Protein phosphatases in chromatin structure and function

Chromatin structure and dynamics are highly controlled and regulated processes that play an essential role in many aspects of cell biology. The chromatin transition stages and the factors that control this process are regulated by post-translation modifications, including phosphorylation. While the role of protein kinases in chromatin dynamics has been quite well studied, the nature and regulation of the counteracting phosphatases represent an emerging field but are still at their infancy. In this review we summarize the current literature on phosphatases involved in the regulation of chromatin structure and dynamics, with emphases on the major knowledge gaps that should require attention and more investigation.     

Large-scale chromatin structure and function.

Recent results in living cells have now established the existence of levels of chromatin folding above the 30 nm fiber within interphase chromosomes. We discuss the potential functional impact of this large-scale chromatin organization, including its possible role in regulating gene expression.     

ATP依赖的染色质改构复合物及其作用机制

染色质高度紧密的折叠阻止了转录因子和辅因子与DNA的结合, 因而通过染色质重塑以解除这样的抑制环境, 对于转录活动的正常进行是至关重要的。目前认为, 染色质重塑至少是通过两种机制来完成的, 一种是通过ATP依赖的染色质改构复合物, 另一种是通过对组蛋白尾部进行共价修饰的组蛋白修饰酶复合物。文章结合近年来的研究进展, 对前者进行染色质重塑的机制及两者在基因转录调控过程中如何相互协作等进行了论述。     

Hypoxia-induced changes in neuronal network properties

Because of their high energetic demand, neurons within the mammalian central nervous system are extremely sensitive to changes in partial pressure of oxygen. Faced with acute hypoxic conditions, an organism must follow a complex and highly dynamic emergency plan to secure survival. Behavioral functions that are not immediately essential for survival are turned off, and critical behaviors (such as breathing) undergo a biphasic response. An augmentation of breathing is initially adaptive, whereas prolonged hypoxic conditions are better served by an energy-saving mode. However, the hypoxic response of an organism depends on many additional factors. Environmental conditions, the animal's age and health, and the pattern (continuous vs intermittent) and duration (chronic vs acute) of hypoxia greatly determine the specific course of a hypoxic response. Different forms of hypoxia can cause pathology or be used as therapy. Therefore, it is not surprising that the hypoxic response of an organism results from widespread and highly diverse reconfigurations of neuronal network functions in different brain areas that are accomplished by diverse hypoxic changes at all levels of the nervous system (i.e., molecular, cellular, synaptic, neuronal, network). Hypoxia-induced changes in synaptic transmission are generally depressive and result primarily from presynaptic mechanisms, whereas changes in intrinsic properties involve excitatory and inhibitory alterations involving the majority of K+, Na+, and Ca2+ channels. This article reviews the response of the nervous system to hypoxia, accounting for all levels of integration from the cellular to the network level, and postulates that a better understanding of the diversity of cellular events is only possible if cellular and network events are considered in a functional and organismal context.     

Hypoxia-induced changes in shivering and body temperature

Experiments were carried out on conscious cats to evaluate the general characteristics and modes of action of hypoxia on thermoregulation during cold stress. Intact and carotid-denervated (CD) conscious cats were exposed to ambient hypoxia (low inspired O2 fraction) or CO hypoxia in prevailing laboratory (23-25 degrees C) or cold (5-8 degrees C) environments. In the cold, both groups promptly decreased shivering and body temperature when exposed to either type of hypoxia. Small increases in CO2 concentration reinstituted shivering in both groups. At the same inspired concentration of O2, CD animals decreased shivering and body temperature more than intact cats. While this difference resulted, in part, from a lower alveolar PO2 in CD cats, a difference between intact and CD cats was apparent when the two groups were compared at the same alveolar PO2. During more prolonged hypoxia (45 min), shivering returned but did not reach normoxic levels, and body temperature tended to stabilize at a hypothermic value. Exposure to various levels of hypoxia produced graded suppression of shivering, with the result that the change in body temperature varied directly with inspired O2 concentration. Hypoxia appears to act on the central nervous system to suppress shivering and sinus nerve afferents appear to counteract this direct effect of hypoxia. In intact cats, this counteraction appears to be sufficient to maintain body temperature under hypoxic conditions at room temperature but not in the cold.     

Role of histone modifications in defining chromatin structure and function

Chromosomes in eukaryotic cell nuclei are not uniformly organized, but rather contain distinct chromatin elements, with each state having a defined biochemical structure and biological function. These are recognizable by their distinct architectures and molecular components, which can change in response to cellular stimuli or metabolic requirements. Chromatin elements are characterized by the fundamental histone and DNA components, as well as other associated non-histone proteins and factors. Post-translational modifications of histone proteins in particular often correlate with a specific chromatin structure and function. Patterns of histone modifications are implicated as having a role in directing the level of chromatin compaction, as well as playing roles in multiple functional pathways directing the readout of distinct regions of the genome. We review the properties of various chromatin elements and the apparent links of histone modifications with chromatin organization and functional output.     

Capacitation, acrosome function and chromatin structure in stallion sperm

In general, fertility in breeding stallions is lower and more variable than in the other farm animal species, primarily because selection is based on pedigree, looks and/or athletic performance, with little consideration of fertility or fertility potential. Moreover, because the average stallion breeds only a limited number of mares per year and in-field fertility is influenced significantly by non-stallion factors such as management and mare fertility, meaningful fertility data are hard to come-by. Unfortunately, generating usable figures would involve impractically high costs, time and numbers of mares. Instead, a breeding soundness examination (BSE), based on assessments of sperm number, motility and morphological normality and of mating ability, is often carried out with the ostensible aim of identifying animals with the "potential for good fertility". In fact, the BSE generally succeeds only in ruling out those stallions with a very clear reason for sub-fertility, and still fails to identify some seriously sub-fertile animals. Thus, the routine BSE has very limited use as a predictor of subsequent fertility. This paper reviews assays developed for identifying capacitated, acrosome-reacted and DNA-damaged sperm, and assesses their utility for improving our ability to predict a stallion's fertility prior to the onset of his breeding career.     

Postirradiation changes in the chromatin structure of thymocytes in irradiated rats

In this work the antibodies were obtained against chromatin isolated from thymocytes of intact and irradiated rats (2 h after exposing to 10 Gy) and against polydeoxyribonucleotides (PDN) extracted from thymus nuclei 6 h following irradiation. All the antibodies under study reacted more readily with the chromatin obtained from the thymus of exposed rats than with the control chromatin. The complexes of DNA with the most firmly bound non-histone proteins, obtained from the three objects under study, reacted with the antibodies with equal efficiency. Thus, a higher reactivity of PDN and chromatin from thymocytes of exposed rats was associated with the decondensation of the latter leading to an increase in availability of a part of antigenic determinants. Using the immunoblotting method we failed to discover any qualitative differences in the protein composition of the chromatin from control and exposed rats.     

Ligand dependence of estrogen receptor induced changes in chromatin structure.

To determine whether the human estrogen receptor requires ligand to bind to its cognate estrogen receptor element (ERE) in vivo, we have examined the structure of chromatin at a chromosomally integrated ERE-URA3 reporter gene in yeast, and the influence of ligand bound and ligand free estrogen receptors on that structure. Using indirect end-labelling to map DNaseI and micrococcal nuclease sensitive sites, we found that receptor induced alterations in chromatin structure were completely dependent upon the presence of estradiol. These same alterations in chromatin structure were induced by a truncated estrogen receptor with both TAF-1 and TAF-2 transactivation functions deleted, suggesting that DNA binding per se disrupts chromatin structure. These results support models in which the estrogen receptor requires ligand to bind to the ERE in vivo.