The centrosome in cells and organisms

The centrosome acts as the main microtubule-nucleating organelle in animal cells and plays a critical role in mitotic spindle orientation and in genome stability. Yet, despite its central role in cell biology, the centrosome is not present in all multicellular organisms or in all cells of a given organism. The main outcome of centrosome reproduction is the transmission of polarity to daughter cells and, in most animal species, the sperm-donated centrosome defines embryo polarity. Here I will discuss the role of the centrosome in cell polarity, resulting from its ability to position the nucleus at the cell center, and discuss how centrosome innovation might have been critical during metazoan evolution.     

东北马鹿脾脏T、B淋巴细胞及真DNA和RNA分布规律的研究

用核酸的组织化学方法，对东北马鹿脾脏Ｔ、Ｂ淋巴细胞及其ＤＮＡ和ＲＮＡ的分布进行研究。结果表明：东北马鹿脾脏Ｂ－淋巴细胞主要分布在脾索中；Ｔ－淋巴细胞分布在脾小结外周、动脉周围淋巴鞘、边缘区和椭球内。ＤＮＡ存在于Ｔ、Ｂ淋巴细胞核内．ＲＮＡ存在于胞浆中。东北马鹿脾脏白髓不发达，脾小结未见生发中心。红髓发达，脾窦宽大。椭球数量较少，但体积较大。脾小梁丰富。     

Autoimmune disease and impaired uptake of apoptotic cells in MFG-E8-deficient mice

Apoptotic cells expose phosphatidylserine and are swiftly engulfed by macrophages. Milk fat globule epidermal growth factor (EGF) factor 8 (MFG-E8) is a protein that binds to apoptotic cells by recognizing phosphatidylserine and that enhances the engulfment of apoptotic cells by macrophages. We report that tingible body macrophages in the germinal centers of the spleen and lymph nodes strongly express MFG-E8. Many apoptotic lymphocytes were found on the MFG-E8-/- tingible body macrophages, but they were not efficiently engulfed. The MFG-E8-/- mice developed splenomegaly, with the formation of numerous germinal centers, and suffered from glomerulonephritis as a result of autoantibody production. These data demonstrate that MFG-E8 has a critical role in removing apoptotic B cells in the germinal centers and that its failure can lead to autoimmune diseases.     

The polarity protein Par-3 directly interacts with p75NTR to regulate myelination

Cell polarity is critical in various cellular processes ranging from cell migration to asymmetric cell division and axon and dendrite specification. Similarly, myelination by Schwann cells is polarized, but the mechanisms involved remain unclear. Here, we show that the polarity protein Par-3 localizes asymmetrically in Schwann cells at the axon-glial junction and that disruption of Par-3 localization, by overexpression and knockdown, inhibits myelination. Additionally, we show that Par-3 directly associates and recruits the p75 neurotrophin receptor to the axon-glial junction, forming a complex necessary for myelination. Together, these results point to a critical role in the establishment of cell polarity for myelination.     

Asymmetric inheritance of mother versus daughter centrosome in stem cell division

Adult stem cells often divide asymmetrically to produce one self-renewed stem cell and one differentiating cell, thus maintaining both populations. The asymmetric outcome of stem cell divisions can be specified by an oriented spindle and local self-renewal signals from the stem cell niche. Here we show that developmentally programmed asymmetric behavior and inheritance of mother and daughter centrosomes underlies the stereotyped spindle orientation and asymmetric outcome of stem cell divisions in the Drosophila male germ line. The mother centrosome remains anchored near the niche while the daughter centrosome migrates to the opposite side of the cell before spindle formation.     

Imaging of germinal center selection events during affinity maturation

The germinal center (GC) is an important site for the generation and selection of B cells bearing high-affinity antibodies, yet GC cell migration and interaction dynamics have not been directly observed. Using two-photon microscopy of mouse lymph nodes, we revealed that GC B cells are highly motile and extend long cell processes. They transited between GC dark and light zones and divided in both regions, although these B cells resided for only several hours in the light zone where antigen is displayed. GC B cells formed few stable contacts with GC T cells despite frequent encounters, and T cells were seen to carry dead B cell blebs. On the basis of these observations, we propose a model in which competition for T cell help plays a more dominant role in the selection of GC B cells than previously appreciated.     

Nanos maintains germline stem cell self-renewal by preventing differentiation

Despite much progress in understanding how extrinsic signaling regulates stem cell self-renewal, little is known about how cell-autonomous gene regulation controls this process. In Drosophila ovaries, germline stem cells (GSCs) divide asymmetrically to produce daughter GSCs and cystoblasts, the latter of which develop into germline cysts. Here, we show that removing the translational repressor Nanos from either GSCs or their precursors, the primordial germ cells (PGCs), causes both cell types to differentiate into germline cysts. Thus, Nanos is essential for both establishing and maintaining GSCs by preventing their precocious entry into oogenesis. These functions are likely achieved by repressing the translation of differentiation factors in PGCs and GSCs.     

Asymmetry and aging of mycobacterial cells lead to variable growth and antibiotic susceptibility

Cells use both deterministic and stochastic mechanisms to generate cell-to-cell heterogeneity, which enables the population to better withstand environmental stress. Here we show that, within a clonal population of mycobacteria, there is deterministic heterogeneity in elongation rate that arises because mycobacteria grow in an unusual, unipolar fashion. Division of the asymmetrically growing mother cell gives rise to daughter cells that differ in elongation rate and size. Because the mycobacterial cell division cycle is governed by time, not cell size, rapidly elongating cells do not divide more frequently than slowly elongating cells. The physiologically distinct subpopulations of cells that arise through asymmetric growth and division are differentially susceptible to clinically important classes of antibiotics.     

Ethylene modulates stem cell division in the Arabidopsis thaliana root

The construction of multicellular organisms depends on stem cells-cells that can both regenerate and produce daughter cells that undergo differentiation. Here, we show that the gaseous messenger ethylene modulates cell division in the cells of the quiescent center, which act as a source of stem cells in the seedling root. The cells formed through these ethylene-induced divisions express quiescent center-specific genes and can repress differentiation of surrounding initial cells, showing that quiescence is not required for these cells to signal to adjacent stem cells. We propose that ethylene is part of a signaling pathway that modulates cell division in the quiescent center in the stem cell niche during the postembryonic development of the root system.     

Critical roles of activation-induced cytidine deaminase in the homeostasis of gut flora

Activation-induced cytidine deaminase (AID) plays an essential role in class switch recombination (CSR) and somatic hypermutation (SHM) of immunoglobulin genes. We report here that deficiency in AID results in the development of hyperplasia of isolated lymphoid follicles (ILFs) associated with a 100-fold expansion of anaerobic flora in the small intestine. Reduction of bacterial flora by antibiotic treatment of AID-/- mice abolished ILF hyperplasia as well as the germinal center enlargement seen in secondary lymphoid tissues. Because an inability to switch to immunoglobulin A on its own does not lead to a similar phenotype, these results suggest that SHM of ILF B cells plays a critical role in regulating intestinal microflora.     

Phospholipid methylation and biological signal transmission

Many types of cells methylate phospholipids using two methyltransferase enzymes that are asymmetrically distributed in membranes. As the phospholipids are successively methylated, they are translocated from the inside to the outside of the membrane. When catecholamine neurotransmitters, lectins, immunoglobulins or chemotaxic peptides bind to the cell surface, they stimulate the methyltransferase enzymes and reduce membrane viscosity. The methylation of phospholipids is coupled to Ca2+ influx and the release of arachidonic acid, lysophosphatidylcholine, and prostaglandins. These closely associated biochemical changes facilitate the transmission of many signals through membranes, resulting in the generation of adenosine 3',5'-monophophate in many cell types, release of histamine in mast cells and basophils, mitogenesis in lymphocytes, and chemotaxis in neutrophils.     

Human lymphocytes making rheumatoid factor and antibody to ssDNA belong to Leu-1+ B-cell subset

B lymphocytes bearing the Leu-1 cell-surface antigen (Leu-1+), the human equivalent of mouse Ly-1+ B lymphocytes, have been detected in human peripheral blood, but there is little information on their frequency and properties. Analysis by fluorescence-activated cell sorter and double immunofluorescence showed that Leu-1+ B cells are consistently present in the peripheral blood and spleens of healthy subjects and constitute 17.0 +/- 5.0% (mean value +/- standard deviation) and 17.3 +/- 3.9%, respectively, of total B cells. When purified Leu-1+ and Leu-1- B lymphocytes were transformed into immunoglobulin-secreting cells by infection with Epstein-Barr virus and the culture fluids were tested for reactivity with self-antigens, at least two important autoantibodies, antibody to the Fc fragment of human immunoglobulin G (rheumatoid factor) and antibody to single-stranded DNA, were found to be made exclusively by Leu-1+ B cells. It is concluded that the Leu-1+ lymphocytes represent a major subset of the normal human B cell repertoire and include the B cells capable of making autoantibodies similar to those found in systemic lupus erythematosus and rheumatoid arthritis.     

Immunology. Long live the mature B cell--a baffling mystery resolved

What determines whether transitional B cells newly emerged from the bone marrow will differentiate further to become mature, long-lived, circulating B lymphocytes? In a Perspective, Waldschmidt and Noelle discuss new findings showing that the TNF family ligand BAFF and its receptor BAFF-R are crucial for selecting transitional B cells into the mature B cell pool (Thompson et al., Schiemann et al.).     

球虫感染后兔圆小囊病理组织学变化

为研究圆小囊在家兔球虫感染后的病理变化,试验收集自然感染病例的圆小囊,按HE染色石蜡切片方法制作病理组织切片,观察自然感染球虫后家兔圆小囊的组织病理学变化,并与正常圆小囊组织进行比较。结果表明,球虫感染病例的圆小囊集合淋巴小结生发中心反应活跃,粘膜杯状细胞和固有层散在淋巴细胞明显增多,淋巴小结周边及中心多见吞噬细胞成团聚集,或形成大的多核巨细胞,而集合淋巴小结结构基本完整.该研究结果说明圆小囊的局部免疫机制可能在机体抗球虫免疫中起着重要作用。     

1例转基因克隆牛脑和免疫器官组织形态观察报告

【目的】对1例新生死亡的转基因克隆牛的脑和免疫器官进行观察,分析和评价克隆牛脑和免疫器官的发育状况。【方法】采用病理解剖、组织学常规石蜡切片-HE染色技术,对转基因克隆牛的脑部和免疫器官的组织结构进行系统观察与分析。【结果】该克隆牛体格偏大,头颅骨质较一般犊牛坚硬,免疫器官外观无明显变化。镜下观察显示,大脑皮质层次清晰,有轻度充血和1处明显的点状出血;小脑皮质可分为明显的3层结构,蛛网膜有轻度充血;脾脏红髓轻度淤血并有散在含铁血黄素沉着;白髓中央动脉周围淋巴鞘清晰可见,脾小体数量较多,但脾小体和动脉周围淋巴鞘内淋巴细胞数量较少;淋巴结被膜薄,小梁不发达,皮质淋巴细胞多而密集,但典型的具有生发中心的淋巴小结较少;髓索不明显,髓质部毛细血管扩张充血;胸腺被膜和小梁上血管可见充血现象;皮质部淋巴细胞丰富,可见散布的胸腺细胞;髓质淋巴细胞密集,细胞间可见数量较多的巨噬细胞;嗜酸性胸腺小体易于辨认,但发育不完全。【结论】该例克隆牛脑发育较为成熟,但免疫器官发育不均衡。     

Cell cycle dependence of chloride permeability in normal and cystic fibrosis lymphocytes

Cystic fibrosis (CF) is a genetic disease characterized by abnormal regulation of epithelial cell chloride channels. Nonepithelial cells, including lymphocytes and fibroblasts, may exhibit a similar defect. Two independent techniques were used to assess the macroscopic chloride permeability (PCl) of freshly isolated B lymphocytes and of B and T lymphocyte cell lines. Values for PCl increased specifically during the G1 phase of the cell cycle and could be further enhanced by increasing intracellular adenosine 3',5'-monophosphate (cAMP) or calcium. In lymphocytes from CF patients, regulation of PCl during the cell cycle and by second messengers was absent. Characterization of the cell cycle-dependent expression of the chloride permeability defect in lymphocytes from CF patients increases the utility of these cells in the analysis of the functional consequences of mutations in the CF gene.     

Mcl-1 is essential for germinal center formation and B cell memory

Lymphocyte survival during immune responses is controlled by the relative expression of pro- and anti-apoptotic molecules, regulating the magnitude, quality, and duration of the response. We investigated the consequences of deleting genes encoding the anti-apoptotic molecules Mcl1 and Bcl2l1 (Bcl-x(L)) from B cells using an inducible system synchronized with expression of activation-induced cytidine deaminase (Aicda) after immunization. This revealed Mcl1 and not Bcl2l1 to be indispensable for the formation and persistence of germinal centers (GCs). Limiting Mcl1 expression reduced the magnitude of the GC response with an equivalent, but not greater, effect on memory B cell formation and no effect on persistence. Our results identify Mcl1 as the main anti-apoptotic regulator of activated B cell survival and suggest distinct mechanisms controlling survival of GC and memory B cells.