptional activator to this site. Spot redistribution is perturbed by specific actin or nuclear myosin I mutants. Extended periods of chromosome immobility are interspersed with several minute periods in which chromosomes move unidirectionally along curvilinear paths oriented roughly perpendicular to the nuclear envelope at velocities of 0.1-0.9 microm/min over distances of 1-5 microm. Our results suggest an active mechanism for fast and directed long-range interphase chromosome movements dependent directly or indirectly on actin/myosin.

Actin is abundant in the nucleus and has been implicated in transcription; however, the nature of this involvement has not been established. Here we demonstrate that beta-actin is critically involved in transcription because antibodies directed against beta-actin, but not muscle actin, inhibited transcription in vivo and in vitro. Chromatin immunoprecipitation assays demonstrated the recruitment of actin to the promoter region of the interferon-gamma-inducible MHC2TA gene as well as the interferon-alpha-inducible G1P3 gene. Further investigation revealed that actin and RNA polymerase II co-localize in vivo and also co-purify. We employed an in vitro system with purified nuclear components to demonstrate that antibodies to beta-actin block the initiation of transcription. This assay also demonstrates that beta-actin stimulates transcription by RNA polymerase II. Finally, DNA-binding experiments established the presence of beta-actin in pre-initiation complexes and also showed that the depletion of actin prevented the formation of pre-initiation complexes. Together, these data suggest a fundamental role for actin in the initiation of transcription by RNA polymerase II.

Gpm6a was identified as a stress-responsive gene in the hippocampal formation. This gene is down-regulated in the hippocampus of both socially and physically stressed animals, and this effect can be reversed by antidepressant treatment. Previously we showed that the stress-regulated protein M6a is a key modulator for neurite outgrowth and filopodium/ spine formation. In the present work, mutational analysis was used to characterize the action of M6a at the molecular level. Glycoprotein M6a is a neuronally expressed member of the proteolipid protein (PLP/DM20) family (1) whose gene has been identified as a stress-responsive gene in the hippocampal formation. In several animal models of chronic stress, expression levels for M6a in hippocampal tissue were found to be diminished by chronic stress exposure and this effect was counteracted by treatment with antidepressants (2-3). Recently, an association of the M6A gene with the subgroup of schizophrenia patients with high levels of depression has been reported (4). These findings suggest that M6a plays a role in the stress-induced hippocampal alterations that are found in psychiatric disorders in general. M6a is prominently expressed in the central nervous system as early as embryonic day 10 and remains detectable in adulthood (5). Originally, it was identified as an antigen reacting with the monoclonal M6 antibody and its role as a modulator of neurite outgrowth was postulated (6). Lagenaur et al. (6) demonstrated that IgG or Fab fragments of M6 antibody interfere with the extension of neurites by cultured cerebellar neurons. A recent study by Zhao et al. (7) shows that M6a expressed in the murine neural retina also regulates neurite extension. The neurite outgrowth of M6a-overexpressing retinal cells was strikingly enhanced, although M6a did not affect differentiation and proliferation.Even though the precise biological function of M6a still remains unclear, there is a growing body of evidence indicating the importance of M6a in the processes of neural development such as neurite extension and differentiation. For example, a study by Mukobata et al. (8) reported that M6a expression enhances nerve growth factor-primed neurite extension in rat pheochromocytoma PC12 cells. They show that it also induces an increase in the intracellular Ca 2ϩ concentration of PC12 cells and that the anti-M6a antibody efficiently interferes with both nerve growth factor-triggered Ca 2ϩ influx and neurite extension (8). Next, inhibition of mouse M6a expression was found to lead to decreased differentiation of neurons derived from mouse embryonic stem cells (9). Furthermore, it has been demonstrated that M6a plays an important role in neurite/ filopodium outgrowth and synapse formation (10). This study shows that M6a overexpression induces neurite formation and increases filopodia density in hippocampal neurons. M6a knockdown with small interference RNA methodology showed that M6a low expressing neurons display decreased filopodia number and a lower density of synaptophysin clusters. Th...