Thrombospondins are astrocyte-secreted proteins that promote CNS synaptogenesis

The establishment of neural circuitry requires vast numbers of synapses to be generated during a specific window of brain development, but it is not known why the developing mammalian brain has a much greater capacity to generate new synapses than the adult brain. Here we report that immature but not mature astrocytes express thrombospondins (TSPs)-1 and -2 and that these TSPs promote CNS synaptogenesis in vitro and in vivo. TSPs induce ultrastructurally normal synapses that are presynaptically active but postsynaptically silent and work in concert with other, as yet unidentified, astrocyte-derived signals to produce functional synapses. These studies identify TSPs as CNS synaptogenic proteins, provide evidence that astrocytes are important contributors to synaptogenesis within the developing CNS, and suggest that TSP-1 and -2 act as a permissive switch that times CNS synaptogenesis by enabling neuronal molecules to assemble into synapses within a specific window of CNS development.     

Neurotrophin signaling among neurons and glia during formation of tripartite synapses

Synapse formation in the CNS is a complex process that involves the dynamic interplay of numerous signals exchanged between pre- and postsynaptic neurons as well as perisynaptic glia. Members of the neurotrophin family, which are widely expressed in the developing and mature CNS and are well-known for their roles in promoting neuronal survival and differentiation, have emerged as key synaptic modulators. However, the mechanisms by which neurotrophins modulate synapse formation and function are poorly understood. Here, we summarize our work on the role of neurotrophins in synaptogenesis in the CNS, in particular the role of these signaling molecules and their receptors, the Trks, in the development of excitatory and inhibitory hippocampal synapses. We discuss our results that demonstrate that postsynaptic TrkB signaling plays an important role in modulating the formation and maintenance of NMDA and GABAA receptor clusters at central synapses, and suggest that neurotrophin signaling coordinately modulates these receptors as part of mechanism that promotes the balance between excitation and inhibition in developing circuits. We also discuss our results that demonstrate that astrocytes promote the formation of GABAergic synapses in vitro by differentially regulating the development of inhibitory presynaptic terminals and postsynaptic GABAA receptor clusters, and suggest that glial modulation of inhibitory synaptogenesis is mediated by neurotrophin-dependent and -independent signaling. Together, these findings extend our understanding of how neuron-glia communication modulates synapse formation, maintenance and function, and set the stage for defining the cellular and molecular mechanisms by which neurotrophins and other cell-cell signals direct synaptogenesis in the developing brain.     

Thrombospondins and tumor angiogenesis

The thrombospondins (TSPs) are a family of five secreted proteins that are widely distributed in the extracellular matrix of numerous tissues. TSPs are multimodular and each domain specifies a distinct biological function through interaction with a specific receptor. TSP1 and TSP2 have anti-angiogenic activity, which, at least for TSP1, involves interaction with the microvascular endothelial cell receptor CD36. Expression of TSP1 and TSP2 is modulated by hypoxia and by oncogenes. In several tumors (thyroid, colon, bladder carcinomas), TSP1 expression is inversely correlated with tumor grade and survival rate, whereas in others (e.g. breast carcinomas), it is correlated with the stromal response and is of little prognostic value. Recent studies suggest that TSPs or TSP-derived peptides retaining biological activity could be developed into promising new therapeutic strategies for the anti-angiogenic treatment of solid tumors.     

Functions of the conserved thrombospondin carboxy-terminal cassette in cell-extracellular matrix interactions and signaling

Thrombospondins (TSPs) are extracellular, multidomain, calcium-binding glycoproteins that function at cell surfaces, in extracellular matrix (ECM) and as bridging molecules in cell-cell interactions. TSPs are multifunctional and modulate cell behavior during development, wound-healing, immune response, tumor growth and in the homeostasis of adult tissues. TSPs are assembled as oligomers that are composed of homologous polypeptides. In all the TSP polypeptides, the most highly-conserved region is the carboxyl-region, which contains a characteristic set of domains comprising EGF domains, TSP type 3 repeats and a globular carboxy-terminal domain. This large region is termed here the thrombospondin carboxy-terminal cassette (TSP-CTC). The strong conservation of the TSP-CTC suggests that it may mediate ancestral functions that are shared by all TSPs. This review summarizes the current knowledge of the TSP-CTC and areas of future interest.     

Invoking the power of thrombospondins: Regulation of thrombospondins expression

Increasing evidence suggests critical functions of thrombospondins (TSPs) in a variety of physiological and pathological processes. With the growing understanding of the importance of these matricellular proteins, the need to understand the mechanisms of regulation of their expression and potential approaches to modulate their levels is also increasing. The regulation of TSP expression is multi-leveled, cell- and tissue-specific, and very precise. However, the knowledge of mechanisms modulating the levels of TSPs is fragmented and incomplete. This review discusses the known mechanisms of regulation of TSP levels and the gaps in our knowledge that prevent us from developing strategies to modulate the expression of these physiologically important proteins.     

Modulation of the extracellular matrix patterning of thrombospondins by actin dynamics and thrombospondin oligomer state

Thrombospondins (TSPs) are evolutionarily-conserved, secreted glycoproteins that interact with cell surfaces and extracellular matrix (ECM) and have complex roles in cell interactions. Unlike the structural components of the ECM that form networks or fibrils, TSPs are deposited into ECM as arrays of nanoscale puncta. The cellular and molecular mechanisms for the patterning of TSPs in ECM are poorly understood. In the present study, we investigated whether the mechanisms of TSP patterning in cell-derived ECM involves actin cytoskeletal pathways or TSP oligomer state. From tests of a suite of pharmacological inhibitors of small GTPases, actomyosin-based contractility, or actin microfilament integrity and dynamics, cytochalasin D and jasplakinolide treatment of cells were identified to result in altered ECM patterning of a model TSP1 trimer. The strong effect of cytochalasin D indicated that mechanisms controlling puncta patterning depend on global F-actin dynamics. Similar spatial changes were obtained with endogenous TSPs after cytochalasin D treatment, implicating physiological relevance. Under matched experimental conditions with ectopically-expressed TSPs, the magnitude of the effect was markedly lower for pentameric TSP5 and Drosophila TSP, than for trimeric TSP1 or dimeric Ciona TSPA. To distinguish between the variables of protein sequence or oligomer state, we generated novel, chimeric pentamers of TSP1. These proteins accumulated within ECM at higher levels than TSP1 trimers, yet the effect of cytochalasin D on the spatial distribution of puncta was reduced. These findings introduce a novel concept that F-actin dynamics modulate the patterning of TSPs in ECM and that TSP oligomer state is a key determinant of this process.     

Thrombospondins function as regulators of angiogenesis

Thrombospondins (TSPs) -1 and -2 were among the first protein inhibitors of angiogenesis to be identified, a property that was subsequently attributed to the interactions of sequences in their type I repeats with endothelial cell-surface receptors. The interactions of TSPs-1 and -2 with cell-surface receptors, proteases, growth factors, and other bioactive molecules, coupled with the absence of direct structural functions that can be attributed to these matrix proteins, qualify them for inclusion in the category of ‘matricellular proteins’. The phenotypes of TSP-1, TSP-2, and double TSP-1/2-null mice confirm the roles that these proteins play in the regulation of angiogenesis, and provide clues to some of the other important functions of these multi-domain proteins. One of these functions is the ability of TSP-1 to activate the latent TGFβ1 complex, a property that is not shared by TSP-2. A major pathway by which TSP1 or TSP2 inhibits angiogenesis involves an interaction with CD 36 on endothelial cells, which leads to apoptosis of both the liganded and adjacent cells. However a homeostatic mechanism, which inhibits endothelial cell proliferation, and may be physiologically preferable under some circumstances, has also been elucidated, and involves interaction with the very low density lipoprotein receptor (VLDLR). The interaction of TSP1with its receptor, CD47, further inhibits angiogenesis by antagonizing nitric oxide signaling in endothelial and vascular smooth muscle cells. Paradoxically, there is also evidence that TSP-1 can function to promote angiogenesis. This apparent contradiction can be explained by the presence of sequences in different domains of the protein that interact with different receptors on endothelial cells. The anti-angiogenic function of TSPs has spurred interest in their use as anti-tumor agents. Currently, peptide mimetics, based on sequences in the type I repeats of TSPs that have been shown to have anti-angiogenic properties, are undergoing clinical testing.     

Thrombospondins in the heart: potential functions in cardiac remodeling

Cardiac remodeling after myocardial injury involves inflammation, angiogenesis, left ventricular hypertrophy and matrix remodeling. Thrombospondins (TSPs) belong to the group of matricellular proteins, which are non-structural extracellular matrix proteins that modulate cell–matrix interactions and cell function in injured tissues or tumors. They interact with different matrix and membrane-bound proteins due to their diverse functional domains. That the expression of TSPs strongly increases during cardiac stress or injury indicates an important role for them during cardiac remodeling. Recently, the protective properties of TSP expression against heart failure have been acknowledged. The current review will focus on the biological role of TSPs in the ischemic and hypertensive heart, and will describe the functional consequences of TSP polymorphisms in cardiac disease.     

Interactions between brain endothelial cells and human T-cell leukemia virus type 1-infected lymphocytes: mechanisms of viral entry into the central nervous system

Human T-cell leukemia virus type 1 (HTLV-1) is associated with a variety of clinical manifestations, including tropical spastic paraparesis or HTLV-1-associated myelopathy (TSP/HAM). Viral detection in the central nervous system (CNS) of TSP/HAM patients demonstrates the ability of HTLV-1 to cross the blood-brain barrier (BBB). To investigate viral entry into the CNS, rat brain capillary endothelial cells were exposed to human lymphocytes chronically infected by HTLV-1 (MT2), to lymphocytes isolated from a seropositive patient, or to a control lymphoblastoid cell line (CEM). An enhanced adhesion to and migration through brain endothelial cells in vitro was observed with HTLV-1-infected lymphocytes. HTLV-1-infected lymphocytes also induced a twofold increase in the paracellular permeability of the endothelial monolayer. These effects were associated with an increased production of tumor necrosis factor alpha by HTLV-1-infected lymphocytes in the presence of brain endothelial cells. Ultrastructural analysis showed that contact between endothelial cells and HTLV-1-infected lymphocytes resulted in a massive and rapid budding of virions from lymphocytes, followed by their internalization into vesicles by brain endothelial cells and apparent release onto the basolateral side, suggesting that viral particles may cross the BBB using the transcytotic pathway. Our study also demonstrates that cell-cell fusion occurs between HTLV-1-infected lymphocytes and brain endothelial cells, with the latter being susceptible to transient HTLV-1 infection. These aspects may help us to understand the pathogenic mechanisms associated with neurological diseases induced by HTLV-1 infection.     

Thrombospondins: structure and regulation of expression.

Thrombospondin (TSP) is a large, trimeric, modular glycoprotein that is a major constituent of platelet alpha granules. TSP is also secreted by a wide variety of epithelial and mesenchymal cells in patterns that reflect developmental changes in the embryo and response to injury in the adult. In addition to its role in blood coagulation, TSP has been reported to serve both adhesive and anti-adhesive functions, to foster neurite outgrowth, stimulate and inhibit cell growth and migration, and inhibit angiogenesis. Although this diversity in apparent function can be attributed, in part, to the ability of a single TSP to interact with several different cell-surface receptors, it is now known that the TSPs are encoded by at least three homologous genes in both human and mouse. TSP1, the commonly recognized protein isolated from platelets, is similar to TSP2 in structure. Both proteins contain NH2-terminal, COOH-terminal, and procollagen homology domains, and type I (TSP or properdin), type II (EGF-like), and type III (Ca(2+)-binding) repeats. However, the two TSPs differ in amino acid sequence and in the regulation of their expression. TSP1 is rapidly induced by serum and growth factors. An SRE and a binding site for NF-Y have been shown to mediate the serum response of the human TSP1 gene. On the other hand, TSP2 is far less responsive to serum than TSP1 and lacks the promoter elements that mediate the serum responsiveness of TSP1. TSP3 resembles TSP1 and TSP2 in its COOH-terminal domain and type III repeats, but contains four rather than three type II repeats and lacks type I repeats and a procollagen homology. The NH2-terminal domain of TSP3 also differs from that of either TSP1 or TSP2. All three TSPs demonstrate characteristic patterns of expression in the developing and adult mouse. It is therefore likely that each protein subserves a discrete function. In the future it will be necessary to distinguish among the three TSPs in addressing the function of these proteins.     

Thrombospondins use the VLDL receptor and a nonapoptotic pathway to inhibit cell division in microvascular endothelial cells

TSPs 1 and 2 function as endogenous inhibitors of angiogenesis. Although thrombospondins (TSPs) have been shown to induce apoptosis in HMVECs, we reasoned that a homeostatic mechanism would also be needed to inhibit EC growth without causing cell death, e.g., in the maintenance of a normal vascular endothelium. HMVECs, cultured in low serum, responded to VEGF with an increase in [(3)H]thymidine incorporation that was inhibited by TSPs and was accompanied by decreases in the phosphorylation of Akt and MAPK, without an increase in apoptosis. RAP, an inhibitor of the low-density lipoprotein (LDL) family of endocytic receptors, and blocking antibodies to VLDLR were as effective as TSPs in the inhibition of thymidine uptake in response to VEGF, and the effects of these agents were not additive. Supportive evidence for the role of the VLDLR in mediating this inhibition was provided by the demonstration of a high-affinity interaction between TSPs and the VLDLR. We propose that TSP1 and TSP2, together with the VLDLR, initiate a nonapoptotic pathway for maintenance of the normal adult vascular endothelium in a quiescent state, similar to that invoked for the regulation of mitogenesis by PDGF, but involving signaling via the VLDLR rather than LRP1.     

Biological Mechanisms of Physical Activity in Preventing Cognitive Decline

In order to guarantee better conditions for competition, the nervous system has developed not only mechanisms controlling muscle effectors, but also retrograde systems that, starting from peripheral structures, may influence brain functions. Under such perspective, physical activity could play an important role in influencing cognitive brain functions including learning and memory. The results of epidemiological studies (cross-sectional, prospective and retrospective) support a positive relationship between cognition and physical activities. Recent meta-analysis confirmed a significant effect of exercise on cognitive functions. However, the biological mechanisms that underlie such beneficial effects are still to be completely elucidated. They include supramolecular mechanisms (e.g. neurogenesis, synaptogenesis, and angiogenesis) which, in turn, are controlled by molecular mechanisms, such as BDNF, IGF-1, hormone and second messengers.     

Control of cell motility by interaction of gangliosides, tetraspanins, and epidermal growth factor receptor in A431 versus KB epidermoid tumor cells

Growth of epidermoid carcinoma cell lines, A431 and KB, has been known to be controlled by the interaction of epidermal growth factor (EGF) and its receptor (EGFR) with tyrosine kinase. Ganglioside GM3 was previously found to interact with EGFR and to inhibit EGFR tyrosine kinase. However, motility of these cells, controlled by EGFR and ganglioside, was not studied. The present study is focused on the control mechanism of the motility of these cells through interaction of ganglioside, tetraspanin (TSP), and EGFR. Key results are as follows: (i) The level of EGFR expressed in A431 cells is 6 times higher than that expressed in KB cells, and motility of A431 cells is also much higher than that of KB cells, yet growth of A431 cells is either not affected or is inhibited by EGF. In contrast, growth of KB cells is enhanced by EGF. (ii) Levels of TSPs (CD9, CD82, and CD81) expressed in A431 cells are much higher than those expressed in KB cells, and TSPs expressed in A431 cells are reduced by treatment of cells with EtDO-P4, which inhibits the synthesis of glycosphingolipids (GSLs) and gangliosides. (iii) These TSPs are co-immunoprecipitated with EGFR in both A431 and KB cells, indicating that TSPs are closely associated with EGFR. (iv) High motility of A431 cells is greatly reduced, while low motility of KB cells is not affected, by treatment of cells with EtDO-P4. These results, taken together, suggest that there is a close correlation between high motility of A431 cells and high expression of EGFR and TSPs, and between ganglioside GM3/GM2 and TSP. A similar correlation was suggested between the low motility of KB cells and low levels of EGFR and TSP. The correlation between high motility and high level of EGFR with the ganglioside–TSP complex in A431 cells is unique. This is in contrast to our previous studies that indicate that motility of many types of tumor cells is inhibited by a high level of CD9 or CD82, together with growth factor receptors and integrins.     

Gap junctions regulate the development of neural circuits in the neocortex

Gap junctions between cells are ubiquitously expressed in the developing brain. They are involved in major steps of neocortical development, including neurogenesis, cell migration, synaptogenesis, and neural circuit formation, and have been implicated in cortical column formation. Dysfunctional gap junctions can contribute to or even cause a variety of brain diseases. Although the role of gap junctions in neocortical development is better known, a comprehensive understanding of their functions is far from complete. Here we explore several critical open questions surrounding gap junctions and their involvement in neural circuit development. Addressing them will greatly impact our understanding of the fundamental mechanisms of neocortical structure and function as well as the etiology of brain disease.     

The role of thrombospondins 1 and 2 in the regulation of cell-matrix interactions, collagen fibril formation, and the response to injury

Thrombospondins (TSPs) 1 and 2 are extracellular modular glycoproteins that are best known for their anti-angiogenic properties and their ability to modulate cell-matrix interactions. However, these proteins, and in particular TSP2, are pleiotropic in function and affect processes as disparate as bone growth and hemostasis. In recognition of their ability to influence a wide variety of cell functions, and in the absence of convincing evidence for their participation as integral components of extracellular structures, the term 'matricellular' has been applied to these and a small group of functionally related proteins. In this review, we focus on the role of TSP1 and 2 in two forms of injury in mice, excisional skin wounds and subcutaneously implanted biomaterials, and take advantage of mice with targeted disruptions of one or both genes to identify likely biochemical mechanisms that could account for the characteristics of the injury response in these knockout mice. In work that stems largely from our own laboratory, we show that pericellular levels of the matrix metalloproteinase, MMP2, are controlled to a large extent by TSP2 (and potentially also by TSP1), and that elevated levels of MMP2 are likely to account in part for defects as diverse as reduced cellular adhesion, abnormal collagen fibril structure, and increased endothelial cell and vascular proliferation.     

The role of thrombospondins in wound healing, ischemia, and the foreign body reaction

Thrombospondin (TSP) 1 and TSP2 have been implicated in the regulation of several processes during tissue repair. Due to their matricellular nature, these proteins are thought to modulate cell-matrix interactions through a variety of mechanisms specific to the spatio-temporal context of their expression. Most notably, TSP1 and TSP2 appear to play distinct, non-overlapping roles in the healing of skin wounds. In contrast, both proteins have been implicated as regulators of ischemia-induced angiogenesis. Moreover, TSP2 has been shown to be a critical regulator of angiogenesis in the foreign body response (FBR). In this review, we discuss the role of TSPs in tissue repair and examine the mechanistic data regarding the ability of the thrombospondins to modulate cell-matrix interactions in this context.     

Assessment of temporal resolution of multi-detector row computed tomography in helical acquisition mode using the impulse method

The purpose of this study was to propose a method for assessing the temporal resolution (TR) of multi-detector row computed tomography (CT) (MDCT) in the helical acquisition mode using temporal impulse signals generated by a metal ball passing through the acquisition plane. An 11-mm diameter metal ball was shot along the central axis at approximately 5 m/s during a helical acquisition, and the temporal sensitivity profile (TSP) was measured from the streak image intensities in the reconstructed helical CT images. To assess the validity, we compared the measured and theoretical TSPs for the 4-channel modes of two MDCT systems. A 64-channel MDCT system was used to compare TSPs and image quality of a motion phantom for the pitch factors P of 0.6, 0.8, 1.0 and 1.2 with a rotation time R of 0.5 s, and for two R/P combinations of 0.5/1.2 and 0.33/0.8. Moreover, the temporal transfer functions (TFs) were calculated from the obtained TSPs. The measured and theoretical TSPs showed perfect agreement. The TSP narrowed with an increase in the pitch factor. The image sharpness of the 0.33/0.8 combination was inferior to that of the 0.5/1.2 combination, despite their almost identical full width at tenth maximum values. The temporal TFs quantitatively confirmed these differences. The TSP results demonstrated that the TR in the helical acquisition mode significantly depended on the pitch factor as well as the rotation time, and the pitch factor and reconstruction algorithm affected the TSP shape.     

Neuregulin／Erb信号转导通路在神经发育中的作用

Neuregulins(NRG)是一在结构上相类似的多肽家族,它们由四个不同的基因编码。NRG有三种异构体（NRG1、2和3）。这些异构体在及脑内有高表达,包括发育中的脑和成熟的脑。这些异构体中,对NRG1研究最为广泛。NRG1对神经系统的发育有多种重要的调节作用。它能促进胶质细胞的生化和分化;也能调节小脑、颗粒细胞沿胶质细胞的迁移。在突触形成过程中,NRG1可诱导以下三种受体的表达：神经肌肉接头和中枢 神经系统内的乙酰胆碱受体的表达;小脑胶质细胞中NMDA受体的NR2C亚单位的表达;小脑胶质细胞中GABAA受体的表达。近年来的研究表明,NRG受体多分布于突触后膜致密区,提示NRG可能对突触的可塑性有重要的作用。本文综述了NRG的研究进展,特别对其功能及其信号转导机制有较多的讨论。     

Dynamic interplay between adhesion surfaces in carcinomas:Cell-cell and cell-matrix crosstalk

Cell-cell and cell-matrix signaling and communication between adhesion sites involve mechanisms which are required for cellular functions during normal development and homeostasis; however these cellular functions and mechanisms are often deregulated in cancer. Aberrant signaling at cell-cell and cell-matrix adhesion sites often involves downstream mediators including Rho GTPases and tyrosine kinases. This review discusses these molecules as putative mediators of cellular crosstalk between cell-cell and cell-matrix adhesion sites, in addition to their attractiveness as therapeutic targets in cancer. Interestingly, inter-junctional crosstalk mechanisms are frequently typified by the way in which bacterial and viral pathogens opportunistically infect or intoxicate mammalian cells. This review therefore also discusses the concept of learning from pathogen-host interaction studies to better understand coordinated communication between cell-cell and cell-matrix adhesion sites, in addition to highlighting the potential therapeutic usefulness of exploiting pathogens or their products to tap into inter-junctional crosstalk. Taken together, we feel that increased knowledge around mechanisms of cell-cell and cell-matrix adhesion site crosstalk and consequently a greater understanding of their therapeutic targeting offers a unique opportunity to contribute to the emerging molecular revolution in cancer biology.