Developmental regulation of T-cell receptor gene expression

In contrast to B cells or their antibody products, T lymphocytes have a dual specificity, for both the eliciting foreign antigen and for polymorphic determinants on cell surface glycoproteins encoded in the major histocompatibility complex (MHC restriction). The recent identification of T-cell receptor glycoproteins as well as the genes encoding T-cell receptor subunits will help to elucidate whether MHC proteins and foreign antigens are recognized by two T-cell receptors or by a single receptor. An important feature of MHC restriction is that it appears to be largely acquired by a differentiating T-cell population under the influence of MHC antigens expressed in the thymus, suggesting that precursor T cells are selected on the basis of their reactivity with MHC determinants expressed in the host thymus. To understand this process of 'thymus education', knowledge of the developmental regulation of T-cell receptor gene expression is necessary. Here we report that whereas messenger RNAs encoding the beta-and gamma-subunits are relatively abundant in immature thymocytes, alpha mRNA levels are very low. Interestingly, whereas alpha mRNA levels increase during further development and beta mRNA levels stay roughly constant, gamma mRNA falls to very low levels in mature T cells, suggesting a role for the gamma gene in T-cell differentiation.     

Sequence of a cDNA clone encoding human preproinsulin-like growth factor II

The insulin-like growth factors (IGF) I and II are single-chain serum proteins of 70 and 67 amino acids, respectively, which are synthesized by the liver and possibly other tissues. They are probably required for normal fetal and postnatal growth and development. They also stimulate the growth of cultured cells, possibly by controlling the progression through the G1 phase of the cell cycle. In contrast to IGF-II whose concentration does not vary during postnatal development, the serum levels of IGF-I increase several-fold to adult levels during puberty. The serum concentration of IGF-I is a sensitive monitor of growth hormone levels and is decreased in individuals with growth hormone deficiency and elevated in those with growth hormone-secreting tumours. As a first step in studying the biosynthesis of these proteins and elucidating their role(s) in normal development and in tumorigenesis, we have isolated and sequenced cDNAs prepared from adult human liver mRNA which encode the precursors to IGF-I and -II. We report here the sequence of a cDNA encoding a 180-amino acid protein which is the precursor to IGF-II.     

细胞骨架在植物细胞周期进程中的动态变化及其调控?

细胞周期,即细胞生长与分裂的周期,是生命得以世代繁衍而生生不息的基础.真核细胞有丝分裂周期进程调控的分子机制高度保守.其间,微管和微丝骨架进行有规律的动态变化,顺次组成各种细胞生长和分裂装置,主动参与细胞周期进程的调节.然而,高等植物细胞周期不同时相分别有着与动物细胞不完全相同的、独特的细胞骨架列阵.而这些列阵的产生和维持直接依赖于众多细胞骨架结合蛋白以及上游信号分子的调控.本文重点综述了植物细胞周期进程中微管和微丝骨架的动态变化规律以及参与植物细胞骨架动态和有丝分裂装置组装调控的细胞骨架结合蛋白的最新研究进展,同时对细胞骨架在植物细胞周期进程中研究进行总结和展望.     

Targets of the cyclin-dependent kinase Cdk1

The events of cell reproduction are governed by oscillations in the activities of cyclin-dependent kinases (Cdks). Cdks control the cell cycle by catalysing the transfer of phosphate from ATP to specific protein substrates. Despite their importance in cell-cycle control, few Cdk substrates have been identified. Here, we screened a budding yeast proteomic library for proteins that are directly phosphorylated by Cdk1 in whole-cell extracts. We identified about 200 Cdk1 substrates, several of which are phosphorylated in vivo in a Cdk1-dependent manner. The identities of these substrates reveal that Cdk1 employs a global regulatory strategy involving phosphorylation of other regulatory molecules as well as phosphorylation of the molecular machines that drive cell-cycle events. Detailed analysis of these substrates is likely to yield important insights into cell-cycle regulation.     

Two rat homologues of Drosophila achaete-scute specifically expressed in neuronal precursors

In vertebrates, the peripheral nervous system is embryologically derived from the neural crest. Although the earliest neural crest cells seem to be multipotent, the molecular mechanisms responsible for the restriction of these cells to different sublineages are not understood. We therefore searched for developmental control genes expressed in crest cells or their derivatives. One important class of regulatory molecules comprises proteins with common DNA-binding and dimerization domains, the basic helix-loop-helix (B-HLH) region. Members of this family include MyoD, a mammalian myogenic determination molecule, and proteins encoded by genes of the achaete-scute complex of Drosophila, which have an important role in neuronal determination. From a sympathetic neuronal precursor cell line derived from the neural crest we have now isolated two different mammalian genes that are homologous to genes of the achaete-scute complex. The sequence of the B-HLH-encoding region of these genes is more similar to that of the genes of the achaete-scute complex than it is to that of other, mammalian members of the B-HLH family. At least one of these genes is transiently expressed in the embryonic rat nervous system, is not detected in non-neuronal tissues or cell lines, and is induced by nerve growth factor in PC12 cells.     

Proteomic analysis of a toxic dinoflagellate Alexandrium catenella under different growth phases and conditions

Alexandrium is a widely spread dinoflagellate genus throughout many regions of the world,which not only causes the harmful algal blooms(HABs) but also results in the paralytic shellfish poisoning(PSP) throughout the world.This study compared protein profiles of A.catenella grown under different growth phases and conditions using a proteomic approach,and identified the differentially expressed proteins.The results showed that the expressions of proteins identified in three different regions of the gels,the groups 1,2 and 3 proteins,varied significantly with the growth phases and conditions.Group 1 proteins and six Group 2 proteins were highly expressed at the initial,exponential and stationary growth phases,eight Group 2 proteins were highly expressed only at the initial phase,and Group 3 proteins were highly expressed at the exponential and/or stationary phases.However,all these proteins were expressed at low levels or were barely visible at the dissipation phase.The expressions of groups 1 and 2 proteins were low or barely visible in various growth conditions except in continuous darkness they were highly expressed.Group 3 proteins,on the other hand,were overexpressed in continuous illumination and expressed at low levels or barely visible in continuous darkness or under nitrate-starvation.The data from MALDI-TOF-TOF mass spectrometry demonstrated that these differentially expressed proteins were associated with macromolecular biosynthesis,photosynthesis,tRNA synthesis and DNA stability,stress response and cell division regulation.Synthetase was the major component of the altered proteins.This is one of the first comprehensive proteomic study of a dinoflagellate,A.catenella,that provides a fundamental understanding of the proteins involved in A.catenella growth and response to environmental stresses,and potential physiological indicator proteins related to growth and environmental stress have been identified.     

Identification of classical minor histocompatibility antigen as cell-derived peptide.

Histocompatibility antigens expressed on tissue grafted between individuals are recognized by host T cells, which reject the graft. The major histocompatibility complex (MHC) antigens have been identified on the molecular level, whereas the molecules representing the remaining ones, the minor histocompatibility antigens, are unknown, apart from some exceptions. The cytotoxic T lymphocyte (CTL) response against minor histocompatibility antigens shares many aspects with that against virus-infected cells. Virus-specific CTL recognize peptides derived from viral proteins produced in the infected cell. These peptides are presented by MHC class I molecules, as indicated by functional and crystallographic data. By analogy, minor histocompatibility antigens have been postulated to be peptides derived from normal cellular proteins presented by MHC class I molecules. Here we report that peptides derived from normal cellular proteins can indeed be recognized by CTL raised in the classical minor histoincompatible mouse strain combination, C57BL/6 against BALB.B. Thus, we have proven the above postulate, and isolated one of the minor histocompatibility molecules elusive for several decades.     

Temporal identity in axonal target layer recognition

The segregation of axon and dendrite projections into distinct synaptic layers is a fundamental principle of nervous system organization and the structural basis for information processing in the brain. Layer-specific recognition molecules that allow projecting neurons to stabilize transient contacts and initiate synaptogenesis have been identified. However, most of the neuronal cell-surface molecules critical for layer organization are expressed broadly in the developing nervous system, raising the question of how these so-called permissive adhesion molecules support synaptic specificity. Here we show that the temporal expression dynamics of the zinc-finger protein sequoia is the major determinant of Drosophila photoreceptor connectivity into distinct synaptic layers. Neighbouring R8 and R7 photoreceptors show consecutive peaks of elevated sequoia expression, which correspond to their sequential target-layer innervation. Loss of sequoia in R7 leads to a projection switch into the R8 recipient layer, whereas a prolonged expression in R8 induces a redirection of their axons into the R7 layer. The sequoia-induced axon targeting is mediated through the ubiquitously expressed Cadherin-N cell adhesion molecule. Our data support a model in which recognition specificity during synaptic layer formation is generated through a temporally restricted axonal competence to respond to broadly expressed adhesion molecules. Because developing neurons innervating the same target area often project in a distinct, birth-order-dependent sequence, temporal identity seems to contain crucial information in generating not only cell type diversity during neuronal division but also connection diversity of projecting neurons.     

Reversible inhibition of translation by Xenopus oocyte-specific proteins

A characteristic of growing oocytes of all animal species is the synthesis and accumulation of messenger RNA which is destined to be used primarily by the early embryo. The mechanism(s) which regulates the translation of this maternal mRNA remains unknown. However, the inability of the oocyte to translate all of its putative mRNA has been attributed to at least three limitations: (1) The rate of translation is limited by the availability of components of the translational apparatus other than mRNA, (2) the structural organization of the mRNA prevents translation, and (3) proteins associated with the mRNA prevent translation. Several investigators have suggested that proteins associated with maternal mRNA suppress translation in sea urchin eggs, although others claim that such results may be due to experimental artefacts. Oocyte-specific proteins have been identified in association with non-translating poly(A)+ mRNAs from Xenopus laevis oocytes, and we report here that when these proteins are reconstituted with mRNAs in vitro the translation of the mRNAs in vitro is reversibly repressed. The implication is that these proteins are involved in the regulation of translation of stored maternal mRNAs.     

Glial cells express N-CAM/D2-CAM-like polypeptides in vitro

The joining together of neurites to form fascicles and the growth of axons along glial surfaces during early development suggest that neurone-neurone and neurone-glial adhesion interactions are of considerable importance for defining nerve tracts. In vitro studies have indicated that adhesion between neurones involves a glycoprotein that has been independently studied under the names of N-CAM (for neural cell adhesion molecule), D2-CAM and BSP-2 (refs 10, 11). As N-CAM/D2-CAM appears to be a homophilic ligand that binds to N-CAM/D2-CAM polypeptide on adjacent cells, this glycoprotein is potentially important in adhesion interactions between any two N-CAM/D2-CAM-expressing cells. While it has been suggested that neurone-glial adhesion involves molecules other than N-CAM/D2-CAM, it is known that N-CAM/D2-CAM antigenic determinants are expressed by glial cells in vivo and that injection of anti-N-CAM antibodies into the eye-cup of chick embryos disrupts normal patterns of neuritic apposition to glial endfeet in the developing optic stalk. Do the molecules expressed by glia share restricted antigenic determinants, or binding domains, with N-CAM/D2-CAM, or are N-CAM/D2-CAM polypeptides expressed by glia? Here we present immunocytochemical evidence which suggests that all classes of macroglia express N-CAM/D2-CAM antigenic determinants on their surfaces and immunochemical analyses which indicate that the molecules expressed by purified astrocytes are closely similar, or identical, to at least some forms of N-CAM/D2-CAM obtained from whole brain or purified neurones. However, our results also suggest that different N-CAM/D2-CAM polypeptides may be separately expressed by neurones and astrocytes.